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Most if not all of the off-the-shelf epoxies I've used have generated a fair amount of heat while they are hardening. This seems to indicate to me that these are exothermic reaction, i.e. they generate heat from the reaction.

The common practice at my last company was to apply heat (heatgun or hairdryer) to epoxied joints to get them to cure faster.

As I understand the chemistry though, adding heat to an exothermic reaction should slow the reaction and effectively take longer for the joint to cure.

Was my company's 'standard practice' wrong to be adding heat to epoxy joints and extending the curing time, or is epoxy some type of anomalous material that cures faster when adding heat to the exothermic reaction. OR, is epoxy actually an endothermic reaction and the heat perceived during curing is something else?

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Per the Arrhenius Equation Wiki; "the reaction rate doubles for every 10 degree Celsius increase in temperature."

The fact that temperature increases the reaction speed is independent of endothermic or exothermic. The reaction rate increases because the motion of particles is greater when the temperature is higher, which increases the likeliness (and speed) that two reactant molecules will meet one another.

I have used a microwave (very briefly) to get a head start on fiberglass epoxy in cold weather, but heating your work space is a much better idea. Since it is exothermic like you mentioned, it is easy for it to have a run-away reaction where the heat it generates further speeds it up. Too fast of cure time will result in a much weaker material; too slow (like days in the cold) and it is possible for the hardener wick into a surface more than the resin and the epoxy will never fully harden.

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As I understand the chemistry though, adding heat to an exothermic reaction should slow the reaction and effectively take longer for the joint to cure.

Higher temperature means higher reaction speed, due to higher molecular kinetic energy. What changes for exothermic vs. endothermic reactions is their equilibrium point, not their speed. In the case of epoxy polymerization, you have an irreversible reaction, so the point is moot.

Epoxy can self-degrade if the heat release is not acounted for; but this heat is, by itself, not enough to even fully polimerize the adhesive, except in very thick parts where heat cannot be conducted away fast enough.

In the aerospace industry, for example, epoxy is cured at high temperatures and pressures, with a careful temperature ramp to avoid local overheats from the exothermic release in thick sections.

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  • $\begingroup$ Do you have a source for this info? $\endgroup$ – Diesel Sep 12 '17 at 1:14
  • $\begingroup$ As @ericnutsch answered, the reaction speed is given by the Arrhenius eq. For the equilibrium effects, see the Van t'Hoff equation. For the aerospace tidbit, it is my field of work. $\endgroup$ – AEhere supports Monica Sep 12 '17 at 7:52

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