Acetals reactions with wood

Figure 3.2 The effect of increasing the concentration of pyridine catalyst upon the reactivity of acetic anhydride with wood, for reaction at 700° C for 30 minutes (from data of (ffetin, 1999).

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. 

Since reaction of wood with acetic anhydride leads to the formation of acetic acid by-product, which must be removed from the wood, there has been some interest in the use of ketene gas for acetylation (Figure 4.4a). Ketene, for reaction with wood, is produced by pyrolysis of diketene. Provided that the wood contains no moisture, no acetic acid by-product is produced. However, ketene presents handling problems it is very toxic and explosive, and it also has a tendency to dimerize. A comprehensive series of studies of ketene-based acetylation has been performed in Latvia and this work has been reviewed by Morozovs etal. (2003). Hardwoods have been found to be more reactive to ketene than softwoods and the optimal temperature for reaction has been determined as 47 °C. Application of vacuum and treatment of wood with ammonia solution has been used to remove the excess ketene. The reaction of wood with liquid diketene was also studied, with a WPG of 35 % being obtained after reaction for 3 hours at 52 °C. 

Scheme 9.4.8 Reaction of acetic anhydride with wood.

X5lenol is an important starting material for insecticides, xylenol—formaldehyde resins, disinfectants, wood preservatives, and for synthesis of a-tocopherol (vitamin E) (258) and i7/-a-tocopherol acetate (USP 34-50/kg, October 1994). The Bayer insecticide Methiocarb is manufactured by reaction of 3,5-x5lenol with methylsulfenyl chloride to yield 4-methylmercapto-3,5-xylenol, followed by reaction with methyl isocyanate (257). Disinfectants and preservatives are produced by chlorination to 4-chloro- and 2,4-dich1oro-3,5-dimethylpheno1 (251). 

Figure 3.1 The reaction of an anhydride with wood, for acetic anhydride (R = CHj) propionic (R = C2Hs), butyric (R = C2Hj), valeric (R = C4H, hexanoic (R = C H-iq) and heptanoic (R = CgH-j ).

Nishino (1991) noted the presence of an induction period in the kinetic profile of reactions of solid wood samples with acetic anhydride vapour. The duration of this induction period decreased and the rate of reaction increased with both increasing temperature and concentration of acetic anhydride vapour in the reaction vessel (Nishino, 1991). The rate of reaction was diffusion limited, with the rate of diffusion being greatest in the longitudinal direction of the wood samples. This phenomenon is only found in solid wood samples, since no induction period was noted for the reaction of wood fibres with acetic anhydride vapour (Loras, 1968). 

Li etal. (2000b) investigated the reaction of wood with acetic, propionic, butyric, isobutyric or hexanoic anhydrides. The pre-extracted samples were vacuum impregnated... 

The production of moisture resistant particleboard by treatment with a maleic anhydride -glycerol mixture and using phenol formaldehyde as the matrix material has been investigated (Fujimoto etal., 1987). Boards prepared from modified wood showed considerable improvements in modulus of elasticity and internal bond strength when compared to control boards. Composites made from aspen fibres modified with SA, MA or acetic anhydride using phenol-formaldehyde (PF) or polypropylene as binder have also been studied (Clemons etal., 1992 Rowell etal., 1993b). The reaction of wood with MA was found to proceed at a slower rate than with SA. The volume increase due to modification... 

Butyl acetate [123-86-4], one of the more important derivatives of -butyl alcohol produced commercially, is employed as a solvent in rapid drying paints and coatings. In some instances, butyl acetate, C.H1202, has replaced ethoxyethyl acetate [111-15-9] due to the latter s reported toxicity and teratogenicity. Butyl acetate is used in leather treatment, perfumes, and as a process or reaction solvent and is also used extensively with wood coatings, maintenance coatings, and in coatings for containers and closures. 

The natural fibers obtained from cotton, wood, flax, hemp, and jute all are cellulose fibers and serve as raw materials for the textile and paper industries. In addition to its use as a natural fiber and in those industries that depend on wood as a construction material, cellulose is used to make cellulose acetate (for making rayon acetate yarn, photographic film, and cellulose acetate butyrate plastics), nitric acid esters (gun cotton and celluloid7), and cellulose xanthate (for making viscose rayon fibers). The process by which viscose rayon is manufactured involves converting wood pulp or cotton Iinters into cellulose xanthate by reaction with carbon disulfide and sodium hydroxide ... 

Acetylation of cellulose to the triacetate has been carried out without breaking down of the structure with acetic anhydride containing pyridine to help open up the cell wall structure and to act as a catalyst (71). This led Stamm and Tarkow (72) to test the liquid phase reaction on wood. High dimensional stabilization without break down of the structure was obtained, but excessive amounts of chemical were used. They hence devised a vapor phase method at atmospheric pressure that proved suitable for treating veneer up to thicknesses of 1/8 inch. Acetic anhydride pyridine vapors generated by heating an 80-20% mixture of the liquids were circulated around sheets of veneer suspended in a box lined with sheet stainless steel. Hardwood veneer,... 

A large number of catalysts and swelling agents have been studied for the acetylation reaction. The best acetylation conditions have been reported in vapor phase treatment with uncatalysed acetic anhydride in xylene solutions at 100-130 C (28). Acetylation reaction between wood and acetic anhydride proceeds as follows 0... 

Of several anhydrides studied, acetic anhydride reacted the most readily. Reactions were carried out by refluxing the wood in a xylene/acetic anhydride solution or with acetic anhydride vapors alone at 120°C. With this system, for each mole of acetate bonded onto the wood a mole of acetic acid is generated as a byproduct. Although this byproduct generation is a disadvantage of the process, the chemical system does penetrate and react quickly with wood, without a catalyst. It is not... 

In the reaction of wood with acetic anhydride, acetylation occurs as follows [Reaction (8)] ... 

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