FI76546C - Process for the preparation of water-repellent plaster molds - Google Patents

Description

1 76546

The present invention relates to a process for the production of water-repellent moldings from gypsum powder, in particular to a process for the production of porous gypsum boards in which polyalkylhydrogen-loxane is added to the gypsum mass.

It is known to make gypsum products, such as gypsum plasterboards, which have a high water absorption capacity, water-repellent by using organosilicon compounds. Hydrophobicization can occur retrospectively after the manufacture of gypsum products, but is most often done by adding suitable organosilicon additives to the gypsum components. This method is also called mass hydrophobization.

Aqueous emulsions based mainly on polyalkylhydrogensiloxanes are currently used in connection with this mass hydrophobization (DE patent 1,223,287). It is also known to use polyalkylhydrogensiloxanes as such (DE-A-2 740 049). In the latter case, however, it is practically impossible to distribute the oil due to its insolubility in water to an aqueous gypsum mass consisting of mixing water and gypsum powder. As a result, despite the addition of organosilicon compounds, water-repellent properties are not obtained.

25 In practice, the use of polyalkylhydrogensiloxanes is substantially cheaper; to date, however, there has been no method of distributing the oil evenly over the gypsum mass.

Therefore, there is a need to find a suitable method for making porous gypsum plasterboards water-repellent by a mass hydrophobization method using dust alkyl hydrogen oxoxane, which method would be easy to implement without changing the production process and would be more economical than hitherto known methods. It is therefore an object of the present invention to provide such a method.

»2 76546 The present invention thus relates to a process for the preparation of water-repellent porous gypsum moldings by treatment with an alkylhydrogenpolysiloxane. The process is characterized in that a foam is formed from water, a surfactant and a polyalkylhydrogen siloxane and this foam is added to an aqueous gypsum mass consisting of mixing water, gypsum powder and possibly other conventional additives.

Compounds known per se are used as foaming surfactants, in particular substances belonging to the groups alkylaryl sulphates and alkylarylsulphonates, which, due to their strong foaming ability, cause the formation of pores.

The surfactant content of the frother is about 0.1 to 0.5 parts by weight per 100 parts by weight of the gypsum powder.

It has surprisingly been found that the described low surfactant content in the foamer is sufficient to emulsify the polyalkylhydrogensiloxane in water and to give this emulsion the stability necessary to distribute the emulsified silicone oil evenly in the gypsum mass. The addition of polyalkylhydrogensiloxane to the foam does not adversely affect the volume or pore size of the foam. This is all the more surprising given that silicone oils have a foam-lowering effect and sulfur-25 hard foams.

Suitable frothers are devices known per se, such as generators, power mixers, etc., which can be used to form foams in a simple manner.

Gypsum moldings made according to the invention, in particular porous gypsum boards, have excellent water-repellent properties.

Suitable substances for the process according to the invention are polyalkylhydrogensiloxanes, preferably polymethylhydrogensiloxanes having a viscosity of 10-50 cP at 25 ° C. The water repellency of the porous gypsum boards produced by the process of the invention appears 3 76546 immediately after the gypsum has hardened. No adverse effects on flexural strength, compressive strength and plate weight are observed.

The invention is further illustrated by the following 5 examples. The term "part" in the following examples means part by weight.

Example 1 150 parts of water, 1 part of ether sulphate ("Zeliquid LP 2") and 1.5 parts of polymethylhydrogensiloxane with a viscosity of 15 cP at 25 ° C are mixed together in a frother at 1200 rpm The surfactant-silicone foam obtained as described is added 700 parts of an aqueous gypsum mass consisting of 200 parts of water and 500 parts of gypsum powder containing 94.5% of calcium sulfate, 5% of impurities and 0.5% of a retarder and a thickener. The stirring time is 5 s at 400 rpm. The mixture thus obtained is poured into a metal cup. The water absorption capacity is determined after 48 hours by a dip test. In this experiment, the obtained porous kip-20 sheet is kept for 2 hours in a water bath. After this two-hour watering, the gypsum board has absorbed 6% by weight of water.

Example 2 (comparative example)

When the polymethylhydrogensiloxane of Example 1 is replaced with water and treated as described in Example 1, products are obtained which increase in weight by 39% by weight upon irrigation.

Example 3 (comparative example)

Addition of a corresponding amount of the polymethylhydrogensiloxane described in Example 1 directly to the aqueous gypsum mass and proceeding as described in Example 1 gives products with a mass increase of 39% upon wetting.

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Example 4 (comparative example)

When the amount of polymethylhydrogensiloxane to be added in Example 1 is increased to 5 parts, this amount of oil is added directly to the aqueous gypsum mass and the procedure is described in Example 5 to give products which increase the mass by wetting by 37% by weight.

Example 5

By reducing the amount of polymethylhydrogensiloxane added in Example 1 by 1 part and proceeding as described in Example 10 1, products are obtained whose weight increases by wetting by 9% by weight.

Claims (3)

5,76546 A process for the preparation of water-repellent porous gypsum moldings by treatment with an alkylhydrogen-5 polysiloxane, characterized in that a foam is formed from water, a surfactant and a polyalkylhydrogensiloxane and this foam is added to an aqueous gypsum mass consisting of a mixture of mixing water and optionally gypsum. Process according to Claim 1, characterized in that 0.1 to 0.5 parts by weight of polymethylhydrogensiloxane per 100 parts by weight of gypsum powder are used. Gypsum moldings, characterized in that they are made hydrophobic by the method according to claim 1 or 2.