U.S. patent application number 11/152317 was filed with the patent office on 2006-12-14 for modifiers for gypsum slurries and method of using them.
This patent application is currently assigned to United States Gypsum Company. Invention is credited to David R. Blackburn, Dennis M. Lettkeman, Qingxia Liu, Brian S. Randall, Michael P. Shake, John W. Wilson.
Application Number | 20060280898 11/152317 |
Document ID | / |
Family ID | 37522959 |
Filed Date | 2006-12-14 |
United States Patent
Application |
20060280898 |
Kind Code |
A1 |
Lettkeman; Dennis M. ; et
al. |
December 14, 2006 |
Modifiers for gypsum slurries and method of using them
Abstract
An improved gypsum slurry that includes water, calcium sulfate
hemihydrate, a polycarboxylate dispersant and a modifier. The
modifier is chemically configured to improve the efficacy of the
polycarboxylate dispersant. Preferred modifiers include cement,
lime, slaked lime, soda ash, carbonates, silicates and
phosphates.
Inventors: |
Lettkeman; Dennis M.;
(Watonga, OK) ; Shake; Michael P.; (Johnsburg,
IL) ; Liu; Qingxia; (Vernon Hills, IL) ;
Wilson; John W.; (Fairview, OK) ; Randall; Brian
S.; (Fairview, OK) ; Blackburn; David R.;
(Barrington, IL) |
Correspondence
Address: |
GREER, BURNS & CRAIN
300 S WACKER DR
25TH FLOOR
CHICAGO
IL
60606
US
|
Assignee: |
United States Gypsum
Company
|
Family ID: |
37522959 |
Appl. No.: |
11/152317 |
Filed: |
June 14, 2005 |
Current U.S.
Class: |
428/70 ; 106/772;
106/778; 106/781; 106/783; 106/785; 106/788 |
Current CPC
Class: |
C04B 28/14 20130101;
C04B 28/14 20130101; C04B 22/16 20130101; C04B 22/10 20130101; C04B
24/003 20130101; C04B 14/04 20130101; C04B 22/064 20130101; C04B
24/003 20130101; C04B 14/04 20130101; C04B 24/2641 20130101; C04B
2103/408 20130101; C04B 22/16 20130101; C04B 22/064 20130101; C04B
22/10 20130101; Y10T 428/232 20150115; C04B 28/14 20130101 |
Class at
Publication: |
428/070 ;
106/778; 106/772; 106/781; 106/785; 106/783; 106/788 |
International
Class: |
C04B 28/14 20060101
C04B028/14; C04B 11/00 20060101 C04B011/00; B32B 1/04 20060101
B32B001/04 |
Claims
1. A gypsum slurry comprising: water; calcined gypsum; a
polycarboxylate dispersant; and at least one modifier chemically
configured to increase the efficacy of said polycarboxylate
dispersant.
2. The gypsum slurry of claim 1 wherein said modifier is selected
from the group consisting of lime, soda ash, carbonates, silicates,
phosphates, phosphonates and combinations thereof.
3. The gypsum slurry of claim 2 wherein said modifier is soda
ash.
4. The gypsum slurry of claim 1 wherein said modifier is present in
concentrations of about 0.15% to about 0.25% by weight based on the
dry weight of said calcined gypsum.
5. The gypsum slurry of claim 1 wherein said polycarboxylate
dispersant comprises a vinyl repeating unit and a repeating unit
comprising at least one of the group consisting of acrylic acid,
methacrylic acid, maleic acid, their salts or esters.
6. The gypsum slurry of claim 1 wherein said polycarboxylate
dispersant has a charge density ratio of about 5 to about 8.
7. The gypsum slurry of claim 1 wherein said polycarboxylate
dispersant is present in amounts of about 0.05% to about 0.5 %
based on the dry weight of said calcined gypsum.
8. The gypsum slurry of claim 1 wherein said water is present in
amounts of about 0.4 to about 0.8 based on the dry weight of said
calcined gypsum.
9. A gypsum panel comprising at least one facing material; a gypsum
core comprising the hydrated gypsum slurry of claim 1.
10. The gypsum panel of claim 9 wherein said modifier is selected
from the group consisting of lime, soda ash, carbonates, silicates,
phosphates, cement and combinations thereof.
11. The gypsum panel of claim 10 wherein said modifier is soda
ash.
12. The gypsum panel of claim 9 wherein said modifier is present in
concentrations of about 0.05% to about 0.25% by weight based on the
dry weight of said calcined gypsum.
13. The gypsum panel of claim 9 wherein said polycarboxylate
dispersant comprises a polyether repeating unit and at least one of
acrylic acid, methacrylic acid and maleic acid repeating units.
14. The gypsum panel of claim 9 wherein said polycarboxylate
dispersant has a charge density ratio of about 5 to about 8.
15. The gypsum panel of claim 9 wherein said polycarboxylate
dispersant is present in amounts of about 0.05 % to about 0.5 %
based on the dry weight of said calcined gypsum.
16. The gypsum panel of claim 9 wherein said water is present in
amounts of about 0.5 to about 0.6 based on the dry weight of said
calcined gypsum.
17. A method of making a gypsum slurry comprising a polycarboxylate
dispersant comprising: selecting a modifier chemically configured
to increase the efficacy of the polycarboxylate dispersant;
combining the modifier, dispersant, water and calcined gypsum to
form a gypsum slurry.
18. The method of claim 17 wherein the modifier is selected from
the group consisting of lime, soda ash, carbonates, silicates,
phosphates and cement.
Description
CROSS REFERENCES TO RELATED APPLICATIONS
[0001] This application is related to co-pending U.S. Ser. No.
11/______ (Attorney Ref. No. 2033.72380), entitled "Fast Drying
Wallboard", U.S. Ser. No. 11/______ (Attorney Ref. 2033.72699),
entitled "High Strength Flooring Compositions", U.S. Ser. No.
11/______ (Attorney Ref. No. 2033.72740), entitled "Gypsum Products
Utilizing a Two-Repeating Unit System and Process for Making Them";
U.S. Ser. No. 11/______ (Attorney Ref. No. 2033.73064), entitled
"Method of Making a Gypsum Slurry with Modifiers and Dispersants"
and U.S. Ser. No. 11/______ (Attorney Ref. No. 2033.73130),
entitled, "Effective Use of Dispersants in Wallboard Containing
Foam", all filed concurrently herewith and all hereby incorporated
by reference.
BACKGROUND
[0002] This invention relates to improved gypsum products. More
specifically, it relates to an improved gypsum slurry that is
flowable at low water concentrations, with less expense and less
retardive effects of using carboxylate dispersants alone.
[0003] Gypsum products are commonly used as building materials for
many reasons, such as wallboard. Wallboard sheets are easily joined
together to make continuous walls of any size and shape. They are
easily patched and have fire and sound proofing properties.
Decorative finishes, such as wallpaper or paint readily adhere to
plaster or wallboard surfaces to allow for a large variety of
decorating options.
[0004] The strength of gypsum products made from full density
slurries is inversely proportional to the amount of water used in
their manufacture. Some of the water that is added to the gypsum
slurry is used to hydrate the calcined gypsum, also known as
calcium sulfate hemihydrate, to form an interlocking matrix of
calcium sulfate dihydrate crystals. Excess water evaporates or is
driven off in a kiln, leaving voids in the matrix once occupied by
the water. Where large amounts of water were used to fluidize the
gypsum slurry, more and larger voids remain in the product when it
is completely dry. These voids decrease the product density and
strength in the finished product.
[0005] 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 the resultant increase in product strength, however, there are
disadvantages known to be associated with use of large doses of
polycarboxylate dispersants. These materials are relatively
expensive. When used in large doses, polycarboxylate dispersants
can be the 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.
[0006] 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 stacked and wrapped for shipping. 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.
[0007] Lime has been used in plaster to improve its workability. It
gives the plaster a good "feel", imparting a smoothness and
plasticity that makes it easy to trowel. Since it is alkaline, lime
acts to make some retarders more efficient, increasing the open
time of the plaster. Finally, the lime present in the plaster
oxidizes over time to form calcium carbonate which gives the
surface a hardness beyond that obtainable with plaster alone.
[0008] U.S. Pat. No. 5,718,759 teaches the addition of silicates to
mixtures of beta-calcined gypsum and cement. In the examples,
lignosulfates or naphthalene sulfonates are used as water-reducing
agents. The addition of pozzolanic materials, including silicates,
is credited with reducing expansion due to the formation of
ettringite. The composition is suggested for use in building
materials, such as backer boards, floor underlayments, road
patching materials fire-stopping materials and fiberboard.
[0009] Luongo, in U.S. Pat. No. 6,391,958, teaches a novel
wallboard composition combining gypsum with sodium silicates and a
synthetic, cross-linking binder. Vinyl acetate polymers were the
preferred cross-linking binder. The addition of sodium silicates
reduces the amount of calcined gypsum that is needed to make a
given number of panels. The weight of the building panel is
reduced, making it easier for workers to move the panels before and
during installations.
[0010] A number of polycarboxylate dispersants are disclosed in
U.S. Pat. No. 6,005,040. In one embodiment of the invention, a
water soluble polymer is disclosed having one repeating unit of the
formula: ##STR1##
[0011] where P is an integer from 1-10 and R.sup.2, R.sup.3,
R.sup.4, R.sup.5 and R.sup.6 are not all hydrogens, but any one of
them can be a hydrogen. The polymer also includes a water-soluble
repeating unit selected from a group containing acrylic acid and
methacrylic acid, among others. The polymer is used in an unfired,
ceramic precursor material. Other polymers are disclosed in the
application, including some for use in the core forming process of
gypsum wallboard or the preparation of gypsum slurries.
[0012] 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.
[0013] 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
[0014] These and other problems are improved by this invention
which includes the addition of an efficacy modifier to a gypsum
slurry that increases the fluidity of slurries made with
polycarboxylate dispersant. When one or more of the modifiers is
used, less dispersant is required to achieve a given fluidity
resulting as lower dispersant cost and generally less
retardation.
[0015] More specifically, the invention relates to an improved
gypsum slurry, that includes water, calcium sulfate hemihydrate, a
polycarboxylate dispersant and a modifier. The modifier is
chemically configured to improve the efficacy of the
polycarboxylate dispersant. Preferred modifiers include cement,
lime, slaked lime, soda ash, carbonates, silicates, phosphonates
and phosphates.
[0016] In another embodiment of this invention, a gypsum panel is
made from at least one facing sheet and a core made from the
improved gypsum slurry. Yet another aspect of this invention is a
method of making the gypsum slurry that includes selecting a
modifier, mixing the modifier with a polycarboxylate dispersant and
adding the calcium sulfate hemihydrate.
[0017] Use of the modifiers herein described improves the efficacy
of the dispersant in fluidizing the gypsum slurry. This allows less
of the dispersant to be used and still obtain high flowability in
low water slurries for strength. Polycarboxylate dispersants are
often one of the most expensive components in products that use
them. Using lower dosages of the dispersant reduces the cost so
that a competitively priced gypsum product can be made.
[0018] Lowering the concentration of the dispersant also minimizes
the disadvantageous effects of the polycarboxylate dispersant. At a
lower dose, there is less retardation of the setting reactions.
Less set accelerator would be needed in the product to overcome the
effects of set retardation, reducing the price paid for the
accelerator.
[0019] Instead of or in addition to reducing the dispersant dosage,
the improved efficacy of the dispersant can also be used to reduce
the amount of water used to make the gypsum slurry. The
manufacturing process can be made more fuel efficient, conserving
fossil fuels and realizing the cost savings. Fuel savings can be
based on either reduced kiln temperatures or shorter residence time
in the kiln.
DETAILED DESCRIPTION OF THE INVENTION
[0020] The gypsum slurry of this invention is made using water,
calcined gypsum, a polycarboxylate dispersant and a modifier.
Although the benefits of this invention are most clear when used in
a slurry for a high strength product, it can be used with any
slurry using a polycarboxylate, even those that already utilize low
doses of polycarboxylate dispersant.
[0021] Any calcined gypsum or stucco is useful in this slurry. It
has unexpectedly been discovered that some gypsum sources are more
responsive to the dispersant and the modifier than other sources.
Laboratory tests indicate that gypsums from different geographical
areas contain different salts and impurities. While not wishing to
be bound by theory, it is believed that the salts present in the
gypsum influence to the fluidity in the slurry.
[0022] Reduction in the amount of water used to make the slurry is
achieved by the addition of a polycarboxylate dispersant. 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 increased product strength
and lower drying costs.
[0023] Any polycarboxylate dispersant that is useful for improving
fluidity in gypsum may be used in the slurry of this invention. A
number of polycarboxylate 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/______ (Attorney Ref. No 2033.72740), 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".
[0024] 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.
[0025] The second repeating unit satisfies Formula I,
R.sup.1--O--R.sup.2 I
[0026] where R.sup.1 is an alkenyl group of Formula II ##STR2##
[0027] 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
[0028] 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.
[0029] 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). ##STR3##
[0030] 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.
[0031] 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/______ filed concurrently herewith, by Degussa Construction
Polymers (Attorney Ref. DCP3) entitled "Polyether-Containing
Copolymer", herein incorporated by reference.
[0032] 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 lower 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 dispensability. However, tests with gypsum indicate
that efficacy of the dispersant is reduced at molecular weights
above 50,000 Daltons.
[0033] 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 %.
[0034] 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).
##STR4##
[0035] 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, Gm bH) are described
for use in wallboard and gypsum slurries in U.S. Ser. No. 11/______
(Attorney Ref. No. 2033.72380), entitled "Fast Drying Wallboard",
previously incorporated by reference.
[0036] 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. ##STR5##
[0037] 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 dependent 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
stucco. Preferably, the dispersant is used in amounts of about
0.05% to about 0.5% based on the dry weight of the stucco. 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.
[0038] 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.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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 hemihydrate. 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.
[0044] 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 hemihydrate. 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 %.
[0045] 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.
[0046] Modifiers appear to be less effective if the calcium sulfate
hemihydrate is wetted with the dispersant before the modifier is
added to the mixture. It is, therefore, preferred that the
dispersant and the modifier be combined prior to mixture with the
stucco. If either the modifier or the dispersant is in a liquid
form, the liquid is preferably added to the process water. The
other of the modifier or the dispersant is then added to the water
prior to addition of the calcium sulfate hemihydrate. Only a few
seconds of mixing is needed to blend the modifier and the
dispersant together. If both the modifier and the dispersant are in
dry form, they can be mixed together and added simultaneously with
the stucco. The preferred method of combining water, dispersant,
modifier and stucco is further described in U.S. Ser. No. 11/______
(Attorney Ref. No. 2033.73064), entitled "Method of Making a Gypsum
Slurry with Modifiers and Dispersants", previously incorporated by
reference.
[0047] 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.
[0048] 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 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.
[0049] Any amount of water may be used to make the slurry of this
invention as long as the slurry has sufficient fluidity for the
application being considered. The amount of water varies greatly,
depending on the source of the stucco, how it is calcined, the
additives and the product being made. For wallboard applications, a
water to stucco ratio ("WSR") of 0.18 to about 0.8 is used,
preferably from about 0.2 to about 0.5. Pourable flooring utilizes
a WSR of from about 0.17 to about 0.45, preferably from 0.17 to
about 0.34. Flooring compositions using this dispersant are
revealed in U.S. Ser. No. 11/______ (Attorney Ref. 2033.72699),
entitled "High Strength Flooring Compositions", previously
incorporated by reference, that utilize water to stucco ratios less
than 0.3. Castable products utilize a WSR from about 0.1 to about
0.3, preferably from about 0.16 to about 0.25. In the laboratory,
water to stucco ratios of less than 0.1 are attainable, however,
commercially, the water to stucco ratio is typically from 0.5 to
0.7. Generally, water to stucco ratios of about 0.2 to about 0.6
are preferred. The amount of water can be diminished compared to
other slurries, resulting in fuel savings and higher product
strength.
[0050] In a second aspect of this invention, the slurry is used to
make gypsum panels or wallboard having increased strength. To form
gypsum panels, the slurry is poured onto at least one sheet of
facing material. Facing materials are well known to an artisan of
gypsum panels. Multi-ply paper is the preferred facing material,
however, single-ply paper, cardboard, plastic sheeting and other
facing materials may be used.
[0051] 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 (9.8g/m.sup.2)) or dry
accelerators (up to about 35 lb./MSF (170 g/m.sup.2)) 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 of
sugar per 100 pounds of calcium sulfate dihydrate. It is further
described in U.S. Pat. No. 2,078,199, herein incorporated by
reference. Both of these are preferred accelerators.
[0052] 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.
[0053] In some embodiments of the invention, additives are included
in the gypsum slurry to modify one or more properties of the final
product. Additives are used in the manner and amounts as are known
in the art. Concentrations are reported in amounts per 1000 square
feet of finished board panels ("MSF"). Starches are used in amounts
from about 3 to about 20 lbs./MSF (14.6 to 97.6 g/m.sup.2) to
increase the density and strengthen the product. Glass fibers are
optionally added to the slurry in amounts of at least 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 lbs./MSF (0.4 kg/m.sup.2) to improve the
water-resistency of the finished gypsum board panel.
[0054] In embodiments of the invention that employ a foaming agent
to yield voids in the set gypsum-containing product to provide
lighter weight, 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. 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. If foam is added to the product, the polycarboxylate
dispersant can be divided between the process water and the foam
water prior to its addition to the calcium sulfate hemihydrate. A
preferred method of incorporating one or more dispersants into the
mixer water and the foam water is disclosed in U.S. Ser. No.
11/______ (Attorney Ref. No. 2033.73130), entitled, "Effective Use
of Dispersants in Wallboard Containing Foam", previously
incorporated by reference.
[0055] A trimetphosphate compound is added to the gypsum slurry in
some embodiments to enhance the strength of the product and to
reduce 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 calcined gypsum. 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 alkalinity 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.
[0056] 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 facing or the gypsum core. When
used, biocides are used in the facings in amounts of less than
about 500 ppm.
[0057] In addition, the gypsum composition optionally can include a
starch, such as a pregelatinized starch and/or an acid-modified
starch. 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. 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.
[0058] 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 polysiloxanes 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.
[0059] While individual gypsum panels can be made in a batch
process, in a preferred process, gypsum board is made in a
continuous process formed into a long panel and cut into panels of
desired lengths. The formed facing material is obtained and put
into place to receive the gypsum slurry. Preferably, the facing
material is of a width to form a continuous length of panel that
requires no more than two cuts to make a panel with the desired
finished dimensions. Any known facing material is useful in making
the wallboard panels, including paper, glass mat and plastic
sheeting. Facing material is continuously fed to the board
line.
[0060] The slurry is formed by mixing the dry components and the
wet components together in any order. Typically, liquid additives
are added to the water, and the mixer is activated for a short time
to blend them. Water is measured directly into the mixer. If
modifiers are used, preferably the modifiers and dispersants are
predissolved in the mixer water prior to introduction of the
stucco. Dry components of the slurry, the calcined gypsum and any
dry additives, are preferablyblended together prior to entering the
mixer. The dry components are added to the liquid in the mixer, and
blended until the dry components are moistened.
[0061] The slurry is then mixed to achieve a homogeneous slurry.
Usually, an aqueous foam is mixed into the slurry to control the
density of the resultant core material. Such an aqueous foam is
usually generated by high shear mixing of an appropriate foaming
agent, water and air to prior to the introduction of the resultant
foam into the slurry. The foam can be inserted into the slurry in
the mixer, or preferably, into the slurry as it exits the mixer in
a discharge conduit. See, for example, U.S. Pat. No. 5,683,635,
herein incorporated by reference. In a gypsum board plant,
frequently solids and liquids are continuously added to a mixer,
while the resultant slurry is continuously discharged from the
mixer, and has an average residence time in the mixer of less than
30 seconds.
[0062] The slurry is continuously dispensed through one or more
outlets from the mixer through a discharge conduit and deposited
onto a moving conveyor carrying the facing material and formed into
a panel. Another paper cover sheet is optionally placed on top of
the slurry, so that the slurry is sandwiched between two moving
cover sheets which become the facings of the resultant gypsum
panel. The thickness of the resultant board is controlled by a
forming plate, and the edges of the board are formed by appropriate
mechanical devices which continuously score, fold and glue the
overlapping edges of the paper. Additional guides maintain
thickness and width as the setting slurry travels on a moving belt.
While the shape is maintained, the calcined gypsum is kept under
conditions sufficient (i.e. temperature of less than about
120.degree. F.) to react with a portion of the water to set and
form an interlocking matrix of gypsum crystals. The board panels
are then cut, trimmed and passed to dryers to dry the set but still
somewhat wet boards.
[0063] Preferably, a two-stage drying process is employed. The
panels are first subjected to a high temperature kiln to rapidly
heat up the board and begin to drive off excess water. The
temperature of the kiln and the residence time of the board vary
with the thickness of the panel. By way of example, a 1/2-inch
board (12.7 mm) is preferably dried at temperatures in excess of
300.degree. F. (149.degree. C.) for approximately 20 to 50 minutes.
As water at the surface evaporates, it is drawn by capillary action
from the interior of the panel to replace the surface water. The
relatively rapid water movement assists migration of the starch and
the pyrithione salt into the paper. A second-stage oven has
temperatures less than 300.degree. F. (149.degree. C.) to limit
calcination of the board.
EXAMPLE I
[0064] Tests were conducted to determine the effect of the addition
of potassium carbonate on two different dispersants. In each of the
following samples, a gypsum slurry was made from 400 grams of
stucco from Southard, Okla., 180 grams of water and 0.2% dispersant
based on the dry weight of the stucco. The dispersant type and
amount of potassium carbonate are shown in Table I below, together
with the results of the patty size and the stiffening rate tests.
TABLE-US-00001 TABLE I Potassium Stiffening Dispersant Carbonate, g
Patty Size, cm Time 211 0.6 30.3 6:00 211 0.0 19.8 2:05 Melflux
2500L 0.6 26.0 10:30 Melflux 2500L 0.0 15.5 2:35
[0065] As is seen in the data in Table I above, the addition of
potassium carbonate increases the slurry fluidity as evidenced by
the increase patty size. The modifier addition also retarded the
stiffening time compared to the samples where no potassium
carbonate was used.
EXAMPLE 2
[0066] Tests were run to determine the effect of lime on dispersant
MELFLUX 2500L with two different stuccos. TABLE-US-00002 TABLE II
Water to Stucco Ratio at Given Dispersant Dose Stucco Lime 0.0%
0.05% 0.11% 0.22% Shoals 0 0.65 0.64 0.58 0.50 Shoals 0.25% 0.68
0.61 0.52 0.41 Galena 0 0.60 0.53 0.45 0.39 Galena 0.25% 0.64 0.50
0.39 0.30
[0067] Tests presented above show that lime is an effective modifer
with stuccos from Shoals and Galena Park.
EXAMPLE 3
[0068] Soda ash was tested in the laboratory for suitability as a
modifier. The amount of water listed in Table III, the water
demand, was added to 50 cc of board stucco, a beta-calcined
hemihydrate. This amount of water was selected to produce a
standard 3-3/4'' patty when the stucco, dispersant, modifier and
water were combined. The dispersant was added at the rate of
1.5#/MSF on a solids basis as if it were being added to the slurry
for a 1/2'' wallboard. The dispersant was a PCE211, two-repeating
unit dispersant, identified as PCE49.
[0069] Sodium carbonate was added at the concentrations shown in
Table III, ranging from none to 0.6% by weight based on the weight
of the dry stucco. The water demand and the set time are shown in
Table III. TABLE-US-00003 TABLE III Na.sub.2CO.sub.3 cc Water Added
Set Time 0.0% 52 17 0.05% 47 17 0.10% 45 17 0.20% 45 15 0.40% 47 11
0.60% 50 11
[0070] Up to and including, 0.2%, the amount of water needed to
make a patty of standard diameter decreases as the amount of the
modifier increases. At 0.40% and 0.60% levels, the water demand
again rises. The set time consistently drops as the amount of soda
ash increases.
EXAMPLE 4
[0071] A similar study was conducted, except that CaMg(OH).sub.4
was used as the modifier. Water was added in the amounts reported
in Table IV. The same stucco and dispersant were used in the same
amounts. TABLE-US-00004 TABLE IV CaMg(OH).sub.4 cc Water Added Set
Time 0.0% 52 17 0.05% 48 13 0.10% 47 11 0.20% 45 11 0.40% 44.5 11
0.60% 45 9
[0072] Hydroxides act as modifiers, allowing less water to be used
to obtain a standard fluidity which produces a standard patty size.
Although the efficacy is good, hydroxides are not suitable for some
products, such as wallboard, because the slurry becomes very
alkaline, causing loss of efficacy of some preferred additives,
including trimetaphosphate. Even at 0.05% CaMg(OH).sub.4, the pH
was above 10. For products where pH of the product is not a
problem, hydroxides can be used effectively as modifiers.
EXAMPLE 5
[0073] The preferred 211 dispersant was tested with a variety of
modifiers to determine the improvement in efficacy. Reagent grade
tetra sodium phosphate ("TSP"), tetra sodium pyrophosphate ("TSPP")
and sodium carbonate (soda ash) were tested. Dequest 2006 (Solutia,
Inc. St. Louis, Mo.), a penta sodium salt of aminotri (methylene
phosphonic acid), was also tested.
[0074] For all testing samples, the water to stucco ratio was 0.5.
Wet gypsum accelerator (WGA) was based on the dry weight of the
stucco was added. The control sample had only 0.5% by weight WGA.
The amount of each modifier added is shown in Table V, along with
the set time and patty size produced by each sample.
[0075] The modifier and dispersant were added to the water,
followed by addition of the stucco and WGA. The slurry was stirred
until it was consistent. TABLE-US-00005 TABLE V DEQUEST Soda
Modifier Control 2006 TSP TSPP Ash Amount 0 0.05% 0.05% 0.05% 0.15%
Patty Size 20 cm 23.7 cm 21.5 cm 25.5 cm 27.5 cm Stiffening 2:15
2:35 2:15 2:55 2:30 Time
[0076] Even though more soda ash was used to obtain these results,
it is considered to be effective because it costs one third the
price of the other modifiers. Further, it increases the patty size
by 37% while increasing the set time only 11%. DEQUEST 2006 yields
a much smaller patty for about the same set time and TSPP has a
smaller patty size but has a higher set time.
[0077] While a particular embodiment of the modifiers for gypsum
products, it will be appreciated by those skilled in the art that
changes and modifications may be made thereto without departing
from the invention in its broader aspects and as set forth in the
following claims.
* * * * *