FIG. 24 Zisman plot for estimating the cellulose fiber surface tension (the cosines of the contact angles were extracted from the measured normalized wetting rates) using the averaged constant determined from Fig. 23. (From Ref. 118.)

The capillary-rise technique [108] as described above was successfully applied to characterize differently sized glass fibers [91] and modified jute fibers [70] with respect to their surface tensions. Gassan et al. [70] found that the packing density of the fibers influences only slightly the measured contact angle. However, in the case of hydrophilic natural fibers the measured contact angle is affected by the diffusion of the test liquid into the fibers. The measured [m2/t] function contains not only information about the wetting process but also about the absorption of the liquid, the penetration of the test liquid into the fibers. Swelling takes place and therefore the fiber diameter increases, which again influences the geometry factor C, which influences the contact angle. Significant

differences were found comparing the surface-tension values cal-culated from the contact angles determined using the capillary-rise technique and the modified Wilhelmy method [70].