Research On Mechanism Of Wheat Straw Modified By Low Pressure Radiofrequency Cold Plasma Treatment

The straw particleboard has been received much attention in recent years because of theshortage of forest resources and environment concerns. But the difficult bondability remains amajor awkward problem, since the exterior surface of the crop straw contains much cutin andwax, which prevent the resin’s wetting, spreading and penetrating abilities in a way. Themethylene diphenyl diisocyanate (MDI) could bond to the wheat straw well, but it remains someproblems 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 asurea-formaldehyde (UF) and phenol-formaldehyde (PF) resin. An effective technique formodifying the surface of the wheat straw employs the low pressure radiofrequency(RF) plasma inthis study, which makes it possible to improve the surface wettability and bonding strength. Theinfluence on the morphology, chemical composition, surface wettability and interfacial bondingproperties of wheat straw by the low pressure RF plasma was systematically analyzed, and themechanism of modification was clarified.The results are shown as following:1. The structures and chemical compositions of the exterior and interior surface are different.The exterior surface contains plenitudinous cuticle, which prevents the resin’s wetting, spreadingand penetrating abilities and reduces the interfacial bonding strength.2. Significant etching was observed on the exterior and interior surface of wheat straw afterbeing exposed to the low pressure RF plasma. The surface roughness Ra value increased. Inaddition, the etching belongs to the nano-scale etching byAFM. With the prolonging of treatmenttime, and the increasing of treatment power, the surface of wheat straw had higher roughness. Forthe different treatment gas, the effect of treatment was different and the oxygen plasma presentedthe best result among the treatment gases. By analysis of the surface morphology of untreated andplasma-treated wheat straw, the better plasma process was processing time150second, power200W~300Wand the oxygen plasma.3. There was great changes in the chemical composition between the untreated andplasma-treated wheat straw. The spectra of EPR showed that a great deal of free radicals onsurface could be generated after being exposed to plasma.The XPS showed that oxygen contentincreased significantly while the carbon content decreased significantly. Through the surveyspectra of C1s, it was found that C1content reduced and C2,C3content increased. Besides, thesilicon content on the exterior surface was also increased after plasma treatment. It could beproved by the analysis of SEM-EDX. With the prolonging of treatment time, and the increasingof treatment power, the surface of wheat straw had higher O/C ratio. For the different treatmentgas, the effect of treatment was different and the oxygen plasma presented the best result. The Nelement content of the surface increased after being treated by Ammonia plasma. Incomprehensive view, by analysis of the chemical composition of untreated and plasma-treated wheat straw, it was obtained that the better plasma process was processing time150second,power200W~300W and the oxygen plasma.4. The dynamic wettability of different adhesives (UF, PF and MDI) on the exterior andinterior surfaces of wheat straw was remarkably improved after being exposed to the lowpressure RF plasma by measuring the contact angles, the surface free energy and the K-value.With the prolonging of treatment time, and the increasing of treatment power, the wettability hadbeen greater improved, and the wettability presented moderate growth while the power reached acertain extent. For the different treatment gas, the effect of treatment was different and theoxygen plasma presented the best result. In general, by analysis of the dynamic wettability ofuntreated and plasma-treated wheat straw, the better plasma process was processing time150second, power200W~300W and the oxygen plasma, and the equlibrium angle decreased55.17%~85.09%.5. The wheat straw specimens with different gluing surfaces were bonded together with UFand PF and then hot-pressed in order to assess the bonding strength. The results indicated that thebonding strength was remarkably improved after being treated by plasma, especially the interfaceof exterior-exterior surfaces. With the prolonging of treatment time, and the increasing oftreatment power, the bonding strength had been greater improved. For the different treatment gas,the effect of treatment was different and the oxygen plasma presented the best result among thetreatment gases. In general, the better plasma process was processing time150second, power200W~300W and the oxygen plasma, and the bonding strength of exterior-exterior interfaceincreased531.43%~846.77%.6. The mechanism of wheat straw modified by low pressure RF plasma treatment wasshown as following. The high energy particles in low pressure RF plasma produced the sputteretching and chemical etching on the surface of wheat straw. The etching exposed the polarcomposition under the surface of wheat straw, and effectively improved the roughness andincreased the interface area between the straw and resins, which can remarkably improved thebonding strength. The degree of etching changes with the plasma treatment parameters, such asthe treatment time, treatment power or gas. Besides, the plasma can also break the chemicalbonds of wheat straw, produced a great deal of free radicals and had a series of chemical reactionswith wheat straw. In general, the plasma can effectively changes the chemical structure andcomposition of wheat straw by introducing some polar groups, such as hydroxyl and carbonylgroups, which makes a great contribution to bonding between the wheat straw and resins.