EP0276476A1 - Method for forming coatings on aluminium surfaces - Google Patents

Abstract

A method for producing hydrophilic coatings, wherein an aqueous solution or dispersion containing a water-soluble polymer or copolymer A formed from a monomer of the formula <IMAGE> in which R1 is H or CH3 and R2 and R3 are H, a C1-C4-alkyl group, a benzyl group or a C2-C3-alkanol group, and a water-soluble crosslinking agent B are applied to the aluminium surface and dried, and the polymer is then crosslinked. In preferred embodiments of the method, a boehmite layer or a chemical conversion coating, such as a chromate coating, are applied before the polymer coating is produced and/or a water glass solution is applied to the polymer coating and dried on.

Description

The invention relates to a method for forming hydrophilic coatings on aluminum surfaces using aqueous polymer solutions or dispersions.

In the previous technology for the surface treatment of heat exchangers made of aluminum or aluminum alloy (hereinafter referred to as aluminum), the aim of which was to prevent the development of white rust on the fin surfaces, etc., anodic oxidation layers, boehmite layers, resin treatments etc. were used. However, these methods result in layers whose surfaces have almost no water wettability and are even more water-repellent. In order to prevent white rust formation, chromate conversion layers are also used in practice, which, although giving low water wettability, only for a short time after the layer formation. Chromating alone therefore does not lead to the desired degree of hydrophilic properties. In addition, the chromate conversion layer tends to transform its hydrophilic property into a hydrophobic one over time, especially when heated / dried. Consequently, it is problematic to use chromate layers for the surface treatment of heat exchange fins.

On the other hand, from the structural aspect of a heat exchanger, the effectiveness of the heat radiation / cooling should be as high as possible. In most cases, the radiation and cooling components are therefore designed with the largest possible surface area and the distance between them is kept extremely short. If a heat exchanger is used for cooling, this leads to condensation of air humidity on the heat exchanger surface, especially in the spaces between. Water condensed in this way forms droplets, and the more hydrophobic the surface of the ribs, the more so is. The water droplets accumulate in the interstices and prevent air from passing through to an increased extent, which ultimately leads to reduced heat exchanger performance.

In addition, heat exchanger fans cause the droplets to splash around in the interstices of the fins, so that the drip catchers fitted under the heat exchangers cannot prevent the droplets from falling onto adjacent areas and contaminating them.

In order to avoid water droplets remaining in the interstices of the ribs, where they hinder the passage of air, the proposal was made to give the aluminum surface hydrophilic properties in order to improve the wettability. Treatment of the rib surface with silicate, e.g. Water glass enables an effective increase in water wettability and heat resistance at a low cost, so that a number of methods have been proposed in this regard.

• a Phosphatierte Aluminiumoberflächen werden direkt mit wässrigem Silikat beschichtet und dann getrock­net. Dieses Verfahren ist Gegenstand des offenge­legten japanischen Patents Sho 50-38645 (1975).
• b Die offengelegte japanische Patentpublikation Sho 60-221582 (1985) behandelt und betrifft ein Rippenmaterial, bei dem eine hydrophile anorganische Schicht bestehend aus Silikat, Böhmit etc. auf Aluminiumblech produziert wird, auf die dann ein hydrophiles organisches Polymer mit einem Polymerisationsverhältnis von mehr als 50 aufge­bracht wird.
• c Dieser Vorschlag bezieht sich auf ein Verfahren, bei dem eine zunächst mit einer organischen Polymerschicht versehene Aluminiumoberfläche anschließend mit Silikatlösung oder -dispersion beschichtet und getrocknet wird; (offengelegte japanische Patentpublikation Sho 59-205596) (1984)).
• d Dieser Vorschlag befaßt sich mit einer Methode zur Beschichtung von Aluminiumoberflächen mit einer Mischung aus organischem Polymer und anorganischem Silikat. Er wird in den folgenden Patenten behan­delt.

The processes previously used for layer formation can be divided into the following four categories, on the one hand based on the use of inorganic compounds such as silicate or organic compounds and on the other hand on the application method:

• a Phosphated aluminum surfaces are coated directly with aqueous silicate and then dried. This method is the subject of Japanese Patent Laid-Open Sho 50-38645 (1975).
• b The published Japanese patent publication Sho 60-221582 (1985) deals with and relates to a rib material in which a hydrophilic inorganic layer consisting of silicate, boehmite, etc. is produced on aluminum sheet, on which a hydrophilic organic polymer with a polymerization ratio of more than 50 is then applied is applied.
• c This proposal relates to a method in which an aluminum surface initially provided with an organic polymer layer is subsequently coated with silicate solution or dispersion and dried; (Japanese Patent Laid-Open Publication Sho 59-205596) (1984)).
• d This proposal deals with a method for coating aluminum surfaces with a mixture of organic polymer and inorganic silicate. It is covered in the following patents.

Japanese Patent Laid-Open Publication Sho 61-8593 (1986) deals with a rib material which is mixed with a mixture of styrene / maleic acid copolymer, polyacrylamide, butylene / maleic acid copolymer, polyacrylic acid or their salts and silicates, represented by xM₂O · ySiO₂ (M = Li, Na or K , y / x 2) is coated. Japanese Patent Laid-Open Sho 60-101156 (1985) relates to a chemical for forming a hydrophilic layer on aluminum containing alkali silicate and carbonyl compounds (aldehydes, esters, amides, etc.).

In connection with the conventional technology using organic compounds for the hydrophilic treatment of aluminum, Japanese Patent Laid-Open Sho 59-205596 describes a method in which organic compounds are applied using an organic solvent. The organic compounds are, in particular, acrylic resins, resins based on epoxy or urethane, vinyl resins, such as, for example, polyvinyl chloride, polyvinyl acetate, polyethylene, polypropylene, resins based on styrene, phenolic resins, fluorine resins, silicone resins, diaryl phthalate resins, polycarbonate resins, polyamide resins, alkyd resins, polyester resins , Urea resins, melamine resins, polyacetal resins and cellulose resins. On the other hand, published patent Sho 60-101156 states that low molecular weight organic compounds with carbonyl groups (e.g. glyoxal) and water-soluble organic polymers (e.g. copolymer of acrylamide and acrylic acid), contained in a hydrophilic treatment chemical for aluminum, diluted with water, applied to aluminum and then heated and dried.

The use of polyacrylamide as a chemical for the hydrophilic treatment is known from the Japanese patents Sho-60-101156 and Sho 61-8598. This compound can be mixed uniformly with water as long as the polyacrylamide content is low. With increasing concentration, which is caused by the drying process etc., the alkali silicate and polymer split into two phases, which often leads to non-reproducible results.

According to Japanese Patent Laid-Open Sho 60-221582, polyacrylamide is applied to the hydrophilic inorganic film to form a hydrophilic organic layer. The degree of polymerization of the polyacrylamide is adjusted so that removal of the lubricating oil used for processing by means of a solvent is possible without problems and that the organic polymer layer remaining on the hydrophilic inorganic layer after degreasing fills any specks in said inorganic layer. In addition, a crosslinking agent composed of Zr, Ti compounds etc. can bring about a crosslinking between the polyacrylamide and the hydrophilic group, but in such a way that there is no total crosslinking of said group. In the case of the fin material according to the invention, the last layer remaining after solvent cleaning of the heat exchanger fins is an inorganic hydrophilic layer, which is achieved by applying a silicate or boehmite layer as the base layer.

• a: Mit Phosphat behandeltes Aluminium wird direkt mit einer wässrigen Silikatlösung beschich­tet, wodurch der Schicht hydrophile Eigenschaften, aber keine Korrosionsbeständigkeit verliehen werden. Daher kann eine solche Schichtbildung die Korrosionsbeständigkeit geradezu verschlechtern und hat den Nachteil, daß eine Neigung zu Weiß­rostbildung besteht.
• b: Organische Polymerbeschichtung einer zuvor aufgebrachten anorganischen Schicht, wobei die hydrophilen Eigenschaften im wesentlichen von den anorganischen Komponenten wie Böhmit und Silikat herrühren. Hauptzweck der organischen Beschichtung ist die Bewahrung der anorganischen Schicht vor einer Verschmutzung durch Schmieröl, wodurch sie wasserabstoßend würde; sobald diese Funktion erfüllt ist, wird die organische Schicht zusammen mit dem Schmieröl in der darauffolgenden Entfet­tungsstufe entfernt. Das Ergebnis ist, daß weder die Korrosionsbeständigkeit, noch die Hydrophilität zufriedenstellend sind.
• c: Die Bildung einer Silikatschicht auf der organischen Polymerschicht ist in der Anfangszeit hinsichtlich Korrosionsbeständig­ keit und hydrophiler Eigenschaften zufrieden­stellend; der Nachteil besteht jedoch darin, daß das Silikat der Deckschicht leicht mit Kondens­wasser abgewaschen wird, d.h. die Stabilität der hydrophilen Eigenschaften ist unzureichend.
• d: Beschichtung mit einer Mischung aus organi­schem Polymer und Silikat; da das Silikat in der auf dem Aluminium gebildeten Schicht hydrophile Eigenschaften hat, tendiert die so behandelte Aluminiumrippe zu einer beschleunigten Weißrostbil­dung. Außerdem kann die Trocknung der mit besagter Mischung auf dem Aluminium gebildeten Schicht zur Spaltung des Silikats und des organischen Polymers in verschiedene Phasen führen, so daß die Ergebnisse stark in Abhängigkeit von den Herstellungsbedingungen schwanken und die so behandelten Rippen in vielen Fällen unzureichend hydrophil sind.

The aforementioned known methods for forming hydrophilic coatings have the following disadvantages.

• a: Aluminum treated with phosphate is coated directly with an aqueous silicate solution, which gives the layer hydrophilic properties but no corrosion resistance. Therefore, such a layer formation can almost deteriorate the corrosion resistance and has the disadvantage that there is a tendency to form white rust.
• b: Organic polymer coating of a previously applied inorganic layer, the hydrophilic properties deriving essentially from the inorganic components such as boehmite and silicate. The main purpose of the organic coating is to protect the inorganic layer from contamination by lubricating oil, which would make it water-repellent; as soon as this function is fulfilled, the organic layer together with the lubricating oil is removed in the subsequent degreasing step. The result is that neither corrosion resistance nor hydrophilicity are satisfactory.
• c: The formation of a silicate layer on the organic polymer layer is initially resistant to corrosion satisfactory and hydrophilic properties; the disadvantage, however, is that the silicate of the top layer is easily washed off with condensed water, ie the stability of the hydrophilic properties is insufficient.
• d: coating with a mixture of organic polymer and silicate; since the silicate in the layer formed on the aluminum has hydrophilic properties, the aluminum rib treated in this way tends to accelerate white rust formation. In addition, drying the layer formed with said mixture on the aluminum can lead to the splitting of the silicate and the organic polymer into different phases, so that the results vary widely depending on the production conditions and the ribs treated in this way are in many cases insufficiently hydrophilic.

The object of the invention is to provide a process for producing hydrophilic coatings on aluminum surfaces which does not have the disadvantages of the known, in particular the aforementioned processes, leads to coatings of high hydrophilicity which, even over a long period of time, are adherent and corrosion-protecting. In addition, the coatings should be processable in a simple manner with coating agents, which are in particular free of organic solvents, and be able to withstand any subsequent treatments (the effects of forces and heat).

- Wobei R₁ = H oder CH₃
R₂ und R₃ = H,
eine Alkylgruppe mit C₁ bis C₄, eine Benzyl- oder Alkanolgruppe mit C₂ - C₃

bedeuten,
gebildetes wasserlösliches Polymer oder Copolymer A und ein wasserlösliches Vernetzungsmittel B enthält, diese auftrocknet und das Polymer ver­netzt.The object is achieved by designing the method of the type mentioned at the outset in accordance with the invention in such a way that an aqueous solution or dispersion is applied to the surfaces, which comprises a monomer of the general formula

- Where R₁ = H or CH₃
R₂ and R₃ = H,
an alkyl group with C₁ to C₄, a benzyl or alkanol group with C₂ - C₃

mean,
contains water-soluble polymer or copolymer A and a water-soluble crosslinking agent B, dries them up and crosslinks the polymer.

A preferred embodiment of the invention consists in subsequently applying and drying a water glass solution on the polymer coating.

The water-soluble crosslinking agent B can be inorganic, organic or also combined inorganic / organic in nature. Suitable inorganic crosslinking agents are, in particular, metal compounds which can form a complex compound with the water-soluble polymer or copolymer. Such compounds, of which those with a coordination number of more than 4 are particularly effective, can be found in Table 1. Among the Cr, Ti, Al and Zr compounds, those with a very high water solubility are particularly effective, for example chromic acid, Bichromic acid and its salts, di-isopropoxy-titanium-bis-acetylazetone, the reaction product of lactic acid and alkoxy-titanium compound, zirconyl nitrate, zirconyl acetate, zirconylammonium carbonate, fluorozirconic acid and its salts, aluminum sulfate etc.

Suitable water-soluble organic crosslinking agents are water-soluble, neutralized polyisocyanate and / or water-soluble polymethylol, polyglycidyl, polyaziridyl compound, for example polyisocyanate neutralized with NaHSO₃ (for example ELASTRON: product from DAI-ICHI KOGYO SEIYAKU Co., Ltd.), methylolmelamine, Methylol urea, methylol polyacrylamide, diglycidyl ether of polyethylene oxide, diaziridyl polyethylene oxide etc.

Suitable combined inorganic / organic crosslinking agents are, for example, compounds of Cr, Ti, Al and Zr as inorganic water-soluble compounds and neutralized polyisocyanates, polymethylol / polyglycidyl / polyaziridyl compounds as organic water-soluble compounds.

The amount of crosslinking agent (B) used depends on its type. It also plays a role whether the (co) polymer layer (A) serves as a base layer and mainly for corrosion protection or is provided as a single layer. In general, the amount used per 100 parts by weight of polymer or copolymer will be 1 to 400 parts by weight, preferably 5 to 200 parts by weight.

Drying is generally carried out at 90-300 ° C, preferably 100-250 ° C.

According to a preferred embodiment of the invention, the polymer used should have a molecular weight of more than 5,000. If the weight is less than 5,000, and especially if the polymer has a high hydrophilicity, an increased crosslinking agent ratio is required for the purpose of water insolubility.

• (i) Ein Polymer oder Copolymer von Acryl­amid, Metacrylamid, N-Methylacrylamid, N-Dimethylacrylamid (Iʹ).
• (ii) Entweder ein nicht ionisches Polymer, d.h. ein Copolymer von (Iʹ) und ein ungesättigtes nicht ionisches Monomer (II), das mit (Iʹ) copolymerisiert werden kann, oder ein kationisches Polymer, d.h. ein Copolymer von (Iʹ) und (II) und/oder ein ungesättigtes Monomer (III) mit einer Aminogruppe.
• (iii) Ein Polymer, das man als Reaktionsprodukt der oben mit (i) und (ii) bezeichneten Polymere oder Copoly­mere durch eine Hofmann oder Mannich­reaktion erhält, oder ein Polymer, bei dem das vorgenannte Reaktionsprodukt bis zu einer quaternären Aminoverbindung alkyliert wird.
• (iv) Ein Reaktionsprodukt aus Alkylenpolyamin sowie dem Copolymer von (Iʹ) und Acryl­säureester.
• (v) Ein anionisches Polymer, d.h. ein Copolymer aus (Iʹ) und (IV) (s. unten), das mit (Iʹ) polymerisiert werden kann, und ein anionisches Polymer, das ein Copolymer von (Iʹ), (IV) und (II) darstellt.
• (vi) Ein Copolymer von (Iʹ) mit Malein­anhydrid oder Itakonsäure.
• (vii) Ein anionisches Polymer, das ein Hydrolyseprodukt von (i), (v) und (vi) o.ä. darstellt.

The following products are suitable as (co) polymer (A):

• (i) A polymer or copolymer of acrylamide, methacrylamide, N-methylacrylamide, N-dimethylacrylamide (Iʹ).
• (ii) Either a non-ionic polymer, ie a copolymer of (Iʹ) and an unsaturated non-ionic monomer (II) that can be copolymerized with (Iʹ), or a cationic polymer, ie a copolymer of (Iʹ) and (II ) and / or an unsaturated monomer (III) with an amino group.
• (iii) A polymer obtained as a reaction product of the polymers or copolymers designated by (i) and (ii) above by a Hofmann or Mannich reaction, or a polymer in which the aforementioned reaction product is alkylated to a quaternary amino compound.
• (iv) A reaction product of alkylene polyamine and the copolymer of (Iʹ) and acrylic acid ester.
• (v) An anionic polymer, ie a copolymer of (Iʹ) and (IV) (see below) which can be polymerized with (Iʹ) and an anionic polymer which is a copolymer of (Iʹ), (IV) and (II).
• (vi) A copolymer of (Iʹ) with maleic anhydride or itaconic acid.
• (vii) An anionic polymer which is a hydrolysis product of (i), (v) and (vi) or the like. represents.

Non-ionic or cationic polymers with the following general formula may be mentioned as concrete examples of (ii):

(Iʹ) _{l - (III) m - (II) n}

means

l + m + n = 100
l> 40
m = 0 to 60
n = 0 to 60

(II) is a non-ionic monomer that can be copolymerized; concrete examples are:

2-hydroxy (metha) acrylate
Diazetone acrylamide
Methylolacrylamide
Acryloylmorpholine
Acrylonitrile
(Metha) acrylic ester
Styrene
Vinyl acetate

wobei M für eine der nachfolgenden Formeln a) bis e) steht.

R_a, R_b, R_c = H, Alkyl, Hydroxyalkyl, Phenyl, Benzyl

r = 1 bis 3

x⁻ = Säuregruppe einer anorganischen oder organischen Säure
In the above formula, (III) is represented by the general formula

where M stands for one of the following formulas a) to e).

R _a , R _b , R _c = H, alkyl, hydroxyalkyl, phenyl, benzyl

r = 1 to 3

x⁻ = acid group of an inorganic or organic acid

Examples of the Mannich and Hofmann reaction for (iii) include:

dabei steht N für die nachstehende Formel f) bis ℓ)

An example of (v) is an anionic polymer with the following general formula:

(Iʹ) l - (IV) o - ((II) p

means

ℓ + o + p = 100
ℓ> 40
o = 1 to 60
p = 0 to 59

(IV) is represented by the general formula:

where N stands for the following formula f) to ℓ)

The hydrolysis reaction of (vii) is expressed as follows:

As for the coating method, the application can be by dipping, spraying, brushing, rolling or flow, the molecular weight should be set to less than 2,000,000, preferably 1,000,000, to avoid that the coating agent pulls strings . With regard to the choice of concentration and viscosity, suitable values have to be determined according to the coating method used and the desired layer thickness. In the case of heat exchangers, especially if their heat output is to be improved and a contribution to corrosion resistance is to be made, a layer thickness of 0.1 to 10 μ, preferably 0.2 to 2 μ, is advisable.

If pre-degreased aluminum surfaces are to be provided with a coating, a further advantageous embodiment of the invention provides for a boehmite layer or a chemical conversion coating, such as a chromate coating, to be applied beforehand. In the case of direct coating, the addition of chromic acid, bichromic acid or their salts to the polymer solution is particularly effective. The aforementioned (co) polymer (A), which is crosslinked with a water-soluble crosslinking agent (B), has sufficient mixing stability so that two processes by which the aluminum surface is corroded with the aid of chromate made permanent and a (co) polymer layer is formed - can be carried out in one operation. In addition, these forms of treatment may have a synergistic effect by producing a particularly excellent surface quality.

An improvement in the hydrophilic properties of the rib can also be achieved if such an organic polymer layer is applied to a silicate film. If necessary, an organic layer of high hydrophilicity can also be applied to a double layer consisting of a base layer with high corrosion resistance and a sufficiently hydrophilic, even top layer. In this way it is avoided that the formers are exposed to hydrophilic layers such as silica gel and water glass, so that associated problems of tool wear during subsequent processing are eliminated.

Other additives, such as Corrosion inhibitors, fillers, pigments, surfactants, foam inhibitors, leveling agents, antibacterial / antifungal agents, etc. can be added as long as they do not interfere with the results intended by this invention.

In order to accelerate the drying process and to improve the quality of the film, it may be advantageous to add water-soluble solvents such as alcohol, ketone and cellosolve in small amounts.

The stability of the coating solution varies depending on the composition. In general, it is advisable to design the solution or dispersion so that when a cationic polymer is used, it is used in the neutral to acidic range and when an anionic polymer is used, it is used in the neutral to alkaline range.

The crosslinking agents are used under the conditions e.g. applied in terms of pH, which are common for them.

In the embodiment of the invention with subsequent application of water glass, water glass with an SiO₂ / M₂O ratio (M stands for Na, K or Li) of 2: 5 is generally used, although there are no special limits here. The concentration of the aqueous silicate solution can be set as long as long as an effective hydrophilic surface, i.e. a simple coating is guaranteed.

In connection with the amount of the aqueous silicate solution to be applied, it is desirable to design the process in such a way that a 0.1 - 5 μ silicate layer is obtained after heating / drying. A thickness of less than 0.1 µ does not result in sufficient long-term hydrophilicity, while more than 5 µ often leads to insufficient hardening (water insolubility) or cracking on the layer, which affects the performance of the heat exchanger.

The addition of polymer, i.e. a water-soluble acrylate to water glass effectively prevents cracking.

The heating / drying conditions for silicate should be in the range of 100 - 250 ° C and 20 s - 10 min, with shorter times for higher temperatures and longer times for lower temperatures.

The present invention makes it possible to use polymer crosslinked with water-soluble crosslinking agent for coating aluminum because the film formed in the process becomes water-insoluble. In addition to the hydrophilicity conferred by the polymer, excellent corrosion resistance is achieved. A coating with the aforementioned excellent properties is particularly suitable for the treatment of heat exchanger surfaces which are made from aluminum.

The present invention is explained in more detail below and by way of example using examples.

Examples

The following test method was used in the examples:

. Contact angle

Water droplets of 1 to 2 mm in diameter were placed on a coated surface and the contact angle using a device for measuring the surface contact angle, model CA-P, manufactured by Kyowa Kaimenkagaku Co., Ltd. certainly. Both fresh layers shortly after application and those that had been immersed in sea water for a week were tested.

. Corrosion resistance

Salt spray test according to JIS Z-2371 until white rust formation on 5% of the surface.

. Resistance to running water

The test specimen was immersed in flowing water at room temperature for 8 hours and then dried at 80 ° C. for 16 hours. After repeating this cycle five times, the water contact angle was measured.

example 1

An aqueous mixture of 40 g / l polyacrylamide with a molecular weight of 800,000, 10 g / l water-soluble polyurethane (ELASTRON A-42, a product of Daiichi Kogyo Seiyaku Co., Ltd.) as a dry substance and 30 g / l phosphoric acid was placed on aluminum sheet (A1100), which had been cleaned beforehand and provided with a chromium phosphate coating (Cr content in the layer: 70 mg / cm²).

The layer thus obtained was dried in an electric oven at 180 ° C. for 3 minutes. The film thickness averaged 0.5 µ. An aqueous solution of sodium silicate No. 3 was applied to achieve an average thickness of 0.5 μ in the dry state, and then dried.

Examples 2 to 10

According to the process conditions of Example 1, but with pretreatments and coating agents according to Table 2, further coatings were applied. The results are summarized in Table 3.

Claims (4)

1. A process for the production of hydrophilic coatings on aluminum surfaces using aqueous polymer solutions or dispersions, characterized in that an aqueous solution or dispersion is applied to the surfaces, which consists of a monomer of the general formula

- Where R₁ = H or CH₃
R₂ and R₃ = H,
an alkyl group with C₁ to C₄, a benzyl or alkanol group with C₂ - C₃

mean,
contains water-soluble polymer or copolymer A and a water-soluble crosslinking agent B, dries them up and crosslinks the polymer. 2. The method according to claim 1, characterized in that a water glass solution is then applied to the polymer coating and dried. 3. The method according to claim 1 or 2, characterized in that an aqueous solution or dispersion is applied to the surfaces, the polymer A has a molecular weight of more than 5000. 4. The method according to claim 1, 2 or 3, characterized in that before the production of the polymer coating, a boehmite layer or a chemical conversion coating, such as a chromate coating, is applied.