NO791998L - PROCEDURE FOR MAKING A PLASTERBOARD - Google Patents

Description

"Procedure for the manufacture of

a plasterboard".

This invention relates to improvements in the manufacture of plasterboard. '

Gypsum, or calcium sulfate hemihydrate, exists in several forms; the present description deals with two of these, namely a-hemihydrate and B-hemihydrate. They can be produced from natural or chemically produced gypsum, either by calcination, whereby the (3-hemihydrate is obtained), or by treatment in an autoclave, whereby the α-hemihydrate is obtained.

The production of a-hemihydrate requires less energy input than the production of B-hemihydrate, the latter being correspondingly more expensive than the a-form. It will thus be economically advantageous to produce plasterboard from the a-hemihydrate, but when attempting to do this in a similar way to the conventional method, i.e. the B-hemihydrate method, several difficulties arise, as explained below.

It is common to produce the plasterboard material

from a slurry of B-hemihydrate. It is then appropriate to prepare a slurry that contains 0.7-0.9 parts by weight of water per 1.0 part by weight of B~hemihydrate with the addition of foam. Such a slurry contains sufficient water for the development of strong paper-to-core bonds in the plasterboard. It also has a suitable viscosity for maintaining the added foam in a homogeneous base mass. It is conventional to incorporate foam into the slurry so that the resulting plasterboard has an acceptable core density, i.e. that a finished

plate with a thickness of 9.5 mm has a weight per square meter of 6.5-10.5 kg (the values have been obtained as a combination of the regulations according to B.S.S. 1230 and DIN 18180).

When α-hemihydrate is used instead of 3-hemihydrate, a vanh/gypsum ratio in the aforementioned range of 0.7-0.9 to 1.0 will give an insufficiently viscous slurry for foam retention. As a result, part of the added foam will be lost, the board density will be higher than desired, and the board will be non-homogeneous over its thickness. A solution to the difficulty is to use a somewhat higher water/gypsum ratio (above 1:1) to reduce the density of the final board to an acceptable level. This measure, however, has the disadvantage that it entails increased energy costs due to the increased amount of free water in the hardened mass, which water must be removed during drying to obtain the final plasterboard. The fact that the resulting slurries will be very fluid also makes them difficult to work with. On the other hand, if the water/gypsum ratio is reduced below 0.4, considerable difficulty is encountered in developing and maintaining acceptable paper-to-core bonds, while on the other hand very satisfactory foam retention and appropriate slurry density can be achieved. However, low water/gypsum ratios with consequent low drying costs are advantageous compared to the high ratios required when using (3-hemihydrate.

It is an object of the invention to remove the difficulties associated with plasterboard production from α-hemihydrate, by specifying preferred process conditions which reduce the above-mentioned difficulties.

It has now been found that a plasterboard with acceptable board density as well as good plaster-to-paper adhesion can be produced by using a foamy slurry comprising water and α-hemihydrate in a weight ratio in the range of 0.4:1 to 1:1 provided that the slurry, in cases where the range is 0.65:1 to 1:1, also contains a clay capable of making the slurry thixotropic or pseudoplastic, which contributes to the retention of the foam in a homogenous base mass.

The invention thus provides a method for the production of gypsum boards, which method involves preparing a gypsum slurry by mixing water with α-hemihydrate in a ratio in the range of 0.4-1.0 parts by weight of water per part by weight α-hemihydrate and, when said ratio is in the range 0.65-1.0, furthermore incorporating into the slurry a clay capable of making the slurry thixotropic or pseudoplastic, also incorporating a conventional foam into the slurry.

The clay is preferably a gelling clay. Particularly suitable clays can be selected from the class consisting of attapulgite, bentonite and sepiolite. The amount of clay, depending on the weight of the slurry, is appropriate in the range 0.1-5%.

A preferred method of the invention includes the successive steps of adding the clay to the calculated amount of water, agitating the resulting mixture to produce a uniform suspension, mixing in the α-hemihydrate and incorporating an appropriate volume of foam. At ratios between water and the hemihydrate below 0.65 (ie 0.4-0.65), clay is not necessary, and drying costs are low. When clay is used, the ratio range 0.65-1.0 becomes available for plasterboard production.

The foam is preferably incorporated in a proportion that is in the range of 0.1-0.7 liters of foam per kg slurry. Suitable foams, or foam-forming surfactants, are commercially available.

The following examples of embodiments of the invention will further illustrate this.

EXAMPLE 1

Gypsum board material was produced as follows:

700 kg of calcium sulfate-α-hemihydrate, which had been prepared by treatment in an autoclave, was mixed with 350 kg of water using a mixing apparatus common for slurries. The weight ratio between water and gypsum was thus 0.5. Foam with a specific gravity of 0.12 was added to the slurry in amounts corresponding to 0.3 liters of foam per kg a-hemihydrate (ie 0.2 liters per kg slurry, or 0.2 m per 1,000 kg slurry) whereby a homogeneous foam-gypsum slurry is obtained.

The slurry was applied in a conventional manner

a continuously traveling paper path. Another paper web was laid on top of the layer of slurry in a continuous manner, whereby a continuous strip of wet plasterboard material was obtained. The rest of the process is well known to those skilled in the art.

The final plate had a weight of 7.5 kg/m 2, which shows that approx. 100% by volume additional foam was formed in the slurry mixer. The paper-to-core bond was very good, and the sheet material was undulating.

The plate dimensions were 2480 mm x 1200 x 9.5 mm. The distribution of bubbles through the plaster core was particularly good.

The average breaking strength for the plate material was

650 Newtons in the L direction and 210 Newtons in the X direction.

EXAMPLE 2

100 kg of attapulgite clay (sold by Floridin Inc., USA under the trade name "Min-U-Gel" 200) was added to 4300 kg of water in a vessel equipped with agitation equipment. The suspension was vigorously agitated for approx. 1 hour and was then transferred to a slurry mixing facility, where 6300 kg of α-hemihydrate, retarded to a predetermined degree, was added and the whole mixed into a homogeneous slurry in a continuous manner, while it was carried forward to a board manufacturing plant and there converted into gypsum board material by a process known in and of itself. The water/gypsum ratio used in the production of the slurry was thus 4300/6300 or 0.68.

Using this method, 285 boards were produced with a width of 1200 mm, a thickness of 9.5 mm and a length of 2480 mm, and with a weight of 9.0 kg/m 2.

The advantageous physical condition of the slurry was evident immediately downstream of the slurry mixer, as the slurry remained motionless on the lower layer of paper, ready for application of the upper layer. It had little or no tendency to flow over the edges of the paper. The thickness of the slurry was uniform and did not change

later.

For comparison purposes, the procedure described in the example above was repeated with a smaller charge / where the addition of the gelling clay and the subsequent agitation of the clay suspension were omitted. The viscosity of the slurry at the time it was agitated and applied to the paper was found to be about the same as in this example. However, the viscosity after passage through the mixer was found to have increased to a lesser extent than the viscosity of the slurry produced using clay,

due to the far higher pseudoplasticity of the latter slurry. The low pseudoplasticity caused the slurry to flow over the edges of the paper.

EXAMPLE 3

The procedure described in the first paragraph in example 2 was repeated. In the final or slurry mixing stage, and immediately before the slurry was applied to the paper web on the forming table, a foam was produced by blowing air through a 1.34% solution of Millifoam, a surfactant, added to the slurry in a

-2 3

amount corresponding to 16.2 x 10 m of foam per 1000 kg slurry.

The finished plasterboard in this case had a weight of 7.5 kg/m 2 and the dimensions 2480 mm x 1200 mm x 9.5 mm, as before. The air bubbles were evenly distributed throughout the thickness of the plate. The upper layer of paper was glued as firmly as the lower one.

For comparison purposes, a smaller batch of slurry was prepared and treated as described in the example above, however without the use of attapulgite clay. The viscosity of the slurry on the forming table was significantly lower than the viscosity of the slurry in Example 3, so that some floated over the edges of the lower layer of paper. The foam underwent considerable separation under the action of gravity before curing could take place, and the upper layer of paper adhered poorly. Some foam that reached the upper surface of the slurry was lost, and the thickness of the plate began to decrease. To counteract this, the belt speed was reduced until the desired plate thickness of 9.5 mm was re-established, but the formed plates no longer had the intended weight

of 7.5 kg/m , but a significantly higher weight. Thus, an unnecessarily large amount (on a weight basis) of gypsum was used for one

equivalent production expressed as plates per time unit, and additional production time per plate went with compared to the results in example 3.

To further elucidate the effect of adding clay to plaster slurry, the water requirement was determined by the method according to British Standard Specification No. 1191: Part 1, for a gypsum requiring little water, with varying proportions of attapulgite clay. Before making the gypsum-water slurry, the clay was mixed with the water in a powerful mixer. The results are shown in the table below.

It will be seen that the water requirement increases with the proportion of gelling clay in the slurry.

Further determination of the proportions of intentionally thickened slurries, including slurries prepared according to the invention, was performed as follows: Gypsum slurries were prepared from autoclaved gypsum (water requirement, 39) and water at a water/gypsum weight ratio of 0.65. In all experiments, the slurries were retarded with sodium citrate to prevent hardening during the viscosity measurement. Before the formation of the slurry, various additives were mixed with the water as shown in the table below.

The viscosity was then measured using a Brookfield viscometer over a range of spindle rotation speeds. The results are shown in the table, where the additive quantities are given in weight% of dry α-hemihydrate.

The far faster increase of the apparent . viscosity with decreasing spindle rotation speed is indicative of the higher pseudoplasticity of gypsum slurries containing attapulgite clay, compared to slurries containing no additive. Other gelling clays, including bentonite and sepiolite, are essentially as effective as attapulgite.

Methyl cellulose is a known thickener for plaster slurries. For this reason, a slurry containing 0.25% methylcellulose (MC-3000) was included. The results show that this material has a similar effect on the pseudo-plasticity of gypsum slurries such as attapulgite. However, attapulgite is available at a price of approx. 1/20

of the price of the MC-3000. It will be understood from this that the method according to the invention is of considerable technical-economic value.

Millifoam, which is mentioned in example 3, is supplied

of Millimaster-Onyx International Inc., 23 Just Road, P. 0.

Box 832, Fairfield, New Jersey, USA.

Claims (9)

1. Method for the production of gypsum board material, characterized in that a gypsum slurry is produced by mixing water with α-hemihydrate in a ratio in the range of 0.4-1.0 parts by weight of water per part by weight α-hemihydrate and, when said ratio is in the range of 0.65-1.0, further incorporating into the slurry a clay capable of making the slurry thixotropic or pseudoplastic, a conventional foam also being incorporated into the slurry. 2. Method according to claim 1, characterized in that the clay is a gelling clay. 3. Method according to claim 1 or 2, characterized in that the clay is attapulgite and/or bentonite and/or sepiolite. 4. Method according to one of the preceding claims, characterized in that the proportion of clay is in the range 0.1-5% by weight. of the slurry. 5. Method according to one of the preceding claims, characterized in that clay is added to the calculated amount of water, the resulting mixture is agitated to produce a uniform suspension and then the α-hemihydrate is mixed in. 6. Method according to one of the preceding claims, characterized in that the ratio between water and hemihydrate is in the range of 0.4-0.6 parts by weight of water per weight part hemihydrate, and that clay is not incorporated into the slurry. 7. Method according to one of the preceding claims, characterized in that the foam is mixed with the slurry in a ratio chosen so that it gives a predetermined plate density. 8. Method for producing plasterboard material mainly as described with reference to the examples. 9. Plasterboard material produced by a method according to one of the preceding claims.