Thermoset Plastics
Thermoset plastics are made quite differently from thermoplastics. Thermoset, or thermosetting, plastics are synthetic materials that strengthen when heated, but cannot be successfully remolded or reheated after their initial heat-forming. This is in contrast to thermoplastics, which soften when heated and harden and strengthen after cooling.
Thermosetting plastics have a number of advantages. Unlike thermoplastics, they retain their strength and shape even when heated. This makes thermosetting plastics well-suited to the production of components that require tight tolerances and excellent strength attributes under elevated temperatures.
Curing of thermoset plastics is done in two steps, partly by the material supplier and partly by the molder. For example, phenolic (a typical thermoset plastic) is first partially polymerized (cured) by reacting phenol with formaldehyde under heat and pressure. The reaction is stopped at the point where mostly linear chains have been formed. The linear chains still contain unreacted portions, which are capable of flowing under heat and pressure.
The final step of polymerization (curing) is completed in the molding press by heating liquid or powder within a mold, allowing the material to cure into its hardened form, producing a crosslinking reaction between molecular chains. Unlike a thermoplastic monomer, which has only two reactive ends for linear chain growth, a thermoset monomer must have three or more reactive ends so that it’s molecular chains crosslink in three dimensions.
After it has been molded, a thermoset plastic has virtually all of its molecules interconnected with strong, permanent, physical bonds, which are not heat reversible. Theoretically, the entire molded thermoset part could be a single giant molecule. In a sense, curing a thermoset is like cooking an egg. Once it is cooked, reheating does not cause remelting, so it cannot be remolded. But if a thermoset is heated too much or too long, the chains break and properties are degraded.
In general, thermoset plastics, because of their tightly crosslinked structure, resist higher temperatures and provide greater dimensional stability than do most thermoplastics. Examples of thermoset plastic products include glass-reinforced-polyester boat hulls and circuit-breaker components, epoxy printed-circuit boards, and melamine dinnerware.
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