Background of the Invention

The present invention relates to a silicone coating composition, more particularly a composition that is resistant to cracking and dielectric degradation at high temperatures, and a method for preparing the composition. High temperature protective coatings and insulating materials to protect a variety of equipment and devices against extremely high temperatures. Heater elements for electric vehicles, exhaust systems for automotive engines, power plants, and top coatings for stoves, for example, all benefit from such protective coatings. In many applications, the coating layers must withstand temperatures exceeding 300 °C over several months without cracking or losing dielectric and insulating properties and must pass aggressive thermal shock tests over a broad temperature range.

High temperature resistance of silicones ostensibly makes them promising candidates as high temperature protective coatings and sealants; nevertheless, silicone rubbers are not resistant to cracking above 250 °C beyond 3 weeks. The combination of silicone and inorganic filler such as SiO2, TiO2, and AI2O3 provides a composition with long term high temperature resistance; however, coatings prepared from such compositions require aging at temperatures exceeding 500 °C to form ceramic-like coatings. At such extreme temperatures, the coatings are likely to crack and suffer thermal shock failure; moreover, electronic elements beneath the surface of the coating are vulnerable to damage. It would therefore be an advance in the field of high temperature protective coatings to develop a composition that provides a coating that is resistant to cracking, delamination, and thermal shock failure, while maintaining acceptable dielectric properties at temperatures exceeding 300 °C for an extended period.

Summary of the Invention

In one aspect, the present invention relates to a composition comprising an MQ resin, a ZO-terminated poly(dimethylsiloxane), and mica, wherein the weight-to-weight ratio of the MQ resin to the ZO-terminated poly(dimethylsiloxane) is in the range of from 70:30 to 10:90, and the weight-to-weight ratio of the mica to the sum of the MQ resin and the ZO-terminated poly(dimethylsiloxane) is in the range of from 70:30 to 30:70, where each Z is independently H, Ci-C4-alkyl, or C(O)CH3. The composition of the present invention is useful as a coating for a substrate, wherein the coating, when cured, exhibits adhesion and crack-resistance when subjected to high temperatures for hundreds of hours. Detailed Description of the Invention

The present invention relates to a composition comprising an MQ resin, a ZO-terminated poly (dimethylsiloxane-), and mica, wherein the weight-to- weight ratio of the MQ resin to the ZO-terminated poly(dimethylsiloxane) is in the range of from 70:30 to 10:90, and the weight-to- weight ratio of the mica to the sum of the MQ resin and the ZO-terminated poly(dimethylsiloxane) is in the range of from 70:30 to 30:70, where each Z is independently H, Ci-C ₄ -alkyl, or C(O)CH ₃ .

As used herein, the term “MQ resiri’ refers to a kinetically stable three-dimensional polymer having repeat units of SiO ₄ /2 (Q), and a plurality of tri-Ci-C ₄ -alkylsilyl, preferably trimethylsilyl capping groups (M). The resin may include additional capping groups such as CiC ₄ -alkyl, dimethylhydroxysilyl, and dimethylvinylsilyl capping groups. An example of commercially available MQ resins are DOWSIL™ MQ-1600, MQ-1601, and MQ-1640 Resins (A Trademark of The Dow Chemical Company or its Affiliates).

The ZO-terminated poly(dimethylsiloxane) (ZO-PDMS-OZ) can be illustrated by the following structure: where n is preferably from 20 or from 40 or from 70 or from 100, to 300 or to 250 or to 200.

The weight-to-weight ratio of the MQ resin to the ZO-terminated poly(dimethylsiloxane) is in the range of from 70:30 or from 50:50 or from 40:60 to 10:90 or to 20:80 or to 25:75.

Micas are hydrated aluminum silicate minerals including muscovite, biotite, fuchsite, phlogopite, margarite, glauconite, and lepidolite micas, of which muscovite mica and phlogopite mica are predominant. Muscovite mica has a typical composition of K2Al ₄ (AhSi602o)(OH) ₄ . The w/w ratio of the mica to the sum of the of the MQ resin and the ZO-terminated poly(dimethylsiloxane) is in the range of from 70:30 or from 60:40, to 30:70 or 30:60.

The composition of the present invention advantageously further comprises a crosslinking agent such as a Ci-C ₄ -alkyl tri-Ci-C ₄ -alkoxy silane, preferably methyltrimethoxysilane (MTMS), and a moisture cure catalyst to promote curing of the composition after it is applied as a coating onto a substrate. Examples of moisture cure catalysts include organotin and organotitanate catalysts such as tin octanoate, tin butanoate, tetraisopropyl titanate, tetra-/?-butyl titanate, and tetra-z-butoxy titanate. This curable composition may be prepared by first blending the MQ resin, the ZO-PDMS-OZ, the crosslinking agent, and the moisture cure catalyst in the presence of a solvent to tune the viscosity to a desired level, preferably in the range of from 20 cP or from 50 cP or from 100 cP, to 20,000 cP or to 10,000 cP or to 5,000 cP, or to 1200 cP. Examples of suitable solvents include aprotic solvents such as ethyl acetate, propyl acetate, butyl acetate, propyl propionate, and hexamethyldisiloxane (HMDS). The blend is then advantageously contacted with the mica with additional blending, then applied to a substrate, such as a metal, metal oxide, ceramic, or glass substrate, at a desired coating thickness, typically in the range of from 10 pm or from 20 pm or from 50 pm to 200 pm or to 100 pm. The coatings are then dried and subjected to thermal aging.

During thermal aging, at least some portion of the MQ resin is observed to react with at least some portion of ZO-PDMS-OZ to form an MQ-PDMS copolymer. Thus, in another aspect, the present invention is a substrate coated with a composition comprising an MQ-PDMS copolymer and mica.

The composition of the present invention provides a tack-free coating in minutes that is thermally stable to cracking for hundreds or even thousands of hours.

Examples 1-5 - Preparation of Blend of MQ resin, Silanol-terminated PDMS, and Mica

DOWSIL™ MQ-1600 resin (Mo.45 Q0.55, 11.0 mole% SiOH), silanol-terminated PDMS (HO-PDMS-OH dp = 80), methyltrimethoxysilane (10 wt %, based on the total weight of the MQ-1600 resin, the silanol-terminated PDMS, and methyltrimethoxy silane), and a sufficient amount of hexamethyldisiloxane to adjust the viscosity of the mixture to 500 cp to 2000 cp were added to a dry flask under nitrogen. The mixture was stirred for 30 min, after which time mica, which had been dried in vacuo at 120 °C for 3 to 20 h then cooled to room temperature under nitrogen, was added to the mixture with stirring under nitrogen. MRX muscovite mica (MRX, median particle size 11.4 pm, obtained from Arctic Minerals) was used for Examples 1, 2, and 3, and C-4000 muscovite mica (C-4000, median particle size 10.8 pm, obtained from IMERYS) was used for examples 4 and 5. Tetraisopropyl titanate (1 wt% based on the weight of the formulation) was added to each sample with stirring. The comparative example formulation (Cl) did not include mica. Long Term High Temperature Resistance Testing

Aluminum panels (3” x 6”) were washed with toluene and acetone, then dried. A portion of the composition was applied at a thickness of 50 pm to 100 pm using a 4-mil drawdown bar. The panels coated with the example formulations containing mica became tack free in 10 min at room temperature, while the panel coated with the mica-free formulation (Cl) became tack free in 1 h. Each sample was then heated in an oven at 300 °C. Film cracking time was recorded (in days) as the first instance of visible cracks in the coatings.

Table 1 shows the thermal stability of coatings as measured by crack time. The mica weight percent is based on the sum of the weights of the MQ resin, the HO-PDMS-OH, and the mica. MQ resin and HO-PDMS-OH weight percentages are based on the sum of the MQ resin and HO-PDMS-OH.

Table 1 - Thermal Stability of Coatings

The data show that the cured coatings containing mica exhibit a dramatic resistance to cracking and delamination. Also, samples containing mica cured much faster than the sample without mica. The combination of the MQ resin and mica alone was found to fail the cracking test within 2 d, while the combination of the HO-PDMS-OH and mica alone delaminated readily from the substrate at 300 °C. Moreover, of the fillers tested - silica, calcium carbonate, aluminum silicate, calcium silicate, alumina, ferric oxide, and mica - mica was found to be the only class of fillers to exhibit crack times beyond 120 h.