It can be seen from Fig. 1 that after the pyrolysis under 600 �C, the asymmetrical O–H stretching vibration, the aliphatic C–H stretching vibration, and the aromatic C¼C vibration of poplar have disappeared. It is clear from the Fig. 3 that the number of these three functional groups decreased with the addition of PB, which is the result of a reduction in the weight fraction of HDPE; there is no C–O stretching vibration in neat HDPE (Fig. 3) but found in PB (Fig. 1), which indicates that this functional group was caused by PB. The above analysis shows that all the functional groups came from PB or HDPE. No new functional groups were produced in the forming process of the PB/HDPE composites, which indicated that there were no chemical reactions between PB and HDPE [34].