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Polymer Structure
The structures of polymeric repeat units determine their stiffness, polarity and intermolecular bonding. Incorporation of stiff side groups or rigid aromatic rings into a polymer repeat increase the modulus (stiffness) of the molten and solid polymer as well as increasing its critical transition temperature. Thus, polyethylene’s Tg is about -120°C, while the Tg valu
es for polystyrene and polycarbonate are 100 and 150°C, respectively. More polar repeat units also allow for stronger intermolecular bonding. For example, polyethylene which has only van der Waals interactions has a Tm of 110°C and a flexural modulus of 1110 MPa, whereas the hydrogen bonding in polyamide-6,6 provides a Tm of about 200°C and a flexural modulus of 2800 MPa.
Table 1 summarizes structure-property relationship for polymers. In general, higher molecular weight can improve the tensile strength, toughness, hardness, chemical resistance, and softening temperature of plastic injection mold, whereas broad molecular weight distributions reduce most of these properties. Greater levels of crystallinity and cross-linking enhance the tensile strength, modulus, hardness, chemical resistance, and softening temperature of polymeric materials. Finally, the strength of the primary and secondary bonds, the location of transition temperatures, and the morphology are determined by the chemical and steric structures[41].
Table 1. Structure-Property Relationships for Polymers[42]
Polymer resins can be homopolymers, copolymers, or blends. Homopolymers contain one type of repeat units, while copolymers are polymerized using two or more monomers. Low density polyethylene (LDPE), polypropylene, and other homopolymers may actually contain a small amount of a second repeat unit, but typically have a well-defined processing window. In contrast, the processing conditions for copolymer depend on the ratio of components and their arrangement within the polymer chain. Random copolymers, like poly(ethylene-co-vinylacetate) and poly(styrene-co-acrylonitrile), consist of a random arrangement of the two mers and are generally single phase systems. Since block copolymers include long segments of each mer, they tend to phase separate (i.e., form separate regions containing each component). Block copolymers include thermoplastic polyurethane elastomers, polyetheramides, and styrenic block copolymers such as SEBS.
Blends, such as poly(acrylonitrile-butadiene-styrene) (ABS), are mixtures of two or more polymers. Since the polymers do not mix well, most blends exist as phase separated systems. This behavior improves the impact resistance of polystyrene in impact modified polystyrene (HIPS), but can produce complex morphologies that are sensitive to part design and processing conditions. For example, polycarbonate/ABS, a three component blend in which the three phases are polycarbonate, polystyrene-co-acrylonitrile, and polybutadiene, shows significant differences in mechanical properties and surface morphology (which affects painting and plating) when the processing and design produce high residual stresses[43].
Few commercial plastics resins are solely polymer. Additives are incorporated into the polymer to alter the bulk properties, either for a wide range of end uses (e.g., antioxidants) or for specific applications. The main categories of additives are the
fillers, plasticizers and softeners, lubricants and flow promoters, anti-aging additives, flame-retardants, colorants, blowing agents, cross-linking agents, and UV stabilizers. Fillers, such as glass fibers, carbon black, and talc, enhance the mechanical properties. Plasticizers aid in the flowability of the polymer while the lubricants reduce the friction of the moldings. Flame retardants prevent or limit the burning of polymers. Blowing agents are used to produce gas-filled cells in the polymer matrix which enhance the properties and lower the weight of the polymer.
3.5. Melt Viscosity
Melt viscosity, the resistance of a polymer melt to flow, varies with processing conditions, particularly flow rate (i.e., shear rate), temperature, and pressure, and with the polymeric material, i.e., the structure, molecular weight, and additives. Viscosity increases for more rigid repeat units, with molecular weight, and when filler and fibers are incorporated into the polymer.
As shown in Figure 14, low shear rates do not affect melt viscosity. When the shear rate reaches a critical value, however, the polymer chains begin to align in the direction of flow and the melt viscosity begins to decrease with increasing shear rate. The linear decrease (on a log-log curve) in viscosity with increasing shear rate is known as the power law region and can be modeled using:
 (3)
where k is the consistency index and n is the power law index. Not all polymers have the same sensitivity to shear rate, and so the power law index varies from about 0.15 for thermoset rubbers to 0.30 for polystyrene and polyvinyl chloride to 1.0 for Newtonian fluids like water.
Melt viscosity decreases with increasing temperature. For narrow ranges of melt temperature, this decrease can modeled using an Arrhenius equation:
 (4)
where hR is a reference viscosity, Ea is the activation energy for flow, R is the universal gas constant, and T is the absolute temperature. Activation energies typically vary from about 7 kJ/mol for thermoplastic vulcanizates to 100 kJ/mol for materials like PMMA and polycarbonate. The WLF equation is employed when the temperature dependence of melt viscosity is modeled over larger temperature ranges. Higher pressures produce in an increase melt viscosity that can be expressed as:
 (5)
where a is an empirical constant.
Figure 14. Effect of shear rate on melt viscosity[44].
3.6 Degradation
Thermal stability, the ability of a polymer to withstand processing and service (use) temperatures, is a function of the polymer structure and molecular weight. Aromatic rings in the polymer backbone tend to enhance stability whereas some functional groups like halides (e.g.., chloride in PVC) often leave the chain easily. High molecular weight causes more entanglement, which limits the mobility needed for degradation, and reduces the number of unstable end groups.
During processing, thermal, mechanical, chemical, and r
fillers, plasticizers and softeners, lubricants and flow promoters, anti-aging additives, flame-retardants, colorants, blowing agents, cross-linking agents, and UV stabilizers. Fillers, such as glass fibers, carbon black, and talc, enhance the mechanical properties. Plasticizers aid in the flowability of the polymer while the lubricants reduce the friction of the plastic injection mold. Flame retardants prevent or limit the burning of polymers. Blowing agents are used to produce gas-filled cells in the polymer matrix which enhance the properties and lower the weight of the polymer.
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