GB2248448A

GB2248448A - Lightweight gypsum product and apparatus for manufacturing the same

Info

Publication number: GB2248448A
Application number: GB9119878A
Authority: GB
Inventors: Robert Watt Hughes
Original assignee: MITCHELSTON BUILDING PRODUCTS
Current assignee: MITCHELSTON BUILDING PRODUCTS
Priority date: 1990-10-04
Filing date: 1991-09-18
Publication date: 1992-04-08
Anticipated expiration: 2011-09-18
Also published as: GB9119878D0; GB2248448B

Abstract

A gypsum product comprises lightweight porous filler and hydrated calcium sulphate. The filler used is preferably pumice, has a dry bulk density of less than 550 kg/m<3> and comprises 50% or more of the volume of the product. A process for producing the gypsum product comprises adding the filler to a slurry of calcined gypsum and water. The filler is preferably added in the damp or dry state. Apparatus for forming a building board from the product comprises a mixer 10 with a screw conveyor / mixes 14. Inlets 16, 18, 20, 22, 24 are provided for water, foam, calcined gypsum, accelerator and filler, respectively. The mixture is extruded on to carrier paper 28 and passes beneath roller 30 to yield a board, which may comprise a top paper 32. <IMAGE>

Description

LIGHTWEIGHT GYPSUM PRODUCT AND PROCESS FOR MANUFACTURING THE SAME The present invention relates to a lightweight gypsum product, particularly a fire-resistant gypsum building board, and to a process for manufacturing such a product.

Plasterboard is commonly used in the building industry.

It is usual to manufacture plasterboard by mixing calcined gypsum, so called stucco gypsum, which consists mainly of calcium sulphate hemihydrate (CaS04.

it20), with a filler and/or additives and sufficient water to produce a slurry, spreading the slurry onto a carrier to form a board, allowing the gypsum to hydrate, and thereafter heating the board to drive off excess water.

The known processes for making plasterboard require that sufficient water be added to calcined gypsum to form the slurry so that fully hydrated gypsum (CaS04.2H20) is formed. However, in order to produce a slurry which is sufficiently liquid to allow it to be poured and spread on to a carrier, excess water in an amount many times the quantity of water required for the hydration of gypsum is required. The excess water must then be removed in a drying process which is a costly part of the production process.

It is known that if more of the gypsum is replaced with inert filler correspondingly more water must be added to produce a slurry which has the desired fluidity. The consequence is that still more water needs to be dried off since a smaller quantity is consumed in the hydration of gypsum.

Plasterboard made only of stucco gypsum, in spite of having good short term fire-resistance, decomposes with prolonged heating. On the other hand, it is thought that high proportions of filler would reduce fireresistance of the board because there would be less water of crystallisation in the board.

When a filler is used which has a density which deviates from the density of the gypsum/water suspension, unwanted settling of the heavier filler tends to occur whereas the lighter filler floats up in the mixture.

In order to improve the mechanical properties of the board it is common to include additives, such as chopped fibres to improve the strength of the board. However, such additives are expensive and therefore add to the cost of the final board.

An object of the present invention, therefore, is to provide an improved gypsum product which at least reduces the problems encountered with the prior art and to provide a simplified process for the manufacture of such a gypsum product.

Accordingly, the present invention provides a gypsum product comprising a lightweight porous filler and hydrated calcium sulphate. Preferably, the filler has a dry bulk density of less than 550 kg/m3, more preferably less than 450kg/m2.

Advantageously, the dry bulk density of the filler lies between 300 and 400 kg/m3. The weight per volume of the filler is lower than the weight per volume of the gypsum since a significant volume of the filler is occupied by voids in the form of open and closed vesicles. In general, however, the filler is a substantially open cellular material.

Generally, the filler comprises 50% or more of the volume of the product or 30% or more by weight (dry material) of the finished product, and preferably up to 60% by weight of the finished product. With such proportions of filler to gypsum, the gypsum acts as a binder such that the filler is bonded together with the gypsum in the finished product.

The filler is preferably particulate with grain size in the board lying between 0.1 and 5.0 mm, preferably 0.5 and 3.0mm. Maximum grain size used depends upon the thickness of the board. For-example, a 13 mm thick board would tolerate a maximum grain size of 5mm, and a 6mm thick board would tolerate a maximum grain size of 2.5 mm. The particular grain size or mixtures of grain sizes used can be varied to achieve optimum saturation of the board with the inert porous material and selected according to the required thickness, weight, flexibility and fire resistance of the board.

The filler may be pumice or other porous volcanic rock.

A most preferred material for the filler is palagonite or tephra which is a super light pumice found in Iceland having essentially open vesicles. The porosity of this pumice is such that it can absorb up to its own dry weight in water but in its natural environment tephra usually contains about 46 to 50% saturation water by weight of wet pumice. The use of tephra as mined is particularly suitable for the present invention. The pore volume of raw or unprocessed tephra is related to its grain size. On average, a grain having a size of between 0.5mm and 4.0 mm comprises approximately 60% open pore volume and approximately 10% closed pore volume. The surface of the pumice is open and this open surface allows good adhesion or bond strength between filler and binder. Moreover, the pumice has relatively few impurities in comparison to other pumices.

The present invention further provides a process for producing a gypsum product, comprising mixing calcined gypsum, stucco or the like, with water to form a slurry and adding a porous filler to the slurry. Preferably, the porous filler has a dry bulk density of less than 550kg/m3, more preferably less than 450kg/m3.

The filler may be in a damp or dry state. When in the damp state, the filler may contain water in an amount of up to 50% by weight of the damp filler, preferably from 0 to 35%.

The ratio of the weight of gypsum to added water in the process is generally 1:1. The amount of water present in the damp filler is believed not to be sufficient to migrate to contribute to the hydration of gypsum.

Further steps in the process include extruding the mixture onto a carrier to form a board and drying the mixture after hydration of the calcined gypsum. The core material of the board may be faced on one or both major surfaces with a porous or semi-porous paper or the like.

The apparatus used for carrying out the above process provides yet a further aspect of the present invention and comprises mixer means comprising a body having inlet means for the ingredients to be mixed and outlet means for the mixed ingredients, wherein the gypsum and water inlet means and the filler inlet means are provided at different parts of the mixer body.

Preferably, the mixer means is in the form of a screw conveyor and individual inlet means are spaced longitudinally on the mixer body for each of the ingredients to be mixed. The screw conveyor may have a fine pitch at the water and gypsum inlet portions of the body and a coarse pitch at the filler inlet portion of the body. A plurality of mixer means may be used together.

By replacing a substantial proportion of gypsum building board with filler, the amount of water required for the hydration of gypsum is reduced. When using a damp filler, it has been found that the proportion of added water can be significantly reduced in comparison to when dry filler is used. Thus, a proportionate part of the excess water is eliminated, reducing the drying heat requirements. Because the filler used has a lower weight per volume in comparison to the gypsum, the resultant product is lighter than conventional gypsum products.

Thus, whereas conventional gypsum plasterboard has a density in the region of 950kg/m3, the gypsum product defined above containing 30%, 45% and 60% dry weight tephra has a density in the region of 780, 700 and 610kg/m3, respectively.

Moreover, the fire resistant and mechanical characteristics of the product are far superior to those of conventional gypsum products. The porous filler counteracts the tendency of the hydrated gypsum to decompose upon repeated heating and also is bound with the gypsum so as to provide rigidity and stability of the product which results in reduced shrinkage of the product in a fire condition.

Since a large proportion of the gypsum is replaced with a less expensive material and the product has sufficient strength so as not, in most cases, to require additives to improve its strength, the resultant board is cheaper than its prior art counterpart. Moreover, it has been found that when the volume of the filler is more than 50% of the total volume of the mixture, unwanted settling of the filler in the mixture does not occur.

An embodiment of the invention will now be described, by way of example, with reference to the single drawing which is an apparatus for use in manufacturing a gypsum product in accordance with the present invention.

Referring to the single Figure, apparatus for forming a building board comprises a mixer, indicated generally by reference numeral 10, having a longitudinal body 12 and a screw conveyor/mixer 14 operable within the body 12.

The pitch of the screw conveyor 14 varies along the length of the body 12, having a fine pitch 14A at one end portion of the body 12 and a course pitch 14B at the other end portion of the body 12. Inlets 16, 18, 20, 22 and 24 are provided on the body for water, foam, gypsum, accelerator and filler, respectively. A manifold 26 is provided at the delivery end of the body 12 which serves to shape and size the mixture as it is extruded under pressure by the screw conveyor onto the carrier paper 28.

In operation of the apparatus, a metered amount of water capable of activating the gypsum and providing flow to the final slurry is added through inlet 16 to the body 12. Wet additives such as retarder and foam are added through inlet 18 to the water in the body 12 which passes through the mixer body 12 via the screw conveyor 14 to be dosed with calcined gypsum through inlet 20 and thereafter dry additives such as accelerator are added through inlet 22. Other possible additives include chopped strand mineral fibre (alkaline resistant), mineral fibre tissue, netting or scrim (alkaline resistant), detergent, starch, and synthetic oils. The additives are used in small quantities either independently or in combination and serve to give the board better flow or drying during manufacture, additional strength and moisture resistance. This composition of slurry is progressed along the screw conveyor 14 where it is dosed with the filler (graded as required) through inlet 24. The final slurry mixture is of a paste-like consistency and is extruded through manifold 26 onto the carrier paper 28 and beneath a roller 30 to produce a board of the required size. A top paper 32 is applied where appropriate and thereafter the board is transported to curing, cutting and drying apparatus.

The process as defined above requires an addition of water corresponding to approximately four times the amount of water required for the hydration of gypsum whereas prior art methods require approximately double this amount of water. Therefore, the energy requirements for drying are considerably reduced with the above process.

Claims

1. A gypsum product comprising a lightweight porous filler and hydrated calcium sulphate.

2. A process for producing a gypsum product, comprising mixing calcined gypsum, stucco gypsum or the like with water and a lightweight porous filler.

3. A product according to claim 1 or a process according to claim 2, wherein the filler has a dry bulk density of less than 550kg/m3.

4. A product or process according to claim 3, wherein the filler has a dry bulk density of less than 450kg/m3.

5. A product or process according to claim 4, wherein the filler has a dry bulk density of between 300 and 400kg/m3.

6. A product according to any one of claims 1, or 3 to 5, comprising 30% or more by weight of filler.

7. A product according to any one of claims 1, or 3 to 6, comprising up to 60% by weight of filler.

8. A process according to any one of claims 2 to 5, wherein the filler contains from 0 to 50% of water by weight of the damp filler.

9. A process according to claim 8, wherein the filler contains from 0 to 35% of water by weight of the damp filler.

10. A product or process according to any one of the preceding claims, wherein the filler is particulate.

11. A product or process according to claim 10, wherein the filler has a grain size of between 0.1 and 5.0 mm.

12. A product or process according to any one of the preceding claims, wherein the filler is pumice.

13. A product or process according to claim 12, wherein the filler is tephra.

14. A process for producing a plasterboard according to any one of claims 2 to 5 or 8 to 13, further comprising extruding the mixed ingredients onto a carrier to form a board and subsequently drying the mixture after hydration of the gypsum.

15. A process for producing a gypsum product substantially as herein described with reference to the drawing.

16. An apparatus for carrying out the process according to any one of claims 2 to 5 and 8 to 15, comprising mixer means including a body having inlet means for the ingredients to be mixed and outlet means for the mixed ingredients, wherein the calcined gypsum and water inlet means and the filler inlet means are provided at different parts of the mixer body.

17. An apparatus according to claim 16, wherein the mixer means is in the form of a screw conveyor and individual inlet means are spaced longitudinally on the mixer body for each of the main ingredients to be mixed.

18. An apparatus for carrying out a process for producing a gypsum product substantially as herein described with reference to the drawing.