Analytical Testing Methods for Quantifying Recycled Content in Polyolefin Compounds
Accurate quantification of recycled polyolefins requires combining DSC thermal integration, high-temperature GPC, ICP-MS trace metals, and melt rheology sweeps.
Catalyst Residue
Meaning
Inorganic chemical leftovers from polymer synthesis remain in the polymer matrix as an unavoidable trace by-product. Excess catalyst residue initiates thermal degradation and colour shifts during subsequent high-temperature melt processing. This material property governs the thermal stability limit of the virgin resin during the compounding and moulding stages.
The influence of the trace material ceases once the polymer has cooled below its crystallisation temperature.
Chain Cleavage
Transition metal compounds left over from polymerization accelerate the oxidation of polyolefin chains when heat is applied. This catalyst residue reacts with trace oxygen inside the injection barrel to generate hydroperoxides. Chemical breakdown generates shorter segments and lowers the molecular weight of the resin.
The resulting lower viscosity leads to flash at the parting line and reduces the load-bearing capacity of the final part.
Colour Deterioration
Yellowing of clear films and natural resin occurs when these metallic compounds undergo thermal oxidation in the melt stream. Highly active catalyst residue reacts with common phenolic antioxidants added to the resin to protect it from heat. This interaction creates coloured complexes that turn the melt from clear or white to a distinct yellow or grey hue.
The cosmetic defect cannot be reversed by compounding or adding pigment without altering the specified base colour of the product.
Cleaning Boundary
Post-reactor washing removes the majority of these reactive salts before the resin is pelletised at the production plant. High-performance grades require multiple wash cycles to bring the catalyst residue below a few parts per million. This intensive cleaning process increases the cost of the virgin resin and limits the volume that can be produced per hour.