Mica-containing silicone coating composition

Abstract

The present invention relates to a composition comprising an MQ resin, a ZO-terminated poly (dimethylsiloxane), and mica. The composition can be used as a coating on a substrate wherein the coating exhibits adhesion, uniformity and crack resistance when subjected to high temperatures for hundreds or even thousands of hours.

Description

Background Art

[0001] The present invention relates to a silicone coating composition, more specifically 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 protect various equipment and devices from extremely high temperatures. For example, heater elements for electric vehicles, exhaust systems for automotive engines, power plants, and top coatings for stoves all benefit from such protective coatings. In many applications, the coatings must withstand temperatures in excess of 300°C for months without cracking or loss of dielectric and insulating properties, and must pass aggressive thermal shock tests over a wide temperature range.

[0002] The high temperature resistance of silicones on surfaces makes them promising candidates as high temperature protective coatings and sealants; however, silicone rubber is not resistant to cracking at temperatures above 250°C for more than 3 weeks. 2 、TiO 2 and Al 2 O 3 The combination of provides a composition with long-term high temperature resistance; however, coatings prepared from such compositions need to be aged at temperatures exceeding 500°C to form ceramic-like coatings. At such extreme temperatures, the coatings may crack and suffer thermal shock failure; in addition, electronic components under the coating surface are easily damaged. Therefore, it would be an advancement 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 for a long time at temperatures exceeding 300°C. 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) is in the range of 70:30 to 10:90, and the weight ratio of the mica to the sum of the MQ resin and the ZO-terminated poly(dimethylsiloxane) is in the range of 70:30 to 30:70, wherein each Z is independently H, C 1 -C 4 -alkyl or C(O)CH 3 The compositions of the present invention are useful as coatings for substrates where the coatings, when cured, exhibit adhesion and crack resistance when subjected to elevated temperatures for hundreds of hours. DETAILED DESCRIPTION

[0004] The present invention relates to a composition comprising MQ resin, ZO-terminated poly(dimethylsiloxane-) and mica, wherein the weight ratio of MQ resin to ZO-terminated poly(dimethylsiloxane) is in the range of 70:30 to 10:90, and the weight ratio of mica to the sum of MQ resin and ZO-terminated poly(dimethylsiloxane) is in the range of 70:30 to 30:70, wherein each Z is independently H, C 1 -C 4 -alkyl or C(O)CH 3 .

[0005] As used herein, the term "MQ resin" refers to a kinetically stable three-dimensional polymer having SiO 4 / 2 Repeating unit (Q) and multiple tri-C 1 -C 4 The resin may include additional end-capping groups such as C 1 C 4 -alkyl, dimethylhydroxysilyl and dimethylvinylsilyl end capping groups. Examples of commercially available MQ resins are DOWSIL TM 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 illustrated by the following structure:

[0007]

[0008] wherein n is preferably 20 or 40 or 70 or 100, to 300, to 250, or to 200.

[0009] The weight ratio of MQ resin to ZO-terminated poly(dimethylsiloxane) ranges from 70:30 or 50:50 or 40:60 to 10:90 or to 20:80 or to 25:75.

[0010] Mica is a hydrated aluminum silicate mineral, including muscovite, biotite, chrome mica, phlogopite, pearl mica, glauconite and lepidolite, among which muscovite and phlogopite are the main ones. Muscovite has a typical composition of K 2 Al 4 (Al 2 Si 6 O 20 )(OH) 4 The w / w ratio of mica to the sum of the MQ resin and the ZO-terminated poly(dimethylsiloxane) is in the range of 70:30 or 60:40 to 30:70 or 30:60.

[0011] The composition of the present invention advantageously further comprises a crosslinking agent, such as C 1 -C 4 -Alkyltri-C 1 -C 4 Alkoxysilane, preferably methyltrimethoxysilane (MTMS) and moisture curing catalyst, to promote the curing of composition after it is applied as coating on substrate.The example of moisture curing catalyst includes organotin and organotitanate catalyst, such as tin octoate, tin butanoate (tin butanoate), tetraisopropyl titanate, tetra-n-butyl titanate and tetra-tert-butyl titanate.This curable composition can be by first MQ resin, ZO-PDMS-OZ, crosslinking agent and moisture curing catalyst are blended in the presence of solvent to adjust viscosity to the desired level, preferably in the range of 20cP or 50cP or 100cP to 20,000cP or to 10,000cP or to 5,000cP or to 1200cP to prepare.The example of suitable solvent includes aprotic solvent, such as ethyl acetate, propyl acetate, butyl acetate, propyl propionate and hexamethyldisiloxane (HMDS). The blend is then advantageously contacted with mica and additionally blended before being applied to a substrate, such as a metal substrate, metal oxide substrate, ceramic substrate or glass substrate, at a desired coating thickness, typically in the range of 10 μm or 20 μm or 50 μm to 200 μm or to 100 μm. The coating is then dried and heat aged.

[0012] During heat aging, at least some portion of the MQ resin was observed to react with at least some 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.

[0013] The compositions of the present invention provide tack-free coatings within minutes that are thermally stable without cracking for hundreds or even thousands of hours.

[0014] Example 1-5—Preparation of blends of MQ resin, silanol-terminated PDMS and mica

[0015] 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 a sufficient amount of hexamethyldisiloxane to adjust the viscosity of the mixture to 500 cp to 2000 cp were added to a dry flask. The mixture was stirred for 30 minutes, and then mica was added to the mixture while stirring under nitrogen. The mica had been dried in a vacuum at 120°C for 3h to 20h and then cooled to room temperature under nitrogen. MRX white mica (MRX, median particle size 11.4μm, available from Arctic Minerals) was used for Examples 1, 2 and 3, and C-4000 white mica (C-4000, median particle size 10.8μm, available 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.Comparative Example Formulation (C1) did not include mica.

[0016] Long-term high temperature resistance test

[0017] Aluminum panels (3" x 6") were washed with toluene and acetone and then dried. A portion of the composition was applied at a thickness of 50 μm to 100 μm using a 4 mil drawdown bar. Panels coated with the example formulations containing mica became tack-free within 10 minutes at room temperature, while panels coated with the mica-free formulation (C1) became tack-free within 1 hour. Each sample was then heated in an oven at 300°C. The time to film cracking (in days) was recorded as the first instance of visible cracks in the coating.

[0018] Table 1 shows the thermal stability of the coatings measured by cracking time. The mica weight percentage is based on the sum of the weights of MQ resin, HO-PDMS-OH and mica. The MQ resin and HO-PDMS-OH weight percentages are based on the sum of the MQ resin and HO-PDMS-OH.

[0019] Table 1 - Thermal stability of coatings

[0020] Example MQ wt% HO-PDMS-OH weight% Mica / weight% Cracking time 1 35 65 MRX / 40 >90 days 2 30 70 MRX / 40 >90 days 3 30 70 MRX / 50 >100 days 4 30 70 C-4000 / 40 >90 days 5 30 70 C-4000 / 50 >100 days C1 30 70 No Mica 3 days

[0021] The data show that the cured coatings containing mica exhibited significant resistance to cracking and delamination. In addition, the samples containing mica cured much faster than the samples without mica. It was found that the combination of MQ resin and mica alone failed the cracking test within 2 days, while the combination of HO-PDMS-OH and mica alone easily delaminated from the substrate at 300°C. In addition, among the fillers tested (silicon dioxide, calcium carbonate, aluminum silicate, calcium silicate, aluminum oxide, iron oxide and mica), mica was found to be the only type of filler that exhibited a cracking time of more than 120 hours.

Claims

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 sum of the MQ resin and the ZO-terminated poly(dimethylsiloxane) is in the range of 70:30 to 30:70, wherein each Z is independently H, C 1 -C 4 -alkyl or C(O)CH 3 .

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

3. The composition of claim 2, wherein the mica is muscovite or phlogopite, and wherein 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. The composition of claim 3, wherein the weight ratio of the MQ resin to the HO-terminated poly(dimethylsiloxane) is in the range of 50:50 to 20:

80.

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

75.

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

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 according to claim 6.

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

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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