Why do thermosetting plastics not melt when heated?
Properties of plastics:
Thermosets (also called thermosets) are in some respects the opposite of thermoplastics: they do not become soft when heated and do not melt, but instead decompose because their melting temperature is higher than the decomposition temperature.
Furthermore, they do not deform when you pull them, rather they break. Overall, they are significantly harder and more brittle than thermoplastics. If you want to process them, you have to do this mechanically - as is the case with wood, for example - by sawing, filing or rasping (in practice, however, this is avoided by using them straight away in the desired shape).
From a chemical point of view, thermosets are highly cross-linked plastics, as can be seen in the following diagram.
A piece of thermoset such as a piece of cast resin (the transparent plastic blocks in which holiday souvenirs such as seahorses and mussels are preserved) is basically a single giant molecule: no wonder it doesn't melt!
Thermosets are formed when trifunctional monomers (with three reacting functional groups) react with one another, e.g. glycerine (trihydroxypropane) and a dicarboxylic acid to form a polyester or glycerine and a diisocyanate to form a polyurethane.
But thermosets are also created when, for example, using a unsaturated dicarboxylic acid (e.g. maleic or fumaric acid = cis- or trans-butenedioic acid) produces an unsaturated polyester and then crosslinks this at the unsaturated bonds in a polymerization reaction.
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