SILICONE RESINS

Silicone resins are synthetic materials capable of cross-linking or polymerizing to form films, coatings, or molded shapes with outstanding resistance to high temperatures.

They are a group of resinlike materials in which silicon takes the place of the carbon of the organic synthetic resins. Silicon is quadrivalent like carbon. But, while the carbon also has a valence of 2, silicon has only one valence of 4, and the angles of molecular formation are different. The two elements also differ in electronegativity, and silicon is an amphoteric element, with both acid and basic properties. The molecular formation of the silicones varies from that of the common plastics, and they are designated as inorganic plastics as distinct from the organic plastics made with carbon.

In the long-chain organic synthetic resins the carbon atoms repeat themselves, attaching on two sides to other carbon atoms, while in the silicones the silicon atom alternates with an oxygen atom so that the silicon atoms are not tied to each other. The simple silane formed by silicon and hydrogen corresponding to methane, CH4, is also a gas, as is methane, and has the formula SiH4. But, in general, the silicones do not have the SiH radicals, but contain CH radicals as in the organic plastics.

Composition

Silicones are made by first reducing quartz rock (SiO2) to elemental silicon in an electric furnace, then preparing organochlorosilane monomers (RSiCl3, R2SiCl2, R3SiCl) from the silicon by one of several different methods. The monomers are then hydrolyzed into cross-linked polymers (resins) whose thermal stability is based on the same silicon-oxygen-silicon bonds found in quartz and glass. The properties of these resins will depend on the amount of crosslinking, and on the type of organic groups (R) included in the original monomer. Methyl, vinyl, and phenyl groups are among those used in making silicone resins.


Properties

Silicone resins have, in general, more heat resistance than organic resins, have higher dielectric strength, and are highly water resistant. Like organic plastics, they can be compounded with plasticizers, fillers, and pigments. They are usually cured by heat. Because of the quartzlike structure, molded parts have exceptional thermal stability. Their maximum continuous-use service temperature is about 260°C. Special grades exceed this and go as high as 371 to 482°C. Their heat-deflection temperature for 1.8 MPa is 482°C. Their moisture absorption is low, and resistance to petroleum products and acids is good. Nonreinforced silicones have only moderate tensile and impact strength, but fillers and reinforcements provide substantial improvement. Because silicones are high in cost, they are premium plastics and are generally limited to critical or high-performance products such as high-temperature components in the aircraft, aerospace, and electronics fields.

Common characteristics shared by most sil-icone resins are outstanding thermal stability, water repellency, general inertness, and electrical insulating properties. These properties, among others, have resulted in the use of sili-cone resins in the following fields:

1. Laminating (reinforced plastics), molding, foaming, and potting resins

2. Impregnating cloth coating, and wire varnishes for Class II (high-performance) electric motors and generators

3. Protective coating resins

4. Water repellents for textiles, leather, and masonry

5. Release agents for baking pans

Laminating Resins

Laminates made from silicone resin and glass cloth are lightweight, strong, heat-resistant materials used for both mechanical and dielectric applications. Silicone-glass laminates have low moisture absorption and low dielectric losses, and retain most of their physical and electrical properties for long periods at 260°C.

Laminates may be separated into three groups according to the method of manufacture: high pressure, low pressure, and wet layup.

High pressure. Silicone-glass laminates (industrial thermosetting laminates) have excellent electric strength and arc resistance, and are normally used as dielectric materials. To prepare high-pressure laminates, glass cloth is first impregnated by passing it through a solvent solution of silicone resin. The resin is dried of solvent and precured by passing the fabric through a curing tower. Laminates are prepared by laying up the proper number of plies of preimpregnated glass cloth and pressing them together at about 6.8 MPa and 177°C for about 1 h. They are then oven-cured at increasing temperatures, with the final cure at about 249°C. The resulting laminates can be drilled, sawed, punched, or ground into insulating components of almost any desired shape. Typical applications include transformer spacer bars and barrier sheets, slot sticks, panel boards, and coil bobbins.

Low pressure. Silicone-glass laminates made by low-pressure reinforced plastics molding methods usually provide optimum flexural strength, e.g., about 272 MPa even after heat aging. They are used for mechanical applications such as radomes, aircraft ductwork, thermal barriers, covers for high-frequency equipment, and high-temperature missile parts. In making low-pressure laminates, glass cloth is first impregnated and laid up as described above. Since the required laminating pressure can be as low as 0.068 MPa, matched-metal-molding and bag-molding techniques can be used in laminating, making possible greater variety in laminated shapes. Lamination should be after-cured as already described.

Wet layup. Silicone-glass laminates can now be produced by wet layup techniques because of the solventless silicone resins recently developed. Such laminates can be cured without any pressure except that needed to hold the laminate together. This technique should prove especially useful in making prototype laminates, and in short production runs where expensive dies are not justified. Laminates are prepared by wrapping glass cloth around a form and spreading on catalyzed resin, repeating this process until the desired thickness is obtained. The laminate surface is then wrapped with a transparent film, and air bubbles are worked out. Laminates are cured at 149°C and, after the transparent film is removed, post-cured at 204°C.

Molding Compounds

Silicone molding compounds consist of silicone resin, inorganic filler, and catalyst, which, when molded under heat and pressure, form thermo-setting plastic parts. Molded parts retain exceptional physical and electrical properties at high temperatures, resist water and chemicals, and do not support combustion. Specification MIL-M-14E recognizes two distinctly different types of silicone molding compounds: type MSI-30 (glass-fiber filled) and type MSG (mineral filled). Requirements for both are as follows.

Type MS1-30. Glass-filled molded parts have high strengths, which become greater the longer the fiber length of the glass filler. Where simple parts can be compression-molded from a continuous fiber length compound, strengths will be approximately twice as great. Properly cured glass-fiber-filled molded parts can be exposed continuously to temperatures as high as 371°C and intermittently as high as 538°C. The heat-distortion temperature after postcure is 482°C. Because of the flow characteristics of these fiber-filled compounds, their use is generally limited to compression molding.

Type MSG. Mineral-filled compounds are free-flowing granular materials. They are suitable for transfer molding, and can be used in automatic performing and molding machines. They are used to make complex parts that retain their physical and electrical properties at temperatures above 260°C, but that do not require impact strength. Silicone molding compounds are excellent materials for making Class H electrical insulating components such as coil forms, slot wedges, and connector plugs. They have many potential applications in the aircraft, missile and electronic industries.

Foaming Powders

Silicone foaming powders are completely formulated, ready-to-use materials that produce heat-stable, nonflammable, low-density sili-cone foam structures when heated. Densities vary from 160 to 288 kg/m3, compressive strengths from 0.68 to 2.23 MPa. Electrical properties are excellent, and water absorption after 24-h immersion is only 2.5%. The maximum continuous operating temperature of these foams is about 343°C.

Foams are prepared by heating the powders to between 149 and 177°C for about 2 h. The powder can be foamed in place, or foamed into blocks and shaped with woodworking tools. Foams are normally after-cured to develop strength, but can often be cured in service.

Silicone foams are being used in the aircraft and missile industries to provide lightweight thermal insulation and to protect delicate electronic equipment from thermal shock. They can also be bonded to silicone-glass laminates or metals to form heat- and moisture-resistant sandwich structures.

Potting Resins

Solventless silicone resins can be used for impregnating, encapsulating, and potting of electrical and electronic units. Properly catalyzed, filled, and cured, they form tough materials with good physical and electrical properties, and will withstand continuous temperatures of 204°C and intermittent temperatures above 260°C.

Typical physical properties of cured resins include flexural strength of 48.6 MPa, compressive strength of 117 MPa, and water absorption of 0.04%.

Before use, resins are catalyzed with dicumyl peroxide or ditertiary butyl peroxide.

Resins can be simply poured in place, although vacuum impregnation is suggested where fine voids must be filled. Fillers such as glass beads or silica flour are added to extend the resin; their use increases physical strength and thermal conductivity, but decreases electrical properties. The resin is polymerized by heating it to about 149°C, and postcured, first at 204°C, then at the intended operating temperature if higher.

Electrical Varnishes

Silicone varnishes (solvent solutions of silicone resins) have made possible the new high-temperature classes of insulation for electrical motors and generators. Silicone insulating varnishes will withstand continuous operating temperatures at 177°C or higher.

Electrical equipment that operates at higher temperatures makes possible motors, generators, and transformers that are much smaller and lighter, or equipment that delivers 25 to 50% more power from the same size and still has a much longer service life.

The resinous silicone materials used in Class H electrical insulating systems include the following:

1. Silicone bonding varnish for glass-fiber-covered magnet wire.

2. Silicone varnishes for impregnating and bonding glass cloth, mica, and asbestos paper. Sheet insulations made of these heat-resistant materials are used as slot liners for electric motors and as phase insulation.

3. Silicone dipping varnish that impregnates, bonds, and seals all insulating components into an integrated system.

Other silicone materials used in electrical equipment include silicone rubber lead wire, silicone-adhesive-backed glass tape, and temperature-resistant silicone bearing greases. Sil-icone insulated motors, generators, and transformers are now being produced by the leading electrical equipment manufacturers.

Uses

The wide range of structural variations of sili-cone resins makes it possible to tailor compositions for many kinds of applications. Low-molecular-weight silanes containing amino or other functional groups are used as treating or coupling agents for glass fiber and other reinforcements to cause unsaturated polyesters and other resins to adhere better.

The liquids, generally dimethyl silicones of relatively low molecular weight, have low surface tension, great wetting power and lubricity for metals, and very small change in viscosity with temperature. They are used as hydraulic fluids, as antifoaming agents, as treating and waterproofing agents for leather, textiles, and masonry, and in cosmetic preparations. The greases are particularly desired for applications requiring effective lubrication at very high and at very low temperatures.

Silicone resins are used for coating applications in which thermal stability in the range 300 to 500°C is required. The dielectric properties of the polymers make them suitable for many electrical applications, particularly in electrical insulation that is exposed to high temperatures and as encapsulating materials for electronic devices.

Silicone enamels and paints are more resistant to chemicals than most organic plastics, and when pigmented with mineral pigments will withstand temperatures up to 538°C. For lubricants the liquid silicones are compounded with graphite or metallic soaps and will operate between -46 and 260°C. The silicone liquids are stable at their boiling points, between 399 and 427°C, and have low vapor pressures, so that they are also used for hydraulic fluids and heat-transfer media. Silicone oils, used for lubrication and as insulating and hydraulic fluids, are methyl silicone polymers. They retain a stable viscosity at both high and low temperatures. As hydraulic fluids they permit smaller systems to operate at higher temperatures. In general, silicone oils are poor lubricants compared with petroleum oils, but they are used for high temperatures, 150 to 200°C, at low speeds and low loads.

Silicone resins are blended with alkyd resins for use in outside paints, usually modified with a drying oil. Silicone-alkyd resins are also used for baked finishes, combining the adhesiveness and flexibility of the alkyd with the heat resistance of the silicone. A phenyl ethyl silicone is used for impregnating glass-fiber cloth for electrical insulation and it has about double the insulating value of ordinary varnished cloth.

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