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With society focusing more and more on environmental issues, the recycling of materials of all types has become an important concern. As a result of previous processing and usage, the properties of recycled materials are often significantly inferior when compared to their unrecycled counterparts. Recycled polymeric materials, for example, have lowered molecular weights caused by previous thermal and/or shear histories. Thus, the scope of application for recycled materials has traditionally been very limited. With the aid of a new processing concept known as vibration-assisted injection molding (VAIM),the mechanical properties of products containing recycled polymeric materials could be dramatically improved. The current paper presents the results of an initial experimental investigation into the application of VAIM for recycled polystyrene (Recycled-PS) products. A number of recycled/new material blends were studied, and in all cases significant product mechanical property enhancements were realized through the use of VAIM. It was concluded that VAIM molded products containing as much as 50% low-grade recycled material could be made stronger than products made using traditional molding with
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100% new polymer. Detailed results are presented in the paper, along with some insight into the likely fundamental mechanism underlying the utility of the VAIM technique.
Introduction 23979
The recycling of plastic materials, which has been explored for a long time for ecological and economical reasons, has been accomplished to some degree . A strong demand exists, however,for much more success in this area. Recycling can reduce the quantity of municipal solid waste （MSW） that requires further treatment and ultimate disposal, and regulating bodies across the U.S. have adopted goals for the amount of MSW to be recycled in an effort to reduce the dependence on landfills and incinerators.
In general, recycled materials are considered to be of lower quality than their unrecycled counterparts . This is because the properties of materials(later to be recycled ) deteriorate during processing and usage. The major causes of quality losses in recycled polymer materials are UV exposure and degradation due to thermal, chemical, and shear history. Depending on the chemical structure and existing degradation condition, polymers degrade by chain scission. This can occur randomly along the chain, at the chain midpoint, and/or at the chain end. The net result is that the fundamental polymer chains are broken, the molecular weight becomes lower, and hence the associated material properties are reduced.
Recycled polymers can be categorized into two types—postindustrial and postconsumer plastics. Post-industrial recycled plastics are usually runners and sprues resulting from injection molding processes. These recycled plastics are used frequently by blending them in with new or virgin polymer. Typical virgin/recycled material blends utilize recycled plastic levels of up to 30% by weight. The major reasons for using recycled materials during injection molding are better flo w, lower cost, and a minimization of final material waste levels. Better flow into molds is achieved because recycled polymers usually have lower viscosities than their unrecycled counterparts. The lowered viscosity is the result of fewer entanglements associated with the lowered molecular weight. While this can have a beneficial effect as far as mold filling is concerned, it likewise reduces the mechanical performance of the final products. In the case of general-purpose polystyrene materials, lower molecular weights lead to more brittle behavior and, hence, a loss of material toughness.