Surface Structure and Dynamic Adhesive Wettability of Wheat Straw


  • Zhi-Ming Liu
  • Feng-Hu Wang
  • Xiang-Ming Wang


Wheat straw, surface structure, wettability, adhesive, penetrating, spreading, OM, SEM


The structural features of wheat straw differ from 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.


Berg, J. C. 1993. Role of acid-base interactions in wetting and related phenomena. Pages 76-148 in J. C. Berg, ed. Wettability. Marcel Dekker, Inc. New York, NY.nChen, C. M. 1970. Effect of extractive removal on adhesion and wettability of some tropical woods. Forest Prod. J.20(1):36-41.nGardner, D. J. 2002. Wood Surface Properties. In 36th International Wood Composites Materials Symposium. Washington State Univ., April 8. 1-24, Pullman, WA.nHan, G.-P., C.-W. Zhang, D.-M. Zhang, K. Umermura, and S. Kawai. 1998. Upgrading of UF-bonded reed and wheat straw particleboards using silane coupling agents. J. Wood Sci.44:282-286.nHan, G.-P., K. Umenmura, S. Kawai, and H. Kajata. 1999. Improvement mechanism of bondability in UF-bonded reed and wheat straw boards by silane coupling agent and extraction treatments. J. Wood Sci.45:299-305.nHerczeg, A. 1965. Wettability of wood. Forest Prod. J.15(11):499-505.nHse, C. Y. 1972. Wettability of southern pine veneer by phenol formaldehyde wood adhesives. Forest Prod. J.22(1):51-56.nJohn, Z. and R. Robert. 1995. The second forest: Filling the wood source gap while creating the environmental performance board of the 21st century. Pages 225-231 in Proc. 29th International Particleboard/Composite Materials Symposium, Washington State Univ. Pullman, Washington.nLiu, F. P., D. J. Gardner, and M. P. Wolcott. 1995. A model for the description of polymer surface dynamic behavior, I. Contact angle vs. polymer surface properties. Lanmuir11(7):2674-2681.nMaldas, D. C., and D. P. Kamdem. 1998. Surface tension and wettability of CCA-treated red maple. Wood Fiber Sci.30(4):368-373.nMarkessini, E. E. Roffael, and L. Rigal. 1997. Panels from annual plant fibers bonded with urea-formaldehyde resins. Pages 147-160 in Proc. 31st International Particleboard/Composites Materials Symposium, April 8-10. Washington State Univ., Pullman, WA.nNguyen, T., and W. E. Johns. 1978. Polar and dispersion force contributions to the total surface free energy of wood. Wood Sci. Technol.12:63-74.nPaper and Pulp Manual (Part one). 1987. Cellulose Materials and Chemical Industry Materials. Beijing: Light Industry Publication.4:74-116, 130-176.nRowell, R. M., R. A. Young, and J. K. Rowell. 1997. Paper and composites from agro-based resources. CPC Press, Boca Raton, FL. Pp. 383-387.nSauter, S. L. 1996. Developing composites from wheat straw. Pages 197-214 in Proc. 30th International Particleboard/Composite Materials Symposium, Washington State Univ., Pullman, WA.nScheikl, M., and M. Dunky. 1998. Measurement of dynamic and static contact angles on wood for the determination of its surface tension and the penetration of liquids into the wood surface. Holzforschung52(1):89-94.nShi, Q., and J. Z. Wang. 1997. Utilization of polymer automobile fluff in wood fiberboard. J. Solid Waste Technol. Mgmt.24(4):188-195.nShi, Q., and D. J. Gardner. 2001. Dynamic adhesive wettability of wood. Wood Fiber Sci.33(1):58-68.nSun, B. C. H., R. N. Hawke, and M. R. Gale. 1994. Effect of polyisocyanate level on strength properties of wood fiber composite materials. Forest Prod. J.44(3):34-40.nWu, Y. 1991. The cellulose chemistry on plant (2nd ed.). Beijing: Light Industry Publication. 47-59.nZisman, W. A. 1962. Pages 176-208 in P. Weisis, ed. Constitution effects on adhesion and adhesion. Adhesion and cohesion. Elsevier Publ. Co. Amsterdam, London, New York.n






Research Contributions