HYDRODYNAMIC DETERMINATION OF CRITICAL SURFACE TENSION OF FIBERS

Abstract

The critical surface tension (CST) pointed out by Zisman is a practical parameter related to complete wetting of fibers. In the form of fiber, in general, the value of CST is estimated with some difficulties because of complex contact of fiber with liquid or unevenness of fibersIt was found that the value of CST could also be estimated, using the previously reported hydrodynamic apparatus on estimation of wettability of fiber assemblies. This study is mainely concerned with the principles and methods of determination of CST in the fiber assembly.The measurement of CST was based on the calculations of penetrating velosity (dh/dt) of dipsolution through capillaries in fiber assembly under forced flowing of dip-solution. Continious record of water pressure during the wetting process permitted the calculations of average water level in sample chamber and two curves of pressure and water level on the same time-scale permitted the comparision of amounts of pressure occured naturally in capillaries and of pressure loss due to forced flowing. The recorded pressure, when the two kinds of pressure just compensated, gave the value of dh/dt. By determination of the relation between dh/dt and water level in chamber, the product of radius of capillary by cosine of contact angle was obtained as an average value on numerous capillaries. Further, the plotting of these products vs surface tension of dip-solutions gave the value of CST of sample fibers in accordance with Zisman's method.Using above described method, value of CST of merino '64 wool fiber was determined at about 30 dyne/cm. This value was much smaller than the values reported in literatures, but it might be considered that this represented the more real value on undamaged clean wool fiber.Consequently, the merits of this hydrodynamic estimation were summarized as follows; 1) CST was determined as an average value of numerous fibers, 2) not only the CST, but the average radius of capillaries in fiber assembly could be estimated, 3) convenient determination of CST was also possible, by comparision of the shapes of pressure-time curves obtained on a series of solutions having various surface tensions.