Synthesis, Recycling, and Modification of Thermoset Silicone Resins via Fluoride Ion Catalyzed Rearrangement

Abstract

Silicones are unique polymers with inorganic backbones that, in part, afford properties that cannot be matched by organic polymers. A short list of their many attractive properties includes thermal stability, chemical resistance, hydrophobicity, and physiological inertness. Such an intriguing combination of properties lends itself to an abundant and diverse array of applications ranging from aerospace sealants to automotive molding release agents to marine anti-corrosion/fouling coatings to medical prosthetic devices. Despite, and in some cases because of, their excellent properties there are challenges associated with this important class of polymers. This dissertation aims to explore some of these opportunities and offer new solutions. First, we focus on using fluoride ion (F-) catalyzed rearrangement of siloxane and silsesquioxane bonds as a new route to synthesize multi-functional and highly cross-linked silicone resin networks. The resulting monoliths and coatings exhibit high thermal and oxidative stability, up to >460°C. Our approach also allows for the typically difficult combination of hydrophobicity and wear resistance. Thereafter, we employ a mixed phenyl/methyl silicone resin synthesized via F- catalyzed rearrangement as a model system for our new recycling technique. Silicones, especially resins, are some of the most challenging polymers to recycle. Not only are they cross-linked thermosets, but their high temperature stability and inertness make them even more difficult to degrade by conventional recycling methods, such as pyrolysis. Here, we present the facile recycling of highly cross-linked silicones at ambient temperature and pressure via F- catalyzed rearrangement in solvent. Rigid virgin silicone resin cured at 250°C becomes soluble in less than 24 h in the presence of F- and can be reapplied or cast easily. The recycled silicones maintain nearly 100% of key properties such as thermal and wear resistance. We also show this technique works for commercially relevant silicone rubber and resin made by conventional means. In some cases, the properties of the recycled material is higher than the virgin material, which suggests the potential for up-cycling. Lastly, we explore a discovered opportunity presented by the nature of our recycling process. When the fully cured silicone resin dissolves in the presence of F-, the ratio of starting materials can be adjusted by the addition of new silicone building blocks. For example, increasing the phenyl content and cross-link density yields a silicone resin with a thermal stability of >530°C. The introduction of wholly new functionalities can modify the virgin silicone such that the recycled material can be used in a different, and possibly more demanding, application. For example, the introduction of 10 wt% of a phenyl functionalized silsesquioxane to a recycled commercial silicone resin via F- catalyzed rearrangement increased the Tg and thermal stability by 115°C and 30°C, respectively.