Method for the Production of Gypsum-Based Boards and Stucco Slurry Comprising Non-Pregelatinized Migratory Starch for Use Therewith

Abstract

A method for continuously forming gypsum-based panels of high fixing strength comprises the steps of: forming a mixture comprising stucco, non-pregelatinized migratory starch, glass fibre, fluidizer and water; casting the mixture in a continuous band; maintaining the band under conditions sufficient for the stucco to form an interlocking matrix of set gypsum; cutting the band to form one or more wet panel precursors; and drying the wet panel precursor to form one or more gypsum-based panels. The weight ratio of water to stucco in the mixture is less than 0.7; the stucco is present in the mixture in an amount of over 60 wt % relative to the total solids content of the mixture; the starch is present in the mixture in an amount of over 3 wt % relative to the the stucco; the glass fibre is present in the mixture in an amount of over 1 wt % relative to the stucco; the fluidizer is is present in the mixture in an amount of at least 0.1 wt % relative to the stucco; and the density of the gypsum-based panel is greater than 700 kg/m.

Claims

1. A method for continuously forming gypsum-based panels, comprising: forming a mixture comprising stucco, migratory starch, glass fibre and water; casting the mixture in a continuous band; maintaining the band under conditions sufficient for the stucco to form an interlocking matrix of set gypsum; cutting the band to form one or more wet panel precursors; and drying the wet panel precursor to form one or more gypsum-based panels; wherein the weight ratio of water to stucco in the mixture is less than 0.7; the stucco is present in the mixture in an amount of over 60 wt % relative to the total solids content of the mixture; the starch is present in the mixture in an amount of over 3 wt % relative to the the stucco; the glass fibre is present in the mixture in an amount of over 1 wt % relative to the stucco; and the density of the gypsum-based panel is greater than 700 kg/m.sup.3.

2. A method according to claim 1, wherein the weight ratio of water to stucco in the mixture is less than 0.65.

3. A method according to claim 1, wherein the mixture additionally comprises a fluidiser.

4. A method according to claim 3, wherein the fluidiser is one of a polycarboxylate-based fluidiser, a phosphate polycondensate fluidiser, and a naphthalenesulphonate-based fluidiser.

5. A method according to claim 3, wherein the fluidiser is present in an amount of at least 0.1 wt % relative to the stucco.

6. A method according to claim 1, further comprising the step, before the step of forming a mixture, of conditioning the stucco by subjecting it to an annealing treatment to reduce the extent of microcracking in the stucco.

7. A method according to claim 1, wherein the glass fibres have an average length in the range 3-12 mm.

8. A method according to claim 7, wherein the glass fibres have an average diameter in the range 5-50 micron.

9. A method according to claim 1, wherein the starch is present in an amount of at least 5 wt % relative to the stucco.

10. A method according to claim 1, wherein the starch is an acid-thinned starch or an oxidised starch.

11. A stucco slurry for use in a continuous method for preparing a gypsum-based panel having a density of at least 700 kg/m.sup.3, the slurry comprising: stucco, non-pregelatinised migratory starch, glass fibre and water; wherein the weight ratio of water to stucco in the mixture is less than 0.7; the stucco is present in the slurry in an amount of over 60 wt % relative to the total solids content of the slurry; the starch is present in the slurry in an amount of over 3 wt % relative to the the stucco; the glass fibre is present in the slurry in an amount of over 1 wt % relative to the stucco.

12. A panel having a gypsum-based core that comprises a gypsum matrix having embedded therein a non-pregelatinised migratory starch in an amount of at least 3 wt % relative to the gypsum and glass fibre in an amount of at least 1 wt % relative to the gypsum, the panel having a maximum dimension greater than 1 m, wherein the density of the gypsum-based core is greater than 700 kg/m.sup.3, and the amount of gypsum in the gypsum-based core is greater than 60 wt %, and further wherein the total volume of water voids in the gypsum-based core is less than the total volume of the gypsum in the panel core.

13. A method according to claim 1, wherein the weight ratio of water to stucco in the mixture is less than 0.55.

14. A method according to claim 1, wherein the weight ratio of water to stucco in the mixture is less than 0.65; the mixture additionally comprises a fluidiser present in an amount of at least 0.1 wt % relative to the stucco. the glass fibres have an average length in the range 3-12 mm and an average diameter in the range 5-50 micron. the starch is present in an amount of at least 5 wt % relative to the stucco.

15. A method according to method according to claim 14, further comprising the step, before the step of forming a mixture, of conditioning the stucco by subjecting it to an annealing treatment to reduce the extent of microcracking in the stucco.

Description

DETAILED DESCRIPTION

[0057] The invention will now be described by way of example only.

[0058] Gypsum-based boards having a weight of 12 kg/m.sup.2 were prepared from stucco slurries according to the recipes set out in Table 1. The amounts of the ingredients are given as a percentage relative to the stucco amount. The slurry was cast as a continuous band overlaying a first paper liner. A second paper liner was overlaid on the deposited slurry.

[0059] The gypsum was allowed to set and was cut into sections which were then dried to form gypsum-based boards.

[0060] Screw pull-out tests were carried out on samples measuring 100 mm by 100 mm that had been conditioned at a temperature of 23 C. and a relative humidity of 50%. A 50 mm single thread wood screw was inserted into the sample, passing through a metal load transfer element positioned on the surface of the sample. The load transfer element has a first portion that is configured to lie between the screw head and the surface of the sample, and a second portion that is configured to engage with a testing machine so as to allow a load to be applied to the screw along the axis of the screw.

[0061] The specimen was then mounted in a Zwick Universal Testing Machine and a 10N pre-load applied to the screw along the axis of the screw. Subsequently, the load was increased by setting a constant cross-head speed of 10 mm/minute until pull out was achieved.

[0062] The results are set out in Table 1. These are averages, each taken from 16 samples.

[0063] It can be seen that screw pull-out strength increases with decreasing water gauge. This is thought to be due to the fact that starch is more evenly distributed in boards prepared at a lower water gauge, that is, the starch has a lower tendency to migrate to the board surface.

TABLE-US-00001 TABLE 1 Examples Comparative Examples 1 2 3 4 5 6 7 8 1 2 3 Water gauge (%) 61 61 55 61 61 58 64 60 72 76 76 Glass fibre, 6 mm 2 2 2 2 2 2 2 2 2 2 2.5 length (%) Acid modified starch 5.sup.1 5.5.sup.2 5.sup.1 5.sup.2 6.sup.2 6.sup.2 6.sup.2 6.sup.2 5.sup.1 6.sup.2 6.sup.2 (%) Retarder.sup.3 (%) 0 0 0 0.01 0.01 0.01 0.02 0.02 0 0.02 0.02 Fluidizer (%) 0.2.sup.4 0.2.sup.4 0.2.sup.4 0.2.sup.4 0.2.sup.4 0.3.sup.4 1.sup.5 1.3.sup.5 0.2.sup.4 0.8.sup.6 0.8.sup.6 Setting accelerator 0.1 0.1 0.1 0.3 0.3 0.3 0.2 0.2 0.1 0.1 0.1 (%) Stucco 100.sup.7 100.sup.7 100.sup.7 100.sup.7 100.sup.7 100.sup.7 100.sup.8 100.sup.8 100.sup.7 100.sup.8 100.sup.8 Line speed (M/min) 27 27 27 22 22 22 22 22 27 22 22 Screw Pull (N) 729 735 793 627 666 734 617 701 495 572 600 .sup.1Merifilm 102 corn starch from Tate & Lyle .sup.2Fluitex MB065 corn starch from Roquette .sup.3PlastRetard from Sicit 2000 .sup.4Ethacryl M copolymer fluidiser .sup.5Phosphated polycondensate fluidiser comprising side chains from BASF .sup.6Bozzeto CA40 .sup.7Desulphogypsum .sup.8Natural gypsum