Effects of polyvinyl alcohol on leachability and efficacy of boron wood preservatives against fungal decay and termite attack
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- Mohareb, A., Thévenon, M.F., Wozniak, E. et al. Wood Sci Technol (2011) 45: 407. doi:10.1007/s00226-010-0344-4
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In this study, the use of polyvinyl alcohol (PVA) as boron fixative agent was investigated. Two levels of PVA (2.5 and 4%) were evaluated with disodium octaborate tetrahydrate (DOT) at three concentrations (1, 2 and 4% boric acid equivalent (BAE)) using a double vacuum impregnation process on Scots pine sapwood specimens. Leaching was performed according to a laboratory leaching procedure. Boron analysis using atomic absorption spectrometer showed a significant reduction in boron leachability for the samples treated with both concentrations of PVA when compared to the stand-alone boron treatment leading to boron retentions capable of preventing wood biological degradations. Decay resistance of the leached specimens was evaluated using the brown rot fungus Poria placenta. Even if complete protection was not fully achieved, an improvement in decay resistance was observed for the samples treated with DOT in presence of PVA. This leak of efficacy was attributed to a decrease in the biological activity of the complexed boron against fungi. Durability of treated wood against termite attack, evaluated using Reticulitermes santonensis, indicated a significant enhancement for the samples treated in presence of the fixative agent compared to the pure boron treatment.
Boron preservatives have been described as valuable alternatives for non-ground contact applications. Products such as disodium octaborate tetrahydrate (DOT), boric acid and borax are the most widely used boron-based wood preservatives. They possess many advantages such as being colourless, odourless, non-corrosive, non-flammable, inexpensive and having low vapour pressure but suffer of an important drawback due to their high susceptibility to leaching that limits their significance for outdoor applications. Boron has been considered a relatively safe wood preservative up to now. However, there are currently important discussions about the future uses of boron-based wood preservatives. Indeed, some boron compounds have been recently classified as toxic for reproduction in Europe, which will considerably restrict their field of application except for boron concentrations below the authorized levels. However, the key to allow the use of boron preservatives in outdoor conditions is to enhance their resistance to leaching (Obanda et al. 2008). To counteract the problem of boron leachability, several investigations have been performed to improve the fixation of boron in wood using several approaches. Several authors described the use of water repellents or polymerizable monomers to reduce boron leachability (Baysal et al. 2004; Kartal et al. 2009; Temiz et al. 2008; Mourant et al. 2009). Other studies involved the use of organic chemicals to reduce water solubility of boron through formation of insoluble complex (Kartal and Green 2003; Kartal and Imamura 2004; Kartal et al. 2004). Formation of protein borates has also been described to decrease boron leachability (Thévenon et al. 1997, 1998; Thévenon and Pizzi 2003). Different alcohols such as glycol, glycerol or polyethylene glycol able to form organic borates with boron have been investigated with or without wood coupling agents to bound biocide to wood structure (Gezer et al. 1999; Weining and Kamdem 1999, Toussaint-Dauvergne et al. 2000; Mohareb et al. 2002). A similar beneficial effect has been stated for the hydrophobic organo-boron compounds that have been reported to increase the retention of boron in the treated wood (Mohareb et al. 2004; Lyon et al. 2007). Knowing the ability of polyvinyl alcohol (PVA) to form complex with the borate ions (Lin et al. 2002), the aim of this work is to investigate the effect of PVA on boron fixation to develop new formulations for wood preservation and to evaluate the efficacy of these treatments against fungal decay and termites.
Materials and methods
Tim-bor® (disodium octaborate tetrahydrate, DOT) was obtained from the Borax Company (Guildford, UK). Elvanol® 90-50, a fully hydrolysed polyvinyl alcohol, was generously furnished by DuPont (Paris, France). This product is commonly used in the paper industry to give a moisture barrier for protecting the hydrophilic paper fibres. Elvanol® 90-50 is insoluble in cold water but slightly soluble in hot water allowing for preparation of aqueous solutions up to 4%.
Mini-blocks (15 mm × 5 mm in cross section by 50 mm along the grain) of Scots pine sapwood (Pinus sylvestris L.) were used in this study. Twelve replicates were used for each treatment. Samples were oven-dried at 103°C for 48 h and weighed to a precision of 0.001 g.
Except for the treatments involving only DOT impregnation, all treatments were realized with double impregnation processes. Disodium octaborate tetrahydrate concentrations were expressed as Boric Acid Equivalent (BAE). Three BAE concentrations (1, 2 and 4%) and two polyvinyl alcohol concentrations (2.5 and 4%) were tested.
Wood specimens were vacuum-treated at 5 mbar for 30 min, impregnated with the boron test solutions and kept immersed for 2 h at atmospheric pressure and finally reweighed to determine solution uptake. Boron retention (kg BAE/m3) was calculated for each specimen. Wood samples were kept for 16 h at ambient laboratory temperature and finally dried at 103°C for 48 h. Wood specimens were re-treated with the fixative additives according to the same sequence as described above.
Boron leaching procedure
Leaching was performed according to a procedure adapted from the European standard ENV 1250-2 (1994). Six samples were immersed in 70 mL of distilled water and subjected for six leaching periods of increasing duration under continuous shaking at 20°C. Water was replaced for each leaching period after 1, 2 and 4 h. Samples were then removed and air-dried for 16 h. Other leaching periods conducted were 8, 16 and 48 h with change of water between each. All leachates were collected and kept for boron analysis.
Boron retention in the treated wood samples was determined using the water bath extraction method described by Kartal et al. (2009) with some modification. In this method, samples were milled to pass through a 40-mesh screen and oven-dried at 103°C, and 1.5 g of ground wood was weighed to the nearest 0.001 g into a 250-ml flask. For each treatment, two specimens were ground and analysed. One hundred millilitres of deionized water was added to each flask containing the ground wood. The flasks were placed in a water bath at 90–95°C for 6 h with continuous agitation. After cooling, the contents of the flasks were filtered through Whatman # 4 filter paper, washed three times with 20 ml of hot deionized water and diluted to 200 ml in a volumetric flask. The boron contents of the treated wood and the leachates from different leaching cycles were determined using a Varian SpectrAA 220 FS atomic absorption spectrometer with standard solutions comprising between 100 and 1,000 mg L−1.
Boron complexation biological efficacy test
Resistance tests of treated wood against termites were carried out on Scots pine sapwood specimens (15 mm × 5 mm in cross section by 50 mm along the grain) using the European standard EN 117 (1990). The termite exposure time was five weeks instead of eight in the standard due to the size of the tested samples. Percentage of survival termites and weight loss (WL) of wood blocks were determined at the end of the experiment time. Each experiment was performed on four replicates.
Results and discussions
Boron retention of Scots pine sapwood blocks before and after leaching
BAE retained (%)
Weight loss, percentage of termite survival and attack rate of Scots pine sapwood blocks after termite exposure
38.3 ± 6.2
78.3 ± 5.2
41.2 ± 3.0
61.6 ± 8.9
6.9 ± 3.9
35.5 ± 11.4
79.0 ± 9.1
27.4 ± 3.9
39.2 ± 21.8
3.2 ± 0.6
47.5 ± 4.5
76.0 ± 2.4
10.2 ± 1.5
2.9 ± 0.1
35.1 ± 5.7
65.8 ± 12.4
Elvanol® 90-50, a fully hydrolysed polyvinyl alcohol, allows a reduction in boron leachability. Treatments are based on a double impregnation process involving a first impregnation with DOT followed by a second impregnation with polyvinyl alcohol. Durability of Scots pine sapwood blocks treated with 1, 2 or 4% BAE followed by a second impregnation with a 2.5 or 4% solution of PVA was evaluated against Poria placenta after leaching. The results revealed that wood specimens treated successively by DOT and PVA were more resistant to fungal decay as demonstrated by the more or less important inhibition of mycelium development during the first weeks of the decay tests. Efficacy of the treatment was however not sufficient to secure the full protection for the wood blocks after 12 weeks of fungal exposure. Growth inhibition assays performed with DOT and PVA alone or mixtures of these latter ones indicated that biological activity of complexed boron is lower than that of its free form, explaining the leak of efficacy observed. Durability of treated wood against termite attack was evaluated in laboratory conditions against Reticulitermes santonensis. Results indicated a significant improvement in wood resistance for the samples treated with DOT in the presence of PVA, confirming its ability to prevent boron depletion. Even if the protection against fungi was not completely achieved, the gathered data clearly indicated a reduction in boron leachability leading to a significant improvement of wood durability against termites.
The authors would like to thank Mr. Stéphane Parant from UMR UHP SCRSMC for his assistance during utilization of the atomic absorption spectrometer. The authors also gratefully acknowledge the financial support of the CPER 2007-2013 “Structuration du Pôle de Compétitivité Fibres Grand’Est” (Competitiveness Fibre Cluster). Finally, the authors would like to thank the Agence Universitaire de la Francophonie (AUF) for the post-doctoral fellowship granted to the first author.