What Tg really is
Below its glass transition temperature, a polymer’s chain segments are frozen in place: the material is hard and glassy. Above Tg, segments gain enough energy to wriggle, and the material becomes rubbery and flexible. The change happens over a range of temperatures and depends on how fast the material is measured or loaded.
Why it matters in coatings
- Hardness and block resistance: a coating used above its Tg feels soft and can pick up dirt or stick to itself.
- Flexibility and cracking: a coating used well below its Tg may crack when the substrate flexes or temperatures swing.
- Chemical resistance: rubbery films let chemicals and water diffuse in faster.
- Cure in the cold: as a two-component film cures, its Tg rises. If it approaches the cure temperature, the network “vitrifies” and the reaction slows dramatically, which is why epoxies cured in the cold can stay under-cured until warmed.
Two very different design goals
| System | Tg design | Result |
|---|---|---|
| Spray polyurea, elastomeric membranes | Soft-segment Tg well below service temperature | Flexible, crack-bridging, high elongation |
| Epoxy linings, novolacs | Network Tg well above service temperature | Hard, chemically resistant, dimensionally stable |
| Latex paints | Tg near room temperature, assisted by coalescent | Films form in the cold yet resist blocking |
What moves Tg
Rigid, aromatic backbones (bisphenol A, MDI hard segments) and dense crosslinking raise Tg. Long flexible chains (polyether polyols, polyetheramines), plasticizers and retained solvent lower it. In acrylics, formulators blend hard monomers like MMA with soft ones like butyl acrylate to hit a target Tg.
Phase-separated polymersPolyureas and polyurethanes often show two transitions: a low soft-segment Tg (well below zero) and a much higher hard-segment transition. That combination is what makes them both flexible and strong. Research on polyurea under blast loading shows the soft segments can even pass through a glass transition at very high strain rates.