Compatibilizers — Making Incompatible Polymers and Fillers Work Together
Direct answer
Most polymer pairs are immiscible: blended without help, they form coarse domains with no adhesion across the interface, and the compound fails at the domain boundary under impact. A compatibilizer is a molecule with affinity for both phases — most commonly a maleic anhydride grafted polyolefin — that sits at the interface, reduces domain size, and transmits stress across it. The same chemistry couples polar fillers and glass fibre to a non-polar matrix. The symptom that tells you a compatibilizer is missing is delamination, low impact strength, or a blend that looks fine at moulding and splits in service.
Where it is genuinely required
- PP with recycled PE, or PE with recycled PP. Mixed-polyolefin recyclate is the most common case in Canada today. MA-grafted PP or PE at a few percent restores impact strength that sorting alone cannot.
- Glass-fibre reinforced PP. Without MA-g-PP coupling, the fibre carries almost no load and the reinforcement is cosmetic.
- Mineral-filled polyolefins at high loading. Coupling raises tensile and flexural strength instead of merely stiffening a brittle compound.
- Polyamide or EVOH contamination in a polyolefin stream — for example reprocessed multilayer packaging scrap. Compatibilization converts a delaminating reject into a usable grade.
- Engineering blends (PC/ABS, PA/PP) where an impact modifier alone does not hold the phases together.
What decides the grade and the dose
Grafting level, base polymer and melt flow all matter. The base polymer of the compatibilizer must match the continuous phase, and its melt flow must sit in the processing window of the compound, or it will not distribute. Higher graft level means more interfacial activity per kilogram but also more moisture sensitivity — anhydride functionality hydrolyses, so these grades usually need drying and sealed storage. Typical dose ranges from 1 % to 5 %; more is rarely better, because unreacted compatibilizer plasticises the matrix.
Test the result mechanically, not visually. Notched impact and elongation at break reveal interfacial adhesion; a smooth-looking pellet does not.
What to specify on your enquiry
- Continuous phase polymer and grade.
- Dispersed phase or filler, and its percentage.
- Recycled content share and its source stream.
- Property target — impact, tensile, delamination resistance.
- Process and melt temperature.
- Drying capability on site.
- Annual volume.
Related knowledge
- Antioxidants for recycled streams that have already seen heat history.
- Lubricants and processing aids when the problem is flow rather than adhesion.
- Mineral filler selection for coupled, highly filled compounds.