Note: Descriptions are shown in the official language in which they were submitted.
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GYPSUM WALLBOARD CORE, AND
METHOD AND APPARATUS FOR MAKING THE SAME
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention generally relates to the production of
gypsum board materials and, more specifically, the invention relates to the
manufacture of gypsum wallboard utilizing an extrusion technique to prepare a
gypsum core.
Brief Description of Related Technology
A common method of constructing walls and ceilings includes
the use of inorganic wallboard panels or sheets, such as gypsum wallboard,
often referred to simply as "wallboard" or "drywall." Wallboard can be
formulated for interior, exterior, and wet applications. The use of wallboard,
as opposed to conventional boards made from wet plaster methods, is
desirable because the installation of wallboard is ordinarily less costly and
less
cumbersome when compared to the installation of conventional plaster walls.
Walls and ceilings made with gypsum wallboard panels
typically are constructed by securing, e.g., with nails or screws, the
wallboard
panels to structural members, such as vertically- and horizontally-oriented
pieces of steel or wood often referred to as "studs." Because wallboard
typically is supplied in standard-sized sheets or panels, when forming a wall
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from the sheets, there will generally be a number of joints between adjacent
sheets. In most wallboard construction, these joints typically are filled and
coated with an adhesive material called a joint compound so that the wall will
have a smooth finish similar to that obtained with conventional plaster walls.
Generally, wallboard is produced by enclosing a core of an
aqueous slurry of calcined gypsum and other materials between two large
sheets of board cover paper. Various types of cover paper are known in the
art. After the gypsum slurry has set (i.e., reacted with water present in the
aqueous slurry) and dried, the formed sheet is cut into standard sizes.
Methods
for the production of gypsum wallboard generally are described, for example,
by Michelsen, T. "Building Materials (Survey)," Encyclopedia of Chemical
Technology, (1992 4th ed.), vol. 21, pp. 621-24, TP9.E685, the disclosure of
which is hereby incorporated herein by reference.
Gypsum wallboard is manufactured utilizing commercial
processes that are capable of operation under continuous, high-speed
conditions. A conventional process for manufacturing the core composition of
gypsum wallboard initially includes the premixing of dry ingredients in a high-
speed mixing apparatus. The dry ingredients can include calcium sulfate
hemihydrate (stucco), an accelerator, and an antidesiccant (e.g., starch). The
dry ingredients are mixed together with a "wet" (aqueous) portion of the core
composition in a pin mixer apparatus. The wet portion can include a first
component, commonly referred to as a "paper pulp solution," that includes a
mixture of water, paper pulp, and, optionally, one or more fluidity-increasing
agents, and a set retarder. The paper pulp solution provides a major portion
of
the water that forms the gypsum slurry of the core composition. A second wet
component can include a mixture of the aforementioned strengthening agent,
foam, and other conventional additives, if desired. Together, the
aforementioned dry and wet portions comprise an aqueous gypsum slurry that
eventually forms a gypsum wallboard core.
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A major ingredient of the gypsum wallboard core is calcium
sulfate hemihydrate, commonly referred to as "calcined gypsum," "stucco," or
"plaster of Paris." Stucco has a number of desirable physical properties
including, but not limited to, its fire resistance, thermal and hydrometric
dimensional stability, compressive strength, and neutral pH. Typically, stucco
is prepared by drying, grinding, and calcining natural gypsum rock (i.e.,
calcium sulfate dehydrate). The drying step in the manufacture of stucco
includes passing crude gypsum rock through a rotary kiln to remove any free
moisture present in the rock from rain or snow, for example. The dried rock
then is passed through a roller mill (or impact mill types of pulverizers),
wherein the rock is ground or comminuted to a desired fineness. The degree
of comminution is determined by the ultimate use. The dried, fine-ground
gypsum can be referred to as "land plaster" regardless of its intended use.
The
land plaster is used as feed to calcination processes for conversion to
stucco.
The calcination (or dehydration) step in the manufacture of
stucco is performed by heating the land plaster, and generally can be
described
by the following chemical equation which shows that heating calcium sulfate
dehydrate yields calcium sulfate hemihydrate (stucco) and water vapor:
CaS04~2H20 + heat ---~ CaS04~%ZHzO + 1 %Z HZO.
This calcination process step is performed in a "calciner," of which there are
several types known by those of skill in the art.
Uncalcined calcium sulfate (i.e., land plaster) is the "stable"
form of gypsum. However, calcined gypsum, or stucco, has the desirable
property of being chemically reactive with water, and will "set" rather
quickly
when the two are mixed together. This setting reaction is actually a reversal
of
the above-described chemical reaction performed during the calcination step.
The setting reaction proceeds according to the following chemical equation
which shows that the calcium sulfate hemihydrate is rehydrated to its
dehydrate
state:
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CaSOy'/ZH20 + 1'/Z H20 -~ CaS04~2Hz0 + heat.
The actual time required to complete the setting reaction generally depends
upon the type of calciner and the type of gypsum rock that are used to produce
the gypsum, and can be controlled within certain limits by the use of
additives
S such as retarders, set accelerators, and/or stabilizers, for example.
Generally,
the rehydration time period can be in a range of about two minutes to about
eight hours depending on the quantity of retarders, set accelerators, and/or
stabilizers present.
After the aqueous gypsum slurry is prepared, the slurry and
other desired ingredients are continuously deposited to form a gypsum
wallboard core (hereinafter "wallboard core" or "core") slurry between two
continuously-supplied moving sheets of cover paper. The two cover sheets
comprise a pre-folded face paper and a backing paper. As the slurry is
deposited onto the face paper, the backing paper is brought down atop the
deposited core slurry and bonded to the prefolded edges of the face paper. The
whole assembly then is sized for thickness utilizing a roller bar or forming
plate. The deposited core slurry is then allowed to set between the two cover
sheets, thereby forming a board. The continuously-produced board is cut into
panels of a desired length, which are vertically-stacked, and then passed
through a drying kiln where excess water is removed from the board to form a
strong, dry, and rigid building material.
The cover sheets used in the process typically are multi-ply
paper manufactured from re-pulped newspapers. The face paper has an
unsized inner ply which contacts the core slurry such that gypsum crystals can
grow up to (or into) the inner ply-this, along with the starch present in the
slurry, is the principal form of bonding between the core slurry and the cover
sheet. The middle plies are sized and an outer ply is more heavily sized and
treated to control absorption of paints and sealers. The backing paper is also
a
similarly constructed mufti-ply sheet. Both cover sheets must have sufficient
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permeability to allow for water vapor to pass therethrough during the
downstream board drying step(s).
Standardized sheets (or panels) of wallboard typically are about
four feet (about 1.22 meters) wide and about 8 feet to about 16 feet (about
2.4
meters to about 4.9 meters) in length. Sheets typically are available in
thicknesses varying in a range of about '/4 inch to about one inch (about 0.6
centimeters to about 2.6 centimeters).
In order to provide satisfactory strength, commercially-
available gypsum wallboard generally requires a density of about 1600 to
about 1700 pounds (about 726 to about 772 kilograms) per thousand square
feet (lbs/MSF) of one-half inch board. Heavy or high-density gypsum
wallboards are more costly and difficult to manufacture, transport, store, and
manually install at job sites, compared to lighter or low-density boards. It
is
possible to formulate wallboard having reduced densities through the inclusion
of lightweight fillers and foams, for example. Often, however, where
wallboard is formulated to have a density less than about 1600 lbs/MSF of
one-half inch board, the resulting strength is unacceptable for commercial
sale.
Because high-density or heavy gypsum wallboard generally is not desirable,
various attempts have been made to reduce board weight and density without
sacrificing board strength. However, while lighter and less dense wallboard
products can be produced, many of the wallboard products may be of a quality
ill-suited for commercial use.
Moms et al. U.S. Patent No. 5,482,551 discloses an extrudable
composition for use in production of articles for building and construction
made of about 45-85 weight percent calcium sulfate dihydrate and a filler to
control density.
In view of the foregoing, it would be desirable to produce high-
strength gypsum wallboard having weights and densities generally equal to or
slightly less than that produced by conventional methods. Reduced weight and
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density boards, however, should meet industry standards and have strengths
similar to, or greater than, conventional wallboard. Such wallboard also
should be able to be manufactured using high-speed manufacturing apparatus
and not suffer from other negative side-effects. For example, such high-
strength wallboard should be able to set and dry within a reasonable period of
time.
Further, the use of conventional ingredients in the preparation
of the aqueous slurry can cause fouling and undesired plugging of mixers and
tubing used to prepare and convey, respectively, the aqueous slurry onto the
paper cover sheet(s). For example, admixing set accelerators into the slurry
in
an upstream mixer, such as a pin mixer, can cause the slurry to begin setting
(i.e., calcine or harden) before its deployment onto the cover sheet(s). Thus,
it
would be desirable to produce gypsum wallboard utilizing processes that do
not require certain ingredients in the gypsum slurry which could cause fouling
or premature setting of the slurry, or impart other undesired effects.
SUMMARY OF THE INVENTION
One aspect of the invention is a method of forming a gypsum
wallboard core, which includes the method steps of preparing a gypsum slurry
in a mixer, and introducing the slurry into an extrusion die, the die
preferably
having provisions at its periphery for the introduction of at least one gypsum
slurry additive selected from the group consisting of slip agents and strength-
enhancing agents. The method also preferably includes the steps of
introducing the slurry additive into the slurry as the slurry exits the die,
and
extruding the additive-containing gypsum slurry onto a substantially flat,
moving surface. Thereafter, the slurry can be dried to in a hybrid dryer by
convection heating and microwave drying techniques. Therefore, another
aspect of the invention is a gypsum wallboard core prepared according to the
foregoing method.
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Still further, another aspect of the invention is an apparatus for
preparing a gypsum wallboard core. Generally, the apparatus includes a mixer
that is in fluid communication with a die inlet of a die. The die comprises
the
die inlet and a die exit and a manifold disposed between the inlet and outlet.
The apparatus also includes a substantially flat, movable surface disposed
adjacent the die exit and a dryer.
Advantages of the invention may become apparent to those
skilled in the art from a review of the following detailed description, taken
in
conjunction with the drawings, the examples, and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
For a complete understanding of the invention, reference should
be made to the following detailed description, examples, and accompanying
drawings wherein:
Figure 1 is a schematic diagram illustrating various process
equipment that may be useful in carrying out a method of making gypsum
wallboard according to the invention; and,
Figure 2 is a schematic diagram illustrating in more detail
features of the equipment and method illustrated in Figure 1.
While the invention is susceptible of embodiment in various
forms, there are illustrated in the drawing figures and will hereafter be
described specific embodiments of the invention, with the understanding that
the disclosure is intended to be illustrative, and is not intended to limit
the
invention to the specific embodiments described and illustrated herein.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Generally, the invention is directed to a method of forming a
gypsum wallboard core, which includes the method steps of preparing a
gypsum slurry in a mixer and introducing the slurry, preferably under positive
displacement into an extrusion die, the die preferably having provisions at
its
periphery for the introduction of at least one gypsum slurry additive selected
from the group consisting of slip agents and strength-enhancing agents. The
method also preferably includes the steps of introducing the slurry additive
into the slurry as the slurry exits the die, and soft-extruding the additive-
containing gypsum slurry onto a substantially flat, moving surface.
Thereafter,
the slurry can be dried to in a hybrid dryer by convection heating and
microwave drying techniques.
For a general overview of the invention, reference should be
made to the drawing figures wherein like reference numbers designate the
same or similar structure throughout the various figures. Figure 1 illustrates
an embodiment of a process 10 of the present invention. The process 10
includes a mixer 12 which mixes dry and wet ingredients making up a gypsum
slurry (not shown) and discharges the formed slurry through a die 14,
described in greater detail below. The slurry exits the die 14 and is extruded
directly onto a smooth surface of a conveyor belt 16. A top conveyor belt 18,
optionally, also can be used. Downstream of the conveyor belt 16 is a hybrid
dryer 20 that includes microwave heating and convection drying sections (not
shown). Downstream of the hybrid dryer 20 is a convection dryer 22 wherein
any excess water present in the extruded slurry is removed via convection
drying to result in a dry wallboard core. Both of the hybrid dryer 20 and
convection dryer 22 are described in greater detail below.
With continued reference to Figure 1, downstream of the hybrid
dryer 20 and convection dryer 22, the dried wallboard core can undergo
optional core enhancing treatments in a suitable apparatus 24 known to those
skilled in the art. To the extent such optional core enhancing treatments
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include the application of water to the wallboard core, an optional surface
treatment dryer 26, typically a small convection dryer, can be used to remove
the water. The wallboard core, optionally, can be surface treated to include
various coverings (i.e., paper cover sheets), decorative coverings, and/or
lamination in a suitable surface treatment apparatus 28 before being trimmed
and cut with trimming equipment 30. After the wallboard has been cut and
shaped, the wallboard can be stacked and bundled utilizing stacking equipment
32 known to those of skill in the art and stored in a warehouse 34, or using
other suitable storage means.
Figure 2 illustrates a more detailed view of the process 10.
Specifically, the mixer 12 is shown in Figure 2 connected at a mixer discharge
end 36 to the die 14. The die 14 includes a die inlet 38, a die exit 40, and a
die
manifold 42 disposed between the inlet 38 and exit 40. The die 14 preferably
includes a plurality of conduits, such as for example, one or more conduits
44,
for introducing various additives into the discharging slurry. The slurry is
discharged directly onto the conveyor belt 16 which carries the deposited
slurry to downstream processing equipment (described and shown in Figure 1 )
where preparation of the wallboard product can be completed.
A preferred embodiment of the invention includes the
preparation of gypsum slurry in a mixer (described in more detail below),
other than a pin mixer. The slurry includes gypsum, water, slip agents, water-
reducing agents, surfactants and, optionally, binders, set retarders (setting
agents), paper pulp, glass fibers, fly ash spheres, mica, paraffin granules,
perlite, and/or vermiculite. Preferably, a pregenerated foam also is
introduced
into the mixer in a manner such that the foam is homogeneously distributed
throughout the slurry; the foam is not injected into the slurry as the slurry
exits
the mixer or die because foam injection does not necessarily ensure
homogenous distribution.
A suitable mixer for use in accordance with the invention
should be capable of delivering a viscous gypsum slurry to a die under
positive
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displacement. The term "positive displacement" is intended to mean that a
gypsum slurry, or ingredients comprising a gypsum slurry, are passed through
a mixer having no (or no excess) void spaces therein. The phrases "void
spaces" and "excess void spaces" are intended to mean spaces existing within
a mixer during mixer operation that are not filled with a gypsum slurry or
ingredients comprising a gypsum slurry. Void spaces (or excess void spaces)
within the mixer provide areas for portions of the slurry to undesirably
collect
or "hang up." The collected slurry sits in these void spaces and eventually
hydrates (hardens). In time, the hardened gypsum is likely to become
dislodged from the void spaces and undesirably flow in a mixture of the slurry
out of the mixer. Alternatively, the dislodged and hardened gypsum can
damage and/or plug the mixer and, thereby, force the operator to shut down
the mixer for a period of time sufficient to fix the damaged mixer components
and/or to remove the hardened gypsum material. A mixer operating to provide
a gypsum slurry under positive displacement, by the foregoing definition, has
a
minimal amount or no void spaces therein.
By way of example, a suitable mixer for use in the invention
includes a twin screw continuous mixer modified to ensure a positive
displacement. A twin screw mixer (hereinafter a "TSC mixer") can be
obtained from Readco Mfg. Co. of York, Pennsylvania, and is described in a
product bulletin for Readco's Continuous Processor (entitled the "Readco
Continuous Processor," undated) and in U.S. Patent No. 5,000,900, the
disclosures of which are hereby incorporated herein by reference. While it is
not recommended that such a TSC mixer be used in the present invention
without modifications (described below), the general design of the TSC mixer
is informative nonetheless.
A TSC mixer generally includes adjacent, co-rotating shafts
(screws) axially disposed within a single barrel or housing. The screws rotate
in opposite directions with respect to each other. Each shaft includes paddles
(or screw threads) which extend from a screw shaft to almost contact or touch
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the inside surface of the barrel and to almost contact or touch the other
shaft
and its associated paddles (i.e., there should be no contact). During
operation,
the screws and associated paddles convey or push the materials (ingredients
and/or slurry) from one end of the mixer to the other end of the mixer, which
is preferably connected to a die. Furthermore, during operation, the close
proximity between the paddles of one shaft and its neighboring shaft, and
between the paddles and the inside barrel surface serve, to provide the TSC
mixer with a self wiping action which reduces the likelihood of fouling or
plugging of the mixer.
Generally, a TSC mixer is divided into multiple sections which
can be used to perform various functions including, but not limited to, pre-
mixing dry ingredients, conveying dry ingredients, pre-mixing wet ingredients,
mixing dry and wet ingredients to form a mixed slurry, and conveying the
slurry. In other words, sections of the TSC mixer are desirably designed to
perform different functions as the mixed slurry is formed and passes through
each section.
The TSC mixer is described hereafter with respect to imaginary
sections or locations along the screw shaft where raw materials are
introduced.
The sections generally are not separated by structure, and instead are defined
by the locations at which the operator determines that various components will
be added. For example, a first section of the TSC mixer preferably serves to
introduce and convey various dry ingredients, such as stucco and, optionally,
vermiculite and fiberglass, for example, which comprise a dry mixed blend. A
second section downstream of this first section preferably serves to convey
mix the dry mixed blend. A third section downstream of the first and second
sections preferably is designed to introduce pulp water (preferably containing
potash and pregenerated foam) into the dry mixed blend. Here, the
mixing/blending action of the TSC mixer is relatively high shear ensuring
thorough blending of all of the ingredients (wet and dry) in a short time and
over a screw shaft length. Additionally, a suitable set accelerator, such as a
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ball mill accelerator, can be added into the mixer and combined with the
mixed material to form the slurry. The slurry preferably is expelled from this
mixing/blending section and discharged into a die (described herein in greater
detail). Accordingly, the mixing/blending section of the TSC mixer also is
designed to convey the slurry into the die.
A commercially-available TSC mixer preferably is modified in
order to ensure positive displacement of the gypsum slurry. One such
modification includes ensuring that the cross-sectional area for flow
throughout the mixer remains constant. As previously noted, positive
displacement is believed to be ensured where the mixer contains a small
amount or no void spaces. With a constant flow of raw materials into the
mixer and a correspondingly consistent rotation of the screws, it is not
likely
that the slurry will undesirably collect and harden within a mixer having a
constant cross-sectional area.
Another preferred modification to the commercially-available
TSC mixer includes provisions (e.g., conduits or injection ports) throughout
the mixer for the introduction of the various dry and wet ingredients that
comprise the gypsum slurry. As previously noted, the dry and wet ingredients
are preferably introduced into the mixer in a sequential manner. In order to
introduce these ingredients in sequence and to ensure a proper mixing of all
slurry ingredients, the TSC mixer should be designed or modified to include a
plurality of conduits or injection ports for introduction of these
ingredients.
More preferably, the mixer should be modified to include multi-injection ports
46 (see Fig. 1 ) to ensure that water is properly mixed with the dry
ingredients.
The water introduced into these ports 46 can be in the form of foam water
and/or pulp water.
Other suitable mixers for use in the present invention include
modified versions of the mixers disclosed in U.S. Patent Nos. 5,304,355 and
5,607,233, which are assigned to Quantum Technologies, Inc., of Twinsburg,
Ohio, and are hereby incorporated herein by reference. The mixers described
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in the '355 and '233 patents generally are single-screw mixers which provide a
mixing of liquids, solids, and/or gases, and a self wiping action akin to that
characteristic of the TSC mixer described above. It is believed that the
mixers
disclosed in the '355 and '233 patents, when designed to include the
modifications mentioned above with respect to the TSC mixer, will provide a
viscous gypsum slurry to a die inlet under positive displacement.
Use of an appropriately modified mixer in the present invention
is particularly attractive because the screw of the single screw extruder (and
the co-rotating screws of the TSC mixer) and close clearances between mixer
paddles on the screw shaft(s), as well as between the paddles and the barrel
provide efficient, uniform mixing to result in a viscous gypsum slurry. For
example, throughput in the TSC mixer preferably is engineered such that an
average residence time of the material in the mixer preferably is less than
about 20 seconds, more preferably less than about 15 seconds, and even more
preferably less than about 10 seconds. Residence time, for purposes of the
present invention, can be defined as the time period beginning when water
first
contacts "dry" stucco and ending when that water/stucco (i.e., gypsum slurry)
mix exits the TSC mixer and is discharged into the die inlet or die manifold.
The more efficient mixing and short residence times prevent material buildup
(fouling or plugging) inside the barrel and result in a self wiping action
that
may reduce clean-up time by as much as 90%.
The gypsum slurry is soft-extruded through a die onto a
substantially flat, smooth, moving surface, such as a Teflon'material-covered
conveyor belt. A preferred die 14 is shown in Figure 2 and includes a die
inlet
38 connected to a discharge end of the mixer 12, a die manifold 42, and a die
exit 40. Inside the die manifold 42, the slurry occupies and takes on the
shape
of the manifold 42 and, therefore, obtains its final cross-sectional shape
before
exiting the die 14 through the die exit 40. Desirably, the die 14 and, more
specifically, the die manifold 42 reshape the slurry exiting the mixer 12 from
a
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rounded cross-sectional area to the thickness and width of the finished
product.
A gypsum slurry exiting the mixer at the mixer discharge end
36 will encounter various die passages as it passes through the die 14. The
incremental cross-sectional area, perpendicular to the direction of slurry
flow,
of the various die passages (i.e., die inlet 38, die exit 40, and die manifold
42)
preferably are constant (or are constantly increasing or decreasing) to
prevent
gypsum slurry from undesirably collecting and hardening within the die.
Furthermore, the die inlet 38 preferably has a cross-sectional area that is
substantially the same as that of the mixer discharge end 36.
The die specified for use in accordance with a preferred
embodiment of the invention deposits a gypsum slurry in a manner such that
the slurry exiting the die has cross-sectional dimensions substantially
identical
to that of the hardened wallboard core. This is accomplished by utilizing a
die
having a die exit (or opening), through which the slurry discharges onto the
moving conveyor belt, having cross-sectional dimensions substantially
identical to that of the hardened wallboard core. For example, a hardened
wallboard core (and the die exit both) can be about '/4 to about one inch
thick
(high) (about 0.64 to about 2.54 centimeters (cm)), which includes specific
board core thicknesses of about 5/16, 3/a, '/2, 5/a, and about'/4 inches
(about
0.79, 0.95, 1.27, 1.59, and about 1.90 cm, respectively). Additionally, a
hardened wallboard core (and the die exit both) can be about 4 to about 4.5
feet (about 1.22 to about 1.38 meters) wide. Thus, the die may have a width to
height ratio of about 48:1 to about 216:1. In contrast, discharge boots used
in
conventional gypsum wallboard manufacturing processes do not possess cross-
sectional dimensions in this range.
The die preferably is designed to include a die exit having a
substantially rectangular geometry defined by top and bottom forming plates
and side plates. These plates can be arranged to provide die exit dimensions
described in the foregoing paragraph. Preferably the bottom and side plates
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are fixed (i.e., immovable) and the top plate is adjustable to provide the
operator with flexibility in preparing gypsum wallboards of a variety of
thicknesses. Depending upon the downstream drying capacity (discussed in
greater detail below), the thickness of the desired wallboard and,
consequently
the thickness of the die exit opening, will aid in determining the speed at
which wallboard can be produced (i.e., line speed). Generally, as the cross
sectional area of the board decreases the line speed will increase, and vice
versa. The throughput (i.e., the residence time of the slurry ingredients) in
the
upstream mixer can be adjusted to accommodate varying line speeds,
wallboard cross-sectional areas, and downstream drying capacity.
The die preferably has one or more (preferably a plurality of)
provisions or conduits for the introduction (via injection means, for example)
of set accelerators and additional slip agents into the slurry, and provisions
or
conduits at its lateral outer edges for the introduction of a strength-
enhancing
acrylic polymer (described below) capable of providing reinforcement to edge
portions of the formed board product. Suitable dies for use in accordance with
the invention can be obtained from a number of die manufacturers and,
thereafter, modified to include the provisions or conduits. For example, a
suitable die with modifications can be obtained from the Phoenix Engineering
Group of National Gypsum Company, of Phoenix, Arizona. Because the
gypsum slurry mixer, described above, typically discharges the slurry product
in a downward fashion, a transition conduit preferably connects the mixer
discharge to the die inlet or manifold.
Use of the phrases "soft-extrusion" and "soft-extruded," are
intended to mean that the gypsum slurry exits a suitable die without the
assistance or the application of high pressure, which typically is required to
extrude highly viscous materials, such as thermoplastic and thermoset resins
used in other fields. While the invention is not bound by any particular
mechanism or theory, it is believed that ingredients (both wet and dry)
entering
the mixer help force the slurry through the mixer, into the manifold, and
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through the die exit. The ingredients introduced into the mixer along with the
positive displacement of slurry by the mixer, exert a force on the slurry
passing through the die, such that the slurry exits the die under a pressure
of
about 5 to about 100 pounds per square inch (psi) (about 35 to about 690
kilopascals (kPa)), and preferably about 5 to about 30 psi (about 35 to about
207 kPa), more preferably about 10 to about 20 psi (about 69 to about 137
kPa).
Generally, the physical appearance and consistency of the soft-
extruded slurry resembles that of a soft ice cream, as opposed to that of a
cake
or pancake batter. Thus, the deposited gypsum slurry desirably has a viscosity
in a range of about 19,000 millipascal~seconds (mPa~s) to about 22,000 mPa~s.
Preferably, however, the deposited gypsum slurry has a viscosity in a range of
about 19,500 mPa~s to about 21,500 mPa~s. More preferably, the deposited
gypsum slurry has a viscosity in a range of about 19,500 mPa~s to about
21,000 mPa~s. The foregoing viscosity measurements can be obtained using a
Brookfield DV-III Programmable Rheometer with a No. 6 spindle operating at
about 20 rotations per minute (RPM).
Once extruded onto the conveyor belt, the slurry is chemically-
activated to set and form a hardened board core which may be easily removed
from the conveyor belt, dried via a combination of microwave heating and
convection drying means, laminated, and cut to a desired shape and size, as
described in detail below.
In accordance with the invention, the gypsum slurry desirably
has an open time (i.e., a working time) of less than about 30 seconds,
preferably less than about 20 seconds, and more preferably less than about 10
seconds. The term "open time" as used herein refers to the time that elapses
between (a) the exposure to the atmosphere of the gypsum slurry, and (b) the
point where the calcined gypsum has reacted sufficiently with the water
present in the slurry to form the dihydrate. The open time of a gypsum slurry
can be measured by conventional procedures known by those of skill in the art.
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The preferred ingredients of the wallboard core composition
will now be described in more detail. Generally, a preferred method for
manufacturing the core composition and wallboard of the invention initially
includes the premixing of dry ingredients in a mixing apparatus (e.g., the
modified TSC mixer). The dry ingredients can include calcium sulfate
hemihydrate (stucco), an accelerator and, optionally, an antidesiccant (e.g.,
starch), as described below in greater detail.
The dry ingredients are mixed together with a "wet" (aqueous)
portion of the core composition in the mixer apparatus. The wet portion can
include a first component (referred to as a "paper pulp solution") that
preferably includes a mixture of water, paper pulp, and, optionally, fluidity-
increasing agents. A set retarder also can be included. A majority of the
water
present in the slurry is introduced via the pregenerated foam. Another source
for water in the slurry is the paper pulp solution. Other sources for the
introduction of water into the slurry include, but are not limited to,
mixtures of
the aforementioned strengthening agent and other conventional additives,
when used.
A principal ingredient of the wallboard core composition of the
invention is calcium sulfate hemihydrate, or stucco (CaS04~'/ZH20). Calcium
sulfate hemihydrate generally is described by Petersen, D.J., et al. "Calcium
Compounds (Calcium Sulfate)," Encyclopedia of Chemical Technology, (1992
4th ed.), vol. 4, pp. 812-26, TP9.E685, the disclosure of which is hereby
incorporated herein by reference, and can be produced by the methods
described above. As is known by those of skill in the art, there are two types
or forms of calcium sulfate hemihydrate, an a-hemihydrate form and a ~i-
hemihydrate form. These two forms typically are produced by different types
of calcination processes and differ structurally to some extent. Either type
of
calcium sulfate hemihydrate, however, is suitable for use in the present
invention. Stucco preferably is present in the gypsum slurry composition
extruded from the die exit in an amount of about 59 to about 64 wt.% based on
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the total weight of the gypsum slurry composition, more preferably about 60 to
about 63 wt.%, for example about 61 to about 62 wt.%.
An optional antidesiccant, such as starch, also can be included
to provide added strength to the finished wallboard product, however, it is
not
necessary to the success of the invention. Accordingly, the antidesiccant can
be present in an amount of less than about 5 lbs/MSF. In some products,
optional lightweight aggregates (e.g., treated expanded perlite or
vermiculite)
also can be included.
An aqueous slurry or solution of paper pulp also can be
included in the core composition. The pulp solution comprises water and
paper fibers ("paper pulp"), and may also include an optional binder, a set
retarder (or setting agent), corn starch, and/or potash. Optional binders that
can be used include, but are not limited to, inorganic binders such as, for
example, colloidal silica and colloidal alumina. It may be desirable to
utilize a
suitable set retarder when an inorganic binder is present in order to provide
moisture resistance and, thereby, avoid sacrificing strength due to the
presence
of moisture. The set retarder can be used to tailor the set time of the core
composition. Set retarders typically are used in the invention at very low
rates
(if at all), for example at about 0.0007 weight percent, based on the weight
of
the core composition.
The terms "set retarder" and "setting agent" are used herein to
include any substance which will react with the stucco to form an insoluble
complex. One class of such setting agents which can be used in the present
invention comprises divalent or trivalent metal compounds, such as
magnesium oxide, zinc oxide, calcium carbonate, magnesium carbonate, zinc
sulfate, and zinc stearate.
The paper pulp solution also can include one or more of a
number of additives that increase the fluidity of the slurry and/or reduce the
water requirements of slurry. Materials used as fluidity-enhancing and/or
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water-reducing agents include "lignosulfonates" which are commercially-
available either in liquid or powder form. Fluidity-enhancing and/or water-
reducing agents supplied in liquid form can be either incorporated in the pulp
solution or added directly to the mixing operation.
The paper pulp solution can be prepared by blending or mixing
the above ingredients with water in a blending apparatus. Alternatively, a
concentrated pulp solution using only a small volume of water can be
produced. In this case, the remainder of the core mix water requirement is
made up with water from a separate water source, such as pregenerated foam,
for example. In contrast to conventional gypsum slurries, a large excess of
water with respect to the above-described rehydration reaction preferably is
not included. Typically, about 15 to about 20 weight parts pulp water are used
per 100 weight parts stucco. In contrast, conventional gypsum wallboard
manufacturing processes utilize a large excess of more than about 40 weight
parts pulp water per 100 weight parts stucco.
Preferably, high shear mixing "pulps" the material, forming a
homogenous solution or slurry. The pulp solution can be transferred to a
holding vessel, from which it can be continuously added to the core
composition mix. The paper fibers in the pulp solution can serve to enhance
the flexibility of the gypsum wallboard. Gypsum wallboard made without
fibers can be very brittle and more susceptible to fracture during handling.
The paper fibers also aid in evenness of drying during manufacture, as well as
enhance the ability of the final wallboard product to accept and hold nails
during installation.
As indicated above, the wet portion of the core composition
also can include a component that incorporates a pregenerated foam (e.g., air
encased in soap bubbles). Foam introduces air voids into the core through the
use of a foam that contains very little solid material, but is resilient
enough to
resist substantial breakdown in the mixing operation. In this manner, the
density of the core can be controlled. Known foaming agents may be supplied
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in either liquid or flake (powdered) form, and may be produced from soaps
known in the art. Methods for preparing a pregenerated foam are generally
known by those of skill in the art. It is believed that foam can be generated
in
situ within a modified version of mixers disclosed in U.S. Patent Nos.
5,304,355 and 5,607,233, described above, if desired, by appropriate addition
of soaps and surfactants.
One or more slip agents are preferably included to reduce drag
as the slurry is discharged into the die and extruded therefrom. The slip
agent
is added to the core mixture in an amount of about 0.4 wt.% to about 1.5 wt.%,
based on the weight of stucco present in the core mixture. Preferably, the
slip
agent is added to the core mixture in an amount of about 0.4 wt.% to about 1.0
wt.%, based on the weight of stucco present in the core mixture. Even more
preferably, the slip agent is added to the core mixture in an amount of about
0.7 wt.% to about 0.8 wt.%, based on the weight of stucco present in the core
mixture. The slip agent also can be introduced into the slurry as it passes
through the die via injection parts or conduits of the die. A suitable slip
agent
for use in the present invention is Zelec NE which is commercially-available
from Stepan Chemical Co. of Fieldsboro, New Jersey.
A strength-enhancing agent is preferably added to the gypsum
slurry as it passes through the die exit. Preferably, the strength-enhancing
agent is added to that slurry portion which will eventually form the edge
portions of the wallboard. The strength-enhancing agent of the invention
preferably includes, and may consist essentially of, an acrylic polymer
emulsion having certain preferred properties, as described below. When added
to the gypsum wallboard core composition, the acrylic polymer emulsion can
provide significantly increased core strength, paper-to-core bond, and other
physical properties. Consequently, the board density can be reduced while
still
maintaining other desirable board physical properties. Furthermore, when
added in concentrated amounts to edge portions of the board core, the agent
can serve to enhance greatly the strength of said edge portions. Strong edge
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portions are particularly desirable as it is at these edge portions where the
wallboards are secured to studs by nails or screws, for example. Although the
invention is not limited by any particular mechanism and the mechanisms that
achieve the benefits of the invention are not presently clearly understood, it
is
believed that the acrylic polymer deposits itself in the contact areas between
crystals of calcium sulfate dihydrate.
Suitable additives and methods of using the same are described
in U.S. Patent No. 5,879,825, assigned to the assignee of the present
application, the disclosure of which is hereby incorporated herein by
reference.
One important factor in selecting the acrylic polymer emulsion is the glass
transition temperature, or "Te", of the acrylic polymer emulsion. The glass
transition temperature is the temperature at which an amorphous material
changes from a brittle vitreous state to a plastic state. Many polymers such
as
acrylics and their derivatives have this transition point, which may, at least
in
some cases, be related to the number of carbon atoms in their ester groups.
The polymer emulsion should have a glass transition
temperature (Te) of about 15°C or greater, and preferably in the range
of about
15°C to about 60°C, more preferably in the range of about
20°C to about 60°C,
and most preferably in the range of about 35°C to about 60°C.
It has been found that polymer emulsions having T~ values
substantially below about 15°C undesirably provide a core that forms a
moisture vapor transmission barrier at the plane of evaporation. The plane of
evaporation is the location at or below the core surface where the water drawn
thereto evaporates during the drying process. A moisture transmission barrier
is formed if the polymer forms a film that inhibits the water within the
gypsum
wallboard from evaporating in a reasonable period of time. Such a film would
make it substantially more difficult to dry gypsum wallboard cores, causing
increased energy and cost requirements for the drying process. Therefore, it
is
not desirable to form a film in the wallboard core. The invention therefore
allows use of commercial manufacturing apparatus and facilities.
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Certain acrylic polymer emulsions are more stable than others
in the aqueous calcium sulfate environment encountered during the gypsum
wallboard production process. Since the divalent calcium ions in the aqueous
slurry can adversely affect the performance of some polymer emulsions, the
polymer emulsion should be formulated to be stable to calcium ions.
By way of example only, the acrylic polymer may have a
molecular weight in the range of about 300,000 to about 700,000, although
this range is believed to be variable. Acrylic polymers having other molecular
weights are useful with the invention. The acrylic polymer can be crosslinked
or noncrosslinked.
The polymers used with the invention are preferably neutralized
with sodium hydroxide (NaOH) or other nonvolatile neutralizing agent, and
more preferably neutralized with an agent consisting essentially of a
nonvolatile neutralizing agent. Ammonium hydroxide is preferably not
included in any substantial amount in the neutralizing agent for the acrylic
polymer, since substantial amounts may adversely affect the product. Most
preferably, the neutralizing agent is substantially free of ammonium salts or
other source of ammonia.
Various acrylic polymer emulsions suitable for use with the
invention are commercially available. For example, suitable polymer
emulsions are available from Rohm & Haas Company of Philadelphia,
Pennsylvania under the trade name Rhoplex (e.g., Rhoplex SS-521, Rhoplex
E-2409, and Rhoplex B-1162). Other polymer emulsions in the Rhoplex line
have been designated by Rohm & Haas as RG 2718, RG 2719, RG 2721, and
KAK 1868. Other suitable polymer emulsions are available from The Dow
Chemical Company of Midland, Michigan.
The polymer emulsions can include about 20 to about 80
weight percent of an acrylic polymer, about 20 to about 80 weight percent
water, about 0.3 weight percent or less aqua ammonia, and less than about 0.1
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weight percent residual monomers. The emulsions can have a pH in a range of
about 2.1 to about 11.0, and a specific gravity in a range of about 1.0 to
about
1.2.
The strength-enhancing agent of the invention is preferably
included at a rate in a range of about 0.25 to about 2.5 percent solids, more
preferably about 0.5 to about 2.0 percent solids, and most preferably about
0.5
to about 1.0 percent solids, based on the weight of the rehydrated gypsum in
the final product.
One advantage of the invention is that the slurry may be
prepared in the mixer without incorporation of a set accelerator into the
slurry
until the mixing process is substantially complete. The set accelerator is
introduced into the gypsum slurry composition substantially near the discharge
end of the mixer (e.g., after all other slurry ingredients have been added to
the
mixer). The set accelerator is used to control, within certain limits, the
crystal
1 S growth rate and set time of the stucco. Examples of suitable accelerators
include ball mill accelerators ("BMA") and potassium sulfate, although many
others are known by the skilled artisan. In some cases, the invention may
require increased amounts of accelerator due to the retarding effect of some
of
the strength-enhancing additives.
Addition of the set accelerator downstream of the mixing
apparatus obviates the likelihood that the gypsum setting reaction will occur
prematurely within an upstream portion of the mixing apparatus. Furthermore,
the likelihood that process equipment upstream of the extrusion die will foul
and/or plug with the slurry also is diminished greatly by addition of the set
accelerator towards the discharge end of the mixer.
Another advantage of the invention is that the gypsum slurry
prepared within the mixing apparatus can be supplemented by the addition of
further gypsum slurry additives via the die periphery. These agents include,
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but are not limited to, the aforementioned slip agents and the strength-
enhancing acrylic polymer.
Additional advantages of the invention include, but are not
limited to, the elimination of conventional slurry-discharge means, such as
discharge boots, hard-edge mixers, forming plates, powered rolling or
smoothing (doctor) bars, and edge tape. Furthermore, because the slurry may
be soft-extruded directly onto the conveyor belt, there is typically no need
for
sheets of cover paper to encase the slurry, which are required when using a
conventional, less viscous (cake batter-like) gypsum slurries. According to
the
invention, use of these sheets of cover paper can be deferred to a point in
the
wallboard manufacturing process where the wallboard has already been dried.
The produced core composition (i.e., aqueous gypsum slurry) is
deposited directly onto a smooth, continuously moving conveyor belt. The
core composition is allowed to cure or set, whereby calcium sulfate
hemihydrate is converted to calcium sulfate dihydrate. In one embodiment, as
the core slurry is deposted on the moving, smooth-surface conveyor belt, the
top surface of the exposed core slurry can be sprayed with a chemical
activator
(or accelerator). The chemical activator accelerates the setting reaction in a
manner similar to that used with plaster to "brown" plaster. The chemical
activator preferably acts fast enough to accelerate setting of the mass of
deposited slurry and should provide an induction time before the slurry begins
to from crystals, yet allow rapid crystallization to occur after the induction
time.
Thereafter, the product preferably is dried to remove water not
consumed in the setting reaction (i.e., the reaction forming the calcium
sulfate
dihydrate). When the board is removed from the smooth-surface conveyor
belt, the board preferably has a dense, glassy surface that, when dry,
provides
an excellent surface for lamination of desired surface materials as described
in
more detail below.
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In contrast to conventional methods, the present invention
preferably does not require, and preferably does not utilize, high levels of
process water to decrease the viscosity of the slurry during production.
Because the present invention utilizes a soft-extrusion step, the necessity of
high levels of water is obviated, as fluidity and viscosity are not as
significant
of a problem as in conventional processes. Furthermore, the present invention
obviates the use of sheets of cover paper to encase the gypsum slurry and
defers application of these sheets, if at all, until after the slurry has been
dried
to form a hardened wallboard core.
Conventional wallboard forming processes typically require
removal of about 800 lbs/MSF (about 3900 kilograms per one thousand square
meter (kg/MSM)) to about 850 lbs/MSF (4150 kg/MSM) water from a slurry
encased by sheets of cover paper. This water has to be evaporated through the
paper cover sheets in energy-intensive, expensive drying steps which require
about 36 to about 40 minutes to remove the water.
In contrast, in a preferred embodiment of the invention, less
than about 700 pounds of water per MSF (about 3417 kg/MSM) must be
removed. More preferably, less than about 600 lbs/MSF (about 2925
kg/MSM), and even more preferably less than about 400 Ibs/MSF (about 1950
kg/MSM) must be removed. Thus, the invention provides a reduction in the
use of process water by about 12.5% to about 53%, preferably about 12.5% to
about 25%, when compared to conventional gypsum wallboard manufacturing
processes.
With less water to be evaporated, energy-intensive, expensive
dryers preferably are no longer necessary. Instead, it has been discovered
that
with only about 600 lbs/MSF water to be evaporated, a hybrid dryer employing
microwave heating techniques in combination with convection drying can be
used. This advanced hybrid dryer provides a substantial cost and energy
savings over the conventional dryers used to evaporate large amounts of water.
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The hybrid dryer includes microwave heating and concurrent
convection drying capabilities. The microwave heating serves to heat the
water present in the wallboard core to a temperature of about the vaporization
temperature of the water, and it preferably causes the heated water to migrate
from the core to outer surfaces of the wallboard core. As the water migrates
to
the outer surfaces, the water can be more easily removed by convection drying
in the hybrid dryer, and when the wallboard core passes through the
downstream convection dryer. These hybrid dryers are typical in cereal
manufacturing processes where they are used to dry wet cereals. Hybrid dryers
cannot generally be used in conventional gypsum wallboard manufacturing
processes because the microwave heating would likely cause the paper cover
sheets encasing the wet gypsum core to blow off of the core. In the present
invention, however, preferably no such paper cover sheets are used to encase
the viscous gypsum slurry and, therefore, use of a hybrid dryer is possible.
A downstream convection dryer also can be used in-line to dry
the uncut gypsum wallboard core. The convection dryer preferably includes a
number of drying zones each of which can provide heat at different
temperatures depending upon the amount of moisture present in the wallboard
core. Infrared moisture sensors can be positioned within the various zones of
the convection dryer to control the temperatures encountered in each zone.
Preferably, the board drying is computer controlled. Conventional gypsum
wallboard manufacturing processes utilized convection drying kilns which
were responsible for simultaneously drying a number of pre-cut and vertically-
stacked gypsum wallboards. Because the wallboard core prepared according
to the present invention is dried in the absence of conventional paper
covering
and prior to cutting, better drying control can be achieved.
According to the present invention, utilizing the
aforementioned hybrid dryer and convection dryer, the time required to
remove about 600 lbs/MSF water can be accomplished in less than about 1 S
minutes, preferably less than about 12 minutes, more preferably in about 10 to
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about 12 minutes, and even more preferably about 6 to about 7 minutes. The
lack of paper cover sheets and the use of a hybrid dryer enable one to achieve
such short drying times.
Gypsum wallboard can be adapted for wet and exterior
applications, in addition to use in constructing interior walls and ceilings.
In
the production of exterior sheathing and moisture-resistant board cores,
various materials can be incorporated into the core to impart, for example,
fire
resistance and/or increased water absorption resistance to the board. Useful
materials include silicone water repellents, mica, paraffin granules, fly ash
spheres, perlite, vermiculite, waxes, and asphalt emulsions. These materials
are typically supplied as water emulsions to facilitate ease of incorporation
into the board core. These materials can be added directly into the mixing
apparatus or incorporated into the pulp solution prior to addition to the
mixing
apparatus. Furthermore, some core treatments, such as, for example, silicone
water treatments, may be applied to the wallboard core downstream of the
hybrid dryer and convection dryer. To the extent that these core treatments
introduce water into the wallboard core, a convection dryer downstream of the
treatment station can be used to drive off the undesired water.
The soft-extruded and dry gypsum core comprises calcium
sulfate dehydrate in an amount in the range of at least about 90 percent by
weight (wt.%), based on the total weight of the core. Preferably, the dry core
comprises at least about 95 wt.% calcium sulfate dehydrate, and more
preferably at least about 99 wt.%, based on the total weight of the core.
Accordingly, the dry density of the core, which can be determined by standard
techniques known by those of skill in the art, is about 50 pounds per cubic
foot
(pcf) or less, preferably about 45 pcf or less, and more preferably about 40
pcf
(i.e., 1575 lbs/MSF) or less.
High-speed lamination equipment can be used to attach desired
surface materials to the hardened, dry core. This can be accomplished in-line
simultaneously to both surfaces of the board. Suitable surface materials can
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include a variety of films and decorative papers depending on the end use.
Furthermore, prior to lamination, the core can be impregnated with strenth-
enhancing agents, water-repelling agents, and/or other treatments. As
previously noted, to the extent that these agents and/or treatments introduce
water into the core, a downstream convection dryer can be used to remove the
water prior to lamination and cutting/trimming.
Modern trimming equipment which typically could not be used
in prior board forming processes due to the presence of cover sheets and the
associated problems of (cover sheet) end peel, are suitable for use in the
present invention. Furthermore, modern trimming equipment is preferred to
cut and trim the boards made by the method of the present invention because
the boards do not have burnt (i.e., over-calcined) edges which can damage the
equipment.
Use of conventional cover sheets to enclose the gypsum slurry
core is obviated by the present invention. These cover sheets, however, can be
applied with a suitable adhesive (e.g., polyvinylacetate binder) to a dried
wallboard core. When the board is to be covered by a vinyl or metallic finish,
no paper cover sheets are necessary and the vinyl or metallic finish can be
applied directly onto the surface of the dried wallboard core. Thus, the
present
invention reduces material costs by not requiring the use of cover sheets in
certain instances. In addition to the cost savings, there are other process
advantages that can be realized by reducing or eliminating the need for paper
cover sheets. For example, conventional continuous processes often
encountered periods where the process had to be shut down temporarily to
attend to torn or improperly deployed cover sheets, cockles, cobbs, poor
porosity, and mismatched papers for drying. Because these cover sheets are no
longer necessary in certain applications, such temporary shut down periods are
not likely to occur. Even when the paper cover sheets are desired, their
application to a wallboard core is deferred until after the core has been
dried,
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thereby avoiding the complexities associated with applying a paper cover sheet
to a wet cake-batter like slurry.
Additional advantages of the invention include, but are not
limited to, the elimination of conventional slurry-discharge means, such as
discharge boots, hard-edge mixers, forming plates, powered rolling or
smoothing bars, and edge tape. Without the need for such conventional slurry-
discharge means, the present invention provides a reduction in capital costs
associated with the manufacture of gypsum wallboard.
EXAMPLES
The following examples are provided to illustrate the invention
but are not intended to limit the scope of the invention.
Examples 1-3 describe viscous gypsum slurries that can be
prepared in a suitable mixer and provided to a die under positive displacement
for extrusion onto a moving smooth-surface conveyor belt in accordance with
the present invention. Additionally, Examples 1-3 are illustrative of various
wallboard cores and core weights that can be achieved, and the low amount of
water present in the gypsum slurries. In contrast, Comparative Example 4
describes a low-viscosity gypsum slurry prepared according to conventional
gypsum wallboard manufacturing processes. Example 5 illustrates acceptable
viscosities for gypsum slurries.
Example 1
This example is directed to a viscous gypsum slurry
composition that can be prepared in a suitable mixer and to the water content
therein. This example illustrates an embodiment of the invention using a high
level of stucco (i.e., 63.08 wt.% stucco, based on the weight of the gypsum
slurry).
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Gypsum rock having a purity of 89.0 is used to prepare a
wallboard having a core weight of about 1653 lbs/MSF of 0.472 inch thick
board. The (gram) molecular weight of the land plaster corresponding to a
gypsum rock purity of 89.0 is 193.45. Table I, below provides the
composition of a gypsum rock (i.e., the (gram) molecular weights for each of
the dihydrate, hemihydrate, anhydrite forms of calcium sulfate, and also for
any inerts present in the mix).
Table I
Constituent Mol. Amount Amount
Wt.
(g/gmole)(parts/100 (parts/100
parts parts
Land Plaster)Stucco)
Land Plaster 193.45 100.00 116.74
Dihydrate 172.17 89.00 103.90
Inerts 21.28 11.00 12.84
Hemihydrate 145.15 68.99 80.54
Anhydrite 136.14 5.66 6.61
Stucco 85.66 100.00
Based on the foregoing gypsum rock purity and target
wallboard core weight, a gypsum slurry is prepared containing the ingredients
listed in Table II, below:
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Table II
Board/Slurry Amount Wt.% Dry Weight
Composition (lbs/1000 lbs (lbs/1000 lbs
of Stucco)' of Stucco)$
Dry Ingredients
Stucco 1000.0 63.08 1167.4
Starch 0.0 0.00 0.0
Accelerator 0.9 0.06 0.9
Wet Ingredients
Pulp Ingredients
Paper 2.3 0.15 2.3
Pulp Water 157.7 9.95 0.0
Water Reducing 4.6 0.29 1.8
Agent
Potash 0.6 0.04 0.6
Foam Ingredients
Surfactant 0.5 0.03 0.5
Foam Water 418.6 26.41 0.0
Totals 1585.3 100.00 1173.6
i~ = Weight is based upon 1000 lbs. of stucco.
$ = Weight after conversion to gypsum, based upon 1000 lbs. of stucco.
The above formulation is calcined to convert all of the
dihydrate to hemihydrate with slight overburn and minimal underburn.
Accordingly, a rehydrated combined moisture level of the slurry is reduced
from 18.63% to a stucco combined moisture level of about 4.91%, which
corresponds to a water loss of about 579 lbs/MSF of 0.472 inch thick board.
This water was removed in accordance with the invention in a convection
dryer in less than about 12 minutes. This water can be removed in accordance
with the invention in a hybrid dryer (containing microwave heating and
convection drying means) followed by a multi-zone convection dryer is less
than about 10 minutes.
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Example 2
This example is directed to a viscous gypsum slurry
composition that can be prepared in a suitable mixer and to the water content
therein. This example illustrates another embodiment of the invention using a
particularly preferred amount of stucco (i.e., 61.62 wt.% stucco, based on the
weight of the gypsum slurry).
Gypsum rock having a purity of 89.0 is used to prepare a
wallboard having a core weight of about 1575 lbs/MSF of 0.472 inch thick
board. The (gram) molecular weight of the land plaster corresponding to a
gypsum rock purity of 89.0 is 193.45. Table I in Example l, above, the
composition of the gypsum rock.
Based on the foregoing gypsum rock purity and target
wallboard core weight, a gypsum slurry is prepared containing the ingredients
listed in Table III, below:
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Table III
Board/Slurry Amount Wt.% Dry Weight
Composition (Ibs/1000 lbs (Ibs/1000 lbs
of Stucco)' of Stucco)$
Dry Ingredients
Stucco 1000.0 61.62 1167.4
Starch 0.0 0.00 0.0
Accelerator 0.9 0.06 0.9
Wet Ingredients
Pulp Ingredients
Paper 2.3 0.14 2.3
Pulp Water 177.0 10.91 0.0
Water Reducing 4.6 0.28 1.8
Agent
Potash 0.6 0.04 0.6
Foam Ingredients
Surfactant 0.5 0.03 0.5
Foam Water 436.8 26.92 0.0
Totals 1622.7 100.00 1173.6
~ = Weight is based upon 1000 lbs. of stucco.
$ = Weight after conversion to gypsum, based upon 1000 lbs. of stucco.
The above formulation is calcined to convert all of the
dihydrate to hemihydrate with slight overburn and minimal underburn.
Accordingly, a rehydrated combined moisture level of the slurry is reduced
from 18.63% to a stucco combined moisture level of about 4.91%, which
corresponds to a water loss of about 601 lbs/MSF of 0.472 inch thick board.
This water was removed in accordance with the invention in a convection
dryer in less than about 12 minutes. This water can be removed in accordance
with the invention in a hybrid dryer (containing microwave heating and
convection drying means) followed by a mufti-zone convection dryer is less
than about 10 minutes.
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Example 3
This example is directed to a viscous gypsum slurry
composition that was prepared, and to the water content therein. This
example illustrates yet another embodiment of the invention using a low
S amount of stucco (i.e., 59.39 wt.% stucco, based on the weight of the gypsum
slurry).
Gypsum rock having a purity of 90.0 was used to prepare a
wallboard having a core weight of about 1459 lbs/MSF of 0.472 inch thick
board. The (gram) molecular weight of the land plaster corresponding to a
gypsum rock purity of 90.0 is 191.3. Table IV, below provides the
composition of the gypsum rock (i.e., the (gram) molecular weights for each of
the dihydrate, hemihydrate, anhydrite forms of calcium sulfate, and also for
any inerts present in the mix).
Table IV
Constituent Mol. Amount Amount
Wt.
(g~gmole)(parts/100 (parts/100
parts parts
Land Plaster)Stucco)
Land Plaster 191.30 100.00 117.27
Dihydrate 172.17 90.00 105.54
Inerts 19.13 10.00 11.73
Hemihydrate 145.15 66.17 77.60
Anhydrite 136.14 9.1 10.67
Stucco 85.27 100.00
Based on the foregoing gypsum rock purity and target
wallboard core weight, a gypsum slurry was prepared containing the
ingredients listed in Table V, below:
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Table V
Board/Slurry Amount Wt.% Dry Weight
Composition (Ibs/1000 Ibs (lbs/1000 lbs
of Stucco)t of Stucco)$
Dry Ingredients
Stucco 1000.0 59.39 1172.7
Starch 0.0 0.00 0.0
Accelerator 0.9 0.06 0.9
Wet Ingredients
Pulp Ingredients
Paper 2.3 0.14 2.3
Pulp Water 239.1 14.20 0.0
Water Reducing 4.6 0.27 1.8
Agent
Potash 0.6 0.03 0.6
Foam Ingredients
Surfactant 0.5 0.03 0.5
Foam Water 435.6 25.87 0.0
Totals 1683.6 100.00 1178.9
~- = Weight is based upon 1000 lbs. of stucco.
$ = Weight after conversion to gypsum, based upon 1000 lbs. of stucco.
The above formulation is calcined to convert all of the
dehydrate to hemihydrate with slight overburn and minimal underburn.
Accordingly, a rehydrated combined moisture level of the slurry was reduced
from 18.83% to a stucco combined moisture level of about 4.68%, which
corresponded to a water loss of about 623 lbs/MSF of 0.472 inch thick board.
This water was removed in accordance with the invention in a convection
dryer in less than about 12 minutes. This water can be removed in accordance
with the invention in a hybrid dryer (containing microwave heating and
convection drying means) followed by a mufti-zone convection dryer is less
than about 10 minutes.
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Comparative Example 4
This example is directed to a viscous gypsum slurry
composition that was prepared in a pin mixer according to conventional
gypsum wallboard manufacturing practices, and to the water content therein.
Gypsum rock having a purity of 89.0 was used to prepare a wallboard having a
core weight of about 1664 lbs/MSF of 0.470 inch thick board. Each of two
sheets of cover paper which encase the wet gypsum slurry prior to setting have
a thickness of about 0.0135 inch. Accordingly, a finished wallboard of this
example had a thickness of about 0.500 inch. Based on the foregoing gypsum
rock purity and target wallboard core weight, a gypsum slurry was prepared
containing the ingredients listed in Table VI, below. The gypsum composition
had a viscosity too low for use in a positive displacement mixer of the type
described herein and is not suitable for extrusion via a die.
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Table VI
Board/Slurry Amount Wt.% Dry Weight
Composition (lbs/10001bs (lbs/1000 lbs
of Stucco)t of Stucco)$
Dry Ingredients
Stucco 1000.0 55.20 1167.4
Starch 4.9 0.27 4.9
Accelerator 1.7 0.09 1.7
Wet Ingredients
Pulp Ingredients
Paper 2.3 0.13 2.3
Pulp Water 424.9 23.46 0.0
Water Reducing 3.4 0.19 1.4
Agent
Potash 0.6 0.03 0.6
Foam Ingredients
Surfactant 0.6 0.03 0.6
Foam Water 373.1 20.60 0.0
Totals 1811.6 100.00 1178.9
j' = Weight is based
upon 1000 lbs. of stucco.
$ = Weight after conversion 1000 lbs.
to gypsum, based upon of stucco.
The above formulation is calcined to convert all of the
dihydrate to hemihydrate with slight overburn and minimal underburn.
Accordingly, a rehydrated combined moisture level of the slurry was reduced
from 18.63% to a stucco combined moisture level of about 4.91%, which
corresponded to a water loss of about 849 lbs/MSF of 0.500 inch thick
wallboard, in this case. In order to remove this amount of water utilizing a
conventional gypsum wallboard manufacturing process (i.e., a conventional
convection drying kiln), the drying time was about 36 to about 40 minutes.
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The following Table VII summarizes the composition of the
various slurnes prepared in each of Examples 1-4, and comparative Example
5, including the board core weight and water content.
Table VII
Board/Slurry CompositionEx. 1 Ex. Ex. Ex.
2 3 4
Comp.
Dry Ingredients
Stucco 63.08 61.62 59.39 55.20
Starch 0.00 0.00 0.00 0.27
Accelerator 0.06 0.06 0.05 0.09
Wet Ingredients
Pulp Ingredients
Paper 0.15 0.14 0.14 0.13
Pulp Water 9.95 10.91 14.20 23.46
Water Reducing 0.29 0.28 0.27 0.19
Agent
Potash 0.04 0.04 0.03 0.03
Foam Ingredients
Surfactant 0.03 0.03 0.03 0.03
Foam Water 26.41 26.92 25.87 20.60
Totals 100.00 100.00 100.00100.00
Finished Board Core Wt. (lbs/MSF) 1653 1575 1459 1664"
Water Loss (lbs/MSF) 579 601 623 849
* - Weight includes 100 lbs/MSF of paper cover sheets which are required in
conventional wallboard forming operations.
Based on Examples 1-3 and Comparative Example 4, it is
apparent that gypsum wallboard cores made in accordance with the present
invention contain much less water (e.g., about 579 to about 623 lbs/MSF)
compared to gypsum wallboards made in accordance with conventional
processes (e.g., about 849 lbs/MSF). The reduced water content is an
advantage that aids downstream drying operations and improves the overall
efficiency of the wallboard manufacturing process.
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Example 5
This example demonstrates the viscous nature of a gypsum
slurry that can be prepared in accordance with the present invention utilizing
a
suitable mixture and low water content and compares the viscosity of the
slurries to that of gypsum slurries ill-suited for use in the present
invention.
Slump tests were performed to determine the viscosity of
gypsum slurnes capable of being prepared in a suitable mixer operating under
conditions sufficient to provide the slurry to a die inlet under positive
displacement.
The test was conducted after dipping a brass cylinder having a
wall thickness of about 0.07 inches, a height of about 4 inches, and an inner
diameter of about 2 inches into a low-viscosity lubricating oil bath. The
cylinder was removed and excess oil was drained off the surfaces of the
cylinder. The cylinder then was placed upright onto a center portion of a
clean
(i.e., no scratches), dry glass plate having the following dimensions: about
10
inches in length, about 10 inches in width, and about 0.1875 inch thick.
The mixed slurry was poured into the cylinder such that the
cylinder was completely filled with a slight excess. The scoop can be a clean
metal or plastic scoop of convenient size or can be formed from disposable
gypsum board paper. The excess was screened off to a level with the top of
the cylinder without dropping any of the slurry onto the surface of the glass
plate.
Within about 10 seconds of removing excess slurry, the
cylinder was raised vertically with a smooth and uniform motion, and the
slurry contained within the cylinder was allowed to slump to a circular patty
onto the surface of the glass plate. After the slurry had solidified, the
glass
plate was turned over and the diameter of the slump in contact with the glass
plate was measured to the nearest'/s inch.
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A Brookfield DV-III Programmable Rheometer with a No. 6
spindle was used to determine the viscosity of the various gypsum slurry
slumps. The viscosity of the various slumps are reported in Table VIII, below,
based on rotations per minute (RPM) of the spindle and the % torque:
Table VIII
Viscosity
(mPas)
of Various
Slump
Sizes
(%Torque)
RP 7'/4 6'/4 55/e 3'/<
M Slump Slump Slump Slump
10 23,800 22,800 26,200 29,300
(24.1%) (23.0%) (26.3%)(28.5%)
12,250 12,550 14,000 19,000
(24.0%) (27.1%) (26.0%)(32.3%)
40 6,425 6,3.75 7,125 8,800
(25.7%) (24.9%) (28.5%)(34.5%)
60 4,583 4,833 5,000 6,200
(27.3%) (27.1%) (30.2%) (37.5%)
15 Generally, gypsum slurry compositions suitable for use in the
present invention include those having a viscosity of at least about 19,000
mPa~s, preferably about 19,000 to about 22,000 mPa~s, and more preferably
about 20,000 mPa~s as measured above at 20 RPMs. Furthermore, gypsum
slurry compositions suitable for use in the present invention include those
20 having a viscosity of at least about 29,300 mPa~s as measured at 10 RPM, at
least about 8,800 mPa~s as measured at 40 RPMs, and at least about 6,200
mPa~s as measured at 60 RPMs.
The foregoing description is given for clearness of
understanding only, and no unnecessary limitations should be understood
therefrom, as modifications within the scope of the invention may be apparent
to those having ordinary skill in the art.
