Acetylated wood, properties

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. 

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. 

Mahlberg etal. (2001) studied the effect of acetylation of wood fibres (20 % WPG) upon the mechanical properties of wood fibre PP fibre composites, with and without the addition of a novalak PF. Acetylation resulted in significant improvements in MOR, MOE and IBS in a composite containing 20 % by weight of PP fibre. With a combination of PF and PP (both 10 % by weight) and acetylated wood fibre, MOR was unchanged, MOE decreased and only IBS showed a significant improvement. Acetylation also contributed to a better dispersion of the wood fibres in the PP matrix. 

Arora, M., Rajawat, M.S. and Gupta, R.C. (1981). Effect of acetylation on properties of particleboards prepared from acetylated and normal particles of wood. Holforschung und Holzyerwertung, 33( 1), 8-10. 

Dreher, W.A., Goldstein, I.S. and Cramer, G.R. (1964). Mechanical properties of acetylated wood. Forest Products Journal, 14(2), 66-68. 

Rowell, R.M., Imamura, Y., Kawai, S. and Norimoto, M. (1989). Dimensional stability, decay resistance, and mechanical properties of veneer-faced low-density particleboards made from acetylated wood. Wood and Fiber Science, 21(1), 67-79. 

Yano, H., Norimoto, M. and Rowell, R.M. (1993). Stabilization of acoustical properties of wooden musical instruments by acetylation. Wood and Fiber Science, 25(4), 395-403. 

Recently, particleboards have been prepared from mixtures of acetylated and untreated wood chips [55]. Thickness swelling and water absorption after soaking in water for 24 h decrease as the number of acetylated chips increases. The specimens containing 100% of acetylated chips show no sign of decay. Further, particleboards from acetylated wood particles have been reported to have higher mechanical properties than those from PO-treated particles [56]. 

It is noteworthy that acetylated wood meals prepared by the TFAA method melted clearly at 320°C under a pressure of 0.29 MPa [4,5]. Other methods of acetylation resulted in products that did not undergo complete flow. However, thermal properties of the acetylated wood were enhanced by mixed esterification with other acyl groups. That is, esterified woods containing either propio-nyl or butyryl groups in addition to acetyl revealed meltable properties [4,5]. A film prepared from the acetylated-butyrylated wood meal has a tensile strength of 41.0 MPa and an elongation of 12.5% [41. 

The mechanical properties of acetylated wood are generally equal to those of untreated wood. However, shear strength parallel to the grain decreases in treated wood (47), and the modulus of elasticity decreases slightly (54). Impact strength (38) or modulus of elasticity (or stifihess) are unchanged (47). Wet and dry compressive... 

Many of the properties of acetylated wood depend on the method of acetylation. The temperature of treatment, time of reaction, and type and amount of catalysts play a significant role in the extent that fibers degrade during treatment. The amount of moisture present in the wood also is important. Some moisture (2-5%) seems to be needed to obtain the best reaction, but above this level the water hydrolyzes the acetic anhydride to acetic acid. This loss by hydrolysis accounts for a 5.7% loss of anhydride with each 1% of water in the wood (36). The rate of acetylation decreases as moisture content increases (37). 

USDA, Forest Service, Forest Products Laboratory, FPL GTR-55 Rowell RM, Imamura Y, Kowai S and Norimoto M (1989) Dimensional stability, decay resistance and mechanical properties of veneer faced low-density particlehoards made from acetylated wood. Wood and Fiber Science, 27(1)67-79 Rowell RM, Kawai S and Inone M (1995) Dimensionally stabilized, very low density fibreboard. Wood and Fiber Science, 27(4) 428-36 Rozsa AN (1994) Dielectric vacuum drying of hardwood. Proceedings 4th lUFRO International Drying Conference, Rotorua, New Zealand, 271-8 Ruddick JNR (1987) Proceeding of the incising workshop, Richmond, British Colombia, 1986. Special Publication 28. Forinteck Canada Corp., Vancouver, BC... 

A commercial plant (Titan Wood) has started to produce acetylated wood and has given the acetylated wood the name of a new wood species Accoya . Improved properties are high durability, high dimensional stability and high UV-resistance. ... 

Cellulose acetate with improved solubiUty properties can be prepared from low quaUty wood pulps by multistage addition of the components (97) or by intermpting the reaction in the early stages, filtering, and continuing the acetylation with fresh reactants (98,99). 

There have been reports in which acetylation has a detrimental affect upon mechanical properties. Reachon of Scots pine in an acetic anhydride/xylene solution for 4 hours at 145 °C resulted in a 50 % decrease in the tensile modulus (Ramsden etal., 1997). Spruce modified in acetic anhydride at 100 °C was found to exhibit a reduction in toughness of about 20 %, compared to unmodified wood (Reiterer and Sinn, 2002). 

Popper and Bariska (1972) studied the moisture sorption properties of wood chemically modified with acetic (or phthalic) anhydride and analysed the results using Brunauer-Emmett-Teller (BET) theory and the H-H model. Acetylation was found to reduce the number of sorption sites, whereas little effect was noted with phthaloylation. By dividing the sorption isotherm into a monolayer component and a multilayer component using the H-H model, it was shown that there was a large reduction in the... 

Titan Wood limited was formed in April 2003, by Accys Chemicals PTC, a UK-based company. The company is in the process of building a full-scale production facility for the acetylation of solid wood located at Arnhem in The Netherlands, which will become operational at the end of 2005. The company acquired equipment and intellectual property owned by the former AKBV, based in Arnhem. AKBV developed a pilot plant acetylation reactor of 2 300 litre capacity, 85 cm in diameter and 4 m long, capable of acetylating 0.9 m of solid wood per batch. 

Bongers, H.P.M. and Beckers, E.P.J. (2003). Mechanical properties of acetylated solid wood treated on pilot plant scale. In Proceedings of the First European Conference on Wood Modification, Ghent, Belgium, Van Acker, J. and Hill, C.A.S. (Eds.), pp. 341-350. 

---