Solid-state polymerization of PET: influence of nitrogen sweep and high vacuum

Abstract

We have examined the influence of reaction environment on the solid-state polymerization (SSP) of thin (180 μm) poly(ethylene terephthalate) (PET) chips at 250 °C by following the intrinsic viscosity (IV) increase and the end-group depletion. When the SSP reaction is carried out in vacuum, IV increases from 0.58 to 2.4 dl/g in 2.5 h of reaction. The initially rapid reaction slows considerably with time, and IV rise nearly stops at 2.75 dl/g at 6 h, though we still detect the acid and the hydroxyl end-groups at concentrations of 3 and 5 mequiv./kg, respectively. This suggests a role of crystallization in limiting the approachability of the end-groups to each other, thereby temporarily rendering them inactive. At this stage, raising the temperature to 270 °C to melt the PET in vacuum again increases the IV to 2.97 dl/g in 1.5 h, perhaps due to the release of crystalline restraints in the melt allowing some these inactive end-groups to approach each other. We find that accounting for these temporarily inactive end-groups is a must for a good kinetic description of SSP to IV>1.3 dl/g. When nitrogen is used as a carrier gas, the reaction rate and the extent of molecular weight build-up are somewhat lower compared to SSP under vacuum. A sublimate is collected during SSP under vacuum, and we find it to be made up of terephthalic acid, monohydroxyethyl terephthalate, bishydroxyethyl terephthalate and cyclic oligomers. This indicates the presence of a new condensation mechanism during SSP under vacuum.