Silicone rubber in its many forms with a list of high-quality manufacturers.
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Silicone rubber is a synthetic rubber elastomer that is widely used due to its resistance to high temperatures, excellent durability, and non-toxicity. It is a flexible material that is made from a combination of linear silicone polymers that are reinforced by carbon, hydrogen, or oxygen mixed with a crosslinker and catalyst. Silicone rubber can be stretched, compressed, and deformed, after which it returns to its original shape.
The variations in viscosity and structural features of silicone rubber are due to how the straight chain silicone polymer has been combined and its processing temperature. In addition, the reinforcement of the various elastomers creates different properties for different applications. Silicone rubber's flexibility, tear strength, and resistance to temperature variations are determined by the type of reinforcement used during the manufacturing process.
The chemical structure of silicone rubber differentiates it 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 precise 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 as well as the inclusion of additives to create specific characteristics and enable silicone 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. 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 includes extracting high performance polymers silicone raw 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 products.
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, silica sand is the foundational element for the production 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 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 is ground into a fine powder. The grinding process produces 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.
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 is allowed to stand 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, 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 final product. 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 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 its wide range of features. The silicone to oxygen structure of silicone gives it its 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 classification 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 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 push buttons and keyboard keys.
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 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.
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.
Since its introduction in the latter part of the 20th century, silicone rubber has become a critical part of a wide range of industries, which includes commercial and residential products. In all its varied forms, silicone rubber has become the perfect choice for applications that operate at temperatures over 100°C (212°F), which other rubbers cannot withstand. In addition, its performance as electrical insulation is unequaled as well as its ability to endure high steam sterilization.
In the auto industry, silicone rubber is used for sealants, gaskets, connectors, spark plugs, tires, radiators, heat exchangers, and water pumps, to name a few. The strength and durability of silicone rubber ensures longevity and radically reduces part failures. It has become the go to material for automobile production.
Electrical and electronics are industries that rely on the dependability of silicone rubber due to its ability to protect against random electrical charges and provide for protection. Cable terminations, insulators, energy transmission parts, and engine gaskets are all made from silicone rubber. Its stability with temperature fluctuations, aging properties, and resistance to kinks and vacuum collapse make silicone rubber the ideal material.
Silicone rubber is a critical part of the manufacture of medical instruments due to its inertness and biocompatibility. Several forms of instruments are made with silicone rubber, especially those that come in contact with the skin. The list of instruments includes catheters, tubing, drainage, seals, gaskets, o-rings, valves, membranes, respiratory care, anesthetics, masks, cables, textile coatings, and baby bottle nipples. The reliability and safety of silicone rubber has enhanced the performance of personnel and helped relieve suffering.
The list of leisure products made from silicone rubbers is long and unending. It includes toys, containers, masks, athletic equipment, and nearly every type of implement that is used for a wide variety of activities. In the area of deep sea diving, masks and snorkels are made from silicone due to its ability to endure changes in temperature and pressure. The use of silicone has become a boon to the athletic equipment market. One of the factors that has led to the wide use of silicone rubber for leisure devices is its biocompatibility. Regardless of the amount of silicone rubber being used, it does not irritate the skin or cause rashes.
Aside from its use for leisure products, silicone rubber is used to produce a wide assortment of kitchen tools and cookware. Since it is non-toxic, long lasting, and food safe, it is widely used in food preparation from packaging equipment to vending machines. Silicone rubber has been approved by the Food and Drug Administration (FDA) as a safe material for the handling and preparation of food. It is resistant to cooking fats and is used for sterilizing fluids and instruments, making it suitable for a wide range of food applications.
The list of uses for silicone rubber described above is a minimal list of the many places where silicone rubber is used. In essence, silicone rubber can be found in many aspects of life due to its longevity, dependability, and stability. It is a safe material that modern society depends on for its performance.
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