Silicone: Uses, Types, and Materials
Learn more about this material's composition, uses, properties, and common types.
A silicone (or polysiloxane) is a polymer made up of siloxane (−R2Si−O−SiR2−, where R = organic group). They are typically colorless oils or rubber-like substances. Silicones are used in sealants, cooking utensils, adhesives, lubricants, medicine, and thermal and electrical insulation. Some common forms include silicone oil, grease, rubber, resin, and caulk.
The history of silicone begins with F. S. Kipping creating the term silicone in 1901 to describe the combination of polydiphenylsiloxane, Ph2SiO (Ph denoting phenyl, C6H5), by analogy with the formula of the ketone benzophenone, Ph2CO. His term was originally silicoketone. The scientist knew that polydiphenylsiloxane is polymeric, whereas benzophenone is monomeric and noted the contrasting properties of Ph2SiO and Ph2CO. However, discovering the structural differences between Kipping's molecules and the ketones means that silicone is no longer the correct term, although it remains in common usage. Siloxane is the preferred term used in chemistry today.
Silicone is often confused with silicon, but they are two distinct substances. Silicon is a chemical element. Silicone is a compound that contains silicon, carbon, hydrogen, oxygen, and other kinds of atoms. This article explains what silicone is, the composition of silicone, how it is made, what silicone is used for, the properties and types, and whether or not silicone is safe for the environment.
The composition of silicone is an inorganic silicon–oxygen backbone chain with two organic groups attached to each silicon center. Typically, the organic group includes methyl. The materials can be cyclic or polymeric. By varying the −Si−O− chain lengths, side groups, and crosslinking, silicones can be synthesized with various properties and compositions.
Silicones can vary in consistency from liquid to rubber to gel to hard plastic. The most common siloxane is linear polydimethylsiloxane (PDMS), which is silicone oil.
Silicone is made by extracting silicon from silica and passing it through hydrocarbons. The material is then mixed with the other chemicals to create silicone. Silicone rubber combines an inorganic Si-O backbone with organic functional groups attached. The silicon-oxygen bond gives silicone its high-temperature resistance and flexibility over a wide range of temperatures.
The silicone polymer is mixed with reinforcing fillers and processing aids to form a stiff gum, which can then be crosslinked at elevated temperature using either peroxides or polyaddition curing. Once crosslinked, the silicone becomes a solid, elastomeric material.
Silicone is used for many different applications, including automotive, aerospace, coatings, construction, cookware, defoaming, lubricants, dry cleaning, electronics, firestops, mold making, jewelry, medicine and cosmetic surgery, toys, and hobbies.
Electronics applications include some of the most highly purified silicone rubber for insulating tape, sealant, varnish, lubricants, keyboards, and housings. In addition, high-thickness rubber can be injection molded into insulators and arrestors.
Aerospace silicone rubber is used in spacesuit fabrics, tooling materials, seals, gaskets, and other products used in aircraft manufacturing and maintenance.
Construction uses silicone rubber for adhesives, sealants, and coatings, thanks to its weathering properties and ability to bond to metal.
Automotive applications include vibration and weather-resistant parts and coatings and varnishes, but only the most heat, oil, and fuel resistant types are used for these purposes. Medical silicone rubber can be found in tubing, adhesives, and defoamers.
Consumer products that use this material include anti-stick bakeware, food containers, utensils, toys, jewelry, and silicone rubber band bracelets. However, it can also be an ingredient in shampoo, laundry soap, cosmetics, and deodorant.
The properties of silicone include temperature resistance and flexibility. Depending on the variant, this material can handle temperatures between -150 degrees and 550 degrees F before turning brittle or melting. Its tensile strength is between 200 and 1500 PSI, and its maximum elongation is 700%.
Silicone rebounds and compresses fairly well, is heat and flame resistant and an electrical insulator, and bonds well with metals. Silicone rubber works well outdoors, thanks to its UV, ozone, and general weather resistance. It’s also hypoallergenic and water-resistant while staying permeable to gases, making it popular in medical applications.
Because silicone is more chemically inert than most plastics, is nonstick, and doesn’t stain, it can be found in consumer and industrial food and beverage applications.
However, this material also has some drawbacks. For example, silicone is not oil resistant for long periods, and it can swell after long periods of contact with oil or petroleum, although there are more oil-resistant varieties. Silicone is also not abrasion or tear-resistant and can get stiffer at higher temperatures.
Silicone rubber has several variants with traits that differ depending on the application for which it’s used.
Methyl groups go under the name MQ. These are the basic silicone rubbers that were first developed.
Methyl vinyl groups, which feature good compression traits, are called VMQ. These are the most commonly used types of silicone rubber since the vinyl helps with vulcanization.
Methyl phenyl (MPQ) and methyl phenyl vinyl (PVMQ) groups increase the rubber’s capabilities in low temperatures.
Flouro, vinyl, and methyl (FVMQ) groups are more chemically resistant to oil, fuel, and solvent.
Among the families of silicone rubber variations, there are three primary forms of silicone rubber, depending on the processing method: liquid, room temperature vulcanized, and high temperature vulcanized.
Liquid silicone rubber has the smallest molecular chains out of the three types, making its polymers have a low molecular weight. As a result, it can be extruded or injection molded with specialized machinery. In addition, this type of silicone rubber is more weather resistant and transparent.
RTV (room temperature vulcanized) silicone rubber can be from one or a mixture of two components and hardens at room temperature to soft or medium hardness. It’s generally used for potting, sealant, and encapsulations. Silicone rubber molds are also made from RTV silicone rubber.
Solid or HTV (high temperature vulcanized) silicone rubber has long-chained polymers with the heaviest molecular weights. This type usually comes in an unprocessed form and must be vulcanized at the same temperature as more traditional types of rubber.
For more information, see our full article on the Types of Silicone.
Is silicone rubber or plastic? Scientifically, silicone lies in a gray area but is generally considered a rubber. It's a human-made elastomer with both plastic and rubber properties. Silicone can be used to form liquids, hard solids, and rubbery products, but since elastomers are generally referred to as rubbers, it's also usually referred to as rubber. Silicone is also easily confused with silicon, though they are different materials. Silicon is an element, while silicone is a polymer (a large molecule with repeating units).
Silicone is safer for the environment than plastic. It is made from silica derived from sand, and manufacturing silicone does not involve mining for crude oil, from which most plastics are made. It is also more durable than most plastic, so it isn't single-use like many plastics. Silicone is recyclable, but unfortunately, it is not biodegradable. However, it does not break down into micro-plastics, making it ocean-friendly and less harmful than plastic.
This article reviewed the properties, uses, composition, and different types of silicone.
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