In recent years, silicone materials have become an increasingly popular synthetic material in various applications, from medical devices to consumer products. Generally, some common forms of silicone materials include silicone oil, grease, rubber, resin, and caulk. Today, we mainly talk about silicone rubber. Silicone rubber is a synthetic elastomer made from silicon, oxygen, carbon, and hydrogen and has a stable structure resistant to heat, chemicals, and extreme weather conditions. Its excellent properties make it a top choice for different applications.
This article provides a basic guide about silicone rubber and discusses its definition, types, applications, etc. You will learn more about silicone rubber and some of this compound’s notable advantages and disadvantages.
What is Silicone Rubber?
Silicone rubber is an elastomeric material made from silicone polymers containing silicon, oxygen, hydrogen, and carbon. It has a rubber-like elasticity but retains some of the silicones’ extreme heat and chemical-resistant properties. Reinforcing fillers are frequently added to improve properties like tear strength, elongation, and compression set resistance.
Silicone rubber is typically non-reactive, stable, and resistant to severe environments and temperatures ranging from −55 to 300 °C (−70 to 570 °F) while retaining its beneficial properties. Because of its useful properties and ease of production and forming, silicone rubber can be found in many products, including voltage line insulators, food storage products, apparel, electronic devices, medical devices, implants, silicone sealants, etc.
It should be noted that silicone rubber is usually a highly adhesive gel or liquid in its uncured form. Therefore, it must be cured, vulcanized, or catalyzed to become a solid. This is often done in a two-stage procedure at the fabrication time into the required shape, followed by a lengthy post-cure phase. Generally, silicone rubber can be cured using a platinum-catalyzed cure system, a condensation cure system, a peroxide cure system, or an oxime cure system. And the platinum-catalyzed cure system’s curing process can be sped up by applying heat or pressure.
A Brief History of Silicone Rubber
Here is a brief history of silicone rubber:
The mid-1900s – Silicone rubbers were first developed in the mid-1900s by chemical companies like Dow Corning and General Electric. They were discovered as an outgrowth of early silicone research conducted in the first half of the 20th century.
The 1940s – The first silicone rubbers were introduced in the 1940s. During World War II, silicones were critical for applications like insulation, seals, and cushions in aircraft and submarines. This spurred an early interest in silicone materials and investment in new formulations.
The 1950s – In the 1950s, improvements were made to simplify processing silicones with better physical properties for consumer applications. Room-temperature vulcanized (RTV) silicones were introduced, eliminating the need for high temperatures during production.
The 1960s and 1970s – The 1960s and 1970s saw rapid growth and product diversification of the silicone market. Liquid injection molding and extrusion methods were developed to produce complex parts for automotive and electrical uses. Medical-grade silicones enabled major healthcare innovations like implantable devices.
The 1980s – Global expansion accelerated in the 1980s, especially in Asia and Latin America. Silicone rubber became widely used worldwide for automotive parts, cookware products, adhesives, sealants, and gaskets.
The 1990s and 2000s – In the 1990s and 2000s, liquid silicone and high-consistency rubber materials were introduced for specialized applications like electronics, medical devices, and high-performance seals. However, rising environmental concerns also led to more silicone manufacturing and compound regulations.
Today, silicone rubber enables emerging technologies like alternative energy, electric vehicles, and advanced materials. While silicones have been criticized on sustainability grounds, the industry responds with “greener” chemistries and improved recycling capabilities.
How is Silicone Rubber Made?
Silicone rubber is an elastomer of cross-linked polymer chains containing silicon and oxygen. The basic steps in silicone rubber manufacturing include the following:
Step 1: Mixing
Silicone polymers, fillers, and additives are mixed into a homogenous raw material compound. The specific ingredients are customized based on the desired properties of the end product.
Step 2: Compounding
The raw material is kneaded under heat and pressure to disperse all ingredients adequately. This produces an uncured plastic material called a base compound that can be molded into different forms.
Step 3: Shaping
The base compound is shaped by extrusion (for hose, tubing, etc.), rolling out into sheets, or molding into more complex parts using injection or compression molding processes. These shapes will become the final product after vulcanization.
Step 4: Vulcanization
The shaped material undergoes vulcanization, which uses a curing agent that generates heat to cross-link polymer chains. This may be done at either high temperatures (in a process called High-Temperature Vulcanization or HTV) or at room temperature (called Room Temperature Vulcanization or RTV).
In HTV, parts are vulcanized in a heated press for several minutes. This produces high-strength silicone rubber best for industrial applications. RTV uses a platinum-based catalyst allowing cross-linking to occur at ambient temperatures. RTV silicones can cure relatively quickly, within 24 hours, and are suitable for sealing and bonding.
Step 5: Post-treatment
The vulcanized parts sometimes undergo final stabilizing or surface treatments like waxing or coating before being cut and finished into end products. These post-treatments improve durability and appearance.
Types of Silicone Rubber
Regarding the types of silicone rubber, we can take a category according to its organic groups and molecular structure. Let us take a closer look at them.
Classified Based on Organic Groups
The organic groups of silicone rubbers may contain methyl, vinyl, phenyl, etc. When silicone rubbers are classified based on the organic groups, the categorization comprises the following terms:
Methyl Group – Methyl Group is also called dimethylsilicone elastomer/rubber or methyl silicone rubber. It goes under the name MQ. They are the basic silicone rubbers that were first developed.
Methyl and Phenyl Groups – Methyl and Phenyl Groups are also called methyl-phenylsilicone elastomer/rubber or phenylsilicone rubber. It goes under the name PMQ. It performs excellently at low temperatures.
Methyl and Vinyl Groups – Methyl and Vinyl Groups are also known as methylvinylsilicone elastomer/rubber. It goes under the name VMQ and features good compression traits. This type of silicone rubber is the most commonly used because the vinyl helps with vulcanization.
Methyl, Phenyl, and Vinyl Groups – Methyl, Phenyl, and Vinyl Groups are also called PVMQ. It is also well-known for its good low-temperature performance.
Fluoro, Vinyl, and Methyl Groups – Fluoro, Vinyl, and Methyl Groups are also called fluorinated or fluorosilicone rubber. FVMQ also refers to it. They are more chemically resistant to oil, fuel, and solvent.
Classified Based on Molecular Structure
When silicone rubbers are classified based on their molecular structure, they are available in three main types: high-temperature vulcanized (HTV) silicone rubber, room-temperature vulcanized (RTV) silicone rubber, and liquid silicone rubber (LSR).
Solid Silicone Rubber or High-Temperature Vulcanized, HTV
HTV silicone rubbers comprise polymers with a high molecular weight. They possess comparatively lengthy polymer chains and are available in an uncured state. Usually, they require conventional processing techniques for rubber.
Heat-curable elastomers are rubbers with a higher viscosity. They are processed and combined similarly to other elastomers. Organic peroxides or platinum catalysts cure these substances at elevated temperatures.
Solid silicone rubbers are used for high-performance parts like gaskets, seals, hoses, and wire insulation. Especially suitable for automotive and aerospace applications. However, it’s more difficult to process due to high vulcanization temperatures and the need for high-pressure molding equipment. So it is limited to simple part shapes.
Room Temperature Vulcanized, RTV
This type of silicone rubber is vulcanized at room temperature using a platinum catalyst, and no heating is required. RTV silicone rubber is a kind of silicone rubber that is made from one-part (RTV-1) or two-component (RTV-2) systems. Their hardness varies from very soft to medium. Generally, room-temperature vulcanized silicone rubber is easier to process since it can be molded or extruded using simple equipment and cast into more intricate shapes. They are offered for potting, encapsulating, adhesives, sealants, conformal coatings, low-temperature gaskets, industrial and consumer industries, and other applications.
Liquid Silicone Rubber, LSR
Liquid silicone rubber contains the shortest molecular chains of the three types, resulting in low molecular weight polymers. Consequently, specialized equipment may be used to extrude or injection mold it. Furthermore, this silicone rubber is more weather resistant, transparent, and has better flow properties.
LSR is a low-viscosity and high-purity thermoset elastomer. It can perform injection molding at 150-210°C to produce high-precision parts with complex geometries impossible with other silicones. This heat-cured elastomer offers excellent optical clarity and long-term durability in extreme environments. LSR highly boosts productivity by decreasing cycle time, reducing material waste, and using smaller machines. It can be used for consumer electronics, medical devices, automotive sensors, and food contact applications.
Advantages and Disadvantages of Silicone Rubber
While the performance benefits of silicone rubber are unmatched for high heat and chemically demanding applications, its disadvantages drive ongoing research into improved and more sustainable material options. Here are the main advantages and disadvantages of silicone rubber:
Exceptional Temperature Resistance and Flexibility
Temperature resistance and flexibility are two of silicone’s excellent properties. Based on the variant, this material may withstand temperatures ranging from -150 to 550 ℉ before becoming brittle or melting. It has a tensile strength of 200 to 1500 PSI and a maximum elongation of 700%.
Low Compression Set
Due to low-stress relaxation, silicone rubber retains its original shape when compressed for long periods. Upon release, it will return to nearly 100% of normal dimensions with minimal permanent deformation. This is important for seal and closure applications where memory is critical.
Excellent Weatherability and UV Resistance
Silicone rubber is highly resistant to damage from exposure to sunlight, ozone, and weather. It can last 30-50 years outdoors without breaking down. The silicon-oxygen bonds are too stable to be easily broken by UV radiation. Some surface dulling or chalking may occur but will not impair physical properties.
Silicone rubber is unaffected by exposure to most chemicals and solvents, including acids, bases, oils, and greases. Only strong alkalis or polar solvents like acetone may cause swelling or damage over long-term contact. The non-polar silicon-oxygen bonds and stable polymer structure provide excellent chemical resistance.
Silicone rubber is an effective electrical insulator and does not readily conduct heat or electricity. Due to the lack of free electrons in the silicon-oxygen atomic bonds, it has high electrical resistivity.
Certain silicones are biologically inert and compatible with medical devices and healthcare applications. Platinum-cured silicones that meet USP Class VI and ISO 10993 standards are commonly used for products like catheters, tubing, and bandages that contact the body.
Poor Oil and Tear Resistance
In general, silicone is not oil resistant for lengthy periods and can swell after prolonged contact with oil or petroleum. In addition, silicone is neither abrasion nor tear resistant and becomes stiffer at higher temperatures.
Due to its low surface energy and non-polar nature, silicone rubber does not adhere or bond well to most surfaces. Special primers and surface treatments are required to achieve adequate adhesion for bonding or coatings.
Difficult to Process
Silicone rubber can be challenging to manufacture compared to organic rubbers. It often requires specialized high-temperature equipment and molds and technical expertise to handle. Recycling of silicone rubber is also problematic. These factors contribute to its overall higher cost.
Applications of Silicone Rubber
Silicone rubber is an exceptionally versatile material that serves many critical functions across various industries due to its ability to operate in extreme environments with minimal deterioration. Some of the major applications of silicone rubber include:
Silicone rubber is an excellent insulator used extensively for wire and cable insulation, connector seals, and switch boots. The high-temperature resistance and weather ability allow it to insulate electronics in extreme conditions that would cause other materials to melt or degrade. Silicone insulation remains flexible, waterproof, and resistant to arcing and cracking to protect sensitive components.
Silicone hoses, gaskets, seals, and other parts are common under the hood and throughout vehicles. As radiator, heater, and vacuum hoses, silicone rubber lasts the vehicle’s lifetime and resists engine heat, oils, and fuels that would cause ordinary rubber to fail. Silicone seals and gaskets also provide reliable, long-term sealing of fluid and air systems in harsh automotive environments.
Ovens, stovetops, microwaves, refrigerators, dishwashers, and other appliances utilize silicone door and lid seals, tubing, pumps, valves, and wiring. Silicone is ideally suited to withstand appliance operating temperatures. It puts up with constant opening and closing and exposure to water, chemicals, and grease over many cycles with minimal change in properties.
Space and Aviation
Silicone rubber and coatings are essential for components and parts in space and aviation, where extreme temperatures, radiation, and chemical exposure are routine requirements. Their unparalleled high performance makes them uniquely suited for these demanding applications.
Silicone rubber is chemically inert, biocompatible, and sterilizable. It is well-suited for tubing, catheters, wound care products, orthopedic pads, and other healthcare applications that contact or even enter the body. Its flexibility, clarity, and lubricity also provide comfort and visibility benefits. Implanted silicone parts have remained intact for over 40 years without irritation or rejection by the body’s tissues.
Adhesives and Sealants
Silicone adhesives, sealants, and caulks create waterproof, weather-resistant, and flexible bonds for tile, glass, concrete, and plastics applications. They adhere well even when applied over damp or greasy surfaces where other sealants fail. Silicone can accommodate significant joint movement without losing adhesion or cracking for long-lasting protection from water damage. It is popular in bathrooms, kitchens, exteriors, and pools/spas.
Silicone coatings provide durable, non-stick, protective finishes resistant to high temperatures, chemicals, and weathering. Use in place of conventional paints or powder coats, silicone coatings prevent sticking, corrosion, and abrasion on surfaces like molds, ovens, fabrics, ceramics, and composites. Some add friction or abrasion resistance, and others improve release or hydrophobicity. Room temperature and high-temperature cure systems are available.
Silicone release agents prevent other materials from bonding to molds during high-temperature processing. Applied as sprays, pastes, or coatings, they create an easy-release surface so parts can be quickly de-molded without damage. Unlike powder or wax-based releases, silicone provides multiple releases from a single application and does not transfer or contaminate parts. It produces plastics, composites, rubber, and metal parts.
Silicone oils and greases are used as lubricants where high temperatures or harsh chemicals are present. They can withstand environments that would break down most organic lubricants.
Silicone rubber is used in consumer products like non-stick bakeware, food containers, cutlery, toys, jewelry, and silicone rubber band bracelets. In addition, it can also be an ingredient in laundry soap, cosmetics, shampoo, and deodorant.
Silicone rubber is a high-performance elastomer with an uncommon combination of properties covering high-temperature performance, durability, electrical insulation, transparency, etc. Because of these properties, silicone rubber has become a high-performance material in various industries.
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Silicone rubber – From Wikipedia
Silicone Rubber: Complete Guide on Highly Durable Elastomer – From SpecialChem
Silicone is sometimes mistaken for silicon. However, the two are not the same. Silicon is a chemical element. Silicone is a compound that comprises silicon, carbon, hydrogen, oxygen, and other atoms.
No, silicone is not plastic. Plastic and silicone are durable and malleable materials with a similar appearance and feel. While the two are quite similar, their differing chemical and molecular composition distinguish them.
Plastics contain a molecular backbone created with carbon and hydrogen. Silicone is an extensive class of polymers comprising a siloxane bond (chemical formula -Si-O-Si-) with numerous organic compounds.
Silicone rubber recycling possibilities are limited. Pure silicone rubber is not readily recyclable. Uncontaminated silicone rubber can be ground and reused on occasion. Another approach is to reuse silicone rubber products rather than recycle them. As a result, waste is reduced, and resources are conserved.