Particle-mediated anodizing process for white anodized surfaces

Abstract

A method of anodizing an aluminum alloy, comprising the steps of: providing an anodizing bath containing tert-butoxide, primarily oxalic acid; aging the anodic oxidation bath to condense the metal alkoxide to form metal oxide particles and an acid ester; heating the bath to 40 DEG C to 80 DEG C; stirring the bath at a speed of 200 to 700 rpm (revolutions per minute); immersing the workpiece in a bath and anodizing at a constant current density of 1 A / dm2 to 4 A / dm2 for 1 to 20 minutes or until the bath voltage reaches a threshold value of 100 to 200 V; the workpiece is then anodized at a constant voltage for 20 minutes to 200 minutes to produce an anodized surface having a thickness of 5 microns to 100 microns, the anodized surface comprising an optically discontinuous pore structure that produces a white or near-white appearance.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to an improved method of anodizing aluminum or aluminum alloys using an aqueous anodizing bath containing substantive organic acids. More specifically, the present invention relates to a method of anodizing aluminum substrates using aqueous solutions of carboxylic acids and boric acid under controlled conditions, the aqueous solutions containing particulate media, including media derived from hydrolyzed metal alkoxides, or long chain polymers, or leached intermetallic compounds or alloying elements from different aluminum substrates, which media alter the anodizing process to produce the desired anodized coating morphology and create a hard, generally white surface. BACKGROUND

[0002] U.S. Patent Application No. 2022 / 0364253 Al (Hou et al.) “Method to Apply Coloured Coatings on Alloys” describes a method of anodizing light metal substrates in an anodizing bath containing primarily phosphoric acid to form an anodized film structure that develops black to red colors structurally when filled with nickel or other metals by electroplating. The anodizing solution described by Hou et al. produces a regular pore side-pore structure that develops narrow bandwidth photonic resonators when filled with metal that reflect a single color. Such surfaces cannot display white color because white surfaces inherently require broadband reflectors.

[0003] PCT Publication WO 2012 / 119306 (Nano Institute) describes a method of making a white anodized aluminum coating by anodizing to form a pore structure, infiltrating a first chemical into the pore structure, and then infiltrating a second chemical into the pore structure. The two chemicals react in the pore structure to form a white compound. A third acidic process alters the anodized aluminum pore structure to increase the opacity of the anodized layer and improve the whiteness of the surface. The method is too complex to implement in an industrial environment, and the surface whiteness is heavily dependent on whether the second anodizing step forms an anodized layer that is sufficiently opaque.

[0004] U.S. Patent 10,760,175 B2 (“Apple patent”) “White Anodic Films with Multiple Layers” teaches a multi-step process to produce a white anodized film comprising at least two anodization steps. The first step produces a dense surface pore structure to improve durability, the second step develops an irregular pore structure to provide diffuse reflection and introduces particles such as Ti02 into the pores and seals the pores. This method produces a white surface by managing reflection through multiple layers, however this process is complex and difficult to produce surfaces with whiteness exceeding L of 90 when measured using the CIELab* method. This multi-step process is complex and difficult to control in an industrial setting. Both the Nano Institute disclosed technology and the Apple patent rely on altering the anodization pore structure and introducing secondary materials into the anodization pore structure to form the surface color. This multi-step process is complex and difficult to control in an industrial setting.

[0005] Korean Patent KR102244376B1, entitled “White Anodizing Method Using Nanoparticles,” teaches a similar anodization structure forming method as the Apple patent, but employs vacuum technology to facilitate the infiltration of the nanomaterials within the anodization structure. SUMMARY

[0006] The present disclosure describes various aspects and embodiments related to the development of novel anodization films for aluminum or aluminum alloys that exhibit a glossy white, matt white, or opaque white appearance. In particular, in one aspect, an anodization process mediated by particulate or particulate-like compounds is described for the production of white anodized surfaces.

[0007] In one aspect, an aluminum anodization method for producing a white or substantially white nanocrystalline alumina surface on an aluminum substrate is provided, the method comprising the steps of: i. pre-treating an aluminum substrate; ii. immersing the aluminum substrate in an aqueous anodization bath comprising: - a buffering agent, - one or more particulate sources, and - one or more carboxylic acids; and iii. anodizing the aluminum substrate by following anodization current and voltage specifications to provide an anodized substrate comprising an anodized surface.

[0008] In one embodiment, the anodization current and voltage specifications comprise: i. a first time period at a first constant current; ii. increasing the current to a second constant current for a second time period; iii. anodizing at the second constant current until the voltage reaches a threshold value, then switching to constant voltage anodization; and iv. constant voltage anodization for a third time period.

[0009] In another aspect, an anodized substrate is provided, comprising an anodized surface comprising a compact nanocrystalline underlayer, a nanotube layer substantially filled with nanocrystals, and a porous nanotube layer having nanocrystalline walls. In one aspect, the anodized substrate is an aluminum substrate.

[0010] In one aspect, a method of forming an anodized surface comprising a white oxide film is provided. The method comprises pretreating an aluminum or aluminum alloy substrate; anodizing the substrate in an anodizing bath comprising an organic acid electrolyte, the anodizing bath containing a particle source from hydrolysis of an alkoxide or other acid activity modifier. The method can further comprise the step of polishing the anodized surface to increase gloss. The method can further comprise the step of sealing the anodized surface to improve coating durability.

[0011] In one embodiment, the anodized surface produced by the method comprises a layered anodized structure having a random pore structure and a periodic optical discontinuity. By random pore structure is meant a structure that does not have the regular geometric pore arrangement typical of anodized aluminum. The random pore structure is capable of diffusely reflecting all visible light frequencies, thereby producing a surface appearance that is white or substantially white.

[0012] In one embodiment, the method further comprises impregnating the anodized structure having a random pore structure with particles of 40 nm or less to increase the diffuse reflection provided by the anodized surface.

[0013] According to another embodiment, the aluminum alloy or aluminum substrate is preformed into an enclosure, a device housing, an apparatus housing, a window frame, or other architectural, commercial, aerospace, or industrial component, where a white or near-white anodized surface of the component provides a useful aesthetic effect, improves surface performance, or provides other functional properties.

[0014] In one embodiment, the particle source in the anodizing bath and the alcohol produced by hydrolysis of the metal alkoxide is replaced by a combination of metal oxide nanoparticles having a particle size of about 100 nm and alcohol.

[0015] In one embodiment, the particle source in the anodizing bath and the alcohol produced by hydrolysis of the metal alkoxide is replaced by metal ions or compounds leached out during the anodizing process and alcohol.

[0016] In one embodiment, the source of particles in the anodization bath and the alcohol produced by hydrolysis of the metal alkoxide is replaced by long chain organic acids or other long chain organic molecules.

[0017] It is to be understood that while the foregoing describes certain aspects and embodiments, others will become apparent from the following specific description and drawings. BRIEF DESCRIPTION OF DRAWINGS

[0018] The present disclosure will become readily apparent from the following specific description and drawings, wherein the same reference numerals are employed to designate like elements throughout.

[0019] Figure 1 A cross-sectional view of a substrate having a standard anodized surface providing a translucent or semi-translucent colored surface is shown.

[0020] Figure 2 A cross-sectional view of an optically discontinuous anodized surface is shown, according to certain embodiments, wherein the structure provides a white appearance.

[0021] Figure 3 A flow chart outlining a method for forming an anodized film having a white appearance, according to certain embodiments, is shown.

[0022] Figure 4 A plot of anodization voltage and current, according to certain embodiments, is shown.

[0023] Figures 5A to 5H SEM surface and cross-sectional images of an anodized film manufactured according to certain embodiments are shown.

[0024] FIGS. 6A-6C show XRD spectra of white anodized films, according to certain embodiments.

[0025] Figure 7 is a simplified diagram of an anodization method apparatus showing the alignment direction of nanoparticles in the formation of a coating.

[0026] Figure 8 is a plot of coating color, thickness, and surface pore size as a function of anodization time.

[0027] Figure 9 is a plot showing the optical effect produced by the nanoporous regions in a white anodized coating. DETAILED DESCRIPTION

[0028] The detailed description is described with reference to the accompanying drawings.

[0029] In the following description, the word "aluminum" is understood to include all types of aluminum and wrought aluminum alloys, such as pure 1000 series aluminum, high strength 2000 series aluminum alloys, high ductility 3000 series aluminum alloys, malleable 5000 series alloys, high formability 6000 series alloys, and 7000 series aerospace alloys. The word "aluminum" can also include die cast aluminum alloys, such as A360.0 series, A380.0 series, C443.0 series, and B390.0 series.

[0030] In the description and claims, the preformed aluminum or aluminum alloy substrate can include, but is not limited to, window frames, window frame extrusions, housings for portable personal devices, such as watch housings or cell phone housings; automotive components and extrusions, architectural components and extrusions; aerospace components, aerospace extrusions, marine components, marine extrusions; die cast or vacuum cast components, and the like.

[0031] In the description, the word "about" used herein in conjunction with a reference number indicates the reference number plus or minus up to 10% of the reference number. For example, "about 10 g / L" encompasses a range of 9 g / L to 11 g / L.

[0032] In the description, the terms "comprises", "comprising", "includes", "including" and the like are to be construed as being inclusive (i.e., to include, and does not exclude) rather than

[0033] The term "substantially" as used herein means mostly, largely, essentially, or to a great or significant degree.

[0034] The terms "white" and "white color" as used herein with respect to a surface means L* of 90, and a* and b* of -2 to +2.

[0035] The term C1-C6 alcohol as used in the description refers to a straight chain or branched carbon entity having a single hydroxyl fatty alcohol. In one embodiment, the C1-C6 alcohol is derived from in situ hydrolysis of a metal alkoxide added to the anodization bath. In another embodiment, a combination of metal oxide nanoparticles and any C1-C6 alcohol can be directly added to the anodization bath.

[0036] The methods described herein provide white hard anodic oxide films on aluminum substrates. The anodic oxide films form a protective layer on components or parts formed from aluminum or aluminum alloys by providing a relatively dense and thick metal oxide surface. The properties of these anodic oxide films are desirable because they provide substantial hardness, chemical resistance, and optical properties.

[0037] Figure 1 An illustrative representation of a typical anodized surface is shown, for example, film 100 fabricated on aluminum substrate 101. Film 100 has a plurality of pores 110 with a pore diameter 102 that can range from about 2 nm (formed primarily in a sulfuric acid anodization bath) to about 200 nm (formed primarily in a phosphoric acid anodization bath). For hard anodization, the thickness 103 of the anodized surface 100 can range from sub-micron to hundreds of microns. Film 100 also includes a barrier layer with a thickness 104 that can vary from about 2 nm to about 200 nm or more depending on the anodization voltage. The pore wall thickness 105 and pore diameter 103a can range from about 5 nm to about 100 nm. In all cases, the optical properties of the coating are driven primarily by the substantially transparent anodic oxide film 100 that minimally interferes with the incident light, as shown by light rays 106, 107, which is mostly reflected by the underlying substrate. The anodized surface refracts light rays 106, 107, reflecting a small portion of the light, as shown by reflected light rays 108 and low-angle light rays 109, depending on the angle of incidence. The optical properties of the anodized surface, for example, film 100, result in the typical silver / yellow iridescent luster appearance of anodized surfaces.

[0038] Figure 2 A cross-sectional pictorial representation of an anodized surface 205 on an AL6061 aluminum alloy substrate 201 according to certain embodiments of the present disclosure is shown. An exemplary SEM image of the cross-sectional morphology of the anodized surface is also shown. The anodized structure and its interaction with light can be understood with reference to Figure 2 , 200 and the associated SEM image segments 208, 209, 210. Here, the anodized surface 200 includes an aluminum substrate 201 and a surface film 205. The surface film 205 includes a plurality of layers 202, 203, and 204. Layer 202 is a substantially dense nanoporous layer in contact with the aluminum substrate 201. Layer 203 is a multilayer stack including a plurality of discontinuous nanoporous anodic oxide structures, and layer 203 is positioned between and in contact with layers 202 and 204. Layer 204 is a branched nanoporous layer positioned furthest from 201. Depending on the anodization parameters and substrate, each layer can have a variety of thicknesses. However, in one described embodiment (Figure 5), the thickness of layers 202, 203, and 204 are nearly equal, about 10 μιη. Reference is made to Figure 2, incident light rays 206 can have any angle of incidence without significantly changing the effect and first strike surface layer 204. Layer 204 has a random porous structure, see SEM 208, which resembles ground glass in appearance. Thus, incident light rays 206 are scattered, only a small fraction of light 207 is reflected and scattered. Diminished scattered light rays 206 then reach layer 203, which has a random discontinuous structure. See SEM 209, the random discontinuous structure of this layer provides multiple nanopore / alumina interfaces and layer 203 reflects or scatters a substantial proportion of the incident light. Light rays 206 then enter layer 202. Layer 202 is the densest of the layers, see SEM 210, and provides additional pore / alumina interfaces which substantially or completely reflect or scatter the remaining incident light rays 206. Thus, only a small fraction of the incident light is reflected from the substrate. Figure 2 The overall scattered and reflected light rays 207 in Figure 2 cause the anodized surface to appear substantially white in appearance. Figure 9 is an image of a sample on which the anodized surface 200 was electrochemically stripped to understand the effect of the nanopores on the appearance of the coating. Here, Figure 9 901 shows that at least the top two layers (compare to Figure 2 203 and 204 in Figure 2 ) have a white appearance. The remaining coating is sufficiently transparent to view the underlying aluminum substrate (902, compare to substrate 201 and layer 202 in

[0039] ). Figure 3 A flowchart showing a method of developing a white anodized surface according to certain embodiments of the present disclosure is shown. The method begins at step 301.

[0040] At step 302 of Figure 3 , the method requires the preparation of an anodizing bath. In certain embodiments, the anodizing bath comprises an aqueous solution of oxalic acid, boric acid, and a carboxylic acid. In one embodiment, the oxalic acid is the primary bath acid at a concentration of about 10 g / L to about 140 g / L, or about 20 g / L to about 60 g / L, or preferably about 25 g / L to 50 g / L, more preferably about 42 g / L. In one embodiment, the boric acid is a bath stabilizer at a concentration of about 5 g / L to about 20 g / L, or about 8 g / L to about 16 g / L, preferably about 10 g / L. In one embodiment, the carboxylic acid is a conductivity enhancer at a concentration of about 0.1 g / L to about 5 g / L, preferably about 1 g / L. The carboxylic acid can be selected from a wide range of acids, however citric acid is preferred. Attempts to anodize using an anodizing bath to produce an anodized surface did not result in an increase in the depth of anodization as the rate of alumina formation was approximately the same as its dissolution.

[0041] One embodiment uses a metal alkoxide compound to modify the anodization bath. Metal alkoxides are a source of metal microparticles and metal nanoparticles. Many metal alkoxides can be used, including titanium butoxide, aluminum butoxide, zirconium propoxide, and the like. The amount of metal alkoxide used is not particularly limited, however, the amount of metal alkoxide required is sufficient to produce a concentration of nanoparticles to microparticles that allow the anodization process to proceed. In one embodiment, the alkoxide selected is a metal butoxide, preferably titanium butoxide, at a concentration of about 1 g / L to about 80 g / L, or about 10 g / L to about 60 g / L, or about 20 g / L to about 40 g / L. In a preferred embodiment, the bath contains about 28 g / L of titanium butoxide.

[0042] In one embodiment, the bath can be aged to produce a source of particles. The source of particles is generated in situ from a dispersion of metal oxide nanoparticles or microparticles, and alcohol is generated by condensation of the metal alkoxide. Condensation of the metal alkoxide typically occurs naturally in the bath in the presence of water; however, the presence of acid in the bath can hinder the natural condensation, and thus catalyzing the process is required. In one embodiment, the formation of nanoparticles and butanol is catalyzed by the formation of hydroxide at the cathode during anodization. In an alternative embodiment, catalysis is achieved by directly adding a hydroxide, such as potassium hydroxide or sodium hydroxide, to the bath.

[0043] In one embodiment, the metal alkoxide can be titanium butoxide. Chemical reaction (1) summarizes the metal alkoxide condensation reaction of titanium butoxide (as shown) in an anodization bath to generate metal oxide particles of about 80 nm to about 3000 nm and release butanol.

[0044] Ti(OBu)4+ 2H2O → TiO2+ 4HOBu ——(1) Without being bound by any theory, the inventors believe that in the presence of one or more metal oxides and one or more organic acids, alcohol catalyzes the formation of one or more stable esters that act as acid activity modifiers to support the development of white anodized surfaces and anodized structures on substrates. Examples of suitable acid activity modifiers include stable polar esters, such as butyl oxalate, butyl citrate, and combinations thereof.

[0045] The source of metal oxide nanoparticles or microparticles can be an oxide of any metal produced by condensation of a metal alkoxide. However, titanium dioxide nanoparticles or titanium dioxide microparticles are preferred. Preferably, the metal oxide microparticles or nanoparticles have a negative zeta potential. In this way, the particles are attracted to the anode during anodization, however, the zeta potential can be any potential less than -0.2 mV.

[0046] In one embodiment, the anodization bath also includes a surfactant. The surfactant stabilizes the metal oxide micro- or nano-sized particles by functions such as adsorption to the particle surface. In principle, any surfactant can be suitable. However, non-ionic surfactants such as Tween® 20 (polyoxyethylene sorbitol ester), and cationic surfactants such as SDS (sodium dodecyl sulfate) are preferred. The concentration of the surfactant is preferably from about 0.1 mL / L to about 5 mL / L, more preferably about 1 mL / L.

[0047] In one embodiment, the anodization process condenses the metal alkoxide to form the hydroxyl- linked metal oxide particles.

[0048] The hydroxyl-linked metal oxide particles can be of any size; however, the optimal size is large enough to reduce adsorption to the anodization structure and small enough to remain easily suspended in the anodization bath. In one embodiment, the optimal size of the hydroxyl-linked particles is from about 50 nm to about 5 microns. In one embodiment, the hydroxyl-linked particles are titanium dioxide particles and the optimal size is from about 500 nm to about 4 microns, or preferably about 2.5 microns. In one embodiment, the hydroxyl-linked particles have a size of about 3000 nm as measured by dynamic light scattering. In an alternative embodiment, the metal particles are zirconium dioxide and the optimal size is from about 50 nm to about 250 nm, preferably about 100 nm.

[0049] In an alternative embodiment, the anodization bath at 302 is prepared without the addition of metal alkoxide, but rather by the addition of an amount of pre-prepared nano- or micro-sized particles and alcohol (preferably butanol). The nano- or micro-sized particles added can in principle be any nano- or micro-sized particle, preferably metal oxide nanoparticles, such as Titanium (IV) oxide, anatase, 25 nm nanopowder from Sigma Aldrich. In one embodiment, the amount of nanopowder added is from about 1 g / L to about 20 g / L, preferably from about 5 g / L to about 10 g / L. The amount of alcohol required is that which would be released by the hydrolysis of the metal alkoxide, from about 30 mL / L to about 100 mL / L, preferably about 65 mL / L. In a more preferred embodiment, pre-prepared nanoparticles are used to generate the white surface from the anodization bath, rather than the micro- and nano-particles formed in situ from the metal alkoxide in the anodization bath. Without being bound by theory, the inventors believe that the in situ formation of micro- and nano-particles can create inconsistencies in the particle size and the acid active ester formed, affecting the surface generated.

[0050] In another alternative embodiment, the anodization bath at 302 is prepared without the addition of metal alkoxide. The metal alkoxide can be replaced by metal oxide powder, such as magnesium oxide powder. Preferably, about 1 g / L to 2 g / L of magnesium oxide powder from Sigma Aldrich. Or alternatively, the metal oxide micro or nano particles formed in situ by leaching out during anodization of the aluminum alloy in the anodization bath comprising carboxylic acid and boric acid. Without being bound by theory, the inventors believe that the specific sub-micron metal oxide acts as an acid regulator by participating in the dissociation of the carboxylic acid in a similar manner as the stable polar ester.

[0051] In an alternative embodiment, the particle source and alcohol are replaced by succinic acid, long chain organic acid or other long chain organic molecules. A bath prepared using 2-propanol and polyethylene glycol (PEG) with a molecular weight of 400 g-mol -1 ) produces a white surface. The alcohol, preferably 2-propanol, is at a concentration of about 20 mL / L to about 100 mL / L, preferably about 40 mL / L to about 60 mL / L. The PEG MW 400 is at a concentration of about 5 mL / L to about 30 mL / L, preferably about 15 mL / L. Without being bound by theory, the inventors believe that the long chain organic molecules of PEG MW 400 interact with the carboxylic acid in a similar manner as the stable polar ester derived from the metal alkoxide to regulate the acid activity and growth of the nanostructure in a white appearance.

[0052] Referring to Figure 3 , step 303 of method 300 involves pre-treating the substrate to prepare the substrate for anodization. In certain embodiments, the pre-treatment comprises a step of soaking the substrate in an alkaline bath to remove surface contaminants. A commercial alkaline cleaner such as METACLEAN ZX from CMP Limited, India can be used. Here, the bath is operated at about 60 °C for about 5 minutes to about 30 minutes. An acid etch step can be performed after the alkaline soak. The acid etch can comprise a solution of 10 volume % sulfuric acid and 5 volume % hydrofluoric acid in deionized water (DI water). The acid etch step in this case can be performed at a temperature of about 20 °C to 30 °C for about 3 to about 5 minutes, preferably about 4 minutes. Alternatively, any commercial solution containing ammonium bifluoride can be used as the active chemical. After the acid etch, a de-smut step can optionally be performed in a solution of about 50% nitric acid. If needed, the de-smut step involves immersing the substrate in the nitric acid solution for about 1 minute.

[0053] In alternative embodiments, the pre-treatment step 303 of the substrate can involve polishing, preferably mechanical or electro-polishing treatment. The electro-polishing or mechanical polishing treatment reduces the substrate surface roughness and improves the uniformity of the anodization layer. The electro-polishing step can employ one of many electro-polishing methods known for aluminum substrates and is continued to provide a mirror or near mirror surface on the substrate. The mirror or near mirror surface has a roughness Ra of about 0.1 μm to about 0.5 μm, preferably about 0.2 μm or less. The mirror bright substrate can be preferred in certain applications to form a reflective rather than a matte anodized surface.

[0054] In Figure 3 the pre-treated substrate is immersed in a heated and agitated anodization bath. One preferred embodiment is to heat the anodization bath to about 20°C to about 90°C, or about 30°C to about 80°C, or more preferably about 45°C. A temperature of at least about 35°C ensures that the anodization process functions to form a dense nanocrystalline structure resulting in a white surface.

[0055] In one embodiment, the anodization bath is agitated. Many agitation methods are suitable as long as the agitation is sufficient to keep the metal nanoparticles suspended. In one embodiment, the bath is agitated by a magnetic stirrer at a speed of about 300 rpm to about 800 rpm, or about 400 rpm to about 700 rpm, or preferably about 600 rpm. In alternative embodiments, the agitation of the bath is achieved by the use of an external solution pump. Another alternative embodiment is to use low pressure compressed air to agitate the bath. In a preferred embodiment, sufficient agitation is maintained to achieve uniform suspension of the metal micro- and nanoparticles while maintaining uniform bath temperature during anodization.

[0056] In Figure 3 step 305, a first anodization current is applied between the substrate (anode) and a cathode. The cathode can be any material that remains substantially inert in the bath while at the same time being highly conductive. Suitable materials include carbon, titanium, stainless steel, and the like. In a preferred embodiment, the cathode is stainless steel. Suitable cathode materials will be apparent to those skilled in the art.

[0057] In a preferred embodiment, constant current DC anodization is performed. Constant current DC refers to a constant (steady) time independent current that does not vary in intensity over time. Alternatively, the method can also use pulsed DC anodization, which can be beneficial when anodizing certain Al alloys. Pulsed DC refers to a periodic current that varies in value but not in direction. In another embodiment, constant voltage anodization can also be used.

[0058] In Figure 3In step 305, a first anodic oxidation current density is applied. Preferably, the first anodic oxidation current density is about 0.5 A / dm³. 2 Approximately 3 A / dm 2 or approximately 1 A / dm 2 To approximately 2.5 A / dm 2 Or preferably about 2 A / dm 2 The method initially preferably maintains the first anodic oxidation current density for about 1 second to about 2 minutes, or about 30 seconds to about 1.5 minutes, or preferably about 1 minute.

[0059] exist Figure 3 In step 306, the anodic oxidation current is optionally increased from a first anodic oxidation current density to a second anodic oxidation current density. The second anodic oxidation current density is greater than the first current density. Preferably, the second anodic oxidation current density is about 2 A / dm³. 2 Approximately 8 A / dm 2 or approximately 3 A / dm 2 Approximately 6 A / dm 2 Or preferably 4 A / dm 2 Preferably, when the second anodic oxidation current increases from the first anodic oxidation current density to the second anodic oxidation current density, this increase is adjusted over a period of about 2 minutes to about 4 minutes (or optionally about 3 minutes). The nano- or micron-sized particle content of the substrate and the anodic oxidation bath affects the required second anodic oxidation current density. For example, titanium dioxide particles require about 4 A / dm³. 2 To produce the optimal color, alumina particles require approximately 6 A / dm². 2 Approximately 8 A / dm 2 Preferably, in each embodiment, the second anodic oxidation current density has a maximum current density, which is selected to achieve optimal color and / or optimal porosity of the anodic oxidation surface.

[0060] In a preferred embodiment, the second anodic oxidation current density is maintained until the voltage reaches the desired anodic oxidation voltage threshold. The anodic oxidation voltage threshold is affected by the bath composition, the type and size of the bath particles, the bath temperature, and the substrate. Preferred anodic oxidation voltage thresholds may be about 80V to about 300V, or about 120V to about 250V, or about 140V to about 180V, or preferably about 150V. In one embodiment, when the particles in the anodic oxidation bath are titanium dioxide particles with a particle size of about 3000 nm, the anodic oxidation bath composition includes about 27 g / L of oxalic acid, about 10 g / L of boric acid, about 1 g / L of citric acid, and about 24 g / L of titanium dioxide; the substrate is 6061-T6 aluminum, and the anodic oxidation voltage threshold is about 150V.

[0061] In an alternative embodiment, this embodiment produces a glossy white surface instead of a matte white surface, and the anodizing steps (304, 305, 306, 307) are performed on a polished or electropolished substrate. In this embodiment, the first current density and the first time period are divided into multiple smaller portions, including a first step, a second step, and a third step. Preferably, the first step with the first current density is approximately 0.5 A / dm. 2 Approximately 3 A / dm 2 Or preferably about 2 A / dm 2 Preferably, the first current step lasts for about 5 minutes to about 25 minutes, or more preferably about 15 minutes. Preferably, the second step of the first current density is about 2 A / dm³. 2 Approximately 4 A / dm 2 Preferably about 3 A / dm 2 Preferably, the second step is maintained for about 5 minutes to about 25 minutes, more preferably about 15 minutes. Preferably, the third step with the first current density is about 3 A / dm³. 2 Approximately 5 A / dm 2 Preferably about 3.4 A / dm 2 Preferably, the third step is maintained for about 5 minutes to about 25 minutes, more preferably about 15 minutes. Preferably, the second current density is about 3 A / dm³. 2 Approximately 6 A / dm 2 Preferably about 4 A / dm 2 Maintain this voltage for a certain period of time until it reaches the threshold anodizing voltage.

[0062] exist Figure 3 In step 307, the anodizing rectifier can switch from a constant current mode to a constant voltage mode, and maintain the anodizing voltage at an anodizing voltage threshold for a voltage control period of approximately 10 minutes to approximately 200 minutes, or approximately 50 minutes to approximately 150 minutes, or approximately 80 minutes to approximately 130 minutes, preferably approximately 90 minutes to approximately 120 minutes, or until the current drops below 10% of a first current density threshold. This threshold is below approximately 0.5 A / dm². 2 or below approximately 0.2 A / dm 2 Preferably below approximately 1 A / dm 2 .

[0063] In a preferred embodiment, the anodic oxidation current density gradually decreases as a dense optical discontinuous oxide film providing a white surface is formed.

[0064] In an alternative embodiment, the rectifier is not switched to constant voltage mode, and the current density remains at the second current density for the remainder of the anodizing period. Maintaining a high current density promotes the growth of the anodized surface (e.g., a film) and shortens the time required to form a white anodized surface. However, in order to maintain... Figure 4 The maximum anodizing voltage is 405V, requiring control of the anodizing bath temperature. Lowering the temperature increases the voltage, and vice versa. Therefore, to maintain the voltage at its optimal point, the temperature is increased when the voltage is too high and decreased when the voltage is too low.

[0065] Reference Figure 4 A voltage-current profile 401 is shown for a preferred embodiment, but it should be understood that other voltage-current profiles may also produce a white surface. Figure 4 Line 402 shows the applied current density, while line 403 shows the anodizing cell voltage. In this embodiment, an excitation voltage 404 is applied to the anodizing cell at t=0, accompanied by approximately 2 A / dm. 2 A constant current 402 is maintained, and the anodizing tank voltage 403 is approximately 52V. This constant current 402 is maintained for approximately 3 minutes, while the anodizing tank voltage 403 rises to approximately 60V. Subsequently, over a period of approximately 3 minutes, the current density 402 increases to approximately 4 A / dm². 2 The value of the cell voltage 403 increases slowly as the thickness of the anolyte film grows. The time period 406 is related to the generation... Figure 2 The porous surface 204 is highly relevant, but further etching of this exposed surface by bath acid partially leads to this structure. Figure 4 At point 405, once the tank voltage reaches the threshold voltage 405 required for the growth of the white anodic oxide layer, the rectifier is switched to constant voltage mode. Afterward, as the thickness of the anodic oxide film increases, the current density 402 slowly decreases until anodizing is complete, a process that takes a total of two hours. Time periods 407 and 408 reflect the same conditions... Figure 2 The generation process of layers 203 and 202 is described.

[0066] Figure 7 A schematic diagram of an anodizing bath 700 is shown. The anodizing bath 700 includes a bath container 701, a heater 702, a stirrer 703, a power supply 704, a cathode 705, and a workpiece / anode 706. The anodizing bath also includes a bath 710 containing acid and a surfactant, and metal microparticles or nanoparticles 708 coated with a surfactant.

[0067] Without being limited by any theory, the inventors believe that the metal microparticles or nanoparticles in the bath form a loosely coupled electrophoretic layer 709 on or inside the anodic oxide surface (see...). Figure 7This loosely coupled layer 709 significantly slows down the diffusion of acid from the anodic oxidation bath 710 to the surface of the substrate 707, which alters the anodic oxidation process and forms the optically discontinuous nanocrystalline anodic oxidation structure of the present invention.

[0068] In a preferred embodiment, anodizing is performed in three stages. The first stage involves the formation of a substantially nanocrystalline layer. Here, the native oxide layer on the aluminum substrate is bonded to a surfactant acid film to support an initial anodizing voltage of approximately 50V. In some embodiments, the surfactant and organic acid (e.g., citric acid) alter the wetting behavior of the native oxide surface of the aluminum, thereby modulating the ability of the anodizing bath to dissolve the native oxide surface, preventing pitting and promoting coating growth. The acid in the anodizing bath promotes the electrochemical dissolution of the aluminum substrate material (Equation 2) and constructs a barrier layer and pore walls (Equation 3).

[0069] Al → Al 3+ + 3e - —(2) 2Al 3+ + 3OH - → Al2O3 + 3H + —(3) While formulas (2) and (3) represent standard anodizing processes, the actual chemical exchange at the substrate is more complex. The inventors have determined that anodizing baths lacking condensed alkoxides, or metal microparticles and nanoparticles, and alcohols cannot produce any significant anodizing depth. Without being limited to any particular theory, the inventors believe that interactions between carboxylic acids; organic compounds (including alcohol-derived compounds); and metal microparticles and nanoparticles are desirable to enable the formation of durable anodic structures that form and dissolve at equivalent rates in the absence of these interactions. Figure 5B Figure 507 shows a surface SEM of an anodized aluminum sample in a bath free of metal alkoxides, revealing a typical structure associated with excessive acid dissolution of alumina. A porous structure is visible beneath the thin, soft surface. The inventors believe that the adsorption of metal microparticles, including alcohols, and nanoparticle-derived compounds onto the anodized surface can delay the dissolution of the anodic structure.

[0070] The first anodizing stage occurs Figure 4 During the period of 406, a nanoporous outer layer was developed, such as Figure 5A Zhong 502 and Figure 2 As shown in 202. In Figure 4By the end of time period 406, sufficient metal microparticles and nanoparticles had aggregated onto the anodic oxide surface 201 via electrophoretic transport, thereby influencing the chemical reactions of the anodic oxide process. In the absence of sources of metal microparticles and nanoparticles and related materials, conventional anodic oxide structures continue to evolve. Figure 5B Figure 508 shows a cross-sectional SEM image of the coating produced from the anodic oxidation bath, where the particles are maintained at the nanoscale, i.e., <100 nm. Here, the anodic oxidation structure comprises porous nanotubes from the substrate to the surface (i.e., Figure 2 Layer 202). When the size exceeds 100 nm, metal microparticles and nanoparticles can form an attached electrophoretic deposition (EPD) layer on the anodic oxide film, while still allowing the formation of a generally uniform porous nanotube structure. It has been found that particles larger than 1 micrometer form a non-attached layer on the anodic oxide surface, which limits the diffusion of chemicals into the pores. The inventors have found that with the presence of this layer, the voltage drop of the anodic oxide bath is reduced by the voltage drop of the anodic oxide process (e.g., ...). Figure 4 As shown, this is caused by the voltage drop due to ion diffusion restriction in the nanotubes and the voltage drop on the particle surface, which can reach about 100V for titanium dioxide particles and those with Tween 20 on the surface.

[0071] The second anodizing stage is in Figure 4 It took place during time periods 407 and 408, and developed Figure 5A Layers 503 and 504 (which are also) Figure 2 Layers 203 and 204). Here, the increased surface particle density restricts the interdiffusion of chemicals between the pores and the anodic oxidation bath. Hindering the anodic oxidation process through the interaction of nanoparticle materials or the interaction of alloying elements from the aluminum substrate initially favors the growth of adjacent unobstructed pores, leading to pore bifurcation, such as... Figure 2 As shown in 209 and 210, once a large number of pores become partially, substantially, or completely obscured by metal oxide nanoparticles, the pores become pH polarized, changing from strongly acidic near the aluminum substrate to alkaline near the anodic oxide surface. Excess Al at the substrate of the pores... 3+ That is, aluminum ions that were not immediately incorporated into the anodic oxide structure ( Figure 2 (202 to 204) migrate to the alkaline pH region under the influence of an electric field, where they combine with hydroxide ions to form aluminum hydroxide. Aluminum hydroxide adsorbs onto the pore walls and develops into a nanotube intermediate layer that is largely filled with nanocrystals, such as... Figure 5A 503 and Figure 2 As shown in 203 and 209. These structures are primarily responsible for scattering light, which produces the white surface.

[0072] Intermediate nanostructures 203 / 503 and 204 / 504 ( Figure 2 The growth of (Figure 5) further restricts diffusion through the layer, thereby reducing the anodic oxidation current. When the current drops below the threshold, such as Figure 4 As shown between 407 and 408, the final structure of layers 203 / 503 and 204 / 504 is formed. The thickness ratios of the three observed layers are as follows: Figure 5A 502:503:504 in Figure 2 The ratios of 202, 203, and 204 in the equation are approximately 1:0.4:0.5 to approximately 1:0.8:1.2.

[0073] Elements in the alloy can influence surface formation during anodizing of 406 to 408 alloys. Insoluble and non-anodizable alloying elements, such as silicon or silicon-magnesium precipitates, will remain as inclusions in the anode layer. The presence of these alloying elements in the alloy may hinder the growth of the anode film but may increase light scattering points. Alloying elements (such as copper) can leave pores in the anode layer formation, thus promoting the formation of side holes. Alloying elements (such as magnesium and zinc) will oxidize, but not as much as aluminum, leaving magnesium and zinc oxides in the anode structure.

[0074] In one embodiment, the anodic oxidation surface depends on the maximum anodic oxidation voltage, which is determined in part by the size of the nanoparticles in the bath. Coatings formed at 85V, such as... Figure 5C 509 in the figure is associated with small particles 510 ranging from 20 nm to 100 nm, which may be oxides and hydroxides of metal nanoparticles, such as titanium dioxide particles produced by the condensation of tert-butoxide. These small particles allow the metal to bind to the growing anode surface 511 in the form of an aluminum-metal compound (Figure 6B) complex (TiAlO2). Larger particles ranging from approximately 100 nm to approximately 500 nm ( Figure 5D The 512 particles can also be adsorbed onto the anodic oxide surface as an attached electrophoretic layer of titanium dioxide. The largest particles (>500 nm) only participate in the formation of the anodic surface but are not integrated into the coating. Figure 5E 513 describes an anodized surface formed in a bath containing particles of approximately 1700 nm, with a maximum sustained anodizing voltage of approximately 85 V to approximately 100 V. This significantly affects the surface and bulk nanostructure, producing a blue / white surface. Figure 5E 514 depicts a SEM image of a generally white anodized surface prepared at 150V in a bath containing particles of approximately 3000 nm.

[0075] In another embodiment, the source of nanoparticles is zirconium propoxide. Smaller particles, typically less than 200 nm, can also produce the same anodization effect due to different interactions between the bath and the nanoparticles.

[0076] In Figure 3 At step 308, the aluminum substrate is anodized to form an anodized substrate having an anodized surface. The anodized substrate is removed from the anodizing bath and rinsed. After rinsing, the surface can be matte or glossy white, depending on the pre-treatment of the aluminum substrate. However, the incorporation of additional white material to the anodized surface and / or sealing of the anodized surface can further enhance the whiteness.

[0077] Many methods of surface coloring known in the art can be used on the present anodized surface to improve or change the surface color. The porosity of the white anodized surface makes it suitable for further coloring by methods including immersion of the surface in organic dyes, electrophoretic deposition of metal oxides or other materials, and electrolytic coloring using metal salts.

[0078] One embodiment to enhance the whiteness of the anodized surface involves immersing the anodized substrate in a bath suitable for enhancing whiteness, such as an aqueous dispersion containing barium sulfate particles. Preferably, the barium sulfate particles are barium sulfate nanopowder. Preferably, the bath suitable for enhancing whiteness comprises about 0.01 mol / L to about 0.5 mol / L of barium sulfate nanopowder, or about 0.05 mol / L to about 0.3 mol / L of barium sulfate nanopowder, or preferably about 0.2 mol / L of barium sulfate nanopowder. Preferably, the barium sulfate nanopowder has an average particle size of about 100 nm or less (less than the pore size of the coating). An equimolar amount of disodium EDTA is sufficient to stabilize the particles in suspension. The bath for enhancing whiteness is agitated to significantly reduce particle agglomeration. The method also applies a voltage corresponding to an electric field of about 1 V / cm to about 30 V / cm, or about 5 V / cm to about 25 V / cm, preferably about 20 V / cm, between the substrate and an inert counter-electrode. The method employs a voltage polarity according to the surface charge on the particles, which can be influenced by the particle size and surfactant properties, and applies a constant voltage specification for about 1 minute to about 30 minutes, or about 5 minutes to about 20 minutes, preferably about 10 minutes, such that the barium sulfate particles are electrophoretically immersed into the porous surface. Those skilled in the art will appreciate the proportional relationship between the applied electric field and the depth of penetration of the nanoparticles.

[0079] The present inventors have determined that white anodized substrates having a porous nano-morphology on the anodized surface exhibit hydrophobic properties. These properties reduce the effectiveness of aqueous post-treatments. An alternative embodiment takes advantage of a dispersion of barium sulfate nano-powder in a suitable mixture of organic solvents at a concentration within the range defined above. In this embodiment, the voltage corresponds to an electric field range of about 50 V / cm to about 200 V / cm, or about 60 V / cm to 80 V / cm, preferably about 75 V / cm.

[0080] In an alternative embodiment, to change the color from white to a tint, the method comprises immersing the anodized substrate in a solution containing an organic dye and an organic solvent. The anodized substrate is immersed for a period of time ranging from about 10 minutes to about 20 hours. It is then dried. The drying method can include blowing under compressed air, or baking in an oven to remove the solvent, resulting in a dyed anodized surface. Suitable drying methods will be apparent to those skilled in the art. In one embodiment, the color of the dyed anodized surface depends on the dye density in the pores of the anodized surface, as well as the depth of penetration of the dye.

[0081] In Figure 3 In step 309, the method optionally includes a step of sealing the anodized surface. In one embodiment, a polishing step can be performed prior to the sealing step to produce a uniformly smooth surface, enhancing the aesthetic appearance of the surface. Polishing can be performed using any method commercially available, depending on the size and shape of the anodized article. Such methods include buffing, burnishing, barrel finishing, vibratory finishing, soda blasting, and the like. Polishing the surface prior to sealing can improve the effectiveness of the sealing process.

[0082] Sealing the anodized surface can improve its durability, increase its whiteness, preserve secondary colors, and improve its resistance to chemical attack, among other properties. Methods of sealing anodized surfaces are well known in the art, including boiling water sealing, metal acetate sealing, and polymer sealing, among others, and most methods can be successfully applied to anodized surfaces produced according to the present method.

[0083] One preferred embodiment of sealing an anodized substrate comprising an anodized surface comprises: contacting the anodized substrate with a sealing solution for a contact period of time to provide a sealed anodized substrate; rinsing the sealed anodized substrate with an alcohol; and curing the sealed anodized substrate under curing conditions. Preferably, the anodized substrate is contacted with a sealing solution. Preferably, the sealing solution is a sol-gel. Preferably, the sol-gel is a silica-siloxane metal oxide sol-gel. Preferably, a C 1-6An alcohol, preferably a C3 alcohol, more preferably isopropanol, is used to prepare the sol-gel. Preferably, contacting the anodized substrate with the sol-gel comprises electrophoretic deposition. Preferably, the electrophoretic deposition is performed at 30 V to 200 V, preferably 170 V, preferably for 10 minutes to 120 minutes, preferably 30 minutes. Preferably, the sol-gel penetrates into the anodized substrate during the contacting. The contacting can be for 10 minutes to 120 minutes, preferably 30 minutes. Preferably, rinsing the sealed anodized substrate with an alcohol comprises an alcohol selected from the group consisting of: methanol, ethanol, propanol, isopropanol, butanol. Preferably, the curing conditions comprise a temperature of about 50 °C, at least 50 °C, or less than 80 °C. Preferably, the curing conditions comprise a relative humidity of more than 80%.

[0084] One preferred embodiment is to seal a surface using a mixed silica and siloxane-metal oxide sol-gel. The sealing uses electrophoretic deposition at 30 V to 200 V, preferably 170 V, for 10 minutes to 120 minutes, preferably 30 minutes, with an ethanol rinsed white anodized surface as the cathode and an inert titanium anode to penetrate the sol-gel into the coating; the sealed surface is cured at a temperature higher than 50 °C and a relative humidity higher than 80% for more than one hour. A mixed silica sol-gel is prepared using isopropanol, using 0.1 M to 1 M of silicon, and a mixture of 70% to 85% TEOS (tetraethyl orthosilicate), 10% to 20% OTES (octyltrimethoxysilane), 5% HDTMS (hexadecyltrimethoxysilane), and 10% to 20% FAS (1H, 1H, 2H, 2H-Perfluorooctyltriethoxysilane). The solution is peptized using 5 mL / L to 15 mL / L of hydrochloric acid (37% by volume) or phytic acid (50% by volume).

[0085] In an alternative embodiment, the color and hardness of an anodized surface can also be modified by a siloxane-metal oxide sol-gel prepared by adding a metal alkoxide to a mixed silica sol-gel. In one embodiment, 50 mL / L to 100 mL / L of aluminium tri-sec-butoxide is added to the mixed silica sol-gel, and the solution is peptized using 10 mL to 30 mL of hydrochloric acid (37% by volume).

[0086] In an alternative embodiment, the anodized surface can be sealed using a benign material, such as a phytic acid solution. A PA solution of approximately 2.5 wt% is prepared using phytic acid (PA, 70 wt% in water) from Sigma-Aldrich and deionized water, with a small amount of triethylamine from Sigma-Aldrich sufficient to adjust the pH of the phytic acid solution to approximately 1.2 to 2.0, preferably to approximately 1.5, to provide a sealing solution. The phytic acid sealing solution is heated to approximately 90°C. The anodized substrate is immersed in the sealing solution for approximately 15 minutes, which produces a deposited film (sealing film) of approximately 3 μm to 4 μm on the surface of the anodized substrate.

[0087] In an alternative embodiment, the anodized surface can be sealed using a commercially available sealant, such as Lyndar Clearcoat Aerosol. The Lyndar Clearcoat Aerosol compound is sprayed onto the anodized surface of the anodized substrate to deposit approximately 20 μm of clear sealant. The clear sealant is cured for approximately 24 hours. The resulting coating exhibits improved durability and nearly 100 times higher gloss than the originally prepared coating. The inventors observed that this sealing technique does not affect the L* value of the white anodized coating while imparting a high gloss to the surface.

[0088] In an alternative embodiment, a metal-based transparent coating sealant can be used to seal the anodized surface. Here, metal deposition techniques such as physical vapor deposition (PVD), chemical vapor deposition (CVD), atomic layer deposition (ALD), and pulsed laser deposition (PLD) can be used to provide a uniform nanoscale seal on the anodized surface. In a preferred embodiment, a thin metal-based coating is deposited on the white anodized surface via PLD. On the originally prepared white anodized coating, under low oxygen conditions at approximately 1 J / cm²… 2 Deposition is performed using laser flux.

[0089] The metal oxide layer is thick enough to completely seal the nanoporous structure of the anolyte coating without altering its aesthetic appearance or tactile behavior. Metal-based sealants include, but are not limited to, indium tin oxide, vanadium oxide, and zinc oxide.

[0090] exist Figure 3 The method ends at step 310.

[0091] Example The following embodiments describe specific operating conditions and demonstrate the practice of this disclosure. However, these embodiments should not be considered as limiting the scope of this disclosure. The selected embodiments specifically illustrate aspects of the anodic oxidation bath and the method for producing a white anodic oxide surface.

[0092] In various embodiments, the results are determined by the degree of whiteness of the method. Colour measurements were generally made using the CIE 1976 L*a*b* colour space standard, which is used herein to measure the colour of the anodised surface. The measuring instruments used were a NR10QC colour difference meter and a GC268 reflectance meter. There is a significant error in measuring light colours and white colours with CIE L above 85, especially for structurally coloured surfaces, and the measurement can vary with the angle of light contact, so the measurements were cross-checked against Resene Paints colour swatches to improve accuracy.

[0093] Example 1 - Effect of bath composition To understand the theoretical basis of the anodising process, various anodising bath compositions and recipe methods were tested. The anodising bath composition comprised 25 g / L oxalic acid, 10 g / L boric acid, 1 g / L citric acid, and 1 mL / L of Tween 20 surfactant. In all experiments, the anodising bath was stirred at 300 rpm at 70°C, held on a hot plate stirrer, and a water bath was used. In experiments including butoxide, the concentration of butoxide was 47 mL / L. The pH of the anodising bath was 2 to 2.4 pH units in all cases. Table 1 shows the range of bath additives and methods.

[0094] Table 1

[0095] All substrate samples were 2 x 3 cm 6061-T6 aluminium sheets, connected using a 2 mm insulated 4000 series aluminium solder wire clamp. The solder wire was forced into a 1.8 mm hole drilled into the centre top of the aluminium substrate sample. The aluminium substrate was pre-treated using Activax caustic soak for 8 minutes at 60°C, followed by a 1 minute decontamination in 50% nitric acid.

[0096] Referring to anodising bath 1.1 in Table 1, the results show that the mixture of organic acids alone is not sufficient to produce a sufficient anodised layer. The maximum voltage achieved was about 30V, and the resulting anodic structure showed an incomplete collapsed anodic morphology (see Figure 1). The inventors also confirmed that a substrate without a sufficient native oxide layer was not even able to reach an initial voltage of 30V, which indicates that the initial barrier layer cannot be established by the anodising bath alone, and the aluminium substrate needs a native oxide layer for anodising. Figure 5H

[0097] ​Referring to anodizing bath 1.2 in Table 1, the results show that a ripened bath, containing 25 g / L of oxalic acid, 10 g / L of boric acid, 1 g / L of citric acid, 1 mL / L of Tween 20 surfactant, and hydrolyzed t-butoxide, produced a "white" coating with L* values > 90. The maximum voltage reached 150 V. The resulting anodized surface showed non-uniform nanoscale pores on its surface ranging from 50 nm to 100 nm (see Figure 5A , 506). Cross-sectional images (see Figure 5A , 501) show that the coating, which is about 38 mm, exhibits a three-stage morphology development. Without being bound by theory, the inventors believe that this is the cause of the scattering and diffusing of the incident light, thereby providing an anodized substrate comprising an anodized surface having a substantially white appearance. The inventors believe that the combination of particles, esters, and butanol from the hydrolyzed t-butoxide with the organic acids can be the cause of inducing the anodic coating to appear white.

[0098] Referring to 1.3 in Table 1, this investigated the effect of butanol on the anodic structure of the anodized surface. The condensation of the butoxide in the bath releases butanol (as shown in Formula 1 above). The results of this experiment show that butanol alone does not produce a significant anodic layer because the voltage does not rise. However, the presence of butanol does change the anodized surface and its structure because the surface changes color. Figure 5C , 519, 510, 511 show the pore structure of the anodized surface when generated from anodizing bath 1.3. Pores of about 30 nm are characteristic of the anodizing bath and the anodizing voltage.

[0099] In the experiment using bath 1.4, the addition of 25 nm Ti02powder supported the generation of a colored anodic film, as shown by the rise in voltage and the surface appearance of the anodized surface. Thus, the nanoparticles in the anodizing bath and the compounds derived from butanol are believed to be very important to the anodizing process.

[0100] Another experiment, not included in Table 1, also shows that Ti02nanoparticles alone without butanol are not sufficient to form an anodized surface. Figure 5F , 515 are exemplary images showing the surface morphology of aluminum anodized in an anodizing bath containing Ti02nanoparticles but no butanol. The coating appears discontinuous and has distinct grain boundaries. Higher magnification image 516 shows that this anodizing bath composition produces a poor anodic structure of the anodized surface with collapsed nanotubes.

[0101] In the experiment of Table 1 using Bath 1.5, the particles from the anodizing bath of Experiment 1.2 were filtered. The remaining liquid components were sufficient to develop a relatively thick anodized structure, as the anodizing voltage reached 150 V. However, the anodizing bath did not support the formation of a white anodized surface. Instead, the anodizing current dropped to zero within a period of about 20 minutes, resulting in a gray surface with a transparent nanoporous surface, as shown in Figure 5. Figure 5G

[0102] The orthogonal experiment showed that a wide range of anodizing bath acid concentrations produced white anodic surfaces. Table 2 presents a series of anodizing bath formulations and the color of the anodized surfaces they produced.

[0103] Table 2

[0104] Table 2 shows the results of anodizing using the standard anodizing voltage current profile, unless otherwise noted. Each experiment began with a fresh bath having the target chemical composition. The experiments initially anodized pre-treated aluminum substrate sheets (as described above) that were 5 cm x 3 cm in size until the voltage reached 150 V. This process allowed the bath to fully mature to produce a suspension of about 3 micron titania particles and the associated butanol-derived esters.

[0105] The process initially anodized at a constant current of 2 A / dm 2 for 1 minute, followed by a linear increase in current density to 4 A / dm 2 for a period of 3 minutes. When the voltage reached 150 V, the DC power supply was switched to constant voltage, and the anodizing continued until the total time reached 120 minutes.

[0106] In each experiment, the bath temperature was maintained at about 60°C to 80°C.

[0107] Although the data showed that the whitest surface was produced with an oxalic acid concentration of about 28 g / L, a boric acid concentration of about 5 g / L, and a butoxide concentration of about 24 g / L, the oxalic acid concentration appeared to be the most critical factor.

[0108] The ratio of oxalic acid to boric acid also appeared to be important at low concentrations of oxalic acid, with the whitest surface produced at a ratio of about 3:1. Ratios below 2:1 increased the initial anodizing rate, and the process reached the maximum anodizing voltage in a short time. When the oxalic acid to boric acid concentration was about 1:1, the bath was unable to support the 4 A / dm 2 .

[0109] Although the amount of butoxide appeared to not affect the surface brightness, lower amounts appeared to improve the purity of the color.

[0110] ​Example 2 - Tert-butoxide particle development Table 3 shows various metal butoxide particle forms and the resulting anodized surface color. Anodizing tank 3.2, 3.3 and 3.4 used 47 g / L of titanium butoxide. Anodizing tank 3.1 contained no butoxide.

[0111] Table 3

[0112] The sample was pre-treated and prepared according to the procedure outlined in Example 1 above.

[0113] The bath temperature used was 60°C with magnetic stirring at 600 rpm.

[0114] Experiment 3.1 in Table 3 was anodizing a sample in a bath containing no tert-butoxide. The bath remained clear and the anodizing structure was insufficient to color the surface.

[0115] Experiment 3.2 added 47 g / L of tert-butoxy titanium from Sigma Aldrich to the bath. The experiment controlled the current density to slow the condensation reaction of the butoxide and allow for the study of various anodic surface structures developed at different stages of particle development. The surface was formed from a generally clear butoxide containing anodizing bath, i.e. a bath containing less than 100 nm of nascent particles. The results show that the state of particle development of the anodizing bath appears to be important to developing a white anodized surface. Typically, the addition of tert-butoxide to a hot acidic solution would immediately complete condensation, but this experiment appears to show that although the condensation reaction can initially occur, the particles stabilize at the nanoscale in the anodizing bath. The surface formed is relatively thick and appears semi-transparent purple, Figure 5E 513.

[0116] The results of Experiment 3.3 show that partially developed oxide particles support the formation of an anodic surface, however the structure of the oxide particles still limits the voltage growth by allowing free exchange of material between the anodizing bath and the pore structure. The surface structure developed is similar to a coating from an anodizing bath containing fully formed particles, but some electrophoretic Ti02deposition is combined to the anodic surface (see Figure C 510) and the resulting surface is blue / white.

[0117] Experiment 3.4 shows that a fully formed anodizing bath with particles of approximately 3000 nm in size produces a white anodized surface on an aluminum substrate. Experiment 4 used a current controlled voltage increase followed by a voltage controlled current increase. The process employed a first current of 2 A / dm 2 for 3 minutes followed by a second current of 3 A / dm 2 for 3 minutes. The current was increased to 4 A / dm 2third current and held at this value until the voltage reached 150 V. At this stage, the method switched to a constant voltage control stage, where 150 V was supplied for 120 minutes. The resulting anodized surface was white and had an L* value of 91.3. The coating of about 35 mm presented a non-uniform nanoporous morphology, as shown in Figure 514. The results also showed that large 3037 nm size particles were not incorporated into the anodized surface, but significantly affected the surface and bulk nanostructure, resulting in a white surface. Figure 5E

[0118] Example 3 - Anodized surface development Degreased 3 x 5 cm Al 6061-T6 samples were used and immersed in pre-prepared white anodizing baths prepared according to Table 2 above. The aluminum substrates were anodized for various periods of time from 5 minutes to 2 hours, then the samples were removed from the anodizing bath and rinsed with deionized water.

[0119] After color measurements were taken using a colorimeter, cross sections were prepared for SEM analysis.

[0120] SEM analysis determined the development of pores and surface and the coating thickness.

[0121] Table 4

[0122] * UNP = uniform nanoporous, BNP = branched nanoporous, DNP = discontinuous nanoporous Figure 8 Figure 801 shows the evolution of the sample color as a function of anodizing time, using the data in Table 4, which also shows the evolution of the anodizing morphology that leads to a white surface.

[0123] Examples 4.2 to 4.4 show that the surface color of the anodized surface is a result of the combination of the structure and thickness of the surface. When the total surface thickness is less than 20 microns, it appears that there is insufficient interaction between the incident light and the pore / alumina interface to reflect a significant percentage of the incident light. However, it was observed that as the surface thickness increased, the number of interactions increased and the coating became whiter.

[0124] It was observed that the thinner samples appeared to have a bluer hue, which was thought to be characteristic of the nanopore size (sample number 1). However, as the coating became thicker, it was observed that the color became greener, the b* became less negative and the a* value became more negative. The color change was thought to be due to the larger pore size at the surface of the coating as a result of the acid dissolution of the alumina.

[0125] Example 4 - Glossy surface The pre-treatment and preparation of the samples were carried out according to the procedure described in Example 1 above.​

[0126] The electropolishing step can employ a variety of well-known electropolishing methods for aluminum substrates. The electropolishing bath composition herein comprises 800 mL / L phosphoric acid, 30 mL / L hydrofluoric acid, 70 mL / L sulfuric acid, and 100 mL / L glycerol. The temperature of the electropolishing bath is maintained at 80 °C. A constant DC voltage of 12 V is applied for 2 to 3 minutes, followed by a rinse with deionized water. This method develops a mirror surface on the aluminum alloy surface with an Ra of about 0.1.

[0127] The electropolished samples were anodized in the burned-in bath 2.5 in Table 2 of Example 1. The anodization constant current density was 2 A / dm 2 for 8 minutes, then increased to 3 A / dm 2 for 5 minutes, again increased to 3.5 A / dm 2 for another 5 minutes. Then, once the voltage reached 150 V at the final constant current time period, a constant voltage anodization was applied at 150 V and the anodization continued for a total of 75 minutes.

[0128] The resulting surface was generally white with a measured color of 87.3, -2.77, -1.63, L*a*b*, and a surface gloss measured at 85° of 88.5 GU (gloss units).

[0129] Example 5 - Colored Surface Preparation A white anodized sample was prepared using the 1.2 bath and method in Example 1 above.

[0130] The anodized sample was fully dried by baking in a temperature controlled oven at 120 degrees for 2 hours.

[0131] The dried sample was suspended in a solution of 0.01 g / L methylene blue in either deionized water or ethanol for 20 hours.

[0132] The sample was removed from the methylene blue solution and dried using compressed air.

[0133] The color of the sample was measured using a color difference meter and the results are shown in Table 5.

[0134] The methylene blue in water reduced the reflectivity of the surface, as indicated by the lower L* value, and produced a light blue colored surface. However, the methylene blue in alcohol produced a significantly bluer surface.

[0135] The inventors believe that the surface was only partially wetted, which limited the penetration of the water into the pores, whereas the ethanol more easily penetrated the pores. Figure 9Figure 903 shows the wetting behavior of a white anodized sample using deionized water. The water contact angle of the droplet was about 40°.

[0136] Table 5

[0137] Example 6 - Sealing of White Anodic Surfaces with a Clear Coat A white anodized substrate was prepared using the 1.2 bath and method detailed in Example 1 above. The surface of the anodized substrate exhibited an L* value of about 92.16.

[0138] The surface of the anodized substrate was sealed using a commercially available clear coat sealer, such as Lyndar Clear Coat Aerosol. The application of the clear sealer initially made the surface slightly translucent, significantly reducing the L* value, as the solvent in the sealer entered the anodization pores and altered the optical properties of the surface.

[0139] The clear coat was sprayed onto the anodized surface and allowed to cure for about 2 hours. Once the solvent evaporated from the sealer, the whiteness of the surface returned to the whiteness consistent with before the sealer was applied, with an L* value of 92.1 measured. The sealer was fully cured after 24 hours.

[0140] The sealed anodized surface was white and exhibited a higher gloss than the unsealed anodized surface due to the nature of the sealer. The unsealed surface measured about 1.3 GU (gloss units) at 85°, while the GU increased to 90.1 after sealing.

[0141] Example 7 - Silica Alumina Sealing of White Anodic Surfaces A white anodized substrate was prepared using the 1.2 bath and method detailed in Example 1 above. The color of the prepared white surface was measured using an optical colorimeter, with an L* value of about 92.75.

[0142] A silica alumina sol-gel was prepared in about 400 mL. A mixture of 21 mL TEOS (tetraethyl orthosilicate), 2 mL HDTMS (hexadecyltrimethoxysilane) in 360 mL of isopropyl alcohol was stirred vigorously; 25 mL of aluminum sec-butoxide was then added using a syringe; and 12 mL of hydrochloric acid (37% by volume) was then added dropwise to gel the sol. After stirring for 5 to 10 minutes, the suspension changed from cloudy to clear to provide a sealer suitable for use on anodized surfaces. However, stirring for a longer period of time (e.g., 4 hours or more) can yield better results to substantially complete the hydrolysis and condensation reactions.

[0143] The white anodized surface was rinsed with ethanol in preparation for electrophoretic deposition.

[0144] Electrophoretic deposition for about 30 minutes provided penetration of the silica alumina sol-gel into the anodized surface. Deposition was performed at 170 V DC using the white anodized surface as the cathode and using a parallel titanium anode to provide a sealed anodized surface.

[0145] The sealed anodized substrate with an anodized surface was initially air dried for 30 minutes and then cured at about 50 °C for 1 hour in a humidity controlled environment with RH over 80%.

[0146] Colorimetric measurements of the sealed anodized surface showed a slight increase in L* value of about 0.5 units to 93.23.

[0147] Post-sealing heat treatment at 200 °C for 4 hours increased the hardness of the sealed anodized surface to over 206 HV 0.1 Without being bound by theory, the inventors believe that the alpha alumina in the anodized pore walls catalyzes the conversion of amorphous alumina in the sol / gel to the more robust boehmite form.

[0148] The sealed anodized surface remained white, exhibiting superhydrophobic properties with a contact angle over 150°, see Figure 9 , 904, and had excellent chemical resistance and wear resistance due to the silica-alumina seal.

[0149] Example 8 - Organic Particle Source Substitution A bath in which the alkoxide component was replaced by high molecular weight polyethylene glycol produced a near-white anodized surface on an aluminum substrate.

[0150] An Al 7075 series substrate of 50 mm x 30 mm x 1.2 mm was pre- treated and prepared following the procedure outlined in Example 1 above.

[0151] A white anodizing bath was prepared comprising 42.5 g / L oxalic acid, 10 g / L boric acid, 1 g / L citric acid, 42 mL / L 2-propanol, and 15 mL / L PEG MW 400.

[0152] A pure organic bath requires a bath temperature of 30 °C ± 4 °C. Magnetic stirring is used to stir the bath at 600 rpm.

[0153] Constant current anodizing was performed at 4 A / dm 2 for about 60 minutes until the anodizing voltage reached 150 V. The rectifier was switched to constant voltage mode for the remainder of the anodizing period. The sample was anodized for an additional 120 minutes until a final current density of 2 A / dm 2The anodized sample was removed from the bath, rinsed and dried with compressed air.

[0154] Color measurements were made using a 3nh ASTM D1500 color difference meter from Shenzen ThreeNH Technology. The L*a*b* measurements for the surface were 88.65, -3.66 and 0.22, respectively, showing that the bath with no particles produced a white surface.

[0155] Example 9 - Organic bath with particles An organic bath containing a small amount of metal alkoxide was able to produce a white surface.

[0156] A 50 mm x 30 mm x 3 mm Al 7075 series aluminum substrate was pre- treated and prepared following the procedure outlined in Example 1 above.

[0157] An organic and particle bath was formulated using long chain organic particles and a small amount of metal alkoxide. The bath contained 42.5 g / L oxalic acid, 10 g / L boric acid, 1 g / L citric acid, 42 mL / L 2-propanol, 15 mL / L PEG MW 400, and 10 mL / L zirconium propoxide.

[0158] Constant current anodization was performed using a stainless steel cathode at 4 A / dm 2 for approximately 30 minutes until the anodization voltage reached 150 V. The rectifier was switched to constant voltage mode for the remainder of the anodization period. The sample was anodized for an additional 120 minutes until a final current density of 2 A / dm 2 The anodized substrate was removed from the bath, rinsed and dried with compressed air.

[0159] Color measurements were made using a 3nh ASTM D1500 color difference meter from Shenzen ThreeNH Technology. The L* measurements for all surfaces were in excess of 91, showing that a bath containing a small amount of alkoxide and long chain organic was able to produce a white surface.

[0160] As shown and detailed in the results above, the above method is advantageous in that it provides a single step anodization method using a bath containing one or more organic acids, including a hydrolyzed metal alkoxide that is able to directly form a substantially random anodized surface structure. The anodized surface structure itself, not any secondary material, reflects a broad bandwidth of visible light frequencies, produces a surface brightness (L*) of greater than 91 measured on the CIE Lab* scale, and a color component (a*, b*) of less than 2, i.e. a substantially white surface.

[0161] In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge was at any time prior to the filing date of the application at issue at prior art or part of the common general knowledge in the art.

[0162] The application and the embodiments thereof have been described in detail. However, the scope of the application is not intended to be limited to the particular embodiments of the processes, machines, manufactures, compositions of matter, means, methods and / or steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods and / or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the examples described herein can be utilized according to such embodiments of the present application. Accordingly, the appended claims are intended to cover all modifications, alternatives and variations of the features, components, kits, means and / or methods disclosed herein.

Claims

1. An aluminum anodizing process for producing a white or substantially white anodized surface on an aluminum substrate, the process comprising the steps of: i. pretreating an aluminum substrate; ii. immersing the aluminum substrate in an aqueous anodizing bath comprising: - a buffering agent, - one or more particle sources, and - one or more carboxylic acids; and iii. anodizing the aluminum substrate by following anodizing current and voltage specifications to provide an anodized substrate comprising an anodized surface.

2. The process of claim 1, wherein the aluminum substrate comprises pure aluminum or an aluminum alloy pre-formed into a component.

3. The process of claim 1 or 2, wherein the pretreating step comprises an alkaline degreasing step.

4. The process of claim 3, wherein pretreating the aluminum substrate comprises an acid cleaning step.

5. The process of any one of claims 1 to 4, wherein the pretreating step is an electropolishing step.

6. The method of any one of claims 1 to 5, comprising adding C 1-6 alcohol to the aqueous anodizing bath.

7. The method of claim 6, wherein the C 1-6 alcohols are selected from the group consisting of methanol, ethanol, propanol, and butanol.

8. The process of any one of claims 1 to 5, wherein the one or more particle sources are selected from the group consisting of one or more metal oxide particles, one or more hydrolysable metal alkoxides, one or more low molecular weight polymers, one or more long chain organic compounds, succinic acid, and combinations thereof.

9. The process of claim 8, wherein the hydrolysable metal alkoxide particles are selected from the group consisting of titanium butoxide, titanium isopropoxide, aluminum sec-butoxide, zirconium butoxide, and combinations thereof.

10. The process of any one of claims 8 to 9, wherein the one or more hydrolysable metal alkoxides have a concentration in the aqueous anodizing bath of about 10 g / L to about 70 g / L.

11. The method of any one of claims 6 to 10, wherein the adding C 1-6 alcohol is achieved by hydrolyzing a source of hydrolyzable metal alkoxide particles to form one or more metal oxide particles and the C 1-6 alcohol.

12. The process of claim 11, wherein the hydrolysis is performed in situ during the anodizing step.

13. The process of any one of claims 8, 11, 12, wherein the metal oxide particles are microparticles or nanoparticles.

14. The process of any one of claims 1 to 13, wherein the one or more carboxylic acids are selected from the group consisting of oxalic acid, citric acid, and combinations thereof.

15. The process of claim 14, having a concentration of oxalic acid of about 10 g / L to about 70 g / L.

16. The process of claim 14 or 15, having a concentration of citric acid of about 1 g / L to about 10 g / L.

17. The process of any one of claims 1 to 16, wherein the buffering agent is boric acid.

18. The process of claim 17, wherein the boric acid buffering agent has a concentration of about 5 g / L to about 20 g / L of boric acid.

19. The process of any one of claims 1 to 18, wherein the aqueous anodizing bath further comprises a surfactant.

20. The process of claim 19, wherein the surfactant is selected from the group consisting of non-ionic surfactants and cationic surfactants.

21. The process of claim 20, wherein the non-ionic surfactant has a concentration of about 1 g / L to about 5 g / L.

22. The method of any one of claims 20-21, wherein the non-ionic surfactant is Tween 20.

23. The method of any one of claims 6 to 22, wherein the C 1-6 alcohol is present at a concentration of about 30 mL / L to about 100 mL / L.

24. The method of any one of claims 6 to 23, wherein the C 1-6 The alcohol is substantially evaporated from the bath and / or converted to oxalate or oxalic acid ester.

25. The method of any one of claims 8-24, wherein the metal oxide nanoparticles are added to the anodization bath.

26. The method of claim 25, wherein the concentration of the metal oxide nanoparticles in the aqueous anodization bath is about 1 g / L to about 10 g / L.

27. The method of any one of claims 1-26, wherein the one or more particle sources comprise a low molecular weight polymer.

28. The method of claim 27, wherein the low molecular weight polymer comprises polyethylene glycol 400.

29. The method of any one of claims 1-26, wherein the one or more particle sources comprise a combination of succinic acid and a metal oxide powder.

30. The method of claim 29, wherein the metal oxide powder is a magnesium oxide powder.

31. The method of any one of claims 1-30, wherein the anodization current and voltage specifications comprise: i. a first anodization stage comprising applying a first current density for a first time period; ii. increasing the first current density to a second current density and voltage for a second time period to provide a second anodization stage, wherein the second current density is higher than the first current density; iii. anodizing during the second anodization stage until the voltage reaches an anodization voltage threshold; iv. switching the second current density to a voltage control stage for a voltage control time period, the voltage control stage comprising a constant anodization voltage and anodization current density.

32. The method of claim 31, wherein the first current density is about 0.5 A / dm 2 to about 3 A / dm 2 .

33. The method of claim 31 or 32, wherein the first time period is about - 1 second to about 1.5 minutes; - about 15 minutes to about 75 minutes.

34. The method of any one of claims 31 to 33, wherein the second current density is about 2 A / dm 2 to about 8 A / dm 2 .

35. The method of any one of claims 31 to 34, wherein the second current density is about 4 A / dm 2 .

36. The method of any one of claims 31-35, wherein the second time period is about 5 minutes to about 25 minutes.

37. The method of any one of claims 31-36, wherein the second time period lasts for about 15 minutes.

38. The method of any one of claims 31-37, wherein the anodization voltage threshold is about 80 V to about 300 V.

39. The method of any one of claims 31-38, wherein the voltage control stage is maintained at a constant anodization voltage and anodization current for a voltage control time period of about 10 minutes to about 120 minutes.

40. The method of any one of claims 31-39, wherein the voltage control stage continues until the anodization current drops to a value that is 10% lower than the first current density.

41. The method of claim 31, wherein the first current density comprises a first step, a second step, and a third step.

42. The method of claim 41, wherein the first step of the first current density comprises a current density of about 0.5 A / dm 2 to about 3 A / dm 2 , preferably about 2 A / dm 2 .

43. The method of any one of claims 41-42, wherein the first step of the first current density lasts for about 5 minutes to about 25 minutes, preferably about 15 minutes.

44. The method of any one of claims 41 to 43, wherein the second step of the first current density is from about 2 A / dm 2 to about 4 A / dm 2 , preferably about 3 A / dm 2 .

45. The method of any one of claims 41 to 44, wherein the second step of the first current density is for about 5 minutes to about 25 minutes, preferably about 15 minutes.

46. The method of any one of claims 41 to 45, wherein the third step of the first current density is about 3 A / dm 2 to about 5 A / dm 2 , preferably about 3.4 A / dm 2 .

47. The method of any one of claims 41 to 46, wherein the third step of the first current density is for about 5 minutes to about 25 minutes, preferably about 15 minutes.

48. The method of any one of claims 41 to 47, wherein the second current density of the second anodization stage is about 3 A / dm 2 to about 6 A / dm 2 , preferably about 4 A / dm 2 .

49. The method of any one of claims 30 to 48, wherein the first anodization stage, the second anodization stage, and the voltage control stage each provide a crystalline layer in the anodized surface, and wherein each crystalline layer has a different crystalline morphology.

50. The method of claim 49, wherein the crystalline layers are nanocrystalline layers.

51. The method of any one of claims 31 to 50, wherein i) the first anodization stage provides an outermost layer of substantially branched random nanoporous layer having nanocrystalline walls located at a position furthest from the substrate; ii) the second anodization stage provides a substantially nanocrystalline filled nanotubular intermediate layer; iii) the voltage control stage provides a nanocrystalline bottom layer in substantial contact with the substrate and having a density greater than the outermost layer and intermediate layer; and wherein the substantially nanocrystalline filled nanotubular layer is located between and in substantial contact with the substantially branched random nanoporous layer having nanocrystalline walls and the nanocrystalline bottom layer having a density greater than the outermost layer and intermediate layer.

52. The method of any one of claims 1 to 51, further comprising a step of dyeing the anodized surface of the anodized substrate.

53. The method of any one of claims 1 to 52, wherein the method further comprises a step of sealing the anodized surface of the anodized substrate.

54. The method of any one of claims 1 to 53, wherein the anodized surface is separated from the aluminum substrate and processed into a dense nanoporous element.

55. An anodized surface disposed on an aluminum substrate, the anodized surface comprising: i) an outermost layer of substantially branched random nanoporous layer having nanocrystalline walls located at a position furthest from the substrate; ii) a substantially nanocrystalline filled nanotubular intermediate layer; iii) a nanocrystalline bottom layer in substantial contact with the substrate and having a density greater than the outermost layer and intermediate layer; and wherein the substantially nanocrystalline filled nanotubular layer is located between and in substantial contact with the substantially branched random nanoporous layer having nanocrystalline walls and the nanocrystalline bottom layer having a density greater than the outermost layer and intermediate layer. ​ ​

Images