Composites Utilizing Acetic Anhydride Modified Wood

10 Composites Utilizing Acetic Anhydride Modified Wood 

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, 

An early study by Rudkin (1950) showed that substitution of OH groups with acetyl resulted in a significant decrease in bond strength between the wood and a UF resin in lap-joint tests. Vick and Rowell (1990) studied the adhesive bonding of acetylated yellow poplar, with 18 different thermoplastic and thermosetting adhesives. The effectiveness of the adhesives was examined by determination of bond shear strength (and wood failure) of 6 mm thick, bonded wood strips after conditioning at 27 °C and 

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 

The generally poorer mechanical properties exhibited by acetylated lignocellulosic material in composites bonded using aqueous resin systems was considered by Korai etal. (2001). Fibres of yellow cedar (Chamaecyparis nootkatensis) were acetylated to a WPG of 24.8 % and then ozonated to different extents to increase the hydrophilicity of the fibre surface. Boards were fabricated from the fibres using an aqueous MF resin. Ozonation improved IBS of boards fabricated from acetylated fibres, proportional to level of ozone charge, and resulted in IBS values comparable to those of nonacetylated controls at higher levels of ozonation. However, although ozonation also improved MOR, the values obtained for acetylated fibres were always less than those obtained with unmodified fibres. 

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