Wood flour is used widely as filler for plastic materials due to its low price, easy availability and bio-based nature. However, there is an increasing interest in using wood fibres instead of wood flours: the fibre shape can provide reinforcement instead of just filling and therefore increase mechanical properties of the composite material [1]; [2]. Typical wood plastic composites (WPC’s) are based on polypropylene (PP), polyethylene (PE), or polyvinyl chloride (PVC) matrix filled with wood flour [3]. In addition to using conventional WPC matrix polymers, there are also studies related to, e.g. polystyrene (PS) [4] and polylactic acid (PLA) [5]; [6] matrixes, from which especially PLA as a bio-based and compostable polymer has been of growing interest.
The characteristics of wood fibres depend mostly on their source. The average lengths of softwood and hardwood fibres are 3.3 and 1.0 mm, diameters 33 and 20 lm, stiffness 10–50 and 10–70 GPa, and strength 100–170 and 90–180 MPa, respectively [7]. The fibre pulps produced by paper making industry also differ from their properties depending on how the fibres are separated. Two common pulp types utilized in the paper industry are chemical and mechanical pulp. In chemical pulping, the fibres are separated by dissolving and removing the lignin that bonds fibres together resulting for long and less damaged fibres, whereas in mechanical pulping lignin is softened with heat and repeated mechanical stress. Therefore, mechanical pulps contain a high amount of lignin whereas chemical pulp is almost lignin free. In addition, the fibre length distribution is wider and fines content is higher in mechanical than chemical pulps. In the literature, mechanical pulp fibres are described as more stiff, coarse and straight than chemical pulp fibres [8].
The reinforcement capability of fibres depends on the fibre properties, the interfacial bond between the fibre and the polymer matrix, and the critical fibre length [9]. The critical fibre length lc is defined by fibre diameter d, fibre tensile strength rf and fibre– matrix bond strength sc, according to