Silicone resins laminates

Uses of silicone resins laminates, moulding compositions ... 

Fig. 25. Resistivity of laminate, 6.2 mm thick, at 96% rb and 23°C. A, Volume resistivity perpendicular to laminations B, surface resistivity in machine direction C, surface resistivity across machine direction D, volume resistivity along laminations in machine direction E, volume resistivity along laminations across machine direction, (a) Paper-phenolic resin laminate (b) cotton cloth-phenolic resin laminate (c) coarse glass cloth-silicone resin laminate and (d) fine-weave, glass cloth-silicone resin laminate.

Because of their favourable price, polyesters are preferred to epoxide and furane resins for general purpose laminates and account for at least 95% of the low-pressure laminates produced. The epoxide resins find specialised uses for chemical, electrical and heat-resistant applications and for optimum mechanical properties. The furane resins have a limited use in chemical plant. The use of high-pressure laminates from phenolic, aminoplastic and silicone resins is discussed elsewhere in this book. 

Figure 29.4. Influence of resin content on the flexural strength of glass-cloth laminates made with two silicone resins A and B. (After Gale " )...

In recent years the dominance of shellac in mica-based laminates has met an increasing challenge from the silicone resins. 

In addition to the polymer substrate, a series of binding resins made from polyamide and polyimide, polyphenylsulfide (PPS), polyether sulfone (PES) and/or silicone resin are necessary for applying the coating. Such substances like laminating agents (lithium polysilicate, aluminum phosphate and phosphoric acid) and various additives are also used. Included in these additives are emulsifiers and further processing aids (e.g. silicone oil). 

The combination of an adhesive and adherend is a laminate. Commercial laminates are produced on a large scale with wood as the adherend and phenolic, urea, epoxy, resorcinol, or polyester resins as the adhesives. Many wood laminates are called plywood. Laminates of paper or textile include items under the trade names of Formica and Micarta. Laminates of phenolic, nylon, or silicone resins with cotton, asbestos, paper, or glass textile are used as mechanical, electrical, and general purpose structural materials. Composites of fibrous glass, mat or sheet, and epoxy or polyester resins are widely employed as reinforced plastic (FRP) structures. 

Many t3ipes of silicone resins are prepared for many different end uses. Of these, their application in the electrical held, where tiiey impart hi temperature resistance to electrical and electronic parts and equipment, is of particular importance. Silicone-glass laminates retain their strength after, long exposure to temperatures of 600 F (260 C). Certiun reans are capable of being foamed in place at either room temperature or elevated temperature. They have excellent heat-insulating properties, with a heat-distortion temperature of above 700 F (371.1°C). 

Silicone laminates are used principally in electrical applications such as printed circuit boards, transformers, and slot wedges in electric motors, particularly class H motors. Compression-molding powders based on silicone resins are available and have been used in the molding of switch parts, brush ring holders, and other electrical applications that need to withstand high temperatures. 

Glass fabric laminates are made by dipping the glass fabric into the oligomer solution, impregnating it with 25 to 45 percent silicone resin, and evaporating the solvent. Layers of impregnated fabric are then plied to the desired thickness and press-cured. Flat sheets are cured 30 to 60 minutes at 1000 psi and 170°C. Complex shapes can be made by lower-... 

Silicone elastomers have exceptional stability to both high as well as low temperatures. Thus, silicone rubbers are quite flexible at temperatures as low as -60 to -70 °C. They do not seem to suffer much degradation up to 300 °C. Silicone resins find electrical and non electrical applications. Thus, these are used in the insulation of electrical equipment and for laminating printed circuit boards. 

Silicone resin, SI. Applications molding compounds, laminates. 

The principal uses of silicone resins are in surface coatings and glass-fibre laminates. Thus it is appropriate to consider both film and bulk properties. 

Historically, copper-clad laminates with acrylic or epoxy adhesives have been the major materials for flexible circuits. Each manufacturer has developed a special resin grade or special additives to ensure reliable flexibility and bond strength. Other adhesive materials such as phenol resin or silicon resin have been developed however, they have not become standard adhesive materials in flexible circuits. 

Low and stable values of dissipation factor and permittivity under varying temperatures and also varying frequency became apparent when electronic applications for silicones were first developed over 30 years ago. Von Hippel measured very stable values of dielectric characteristics for silicone resin and silicone resin bonded glasscloth laminates at temperatures up to 200°C and at frequencies from 100 Hz to 100 MHz... 

Many fabricated materials made from or based on silicones are also available for the construction of electronic equipment, including silicone resin bonded glasscloth laminates, insulating sleeving cables or wiring and a wide variety of moulded or extruded components. 

Silicone resins contain, in addition to the D integral parts —O—Si(R2)—O—, some T parts —O—Si(R)(0)—O—, and on both ends of the chains they have OH or OR residues. Pure methyl silicone resins are comparatively brittle and moderately heat resistant. Industrial silicone resin solutions consist of cross-linked methylphenyl polysiloxanes. The heat resistance of the resins is increased by halogenating the phenyl residues. For the manufacture of moulded plastics and laminates, silicone resins may be mixed with filler, such as glass fiber, quartz powder, glimmer, pigments, etc. 

Glass Fiber Base—Epoxy Resin Low Pressure Laminated Plastic Silicone Resin, Glass Fiber Base, Low-Pressure Laminated Plastic Plastic Sheets and Parts, Modified Acrylic Base, Mono-lithic, Crack Propaqation Resistant—Covers Stretched Acrylic.. 060" Thru. 675" in Thickness... 

Silicone resins can also be used with glass reinforcement to produce laminates which are electrically superior to phenolic resin and amino resin based laminates but are mechanically weaker than other thermosets. Silicone laminates are used where very good electrical properties are required under high humidity conditions. 

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