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Jang, E.-S., and Kang, C.-W. (2023). "Effect of steam explosion treatment on impregnation of three species of softwoods: North American spruce, Korean pine, and Japanese larch," BioResources 18(1), 1454-1464.

Abstract

Effects of steam explosion were investigated relative to impregnation of wood. Three types of softwoods [North American spruce (Picea orientalis), Korean pine (Pinus koraiensis), and Japanese larch (Larix kaempferi)] were prepared and subjected to steam explosion treatment. The cross-sectional surfaces of the samples were observed with SEM, and their open-pore and closed-pore porosities were determined using a gas pycnometer. The softwoods were vacuum impregnated using ACQ-2 (Alkaline Copper Quaternary), a commercial preservative. After steam explosion treatment, the impregnation amount increased by 42.9% in the spruce and 155% in the Korean pine. However, there was no significant difference in the Japanese larch. The results from this study indicated that the steam explosion treatment helped to improve open-pore porosity by generating micro-cracks in the cell walls of softwoods, which improved the impregnation process. However, the degree of improvement in impregnation process differed by the species type.


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Effect of Steam Explosion Treatment on Impregnation of Three Species of Softwoods: North American Spruce, Korean Pine, and Japanese Larch

Eun-Suk Jang and Chun-Won Kang *

Effects of steam explosion were investigated relative to impregnation of wood. Three types of softwoods [North American spruce (Picea orientalis), Korean pine (Pinus koraiensis), and Japanese larch (Larix kaempferi)] were prepared and subjected to steam explosion treatment. The cross-sectional surfaces of the samples were observed with SEM, and their open-pore and closed-pore porosities were determined using a gas pycnometer. The softwoods were vacuum impregnated using ACQ-2 (Alkaline Copper Quaternary), a commercial preservative. After steam explosion treatment, the impregnation amount increased by 42.9% in the spruce and 155% in the Korean pine. However, there was no significant difference in the Japanese larch. The results from this study indicated that the steam explosion treatment helped to improve open-pore porosity by generating micro-cracks in the cell walls of softwoods, which improved the impregnation process. However, the degree of improvement in impregnation process differed by the species type.

DOI: 10.15376/biores.18.1.1454-1464

Keywords: Steam explosion; Impregnation; Spruce; Korean pine; Japanese larch

Contact information: Department of Housing Environmental Design, and Research Institute of Human Ecology, College of Human Ecology, Jeonbuk National University, Jeonju 54896, South Korea;

* Corresponding author: kcwon@jbnu.ac.kr

INTRODUCTION

Overcoming global warming will require substantial reductions in carbon dioxide (CO2) emissions worldwide (Azadi et al. 2020; Deng et al. 2020). Trees efficiently pull CO2 from the atmosphere and store it in wood, thus reducing its presence in the atmosphere (Werner et al. 2006; Russell and Kumar 2017). Historically, deforestation was assumed to be detrimental to the environment (Hughes 2011). However, the subsequent planting of new trees was found to benefit efforts to overcome global warming because older trees have a reduced carbon absorption capacity, whereas young trees act as vigorous carbon sinks (Ney et al. 2019).

Wood is an eco-friendly material with excellent durability and mechanical performance (Sandberg et al. 2017). Its flexibility as a building material allows it to be used as a structural material, interior material, or exterior material in the construction field (Werner et al. 2006). When wood is left outdoors, it is attacked by microorganisms, which slowly deteriorates it (Feist 1990). These characteristics of wood are advantages in terms of natural purification but disadvantages in terms of durability. For this reason, wood used in the outdoors must be preserved (Kim 2013).

There are several methods for preserving wood, including heat treatment (Candelier et al. 2016; Kim 2016), coating (Nejad and Cooper 2011), immersion (İlker 2021), and impregnation processes (Soulounganga et al. 2004). Of these, the most reliable method is impregnation, in which chemicals are forced into the pores of wood using vacuum or high pressure. Wood has different pore shapes, and its content varies depending on the species.

Therefore, to predict impregnation, the pore-filling ratio (open-pore porosity) by skeletal density is an important parameter (Wu et al. 2017). Generally, a higher open-pore porosity is associated with higher gas permeability (Jang and Kang 2019; Jang et al. 2020). Therefore, the impregnation process can be facilitated through pre-treatment that increases the permeability of wood (Lehringer et al. 2009).

Pretreatment methods for improving permeability include treatments using microwave (He et al. 2017), ultrasonic wave (Kang et al. 2021a), alkaline enzyme (Durmaz et al. 2015), delignification (Kang and Lee 2005; Jang and Kang 2021), and steam explosion methods (Kolya and Kang 2021a). This study focuses on steam explosion treatment, which creates high steam pressure in a short time and destroys the cell lumen of wood (Kolya and Kang 2021a,b).

The steam explosion treatment results in a more open pore structure of the wood, as a result increasing its gas permeability (Kolya and Kang 2021a,b). The main purpose of this study was to determine whether steam explosion pretreatment can improve the impregnation of wood.

EXPERIMENTAL

Sample Preparation

This study examined air-dried timbers of North American spruce (Picea orientalis), Korean pine (Pinus koraiensis), and Japanese larch (Larix kaempferi) that were approximately 25 years old (Jeon-il Timber Co., Ltd; Gimje, Jeollabuk-do, Korea). After the timbers were cut to sample sizes of 2 × 2 × 29 cm, 10 intact samples without cracks and knots were selected from each species. The moisture content was 7.2% for spruce, 7.6% for Korean pine, and 7.3% for Japanese larch, as measured by KSF-2198 (2001). The densities of the samples were 0.52 ± 0.03 for spruce, 0.40 ± 0.03 for Korean pine, and 0.54 ± 0.06 for Japanese larch, as measured by KSF-2199 (2016) . The samples were cut in half and divided into control and steam explosion treatment groups. Figure 1 shows the samples from the three softwood species prepared for this study.

Steam Explosion Treatment

Samples from the three softwood species in the steam explosion treatment group were immersed in ultrapure water first and then decompressed at -0.1 MPa in a vacuum chamber for 4 weeks. Figure 2-a shows the schematic representation of the steam explosion treatment. A steam explosion machine (Model CK-0533579092, Chilgok, Korea) was used, as in previous studies (Kang et al. 2021b; Kolya and Kang 2021a,b). This machine was designed originally for production of puffed rice snacks; however, in this study, it was used for steam explosion of wood.

First, water-saturated specimens were sealed in the chamber. When the lower part of the chamber was heated, the pressure in the chamber increased. After approximately 13 min, the pressure had been increased to 10 bar. Then, the operator opened the chamber to release the steam. After that, the specimens were collected and naturally dried in the laboratory at 20 °C for 2 weeks. The steam explosion treatment was conducted by an operator from a Korean traditional puffed rice store (Moraenae puffed rice, Jeonju. Korea).

Fig. 1. Sample preparation for steam explosion and impregnation

Cross-sectional Morphology Before and After Steam Explosion

To observe the cross-sectional surfaces of the three softwoods before and after steam explosion, SEM analysis (Genesis-1000, Emcrafts, Korea) was used. The softening, microtome cutting, drying, and ion-coating processes were performed according to the reported pretreatment method in the SEM measurement of wood specimens (Jansen et al. 1998). The SEM analysis used 15 KV in high vacuum mode and 1,000× magnification for observation.

Analysis of Porosity and Open-Pore Porosity

Each sample density ρ (g/cm3) was measured by its weight and volume. Sample porosity φt (g/cm3) was calculated from wood substance density and sample density (Eq. 1). According to previous studies, the wood substance density ρws (g/cm3) was assumed to be 1.50 g/cm3 (Lindgren 1991; Moore et al. 2007; Tanaka et al. 2014).

(1)

Open-pore porosity (φo) can be analyzed by gas pycnometry per ISO 12154 (2004) . The skeletal density of each sample was obtained through a gas pycnometer (PYC-100A-1, PMI, USA), and the porosity was calculated using Eq. 2:

(2)

The closed-pore porosity (φc) was calculated by subtracting the open-pore porosity from the total porosity, as in Eq. 3:

(3)

Impregnation

Figure 2-b shows a schematic representation of the impregnation test used in this study. Specimens were placed in a water bath. Then impregnation solution was added, and a heavy mesh was used to prevent the specimens from floating. The impregnation solution was prepared by diluting a commercial outdoor wood preservative (ACQ-2, Jeonil Timber Co. Ltd., Jeonju, Korea) at 8% (w/v) in distilled water (Ra et al. 2017; Pang et al. 2017).

ACQ-2 is widely used as a wood preservative for impregnation in Korea. It is composed of CuO 8.0% and dodecyldimethyl-ammonium chloride <8.0%. Typically, when impregnation treatment is finished, the specimen is left for about 10 minutes to recover the surplus ACQ-2. Then, the active preservative is cured for settlement in the treated wood for 4 weeks at room temperature (Kim et al. 2015).

In this study, impregnation was performed in a laboratory vacuum oven (model: OV4-30, Jeotech, Korea). The vacuum oven was depressurized to -0.1 MPa at 25 °C. Weights were measured every hour up to a maximum of 4 h. The amount of impregnation was calculated as in Eq. 4.

(4)

Here, I represents the impregnation amount (g/cm3), m1 is wood mass before impregnation (g), m2 is wood mass after impregnation (g), and V denotes wood volume (cm3).

Fig. 2. Schematic diagram of the steam explosion chamber and impregnation test

RESULTS AND DISCUSSION

SEM Images of Cross-Sectional Surfaces

Figure 3 shows the SEM images of the cross-sectional surfaces of the three softwood specimens before and after steam explosion. After steam explosion, the cell walls of the spruce and Japanese larch were thinned and appeared flabby. In the Korean pine, a number of cracks were apparent in the cell wall.

Fig. 3. SEM images of the cross-sectional surfaces of the three softwood specimens before and after steam explosion

Porosity Analysis

Figure 4 shows the open-pore and closed-pore porosities before and after steam explosion treatment. Before steam explosion, the open-pore porosity of spruce was 68.11 ± 0.76 % (± standard deviation), Korean pine was 43.10 ± 0.72%, and Japanese larch was 14.76 ± 0.38%. After steam explosion treatment, the open-pore porosity of spruce was 72.19 ± 0.38 %, Korean pine was 54.84 ± 0.29 %, and Japanese larch was 16.94 ± 0.39 %. The resulting increases were 6.0 % (t = -14.11, P < 0.001, paired T-test, n=20) for spruce, 24.9% (t = -25.40, P < 0.001, n=20) for Korean pine, and 15.5% (t = -10.36, P < 0.001, n=20) for Japanese larch, respectively after steam explosion treatment.

The pore structure changes and the improvements to the impregnation process for the three softwood species were observed. Previous studies (Kolya and Kang 2021a,b) reported that steam explosion of wood caused changes in the cellulose crystallinity of wood. Also, it can be assumed that the presence of rays (ray parenchyma cells) and resin canals changed to an open structure due to the steam explosion. These effects affect the increase in permeability. Modification of wood cell walls can be effective at improving air permeability. The results from this study showed that steam explosion caused micro-cracks in the cell walls, and the pore structure of the wood was opened.

Fig. 4. Porosity analysis before and after steam explosion treatment (C: control, T: steam explosion treated)

Results of Impregnation

Figure 5 shows the impregnation amount before and after steam explosion for each of the softwood species examined. The impregnation patterns of the three species indicated that the wood samples were intensively impregnated during the first hour and then gradually increased thereafter. The impregnation amount was greatest for spruce, followed by Korean pine and Japanese larch. In summary, the impregnation performance of wood species is related to their open-pore structure but not linked to their porosity (Wu et al. 2017).

After one hour of steam explosion, the impregnation amount of Japanese larch increased by approximately 42.9% (t = -4.733, P < 0.001, n=20), from 0.07 to 0.10 g/cm3. That of Korean pine increased by approximately 155 % (t = -6.928, P < 0.001, n=20), from 0.20 to 0.51 g/cm3. However, spruce did not show a statistically significant increase (t = 0.898, P = 0.392, n=20).

After steam explosion, the Korean pine had the greatest increase in open-pore porosity among the three species. Accordingly, Korean pine showed the best impregnation performance of the three species examined. Conversely, Japanese larch had the lowest open-pore porosity among the three species. Although the open-pore porosity increased after steam explosion, it remained at a low level and was presumed to not affect the impregnation process. Accordingly, no improvement in impregnation was found in Japanese larch after steam explosion.

Permeability has a decisive influence on impregnation. There are various methods for improving the permeability of wood. Heat treatment effectively improves permeability, but processing time is long and energy consumption is high. The chemical treatment method requires chemical treatment and disposal facilities and might not be environmentally friendly. On the other hand, steam explosion treatment has the advantage of a simple facility and can change the wood’s pore structure to an open type in a relatively short time.

As the result of this study, steam explosion changed the pore structure of the wood to be more open, which improved the gas permeability. This open pore structure helped to improve impregnation. However, steam explosion treatment may reduce strength. So, it is necessary to establish steam explosion treatment conditions that minimize the decrease in strength in the future. These results suggest that various parameters should be selected for steam explosion treatment based on the species. Future studies should evaluate patterns in impregnation resulting from the steam explosion in additional species, which can help to improve the impregnation process for the wood industry.

Fig. 5. Difference in impregnation amount before and after steam explosion treatment

CONCLUSIONS

  1. After steam explosion treatment, the open-pore porosity of each species increased: 6.0% for spruce, 24.9% for Korean pine, and 15.5% for Japanese larch.
  2. After steam explosion treatment, the impregnation amount increased by 42.9 % for spruce and 155% for Korean pine. However, Japanese larch did not show any significant improvements.
  3. Steam explosion treatment helped improve open-pore porosity by creating microcracks in the cell walls, changing ray parenchyma cells, and resin canals softwoods.

ACKNOWLEDGEMENTS

This research was supported by a grant from the Basic Science Research Program of the National Research Foundation of Korea (NRF), and funded by the Ministry of Education (NRF-2019R1I1A3A02059471). It was also supported by a grant from the international cooperation program framework managed by the NRF (NRF-2020K2A9A2A08000181). This study was also supported by Invitational Fellowships for Research in Japan (Chun-Won Kang).

Authors’ Contributions

Eun-Suk Jang is the first author of this study. He designed the study, conducted all experiments, and was a major contributor in the original writing, reviewing, and editing of the manuscript. Chun-Won Kang is the corresponding author, he was the supervisor of this project and contributed by reviewing and editing. All authors read and approved the final manuscript.

REFERENCED CITED

Azadi, M., Northey, S. A., Ali, S. H., and Edraki, M. (2020). “Transparency on greenhouse gas emissions from mining to enable climate change mitigation,” Nature Geoscience 13(2), 100-104. DOI:10.1038/s41561-020-0531-3

Candelier, K., Thevenon, M. -F., Petrissans, A., Dumarcay, S., Gerardin, P., and Petrissans, M. (2016). “Control of wood thermal treatment and its effects on decay resistance: a review,” Annals of Forest Science 73(3), 571-583. DOI:10.1007/s13595-016-0541-x

Deng, Q., Alvarado, R., Toledo, E., and Caraguay, L. (2020). “Greenhouse gas emissions, non-renewable energy consumption, and output in South America: The role of the productive structure,” Environmental Science and Pollution Research 1-15. DOI:10.1007/s11356-020-07693-9

Durmaz, S., Yildiz, U. C., and Yildiz, S. (2015). “Alkaline enzyme treatment of spruce wood to increase permeability,” BioResources 10(3), 4403-4410. DOI:10.15376/biores.10.3.4403-4410

Feist, W. C. (1990). “Outdoor wood weathering and protection.” Archaeological Wood, Properties, Chemistry, and Preservation-Advanced in Chemistry Series 225, 263-298.

He, X., Xiong, X., Xie, J., Li, Y., Wei, Y., Quan, P., Mou, Q., and Li, X. (2017). “Effect of microwave pretreatment on permeability and drying properties of wood,” BioResources 12(2), 3850-3863. DOI:10.15376/biores.12.2.3850-3863

Hughes, J. D. (2011). “Ancient deforestation revisited,” Journal of the History of Biology 44(1), 43-57. DOI:10.1007/s10739-010-9247-3

İlker, U. (2021). “Suggestions for designing test specimens and testing procedures to impregnate wood material by pre-vacuumed immersion method realized using a special device.” Sakarya University Journal of Science 25(4), 1020-1030. DOI:10.16984/saufenbilder.749447

ISO 12154 (2014). “Determination of density by volumetric displacement – skeleton density by gas pycnometry,” International Organization for Standardization. Geneva, Switzerland.

Jang, E.-S., and Kang, C.-W. (2019). “Changes in gas permeability and pore structure of wood under heat treating temperature conditions,” Journal of Wood Science 65(1), 1-9. DOI:10.1186/s10086-019-1815-3

Jang, E.-S., Yuk, J.-H., and Kang, C.-W. (2020). “An experimental study on change of gas permeability depending on pore structures in three species (hinoki, Douglas fir, and hemlock) of softwood,” Journal of Wood Science 66(1), 1-12. DOI:10.1186/s10086-020-01925-9

Jang, E.-S., and Kang, C.-W. (2021). “Delignification effects on Indonesian momala (Homalium foetidum) and Korean red toon (Toona sinensis) hardwood pore structure and sound absorption capabilities,” Materials 14(18), 5215.

Jansen, S., Kitin, P., De Pauw, H., Idris, M., Beeckman, H., and Smets, E. (1998). “Preparation of wood specimens for transmitted light microscopy and scanning electron microscopy,” Belgian Journal of Botany 41-49.

Kang, C.-W, and Lee, N.-H. (2005). “Changes of sound absorption capability and anatomical features of wood by delignification treatment,” Journal of The Korean Wood Science and Technology 33(4), 9-14.

Kang, C.-W., Jang, E.-S., Lee, N.-H., Jang, S.-S., and Lee, M. (2021a). “Air permeability and sound absorption coefficient changes from ultrasonic treatment in a cross section of Malas (Homalium foetidum),” Journal of Wood Science 67(1), 1-5. DOI: 10.1186/s10086-020-01940-w

Kang, C.-W., Kolya, H., Jang, E.-S., Zhu, S., and Choi, B.-S. (2021b). “Steam exploded wood cell walls reveals improved gas permeability and sound absorption capability,” Applied Acoustics 179, article 108049. DOI: 10.1016/j.apacoust.2021.108049

Kim, Y.-S. (2013). “Current research trends in wood preservatives for enhancing durability-a literature review on non-copper wood preservatives,” Journal of the Korean Wood Science and Technology 41(3), 187-200. DOI: 10.5658/WOOD.2013.41.3.187

Kim, M.-J., Choi, Y.-S., and Kim, G.-H. (2015). “Evaluation of pretreatment moisture content and fixation characteristics of treated wood for pressure treatment of yellow poplar skin timber with ACQ, CUAZ and CuHDO,” Journal of the Korean Wood Science and Technology 43(6), 810-817. DOI: 10.5658/WOOD.2015.43.6.810

Kim, Y.-S. (2016). “Research trend of the heat-treatment of wood for improvement of dimensional stability and resistance to biological degradation,” Journal of the Korean Wood Science and Technology 44(3), 457-476. DOI: 10.5658/WOOD.2016.44.3.457

Kolya, H., and Kang, C.-W. (2021a). “Effective changes in cellulose cell walls, gas permeability and sound absorption capability of Cocos nucifera (palmwood) by steam explosion,” Cellulose 28(9), 5707-5717. DOI: 10.1007/s10570-021-03891-x

Kolya, H., and Kang, C.-W. (2021b). “Effective changes in softwood cell walls, gas permeability and sound absorption capability of Larix kaempferi (larch) by steam explosion,” Wood Material Science and Engineering 1-9. DOI: 10.1080/17480272.2021.1915864

KSF-2198 (2001). “Determination of density and specific gravity of wood,” Korean Standards Association Seoul, Korea.

KSF-2199 (2016). “Determination of moisture content of wood,” Korean Standards Association. Seoul, Korea.

Lehringer, C., Richter, K., Schwarze, F. W., and Militz, H. (2009). “A review on promising approaches for liquid permeability improvement in softwoods,” Wood and Fiber Science 41(4), 373-385.

Lindgren, L. (1991). “Medical CAT-scanning: X-ray absorption coefficients, CT-numbers and their relation to wood density,” Wood Science and Technology 25(5), 341-349. DOI: 10.1007/BF00226173

Moore, G. R., Kline, D. E., and Blankenhorn, P. R. (2007). “Impregnation of wood with a high viscosity epoxy resin,” Wood and Fiber Science 15(3), 223-234.

Nejad, M., and Cooper, P. (2011). “Exterior wood coatings. Part-1: Performance of semitransparent stains on preservative-treated wood,” Journal of Coatings Technology and Research 8(4), 449-458. DOI: 10.1007/s11998-011-9332-3

Ney, P., Graf, A., Bogena, H., Diekkrüger, B., Drüe, C., Esser, O., Heinemann, G., Klosterhalfen, A., Pick, K., and Pütz, T. (2019). “CO2 fluxes before and after partial deforestation of a Central European spruce forest,” Agricultural and Forest Meteorology 274, 61-74. DOI: 10.1016/j.agrformet.2019.04.009

Pang, S. J., Oh, J. K., Lee, S. J., Park, J. H., Jang, S. I., and Lee, J. J. (2017). “Surface checking reduction effect of preservative-treated Korean larch round-woods with various physical treatments,” Journal of the Korean Wood Science and Technology 45(1), 107-115. DOI: 10.5658/WOOD.2017.45.1.107

Ra, J. B., Ingram, J., Wang, J., and Morris, P. I. (2017). “Evaluation of preservative efficacy for refractory wood species in field tests and its implication for Korean wood preservation industry,” Journal of the Korean Wood Science and Technology 45(5), 544-558. DOI: 10.5658/WOOD.2017.45.5.544

Russell, A. E., and Kumar, B. M. (2017). “Forestry for a low-carbon future: Integrating forests and wood products into climate change strategies,” Environment: Science and Policy for Sustainable Development 59(2), 16-23. DOI: 10.1080/00139157.2017.1274580

Sandberg, D., Kutnar, A., and Mantanis, G. (2017). “Wood modification technologies – A review,” Iforest-Biogeosciences and Forestry 10(6), 895. DOI: 10.3832/ifor2380-010

Soulounganga, P., Loubinoux, B., Wozniak, E., Lemor, A., and Gérardin, P. (2004). “Improvement of wood properties by impregnation with polyglycerol methacrylate,” Holz als Roh-und Werkstoff 62(4), 281-285. DOI: 10.1007/s00107-004-0485-y

Tanaka, S., Shiraga, K., Ogawa, Y., Fujii, Y., and Okumura, S. (2014). “Effect of pore conformation on dielectric anisotropy of oven-dry wood evaluated using terahertz time-domain spectroscopy and eigenvalue problems for two-dimensional photonic crystals,” Journal of Wood Science 60(3), 194-200. DOI: 10.1007/s10086-014-1390-6

Werner, F., Taverna, R., Hofer, P., and Richter, K. (2006). “Greenhouse gas dynamics of an increased use of wood in buildings in Switzerland,” Climatic Change 74(1), 319-347. DOI: 10.1007/s10584-006-0427-2

Wu, G., Shah, D. U., Janeček, E.-R., Burridge, H. C., Reynolds, T. P., Fleming, P. H., Linden, P., Ramage, M. H., and Scherman, O. A. (2017). “Predicting the pore-filling ratio in lumen-impregnated wood,” Wood Science and Technology 51(6), 1277-1290. DOI: 10.1007/s00226-017-0933-6

Article submitted: September 28, 2021; Peer review completed: November 30, 2021; Revised version received and accepted: December 31, 2022; Published: January 9, 2023.

DOI: 10.15376/biores.18.1.1454-1464