Wettability of Wood

The purpose of this study was to check the application of defined physico-chemical concepts to the wettability of wood surfaces. Specific objectives were to determine 1) the critical surface tension of wetting for Douglas-fir, 2) the effect of surface aging upon contact angles, and 3) the influence of complete wetting or spreading on adhesion strength. Zisman’s linear relationship, cosine contact angle versus surface tension was confirmed on Douglas-fir wood. Zisman’s parameter “critical surface tension” also applies to Douglas-fir and proved to be a characteristic that varies with the solid surface condition, i.e age, roughness, etc. The critical surface tensions of Douglas-fir summer and springwood were 44 and 50 degrees per centimeter, respectively. Maximum work of adhesion values were in agreement with previously published findings. it was shown that aging increased contact angles, i.e. reduced wettability. Bond strength of cross lap specimens, bonded with a urea-resin adhesive with different surface tensions, was found to be maximum when the surface tension of the adhesive approximated the critical surface tension of the wood. High surface tension adhesive bonds apparently had lower strength due to poorer wetting while adhesives with lower surface tension penetrated the porous wood surface excessively. This suggests a strong dependence of bond strength on wetting, spreading, and surface tension of the adhesive.

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