Michael A. Brook

Highly reticulated silsequioxanes from sol-gel processes of coupling agents bind waste organic rubber to make resilient composites.

Improved circularity in elastomers and composites requires better opportunities for reusing/repurposing before ultimate disposal, and better degradability once placed in the environment. We report that tannins—commonly used for enhancing fermentation and flavors in wines—are excellent crosslinkers for aminosilicones. Simply mixing a solution of the tannin in alcohol/water with aminosilicones provided an elastomeric composite after evaporation. The physical properties could be tuned simply by varying the quantity of food grade tannin and the molar mass of the telechelic aminosilicones. The products were thermoplastic and could be (re)processed at 140 °C or completely depolymerized by the addition of butylamine to regenerate both the aminosilicone and, after evaporation of butylamine, the tannin, consistent with an enhanced life cycle for these materials. An alternative phenolic filler, lignin, did not lead to composites with useful physical properties.

Covalent grafting of sugars onto silicones imparts viscoelasticity via hydrogen-bonding among sugar moieties.

Silicones underpin an enormous range of simple and advanced technologies. Often, only small quantities of silicone are used to enable a technology such that, on a “per use” basis, one might suppose the environmental impact is low. However, silicone preparation processes have a very high carbon footprint, and billions of kg are produced each year. To provide context to the consideration of new strategies to improve silicone sustainability, we first outline traditional silicone chemistry and then describe strategies to improve the degree to which silicones are green, sustainable and circular. One strategy involves dilution of the silicone oil or elastomer by tethering organic entities, particularly natural products, that may provide new properties including facilitated degradation in nature at end-of-life. A greater focus is given to strategies that permit extensive reuse and repurposing of oils and elastomers (e.g., with thermoplastic elastomers), before the silicone undergoes recycling. Each reuse, repurposing or recycling step reduces the net carbon footprint. These mostly involve straightforward, high-yielding organic chemical processes that work efficiently in a silicone milieu. Silicones will eventually end up in the environment, where linear oils are known to rapidly degrade, particularly when compared to organic polymers. Alternative strategies that permit triggered or biological degradation of oils and, more importantly elastomers, are described, including enzymatic degradation and composting.

Correction for ‘Cleaning steel by devulcanizing rubber from used automotive tires' by Yang Chen et al. , RSC Sustain. , 2023, 1 , 2006–2013, https://doi.org/10.1039/D3SU00218G.

Silicone-protein elastomers form with crosslinking provided by HCHO. The resulting product undergoes enzymatic degradation in 1–2 weeks to give silicone oils and protein fragments.

Increasing demand for fully recyclable polymers has prompted an interest in materials crosslinked via non-covalent interactions. Aggressive depolymerization can cleave backbone SiO bonds to generate linear oils or cyclic monomers from silicone rubbers, but one would rather recover the starting material oils simply by selectively breaking crosslinks. We demonstrate that ligand binding to metals can be used to reversibly crosslink silicone chains. Aminopropylsilicones were crosslinked via complexation with copper (II) acetate to form blue silicone oils. These oils slowly cured in air at room temperature to form soft, green elastomers over a month, or overnight at 50–55 °C in air to give robust, hard red elastomers. Potential explanations for the observed color changes are discussed. Elastomers prepared by either pathway underwent ready degradation by removal of the copper ions via competitive ligand binding using ethylenediamine, allowing for recovery of the unmodified silicone oil; the recovered amine could be reused with more copper to form a (softer) elastomer. This process provides a method for the synthesis of stable elastomeric silicones that are degradable on demand.

In commerce, silicone elastomers are typically cured by platinum-catalyzed hydrosilylation or radical induced crosslinking (high temperature vulcanization (HTV)), both of which lead to crosslinks comprised of 2 or 3 carbon fragments. Alternatively, room temperature vulcanization (RTV) utilizes catalyzed nucleophilic substitution to give elastomers with Si–O–Si crosslinks. At end of life, the best current recycling protocols use aggressive acid/base conditions to regenerate cyclic monomers and mixtures of crosslinkers/fillers, etc. Over longer time periods all three types of elastomers undergo acid-catalyzed depolymerization initiated by HBr derived from bromine. However, we report that oxidative de-crosslinking with bromine of the Pt-cured elastomers occurs in minutes to hours to regenerate long chain silicone polymers, terminated with SiOH groups. It is therefore possible to devulcanize Pt-cured elastomers in the presence of RTV elastomers and reuse the high molar mass materials directly in new RTV elastomers, avoiding complete depolymerization, and thereby increasing the circularity of silicones.

The incorporation of natural amino acids onto a silicone framework is efficiently and simply achieved, without protecting groups or solvents, by using aza-Michael addition to acrylate-modified silicones.