Glutaraldehyde is well-known disinfector and cross-linker. The aldehyde groups in glutaraldehyde molecules can react with the hydroxyl groups of wood polymer forming stable acetal bonds. This study is devised to investigate the effects of chemical modification with aqueous solutions of glutaraldehyde (GA) and magnesium chloride on the properties of Scots pine sapwood (Pinus sylvestris L.) and European beech wood (Fagus sylvatica L.).
Compared to the untreated controls, GA treatments reduced the equilibrium moisture content up to 30% at 90% relative humidity (RH). Capillary water uptake of wood was also restrained by GA treatment (8.6% WPG) resulting in water reduction effectiveness of approximately 50% in both radial and tangential direction after 244h water immersion. Wood blocks treated to 22% WPG attained 70% anti-swelling efficiency (ASE). After 10 water submersion and drying cycles, the wood treated to 22% WPG lost approximately 10% ASE. The study on the optimum treating parameters showed that at pH 4.0 GA was fixed in the wood maximally. The tensile strength of GA treated wood was reduced with decreasing the pH value of the treating solution in the range of 3.5–5.5. Sole treatment with MgCl2 can cause considerably decrease in the tensile strength (up to 25%) under acidic conditions. GA treatment did not affect the modulus of rupture and the modulus of elasticity, but significantly reduced work to maximum load in bending and impact bending strength; the compression strength, however, increased with increasing the WPG of wood.
GA treatments to a WPG above 7% obviously restrained the growth of the blue stain fungus Aureobasidium pullulans on both pine and beech wood in a test according to the standard of EN 152. With the white rot fungus Trametes versicolor, a WPG of 7% was the threshold to protect beech, while the threshold to prevent decay of beech and pine specimens towards the brown rot fungus Coniophora puteana was at a WPG of only 3%. The Scots pine stakes treated with GA to a WPG over 6% were protected from soft rot decay during 32 weeks exposure according to the standard of ENv 807.
Artificial weathering revealed that GA treatment caused an increased photo-stability of lignin as shown by infrared spectroscopy; wood treated to higher WPG level exhibited lower reduction in tensile strength and cell wall deterioration through weathering. During 18 months of outdoor exposure, GAtreated Scots pine sapwood panels exhibited lower moisture content and water uptake than the untreated ones. Blue stain colonization on the surface was also reduced strongly. The GA-treated panel surface was significantly smoother due to less erosion, cracking and minor peeling of tracheids. Scanning electron microscopy further revealed that individual tracheids were detached from the cell compound and then washed away from the untreated wood surface; whereas tracheids on surfaces of GA-treated wood remained in the tissue compound, although displaying many axial and transversal cracks.
Several commercial coatings were applied on the untreated and GA-treated wood surfaces and the dry time and wet adhesion strength of waterborne coatings were increased due to GA modification, but were not influenced or slightly negatively influenced for solvent-borne coatings. When the coated panels were exposed outdoors, the modified panels exhibited less fluctuation in moisture content; the deterioration of the coating film and fungal staining below the coating film did not, however, diminish due to wood substrate modification.