Silicone is one element that is very abundant and is top second on Earth, which is 25.8% on the earth’s surface. Most people know the function of metal silicone. Silicon semiconductor materials have boosted the development of the modern electronic industry. In addition, silicate-based inorganic silicon compounds are widely existing in nature, which is very convenient to use. Over thousands of years, people use silicone-related materials to produce cement, glass, ceramics, and other products.
Inorganic silicone compounds have been applied very early to produce products such as glass and ceramics. But organic silicone compounds do not exist in nature. It is mainly by synthesis and was synthesized around 50 years ago. Since industrialization in the 1940s, organic silicone compounds have developed rapidly.
Organic silicon, also known as silicone or siloxane, is a siloxane organic polymer cross-linked from silicon and oxygen. It has good characteristics with excellent heat resistance, cold resistance, oxidation resistance, and electrical insulation, which are not available from other general organic polymers. Among the organic silicon compounds, Poly siloxane has wide applications with its special structural characteristics.
Silicone materials are mainly divided into silicone oil (Dimethicone/PDMS/ Dimethyl Silicone Fluid, silicone defoamer, silicone leveling agent, polyether-modified silicone oil), silicone rubber, silicone resin, and silane coupling agent, etc. And there are various silicone material-related products. For example, Dow Corning has more than 4000 kinds of types of silicone materials. If converted into Poly siloxane, the total global consumption of various silicone products is about 650,000 tons, which accounts for 0.65% of the global total output of different synthetic resin products. In addition, the sales amount of silicone products is as high as $6.5 billion, which accounts for 7% of the global total sales amount of different synthetic resin products.
Silicone can be widely used in the production of silicone fluid, agriculture silicone adjuvant, advanced lubricants, adhesives, dielectric oil, adhesives, paint, anti-foaming agents, gasket, seals as well as spare parts for rockets and missiles. In recent years, silicone material has been applied gradually from military or national defense to people’s daily life. Products for daily application include conductive buttons for computers, mobile phones, and electrical keyboards; contact lenses, swimming glasses, and swimming caps; nipple; silicone sealants for curtainwalls; finishing agent for high-end leather or fabric; silicone oil for shampoo. Silicone material has become a necessary part of people’s daily life. It also becomes a new chemical material with development on its rise.
With the prospects of silicone material, many developed countries put silicone materials as one of the most important new materials for the key development of the new century. Organic silicon is new material itself, meanwhile, it is also the new material foundation for the development of related industrial areas. Given the changing nature of organic silicon material and with the characteristics of small volume wide application, it is reputed as the catalyst for scientific and technological development. In the silicone industry, only a few upstream enterprises are on a large scale and most of them are small and medium-sized private enterprises engaged in the production of consumer products and additives.
Main silicone products and their applications
Silicone materials are mainly grouped into four categories: silicone oil with its secondary processing products, silicone rubber, silicone resin, and silane coupling agent. It is known as industrial MSG as silicone products have the properties of electrical insulation, corrosion resistance, radiation resistance, flame resistance, high and low-temperature resistance as well as physiological inertia. They are widely applied in building materials, electronic and electrical products, light industry, textile, plastics, rubber, machinery, transportation, medical and other industries. At present, the global annual production capacity of silicone products has reached more than 1.2 million tons. There are around 5,000 to 10,000 products, and the total market sales can reach US $7 billion.
Among the more than 10,000 kinds of silicone products can be roughly divided into three categories: raw materials, intermediates, and products. Silicone monomer refers to organic silicon polymer monomers synthesized by organic chlorosilane, such as raw materials like benzene chlorosilane, methyl chlorosilane, vinyl dichlorosilane, and some other materials.
Organic silicon intermediates refer to line-type or ring-type siloxane oligomers, such as octamethyl cyclotetra siloxane (D4), hexamethyldisilane (MM), dimethyl cyclosilane mixture (DMC), etc. Silicone products are the products produced by the polymerization reaction with add of inorganic fillers or modified additives. It mainly includes silicone rubber (high-temperature vulcanized silicone rubber and room-temperature vulcanized silicone rubber), silicone oil and its secondary processing products, silicone resin, and silane coupling agent. And through the sulfur molding process, silicone rubber can be made into conductive keys, seal gaskets, swimming caps, and many other consumer products.
Although there are many varieties of silicone products, its starting raw materials for production are limited to a few organic silicon monomers. The top amount is trimethylchlorosilane, followed by phenyl chlorosilane. In addition, trimethylchlorosilane, ethyl and propyl chlorosilane, vinyl chlorosilane, etc., are also necessary raw materials in the production of silicone products.
The production of silicon monomers is not complicated. The main raw material for silicon monomer is silicon block, methanol, and hydrogen chloride. At present, methyl hydrochloride monomer is synthesized in a boiling bed reactor. Silicon powder and chloromethane are reacted under a catalyst at a high temperature which makes a methyl hydrochloride mixture. And through efficient fractionation, the target fraction is obtained. Silicone monomers are made by hydrolysis, lysis, and condensation to get different products. With silicon monomer as the raw material and using different polymerization means and processes, different silicone products can be produced by adding various fillers and additives. The basic raw materials for the production of silicone oil, silicone rubber, silicone resin, and silane coupling agent are different kinds of organic silicon monomers. From these basic silicone monomers, thousands of organic silicon products can be produced. Organic silicon monomers mainly include methyl chlorosilane (methyl-monomer), phenyl chlorosilane (benzene-monomer), methyl vinyl chlorosilane, vinyl trichlorosilane, ethyl trichlorosilane, propyl trichlorosilane, γ-chloropropyl trichlorosilane, and fluor silane monomer. Among them, methyl chlorosilane accounts for the top amount, which is 90% of the total monomer, followed by phenyl chlorosilane.
Organo chlorosilane (methyl chlorosilane, phenyl chlorosilane, vinyl chlorosilane) is the basis of the whole silicone industry, and methyl chlorosilane is the pillar in the organic silicone industry. Most of the silicone polymers are Poly dimethyl siloxane made from trimethylchlorosilane. After adding other groups such as phenyl, vinyl, chlorophenyl, fluoroalkyl, etc., the product can meet special needs. The production process of methyl chlorosilane is very long and difficult for production technicians. This industry is technology-intensive and capital-intensive. Therefore, the basic manufacturing site of the major foreign companies is on a large scale and under centralized construction. While the manufacturing of the downstream product is distributed as per their use and market conditions.
The key to the development of any polymer material is monomer technology development. The feature of the silicone industry is that the monomer production is concentrated, and the further process of silicone products are in different places. Therefore, the production of monomers plays an important role in the silicone industry. And the level of monomer production is a direct reflection of the development of the silicone industry.
Organic silicon monomers can be made into different silicone intermediates by hydrolysis (or lysis) and lysis. Silicone intermediates are the direct raw material to form silicone rubber, silicone oil, and silicone resin. Silicone intermediates include hexamethylcyclotrisiloxane(D3), octa-ethyl cyclic tetrasiloxane (D4), hexamethyldisilane (MM), dimethyl cyclosiloxane mixture (DMC), and other linear or cyclic dioxide series.
Silicone rubber is one of the important products among silicone polymers. Among all rubber, silicone rubber can work under a wide temperature range, from -100℃~350℃. It is excellent in high and low-temperature resistance. According to its ionization mechanism, silicone rubber has three categories: organic peroxide-induced free radical crosslinking type (High temperature vulcanized silicone), condensation reaction type (Room temperature vulcanized silicone), and addition reaction type. High-temperature vulcanized silicone rubber (HTV): thermal vulcanized silicone rubber is also called high-temperature vulcanized silicone rubber. All the direct chain siloxane with molecular weight between 500,000-800,000 is grouped as high-temperature vulcanized rubber. Usually, gum can be made with octa ethyl cyclo tetrasiloxane (D4) as the main raw material and get polymerized under the catalysis of acid or alkali. Then with oxides as a crosslinking agent and combined with different additives (such as reinforcing filler, thermal stabilizer, structural control agent, etc.), it can be made into homogeneous rubber additives. Various rubber products can be vulcanized by molding, extrusion or calendaring, etc.,
Room temperature vulcanized silicone rubber (RTV) and addition reaction silicone rubber (LSR). Room temperature vulcanized silicone rubber refers to the one which uses low molecular weight active Poly Organo siloxane as the base material and can be formed with the crosslinker and catalyst at room temperature.
Addition reaction silicone rubber refers to the silicone rubber which is made under the addition reaction with the platinum compound as catalyst. It has no by-products during the reaction. Usually, it is composed by vinyl terminated polydimethylsiloxane, silicone resin, low molecular weight polymethyl hydro siloxane, a platinum catalyst, and reaction inhibitor, etc., It is also known as liquid silicone rubber or LSR. Usually, additional reactive silicone rubber is also vulcanized at room temperature (medium temperature), so it is also classified as room temperature sulfide silicone rubber. The above two of these are referred to as room-temperature vulcanized silicone rubber.
Room-temperature vulcanized silicone rubber has a low molecular weight (from 10,000 to 80,000), and it is a viscous liquid. As per different package forms, it can be divided into single-component RTV silicone rubber and two-component RTV silicone rubber. Single-component RTV silicone rubber mixes the raw rubber evenly with filler, crosslinker, or catalyst in anhydrous conditions. And it will react with the moisture in the atmosphere after opening. While two-component RTV silicone rubber usually puts the raw rubber and crosslinker agent or catalyst in separate packages. They will link together when mixed in a certain ratio. Its reaction is not related to moisture.
Silicone Oil or Silicone Fluid
silicone oil is a kind of Poly siloxane liquid oil with different viscosity. It is not toxic, not corrosive, has no smell, and is not easy to burn. According to the chemical structure, silicone oil can be divided into methyl silicone oil, phenyl silicone oil, ethyl silicone oil, methyl hydrogen silicone oil, methyl chlorophenyl silicone oil, methyl phenyl silicone oil, methyl ethoxy silicone oil, and methyl trifluoro propane, etc., Among them, methyl silicone oil is the top item. By changing the polymerization degree of Poly siloxane and the type of organic group, or making Poly siloxane polymerized with other organic compounds, silicone oil with different properties can be made, such as waterproofing, anti-adhesion, demolding or defoaming. Silicone oil can also be grouped into linear silicone oil and modified silicone oil.
Silicone oil has many special properties. Its features include low viscosity-temperature coefficient, antioxidant resistance, good resistance to high and low temperature, high flash point, excellent insulation, low volatility, low surface tension, no corrosion to the metal, non-toxic, etc., With these properties, silicone oil has excellent effects in different applications. Among all kinds of silicone oils, methyl silicone oil is the most widely applied and is the most important type followed by methyl phenyl silicone oil. Different functional silicone oils and modified silicone oils are mainly used for special purposes.
Silicone resin is a semi-inorganic polymer with silicon-oxygen-silicon as the main chain and with silicon atoms cross-linked with organic groups. It emerged along with the silicone monomer produced by the silicone monomer. Its production is a half year ahead of silicone oil and silicone rubber.
Silicone resin has outstanding weather resistance, which is better than any other organic resin. Even under strong ultraviolet irradiation, silicone resin still has good yellowing resistance. Silicone also has superior dielectric properties. The property can remain stable in a wide range of temperatures, humidity, and frequency. In addition, it also has good oxidation resistance, irradiation resistance, smoke resistance, waterproof, mildew prevention, and other characteristics.
Silane coupling agent
The general formula of the silane coupling agent can be Y-R-SiX3. X and Y are two active groups with different reaction characteristics. X is easy to bind strongly with clay, glass, silica, metal, and metal oxides, while Y is easy to bind with resin and rubber in organic material. With both functional groups that can have a good reaction with organic and inorganic material, a silane coupling agent can bond organic and inorganic material together to reach satisfactory bonding. According to the number of hydrolyzed groups (X groups) connected to silicon atoms, the silane coupling agents can be divided into two groups: trifunctional and difunctional types. In recent years, the production of silane coupling agents is controlled by several giant companies. To form a monopoly, every company has named its product types, and for the same product, there are various names and types on the market. Union Carbide Corporation (UCC) is the world’s largest silane coupling agent manufacturer and has the largest number of product varieties. Silane coupling agent was first developed as a glass fiber treatment agent for glass fiber-reinforced plastics. The silane coupling agent has improved the adhesion between the glass fiber and the resin, thus the mechanical properties of the reinforced plastics are significantly improved. With the rapid development of composites, silane coupling agent also develops rapidly both in variety and output. In recent years, silane coupling agent has adopted some specific functional groups and this can improve the surface properties of materials, which get new properties such as antimildew, antistatic, anticoagulation, and physiological inert. This has become a new application for silane coupling agents. Along with the new development, silane coupling agents become an important branch of organic silicone.
Development history of silicone
All compounds containing Si-C bonds are generally called organic silicon compounds. Those that connect organic groups like oxygen, sulfur, and nitrogen to silicon atoms are also regarded as organic silicon compounds. Among them, the Poly siloxane composed of silicone oxygen bond (-Si-0-Si-) accounts for the top amount in silicone compounds. It is the most studied and widely applied type in silicone compounds, which is more than 90% of the total. Silicone material has both properties of organic materials and inorganic materials. It has many good properties such as high and low-temperature resistance, ozone resistance, electrical insulation, radiation resistance, flame retardant, water resistance, non-toxic and physiological inertia, and other excellent characteristics. The silicone material is widely applied in electrical, electronics, construction, chemical, textile, medical and other industries. The main functions of silicone include sealing, adhesion, encapsulation, lubrication, coating, lamination, surface activity, releasing, anti-foaming, crosslinking, waterproofing, penetrating, insertion and filling, etc. With the continuous development of the quantity and varieties of silicone material, it has become one of the most important materials in the new chemical material field. Many varieties of silicone are unable to be replaced by other chemicals.
Different phases of silicone chemistry
Beginning period: In 1863, French scientists Fiddle and Kraft heated silicon tetrachloride and zinc diethyl to 160℃ in a sealing tube and synthesized the first silicone compound, which is tetraethyl silane with Si-C bonds. Since then, much more tetraethyl silane derivatives have been synthesized. The forty years from 1863 to 1903 was the founding period of silicone chemistry, which was also referred to as the first phase
Growing period: From 1904 to 1937, there are many simple silicone compounds were synthesized. Meanwhile, some ring and linear Poly siloxane (with-Si-0-Si-bonds as the backbone). From the theoretical aspect, the synthesis of asymmetric silicon atomic compounds has been started, which created circumstances for the study of silicone photoactive isoforms. These 30 years were referred to as the growing period of silicone chemistry, which is also called the second phase.
Development period: Chemist Hyde from Corning as well as Patnode and Rojo from General Electronics realized that silicone polymer would have a good future, and they have actively improved the synthesis of silicone monomer. This helped the industrialization of silicone. In particular, Rojo invented the direct synthesis method for methyl chlorosilane in 1941. This was a revolution in the production of silicone and was a good foundation for the large-scale production of silicone compounds. In the 1940s, when some of the major countries realized industrialization, the equilibrium reaction methods of Poly organic siloxane were invented. And a complete system of industrial technologies was established. Different types of silicone oil, silicone rubber, silicone resin, and silane coupling agent with excellent performance have emerged. The development of the silicone industry was accelerated. The period from 1938 to1965 was called the third phase.
Boom period: Since 1966, further, consolidating, developing, improving, and utilizing the existing achievements, silicone was also developing in the new field. Some compounds which were impossible to make can also be synthesized. One of the groups with the fastest development is silicon —— metal bonding compounds. Especially chemical products formed by silicon and transition elements, and this has more practical value. And silicone chemistry has yielded fruitful results. So, the period from 1966 has been called the fourth phase.
The development of science has promoted the development of production and construction. And the production in turn has new requirements for scientific research. In many industries, thermal resistance is needed. But normal organic polymers cannot meet the demands. Natural silicate is long known but its -Si-0-Si- bond is fragile. Therefore, chemists introduced silicon atoms with an organic group, which can make them a linear structure or low-degree crosslinked polymer. Thus, the silicone material can form flexible or elastic material, and the application would be further enlarged. The study on Organo Poly siloxane also started. Chemist Hyde from Corning Glass first combined silicone and polymer chemistry and gained experience in silicone polymerization. Under his guidance, silicone resin was produced for electronic insulated glass cloth. From 1938 and 1941, Hyde and his collaborators developed many more Poly organic siloxane products. Meanwhile, Dow Chemical also started the study and production of Poly organic siloxane. In 1942, a dimethyl silicone oil and toluene silicone pilot plant was established.
In 1943, Dow combined with Corning glass and established the world-famous Dow-Corning Chemical. They built the synthetic Poly siloxane factory in Midland. And soon they developed DC4 ignition sealing material, which was applied on aerial aircraft in World War II. Then Dow Corning Chemical gradually became one of the world’s largest professional manufacturers of silicone products.
In the past 20 years, the application of silicone technology has achieved a brilliant level. Room temperature vulcanized silicone rubber and silane coupling agents got comprehensive development. Silicone polymer for biological and medical use also emerged. Now, many more silicone compounds were invented for a wide application. Many impossible things can be realized now due to the development of silicone material
From an annual output of 10,000 tons in 1997 to about 850,000 tons per year in 2015, methyl chlorosilane has a rapid development, which is top among all the monomers. In the future, with methyl chlorosilane production capacity increasing, the organic silicon industry will have further development in technology, with both capacity and quality getting onto a new level.