Silicones possess a number of desirable mechanical and chemical properties, including their unmatched flexibility at low temperatures, high thermal stability and chemical resistance, and excellent biocompatibility1. As a result, silicones are used in a wide variety of applications from simple sealants to implantable medical devices. However, one weakness of silicones has always been their low tear and tensile strengths, which for unfilled silicone elastomers is usually less than 10 N/mm and 3 MPa, respectively, at 25 °C.
The use of fillers, typically fumed silica, to strengthen silicone elastomers is a traditional technique for increasing the tear and tensile strengths of silicones.1 Consequently, a wide variety of silicone elastomers with tear strengths ranging from less than 10 N/mm to 50 N/mm are commercially available, with hardness ranging from 20-75 on the Shore A scale. Nonetheless,despite decades of study, there are still several gaps in the materials that are being sold commercially or have been published in the open literature. One challenge is that while for materials with hardnesses in the middle of the aforementioned range (approximately 50-65 Shore A), a number of silicones with > 45 N/mm tear strengths exist, both above 65 hardness and below 50 hardness few or no silicones with extremely high tear can be found. Another problem with currently available materials is that the high filler loadings used to provide good tear strength (typically 20-40% by weight) result in elastomers with much higher moduli than the unfilled silicones from which they are made. Depending on the application, higher modulus may or may not be desirable. In particular for a number of medical applications, softer, more flexible materials are preferred.
The production of high tear silicone rubbers is not a simple phenomenon,requiring precise control over the structure of the base silicone resin2-6, filler structure and surface functionalization6, and crosslinker structure and density8-10, as well as thorough and repeatable mixing protocols that insure an intimate blend of filler and base elastomer7. Additionally, a number of additives is required to catalyze, inhibit, plasticize, or stabilize the resin, all of which in turn can affect the desired mechanical properties. In this report, we discuss a method to produce silicones with high tear strength (> 45 N/mm) across the entire hardness range from 30-70 Shore A, as well as possessing lower modulus and higher elongation at any given hardness than is currently available commercially.