Low Frequency Rheological Testing for Hydrolyzed Fraction Identification in Resins
Low frequency oscillatory rheology unmask hydrolyzed resin fractions by measuring zero-shear viscosity drop and storage modulus slope collapse near zero frequency.
Polyethylene Terephthalate Degradation
Meaning
The breakdown of polyester chain structures due to thermal or hydrolytic stress during melt processing represents a major source of quality loss in resin recycling and bottle production. This process of polyethylene terephthalate degradation occurs rapidly when the polymer is processed in the presence of moisture or at temperatures above three hundred degrees Celsius. It limits the reuse of post-consumer resin in high-performance applications like carbonated soft drink bottles.
Moulders must monitor this breakdown to prevent structural failures in the blown containers.
Reaction Pathway
Hydrolysis is the most rapid mechanism of polymer chain breakage in this material. When heated, the residual moisture in the polymer reacts with the ester linkages, causing polyethylene terephthalate degradation and generating shorter chains. Thermal degradation also occurs, producing acetaldehyde as a byproduct, which can migrate into packaged beverages and alter their taste.
Mechanical shear in the extruder screw further accelerates these reactions by generating localized hot spots.
Molecular Consequence
Lower intrinsic viscosity of the polymer is the direct result of a reduction in molecular weight. This lower viscosity reduces the melt strength, making the parison unstable during blow moulding. The degraded polymer chains are also more prone to crystallization, which increases the opacity of the bottle and makes it brittle.
These changes reduce the impact strength of the finished bottle, leading to cracks during shipping or storage.
Process Mitigation
Preventing this chain breakdown requires strict control of the drying process and melt temperatures. Moulders dry the polymer to a moisture content below fifty parts per million using desiccant dryers operating at high temperatures before extrusion. Solid state polymerization can be used to rebuild the molecular weight of the recycled flakes by heating them under a vacuum.
This process allows the recycled material to achieve the necessary viscosity for bottle production. During moulding, screw speeds and melt temperatures are kept as low as possible to minimize both shear and thermal stress on the material, which protects the polymer from further degradation.