Meaning
Physical or chemical breakdown of polymer chains caused by repeated heat history during recycling and remolding limits material reuse. Regrind degradation is caused by heat, shear, pressure and oxidation during each pass through an extruder or molding machine. This process eventually makes the material too brittle for structural use.
Chain Scission
High temperatures and mechanical stress break the long polymer chains into shorter pieces. As regrind degradation progresses, the melt viscosity of the material decreases, which can lead to flashing in the mold. The shorter chains cannot tangle as effectively, resulting in a loss of impact strength.
This molecular shortening is often irreversible and requires the addition of virgin resin to maintain the integrity of the part.
Property Loss
Parts made with a high percentage of recycled material often show lower elongation at break and reduced toughness. Testing for regrind degradation helps manufacturers determine the maximum amount of scrap they can safely blend with virgin resin. If the material is too degraded, it may only be suitable for low-value applications like flowerpots or pallets.
Process Control
Adding stabilizers and antioxidants can slow the rate of breakdown during remolding. Careful monitoring of the melt temperature and residence time is the best way to minimize regrind degradation in a production environment. By keeping the heat history as low as possible, a molder can maximize the number of times a material is reused.
