The injection molding process of polyethylene terephthalate (PET) at 270 to 290 degree C generates parts per million concentration of acetaldehyde (AA), an undesirable chemical residual in food packaging applications. Over the last 60 years, researchers have been looking for the chemical reaction mechanisms of acetaldehyde generation behavior and the relationship with PET end groups such as carboxyl end group, hydroxyl end group, vinyl ester end group, and DEG components.
Here, we develop techniques to quantitative determining end groups at low concentrations and establish the contributions of specific end groups to AA generation rates in PET resins. In order to study the effects of the contribution of hydroxyl and carboxyl end groups, we also produce and analyze end capped PET samples.
As a result, it is concluded that the hydroxyl end group is one of the most significant factors that affect the acetaldehyde generation behavior, while the others groups such as carboxyl end group, vinyl ester end group, and DEG group do not affect AA generation directly. For short processing time (less than 20 minutes), acetaldehyde may come from two reactions: (1) Hydroxyl end groups reacting with vinyl end groups. Or (2) direct degradation of hydroxyl end groups. For long processing time (35 to 55 minutes), AA generation is mainly controlled by chain scission.