A filling composition

Technical field

The present invention relates to a filling composition containing a filler which comprises gypsum and a solvent.

The object of the invention is to provide a substantially shrink-free filling composition for the purpose of rationalizing the work of filling cracks, smoothing surfaces and the like in different structures, such as building structures and wooden objects.

Background art

When treating the walls of new building structures, for example in the erection of new buildings, prior to painting and/or wall-papering the walls, and when renovating old walls prior to applying fresh paint and wall-paper, it is known to fill in cracks and hollows etc., so as to obtain a smooth undersurface. To this end there is used a product which, although containing no sand, is called a sand-filler. It is a matter of fact that when a filler is applied too thickly, the filler will shrink excessively and will sink and/or move away from the surfaces defining the crack or hollow. Furthermore, the applied filler will often crack quite profusely, when drying out. This means that the crack or hollow must be built-up gradually, with a multiplicity of superposed filler layers, if a satisfactory finish is to be obtained. The work of filling such cracks and hollows is done manually and is consequently very expensive. Moreover, such work is extremely time demanding and can delay completion of the preparation work by several days, since the filler is slow to dry out.

Consequently, there is a need for a filling composition, which exhibits the minimum shrinkage tendencies, and which will dry out more quickly than known filling compositions.

Description of the present invention

It has now surprisingly been found possible to produce, in accordance with the invention, a filling composition which will fulfil these needs. According to the invention, this composition is characterized by a mixture of calcium sulphate-dihydrate and calcium sulphate-anhydrite in ratios of 90-0:10-100, to which there is added water or an aqueous solution in an amount of 50-110 parts by weight per 100 parts by weight of calcium-sulphate mixture.

Other characterizing features of the invention are set forth in the following claims.

The calcium sulphate-dihydrate used may either be in the form of particulate dihydrate gypsum or acicular dihydrate gypsum, so-called fibrous gypsum.

Dihydrate fibrous gypsum can be produced by dissolving gypsum to saturation in a sulphuric acid solution containing 5-15% H₂SO₄ at an elevated temperature of 70-100⁰C, whereafter the saturated solution is permitted to cool to ambient temperature, to crystallize-out acicular calcium sulphate-dihydrate crystals, having lengths of 10-100_/um and thicknesses of about 1_/um, whereafter the crystals are isolated and dried.

The anhydrite gypsum, which is present in particle form, with a particle size of less than 30_/um, more often 2-15/um, is prepared by dissolving dihydrate gypsum and/or hemihydrate gypsum to supersaturation in heated sulphuric acid solution having a concentration greater than 35% H₂S0₄, whereafter the anhydrite gypsum is permitted to precipitate, and is then isolated.

The particulate anhydrite gypsum can also be replaced with an anhydrite fibrous gypsum, in which case the dihydrate gypsum should be present in particle form. Such anhydrite fibres can be produced by calcining acicular <x-hemihydrate gypsum crystals, these being obtained by oxidizing calcium sulphite-hemihydrate (DE-A-2 613 651).

In order to obtain a higher setting effect, i.e. a shorter setting time, 4% potassium sulphate and/or ammonium sulphate calculated on the amount of gypsum used can be mixed in. In this case, the potassium sulphate and/or some other hardening salt, such as aluminium sulphate, is suitably dissolved in the water used for preparing the paste/composition.

It has showed particularly preferable to use phosphoric acid as a corrosion inhibitor, when an acidic hardening agent is used. One known disadvantage with prior known and used filling materials is that nails and screws coming into contact with the filling material will rust and thereby miscolour the filling material and/or any wall paper or paint covering it.

The amount of phosphoric acid used is depending on the pH of the gypsum used. Thus, phosphoric acid can be added to a pH of down to 3, but an adjustment of pH down to 4-5 is often enough.

In order to increase the ability of the composition to adhere to the surfaces to which it is applied, wood fibres, for example in the form of groundwood pulp (WP) can be incorporated in the composition. These wood fibres will also bind the gypsum to some extent.

The wood pulp is suitably added in an amount of up to 1% of the amount of gypsum used.

For the purpose of inhibiting the setting reaction and of controlling the same, a polyelectrolyte, such as polyamide resin, can be incorporated in the composition. One such suitable product in this respect is BOZEFLOC®, which is mixed with the composition in an amount of up to 1% of the amount of gypsum used. The polyelectrolyte is suitably contained in the aforementioned aqueous solution which, as beforementioned, may also contain potassium sulphate. The groundwood pulp may also be incorporated in the aqueous solution.

For the purpose of preventing attack by mould fungi, a suitable fungicide can be added, such as sorbic acid or sodium bensoate or some other suitable fungicid. Instead of using a polyelectrolyte, or in combination therewith, cellulose starch, microcrystalline cellulose, carboxyethyl cellulose and hydroxyethyl cellulose can be admixed in the composition, such as MODOCOLL ^R .

In such cases, a carrier in the form of dolomite lime may be admixed with the composition.

When wishing to produce a fine filler, a suitable water-based acrylate pigment and/or latex pigment can be used as a base.

Example

A dry mixture was prepared from 65 g calcium sulphate-dihydrate and 35 g of calcium sulphate (anhydrite) having a particle size 2-20_/um. 80 ml of a ₁%-BOZEFLOC ^R -solution containing 1.5 g of potassium sulphate were added to this mixture. The total mixture was carefully stirred to form a smooth paste.

The shrinkage tendencies of this filling composition were tested in an annular test mould having an inner diameter of 45 mm and a height of 10 mm. Two commercially available products A and B were also tested for comparison purposes, the product B being a so-called latex filler.

The dihydrate gypsum used in the above example can be replaced with a dihydrate fibrous gypsum, which affords slightly stronger binding properties and produces a more porous product.

The anhydrite enables a coarse-grain composition of dihydrate gypsum to be used, while still obtaining a highly smooth composition.

The cellulose, the groundwood pulp, can be admixed with the substance above, whereupon the substance would adhere more strongly to paper, board and wood.

The polyelectrolyte increases the smoothness of the composition, and enables a plastics trowel to be used, without the composition adhering thereto.

One advantage afforded by the filling composition according to the invention is that it is not sensitive to temperature, i.e. its drying rate is not affected by the ambient temperature.

During the reaction between dihydrate gypsum and any potassium sulphate present water is formed and absorbed by the anhydrite gypsum, to cause swelling, which is one reason why the composition exhibits reduced shrinkage tendencies.

The filling composition dries to form a hard surface, which can readily be smoothed down.