Acetylated wood

Whereas the majority of reactions of acetic anhydride with wood are thermally assisted, there has been some interest in using other methods for delivering energy. Larsson Brelid (2002), Larsson Brelid and Simonson (1999) and Larsson Brelid etal. (1999) studied the use of microwave heating to acetylate wood in order to reduce reaction times, improve the distribution of bonded reagent within the wood and achieve more efficient removal of process chemicals and by-products. 

SEM observations of acetylated wood have also shown that at WPGs in the region of 20 %, expansion of the cell wall into the lumen occurs (Evans etal., 2000 Mohebby and Militz, 2002 Sander etal., 2003). The decrease in lumen cross-sectional area may be related to the observation by Kumar etal. (1979) that the axial permeability of acetylated... 

A comprehensive study of the properties of acetylated wood was undertaken by the Forest Products Laboratory in Madison, Wisconsin, and included an investigation of the mechanical properties of acetylated wood (Tarkow etal., 1946). The property changes were not significant, but there was some variation between species. For example, Sitka spruce and basswood exhibited increases in strength and MOE upon acetylation to about 20 % WPG, whereas yellow birch showed a decrease in these properties at 16 % WPG. 

Larsson and Simonson (1994) studied the mechanical properties of acetylated Pinus sylvestris and Picea abies. The MOR and MOE decreased by about 6 % for pine, but increased by about 7 % with spruce samples after acetylation. Samples for this study were vacuum/pressure impregnated with acetic anhydride, excess anhydride was then drained off and samples were heated at 120 °C for 6 hours. The hardness of the acetylated wood samples was also found to increase, which was considered to result from the lower MC of the modified wood. Acetylated samples were also found to be less susceptible to deformation when subjected to varying RH. 

The dynamic viscoelastic properties of acetylated wood have been determined and compared with other wood treatments in a number of studies. Both the specific dynamic Young s modulus (E /j) and tan S are lower in acetylated wood compared with unmodified wood (Akitsu etal., 1991, 1992, 1993a,b Korai and Suzuki, 1995 Chang etal., 2000). Acetylation also reduces mechanosorptive creep deformation of the modified wood (Norimoto etal., 1992 Yano etal, 1993). In a study of the dynamic mechanical properties of acetylated wood under conditions of varying humidity, it was concluded that the rate of diffusion of moisture into the wood samples was not affected by acetylation (Ebrahimzadeh, 1998). 

The acoustic properties of wood are changed by acetylation and there have been several reports of the utilization of acetylated wood in musical instruments (Yano etal., 1986a, 1988, 1993 Obataya, 1999). Both the sound velocity and sound absorption decrease as the WPG of wood is increased. 

Table 3.1 Table summarizing experiments on decay resistance of acetylated wood exposed to brown rot fungi... 

A few studies have been performed examining the resistance of acetylated wood to mould and stain fungi in a laboratory environment (Wakeling etal., 1991, 1992). These studies found that acetylation reduced the rate of mould growth, but at the end of the test there was no difference in the extent of colonization. Furthermore, samples modified to a WPG of 10 % exhibited a higher rate of colonization for reasons that were not clear. Samples acetylated up to a WPG of 20 % did not exhibit any improvement in performance in terms of mould growth, compared to unmodified samples, when exposed in a field trial. 

Imamura etal. (1986) exposed particleboard made from acetylated wood to the termite species Coptotermes formosanus and Reticulitermes speratus. A forced feeding test according to the JWPA standard 11-1981, where the untreated or acetylated wood was the only food source, and a choice feeding test (where wood specimens were randomly placed on a termite breeding colony for 30 days) were used. With C. formosanus, there was limited attack of the fully acetylated boards, with about 50 % termite mortality after 3 weeks in forced feeding tests, whereas with R. sparatus there was virtually no attack and 100% mortality. 

Johnson and Rowell (1988) found that some protection against marine borers was obtained by acetylating wood, but this was lower than that obtained with CCA or creosote treatment. Larsson Brelid etal. (2000) also found that acetylation provided only slight protection against attack in a marine environment. However, Westin etal. (2004b) exposed five acetylated pine samples (21 % WPG) at a marine test site, and found that four of these were free from attack after 4 years exposure. Untreated pine sapwood samples had failed after 1 year due to heavy attack by Teredo navalis. 

In measurements of the dielectric relaxation of water adsorbed on acetylated wood, a large change in the activation enthalpy and entropy of dielectric relaxation was found to occur at 6 % moisture content (Zhao etal., 1994), this presumably being attributable to the onset of formation of capillary water in the cell wall. 

Research on resin-bonded boards has used wood chips, or fibres, that are acetylated prior to composite manufacture. In almost all cases, acetylation has been achieved by using uncatalysed acetic anhydride. Significant improvements in dimensional stability proportional to the WPG of the wood particles have been reported when acetylated wood is used, and there is also a reduction in irreversible swelling (e.g. Bekere etal., 1978 Arora etal., 1981 Rowell etal., 1986a-e, 1989, 1990, 1991,1995 Yoshida etal., 1986 Youngquist etal., 1986a,b, 1988 Larsson and Tillman, 1989 Subiyanto etal., 1989 Vick etal., 1991 Clemons etal., 1992 Kajita and Imamura, 1993 Sasaki and Kawai,... 

Youngquist etal. (1988) found improvements in both resin distribution and IB values when suitable emulsifiers were used in conjunction with waterborne resins, but considered it unlikely that the improvement in performance could be justified on cost grounds. It was postulated that acetylated wood interfered with the polymerization of the resin, so that it was not fully cured. It has also been suggested that acetic acid, which may be released during board pressing, could accelerate resin curing of resol type resins. In a study to determine whether this was so, acetylation of wood was found to slightly reduce... 

Okino and co-workers produced composites from acetylated rubberwood (Okino etal., 2001) and acetylated cypress (Okino etal., 2004), bonded using a UF resin. The composites exhibited inferior mechanical properties compared to those produced from unmodified wood. The reduction in mechanical properties is undoubtedly due to poor wetting and weak interaction of the surface of the modified wood with the UF resin due to a reduction of H-bonding sites on the acetylated wood surface. 

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