Silicone rubber, with its exceptional properties including high-temperature performance, durability, electrical insulation, and transparency, is a superior elastomer widely used in multiple sectors. Learn how to meet your end product’s demanding performance specifications!
What is Silicone Rubber?
Its molecular structure is characterized by the following:
- A siloxane backbone (a chain of silicon and oxygen)
- An organic component is attached to the silicon.
These inorganic and organic groups within its structure contribute to the distinctive properties of silicone rubber.
Classification of Silicone Rubbers
They are classified into the following two categories:
1. Classification of Silicone Rubbers based on Organic Group
It includes the following classifications:
- Methyl Group: Also known as dimethylsilicone elastomer/rubber or methyl silicone rubber, sometimes abbreviated as MQ.
- Methyl and Phenyl Groups: Also known as methyl-phenylsilicone elastomer/rubber or phenylsilicone rubber, often referred to as PMQ. It exhibits excellent performance at low temperatures.
- Methyl and Vinyl Groups: Also known as methylvinylsilicone elastomer/rubber, commonly referred to as VMQ.
- Methyl, Phenyl, and Vinyl Groups: Referred to as PVMQ, this type is recognized for its outstanding low-temperature performance.
- Fluoro, Vinyl, and Methyl Groups: Also known as fluorinated rubber or fluorosilicone rubber, it is referred to as FVMQ. These rubbers exhibit high resistance to chemical attacks from fuels, oils, solvents, and more.
2. Classification of Silicone Rubbers based on Molecular Structure
Silicone rubbers are classified based on their molecular structure, viscosity, and processing methods. They are typically available in three main forms:
Solid Silicone Rubber or High-Temperature Vulcanized (HTV)
Silicone solid rubbers consist of high-molecular-weight polymers with long chains. They are uncured and require traditional rubber processing techniques. Heat-curable elastomers, with higher viscosity, are processed similarly to other elastomers. They cure at elevated temperatures using organic peroxides or a platinum catalyst.
Liquid Silicone Rubber (LSR)
Liquid silicone rubbers (LSRs) contain polymers with lower molecular weight, resulting in shorter chains and improved flow properties.
LSRs are low-viscosity and high-purity thermoset elastomers that maintain mechanical properties across a wide temperature range (-50°C to 250°C).
They offer several advantages:
- Excellent optical clarity
- Long-term durability in harsh environments (high temperature, UV, etc.)
- Design freedom
These heat-cured elastomers significantly enhance productivity by reducing cycle time, minimizing material waste, and enabling the use of smaller machines. They are specifically processed using injection molding and extrusion equipment.
It can integrate multiple parts into a single component. This leads to substantial cost reduction. This transparent and innovative material finds applications in various industries, including high-power LED lighting, electronics, automotive lighting, and more.
Room Temperature Vulcanized (RTV)
RTV silicone rubber is available as RTV-1 or RTV-2 systems. It is a type of silicone rubber with a hardness range from very soft to medium. It is commonly used for potting, encapsulation, sealants, and other applications.
Methods to synthesize Silicone Rubbers
The synthesis of silicone rubbers typically involves three steps: preparation of chlorosilanes, hydrolysis, and polymerization resulting in silicone elastomers.
Silicones are commonly produced from chlorosilanes through the Rochow direct process. This involves a reaction in a fluidized bed containing silicon metal powder, where methyl chloride is passed through. The reaction occurs at temperatures of 250 to 350°C and pressures of 1 to 5 bars, utilizing a copper-based catalyst.
The resulting mixture of silanes primarily contains dimethyldichlorosilane (Me2SiCl2) along with other silanes.
Dimethyldichlorosilane, obtained from the mixture, is isolated through distillation. It serves as the monomer for the production of Polydimethylsiloxanes. This is achieved by hydrolyzing dimethyldichlorosilane in the presence of excess water.
The hydrolysis of dimethyldichlorosilane, which is a heterogeneous and exothermic reaction, produces a disilanol compound (Me2Si(OH)2). This disilanol readily undergoes condensation with the presence of HCl acting as a catalyst. As a result, a mixture of linear or cyclic oligomers is obtained through inter- or intramolecular condensation.
For most applications, the linear and cyclic oligomers obtained from the hydrolysis of dimethyldichlorosilane have short chains. To achieve elastomers, these oligomers need to be condensed or polymerized and crosslinked.
Properties of Silicone Rubber
Silicone rubbers possess unique performance properties due to their strong Si-O chemical structure and high bond energy. They offer several advantages which are as follows:
- Wide service temperature range with excellent thermal and thermoxidative resistance (Si-O-Si bond energy is higher than C-C bonds).
- Excellent resistance to oxygen, ozone, and sunlight.
- Resistant to electromagnetic and particle radiation (UV, alpha, beta, and gamma rays).
- Non-stick and non-adhesive properties.
- Low toxicity.
- Flexibility at low temperatures due to low glass transition temperature (Tg).
- Optical transparency.
- Excellent insulation properties.
- Low chemical reactivity.
- High bio-compatibility.
- Outstanding mechanical properties with high tear strength and high elongation.
The properties of silicone rubber can vary significantly based on the molecular structure and organic groups present.
Silicone rubbers can withstand temperatures: -50°C to 350°C. This depends on the duration of exposure. Even when exposed to wind, rain, and UV rays for extended periods, silicone rubber parts experience a minimal change in their physical properties. Additionally, unlike most organic rubbers, silicone rubber is not affected by ozone.
Compared to organic rubber, silicone rubber, with its Si-O bond, exhibits superior performance in terms of:
- Heat resistance
- Chemical stability
- Electrical insulation
- Abrasion resistance
- Ozone resistance
Effect of Additives on Silicone Rubbers
Silicone rubbers typically incorporate additives to achieve high performance for specific applications.
Crosslinking: Silicone rubbers are cured or crosslinked using either peroxide crosslinkers (such as benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, t-butyl perbenzoate, and dicumyl peroxide) or platinum catalysts, resulting in a mechanically stable cured product.
Fillers: Pyrogenic silica is a reinforcing filler with a high BET surface area, while quartz is a non-reinforcing filler. These fillers are used to enhance tear strength or increase conductivity, such as with the addition of carbon black.
Stabilizers: Stabilizers are added to silicone rubber to improve its heat resistance.
Flame Retardants: Flame retardants are employed to enhance the fire resistance of silicone rubbers. These additives can include carbon black, aluminum trihydrate, zinc, platinum, and ceric compounds.
Pigments and Colors: Unlike other rubbers, which are typically black, silicone rubbers offer high transparency, making them easily colorable with pigments to meet specific application requirements.
Difference between Silicon & Silicone
Silicone rubber poses challenges for recycling due to limited opportunities for reuse. Pure silicone rubber is not easily recyclable, although uncontaminated silicone rubber can sometimes be ground and reused. Alternatively, reusing silicone rubber products instead of recycling them can help reduce waste and conserve resources.
Silicone rubbers are generally safe and non-toxic for food contact applications due to their inert nature. They do not react with most chemicals, making them popular for food-grade and medical uses. However, some additives in silicone rubber can be harmful if mishandled. It is important to follow safety guidelines and ensure proper ventilation when working with silicone rubber to minimize potential risks.