Evaluation of the processing conditions on the production of expanded or plasticized wood plastic composite with cashew nutshell powder

Abstract In this work we investigate the influence of closed (“C”) or open (“O”) extrusion degassing, and low (5 MPa – “L”) or high (90 MPa – “H”) holding pressure during injection molding on the properties of wood-plastic composite (WPC) based on high-density polyethylene (HDPE) flakes compatibilized with 5 wt% maleic anhydride-modified polyethylene (MAPE), 5 wt% lubricant (Struktol® TPW 113), and 40 wt% cashew nutshell powder (CNSP). Two reference compounds were extruded with closed degassing and injected at 90 MPa of holding pressure: (1) HDPE flakes extruded with 5 wt% MAPE and 5 wt% Struktol® TPW 113 (HDPEad_C-H) and (2) HDPE flakes previously melt mixed in an internal mixer with 5 wt% CNSL (HDPEr/5CNSL) and then extruded with the same additive content (HDPEad/5CNSL_C-H). At 90 MPa holding pressure, the extract content of WPC slightly increased and CNSL acted as a lubricant and poor plasticizing agent, reducing the crystallinity and density of the additivated matrix (HDPEad_C-H), increasing the flow rate without significantly changing the tensile strength. On the other hand, low holding pressure (5 MPa) favored the volatilization and expansion of the residual CNSL in the WPC, which acted as a blowing agent improving filling of the mold cavity and avoiding sink marks. Sample extruded with open degassing (atmospheric pressure) did not significantly change the extract content, but favored CNSL diffusion from particles to matrix, which subsequently during injection molding led to its entrapment at the interface, resulting in low adhesion, especially at high holding pressure that hinders CNSL expansion and vaporization. These findings contribute to understand the role of residual CNSL of CNSP in WPC properties and thus, to strengthen the plastics recycling chain and reduce carbon footprint.