Thermal mechanical analyzer (TMA) is frequently used for coefficient of thermal expansion (CTE) measurement. In a previous blog CTE by TMA I mentioned that the expansion probe is used to measure linear CTE of a solid block of sample and the through-thickness expansion of a film sample. To measure in-plane behaviors such as thermal expansion and shrinkage of a film sample, the tension probe with film sample assembly is used, see the photograph below.
Depending on how a film or a sheet is made it is likely to have directional differences in its machine direction (MD) and transverse direction (TD). One common example that everyone can relate to is the shrinkage of a T-shirt after wash and dry. I’ve had T-shirts that did not change size or shape after wash and dry, and I’ve had T-shirts that became smaller all around afterwards. There was one T-shirt I had that became shorter but not narrower after wash and dry. All these differences point to the various materials and processes that can be used to weave a fabric and to make a T-shirt which cause a difference in how the shirt behaves after wash and dry cycles.
This means the film or sheet sample should be tested in both directions and care should be taken to align directions during sample preparation. One problem that can happen when sample is not aligned or when a sample is particularly heterogeneous is that the thermal stress differences in directions might cause the sample to twist during thermal testing.