U.S. patent application number 13/092533 was filed with the patent office on 2011-08-11 for lightweight plasterboard and plaster slurry composition used for its manufacture.
This patent application is currently assigned to LAFARGE PLATERS. Invention is credited to Frederic Peronnet, Michel RIGAUDON.
Application Number | 20110192520 13/092533 |
Document ID | / |
Family ID | 37450839 |
Filed Date | 2011-08-11 |
United States Patent
Application |
20110192520 |
Kind Code |
A1 |
RIGAUDON; Michel ; et
al. |
August 11, 2011 |
LIGHTWEIGHT PLASTERBOARD AND PLASTER SLURRY COMPOSITION USED FOR
ITS MANUFACTURE
Abstract
A plaster slurry composition includes plaster, water and starch,
the starch having a Brookfield viscosity of less than 60 cps at a
temperature below 60.degree. C. and a Brookfield viscosity greater
than 10,000 cps at a temperature of 70.degree. C.
Inventors: |
RIGAUDON; Michel; (Communay,
FR) ; Peronnet; Frederic; (Shanghai, CN) |
Assignee: |
LAFARGE PLATERS
Avignon Cedex 9
FR
|
Family ID: |
37450839 |
Appl. No.: |
13/092533 |
Filed: |
April 22, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12295475 |
Sep 30, 2008 |
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PCT/IB07/01684 |
Mar 27, 2007 |
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13092533 |
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Current U.S.
Class: |
156/45 ;
156/242 |
Current CPC
Class: |
C04B 24/38 20130101;
Y10T 428/31971 20150401; Y10T 428/249921 20150401; C04B 24/383
20130101; Y10T 428/27 20150115; C04B 28/14 20130101; Y10T 442/60
20150401; Y10T 156/1052 20150115; C04B 28/14 20130101; C04B 24/383
20130101; C04B 28/14 20130101; C04B 24/38 20130101 |
Class at
Publication: |
156/45 ;
156/242 |
International
Class: |
B32B 37/24 20060101
B32B037/24; B32B 37/02 20060101 B32B037/02; B32B 37/06 20060101
B32B037/06; B32B 37/08 20060101 B32B037/08; B32B 38/10 20060101
B32B038/10; B32B 38/16 20060101 B32B038/16 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 30, 2006 |
FR |
06/02758 |
Claims
1. A method for preparing a plasterboard consisting of a core based
on plaster and at least one facing, said method comprising the
following steps: preparation of a slurry of a plaster composition
comprising plaster, water and starch, said starch having a
Brookfield viscosity of less than 60 cps at a temperature below
60.degree. C. and a Brookfield viscosity greater than 10,000 cps at
a temperature of 70.degree. C., by mixing the various constituents
of the composition with water in a mixer; deposition of the slurry
thus prepared on at least one facing, followed by shaping, into a
ribbon, with optional covering of the upper face of the slurry
using a second facing; where appropriate, shaping the edges of the
ribbon of the board obtained previously by moulding of the ribbon
on profiled bands; hydraulic setting of the plaster on a
manufacturing line while the ribbon of board runs along a conveyor
belt; cutting the ribbon at the end of the line into predetermined
lengths; and drying the boards obtained.
2. The method according to claim 1, wherein the plasterboard
consists of a core based on plaster and two facings.
3. The method according to claim 1, wherein the starch has a
Brookfield viscosity of less than 20 cps at a temperature of
60.degree. C.
4. The method according to claim 1, wherein the starch has a
Brookfield viscosity of greater than 20,000 cps at a temperature of
70.degree. C.
5. The method according to claim 1, wherein the starch has a
Brookfield viscosity of greater than 30,000 cps at a temperature of
70.degree. C.
6. The method according to claim 1, wherein the starch has a
Brookfield viscosity of greater than 60,000 cps at a temperature of
80.degree. C.
7. The method according to claim 1, wherein the starch is
non-retrograding.
8. The method according to claim 1, wherein the starch has a
viscosity at 25.degree. C., after cooling following heating of the
starch solution up to 90.degree. C., which is greater than the
maximum viscosity obtained during heating of said starch
solution.
9. The method according to claim 1, wherein the starch has a
viscosity at 25.degree. C., after cooling following heating of the
starch solution up to 90.degree. C., of at least 100,000 cps.
10. The method according to claim 1, wherein the starch has a
viscosity at 25.degree. C., after cooling following heating of the
starch solution up to 90.degree. C., of at least 300,000 cps.
11. The method according to claim 1, wherein the slurry comprises,
relative to the weight of plaster, between 0.05 and 1% by weight of
starch.
12. The method according to claim 1, wherein the slurry comprises,
relative to the weight of plaster, between 0.1 and 0.5% by weight
of starch.
13. The method according to claim 1, wherein the slurry has a pH
between 5.5 and 8.5.
14. The method according to claim 1, wherein the slurry has a pH
between 6 and 7.5.
15. The method according to claim 1, wherein the slurry has a W/P
ratio ranging from 0.55 to 0.95.
16. The method according to claim 1, wherein the slurry has a W/P
ratio ranging from 0.65 to 0.80.
17. The method according to claim 1, wherein the slurry further
comprises a thin-boiling starch.
18. The method according to claim 1, wherein the core has a density
varying from 540 kg/m.sup.3 to 1100 kg/m.sup.3.
19. The method according to claim 1, wherein the core has a density
varying from 750 kg/m.sup.3 to 950 kg/m.sup.3.
20. The method according to claim 1, wherein the facing is paper,
or a mat or nonwoven, said facing being optionally filled with a
fine filler and/or a filler that has optionally received a surface
treatment.
21. The method according to claim 20, wherein the facing is a mat
of glass fibres, or of synthetic or cellulose fibres.
22. A method for manufacturing a plasterboard, the method
comprising utilizing starch having a Brookfield viscosity of less
than 60 cps at a temperature below 60.degree. C. and a Brookfield
viscosity greater than 10,000 cps at a temperature of 70.degree.
C.
23. The method according to claim 22, wherein the starch has a
Brookfield viscosity of less than 20 cps at a temperature of
60.degree. C.
Description
[0001] This application is a divisional of application Ser. No.
12/295,475, filed Sep. 30, 2008, which is the National Stage filing
under .sctn.371 of PCT/IB2007/001684, filed Mar. 27, 2007, which,
In turn, claims priority to French Patent Application No. 06/02758,
filed Mar. 30, 2006, the entire content of each of which is
incorporated herein by reference.
TECHNICAL FIELD
[0002] One subject of the invention is a novel lightweight board or
a novel board having better mechanical compression properties, and
also a plaster slurry composition used in its manufacture. The
invention also provides a method for manufacturing such a board and
use of a specific starch in the manufacture of a plasterboard.
PRIOR ART
[0003] Plasterboards have been known for many years. These boards
generally comprise two sheets or facings, generally made of paper
or paperboard, between which a plaster core is sandwiched. The
facings have a certain tensile strength whereas the core has a
certain compressive strength.
[0004] Furthermore, it is sought to lighten the plasterboards,
while still having good mechanical properties. A critical aspect is
the compressive strength of the core of the board. A method is
therefore sought for preparing a plasterboard with a core having a
higher compressive strength for densities that are identical to
current densities, which would allow the board strengths to be
obtained for lower densities.
[0005] One feature that has been the subject of numerous studies is
the adhesion of the paper onto the plaster core of the board. In
order to improve the adhesion, starch is generally added to the
plaster slurry and during drying of the board the starch migrates
towards the paper and concentrates at the core/facing interface.
This is obtained by selecting a starch that has a particular
rheology as a function of the temperature.
[0006] FR-A-I 429 406 and EP-A-0 172 773 describe the use of
natural or modified starches for manufacturing boards with a paper
facing, having an improved core/paper adhesion.
[0007] EP-A-O 936 201 describes the use of starch mixtures that are
modified with respect to the long-term strength of mixtures of
cement-type material, including plaster. The objective in this
document is to control the setting and the slump value of the mixed
slurry without there being any effect on the long-term
strengths.
[0008] EP-A-O 807 612 describes the use of cationic starch in
plaster slurries, which has a nitrogen content of less than 0.15%
and a solubility greater tan 50%. The starch is presented as a
thickener for the plaster slurry.
[0009] EP-A-O 172 773 describes the use of starch having a very
precise particle size, as an adhesive in the core for the facings.
The advantage presented by the specific particle size is a better
migration of the starch towards the core/facing interface.
[0010] SU-A-1 100 264 describes the use of alkyl sulphates and
fibres for increasing the shock resistance and impact strength. The
presence of starch is optional.
[0011] Thus, the starch in the plasterboards is very generally used
only at the core/facing interface.
[0012] US 2004/0 045 481 and U.S. Pat. No. 6,783,587 describe a
plaster slurry for a lightweight board comprising hemihydrate,
starch, foam and water, and in which the starch represents from 1.5
to 3% by weight relative to the weight of the hemihydrate and the
W/P ratio is between 0.7 and 0.95. It is indicated that the
resulting boards have a reduced density. The starch used is any
starch, but the example uses an acid-modified starch, available
from Staley Manufacturing. The starch from this document is of the
thin-boiling type.
[0013] US 2003/0 084 980 describes a plaster slurry for a
lightweight board comprising hemihydrate, acid-modified starch, a
starch crosslinking agent and water, the pH of the slurry being
around 9 to 11. The acid-modified starch is of the same type as
that mentioned in documents US 2004/0 045 481 and U.S. Pat. No.
6,783,587. It is indicated that the starch used has a lower
gelatinization temperature, which facilitates its migration towards
the core/facing interface, while the crosslinking makes it possible
to prevent the starch from completely migrating out of the core. It
is furthermore indicated that the starch also has a use as a binder
in the core of the board. It is indicated that the acid-modified
starches have gelatinization temperatures typically of 70 to
77.degree. C., for example 72.degree. C. In the cooked form, in the
board at the time it passes into the dryer, the starch is said to
be thin boiling, that is to say that it is fluid, in the form of a
syrup, but it turns to a gel when the temperature decreases. In the
invention that is the subject of the patent, increasing the pH
value of the plaster slurry reduces the gelatinization temperature
to values around 60-66.degree. C., typically 61.degree. C. In the
cooked form, the acid-modified starch thus treated (i.e. thus
crosslinked) is solid, that is to say that it is more viscous than
a thick-boiling starch, and a syneresis effect (separation of the
water and of a solid from a gel) appears. In this document,
increasing the pH within a given range is necessary in order to
obtain the crosslinking and the associated effect.
[0014] US 2005/0 126 437 describes the use, for increasing the
strength, of a modified starch that is insoluble in the plaster
slurry during mixing, but which is dissolved when the temperature
increases. The starch is modified, for example, by
hydroxyalkylation or acetylation.
[0015] Nothing in the prior art describes or suggests the
invention, which satisfies the requirement of providing a
lightweight board having good mechanical characteristics.
SUMMARY OF THE INVENTION
[0016] The addition of a certain type of starch to the core of the
board makes it possible to increase the compressive strength by up
to 30%, or even 50%, the starch having a particular rheological
profile.
[0017] The invention therefore provides a plaster slurry
composition comprising plaster, water and starch, said starch
having a Brookfield viscosity of less than 60 cps at a temperature
below 60.degree. C. and a Brookfield viscosity greater than 10,000
cps at a temperature of 70.degree. C.
[0018] According to one embodiment, the starch has a Brookfield
viscosity of less than 20 cps at a temperature of 60.degree. C.
[0019] According to one embodiment, the starch has a Brookfield
viscosity of greater than 20,000 cps, preferably greater than
30,000 cps, at a temperature of 70.degree. C.
[0020] According to one embodiment, the starch has a Brookfield
viscosity of greater than 60,000 cps at a temperature of 80.degree.
C.
[0021] According to one embodiment, the starch is of the
non-retrograding type.
[0022] According to one embodiment, the starch has a viscosity at
25.degree. C., after cooling following heating of the starch
solution up to 90.degree. C., which is greater than the maximum
viscosity obtained during heating of said starch solution.
[0023] According to one embodiment, the starch has a viscosity at
25.degree. C., alter cooling following heating of the starch
solution up to 90.degree. C., of at least 100,000 cps,
advantageously of at least 300,000 cps.
[0024] According to one embodiment, the composition comprises,
relative to the weight of plaster, between 0.05 and 1%, preferably
between 0.1 and 0.5% by weight of starch.
[0025] According to one embodiment, the composition has a pH
between 5.5 and 8.5, preferably between 6 and 7.5.
[0026] According to one embodiment, in the composition, the W/P
ratio varies from 0.55 to 0.95, preferably from 0.65 to 0.80.
[0027] According to one embodiment, the composition comprises, in
addition, a thin-boiling starch.
[0028] Another subject of the invention is a plasterboard
comprising a core obtainable by setting of a plaster slurry
according to the invention.
[0029] According to one embodiment, the plasterboard has a core
density varying from 540 kg/m.sup.3 to 1100 kg/m.sup.3, preferably
from 750 kg/m.sup.3 to 950 kg/m.sup.3.
[0030] According to one embodiment, the plasterboard has one,
preferably two, facing(s).
[0031] According to one embodiment, the facing is paper, or a mat
or nonwoven, preferably a mat of glass fibres and/or of synthetic
and/or cellulose fibres, said facing being optionally filled with a
fine filler and/or a filler that has optionally received a surface
treatment.
[0032] One further subject of the invention is a method for
preparing a plasterboard consisting of a core based on plaster and
at least one, advantageously two, facing(s), said method comprising
the following steps: [0033] preparation of a slurry of the plaster
composition according to the invention by mixing the various
constituents of the composition with water in a mixer; [0034]
deposition of the slurry thus prepared on at least one facing,
followed by shaping, into a ribbon, with optional covering of the
upper face of the slurry using a second facing; [0035] where
appropriate, shaping the edges of the ribbon of the board obtained
previously by moulding of the ribbon on profiled bands; [0036]
hydraulic setting of the plaster on a manufacturing line while the
ribbon of board runs along a conveyor belt; [0037] cutting the
ribbon at the end of the line into predetermined lengths; and
[0038] drying the boards obtained.
[0039] One further subject of the invention is the use, for
manufacturing a plasterboard, of starch having a Brookfield
viscosity of less than 60 cps at a temperature below 60.degree. C.
and a Brookfield viscosity greater than 10,000 cps at a temperature
of 70.degree. C.
[0040] One further subject of the invention is the use, for
increasing the compressive strength of a plasterboard, preferably
by at least 0.5 MPa, of starch having a Brookfield viscosity of
less than 60 cps at a temperature below 60.degree. C. and a
Brookfield viscosity greater than 10,000 cps at a temperature of
70.degree. C.
[0041] According to one embodiment, the uses according to the
invention employ the starch described herein.
BRIEF DESCRIPTION OF THE FIGURES
[0042] FIG. 1 is a graph representing the viscosity as a function
of the temperature for a starch according to the invention and for
several thin-boiling starches. The point at 125.degree. C. in fact
represents the point for the temperature of 25.degree. C., after
heating up to 90.degree. C.; and
[0043] FIG. 2 is a graph representing the viscosity as a function
of the temperature for three starches according to the invention.
The point at 125.degree. C. in fact represents the point for the
temperature of 25.degree. C., after heating up to 90.degree. C.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
Starch
[0044] The starch used in the invention has a fundamentally
different rheology as a function of temperature, which enables the
starch to be dispersed in the core at low temperatures in order to
penetrate into the inter-crystalline spaces (without the Applicant
being tied to one theory). As soon as the temperature is above
60.degree. C., the viscosity of the starch increases rapidly up to
a very high level to ensure that the starch actually remains in the
core and does not migrate to the core/facing interface. In the
invention, an effect more or less opposite to the thin-boiling
starches is sought. Once the starch is thickened, it remains at a
high viscosity at the high temperatures of the dryer. As can be
seen in FIG. 1, for the thin-boiling starches of the prior art, the
viscosity in fact decreases after a peak around 80-85.degree. C.
The starch is said to be a retrograding starch. According to one
embodiment, the starch according to the invention has a viscosity
that remains high and does not have a peak; the starch is
preferably a non-retrograding starch.
[0045] According to one embodiment, the starch also has the
characteristic that the viscosity remains high even after the
starch is cooled, indeed is even higher. The starch used in the
invention may be, for example, (1) of the type whose viscosity
increases further during cooling or (ii) of the type whose
viscosity remains constant during cooling, or else (iii) of the
type whose viscosity only decreases by a maximum of 15% during
cooling.
[0046] Without wanting to be tied to one theory, the Applicant
believes that the starch which is viscous at the gypsum crystal
interfaces contributes to the increase of the compressive strength.
The starch is chosen by measuring the Brookfield viscosity of a
solution obtained by dissolving 100 g of starch in 600 ml of water
at a temperature of 20.degree. C. The solution is brought to
60.degree. C. and then heated at a rate of 1.degree. C./min up to
90.degree. C. The solution is stirred during the heating. The
viscosity is measured at various temperatures (for example: 20, 40,
60, 65, 70, 75, 80, 85, 90.degree. C. etc.). The viscometer is a
Brookfield viscometer adapted for is measuring viscosities from 1
to 100,000 cps in particular. The testing is made with the number 6
spindle at the speed setting of 10, allowing the maximum result to
be directly read off the Brookfield viscometer between 50 and 80%
of the range on the scale. Outside that range, another spindle may
be selected.
[0047] In contrast with a thin-boiling starch whose viscosity
generally remains less than or equal to 1000 cps up to 90.degree.
C., the starch used in the invention is of the thick boiling type,
and according to one embodiment it is non-retrograding. In general,
the starch that can be used in the invention has a Brookfield
viscosity of less than 60, preferably of less than 20 cps at a
temperature below 60.degree. C. and a Brookfield viscosity greater
than 20,000 cps at a temperature of 70.degree. C., preferably
greater than 30,000 cps at a temperature of 70.degree. C.,
advantageously greater than 60,000 cps at a temperature of
80.degree. C. The starch that can be used in the invention also
preferably has a high viscosity up to high temperatures, for
example 90.degree. C., whereas the thin-boiling starches generally
have a drop in the viscosity at these high temperatures (these
thin-boiling starches are, as already indicated, retrograding).
Advantageously, the starch according to the invention will retain a
high viscosity for the entire heating and cooling duration.
According to one embodiment, the viscosity of the solution at
25.degree. C. after cooling following heating to 90.degree. C. will
be greater than the viscosity peak during heating. When the starch
is of the non-retrograding type, there is no actual peak to speak
of and the peak is then considered to be the highest value for the
temperature of 90.degree. C. For example, the Brookfield viscosity
of the starch at 25.degree. C., after cooling following heating of
the solution up to 90.degree. C., is at least 100,000 cps,
advantageously at least 300,000 cps.
[0048] The amount of starch used in the invention is variable. In
general, the amount of starch, expressed relative to the amount of
hemihydrate involved, may be between 0.05 and 1%, preferably
between 0.1 and 0.5%, by weight. In general, the amount, for a
board of density around 8.5 kg/m.sup.2, is from 5 to 50 g/m.sup.2,
in particular from 10 to 30 g/m.sup.2.
[0049] These starches are commercially available, especially from
Cerestar under the references RG 03408, PT 20002 and C*size
05903.
[0050] FIG. 1 gives a comparison between a thin-boiling starch from
the prior art and a thick-boiling starch according to the
invention; as indicated previously.
[0051] Finally, it should be noted that the use of the starch
according to the invention does not in any way prevent the use, at
the same time, of a thin-boiling starch for improving the
core/facing interface. Thus, according to one embodiment, the
plasterboard also contains a thin-boiling starch for the adhesion
at the core/facing interface.
Components of the Plasterboard.
[0052] The other components of the plasterboard are conventional
components. They are in the first place hydratable calcium
sulphate. The twin "plaster slurry" is misunderstood here to mean a
conventional plaster composition, that is to say for the most part
composed of plaster. The pH of such a shiny is typically between
5.5 and 8.5, in particular 6 to 7.5. According to one variant, no
additive capable of changing the pH towards basic values is added
during the preparation of the slurry.
[0053] The term "plaster" is understood, in the present
description, to mean the product resulting from the hydraulic
setting and hardening of hydratable calcium sulphate, that is to
say anhydrous calcium sulphate (anhydrite II or III) or calcium
sulphate hemihydrate (CaSO.sub.4, 1/2H.sub.2O) in its .alpha. or
.beta. crystalline form. These compounds are well known to a person
skilled in the art and are generally obtained by calcining gypsum.
It is possible to use natural or synthetic (FGD type) gypsum.
[0054] The composition may also comprise other hydraulic binders in
small quantities.
[0055] The core density may vary from 540 kg/m.sup.3 to 1100
kg/m.sup.3, especially from 750 kg/m.sup.3 to 950 kg/m.sup.3.
[0056] The core material may also comprise aggregates and/or
fillers, such as for example fumed silica, fly ash, blast furnace
slag, lime, vermiculite, perlite, microspheres, limestone, recycled
components, etc.
[0057] The plaster-based composition according to the invention may
comprise, in addition, additives that are conventionally used in
plaster-based compositions and are well known to a person skilled
in the art. In this respect, mention may be made of setting
accelerators, setting retarders, binding agents, adhesive agents,
plasticizers, water-retaining agents, air-entraining agents,
thickeners, bactericides, fungicides, reinforcing materials, flame
retardants and/or fillers. It is also possible to add additives for
obtaining boards having water-resistant properties (waxes,
silicones, etc.) or fire resistant properties.
[0058] Resins may also be used to further reinforce the board, such
as polyacrylic, polystyrene, polyvinyl chloride, polyolefin,
polyurethane, cellulosic, polyalcohol, polyamide, polyester,
polyether, polyphenolic, polysulphide, polysulphone, silicone or
fluoropolymer resins.
[0059] Examples of retarder/accelerator pairs are conventional
retarder/BMA, sodium polyacrylate/aluminium sulphate and sodium
phosphonate/zinc sulphate.
[0060] A bubble-stabilizing agent may also be used.
[0061] Agents that modify viscosity may also be added. Examples are
polymers of variable nature, clays, or additives having a modified
surface.
[0062] In a very conventional manner, a foaming agent is added to
the plaster slurry composition before it sets. In general, alkyl
sulphates, alkyl ether sulphates or mixtures thereof may be used.
Examples may be found in the following documents: U.S. Pat. No.
4,676,335, U.S. Pat. No. 5,158,612, U.S. Pat. No. 5,240,639, U.S.
Pat. No. 5,085,929, U.S. Pat. No. 5,643,510, WO-A-95 16515, WO-A-97
23337, WO-A-02 70427 and WO-A-02 24595. The amount is standard and
may be from 0.01 to 1 g/l of slurry (expressed in dry weight/dry
weight of slurry).
[0063] According to one embodiment of the invention, air is
introduced, in addition, to the plaster-based composition by
adding, for example, a foam. This foam may be produced by using any
suitable foaming agent, for example, the foaming agent of formula
ROSO.sub.3M, as defined in page 14, line 20 to page 15, line 16 of
international application WO 99/08978.
[0064] The plaster composition may, in addition, comprise fibres,
in particular glass fibres, if necessary.
[0065] The void volume in the core may vary within wide limits,
depending on the type of aeration in the mixture and on the W/P
ratio. Typically, the W/P ratio varies from 0.55 to 0.95,
preferably from 0.65 to 0.80. The facings used in the invention are
typically made of paper or paperboard, but facings may also be used
in the form of mats or nonwovens, for example made from a mat of
glass fibres and/or synthetic and/or cellulose fibres. Facings may
also be used comprising mixtures of different types of fibres,
especially glass fibres and organic fibres and/or facings
containing a fine filler dispersed in the fibres forming the mat.
Surface treatments to make the facing hydrophobic or to make it
fire resistant may also be provided.
[0066] The plasterboard thus obtained in the invention has: (1) for
equal compressive strengths, a lower density (and therefore a lower
surface weight) or (2) for equal densities (and therefore for equal
surface weights), an increased compressive strength, advantageously
increased by at least 0.5 MPa.
[0067] The board according to the invention is available in
conventional dimensions and thicknesses, especially in the form of
a standard so-called BA13 board. Thicknesses other than 12.5 mm are
possible, for example 10 mm or less, or 15 mm or more.
Method for Manufacturing Plasterboards
[0068] The final subject of the invention is a method for
continuously manufacturing plasterboards, essentially comprising
the following steps: [0069] preparation of a slurry of the plaster
composition according to the invention by mixing the various
constituents of the composition with water in a mixer; [0070]
deposition of the slurry thus prepared on at least one facing,
followed by shaping, into a ribbon, with optional covering of the
upper face of the slurry using a second facing, especially shaping
consisting in feathering the edges of the board; [0071] where
appropriate, shaping the edges of the ribbon of the board obtained
previously by moulding of the ribbon on profiled bands; [0072]
hydraulic setting of the plaster on a manufacturing line while the
ribbon of board runs along a conveyor belt; [0073] cutting the
ribbon at the end of the line into predetermined lengths; and
[0074] drying the boards obtained.
[0075] The plaster slurry composition is obtained conventionally by
mixing the hemihydrate with water. The additives may be introduced
with the hemihydrate, in particular when they are in powder form or
with a portion of the water for mixing when they are water soluble
or available in liquid form. If a foam is injected into the mixer,
the additives may also optionally be present in that foam.
[0076] The drying step is carried out at standard temperatures in
the plasterboard manufacturing lines; from this point of view, the
use of starch according to the invention does not change the
conventional processes. Likewise, the other properties of the board
are acceptable in every aspect. There is no setting delay effect
and no thickening of the slurry which could result in a variation
of the size of the board. There is no problem with drying the board
under the normal conditions in the dryer. The adhesion of the
facing to the core in the wet state and in the dry state is good in
every case.
EXAMPLES
[0077] The following examples illustrate the invention without
limiting it.
Example 1
[0078] Laboratory miniboards were prepared with the following
composition. Two different starches were used, Cerestar PT 20002
and C*size 05903, A and B respectively. FIG. 2 shows the viscosity
as a function of temperature, FIG. 2 also shows the viscosity of a
third starch capable of being used in the invention, RG 03408. The
starches presented in the figures are given solely by way of
illustration.
[0079] The miniboards are obtained from a slurry comprising (in
grams):
TABLE-US-00001 Plaster 1017 Water 590 Foaming agent (alkyl
sulphate) 0.5 BMA (Ball Mill Accelerator) 3 K.sub.2SO.sub.4 1
Plasticizer 3 Retarder 0.028
[0080] The volume of the foam was 770 ml, the amount of water used
to form the foam being 180 g.
[0081] The paper was a conventional facing paper, with a basis
weight of 205 g/m.sup.2. Thus, boards having a thickness of 12.5 mm
and a density of 8.5 kg/rn.sup.2 were prepared.
[0082] Next, a starch was added, of which the theological profile
is given in the graph from FIG. 2, 10 and 20 g/m.sup.2.
[0083] The results are given in the table below.
TABLE-US-00002 Without starch Starch A Starch A Starch B Starch B
Starch dose 0 10 20 10 20 (g/m.sup.2) Compressive 5.02 5.71 5.62
6.3 6.46 strength (MPa) Increase in the 13.7 10.8 25.5 28.7
compressive strength (%)
Example 2
[0084] An industrial scale process was used on a plasterboard
manufacturing line. The board manufactured was the standard BA13
board, with a density of 9.15 kg/m.sup.2, a W/P ratio of 0.59, a
line speed of 68 m/min, using a conventional starch for adhesion of
the paper facing. The starch B was used at a dilution of 50% in
water. The diluted starch solution was added to the mixing water in
two doses, 8 and 25 g/m.sup.2. The line was kept in production for
1 minute. The following results for the compressive strength (dry
conditions) were obtained, averaged for 5 values. The variations
recorded were at most 6%.
TABLE-US-00003 Starch concentration Compressive strength Increase
Sample (g/m.sup.2) of the board (MPa) (%) 1 0 2.90 -- 2 8 3.18 9.7
3 25 3.85 32.8 4 37 4.37 50.7
[0085] A substantial increase was therefore observed on adding the
starch according to the invention.
* * * * *