639 Agronomy Research 13(3), 639–653, 2015 Effect of chemical modification of wood flour on the mechanical properties of wood-plastic composites H. Kallakas * , M.A. Shamim, T. Olutubo, T. Poltimäe, T.M. Süld, A. Krumme and J. Kers Department of Polymer Materials, Tallinn University of Technology, Ehitajate tee 5, EE19086 Tallinn, Estonia; * Correspondence: heikko.kallakas@ttu.ee Abstract. The poor compatibility between the highly hydrophilic wood fibres and the hydrophobic polymers is associated with a loss of mechanical properties. Therefore, to improve the interfacial adhesion between the polymer matrix and wood flour (WF), a chemical modification of WF is an appropriate solution. This study analyzes the influence of different chemical modifications of WF on the mechanical properties of wood-plastic composites (WPCs). WPC test samples were prepared from birch (Betula) WF with a mesh size of 0.63 mm as the filler material and polypropylene (PP) as the matrix material. WF was chemically modified by six different methods to increase its adhesion to, and compatibility with, the polymer matrix. The six chemical methods used were: alkaline (NaOH) modification, polyvinyl alcohol (PVA) modification, silane treatment with 3-aminopropyltriethoxysilane (APTES), acetylation with acetic anhydride, cyanoethylation, and wood fibre esterification. The composites were produced using a twin-screw extruder and the test samples were prepared by injection moulding. The composites’ mechanical properties (three-point bending test), Charpy impact strength and thermal properties were tested. In addition, SEM micrographs of WPC surfaces were generated. WF as a filler material enhanced the flexural properties, while impact strength decreased, making the material more rigid and brittle. The test results revealed that the chemical modifications of WF improved the mechanical properties and crystallinity of WPC materials, while the melting temperature decreased. However, the influence of the chemical modification on the mechanical and thermal properties of WPC varied by method. Key words: wood-plastic composite, wood flour, chemical modification, mechanical properties, thermal properties. INTRODUCTION In recent years, wood-based renewable composites have garnered much attention, both scientific and commercial, due to their light weight, eco-friendliness and low cost, as well as worldwide environmental awareness. Natural fibres are considered potential alternatives to glass or carbon fibres in fibre-reinforced thermoplastic composites because of their nature-friendliness, recyclability, biodegradability and low cost compared to mineral fillers (Ashori, 2008; Lu et al., 2008). According to previous research, natural fibres are less abrasive than inorganic mineral fillers and therefore cause little damage to moulding and mixing equipment (Acha et al., 2006). However, the usability of natural fibres in composite materials is still limited due to their hydrophilic nature, which leads to poor compatibility with the hydrophobic