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The Simultaneous Treatment of Compression Drying and Deformation Fixation in the Compression Processing of Wood

September 13, 2008

By Fukuta, Satoshi Asada, Fumihito; Sasaki, Yasutoshi

Abstract The impact of holes drilled into air-dried wood specimens was examined in a closed heating system on the deformation fixation process of compressed wood. Holes were drilled into wood to improve steam permeability, and therein, improve manufacturing efficiency in comparison to conventional processes. The impact of drilled holes on the permeability of water in wood was also examined. The deformations of the wood specimens in the early stages of the compressive deformation were closely analyzed, and the removal of water from wet specimens during compressive deformation (compression drying) was examined. Drilled holes were observed to decrease the moisture content and the compressive stress of specimens during the compression drying process. The decrease in the observed compressive stress was due to the decrease in the hydraulic pressure in the compressively dried specimens. The compressive deformation drying process of wetwood specimens in a closed heating system elucidated the possibility of simultaneously compression drying and deformation fixing wood during the compression process.

Compressive deformation processes are a technology that improves the mechanical properties of wood by increasing the material density. This process is of increasing interest in the Japanese cedar and soft wood industries, as these types of wood are common in forested areas of Japan, and the products derived from these woods are popular as specially textured floor materials.

In the compressive deformation process of wood, deformation fixation is an important technical problem. Investigations of fixation in literature have reported that deformation recovery is controlled by steam treatment (Norimoto 1993, Inoue et al. 1993a, Norimoto 1994, Ito et al. 1995), and thereby, a closed heating system was reported as a method for applying the steam treatment using a simple metal device (Inoue et al. 1993b), consisting of a sealing mechanism and a hot-press. These initial reports attracted considerable attention to the compressive deformation process from industry.

Closed heating is a method of using a gasket around a specimen to seal the specimen when under compression in a hotpress. Unfortunately, closed heating is a batch process, wherein the processing time is long, and requires significant amounts of energy because the press is required to be cooled and reheated for every press execution. As such, the manufacturing efficiency is low, which detracts from the overall productivity of the process.

We have attempted to improve the inefficiency of contemporary compressive deformation processes by predrilling holes into the wood specimens, potentially resulting in a process that is continuous and does not require press cooling. The permeability of water in the wood specimens, as a function of the predrilled holes, and the water removal process from wet specimens during compressive deformation (compressive drying) were investigated. In summary, the simultaneous treatment of compression drying and deformation fixation in the compression process of wood was investigated. In this treatment, wood can be quickly and efficiently dried by the removal of water using the heat transfer from the hot-press plates. Thereby, we expect an efficient, low energy wood drying process after lumbering.

In this study, we examined the effect of drilled holes on the heating of air-dried specimens in a closed heating system. We also investigated the drilling and pressing conditions that potentially impact the dehydration and deformation behavior of wood specimens. Finally, simultaneous compression drying and deformation fixation as components of the compression process were examined.

Materials and methods

Drilling process

The drilling of wood is a unique processing feature used in this study. The drilled holes resemble the incisions ordinarily used in the preservative treatments of refractory wood. Incisions can improve the surface permeability to water, while deep drilling can markedly improve the internal permeability to water. Holes were drilled into wood samples at a machining center, as depicted in Figure 1. The drilled holes were positioned with a 3 mm offset for each row, and were produced regularly according to a prescribed depth and interval distance. The drilled holes were 1.0 mm in diameter for the airdried specimens, and were 1.3 mm in diameter for the wet specimens.

The compressive deformation process in a closed heating system

Figure 2 depicts the schematic of the compressive deformation process in a closed heating system. Figure 2(a) depicts the conventional process applied to an air-dried specimen. A hygrothermal reaction is induced without evaporating the moisture contained in the specimen by the pressure in the sealed space chamber. Therein, steam can be introduced in the space. The steam in the space is released after a prescribed time, and the specimen is obtained. Unfortunately, the specimen often swells or ruptures when removed from the sealed space chamber because it maintains a high steam pressure within it even after removal. Therefore, the press and the device need to be cooled, and the steam pressure inside the specimen lowered. Once the specimen is depressurized and sufficiently low enough in temperature, it can be removed from the press. The next specimen to be processed is required to be reheated as to build enough pressure for the creation of steam.

In this study, holes were predrilled into specimens in order to permit the outward permeation of steam, and hence, reduce the risk of specimen swelling or rupture. Figure 2(b) depicts a schematic of the process. The specimen can be removed without cooling the press, since the steam inside the drilled specimen can be released at the same time as when the steam pressure in the space is released. Thereby, manufacturing efficiency can be markedly improved because continuous processing becomes feasible.

The permeability of water as a function of the drilled holes, and the simultaneous treatment of compression drying and deformation fixation in the compression process of wood, were investigated. Figure 2(c) depicts a schematic of the simultaneous treatment of compression drying and deformation fixation in the compression process of wood. Although the process depicted in Figure 2(c) closely resembles that of Figure 2(b), specimens can be processed from their green-wood state using the process of Figure 2(c). The objectives of the simultaneous treatment shown in Figure 2(c) are to remove water during the compressive deformation process, and to dry the specimen during the fixation and steam discharging processes.

Materials and conditions for the compressive deformation process

Specimens were obtained from the heartwood of Japanese cedar (Cryptomeria japonica D.Don), with dimensions of 320 by 38 by 100 mm (longitudinal by radial by tangential). The moisture contents (MC) were 12 percent when air-dried, and 70 to 250 percent when wet.

The specimens were compressed in the radial direction at a temperature of 180 [degrees]C, at a loading speed of 30 mm/min, and at a compression ratio (CR) of 50 percent. A 0.5 mm in diameter wire mesh was placed between the drilled specimen surface and the press. The wire net was used to release the steam that was horizontally exiting the drilled holes. A control experiment was similarly established for specimens without drilled holes. The heating time for the steam treatment was 15 minutes.

As depicted in Figure 2(b), a sufficient amount of steam was fed into the sealed space chamber, resulting in a poststeam treatment pressure of 1.0 MPa after 15 minutes, that is, a saturated steam pressure at 180 [degrees]C. After a steam discharging time of 5 minutes, the specimen was removed without cooling the press. The specimens were visually inspected for swelling or rupture after the pressing process. Additionally, a seal was applied to the end grain of some specimens with an epoxy adhesive to evaluate the impact of end grain evaporation.

The device depicted in Figure 2(c) had a poststeam treatment pressure of 1.0 MPa after 15 minutes. The steam discharging times were 10 and 40 minutes

Compression test for removing water from the specimen

Specimens were obtained from the heartwood of Japanese cedar (Cryptomeria japonica D.Don), with dimensions of 100, 320, or 1,000 by 38 by 100 mm (longitudinal by radial by tangential). The specimen MC was 40 to 270 percent. Holes were drilled in these specimens at intervals of 10 or 16 mm, at depths of 30.4 or 35 mm. The specimens were compressed in the radial direction at room temperature using a load testing machine or a press. The investigated specimen loading speeds were 10, 30, 50, or 70 mm/ min, and the CRs were 30, 50, or 70 percent. A wire net was used in the same manner as in the compressive deformation process investigation.

The compression stress at the prescribed CR was determined. The specimen was removed immediately, and the weight of the water removed from the specimen as a function of compressive deformation was measured by the change in specimen mass. The deformation behavior and the external appearance were also evaluated.

Results and discussion The compressive deformation process of an air-dried specimen in a closed heating system

Table 1 summarizes the results of the compressive deformation process for air-dried specimens in a closed heating system. Table 1(a) summarizes the results of specimens without end grain sealants, while Table 1(b) summarizes the results of specimens with sealing. The conditions at which swelling or rupture occurred are depicted by the symbol “x,” while the conditions where specimens were normally processed are depicted by the symbol “O.” The control specimens convexly swelled from the end grain to the specimen center, wherein the center swelling was significantly larger than that of the end grain. This observed swelling behavior was caused by the steam pressure inside the specimen, suggesting that steam evaporation from the end grain during the discharging process contributed to less swelling at the end grain in comparison to the specimen center. Drilled specimens at fixed conditions did not swell, as depicted in Table 1(a), suggesting that the steam inside the specimen was released through the drilled holes during the discharging process.

Epoxy adhesive seals were applied to the end grains to more closely evaluate the impact of grain end evaporation on specimen swelling. Therein, grain end-sealed specimens without drilled holes exhibited a significantly larger swelling compared to unsealed specimens, that is, the recovery ratio in the sealed specimen center was approximately 2.4 times larger than the unsealed specimen. In contrast, the specimens with drilled holes did not swell, as depicted in Table 1(b). The possible conditions for normal processing detailed in Table 1(b) are more limited than those of Table 1(a) in terms of drilled hole depth, suggesting that deeper drilled holes effectively prevent swelling.

From these results, we can conclude that the internal steam pressure of the wood specimens was effectively released through the drilled holes, enabling continuous processing. In conventional compressive deformation processes, the hot-press is cooled every processing cycle, as depicted in Figure 2(a), to mitigate internal steam pressure-induced specimen swelling and rupture. This cooling requirement has contemporarily made the industrialization of this process difficult; however, the results obtained in this study, that is, the ability to maintain a continuous pressing process, potentially facilitate a more industrially implementable process.

Compression test for removing water from the specimen

The impact of drilled holes on specimen water permeability was examined. Water removal is a function of specimen water permeability, and therein, if the permeability is sufficiently improved, compressive deformation-induced water removal (compression drying) is more easily facilitated. The deformation behaviors of wet specimens were subsequently investigated to evaluate the potential of compression drying. Unfortunately, the amount of water flowing out from the end grain cannot be disregarded when the longitudinal length is small; hence, several longitudinal lengths (L) were investigated.

L = 1000 mm

Water inside the aforementioned control specimens was forced out from the end grain during deformation, while in contrast, the drilled specimens exhibited a substantial water flow from the drilled holes out from the back of the specimen, with limited flow from the end grain. Figure 3 depicts the specimens after the water removal compression drying process. The induced specimen deformations recovered to their former shapes due to them being wet. At a high MC, cracks along the longitudinal direction over the total length of the control specimens were observed (Fig. 3(a)-l, Fig. 3(b)-l), while in contrast, no cracks were observed in drilled specimens (Fig. 3(a)-2, Fig. 3(b)-2). Moreover, finer cracks were observed in the cross sections of the control specimens, which were attributed to deformation-induced increases in hydraulic pressure.

Figure 4 depicts the MC, before and after compression, for specimens with a longitudinal length of 1000 mm. In the control specimens (D), a crack was observed when the MC exceeded approximately 50 percent, wherein the crack size was observed to be a function of increased MC. The compression drying effect was observed when the specimen MC was approximately equal to or greater than 100 percent; however, the effect appeared to be influenced by water from the crack or the end grain. In contrast, drilled specimens ([black square]) exhibited a compression drying effect, which was effectively about 2/3 of the MC of the undrilled specimens. Specimens drilled with shallow holes at a wide interval ([black circle]) exhibited a similar effect as to the previously mentioned drilled holes. No cracks appeared in the drilled specimens for all of the investigated conditions.

Figure 5 depicts the relationship between the specimen MC and compressive stress at a CR of 50 percent. The plots connected by the line are the results from specimens gathered from the same lumber by end matching. A compressive stress of approximately 4 MPa has been reported for a similar small test piece under the same conditions (Fukuta et al. 2007); however, the stresses obtained in this study are generally higher, and are most likely due to impeded moisture movement along the longitudinal direction, increasing the hydraulic pressure, and subsequently, the compression stress as well. Drilled specimens ([black square]) exhibited a lower stress than the control ([white square]), although the stresses of the control specimens were most likely decreased due to cracking. The lower stress exhibited by the drilled specimens is a direct result of a more efficient outflow of water through the drilled holes, resulting in a decreased internal hydraulic pressure.

The drilled specimens were expected to have an internal stress that increase directly to increases in MC; however, this relationship was unclear since the effects of other physical properties, such as density and permeability, were significant.

L = 320 mm

Figure 6 depicts the MCs of specimens with longitudinal lengths of 320 mm, before and after compression. In the case of a CR of 30 percent (Fig. 6(a)), no differences between drilled ([black square]) and control specimens ([white square]) were observed, and no significant compression drying was obtained. In the case of CR values of 50 and 70 percent (Figs. 6(b) and (c)), compression drying was obtained. In drilled specimens, the compression drying was obtained for MCs of 100 percent or more at a CR of 50 percent, and for MCs of 70 percent or more at a CR of 70 percent. The MC after compression was asymptotic at approximately 90 percent and 70 percent, respectively. In contrast, cracks occurred for MCs of 200 percent or more at a CR of 50 percent, and for MCs of 120 percent or more at a CR of 70 percent, in the control specimens. Moreover, the MC after compression was higher in the control specimens than in the drilled specimens.

Next, the difference of longitudinal length on the same condition of CR of 50 percent was considered in Figures 4 and 6(b). In the control specimens, the MCs after the compression of specimens with longitudinal lengths of 320 mm (Fig. 6(b)) were lower than that for specimens with lengths of 1000 mm (Fig. 4). The cracking area was limited in specimens with lengths of 320 mm; hence, the impact of specimen longitudinal length on MC is significant. In contrast, drilled specimens only exhibited a minor fluctuation in MC as a function of longitudinal length, as depicted in Figure 4 (L = 1000 mm), and in Figure 6(b) (L = 320 mm). The effect of the water outflow from the end grain is smaller than the outflow from the drilled holes, and can therefore be neglected.

Figure 7 depicts the relationship between the specimen MC and compressive stress for each CR. For a CR of 30 percent (Fig. 7(a)), no differences in compressive stress were observed between drilled and control specimens. For a CR of 50 percent (Fig. 7(b)), the compressive stress of drilled specimens was lower than that of the control specimens. For a CR of 70 percent (Fig. 7(c)), the compressive stress of the control specimens was approximately 50 percent greater than the drilled specimens. In the compressive deformation process of wood, the size of cell pores decreases as the CR increases. Since the hydraulic pressure in the cell pores is expected to grow simultaneously with deformation, the impact of drilled holes is clearly observed in the case of a CR of 70 percent (Fig. 7(c)). The relationship between specimen MC and compressive stress was unclear for specimens with a longitudinal length of 1000 mm, as depicted in Figure 5.

L = 100 mm

Figure 8 depicts the MCs of specimens with longitudinal lengths of 100 mm, before and after compression. No marked difference between drilled and control specimens was observed between the investigated conditions, and compression drying was obtained at MCs of 100 percent or more under all conditions. No differences were observed because the outflow of water from the end grain is easy in specimens with longitudinal lengths of 100 mm.

Specimen loading speed and compressive stress were observed to be related. Figure 9 depicts the relationship between loading speed and compressive stress at a CR of 50 percent. The compressive stress is shown as a ratio to the stress value at the loading speed of 10 mm/ min (with drilled holes). The specimens for each test condition were continuously obtained from a piece of lumber 1-m long, for a total of seven lumber specimens. The compressive stress was observed to increase as the loading speed was increased. The compressive stress of drilled specimens was lower than that of control specimens at each loading speed. Figure 10 depicts an example of the relationship between the specimen CR and the compressive stress. The reason why the difference in compressive stress grows as per the CR for each investigated condition is most likely related to the increase in the hydraulic pressure as a function of the deformation, as depicted in Figure 7. When specimen permeability to water is poor, the hydraulic pressure appears to increase proportionally to the loading speed, resulting in a subsequent increase in compressive stress proportionally to the loading speed. The compressive stress of the drilled specimens was lower than that of the control specimens because the hydraulic pressure was released through the drilled holes. It has been reported that strength generally increases proportionally to the loading speed (Okuyama and Asano 1970, Okuyama et al. 1970). Therefore, the increase in compressive stress proportionally to the loading speed is considered most likely the outcome of a combination of viscoelasticity and increase in hydraulic pressure. Simultaneous treatment of compression drying and deformation fixation in a closed heating system

Table 2 depicts the results of a simultaneous treatment of compression drying and deformation fixation in a closed heating system. Tables 2(a) and 2(b) depict the results for steam discharging times of 10 and 40 minutes, respectively, and the results therein were examined similarly as in the case of Table 1. A longer steam discharging time than that used for air-dried specimens was used due to a large MC. As depicted in Table 2(a), normal processing was possible under limited conditions. The MC after the simultaneous treatment of compression drying and deformation fixation in a closed heating system was approximately 25 percent. From Table 2(b), it can be observed that processing was possible under a wide variety of conditions, and the average specimen MC after processing was approximately 6 percent, hence demonstrating that the simultaneous treatment of compression drying and deformation fixation in the compression process of wood is feasible. Wood drying effects, such as drying checks and cupping, were not observed after the process. A noticeable swelling and a crack in the longitudinal direction were observed for the control specimens, which can most likely be explained by the aforementioned hydraulic pressure. Under the conditions denoted by the symbol “x” in Tables 2(a) and 2(b), swelling was observed, but a longitudinal crack did not form.

Conclusions

The effect of holes drilled into air-dried specimens was examined in a closed heating system on the deformation fixation of compressed wood. Specimens were drilled with holes to improve the permeability of steam in wood, and to reduce the press cooling-reheating process requirements that are problematic to contemporary deformation fixation. This investigation realized a basis for continuous press processing, which is expected to have important productivity and industrial implications for the lumbering industry.

The impact of drilled holes on the permeability of water in wood was examined using wet specimens. The impact of drilled holes and processing conditions on specimen compressive deformation and compression drying was investigated. The effect of the drilled holes on specimen permeability to water was observed from changes in specimen MC, compressive stress, and appearance. Water removal rates were markedly increased for drilled specimens, and the MC decreased correspondingly. Further, the MC was asymptotic at a constant value, depending on the CR. The higher the specimen MC prior to compression and the greater the CR, that is, the effect of removal was appreciable. The compressive stress of the drilled specimens was lower than that of the control specimens, and the hydraulic pressure- induced cracking was controlled by the drilled holes. The deformation speed was not observed to affect the MC change as a function of MC in this study, while the compressive stress increased proportionally to the deformation speed.

From the results obtained from processing wet specimens, it is clear that the simultaneous treatment of compression drying and deformation fixation is feasible, that is, wood can be dried using compression and fixation. This finding is expected to have important implications for reduced processing energy requirements, and improve the over efficiency of lumber processing. Unfortunately, the investigated pressing conditions were not suitable for the simultaneous treatment of compression drying and deformation fixation. Future work will investigate improved processing conditions or methods in the vein of compression drying.

Literature cited

Fukuta, S., F. Asada, and Y. Sasaki. 2007. Compressive deformation process of Japanese cedar (Cryptomeria japonica). Wood and Fiber Sci. 39(4):548-555.

Ito, Y., M. Tanahashi, M. Kawai, M. Shigematsu, and Y. Shinoda. 1995. Compressively molded wood and practical processing by high- pressure steam treatment. Res. Bulletin of the Faculty College of Agriculture, Gifu Univ. 60:121-127. (in Japanese).

Inoue, M., M. Norimoto, M. Tanahashi, and R.M. Rowell. 1993a. Steam or heat fixation of compressed wood. Wood and Fiber Sci. 25(3): 224-235.

__________, N. Kadokawa, J. Nishio, and M. Norimoto. 1993b. Permanent fixation of compressive deformation by hygro-thermal treatment using moisture in wood. Wood Res. and Technical Notes. 29:54-61. (in Japanese).

Norimoto, M. 1993. Large compressive deformation in wood. Mokuzai Gakkaishi 39(8):867-874. (in Japanese).

Norimoto, M. 1994. Transverse compression of wood and its application to wood processing. Wood Res. and Technical Notes. 30:1- 15. (in Japanese).

Okuyama, T. and I. Asano. 1970. Effect of strain rate on mechanical properties of wood. I. On the influence of strain rate to compressive properties parallel to the grain of wood. Mokuzai Gakkaishi 16(1): 15-19. (in Japanese).

__________, K. Tsuzuki, and I. Asano. 1970. Effect of strain rate on mechanical properties of wood. II. On the empirical equation of stress-strain relation in compression. Mokuzai Gakkaishi 16(1):20- 25. (in Japanese).

The authors are. respectively, Research Scientists, Aichi Industrial Technology Inst., Aichi, Japan (satoshi_2_fukuta@pref.aichi. lg.jp, furnihito_asada@pref.aichi.lg.jp); and Professor, Graduate School of Bioagricultural Sci., Nagoya Univ., Nagoya-city, Aichi, Japan (gasteig@agr.nagoya-u.ac.jp). This paper was received for publication in August 2007. Article No. 10397.

(c) Forest Products Society 2008.

Forest Prod. J. 58(7/8):82-88.

Copyright Forest Products Society Jul/Aug 2008

(c) 2008 Forest Products Journal. Provided by ProQuest LLC. All rights Reserved.




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