What is POLYETHYLENE TEREPHTHALATE (PET)?
Polyethylene terephthalate (PET) is a thermoplastic polymer of the polyester family that is produced by stepwise polymerization by the reaction between pure terephthalic acid (PTA) and monoethylene glycol (MEG). The glass transfer temperature of this polymer is about 80 ° C. There is a great variety in the properties of this polymer, depending on the application and the orientation of the polymer chains. The polymer melts at a temperature of about 255 ° C and if cooled rapidly, it becomes almost completely amorphous and its density will be about 31.33 gr / cm. Due to slow molecular motions, crystallization begins at about 80 ° C and at 130 ° C its density increases by 1.37 gr / cm3. Also, with proper orientation and application of heat, the density increases up to 1.40 gr / cm3.
Polymer polyethylene terephthalate
Polyethylene terephthalate is a polymer that is used in most countries to produce 70% polyester fiber, 22% bottle resin, 6% film resin and 2% engineered polyester resin, but in Iran, this polymer is mostly used to make all kinds of beverage bottles. . Due to the high resistance of these bottles against breakage, temperature and penetration of gases, having a 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.
As a result, used PET bottles enter the municipal waste stream. The high volume of these bottles in relation to the weight, which take up a lot of space during transportation and burial, and its very long decomposition in nature (about 300 years), has caused the problem of recycling these bottles, especially in In recent years, it has attracted everyone’s attention. According to some reports, global demand for PET has resulted in an annual production volume of 56 million tons. In Iran, Shahid Tondgovian Petrochemical Company is a producer of various grades of polyethylene terephthalate.
Production of polyethylene terephthalate (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 concentration reaction, in which molecules react and water is released, which is followed by a second polymerization reaction that occurs in the solid phase. Applied intermediates in the production of PET are pure terephthalic acid and ethylene glycol, which are derivatives of crude oil. When the materials are heated together, the first product produced is a monomer (hydroxyethyl terephthalate base) mixed with low molecular weight polymers (oligomers). Be.
Pet at this stage is a molten viscous liquid that is pressurized and cooled by water to form amorphous glass. PET based on dimethyl ester is sometimes produced from phthalic acid. High molecular weight PET is produced by solidification by the second stage of polymerization at low temperature. This method removes all volatile impurities such as acetaldehyde, free glycol and water. In the preparation of PET, reaching high molecular weight is necessary to create mechanical properties, hardness and creep resistance to have sufficient flexibility against breaking.
Purification of polyethylene terephthalate polymer
Purification of the formed polymer is very difficult, so the raw material is the key to achieving high quality polymer for food packaging. Ethylene glycol is readily purified by vacuum distillation and acid by repeated crystallization of the solvent. Such materials with high purity and molecular weight are required for functional food packaging. Catalysts are used in very low concentrations to develop reactions and ensure economical use. The most common catalyst is antimony trioxide, but salts of titanium, germanium, cobalt, manganese, magnesium, and zinc are sometimes used, with small amounts remaining in the polymer mold or in the polymer chain itself.
Classification
PET is classified as a semi-crystalline polymer and changes from a hard glass to an elastic form when heated above 72 ° C. When polymer molecular chains are stretched and placed in one direction, they take the form of filaments or in two directions in the form of films and sheets.
If the molten material cools rapidly while still being held in tension, the chains will freeze in a directional direction and remain intact. Being in a state of tension causes a characteristic of strength. (Features found in PET bottles.)
If the pet is kept in elongated form at 72C and slowly crystallized, the material tends to become opaque, resulting in greater hardness and less flexibility. (Later known as crystalline PET) This form is able to withstand higher temperatures and can be used in the production of trays and containers with mild gas temperatures. This is a heat setting technique that often improves the washing and wrinkling properties of polyester textiles. Our modification between each of these forms produces a wide range of different products, all of which are from the same original PET chemical formula.
