06 thermoplastics resins

THERMOPLASTIC RESINS 4 Lars A. Berglund

6.1 INTRODUCTION posites offer advantages. They have very low toxicity since they do not contain reactive chemicals (therefore storage life is infinite). Because it is possible to remelt and dissolve such thermoplastics, their composites are also easily recycled or combined with other recycled materials in the market for molding compounds.

In the aerospace market, composites based on toughened epoxies dominate. The poten- tially cheaper manufacturing of thermoplastic

extent necessary to motivate large-scale investment in new manufacturing equipment. However, for the next generation of aircraft, interest in thermoplastic composites is high. Higher flying speeds require higher temperatures in the materials than the maximum temperature available from epoxy-based composites in use today. Since the release of gases during processing and inherent brittleness are serious disadvantages of thermoset polyimides, thermoplastic composites are of great interest.

In the automotive market, thermoplastic composites are used extensively. Matched-die compression molding of glass mat thermoplastics (GMT), primarily based on glass fiber/polypropylene (GF/PP), is common, because it permits fast processing cycles for fairly large components. In the established field of injection molded components, materials are used with long fibers (5-10mm) in molding pellets. This leads to improved

Thermoplastic composites form a fairly new group of materials. Commercial prepreg tape such as CF/PEEK (carbon fiber/polyether etherketone) and later CF/PPS (polyphenyle- nesulfide) was introduced in the early 1980s. However, as early as 1966, Menges reported on improved static strength and fatigue resistance when epoxy was replaced by polyamide 6 as a composite matrix (Menges, 1966). In the mid

fone) due to expectations of better processing methods and improved toughness characteris- tics. However, solvent resistance was found to be a problem. Composites later introduced based on semi-crystalline thermoplastics, such as PEEK and PPS, which have been introduced more recently, have excellent chemical resistance and are superior to epoxy-based composites in this respect.

Enthusiasm for thermoplastic composites is generated for, basically, three different reasons. First, processing can be faster than for thermoset composites since no curing reaction is required. Thermoplastic composites only require heating, shaping and cooling. Secondly, the properties are attractive, in particular, high delamination resistance and damage tolerance, low moisture absorption and the excellent chemical resistance of semi- crystalline polymers. Thirdly, in light of environmental concerns, thermoplastic com-

1970s there was interest in CF/PSU (Po1YSu1- composites has not yet been realized to the

mechanical properties compared with materials based 0; shorter fibeis (Truckenmueller and Handbook of Composites. Edited by S.T. Peters. Published

in 1998 by Chapman &Hall, London. ISBN 0 412 54020 7 1991).

116 Thermoplastic resins

6.2 MANUFACTURING METHODS processing, for conversion into a high-viscosity melt. Shaping then takes place and the material solidifies on cooling. In Table 6.1, different manufacturing methods used for thermoplastic composites are outlined. Further discussion of thermoplastic composites processing is available in Chapter 24 of this book and in previous reviews (Carlsson 1991; Cogswell, 1992; Kausch, 1993).

As with thermoset composites, materials with low fiber volume fractions show ease of

The principles for thermoplastic composites processing are very different from those for thermoset composites. During the processing of thermosets, the polymer is initially a liquid which then solidifies due to the formation of a three-dimensional molecular network from chemical reactions. Thermoplastics are in the solid state before processing because of their high molecular weight. They are heated above their softening temperature during

Table 6.1 Manufacturing routes for composites based on thermoplastic resins -

Manu fact zi ring ro ii te Outline of fabrication and processing methods

Open mold processes 1. Autoclave

2. Filament winding

3. Folding

Closed mold processes 4.

5.

6.

7.

8.

9.

Injection molding (short fibers, 0.1-10 mm)

Compression molding (short fibers, 5-50 mm)

Compression molding (continuous fibers)

Diaphragm forming

Pultrusion

Resin injection

Unidirectional or woven fibers pre-impregnated by the resin (prepreg) are used. Other forms of prepreg have reinforcing fibers in combination with the resin as fibers or as powder. The prepreg layers are stacked on the mold surface and covered with a flexible bag. Consolidation is obtained by external pressure applied in an autoclave at elevated temperature. Prepreg tape or tape with the resin as fibers or powder are wound onto a mandrel at pre-determined angles. Heat and pressure are applied to the tape in order to continuously weld it onto the underlying material. Preconsolidated sheets are heated. Simple fixtures are then used to shape the sheets into the desired geometry.

A mixture of molten thermoplastic and short fibers is injected into a colder metal mold at very high pressure. The component is allowed to solidify and is automatically ejected. Semi-finished sheets of glass mat thermoplastics are heated and placed in the lower part of the mold in a fast press. The press is quickly closed and pressure is applied so that the material can flow to fill the mold. Technology is also available where the hot molding compound reaches the mold from an extruder. The same principle as for short fiber materials. Continuous fibers require special clamping fixtures for the sheets and can primarily be used for simple geometries. A stack of prepreg is placed inbetween two diaphragms (superplastic aluminium or polymer film). The diaphragms are fixed whereas the prepreg can move freely. The material is slowly deformed by external pressure and the mold. Prepreg tape or tape with the resin as fibers or powder is pulled through a heated die to form beams or similar continuous structures with constant cross-section geometry. The material is allowed to cool and solidify. Dry reinforcing fibers are placed in the mold. Monomers and/or low molecular weight polymer with low viscosity are injected, the reinforcement is impregnated. Polymerization to a high molecular weight thermoplastic occurs by mixing of reactive components and/or thermal activation.

Material forms 117

processing but low stiffness and strength. On the other hand, materials with high fiber content have high stiffness and strength but require slow processing and are difficult to shape into geometrically complicated structures. For high fiber content materials, the high viscosity of a molten thermoplastic usually requires some kind of prepreg fabrication step before final processing. The prepregs may need to be combined into the consolidated, semi-finished sheets before the final processing step.

Regular autoclave processing can be used for thermoplastic composites. For most high- performance thermoplastics, however, temperatures have to be higher than the typical 177°C used for epoxy-based composites. Often, the composite manufacturer must pur- chase a new autoclave if this is the preferred processing route. Autoclave processing of thermoplastics has been modeled (Lee and Springer, 1987). Consolidation of the prepreg layers is an important issue. At a given temperature, sufficient time must be available for the polymer molecules to diffuse from one prepreg layer into the other and form strong physical entanglements (Howes, Loos and Hinkley, 1989). In addition, the air initially present in the material must be displaced.

For thermoplastic composites, filament winding has demonstrated good economic potential (Egerton and Gruber, 1988). The major problem is in the welding of filaments or the tape onto the underlying composite layers. Heat has been applied by means of a gas flame, IR, laser beam or simply from a hot metal surface. Pultrusion of thermoplastic composites offers potential for faster processing than with thermoset composites (Astrom, Larsson and Pipes, 1991), due to the absence of exothermal heat generation from chemical reactions. Profiles may also be produced by roll-forming techniques similar to those used in metal- working. The shape of existing profiles can be changed. The low-cost folding technique (GE Plastics, 1990) has been used commercially by Fokker and TenCate in Holland for quite large components of fairly simple geometry.

Compression molding of glass mat thermoplastics (GMT) is a wide-spread process of great sigruficance in the automotive industry (Berglund and Ericson, 1994). Resin injection of polymerizing prepolymer molecules of low viscosity is in principle the same process as for thermosets although the chemical reactions lead to increased molecular weight rather than to cross-linking. Such a process does not provide the advantages of infinite storage life materials with low toxicity. Diaphragm forming is a processing route where the problem of low extensibility of prepreg-based materials is addressed (Mallon, O’Bradaigh and Pipes, 1989).

6.3 MATERIAL FORMS

Thermoplastic composites are usually supplied as semi-finished materials, with the exception of resin injection materials. In Table 6.2, material forms for thermoplastic composites are presented. Prepregs of high fiber volume fractions (V, = 0.6) may be prepared by solvent-, melt-, prepolymer- or powder- impregnation of the reinforcing fibers. Solvent-impregnation is limited to amorphous resins with high solubility. Melt-impregnation is a technique successfully developed by IC1 (Cogswell, Hezzell and Williams, 1981) pro- ducing high-quality prepreg. The resulting prepreg is considered too stiff, for some processing situations with little drapability in comparison with CF/EP (epoxy) prepreg. This problem is addressed in prepolymer- and powder-impregnated prepreg. One example is the FIT-technology where small tubes containing reinforcing fibers and polymer powder are used (Thiede-Smet, 1989). In addition, commingled weaves (prepregs) are available. The resin is present in the form of fibers which are melted during processing to form a matrix. Composites produced from commingled material forms may have a fairly inhomoge- neous distribution of fibers (Olson, 1990).

Film-stacking is a simple method often used for preparation of laboratory samples


Table 6.2 Material forms for composites based on thermoplastic resins

Material forms Outline of preparation procedure

Prepregs 1. Solvent impregnated

2. Melt impregnated

3. Prepolymer impregnated

4. Powder impregnated

Other material forms 5. Film stacked composites

6. Fiber hybridized weaves

7. Prepolymer liquid and

8. Semi-finished glass mats

and roving

dry reinforcement

(low fiber content)

9. Pellets (low fiber content)

Reinforcing fibers are impregnated by a mixture of solvent and thermoplastic. The solvent is removed by evaporation. Reinforcing fibers are impregnated by thin molten films to which pressure is applied. Low viscosity prepolymers are used to impregnate the fibers. Polymerization to high molecular weight thermoplastic takes place during processing. Fibers are enclosed by thermoplastic powder, either in small tubes containing reinforcing fibers and powder or by powder particles adhering to the fibers from partial melting in a fluidized bed.

Stacks with alternating layers of dry fibers and polymer film are heated and compressed. Roving of commingled reinforcing fibers and the matrix in fiber form. Weaves are produced from the roving. Reinforcing weaves are impregnated by prepolymer liquid and polymerized to a thermoplastic composite. Supplied as sheets prepared in belt press by extrusion and melting of films which impregnate fiber mats. Porous sheets are produced from slurry of fibers and thermoplastic powder in water, by technology similar to manufacturing of paper. Recycled material can be used. Often prepared by pultrusion of unidirectional fibers and matrix followed by chopping into pellets. Used in injection molding or in plasticizing and/or compounding unit combined with compression moulding. Recycled material is easily incorporated.

(Hartness, 1982) although the technique has also been used commercially. In the category of materials with low fiber volume fraction (V, =: 0.2), semi-finished sheets of GMT-materials are available. They usually have random, chopped or continuous fiber mat reinforce- ments. Unidirectional prepreg may be used in order to selectively provide additional stiffness, strength and creep resistance. An interesting step forward is provided by extrusion compounded GMT (Composite Products Inc, 1994; Hoechst AG, 1994). No semi-finished sheets are used, instead a special extruder is used to produce a hot, soft 'cake' constituted of chopped fibers and the polymer matrix, often PP. The cake is placed in a press and molded. The investment in technology is higher than for conventional GMT molding.

On the other hand, the material cost and energy consumption is reduced, greater free- dom in materials selection is obtained and recycling is facilitated. Suppliers of thermoplastic composites are listed in Table 6.3.

6.4 THERMOPLASTIC RESINS

Thermoplastics have either amorphous or semi-crystalline structure (Sperling, 1992). The large, chain-like polymer molecules do not show long-range order in amorphous thermoplastics, which may be viewed as polymer glasses and, in the absence of color pigments, are usually transparent. Thermosets are also amorphous. In contrast, crystalline polymers have regions of molecular order. In melt- processed crystalline polymers, a spherical

Thermoplastic resins 119

Table 6.3 Suppliers of thermoplastic composites

Supplier Materials

Ba ycomp Burlington, Ontario, Canada

CYTEC, Anaheim CA, USA

DuPont de Nemours Bad Homburg, Germany and Newark, DE, USA

Electrostatic Technology Branford, C, USA

GE Plastics Amsterdam, Netherlands and Pittsfield, MA, USA

Hoechst Frankfurt, Germany

Huls Marl, Germany

ICI/Fiberite Monchengladbach, Germany and Laguna Hills, CA, USA

Porcher Textile Lyon, France

Quadrax Corp Portsmouth, RI, USA

Schappe Techniques Charnoz, France

Symalit AG Lenzburg, Schweiz

TenCate Advanced Composites Nijverdal, Netherlands and Fountain Valley, CA, USA

Unidirectional tapes. Matrices PP, HDPE, PA12 PC, PEI, PBT, PES, PPS, PEEK, ABS, PPO. Fibers: glass, carbon, aramid and stainless steel.

Commingled yams. Carbon fiber with PEEK, PEKEKK, PA6,6, TPI (Aurum@). GF/PA6,6.

Prepreg based on Avimid@K, thermoplastic polyimide, and carbon fiber. Sheets laminated of continuous fiber thermoplastic composites or unidirectional discontinuous fibers. Molding compounds of lower fiber content. Matrices: PA6,6, PEKK, PET and others. Fibers: carbon, glass and aramid.

Prepreg fabrication by deposition of polymers in powder form on tow and fabrics. Wide variety of resins and fibers.

GMT-materials based on glass fiber mats and PP, PBT, PC and blends PC/PBT. Unidirectional GF/PP.

Unidirectional prepreg of GF/PP, GF/PA6, GF/PE, CF/PPS, CF/PA6. Pellets > 12 mm for use in plasticating extruder combined with compression moulding.

GF/PA12 fabric prepreg.

APC-2 (CF/PEEK) prepreg tape and tow and developmental materials, primarily for high-temperature applications.

FIT-weaves (Thiede-Smet, 1989). Matrices PA12, PEI, PEEK. Glass and carbon fibers. Enichem, Milano, Italy reportedly produces GF/PP, PET, PBT with FIT-technology.

Prepreg fabrics and unidirectional tape, consolidated sheets. Matrices PA6,6, PMMA, PEI, PPS and PEEK. Carbon, glass and aramid fibers.

Spun yarns combining reinforcing and matrix fibers for subsequent weaving. Matrices PP, PA6, PA6,6, PPS, PC, PEI, PEEK. Carbon, glass and aramid fibers.

Glass mat thermoplastic sheets based on GF/PP.

Prepreg fabrics and unidirectional tape, consolidated sheets. Matrices PES, PEI and PA12. Carbon, glass and aramid fibers.


morphology, termed spherulitic, is often observed (Bassett, 1981). Crystalline lamellae are present within the spherulites although disordered regions exist between and within the lamellae. This is because the large size of polymer molecules inhibits perfect crystallization. The crystalline thermoplastics are therefore more correctly described as semi- crystalline, since the degree of crystallinity never reaches 100%. Semi-crystalline thermoplastics can be viewed as two-phase materials with a crystalline and an amorphous phase.

To illustrate the difference in behavior of semi-crystalline and amorphous thermoplastics, polyethylene terephthalate (PET), may be used as an example. PET is a thermoplastic polyester which crystallizes fairly slowly. Therefore, upon rapid cooling from the molten state, crystallization can be sup- pressed and an amorphous polymer is obtained (similar behaviour is shown by PEEK). Samples of PET with different degrees of crystallinity can be produced by changing the conditions of cooling. The shear modulus G' (obtained from dynamic mechanical thermal analysis, DMTA) is plotted against temperature for such samples in Fig. 6.1. The

amorphous sample shows a dramatic drop in modulus at the Tg (glass transition temperature). The drop in modulus for semi-crystalline samples is less dramatic: the higher the crystallinity, the slower the drop. Above T,, material modulus is maintained by the crystalline phase (although strength usually decreases dramatically). Another effect of reduced degree of crystallinity is reduced chemical resistance.

A disadvantage with semi-crystalline polymers is the high processing temperature, see Tables 6.4 and 6.5, compared with the heat deflection temperature (see next section, Table 6.11). The melting temperature, Tm, of the crystalline phase must be exceeded during processing, although the maximum use temperature, as for amorphous polymers, is still below T,.

Characteristic temperatures of thermoplastics used in applications where only moderate temperatures are experienced are presented in Table 6.4. These materials are available from many different chemical companies, therefore trade names and suppliers are not listed. In Table 6.5, thermoplastics for applications at higher temperatures are listed. These polymers

I I I I I I I I

50 100 150 200 250 Temperature ("C)

Fig. 6.1 Shear modulus (G') compared with temperature for PET of different degrees of crystallinity.

Thermoplastic resins 121

Table 6.4 Characteristic temperatures for thermoplastic resins with T, < 90°C

Polymer Chemical Structure T T,,, Processing temp. ("C)

___ name ("C) ( "C)

- ~ ___ type Polyolefin Polypropylene Crystalline -10 165 200-240

(PP)

Polyamides Polyamide 6,6 Crystalline 55 265 270-320

Polyamide 12 Crystalline 35 180 220-260 (pA6,6)

(PA12)

Polyesters Polyethylene Crystalline 70 265 280-310 terephthalate (PET)

(PBT)

Polybutylene Crystalline 20 240 260-290 terephthalate

Table 6.5 Characteristic temperatures for thermoplastic resins with T, 290°C

Polymer

fYPe Polyester

Polyarylene ether or sulfide

Poly- sulfones

Polyamide- imides

Polyimides

Chemical name

Polycarbonate (PC) Polyphenylene sulfide (PPS) Polyarylene sulfide PEEK PEEKK PEKK PEKEKK Polyketone

Polysulp hone (PSU) Polyether- sulfone (PES)

Polyamide- imide (PAI)

~~ ~~

Polyetherimide

Polyimide (TPI) Polyimide (TPI)

(PW

Trade name and supplier

Lexan, GE Makrolon, Bayer Ryton, Phillips

~~

PAS-2, Phillips

Victrex PEEK, IC1 Hostatec, Hoechst Declar, DuPont Ultrapek, BASF Victrex HTX, IC1

Udel P1700, Amoco

Victrex 4100G, IC1 Ultrason E, BASF

Torlon C, Amoco Torlon AIX-159 Amoco

Ultem, GE

Avimid KIII, Du Pont Aurum, Mitsui Toatsu

Structure T, ("Ci

Amorphous 150

crystalline 90

Amorphous 215

Crystalline 143 Crystalline 165 Crystalline 155 Crystalline 175 Crystalline 205

Amorphous 190

Amorphous 220

Amorphous 275 Amorphous 290

Amorphous 217

Amorphous 250 Crystalline 260

none

280

none

343 365 340 3 75 385

none

none

none none

none

none 390

Processing temp. ("C)

280-330

300-340

330

380400

380400 400420 420430

390-415

300-350

300-320

350400 350400

335420

340-360 400420


are more expensive and are often termed ’high- performance’ resins. The higher cost of these materials is due to small material volumes, more expensive monomers and more difficult polymerization procedures.

Many resins used in injection molding are so called blends, physical mixtures between two thermoplastics. In the field of commercial composite materials, this technology is primarily used for GMT-materials, where composites based on PC/PBT blends are available (Table 6.3). However, for high-performance resins, blending amorphous with semi-crystalline thermoplastics is an interesting route to improved chemical resistance. Most polymer mixtures form immiscible two- phase structures although PEEK and PEI may be mixed to form a miscible blend (Crevecoeur and Groeninckx, 1992).

6.5 PROPERTIES AND DESIGN CONSIDERATIONS

In contrast to thermoset resins, thermoplastics can be dissolved and melted. In general, viscoelastic and plastic effects are more pronounced in thermoplastics. A presentation

of static mechanical properties of the resins is given in Table 6.6. Resins with low Tg, such as PP and PA12, have lower modulus and strength. Their fracture toughness is high and valid data according to linear elastic fracture mechanics are difficult to obtain. Among polymers with T, well above room temperature, the modulus is fairly similar. It is controlled by weak physical forces between the molecules. Viscoelastic effects such as creep and stress relaxation during loading will affect the data. Tensile strength varies more widely than modulus between different resins. As a material property it is unfortunately not very reliable. It is sensitive to loading rate, specimen geometry, specimen preparation and the presence of microscopic flaws on the specimen surface. In addition, uniaxial resin tensile strength is different from resin strength in the composite where the stress state is different. Thermoplastics have higher fracture toughness than epoxy and other thermosets, although epoxy fracture toughness can be improved by addition of a thermoplastic (Bucknall and Gilbert, 1989) or other means. Although not apparent from the table, epoxy modulus is usually slightly higher than for thermoplastics.

Table 6.6 Mechanical properties of thermoplastic resins

Material Tensile modulus E (GPu)

-__ PP 1 .l-1.6 PA6,6 2.5-3.8 PET 2.74.0

Amorph. PA a-2) 3.2

PAS (PAS-2) 3.2

PSU (Udel P1700) 2.5 PES (Victrex) 2.6 PA1 (Torlon) 2.84.4 PEI (Ultem) 3.0 TPI (Avimid K-111) 3.5

PC 2.3-3.0

PPS (Ryton) 3.5

PEEK 3.1-3.8

TPI (LaRC-TPI) 3.7 EP(thermoset) 2.8-3.5

Tensile strength 0, ( M W

Fract lire toughness G,? (kJ m-2)

3040 50-80 50-70 60-70 100 80 100

90-100 70 80

90-190 105 100 120

40-120

-

- - -

1.6 0.5-0.9

- 4.0 2.5 1.9 3.4 3.3 1.5 1.8

0.1-0.5

Properties and design considerations 123

Many mechanical properties of composites are dominated by the influence of fiber modulus, fiber strength and fiber volume fraction. This is usually true for longitudinal tensile modulus and strength as well as flexural modulus and strength. For thermoplastic composites based on AS-4 carbon fiber, typical tensile data are: longitudinal tensile modulus E,= 130 GPa, longitudinal tensile strength (5, = 1950 MPa. In the present con- text, we are more interested in properties dominated by the matrix and the fiber/matrix interface. One such property is the transverse tensile strength of unidirectional laminates. When a multidirectional laminate is loaded in in-plane tension, the first major damage mechanism is likely to be matrix cracking in the plies with transverse orientation to the maximum load direction. This reduces laminate stiffness and initiates other damage mechanisms such as delamination. In Table 6.7, transverse strength and modulus are presented for different thermoplastic composites. Fiber volume fractions are high, V , = 0.5-0.6, For composites based on brittle epoxies, typical transverse strength is 40 MPa. Composites

based on LaRC-TPI, J-2, PAS-2 and K-111 show transverse strengths in the range 3241 MPa. Otherwise, typical transverse strengths for thermoplastic composites are in the range 60-90 MPa. Toughened epoxy composites also show fairly high transverse tensile strengths, typically around 75 MPa.

The use of transverse tension data in failure criteria will lead to conservative estimates. Data are higher for transverse plies in multidirectional laminates (Berglund, Varna and Yuan, 1991). The modulus data for carbon fiber composites in Table 6.7 appear insensi- tive to small differences in matrix modulus. Variations in fiber volume fraction and transverse fiber modulus between the materials mask any such effect. The detrimental effect of glass fiber as opposed to carbon fiber is apparent from the GF/PA6,6 and CF/PA6,6 data. GF/PP shows very poor performance, probably due to poor fiber/matrix interfacial adhesion (note the low V, ). Interfacial weak- ness is also likely to explain the low strength for Kevlar / PEKK. For thermoplastic composites based on AS-4 carbon fiber, Table 6.7 can be used to estimate typical data: transverse

Table 6.7 Transverse tensile properties of thermoplastic composites

Material Transverse modulus

- ET (GPa)

~~~

GF/PP Plytron (V, = 0.35) 4 GF/PA6,6 E-glass/Ultramid 8.6 CF/PA6,6 G30-500/Ultramid 7.2 CF/Am. PA AS-4/J-2 9 CF/PPS AS-4/Ryton 7.6 CF/PAS T 650-42/Rade18320 8.4 CF/PAS AS-4/PAS-2 8.3 CF/PSU AS-4/Udel P1700 7 K49/PEKK Kevlar/PEKK LDFTM 6.2 CF/PEKEKK G30-500/Ultrapek 10.3 CF/PEEK AS-4/PEEK (ICI) 8.9 CF/PAI T650-42/PAI-696 7.6 CF/TPI AS-4/K-III 9 CF/TPI G30-500/’NewTPI’ 8.3 CF/TPI AS-4/LaRC-TPI CF/EP AS-4/3501 (thermoset) 9 CF/EP HTA/6376C (thermoset) 9.9

Transverse strengf h

11 48 72 35 72 61 32 59 21 90 80 67 41 59 33 52 75

(MPa)


tensile modulus E, = 8.6 GPa, transverse tensile strength (T, = 75 MPa.

High interlaminar toughness is desirable since this suppresses the tendency for delamination crack formation during loading. Interlaminar fracture toughness is determined on double cantilever beam specimens (DCB), usually unidirectional materials are used (Whitney, Browning and Hoogsteden, 1982). In Table 6.8 such data are presented. CF/PEEK shows the highest fracture toughness. All thermoplastic composites show higher toughness than the thermoset composites. There is a difference between crack initiation and crack propagation data (Davies, Benzeggagh and de Charentenay, 1987). The data presented here are crack propagation data; crack initiation data are in general much lower. For tough matrices one may question the applicability of the data to design prob- lems. In DCB experiments, the crack opening displacement (COD) is very high, whereas the COD at small central cracks in stiff laminates is much smaller. Local stress fields and damage mechanisms may therefore be different and affect the measured fracture toughness. At present, delamination fracture toughness from DCB tests are therefore preferentially used to compare material. It has been pointed out that composite data are significantly lower than resin data (Hunston, 1984). In tests

on neat resins, energy is absorbed by yielding and other types of damage when the volume of material is relatively large. In a composite, the presence of fibers tends to limit this material volume. For shear strength, no comparable data for different thermoplastic composites appear to be available in the liter- ature. The interlaminar fracture toughness in mode I1 shear loading, Glrc, is higher for thermoplastic than for comparable thermoset composites (Cantwell and Davies, 1993). This also indicates a higher shear strength for the thermoplastic composites. A typical value for the in-plane shear modulus of thermoplastic composites based on AS-4 carbon fiber is 4.8 GPa which is similar to toughened CF/EP systems but slightly lower than for brittle matrix CF/EP composites.

Compressive strength is lower for thermoplastic than for thermoset composites (Table 6.9). Most of the thermoplastic composites are in the range 900-1100MPa whereas typical thermoset composite data are 1700 MPa. Fiber misalignment, shear stiffness and strength have been shown to affect compression strength based on plastic kink band formation (Budiansky, 1983). Compression modulus data in Table 6.9 are similar, in support of similar fiber volume fractions for the materials compared. The composite based on PA6,6 has the lowest strength; PA6,6 also has

Table 6.8 Interlaminar fracture toughness of thermoplastic composites

Material Trade name Fracture toughness

CF/Amorph. PA AS-4/ J-2 1.3 CF/PPS AS-4/Ryton 0.9 CF/PEEK AS-4/Victrex PEEK 2.1 CF/PEEK IM7/Victrex PEEK 2.5 CF/PSU AS-4/Udel P1700 1.2 CF/PAI T650/Torlon AIX-159 1.3 CF/PEI T300/Ultem 1000 0.9 CF/TPI AS-4/Avimid K-I11 1.8 CF/TPI AS-4/ LaRC-TPI 0.8

qc (kJ m-2) _ _ ~ ~ ~ ~ -~

CF/EP AS-4/3501-6 (thermoset) 0.2 CF/EP IM7/8551-7 (thermoset) 0.5

Properties and design considerations 125

Table 6.9 Compressive properties of unidirectional thermoplastic composites

Material

CFDA6.6 CF/Amorph. PA CF/PPS CF/PAS CF /PEEK CF/PEEK CF /PEKEKK CF/PSU CF/PAI CF/TPI CF/EP CF/EP

Trade name

G30-500 /Ultramid

AS-4 / Ryton

AS4/Victrex PEEK IM7/Victrex PEEK AS-4/Ultrapek AS-4/Udel P1700 C-3000/Torlon C AS-4/Avimid K-111 AS-4/ 3501 -6 (thermoset) HTA/6376C (thermoset)

AS-4/ J-2

AS-4/ PAS-2

Compression strength (MPa)

700 1100 940 900 1100 1140 1310 1040 1380 1000 1720 1720

__

Modulus (MPa)

110

130 120 120

127

-~

-

-

- -

110 140 130

the lowest creep modulus and yield stress of the investigated matrices. It is interesting to note that AS-4/PEEK and IM-7/PEEK have roughly the same strength although the IM-7 fiber has higher modulus. The smaller diame- ter of the IM-7 fiber appears to have a negative effect as expected from Euler-buckling considerations.

Compression strength after impact, a mea- sure of laminate and material damage tolerance (Dorey, 1989), is presented in Table 6.10. A quasi-isotropic laminate of given lay-up and geometry is subjected to impact of a certain energy. Internal damage mechanisms such as matrix cracking and delamination occur in the laminate. The plate is then subjected to compressive load and the stress and strain at failure can be determined. The data show that thermoplastic composites have

superior performance to first generation thermoset composites (AS-4/3501-6). This is because the delaminated area due to the impact event is more limited for the thermoplastic composites. However, toughened epoxy resin composites combined with toughened interlayers between the plies do in general show as good compression strength after impact as thermoplastic composites. In fatigue, delamination resistance is higher for AS-4/PEEK compared with epoxy composites (Gustafsson, 1988). However, in uniaxial tension, brittle CF/EP was found to be superior to both toughened CF/EP and the thermoplastic composite (Curtis, 1987). Claims have been made that this observation is due to heating effects in the thermoplastic composite specimens from testing at high frequency (Moore, 1991).

Table 6.10 Compression strength after impact of thermoplastic composites

Material Trade name Compression strength after impact

(28 J) (42 1) (571) (impact energy)

o (MPa) o (MPa) o (MPa)

CF / PPS AS-4 /Ryton 221 179 - CF/PEEK AS-4/Victrex PEEK 331 310 290 CF / PA1 C-3000/Torlon C 365 345 317 CF/EP AS-4/3501-6 (thermoset) 1 79 145 131 CF/EP AS-4/8551-7 (thermoset) - 303 -


Table 6.11 Glass and heat deflection temperatures for amorphous thermoplastics

Table 6.12 Glass melting and heat deflection temperatures for semi-crystalline thermoplastics

Material T, ("C) HDT ("C) Material T, ("C) T,,, ("C) HDT ("C)

PC 150 132 Amorph. PA 160 154 PSU 190 175 PES 220 203 PA1 290 278 PEI 217 200 EP (thermoset) 200 180

PP -10 165 60 PA6.6 55 265 75 PET 70 265 41 PPS 90 280 135 PEEK 143 343 160 TPI (Aurum) 250 388 238

Increased market need for polymer composites with good performance at elevated temperature has generated interest in thermoplastic composites. Materials with continuous use temperatures above 150°C are of particular interest since they perform better than epoxies. One question is how maximum use temperature relates to Tg. In Table 6.11, Tg and the heat deflection temperature (HDT) for amorphous thermoplastics are presented. HDT is determined by subjecting the material to static load (typically 1.8 MPa) and slowly increasing the temperature. HDT is determined as the temperature at which a critical deflection of the sample is obtained. Table 6.11 shows HDT to be 620°C below the T of amorphous polymers. In comparison with room temperature strength, the strength of the composite is significantly reduced above the HDT. The reason

6

is that molecular mobility in the polymer is increased dramatically as the temperature approaches T .

In Table 8.12, similar data to those in Table 6.11 are presented for semi-crystalline thermoplastics. HDT is usually somewhat higher than Tg. However, for some semi- crystalline polymers, HDT is below T (as for amorphous polymers). Creep effects 'will be very strong close to and above Tg. For this reason the maximum temperature for continuous service under significant load is unlikely to exceed a temperature of 20°C below T' for semi-crystalline thermoplastics. The primary advantage of crystallinity is therefore chemical resistance. This is apparent from Table 6.13, where chemical resistance for different thermoplastics is indicated in a qualitative way. Semi-crystalline thermoplastics have much

Table 6.13 Chemical resistance of thermoplastics

Material Structure Hydraulic Chlorinated Ketones Esters H,O abs. fluid kydro- (Yo)

carbons

PA6,6 Crystalline - 0 0 0 PEEK Crystalline 0 0 0 0 PPS Crystalline 0 0 0 0 PEI Amorphous 0 D A A PSU Amorphous A D A A PA1 Amorphous 0 0 0 0 PES Amorphous A D A A Am.PA Amorphous A A A PC Amorphous - D A A

0 = no effect, A = is absorbed, D = is dissolved

8 0.5 0.5 1.2 0.9 2 4 0.3 5 -

Applications 127

better chemical resistance than the amorphous polymers. A notable exception is the high water absorption in PA6,6 caused by hydrophilic groups in its chemical structure. The solvent resistance of a large selection of different thermoplastic composites has been reported (Johnston, Towel1 and Hergenrother, 1991). For most thermoplastics, e.g. PEEK, moisture expansion coefficients of the carbon fiber composite may be taken as 0. Thermal expansion coefficients have been characterized (Barnes et al., 1990) and are similar to epoxy composites. For AS-4/PEEK, the composite density is 1600 kg m-3.

For polymers with high Tg, exposure to elevated temperature may lead to increased density not connected with crystallinity but with the amorphous state. The phenomenon is termed physical aging and leads to a more brittle behavior of the polymer (Kemmish and Hay, 1985). Further work is needed to eluci- date the importance of physical aging to composite fracture behavior under practical service conditions. For composites processed at high temperatures, residual stresses will also affect fracture behavior. The magnitude of the residual stresses and, consequently, detrimental effects will increase with increasing cooling rate (Manson and Seferis, 1992).

Thermoplastic composites can be joined by the same methods as thermoset composites. Bolted joint performance has been compared for thermoset and thermoplastic composites (Walsh, Vedula and Koczak, 1989) with results in favor of thermoplastic composites. With the semi-crystalline thermoplastics, adhesive bonding requires careful surface preparation (Kinloch and Taig, 1987). This is because of the good chemical resistance and limited solubility of these polymers. However, with careful surface preparation, as good adhesive bonds are obtained as with thermoset composites.

The thermoplastic nature of the matrix offers another possibility: fusion bonding. Different methods have been compared (Davies and Cantwell, 1993), including hot gas, IR, laser, ultrasonic, vibration, electrical resistance and

induction welding. There are difficulties in con- trolling the processes and very few methods offer the promise of portable equipment. Various bonding technologies for PEEK composites have been compared (Silverman and Criese, 1989). The study favored a technique for fusion bonding with a polyetherimide film. This approach can also be used in repair of damaged structures, since the temperature needed is below the T, of PEEK.

6.6 APPLICATIONS

Thermoplastic composites can be used in similar applications to thermoset composites. The following examples will demonstrate some of the reasons for choosing a thermoplastic composite material. In Europe, the automotive market for GMT composites is significant. Rapid processing by compression molding, cycle times of typically 30s even for large structures, results in cheaper components. In addition, more functions can be integrated into each component compared with sheet metal structures. Bumper beams dominate the automotive market in the US whereas, in Europe, a wider variety of applications are in commercial production. Typical components are subjected to minor loads or impact and surface appearance is not important. Battery trays, beams supporting the hood, seat sup- ports, oil trays, engine shields and even the complete front end have been produced for Volvo, Volkswagen and others.

In the aerospace market, DuPont has supplied thermoplastic polyimide composites to the prototype programs for the F-22 fighter aircraft. In the supersonic civil aircraft pro- gram, the same thermoplastic polyimide is considered for wing skins. The main reason for this particular thermoplastic composite is a continuous high maximum temperature. AS-4/PEKK (unidirectional discontinuous fibers) was used by Bell Helicopter Textron in a V-22 tiltrotor thermoplastic wing rib for better open hole compression behavior than thermoset composites for high proportions of


Fig. 6.2 Bike helmet, an example of a commercial application of thermoplastic composites (Courtesy: DuPont Europe).

45" ply angles (probably for decreased tendency to delamination). A female steel tool and matched-die press forming was used (Chang, 1992). A stretch forming process was used to fabricate C-section curved fuselage frames for Boeing Helicopter from AS- 4/PEKK (Chang, 1992). TenCate in Holland supplies materials for Fokker Special Products. Landing flap ribs and impact resis- tant ice-protection plates are produced for the Dornier 328 aircraft.

I

Bike helmets are also produced (Fig. 6.2). A semi-finished sheet is heated by IR, transfered to heated dies where a selective clamping sys- tem is used to hold the sheet. Forming and consolidation pressure is applied and the mold is cooled before demolding. Hoechst in Germany present applications made by winding, a pressure vessel, tubes, sealings and support rings (Fig. 6.3). Matrices include PA6 and PPS with carbon and glass fibers.

In the field of biomedical applications, such as hip prostheses, semi-crystalline thermoplastic matrices offer good potential due to their chemical stability (Williams and McNamara, 1987).

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Fig. 6.3 Thermoplastic composite applications New York: Elsevier. made by winding: pressure vessel, tubes, sealing and support rings. (Courtesy: Hoechst AG)

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06 thermoplastics resins

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