Cure Terms Glossary

Time-Temperature-Transformation

The time-temperature-transformation (TTT) cure diagram is a device for setting out the different sequence of changes which are possible in various resin cures. For example, a given cure system may undergo gelation before vitrification - or perhaps pass straight to vitrification, e.g.,
either,

elastoviscous liquid → (gelation) → viscoelastic solid → (vitrification) → glass

or,

elastoviscous liquid → (vitrification) → glass

Which sequence is followed depends on the cure temperature. In the case of epoxies, an ambient temperature cure might take the latter course whilst a heated cure could gel before vitrifying. The TTT cure diagram, which is specific to an individual cure system, sets out the changes for isothermal cures. A typical format uses the terms liquid (for elastoviscous liquid) and rubber (for viscoelastic solid) and the cure diagram might look something like the following.

 


Schematic TTT cure diagram for resin cures


The diagram is divided into different regions by the gelation and vitrification lines, and into temperatures ranges determined by values of glass transition temperature. The latter are:
(i) Tg0 and Tg∞, the glass transition temperatures of the uncured and fully cured resin and,
(ii) gelTg is the temperature at which gelation coincides with vitrification.

The useful temperature range for cure sits between Tg0 and Tg∞. Within this range, curing behaviour is critically dependent on gelTg. If Tcure is the temperature of cure, then,

  • if Tcure > gelTg, gelation precedes vitrification,
  • if Tcure < gelTg, vitrification occurs first.