Effect of Wood Preservatives on Surface Properties of Coated Wood

Published Date

Advances in Materials Science and Engineering

Volume 2015 (2015), Article ID 631835, 6 pages
http://dx.doi.org/10.1155/2015/631835

Research Article

Effect of Wood Preservatives on Surface Properties of Coated Wood

Turgay Ozdemir, Ali Temiz, and Ismail Aydin

Department of Forest Industry Engineering, Faculty of Forestry, Karadeniz Technical University, 61080 Trabzon, Turkey

Received 17 April 2015; Revised 21 June 2015; Accepted 23 June 2015

Academic Editor: Simon C. Potter 

Copyright © 2015 Turgay Ozdemir et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Effect of wood preservatives (waterborne and organicborne) on the performance of surface finishing properties is investigated. Sapwood of scots pine, (Pinus sylvestris L.), oriental beech (Fagus orientalis Lipsky), and chestnut (Castanea sativa Mill.) specimens (300 × 100 × 15 mm along the grain) were impregnated with aqueous solution of 2% CCA, 2% Tanalith E, 1% boric acid, and Immersol aqua. Surface roughness, dry film thickness, adhesion strength, gloss measurement, scratch, and abrasion resistance were determined according to related standards for treated and untreated samples. The results indicated that surface roughness and adhesion strength depended on wood species and the chemical composition of preservatives. Generally, waterborne wood preservatives increased the surface roughness of wood while the organic-based wood preservatives decreased it. The organic-based wood preservatives decreased adhesion but they increased gloss value. Wood preservatives did not affect the scratch resistance which was found to depend on properties of the coating. All the wood preservatives increased abrasion resistance.

1. Introduction

Wood and wood-based materials have gained extensive usage areas in outdoor and indoor applications due to their abundance and versatility. Wood, commonly used in outdoor applications, is preferred for proper protection and best service life against decay (rot), insects, weathering, and dimensional instability [1, 2]. The term of wood weathering describes combination of UV-degradation, moisture, heat, and atmospheric pollutants (e.g., acid rain, ozone, nitrogen oxides, and sulphur dioxide) [3]. Wood has several chromophoric functional groups and sites such as hydroxyl, carbonyl, carboxyl groups, and aromatic and phenolic groups [3]. The effect of wood weathering is manifested by initial wood surface colour changes, followed by loss of gloss, roughening, and checking [1, 3]. These changes are due to modification of chromophoric groups of wood and formation of coloured quinines-like component [1, 3, 4].

Wood materials can be protected against these factors by applying wood treatment process and/or wood finishing. The first option is the treatment with wood preservatives. The major wood preservatives used for treatment process are creosote, pentachlorophenol, CCA (chromated copper arsenate), and other copper-based wood preservatives such as alkaline copper quaternary (ACQ) and amine copper azole (CA) [1, 2, 5].

The second option is applying coatings. A variety of finishes or coatings can be applied to wood. Finishes can be divided into two groups: (1) opaque coatings, such as paints and solid-color stains and (2) natural finishes, such as water repellents, oils, and semitransparent penetrating stains [6]. Performance of wood coatings depends on several factors such as wooden substrate itself, humidity, moisture content of wood, temperature, environmental pollutants, and microorganisms [7]. Photo-induced degradation of wood caused shorter service life of wood coatings. This is due to penetration of UV light through transparent finishes and degradation of the bonding mechanism between finishes and wood [7]. The performance of wood coatings can be improved by different strategies. One option is to add UV absorbers and radical scavengers. The second strategy is treatment of wood materials with wood preservatives since wood preservatives can inhibit photo-induced degradation and increase durability against fungi and insects [7]. The most common wood preservative used is the chromated copper arsenate (CCA) but it contains pentavalent arsenic which is hazardous to human and environment [2]. The effect of chromated copper arsenate (CCA) on the wood weathering and its combination with the wood coatings are well-established [3, 8]. However, there is still research needed on the performance of new generation preservatives which are copper based but chrome- and arsenic-free.

In this study, it was aimed at determining the effect of new generation wood preservatives on the performance of wood coating. Scots pine, beech, and chestnut samples were treated with CCA, Tanalith E, boric acid, and Immersol and coated with cellulosic-based finishing.

2. Material and Methods

2.1. Treatment Process

Sapwood of scots pine, (Pinus sylvestris L.), oriental beech (Fagus orientalis Lipsky), and chestnut (Castanea sativa Mill.) specimens (300 × 100 × 15 mm along the grain) were impregnated with aqueous solution of 2% CCA, 2% Tanalith E, and 1% boric acid and Immersol aqua vacuum (625 mm Hg) over the blocks was applied for 45 min. After vacuum, the wood samples were kept in the treatment solution for 60 min in atmospheric conditions. The wood samples were then removed from the treatment solution, wiped lightly to remove solution from the wood surface, and weighed to determine gross retentions for sample. Retention values were chosen according to producers’ suggestions. The retention for each treatment solution was calculated according to the following formula:

where

  are the grams of treating solution absorbed by the block,  are grams of preservative or preservative solution in 100 g of the treating solution,  is volume of block in cubic centimeters.

After the treatment process, the samples treated with copper-based wood preservatives were then wrapped in plastic bag for a week at the room temperature for fixation.

2.2. Surface Coating Process

The surfaces of the samples were sanded with 100 and then with 150 grid size sandpaper. Cellulose varnish type was commercially obtained from supplier. The coating was applied by spraying with approximate varnish application amount of 120 g/m2. Viscosity for application was 20 s, DIN cup/4 mm/20°C. The coating was applied to surface as 2 bases and 1 top layer. The component details of the coatings used are given in Table 1.

Table 1: Mixture portion of coatings.

2.3. Surface Roughness

Surface roughness values of the treated and untreated samples were determined by using Mitutoyo Surftest SJ-301, a stylus type profilometer. Detector tip radius was 5 μm while cut-off length was 2.5 mm and sampling length was 12.5 mm in the surface roughness measurements. Three roughness parameters including the average roughness (), the mean peak-to-valley height (), and the maximum roughness () were used to evaluate surface roughness of the samples according to DIN 4768 [9]. A total of fifteen replicate measurements were taken from each untreated and treated wood. The dimensions of specimens used for surfaces roughness measurements were 10 cm by 10 cm. All measurements were made across the grain orientation.

2.4. Dry Film Thickness

Dry coating thickness of samples treated wood preservatives and untreated (control) was determined by using the dry film thickness apparatus (Erichsen P.I.G 455) according to ASTM D 4138 [10].

2.5. Adhesion Strength

Adhesion strength based on the Pull-off methods was determined according to ASTM D 4541 [11]. Twenty different points with a contact area of 20 mm circles were taken from each side of the treated and untreated samples. The Ericksen Adhesion-525 MC tester with a head glued to the surface of the samples was employed for the test. The testing device runs at a constant speed of 10 cm/min and applies tension force to the surface layer by pulling the coating from the wood surface.

2.6. Gloss Measurement

Gloss measurement was carried out according to standard test of ISO 2813 [12]. Ten replicates with dimensions of 100 by 100 by 15 mm were used. During the measurements, the 60° geometry was applied to all coated samples.

2.7. Scratch Resistance

Scratch resistance test was done according to DIN 68861 [13]. Rotation was 5 r/min ± 1 r/min. A special scratch diamond with a hemispherical point was used with different weight forces (1.5–4.0 N). Five replicates (100 × 100 × 15 mm) for each group were used for determining the resistance of scratch.

2.8. Abrasion Resistance

Abrasion test was carried out according to DIN 68861 [14]. In this test, the taber abraser consists of a horizontal plate on which the test samples with the dimensions of 100 by 100 by 15 mm were placed flat. Five replicates were used. The plate turned at a speed of 55 ± 6 min−1. Above the plate, two abrasive discs with rubber coating were mounted to a press that can exert a force of 5.5 ± 0.2 onto the test piece. Strips of sandpaper were attached to the rubber coating. The number of revolutions necessary to reach the final point then determines the abrasion as follows:

(a)Meaning up to 50% of the wood has been worn away.(b)In case of self-colored surface, the color disappears, and/or the under surface was visible.

2.9. Statistical Analysis

The multifactor analysis of variance was used for comparing the results obtained from experiments. The Duncan test with 95% confidence level was used to compare mean values of variance sources.

3. Result and Discussion

The average retention and surface roughness values of pine, beech, and chestnut wood samples treated with CCA, Tanalith E, boric acid, and Immersol aqua are presented in Table 2.

Table 2: Average retention and surface roughness results of untreated and treated samples.

According to Table 2, the surfaces of pine control samples were found to be smoother than those of beech and chestnut control samples. The surface roughness differences between pine wood as softwood and beech and chestnut woods as hardwoods could be related to their density differences [15]. Among hardwoods, chestnut samples showed higher surface roughness than beech wood. Chestnut is ring-porous tree while beech wood is diffuse-porous tree. Therefore, vessels in chestnut tree are located mainly in earlywood section which causes higher surface roughness while vessels in beech wood are located throughout cross section which causes smother surface.

The highest average surface roughness value () was measured on chestnut samples treated with boric acid (77.37 μm) and the lowest result was obtained for pine samples treated with solvent-based wood preservative (16.44 μm). Generally, waterborne wood preservatives increased surface roughness of wood. While, organic-based ones decreased it (Table 3).

Table 3: Statistical results of Duncan test.

Effects of wood preservatives on the surface roughness of wood depend on wood species and wood preservative types used due to anatomical differences between softwood and hardwood, wood preservatives’ chemical composition, and bonding mechanism with wood. It was reported that waterborne wood preservatives increased surface roughness of wood due to increased surface porosity and raised fibres on the surface of wood. However, organicborne wood preservatives decreased surface roughness of wood due to the filled surface cavity and decreased surface porosity [16].

The highest surface roughness value is obtained for samples treated with boric acid ( μm) since boric acid crystals were formed on the wood surfaces after impregnation [17].

According to a study conducted by Maldas and Kamdem [18], the surface of CCA-treated red maple wood was found to be rougher than that of untreated wood. However, no marked changes in the surface roughness of alder and beech wood after borax and boric acid treatment were found by Aydin and Colakoglu [19]. Temiz et al. [20] also reported that impregnation with copper-based preservatives (CCA, ACQ, Tanalith E, and Wolmanit CX) increased the values of surface roughness when compared with untreated (control) samples.

Table 3 shows statistically the effects of wood species and wood preservatives on finishing properties.

Dry film thickness of treated and untreated wood species was found as 90 μm for all group and wood species. No differences were found between untreated and treated wood samples. These results agreed with those obtained by others [16, 21–23].

Figures 1 and 2 illustrate the effect of treatment on adhesion strength and gloss values.

Figure 1: Adhesion strength of untreated and treated samples.

Figure 2: Gloss values of untreated and treated samples.

The highest adhesion strengths were determined on beech-treated and beech-untreated wood samples while the lowest adhesion strength was obtained on pine samples. These differences can be attributed to wood properties and density differences (higher density of beech wood). Beech wood which has diffuse-porous structure showed better absorption of the varnish, which, in turn, caused higher magnitude of interaction between wood and varnishes. In addition, varnishes applied on beech and chestnut samples had deeper penetration than that of pine samples due to their more porous structures. Similar findings were reported by previous studies [24, 25]. According to the adhesion strength test, organic-based wood preservative showed the lowest adhesion result among the other wood preservatives due to decreasing bonding capacity of coatings and wood. However, boric acid treatment provided increasing adhesion strength for all wood species used. The increasing adhesion strength of boric acid treatment could be attributed to higher mechanical interlocking mechanism of adhesion caused by increased surface roughness of boric acid-treated wood.

The decreasing adhesion test for treated samples could be related to poor wetting properties after impregnation. The fact that the poor wettability resulted in poor adhesion was also observed by Gray [26], Shupe et al. [27], and Aydin [28]. Poor wettability for CCA-treated wood was also reported by Maldas and Kamdem [18].

The highest value of the gloss measurement was determined on organic-based wood preservative treatment. The waterborne wood preservatives of control groups (untreated) decreased gloss values while organic-based wood preservative increased them. The reason for higher gloss value of organicborne wood preservative could be better filled surface cavity that resulted in better reflection of light. However, water-based wood preservatives increased surface porosity and raised fibres decreased gloss value. Regarding to wood species, pine untreated samples showed highest gloss value while the lowest gloss value was obtained for beech-untreated samples. The differences between gloss values are due to the anatomical and chemical structure of softwood and hardwoods.

The scratch resistance and the abrasion resistance of the untreated and treated samples are shown in Figures 3 and 4.

Figure 3: Scratch resistance of untreated and treated samples.

Figure 4: Abrasion resistance of untreated and treated samples.

As expected, there were no major differences between wood species and treatment preservatives because scratch resistance depended on only properties of the coating, that is, components, hardness of coating film, and so forth (Table 3). Similar findings were reported by other researchers [22, 29].

Regarding to the abrasion resistance test the untreated beech samples had higher abrasion resistance values (37.50 rpm) than those of the untreated pine (33.00 rpm) and chestnut (35.83 rpm). This is due to the wood properties such as density and permeability. Wood preservatives increased abrasion resistance for all groups. Remaining film layer on the surface caused to an increase in the abrasion resistance.

4. Conclusions

This study dealt with the effect of wood preservatives on the performance of finishing. Waterborne and organic-based wood preservatives were tested. The major findings of the study can be listed as follows:

(1)Surface roughness values changed depending on wood species due to the anatomical differences between softwood and hardwood.(2)Adhesion properties of treated wood depended on the chemical composition of preservatives. Organic-based wood preservative (Immersol aqua) decreased the adhesion properties because of the poor adhesion properties after impregnation. While organic-based wood preservative increased the gloss value, the waterborne wood preservatives decreased the gloss value.(3)Wood preservatives did not affect the scratch resistance which was disclosed to depend on the coating properties.(4)Treatment with both waterborne and organic-based wood preservatives increased the abrasion strength significantly.

Disclosure

Some part of this study was presented in International Research Group of Wood Protection Annual Meeting at Istanbul, Turkey, in 2008 (IRG-WP 08-40405).

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

References

1. A. Temiz, N. Terziev, M. Eikenes, and J. Hafren, “Effect of accelerated weathering on surface chemistry of modified wood,” Applied Surface Science, vol. 253, no. 12, pp. 5355–5362, 2007. View at Publisher · View at Google Scholar · View at Scopus
2. A. Temiz, G. Alfredsen, U. C. Yildiz et al., “Leaching and decay resistance of alder and pine wood treated with copper based wood preservatives,” Maderas: Ciencia y Tecnologia, vol. 16, no. 1, pp. 63–76, 2014. View at Publisher · View at Google Scholar · View at Scopus
3. J. Zhang, D. P. Kamdem, and A. Temiz, “Weathering of copper-amine treated wood,” Applied Surface Science, vol. 256, no. 3, pp. 842–846, 2009. View at Publisher · View at Google Scholar · View at Scopus
4. A. Temiz, U. C. Yildiz, and T. Nilsson, “Comparison of copper emission rates from wood treated with different preservatives to the environment,” Building and Environment, vol. 41, no. 7, pp. 910–914, 2006. View at Publisher · View at Google Scholar · View at Scopus
5. R. Stirling and A. Temiz, “Fungicides and insecticides used in wood preservation,” in Deterioration and Protection of Sustainable Biomaterials, American Chemical Society Books, 2014. View at Google Scholar
6. W. C. Feist and A. S. Ross, “Performance and durability of finishes on previously coated CCA-treated wood,” Forest Products Journal, vol. 45, no. 9, pp. 29–36, 1995. View at Google Scholar · View at Scopus
7. Y. Xie, A. Krause, C. Mai et al., “Weathering of wood modified with the N-methylol compound 1,3-dimethylol-4,5-dihydroxyethyleneurea,” Polymer Degradation and Stability, vol. 89, no. 2, pp. 189–199, 2005. View at Publisher · View at Google Scholar · View at Scopus
8. W. C. Fiest and D. N. S. Hon, “The chemistry of solid wood,” in Advances in Chemistry Series, No: 207, R. M. Rowell, Ed., pp. 401–451, American Chemical Society, Washington, DC, USA, 1984.View at Google Scholar
9. DIN 4768, “Determination of values of surface roughness parameters Ra. Rz. Rmax using electrical contact (stylus) instruments,” Concepts and Measuring Conditions, Deutsches Institut für Norming, Berlin, Germany, 1990. View at Google Scholar
10. ASTM International, ASTM D 4138, Test Method for Measurement of Dry Film Thickness of Protective, ASTM International, Philadelphia, Pa, USA, 1971.
11. ASTM International, ASTM D 4541, Test Method for Pull-Off Strength of Coatings Using Portable, ASTM International, Philadelphia, Pa, USA, 1978.
12. ISO, ISO 2813, Paints and Varnishes—Determination of Specular Gloss of Non-Metallic Paint Films at 20 Degrees, 60 Degrees and 85 Degrees, International Organization for Standardization, Geneva, Switzerland, 1994.
13. DIN, “Furniture surfaces; behavior at scratches,” DIN 68861-4, Deutsches Institut für Norming, Berlin, Germany, 1981. View at Google Scholar
14. Deutsches Institut für Norming, DIN 68861-2, Furniture Surfaces; Behavior at Abraison, Deutsches Institut für Norming, Berlin, Germany, 1981.
15. T. Ozdemir, S. Hiziroglu, and M. Kocapinar, “Adhesion strength of cellulosic varnish coated wood species as function of their surface roughness,” Advances in Materials Science and Engineering, vol. 2015, Article ID 525496, 5 pages, 2015. View at Publisher · View at Google Scholar
16. T. Ozdemir and S. Hiziroglu, “Evaluation of surface quality and adhesion strength of treated solid wood,” Journal of Materials Processing Technology, vol. 186, no. 1–3, pp. 311–314, 2007. View at Publisher · View at Google Scholar · View at Scopus
17. I. Aydin, “Effects of borate treatments on the properties of spruce veneers and plywood panels,” in Handbook on Borates: Chemistry, Production and Application, chapter 11, pp. 349–366, Nova Science Publishers, New York, NY, USA, 2009. View at Google Scholar
18. D. C. Maldas and D. P. Kamdem, “Surface characterization of chromated copper arsenate (CCA)-treated red maple,” Journal of Adhesion Science and Technology, vol. 12, no. 7, pp. 763–772, 1998.View at Publisher · View at Google Scholar · View at Scopus
19. I. Aydin and G. Colakoglu, “Variation in surface roughness, wettability and some plywood properties after preservative treatment with boron compounds,” Building and Environment, vol. 42, no. 11, pp. 3837–3840, 2007. View at Publisher · View at Google Scholar · View at Scopus
20. A. Temiz, U. C. Yildiz, I. Aydin, M. Eikenes, G. Alfredsen, and G. Çolakoglu, “Surface roughness and color characteristics of wood treated with preservatives after accelerated weathering test,” Applied Surface Science, vol. 250, no. 1–4, pp. 35–42, 2005. View at Publisher · View at Google Scholar · View at Scopus
21. E. Zavarin, “Activation of wood surface and nonconventional bonding,” in The Chemistry of Solid Wood, R. Rowell, Ed., American Chemical Society (ACS), Washington, DC, USA, 1984.View at Google Scholar
22. A. Shakri and M. Seman, “Finishing properties of Acacia mangium, Paraserianthes falcataria and Gmelina arborea timbers: some important parameters,” Journal of Tropical Forest Products, vol. 1, no. 1, pp. 83–89, 1995. View at Google Scholar
23. Z. Wicks, F. Jones N, and S. Pappas P, Organic Coatings Science and Technology, vol. A of SPE Monograph Series, Willey Interscience, New York, NY, USA, 2nd edition, 1999.
24. E. Liptáková, J. Kúdela, and O. Paprzycki, “The adhesion of polystyrene to wood,” Holz als Roh-und Werkstoff, vol. 49, no. 1, pp. 31–37, 1991. View at Publisher · View at Google Scholar
25. J. Vitosyto, K. Ukvalbergieno, and G. Keturakis, “The effects of surface roughness on adhesion strength of coated ash (Fraxinus excelsior L.) and Birch (Betula L.) wood,” Materials Science, vol. 18, no. 4, pp. 347–351, 2012. View at Publisher · View at Google Scholar · View at Scopus
26. V. R. Gray, “The wettability of wood,” Forest Products Journal, vol. 12, pp. 452–461, 1962. View at Google Scholar
27. T. F. Shupe, C. Y. Hse, and W. H. Wang, “An investigation of selected factors that influence hardwood wettability,” Holzforschung, vol. 55, no. 5, pp. 541–548, 2001. View at Publisher · View at Google Scholar · View at Scopus
28. I. Aydin, “Activation of wood surfaces for glue bonds by mechanical pre-treatment and its effects on some properties of veneer surfaces and plywood panels,” Applied Surface Science, vol. 233, no. 1–4, pp. 268–274, 2004. View at Publisher · View at Google Scholar · View at Scopus
29. T. Ozdemir, The investigation of varnishes features at some tree species grown in Turkey [Ph.D. thesis], Karadeniz Technical University, Trabzon, Turkey, 2003.

For further details log on website :
http://www.hindawi.com/journals/amse/2015/631835/

ENVIRONMENTALLY FRIENDLY WOOD PRESERVATIVE SYSTEM BASED ON POLYMERIZED TANNIN RESIN-BORIC ACID FOR OUTDOOR APPLICATIONS

Author

Marie-France Thévenon¹, Gianluca Tondi², Antonio Pizzi²

¹Wood preservation laboratory, UR40, CIRAD, TA B 40/16, 34398 Montpellier Cedex 5, France  

²ENSTIB-LERMAB, Nancy University, 27 rue du Merle blanc, BP 1041, 88051 Epinal, France

ABSTRACT

Boron compounds are used as wood preservatives as they are both fungicide and insecticide, relatively inexpensive and environmentally acceptable. Nevertheless, in the field of wood protection, borates are only used for indoor non-exposed applications or in association with other biocides, due to their main disadvantage of being readily leachable from treated wood. To overcome this problem, boric acid was fixed into wood with condensed tannins and hexamine through a non-formaldehyde emission polymer network. Treated mixtures were tested with different proportions of mimosa tannins and hexamine, with and without co-added boric acid. The treated beech samples were leached and tested according to European Standard EN 113 against Pycnoporus sanguineus in tropical conditions. The systems had minimal boron depletion and good fungal decay efficacy, meeting the efficacy requirements of EN 113.

Thus, these associations could be envisaged to treat timber with insufficient natural durability for above ground outdoor use, and for a long service-life of the wooden commodities. 

Keywords: Boric Acid, Tannins, Hexamine, Leaching, EN 113, Pycnoporus sanguineus

INTRODUCTION

Borates such as boric acid, borax or disodium octaborate tetrahydrate (DOT) have proved their efficiency for many years as wide spectrum wood preservatives (Lloyd 1997, Drysdale 1994). They have many advantages including being inexpensive, odourless, colourless and non flammable. They are also soluble in water allowing them to be introduced in wood by conventional methods like dipping-diffusion or vacuum-pressure treatments (Byrne and Morris 1997, Lebow and Morrell 1989). Boron compounds have been shown to have a lower human toxicity (Teshima et al. 2001, Usuda et al. 1998, Jansen et al. 1984) than for some animals’ species (Hamilton and Buhl 1990, Maier and Knight 1991), and boric acid has been considered environmentally acceptable for many years. On the other hand, this high water solubility makes boron compounds easily leachable from treated wood and thus boron treated wood is not suitable for outdoor application (Lloyd 1998, Peylo and Willeitner 1997). The key issue to expand boron’s use for wood protection appears to be their fixation into wood but allowing for sufficient mobility so they remain fungicidal (Obanda et al. 2008). Several different methods have been tried to decrease borate leachability from wood, including forming complexes of boron with flavonoid tannins (Pizzi and Baecker 1996). Another system involved a 2 step impregnation of copper, zinc and boron with tannins, with this system having good efficacy against wood destroying fungi (Scalbert et al. 1998). Water borne solutions of boric acid, gelatin and tannins can also be used to treat wood (Thevenon, 1999). Gelatin, as a protein, can partially fix boric acid, and tannins can waterproof the protein-boric acid polymer leading to an insoluble network which minimized depletion of boron when treated wood is leached. 
Another tannin system might be to harden a tannin resin with hexamine, a non emission substance when in presence of a fast reacting polyflavonoid tannin (Pichelin et al. 2006; Kamoun and Pizzi 2000 a,b; Kamoun et al.2003). This paper presents some results obtained with tannin/hexamine/boric acid used as wood preservatives. 

MATERIALS AND METHODS

Beech (Fagus sylvatica) wood specimens, 50x25x15 mm³, were treated with experimental wood preservatives. The different treatments used were (% w/w): (1) mimosa tannin extract in 10%, 20% and 35% solution and hexamine at 6% by weight on dry tannin extract weight; (2) mimosa tannin extract in 10%, 20% and 30% solution, with 5% boric acid on total tannin solution, and hexamine at 6% by weight on dry tannin extract. Dowanol (Dow Chemicals) (5% on dry tannin extract weight), a polyether, was added to the tannin solutions to decrease the viscosity of the tannin solution and facilitate wood penetration by the preservative solution. 

Ten specimens for each case were treated by vacuum-pressure application (60 min at 10 mbar, introduction of the treatment solution, then 2 hrs at atmospheric pressure). The treatment was followed by 24 hours drying in oven at 103°C. After drying, half of the specimens were leached in 500 ml of water (1 vol of wood / 6.7 vol of water) at 20°C. The water was changed daily for 5 days. Once the samples were air dried, anhydrous weight of the samples (weight after drying at 103°C) was recorded after the treatment and the leaching, for the un-leached and leached samples respectively. This anhydrous weight was considered as "initial weight" for weight loss calculations (M0), and retention of the total treatment product was calculated after leaching. 

The treated specimens were tested for resistance to biological attack according to the EN113, 1996, against Pycnoporus sanguineus (tropical brown rot, strain CTFT 270) grown on malt/agar medium (malt 40g/l, agar 20g/l). All wood specimens were sterilized by gamma radiations prior to fungal exposure. In each culture flask, one treated specimen and a control (untreated beech) were introduced. Virulence control was also performed on 6 untreated beech specimens. The specimens were incubated for 16 weeks at 27°C, 75% RH (tropical conditions to allow high fungal virulence). After this, the mycelium was removed and the specimens were weighted (M1) to determine their moisture content at the end of fungal exposure. The specimens were then dried at 103°C and their final weight was recorded (M2).

The following data were calculated, according to the formulas (1) and (2):

Humidity (end of fungal exposure, treated samples) % = [(M1 – M2)/M2] x 100	(1)
Weight loss % (treated samples) = [(M0-M2)/M0] x 100	(2)

RESULTS AND DISCUSSION

The results of the biological tests are shown in Table 1. The average mass loss of the specimens used for virulence controls was 35.47±6.86 %. 

Table 1. Biological results of beech specimens treated with tannins + hexamine ± boric acid, unleached and leached, exposed to Pycnoporus sanguineus (* Tannin+ hexamine +/- boric acid ** Standard deviation)

The formulations based on condensed tannin (mimosa tannin extract) and hexamine only can be considered as the matrix without the presence of any active ingredient. They provide a slight protective effect only when tannins are used at 35% and for unleached samples. 

The formulations based on the complex formed by boric acid with condensed mimosa tannin being networked and hardened by reaction with hexamine induce a high biological resistance to the wood, considering the severity of this test (the virulence of the fungus being very strong). For the formulations containing boric acid, and for each mimosa tannin concentration, the non-leached specimens present a higher average mass loss than the leached ones. This is probably due to some leaching with no correction factors determined (according to EN113 standard). For each formulation and concentration, the correction factor corresponds to the average mass loss of treated specimens (at least 4 specimens) on malt/agar medium only, which would have account for non-fungal mass loss due to leaching.

The chemical mechanism(s) of these systems are still to be explored, but it is clear that the resinification of the hydrophobic tannin + hexamine system greatly reduces leaching of the boric acid. The boric acid is then still most non-covalently bonded to the tannin resin but retains sufficient mobility that allows it to work as a fungicide. 

CONCLUSIONS

Wood preservatives based on the cross-linking and hardening of condensed polyflavonoid tannins by hexamine, onto which boric acid is added and complexed, had greatly reduced boron leaching. Further, wood treated with these combinations had increased fungal durability before and after leaching according to European standard EN 113. Further work with this system might lead to environmentally-benign wood protection systems. 

NOTE

*This paper was originally presented at the 2010 EC-IAWS/ESTB7 meeting, Rabat-Morocco, March 2010, and has been updated.

REFERENCES

Byrne, A.; Morris, P.I. 1997. Recent Research in Boron Treatment of Canadian Wood Species. The Second International Conference on Wood Protection with Diffusible Preservatives and Pesticides, Forests Products Society55-61        [ Links ]

Drysdale, J.A. 1994. Boron Treatments for the Preservation of Wood. A review of Efficacy Data for Fungi and termites. The International Research Group on Wood Preservation. Stockholm, Sweden. Doc IRG/WP 94-30037        [ Links ]

European Norm. 1996. EN113. Wood preservatives. Method for determining the protective effectiveness against wood destroying Basidiomycetes. Determination of toxic values.         [ Links ]

Hamilton, S.J.; Buhl, K.J. 1990. Acute toxicity of boron, molybdenum and selenium to Fry of Chinook salmon and Coho salmon. Archives of Environmental Contamination and Toxicology 19: 366-373        [ Links ]

Jansen, J.A.; Andersen, J.; Shou, J.S. 1984. Boric acid single dose pharmacokinetics after intravenous administration to man. Archives of Toxicology 55:64-67        [ Links ]

Kamoun, C.; Pizzi, A. 2000a. Mechanism of hexamine as a non-aldehyde polycondensation hardener, Part 1:mechanisms. Holzforschung Holzverwertung 52(1):16-19        [ Links ]

Kamoun, C.; Pizzi, A. 2000b. Mechanism of hexamine as a non-aldehyde polycondensation hardener, Part 2: recomposition of intermediate reactive compound. Holzforschung Holzverwertung 52(3):66-67        [ Links ]

Kamoun, C.; Pizzi, A.; Zanetti, M. 2003. Upgrading of MUF resins by buffering additives – Part 1: hexamine sulphate effect and its limits. Journal of Applied Polymer Science 90(1): 203-214        [ Links ]

Lebow, S.T.; Morell, J. 1989. Penetration of boron in Douglas-fir and western hemlock lumber. Forest Products Journal 39(1): 37-70        [ Links ]

Lloyd, J.D. 1997. International borate status of borate preservative systems.  The second international conference on wood protection with diffusible preservatives and pesticides 45-54        [ Links ]

Lloyd, J.D. 1998. Borates and their biological applications. The International Research Group on Wood Preservation, Stockholm, Sweden. Doc IRG/WP 98-30178        [ Links ]

Maier, K.J.; Knight, A.W. 1991. The toxicity of waterborne boron to Daphnia magnia and Chironomus decorus and the effect of water hardness and sulphate on boron toxicity. Archives of Environmental Contamination and Toxicology  20: 282-287        [ Links ]

Obanda, N.D.; Shupe, F.T.; Barnes, M.H. 2008. Reducing leaching of boron based wood preservatives – A review of research. Bioresource Technology 99:  7312-7322        [ Links ]

Peylo, A.; Willeitner, H. 1997. Leaching of Boron more than 3 years after exposure. The International Research Group on Wood Preservation, Stockholm, Sweden. Doc IRG/WP 97-30143        [ Links ]

Pichelin, F.; Nakatani, M.; Pizzi, A.; Wieland, S.; Despres, A.; Rigolet, S. 2006. Thick wood panels bonded industrially with formaldehyde free tannin adhesives.  Forest Products Journal 56(5): 31-36        [ Links ]

Pizzi, A.; Beacker, A. 1996. A new boron fixation mechanism for non-toxic wood preservatives. Holzforschung50: 507-510        [ Links ]

Scalbert, A.; Cahill, D.; Dirol, D.; Navarrete, M.A.; de Troya, M.T.; Van Leemput, M. 1998. A tannin/copper preservation treatment for wood. Holzforschung  52:133-138         [ Links ]

Teshima, D.; Taniyama, D.; Oishi, R. 2001. Usefulness of forced diuresis for acute boric acid poisoning in an adult. Journal of clinical Pharmacy and Therapeutics 26: 387-390         [ Links ]

Thevenon, M.F. 1999. Formulation of long-term, heavy-duty and lowtoxicwood preservatives. Application to the associations boric acid-condensed tannins and boric acid-proteins. Ph.D. Thesis, University of Nancy I, France        [ Links ]

Usuda, K. ; Kono, K. ; Orita, Y. ; Dote, T.; Iguchi, K.; Nishiura, H.; Tominaga, M.; Tagawa, T.; Goto, E.; Shirai, Y. 1998. Serum and urinary boron levels in rats after single administration of sodium tetraborate. Archives of Toxicology 72 (8): 468-474         [ Links ]

Received: 08.06.2010 Accepted: 25.09.2010
Corresponding author: marie-france.thevenon@cirad.fr

For further details log on website :
http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0718-221X2010000300009