Making Silicone Chemistry More Sustainable!

Green chemistry is a becoming one of the most popular and relevant branches of chemistry, and this article highlights some of the amazing green research done at McMaster. Dr Brook is a professor who helped found the Sustainable Chemistry program, and his work focuses on increasing the sustainability of silicone chemistry.

Silicon is an element that has many purposes and is commonly ingrained into our daily lives. It is the primary component of silicone, a polymer that is often found in adhesives and coatings. Fundamentally, silicone is a polymer composed of repeating siloxane bonds (Figure 1) which form between a silicon atom and an oxygen atom. In this article, we’ll take a look at how the Brook Lab aims to make silicone more sustainable.

Figure 1: A siloxane bond is characterized by a silicon atom bound to an oxygen atom

Materials containing silicone have unique mechanical properties due to their inherent elasticity. These properties allow them to be used in many energy-saving applications, such as in the rubber of car tires. Additionally, since silicone oils can be environmentally degraded using merely water, carbon dioxide, and sand (a natural source of silicones), they are a prime candidate to be used in sustainable applications. However, what some fail to consider is that creating silicones from sand is a highly energetic process. The reduction of bonds in order to synthesize repeating siloxane units often requires the use of fossil fuels or other energy sources, thus, leading to the production of greenhouse gases. Simply said, the more siloxane bonds present, the more energy silicone creation requires. Evidently, this counteracts the intended sustainable use of silicones, and this gives way to the Brook Lab where they aim to remove this costly process and apply green chemistry principles to silicone creation.

Simple silicones containing repeating units of only silicon-oxygen bonds, however, this leads to silicon density that is greater than needed to fulfil its purpose and increases the amount of energy needed to form a single polymer. But what if there was a renewable compound that could space out siloxane bonds and reduce the energy needed? The compound that Chen et al. investigated was starch. This compound is common to many of us, being present in foods such as potatoes; however, it is considerably less present in chemistry labs.

What they found was that starch was able to successfully mesh into the network of siloxane bonds (Figure 2). This spaces out the high-energy bonds and therefore decreases their density and the overall amount of energy required to form the polymer. This then presents a considerable replacement option to what is used in traditional silicones, but it poses the question: do starch-silicone composites maintain the properties that make traditional silicones so useful?

Figure 2: The reaction scheme shows silicone (A) being combined with starch (B) to create starch-silicone polymers (C)

After lengthy research, Chen et al. found promising results. They discovered that starch-silicones replicate many of the sought-after elastic properties of traditional silicones, although simply to a lesser extent. They also found that it was possible to manipulate the physical properties of the silicone, altering it from a rigid elastomer to a soft gel, depending on the concentration of starch used. Seemingly, this alternative method has some potential.

Upon analysis, it is apparent that while imposing lower-quality, starch can replace up to 75% of silicones present while maintaining essential elastic properties. This introduces a fantastic lower-energy alternative to applications that only require a mild elasticity. It is evident that the Brook lab was successful in creating a more sustainable silicone that can replace certain instances of traditional silicone.

This approach is still juvenile, but further research may extend their applicability and allow them to replace even more of the silicones present in our everyday lives.

The finding of this work has been published in Sustainability: Chen, Y.; Valentini, D. A.; Brook, M. A. Starch/Silicone Elastomers and Foams. Sustainability. 2023, 15 (13), 9941. https://doi.org/10.3390/su15139941.