Potassium acetate-catalyzed acetylation of wood at low temperatures II: vapor phase acetylation at room temperature

Ezomatsu wood blocks were impregnated with potassium acetate (KAc) and then exposed to acetic anhydride vapor at 25°C and 120°C. The KAc-impregnated wood was rapidly acetylated at 120°C, and only 6 min was needed to achieve 20% weight percent gain (WPG). The WPG increased with increasing catalyst loading (CL), but it turned to decrease above 20% CL probably because the diffusion of acetic anhydride vapor was hindered by excess KAc depositing in the cell lumina. Thus, careful control of CL is necessary in the vapor-phase acetylation. KAc was also effective in catalyzing the vapor-phase acetylation at 25°C: the KAc-impregnated wood attained 20% WPG within 7 days, whereas the WPG did not exceed 10% even after 1 month in the uncatalyzed system. Irrespective of treatment methods, the hygroscopicity of wood was reduced and its dimensional stability was improved with an increase of WPG. These results confirm that the use of KAc simplifies the acetylation process at room temperature with minimal loss of acetic anhydride.     

Potassium acetate-catalyzed acetylation of wood: extraordinarily rapid acetylation at 120°C

The catalytic effect of potassium acetate (KAc) on wood acetylation was investigated. Spruce wood specimens were impregnated with KAc and then heated in acetic anhydride at 120°C. The degree of acetylation was evaluated by the weight percent gain (WPG). In the presence of KAc, the reaction time to achieve a 20% WPG decreased by a factor of 200: 2 min was required in the KAc-catalyzed acetylation, while the uncatalyzed acetylation required at least 5 h. The hygroscopicity and dimensional stability of acetylated wood depended on the WPG irrespective of the treatment methods. This fact proved that KAc had no adverse influence on the dimensional stability of acetylated wood. As KAc is a cheap, water-soluble and non-toxic salt it can be a useful catalyst for the extraordinarily rapid acetylation of wood.     

Potassium acetate-catalyzed acetylation of wood: reaction rates at low temperatures

Spruce wood blocks were acetylated in the presence of potassium acetate (KAc) at 20, 40, 60, 80 and 120°C. At 20°C, the weight percent gain (WPG) due to the KAc-catalyzed acetylation reached 20% in 18 days, whereas that due to pyridine-catalyzed acetylation did not exceed 8%. The hygroscopicity and dimensional stability of the KAc-acetylated wood were the same as those of conventionally acetylated wood at the same WPG, irrespective of reaction temperature. These facts suggest that the KAc enables simplified acetylation of wood at room temperature. The activation energy (E _a) of the KAc-acetylation in the lower temperature range (20–40°C, 121–131 kJ/mol) was comparable to that of the acetylation of wood meal (140–146 kJ/mol). It was speculated that diffusion became a minor factor at reduced reaction rates in the lower temperature range, thus requiring a greater E _a.     

Preparation of acetylated wood meal and polypropylene composites I: acetylation of wood meal by mechanochemical processing and its characteristics

Wood meals of Sugi (Cryptomeria japonica D.Don) passing 2.0 mm and retained on 1.0 mm mesh screens were milled along with acetic anhydride (AA) and pyridine as a catalyst in a high-speed vibration rod mill at ambient temperature. The weight percent gain (WPG) of the chemically modified wood was calculated based on the yield after washing with deionized water. The effects of amounts of AA and catalyst added, pulverization time, and saponification of the acetylated wood on WPG were examined. In addition, FT-IR analysis, and water vapor adsorption and desorption tests were performed as functions of the WPG. Increases in WPG, the acetyl contents of the acetylated wood after saponification, changes in the FT-IR spectra after pulverization, and the water vapor sorption isotherms showed that the one-step acetylation systematically modified the hydroxyl groups of the wood into acetyl groups. Up to 38 % WPG was obtained at 100 phr AA and 15 phr catalyst, and 120 min pulverization. Pulverization time and the amounts of AA and catalyst added to the wood meals could be adjusted to obtain acetylated wood meal with the desired WPG. These demonstrated that the mechanochemical acetylation is a method to prepare acetylated wood meals with high WPG at less reaction time and required AA addition.     

Effects of previous solvent exchange on acetylation of wood

Wood specimens were prepared in a swollen state using solvent exchange (PS) treatment. The swollen wood specimens were acetylated using acetic anhydride by heating at 80–120°C. At the beginning of heating, the weight percent gain (WPG) of PS-treated wood was greater than that of conventionally acetylated wood. This acceleration effect of the PS treatment was explained by the introduction of treating reagent into the wood polymers where the intermolecular hydrogen bonds were previously broken. On the other hand, the PS treatment had no influence on the final WPG and moisture sorption characteristics of acetylated wood. This indicated that the intrinsic reactivity of wood constituents was unaffected by the PS treatment. The acetylation of PS-treated wood produced greater bulking and slightly higher dimensional stability than that in the case of conventional acetylation at the same WPG. It was speculated that the expansion of cell lumina due to the PS treatment resulted in greater bulking on acetylation and lesser swelling of acetylated wood with moisture sorption.     

Non-uniform reaction of solid wood in vapor-phase acetylation

To clarify the non-uniform reaction of wood during vapor-phase acetylation, spruce wood blocks were exposed to acetic anhydride vapor at 120°C. Weight percent gain (WPG) due to the acetylation was estimated from the equilibrium moisture content at 25°C and 60% relative humidity. The diffusion of reagent vapor was much faster along the longitudinal direction than along the tangential direction. When the end surface was exposed to the reagent vapor for 48?h, 20% WPG, which was known to have sufficient stability and durability, was achieved to a depth of 42.5?mm. However, this depth was only 6.5?mm when the straight-grain surface was exposed. The reaction profiles were successfully approximated using reaction time (t), reaction rate (k′), delay time (t _d′), and a parameter n reflecting the diffusion-controlled reaction. The t _d′ value increased almost linearly as the depth increased from the surface. The k′ value ranged from 0.02 to 0.03?h^?1, regardless of the depth and direction of diffusion. The n value decreased with an increase in the depth and approached 1–2. These values enabled the prediction of the degree of acetylation at any reaction time and positions of wood during vapor-phase acetylation.     

A novel method of acetylation of wood using supercritical carbon dioxide

Sugi heartwood was acetylated with acetic anhydride in supercritical carbon dioxide (CO₂) (120°C or 130°C, 10–12 MPa). As a result, the weight percent gain increased with increasing acetylation time up to 16%–20% at 1 h and 24%–28% at 24 h. The antiswelling efficiency of the acetylated specimens reached 75%–80% at 3–4 h of acetylation. It is supposed that the acetylation in supercritical CO₂ has a high bulking effect compared with liquid-phase and vapor-phase acetylation with uncatalyzed acetic anhydride. The results showed that the acetylation progressed rapidly because supercritical CO₂ and acetic anhydride formed a single phase at more than 90°C, and the acetic anhydride reached the reaction sites in the wood quickly.     

Acetylation of wood in combination with polysiloxanes to improve water-related and mechanical properties of wood

Scots pine sapwood was acetylated with ethyltriacetoxysilane using acetic acid as a solvent and sulfuric acid as a catalyst. A weight percent gain (WPG) of 14 % and cell wall bulking of 7 % were obtained after 5 h of reaction time. Pine specimens were acetylated with acetic anhydride in the presence of 1 % ethyltriacetoxysilane, dihydroxy-functional siloxane, acetoxy-functional siloxane, amino-functional siloxane and non-functional siloxane, respectively. Acetoxy-functional siloxane induced the greatest reduction in water uptake with a water repellent effectiveness after 24 h of up to 62 % as compared to acetylated wood. WPG and cell wall bulking increased compared to solely acetylated wood with increasing concentrations of acetoxy-functional siloxane in acetic anhydride; anti-shrink efficiency, however, did not increase. Fungal resistance of pine sapwood and beech as well as mechanical strength properties did not change when 20 % acetoxy-functional siloxane was added to acetic anhydride compared to solely acetylated specimens.     

Compressive deformation of wood impregnated with low molecular weight phenol formaldehyde (PF) resin V: effects of steam pretreatment

This study evaluated the potential of steam pre-treatment for making highly compressed phenol-formaldehyde (PF) resin-impregnated wood at a low pressing pressure. Sawn veneers of Japanese cedar (Cryptomeria japonica) were first subjected to saturated steam at different steaming temperatures (140°-200°C), followed by impregnation with a 20% low molecular weight PF resin aqueous solution resulting in a weight gain of around 50%-55%. Four oven-dried treated veneers were laminated and compressed up to a pressing pressure of 1 MPa at a pressing temperature of 150°C and pressing speed of 5 mm/min, and the pressure was held for 30 min. Steam treatment, causing partial hydrolysis of hemicellulose, accelerated the compressibility of Japanese cedar in the PF resin-swollen condition. As a consequence, a discernible increment in density was achieved at a pressing pressure of 1 MPa due to steam pretreatment between 140° and 200°C for 10 min. It was also found that even a short steaming time such as 2 min at 160°C is sufficient for obtaining appreciable compression of PF resin-impregnated wood. The density, Young’s modulus, and bending strength of steam-treated (200°C for 10 min) PF resin-impregnated wood composite reached 1.09 g/cm³, 20 GPa, and 207MPa, respectively. In contrast, the values of untreated PF resin-impregnated wood composite were 0.87 g/cm³, 13 GPa, and 170MPa, respectively.     

Dependence of reaction kinetics and physical and mechanical properties on the reaction systems of acetylation I: reaction kinetic aspects

The dependence of the reaction parameters of acetylation on the reaction mixture was compared among uncatalyzed, acetic anhydride-xylene mixed, and acetic anhydride-pyridine mixed solutions. Wood meal and blocks were used to examine the effect of sample size. A first-order rate equation was applied to the data, and a rate constant and leveling off value of weight gain (WG) were estimated. The rate-determining step was examined from the viewpoint of activation energy. The results were as follows: (1) Regarding the magnitude of the rate constant, the order was pyridine system > uncatalyzed system > xylene system. (2) The ultimate value of WG was lower in the uncatalyzed and xylene systems than the pyridine system, probably because of the swelling ability of pyridine. (3) The activation energies of acetylation estimated for wood meal were 120, 135, and 110kJ/mol for the uncatalyzed, xylene, and pyridine systems, respectively. (4) The characteristics of the diffusion-controlled reaction became marked when the acetylation was carried out in the pyridine system, at elevated temperature, and for wood blocks. Under these conditions, the supply of reagent to the reaction site might not be sufficient to fuel the reaction.Part of this report was represented at the 52nd Annual Meeting of the Japan Wood Research Society, Gifu, April 2002     

Chemical modification of wood by anhydrides without solvents or catalysts

Acetylated, propionylated, butyrylated, isobutyrylated, and hexanoylated woods were prepared at several temperatures. The reaction rate, dimensional stability, and changes in the ratios of specific gravity and dimensions in the tangential and radial directions were estimated. The reaction rate of propionylation was slow at temperatures under 90°C but increased with the temperature. The butyrylated, isobutyrylated, and hexanoylated woods showed little or no weight percent gain (WPG) and little or no antiswelling efficiency (ASE) below 110°C even for 24h, but they achieved significant WPG and ASE values at 140°C with a longer reaction time. The acetylated, propionylated, and butyrylated woods showed almost the same values for dimensional stabilization efficiency based on WPG (DSE). The specific gravity and dimensions ratios for acetylated, propionylated, and butyrylated woods compared to those of untreated wood increased with an increase in WPG.     

Characterization of acetylated wood decayed by brown-rot and white-rot fungi

The objective of this study was to characterize the decay of acetylated wood due to brown-rot and white-rot fungi by analysis of chemical composition, X-ray measurements, and¹³C-NMR spectroscopy. The decay by brown-rot fungus became inhibited at a weight percent gain (WPG) due to acetylation of more than 10%, and the mass loss (LOSS) due to decay became zero at a WPG of about 20%. The LOSS due to white-rot fungus decreased slowly with the increase in WPG, reaching zero at a WPG of about 12%. The losses of lignin by brown-rot decay increased initially with the decrease in LOSS owing to the progressing acetylation and then decreased at a LOSS of less than 60%. Polysaccharides were more easily decomposed than lignin during the decay of acetylated wood due to brown-rot fungus. The losses of both components due to white-rot decay decreased as the LOSS decreased with progressing acetylation. The white-rot fungus tended to preferentially decompose the lignin during the decay of acetylated wood. The brown-rot fungus decomposed the cellulose in the crystalline region to a large degree when the LOSS was more than 40%, whereas the white-rot fungus decomposed the crystalline region and the noncrystalline region in acetylated wood to the same degree. The brown-rot fungus preferentially decomposed unsubstituted xylose units in acetylated wood and partly decomposed the mono-substituted xylose units. It was suggested that the mono- and disubstituted cellulose were partly decomposed by brown-rot fungus.This paper was presented at the 46th and 47th annual meetings of the Japan Wood Research Society at Kumamoto and Kochi in April 1996 and April 1997, respectively     

Effects of high temperature kiln drying on the practical performances of Japanese cedar wood (Cryptomeria japonica) I: changes in hygroscopicity due to heating

The effect of heating on the hygroscopicity of Japanese cedar wood was investigated as a simple evaluation of thermal degradation in large-dimension timber being kiln-dried at high temperatures (>100°C). Small wood pieces were heated at 120°C in the absence of moisture (dry heating) and steamed at 60°, 90°, and 120°C with saturated water vapor over 2 weeks, and their equilibrium moisture contents (M) at 20°C and 60% relative humidity (RH) were compared with those of unheated samples. No significant change was induced by steaming at 60°C, while heating above 90°C caused loss in weight (WL) and reduction in M of wood. The effects of steaming were greater than those of dry heating at the same heating temperature. After extraction in water, the steamed wood showed additional WL and slight increase in M because of the loss of water-soluble decomposition residue. The M of heated wood decreased with increasing WL, and such a correlation became clearer after the extraction in water. On the basis of experimental correlation, the WL of local parts in large-dimension kiln-dried timber was evaluated from their M values. The results indicated that the thermal degradation of inner parts was greater than that of outer parts.     

Dimensional stability of wood acetylated with acetic anhydride solution of glucose pentaacetate

Five wood species were acetylated with acetic anhydride (AA) solution of glucose pentaacetate (GPA) at 120°C for 8h, and the effect of GPA on the dimensional stability of the acetylated wood was investigated. Some GPA was introduced into the wood cell wall during acetylation. The GPA remaining in the cell lumen penetrated the cell wall effectively after heating to more than 140°C for 10min. The bulking effects of GPA resulted in a 10%–30% increase in the anti-swelling efficiency of the acetylated wood with 20% GPA/AA solution in place of AA. Hydrophobic GPA did not deliquesce under highly humid conditions and it remained in the cell wall after boiling in water.Part of this paper was presented at the 51st Annual Meeting of the Japan Wood Research Society, Tokyo, April 1988     

Influence of heating history on dynamic viscoelastic properties and dimensions of dry wood

To obtain new information about the mechanical and physical properties of dry wood in unstable states, the influence of heating history on viscoelastic properties and dimensional changes of dry wood in the radial, tangential, and longitudinal directions was studied between 100° and 200°C. Unstable states of dry wood still existed after heating at 105°C for 30 min and were modified by activated molecular motion in the first heating process to temperatures above 105°C. This phenomenon is thought to be caused by the unstable states reappearing after wetting and drying again. Dry wood components did not completely approach the stable state in the temperature range tested, because they did not entirely surpass the glass transition temperatures in most of the temperature range. In constant temperature processes at 135° and 165°C, E′ increased and E″ decreased with time regardless of the direction. This indicated that the unstable states of dry wood components were gradually modified with time at constant temperatures. On the other hand, anisotropy of dimensional change existed and dimension increased in the longitudinal direction, was unchanged in the radial direction, and decreased in the tangential direction with time at constant temperatures. Part of this report was presented at the 13th Annual Meeting of the Chubu Branch of the Japan Wood Research Society, Shizuoka, August 2003     

An investigation on the transition characteristics of the wood cell walls during carbonization

The structural changes of the cell wall and crystalline cellulose of Quercus variabilis wood in a pyrolysis system at several temperatures ranging from 250 to 500°C were investigated to examine the wood carbonization characteristics. The volume of the wood sample was decreased and the weight loss was increased by increasing the carbonization temperature. Vessels collapsed severely in tangential direction during the charring process above 350°C. SEM observation indicated that the layering structure of the walls in wood fibers and parenchyma cells were retained below 300°C. However, the cell walls above 350°C changed to an amorphous-like structure without cell wall layering. X-ray diffraction confirmed that the cellulose crystalline substance was still remained at the carbonization temperature of 300°C but was not detected above 350°C. It can be concluded that the transition from Q. variabilis wood to charcoal might occur at approximately 350°C.     

Modification and characterization of fibers of three sandy willow shrub species

1 Introduction China, as a new member country of the World Trade Organisation, the club of market economies, is paying more attention on sustainable use of internal resources instead of dependence on the import of fossil fuels. The utilization of wood and other forms of biomass, such as shrub and cereal straws, offers possibilities to solve this problem. Forests are the main natural resource in China. However, the wood resource in China is grossly in- adequate and accounting for only 7% of d…     

Analysis of the factors influencing the acetylation rate of wood

Factors influencing the rate of acetylation were examined based on the swelling of wood in the reaction solution and the dimensions of the wood sample. The activation energy of acetylation was also estimated. In a swelling test, it was found that wood swells thoroughly in acetic anhydride even without pyridine above 60°C. Therefore, pyridine may facilitate the acetylation process as a catalyst and not as a swelling agent. The weight gain, x (%), attained at reaction time t (h), for various compositions of acetylation solution or dimensions of wood sample were analyzed by applying an original rate equation [x = a × (1 – e^–kt )^1/n ], where a is the ultimate weight gain (%), k is the rate constant (h^–1), and n is a measure of the hindrance against the diffusion of reagent. The optimum volume fraction of pyridine in the pyridine-catalyzed acetylation was about 0.2. Accompanied by a rise in pyridine content, the reaction showed increased diffusion-controlled behavior. The rate constant, which is not affected by the dimensions of the wood sample, was estimated from which an activation energy of about 130kJ/mol was calculated.     

Thermal-death kinetics of the bark beetle (Dendroctonus armandi; Coleoptera: Scolytidae)

Data on thermal-death kinetics of bark beetles are essential to develop phytosanitary heat treatments for pine wood and pine wood packaging materials. Using a heating block system, effects of different heating rates between 44 and 50°C at 2°C intervals on destruction of Dendroctonus armandi adult insect were examined. Heat resistance of the insects was found to increase at low heating rates (0.1 or 0.5°C/min). Therefore, the thermal-death kinetics of the beetles were determined at a high heating rate of 5.0°C/min which simulated the rapid dielectric heating of wood products. Results showed that the thermal death curve of D. armandi followed a zero-order reaction kinetic model, indicating the heat destruction rate of the beetle at different treatment temperatures to be independent of their population size. The required thermal holding times to result in destruction of the entire population were 40, 8, 4, and 2?min at 44°C, 46°C, 48°C, and 50°C, respectively. The evaluated thermal-death kinetic data are useful in developing effective beetle elimination quarantine protocols for the wood. A 50°C ?2?min heat treatment with a heating rate of ～5°C/min can be effectively used for disinfesting bark wood materials.     

Interaction between glycerin and wood at various temperatures from stress relaxation approach

In order to understand the reason why glycerin pre-treatment can accelerate the deformation fixation of compressed wood, the interaction between glycerin and wood at various temperatures was investigated in this study from stress relaxation approach. The compression stress relaxation curves of poplar (Populus cathayana Rehd.) samples impregnated with glycerin were measured at temperatures ranging from 25 to 180°C, together with the curves of oven-dry wood at temperatures between 100 and 180°C for comparison. The activation energy was calculated according to the Eyring’s absolute rate reaction theory. The results showed that temperature had very obvious effect on stress relaxation for both glycerin-treated wood (GTW) and oven-dry wood. The stress released very fast at higher temperatures. Glycerin showed an accelerating effect on stress relaxation. At temperatures exceeding 120°C, a complete relaxation of the stress could be expected. While for untreated wood, it cannot be reached until 160°C. By calculating the apparent activation energy (ΔE) of GTW at different temperatures, it is clear that two mechanisms are responsible for different temperature ranges. From 40 to 100°C, ΔE is only 8.24 kJ/mol, which corresponds to the hydrogen bonds formed between wood and glycerin molecules; from 120 to 180°C, ΔE reached 81.38 kJ/mol, which corresponds to the degradation of hemicelluloses or lignin, and during this process, new cross-linking would happen.