Microbubble on fiber method to determine the contact angle between steel substrates and highly viscous molten PEKK and PA 6

Determining the contact angle between a molten thermoplastic and a solid is important for the processing of thermoplastics and their composites. The well-known sessile drop method can be used to determine the contact angle of thermoplastics. However, complex instrumental systems are needed due to the high viscosity and high melting point of thermoplastics. Inspired by the captive bubble method, a simple method based on the system of an air bubble on a substrate in the molten thermoplastic was proposed. This system is prepared by melting fibers and thermoplastic powder materials mixtures in between two glass plates using a hot stage. The contact angle of a microbubble in contact with fiber in molten thermoplastic is measured using an optical microscope. The system of a microbubble in molten thermoplastic can easily reach the equilibrium state. Two types of highly viscous thermoplastics in contact with stainless steel fibers are studied and the contact angle is sensitive to both the physicochemical properties of the fiber surface and the type of polymer matrix materials, which demonstrates the applicability of this method. Our proposed method is promising to be further developed into a general method to determine the contact angle between thermoplastics and solid surfaces.