Polysiloxane Mica Blend

Abstract

The present invention is based on the use of mica, T Me A composition comprising a mixture of a resin and a polydimethylsiloxane is useful as a coating for a substrate, the coating exhibiting good adhesion and crack resistance when exposed to high temperatures for hundreds 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] Although the high-temperature resistance of silicones ostensibly makes them promising candidates for high-temperature protective coatings and sealants, silicone rubbers are nevertheless not resistant to cracking above 250°C for more than two weeks. Combining silicones with inorganic fillers such as SiO2, TiO2, and Al2O3 provides compositions with long-term high-temperature resistance. 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 thermal shock fracture, and furthermore, 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 fracture 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] The present invention is based on the use of mica, T Me A composition comprising a mixture of mica and T MeThis needs in the art is addressed by providing a composition in which the weight to weight ratio of resin to polydimethylsiloxane is in the range of 10:90 to 90:10.The composition of the present invention is useful as a coating for substrates, which coating exhibits good adhesion and crack resistance when exposed to high temperatures for hundreds of hours. DETAILED DESCRIPTION OF THE INVENTION

[0004] The present invention is based on the use of mica, T Me A composition comprising a mixture of mica and T Me The composition has a weight-to-weight ratio of resin to polydimethylsiloxane in the range of 10:90 to 90:10.The composition of the present invention is useful as a coating for a substrate, and the coating exhibits good adhesion and crack resistance when exposed to high temperatures for hundreds of hours.

[0005] "T Me The term "resin" refers to the Me-SiO 3 / 2 , Me-SiO 2 / 2 (OZ), and optionally 、 Me-SiO 1 / 2 It refers to a kinetically stable three-dimensional polymer having repeating units of the formula (OZ)2, where Me-SiO 3 / 2 The unit is represented by the following structure:

[0006] [ka] Me-SiO 2 / 2 (OZ) is represented by the following structure:

[0007] [ka] Z is H, C1-C4-alkyl, or C(O)CH3, and MeSiO 1 / 2 The (OZ)2 unit is represented by the following structure:

[0008] [ka]

[0009] T Me Each Z in the resin is preferably methyl. Me Resins include DOWSIL™ RSN-2403 and DOWSIL™ RSN-2405 Flake Resins (trademarks of The Dow Chemical Company or its affiliates).

[0010] Polydimethylsiloxane (PDMS) contains repeating units of dimethylsiloxane groups, as shown below:

[0011] [ka] In the formula, n (alternatively, the degree of polymerization) is preferably 2, 5, 20, 40, 70, 100 to 800, 500, 300, or 200. Me The weight to weight ratio of resin to PDMS preferably ranges from 20:80 or 30:70 to 80:20 or 70:30.

[0012] Micas are hydrated aluminum silicate minerals that include muscovite, biotite, chromite, phlogopite, nacre, glauconite, and lepidolite, of which muscovite and phlogopite are predominant. Me The total w / w ratio of resin to PDMS ranges from 90:10, 80:20, 70:30, or 65:35 to 10:90, 20:80, 30:70, or 35:65.

[0013] In another aspect of the invention, the composition comprises T Me Blend of resin and PDMS; Mica; R is C1-C 12-alkyl or aryl, and R' is 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 stannous octoate or stannous butanoate, or a titanium-based catalyst, such as tetraisopropyl titanate, tetra-n-butyl titanate, and tetra-t-butoxy titanate.

[0014] The amount of aprotic solvent is sufficient to achieve a Brookfield viscosity at 25°C ranging from 20 cP, or 50 cP, or 100 cP to 20,000 cP, or 10,000 cP, or 5,000 cP, or 1200 cP. Alternatively, the concentration of the aprotic solvent ranges from 5, or 10, or 20 weight percent to 90, or 75, or 60 weight percent, based on the weight of the composition.

[0015] 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 coating thickness generally ranges from 20 μm to 300 μm.

[0016] The compositions of the present invention provide coatings for substrates, such as metal, metal oxide, ceramic, or plastic substrates, that are non-tacky in less than 30 minutes at ambient temperature and thermally stable against cracking for hundreds or thousands of hours. [Example]

[0017] In the following examples, pbw refers to parts by weight.

[0018] Intermediate Example 1-T Me Preparation of resin and PDMS blends DOWSIL™ RSN-2403 Flake Resin (2403 resin, 40 pbw), silanol-terminated PDMS (60 pbw, n=80), methyltrimethoxysilane (5 pbw), tetraisopropyl titanate (1 pbw), and butyl acetate (30 pbw) were added to a dry flask under N to form a blend with a solids content of 69%. The mixture was stirred for 30 minutes.

[0019] Intermediate Example 2-T Me Preparation of resin and PDMS blends T Me The procedure of Intermediate Example 1 was repeated, except that DOWSIL™ RSN-2405 Flake Resin (2405 resin, 35 g) was used as the resin.

[0020] Comparative Examples 1 and 2 were prepared by adding tetraisopropyl titanate (1 pbw) to a vessel containing the contents of either Intermediate Example 1 (100 pbw) or Intermediate Example 2 (100 pbw).

[0021] Example 1 and Example 2-T Me Preparation of resin, PDMS, and mica blends Examples were prepared by the following general procedure: C-4000 muscovite (KAl(AlSiO 20 )(OH)4 (median particle size 10.8 μm, obtained from IMERYS) was dried in vacuo at 120° C. for 10 hours and then cooled to room temperature under N2. Dried mica (100 pbw) was added to a vessel containing the contents of either Intermediate Example 1 (100 pbw) or Intermediate Example 2 (100 pbw), tetraisopropyl titanate (1 pbw), and butyl acetate (30 pbw). The contents of the vessel were mixed with a mechanical stirrer under N2. The mixture was then poured into a bottle and sealed for further use.

[0022] Preparation of coatings Aluminum panels (Gardco Type A, 3" x 6") were cleaned with toluene and acetone and dried under airflow before use. A portion (2 g) of the prepared formulation was coated onto the panel using a 4-mil drawdown bar to form a film with a thickness ranging from 50 μm to 100 μm. The coated film was dried under airflow at 70°C for 30 minutes to remove the solvent. The dried film was cured at room temperature or at 200°C for 10 to 60 minutes, followed by heat aging at 300°C.

[0023] Cracking time measurement The cured coatings were aged in an oven at 300°C. Each sample was checked for cracking every other day for the first 14 days (day, d), then once a week. The cracking time was recorded when cracking was observed in the coating.

[0024] Thermal cycling test: Each formulation was coated as a 100 μm thick film, subsequently cured at room temperature or 150°C, and then aged at 300°C for 10 days. Using a Tenney Thermal Chamber, the samples were then subjected to 100 cycles of temperature cycling from -50°C to 150°C at a temperature ramp rate of 20°C / min, 10 minutes per cycle. If no cracking or delamination was observed after the test was completed, the coated sample was considered to have passed the thermal cycling test.

[0025] Table 1 shows the time-to-cracking results for samples with and without mica. TCT refers to thermal cycling test. Mica / wt% refers to the weight percent of mica based on the weight of the blend and mica or copolymer and mica. C-4000 refers to C-4000 muscovite.

[0026] [Table 1] a The time-to-cracking test was stopped at 1200 hours.

[0027] The results show a dramatic difference in cracking time and thermal cycling test results between the blends containing mica versus the samples without mica. Me It was discovered that excellent crack times could also be achieved for samples prepared by simply blending resin, PDMS, and mica. Me The combination of resin and mica alone was found to fail the crack test within 2 days, whereas the combination of PDMS and mica alone easily 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 that exhibited a crack time greater than 120 hours.

Claims

1. Mica, T Ｍｅ A composition comprising a mixture of a mica and a resin, and a polydimethylsiloxane, Ｍｅ A composition wherein the weight to weight ratio of the total of resin and said polydimethylsiloxane ranges from 10:90 to 90:

10.

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

3. The mica vs. Ｍｅ 3. The composition of claim 2, wherein the weight to weight ratio of resin to polydimethylsiloxane in total is in the range of 20:80 to 80:20, the polydimethylsiloxane has a degree of polymerization in the range of 2 to 800, and the concentration of the aprotic solvent is sufficient to achieve a Brookfield viscosity at 25° C. in the range of 50 cP to 5,000 cP.

4. The mica vs. the T Ｍｅ 4. The composition of claim 3, wherein the weight to weight ratio of resin to polydimethylsiloxane in total is in the range of 30:70 to 70:30, the mica is muscovite, biotite, chromite, phlogopite, nacreous mica, glauconite, or lepidolite, and the concentration of the aprotic solvent is sufficient to achieve a Brookfield viscosity at 25° C. in the range of 50 cP to 5,000 cP.

5. The degree of polymerization of the polydimethylsiloxane is in the range of 5 to 300, the mica is muscovite or phlogopite, and the ratio of the mica to the T Ｍｅ 5. The composition of claim 4, wherein the weight to weight ratio of resin to polydimethylsiloxane combined is in the range of 65:35 to 35:65, and the concentration of the aprotic solvent is sufficient to achieve a Brookfield viscosity at 25° C. in the range of 100 cP to 1200 cP.

6. The composition of claim 5 wherein the mica is muscovite.

7. R is C １ -C １２ - alkyl or aryl, and R' is C １ -C ４ -alkyl, RSi(OR') ３ and a moisture cure catalyst.

8. RSi(OR') ３ is 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 titanium-based catalyst.

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

10. 10. An article comprising a substrate and a coating disposed thereon having a thickness in the range of 20 μm to 300 μm, wherein the coating comprises the cured composition of claim 7.

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