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Some physical, mechanical properties and termite resistance of ammonium pentaborate-treated strand board Wei Gao, Jinzhen Cao, Jianzhang Li Beijing Forestry University, College of Material Science & Technology Haidian, Beijing, China
ABSTRACT In order to improve termite resistance and reduce formaldehyde emission level of strand board, ammonium pentaborate (APB) was introduced into strand board with phenol formaldehyde (PF) resin as its adhesive. The physical and mechanical properties, termite resistance and formaldehyde emission of panels were evaluated. The results showed that APB was very effective on improving the termite resistance even at low APB loadings. Its significant effect on reducing formaldehyde emission of strand boards was also obtained as expected, which can be explained by the reaction of ammonium and acetic acid with formaldehyde caused by APB during hot pressing. As same as the other borates such as disodium octaborate tetrahydrate (DOT) and zinc borate (ZB), APB also showed a negative effect on physical and mechanical properties of strand board. The adverse effect could be compensated by a 3-layer structure which contained APB only in the surface layers of the strand board. Compared with DOT and ZB, APB showed the least influence in 3-layer strand board at same BAE level of 1.8%. KEY WORDS: ammonium pentaborate; strand board; termite resistance; physical and mechanical properties; formaldehyde emission.
INTRODUCTION The excellent mechanical and physical properties of oriented strand board (OSB) make it a construction material for roof, wall and floor in the residential house and components for frame furniture (Biblis 1985, Guss 1995, Laks 2002). But the majority of OSB is normally made from non-durable wood species such as aspen and susceptible to be attacked by mould, decay fungi and termites in increasingly challenging environments. Therefore, a large number of investigations have been done to protect OSB from biological attacks (Sean et al. 1999, Curling et al. 2003, Goroyias and Hale 2004, Smart and Wall 2006, Amusant et al. 2008). Boron compounds are environmentally-friendly preservatives that are effective against both fungi and insects, inexpensive, colorless, odorless and non-corrosive (Bum Ra 1999, Yalinkilic 2000, 61
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Yıldız et al. 2009). Thus, many boron compounds such as boric acid, borax, disodium octaborate tetrahydrate (DOT), or some metal-containing borates are used as fungicide, mouldicide, and insecticide for wood composites. Vapor boron treatment has been indicated as an effective approach to apply trimethyl borate on wood composites (Turner et al. 1990). Myles (1994) investigated the efficacy of DOT in aspen waferboard against the eastern subterranean termite. The results showed that the termites preferred to attack untreated samples and a 100 % mortality of termites was reached within one month. Smart and Wall (2006) suggested that 10% copper hydroxide formulation of copper borate (CuB) provided superior protection against mould, while all formulations tested gave adequate protection against fungal decay and formosan subterranean termite (FST) attack. Except CuB, zinc borate (ZB) is most widely used to increase the durability of composites (Lee and Wu 2002, Wu et al. 2003, Lee et al. 2004, Yıldız et al. 2009). Wu et al. (2003) indicated that the incorporation of ZB and calcium borate (CaB) into the OSB provided suitable protection against brown- and white-rot fungi, which showed no significant weight loss in treated samples even at 1.0% BAE. Lee et al. (2004) reported that ZB and CaB treatments at BAE levels higher than 1.0% provided sufficient protection from FST attack. However, many works (Sean et al. 1999, Dönmez and Kalaycıoğlu 2006, Çavdar et al. 2008) indicated that both ZB and CaB addition to wood-based panels caused significant reduction of the mechanical and physical properties. Ammonium pentaborate (APB, NH4B5O8•4H2O) is a metal-free boron compound, which is a kind of fire retardant used in polymers (Myers 1985). At temperatures higher than 90 °C, APB begins to release ammonium (Zhou 2004). After further heating at high temperatures, APB will be transformed to B2O3 containing ammonium. Boron oxide from APB will exist in the form of boric acid which plays the roles of fungicide and insecticide in wood composites. The ammonium released from APB will react with free formaldehyde from phenol formaldehyde (PF) resin which may reduce the formaldehyde emission level. Based on above considerations, we consider APB as a potential additive for wood composites, especially for those structural composites. Our previous research focused on the effect of APB on the curing of PF resin by using differential scanning calorimetry (DSC). The results showed that APB caused a second curing process in PF resin and made the cure reaction more completely in prolonged time (Gao et al. 2008). This phenomenon has not been observed in other borates incorporated with PF resin, so the conclusion can be drawn that the influence of APB on the properties of wood composites would differ from other borates such as ZB. The objective of this study was to determine the effect of APB on improving termite resistance and reducing formaldehyde emission of the PF bonded strand board, and also to investigate the influence of APB on physical and mechanical properties. Two kinds of widely used borates, DOT and ZB, were also used for comparison.
MATERIAL AND METHODS Manufacturing of strand boards Chinese white poplar (Populus tomentosa) strands were obtained from a local manufacturer, which were of 90-120 mm length, 10-20 mm width and 0.4-0.7 mm thickness and dried to approximately 3% moisture content before manufacturing. PF resin was synthesized in laboratory with solid content of 46.4% and pH value of 10.5-11.5. The target thickness and specific density of the strand boards was 12 mm and 700 kg m-3 respectively, while the loading of PF resin was 54 kg. m-3. APB, DOT and ZB (Zn3B2O6•3.5H2O) are all analytical reagents. During the blending process, they were sprayed into the blender together with 1.0 % (based on oven dried stand weight) waterproofing agent. 62
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The target BAE level of APB was 0.7, 1.0, 1.8 and 3.5% based on the whole oven dry strand weight. APB distributed evenly in 1-layer structure, while for 3-layer structure APB was applied only on both surface layers with a thickness of 2 mm. The effects of APB, ZB and DOT were compared at BAE level of 1.8% in 3-layer structure. After blending, the strands were hand-formed with random orientation into 450 × 450 mm billets. The temperature, pressure, and duration used in hot pressing are 160 °C, 1.5 MPa, and 40 sec mm-1, respectively. After hot pressing, the strand boards were cooled down and conditioned at 22±1 °C, 65±2% relative humidity (RH) till constant weight prior to test. For each BAE level, three replicate panels were prepared.
Determination of physical and mechanical properties
The water absorption (WA), thickness swelling (TS), internal bond strength (IB), modulus of rupture (MOR), and modulus of elasticity (MOE) of the strand boards were tested according to the Chinese standard (GB/T 17657-1999) “Test Methods of Evaluating the Properties of Wood-Based Panels and Surface Decorated Wood-Based Panels”. According to the standard, the specimens with a dimension of 50±1 mm (L) × 50±1 mm (W) × 12±1 mm (T) were completely immersed into water at 20±2 °C for 24 h, then WA (%) and TS (%) after 24 h were calculated based on the weight and mid-span thickness changes. Same size specimens were used to evaluate IB. Three point bending method was applied to test MOR and MOE with specimen of 350±2 (L) × 50±1 (W)× 12±1 (T) mm in a mechanical testing machine (Jinan Shijin, Made in China).
Determination of formaldehyde emission
One specimen with a dimension of 40±1(W) × 100±2(L) × 12±1(T) mm was edge sealed with tinfoil adhesive tape, and then placed into a 500 ml jar at 20 °C and 40 % RH for 48 hours. The formaldehyde concentration in the jar was measured using a F400 formaldehyde-meter (Wales). The formaldehyde emission value was defined as follow: where, V F is the specific formaldehyde emission (ppm mm-2), Vt is the measured value of formaldehyde emission (ppm), S is the surface area of the specimen (mm 2). For each experiment condition, 8 replicates were measured and their average value was recorded as the final formaldehyde emission.
Testing the reaction between formaldehyde and acetic acid
0, 5, 10, 15, 20 % acetic acid solutions with concentrations of 20, 100 and 300 mg were mixed with 200 ml formalin solution with corresponding concentrations of 2, 10 and 30 mg.ml-1. The sealed mixture was heated at 95 °C for 1 h. After cooled to room temperature, the absorbance was tested at 412 nm by a visible spectrophotometer (type of 721-100).
Formosan subterranean termite resistance test
Five samples with a dimension of 25±1 × 25±1 × 12±1 mm for each experiment condition and five untreated controls were taken for no-choice laboratory termite tests according to the American Wood Protection Association Standards (AWPA. 2008). Prior to each termite test, the blocks were oven-dried at 60 °C for 72 h and sample weight (W1) was measured. Each test bottle (80 mm diameter × 100 mm height) was autoclaved for 60 min under 1.01 MPa and dried. Autoclaved sand (150 g) and distilled water (30 ml) were added to each bottle. Finally, four hundred termites 63
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(Coptotermes Formosanus Shiraki), 360 workers and 40 soldiers, were added to the opposite sides of the test block in the container. All the containers were maintained at 24 °C. After 4 weeks, each bottle was dismantled. Live termites were counted, and test blocks were removed and cleaned. Each block was oven-dried again at 60 °C for 72 h to determine the dry sample weight (W2). From the measurements, sample weight loss [(W1-W2)/W1] and termite mortalities were determined.
RESULTS AND DISCUSSION Effect of APB loading level and distribution on the physical and mechanical properties of strand board
Fig. 1 show the physical and mechanical properties of strand boards treated with different BAE level of APB for both 1-layer and 3-layer structures. It is apparent from Fig. 1 (a) that APB caused increasing of WA and TS, suggesting that the dimensional stability of the strand board would become worse after addition of APB. For 1-layer strand board, IB decreases dramatically at a lower BAE of 0.7 %, increases and reaches maximum at BAE of 1.0 %, and then decreases again with further addition of APB. MOR and MOE reduce dramatically as well at BAE of 0.7 %, and then increase with increasing APB. The increasing trend becomes gradual after BAE of 1.8 %. This result is consistent with our previous report that APB caused a second curing process of PF resin within a certain range of APB loading (Gao et al. 2008). After the second curing process, the PF resin would cure more completely and thus the mechanical properties could be improved. It should also be noted that the second curing process appeared at higher temperatures than normal PF resin. However, it could not happen or complete if the hot pressing time is limited and the pressing temperature is relatively lower, which may explain the reduction of IB at higher BAE of APB such as 1.8 and 3.5 %. To alleviate the adverse effect of APB on physical and mechanical properties, a 3-layer structure was proposed in this study. As shown in Fig. 1, 3-layer strand boards reduced WA and TS, and improved IB, MOR, MOE compared to 1-layer strand boards. The MOR and MOE of the 3-layer APB treated strand boards are comparable to the control panels. It suggests that the 3-layer structure is an effective approach to compensate the loss in mechanical properties caused by APB.
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Fig. 1: Water absorption, thickness swelling (a), internal bond strength (b), MOR and MOE (c) of strand boards using ammonium pentaborate (APB) modified phenol-formaldehyde (PF) resin as adhesive.
Effect of different borates on the physical and mechanical properties of strand board
The physical and mechanical properties of strand boards treated with APB, DOT and ZB at 1.8% BAE level are compared in Fig. 2. The results in Fig. 2a show that all the three types of borates increase the WA and TS to different extents, while APB displays the least influence on these two index. It indicates that the APB-treated strand board has a better dimensional stability than the DOT and ZB-treated ones, which may be caused by the significantly different target weight ratios of these three borates based on aqueous PF resin, that is, 8.3 % for APB, 15.1 % for DOT, and 55.2 % for ZB at the same BAE of 1.8 %. In addition, the extremely lower water solubility of ZB at room temperature may lead to deep penetration of water into sample and even getting through the glue line of the strand boards. The mechanical properties of the strand boards treated with three kinds of borates are demonstrated in Figs. 2b, c. APB caused the least loss of IB compared with DOT and ZB (Fig. 2b), while DOT occupies the lowest IB. The inferior IB of DOT than ZB is consistent with the results reported in previous study (Çavdar et al. 2008). Actually, the reduction of IB caused by APB is insignificant as compared with the untreated control considering the standard deviation. For MOR and MOE, the strand boards treated with APB and DOT show similar values as the untreated control, but ZB displays an obvious negative effect. Above all, APB possesses the best mechanical performance in 3-layer strand boards at the same BAE level of 1.8 %. 65
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Fig. 2: Water absorption, thickness swelling (a), internal bond strength (b), MOR and MOE (c) of 3-layer strand boards using phenol-formaldehyde (PF) resin as adhesive and treated with ammonium pentaborate (APB), disodium octaborate tetrahydrate (DOT) and zinc borate (ZB) at a same BAE level of 1.8%.
Formaldehyde release
The effects of APB, ZB and DOT on the formaldehyde emission of treated strand board are listed in Tab. 1. The VF of untreated strand board is 4.00 × 10-5 ppm mm-2. After addition of APB or other two borates, the average VF values decrease significantly. The reduction of formaldehyde emission caused by APB may be explained by two mechanisms. First, ammonium in APB will react with free formaldehyde in PF resin during hot pressing, which is also the mechanism of a traditional way to reduce formaldehyde emission from wood composites known as ammonia gas fumigating method (Liu and Liu 2009). Another possible reason is that the acetic acid reacted with free formaldehyde in resin. The previous research indicated that borax may have caused hydrolysis of acetyl groups with the existence of the thermal effects like drying or press temperature in panel production (Colak and Colakoglu 2004). Thus APB is likely to possess the similar effect to borax that caused hydrolysis of acetyl groups during hot pressing and result in the reaction of acetic acid with free formaldehyde. In order to confirm this hypothesis, different concentrations of acetic acid and formalin solution was mixed and reacted at 90 °C for 1 hour. The residual formaldehyde in samples was tested and the results are shown in Fig. 3. It is clear from this figure that with the increasing acetic acid concentration, the absorbance at 412 nm decreased significantly especially for higher concentration samples. This trend is consistent with the results in Tab. 1.
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Tab. 1: Formaldehyde emission levels of 11 types of strand boards. Sample Control
APB 0.7 % (1 layer) APB 1.0 % (1 layer)
APB 1.8 % (1 layer)
APB 3.5 % (1 layer)
APB 0.7 % (3 layer) 1 2
VF 1 (10 -5 ppm mm-2) 4.00 (0.18)2 3.91 (0.09)
Sample APB 1.0 % (3 layer)
APB 1.8 % (3 layer)
VF (10 -5 ppm mm-2) 2.56 (0.37) 2.13 (0.18)
3.88 (0.35)
APB 3.5 % (3 layer)
2.04 (0.27)
2.95 (1.33)
DOT 1.8 % (3 layer)
3.56 (0.09)
3.15 (0.09)
2.63 (0.18)
ZB 1.8 % (3 layer)
3.44 (0.27)
Values are the average of 8 replicates. Numbers in parentheses are the standard deviations.
mg.L-1 mg.L-1 mg.L-1
mg.L-1 mg.L-1 mg.L-1
Fig. 3: The absorbance at 412 nm of acetic acid and formalin mixed solutions with different concentrations..
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Formosan subterranean termite resistance
The results of laboratory no-choice termite tests are shown in Tab. 2. It indicates that APB has a very good effect on improving termite resistance of strand board. After 4 weeks’ tests, the weight loss and termite mortality was respectively 16.22% and 65.70% for strand board control. The mortality value is higher than 33% reported by Lee et al. (2004) and 13.0% by Smart and Wall (2006), which may due to the different wood species, various PF formulation and even different termites activities. After adding APB, the weight loss obviously decreases from 16.22% to less than 5.84%, and the mortality increases from 65.70% to higher than 93.21 %. The AWPA rating also rises from 5 (severe attack) to 8~9.5 (slight attack). While the BAE of APB is higher than 1.8%, all the mortality values and the AWPA rating reach to 100% and 9 respectively. There is no significant difference between the termite resistance of 1-layer structure and 3-layer structure. The weight loss of 3-layer strand boards is slightly higher than that of 1-layer strand boards, which may be caused by the termite attack to the APB-free core layer of the 3-layer strand board. The termite resistance of 3-layer strand boards treated with APB, ZB, and DOT at same BAE level of 1.8 % was also compared in Tab. 2. All the three types of borates performed promising termite resistance. The weight loss of APB treated strand board is slightly higher than that of ZB and DOT. But the difference is very small. The mortality and the rating levels are all similar. Thus, it is reasonable that APB can also be applied to protect wood composites against termite attack. Tab. 2: AWPA rating, weight losses and termite mortality of untreated and treated strand board blocks according AWPA E1-06 test. Sample Control
APB 0.7 % (1 layer) APB 1.0 % (1 layer)
APB 1.8 % (1 layer)
Weight loss1 ( %)
Mortality2 ( %)
Mean AWPA rating3
5.33 (0.55)
93.21 (4.50)
8
3.27 (0.08)
100
9
100
8
100
9
100
9.5
16.22 (1.96) 3.91 (0.61)
65.70 (8.50)
5
99.22 (0.91)
8
APB 3.5 % (1 layer)
2.87 (0.15)
APB 1.0 % (3 layer)
5.84 (0.67)
97.63 (3.67)
APB 3.5 % (3 layer)
4.28 (0.19)
100
APB 0.7 % (3 layer) APB 1.8 % (3 layer) ZB 1.8 % (3 layer)
DOT 1.8 % (3 layer)
5.06 (1.30)
4.15 (0.24) 2.57 (0.41) 3.13 (0.19)
100
100
9
8
9
9
Weight losses are the average value of 5 samples, and numbers in parentheses are the standard deviations. Mortality is the average value of 5 samples, and numbers in parentheses are the standard deviations. 3 AWPA E1-06 visual rating scale of 10 (sound), 9 (slight attack), 7 (moderate attack, penetration), 4 (very severe attack), 0 (failure). 1 2
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CONLUSIONS The effect of APB on physical and mechanical properties, termite resistance, and formaldehyde emission of PF bonded strand board was investigated in this study. As expected, APB showed positive effect on termite resistance. The adverse influence on physical and mechanical properties can be overcome by using a 3-layer structure, and finally the IB, MOR and MOE of the strand boards treated with APB could be comparable with the untreated strand board control. All these results confirmed that APB could be a promising additive introducing to structural wood composites. In this study, APB showed significant effect on reducing formaldehyde emission level, which can be interpreted by the reaction between ammonium and acetic acid with formaldehyde caused by APB during hot pressing.
ACNOWLEDGEMENTS This study is financially supported by the Eleventh Five-Year Supporting Program of National Science and Technology, China (No. 2006BAD18B0901).
REFERENCES 1. American Wood Preservers Association (AWPA), 2008: Standard method for laboratory evaluation to determine resistance to subterranean termites. E1-06 2. Amusant, N., Arnould, O., Depres, A., Mansouris, R. H., Pizzi, T., Baudassé, C., 2008: Utilizing Cypress to improve the decay and termite resistance of OSB panels. IRG/ WP 08-10658. The 39th Annual Meeting of International Research Group on Wood Preservation, Istanbul, Turkey 3. Biblis, E. J., 1985: Prosperities of three-layer oriented strandboard from southern hardwoods. Forest Product Journal 35(2): 28-32 4. Bum Ra, J., 1999. Diffusion of boron in wood. Mississippi State Univ. PhD thesis: 132 pp. 5. Çavdar, A. D., Kalaycıoğlu, H., Nemli, G., 2008: The effects of zinc borate treatment of wood strands on some technological properties of oriented strandboard (OSB). IRG/ WP 08-40419. The 39th Annual Meeting of International Research Group on Wood Preservation, Istanbul, Turkey 6. Colak, S., Colakoglu, G., 2004: Volatile acetic acid and formaldehyde emission from plywood treated with boron compound. Building and Environment 39(5): 533-536 7. Curling, S. F., Winandy, J. E., Carll, C., Micales, J. A., Tenwolde, A., 2003: How variability in OSB mechanical properties affects biological durability testing. Holzforschung 57(1): 8-12 8. Dönmez, A., Kalaycıoğlu, H., 2006: Some technological properties of OSB panels treated zinc borate and borax. The 3rd International Borate Symposium, Ankara 9. Gao, W., Cao, J., Li, J., 2008: Cure kinetics of powdered phenol-formaldehyde resins with wood preservatives: ammonium pentaborate. The 39th Annual Meeting of International symposium on wood science and technology, Harbin, P. R. China 10. Goroyias, G. J., Hale, M. D., 2004: The mechanical and physical properties of strand boards treated with preservatives at different stages of manufcture. Wood Science Technology 38(2): 93-107 69
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11. Guss, L. M., 1995: Engineered wood products: The feature is bright. Forest Product Journal 45(7/8): 17-24 12. Laks, P. E. 2002. Biodegradation susceptibility of untreated engineered wood products. In: Enhancing the Durability of Lumber and Engineered Wood Products. FPS Symposium Proceedings No. 7249. Forest Products Society: Madison, WI., Pp. 125-130 13. Lee, S., Wu, Q., 2002: Leachability of borate-modified oriented strandboard: A comparison of zinc and calcium borate. IRG/WP 02-40232. The 33rd Annual Meeting of International Research Group on Wood Preservation, Cardiff, Wales 14. Lee, S., Wu, Q., Smith, W. R., 2004: Formosan subterranean termite resistance of boratemodified strandboard manufactured from southern wood species: A laboratory trial. Wood and Fiber Science 36(1): 107-118 15. Liu, Y., Liu J., 2009: Research of amine formaldehyde scavenger to scavenge volatile formaldehyde. Applied Chemical Industry 38(2): 256-259 16. Myers, R. E., 1985: Ammonium pentaborate: an intumescent flame retardant for hermoplastic polyurethanes. Journal of Fire Science 3(6): 432-449 17. Myles, T. G., 1994: Use of disodium octaborate tetrahydrate to protect aspen waferboard from termites. Forest Product Journal 44(9): 33-36 18. Sean, T., Brunette, G., Côté, F., 1999: Protection of oriented strandboard with borate. Forest Product Journal 49(6): 47-51 19. Smart, R., Wall, W., 2006: Copper borate for the protection of engineered wood composites. IRG/WP 06-40334. The 37th Annual Meeting of International Research Group on Wood Preservation, Tromsø, Norway 20. Turner, P., Murphy, R. J., Dickinson, D. J., 1990: Treatment of wood-based panel products with volatile borate. IRG/WP 90-3616. The 21st Annual Meeting of International Research Group on Wood Preservation, Rotorua, New Zeeland 21. Wu, Q., Lee, S., Jones, J. P., 2003: Decay and mold resistance of borate modified oriented strandboard. IRG/WP 03-40260. The 34th Annual Meeting of International Research Group on Wood Preservation, Brisbane, Queensland 22. Yalinkilic, M. K., 2000. Improvement of boron immobility in the borate treated wood and composite materials. Kyoto Univ. Ph. D. Thesis, Kyoto, Japan 23. Yıldız, Ü. C., Kalaycıoğlu, H., Yıldız, S., Temiz, A., Tomak Dizman, E., Çavdar Dönmez, A., 2009: Biological performance of boron-based chemicals treated wood composites. IRG/ WP 09-40464. The 40th Annual Meeting of International Research Group on Wood Preservation, Beijing, P. R. China 24. Zhou, G. Dictionary of Chemistry. Chemical Industry Press, Beijing, 2004
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Wei Gao Beijing Forestry University, College of Material Science and Technology Qinghu Eastroad 35, Haidian, Beijing China 100083 Jinzhen Cao College of Material Science and Technology Beijing Furestry University Qinghu Eastroad 35 Haidian, Beijing China 100083 E-mail:
[email protected] Tel: 86 010 62337381. Fax: 86 010 62337381. Jianzhang Li College of Material Science and Technology Beijing Furestry University Qinghu Eastroad 35 Haidian Beijing China 100083
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