7 巻 (2011) 1 号 p. 96-107
Background and Objective. Material recycling of waste plastics is generally categorized into “closed-loop recycling”, where waste plastic products are recycled into resin material used for the same kind of products, and “open-loop recycling”, where waste plastic products are recycled into resin material used for other kinds of products. When life-cycle assessment (LCA) is applied to evaluate material recycling of waste plastics, a crucial factor is what material is actually substituted by recycled material from the investigated products: (i) virgin material, (ii) recycled material from other kinds of products, or (iii) no material at all in case recycled material is used for functions that would otherwise not be generated. The mix of materials (i)-(iii) substituted by an increased outflow of recycled material depends on how the market reacts to it. In this study, the methodology of LCA for closed-loop and open-loop recycling of waste plastics was summarized according to the substituted materials (i)-(iii) in terms of simplified system boundaries, and then a framework of LCA for material recycling of waste plastics based on market substitutability were presented. The framework was applied to a case study of material recycling of post-consumer polyethylene terephthalate (PET) bottles.
Results and Discussion. The mixed share of substituted materials (i)-(iii) affected by an increased outflow of closed-loop or open-loop recycled material was formulated on the basis of the price elasticity of supply of and demand for recycled and virgin materials. The results of the case study showed that the share of open-loop recycled material which substitutes no material depended on the price elasticity of supply of and demand for non-bottle grade PET resin, and as a result, CO2 emission and fossil resource consumption of open-loop recycling could be larger than those of closed-loop recycling and incineration of post-consumer PET bottles. In addition, the uncertainty regarding the final products of open-loop recycling for which recycled PET resin were used could be important for the results of LCA based on market substitutability.
Conclusions. The essential difference between closed-loop and open-loop recycling in the framework of LCA based on market substitutability lies in whether the demand for the whole material including recycled and virgin material is increased or not with an increase in the outflow of recycled material. If the demand for the material can be assumed to be constant against an increase in the outflow of closed-loop recycled material, closed-loop recycling which substitutes virgin material of the same kind has advantage over open-loop recycling which possibly substitutes no materials.