Effects of Surface Roughness on Direct Plasma Bonding between Silicone Rubbers Fabricated with 3D-Printed Molds

This study presents the effects of surface roughness on the adhesion strength of plasma-treated rubbers that are widely used in soft robotics. The rubbers are designed with 11 molds of different patterns and fabricated from liquid silicones for mutual comparison. Several specimens with nonperiodic and periodic surface waveforms are quantitatively analyzed based on the correlation between surface roughness and adhesion strength. The surface roughness of three-dimensional (3D) printed molds under different printing conditions is compared to that of the standard specimens molded by a smooth acrylic plate and four sandpapers. The surface profiles are measured by a stylus profiler, analyzed using fast Fourier transform, and subsequently quantified using the experimental roughness parameters, Ra and Rku*. The kurtosis ratio Rku* is proposed to simultaneously evaluate the sharpness, total height, and peak density to identify contact surfaces. A 90 degrees peel test is also conducted to evaluate the adhesion strength, considering the designed pattern and printing orientation relative to the peeling direction. Microstructural analysis of the specimens is performed to investigate the peeling mechanism and molding quality using scanning electron and digital microscopes. Correlations between adhesion strength and surface roughness are obtained through the evaluation of the plasma-treated silicone specimens. Rku* is significant in determining the surface properties of the effective contact area, particularly for rough surfaces, and further contributes to an effective evaluation when the parameter Ra is used simultaneously. The results suggest that the plasma bonding of silicone rubbers fabricated with 3D-printed molds is effective in enhancing the adhesion strength of soft robots or stretchable devices.