Thermosets versus thermoplastic polymers

Polymers are frequently classified in terms of bonding in one dimension versus bonding in two or three dimensions. Bonding in one dimension results in linear polymers with single-strand chains. Bonding in two or three dimensions results in cross-linked polymers having infinite sheets or three-dimensional networks. Linear polymers are produced by addition polymerization if the reactant has only one double bond or by condensation polymerization if the reactant or reactants each have two reactive sites. Such polymers are usually soluble in suitable solvents. Since they also tend to soften when heated, they are called thermoplastic polymers. Cross-linked polymers may be produced by addition polymerization if the reactant has more than one double bond, or by condensation polymerization if the reactant or reactants each have more than two reactive sites. Such network polymers are usually insoluble and Infusible and are called thermosetting polymers. 

Cross-linked polymers, such as thermosets and elastomers, behave completely different than their counterparts, thermoplastic polymers. In cross-linked systems, the mechanical behavior is also best reflected by the plot of the shear modulus versus temperature. Figure 1 compares the shear modulus between highly cross-... 

Polymer-matrix materials include a wide range of specific materials. Perhaps the most commonly used polymer is epoxy. Other polymers include vinyl ester and polyester. Polymers can be either of the thermoset type, where cross-linking of polymer chains is irreversible, or of the thermoplastic type, where cross-linking does not take place but the matrix only hardens and can be softened and hardened repeatedly. For example, thermoplastics can be heated and reheated, as is essential to any injection-molding process. In contrast, thermosets do not melt upon reheating, so they cannot be injection molded. Polyimides have a higher temperature limit than epoxies (650°F versus 250°F or 350°F) (343°C versus 121°C or 177°C), but are much more brittle and considerably harder to process. 

In fact, RIM was the first plastic to be approved for bumper fascia in North America to meet the low-temperature crashworthiness demanded by some OEMs. Unfortunately, due to process complexity, RIM could not be fabricated at a high enough rate to meet the demands of large car platforms. Faster cycle time for part production is delimited by the speed of curing chemistry and the ability of the resin to flow into molds. Furthermore, the automotive industry wanted to move away from the use of thermoset polymer systems that cannot be remelted. This attribute of thermosets was labeled by the industry as non-recyclable, and the industry made room for other thermoplastics such as TPO that can be remelted. Reaction-injection-molded parts are made from urethane epoxies, polyesters, and polyamides. A study of cost versus performance reported that RIM gives the best cost-performance characteristic for composite materials and is competitive with steel. ... 

Besides the chemical structures of the polymers in the plastics, there are several other characterizations that are important, including molecular weight, thermoplastics versus thermosets, and crystallinity. 

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