Maximum Compression Ratios of Softwoods Produced in Eastern Canada

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1 Maximum Compression Ratios of Softwoods Produced in Eastern Canada Summary Meng Gong Research Scientist and Adjunct Professor Wood Science and Technology Centre, University of New Brunswick Fredericton, NB, Canada Makoto Nakatani Post Doctoral Research Fellow Institute of Wood Technology, Akita Prefecture University Noshiro, Akita, Japan Yin Yang Graduate Research Assistant Faculty of Forestry and Environmental Management, University of New Brunswick Fredericton, NB, Canada Muhammad T. Afzal Associate Professor Faculty of Forestry and Environmental Management, University of New Brunswick Fredericton, NB, Canada This study investigated the compressibility of two low-density softwoods, eastern white pine (Pinus strobes) and balsam fir (Abies balsamea). It was found that: (1) Both temperature and compression ratio have a strong effect on the relative change in thickness and the peak load applied during compression. (2) Unrecovered set increases with increasing compression ratio and temperature. (3) High temperature helps in softening water-saturated wood. (4) Minute buckling and cracks exist in the recovered specimens, even at a compression ratio of 5% and at room temperature, even though no visible cracks appear during compression. (5) Balsam fir seems more compressible than white pine. (6) For both species tested, the maximum compression ratio is approximately 60% at watersaturated condition and temperature ranging from 20 to 90 o C. 1. Introduction A large proportion of the conifers that grow in Canada have relatively wide growth rings resulting in low density and thin-walled cells. To better utilize what is often thought of as junk wood for structural purposes, this type of wood can be mechanically densified under appropriate combination of temperature and wood moisture content. The densification of wood is a good approach to add value to underutilized species because a higher strength can be achieved. Surface-densified lumber (Inoue et al. 1990) and compressively squared logs (Ito et al. 1998) are two pioneering products of this kind. A wide range of research has been performed to understand wood compressibility in order to provide a foundation for applying wood densifying techniques (Norimoto 1993, Uhmeier et al. 1998, Ellis and Steiner 2002, Wang and Cooper 2005). However, limited publications are available regarding the densification of Canadian wood species. The objective of this study was to investigate the compressibility of low-density softwoods produced in eastern Canada and to estimate their maximum compression ratios. The term compressibility refers to the degree of structural change in wood without sustaining unrecoverable

2 deformation in cell walls during densification. Compression ratio is the ratio of the reduced dimension to the original dimension. The maximum compression ratio is the critical value below which no damage is induced into wood cell walls during mechanical densification at a particular temperature and wood moisture content. 2. Methodology The species tested were eastern white pine (Pinus strobes) and balsam fir (Abies balsamea). The average oven-dried relative density values of pine and fir used in this study are about and 0.375, respectively. The dimension of a specimen was 30mm (grain direction) x 30mm (tangential direction) x 25mm (radial direction). Specimens were grouped based on density with each group having a similar average and standard deviation. Five specimens were tested at a given condition indicated in Table 1. Specimens were vacuum-soaked in water at room temperature (about 20 o C) until they were saturated. The water-saturated specimens of pine and fir had an average initial moisture content of about 180% and 200%, respectively. The specimens tested at higher temperatures were heated prior to compression. After densification, the specimens were stored in a conditioning chamber of 20 o C and 65% relative humidity until they reached equilibrium moisture content. Then, the specimens were oven-dried at 103 o C to measure their oven-dried dimension and mass values. Finally, the specimens were vacuum-soaked again in room temperature water, following by a 2-hour boiling treatment in water, with an aim at recovering the dimensions of the specimens. Table 1 Nominal and actual compression ratios used water-soaked specimens Species Temperature Compression ratio (%) ( o C) Nominal Actual Pine 50 Actual Actual Actual Fir 50 Actual Actual A specimen was compressed using an Instron machine in the radial direction to a pre-set deformation, held and unloaded as shown in Fig. 1. Two linear variable differential transducers (LVDTs) were used to measure the average deformation. A compressing device with two temperature-controlled hot platens was used to maintain a constant temperature during pressing. The nominal compression ratios ranged from 5% to a value not causing visual cracks. The actual compression ratios slightly differ from the nominal values as shown in Table 1. To facilitate discussion, only nominal compression ratios are used in this paper. The thickness and mass of each specimen was measured after each of abovementioned treatments. Relative change in thickness (radial dimension) was calculated using equation 1. initial thickness thickness at test relative change in thickness (%) = 100 [1] initial thickness An unrecovered set means the relative change in thickness after all treatments in this study, which reflects the damage (permanent structural changes in cell walls) in wood during densification. Selected specimens were used for damage examination using electronic scanning microscopy.

3 Fig 1 Experimental set-up (left) and compressing history (right) 3. Results and Discussion 3.1 Effect of species, temperature and compression ratio Fig 2 Main effects plot for average relative change (%) in thickness after air-drying treatment (AD) Fig 3 Main effects plot for average peak load applied during compression The commercial statistical software Minitab was used to analyze the effect of species, temperature and compression ratio (Minitab Inc. 2005). Two species (pine and fir), three temperatures (20, 50 and 90 o C) and three compression ratios (10, 25 and 50%) were used to determine their effects on the relative change in thickness at various conditions. Fig. 2 illustrates the main effect species, temperature and compression ratio have on the average relative change in thickness after the treatment AD (i.e. air drying). An analysis of variance (ANOVA) shows that temperature (P-value = 0.000) and compression ratio (P-value = 0.000) have a significant effect on the relative change in thickness at AD, but the effect of species (P-value = 0.858) is not significant. A P-value greater than 0.05 indicates no significant effect. Similar conclusions can be drawn regarding the effect these three influencing factors have on the average relative change after other treatments mentioned in section 2. The influence of the compression ratio on the relative change in thickness is more

4 significant than temperature. However, the ANOVA analysis indicates all three selected factors have significant effects on the peak load applied during compression tests as indicated in Fig. 3. The temperature has the strongest impact on the peak load, and the compression ratio is more influential than species. 3.2 Unrecovered set It can be seen from Fig. 4 that the unrecovered set increases with increasing temperature or/and compression ratio. Also, shown in here is that the compressed deformation could not be fully recovered after the treatments used in this study, suggesting that some permanent structural changes in wood cell walls (damage) might exist even for a compression ratio as low as 5% at room temperature. Fig. 4 also shows that the unrecovered sets of pine specimens are systematically larger than those of fir at various levels of temperature and compression ratio, suggesting that the compressibility of fir is better than pine. It would be reasonable to deduce that the compressibility of wood is largely dependent on its porosity at a given test condition. The porosity of pine and fir is estimated to be 73% and 75%, respectively, based on their oven-dried relative density values. Fir has a slightly higher porosity value, exhibiting a lower unrecovered set value. Unrecovered set (%) Fir: 20C Fir: 50C Fir: 90C Pine: 20C Pine: 50C Pine: 90C Compression ratio (%) Fig 4 Effect of temperature and species on unrecovered set at various compression ratios 3.3 Peak load applied during compression Fig. 5 shows that the peak load applied during compression increases with increasing compression ratio. This is a result of the elimination of cell lumens. The peak load decreases with increasing temperature. The water-saturated wood behaves much softer as temperature increases. It is reported by Irvine (1984) that the softening of water-saturated wood is mainly governed by the lignin response. Glass transition temperature (T g ) of native lignin ranges from o C at wet condition (Irvine 1984). A water-saturated wood specimen tested at 90 o C may reach T g, resulting in a lower peak load. At the same testing condition, pine achieves a higher peak load than fir because the density of pine is slightly greater than fir. Denser wood has a greater modulus of elasticity and achieves higher stress levels (Ellis and Steiner 2002). In addition, the difference in the volume of wood rays between pine and fir might, from the point of view of wood anatomy, have an impact on the peak load applied during compression. This will be examined in the future study.

5 14.00 Peak load applied (kn) Fir: 20C Fir: 50C Fir: 90C Pine: 20C Pine: 50C Pine: 90C Compression ratio (%) Fig 5 Effect of temperature and species on peak load applied during tests at various compression ratios 3.4 Observation of structural change in cell walls It was found in both pine and fir specimens that the visual cracks appeared during compression when the nominal compression ratios exceeded 65%. Visual cracks did occur, however, at a compression ratio of greater than 60% for the pine specimens tested at room temperature. No visual cracks were observed at compression ratios below the abovementioned values. However, buckling and possible minute fracture of cell walls were observed under an electronic scanning microscope (SEM) in those specimens experiencing all treatments (after re-soaking and 2-hour boiling) as illustrated in Figs. 6 and 7. The permanent micro-structural changes (i.e. buckling and minute cracks) were discovered in the cell walls of those specimens compressed at compression ratio of as low as 5% and room temperature. The SEM observation supports the existence of unrecovered set shown in Fig. 4, suggesting that compressed wood can not be fully recovered for pine and fir tested in this study. Fig 6 Balsam fir: uncompressed cell walls (left) and unrecovered cell walls (right, after recovery treatment) at compression ratio of 50% and room temperature

6 Fig 7 White pine: uncompressed cell walls (left) and unrecovered cell walls (right, after recovery treatment) at compression ratio of 50% and room temperature 3.5 On-going work It appears that a higher initial moisture content will increase the softening of wood, thus leading a lower modulus of elasticity and lower levels of stress during compression. Increasing moisture content, however, will also cause greater springback after the release of stress and a greater recovery after swelling in water (Ellis and Steiner 2002). The findings in this study support this. The drying of compressed water-saturated wood is a time and energy consuming process. It has been reported that the mechanical behaviour of Norway spruce (Piece abies) is similar at the watersaturated condition and fibre saturation point, suggesting extra water in cell lumens does not aid in the softening of wood (Uhmeier et al. 1998). To increase the efficiency of wood densification technology, a reduction in the initial moisture content of wood is suggested. Research on the compressibility of air dried softwoods produced in eastern Canada is in progress. 4. Conclusions Based on the above findings, the following conclusions would be drawn: (1) Species has minimal effect on relative change in thickness, and compression ratio has a stronger influence than temperature. (2) Temperature has the most significant impact on the peak load applied during compression, and compression ratio has stronger effect than species. (3) Unrecovered set increases with increasing compression ratio and temperature. (4) High temperature assists in softening watersaturated wood. (5) Minute buckling and cracks exist in the recovered specimens, even at a compression ratio of 5% and at room temperature. (6) Compressibility of the two tested species is similar, although balsam fir seems slightly more compressible than white pine. (7) The maximum compression ratio of white pine and balsam fir is about 60% at the water-saturated condition and temperature up to 90 o C. 5. References [1] Inoue M., Norimoto M., Otsuka Y., and Yamada T Surface compression of coniferous wood lumber. 1: A new technique to compress the surface layer. Mokuzai Gakkaishi Vol. 36, No. 11, pp [2] Ito Y., Tanahashi M., Shigematsu M., Shinoda Y., and Ohta C Compressive molding of wood by high-pressure steam-treatment. Part 1. Development of compressively molded squares from thinnings. Holzforschung Vol. 52, pp

7 [3] Norimoto M Large compressive deformation in wood. Mokuzai Gakkaishi Vol. 39, No. 8, pp [4] Uhmeier A., Morooka T., and Norimoto M Influence of thermal softening and degradation on the radial compression behaviour of wet spruce. Holzforschung Vol. 52, pp [5] Ellis S. and Steiner P The behaviour of five wood species in compression. IAWA Journal Vol. 23, No. 2, pp [6] Wang J. and Cooper P.A Vertical density profiles in thermally compressed balsam fir wood. Forest Products Journal Vol. 55, No. 5, pp [7] Minitab Inc Minitab Statistical Software (Release 14). [8] Irvine, G The glass transition of lignin and hemicellulose and their measurement by DTA. TAPPI Vol. 67, No. 5, pp Acknowledgements This study was supported by Natural Resources of Canada under its Value to Wood Program and Natural Sciences Engineering Research Council of Canada (Grant #: RGPIN ).