Full factorial studies were conducted to determine the effects of a coupling agent (a low molecular weight maleated polypropylene (MAPP)) and other composition and processing variables on the mechanical properties of a wood-flour-filled polypropylene (PP) composite. Effects of MAPP on the bonding between PP and wood veneer were also examined. At less than 1 percent by weight, MAPP produced useful increases in strength and modulus properties of the composite, and this effect was somewhat enhanced by small-particle-size wood flour and multiple extrusions. However, MAPP caused small losses in notched impact energy. High extrusion temperature (190°C to 250°C) had little influence on strength, but it decreased notched impact energy. Peel force between PP and wood veneer was increased by pretreatment with MAPP for aspen, but not for birch, aspen being more porous than birch. The effectiveness of MAPP may therefore be related to its ability to penetrate the wood and form a strongly held hydrophobic layer that is attractive to the PP, thereby increasing both the effective bonding area and mechanical interlocking.

Polypropylene (PP)-wood veneer laminates were used as a model system to investigate adhesion in wood-polypropylene composites. Wood veneers were treated with maleated polypropylene waxes (MA-PP). PP films were then compression molded to the wood surfaces and peel forces were measured.Low MA-PP treatment levels increased the peel adhesion over that for untreated surfaces. High MA-PP treatment levels decreased the peel adhesion and intermediate MA-PP levels had no effect on the peel adhesion. Microscopy of the fracture surfaces indicated PP penetration into lumens in both treated and untreated wood veneer. Untreated surfaces also exhibited PP penetration into pits and intercellular spaces, while treated surfaces exhibited only hindered penetration on this scale. The penetrated PP formed tendrils during fracture. DSC of PP on wood and cellulose surfaces showed higher PP crystallization temperatures on untreated surfaces than on treated surfaces.

Wood strip surfaces (Aspen and Birch) and cellulose fibers (Birch, pulped paper fibers) were treated with azidosilane coupling agent, via immersion in methylene chloride and methanol solutions respectively, and then subsequently dried.Polypropylene films were compression molded to the wood surfaces and peel forces were measured.The treated cellulose fibers were suspended and mixed with polypropylene fibers in water, formed into thin sheets by wet forming, dried, and compression molded to melt the polypropylene.Treated wood surfaces gave two to six times the dry peel force versus untreated wood surfaces.However, the wet peel force of treated wood degraded to that of untreated wood.The elongation, fail stress, and tensile energy absorption of treated molded sheets were all increased over the untreated fibers for both dry and wet testing; the wet-tested properties were especially enhanced by the silane treatment.