Development of composite materials by mechanochemical treatment of post-consumer plastic waste
Introduction
Post-consumer mixed plastic waste consists of a wide variety of polymer types which originate from an almost random selection of suppliers and processes. Recent studies on both UE and national MSW composition have surveyed the volumes and contents of plastic waste. The largest fraction of waste consists of polyolefins, such as polyethylene (PE) and polypropylene (PP) (60–70% and the remaining components include polystyrene (PS 10–15%), polyvinylchloride (PVC 15%) and polyethyleneterephtalate (PET 5%).
The low recycling rate for mixed plastic waste is largely due to the lack of cost-effective recycling technology; in addition, contaminants such as paper and glue additives can potentially affect the physical properties of recycled polymers as well as their prospects for application. The main drawbacks of mechanical recycling are related to the selection of waste into polymer types and to loss of properties during final reprocessing.
Low temperature, high energy ball milling of mixed polymers is an effective technique which can be used to co-pulverize polymer powder particles or shredded polymer films, thus promoting a substantial size reduction and creating mechanochemical effects on the milled material (Shaw, 1998, Bai et al., 2000, Nesarikar et al., 1997). In fact, the energy transfer promoted by the milling system causes impact, compressive and shear forces on the different polymer powder particles and the combination of such mechanical effects induce chain scission with free radical formation (Casale and Porter, 1996). Reaction of free radicals from different chain species of intrinsically incompatible polymers can easily couple and produce a stable blend.
Recently a new, near room temperature, high energy ball milling technique has been developed (Padella et al., 1998, Padella et al., 1999). The method consists in enhancing mechanochemical effects promoted by the milling action through the insertion of a quantity of liquid CO2 in the milling vial. In this condition, the energy transfer from the milling device to the milled system, that is achieved at each hit event, promotes repeated microexplosive evaporation of liquid CO2 which is trapped between the ball and the vial wall. The described phenomena enhance the effectiveness of energy transfer and the mechanochemical compatibilization of the blend occurs in a very short milling time (10–60 min) (Padella et al., 1998).
In this work, such a technique has been applied on polymeric materials. Post-consumer mixed plastic waste as well as selected mixed and single phase plastics have been investigated in order to better understand the chemical phenomena which are induced by the process and evaluate the possibility of utilizing compatibilized mixed polymeric waste as a matrix for composite, low cost, high strength material. Short polyester fibers (coming from tire dismantling) have been tested as a reinforcing material.
Section snippets
Experimental
As with the procedure which is elsewhere described (Padella et al., 1998, Padella et al., 1999, Cavalieri et al., 2000), a single milling treatment consisted in 1 g of polymeric material which was placed in a suitable high pressure cylindrical stainless steel vial (60 cm3) with 150 g tungsten carbide balls (8–15 mm diameter) and 20 g of carbon dioxide. To facilitate CO2 manipulation, the compound was put into the vial in a solid state. The vial was closed immediately after its filling. After a
CO2 enhanced high energy ball milling
Usually cryogenic condition and long milling time (12–24 h) are needed to mill polymer blend (Pan and Shaw, 1994). As well known, in a ball milling device an energy transfer to the milled powder occurs during repeated hits between balls and between ball and vial walls (Magini and Iasonna, 1995). Reported process uses liquid CO2 as energy transfer medium, overcoming the necessity to weaken milled polymers, as in cryogenic process. Unlike the traditional ball milling process, when a single hit
Conclusions
Using a very fast mechanochemical process, a composite polymeric material has been successfully developed starting from post consumer mixed polymeric waste and short polyester fibers which come from tire dismantling. The mechanical properties were investigated. Resulting values show that the obtained material is characterized by properties that approximate and, in some cases, overcome the property of virgin material. The material can be utilized for non-critical structural applications, and its
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