In the present study, we investigated the effects of mixing time on the dispersion and degradation behaviors and mechanical properties of high density polyethylene (HDPE) / ultra high molecular weight polyethylene (UHMWPE) blends. Optical microscopy revealed that the dispersion of UHMWPE in HDPE was improved by increasing mixing time, and the UHMWPE was almost homogeneously dispersed with HDPE at the mixing time longer than 250 s. Thermal analysis revealed that molecular dissolution of UHMWPE into HDPE occurred even in the early stage of mixing below 250 s. On the other hand, gel permeation chromatography and infrared spectroscopy suggested that as the mixing time increased, the thermal decomposition producing trans-vinylene group and the oxidation reaction producing carbonyl group proceeded simultaneously, resulting in the progress of chain scission. The chain scission was noticeable above 250 s. In the tensile test, the influence of dispersibility on the tensile properties became dominant at the initial stage of mixing, and both the strength and strain at break were improved by increasing the mixing time up to 250 s, although these properties decreased for the mixing time longer than 250 s. In addition, from the discussion of mixing parameter, it is suggested that the improvement in dispersion and the progress of degradation accompanying the increase in mixing time occurred due to an increase in total rotation number and thermal history. Therefore, it is important to select an appropriate mixing time to improve the dispersion of UHMWPE without degradation.
