Karl Fischer Titration Interference Mechanisms in Recycled Polyester Flake Qualification
Karl Fischer moisture testing of recycled PET flake requires thermal desorption at 170 C with methanol-free reagents to eliminate volatile interference.
Chain Degradation
Meaning
Chemical breakdown of polymer backbone structures resulting in reduced molecular weight and weakened physical properties. Injection moulding of damp hygroscopic resins causes chain degradation when heat and water combine to sever covalent bonds in the polymer melt. This reaction reduces the length of the molecular chains, destroying the polymer’s ability to withstand mechanical stress.
Hydrolytic Cleavage
Polyesters and polyamides are highly susceptible to this water-induced reaction at high temperatures. Even a tiny amount of moisture causes rapid chain degradation during the plastification stage in the extruder barrel. The water molecules attack the ester or amide linkages, splitting the long molecules into shorter fragments.
This reaction occurs in seconds under the intense heat and pressure of the injection screw.
Mechanical Loss
Shortened polymer chains result in a drastic drop in impact strength and elongation at break. This structural weakness means the moulded parts will fail under load even if they look cosmetically perfect. The melt flow index of the degraded resin rises because the shorter molecules flow more easily.
Such an increase in flow rate often causes flashing at the mould parting line.
Processing Prevention
Rigorous desiccant drying of the resin before moulding prevents this chemical breakdown. This drying step ensures that the polymer retains its original molecular weight and design strength throughout the run.