2. Properties

2.1 Structure

PBT and PET are semi-crystalline polymers (in the solid state, part of the material is amorphous and the rest is crystallized).

The crystallinity rate does not exceed 40-50% by volume. A number of factors influence the rate of crystallization:

• the chemical nature of the polymer: PBT crystallizes much faster than PET ;

• the molecular weight of the polymer: the higher the molecular weight, the lower the crystallization rate;

• temperature.

For PET, the crystallization rate is highest at around 175 C. Still sufficient between 130 and 140 C to enable molding cycle times of less than 1 min, it becomes too...