**Detailed Introduction of Polyethylene Terephthalate (PET)** PET is the abbreviation of the English name of polyethylene terephthalate (polyethylene terephthalate or poly(ethylene terephthalate)), and it is a crystalline saturated polyester. Its chemical formula is (C₁₀H₈O₄)n, and it is a colorless and transparent (amorphous) or milky white or light yellow solid (crystalline). The main characteristics of PET include: - **Good physical properties**: The density is 1.3-1.4g/cm³, non-toxic, odorless, good hygienic safety, and can be directly used for food packaging; it has good barrier properties and has a high barrier effect on oxygen, water and carbon dioxide; the water absorption rate is low, and it can maintain good dimensional stability; the strength and rigidity are larger among common thermoplastic plastics, the tensile strength of the film can be comparable to that of aluminum foil, it has good folding resistance, but the tear resistance and impact resistance are poor; as the degree of polymerization increases, the melting point will increase. - **Good optical properties**: Good luster, high transmittance, and can block ultraviolet light; amorphous PET has good optical transparency. - **Excellent high and low temperature resistance**: It can be used for a long time within the temperature range of 120°C, and can withstand a high temperature of 150°C for short-term use and a low temperature of -70°C. Moreover, it has little effect on its mechanical properties at high and low temperatures. - **Chemical stability**: It is very stable to dilute acids, especially organic acids, but it is not resistant to the long-term action of concentrated sulfuric acid and concentrated nitric acid; the ester group is extremely prone to hydrolysis under alkaline conditions, so it is not resistant to dilute alkali at high temperatures, and concentrated alkali can cause it to be damaged at room temperature; it is relatively stable to general non-polar organic solvents and has a certain tolerance to polar organic solvents at room temperature. Some phenolic compounds and mixed solvents can dissolve it under heating conditions. The molecular structure of PET is a linear macromolecule, the functional groups on the molecular chain are neatly arranged, there are no branched chains, it is easy to orient, and it has a certain fiber-forming and film-forming property. Among its repeating units, the flexible alkyl group, the difficult-to-move benzene ring and the polar ester group, a large conjugate system is formed between the ester groups at both ends and the benzene ring, which increases the rigidity of the molecular chain, makes the toughness of the polymer material poor, showing a certain brittleness, and has a higher glass transition temperature and melting point. At the same time, the benzene rings are almost in the same plane, so that the concave and convex parts on adjacent macromolecules can be inlaid with each other, which is conducive to the generation of crystals, and the regularity and non-curling property of the molecular chain can also promote crystallization. The preparation method of PET is usually a chemical synthesis method, using terephthalic acid and ethylene glycol as raw materials. Generally, the intermediate bis-β-hydroxyethyl terephthalate (BHEt) is synthesized first, and then the polymer is obtained by polycondensation from this intermediate. The synthesis methods of the intermediate BHEt include transesterification method, direct esterification method and ethylene oxide method. The application field of PET is very wide, and the common related products are: - **Bottle resin**: It can be made into bottle containers and used for the packaging of carbonated beverages and purified water, etc. It is characterized by light weight, high strength, good toughness, high transparency, and can withstand higher internal pressure after tensile orientation. It has good chemical stability and high barrier properties. PET resin with higher crystallinity is a better heat-resistant packaging material, suitable for food containers for frozen foods and microwave-treated foods, as well as packaging for hot-filled foods. However, common transparent PET products have low crystallinity and poor temperature resistance. - **Fiber products**: PET materials are readily available and inexpensive, the production process is short and can be produced on a large scale and fully automatically, so the cost is low. Its products include drawn yarn, low elastic deformation yarn, air textured yarn, high strength yarn, staple fiber and drawn sliver, etc. It can be used to make various clothing items, indoor decorations, bedding, etc. The clothes or articles made have a crisp appearance, good shape retention, are easy to wash and dry quickly, and do not need ironing. - **Electronic and electrical aspects**: Reinforced PET can be used to manufacture the shells of connectors, coils, bobbin windings, capacitors, transformers, etc., TV accessories, tuners, switches, automatic fuses, motor brackets and relays, etc. The composite film made of PET can be used as the base material of audio tapes, video tapes, movie films, computer floppy disks, various optical discs, X-ray films, photosensitive films, etc., as well as materials used in the electronic field such as electrical insulation materials, capacitor films, flexible printed circuit boards and film switches. - **Mechanical parts**: PET has the advantages of stable size, high precision, good wear resistance, high strength, light weight, low price and long service life. It can be used to manufacture gears, cams, pump housings, pulleys, motor frames, watch parts, etc. PET is also widely used in the structural parts and electrical accessories of automobiles. It can be used to manufacture distribution panel covers, ignition coils, various valves, exhaust parts, distributor covers, measuring instrument covers, small motor covers, etc. - **Other fields**: It can be used in photovoltaic modules, LED lights, 3D printing consumables, etc.; for manufacturing exterior parts of automobiles; PET sheet is a sheet used for pharmaceutical packaging after polyvinyl chloride sheet. PET film can be used as the base material for medical appliances, or it can be aluminized or coated with polyvinylidene chloride and then compounded with other films to make a composite film for packaging materials of medicines.