Polyester is formed by polycondensation of terephthalic acid and ethylene glycol, following the general rule of linear polycondensation. In the production of polyester polyester, two synthetic technologies have been developed successively: transesterification and direct esterification.
(1) Transesterification method or indirect esterification method
This is a traditional production method consisting of three steps of methyl esterification, transesterification and final polycondensation. The purpose of methyl esterification is to facilitate the purification and purification of dimethyl terephthalate.
① Methyl esterification: Terephthalic acid reacts with a slight excess of methanol and first esterified to dimethyl terephthalate. The water, excess methanol, methyl benzoate and other low-boiling substances are evaporated, and then rectified to obtain pure dimethyl terephthalate.
② Transesterification: At 190~200℃, cadmium acetate and antimony trioxide are used as catalysts to make dimethyl terephthalate and ethylene glycol (molar ratio about 1:2.4) undergo transesterification to form polyester oligomerization Things. Distill off methanol to make the transesterification sufficient.
③ Final polycondensation: at a temperature higher than the melting point of polyester, such as 283℃, antimony trioxide is used as a catalyst to make ethylene terephthalate self-condensation or transesterification, and continuously distill out the by-product ethylene distillate under reduced pressure and high temperature. Alcohol gradually increases the degree of polymerization.
In the stages of methyl esterification and transesterification, equal group ratios are not considered. In the final polycondensation stage, according to the distilled amount of ethylene glycol, the ratio of the numbers of the two groups is naturally adjusted, gradually approaching the amount of the same substance, slightly excess ethylene glycol, blocking the two ends of the molecule, and reaching the predetermined degree of polymerization.
(2) Direct esterification method
After terephthalic acid purification technology is solved, this is the preferred economic method. Terephthalic acid and excess ethylene glycol are first esterified into polyethylene terephthalate with low degree of polymerization (such as X=1~4) at 200°C, and then finally polycondensed at 280°C to form a final high degree of polymerization. For polyester products (n=100~200), this step is the same as indirect esterification.
As the degree of polycondensation increases, the viscosity of the system increases. In engineering, it is more advantageous to segment the condensation polymerization in two reactors. Pre-polycondensation in the front stage: 270℃, 2000~3300Pa. Final polycondensation in the latter stage: 280~285℃, 60~130Pa.