reinforced plastics handbook || reinforcements

Reinforcements

Overview

Many combinations of reinforcements and plastics are used by the plastic industry to affect a diversity of performance and cost characteristics. These may be in layered form, as in typical thermoset (TS) polyester impregnated glass fiber mat, fabric and melamine-phenolic impregnated paper sheets, or molding compound form such as in glass fiber or cotton-filled/TS polyester, phenolic, urea, or nylon RPs. Inline compounds are prepared by injection molding or extruding with short and long glass (and other) fibers. As an example, chopped glass fibers (rovings, etc.) can be fed into an injection-molding machine or a single to twin-screw extruder where principally TP is melted and bonded to the fibers providing an excellent mix. All these resulting plastic RPs have many properties superior to the component materials (Chapter 4).

Reinforcements can significantly improve the structural characteristics of a TP or TS plastics. They are available in continuous forms and chopped forms having different lengths, or discontinuous in form (whiskers, flakes, spheres, etc.) to meet different properties and/or processing methods. Glass fiber represents the major material used in RPs worldwide. Others provide higher structural performances, etc. The reinforcements can allow the RP materials to be tailored to the design, or the design tailored to the material (Figures 2.1 and 2.2 and Tables 2.1 to 2.3).

The large-production reinforcing fibers used today are glass, cotton, cellulosic fiber, sisal, jute, and nylon. Specialty reinforcing fibers are carbon, graphite, boron, aramid, whiskers, and steel. They all offer wide variations in properties, weight, and cost.

2. Reinforcements 25

Figure 2, I Comparison of specific strength vs. specific modulus of RPs. Specific properties are normalized by RP density (Pa or N/m 3 divided by kg/m 3)

S.esl

Epoxy 60% c a d ~

Steel I~polly ~en

Aluminum

Epoxy

Epoxy

Su~in

Figure 2,2 Tensile stress-strain curves for different fiber/epoxy and aluminum and steel materials

Fibers in RPs are primarily used to reinforce a resin by transferring the stress under an applied load from the weaker resin matrix to the much stronger fiber. Plastics provide valuable and versatile materials for use as matrices, but other materials, such as metals, ceramics, and cements, are

Table 2.1 Properties of synthetic and natural-inorganic or organic and metallic fibers

Diameter, Fiber Sp. Gr. Length, in. IJ.

Tensile strength

x 10 -3, PSI

Modulus of elasticity

x 10 -6, PSI

Heat resistance,

o F

Coeff.of linear

expansion

a ~

Synthetic-lnorganic Conventional glass (Type E) 2.6 _a,~ 5-15

Beryllium glass 2.6 _a,~ 5-15 Quartz (fused silica) 2.2 _a,b 8-10 Carbon 1.8 _a,~ 1-100 Aluminum silicate 2.7 up to 10 2-20

3.9 Graphite 1.6 up to 4 2-30

2.2 Rock wool 2.8 up to 4 1-22

Natural-inorganic asbestos 2.5 up to 4 I -3

Metals and refractories Steel 7.8 _a,b 1-25 Aluminum 2.8 _a,~ 4-20 Tungsten 19.3 up to I 20 Tantalum 16.6 up to 0.5 5 Molybdenum 10.2 up to 0.5 5-20 Magnesium 1.8 _a,~ 6-15

Synthetic-organic Fluorocarbon 2.2 _a,b 20 Polyester 1.4 _a,~ 10-25 Acrylic 1.2 _a,~ 10-25 Polyamide 1.1 _a,~ 10-40 Cellulose acetate 1.3 _a,~ 11-44 Regenerated cellulose (rayon) 1.5 _a,~ 10-40

Natural organic Cotton 1.6 up to 2 17 Sisal 1.3 up to 24 19 Wool 1.3 up to 15 28

4OO

280 100-350 20

100-600

2-20

0.02

200-400 60-90

200 70-90

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47 100 50 70-120 25 30-105

50-100 120 29

10.5

12-20 10-25

1-4 2-15

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100-200

20-30 10 58 28 42

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600 c 1500 d

1500 d

3500 c 6200 e 3300 d

6764 f

2800 d

2770 d

2920 e 1212 e 6150 e 5390 e 4700 e 1200 e

525 f 480 e 4 5 0 e

480 e 500 e 400 d

275g 212g 212g

2.8

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28 Reinforced Plastics Handbook

Table 2.3 Cost comparison of type fibers and mechanical properties (glass cost = 100)

Structural E-glass Carbon (1997) Carbon (2000) requirement

Weight Cost Weight Cost Weight Cost

Compressive strength 1000 1.00 419 6.91 419 2.88 Tensile strength 1000 1.00 267 4.40 267 1.84

Tensile modulus 1000 1.00 147 3.14 147 1.30

Source: Reinforced Plastics

also used as matrices for fibrous reinforcement composites. For an efficient RP under stress, the elongation of the fiber must be less, and its stiffness modulus higher, than that of the matrix. Stress transfer along the all-important fiber/matrix interface can be improved by use of sizings, binders, or special coupling agents. The diameter of the fiber also plays an important part in maximizing stress transfer. Smaller diameters give a greater surface area of fiber per unit weight, to aid stress transfer in a given reinforcement context.

Glass Fibers

Glass fibers, the most widely used at over 90% of all reinforcements with TSs or TPs matrixes, arc available in many forms for producing different commercial and industrial products. They also include parts in aircraft to space vehicles, and surface water to underwater vehicles. The older and still most popular form is E-glass. Other forms of glass fiber are used that meet different requirement such as S-glass that produces higher strength properties.

Materials in the form of fibers are often vastly stronger than the same materials in bulk form. Glass fibers, for example may develop tensile strength of 7 MPa (1,000,000 psi) or more under laboratory conditions, and commercial fibers attain strengths of 2,800 to 4.8 MPa (400,000 to 700,000 psi), whereas massive plate glass breaks at stresses of about 7 MPa (1000 psi). The same is true of many other materials whether organic, metallic, or ceramic. Compression wise there are plate glasses that are the strongest of any material (steel, etc.) however very weak under other loads.

Acceptance and use of nonwoven fabrics as reinforcement of structural plastics continues to increase. Theoretically only with nonwoven fiber


sheet structures can the full potential of fiber strength be realized. Great advances have been made in developing new fibers and plastics, in new chemical finishes given to the fiber, in methods of bonding the fiber to the plastic, and in mechanical processing methods. Nonwoven fabrics are inherently better able to take advantage of these developments than are woven products.

Strength of commercial RPs is far below any theoretical strength. Ordinary glass fibers are three times stronger and stiffer for their weight than steel. Nonwoven glass fiber structures usually have strength about 40 to 50% below that of woven fabric lay-ups. In special constructions, properly treated fibers have produced products as strong as the woven product, better in some cases.

RPs are usually applied as laminates of several layers. Many variables are important in determining the performance of the finished product. Some of the important ones arc orientation of plies of the laminate, type of plastic, fiber-plastic ratio, type or types of fibers, and directional orientation of fibers (Chapter 7). Nonwoven fabrics are fibrous sheets made without spinning, weaving, or knitting. They include felts, bonded short to long fiber fabrics, and papers. The interlocking of fibers is achieved by a combination of mechanical work, chemical action, moisture, and heat by either textile or paper malting processes.

Still stronger and stiffer forms of fibrous materials are the unidirectional crystals called whiskers. Under favorable conditions, crystal-forming materials will crystallize as extremely fine filamentous single crystals a few microns in diameter and virtually free of the imperfections found in ordinary crystals. Whiskers are far stronger and stiffer than the same material in bulk form. To date their use is limited principally due to special handling requirement during fabrication into RPs and cost.

Fine filaments or fibers by themselves have limited engineering use. They need support to hold them in place in a structure or device. This is accomplished by embedding the fibers in a continuous supporting matrix (plastic) sufficiently rigid to hold its shape, to prevent buclding and collapse of the fibers, and to transmit stress from fiber to fiber. The matrix may be, and usually is, considerably weaker, of lower elastic modulus, and of lower density than the fibers. By itself, it would not withstand high stresses. When fibers and matrix are combined into a plastic composite, a synergistic effect occurs; combination of high strength, rigidity, and toughness frequently emerges that far exceed the properties in the individual constituents.

Glass fibers are a family of short (staple, chopped, milled), long chopped, or continuous fiber reinforcement, used widely with both TSs


and TPs for increased strength, dimensional stability, thermal stability, corrosion resistance, dielectric properties, etc. (Figures 2.3 and 2.4).

~IL HIGHER ~ ~ J CONCENTRATION I /

/ MORE ~ / ORIENTATI~~

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~ 1 GER BETTER T FIBERS ~~176

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~ , , 0.01 O. I I.O IO

FIBER LENGTHS (mm)

Figure 2.3 Short to long fibers influence properties of RPs

Flexural modulus, million psi

4.

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(~0J 0

0 t~5~ 0 IdOl

0 c55~

c401 D O (4o)

Long-fiber compounds

P" Ops' O"y'~ % Gloss shown in porenlheses

I i

notched Izod Impact Strength, (ft-lb/in.)

10

Figure 2.4 Mechanical properties of short and long fiber/thermoplastic compounds (BMCs)

Glass fibers have high tensile strength combined with low extensibility (3.5%), giving exceptional tensile, compression and impact properties, with a relatively high modulus of elasticity and good bend strength. It also has high temperature resistance and low moisture pick-up, giving good dimensional stability and weather resistance. Finally, low moisture

2 �9 R e i n f o r c e m e n t s 31

absorption makes it possible to produce moldings with good electrical properties that do not deteriorate, even under adverse weather conditions. However proper bond between fiber and resin matrix has to exist otherwise properties of the RP will be significantly reduced or destroyed with the migration of moisture between the fiber and resin.

The fiber also exhibits virtually elastic behavior. It will stretch uniformly under stress to its breaking point without yielding and, on removal of the tensile load short of breaking point, the fiber will return to its original length. This lack of hysteresis (which is not found in conventional metal and organic fibers), together with high mechanical strength, makes it possible for glass fiber to store and release large amounts of energy without loss. If protected against abrasion, this capability, together with dynamic fatigue resistance is put to effective use in applications such as springs for automobiles, trucks, trailers and furniture.

The fibers are made by the melt drawing of various grades or types to be reviewed (electrical, chemical, high tensile strength, etc.) of glass and are comprised of strands of filaments that can be further processed by size reduction, twisting, or weaving into fabrics or mats (Figure 2.5). The fiber is produced by blending the raw materials (sand, kaolin, limestone and colemanite) and feeding the mix into a batch oven heated to about 1600C. The liquid glass flows into channels and the fibers are drawn through electrically heated bushings, each of which can produce thousands of filaments of 10-24 pm diameters. The filaments are coated with size, to ensure cohesion and protect them from abrasion (also providing properties essential for subsequent processing operations). Finally, the wet fiber is dried and processed into its finished form.

To be effective, the reinforcement must form a strong adhesive bond with the plastics; for certain reinforcements special cleaning, sizing, coupling agents, finishing, etc. treatments are used to improve bond. They are often surface modified to provide a special property such as electrical conductivity (by coating with nickel, etc.). Also used alone or in conjunction with fiber surface treatments are bonding additives in the plastic to promote good adhesion of the fiber to the plastic.

Different types of reinforcement construction are used to meet different RP properties and/or simplify reinforcement layup fabricating processes to meet design performance shape requirements. They include woven, nonwoven, rovings, prcforms, and others. These different constructions are used to provide different processing and directional properties.


Production

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Sizing

Basic fiber linear weight 2.5 to 4800 tex

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To Weaving and Textile Fabrication - or Direct to Wire and Cable Manufacturers

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Figure 2 .5 Condensed and detailed schematics for production of glass fiber


Manufacturers are moving to a policy of making a product of identical specification available anywhere in the world, from a platform of a common technology. There is also an industry-wide initiative to convert the standards for glass from classification by composition to classification by properties.

Long Fibers

In production of injection molding compounds, some amount of mechanical work occurs. If proper equipment is not used, fiber reinforcement is inevitably broken up into very short lengths. Because of this situation, TP molding compounds can contain very short lengths of fiber (typically 0.3 mm). The mechanical properties of the compound are closely related to the length of the reinforcing fiber. With the proper equipment, longer fibers are retained if not complete lengths retained.

In an ideal nylon 6 /6 compound reinforced with 50 wt% glass fiber and with all fibers aligned along the length of the molding, the flexural and tensile moduli increase rapidly as the fiber length is increased from 0.1 mm to 2.0 mm. These are produced, not by classical physical mixing, but by a process analogous to the TS resin pultrusion process, with internal lubrication additives to counteract the chopping effect of injection molding plasticizing screw action (Chapter 5). However, with the proper injection molding technology (Ingersoll-Rand IMPCO machine, etc.) used since at least the 1970s, RTPs can be processed retaining long fiber lengths.

Notched Izod impact strength tests have been conducted on the effect of fiber length in a nylon 6 /6 compound reinforced with 50 wt% glass fiber. It shows that impact strength of reinforced TPs (RTPs) increases significantly, as fiber length is increased. Similar effects have been measured with other fibers, such as aramid and carbon, and with other matrices, such as polypropylene and polyphenylenc sulphide.

The theory has shown that an improvement of some 50% in mechanical properties should be produced by increasing fiber length from 0.3 mm to 2 mm. Several producers and specialist compounders of TPs have long fiber technology using nylons, polypropylenes, TP polyesters, and polyphenylene sulphide (PPS).

As an example LNP Engineering Plastics, USA has been publishing results of tests that include a selection of die-cast metals compared with a 60 wt% long glass fiber reinforced nylon 6 /6 compound (Verton RF700-12EM). The conclusion was that (for moisture-conditioned samples) with a density lower than metals, with the exception of certain


magnesium alloys, a long fiber-reinforced nylon 6 /6 compound shows reasonable tensile strength, high impact, and good elongation. Flexural modulus, however, is low. When values are calculated relative to density, the specific tensile strength of the nylon compound exceeds all alloys tested and the modulus is a little below that of the zinc alloys (Zamak). Shear strength values are close to those of magnesium and aluminum and about twice those of Zamak. Specific impact favors nylon 6 /6 by a factor of nearly seven compared with Zamak, and 30 compared with aluminum.

With long fiberglass reinforcement, the mechanical values of the nylon compound are less influenced by temperature. The effect is seen particularly at low temperatures, where impact strength falls by only 5% at --40C, where Zamak alloys fall from about 60 Joules at room temperature to 5 Joules a t -20C. At elevated temperatures (120-170C), the performance of nylon is at least comparable with that of most of the die-cast alloys. Compared with short fiber reinforced TPs, resistance to creep is outstanding, up to 140C. The LNP report concludes that long fiber TPs offer a viable alternative to metals in many structural applications, at temperatures up to 175C.

Of the long fiber RTPs, polypropylene is the most interesting due to the relatively low cost of the matrix material. A typical range is in the form of 15 mm (0.59 in) chips with 20-50 wt% glass content. Pro- perties include high dimensional stability, low warping, good surface finish and elimination of the usual effects of shrinkage. The compound offers high impact, especially a t -20C to + 30C (-4F to + 86F) and is free of ductile fracture (shatter). Stability at elevated temperature is good: a 40% long glass fiber compound withstands 150C (300F) under 1.8 MPa loading.

Long-fiber nylon 11 and 12 (30/50 wt% glass fiber) shows up to 200% improvement in impact, dimensional stability, good surface appearance, low moisture content and improved abrasion resistance. Table 2.4 provides short- and long-glass fiber/nylon 6 /6 RPs properties at elevated temperatures.

In-Line Compounding

Automobile underbody shields of glass fiber filled polypropylcne (PP) produced on an unusually elaborate, multi-step system for direct molding of long fiber thermoplastics with in-line compounding. Sources at machine builder Dieffenbacher GmbH & Co. in Germany reported this was its most complex installation ever for auto-parts production. The system was delivered to Menzolit-Fibron GmbH of

2 . Reinforcements 35

Table 2.4 Example of short- and long-glass fiber/nylon 6/6 RPs properties at elevated temperatures

Short fiber Long fiber

Property 300/o 50% 30% 50%

At 300~

Tensile strength (10 3 psi) 12.8 13.8 14.3 19.2 Elongation (%) 9.3 7.8 5.3 5.6 Flexural strength (103 psi) 13.8 14.3 17.4 23.7 Flexural modulus (105 psi) 4.64 5.27 5.50 8.90

At 400~

Tensile strength (10 3 psi) 6.3 7.3 7.8 8.3 Elongation{O/o) 8.6 9.5 6.2 6.8 Flexural strength (103 psi) 6.9 7.4 8.9 10.0 Flexural modulus (10 s psi] 3.96 4.80 5.19 7.51

Data on long-fiber glass-reinforced grades are for Verton compounds * To convert psi to pascals (Pa), multiply 6.895 x 103

Bretten, Germany, and on start up fabricated parts for the Volkswagen Golf, AudiA3, and other cars.

The plant starts with twin-screw compounding of PP and glass fiber to produce a hot sheet, which is robotically transferred to a vertical compression press of 3300 tons. The fast-stroke press has automatic parallelism control. Another robot transfers parts to a l l0- ton press that punches out attachment holes in the parts. Finally a third robot transfers parts to a machine that inserts hole reinforcements and inspects the parts.

With double tools, the line can produce two parts every 22 s. Dieffenbacher reports the line is designed to operate on cycles as short as 20 sec. A single supervisory computer oversees the entire process. A barcode printer records critical process data for each molding cycle on each part. (Contact Tel: 519-979-6937, www.dieffenbacher.com)

Long Aligned Discontinuous Fiber Technology LDF (long discontinuous fiber) technology is a proprietary technology, employing reinforcement of long aligned discontinuous fibers, including carbon, aramid, or glass. In high-performance TP matrices, such as the polyether ketone group of materials [polyetheretherketone (PEEK), etc.], aligned, discontinuous fibers provide a drawable feature that can


overcome some of the thermoforming limitations encountered with continuous fiber systems (Table 2.5). Fabrication processes such as stretch forming and press forming can be used. The technology is claimed to be cost-effective in manufacture of complex shape parts of aerospace structures. Composites demonstrate excellent mechanical properties comparable with those of continuous fibre-reinforced products.

Table 2.5 LDF technology applied to aramid/PEEK reinforced thermoplastics

Property Unit LDF technology Continuous fiber

Tensile: Strength (0 ~ MPa 1100 1240 Modulus (0 ~ GPa 725 76 Strength (90 ~ MPa 21 18 Modulus (90 ~ GPa 6.2 5.5

Compressive: Strength (0 ~ MPa 269 255

Flexural: Strength (0 ~ MPa 656 760 Modulus {0 ~ GPa 63 64

Shear: Inplane strength MPa 64 55 Inplane modulus GPa 2.1 2.1 Short beam shear strength MPa 55 55

QBT Technology An advanced reinforcements system identifies QBT. It is the name for a biaxial thermoplastic (PA, PBT, PET, PP/PPS, PE1, APC-2, Radel), carbon, aramid, and glass pre-impregnated tape. It is unidirectional, interlaced in biaxial form, in continuous lengths and very large width (up to 10 ft). It maintains and improved properties of unidirectional cross-ply laminates, giving the benefit of unidirectional tape in larger and more easily processed formats. It is claimed to be first real alternative to a woven composites compromise, with good drape.

Pushtrusion/Injection Molding Processes The Pushtrusion direct inline process for molding small to large products is accomplished by compounding and injection molding (IM) in a single operation long and/or short fibers from continuous fiber reinforcements. Details are reviewed in Chapter 5 Injection Molding.

Pushtrusion/Extrusion Processes The Pushtrusion direct inline process for extruding small to large profile products is accomplished by an in-line, single operation compounding

2 �9 Reinforcements 37

extruder and injection molding (IM) of long and/or short fibers from continuous fiber reinforcements. Details are reviewed in Chapter 5 Extrusion.

Aspect Ratios

Aspect ratio of fibers is applicable to RPs. It is the ratio of length to diameter (L /D) of a fiber (also the ratio of the major to minor axis lengths of a material such as a particle). In RPs fiber L / D will have a direct influence on the RP performance. High values of 5 to 10 provide for good reinforcements. Lining up and overlapping fibers (disks, etc.) takes advantage of directional properties. Theoretically, with proper lay- up the highest performance RPs (using fibers such as glass, graphite and/or boron) could be obtained when compared to other materials. These ratios can be used in determining the effect of dispersed additive fibers and/or particles on the viscosity of a fluid/melt and in turn on the performance of the compound based on L / D ratios.

Woven Constructions

Woven reinforcement material constructed is by interlacing fibers, yarns, or filaments to form such fabric patterns as basket, plain, harness, satin, leno weaves, scrim, etc. These different weaving patterns are used to provide different processing and/or directional properties. There are filling threads that represent threads in the so-called machine direction; warp threads represent those in the transverse direction or at 90 ~ to the filling threads.

Nonwoven Constructions

There are certain types of so-called nonwovcn fabric that are directly formed from short or long chopped fibers as well as continuous filaments that may be in circular or other patterns. They are produced by loosely compressing together fibers, yarns, rovings, etc. with or without a scrim cloth carrier; assembled by mechanical, resin, chemical, thermal, and/or solvent methods. Products of this type include melted and spun-bonded fabrics. The nonwoven spun-bonded integrates the spinning, lay-down, consolidation, and bonding of continuous filaments to form fabrics. Felt is the term used to describe nonwoven compressed fabrics, mats, and bats prepared from staple fibers without spinning, weaving, or knitting; made up of fibers interlocked mechanically.

A fibrous material extensively used in RPs is the mat constructions. It consists of different randomly and uniformly oriented products:


1 chopped fibers with or without carrier fibers or binder plastics

2 short fibers with or without a carrier fabric

3 swirled filaments loosely held together with a plastic binder

4 chopped or short fiber with long fibers included in any desired pattern to provided addition mechanical properties in specific directions

5 chopped or long fibers included in any desired pattern to provided maximum mechanical properties for mats in specific directions;

and so on.

There are reinforcement preform constructions. A preform is a method of making chopped fiber mats of complex shapes that are to be used as reinforcements in different RP molding fabricating processes (hand lay- up, injection, etc.). Oriented fiber patterns can be incorporated in the preforms (Chapter 5).

When conventional flat mats are used, they may tear, wrinkle, or give uneven glass distribution when producing complex shapes. To alleviate this problem, it is necessary to take great care in tailoring the mat and in placing it properly in or on the mold cavity. Otherwise, mats may cause poor products or poor production rates. Preforms are used to overcome these problems. They are slightly more expensive for short production runs. However, they are used when mats are considered impractical, or a relatively high production run exists that offset the higher cost.

Glass for Special Reinforcements

Special types of glass fiber for specific processes (Chapters 4 and 5), such as sheet molding compound (SMC), pultrusion, and reinforcement of thermoplastics, is a marked trend in current development. Examples follow:

For SMC, a needled mat reinforcement from PPG (MatVantage SMC) reduces overall system costs while providing compounders with a product with higher level of uniformity, using expertise in producing needled mat for glass mat thermoplastics. As a prefabricated needled glass fiber mat, it removes a number of subsidiary systems from manufacture of SMC, and the removal of roving adds the benefit of eliminating fuzz and fly issues, strand entanglement problems, and chopper investment and maintenance. It allows reinforcement levels of up to 50 wt%, which could support the use of SMC in structural applications such as bumper beams.


Owens Coming has also developing a structural pigmentable sheet- molding compound roving for general purpose and semi-structural applications.

Also for SMC (and described as the next generation of reinforcement) is Roving 23C from Vetrotex. It is easily chopped with good fiber distribution, fast wet-through, wet-out, and generating low fuzz and fly. It is particularly suited to molding of automobile doors, wings, boot lids, spoilers, cross-car beams and other components.

For wet lay-up, resin transfer molding (RTM) and SCRIMP molding processes, a stitched mat which combines some of the advantages of mat and woven roving (Fabmat from Fiber Glass Industries, USA) improves consistency, enabling processors to maintain good control of laminate weight and thickness. It can be formed to tight radii without any tendency to spring back and, with rapid wetting out, can reduce molding cycle times.

For pultrusion, a heavier roving from Owens Corning (366 high rex type 30) cuts costs. It has a nominal weight of 9600 tex, doubling the weight of any previously available single end roving, so that the product requires less than half the number of ends to produce a part of equal glass content.

Flexible knitted reinforcement for complex shapes Syncoflex (from Syncoglas), provides a regular strength distribution across the whole surface (virtually equal values in all directions). The reinforcement has many holes allowing quick and thorough wetting out.

For TPs, improved chopped strand glass fiber grades for reinforcement of nylon and TP polyester (PET) are from PPG Industries, in response to the increasing use of glass fiber-reinforced thermoplastic RPs, in automotive components especially. They include:

Type 3660 chopped strand, for use with hydrolysis-resistant nylon 66 compounds, aimed particularly at glass-reinforced nylons for radiator and other under-bonnet automotive applications. It has improved elongation and toughness after exposure to ethylene glycol engine coolant, giving up to 10-30% higher impact resistance in impact-modified compounds.

Type 3563 chopped strand arises from a breakthrough in sizing, giving 50% better resistance to generation of fuzz during transit, while superior strand integrity allows the fiber to be conveyed horizontally for more than 130 m in a dense-phase conveying system. It is suitable for PET grades including glass filled, glass/mineral (where a 20% improvement in impact is claimed), and high-glass


content, flame-retardant, and impact-modified. The novel sizing chemistry also makes it a candidate for polymer blends and alloys containing compatibilizers.

High strength and brilliant whiteness is provided by PPG glass fiber reinforcement with PE Maxichop 3298 chopped strand uses new sizing chemistry that gives color control during compounding and molding. By maintaining whiteness during processing, the amount of white pigment required is substantially reduced, allowing mechanical properties to be maintained.

Fiber-directed preforms development, where new glass fiber reinforcements have been designed specifically for liquid RP molding applications such as resin transfer molding (RTM) with TS polyesters. A new E-glass roving gives fast wet-out with TS polyester, vinyl ester, epoxy, phenolic, urethane, and furan resin systems, due to low sizing content. Low loss on ignition means that there is not an excessive amount of sizing to be broken down by the styrene in the resin.

Fiber reinforcement ofpolypropylene (PP) has attracted a number of developments, both for injection molding compounds and for glass mat TP products. For molding compounds, a new range of fiber products for homopolymer and chemically coupled PP has been introduced by Owens Corning under the name Cratec 146A chopped strand. It aims to enhance the properties of the RP and improve its whiteness, while maintaining a good balance of cost and performance. In chemically coupled PP RPs, it is possible to improve properties with the same or a lower amount of coupling agent. Glass fiber in both 14 and 17 micron diameters is available (but the company's research so far shows little difference between them in PP compounds).

Other new forms of glass reinforcement (from Chomarat) include Rovimat (woven roving/chopped fiber fabric), Aramat (glass/ aramid hybrid), Diagonap (muhiaxial fabric for isotropic strength where required, such as in windmill blades) and Rovicore (woven fabric/chopped fiber fabric for closed molding). Rovicore is a sandwich of chopped strand or woven mat with a core layer of large diameter unidirectional glass fibers that can easily be stretched around shapes, to eliminate preforming. A modification is Rovicore mat, a nonwoven core of large diameter fibers in the sandwiched construction without chemical binder or woven mat, the whole sandwich mechanically stitched together. It has high deformability and a surface veil gives good finish, while the molded part has good


stiffness from the sandwich structure. It is particularly suitable for RTM and vacuum molding.

Glass, Silica, Quartz Fibers

The term high silica is used to describe any high-purity glass. For use in RPs, it is at least 95% pure silicon dioxide (SiO2) produced by a leaching process; glass fiber, with a silica content of 65%, is subjected to a hot-acid treatment that removes virtually all the impurities while leaving the silica in tact. High silica fibers and fabrics are flexible materials that are similar in appearance as conventional E-glass fibers. Quartz, somewhat similar to glass and high silica, has 99.95% SiO2.

High silica and quartz are both used in a wide variety of similar products. The selection of what type to use is generally dictated by a combination of performance requirements, manufacturing needs, and cost. Quartz has about five times the tensile strength of high silica and both have similar thermal characteristics. The major difference is the higher melt viscosity of quartz because of its higher silica content. Both do not melt and vaporize until a temperature exceeds 3000F (1649C). At continuous temperature in excess of 1800F (982C), both forms will begin to denitrify into a crystallized form known as cristobalite. This conversion tends to stiffen the materials, but causes no change in their physical form. Their products can be heated to 2000F (1093C) and rapidly quench in water without any apparent change.

Glass Characteristics

In designing fibrous-RPs it is necessary to take into account the combined actions of the fiber and the plastic. At times, the combination can be considered homogeneous; in many cases, homogeneity cannot be assumed. Information on glass fiber compositions, behaviors, properties, and terminology follows:

Glass, borate A glass in which the essential glass former is boron oxide instead of silica

Glass chopper A chopper gun cuts long glass fibers into strands and shorter fibers to be used as reinforcements in preforms, spray, etc.

Glass cloth Woven glass fiber material.

Glass collet The drive wheel that pulls glass fibers from the bushing/spinneret; a forming tube is placed on the collet and a package of strand is wound upon the tube.


Glass composition Glass is an inorganic product of fusion that has cooled to a rigid condition without crystallizing. Glass is typically hard and relatively brittle, and has a conchoidal fracture. It contains the most abundant elements of the earth that is sand. Although basically a ceramic product, glass is an amorphous inorganic plastic. Glass is always used in its elastic range; below its glass transition temperature (Tg). Most glass is based on the silicate system and is made from the three major constituents of silica (SiO), lime (CaCO3), and sodium carbonate (NaCO3). Various oxides are added to tailor the glass to meet specific requirements. Families of glass include soda- lime (most common: windows, bottles, drinking glasses, etc.), borosilicate (thermal and chemical shock resistance), lead-alkali (optical applications, better electrical), alumino-silicate (high operating heat), silica (formable), and fused silica (high properties; expensive).

Glass, continuous Also called long glass fiber, continuous strand roving, continuous roving, or continuous glass roving. They are strands of filaments (rovings) that can be twisted and used alone or in many different configurations for use in reinforcing plastics and elastomers.

Glass devitrification Formation of crystals (seeds) in a glass melt, usually occurring when the melt is too cold. These crystals can appear as defects in glass fibers.

Glass devitrified Glass with controlled crystallization.

Glass fiber Also called glass fiber and Owens-Corning trade name Fiberglas TM. Glass fibers represent the major material used in RPs. They are a family of short (staple, chopped, milled) or continuous fiber reinforcement, used widely with both TSs and TPs for increased strength, dimensional stability, thermal stability, corrosion resistance, dielectric properties, etc. Available in different forms such as mat, fabric, roving, etc. The fibers are made by the melt drawing of various grades (electrical, chemical, high tensile strength, etc.) of glass and are comprised of strands of filaments (rovings) that can be further processed by size reduction, twisting, or weaving into fabrics or mats. They are often surface modified with coupling agents to improve bonding with plastic matrix and/or to impart special properties such as electrical conductivity (by coating with nickel).

Glass fiber, bare It is glass fiber from which the sizing or finish have been removed. It is also the glass fibers prior to the application of sizing or finishing.


Glass fiber, bilobe Fibers of non-round cross section are prepared by different methods in various geometries. The shapes provide a different fiber packing.

Glass fiber b inder /s iz ing coupling agent This treatment is used on different types of fibers to provide meeting their specific requirements such as bonding capabilities and/or protection of fibers. A major requirement for these agents involves treating glass fibers. Continuous glass fiber (as well as other fiber) strands intended for weaving are treated at their forming bushing during their manufacture with starch-oil binders. These binders protect the fibers from damage by lubrication during their formation and such subsequent textile operations as twisting, plying, and weaving. Usually they are satisfactory when used with TPs but are not compatible with most TSs. The hydrophilic character of the binders allows moisture to penetrate the glass-plastic interface, which leads to degradation of TS-RPs in wet and humid environments. Binder is removed via heat treatment before being used with these plastics. This is accomplished by exposing the reinforcing material (fabric, etc.) to carefully controlled time-temperature cycles. To protect the weak heat- cleaned fibers, chemical sizing coupling agents are used such as methacrylic chromic chloride complex, organosilanes, etc.

Glass fiber bushing The spinneret platinum unit through which molten glass is drawn in making glass filament.

Glass fiber cheese A supply of glass fiber wound into a cylindrical mass.

Glass fiber chopped Fiber glass can be cut to deferent lengths of fibers or produced into short fibers. Their length can range from milled to any short length with the usual about 1/8 to 2 in. (3.2 to 50 mm) or to long lengths of 2 to 5 in. (5 to 12.7 cm) for use in molded RP products. See Glass fiber milled.

Glass fiber, continuous Also called long glass fiber, continuous strand roving, continuous roving, or continuous glass roving. They are strands of filaments (rovings) that can be twisted and used alone or in many different configurations for use in reinforcing plastics and elastomers..

Glass fiber diameter The industry standard provides for the RP industry letter designations that range from about 1.5 to 5.1 x 10 -4 in (3.8 to 13 l~m).

Glass fiber forming package A single glass strand gathered on a thin- wall paper or plastic tube to be used in manufacture.


Glass fiber milled Also called milled glass filler. Usually produced by hammer milling equipment continuous glass ends into very short fibers ranging from 1/64 to 1/4 in. (0.40 to 6.35 mm). They produce lower stiffness and strength than chopped fiber, but controlling heat distortion and improving surface finish. Use includes alone or as an additive in liquid component mixing/injection processes such as reinforced reaction injection molding (RRIM) of polyurethanes (Chapter 5). Used also as anti-crazing reinforcing fillers for plastics and adhesives. Because of fiber-length and lightness, they can cause dust and irritation in the production shop and should only be handled in closed systems.

Glass fiber production Both continuous and staple fibers are manufactured by the same basic process up until fiber drawing. Temperature of glass melt and actual production method vary depending on glass composition; generally about 1260C (2300F) with melts extruding through platinum multi-opening bushings (spinnerets). Two principal manufacturing processes are used namely the glass marble (batch) method or the direct melt method. See Glass Fibers in this chapter.

Glass fiber slug A particle of glass sometimes taking the form of a glass bead, which is imperfection in glass fibers.

Glass fiber tempered Glass with surface compressive stresses induced by heat treatment, resulting in toughened glass.

Glass fiber texturizing For special applications, fibers are subjected to a jet of air impinging on their surfaces, which causes random controlled breakage or fluffing of their surfaces. Although mechanical damage occurs weakening the fibers, its bulkiness allows greater plastic absorption.

Glass fiber types

A-glass (alkali): the original type, a high alkali-content material, with a chemical composition similar to that of window glass; it has been largely replaced by other forms.

C-glass (chemical): a grade with improved resistance to chemical attack, mainly used for surface tissue.

D-glass (dielectric): particularly good dielectric characteristics and used mainly in the electronics industry.

E-glass (electrical): a calcium-alumino-borosilicate composition, low in alkali content and stronger than A-glass. This is regarded as the

2 �9 Re in forcements 4 5

pioneer type and is usually specified for reinforcement of plastics, unless operating stresses are relatively low (Ta Hanser 2.16 p70++). It has good tensile and compressive strength and stiffness, good electrical properties and relatively low cost, but impact resistance is relatively poor.

Although E-glass fiber is widely used for its high strength/cost ratio, glass generally is not totally inert in chemically corrosive environments and many design codes require a corrosion barrier or liner to be incorporated in a laminate, to protect the structural integrity of the glass-reinforced substrate. This usually consists of a resin-rich layer supported by C-glass, or an organic fiber veil such as polyester or acrylic, to act as an impermeable protective layer. Sustained stresses and corrosive attack by strong acids or alkalis act synergistically, gradually deteriorating E-glass fibers (Table 2.6 and 2.7).

Table 2.6 Mechanical properties of E-, S-, and (3- glass fiber RPs

High temperature, Chemical

Physical properties Electrical 'E' 'S'-glass 'C'

Tensile strength (psi) at 80~ 500,000 650,000 450,000 at 500~ 430,000 610,000 -

at IO00~ 250,000 353,000 - Modulus of elasticity, x 106 psi

at 80~ 10.5 12.33 - Density (grams/cm3) at 80~ 2.55 2.49 2.49 Coefficient of thermal expansion

-l inear (F) 2.8 x 10 -6 - 4.0 x 10 -6 Coefficient of thermal

Conductivi ty-

Btu-in. 6-6.4 - 7-7.3 hours-square feet-~

Specific heat (bulk glass) at 80~ 0.185 - 0.188 at 500~ 0.244 - 0.52 at IO00~ 0.275 - 0.290

Index of refraction at 80~ 1.547 1.523 1.54 Dielectric constant

at 104 cycles 6.4-6.5 - 7.2-7.5 at 101~ cycles 6.1-6.4 5.6 6.8-6.9

Dissipation factor at 104 cycles 0.001-0.002 - 0.008-0.009 at 10 l~ cycles 0.005-0.006 - 0.010-0.013

Volume resistivity (Ohms/cc) at 72~ 2 - 5 x 1012 - - at 1320~ 107 - - at 1600~ 105 - - at 2300~ 102 - -

Table 2.7 Thermal and electric properties of E-glass fiber RPs

4~

m ,

Plastic Glass-fiber

content, wt O/o

Heat deflection temp at 1.7 MPa a,

~ D 648

Coefficient of linear thermal

expansion, 10 -5 cm/cm/~

D 696

Maximum temp,

continuous use, ~

Water absorption,

24h, O/o, D570

Volume resistivity,

cm,

D257

Dielectric strength

dry, V/pm, D 149

Mold shrinkage,

cm/cm, D 955

"-h 0

, , m ,

-!-

ABS

Acetal

Nylon-6

Nylon-6,6

Nylo n-6,12

Polycarbonate

Polyester,

thermoplastic

Polyethylene

10

20

30

10

30

15

30

13

30

30

10

30

30

10

30

98

99

100

124

163

196

204

246

252

199

138

143

213

110

124

4.1

3.8

3.1

5.2

4.3

3.1

2.7

2.7

2.3

2.2

3.2

2.3

2.5

5.4

3.8

77

82

82

110

127

93

110

107

127

110

127

132

121

82

93

0.3

0.3

0.2

0.22

0.2

1.8

1.3

1.0

0.9

0.2

0.14

0.12

0.06

0.08

0.06

1015

1016

1016

10 TM

10 TM

1015

1015

1016

1016

1013

1015

1015

1016

1016

1016

17.7

18.3

18.9

20.0

18.9

16.5

16.1

20.9

19.7

19.7

17.3

18.9

23.6

26.8

24.0

0.003

0.002

0.002

0.006

0.005

0.007

O.O04

0.007

0.004

0.004

0.005

O.OO3

0.003

0.005

0.003

0 0


E-CR glass (corrosion-resistant): developed to meet the demand for improvement in long-term resistance to chemicals. The resistance to acid corrosion is significantly better than that of E-glass although its composition does not differ greatly, the main difference being that it does not contain boron oxide. It is listed (in ASTM D578 and ISO 2078) for improved resistance to acidic corrosion and, under DIN 1259, is classified as aluminum-lime-silicate glass, which is particularly designed for reinforcement of plastics, submitted to acidic environments. Grades of this glass have Lloyds approval and are certified to meet the Boeing BMS-8-79 specification.

The slightly higher density of E-CR glass is not a serious factor, as the diameter ranges are within the tolerances of traditional E-glass. A slightly higher refractive index may give E-CR glass laminates a slightly more yellowish tint, but this is barely distinguishable. The moduli and stiffness of laminates made with E- and E-CR glass are identical. Tensile, flexural and shear strengths are generally equal or slightly higher with E-CR. The long-term behavior (tension-creep in air) is also identical.

R- and S-glass: have a different chemical composition, giving a higher tensile strength and modulus and better-wet strength retention. R- glass is the type produced in Europe and S-glass in the USA: their properties are broadly similar and the density is the same as E-glass. They were developed to meet demand for higher technical performance from the aerospace and defense industries. They have smaller filament diameters that increase the surface area, so improving inter-laminar strength and wet-out properties.

Advantex-glass: A replacement for E-glass with the properties of ECR- glass is available from Owens Corning under the name Advantex. It combines the electrical and mechanical properties of the current industry standard, E-glass, with the higher heat resistance and acid corrosion properties of ECR-glass glass fibers. It is based on a new boron-free formulation developed by the company, which minimizes production of air pollutants during the manufacturing process (and so contributes to meeting increasingly strict environmental standards without the need to resort to cosily control systems). It is available in the form of both continuous filament and chopped strand. At the time of writing, some 10% of Owens Corning capacity has been converted to the new glass.

ZenTron glass: Claimed to be the strongest glass fiber yet is ZenTron developed by Owens Corning. This combines a revolutionary glass composition with Type 30 single end technology and offers 15-20%


higher tensile strength than standard glass and 50% improvement in impact resistance. It is compatible with epoxy and vinyl ester systems because of unique coating chemistry, and offers excellent adhesion. The impregnated tensile strength is quoted as 3789.5 MPa and intralaminar shear strength is over 68.9 MPa.

Glass fiber wear Tooling (molds, dies, cutters, etc.) will wear when processing glass fibers. As an example, wear in screw melting plasticators generally causes an increase in the clearance between screw flight and barrel. It often occurs toward the end of the compression section. This type of wear is more likely to occur when the screw has a high compression ratio. Regardless of where this erosion of metal occurs, the plasticators melting capacity is reduced. If the wear is serious enough it will effect the process so that products are exiting at a slower rate or more likely a lower quality product at the end of the line. The mechanism that causes wear include abrasive wear due to glass, etc. (galling), adhesive wear (metal to metal contact under high stress), laminar wear (thin outer layers of metal interface wear), surface-fatigue wear (micro- or macroscopic separation from the surfaces), and corrosion wear (chemical reaction and mechanical attack of the sliding surfaces.

Glass filament A form of glass that has been drawn to a small diameter and extreme length. Most filaments are less than 0.005 in. (0.013 cm) in diameter.

Glass filament liquid temperature The maximum temperature at which equilibrium exists between the molten glass during its manufacture and its primary crystalline phase.

Glass filler These are a widely used family of fillers in the form of beads, hollow spheres, flakes, milled particles. They increase dimensional stability, mechanical strength, chemical resistance, moisture resistance, and thermal stability of the plastic matrix.

Glass flake Thin, irregular shaped flakes of glass used as non-fibrous reinforcements. They are used especially in resin-based coatings, to reduce permeability to moisture, vapors, and solvents. Also used in reaction molded polyurethanes to improve molded product surface finish. Methods of application have in the past involved mixing the flake with resin and other fillers and either spraying or trowelling the mix onto the lay-up. This had the disadvantage of losing control over quality and orientation of the glass, leading to inconsistent properties across the surface, and between moldings. A more controlled method of use is to use flake glass in the form of a surfacing veil, together with corrosion resistant fiber. When applied

2-Reinforcements 49

as a surfacing veil, the flakes are mainly parallel to the surface of the lay-up.

Glass former Oxide that forms glass easy and contributes to the network of silica glass when added to it.

Glass forming package A single glass strand gathered on a thin-wall paper or plastic tube to be used in manufacture.

Glass form For reinforcement of plastics, glass fiber is available in continuous and discontinuous forms, for use as roving, bonded mats, or a wide variety of woven or knitted textile forms (Table 2.8). An increasing amount of glass fiber is supplied as continuous roving, which is chopped into small lengths in resin mixing and spraying units, for fiber-directed pre-forming. Using roving rather than continuous strand mat has an important economic benefit, because the glass costs less and there is less waste. A key advantage of the fiber-directed preform process is the ability to change the geometry of the reinforcement strands simply by changing the input. Impact strength increases as the roving strand geometry becomes coarser, while tensile and flexural strengths are only minimally affected. An important area of current development is the design of 3-D fabrics and other forms, which will either provide bulk and/or lend themselves easily to the shape of a require molding without the need for expensive pre-forming stages.

Table 2.8 Typical properties of moldings with various glass reinforcement forms

Woven C h o p p e d Continuous Property Unit cloth strand met roving

Glass content Olo 55 30 70 Specific gravity 1.7 1.4 1.9 Tensile strength MPa 300 100 800 Compressive strength MPa 250 150 350 Bend strength MPa 400 150 1000 Modulus in bend GPa 15 7 40 Impact strength: kJlm 2 150 75 250 Izod (unnotched) Coefficient of x 10 -6 linear thermal per ~ 12 30 10 expansion

Thermal conductivity W/mK 0.28 0.2 0.29


Glass liquidus temperature Maximum temperature at which equilibrium exists between molten glass and its primary crystalline phase.

Glass marble Small spheres of glass used for melting and subsequently drawing into glass fibers.

Glass production Both continuous and staple fibers are manufactured by the same basic process up until fiber drawing. Temperature of glass melt and actual production method vary depending on glass composition; generally about 1260C (2300F) with melts extruding through platinum multi-opening bushings (spinnerets). Two principal manufacturing processes are used namely the glass marble (batch) method or the direct melt method.

Glass roving They are bundled glass filament strands supplied in cylindrical packages (called cheeses), classified by the number of strands in the bunch (such as 6 to 150 end rovings). This is the lowest cost form of glass fiber reinforcement. It can be used to give very high strength in the direction of the fiber. Extensively used in filament winding of hollow symmetrical structures, such as pipes, tanks or high-pressure containers. It is increasingly used in spray deposition techniques, where it is chopped at the point of molding to give isotropic orientation. Different forms are available.

Bulked continuous roving can include loops in both the axial and transverse direction

Gun roving is continuous roving processed in chopper/spray gun: the process offers good lay-down without falling or sliding from the mold, fast wet-out and wet-through to aid roll-out, and air removal adaptability to different gun and transfer systems, together with good laminate properties.

Unidirectional roving cloth (orientated) is a variation of woven roving where the fibers are arranged to provide greater strength in a specific direction or directions (such as by using heavier roving for the warp and lighter for the weft/Chapter 7).

Woven roving can be supplied in a variety of weights and types for use where both strength and bulk are needed. Woven roving is a fabric that can be heavier or lighter according to number and density of strands. It is difficult to produce a good surface finish with woven' roving alone and inter-laminar cohesion between layers is not good. It is often used in conjunction with chopped strand mat to give bulk and additional strength. Densities are measured in weight per unit area. Fibers are usually arranged at right angles to

2 �9 Re in forcements 51

each other or in other positions so that their orientation gives a uniform and well-balanced strength (dcscribed as directional pro- pcrties/Chapter 7). Woven roving (continuous roving woven into heavy drapeable fabric) offers rapid wet-out of fabric in the mold and fast build-up of strength, with rigidity and smoothness in the finished product.

Woven raving mat A ply of woven roving is joined with a chemical binder to a layer of chopped strand mat (CSM). It forms strong drapeable reinforcement that combines the strengths of bidirectional and multi-directional orientations. It is laborsaving in pattern cutting, giving reduced laminate build-up time and excellent conformability to the shape of thc mold, making a light, strong, smooth laminate, with high stiffness plus impact strength, and excellent appearance on the visible surface.

Glass slug A particle of glass sometimes taking the form of a glass bead, which is imperfection in glass fibers.

Glass sphere Spheres, in addition, called beads, are uses as fillers and reinforcements. They are available in different forms and a wide range of dimensions (5 to 1000 ~tm). Their smooth shapes reduce abrasive and viscosity effects.

Salid sphere Microscopic solid glass spheres addcd to an RP compound gives smoothness, hardness, and excellent chemical resistance. The spheres lower the viscosity of most resin mix systems. They act as miniature ball bearings to improve flow. They can be used in combination with fibcrs and other particle shapes such as flakes, reducing product dcfccts. Precise geometry allows even dispersion, close pacldng and easy wetting out in the compound, for high filler loadings. High loadings add significantly to the dimensional stability of finished products, by reducing shrinkage and improving part flatness. High loadings can increase flexural modulus, abrasion resistance, surface hardness, and improve stress distribution.

Better stress distribution is given by spherically shaped particles: the stress pattern around the particle is regular and predictable, with fewer localized stress concentrations. With conventional fibre reinforcement, shrinkage is generally very low along the fibre but very high across it, so that the dimensional stability of the molded part is dependent on flow. The non-directional orientation of spheres, however, gives a more uniform shrinkage rate throughout the part and the isotropic nature of spheres results in more predictable manufacturing quality. Specially formulated coupling agents are incorporated in the coatings


on spheres, designed for optimum performance in specific resin systems, applied in molecular layers to obtain maximum sphere/ resin interfacial bonding (Table 2.9).

Table 2.0 Improved properties with coated glass solid sphere-filled nylon 6/6 compounds

Fiber-reinforced Solid glass Unfilled (40% by weight) spheres

Flexural strength psi dry 14,300 14,200 13,000 wet 8,900 8,700 12,100

Flexural modulus psi x I0 s dry 3.2 4.9 5.4 wet 1.7 2.7 3.1

Tensile strength psi dry 9,400 7,100 11,100 wet 8,000 5,500 9,400

Heat distortion temperature ~ @ 264 psi 75 127 126

Note: Samples were conditioned 16 h in water at 50~ prior to testing.

The blow molding (BM) process can take advantage of improving melt flow with spheres (Chapter 5). Elongational flow is one of the main processing criteria and plastics, which in their elongational viscosity exhibit strong strain hardening thus tend to have good processability by BM.

A team at Yamagata University, Japan has studied the effect of glass beads in a high-density polyethylene (HDPE) compound for BM. A high molecular weight HDPE was examined, both unfilled and reinforced with untreated glass beads of 18 mm diameter and 2.6 specific gravity. Modulus increased with glass bead content at both low and high frequencies and it was shown that Trouton's law (that, for homogeneous plastics/polymers, elongational viscosity in the strain rate independent region is very close to three times the shear viscosity) holds good for RP systems as well as for the virgin material.

Strain hardening has an anomalous dependency on strain rate, and is more marked at lower strain rate. In composites, strain rate dependence of strain hardening is similar to that of virgin HDPE. The hardening phenomenon appears at large strain and is generally believed to be caused by elastic behavior of elongated polymer chains. The glass beads suppress the large deformation of matrix polymer chains around them (which may possibly be one of the causes of the suppression of chain hardening by the glass beads).

In BM, it is important for the parison to be able to resist drawdown and, since this occurs slowly, it can be expected that a compound


with a strong strain hardening at low rates of strain (as exhibited by the RPs tested) will have good processability (reference my BM book).

Hollow sphere Hollow microscopic spheres of a chemically stable soda lime borosilicate glass are used in plastics compounds, for reinforcement and weight reduction of both TSs and TPs. They have a density of about one-fifth that of most solid spheres or resin. For an equal weight, hollow spheres occupy about five times the volume of the resin, resulting in reducing the cost and weight of a compound. They can also produce/improve other useful properties, such as resistance to impact and thermal shock, and the surface finishing characteristics. They can be used in formulations for spray- up and casting, and in molding compounds. In SMC or BMC compounds, weight can be reduced by up to 30% (to 1.3 g / cm 3 or less). A typical range runs from very low density (0.125 g /cm 3) with moderate pressure strength (17 bar) to moderate density (0.60 g /cm 3) with high-pressure strength (690 bar).

Other grades include 8 pm diameter boro-silicate spheres, which are white in color and can be used at injection molding pressures. The density of a typical range is 1.1 g /cm 3 (mineral fillers have a density of 2.4-2.9). On an equal volume basis, the amount of a typical hollow glass sphere grade compounded on weight addition would be (1.1/2.5) x 100 or 44% by weight. If a compound calls for 40% by weight of mineral filler, then 17.6% (44% of 40) by weight should be added, to get the same volume loading.

The spheres are moderately alkaline and prolonged exposure may irritate the respiratory tract. In dusty environments, it is recommended to use a NIOSH-approved mask or respirator. Safety data sheets are available from suppliers. Hollow spheres also produce opacity and whiteness, allowing titanium dioxide to be replaced. Weight reductions of 20-25% can be achieved compared with mineral-filled polymers.

Syntactic sphere core Syntactic cellular core plastics are also called RP syntactic foam or syntactic foam. An RP compound made by mixing hollow microspheres of glass, epoxy, phenolic, etc. into a fluid plastic with its additives and curing agents. It forms a moldable, curable, lightweight mass, as opposed to foamed plastics in which its cells are formed by gas bubbles, etc.

A syntactic core material made of a 120C curing epoxy film adhesive filled with glass micro spheres and supplied with a lightweight carrier scrim is also available. In the uncured state, the material is in


1 and 1.5 mm thick sheet-form, pliable at room temperature with respective surface weights of 570 g / m 2 and 855 g / m 2. In use, it is taken from cold storage (it will store for up to six months a t - 1 8 C in sealed polyethylene bags) and allowed to reach room temperature. It is then trimmed to the required shape. The release paper is removed from one side, the trimmed sheet is positioned and the other release paper is removed. Gel time is 13 rain at 120C (248F), curing is one hour at the same temperature, using a heat up rate of 2C (3.6P) per minute. The cured material has a density of 0.57 g / c m 3 .

Expandable microsphere Thermoplastic microspheres are droplets of liquid hydrocarbon encapsulated in a shell of a thermoplastic polymer. When exposed to heat, the shell softens and the hydrocarbon gasifies, and the microsphere expands from, typically, 12 to 40 pm and the density drops from 1000 to 30-40 k g / m 3. The microspheres can be used either as a form of blowing agent, or may be supplied in expanded form for use as a lightweight filler. The activating temperature of mold and material is 100C (212F).

The expansion creates an internal force in the molding compound, which is maintained until gel or cure takes place. This results in reduction of surface defects, voids and hollow parts. It will also reduce resin shrinkage while a syntactic foam core is established. Parts containing hollow microspheres can be deflashed and trimmed more easily and with less work. They are also easier to grind, drill, tap, and thread with increased holding power, which can be attributed to the syntactic foam, which will exhibit compression/ rebound properties.

When a resin or heavy filler is replaced with the microspheres, most physical properties are reduced, based on constant volume (lower density). Stiffness is reduced, due to the resilient characteristics. Strength/weight ratio, fatigue, stress and resilience can offer useful product enhancement, with relatively small additions.

The pre-expanded form can be used with open or closed mold applications. At moderate pressure of 7-14 bar (100-200 psi) the expanded spheres are compressed, increasing the volume of liquid displaced through the system. When pressure is released, the spheres regain their original volume, increasing viscosity and reducing density. In the cured matrix, the microspheres retain their resilient properties, and are able to compensate stress from curing as well as differential thermal expansion in the composite. They also improve matrix resilience by absorbing energy without suffering permanent set and


reduce fatigue by allowing movement of the matrix in combination with good adhesion to the resin, so reducing crack formation.

There are phenolic-based prepregs in which glass fiber is continuously impregnated with a binder, a water-based phenolic resin, and unexpanded microspheres. When the water is dried, it either puts the resin into a semi-cured stage allowing the microspheres to be expanded (producing a thick low-density prepreg) or left unexpanded (for expansion during the molding process). The product is a syntactic foamed or foamable phenolic-based material, of 100-1000 k g / m 3 density, which can be molded to various shapes, offering useful mechanical properties, and a good level of flame retardancy and low smoke evolution. Applications are in high-speed railway trains (3 k g / m 2 laminated prepreg is used in ceilings), automobile interiors (800 g / m 2 x 4 mm thick material meets requirements for headliners), etc.

Glass strand It is a primary bundle of continuous filaments combined in a single compact unit without the usual twist of a fiber. These filaments that number usually 51, 102, or 204, are gathered together during the fiber forming operation. The filament strand end is the group of filaments at an end. Filament strand integrity is the degree to which the applied sizing holds the individual filaments making a strand or end together.

Glass strand is normally measured by the number of 100 yards in 1 lb weight (for example, a 130 s count contains 13,000 yards per pound weight or the number of grams per kilometer, under the international unit rex. Tex is a unit for expressing linear density equal to the mass of weight in grams of 1000 m of strand, fiber, filament, yarn, or other textile strand.

Nylon Fibers

Also called aramid fibers. See High Performance Reinforcements, Aramid Fibers in this chapter.

Polyester Fibers

TP polyester offers a low density, high tenacity fiber with good impact resistance but low modulus. It is used in areas where high stiffness is not required, but where low cost, lightweight, and high impact or


abrasion resistance are important. Polyester is used mainly in surface tissue for laminates, but also offers high impact resistance, good chemical resistance and good abrasion resistance.

The advantages of polyester are that it does not use binders that have to be dissolved in the resin matrix, has high conformability and excellent strength/weight ratio. As a surfacing material, the fiber is easy to sand. Fabrics are half the weight of the equivalent glass, with excellent energy absorption, chemical resistance, and dielectric/electrical insulating properties. Lloyd's Register of Shipping and the American Bureau of Shipping internationally certify them.

Continuous needle-punched nonwoven polyester fabrics have been developed specifically for the fiber-RPs industry and are designed to saturate easily with all resin systems. Individual grades are used as a surface layer, to reduce print-through of the gel coat, as a superior bedding substrate for thick cores and reinforcing materials, or as a core to increase structural thickness and stiffness. Polyester fiber mats can also reduce laminate density, reducing total weight and can be used with, or as a replacement for, glass fiber in sheet molding compounds (SMCs). The same impact and density advantages are obtained in resin transfer moldings (RTMs).

A polyester surfacing veil is also designed for the outer layer for filament winding. It uses specially modified spun laced polyester, 0.076 mm (0.003 in) thick, which is very smooth and flat. Typical applications include print blocker, thin core, core bedding, and toughening layer in marine applications; surfacing veil, thin core, and exterior abrasion-resistant layer in pultrusion and compression molding; corrosion barrier in anti-corrosion applications, and as thin core and surfacing veil in panels.

Polyethylene Fibers

Very low-density fiber can be produced from ultra-high molecular weight polyethylene (UHMWPE), offering strengths which (for the density of the fiber) are among the highest to be found anywhere. It is made up of aligned polymer chains with high elongation and good impact resistance. However, although the fiber has remarkable properties, its low modulus and ultimate tensile strength and the relatively high cost of treating the surface to improve the fiber/matrix bond mean that PE fiber is not often used in RP structures.

The specific gravity is very low, at 0.97 (aramid is 1.44, polyester 1.38). It is 35% stronger than aramid and has a high energy/break ratio, giving


remarkable ballistic properties. It exhibits impact energy absorption in RPs 20 times that of glass, aramid, graphite, and also has excellent vibration damping. The melting point is 147C (300F). Possible applications for composites include boat hulls, sports equipment, radomes, and structural components, pressure vessels, aerospace and industrial.

Hybrid Fibers

There are inter-ply hybrid RPs that have two or more fiber reinforcements embedded in the plastic matrix. They have evolved as a logical sequel to conventional single-fiber RPs. Hybrids provide unique combinations of performance features to meet different and competing requirements in a more cost effective way. This cost advantage has been found principally in using glass fibers with the more expensive fibers, since the 1950s.

For example, regarding the cost of hybrid RPs, high modulus graphite- resin is about 60 times more expensive than E-glass laminates. Intermediate graphite modulus-resin is about 36 times more expensive. Aramid-resin is about 8 times as costly. Thus, when performance standards of hybrid RPs can be met, cost advantages occur.

An almost unlimited field of possibilities continues to exist with the combination of different fibers as hybrids that, with an appropriate resin matrix, can most closely fill a specific closely identified application. In most cases, however, this is a matter for specialists, backed by an exhaustive database of fiber forms and properties.

A typical example of an off-the-shelf hybrid is a boron/graphite prepreg, composed of small diameter graphite fibers dispersed between 76-100 pm diameter boron fibers, in an epoxy matrix, to 70-80 wt% total fiber content. This prepreg provides properties superior to RPs based on either fiber. The flexural stiffness and strength is twice that of carbon and 40% higher than boron. Intralaminar shear strength also exceeds carbon and boron. The resin matrix can be a toughened epoxy or a polyimide.

Other Fibers and Reinforcements

Overview

Other reinforcing fibers, of less commercial importance at the present time, include asbestos, ceramic fibers, silicon carbide fibers, whisker


fibers, natural fibers, and mineral fibers. See Fiber /Fi lament Characteristics that includes more information on different fibers.

Asbestos Fibers They have been used extensively in the past in many different applications including RPs. The fibers offer advantages such as excellent/high strength and stiffness, good rigidity, chemical resistance, and particularly fire resistance. However its use has ceased in all but closely controlled applications, following realization of the health hazards associated with it.

Ceramic Fibers They posses unique wear and corrosion resistance, and high temperature stability. They consist of approximately 50 wt% alumina and 50% silica with traces of other inorganic materials. The fibers are made by atomizing a molten ceramic stream using high-pressure air or spinning wheels; also use chemical vapor deposition, melt drawing, and special extrusion processes. Although glass fibers are also ceramic material, they are not generally categorized as ceramic fiber; they are called glass fibers.

Silicon Carbide Fibers A reinforcing fiber with high strength and modulus with 2.7 density. Primary purpose for this development was for the reinforcement of metal matrix and ceramic matrix composite structures used in advanced aerospace applications by the military. SiC fibers were developed to replace boron fibers in these RPs, where boron had its drawbacks; principally degradation of mechanical properties at temperatures greater than 540C (1000F) and very high cost.

Whisker Fibers Whiskers are metallic and nonmetallic single crystals (micrometer size diameters) of ultrahigh strength and modulus. Their extremely high performances (high melting points, resistance to oxidation, low weights, etc.) are attributed to their near perfect crystal structure, chemically pure nature, and fine diameters that minimize defects (Figure 2.6). They exhibit a much higher resistance to fracture (toughness) than other types of reinforcing fibers. Many different materials have been used (literally hundreds) with diameters ranging from 1 to 25 ~am (40 to 980 lain.) and having aspect ratios between 100 to 15,000. Processes used to manufacture whiskers include growing them by condensation from supersaturated vapor, from chemical solution, and by electrodeposition. Because of their extremely high costs and not easy to process with present technology, use has been in specialty applications such as aerospace, medical/dental, etc. Matrices include plastics, ceramics, and metals.


Figure 2.6 Range of properties for different materials including glass fiber and whisker reinforced plastics

Natural Fibers

They are usually derived from vegetable matter and since the Ford Model T automobile (1903) continues to receive increasing attention worldwide, where there is a strong urge to use materials that are thought to be friendlier to the environment. There is growing interest in the possible use of natural fibers in RPs, not only in the developing countries that produce them, but also in the industrialized countries, where some believe they might help in solving recycling problems. However, they do not match the performance and consistency offered by existing reinforcement materials (such as glass). Natural fibers may well have to be treated with coatings that may militate against easy recycling of the base fiber.

Mcrcedes-Benz (among other companies) has studied many materials and is using animal hair and fibers made from flax, sisal, coconut, and cotton, in upholstery, door panels and rear shelves of its cars. The company is looking to replace glass fiber with natural fiber alternatives, but has found it difficult. Not only arc natural materials usually sensitive to temperature, but they also tend to absorb water and often exhibit extreme variation in quality that is not good for an automobile manufacturer. For over a century results of studies and evaluations of the different natural fibers continue not to be practical for use in the RP industry.


As well as biodegradability, the advantages of natural fibers include low density and cost, better damage tolerance in RPs, and high specific strength to stiffness. These advantages make natural fibers interesting for applications with low load requirements. On the negative side, however, there continues to be basic problems of degradation by moisture, poor surface adhesion to hydrophobic resins, and susceptibility to fungal and insect attack. As reviewed properties can be improved by treatment of the fibers, one obvious route being to reduce the water absorption, which has a direct effect on physical performance such as tensile properties.

Flax Fibers It is reported to be able to withstand the same tensile force as ramie, and can out-perform glass fiber on a weight-for-weight basis. However, to counter the other drawbacks of natural reinforcements, various forms of resin matrix are being studied. Polyurethane, processing at low temperatures, is promising. Another development approach is with TPs such as PP , using an extrusion/compression molding technique developed by DaimlerBenz at Ulm (Chapter 5).

Hemp Fibers Environmental considerations (including clean production and reduced skin irritation when handling at the workplace) were key factors in the decision by Ford UK to investigate the use of hemp (Cannabis Sativa) in place of glass fiber for reinforcement of some components on the Transit van. Hemcore Ltd processes the hemp (which is grown under close government scrutiny, although the strain has negligible narcotic content) in conjunction with J E Plant Fibres as a needle-punched mat, Hempmat 250.

Ford is using hemp fibers as a replacement for chopped strand glass mat in the parcel shelf for the high roof model of the Transit. It is molded by resin injection molding (RTM). Depending on results, the auto company sees a wide range of components in both vans and cars using hemp as the reinforcement.

The hemp is grown free of pesticides and is separated by a completely mechanical process. In the molding shop, it is said to have lower skin irritability than glass fiber. However, the environmental advantages are really a bonus for Ford, which has also identified significant savings in both cost and weight. The mat is also being tested as a core material for thick RPs. Results to date point to properties superior to standard core products, suggesting that there might be significant cost-saving as well as increased stiffness. B IP Plastics Ltd is testing a short chopped hemp fiber as reinforcement for TS polyester compounds for injection and compression molding.


Hemcore does not see the product as meeting every application need of the fiber-RPs industry, but it considers that the fiber could find some price/performance gaps. No yield figures have yet been published, but it is possible that, in common with other natural fibers, a very large area of land will be needed to produce viable quantifies.

Jute Fibers This is one of the most important natural fibers. It is produced in India, Bangladesh, Thailand, Vietnam, and other countries. It contains 56-64 wt% cellulose, 29-25% hemicellulose, 11-14% lignin, and a small proportion of fats, pectin, ash, and waxes. Application of jute fiber in RPs with matrices of TS resins such as unsaturated polyester or vinyl ester resins has been widely studied. To date the poor adhesion to hydrophobic TPs, such as polyethylene and polypropylene, has to date limited application in TPs.

Jute fiber (JF) has been popular since the start of this century, principally as a filler and as a reinforcement with TP matrices. These low-cost natural fibers consist mainly of cellulose and hemicellulose chains running parallel to the fiber direction and lignin. High performance, average unidirectional-oriented tensile strength (Ts) is 500 MPa, elastic modulus (E) is 40 GPa, and elongation is 1.7%. Other fiber properties are density 1.45 and weight 0.21 g /m.

Testing of JFRPs has been conducted using polypropylene (PP) film (T~ at 17 MPa, E at 0.7 GPa, and 100% elongation). Prepregs were made by pressing unidirectional-oriented layers of JFs between PP films. No fiber pretreatment was used. Eight layers of prepregs were compression molded (at 190C and 20 MPa) to produce test specimens. One series was unidirectional and another isotropic (0~176 4 5 ~ 1 7 6 1 7 6 +45~176176 JFRPs had a density of 1.11 with fiber content of 50 wt% (40 vol %). Unidirectional and isotropic properties were Ts of 140 and 50 MPa, E of 13.2 and 5.7 GPa, and elongation Ts yield of 2.7 and 3.0%, respectively. In comparison to the unidirectional Ts at 0 ~ of 140 MPa, the 10 ~ was 70 MPa, and 45 ~ or less it was 10 MPa. With the isotropic RPs, Ts in all directions was 50 MPa. No brittle behavior occurred during testing. When the tensile yield strain is exceeded (for unidirectional RPs), about 50% of Ts remains at 5% elongation and about 20% at 8% elongation (Table 2.10).

Work on production of RPs from jute fiber by hot press molding with TP films (at Ho Chi Mihn City University of Technology, Polymer Research Centre, Vietnam) has shown that they have a potential use in replacement of wood and also glass fiber RPs, provided that the high water absorption and poor interfacial adhesion can be eliminated. A


Table 2.10 Jute fiber (40 wt%) isotropic lay-up RP tensile properties with thermoplastics

Matrix Fiber

Tensile Tensi le Flexural Flexural strength modulus strength modulus

(MPa) (GPa) (MPa) (GPa)

LDPE Untreated fiber 94 5.25 19 1.16

NaOH-treated fiber 101 5.28 22 1.22

Cardanol-treated fiber 114 5.74 28 1.27

HDPE Untreated fiber 126 7.86 43 2.81


Cardanol-treated fiber 142 8.39 56 3.22

PP Untreated fiber 148 11.45 73 3.13


Carda hal-treated fiber 172 12.15 82 3.86

Source: Polymers Et Polymer Composites.

number of chemical treatments can be used, including those used with glass fibers that are silane coupling agents, polyisocyanate, vinyl monomer linkages, or compatibilizers.

A particularly promising route is to treat the fiber with a phenolic resin based on cardanol formaldehyde (CF). This is a natural alkyl-phenol, in which the methylol groups are able to react with the hydroxyl groups of the cellulose, while the long C-15 alkyl group of cardanol facilitates the formation of an adhesive bond with non-polar TPs. It is concluded that Jute fiber thermoplastic RPs can bear comparison with glass fiber RPs. However, the best solution may well be a combination of treated jute with glass.

Ramie Fibers It is derived from a 2.5 m high Chinese relative of the common stinging nettle (more correctly known as Boehmeria Nivea). It yields fibers that are almost as resistant to tearing as glass and has been evaluated in the past by different organizations. Recently Daimler-Benz Aerospace specialists in Bremen are studying it as a possible replacement for glass in interior fittings of the Airbus, where it also satisfies a crucial requirement in that it is fireproof. Processing the ramie fiber proved a problem until the team in Bremen found the right technology in Switzerland (with a manufacturer of cheesecloth).

Sisal Fibers A white fiber produced from the leaves of the agave plant found in Central America, West Indies, and Africa. Used primarily for at least half of the 20th century as cordage, binder twine, RPs, etc. When

2-Reinforcements 63

chopped, it is used as a low cost filler. This natural fiber is used in some bulk (dough) molding compounds and more with phenolic matrices than with TS polyesters or epoxies.

Soya Bean/Cellulose Fibers These fibers form a range of materials also claimed to be a viable alternative to glass fiber in RPs. The basis is waste cellulosic fibers bound with soy protein/phenolic binder systems. The fibers have been developed by United Soybean Board (USB) under the name Proteinol Composites and can be formed into extruded shapes and compression molded sheets and can be nailed, sawed and machined. The cellulosic fibers are essentially wastes from agricultural crops, forest products, and paper. Future development will turn on growing understanding of the chemistry of soy proteins. Improvements are being sought in moisture resistance, fiber/binder compatibility, and processing efficiency.

Mineral Fibers

The mineral fillers are a large subclass of inorganic fillers comprised of ground rocks as well as natural, refined, or synthetic minerals. Com- modity minerals are relatively inexpensive and are used mostly as additive extenders. Other fillers, so-called specialty minerals, are usually the reinforcing types. There are also inherently small particle size fillers such as talc and surface chemically modified fillers. The inert filler are those added to plastics to alter the properties of a product through physical rather than chemical means.

Wollaston i te Fibers These fibers played a key role in the development of a prototype automobile wing molded in reinforced reaction injection molded (RRIM) polyurethane which can resist temperatures of up to 190C (Tremin 939-100 USST wollastonite fiber from Quarzwerke GmbH, Frechen, Germany). In the PU RP, it produced a wing with surface quality similar to that of sheet metal, while permitting wall thicknesses as low as 2.5 mm in series production. It was claimed to be interesting to the automotive industry, which requires plastics materials for bodywork panels that can be coated inexpensively in an on-line f~shing process, using a cathodic electrodeposition primer. Temperatures of around 190C level are encountered during cathodic electrodeposition priming and it is important that plastics panels should be able to resist this without requiring separate assembly.

Intumescent Graphite Fibers A range of graphite-based mineral fiber stabilized flexible mats, which can be incorporated in a laminate and Technical Fiber Products


(Technofire), UK, have developed give intumescent performance in the event of fire. The thermally active constituent is exfoliating graphite, which increases its volume considerably in fire conditions, leaving a stable insulating layer of mineral fibers. The material starts activation at 190C, developing a peak expansion pressure of about 1.5 kgf/cm 2 at about 400C. The average expansion ratio is 9:1. The mat is available in standard sizes in 1.5, 1.8, 3.0, and 4.0 mm thickness: non-standard thicknesses between 0.5 and 5.0 mm can be supplied.

Mica Mica is a generic name to describe a group of complex hydrous potassium aluminate silicate materials, differing in chemical composition, but sharing a unique laminar crystalline structure. In nature, mica develops in a book-like form. The individual platelets can be delaminated into very thin high aspect ratio particles that are tough and flexible. Of the commercially important forms, muscovite and phlogopite are used as reinforcements for plastics.

Mainly used in TPs, mica reinforcement improves the tensile and flexural strength and flexural modulus. Heat distortion temperature is increased and the coefficient of linear thermal expansion is reduced. Shrinkage and creep are significantly reduced, and warpage is virtually eliminated. Chemical resistance is high and permeability is reduced. Mica can also help to produce a Class A surface finish (Table 2.11).

Table 2. I I Effect of various reinforcements with 40 wt% polypropylene

Type

Tensile F lexura l Notched strength modulus Izod

(psi) (kpsi) (ft-lb/in)

HDT Shrinkage (264 psi ~ (%)

100% PP 4580 240 2.90 150 2.3

Calcium carbonate 3250 430 0.41 157 1.5

Glass: flake 1/64 3370 710 0.43 183 0.6

Glass: milled 1/8 3580 670 0.42 200 0.4

Mica, HiMod-360 4435 1110 0.39 238 0.8

Mica, L-135 3710 1010 0.46 244 0.8

Silica 3090 380 0.51 154 1.5

Talc 4220 660 0.44 181 1.1

Wo I Iasto n ire 3690 740 0.41 190 0.6

Source: Franklin Industrial Minerals

Because of the flexibility and softness of mica, injection molding and extrusion is possible with very little change in particle size (whereas


milled glass fiber, glass flake, and wollastonite tend to break during processing) and there is less wear on equipment than with other minerals. To avoid size reduction, however, these materials should be processed under low shear conditions, and material of larger particle size should be added to an extruder through a side feeder at a stage after the resin has been softened.

In PP compounds (which is the main use of mica in TP reinforcement), the mechanical and thermal properties are considerably enhanced by modifying the PP with a maleic anhydride compatibilizer, which improves adhesion. Test results suggest that the improvement continues with increasing amounts of maleic anhydride.

Forms of Reinforcements

The available forms of reinforcement follow terminology and technology borrowed from the textile industry. The basic forms described throughout this chapter and other chapters for glass and other fibers including hybrid mixtures are summarized in this chapter. They are continuous filament, woven fabrics, nonwoven fabrics, knitted fabrics, braids, and tapes.

Three-Dimensional Reinforcements

The introduction of what is a 2-D (two-dimensional) fiber reinforcement into a molding that is intended to have 3-D produces a conflict. Depending on the shape of the intended product, and the molding technology employed, the fiber reinforcement will be in the form of short or long chopped filaments/strands, mats made of random chopped strands, or woven fabrics of varying density. Woven and nonwoven fabrics can be also used to improve surface qualities such as appearance, impact resistance, abrasion and chemical resistance. To improve the distribution and orientation of fibers in a 3-D molding, there have been produced machine-made 3-D arrangements of fiber that offer better drape in a mold.

Surface Tissues

Nonwoven surfacing fabrics are used, with special gel coats, to give RPs additional resistance to abrasion and corrosion, optically smooth surface, and stability under load. They have been used for over a half century, meeting extreme fluctuations in temperature, chemically aggressive substances, mechanical stresses, high UV radiation, and


high-pressure loads. They are based on textile glass fibers (C-, E- and E-CR-glass), polyacrylonitrile, and TP polyester. Standard weights range from 14 to 40 g / m 2 and special products from 18 to 60 g / m 2. There arc grades suitable for all types of RP processes. Special products are printable or electroconductive.

Overlay mats and surfacing tissues are widely used to produce a high- quality surface to RPs. These are thin tissues of staple fiber with a binder that wets out rapidly and are designed to absorb a high percentage of resin, to produce a resin-rich surface (or to cover a coarse pattern of chopped strand mat on the inner surface of a contact molding). Nonwoven tissues for surfacing veil are available in glass, TP polyester, carbon, and aramid fiber.

Conductive Nonwovens

Nickel-coated carbon fiber nonwovens offer very high electrical conductivity (Table 2.12). They are used for many applications such as in shielding, panel heaters, aircraft lightning strike protection, electromagnetic interference/radio frequency interference (EMI/RFI) shielding and layers in de-icing systems, automotive E M I / R F I shielding and seat heaters, architectural anti-surveillance systems and anti-icing systems, and industrial conductive tapes and resistive heating. They can be used alone or mixed with other fibers.

A nonwoven mat of smooth flexible aluminum coated oriented glass fibers, Metafil, has been developed by Tracor Aerospace and BGF Industries, USA, for electrically or thermally conductive applications, such as parabolic dish antennae. It is compatible with most resin systems and processing is said to be similar to uncoated glass mat, but cost is lower than other conductive materials.

University of Maryland physicists have found that semiconducting carbon nanotubes have the highest "mobility" of any known material at room temperature. Mobility refers to how well a semiconductor conducts electricity. A semiconducting transistor made from a single carbon nanotube showed mobility more than 70 times greater than the silicon used today in computer chips. The researchers had to grow extremely long carbon nanotubcs, up to 0.3 mm in length, and had to place metal wires precisely on each end of a single tube to make the measurements. The technology holds promise as a replacement for silicon chips, if production and substrate issues can be resolved. Semiconductors arc just one of many potential applications of single- wall carbon nanotubes, which include use as RPs.

Table 2,12 Typical properties of conductive fiber reinforced thermoplastics

Specific Base polymer Reinforcement, wt. % gravity

Tensile modulus,

GPa (Mpsi)

Notched Izod impact strength, Jim (6.4ram bar) (ft-lb/in. ( 7/4" bar)

Surface resistivity, ohms/sq.

ASTM D792 PC 5% Stainless steel 1.28

PA 6/6 5% Stainless steel 1.22

PPS

PA 6/6

15% Carbon fiber nickel, coated 1.45

15O/o Carbon fiber nickel, coated 1.20

PPS 400/0 Carbon fiber 1.49

PA 6/6 500/0 Carbon fiber 1.38

PA 6/6 400/0 Carbon fiber 1.33

PA 6/6 30% Carbon fiber 1.28

PC 300/0 Carbon fiber 1.38

D638 4.83 (0.70) 4.41 (0.64) 6.89 (1.00) 7.58 (1.10) 30.30 (4.40) 34.5 (5.00 29.3 (4.10) 20.7 (3.00) 15.9 (2.30)

D256 80.1 (1.5) 42.7 (0.8) 32.0 (0.6) 37.4 (0.7) 80.1 (1.5) 106 (2.0) 118 (2.2) 107 (2.0) 96.1 (1,8)

D257 102

102

103

101

102

101

102

102

102

Shielding effectiveness db attenuation @

lOSMHz, 6.4mm (7/8") thick.

ES7 40

40

20

55

30

50

40

30

40

I'D m ,

"-h 0

I'D 3 I'D

I l l


High Performance Reinforcements

Aramid Fibers

Aramid fiber (AF) is the generic name for the organic aromatic polyamide fibers. It has a long chain synthetic polyamides (nylon) in which over 85 wt% of the amide linkages are attached directly to two aromatic rings. DuPont's trade name is Kevlar TM. They have excellent properties such as high strength, modulus of elasticity, lightweight, impact resistance, abrasion resistance, creep-rupture characteristics, and chemical and mechanical stability over a wide temperature range (Table 2.13). Their use includes RPs, high performance fabrics (boat sails, bulletproof vests, etc.), medical devices, etc.

AFs have low density/high tensile strength and are produced by spinning liquid crystal polymer, usually as filament yarns, rovings or chopped fibers. They have a characteristic of bright golden yellow color. All grades are particularly good in resistance to high impact; lower modulus grades are widely used in anti-ballistic applications. The compressive strength, however, is unexceptional and only equivalent to that of glass fiber RPs.

Applications are not restricted to the obviously high performance sector. In fact, a large part of the business of aramid fibers is in combinations with other reinforcements as hybrids, giving predictable properties precisely where they are required. For example, the PH11 Hovercraft boat was designed with a hull of aramid/glass hybrid/TS polyester RP to give a wear- and impact-resistant surface with high tensile strength and limiting bending in the horizontal direction. The gangways were an aramid/glass RP and, around the propeller, an aerodynamic tunnel of aramid/epoxy was used to increase thrust, stop flying parts (in the event of the propeller fracturing), and ensure a rigid construction, preventing contact between propeller and tunnel in starting and stopping.

Aramid fiber is also used in production of reinforced thermoplastics (RTPs), as commingled yarns or a hybrid yarn or fabric, suitable for high drapability and coping with very sharp fillet radii, and in specialty tires.

A significant development is a technique for pulping or fibrillation that greatly increases the surface area of short-length fibers of para-aramid, and renders them suitable for reinforcement of plastics and elastomers. A typical staple fiber will have a surface area of about 0.1 m2/g, but the compounding process increases this to 7-9 m2/g, so increasing the area available for adhesion to the matrix plastic. The bond achieved will

Table 2.13 Properties of aramid fiber/thermoplastic RPs

Base resin

Tensile Flexural Mold Water strength modulus

Specific shrinkage absorption, 10 3 psi 10 6 psi gravity [in./in.) 24hr. {O/o) {MPa) {GPa) D 792 D 955 D 570 D 638 D 790

Nylon 6/6 1.19 0.008 0.90 (1.14) (0.016) (1.50)

Nylon 6/6 1.29 0.008 0.6 Nylon 6 1.19 0.008 1.0

(1.14) (0.016) (1.8) Polyester (PBT) 1.33 0.013 0.06

(1.31) (0.020) (0.08) Polyca rbon ate 1.23 0.005 O. 12

(1.20) (0.006) (0.15)

Impact Strength, Izod (ft.-Ib./in.]Thermal

14.5 100.0 (11.8) 81.4 (13.5) 93.1 13.0 89.6

(11.8) 81.4 9.5 65

(8.5) 59 11.0 75.8 (9.0) 62

Notched Unnotched D256 D256

0.64 (4.4) 1.0 6.7 (0.41) (2.8) (0.9) - 0.55 (3.8) 1.0 8.5 0.58 (4.0) 1.1 9.0

(0.40) (2.8) (1.0) - 0.60 (4.1) 0.8 9.0

(0.34) (2.3) (1.2) - 0.54 (3.7) 0.9 11

(0.33) (2.3) (2.7) (60)

Deflection expansion

( l O-S in./in. -~ D 696

2.4 (4.5) 3.1 3.0

(4.6) 3.0

(5.3) 3.0

(3.7)

temperature, 264 psi ~ (~ D 648

450 (232) (170) (76.7) 465 (240) 390 (199)

(167) (75) 380 (193)

(130) (54.4) 280 (138)

(265) (129)

, m ~

"-h 0

3

I l l

tad

Table 2.1 4 Examples of different carbon fibers

Type

No. of Tensile strength Tensile modulus a Mass per filaments Elongation unit length per tow ksi MPa kgf/mm 2 msi GPa kgf/mm 2 0/0 Tex [g/lO00 m)

T300 1,000-12,000 514 3530 360 33.4 230 23,500 1.5 66-800

T3OOJ 3,000-12,000 611 4210 430 33.4 230 23,500 1.8 198-800

T4OOH 3,000-6,000 640 4410 450 36.3 250 25,500 1.8 198-396

T 7 0 0 S 12,000-24,000 b 711 4900 500 33.4 230 23,500 2.1 800-1650

T8OOH 6,000-12,000 796 5490 560 42.7 294 30,000 1.9 223-445

TIOOOG 12,000 924 6370 650 42.7 294 30,000 2.2 485

M35J 6,000-12,000 683 4700 480 49.8 343 35,000 1.4 225-450

M40J 6,000-12,000 640 4410 450 54.7 377 38,500 1.2 225-450

M46J 6,000-12,000 611 4210 430 63.3 436 44,500 1.0 223-445

M50J 6,000 597 4120 420 69.0 475 48,500 0.8 216

M55J 6,000 583 4020 410 78.2 540 55,000 0.8 218

M60J 3,000-6,000 555 3820 390 85.3 588 60,000 0.7 100-200

M30S 18,000 797 5490 560 42.7 294 30,000 1.9 745

M40 1,000-12,000 398 2740 280 56.9 392 40,000 0.7 61-728

a Measured using the impregnated strand test method

b T700S - 24,000 is temporary value, subject to change.

This information can be used just for material selection purpose

Source: Toray Industries Inc.

Density g/cm~

1.76

1.78

1.8O

1.8O

1.81

1.80

1.75

1.77

1.84

1.88

1.91

1.94

1.73

1.81

",,4 o

' - h 0

"1"

0 0

2 �9 Re inforcements 71

improve properties of the compound, particularly abrasion resistance. High strength/low weight, mechanical stiffness and resistance to thermal and chemical attack are other advantages. The development has been commercialized (by DuPont) as a masterbatch for elastomers used in applications for power transmission beltings, hoses, tire beads and tread areas, beatings, packings, and seals.

With advances in technology, there is cooperation between some manufacturers on product and process development of higher-performance aramid fibers. Different production processes use different solvent systems, making it possible to modify product properties by changing the basic polymer composition with additives and/or fillers.

Techniques for production of 3-D structures of high tenacity aramid fiber have also been developed, offering excellent fatigue resistance to abrasion, flexure, and stretching. One such system is on a wire frame basis, with mechanized frame building, and it is proposed as reinforcement for concrete pillars and other structures. As well as the strength of the fiber, this application exploits the high chemical resistance of aramid to acids, alkalis, and cement.

Carbon Fibers

Various organizations worldwide have been involved in developing carbon fibers (CFs). A few of these organizations are reviewed as well as performances of carbon fibers (Table 2.14). Carbon fibers based on rayon (cellulose) were first investigated during 1880 in USA. It was during this year that Thomas Edison and Joseph Swan first patented the incandescent electric lamp (after years of trial and error with literally thousands of materials). The filaments that he used in the lamp were made of carbon and had been produced by pyrolyzing natural and regenerated cellulose fibers. The carbon filaments themselves were very fragile. However, the invention of the tungsten filament overtook this development.

Initial modern carbon fiber development occurred during 1944-1960 in the research and development Materials Laboratory of the Wright- Patterson Air Force Base (WPAFB), Dayton, OH, USA. The UK Royal Aircraft Establishment developed carbon fiber-RPs in the late 1950s. During this period carbon fiber produced from cotton and viscose rayon fabrics were principally used in military applications such as rocket nozzle cones and ablative surface panels on outer space vehicles (other fibers were also used). Barnaby-Cheney and National Carbon manufactured a small amount of carbon fiber from these fibers.

Starting during 1951 new ablative plastics were developed in Materials Laboratory, WPAFB; they are materials that absorbs heat, while part of


it is being consumed by heat through a decomposition process that takes place near its surface when exposed to excessive heat. Their use is to prevent heat build-up under the ablative material. An example is the initial use of asbestos fibers/phenolic RPs used as ablative materials on the nose cones and exhaust cones on rockets and missiles. Other materials were latter used that included carbon fiber-phenolic RP. These materials are exposed to a temperature of 1650C (3000F) for a time of less than a second. It is the surface material on a reentry into the earth's atmosphere from outer space of a rocket or space vehicle that is subjected to the high heat when entering the earth's atmosphere (D. V. Rosato involved in R&D, preparing compound, and fabrication of nose cones, etc.)

Sohes and Abbot of USA during 1955 developed processes for converting both natural cellulose and rayon into fibrous carbon. Essentially the carbon fibers were produced by heat-treating the precursors to temperatures about 1,000C (1,832F) in inert atmosphere. Fiber tensile strengths were as high as 40 ksi (275 MPa).

Union Carbide, USA, begins production of carbon fiber cloths, felts, yarns, and battings using rayon fibers as the precursor material during 1959. Carbon fibers were batch processed by heat-treating the rayon in an inert atmosphere at about 900C (1652F), subsequently carbonizing the fibers at temperatures generally over 2,500C (4,532F). Tensile strength of fibers is 48 to 130 Ksi (300 to 900 MPa). Use included reinforcing plastics.

During 1961 A Shindo, of the Japanese Government Industrial Research Institute, Osaka, produced carbon fiber from polyacrylonitrile fiber, and started the development of PAN type-high performance carbon fiber.

In 1967, Rolls Royce, UK, experienced an unsuccessful project to use carbon fiber (CF) fan blades in jet engines. Problem was due primarily to the RP's coefficient of thermal excessive expansion that occurred during the heating operations causing blades to expand. The heated rotating blades would contact its enclosed peripheral wall and in turn cause engine failure. Prior work during 1944 and 1953-1954 on RP jet engine blades had a similar problem. Work was done at Air Force R&D Laboratory with D. V. Rosato involved in design, fabricating, and testing RPs blades. This unsuccessful program was caused by not being able to restrict expansion of the RP blades using different fibers and resins available at that time.

RP blades for aircraft jet engines are being designed now by NASA, USA. They are essentially hollow with an internal rib structure. These

2 �9 Reinforcements 7 3

rib like vents direct, mix, and control airflow more effectively and reduce thermal expansion that reduces the amount of energy needed to turn the blades and cuts back on heat and noise. Most engine noise actually comes from wind turbulence that collides with the nacelle. By directing air out the back of the fan blades, the noise can be reduced by a factor of two. By drawing more air into the blades, engine efficiency is improved by 20%. There also exist embedded elastic dampening materials in the blades, which minimizes vibrations and expansions to improve resiliency, etc. Because the blade is lighter and experiences lower centrifugal forces further enhanced the blade's durability occurs. Small-scale wind tunnel tests show they last 10 to 15 times longer than any existing blade. The No. I maintenance task is the constant process of taking engines apart to check the blades.

These new blades should lend themselves to more efficient production techniques. If you use titanium, you need to buy a big block of it and machine it down to size, wasting a lot of material. As reported, this is very time consuming, and one has to worry about thermal warping. The RP allows for mass production. It is fabricated into a mold, making the process more precise and ensuring the blades are identical. NASA will test the new blades in large-scale wind tunnels at the NASA Glenn Research Center in Cleveland.

Today, high cost carbon fiber alone and in hybrid form (Table 2.15) is widely used in high performance applications were performance to cost advantages exist. Depending on the manufacturing method the types of fiber available range from amorphous carbon to crystalline graphite. Its stiffness or modulus of elasticity can range from less than that of glass to three times that of steel; the most widely used types have a modulus of 230-392 GPa. The fiber is available as short-length fibers, twisted and non-twisted yarns, continuous filament and tows. A fiber tow is an untwisted bundle of continuous filaments, usually referring to any synthetic fibers. As an example, a tow designated as 140 K has 140,000 filaments.

CFs arc the predominant high strength, high modulus fiber used in the manufacturer of advanced RPs products. They can be made by the pyrolytic degradation of a fibrous organic precursor. Most CFs arc obtained by the pyrolysis of polyacrylonitrilc (PAN); this old basic technology subjects the PAN to temperatures up to 1080F (2000C). Other methods include pyrolysis of cellulose (rayon) and acrylic fibers, burning-off binder from the pitch precursor, and growing single crystals (whiskers) via thermal cracking of hydrocarbon gas. CFs are essentially crystalline carbon (graphite) having high mechanical and physical performances in reinforcements. Their benefits also extend to


Table 2.1 5 Properties of unidirectional carbonlaramid hybrid/epoxy RPs

Ratio of carbon / Specific aramid fibers gravity

Tensile modulus,

10 6 psi

Stress at 0.02% offset, 10 3 psi

Compression Flexure

0/100 1.35 11.2 26.4 49.2 50/50 1.51 15.7 59.9 120 75/25 1.56 17.4 68.8 181 100/0 1.60 21.1 98.4 233

Ratioof aramid/ carbon

Dynamic flexure Impact energy, strength, 10 3 psi ft.- lb./ in. 2

100/0 63 48 50/50 82 44 25/75 82 34 0/100 99 28

high thermal stability, electrical conductivity, chemical resistance wear resistance, and relatively low weight.

MEC (London U.K. and Shanghai, China), an international engineering services company, was awarded a $25 million contract by China Worldbest Group Co. Ltd. (Changzhou, China) to engineer and construct a polyacrylonitrile (PAN) and carbon fiber production plant in Bengbu, a city in Anhui province, located in eastern mainland China. The plant will be the first combined polyacrylonitrile and carbon fiber manufacturing facility built in China. The operation will include production of PAN precursor, which then will be converted into carbon fiber tow at the same location.

The plant will house all the facilities necessary for raw material preparation and storage, batch polymerization processing (conversion of acrylonitrile monomer into polyacrylonitrile), the spinning of the polymerizcd product into yarn, all further processing and drying necessary to form the PAN precursor, collection of the precursor on bobbins, and the final pyrolization process that forms the carbon fiber product.

Project completion is scheduled for February 2005. China Worldbest Group is China's largest textile/pharmaceutical group, with 50,000 employees working in 34 subsidiaries and affiliates in China and elsewhere in the world.


In the ongoing search for a lower-cost carbon fiber precursor, researchers from Virginia Tech's Materials Research Institute (MRI) and Clemson University have worked over the past three years, with Department of Energy funding, to develop cheaper, more environ- mentally friendly materials. During laboratory-scale trials, the team has developed acrylic fibers that can be spun directly from the melt, from 100% solids and without the addition of solvents. Elimination of solvents cuts processing costs and avoids hazardous waste handling. In addition, by adding a molecular component to the polymer that reacts with ultraviolet (UV) light, the group hopes to use photo crosslinking and UV energy to cut fiber carbonization time down to one or two hours. It is expensive to process material for 10 or more hours at very high temperatures. If the process can be successfully scaled up, lower- cost carbon fiber may be the result.

R&D is being conducted on a new generation of carbon fibers. See Chapter 10 Nanotechnology Successes for details concerning the potential of this new generation carbon fibers.

Graphite Fibers

Graphite is a relatively soft, black material that is naturally occurring. It is made synthetically via graphitization principally to produce graphite fibers for high performance RPs. It is a crystalline allotropic form of carbon. This allotropic structure exists as a substance, especially an element, in two or more physical states such as crystal and graphite.

Graphitization is the formation of graphite-like material from organic compounds by pyrolyization. Pyrolysis is the technology of decom- posing organic materials at high temperatures where chemical change is brought about by the action of heat occurring during carbonization. An intermediate phase in the formation of carbon from a pitch precursor is called mesophase. This is a liquid crystal phase in the form of microspheres which upon prolonged heating above 400C (750F) coalesce, solidify, and form regions of extended order. Heating above 2000C (3630F), forms graphite structures. Fiber properties relate to degree of heat.

The process of pyrolyization occurs in an inert atmosphere at temperatures in excess of 2000C (3630F), usually as high as 2480C (4500F), and sometimes as high as 9750C (5400F), converting carbon to its crystalline allotropic form. Temperature depends on precursor and final properties desired. These carbon fibers develop higher modulus of elasticity and the product is usually identified as graphite fiber. During pyrolysis, molecules containing oxygen, hydrogen, and


nitrogen are driven from the precursor fibers, leaving continuous chains of carbon.

The terms graphite and carbon are often used interchangeably, though they differ. The basic differences lie in the temperature at which the fibers are made and heat-treated, in the amount of elemental carbon produced, and mechanical properties. Carbon fiber is produced by controlled oxidization and carbonization of fiber-form precursors. The usual precursors are cellulose, polyacrylonitrile (PAN), lignin, and pitch, of which PAN is most commonly used as it has high carbon content. Oxidization and carbonization at temperatures of up to 2600C (4500F) produces a high-strength fiber, and graphitization by increasing the temperature to 3000C (5500F) produces a high modulus graphite fiber. They assay at more than 99% elemental carbon.

As reviewed, this heat chemically changes the fiber yielding high strength/weight and high stiffness/weight fibers. Resistance to fatigue and creep is high and the fiber has good resistance to wear, together with properties of vibration damping, thermal stability and high long- term resistance to corrosion. As it is carbon, the fiber also imparts electrical conductivity and it is permeable to X-rays. Surface treatment and sizing is used to improve bonding and handleability. The latest types of fiber also offer lower fuzz and greater spreadability.

The resulting fiber is stronger than steel, lighter than aluminum, and stiffer than titanium. Carbon fiber (CF) can also be produced from a pitch precursor, but the elongation of these fibers tends to be low. Pitch is the high molecular weight residue from the destructive distillation of petroleum and coal products.

The usual grades of fiber (indicated by their initials) are high strain (HS), low modulus (LM), high modulus (HM), ultra high modulus (UHM), high strength (HT), and intermediate grades, such as intermediate modulus (IM). The most common form is a high tensile strength fiber, produced by most suppliers. CFs offers the highest specific stiffness and very high strength in both tension and compression. Their impact strength is lower than that of glass or aramid fibers, and carbon is often combined with these to form hybrid materials.

Examples of performance in TP matrix with carbon or graphite fibers include the use of epoxy and nylon (PA). Nylon 6 (DuPont) with a 30 wt% fiber content will increase flexural strength by about three times, and flexural stiffness may be raised by a factor of seven. Electrical properties, friction behavior and wear resistance may also be improved. The electrical applications largely fall into two categories: to impart conductivity and prevent build-up of electrostatic discharge (which may


cause short circuits or explosions when handling hazardous materials), and to screen components from electromagnetic interference. For engineering applications, frictional and wear properties are very good in comparison with non-reinforced or glass fiber-reinforced compounds. Unreinforced nylon has static and dynamic friction coefficients about 25% and 40% higher than those of a carbon-reinforced PA 6, while the abrasion factor is ten times higher. Combined with the higher thermal conductivity of carbon-reinforced compounds, this produces higher pv values. These are a measure of the heat generated by parts sliding in contact with each other (p = bearing pressure, v = sliding velocity).

Graphite or carbon fiber is supplied as a continuous or as a chopped fiber. Continuous fiber can be combined with virtually all thermoset and thermoplastic resin systems and is used for weaving, braiding, prepreg manufacture and filament winding. Chopped fibers can be used in molding compounds for compression and injection molding, giving parts with high resistance to corrosion, creep and fatigue, with high strength and stiffness (Tables 2.16 and 17).

Boron Fibers

Boron fibers (BFs) were the first high strength, high modulus fiber to be produced. They are produced by chemical vapor deposition from a gaseous mixture of hydrogen (H2) and boron trichloride (BCI3) on primarily an electrically heated tungsten substrate of 0.5 rail (12.5 pro) diameters. The resulting amorphous boron has excellent properties, but the process is very costly. The final filament diameter is 4 mil (100 ~am), 5.6 mil (140 pm), or 8 mil (200 }am) in descending order of production quantifies; however, both small and large diameters have been produced in experimental quantities. Performance wise, they have exceptionally high tensile strength and modulus of elasticity with a relatively low density.

High and uniform modulus and tensile strength are attainable [modulus of 380-400 GPa (55-58 x 106 psi) and tensile strengths of 3.5-3.65 GPa (500,000-530,000 psi)]. There are lower-cost fibers but they are not as consistent or high performance. Upper temperature limit in an oxidizing atmosphere is 250C (480F). This high cost material usually has a matrix of expensive high performance epoxy plastic. This was the first of the high strength, high modulus fibers to be produced; U.S. Air Force Materials Laboratory, Dayton, OH was very influential in its development during the early 1950s. Use includes aerospace structural parts, tennis rackets, fishing rods, golf clubs, etc.

Table 2.16 Examples of graphite fiber/thermoset and thermoplastic RPs

',,4

r ,m,

m l

t'1)

Graphite-epoxy RC = 29-33%

Grephite-epoxy VC = I. 7-2.40/0

Property Units 22~ {72~ 277~ {350~ 22~ (72~ 177~ {350~

Graphite-polyimide RC = 350/0 VC= 00/0

22~ (72~ 177~ {350~

Graph ite-polyimide RC= 27.5-31o/o

VC= 0~

22~ (72~ 177~ (350~

Graphite-polysulfone RC = 33-34o/o VC= 0-1.9O/o

22~176 117~176

I / I

m ,

I / I

, - r

Unidirectional laminate Longitudinal (0 ~ properties Tensile strength Ksi(MPa) 218 (1502) 208 (1433) 197.7 (1362) 141.7 (976) Tensile modulus of elasticity Msi(GPa) 26.3 (181) 28.5 (196) 20.3 (140) 19.3 (133) Compressive strength Ksi(MPa) 218 (1502) 206 (1419) 157.4 (1084) 148.1 (1020) Compressive modulus of elasticity Msi(GPa) 23.0 (158) 22.5 (155) 19.8 (136) 25.0 (172) Flexural strength Ksi(MPa) 247 (1702) 196 (1350) 200.7 (1383) 96.4 (664) Flexural modulus of elasticity Msi(GPa) - - 17.77 (122) 16.29 (112) Interlaminar shear strength (short beam) Ksi(MPa) 15.9 (110) 8.9 (61) 12.69 (87.4) 7.18 (49.5) Transverse (90 ~ ) properties Tensile strength Ksi(MPa) 3.85 (26.5) 2.89 (19.9) 4.9 (33.8) 3.7 (25.5) Tensile modulus of elasticity Msi(GPa) 1.50 (10.3) 1.78 (12.3) 1.3 (9.0) 1.05 (7.2)

156.7 (1080) 152.2 (1049)

20.25 (140) 19.56 (135) 180.0 (1240) 120.0 (827)

18.2(125) 20.5(141) 204.0 (1406) 179.1 (1234)

16.89 (116) 18.43 (127)

14.8 (102) 10.2 (70.3)

2.82 (19.4) 2.97 (20.5)

1.50 (10.3) 1.15 (7.9)

203.3 (1401) 187.4 (1291)

18.3 (126) 18.9 (130) 206.1 (1420) 164.6 (1134)

18.7 (129) 19.1 (132) 224.4(1546) 178.8 (1232)

18.4 (127) 17.3 (119)

13.62 (93.8) 9.79 (67.5)

5.37 (37) 3.81 (26.3)

1.39 (9.6) 1.09 (7.5)

187.9 (1295) 179.1 (1234)

16.3 (112) 17.5 (121) 102.1 (703) 90.2 (621)

17.3 (119) 18.5 (127) 191.5 (1319) 135.2 (932)

17.8(123) 20.0(138)

11.6 (79.9) 8.4 (57.9)

5.02 (34.6) 5.39 (39.1)

1.15 (7.9) 1.07 (7.37)

a " 0 0

RC = resin content by weight

VC = void content by volume

2-Reinforcements 79

Table 2.17 Properties of carbon and graphite fabriclthermoset resin RPs

Material composition

Epoxy- Epoxy Phenolic (330/o), Phenolic (31O/o), novolac (370/0), novolac (350/0)

carbon graphite carbon graphite fabric (670/o) fabric (690/0) fabric (63O/o) fabric (650/0)

Density, Ib/ft 3 84 89 85 85

Tensile strength, psi

Parallel

75~ 21,600 12,900 18,500 10,100

350~ 19,250 12,325 2,620 3,820

Perpendicular

75~ 2,450 680 4,000 1,000

350~ 230 574 160 100

Tensile elastic modulus, million psi,

Parallel

75~ 3.50 1.58 2.63 2.16

350~ 2.80 1.70 0.64 0.58

Perpendicular

75~ 2.39 0.81 1.46 1.13

350~ - 0.48 0.09 0.14

Tensile elongation at failure, %

Parallel

75~ 1.08 0.97 1.15 0.77

350~ 1.14 1.10 0.67 0.47

Perpendicular

75~ 0.10 0.11 0.29 0.10

350~ 0.32 0.16 0.19 0.10

Shear strength, psi

Parallel

75~ 4,000 1,820 - 2,600

350~ 3,230 1,590 - 1,500

Compressive strength, psi 35,000 10,800 16,700 -

Compressive elastic modulus, million psi 1.57 1.29 1.19 -

Flexural strength, psi 30,000 23,000 16,000 -

Flexural elastic modulus, million psi 2.4 1.9 0.93 -

Hardness, Barcol 73 52 - -

Specific heat, Btu/Ib-~ 0.24 0.26 - -

Thermal conductivity, Btulhrlft3-~ Pa ra Ilel 0.82 1.30 0.59 1.50

Perpendicular 0.27 0.80 0.35 0.66

Thermal expansion coefficient, 10 -6 in./in./~

Parallel 3.8 2.4 7.5 7.0

Perpendicular 8.5 9.0 60.0 53.0

Emittance 0.8 0.8 0.8 0.8

All properties are parallel to the fabric warp unless otherwise noted.


Silica Fibers

Silica fibers are smooth-surfaced, glasslike fibers, with a nearly round cross section. They are spun from silicon dioxide, which may be pure or contain a small amount of other materials. Silica fibers can be produced indirectly from glass filaments from which all constituents other than silica have been removed, or through spinning a viscous filament that contains a high amount of silica. The organic materials are burned away, leaving a porous silica filament. Silica fibers are principally used in chemical engineering, high temperature electrical insulation, and high temperature thermal insulation.

Quartz Fibers

Quartz fibers are made from natural quartz crystals by softening quartz rods in an oxy-hydrogen flame and drawing rods into filaments. Because high purity quartz crystals are rare, the cost of quartz fibers is considerable higher than that of glass fiber and high silica fibers.

Quartz filaments have assumed a role as high-temperature resistant fibers, and are produced in considerable quantities for high temperature and corrosion-resistant applications. They are widely used as filtration and insulation materials at temperatures above those of mineral silicate fibers. Quartz and silica fiber RPs are used in jet aircrafts, rocket nozzles, nose cones, and reentry heat shields for spacecraft.

Quartz fiber tensile strength at room temperature is 130 x 103 psi (896 MPa) and at 204C (400F) is 99 x 103 psi (682 MPa). Tensile modulus at room temperature is 10 x 106 psi (6.89 GPa).

High silica (s.g. 1.74) and quartz (s.g. 2.2) fibers have higher strength- to-weight ratios than most other high temperature materials. Mso, quartz has about 5 times the tensile strength of high silica. Both types of fibers are almost perfectly elastic and their elongation at break is about 1%.

Fiber/Filament Characteristics

Information on fibers applicable to different materials of construction and use that relates to compositions, surface treatments, performances, processing, and terminology follows:

Fiber Fiber is a general term used to refer to filamentary materials. Often it is used synonymously with filament, monofilament, whisker, and yarn. It is any material in a form such that it has a


minimum length of at least 100 times its diameter. Diameters are usually 0.004 to 0.005 in. (0.10 to 0.13 mm). Fibers can be continuous or reduced to short lengths (discontinuous) where the industry lists less than as having a specific length such as 0.125 in (3.2 mm). A filament is the smallest unit of a fibrous material and usually not used alone. They are the basic units formed during manufacture that are gathered into strands of fiber. Their diameters are less than 0.001 in. (0.025 mm). Its denier also identifies the fineness of a fiber.

Fiber, abaca The plant Musa textiles provides 6 to 12 ft (1.8 to 3.6 m) long fiber bundles used in the manufacture of ropes, cables, and RPs.

Fiber, acrylic Filaments made from any long chain synthetic plastic containing 85 wt% or more acrylonitrile.

Fiber, alumina-silica Amorphous structure with excellent resistance to all chemicals except hydrochloric acid, phosphoric acid, and concen- trated alkalis. Tensile strength is 400,000 psi (2,000 MPa), modulus of elasticity 16 million psi (110 GPa), upper temperature limit in oxidizing atmosphere at 1,470F (800C), noncombustible, low heat conductivity, and thermal shock resistance.

Fiber, aramid See Aramid Fibers previously reviewed in this chapter.

Fiber, areal weight The weight of fiber reinforcement per unit area (width x length) of tape or fabric.

Fiber at tenuation The process for making thin or slender. It applies to the formation of fiber from molten glass.

Fiber biconsti tuent A hybrid or composite fiber comprising a dispersion of fibrils of one synthetic plastic within and parallel to the longitudinal axis of another; also a construction of plastic and metal or alloy filaments.

Fiber binder See Glass Characteristics, Glass fiber b inder /s iz ing coupling agent in this chapter.

Fiber bobbin Sometimes also called a package. It is the smallest production unit of yarn or roving, including its appropriate (usually cardboard or plastic tube) support.

Fiber, boron See Aramid Fibers previously reviewed in this chapter.

Fiber braided/direct ional Weaving fibers into a tubular shape. As an example, A&P Technology, Cincinnati, OH, specializes in developing braided reinforcements using a variety of material types, braid forms, and braid architectures for applications ranging from prostheses to


airframe structures to hockey sticks to boat hulls. Their braided reinforcements allow the molder to optimize fiber architectures better than conventional fabric reinforcements, while enabling an easy method of manufacture. Standard reinforcements include carbon, Kevlar, and glass fiber sleevings, broadgoods, and tapes, and its most recent additions are Bimax TM biaxial fabric, Trimax TM triaxial broadgoods and Zero TM unidirectional carbon fabrics.

Bimax biaxials are constructed to provide the molder with a _+45 ~ fiber orientation fabric that does not need to be cut, stitched, or manipulated. Its design makes it highly drapeable and conformable, which reduces processing time and cost by making the lay-up process easy and consistent. Fine hot melt yarns are incorporated in the axial direction to enable better handling.

Zero unidirectional carbon fiber fabric was originally developed for Lockheed Martin's F-22 air fighter program. Designers at Lockheed were looking for a unidirectional fabric with increased compression values and to meet these needs, A&P designed a nonwoven unidirectional reinforcement. Since it is nonwoven there is little crimp in the individual yarns, which increases the fabric's compression values. In the Spring of 2003, intermediate modulus Zero fabrics were qualified for applications on the F-22, which involved testing against a baseline of unidirectional prepreg and dry fabric lay-up. In all areas, including compression, Zero outperformed the baseline. It has since been specified for use throughout the airframe where it will replace other dry unidirectional fabrics a n d / o r prepregs.

Zero's use is not restricted to high cost aerospace applications as A&P designed the fabric to provide affordable but superior performance. It can be used in a wide range of RP molding processes including resin transfer molding (RTM), vacuum assisted RTM (VARTM), resin film infusion, and hand lay-up (Chapter 5).

Fiber breakout Fiber separation or break on surface plies at drilled or machine edges.

Fiber bridging Reinforcing fiber that bridges an inside radius that is caused by shrinkage stresses around such a radius during curing.

Fiber bristle A generic term for a short stiff, coarse fiber.

Fiber bundle Identifies a collection of essentially parallel fibers or filaments.

Fiber, buttress A type of thread used for transmitting load in only one direction. It has the efficiency of the square thread and the strength of the V-thread.

2" Reinforcements 83

Fiber capillarity action The attraction between molecules, similar to surface tension, which results in the rise of a liquid in fibers, as could occur in RPs, etc.

Fiber, carbon See Aramid Fibers previously reviewed in this chapter.

Fiber, carbon stabilization During forming, it is the process used to render the carbon fiber precursor infusible prior to carbonization.

Fiber carding A process of untangling and partially straightening fibers, such as cotton and asbestos, by passing them between two closely spaced surfaces which are moving at different speeds, and at least one of which is covered with sharp points. Carding machine converts a tangled mass of fibers to a filmy web. Use includes reinforcements in reinforcing plastics.

Fiber, cellulose acetate Acetyl derivative of cellulose. Triacetate designation can be used when not less than 92% of the cellulose groups are acetylated.

Fiber, ceramic See Other Fibers and Reinforcements in this chapter.

Fiber count The number of warp fiber/yarn (ends) and filling fiber/yarn (picks) per inch. Cross section or thickness of fiber, yarn or roving expressed as denier. See Fiber decitex.

Fiber creel A spool, along with its supporting structure, that holds the required number of fibers or roving balls for supply packages in a desired position for unwinding into the next processing step such as weaving, braiding, filament winding, RP fabrication, etc.

Fiber crimp The waviness of a fiber or fabric responsible for void formations. It determines the capacity of fibers to cohere under light pressure. Measure either by the number of crimps, waves per unit length, or the percent increase in extent of the fiber on removal of the crimp.

Fiber decitex Also called dtex or (deci)tex. This is a property unique to the fiber industry to describe fineness (and conversely the cross sectional area) of a filament, yarn, rope, etc. It is defined as the weighting of 10,000 m of the material. One decitex-- 0.9 denier.

Fiber denier It is a unit of weight expressing the size or coarseness but particularly the fineness of a continuous fiber or yarn. The weight in grams of 9000 m (30,000 ft) is one denier. The lower the deniers, the finer the fiber, yarn, etc. One denier equals about 40 micron. Sheer women's hosiery usually runs 10 to 15 denier. Commercial work of 12 to 15 denier fiber is usually generated.


Fiber desizing Process of eliminating sizing from gray or greige goods before applying special finishes or bleaches. Also removing lubricant size following weaving of a cloth.

Fiber directional property See Chapter 7.

Fiber drawing Fiber with a certain amount of orientation imparted by the drawing process when the fibers are formed. Result is significant increase in strength and other properties.

Fiber end It is an individual fiber, thread, roving, yarn, or cord. In fabric, an end is a warp yarn, running the length of the fabric. A strand of roving consisting of a given number of filaments gathered. The group of filaments is considered an end or strand prior to twisting.

Fiber, felt A fibrous material made up of interlocking fibers by mechanical or chemical action, moisture, and/or heat. They can be made of many different fibers, including glass, cotton, nylon, etc.

Fiber fibrillation Production of fiber from film.

Fiber finish The surface treatment applied to processed fibers, particularly glass fibers.

Fiber f'mish, satin Type of finish having a satin or velvety appearance that is midway glossy (or bright) and mat. It behaves as a diffuse reflector that is lustrous but not mirror-like.

Fiber, flax Natural fiber obtained from the inner bark of the flax plant. Use includes as filler and in producing of high strength reinforced or laminated plastics.

Fiber float A warp or filling fiber that lies on top of the opposite series of yarn for a distance of several fibers.

Fiber flock Very short fibers used as fillers in plastic materials that can improve processing, properties, and/or reduce cost. Reducing fibers to these fragments is by cutting, tearing, or grinding producing different forms that include entangled fibers, small bead size, or usually broken fibers.

Fiber flocking Also called flock spraying. It is a method of coating by spraying finely dispersed fibers or powders by pneumatic or electrostatic means on adhesive coated surface producing a velvety surface. Another method takes preheated parts that are dropped into a bed of fibers or powders. Fibers used include nylon, rayon, cotton, or TP polyester. It provides an attractive/decorative surface, sound absorber, etc.

2-Reinforcements 85

Fiber fuzz Accumulation of short, broken filaments after passing strands, yarns, rovings, etc. over a contact point.

Fiber, graphite See Aramid Fibers previously reviewed in this chapter.

Fiber, hollow These plastic fibers can produce high bulk, low-density fabrics. Other fiber configurations can be produced such as trilobal cross section. Annular dies are used to produce the desired hollow cross section shape. Fiber spinning methods used are: (1) wet from a plastic solution into a liquid coagulant, (2) dry from a plastic solution in a volatile solvent with an evaporative column, and (3) conventional melt systems.

Fiber, hybrid Two or more different types of fibers are used to provide different RP performances.

Fiber, inorganic Fibers used in RPs, etc. include glass (different types), aluminum silicate, beryllium glass, carbon, ceramic, graphite, and quartz (fused silica).

Fiber, jute See Jute Fibers previously reviewed in this chapter.

Fiber, Kevlar See Aramid Fiber previously reviewed in this chapter.

Fiber kink Also called curl yarns, looped yarn, or snarl in a fiber. In fabric, a short length of yarn that has spontaneously doubled back on itself to form a loop. It can be a type of a waviness occurring as interior edges, not to be confused with the more abrupt departures as ridges or surface marks.

Fiber length, critical Minimum fiber length required for shear loading to its ultimate strength by the matrix.

Fiber linter Short, fuzzy fibers that adhere to the cotton seed after ginning. Use includes in rayon manufacture, as fillers for plastics, as a base for the manufacture of cellulosic plastics etc.

Fiber manufacturer Originator of commercially produced glass fibers Owens Corning is a world leader in building materials systems and RP solutions. Founded in 1938 by Owens Illinois and Dow Coming, the company had sales of $4.8 billion in 2001 and employs approximately 19,000 people worldwide. Additional information is available on Owens Corning's Web site at www.owenscorning.com or by calling the company's line 1-800-GETPINK.

Fiber mat A fibrous material used in RPs; consists of randomly and uniformly oriented: (1) chopped fibers with or without cartier fibers or binder plastics; (2) short fibers with or without a cartier fabric; (3) swirled filaments loosely held together with a plastic binder; (4)


chopped or short fiber with long fibers included in any desired pattern to provided addition mechanical properties in specific directions; and so on. They are produced in flat and curved blanket sheets, tape forms, etc. for use in the different RP processes.

Fiber mat, needled A mat felted together in a needle loom with or without a cartier.

Fiber mat veil An ultra-thin mat similar to a surface mat, often composed of organic fibers as well as glass fibers (Table 2.18).

Fiber material, plastic Different TPs are used to produce fibers. The more important production wise materials are PP, nylon 66, polyester, and PETP; other plastics are also used. Each plastic family has different grades to provide different properties during and after being processed. Their plastic fiber structures have different levels of molecular organizations with each relating to certain aspects of fiber behaviors and properties. As an example, their organochemical structure defines the chemical composition and molecular structure. This molecular structure is directly related to the fibers chemical properties, dye ability, moisture sorption, swelling characteristics, and indirectly related to all physical properties. The physical properties of fibers are influenced by the processing techniques used on-line where factors from melt conditions to windup speed. However, they are strongly influenced by the plastic morphology. All plastic fibers that are useful in textile applications are usually semicrystalline (usually referred to as crystalline), irreversibly in an oriented pattern.

Fiber, metallic Manufactured fiber used in RPs includes metal, plastic- coated metal, metal-coated plastic, or a core completely covered by metal. Included are steel, aluminum, magnesium, and tungsten. The latest steel reinforcement (Hardwire TM 3S and 3SX) from Hardwire, of Pocomoke, MD, USA, provides a significantly greater flexural strength and modulus with ductility than its previous products. The new steel cords grades have been specially designed by Hardwire in partnership with cord manufacturer Goodyear. The fibers can either be used as reinforcement in their own right or together with other fibers such as glass or carbon to produce fiber RP structures that fail in a ductile manner rather than catastroph- ically. They can be used with various resin systems including TS polyester, vinyl ester, modified acrylic, urethane and epoxy. It permits fabricating RP boats that dent instead of tear open, concrete repairs that survive fire and earthquakes, pressure vessels, pipe that exhibit no long-term creep or stress rupture, and wind turbine blades that are lower in weight, faster to make and lower in cost.


Table 2.18 Characteristics and applications of nonwoven surfacing veils

Type Physical data Characterist ics Applications

Aramid: para-aramid, 10-150 glm 2 2.8 Pa strength, Tensile strength: 70 GPa modulus; up to 8 Nlmm 2 chopped staple Density: or fibrid (pulp) 80 kg/mm 3

Improved impact resistance Smooth finish Good wear resistance Can be blended with conductive fiber

Superior temperature resistance

Aerospace: adhesive carriers Automotive: improved stone impact resistance Defence: radar cross section Recreation: skis, snow and surf boards, surf boards, canoes

Industrial: substrata for friction products; wear resistance for high-speed rolls

Electrical: printed circuit boards

Carbon: PAN-based, 8-200 glm 2 Integral electrical

200 or 250 GPa Tensile strength: conductivity modulus 3-25 mm up to 20 Nlmm 2 Corrosion resistance fiber length Density: Improved strength,

80 kg/mm 3 stiffness, surface finish

Chemical vessels and pipework: electrical grounding, improved

corrosion resistance; spark lasting tank liners Computer cases: high strength, integral EMI/RFI shielding Electronics Fuel cells Sports goods Pultrusion: increased hoop strength, better surface finish

Glass: 10-200 glm 2 C-glass Tensile strength: (chemical resistance) up to 12 Nlmm 2

E-glass Density: (electrical properties) 140 kg/mm 3

Improved impact, Aerospace: interior surface finishing wear resistance Defence: blended with conductive fiber Can be blended with to reduce radar cross section conductive fiber to Recreation Industrial: C-glass improves reduce static discharge corrosion resistance; better wear Reduced weight and cost with less paint and resin can prevent galvanic corrosion by separating dissimilar conductive materials Superior temperature resistance

resistance; blended with conductive fiber to reduce static discharge Electronics: printed circuit cards

Polyester 10-100 g/m 2

Tensile strength: up to 10 N/mm 2

Density: 120 kg/mm 3

Improved impact, wear resistance Electrical transparency

Superior drape to glass surfacing

Opacitylwhiteness Good acid resistance

Aerospace: adhesive carriers surfacing layer for radomes Defence: blended with conductive fiber

to reduce radar cross section Recreation: smooth complex shapes; screen printable Industrial: reaction vessels, double curvature; flexible laminates

Source: Based on data from Technical Fibre Products


Its improved stiffness, strength, and ductility is targeted to enable fabricators in molding extremely strong parts like conventional RPs yet act more like metal. Working with the US Navy as the testing partner, Hardwire and Goodyear worked on material, configuration, surface chemistry, and production machinery developments to achieve the desired performances (website: www.hardwirellc.com).

Fiber, micro- Fiber whose individual filaments are less than 1.0 denier or 1.0 tex. They are four times finer than the average human hair; at least three times finer than cotton fiber; and finer than natural silk. They are TP polyester spun and oriented to produce incredibly lightweight, durable, and water resistance.

Fiber, nylon See Aramid Fibers previously reviewed in this chapter.

Fiber, nanotube See Chapter 10 Nanotechnology Successes.

Fiber, one-ended A fiber so short in length it appears that it only has one end. Examples include very short length milled glass fibers and asbestos fibers.

Fiber orientation See Chapter 7 Directional Properties.

Fiber optics Fiber optics may be defined as the guidance of electromagnetic radiation along transparent dielectric hair-thin glass fibers. More specifically, the guidance usually involves the mechanism known as total internal reflection. If the fibers are of dimensions comparable to the wavelength of light, the fiber will act as a wave- guide to conduct the radiation in discrete modes. Glass fibers with extruded plastic coating, usually PE, are used. A fine-drawn silica (glass) fiber or filament of exceptional purity and specific optical properties (refractive index) that transmits laser light impulses almost instantaneously with high fidelity is used.

Fiber, other They include natural/vegetable, sisal, asbestos, ramie, flax, soya bean/cellulose, and hemp types.

Fiber pattern The pattern formed by the fibrous strands.

Fiber pencil A rod-like assemblage of fibers in close packed parallel orientation of generally uniform diameter that can be fiberized readily.

Fiber pick Also called fill, woof, or weft. An individual filling yarn running the width of a woven fabric at right angles to the warp.

Fiber, polybenzimidazole High strength, heat resistance fiber made from polybenzimidazole plastic

Fiber, polyethylene terephthalate Plastic fiber identified as XTC.


Fibers processing Thermoplastic fibers or filaments can be produced by screw extruders. They are manufactured using the three common methods of melt spinning, dry spinning, and wet spinning. There are many variations and combinations of these basic processes. Other types of fiber-forming processes include: (1) reaction spinning; (2) dispersion, emulsion, and suspension spinning; (3) fusion-melt spinning; (4) phase-separation spinning; and (5) gel spinning. The processes force molten plastic by an extruder and/or gear pump through fine holes in a spinneret (or spinaret) die. In turn they are immediately stretched or drawn (oriented), cooled, and collected at the end of the line. During this process, they may be subjected to other operations such as: (1) thermal setting and thermal relaxation processes to provide dimensional stability; (2) twisting and interlacing to provide cohesion of the filaments with or without sizings; (3) texturing; and (4) crimping and cutting to provide staple products. Speeds of certain lines using the melt and dry spinning processes can go from 6,600 to 13,000 ft/min (2,000 to 4,000 m/min).

Numerous techniques for producing fibers without using the spinneret have been used. They include centrifugal spinning, electrostatic spinning, tack spinning, and solid-state extrusion (SSE). The SSE process extrudes through a capillary remoter with a conical die; the processing temperature is close to the melting point of the plastic.

In the manufacture of fibers a relatively isotropic plastic with properties similar in all directions converts into an orthotropic plastic where most of the plastics strength is in the direction of the fiber axis. This desirable effect provides a certain degree of fiber strength in their longitudinal direction, but usually not enough. So the fibers arc made stronger by stretch-orientation during or after processing. These spinning lines can include a variety of operations useful for the fibers different applications. A finish can be applied after cooling in-line. Rather than keeping them straight, texturing techniques are used. Texturing introduces crimp, whereby the straight filaments are given a twisted, coiled, or randomly kinked structure. A yarn made up of these filaments is softer and more open in structure; it is more pleasing to the touch. Finishes are used to improve the processing and handling of fibers. The finishing mix can include a lubricant.

Fiber processing development Unusual plastic fibers such as poly- olefin fibers are produced by a spurted or melt-blown spinning technique. A variety of directly formed nonwovens with excellent filtration characteristics is produced. Original development was by


Exxon Corp. that produced very fine, sub-micrometer filaments. Pulp like olefin fibers are produced by a high-pressure spurting process developed by Hercules, Inc. and Solvay, Inc. A high modulus commercial PE fiber with properties approaching those of aramid and graphite fibers is prepared by gel spinning. Higher tensile strengths are also available from gel spinning or fibrillar crystal growth.

Fiber processing filtration Many processes require a plastic melt free of contaminates larger than a specific size. The fiber processors usually filter down to 5 micron particle size to protect the melt spinning machines from filament breaks. The fiber process typically operates at very high speeds. A filament break at this speed is costly to both the product quality and the process efficiency. The media used for filtration has included sand packs, wire screens, sintered metal powder sheets, and sintered metal fiber sheets. There are also different sandwich combinations such as wire screens and sintered metal fiber sheets that in theory provided the best properties of each component. With a gear pump running clearance can be as low as 0.00025 in. (0.006 ram) about its periphery and on either side of the metering gear. Any slight burr, nick, or particle of any "foreign" matter will cause scoring and possible seizure of the pump. Recognize that 0.001 in. (0.025 ram) equals 25 microns, so filtration down to just the pump has to be down to 6 microns or less.

Fiber processing, solid-state extrusion SSE is a means for the deformation and evaluation of uniaxial molecular chain orientation and product extension for a wide range of plastics. Developments produced HDPE drawn into fibers with some of highest specific tensile moduli and strengths. A two-step drawing process is used for the preparation of polyoxyethylene, PP, and PE fibers. The plastic is first drawn to their natural draw ratio at a fast rate and subsequently slowly super-drawn at a temperature that depends on the crystalline dispersion temperature. A highly oriented extrudate can be obtained by extruding through a capillary rheometer with a conical die at temperatures close to the melting point. Initial work led to the development of transparent and fibrous linear PE extrudates. These were obtained by extruding HDPE from the molten state [55 to 58F (132 to 136C)] above a critical shear rate in a capillary rheometer and through a conical die. This procedure was subsequently modified by processing HDPE exclusively in the solid state where the plastic is semicrystalline before extruding through the die. This modification produced continuous transparent fibers with moduli in the range of 4,400 to 10,200 psi (30 to 70 MPa).

2 �9 Re in forcements 91

Fiber processing, spinning The processes principally used are wet spinning, dry spinning, and melt spinning. The plastic and ingredients (such as primarily stabilizers, pigments, and rheological modifiers) are fed into a screw extruder. The gear pump accurately meters melt through a filter pack of graded sand or porous metal and a spinneret. The multihole spinneret represents the die. Upon leaving the spinneret, the molten filaments pass at very high speed usually vertically downwards into water and/or a counter current of air where they are cooled and solidified. At the same time, after leaving the spinneret, they are stretched to the desired diameter. Finish can be applied prior to the fiber reaching the end of the line where it is wound on bobbins or other windup rolls. To obtain the required high performance properties, reheating and drawing orient the fibers. This operation is usually a separate operation since it requires much higher linear speeds than melt spinning.

The slit-film or film-to-fiber technology produces a substantial volume of polypropylene (PP) fibers. Cutting or slitting the film produces fibers. Stretching before or after the cutting process orients the fiber. Also used is mechanical or chemo-mechanical fibrillation. In this procedure film is created to be anisotropy by stretching before fibrillation. This is the phenomenon wherein filament or fiber shows evidence of basic fibrous structure or fibrillar crystalline nature. It occurs by a longitudinal opening up of the filament under rapid load with excessive tensile or shearing stresses. Separate fibrils can then often be seen in the main filament trunk. The whitening of the plastic when unduly strained at room temperature is a manifestation of fibrillation. Applications for this product are primarily for carpet backing, rope, and cordage.

Fiber processing, spinning, dry In dry spinning a plastic solution is extruded (metering pumped) through a spinneret. The filaments exit the spinneret through a gas-heated cabinet where the solvent is rapidly removed from the plastic filaments. The suitable solvent is filtered and recovered for further use in-line. Filaments end up at the driven haul-off roll.

Fiber processing, spinning, gel Basically the world's strongest commercial fiber, the polyethylene fiber strength is increased by 30% via gel spinning, 15 times stronger than steel, and twice as strong as aramid fiber. It uses ultrahigh molecular weight PE (patented by DSM High Performance Fibers, Heerlen, Netherlands). In this process, UHMWPE molecules are dissolved in a volatile solvent. By cooling and solvent removal, a gel-like fiber is spun from this solution. It is then subjected to a drawing-orienting operation. In


the gel-like fiber, the molecules lie folded in crystals that are at fight angles to the fiber length. In the drawing process, these molecules are tipped over so they unfold in longitudinal direction, which is what gives the fiber its high strength.

Fiber processing, spinning, jet For most purposes this process is similar to fiber spinning. Hot gas jet spinning uses a directed blast or jet of hot gas to pull molten plastic from a die lip and extending it into fine fibers.

Fiber processing, spinning, raw nucleation The mechanism by which stress-induced crystallization is initiated usually during fiber spinning or hot drawing.

Fiber processing, spinning, reaction A liquid polymer is extruded through a spinneret plate and encounters a chain extending crosslinking component producing a filament.

Fiber processing, spinning, solution Process is a used to produce high modulus polyethylene fibers; fibers are called extended-chain PE (ECPE). Fibers have tensile strengths of 3.75 to 5.60 x 10 s psi (2890 to 3860 MPa) and moduli of 15 to 30 x 106 psi (103 to 207 MPa). In this process, a high molecular weight PE is used. The process begins with the dissolution in a suitable solvent of a polymer of about 1 to 5 million molecular weight. The solution serves to disentangle the polymer chains, a key step in achieving an extended chain polymer structure. The solution must be fairly dilute but viscous enough to be spun using conventional spinning equipment. The cooling of the extrudate leads to the formation of a fiber that can be continuously dried to remove the solvent or latter extracted by an appropriate solvent. The fibers are generally post-drawn or stretched oriented.

Fiber processing, spinning, wet In this process, also called reaction spinning, a plastic solution is extruded from a spinneret and immersed into a spin bath tank containing a circulating non-solvent solution that coagulates (precipitates) the plastic filaments. Both the solution and the precipitation stages involve chemical reactions. After passing through the spin bath tank, they are washed prior to the windup. Conventional wet spinning has the slowest line speed compared to the other lines. However, since it permits very short distances between the holes in the spinneret face, a single spinneret may carry a very large number of holes. With this single spinneret, high production rates can still be obtained.

Fiber proper ty Different grades of each fiber exist so that properties can change. Typical tensile strength values in lb/ in 3 (g /cm 3) are:


aramid 90.4 (2.55) and E-glass 159.0 (2.55). Their modulus of elasticity in 106 psi (104 MPa) are: aramid 27 (18.6), E-glass 10 (6.9), and S-glass 12 (8.6).

Fiber, quartz See Quar tz Fibers previously reviewed in this chapter.

Fiber, ramie A strong natural fiber of vegetable origin, sometimes used as a filler or reinforcing material providing high shock resistance and strength.

Fiber, rayon The generic term for fibers, staples, and continuous filament yarns composed of regenerated cellulose but also frequently used to describe fibers obtained from cellulose acetate or cellulose triacetate. Rayon fibers are similar in chemical structure to natural cellulose fibers (cotton) except that the synthetic fiber contains short plastic units. Most rayon is made using the viscose process.

Fiber, rayon viscose A regenerated cellulosic fiber made by treating wood pulp with caustic soda, and with carbon disulfide to form cellulose xanthate that is then dissolved in a weak caustic solution. It is from the latter that extrusion and coagulation forms the fiber.

Fiber reinforced plastic FRP is a term that is usually used as a generic term for all fiber RPs, regardless of process and type of fiber.

Fiber, silk A natural fiber secreted as a continuous filament by the silkworm.

Fiber skein A continuous fiber, filament, strand, yarn, or roving wound up to some measurable length and usually used to measure various mechanical and physical properties.

Fiber sliver A number of staple or continuous filament fibers aligned in a continuous strand without twist.

Fiber, spandex Elastomeric fibers are principally made from segmented polyurethanes (spandex) and polyisoprene (natural rubber). The elastomeric fibers consist of plastics with a main glass transition temperature (Tg) well below room temperature. This criterion excludes some fibers with elastic properties. The fibers are produced primarily using dry spinning and wet spinning with a few producers using melts spinning. For the natural rubber, a latex mixture is continuously forced through a capillary tube into an acid bath, where it is coagulated; the thread-like coagulum is pulled from the bath followed simultaneously with washing, drying, and curing.

Fiber, spider silk These DuPont fibers, on an equal weight basis, are stronger than steel. They are also very elastic and tough. Their combination of strength and stretch makes the energy-to-break very


high. These biosynthetic plastics provide a very broad range of mechanical properties.

Fiber spinneret The spinneret (or spinnaret) is a type of die principally used in fiber manufacture. It is usually a metal plate with many small holes (or oval, etc.) through which a melt is pulled and /o r forced. They enable extrusion of filaments of one denier or less. Con- ventional spinneret orifices are circular and produce a fiber that is round in cross section. They can contain from about 50 to 110 very small holes. A special characteristic of their design is that the melt in a discharge section of a relatively small area is distributed to a large circle of spinnerets. Because of the smaller distance in the entry region of the distributor, dead spaces are avoided, and the greater distance between the exits orifices make for easier threading. Precision machining of the orifices is required in order to avoid differences in thickness between the filaments being pulled. Note that the volumetric discharge from a cylindrical die increases with the fourth power of the diameter. An error of 10% in diameter will cause a 47% error in output. Because these differences in spinneret heads cannot be balanced out by adjusting the individual filament haul-off speeds, the diameter of the monofilament is altered by 21%.

Fiber spool Holder for fibers.

Fiber, staple Staple fibers are made up of a very large number of discontinuous, randomly oriented, individual fibers normally shipped in a box or bale. The fibers can be obtained by cutting continuous filament into 1/2 to 2 in. (12.7 to 50 mm) lengths and 1 to 5 denier or manufactured directly into desired lengths. They are usually subjected to a series of processes, culminating in textile spinning to yarn and are processed like natural fibers, such as wool and cotton, with which they can be blended.

Fiber, s traw A fibrous, cellulosic component of certain plants (wheat, rice, etc.). Its fibers are 1 to 1.5 mm long, similar to those of hardwoods. Straw can be used as filler in plastics. Its main use is preparing a pulp by the alkaline process to yield specialty papers of high quality. Use of straw for conventional papermaking in USA is of limited importance due to the abundance of pulpwood.

Fiber s t res s , m a s s Force per unit mass per unit length in grams per linear denier. Used the same way as force per unit area.

Fiber stretch, cold Pulling operation with little or no heat on fibers to increase tensile strength.

Fiber tenacity Also called breaking strength. The term generally used


in yarn manufacture and textile engineering to denote the strength of a yarn or a filament for its given size. Numerically it is the grams (of breaking force) per denier unit of yarn or filament size (gpd). The yarn is usually pulled at the rate of 5 c m / m . Tenacity equals breaking strength (g) divided by denier.

Fiber tex A unit for expressing linear density equal to the mass of weight in grams of 1000 m of fiber, filament, yarn, or other textile strand.

Fiber tow The precursor of staple fibers is tow, which consists of large numbers of roughly parallel, continuous filaments. They are converted by cutting or breaking into staple fibers or directly into slivers, intermediate stages between staple fibers and yarns. In the latter case, the filaments remain parallel.

Fiber, textile Fibers or filaments that can be processed into yarn or made into a fabric by interlacing in a variety of methods, including weaving, knitting, and braiding. These forms of textile are used with plastics to fabricate parts such as high strength tubes/pipes, electrical and medical devices, etc.

Fiber tow An untwisted bundle of continuous filaments, usually referring to fabricated fibers. As an example, a tow designated as 140 K has 140,000 filaments.

Fiber tu rn per inch Tpi is a measure of the amount of twist produced in a fiber, yarn, roving, etc. during its processing.

Fiber tracer A fiber or yarn added to a prepreg for verifying fiber alignment, in the case of woven materials for distinguishing warp fibers from fill fibers, etc.

Fiber twist In the yarn or other textile strand, it is the spiral turns about its axis per unit of length. Twist may be expressed as turns per inch (tpi). The letters S and Z indicated the direction of the twist, in reference to whether the direction conforms to the middle-section slope of the particular letter. A yarn or strand has what is known as an "S" twist if when held in a vertical position, the spirals conform in slope to the central position of the letter S. It has a "Z" twist if the spirals conform in slope to the central portion of the letter Z. Strands that are simply twisted (greater than 1 turn/ in , or 40 tu rns /m) will kink, corkscrew, and /o r unravel because of their twist is only in one direction. The plying operation normally eliminated this problem. For example, single yarns having a "Z" twist are plied with an "S" twist, thus resulting in a balanced yarn. Depending on the twisting and plying operations, different yarn strengths,


diameter, and flexibility can be obtained. This action provides the different shaped and handling fabrics that are used to meet different performance requirements of plastic materials such as coated fabrics, RPs, pultrusions, etc.

Fiber, V Fiber with its leading flank intersecting with flowing flank of adjacent fiber at the fiber root.

Fiber, vegetable Different vegetable fibers are used in RPs, etc. They include: (1) seed-hair-cotton, kapok, milkweed floss; (2) bast-flax, hemp, jute, ramie; and (3) leaf-abaca, sisal.

Fiber, vulcanized There are natural plastics such as gutta percha and shellac; the synthetics include many such as nylon and phenolics. There has been, patented in 1871, one that seems to be between the two and is known as vulcanized fiber which is processed regenerated cellulose fibers, viscose rayon, etc. In the past, this material was popular but now it is almost obsolete.

Fiber wadding A loose cohering mass of fibers in sheet or lap form.

Fiber warp Identifies the yarn running lengthwise in a woven fabric. Also a group of yarns in long lengths and approximately parallel, put on beams or warp reels for further textile processing including weaving, knitting, dyeing, etc.

Filament A single, thread-like fiber or a number of these fibers put together. A variety of fiber characterized by extreme length, which permits their use in yarn with little or no twist and usually without the spinning operation required for fibers. As an example, it is a form of glass that has been drawn to a small diameter and extreme length. Most filaments are less than 0.005 in. (0.013 cm) in diameter.

Filament greige Also called gray goods. It is any filament, fiber, yarn, fabric, etc. before finishing, sizing, dyeing, etc.

Filament lay Length of twist produced by stranding filaments, such as fibers, wires, or rovings; angle that such filaments make with the axes of the strand during a stranding operation. The length of twist of a filament is usually measured as the distance parallel to the axis of the strand between successive turns of filaments.

Filament, mono- Mso called monofill. A monofilament is a single filament of relatively indefinite length. They are generally produced by extrusion.

Filament, multi- A continuous thread comprising of several individual monofilaments.


Filament shoe A device for gathering the numerous filaments into a strand in glass fiber forming.

Filament sliver A number of staple or continuous filament fibers aligned in a continuous strand without twist.

Filament strand A primary bundle of continuos filaments combined in a single compact unit without twist. These filaments, usually 51, 102, or 204, are gathered together in their forming operation.

Filament strand end The group of filaments is considered an end.

Filament strand integrity The degree to which the individual filaments making a strand or end are held together by the applied sizing.

Filament, virgin An individual filament, which has not been in contact with any other fiber or any other hard material.

Finish The surface treatment applied to processed fibers, particularly glass fibers.

Finish, satin Type of finish having a satin or velvety appearance that is midway glossy (or bright) and mat. It behaves as a diffuse reflector that is lustrous but not mirror-like.

Flax Natural fiber obtained from the inner bark of the flax plant. Use includes as filler and in producing of high strength reinforced or laminated plastics.

Float A warp or filling fiber that lies on top of the opposite series of yarn for a distance of several fibers.

Flock Very short fibers used as fillers in plastic materials that can improve processing/properties and /or reduce cost. Reducing fibers to fragments makes them by cutting, tearing, or grinding producing different forms that include entangled fibers, small bead size, or usually broken fibers.

Fuzz Accumulation of short, broken filaments after passing strands, yarns, rovings, etc. over a contact point.

Reinforcement Fabrics and Forms

Fabric identifies any woven, nonwoven, knitted, felted, bonded, braided, knotted, three-dimensional (3-D), chopped mat, etc, textile material used to fabricate RP products. Information on fabrics applicable to different materials of construction and uses that relates to


compositions, surface terminology follows:

treatments, performances, processing, and

Batt Term used to describe felts. They are nonwoven compressed fabrics, mats, and bats prepared from staple fibers without spinning, weaving, or knitting; made up of fibers interlocked mechanically.

Bias Fabric consisting of warp and fill fibers at an angle to the length of the fabric.

Bias cut Cutting material at 450 from the weave pattern.

Bonded A web of fibers held together by an adhesive medium that does not form a continuous film.

Braids Is used to give high strength three-dimensional (3-D) reinforcement, incorporating more than one type of fiber, if required. Conventional woven fabrics are limited to providing reinforcement at orthogonal orientations, but many reinforced plastics structures are loaded in non-orthogonal fashion. Woven fabrics are, therefore, not necessarily mechanically efficient.

Braids offer the designer an opportunity to specify a non- orthogonal reinforcement, but 2-D laminated braided structures have inherent weakness in the out-of-plane direction, analogous to 2-D woven structures. Therefore, there has been development of 3- D braided preforms, the first step being so-called track-and-column braids, where most of the reinforcement is out of the plane of the general loading. However, these types might not withstand the same degree of in-plane loading as a 2-D braided preform, while the production equipment does not easily allow introduction of axial yarn reinforcement as is a common feature of 2-D constructions.

An important variant has been introduced: multilayer interlock braiding, with interlocking contiguous layers of braid, offering the possibility of a varying amount of axial yarns (or none) and a cost- effective production method. The interlocking yarns are mainly in the plane of the braided structure and thus do not significantly compromise in-plane properties of RPs. Energy absorption and residual compression strength after impact is higher than for comparable conventionally braided materials such as carbon/epoxy RPs. Multilayer interlocked braids can be produced in tubular or solid configuration.

Broad good Fiber woven into fabric usually 50 in. (1.27 ram) wide. It can be impregnated with plastic and is usually furnished in rolls of 50 to 300 lb (25 to 140 kg).


Burlap A coarse, loose woven fabric made from jute or similar fiber. Used in low cost, low performance laminated or RPs.

Canvas A closely woven cloth of flax, hemp, or cotton, which is sometimes used in industrial laminated plastics. It usually represents fabric weighing more than 4 oz /yd 2 (0.14 kg/m2).

Coun t In fabric, it is the number (count units) of warp fibers (ends) and filling fibers (picks) per unit of length (cm or in).

Cowoven A fabric woven with two different types of fibers in individual yarns. For example, TP fibers woven side by side with glass fibers.

Crimp Cloth woven with about equal corrugations in both the warp and fill.

Desizing The process of eliminating sizing, which is generally starch, from gray goods prior to applying special finishes or bleaches for fibers such as glass or cotton.

Drape The ability of a fabric weave to conform to a contoured surface.

Elastic Fabric made from an elastomer either alone or in combination with other textile materials. At room temperature, it will stretch under tension and will return quickly to its original dimensions and shape when tension is removed.

Fabric, closed molding Multicore Saint-Gobain Technical Fabrics (SGTF) has a new fabric for use in closed mold processes such as resin transfer molding (RTM), pressure and vacuum injection molding, infusion molding, and compression molding. The fabric Multicore | consists of stitched on both sides of a synthetic core. The outer layers give the fabric its strength while the core facilitates resin flow through a part. Advantages of using the product are said to include: good conformability; fast wet-out; quicker processing; compatibility with most resins; and excellent surface finish. The fabric is available in a number of weights that allow selection of a single product for varying cavity thicknesses. This also reduces cutting and installation labor and material scrap.

Fabric, dosed molding OC | FlowTex TM Owens Corning Composite Solutions business launched its latest fabric reinforcement during the 2003 October's boatbuilding exhibition in Florida, USA. With emissions standards forcing many boat builders to change from traditional open molding techniques to cleaner, closed mold processes, OC has produced OC | FlowTcx TM fabrics that can help the transition to be smooth and cost effective. Since closed molding processes are automatically MACT clean air compliant (Chapter 3),


many manufacturers are looking at ways to replace at least a portion of their open mold processes. At first glance, a switch can appear to be capital-intensive, so changeovers in the industry have been slow. FlowTex fabrics can enable boat builders (and others) who do not have the resources, but want to convert to a closed molding process, to do so in an economical way.

The fabric's unidirectional fibers are constructed in a way that increases resin flow during molding, and wet-out is up to 40% faster than with other products. So that molders don not have to modify their existing laminate designs, the fabrics are based on traditional knitted fabric technology and have comparable properties. Channels are built into the fabric structure to ensure a fast, even resin distribution. The faster flow rate can lead to higher production and mold turnover and because there is no need for local resin distribution media, the fabric can potentially decrease molding costs. A continuous filament mat version of the fabric is said to offer even faster surface flow.

Fabric, cut Distributor Composites One Co. highlights its Kit Concepts, a new service that provides manufacturers with prepackaged glass fiber material cut to size and nested in the order needed, saving the manufacturer labor-intensive cutting and loading time. The pre-cut glass fiber is said to be ideal for customers using closed mold processes such as RTM and closed cavity bag molding. Each kit contains material cut exactly to the customer's patterns and computer aided design (CAD) drawings. Cut material is then packed together in a single kit, in the order that it is used in the manufacturing process.

Fabric, gray Also called greige goods. It is any fabric, yarn, fiber, etc. before finishing, sizing, dyeing, etc.

Fabric, OC Owens Corning introduces innovative fabrics for use in closed molding, a process being increasingly utilized in the marine industry. OC Molding Mat Fabrics are unique in that they combine the infusion capability of traditional molding mat with the structural stability and visual quality associated with knitted OC TM Knytex | fiber fabrics. Designed for use in various closed molding processes including resin transfer molding (RTM) and vacuum infusion, OC Molding Mat Fabrics are a composite reinforcement comprised of a non-woven synthetic core, stitch-bonded between two layers of binder free chopped glass fiber, and/or continuous reinforcement, including a surfacing veil. The combination of these materials results in highly conformable reinforcement fabrics that boast high

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resin permeability, fast wet-out and variable part thickness capabilities.

OC Molding Mat Fabrics bring numerous benefits to the marine industry. More structurally sound than traditional molding mat, OC Knytex fabrics offer greater stability. The flexibility and versatility of the product enables it to better accommodate boat design, thereby creating new opportunities for marine designers. Additionally, OC Molding Mat Fabrics impart a surface on finished parts that is superior to traditional woven and knitted reinforcements, and their unique composition provides for reduced processing time and increased ease of handling. Combined with the corrosion advantages of OC Advantex | glass fibers, the fabrics are a unique reinforcement.

These fabrics are well suited for use in closed molding applications. They allow reduction in both the kit cutting time and the part loading time, by combining all layers of a laminate into one stack. As the marine industry is moving into closed molding, it needs to rethink the laminate composition and move away from a layer-by- layer approach.

Fabric, three-dimensional See Three-Dimensional Reinforcements in this chapter.

Fill Also called weft or woof. It is the transverse threads or fibers in a woven fabric; those fibers running perpendicular to the warp.

Filler Pieces of chopped cloth or other fabric to improve properties and /or reduce cost.

Fill face That side of a woven fabric on which the greatest number of yarns is perpendicular to the selvage.

Fourdrinier The machine most widely used for papermaking. Includes use of plastic fibers to produce "plastic" papers or nonwoven fabrics.

Gigg A machine for raising a nap on fabrics.

Glassine A thin, transparent, and very flexible paper obtained by excessive beating of wood pulp. It may contain an admixture such as urea-formaldehyde plastic to improve strength.

Gout Foreign matter, usually lint or waste fibers, woven in a fabric by accident.

Gray Also called greige goods. It is any fabric, yarn, fiber, etc. before finishing, sizing, dyeing, etc.

Hand The softness of fabric as determined by a touch, subject to the persons judgment.


Impregnated A fabric in which the interstices between yarns are completely filled with the impregnating compound throughout the thickness of the fabric, as distinguished from sized or coated fabrics where these interstices are not completely filled.

Kni t ted This woven fabric has an interlacing (interloping) yarn or thread in a series of connecting loops with needles. This is a rather compact woven construction.

Kni t ted textiles Knitted fiber reinforcement textiles can give properties more precisely tailored to the application, plus improvements in processing. These multi-axial reinforcement textiles differs from conventional materials in that flat straight fiber assemblies are knitted and cross-stitched with fine high-strength resin-compatible yarn with all needlework carried out between individual fiber assemblies, to prevent fiber damage. Layer-fiber orientations are often at + 45 ~ and -45 ~ and angles may be set as required between 30 ~ and 60 ~ . This helps to achieve the required directional and multidirectional strengths, yet gives a drapability tailored to the individual application, which is particularly important in resin transfer molding and other fabricating processes. Unlike woven reinforcement, however, the resulting textile is virtually flat, without the risk of fraying when cut and laid up. It can be more uniformly wetted out without resort to excessive resin and the resulting finish is superior, without weave pattern or wash-up of fibers to the surface of the laminate.

The Norwegian naval authorities, initially for construction of minehunters and minesweepers, have approved an advanced multi- axial range of textile reinforcement. Among the first civil applications was the hull of the 35 m luxury yacht Moonraker (which was designed to be the fastest large yacht in the world, utilizing the weight saving, strength and structural integrity of the knitted reinforcement).

Mats This is a form of glass fiber reinforcement. Supplied in roll form, it is a mat of cuffed continuous or random chopped strand held together by a light binder. Popular is the chopped strand mat (CSM) that offers uniformity of weight, improved drape ability, exceptionally fast wet-out, easy workout of entrapped air, and good surface finish. It is used to fabricate different products by different processes such as open contact molding and closed press molding, and for production of sheet (Chapter 5). The nature of the binder is important. For contact molding it must be readily soluble to facilitate rapid wetting-out and conform to the mold contours. For

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press molding it should be less soluble, to avoid the possibility of the mat being pulled apart as the mold closes. For translucent sheet production, the binder should dissolve completely in the resin, to avoid blemishes. The length of the chopped strand usually ranges from 1/8 in. (3.2 mm) to 2 in. (50 mm) and the quality of mat is expressed as weight per unit area, ranging from 300 g / m 2 (10 oz / f t 2) to 750 g / m 2 (2.5 oz/ft2). Moldings made from CSM/TS polyester resin may have 30-60% of the tensile strength of a laminate made of woven glass cloth but have exceptionally good inter-laminar cohesion and impact strength. On a weight basis, CSM is considerably less expensive and has now largely replaced cloth except where very high strength is needed, justifying the additional cost.

Mechanical fabrics Identifies nonwoven fabric.

Naps Little lumps of tangled fibers or small thickened places found in fabric or yarn.

Nesting Also called nested cloth. Placing plies of fabric so that the fibers of one ply lie in the valleys between the fibers of the adjacent ply developing laminated constructions

Nonwovens The textile and paper industries are based on the two oldest (wet and dry) processes. Manufacturers of nonwovens for plastics draw on both. With the wet, there are basically two types namely the Fourdrinier and cylinder machine types that have been modified. In addition, two basic types exist for the process; formation of the web and application of the bonding agent or system where mechanical carding of fibers is used. The particular equipment and method of operation to be used, with their many modifications, is influenced by desired requirements such as mechanical properties, softness, surface condition, tenacity, etc. There are certain types of so-called nonwoven fabric that are directly formed from short or chopped fiber as well as continuous filaments. They are produced by loosely compressing together fibers, yarns, rovings, etc. with or without a scrim cloth carrier; assembled by mechanical, chemical, thermal, or solvent methods. Products of this type include melted and spun-bonded fabrics.

Nonwoven flash-spuns Flash-spinning is a radical departure from the conventional melt spinning methods to produce nonwoven fabrics. In flash-spinning a 10-15 wt% solution of, for example HDPE in trichlorofluoromethane or methylene chloride. It is heated to 200C (392F) and pressurized to at least 4500 kPa (650 psi). This pressurized vessel is connected to a spinneret containing a single


hole. When the pressurized solution is permitted to expand rapidly through the single hole, the low boiling solvent is instantaneously flashed off, leaving a 3-D film-fibril nonwoven network referred to as a plexifilament. This process with precise conditioning can result in film thickness is 4 lum.

Nonwoven mechanicals The general paper product processed through Fourdrinier cylinder wet machines is very dense, so the saturation with plastics is very difficult. Saturability is improved by reducing paper thickness, including plastics in the pulp mix, using foaming or dispensing agents in the pulp, air-blowing paper during drying, or increasing hole diameters or porosity in wire screen or felt carriers used in the processing. In the dry process sheets are formed by mechanical carding of fibers, air-laying system, or air-floatation system. The techniques provide latitude in orientation fibers including continuous swirl fiber patterns, fibers can be roughly parallel in the machine direction, or other patterns such as orthotropic and isotropic lay-ups.

Nonwoven melt-blown These fibers are composed of discontinuous filaments and are smaller than those of spun-bonded fabrics. Fibers produced are very fine with a typical diameter of 3 lum. Most commercial products are made of polyester or high melt-flow polypropylene plastic.

Nonwoven spun These fabrics include spun-bonded, flash-spun, melt- blown, and mechanical nonwoven swirl. They are used in durable and disposable products that include interlining-interfacing (apparel), carpet backing, geotextile, roofing, industrial filtration media, surgical apparel, medical dressing, and diaper.

Nonwoven spun-bonded They are distinguished from other nonwoven fabrics by a one-step operation that provides a complete chemical to fabric route. The process integrates the spinning, lay- down, consolidation, and bonding of continuous filaments to form fabrics. Its largest growth area is disposable diaper cover stock.

Reinforced plastics, advanced The advanced RP (AR~) refers to a plastic matrix reinforced with very high strength, high modulus fibers that include carbon, graphite, aramid, boron, and S-glass. They can be at least 50 times stronger and 25 to 150 times stiffer than the matrix. ARPs can have a low density (1 to 3 g/cm3), high strength (3 to 7 GPa) and high modulus (60 to 600 GPa).

Scrim A low cost reinforcing nonwoven fabric made from continuous filament yarn in an open-mesh construction. Used includes surfacing RPs to produce a smooth surface. Mso used as a carrier of adhesives for use in secondary bonding of RPs, etc.

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Selvage That edge of a woven fabric that runs parallel to the direction of the warp threads.

Sheer A fabric that is transparently thin or diaphanous. Uses include as overlayer for plastic protection or provide decorative effects.

Tyvek DuPont 's trade name for a spun bonded, tough, strong H D P E fiber sheet product. Its use includes mailing envelopes (protects contents, etc.), medical devices, wrapping around buildings to completely seal off cracks and seams to prevent drafts and cut airflow penetration between the outside and inside (allows moisture to escape from the walls, eliminating or minimizing the prospect of harmful condensation damage), etc.

Vent cloth Also called breather cloth. A layer or layers of open weave cloth used to provide a path for vacuum to reach the area over the RP being cured during fabrication so that plastic volatiles and air can be removed. This action also provides a means of applying pressure to the complete RP.

Warp It is the yarn running lengthwise in a woven fabric. Also identifies a group of yarns in long lengths and approximately parallel, put on beams or warp reels for further textile processing including weaving, knitting, dyeing, etc.

Warp face Fabric side that has the greatest number of yarns that are parallel to the selvage.

Weave The particular manner in which a fabric is formed by interlacing yarns.

Weft The threads of a woven structure that can run across the fabric from selvedge to selvedge at right angles to the warp threads.

Woven Glass woven cloth is produced by conventional textile methods in virtually any variation (Figure 2.7). Thinner cloths make laminates of very high tensile strength and modulus, but generally poor inter-laminar cohesion and tend to be less economical than heavier fabrics, on a basis of weight. The tensile strength offered by woven fabrics is often far higher than is actually needed, and care should be taken not to over design or over specify. Standard types of weave include:

Basket weave Two or more warp fibers go over and under two or more filing fibers in a repeat pattern.

This weave is less stable than the plain weave but produces a flatter and stronger fabric. It is also a more pliable fabric and will conform more readily to simple contours. It maintains a


Figure 2~ Examples of types of weaves: (a) plain, (b) twill, (c) satin, (d) unidirectional, (e) gauze, {f] bidirectional deformable pattern, and {g) knit

certain degree of porosity wi thout lack of too much firmness but not as much as the plain weave.

Bias weave Consists of warp and fill fibers at an angle to the length of the fabric.

Bidirectional deformable pat tern weave Any crimp in the threads is eliminated. The threads are arranged in plies placed at 90 ~ to each other and bound together by a thin thread, representing less than 10% of the total.

Cowoven weave Two different types of fibers in individual yarns, such as thermoplastic fibers woven side by side with glass fibers.

Crowfoot weave It is a three-by-one weave, that is, a filling thread floats over three warp threads and then under one. This type fabric looks different on one side than the other. Fabrics with this weave are popular since they are more pliable than either the plain or basket weave. It is easier to form around curves and provide 3-D forming.

Eight-harness satin weave It is a seven-by-one weave where a filling thread floats over seven warp threads and then under one. Like a crowfoot weave, it looks different on one side from the other side. This weave is more pliable than others are and is especially adaptable to forming around the more complex shapes.

Four-harness satin weave Also called crowfoot satin because the weaving pattern resembles the imprint of a crow's foot. It is a


three-by-one weave. The filling thread floats over three warp threads and then under one thread. The two sides of this fabric have different appearances. As with other satin weaves, it provides some flexibility to form around shapes.

G a u z e weave This is an example of a special weave pattern where two warp threads are taken around the weft threads, to the right and left alternately. A wide variety of fabrics can be held together more or less closely by points (gauze weave) with plain links. These patterns produce a decorative effect used mainly in veils.

Geotextile weave Also called geosynthetic. Geotextiles, as well as geonets, geogrids, and geomembranes, represent a major market for plastics. They appear in all manners of civil works, from roads to canals, from landfills to landscaping. They often prove more cost-effective than nature and other man-made products. The primary plastics are polyester, nylon, PP, and HDPE filaments. The fabrics are made in both woven and nonwoven varieties. The former are characterized by high-tensile, high modulus, and low- elongation traits; the latter by high-permeability and high- elongation. There are those impregnated with plastic to eliminate permeation of water and other liquids.

K n i t weave Also known as unidirectional fabrics and non-woven roving are made by gathering continuous roving into unidirectional two-layer (biaxial) or three-layer (triaxial) orientation and knitting them together with plastic thread. The result is higher tensile strength, reduced laminate weight and thickness, easy wet-out and good mould conformability, with minimal pattern print-through.

Leno weave A locking-type weave in which two or more warp threads cross over each other and interlace with one or more filling threads. It is used primarily to prevent the shifting of fibers in open-weave fabrics.

Leno mock weave Open weave that resembles a leno. It uses a system of interlacing that draws a group of threads together and leaves a space between one group and the next group. The warp threads do not actually cross each other as in a real leno and, therefore, no special attachments are required for the loom. Used when a high strength is required and the fabric is to remain porous.

Pick coun t weave Also called woof or weft count. It is an individual filling yarn running the width of a woven fabric at right angles


to the warp. Call out is the number of filling yarns per inch (cm) of woven fabric.

Pla in weave (linen weave): the weft thread passes successively above and then below each warp thread, and then inversely in the following pass. It is a one-by-one weave where one filling thread floats over one warp thread providing bidirectional strength properties.

Sa t in weave The warp and weft threads are crossed in a programmed order and frequency to obtain a flat appearance. As a result, one side of the fabric has more warp threads, while the back appears to consist mainly of weft threads. The higher the satin number (7 satin, 8 satin), the higher the count of warp and weft threads. Satin weaves allow production of fabrics with high mass per unit of surface area, and good drapability over molds. Different satin patterns are used. See Eight-harness satin and four-harness satin.

Twil l weave A basic weave characterized by a diagonal rib or twill line. Each end floats over at least two consecutive picks, allowing a greater number of yarns per unit area than in a plain weave, while not losing a great deal of fabric stability. This pattern has better drapability than either plain or basket w e a v e s .

Unid i rec t iona l weave The number of threads is considerably higher in one direction than in the other (unidirectional warp fabric or unidirectional weft fabric). The threads are parallel and simply held together.

Woven, shutt le mark A fine filling line caused by damage to a group of warp yarns by weaving shuttle abrasion.

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