2.3Thermoplastics – PropertiesJ. D. MuzzyGeorgia Institute of Technology, Atlanta, GA, USA__________________________________________________________________________2.3.1 INTRODUCTION2.3.2 GENERAL FEATURES2.3.3 GUIDELINES FOR SELECTION OF POLYMERS AND PROPERTIES2.3.4 THERMOPLASTIC GROUPS2.3.4.1.1 Polyolefins2.3.4.1.2 Styrenics2.3.4.1.3 Vinyls2.3.4.1.4 Acrylics2.3.4.1.5 Fluoropolymers2.3.4.1.6 Polyesters2.3.4.1.7 Polyamides (Nylons)2.3.4.1.8 Polyimides2.3.4.1.9 Polyethers2.3.4.1.10 Sulfur Containing polymers2.3.4.1.11 Additional Thermoplastics2.3.5 ACKNOWLEDGEMENTS2.3.6 REFERENCES2.3.1 INTRODUCTIONThe primary objective of this chapter is to present representative properties of thermoplastics.First, some general comparisons with thermosetting matrices are presented since most highperformance composites have thermosetting matrices. Next, eleven groups are established fordividing the presentation of 40 different types of thermoplastics. Then the properties of thesethermoplastic types are presented.Not all types of thermoplastics are included in this chapter. Thermoplastic rubbers have beenexcluded since these rubbers have low elastic moduli. Usually the reason for reinforcing athermoplastic is to increase its stiffness and strength. From this perspective a thermoplasticrubber represents a low starting point. If the property desired is high impact strength ordurability, then reinforcing a thermoplastic rubber should be considered.2.3.2 GENERAL FEATURESThe mechanical properties of polymers are sensitive to temperature changes. Figure 1 illustratesthe change in modulus with respect to temperature for an amorphous thermoplastic. Below itsglass transition temperature (Tg) the modulus is relatively constant with a value close to 2.8 GPa(0.4 msi). As the temperature increases above Tg the modulus drops roughly three orders ofmagnitude to 0.28 GPa (0.4 ksi) as the polymer becomes rubbery. If the molecular weight of the2polymer is high, the polymer becomes a viscoelastic fluid about 100°C above its glass transitiontemperature. At this temperature the polymer can be processed as a melt.A crystalline thermoplastic has a modulus similar to an amorphous thermoplastic if both arebelow their Tg. As shown in Figure 2, above Tg a crystalline polymer has an intermediatemodulus depending on the degree of crystallinity present. This crystallinity disappears duringmelting, leading to a rapid drop in modulus as the polymer becomes a viscoelastic fluid.In comparing crystalline polymers and thermosetting polymers a similar trend is observed aboveTg. For thermosets the modulus above Tg is dependent on the crosslink density, as shown inFigure 3. However, crosslinks are permanent covalent bonds which cannot be "melted" likepolymer crystals. From this perspective, crystalline polymers have thermally reversiblecrosslinks which enable them to be melt processed.When polymers are reinforced, the modulus is raised, as shown in Figure 4. The transitiontemperatures, Tg and Tm, are not changed significantly. However, the heat deflectiontemperature (HDT) determined according to ASTM D 648 can change substantially, particularlyin reinforced crystalline polymers. In accordance with ASTM D 648, a flexural test sample issubjected to a stress of 0.46 Mpa (66 psi) or 1.8 MPa (264 psi) while the sample is heated at2°C/min. When the deflection reaches 0.25 mm the HDT has been attained. These conditionscorrespond to apparent flexural moduli of 0.9 GPa and 3.6 GPa for stresses of 0.46 MPa (66 psi)and 1.8 MPa (264 psi) respectively. These moduli are shown as horizontal lines in Figure 4.Thus, a large increase in HDT for a crystalline polymer through reinforcement is due to raisingits modulus above the HDT lines in Figure 4.Usually there are some performance differences between thermoplastic and thermosettingmatrices. As shown in the following property tables, thermoplastics often have tensile strengthsand moduli, which are less than those of many thermosets. In many cases this tendency is due tothe Tg of the thermoplastic being close to or below room temperature. In contrast, these samethermoplastics often have very high strains to failure compared to thermosets. This high failurestrain capability also leads to high fracture toughness and high impact strength. Becausethermoplastics are not crosslinked they often exhibit high creep strains.Compared to thermosetting composites, achieving good interfacial adhesion in thermoplasticcomposites can be more challenging. The combination of high melt vicosities and the lack ofreactive groups make it difficult for a thermoplastic to wet and bond to reinforcing fibers. Oftenthe reinforcing fibers must be coated (sized) with hybrid polymers, sometimes calledcompatibilizers, which can associate with the fibers well and blend with the matrix.Alternatively, the compatibilizers migrate to the fiber-matrix interface. A common example isthe use of maleated polypropylene in glass/polypropylene composites. The maleic anhydridegraft is expected to associate with the glass fibers well while the more extensive propylenecomponent of this graft copolymer blends with the polypropylene matrix.Since thermoplastics are fully polymerized before they are combined with reinforcing fibers theglass transition temperature and melting temperature are fully developed. Consequently, thethermoplastic must be melted at high temperature in order to wet the fiber. Furthermore, once3the thermoplastic is melted, its viscosity is orders of magnitude higher than the low molecularweight prepolymers used in thermoset composite processing. Thus, combining thermoplasticmatrices with reinforcements and insuring good fiber wetout and bonding is a significanttechnical challenge.Since thermoplastics aren't cured during the fabrication of parts, rapid cycle times can berealized. The ultimate limitation on cycle time is the heat transfer required to solidify thethermoplastic. For thin cross-sections cycle times in the range of seconds are possible.However, many thermoplastic preforms consist of unconsolidated commingled fibers or powdercoated fibers. In these cases the time required to complete fiber wetout would extend the cycletime. In these cases cycle times are in the range of minutes. In contrast, the cure times for somethermosets can be hours long.2.3.3 GUIDELINES FOR SELECTION OF POLYMERS AND PROPERTIESThere are well over fifty types of thermoplastics based on chemical