Effects of injection‐molding processing parameters on acetaldehyde generation and degradation of poly(ethylene terephthalate)

Abstract

Abstract The acetaldehyde (AA) generation behavior of poly(ethylene terephthalate) (PET) has been investigated in terms of its relationship to changes in various processing conditions. A single‐cavity injection‐molding machine was used to prepare preforms in order to relate changes in barrel temperature, screw shear rate, back pressure, cooling time and total residence time to levels of AA generated during processing. Within the temperature range 280–300 °C, a 10 °C rise in processing temperature causes AA production to double. Shear rate increases from 20 to 40 m min −1 result in 13–21 % increases in AA generation at temperatures from 300 to 280 °C. Increased back pressures from 0 to 200 bar result in AA concentration increases of about 1.2 ppm for each 50 bar pressure increase. The majority of this change is caused by increased polymer residence time. Longer cooling times also increase overall cycle times and result in higher levels of AA generation, at the rate of almost 7 ppm per additional minute at 290 °C processing conditions. Apparent activation energies of 167 kJ mol −1 were calculated for samples prepared at various shear rates. These results are in agreement with literature values obtained under conditions of static mixing and indicate that shear rate and plastication do not significantly affect reaction mechanisms for AA generation. Copyright © 2005 Society of Chemical Industry