Silicone rubber is a special synthetic elastomer that can create a unique balance between chemical and mechanical properties and has many industrial applications today.
From its initial creation in the 1940s in the Grignard laboratory to its final commercial form today, the superiority of silicone rubber has been proven in the following detailed properties:
- High temperature stability
- Low temperature flexibility
- Chemical resistance
- Environmental compatibility
- Good electrical performance
Sealing ability
In addition, due to its relative purity and chemical composition, silicone rubber has shown exceptional biocompatibility, which makes it suitable for many care, health and pharmaceutical needs. Also, compared to many organic elastomers, silicone rubber has a high ease of manufacture, which leads to its high productivity and cost-effectiveness for long-term use and reliable service.
It is compatible with the human body and the environment. A good example of this claim is the sealing gaskets and connectors used in the automotive industry, which can have a long life in the face of diverse environmental conditions and extreme temperature changes.
High-temprature Resistance Elastomers
Silicone Rubber Chemistry
In the field of specialty elastomers, the distinguishing feature of silicone rubber is its mineral origin, which is ordinary beach sand. The result is a unique chemistry that, due to the s-o-s linkage, can be the source of a wide range of services. Thermal capability, as well as its exceptional resistance to oxidation and degradation by ozone, represent the fundamental difference between silicone polymers and other organic polymers.
The performance of silicones and their general resistance to corrosive agents such as ozone and the corona electric spectrum, environmental erosion and radiation. This is while many organic polymers that have double bonds in their main chain are subject to ozone and oxidation attack.
The lack of any unsaturation in the silicone main chain makes them exceptionally resistant to environmental degradation. The silicone molecule can be tuned by the chemist to optimize specific properties for specific needs.
Among the several organic side branches of the silicone chain that can be used, methyl, vinyl, phenyl and trichloropropyl are the most common and of high commercial importance.
The incorporation of vinyl branching (generally less than 1 mol%) is greatly enhanced by crosslinking with organic peroxides, resulting in low pressure stability and improved resistance to hot oil. For this reason, almost all silicones on the market today contain some amount of vinyl.
Although dimethyl silicone is flexible at -60 degrees Celsius, partial substitution of the phenyl branch can increase the flexibility performance to -93 degrees Celsius and prevent the strategic linkage of the polymer chains.
Such rubber compounds are used in aerospace applications where low temperature performance is very important.
Finally, the inclusion of the trifluoropropyl side branch gives the silicone a polar nature that ultimately makes the fluorosilicone resistant to fuels and their derivatives.
Like other silicones, fluorosilicone rubber has high and low temperature tolerance and is used against harsh chemicals and the most demanding applications.
More than one side branch in the main silicone chain can provide specific advantages in the same polymer composition.
For example, a polymer containing vinyl and phenyl branches can improve crosslinking and flexibility at low temperatures.
The chemical structure of various types of silicone polymers along with their ASTM D14118 classification is shown in the figure below.
Commercializing Silicone Polymer
Most silicone products, including liquid silicones such as RTV and silicone rubber, are derivatives of the chemical raw materials of silicones, which are then differentiated by changes in viscosity or degree of polymerization.
The process begins with the conversion of sand into elemental silicon metal. The silicon metal is mechanically ground and chemically reacted with methyl chloride at 300 degrees in the presence of a copper catalyst.
This results in the formation of reactive methylchlorosilanes, which are distilled and their derivatives are monochlorosilane, dichlorosilane, and trichlorosilane. The resulting products lead to the formation of reactive methylchlorosilanes, which are separated by a small amount of distillation into the three types of mono, di, and trichlorosilane.
Note that the dichlorosilane species are of great importance for the formation of long linear polymer chains, because they can chemically grow in two dimensions.
Trichlorosilane species form a three-dimensional cross-linking network, although they are important for rigid, non-elastomeric molding resins, but must be separated from the Di stream to prevent polymer gelation.
After distillation, the dimethylchlorosilanes are hydrolyzed to form silanols, which rapidly condense to form cyclic siloxanes and low molecular weight linear siloxanes.
The latter are reacted with caustic soda to produce cyclic siloxanes, particularly dimethyltetramer or D4, which is the starting material for all dimethyl silicone rubbers. It is a clear, low-viscosity liquid.
Ring opening polymerization, D4, is completed by converting the base into a linear polymer whose viscosity is increased by the addition of monofunctional siloxanes whose function is to terminate the polymer molecular chain.
Compounding silicone rubber Termiator(stopper)
Unlike organic polymers, silicone polymers are inherently relatively weak and their tensile strength after crosslinking is only 1 MPa. To obtain useful engineering properties, it is necessary to use very fine reinforcing fillers with a high dispersion surface area that have good chemical compatibility with silicone to reinforce the polymer. Such as fumed silica and precipitated silica. In addition, functional fluids, known as process aids, are required to control sufficient durability and also good processability.
And curing agents are required for the vulcanization of the polymer. Various formulas have been designed for silicone rubber compounds as follows:
Fillers
The most common reinforcing material used in silicone compounds is fumed silica, which is obtained by burning silicon tetrachloride in the presence of hydrogen and oxygen. The silica particles produced are fumed, very fine and spherical, and their surface area can reach up to 325 m2/g.
These particles are amorphous but are linked in filamentary clusters and can be chemically linked to the Si-O chain, giving the polymer desirable reinforcing properties.
Crosslinked silicone compounds containing fumed silica can have tensile strengths higher than 10 MPa, which is ten times stronger than pure silicone.
Precipitated silica, which is obtained by acidifying and precipitating sodium silicate, can also be used as a filler to reinforce silicone rubber, but it has lower strength than fumed silica.
Transparent Silicone
Silicone-based polymers have a unique set of properties that make them well-suited for optical applications. In addition to excellent thermal stability, mechanical properties, and ease of processing, they are transparent in the ultraviolet, visible, and near-infrared bands. Commonly used applications for these types of silicones, which are also commonly used for hygiene purposes, include respirators and swimming goggles.
The company is capable of producing transparent silicone in the 30-70shore A hardness range with a tensile strength of 7-8 Mpa.
High tear strength silicone
Silicone elastomers are used in numerous applications due to their high thermal and chemical stability and flexibility over a wide range of temperatures. However, silicones are highly susceptible to tearing
and have much lower tear strength than other elastomeric polymers such as polyethylene or polypropylene.
It is well known that the tear strength of silicone elastomers can be easily improved by adding fillers such as silica, but the addition of filler also causes significant changes in other mechanical properties such as increased stiffness and reduced elongation, which may not be desirable in all applications.
WireIran is able to significantly increase the tear strength by up to 50 N/mm while maintaining other desirable properties. In the hardness range of 30-70 Shore A.
Fire Ceramic Silicone (Flame Resistant)...
While fires are becoming increasingly complex, causing explosions and greater damage from fires that are difficult to extinguish, fires caused by electrical equipment can cause irreparable financial losses.
Science and technology, high-power electromechanical equipment is increasingly increasing, which can increase the risk of fire. Therefore, flame retardant materials are needed to prevent the spread of fire, and formulas have been designed and manufactured to manufacture fire-resistant silicones that can well meet the need for fire resistance.