Wood-inorganic composites

Miyafuji, H. and Saka, S. (1996). Wood-inorganic composites prepared by the sol-gel process V. Fire-resisting properties of the SiO,-P,Oc-B,Oo wood-inorganic composites. Mokuzai Gakkaishi,... 

Miyafuji, H., Saka, S. and Yamamoto, A. (1998). Si02-P205-B203 wood-inorganic composites prepared by metal alkoxides ohgomers and their fire-resisting properties. Holzforschung, 52(4), 410-416. 

Saka, S. and Ueno, T. (1997). Several Si02 wood-inorganic composites and their fire-resisting properties. Wood Science and Technology, 31(6), 457-466. 

Tanno, F., Saka, S. and Takabe, K. (1997). Antimicrobial TMSAC-added wood-inorganic composites prepared by the sol-gel process. Materials Science Research International, 3(3), 137-142. 

Saka S., Miyafuji H., Tanno F. Sol-gel products news wood-inorganic composites prepared by the sol-gel process. J. Sol-Gel Sci. Technol. 2001 20 213-217 Sakagami T. Organic-inorganic hybrid coating. Indust. Mater. 1998 46 57-61 (in Japanese) Sakka S. The current state of sol-gel technology. J. Sol-Gel Sci. Technol. 1994 3 69-81 Sakka S. Sol-Gel films for optical and electronic application. In Structure and Bonding, Vol. 85. 

Saka S. Wood-inorganic composites as prepared by the sol-gel process. In Wood and Cellu-losic Chemistry, Hon D., Shiraishi A.S., eds. New York Marcel Dekker, 2001, Chapter 20, p. 781... 

Wood has been used for various purposes since ancient time. However, it has some unfavorable end-products properties in dimensional instability, flammability, and biodeterioration. A recent trend in wood research is to remove such defects and add the value to wood. A study of wood-inorganic composites is one of its trials and has been actively carried out to develop new functional woody materials (Nishimoto, 1987, 1988). 

For this purpose, we have developed a method using a metal alkoxide to prepare wood-inorganic composites by sol-gel process. For this preparation, efforts have been made to add the value to the wood without losing its characteristic properties such as porous structure. 

However, the gels in this study must be treated under the temperature at which wood is not thermally degraded. Therefore, in a reaction medium of the metal alkoxide/alcohol (solvent)/acetic acid (catalyst), the moisture-conditioned wood or water-saturated wood is soaked at ambient temperature under reduced pressure or atmospheric pressure. The water present within the wood cells initiates the reaction of the hydrolysis and polycondensation of metal alkoxide. The soaked wood is subsequently treated at a temperature between 50 and 60 C for 24h, and at 105°C for another 24h to prepare wood-inorganic composites (Saka, 1992 Ogjso, 1993). 

Figure 27-1. Distribution of inorganic substances in wood-inorganic composites (Saka, 1993a).

A difference in the hydrolysis/polycondensation rate seems to result in the difference in the distribution of inorganic substances within the wood cells (Saka, 1993a). Evidence for this concept can be found in a study of Ti02 wood-inorganic composites prepared with titanium alkoxides and titanium chelates (Miyafuji, 1997). These agents have different rates of hydrolysis and polycondensation, and accordingly result in different distribution of TiOi gels in wood cells. 

Figure 27-3 shows relationship between antiswelling efficiency (ASE) and weight percent gain (WPG) in various wood-inorganic composites. Here, the ASE is a measure to evaluate the dimensional stability, i.e., an ASE of 0% refers to no control of dimensional stabilization, whereas 100% ASE refers to its complete control. 

Figure 27-5. Changes of water absorption ratio in Si02 wood-inorganic composites prepared with and without a water-repellent agent 3,3,3-trifluoropropyltrimethoxysilane (TFPTMOS), decyltrimethoxysUane (DTMOS), 2-heptadecafluorooctylethyltrimethoxysilane (HFOETMOS) (Saka, 1996).

For various kinds of wood-inorganic composites, a leaching test for inorganic substances was made under the severe conditions of water stirred 160 times per minute in... 

Creosote and chromated copper arsenate (CCA) have been widely used as preservatives for antibacterial treatment of wood. In spite of their excellence in this property, they have some drawbacks in toxicity. Therefore, less toxic and environmentally-benign chemicals are expected to be used. Quaternary alkylammonium salts are, thus, one of the candidates for the anti-microbial treatment of wood. In this study of wood-inorganic composites, trimethoxysilylpropyldimethyloctadecyl ammonium chloride (TMSAC), shown below, was used as a property enhancer to add an anti-bacterial property to wood (Tanno, 1997). 

Figure 27-10. Changes of the color difference ( E ) for BP added wood-inorganic composites by UV irradiation ( ) untreated wood, (t ) Si02 composites (3.5 WPG), (o) BP wood (10.3 WPG), (< ) BP-SiC>2 composites (14.3 WPG) (Miyafuji, 2004).

Figure 27-11 shows the combustibility test after 30 s ignition of model houses made from 2 mm-thick veneers of hinoki (Chamaecyparis obtuse Endl.). It is apparent that the Si02 wood-inorganic composites show much higher resistance to burning, compared with the untreated wood. 

To improve the properties of these Si02 composites further, a study of multi-component Si02 wood-inorganic composites has been made with two or more kinds of metal alkoxides. 

Figure 27-13. Thermogravimetric analyses of various wood-inorganic composites (a) untreated wood (b) SiOi composites (8.4 WPG) (c)Si02 P20s composites (16.9 WPG) (d) 8102- 203 composites (38.7 WPG), (e) Si02-P20s-B203 composites (34.3 WPG) (Miyafuji, 1996).

Figure 27-14. Comparisons of various wood-inorganic composites by combustibility test. Untreated wood veneer (left), wood-inorganic composites corresponding to those in Figure 27-13 (right) (Miyafuji, 1996).

WOOD-INORGANIC COMPOSITES FROM WATER REACTION SYSTEM... 

In the preparation of the wood-inorganic composites by sol-gel process as mentioned above, organic solvents such as methanol and ethanol were used in the reaction medium. However, for commercial applications, the use of such solvents should be avoided because of the danger of explosion during the heating process. Therefore, the preparation of the wood-inorganic composites in the reaction medium of water was proposed. In the proposed water reaction system, the silicon alkoxide oligomers which can be stable in water was used for the preparation of the composites. 

Figure 27-19. Changes of water absorption ratio (WAR) for wood-inorganic composites from water-soluble oligomers with water-repellency (9) untreated wood, (O) composites from water-soluble oligomer immediately cfter its preparation (0.9 WPG), (h) composites from water-soluble oligomers after 15 days of its preparation (0.9 WPG), (o) composites from water-soluble oligomers after SO days of its preparation (1.9 WPG) (Fujita, 2003).

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