Tg marks the temperature range where a polymer's stiffness drops sharply. Structural adhesives lose much of their strength and creep resistance above it, so the service temperature has to stay safely below Tg. It is measured by DSC or DMA and rises with fuller cure and post-cure. Flexible adhesives such as silicones and many urethanes have Tg well below zero and are used above it on purpose.
Housing sees 90 C in the engine bay and this epoxy's Tg is only 70, that bond will creep.
Service temperature should stay well below Tg, often by 15 to 25 C, because strength drops off before the full transition.
Also heard as
- glass transition
- glass transition temperature
Related terms
Post-cure
Extra time at elevated temperature after the first cure, which finishes crosslinking and raises Tg and heat resistance.
Creep
Slow, continuing deformation of an adhesive under a constant load, which can end in failure over time.
Cure
The chemical reaction or physical change that turns a liquid adhesive into a solid that carries load.
Epoxy
A thermoset adhesive cured by reacting epoxy resin with a hardener, the benchmark for strong, rigid structural bonds.