Decay resistance of modified wood
Different modifications, namely hexanolylation, acetylation, thennal modification and silylation by trimethoxy vinyl silane (VTMS,) and by 'Y-methacryloxy propyl silane (TMPS) were applied to impart decay resistance to Corsican pine sapwood, which is a non-durable wood. The possible mechanisms by which the modifications impart decay resistance to the wood were also investigated. The silanes applied in this study showed completely different reactivity so that the vinyl group of VTMS remained un-reacted and the vinyl group of TMPS reacted (but its sHanol was not very reactive). The decay resistance of the modified woods were assessed by soft rot, brown rot and white rot fungal tests. The decay resistance against basidiomycetes and soft rot fungi was improved by all the wood modifications applied in this study, but the wood treated with methanolic solutions of the silanes did not show complete decay resistance against brown rot fungi. The failure of the silanes to impart complete protection of wood against brown rot fungus (c. puteana) was attributed to the restriction of the silanes in penetrating into the cell wall. In the soft rot tests, VTMS modified stakes showed high decay resistance but TMPS modified wood showed a moderate decay resistance. This was attributed to the uneven distribution of TMPS in wood. An industrially acceptable treatment method (by using aqueous solutions of silane instead of methanolic solutions and pressure treatment instead of vacuum treatments was established). The perfonnance of the silane treated wood was assessed by field tests. The method worked well with TMPS (the wood showed significant decay resistance in the test) but it didn not work with VTMS. Since both anhydrides (hexanoic and acetic anhydride) used in this study, showed nearly the same perfonnance against C. puteana and soft rot fungi, it was concluded that the improvement in decay resistance against the brown rot fungi and soft rot depends on the WPGonly. Heat treatment above 200°C was recognised to be an effective treatment in improving the decay resistance of wood. It was shown that heat treatment temperature plays a more important role in the improvement of decay resistance than the treatment time. Heat treatment at 250°C for 2 hours, imparted complete decay resistance to the wood against basidiomycetes and soft rot. No significant difference between decay resistance of heated wood post-extracted and heated wood without any extraction against basidiomycet and soft rot fungi was obtained, suggesting that extractable fungicidal is not the reason for the improved decay resistance. In addition to the decay tests, dimensional stability, pore cell wall pore accessibility and hygroscopicity of the modified wood were also studied to find the mechanism by which the modification imparts decay resistance to the wood. It was suggested that hexanoylation and acetylation reduces hygroscopicity so that not enough water is available for the diffusion of brown rot degrading agents into the cell wall, while lignin substitution might be the main reason for the improved decay resistance against white rot fungi. For heated wood, a good correlation between a reduction in FSP and WL due to decay was obtained. Thus, it was suggested that a reduction in the hygroscopicity of wood could be the main reason for the improved decay resistance. By using the Hailwood Horrobin model it was shown that a reduction in poly-molecular adsorption of heated wood is the main reason for the reduced hygroscopicity. Since good correlation between an increase in the lignin content and a reduction in the poly-molecular sorption was obtained, a reduction in the hygroscopicity of heated wood was suggested to be due to a reduction in the swelling of the wood cell wall in which micro fibrils were placed in a matrix of condensed lignin and the hemicellulose residue, rather than a reduction in the hydroxyl groups of wood. Lignin modification is thought to be the main reason for the reduced hygroscopicity.