Polysiloxane mica blend

Abstract

The present invention is a composition comprising a mixture of mica, TR-DR'R' resin, and a silanol- or alkoxyl-(PhMeSiO2 / 2)n polymer, where R, R', and n are defined herein. The composition is useful as a coating for a substrate, which coating exhibits good adhesion and crack-resistance when subjected to high temperatures for several weeks.

Description

[0001] Polysiloxane Mica Blend 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 2 weeks. The combination of silicone and inorganic filler such as SiO2, TiO2, and Al2O3provides 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 The present invention addresses a need in the art by providing a composition comprising a mixture of a mica, a TR-DR′R′resin, and a silanol- or C1-C4-alkoxy-terminated (PhMeSiO2 / 2)n polymer, where each R and R′ are independently methyl or phenyl, wherein n is in the range of from 20 to 800, wherein the weight-to-weight ratio of the sum of the TR-DR′R′resin and the (PhMeSiO2 / 2)npolymer to the mica is in the range of from 10:90 to 90:10. The composition of the present invention is useful as a coating for a substrate, wherein the coating exhibits good adhesion, and crack-resistance when subjected to high temperatures for several weeks. Detailed Description of the Invention The present invention is a composition comprising a mixture of a mica, a TR-DR′R′resin, and a silanol- or C1-C4-alkoxy-terminated (PhMeSiO2 / 2)npolymer, where each R and R′ are independently methyl or phenyl, wherein n is in the range of from 20 to 800, wherein the weight-to-weight ratio of the sum of the TR-DR′R′resin and the (PhMeSiO2 / 2)npolymer to the mica is in the range of from 10:90 to 90:10. The term “TR-DR′R′resin” refers to a kinetically stable three-dimensional polymer having repeat units of R-SiO3 / 2, R-SiO2 / 2(OZ), and optionally R-SiO1 / 2(OZ)2(collectively TR); and R′2SiO2 / 2(DR′R′). A unit of R-SiO3 / 2 is represented by the following structure: where each R is methyl or phenyl. R-SiO2 / 2(OZ) is represented by the following structure: ; where Z is H, C1-C4-alkyl, or C(O)CH3; and a unit of RSiO1 / 2(OZ)2is represented by the following structure: Each Z in the TRportion of the resin is preferably methyl or H. A unit of R′2SiO2 / 2 is represented by the following structure: where each R′ is independently methyl or phenyl. The term "silanol- or C1-C4-alkoxy-terminated (PhMeSiO2 / 2)n polymer” refers to a polyphenymethylsiloxane (PPhMS) that contains repeat units of PhMe siloxane groups, as illustrated: where X is H, or C1-C4-alkyl, n (alternatively, the degree of polymerization or DP) is preferably from 2 or from 5 or from 20 or from 40 or from 70 or from 100, to 800 or to 500 or to 300 or to 200. The weight-to-weight ratio of the TR-DR′R′resin to the (PhMeSiO2 / 2)n polymer is preferably in the range of from 20:80 or from 30:70 to 80:20 or to 70:30. 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. The w / w ratio of the mica to the sum of the TR-DR′R′resin and the (PhMeSiO2 / 2)n polymer is in the range of from 90:10 or from 80:20 or from 70:30, or from 65:35, to 10:90 or to 20:80 or to 30:70 or to 35:65. In another aspect of the present invention, the composition comprises a blend of a TR-DR′R′resin; a (PhMeSiO2 / 2)npolymer; a mica; R2Si(OR3)3, where R2is C1-C12-alkyl or aryl, and R3is C1-C4- alkyl, such as methyltrimethoxysilane, phenyltrimethoxysilane, methyltriethoxysilane, and ethyltrimethoxysilane; an aprotic solvent such as propylene glycol methyl ether acetate, ethyl acetate, propyl acetate, butyl acetate or propyl propionate; and a moisture cure catalyst, for example, a tin-based catalyst such as tin octanoate or tin butanoate, or a titanium-based catalyst such as tetraisopropyl titanate, tetra-n-butyl titanate, and tetra-t-butoxy titanate. The amount of aprotic solvent is sufficient to achieve a Brookfield viscosity at 25 °C 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; alternatively, the concentration of aprotic solvent is in the range of from 5 or from 10 or from 20 weight percent, to 90 or to 75 or to 60 weight percent, based on the weight of the composition. In yet another aspect, the present invention is a process for preparing a cured coating on a substrate. The present invention is also an article comprising a substrate coated with the cured composition. The thickness of the coating is generally in the range of from 20 µm to 300 µm. The composition of the present invention provides a coating for a substrate such as a metal, a metal oxide, a ceramic, or a plastic substrate, that is tack-free in less than 30 minutes at ambient temperature, and thermally stable to cracking for hundreds or even thousands of hours.

[0002] Examples Intermediate Example 1 – Preparation of a TRDR′R′Resin 1, DMeMe0.15DPhPh0.05T0.10 TPh0.70 Water (2.5 moles) was added slowly at room temperature to a vessel containing a mixture of Me2SiCl2(0.15 mole), Ph2SiCl2(0.05 mol), MeSiCl3(0.10 mole), and PhSiCl3(0.70 mole) in toluene. The mixture was heated at 60 °C for 4 h to obtain a solution containing the TRDR′R′resin. The solution was washed with water to remove acid, then stripped under vacuum and heat to produce a solid resin, which was then converted to a resin flake by crushing. Intermediate Example 2 – Preparation of a TRDR′R′Resin 2, DMeMe0.15TMe0.40 TPh0.45 Water (2.5 moles) was added slowly at room temperature to a vessel containing a mixture of Me2SiCl2 (0.15 mole), PhSiCl3 (0.45 mole), and MeSiCl3 (0.40 mole) in toluene. The mixture was heated at 60 °C for 4 h to obtain a solution containing the TRDR′R′resin. The solution was washed with water to remove acid, then stripped under vacuum and heat to produce a solid resin, which was then converted to a resin flake by crushing. Comparative Example 1 – Preparation of TRDR′R′Resin 1 and Silanol-terminated (PhMeSiO2 / 2)120 Blend Resin 1 flake (45 pbw), silanol-terminated PPhMS (55 pbw, DP = 120), butyl acetate (30 pbw), and methyltrimethoxy silane (5 pbw) were added to a flask under N2followed by the addition of tetraisopropyl titanate (1 pbw). The contents of the flask were stirred for 1 h, then poured it into a glass bottle and sealed under N2. Comparative Example 2 – Preparation of TRDR′R′Resin 2 and Silanol-terminated (PhMeSiO2 / 2)120 Blend Resin 2 flake (90 pbw), silanol-terminated PPhMS (10 pbw, DP = 120), butyl acetate (30 pbw), and methyltrimethoxy silane (5 pbw) were added to a flask under N2followed by the addition of tetraisopropyl titanate (1 pbw). The contents of the flask were stirred for 1 h, then poured it into a glass bottle and sealed under N2. Examples 1 and 2 – Preparation of TRDR′Resins, (PhMeSiO2 / 2)120 Polymer, and Mica Blend C-4000 muscovite mica (K2Al4(Al2Si6O20)(OH)4 (median particle size 10.8 µm, obtained from IMERYS) was dried in vacuo at 120 °C for 10 h, then cooled to room temperature under N2. The dried mica (100 pbw) was added to a vessel containing the contents of either Comparative Example 1 (100 pbw) or Comparative Example 2 (100 pbw), and butyl acetate (30 pbw). The contents of the vessel were mixed by mechanical stirring under N2. The mixture was then poured into a bottle and sealed for further use. Preparation of Coatings Aluminum panels (Type A from Gardco, 3” x 6”) were washed with toluene and acetone and dried by air flow before use. A portion of the prepared formulation (2 g) was coated on the panel to form a film with a thickness in the range of from 50 µm to 100 µm films using a 4-mil drawdown bar. The coated films were dried at 70 °C for 30 min under air flow to remove solvents. The dry films were cured at room temperature or 200 °C for 10 min to 60 min, followed by thermally aging at 300 °C. Measurement of Cracking Time The cured coatings were aged in an oven at 300 °C. In the first 14 days (d), sample cracking was checked every other day for each sample, and then checked once per week. The cracking time was recorded when some cracks were observed to form in the coatings. Thermal cycle test Each formulation was coated as 100-µm thick film, followed by curing at room temperature or 150 °C, then aged at 300 °C for 10 d. Then, the samples were subjected to 100 cycles of temperature cycling between -50 °C and 150 °C at a temperature ramping rate 20 C° / min, 10 min per cycle, using a Tenney Thermal Chamber. A coated sample was deemed to pass the thermal cycle test if no cracks or delamination were observed after completion of the test. Table 1 illustrates the results of cracking time for samples with and without mica. TCT refers to thermal cycle test; pbw refers to parts by weight of each component in the formulation. Mica refers to C-4000 muscovite mica. Table 1 – Crack time and Thermal Cycle Test Results for Coatings R ExampleTDR′R′Resin PPhMS Mica CraTCT (pbw) pbwpbwck timeThe r r blends that contained mica versus mica free samples. It has been surprisingly discovered that excellent crack times can also be achieved for samples prepared by merely blending the TRDR′R′Resin, silanol-terminated PPhMS, and mica. The combination of the PPhMS 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.

Claims

Claims:

1. A composition comprising a mixture of a mica, a TR-DR′R′resin, and a silanol- or C1-C4-alkoxyl-terminated (PhMeSiO2 / 2)npolymer, where each R and R′ are independently methyl or phenyl, wherein n is in the range of from 20 to 800, wherein the weight-to-weight ratio of the sum of the TR-DR′R′resin and the (PhMeSiO2 / 2)npolymer to the mica is in the range of from 10:90 to 90:

10.

2. The composition of Claim 1 which further comprises a sufficient concentration of an aprotic solvent to achieve a composition Brookfield viscosity at 25 °C in the range of from 20 cP to 10,000 cP.

3. The composition of Claim 2 wherein the weight-to-weight ratio of the mica to the sum of the TR-DR′R′resin and the silanol- or C1-C4-alkoxyl-terminated (PhMeSiO2 / 2)n polymer is in the range of from 20:80 to 80:20; wherein the concentration of the aprotic solvent is sufficient to achieve a Brookfield viscosity at 25 ° in the range of from 50 cP to 5,000 cP.

4. The composition of Claim 2 wherein the weight-to-weight ratio of the mica to the sum of the TR-DR′R′resin and the silanol- or C1-C4-alkoxyl-terminated (PhMeSiO2 / 2)n polymer is in the range of from 30:70 to 70:30; wherein the C1-C4-alkoxyl-terminated (PhMeSiO2 / 2)n polymer is a silanol-terminated (PhMeSiO2 / 2)n polymer; and wherein the mica is muscovite, biotite, fuchsite, phlogopite, margarite, glauconite, or lepidolite; wherein the concentration of the aprotic solvent is sufficient to achieve a Brookfield viscosity at 25 °C in the range of from 100 cP to 1200 cP.

5. The composition of Claim 4 wherein n is in the range of from 5 to 300; the mica is muscovite or phlogopite; and the weight-to-weight ratio of the mica to the sum of the TR-DR′R′resin and the silanol-terminated (PhMeSiO2 / 2)n polymer is in the range of from 65:35 to 35:

65.

6. The composition of Claim 4 wherein the mica is muscovite.

7. The composition of any of Claims 1 to 6 which further comprises R2Si(OR3)3, where R2is C1-C12-alkyl or aryl; and R3is C1-C4-alkyl; and a moisture cure catalyst.

8. The composition of Claim 7 wherein R2Si(OR3)3is methyltrimethoxysilane or phenyltrimethoxysilane; the aprotic solvent is propylene glycol methyl ether acetate, ethyl acetate, propyl acetate, butyl acetate or propyl propionate; and the moisture cure catalyst is a tin-based or a titanium-based catalyst.

9. A process comprising the steps of coating a substrate with the composition of Claim 7 then curing the coating.

10. An article comprising a substrate and a coating having a thickness in the range of from 20 µm to 300 µm disposed thereupon, where the coating comprises a cured composition of Claim 7.

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