Recyclable silicone elastomer achieved by a simple method

Crosslinked elastomers, or rubbers, have a network structure formed by chemical and/or physical crosslinks between the polymer chains. The density and type of crosslinks will determine the characteristic mechanical properties of the system. Recycling rubber materials is very challenging, mainly due to the crosslinks being irreversible chemical bonds, which will prevent reprocessing methods that traditionally are being used for thermoplastic materials. [1] The task is indeed so complex that there is no rubber material on the market today that is fully recyclable into its starting polymers. Recycling techniques commonly being investigated today are pyrolysis, methods using co-rotating twin extruders at different temperatures, and microwave technologies, to mention a few. A major drawback of most techniques investigated today is that they do not result in polymers structurally identical to the starting polymers, which complicates the re-processing and re-using of the recycled material. Further research within this area is therefore vital to introduce circularity into these types of materials and, in the end, to the economy. [2] [3]

In this first work, aiming at developing recycling methods for crosslinked elastomers, a recyclable silicone elastomer is presented. Silicone elastomers are widely used in a wide variety of applications ranging from biomedical to construction machinery due to their chemical and thermal stability, unique mechanical properties as well as biocompatibility. Previous work at the department resulted in a novel silicone elastomer which is proven to be recyclable by a simple method in this work. The concept consists of a hydroxyalkyl modified polydimethylsiloxane (PDMS) crosslinked with a polysilazane. The polysilazane can react with the chain-end hydroxyl groups and form chemical crosslinks in the form of silyl ethers. [4] Silyl ethers are commonly used as protecting groups for alcohols in organic synthesis, and they are generally sensitive to hydrolysis, which is utilised in their deprotection. [5] Interestingly, they can be stable for months as crosslinks in the network of the hydrophobic PDMS-based elastomer, and at the same time, they have mechanical properties comparable to commercial silicones. The crosslinked polymers can return to their original form with hydroxyl end groups by using a method where the silicone network is swelled with a good solvent in the presence of an acid. Swelling of the network allows the acid to access the silyl ethers and hydrolyse them, re-forming the hydroxyl groups, ultimately leading to polymers very similar to the starting materials. The polymers can then be crosslinked again using new polysilazane to form a new silicone elastomer. The results so far in the study demonstrate a highly recyclable elastomer, where the recycled polymers are chemically similar to the original polymers, but further research is needed. Investigations of how the mechanical properties of the elastomer are affected by the recycling are still ongoing.