11. polymers and plastics

11 Polymers and Plastics

Thermoforming uses plastic sheet that is heated, stretched, cooled, and mechanically cut. For the most part, amorphous plastic sheet is manipulated as a rubbery solid. Crystalline plastic sheet is manipulated either as a rubbery solid or as an elastic liquid close to the polymer melting temperature. As a result, the solid or elastic liquid properties of polymers are more important than pure viscous properties. In this chapter, important polymer characteristics are examined. Then the characteristics of specifi c thermoformable polymers, including biopolymers are discussed. Filled, reinforced, multilayer, and foamed polymer characteristics are considered.

11.1 Polymer Characterization

Polymers are pure organic molecules consisting of long chains of simple molecules. As an example, polyethylene, the polymer most widely used globally, is made by reacting ethylene gas at high temperature and pressure, in the presence of a catalyst. Ethylene has a chemical composition of H2C=CH2, where C is carbon, H is hydrogen, and the symbol “=” indicates a double bond or a reactive link between the carbons. The ethylene molecule is called a monomer.It has a melting temperature of –169 °C and a boiling temperature of –104 °C.

The structure of polymerized ethylene or polyethylene (PE) is often written as H2C–(CH2–CH2)x–CH2, where x represents the number of ethylenic segments or mers in the polymer. If the value of x is relatively small, say on the order of 100, the polymer is a high-temperature wax. If the value of x is relatively high, say on the order of 15,000, the polymer has a melting temperature of approx. 130 °C. It degrades before it boils. This polymer is generally processed by one of the standard plastics processing methods such as injection molding, blow molding, rotational molding, extrusion, or thermoforming. If the value of x is very high, say on the order of 300,000, the polymer, called ultrahigh molecular weight polyethylene (UHMWPE), is usually intractable in normal processing equipment. Techniques such as compression molding or compaction-sinter-fusion of powder may be required.

11.1.1 Plastic vs. Polymer

Most plastics practitioners generally consider the words plastic and polymer to be inter-changeable. Technically, they are not. Polymers are by defi nition the result of chemical reaction of organic monomers. Nearly all polymers are mixed or compounded with additives such as thermal stabilizers, antioxidants, color correcting dyes, internal and external processing aids, and product-specifi c additives such as fi re retardants, colorants, UV stabilizers, fi llers, reinforcing agents, and others. The term plastic refers to the polymer and its additives, delivered to the

Long chains of organic molecules

Polymer + additives = plastic

172 11 Polymers and Plastics

processing equipment as resin pellets, powders, or sheet. As noted earlier, the terms plastic and polymer are used interchangeably here.

11.1.2 Thermoset and Thermoplastic Defi nitions

There are two general categories of polymers – thermoplastic and thermosetting. When the polymer can be heated and shaped many times without substantial change in its physical characteristics, it is a thermoplastic. Polyethylene (PE), polystyrene (PS), and polycarbonate (PC) are examples of thermoplastics. When the polymer cannot be reshaped after being heated and shaped for the fi rst time, it is a thermoset. Epoxies and phenol-formaldehyde or phenolic are examples of thermosetting polymers. Thermoforming is primarily concerned with conversion of thermoplastics.

11.1.3 Crystalline and Amorphous Defi nitions

There are two general categories of thermoplastics – amorphous and crystalline or semi-crystalline. When any thermoplastic polymer is heated from a very low temperature, it undergoes a physical transition from its low-temperature glassy state to a rubbery state. Although this transition occurs over a temperature range of several degrees, usually only one temperature is

Defi nitions

Table 11.1: Transition Temperatures of Some Thermoformable Polymers

Polymer Glass transition temperature

[°F (°C)]

Meltingtemperature

[°F (°C)]

Heat distortion temperature 66 psi(0.46 N/mm2)[°F (°C)]

Polystyrene 200 (94) – (–) 155–204 (68–96)

PMMA 212 (100) – (–) 165–235 (74–113)

PMMA/PVC 221 (105) – (–) 177 (81)

ABS 190–248 (88–120) – (–) 170–235 (77–113)

Polycarbonate 300 (150) – (–) 280 (138)

Rigid PVC 170 (77) – (–) 135–180 (57–82)

PETG 180 (82) – (–) 158 (70)

LDPE –13 (–25) 239 (115) 104–112 (40–44)

HDPE –148 (–100) 273 (134) 175–196 (79–91)

Cellulose acetate 158,212 (70,100) 445 (230) 125–200 (52–93)

Polypropylene 41 (5) 334 (168) 225–250 (107–121)

Copolymer polypropylene –4 (–20) 302–347 (150–175) 185–220 (85–104)

PET 158 (70) 490 (255) 120 (49)

173

reported as the glass transition temperature. Polymers that only have glass transition tempera-tures are called amorphous polymers. Polystyrene (PS), polyvinyl chloride (PVC), and polycar-bonate (PC) are examples of amorphous polymers. Amorphous polymers constitute about 80% of all polymers that are thermoformed. Furthermore, about 70% of all amorphous polymers that are thermoformed are styrenic in nature – polystyrene (PS), impact polystyrene (HIPS), ABS, styrene-acrylonitrile (SAN), and others.

When polymers exhibit a second physical transition, from a rubbery state to a fl uid, molten or melt state, they are called crystalline or semi-crystalline polymers. This second transition usually occurs over a temperature range of several degrees, although usually only one temperature is reported as the melting temperature. Polyethylene (PE) and polypropylene (PP) are crystalline polymers. Generic transition temperature values for several thermoformable polymers are presented in Table 11.1.

All polymers have glass transition temperatures. Only crystalline polymers have melt tempera-tures.

11.1.4 Homopolymers, Copolymers, Terpolymers, and Blends

If only one polymer specie or moiety is used in a given plastic recipe, the polymer is called a homopolymer. Low-density polyethylene (LDPE) and general purpose polystyrene (GPPS) are examples of homopolymers. Polyethylene terephthalate (PET) is made by reacting two moieties, ethylene glycol and terephthalic acid, but only one type of repeat unit or mer – ethylene terephthalate – is produced. Therefore, PET is a homopolymer. Polycarbonate (PC) is another homopolymer, made by reacting two moieties, bisphenol A and phosgene, to produce a polymer with only one repeat unit.

If one polymer is reacted with another, the polymer is called a copolymer. Impact polystyrene (HIPS) is an example, where styrene monomer is reacted with butadiene monomer. Many copolymers are used in thermoforming. Other examples include polypropylene-polyethylene (coPP), polyvinyl chloride-polymethyl methacrylate (PVC-PMMA), and styrene-acrylonitrile (SAN). If three polymers are reacted together, the polymer is called a terpolymer. The classic terpolymer is ABS, which is a reacted product of acrylonitrile, butadiene, and styrene.

Occasionally, two polymers are extrusion or melt blended together to produce a specifi c plastic recipe. The classic blended polymer is modifi ed polyphenylene oxide (mPPO). It is a near-equal blend of polystyrene and polyphenylene oxide, with good impact resistance and fi re retardancy properties. Because these polymers are miscible, the blend is often called an alloy or an interpenetrating network polymer. A blend version of ABS is also produced by compounding SAN and HIPS.

11.1.5 Additives, Fillers and Reinforcements

Additives are used with most polymers. Some of these additives are required to make the polymer processible. Polyvinyl chloride (PVC) must be compounded with many additives to make it processible and useful for many applications. Octylphthalates are plasticizers that lower

Glass transition temperature

Melting temperature

Homopolymers

Copolymers

Terpolymers

PVC additives

11.1 Polymer Characterization


PVC fl exural modulus and glass transition temperature, producing fl exible vinyl (FPVC) sheet that is thermoformed into interior automotive door panel and instrument panel skins.

Other additives, called anti-block agents and slip agents, are needed to prevent rolled sheet from sticking together. Additives such as antimony oxide are added when fi re retardancy is needed. Odor suppressants are used with most polypropylenes, particularly when the product is to be used for rigid food containers. Anti-static agents are used to minimize static discharge when the product is to be used in electronic packaging. Additives are often used in concentrations of 1% (wt) or less.

A list of common additives is given in Table 11.2.

Even though there is growing interest in forming fi lled and reinforced plastic sheet, nearly all thermoplastics that are thermoformed are neat, meaning that they contain no fi llers or reinforcing elements. However, many types of fi llers can be compounded into polymers, as noted in Table 11.2. Most fi llers are inexpensive inorganic minerals such as calcium carbonate and talc. In general, fi llers increase polymer stiffness and decrease polymer impact strength. Fillers do not appreciably alter transition temperatures. As a result, increased fi ller loading implies increased applied forming forces at conventional forming temperatures. Because fi llers cannot be deformed, the degree of drawdown is restricted when forming relatively high fi ller-loaded sheet.

Glass is the predominant fi ber used with thermoplastics. Mineral, metal, and carbon fi bers are also used, as noted in Table 11.2. Organic fi bers such as sisal and cotton lintel are diffi cult to compound into thermoplastics and so are not often used. Short-length reinforcing fi bers behave somewhat like fi llers, increasing polymer stiffness and decreasing polymer impact strength.

Anti-block agents

Fillers

Fibers

Table 11.2: Fillers, Fibers, and Additives to Thermoformable Polymers

Additives Fillers Fibers

Antioxidants•– Lubricants– Internal

Viscosity suppressants•

Antiblocking agents•

Plasticizers•

Antistatic agents•

Pigments•

Heat stabilizers•

Ultraviolet stabilizers•

Nucleating agents for •crystallization

Nucleating agents for foam•

Antimicrobials•

Flame retardants•

Fragrance enhancers•

Foaming agents•

Silica and silicate minerals•

Glass•

Calcium carbonate•

Metallic oxides•

Other inorganics such •as barium sulfate, silicon carbide

Metal powders•

Carbon•

Cellulosics such as wood •and shell fl our

Cellulosics such as cotton, •jute, sisal

Synthetic fi bers such as •nylon

Carbon fi bers•

Fiberglass•

Glass yarn•

Whiskers•

Metallic fi bers•

175

However, when short-fi ber fi lled sheet is formed, the fi bers locally orient in the local direction of stretch. As a result, the mechanical performance – stiffness, impact strength, elongation to break – of the formed part may vary greatly across the part.

Long-length reinforcing fi bers greatly inhibit the extensibility of the sheet. Typical problems include fi ber prominence and resin-rich/resin-poor regions, particularly in small radii regions. As a result, high fi ber-loaded sheet often requires special forming equipment such as heated, matched steel molds and high pressure forming presses. Autoclaves similar to those used for glass- and carbon-fi ber thermoset composite forming have been used with high fi ber-loaded engineering plastics.

Nanoparticles are extremely fi ne inorganics such as intercalated clay or kaolin that are compounded into polymers to improve stiffness without dramatically affecting impact strength or transparency. Typically, the compounding level is less than about 3% (wt). Nanoparticles should have at least one dimension that is less than 100 nanometers or 0.1 μm. By comparison, human hair is on the order of 20 to 100 μm in diameter22. The keys to successful nanocom-posite compounding are the additives that minimize particle agglomeration. Commercial experience with thermoforming nanocomposite sheet is very limited. Because of this, little is known about formability of polymers containing appreciable amounts of nanoparticles. It is anticipated, however, that nanocomposite sheets, like fi lled plastic sheets, will be stiffer at plastic forming temperature and, as a result, pressure forming will be preferred over simple vacuum forming.

11.2 The Thermoforming Window

The thermoforming molding diagram or the interaction of sheet extensibility, sheet temper-ature, and applied force was discussed in some detail in Chapter 9.

The thermoforming window is a plastic-specifi c criterion. It is considered to be the temperature range over which the polymer is suffi ciently supple or deformable for stretching and shaping into the desired shape at a given applied force. Typically, amorphous polymers have broader thermoforming windows than crystalline polymers. Polystyrene, for example, can be formed at temperatures as low as 260 °F (127 °C) or about 50 °F (30 °C) above its glass transition temperature. It can be formed at temperatures as high as 360 °F (180 °C) or only a few degrees below the temperature at which it is injection-moldable. On the other hand, homopolymer polypropylene (homoPP) is so fl uid above its melting temperature of 330 °F (165 °C) that its thermoforming window may be no more than one degree wide. Because thermoforming equipment is not designed to control plastic sheet temperature this accurately, homoPP is frequently formed at temperatures just below its melting temperature23. Even then, the forming window range may be only two or three degrees.

Nanoparticles

Material specifi c characteristic

22 “Human hair”, Wikipedia, May 2007.23 When homopolymer polypropylene (homoPP) is formed below its melting temperature, the process

is sometimes called Solid Phase Pressure Forming (SPPF), to contrast melt phase forming or forming crystalline polymers above their melting temperatures.

11.2 The Thermoforming Window


11.3 Thermoformable Polymers

As stated earlier, the majority of thermoformable polymers are amorphous, and the majority of amorphous polymers that are thermoformed are styrenics. Other amorphous polymers include vinyls, acrylics, cellulosics, polycarbonate, certain polyesters, and many biopolymers.

11.3.1 Polystyrene and Other Styrenics

Typical physical properties for styrenics are given in Table 11.3. About twenty years ago, polystyrene (PS) and the family of styrenics such as HIPS, ABS, SAN, ABA, and OPS, dominated the thermoforming industry. For example, in 1983–1984, nearly 80% of all thermoformed products were styrenics.

Unmodifi ed PS is characterized as having a high modulus, a low room temperature elongation at break, excellent clarity, superior hot strength drawability, and a very broad thermoforming window. Although PS is easy to thermoform, its trim dust is tenacious and improper trimming can cause edge microcracks that ultimately lead to cracking and part failure. Highly stretched regions in formed parts tend to be very brittle.

To improve PS impact resistance, it is either melt-blended or co-reacted with butadiene, a synthetic rubber, to form impact polystyrene (HIPS). The rubber forms a second phase, rendering the polymer hazy to translucent to opaque, depending on the size of the rubber moiety and the level of rubber, which typically ranges from 2–10% (wt). Rubber-modifi ed polystyrene has improved impact strength but reduced modulus, tensile strength, and formability.

Acrylonitrile is co-reacted with polystyrene to produce styrene-acrylonitrile (SAN), a tough, transparent polymer that fi nds extensive use in appliance manufacture. Acrylonitrile-butadiene-styrene (ABS) is a tough, high-impact terpolymer that is used in electronic and medical cabinetry, appliances, and interior and exterior truck cab components.

Because rubber-modifi ed styrenics yellow when overheated, care must be taken to avoid excessive heating during thermoforming. ABS is prone to moisture absorption. As a result,

PS properties

Imroving properties

Table 11.3: Physical Properties for Styrenic Family of Polymers

Property PS HIPS ABS SAN 35% GR SAN

Density [kg/m3] 1050 1040 1050 1080 1360

Elongation at break [%] 3 40 15 5 2.7

Glass transition [°C[ 105 100 105 106 106

Processing temp [°C] 160 155 165 165 200

HDT at 1.82 MPa [°C] 100 90 95 102 105

COE [10–6/°C] 60 60 65 60 3

Tensile strength [MPa] 40 30 50 75 110

Flex strength [MPa] 80 80 75 135 150

177

ABS sheet must be kept dry prior to thermoforming to minimize moisture bubbles and surface defects in fi nished parts.

All styrenics are ultraviolet (UV)-sensitive. As a result, styrenic sheet is usually laminated with acrylic (PMMA) or fl uoropolymer (FEP) fi lm prior to forming products for exterior applications. Solvent-based paints and certain acrylic paints adhere very well to styrenics. As a result, fi nished parts are often painted for exterior applications. Nearly all styrenics are solvent-weldable.

Oriented polystyrene (OPS) fi nds extensive application in quality bakery containers such as cake covers. OPS is made by stretching hot light-gauge polystyrene sheet in both the machine direction and the cross-machine or transverse direction in a post-extrusion step. Orientation in the machine direction is achieved by differentially changing the speeds of the take-off rolls. Orientation in the transverse direction is achieved by reheating the sheet, gripping the sheet edges and pulling it at right angles to the machine direction in a technique known as tentering.This is shown schematically in Fig. 11.1. The sheet can be oriented up to a four-fold factor in each direction. Oriented polystyrene sheet is quite expensive to produce but is very tough and has exceptional clarity. The sheet must be heated very carefully to minimize loss of this orientation. Contact heat thermoformers are usually used.

11.3.2 Polyvinyl Chloride and Other Vinyls

Vinyl chloride was polymerized in the 1800s. The resulting polymer was intractable, degrading before it could be melted and molded. In the 1930s, polyvinyl chloride (PVC) was masticated with processing aids such as dioctylphthalate (DOP) on conventional two-roll rubber mills. Depending on the types and concentrations of processing aids, the fi nal product properties could be changed from rigid and tough (rigid PVC or RPVC) to extremely fl exible and trans-parent (fl exible PVC or FPVC). The fi rst sheet used in thermoforming was calendered from the rubber mills. Further additive development produced recipes that could be extruded in near-conventional equipment without serious degradation or property loss. The production of extruded sheet for thermoforming followed the extensive development of PVC in wire and cable applications.

UV sensitive

Orientation

Rigid and tough to fl exible

Catingroll

Cross directionstretch

From extruder

Heating zone Cooling zone

Oriented film

Machinedirectionstretch

Figure 11.1: Sequential biaxial orientation of light-gauge sheet, using a tenter to achieve cross-machine orientation



PVC is very sensitive to overheating. Early PVC was stabilized with lead. The toxicity of lead eventually led to its replacement, fi rst with other heavy metals, and now with tin, zinc, and organic stabilizers.

PVC is inherently fi re-retardant with a limiting oxygen index (LOI) greater than 30. As a result, it fi nds heavy-gauge thermoformed applications in equipment cabinetry. It has excellent UV stability and so it is used for exterior products such as siding, window fascia, and shutters. Semi-rigid, light-gauge sheet is thermoformed into containers in the packaging industry. Flexible, light-gauge sheet is thermoformed into interior automotive components such as door panel skins, arm and head rests, air bag covers, and instrument panel skins.

As noted, care must be taken when processing PVC. The fi rst indication of excessive heat is color shift or discoloration. Ultimately, PVC degrades to a dark brown color and generates hydrogen chloride (HCl) gas. HCl is a corrosive acid, particularly in combination with moisture. It is a mucus membrane irritant. Because HCl is so corrosive, processing equipment in contact with PVC melt is usually stainless steel or chrome-plated steel. Although thermoforming equipment is less susceptible to corrosion than is extrusion equipment, heaters and electrical connections should be protected from potential damage when processing PVC. In light-gauge thermo-forming, where a substantial portion of the sheet is trim, reprocessability must be carefully monitored and restricted to minimize contamination, usually in the form of black specks and gels, sometimes called fi sh-eyes. Although nearly all PVCs have some level of crystallinity, the crystalline level is quite low and the crystalline structure does not interfere with the formability of PVC. PVC thermoforms as if it is completely amorphous.

PVC is incompatible with many polymers. It is compatible with acrylates, however. Melt blends of PVC and polymethyl methacrylate (PMMA) are thermoformable into heavy-gauge products having good thermal stability, excellent weatherability, good scratch resistance, and fi re retardancy. PVC is solvent-weldable.

11.3.3 Acrylics

Polymethyl methacrylate (PMMA) is often the polymer of choice for transparent and trans-lucent sky domes and outdoor signs. It can be drawn better and more consistently than any other polymer. In general, acrylics have superior UV resistance and high modulus but tend to be brittle. Rubber-modifi ed acrylics have improved toughness but are often hazy to translucent to opaque. Acrylics are amorphous and are solvent-weldable.

There are two primary methods for making PMMA sheet. Cell-cast PMMA is made by pouring reactive resin syrup into a frame, surfacing the frame with fl oating plates of highly polished metal or glass, and warming the assembly to allow the resin to react. Extruded PMMA sheet is produced by the conventional extrusion of acrylic pellets.

Cell-cast PMMA has a greater molecular weight than extruded PMMA. It is tougher, more scratch-resistant, and more diffi cult to thermoform. Unlike extruded PMMA, it cannot be reground and recycled.

Care must be taken to ensure that the PMMA sheet is hot enough when forming it into three-dimensional corners. Cold-formed corners can be brittle and can exhibit severe stress-cracking. This is more critical with cell-cast PMMA than with extruded PMMA.

Sensitive to heat

Fire-retardant

Degradation hazards

Transparent and translucent

179

PMMA will absorb moisture. It is recommended that heavy-gauge sheet be protected from atmospheric moisture when produced. Sheet that has been exposed to the atmosphere for some time should be thoroughly oven-dried before forming. Absorbed moisture can result in haze formation during heating in the thermoforming process. Microbubbles act as stress concentration points during product use.

11.3.4 Cellulosics

Cellulosics are often called the earliest synthetic polymers. In actuality, they are semi-synthetic in that the primary building block, cellulose, is a natural polymer. Cellulose nitrate or nitro-cellulose (CN) was developed in the mid-1800s in England and the United States by reacting cotton with nitric acid. In the US, John Wesley Hyatt found that a moldable plastic was formed when cellulose nitrate was reacted with camphor. Sheets skived from camphorated nitrocel-lulose blocks, trademarked celluloid, were heated with steam and formed against steel molds to produce hollow products such as mirror cases and baby rattles [32]. The standard recipe for celluloid is 80 parts cellulose nitrate with at least 10% nitrogen, 30 parts of camphor, and 1 part ethanol [33].

Because cellulose nitrate (CN) is highly fl ammable, it was replaced in the early 20th century with cellulose acetate, being the reaction of cellulose with acetic acid. Cast cellulose acetate (CA) fi lm is quite transparent and very tough. It was the fi rst bread wrapper and was the inner layer between two sheets of glass to produce safety glass. It was also used extensively as thermoformed rigid packaging. Heavy-gauge cellulose acetate sheet was used to produce aircraft windscreens in World War II.

Although cellulosics are nominally crystalline, they process as if they are amorphous. Cellu-losics, in general, absorb water and transmit water vapor. Unless the cellulosic sheet is kept dry, the fi nal product may contain microbubbles or pinholes. CA fi lm, like CN fi lm, is prone to splitting when bent sharply or creased.

Cellulose acetate has been replaced in many applications by cellulose acetate butyrate (CAB) and cellulose acetate propionate (CAP). These polymers are much tougher than either CA or CN, but they are also more expensive to manufacture. As a result, they are fi nding limited use, particularly as formable sheet and fi lm. However, with growing emphasis on renewable resources for polymers, this may change.

11.3.5 Polycarbonate

Polycarbonate (PC) is a very tough, high-temperature, amorphous transparent plastic. PC, like PMMA, fi nds extensive use in heavy-gauge signage and skylights. It is more UV-sensitive than acrylic and tends to yellow with extended outdoor exposure. It is more diffi cult to thermoform, requiring long oven times to achieve uniform internal sheet temperature. Pressure forming yields the best products. Polycarbonate picks up substantial amounts of atmospheric moisture. Extensive moisture bubble fi elds develop at elevated forming temperatures. Microbubbles in fi nished parts act as stress concentrators that dramatically reduce impact strength. To minimize moisture pick-up, sheet should be kept wrapped in polyethylene fi lm from the time of extrusion

Moisture absorbent

Highly fl ammable

Tough and transparent



to the time of forming. Sheet that has been exposed to atmospheric moisture for a few hours should be thoroughly oven-dried. Similarly, trim that is reground for recycle must be dried thoroughly before extrusion. Small amounts of absorbed moisture in the recycle stream will react with and degrade the polycarbonate by cleaving the polymer backbone.

11.3.6 Polyesters

Polyesters are formed by reacting difunctional alcohols with difunctional acids to form long-chain esters. Although many polyesters are manufactured, the dominant polymer is polyethylene terephthalate (PET), being the product of the reaction of diethylene glycol with terephthalic acid. Water is the dominant small molecule that is extracted from the PET reaction.

The level of PET molecular weight is determined through wet chemistry. A measured amount of PET is dissolved in a measured amount of solvent. The viscosity of the solution is then deter-mined. An additional measured amount of solvent is added to the solution and the viscosity is again determined. This dilution is repeated, the viscosities are plotted against the polymer concentration, and the curve is extrapolated to zero polymer concentration. The intercept value is then divided by the known viscosity of the solvent and the value is reported as the intrinsic viscosity (IV) of the polymer. The IV is directly related to the molecular weight of the polymer. PET molecular weight or IV affects many physical properties, which in turn affect PET processibility. As examples, for fi ber forming, PET IV is around 0.7, in sheet production and thermoforming, it should be around 0.8, while for PET beverage bottle production, it is around 0.9. For low-density PET foam production, the IV should be greater than about 1.2. Traditional condensation reactor technology is normally used to achieve IVs of less than approx. 0.9. To achieve IVs in excess of approx. 0.9, solid-state reaction under very high vacuum can be used but more recently chain extenders such as pentaerythritol (PAE) and para-mellitic dianhydride (PMDA) are employed. As expected, the cost of PET production increases as the IV increases.

PET is a relatively stiff molecule. PET has a glass transition temperature of around 160 °F (70 °C). Under ideal thermal conditions, however, PET will crystallize, albeit quite slowly, to a maximum of about 40% (wt) and a melting temperature of about 510 °F (265 °C). The rate of crystallization of PET is governed by IV and its instant temperature. A measure of the rate of crystallization is the halftime of crystallization or the time it takes for PET to reach half its fi nal crystallinity. This is shown as a function of temperature in Fig. 11.2. As is apparent, the rate of crystallization is extremely slow at low temperatures, just above the glass transition temperature, and the rate of crystallization is extremely slow at high temperatures, just below the polymer melt temperature. The rate of crystallization is maximum in a temperature range of ~340 °F (~170 °C) or about halfway between the glass transition temperature and the melt temperature. If a polymer is cooled very rapidly through this region, few crystallites can form before the polymer is too cold to allow further chain mobility.

Amorphous PET (APET) is formed by quickly cooling an extruded sheet from the melt to room temperature. APET has a reasonably high modulus, and is a tough, transparent to slightly hazy polymer.

Thin APET sheet is frequently thermoformed into rigid packages. Because PET is a commonly post-consumer recycled plastic, APET competes quite well with other light-gauge transparent

PET

Intrinsic viscosity

Rate of crystallization

APET

181

package plastics such as polystyrene (PS) and PVC. Even though APET is not recommended for steam sterilization, it is used in medical packaging where gamma sterilization is used. In thermoforming, care must be taken to prevent overheating APET sheet as it will begin to crystallize in the oven. This will lead to poorly formed parts that may be hazy rather than water-white. APET regrind must be thoroughly dried and recrystallized before it can be mixed with virgin PET as feed to the extruder. APET cannot be heat-staked and is diffi cult to solvent weld. Pressure-sensitive adhesives (PSAs) have been developed that work well with APET.

PET is not normally extruded in thick sheets, simply because the sheet cannot be cooled rapidly enough to prevent centerline crystallization. Copolymer PETs have been developed to meet the heavy-gauge sheet and thermoforming markets. Typically, a portion of the diethylene glycol is replaced with a longer-chain aliphatic glycol. One of the earliest was neopentyl glycol. The copolymer PET, generically called PETG, is amorphous as the longer-chain glycol prevents the PET from crystallizing. PETG is more expensive than PET, but its thermal stability is desired for thick sheets and applications such as medical packaging where processibility is more important than cost.

In general, polyesters are quite moisture sensitive. Moisture does not normally manifest itself as moisture bubbles in the fi nal part. Rather, at high temperature, water attacks the polymer backbone, degrading it. This results in excessive sag during heating in the thermoforming oven and in a diffi cult-to-detect loss in mechanical properties such as impact strength. PET polymer producers recommend moisture levels of no more than 50 parts per million (50 ppm) in the feed stream to processing equipment.

Light-gauge polyester tends to be very diffi cult to trim cleanly, necessitating very sharp, heated trim dies. Improper trimming can result in fuzz, angel hair, and substantial trim dust. Trim dust is the primary problem when trimming heavy-gauge polyester products.

Biaxially oriented polyethylene terephthalate (OPET) fi lm is produced in blown fi lm processes and match-metal formed into speaker cones and microphone diaphragms.

Copolymer PET for heavy-gauge sheet

Moisture sensitive

Figure 11.2: Temperature-dependent halftime of crystallization for PET. Parameters include IV and the effect of talc as a nucleant



Beginning in the 1980s, the wide use of microwave ovens – both commercial and residential – led to the development of high-temperature food packaging that could be used in either microwave ovens or conventional hot air ovens. The fi rst polymer to meet this criterion was crystallizable PET (CPET). CPET has essentially the same polymer base as APET but materials are added that accelerate the formation of crystallites. The additives are organics such as polypropylene or inorganics such as talc. CPET containers are formed by heating a light-gauge PET sheet rapidly enough to minimize crystallization, and forming the crystallizing sheet against a mold heated to about 340 °F (170 °C). In other words, the optimum cycle time occurs when the mold is about the temperature at which the maximum rate of crystallization occurs, see Fig. 11.2.

The formed part is held under pressure against the mold until the PET has a crystalline level of about 20% (wt). At this level of crystallinity, the formed container will not distort when placed in a 400 °F (200 °C) hot air convection oven for up to one hour. Because PET is transparent to microwave energy, the formed container can also be used in microwave ovens.

Because PET density greatly increases as the polymer crystallizes, fi nal part dimensions are dramatically altered as the part crystallizes. Because the wall thickness is not uniform across the part, differential crystallinity often leads to part warpage and distortion. To minimize these effects, the part is often fi xtured after it exits the mold. As with other polyesters, the CPET recipe requires substantial drying to prevent moisture absorption that can ruin the formability and mechanical properties of the formed product. Pressure sensitive adhesives have been developed for lidding CPET containers.

11.3.7 Polyethylene

Amorphous polymers account for about 70% (wt) of all polymers thermoformed. Because crystalline polymer sheet must be heated above the polymer melt temperature to be formed, the polymer must have suffi cient strength in the melt. Otherwise, the sheet will come apart and fall into the heater.

Polyethylene (PE) is the crystalline polymer most often used in heavy-gauge thermoforming, primarily because of its very high melt strength or hot strength. PE melt strength is demonstrated in blow molding to produce a hollow container. A tube of PE, called a parison, is extruded and allowed to hang vertically as mold halves close around it. Parison blow molding of other crystalline polymers such as nylon (PA), polypropylene (PP), and PET, is impossible because of their poor melt strengths.

High-density polyethylene (HDPE) has a density of about 960 kg/m3. It has exceptional impact strength, chemical resistance, and excellent outdoor weatherability. As a result, it is thermoformed into pallets, dunnage, totes, marine applications, and many outdoor products. Low-density polyethylene (LDPE) has a density of about 920 kg/m3, has a much lower modulus than HDPE, and competes with fl exible PVC in soft packaging and other non-transportation applications. Care must be taken when heating LDPE to its forming temperature as it can sag very quickly in the forming oven.

Linear low-density polyethylene (LLDPE) is tougher than LDPE but softer than HDPE. It is far more diffi cult to thermoform than either of these. The reason for this will be discussed in the following when the problems with formability of homopolymer polypropylene (homoPP) are discussed.

CPET

Warpage and distortion

Most often used in heavy-gauge

forming

Density

183

11.3.8 Polypropylene

Polypropylene (PP) has a density of 910 kg/m3. Until recently, it was the least expensive, in terms of cost per unit volume, of the four commodity polymers – Polystyrene (PS), PVC, polyeth-ylene (PE), and PP. PP is sought for its high melting temperature, its excellent chemical and moisture resistance, high modulus, and good impact strength. Homopolymer PP (homoPP) is semi-crystalline with a glass transition temperature of around 32 °F (0 °C) and a melting temperature around 330 °F (165 °C). The crystallites in homoPP are quite large. As a result, light-gauge homoPP sheet is hazy to translucent. Furthermore, homoPP has very poor melt strength, meaning that it is not normally thermoformed in a melt state. Instead, it is carefully heated to within a few degrees below its stated melt temperature and pressure formed into the mold. Because the crystallites do not completely melt, the formed part retains much of the haze present in the sheet.

For years, PP polymer chemists strived to produce a PP that could be melt-phase formed in a fashion similar to HDPE. One early success involved copolymerization of around 10% (wt) PE with PP. Copolymer PP (coPP) has a glass transition temperature of around –5 °F (–20 °C) and a melting temperature around 310 °F (155 °C). It has lower modulus and lower crystallinity but improved melt strength. This allows it to be heated to forming temperatures a few degrees above its melt temperature. Formed parts have substantially less haze than those formed with homoPP. More importantly, heavy-gauge parts can now be formed from coPP.

HDPE has few side chain branches and so appears as a rather smooth molecule when compared with highly branched LDPE. However, the HDPE polymer chain is quite fl exible. This allows for extensive entanglements that increase the polymer resistance to shearing force. This provides its great melt strength.

Homopolymer PP is also a rather smooth molecule, with the backbone rotating to protect the pendant methyl group. The polymer chain is quite stiff when compared with HDPE and few entanglements occur. This implies that the polymer has little resistance to shearing force and therefore poor melt strength. Measurement of the time-dependent extensional viscosity of polyolefi ns reveals this. If the extensional viscosity increases with time, the polymer is considered to be strain rate hardening. This is seen in Fig. 11.3 for HDPE, where entanglements cause the time-dependent increase in viscosity and in Fig. 11.4 for LDPE, where long side chain branches cause the time-dependent increase in viscosity. On the other hand, as shown in Fig. 11.5, homopolymer PP shows a dramatic reduction in viscosity with increasing time under load [34].

Recent work has focused on developing short side chain branches on PP, as seen in Fig. 11.6 for high-melt strength PP (HMS-PP). Improved melt strength implies less sag while the sheet is in the oven, wider forming windows, and deeper draws and more uniform wall thickness in the forming station. Again, these improvements have spurred interest in thermoforming heavy-gauge parts of PP.

PP is a slowly crystallizing polymer. This is best demonstrated by examining its recrystallization temperature. Differential scanning calorimetry (DSC) is used to measure material transitions [35]. The technique involves side-by-side constant rate of heating (or cooling) of a small sample of polymer and a reference sample. If the polymer is not undergoing a transition, its rate of heating parallels that of the reference sample. As the polymer reaches a transition temperature

Properties

coPP

Strain rate hardening



Figure 11.3: Time-dependent extensional viscosity for high-density polyethylene (HDPE), showing the effect of chain entanglement to produce strain rate hardening

Figure 11.4: Time-dependent extensional viscosity for low-density polyethylene (LDPE), showing the effect of long chain branching to produce strain rate hardening. Strain rate values in s–1: A = 0.1, B = 0.3, C = 0.5, D = 0.7, E = 1.0

185

Figure 11.5: Time-dependent extensional viscosity for homopolymer polypropylene (homoPP), showing the absence of strain rate hardening. Strain rate values in s–1: A = 0.1, B = 0.3, C = 0.5, D = 0.7, E = 1.0

such as its crystalline melting temperature, its rate of heating slows, because the inputted energy is used to melt the crystallites. Once the crystallites are melted, the polymer rate of heating once again parallels that of the reference sample. When the polymer is cooled from its melt state, its rate of cooling parallels that of the reference sample until the polymer undergoes a transition, such as recrystallization. The rate of cooling increases as the polymer recrystallizes. Again, once the polymer has completed its transition, the rate of cooling parallels that of the reference sample. Figure 11.7 shows heating and cooling profi les for three polyolefi ns.

The heating and cooling curves for HDPE are depicted to the left in Fig. 11.7. HDPE melts around 280 °F (137 °C) and recrystallizes around 265 °F (130 °C). The heating and cooling curves for LDPE are depicted in the center of Fig. 11.7. LDPE melts in the general range of 230 °F (110 °C) and recrystallizes around 210 °F (100 °C). The heating and cooling curves for homoPP are depicted to the right in Fig. 11.7. Homopolymer PP melts around 320 °F (160 °C) and recrystallizes around 250 °F (120 °C).

Polyethylenes characteristically recrystallize very rapidly. As a result, their recrystallization temperatures are very close to their melting temperatures. On the other hand, PP charac-teristically recrystallizes slowly. As is apparent in Fig. 11.7, recrystallization temperatures are



Figure 11.6: Time-dependent extensional viscosity for high-melt strength polypropylene (HMS-PP), showing the presence of strain rate hardening. Strain rate values in s–1: A = 0.1, B = 0.3, C = 0.5, D = 0.7, E = 1.0

Figure 11.7: Differential scanning calorimetry heating and cooling curves for three polyolefi ns. a: HDPE. b: LDPE. c: Homopolymer PP. The heating and cooling rates are 7 °F/min (4 °C/min)

187

substantially below PP melting temperatures. The slow crystallization rate often dramatically affects the fi nal dimensions of thermoformed parts. Crystallization may continue for hours after the parts have been trimmed from their web and even after the parts have been inspected, packaged and warehoused. This often results in unwanted part distortion and warpage and wholesale rejection of fi nished products.

Small amounts (~0.1% wt) of additives such as sorbitols and phosphanates increase the rate of PP recrystallization by nucleating microcrystallites, as seen in Table 11.4. The crystallites thus formed are much smaller than those formed without the additives. As a result, the fi nished product has dramatically improved clarity, with haze levels typically below 5%.

11.3.9 Other Polyolefi ns

There is growing interest in the general family of thermoplastic olefi ns (TPOs). TPOs usually consist of a polyolefi n such as polyethylene, homopolymer PP, or copolymer PP, and a synthetic rubber such as ethylene-propylene-diene rubber (EPDM), or an ethylene-styrene-butadiene rubber (ESBR). For many applications such as automotive interior fascia, the TPO also includes fi ller such as talc or milled glass fi ber. If the olefi n is the greater organic component, it is the continuous phase and the rubber is the discrete phase. The resulting product is tough and semi-rigid to rigid. If the rubber is the greater organic component, it is the continuous phase and the olefi n is the discrete phase. The resulting product is fl exible and is often referred to as a thermoplastic rubber (TPR). If the rubber is reactive and can be crosslinked, the fi nal product is essentially a thermoset and is called a thermoplastic vulcanizate (TPV).

TPOs are thermoformable. The level of required applied forming force increases with increasing fi ller content. TPRs are also thermoformable, but the depth of draw is restricted and formed parts may experience short-term rebound or recovery once the forming pressure is removed. They often experience long-term stress relaxation as well. Most TPVs have some degree of mobility at elevated temperatures and so are thermoformed into relatively shallow-draw parts.

TPOs

Table 11.4: Additive Effect on PP Recrystallization Temperature (ppm = parts per million)

Copolymer melting temperature 155 °C

Recrystallization temperature, °C No clarifi er• Dibenzylidene sorbitol (DBS)• Methyl dibenzylidene sorbitol (MDBS)• Millad 3988 (unknown chemistry)•

92105 @ 1800 ppm107 @ 1200 ppm108 @ 600 ppm

Homopolymer melting temperature 165 °C

Recrystallization temperature, °C No clarifi er• Dibenzylidene sorbitol (DBS)• Methyl dibenzylidene sorbitol (MDBS)• Millad 3988 (unknown chemistry)•

102115 @ 1800 ppm120 @ 1800 ppm121 @ 1200 ppm



Cycloolefi ns (COCs) have been developed recently by reacting ethylene with cyclopentadiene to produce norbornene. Norbornene is then reacted with ethylene using a metallocene catalyst to produce the amorphous polymer, COC. The temperature-dependent elastic modulus of COC is shown in comparison with other thermoformable polymers in Fig. 11.8. Unlike other polyolefi n polymers, the glass transition temperature of COC can be dramatically altered by changing the ratio of norbornene to ethylene, as seen in Fig. 11.9. Because COCs are olefi nic, they have good UV resistance and excellent chemical resistance. Because they are amorphous, they have wide forming windows as seen by the very fl at plateau at temperatures above 100 °C. So far, they are quite expensive.

COCs

Figure 11.9: The effect of norbornene concentration on glass transition temperature of cycloolefi n, from Ticona Topas Advanced Polymers, Florence, KY

Figure 11.8: Temperature-dependent storage moduli of homopolymer polypropylene (homoPP), polyvinyl chloride (PVC) and cycloolefi n (COC) from Ticona Topas Advanced Polymers, Florence, KY

189

Ethylene vinyl acetate (EVA) is a relatively low-melting (~200 °F or 95 °C) very fl exible polyolefi n containing about 10–15% (mol) vinyl acetate. Although it is normally used as a fi lm, it has been thermoformed into fl exible semi-rigid packages. Because it is FDA-approved, it competes well with fl exible PVC and nylon in food packaging. Although it has exceptional elongation to break, low permeability to most oil-based essences, and excellent gloss, it has very high surface adhesion and friction. As a result, it is very diffi cult to handle in roll form without antistatic and antiblocking agents.

11.3.10 Formable Biopolymers

The global success of plastics as disposable light-gauge protective packaging is also causing consternation and concern. Once used, a substantial portion of the packaging resides in landfi lls or is discarded into the environment. As a result, there is growing interest in biodegradable and compostable polymers [36]24. Nearly all such polymers contain, at least in part, some type of biological polymer and so are called biopolymers.

Most biopolymers tend to be moisture sensitive, have low melting or softening points, have poor mechanical properties, and are more expensive than the hydrocarbon-based polymers with which they compete.

There are many types of biopolymers. The very earliest were based on cellulose – camphorated cellulose nitrate, cellulose acetate, and others, as described above. Starches have been polym-erized for many decades. And more recently, polylactic acid (PLA) has been commercialized.

To produce PLA, lactic acid is extracted from bacterial fermentation from natural crops such as corn and sugar cane. The lactic acid is then catalytically oligomerized, dimerized, and polym-erized. There are two stereospecifi c forms of PLA. L-PLA is about 35% crystalline, with a glass transition temperature of about 160 °F (70 °C) and a melting temperature of 350 °F (175 °C). These properties are increased by melt-blending up to 40% (wt) D-PLA. Blends of L-PLA and D-PLA are transparent. As with most biopolymers, PLA is moisture sensitive and must be thoroughly dried before extrusion. Rollstock and regrind must also be moisture-protected. PLA is also thermally sensitive. Extrusion temperatures must be carefully monitored. Low D-PLA concentration PLAs extrude like stiff polystyrenes. Gels are potential problems with light-gauge sheet and fi lm. PLA does not degrade in a traditional manner. It continues to crystallize in landfi lls. At around 150 °F (65 °C), it crumbles to powder in about 30 days.

Although polystarches have been produced in laboratories for decades, they tend to be very moisture sensitive and very diffi cult to extrude into quality sheet. Recently, polystarches have been combined with poly-e-caprolactone to produce a thermally stable polymer. Poly-e-capro-lactone is produced by oxidizing cyclohexanone with peracetic acid to produce e-caprolactone,

EVA

Based on cellu lose, starch, or poly-lactic acid

PLA properties

24 The terms biodegradable and compostable are not identical. ASTM defi nes a biodegradable plastic as one “in which the degradation results from the action of naturally occurring micro-organisms such as bacteria, fungi, and algae”. ASTM defi nes a compostable plastic as one “that undergoes biological degradation during composting to yield carbon dioxide, water, inorganic compounds and biomass at a rate consistent with other known compostable materials and leaves no visually distinguishable or toxic residues”. For more details, see L. F. Doty, “Compostable, biodegradable not identical”, Plastics News, July 16, 2007.



then reacting e-caprolactone with a catalyst to produce the polymer. Both polystarch and poly-e-caprolactone are biopolymers but may not necessarily be biodegradable. Degradation occurs by hydrolysis.

There is growing interest in genetically modifi ed microorganism- and plant-generated biopol-ymers. Two biopolymers of current interest are polyhydroxybutyrate (PHB) and polyhydroxy-valerate (PHV). Combinations of these polymers, called PHA or PHBV, are also of interest. PHB is quite stiff and very brittle. It has a glass transition temperature of about 50 °F (10 °C), melting temperature of about 340 °F (170 °C), and is transparent. Currently, the dried leaves of a genetically modifi ed watercress produce about 15% (wt) PHB. Genetically modifi ed e. coli bacteria also produce substantial amounts of PHB.

Other biopolymers include those synthesized from polypeptides. Two very early polymers were based on amino-acids obtained from gluten or animal protein and gliadin or vegetable protein.

PHB and PHV

Table 11.5: Biopolymer Glass Transition and Melt Temperatures(Courtesy of Engineer Anne-Marie Clarinval, CRIF-WTCM, Brussels, Belgium, 2001, www.CRIF.be)

Biopolymer Name Tg [°C] Tm [°C]

Cornfl our 1 NA 63

Cornfl our 2 NA 75

Cornfl our + Wax 1 NA 62

Cornfl our + Wax 2 NA 74

Corn amylase NA 80

Wheatfl our 1 NA 51

Wheatfl our 2 NA 64

Potato starch 1 NA 52

Potato starch 2 NA 73

EcoPLA 4040D 51 130

Galactic LA33JZE 42 168

Galactic LA07JZE 72 180

Biopol PHB ~0 177

Biopol PHBV 8% HV ~0 152

Biopol PHBV 12% HV ~0 143

PHB Metabolix 5 178

PHO Metabolix –30 62

Capa 650 (Solvay) –65 63

EastarBio 14766 –29 108

Bionolle PBS –30 112

Bionolle PBSA –45 90

191

Casein was fi rst synthesized from the amino acids in milk. Zein is synthesized from maze gluten. Polymers have been synthesized from vegetable proteins in soy bean, castor bean, and blood and from hair, wool, and silk. Nearly all of these polymers were reacted with formaldehyde or other crosslinking agents to produce moldable thermoset products. And most or all of these early polymers were supplanted by synthetic ones with superior properties and lower costs. All of these polymers could be biodegradable if they are not crosslinked. The high cost of extraction and the low yield of the basic building block from the animal or plant structure remain the primary economic barriers to more development and wider application of these biopolymers. The temperature characteristics of several biopolymers are given in Table 11.5.

11.3.11 Other Formable Polymers

If a polymer can be produced in sheet form, it can usually be thermoformed into a functional product. Although the great majority of polymers are identifi ed in detail above, there are niche applications for many other polymers. Some polymers that have found commercial success include:

• Fluoropolymers such as polyethylene terephthalate (PTFE) and fl uoroethylene polymers (FEPs). Despite their high cost, these highly crystalline polymers are sought for their extreme chemical resistance and elevated temperature applications. The sheet is usually cast rather than extruded. Fluoropolymers require very high forming temperatures and must be heated very carefully. Because sheet at elevated temperature cools very quickly in environmental air, forming must take place very quickly. Fluoropolymers are quite soft. As a result, trimming is very easy.

• Polyamides (PAs) or nylons are high-temperature crystalline polymers that have excellent chemical resistance. However, most have very poor melt strengths and are therefore not thermoformable. Recent development work has focused on improving nylon melt strength for both blow molding and thermoforming. Toughened or rubberized nylon is a copolymer of EPDM and polyamide with a melting temperature of about 315 °F (157 °C). A block copolymer of polyphenylene oxide and poly-e-caprolactam (nylon 6 or PA6) exhibits two crystalline melting temperatures indicative of the two crystalline moieties. To date, these nylons have found limited use because of their relatively high cost.

• Thermoplastic polyurethane (TPU) is a segmented block copolymer thermoplastic elastomer containing the traditional urethane hard and soft sectors. The hard segment is typically an aliphatic isocyanate. The soft segment is either a polyester or a polyether. TPUs used in aqueous applications are polyester-based. TPUs used in oil applications are polyether-based. TPUs are weldable and can be colored, printed, and sterilized. They have low temperature fl exibility and some grades have moderate UV resistance and optical clarity. Their chemical inertness in contact with human skin allows them to be thermoformed into medical devices such as gloves and condoms that compete with natural rubber latex products.

• High-nitrile resin (HNR) is usually an amorphous copolymer of modifi ed acrylonitrile and ethyl or methyl acrylate, with a glass transition temperature of about 210 °F (100 °C). It fi nds application as a thermoformed rigid package where moderate to good oxygen

Fluoropolymers

Polyamides

TPUs

HNR



and carbon dioxide gas barrier properties are needed. While its fi lm barrier properties are not as good as those of polyvinyl dichloride (PVDC) or ethylene vinyl alcohol (EVOH), it provides a barrier in a single layer sheet rather than in a diffi cult-to-recycle multilayer sheet. It extrudes and thermoforms as a relatively stiff SAN.

• Polysulfone (PSO) is a high-temperature amorphous polymer with a glass transition temperature of 365 °F (185 °C) and a continuous use temperature in air of about 285 °F (140 °C). It is a tough, ductile polymer with an elongation at break of 50%. It is chemically resistant to most acids, bases, and hydrocarbons but is attacked by ketones and certain chlorinated hydrocarbons. Although PSO is transparent, it typically has a yellow-to-amber tint, even in thin gauges. PSO sheet should be heated to at least 579 °F (300 °C) and should be formed against molds heated to around 210 °F (100 °C). Steel tooling is usually recom-mended. Because PSO is quite notch sensitive, thermoformed parts should be designed to have generous radii, particularly in potential impact areas.

Polyimides (PIs) have the following basic structure:•

R' C N C R''

R

O O

where R and R are linear aliphatic chains for linear PI or they are aromatic or cyclic chains for heterocyclic PI. Polyether-imide (PEI) is amorphous with a glass transition temperature of 783 °F (417 °C). If it is available in sheet form, it can be thermoformed using hot matched steel tooling.

When PI is copolymerized with aromatic polyamide, the result is an amorphous high-temperature polyamide-imide having a glass transition temperature of 525 °F (275 °C). It is normally thermoformed in a pre-imidized state and the trimmed part is then fully imidized in a controlled temperature oven.

• Polyphenylene sulfide (PPS) is a crystalline high-temperature polymer that can be thermoformed, albeit with diffi culty, provided that sheet product is available. Its melting temperature is 545 °F (285 °C). It has moderate-to-poor melt strength. As a result, it is often glass or mineral fi lled. Highly fi lled PPS must be pressure-formed. It is one of the most chemically resistant polymers known, being inert to most solvents and organic acids and alkalis, even at elevated temperatures. Unfi lled PPS structures pass FDA and NSF25

regulations for food and potable water contact.

Again, there are many other polymers that, when available in sheet form, are thermoformed into products that have niche applications.

PSO

PI

PPS

25 NSF = National Sanitary Foundation

193

11.4 Multilayer Polymers

Vinyl-acrylic blend polymers were discussed earlier. Heavy-gauge laminated PVC-PMMA sheet is often recommended for sanitary products, including shower stalls, soaking tubs, bathtubs, whirlpools, and spas. Because both of these polymers are amorphous with glass transition temperatures between about 175 °F (80 °C) and 210 °F (100 °C), the laminate thermoforms quite easily. The greatest concern is delamination, caused by differential expansion at the laminate interface. This is best controlled by reducing the rate of heating to achieve a more uniform temperature profi le through the sheet. Drawdown into sharp corners must be minimized to prevent delamination and read-through of the backing sheet.

Acrylic (PMMA) is extensively used as a cap sheet for ABS in heavy-gauge exterior applications such as construction products and vehicular components. The cap sheet is usually greater than about 0.006 in (0.15 mm) in thickness. As the cap sheet is stretched during forming, it may thin substantially. If the cap sheet is not colored the same as the substrate, the substrate color may read through, particularly in sharply radiused corners. Fluoropolymers (FEPs) are also used as cap sheets on HIPS or ABS substrates on products where abrasion or solid particle erosion may be a problem. Fluoropolymer cap sheets are typically less than about 0.012 in (0.3 mm) in thickness.

Many types of multilayer structures are used to produce light-gauge products. Often dissimilar polymers are laminated to provide characteristics that are not achievable with a single polymer. For example, to achieve a combination of stiffness, moisture barrier and oil resistance, polystyrene (PS) may be laminated with a polyolefi n (PO). To achieve small molecule diffusion resistance, a barrier polymer such as ethylene vinyl alcohol (EVOH) may be sandwiched between two polyolefi n (PO) sheets. Often, tie layers such as ethylene vinyl acetate (EVA) or amines are employed between plies to ensure adhesion during heating, shaping, trimming, and end use. As a result, light-gauge multilayer sheet may have three layers, at a minimum, to perhaps as many as seven or nine layers of dissimilar polymers.

When forming multilayer sheet, all layers draw locally to the same extent, regardless of their individual thicknesses. As an example, consider thermoforming a 0.006 in (0.15 mm) thick cap sheet on a 0.120 in (3 mm) substrate. If the sheet is drawn locally to 33% of its original thickness, the local substrate thickness will be 0.040 in (1.0 mm) and the local cap sheet thickness will be 0.002 in (0.050 mm). If, as is the case of acrylic-capped ABS, the cap sheet is there to protect the substrate from ultraviolet or UV damage, the barrier may be compromised if the cap sheet is too thin.

In some cases, care must be taken to ensure that very thin plies of high-forming temperature polymers are at their forming temperatures. If not, the higher temperature polymers may delaminate, form microcracks, or microscopic pinholes may form as the plies are drawn. In any case, the barrier resistance may be compromised.

The formability of multilayer sheet depends on a combination of factors such as the forming temperature range of the higher-modulus polymer, the thickest ply, and the ply requiring the highest forming temperature. As an example, consider a multilayer sheet consisting of a moderate temperature amorphous polymer of substantial thickness, a very thin high-temper-ature amorphous barrier layer, and a moderately thick ply of high-temperature crystalline polymer. It is apparent that the forming temperature of the entire sheet will depend on the

Cap sheet

Achieving specifi c property combina-tions by upto 9 layers of dissimilar polymers

Example

11.4 Multilayer Polymers


formability of the crystalline polymer, even though the temperature of the amorphous polymer may be substantially higher than its normal forming temperature. The thin barrier layer fi lm will stretch when the crystalline polymer is at its forming temperature.

11.5 Foamed Plastics

Thermoplastic foams are produced by adding an appropriate foaming agent to the polymer during the extrusion process. There are three general classes of thermoplastic foams. High-density foams typically are more than about 70% of the density of their respective unfoamed polymers. About 20% (wt) of all thermoplastic foams are considered high-density foams. Medium-density foams have densities in the range of 20 to 70% of those of the unfoamed polymers. About 5% (wt) of all thermoplastic foams are considered medium-density foams. Low-density foams typically are less than about 20% of the density of their respective unfoamed polymers. About 75% (wt) of all thermoplastic foams are considered low-density foams [37].

11.5.1 High-Density Foams

High-density and medium-density foam sheet are usually produced by adding a chemical foaming agent to the extruder hopper. Chemical foaming agents are pure chemicals that are thermally unstable above a very specifi c temperature range. There are two general classes of chemical foaming agents. Exothermic foaming agents generate heat when they decompose. Azodicarbonamide (AZ) is the most widely used exothermic foaming agent. It decomposes around 400 °F (205 °C) to produce about 220 cm3 (STP)26 of gas per gram of agent. The primary liberated gas is nitrogen.

Endothermic foaming agents require heat to decompose. Sodium bicarbonate (baking soda or NaHCO3) is the most widely used endothermic foaming agent. It decomposes around 250 °F (120 °C) to produce about 135 cm3 (STP) of gas per gram of agent. The primary liberated gases are carbon dioxide and water vapor. Endothermics are often required when the thermoformed foam product is used in contact with food or medical products.

Endothermics and exothermics are often combined with total concentrations of 0.5% (wt) to 2.0% (wt) to produce foam sheet having densities of about 70% of those of the unfoamed polymers. Foamed impact modifi ed polystyrene (HIPS), ABS, polyethylene terephthalate (PET), polyvinyl chloride (PVC), and polycarbonate (PC) are commercially available as sheet and other polymers can be custom foamed by several extrusion houses. Conventional fl at extrusion dies and roll stacks are often used to produce high-density foam sheet. The greatest concern during extrusion is gauge control as the hot sheet may continue to expand for some distance after leaving the extrusion die. High-density foam sheet often does not have a quality surface. As a result, the foamed product may be cap-sheeted on one or both sides for appearance applications.

Foaming agents

Gauge control

26 STP = Standard Temperature and Pressure, usually 77°F (25°C) and 1 atmosphere (0.1 MPa).

195

Typically, high-density foam sheet thermoforms in a manner similar to the unfoamed polymer. However, care must be taken to prevent overheating the sheet as excessive heat causes the cell gas to expand and in the extreme, to catastrophically collapse. Care must also be taken when pressing the heated sheet against the mold surface. Excessive pressure will cause cell collapse, resulting in an increase in local part density and reduction in local part thickness. This is particularly true in two- and three-dimensional corners. Chamfered corners are always preferred over radiused corners when thermoforming foam of any density.

High-density foam products are often called structural products, meaning that they are usually permanent products designed to carry loads. Very frequently, one of the primary design criteria is stiffness. Stiffness, S, is the product of the material modulus, E, and I, the moment of inertia or geometry of the structure. It is written as:

=S E I (11.1)

The moment of inertia is a function of the product design. For many structures, the moment of inertia is approximated by that for a fl at panel:

= 3I G t (11.2)

Where G is a geometric parameter and t is the panel thickness. When the polymer is foamed, its modulus decreases in proportion to the square of the density:

⎛ ⎞= ⎜ ⎟⎝ ⎠

2

ff o

o

E E (11.3)

Where Ef is the modulus of the foam, Eo is the modulus of the unfoamed polymer, f is the density of the foam and o is the density of the unfoamed polymer.

There are two general reasons for producing high-density foam products. If the product weight is reduced at the same product thickness, the foamed product stiffness is:

=f fS E I (11.4)

Where Sf is the stiffness of the foamed product. This stiffness is related to the unfoamed stiffness as:

⎛ ⎞ ⎛ ⎞= =⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠

2 2

f ff o o

o o

S E I S (11.5)

The stiffness reduces in proportion to the square of the foam density. As an example, if the foam density is 70% of that of the unfoamed polymer, the foamed product stiffness is 49% of that of the unfoamed product.

If the product weight is to remain the same, the product thickness increases in proportion to the decrease in density. The foamed product stiffness is:

=f f fS E I (11.6)

Structural products

Stiffness

Moment of inertia

Modulus

Stiffness of foamed product



Where If is the moment of inertia of the foamed product. The moment of inertia of the foam is given as:

⎛ ⎞= ⎜ ⎟⎝ ⎠

3

of o

f

I I (11.7)

The stiffness is related to the unfoamed stiffness as:

⎛ ⎞ ⎛ ⎞ ⎛ ⎞= =⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎝ ⎠ ⎝ ⎠⎝ ⎠

2 3

o off o o o

o f f

S E I S (11.8)

The stiffness increases in inverse proportion to the decrease in foam density. As an example, if the foam density is 70% of that of the unfoamed product, the foamed product stiffness is 143% of that of the unfoamed product.

11.5.2 Low-Density Foams

Thermoformed low-density foam products are used primarily as thermal insulators such as pizza boxes and as shock mitigators such as egg cartons. Polystyrene (PS) and modifi ed polystyrene (HIPS) dominate low-density foam thermoforming. Polyethylene (PE), crosslinked polyethylene (XLPE), polypropylene (PP), and polyethylene terephthalate (PET) foams are also thermoformed. Low-density foams are characterized by densities from 2 lb/ft3 (30 kg/m3) to at least 10 lb/ft3 (160 kg/m3). Although these foams can be extruded using modifi ed extrusion equipment using fl at dies, most commercial foams are produced on highly specialized extruders, annular dies, and shaping and take-off equipment [38]. These extrusion units are described in more detail in Chapter 12.

Physical foaming agents are used to produce low-density foams. Aliphatic hydrocarbons such as butanes and pentanes, hydrochlorofl orocarbon (HCFC) refrigerant gases such as chloro-difl uoroethane (R-142b), and hydrofl uorocarbon (HFC) refrigerant gases such as tetrafl uoro-ethane (R-134a), are volatile liquids that vaporize to produce cell gases27. Carbon dioxide is the primary atmospheric gas used to produce foams. Argon and nitrogen are atmospheric gases that fi nd limited use. Chemical foaming agents, as described earlier, are often added to the extruder. Although they also produce gas, they act primarily as nucleating agents for bubble production.

Throughout the extrusion process, the gases generated either from decomposition of chemical foaming agents or volatilization of physical agents, or as atmospheric gases that have been dissolved in the polymer, are held in solution in the polymer melt by high melt temperature and pressure. Foam is produced as the gas-laden melt issues from an appropriately shaped extrusion die. Individual foam cells are formed by dissolution of the gas from the polymer melt and cell walls are created by biaxial stretching of the polymer. As a result, low-density thermoplastic foams are nearly always closed-cell foams.

Low-density foam materials

Foaming agents

Closed-cell foams

27 The Montreal Protocol restricts the use of chlorofl uorocarbons (R-11 and R-12) and by 2030 the use of hydrochlorofl uorocarbons (such as R-142b) in foams. The Kyoto Protocol may eventually restrict hydrofl uorocarbons (such as R-134a).

197

Once the foam is produced, it is usually aged. Aging allows air to diffuse into the cells while a portion of the blowing gases diffuses out. This gas interchange is important in thermoforming. As the foam sheet is heated prior to forming, the internal cell gas pressure increases and the polymer softens. This causes the foam thickness to increase in the oven by 50% to as much as 100%. This secondary expansion allows the extruder to produce a higher-density and potentially a more stable foam. It also allows for increased stiffness in the fi nal product.

Low-density foamed polymers are far more diffi cult to form than their unfoamed equiva-lents. Because the foam cell walls retain a portion of the blowing gas, amorphous polymer glass transition temperatures and crystalline polymer melting temperatures are depressed. As shown in Fig. 11.10, polystyrene (PS) glass transition temperature is depressed as much as 14 °F (8 °C) per 1% (wt) foaming gas. As a result, foams soften at temperatures below the polymer transition temperatures. As the foam is heated prior to forming, the polymer softens and the internal cell gas pressure increases. The effect is biaxial stretching of the cell membranes. Excessive temperature results in membrane rupture and catastrophic cell collapse. As a result, thermoplastic foams are heated quite slowly and only until secondary expansion is nearly complete. Because the foams are quite stiff at this temperature, they are nearly always formed between matched metal molds.

Aging

Thermoforming challenges

CO2

C4

134a

142b

142b

0 5 10 15 2025

Gla

ss tr

ansi

tion

tem

per

atur

e, °

C

Foaming agent conc., wt %

50

75

100

Figure 11.10: The effect of small gas molecule concentration on the glass transition temperature of polystyrene (PS)



Relatively deep draws are achieved by compressing the foam cells between the two halves of the mold rather than by biaxial stretching as is the case with unfoamed polymers.

The surfaces of low-density foams usually consist of a thin layer of higher-density, compressed cells. The surfaces are usually quite matte. To improve surface quality for graphical appearance or cut resistance, foams are laminated with unfoamed sheet stock typically 0.005 in (0.125 mm) in thickness.

11.6 Thermal Properties

When compared with metals and ceramics, polymers are thermal insulators. Thermoformers need to effi ciently heat plastic sheet to a formable temperature. Thermoformers need to cool the formed part to a temperature at which the part retains the shape of the mold. Designers of thermoformed parts need to know how those parts expand and contract with temperature. Five thermal properties are important to thermoformers:

• Enthalpy or heat capacity, its derivative

Thermal conductivity•

Temperature-dependent density•

Thermal diffusivity, being a function of heat capacity, thermal conductivity, and density•

Thermal coeffi cient of expansion or COE•

Table 11.6 gives representative values of four of these properties for several thermoformable polymers.

Thermal proper-ties important to

thermoformers

Table 11.6: Physical Properties of Thermoformable Polymers

Polymer Densitylb/ft3

(kg/m3)

Thermal conductivityBtu/ft h °F

( 10–3 cal/cm s °C)

Heat capacityBtu/lb °F or cal/g °C

Thermal ex pansion coeff.

10–6 °F–1

( 10–6 °C–1)

PS 65.5 (1050) 0.105 (0.18) 0.54 40 (70)

ABS 65.5 (1050) 0.070 (0.12) 0.40 50 (90)

PC 74.9 (1200) 0.121 (0.207) 0.49 40 (70)

RPVC 84.2 (1350) 0.100 (0.171) 0.365 45 (80)

LDPE 57.4 (920) 0.23 (0.39) 0.95 140 (250)

HDPE 59.9 (960) 0.29 (0.50) 1.05 110 (200)

HomoPP 56.2 (900) 0.11 (0.19) 0.83 85 (150)

PET 85.5 (1370) 0.138 (0.236) 0.44 40 (70)

Low-density PS foam 4.0 (64) 0.016 (0.027) 0.5 110 (200)

199

11.6.1 Heat Capacity

Heat capacity, sometimes called specifi c heat, is a measure of the amount of energy needed to raise the temperature of the polymer by a specifi c amount. The fi eld of study that focuses on energy uptake of materials is called thermodynamics. Enthalpy is the fundamental measure of energy uptake. The enthalpy of a material increases with increasing temperature. When a material passes through a primary transition such as melting, the shape of the temperature-dependent enthalpic curve changes dramatically. When a material passes through a secondary transition such as the glass-to-rubber transition, the shape of the temperature-dependent enthalpic curve changes subtly, if at all.

Temperature-dependent enthalpic curves for several thermoformable polymers were shown in Fig. 8.5. As expected, far more energy is required to heat a crystalline polymer from room temperature, say, to a temperature above its melting temperature, than is required to heat an amorphous polymer over the same temperature range. For example, it takes about twice as much energy to heat polyethylene (PE) from room temperature of 77 °F (25 °C) to 356 °F (180 °C) than it does to heat polystyrene (PS) over the same temperature span. Furthermore, because the thermoformed shape must be cooled, twice as much energy must be removed to cool PE to a given temperature than to cool PS to that same temperature.

As seen in Table 11.3, specifi c heat or heat capacity for a given polymer is usually given as a single value. This value is the slope of the enthalpy-temperature curve and is determined by dividing the enthalpy difference by the appropriate temperature difference. This method is usually acceptable for amorphous polymers but should be used with caution for crystalline polymers. As is apparent with any crystalline polymer, the slope of its temperature-dependent enthalpy curve – and therefore its specifi c heat – changes dramatically in the polymer melting temperature range.

11.6.2 Thermal Conductivity

Thermal conductivity is the measure of steady-state energy transmission through a material. Thermal conductivity values for organic chemicals, including plastics, are in general orders of magnitude lower than those for metals, for examples. In other words, plastics are thermal insulators. As an example, the thermal conductivity of aluminum, the common thermoforming mold material, is nearly one thousand times greater than that for polystyrene (PS).

Even though thermal conductivity values for polymers are low, there are differences in values among polymers. For example, the thermal conductivity of high density polyethylene (HDPE) is about three times larger than that of polystyrene (PS) or ABS. The rate at which energy is conducted through a material is important when heating heavy-gauge sheet and cooling heavy-gauge formed parts. For very thick sheets, the rate of energy transfer into the sheet and out of the formed part into the mold and environment often controls the total cycle time. Although thermal conductivity values typically decrease slightly with increasing temperature, the values can be considered constant for most processing purposes.

Specifi c heat

Enthalpy-temperature curve

Plastics are thermal insulators

11.6 Thermal Properties


11.6.3 Polymer Density

Although density is not truly a thermal property, its role is quite important during the thermo-forming process. For most materials, space between molecules increases when they heat. The result is manifest as an increase in specifi c volume – the volume per unit mass. Density, the reciprocal of specifi c volume, decreases with increasing temperature. Near the polymer glass transition temperature, the slope of the temperature-dependent specifi c volume curve changes perceptively. Near the crystalline polymer melting temperature, the slope changes dramatically. Typically, the density of an amorphous polymer at its forming temperature may be 10 to 15% less than that at room temperature. The density of a crystalline polymer at its forming temperature may be as much as 25% less than that at room temperature. Obviously, as a formed polymer shape cools from its forming temperature, its density increases and its volume decreases. As a result, fi nal part dimensions decrease and the part exhibits thermal shrinkage.

11.6.4 Thermal Diffusivity

Thermal diffusivity is a measure of time-dependent energy transmission through materials. It is the ratio of thermal conductivity to the product of density and specifi c heat, as shown:

=p

k

c (11.9)

Where is thermal diffusivity, k is thermal conductivity, is density, and cp is specifi c heat. Because of the unique bundling of temperature-dependent characteristics of these polymer properties, thermal diffusivity is nearly temperature-independent for any given polymer.

11.6.5 Thermal Coeffi cient of Expansion

Thermal coefficient of expansion (COE) is a measure of the dimensional change of a material with temperature. There are two types of COEs – linear COE and volumetric COE. Linear COE is usually used to determine the temperature-dependent change in a specifi c solid product dimension. If the product is isotropic, meaning that the product properties are the same in all principal directions, the linear coeffi cient of expansion is approximately one-third the volumetric coeffi cient of expansion. Typically, polymers have greater COEs than metals. For example, the linear COE for high density polyethylene (HDPE) is 200 mm/m °C (200 10–6 °C–1) compared with 23 10–6 °C–1 for aluminum and about 12 10–6 °C–1 for steel. Crystalline polymers tend to have higher COEs than amorphous polymers. For example, the linear COE for polystyrene (PS) is 70 10–6 °C–1, compared with 200 10–6 °C–1 for HDPE.

Keep in mind that COE is a parameter that designers must consider only after all product dimensional changes owing to effects such as recrystallization and stress relaxation have been accounted for.

Specifi c volume

Linear and volumetric COE

201

11.6.6 Thermal Properties of Multilayer Structures, and Filled and Reinforced Polymers

The thermal properties of multilayer structures depend strongly on the characteristics of the dominant layer and on the quality of the adhesion between layers. If the surface layer has a high thermal diffusivity value and the substrate has a low value, the surface layer may heat far faster than the substrate. If the value of COE of the surface layer is greater than that of the substrate, interfacial delamination may occur.

In general, for fi lled and reinforced polymers, the polymer density increases nearly linearly with increasing mineral loading. Thermal conductivity tends to increase with increasing mineral loading. Heat capacity usually remains constant or increases slowly. In general, thermal diffusivity, as defi ned as a ratio of these properties, usually does not change with the level of mineral loading. On the other hand, the COE value usually decreases rapidly with small levels of mineral loading. As the loading level increases, the decrease in value is not as rapid.

Keep in mind that for most fi lled polymers, particle distribution is rather uniform throughout the polymer sheet, meaning that fi lled polymers can be considered as isotropic. This is usually not the case for fi ber-reinforced polymers. Thermal properties can differ quite dramatically in the three principal directions, depending on the fi ber orientation in the polymer. Thermal properties in the machine direction or the dominant fi ber direction may differ greatly from those in the cross-machine direction and the through-thickness direction. This implies that the heating and cooling characteristics of fi lled sheet may differ substantially across and down the sheet.

Keep in mind that a fi lled, reinforced, or even heavily pigmented sheet may appear to radiantly heat faster than that sheet having no such additives. Minerals on the sheet surface block and absorb a portion of the radiant energy that would otherwise be absorbed by the polymer within the fi rst few microns of the sheet surface. Infrared thermometers only measure surface temperature. Although the sheet absorbs the same amount of energy, increased mineral loading will indicate increased surface temperature.

11.7 Infrared Energy Absorption for Specifi c Polymers

Essentially all light-gauge sheet and a substantial portion of heavy-gauge sheet are radiantly heated. The electromagnetic radiation bandwidth or wavelength is usually given in microns or μm. The entire electromagnetic radiation band, from the very short radio and microwave wavelengths to the extremely long wavelengths of nuclear and cosmic rays, was depicted in Fig. 8.1. The visible wavelength spectrum is very narrow, from 0.4 μm to about 0.7 μm. The near-infrared wavelength spectrum is from about 0.7 μm to about 2.5 μm. The far-infrared wavelength spectrum is from about 2.5 μm to about 100 μm. Most commercial thermoforming heaters emit energy in the far-infrared wavelengths. The usual far-infrared wavelength range for heating polymer sheet is from about 2.5 μm to around 15 μm.

Dominant layer determines properties

Fiber orientation important

Electromagnetic radiation band width



The chemical make-up of a polymer determines how much radiant energy a polymer absorbs and how much is transmitted through the polymer. Figure 11.11 is a transmission spectrum for polystyrene (PS). It is instructive to examine this fi gure in some detail. First, the data are obtained using a Fourier transform infrared (FTIR) scanning device [39]. A thin fi lm of polymer is scanned with infrared energy from a black body source. The beam passes through an interferometer where it is encoded. It then passes through the polymer fi lm. As the frequency of the beam energy is altered, the amount of energy absorbed by the polymer and the amount transmitted through the polymer at that frequency are recorded electronically. The frequency spectrum data are then analyzed using Fourier transformation mathematics. The results are presented in graphical form, as seen in Fig. 11.11, where the amount of energy transmission through the polymer is given as a function of beam wavelength.

FTIR analysis is a fundamental analysis tool in organic chemistry. By electronically comparing specifi c wavelength peaks with databases containing literally thousands of pure organic substances, polymers, additives, and organic contaminants can be identifi ed and their concen-trations determined in minutes.

FTIR analysis is also an important teaching tool for the thermoformer. Re-examination of Fig. 11.11 shows that two curves are shown. The thinner top curve is for 0.001 in (25 μm) thick fi lm. Except for the regions around 3.5 μm and 6.5–7 μm, the fi lm is approx. 90% transparent. The heavier lower curve is for 0.010 in (250 μm) thick fi lm. Again, except for regions around 3.5 μm and 6.5–7 μm, the fi lm is 30–50% transparent. In the 3.5 μm and 6.5–7 μm regions, the fi lm has zero or near-zero transparency. Chemically, these regions represent carbon-hydrogen bond activity28. In other words, all carbon-hydrogen polymers should show complete or nearly complete energy absorption in these wavelengths.

Now consider regions away from these regions. It is apparent that as the PS fi lm gets thicker, it absorbs more far-infrared energy and therefore transmits less far-infrared energy. In fact, it has been mathematically verifi ed that the amount of energy absorbed increases exponentially with fi lm thickness. This also implies that during heating, the sheet is absorbing radiant energy volumetrically and not just on the surface.

Now compare the FTIR curves in Fig. 11.11 for PS with Fig. 11.12 for polyethylene (PE), Fig. 11.13 for polyvinyl chloride (PVC), and Fig. 11.14 for polyethylene terephthalate (PET).

All three sets of curves show zero or near-zero fi lm transparency in the 3.5 μm and in the 6.5–7 μm regions. Note, however, that 0.010 in (250 μm) thick PE fi lm is still nearly 70% transparent in the far-infrared energy range. This is why it is more diffi cult to radiantly heat light-gauge PE sheet than to radiantly heat light-gauge PVC sheet of the same thickness.

As noted earlier, opaque substances such as pigments, fi llers, and reinforcing elements, block a portion of inbound radiant energy. The amount of blocking is proportional to the opaque substance loading level. As expected, blocking alters the energy distribution to the sheet, meaning that more energy is absorbed on the surface and less transmitted to the sheet interior. The result is an altered temperature profi le in the polymer residing near the sheet surface.

Keep in mind that the FTIR scan presents energy transmission as a function of wavelength in microns (μm). Any solid or semi-solid particles having dimensions in the range of 2.5 μm to, say, 15 μm will interfere with far-infrared radiant energy transmission. Such particles

FTIR

Example

Infl uence of fi llers, pigments, and

reinforcing agents

28 –CH stretch occurs in the 3.0–3.7 μm region and –CH bend occurs in the 6.5–7 μm region.

203

Figure 11.11: Infrared transmission spectrum for polystyrene (PS)

Figure 11.12: Infrared transmission spectrum for polyethylene (PE)

Figure 11.13: Infrared transmission spectrum for polyvinyl chloride (PVC)

Figure 11.14: Infrared transmission spectrum for polyethylene terephthalate (PET)



might include pigments, nucleating agents, gelled and/or crosslinked polymers, and other detritus. Dyes and tints are normally organics and should appear in FTIR scans as specifi c peaks. Unless they agglomerate, particles with dimensions less than, say, 2.5 μm, will usually not interfere with far-infrared radiant energy transmission. In particular, particles that are added to alter the polymer appearance in the visible wavelength range usually will not alter the heating performance of the plastic sheet. Opacifi ers such as titanium dioxide are additives in this class.

There is a strict mathematical equation relating peak radiation wavelength with temperature. The equation is:

+ =max

2897.6273 °CT (11.10)

Where T is the temperature of the energy source and max is the peak radiation wavelength. At 2.5 μm peak wavelength, for example, the temperature of the energy source is 1625 °F (885 °C). At 3 μm, it is 1280 °F (695 °C) and at 4 μm, it is 845 °F (450 °C). These represent the peak wavelengths generated by thermoforming heaters at their respective temperatures.

The relative heating characteristics of polymers can be surmised from examining their FTIR scans, side by side. Compare, for example, the FTIR scan for 0.010 in (0.25 mm) thick PE (Fig. 11.12) with that for 0.010 in (0.25 mm) thick PET (Fig. 11.14). It is apparent that the transmission rate through each fi lm at a peak wavelength of 3.5 microns (3.5 μm) is nearly zero. As a result, the two sheets should heat at relatively the same rates if their heater temperatures are both set at 1030 °F (555 °C). On the other hand, at 490 °F (255 °C), the peak radiant energy wavelength is about 5.5 microns (5.5 μm). As a result, at this heater temperature, transmission through PE will be about 65% while that for PET will be nearly zero. In other words, PET will absorb nearly 3 times more energy than PE will at this lower temperature. In short, the energy absorption character of the polymer must be taken into account whenever initial heater temperature is determined and any time heater temperatures are to be adjusted.

Relation between peak radiation

wavelength and temperature

11. polymers and plastics

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