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

12. 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.

13. A method according to method according to claim 12, 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.

14. A method according to claim 1, wherein the stucco is present in the mixture in an amount of over 70 wt % relative to the total solids content of the mixture.

15. A method according to claim 1, wherein the glass fibre is present in the mixture in an amount of over 2 wt % relative to the stucco.

16. A method according to claim 6, wherein the conditioning the stucco comprises exposing the stucco to water vapour at raised temperatures and/or pressures.

17. A method according to claim 6, wherein the conditioning the stucco comprises the stucco in the presence of small quantities of water or an aqueous solution.

18. A method according to claim 1, wherein the mixture further comprises a retarder.

Description

DETAILED DESCRIPTION

(1) The invention will now be described by way of example only.

(2) 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.

(3) The gypsum was allowed to set and was cut into sections which were then dried to form gypsum-based boards.

(4) 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.

(5) 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.

(6) The results are set out in Table 1. These are averages, each taken from 16 samples.

(7) 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.

(8) 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