Waterproof Gypsum Molded Product

The present invention relates to a waterproof gypsum molded product.

As gypsum molded products have excellent fire resistance and adiabatic properties as well as comparatively high strength, they are particularly useful as building materials. However, gypsum molded products can not be applied to a place exposed to rain and liable to be drenched and condensed, such as a roof, outside wall, bath room, kitchen and toilet, because of its poor waterproof properties; the product is merely applied to such places as ceiling, floor, inside wall and the like.

Recently, a large amount of gypsum was produced as a by-product in desulfurization of an exhaust gas and enlargement of effective use of the gypsum is required. For the sake of the noted enlargement, several processes have been proposed to impart waterproofness to gypsum molded products. For example, Japanese Patent Publication (unexamined) No. 136224/77 discloses combining an alkylsilicate compound with a slurry of a gypsum mixture as formed by adding an .alpha.-hemihydrate gypsum and/or II-type anhydrous gypsum to .beta.-hemihydrate gypsum to make a hydrated product. Japanese Patent Publication No. 1224/78 shows mixing a gypsum or a gypsum composite with a water-soluble alkaline salt, such as a sodium or potassium salt, of silicone polymeric material. Japanese Patent Publication (unexamined) No. 33524/79 discloses using an aqueous solution of metal alkylsiliconate or phenylsiliconate for a building material containing gypsum as a principal component.

The aforementioned silicone compound imparts excellent waterproofness to concurrently decreases mechanical strength of gypsum molded products. Therefore, when the silicone compound is used as a water repellent or waterproofing agent, a sufficient improvement in waterproofness must be accompanied by a minimal decrease in mechanical strength. However, a sufficiently improved method had not been known.

Silikattechn. 13 (1962) Nr. 12 by K. Bergt discloses imparting waterproofness to gypsum products containing calcium oxide by the addition of potassium methylsiliconates. However, the calcium oxide content is too large to produce a molded waterproof gypsum product without spoiling mechanical strength, particularly in production of foamed molded waterproof products.

It has been found that addition of a calcium hydroxide or a calcium oxide to gypsum in combination with the silicone compound or an additional use of poly(.alpha.,.beta.-unsaturated acid ester) emulsion solved the preceding problems.

The present invention relates to a waterproof gypsum molded product formed by a combination of hemihydrate gypsum, aqueous solution of alkaline metal alkylsiliconates or phenylsiliconates and a small amount of calcium hydroxide or calcium oxide as essential components, and also a waterproof gypsum molded product formed by a combination of hemihydrate gypsum, aqueous solution of alkaline metal alkylsiliconates or phenylsiliconates, poly(.alpha.,.beta.-unsaturated carboxylic acid ester) emulsion and a small amount of calcium hydroxide or calcium oxide.

In the present invention, the gypsum may be preferably .alpha.- or .beta.-hemihydrate gypsum as produced by calcining a gypsum selected from the group consisting of natural gypsum, chemical gypsum, and a gypsum from desulfurization of exhaust gas.

As alkaline metal alkylsiliconates or phenylsiliconates, there are sodium methylsiliconate, sodium ehtylsiliconate, sodium propylsiliconate, potassium methylsiliconate, potassium ethylsiliconate, potassium propylsiliconate, sodium phenylsiliconate, sodium benzylsiliconate, potassium phenylsiliconate, potassium benzylsiliconate. Such alkaline metal siliconates are used in aqueous solution form.

The amount of the aqueous solution of alkaline metal siliconates to be used is not critical, but it is preferably from 0.5 to 2 parts by weight based on 100 parts by weight of gypsum. The use more than 2 parts by weight is not economical and shows a tendency of decrease of the strength of the molded product.

Examples of the poly(.alpha.,.beta.-unsaturated carboxylic acid ester), are poly(methyl acrylate), poly(ethyl arcylate), poly(butyl acrylate), poly(methyl methacrylate), poly(ethyl methacrylate), poly(butyl methacrylate), copolymer of acrylic acid or methacrylic acid with other monomers, and partially-hydrolized materials, which may be emulsified on or after of polymerization. Particularly an emulsion of after polymer having excellent alkaline resistance and the lowest film-forming temperature (less than 45.degree. C.) is preferable.

An emulsion of poly(.alpha.,.beta.-unsaturated carboxylic acid) imparts weathering stability, alkaline resistance, water repellency, waterproofness and the like to a gypsum molded product. Furthermore, in case of foamed materials, said emulsion not only prevents powdering or whitening on the surface of the foamed materials but also stabilizes.

The preferred amount of poly(.alpha.,.beta.-unsaturated carboxylic acid ester) emulsion is from 5 to 20 parts by weight based on 100 parts by weight of gypsum.

In the present invention, the calcium hydroxide or the calcium oxide is used in about 0.07 to about 3.5 parts by weight (as CaO), particularly more than about 0.14 to about 3.0 parts by weight (as CaO), based on 100 parts by weight of gypsum.

Use of the calcium oxide or calcium hydroxide in more than about 3.5 parts by weight (as CaO) lowers waterproof ability of the molded product, and use of large excess of it, for instance, about more than 10 or 20 parts by weight, lowers mechanical strength. Particularly, a foamed molded product produced from a gypsum composition containing the large excess of calcium oxide or calcium hydroxide is very apt to crack.

The noted essential components are blended with a suitable amount of water, such as from 30 to 80 parts by weight based on 100 parts by weight of gypsum, to make a slurry and then cast, molded and dried to give a waterproof gypsum molded product.

The gypsum molded product of the present invention may be lightened in accordance with the purpose. A method of lightening is not critical, but a foaming method comprising generation of gases in the slurry during solidification is preferable, because such method yields a uniform lightweight molded product. The lightweight gypsum molded product may be produced by foaming slurry in the presence of hydrogen peroxide and degradation catalyst. The amount of hydrogen peroxide used as a foaming agent may be varied according to the required density of the molded product. In general, it is preferable to use 0.4 to 2.0 parts by weight of hydrogen peroxide (35% by weight) based on 100 parts by weight of the gypsum.

The degradation catalyst for hydrogen peroxide as used in the present invention, is preferably selected from cobalt compounds, such as cobalt sulfate, cobalt nitrate, cobalt chloride and the like, considering activity of the catalyst, condition of foaming, density of foamed molded product, slurry setting time and the like. Of course, the catalyst is not restricted to a cobalt salt. If desired, an agent to control the pot life of the slurry, such as citric acid and potassium sulfate, may be combined.

According to the present invention, the problem that the aforementioned silicone compound (having an excellent ability as a water repellent or a waterproofing agent) weakens a gypsum molded product can be solved.

That is, the gypsum molded product according to the present invention has excellent waterproofness and mechanical strength and thus is useful for various applications, particularly as building material.

The present invention enlarges the application of gypsum and contributes to effective use of gypsum which is produced in large amount as a by-product in desulfurization of exhaust gas, which is widely employed for preventing pollution.

The invention will be described further in conjugation with examples showing the practice of the present invention. These examples, however, are not to be construed as limiting the invention to their details.

Seven kinds of molded product (including two comparatives) are prepared according to the formulations as shown in Table 1 (molded products A and B are comparative products).

Water, citric acid, calcium hydroxide, potassium alkylsiliconate, emulsion of poly(acrylic acid ester), hemihydrate gypsum and cobalt (II) sulfate are mixed with stirring to prepare gypsum slurries. After the hydrogen peroxide (6% by weight) is added to the resultant slurry with sufficient stirring, the mixtures are cast into molds (4 cm.times.4 cm.times.16 cm) and removed from the molds after one hour. The molded products are permitted to stand at 50.degree. C. for 24 hours to prepare gypsum molded products.

The physical properties of the gypsum molded products are shown in Table 2.

The relation between bending strength, the compressive strength and the wettability in Table 1 and the amount of calcium hydroxide to be used in Table 2 is illustrated in Figure, wherein the figure of drawing which shows the relationship between the amount of calcium hydroxide (abscissa) and the bending strength, the compressive strength and the wettability by (1), (2) and (3), respectively (the strength is expressed by the left longitudinal axis and the wettability is expressed by the right axis).

Four kinds of molded product are prepared according to formulations of Table 3 (molded products C and D are comparative products).

The components in Table 3 are mixed with stirring to give gypsum slurries. The slurries are cast into molds (4 cm.times.4 cm.times.16 cm) and removed after one hour. Standing for one day and night at room temperature, the products are dried for 24 hours at 50.degree. C. to give gypsum molded products, the physical properties of which are shown in Table 4.

This Example indicates the relation between the amount of calcium oxide and waterproofness and mechanical strength of foamed molded products.

Seven kinds of molded product are prepared according to the formulation as shown in Table 5 (molded products E, F and G are comparatives; E does not include potassium methyl siliconate).

Preparation of these gypsum products and the physical properties of the products are determined according to Example 1. The results are shown in Table 5.

This Example illustrates the relation between the amount of calcium hydroxide and waterproofness and mechanical strength of foamed molded products.

Eight kinds of molded product are prepared according to the formulations as shown in Table 6 (molded products H, I, J and K are comparative molded products; E which is prepared in Example 3 is listed in this Table as a reference).

Preparation of these gypsum products and the physical properties of the products are determined according to Example 1. The results are shown in Table 6.

Eight kinds of molded product are prepared according to Example 4 provided that potassium methylsiliconate is used in 2 parts by weight (1 part by weight as solid) per 100 parts by weight gypsum (molded products L, M, N and P are comparatives). Formulations and physical properties of the products are shown in Table 7.

Example 6 (non-foamed molded gypsum products)

All components indicated in each column of Table 8 are mixed together and the mixtures are cast into molds (4 cm.times.4 cm.times.16 cm) and removed from the molds after one hour. The molded products are permitted to stand at 50.degree. C. for 24 hours to give ten kinds of non-foamed gypsum molded product including five comparatives (Product Nos. Q, R, S, T and U).

The physical properties of the gypsum molded products are determined and the results are shown in Table 8.

FIG. 1 shows an effect obtained by the addition of calcium hydroxide to the bending strength (1), the compressive strength (2) and the wettability (3) of the molded foamed gypsum product of the present invention.