Coating compositions

Abstract

This invention relates to lubricating coating compositions containing a binder resin and a solid lubricant with a specific ratio, having excellent sliding property and insulating property without generating blister.

Description

COATING COMPOSITIONS

[0001] This invention relates to lubricating coating compositions having excellent sliding properties and insulating properties, as well as a sliding member with a coating film formed from the coating compositions.

[0002] Insulating coatings are applied on metal surfaces of many industrial materials. However, when the coatings are used for a sliding member such as a stator for rotor or a winding in a motor, the coatings tend to be damaged because the surface of such sliding member is frequently rubbed with another component.

[0003] To increase wear resistance of a coating film, a coating composition containing a hard particle is known. For example, JP2020090562A discloses a coating composition containing a binder resin, a lubricant and a hard particle.

[0004] However, it is difficult to get insulating coatings with both excellent sliding property and high durability. When increasing the amounts of lubricants or hard particles in a coating composition, the amount of binder resin is relatively decreased thus the insulating property of a coating film would decrease. In addition, other resin-oriented properties such as adhesion property is also decreased. Therefore, insulating coatings with excellent sliding property as well as high durability is still required.

[0005] When a coating is applied to a substrate, ‘blistering’ or ‘bubbling’ is observed on the surface of the coating. In this specification, the words ‘blistering’ and ‘bubbling’ are used synonymously. Blistering means the formation of bubbles which is under or within a coating film, and it is known as a coating failure. One expected mechanism of the blister formation is, when coating is applied on a porous substrate or a rough substrate, air trapped in the pore of the substrate or valley of the rough substrate is sealed by the applied coating. When a coating composition is stirred excessively, air is also included in the coating composition. Normally, such air is evaporated together with a solvent during cure process, but when the surface of the coating film is dried first and a skinning layer is formed on the surface of the coating for some reason, such air cannot be evaporated and forms blistering. When the thickness of a coating film increased, or when a rough substrate is used, blister is often observed and causing a problem.

[0006] Normally, defoamer is added in a coating composition to reduce bubbles in the coating composition, but such defoamer cannot fully suppress the blister generation during cure process of the coating composition.

[0007] Specific formulation of the coating compositions, especially for the volume ratio of binder resin and solid particles / lubricants improve both sliding property and durability of the coating compositions. In addition, specific amount of epoxy resin works to reduce blister generation significantly, even if the coating film is formed on a rough surface.

[0008] Therefore, one aspect of the invention is a coating composition comprising: (A) at least one binder resin comprising: (i) a resin selected from the group consisting of polyamideimide, polyimide and benzocyclobutene, (B) a solid lubricant, and (C) optionally another solid particle excepting for the solid lubricant, in which the volume ratio of [(B)+(C)] / (A) is 0.1 to 0.3.

[0009] Another aspect of the invention is, a coating film prepared from the coating composition.

[0010] Further another aspect of the invention is, a sliding member having the coating film.

[0011] The invention also directed to a method for forming the coating film on a sliding member.

[0012] Compounds A) Binder resins (i) Binder resin used in the invention is a resin which forms a lubricating film as a insulating resin and has a function as a binder for supporting a solid lubricant described later. Resins which can be used include, for example, polyamideimide (PAI), polyimide (PI), benzocyclobuten (BCB), phenol resin, polyamide, polybenzimidazole, polyphenyl sulfonate and polyether ether ketone, and one or more of these. Preferably, the binder resin contains polyamideimide, polyimide and benzocyclobuten. More preferably, the binder resin contains polyamideimide. The content of the binder resin is 60 to 95 % by weight, preferably 70 to 90 % by weight, based on the weight of the solid contents of the whole coating composition.

[0013] (ii) Epoxy resin Binder resin can optionally comprise epoxy resin. When a binder resin comprises epoxy resin, blistering is significantly reduced and the surface of the coating film can be smooth. Although not bound by theory, inventors consider that: epoxy resin can soften the skinning layer of the coated composition or delayed the speed of the formation of skinning layer. As a result, air bubbles inside of a coating composition are easily evaporated together with solvent and do not generate blister during curing step. The amount of epoxy resin is, normally from 1 to 40, preferably from 10 to 30, more preferably from 15 to 25 parts by weight based on 100 parts by weight of the binder resin (A)(i).

[0014] The epoxy resin is not particularly limited, and one or more types may be selected and used including bisphenol based epoxy resin, amine based epoxy resin, phenol novolac based epoxy resin, cresol novolac based epoxy resin, resorcinol based epoxy resin, phenol aralkyl based epoxy resin, naphthol aralkyl based epoxy resin, dicyclopentadiene based epoxy resin, epoxy resin having a biphenyl backbone, isocyanate modified epoxy resin, tetraphenyl ethane based epoxy resin, triphenyl methane based epoxy resin and fluorene based epoxy resin.

[0015] Generally, bisphenol based epoxy resin is an epoxy resin for which two phenolic hydroxyl groups of a bisphenol compound have been glycidylated, and examples include a bisphenol A type, a bisphenol F type, a bisphenol AD type, a bisphenol S type, or halogen or alkyl substitutes, hydrogenated products, dimer acid-modified products, or the like of these bisphenols. Moreover, the bisphenol based epoxy resin is not limited to monomers, and polymers having a plurality of repeating units can also be favourably used.

[0016]

[0017] B) Solid lubricants The coating compound of the invention comprises one or more solid lubricants (B). The solid lubricants can provide sliding property to a coating film formed from the composition. Solid lubricants which can be used include, for example, polytetrafluoroethylene (PTFE), polyamide (PA), polyether ether ketone (PEEK), polyethylene (PE), polypropylene (PP), and acryl resin. Preferably, solid lubricant contains PTFE. The amount of solid lubricants is, normally from 10 to 50 parts by mass, preferably from 20 to 40 parts by mass based on the binder resin (A).

[0018] In addition, the amount of solid lubricants is determined to satisfy the condition that the volume ratio of [(B)+(C)] / (A) is 0.1 to 0.3, as disclosed later.

[0019] (C) Other solid particles The coating composition of the invention optionally comprises one or more other solid particles in addition to the solid lubricants (B).

[0020] One example of the other solid particles is hard particles. Hard particles can provide wear resistance to a coating film formed from the composition. Hard particles mean that particles having a new Mohs hardness of more than 4, preferably more than 9. Hard particles include tungsten carbide, titanium nitride, zirconia, alumina, and silicon carbide.

[0021] Examples of other solid particles include; coloring materials inculding colorants such as pigments (inorganic colorants and organic colorants); carbon (for example, carbon black, fullerene and carbon nanotube); ultraviolet absorption (or blocking) include metal oxides (or metal oxide particles) such as titanium oxide and zinc oxide; fibrous (for example, glass fibers, carbon fibers, metal fibers and whiskers); ferromagnetic materials such as ferromagnetic metals (powder) such as iron, cobalt and nickel; ferromagnetic alloys (powder) such as magnetite and ferrite; ferromagnetic metal oxides such as magnetic iron oxide (powder); metal hydroxides (for example, aluminum hydroxide); metal salts (for example, metal sulfates and carbonates such as calcium carbonate, metal phosphates such as calcium phosphate and titanium phosphate); mica; calcium silicate; and silicates such as bentonite, zeolite, talc and montmorillonite.

[0022] When the coating composition comprises other solid particles, the amount of such solid particles is, normally from 0.1 to 5 parts by mass, preferably from 0.2 to 1.0 parts by mass based on the binder resin (A).

[0023] The key of the invention is that; the volume ratio of [(B)+(C)] / (A) is from 0.1 to 0.3. When the volume ratio is within the range, a coating film having both excellent sliding property and insulating property is obtained. When the ratio is less than 0.1, the amount of solid lubricants is not enough thus the COF (Coefficient of friction) of a coating film becomes high. Consequently, the coating film would be worn in a short time. On the other hand, when the ratio is larger than 0.3, adhesion between the coating film and a substrate becomes low, or the coating film cannot retain the solid lubricants well thus the coating film would be worn in a short time. In addition, the air space between binder resin and solid lubricants / other solid particles would increase thus the insulating property of the coating film becomes low. Such air space can be observed by SEM analysis after vertical / hodizontal direction cutting of the coating film.

[0024] (D) Solvents The coating compounds described herein may additionally comprise, solvents and liquids additives. Examples of solvents are; ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone; esters such as methyl acetate and ethyl acetate; aromatic hydrocarbons such as toluene and xylene; organic halogen compounds such as methyl chloroform, trichloroethylene and trichlorotrifluoroethane; N-methyl-2-pyrrolidone (NMP); N-ethyl-2-pyrrolidone (NEP); 1,3-Dimethyl-2-imidazolidinone (DMI); γ-butyrolactone (GBL); 3-methoxy-N, N-dimethylpropanamide (MPA); methylisopyrrolidone (MIP); dimethylformaldehyde (DMF); dimethylacetaldehyde (DMAC) and 1-Butyl-2-pyrrolidone (NBP). The solvent may be one or mixture of two or more. Particularly preferred solvents are NEP, DMI, GBL and MPA.

[0025] (E) Other ingredients The coating compounds described herein may additionally comprise liquid additives different from the solid particles (C). Examples of liquid additives are; UV absorber, a light stabilizer, an antioxidant, a thermal polymerization inhibitor, a leveling agent, a deformer, a thickener, an anti-settling agent, infrared absorbers, fluorescent whitening agents, dispersants, antistatic agents, antifogging agents, coupling agents and thixotropic agent. Thixotropic agents include, for example, fillers such as bentonite, calcium carbonate, kaolin, and clay, polymerized linseed oil, castor oil wax, polyolefin, silica gel, aluminum organic acid salts, cellulose, amide waxes, and polyamide-based thixotropic agents. Polymer-type leveling agents can be used, such as acrylic-based and silicone-based leveling agents, Polyoxyalkylene fatty acid alkanolamide derivative.

[0026] Process for forming a coating film on a sliding member The process comprising three steps, i.e. (a) preparing a coating composition comprising (A), (B) and optionally (C) disclosed above, (b) applying the coating composition on the surface of a sliding member, then (c) curing the coated composition and forming a coating film on the surface of the sliding member.

[0027] Step (a) A coating composition comprising (A), (B) and optionally (C) can be prepared by mixing the composition using a propeller mixer, a rotation / revolution mixer, or any other mixing equipments. Solvent can be added to get a coating composition with an appropriate viscosity, to coat the composition on a sliding member.

[0028] Step (b) The coating composition is applied on the surface of a sliding member, by dipping, spin coating, flow coating, spraying, bar coating, gravure coating, roll coating, blade coating, screen printing, air knife coating and any other methods. The thickness of the coating film is not particularly limited, but a thickness of 1 to 100 μm is preferable, and a thickness of 40 to 80 μm is more preferable. Optionally, pre-treatment of a sliding member can be conducted before applying the coating composition.

[0029] Step (c) The coated composition is cured and form a coating film on the surface of a sliding member. Normally thermal curing is conducted, by heating in an oven or any other equipments. During thermal curing of the applied composition, the solvent in the composition can be removed in the first phase of heating, and it can be cured by crosslinking reaction in the second phase of heating. For example, the first phase of heating may be performed at 60 to 100 °C for 5 to 30 minutes, and then the second phase of heating may be performed at 180 to 250 °C. for 20 to 120 minutes.

[0030] Sliding member The coating composition is useful for insulating coatings on metal surfaces of a sliding member. Examples of such sliding member include; inner or outer ring for bearing, bearing shaft, EV stator core, battery tray, PCU (power control unit) and housing (including inverter case and DC / DC converter).

[0031] This coating composition can be used for metal surfaces of sliding member even if the surface is rough, for example, the surface roughness is 10 μm or more. The Surface roughness (Ra) is defined as the arithmetic average roughness based on JIS B0601 (2001). Ra of the surface of the sliding member to be coated can be preferably 0.1 to 20 μm, alternatively, Ra of the surface of the member can be 1 to 10 μm. Normally, it was difficult to get smooth coating surface without blistering when the surface of a member is rough. This coating composition, especially for the coating composition comprising epoxy resin, can provide smooth coating surface without blister generation.

[0032] Materials 1. Binder resins [A] PAI-A: polyamideimide resin dissolved in NEP (solid content is 35 wt%) (Specific gravity：1.4) PAI-B: polyamideimide resin dissolved in GBL (solid content is 36 wt%) (Specific gravity：1.4) PAI-C: polyamideimide resin dissolved in MPA (solid content is 35 wt%) (Specific gravity：1.4) PI-A: Polyimide resin dissolved in NEP (solid content is 20 wt%)(Specific gravity：1.4) Epoxy-A: Epoxy resin, product name is jER828, Bishenol A type epoxy resin, available from Mitsubishi chemical (Specific gravity：1.4) Epoxy-B: Epoxy resin, product name is jER1007, Bishenol A type epoxy resin, available from Mitsubishi chemical (Specific gravity：1.4) BCB: Benzocyclobutene resin dissolved in Mecytylene, Cyclotene 3022-63 available from DuPont, solid content is 63wt%. (Specific gravity：1.4) Phenol: Phenol resin, product name is TD-2495, available from DIC Corporation. (Specific gravity：1.4)

[0033]

[0034] 3. Solid additives [C] CB: Carbon black MA-100, available from Mitsubishi Chemical, having a median diameter of 1 to 2 μm as measured by laser diffraction scattering particle size distribution measurement (Specific gravity：1.7)

[0035] 4. Solvents [D] NEP: N-ethyl-2-pyrrolidone available from Maruzen Chemical GBL: Gammabuthyrolactone available from Mitsubishi Chemical MPA: 3-methoxy-N, N-dimethylpropanamide available from KJ Chemicals Xylene: Xylene available from Kyoei Chemical DAA: Diacetone Alcohol available from Sankyo Chemical MEK: Methyl ethyl ketone available from Sankyo Chemical

[0036] 5. Other ingredients [E] Defoamer: Xiameter OFX-0230 Fluid available from Dow chemicalExamples 1-15

[0037] The ingredients disclosed Tables 1-4 were mixed by rotation / revolution mixer at room temperature. The mixture was diluted with solvent then the solid content concentration was adjusted about 23-26% by mass. The coating compositions have been coated by spray coating on a test sample (material: steel plate cold commercial (SPCC)-SB plate, size: 150mm x 70mm x 0.6mm, surface roughness Ra=0.1). The thickness of the spray coated layer is 0.07 mm. Then the test samples were put in an oven at 230 degree C for 60 minutes to evaporate solvent and cure. The thickness of the coated film was 0.05 mm. The following evaluation tests were conducted.

[0038] Analytical Methods Dielectric strength: The coated SPCC-SB plate was measured dielectric strength by ASTM D149-09 (JIS C2010-1) with Automatic insulation voltage tester 3153 by HIOKI. Trigger to leakage current 0.5mA, Step up by 0.5kV (AC) and keep 20 seconds to see OK or Fail. If OK, the voltage will be changed to 1.0kV, then keep 20 seconds to continue until fail.

[0039] Durability: The coated ring on plate was carried out by EFM-III-1010 available from Orientec Corporation. Rotation speed: 2000rpm, Load 1000N until seizure.

[0040] Surface roughness: Surface roughness was measured according to JIS 2001:2013. Surface texture and contour measuring instruments Surfcom 1400D available from Accretech. was used. Measurement speed for needle: 0.6mm / seconds.

[0041] Blister: The appearance of coated SPCC-SB was observed by microscope Olympus DP28 to confirm the existence of blister.

[0042] Numbers are described as parts by weight for each ingredient at Tables 1-4. *[(B) + (C)] / (A) volume ratio was calculated by converting units from weight to volume by using Specific gravity of each ingredient. For example, Ex 2: 0.11= (8 / 2.0 + 8 / 2.0 + 0.5 / 1.7) / 100 / 1.4. **Durability (minutes) : more than 5 minutes is desirable. **Blister : 〇 indicates no existence of blister, × indicates some existence of blister.

[0043]

[0044]

[0045]

Claims

1. A coating composition comprising: (A) at least one binder resin comprising: (i) a resin selected from the group consisting of polyamideimide, polyimide and benzocyclobutene, (B) a solid lubricant, and (C) optionally another solid particle excepting for the solid lubricant, wherein the volume ratio of [(B)+(C)] / (A) is 0.1 to 0.3.

2. The coating composition of claim 1, wherein the solid lubricant is polytetrafluoroethylene.

3. The coating composition of claim 1, wherein the solid particle is carbon black.

4. The coating composition of claim 1, wherein the binder resin (A) further comprises (ii) epoxy resin in addition to (i).

5. The coating composition of claim 4, wherein the weight ratio of (A)(i) / (A)(ii) is from 100 / 1 to 100 / 40.

6. A coating film prepared from the coating composition of claim 1, wherein the coating film has 50 kV / mm or more of dielectric strength.

7. A sliding member, having a coating film that, before curing, comprised: (A) at least one binder resin comprising: (i) a resin selected from the group consisting of polyamideimide, polyimide and benzocyclobuten, (B) a solid lubricant, and (C) optionally another solid particle excepting for the solid lubricant, wherein the volume ratio of [(B)+(C)] / (A) is 0.1 to 0.3, and the coating film has 50 kV / mm or more of dielectric strength.

8. A sliding member, having a coating film that, before curing, comprised: (A) at least one binder resin comprising: (i) a resin selected from the group consisting of polyamideimide, polyimide and benzocyclobutene, and (ii) epoxy resin, (B) a solid lubricant, and (C) optionally another solid particle excepting for the solid lubricant, wherein the volume ratio of (B)+(C) / (A) is 0.1 to 0.3, and the surface of the coating film has 1 or less of Ra measured by according to JIS 2001:2013.

9. The sliding member of claim 8, wherein the surface of the sliding member to be coated with the coating film has 10 or more of Ra measured by according to JIS 2001:2013.

10. The sliding member of claims 7 or 8, in the form of an inner or outer ring for bearing, a bearing shaft, an EV stator core, a battery tray, power control unit, housing for inverter case, and DC / DC converter.

11. A method for forming a coating film on a sliding member, comprising the steps of: (a) preparing a coating composition comprising (A) at least one binder resin comprising: (i) a resin selected from the group consisting of polyamideimide, polyimide and benzocyclobutene, (B) a solid lubricant, and (C) optionally another solid particle excepting for the solid lubricant, (b) applying the coating composition on the surface of a sliding member, then (c) curing the coating composition and forming a coating film on the surface of the sliding member.

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