U.S. patent application number 11/217039 was filed with the patent office on 2007-03-01 for modified landplaster as a wallboard filler.
Invention is credited to David R. Blackburn, Qingxia Liu, Michael P. Shake.
Application Number | 20070044687 11/217039 |
Document ID | / |
Family ID | 37802265 |
Filed Date | 2007-03-01 |
United States Patent
Application |
20070044687 |
Kind Code |
A1 |
Blackburn; David R. ; et
al. |
March 1, 2007 |
Modified landplaster as a wallboard filler
Abstract
A gypsum slurry includes calcium sulfate hemihydrate, water and
calcium sulfate dihydrate is coated with a hydrophilic, dispersible
coating. The coating is less soluble than the calcium sulfate
hemihydrate to delay exposure of the landplaster to the remainder
of the slurry, preventing premature crystallization and the early
stiffening that accompanies it. Another embodiment is a gypsum
slurry that includes calcium sulfate hemihydrate, a polycarboxylate
dispersant, water and coated calcium sulfate dihydrate. In this
case, the hydrophilic, dispersible coating is selected to serve as
a modifier to enhance the ability of the dispersant to fluidize the
gypsum slurry.
Inventors: |
Blackburn; David R.;
(Barrington, IL) ; Liu; Qingxia; (Vernon Hills,
IL) ; Shake; Michael P.; (Johnsburg, IL) |
Correspondence
Address: |
GREER, BURNS & CRAIN, LTD.
300 SOUTH WACKER DRIVE
SUITE 2500
CHICAGO
IL
60603
US
|
Family ID: |
37802265 |
Appl. No.: |
11/217039 |
Filed: |
August 31, 2005 |
Current U.S.
Class: |
106/772 ;
106/791 |
Current CPC
Class: |
Y10T 428/2993 20150115;
C04B 14/365 20130101; C04B 20/1066 20130101; C04B 20/107 20130101;
C04B 28/145 20130101; C04B 14/365 20130101; C04B 24/2647 20130101;
C04B 28/145 20130101; C04B 20/1066 20130101; C04B 20/107 20130101;
C04B 28/145 20130101; C04B 20/1066 20130101; C04B 28/145 20130101;
C04B 2111/0062 20130101 |
Class at
Publication: |
106/772 ;
106/791 |
International
Class: |
C04B 28/14 20060101
C04B028/14; C04B 28/08 20060101 C04B028/08 |
Claims
1. A gypsum slurry comprising: calcium sulfate hemihydrate; water;
and calcium sulfate dihydrate coated with a hydrophilic,
dispersible coating, wherein said coating is less soluble than
calcium sulfate hemihydrate.
2. The slurry of claim 1 wherein the temperature of said water is
greater than 100.degree. F.
3. The slurry of claim 1 wherein said coating is at least one of
the group consisting of a phosphate, a phosphonate, a hydroxide and
a carbonate coating.
4. The slurry of claim 1 wherein said calcium sulfate dihydrate has
been treated with at least one of the group consisting of soda ash
and hydrated lime.
5. The slurry of claim 1 wherein said coating is applied at a rate
of up to 0.2% based on the total dry weight of calcium sulfate.
6. The slurry of claim 1 wherein said coated calcium sulfate
dihydrate is present in amounts of up to about 10% by weight based
on the combined weight of said calcium sulfate hemihydrate and said
calcium sulfate dihydrate.
7. The slurry of claim 1 further comprising a polycarboxylate
dispersant.
8. The slurry of claim 7 wherein said coating comprises a modifier
that enhances the efficacy of the dispersant.
9. The slurry of claim 7 wherein said modifier comprises at least
one of the group consisting of soda ash, trisodium phosphate, lime,
calcium carbonate and tetrasodium pyrophosphate.
10. A method of making a gypsum slurry, comprising: selecting a
hydrophilic, dispersible coating that is less soluble than calcium
sulfate dihydrate; coating calcium sulfate dihydrate with said
coating; combining the coated calcium sulfate dihydrate with
calcium sulfate hemihydrate and water to form the slurry.
11. The method of claim 10 wherein said combining step comprises
mixing.
12. The method of claim 10 wherein said coating step comprises
precipitating, spraying or dipping.
13. The method of claim 10 further comprising adding a
polycarboxylate dispersant to the slurry.
14. The method of claim 13, wherein said coating comprises a
modifier for enhancing performance of said dispersant.
15. A method of making a wallboard panel comprising: selecting a
hydrophilic, dispersible coating material that is both less soluble
than calcium sulfate dihydrate and enhances the efficacy of a
polycarboxylate dispersant; coating calcium sulfate dihydrate with
said coating material; combining the coated calcium sulfate
dihydrate with calcium sulfate hemihydrate, the polycarboxylate
dispersant and water to form a slurry; depositing the slurry on a
facing material; forming the panel; and allowing the slurry to
set.
16. The wallboard panel of claim 15, wherein said coating material
comprises at least one selected from the group consisting of
carbonates, hydroxides, phosphates and phosphonates.
17. The wallboard panel of claim 15, wherein said coating material
is applied in amounts of up to 0.2% based on the combined dry
weight of the calcium sulfate hemihydrate and calcium sulfate
dihydrate.
18. The wallboard panel of claim 15, wherein the calcium sulfate
dihydrate comprises from about 3% to about 10% of the combined
weight of the calcium sulfate dihydrate and the calcium sulfate
hemihydrate.
Description
CROSS REFERENCES TO RELATED APPLICATIONS
[0001] This application is related to co-pending U.S. Ser. No.
11/152,317, entitled "Modifiers for Gypsum Slurries and Method of
Using Them", U.S. Ser. No. 11/152,418 entitled "Gypsum Products
Utilizing a Two-Repeating Unit System and Process for Making Them",
and U.S. Ser. No. 11/152,661, entitled "Fast Drying Wallboard", all
filed Jun. 14, 2005, and herein incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention is directed to a composition utilizing
a coated landplaster as a filler in gypsum slurries. More
specifically, landplaster is coated with a coating that is less
soluble than stucco to reduce or delay its ability to catalyze
crystallization reactions.
BACKGROUND OF THE INVENTION
[0003] Gypsum-based building products are commonly used in
construction. Wallboard made of gypsum is fire retardant and can be
used in the construction of walls of almost any shape. It is used
primarily as an interior wall and ceiling product. Gypsum has
sound-deadening properties. It is relatively easily patched or
replaced if it becomes damaged. There are a variety of decorative
finishes that can be applied to the wallboard, including paint and
wallpaper. Even with all of these advantages, it is still a
relatively inexpensive building material.
[0004] Gypsum is also known as calcium sulfate dihydrate, terra
alba or landplaster. Plaster of Paris is also known as calcined
gypsum, stucco, calcium sulfate semihydrate, calcium sulfate
half-hydrate or calcium sulfate hemihydrate. Synthetic gypsum, for
example, that which is a byproduct of flue gas desulfurization
processes from power plants, may also be used. When it is mined,
raw gypsum is generally found in the dihydrate form. In this form,
there are two water molecules associated with each molecule of
calcium sulfate. To produce the hemihydrate form, the gypsum is
calcined to drive off some of the water of hydration by the
following equation:
CaSO.sub.4.2H.sub.2O.fwdarw.CaSO.sub.4.1/2H.sub.2O+3/2H.sub.2O
[0005] A number of useful gypsum products can be made by mixing the
stucco with water and permitting it to set by allowing the calcium
sulfate hemihydrate to react with water to convert the hemihydrate
into a matrix of interlocking calcium sulfate dihydrate crystals.
As the matrix forms, the product slurry becomes firm and holds a
desired shape. Excess water must then be removed from the product
by drying.
[0006] Significant amounts of energy are expended in the process of
making gypsum articles. Landplaster is calcined to make stucco by
heating it to drive off water of hydration. Later the water is
replaced as the gypsum sets by hydration of the hemihydrate to the
dihydrate form. Excess water used to fluidize the slurry is then
driven from the set article by drying it in an oven or a kiln.
Thus, reducing the amount of water needed to fluidize the slurry
turns into a monetary savings when fuel requirements are decreased.
Additional fuel savings would result if the amount of material that
required calcining were reduced.
[0007] Attempts have been made to reduce the amount of water used
to make a fluid slurry using dispersants. Polycarboxylate
superplasticizers are very effective in allowing water reduction
and where water reduction results in increased density, a strength
increase is achieved. These materials are relatively expensive.
When used in large doses, polycarboxylate dispersants can be one of
the single, most expensive additives in making gypsum products. The
high price of this component can overcome the narrow margins
afforded these products in a highly competitive marketplace.
[0008] Another disadvantage associated with polycarboxylate
dispersants is the retardation of the setting reaction. Gypsum
board is made on high speed production lines where the slurry is
mixed, poured, shaped and dried in a matter of minutes. The board
must be able to hold its shape to be moved from one conveyor line
to another to put the board into the kiln. Damage can occur if the
boards have not attained a minimum green strength by the time they
are cut to length and handled during the manufacturing process. If
the board line has to be slowed down because the board is not
sufficiently set to move on to the next step in the process,
production costs are driven up, resulting in an economically
uncompetitive product.
[0009] Modifiers have been found that increase the efficacy of the
dispersant in fluidizing the slurry, allowing the modifier to
replace a portion of the expensive dispersant while still reducing
water demand. However, it has been found that the modifier does not
work consistently, depending on how and when it is added to the
slurry. Thus, there is a need for a delivery vehicle to carry the
modifier to the slurry in a manner that allows it to perform
consistently so that the amount of dispersant can be reduced.
[0010] The use of fillers that are easily fluidizable in water have
been considered as another method of reducing fuel demand. However,
one of the important properties of gypsum products, and especially
gypsum panels or wallboard, is its fire resistance. Calcium sulfate
dihydrate is approximately 20% water by weight. Replacing a portion
of the calcined gypsum with fillers that are less fire retardant
diminishes this property in the finished product. Many fillers also
reduce the compressive strength and the nail pull strength of
wallboard.
[0011] Landplaster has been used as a filler in gypsum products. It
is also fire retardant, inexpensive, readily available and reduces
the amount of calcined gypsum that is needed, but it also has
disadvantages. Calcium sulfate dihydrate used in sufficient
quantities to act as a filler also acts as a set accelerator for
the hemihydrate by providing seed crystals that start the
crystallization process more quickly. This leads to premature
stiffening of the slurry.
[0012] Thus there is a need in the art for a filler for use in
gypsum articles, particularly wallboard, that reduces fuel
consumption by replacing calcined gypsum, by reducing the amount of
water driven from the set product or both. The filler should have
fire retardancy approximately equal to set gypsum and it should be
inexpensive, readily available and should not decrease the strength
of the finished product.
[0013] The prior art has failed to adequately address the problem
of improving the efficacy of a given polycarboxylate dispersant.
Improving the efficacy of a dispersant would reduce the cost of the
dispersant and maintaining the reasonable price of gypsum
products.
[0014] Thus, there is a need in the art to reduce the dosage of
dispersants used in a gypsum slurry while maintaining flowability
of the slurry. Reduction in dispersant use would result in saving
of costs spent on the dispersant and would reduce adverse side
effects, such as set retardation.
SUMMARY OF THE INVENTION
[0015] These and other needs are met or exceeded by the use of the
present invention which utilizes an improved, coated landplaster as
a filler in gypsum products. The coated landplaster also optionally
serves as a delivery vehicle for a modifier that enhances
performance of polycarboxylate dispersants.
[0016] One embodiment of this invention is drawn to a gypsum slurry
that includes calcium sulfate hemihydrate, water and calcium
sulfate dihydrate coated with a hydrophilic, dispersible coating.
The coating is less soluble than the calcium sulfate dihydrate to
delay exposure of the landplaster to the remainder of the slurry,
preventing premature crystallization and the early stiffening that
accompanies it.
[0017] Another embodiment of this invention is a gypsum slurry that
includes calcium sulfate hemihydrate, a polycarboxylate dispersant,
water and coated calcium sulfate dihydrate. In this case, the
hydrophilic, dispersible coating is selected to serve as a modifier
to enhance the ability of the dispersant to fluidize the gypsum
slurry.
[0018] A method of making the slurry includes selecting a coating
that is less soluble than calcium sulfate hemihydrate. Calcium
sulfate dihydrate is coated with the hydrophilic, dispersible
coating, then combined with water and calcium sulfate
hemihydrate.
[0019] Replacement of a portion of the calcined gypsum with coated
landplaster results in lower requirements for calcined gypsum,
resulting in savings realized from a reduction in fuel and power
consumed by the calcining process. Plants that are limited by
stucco production may also achieve an increase in capacity since
more wallboard can be made with the same amount of stucco.
[0020] Coating of the landplaster reduces its ability to act as a
set accelerator. By covering the landplaster crystal, the
hemihydrate molecules do not have access to the seed crystals as
long as the coating remains in place. As the coating dissolves into
the slurry water, the landplaster is exposed and begins to catalyze
the hydration reactions. However, removal of the coating takes time
that delays initiation of the setting reactions so that premature
stiffening of the slurry is minimized or avoided. Another
possibility it that the coating is insoluble and merely renders the
landplaster inert. The ability to control when the landplaster is
available to initiate setting reactions allows reduction the usage
of set accelerator, resulting in a cost savings.
[0021] If the coating is highly dispersible, the landplaster may
disperse in the slurry more easily than the calcined gypsum it
replaces, allowing a further reduction in the water needed to
fluidize the slurry. Less fuel will be required for the kiln where
there is less water to dry from the product. Instead of the energy
savings, plants that are kiln-limited can realize additional
capacity from increased line speeds and sale of additional
product.
[0022] Where there is a capacity increase, it is obtained without a
significant increase in capital spending. This capital becomes
available for other projects or interest that may have been paid
could be saved. Since a large number of plants are limited by
either stucco production or by kiln drying, use of this coating
could have wide application.
[0023] In some embodiments, the loss in strength is avoided
entirely. Landplaster results in higher strengths than many other
fillers. At least one of the preferred coatings results in a
product where there is no loss in strength at all. This produces a
particularly good product, having many of the properties of gypsum
set from 100% calcined gypsum.
[0024] When used in formulations with polycarboxylate dispersants,
the coated landplaster is also usable as a vehicle to deliver a
modifier for enhancement of the dispersant. A number of modifiers
are known and are suitable for deposition onto the landplaster
particle.
DETAILED DESCRIPTION OF THE INVENTION
[0025] The gypsum slurry of this invention is made using water,
calcined gypsum and a coated landplaster. Although the benefits of
this invention are most clear when used in a slurry that includes a
polycarboxylate, it is useful in any embodiment where it is
desirable to utilize landplaster as a filler but premature
thickening is to be avoided.
[0026] Any calcined gypsum or stucco is useful in this slurry.
Either alpha or beta calcined stucco is useful. Stuccos from a
variety of sources can be used, including synthetic gypsum. As
discussed below, average or low salt stuccos are preferred in
embodiments where polycarboxylate dispersants are used due to
possible interaction.
[0027] Landplaster is used as a filler to replace a portion of the
stucco. Since landplaster is already in the dihydrate form, it
requires no water of hydration and thus has less of a water demand
than stucco. However, landplaster does not participate in the
crystal formation reactions, and therefore does not become bound
into the crystal matrix to the same degree as the hemihydrate. Some
loss in strength occurs, particularly if the amount of landplaster
exceeds 10% of the total amount of gypsum materials. Any amount of
landplaster may be used, but preferably, the amount of landplaster
is about 3-10% of the total calcium sulfate materials on a dry
basis. As used in this application, the term "calcium sulfate
materials" includes calcium sulfate in all of its forms, including
the anhydrate, hemihydrate and the dihydrate forms.
[0028] The landplaster is coated with any applicable coating that
prevents early onset of thickening of the gypsum slurry.
Preferably, the coating is less soluble than the stucco, providing
time for mixing and incorporation of other additives before the
landplaster is exposed. The coating is applicable to the
landplaster in any suitable coating method. Preferably the
landplaster is added to a coating solution. Once coated, the
landplaster is optionally dried for later use. However, in a
preferred coating method, the coating is precipitated onto the
landplaster while the landplaster remains slurried with the coating
solution. Energy required to dry the landplaster is saved. The
coating slurry with the coated landplaster is then incorporated
with the stucco slurry before the product is formed. Coated
landplaster, water, excess coating and/or byproducts are all added
to the stucco slurry prior to final mixing.
[0029] Many coatings are useful in the present invention. Preferred
coatings include DEQUEST particularly DEQUEST 2006, phosphonate
dispersants (Solutia, St. Louis, Mo.) or calcium carbonate. Other
coatings made of trisodium phosphate or tetrasodium pyrophosphate
are also useful. Any material is usable that is capable of being
coated onto the landplaster particles, that is less soluble than
the landplaster and reduces the active sites of nucleation.
[0030] The coating that is particularly useful is calcium
carbonate. The coating is preferably formed by precipitation of the
calcium carbonate onto the calcium sulfate dihydrate, or
landplaster, from solution. One embodiment of the coating is
obtained by combining hydrated lime, such as calcium magnesium
hydroxide, and soda ash or sodium carbonate. Next the calcium
sulfate dihydrate is added. A replacement reaction occurs, bringing
calcium carbonate together to form a solid. The addition of lime
also causes the calcium carbonate to precipitate onto the
landplaster specifically, rather than on the interior of the mixer
or other equipment. After the coated landplaster has been prepared,
the stucco and any other additives are added to the slurry. When
10% by weight of the total calcium sulfate material is in the form
of landplaster coated with calcium carbonate and 90% by weight of
the calcium sulfate material is in the form of hemihydrate, almost
10% water reduction is achieved compared to 100% hemihydrate.
[0031] Preferably, the water is warm when the lime and soda ash are
added to it. Use of warm water appears to improve the efficacy of
the coating process. Water temperatures up to 120.degree. F. are
especially useful for dissolving the salts, and the use of higher
temperatures is contemplated.
[0032] In some embodiments, reduction in the amount of water used
to make the slurry is achieved by the addition of a dispersant,
such as a polycarboxylate or naphthalene sulfonate. The dispersant
attaches itself to the calcium sulfate, then charged groups on the
backbone and the side chains on the branches of the polymer repel
each other, causing the gypsum particles to spread out and flow
easily. When the slurry flows more easily, the amount of water can
be reduced and still obtain a flowable fluid. In general, reduction
in water results in lower drying costs.
[0033] Any polycarboxylate dispersant that is useful for improving
fluidity in gypsum is preferred in the slurry of this invention. A
number of polycarboxlate dispersants, particularly polycarboxylic
ethers, are preferred types of dispersants. One of the preferred
class of dispersants used in the slurry includes two repeating
units. It is described further in co-pending U.S. Ser. No.
11/152,418, entitled "Gypsum Products Utilizing a Two-Repeating
Unit System and Process for Making Them", previously incorporated
by reference. These dispersants are products of Degussa
Construction Polymers, GmbH (Trostberg Germany) and are supplied by
Degussa Corp. (Kennesaw, Ga.) (hereafter "Degussa") and are
hereafter referenced as the "PCE211-Type Dispersants".
[0034] The first repeating unit is an olefinic unsaturated
mono-carboxylic acid repeating unit, an ester or salt thereof, or
an olefinic unsaturated sulphuric acid repeating unit or a salt
thereof. Preferred first repeating units include acrylic acid or
methacrylic acid. Mono- or divalent salts are suitable in place of
the hydrogen of the acid group. The hydrogen can also be replaced
by hydrocarbon group to form the ester.
[0035] The second repeating unit satisfies Formula I,
R.sup.1--O--R.sup.2 I
[0036] where R.sup.1 is an alkenyl group of Formula II ##STR1##
[0037] and R.sub.2 is derived from an unsaturated (poly)alkylene
glycol ether group according to Formula III.
--(C.sub.mH.sub.2mO).sub.x--(C.sub.nH.sub.2nO).sub.y--R.sup.4
III
[0038] Referring to Formula II, the alkenyl repeating unit
optionally includes a C.sub.1 to C.sub.3 alkyl group between the
polymer backbone and the ether linkage. The value of p is an
integer from 0-3, inclusive. Preferably, p is either 0 or 1.
R.sup.3 is either a hydrogen atom or an aliphatic C.sub.1 to
C.sub.5 hydrocarbon group, which may be linear, branched, saturated
or unsaturated. Examples of preferred repeating units include
acrylic acid and methacrylic acid.
[0039] The polyether group of Formula III contains multiple
C.sub.2-C.sub.4 alkyl groups, including at least two different
alkyl groups, connected by oxygen atoms. M and n are integers from
2 to 4, inclusive. Preferably, at least one of m and n is 2. X and
y are integers from 55 to 350, inclusive. R.sup.4 is hydrogen or an
aliphatic C.sub.1 to C.sub.20 hydrocarbon group, a cycloaliphatic
C.sub.5 to C.sub.8 hydrocarbon group, a substituted C.sub.6 to
C.sub.14 aryl group or a group conforming at least one of Formula
IV(a), IV(b) and VI(c). ##STR2##
[0040] In the above formulas, R.sup.5 and R.sup.7, independently of
each other, represent an alkyl, aryl, aralkyl or alkylaryl group.
R.sup.6 is a bivalent alkyl, aryl, aralkyl or alkylaryl group. A
particularly useful dispersant of the PCE211-Type Dispersants is
designated PCE211 (hereafter "211"). Other polymers in this series
known to be useful in wallboard include PCE111. PCE211-Type
dispersants are described more fully in U.S. Ser. No. 11/152,678,
filed Jun. 14, 2005, by Degussa Construction Polymers, entitled
"Polyether-Containing Copolymer", herein incorporated by
reference.
[0041] The molecular weight of the PCE211 Type dispersant is
preferably from about 20,000 to about 60,000 Daltons. Surprisingly,
it has been found that the higher molecular weight dispersants
cause less retardation of set time than dispersants having a
molecular weight greater than 60,000 Daltons. Generally longer side
chain length, which results in an increase in overall molecular
weight, provides better dispersibility. However, tests with gypsum
indicate that efficacy of the dispersant is reduced at molecular
weights above 50,000 Daltons.
[0042] R.sup.1 preferably makes up from about 300 to about 99 mole
% of the total repeating units, more preferably from about 40 to
about 80%. From about 1 to about 70 mole % of the repeating units
are R.sup.2, more preferably from about 10 to about 60 mole %.
[0043] Another class of polycarboxylate compounds that are useful
in this invention is disclosed in U.S. Pat. No. 6,777,517, herein
incorporated by reference and hereafter referenced as the
"2641-Type Dispersant". Preferably, the dispersant includes at
least three repeating units shown in Formula V(a), V(b) and V(c).
##STR3##
[0044] In this case, both acrylic and maleic acid repeating units
are present, yielding a higher ratio of acid groups to vinyl ether
groups. R.sup.1 represents a hydrogen atom or an aliphatic
hydrocarbon radical having from 1 to 20 carbon atoms. X represents
OM, where M is a hydrogen atom, a monovalent metal cation, an
ammonium ion or an organic amine radical. R.sup.2 can be hydrogen,
an aliphatic hydrocarbon radical having from 1 to 20 carbon atoms,
a cycloaliphatic hydrocarbon radical having from 6 to 14 carbon
atoms, which may be substituted. R.sup.3 is hydrogen or an
aliphatic hydrocarbon radical having from 1 to 5 carbon atoms,
which are optionally linear or branched, saturated or unsaturated.
R.sup.4 is hydrogen or a methyl group, depending on whether the
structural units are acrylic or methacrylic. P can be from 0 to 3.
M is an integer from 2 to 4, inclusive, and n is an integer from 0
to 200, inclusive. PCE211-Type and 2641-Type dispersants are
manufactured by Degussa Construction Polymers, GmbH (Trostberg,
Germany) and marketed in the United States by Degussa Corp.
(Kennesaw, Ga.). Preferred 2641-Type Dispersants are sold by
Degussa as MELFLUX 2641F, MELFLUX 2651F and MELFLUX 2500L
dispersants. 2641-Type dispersants (MELFLUX is a registered
trademark of Degussa Construction Polymers, GmbH) are described for
use in wallboard and gypsum slurries in U.S. Ser. No. 11/152,661,
entitled "Fast Drying Wallboard", previously incorporated by
reference.
[0045] Yet another preferred dispersant family is sold by Degussa
and referenced as "1641-Type Dispersants". This dispersant is more
fully described in U.S. Pat. No. 5,798,425, herein incorporated by
reference. A particularly preferred 1641-Type Dispersant is shown
in Formula VI and marketed as MELFLUX 1641F dispersant by Degussa.
This dispersant is made primarily of two repeating units, one a
vinyl ether and the other a vinyl ester. In Formula VI, m and n are
the mole ratios of the component repeating units, which can be
randomly positioned along the polymer chain. ##STR4##
[0046] These dispersants are particularly well-suited for use with
gypsum. While not wishing to be bound by theory, it is believed
that the acid repeating units bind to the hemihydrate crystals
while the long polyether chains of the second repeating unit
perform the dispersing function. Since it is less retardive than
other dispersants, it is less disruptive to the manufacturing
process of gypsum products such as wallboard. The dispersant is
used in any effective amount. To a large extent, the amount of
dispersant selected is dependant on the desired fluidity of the
slurry. As the amount of water decreases, more dispersant is
required to maintain a constant slurry fluidity. Since
polycarboxylate dispersants are relatively expensive components, it
is preferred to use a small dose, preferably less than 2% or more
preferably less than 1% by weight based on the weight of the dry
calcium sulfate material. Preferably, the dispersant is used in
amounts of about 0.05% to about 0.5% based on the dry weight of the
calcium sulfate material. More preferably, the dispersant is used
in amounts of about 0.01% to about 0.2% on the same basis. In
measuring a liquid dispersant, only the polymer solids are
considered in calculating the dosage of the dispersant, and the
water from the dispersant is considered when a water/stucco ratio
is calculated.
[0047] Many polymers can be made with the same repeating units
using different distributions of them. The ratio of the
acid-containing repeating units to the polyether-containing
repeating unit is directly related to the charge density.
Preferably, the charge density of the co-polymer is in the range of
about 300 to about 3000 .mu.equiv. charges/g co-polymer. It has
been found that the most effective dispersant tested for water
reduction in this class of dispersants, MELFLUX 2651F, has the
highest charge density.
[0048] However, it has also been discovered that the increase in
charge density further results in an increase in the retardive
effect of the dispersant. Dispersants with a low charge density,
such as MELFLUX 2500L, retard the set times less than the MELFLUX
2651F dispersant that has a high charge density. Since retardation
in set times increases with the increase in efficacy obtained with
dispersants of high charge density, making a slurry with low water,
good flowability and reasonable set times requires keeping of the
charge density in a mid-range. More preferably, the charge density
of the co-polymer is in the range of about 600 to about 2000
.mu.equiv. charges/g co-polymer.
[0049] Modifiers are optionally added to a gypsum slurry to enhance
performance of a polycarboxylate dispersant. The modifier can be
any substance, liquid or solid, which when combined with a
polycarboxylate dispersant in a gypsum slurry, leads to an
improvement the efficacy of the dispersant. Modifiers are not
intended to be dispersants in themselves, but serve to allow the
dispersant to be more effective. For example, at constant
concentrations of dispersant, better fluidity is obtained when the
modifier is used compared to the same slurry without the
modifier.
[0050] Although the exact chemistry involved in the use of
modifiers is not fully understood, at least two different
mechanisms are responsible for the increase in dispersant efficacy.
Lime, for example, reacts with the polycarboxylate in the aqueous
solution to uncoil the dispersant molecule. In contrast, soda ash
reacts on the gypsum surface to help improve the dispersant effect.
Any mechanism can be used by the modifier to improve the efficacy
of the dispersant for the purposes of this invention.
Theoretically, if the two mechanisms work independently,
combinations of modifiers can be found that utilize the full effect
of both mechanisms and result in even better dispersant
efficacy.
[0051] Preferred modifiers include cement, lime, also known as
quicklime or calcium oxide, slaked lime, also known as calcium
hydroxide, soda ash, also known as sodium carbonate, potassium
carbonate, also known as potash, and other carbonates, silicates,
hydroxides, phosphonates and phosphates. Preferred carbonates
include sodium and potassium carbonate. Sodium silicate is a
preferred silicate.
[0052] When lime or slaked lime is used as the modifier, it is used
in concentrations of about 0.15% to about 1.0% based on the weight
of the dry calcium sulfate material. In the presence of water, lime
is quickly converted to calcium hydroxide, or slaked lime, and the
pH of the slurry becomes alkaline. The sharp rise in pH can cause a
number of changes in the slurry chemistry. Certain additives,
including trimetaphosphate, break down as the pH increases. There
can also be problems with hydration and, where the slurry is used
to make wallboard or gypsum panels, there are problems with paper
bond at high pH. For workers who come in contact with the slurry,
strongly alkaline compositions can be irritating to the skin and
contact should be avoided. Above pH of about 11.5, lime no longer
causes an increase in fluidity. Therefore, it is preferred in some
applications to hold the pH below about nine for maximum
performance from this modifier. In other applications, such as
flooring, a high pH has the benefit of minimizing mold and mildew.
Alkali metal hydroxides, especially sodium and potassium hydroxides
are preferred for use in flooring.
[0053] Other preferred modifiers include carbonates, phosphonates,
phosphates and silicates. Preferably, the modifiers are used in
amounts less than 0.25% based on the weight of the dry calcium
sulfate material. Above these concentrations, increases in the
amount of modifier causes a decrease in the dispersant efficacy.
These modifiers are preferably used in amounts of from about 0.05
to about 0.2 weight %.
[0054] Many of the modifiers disclosed above are advantageously
applied as the landplaster coating. In such cases, the coated
landplaster serves two functions, that of reducing premature
thickening of the slurry, as well as a delivery vehicle for the
modifier. Water demand of the slurry is reduced by permitting use
of a dihydrate filler, as well as delivering the modifier that
enhances the efficacy of the dispersant. The resulting slurry
utilizes water very efficiently.
[0055] The charge density of the dispersant has also been found to
affect the ability of the modifier to interact with the dispersant.
Given a family of dispersants with the same repeating units, the
modifier causes a greater increase in efficacy in the dispersant
having the higher charge density. It is important to note that
although the general trend is to obtain a higher efficacy boost
with higher charge density, when comparing the effectiveness of
dispersants having different repeating units, the effectiveness of
the dispersants may be considerably different at the same charge
density. Thus, adjustment of the charge density may not be able to
overcome poor fluidity with a particular family of dispersants for
that application.
[0056] It has also been noted that the reaction of the
polycarboxylate dispersants and the modifiers react differently
when used in different gypsum media. While not wishing to be bound
by theory, the impurities present in gypsum are believed to
contribute to the efficacy of both the dispersant and the modifier.
Among the impurities present in stucco are salts that vary by
geographical location. Many salts are known to be set accelerators
or set retarders. These same salts may also change the efficacy of
the polycarboxylate dispersant by affecting the degree of fluidity
that can be achieved. Some preferred polycarboxylates, including
the PCE211-Type Dispersants, are best utilized with a low salt
stucco. Other dispersants, such as the 2641-Type Dispersants are
suitable for use with high-salt stuccos.
[0057] As a result of the use of fluidity enhancing dispersants and
modifiers to boost their performance, the amount of water used to
fluidize the slurry can be reduced compared to slurries made
without these additives. It must be understood that the stucco
source, the calcining technique, the dispersant family, the charge
density and the modifier all work together to produce a slurry of a
given fluidity. In the laboratory, it is possible to reduce the
water level close to, equal to, or even below that theoretically
required to fully hydrate the calcium sulfate hemihydrate. When
used in a commercial setting, process considerations may not allow
water reduction to this degree.
[0058] When used to make gypsum board, a number of additives are
useful to improve the properties of the finished article.
Traditional amounts of additives are used. Amounts of several
additives are reported as "lbs/MSF," which stands for pounds of
additive per one thousand square feet of board.
[0059] Some embodiments of the invention employ a foaming agent to
yield voids in the set gypsum-containing product to provide lighter
weight. In these embodiments, any of the conventional foaming
agents known to be useful in preparing foamed set gypsum products
can be employed. Many such foaming agents are well known and
readily available commercially, e.g. the HYONIC line of soaps from
GEO Specialty Chemicals, Ambler, Pa. Foams and a preferred method
for preparing foamed gypsum products are disclosed in U.S. Pat. No.
5,683,635, herein incorporated by reference.
[0060] Dispersants are used to improve the flowability of the
slurry and reduce the amount of water used to make the slurry. Any
known dispersant is useful, including polycarboxylates, sulfonated
melamines or naphthalene sulfonate. Naphthalene sulfonate is
another preferred dispersant, and is used in amounts of about 0
lb/MSF to 18 lb/MSF (78.5 g/m.sup.2), preferably from about 4
lb/MSF (17.5 g/m.sup.2) to about 12 lb/MSF (52.4 g/m.sup.2). A
preferred naphthalene sulfonate dispersant is DAXAD Dispersant (Dow
Chemical, Midland, Mich.). Even where dispersants are used in the
coating, it maybe advantageous to have additional dispersant to
further improve the fluidity of the slurry.
[0061] A trimetaphosphate compound is added to the gypsum slurry in
some embodiments to enhance the strength of the product and to
improve sag resistance of the set gypsum. Preferably the
concentration of the trimetaphosphate compound is from about 0.07%
to about 2.0% based on the weight of the calcium sulfate material.
Gypsum compositions including trimetaphosphate compounds are
disclosed in U.S. Pat. Nos. 6,342,284 and 6,632,550, both herein
incorporated by reference. Exemplary trimetaphosphate salts include
sodium, potassium or lithium salts of trimetaphosphate, such as
those available from Astaris, LLC., St. Louis, Mo. Care must be
exercised when using trimetaphosphate with lime or other modifiers
that raise the pH of the slurry. Above a pH of about 9.5, the
trimetaphosphate loses its ability to strengthen the product and
the slurry becomes severely retardive.
[0062] Other additives are also added to the slurry as are typical
for the particular application to which the gypsum slurry will be
put. Set retarders (up to about 2 lb./MSF (9.8 g/m2)) or dry
accelerators (up to about 35 lb./MSF (170 g/m2)) are added to
modify the rate at which the hydration reactions take place. "CSA"
is a set accelerator comprising 95% calcium sulfate dihydrate
co-ground with 5% sugar and heated to 250.degree. F. (121.degree.
C.) to caramelize the sugar. CSA is available from USG Corporation,
Southard, Okla. plant, and is made according to U.S. Pat. No.
3,573,947, herein incorporated by reference. Potassium sulfate is
another preferred accelerator. HRA is calcium sulfate dihydrate
freshly ground with sugar at a ratio of about 5 to 25 pounds (2.2
to 11.4 kg) of sugar per 100 pounds (4.5 kg) of calcium sulfate
material. It is further described in U.S. Pat. No. 2,078,199,
herein incorporated by reference. Both of these are preferred
accelerators.
[0063] Another accelerator, known as wet gypsum accelerator or WGA,
is also a preferred accelerator. A description of the use of and a
method for making wet gypsum accelerator are disclosed in U.S. Pat.
No. 6,409,825, herein incorporated by reference. This accelerator
includes at least one additive selected from the group consisting
of an organic phosphonic compound, a phosphate-containing compound
or mixtures thereof. This particular accelerator exhibits
substantial longevity and maintains its effectiveness over time
such that the wet gypsum accelerator can be made, stored, and even
transported over long distances prior to use. The wet gypsum
accelerator is used in amounts ranging from about 5 to about 80
pounds per thousand square feet (24.3 to 390 g/m.sup.2) of board
product.
[0064] Other potential additives to the wallboard are biocides to
reduce growth of mold, mildew or fungi. Depending on the biocide
selected and the intended use for the wallboard, the biocide can be
added to the covering, the gypsum core or both. Examples of
biocides include boric acid, pyrithione salts and copper salts.
Biocides can be added to either the covering or the gypsum core.
When used, biocides are used in the coverings in amounts of less
than 500 ppm. Pyrithione is known by several names, including
2-mercaptopyridine-N-oxide; 2-pyridinethiol-1-oxide (CAS Registry
No. 1121-31-9); 1-hydroxypyridine-2-thione and 1
hydroxy-2(1H)-pyridinethione (CAS Registry No. 1121-30-8). The
sodium derivative (C.sub.5H.sub.4NOSNa), known as sodium pyrithione
(CAS Registry No. 3811-73-2), is one embodiment of this salt that
is particularly useful. Pyrithione salts are commercially available
from Arch Chemicals, Inc. of Norwalk, Conn., such as Sodium OMADINE
or Zinc OMADINE.
[0065] In addition, the gypsum composition optionally can include a
starch, such as a pregelatinized starch or an acid-modified starch.
Starches are used in amounts of from about 3 to about 20 lbs/MSF
(14.6 to 97.6 g/m.sup.2) to increase paper bond and strengthen
product. The inclusion of the pregelatinized starch increases the
strength of the set and dried gypsum cast and minimizes or avoids
the risk of paper delamination under conditions of increased
moisture (e.g., with regard to elevated ratios of water to calcined
gypsum). One of ordinary skill in the art will appreciate methods
of pregelatinizing raw starch, such as, for example, cooking raw
starch in water at temperatures of at least about 185.degree. F.
(85.degree. C.) or other methods. Suitable examples of
pregelatinized starch include, but are not limited to, PCF 1000
Starch, commercially available from Lauhoff Grain Company and
AMERIKOR 818 and HQM PREGEL starches, both commercially available
from Archer Daniels Midland Company (Decatur, Ill.). If included,
the pregelatinized starch is present in any suitable amount. For
example, if included, the pregelatinized starch can be added to the
mixture used to form the set gypsum composition such that it is
present in an amount of from about 0.5% to about 10% percent by
weight of the set gypsum composition. Starches such as USG95
(United States Gypsum Company, Chicago, Ill.) are also optionally
added for core strength.
[0066] Other known additives may be used as needed to modify
specific properties of the product. Sugars, such as dextrose, are
used to improve the paper bond at the ends of the boards. Wax
emulsions or siloxanes are used for water resistance. If stiffness
is needed, boric acid is commonly added. Fire retardancy can be
improved by the addition of vermiculite. These and other known
additives are useful in the present slurry and wallboard
formulations. Glass fibers are optionally added to the slurry in
amounts of up to 11 lb./MSF (54 g/m.sup.2). Up to 15 lb./MSF (73.2
g/m.sup.2) of paper fibers are also added to the slurry. Wax
emulsions are added to the gypsum slurry in amounts up to 90
lb./MSF (0.439 kg/m.sup.2) to improve the water-resistency of the
finished gypsum board panel.
EXAMPLE 1
[0067] A number of coatings were added to landplaster and tested in
the laboratory for their ability to improve the fluidity of the
samples. Components and amounts used in each sample are shown in
Table I.
[0068] Forty grams of landplaster and water as shown were added to
a Hobart Model N-50 mixer followed by addition of the additive. The
mixer was turned on low (setting 1) for five minutes. The amount of
dispersant was sufficient to yield 0.6 grams of solids. The
dispersant was weighed out on a small plastic boat and added to the
mixture by hand. Three hundred sixty grams of stucco was then added
to the mixer and allowed to soak for 15 seconds. The slurry was
mixed on medium speed (setting 2) for 15 seconds. TABLE-US-00001
TABLE I Patty Disper- Water, Amt., Size, Stiff Vicat sant grams
Additive grams cm Time Set 211 200 Control 0.0 22.8 2:55 9:00 211
200 DEQUEST 0.80 28.0 22:00 >30:00 .sup. 211 200 SodaAsh 0.80
26.5 2:35 7:50 211 200 Lime 0.80 26.5 3:15 10:20 Daxad 240 Control
0.0 18.8 2:55 9:00 Daxad 240 DEQUEST 0.80 26.0 12:00 23:00 Daxad
240 SodaAsh 0.80 12.2 <1:30 7:00 Daxad 240 Lime 0.80 17.5 2:30
7:10
[0069] For testing, a portion of the slurry was transferred to a
Slump Cylinder 2 inches (5 cm) in diameter and four inches (10 cm)
tall and a 7 ounce (207 cc) cup. Contents of the cylinder were
screeded flush with the top of the cylinder. If compression
strength and temperature rise set measurements were taken,
additional slurry was poured into brass two inch cube molds and an
insulated cup. Sixty seconds from the start of the stucco soak, the
slump cylinder was raised with a pneumatic mechanism. The diameter
of the resultant patty was measured in at least two directions and
recorded as the average of the two readings. "Stiffening time" is
measured as the elapsed time from the beginning of the stucco soak
to when a Vicat needle drawn through the slurry left a definite
line that did not flow back. The stiffening time is a measure of
the hydration of the slurry. "Vicat set" refers to the elapsed time
from the onset of the stucco soak until a 300 gram Vicat needle
positioned at the surface of the 7 ounce cup fails to penetrate to
the bottom of the sample.
[0070] As shown in the data above, the addition of DEQUEST 2006 and
Soda Ash each resulted in an increase in fluidity of the slurry as
indicated by the increase in patty size over the control sample.
DEQUEST 2006 increased patty size with both 211 (polycarboxylate)
and Daxad (naphthalene sulfonate) dispersants.
EXAMPLE 2
[0071] Additional laboratory tests were conducted where accelerator
was added to reduce the retardive effects of the dispersant. The
slurry was made more similar to a wallboard slurry by the addition
of foam.
[0072] One hundred twenty grams of landplaster and water as shown
were added to a Hobart Model N-50 mixer followed by addition of the
additive. The mixer was turned on low (setting 1) for five minutes.
The amount of dispersant was enough to give 1.8 grams of solids.
The dispersant was weighed out on a small plastic boat and added to
the mixture by hand. One thousand eighty grams of stucco were then
added to the mixer and allowed to soak for 15 seconds. The slurry
was mixed on medium speed (setting 2) for 15 seconds.
TABLE-US-00002 TABLE II Patty Disper- Water, Amt., Size, Stiff sant
grams Additive grams cm Time 211 600 Control 0.0 20.7 2:15 211 600
DEQUEST 0.60 25.5 2:50 211 600 TSP 0.60 22.5 2:30 211 600 TSPP 0.60
25.0 2:40 Daxad 720 Control 0.0 13.2 <1:50 Daxad 720 DEQUEST
0.60 17.1 1:45 Daxad 720 TSP 0.60 14.0 1:20 Daxad 720 TSPP 0.60
15.2 2:10
[0073] Other dry additives, such as set accelerators or starches,
are preferably combined with the stucco prior to entry to the
slurry. Wet additives are generally added directly to the mixer
prior to introduction of the dry components. When all components
are added, the resulting slurry is mixed until a homogeneous slurry
is obtained.
* * * * *