The common processing method of PET, polyethylene terephthalate, is COC6H4COOCH2CH2O. (English: Polyethyleneterephthalate, referred to as PET), by the ester exchange of dimethyl terephthalate and ethylene glycol or the esterification of terephthalic acid and ethylene glycol to synthesize terephthalic acid bis-hydroxyethyl ester, and then polycondensation reaction. It is a crystalline saturated polyester, a Milky White or light yellow, highly crystalline polymer with a smooth and glossy surface. It is the 1 common resin in life. Polyester is made of terephthalic acid and ethylene glycol polycondensation, which follows the general law of linear polycondensation. In the production of polyester, two synthetic technologies have been developed: transesterification and direct esterification.

(1) Transesterification or indirect esterification

This is a traditional production method, consisting of methyl esterification, ester exchange, final polycondensation 3 steps, the purpose of methyl esterification is to facilitate the purification of dimethyl terephthalate.

① Methylated terephthalic acid reacts with a slight excess of methanol and is first esterified to dimethyl terephthalate. Water, excess methanol, benzoic acid methyl ester and other low boiling substances, and then distilled, that is, pure dimethyl terephthalate.

② Transesterification At 190~200 ℃, using cadmium acetate and antimony trioxide as catalysts, dimethyl terephthalate and ethylene glycol (molar ratio about 1:2.4) were transesterified to form polyester oligomers. Methanol was distilled off to allow sufficient transesterification.

③ The final polycondensation is higher than the melting point of polyester, such as 283 ℃, with antimony trioxide as catalyst, the ethylene terephthalate self-polycondensation or ester exchange, by decompression and high temperature, continuously distilling off the by-product ethylene glycol, and gradually increasing the degree of polymerization.

The methyl esterification and transesterification stages do not take into account equal group number ratios. In the final polycondensation stage, according to the distillate amount of ethylene glycol, the ratio of the number of two groups is naturally adjusted, and the amount of equal substances is gradually approached, so that ethylene glycol is slightly excessive, both ends of the molecule are blocked, and the predetermined polymerization degree is reached.

(2) Direct esterification method

After the terephthalic acid purification technology is solved, this is the preferred economic method. Terephthalic acid and excess ethylene glycol are first esterified at 200°C to form polyethylene terephthalate with a low degree of polymerization (e. g., X = 1-4), and then finally polycondensed at 280°C to form a final polyester product with a high degree of polymerization (n = 100-200). This step is the same as the indirect esterification method. As the degree of polycondensation increases, the viscosity of the system increases. In engineering terms, it is more advantageous to carry out the polycondensation in stages in two reactors. Pre-condensation of the front section: 270 ℃,2000~3300Pa. Post-stage final polycondensation: 280~285 ℃,60~130Pa