Coating composition

Abstract

The present invention relates to a lubricating coating composition containing a binder resin and a solid lubricant in a specific ratio, having excellent sliding characteristics and insulating characteristics without generating blisters.

Description

Technical Field

[0001] The present invention relates to a lubricating coating composition having excellent sliding and insulating properties, and a sliding member having a coating film formed from the coating composition. Background Technology

[0002] Insulating coatings are applied to the metal surfaces of many industrial materials. However, when these coatings are used on sliding components (such as the stator or windings of a motor rotor), they are prone to damage because the surface of such sliding components is frequently rubbed against another component.

[0003] To increase the wear resistance of coating films, a coating composition containing hard particles is known. For example, JP2020090562 A discloses a coating composition containing a binder resin, a lubricant, and hard particles.

[0004] However, it is difficult to obtain an insulating coating that simultaneously possesses excellent sliding properties and high durability. When the amount of lubricant or hard particles in the coating composition is increased, the amount of binder resin relatively decreases, thus reducing the insulating properties of the coating film. Furthermore, other resin-oriented properties, such as adhesion properties, also decrease. Therefore, an insulating coating with both excellent sliding properties and high durability remains in demand.

[0005] When a coating is applied to a substrate, “bubbling” or “blistering” is observed on the coating surface. In this specification, the terms “bubbling” and “blistering” are used synonymously. Bubbling refers to the formation of air bubbles under or within the coating film, and it is considered a coating failure. One expected mechanism for bubbling formation is that when a coating is applied to a porous or rough substrate, air trapped in the pores of the substrate or the valleys of the rough substrate is sealed by the applied coating. Air can also be incorporated into the coating composition when it is over-stirred. Normally, this air evaporates with the solvent during the curing process; however, when the surface of the coating film dries first and a skin layer forms on the coating surface for some reason, this air cannot evaporate and bubbling occurs. Bubbling is frequently observed and causes problems when the thickness of the coating film increases, or when a rough substrate is used.

[0006] Typically, defoamers are added to coating compositions to reduce air bubbles, but such defoamers cannot completely suppress foaming during the curing process of the coating composition. Summary of the Invention

[0007] The specific formulation of the coating composition, especially the volume ratio of binder resin and solid particles / lubricant, improves both the sliding properties and durability of the coating composition. Furthermore, even when forming a coating film on rough surfaces, a specific amount of epoxy resin can significantly reduce bubbling.

[0008] Therefore, one aspect of the present invention is a coating composition comprising:

[0009] (A) At least one adhesive resin comprising: (i) a resin selected from the group consisting of: polyamide-imide, polyimide, and benzocyclobutene,

[0010] (B) Solid lubricants, and

[0011] (C) Optionally, another solid particle besides the solid lubricant.

[0012] The volume ratio of [(B) + (C)] / (A) is between 0.1 and 0.3.

[0013] Another aspect of the present invention is a coating film prepared from a coating composition.

[0014] Another aspect of the present invention is a sliding member having a coated film.

[0015] The present invention also relates to a method for forming a coating film on a sliding member. Detailed Implementation

[0016] Blend

[0017] A) Adhesive resin

[0018] (i) The binder resin used in this invention is a resin that forms a lubricating film as an insulating resin and functions as a binder for supporting the solid lubricant described below. Resins that can be used include, for example, polyamide-imide (PAI), polyimide (PI), benzocyclobutene (BCB), phenolic resins, polyamides, polybenzimidazoles, polyphenylene sulfonate, and polyetheretherketones, and one or more of these. Preferably, the binder resin includes polyamide-imide, polyimide, and benzocyclobutene. More preferably, the binder resin includes polyamide-imide.

[0019] The content of the binder resin is 60 to 95% by weight, preferably 70 to 90% by weight, based on the total solids content of the coating composition.

[0020] (ii) Epoxy resin

[0021] The binder resin may optionally contain epoxy resin. When the binder resin contains epoxy resin, foaming is significantly reduced and the surface of the coating film can be smooth. Although not bound by theory, the inventors believe that epoxy resin can soften the skin layer of the coated composition or delay the formation rate of the skin layer. As a result, air bubbles within the coating composition easily evaporate with the solvent and no foaming occurs during the curing step.

[0022] The amount of epoxy resin is typically 1 to 40, preferably 10 to 30, more preferably 15 to 25 parts by weight of adhesive resin (A)(i) based on 100 parts by weight.

[0023] Epoxy resins are not particularly limited, and one or more types can be selected and used, including bisphenol-based epoxy resins, amine-based epoxy resins, phenol-formaldehyde varnish-based epoxy resins, cresol-formaldehyde varnish-based epoxy resins, resorcinol-based epoxy resins, phenol-aralkyl-based epoxy resins, naphthol-aralkyl-based epoxy resins, dicyclopentadiene-based epoxy resins, epoxy resins with a biphenyl backbone, isocyanate-modified epoxy resins, tetraphenylethane-based epoxy resins, triphenylmethane-based epoxy resins, and fluorene-based epoxy resins.

[0024] Typically, bisphenol-based epoxy resins are epoxy resins in which the two phenolic hydroxyl groups of the bisphenol compound have been glycidylated, and examples include bisphenol A, bisphenol F, bisphenol AD, bisphenol S, or halogenated or alkyl-substituted derivatives, hydrogenated products, dimer acid-modified products, etc. of these bisphenols. Furthermore, bisphenol-based epoxy resins are not limited to monomers, and polymers having multiple repeating units can also be advantageously used.

[0025] Examples of commercially available bisphenol A type epoxy resins include jER® 825, 828, 834, 1001, 1002, 1003, 1003F, 1004, 1004AF, 1005F, 1006FS, 1007, 1009, and 1010, which are commercial products of Mitsubishi Chemical Corporation. Examples of brominated bisphenol A type epoxy resins include jER® 505, 5050, 5051, 5054, and 5057, which are commercial products of Mitsubishi Chemical Corporation. Examples of commercially available hydrogenated bisphenol A type epoxy resins include ST5080, ST4000D, ST4100D, and ST5100, which are commercial products of Nippon Steel Chemical Co., Ltd.

[0026] B) Solid lubricants

[0027] The coating formulations of the present invention comprise one or more solid lubricants (B). The solid lubricants can provide sliding properties to the coating film formed from the composition. Suitable solid lubricants include, for example, polytetrafluoroethylene (PTFE), polyamide (PA), polyetheretherketone (PEEK), polyethylene (PE), polypropylene (PP), and acrylic resins. Preferably, the solid lubricant comprises PTFE. The amount of solid lubricant is typically 10 to 50 parts by weight, preferably 20 to 40 parts by weight, based on the binder resin (A).

[0028] Furthermore, the amount of solid lubricant was determined to satisfy the condition that the volume ratio of [(B) + (C)] / (A) is 0.1 to 0.3, as disclosed below.

[0029] (C) Other solid particles

[0030] In addition to the solid lubricant (B), the coating composition of the present invention may optionally contain one or more other solid particles.

[0031] Another example of other solid particles is hard particles. Hard particles can provide abrasion resistance to the coating film formed from the composition. Hard particles are defined as particles having a new Mohs hardness greater than 4, preferably greater than 9. Hard particles include tungsten carbide, titanium nitride, zirconium oxide, alumina, and silicon carbide.

[0032] Other examples of solid particles include: coloring materials, including colorants such as pigments (inorganic and organic colorants); carbon (e.g., carbon black, fullerenes, and carbon nanotubes); ultraviolet-absorbing (or blocking) solid particles, including metal oxides (or metal oxide particles), such as titanium oxide and zinc oxide; fibers (e.g., glass fibers, carbon fibers, metal fibers, and whiskers); ferromagnetic materials, such as ferromagnetic metals (powders), such as iron, cobalt, and nickel; ferromagnetic alloys (powders), such as magnetite and ferrite; ferromagnetic metal oxides, such as magnetic iron oxide (powder); metal hydroxides (e.g., aluminum hydroxide); metal salts (e.g., 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.

[0033] When the coating composition contains other solid particles, the amount of such solid particles is typically 0.1 to 5 parts by weight, preferably 0.2 to 1.0 parts by weight, based on the binder resin (A).

[0034] The key to this invention lies in the volume ratio of [(B) + (C)] / (A) being between 0.1 and 0.3. When the volume ratio is within this range, a coating film exhibiting both excellent sliding and insulating properties is obtained. When the ratio is less than 0.1, the amount of solid lubricant is insufficient, resulting in a high COF (coefficient of friction) of the coating film. Consequently, the coating film will wear out quickly. On the other hand, when the ratio is greater than 0.3, the adhesion between the coating film and the substrate becomes lower, or the coating film cannot retain the solid lubricant well, thus the coating film will also wear out quickly. Furthermore, the air space between the binder resin and the solid lubricant / other solid particles increases, thus reducing the insulating properties of the coating film. Such air spaces can be observed by SEM analysis after the coating film is cut in the vertical / horizontal direction.

[0035] (D) Solvent

[0036] The coating formulations described herein may additionally include solvents and liquid additives.

[0037] 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; organohalogen compounds, such as methyl chloroform, trichloroethylene, and trichlorotrifluoroethane; N-methyl-2-pyrrolidone (NMP); N-ethyl-2-pyrrolidone (NEP); 1,3-dimethyl-2-imidazolinone (DMI); γ-butyrolactone (GBL); 3-methoxy-N,N-dimethylpropionamide (MPA); methyl isopyrrolidone (MIP); dimethylformaldehyde (DMF); dimethylacetaldehyde (DMAC); and 1-butyl-2-pyrrolidone (NBP). Solvents may be one, or mixtures of two or more. Particularly preferred solvents are NEP, DMI, GBL, and MPA.

[0038] (E) Other ingredients

[0039] The coating formulations described herein may additionally contain liquid additives other than the solid particles (C). Examples of liquid additives include: UV absorbers, light stabilizers, antioxidants, thermal polymerization inhibitors, leveling agents, defoamers, thickeners, antisettling agents, infrared absorbers, optical brighteners, dispersants, antistatic agents, antifogging agents, coupling agents, and thixotropic agents. Thixotropic agents include, for example, fillers such as bentonite, calcium carbonate, kaolin, and clay; polymerized linseed oil; castor oil wax; polyolefins; silica gel; aluminocarboxylate salts; cellulose; amide waxes; and polyamide-based thixotropic agents. Polymer-based leveling agents, such as acrylic-based and silicone-based leveling agents, and polyoxyethylene fatty acid alkanolamide derivatives, may be used.

[0040] Method for forming a coating film on a sliding component

[0041] The method comprises three steps: (a) preparing a coating composition comprising (A), (B) and optionally (C) disclosed above; (b) applying the coating composition to the surface of a sliding member; and (c) curing the coated composition to form a coating film on the surface of the sliding member.

[0042] Step (a)

[0043] A coating composition comprising (A), (B), and optionally (C) can be prepared by mixing the composition using a propeller mixer, a rotary / revolutionary mixer, or any other mixing equipment. A solvent can be added to obtain a coating composition with a suitable viscosity for application onto sliding components.

[0044] Step (b)

[0045] The coating composition is applied to the surface of the sliding member by dip coating, spin coating, flow coating, spray coating, bar coating, gravure coating, roller coating, doctor blade coating, screen printing, air knife coating, and any other method. The thickness of the coating film is not particularly limited, but a thickness of 1 to 100 μm is preferred, and a thickness of 40 to 80 μm is more preferred. Optionally, the sliding member may be pretreated before the coating composition is applied.

[0046] Step (c)

[0047] The applied composition is cured and forms a coating film on the surface of the sliding component. Typically, thermal curing is performed by heating in an oven or other equipment. During the thermal curing of the applied composition, the solvent in the composition can be removed in a first stage of heating, and the applied composition can be cured by a crosslinking reaction in a second stage of heating. For example, the first stage of heating can be carried out at 60°C to 100°C for 5 to 30 minutes, and then the second stage of heating can be carried out at 180°C to 250°C for 20 to 120 minutes.

[0048] sliding member

[0049] This coating composition can be used as an insulating coating on the metal surface of sliding components. Examples of such sliding components include: inner or outer rings of bearings, bearing shafts, EV stator cores, battery trays, PCUs (Power Control Units), and housings (including inverter housings and DC / DC converters).

[0050] This coating composition can be applied to the metal surface of sliding components, even if the surface is rough, for example, with a surface roughness of 10 μm or greater. Surface roughness (Ra) is defined as the arithmetic mean roughness based on JIS B0601 (2001). The Ra of the surface of the sliding component to be coated can preferably be from 0.1 to 20 µm, or alternatively, the Ra of the surface of the component can be from 1 to 10 µm. Generally, when the surface of the component is rough, it is difficult to obtain a smooth coating surface without bubbling. This coating composition, especially a coating composition containing epoxy resin, can provide a smooth coating surface without bubbling.

[0051] Material

[0052] 1. Adhesive resin [A]

[0053] PAI-A: Polyamide-imide resin dissolved in NEP (solids content 35 wt%) (specific gravity: 1.4)

[0054] PAI-B: Polyamide-imide resin dissolved in GBL (solids content 36 wt%) (specific gravity: 1.4)

[0055] PAI-C: Polyamide-imide resin dissolved in MPA (solids content 35 wt%) (specific gravity: 1.4)

[0056] PI-A: Polyimide resin dissolved in NEP (20 wt% solids) (specific gravity: 1.4)

[0057] Epoxy Resin-A: Epoxy resin with product name jER828, bisphenol A type epoxy resin, available from Mitsubishi Chemical Corporation (specific gravity: 1.4).

[0058] Epoxy Resin-B: Epoxy resin with product name jER1007, bisphenol A type epoxy resin, available from Mitsubishi Chemical Corporation (specific gravity: 1.4).

[0059] BCB: Benzocyclobutene resin dissolved in Mecytylene, Cyclotene 3022-63, available from DuPont, with a solids content of 63 wt%. (Specific gravity: 1.4)

[0060] Phenol: Phenolic resin, product name TD-2495, available from DIC (specific gravity: 1.4).

[0061] 2. Solid lubricants [B]

[0062] PTFE-A: Spherical polytetrafluoroethylene resin particles with a median diameter of 3 to 7 μm (specific gravity: 2.0), as measured by laser diffraction scattering particle size distribution.

[0063] PTFE-B: Spherical polytetrafluoroethylene resin particles with a median diameter of 4 to 8 μm (specific gravity: 2.0), as measured by laser diffraction scattering particle size distribution.

[0064] MoS2: Industrial fine MoS2 powder, available from Climax Molybdenum Company, consisting of molybdenum disulfide particles with a median diameter of 4 to 6 μm (specific gravity: 4.8), measured by laser diffraction scattering particle size distribution.

[0065] Gra-A: Graphite UF-2, available from Graphit Kropfmühl GmbH, consists of graphite particles with a median diameter of 4 to 5.5 μm (specific gravity: 2.2) measured by laser diffraction scattering particle size distribution.

[0066] 3. Solid additives [C]

[0067] CB: Carbon black MA-100, available from Mitsubishi Chemical Corporation, with a median diameter of 1 to 2 μm (specific gravity: 1.7) as measured by laser diffraction scattering particle size distribution.

[0068] 4. Solvent [D]

[0069] NEP: N-ethyl-2-pyrrolidone, available from Maruzen Chemical Co., Ltd.

[0070] GBL: γ-Butyrolactone, available from Mitsubishi Chemical Corporation.

[0071] MPA: 3-Methoxy-N,N-Dimethylpropionamide, available from KJ Chemicals.

[0072] Xylene: Xylene is available from Kyoei Chemical Co., Ltd.

[0073] DAA: Diacetone alcohol, available from Sankyo Chemical Co., Ltd.

[0074] MEK: Methyl ethyl ketone, available from Sankyo Chemical Co., Ltd.

[0075] 5. Other ingredients [E]

[0076] Defoamer: Xiameter OFX-0230 Fluid, available from Dow Chemical.

[0077] Example 1-15

[0078] The components disclosed in Tables 1-4 were mixed at room temperature using a rotation / revolution mixer. The mixture was diluted with solvent, and the solids concentration was then adjusted to approximately 23-26% by mass. The coating composition was applied to the test sample (material: cold-rolled steel sheet (SPCC)-SB sheet, dimensions: 150 mm × 70 mm × 0.6 mm, surface roughness Ra = 0.1) by spraying. The thickness of the sprayed coating was 0.07 mm. The test sample was then placed in an oven at 230°C for 60 minutes to evaporate the solvent and cure. The thickness of the coating film was 0.05 mm. The following evaluation tests were performed.

[0079] Analytical methods

[0080] Dielectric strength: The dielectric strength of the coated SPCC-SB board was measured using an automatic insulation withstand voltage tester 3153 from HIOKI Corporation, in accordance with ASTM D149-09 (JIS C2010-1). A leakage current of 0.5 mA was applied, followed by a voltage increase of 0.5 kV (AC) and held for 20 seconds to observe either OK or fault. If OK, the voltage was increased to 1.0 kV and held for another 20 seconds until a fault occurred.

[0081] Durability: The coating rings on the plate were applied using EFM-III-1010, available from Orientec. Rotation speed: 2000 rpm, load 1000 N until seizure.

[0082] Surface roughness: Surface roughness was measured according to JIS 2001:2013. A Surfcom 1400D surface texture and profile measuring instrument, available from Accretech Ltd., was used. The needle measurement speed was 0.6 mm / s.

[0083] Bubbling: The appearance of the coated SPCC-SB was observed using an Olympus DP28 microscope to confirm the presence of bubbling.

[0084] Table 1

[0085]

[0086] The numbers in Table 1-4 describe the weight parts of each ingredient.

[0087] The volume ratio [(B) + (C)] / (A) is calculated by converting the units from weight to volume using the specific gravity of each component. For example, in Example 2: 0.11 = (8 / 2.0 + 8 / 2.0 + 0.5 / 1.7) / 100 / 1.4.

[0088] Durability (minutes): More than 5 minutes is desirable.

[0089] Bubbling: 〇 indicates no bubbling, × indicates some bubbling.

[0090] Table 2

[0091]

[0092] Table 3

[0093]

[0094] Table 4

[0095] .

Claims

1. A coating composition comprising: (A) At least one adhesive resin, the adhesive resin comprising: (i) Resins selected from the group consisting of: polyamide-imide, polyimide, and benzocyclobutene, (B) Solid lubricants, and (C) Optionally, another solid particle besides the solid lubricant. The volume ratio of [(B) + (C)] / (A) is between 0.1 and 0.

3.

2. The coating composition according to claim 1, wherein, The solid lubricant is polytetrafluoroethylene.

3. The coating composition according to claim 1, wherein, The solid particles are carbon black.

4. The coating composition according to claim 1, wherein, In addition to (i), the adhesive resin (A) further comprises (ii) an epoxy resin.

5. The coating composition according to 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 a dielectric strength of 50 kV / mm or greater.

7. A sliding member having a coating film, said coating film comprising, prior to curing: (A) At least one adhesive resin, the adhesive resin comprising: (i) Resins selected from the group consisting of: polyamide-imide, polyimide, and benzocyclobutene, (B) Solid lubricants, and (C) Optionally, another solid particle besides the solid lubricant. The volume ratio of [(B) + (C)] / (A) is between 0.1 and 0.3, and The coating film has a dielectric strength of 50 kV / mm or greater.

8. A sliding member having a coating film, said coating film comprising, prior to curing: (A) At least one adhesive resin, the adhesive resin comprising: (i) resins selected from the group consisting of: polyamide-imide, polyimide, and benzocyclobutene, and (ii) epoxy resins. (B) Solid lubricants, and (C) Optionally, another solid particle besides the solid lubricant. The volume ratio of (B)+(C) / (A) is 0.1 to 0.3, and the surface of the coating film has a Ra of 1 or less as measured according to JIS 2001:2013.

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

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

11. A method for forming a coating film on a sliding member, the method comprising the steps of: (a) Preparing a coating composition, said coating composition comprising (A) At least one adhesive resin, the adhesive resin comprising: (i) Resins selected from the group consisting of: polyamide-imide, polyimide, and benzocyclobutene, (B) Solid lubricants, and (C) Optionally, another solid particle besides the solid lubricant. (b) Apply the coating composition to the surface of the sliding member, and then (c) Curing the coating composition and forming a coating film on the surface of the sliding member.

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