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The objective of this study was to investigate the mechanical properties of wood/plastic composites (WPCs) produced using wood flour (WF) prepared by wet ball-milling under various milling times (0-120 min) and drying methods (freeze-or heat drying). The drying method did not affect the particle size distribution, shape, or specific surface area of WF at milling times shorter than 40 min. At milling ≥ 40 min, freeze-dried ball-milled WF (FDWF) had smaller particle sizes and higher specific surface area than heat-dried ball-milled WF (HDWF). The highest tensile strength and modulus of rupture (MOR) were observed in WPCs made from freeze-and heat-dried WF at a milling time of 30 min. At milling time of 30 min, the amount of 100-300 µm FDWF and HDWF was 37% and 36%, respectively. The impact strength of WPCs increased, as the milling time increased. The amount of small freeze-and heat-dried WF particles increased due to an increase in the amount of 17 µm particles and specific surface area with increased milling time. Thus, impact strength of WPCs increased as particle size decreased. At milling times ≤ 60 min, there were no significant differences in mechanical properties between WPCs containing freeze-and heat-dried WF under the condition of this study.

Commercial wood flour of pine (Pinus densiflora) was used as an experimental material. It was milled with different milling times 0, 10, 20, 30, 40, 60, and 120 min to investigate the effect of their particle size distribution on the mechanical properties of the wood-plastic composite. Two kinds of drying conditions, 7 days of freeze-drying at a temperature of −45 °C and heat drying (80 °C) for 24 h were applied. Polypropylene and maleic anhydride-grafted polypropylene were used as a matrix and the compatibilizer of the wood-plastic composite compound, respectively. Particle size analysis showed that an increase in the time of ball milling decreased the wood flour size. However, an excessive milling period of time longer than 40 min corresponded to an increase in the amount of aggregation. Scanning electron microscope images showed the existence of aggregation for the wood flour with 120 min of wet milling time. As the particle size decreased, mechanical properties of wood-plastic composite increased for up to 30 min of wet milling time and then slightly decreased. This behavior might be due to aggregation. The optimum mechanical property was obtained at 30 min of wet milling time under freeze-drying conditions.

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