Calcium sulphate-based products

Abstract

This invention relates to improved high temperature resistant calcium sulphate-based products e.g. gypsum wallboard products and, in particular, to products having reduced shrinkage at high temperatures. The invention provides calcium sulphate-based product comprising gypsum and a shrinkage resistance additive. The shrinkage resistance additive is melamine polyphosphate or melamine pyrophosphate.

Claims

1. A calcium sulphate-based product formed by drying an aqueous slurry comprising: gypsum that is calcined and a shrinkage resistance additive, wherein: the shrinkage resistance additive is melamine polyphosphate or melamine pyrophosphate, and the calcium sulphate-based product has an area shrinkage of between 8-14% after heating to a temperature of 1000? C. over 120 minutes.

2. The product according to claim 1, wherein the shrinkage resistance additive is provided in an amount from 0.1 to 20 wt %.

3. The product according to claim 2, wherein the shrinkage resistance additive is provided in an amount from 2 to 5 wt %.

4. The product according to claim 1, wherein the product is a wallboard, panel, tile, joint filler material or mould for metal casting.

5. The product according to claim 4, wherein the product is a composite wallboard comprising a core of the gypsum and shrinkage resistance additive sandwiched between two liners.

6. The product according to claim 1, wherein the shrinkage resistance additive is melamine polyphosphate.

7. A method for manufacturing the calcium sulphate-based product according to claim 1, the method comprising: mixing the gypsum that is calcined and the shrinkage resistance additive with water to provide an aqueous slurry; and drying the aqueous slurry.

8. The calcium sulphate-based product comprising: gypsum and a shrinkage resistance additive, wherein: the shrinkage resistance additive is melamine polyphosphate or melamine pyrophosphate, and the calcium sulphate-based product has a linear shrinkage between 5% and 10% at a temperature of 1000? C.

9. The product according to claim 8, wherein the shrinkage resistance additive is provided in an amount from 0.1 to 20 wt %.

10. The product according to claim 9, wherein the shrinkage resistance additive is provided in an amount from 2 to 5 wt %.

11. The product according to claim 8, wherein the product is a wallboard, panel, tile, joint filler material or mould for metal casting.

12. The product according to claim 11, wherein the product is a composite wallboard comprising a core of the gypsum and shrinkage resistance additive sandwiched between two liners.

13. The product according to claim 8, wherein the shrinkage resistance additive is melamine polyphosphate.

14. A method for manufacturing the calcium sulphate-based product according to claim 8, the method comprising: mixing the gypsum that is calcined and the shrinkage resistance additive with water to provide an aqueous slurry; and drying the aqueous slurry.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a graph of area shrinkage for the control and MPP samples after heating to 1000? C. and subsequent cooling;

(2) FIG. 2 shows a graph of linear shrinkage for the control and MPP samples during heating to 1000? C.; and

(3) FIG. 3 shows a graph of linear shrinkage for the control, MPP and melamine pyrophosphate samples during heating to 1000? C.

EXPERIMENTAL

(4) The following examples are given by way of illustration only.

(5) Control Sample 1

(6) 1500 g of stucco was blended with 0.1 wt % (relative to the weight of the stucco) ground gypsum accelerator (GMNGround mineral NANSA) and added to 1350 g of water at 40? C. This was mixed for 10 seconds in a large Waring blender and the resulting slurry was poured 100?50?11 mm and 200?200?12.5 mm brass moulds to harden. The thumb set was less than 10 minutes. The thumb set is taken by depressing a thumb end onto a portion of the setting gypsum. The time is recorded when sufficient strength is attained such that an impression can no longer be made in the setting gypsum. After leaving the samples to hydrate for an hour, they were transferred to an oven at 40? C. and left to dry overnight (at least 12 hours).

(7) Control Sample 2

(8) 1500 g of stucco was blended with 0.1 wt % ground gypsum accelerator. 0.5 wt % (based on the weight of the stucco) (i.e. 7.5 g) Johns Manville glass fibres were dispersed in 1350 g of water at 40? C. for 10 seconds and then the dry blend was added. This was mixed for 10 seconds in a large Waring blender and the resulting slurry was poured 100?50?11 mm and 200?200?12.5 mm brass moulds to harden. The thumb set was less than 10 minutes. After leaving the samples to hydrate for an hour, they were transferred to an oven at 40? C. and left to dry overnight (at least 12 hours).

(9) MPP Sample 1

(10) 1500 g of stucco was blended with 0.3 wt % (based on the weight of stucco) ground gypsum accelerator. 2.5 wt % MPP (based on weight of stucco) was dispersed in 1350 g of water at 40? C. for 10 seconds and then the dry blend was added. This was mixed for 10 seconds in a large Waring blender and the resulting slurry was poured 100?50?11 mm and 200?200?12.5 mm brass moulds to harden. The thumb set was less than 10 minutes. After leaving the samples to hydrate for an hour, they were transferred to an oven at 40? C. and left to dry overnight (at least 12 hours).

(11) MPP Sample 2

(12) 1500 g of stucco was blended with 0.3 wt % (based on the weight of stucco) ground gypsum accelerator. 0.5% Johns Manville glass fibres and 2.5 wt % MPP (based on weight of stucco) were dispersed in 1350 g of water at 40? C. for 10 seconds and then the dry blend was added. This was mixed for 10 seconds in a large Waring blender and the resulting slurry was poured 100?50?11 mm and 200?200?12.5 mm brass moulds to harden. The thumb set was less than 10 minutes. After leaving the samples to hydrate for an hour, they were transferred to an oven at 40? C. and left to dry overnight (at least 12 hours).

(13) MPP Sample 3

(14) 1500 g of DSG Stucco was blended with 0.5 wt % (based on the weight of stucco) ground gypsum accelerator. 5 wt % MPP (based on weight of stucco) was dispersed in 1350 g of water at 40? C. for 10 seconds and then the dry blend was added. This was mixed for 10 seconds in a large Waring blender and the resulting slurry was poured 100?50?11 mm and 200?200?12.5 mm brass moulds to harden. The thumb set was less than 10 minutes. After leaving the samples to hydrate for an hour, they were transferred to an oven at 40? C. and left to dry overnight (at least 12 hours).

(15) MPP Sample 4

(16) 1500 g of DSG Stucco was blended with 0.5 wt % (based on the weight of stucco) ground gypsum accelerator. 0.5% Johns Manville glass fibres and 5 wt % MPP (based on weight of stucco) were dispersed in 1350 g of water at 40? C. for 10 seconds and then the dry blend was added. This was mixed for 10 seconds in a large Waring blender and the resulting slurry was poured 100?50?11 mm and 200?200?12.5 mm brass moulds to harden. The thumb set was less than 10 minutes. After leaving the samples to hydrate for an hour, they were transferred to an oven at 40? C. and left to dry overnight (at least 12 hours).

(17) TABLE-US-00001 TABLE 1 Summary of MPP Samples Control Control MPP MPP MPP MPP 1 2 1 2 3 4 Calcined 1500 1500 1500 1500 1500 1500 gypsum/g Water/g 1350 1350 1350 1350 1350 1350 Accelerator/g 1.5 1.5 4.5 4.5 7.5 7.5 Glass Fibres/g 7.5 7.5 7.5 MPP/g 37.5 37.5 75 75
Melamine Pyrophosphate Sample 1

(18) 2.5 wt % melamine pyrophosphate (based on weight of stucco) was dispersed in 140 mL of tap water for 5 minutes using an Ultra-Turrax high shear mixer and then 200 g of stucco was added. This was mixed by hand for 1 minute and the resulting slurry was formed into 12.5 mm diameter gypsum cylinders. They were transferred to an oven at 40? C. and left to dry overnight (at least 12 hours).

(19) For comparison with this melamine pyrophosphate, gypsum cylinders a) as above but with no melamine pyrophosphate, b) as above but with 2.5 wt % MPP instead of melamine pyrophosphate and c) as above but with no melamine pyrophosphate and 2.0 wt % (based on weight of stucco) micro silica. The results of the comparison are discussed below and shown in FIG. 3.

(20) Area Shrinkage

(21) For each of the 100?50?11 mm samples, the initial measurements (length and width) were recorded and then the samples heated to around 1000? C. over 120 mins (at 20? C./min up to around 200? C. and thereafter at a steadily and slowly decreasing rate). After cooling, the sample's dimensions were re-measured. The area shrinkage was calculated as the difference between the initial area of the sample and the heat treated sample and is shown in FIG. 1.

(22) It can be seen that all samples containing MPP showed a considerable reduction in area shrinkage compared to the control samples containing no MPP. The reduction in shrinkage is achieved with as little as 2.5 wt % MPP. Indeed, doubling the amount of MPP to 5 wt % does not show a significant further reduction in area shrinkage.

(23) The samples were inspected for cracks and the results are shown below in Table 2.

(24) TABLE-US-00002 TABLE 2 Observations after heating to 1000? C. Observations Control sample 1 Numerous visible cracks - some very wide - sample disintegrated Control sample 2 Numerous visible cracks MPP sample 1 A couple of very fine cracks MPP sample 2 A couple of very fine cracks MPP sample 3 A couple of very fine cracks MPP sample 4 A couple of very fine cracks
Linear Shrinkage

(25) The linear shrinkage of the 200?200?12.5 mm samples was measured using a ceramic rod attached to a linear displacement transducer. The samples were supported by other ceramic rods and the heated in a furnace to 1000? C. at an initial rate of around 44? C./min up to around 600? C. and then at a steadily and slowly decreasing rate (in line with ISO 834). The results are shown in FIG. 2.

(26) It can be seen that the linear shrinkage at 1000? C. is reduced to around 5% for all samples containing MPP. The biggest reduction in linear shrinkage was seen in the samples containing 5% MPP.

(27) FIG. 3 shows the linear shrinkage results for melamine pyrophosphate. It can be seen that the reduction in shrinkage is comparable to that obtained with the MPP i.e. a shrinkage of around 10% compared to around 19% for the control sample (with no melamine pyrophosphate).