JP3818882B2 - Method for producing hydrophilic silica coating - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a method for producing a hydrophilic silica coating that maintains corrosion resistance and hydrophilicity for a long period of time, particularly in metal substrates used for fins and the like in evaporators of heat exchangers such as room air conditioners and car air conditioners.
[0002]
[Prior art and its problems]
In general, in a heat exchanger, particularly an evaporator of an air conditioner, water is condensed on the surface of the fin because the surface temperature of the fin is below the dew point of the atmosphere. Such adhesion of moisture causes various problems such as a decrease in heat exchange performance due to an increase in flow resistance of air to be heat exchanged, generation of noise due to air blowing, and scattering of water droplets. .
[0003]
In order to solve these problems, it is a common practice to prevent blockage of the air flow path by imparting hydrophilicity to the fin surface and improving the flow of the attached water. For example, in Japanese Patent Application Laid-Open No. 50-38645, the surface of aluminum is dipped in a solution containing an alkali metal carbonate and an alkali metal chromate, coated, and further contains an alkali metal oxide and silicon dioxide. A method of immersing in a solution and forming a continuous, porous, hydrophilic coating on an aluminum surface is disclosed.
[0004]
Japanese Patent Application Laid-Open No. 62-235477 discloses a method of forming a hydrophilic film by coating an aluminum surface with a solution containing an alkali metal silicate, an inorganic curing agent, and a water-soluble organic polymer compound. .
[0005]
Japanese Patent Application Laid-Open No. 62-272099 discloses a method for imparting hydrophilicity by immersing a surface of a part made of aluminum in an aqueous medium containing a compound having a silanol group and polyvinylpyrrolidone.
[0006]
In Japanese Patent Application Laid-Open No. 1-208475, chromate treatment is performed, and then an alkali metal silicate aqueous solution containing normal phosphoric acid is applied, then a normal phosphoric acid solution is further applied, and then heated and dried. A method for forming a conductive film is disclosed.
[0007]
As described above, the specific methods described in the published patent specifications have been clarified, but each method has an affinity with water exhibited by alkali metal silicate as its fundamental principle, and is a coating that is practically required. In order to improve the durability, it can be said that the durability is improved by adding an inorganic curing agent or an organic polymer component. At the same time, attempts have been made to reinforce the adhesion between the substrate having a porous surface and a hydrophilic film and a substrate.
[0008]
The hydrophilicity imparted by utilizing such characteristics of the alkali metal silicate has sufficiently achieved its purpose and is widely used in practice. However, if such a hydrophilic coating is kept in contact with water for a long time, the portion that exhibits the hydrophilic function of the coating disappears due to the excellent solubility of the alkali metal silicate in water. May cause problems. Also, when used in highly closed spaces such as car air conditioners, the membrane changes due to the formation of carbonates or organic degradation products due to the reaction between carbon dioxide or organic substances contained in the breath of the human body and alkali metal silicates. It is also a problem to generate an off-flavor due to. Further, in the case of using an organic polymer compound as a part of the constituent material of the film, although it is not exposed to such a high temperature under normal use conditions, there remains anxiety in heat resistance over a long period of time.
[0009]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present inventors have disclosed a method for forming a coating film on a metal surface without using an organic compound as a constituent material of the coating film and having essentially no change in hydrophilicity over a long period of time. As a result of intensive studies, a coating film containing titanium dioxide having an average particle size of 5 μm or less and at least silica is practically necessary for hydrophilicity and stable under severe conditions such as heated water for a long time. And found the present invention.
[0010]
That is, this invention is a manufacturing method of the hydrophilic silica film which apply | coats the coating solution for hydrophilic silica film formation to a base material, and is dried and temporarily baked at 50-200 degreeC, and baked at 300 degreeC or more , The said hydrophilic The coating solution for forming a conductive silica film is represented by the general formula Si (OR ¹ ) (OR ² ) (OR ³ ) (OR ⁴ ), SiCl (OR ¹ ) (OR ² ) (OR ³ ), SiCl ₂ (OR ¹ ) (OR ² ) or SiCl ₃ (OR ¹ ) (wherein R ¹ , R ² , R ³ and R ⁴ are methyl group, ethyl group, normal propyl group, isopropyl group, normal butyl group, secondary butyl group, methoxyethyl group) , An ethoxyethyl group or a phenyl group.) In a solution of an alkoxy compound of silicon represented by the following formula or a hydrolyzate thereof dissolved in an organic solvent, titanium dioxide having an average particle size of 5 μm or less is coated. In the film, the composition of titanium dioxide is contained so as to be 3 to 40 wt%, and the concentration of the silicon in the solution is 0.1 to 5 mol / l in terms of SiO ₂ . It is a method for producing a hydrophilic silica coating, characterized in that a hydrophilic porous film having a thickness of 0.1 to 10 μm with fine pores or cracks generated along with unevenness caused by titanium dioxide is formed on the surface. .
[0011]
By the way, applying a metal alkoxide to the surface of a base material and firing has been conventionally conducted for various purposes and has been used for various purposes, and most of them are the method of the present invention. Although it seems that it is the same, there is also an object of completely opposite object, that is, an object of imparting antifouling property by hydrophobizing the surface of the substrate. Therefore, in the present invention, it is necessary to use a film-forming solution having a specific composition containing a metal alkoxide, which includes titanium dioxide having an average particle diameter of 5 μm or less and a general formula Si (OR ¹ ) (OR ² ). (OR ³ ) (OR ⁴ ), SiCl (OR ¹ ) (OR ² ) (OR ³ ), SiCl ₂ (OR ¹ ) (OR ² ) or SiCl ₃ (OR ¹ ) (wherein R ¹ , R ² , R ³ and R ⁴ represent a methyl group, an ethyl group, a normal propyl group, an isopropyl group, a normal butyl group, a secondary butyl group, a methoxyethyl group, an ethoxyethyl group, or a phenyl group. And a mixture containing the alkoxy compound or a hydrolyzate thereof .
[0012]
The coatings of the present invention do not use alkali metals as well as alkali metal silicates, and do not cause long-term lack of stability in water due to their elution. The coating film of the present invention is a hydrophilic film having high mechanical strength, porosity, and long-term stability. It is recognized that the surface of the coating is covered with fine irregularities as shown in FIG. 1, and this irregularity is considered to be involved in the expression of hydrophilicity. However, only the irregularities caused by the fine particles observed in FIG. 1 do not contribute to the hydrophilicity of the present invention, but rather fine pores or cracks that are difficult to confirm depending on FIG. 1 are hydrophilic. It is thought to be the main cause. The cause of the formation of these fine pores is not clear, but the silica component shrinks when the film is formed by firing, but the solid content does not shrink, so the film remains partially while leaving a stress. In addition, it is considered that such fine pores are generated at a large density on the surface.
[0013]
In porous materials with pores, water penetrates into the inside due to its surface tension, and macroscopically, the contact angle is low when a liquid is brought into contact with the surface, or the spread when water drops are dropped on the surface. As being hydrophilic. The coating of the present invention is
In addition to exhibiting hydrophilicity by such a mechanism, it also has hydrophilicity due to the surface energy of the film made of silicon itself.
[0014]
In the present invention, the composition of the titanium dioxide film having an average particle diameter of 5 μm or less is preferably 3 to 40 wt%, more preferably 10 to 30 wt%. When the content is 3 wt% or less, the film does not form sufficient irregularities and the above-described micropores, and does not exhibit desirable hydrophilicity. When the content is 40 wt% or more, the strength of the film decreases and particle peeling occurs. Therefore, it is not preferable in handling.
[0015]
As titanium dioxide having an average particle size of 5 μm or less, which is a solid content in the present invention, a fibrous material of TiO ₂ (rutile, anatase), a mixture thereof, or the like can be used, and those finely divided as a pigment are preferable. The particle size distribution of the fine particles does not have to be particularly narrow, and it is preferable that the particle size is widely distributed to about 0.05 to 5 μm. In the case of a fibrous material, the diameter is preferably about 0.05 to 5 μm, and the length is preferably 100 μm or less.
[0016]
The raw materials of the components other than the solid content used to form the coating film of the present invention are represented by the general formula Si (OR ¹ ) (OR ² ) (OR ³ ) (OR ⁴ ) that can form an oxide substantially by the firing treatment. ), SiCl (OR ¹ ) (OR ² ) (OR ³ ), SiCl ₂ (OR ¹ ) (OR ² ) or SiCl ₃ (OR ¹ ) (wherein R ¹ , R ² , R ³ , R ⁴ are An alkoxy compound represented by a methyl group, an ethyl group, a normal propyl group, an isopropyl group, a normal butyl group, a secondary butyl group, a methoxyethyl group, an ethoxyethyl group or a phenyl group) or a hydrolyzate thereof. If it is .
[0017]
In particular, tetraethoxysilane, tetramethoxysilane, tetraisopropoxysilane, tetranormalpropoxysilane, tetranormalbutoxysilane, tetratertiarybutoxysilane, or the like, or a hydrolyzate thereof is preferable .
[0018]
In addition, the silicon oxide soluble in organic solvents such as alcohols in the present invention is generally known as colloidal silica, but it is dissolved or uniformly dispersed when dissolved in alcohols. I just need it.
[0019]
In the present invention, since silicon is the main component for film formation, it is excellent in film forming property, adhesion of the film to the substrate, and stability of the film, but it becomes an oxide corresponding to firing when necessary. It is also effective to add zinc and lead compounds.
[0020]
The concentration of the coating chemical used for forming the hydrophilic film is preferably 0.1 to 5 mol / l in terms of SiO ₂ as the concentration of silicon other than the solid content in the solution, but more preferably 0.4 to ₂ mol / l. . If the concentration is lower than 0.1 mol / l, the film formed in one application operation becomes thin, and the number of times required for repeated application increases, which is not preferable for practical use. On the other hand, if it is higher than 5 mol / l, it is not preferable because large cracks that cause peeling may occur in the film due to choking (powder blowing phenomenon) or extreme volume shrinkage. However, this concentration condition does not necessarily have to be adjusted, and adjustment is possible depending on the type of additive and solvent.
[0021]
Alcohols should be used as the solvent for concentration adjustment, and examples include methanol, ethanol, i-propanol, n-propanol, n-butanol, i-butanol, t-butanol, methoxyethanol, ethoxyethanol, and ethylene glycol. It is also possible to use a combination of two or more of these.
[0022]
The thickness of the hydrophilic coating obtained at this time is preferably from 0.1 to 10 μm, more preferably from 0.3 to 5 μm. Since the pore volume is almost proportional to the film thickness, the effect of imparting hydrophilicity to the metal surface is not sufficient when the film is thinner than 0.1 μm, while the effect of generating irregularities due to fine particles is obtained when the film is thicker than 10 μm. Is not sufficient, and peeling is likely to occur. The hydrophilic film can be formed by a single application or a plurality of times.
[0023]
The chemical solution is applied onto the substrate by a method such as dipping, spraying, roller coating, flow coating, screen printing, or brush coating. The type of metal with a hydrophilic surface is selected in consideration of the use conditions based on thermal conductivity, chemical stability, mechanical strength, etc., but iron, aluminum, aluminum alloy, copper, brass, stainless steel These are generally plate-shaped or hollow-coiled, but it is naturally possible to impart hydrophilicity to a laminate or composite of various metals.
[0024]
In the method for producing a film of the present invention, it is useful to perform various pretreatments on the surface of the substrate. Mechanical polishing, electropolishing, acid cleaning, alkali cleaning, water cleaning or degreasing cleaning with an organic solvent are effective for any metal. In the case of aluminum or aluminum alloy, alumite treatment, chromate treatment, or chemical conversion treatment with phosphates such as zinc, titanium, zirconium, etc. are used for the purpose of improving corrosion resistance and coating adhesion. You can also Furthermore, pretreatment with an organic polymer compound may be applied within a range not departing from the object of the present invention.
[0025]
The coating film formed by various methods is dried and temporarily fired at 50 to 200 ° C. for 5 to 30 minutes, and then fired at 300 ° C. or higher for 10 to 60 minutes in an electric furnace to obtain a metal having an excellent hydrophilic surface. be able to. The upper limit of the firing temperature is not particularly limited, but is preferably about 500 ° C. or less in stainless steel.
[0026]
Hereinafter, the present invention will be described in more detail with reference to examples.
[0027]
【Example】
Example 1
In a 2 L three-necked flask equipped with a stirrer, 500 g of a 20.0 wt% tetraethoxysilane ethanol solution (manufactured by Colcoat Co.) in terms of SiO ₂ was added, and about 1 cc of 35% hydrochloric acid and 50 cc of water were added thereto at 50 ° C. The mixture was stirred for 3 hours under reflux. Next, 16.7 g of rutile titanium oxide having an average particle diameter of 0.2 μm was added while stirring, and the mixture was further added with 70% ethanol and 30% methoxyethanol so that the SiO ₂ concentration became 1.0 mol / l. diluted with a mixed solvent of about 1100cc, the white coating solution was made of tone.
[0028]
Take this coating solution in a 500 cc glass beaker, put a 50 × 100 × 0.2 mm SUS304 sample piece into it, then pull the sample piece out of the coating solution at a constant rate of 4 mm / sec, in air at 70 ° C. For 20 minutes, and further calcined in air at 150 ° C. for 20 minutes and cooled to room temperature.
[0029]
Next, the same coating and drying were repeated again, and then the temperature was further raised from 200 ° C. to 450 ° C. at a rate of 5 ° C./min, kept for 20 minutes, and taken out from the electric furnace and allowed to cool to form a white film. Sample pieces were obtained. A scanning electron micrograph of the surface of the coating is shown in FIG.
[0030]
The following evaluation test was performed on the coated sample, and the hydrophilicity and chemical stability of the film were confirmed.
1) Acid resistance test: A 1 wt% hydrochloric acid solution was placed in a glass 1 L-ker, the sample was immersed therein, and allowed to stand at room temperature for 24 hours. After a lapse of time, the sample was washed with running water, and the surface state was observed visually. In the evaluation, ○ indicates that there is no change at all, × indicates that crack or peeling is observed, and ○ indicates the middle.
[0031]
2) Solvent resistance test: Acetone was placed in a 1 L glass beaker, the sample was immersed in it, and allowed to stand at room temperature for 24 hours. After a lapse of time, the sample was washed with running water, and the surface state was observed visually. In the evaluation, ○ indicates that no change is observed, × indicates that a crack or peeling is observed, and ○ indicates an intermediate.
[0032]
3) Water cracking test: Immerse the sample in a beaker containing distilled water for 10 seconds and evaluate how it drains quickly when it is pulled up. The case where no part was recognized was marked with ×, and the middle of the marked portion was marked with Δ.
[0033]
4) Measurement of wetting and spreading: 5 μl of distilled water was gently dropped on the surface of the sample with a microsyringe, and the spreading of the water droplets after 5 seconds was measured with calipers. In addition, the hot water immersion test as a hydrophilic acceleration test is immersed in distilled water at 45 ° C. for changes over time, taken out from the tank after a predetermined time, then dried to a constant weight at 80 ° C., and cooled to room temperature. Then, the above measurement was performed.
[0034]
Each measurement result is shown in Table 1. It is obvious that the coating has wettability and water cracking test and has extremely superior hydrophilicity as compared with the comparative example.
[0035]
[Table 1]
-------------------------------------------------- ------------------------------
Film-forming composition Wet spread
-------------------------------------------------- ------------------------------
* 1 * 2 Concentration Dilution Stability Water cracking mmφ
Raw material wt% mol / l Volume ratio Appearance Acidic water Acetone Initial 24hr
-------------------------------------------------- ------------------------------
Example 1 TEOS 85.7 1.0 EtOH 7 ○ ○ ○ ○ 12.6 9.1
TiO ₂ 14.3 EGME3
-------------------------------------------------- ------------------------------
Comparative Example 1 TEOS 100 1.0 EtOH ○ ○ ○ × 4.2 2.4
-------------------------------------------------- ------------------------------
EtOH: Ethanol, EGME: Ethoxyethanol
* 1 wt% of each component when oxide
* 2 Concentration of components other than solids mol / l
[0036]
Comparative Example 1
The terms of SiO ₂ 20.0 wt% ethanol solution of tetraethoxysilane (manufactured by Colcoat) concentration of SiO ₂ is adjusted to a concentration of 1.0 mol / l, it was subjected to film formation in the same manner as in Example 1. The results are shown in Table 1.
[0037]
【The invention's effect】
As is apparent from Table 1 showing the results of the examples, the silica film produced using the film forming solution of the present invention has hydrophilicity and water flow properties required for a condenser of a heat exchanger. In addition, since it does not contain alkali metal silicate or organic matter, it has an effect that it can form a film that exhibits hydrophilicity whose performance does not change over a long period of time even under severe use conditions.
[Brief description of the drawings]
1 is a drawing-substituting electron micrograph of a thin film formed on a substrate of Example 1. FIG.

Claims (1)

The hydrophilic silica film-forming coating solution was applied to a substrate, dried and calcined at 50 to 200 ° C., a method for producing a sintered hydrophilic silica coating at 300 ° C. or higher, the coating the hydrophilic silica coating formation The solution may be represented by the general formula Si (OR ¹ ) (OR ² ) (OR ³ ) (OR ⁴ ), SiCl (OR ¹ ) (OR ² ) (OR ³ ), SiCl ₂ (OR ¹ ) (OR ² ) or SiCl ₃ ( OR ¹ ) (wherein R ¹ , R ² , R ³ and R ⁴ are methyl, ethyl, normal propyl, isopropyl, normal butyl, secondary butyl, methoxyethyl, ethoxyethyl or phenyl) In a coating film, titanium dioxide having an average particle size of 5 μm or less is added to a solution obtained by dissolving a silicon alkoxy compound or a hydrolyzate thereof in an organic solvent. The silicon composition is contained in an amount of 3 to 40 wt%, and the silicon concentration in the solution is 0.1 to 5 mol / l in terms of SiO _2. A method for producing a hydrophilic silica film, comprising forming a hydrophilic porous film having a film thickness of 0.1 to 10 μm in which fine pores or cracks are generated together with unevenness caused by the unevenness.

Images