Elastomers and Silicones: Enhancing Degradation at End of Life

Date: November 30, 2023 2:00 pm (ET)


  • Michael Brooks
    McMaster University

Bio: Mike Brook is a Professor of Chemistry and Chemical Biology, Distinguished University Professor at McMaster University in Hamilton, Ontario, Canada and Faculty of Science Chair in Sustainable Silicone Polymers. Brook is an expert in silicon, silica and silicone chemistry. He wrote the solely authored book Silicon in Organic, Organometallic and Polymer Chemistry (Wiley) in 2000 and has published over 300 papers and 6 patents granted. His main interests revolve around the strategies to improve the sustainability of polymers using silicon chemistry. Currently, his research has three main axes: using HSi-containing silicones in the degradation of elastomers, particularly used automobile rubber; incorporation of natural materials in silicones to convey new properties and more facile breakdown at end of life; and developing synthetic strategies to create high molecular weight silicones with explicit structural control and very narrow ranges of properties, so less material is required for a given application.

Brook won the Canadian Institute for Chemistry Award in 2023, Macromolecular Science & Engineering Award from the CIC in 2017, the Frederic Stanley Kipping Award for Silicon Chemistry (ACS) in 2016, was a Canadian Killam Research Fellow and, with Mark McDermott (McMaster University), won the Synergy Prize for Industrial Collaboration with Connaught Laboratories.


Abstract: It is inarguable that there is a problem with the rate of degradation of petroleum-derived polymers; it’s too slow. The problem is normally exacerbated when crosslinks are added to the material making them more robust per se, and harder to break down chemically or mechanically to a size where high surface area and/or biological entities facilitate degradation. A classic example of this is used automobile tires that, at end of life, retain ~85% of their mass but are rarely devulcanized; there is significant concern currently with tire wear debris. Two strategies will be discussed to address enhanced degradation of elastomers: silicone-enabled devulcanization of sulfur-cured organic elastomers and natural material-containing silicone elastomers that possess a weak link to facilitate degradation in the environment.

The S-S bonds arising from vulcanization are readily (100 °C, 1 hour) cleaved using silicones bearing HSi groups including the homopolymer (MeHSiO)n, which is one of the cheapest silicone polymers available. However, it is even more interesting to only partially degrade tire crumb and used the resulting silicone-coated organic rubbers as reinforcing agents in silicone foams and elastomers.

Silicone elastomers are surprisingly resilient to induced stress from high voltage, irradiation with UV light, high temperatures, aggressive biological conditions, etc. While this makes them extremely valuable materials for many applications, it also complicates their degradation. Four strategies will be described that can improve the degradability of silicones once they move into the environment: readily reversible reactions to facilitate degradation; biologically degradable crosslinkers; reactions that do not require catalysts; and avoiding covalent bonds altogether. Examples will be provided which show that H-bonding and ionic crosslinks lead to robust thermoplastic elastomers with unusual properties. Catalyst free reactions including imine forming reactions create underwater sealants. Elastomers formed using thiol/disulfide crosslinks are almost perfectly recycled and silicone/protein elastomers are readily degraded by enzymes.