Using a vacuum bagging process and scanning electron microscopy (SEM), the mechanical interlocking between wood species from small-diameter logs and high-density polyethylene (HDPE), without coupling agent and additives, has been studied. The pressure and high temperatures assist melting and flowing of the HDPE mainly through the radial face (tangential direction) in small softwood samples, generating a three-dimensional interpenetration of the thermoplastic into the cell wood structure. As a result, a contact interfacial area HDPE-cell wall appears. According to SEM analysis, the presence of simple pits, their size, and distribution on the cell wall create a potential path for the transverse movement of HDPE. Also, the collapse of cell walls under pressure during the vacuum bagging experiment was identified as a competing phenomenon avoiding the free flow of the molten thermoplastic. Penetration and interface area are significantly affected by the presence of earlywood or latewood in the wood sample. Wood species that presented a high interface area have the potential for better mechanical interlocking. The interface area was directly related to the viscous constant of the Maxwell model which may describe the interlocking mechanism of wood-plastic composites. It was observed that anatomical features of wood species play an important role in a real extrusion trial. The relation of cell wall thickness/lumen diameter for each wood specie plus the interconnectivity between wood cells in a wood flour particle through flow paths generated by pits, domain the collapse and potential penetration of the thermoplastic into the natural filler, was analyzed as short fiber.
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