Laboratory Evaluation of Borate:Amine:Copper
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LABORATORY EVALUATION OF BORATE:AMINE:COPPER DERIVATIVES IN WOOD FOR FUNGAL DECAY PROTECTION George Chen Research Chemist Performance Enhanced Biopolymers USDA Forest Service, Forest Products Laboratory 1 Gifford Pinchot Drive Madison, WI 53726 (Received December 2010) Abstract. This study aimed to evaluate borate:amine:copper derivatives in wood for fungal decay protection as well as the permanence of copper and boron in wood. Each of four derivatives of borate: amine:copper prevented fungal decay in wood. Disodium tetraborate decahydrate (borax):amine:copper derivatives with 0.61-0.63% retention after water leaching prevented decay by Gloeophyllum trabeum (Gt) and with 0.64% retention prevented decay by Trametes versicolor (Tv). Leaching did not decrease decay resistance for either Gt or Tv. Disodium octaborate tetrahydrate (DOT):amine:copper derivatives with 1.14-2.93% retention after water leaching prevented decay by Gt and with 0.54-1.19% retention prevented decay by Tv. Leaching decreased decay resistance to Gt but not to Tv. Higher copper and boron in disodium borax:amine:copper derivatives contributed to more decay resistance to Gt and Tv than that of DOT:amine:copper derivatives as evidenced by elemental analysis. IR spectra of wood treated with 5% borate:amine:copper derivatives after water leaching showed increased absorption at 1632-1635 cmÀ1 compared with the control. This increased absorption was partly attributable to carbonyl of copper carboxylates from oxidation of hemiacetals of hemicelluloses and cellulose by copper (II) ions and carbonyls of copper (II) quinone methides by oxidation of guaicyls by copper (II) ions. It was also partly attributable to carbonyls of copper carboxylates from hemicelluloses and phenolates from lignin through ion exchange reactions. These oxidation and ion exchange reactions of copper with wood components may account for their efficacy and long-term performance. Keywords: Borate:amine:copper derivatives, fungal decay protection, fixation of copper. INTRODUCTION extracellular substrate sequestration, intercellular substrate sequestration, enzyme inhibition, and Boron compounds, including boric acid, diso- change in membrane function (Lloyd et al 1990) dium tetraborate (borax), and disodium octaborate and interaction of borate in xylose moiety of the (DOT), are broad-spectrum biocides (Pickard oxidized coenzymes NAD+,NMN+,andNADP+ 1948) with low toxicity to mammals and aquatic (Lloyd 1997). Borates are used in large quantities life (Lloyd 1997) and have been used as fungi- in building products in Asia and North America, cides, insecticides, bactericides, herbicides, and in wood composites and pest control in North fire retardants (Pickard 1948). The efficacy of America, and in formulation of exterior and reme- boric acid and borates against wood decay fungi, dial treatments in Europe (Lloyd 1997). Because termites, and fire has been well established in boric acid and borates are water-soluble, their use wood products for the past 60 yr (Croft and Levy in outdoor wood products is limited. Efforts to 1973; Lloyd 1997). The mechanisms of boric acid minimize borate leaching from wood products by and borate inhibition to fungi and plants are not impregnating hydrophobic polymers, including well investigated but are postulated to inhibit polyethylene glycol, polyvinyl acetates, and pentose pathway (Lee and Aronoff 1967). The acrylic polymers, were not successful (Murphy tetrahydroxyborate ion acts by complexation with et al 1995; Gezer et al 1999a; Williams and poly-ols and probably attacks decay fungi through Bergstrom 2005). Research to find more leach- resistant borates for wood products has been per- Corresponding author: [email protected] formed in recent years. Chromate copper borates Wood and Fiber Science, 43(3), 2011, pp. 271-279 # 2011 by the Society of Wood Science and Technology 272 WOOD AND FIBER SCIENCE, JULY 2011, V. 43(3) (Ochrymowych and McOrmond 1978) performed Table 1. Weight (g) of 5% borate:amine:copper in 1:2:4 well in field tests; however, chromium may not be molar ratio in 1-kg solution dissolved in 8% ammonium used in products because of environmental con- hydroxide. Formulationb cerns. Copper borates dissolved in ammonium a hydroxide in wood (Johnson 1983; Johnson Treatment Amine Copper and Foster 1991) performed well after 6 yr in B1A1 Cu 12.316 5.431 32.253 B1A2 Cu 12.711 4.003 33.286 the field, but boron was leached from wood. B2A1 Cu 13.060 5.323 31.616 Effectiveness of copper borates was attributed to B2A2 Cu 13.470 3.922 32.608 permanence of copper in wood. Albumis protein a B1, borax; B2, DOT; A1, dicyandiamide; A2, Urea; Cu, copper sulfate borates (Thevenon et al 1997, 1998a, 1998b) pentahydrate. b Weight (g) of 5% borate:amine:copper in a 1:2:4 molar ratio per 1-kg were effective in laboratory and field tests. Their solution. long-term field performance remains to be as- sessed. Organoborates, including borate esters and aminoborates (Carr et al 2005; Chen 2008), Impregnation in treated wood after water leaching showed Each concentration (0.5, 1.0, 2.5, and 5%) of effectiveness in laboratory fungal decay tests. each borate:amine:copper solution was impreg- This led us to prepare borate:amine:copper deriv- nated into wood by full cell pressure. For each atives. Borate derivatives may form stable, leach- concentration, 32 sapwood blocks in anatom- resistant compounds in wood through covalent ical directions were used, including 20 blocks bonding of boron with nitrogen in amines, which for fungal decay tests—10 loblolly pine (Pinus then forms a complex with copper (II). taeda L.) and 10 sweet gum (Liquidambar Copper compounds are used for controlling styraciflua L.). The remaining 12 blocks (6 lob- algal growth, wood decay fungi, marine borers, lolly pine and 6 sweet gum) were used for the and crop fungi and have been used successfully weathering test. The treatment with one concen- for more than a century (Richardson 1997). tration for fungal decay and weathering tests is Copper oxidizes proteins, enzymes, and lipids described subsequently. and interferences with enzymatic processes Thirty-two sapwood blocks (16 loblolly pine (Eaton and Hale 1993; Rui and Morrell 1994). and 16 sweet gum), 19 mm in all anatomical This study aims to evaluate borate:amine:copper directions, were conditioned at 27C and 30% in wood for fungal decay protection as well as RH for 3 wk and weighed. The blocks reached permanence of copper and boron in wood and to equilibrium moisture content after 3 wk. They describe the complex formation of copper with were placed in a 1-L beaker in a desiccator wood components. under vacuum at 0.27-0.4 KPa for 30 min and then impregnated with one of four concentration levels in 8% ammonium hydroxide solution MATERIALS AND METHODS (500 mL). After impregnation, the pressure was released and another 100 mL of solution was Formulation added. Treated blocks were soaked for 24 h. Four derivatives of borate:amine:copper were Blocks were then removed from the solution. formulated using two borates, borax and DOT, Excess solution was wiped off, and blocks and two amines, dicyandiamide and urea and were weighed to determine the amount of solu- copper sulfate penta hydrate, in a 1:2:4 molar tion absorbed. Blocks were then air-dried for ratio at four concentration levels (0.5, 1.0, 2.5, 1 wk under a chemical hood. Half the blocks and 5%). Borate:amine:copper derivatives were (16) were leached in distilled water (800 mL, dissolved in 8% ammonium hydroxide solution. 50 mL/block) for 6, 24, and 48 h and then every Each borate derivative was prepared in a 1-kg 24 h for 2 wk according to AWPA E10-91 solution (Table 1). (AWPA 1996). Leached and nonleached blocks Chen—EVALUATION OF BORATE:AMINE:COPPER DERIVATIVES IN WOOD 273 were conditioned at 27C and 30% RH for 3 wk Another 24 loblolly pine sapwood blocks and weighed before fungal decay tests. Two (6 blocks, including 3 leached and 3 nonleached, fungi, Gloeophyllum trabeum (Gt), a brown-rot for each derivative) were treated with 5% of fungus, and Trametes versicolor (Tv), a white- each of four borate:amine:copper derivatives. rot fungus, were used in a 12-wk laboratory soil- The procedures and leaching were the same as block fungal decay test. Boric acid was used as a described previously except one concentration reference. (5%) was used. These blocks were used for ele- mental and IR analyses. Weathering tests were used to determine the chemical losses caused by evaporation without Chemical retention in the blocks is expressed in presence of fungi. Six blocks (3 leached and kilograms per cubic meter (kg/m3) obtained by 3 nonleached) from two species (loblolly pine multiplying solution concentration with block and sweet gum) were placed in a soil-block weight. Chemical retention in wood for fungal decay bottle on top of a feeder strip not inocu- decay tests is based on solution absorbed by lated with fungal mycelia, and the bottles were blocks and conditioning at 27C and 30% RH placed in a growing chamber (27C and 80% for 3 wk, before and after decay tests. Because RH) for 12 wk. After 12 wk, the weathering too few data points (four concentrations) existed blocks were conditioned at 27C and 30% RH to establish a threshold relationship, nonlinear for 3 wk and weighed to determine weight loss regression analysis to determine threshold reten- caused by evaporation of volatile compounds tion would not be accurate (Steel and Torrie from wood and dehydration of chemicals. 1960; Nance and Amburgey 1976; Gezer et al Tables 2 and 3 show weight losses by decay 1999b). Weight losses greater than 2% were fungi after correction for weathering losses. considered to be of fungal origin, and weight Table 2. Effect of borate:amine:copper derivatives on weight losses of loblolly pine wood decayed by Gloeophyllum trabeum in a 12-wk soil-block fungal decay test of nonleached and leached specimens.