Research on Wood Treatment with A Dispersion of Aqueous Styrene/Acrylic Acid Copolymer
Author:Che Wen Bo
Supervisor:xie yan jun xiao ze fang
At present,efficient utilization of fast-growing wood is the main way to solve the shortage of forest resources.However,due to the short growth cycle of plantation timber,there are some shortcomings such as soft quality,low strength and easy deformation.Chemical modification can improve the mechanical strength,dimensional stability and durability of wood,which increased the value of wood and expand its application scope.Treatment wood through in-situ polymerization of organic monomers can significantly improve wood density,hardness,mechanical strength and moisture resistance.However,it is difficult to achieve industrial production by using organic monomers to treatment wood because the volatilization of organic monomers caused air pollution and increased cost.Besides,the recovery process of organic monomers is complex and has potentially risk of explosive.In order to avoid the volatilization and chemical instability of monomers in wood treatment,the use of formaldehyde-free and water-soluble olefin oligomer dispersion as wood treatment agent is a new choice.In this paper,water-soluble styrene/acrylic(SA)copolymer dispersion was used to impregnate wood under vacuum-pressure.The physical,mechanical properties,dimensional stability,weathering and microbial decay resistance of the treated wood were evaluated.Moreover,the distribution of SA copolymer in wood cells and the reaction mechanism with wood components were discussed.Furthermore,the strengthening mechanism of SA copolymer on wood was clarified.Here,the main contents and results of this study are as follows:(1)The process of treating radiation pine wood with SA copolymer was optimized,and the effects of SA copolymer on the physical and mechanical properties of the treated wood were evaluated.The optimum curing process of SA copolymer treated wood was selected as follows,curing temperature and time under 120℃ and 24 h,and added 0.2 wt%catalyst.The main physical and mechanical properties,dimensional stability and thermal stability of the treated wood were evaluated and analyzed.The results showed that the maximum weight percent gain(WPG)of the treated wood reached 42%,and oven-dry density reached 0.67 g/m3.Due to the slight degradation of hemicellulose in alkaline environment during high temperature,the dimensional stability of treated wood was not positive,and the hygroscopicity was slightly reduced.Moreover,the water absorption of treated wood is 40%lower than untreated due to the deposition of copolymer in the wood cell lumens that occupying the channel of water transfer.The mechanical strength of treated wood was significantly improved;both the MOR and MOE of bending were increased by 25%;the surface hardness and compressive strength were increased by 32%and 44%,respectively.Importantly,the impact strength of treated wood was almost unaffected by treated with SA copolymer.The thermal stability and crystallinity of treated wood remained almost unchanged.Fourier transform infrared spectroscopy(FTIR),X-ray diffraction(XRD)and scanning electron microscope(SEM)were used to analyze the treated wood too,which was concluded that SA coplymer could not permeate into cell wall of wood.Although it just deposit in the cell lumen because of the high molecular weight of SA copolymer,it could bind closely to the cell wall and firmly exist in the wood.(2)The effect of aqueous SA copolymer treatment on weathering of wood was evaluated.FTIR and X-ray photoelectron spectroscopy(XPS)suggested that the SA treatment delayed the lignin degradation during artificial weathering;the protection effect increased with the SA concentration.The water contact angle of the treated wood decreased slower than the untreated control through the artificial weathering.SEM further revealed that the SA polymer deposited in the wood cell lumen,which facilitates to inhibit the moisture diffusion and light(especially ultraviolet light)penetration into wood cell walls.During 12 months of outdoor exposure,the SA-treated wood panels exhibited lower moisture content and water uptake than the untreated controls.Treatments with the SA can stabilize the surface color and reduce the discoloration caused by staining fungi and contaminants.The treated wood panels showed less earlywood detachment than the untreated panels.The results suggest that treatment with SA can substantially improve wood durability to weathering.(3)The effects of SA copolymer treatment on the decay of fast-growing poplar and radiata pine wood were evaluated.The brown and white rot decay resistances of untreated wood and SA-treated wood were evaluated by the mass loss and observed by SEM.The decay resistance of wood improved after SA treatment,thus protecting wood from degradation by brown and white rot-causing fungi.The mass loss of SA-treated wood was 8.5%after 12 weeks of brown rot decay compared with the mass loss of untreated wood,which was 53.8%.This is attributed to SA copolymers filled in wood cell lumen resulting in the blockage of passages for microorganisms and moisture to cell walls.The mass loss of untreated and SA treated poplar wood reached 52%and 35%after 40 weeks of soft rot,respectively.Although SA treatment delayed the soft rot decay of wood,it failed to improve the soft rot resistance,because it had no positive effect on the cell walls.Furthermore,the growth of mold fungi on the wood surface was investigated in petri dish and tank chambers,respectively.The results showed that SA treatment delayed the growth of mold fungi on wood surfaces.As the growth time prolongs,there was no restriction on the propagation of mold fungi,because SA is a non-toxic copolymer.As a result,SA treatment improved the brown and white rot resistance of wood by fungi,and delayed the soft rot decay and mold growth on wood.(4)The effects of modification of SA copolymer mix with low-molecular-weight agents(LMWAs)on dimensional stability and mechanical properties of wood were evaluated.Aforementationed study revealed that SA copolymer is filling in cell-lumen,and the treated wood exhibited negative values for cell wall bulking efficiency(BE)and anti-swelling efficiency(ASE).Thus,three LMWAs,1,3-dimethylol-4,5-dihydroxyethyleneurea(DM),glutaraldehyde(GA),and n-methylol acrylamide(NMA),were separately combined with SA and used to modify radiata pine wood at the levels of cell walls and lumens.The WPG and density of the treated wood increased with the increase of the concentration of SA.When the composite treatment solution was 10%DM/20%SA copolymer,the WPG of the treated wood reached 54%,and oven-dry density reached 0.68 g/cm3.The combinative treatments caused positive BE and ASE values,indicating restrained wood deformation,likely due to the diffusion and reaction of LMWAs in the wood cell walls.Infrared spectroscopy and dynamic mechanical analysis showed that LMWAs exhibit little reaction with SA,and the SA copolymer retains thermoplasticity in the wood cell lumen.The combinative treatments resulted in considerable improvement in bending modulus,bending strength,and compression strength of wood of 36,36,and 124%,respectively,but there was little effect on impact strength.The findings through FTIR,XRD,and SEM confirmed that LMWAs can act as cell wall modifying agents to synergistically improve wood properties,especially the dimensionally stability,when used together with SA.In summary,the moisture resistance,mechanical properties,weather resistance and anti-microbial decay performance of wood were improved after treatment with aqueous SA copolymer.Besides,it has the advantages of environmental protection because of its water-borne characteristics.The SA mixed with LMWAs treatment can slightly improve the dimensional stability of wood.However,more research work is needed on how to simply and efficiently eliminate negative effects and realize its industrial application.