Stress relaxation during bond formation and adhesion of pressure-sensitive adhesives

Relaxation properties and adhesion of pressure-sensitive adhesives (PSAs) have been studied with the Probe Tack method under the conditions corresponding to the adhesive bond formation. Typical representatives of various PSA classes are examined: adhesives based on the styrene-isoprene-styrene (SIS) block copolymer, polyisobutylene of two molecular weights, acrylic and silicone PSAs. By comparison of the adhesive and relaxation behaviors of different PSAs it has been established that the PSA relaxation contributes appreciably to the strength of the adhesive bond and underlies the impact of contact time on adhesion. Direct correlation has been established between the compressive stress relaxation in the course of bond formation and the mechanism of debonding. All the examined PSAs can be classified into two groups: 1) the fluid-like PSAs that are capable of relaxing fully under compression (PIB, silicone adhesives) and 2) the PSAs, which reveal a residual unrelaxed stress. Physically crosslinked SIS and chemically crosslinked acrylic adhesives exemplify the PSAs of the second group. The occurrence of two peaks on the debonding stress-strain curves is typical of the PSAs of the second group. High adhesive strength requires the contribution of the longer relaxation times that vary for different PSAs in the range from 150 to 800s. Minimum values of the longer relaxation times are featured for fluid adhesives, whereas the maximum values are found for crosslinked, network, and entangled adhesives. The adhesive strength achieves its maximum when the slow relaxation processes become dominating. Relative contributions of viscous and elastic deformations to relaxation properties of PSAs are assessed in terms of the Deborah number.