JPH0116869B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention also relates to anticorrosion pigments comprising compositions containing aluminum phosphite. The lead-based and chromate-based antirust pigments that are currently typically used contain lead or hexavalent chromium, and are toxic and problematic for use in food-related equipment, food factories, and the like. Therefore, these lead-based or chromate-based pigments exhibit extremely excellent rust-preventing properties, but due to their toxicity, it has been desired to develop alternative rust-preventing pigments that are less toxic and non-toxic. Therefore, as non-polluting rust preventive pigments, (a) metal condensed phosphates such as condensed aluminum phosphate, (b) metal phosphates such as silicon phosphate, titanium phosphate, and (c) metal phosphites have been used. salt systems such as zinc phosphites, barium salts, magnesium salts, manganese salts, (d) metal hypophosphites such as calcium hypophosphite or iron hypophosphite, and (e) metal molybdates such as zinc. Many proposals have been made, such as salts and calcium salts, but these non-polluting rust-preventing pigments are not definitive as rust-preventing pigments because their rust-preventing properties are not as good as those of lead-based and chromate-based pigments. Therefore, at present, these proposed pigments are even used in combinations of two or three in order to obtain the necessary rust prevention properties. The present inventors have conducted various studies in order to solve the drawbacks of the conventionally known anti-rust pigments mentioned above, and found that they contain aluminum phosphite, which has a novel structure and is a reaction product of phosphorous acid and alumina hydrate. The present invention has been completed based on the discovery that the composition has anti-rust properties that are as good as, if not superior to, lead-based or chromate-based compositions. That is, the present invention is based on the general formula Al ₂ (PHO ₃ ) _3.x [Al ₂ O ₃
yH ₂ O] (in the formula, x = 0.02 to 1, y = 1 to 3). The aluminum phosphite-containing composition of the present invention is non-polluting and therefore can also be used for food-related rust prevention. Conventional methods for producing aluminum phosphite include (a) adding ammonium phosphite to an alum aqueous solution, and (b) boiling an aluminum phosphite aqueous solution obtained by dissolving Al(OH) ₃ in H ₃ PO ₃ . A method of precipitating crystals using a method is known (Inorganic Chemistry Complete Book, Aluminum).
In method (a), salts such as potassium sulfate and ammonium sulfate are produced as by-products, and washing with water is necessary to remove them, and it is not industrially practical from a raw material standpoint.
On the other hand, method (b) requires an excess of phosphorous acid (it does not dissolve in an equivalent amount), and the phosphorous acid must be removed by washing with water to obtain the product. Moreover, Al(OH) ₃ must be well washed and activated immediately after its formation. In addition, general alumina hydrate that has changed over time does not dissolve in phosphorous acid. There were restrictions such as the fact that freshly made aluminum hydroxide had to be relatively well soluble in phosphorous acid. However, the present inventors found that aluminum phosphite was obtained by reacting alumina hydrate and phosphorous acid under heating, and that the composition containing the obtained aluminum phosphite as an active ingredient was not as effective as conventional lead. It has been found that the anti-rust properties are as good as those of the chromate-based chromate-based chromate-based anti-corrosion products. The reaction between alumina hydrate and phosphorous acid generally proceeds as follows: Al ₂ O ₃ .nH ₂ O + 3H ₂ PHO ₃ →Al ₂ (PHO ₃ ) ₃ + (n
+3) H ₂ O The reaction according to the above general formula is carried out by dispersing alumina hydrate in an aqueous phosphorous acid solution and heating and stirring (under pressure). The reaction conditions depend on the type of alumina hydrate, particle size, molar ratio of phosphorous acid and alumina hydrate, etc. Alumina hydrates used as raw materials include, for example, alumina gel [2Al(OH) ₃ ], gibbsite, bayerite (Al ₂ O ₃・3H ₂ O), boehmite (Al ₂ O ₃・H ₂ O), and other materials. Alumina hydrate can be used. These react as follows: 2Al(OH) ₃ +3H ₂ PHO ₃ →Al ₂ (PHO ₃ ) ₃ +6H ₂ O
…(1) Al ₂ O ₃・3H ₂ O＋3H ₂ PHO ₃ →Al ₂ (PHO ₃ ) ₃ ＋
6H ₂ O …(2) Al ₂ O ₃・H ₂ O＋3H ₂ PHO ₃ →Al ₂ (PHO ₃ ) ₃ ＋
4H ₂ O...(3) Reactivity generally tends to decrease in the order of (1), (2), and (3), although it depends on the conditions. The closer the molar ratio of 3H ₂ PHO ₃ /Al ₂ O ₃ is to 1, the more
Also, although it takes a long time to react to the core of the Al ₂ O ₃ particles, the rust prevention effect is achieved by the aluminum phosphite generated on the particle surface, so the Al ₂ O ₃
There is no need to react to the core of the particles,
If the 3H ₂ PHO ₃ /Al ₂ O ₃ molar ratio is 0.5 or more, sufficient anti-corrosion ability will be exhibited. If the molar ratio is less than 0.5, the antirust ability will be poor. Further, when the above molar ratio exceeds 1, the reaction takes place in an acidic solution, so the reaction rate becomes faster, but washing with water is required to remove excess phosphorous acid after the reaction is completed. Therefore, in the present invention, the amount is often lower than the stoichiometric amount, but it is preferable to limit the amount to an excess of 20 to 30% at most. There is no need to exceed the stoichiometric amount, especially when producing it as a rust-preventing pigment. Therefore, the product according to the invention
It is expressed in the form Al ₂ (PHO ₃ ) ₃ ·x [Al ₂ O ₃ ·yH ₂ O]. Here x is less than 1, practically 0.02 to 1,
Although it is possible to set x to 0, the antirust ability will not improve as much as expected. Moreover, y generally has many changing factors, but 1 to 3 is preferable. The product according to the invention can be described as a particle of alumina hydrate whose surface is reacted and surrounded by aluminum phosphite. The reaction between alumina hydrate and phosphorous acid does not proceed at room temperature. Alumina gel and alumina hydrate, which have good reactivity, start reacting at 60°C to 70°C. However, when a relatively low-reactivity alumina hydrate is used, the reaction may proceed only after heating to a temperature of, for example, 200° C. under pressure, for example, 4 to 5 kg/cm ² . Therefore, the reaction temperature ranges from 70℃ to 200℃ depending on the type of alumina hydrate used.
Preferably, the temperature is generally 80°C to 150°C (approximately 5 kg/cm ² ). However, this does not exclude a reaction at 200°C or higher. It goes without saying that the reaction is carried out under pressure at temperatures above the boiling point of the reaction system. As the reaction progresses, free phosphorous acid decreases, so the end point of the reaction can be determined by measuring the pH. Generally, the finer the size of the alumina hydrate particles, the better the reactivity. However, when producing alumina hydrate (Bayer method), there are problems such as fine particles make it difficult to wash with water, while large particles are easier to wash with water, but are often unsuitable as pigments as they are. In addition, many commercially available alumina hydrates have large particles, and a large amount of energy is required to grind them into fine particles on the order of micrometers. However, we discovered that pulverization becomes easier when alumina hydrate is reacted with phosphorous acid to some extent. This is thought to be because phosphorous acid acts on the cracks of alumina hydrate, making the particles brittle. Further, since the crushed alumina hydrate particles are activated by mechanochemical action, the subsequent reaction with phosphorous acid becomes easy. Therefore, it is often more efficient to carry out the process as follows: primary reaction -> pulverization -> secondary reaction. In this way, the following reaction procedure can be taken depending on the particle size of the raw material alumina hydrate.

[Table] Reaction-Crushing-Reaction-
Filtration - Drying - Grinding - Product
／ ／
phosphorous acid

Claims (1)

_{[ Claims ] 1 . _} A rust-preventing pigment containing an aluminum phosphate-containing composition as an active ingredient.