Wood Science and Technology

, Volume 45, Issue 3, pp 407–417

Effects of polyvinyl alcohol on leachability and efficacy of boron wood preservatives against fungal decay and termite attack

Authors

  • Ahmed Mohareb
    • Department of Forestry and Wood Technology, Faculty of Agriculture, 21545-El-ShatbyAlexandria University
  • Marie France Thévenon
    • Laboratoire de préservation des boisCIRAD PERSYST, UPR 40
  • Edmond Wozniak
    • Dyrup S.A.S, Zone Industrielle Montplaisir
    • Laboratoire d’Etudes et de Recherche sur le Matériau Bois, EA 4370, IFR 110Nancy Université
Original

DOI: 10.1007/s00226-010-0344-4

Cite this article as:
Mohareb, A., Thévenon, M.F., Wozniak, E. et al. Wood Sci Technol (2011) 45: 407. doi:10.1007/s00226-010-0344-4

Abstract

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.

Introduction

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

Material

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.

Blocks impregnation

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 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.

Decay test

Sterile culture medium (20 ml), prepared from malt (40 g) and agar (20 g) in distilled water (1 L), was placed in 9-cm Petri dishes, inoculated with a small piece of mycelium of a freshly grown culture of Poria placenta and incubated for 2 weeks at 22°C and 70% HR to allow full colonization of the medium by the mycelium. Three blocks (two treated and one control) were placed in each Petri dishes under sterile conditions, and all treatments were duplicated. Incubation was carried out for 12 weeks at 22°C under controlled humidity conditions of 70% RH in a climatic chamber WTB BINDER TYP KBF 240. At the end of the test period (12 weeks), mycelia were removed, and all specimens were oven-dried to constant mass at 103°C and weighed. Weight loss (WL) was expressed as a percentage of the initial oven-dried weight of wood sample according to the formula:
$$ {\text{WL}}\,\left( \% \right) = 100 {\kern 1pt} \times {\kern 1pt} \left( {m_{0}-m_{1} } \right)/m_{0} $$
where m0 and m1 are the initial and final dry mass of wood samples before and after the fungal exposure, respectively. Simultaneous with the test series, control wood samples were examined with the fungus species tested (Poria placenta) to act as virulence control specimens and hence to validate the test.

Boron complexation biological efficacy test

To determine the biological influence for the complexation of boron and PVA, an inhibition zone test was carried out. Mycelium was grown in 9-cm Petri dishes filled with 20 ml of malt agar medium prepared as above containing 50, 100, 250, 500 or 1,000 ppm of BAE and Elvanol® 90-50. Also, mixtures of BAE and PVA were prepared at different levels of both agents. Introduction of the tested solution was carried out after medium sterilization (20 min, 120°C, 1 bar) by addition of the necessary quantity of solution. Plates were inoculated in their centre with a small portion of a malt agar freshly grown Poria placenta colony. The cultures were kept at 22°C and 70% RH. Growth inhibition was determined when the diameter of the control culture reached 9 cm by measuring the diameter of the colony estimated from the mean of three diameters and calculated according to the following formula:
$$ {\text{Growth inhibition}}\,\left( \% \right) = 100\, \times \,\left( { 1-d_{ 1} /d_{0} } \right) $$
where d0 is the diameter of the control culture and d1 the diameter of the culture in the presence of tested solution.

Termite tests

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 retentions after impregnation and leaching are reported in Table 1. For the stand-alone boron treatment, severe boron depletion from the treated samples is detected at the end of leaching procedure. An interesting point to note is that both the boron retention estimated from the solution uptake after impregnation and the measured content by atomic absorption in the accumulated leachates are quite similar. The data recorded for treatments with fixative agent show a significant reduction in leachability of boron. Indeed, it is well known that polyvinyl alcohol forms complex with the borate ions to give a gel (Fig. 1) (Lin et al. 2002). The gelification process can be easily observed in the solution by the increase in the viscosity until total gelification of the reaction medium, when boron compound and PVA are mixed together. Moreover, Elvanol® 90-50 is insoluble in cold water, and it was postulated that drying of the gel formed with DOT will lead to a hydrophobic film, which will be retained in the wood. To confirm accuracy of the determination of boron retention based on the solution uptake after the first impregnation, wood samples were crushed to sawdust, extracted according to the procedure reported by Kartal et al. (2009) and analysed to determine the quantity of boron initially present in wood. Results indicated that the values obtained from solution uptake are quite similar to those obtained after boron extraction from sawdust, which allows for validation of the determination method of the initial boron retentions. In the mean time, the second impregnation with the polyvinyl alcohol Elvanol® 90-50 showed an important reduction in boron leachability from the wood samples compared to the pure DOT treated samples. Considering that the initial boron quantities for a given BAE concentration were more or less similar, the lower quantities of boron leached out from the wood in the presence of PVA indicate the efficiency of this latter one to improve boron fixation. However, the increase in PVA concentration slightly improved the detected boron retention. Independent of the PVA and DOT concentrations used, about 22–32% of the initial boron content is retained in the wood specimens treated with the fixative agent tested after 79 h of leaching. It seems that increasing of the PVA concentration has a limited effect on the resistance of boron leachability. In all cases, the boron retention after leaching is above the toxic limit of 1 kg BAE/m3 set for wood protection under outdoor conditions (Drysdale 1994; Schoeman and Lloyd 1998).
Table 1

Boron retention of Scots pine sapwood blocks before and after leaching

Treatment

BAE (kg/m3)

BAE retained (%)

BAE (%)

Additive

Before leachinga

After leachingb

1

None

7.65

0.04

0.52

1

Elvanol 2.5%

8.12

2.05

25.25

1

Elvanol 4%

8.19

1.80

21.98

2

None

16.98

0.04

0.24

2

Elvanol 2.5%

16.95

4.62

27.26

2

Elvanol 4%

16.2

5.11

31.54

4

None

32.52

0.18

0.55

4

Elvanol 2.5%

32.15

7.61

23.67

4

Elvanol 4%

32.18

9.61

29.86

aObtained from solution uptake

bCalculated by difference between the quantity of boron from solution uptake after the first impregnation and the quantity of boron measured in accumulated leachates

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Fig. 1

Complexation of borate ions with polyvinyl alcohol

Figure 2 presents the amount of boron leached out from the treated wood samples at different BAE and PVA concentrations. For all samples, the unfixed boron is rapidly leached out from wood during the first stages of the leaching process.
https://static-content.springer.com/image/art%3A10.1007%2Fs00226-010-0344-4/MediaObjects/226_2010_344_Fig2_HTML.gif
Fig. 2

Effect of polyvinyl alcohol on boron leachability from treated blocks

To confirm the previous data on boron leachability, biological assays were performed on leached samples to evaluate the effect of PVA addition on wood durability and indirectly to confirm the presence of boron in treated samples. Figures 3 and 4 illustrate the effect of the tested treatments on wood durability against the brown rot fungus Poria placenta. After one month, the fungal growth development is obvious at the surface of the controls and stand-alone boron–treated samples compared with the fixative agent–treated specimens, which clearly pointed out the positive effect of PVA on boron fixation. Similar to the control samples, the pure boron–treated specimens are deeply colonized by the mycelium, while those treated with Elvanol® 90-50 are only slightly colonized. This behaviour was not observed at the end of the 12 weeks of exposure to fungal decay, where all the samples were fully colonized. Determination of weight losses indicated more or less important degradation of wood samples according to the tested treatments. The best performances were obtained with treatment realized with 4% BAE and PVA, which showed a significant reduction in the weight losses compared to the control and the stand-alone boron–treated specimens. Indeed, weight losses of blocks treated with 4% BAE and 2.5% PVA were of approximately 10%, while weight losses of controls and pure boron–treated blocks with 4% BAE were of 27 and 33%, respectively. Based on the results of boron leachability, the results of the biological efficacy are, however, somewhat disappointing. Despite the presence of boron retention above the toxic limit of the basidiomycetes fungi (1 kg BAE/m3), wood blocks were more or less degraded by Poria placenta. These results can be explained by either loss of boron through diffusion during the second impregnation process or a decrease in its fungicidal activity due to complexation. To evaluate the quantity of the released boron during the second impregnation, boron analysis was performed on PVA solutions after 2 h of soaking. The results indicated that less than 10% of the quantity of boron impregnated after the first impregnation was leached out by the end of the second impregnation. Therefore, it seems that complexation of boron with PVA decreases the biological activity when compared to its free form. Similar results have been previously described in literature suggesting that complexation reduces biological activity of boron (Lloyd et al. 1990; Lloyd 1998; Obanda et al. 2008). Borate biological activity was primarily due to tetrahydroxyborate ions [B(OH)]4−, which are able to form complexes with polyols in wood-destroying fungi through extracellular and intracellular substrate sequestration, enzyme inhibition and change in membrane function. It was shown that effect of borate on fungal enzymes as well as on wood-decaying fungi was completely negated by the addition of chelators. To confirm this hypothesis, growth inhibition tests were performed with different concentrations of DOT with or without PVA (Fig. 5). At 1,000 ppm BAE, DOT totally inhibited the growth of Poria placenta and a gradual inhibition effect was detected at its lower concentrations. When tested at 500 ppm BAE, DOT inhibits the fungal growth by 82%. The addition of different concentrations of PVA significantly decreases the fungicidal properties of DOT. Indeed, while addition of 50 ppm of PVA has no effect on fungal growth, addition of higher concentrations (250 and 1,000 ppm) considerably reduces boron biological activity as demonstrated by the weak growth inhibition observed. Similar effects were observed for lower BAE concentrations. The results of this study are similar to those obtained with glycol (Gezer et al. 1999) allowing reduction in boron leachability without significant improvement of wood decay resistance.
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Fig. 3

Growth of Poria placenta after one month on Scots pine sapwood blocks treated with 4% BAE with or without PVA after leaching according to ENV 1250-2

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Fig. 4

Weight losses of Scots pine sapwood blocks treated with different concentrations of BAE and PVA and leached according to ENV 1250-2 after 12 weeks of fungal exposure

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Fig. 5

Effect of different BAE concentrations with or without Elvanol® 90–50 on the growth of Poria placenta

To assess resistance to termites and to check the influence of PVA on boron retention, leached samples treated with DOT with or without the fixative agent were subjected to termite attack using a laboratory test for five weeks. The results are reported in Table 2. Independent of the BAE concentration used, weight losses of samples treated without PVA were high, and the mortality rates of termites at the end of the experiment were low. Similar results were obtained for untreated pine samples. The performance of the samples treated with DOT and PVA was different according to their used concentrations. Indeed, even if samples treated at low BAE and PVA concentrations show a weak improvement in wood durability, samples treated with higher BAE and PVA concentrations present significant improvements. Weight losses were low and termite mortality rates were high, with no insects surviving at the end of the experiment. The rate of attack was low for samples treated with high concentrations of BAE and PVA, while it was higher for samples treated with low concentrations. In the mean time, control samples were strongly degraded.
Table 2

Weight loss, percentage of termite survival and attack rate of Scots pine sapwood blocks after termite exposure

Treatment

WL (%)

Survival (%)

BAE (%)

Additive

1

None

38.3 ± 6.2

78.3 ± 5.2

1

Elvanol 2.5%

41.2 ± 3.0

61.6 ± 8.9

1

Elvanol 4%

6.9 ± 3.9

0

2

None

35.5 ± 11.4

79.0 ± 9.1

2

Elvanol 2.5%

27.4 ± 3.9

39.2 ± 21.8

2

Elvanol 4%

3.2 ± 0.6

0

4

None

47.5 ± 4.5

76.0 ± 2.4

4

Elvanol 2.5%

10.2 ± 1.5

0

4

Elvanol 4%

2.9 ± 0.1

0

Control

35.1 ± 5.7

65.8 ± 12.4

Conclusion

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.

Acknowledgments

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.

Copyright information

© Springer-Verlag 2010