Research On The Process And Mechanism Of Wood Acetylation

Wood acetylation is an effective treatment to improve wood dimensional stability, biodegradability and photostability. Currently, acetylated wood has been commercialized in many countries; however, there are no proprietary technologies and instruments for wood acetylation in China. Non-catalyzed wood acetylation with acetic anhydride is a cost efficient process. It simplified the waste solvent recycle after wood acetylation. The properties of acetylated wood are dependent on the pre-treatment, acetylation temperature, time, the acetylation solution ratio and other processing factors. It is still unclear how these factors affected the properties of acetylated wood and wood acetylation mechanism. Therefore, in this thesis, we systematically investigate the non-catalyzed wood acetylation with acetic anhydride. The relationship between the acetylation processing factors and properties of acetylated wood and wood acetylation mechanism were studied.Plantation wood of poplar (Populus ussuriensis Kom.) and pine (Pinus sylvestris var. mongolica Litv.) were acetylated with acetic anhydride without using any catalysts. The effects of acetylation processing factors such as pre-treatment time, acetylation temperature, time and acetylation solution ratio on the properties of acetylated wood were investigated. The weight percentage gain (WPG), dimensional stability, volumetric swelling coefficient (S) anti-shrink/anti-swell efficiency (ASE) as well as other physical and mechanical properties of acetylated wood with various acetylation levels were studied. Meanwhile, advanced analytical instruments including Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Solid-state Nuclear Magnetic Resonance (NMR, Carbon-13 Nuclear Magnetic Resonance Spectroscopy in the solid state using cross polarization and magic angle spinning (13C-NMR CP/MAS) and X-ray Photoelectron Spectroscopy (XPS) were used to characterize the microstructure, crystallanity, chemical compositions and functional groups of wood before and after acetylation. Wood acetylation mechanism was explored through the analysis on the properties of acetylated wood at macro-and micro-level. The main findings and conclusion were summarized as follows:1. The weight percentage gain (WPG) of acetylated poplar and pine increased with the prolonged pre-treatment and then leveled off. The WPG of acetylated wood increased with elevated acetylation temperature and prolonged acetylation time. Under the same acetylation temperature and time, the WPG of acetylated wood first slightly increased and then gradually decreased with the increased acetic acid concentration of the acetylation solvent.2. After acetylation, wood cell swelling rate, anti-shrink/anti-swell efficiency (ASE) and density of acetylated poplar and pine increased, while the moisture adsorption, water uptake, and the volumetric swelling coefficient (S) decreased with the increasing WPG. The ASE of acetylated poplar and pine increased with elevated acetylation temperature and prolonged acetylation time. When the acetylation temperature was higher than 120℃,acetylation might degrade the modulus of elasticity (MOE) and modulus of rupture (MOR) of acetylated wood.3. Considering the effects of acetylation process parameters on the WPG, ASE, MOE and MOR of acetylated wood, the further optimum acetylation condition for poplar and pine is acetylation temperature of 100℃ for 4h. Under the optimized process conditions, the WPG of acetylated poplar reached 19.2%, the WPG of acetylated pine reached 22.6%, the ASE of both wood specials were more than 60%, the MOE and MOR were equivalent to the untreated samples.4. SEM and XRD showed that the microstructure of acetylated poplar and pine did not have any damage, and the crystallinity of acetylated wood barely changed when the acetylation temperature was lower than 120℃. While, when the acetylation temperature was higher than 120℃, the cell wall of acetylated poplar and pine had certain defects such as deformation and damages, meanwhile, the poplar and pine acetylated at the temperature higher than 120℃ had significant lower crystallinity than the untreated poplar and pine as well as those acetylated at the temperature lower than 120℃. Compared with the untreated wood, the acetylated poplar and pine had a lower lignin and alpha-cellulose content, but a higher hemicellulose content. Meanwhile, the ethanol-toluene and hot water extractives of acetylated wood decreased after acetylation.5. XPS, FTIR and the solid-state 13C NMR CP/MAS revealed the mechanism of wood acetylation. Wood acetylation occurred mainly in the C-6 of amorphous cellulose, the C-2, C-3 and C-5 of holocellulose, the C-6 of guaiacyl lignin and C-4 of syringyl lignin as well as the lignin methoxyl. The acetylation of hemicellulose was most notable. The swelling effect of acetyl groups on the cell wall of wood and the reduction of free hydroxyl groups in the wood components especially in hemicellulose, could be the mechanism of improved dimensional stability through wood acetylation.The results and major findings of this thesis provided valuable insight for wood acetylation in terms of theory improvement and completion, practical application and optimization of processing factors. This thesis proposed a novel value-added application of plantation wood. It also provided significant support and insightful guidance for the future commercialization of wood acetylation technology and commercially adoption of acetylated wood in China.