What is Silicone rubber?
Silicone rubber is a rubber-like material composed of silicone—itself a polymer containing silicon together with carbon, hydrogen, and oxygen.
Silicone rubber is generally non-reactive, stable, and resistant to extreme environments and temperatures from -55°C to +300°C while still maintaining its useful properties. Due to these properties and its ease of manufacturing and shaping, silicone rubber can be found in a wide variety of products, including: automotive applications; cooking, baking, and food storage products; apparel such as undergarments, sportswear, and footwear; electronics; medical devices and implants; and in home repair and hardware.
During manufacture heat is required to vulcanize (set or cure) the silicone into its rubber-like form. This is normally carried out in a two stage process at the point of manufacture into the desired shape, and then in a prolonged post-cure process. It can also be injection molded.
Properties
Silicone rubber offers good resistance to extreme temperatures, being able to operate normally from -55°C to +300°C. At the extreme temperatures, the tensile strength, elongation, tear strength and compression set can be far superior to conventional rubbers although still low relative to other materials. Organic rubber has a carbon to carbon backbone which can leave them susceptible to ozone, UV, heat and other ageing factors that silicone rubber can withstand well. This makes it one of the elastomers of choice in many extreme environments.
Compared to other organic rubbers, however, silicone rubber has a very low tensile strength. For this reason, care is needed in designing products to withstand even low imposed loads. Silicone rubber is a highly inert material and does not react with most chemicals. Due to its inertness, it is used in many medical applications and in medical implants.
Structure
Polysiloxanes differ from other polymers in that their backbones consist of Si-O-Si units unlike many other polymers that contain carbon backbones. One interesting characteristic is an extremely low glass transition temperature of about -127˚C (Fitzpatrick 1999:428). Polysiloxane is very flexible due to large bond angles and bond lengths when compared to those found in more basic polymers such as polyethylene. For example, a C-C backbone unit has a bond length of 1.54 Å and a bond angle of 112˚, whereas the siloxane backbone unit Si-O has a bond length of 1.63 Å and a bond angle of 130˚.
The siloxane backbone differs greatly from the basic polyethylene backbone, yielding a much more flexible polymer. Because the bond lengths are longer, they can move further and change conformation easily, making for a flexible material. Another advantage of polysiloxanes is in their stability. Silicon is in the same group (IV) on the periodic table as carbon, but the properties of these elements are quite different. Silicon has the same oxidation state as carbon, but has the ability to use 3d orbitals for bonding by expanding its valence shell. Si-Si bonds have far less energy than C-C bonds and so are more stable, though in practice Si-Si-bonds are very hard to create.
Mechanical properties
| Hardness, shore A |
10–90 |
| Tensile strength |
11 N/mm² |
| Elongation at break |
100–1100% |
| Maximum temperature |
+300°C |
| Minimum temperature |
-120°C |
Liquid Silicone
Liquid silicone rubber is a high purity platinum-cure silicone. Medical device manufacturers favour liquid silicone because of its biocompatibility and excellent part quality. Medical grade silicone combined with a lack of human contact reduces risk of contamination, especially when manufactured in a clean room environment. However, failures of medical implants have occurred due to poor manufacture or poor design.
It is typically supplied in two parts with one of the parts containing the platinum catalyst. These are then automatically mixed with any colours and ingredients which may be required. The mixing produces a very homogeneous material that leads to products that are not only very consistent throughout the part, but also from part to part.
Applications
Once milled and coloured, silicone rubber can be extruded into tubes, strips, solid cord or custom profiles according the size restrictions of the manufacturer. Cord can be joined to make O-rings and extruded profiles can be joined to make seals. Silicone rubber can be moulded into custom shapes and designs.
Becoming more and more common at the consumer level, silicone rubber products can be found in every room of a typical home. From automotive applications; to a large variety of cooking, baking, and food storage products; to apparel, undergarments, sportswear, and footwear; to electronics; to home repair and hardware, and a host of unseen applications.
Non-dyed silicone rubber tape with an iron-oxide additive (making the tape a red-orange colour) is used extensively in aviation and aerospace wiring applications as a splice or wrapping tape due to its non-flammable nature. The iron-oxide additive adds high thermal conductivity but does not change the high electrical insulation property of the silicone rubber. This type of tape self-fuses or amalgamates without any added adhesive.
Self-healing
Recently, silicone rubber formed the matrix of the first autonomic self-healing elastomer.[1] The microcapsule-based material was capable of recovering almost all of the original tear strength. Additionally, this material had improved fatigue properties as evaluated using a torsion-fatigue test.[2]
References
- Keller et al., A Self-Healing Poly(dimethyl siloxane) Elastomer, Advanced Functional Materials, v. 17, p. 2399–2404 (2007).
- Keller et al., Torsion Fatigue Response of Self-Healing Poly(dimethyl siloxane) Elastomers, Polymer, v.49 p. 3136–3145 (2008).
- Brydson, John, Plastics Materials, Butterworth, 9th Ed (1999).
- Lewis, PR, Reynolds, K and Gagg, C Forensic Materials Engineering: Case Studies, CRC Press (2004).
- Primasil Silicones
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