☑️ What is PET?

Polyesters are produced by the two-factor reaction of acids and alcohols in the presence of a metal catalyst. The key polymerization step is known as a condensation reaction, where the molecules react and release water, which is followed by a second polymerization reaction that occurs in the solid phase. Intermediates used in PET production are pure terephthalic acid and ethylene glycol, which are derivatives of crude oil. When the materials are heated together, the first product that is produced is the monomer (bishydroxyethyl terephthalate) which is mixed with low molecular weight polymers (oligomer), then the mixture reacts and the excess ethylene glycol is distilled off and PET is formed. . At this stage, PET is in the form of a molten viscous liquid, which is removed by pressure and cooled by water, and becomes an amorphous glass material. Sometimes PET is also produced based on dimethyl ester of phthalic acid. PET with high molecular weight is produced by the second stage of polymerization in solid form at low temperature. This method removes all volatile impurities such as acetaldehyde, free glycol and water. In the preparation of PET, it is necessary to reach a high molecular weight to create mechanical properties, hardness and creep resistance so that it has enough flexibility against breaking. Purification of the formed polymer is very difficult, that's why raw materials are the key to achieving high quality polymer for food packaging. Ethylene glycol is easily purified by vacuum distillation and acid by repeated crystallization from the solvent. Such materials with purity and high molecular weight are required for practical food packaging. Catalysts are used in very low concentrations in order to develop reactions and ensure economic use. The most common catalyst is antimony trioxide, but salts of titanium, germanium, cobalt, manganese, magnesium, and zinc are sometimes used, a small amount of which remains inside the polymer mold or in the polymer chain itself. PET is classified as a semi-crystalline polymer and when it is heated at a temperature higher than 72C, it changes from a hard glassy state to an elastic form. When the molecular chains of the polymer are stretched and aligned to They are in the form of strings or in two directions in the form of films and sheets. If the molten material is cooled rapidly while still being held in tension, the chains freeze in a directionally oriented manner and remain intact. Being in a state of tension causes the characteristic of strength to appear. (Characteristics that can be seen in PET bottles.) If PET is kept in the stretched form at a temperature of 72C and crystallizes slowly, the material tends to become opaque, resulting in greater hardness and less flexibility. and can be used in the production of trays and containers with mild gas temperature tolerance. This is a heat setting technique that often improves the washing and wrinkle resistance of polyester textiles. Our modification between each of these forms produces a wide range of different products, all of which are of the same basic chemical formula as PET.

Today, PET is mainly obtained from the combination of ethylene glycol with terephthalic acid with the help of heat and catalyst, which is a linear polymer and resistant to solvents. Special physical and chemical properties of PET, such as high mechanical strength, joint strength, transparency, light weight, Its danger in terms of toxicity and impenetrability against CO 2 has caused it to be widely used in making photographic films, beverage bottles, and fibers. It is also used in the preparation of BCF continuous fibers and industrial yarn and Staple, POY for textiles and clothing. Due to the lack of effect of PET on taste and stability against heat, it is also used as food packaging.