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.
Flake Wash Residues
Meaning
Chemical contaminants and surfactant traces left behind on post-consumer recycled plastic flakes after the industrial washing process. Compounding and moulding of recycled resins must account for flake wash residues because these substances can catalyze polymer degradation during processing. This contamination includes sodium hydroxide, detergents, and organic acids that are not completely rinsed away.
Contaminant Ingress
Residual salts and surfactants attract ambient moisture, making the recycled flakes highly hygroscopic. These flake wash residues can also alter the chemical reaction during Karl Fischer moisture testing, causing false water readings. If the titration solution is not buffered, these alkaline or acidic species interfere with the endpoint detection.
Moulders run the risk of processing wet material if they cannot trust the moisture values of their recycled feedstocks.
Melt Processing
High temperatures in the extruder cause these chemical residues to react with the polymer chain. This reaction results in rapid chain cleavage, severe yellowing, and a loss of mechanical toughness. The presence of wash surfactants can also cause foaming and gas build-up in the melt, which leads to splay defects on the surfaces of finished parts.
These surface imperfections spoil the appearance and lower the impact resistance of the product.
Recycling Value
Proper rinsing protocols minimize the concentration of these harmful residues to preserve polymer properties. This thorough cleaning process increases the market value of recycled resin lots.