Recycling Silicone-Based Materials: An Overview of Methods

Abstract

Since the early 1800s, siloxane has been an industrial staple due to its remarkable structure, but even though there are many benefits for using siloxanes, there are significant environmental implications, one of which being the lack of recyclability. As the first step to polymerization or the depolymerization of polymers, the scission of the silicone bond is essential. While condition-specific reactions investigating what triggers polymerization have been extensively studied, traditional synthesis methods are unfortunately not ideal due to their high cost and detrimental release of greenhouse gases. Since the 1950s, several studies have related to rupturing the siloxane bond, including hydrolysis, catalytic depolymerization, thermal depolymerization, and radical extractions. This work has resulted in new polymers, cyclics, and monomeric silanes. However, only a few studies have focused on how to build new silicone-based materials from the primary siloxane cyclic forms. Thus, more investigation into better methods for recycling siloxanes is needed. This chapter summarizes and categorizes the published data on the degradation and depolymerization of polysiloxanes based on their reaction temperature up to July 2021.

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“…4) are polymers consisting of repeat units of silicon-oxygen bonds readily formed through the condensation of M (mono), D (di), T (tri), and Q (tetra) type alkoxy/chlorosilanes, denoting the number of oxygens bound to a silicon, and may include organic side chains. 121 Two types of siloxane polymers are commonly formed depending on the monomers used: linear chains derived from M and D units and three-dimensional networks with silsesquioxanes formed from T types or silica components from Q units. However, these units are often combined to install unique side chains or crosslinking sites to modify properties.…”

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“…Depolymerization of PDMS typically occurs through Si–O bond cleavage, which can be mediated by catalytic or stoichiometric acids and bases. − Initially, hydrolytic cleavage of a main chain Si–O bond forms a silanolate, which undergoes intramolecular σ-bond metathesis, known as “backbiting”, releasing cyclic siloxanes as products. This mechanism can also be initiated by nucleophiles and electrophiles or by heating above 350 °C in the presence of catalysts. , The thermodynamically favored product is hexamethylcyclotrisiloxane (D 3 ); however, octamethylcyclotrisiloxane (D 4 ), decamethylcyclotrisiloxane (D 5 ), and dodecamethylcyclotrisiloxane (D 6 ) can also be formed at lower temperatures under specific conditions.…”