Polyethylene terephthalate is the most important variety of thermoplastic polyester, commonly known as polyester resin. It is formed by transesterification of dimethyl terephthalate and ethylene glycol or esterification of terephthalic acid and ethylene glycol to synthesize dihydroxyethyl terephthalate, and then polycondensation. Together with PBT, it is called thermoplastic polyester, or saturated polyester.
1946, the first patent for preparing PET was published in Britain, and ICI Company in Britain completed the pilot test in 1949. However, after purchasing the patent, DuPont Company of the United States set up a production device at 1953, which took the lead in realizing industrial production in the world. In the early days, almost all PET was used for synthetic fibers. Since 1980s, PET has made a breakthrough as an engineering plastic, and successively developed nucleating agent and crystallization promoter. At present, PET and PBT, as thermoplastic polyester, have become one of the five major engineering plastics.
PET is a kind of high crystalline polymer with milky white or light yellow, and its surface is smooth and shiny. It has excellent physical and mechanical properties in a wide temperature range, the service temperature can reach 120℃, and its electrical insulation performance is excellent. Even at high temperature and high frequency, its electrical properties are still good, but its corona resistance is poor, and its creep resistance, fatigue resistance, friction resistance and dimensional stability are good.
Processing method of polyethylene terephthalate;
1. Transesterification or indirect esterification:
1. Methylation: Terephthalic acid reacts with a little excess methanol and then esterifies to dimethyl terephthalate. Low-boiling substances such as water, excess methanol and methyl benzoate are distilled, and then purified dimethyl terephthalate is obtained by rectification.
2. Transesterification: at the temperature of 190~200℃, using cadmium acetate and antimony trioxide as catalysts, dimethyl terephthalate and ethylene glycol (molar ratio is about 1: 2.4) are subjected to transesterification reaction to produce polyester oligomer. Distilling methanol to make transesterification complete.
3. Final polycondensation: at a temperature higher than the melting point of polyester, such as 283℃, using antimony trioxide as a catalyst, ethylene glycol terephthalate is self-polycondensed or transesterified, and by means of decompression and high temperature, the by-product ethylene glycol is continuously distilled to gradually improve the degree of polymerization.
Second, direct esterification method:
After the purification technology of terephthalic acid is solved, this is the preferred economic method. Terephthalic acid and excess ethylene glycol were esterified at 200℃ to form polyethylene terephthalate with low polymerization degree, and finally condensed at 280℃ to form the final polyester product with high polymerization degree. This step is the same as indirect esterification. With the increase of polycondensation reaction degree, the viscosity of the system increases. In engineering, it is more advantageous to carry out polycondensation in two reactors in stages.
The above contents refer to Baidu Encyclopedia-Polyethylene Terephthalate.