Investigating Adhesion and Degradation of Polymer Materials for Industrial Applications

Polymeric materials are essential in industrial production, fulfilling needs from basic packaging to complex structural requirements. It is evident that polymer materials have significantly facilitated various aspects of daily life. On the other hand, plastic pollution has also emerged as a growing environmental concern. This thesis research focuses on investigating various industrial polymer materials to understand adhesion mechanisms in sealing applications (Chapter 2 to Chapter 4) and environmental degradation in polyethylene packaging materials (Chapter 5). Sum frequency generation (SFG) vibrational spectroscopy, a second-order nonlinear optical spectroscopy with submonolayer surface/interface sensitivity, was employed to study molecular behaviors at surfaces and buried interfaces in situ nondestructively, providing insights into adhesion fundamentals. For the degradation study of polyethylene, a combination of analytical techniques and advanced statistical analysis techniques were introduced, contributing to a more comprehensive picture of the degradation process. Potting compounds and sealants are designed to seal gaps and encapsulate substrates as protective barriers. Polyurethane potting compounds are usually applied after pre-coating substrate surfaces with primers to improve adhesion, but fundamental mechanisms of adhesion enhancement are not fully understood. In Chapter 2, SFG was used to explore the interfacial molecular structures between a polyurethane potting compound and an isocyanate-based primer. The findings suggest that isocyanate groups in primer at the interface were consumed by their reaction to amino groups from potting compound, and the formation of urea chemical bonds at the interface contributed to the enhanced adhesion. Along with the study of the buried interface between the primer and potting compound, it was observed that extended primer exposure to the atmosphere before applying the polyurethane potting compound reduces adhesion enhancement noticeably. In Chapter 3, SFG was utilized to examine the surface of the isocyanate-based primer under varying environmental conditions. The results indicate a decreased number of isocyanate groups on the surface due to the reaction with moisture, and the reduction in adhesion was more pronounced with higher relative humidity and longer exposure times. In addition to exploring the adhesion system of the polyurethane potting compound and isocyanate-based primer, Chapter 4 focuses on a thiol-epoxy model fuel tank sealant in contact with a silane-based adhesion promoter. The immersion effect in saltwater at elevated temperatures was studied by SFG. The results indicate that immersion in saltwater led to disordering at the interface between the adhesion promoter and sealant, and the weaker interfacial interactions can play a role in affecting the adhesion. Understanding the fate of plastics in the environment is crucial for finding sustainable solutions to plastic waste challenges. In Chapter 5, a combination of analytical techniques (attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), water contact angle measurement, Raman, GIXRD, nanoindentation) was applied to systematically measure samples exposed to simulated solar exposure (phase 1). Then, an in-depth study involving lake deployment was conducted in phase 2 using ATR-FTIR, and the results were further studied using statistical analysis to reveal their relationships to a variety of aging conditions. The study suggests that polyethylene with lower densities, longer UV aging, and their interactions led to noticeable polyethylene structural changes, whereas lake deployment showed insignificant effects. In summary, this thesis research offers molecular and structural insights into diverse polymer materials for industrial applications. These methodologies aim to inspire and guide the exploration of a wider range of polymer materials in future development endeavors.