Silicon resins

The silicone oils and silicone resins find application as (i) lubricants (their change of viscosity with temperature is small), (ii) hydraulic fluids (they are unusually compressible), (iii) dielectric fluids, (iv) for the pro duction of water-repellant surfaces, and (v) in the electrical industry (because of their high insulating properties). 

Elastomeric shield materials (ESM) have been developed as low density flexible ablators for low shear appHcations (49). General Electric s RTV 560 is a foamed silicone elastomer loaded with silicon dioxide [7631-86-9] and iron oxide [1317-61 -9] particles, which decomposes to a similar foam of Si02, SiC, and EeSiO. Silicone resins are relatively resistant to thermal decomposition and the silicon dioxide forms a viscous Hquid when molten (50) (see... 

Silicone resins with high phenyl contents may be used with medium or short oil alkyds as blends in air-dried or baked coatings to improve heat or weather resistance the alkyd component contributes to adhesion and flexibiUty. AppHcations include insulation varnishes, heat-resistant paints, and marine coatings. 

Silicone Resins. Sihcone resins are an unusual class of organosdoxane polymers. Unlike linear poly(siloxanes), the typical siUcone resin has a highly branched molecular stmcture. The most unique, and perhaps most usehil, characteristics of these materials are their solubiUty in organic solvents and apparent miscibility in other polymers, including siUcones. The incongmity between solubiUty and three-dimensional stmcture is caused by low molecular weight < 10, 000 g/mol) and broad polydispersivity of most sihcone resins. 

This comprises composite materials on mica, glass fibre and asbestos bases, impregnated or glued together with silicone resins or silicone elastomer. These materials must not contain any organic fibrous materials such as paper or cloth backing, which is covered under class B and even F insulation systems. 

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. 

Although phenolic and amino moulding powders remain by far the most important of the thermosetting moulding compositions a number of new materials have been introduced" over the last 30 years based on polyester, epoxide and silicone resins. 

The silicones have established their value as water-repellent finishes for a range of natural and synthetic textiles. A number of techniques have been devised which result in the pick-up of 1-3% of silicone resin on the cloth. The polymer may be added as a solution, an emulsion or by spraying a fine mist alternatively, intermediates may be added which either polymerise in situ or attach themselves to the fibre molecules. 

On the commercial scale silicone resins are prepared batchwise by hydrolysis of a blend of chlorosilanes. In order that the final product shall be cross-linked, a quantity of trichlorosilanes must be incorporated into the blend. A measure of the functionality of the blend is given by the R/Si ratio (see Section 29.3). Whereas a linear polymer will have an R/Si ratio of just over 2 1, the ratio when using trichlorosilane alone will be 1 1. Since these latter materials are brittle, ratios in the range 1.2 to 1.6 1 are used in commercial practice. Since chlorophenylsilanes are also often used, the CH3/CgH5 ratio is a further convenient parameter of use in classifying the resins. 

High phenyl content resins are compatible with organic resins of the P-F, U-F, M-F, epoxy-ester and oil-modified alkyd types but are not compatible with non-modified alkyds. Silicone resins are highly water repellent. 

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 " )...

Like the fluids, the silicone resins form useful release agents and although more expensive initially are more durable. The resin is applied in solution form and the coated surface is then dried and the resin cured by heating for about two hours at 200-230°C. The bakery industry has found a particular use for these materials in aiding the release of bread from baking pans. 

The heat resistance and water resistance of the resins are attractive properties for surface coatings but the poor scratch resistance of the materials has limited applications of straight silicone resins. 

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

LeGrow, G.E., Solventless silicone resins. Relation between polymer structure and engineering properties. Soc. Plast. Eng., Tech. Pap., 21, 445-446 (1975). 

Silicone alkyds These are alkyds modified with silicone resin. They have superior resistance to weathering (particularly gloss retention) than pure alkyds but are generally significantly more expensive. They are useful for exterior use where appearance is important. 

Modified or pure silicone resin with aluminum Zinc silicate... 

Modified or pure silicone resin with aluminum These silicone-based aluminum paints can be used for temperatures between 260° and 540°C. They require a minimum temperature for curing (usually about 260° C) and in general have poor corrosion and weather resistance. 

The increased durability obtained by siliconising a polyster resin comes from reacting a high hydroxyl value polyester with 20-30% of appropriate silicone resin. 

Saturated polyesters and saturated alkyds cannot undergo such modification with vinyl monomers but can be modified with other polymers such as silicone resins by alcoholysis. Here outdoor durability is considerably improved. 

Semiconducting devices, switches and miniaturised v.h.f. circuits are all particularly sensitive to the slightest reaction on critical surfaces, and in devices calling for the highest levels of reliability even the most inert of the phenolic, epoxide and silicone resins are not considered to be fully acceptablecorrosion of electronic assemblies may often be enhanced by migration of ions to sensitive areas under applied potentials, and by local heating effects associated with current flows. 

A patent (FR 2474891 see CA 96 124298c) assigned to B.A. Ashby, GE Company, claims a platinum-siloxane complex which catalyzes hydrosilation of vinylsiloxanes or SiOH groups in the preparation of silicone resins and rubbers. 

Silicone paints are formed by controlled hydrolysis and condensation of alkyl alkox-ysilanes, and may be encountered either alone or in formulations with other synthetic resins. The typical structural unit in the polymer chain is dimethyl siloxane, and pyrolysis of such resins takes place with random chain scission and the extended formation of stable cyclic fragments. In Figure 12.14 the pyrogram of a silicone resin is shown, with cyclic siloxane oligomers eluting at the shorter retention times, followed by the linear siloxane fragments. 

Silicone resins, which after being sprayed onto the mould surface form a resin under the influence of the heat of the metal mould, are effective and relatively long lasting release agents. A single application of the resin will remain viable for a number of products lifts before requiring reapplication. 

Organosiloxane oligomers, in silicone polymerization, 22 555-556 Organosiloxane polymers, silicone resins as, 22 586-590... 

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