Study On Preparation And Liquid-state Polycondensation Of PET Copolyester

Poly（ethylene terephthaltae）（PET）is one kind of semi-crystalline polyester with good fiber-forming properties,mechanical properties,chemical resistance,light transmittance and electrical insulation.It has been used in fields as fiber,film and packaging bottle.However,the tendency of PET to crystallize reduced the transparency of film products,which limited the application of PET in thicker transparent materials.The high crystallinity also resulted in poor dyeability,low hydrophilicity,generation of static charges and low wearing comfort property.In this study,the method of copolymerization modification was used to reduce the crystallinity of PET and change its glass transition temperature and melting point to expand the use range of PET.PENT copolyesters were synthesized through melt polycondensation reaction using pure terephthalic acid（PTA）and ethylene glycol（EG）as main monomer,as well as by adding different amounts of the comonomer neopentyl glycol（NPG）.The structure and chemical compositions,intrinsic viscosities,thermal properties,crystallization properties and mechanical properties of copolyesters were investigated.The results exhibited that the glass transition temperature,melting point and crystallization properties gradually decreased as the increase of the contents of NPG units.The diethylene glycol（DEG）content and terminal carboxyl group content in copolyester decreased.The PENT copolyester had excellent thermal stability.The copolyester was more flexible than PET material.Liquid-state polycondensation of PENT copolyester was researched on the basis of the synthesis of the copolyester.The law of change of intrinsic viscosities with different reaction time was analyzed.The color value,thermal properties and rheological properties of the copolyester samples before and after melt polycondensation were analyzed and compared.The results showed that the intrinsic viscosity of PENT copolyesters increased significantly with the increase of reaction time,and the growth rate increased with the increase of NPG content.As the reaction time increased,the L value of the copolyester sample gradually decreased and the b value increased.The copolyester appeared as shear thinning non-newtonian fluid.Under the same test conditions,the complex viscosity of the copolyester samples gradually decreased with the increase of NPG content.In the end,to improve the glass transition temperature of PENT copolyester,while increasing the degree of amorphous structure,PENTB copolyester was synthesized by melt polycondensation using 5-tert-butylisophthalic acid（^tBI）as a comonomer.The chemical structure and composition of copolyester,thermal properties,crystallization properties,dynamic thermomechanical properties were studied.The effects of different composition ratios on the properties of copolyesters were analyzed.The results exhibited that when the contents of ^tBI and NPG reached13%or more,the copolyesters were amorphous.The addition of the ^tBI unit increased the glass transition temperature of the copolyesters.The initial degradation temperature(T_5%)of the copolyesters were higher than that of the PET material.