Improving the Properties of Polyolefin Waste by Reactive Processing

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A substantial portion of plastics waste consists of a mixture of polyolefins, i.e., polyethylenes of low and high density and polypropylene. These materials are immiscible and generally incompatible. To achieve good properties of the mixtures suitable for more demanding applications, the blends need to be compatibilized. The possibility of formation of compatibilizers in situ was investigated via reactive processing initiated by thermal decomposition of organic peroxides. The idea is to create propylene–ethylene block copolymers via controlled cocrosslinking on the phase boundaries. The first problem to be dealt with is a degradation of polypropylene macro radicals in radical initiated reactions. It is demonstrated that an addition of appropriate coagent of cross–linking together with peroxide results in a substantial suppression of fragmentation reaction in favor of macroradicals recombination process. Rather high processing temperature of polypropylene results in difficulties if peroxide is mixed into the blend, since premature peroxide decomposition resulting in crosslinking to a high degree may occur. Two attitudes have been developed, one consisting of a modification enabling the process to be performed at a lower temperature, another based on reactive crosslinking during processing. The first technology results in a thermosetting, highly crosslinked material with excellent toughness while the latter process leads to a thermoplastic blend with acceptable reprocessability. The procedure outlined above may also be used for compatibilizing the mixtures of polyolefins (PE, PP, or their blend) with organic fillers, such as wood flour, switch grass, or recycled paper. Interesting materials with versatile properties and a great application potential are formed by such a way.