Ceramics-Silikáty 52 (4) 268-277 (2008)

Silicification of wood in the laboratory

J. Götze, R. Möckel, N. Langhof, M. Hengst, M. Klinger

Silicification of wood was performed in the laboratory to improve the physical properties (e.g. wear resistance, water absorption, hardness) of the material. Experimental studies were carried out using two types of wood (oak and spruce) and five different sources of silica including sodium metasilicate (Na₂SiO₃), a colloidal suspension of silica, tetraethoxysilane (TEOS), a mixture of TEOS with methyltriethoxysilane (MTEOS), and silica sol with three different particle sizes. Additionally, TEOS mixed with the alkoxides tetrapropyl zirconate (TPOZ) and tetrabutyl orthotitanate (TBOT), respectively was tested. Experiments were performed in the temperature range of 20°C and 80°C under normal pressure in glass beakers and at higher temperatures (up to 138°C) and pressures (up to 12 bar) in autoclaves. A special experimental setup with vacuum impregnation was developed to increase the penetration depth of the silica into the wood material. The properties of the impregnated wood were investigated using different analytical techniques. Besides microscopic investigations (binocular, polarizing microscopy, SEM, cathodoluminescence), XRD analysis, FTIR spectroscopy and measurements of water absorption and hardness were performed. XRD studies revealed that the silica coatings completely consist entirely of X-ray amorphous silica (opal-A similar to hyalite). CL microscopy proved to be the most effective method to reveal the distribution of SiO₂ within and on the surface of the wood samples. FTIR studies showed that deposited silica may contain organic residues of the source materials. The technical properties of wood could significantly be improved by the silicification. Material silicified with colloidal silica and TEOS, respectively absorbed up to 40% less water than untreated specimens. Likewise, the Brinell hardness of treated samples could be increased by nearly 100% compared to untreated samples. This is mainly caused by vitreous silica coatings (generated by sol-gel transformation) at the sample surface.

Keywords: Wood, Silicification, Silica sol, TEOS, MTEOS, Microscopy, FTIR spectroscopy