What is the most important is that the absence of the O–H and C–H in the PB (Fig. 1) could improve the compatibility of PB/HDPE composites, which was the important reason for the better flexural strength compared to fiber-polymers composites [37]. It is worth noting that the flexural strength of PB/HDPE composites decreased to 21.04 MPa sharply with the 70 wt% PB content. This could be attributed to the substantial reduction of HDPE, which significantly weakened the bonding effect. Higher PB contents agglomerate in less HDPE matrix and the stress concentration was easy to occur. Furthermore, the preparation of the composites with the 70 wt% PB content is more difficult than that of others, which proved again that the 70 wt% content of PB was not suitable for the molding of PB/HDPE composites.