Acid-base properties of polydimethylsiloxane based adhesives and their influence on adhesion to polymer surfaces

This work aimed to investigate the adhesive properties of polysiloxanes (silicones) on polymer surfaces in relation to their acid-base properties. To adjust the acid-base properties of polydimethylsiloxane (PDMS) based formulations, (3-aminopropyl)-trimethoxysilane (AMMO) and [(3-triethoxysilyl)-propyl]succinic anhydride (SAS) were added as silane coupling agents. The acid-base properties of the resulting formulations were investigated by measuring the isoelectric point (IEP) via zeta potential measurements as well as by measuring the contact angle on cured samples with probe liquids of different pH. The IEP could be increased gradually from 5.7 to 9.3 by addition of AMMO and decreased gradually from 5.7 to 2.7 by addition of SAS. The adhesion to various polymers was determined by tensile lap shear strength measurements on HDPE, PET, PMMA, PVC and PA as well as on glass. Silicones containing AMMO showed a higher shear strength on all polymers compared to the acidic SAS containing silicones. The IEP leading to maximum shear strength depends on the respective polymer surface, which makes the adjustment of the acid-base properties a useful tool for optimizing adhesion. Furthermore, measuring the zeta potential as well as the surface energy of the substrate surfaces revealed significant differences in their polarity. In addition, an approximately linear increase of the maximum shear strength measured on each substrate could be observed with increasing zeta potential of the substrate at pH 9.7. These findings show that zeta potential measurements of both adhesive and substrate have the potential to be a powerful tool for predicting adhesive interactions. • Preparation of PDMS based adhesives with adjustable surface acid-base properties. • Analysis of surface acid-base properties by determining the isoelectric point. • The adhesives isoelectric point notably influences the adhesion to polymer surfaces. • The adhesion correlates with the zeta potential measured on the polymer surfaces.