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Annual Report: CIRAD 2007

Borates and vegetable oils—promising wood preservatives

Borates have wood preservative properties and a low environmental impact. These active substances are, however, rapidly leached out when used alone to treat outdoor wood, therefore reducing their efficacy. CIRAD and partners have combined boron with vegetable oils with the aim of enhancing the fixation of this substance within treated timber. The results of this research could give rise to environment-friendly wood preservatives that effectively control termites and fungi.

Measuring the resistance of treated wood to wood-rotting fungi © F. Lyon

Borates are efficient environment-friendly wood preservatives. Unfortunately they may be rapidly leached away when used to treat outdoor wood. CIRAD thus investigated different combinations of these borates with vegetable oils to improve boron fixation in the treated wood, including: boratevegetable oil double treatments, synthesis of an ammonium borate-oleate salt, borate-oil emulsions and grafting of an oil on borate-impregnated wood. These different combinations were tested for their boron fixation capacity and also for their protective effect against termites and wood-rotting fungi (basidiomycetes). The results of this research could ultimately give rise to efficient environment-friendly wood preservatives that comply with the requirements of the EU biocide products directive (98/8/EC).

Improving boron fixation

In the double treatments, the high drying capacity of vegetable oils increased boron retention, ie 30-40% of the initial boron was retained. Linseed oil was more efficient than rapeseed or soybean oil. Ammonium salt synthesized from boric acid and oleic acid retained 50% of the initial boron. Emulsion combinations were found to have exceptional boron retention properties (up to 90% retention). However, these emulsions are highly viscous, which reduces their retention potential—they do not penetrate deep enough into the wood and are thus surface leached. Finally, grafting of an oil on wood did not give interesting results under any of the tested reaction conditions.

Efficient rot and termite protection

Following the boron fixation tests, the efficacy in providing protection against wood-rotting fungi and termites was confirmed for double boron-oil treatments and for the ammonium borate-oleate salt. This efficacy was linked especially with the quantity of boron remaining in the wood after the leaching trial.

In the double treatments, the treatment efficacy depended mainly on the extent of oil unsaturation, ie the number of carbon-carbon double bonds in the fatty acids. The oil acted as a temporarily effective barrier against rot and boron was highly effective in controlling termites. However, these treatments were inefficient against mould fungus (Aspergillus niger), which aesthetically depreciated the oil-treated wood. The ammonium borate-oleate salt was found to be non-toxic to daphnia (small susceptible crustaceans used in the toxicity tests). This wood preservative product thus has highly promising applications.

There are still many yet unanswered questions. The addition of boron to the oil treatments does not reduce the wood flammability. The fire-hazard aspect of oiled wood hampers the development of these treatments. The possibility of adding fire retardants has been considered. Further studies are under way on the double borate-oil treatments and on the borate-oleate salts, with the aim of improving their performances against degradation agents while also boosting the fire resistance of treated woods.

Contact

Marie-France Thévenon, e-mail , Production and Processing of Tropical Woods (UPR)

Partners

  • Agence de l’environnement et de la maîtrise de l’énergie (ADEME, France)
  • Kyoto University (Japan)
  • Oléobois (France)
  • Languedoc-Roussillon Regional Council (France)
  • University of Montpellier II (France)
  • University of Nancy (France)

For further information

  • Lyon F., Pizzi A., Imamura Y., Thévenon M.F., Gril J., 2007. Synthesis of boric acid ammonium oleate salt for wood preservation: leachability and termite resistance test. In : International Research Group on Wood Protection, 20-24 mai 2007, Jackson Hole, Etats-Unis, Doc IRG-WP-07-30435, 14 p.
  • Lyon F., Pizzi A., Imamura Y., Thévenon M.F., Kartal S., Gril J., 2007. Leachability and termite resistance of wood treated with a new preservative: ammonium borate oleate. Holz als Roh- und Werkstoff, 65 : 359-366.
  • Lyon F., Thévenon M.F., Hwang W.J., Imamura Y., Gril J., Pizzi A., 2007. Effect of an oil heat treatment on the leachability and biological resistance of boric acid impregnated wood. Annals of Forest Science, 64 : 673-678.
  • Lyon F., Thévenon M.F., Imamura Y., Gril J., Pizzi A., 2007. Development of boron/linseed oil combined treatment as a low-toxic wood protection. Evaluation of boron fixation and resistance to termites according to Japanese and European standards. In : IUFRO conference, forest products and environment: a productive symbiosis, 29 octobre - 2 novembre 2007, Taipei, Taiwan, Doc IRG-WP-07-30448, 13 p.

Florent Lyon was awarded, within the framework of his PhD thesis Amélioration de la durée de vie d’essences peu durables à l’aide d’un traitement combiné à base de borates et d’huiles , the Ron Cockroft Award from the International Research Group on Wood Protection in 2007, the Gareth Williams Scholarship Award (funded by Arch Chemical Inc.) granted by a panel of scientists and commercial stakeholders, members of the International Research Group on Wood Protection in 2007, and the Prix Doctiss 2007 for his oral presentation within the framework of the Ecole doctorale I2S, University of Montpellier II.