Research On Surface Performance Of Wheat Straw Modified By Atmospheric Pressure Dielectric Barrier Discharge(DBD) Plasma Treatment

China is experiencing many challenges in the context of natural forests, which have been eradicated or severely diminished, resulting in a shortage of wood. Many researchers are discussing possible ways to help China maintain a sustainable forest sector, while at the same time, helping to bridge the gap between wood demand and supply. So the straw particleboard has been received much attention in recent years.But the difficult bondability remains a major awkward problem, since the exterior surface of the crop straw contains much cutin and wax, which prevent the resin’s wetting, spreading and penetrating abilities in a way. The methylene diphenyl diisocyanate(MDI) could bond to the wheat straw well, but it remains some problems such as the higher cost and bonding to the plate. In order to meet the demand of market, it is necessary to make the straw-board glued with the traditional resins such as urea-formaldehyde(UF) and phenol-formaldehyde(PF) resin. An effective technique for modifying the surface of the wheat straw employs the atmospheric pressure dielectric barrier discharge(DBD) plasma in this study, which makes it possible to improve the surface performance.The influence on the surface wettability, morphology,chemical composition and bonding strength of wheat straw by the atmospheric pressure DBD plasma was systematically analyzed, and the mechanism of modification was clarified.The results are shown as following:1.The wettability of different adhesives(UF, PF and MDI) on the exterior and interior surfaces of wheat straw was remarkably improved after being exposed to the atmospheric pressure DBD plasma by measuring the contact angles, the surface free energy and the K-value. The instantaneous and equilibrium contact angles of the UF、PF and MDI resin on plasma treated wheat straw were much lower than those of the untreated specimens on both the exterior and interior surfaces. The K-value of the UF、PF and MDI resin on plasma treated wheat straw were much higher than those of the untreated specimens on both the exterior and interior surfaces. And the surface free energy were increased after plasma treatment. Within a certain range, with the increase of plasma discharge voltage, the decrease of the electrode space and the extension of treatment time, the surface wettability of wheat straw increased, but when the discharge voltage and electrode space and treatment time reach a certain value, the surface wettability of wheat straw instead of down. The better plasma processing conditions is discharge voltage 30 v, electrode space 6 mm, treatment time 60 s. On the exterior surface the the instantaneous contact angles can be decreased by 33% and the equilibrium contact angles decreased by 68%under the better plasma processing conditions2.The morphologies of the wheat straw were completely different before and after DBD plasma treatment, as shown by the SEM and AFM. Obvious etching was observed on the exterior and interior surfaces of the wheat straw samples after plasma treatment, especially when they were treated for 60 s and the discharge voltage is 30 V and the electrode space is 6mm, which is favorable for resin wetting, spreading, and penetrating. Within a certain range, with the increase of plasma discharge voltage, the decrease of the electrode space and the extension of treatment time, the surface morphologies of wheat straw were more obvious, but when the discharge voltage and electrode space and treatment time reach a certain value, The surface of wheat straw will be breakdown by the high-energy particles.3.There was great changes in the chemical composition before and after plasma treatment on wheat straw surfaces. The spectra of EPR showed that a great deal of free radicals on surface could be generated after being exposed to DBD plasma. The XPS showed that oxygen content increased significantly while the carbon content decreased significantly. And there was an increase in the O/C ratio along with an increase in the C2, C3 and C4 proportions based on the XPS analysis after plasma treatment, which means the number of oxygen-containing functional groups, such as hydroxyl, carbonyl, and carboxyl groups, increased. Besides, the silicon content on the surface was also increased after plasma treatment. Within a certain range, with the increase of plasma discharge voltage, the decrease of the electrode space and the extension of treatment time, the surface of wheat straw had higher O/C ratio, and O/C ratio amplitude decreased after the processing conditions reached a certain extent. In comprehensive view, by analysis of the chemical composition of untreated and plasma-treated wheat straw, it was obtained that the better plasma processing conditions was discharge voltage 30 v, electrode space 6 mm and treatment time 60 s.5. The wheat straw specimens with different gluing surfaces were bonded together with UF and PF and then hot-pressed in order to assess the bonding strength. The results indicated that the bonding strength was remarkably improved after it was exposed to the DBD plasma, especially the interface of exterior-exterior surfaces, which indicates that atmospheric pressure DBD plasma treatment was an effective method for improving the interfacial adhesion between wheat straw and resins. Within a certain range, with the increase of plasma discharge voltage, the decrease of the electrode space and the extension of treatment time, the bonding strength had been greater improved. In general, the better plasma processing conditions was discharge voltage 30 v, electrode space 6 mm and treatment time 60s。6. The mechanism of wheat straw modified by atmospheric pressure DBD plasma treatment was shown as following. The high energy particles in atmospheric pressure DBD plasma produced the physical etching and chemical etching on the wheat straw surfaces, and the degree of etching changes with the plasma treatment parameters, such as discharge voltage,electrode space and treatment time.Besides, the high energy particles can also break the chemical bonds of wheat straw, produced a great deal of free radicals and had a series of chemical reactions with wheat straw. In general, the plasma can effectively changes the chemical structure and composition of wheat straw by introducing some polar groups, such as hydroxyl and carbonyl groups. The etching can expose the polar composition under the wheat straw surface.In addition, the etching can effectively improved the roughness and increased the interface area between the wheat straw and resins, which can remarkably improved the bonding strength. On the other side, the polar groups introduced by plasma increased the reactional activity, which makes a great contribution to bonding between the wheat straw and resins.In a word,they indicated that atmospheric pressure DBD plasma treatment was an effective method for improving the surface performance of wheat straw, which were helpful to UF and PF and MDI resins penetrating...