Manufacturing method of a copolyester containing ethylene naphthalate unit (EN) and its application

A manufacturing method of a copolyester containing ethylene naphthalate unit, whereby the copolyester is obtained from one of the following manufacturing methods:

• (1) Add 2,6-naphthalenedicarboxylic acid (NDA) into the thick liquid or esterification stage of purified terephthalic acid (PTA) process and when the esterification ratio reaches 95 percent, add a polymerization catalyst so as to proceed with polycondensation;
• (2) Add dimethyl 2,6-naphthalenedicarboxylate (NDC) into dimethyl terephthalate (DMT) solution before transesterification taking place in DMT process, additionally adding a transesterification catalyst and when the side product-methyl alcohol is completely removed, add the polymerization catalyst so as to proceed with polycondensation;
• and
• (3) Add dimethyl 2,6-naphthalenedicarboxy atc while the esterification ratio of PTA process reaches85%˜998%, concurrently adding the transesterification catalyst and when the side product-methylalcohol is completely removed, add the polymerization catalyst so as to proceed with polycondensation, and its product, comprising:
• the copolyester obtained from copolymerization has 5˜15 mole % of the NDA or NDC and has the following structure:
• and its intrinsic viscosity is between 0.3˜0.7 dl/g.