Polyethylene terephthalate is a polymer that is used in most countries to produce polyester fibers (70%), bottle resin (22%), film (6%) and engineering polyester resins (2%), but in Iran This polymer is mostly used to make all kinds of plastic bottles with PET blowers.
Due to the high resistance of these bottles against breakage, temperature and gas penetration, low weight and cheap price compared to other packaging materials such as glass and metal, the production of plastic bottles from this resin has been widely used. which consequently causes the entry of disposable PET bottles into the municipal waste stream.
The high volume of these bottles compared to their weight, which take up a lot of space during transportation and burial, and their very long decomposition in nature (about 300 years), has caused the problem of recycling these bottles, especially in to attract everyone's attention in recent years.
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.
PET is biologically neutral, skin safe to handle. It is not dangerous if swallowed or inhaled. No evidence of toxicity was found in the nutritional studies conducted on animals. Negative results from Ames tests and studies on synthetic DNA (non-programmable) showed that PET has no toxicity on genes.
Similar studies on PET monomers and intermediate compounds show that these substances are not necessarily toxic and pose no threat to humans. One of the special substances used on a large scale is antimony trioxide, which causes cancer. However, complete studies on animal nutrition showed that there is no risk due to the use of antimony in PET production. Studies on diets containing 20 g/kg of antimony trioxide had no detectable toxic effects on animals, and genotoxin tests were mostly negative.
Endocrine glands of the human body that control vital human processes such as reproduction, regulation of metabolism and other aspects of development before birth. Man-made chemicals, if absorbed in the body, cause the breakdown of estrogen hormone and disrupt the endocrine cycle. endocrinology, reproduction disorder and decrease in sperm count. So it is important that the compounds used in PET do not have any evidence of estrogenic activity. There is an important evidence that shows that these substances are non-cancerous and relatively safe. And.
The advantage of using PET bottles is that the bottle cap is easy to open and can be closed again, and they are also light and cheap. PET inherently has the characteristics of biodegradable plastics and within two years Every year, more than 30 percent of it is destroyed under the soil, so recycling PET and turning it into consumable products is a more appropriate solution.
