Determining the wettability of carbon fiber tows from single fiber contact angle data

The present Application Report contains a summary of the methodology and the results of investigations on carbon fiber samples which were published in the scientific literature (see references on the last page). We at KRÜSS would like to thank our guest authors for sharing their insights and results and for providing this report.

Physical adhesion between carbon fibers (CFs) and polymer matrices as well as the formation of voids at the interface between these two materials are mostly determined by the wetting properties of the fibers. Due to the hierarchical structure of CF reinforcements, it is essential to study their wetting behavior at different scales: from the single fiber (microscale) to the fabric (macroscale) via the tow scale (mesoscale). Whereas a direct measurement of the contact angle of single CFs by tensiometric means is well established, a direct measurement of the wettability of CF tows is hampered by their porous structure due to densification and liquid take up phenomena. An accurate characterization of their wettability is therefore still highly challenging.

In this Application Report we present a method for determining the wettability of CF tows based on contact angles measured on single fibers. For this we performed a combined and synchronized analysis of tensiometrically and optically determined contact angles. The wettability of single CFs and CF tows composed of unsized and sized CFs were measured using a Force Tensiometer – K100SF and optical means. The contact angles of CFs at meso- and microscales have then been successfully linked using a modified Cassie-Baxter model.