Plastic material out of the family of polyesters applied in hugh quantities for food packaging. In particular almost all plastic bottles are made of PET. Additionally all "Polyester"-fibres and -fabrics are PET. Due to this fact that all Plastic bottles and all Polyester-fabrics are PET, it is the material with the highest quantities beeing recycled, because it is easy to collect 100% pure material.
It is popular to use the designation "Polyester" for PET even if used precisely the designation concerns a larger group of polymers.
PET is semicrystallin. The degree of crystallisation can be controlled by the process parameters when producing final products. Slow cooling of the melt gives time to the molecules to form crystalline structures while shock cooling proides almost 100% amorphous structure. When amorphous the material is completely transparent while it is opaque in case of a higher crystallinity.