Full Circle Recycling of Polysiloxanes via Room-Temperature Fluoride-Catalyzed Depolymerization to Repolymerizable Cyclics

Abstract

Siloxane-based polymeric materials are widely used all over the world because of their chemical, mechanical, and thermal stability and due to their low toxicity. Despite their usefulness, the high energy cost used to make them is lost when they are discarded, wasting resources and energy. Practical and simple methods of recycling these materials are therefore required. However, the simplification of these methods is underdeveloped. The present techniques used to recycle silicone-based materials, such as polydimethylsiloxanes, often involve high temperatures/pressures and complicated setups. To address this issue, we have established an efficient room-temperature technique for the depolymerization of silicone-based polymers, elastomers, and resins in the presence of low catalytic amounts of fluoride in specific high swell organic solvents. The products primarily contain cyclic siloxane units (D 4 , D 5 , and D 6 ) as verified by GCMS and 29 Si nuclear magnetic resonance. Nearly any silicone resin can be depolymerized rapidly using these methods. Silicone-rich systems result in the best conversions and the highest quantity of identifiable cyclics, while complex resins resulted in complicated products alongside discernible cyclics. We have also repolymerized the products from this process to reform silicones via acid, base, and fluoride catalysis. This process has the potential for large-scale industrial processing because of the use of mild conditions and solvent recycling ability.

Recycling Silicone-Based Materials: An Overview of Methods

“…Recently, Rupasinghe et al explored an efficient room temperature and low mol% (0.5) catalytic fluoride (Bu 4 N + F − ) technique to depolymerize rapidly nearly any siliconebased polymers, elastomers, and resins in the presence of high-swell organic solvents such as Tetrahydrofuran THF (Figure 5) [63]. The main products observed were an equilibrium mixture of D , D 5 , and D 6 verified by 29 Si NMR and Gas chromatography-mass spectrometry (GCMS).…”

“…Even though the disruption of the Si-O bond from one location is enough to initiate degradation in linear siloxane systems, when the siloxane polymers have a Recycling Silicone-Based Materials: An Overview of Methods DOI: http://dx.doi.org/10.5772/intechopen.108051 3D network structure, for degradation, it is imperative to rupture from two or more points [51]. Backbiting reaction mechanisms can explain hydrolysis and catalytic depolymerization reactions [52] and these are preferable in the range of 110-250°C and, in some instances, at room temperature [53]. Additionally, backbiting reactions, radical scission, and calcination reactions can explain thermal depolymerizations.…”

Synthesis and characterization of novel poly(sulfone siloxane)s with good solubility and recyclability based on siloxane units

“…Practical and simple methods of recycling siloxane-containing polymers have been continuously studied. 8,46,47 As shown in Scheme 1c, poly(sulfone siloxane)s were depolymerized using KHSO 4 in THF aqueous solution at room temperature for 24 h. Then, the crude products were purified by column chromatography to collect the initial cyclic monomers, and the depolymerization results were shown in Fig. 7.…”

“…[1][2][3] Considerable studies have been performed on recyclable materials, such as polycarbonates, 4,5 polythioesters, 6 polyamides, 7 and polysiloxanes. 8,9 Aliphatic polysulfones are an interesting class of industrial polymers including sulfonyl groups (-SO 2 -) connected to aliphatic groups in the main chain. Poly(olefin sulfone)s and poly(alkylene sulfone)s are alternating copolymers usually prepared by free-radical copolymerization of sulfur dioxide and olefin monomers.…”

Silicone-recycled pyrolyzed fillers for enhanced thermal - and flame - resistant silicone elastomers

“…The production of silicones has severely increased during the last decades [2] and the end-of-life of these complex composite materials remains an open question nowadays. Usually, most silicones end up as landfill waste [3] where they depolymerize within few days to few weeks [4][5][6] . They breakdown naturally into harmless byproducts such as oligomers containing silanol groups, silicates, CO 2 and H 2 O [4][5][6][7] .…”