| Plywood has long been studied in the world and widely used in various applicationsbecause of their high performance and simple technology. However, adhesives mainly used forplywood are largely formaldehyde-based materials, such as phenol-formaldehyde resins,urea-formaldehyde resins and melamine-formaldehyde resins. The emission of formaldehydein the production and use of plywood poses a great hazard to human health and our livingenvironment. As a result, there is an urgent need for developing formaldehyde-free woodadhesives, which will bring great economic and environmental value for our society.In order to evaluate the bonding ability of plastic films, high density polyethylene (HDPE)films were selected as formaldehyde-free wood adhesive to manufacture HDPE film/poplarveneer plywood (wood-plastic plywood) by improved hot-pressing in this study. The effects ofsurface modifications and technological factors on the physical-mechanical properties andinterfacial adhesion of wood-plastic plywood, as well as the wet cycle resistance of plywoodwere studied. The main researches and results are summarized below:(1) Response surface methodology was used to optimize the processing technologyincluding pressing temperature (140、155、170oC), pressing time (0.4、0.8、1.2min/mm) andlayers of HDPE film (1、3、5). Results showed that shear strength of the plywood wassignificantly affected by pressing temperature, pressing time, layers of HDPE film as well asthe interaction effects between pressing temperature and pressing time. When plywood wasmanufactured under the pressing temperature of152oC, pressing time of1.1min/mm, HDPEfilms of4to5layers (264.92g/m2), shear strength (soaking in hot water) could reach themaximum value of1.68MPa, which totally met the requirement of type II plywood accordingto the National Standard of GB/T9846.3-2004.(2) In order to evaluate the feasibility of using HDPE film to replace UF resins as woodadhesives, performance of two kinds of plywood, made with HDPE film and UF resinsrespectively, were investigated by dynamic mechanical analysis (DMA) and scanning electronic microscope (SEM). Results showed that both HDPE film and UF resins canpenetrate into the porous structures of poplar veneer and form mechanical interlock duringhot-pressing, which endowed these two adhesives comparable bonding ability. HDPE film hasbetter water resistance than UF resins because of its hydrophobic character. Water absorption(WA) and thickness swelling (TS) values after soaking in water for168hours of the plywoodbonded with HDPE film were85.75%and7.65%, which was18.8%and4.9%lower thanthose with UF resins respectively. Dynamic mechanical properties of wood-plastic plywoodwere greatly affected by the melting behaviour of HDPE film. When experimental temperatureincreased to130oC, the bondline in the wood-plastic plywood began to melt and slip, and thebonding structure failed, indicating by the rapidly decreased E’ value and increased tan value.(3) Surface properties of poplar veneers either thermally modified in an oven at differenttemperatures (130,160,180,200oC) or chemically modified by vinyltrimethoxysilane (1,2,5%) were evaluated by Fourier transform infrared spectroscopy (FTIR), contact angle (CA),X-ray photoelectron spectroscopy (XPS), dynamic vapour sorption (DVS) and SEM. Theresults indicate that polarity of veneers lowered because of the hemicelluloses degradationafter thermal treatment, or the deposition of hydrophobic silane coating and the formation ofSi-O-C covalence after silane treatment, which improved the interfacial compatibility betweenpoplar veneer and HDPE film. The untreated veneer had lower contact angel (42o) and higherequilibrium moisture content (15.36%) compared with that thermally modified at200oC (122oand13.3%) or modified by2%silane A-171(114oand14.71%). The veneer became morebreakable and fragile after thermal-treatment, with obvious breakages on its surface. While theveneer surface became much smoother after silane-treatment, because of the deposition ofsilane coating.(4) Both thermal treatment and silane modification resulted in enhancement of shearstrength and water resistance of wood-plastic plywood as well as the interfacial adhesionbetween wood and plastic. When veneers were heated at200oC or spraying by2%silaneA-171, shear strength of wood-plastic plywood reached1.24MPa or1.80MPa (treated by‘boiling-dry-boling’ procedure),51%or128%higher than the untreated sample. It also caused 24-hour WA value decreased from65.81%to49.33%or52.97%respectively. However,bending strength were lowered due to thermal treatment. MOR and MOE values decrease by41%and32%respectively after veneers heated at200oC.The modified plywood exhibited higher E’ and lower tan maxvalues with respect tountreated sample, indicating much more rigid interface The eventual retention rate of E’increased from4.48%to27.42%after heated at200oC and to30.48%after treated by2%silane as shown by DMA results. Higher retention rate of E’ corresponded to the tan maxvalues,which decreased from0.234for untreated sample to0.211for thermal-treated and0.115forsilane-treated sample. SEM results showed that poplar veneer and HDPE film were closelyentangled by means of a chemical bond thanks to bifunctional structural silane A-171and theoxy radicals generated by dicumyl peroxide, which contributed to forming enhanced interlockand a stronger interface between the two phases. This kind of bonding structure in turnresulted in much higher shear strength and wood failure (almost above90%).(5) High performance HDPE film/Poplar veneer plywood were made using silane A-171and DCP as coupling agent. The effects of pressing temperature, HDPE layer, dosage of A-171and DCP on wood/plastic interface and its performance were studied. When pressingtemperature increased from140oC to160oC, shear strength (treated by ‘boiling-dry-boling’procedure), MOR and MOE value increased from1.27MPa,63.9MPa,5970MPa to1.89MPa,72.2MPa,6710MPa. However, higher pressing temperature would adversely affect theshear strength, MOR and MOE. Interfacial rigidity at higher temperature strengthened withincreased pressing temperatures. The The retention rate of E’ at130oC increased from62.31%to92.01%when pressing temperature rose from140oC to170oC. This trend also applied to thetemperature for tan max, which lagged from141oC to200oC. DCP dosage greatly affected thechemical reaction between silane-treated veneer and HDPE film. When the addition of DCPraised from0to0.15%, shear strength, MOR and MOE values of the plywood reached2.07MPa,77.2MPa and6822MPa respectively. Also the retention rate of E’ at130oC and thetemperature for tan maxincreased to88.34%and194oC respectively. Optimal interfacialcompatibility can be obtained when only1layer HDPE film was used, with shear strength of 1.89MPa, retention rate of E’ at130oC of85.58%and180oC for tan max.The plywood had thestrongest water resistance when4layer films were used as wood adhesives with24-hour WAand TS values of48.86%and3.40%respectively.(6) Properties of wood-plastic plywood and its bonding structure after bond durabilitytests were studied by DMA and SEM. Both cohesive and interfacial failure existed in thewood-plastic plywood after3cyclic water-freezing-dry heat experiments by SEM results.Obvious cracks along the interface and the fragments of HDPE were observed in bothuntreated and thermal-treated samples, while there were only a little bit of break forsilane-treated sample after the bond durability tests. Different kinds of failures of the bondingstructure led to the decrease in mechanical properties of wood-plastic plywood at differentlevels. The retention rate of shear strength for untreated, thermal-treated and silane-treatedafter3cyclic tests were57%,72%and84%respectively while the retention rate of MOR andMOE values were both higher than80%. Thermal stability of wood-plastic plywood alsoweakened after bond durability tests by DMA results. Plastic film was totally separated fromboth the untreated and thermal-treated poplar veneers when plywood were heated at up to200oC. However, HDPE film could still closely tethered with silane-treated veneer in thewhole stage and the bonding structure did not destruct. |
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