Mechanical, properties acetylation

The HM and LM pectins give two very different types of gels the mechanisms of stabilization of the junction zones in the two cases are described and few characteristics given. The different molecular characteristics (DE, distribution of methoxyl or acetyl substituents, neutral sugar content or rhamnose content) play an important role on the kinetic of gelation, mechanical properties of the gel formed and also on the experimental conditions to form the stronger gels. All these points were briefly discussed. 

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. 

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). 

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 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. 

Korai (2001) also considered the importance of density profile of composites made from acetylated fibres in determining mechanical properties. Fibres of yellow cedar were acetylated with vapour-phase acetic anhydride and fibreboards were made from these, bonded with melamine formaldehyde resin. The results from this study indicated that bonding between fibres was the most important property determining mechanical properties. 

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. 

Birkinshaw, C. and Hale, M.D. (2002). Mechanical properties and fungal resistance of acetylated fast grown softwoods. I. Small specimens. Irish Forestry, 59(1-2), 49-58. 

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. 

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

Korai, H. (2001). Effects of low bondability of acetylated fibers on mechanical properties and dimensional stability of fiberboard. Journal of Wood Science, 47(6), 430-436. 

Okino, E.Y.A., Santata, M.A.E., Souza, M.R. de and Sousa, M.E. de (2001). Dimensional stability and mechanical properties of acetylated Hevea brasiliensis flakeboards. Journal of Tropical Forest Products, 7(1), 96-102. 

Ramsden, M.J., Blake, F.S.R. and Fey, N.J. (1997). The effect of acetylation on the mechanical properties, hydrophobicity and dimensional stability of Pinus sylvestris. Wood Science and Technology, 31(2), 97-104. 

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. 

Subiyanto, B., Yusuf, S., Kawai, S. and Imamura, Y. (1989). Particleboard from acetylated albizzia particles. 1. The effect of acetyl weight gain on mechanical properties and dimensional stability. Mokuzai Gakkaishi, 35(5), 412-418. 

Vick, C.B., Krzysik, A. and Wood, J.E. (1991). Acetylated isocyanate-bonded flakeboards after accelerated aging. Dimensional stabihty and mechanical properties. Holz als Roh- und Werkstoff, 49(6), 221-228. 

Youngquist, J.A., Rowell, R.M. and Krzysik, A. (1986b). Mechanical properties and dimensional stability of acetylated aspen flakeboard. Holz als Roh- und Werkstoff, 44(12), 453 57. 

The copolymer poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHB-co-PHV) produced by A eutrophus has generated more interest than poly-(R)-3-hydroxybutyrate (PHB) homopolymer. Since these bacterial polyesters are biodegradable thermoplastics, their mechanical and physical properties have received much attention. PHB is a relatively stiff and brittle material because of its high crystallinity. However, the physiochemi-cal and mechanical properties of [P(HB-HV)] vary widely and depend on the molar percentage of 3-hydroxyvalerate (HV) in the copolymer (4,5) as shown inTable 1. Propionic acid is converted by a synthetase to propionyl-CoA, and the biosynthetic P-ketothiolase catalyzes the condensation of propionyl-CoA with acetyl-CoA to 3-ketovaleryl-CoA by the acetoacetyl-CoA reductase. The hydroxyvaleryl moiety is finally covalently linked to the polyester by the PHA synthase (6). 

Yano et al. [53] studied acoustic properties of acetylated Sitka spruce by specific dynamic Young s modulus and by logarithmic decrement. For oven-dried specimens, both the modulus and the decrement have been found to increase. Meanwhile, mechanical properties are generally unchanged and adhesive strength is reduced by acetylation [2]. Furthermore, creep deformation of wood under humidity change is remarkably reduced by acetylation [54]. 

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]. 

Using an isocyanate resin, at 3% solids content, on the same aspen flakes as described above, dry internal bond strength decreased by only 2%, MOR decreased by 23%, and MOE decreased by 15% on boards made from acetylated flakes as compared to boards made from nonacetylated flakes [37]. This is just one more example of how the type and level of the resin used influenced composite board mechanical properties. 

There have been several studies on the changes in mechanical properties of fiberboards made from chemically modified wood fiber. Hardboards made from control and acetylated hemlock fiber using 7% phenyl-formaldehyde adhesive were tested. In static bending, MOR was reduced by 23% and MOE reduced by 16% in acetylated boards as compared to control boards [38]. Tensile strength parallel to the surface was reduced by 5% but there was no... 

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