ENVIRONMENTAL DEGRADATION OF ELASTOMERIC ADHESIVES AND RUBBERS IN HYGROTHERMAL AND HYDROLYTIC AGING CONDITIONS

Corrosion of elastomeric adhesives and rubbers in electrical and structural components presents significant challengesto various industries, with the automotive sector being particularly affected. Cross-linked polymeric adhesives serve numerous purposes, including functioning as joints for dissimilar material interfaces, protective matrices against environmental factors, and thermal interfaces for electronics. As a result, adhesive degradation due to environmental factors can negatively impact a vehicle’s performance and reliability. This dissertation investigates the individual and combined effects of environmental aging on elastomeric adhesives, concentrating on hygrothermal, hydrolytic, and thermo-oxidative aging. To achieve this, a series of experiments were conducted at different temperatures and durations, using a variety of viscoelastic materials such as rubbers and structural adhesives. The primary goal of this study was to develop a more comprehensive understanding of aging mechanisms and their impacts on polymeric materials, which would provide valuable insights for the development of predictive models and the assurance of the reliability and performance of numerous components. Initially, the research focused on exploring thermo-oxidation and hydrolytic aging environments, both individually and in combination. This analysis was further broadened to include the behavior of silicone-based adhesives in distilled water and seawater hydrolytic environments, uncovering the distinct responses of these materials under different conditions. Through a set of experiments involving PDMS-based silicone adhesive aged in both distilled and saltwater environments, it was revealed for instance, that even though the initial corrosion rate was higher for saltwater, in the long run, distilled water proved to be more devastating for the material. This was due to the higher absorption of water, which initiated chain scission and leaching processes. In contrast, saltwater conditions formed a protective salt barrier on the surface, which retarded further absorption of water into the polymer matrix, thus protecting the material compared to distilled water aging environments. The decay mechanisms in thermo-oxidation and hydrolytic aging were found to involve simultaneous cross-link formation/reduction and chain scission. Chemical and mechanical characterization methods were employed to demonstrate how environmental conditions synergize and cause damage accumulation through multiple sources. Building upon these findings, the study proceeded to examine damage evolution kinetics in polyurethane adhesives, delving deeper into the nature of each environmental condition. This exploration raised further curiosity to investigate the possibilities and scenarios when multiple environmental agents interact, either in isolation or in combination. Through these investigations, it was discovered that the hygrothermal aging condition presented a competitive environment between sub-aging conditions, specifically thermal-oxidation and hydrolytic environments. This competitive behavior could lead to different material responses and degradation pathways, depending on the balance of environmental factors at play. The study identified that certain factors could accelerate or decelerate the degradation process, depending on the specific combination of environmental conditions. This complex interplay was found to have a profound impact on the overall performance and longevity of elastomeric adhesives. Furthermore, it was revealed that mechanical damage and environmental degradation are independent mechanisms, indicating that the effects of mechanical stress and environmental factors should be considered separately when assessing material performance. This finding was crucial in further understanding the performance of elastomeric adhesives under various conditions and allowed for more targeted and accurate predictions of material behavior in different applications. By examining the damage evolution kinetics in polyurethane adhesives, this research provided valuable insights into the interplay between different environmental agents and the resulting effects on material performance. These insights contribute to a more comprehensive understanding of the factors influencing the degradation and performance of elastomeric adhesives, paving the way for the development of improved predictive models and enhanced material selection strategies in the automotive industry and beyond. The investigation then expanded to explore the effects of dual environmental exposure on polyurethane adhesive, delving into the intricate interplay between multiple environments and their impact on material behavior. The study was conducted in two phases, focusing not only on the combined effects of environmental factors but also on the order of exposure. For instance, the research examined the impact of dual aging involving thermal-oxidation followed by hydrolytic aging and compared it to the reverse order, where hydrolytic aging occurred in phase-1 followed by thermal-oxidation. This two-phase approach enabled a thorough evaluation of the synergistic and antagonistic effects between different environmental conditions, as well as the role of exposure sequence in the material’s degradation process. The study found that the order of exposure played a significant role in determining the material’s response to dual environmental aging, with specific sequences potentially leading to accelerated degradation or even the emergence of unique degradation pathways. Moreover, the research revealed that the combined effects of multiple environmental factors could result in non-linear material behavior, making it challenging to predict the material’s performance based on the individual effects of each factor alone. This finding highlighted the importance of considering the complex interactions between environmental factors when assessing the long-term performance and reliability of elastomeric adhesives. This comprehensive approach allowed for a more profound understanding of the complex relationships between multiple environmental factors and the performance of elastomeric adhesives. The study’s findings contribute to a better appreciation of the material’s degradation behavior under various conditions, ultimately leading to improved predictive models and material selection strategies for applications in the automotive industry and beyond. Towards the end of the research, the degradation of tetrafluoroethylene propylene rubber (FEPM) under sour high-pressure, high-temperature (HPHT) downhole conditions was explored. In order to investigate the changes in chemical structure due to environmental degradation, a combination of analytical techniques was employed, including gravimetric analysis, Fourier-transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (NMR) spectroscopy. This examination provided vital information regarding the material’s performance in extreme environments, further contributing to the broader understanding of elastomeric adhesives and their applications. The study highlighted the significance of comprehending the material properties and degradation mechanisms of FEPM materials when exposed to varying thermal conditions, as this knowledge is crucial for the development of more resilient and durable materials. The findings from this research offer essential theoretical guidance for the development of fluoroelastomers, emphasizing the importance of understanding the impact of different environmental factors on material performance. In light of these results, it is clear that identifying optimal coagents or other additives that can improve the mechanical properties and durability of FEPM elastomers could be beneficial. This study has shed light on the degradation mechanisms of tetrafluoroethylene propylene rubber under extreme HPHT conditions, providing crucial insights into the material’s performance and potential areas for improvement. These findings will be instrumental in guiding the development of fluoroelastomers and ensuring the long-term reliability and performance of components that utilize these materials across various industries. In conclusion, this experimental research offers an in-depth understanding of the constitutive behavior of elastomeric materials during various types of environmental aging exposures, including single and combined long-term effects. By systematically investigating the influence of temperature, water, and relative humidity in a range of environments such as distilled water, salt water, and sour solvents, the study sheds light on the intricate interplay of environmental factors on elastomers’ behavior. This comprehensive understanding is essential for the development of predictive models that can ensure the reliability and performance of automotive components and other applications that rely on elastomeric materials. The findings of this research have significant implications not only for the design and manufacturing processes of elastomeric adhesives and rubbers but also for the development of novel materials, additives, and coatings that can enhance their resistance to environmental degradation. By providing a deeper knowledge of the complex relationships between various environmental factors and the performance of elastomeric materials, this research ultimately paves the way for improvements in the safety, performance, and longevity of these materials across multiple industries. This, in turn, will lead to more cost-effective, environmentally friendly, and sustainable solutions for applications that demand the unique properties and characteristics of elastomers.