Research On Mechanism And Aging Of Wheat Straw Modified By Low Pressure Plasma Graft Treatment

As a traditionally agricultural country without abundant forest resources, take advantage of crops straw to produce wood-based panels is an effective way to solve the shortage of wood resources. But the main technical challenge is that the exterior surface of crop straw, which covered with a cutin layer of wax and silica, severely reducing the bondability of materials. In this study, an effective graft technique for modifying the surface of wheat straw employs the low-pressure dielectric barrier discharge plasma, with a aim to investigate the effects of cold plasma parameters, such as reaction chamber pressure, discharge power, treatment time and grafting monomer, on surface properties(for example, surface morphology and microroughness, chemical compositions, wettability and so on) and interfacial bonding characteristics of wheat straw. In addition, aging behaviors of plasma grafting modification were analyzed. The results are shown as following:1. The microscopic surface topography and chemical compositions of wheat straw were completely different before and after plasma graft modification. As shown by SEM and AFM, obvious etching morphology could be observed on the interior or exterior surface of wheat straw, the surface roughness and specific surface area increased significantly after grafted by low pressure DBD plasma. The spectra of EPR and XPS proved that, the concentration of free radicals and oxygen content improved, while the carbon content decreased. Through curve fitting of C1 s, it was found that C1 content reduced while C2, C3 and C4 content increased, which means a large number of oxygen-containing functional groups grafted to the surface of wheat straw. Within a certain range, as the increase of reaction chamber pressure or plasma discharge power, and the extension of treatment time, the etching effect or roughness became more and more apparent, and the surface of wheat straw had higher O/C ratio. But excessive etching could reduced the intrinsic strength of wheat straw and O/C ratio diminished gradually when these conditions out of range.2. The surface wettability of wheat straw was remarkably improved after plasma graft modification by measuring the contact angles, the K-value and the surface free energy. Compared with those untreated specimens, the instantaneous and equilibrium contact angles of UF or PF resin on plasma treated wheat straw were much lower, the K-value and free energy were much higher, on both interior and exterior surface. Within a certain range, as the increase of reaction chamber pressure or plasma discharge power, and the extension of treatment time, the total surface free energy and its polar components and K-value kept increasing, while the contact angles kept decreasing. But the wettability of wheat straw began to decline instead when these plasma parameters out of range. The better plasma process ing conditions were chamber pressure 20 KPa, discharge power 40 W and treatment time 180 s, and the equilibrium contact angles of UF on interior and exterior surface decreased to 18.8°and 25.4°, respectively. In addition, with different grafting monomer, the effect of wettability was different and glycidyl methacrylate presented the best result.3. The wheat straw specimens were bonded together used UF and PF with three different gluing types and then hot-pressed in order to assess the shear strength. The results indicate that the shear strength was notably improved after plasma graft modification, especially the type of exterior-exterior surface. With the increase of reaction chamber pressure or plasma discharge power, and the extension of treatment time, the shear strength increased firstly and then decreased. At the optimal plasma process, the strength of UF reached the highest, increased by 1277%?772% and 124% with three different gluing types respectively.4. Aging behaviors of plasma graft modification had obvious influence on treatment effect of wheat straw. Being exposed in air for 1 day, the concentration of free radicals decreased by 5%, while the surface free energy had a slight degradation and the contact angle increased a few degrees, which means decline in the wettability and shear strength of wheat straw is insignificantly. But after 3 days, aging behavious result in a 44% degradation of concentration of free radicals, while the wettability and shear strength decreased distinctly. When storage in air for 7 days or longer, it returned to the untreated state with the lowest shear strength.5. The microstructure transformation of wheat straw surface grafted by low pressure DBD plasma reflected in two aspects as following. Firstly, the high energy particles in DBD plasma system produced the physical sputtering and chemical etching on the surface of wheat straw. This etching process not only removed the wax layer on exterior surface, but also effectively increased the roughness and specific surface area, which was advantageous to produce glue nails and active center for grafting reaction. Secondly, the plasma can also break the chemical bonds of fiber surface, produced a great deal of free radicals which initiate a series of reactions between active center and polar groups, such as grafting, crosslinking and aggregation. The formation of branched chain was beneficial to increase the surface polarity of wheat straw. Through the synchronization of these two process, the bonding performance of wheat straw was remarkably enhanced.