Process for continuously manufacturing composites of polymer and cellulosic fibres, and compounded materials obtained therewith

A polymer is fed upstream into an extruder. The polymer is melted and mixed in a zone (A). Cellulosic fibres are fed into the extruder in a zone (B). A mixture of the polymer and the fibres obtained in zone (B) is transported through a degassing zone (C) and then through a pressure building zone (D). The mixture is released from zone (D) into a die. Each of zones (A), (C) and (D) includes at least one positive transportation screw element. The zone (B) includes at least one positive transportation screw element, at least one kneading section and at least one negative transportation screw element such that the cellulosic fibres are fibrillated to obtain cellulosic fibres with an aspect ratio as high as possible while simultaneously mixing the cellulosic fibres with the melted polymer.