GB2081242A

GB2081242A - Hydrophobic gypsum

Info

Publication number: GB2081242A
Application number: GB8123346A
Authority: GB
Original assignee: DESSAU ZEMENTKOMBINAT
Current assignee: DESSAU ZEMENTKOMBINAT
Priority date: 1980-08-06
Filing date: 1981-07-30
Publication date: 1982-02-17
Also published as: JPH0127013B2; FR2495131B1; DE3122009A1; SE8104708L; BE889867A; DD160516A3; GB2081242B; FR2495131A1; CS247105B1; ATA253981A; JPS5751161A; SE454354B; AT378950B

Abstract

Hydrophobic gypsum for making gypsum cardboard sheets, is produced by introducing a mixture of hemihydrate and/or anhydrite and a hard wax or a hard wax-solid hydrocarbon mixture in solid, finely-divided form into water and, after setting of the gypsum, subjecting to a chemical reaction by means of a thermal treatment in which partially chemical and partially physical bonds are formed between the polar groups of the hard wax and the cations of the binding agent.

Description

SPECIFICATION Process for the production of hydrophobic gypsum The present invention is concerned with a process for the production of hydrophobic gypsum for making gypsum cardboard sheets.

In the production of hydrophobic gypsum cardboard sheets for the construction industry, a gypsum-water slurry is applied to a continuously horizontally moving cardboard strip and covers the front side and the edges of the gypsum constructional sheet. The upper cardboard for the rear side of the gypsum constructional sheet is laid on to the gypsum-water slurry mass and stuck on to the projecting side regions of the lower cardboard. The gypsum-water slurry used is admixed, inter alia, with additives which impart water-repellent properties to the gypsum cardboard sheets after heating and drying so that these sheets can be exposed to higher moisture stressings than conventional constructional sheets.

Making constructional materials, such as gypsum, water-resistant is already a very well known problem. According to numerous publications, water-repellent materials are added to the gypsum and also, for example, to lime. The agents employed in recent years include, in particular, silicones, paraffin wax emulsions, ceresin, ozokerite, bitumen, resin emulsions and also water-soluble silicates. Apart from the watersoluble silicates, their effectiveness in imparting water stability is mainly attributable to their ability to make the movement of water mechanically difficult. The action of these additive materials depends mainly upon the size and distribution of the emulsion spheroids.In numerous Patents, water-repellent actions have been described for gypsum which are said to be achieved by combinations of added materials, for example wax and/or asphalt, as well as maize meal with potassium permanganate; water-insoluble thermoplastic materials with mineral oil and natural asphalt, coal tar and polymers based on vinyl acetate and vinyl chloride; a mixture of alkali metal resin soaps, a water-soluble alkaline earth metal salt and heating oil residue, pine resin with coal tar; a mixture of heating oil residues and resin; aromatic isocyanates and diisocyanates; organo hydrogen polysiloxanes; wax-asphalt emulsions; polyvinyl alcohol additive and metal components, as well as the addition of an aqueous emulsion of asphalt/wax in combination with calcined gypsum and small amounts of a borate compound.

In the case of the processes for the impregnation or hydrophobing of gypsum cardboard sheets, the adhesion problems of cardboard and gypsum additionally play an important part. It has been shown that the preponderant number of the hydrophobing agents counter a firm bond or, by the action of moisture, subsequently lead to peeling phenomena of the cardboard from the gypsum core and, furthermore, besides the adhesion problems, unfavourably influence the quality of the cardboard. In addition, in the case of gypsum cardboard sheets, reduced strength values are ascertainable and the water resistance decreases in the course of time.

Federal Republic of Germany Patent Specification No. 1 266,200 describes a process for the production of paper-coated, waterrepellent gypsum constructional sheets. A plaster slurry is shaped between paper strips and a waterrepellent mass used therefor comprising an emulsion of tall oil, a wax-like aliphatic hydrocarbon, as well as a drying oil and/or a fatty acid ester of pentaerythritol. This emulsion is applied to the papers and heated in an oven with a temperature of at least 1 300C. However, this surface impregnation involves certain disadvantages since the gypsum, in the case of damage to the paper layer or due to seals of pipes passing through the constructional sheet, is disadvantageously affected by atmospheric moisture or condensed water.

According to the processes described in German Democratic Republic Patent Specifications Nos. 125,861 and 130,133, finished parts based on concrete, gypsum and clay are, after drying and before cooling, impregnated with paraffin in order to make them watertight.

A process of melt impregnation, with the same object in view, is disclosed in German Democratic Republic Patent Specification No. 132,426.

Paraffin particles or paraffin-bitumen granulate is added to the starting materials and the finished parts are dried after production. The molten impregnation agent penetrates into the pores, cools and blocks up the pores. However, in practice it has been found that a sufficient water resistance is, in the case of gypsum, only achieved by the addition of about 1 5% by weight of paraffin. Due to the high material costs, this process is uneconomic.

Federal Republic of Germany Patent Specification No.2,614,394 also uses, inter alia, paraffin. In order to guarantee a more economic use, emulsions are prepared and employed. The proportion of solid materials in the hydrophobing agent, referred to the gypsum hemihydrate, is 5% by weight. Inter alia, paraffin emulsions are used, the production and composition of which are claimed. For the gypsum cardboard sheet producer, the disadvantages include having to provide an emulsigying plant, having to store at least five starting chemicals and having to use qualified personnel for the complicated technological process of measuring out the components.If, however, the emulsion is obtained from a chemical factory, up to 65% of water is to be additionally transported; furthermore, the stability and storability of these emulsions is limited and the storage tanks must be insulated during cold weather.

It is an object of the present invention economically to produce, with the smallest possible use of hydrophobing additives, a completely hydrophobic gypsum cardboard sheet which possesses construction-technical properties which are at least as good as those of conventional gypsum cardboard sheets and which, in addition, can stiil be used in places with relative atmospheric humidities of > 75%.

The problem forming the basis of the present invention is to provide a process for hydrophobing gypsum, especially for the production of gypsum cardboard sheets, which ensures a good adhesion between the gypsum and the cardboard and also provides a good resistance to occasional high moisture contents of the surrounding air.

Thus, according to the present invention, there is provided a process for the production of a hydrophobic gypsum for making gypsum cardboard sheets, wherein a mixture of hemihydrate and/or anhydrite and a hard wax or a hard wax-solid hydrocarbon mixture is introduced in solid, finely-divided form into water and, after setting of the gypsum, is subjected to a chemical reaction by means of a thermal treatment in which partially chemical and partially physical bonds are formed between the polar groups of the hard wax and the cations of the binding agent.

The gypsum setting process and the wet adhesion of gypsum/cardboard are unaffected.

Dihydrate, hard wax and water are simultaneously present. In the thermal treatment, the temperature in the drier used is so controlled that, in the case of a temperature of the material of up to about 600C., the gypsum cardboard sheet dries out almost completely, with the formation of hollow spaces. The sheets are then briefly heated to about 85"C., in order to melt the hard wax or the hard wax-containing mixture which then, assisted by the vapour pressure of the residual water, penetrates into the hollow spaces present.

Chemical reactions result which lead to the formation of bonds between the hydrophobing and binding agents. These reactions involve a partial saponification, which preferably takes place at a temperature above 600 C., in which the carboxyl groups of the hydrophobing agent react with cations from the binding agent under the conditions of the short heating phase. An additional improvement of the adhesive bond is achieved by the formation of physical bonds between the hydrophobing additive and the binding agent, due to the polar character of the ester grouping, the good impregnation effect due to the use of mixtures corresponding, in the case of the combination, to the amount ratio, hydrophobic as well as hydrophilic groups being bound.

The water-repellent action results from an envelopment of the dihydrate structure by the hydrophobing agent, the capillary absorbency which is typical for gypsum thereby being considerably reduced or largely suppressed.

The additive used according to the present invention are hard waxes, such as montan wax and/or carnauba wax, alone or in combination with solid hydrocarbons, the solidification point of which is > 500C. Having regard to the process used for the production of gypsum cardboard sheets, we have found that the hydrophobing agent should be used in the form of a fine granulate, preferably with a particle size range of from 0.063 to 0.8 mm. and more preferably of from 0.1 to 0.5 mm. Special hydrophobing effects are achieved when using mixtures containing at least 109/0 of solid hydrocarbons, since the lowering of the melt viscosity which this causes provides a favourable prerequisite for the abovedescribed reactions.

The hard wax or hard wax-containing mixture used has a melting point of preferably from 65 to 900C. and more preferably of from 75 to 800C.

with a volume expansion in the melting region of preferably from 5.0 to 12.8% and more preferably of from 6.5 to 11.5%, with a melt viscosity at 900C. of preferably from 1 5 to 75 cP and more preferably of from 25 to 55 cP. Furthermore, the hard wax or hard wax-containing mixture used preferably has an acide number of > 5 or more preferably of from 10 to 35.

For the controlled achievement of favourable constructional physical characterising values of the gypsum cardboard sheets, the composition of the hydrophobing additive can be varied within the range of 90 to 25% by weight of hard wax and 10 to 75% by weight of solid hydrocarbons, depending upon the modifications used of the CaSO4-H20 system. The proportions used in each case are melted, mixed in a liquid state and sprayed to give a fine granulate.Due to the use of a fine granulate of a mixture of hard wax with solid hydrocarbons, for example in the case of a paraffin wax mixture, the melting point is brought into a thermal treatment range which is favourable for the gypsum cardboard sheet production, as described, the melt viscosity of the hydrophobing agent is lowered, a penetration into the hollow spaces formed by the escaping water is made easier and favourable conditions for a rapid saponification are initiated. The amounts of additive necessary are thereby reduced, in comparison with the amounts of hydrophobing agents which are conventional in practice and described in the literature, in the case of the treatment or production of gypsum cardboard sheets to a hitherto unusually low amount of < 4% by weight. Hydrophobic gypsum cardboard sheets were produced according to the present invention with proportions of about 3% by weight of paraffin wax mixture, referred to the amount of gypsum hemihydrate employed. The properties of the sheets so produced correspond to those of a conventional constructional sheet and, in addition, they had an excellent water resistance. This was confirmed by measurements on 40 samples (400 mm.x 300 mm.) which were taken at different places from about 100 m2 of gypsum cardboard sheet removed during the course of a normal production. The samples were placed in water and the average take up of water was 1.48% after 2 hours, 4.1% after 24 hours, 6.5% after 96 hours and 6.8% after 120 hours, the' water being changed every 24 hours.In addition to these favourable values, the hydrophobing agent simultaneously provided a good edge strength in the case of brief overheating in the drier. Further improvement of the strength, water resistance and like properties are possible under the previously mentioned conditions when the amount of water of the gypsum binding agent used for the achievement of the working consistency is lowered and thus a thicker texture is achieved. This is, for example, possible by the use of a-hemidydrate or anyhydrite-ll binding agents or by the addition of liquefiers, for example melamine resins, to the normal hemihydrate gypsum. However, positive results are also achieved when foam or poreforming materials are added to the gypsum slurry and the gross density of the finished sheet is reduced. Admittedly the bending strength is somewhat reduced in comparison with conventional constructional sheets but the hydrophobic character of the light sheets so produced completely satisfies the requirements for using the sheets in moist conditions.

Claims

1. Process for the production of a hydrophobic gypsum for making gypsum cardboard sheets, wherein a mixture of hemihydrate and/or anhydrite and a hard wax or a hard wax-solid hydrocarbon mixture is introduced in solid, finelydivided form into water and, after setting of the gypsum, is subjected to a chemical reaction by means of a thermal treatment in which partially chemical and partially physical bonds are formed between the polar groups of the hard wax and the cations of the binding agent.

2. Process according to claim 1, wherein the hard wax-containing mixture consists of 90% to 25% by weight of hard wax and 10% to 75% by weight of solid hydrocarbons.

3. Process according to claim 1 or 2, wherein, in the case of hard wax-containing combinations, the individual components are mixed in a molten state.

4. Process according to any of the preceding claims, wherein the melt of hard wax or of the hard wax-containing mixture is sprayed to give a fine granulate.

5. Process according to any of the preceding claims, wherein the hard wax or hard waxcontaining mixture used has an acid number of > 5 and, under the conditions of the thermal treatment of the hardened out gypsum, undergoes a partial saponification reaction with the binding agent.

6. Process according to claim 5, wherein the hard wax or hard wax-containing mixture used has an acid number of from 10to35.

7. Process according to claim 6, wherein the partial saponification takes place at a temperature above 600C.

8. Process according to any of the preceding claims, wherein the hard wax or hard wax-solid hydrocarbon mixture is used in granulated form with a particle size range of from 0.063 to 0.8 mm.

9. Process according to claim 8, wherein the hard wax or hard wax-solid hydrocarbon mixture is used in granulated form with a particle size range of from 0.1 to 0.5 mm.

10. Process according to any of the preceding claims, wherein the hard wax or hard waxcontaining mixture has a melting point of from 65 to 900C. with a volume expansion in the melting region of from 5.0 to 12.8% and a melt viscosity at 900C. of from 15 to 70 cP.

11. Process according to claim 10, wherein the hard wax or hard wax-containing mixture has a melting point of from 75 to 880C.

12. Process according to claim 10 or 11, wherein the hard wax or hard wax-containing mixture has a volume expansion in the melting region of from 6.5 to 11.5%.

13. Process according to any of claims 10 to 12, wherein the hard wax or hard wax-containing mixture has a melt viscosity at 900C. of from 25 to 55 cP.

1 4. Process according to any of the preceding claims, wherein, for the achievement of a particularly dense structure, a liquefier is additionally used.

1 5. Process according to claim 14, wherein the liquefier is a melamine resin.

1 6. Process according to any of the preceding claims, wherein, for the production of light, hydrophobic gypsum cardboard sheets, a pore- or foam-forming material is additionally used.

17. Process according to claim 1 for the production of a hydrophobic gypsum, substantially as hereinbefore described.

1 8. Hydrophobic gypsum, whenever produced by the process according to any of claims 1 to 1 7.

1 9. Gypsum cardboard sheets, whenever produced with a hydrophobic gypsum according to claim

18.