The effect of surface roughness on adhesive properties of acrylate copolymers

The effect of surface roughness on the adhesive properties of statistical, uncrosslinked butyl acrylate-methyl acrylate copolymers with different molecular weights (M_w=54 000, 192 000, and 600 000 g/mol) has been investigated using a combination of probe tack test and simultaneous video-optical imaging. Steel probes with different average surface roughnesses (R_a=2.9, 41.2, and 291.7 nm) have been used. The debonding process in a tack experiment is mainly controlled by the viscoelastic properties of the polymer, which control deformation and break of fibrils. However, increasing the probe surface roughness leads to a decrease of the maximum force during debonding and, correspondingly, the work of adhesion in a tack experiment decreases. Surface roughness has a strong effect on the initial cavitation process. The total number of cavities increases with increase in roughness, while their size decreases. The number of cavities increases slowly at the beginning of debonding, then rapidly increases as the force increases, and finally levels off, when the maximum force is reached. Two types of cavities are observed during debonding. Cavities of the first type appear at the beginning of debonding and their size increases slowly, while cavities of the second type appear at a higher stress level, when peak in force is approached, and their growth rate is about five times higher than that of cavities of the first type. Cavities even grow when the force has passed its maximum and eventually stop growing when the characteristic stress plateau is reached. Nevertheless, the growth rate for both cavity types is found to be independent of the surface roughness, but it is controlled by the viscoelastic properties of the polymers used and, accordingly, it decreases significantly with increase in molecular weight.