Access the full text.
Sign up today, get DeepDyve free for 14 days.
To improve the impregnation of wood, the pre-treatment by compression was systematically studied in terms of effects of compression ratio, compression direction, compression speed and compression-unloading place on the liquid impregnation in poplar and Chinese fir. The results showed: the impregnation increased 0.0065 or 0.0074 g/cm for every 1% increase of compression ratio when the compression ratio was lower or equal to 50 and 40% for poplar and Chinese fir, respectively; it continued to increase afterwards while the variation was quite big. There existed a significant difference of the impregna- tion of wood compressed at different directions in Chinese fir, but not in poplar. There existed a significant difference of the impregnation of wood compressed at different speed in both species. The impregnation of wood is likely to be in favor of radial compression in terms of the amount of impregnation. 5 and 10 mm/min were recommended as a compromise of impregnation and pre-treatment efficiency. The impregnation of wood that the compression unloaded in water was about 18.2 (poplar) and 9.2% (Chinese fir) higher in amount and was much quicker in speed than that the compression unloaded in air, and the difference between them was significant, suggesting that compression unloaded in water is significant to improve the impregnation. Keywords Pre-treatment · Compression · Wood impregnation · Penetration Introduction very limited due to very limited impregnation; therefore, a lot of studies have been done around the improvement Liquid impregnation is one of the most important opera- of the liquid penetration of wood. Of all the methods stud- tions in wood industry, by which the dimension stability, ied, the pre-treatment by compression is believed to be one the strength, the durability, the fire retardant properties, etc. of the most important methods because of high efficiency of wood can be improved accordingly with related function- and easy industrialization compared with other treatments improved chemicals. However, the improvement is usually such as microwave treatment, chemical extraction treatment and biological treatment. Pre-treatment by compression is becoming more significant when it comes to a low or no * Juan Guo email@example.com strength loss after the treatment. Pre-treatment by compression to improve intake of pre- Research Institute of Forestry New Technology, servatives was as early as around 1970s [1, 2] and a rather Chinese Academy of Forestry, Qing Long Qiao, Dong narrow compression levels at 2.5, 5.0, 7.5, 10.0, 12.5 and Xiao Fu No.1, Hai Dian District, Beijing 100091, People’s Republic of China 15.0% were tested in terms of retention, strength and dimen- 2 sion change of spruce wood. More studies around compres- Research Institute of Wood Industry, Chinese Academy of Forestry, Qing Long Qiao, Dong Xiao Fu No.1, Hai Dian sion treatment for the improvement of liquid penetration District, Beijing 100091, People’s Republic of China have been done in the recent years. Observation by scanning Laboratory of Wood Technology, Kyoto Prefectural electron microscope (SEM) on the compression wood by University, Shimogamo Nakaragi-cho, Sakyo-ku, Iida  found that even at a compression ratio of 68.4%, any Kyoto 606-8522, Japan Vol.:(0123456789) 1 3 552 Journal of Wood Science (2018) 64:551–556 failure and separation of the cell walls could not be detected, Jiangxi Province, respectively. The specimens were prepared the dimension and cell shapes of the compressed specimens with the size of 30 mm (compression direction) × 50 mm were almost restored to their original state, and the inter- (direction perpendicular to the compression) × 100 mm (lon- pretation on the mechanism was given in terms of cellulose, gitudinal direction) and tested in radial compression and 45° lignin and hemicelluloses. Pit fractures caused by compres- compression. Tangential compression was ignored because sion explain somehow the mechanism of the improvement of of obvious shape deformation of wood after the recovery penetration in anatomy aspect . A series studies of liquid compared with radial and 45° compression. The radial com- penetration of precompressed wood have been done in terms pression was carried on the flat grain specimens with the of the effects of sorts of factors on liquid penetration, includ- compression direction perpendicular to the annual ring; and ing the effects of compressive deformation and recovery [5 ]; the 45° compression was carried on in-between tangential thickness and length of specimen , moisture content (MC) and radial specimens with the compression face having 45° and temperature while compression , cyclic loading, type with the annual ring. The 45° compression specimens were of impregnated chemicals and annual ring angles ; and tested because in the industry practice, most boards are nei- combined treatment  on liquid penetration. All these stud- ther vertical nor flat grain, but in-between vertical and flat ies showed the pre-treatment by compression could signifi- grain. cantly improve the liquid penetration. In addition to this To minimize the effects of specimens variation on the test conclusion, the recovery rate and mechanical properties of results, all the specimens were oven-dried first so that the pre-compressed wood could be well retained after the treat- growth stress was released in some extent, and then vacuum- ment [10–12]. The course of liquid uptake of compressed pressure treated so that all the specimens were fully water wood by confocal laser scanning microscope  showed saturated having a similar MC, and would be easily com- the uptake of water was detected first between deformed and pressed. The compression ratio at 10, 20, 40, 50 and 60%, undeformed regions of compressed wood at all compression and compression speed at 0.5, 1, 3, 5 and 10 mm/min were ratios tested. For the industrial practice, many researches on easily, respectively, controlled by fully computer-controlled roller-pressing method were studied to improve the impreg- Instron 5582 Universal Test Machine, whose compression nation of wood [14, 15]. head was connected with a special adapter. The specimen Despite the aforementioned studies, the studies on the was put between two stainless plates of the adapter and was improvement of liquid penetration are not enough. Par- compressed by the plate driven by the compression head ticularly, the effects of compression direction, compres- of the machine. The compression was fixed or released sion speed and compression-unloading place on the liquid by means of tightening or loosening the nuts on four bolts impregnation are not clear. The compression direction deter- located at the corner of the adapter. Five repeats (each repeat mines the way the wood is compressed; the compression uses a specimen) for each compression ratio level at given speed and the compression ratio directly affect the process- compression speed and direction, as well as for each com- ing efficiency; the compression unloaded in impregnated pression speed level at given compression ratio and direction solution compared with that in air, as well as the compres- were tested, respectively. sion in high ratio compared with low ratio, are more difficult The impregnation was carried out by free immersion in operation, but are assumed more in amount of impregna- method, which needs the specimens to be totally immersed tion; however, it is not clear how big the exact die ff rences are in water (as the impregnated liquid) during the impregna- and if the differences are significant. Therefore, the objective tion. The weight of the wood at different states was meas- of this study is to systematically study the effects of com- ured. The impregnation was finished when the difference of pression ratio, compression direction, compression speed weight in two consecutive measures was less than 0.2 g in and compression-unloading place on liquid impregnation 1 h and was expressed by the weight gained per unit volume of wood. of wet wood. Materials and methods Results and discussion 15 trees of 25-year-old poplar (Populus tomentosa) planta- Eec ff ts of compression ratio on the impregnation tion with the diameter at the breast height of 25–33 cm and the air-dried density of 0.43 g/cm , and five trees of 25-year- To limit the variation, all the specimens for studying the old Chinese fir (Cunninghamia lanceolata) plantation with effects of compression ratio on the impregnation were radi- the diameter at the breast height of 22–26 cm and the air- ally compressed at a speed of 3 and 5 mm/min for poplar dried density of 0.36 g/cm were collected from Guanxian and Chinese fir, respectively. The compression was unloaded County of Shandong Province and Suichuan County of in water. 1 3 Journal of Wood Science (2018) 64:551–556 553 restored after the release of compression. The voids in 0.50 Poplar Chinese fir Chinese fir was about 10% more than that in poplar derived from the data of cell wall ratio of both species . That 0.40 well explained why the impregnations in Chinese fir at all the same compression ratio were higher than those in 0.30 poplar except that at a low ratio of 10%. Linear relationship between impregnation and com- 0.20 pression ratio (Fig. 2) was found when the compression ratio was lower or equal to 50 and 40% for poplar and 0.10 Chinese fir, respectively, suggesting that the impregnation increased 0.0065 or 0.0074 g/cm for every 1% increase 0.00 of compression ratio for poplar and Chinese fir, respec- 10 20 40 50 60 Compressionratio˄ %˅ tively. The impregnation continued to increase afterwards for poplar and Chinese fir, respectively, while the variation was quite big (Fig. 2). Fig. 1 Impregnation of specimens after the pre-treatment by com- pression at different compression ratio It is worth to notice that, the compression ratio ranges that existed the linear relationship for impregnation for poplar and Chinese fir were exact the same with those Impregnation at different compression ratio (Fig. 1) existed the linear relationship for recovery rate . Over showed it increased remarkably with the increase of com- these ranges, the recovery rate also had a quite big varia- pression ratio. Impregnation was only 0.053 and 0.025 g/ tion. From this, it concludes that the recovery of wood, the cm at 10% compression ratio, while was 0.400 and 0.429 g/ intrinsic character of wood, is one of the most important cm at 60% compression ratio for poplar and Chinese fir, driving forces for the impregnation of wood resulting from respectively. Compression reduced the wood size along the extrinsic pre-treatment by compression. The other intrin- compression direction, causing the volume shrink during sic change of wood itself was the pit fractures caused by the compression which would resulted in the MC reduction extrinsic compression , resulting in more flow paths  and compression energy was supposed to be stored in for the transportation of water. The more the wood was wood. With the increase of compression ratio, more space, compressed, the more driving force and flow paths were especially the cavity of vessel in poplar and tracheid lumen created. in Chinese fir, would be saved for potential impregnation The above results were based on water saturated condi- resulted from MC reduction, and more compression energy tion, the effects of compression on different MC of wood was supposed to be stored in wood. As soon as the compres- on the impregnation will be report later. sion was released in water, wood was recovered in a very short time, almost simultaneously by the spring back force of microb fi rills, which enlarged the wood cavity volume, and was believed to cause the temporary low pressure within the 0.50 wood cavity compared with the atmosphere pressure plus the water pressure outside the wood. Under this pressure gradi- 0.40 ent, the water was absorbed into wood . The bigger the y = 0.0065x -0.0244 compression ratio was, the more energy was supposed to be R² = 0.9807 0.30 stored in wood and more space was saved for the absorption. This explains the results in Fig. 1 why the impregnation y = 0.0074x -0.0446 0.20 R² = 0.9922 increased with the increase of compression ratio. The absorption of water, especially at low compression Poplar 0.10 ratio, was easily first occurred in vessels rather than fibers Chinese fir in poplar because of the large diameter and the perforate plate in vessels, while that always steadily occurred in 0.00 lumen in Chinese fir. At high compression ratio, after the release of compression, more water occupied the space Compression ratio (%) in vessels, the absorption in fibers begin to occur, which was harder than that in lumen of tracheid. The impregna- Fig. 2 Linear relations between the impregnation and the compres- tion capacity at high compression ratio was determined by sion ratio when the compression ratio ≤ 50% (poplar) or ≤ 40% (Chi- the compressible voids since they could be almost totally nese fir) 1 3 Impregnation (g/cm ) Impregnation (g/cm ) 554 Journal of Wood Science (2018) 64:551–556 Table 1 ANOVA: Directions versus speed Eec ff ts of compression direction and compression speed on the impregnation Source SS df MS F Sig. Poplar To limit the variation, all the specimens for studying the Directions 0.0006 1 0.0006 0.7256 0.3994 effects of compression direction and compression speed on Speed 0.0245 4 0.0061 8.0559 0.0001 the impregnation were compressed at the compression ratio Direction × speed 0.0010 4 0.0003 0.3400 0.8493 of 60 and 40% for poplar and Chinese fir, respectively. The Error 0.0304 40 0.0008 compression was unloaded in water. Total 0.0566 49 Impregnation of wood compressed at different directions Chinese fir and at different speed is shown in Fig. 3. For poplar, the Directions 0.0207 1 0.0207 28.5267 0.0000 impregnation of wood compressed radially at 0.5 and 1 mm/ Speed 0.0368 4 0.0092 12.6738 0.0000 min was almost the same or a little bit lower than those com- Direction × speed 0.0092 4 0.0023 3.1814 0.0232 pressed 45°, while the impregnation of wood compressed Error 0.0291 40 0.0007 radially at 3, 5 and 10 was higher than those compressed Total 0.0958 49 45°. Analysis of variance (ANOVA) (Table 1) showed the difference in impregnation of wood compressed in differ - SS sum of square, df degree of freedom, MS mean square, F F test ent directions was not statistically significant in poplar; for statistic, Sig. significance Chinese fir, the impregnation of wood compressed radially Significant at 0.01 level at all compression speed was higher than that compressed 45°, respectively, and the difference in impregnation of wood between radial compression and 45° compression. ANOVA compressed in different directions was statistically signifi- (Table 1) showed the difference in impregnation of wood cant. This can be explained from the early study  that, compressed in different speed was statistically significant in for Chinese fir, the recovery rate of 45° compression was both poplar and Chinese fir. As a compromise of impregna- lower than that of radial compression: the microfibrils were tion and pre-treatment efficiency, 5 and 10 mm/min were first bent in the corner of near rectangle shaped wood cell recommended for both poplar and Chinese fir. when it was compressed 45°; while the microfibrils in radial wall were firstly bent when it was compressed radially. The Eec ff ts of the compression‑unloading place microfibrils in the corner were more tolerant to this bend on the impregnation than that in radial wall because microfibrils in the corner intrinsically had a sharp bend and therefore were readily To limit the variation, all the specimens for studying the used to this sharp bend. Consequently, the wood recovered effects of compression-unloading place on the impregna- less, and impregnated less in 45° compression. Much uneven tion were radial compressed at the speed of 5 mm/min. A structure of poplar compared with even structure of Chinese compression ratio of 60 and 40% for poplar and Chinese fir may contribute to the impregnation trend of the former fir was, respectively, adopted for no or not obvious shape more complicated than that of the later. After all, the impreg- deformation after impregnation. nation was likely to be in favor of radial compression. The impregnation of wood that the compression was Impregnation of wood compressed at different speed did unloaded in water was about 18.2 and 9.2% higher than that not show a clear pattern although it showed a similar trend Fig. 3 Impregnation of wood Poplar Chinese fir after pre-treatment by compres- sion at different compression Radial 45° Radial 45° 0.40 0.50 directions and different com- pression speed 0.40 0.30 0.30 0.20 0.20 0.10 0.10 0.00 0.00 0.51 35 10 0.51 35 10 Compression speed (mm/min) Compression speed (mm/min) 1 3 Impregnation (g/cm ) Impregnation (g/cm ) Journal of Wood Science (2018) 64:551–556 555 0.50 in Chinese fir were considered to have more air stay in wood. water air 0.45 When the compression was released, the wood was found to spring back quickly, the water together with the air stayed 0.40 in wood would redistribute in wood, along with the water 0.35 impregnating into wood. More air stayed in wood was sup- 0.30 posed to have an influence to deter the water absorption 0.25 even when the compression was unloaded in water. This 0.20 may explain that the impregnation difference between 0.15 unloaded in water and in air in Chinese fir was smaller than 0.10 that in poplar. In this case, lower compression ratio of 40% 0.05 for Chinese fir, compared 60% for poplar, would result in 0.00 Poplar Chinese fir more air to stay in wood, and consequently enlarge the dif- ference of impregnation difference unloaded in water and in Fig. 4 Impregnation comparison of wood after pre-treatment by com- air between two species. pression and unloaded in water and in air Conclusions the compression was unloaded in air and then immersed in water for poplar and Chinese fir, respectively (Fig. 4). The 3 The impregnation increased 0.0065 or 0.0074 g/cm for former (unloading in water) was 0.44 and 0.27 g/cm and every 1% increase of compression ratio when the com- the later (unloading in air) was 0.38 and 0.25 g/cm for pop- pression ratio was lower or equal to 50 and 40% for poplar lar and Chinese fir, respectively. Analysis on the variance and Chinese fir, respectively. The impregnation continued showed that the difference between them was significant at to increase afterwards, while the variation was quite big. 0.01 and 0.05 level for poplar and Chinese fir, respectively. It Impregnations in Chinese fir at all the same compression concludes that compression-unloading in water is significant ratio were higher than those in poplar except that at low to improve the impregnation. ratio of 10%. It was observed that, even the compression was released There existed a significant difference of impregnation in a few seconds, the wood was restored to almost the final of wood compressed at different directions in Chinese fir, size in compression direction as soon as the completion of but not in poplar. There existed a significant difference of unloading. When it was unloaded in water, quick swollen the impregnation of wood compressed at different compres- cell was supposed to cause temporary low pressure within sion speed in both poplar and Chinese fir. The impregnation the cell lumen, and therefore would absorb the water into of wood is likely to be in favor of radial compression in the lumen due to the pressure gradient  between pres- terms of the amount of impregnation. 5 and 10 mm/min sure outside of wood (atmosphere pressure + water pres- were recommended as a compromise of impregnation and sure) and inside of wood. It accounted for almost all of the pre-treatment efficiency. impregnation by the end of the moment of the completion The impregnation of wood that the compression was of the unloading in water, since there was almost no more unloaded in water was about 18.2 (poplar) and 9.2% (Chi- impregnation even after a long time immersing in water. nese fir) higher in amount and was much quicker in speed When it was unloaded in air, quick swollen cell was sup- than that the compression was unloaded in air and then posed to absorb the air into the lumen in some extent, at impregnated in water, and the difference between them was least in the cell of outside part of wood. At this condition, significant, suggesting that compression unloaded in water the air was believed to deter the impregnation of water, and is significant to improve the impregnation. the impregnation would not become steady until a rather long time period compared with that compression unloaded Acknowledgements This work was supported by the Fundamental in water. All these were in favor of the unloading compres- Research Funds of Research Institute of Forest New Technology, CAF sion in water in terms of the amount of impregnation and the (Grant no. CAFYBB2017SY037). time required for impregnation. Therefore, for the industry, it is recommended to pre-treat the wood by compression and unload the compression directly in the impregnation liquid. References As aforementioned, the voids in Chinese fir were higher than those in poplar. At the same compression ratio, the 1. Cech MY, Huffman DR (1970) Dynamic transverse compression voids reserved in Chinese fir were supposed to be higher treatment of Spruce to improve intake of preservatives. For Prod than that in poplar. Higher voids reserved after compression J 20:47–52 1 3 Impregnation (g/cm ) 556 Journal of Wood Science (2018) 64:551–556 2. Cech MY (1971) Dynamic transverse compression treatment to 11. Zhao Y, Wang Z, Iida I, Huang R, Lu J, Jiang J (2016) Studies improve drying behavior of Yellow Birch. For Prod J 21:41–50 on pre-treatment by compression for wood drying II: studies on 3. Iida I, Norimoto M, Imamura Y (1984) Hygrothermal recovery of pre-treatment by compression for wood drying II: effects of com- compression set (in Japanese). Mokuzai Gakkaishi 30:354–358 pression ratio, compression direction and compression speed on 4. Watanabe U, Imamura Y, Iida I (1998) Liquid penetration of the recovery rate and mechanical properties of wood. J Wood Sci precompressed wood VI: anatomical characterization of pit frac- 62:226–232 tures. J Wood Sci 44:158–162 12. Zhao Y (2017) Studies on pre-treatment by compression for wood 5. Iida I, Takayama C, Miyagawa O, Imamura Y (1992) Liquid pen- drying III: the reduction of moisture content, the recovery rate, etration of precompressed wood I: effects of compressive defor - and mechanical properties of wood compressed at different mois - mation and recovery upon liquid uptake (in Japanese). Mokuzai ture content conditions. J Wood Sci 63:209–215 Gakkaishi 38:233–240 13. Abe H, Funada R, Kuroda N, Furusawa O, Shibagaki M, Fujii 6. Iida I, Imamura Y, Kashiwa N, Nakamura Y (1992) Liquid pen- T (2001) Confocal laser scanning microscopy of water uptake etration of precompressed wood II: effects of thickness and length during the recovery of compressed and drying-set wood. Iawa J of specimen on liquid uptake (in Japanese). Mokuzai Hozon 22(1):63–72 (Wood Preserv) 18:31–37 14. Adachi K, Inoue M, Kawai S (2005) Liquid impregnation of green 7. Iida I, Ikeuchi A, Imamura Y (1995) Liquid penetration of wood using a roller-pressing method (in Japanese). Mokuzai Gak- precompressed woods 3: ee ff cts of moisture contents of specimens kaishi 51:159–165 and ambient temperatures while compression on liquid uptakes 15. Günzerodt H, Walker JCF, Whybrew K (1988) Compression roll- of softwoods and hardwoods (in Japanese). Mokuzai Gakkashi ing of sitka spruce and Douglas fir. For Prod J 38:16–18 41:811–819 16. Zhao Y, Wang Z, Iida I, Huang R, Lu J, Jiang J (2015) Stud- 8. Iida I, Mori S, Nakamura Y, Sakai H, Imamura Y (1996) Liquid ies on pre-treatment by compression for wood drying I: effects penetration of precompressed wood V: effects of cyclic loading, of compression ratio, compression direction and compression type of impregnated chemicals and annual ring angles on the speed on the reduction of moisture content in wood. J Wood Sci uptake of water or oily solvents (in Japanese). Mokuzai Gakkaishi 61:113–119 42:581–588 17. Siau JF (1995) Wood: influence of moisture on physical proper - 9. Iida I, Yusuf S, Watanabe U, Imamura Y (2002) Liquid pene- ties. Department of Wood Science and Forest Products, Virginia tration of precompressed wood VII: the combined treatment of Polytechnic Institute and State University, Blacksburg precompression and extraction in hot water on the liquid penetra- 18. Jiang X, Yin Y (2008) Quantative anatomy of Chinese fir and pop- tion of wood. J Wood Sci 48:81–85 lar. In: Jiang Z, Jiang X (eds) Wood structures and their relations 10. Iida I, Imamura Y (1995) Liquid penetration of precompressed with the wood properties (in Chinese). Science Press, Beijing, wood 4: mechanical properties of set-fixed wood before and after pp 81–82 recovery (in Japanese). Mokuzai Gakkaishi 41:1165–1172 1 3
Journal of Wood Science – Springer Journals
Published: Jun 4, 2018
Access the full text.
Sign up today, get DeepDyve free for 14 days.