Silicone rubber, scientifically known as polysiloxane is an elastomer made up of silicon, carbon, hydrogen, and oxygen. It is strong enough to survive the demands of jet engines while being healthy to use for baby bottle teats.
Silicone rubber is a high-performance polymer manufactured from silicon metal and silica powder. It was first created to insulate electric motors and generators.
DIFFERENT TYPES OF SILICONE RUBBER
Silicone rubbers can contain methyl, vinyl, phenyl or other organic groups. Silicone rubbers are graded as follows by the ASTM D1418 standard which covers a scheme of general classification or nomenclature for rubber and rubber lattices
- Dimethyl Silicone elastomer or simply methyl silicone rubber belongs to the Methyl Group. MQ also refers to it.
- Methyl and phenyl groups are also known as methyl phenyl silicone elastomer or phenyl silicone rubber. It’s known as PMQ and it’s recognized for its low-temperature efficiency.
- Methyl vinyl Silicone elastomer is also known as VMQ
- Groups of Methyl, Phenyl, and Vinyl – It’s also known as PVMQ and is well-known for its low-temperature efficiency
- Fluoro, Vinyl, and Methyl Groups – Fluorinated rubber, also known as Fluorosilicone rubber. It’s known as FVMQ and it’s highly resistant to chemical attachment (fuel, tar, solvent, etc.)
Aside from its molecular structure, the viscosity and manufacturing system used to identify silicone rubber are also important considerations. Silicone rubber comes in three different types:
- Strong Silicone Rubber is also known as HTV (High-Temperature Vulcanized) is made up of polymers with a high molecular weight and long polymer chains. They are available in an uncured condition and must be treated using conventional rubber processing methods.
- Liquid Silicone Rubber or LSR is made up of polymers with a lower molecular weight and, as a result, shorter chains. It has improved flow characteristics. It’s made with injection molding and extrusion equipment that’s been designed specifically for it.
- RTV Silicone rubber is a type of silicone rubber that is made from one-part (RTV-1) or two-component (RTV-2) systems and has a soft to medium hardness range. They can be used for potting, encapsulation, and sealants, among other things.
Over a broad temperature range from -50°C to 250°C, Liquid Silicone Rubber retains mechanical properties. The optical clarity, longevity, and design freedom of this heat-cured elastomer are exceptional.
This cutting-edge transparent material has a wide variety of uses, including high-power LED lighting, electronics, automotive lighting, and many more.
WHAT IS SILICONE RUBBER MADE FROM?
Silicone is a compound that consists of silicon, oxygen, hydrogen, and carbon. It’s a man-made substance with viscoelasticity, which means it’s viscous and elastic at the same time.
Since silicone polymer is both man-made and has elastic properties, it is classified as a synthetic elastomer. Silicones are molecules made of siloxane repeating units.
Siloxane is a molecule made of an alternating chain of silicon-oxygen atoms, as well as hydrogen and carbon.
MANUFACTURING OF SILICONE RUBBER
Silicone rubber can be manufactured using a variety of methods:
- Silicone rubber may be formed using injection molding, compression molding, or transfer molding techniques. Injection molding is used to make LSR.
- Silicone rubber extrusion and coextrusion is a continuous method for making wires, profiles, and tubes. For various processes, silicones are sold with various curing systems.
- Calendering for silicone rubber: Calendering is a method of producing uniform silicone and textile sheets. Silicones come in a variety of formulations to suit your needs.
CHEMICAL STRUCTURE OF SILICONE RUBBER
Silicones are made up of an inorganic silicon-oxygen backbone chain with two organic groups attached to each silicon core and are also known as polymerized siloxanes or polysiloxanes.
Methyl groups are the most common organic groups. Cyclic or polymeric materials may be used. Silicones can be synthesized with a variety of properties and compositions by changing the lengths of the SiO chains, side groups, and crosslinking.
They can be liquid, gel, rubber, or hard plastic in texture. Linear polydimethylsiloxane, silicone oil is the most widely used siloxane.
CURING METHODS FOR SILICONE RUBBER:
Silicone rubber is a highly sticky gel or liquid in its uncured state. It must be cured or vulcanized to become solid. This is usually done in two stages: at the point of manufacture to achieve the desired form and then in a lengthy post-cure process.
Injection molding is another choice. Platinum catalyzed cure system, a condensation cure system, a peroxide cure system or an oxime cure system may all be used to cure silicone rubber.
Heat or pressure may be used to speed up the curing process in the platinum-catalyzed cure system.
1. PLATINUM BASED CURING
A hydride and a vinyl functional siloxane polymer react in the presence of a platinum complex catalyst in a platinum-based silicone cure system, also known as an additional system because the primary reaction building polymer is an addition reaction, forming an ethyl bridge between the two.
There are no byproducts of the reaction. The rate of cure or even the ability to cure such silicone rubbers is easily inhibited in the presence of elemental tin, sulfur, and many amine compounds.
2. CONDENSATION CURE SYSTEM
One part or two-part condensation curing systems are available. A cross-linker exposed to ambient humidity undergoes a hydrolysis step and is left with a hydroxyl or silanol group in one part or RTV method. The silanol begins to condense with another hydrolyzable group on the polymer or crosslinker until the system is fully cured.
Such a system can cure on its own at room temperatures and unlike the platinum-based system will not be easily blocked by interaction with other chemicals but it can be hampered by contact with certain plastics or metals and may not occur at all if it comes into contact with silicone compounds that have already been cured.
Alkoxy, acetoxy, ester, epoxy or oxime silanes such as methyl trimethoxy silane for alkoxy curing systems and methyl triethoxysilane for acetoxy curing systems are commonly used as crosslinkers in condensation cure systems.
To completely cure the RTV system and achieve a tack-free surface, an additional condensation catalyst is frequently added. In alkoxy cured systems, organo titanate catalysts such as tetra alkoxy titanates or chelated titanates are used.
In oxime and acetoxy cured systems, tin catalysts such as dibutyltin dilaurate can be used. Acetoxy tin condensation is one of the oldest cure chemistries for silicone rubber and it’s what you’ll find in bathroom caulk.
Silicone systems may be classified as acidic, neutral or alkaline depending on the form of a disconnected molecule. The cross-linker and condensation catalyst are packaged together in one part, while the polymer and any fillers or pigments are packaged in the second part.
The curing takes place when the two pieces are mixed. Condensation systems can be used as sealants and caulk in plumbing and construction, as well as molds for casting polyurethane, epoxy, and polyester resins, waxes, gypsum, and low melting temperature metals like lead until fully cured.
They usually have a high tear strength and are very flexible. Since silicones are non-stick, they don’t need the use of a release agent.
3.PEROXIDE CURE SYSTEM
Silicone rubber is sometimes cured with peroxide. Byproducts of the curing process can cause problems in food contact and medical applications.
These products on the other hand are normally processed in a post-cure oven, which significantly decreases the peroxide breakdown product content.
Dicumyl peroxide is one of the two major organic peroxides used and its main breakdown products are acetophenone and phenyl 2 propanols. Dichlorobenzoyl peroxide is the other with dichlorobenzoic acid and dichlorobenzene as the main breakdown products.
WHY CHOOSE LIQUID SILICONE RUBBER?
LSR has many benefits depending on the application or industrial use. To begin with, it is extremely biocompatible, meaning it can be used in sections that come into contact with humans such as organ parts or prostheses.
Second, it is extremely long-lasting ensuring long-term chemical tolerance and stability. Third, it can withstand a wide range of temperatures from -60° to 250°C, while retaining its high mechanical performance.
Fourth, its electrical properties make it suitable for conductivity safety and insulation. Finally, it is transparent and pigments can fit any color requirement, including human skin tones.
LSR is also the material of choice for industries such as automotive and medical devices, which require small and complex elastomeric parts to be manufactured at high speed and with maximum productivity.
Liquid Injection Molding of LSRs becomes one of the most effective processes for fabricators in such situations.
PROPERTIES OF SILICONE RUBBER
Temperature tolerance and durability are the most important mechanical properties of silicone rubber. Depending on the variant, this material can withstand temperatures ranging from -150 to 550 degrees Fahrenheit before becoming brittle or melting.
It has a tensile strength of 200 to 1500 PSI and a gross elongation of 700 percent, both of which are remarkable for rubber.
It also has good rebound and compression properties as heat and flame resistance act as an electrical insulator, and bond well with metals. Because of its Ultra Violet, ozone, and general weather resistance, silicone rubber is ideal for use outdoors.
It’s also hypoallergenic and water-resistant while remaining gas permeable making it ideal for medical use. It’s used in consumer and industrial food and beverage applications because it’s more chemically inert than most plastics, nonstick, and doesn’t stain.
Silicone rubber in its sponge shape is lightweight and reduces friction and noise making it ideal for use in transportation.
Also Check:- What is Silicone Melting Point? [With Temperature]
APPLICATIONS AND USES OF SILICONE RUBBER
- For insulating tape, sealant, varnish, lubricants, keyboards, and housings, electronics applications use some of the most highly purified silicone rubber. Insulators and arrestors made of high thickness rubber can be injection molded.
- Because of its weathering properties and ability to adhere to metal, silicone rubber is used in construction as adhesives, sealants, and coatings.
- Spacesuit fabrics, tooling components, seals and gaskets and other products used in aircraft production and servicing are all made of aerospace silicone rubber.
- Vibration and weather-resistant components, as well as coatings and varnishes, are used in automotive applications but only the most heat, oil, and fuel resistant forms are used.
- Anti stick bakeware, food containers, utensils, toys, and jewelry, such as silicone rubber band bracelets are all examples of consumer goods that use this material. It can however be used in shampoos, laundry soaps, cosmetics, and deodorants.
ADVANTAGES AND DISADVANTAGES OF SILICONE RUBBER
The heat tolerance of silicone rubber is one of its key advantages. It has excellent temperature tolerance with compounds able to withstand temperatures of up to 305 degrees Celsius.
Silicone’s thermal stability ensures that its structure and properties are maintained over a large temperature range. This applies to low temperatures as well, even at -100°, this rubber retains its versatility.
Have you ever wondered why silicone is used in so many kitchen utensils? That’s because another benefit of silicone rubber is that it’s non-porous.
This makes silicone immune to bacterial growth making it ideal for food and beverage applications where food-grade compounds can provide complete peace of mind. Silicone is non-toxic and recyclable in specialized facilities
The low tear power of silicone rubber is one of its main drawbacks. This can be enhanced with the addition of other ingredients but it lacks natural strength and abrasion resistance, so the application’s suitability for silicone as a solution must be carefully evaluated.
Another drawback of silicone elastomers is that, as a high-end product, they can be expensive when opposed to other rubber products. However, increases in uptime and quality management often outweigh this initial investment.
FREQUENTLY ASKED QUESTIONS
1. WHAT IS THE MELTING POINT OF SILICONE RUBBER?
Other elastomers tend to melt at high temperatures but silicone has no melting point and stays solid until it is combusted.
Silicone rubber can gradually lose its mechanical properties and become brittle at high temperatures of 200-450 degrees celsius.
2. CAN YOU LASER CUT SILICONE RUBBER?
Yes, Silicone rubber is an excellent laser cutting and engraving material. Without the use of chemicals, intricate art and text can be laser etched to make rubber stamps quickly and easily.
3. IS SILICONE RUBBER SAFE?
Yes, Silicone is safe. The low toxicity, low chemical reactivity, and low thermal conductivity of this flexible material are only a few of its advantages.
It also contains no toxic fumes and is non-biodegradable making it recyclable though sadly, many recycling centers still refuse to allow it.
We hope that now we have covered the fundamentals of silicone rubbers, such as forms, properties, applications and uses, you’ll be able to source more effectively in your work.
Silicone rubber is only one of the certain types of rubber used in today’s industry. Check out our Types of Rubber and Their Properties article for more detail or follow us on Instagram to stay up to date