Silicone Rubber is one of a very few classes of elastomers with high molecular weight that is considered high-performance. Unlike ordinary (carbon based) polymers e.g. Polyethylenes or Polypropylene's, it is based upon a skeleton that consists primarily of silicon-oxygen (Si-O) linkage. This provides silicone rubber with both excellent heat resistance and the ability to be processed easily like organic materials. It is this "semi-organic, semi-inorganic" chemical structure that gives silicone rubber properties required to solve several exceptionally difficult problems facing the aerospace, medical device, and new energy industries.
Structural Advantages at the Molecular Level
Silicone rubber's superior performance characteristics are a direct result of its unique molecular structure, with a polymeric backbone containing alternating atoms of silicon and oxygen; specifically, the polymer's repeating unit includes both Si and O (i.e. -Si-O-Si-).
This type of structure has an exceptionally high bond dissociation energy (BDE), at approximately 451 kJ/mol, which is much higher than the BDE of the C-C bonds typical of other types of synthetic rubbers (348 kJ/mol). A very important property of silicone rubber is its high bond energy, which provides the material with thermal stability and chemical inertness (due to the fact that significant energy is needed to break the chemical bonds). Silicone rubber also has a high level of flexibility due to the low energy required to rotate the organic side groups (i.e. methyl and vinyl groups) attached to each silicon atom at the ends of the main chain. This flexibility is what gives silicone rubber its remarkable elasticity and gas permeability. In addition, by modifying the organic side groups of silicone rubber (for example, by adding phenyl or trifluoropropyl side groups), a materials scientist can tailor the performance of the rubber with respect to certain conditions, such as resistance to low temperatures, ablation, and solvents.
Retention of Physical Properties Across a Broad Temperature Window
The reliability of a material when subjected to extreme environmental conditions is generally regarded as the most important factor in determining its worth in engineering applications. Silicone rubber is recognized as having the greatest advantage due to its extremely broad range of operating temperatures.
Most other organic rubbers will undergo oxidative degradation when exposed to high-temperature environments, causing the rubber to become either tacky or harden; while upon cooling, they will experience a rapid decrease in temperature and reach their glass transition temperature (Tg), resulting in brittle failure and loss of elasticity. Silicone rubber maintains nearly constant levels of critical physical properties such as tensile strength, elongation at break, and hardness over the temperature range of -60°C to +200°C. Because of its ability to maintain consistently high levels of physical properties under extreme thermal shock, silicone rubber is the preferred choice for use as seals in automotive engines, seals for aircraft fuselages, and equipment developed for use in polar expeditions.
Dual Assurance: Insulation and Weatherability
The durability of a material when exposed to extreme environmental conditions is often considered the most critical factor in measuring the value of a material for engineering purposes; therefore, silicone rubber is widely regarded as having superior properties due to the large range of temperature at which it can be used.
Organic Rubbers tend to experience oxidative breakdown when heated, and they either become tacky or get harder, after which they cool quickly because they reach their glass transition (Tg) and will become brittle and lose their elasticity. In contrast, silicone rubber will maintain a nearly constant level of tensile strength, elongation and hardness from -60°C to +200°C. As a result of silicone rubber’s ability to preserve high levels of tensile strength and elongation during thermal shock, silicone rubber has been the seal of choice in automotive engines, aircraft fuselages and equipment constructed for use in polar regions.
Biocompatibility and Medical Compliance
In terms of material requirements, Medical devices are subjected to the highest level of stringency, which is Biocompatibility. Silicone rubber has very low surface energy, high chemical stability, and as a result, is very rarely coagulated or rejected by the human immune system when it comes into contact with human tissue.
The use of medical-grade silicone rubber that is purified and manufactured according to the same international standards as USP Class VI and ISO 10993 provides medical professionals and patients with confidence in the product’s performance. Additionally, silicone rubber can withstand being sterilized via a variety of methods, including autoclaving/high temperature steam, (EtO) ethylene oxide, and radiation, allowing for medical instrument reuse. Medical-grade silicone rubber has established itself as a base material in modern medicine, ranging from long-term implantable components for artificial organs to disposable catheters and drug delivery systems.
Conclusion
Silicon rubber has demonstrated through many applications, starting with the molecular design stage, that it's not simply an alternative material, but also a required material to create superior designed engineering products. Due to the increased usage of liquid silicone rubber (LSR) injection molding technology, together with the advances in creating functionalized composites (i.e., thermally conductive and electrically conductive), silicon rubber is transitioning into newer market segments (5G communication and electric vehicle thermal management) and flexible electronics and thus continually pushing the envelope of advanced manufacturing capabilities.