Effect of Magnetite Nanoparticles Content and Surface Modification on Tack of a Pressure-Sensitive Adhesive

In recent years, the use of pressure-sensitive adhesives (PSA) has increased in various fields of technology. Meanwhile, developing high-performance PSAs responsive to various stimuli has received considerable attention. Since development of magneto-responsive PSAs has been overlooked, the aim of this research is to prepare acrylic PSAs containing magnetite nanoparticles. The effect of addition of two levels of unmodified or silane-modified magnetite nanoparticles to an acrylic PSA on tack properties was systematically studied. Magnetite nanoparticles were synthesized by co-precipitation method. The successful synthesis of the magnetite nanoparticles was confirmed by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction, and their diameter was found to be 10 nm. The surface modification of the nanoparticles was accomplished by reacting them with 3-(methacryloxy)propyltrimethoxysilane, which was verified by FTIR after removing the unreacted silane. On the other side, an acrylic copolymer containing 99 wt. % butyl acrylate and 1 wt. % acrylic acid was synthesized by solution polymerization. Subsequently, the PSA was prepared by mixing the synthesized copolymer, 10 or 20 wt. % unmodified or modified magnetite nanoparticles, and 0.1 wt. % crosslinking agent, followed by film casting and drying. The probe tack test was performed on the PSAs using a magnetic probe to determine their instant adhesion properties. The maximum stress (tack) for the unfilled PSA and the PSAs containing 10 wt. % unmodified nanoparticles, 10 wt. % modified nanoparticles, 20 wt. % unmodified nanoparticles, and 20 wt. % modified nanoparticles were measured to be 0.19, 0.21, 0.20, 0.21, and 0.17 MPa, respectively. Moreover, the adhesion energy of these PSAs was found to be 23.39, 20.90, 17.06, 14.23, and 12.87 J/mm2, respectively. The results showed that the addition of the unmodified nanoparticles and increasing their content slightly improved the maximum stress (tack) of the PSAs, which was assigned to the magnetic response of the PSAs with no negative effect of the nanoparticles on the wetting of the PSAs on the substrate. However, this change was accompanied by a decrease in the adhesion energy of the PSAs, which was a consequence of the strong effect of magnetite nanoparticles on the fibrillation of the PSA. The addition of the modified nanoparticles resulted in a reduction in the tack of the PSAs compared to that of the PSAs containing unmodified nanoparticles; so that the PSA containing 20 wt. % modified nanoparticles was of a lower tack compared to the unfilled PSA. These PSAs also had a lower adhesion energy compared to both the unfilled PSA and the PSA with the same nanoparticle content. These results were attributed to the increase in the number density of the interactions between the modified nanoparticles and the copolymer, revealing a role of nanoparticles similar to crosslinkers, which in turn diminished the mobility of the chains and thus reduced both the wettability of the PSA and the viscoelastic loss. Indeed, the attraction of the magneto-responsive PSA to the magnetic probe hindered the amount of stretchability and fibrillation of the PSA, assisting the breaking of the fibrils. Although the adhesion energy of the PSAs containing modified nanoparticles was less than that of the PSAs containing unmodified nanoparticles, interestingly, the increase in the amount of the modified nanoparticles led to a 10% lower adhesion energy loss compared to the unmodified nanoparticles. Hence, it can be concluded that the fibrillation and adhesion energy of a magneto-responsive PSA can better be controlled using modified magnetite nanoparticles.