A recycled blend of unfilled polypropylene and up to 15 percent unfilled acetal copolymer lost no tensile strength compared to virgin PP. Mineral-filled PP and up to 15 percent Celcon actually produced a higher tensile strength, and flex modulus increased in both cases. These data came from researchers at the Advanced Materials Group at Hoechst Celanese, in a paper given at the first Intersociety Polymer Conference in October; this new conference focuses on the creation, utilization, and recycling of multiphase polymer systems.
The typical car today includes 16 percent polypropylene and 1 percent acetal by weight. Those acetal parts are small and difficult to remove; it's easier to leave them commingled. Michel Bitritto, director of resource recovery/recycling at AMG, found that engineering resins like acetal make cost-effective recovery a particular challenge. But this research shows that small acetal parts don't have to be removed from the bulk of automotive plastics; the combination of acetal and PP makes a commercially viable recycled resin. "These findings let the designer continue to use the best material for each separate application without compromising recycling efforts," said Bitritto.
The resins used in the study met the material specs of a major automaker. They were blended by twin-screw extruder and then molded; scanning electron microscopy showed that the acetal copolymer was uniformly distributed through the PP. The findings have implications for auto designers who want to use engineering resins while facilitating recycling of bulk materials.
An actual application further illustrates the potential. A manufacturer needed a small support rod; it wasn't strong enough in virgin PP, but too expensive in virgin acetal copolymer. The solution: an 85/15 blend of unfilled PP waste and 25 percent glass-reinforced reclaimed acetal; it had greater flex strength, greatly increased flex modulus, and a higher notched Izod than straight PP. Bitritto explained that the molder had a lot of PP scrap with no use. "By adding low-cost acetal scrap, the company could improve properties and satisfy a real requirement with a low-cost material."