Evaluation of the wettability of the filler binder during the manufacture of prepreg with ultrasound exposure for three-dimensional printing filaments reinforced with continuous carbon fiber

Background and Objectives: The aim of the research is to study the effect of ultrasound on the wettability of carbon fibers with a thermosetting binder and to determine the work of adhesion in the binder-fiber contact as part of the filament prepreg for additive technologies. Materials and Methods: Carbon fibers and carbon fiber roving with a width of 2 mm GG-200P were used in the research. Impregnation was carried out with ED-20 epoxy resin with a PEP hardener by pulling a harness at a speed of 10 mm/s through a gap of 2-3 mm between the end of the ultrasonic concentrator and the lower surface of the container with a binder. In total, 5 control and 5 experimental samples with a length of 300 mm were impregnated. Ultrasonic processing of samples was carried out on an experimental ultrasonic installation with an experimental ultrasonic generator controlled from a laptop and providing a discreteness of 10 Hz adjustment. The impregnation was carried out at a resonant frequency of 21650 Hz and an oscillation amplitude of the output end of the concentrator – 15 microns. The diameter of the output part of the concentrator was 14 mm, respectively, the length of the fiber bundle section was the same size at each time. After curing of the binder, the surface of the fibers was studied using a digital microscope Bresser LCD 50x–2000x at magnification x40 and x300. When impregnating control samples, the ultrasonic transducer of the installation was not included. Microphotographs were used to evaluate the wettability of the fibers with a binder under the influence of ultrasound and without ultrasound, and also to determine the edge angle of wetting and then - according to the Young - Dupree equation - the adhesion of the binder to the fiber surface. Results: The control bundle of carbon fibers is characterized by incomplete consolidation of fibers into a bundle, there are separate disoriented fibers, as well as separately existing conglomerates of fibers. Experimental bundles impregnated under the influence of ultrasound are monolithic structures with a continuous filling with a binder. At the same time, areas with inflows of the cured binder that violate the geometric shape of the prepreg are noted. Both control and experimental samples of harness are generally fully impregnated consolidated prepregs, there are no individual fibers or groups of fibers, which may be due to the initially regular structure of harness compared to randomly organized individual fibers into a thread. At the same time, in the control samples, the binder is unevenly distributed over the surface, individual inflows are noted, which indicates an insufficiently uniform impregnation of the harness. which is not observed on the prototypes. By calculating the adhesion performance for control and experimental samples using the experimentally obtained values of the wetting angle, it was found that its value is 44.71-48.98 mJ/m$^2$ and 64.46-66.4 mJ/m$^2$ , respectively, for control and experimental samples. Conclusion: A significant improvement in the wettability of the fibers with a binder has been found, manifested in a decrease in the wetting edge angle from 70-77$^{\circ}$ to 35-40$^{\circ}$. Using the Young - Dupree equation, the adhesion of the binder to the fiber has been calculated and it has been shown that the effect of ultrasound during the impregnation of fibers increases the adhesion by an average of 39.7$\%$.