Diffusion of ethylene glycol accompanied by reactions in poly(ethylene terephthalate) melts

Abstract

Diffusion coefficients of Ethylene Glycol (EG) have been measured in poly(ethylene terephthalate) (PET) melts by using a quartz spring sorption apparatus which is designed for high temperature measurements using silicone oil for high temperature as a heat transfer medium. A simple mathematical model developed to consider the effect of reactions because PET-EG system inherently involves a lot of reactions along with diffusional sorption. Diffusion coefficients are deduced by considering an asymptotically thin sample depth in order to minimize the effect of reactions. Diffusivity of EG are strongly dependent on the vapor pressure of EG and temperature but not on the molecular weight of PET in this experimental range(degree of polymerization 80-120). Diffusion coefficient of EG in PET melt at $265\,^\circ\!C$ is $2.5830\times10^{-7}cm^2/\sec$, activation energy for diffusion is 38.4 kcal/gmol and heat of solution for sorption is 44.9 kcal/gmol. Temperature and pressure dependences on solubility in these systems showed that Henry``s law is not valid. By analyzing the volatile materials of PET-EG system with gas chromatography, frequency factors of polymerization reaction k1 and the formation reaction of acetaldehyde was determined to be $5.84\times10^8\,cm^3$/mol-min and $3.90\times10^{11} \min^{-1}$ by numerical iteration of the developed model. Apparatus used in this experiment also could be applied other polymer melt system of which the melting point was less than $300\,^\circ\!C$.