Surface structure and dynamic adhesive wettability of wheat straw

摘要

The structural features of wheat straw differ front those of wood. By means of an Optical Microscope (OM) and a Scanning Electron Microscope (SEM), three kinds Of tissues (epidermis, parenchyma, and vascular tissue) were observed on the cross section of wheat straw. A smooth cuticle was found on the exterior surface. The exterior surface of wheat straw treated by NaOH solution at room temperature appeared to be chemically etched. After this treatment, the wettability of the exterior surface was improved substantially. In this study, using a wetting model describing the dynamic contact angle process, a parameter (K) was used to quantify the adhesive spreading and penetrating during the wetting process. By applying the wetting model, the adhesive wettabilities associated with resin type (UF, PF, and PMDI), drop location on the wheat straw surface (exterior and interior), and grain direction (along and across) were compared. The results of this study showed that PMDI resin had a lower contact angle (both initial and equilibrium) and a greater spreading and penetrating constant compared to UF and PF resins on natural (untreated) wheat straw surfaces. The K value of the interior surface was higher than that of the exterior surface for the same resin on the untreated wheat straw. In addition, the K values of the three resins on the treated wheat straw surfaces were higher than those on untreated wheat straw surfaces. This indicates that the alkali treatment was an effective method for improving the wettabilty of wheat straw surfaces. The wheat straw grain direction also significantly affected the adhesive wetting process. The K values of adhesive wetting along the wheat straw grain direction were always greater than those across the grain direction for the same resin.