Thermal Expansion: Of course it affects photocures

Oops. We’re late. Sorry.

Thermal Expansion is a fundamental property of materials, and despite the occasional comment “Everything expands,” everything really doesn’t. As you cure a material, it shrinks as the chains crosslink and the free volume decreases. You see it in enameling jewelry where the contractive forces can distort the material, in forming epoxy parts where stress cracks from in thick components, and in photocuring thin films where it can cause ripples, etc. in the surface.

This shrinkage can be studied in the Thermomechanical Analyzer (TMA). Normally used for measuring the coefficient of thermal expansion and the Tg of materials, it also detects shrinkage. Setup is often a bit more complicated as shown below.

On the left, a lump of gel-like material is cured in a dilatometer. This small quartz vessel allows us to handle odd-shaped material or those that can’t support their own weight. Since it does measure the shrinkage (or expansion) in all three directions. This is volumetric expansion or shrinkage, and you then divide by 3 for a homogeneous material to get the linear value. The right shows a material run in tension coated on a substrate. In this case, it was a strip of paper, so the shrinkage caused a distortion or bending of the sample.