Silicone rubber

Silicon rubber is a rubber-like material composed of silicone (a polymer containing silicon, oxygen, and hydrogen) through silicone vulcanization to harden the material. It can be first shaped, then cured, but is frequently injection molded as well.

Silicone rubber is fairly non-reactive, stable, and resistant to fairly extreme temperatures (-55°C to +300°C) without significant loss of its core properties. These properties, ease of synthesis, and ease of shaping make it ideal for use in a large range of everyday products.

Technical Details

Silicone itself comes in many different forms. For example, silicone oil, silicone grease, silicone rubber, and silicone resin are common. The term silicone itself is actually a misnomer dating back to its early discovery where it was believed to have a silicon backbone with oxygen atoms attached as side chains to each silicon. We now know that the material contains an alternating structure where each silicon alternates with an oxygen molecule, forming a long chain (-Si-O-Si-O-Si-). The correct term for referring to these materials is "siloxanes".

The most common silicon is polydimethylsiloxane (PDMS), a form of silicone oil. Branching of silicon oils can determine their overall viscosity, from a thin liquid to a hardened plastic. However, silicone rubber is typically a combination of these methods.

In RTV-1 type vulcanization (the resulting material being commonly referred to as CAFs), the silicone reacts with atmospheric humidity to cure. This tends to result in a Shore A hardness between 18 and 60, with an elongation breaking range between 150% and 700%. These materials have excellent aging and UV resistance. and example of this type of silicone would be an aquarium seal or silicone bathroom sealant.

In RTV-2 silicone vulcanization, two components are mixed and cure at room temperature into a solid elastomer, gel, or flexible foam. Most RTV-2 remains flexible between -80 °C to +250 °C. At above 350 °C, the structure will break down, leaving an inert silica deposit. The material is a good dielectric, making it suitable for electrical insulation. RTV-2 is useful for making molds and technical parts.

Silicone rubber tends to have a very low glass transition temperature (~ -127 °C) Mechanical proerties can vary based on components, but in general, Shore A Hardness ranges from about 10-90, Tensile strength is about 11 N//mm², Elongation at break is about 100–1100%, Maximum temperature is +240 °C to +300 °C, and minimum temperature is -80 °C to -120 °C.

Synthesis

Silicones are crafted from silane precursors, such as chlorosilane. For example, by combining Hydrogen Chloride in the presence of copper of a copper catalyst yields:

<math>2Si + 6HCl → 2HSiCl_3 + 2H_2</math>

Primarily this yields trichlorosilane <math>(HSiCl_3)</math>, but with dichlorosilane <math>(H2SiCl_2)</math> and silicon tetrachloride<math>(SiCl_4)</math> as byproducts.

Using a chlorosilane such as dichlorosilane, the material can react with water to form a polysiloxane, like so:

<math>nSi(CH_3)2Cl_2 + nH_2O → [Si(CH_3)_2O]n + 2nHCl</math>

However this method produces poisonous hydrogen chloride gas as a byproduct. Alternatively, the precursor dichlorosilane(<math>(H2SiCl_2)</math>) could have been produced using the much safer acetic acid (<math>C_2H_4O_2</math>), the primary component of white vinegar (about 3% acetic acid, 97% water). As a result, acetic acid is now the biproduct of the reaction.

Also, using different silane precursors with more acid forming groups such as adding methyltrichlorosilane introduce more branches and crosslinks to the polymer, forming a stiffer product.

Most industrial production of silicone resins is done with Tetraethoxysilane which has less toxic byproducts, and is a more mild and controllable reaction.

To create a silicon rubber, the polymerization reaction will have an additive to facilitate the process. For instance, in RTV-2 silicones, one of the parts tends to contain a platinum catalyst.

Crafting

Silicone rubber can be easily molded and cured and lends well to injection molding. Curing can take place in heated environments, although normal room-temperatures are typically suitable. Silicone rubber can be easily cut, but does not shape or sand well after curing.

RTV-1 Silicone Rubber (CAF) is best used for sealants such as joining panes of glass or plastic.

RTV-2 Silicon rubber typically contains 2 or occasionally more components that require mixing. This Silicone Rubber is better suited for creating silicone molds or forming parts.

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