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They normally decompose before melting. Hard, plastic thermosets may undergo permanent or plastic deformation under load. Elastomers, which are soft and springy or rubbery and can be deformed and revert to their original shape on loading release. Conventional thermoset plastics or elastomers cannot be melted and re-shaped after they are cured.
Thermosets do not melt when heated, but typically decompose and do not reform upon cooling. Stress-strain graph of a thermoplastic material Above its glass transition temperature and below its melting point , the physical properties of a thermoplastic change drastically without an associated phase change .
Thermosets, or thermosetting polymers, can melt and take shape only once: after they have solidified, they stay solid and retain their shape permanently. [16] If reheated, thermosets decompose rather than melt. Examples of thermosets include epoxy resin, polyimide, and Bakelite. The vulcanization of rubber is an example of this process.
The cut of polyimides is 450 °F (232 °C), highest of all thermosets, with short term exposure capabilities of 900 °F (482 °C). Normal operating temperatures range from cryogenic to 500 °F (260 °C). Polyimide composites have the following properties: Good mechanical properties and retention at high temperatures; Good electrical properties
Elastomers are usually thermosets (requiring vulcanization) but may also be thermoplastic (see thermoplastic elastomer). The long polymer chains cross-link during curing (i.e., vulcanizing). The molecular structure of elastomers can be imagined as a 'spaghetti and meatball' structure, with the meatballs signifying cross-links.
It is distinct from thermal-oxidation, which can usually take place at less elevated temperatures. [3] The onset of thermal degradation dictates the maximum temperature at which a polymer can be used. It is an important limitation in how the polymer is manufactured and processed.
Polymer morphology is a microscale property that is largely dictated by the amorphous or crystalline portions of the polymer chains and their influence on each other. Microscopy techniques are especially useful in determining these microscale properties, as the domains created by the polymer morphology are large enough to be viewed using modern ...
PMCs matrices are typically either thermosets or thermoplastics. Thermosets are by far the predominant type in use today. Thermosets are subdivided into several resin systems including epoxies, phenolics, polyurethanes, and polyimides. Of these, epoxy systems currently dominate the advanced composite industry. [3] [4] [5]