Polymer Structure
Chemical end groups that form on polyester chains during high temperature melt degradation indicate a loss of molecular weight. These active groups, known as vinyl ester terminals, are produced when the polyester backbone undergoes thermal scission under intense heat. Their presence in the polymer melt acts as a precursor for further degradation, as they readily react to form volatile byproducts like acetaldehyde.
Generation Pathway
Extreme temperatures and long residence times inside the extruder barrel accelerate the thermal cleavage that yields these end groups. The reaction starts with a intramolecular transition that splits the ester linkage, leaving a carboxyl end group and a vinyl ester end group. This molecular breakdown represents a permanent damage to the polymer structure.
Downstream Effect
High concentrations of these degraded end groups reduce the intrinsic viscosity of the polymer, leading to weaker yarns. The resulting fibers are prone to breakage during melt spinning and high-speed texturizing. In addition, the volatile compounds generated from these terminals can cause odor issues in the final fabric, making the yarn unsuitable for high-end apparel.
Analytical Assessment
Nuclear magnetic resonance spectroscopy and infrared spectroscopy are the primary methods used to detect and quantify these reactive end groups. Laboratory technicians measure the ratio of degraded end groups to healthy chain ends to assess the thermal history of the recycled polymer. This test helps spinning mills identify degraded batches before they are processed into fine denier yarns.
Batches with high concentration of degraded terminals are rejected or diverted to non-textile applications where high mechanical strength is not a requirement. This screening ensures that only high-quality material is spun into yarn.
