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I have a question that is bothering me since few weeks. I'm not a chemist, nor I have any experience with polymers theory, but:

Epoxy - is defined on Wikipedia as a thermoset polymer as a process of polimerization occurs that can not be reversed.

PVA - Is defined (again on Wikipedia) as a thermoplastic polymer.

So why the cured epoxy has a glass transition temperature, and therefore when heated above that temperature it softens and its shape can be modified. But when PVA glue hardens (by loss of humidity) it can not be softened by heat?

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First to clear up some terms. Glass transition just means that the intermolecular bonds are weak enough for the material to continue crystallizing—it is not the melting temperature. This can be accompanied by softening of the material but that does not mean the polymer will flow. Thermoplastics can flow at elevated temperature but thermosets (which includes cross-linked thermoplastics) do not by definition.

Epoxy has a glass transition due to a loss of crystallinity but is not usually shapeable unless formulated to or poorly prepared. PVA is a thermoplastic that melts around $\pu{200^\circ C}$ which is also the temperature that it starts to pyrolyze and char. With careful temperature control you can melt and form PVA and there are 3D filaments dissolvable in water based on PVA. Without careful control, you decompose the plastic before melting it.

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  • $\begingroup$ By shapeable (heated epoxy), I ment, that can be bent with a plastic deformation, I assume this is not poorly prepared, as I noticed it on any epoxy I use. Don't you agree? $\endgroup$ – Newbie Jul 2 '18 at 22:09
  • $\begingroup$ Yes thermoplastics can be plastically deformed. One example of a room temperature deformable thermoplastic is PVA slime which is PVA cross-linked with borate groups. The length of the polymer units between cross-linking groups also has a large effect on the amount of strain a polymer may experience without failure, with shorter chains being more rigid. $\endgroup$ – A.K. Jul 2 '18 at 23:11

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