Application of micromechanical models to tensile properties of wood–plastic composites

Wood–plastic composites (WPC) were produced with white birch pulp fibers of different aspect ratios (length-to-diameter), high-density polyethylene, and using two common processes: extrusion or injection molding. Three additive levels were also used: no additive, compatibility agent, and process lubricant. Fiber size was measured with an optical fiber quality analyzer. Tensile properties of WPC were measured and modeled as a function of fiber aspect ratio. Models were fitted to experimental values using the minimum sum of squared error method. A shift from the oriented fiber case (injection molding) to the randomly oriented fiber case (extrusion) was achieved using a fiber orientation factor. Fiber/matrix stress transfer increased with increasing fiber aspect ratio. Stress transfer was reduced with the use of process lubricant. Unexpectedly, the compatibility agent had the same effect. Fiber strength and stiffness contributions to the composite were lower than those of intrinsic fiber properties.