Reaction Kinetics And Crystallization P Erformance Of High Viscosity PBT Polycondensation

Poly(butylene terephthalate)(PBT)has excellent physical and mechanical,electrical,dimensional stability,creep resistance,aging resistance and molding and processing properties.It is widely used in automotive parts,mechanical equipment,electrical insulation boards,cables Coating and food heaters.Increasing the molecular weight of PBT is an inevitable requirement to obtain high mechanical strength and then expand its application range.In recent years,this research group has developed a new technology for melt polycondensation to prepare highly viscous poly(ethylene terephthalate)(PET)melts.Polycondensation of PBT is rarely reported,so this paper systematically studies the solid-state and melt polycondensation reaction processes of high-viscosity PBT,comparatively analyzes the processes and mechanisms of the two,establishes a reaction kinetic model,and studies the crystallization properties of high-viscosity PBT obtained by solid-state polycondensation(SSP).First,two kinds of PBT circular chips with different initial carboxyl group concentrations were placed in a high vacuum atmosphere for melt polycondensation.The effects of polycondensation temperature and time on the intrinsic viscosity,carboxyl group concentration,and hue b value were studied.It was found that the high temperature promoted the polycondensation reaction and exacerbated the degradation reaction,and the extension of the polycondensation time made the effect of the degradation reaction more apparent.Materials with a higher initial carboxyl group concentration have faster reactive polycondensation because they have more active end groups,and can obtain higher viscosity PBT.Therefore,melt polycondensation is a process where the reaction and mass transfer are deeply coupled,and the side reactions and degradation reactions involved in the reaction are more obvious compare with the SSP.Due to the existence of the acid hydrolysis degradation reaction,a reaction kinetic model describing the melt polycondensation process of PBT was constructed based on previous studies.The reaction mechanism and kinetics of PBT SSP under vacuum and nitrogen atmosphere were studied.The effects of polycondensation temperature,polycondensation time,vacuum degree,and nitrogen rate on the intrinsic viscosity,carboxyl terminal concentration,and hue b value of the reaction product were investigated.It was found that with the increase of reaction time,the terminal carboxyl groups of PBT were gradually consumed and the intrinsic viscosity was continuously increased.With the increase of reaction temperature,the rate of consumption of terminal carboxyl groups and the increase of the intrinsic viscosity were accelerated.Increasing the nitrogen rate and vacuum during the reaction also significantly increased the intrinsic viscosity.The increase in intrinsic viscosity under high temperature and high vacuum gradually slowed down and flattened as the reaction time increased.PBT SSP is regarded as a second-order chemical reaction process with active end groups and a model is constructed to obtain the reaction rate constant and reaction activation energy under different process conditions.DSC analysis of the solid-phase polycondensation products under vacuum and nitrogen atmosphere,the results showed that double melting peaks appeared in the low-temperature polycondensation products of PBT under nitrogen atmosphere,the area of the low-temperature peak increased with the extension of the reaction time,their peaks blend into one finally.This is presumably due to the melting recrystallization of the imperfect crystals of the low-temperature peak.Regarding the change of the melting point with the reaction temperature and time,the melting point under the nitrogen atmosphere doesn't follow the rule.In a vacuum atmosphere,the melting point increases as the reaction temperature increases,but it has little to do with the reaction time.The crystallinity test found that at any time,there is a polycondensation temperature with a maximum crystallinity.At a high crystallinity reaction temperature,as the reaction time increases,it follows a linear rise rule.It is inferred that the reaction process and the crystallization behavior in the SSP process are inseparable.