Cure kinetics: how a thermoset hardens
A thermoset doesn't cool to solid like a plastic — it reacts to solid, and once set it can never melt again. What cure is, why it makes its own heat, and how the cure cycle races speed against a runaway exotherm.
Reacting to solid, not cooling to solid
A thermoplastic — polyethylene, nylon, PET — melts and freezes reversibly. Heat softens it, cooling hardens it, and you can do it again and again. A thermoset is different in kind: it begins as a liquid resin and a hardener that chemically react together, growing a permanent three-dimensional cross-linked network. It does not cool to solid, it reacts to solid.
And it is a one-way trip. Once cured, a thermoset can never be melted again — heat it enough and it chars rather than flows. That is exactly why epoxy is used where a thermoplastic would soften, and also why thermoset composites, like wind-turbine blades, are so hard to recycle.
Degree of cure
Cure is a reaction that runs over time, so engineers track how far it has gone with the degree of cure, written α, running from 0 for raw resin to 1 for fully reacted. Strength, stiffness, the glass-transition temperature and even the ability to lift the part out of the mould all depend on α passing a threshold.
An undercured part is weak and rubbery; push α too low and the part is useless. So you cure until α is high enough — and for demanding parts, add a postcure to drive it close to 1.
The exotherm — cure makes its own heat
Cross-linking is exothermic: the reaction gives off heat. In a thin part that heat escapes to the surroundings and cure stays controllable. In a thick part it cannot escape fast enough, and a dangerous feedback begins — the trapped heat speeds up the reaction, which makes more heat, which speeds it up further.
This runaway exotherm can scorch the resin, boil off volatiles into voids, or crack the part as it cools unevenly. It is the reason thick composite laminates are cured slowly and in stages: not to waste time, but to keep the exotherm on a leash.
The cure cycle
A cure cycle is simply the temperature-versus-time schedule the part follows in the oven or autoclave: a ramp up, usually a dwell — a hold — that lets cure progress evenly and lets the exotherm settle, and often a second, hotter dwell (the postcure) that drives α close to 1 and raises the final glass-transition temperature.
Designing the cycle is a balance. Too fast and you risk the exotherm and an unevenly cured part; too slow and you burn time and energy for no benefit. It is a real engineering trade you can feel in the cure-cycle and cure-kinetics simulators.
The Kamal model
The rate of cure, dα/dt, depends on two things: temperature, through an Arrhenius term (hotter reacts faster), and the current degree of cure itself — the reaction is autocatalytic, speeding up as it proceeds before slowing as it runs out of un-reacted resin.
The Kamal model wraps this into a compact equation with a handful of fitted constants, and it is exactly what the cure-kinetics simulator integrates to predict α over a chosen cure cycle. Watch a hotter dwell reach full cure faster — and watch the exotherm climb with it.
1. What is the key difference between a thermoset and a thermoplastic?
2. Why are thick thermoset parts cured slowly and in stages?
3. The degree of cure α runs from…