Mechanical properties of wood/plastic composites formed using wood flour produced by wet ball-milling under various milling times and drying methods
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.
KeywordsWood/plastic composites Wet ball-milling Drying methods Mechanical properties Physical properties
maleic anhydride-grafted polypropylene
freeze-dried ball-milled wood flour
heat-dried ball-milled wood flour
scanning electron microscope
modulus of rupture
modulus of elasticity
Wood/plastic composites (WPCs) are mixtures of wood flour (WF) and thermoplastic resins, such as polypropylene (PP), polyethylene (PE), or polyvinyl chloride (PVC). WPCs can be fabricated from environmentally friendly materials, such as wood waste, unused natural resources, and recycled thermoplastic resins [1, 2]. WPCs have many excellent properties such as high durability, specific strength, specific stiffness, and resistance to wear. They also have high molding performance and a texture similar to that of solid wood. The main application of WPCs is in the manufacture of exterior decking. However, for WPC technology continues to mature as manufacturing processes improve, WPCs can also be used in other industries, such as the automobile and consumer electronics sectors [2, 3].
WF is among the determinants of the mechanical properties of WPCs. Since WF is hydrophilic, it is necessary to overcome its incompatibility to hydrophobic thermoplastic resins to improve the mechanical properties of WPCs. Physical treatments such as corona, plasma, and ionizing radiation, and chemical treatments such as esterification and acetylation, have been applied to WF to improve compatibility [4, 5]. However, these treatments require large amounts of energy or the production of considerable hazardous waste. Therefore, small amounts of compatibilizers (e.g., maleic anhydride-grafted polypropylene, MAPP) are generally added to WPC. WF parameters such as particle species, size, and shape are also important determinants of the mechanical properties of WPCs. In general, commercial WF with particle sizes of 100–300 µm is produced by dry milling by a hammer mill or cutter mill [1, 6, 7]. In a study by Stark and Berger , the tensile strengths of WPCs containing oak, maple, and ponderosa pine WF were shown to be similar. In addition, the tensile and flexural strengths of them had the highest value at 20–40 wt% WF content. In addition, the tensile strength and modulus of rupture (MOR) of WPC containing 235-mesh (> 63 µm) WF were lower than those of WPC containing 70-mesh (> 185 µm) WF. Salemane and Luyt  reported that the tensile strength of WPCs containing WF with particle size < 38 µm was lower than that of WPCs containing WF with particle sizes of 38–150 µm and 300–600 µm. In addition, in the case of same size filler, Stark and Rowlands  reported that WPCs containing high aspect ratio filler had higher mechanical properties than that containing low aspect ratio filler.
WF formed by ball-milling is also used to produce WPCs [11, 12, 13]. The tensile and flexural strengths of WPCs containing wet ball-milling WF had high strength by surface fibrous structure on WF [11, 12]. The strength did not improve in the WPCs containing dry ball-milling WF . Thus, the size and shape of WF prepared by wet ball-milling effectively improve the mechanical properties of WPCs. However, the mechanical properties of WPCs containing WF that had been wet ball-milled shorter than 2 h were not evaluated.
Therefore, the objective of this study was to investigate the effect of size and shape of WF wet ball-milling at times shorter than or equal to 2 h on the mechanical properties of WPCs.
In the previous studies, milled WF was vacuum- and freeze-dried [11, 12, 13]. During heat drying, WF becomes easily aggregated through rapid water vaporization. However, it has been assumed that WF aggregation during heat drying is negligible with short milling times. Therefore, we also evaluated the effect of the drying method on ball-milled WF (BMWF) in this study.
Materials and methods
The raw material for this study was Japanese red pine (Pinus densiflora) WF with a particle size of about 2 mm. PP (J-107G; Prime Polymer Co. Ltd., Japan) as a matrix was a homopolymer with a melt flow rate of 30 g/10 min (230 °C/2.16 kg) and a density of 0.9 g/cm3. MAPP (Kayabrid 006PP-N; Kayaku Akzo Co. Ltd., Japan) as a compatibilizer were used. The MAPP powder contained 2 wt% maleic anhydride and had an average molecular weight of 75,000.
Preparation of wood flour
Ball-milling was performed using a planetary ball mill (Pulverisette 6; Fritsch Japan Co., Ltd., Japan). For each load, WF (13.5 g) and distilled water (200 mL) were milled with 25 balls (diameter: 20 mm) in a cycle consisting of 10 min of milling, followed by a 10 min pause, for the prescribed time. This step was the same as previous study . The ball-milling rotational speed was set to 200 rpm and the milling times were 0, 10, 20, 30, 40, 60, and 120 min. The BMWF was dried under two types of drying processes: heat drying and freeze-drying. The conditions for freeze-drying were − 45 °C for 168 h in a freeze-dryer (FDD1200; Tokyo Rikakikai Co. Ltd., Japan). The conditions for heat drying were 80 °C for 24 h in an oven dryer (SOFW-600; AS ONE Co. Ltd., Japan). The BMWF after drying was crushed using a mixer (IFM-800DG; Iwatani Co. Ltd., Japan). The products of these processes were termed freeze-dried BMWF (FDWF) and heat-dried BMWF (HDWF), respectively.
Wood flour characteristics
A laser-diffraction particle size distribution analyzer (Partica LA-9502; Horiba Ltd., Japan) was used to obtain the particle size distributions of BMWF, FDWF, and HDWF. FDWF and HDWF surface morphology was observed using a scanning electron microscope (SEM) (JSM-6510LV2; JEOL Ltd., Japan). FDWF- and HDWF-specific surface area (Brunauer–Emmett–Teller area) was measured using the nitrogen adsorption method with a specific surface area and pore size distribution analyzer (Gemini 2360BELSORP-mini II,; MicrotracBEL Ltd., Japan). Prior to measurements, WF was dried at 105 °C for 6 h under a flow of N2.
Preparation of WPCs
The materials were blended at a dried BMWF/MAPP/PP ratio of 25/1/74 (wt%). The WF content was lower than general WPC products to avoid interaction WF itself. Compounds of dried BMWF/MAPP/PP were produced at 190 °C for 13 min at a rotary speed of 30 rpm using a twin-screw kneader (Laboplast Mill 30R150; Toyo Seiki Seisaku-sho Ltd., Japan). The compounds were crushed using a low speed axial crusher (SA-23; Stolz Co. Ltd., Japan) into a powder with particle size < 10 mm. The crushed compounds were then melted and mixed using a micro-compounder (Micro5 cc Twin-Screw Compounder; DSM Xplore, The Netherlands) at 190 °C for 5 min at a rotary speed of 50 rpm. An injection molder (Micro5.5 cc Injection Molding Machine; DSM Xplore, The Netherlands) was then used to mold the composites into two types of specimens at 190 °C, with an injection pressure of 1.6 MPa. The dimensions of dumbbell-shaped specimens produced for the subsequent tensile test were about 50 × 4 × 2 mm, and those of rectangular specimens produced for bending and impact tests were about 50 × 6 × 2 mm. All specimens were stored at 20 °C and 65% relative humidity for 1 week before each test. The moisture content of all WPC specimens was about 0.5%.
Mechanical properties of WPCs
WPC performance was evaluated by tensile, bending, and impact tests. The tensile and bending tests were conducted according to the JIS A-5741 protocol . Tensile strength was tested using a universal testing machine (AGS-5kNX; Shimadzu Co. Ltd., Japan) with a crosshead speed of 20 mm/min and a gauge length of 30 mm. A three-point bending strength test was conducted to calculate the MOR and modulus of elasticity (MOE) using a universal testing machine (Yasui Kiki Co. Ltd., Japan) with a crosshead speed of 5 mm/min and a span length of 32 mm. An unnotched Izod impact test was conducted according to the JIS K-7110 protocol  using an impact tester (U–F impact tester; Ueshima Seisaku-sho, Japan) at a speed of 3.5 m/s and impact energy of 2 J. Five specimens were tested for each mechanical test. SEM was used to observe the broken surfaces of WPCs after tensile testing at milling times of 0 and 30 min.
Results and discussion
Evaluation of wood flour
In total, 14 types of FDWF and HDWF under different milling times and drying methods were prepared in this study. However, we could not measure the particle size distribution of HDWF at a milling time of 120 min. Therefore, we will not discuss the mechanical properties of WPCs containing FDWF and HDWF produced at a milling time of 120 min.
Mechanical properties of WPCs
The differences in WF size and shape due to different drying methods did not affect mechanical properties in this study. There are the possibility that size and shape of FDWF and HDWF inside WPCs did not larger differ at same milling time because of defibration of aggregated HDWF or inversely the aggregation of small size FDWF during mixing and molding for WPCs. It will be proven by measuring size and shape of WF inside WPCs. This is a matter for future investigation. However, it is a significant finding that WPCs containing HDWF have the same mechanical properties as those containing FDWF under the condition of this study, because heat drying requires less energy and shorter drying times than freeze-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 times longer than or equal to 40 min, FDWF particles were smaller and had higher specific surface area than HDWF particles.
The highest tensile strength and MOR among WPCs were observed for both FDWF and HDWF at a milling time of 30 min. In the case of FDWF, trends in tensile strength were consistent with trends in 100–300 µm particle size behavior.
Impact strength of WPC increased as milling time increased, possibly due to an increase in the amount of small WF particles.
At milling times shorter than or equal to 60 min, there were no significant differences in the mechanical properties of WPCs containing FDWF and HDWF. Different drying methods did not affect mechanical properties of WPCs under the condition of this study.
KM, TU, and YK have participated sufficiently in the work to take public responsibility for entire of the content of the manuscript. HK, SS, KA, HI, SO, and MO have participated sufficiently in the work to take public responsibility for part of the content of the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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