PET/PBT. The Difference Crystallinity Makes

PBT and PET Polyester: Part 1 The Difference Crystallinity Makes

The differences between PBT and PET are best understood by examining the chemical structure of the repeating unit that makes up the polymer chains. The essential feature that makes the materials distinctive is the terephthalate ester group that lends its name to this family of materials.

If PET preforms get too hot in the preheat cycle, they turn cloudy and brittle with crystallization. The trick is to keep the material above its glass-transition temperature, but below its crystallization temperature. This graph shows that the temperature window may not be very wide.

PBT & PET Polyester: Part 2 The Performance Factor

PET retains its superior properties across the range from room temperature to the point where the two materials begin to melt. Before melting, both materials undergo a step change in modulus (glass transition) that reduces the stiffness of the materials by a little more than 50%.