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.
[1]
G. Han,et al.
Improvement mechanism of bondability in UF-bonded reed and wheat straw boards by silane coupling agent and extraction treatments
,
1999,
Journal of Wood Science.
[2]
M. Dunky,et al.
Measurement of Dynamic and Staue Contact Angles on Wood for the Determination of its Surface Tension and the Penetration of Liquids into the Wood Surface
,
1998
.
[3]
R. Rowell,et al.
Paper and Composites from Agro-Based Resources
,
1996
.
[4]
M. Wolcott,et al.
A Model for the Description of Polymer Surface Dynamic Behavior 1. Contact Angle vs Polymer Surface Properties
,
1995
.
[5]
R. Reen,et al.
The second forest: filling the wood source gap while creating the environmental performance board of the 21st century
,
1995
.
[6]
B. Sun,et al.
Effect of polyisocyanate level on strength properties of wood fiber composite materials
,
1994
.
[7]
J. Berg.
Role of Acid-Base Interactions in Wetting and Related Phenomena
,
1993
.
[8]
C. Hse.
Wettability of southern pine veneer by phenol formaldehyde wood adhesives
,
1972
.
[9]
C. M. Chen.
Effect of extractive removal on adhesion and wettability of some tropical woods.
,
1970
.
[10]
V. Gray.
The wettability of wood
,
1962
.