More precisely called polymerized siloxanes or polysiloxanes, silicones consist of an inorganic silicon–oxygen backbone chain (⋯−Si−O−Si−O−Si−O−⋯) with two organic groups attached to each silicon center. Commonly, the organic groups are methyl. The materials can be cyclic or polymeric. By varying the −Si−O− chain lengths, side groups, and crosslinking, silicones can be synthesized with a wide variety of properties and compositions. Not to be confused with the chemical element silicon.
A general formula for silicones is (R2SiO)x, where R can be any one of a variety of organic groups. Silicones are high-performance polymers that can take a variety of physical forms that may range from solids to water-thin liquids and semi-viscous pastes, Some common forms include silicone fluid, silicone grease, silicone rubber, silicone resin, and silicone caulk.
Manufacture Process
You may have heard that it’s made from sand. That is technically true: Silicone is made from silica, the main constituent of sand. Silica is also known as silicon dioxide, which contains the elements silicon and oxygen.
In fact, Silicone rubber is not “naturally” derived from sand. It is a chemical process, meaning that these products cannot technically be “organic.” Silicones are produced by reacting silicon—one of the earth's most common elements- with methyl chloride and further reaction with water which removes the chlorine atom.
Silicones are essentially organically modified quartz. Silicon quartz consists of silicon and oxygen atoms. Silicones are manufactured from pure silicon which has been obtained by the reduction of silicon dioxide (silica) in the form of sand with carbon at high temperatures:
Silicon is always found in chemically and thermally stable mineral combinations but never in its pure form. Silicon is the key to all silicone chemistry as its atomic structure dictates the properties of silicones. Among them, the production of silicones from silicon takes place in three stages, Respectively :（1）synthesis of chlorosilanes（2）hydrolysis of chlorosilanes（3）condensation polymerization.
Performance
Due to their molecular structure, silicones can be made in a range of forms, including solids, liquids, semi-viscous pastes, greases, resins, rubbers and fluids (oils). The silicone family includes siloxanes and silanes, all of which are widely used in thousands of products and provide essential benefits to key segments of our economy.
Characteristics
Silicones exceptional breadth of chemical and physical properties makes silicones important to achieving innovation in key sectors of the economy. Because they are flexible and resistant to moisture, chemicals, heat, cold and ultraviolet radiation, they also make products more stable, easier to use, more affordable and longer-lasting.
Silicones exhibit many useful characteristics, including:
- Low thermal conductivity.
- Low chemical reactivity.
- Low toxicity.
- Thermal stability (constancy of properties over a wide temperature range of −50 to 250 °C).
- The ability to repel water and form watertight seals.
- Does not stick to many substrates, but adheres very well to others, e.g. glass.
- Does not support microbiological growth.
- Resistance to oxygen, ozone, and ultraviolet (UV) light. This property has led to the widespread use of silicones in the construction industry (e.g. coatings, fire protection, glazing seals) and the automotive industry (external gaskets, external trim).
- Electrical insulation properties. Because silicone can be formulated to be electrically insulative or conductive, it is suitable for a wide range of electrical applications.
- High gas permeability: at room temperature (25 °C), the permeability of silicone rubber for such gases as oxygen is approximately 400 times that of butyl rubber, making silicone useful for medical applications in which increased aeration is desired. Conversely, silicone rubbers cannot be used where gas-tight seals are necessary such as seals for high-pressure gasses or high vacuum.
Silicone can be developed into rubber sheeting, where it has other properties, such as being FDA compliant. This extends the uses of silicone sheeting to industries that demand hygiene, for example, food and beverage and pharmaceutical.
Difference
Silica: When people say silicones are made of sand, they are not incorrect, though that’s too simplistic a description. Silica—or silicon dioxide—is what they are referring to. Silica is the raw material used to make silicone resins. Beach sand is practically pure silica, as is quartz.
Silicon: This is the base element that makes up silica, but silicon is not generally found in nature in this elemental form. It is made by heating silica at very high temperatures with carbon in an industrial furnace.
Silicone (siloxane): The silicon is then reacted with fossil fuel–derived hydrocarbons to create the siloxane monomers (alternating silicon + oxygen atoms) which are bonded together into polymers to form the backbone of the final silicone resin. The quality of these silicones can vary greatly depending on the level of purification done. For example, the silicones used to make computer chips are highly purified.
Benefits & Uses
Silicones impart a number of benefits to the products in which they are used, including enhanced flexibility and moisture, heat, cold and ultraviolet radiation resistance. Silicones have unique properties amongst polymers because of the simultaneous presence of organic groups attached to a chain of inorganic atoms. They are among the world’s most important and adaptable raw materials, used in literally thousands of products and applications – from healthcare, aerospace and personal care, to electronics, transportation, construction and energy.
Personal Care Products: Silicones used in personal care products reduce the white residue and tacky feel of antiperspirants in deodorants. They are also “long-lasting” and help to retain the color and luster associated with cosmetics, shampoos and conditioners, as well as impart better shine, and allow skin care products to be made with stronger SPF. Wetting and spreading qualities provide for smooth and even application of cosmetics, lotions, sunscreens and cleansers.
Energy: Silicone improves the efficiency, durability and performance of solar panels and photovoltaic devices, making them more cost-effective. Because they can withstand the sun for years, silicones are ideal materials for solar panel and photovoltaic applications.
Healthcare: Silicones are also used in a wide range of health care and medical applications. They serve as coatings for hypodermic needles, ensure high oxygen permeability in hydrogel contact lenses, are used in tubing in a wide range of medical devices including insulin pumps, and are particularly suitable in prosthetic devices due to their hypoallergenic properties and a wide range of beneficial physical properties, helping millions of people in their daily lives.
Electronics: Keypads, keyboards and copier rollers are made with sturdy, durable silicones – as are many components of computers, mobile electronics and home entertainment equipment. Silicones also play an essential role in enabling LED lighting technology. Silicones high thermal stability and excellent dielectric properties allow for use in a variety of electrical transmission applications.
Aviation: Because silicones can withstand stress and temperature extremes, silicone adhesives and sealants are used to seal and protect doors, windows, wings, fuel tanks, hydraulic switches, overhead bins, wing edges, landing gear electrical devices, vent ducts, engine gaskets, electrical wires and black boxes.
Construction and Architecture: Silicones are key to construction and renovation of commercial and residential buildings – from enabling glass walled skyscrapers to enabling energy efficient architecture. At home, silicone sealants and caulks are used to reduce energy usage and prevent damage from moisture and bacteria build-up.
Kitchenware: The flexible, non-stick surface of silicone bakeware and cookware is easy to clean and does not impart flavor or odor to food. Cake pans, muffin molds, and baking mats can go from the freezer to the oven, microwave or dishwasher without affecting food taste or quality.
Paints and Coatings: Newer silicone-enhanced paints keep the exterior coatings of houses, bridges and railway cars flexible so they withstand freeze and thaw cycles without cracking. Silicone coatings on highway, oil rig and road surfaces are less likely to corrode due to exposure to oils, gasoline, salt spray and acid rain.
Sporting Goods and Apparel: Silicones seal out water from goggles and diving masks. Silicones enable new techniques to design sportswear that is lightweight, durable, water repellent and high performing, while allowing the fabric to maintain “breathability.”
The most widely used silicones are those which have methyl groups along the backbone. Properties such as solubility in organic solvents, water-repellence and flexibility can be altered by substituting other organic groups for the methyl groups. For example, silicones with phenyl groups are more flexible polymers than those with methyl groups. They are also better lubricants and are superior solvents for organic compounds.
Safety and environmental considerations
Silicone compounds are pervasive in the environment. Particular silicone compounds, cyclic siloxanes D4 and D5, are air and water pollutants and have negative health effects on test animals. They are used in various personal care products. The European Chemicals Agency found that "D4 is a persistent, bioaccumulative and toxic (PBT) substance and D5 is a very persistent, very bioaccumulative (vPvB) substance". Other silicones biodegrade readily, a process that is accelerated by a variety of catalysts, including clays. Cyclic silicones have been shown to involve the occurrence of silanols during biodegradation in mammals. The resulting silanediols and silanetriols are capable of inhibiting hydrolytic enzymes such as thermolysin, acetycholinesterase, however, the doses required for inhibition are by orders of magnitude higher than the ones resulting from the accumulated exposure to consumer products containing Cyclomethicone.
At around 200 °C in oxygen-containing atmosphere, PDMS releases traces of formaldehyde (but less than other common materials such as polyethylene). At 200 °C, silicones were found to have lower formaldehyde generation than mineral oil and plastics (less than 3 to 48 µg CH2O/(g·hr) for a high consistency silicone rubber, versus around 400 µg CH2O/(g·hr) for plastics and mineral oil). By 250 °C, copious amounts of formaldehyde have been found to be produced for all silicones (1,200 to 4,600 µg CH2O/(g·hr)).
