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Polymer Materials
Note: a good introduction to polymers can be found at http://www.pslc.ws/macrog/index.htm
Polymers are long chain molecules made up of simple repeating molecular units (i.e., mers). The net effects of having long chains are chain entanglement, a summation of intermolecular forces, and time scale of motion[33]. Several factors, including the polymer’s molecular weight and molecular weight distribution, its thermoplastic or thermoset nature, its molecular configuration, and its structure affect the perf
ormance of polymers. Melt viscosity and degradation mechanism of plastics are also important when considering the flow of polymer melts through gates.
3.1. Molecular Weight and Mechanical Properties
During polymerization, chain length can be varied and not all chains will have the same length. Molecular weight is a measure of the average chain length while molecular weight distribution (MWD) is a measure of the range of chain lengths. Three molecular weights are typically reported for polymers. The number-average-molecular weight,  , is mean chain length and provides an estimate of intermolecular attraction and the number of end groups in a resin. In contrast, the weight-average-molecular weight,  , “counts” the longer chains, thereby producing an estimate of chain entanglement. Finally, the z-average-molecular weight,  , favors very long polymer chains and has been correlated with melt strength in materials used for blown film extrusion and extrusion blow molding. While the polydispersity index, PI,
 (1)
does not exactly measure molecular weight distribution, PI is generally used to express the range of chain lengths.
As illustrated in Figure 11, the effect of molecular weight on mechanical properties varies with the specific property. Increasing chain entanglement causes properties, such as melt viscosity and Izod impact resistance, to increase with molecular weight. Viscosity, h, has related to weight-average-molecular weight using Mark-Houwink equation[34]:
 (2)
where K and a are empirical constants. For linear polymers, a is 1.0 until chain entanglement occurs (i.e., for oligomers) and a is 3.4 after the molecular weight has exceeded the critical molecular weight. Properties that depend on intermolecular attractions and the number of end groups initially increase with number-average-molecular weight, but remain constant after attaining a threshold molecular weight. These properties include the tensile strength, flexural modulus, and glass and melt transition temperatures (i.e., Tg and Tm).
Figure 11. Effect of molecular weight on the mechanical properties[35].
Polymers typically used in injection molding have molecular weights greater than the critical or threshold molecular weights. In general, polymers prepared via addition polymerization (e.g., polyethylene, polypropylene, polystyrene, polymethylmethacrylate, and polyvinyl chloride) have higher molecular weights than those prepared using condensation polymerization (e.g., polycarbonate, polyacetal, polyamides or nylons). Improperly dried condensation polymers are also more susceptible to chain scission and the subsequent reduction in molecular weight. Consequently, condensation polymers are more likely to show the effects of molecular weight degradation in the measured tensile and flexural properties, but all polymers exhibit these effects as changes in melt viscosity and impact properties. The effects of heat history, however, depe
nd on the specific characteristics of the polymer. In recent studies[36],[37] impact modified polystyrene (HIPS) and polycarbonate showed significant decreases in the melt flow and Izod impact properties with subsequent recycling histories, but much smaller changes in tensile and flexural properties.
Molecular weight distribution (MWD) is usually a function of the polymerization technique and resin’s heat history. Metallocene and Zeigler-Natta provide relatively narrow molecular distributions (i.e., PI ~ 2 to 4) whereas as free radical catalysis and chrome catalysts yield materials broader molecular weight distributions (i.e., PI ~ 20 to 50)[38]. Broad molecular weight distribution improves the processability of many resins, but reduces properties like heat sealability in polyethylene blown films.
3.2. Thermoplastic and Thermoset Polymers
Polymers are classified into two classes: thermoplastics and thermosets. In a thermoplastic resin, the long chain molecules are held together by relatively-weak intermolecular attractions, such as van der Waals forces and hydrogen bonding. When the material is heated, the intermolecular forces weaken and polymer chains separate. Thus, the resin softens, eventually becoming a viscous melt, and the thermoplastic resolidifies upon cooling. This behavior of the thermoplastic is repeatable and allows reprocessing of thermoplastics. In contrast, thermoset resins are initially individual polymer chains that can be dissolved and can flow. Upon heating, however, covalent bonds or cross-links form between the polymer chains. This irreversible cross-linking produces three-dimensional networks, thereby preventing or limiting reprocessing of these resins plastic injection mold.
Thermoplastic resins usually contain long-chain polymers. Thermoset resins, however, can be divided into two groups: thermoset rubber and thermoset oligomers. Thermoset rubbers are long chain polymers with reactive sites that facilitate cross-linking. This group includes polyisoprene, styrene-butadiene rubber (SBR), and nitrile rubber. The second group contains materials like phenol formaldehyde, melamine formaldehyde, unsaturated polyester, and epoxies. These thermoset resins are usually produced in two stage chemical reactions, with chemically reactive shorter chain molecules or oligomers formed in the first stage. When these oligomers are heated, they form reactive sites. For example, phthalic acid and glycerol are initially condensed to form a branched A-stage resin (Figure 12a) and these branched molecules cross-link to produce the three-dimensional structure shown in Figure 12b.
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Figure 12. a) A-stage resin and b) cross-linked three dimensional structure, where P is phthalic acid and G is glycerol[39].
3.3. Polymer Configurations
Figure 13 presents the effect of processing on the configurations (arrangements) o
f thermoplastic polymers. With the exception of thermotropic liquid crystalline polymers (LCPs), all polymers are amorphous in the melt state. In this state, the polymer chains form a random mass (i.e., random coil configuration), which is the most energetically favorable state. LCPs, however, are rod shaped in the melt because of their rigid polymer structures.
Figure 13. Effect of processing on the morphology of thermoplastic resins[40].
Upon cooling, polymers with irregular structures remain amorphous. This amorphous morphology gives the polymers broad softening ranges and low shrinkage because the randomly-ordered chains slowly expand and contract. The random chain ordering also 1) allows penetration of chemicals, thereby giving the polymers low chemical resistance, 2) cannot resistance fatigue and wear, and 3) provides only one index of refraction, allowing the polymers to be transparent. Amorphous plastics include polystyrene, polymethylmethacrylate, polycarbonate, and polysulfone.
Highly ordered polymer structures allow other polymers to form a combination randomly-ordered chain segments and well-organized structures (i.e, crystallites) when the melt cools. This semi-crystalline configuration gives the polymers sharp softening points as the crystallites melt; high shrinkage associated with forming tightly ordered structures; good chemical, fatigue, and wear resistance offered by the crystallites, and two indices of refraction (i.e., one for each phase) to make the plastics opaque. Semi-crystalline plastics include polyethylene, polypropylene, aliphatic polyamides (e.g., polyamide-6,6), polyacetal, polyethylene terephthalate (PET), and polyphenylene sulfide.
When the temperature exceeds a polymer’s glass transition temperature, Tg, the amorphous regions of the polymer soften, but any crystallites remain intact. As the temperature increases, the amorphous regions expand, thereby soften the plastic. With amorphous polymers, the material gradually becomes a fluid enough to melt process. The crystallites in semi-crystalline polymers, however, do not become disordered until the temperature reaches the polymer’s melting transition, Tm. Once the crystallites “break up,” the polymer rapidly soften enough to undergo melt processing. For polymers, like ultra high molecular weight polyethylene (UHMWPE) and polytetrafluoroethylene (PTFE), increasing temperature causes the covalent bonds in the main polymer chains to break before the polymer can flow. Thus, UHMWPE and PTFE can not be processed using convention melt processing methods.
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