After oxygen, silicon is the element most frequently found on earth. It does not occur free in nature, but as an oxide (silica) or silicate (feldspar, kaolinite and so on) in sand, rock and clay. One method of preparation is by heating quartz (SiO2) with carbon; during this process carbon monoxide is emitted and raw silicon (98% pure) remains. This grade is sufficiently pure for incorporation in alloys—for example, of aluminium and iron—in order to make them harder or less brittle. Pure silicon is prepared by heating raw silicon in chlorine. During this process the volatile compound SiCl4 occurs and is separated by distillation. If this liquid is heated together with hydrogen, pure silicon is released. This is shaped into rod form, and the last impurities are “floated” out from the rod by successively heating small portions of it to melting point, in an atmosphere of inert gas, such as argon, compounded with any trace elements to be added, which become dissolved in the liquid silicon.
Siloxanes are compounds which contain oxygen in addition to hydrogen, silicon and, usually, carbon (although there are some inorganic siloxanes). Starting from small molecules, they can be built up into large units (polymers), to which various properties (liquidity, elasticity, stability and so on) can be imparted. Siloxanes exist in the form of resins, elastomers (rubbery compounds) or oils.
Uses
It is used as an alloying agent for steel, aluminium, copper, bronze and iron. It is also widely used in semiconductor manufacture and in the production of silanes and organosilicon compounds.
Organosilicon compounds are used in the form of resins, elastomers (rubbery compounds) or oils. Resins are organosilicon compounds which, when mixed with a number of other substances used in the paint industry (hardeners, accelerators and so on), form very stable layers and are readily applicable even on bases to which other paints generally do not adhere well (such as metal surfaces). In addition they are fairly resistant to momentary heating or attack by oxygen, and do not fade much in sunlight. Among other things, these resins are also used as moulding compounds (plastics), and in the manufacture of foams which display good resistance to high temperatures and are useful thermal insulators. Other resins are used as so-called foils (thin layers applied in the electronics industry) because of their low combustibility and good electrical insulating properties even in a damp environment. Silicon resins have numerous applications because of their heat stability and water repellency, and their resistance to solvents, high temperatures and sunlight. Silicon resins are used in paints, varnishes, moulding compounds (plastics), electrical insulation, pressure-sensitive and release coatings, and laminates.
Methyl silicate is a fairly volatile liquid used in the manufacture of television screens. When it is decomposed in water, a transparent layer of silicic acid results, which secures the screen to the glass wall. Ethyl silicate is used as a binding agent for making moulds in special metal-founding processes or as a starting point in chemical synthesis.
Hazards and Their Prevention
This section discusses the hazards of organosilicon compounds. The reader is referred elsewhere in the Encyclopaedia for discussions of the important health effects of exposure to silicates, particularly crystalline silicates. The effects of silicon carbides are also discussed elsewhere.
The toxicological hazards of metallic silicon are not known. For most regulatory purposes it is considered a nuisance dust. When silicon is prepared and purified in the absence of air, the process takes place in a sealed, gas-tight enclosure which should limit exposures. Hazards may arise from the chemicals which are used in conjunction with silicon in various manufacturing processes. There are three types of silicon compounds considered here: silanes, siloxanes and heterosiloxanes.
Silanes. Silanes contain hydrogen and silicon. Most of them are very stable, oily substances which in themselves find but little practical application. If chlorine, nitrogen and so on are added, however, they can be used for chemical synthesis. Both tetrachlorosilane and trichlorosilane, however, are highly reactive compounds that can emit a highly irritant asphyxiating vapour. When they come into contact with water they are decomposed (hydrolysis), giving off hydrogen chloride. Water in the atmosphere can initiate such hydrolysis. The hydrolysis products can be have intense effects on the eyes and respiratory tract. Moreover, trichlorosilane ignites readily. These liquids are treated as corrosive substances and are shipped in quartz ampoules or stainless steel boxes. Spills can be rendered harmless by anhydrous soda.
The siloxane oil vapours can be irritating to the eyes, and it is reported that extremely high concentrations can have serious effects on the respiratory system. By contrast the silicon resin compounds have been considered to be harmless in the past and were widely used as implants in the body.
Elastomers (rubbery compounds). These substances are characterized by their great stability at high (250 °C) and low temperatures (down to -75 °C), and resistance to attack by chemicals. Their chemical inertness is such that they are often used as implant material for blood vessels and so on. Moreover, they do not dissolve in many organic solvents, such as trichloroethylene or acetone. Silicone rubber membranes are easily permeable by gases such as oxygen, even when these are dissolved in water.
It should be noted that there have been major controversies and legal disputes over the effects of silicon breast implants, with noted authorities divided about any possible long-range health hazards.
Oils. These compounds also retain their stability when exposed to extreme changes in temperature. For this reason they are often used as lubricants, since their viscosity remains substantially constant at different temperatures. They are also used as water-repellents, applied for example on walls, textiles or leather. Pressed parts can be easily removed from moulds smeared with these compounds, and they also act as anti-foaming agents (the latter property is inter alia of assistance to chronic bronchitis sufferers, as inhalation of the vapours of these oils aids the evacuation of phlegm). In experimental animals it has been found that these substances are eliminated very slowly from the lungs, but that their presence there causes no adverse reactions. Ointments prepared with silicones are also very well tolerated and, by virtue of their water-repellent properties, contribute to prevention of—or recovery from—contact eczemas, since they prevent contact with substances causing reactions due to hypersensitivity.
Animal experiments also have indicated that if the vapour is inhaled in very high concentrations a fatal narcosis can result; if the exposed animals survived the narcosis, however, complete recovery ensued. Silicone oils irritate the ocular mucosae to a slight extent, giving rise to redness, painfulness and lacrimation; more serious symptoms are induced only by compounds of low molecular weight.
Heterosiloxanes. In addition to silicon, hydrogen and oxygen, heterosiloxanes contain certain other elements such as metals (aluminium, tin, lead and so on) as well as boron or arsenic, etc. They hydrolyze readily and are therefore dangerous to the human body, a major part of which consists of water. Heterosiloxanes are generally formed as intermediate products in chemical syntheses. Methyl silicate and ethyl silicate occupy a special place in this group. Methyl silicate, a fairly volatile liquid, is used in the manufacture of television screens. When it is decomposed in water, a transparent layer of silicic acid results, which secures the screen to the glass wall. Methyl silicate liquid or vapour which reaches the eyes produces no immediate effect, but after 10 to 12 h gives rise to violent ocular pain, accompanied by redness and tears. The cornea becomes opaque, and ulcers can occur, which may result in blindness. If the vapour is inhaled, fatal damage to the lungs or kidneys can ensue. Since contact with the vapour or liquid produces no immediate warning pain, special precautions are required with this substance. Breakage of flasks must be avoided. The eyes must be protected by gas-tight goggles, and the risk of inhalation of vapours in case of spillage, etc., must be avoided by installing an exhaust ventilation system.
Ethyl silicate, which is used as a binding agent for making moulds in special metal-founding processes or as a starting point in chemical syntheses, has a low vapour pressure; this physical property helps reduce exposure. In high concentrations it irritates the mucous membranes and the skin, and in very high concentrations it has proved fatal to animals.
As the molecular weight of the silicates increases, there is a decrease in reactivity.
Silicon and organosilicon compounds tables
Table 1 - Chemical information.
Table 2 - Health hazards.
Table 3 - Physical and chemical hazards.
Table 4 - Physical and chemical properties.