JPH0534429B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an improvement in the manufacturing method of a protective film formed by an alkali metal silicate solution on a metal plate such as an aluminum reflective mirror. The present invention relates to a method for producing a protective coating that can be made water repellent so that it can be easily wiped off even when exposed to water. A method of forming a protective film on a metal plate such as an aluminum reflecting mirror using an alkali metal silicate solution is already known and is described in detail in, for example, Japanese Patent Publication No. 30521/1983. Protective coatings produced by such methods are generally hard, glassy, adherent, transparent coatings that are also very chemically stable and extremely resistant to attack by chemicals and contaminants. It has the great advantage of being resistant. However, even objects with protective coatings manufactured by the above method tend to corrode when used for many years, and this tendency is particularly noticeable when used in locations where water and dust are likely to adhere. I found out. The reason why such a phenomenon occurs is considered to be due to the following reasons. That is,
When viewed microscopically, the surface of the protective film has small holes called "pinholes" caused by unevenness on the substrate, and microscopic cracks called "cracks" caused by foreign matter adhering to the substrate surface. There are some defects such as: On the other hand, the surface of conventional alkali metal silicate coatings is hydrophilic for the following reasons. In other words, the terminal end of the alkali metal silicate molecule is written as the chemical formula:

[Formula] (where M is an alkali metal such as potassium or sodium). When this is made into a film and baked at high temperature to harden, moisture disappears and the overall result is

Siloxane bonds represented by the formula are formed to form a three-dimensional structure and form a hard film. However, the end of the molecule

As [formula] remains, the alkali metal M in this reacts with carbon dioxide gas in the air and causes clouding. Therefore, by treating the film with an acid such as nitric acid to extract the alkali metals in the film, the ends of the molecules, that is, the surface layer, are removed.

Ion exchange with [Formula] causes the hydrophilic groups [-OH] to face outward, resulting in a hydrophilic film as a whole.
Therefore, if water adheres to the surface of the protective film, the entire film will be extensively covered with water, and the aforementioned defective areas of the film will also be subject to water intrusion, and furthermore, due to its hydrophilic nature, it will be difficult for water to run off. becomes. In such a state, when dust and gas adhere to water, a chemical reaction occurs, corrosion occurs from the defective portion of the coating, and this develops into pitting corrosion. The hydroxide of aluminum or the like generated from this pitting corrosion portion firmly adheres to the hydrophilic coating, cannot be easily removed even by wiping, and significantly stains the surface appearance of the metal plate. To solve this problem, either apply a water-repellent protective film to the surface of the film to prevent water from adhering to it, which can cause corrosion, or thicken the film in areas where pinholes or cracks occur. By doing so, it is possible to prevent contact between water and the substrate. A common method of the former is to apply a water-repellent resin, such as silicone oil or varnish, to the surface of the complete protective coating. However, this method has the major disadvantage that the water-repellent film has poor adhesion, low durability, and low light transmittance, resulting in a decrease in reflectance when applied to a reflecting mirror or the like. A common method for the latter method is to coat the same type of protective film three to five times, but this method reduces the reflectance due to clouding of the film and the damage caused by thermal shock. It has the disadvantage of being prone to cracking. The present invention has been made in view of the above points, and
It has greater water repellency than conventional alkali metal silicate coatings, and is extremely less susceptible to corrosion.
An object of the present invention is to provide a method for producing a protective coating having excellent properties, which allows corrosion products to be easily wiped off even if they adhere. In order to achieve this objective, the inventors conducted various experiments and conducted a certain treatment using a mixture of water-soluble silicone, particularly alkali methyl siliconate, in an alkali metal silicate solution. It was confirmed that the intended purpose could be achieved by applying this method. The above-mentioned alkali methyl siliconate is sold, for example, by Toray Silicon Co., Ltd. under the trade name "Dry Seal E" and is applied to the surface of concrete to prevent cracks due to frost damage, or added to starch, water glass, etc. It is used to make wall materials, insulation materials, etc. water repellent. Although an alkali metal silicate solution is also a type of water glass, in order to produce the protective coating of the present invention using this solution, it is necessary not only to appropriately select the pH of the solution, but also to carry out acid treatment as described below. It is not enough to simply add alkali methyl siliconate. In addition, in the present invention, alkali methyl siliconate was selected as an additive because it dissolves in water, has a high pH and is safe, is stable even when added to alkali metal silicate solutions, and is suitable for metal plates, etc. This is because good transparency can be obtained when applied. Hereinafter, the method for manufacturing a protective film according to the present invention will be explained step by step. First, a stock solution of alkali metal silicate solution is prepared, and a predetermined amount of water and alkali methyl siliconate are added thereto. The alkali metal silicate solution used in the experiment was a potassium silicate solution, and the stock solution had the following composition in terms of weight percent. Potassium oxide K ₂ O 8.3% Silica SiO ₂ 20.8% Water 70.9% Also, as alkali methyl siliconate,
Sodium methyl siliconate was used. As mentioned earlier, this is, for example, "Dry Seal E"
It is generally commercially available under the trade name, and has the following general characteristics: Appearance Pale yellow transparent Specific gravity (25°C) 1.26-1.27 Solid content (%) 30 PH 13 The mixing ratio of the stock solution of potassium silicate solution, water and sodium methyl siliconate in volume % is as follows. Potassium silicate solution stock solution 26% Water 73% Sodium methyl siliconate 1% In this case, the mixing ratio of sodium methyl siliconate can be appropriately selected within the range of 0.5 to 10% by weight. If the amount is less than this lower limit, it will not be possible to obtain water repellency, and if it is more than the upper limit, the transparency will decrease, making it unsuitable for forming a protective coating for a reflective mirror, etc. Furthermore, in the present invention, care must be taken that the pH of the above-mentioned liquid mixture must be 10 or higher. This means that the pH is less than 10, especially 8~
If it is less than 9, the alkali metal silicate solution will cause a polymerization or condensation reaction, and the resulting precipitation and gelation of free silicic acid will make the solution opaque, making it impossible to form a film. . After preparing the above mixed solution, dip a metal plate such as an aluminum reflector into the mixed solution and let the solution adhere to its surface sufficiently, then pull up the metal plate and place it in a drying room. Dry for 10 minutes at a temperature of approximately 32°C. This causes the water to dissipate and the alkali metal silicate to coagulate with the alkali methyl siliconate to begin to form a film. Therefore, next, we put the above metal plate in a heating furnace and heated it to a temperature of 250 to 320℃.
Heat for 10 minutes to almost completely dehydrate the film and harden the film. This heating causes a condensation reaction in the aggregate of the alkali metal silicate and alkali methyl chiliconate, forming strong siloxane bonds [Si-O-Si] and forming a strong coating that is difficult to dissolve in water. . Next, take out the heated metal plate and heat it for about 70 minutes.
After performing acid treatment by immersing it in a 3% nitric acid solution having a temperature of °C for 3 minutes, it is washed with water. By this acid treatment and washing with water, alkali metal ions present in the coating, ie, potassium ions and sodium ions in the examples, are extracted and removed. What is important in the present invention is not simply to add alkali methyl siliconate as in the conventional general usage method, but to utilize a process of extracting and removing alkali metal ions from the alkali metal silicate coating. , the alkaline content present in the alkali methyl siliconate is also extracted and removed at the same time. As mentioned above, when alkali metal ions are extracted and removed from a film formed using only an alkali metal silicate solution, the surface of the film becomes strongly hydrophilic. Even when alkali metal ions were extracted and removed from a film formed using a solution, not only was there no adverse effect, but the surface of the film became highly water-repellent, a result better than expected. As described above, the reason why the protective coating produced according to the present invention exhibits strong water repellency is due to the presence of methyl siliconate having a methyl group that provides water repellency on the surface of the protective coating. This water repellency makes it difficult for water, which causes corrosion, to accumulate on the surface of the metal plate. In addition, the methyl group of methyl siliconate is inert and non-adhesive to corrosion products such as aluminum hydroxide, so corrosion products are difficult to adhere to the surface of the protective film, and even if they do However, it can be easily wiped off with water. Furthermore, even if a protective film is formed using a mixed solution of an alkali metal silicate solution and an alkali methyl siliconate, the film will become cloudy within a short period of time if the acid treatment described above is not performed. Of course, residual alkali metal ions in the coating and water combine to form a corrosive solution such as potassium hydroxide or sodium hydroxide, which also attacks the siloxane bonds in the coating and the methyl groups that provide water repellency. The method for manufacturing the protective coating according to the present invention is almost the same as the conventional protective coating using an alkali metal silicate solution in terms of the manufacturing process, and does not require any additional steps. That is, the protective film is formed by a series of steps of immersing the metal plate in a solution, drying, heating at high temperature, acid treatment, and washing with water, and has the advantage that conventional manufacturing equipment can be used as is. Furthermore, as mentioned above, the protective coating produced according to the present invention has extremely superior properties compared to conventional protective coatings, and this can also be supported by the experimental results described below. Note that the thickness of the protective film formed by performing the above series of steps only once is usually about 1 to 2 μm, which is not necessarily satisfactory in terms of mechanical strength. Therefore, in order to increase the mechanical strength of the coating, colloidal silica may be further added to the mixed solution to improve the adhesion and strength of the coating, or the series of steps described above may be repeated two or more times to form a multilayer coating. It is possible to do so. However, in the latter case, if the first layer of protective film formed on the surface of the metal plate is water repellent, most of the alkali metal silicate solution used to form the second layer of protective film is water repellent. The problem arises that it does not stick. In order to solve this problem, for example, the first layer of protective coating is made hydrophilic by using an alkali metal silicate solution without adding alkali methyl siliconate.
The second protective coating may be made water-repellent by adding alkali methyl siliconate. Also,
In the present invention, it is also possible to use a sodium silicate solution or a mixture of this and a potassium silicate solution in place of the potassium silicate solution.
Potassium methyl siliconate or a mixture of potassium methyl siliconate and sodium methyl siliconate can also be used in place of sodium methyl siliconate. Finally, the effects of the present invention will be explained based on experimental data. The experiment was conducted by applying an alkali metal silicate solution without adding alkali methyl siliconate to the surface of an aluminum reflector, drying it, baking it, post-processing it, and washing it with water only once. The protective coating (sample 1) formed by repeating the above process twice (sample 2), and the process performed only once using an alkali metal silicate solution to which alkali methyl siliconate was added. In particular, the formed protective coating (sample 3) according to the present invention was tested by a cast corrosion resistance test (JIS H8681), and the corrosion resistance by rating number (RN), the degree of pitting corrosion, the appearance after the test, and the corroded area were evaluated. We investigated the wiping properties of water and the reflection rate of the reflector. The results are shown in Table 1.

[Table] As is clear from Table 1, the reflector (sample 3) equipped with a protective coating manufactured according to the present invention has a protective coating manufactured using an alkali metal silicate solution without adding alkali methyl siliconate. It is much superior in terms of corrosion resistance compared to the one with a protective film (Sample 1), and it is also superior to the one with a protective film obtained by repeating the conventional process twice (Sample 2). There is. In particular, the degree of pitting corrosion is smaller than that obtained by conventional methods, and the appearance after the test is also clean. As shown in the table, the reflectance of sample 3 is almost the same as that of samples 1 and 2. In this way, the method for manufacturing a protective coating according to the present invention requires only one manufacturing step.
It is possible to obtain a protective film with better performance than when using the conventional method of repeating the same process twice, and there is also a great merit in terms of productivity.

Claims (1)

[Claims] 1. A step of applying a solution containing an alkali metal silicate and an alkali methyl siliconate and having a pH adjusted to 10 or more on the surface of an article, and drying the solution attached to the surface of the article. and heating to form a coating on the solid surface of the article, and acid treatment to remove alkali metal ions contained in the coating on the surface of the article. Production method. 2. The method for producing a protective film according to claim 1, wherein the alkali metal silicate is a potassium silicate. 3. The method for producing a protective film according to claim 1, wherein the alkali methyl siliconate contains one or both of sodium methyl siliconate and potassium methyl siliconate. 4. The method for producing a protective coating according to claim 1, wherein the solution further contains collodyl silica.