An explanation of the benefits of silicone rubber gaskets and how they are made with a list of silicone gasket manufacturers.
You will learn:
Silicone rubber gaskets are synthetic rubber elastomers that are widely used due to their resistance to high temperatures, excellent durability, and non-toxicity. The silicone from which the gaskets are made is a flexible material that includes a combination of linear silicone polymers reinforced by carbon, hydrogen, or oxygen mixed with a crosslinker and catalyst. Silicone rubber gaskets can be constantly stretched, compressed, and deformed, after which they return to their original shape.
The variations in viscosity and structural features of silicone rubber gaskets are due to how the straight chain silicone polymer has been combined and its processing temperature. In addition, the reinforcement by various elastomers creates different properties for different applications. The flexibility, tear strength, and resistance to temperature variations by silicone rubber gaskets are determined by the type of reinforcement used during the manufacturing process.
The chemical structure of silicone rubber gaskets differentiates them from other similar materials. Silicone’s unique chemical structure is due to the silicone and oxygen molecules having a high bond energy link. Condensation or crosslinking reactions are used to meet processing and performance requirements, which include mechanical properties, such as elasticity, absorption, and tear strength. The addition of mineral fillers further enhances the properties of silicone rubber gaskets. The inclusion of additives creates specific characteristics and enables silicone rubber gaskets to meet performance standards.
Silicone is a synthetic elastomer that displays viscosity and elasticity, a factor that is described as viscoelasticity. The chemical structure of silicone includes carbon, hydrogen, oxygen, and silicon, which comes from silica found in sand. The process for extracting silicon from silica is complex and time consuming, which is the reason for the high cost of silicone rubber gaskets. The backbone of silicone rubber is silicon-oxygen (Si-O) that gives silicone its high temperature resistance and flexibility.
The process for the manufacture of silicone rubber gaskets includes extracting high performance polymers from raw silicone materials, which are manipulated and configured to obtain the different formulations of silicone rubber. The bases for all silicone products are siloxanes, chains of silicon and oxygen atoms in an alkane compound, which are the building blocks for silicone rubber gaskets.
The first step in the manufacture of silicone is the extraction of silicon from silica, which is found all over the earth and is one of the most common elements in the earth’s crust. Silica is in sand, oil, granite, and rocks and is the main component in quartz. Although it is essential to the manufacture of silicone rubber gaskets, silica sand is the foundational element for the manufacture of glass.
The process for separating silicon from silica involves heating silica at up to 1800°C (3272°F). Oxygen is removed during the heating due the reaction of carbon and oxygen, which forms carbon dioxide and leaves pure silicon at the bottom of the furnace. Oxygen is reintroduced to the pure silicon to eliminate calcium or aluminum that are impurities. The result of the process is metallurgical grade silicon with a 99% level of purity.
The extreme temperature of the process is above the melting point of silicon, 1410°C (2570°F), a factor that causes oxygen to separate from silicon. The melted silicon is drawn from the bottom of the furnace and solidifies into polycrystalline chunks. The carbon dioxide from the process is also drawn off and passes through a scrubber.
The pure silicon crystals are ground into a fine powder with micron or nano sized particles. Although physical grinding is common, chemical grinding is also used. The silicon crystals extracted from the furnace are first subjected to mechanical crushing to decrease their size. After achieving the proper shape, ball mills or other types of grinding equipment grind the particles further until they reach the desired fineness.
The plasma method uses high temperature plasma to vaporize the silicon, after which it is cooled to form an ultra-fine powder. After being heated and evaporated, the silicon condenses into a nano sized powder. The chemical method uses silane or silicon tetrachloride that is decomposed to form pure silicon.
The fine powder is mixed with methyl chloride to form methyl chlorosilanes (CH₃Cl), an essential element for forming silicone substances. Heat applied to the mixture activates the reaction between the components. The chlorosilanes provide a hydrolysis reaction that allows for the controlled formation of the polymer molecular framework. The types of chlorosilanes formed are trimethyl chlorosilane, dimethyl di-chlorosilane and methyl trichlorosilane. Of the many forms of chlorosilane, dimethyldichlorosilane is the most important since it is the primary building block of silicone rubber gaskets.
The chemical structure of dimethyldichlorosilane (CH₃)₂SiCl₂, consists of a silicon atom bonded to two methyl groups and two chlorine atoms. The combination forms a tetrahedral geometry around the silicon atom with the chlorine and methyl positioned at the corners of the tetrahedron.
The process of getting dimethyldichlorosilane to silicon is a complex and intricate process that is carefully controlled and planned. The distillation process separates the components of methyl chlorosilane from one another. The chlorosilanes have different temperatures, which requires heating the mixture slowly in a series of temperature increases. For the process to be successful, the dimethyldichlorosilane has to be of the highest purity, at least 99%. The critical factor, and the most difficult, is the different temperatures required for the process, which takes time and requires extreme accuracy.
The hydrolysis process, also known as methanolysis, involves breaking down the compound from distillation using water. The addition of water with dimethyldichlorosilane causes the separation of hydrochloric acid and disilanol. The hydrochloric acid is a catalyst for the disilanol to condense into polydimethylsiloxane, which has a bond with siloxane, the backbone of silicone.
The hydrolysis process is the most important aspect of the manufacture of silicone. The temperature of the mixture is kept at a specified level and allowed to stand. During this aspect of the process, the mixture stands until silica alcohol and acid water separate into layers. The layer of acid water is discharged and the silica alcohol is washed with water.
Siloxane from hydrolysis goes through polymerization to produce silicone polymers. During polymerization, silicone compounds are reacted with straight chained molecules, cross linking agents, and reinforcement to produce the mechanical properties desired for silicone rubber gaskets, which is a type of silicone made from reactive silicone gums. The reactive groups contained in silicone gums, which are straight chains with very high molecular weight, are combinations of methyl, phenyl, and vinyl.
Several different methods are used for polymerization. The choice of method is determined by the properties required for the silicone rubber gaskets. In the case of rubber, the desired properties are temperature, water, and chemical resistance as well as durability. Also included are compression set, electrical properties, high strength, biocompatibility, and flexibility. The characteristics and properties of silicone rubber gaskets are achieved during the polymerization phase when the cross linking agent is introduced.
The strong bonds between the compounds and the organic side groups work together to give silicone rubber gaskets their wide range of features. The silicone to oxygen structure of silicone rubber gives silicone rubber gaskets their stability, tear resistance, and strength.
The different forms of silicone rubber are divided into organic groups and classes. The distinction between the organic groups is in regard to their chemicals, which are methyl, vinyl, phenyl, and fluoro. They are classified in accordance with their molecular structure, viscosity, and method of being processed. The main forms of the classifications are solid, liquid, and room temperature vulcanized.
| Numeration of Organic Silicone Rubbers | ||
|---|---|---|
| ISO Standard | ASTM D2000 | Chemical Structure |
| MQ | GE | Methyl Silicone |
| PMQ or PVMQ | FC | Phenyl Methyl Silicone |
| VMQ | FE | Vinyl Methyl Silicone |
| FMQ or FVMQ | FK | Fluorosilicone |
Silicone rubbers that are part of the organic group contain methyl, vinyl, phenyl, or combinations of these elements. The American Society for Testing and Materials (ASTM) has developed designations to identify the types of silicone rubber that fit into each of the organic groups.
The methyl group of silicone rubbers was the first type of silicone rubber that was developed and is the simplest form of silicone compound. It is referred to as dimethyl silicone rubber and methyl silicone rubber. MQ rubber has repeating units of oxygen, a factor that gives it resistance to ultraviolet (UV) rays, ozone, and weathering. The limited use of the methyl group of silicone rubber is due to its poor elastomeric properties. In many cases, it is used as a foundation. Over the years, to enhance the performance of methyl silicone rubbers, it has been combined with other chemicals and elements to improve its elasticity.
With MPQ, the methyl groups are replaced with phenyl groups, which enhances the low operating temperature of the rubber. The dramatic change of PMQ allows it to operate at 100°C (212°F) lower than MQ, enabling it to work at temperatures of -100°C (-148°F) or as low as -177°C (286.6°F). The low temperature gradient of PMQ differentiates it from all other forms of silicone rubber. It is known as methyl-phenyl silicone rubber or phenyl silicone rubber. The adjustments to MQ enable it to perform at a wide range of temperatures and give it resistance to weathering, moisture, and several chemicals. These properties of PMQ make it ideal for the manufacture of highly resistant and durable silicone rubber gaskets.
Methyl vinyl silicone rubber is processed by replacing the methyl molecules with vinyl molecules. As with PMQ, the resulting silicone rubber has temperature resistance beyond MQ and an improved compression set. The downside of VMQ is its poor tensile strength, which limits its use as an O-ring. The introduction of vinyl into VMQ helps in the vulcanization of silicone rubber. As with all other forms of silicone rubber, VMQ has a name that has been given to it by ATSM, which is methyl vinyl silicone rubber.
The molecular structure of VMQ is built with a backbone of silicone-oxygen, making it an ideal electrical insulation material. The inertness of VMQ allows it to be used for special functions, such as food processing and medical instruments.
The fluorination of MQ creates stronger bonds at the molecular level for silicone, a factor that improves its chemical resistance. FVMQ or fluorosilicone contains trifluoro propyls next to the methyl groups. This gives FVMQ mechanical and physical properties that are similar to VMQ along with its resistance to fuels, chemical attacks, and mineral oil. The downside of FVMQ is its limited resistance to hot air. In addition, FVMQ is limited to applications that do not include abrasions or dynamic factors.
The primary use of fluorosilicone is in fuel systems that reach temperatures as high as 177°C (350°F) and applications where dry heat resistance is required. The main use of FVMQ is in the petroleum based oils and hydrocarbon fuel industries. In low temperature applications, fluorosilicone forms a tight seal at temperatures as low as -73°C (-100°F). A factor that has to be considered in regard to FVMQ use is its low tear strength, high friction, and extremely limited abrasion resistance. These factors limit the use of FVMQ to static applications.
Included in the understanding of silicone rubbers is a study of their molecular structure, viscosity, and processing methods. The three main categories in this method of classification are solid silicone rubber or high temperature vulcanized (HTV), liquid silicone rubber (LSR), and room temperature vulcanization (RTV).
Solid or high temperature vulcanization silicone rubber is vulcanized at very high temperatures, which causes the rubber to take a solid and fixed shape. The vulcanization process causes the creation of a cross-linked structure between the silicone molecules, a factor that enhances elasticity and creates heat resistance.
HTV is the main type of rubber for the manufacture of high temperature resistant products, such as seals, gaskets, and electric wire insulation. When silicone rubber is enduring the high temperatures, a vulcanizing agent is added, the reaction of which causes the formation of a flexible, heat-resistant silicone structure due to the cross-linking.
High heat vulcanization, aside from adding flexibility and heat resistance, also increases the hardness of silicone rubber. As can be ascertained, the curing process at 150°C up to 200°C (302°F up to 392°F) for HTV is very long due to the high heat of vulcanization. This aspect of the manufacture of HTV increases its production time. In addition, HTV requires an extended cooling period, which is necessary to solidify the rubber. The final steps in the processing are trimming, cutting, surface treatments, and other processes to meet product specifications.
LSR is an inorganic polymer that is formed by silicon (Si), oxygen (O), carbon (C) and hydrogen (H) with the main chemical chain, backbone, being siloxane. During the curing process, platinum and peroxide are added. Platinum cured LSR has excellent tensile strength, tear strength, clarity, and uniform consistency and does not leave a peroxide residue. In LSR, the siloxane bond provides better mechanical performance and exceptional strength.
The inorganic aspects of LSR makes it ideal for applications for the medical field where the material will come in contact with the skin. A key characteristic of LSR is its compression set, which is higher than any other elastomer, a factor that makes LSR ideal for applications that involve gaskets.
Injection molding is used to produce products from LSR. Due to its viscous nature, it processes easily and is ideal for high volume production that requires consistent high quality. The unique properties of LSR makes it the perfect silicone rubber for intricate designs and demanding, critical applications.
Room temperature vulcanized silicone rubber is made from a two-component system and can have a soft or medium texture with a shore hardness between 15 Shore A and 40 Shore. It is a silicone adhesive with a unique mix of properties for creating a gasket like adhesive for gluing surfaces and providing a cushioning effect.
RTV begins to cure as soon as it is exposed to the atmosphere, which makes it ideal for use as a sealing agent because of its water repellent property, adhesiveness, and its ability to retain its shape. During the vulcanization of RTV, water is introduced to the mixture that triggers condensation that forms cross links between the polydimethylsiloxane chains.
Once applied, RTV reacts quickly, as long as the temperature is 24°C (75°F) and forms a tight secure seal in 20 minutes. As the amount of RTV is applied, the time it takes to form a seal increases with the maximum drying time being 72 hours. The exact drying time is dependent on the thickness of the application. After RTV is applied, it continues to strengthen for up to two weeks, ensuring a long-lasting seal.
The main use of RTV is in applications that require high temperature and chemical resistance for the sealing and bonding of equipment and machinery. The properties of RTV make it resistant to engine and gear box fluids with exceptional qualities of adhesion to metal and plastic parts. Since RTV does not oxidize when in contact with metals, it can be used to bond and protect electronic components. It is odorless and non-toxic, which makes it safe for use at workstations and closed environments.
When examining the various positive features of silicone, it is important to understand what it is and how it compares to rubber. Silicone rubber is a synthetic rubber with a silicon backbone that gives it superior heat, UV, and ozone resistance. It comes in different forms, which makes it adaptable to a wide range of applications and processes.
During the manufacturing process, sheets of silicone of differing thicknesses are cut into gaskets. Aside from the variations in thicknesses, the sheets of silicone are made up of different types of silicone to create the various types of silicone gaskets. When clients select silicone gaskets, they examine the gaskets for their sealing abilities, which is a necessity for industrial applications.
Part of choosing a gasket is determining whether an application can be served better by a rubber gasket or a silicone gasket. In the majority of cases, silicone gaskets are more resilient, efficient, and provide better performance than rubber gaskets, especially in extreme high temperatures. The normal temperature range for rubber gaskets is between -50°C and 80°C (-58°F and 176°F) while the temperature range for silicone gaskets is between -70°C and 218.89°C (-94°F and 426°F). Natural rubber wears down at 80°C (176°F) and melts at 120°C (248°F).
A major differentiating factor when deciding between rubber and silicone for gaskets is the cost, since SBR rubber is much less than silicone. However, rubber is not resistant to ozone, strong acids, fats, oils, and grease, materials that silicone easily resists. In addition, silicone gaskets can be used in any type of weather conditions without deteriorating. The strength and high resistance of silicone rubber gaskets enables them to have a much longer service life than rubber gaskets.
Solid silicone gaskets are tightly packed dense materials that are designed for demanding conditions, such as water submersion. They are available in different durometer hardnesses with low durometer gaskets being used for low force applications. Higher durometer gaskets are able to endure more force and are commonly used in metal enclosures. Applications requiring a seal rating of IP67 or higher normally necessitate the use of solid silicone rubber gaskets.
One of the challenges of working with solid silicone gaskets is their amount of compression due to their hardness. They require 15% to 20% compression, which means they will require more closing force to compress while solid silicone gaskets with a lower durometer reading require less force.
Sponge silicone gaskets are made from silicone gum material and have a closed cell foam structure. Their density ranges from 21 lbs. up to 43 lbs. per ft2. Although they are referred to as sponge silicone gaskets, they do not absorb water and are ideal for any form of gasketing and sealing application. As with other types of silicone rubber gaskets, sponge silicone gaskets are waterproof and resistant to the effects of high temperatures. They have been approved by the Food and Drug Administration (FDA) for use with food and are widely used in the pharmaceutical industry, electronics, and outdoor applications. The many benefits of silicone sponge gaskets include a flame rating of MIL-R-764310 Type II, tensile strength of 130 psi, operating temperature between -73.33°C and 260°C (-100°F and 500°F), and resistance to UV, ozone, and solvents.
As with most silicone rubber gaskets, sponge silicone gaskets are precision cut from sheets of silicone sponge. The cutting process is completed with exceptional accuracy to ensure the quality of sponge silicone gaskets. Applications that require cushioning, padding, or a soft gasket use sponge silicone gaskets.
Aside from the open cell version of sponge silicone gaskets, there are closed cell sponge silicone gaskets that are filled with natural gas to prevent air and chemicals from passing through at low pressures.
It would seem that silicone foam gaskets and silicone sponge gaskets would be the same type of gasket. The difference between the two gaskets is in regard to the type of silicone that is used to form the gaskets. Sponge silicone gaskets are made from a silicone gum compound that is mixed and milled with pigment, catalyst, and blowing agent. Foam silicone gaskets are cast from low viscosity liquid silicone that is rapidly catalyzed and mixed before being cast on a smooth plastic release liner. During casting, the foam is placed on a release liner to control the thickness of the gaskets as they pass through a curing oven, after which they are post-cured to remove moisture and vapors. The result of the process is a roll of foam with an extremely smooth surface.
Foam silicone gaskets are made from HT-800 silicone and have a low compression rating, high temperature stability, low temperature flexibility, flame rated at UL 94, FAR, and a sealing rating of UL 50 and UL 508. Various methods are used to form foam silicone gaskets including die cutting, water jet cutting, and roll sitting.
Fluorosilicone gaskets are a unique form of gasket that combines the temperature resistance of silicone with the fuel, oil, and solvent resistance of fluorocarbon. The properties of fluorosilicone vary in accordance with the temperature of the environment with special types being capable of resistance to abrasions and electromagnetic interference (EMI). Fluorosilicone gaskets are also used in place of O-rings and protective boots for electrical equipment.
The chemical composition of fluorosilicone makes it more durable and resistant to chemical degradation than silicone. Unlike silicone, fluorosilicone has poor flexibility and is more expensive. Regardless of this one factor, fluorosilicone has most of the properties of silicone with some of the properties being enhanced with fluorosilicone’s unique chemical composition.
Flame resistant silicone gaskets are a form of solid gasket that have been specially treated to resist burning, slow the spread of flames, and are capable of being functional in a fire. In order for a solid silicone gasket to be classified as flame resistant, it has to pass five fire performance tests, which are burning behavior, smoke generation, toxicity, heat emission, and combustion.
The five silicone gaskets described above are a sampling of the many types of silicone rubber gaskets that producers offer. The other factors that define a silicone gasket include how they are produced, FDA approval, and their level of conductivity. Each of these factors are included in the discussion of the five gaskets above. Silicone gaskets are the most popular form of gasket due to their many positive properties. Their strength, resilience, durability, and long useful life are factors that are measured against their cost.
Of the many types of materials that are used for industrial applications, silicone rubber gaskets are one of the most popular and widely used. The long list of the silicone rubber gasket’s positive properties enables them to be a critical part of machine, equipment, and production applications as well as an essential part of consumer products.
Silicone rubber gaskets are used in the food and medical fields due to their insulation properties, resistance to pressure, and their ability to withstand extremes in temperature. They meet the highest standards for hygiene, durability, and safety, regardless of the application. The key determinant for the use of silicone rubber gaskets in the medical field is their non-toxicity and biocompatibility.
In the automotive industry, silicone gaskets improve engine performance by effectively sealing gearboxes and engines, helping avoid fluid leaks. Silicone rubber gaskets are capable of enduring the high temperatures from engine components, ensuring the safety and effectiveness of vehicles. The result of the stability of silicone rubber gaskets is long engine life and an increased lifetime for vehicles.
The industry that places the greatest amount of strain on gaskets is the aerospace industry. The nature of space and air travel necessitate the use of materials that are highly reliable, resilient, and dependable. On all counts, silicone rubber gaskets step up and fulfill the demand. They are used to supply airtight seals that ensure personnel and aircraft safety as well as protection of vital systems.
HVAC systems rely on all of the positive properties of silicone rubber gaskets. The key factors are reliability, resistance to extreme temperatures, weather, UV, and moisture, a factor that improves the efficiency and useful life of an HVAC system. Compressibility and resilience are necessities for HVAC systems due to the workings of the systems. Silicone rubber gaskets maintain a tight seal regardless of the thermal cycling.
In building and construction, silicone gaskets seal doors, windows, and electrical equipment enclosures. They provide a watertight seal and are able to endure outdoor construction applications due their ability to withstand UV rays, weather, ozone, and wide variations in temperature. When used with electronics, silicone rubber gaskets keep water from entering devices and protect components from heat, corrosion, and EMI.
The term silicone rubber covers a set of materials that breaks into three distinct groups. Each of the types of silicone rubber are distinguished by their chemical composition and their method of being manufactured. This aspect of the nature of silicone rubber necessitates describing the positive features according to their type. Although there are general descriptions of silicone rubber, it is better to examine them in their group, which provides more specific information and assists in selecting the correct silicone rubber for an application.
HTV silicone rubber is referred to as solid silicone rubber due the form it takes after vulcanization, curing, and cooling. It is a highly resilient synthesized rubber that is widely used for the manufacture of gaskets and O-rings.
The versatile properties of LSR come from its siloxane bond, an inorganic backbone that does not interact with biological materials but can be combined with organic chemicals. The siloxane bond enables LSR to have exceptional mechanical performance with high strength.
Pigmentation – After manufacturing, LSR is a translucent white color but can have black or gray pigmentation added. Although these are typical, a wide range of colors are used to highlight LSR for a specific application. The main factor that determines the color of LSR is the application for which it is to be used. In certain cases, LSR may be used to differentiate components, provide aesthetic appeal, or meet industry standards.
As with LSR, RTV is important for use with electronic assemblies as a sealant. It enhances the durability of devices and provides stability for the performance of electronic components. In many cases, RTV is adapted to meet the requirements of an application and the needs of an industry.
RTV is like the other forms of silicone rubber in regard to its ability to perform as electrical insulation. It prevents short circuits, guards against static electricity, and protects electrical devices from all forms of electrical issues. In addition, RTV is easy and convenient to apply. Once applied, it begins to cure immediately with the length of time determined by the amount of RTV that is applied.
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