Treatment of solid wood with silanes, polydimethylsiloxanes and silica sols

Abstract

The study can be divided into three sections. In the first section, wood was treated with different commercially available silica sols of varying pH and surface modification. While alkaline silica sols could not be impregnated into the wood due to lowering of pH and precipitation, neutral and acidic silica sols showed good penetration. One of the tested silica sols with an acidic pH value and cationic surface modified with aluminum-oxychloride reduced water uptake and fungal decay due to incubation with the brown rot fungus Coniophora puteana (pine sapwood) and the white rot fungus Trametes versicolor (beech wood) in a fungal decay test according to EN 113. Blue stain test revealed some inhibition of staining by the fungus Areobasidium pullulans, but no absolute resistance if wood had been treated with this silica sol. Even the smallest available particle sizes of the silica sols did not result in positive increase in cell wall volume (chemical swelling, bulking), indicating, that silica sols cannot penetrate the cell wall of wood and induce increased dimensional stability. Since silica sols are not able to penetrate the cell wall and are solely deposited in the lumens of the cell, the treatment cannot be regarded as a real wood modification. Due to the promising results in water uptake and fungal decay test, wood treated with cationic silica sol was further investigated in a thermo gravimetric analysis (TGA) and burning test. In the TGA the silica sol revealed reduced thermal degradation temperature (a common feature of fire retardant salts), but only to a minor extent. The charcoal yield after pyrolysis was not increased, indicating no reduction of flammable volatiles released during pyrolysis. Furthermore oxidation behavior of the charcoal was similar to the control. In the burning test, the burning duration, burning speed and mass loss of the samples were reduced. Glowing of the charcoal was completely prevented. All these effects were, however, small if compared to a commercially available fire retardant. In the second part of the study, acetoxy-functional silane and PDMS with amino-, acetoxy- and hydroxy-function as well as non-functional PDMS were tested in combination with acetic anhydride for the acetylation of wood. Best water repellence was obtained with acetoxy-functional PDMS, which was further investigated in different concentrations. An addition of 1% proved to be sufficient for maximum water repellence, which indicated coverage of the inner surfaces of the wood rendering them water repellent. Decay resistance was only governed by acetyl-content of the acetylated wood; the PDMS did not show an influence. Even though the PDMS treated samples showed a slight over-swelling in water, bending strength and impact bending strength were not affected by the combined treatment. In the third section of the study, water based emulsions of functional PDMS were tested for their suitability to improve decay resistance and water related properties of wood. The α-ω attached functionalities were: amino, carboxy, epoxy and carbobetain. While best results in reducing water uptake were obtained using carbobetain-functional PDMS, decay by Coniophora puteana and Trametes versicolor was most effectively reduced by carboxy-functional PDMS. This material, however, showed no proper reduction in water uptake and additionally exhibited high leaching. Combining carboxy- and amino-functional PDMS will result in salt formation of the two functionalities and was thought to increase fixation of carboxy-functional siloxane. However, the combination did not show promising synergistic effects. Dimensional stability was hardly achieved by the treatments with PDMS emulsions. The PDMS did not penetrate the cell walls of wood sufficiently and did not cause a high and stable bulking, which is needed for dimensional stabilization.