Polyurethane potting compounds and sealants, widely used as encapsulating and protective barriers, are usually applied to substrates after precoating substrate surfaces with primers. While the use of primers is known to improve adhesion properties of polyurethanes, fundamental mechanisms of adhesion enhancement are not fully understood due to the difficulties of direct observations at buried interfaces. Interfacial properties like adhesion are determined by interfacial molecular structures. In this study, sum frequency generation (SFG) vibrational spectroscopy was applied to investigate interfacial molecular structures in situ between a polyurethane potting compound and an isocyanate-based primer. The SFG signals of isocyanate groups from the primer initially were observed at the interface but disappeared after 16 h of polyurethane cure time. The following SFG experiments confirmed that interfacial reaction occurred at the interface between the polyurethane potting compound and the primer. Attenuated total reflectance (ATR)-FTIR spectroscopy and 180° peel tests were utilized as complementary techniques to support the SFG analyses. The results indicate that the formation of chemical bonds at the interface were responsible for the enhancement of adhesion. This study aims to help build on fundamental design principles toward the development of polymer materials with improved mechanical performance and other macroscopic interfacial properties.

A primer coating is engineered to facilitate compatibility between products like adhesives, sealants, and potting compounds and targeted substrates. Prolonged exposure of isocyanate-based primer surfaces to the environment is known to negatively affect the interfacial adhesion between itself and the products subsequently applied on top of it. However, the molecular behavior behind this observed phenomenon remained to be further investigated. In this study, sum frequency generation (SFG) vibrational spectroscopy, a nonlinear optical spectroscopic technique, was applied to study the surface of an isocyanate-based primer exposed to different environments at the molecular level. Atmospheric moisture was considered to be a potential factor in impairing the adhesion performance of the primer, and thus, time-and humidity-dependent experiments were executed to monitor the molecular behavior at the primer surface using SFG. In addition, 180°peel testing experiments were conducted to measure the adhesion properties of primers after being exposed to the corresponding conditions to correlate to SFG results and establish a chemical structure−macroscopic performance relationship. This study on the changes at the primer surface in different environments with varied humidity levels as a function of time aims to provide an in-depth understanding of the moisture effect on isocyanatebased primers. These learnings may also be helpful toward exploring a broader range of coatings and surface layers and improving customer product use guidelines.