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Springback in acetylated wood based composites
Construction and Building Materials 23 (2009) 3103–3106
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Springback in acetylated wood based composites Behbood Mohebby a,*, Maryam Gorbani-Kokandeh b, Mojtaba Soltani c a b c
Department of Wood and Paper Sciences, Faculty of Natural Resources, Tarbiat Modares University, P.O. Box 46414-356, Noor, Iran Department of Wood and Paper Sciences, Agricultural Sciences and Natural Resources University of Sari, Sari, Mazandaran, Iran Department of Wood and Paper Sciences, Azad University, Chaloos, Iran
a r t i c l e
i n f o
Article history: Received 12 November 2008 Received in revised form 7 February 2009 Accepted 8 February 2009 Available online 31 May 2009 Keywords: Acetylation Spring back Particleboards Medium density fibreboard
a b s t r a c t Reasons of the strength loss of the acetylated wood based composites were still under question. This research was considered to study springback in the acetylated particleboard and the medium density fibreboard. Chips and fibres were acetylated by using the acetic anhydride to gain different percentages of the weights (WPG). The boards were made based on the target thicknesses. Thickness of the boards as well as the modulus of elasticity (MOE) and the modulus of rupture (MOR) were determined after conditioning based on three point static bending test. Results showed that the moisture content and the thickness swelling of the boards were reduced as the WPG was increased. However, increase of the springback and reduction of the MOE and the MOR were revealed as the WPG increased in the test boards after condition. It was also revealed that the springback was correlated with the weight gain and increased as the WPG was raised in the test boards. Reduction of the MOE and the MOR was also correlated with the springback in the test boards. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction During the last decades, demands in the market obliged wood technologists to introduce new composites with enhanced properties, such as particleboard, oriented strand board, plywood, and hardboards. Those products are currently known as interested engineering materials for building and construction. Even though, they were fascinating and reached higher technological properties in comparison with solid wood, however such products had similar problems as wood. For example, those products are potentially attacked by the biological agents, they become dimensionally instable when they are exposed to the changing humidities, influenced by the UV-light and burnt when exposed to the fire. Scientists are in challenge to find solutions for the mentioned problems. Acetylation is one of the interesting processes which have been recently industrialized to modify the wood and the wood based composite to enhance properties of the products for application in the severe conditions and also remove the noticed problems. The acetylation is a reaction between the acetic anhydride with the wood polymers, lignin, cellulose and also hemicelluloses. Advantages of the acetylation have been reported by many authors. The wood and the wood based composites become moisture repellent due to the acetylation reaction [1,2]. The reaction is * Corresponding author. Tel.: +98 122 6253101/9111255972; fax: +98 122 6253499. E-mail addresses: [email protected] (B. Mohebby), ghorbani_mary@ yahoo.com (M. Gorbani-Kokandeh), [email protected] (M. Soltani). 0950-0618/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2009.02.007
accompanied by substitution of the hydrophilic hydroxyl groups with the hydrophobic acetyl groups in the wood cell wall polymers and causes increase in the wood bioresistance [3–8]; resistance against the weathering [9] and decrease in the board roughness [9], reduction in the moisture absorption [5,10–13]. In spite of those advantages, there are different reports indicating that the mechanical strengths are diminished in the acetylated wood based composites [10,11,2,14,15]. There are several opinions to explain reasons of the strength loss; e.g. type of the adhesive [16], less press pressure, etc. It has been proposed to apply higher press pressure to increase the strengths [10,11,2]. However, no report has paid detailed attention to springback in the acetylated composites and its correlation with the strength losses. It is known that the springback in the wood based composites occurs usually after manufacture of the boards. The springback is an irreversible thickness swelling which occurs after wetting of the composites and is attributed to the release of applied stresses accompanied by some loss of the glue bonds [17,18]. The springback indicates debonding of the adhesion between the wood elements and the adhesives. Result of the debonding in the boards is loss of the strengths [19], which is occurred in the specimens subjected to different exposures because of a lowering of the board density [19]. The mechanical properties that were presumably most directly affected by the springback are the shear strength, the modulus of elasticity, and the modulus of rupture, because these properties depend somewhat on the strength that is developed by the mechanical interlocking of the compressed particles in the composites [20].
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Current research was focused on relationship between the acetylation and the strength changes in the wood based composites, the medium density fibreboard (MDF) and the particleboard (PB), which is associated with the springback. 2. Experimental program
Static bending test – The test boards were cut into sizes of 50 mm (width) and 300 mm (length) for determination of the static bending according to DIN 987541 [21]. Modulus of the elasticity (MOE) and modulus of the rapture (MOR) were determined based on three point bending test. For each WPG, 20 and 16 samples were used as replicates for the PB and the MDF, respectively. The MOE and the MOR were calculated based on the Eqs. (5) and (6).
MOE ¼
2.1. Acetylation Poplar wood fibers were provided by Khazarchoob Company, manufacture of the MDF in Iran, and required chips were prepared by a laboratory chipper from Oriental beech wood (Fagus orientalis Lipsky). Both fibers and chips were dried in an oven for 24 h at 103 ± 2 °C. Afterwards, the acetylation was carried out in a stainless steel reactor at 120 °C for varying time to obtain different weight percent gains (WPGs). Acetic anhydride was used for the acetylation reaction. Acetylated fibers and chips were washed in rinsed water to remove formed acetic acid as by-product of the reaction. Afterwards, they were oven dried for 24 h to determine the WPGs in the acetylated fibers and chips. The WPGs were determined based on oven dry weights of parallel samples which were treated at the same conditions as the main samples. The WPG was calculated according to Eq. (1):
WPG ¼ ðW act W unt Þ=W unt 100
ð1Þ
where WPG indicates the weight percent gain (%); Wact and Wunt oven dry weight after and before the acetylation (g), respectively. 2.2. Manufacture of boards Sample boards were made based on required target densities with thicknesses of 10 and 15 mm for the MDF and the PB, respectively. Urea formaldehyde (UF) was applied as resin for 10% based on oven dried weights of the used fibers and chips in the board mats. Pressure (30 bar) was applied at press temperature of 170 °C for 10 min. Five boards (40 cm 40 cm) were made for any WPG. The test boards were conditioned at room temperature (25 °C) and a relative humidity of 65% for 2 weeks. Afterwards, samples were cut from the boards according to required sizes of the tests. 2.3. Tests Springback and soaking test – Specimens with sizes of 50 mm 50 mm board’s thickness were cut from the test boards. Their thicknesses were measured at three points to determine the springback of the boards according to Eq. (2). Afterwards, they were weighed and dried in an oven for 24 h at 103 ± 2 °C to determine the moisture contents (MC) of the conditioned test boards. The MC was calculated based on Eq. (3).
S ¼ ðT 1 T 0 Þ=T 0 100
ð2Þ
Where S indicates springback of the board (%); T1 and T0 indicate board thicknesses after and before the conditioning (mm), respectively.
MC ¼ ðW W W 0 Þ=W 0 100
ð3Þ
where MC indicates the moisture contents (%); Ww and W0 indicate wet and dry weights of the samples (g), respectively.Thicknesses of the dried samples were measured at three points and they were then soaked in the water (25 °C) for 24 h. The thicknesses of the wet samples were measured again to determine the thickness swelling according to Eq. (4). For those tests, 26 and 16 samples were used for the PB and the MDF, respectively.
T S ¼ ðT W T 0 Þ=T 0 100
ð4Þ
where TS indicates the thickness swelling (%); Tw and T0 indicate thicknesses after and before soaking in the water (mm), respectively.
L3 ðP 40% P 10% Þ
ð5Þ
4WB3 ðX 40% X 10% Þ
where MOE is modulus of elasticity (MPa), L,W and B are span, specimen’s width and thickness (mm), P10%, P40% load at 10 and 40% of maximum load (N), X10%, X40%, sample displacement at 10 and 40% of maximum load (mm),
MOR ¼
3P max L
ð6Þ
2W B2
where MOR is modulus of rupture (MPa), Pmax, load at maximum (N), L, W and B are span, specimen’s width and thickness (mm).
3. Results Summary of the test results are indicated in Table 1. Determination of the moisture content (MC) in the acetylated boards after conditioning showed that the MC was reduced as the WPG increased (Fig. 1). It was also revealed that the acetylation affected thickness swelling of the boards (Fig. 2) and caused significant reduction as the WPG increased in the boards when they were soaked in the water. Correlation between the springback and the MOR in the acetylated boards is shown in Fig. 3. It was determined that increase of the springback in the acetylated boards caused significant loss in the MOR. As it is shown in the Fig. 3, coefficient of the determination (R2) was 0.996 and 0.980 for the particleboard (PB) and the medium density fibreboard (MDF), respectively. The results indicated that the springback reduced the MOR in the acetylated boards. Comparison between both boards revealed greater influence of the springback on reduction of the MOR in the MDF. Results also revealed high correlation between the modulus of elasticity (MOE) and the springback in the PB (R2 = 0.984) as well as the MDF (R2 = 0.968) (Fig. 4). According to the results, increase of the springback reduced the MOE in the acetylated boards. Fig. 5 shows correlation between the WPG and the springback in the acetylated boards. As it has shown, increase of the WPG caused increase of the springback with higher coefficient of the determination (0.979 for the PB and 0.995 for the MDF). According to above results, it could be expressed that the acetylation affected severely the springback of the acetylated particleboard than the acetylated MDF. 4. Discussion Concerning the results, the acetylation caused the boards absorb less moisture during the conditioning and become more stable dimensionally. Also the acetylated boards do not swell
Table 1 Results of physical and mechanical tests. WPG (%)
MC (%)
Thickness swelling after 24 h soaking (%)
Springback (%)
Modulus of rupture (MPa)
Modulus of elasticity (MPa)
PB 0 5 9 16
5.80 5.71 5.51 4.10
18.79 ± 2.56a 14.69 ± 1.88 11.77 ± 2.86 5.79 ± 0.71
4.40 ± 1.66 6.61 ± 2.61 11.65 ± 4.06 16.89 ± 6.63
21.89 ± 3.43 15.35 ± 2.05 9.88 ± 1.67 4.09 ± 1.70
2691.88 ± 351.15 2418.50 ± 222.64 2079.00 ± 368.27 666.77 ± 518.25
MDF 0 5 9 16
– – – –
28.03 ± 5.36 11.66 ± 6.65 5.33 ± 3.53 3.74 ± 1.45
5.89 ± 3.14 8.75 ± 1.94 9.934 ± 2.28 11.87 ± 3.53
26.08 ± 7.50 15.35 ± 4.29 11.58 ± 3.31 10.01 ± 1.20
1204.33 ± 345.01 811.01 ± 289.57 559.80 ± 133.75 545.54 ± 126.29
a
Mean ± standard deviation.
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3000
MOEPB = -13.049x2 + 124.41x + 2313 R² = 0.9848
PB
6
2500
MDF
5
MOE (MPa)
Moisture content (%)
7
4 3 2 1
2000 1500 1000 500
MOEMDF = 152.63x2 -3858.9x + 29574 R² = 0.9678
0 0
5
9
16
0
0
WPG (%)
2
4
6
8
10
12
14
16
18
Springback (%) Fig. 1. Moisture content in acetylated particleboard during conditioning. Fig. 4. Correlation between springback and modulus of elasticity in acetylated particleboard (PB) and medium density fibreboard (MDF).
PB
30
MDF
25
18
Spring PB = 0.009 WPG 2 + 0.653 WPG + 4.098 R² = 0.979
16
20
Springback (%)
Thickness swelling (%)
35
15 10 5 0 5
0
10
15
20
WPG (%)
14 12 10 8
Spring MDF = -0.013 WPG 2 + 0.585 WPG + 5.955 R² = 0.995
6 4
MDF
2
PB
0
Fig. 2. Thickness swelling in acetylated boards after 24 h of soaking in water; PB: particleboard, MDF: medium density fibreboard.
0
2
4
6
8
10
12
14
16
18
WPG (%) Fig. 5. Correlation between weight percent gain (WPGs) and springback in acetylated particleboard (PB) and medium density fibreboard (MDF).
30
PB
MOE (MPa)
25
MOR MDF = 0.388x 2 -9.623x + 69.35 R² = 0.996
MDF
20 15 10
MOR PB = 0.061x 2 -2.644x + 31.55 R² = 0.980
5 0 0
2
4
6
8
10
12
14
16
18
Springback (%) Fig. 3. Correlation between springback and modulus of rupture in acetylated particleboard (PB) and medium density fibreboard (MDF).
severely when they are exposed to the water as the untreated one. Dimensional stability of the boards during the conditioning was a good indication of the acetylation effect on the thickness changes. According to the results, the moisture could not affect the boards and cause significant variation in the springback. However, the moduli of elasticity and rupture were significantly affected by the acetylation. The results also showed that the acetylation affected significantly the springback of the boards. As it was shown, the springback was highly correlated with the WPGs of the boards. It means that the acetylation significantly affected the springback in the boards. According to the results, it could be expressed that the springback of the boards was affected by the acetylation not by the moisture absorption, because the acetylation reduced the moisture
absorption in the boards. Therefore, the moisture should not be considered as the main reason for dimensional instability of the boards or their springback. It can be expressed that increase of the WPG was the main reason for the springback in the acetylated boards. Therefore, high correlation between the springback and the strength losses of the boards are related to the acetylation and resulted debonding of the wood elements. The springback in the acetylated boards might be related to bulking of the fibers or chips that was occurred during the acetylation. The springback is an indication of the debonding of the wood elements and stress relief [19]. According to Sanders et al. [22], the acetylation causes bulking of the wood cell walls. Acetylated materials become denser and stiffer materials according to authors’ observation. Those materials resist against applied stresses during pressing of the boards matt during the production. After opening the press, accumulated stresses are gradually relieved. Stronger bonding due to suitable resins could cease the stress relief in the composites. However, application of aqueous-based resins; such as the ureaformaldehyde, interfere bonding between the wood elements due to low wettability according to reports [23,16]. As the urea formaldehyde resin was used here to bond the acetylated wood elements; it is likely that the acetylation interfered with adhesion of the aqueous-based adhesive and could not properly bond the wood fibers and the chips depending on level of the acetylation and caused strength losses in the acetylated composites. Vick and Rowell [23] reported also interfere between the acetylation and the aqueous-based adhesives. Vick et al. [16] reported loss of the MOE and the MOR in acetylated flakeboard.
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5. Conclusion Results of the current research showed that the strength loss in the acetylated composites is highly correlated with the springback. And the springback itself is also significantly depended to the extent of the acetylation and causes loss of the strengths followed by the WPG increase. Acknowledgment The authors express their sincere thanks to Khazarchoob Company due to preparing the fibers for the current research. References [1] Rowell RM. Chemical modification of wood. Forest Prod Abstr 1983;4(12):363–82. [2] Rowell RM. Physical and mechanical properties of chemically modified wood. In: Hon DNS, editor. Chemical modification of lignocellulosic materials. New York: Marcel Dekker; 1996. p. 295–310. [3] Nilsson T, Rowell RM, Simonson R, Tillman AM. Fungal resistance of pine particleboards made from various types of acetylated chips. Holzforschung 1988;42(2):123–6. [4] Takahashi M, Imamura Y, Tanahashi M. Effect of acetylation on decay resistance of wood against brown rot, white rot and soft rot. In: International chemistry, congress of pacific basin societies, agrochemistry, sub-symposium on chemical modification of lignocellulosic materialschemical reactions; 1989. p. 16. [5] Militz H. Die verbesserung des schwind- und quellverhaltens und der dauerhaftigkeit von holz mittels behandlung mit unkalalysiertem essigsäureanhydrid. Holz als Roh Werksttoff 1991;49:147–52. [6] Larsson-Brield P, Simonson R, Bergman O. Resistance of acetylated wood to biological degradation: evaluation of field test. In: The international research group on wood preservation, IRG Document No. IRG/WP 97-30139; 1997. [7] Ibach RE, Hadi YS, Nandika D, Yusuf S, Indrayani Y. Termite and fungal resistance in situ polymerized tributyltin acrylate and acetylated Indonesian and USA wood. In: The international research group on wood preservation, Document No. IRG/WP 00-30219; 2000.
[8] Mohebby B. Biological attack of acetylated wood. Ph.D. thesis, Göttingen University, Göttingen, Germany; 2003. p. 167. [9] Evans PD, Wallis FA, Owen NL. Weathering of chemically modified wood surfaces: natural weathering of scots pine acetylated to different weight gains. Wood Sci Technol 2000;34:151–65. [10] Rowell RM, Youngquist JA, Imamura Y. Strength test on acetylated aspen flakeboards exposed to a brown rot fungus. Wood Fiber Sci 1988;20(2): 266–71. [11] Rowell RM, Kawai SIY, Norimoto M. Dimensional stability, decay resistance and mechanical properties of veneer-faced low-density particleboards made from acetylated wood. Wood Fiber Sci 1989;21(1):67–79. [12] Ohmae K, Minato K, Norimoto M. The analysis of dimensional changes due to chemical treatment and water soaking for Hinoki (Chamaecyparis obusta) wood. Holzforschung 2002;56(1):98–102. [13] Mohebby B, Hadjhassani R. Moisture repellent effect of acetylation on poplar fibers. J Agric Sci Technol 2008;10(2):157–63. [14] Fuwape JA, Oyagade AO. Strength and dimensional stability of acetylated Gamelina and spruce particleboard. J Trop Forest Prod 2000;6(2):184–9. [15] Mahlberg R, Paajanen L, Nurmi A, Kivisto A, Rowell RM. Effect of chemical modification of wood on the mechanical and adhesion properties of wood fiber/polypropylene fiber and polypropylene/veneer composites. Holz als Rohund Werkstoff 2001;59:319–26. [16] Vick CB, Krzysik A, Wood Jr JE. Acetylated, isocyanate-bonded flakeboards after accelerated aging: dimensional stability and mechanical properties. Holz als Roh- und Werkstoff 1991;49:221–8. [17] Geimer RL, Price EW. Construction variables considered in fabrication of a structural flakeboard. In: Proceedings of construction variables in fabrication, Kamzas, USA; June 6–8 1978. [18] Palardy RD, Haataja BA, Shaler SM, Williams AD, Laufenberg TL. Pressing of wood composite panels at moderate temperature and high moisture content. Forest Prod J 1989;39(4):27–32. [19] River BH. Outdoor aging of wood-based panels and correlation with laboratory aging. Forest Prod J 1994;44(11/12):55–65. [20] Hann RA, Black JM, Blomquist RF. How durable is particleboard? Part II: the effect of temperature and humidity. Forest Prod J 1963;8(5):169–74. [21] DIN 68754-1. Harte und mittelharte Holzfaserplatten für das Bauwesen – Holzwerkstoffklasse, Deutsches Institut für Normung; 1976. [22] Sander C, Beckers EPJ, Militz H, van Veenendaal W. Analysis of acetylated wood by electron microscopy. Wood Sci Technol 2003;37(1):39–46. [23] Vick CB, Rowell RM. Adhesive bonding of acetylated wood. Int J Adhes Adhes 1990;10(4):263–72.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Springback in acetylated wood based composites Behbood Mohebby a,*, Maryam Gorbani-Kokandeh b, Mojtaba Soltani c a b c
Department of Wood and Paper Sciences, Faculty of Natural Resources, Tarbiat Modares University, P.O. Box 46414-356, Noor, Iran Department of Wood and Paper Sciences, Agricultural Sciences and Natural Resources University of Sari, Sari, Mazandaran, Iran Department of Wood and Paper Sciences, Azad University, Chaloos, Iran
a r t i c l e
i n f o
Article history: Received 12 November 2008 Received in revised form 7 February 2009 Accepted 8 February 2009 Available online 31 May 2009 Keywords: Acetylation Spring back Particleboards Medium density fibreboard
a b s t r a c t Reasons of the strength loss of the acetylated wood based composites were still under question. This research was considered to study springback in the acetylated particleboard and the medium density fibreboard. Chips and fibres were acetylated by using the acetic anhydride to gain different percentages of the weights (WPG). The boards were made based on the target thicknesses. Thickness of the boards as well as the modulus of elasticity (MOE) and the modulus of rupture (MOR) were determined after conditioning based on three point static bending test. Results showed that the moisture content and the thickness swelling of the boards were reduced as the WPG was increased. However, increase of the springback and reduction of the MOE and the MOR were revealed as the WPG increased in the test boards after condition. It was also revealed that the springback was correlated with the weight gain and increased as the WPG was raised in the test boards. Reduction of the MOE and the MOR was also correlated with the springback in the test boards. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction During the last decades, demands in the market obliged wood technologists to introduce new composites with enhanced properties, such as particleboard, oriented strand board, plywood, and hardboards. Those products are currently known as interested engineering materials for building and construction. Even though, they were fascinating and reached higher technological properties in comparison with solid wood, however such products had similar problems as wood. For example, those products are potentially attacked by the biological agents, they become dimensionally instable when they are exposed to the changing humidities, influenced by the UV-light and burnt when exposed to the fire. Scientists are in challenge to find solutions for the mentioned problems. Acetylation is one of the interesting processes which have been recently industrialized to modify the wood and the wood based composite to enhance properties of the products for application in the severe conditions and also remove the noticed problems. The acetylation is a reaction between the acetic anhydride with the wood polymers, lignin, cellulose and also hemicelluloses. Advantages of the acetylation have been reported by many authors. The wood and the wood based composites become moisture repellent due to the acetylation reaction [1,2]. The reaction is * Corresponding author. Tel.: +98 122 6253101/9111255972; fax: +98 122 6253499. E-mail addresses: [email protected] (B. Mohebby), ghorbani_mary@ yahoo.com (M. Gorbani-Kokandeh), [email protected] (M. Soltani). 0950-0618/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2009.02.007
accompanied by substitution of the hydrophilic hydroxyl groups with the hydrophobic acetyl groups in the wood cell wall polymers and causes increase in the wood bioresistance [3–8]; resistance against the weathering [9] and decrease in the board roughness [9], reduction in the moisture absorption [5,10–13]. In spite of those advantages, there are different reports indicating that the mechanical strengths are diminished in the acetylated wood based composites [10,11,2,14,15]. There are several opinions to explain reasons of the strength loss; e.g. type of the adhesive [16], less press pressure, etc. It has been proposed to apply higher press pressure to increase the strengths [10,11,2]. However, no report has paid detailed attention to springback in the acetylated composites and its correlation with the strength losses. It is known that the springback in the wood based composites occurs usually after manufacture of the boards. The springback is an irreversible thickness swelling which occurs after wetting of the composites and is attributed to the release of applied stresses accompanied by some loss of the glue bonds [17,18]. The springback indicates debonding of the adhesion between the wood elements and the adhesives. Result of the debonding in the boards is loss of the strengths [19], which is occurred in the specimens subjected to different exposures because of a lowering of the board density [19]. The mechanical properties that were presumably most directly affected by the springback are the shear strength, the modulus of elasticity, and the modulus of rupture, because these properties depend somewhat on the strength that is developed by the mechanical interlocking of the compressed particles in the composites [20].
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B. Mohebby et al. / Construction and Building Materials 23 (2009) 3103–3106
Current research was focused on relationship between the acetylation and the strength changes in the wood based composites, the medium density fibreboard (MDF) and the particleboard (PB), which is associated with the springback. 2. Experimental program
Static bending test – The test boards were cut into sizes of 50 mm (width) and 300 mm (length) for determination of the static bending according to DIN 987541 [21]. Modulus of the elasticity (MOE) and modulus of the rapture (MOR) were determined based on three point bending test. For each WPG, 20 and 16 samples were used as replicates for the PB and the MDF, respectively. The MOE and the MOR were calculated based on the Eqs. (5) and (6).
MOE ¼
2.1. Acetylation Poplar wood fibers were provided by Khazarchoob Company, manufacture of the MDF in Iran, and required chips were prepared by a laboratory chipper from Oriental beech wood (Fagus orientalis Lipsky). Both fibers and chips were dried in an oven for 24 h at 103 ± 2 °C. Afterwards, the acetylation was carried out in a stainless steel reactor at 120 °C for varying time to obtain different weight percent gains (WPGs). Acetic anhydride was used for the acetylation reaction. Acetylated fibers and chips were washed in rinsed water to remove formed acetic acid as by-product of the reaction. Afterwards, they were oven dried for 24 h to determine the WPGs in the acetylated fibers and chips. The WPGs were determined based on oven dry weights of parallel samples which were treated at the same conditions as the main samples. The WPG was calculated according to Eq. (1):
WPG ¼ ðW act W unt Þ=W unt 100
ð1Þ
where WPG indicates the weight percent gain (%); Wact and Wunt oven dry weight after and before the acetylation (g), respectively. 2.2. Manufacture of boards Sample boards were made based on required target densities with thicknesses of 10 and 15 mm for the MDF and the PB, respectively. Urea formaldehyde (UF) was applied as resin for 10% based on oven dried weights of the used fibers and chips in the board mats. Pressure (30 bar) was applied at press temperature of 170 °C for 10 min. Five boards (40 cm 40 cm) were made for any WPG. The test boards were conditioned at room temperature (25 °C) and a relative humidity of 65% for 2 weeks. Afterwards, samples were cut from the boards according to required sizes of the tests. 2.3. Tests Springback and soaking test – Specimens with sizes of 50 mm 50 mm board’s thickness were cut from the test boards. Their thicknesses were measured at three points to determine the springback of the boards according to Eq. (2). Afterwards, they were weighed and dried in an oven for 24 h at 103 ± 2 °C to determine the moisture contents (MC) of the conditioned test boards. The MC was calculated based on Eq. (3).
S ¼ ðT 1 T 0 Þ=T 0 100
ð2Þ
Where S indicates springback of the board (%); T1 and T0 indicate board thicknesses after and before the conditioning (mm), respectively.
MC ¼ ðW W W 0 Þ=W 0 100
ð3Þ
where MC indicates the moisture contents (%); Ww and W0 indicate wet and dry weights of the samples (g), respectively.Thicknesses of the dried samples were measured at three points and they were then soaked in the water (25 °C) for 24 h. The thicknesses of the wet samples were measured again to determine the thickness swelling according to Eq. (4). For those tests, 26 and 16 samples were used for the PB and the MDF, respectively.
T S ¼ ðT W T 0 Þ=T 0 100
ð4Þ
where TS indicates the thickness swelling (%); Tw and T0 indicate thicknesses after and before soaking in the water (mm), respectively.
L3 ðP 40% P 10% Þ
ð5Þ
4WB3 ðX 40% X 10% Þ
where MOE is modulus of elasticity (MPa), L,W and B are span, specimen’s width and thickness (mm), P10%, P40% load at 10 and 40% of maximum load (N), X10%, X40%, sample displacement at 10 and 40% of maximum load (mm),
MOR ¼
3P max L
ð6Þ
2W B2
where MOR is modulus of rupture (MPa), Pmax, load at maximum (N), L, W and B are span, specimen’s width and thickness (mm).
3. Results Summary of the test results are indicated in Table 1. Determination of the moisture content (MC) in the acetylated boards after conditioning showed that the MC was reduced as the WPG increased (Fig. 1). It was also revealed that the acetylation affected thickness swelling of the boards (Fig. 2) and caused significant reduction as the WPG increased in the boards when they were soaked in the water. Correlation between the springback and the MOR in the acetylated boards is shown in Fig. 3. It was determined that increase of the springback in the acetylated boards caused significant loss in the MOR. As it is shown in the Fig. 3, coefficient of the determination (R2) was 0.996 and 0.980 for the particleboard (PB) and the medium density fibreboard (MDF), respectively. The results indicated that the springback reduced the MOR in the acetylated boards. Comparison between both boards revealed greater influence of the springback on reduction of the MOR in the MDF. Results also revealed high correlation between the modulus of elasticity (MOE) and the springback in the PB (R2 = 0.984) as well as the MDF (R2 = 0.968) (Fig. 4). According to the results, increase of the springback reduced the MOE in the acetylated boards. Fig. 5 shows correlation between the WPG and the springback in the acetylated boards. As it has shown, increase of the WPG caused increase of the springback with higher coefficient of the determination (0.979 for the PB and 0.995 for the MDF). According to above results, it could be expressed that the acetylation affected severely the springback of the acetylated particleboard than the acetylated MDF. 4. Discussion Concerning the results, the acetylation caused the boards absorb less moisture during the conditioning and become more stable dimensionally. Also the acetylated boards do not swell
Table 1 Results of physical and mechanical tests. WPG (%)
MC (%)
Thickness swelling after 24 h soaking (%)
Springback (%)
Modulus of rupture (MPa)
Modulus of elasticity (MPa)
PB 0 5 9 16
5.80 5.71 5.51 4.10
18.79 ± 2.56a 14.69 ± 1.88 11.77 ± 2.86 5.79 ± 0.71
4.40 ± 1.66 6.61 ± 2.61 11.65 ± 4.06 16.89 ± 6.63
21.89 ± 3.43 15.35 ± 2.05 9.88 ± 1.67 4.09 ± 1.70
2691.88 ± 351.15 2418.50 ± 222.64 2079.00 ± 368.27 666.77 ± 518.25
MDF 0 5 9 16
– – – –
28.03 ± 5.36 11.66 ± 6.65 5.33 ± 3.53 3.74 ± 1.45
5.89 ± 3.14 8.75 ± 1.94 9.934 ± 2.28 11.87 ± 3.53
26.08 ± 7.50 15.35 ± 4.29 11.58 ± 3.31 10.01 ± 1.20
1204.33 ± 345.01 811.01 ± 289.57 559.80 ± 133.75 545.54 ± 126.29
a
Mean ± standard deviation.
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B. Mohebby et al. / Construction and Building Materials 23 (2009) 3103–3106
3000
MOEPB = -13.049x2 + 124.41x + 2313 R² = 0.9848
PB
6
2500
MDF
5
MOE (MPa)
Moisture content (%)
7
4 3 2 1
2000 1500 1000 500
MOEMDF = 152.63x2 -3858.9x + 29574 R² = 0.9678
0 0
5
9
16
0
0
WPG (%)
2
4
6
8
10
12
14
16
18
Springback (%) Fig. 1. Moisture content in acetylated particleboard during conditioning. Fig. 4. Correlation between springback and modulus of elasticity in acetylated particleboard (PB) and medium density fibreboard (MDF).
PB
30
MDF
25
18
Spring PB = 0.009 WPG 2 + 0.653 WPG + 4.098 R² = 0.979
16
20
Springback (%)
Thickness swelling (%)
35
15 10 5 0 5
0
10
15
20
WPG (%)
14 12 10 8
Spring MDF = -0.013 WPG 2 + 0.585 WPG + 5.955 R² = 0.995
6 4
MDF
2
PB
0
Fig. 2. Thickness swelling in acetylated boards after 24 h of soaking in water; PB: particleboard, MDF: medium density fibreboard.
0
2
4
6
8
10
12
14
16
18
WPG (%) Fig. 5. Correlation between weight percent gain (WPGs) and springback in acetylated particleboard (PB) and medium density fibreboard (MDF).
30
PB
MOE (MPa)
25
MOR MDF = 0.388x 2 -9.623x + 69.35 R² = 0.996
MDF
20 15 10
MOR PB = 0.061x 2 -2.644x + 31.55 R² = 0.980
5 0 0
2
4
6
8
10
12
14
16
18
Springback (%) Fig. 3. Correlation between springback and modulus of rupture in acetylated particleboard (PB) and medium density fibreboard (MDF).
severely when they are exposed to the water as the untreated one. Dimensional stability of the boards during the conditioning was a good indication of the acetylation effect on the thickness changes. According to the results, the moisture could not affect the boards and cause significant variation in the springback. However, the moduli of elasticity and rupture were significantly affected by the acetylation. The results also showed that the acetylation affected significantly the springback of the boards. As it was shown, the springback was highly correlated with the WPGs of the boards. It means that the acetylation significantly affected the springback in the boards. According to the results, it could be expressed that the springback of the boards was affected by the acetylation not by the moisture absorption, because the acetylation reduced the moisture
absorption in the boards. Therefore, the moisture should not be considered as the main reason for dimensional instability of the boards or their springback. It can be expressed that increase of the WPG was the main reason for the springback in the acetylated boards. Therefore, high correlation between the springback and the strength losses of the boards are related to the acetylation and resulted debonding of the wood elements. The springback in the acetylated boards might be related to bulking of the fibers or chips that was occurred during the acetylation. The springback is an indication of the debonding of the wood elements and stress relief [19]. According to Sanders et al. [22], the acetylation causes bulking of the wood cell walls. Acetylated materials become denser and stiffer materials according to authors’ observation. Those materials resist against applied stresses during pressing of the boards matt during the production. After opening the press, accumulated stresses are gradually relieved. Stronger bonding due to suitable resins could cease the stress relief in the composites. However, application of aqueous-based resins; such as the ureaformaldehyde, interfere bonding between the wood elements due to low wettability according to reports [23,16]. As the urea formaldehyde resin was used here to bond the acetylated wood elements; it is likely that the acetylation interfered with adhesion of the aqueous-based adhesive and could not properly bond the wood fibers and the chips depending on level of the acetylation and caused strength losses in the acetylated composites. Vick and Rowell [23] reported also interfere between the acetylation and the aqueous-based adhesives. Vick et al. [16] reported loss of the MOE and the MOR in acetylated flakeboard.
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5. Conclusion Results of the current research showed that the strength loss in the acetylated composites is highly correlated with the springback. And the springback itself is also significantly depended to the extent of the acetylation and causes loss of the strengths followed by the WPG increase. Acknowledgment The authors express their sincere thanks to Khazarchoob Company due to preparing the fibers for the current research. References [1] Rowell RM. Chemical modification of wood. Forest Prod Abstr 1983;4(12):363–82. [2] Rowell RM. Physical and mechanical properties of chemically modified wood. In: Hon DNS, editor. Chemical modification of lignocellulosic materials. New York: Marcel Dekker; 1996. p. 295–310. [3] Nilsson T, Rowell RM, Simonson R, Tillman AM. Fungal resistance of pine particleboards made from various types of acetylated chips. Holzforschung 1988;42(2):123–6. [4] Takahashi M, Imamura Y, Tanahashi M. Effect of acetylation on decay resistance of wood against brown rot, white rot and soft rot. In: International chemistry, congress of pacific basin societies, agrochemistry, sub-symposium on chemical modification of lignocellulosic materialschemical reactions; 1989. p. 16. [5] Militz H. Die verbesserung des schwind- und quellverhaltens und der dauerhaftigkeit von holz mittels behandlung mit unkalalysiertem essigsäureanhydrid. Holz als Roh Werksttoff 1991;49:147–52. [6] Larsson-Brield P, Simonson R, Bergman O. Resistance of acetylated wood to biological degradation: evaluation of field test. In: The international research group on wood preservation, IRG Document No. IRG/WP 97-30139; 1997. [7] Ibach RE, Hadi YS, Nandika D, Yusuf S, Indrayani Y. Termite and fungal resistance in situ polymerized tributyltin acrylate and acetylated Indonesian and USA wood. In: The international research group on wood preservation, Document No. IRG/WP 00-30219; 2000.
[8] Mohebby B. Biological attack of acetylated wood. Ph.D. thesis, Göttingen University, Göttingen, Germany; 2003. p. 167. [9] Evans PD, Wallis FA, Owen NL. Weathering of chemically modified wood surfaces: natural weathering of scots pine acetylated to different weight gains. Wood Sci Technol 2000;34:151–65. [10] Rowell RM, Youngquist JA, Imamura Y. Strength test on acetylated aspen flakeboards exposed to a brown rot fungus. Wood Fiber Sci 1988;20(2): 266–71. [11] Rowell RM, Kawai SIY, Norimoto M. Dimensional stability, decay resistance and mechanical properties of veneer-faced low-density particleboards made from acetylated wood. Wood Fiber Sci 1989;21(1):67–79. [12] Ohmae K, Minato K, Norimoto M. The analysis of dimensional changes due to chemical treatment and water soaking for Hinoki (Chamaecyparis obusta) wood. Holzforschung 2002;56(1):98–102. [13] Mohebby B, Hadjhassani R. Moisture repellent effect of acetylation on poplar fibers. J Agric Sci Technol 2008;10(2):157–63. [14] Fuwape JA, Oyagade AO. Strength and dimensional stability of acetylated Gamelina and spruce particleboard. J Trop Forest Prod 2000;6(2):184–9. [15] Mahlberg R, Paajanen L, Nurmi A, Kivisto A, Rowell RM. Effect of chemical modification of wood on the mechanical and adhesion properties of wood fiber/polypropylene fiber and polypropylene/veneer composites. Holz als Rohund Werkstoff 2001;59:319–26. [16] Vick CB, Krzysik A, Wood Jr JE. Acetylated, isocyanate-bonded flakeboards after accelerated aging: dimensional stability and mechanical properties. Holz als Roh- und Werkstoff 1991;49:221–8. [17] Geimer RL, Price EW. Construction variables considered in fabrication of a structural flakeboard. In: Proceedings of construction variables in fabrication, Kamzas, USA; June 6–8 1978. [18] Palardy RD, Haataja BA, Shaler SM, Williams AD, Laufenberg TL. Pressing of wood composite panels at moderate temperature and high moisture content. Forest Prod J 1989;39(4):27–32. [19] River BH. Outdoor aging of wood-based panels and correlation with laboratory aging. Forest Prod J 1994;44(11/12):55–65. [20] Hann RA, Black JM, Blomquist RF. How durable is particleboard? Part II: the effect of temperature and humidity. Forest Prod J 1963;8(5):169–74. [21] DIN 68754-1. Harte und mittelharte Holzfaserplatten für das Bauwesen – Holzwerkstoffklasse, Deutsches Institut für Normung; 1976. [22] Sander C, Beckers EPJ, Militz H, van Veenendaal W. Analysis of acetylated wood by electron microscopy. Wood Sci Technol 2003;37(1):39–46. [23] Vick CB, Rowell RM. Adhesive bonding of acetylated wood. Int J Adhes Adhes 1990;10(4):263–72.