Wood used outside deteriorates if it is not protected. Treatments with boron, which has fungicide, insecticide, anticorrosion and fireproofing effects and no environmental impact, are effective, but only in the short term, since the boron, which is highly water-soluble, is rapidely washed away. Hence the idea of combining it with molecules capable of fixing it in wood. CIRAD and its partners have opted for mimosa tannin-based polymer networks, combined with hexamine. Treated woods show good boron retention, excellent rot resistance, and significantly improved termite resistance.
Using wood for building structures or cladding inevitably runs up against problems relating to durability, fire resistance and weathering. To protect wood against insect attacks and improve its fire performance, a team from CIRAD and partner organizations opted for boron, in the form of boric acid.
This active ingredient has many qualities. It has fungicide, insecticide, including termiticide, anticorrosion and fireproofing effects, is odour- and colourless, plentiful, cheap and, above all, has no environmental impact. It is also highly water-soluble, which makes it easy to treat wood by painting, dipping-diffusion or autoclaving, but this makes it highly leachable, which is its main drawback: woods treated with boron are not protected from biological attacks for very long if used outside.
A solution containing mimosa tannins, hexamine and boric acid
The team therefore set out to find a system capable of fixing boron in timber long-term, while maintaining its efficacy and using concentrations of less than 5.5% (m/m), a restriction imposed by EU legislation.
The researchers used mimosa tannin-based polymer networks. The system was simple: the timber was thoroughly impregnated with a tannin and boric acid solution, which in wood treated and dried at temperatures of between 60 and 105°C formed an insoluble natural polymer network that served to fix the boron.
During leaching trials of treated wood, the levels of these substances in the waste water produced prompted the team to improve the system by adding a small proportion of hexamine. Hexamine-tannin systems are already used to make adhesive mixtures for use in wood-based composites and have been proved not to produce formaldehydes.
This combination of mimosa tannins, hexamine and boric acid also made it possible to treat timber with an aqueous solution, after adjusting the viscosity, and t work with boric acid concentrations of between 1 and 0.40%, well within the relevant restrictions.
Fungus- and termite-resistant woods
Woods treated and dried in this way proved to retain boron well after leaching, in the case of both hard and soft woods. After standard leaching tests (applied to all wood protection products for the European market), their resistance to the fungi that cause fibrous and cubic rot, both temperate and tropical strains (in the latter case under tropical exposure conditions) was excellent and identical to that provided by conventional products already on the market.
Their resistance to Saintonge termites, a species commonly found in France, was also markedly improved. However, since the system did not act as a repellent, the termites had to ingest the product before dying, an aspect that requires improvement.
Furthermore, the polymer network improved the mechanical properties of the wood, particularly compression performance and surface hardness, and acted as a flame retardant. On the other hand, the treated wood was a darker colour, and ageing trials are under way to determine how its appearance changes when used outside.
The system, which is highly promising, requires further study and improvement. In particular, there are plans to conduct impregnation tests with other non-durable species, field trials in contact with the soil, and ecotoxicity tests.
For further details log on website :
http://www.cirad.fr/en/research-operations/research-results/2013/tannins-hexamine-and-boric-acid-effective-wood-protection-against-insects-and-fire
No comments:
Post a Comment