High-phenyl silicone resin

Most silicone resins are copolymers of methyl and phenyl substituted monomers. Based on the type of organic substitution, the two main types of pure silicone resins are high-methyl silicone resins and high-phenyl silicone resins. 

Table 2.10 Typical highlighting properties of high-methyl and high-phenyl silicone 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. 

Chem. Descrip. Methyl phenyl silicone resin aq. emulsion Uses Binderfor highly pigmented and gloss paints, clear varnishes, low-VOC coatings food-contact coatings Features Dries to tack-free finish at R.T. rec. for blending with water-based org. resins high shear resist. 

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 alkyds are etherification products of alkoxy-polysiloxane oligomers and the free hydroxyl groups of alkyd resins (67, 68). The property improvements and applications are similar to those of the alkyd-silicone blends, with the added advantage of incorporating the stable —Si—O—C— strucmre into the alkyd molecules. The preferred silicone oligomers are those with high phenyl contents, and... 

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. 

These are cross-linked compositions, containing trifunctional (T) or tetrafunctional (Q) silicon units. The simplest are produced from methyltrichlorosilane, by hydrolysis, either directly or after intermediate conversion to methyl-trialkoxysilanes. Smaller amounts of dimethylsiloxane or trimethylsiloxane units may be introduced, as the corresponding chloro- or alkoxysUanes, to increase flexibility of the resin. Curing is brought about by aqueous solvolysis, usually catalyzed by metal salts, leading to highly cross-linked siloxane structures. More typical silicone resins contain mixtures of phenyl- or alkyltriethoxysilanes, mixed with MeSi(OEt)3 and perhaps Me2Si(OEt)2 or MesSiOEt. 

When is a small number, the structure is that of a silicone oil, whereas silicon rubbers have high values of . when the ratio R/Si is lower than 2, cross-linked polymers are obtained. Properties of silicone polymers are greatly affected by the type of organic radical present. For a given chain length, a methyl silicone can be an oily liquid, but a phenyl silicone is a hard and brittle resin. 

Silicone Resins. Hydrolysis of (CH3)2SiCl2 produces linear flexible molecules for rubber. Hydrolysis of CH3SiCl3 produces highly cross-linked molecules for thermoset plastics. These are too cross-linked and brittle for most purposes. Useful thermoset plastics are prepared by copolymerizing difunctional and trifunctional monomers. In commercial practice, the ratio of difunctional to trifunctional is generally 80/20 to 40/60. For some products, methyl silicon may be partly replaced by phenyl silicon. 

Silicone resins may be combined with other resinous materials to give modified resins which find use in surface coatings. Combination of the two materials may be achieved either by blending or copolymerization. Silicone resins with a high phenyl content are compatible to some extent with various resins such as alkyd, esterified epoxy and phenol-, urea- and melamine-formaldehyde resins and cold blending may be carried out. However, the limited compatibility of the components restricts the scope of the products which may be obtained. Copolymerization has been applied mainly to silicone-alkyd resins, which are prepared by heating a low molecular weight... 

The synthesis and structure of MQ resins have been described above. The poly(siloxane) network is traditionally derived by free-radical cross-linking of a high molecular weight PDMS polymer or gum using a peroxide catalyst, such as benzoyl peroxide or 2,4-dichlorobenzoyl peroxide. The curing reaction is performed immediately after the PSA has been coated onto a tape substrate, such as PET, PTFE, or Kapton. Uncured PSAs are supplied as a solution in an organic solvent. Some silicone PSAs also incorporate phenyl groups onto the gum portion of the adhesive to increase the use temperature. 

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