Silicone coating composition containing mica

Abstract

The present invention relates to a composition comprising an MQ resin, a ZO-terminated poly(dimethylsiloxane), and mica, which is useful as a coating on a substrate, the coating exhibiting adhesion, uniformity, and crack resistance when exposed to elevated temperatures for hundreds or even thousands of hours.

Description

[Technical Field]

[0001] The present invention relates to silicone coating compositions, more specifically compositions that are resistant to cracking and dielectric degradation at high temperatures, and methods for preparing the compositions. High-temperature protective coatings and insulating materials are used to protect various equipment and devices from extremely high temperatures. For example, heater elements for electric vehicles, exhaust systems for automobile engines, power plants, and stove top coatings all benefit from such protective coatings. In many applications, coating layers must withstand temperatures exceeding 300°C for several months without cracking or losing their dielectric and insulating properties, and must pass severe thermal shock tests over a wide temperature range.

[0002] The high-temperature resistance of silicones ostensibly makes them promising candidates for high-temperature protective coatings and sealants; nevertheless, silicone rubbers are not resistant to cracking above 250°C for more than three weeks. Combinations of silicones with inorganic fillers such as SiO2, TiO2, and Al2O3 provide compositions with long-term high-temperature resistance, but coatings prepared from such compositions require aging at temperatures above 500°C to form ceramic-like coatings. At such extreme temperatures, the coatings are likely to crack and undergo thermal shock failure, and electronic components beneath the coating's surface are susceptible to damage. Therefore, developing a composition that provides a coating resistant to cracking, delamination, and thermal shock failure while maintaining acceptable dielectric properties for extended periods at temperatures above 300°C would be an advancement in the field of high-temperature protective coatings. Summary of the Invention

[0003] In one aspect, the invention relates to a composition comprising an MQ resin, a ZO-terminated poly(dimethylsiloxane), and mica, wherein the weight ratio of the MQ resin to the ZO-terminated poly(dimethylsiloxane) ranges from 70:30 to 10:90, the weight ratio of the mica to the combined MQ resin and ZO-terminated poly(dimethylsiloxane) ranges from 70:30 to 30:70, and each Z is independently H, C1-C4-alkyl, or C(O)CH3. The composition is useful as a coating for a substrate, which, when cured, exhibits adhesion and crack resistance when exposed to elevated temperatures for hundreds of hours. DETAILED DESCRIPTION OF THE INVENTION

[0004] The present invention relates to a composition comprising an MQ resin, a ZO-terminated poly(dimethylsiloxane), and mica, wherein the weight ratio of the MQ resin to the ZO-terminated poly(dimethylsiloxane) is in the range of 70:30 to 10:90, and the weight ratio of the mica to the combined MQ resin and ZO-terminated poly(dimethylsiloxane) is in the range of 70:30 to 30:70, and each Z is independently H, C1-C4-alkyl, or C(O)CH3.

[0005] As used herein, the term "MQ resin" refers to a resin containing SiO 4 / 2 (Q) refers to a kinetically stable, three-dimensional polymer having repeating units of (Q) and multiple tri-C1-C4-alkylsilyl, preferably trimethylsilyl, capping groups (M). The resin may contain additional capping groups, such as C1C4-alkyl, dimethylhydroxysilyl, and dimethylvinylsilyl capping groups. Examples of commercially available MQ resins are DOWSIL™ MQ-1600, MQ-1601, and MQ-1640 resins (trademarks of The Dow Chemical Company or its affiliates).

[0006] ZO-terminated poly(dimethylsiloxane) (ZO-PDMS-OZ) can be represented by the following structure:

[0007] [ka] n is preferably from 20, or from 40, or from 70, or from 100, to 300, or to 250, or to 200.

[0008] The weight ratio of MQ resin to ZO-terminated poly(dimethylsiloxane) ranges from 70:30, or from 50:50, or from 40:60, to 10:90, or from 20:80, or to 25:75.

[0009] Micas are hydrated aluminum silicate minerals that include muscovite, biotite, chromite, phlogopite, nacre, glauconite, and lepidolite, of which muscovite and phlogopite are the predominant ones. Muscovite is a mineral composed of K2Al4(Al2Si6O 20 )(OH)4. The weight ratio of mica to the total of MQ resin and ZO-terminated poly(dimethylsiloxane) ranges from 70:30, or 60:40, to 30:70, or 30:60.

[0010] The compositions of the present invention advantageously further comprise a crosslinker, such as a C1-C4 alkyltri-C1-C4 alkoxysilane, preferably methyltrimethoxysilane (MTMS), and a moisture-cure catalyst to promote curing of the composition after application as a coating to a substrate. Examples of moisture-cure catalysts include organotin and organotitanate catalysts, such as stannous octoate, stannous butanoate, tetraisopropyl titanate, tetra-n-butyl titanate, and tetra-t-butoxytitanate. The curable compositions can be prepared by first blending the MQ resin, ZO-PDMS-OZ, crosslinker, and moisture-cure catalyst in the presence of a solvent to adjust the viscosity to the desired level, preferably from 20 cP, 50 cP, 100 cP, 20,000 cP, 10,000 cP, 5,000 cP, or 1,200 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 mica for further blending, and then applied to a substrate, such as a metal, metal oxide, ceramic, or glass substrate, at a desired coating thickness, typically ranging from 10 μm, or from 20 μm, or from 50 μm, to 200 μm, or to 100 μm. The coating is then dried and subjected to heat aging.

[0011] During thermal aging, at least a portion of the MQ resin is observed to react with at least a portion of the ZO-PDMS-OZ to form an MQ-PDMS copolymer. Thus, in another aspect, the invention is a substrate coated with a composition comprising an MQ-PDMS copolymer and mica.

[0012] The compositions of the present invention provide tack-free coatings within minutes that are thermally stable against cracking for hundreds or even thousands of hours. [Example]

[0013] Examples 1-5—Preparation of Blends of MQ Resin, Silanol-Terminated PDMS, and Mica DOWSIL(TM) MQ-1600 resin (M 0.45 Q 0.55 , 11.0 mol% SiOH), silanol-terminated PDMS (HO-PDMS-OH dp=80), methyltrimethoxysilane (10 wt% based on the total weight of MQ-1600 resin, silanol-terminated PDMS, and methyltrimethoxysilane), and sufficient hexamethyldisiloxane to adjust the viscosity of the mixture to 500 cp to 2000 cp were added to a dry flask under nitrogen gas. The mixture was stirred for 30 minutes, after which mica, which had been dried in vacuo at 120 °C for 3 to 20 hours and then cooled to room temperature under nitrogen, was added to the mixture with stirring under nitrogen. MRX muscovite (MRX, median particle size 11.4 μm, obtained from Arctic Minerals) was used in Examples 1, 2, and 3, and C-4000 muscovite (C-4000, median particle size 10.8 μm, obtained from IMERYS) was used in Examples 4 and 5. Tetraisopropyl titanate (1 wt % based on the weight of the formulation) was added to each sample with stirring. Comparative formulation (C1) did not contain mica.

[0014] Long-term high temperature resistance test Aluminum panels (3" x 6") were cleaned with toluene and acetone and then dried. Portions of the compositions were applied at thicknesses of 50 μm to 100 μm using a 4-mil drawdown bar. Panels coated with the example formulations containing mica became tack-free in 10 minutes at room temperature, while panels coated with the mica-free formulation (C1) became tack-free in 1 hour. Each sample was then heated in an oven at 300°C. The film cracking time was recorded (in days) as the first visible cracking event in the coating.

[0015] Table 1 shows the thermal stability of the coatings as measured by time to crack. The mica weight percentage is based on the sum of the weights of the MQ resin, HO-PDMS-OH, and mica. The weight percentages of the MQ resin and HO-PDMS-OH are based on the sum of the MQ resin and HO-PDMS-OH.

[0016] [Table 1]

[0017] The data show that cured coatings containing mica exhibited dramatic resistance to cracking and delamination. Additionally, samples containing mica cured much faster than samples without mica. The MQ resin and mica alone were found to fail the crack test within two days, whereas the HO-PDMS-OH and mica alone readily delaminated from the substrate at 300°C. Furthermore, 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 filler to exhibit a cracking time greater than 120 hours.

Claims

1. 1. A composition comprising an MQ resin, a ZO-terminated poly(dimethylsiloxane), and mica, wherein the weight ratio of the MQ resin to the ZO-terminated poly(dimethylsiloxane) is in the range of 70:30 to 10:90, and the weight ratio of the mica to the combined MQ resin and ZO-terminated poly(dimethylsiloxane) is in the range of 70:30 to 30:70, and each Z is independently H, C, １ -C ４ -alkyl, or C(O)CH ３ The composition.

2. 10. The composition of claim 1, wherein Z is H and the ZO-terminated poly(dimethylsiloxane) has a degree of polymerization in the range of 20 to 300.

3. 3. The composition of claim 2, wherein the mica is muscovite or phlogopite, and the weight ratio of the mica to the sum of the MQ resin and the HO-terminated poly(dimethylsiloxane) is in the range of 60:40 to 30:

60.

4. 4. The composition of claim 3, wherein the weight ratio of the MQ resin to the HO-terminated poly(dimethylsiloxane) ranges from 50:50 to 20:

80.

5. 4. The composition of claim 3, wherein the mica is muscovite and the weight ratio of the MQ resin to the ZO-terminated poly(dimethylsiloxane) ranges from 40:60 to 25:

75.

6. The composition of any one of claims 1 to 5, further comprising a crosslinker, a moisture cure catalyst, and a solvent.

7. 7. The composition of claim 6, wherein the crosslinker is methyltrimethoxysilane, the moisture cure catalyst is an organotin or organotitanate catalyst, and the solvent is ethyl acetate, propyl acetate, butyl acetate, propyl propionate, or hexamethyldisiloxane.

8. An article comprising a substrate coated with the composition of claim 6.

9. The article of claim 8 , wherein the substrate is a metal, a metal oxide, a ceramic, or a glass.

10. 10. The article of claim 9, wherein the crosslinker is methyltrimethoxysilane, the moisture cure catalyst is an organotin or organotitanate catalyst, and the solvent is ethyl acetate, propyl acetate, butyl acetate, propyl propionate, or hexamethyldisiloxane.

11. The article of claim 10, wherein the coating is cured.

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