GYPSUM PLASTER

PLATRE A GYPSE

English Abstract

An extrudable gypsum plaster paste is disclosed comprising: .alpha. gypsum
plaster; water in a substantially stoichiometric amount; a binder; and a clay
or another rheology modifier functionally equivalent to a clay. Such pastes
can have a yield stress sufficient to make the paste self supporting. The
invention also provides a method for making an extrudable gypsum plaster paste
comprising: mixing .alpha. gypsum plaster, water in a substantially
stoichiometric amount and a binder; and mixing into that mixture a clay or
other rheology modifier functionally equivalent to a clay.

French Abstract

L'invention concerne une pâte de plâtre à gypse extrudable renfermant un plâtre à gypse .alpha., de l'eau dans une quantité sensiblement stoechiométrique, un liant, et une argile ou un autre modificateur rhéologique dont la fonction est analogue à celle d'une argile. De telles pâtes peuvent présenter une limite d'élasticité apparente suffisante pour faire en sorte que la pâte s'autosupporte. L'invention concerne également un procédé de fabrication d'une pâte de plâtre à gypse extrudable consistant à mélanger le plâtre à gypse .alpha., l'eau dans une quantité sensiblement stoechiométrique et un liant, puis à ajouter au mélange une argile ou tout autre modificateur rhéologique dont la fonction est analogue à celle d'une argile.

Claims

CLAIMS
1. A gypsum plaster paste comprising:
a gypsum plaster;
water in a substantially stoichiometric amount (as herein
defined);
a binder; and
a clay or another rheology modifier functionally
equivalent to a clay.
2. A gypsum plaster paste comprising:
a gypsum plaster;
water in a substantially stoichiometric amount (as herein
defined);
a binder; and
a rheology modifier such that the paste has a yield stress
sufficient to make the paste self supporting.
3. A paste according to claim 2 in which the rheology
modifier is a clay.
4. A paste according to claim 1, 2 or 3 in which the rheology
modifier is present in an amount of up to 20% weight of plaster.
5. A paste according to any preceding claim in which all the
gypsum plaster in the paste is a gypsum plaster.
6. A paste according to any preceding claim in which the
water is present in an amount of from 70% to 170% of the
stoichiometric amount.
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7. A paste according to any preceding claim in which the
binder is a cellulosic binder.
8. A paste according to any of claims 1 to 6 in which the
binder is at least one of polyvinylalcohol and polyethyleneglycol.
9. A paste according to any preceding claim in which the
binder is present in an amount of at least 0.1%, preferably at
least 1%, by weight.
10. A paste according to any preceding claim in which the
binder is present in an amount of no more than 10%, preferably no
more than 5%, by weight.
11. A paste according to any preceding claim further
comprising any of a setting retarder, lightweight aggregate and
fibre.
12. A paste according to claim 11 in which the lightweight
aggregate is ceramic microspheres or cenospheres.
13. A paste according to claim 11 or 12 in which the
lightweight aggregate is present in an amount of up to 20% by
weight.
14. A paste according to claim 11, 12 or 13 in which the fibre
is glass or polypropylene fibre.
15. A paste according to claim 11 or 14 in which the fibre is
present in an amount of up to 10% by weight.
16. A paste according to any preceding claim containing from
13% to 32% by water by weight of a plaster.
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17. A method for making a gypsum plaster paste according to
any preceding claim comprising:
mixing a gypsum plaster, water in a substantially
stoichiometric amount (as herein defined) and a binder; and
mixing into that mixture a clay or other rheology modifier
functionally equivalent to a clay.
18. A method for making gypsum plaster paste according to any
of claims 1 to 16 comprising:
mixing a gypsum plaster, water in a substantially
stoichiometric amount (as herein defined) and a binder; and
mixing into that mixture a rheology modifier such that the
paste has a yield stress sufficient to make the paste self
supporting.
19. A method according to claim 18 in which a clay is mixed
into the said mixture as the rheology modifier.
20. A method according to claim 17, 18 or 19 further
comprising forming a pre-gel/suspension comprising water and
binder and adding to that the a gypsum plaster.
21. A method according to any of claims 17 to 20 further
comprising adding a setting retarder to the composition.
22. A method according to any of claims 17 to 21 carried out
at a temperature above 60°C.
23. A method according to any of claims 17 to 22 in which the
water is added at between 13 and 32% by weight of a plaster.
24. A method of forming an extruded gypsum product comprising:
forming a paste according to any of claims 1 to 16;
working the paste into a plastic condition; and
extruding the paste through a die.
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25. A method according to claim 24 in which the extruded paste
is supported as it leaves the die.
26. A method of forming a stamped or pressed gypsum product
comprising:
forming a paste according to any of claims 1 to 16;
working the paste into a plastic condition; and
stamping or pressing the paste into a desired shape.
27. A method of forming a rolled gypsum product comprising:
forming a paste according to any of claims 1 to 16;
working the paste into a plastic condition; and
rolling the paste.
28. A method of forming a gypsum product comprising:
forming a paste according to any of claims 1 to 16;
working the paste into a plastic condition; and
injection moulding the paste.
29. A method according to any of claims 24 to 28 in which the
paste is made by a method according to any of claims 17 to 22.
30. A method according to any of claims 24 to 29 comprising
adding a setting retarder to the composition
31. A method according to any of claims 24 to 30 in which the
working is done at a temperature above 60°C.
32. A gypsum plaster paste made by a method according to
claims 17 to 23.
33. An extruded gypsum product made by a method according to
any of claims 24 to 31.
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34. A set gypsum product made from a paste according to any
of claims 1 to 16 and 32.
35. A gypsum plaster paste substantially as described with
reference to the example.
36. A method of forming a gypsum plaster paste substantially
as described with reference to the example.
37. A method of forming an extruded gypsum product
substantially as described with reference to the example.
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Description

CA 02329424 2000-10-18
WO 99/54265 PCT/GB99/01198
GYPSUM PLASTER
The invention relates to the production of formed gypsum
products and to a composition therefor.
Gypsum p:Laster, calcium sulphate hemihydrate, is generally
formed by the calcining of gypsum, calcium sulphate dihydrate,
although it may be formed in other ways. On addition of water,
gypsum plaster rehydrates to gypsum and can be formed into a
coherent body. Gypsum plaster requires 18.60 water by weight of
plaster to fully rehydrate it.
In the conventional production of formed gypsum products,
plaster is mixed with an excess of water to form a slurry of a
creamy, pouring, consistency. The slurry may contain setting
control agents and other additives such as fibres and lightweight
aggregates. The slurry is fed onto a continuous belt and as it
sets it is constrained into the desired shape. The set formed
product contains water, since an excess was used to form the
slurry; the formed product must be dried by heating, at
considerable cost, since set plaster containing as little as 5 o by
weight free water has very low strength.
Attempts have been made to form plaster products from a
plaster/water mix containing only slightly more than the
stoichiometric amount of water, to reduce the energy needed to dry
the formed product. It has not proved possible heretofore to
provide such a mix having satisfactory rheology to enable it to be
formed, for example by extrusion; the mixture tends to be friable
and does not produce a satisfactorily smooth surface to the formed
product. This is in part because since the mixture is not fluid,
adequate mixing of the water with the plaster is not achieved.
This leaves pockets of unrehydrated plaster and an excess of water
elsewhere.
It has now been found that an extrudable gypsum plaster
paste can be formed using a stoichiometric or close to

CA 02329424 2000-10-18
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stoichiometric amount of water.
According to a first aspect of the invention there is
provided a gypsum plaster paste comprising:
a gypsum plaster;
water in a substantially stoichiometric amount (as herein
defined);
a binder; and
a clay or another rheology modifier functionally
equivalent to a clay.
Also according to the first aspect of the invention there
is provided a method for making a gypsum plaster paste comprising:
mixing a gypsum plaster, water in a substantially
stoichiometric amount (as herein defined) and a binder; and
mixing into that mixture a clay or other rheology modifier
functionally equivalent to a clay.
According to a second aspect of the invention there is
provided a gypsum plaster paste comprising:
a gypsum plaster;
water in a substantially stoichiometric amount (as herein
defined) ;
a binder; and
a rheology modifier such that the paste has a yield stress
sufficient to make the paste self supporting. Preferably, the
rheology modifier is a clay. Preferred binders are cellulosic
binders. The stoichiometric amount of water is that amount needed
to achieve complete rehydration of the plaster from the
hemihydrate to the dihydrate, and no more. Preferably, the paste
contains only a gypsum plaster. The paste may include an
extrusion aid. Preferably, the clay or other rheology modifier is
present in an amount up to 20% , preferably of from 10 to 12% for
extrusion purposed by weight of plaster.

CA 02329424 2000-10-18
WO 99/54265 PCT/GB99/01198
Also according to the second aspect of the invention there
is provided a method for making a gypsum plaster paste:
mixing ~ gypsum plaster, water in a substantially
stoichiometric amount (as herein defined) and a binder; and
mixing into that mixture a rheology modifier such that the
paste has a yield stress sufficient to make the paste self
supporting. Preferred rheology modifiers are clays.
Preferred cellulosic binders are cellulose ethers;
particularly preferred are those having a molecular weight between
12000 and 30000. Other suitable binders include polyvinylalcohol
(PVA) and polyethyleneglycol (PEG), either alone or in
combination. Preferably, the binder is present at a level of at
least 0.1%, and more preferably at least 1%, by weight.
Preferably, the binder is present at a level of no more than 10%,
more preferably no more S%, by weight of plaster.
The binder, preferably a cellulosic binder, is preferably
suspended in the water before it is mixed with the gypsum plaster.
An extrusion aid may be included in the plaster/water mixture
before mixing with the clay.
It is preferred that the water content of the mixture is
between 13% and 32% based on the weight of a plaster. This
corresponds to between 70% and 170% of the stoichiometric amount.
In pastes containing less than the stoichiometric amount of water
the unrehydrated plaster acts as a filler; in contrast to other
proposed processes using near stoichiometric amounts of water, the
water and the unrehydrated plaster is easily dispersed throughout
the paste and there is virtually no free water in the paste. It is
preferred that a :retarder such as diethyl triamine pentacetic acid
(DTPA) is employed as a retarder to prevent the paste from setting
during processing. Alternatively, elevated temperatures
preferably above 60°C can be employed; at such temperatures, the
plaster will not set, allowing the mixture to be mixed thoroughly
and avoiding the plaster setting in the mixer or elsewhere in the
process.
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CA 02329424 2000-10-18
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A preferred clay is Grolleg English china clay, but other
clays may be used; examples of other clays are ball, atapulgous
and bentonitic ~~lays. The Theological effect of clays and other
Theology modifiers on the paste will differ depending on the type
of clay or other modifier employed. Those skilled in the art will
be able to select an appropriate clay or other modifier to achieve
a desired Theology, using if necessary simple experimentation.
The paste of the invention is particularly suitable for
forming into a desired shape by extrusion. Although the paste is
self supporting it will normally be necessary to support the
formed paste immediately it leaves the die, for example on a
moving belt. The extended product can be cut to length before or
after setting.
Alternatively, the paste can be formed into product by
other plastic forming processes such as pressing, stamping,
rolling and injection moulding.
In the context of the present invention, yield stress is
the stress which must be exceeded in order to cause measurable
flow in the paste. The measured yield stress will be different
depending on the processing to which the paste is subjected, such
as extrusion or rolling. Yield stress is measured using standard
Theological techniques such as by use of a controlled stress
rheometer.
It has been found that products made from pastes according
to the invention exhibit excellent strength and hardness.
Extruded product made from a paste according to the invention can
exhibit average flexural strength (measured according to ASTM C78)
of 10 to 30 M Pa. This may be due to the excellent Theological
properties of the paste allowing formation of a product
substantially free of macro defects, especially by extrusion.
The inclusion in the paste of lightweight aggregates,
preferably up to 20% by weight of, allows the density of the
formed product to be reduced to 1.5 g/ml or lower. Preferred
aggregates include lightweight ceramic microspheres, and
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cenospheres.
Fibres may be added to the paste, preferably up to 10% by
weight of plaster. Preferred fibres included glass and
polypropylene fibres.
The invention will be further described with reference to
the following example:
A pre-gel/suspension was formed by combining:
Parts by Weight
DTPA (setting retarder) 41% solution: 0.66 %
methylcellulose (Methocel A4M): 3.18
water: 96.16
The pre-gel/suspension was used to make the following
composition:
Parts by Weight
a gypsum plaster: 84.28 %
pre-gel/suspension 15.6
DTPA (setting retarder) 41% solution 0.12
This composition was mixed in a Z-blade mixer at ambient
temperature for about 4 minutes. To this composition was added
9.820 by weight of the total mixture Grolleg English china clay
and the resulting mixture was mixed for a further 3 minutes until
the resulting paste was sufficiently plastic to be extruded. The
paste was then removed from the mixer and introduced into the
pugging section of a Kema extruder. It was then deaired in the
vacuum section of the extruder and extruded through a die, the
extruded paste being supported on a conveyor as it exited the die.
The extruded products were cut to length before they set.
The density of the extruded product was 2.09 g/ml. The
Rockwell hardness was 119 and the modulus of rupture 11.5 M Pa. In
other examples, a modulus of rupture of up to 25 M Pa has been
achieved.