Thermal Changes in Polymers

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3.6.1. Glasslike Solid Phase

        The responses of polymer materials to heat inputs are similar in some ways to those of small molecules, as has been described in the section on solids, liquids, and gases. The input of thermal energy (heat) is both common and very important for polymeric materials. When polymers are heated, the basic nature of the polymer is to move internally to absorb the energy input. This internal motion can be molecular twisting, vibrating, stretching, translation, and other movements. The extent of these motions is detected by a rise in the temperature of the polymer. Some of these motion modes occur at lower energy inputs than others (for instance, vibrating occurs with little energy input), but as the amount of energy input increases, all the motion modes become active and the temperature increases greatly as a measure of this increased molecular motion.

        At low temperatures, polymers are solids. The motions of atoms within a solid polymer are initially limited to small movements (usually vibrations) of a few atoms. As the amount of heat input increases, these motions become both larger in amplitude and involve more atoms. Because of the limited amount of motion allowed in a solid material, the easiest way to absorb the energy is to involve more of the atoms along the polymer chain but to limit the motions to those that take up the least space and energy-vibrations, rotations, and twisting. Hence, with moderate heat input, the majority of the atoms are in a mode of increased vibration, rotation, and twisting. The properties of the polymer during this stage of heating are little changed except for an increase in the temperature of the polymer and a minor increase in the space occupied by the polymer (volume), because the motions take slightly more space and the atoms are forced apart.

3.6.2. Thermal Expansion

        Most materials expand when heated and contract when cooled. Because plastics absorb input energy through internal motions, the polymer chains usually move apart to allow for this motion. Hence, plastics are likely to expand more than metals or ceramics when heated. This change in dimension of a material with input heat is called thermal expansion and is usually measured as the ratio of the change in a linear dimension (such as overall length) to the original dimension per unit change in temperature. The resulting quantity is called the coefficient of thermal expansion (CTE). The tests for coefficient of thermal expansion (ASTM D 696 for linear and D 864 for volume) are conducted by placing a sample inside a tube that is submersed in a temperature-controlled bath. A quartz rod is placed against the sample and held in place by a low-force spring. The movement of the quartz rod is sensed by a dial gauge or by an electronic measuring device. The sample is heated and the movement of the quartz rod against the sample is noted over the temperature range of interest. The apparatus used is called a dilatometer. A diagram of a test apparatus for making CTE measurements is given in Figure 3.4. Test specimens can be from 2 to 5 inches long and round so as to fit easily within the testing device. Results are given as length/length °F. Typical CTE values for plastics and metals are given in Table 3.1. Ceramics have CTEs in the range 0.06 to 2 in./in. °F, which are much lower than for either plastics or metals. Therefore, the addition of ceramics (such as fillers) into a plastic resin can lower the overall CTE of the mixture. The addition of fiber reinforcements, especially ceramic fibers, will generally decrease the CTE for plastics because the polymer chains are not allowed to move freely, being restricted in their movement by the fibers.

Figure 3.4 Test apparatus (dilatometer) used for measuring coefficient of thermal expansion (GTE).

        This expansion of plastics when hot must be taken into consideration when designing a part. For instance, if a tight fit is desired between a metal screw in a plastic part, the use temperatures should be kept in a narrow range, otherwise at high temperatures the plastic material will expand much more than the metal and the screw will become loose. The CTE for plastics must also be considered when designing a mold for plastics. When a plastic part is formed, it is usually at an elevated temperature and will shrink considerably when cooled. Consequently, the mold cavity should be precisely oversized so that the finished part can shrink to the correct dimensions.

Material	Thermal Conductivity	Heat Capacity	Thermal Expansion (CTE)
Plastic	0.03 to 0.06 Btu/h-ft °F (0.05 to 1.0 W/m • K)	0.4 to 0.9 Btu/lb °F (0.4 to 0.9 cal/g °C)	9 to 12 in./in. °F × 10-5 (5 to 7 cm/cm °C × 10-5)
Metal	30 to 60 Btu/h-ft ° F (50 to 500 W/m • K)	0.1 to 0.3 Btu/lb ° F (0.1 to 0.3 cal/g °C )	2 to 8 in./in. °F × 10-5 (1 to 4 cm/cm °C × 10-5)

Table 3.1 Typical Thermal Conductivities, Heat Capacities, and Coefficients of Thermal Expansion for Plastics and Metals

3.6.3. Creep

        As the polymer is heated further, translational movements become more important. Translational movements occur when atoms move from one place to another in space, beyond the movements of vibration, rotation, and twisting, which are centered about a stationary molecular position. These increased movements allow the polymer chains to slowly disentangle and to move apart, thus further increasing the space between the atoms. This increase in space results in a decrease in the strength of any secondary bonds that may be present and a decrease in the entanglement between polymer chains.

        With these changes, the polymer's movement under applied external loads is increased. In other words, the solid polymer slowly moves under applied loads. This phenomenon is called creep. Creep becomes greater as the temperature of the polymer increases and as the amount of secondary bonding and entanglement decreases. Therefore, polymers that have high secondary bonding, such as crystalline polymers, have inherently less creep than do amorphous polymers, assuming other factors are equal. Creep is also reduced by high entanglement, by the presence of large pendant groups that inhibit movement, by crosslinking, and by the inherent stiffness of the backbone of the polymer. These effects are discussed later in this chapter in the section on steric (shape) effects and in Chapter 4.

3.6.4. Heat Distortion Temperature (HDT)

        At some temperature the plastic will become so pliable and so easily distorted under load that it may not perform the function intended, especially if that function is structural. The temperature at which this happens varies widely among different plastics and among different applications. For instance, although one plastic may be "dishwasher safe," that is, it will not distort at dishwasher temperatures (about 120°F, or 50°C), another plastic will curl and deform. Therefore, the first plastic material is thermally stable at dishwasher temperatures, but the second is not. In another instance, two plastics may be very suitable for cassette tape cases in normal use but one will distort when left inside a car in the summer. The designer of plastic parts should therefore, be aware of the maximum structural use temperature. Aromatic content, crosslink density, crystallinity, and secondary bonding can all raise the temperature at which distortion occurs.

        A convenient test, the Deflection Temperature Under Load (ASTM D648), can be used to measure this upper use temperature. In this test, illustrated in Figure 3.5, a sample of the plastic material (5 X ½ X ¼ in.) is placed in a heated bath of mineral oil or some other liquid that is thermally stable at the temperatures to be used, does not react with the plastic, and will transfer heat readily to the sample. A specific weight (designated by the test procedure and dependent upon the type of plastic material being tested) is then placed on the sample. The entire apparatus is then heated, usually with stirring to ensure good heat evenness in the liquid. The temperature at which the sample deflects (bends) a specified distance under these conditions is the Heat Distortion Temperature (HDT). The HDT is often considered the maximum structural use temperature, especially for any application in which the part will be loaded mechanically.

Figure 3.5 Deflection Under Load test to determine heat distortion temperature.

        Note that the HDT is just a measurement made on a particular plastic under certain specified conditions of sample size and weight applied. It should not be considered a fundamental property of the plastic as are the thermal transition properties (such as the glass transition temperature and the melting point), which will be discussed shortly. But the ease of making the HDT measurement and the low cost of the apparatus has led to the widespread use of the HDT in characterizing polymer use temperatures. Normally, HDT occurs at a lower temperature than the glass transition temperature. For some applications, the maximum use temperature may be lower than the HDT because of excessive creep of the plastic material. Plastics with high creep may, given enough time, eventually distort, even under only moderately elevated temperatures or even at room temperatures. The maximum structural use temperature is, in these cases, determined from the lowest temperature at which change in the plastic material occurs that would be detrimental to the particular application.

        The Vicat softening temperature (ASTM D 1525) is also used to measure the maximum structural use temperature of a plastic. The Vicat test measures the temperature at which a flatended needle penetrates a sample to a specific depth. Another test used to determine a maximum use temperature is the UL temperature index (UL 746B). Underwriters Laboratories (UL), an independent organization concerned with consumer safety, has developed a temperature index to assist UL engineers in judging the acceptability of individual plastics in specific applications involving long-term exposure to elevated temperatures. The UL index lists a temperature for each plastic that is considered the maximum long-term use temperature. The index temperature is determined by exposing samples to circulating air at various temperatures for 10,000 hours. The temperature that causes a sample to lose 50% of its mechanical property (usually strength or toughness) is the index rating temperature.

3.6.5. Glass Transition Temperature

        When one atom in a polymer chain moves in translation, it has a tendency to pull surrounding atoms with it. Hence, these translational movements require more energy than do vibrations, rotations, or twisting. With further heating beyond the HDT, several adjacent atoms (perhaps five or six) will eventually have enough combined energy to move (trans late) as a unit, perhaps with a semirestricted motion somewhat like that of a jump rope or a sinusoidal wave, because some segments of atoms in the polymer are very free to move, whereas other nearby segments remain more restricted. This coordinated, long-range translation results in significant changes in key properties of the material in the region where the long-range motion occurs.

        It might be assumed that such increases in long-range movement should gradually increase with temperature, but, in fact, the onset of these long-range movements is relatively sudden (occurring over a few degrees of temperature). Such relatively abrupt changes as a result of changes in temperature are called thermal transitions. The thermal transition associated with the long-range molecular motions is called the glass transition temperature, Tg, because the structure and properties of the polymer below it are reminiscent of those of ordinary glass. Whereas below Tg the polymer is rigid and hard, with the long-range movements of several adjacent atoms that occur above the glass transition temperature, the polymer becomes more flexible. The extent of this change depends, to a large degree, on the amount of crystallinity in the polymer. In amorphous polymers, the change in flexibility is quite dramatic because the amorphous regions have considerable space and relatively unrestricted molecules that can participate in these long-range movements. The amorphous polymer might become leathery or rubbery above Tg. As might be expected, highly crystalline polymers exhibit much less change through the glass transition temperature because the crystalline structure restricts the polymers and limits the long-range movements to only small actions. Nevertheless, some changes can be detected, even in highly crystalline polymers in the amorphous zones.

Figure 3.6 General thermal behavior for thermoplastic and thermoset plastics.

        For thermoset materials, the presence of cross-links restricts the movements of the atoms in the polymers, thus increasing Tg (and HDT) over the values that would occur in otherwise equivalent amorphous or even crystalline thermoplastics. The restrictions from the crosslinks stiffen the thermoset material above Tg so that it is less leathery and more rigid than thermoplastics. This pattern is depicted in Figure 3.6, where this region is labeled semirigid. The behavior of Formica™ countertop material (a thermoset) is a good example of how the properties of a thermoset will change in the region of Tg. It is hard in normal use but can be softened somewhat by heating so that it can be bent and shaped to fit the contour of the counter. As with all thermosets, further heating of the countertop material will not cause it to melt, but rather to degrade or, at extremely high temperatures, to burn.

        The T_g is related to the number of crosslinks formed, because higher crosslinking will give fortifier restrictions to the molecules and require higher temperatures to affect the long-range movements that are characteristic of the glass transition. This direct relationship between T_g and crosslink density has led to the use of T_g as an indicator of the extent of cure (crosslinking). Because T_g is one measure of the maximum use temperature of the plastic for structural applications, the use range can be increased by increasing the number of cross-links, which in turn raises T_g.

        The most important method for determining T_g is the differential scanning calorimetry (DSC) test (ASTM D3417 and D 3418). This test involves measuring the heat absorbed by a sample when that sample goes through thermal transitions. The DSC test allows these transitions to be identified by noting the absorption of heat from a plot of heat versus time as the sample is gradually heated. Results identify the temperatures of the transitions. The changes in physical properties at T_g allow this transition to be determined using a test called thermo-mechanical analysis (TMA). The TMA method places a probe into the sample and measures the changes in the size or mechanical behavior of the sample as the temperature is progressively raised. TMA can determine the coefficient of volume expansion of a sample and can also determine the T_g of a sample because the sample becomes more pliable above the glass transition temperature. TMA data can also be used to determine the maximum use temperature of the material. Results are a plot of mechanical changes versus temperature.

         If the polymer is (1) tightly entangled, (2) extremely stiff, or (3) highly restricted in some other way, T_g will be high, indicating that considerable energy must be imparted in order to induce the characteristic long-range movements. In fact, the backbones of some polymers are so stiff that the polymers appear to be hard and stiff even above T_g. It is, therefore, sometimes difficult to tell from just feeling the polymer whether it is above or below its glass transition temperature.

3.6.6. Melting

        Melting is a common term applied to materials that go from the solid to the liquid state. For polymers, the nature of melting differs foramorphous and for semicrystalline polymers. These differences, along with the differences in behavior of these materials at the glass transition temperature, are shown in Figure 3.6.

        As more and more heat above T_g is put into an amorphous polymer, the polymer continues to soften and become more pliable, because larger and larger segments of the polymer become excited and gain coordinated movements. The polymers continue to disentangle from each other. They become increasingly more rubbery, then gummy and sticky. With increased heating, the polymer eventually has sufficient energy to move with high (but not complete) independence of its neighbors. When that occurs, the material becomes a liquid. Initially, only a few molecules will have liquid like independent motion. The first of these will be the shortest molecules because they have fewer intermolecular attractions to overcome. With increased heating, eventually all the molecules will gain this independent movement and the entire material will be liquid. Hence, there is no specific thermal transition temperature associated with the melting of amorphous polymers as occurs with small molecules and, as will be seen, with semicrystalline polymers. Nevertheless, the term melting point (T_m) or liquid point is often referred to in discussing amorphous polymers and is, generally speaking, the temperature to which the polymer is heated so that it has liquid like behavior, analogous to the properties of a liquid semicrystalline polymer.

        For semicrystalline polymers, melting is much more easily defined. It is the temperature at which sufficient energy is contained within the polymer structure to break apart the crystalline bonds. By the time this temperature is reached, the amorphous regions have already become liquid and so, when the crystalline regions melt, the entire polymer material is a liquid. Even though the melting phenomenon is well defined in semicrystalline polymers, it is not a specific and sharp temperature. Variations in the polymer crystalline structures, effects of the amorphous regions, and other factors result in melting occurring over a few degrees. The specific semicrystalline polymer melting point is usually chosen as the midpoint of this range.

        In crosslinked thermosets the melting point is dramatically increased relative to thermoplastics, as shown in Figure 3.6. The larger the molecule, the greater the energy that needs to be input to melt it, thus raising the melting point. For thermosets, the increase in molecular size is so great that the melting point is raised above the decomposition temperature (T_d), thus creating a situation where there is no real melting point because the thermoset material will start to decompose before it will melt.

        Characteristics of the polymer that raise T_g or T_m have a tendency to raise its maximum structural use temperature as well. Therefore, in general, thermoplastics have lower thermal stability temperatures than thermosets because the cross-links in thermosets raise T_g and T_m· Other polymer characteristics that raise the amount of energy required to impart internal movements and, therefore, raise the maximum use temperature are higher degrees of aromatic character, secondary bonding, and the stiffness of the polymer backbone.

3.6.7. Flexibility

        The flexibilities of plastics are quite different below and above these thermal transitions. An understanding of these changes is important in predicting how and under which conditions the polymers can be used. Generally, a highly crystalline material will soften slightly but retain its shape and generally stiff or brittle characteristics up to temperatures near the melting point. This behavior indicates that most of the energy is in vibrations rather than translations, as would be expected from a compact, crystalline structure. Therefore, the maximum structural use temperature (to maintain stiffness and strength) for crystalline materials is reasonably close to T_m.

         Amorphous materials are relatively rigid, stiff, and brittle at temperatures below T_g, the glassy region, with mechanical properties somewhat like crystalline polymers. Above T_g, the material becomes significantly softer and takes on many physical characteristics that are much like leather (tough, pliable, flexible) and therefore it is called the leathery region. These relationships are illustrated in Figure 3.7.

        The glass transition temperature is a convenient tool for predicting the useful temperature range for a particular polymer. In some cases, a polymer must be flexible when in use. (Some examples would be rubber, a plastic strap, and a plastic flexible hinge.) This flexible behavior is characteristic of the leathery region. Therefore, materials of this type will be largely amorphous and will have a T_g at a very low temperature so that the temperature of use will be above T_g. When the temperature of use is above T_g, the material is pliable. However, as the temperature is lowered and drops below T_g, the material will change to its glassy state and become brittle. Examples of plastic materials becoming brittle at low temperatures are common and include such items as embrittled cold rubber balls, toys that break when used outside in the winter, and indoor trash carts that break from being dropped when used outside in the winter. If you lived in a cold location that had extremely low temperatures, you might need to worry about the temperature dropping below the T_g of the tires on your car. If you tried to drive on the tires when they were below their T_g, you might break the rubber and ruin the tires.

Figure 3.7 Glass transition and melting point relationships for thermoplastics.

        In other uses, a polymer must be rigid and stiff, such as for an electrical wall outlet or a compact disc box. In these cases, the polymer must be used below the glass transition temperature.

        In summary, if the desired properties of the plastic material are pliability and resiliency rather than structural support, then the plastic should be used above the T_g. For instance, rubber materials would almost always be used above their T_g if the desired properties are rigidity and strength, then the material should be used below the T_g.

3.6.8. Decomposition and Thermal Degradation

        In its melted state, the polymer contains a high amount of energy; this is manifested in translation, vibration, rotation, and twisting motions. If additional energy is input, the amplitude and speed of the motions increase. This increase in amplitude is especially important in the case of vibrations, because eventually the vibration amplitude will become large enough that the bonds will break. At this point, a sufficient amount of the input energy, which randomly moves through the molecule, has localized in the bond and equals the bond energy. This breaking of covalent bonds causes a loss of properties and a change in the basic nature of the polymer. This is called decomposition or degradation, and the temperature at which it occurs is the decomposition temperature (T_d). For thermoplastics, decomposition would generally occur in the liquid state; for thermosets, it generally occurs in the solid state, as shown in Figure 3.5.

        When thermoplastics thermally degrade, they often release a gas and may form crosslinks, thus becoming thermoset materials at high temperatures. When thermosets are heated, either those formed from overheated thermoplastics or from intentional crosslinks, a char is formed. These chars are similar to charcoal, which is, in fact, the char of wood. When a char is formed, by-product gases are often released and the polymer may begin to change color, often yellowing or blackening.

        The decomposition temperature can be determined using DSC or TMA but is probably easier to obtain using a test called thermogravimetric analysis (TGA). TGA is a procedure in which the sample is progressively heated and changes in the weight of the material are recorded. The weight changes are usually associated with the volatilization or decomposition of components of the sample. Often, some portions of the sample, such as mineral fillers, are not volatilized or decomposed and the concentrations of these materials can be determined. (To get the weight of these materials without a char being present, the char is usually subjected to heat in an oxygen atmosphere, thus causing the char to burn away.) The results of the TGA test are a plot of weight changes versus temperature.

        In some polymers, degradation can occur at temperatures below the decomposition temperature because of the presence of certain contaminating materials within the polymer mass. The most common of these contaminants are residual catalyst particles, often metals. Just as the catalyst promotes the formation of the polymer, it can also promote the decomposition since many polymerization reactions are reversible under certain conditions. High heat is one of those conditions that allows reverse reactions. Therefore, it is important to minimize the concentration of residual catalyst. Polymer manufacturers usually treat the polymer with various baths and other processes to remove as much catalyst as possible. The amount of residual catalyst or other inorganic contaminant can be determined using the ash test (ASTM D2584). The plastic material is heated in an oxygen atmosphere to burn off the carbon portion and leave the inorganics. If no fillers are present or if the filler content is known, the amount of residual catalyst can be determined.

        At very high temperatures when oxygen is present, the polymer or the gases that may be given off by decomposition may ignite. This is called combustion (burning), a rapid decomposition. In small molecules (smaller than polymers), additional heating of a melted material will cause the material to gain enough translation energy that all the attractions present in the liquid will be overcome and the material will evaporate into a gas, as was discussed earlier in this chapter and illustrated by the example of water turning to steam. But, because polymer chains are so large, the temperature at which the liquid attractions will break is higher than the decomposition temperature. Hence, polymers degrade before they evaporate. When degradation occurs, most mechanical and physical properties of the plastic material are seriously altered. Mechanical strength, stiffness, and elongation often drop precipitously.

        The accumulation of thermal energy, that is, the exposure to high heat, can occur in many ways. For instance, the polymer could be a component of an oven, or a frying pan handle, or could be near an exhaust duct for high-temperature gases. But the most common situation in which a plastic material will be exposed to high temperatures is during processing. Thermosets are heated to cure them so that crosslinks will be developed. Thermoplastics are heated to melt them so that they can be molded. Neither type of plastic material should be subjected to excessive heat during these processing steps or thermal degradation can occur. For thermosets this usually means not extending the thermal cure cycle for longer times or at higher temperatures than are necessary. For thermoplastics this usually means not heating to a higher temperature than is required for melting and proper viscosity control for molding.

        With thermoplastics, however, another problem arises because they can be processed several times. The most common instance occurs when scrap material is reprocessed, usually after chopping or grinding this material into small pieces so that it can be more easily and uniformly fed into the processing equipment. The scrap material being reprocessed is called regrind. Experience has shown that when regrind material is processed; the evidence of thermal degradation is much more apparent than is seen with nonregrind (virgin) material. One way to limit the effect of regrind problems is to mix the regrind with a large amount (usually over 60%) of virgin material. In this way the amount of regrind in any scrap will be continuously diluted and the effect of thermal degradation on part properties will be minimal.

        Each thermal process causes some thermal degradation to occur even at temperatures well below the decomposition temperature. This degradation is the result of random local buildups of energy, which can be sufficient to break one or more of the covalent bonds in the energy concentration area. In most cases this degradation is limited to the breaking of a few bonds and the effects are not detected in the overall behavior of the polymer. If the plastic material is only processed (melted) once, this degradation is usually minimal, but with each subsequent reprocessing the effects become more evident. Therefore, plastics have an accumulated thermal degradation that is a function of the number of times the material has been heated or melted or, in other words, is a function of the accumulated time at elevated temperature. This phenomenon is called a thermal history of the plastic material. These effects are most evident when high temperatures are used for processing. Some plastics, such as polyvinyl chloride (PVC), are especially sensitive to thermal degradation and minimization of their thermal history is important to ensure that good performance is maintained.

        Another problem with thermoplastic melts is their tendency to form small solid masses that cannot be melted. These masses, which are called gels, result from crosslinks formed when side-chain bonds break (allowing the by-product gas to form) and then recombine to form a bond with an adjacent polymer chain. (This is another form of thermal degradation because the desired properties are altered by heat.) These crosslinked masses form most often in continuous processes such as extrusion where some thermoplastic material may get caught in a crevice or on a nonsmooth part of the system and remain there for a long period of time at a high temperature. These gels may eventually break free, causing a defect in the part or may remain in place and, when cooled, cause great difficulty in disassembling the various parts of the system.

        In some thermoplastics, the degradation and melting temperatures are close to each other and substantial degradation could occur before melting. Thermoplastics of this type are, generally, not processable by traditional thermoplastic processing methods like extrusion or injection molding. An example of this type of material is polytetrafluoroethylene (PTFE), which is processed by methods more akin to metal processing, such as powder fusion. (Recent modifications in PTFE have changed the relative positions of the melting and degrading temperatures so that when modified, usually by copolymerization, this polymer can be normally processed.)

        In thermosets the degradation product is a char. This char material usually resists further heating and is, therefore, an excellent thermal insulator. Some plastics (such as phenolic) take advantage of this property in applications requiring a high thermal insulation after exposure to high heat, such as rocket exhaust nozzles and high-temperature insulation.

3.6.9. Aging

        When a relatively low amount of heat is applied to the polymer over a long period of time, the cumulative effects can be similar to high-temperature degradation. That is, the polymer can reach its thermal stability limit and could eventually begin to degrade. This effect is called thermal aging or simply aging, Because of the long-term nature of aging, the effects of this process are much more difficult to detect than the more rapid types of thermal-induced change. The consequences are, however, potentially catastrophic. With aging, the part will gradually lose mechanical strength and elongation, resulting in an embrittlement that can easily lead to failure. Aging cannot really be prevented, but the useful life of plastic parts that may be especially subject to long-term, slightly elevated temperatures can be extended by the use of thermal stabilizers.

        Oxidation is a similar long-term process in which oxygen reacts with the polymer. This reaction causes changes in the polymer's properties similar to aging. Oxidation can also be slowed through the use of additives.

3.6.10. Thermal-protective Additives and Processing Methods

        Several materials have been developed to assist in the reduction of thermal degradation in thermo-plastics. These materials, which preferentially absorb heat, are added to the plastic material and therefore reduce the amount of thermal energy absorbed by the polymer chains. These materials are commonly called thermal stabilizers. Some of the most effective are powdered inorganic minerals such as limestone, talc, and alumina. These materials absorb heat because of their large heat capacities and therefore "protect" the plastic material. In some cases, however, the presence of these inorganic materials is detrimental to other desired properties of the plastic. For instance, they add weight and decrease tensile strength. Organic heat stabilizers are also available, and although more expensive, are preferred where strength, weight, or other problems with inorganic materials may exist. Materials such as PVC that have relatively low bond energies are especially sensitive to thermal degradation. The most commonly used heat stabilizers in PVC are mixed metal salt blends (such as Ba/Cd/Zn stearates), organotin compounds (such as mercaptides), and lead compounds (such as stearates).

        Another technique that is used to protect heat-sensitive plastics is the use of processing aids. These are materials that lubricate the process, often by melting at low temperatures to facilitate the melting of the remainder of the polymer. Waxes are some of the most common processing aids. When they are added to a polymer mix, the processing temperature can be reduced 60° to 90°F (20° to 30°C). Some materials, such as the stearate salts, can serve as both thermal stabilizers and processing aids.

        Thermal degradation can also be reduced if energy is input to the polymer by methods other than heat. Therefore, most plastics processing equipment uses both mechanical and thermal energy to melt the plastic material. The mechanical energy is often put into the system by screw mixing, which has the added benefit of conveying the polymer through the heating zone. (These processing machines will be described in detail later in this text in the chapters that discuss the various processing methods.) Some equipment, such as twin screw extruders, have a very high mechanical input and are, therefore, especially useful in processing heat-sensitive materials such as PVC.

3.6.11. Processing Temperature

        One other important temperature for plastics is the processing temperature. This is the temperature at which a plastic material can conveniently be molded. The processing temperature is determined experimentally and is somewhat different for different processing equipment and conditions. Nevertheless, an approximate value can be useful as a starting place for new processing conditions. Some examples of the important temperatures associated with common plastics are given in Table 3.2.

Polymer	Tg	Tm	Processing Temp
Polyethylene—low density (LDPE)	-130° to -13°F (-90° to -25°C)	208° to 240°F (98° to 115°C)	300° to 450°F (149° to 232°C)
Polyethylene high density	-160°F (-110°C)	266° to 280°F (130° to 137°C)	350° to 500°F (177° to 260°C)
Polypropylene (PP)	-103° to -94°F (-25° to -20°C)	320° to 356°F (160° to 180°C)	374° to 550°F (190° to 288°C)
Acrylonitrile butadiene styrene (ABS)	212°F (100°C)	230° to 257°F (110° to 125°C)	350° to 500°F (177° to 260°C)
Nylon (6,6)	120°F (49°C)	470° to 500°F (243° to 260°C)	500° to 620°F (260° to 327°C)
Polyethylene terephthalate (PET)	150° to 175°F (66° to 80°C)	413° to 509°F (212° to 265°C)	440° to 660°F (227° to 349°C)
Polycarbonate (PC)	300°F (149°C)	284° to 300°F (140° to 149°C)	520° to 572°F (271° to 300°C)
Polyphenylene oxide (PPO)	375° to 428°F (190° to 220°C)	500° to 900°F (260° to 482°C)	400° to 670°F (204° to 354°C)

Table 3.2 Thermal Properties of Selected Plastics.

3.6.12. Nonthermal Energy Inputs

        In addition to thermal sources of energy, energy input could come from a mechanical source (such as impact) or from any other energy source (sound, light, X-rays, etc.). With all these energy inputs, the polymer moves internally to absorb the energy. If the internal motions can dissipate the energy sufficiently, then no breakage of bonds will occur. However, if the energy input is large or very rapid, the polymer may not be able to dissipate the energy sufficiently, and in some area the localized energy can exceed the bond strength. When this occurs, the polymer breaks. This localization of energy can occur with any type of energy input, although mechanical impacts are a very common source of such energy concentrations because these impacts give very large, rapid energy inputs at a single location.

        The energy inputs can add together. Sometimes these additions will create localized energy concentrations that are sufficient to exceed the bond energies, even when the individual inputs would not be sufficient to cause breakage. This may happen, for instance, when internal stresses are introduced into a plastic material as part of the molding operation; and then later thermal stresses, such as rapid cooling, may cause the material to crack.

        The internal motions that accompany all energy inputs can become quite extensive and may result in changes in mechanical or other properties because they may facilitate movement of one polymer chain past another. For instance, the origin of tensile strength is the resistance to motion of one polymer chain past another. If the internal motions are high, less resistance of movement between polymers would occur and the tensile strength would decrease. On the other hand, some mechanical properties may be improved by some energy input. For instance, elongation would be greater when the molecules have more internal freedom of motion. Toughness, which is a combination of both strength and elongation, could be either raised or lowered, depending on the specific conditions.

3.6.13. Thermal Conductivity

        Thermal conductivity is a measure of how quickly or easily heat moves through or along a material. As already discussed, when plastics are heated, they have a tendency to move internally and therefore absorb input energy. If the energy is absorbed, it is not transferred along. Hence, plastics have low thermal conductivities in comparison to metals. Typical thermal conductivities for plastics and metals are given in Table 3.1. These values are determined using the thermal conductivity test (ASTM C 177). Thermal conductivity is defined as the rate at which heat is transferred by conduction through a unit cross-sectional area of a material when a temperature gradient exists perpendicular to the area. The coefficient of thermal conductivity is sometimes called the K factor. The primary technique for measuring thermal conductivity of plastic materials is the guarded hot-plate method, a complex and expensive method. Great care must be taken to ensure that operator technique does not enter into the test results.

        Plastics are, therefore, ideal materials for insulating applications, provided the temperature does not get higher than the thermal stability temperature. Some plastics, especially thermosets, have a combination of high thermal stability temperatures, high degradation temperatures, and low thermal conductivities. Such materials are excellent for thermal insulators at moderately elevated temperatures (up to about 600°F or 300°C) and can be found in products such as handles for frying pans and toasters.

        A property related to thermal conductivity is heat capacity. This is the measure of the temperature rise in a given weight of material for a given amount of heat input. As might be expected, the tendency of polymers to absorb energy internally results in rather high heat capacities, as can be seen in Table 3.1.

3.6.14. Thermal Stresses

        If a plastic part is constrained so that it cannot expand or contract with changes in temperature, internal energy called thermal stresses develop. The magnitude of these stresses will depend upon the temperature change, the method of shape confinement, and the CTE of the plastic material. These stresses (which represent retained energy) can result in lower overall mechanical and thermal properties because the total energy threshold for a particular property is reached at a lower applied energy level. If other stresses are introduced, such as from drilling holes, internal imperfections, or mechanical stresses, the part may fail prematurely.

        In some cases, these thermal stresses can be relieved by heating the material in a manner that allows free chain movement. This stress-relieving process is called annealing. Generally, annealing is done at a moderately elevated temperature for a long time rather than at a high temperature for a short time. When annealed, some materials may actually contract because the molecules are allowed to move to a more energy-favorable configuration that may be closer together. Metals are annealed in much the same way as described here for plastic materials.

        Plastic materials can build up stresses from energy inputs other than thermal. For instance, a part could be stretched and then quickly cooled so that the molecules cannot move to return to their original positions. Alternately, the molecules may be forced into a highly oriented state, as is often done in the extrusion process, and be cooled before the natural randomization of the molecules can occur. Both of these processes result in internal stresses that can generally be relieved by annealing.

3.6.15. Thermal Effects on the Rate of Chemical Reactions

        Some processes that have been discussed involve chemical reactions (such as thermal degradation and aging), whereas others are changes in physical state (such as thermal transitions, thermal expansion, and thermal stresses). The difference between the chemical reactions and the physical changes is that in chemical reactions the bonds between the atoms in a polymer are broken and reformed in new configurations, whereas in physical changes the molecules are merely separated from neighboring molecules.

        The dependence of physical changes on thermal input is additive and linear. That is, the transition is dependent upon the amount of heat input directly. This can be described mathematically as a linear function between heat input and temperature; the transitions are directly related to the temperature.

        With chemical reactivity, the relationship between temperature and the rate of the chemical reaction is more complicated. The rate was shown to be an exponential function of temperature. This relationship was formalized in 1886 by Svante Arrhenius and is called the Arrhenius equation, shown as Equation (3.1):

Rate = Ae^—(E/RT) (3.1)

where A, called the collision factor, is a measure of the effectiveness of collisions between reacting species, e is the natural logarithm base, E is an activation factor that indicates the amount of energy required to make the reaction occur, R is the gas constant, and T is the temperature in absolute units (Kelvin). The rate of the reaction increases as A increases, decreases as E increases, and increases as T increases. The rate of the reaction approximately doubles with every 10 K (or 10 C) rise in temperature.

        The Arrhenius equation is very important for predicting the rate of any chemical reaction. For instance, oxidation and degradation by ultraviolet light can be described by the Arrhenius equation. Several other phenomena that do not involve chemical reactions, such as diffusion, viscous flow, and electrolytic conduction, can also be described by the Arrhenius equation.