We previously reported that the inner structures of thick (>3 mm) injection-molded test pieces of virgin polypropylene and recycled pre-consumer polypropylene were very different, even though their molecular and mechanical properties were almost the same. This difference was thought to be caused by the molding history of preconsumer polypropylene. Due to this difference in their internal structures, thin films (<100 µm) of recycled preconsumer polypropylene were very brittle. By examining the

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... gradation properties of these thin films in response to UV irradiation, we found that the toughness of these samples decreased exponentially in a UV irradiation timedependent manner. This dependence was almost the same in virgin and recycled samples, and we could superimpose of the results. We also found that the UV irradiation time-dependence was associated with the number of molecules that connected domains within the samples. In this study, we investigated the mechanical and UV irradiation timedependent properties of thick samples composed of a blend of virgin and recycled pre-consumer materials. We found that the mechanical properties of these non-irradiated samples were almost the same. However, the mechanical properties of irradiated samples were significantly different. We also found that this difference linearly depends on the blend ratio. These results suggest that specimens containing recycled polymers can exhibit very poor durability even though there are no differences in their non-irradiated mechanical properties.