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Weekly UpdatesABSTRACT
Low-density wood can be mechanically densified to produce value-added wood products. It is vital to have an understanding of the pressing parameters in order to fully utilize low-density wood and optimize its mechanical properties. Three pressing parameters (compression ratio, press temperature, and press closing time) were evaluated, and their effects on surface hardness, modulus of elasticity (MOE), and nail withdrawal resistance were determined. The purposes of this present study are to enhance the surface properties of aspen (Populus tremuloides Michx.) by mechanical densification and to optimize the densification process. Plainsawn specimens were prepared and mechanically densified on one face only using a softening-pressing-cooling technique. It was found that among the three chosen parameters, compression ratio was the most significant factor in influencing surface hardness, MOE, and nail withdrawal resistance. The effect of press temperature was not significant in most of the responses in this study. The optimum pressing conditions were found to be: compression ratio = 24.0 percent, temperature = 145 [degrees]C, and closing time = 7 minutes. After verifying the optimum pressing conditions, the hardness, MOE, and nail withdrawal resistance of surface densified aspen was improved by an average of 140, 23, and 132 percent, respectively. The hardness of surface densified aspen was almost equal to that of uncompressed red maple tested in this study. The surface densified specimens exhibited a glossy and smooth surface, suggesting that no surface coating would be required in end use.
Wood is widely used for furniture and flooring because of its excellent workability, good mechanical properties and beautiful appearance. For most nonstructural applications such as flooring, wood is subjected to indentation and abrasion in one form or another. This requires that wood possess a certain degree of surface hardness in order to reduce maintenance and replacement. Traditionally, dense hardwood species like white oak and sugar maple are used in applications where indentation loading is high. With a decrease in the supply of high-quality hardwoods, wood scientists and manufacturers are seeking alternatives and looking into low-quality materials for value-added uses. Aspen (Populus tremuloides Michx.), a relatively low-density species, if properly treated, could be an alternative material source.
Various efforts have been made to improve surface properties of wood. Inoue et al. (1990) introduced a new technique of producing surface-densified coniferous lumber. They produced lumbers from sugi (Cryptomeria japonica D. Don), hinoki (Chamaecyparis obtuse Endl.) and western hemlock (Tsuga heterophylla Sarg.). The technique they introduced involves making narrow grooves (2 mm wide and 5 mm deep) on the surface across the grain at 150 mm intervals to increase water penetration. They impregnated the surface of the lumber with water then heated specimens by microwave irradiation before compression in the radial direction. They reported an increase in abrasion resistance and hardness of 40 to 50 percent and 120 to 150 percent, respectively.
Wan (2004) studied different chemical treatments and processes to improve performance of Amabilis fir (Abies amabilis Dougl. Forbes), aspen (Populus tremuloides Michx.), Douglas-fir (Pseudotsuga menziesii Mirb. Franco), hard maple (Acer saccharum Marsh.) and western hemlock (Tsuga heterophylla Raf. Sarg.) for flooring applications. He found that chemical treatments increased the density and improved the dimensional stability and water absorption property of the five wood species. He also found that for a particular treatment, western hemlock had the greatest improvement in hardness and was even harder than untreated hard maple. However, the chemical treatment is costly and could raise an environmental issue. Gong et al. (2006) examined the maximum compression ratios of eastern white pine (Pinus strobus L.) and balsam fir (Abies balsamea (L.) P. Mill.) softwoods in Eastern Canada. They found that temperature and compression ratio significantly affected the relative change in thickness and peak load applied during compression. A wide range of research on wood compressibility has been reported (Norimoto 1993, Uhmeier et al. 1998, Ito et al. 1998, Ellis and Steiner 2002, Wang and Cooper 2005). However, limited publications are available regarding the effects of surface-densification parameters and their influence on mechanical properties of densified products.
The long-term objective of this study is to expand wood products markets and extend use of under-utilized, low-density wood species. The specific goal is to enhance the current surface hardening technology by mechanical densification to produce wood with improved surface properties that would suit selected end applications such as flooring.
Materials and methods
The average ovendried density of aspen used in this study varied from 0.37 to 0.45 g/[cm.sup.3]. Plainsawn specimens were prepared with dimensions of 25-mm (1-in) thick (radial direction) by 38-mm (1.5-in) wide (tangential direction) by 280-mm (11 in) long (grain direction). Specimens were air-dried and conditioned at a temperature 20 [degrees]C and a relative humidity of 65 percent prior to mechanical modification. Stratified sampling method was used to sort the specimens based on density distribution, i.e., all specimens with density outside the range [Mean [+ or -] 1.96 x Standard Deviation] were rejected. The sorted specimens were classified into three groups. Lowest density specimens were assigned in Group A while Group C had the highest density specimens. The groupings were used as blocks in the design of experiments.
Experimental design
Three factors were investigated in this study: compression ratio, press temperature, and press closing time. A general full factorial design with three variables was used in this study. Minitab[R] (Minitab Inc. 2005) was used in the design and analysis of experiments. Detailed information on the experimental design is given in Table 1. Compression ratio is defined as the ratio of the change in dimension to the original dimension and expressed in percent (Gong et al. 2006).
Three replicates were prepared for each run. The responses considered were surface hardness, modulus of elasticity (MOE), and nail withdrawal resistance.
Surface densification and mechanical testing
The mass and thickness of each specimen were measured prior to surface-densification. The surface to be densified for each specimen was then soaked (about 1-mm depth) in boiling water for 5 minutes. This process assisted in softening the cell walls in the surface layers of the wood specimen. The tangential surface to be densified was the side close to the bark. After soaking, the specimen was then placed in the laboratory hot-press for compression in the radial direction.
A 305-mm (12-in) by 305-mm (12-in) hot-press was used to compress a specimen. Compression ratio, press temperature and press holding time were given in Table 1. The face of a specimen to be softened was placed in contact with the hot platen of the press, and the other face was in contact with the cold platen. Loading rate was kept constant and press pressure was monitored while compressing a specimen. Press closing time was set at either 4 or 7 minutes. Once the designated closing time was reached, the specimen was kept under pressure and temperature for 5 minutes. Prior to removal of pressure, the platens were cooled to room temperature using the cooling system developed in the Wood Science and Technology Center of the University of New Brunswick. The compressed specimen was then removed from the press.
After compression, mass and thickness were determined. Then specimens were conditioned for at least 3 weeks at 20 [degrees]C and 65 percent relative humidity prior to mechanical properties tests.
After conditioning, specimens were cut to the desired lengths and sizes for moisture contenet (MC), specific gravity (SG), surface hardness, MOE, and nail withdrawal resistance tests with reference to American Society for Testing and Materials (ASTM 2004). The testing procedures and set-up were also referenced to ASTM standard. The span-to-depth ratio used in bending tests was 14.
Statistical analysis
All test results were subjected to analysis of variance (ANOVA). This was done using Minitab[R] software (Minitab Inc. 2005), which allows a statistical analysis of three factors with different levels in order to graphically describe the effect of the three factors on individual response of interest such as hardness strength and MOE.
For factorial designs, main effects, and interaction plots were used to show the effect of all the factors (compression ratio, press temperature or press closing time) at a particular point of the design space. The main effects plots were used to compare the relative strength of the effects across factors. A steep slope shows that the response is sensitive to that factor. A relatively flat line shows insensitivity to change in that particular factor. On the other hand, an interaction plot is a plot of means for each level of a factor with the level of a second factor held constant. An interaction between factors occurs when the change in response from the low level to the high level of one factor is not the same as the change in response at the same two levels of a second factor.
A statistical model for each response was determined based on the results of the statistical analysis. Once the model was established, response surfaces were plotted in a three-dimension graph with two actual factor variables and one actual factor constant.
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Multiple response optimization
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