Silicones resin

Silicone resins are branched polysiloxanes. Hardening proceeds by polycondensation at high temperatures 

Pure silicone resins are poly(organosiloxanes) with a high proportion of branching, i.e. tri-or tetra-functional siloxy groups together with di- and optionally mono-functional siloxy groups. 

Liquid silicone resins or silicone resin solutions are generally fully condensed by several hours heating (curing) at 180 to 250°C to a highly crosslinked solid. The thermally stable coatings formed, particularly if phenyl groups are present, do not lose their transparency, gloss or elasticity even at 200 to 250°C. They are also hydrophobic and extremely weather resistant. 

Methyl- and methylphenyl-resins are utilized as raw materials for paints, binders and in building preservation. In the electrical industry they are utilized as electrically insulating lacquers (wire enamel) and for the bonding of glass filaments or mica insulating materials. Special meltable solid resins are flow aids in the injection molding of porcelain matrices. 

Corrosion protection-stoving enamels are produced upon pigmentation with zinc dust. These are utilized for the enameling of components which operate at high temperatures e.g. metal chimneys. 

Silicone resins are available in several forms. They are used as thermally stable electrical insulation resins, paint vehicles, molding compounds, laminates, impregnating varnishes, encapsulating materials, and as baked-on release agents. Recommended surface preparation is as follows  

Wipe with acetone or other solvent listed in the general discussion for thermosets in Section 7.3 

or vapor-blast, or use steel wool 

Wipe with a clean dry cloth to remove grit and particles 

SiUcone laminates should be treated according to Section 7.4, regarding Reinforced Plastics/Thermosets. 

These are generally bonded with silicone adhesives, either silicone rubber or silicones. Primers should be used before bonding. 

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/C6H5 ratio is a further convenient parameter of use in classifying the resins. 

The chlorosilanes are dissolved in a suitable solvent system and then blended with the water which may contain additives to control the reaction. In the case of methylsilicone resin the overall reaction is highly exothermic and care must be taken to avoid overheating which can lead to gelation. When substantial quantities of chlorophenylsilanes are present, however, it is often necessary to raise the temperature to 70-75°C to effect a satisfactory degree of hydrolysis. 

At the end of the reaction the polymer-solvent layer is separated from the aqueous acid layer and neutralised. A portion of the solvent is then distilled off until the correct solids content is reached. 

The resins are then cooled and stored in containers which do not catalyse further condensation of the resins. 

The cross-linking of the resin is, of course, not carried out until it is in situ in the finished product. This will take place by heating the resin at elevated temperatures with a catalyst, several of which are described in the literature, e.g. triethanolamine and metal octoates. The selection of the type and amount of resin has a critical influence on the rate of cure and on the properties of the finished resin. 

Three methods are available for preparing the cross-linked resins  

Dimethyl silicone may be prepared by hydrolyzing dimethyldichlorosilane or its esters, and then oxidized with air and a catalyst16 to attain the desired CHs/Si ratio. 

Dimethyldichlorosilane may be mixed with methyltrichlorosilane or silicon tetrachloride and then hydrolyzed and the products cocondensed.17 

Silicon tetrachloride may be partially methylated (as with methyl magne- 

As a group, the methyl silicone resins are characterized by excellent thermal stability and good resistance to oxidation. Samples have been heated to 550° in vacuo and to 500° in hydrogen without disintegration, decomposition, or melting a transparent piece heated in air to 200° C. for one year looks the same as another piece of the same disk kept at room temperature.18 Above 300° C. the resins will oxidize slowly if they have free access to air, but for analytical combustions it has been found necessary to heat the samples to 550° C. or more in pure oxygen. The residue from oxidation under these conditions is a white mass of silica. 

Outstanding properties such as thermal stability, weathering resistance, low temperature flexibility, low surface tension, hydropho-bicity and surface activity of silicone resins and their combinations with other resins made them a commercially important category of binders for high performance coatings. 

Monomer Functionality Effect in final structure of silicone Symbol 

These siloxanes are easily hydrolyzed by ambient humidity and water bound to the substrates to generate transient silanols 77 and 78. Subsequent dehydration initially provides a precursor 79 of crosslinked polydimethylsiloxane network, which is formed by further condensation of the silanol groups. With many variants, this process constitutes the base of the RTV silicones that are mainly used as protection shields in the glob top encapsulation process. Low-stress conductive adhesives have been prepared by the condensation reaction of ethylphenylsila-nediol, l,4-phenylenebis(dimethylsilanol), and trimethylsilyl-terminated poly-diethylsiloxane. When loaded with carbon particles and cured at 180°C, this composition exhibits a volume resistivity of 1.2 X 10 flcm and a thermal conductivity of 2.1 W m K.  

Editor s note For more information about silicones and their chemistry, refer to the chapter Silicone Adhesives and Sealants in Volume 3 of this handbook, which provides a very comprehensive study. 

Phenylchlorosilanes can be prepared by a direct process from silicon and chlorobenzene with copper or silver catalyst, but yields are not so good as those of methylchlorosilanes. Alternative routes are Grignard or phenylsodium arylation of SiQ4, and the reaction between benzene and either H2SiCl2 or HSiCla in the presence of platinum or peroxide catalysts. The required intermediates are made from silicon-copper (or-iron) and HQ  

Resins are normally made in two or more stages. The methyl- and phenyl-chlorosilane mixture is first hydrolysed with a mixture of water and toluene. The product is a relatively low molecular weight oligomer 

Miscellaneous silicone products. Most of the major silicone manufacturers sell over 100 different silicone products, and the applications of these cover a very wide range. Here there is space to mention only two more applications the first being the use of silicone oils and emulsions for the prevention of foam in numerous industrial processes, the other being the use of silicone-containing copolymers as surfactants in the manufacture of polyurethane foams from di-isocyanates, ethylene/propy-lene polyethers and a little water. Unless a suitable surface-active surfactant is added, polyurethane foams tend to collapse before the polymerization is complete. One of the best types of surfactant is a poly-siloxane to which polyethers are attached  

Me3SiOSiMe(OSiMe xOSiMe(OSiMe SiMe(OSiMe2) OSiMe3 R R R 

Both reactions need catalysts such as tin(II) or platinum compounds. For some purposes one siloxane chain can be attached to another using the SiH/SiOH condensation  

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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. 

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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