The glass transition point or glass transition temperature TG is a point on the temperature scale at which a polymer, glass or even metal changes into a rubbery, viscous state. However, this is not equivalent to the melting point.
Especially with polymers, the glass transition is a fixed parameter. A glass transition in plastics is not a proper phase transition because only a small part of the sample changes its state of aggregation. More specifically, a displaceability of individual polymer chains arises against each other, but without changing to the molten state.
Since the glass transition can not be assigned exactly to a specific temperature, but depends on the measurement method and the prevailing conditions, it is important to adhere to standards for its determination and to be able to precisely control the measurement parameters.
Polyethylene terephthalat is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers, beverage, food and other liquid containers, thermoforming applications, and engineering resins often in combination with glass fiber. PET max exist both as an amorphous (transparent) and as a semi-crystalline material.
PET shows at about 77 °C a very significant endothermal glass point, when looking at partial crystalline thermo plastic materials. The relation between the exothermal cold crystallization (131 °C) and the endothermal melting peak is a method for the degree of crystallization of the material. In the case of PET the crystalline part is very small, giving the material a very good transparency. This is why drinking bottles (cocacola) are very often produced from PET.