Technological improvement of Portuguese Pine wood by modification

Abstract

Wood has a long tradition as a structural and building material in Portugal. In the last decade, the European trend to improve physical properties of wood by chemical modification allowed extending its performance on durability and improve its performances in properties relating to moisture, in general. The objective of the Technological improvement of Portuguese pinewood by chemical modification was to study Maritime pine (Pinus pinaster Ait.) wood characteristics and the impact of the chemical modification at large range on resistance to marine organisms, physical and mechanical properties. Four modification methods were tested: 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU), N-methylol melamine (MMF), tetra-alkoxysilane (TEOS) and wax. This work comprises two parts: a preliminary and a main-study. In the former, a large variety of material properties was studied - physical and mechanical properties, and their interactions. Particular attention was taken on the marine organisms, where the role of hardness, shape of specimens as well as the toxicity of chemicals were investigated. In the second part of the work, a study on the subject of creep was carried out. Types and levels of the modification together with stress levels, different environmental conditions and mechano-sorptive effect were evaluated. Although stiffness appeared to be statistically unaffected, significant differences in other properties occured. The anti-swell-efficiency and stiffness stabilization increased, on the one hand. On the other hand, the impact bending strength and the equilibrium moisture content appeared to be significantly reduced in the cell wall modification (with DMDHEU and MMF resins) where a close inter-connection between the latter variables was observed. Under high stress level or under the mechano-sorptive effect, the creep was lower for both modifications with cell wall reaction (with DMDHEU and MMF) than for unmodified wood. For both types of resin, relative creep reduced significantly, despite different changes took place in the equilibrium moisture content, stiffness stabilization or strength. The anti-creep efficiency has shown a close correlation between the equilibrium moisture content reduction, anti-swelling efficiency and stiffness stabilization effciency. The lumen fill modification with TEOS solution had no effect in creep as well as in the equilibrium moisture content and stiffness stabilization efficiency. In the marine trials, the main conclusions were the unimportant role of the hardness and the specimens shape against the attack of marine borers (Limnorids and Teredinids). Modified wood with lumen fill (with TEOS and wax) has no effect on the resistance to both marine borers. Modified wood with condensation resins (DMDHEU and MMF) were unaffected by Limnorids attacks. But, to prevent the attacks of Teredinids over 3 years of exposition, only DMDHEU resin was effective with a required minimum level of modification. The use of modified wood as building material is well known in non-loaded bearing components, where applications in moist rooms, wall claddings or garden furniture are the main applications. This study supports the possible extension of its application for structural purposes, mainly if the design is governed by the serviceability limit state, deformation. Therefore, the brittleness behaviour, mainly in the cell wall modification with DMDHEU resin, and design properties for mechanical connections must be determined in order to extend the use of modified wood as load bearing components where often the ultimate limit state govern the design of structural components.