Compounding of Wood-Polymer Composites on Twin Screw Extruders
Wood fillers are mixed with polymers in twin screw extruders to enhance/tailor the final product to achieve desired mechanical properties at a lower cost. This is accomplished through distributive and dispersive mixing of the various components by shear forces.
Wood fibers are heat and shear sensitive. To avoid degradation screw geometries must be selected to perform high mixing, while also minimizing peak shear and viscous heating. As solid resins melt the viscosity is high, thereby producing high stress rates in the early stages of the extruder, which can cause degradation of wood fibers. In the latter stages of the extruder the viscosities have fallen, so that even high strain rates factored against a decreased viscosity do not produce large stress rates. In this region of the screws wood particles can be effectively mixed without degradation. For this reason wood fibers are often introduced into a process melt stream via a side stuffer. The same process analysis also holds true for virtually all heat and shear sensitive materials.
