EP3802914A1

EP3802914A1 - Anti-fingerprint coatings

Info

Publication number: EP3802914A1
Application number: EP19725185.3A
Authority: EP
Inventors: Mutlu-Iskender MUGLALI; Peter Kühlkamp; Philipp Wachter
Original assignee: Atotech Deutschland GmbH and Co KG
Current assignee: Atotech Deutschland GmbH and Co KG
Priority date: 2018-05-24
Filing date: 2019-05-23
Publication date: 2021-04-14
Also published as: WO2019224286A1; JP7326342B2; CN112189061A; JP2021524541A

Abstract

To achieve anti-fingerprint properties to decorative surfaces, a novel method for coating a metal substrate with an anti-fingerprint coating is provided. The method comprises the following method steps: (a) providing the metal substrate; (b) providing a silicone coating mixture containing: (i) at least one first silicone compound selected from the group consisting of mono- or oligo(aminoalkyl)-fluoroalkyl silicones; (ii) at least one second silicone compound selected from the group consisting of aminoalkyl silicones; (iii) at least one acidifier; and (iv) water; further (c) treating the metal substrate with the silicone coating mixture by bringing the metal substrate into contact with the silicone coating mixture; and (d) curing the treated metal substrate at a predetermined temperature.

Description

Anti-Fingerprint Coatings Des c ription

Field of the Invention

The present invention relates to a novel method for coating a metal substrate, in particular a chromium substrate, with an anti-fingerprint coating by coating the metal substrate using a silicone coating mixture and to the use of said silicone coating mixture for coating said metal substrate.

Background of the Invention

Chromium and other metals have found extensive use as decorative materials in a plurality of applications, for example for automotive parts both in the inside of vehicle interior and for parts mounted on the exterior of a car body, for white goods such as refrigerators, stoves, washing machines and dishwashers, for consumer electronics such as mobile phones, in sanitary appliances, as well as for classical chromium layers included in anti-corrosion coatings used for decorative equipment for example. All these surfaces may adversely be affected by dirt adhering to them so that their appearance will be impaired, in particular fatty or greasy contaminants. The latter type of stains cannot be removed easily using hydrous cleaning material and would therefore resistantly affect the appearance of a metal coated article. Fingerprints are frequently applied to decorative surfaces as a consequence of their manual handling during everyday use. Due to the inherent fat material included in human exudation, such soiling of the decorative surfaces is frequently experienced in decoratively designed parts.

Mechanical removal of stains is most often used to re-establish the original superior visual appearance of the decorative surface. If these surfaces which are polluted with fingerprints are thereafter mechanically cleaned to remove this contamination, fatty material of the fingerprints is spread over the surface. If mechanical removal is applied to such surfaces, soil spreading will be the result and in general be sufficient to re-establish good optical appearance, which the surface had prior to contamination. It has been ascertained that mat metal surfaces are more prone to decorative impairment due to fatty or greasy contaminants than glossy metal surfaces and that dark metal surfaces are likewise more prone to such impairment than bright metal surfaces. Although having the same surface roughness the surfaces of chromium coatings which are deposited from Cr(lll) containing plating compositions are more sensitive to such impairment due to fingerprints than the surfaces of chromium coatings which are deposited using Cr(VI) containing plating compositions. This different behavior may be due to the fact that the chromium surfaces produced using Cr(lll) containing plating compositions are darker than the chromium surfaces produced using Cr(VI) containing plating compositions. This problem is even more serious on surfaces of chromium deposits which are produced by using chloride containing Cr(lll) containing plating compositions. In this latter case, fingerprints will be the most sensitive towards impairment as these surfaces are relatively dark.

Recent trends caused by environmental issues and for health and safety at work reasons lead to the need of eliminating Cr(VI) species in the production of chromium deposits. This however, aggravates the problems encountered with manual handling of parts coated with chromium coatings as fingerprints and other fatty or greasy contamination will visually present such soiling even more prominently.

Various attempts have been made to overcome these problems:

US 2008/0131706 A1 teaches the use of polysilazanes as permanent coatings on metal surfaces, stainless steel, aluminium or chromium-plated surfaces for example, for the prevention of the susceptibility to fingerprints. The polysilazane is in the form of a solution of a polysilazane or a mixture of polysilazanes of the general formula -(SiR’R”-NR”’)_n-, wherein R’, R”, R’” are, independently, = H, optionally-substituted alkyl, aryl, vinyl or (trialkoxysilyl)alkyl radical, n = an integer, such that the polysilazane has a number-average molecular weight of from 150 to 150,000 g/mol in a solvent.

Furthermore, DE 10 2005 018 740 A1 teaches a hydrophobic protective coating for metal surfaces which is free of fluorine in order to prevent the formation of fingerprints. The protective coating consists of hydrolysis and condensation products of (a) 50 to 65 % by weight of tetraalkoxy compounds of silicon, titanium, zirconium; (b) 20 to 30 % by weight of methyl trialkoxy silane; (c) 10 to 20 % by weight of alkyl trialkoxy silane comprising C₁₂ to C₁₈ alkyl; (d) 2 to 5 % by weight of polyalkyleneoxide trialkoxy silane.

Further to these prior art references, literature has been published relating to coatings which are based on silicon containing compounds. Two references are discussed as follows:

US 6,251 ,989 B1 teaches an oligomerized polyorganosiloxane co-condensate which may be obtained, in part, by mixing a water-soluble amino-functional organosilane represented with at least one of the following: a fluoro-functional organosilane and one of various types of organosilanes.

US 8,889,812 B2 teaches an aqueous composition based on tris-silylated amino-functional silicon compounds, which is substantially free of organic solvents and which substantially does not release any alcohol even during the cross-linking process.

Objectives of the Invention

It has emerged that mechanical removing fingerprints from metal surfaces, in particular from chromium surfaces, is possible and may be facilitated by post-treating the metal surfaces before exposing to fingerprints with the agents described in US 2008/0131706 A1 and DE 10 2005 018 740 A1 . However, it has still proven that the metal surfaces provided so far are sensitive to fingerprint exposition in that fingerprints are prominently visible. Furthermore, the effort required to completely remove fingerprints is relatively large with conventional post-treatment of the metal surfaces.

Therefore, it is an objective of the present invention to provide means to produce a metal surface, preferably chromium surface, most preferably a surface of a chromium coating produced by using a Cr(lll) containing plating composition, which is largely insensitive to fingerprint generation and wherein any fingerprint being produced thereon is easily removed mechanically.

It is a further objective of the present invention to provide a metal surface, preferably a chromium surface, most preferably a surface of a chromium coating produced by using a Cr(lll) containing plating composition, with a coating which completely conserves genuine optical appearance provided to the metal surface on its production, i.e., its original shade of color, morphology, such as a predetermined roughness, or other property influencing the appearance of the metal surface.

It is a further objective of the present invention to provide a coating to the metal surface, preferably to a chromium surface, most preferably to a surface of a chromium coating produced by using a Cr(lll) containing plating composition, which is easy to produce. More specifically, the method for preparing the metal surface to achieve anti-fingerprint properties shall be very easy, integrable to a plating line without specific equipment and shall not require much effort to be spent.

Definitions

The term“alkyl” as used herein refers to a saturated linear or branched-chain mono- or divalent hydrocarbon radical of one to twelve carbon atoms (CrC₁₂), preferably of one to six carbon atoms (Ci -Ce), wherein one or more hydrogen atoms of the alkyl radical may be optionally substituted independently by a respective number of the substituents described below. Examples of alkyl groups include, but are not limited to, methyl (-CH₃), ethyl (-CH₂CH₃), 1 - propyl (-CH₂CH₂CH₃), 2-propyl (-CH(CH₃)₂),1 -butyl (-CH₂CH₂CH₂CH₃), 2-methyl- 1 -propyl (-CH₂CH(CH₃)₂), 2-butyl (-CH(CH₃)CH₂CH₃), 2-methyl-2-propyl (-C(CH₃)₃), 1 -pentyl

(-CH₂CH₂CH₂CH₂CH₃), 2-pentyl (-CH(CH₃)CH₂CH₂CH₃), 3-pentyl (-CH(CH₂CH₃)₂), 2-methyl-2-butyl (-C(CH₃)₂CH₂CH₃), 3-methyl-2-butyl (-CH(CH₃)CH(CH₃)₂), 3-methyl-1 -butyl (-CH₂CH₂CH(CH₃)₂), 2-methyl-1 -butyl (-CH₂CH(CH₃)CH₂CH₃), 1 -hexyl

(-CH₂CH₂CH₂CH₂CH₂CH₃), 2-hexyl (-CH(CH₃)CH₂CH₂CH₂CH₃), 3-hexyl

(-CH(CH₂CH₃)(CH₂CH₂CH₃)), 2-methyl-2-pentyl (-C(CH₃)₂CH₂CH₂CH₃), 3-methyl-2-pentyl (-CH(CH₃)CH(CH₃)CH₂CH₃), 4-methyl-2-pentyl (-CH(CH₃)CH₂CH(CH₃)₂), 3-methyl-3-pentyl (-C(CH₃)(CH₂CH₃)₂), 2-methyl-3-pentyl (-CH(CH₂CH₃)CH(CH₃)₂), 2,3-dimethyl-2-butyl (-C(CH₃)₂CH(CH₃)₂), 3,3-dimethyl-2-butyl (-CH(CH₃)C(CH₃)₃, 1 -heptyl, 1 -octyl, and the like. Alkyl also includes, but is not limited to, methylene (-CH₂-), ethylene (-CH₂CH₂-), propylene (-CH₂CH₂CH₂-), and the like.

The term“fluoroalkyl” as used herein refers to a saturated linear or branched-chain monovalent hydrocarbon radical of one to twelve carbon atoms (CrC₂₄), preferably of one to six carbon atoms (CrC₁₂), wherein one or more hydrogen atoms of the alkyl radical of fluoroalkyl is substituted by a respective number of fluorine atoms. If all hydrogen atoms of the alkyl radical are substituted by fluorine, the fluoroalkyl is perfluoroalkyl. For the rest, the same definitions and examples as given above for the term“alkyl” apply mutatis mutandis to fluoroalkyl.

The term“fluoroalkylalkyl” as used herein refers to fluoroalkyl which has a perfluoroalkyl moiety and an alkyl moiety, wherein the perfluoroalkyl moiety may, in a first alternative, form a monovalent radical and the alkyl moiety may form a bivalent moiety, i.e., it may have the chemical formula C_nF2_n-i-C_nH_2n-· In a second alternative, the perfluoroalkylalkyl moiety may form a bivalent perfluoroalkyl moiety and a monovalent alkyl moiety, i.e., it may have the chemical formula C_nH2_n-i -C_nF_2n-.

Alkyl may be substituted, wherein one or more hydrogen atoms thereof may be optionally substituted independently by any radical group like aryl, heteroaryl, OR, NR’R”, COOR, CONR’R”, wherein R, R’, and R” are, independently, selected from hydrogen, alkyl, aryl and heteroaryl. Heteroaryl is a monovalent radical comprising an aromatic ring system which includes at least one of N, S and O, such as pyridyl, pyrryl, thiophenyl, furanyl, and the like.

“Aryl” means a mono- or divalent aromatic hydrocarbon radical of 6-20 carbon atoms (C₆-C₂o) derived by the removal of one or two hydrogen atoms, resp., from one or two carbon atoms, resp., of a parent aromatic ring system. Aryl also includes bicyclic radicals comprising an aromatic ring fused to an aromatic carbocyclic ring. Typical aryl groups include, but are not limited to, radicals derived from benzene (phenyl), substituted benzenes, naphthalene and the like. One or more hydrogen atoms of aryl may be optionally substituted independently by a respective number of substituents described herein below.

Unless otherwise defined in the general chemical formulae recited herein, in aryl one or more hydrogen atoms thereof may be optionally substituted independently by a respective number of substituents, wherein at least one hydrogen atom of the aryl moiety is substituted by any radical group like Hal, alkyl, aryl, heteroaryl, OR, NR’R”, COOR, CONR’R”, wherein R, R’ and R” are independently selected from hydrogen, alkyl, aryl and heteroaryl.

The term“halogen” or“Hal” as used herein refers to fluorine (F), chlorine (Cl), bromine (Br), or iodine (I). The term“siloxane” as used herein refers to silicon compounds which contain [-0-Si(0H)(R)]- and/or [-0-Si(R)(R’)]- moieties, wherein R, R’ mean H, alkyl, aryl, including substituted alkyl and aryl and which typically form a [-O-Si]- backbone. In these compounds terminal moieties may be different from the definitions for R, R’.

Summary of the Invention

In a first aspect of the present invention, these objectives are solved by providing a method for coating a metal substrate, such as a chromium, nickel or stainless steel substrate, in particular a chromium substrate, with an anti-fingerprint coating, the method comprising:

(a) providing the metal substrate, preferably a chromium substrate;

(b) providing a silicone coating mixture containing:

(i) at least one first silicone compound selected from the group consisting of mono- or oligo(aminoalkyl)-fluoroalkyl silicones;

(ii) at least one second silicone compound selected from the group consisting of aminoalkyl silicones;

(iii) at least one acidifier; and

(iv) water;

(c) treating the metal substrate with the silicone coating mixture by bringing the metal substrate into contact with the silicone coating mixture; and

(d) curing the treated metal substrate at a predetermined temperature.

If the term "oligo" is used e.g. in oligo(aminoalkyl)-fluoroakyl silicones, oligo is understood as synonym of a small number of units to be read as 2-8, preferably 2-6 more preferably 2 or 3; or to be read as dimer, trimer, tetramer ... octamer.

Aminoalkyl in the mono- or oligo(aminoalkyl) moiety is preferably -N(R’)-alkyl, wherein R’ is preferably H, alkyl, more preferably ^ to C₆ alkyl, aryl, more preferably C₆-aryl, arylalkyl, alkylaryl or alkylarylalkyl. Most preferably, R’ = ethyl (-CH₂CH₂-) and/or propyl (-CH₂CH₂CH₂-).

Fluoroalkyl in the at least one first silicone compound is preferably perfluoroalkylalkyl and more preferably has the chemical formula C_nF_2n__| -C_nH_2n-. Furthermore, fluoroalkyl in the at least one first silicone compound is preferably perfluoro-C₂ to C₅-alkylalkyl. Most preferably, fluoroalkyl in the at least one first silicone compound is preferably perfluoro-C₂ to Cs-alkyl-Ci to C₄-alkyl.

The silicone coating mixture may contain one mono- or oligo(aminoalkyl)-fluoroalkyl silicone or a plurality of mono- or oligo(aminoalkyl)-fluoroalkyl silicones, wherein these silicones may differ from each other by at least one of the chain length of the mono- or oligo(aminoalkyl) moiety, the meaning of aminoalkyl, comprising the meaning of R’ in -N(R’)- and the meaning of fluoroalkyl for example.

Aminoalkyl in the at least one second silicone compound is preferably R’R”N-alkyl, wherein R’, R” are, independently, preferably H, alkyl, preferably Ci to C₆ alkyl, aryl, preferably C₆-aryl, arylalkyl, alkylaryl or alkylarylalkyl. Most preferably, R’ = R” = H.

The silicone coating mixture may contain one aminoalkyl silicone or a plurality of aminoalkyl silicones, wherein these silicones may differ from each other by at least one of the chain length of the silicone chain and the meaning of aminoalkyl, comprising the meaning of R’ and R” in R’R”N-alkyl for example.

In a second aspect of the present invention, these objectives are solved by the use of the silicone coating mixture for coating the metal substrate with the anti-fingerprint coating. Said silicone coating mixture comprises: (i) at least one first silicone compound selected from the group consisting of mono- or oligo(aminoalkyl)-fluoroalkyl silicones; (ii) at least one second silicone compound selected from the group consisting of aminoalkyl silicones; (iii) at least one acidifier; and (iv) water.

By coating the metal substrate with the anti-fingerprint coating, a superior finish of the metal surface is achieved which is very much resistant against soiling with fatty or greasy material, such as against fingerprint formation, and wherein any fingerprint soils are easily removed mechanically from the surface. Such removal may be performed using a microfiber cloth, for example. The anti-fingerprint coating ensures retention of the original appearance of the metal surface as prepared because it does not change the color tone or surface morphology thereof. Furthermore, treatment of the metal surface according to the invention with the silicone coating mixture is very easy and does not afford lengthy and laborious as well as extensive energy consuming processes, but instead makes possible quick treatment at low processing temperature. In a preferred embodiment of the present invention, the chemicals used are are water soluble, they will not require any organic solvents as solubilizer. The use of standard functional alkoxysilanes would require the use of organic solvents with the known disadvantages for health, safety and environment, if VOC are being used. Contrary to standard functional alkoxysilanes no alcolhols are released upon hydrolysis. Therefore it is not necessary to add organic solvents. Instead, totally aqueous liquids may be used, i.e., the silicone coating mixture preferably does not contain any added organic solvent. Therefore it is also preferred that no non-water-soluble silicone compounds or in water hardly soluble silicone compounds as alkylalkoxysilane or derivates thereof are used, because this would need to use organic solvents.

Though not being bound by theory, it is believed that the compounds contained in the silicone coating mixture adsorb to the metal substrate and, during the curing step, undergo a chemical reaction to form a very thin, non-visible and mechanically resistant top-coat thereon. It is believed that the method of the invention is a sol-gel process that is based on adsorption of silicon containing compounds from a mixture of different organo-functional silicon compounds and optionally at least one polyether siloxane copolymer surfactant in a solution. Due to the top coat, surface energy of the substrate is lowered relative to the non-coated state. Therefore, it is possible to monitor formation of the coating by contact-angle measurements.

In a preferred embodiment of the present invention, the silicone coating mixture is a liquid, more preferably an aqueous liquid and most preferably an aqueous solution or aqueous sol (colloid solution).

In a further preferred embodiment of the present invention, the metal substrate is produced in a conventional manner. The metal substrate may be in the form of any work piece made of any material and coated with a deposit of any metal. The work piece material may preferably be a plastics, metal, glass, ceramics material or any other material. Typically and depending on the intended use, the work piece may be provided as any automotive part or sanitary part or any part for building equipment or part for electronic or audiovisual equipment or any other part exhibiting a decorative property. The metal deposit is preferably a nickel coating or stainless steel coating or, most preferably, a chromium coating.

Accordingly, in a further preferred embodiment of the present invention, the metal substrate is a chromium substrate, more preferably a chromium metal layer forming the substrate which is deposited onto the work piece. In this latter case, the chromium substrate is produced by depositing a chromium metal layer on the work piece. In this case, commonly an undercoat is first produced on the work piece prior to depositing the chromium metal layer thereon. Such undercoat may consist of a plurality of metal layers in order to yield optimum decorative (leveling, brightening) and functional (anti-corrosion) properties of the overall metal coating. The undercoat may for example consist of a base copper metal layer and one or a plurality of nickel metal layers which are arranged directly underneath the chromium metal layer. These metal layers are in general electroplated by using appropriate metal plating compositions. Such sandwich metal coating and the deposition methods thereof are well-known to those in the pertinent technical field.

Depositing the chromium metal layer comprises providing the work piece and an electroplating liquid which contains at least one chromium plating species, more preferably a Cr(lll) plating species, and electroplating the chromium metal layer onto the work piece by using the electroplating liquid containing the at least one chromium plating species, more specifically containing the Cr(lll) plating species. The chromium coating can be produced in a conventional manner. In an even more preferred embodiment of the present invention, if the chromium metal layer is produced using an electroplating method, electroplating is performed by using an electroplating liquid (composition) which comprises Cr(lll) species, such as chromium(lll) chloride, chromium(lll) sulfate or basic chromium(lll) sulfate. Such electroplating liquids typically furthermore contain one or more buffering agents as boric acid and carbonic acid, conductivity salts as ammonia sulfates, sodium sulfates, potassium sulfates and halides, one or more complexing agents as carbonic acid and amino acids, and a wetting agent as sulfosuccinates. If dark chromium metal layers are desired darkening agents can be added. Such liquids are commercially available, and a person skilled in the art is well-acquainted in using such liquids for producing chromium coatings on the work piece to be electroplated.

In an even more preferred embodiment of the present invention, the Cr(lll) plating species containing liquid is free of chloride species.

In a further preferred embodiment of the present invention; the at least one first silicone compound selected from the group consisting of mono- or oligo(aminoalkyl)-fluoroalkyl silicones is a water soluble statistical copolymer of an mono- or oligo(aminoalkyl) silicone and a (fluoroalkyl)alkylsilicone. In a further preferred embodiment of the present invention, the at least one first silicone compound is derived from an aqueous co-condensation of at least two monomeric building blocks selected from the group consisting an mono- or oligoaminoalkyltrihydroxysilane compound and a fluoroalkylalkyltrihydroxysilane compound, wherein the aminoalkyltrihydroxysilane compound has the general chemical formula (I):

(HO)₃Si-(CH₂)_x-[NH(CH₂)_y]_z-NH₂ (I)

wherein: x is 1 -8, preferably 1 -6, more preferably 1 -4

y is 1 -6, preferably 1 -4, more preferably 1 -2

z is 0-8, preferably 0-6, more preferably 0-4, most preferably 1 or 2, and wherein the fluoroalkylalkyltrihydroxysilane compound has the general formula (II):

(HO)₃Si-(CH₂)_a-(CF₂)_bCF₃ (II)

wherein: a is 1 -8, preferably 1 -6, more preferably 1 -4

b is 0-20, preferably 0-10, more preferably 0-5, most preferably 2-5.

Molar relation of the aminoalkyltrihydroxysilane compound to fluoroalkylalkyltrihydroxysilane compound ranges preferably from 1 : 10 to 10: 1.

Most preferred is the first silicone compound derived from an aqueous co-condensation, wherein the aminoalkyltrihydroxysilane compound is (HO)₃Si-(CH₂)₃-[NH(CH₂)₂]₂-NH₂ and the fluoroalkylalkyltrihydroxysilane compound is (HO)₃Si-(CH₂)₂-(CF₂)₅CF₃.

In a further preferred embodiment of the present invention, the average of the molecular weight of the at least one first silicone compound is from 200 to 3,000 g/mole, more preferably from 300 to 2,000 g/mole, most preferably from 400 to 1 ,000 g/mole as determined by gel permeation chromatography against polyethylene oxide standards.

Such compound is commercially available. For example, Dynasylan® SIVO 1 12 (CAS No. 1222158-90-8) and Dynasylan® F 8815 available from Evonik, WASF-151 1 available from Gelest Inc., may be used as one of the at least one first silicone compound. A method of producing these compounds above is described e.g. in US 8,889,812 B2, US 6,251 ,989 B1 and EP1 101787B1 , which are hereby incorporated. This compound has proven to be most responsible for providing low surface energy to the metal substrate.

In a further preferred embodiment of the present invention, the at least one second silicone compound is a water soluble polymeric aminoalkylsilicone compound which can be derived from aqueous condensation of at least one monomeric building block selected from the group consisting of an aminoalkyltrihydroxysilane compound, wherein the aminoalkyltrihydroxysilane compound has the general chemical formula (III):

(HO)₃Si-(CH₂)_x-[NH(CH₂)_y]_z-NH₂ (III)

wherein: x is 1 -6, preferably 1 -4, more preferably 1 -3,

y is 1 -6, preferably 1 -4, more preferably 1 -2,

z is 0-6, preferably 0-4, more preferably 0-2.

Most preferred is the second silicone compound derived from an aqueous condensation, wherein the aminoalkyltrihydroxysilane compound is (HO)₃Si-(CH₂)₃-NH₂.

Such compound is commercially available. For example, Dynasylan® SIVO 160 (CAS No. 1443627-61 -9) and Dynasylan® 1 151 (CAS No. 58160-99-9, 29159-37-3) available from Evonik, 3-Aminopropyl-silanetriol (CAS No. 58160-99-9) available from Gelest Inc, Silquest® A 1 106 (CAS No. 58160-99-9) available Momentive Performance Materials Inc., SiSiB®PC1 106 (CAS No. 58160-99-9) available from Power Chemical Corporation, may be used as one of the at least one second silicone compound. A method of producing these compounds above is described in e.g. EP0675128B1 B1 , which are hereby incorporated. This compound is most responsible for the formation of a polymeric network matrix and then being effective as an anti fingerprint coating at low temperature.

In a further preferred embodiment of the present invention, the at least one acidifier may be any organic or inorganic acid. More preferably, the at least one acidifier is selected from the group comprising formic acid, oxalic acid, sulfuric acid, methacrylic acid, methane sulfonic acid and acetic acid. Furthermore, lactic acid, malic acid, glyceric acid, ortho-phosphoric acid, tartaric acid and succinic acid have in principle been proven acceptable to be used either. However, these latter acids are less preferred than formic acid, oxalic acid, sulfuric acid, methacrylic acid, methane sulfonic acid and acetic acid, because these latter acids may cause an impairment of the optical appearance of the chromium surface. Most preferably, acetic acid, formic acid and sulphuric acid are used.

In a further preferred embodiment of the present invention, the silicone coating mixture has a preferred pH of from 3.5 to 4.5, more preferably of about 3.5 (± 0.2). A pH of 4.5 yields lowest surface energy of the coated metal substrate and will therefore provide superior anti-fingerprint properties to the metal substrate. However, lower pH promotes better optical finish of the anti fingerprint coating. During use of the silicone coating mixture, the resulting anti-fingerprint coating outside the preferred pH range of from 3.5 to 4.5 of the used mixture leads to declining fingerprint test results. Also it could be observed, if pH is lower than 3 or higher than 4.5, the life time of the silicone coating mixture is shortened.

In a further preferred embodiment of the present invention, the silicone coating mixture additionally contains at least one siloxane polymer. The at least one siloxane polymer functions as surfactant and is in the following also named as SURF.

In a further preferred embodiment of the present invention, the at least one siloxane polymer is selected from the group consisting of compounds having general chemical formula (IV):

IV, wherein (x+y) is from 1 to 60 where x is from 1 to 30 and y is 0 to 30,

n is from 3 to 4,

a is from 0 to 30;

b is from 0 to 30;

R is a hydrogen or alkyl radical of 1 to 4 carbon atoms.

Preferably (x+y) is from 1 to 20 where x is from 1 to 10 and y is from 0 to 10 and a is from 0 to 15; b is from 0 to 15; such that at least one of a and b is not zero and (a+b) is from 1 to 30. In a further preferred embodiment of the present invention, the at least one siloxane polymer is siloxane block copolymer selected from the group consisting of polyethersiloxane-siloxane copolymers wherein (x+y) is from 2 to 60 where x is from 1 to 30 and y is 1 to 30, more preferably (x+y) is from 1 to 20 where x is from 1 to 10 and y is from 1 to 10 and a is from 0 to 15; b is from 0 to 15; such that at least one of a and b is not zero and (a+b) is from 1 to 30.

The silicone coating mixture may contain one siloxane polymer or a plurality of siloxane polymers, wherein these polymers may differ from each other by at least one of the parameters a, b and n.

The siloxane polymer has preferably a molecular weight of from 1 ,000 to 30,000 g/mole, preferably from 5,000 to 15,000 g/mole.

Such compound is commercially available. For example, TEGO® Wet 280 (CAS No. 68938-54- 5) , TEGO® Wet 240 (CAS No. 67674-67-3), TEGO® Wet 250 (CAS No. 27306-78-1 ) and TEGO® Wet 270 (CAS No. 68938-54-5) available from Evonik® or Dimethylsiloxane-(50-55% ethylene oxide) block copolymer (CAS No. 68938-54-5) available from Gelest Inc., Silwet® L 7600 (CAS No. 68938-54-5) and Silwet® L 77 (CAS No. 27306-78-1 ) available from Momentive Performance Materials Inc., Metolat®342 (CAS No. 27306-78-1 ) available from Munzing Chemie GmbH, may be used as one of the at least one siloxane polymer. This compound is a wetting agent and further lowers surface energy of the coated metal substrate. It promotes drying of the coated metal substrate.

In a further preferred embodiment of the present invention, the silicone coating mixture contains the at least one first silicone compound and the at least one second silicone compound in a predetermined mass ratio, wherein the mass ratio of all first silicone compounds to all second silicone compounds is preferably from 1 .0 to 4.0, more preferably from 1 .0 to 1.0 and most preferably from 3.0 to 4.0.

In a further preferred embodiment of the present invention, the concentration of the at least one first silicone compound in the silicone coating mixture is from 0.05 g/l to 5.00 g/l, preferably from 0.10 g/l to 2.50 g/l and most preferably from 0.30 g/l to 1.50 g/l. In a further preferred embodiment of the present invention, the concentration of the at least one second silicone compound in the silicone coating mixture is from 0.05 g/l to 10.00 g/l, preferably from 0.10 g/l to 3.00 g/l and most preferably from 0.40 g/l to 1.00 g/l.

In a further preferred embodiment of the present invention, the concentration of the at least one siloxane polymer in the silicone coating mixture is from 0.02 g/l to 5.00 g/l, preferably from 0.05 g/l to 1.00 g/l and most preferably from 0.10 g/l to 0.30 g/l.

In a further preferred embodiment of the present invention, bringing the metal substrate into contact with the silicone coating mixture is performed at a temperature of the silicone coating mixture of from 10 °C to 90 °C, more preferably from 20 °C to 70 °C and most preferably at about 50 °C (± 5 °C).

In a further preferred embodiment of the present invention, exposure time of the metal substrate to the anti-fingerprint coating solution is from 0.5 min to 60 min, more preferably from 1 min to 20 min and most preferably from 1 min to 2 min.

In a further preferred embodiment of the present invention, curing of the coated metal substrate is performed at a temperature of from 20 °C to 100 °C, more preferably from 40 °C to 90 °C and most preferably from 60 °C to 80 °C for a duration of 5 min to 120 min, more preferably from 15 min °C to 90 min °C and most preferably from 30 min to 60 min.

In order to coat the metal substrate with the anti-fingerprint coating, it is brought into contact (treated) with the silicone coating mixture. In a first alternative of the method, the treated metal substrate is partially dried and thereafter rinsed with water (wet-in-wet rinsing method) in order to remove excess silicon coating mixture at geometrically disadvantaged parts for avoiding optical defects. The treated and rinsed metal substrate is finally cured. In a second alternative of the method, the treated metal substrate is dried without rinsing it (dry withdrawal method) and finally cured. The first alternative is quick and easy to perform. However, part of the adsorbed silicone species is in this case desorbed again in the rinsing step. The second alternative overcomes this drawback by achieving homogeneous distribution of the coating over the entire surface area of the metal substrate. However, some application parameters must be controlled for achieving a good optical finish. In a further preferred embodiment of the method, the metal substrate is brought into contact with the silicone coating mixture by dipping it into the coating mixture and left therein for a predetermined period of time. Thereafter either the substrate (coating by substrate withdrawal) or the silicone coating mixture (coating by drainage is removed from the plating tank. More preferably, the coating application is performed at a constant (linear) withdrawal or drainage speed. Even more preferably the withdrawal/drainage speed is at least 1 mm/min, more preferably at least 50 mm/min and most preferably at least 10 mm/min. Furthermore, withdrawal speed is preferably at most 1000 cm/min, more preferably at most 500 cm/min and most preferably at most 100 cm m/min.

In principle, the silicone coating mixture may be applied using methods conventional in plating industry, i.e., in a dip tank as described hereinbefore or in a conveyorized treatment plant wherein workpieces to be treated are conveyed from one treatment station to the next one. As the method of the present invention comprises basically the step of treating the metal substrate with the silicone coating mixture and a curing step and optionally also a rinsing step (bringing the metal substrate into contact with water), the conveyorized plant would comprise a first station for treating the metal substrate with the silicone coating mixture and an optional second station wherein the metal substrate is rinsed and, optionally, a third station wherein the coated metal substrate is cured .

Anti-fingerprint action of the anti-fingerprint coating will have an effect on surface energy of the coated metal substrate. The surface energy can be measured indirectly by measuring contact angle of a test liquid which is brought into contact with the coated metal substrate. Contact angle measurement methods are well-known and for example are described in Law and Zhao, Surface Wetting - Characterization, Contact Angle and Fundamentals, Springer Verlag (2016) ISBN 9783319252124.

Another approach for determining the effect of the anti-fingerprint coating on the metal substrate is to assess the ability of the coated metal substrate to withstand soiling of the surface thereof with human exudation and/or human sebum and/or to overcome such soiling when the contaminated surface is tried to be cleaned mechanically. Correspondingly, exemplary testing conditions may be set up to investigate the effect of an anti-fingerprint coating produced with the method of the present invention. For example, an artificial exudation specimen with predetermined composition is applied in a reproducible manner, by stamping same at a predetermined force, to the metal substrate surface, with a silicone stamp for example. Mechanical removal of the exudation / sebum may likewise by reproducibly be tested by wiping the artificial fingerprint with a cloth of predetermined quality, a microfiber cloth for example, at a predetermined force, predetermined wiping speed and movement, a circular movement for example, and for a predetermined number of wiping events. The effect of the contamination of the metal substrate surface and removal efficiency may finally be determined by determining and comparing the color differences using L/a/b coordinates with a spectrophotometer before and after application of the test. A smaller difference in the initial and final colors indicates less sensitivity of the surface against fingerprints. For example a DI_ value larger than 2.5 units and/or Ab value larger than 1 .75 units on a test spot are easily distinguished as a flake by human eye, whereas smaller values become less visible under normal light conditions.

The invention will now be described in the way of examples. These examples do not constitute any limitation to the scope of the invention which is defined in the appending claims.

Description of the Figures

Fig: 1 shows a diagram displaying contact angle values for samples treated in solutions of single compounds; for comparison, a standard composition (Std: silicone coating mixture according to the invention) is included in the graph (0.9 / 0.3 / 0.2 g/l for CMPD B / CMPD A / SURF);

Fig. 2 shows a diagram displaying fingerprint test results for samples OA-OE in the form of AUAa/Ab values measured before and after application and cleaning of a fingerprint;

Fig. 3 shows a diagram displaying contact angle values for samples treated in silicone coating mixtures containing CMPD B and CMPD A with and without surfactant (SURF);

Fig. 4 shows a diagram displaying fingerprint test results for samples treated in silicone coating mixtures containing CMPD B and CMPD A with and without surfactant (SURF) in the form of AUAa/Ab values measured before and after application and cleaning of a fingerprint;

Fig. 5 shows a diagram displaying contact angle values for samples treated in silicone coating mixtures at different pH; pH was adjusted by the addition of acetic acid;

Fig. 6 shows a diagram displaying fingerprint test results for samples treated in solutions at different pH;

Fig. 7 shows a diagram displaying contact angle values for samples treated in silicone coating mixtures that contained different types of acids for pH adjustment; Fig. 8 shows a diagram displaying contact angle values of samples treated in silicone coating mixtures that contained different combinations of CMPD B and CMPD A;

Fig. 9 shows a diagram displaying fingerprint test results for samples treated in silicone coating mixtures that contained different combinations of CMPD B and CMPD A;

Fig. 10 shows a diagram displaying contact angle values on various chromium substrates after treatment with a silicone coating mixture;

Fig. 1 1 shows a diagram displaying fingerprint test results on various chromium substrates after treatment with a silicone coating mixture;

Fig. 12 shows a diagram displaying contact angle values of samples treated in silicone coating mixtures at different temperatures;

Fig. 13 shows a diagram displaying contact angle values of samples treated with a silicone coating mixture and cured successively at 70 °C for various curing durations;

Fig. 14 shows a diagram displaying fingerprint test results on samples treated with a silicone coating mixture and cured successively at 70 °C for various curing durations;

Fig. 15 shows a diagram displaying contact angle values of samples withdrawn from a silicone coating mixture at various speeds;

Fig. 16 shows a diagram displaying fingerprint test results on samples withdrawn from the silicone coating mixture at various speeds;

Fig. 17 shows a schematic workflow for a fingerprint test.

Experimental Details

Substrates:

For all measurements 7 cm x 10 cm copper Hull Cell Plates were used as substrates. The Hull Cell plates were prepared using the following procedure: i. Satin Ni deposition in Satilume® Plus (trademark of Atotech Deutschland GmbH) bath (coating thickness 12 - 15 pm);

ii. Chromium deposition from trivalent chromium bath Trichrome® Plus (trademark of Atotech Deutschland GmbH) (coating thickness 0.4 - 0.6 pm).

Satilume® Plus bath is an electroplating bath for depositing satin nickel coatings. It is based on NiS04, NiCI2, and boric acid as the main components and organic additives to create the satin appearance. Nickel was deposited under the following conditions: T: 51 °C, pH: 4.1 , current density: 4 A/dm², plating time 15 min. Trivalent chromium bath Trichrome® Plus is a chloride containing electroplating bath for depositing light chromium coatings. It is based on basic chromium sulfate, boric acid as buffering agent, a carboxylic acid based complexing agent and halide based conductivity salts... as the main components. Chromium is electrodeposited using this bath complying with the following plating conditions: T: 35 °C, pH: 2.8, current density: 10 A/dm², plating time 2 min.

For tests on various types of chromium surfaces, other baths Cr 843 (Cr(IV) = Hex Cr), Trichrome® ICE, Trichrome® Smoke 2, Trichrome® Graphite (all Cr(lll) electroplating baths)) have likewise been used according to technical data sheet (TDS).

Cr 843 is based on Cr0₃ and on sulfate and SiF₆ containing catalysts. Chromium is electrodeposited using this bath complying with the following plating conditions: T: 40 °C, current density: 10 A/dm², plating time 3 min.

Trichrome® ICE is based on basic chromium sulfate, boric acid as buffering agent, a carboxylic acid based complexing agent and sulfate based conductivity salts. Chromium is electrodeposited using this bath complying with the following plating conditions: T: 55 °C, pH: 3.5, current density: 5 A/dm², plating time 10 min.

Trichrome® Smoke 2 is based on basic chromium sulfate, boric acid as buffering agent, a carboxylic acid based complexing agent and halide based conductivity salts and sulfur containing darkening agents. Chromium is electrodeposited using this bath complying with the following plating conditions: T: 35 °C, pH: 2.8, current density: 10 A/dm², plating time 5 min. Trichrome® Graphite is based on basic chromium sulfate, boric acid as buffering agent, a carboxylic acid based complexing agent and halide based conductivity salts and sulfur containing darkening agents. Chromium is electrodeposited using this bath complying with the following plating conditions: T: 35 °C, pH: 3.2, current density: 10 A/dm², plating time 5 min.

Silicone Coating Mixture:

A standard composition of the silicone coating mixture comprises the following components:

- 0.9 g/l CMPD B - 0.3 g/l CMPD A

- 0.2 g/l SURF

- pH 3.5 (adjusted with acetic acid)

CMPD A is a first silicone type compound derived from an aqueous co-condensation of at least two monomeric building blocks selected from the group consisting an aminoalkyltrihydroxysilane compound and a fluoroalkylalkyltrihydroxysilane compound, wherein the aminoalkyltrihydroxysilane compound has the general chemical formula (I):

(HO)₃Si-(CH₂)_x-[NH(CH₂)_y]_z-NH₂ (I)

wherein: x is 3, y is 2, z is 2, and wherein the fluoroalkylalkyltrihydroxysilane compound has the general formula (II):

(HO)₃Si-(CH₂)_a-(CF₂)_bCF₃ (II)

wherein: a is 2, b is 5.

This silicone compound is used in the form of a 15 wt.-% solution of this compound in water acidified with formic acid to pH 4.

CMPD B is a second silicone type compound and is a water soluble polymeric aminoalkylsilicone compound which is derived from aqueous condensation of at least one monomeric building block selected from the group consisting of a aminoalkyltrihydroxysilane compound, wherein the aminoalkyltrihydroxysilane compound has the general chemical formula (III):

(HO)₃Si-(CH₂)_x-[NH(CH₂)_y]_z-NH₂ (III)

wherein: x is 3 and z is 0.

This second silicone compound is used in the form of a 10 wt.-% solution of this compound in water acidified with formic acid to pH 4.

The siloxane polymer (SURF) is used as a siloxane block copolymer (CAS No. 68938-54-5) in the form of a 10 wt.-% solution of this compound in water. If not stated otherwise, temperature of the silicone coating mixture during the step of treating the metal substrate was fixed to 25 °C.

Application method:

Prior to sol-gel application, the substrates were cleaned in a cathodic degreasing bath UniClean® 256 (trademark of Atotech Deutschland GmbH, Germany; alkaline degreasing bath) for 1 min at 10 ASD (A/dm²) and thoroughly rinsed with deionized water afterwards. Wet substrates were immersed into the silicone coating mixture in a 500 ml glass beaker.

Immersion and withdrawal of samples were done with the aid of a dip coating robot that allowed to control immersion and withdrawal speed (immersion speed: 100 cm/min; immersion time: 1 min; immersion depth: 8 cm (lower edge of a specimen below liquid level); withdrawal speed: 5 cm/min (if not stated otherwise)).

After withdrawal of the samples from the silicone coating mixture, the dry samples were cured in an oven (ambient atmosphere). Standard settings for curing were t_oven = 30 min T_oven = 70 °C. These settings were also varied in a certain set of experiments to study the influence of the curing parameters.

Contact Angle Measurements:

Contact angle measurements were conducted after 24 h minimum sample storage time at ambient atmosphere. Measurements were done by detecting contact angle of 3 mI water on modified substrates (substrates provided with the anti-fingerprint coating). Evaluation of the angle was done with the Laplace-Young method (Law and Zhao, Surface Wetting - Characterization, Contact Angle and Fundamentals, Springer Verlag (2016) ISBN 9783319252124.).

Contact angles of water deposited on the samples as prepared or coated with the respective anti-fingerprint coating were measured. All measurements were taken at 25 °C and at a relative humidity of 40-60 %. All contact angle values given hereinafter consist of average values of 5 measurements. Contact angles were determined with a constant drop volume of 3.0 mI. Fingerprint Tests:

Fingerprint tests were applied to selected sets of samples. The tests were performed with an in- house developed procedure (see Appendix: Fingerprint Test Method). Basically, surface color change (AColor = AUAa/Ab) was detected before and after application and cleaning of an artificial fingerprint on the substrates by measuring color in L/a/b color space with a spectrophotometer (colorimeter).

The color impression of the samples was measured by the colorimeter and the color is described by the L ^* a^* b^* color space system (introduced in 1976 by the Commission Internationale de I'Eclairage). The value L^* indicates lightness and a^* and b^* indicate color directions. A positive value 25 of a^* indicates a red color while a negative value of a^* means a green color. A positive value of b^* indicates a yellow color and a negative value of b^* means a blue color. When the absolute values for a^* and b^* increase the saturation of the colors also increases. The value of L ^* ranges from zero to 100, wherein zero indicates black and 100 means white.

On fingerprint sensitive surfaces, the difference between measured L/a/b values before and after the test ( AUAa/Ab ) was proven to be usually large due to a high amount of residual fat deposited by the artificial fingerprint. Easy-to-clean surfaces (obtained with the method according to the invention) yielded smaller AColor values.

Details of the test procedure are given in the Appendix“Fingerprint Test Method” at the end of the description.

Test Results and Discussion

First Experimental Set / Experiments OA to OF / Influence of Single Components

(Comparative Examples) Table 1 gives the set-up parameters for producing the test samples.

TABLE 1 : Experimental Design of First Experimental Setup (Experiments OA-OE)

All treatment compositions were prepared as given in Table 1 by specifying that the respective components are added to give 1 I, wherein the remainder is water. T_so, was the temperature of the silicone coating mixtures during the tests. t_oven was the curing temperature. Speed [mm/min] was the speed to remove the samples from the silicone coating mixture. The parameter “Substrate” denotes the specified plating bath for depositing the top chromium substrate.

In this Experimental Setup, the effect of single components on coating quality was investigated. The very first sample (OA) was immersed into water only. The sample did not include any other substance. In the following experiments, each component of the silicone coating mixture, including acetic acid, was dissolved in water separately and used for a respective text (OB-OE). After bath immersion, all samples were treated according to the standard procedure. Modification of the surface by the provision of an anti-fingerprint coating was tested by contact angle measurements and fingerprint tests. Water contact angle values on the sample set of experiments OA to OE are given in Table 2. The results are demonstrated graphically in Fig. 1.

TABLE 2: Water Contact Angle Values Measured on First Experimental Set (Experiments

OA-OE)

According to the results of contact angle measurement, treatment with acetic acid only and, alternatively, with SURF only in the given concentrations did not cause significant change in surface energy of the chromium substrate. On the other hand, CMPD A and CMPD B have relatively small but remarkable effect on surface energy. Minimum surface energy is acquired after treatment in CMPD B solution, yielding a contact angle of 55.5°. CMPD A yields a contact angle of around 30° only, even though this component comprises fluorinated functional groups that would be expected to result in low surface energy. Though not being bound by theory, it is believed that CMPD A cannot be adsorbed properly on the surface if this component is not supported by other organosilicon compounds that anchor it on the chromium surface strongly.

Table 3 lists the results from fingerprint measurements:

TABLE 3: AColor Values Measured on the Samples of the First Experimental Set

(Experiments OA-OE)

Results of the fingerprint test are demonstrated in Fig. 2. Basically, the test results are in line with the contact angle measurements. More hydrophobic surfaces repel dirt and are easier to clean. Consequently, L/a/b values have a smaller shift from the original color after application of the artificial fingerprint. The shift is more pronounced for L and b values whereas the a value is less effected in this test. The untreated sample (OA) reveals -4.0 and 2.3 units shift in L and b values respectively. Similar AUAb values were measured on samples that were treated in acetic acid (OB) and SURF (OE) solutions. The color shift became smaller after treatment in CMPD A (OC) and CMPD B (OD) solutions. Treatment in the standard composition that contained all components yielded AUAa/Ab values of -2.4/0.2/1.6, resp.

Second Experimental Set / Experiments 1 and 2/ Influence of Surfactant (Experiments according to the Invention)

In this set of experiments the surfactant SURF was excluded from coating mixture.

Table 4 gives the set-up parameters for producing the samples in this Experimental Set. Again, as described for the First Experimental Set, all treatment compositions were prepared as given in this Table by specifying that the respective components are added to give 1 I, wherein the remainder is water. Table 5 lists the results from contact angle measurements for these samples. Table 6 lists the results from fingerprint measurements in this Experimental Set. Fig. 3 shows the measured contact angle values on the related samples.

TABLE 4: Experimental Design of Second Experimental Setup (Experiments 1 and 21 and

2)

TABLE 5. Water Contact Angle Values Measured on Second Experimental Set

(Experiments 1 and 2)

TABLE 6: AColor Values Measured on the Samples of the Second Experimental Set (Experiments 1 and 2)

Firstly, the silicone coating mixture yielded a lower surface energy compared to the single components (larger contact angle). Contact angle values between 75 - 90° were achieved due to treatment in the silicone coating mixtures.

Secondly, addition of the surfactant SURF did not remarkably change surface energy. But its addition significantly results in more homogeneous coatings as was deduced from the lower standard deviation in the contact angles. Note, that addition of surfactant also positively influenced optical appearance of the coatings by the elimination of flakes, discolorations and other types of defects which occurred otherwise.

Fingerprint tests (Fig. 4) revealed that the standard composition including surfactant had the best score.

Third Experimental Set / Experiments 3a-c / Influence of pH of Coating Mixture

(Experiments according to the Invention pH of a make-up of the silicone coating mixture was originally 4.3 - 4.4 before addition of any acid. At this pH, amino-functionalized organo-silicon molecules are most stable in water due to maximum hydrolysation reaction kinetics. However, the resulting anti-fingerprint coatings at this pH proved to be inhomogeneous and to yield some optical defects. Hence, pH adjustment was done to improve homogeneity and optical quality.

Table 7 gives the set-up parameters, including pH values, for producing the samples in this Experimental Set. Again, as described for the First Experimental Set, all treatment compositions were prepared as given in this Table by specifying that the respective components are added to give 1 I, wherein the remainder is water. Table 8 lists the results from contact angle measurements for these samples. Table 9 lists the results from fingerprint measurements in this Experimental Set. pH of the silicone coating mixture was adjusted by addition of acetic acid.

TABLE 7: Experimental Design of Third Experimental Setup (Experiments 3a-c)

TABLE 8: Water Contact Angle Values Measured on Third Experimental Set (Experiments

3a-c)

TABLE 9: AColor Values Measured on the Samples of the Third Experimental Set

(Experiments 3a-c)

A gradual declining trend in contact angle values was seen with decreasing pH (Fig. 5). Fingerprint test results correlated with contact angle measurements (Fig. 6).

Fourth Experimental Set / Experiments 4a-e / Influence of the Type of Acid for pH Adjustment

(Experiments according to the Invention)

The previous experiments (Third Experimental Set, Examples 3a-d) showed that decreasing pH from 4.4 to lower values proved to help improve coating homogeneity and optical appearance without changing surface energy of the coating drastically. In this set of experiments, besides acetic acid, other commonly used acids were tested. The tests were done by adjusting bath pH to 3.5using the given acids.

Table 10 gives the set-up parameters for producing the samples in this Experimental Set. Again, as described for the First Experimental Set, all treatment compositions were prepared as given in this Table by specifying that the respective components are added to give 1 I, wherein the remainder is water. Table 1 1 lists the results from contact angle measurements for these samples. Table 12 lists the results from fingerprint measurements in this Experimental Set.

TABLE 10: Experimental Design of Fourth Experimental Setup (Experiments 4a-e)

^* H₂S0₄ (96%)

^** ortho-phosphoric acid (85%)

^*** Formic acid (99%) (CFi₂OFI)

# Glyceric acid (70%) (C₃H₆0₄)

## Malic acid (50wt% sol) (C₄H₆0₅)

TABLE 1 1 : Water Contact Angle Values Measured on Fourth Experimental Set (Experiments

4a-e)

TABLE 12: AColor Values Measured on the Samples of the Fourth Experimental Set

(Experiments 4a-e)

As is clearly seen from Fig. 7, at given pH none of the acids have superior influence on coating surface energy compared to acetic acid. On the other hand, some negative influence on the optical properties has been observed with some acids, especially if post-rinsing is applied after substrate withdrawal from the coating mixture. In previous studies, it was found that siloxane condensation / gelation on the substrate occurred only after complete drying of the sol film. If the substrate was water-rinsed before being completely dried, siloxane molecules, being in their hydrolyzed state, diffused into the rinse water while leaving the rinsed areas uncovered. Basically, these uncovered areas reduced anti-fingerprint effect on the substrate locally without forming any visible defect on and around them. However, usage of ortho- phosphoric acid, glyceric acid and malic acid released a white-hazy color on such post-rinsed areas. Hence, usage of these acids in the silicone coating mixture was assessed to be avoided. A similar behavior was observed as well when tartaric acid and lactic acid were used.

Acetic acid, formic acid and sulfuric acid were shown not to cause any discoloration on the surface even if post-rinsing was applied. Among these acids, acetic acid and formic acid yielded slightly lower coating surface energy. Acetic acid was favored due to easier handling.

Fifth Experimental Set / Experiments 5a-f / Influence of Ratio

(Experiments according to the Invention)

As shown above (Experimental Set OA-OE), the two main silicone compounds of the coating mixture, CMPD B and CMPD A, do not perform well one without the other. Combination of these two compounds was shown to yield much better results than if each one was used without the other one. In this Experimental Set, different mixing ratios of these compounds were tested.

Table 13 gives the set-up parameters for producing the samples in this Experimental Set. Again, as described for the First Experimental Set, all treatment compositions were prepared as given in this Table by specifying that the respective components are added to give 1 I, wherein the remainder is water. Table 14 lists the results from contact angle measurements for these samples. Table 15 lists the results from fingerprint measurements in this Experimental Set.

TABLE 13: Experimental Design of Fifth Experimental Setup (Experiments 5a-e)

TABLE 14: Water Contact Angle Values Measured on Fifth Experimental Set (Experiments

5a-e)

TABLE 15: AColor Values Measured on the Samples of the Fifth Experimental Set

(Experiments 5a-e)

Remarkable differences in surface energy of the resulting coating from different combinations of CMPD B and CMPD A are clearly seen in Fig. 8. Keeping CMPD B concentration constant and increasing concentration of CMPD A in the coating mixture (Std: silicone coating mixture comprising 0.9 g/l CMPD B / 0.3 g / I CMPD A) results in decreasing surface energy gradually. Using the standard coating mixture, average contact angle measured on the coating surface was 89°. Using a combination of 0.9 g/l CMPD B / 1.5 g/l CMPD A (which contained four times more CMPD A compared to the standard composition), an average contact angle of 107° was achieved. Beginning from this point, surface energy of the coating was further decreased up to 1 14° by reducing CMPD B content furthermore to about half this amount (0.5 g/l CMPD B / 1.5 g/l CMPD A). Interestingly, further decreasing CMPD B content had a negative influence and increased surface energy as was observed for 0.2 g/l CMPD B / 1.5 g/l CMPD A.

Similar results were obtained with the fingerprint test as shown in Fig. 9. Best results were obtained for the combination of 0.5 g/l CMPD B with 1.5 g/l CMPD A.

In Experiments 5b and 5e different total concentrations were used, but the coating mixture commonly contained 50 % CMPD A of the total silicone compound amount. Experiments 5b and 5e yielded the same surface energy with the combinations 0.9 g/l CMPD B / 1.5 g/l CMPD A and 0.5 g/l CMPD B / 0.8 g/l CMPD A.

These results indicate that, for decreasing surface energy, more CMPD A can be added into the coating mixture. However, it was shown that CMPD B concentration should be kept above a critical value, ideally above 0.3 g/l. It is believed that this would support CMPD A co-adsorption sufficiently. It has proved that surface energy of the coating remains constant independent of film thickness, once a compact coating has formed. Theoretically, film thickness grew by increasing total silicone concentration in the coating mixture.

Sixth Experimental Set / Experiments 6a-d / Influence of Chromium Substrate

(Experiments according to the Invention)

In order to find out whether silicone adsorption is favored on certain types of chromium surfaces, the method of the present invention was tested on different chromium substrates. Substrates were prepared in various electroplating chromium baths and the same anti fingerprint coating was applied on all of them.

Table 16 gives the set-up parameters for producing the samples in this Experimental Set. Again, as described for the First Experimental Set, all treatment compositions were prepared as given in this Table by specifying that the respective components are added to give 1 I, wherein the remainder is water. Table 17 lists the results from contact angle measurements for these samples. Table 18 lists the results from fingerprint measurements in this Experimental Set.

TABLE 16: Experimental Design of Sixth Experimental Setup (Experiments 6a-d)

TABLE 17: Water Contact Angle Values Measured on Sixth Experimental Set (Experiments

6a-d)

TABLE 18: AColor Values Measured on the Samples of the Sixth Experimental Set

(Experiments 6a-d)

According to the contact angle (Fig. 10) and fingerprint test results (Fig. 1 1 ), darker chromium layers have a less pronounced water and dirt repelling effect, such as TC Shadow and TC Graphite, than brighter chromium layers, such as TC Plus and TC ICE. Considering that darker chromium deposits contain more alloying elements, it may be possible that sol-gel film formation is more difficult to achieve on such complicated surfaces. On the other hand, even if the same coating quality should be achieved on bright and dark surfaces, due to optical effects, larger color shift would be expected on darker surfaces after contaminating the surface. Seventh Experimental Set / Experiments 7a-g / Influence of Temperature of the Coating Mixture (Experiments according to the Invention)

Temperature of the coating mixture has proven an important parameter which has shown to be responsible not only for kinetic activity of the components but also for drying speed of the substrate after its withdrawal from the silicone coating mixture. In this Experimental Set, the temperature of the coating mixture was gradually increased to investigate its effect on coating quality. Table 19 gives the set-up parameters for producing the samples in this Experimental Set. Again, as described for the First Experimental Set, all treatment compositions were prepared as given in this Table by specifying that the respective components are added to give 1 I, wherein the remainder is water. Table 20 lists the results from contact angle measurements for these samples.

TABLE 19: Experimental Design of Seventh Experimental Setup (Experiments 7a-g)

TABLE 20: Water Contact Angle Values Measured on Seventh Experimental Set

(Experiments 7a-g)

No AColor values were measured on these samples.

Looking at the contact angle results in Fig. 12, a tremendous influence of temperature of the silicone coating mixture on coating quality was observed. Surface energy of the anti-fingerprint coating was found to be reduced when temperature rises, thereby making it more hydrophobic and dirt repelling. Furthermore, high bath temperature made it possible to withdraw substrates faster from the silicone coating mixture without forming optical defects such as fat edges that basically occurred after slow drying of accumulated solution at geometrically disadvantaged regions.

Eighth Experimental Set / Experiments 8a-f / Influence of Curing Duration

(Experiment according to the Invention)

Cross-linking of the adsorbed anti-fingerprint coatings was proved to be accelerated by curing of the dried coating at elevated temperatures. In this Experimental Set curing temperature was fixed to 70 °C. This temperature was the maximum allowed temperature for polymeric ABS carriers. In the Experimental Set 8a-f, duration of post-curing at 70 °C was varied.

Table 21 gives the set-up parameters for producing the samples in this Experimental Set. Again, as described for the First Experimental Set, all treatment compositions were prepared as given in this Table by specifying that the respective components are added to give 1 I, wherein the remainder is water. Table 22 lists the results from contact angle measurements for these samples. Table 23 lists the results from fingerprint measurements in this Experimental Set.

TABLE 21 : Experimental Design of Eighth Experimental Setup (Experiments 8a-f)

TABLE 22: Water Contact Angle Values Measured on Eighth Experimental Set (Experiments

8a-f)

TABLE 23: AColor Values Measured on the Samples of the Eighth Experimental Set

(Experiments 8a-f)

Positive influence of curing on coating compactness and consequently on the hydrophobic effect was recognized through the contact angle values plotted in Fig. 13. Contact angles were shown to become larger with longer curing duration. A jump was observed between 45 min and 60 min. However, beyond 60 min increase in hydrophobicity proved to be only small.

Fingerprint test results (Fig. 14) also hint for two regions below and above 60 min curing duration. Above 60 min, removal of the artificial fingerprint was shown to be easier and to leave less residues behind as was deduced from AUAa/Ab values.

Ninth Experimental Set / Experiments 9a-i / Influence of Withdrawal Speed

(Experiments according to the Invention)

At the end of immersion time, substrates were withdrawn at a certain speed from the coating mixture. Withdrawal speed was shown to have a direct influence on drying kinetics and consequently on quality of the formed anti-fingerprint coating. In this Experimental Set, withdrawal speed was varied to investigate its influence on the sol-gel process.

Table 24 gives the set-up parameters for producing the samples in this Experimental Set. Again, as described for the First Experimental Set, all treatment compositions were prepared as given in this Table by specifying that the respective components are added to give 1 I, wherein the remainder is water. Table 25 lists the results from contact angle measurements for these samples. Table 26 lists the results from fingerprint measurements in this Experimental Set.

TABLE 24: Experimental Design of Ninth Experimental Setup (Experiments 9a-g)

TABLE 25: Water Contact Angle Values Measured on Ninth Experimental Set (Experiments

9a-g)

TABLE 26: AColor Values Measured on the Samples of the Ninth Experimental Set

(Experiments 9a-g)

Normally, low withdrawal speeds were found to be favored in water borne coatings, especially in order to control the drying process. Interestingly, it was observed that with increased withdrawal speed, more hydrophobic and dirt repelling coatings were formed (Fig. 15 and Fig. 16). This behavior is believed to be assigned to a thicker coating being formed at high withdrawal speed that has theoretically been proposed and experimentally confirmed in 1942 by Landau and Levich: (_Eq. 1 ) wherein h is thickness of liquid layer adhering to the withdrawn substrate [m], U is the withdrawal speed [m/s], c is a constant (0.944 for Newtonian liquids), h is the liquid viscosity of the liquid [Pa.s], y is the surface tension of the liquid against air [N/m],p is the density of the liquid [kg/m³] and g is the gravitational acceleration [m/s²). According to the Landau-Levich equation (Eq. 1 ), film thickness increases with withdrawal speed, which intuitively would not be expected. At high speed, due to the breakage of capillary forces, residing silicone solution volume per unit area is believed to become larger before gelation occurs. Consequently, silicone concentration is enriched on the surface and hence thicker film forms.

On the other hand, we observed that a very low withdrawal speed (10 mm/min) yields better results than slightly increased speeds (20 mm/min). At a first glance, this result may be considered as an experimental error. However, there are some studies in the literature that are in line with our findings that explain this behavior by the existence of two film forming regimes on the substrate during withdrawal: (i) a capillary regime and (ii) a draining regime. The draining regime obeys Landau-Levich equation that does not consider film formation through evaporation. In the capillary regime, evaporation of the solvent (in this case water) occurs at the triple point. Film thickness increases with evaporation speed that becomes higher in relation to withdrawal speed when the substrate is moved very slowly, whereas it becomes lower in relation to withdrawal speed when the substrate is moved out quickly. Assuming below a certain thickness, film thickness and hydrophobicity are directly correlated to each other. Therefore, a good match between proposed theory and the results of the present investigation is found.

Conclusions

Influence of various parameters on the application and quality of the method of the invention has been studied. The following conclusions are drawn from the demonstrated data: Coating quality can be tested by contact angle measurements. Fingerprint test results are usually in line with the contact angle results. However, contact angle measurements are more reproducible and much easier to apply.

Solutions of single components do not reveal the expected coating quality. Best performance is achieved when CMPD B, CMPD A and SURF are mixed.

CMPD B is believed to behave as the matrix of the anti-fingerprint coating. Providing a sufficient amount of CMPD B, the other main component CMPD A is achieved to be better anchored on the chromium surface and better incorporated into the coating network.

CMPD A is believed to be the main hydrophobing agent in the formulation. Increasing CMPD A amount results in formation of more hydrophobic and dirt repelling coatings.

Film thickness seems to be decisive on stability of the coating and levelling effect especially on rough surfaces. Hence, a thicker film seems to be more favorable. In required cases, CMPD A can be added separately to the coating mixture for improving the repelling effect of the coating.

The surfactant SURF improves film homogeneity and reduces the number of optical defects on the coating.

Adjusting bath pH is required by the addition of some acid into the bath. At the original pH of the coating mixture (without the addition of any acid), more defects on the coating were observed. At pH 3.5, the number of defects significantly reduced. Adjusting the pH to more acidic values resulted in better coating quality, but a low pH value negatively influenced stability of the coating mixture by decreasing hydrolysation rate and by increasing condensation rate of the dissolved silicone compounds.

For pH adjustment acetic acid has proven to be the best compound to use. Alternatively, formic acid and sulphuric acid can also be employed. Other tested acids, such as phosphoric acid, malic acid and glyceric acid, also work principally well, but they may cause discoloration on the surface under certain circumstances. The method of the present invention performs better on bright chromium deposits than on dark chromium deposits. When the deposit contains more alloying elements and becomes darker, hydrophobicity and dirt repelling effect gradually decreases. Coating quality significantly increases with increasing temperature of the coating mixture. Another big advantage of high temperature of the coating mixture is the possibility to increase withdrawal speed due to faster drying of the dip coat.

Withdrawal speed has an astonishing effect on coating quality. At a higher speed, thicker and more hydrophobic coatings seem to be formed. However, high withdrawal speed usually results in the formation of defects at the lower parts of a substrate (fat edges, last drop etc.).

The applied sol-gel anti-fingerprint coating must be cured for achieving best performance. A longer curing duration results in a more compact film. Saturation has proven to reach after 60 min.

In none of the experiments a contact angle value above 1 15° was achieved. This degree of hydrophobicity seems to be the upper limit for the application. Combination of two positive effects such as increasing both, bath temperature and withdrawal speed, does not provide any additional advantage.

The coating mixture performs slightly better after heating it. Activation of the coating mixture is kept for some time even after the coating mixture is cooled down again.

Appendix: F ingerprint Tes t Meth od

For imitating a real fingerprint, a certain amount of artificial skin fat is applied onto a test surface with the aid of a silicon stamp. Consequently, the applied fingerprint is removed with a microfiber cloth to evaluate the ability of the surface to be cleaned easily.

Sample preparation: The sample had a minimum surface area of 25 cm². Samples were measured as received, unless they were excessively contaminated. Visible dust particles and other dirt were gently removed from the surface before the application of the test. Only dry samples were measured.

Application of Artificial Fingerprint: For evaluation of the fingerprint test, color values in L/a/b color space (L: lightness, a: green-red and b: blue-yellow axes) with a spectrophotometer on a test spot before the application of the fingerprint (Fig. 18: Measurement of initial color). Another color measurement was done at the end of the test on the same spot for comparison (Fig 17: Measurement of the final color). The illumination area for the color measurements is fixed in a range 010 ± 2 mm with a suitable aperture. Measurements are carried out by firmly pressing the sample towards the spectrophotometer in vertical position.

All measurement steps were carried out at room temperature in the ambient.

A silicon stamp was used to apply an artificial fingerprint onto a sample surface. To prepare the stamp, prior to each measurement the silicon stamp was cleaned by immersion into isopropanol by immersing same into this solvent for 10 seconds minimum and then drying it.

Artificial sebum (acc. to BEY, commercially available from wfk Testgewebe GmbH for example) was uniformly applied to a felt cloth and transferred to the silicone stamp by pressing same onto the cloth using a force of 4.0 N ± 0.5 N for five seconds. The artificial sebum loaded stamp was pressed onto the test (chromium) surface for five seconds by applying a force of 4.0 N ± 0.5 N. Finally, the stamp was drawn back carefully, leaving an artificial fingerprint on the surface of the sample (Fig. 17: Artificial fingerprint). Three different fingerprints minimum were applied onto a test surface following the same procedure. Evaluation of the sample surface:

The ability of the substrate to be cleaned from of the fingerprint is evaluated by rubbing the stamped areas with a microfiber cloth. For the manual cleaning procedure, typical cleaning gesture is mimicked. The cloth was wrapped around the forefinger of the preferred hand. With a constant force and speed, the fingerprint is rubbed with circular movements. Applied force for rubbing is approximately 5 N - finger force was tested on a laboratory scale by targeting 500 g measured weight during movements. Rubbing speed is not significantly influential on the test result. An unused microfiber cloth is always employed for cleaning. 20 rubbing cycles were applied in total (Fig. 17: Cleaning with a cloth. 40 circular cycles). After 20 cycles, L/a/b values were measured on the cleaned area again. Differences in the color parameters, AUAa/Ab, between before soiling / cleaning and thereafter were used for evaluation of the easy-to-clean properties.

Depending on color difference, the result falls into G (Good), S (Sufficient) or I (Insufficient) range.

The same evaluation process is applied on all available fingerprints on the surface (minimum three). Finally, the average result was taken for the overall evaluation of the sample.

Claims

C la ims

1. Method for coating a metal substrate with an anti-fingerprint coating, the method comprising:

(a) providing the metal substrate;

(b) providing a silicone coating mixture containing:

(iii) at least one acidifier; and

(iv) water;

(d) curing the treated metal substrate at a predetermined temperature.

2. Method of claim 1 , characterized in that the metal substrate is a chromium substrate and that the chromium substrate is produced by depositing a chromium metal layer on a work piece, wherein depositing the chromium metal layer comprises providing the work piece and an electroplating liquid containing at least one Cr(lll) plating species and electroplating the chromium metal layer onto the work piece by using the electroplating liquid containing the at least one Cr(lll) plating species.

3. Method of claim 2, characterized in that the electroplating liquid containing the at least one Cr(lll) plating species is free of chloride species.

4. Method of any one of the preceding claims, characterized in that the at least one first silicone compound is a water soluble statistical copolymer of an mono- or oligo(aminoalkyl) silicone and a (fluoroalkyl)alkylsilicone.

5. Method of any one of the preceding claims, characterized in that the at least one first silicone compound is derived from an aqueous co-condensation of at least two monomeric building blocks selected from the group consisting an mono- or oligoaminoalkyltrihydroxysilane compound and a fluoroalkylalkyltrihydroxysilane compound, wherein the mono- or oligoaminoalkyltrihydroxysilane compound has the general chemical formula (I):

(HO)₃Si-(CH₂)_x-[NH(CH₂)_y]_z-NH₂ (l) wherein: x is 1 -8, preferably 1 -6, more preferably 1 -4,

y is 1 -6, preferably 1 -4, more preferably 1 -2,

z is 0-8, preferably 0-6, more preferably 0-4, and wherein the fluoroalkylalkyltrihydroxysilane compound has the general formula (II):

(HO)₃Si-(CH₂)_a-(CF₂)_bCF₃ (II) wherein: a is 1 -8, preferably 1 -6, more preferably 1 -4

b is 0-20, preferably 0-10, more preferably 0-5.

6. Method of any one of the preceding claims, characterized in that the at least one second silicone compound is a water soluble polymeric aminoalkylsilicone compound derived from aqueous condensation of at least one monomeric building block selected from the group consisting of a aminoalkyltrihydroxysilane compound, wherein the aminoalkyltrihydroxysilane compound has the general chemical formula (III):

(HO)₃Si-(CH₂)_x-[NH(CH₂)_y]_z-NH₂ (III)

wherein: x is 1 -6, preferably 1 -4, more preferably 1 -3,

y is 1 -6, preferably 1 -4, more preferably 1 -2,

z is 0-6, preferably 0-4, more preferably 0-2.

7. Method of any one of the preceding claims, characterized in that the acidifier is selected from the group comprising formic acid, sulfuric acid and acetic acid.

8. Method of any one of the preceding claims characterized in that the silicone coating mixture has a pH of from 3.5 to 4.5.

9. Method of any one of the preceding claims characterized in that the silicone coating mixture additionally contains at least one siloxane polymer.

10. Method of claim 9, characterized in that the at least one siloxane polymer is selected from the group consisting of polyethersiloxane-siloxane copolymers.

1 1. Method of any one of claims 9 and 10, characterized in that the at least one siloxane polymer is selected from the group consisting of compounds having general chemical formula (IV):

IV,

wherein (x+y) is from 1 to 60 where x is from 1 to 30 and y is 0 to 30,

n is from 3 to 4,

a is from 0 to 30;

b is about 0 to 30;

R is a hydrogen or alkyl radical of 1 to 4 carbon atoms.

12. Method of any one of the preceding claims, characterized in that the silicone coating mixture contains the at least one first silicone compound and the at least one second silicone compounds in a predetermined mass ratio, wherein the mass ratio of all first silicone compounds to all second silicone compounds is from 1.0 to 4.0.

13. Method of claim 12, characterized in that the concentration of the at least one first silicone compound in the silicone coating mixture is from 0.05 g/l to 5.00 g/l and that the concentration of the at least one second silicone compound in the silicone coating mixture is from is from 0.05 g/l to 10.00 g/l.

14. Method of any one of the preceding claims, characterized in that curing of the coated metal substrate is performed at a temperature of from 10 °C to 90 °C.

15. Use of a silicone coating mixture for coating a metal substrate with an anti-fingerprint coating, said silicone coating mixture comprising:

(ii) at least one second silicone compound selected from the group consisting of aminoalkyl silicones; (iii) at least one acidifier; and

(iv) water.