Meaning
Material breakdown processes involve the loss of molecular weight and the deterioration of properties in polymers such as PET or PBT. Polyester degradation is typically caused by exposure to high temperatures and moisture, which break the ester linkages in the polymer chains. This leads to a reduction in melt viscosity and a decline in the mechanical strength and clarity of the moulded parts.
Thermal Stress
Heat encountered during the melting and shaping phases of production can trigger the cleavage of the polymer backbone. In the absence of water, polyester degradation occurs through a purely thermal mechanism that releases gases like acetaldehyde. Minimising the residence time in the barrel and keeping the processing temperature as low as possible are the primary ways to limit this type of damage.
Molecular Breakdown
Chemical structural changes result in the formation of shorter chains with a higher concentration of acid end groups. This aspect of polyester degradation is self accelerating because the new end groups can act as catalysts for further chain scission. Monitoring the intrinsic viscosity of the resin before and after processing is a standard way to quantify the extent of the damage.
Property Decay
Functional failure of the finished part is the end result of the internal changes in the polymer. Polyester degradation causes a noticeable yellowing of clear parts and a notable increase in brittleness. For a moulder using recycled content, managing this decay is a constant challenge that requires the use of additives like chain extenders to rebuild the molecular weight and restore the original performance characteristics of the material.
This chemical instability necessitates rigorous drying protocols for all polyester resins used in industrial production.
