| The polymer processing technique of pultrusion can be utilized to produce wood-based composites, however, limitations occur in the ability of the composite system to flow through the stationary die. The consolidation and frictional response of the wood composite impose the resistance to flow as the material is conveyed through the die through a pulling action. The objectives of this research is to describe the mechanisms that govern the consolidation and frictional behavior of a wood fiber composite, how these mechanisms respond to the pultrusion conditions and composite design, and their influence on application and process engineering of a pultrusion operation. Utilizing a non-woven wood and wood/polypropylene (PP) fiber mat, the uniaxial compression and sliding frictional response was tested and described. The description of consolidation and friction were then incorporated into a pulling force model and compared to experimental pultrusion runs. The description of stress during consolidation was based on the relative density and instantaneous modulus, while a generalized Maxwell model fit the ensuing relaxation response. The consolidation and friction response was controlled by the viscoelastic nature of the composite, and the mat structure and composition. Through spectral analysis, the mechanisms of the relaxation behavior were found to be interparticle movement and particle deformation. Predicted pulling force at a die temperature of 170°C was found in close agreement with experimental values, however a deviation was observed at higher temperatures. A reduction in pulling load can be obtained with lower densities and higher temperatures with increased PP contents. It was demonstrated that wood and wood/PP composites can be pultruded successfully and with the use of a pulling force model, the mechanisms of pulling resistance can be identified. |
