| Fast-grow wood is one kind of raw materials that applied for wood-based panels. However, its natrual defects such as texture osteoporosis and big voidage which causes low mechanical properties of the composites and limits the scope of use. Therefore, modification for Fast-grow wood becomes hot research topic in wood industry. Poplar(Populus spp.) was selected for this study since it is a fast-growing species and commonly used because of its low cost. The aim of this study is to investigate the effects of cold plasma parameters, such as plasma gases, discharge power and treatment time, on surface properties(for example, surface morphology and roughness, free radicals and chemical compositions and so on) and interfacial adhesion properties of poplar veneers. In addition, aging behaviors of cold plasma treatment were investigated. The results of this study have important practical significance to promote the usage of fast-grow wood. The results were as following:The untreated poplar wood chip had a smooth suface and poplar wood surfaces were notably roughened by the cold plasma treatment. When oxygen, nitrogen, air, argon or ammonia used as cold plasma gas, the poplar wood surfaces became conlicated and surface roughness enhanced, of which, oxygen cold plasma had the best effect on surface roughness of poplar wood. As the cold plasma treatment time from 1 min to 7 min, surface morphologies of poplar wood became more comlicated and surface roughness kept increasing. When the treatment time extended to 9 min, it had a slight change on surface morphologies and surface roughness of poplar wood. Surface morphologies of poplar wood became more complicated, and surface roughness of the wood enhanced almost linerly with the cold plasma power increasing.Free radicals were introduced onto poplar veneer surfaces and the concentration of free radicals w significantly improved by the cold plasma treatment. The concentration of free radicals on oxygen, nitrogen or air cold plasma-treated poplar veneer surfaces was higher than that of argon or ammonia cold plasma treatment. The free radical concentration of poplar veneer surfaces achieved maximum by oxygen cold plasma treatment. The free radical concentration kept increasing when the cold plasma treatment time increasing. When the treatment time was 7 min, the free radical concentration achieved the maximum value. While a decline in free radical concentration with prolonged plasma treatment. Free radical concentration increased first as the plasma power increased from 100 W to 200 W, and then dropped substantially when the plasma power enhance to 300 W, 400 W and 500 W.A newly formed group, which represented for carboxyl groups(O=C-O) appeared on poplar veneer surfaces in the cold plasma treatment process. The oxygen concentration and the ratio of oxygen to carbon atoms(O/C) improved obviously compared to that of untreated. When oxygen, nitrogen, air or ammonia as the cold plasma gas, oxygen or nitrogen element was introduced into poplar veneer surfaces, so that the oxygen or nitrogen concentration of poplar veneer surfaces increased. The concerntration of oxygen and polar groups reached maximum on poplar veneer surfaces treated by the oxygen cold plasma. The oxygen concentration, O/C and oxygen-contaiing functional groups on poplar veneer surfaces kept increasing with cold plasma treatment time increasing from 1 min to 7 min. And they diclined when the treatment time extended to 9 min. The oxygen concentreation, O/C and oxygen-contaiing functional groups kept increasing with the cold plasma power going up to 200 W. However, these parameters had a slight decrease when the cold plasma power enhanced to 300 W, 400 W and 500 W.The surface wettability of poplar veneers improved obviously after cold plasma treatments. After cold plasma gases treatments, the surface free energy improved and the instantaneous and equilibrium contact angles of UF decreased by 9~15% and 65~79%, respectively. At the mean time, the K value with different cold plasma gases treatments was 3~5 times larger than that of untreated. As the treatment time extension, the surface free energy and its polar components and K value kept increasing, while the instantaneous and equilibrium contact angles kept decreasing. When the treatment time was over 7 min, the surface free energy, K value and the UF contact angles had little change. The surface free energy achieved critical value even at low plasma power, and cold plasma power increasing had minor effects on it. K value achieved to a preferable level when the plasma reached to 200 W, which then declined as the plasma power increased to 300 W, 400 W and 500 W. The surface wettability of poplar veneers with different resins was significantly improved by the cold plasma treatment. After cold plasma treatment, the equilibrium contact angles of UF, PF and MDI resins dereased by 73%, 60% and 91%, respectively. And the K value of UF, PF and MDI increased by 653%, 1204% and 38%, respectively.After cold plasma treatment, the penetration depth and area of UF line in poplar veneers and the thickness of adhesive line increased obviously. The effective penetration depth(EP) and the average penetration depth(AP) improved by 70% and 25%, respectively. The penetration depth was different from different plasma gases treatments. The penetration depth with O2 plasma treatment was deeper than that of N2, Air, and Ar plasma treatments. AP was significantly improved after NH3 plasma treatment because adsorbed NH3 in veneer surfaces easily dissolves in water soluble resin. As the extension of cold plasma treatment time, EP and AP increased at the early stage and then decreased. When the treatment time was 7 min, EP and AP achieved the maximum values. The penetration depth kept increasing along with the increasing plasma power.Cold plasma treatment was benefit for plywood bonding using UF resin. After cold plasma treatment, the shear strength of plywood increased by 27%. The shear strength had different performances with different cold plasma gases treatments. For example, the shear strength behaved maximum value after O2 cold plasma treatment, and after NH3 cold plasma treatment, it was lower than that of untreated. While the cold plasma treatment time was prolonged, the shear strength increased firstly and then decreased. When the treatment time extended to 7 min, the shear strength achieved maximum. The shear strength increased firstly and then decreased with cold plasma power increased from 100 W to 500 W. When the power was 200 W, the shear strength reached the maximum value. Too long plasma treatment time and too high plasma power would damage the surface properties of poplar veneers, which then led to the degradation of interfacial adhesion of plywood. The interfacial adhesion of plywood reached to plateau when the O2 cold plasma treatment time is 7 min and the plasma power is 200 W.Aging behaviors of cold plasma treatment had no obvious effect on the surface morphology and surface roughness of poplar fibers. The concentration of surface free radicals decreased significantly when the cold-plasma-treated poplar veneers were exposed in air for the first 3 days and kept nearly unchanged after 7 days. The concentration of O element and polar groups in poplar veneer surfaces decreased distinctly in the first 3 days after cold plasma treatment, which results in a 21% degradation of surface free energy of poplar veneers. After storage in air for 7 days, the surface elements and polar groups had little changes, so did the surface free energies. The shear strength measurements indicated that the interfacial adhesion of plywood degraded because of the aging behaviors of cold plasma treatment. In the first 3 days after cold plasma treatment, the shear strength decreased by 27%, and after storage in air for 28 days, it returned to the untreated state. Aging behaviors of poplar veneers and plywood appeared in the first 3 days after cold plasma treatment, minor aging effects were observed with the duration of the aging time because the surface and interface states became stable with aging time increasing. |
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