Are you a silicone rubber enthusiast finding a genuine answer to what silicone rubber is? This guide is for you. This Comprehensive guide will educate you on the meaning, types, properties, pros and cons, curing time, and other intriguing things you need to know about silicone rubber.
What Is Silicone Rubber
Silicone rubber is a high-temperature synthetic elastomer used in various industries. It’s a preferred rubber compound for manufacturing products mostly in medical and food industries due to its non-toxicity and flexible properties which are high-temperature performance for durability, extreme heat and chemical resistance, excellent electrical insulating for high voltage application, and transparency.
Silicone rubber withstands temperatures from −55 to 300 °C (−70 to 570 °F), making it generally non-reactive, stable, and resistant to extreme environments. In addition, silicone rubber is made of a unique combination of silicon, oxygen molecules, carbon, and hydrogen. They are versatile, have a host of other desirable attributes, and you’ll find them in different formulations.
Properties Of Silicone Rubber
Chemical Resistance
Silicone rubber is resistant to various chemicals like concentrated sulphuric acid, and hydrofluoric acid, as well as solvents and industrial chemicals, making it ideal for sealing and insulation applications.
Additionally, silicone rubber does not degrade when you expose it to ozone, even when it contacts other corrosive substances like Amine, bromine, hydrogen peroxide, etc. However, when silicone rubber has too much contact with Polar solvents and strong Alkalis like acetone it’ll degrade gradually.
Temperature Resistance
Silicone rubber features tear resistance that makes it well-suited for applications in cold climates and cryogenic environments. They resist low and elevated temperatures from -50°C and 350°C, making them suitable for high-temperature and high-stress environments.
In addition, silicone rubber will not degrade physically even when you expose them to wind, rain, UV rays, etc meaning you can apply them to automotive engines, ovens, and other temperature and industrial applications.
Excellent Electrical Insulation
Silicone rubber stands out based on its excellent electrical insulation. You can apply it to various electronics due to their high dielectric strength, thermal stability, moisture resistance, as well as flexibility, and durability.
Versatility and High Biocompatibility
Silicone rubber features a unique combination of thermal, chemical, and mechanical properties, making it a versatile product you can use. They’re usually utilized to achieve specific coloration, durability, and resistance properties.
You can use them safely with living tissues and there’ll not be adverse repercussions. This means you can use them in the medical and healthcare industries for breast implants, skin contact applications, medical devices, etc.
Flame Retardancy
Flame retardancy property makes it possible for silicone rubber to support less combustion or resist burning due to its excellent chemical composition which consists of silicon-oxygen bonds. It features nonconductive silica which serves as an insulating barrier to resist fire, making it a good fit for extreme temperatures and high-risk applications.
Water Resistance
Silicone rubber resists wind and rain due to its high ductility and elasticity after molding. This is possible because the silicon-oxygen polymer chains which are attached to the methyl groups are nonpolar. Instead of water penetrating the silicone surface, it beads up and then runs off. Therefore, silicone rubber is often used as a raw material for gaskets when producing waterproof sealings and O-rings.
Thermal Conductivity
Moderate thermal conductivity in silicone rubber means conducting heat is possible but at a lower rate. This feature enables the molecular structure to transfer heat from one molecule to the other, making it good for oven mitts.
However, if there is no sufficient heat transfer, the required heat transfer for the actual application can be achieved by adding thermal conductive fillers to the silicon formulation for the increment of the heat transfer. It’s good for applications like electronics, automotive parts, and appliances where heat resistance and stability are required.
Classification And Types Of Silicone Rubber
Silicone rubber may be classified into organic groups and molecular structures. The categorization of silicone rubber based on organic groups is explained below.
Based On Organic Groups
- Methyl Groups: This is the oldest silicone rubber. They go by the abbreviation MQ. You can also call them dimethyl silicone elastomer/rubber or methyl silicone rubber.
- Methyl and Phenyl Groups: They are also referred to as phenyl silicone elastomer/rubber or phenyl silicone rubber. They are a good fit for low temperatures. They are also referred to as PMQ.
- Methyl and Vinyl Groups: You can also call them methyl vinyl silicone elastomer/rubber. It has good comparison traits. Their abbreviation is VMQ. They are commonly used because vinyl helps with vulcanization.
- Methyl Phenyl Vinyl Groups: Also called PVMQ. They are excellent for low-temperature performance.
- Fluoro Vinyl and Methyl Groups: They are also known as fluorinated or fluorosilicone rubber. They go with the abbreviation FVMQ. They resist chemicals like oils and solvents.
Based On Molecular Structure
Classification of molecular structure consists of three main types which are:
Solid Silicone Rubber
Solid silicone rubber is a saturated polymer with a high molecular weight and comparatively lengthy polymer chains. They have excellent electrical insulation, solvent resistance, high-temperature resistance, cold resistance, higher viscosity, etc. They’re Heat-curable elastomers, available in an uncured state usually cured at high temperatures by organic peroxide or platinum catalysts.
Although they are processed and combined similarly to other elastomers, their processing techniques are conventional due to high vulcanization temperatures. Silicone rubber is perfect for making wire insulation, gaskets, seals, and hoses.
Liquid Silicone rubber (LSR)
They have low molecular weight because of their short molecular chains. Some of their properties are long-term durability In harsh environments, highly biocompatible, low viscosity, high-purity thermoset elastomer, and optical clarity, thus giving it stability and chemical resistance for longer term.
In addition, mechanical properties are maintained in a wide range of temperatures from -50°C to 250°C. Extraction, injection molding, and compression molding are ways silicone rubber can be molded. You’ll find them mostly in rubber rollers, drug delivery devices, diving supplies, etc.
Room Temperature Vulcanized (RTV) Silicone Rubber
RTV silicone rubber cures at normal room temperature. Materials like catalysts that originate from tin-based or platinum substances like dibutyltin dilaurate are used to produce RTV silicone rubber.
They come in two types which are one-component products (RTV-1) or mixed from two components (RTV2) usually a base and a curative. Their hardness ranges from very soft to medium, usually from 15 to 40 Shore A, making them a good fit for potting, encapsulations, sealants, etc.
RTV silicone rubber properties include heat resistance, temperatures up to 356°F, flexible, durable, and very easy to work with. They’re a good fit for various scenarios like the automotive industry as an adhesive and sealant for creating gaskets in place, low-temperature over-molding, sealing, lens applications for some optically clear grades, bonding, molds for reproducing, etc.
Types Of Silicone Rubber
White Silicone Rubber
White silicone rubber is the type that’s safe for food and medical purposes. White pigment or fillers are added to the silicone rubber base to get an aesthetic appearance.
This type of silicone rubber features hardness up to 60°±5 rating shore, excellent resistance properties, tensile strength of 7.9 Mpa, elongation at a break of 340%, excellent electrical insulation performance, and resistance to ozone, oxidation, UV light, and weathering.
In addition, white silicone rubber operates at a wide range of temperatures from -60°C to +230°C. Offer good resistance to most chemicals, including oils, solvents, etc making them a good fit for sealing parts in chemical equipment.
They also provide a balance between flexibility and firmness, meaning you can stretch and flex them without them breaking, thus helping them maintain integrity for a longer time. White silicone rubber is perfect for use in aerospace, automotive, as well as food and beverage industries.
Flame Retardant Silicone Rubber
Materials like silicon oil, and plasticizers with other fire retardant additives like halogen compounds, intumescent chemicals, and other substances are utilized in the manufacturing process to enable the slow spread of fire and/or prevent it.
Their features include low smoke, zero halogen, low toxicity grade with excellent fire resistance to resist burning and the spread of flames. Additionally, Flame retardant silicone rubber operates within a temperature range of -70°C to +250°C, making them Ideal for enclosed and confined spaces like transit systems, aircraft, as well as ships and submarines.
They also have a hardness of 68°±5 Shore A, long-lasting performance in harsh conditions, excellent mechanical properties up to 9 Mpa tensile strength, and elongation at a break of 270%, making them a good fit for situations where safety is paramount.
In conclusion, Flame retardant silicone rubber is perfect for industrial applications such as building construction and aerospace. The automotive, aircraft, ship and submarines, rain, and military and defense industries also use flame-retardant silicone rubber to resist fire or high heat.
Electrically Conductive Silicone Rubber
Electrical conductive grade silicone rubber is a premium-grade silicone compounded with carbon, silver, and copper particles that allows the flow of electricity for balance between conductivity and insulation. You will find them in various shapes like O-rings, tapes, sheets, and other shapes for electromagnetic shielding and antistatic rubber.
They ensure durability in outdoor and harsh environments due to their weather-resistant property. Additionally, they have excellent properties such as tensile strength of 700 psi and a break of 200% elongation to maintain integrity under high stress, up to hardness of 70°±5 Shore A, operates within a temperature range of -62°C to +260°C, resistant to weathering, ozone, and UV light, making them suitable for high-temperature sealing applications and defense industries where both electrical conductivity and high-temperature resistance is important.
Lastly, electrically conductive silicone rubbers are compatible with EMI shielding, electrical connectors, and cable insulation where there’s a need for conductive properties.
Sponge Silicone Rubber
Sponge silicone rubber is an outstanding general multipurpose silicone featuring a cellular sponge-like structure. A gas bubble is introduced into the silicone with a soft and resilient material to create a sponge-like structure. They are perfect for sealing, cushioning, vibration dampening, and thermal insulation.
Although they offer minimal water absorption (IP65-66 achievable) you can still use them to create watertight seals to stop water ingress. The material compound is RoHS compliant and It’s approved by the FDA, meaning you can use it for food-contact purposes.
Sponge silicone rubber is lightweight, compressible, has chemical resistance, water and moisture resistance, and excellent flexibility, making it ideal for applications where cushioning, sealing, and thermal insulation are needed.
In addition, they withstand different temperatures from -60°C to +230°C (-76°F to +446°F). You can extrude them in a range of densities between 10 lb/ft and 33 lb/ft. They are useful for renewable energy, gaskets and seals, industrial manufacturing, electronics, construction, and marine industries.
Metal Detectable Silicone Rubber
Metal-detectable silicone rubber helps to identify foreign particles in food and pharmaceutical products. They have outstanding materials which are peroxide-cured metal-detectable additives like iron powder stainless, and aluminum that help them identify the foreign particles.
Metal-detectable outstanding features include UV light, weather resistance, tensile strength of 7.0 N/mm2, 400% elongation, and excellent mechanical properties, making them durable and perform well in harsh environments.
In addition, they have temperature resistance up to -60°C to +200°C, which means they are perfect for high temperatures. Metal-detectable silicone rubber is an excellent solution for food and beverage industries.
Advantages Of Silicone Rubber
Elasticity and Flexibility: They fit perfectly for sealing, tubing, and gasket due to the fact that they can easily stretch or bend without breaking or deforming.
Weathering: Irrespective of how much you expose silicone rubber to harsh environments, it maintains durability and stability due to its resistance to UV radiation, ozone, rain, and wind.
Low manufacturing cost: Silicone rubber manufactured in synthetic compounds requires low manufacturing cost compared to those in extrusions, therefore they require a very low amount to produce. So if you need a small quantity, the overall price for the extrusion is reduced. As you know silicone rubber is durable and resists deformation, so you do not need to change the seal manufactured from the material all the time, thus resulting in reduced cost.
Matching Colors: You can have a silicone rubber in any color that you want. In the production process, colorants will be included to achieve a specific color that matches the seal or RAL color number. This is good for food and drink manufacturing environments or if you need an extrusion that matches your brand’s color.
Safe For Food: Silicone rubber is good for food industries because they have additives that can make them be manufactured into metal detectable. This helps you to easily identify foreign particles. Plus, silicone rubber is FDA-approved.
Disadvantages Of Silicone Rubber
Oil and Fuel Resistance: Silicone rubber cannot withstand oil and fuel for a longer period in some conditions and temperatures. They will swell and deteriorate if you use them often to contact oil and fuel. If you want to have contact with oil and fuel, it’s recommended to use nitrile or Viton.
Premature tear and abrasion resistance: Unlike other elastomers, silicone-manufactured seals are not highly tear resistant. Therefore, silicone rubber is not suitable for instances where tearing and puncturing is possible.
Minimal Adhesion: You cannot easily bond silicone rubber to other materials due to its non stickable surface. Sufficient adhesion can only be achieved through adhesives like silicone rubber or special primers. This means silicone extrusion use is limited to not very strong adhesion applications.
Applications Of Silicone Rubber
Automotive: Silicone rubber is used for producing hoses, gaskets, seals, and other electrical components.
Electronics: They are used to produce insulators, gaskets, and connectors in the electronics industry.
Cooking and Baking: They are used in the cooking and baking industry for baking mats, molds, and spatulas.
Consumer Products: Sportswear, storage containers, electronics, cosmetics, and footwear are produced with silicone rubber.
Medical Products: Implants, tubing, syringe components, and fluid management devices are produced through silicone rubber.
Construction and Aerospace: In construction and aerospace Seals, gaskets, and insulators are produced with silicone rubber.
Curing Processes Of Silicone Rubber
Silicone rubber is present as a highly adhesive gel or liquid in its uncured state. They must pass through the curing process which is curing, vulcanizing, or catalyzing to form a solid silicone rubber. Here’s the breakdown:
Platinum Cure
The platinum cure is also called an additional system due to its key reaction-building polymer. There is the reaction of a hydride and a vinyl-functional siloxane polymer in platinum complex catalysts to create an ethyl bridge between the two.
There are no byproducts when creating bonds between the two silicone molecules. The platinum curing method is considered the fastest for curing all kinds of silicone rubber. However, you should remember that elements like sulfur and tin can inhibit the cure, meaning you should be more careful during this process.
Peroxide Cure
System edit for peroxide cure Silicone rubber can be cured extensively using peroxide curing. Byproducts from the curing process might cause problems in applications involving food contact and medicine.
But these goods are typically processed in a post-cure oven, which significantly lowers the amount of product that breaks down peroxide. The chief breakdown products of dicumyl peroxide, one of the two main organic peroxides employed, are 2-phenyl-2-propanol and acetophenone.
The other is 2,4-dichlorobenzoyl peroxide, which breaks down mostly into 1,3-dichlorobenzene and 2,4-dichlorobenzoic acid.
Condensation Cure
There are one-part and two-part condensation curing techniques. A hydrolysis step is experienced by a cross-linker subjected to ambient humidity, such as water, in a one-part or room-temperature vulcanizing system, leaving it with a silanol or hydroxyl group.
Continued until the system is completely cured, the silanol condenses with another hydrolyzable group on the polymer or cross-linker. This type of system can cure naturally at room temperature and, unlike additional cure systems based on platinum, is not readily hampered by other substances.
However, it can be impacted by certain metals or plastics, and it might not even initiate if it comes into contact with silicone compounds that have already undergone curing.
In condensation cure systems, crosslinkers such as methyl trimethoxy silane for alkoxy-curing systems and methyl triacetoxysilane for acetoxy-curing systems are commonly utilized. To completely cure the RTV system and produce a tack-free surface, an extra condensation catalyst is frequently used.
In alkoxy-cured systems, organic titanate catalysts such as chelated titanates or tetra alkoxy titanates are employed. Oxime and acetoxy-cured systems can benefit from the use of tin catalysts, such as dibutyl tin dilaurate (DBTDL). The cure chemical used in household bathroom caulk, acetoxy tin condensation, is one of the earliest methods for curing silicone rubber.
Depending on the kind, Silicone systems can be categorized as acidic, neutral, or alkaline based on the type of detached molecule. In two-part condensation systems, the polymer and any fillers or pigments are packaged in one part, and the cross-linker and condensation catalyst are packaged in another.
The two components must be mixed for the curing to occur. A common filler used to regulate the sealant’s flow characteristics is called silica, sometimes referred to as pyrogenic silica.
Desiccation systems work well as caulks and sealants when fully cured, and they can also be used as molds to cast low-melting-temperature metals like lead, polyurethane, epoxy, and polyester resins, waxes, and gypsum. Generally speaking, they have a high tear strength and are highly flexible.
Factors To Consider Before You Choose Silicone Rubber
Silicone rubber has excellent properties but it’s only those that are made through the right process and material that can meet your requirements. Nevertheless, below is a list of factors you can consider before you choose a type of silicone rubber.
Cost: Silicone rubber has different grades, types, and specifications each selling for different prices. Their costs are not stable due to their varying excellent properties and use. Therefore consider choosing a type of silicone that aligns with your needs as well as your cost.
Tear resistance: Consider the tear resistance if the silicone rubber will be undergoing harsh mechanical conditions. This is because all silicone rubber might not offer you the same tear resistance. Therefore, you refer to the ‘Types of silicone rubber” section above to make an informed decision.
Flexibility: Choose a type of silicone rubber that offers good bend and twist features if your product needs to be manipulated often.
Customization: Consider choosing flame retardant and metal-detectable silicone rubber as they are the types of silicone rubber that offer customized solutions if you want a customized solution.
Chemical Compatibility: Choose a type of silicone rubber that is most suitable for your type of chemical. As you know different types of silicone rubber offer where you can apply them.
Temperature Range: Consider choosing a type of silicone rubber that’s in complaint with your temperature. For example, RTV silicone rubber resists temperatures up to 356°F, sponge silicone rubber from -60°C to +230°C, etc.
How Silicone Rubber Is Manufactured
Before you learn about how silicone rubber is manufactured, it’s important you first see what materials are utilized and their purpose in the manufacturing process. Here are they:
- Silica Quartz is extracted from quartz sand.
- Methyl chloride for creating polydimethylsiloxane
- curing Agent for the curing process of final properties
- Other additives like pigments, fillers
You can produce silicone rubber through a complex chemical process. What you need is to extract silicon from silica, which is a material that’s generally used.
- Firstly, you need a temperature of 1800°C to heat quartz sand. You can do this by first changing the silica into pure silicon, after which it will react with methyl chloride to produce dimethyldichlorosilane (DMDCS).
- Polydimethylsiloxane (PDMS) chains are then produced through the hydrolyzed DMDCS. You can achieve this by adding water to the DMDCS and mixing it well until you notice the formation of the PDMS chains.
- The next thing is to cross-link the PDMS chains with heat or radiation methods once they are formed. To cross-link PDMS chains, you need to create covalent bonds between adjacent PMDS chains, which is why it’s called a three-dimensional network structure.
- During PDMS chain synthesis, phenyl groups or vinyl group compounds are added to produce silicone, thus making it possible to have various types of silicone with unique properties.
- Lastly, you need to determine if you want the final form to be liquid, elastomer, gel, or rubber. You may need to add fillers or additives to improve hardness, flexibility, or color which are its actual properties.
Methods Of Molding Silicone Rubber
Extraction
The silicone rubber compound is pushed through a die in this method of molding. A stainless steel disc with a specific pattern is the die. Various silicone forms are created in this process.
Silicone rubber is shaped into various profiles like tubes, strips, and solid cords with cross-sections that you create when you are manufacturing through conventional methods. Check the process below:
- Feed the silicone rubber into the extruder.
- Then, put the material into a feed hopper. The silicone begins transforming through this entry point.
- Next, put the rubber compound in another flute of the revolving screw in the extruder. Rotational and forward forces are applied by this component, which helps the silicone to be processed by the machine.
- The entire process warrants the silicone to be forced through the extrusion die to have an advanced design, thus maintaining a desired shape.
- The last step involves putting it inside an oven. Here, the silicone rubber properly cures through the oven’s radiant heat, thus giving it resilience and durability as well as setting its shape. After which you can use the silicone rubber for consumer and industrial applications.
Compression Molding
This process involves first heating the silicone, then getting a big chunk and pressing it into the mold. The material is transformed through the heat obtained from pressing it. Pressing down a sponge for it to soak up heat is what this process looks like, it transforms the sponge giving it a new permanent shape after going through the heating and forming process.
Injection molding
In the injection molding process, your silicone must be malleable. This is achievable when it’s heated. Next, pressure is applied for it to squirt through a nozzle into a mold. The exact shape will be molded when the nooks and crannies of the hot silicone are filled. The rubber hardens into the actual design and is ready for use after it has cooled down.
Step-By-Step Process Of Molding Silicone Rubber
There are stages in molding silicone rubber. They are:
Mixing
This stage involves mixing silicone polymer, fillers, and additives into homogeneous raw material compounds. The desired properties of the end product are what determines the ingredients.
Molding
Ingredients are dispersed accurately kneading the raw material under heat and pressure. The purpose of this is to get uncured plastic material (base compound ) that you can mold into different forms.
Shaping
You can use extraction, or rolling out into sheets to shape the compound base, then use injection or compression processes to mold the compound base into more complex parts. After vulcanization, the shapes will be the final product.
Curing/Vulcanization
In this stage, heat to cross-link polymer chains is generated with a curing agent. You can do this in either High-temperature vulcanization (HTV) or Room-temperature vulcanization (RTV).
In the HTV curing process, parts are heated press many times for vulcanization for the silicone rubber to have high strength to withstand industrial applications.
In the RTV curing process, cross-linking occurs at ambient temperatures with the use of a platinum-based catalyst. In a day (24hrs) RTV can be cured. They are perfect for sealing and bonding.
Finishing
Score your cut and finalize into the end product, consider waxing or coating the surface for the vulcanized parts to have final stabilizing.
How To Maintain Silicone Rubber
- Constant Cleaning: Clean your silicone rubber often with soap and water for a dazzling appearance.
- Avoid Extreme Cold Or Heat: Don’t expose your silicone rubber to direct heat sources for too long to avoid discoloration and reduced flexibility.
- Lubricate When Necessary: Lubricate your silicon rubber often to bypass decreased performance and shortened lifespan.
- Store Properly: Do not store your silicone rubber by bending or folding it to avoid deformation. Instead, store them in a cool and dry place.
- Avoid Sharp objects: Kepl your silicone rubber away from scratch or cut to avoid compromised integrity.
- Avoid Harsh Chemicals: Avoid harsh chemicals like alkaline or acetone because they can degrade your silicone rubber.
FAQs
Are Silicone Rubber Recyclable?
The probability that recycling silicone rubber would work is low because you cannot easily recycle pure silicone rubber. But sometimes, uncontaminated silicone rubber can be grounded and reused. However, instead of recycling silicone rubber, you can reuse them.
Is Silicone Rubber Toxic?
No, silicone rubber is not toxic. In fact, they can have contact with food, and you can use them in medical applications.
Can You Adjust the Silicone Rubber?
Yes, you can. In the process of manufacturing, silicone rubber materials can be adjusted to achieve a certain color.
Conclusion
Silicone rubber is a great product that offers great strength, flexibility, chemical resistance, and good temperature resistance and it can serve you for a longer period of time without deformation if they don’t contact with harsh or hazardous chemicals.
More Resources:
Ultimate Guide To HCR Silicone? – Source: TOGOHK
What Is Fluorosilicone? – Source: TOGOHK
Electric outlook: Is silicone conductive material? – Source: TOGOHK
Understanding UL94V-0 Silicone Rubber for Safety Standards – Source: TOGOHK
Silicone vs Rubber: Which Material is Right for You? – Source: TOGOHK