Note: Descriptions are shown in the official language in which they were submitted.
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COATED GYPSUM BOARD PRODUCTS AND METHOD OF
MANUFACTURE
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION:
[0001] The present invention relates generally to coatings for gypsum board
products, and the production thereof. More particularly, the present invention
relates to the composition of coatings for gypsum board products, and methods
of
producing and applying the coatings in one continuous gypsum board
manufacturing process.
DISCUSSION OF RELATED ARTS:
[0002] In the discussion of the related art that follows, reference is made to
certain
structures and/or methods. However, the following references should not be
construed as an admission that these structures and/or methods constitute
prior art.
Applicant expressly reserves the right to demonstrate that such structures
and/or
methods do not qualify as prior art against the present invention, if
appropriate.
[0003] The product density and surface type and finish of traditional gypsum
board products are manufactured to have a desired strength and appearance
while
minimizing product weight and costs. Generally, calcined gypsum slurry is
agitated in a mixer with aqueous foaming agents and deposited onto a conveyor
carrying a facing sheet. A backing sheet is applied over the slurry and the
wet
gypsum board product is formed to the desired thickness between forming
rollers.
The wet gypsum sets during the conveying process, is cut to the desired
length,
and is dried by passing the gypsum board product through a drying oven.
[0004] Coatings for gypsum board products may be used to impart surface
homogeneity and improved surface appearance, both in the manufactured
condition and after a decorative coating has been applied. An example of a
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previously known two-step manufacturing method for coated gypsum board
products is disclosed in Australian Patent Application No. 19322/92, the
disclosure of which is incorporated herein by reference. A surface coating is
applied to the facing sheet and dried before the gypsum is applied to the
facing
sheet in the gypsum board manufacturing process.
[0005] An alternative method for applying a coating utilizes a separate
coating
process subsequent to the manufacture and drying of the gypsum board product.
[0006] U.S. Patent No. 6,105,325, issued to Zuber et al., the disclosure of
which
is also incorporated herein by reference, discloses the use of matching the
finished
appearance of a joint-pointing coat to that of the lining of the paper of the
plaster
boards jointed.
[0007] In the above processes, the coating imparts desirable aesthetic
qualities
including surface coloration, ret7ectance, and absorbency. However, the
methods
are capital intensive, requiring separate and dedicated production facilities
in
which to apply the coating and suffering inefficiencies by not being part of a
larger
in-line production facility.
SUMMARY
[0008] The present invention provides a coated gypsum board which can be
produced in one continuous gypsum board manufacturing process. Such gypsum
board can be made either with paper on both sides thereof, paper on one side
thereof, or without paper on either side thereof. Further, such gypsum board
coatings may be applied to wet gypsum board prior to drying.
[0009] In an exemplary embodiment, a coated gypsum board comprises a gypsum
core having a first side and second side and a facing sheet disposed on the
first
side. A coating is disposed on at least a portion of the facing sheet and at
least a
portion of the coating penetrates into the gypsum core. In an additional
embodiment, the gypsum board further comprises a backing sheet on the second
side of the gypsum core. In one aspect, the coating may penetrate into the
gypsum
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core to a substantially uniform depth across an area of the gypsum board. A
gypsum board with such a coating exhibits a nail hold value of greater than 80
pounds.
[0010] In a method of producing a coated gypsum board, a gypsum slurry is
deposited to form a wet gypsum board, a coating is applied to the wet gypsum
board, and the wet gypsum board is dried. The coating can be applied directly
to
the gypsum core or to a facing sheet applied over the gypsum slurry. When the
coating is applied to the gypsum board prior to drying, the coating can
penetrate
into the facing sheet and/or the gypsum core and forms a coating that is up to
30
mils in thickness.
[0011] In one exemplary embodiment, a coating to be applied to a gypsum board
is a joint compound or a diluted joint compound.
[0012] In another exemplary embodiment, the coating has a composition of 25-75
vol. % water, 30-70 wt. % calcium carbonate, 0-30 wt. % fillers, 2-10 wt. %
latex
emulsion, and 0-10 wt. % other additives. Fillers can comprise one or more of
mica, talc, clay and limestone. 2-8 wt. % perlite can be added to the coating
to
form a lightweight product and pigment can also be added in an amount
effective
to provide a desired tint to the coating.
[0013] In a further embodiment, a coating to be applied to a gypsum board has
a
composition of 10-60 vol. % water, 50-90 vol. % calcined gypsum; 0.1-10 vol. %
binder; 0-50 vol. % limestone; 0-10 vol. % clay; 0-30 vol. ~ other fillers;
and 0-10
vol % additives. Fillers can comprise one or more of mica and talc. 2-15 vol.
%
perlite can be added to the coating to form a lightweight product and pigment
can
also be added in an amount effective to provide a desired tint to the coating.
[0014] An exemplary method of making a wall deposits a gypsum slurry to form a
wet gypsum board having a gypsum core, applies a coating to the wet gypsum
board, and dries the wet gypsum board. The step of applying the coating occurs
prior to the step of drying the wet gypsum board. The coated gypsum board is
then fastened to a support structure to form the wall and the seams between
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adjacent coated gypsum boards are taped and finished using a joint compound
having a composition substantially similar to the composition of the coating.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
[0015] Aspects and advantages of the invention will become apparent from the
following detailed description of preferred embodiments thereof in connection
with
the accompanying drawings in which like numerals designate like elements and
in
which:
[0016] Figure 1 is a cross-sectional view of a coated gypsum board with a
coating
applied to a facing sheet.
[0017] Figure 2 is a cross-sectional view of a coated gypsum board with a
coating
applied to the gypsum core.
[0018] Figure 3 is a schematic of an embodiment of a production line
incorporating a coating step prior to drying a gypsum board.
[0019] Figure 4 is a schematic plan view of the coating step showing a coating
machine.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] Although the description herein may from time to time refers to either
a
wallboard, a ceiling tile, a ceiling board, or collectively to a gypsum board,
in
each case, the concepts of the present invention may be applied generally to
wallboards, ceiling boards, or ceiling tiles.
[0021] Generally, gypsum board is installed during construction on surfaces
such
as wall, ceilings, and the like, in edge abutting engagement. After affixing
the
gypsum board to the surface with suitable attachments (e.g., nails, screws,
epoxy,
and so forth), the seams and/or artifacts from the attachments (e.g.,
depressions
from nails, nail heads, screw holes, and so forth) are covered with a coating
to
present a uniform and smooth finish. For seams, said coatings can be used in
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combination with tape. The coating material is sometimes referred to as joint
compound.
[0022] Coatings for use in the gypsum board industry can be of one of two
types.
In a first type, the coating is applied and the evaporation of water over time
produces a hard coating. In a second type, the coating is applied and allowed
to
set for a suitable time during which the components of the coating solidify
into a
hard gypsum-based coating.
[0023] A coating of either of the first type or the second type can be applied
to
one side of a gypsum board (e. g. , applied to a facing sheet or directly to a
gypsum
core) during the manufacturing process to form a coated gypsum board. The
selection of the coating can be made such that the coating of the coated
gypsum
board substantially matches the coating or joint compound intended to be used
to
cover the seams and/or artifacts of attachment associated with the affixing of
the
coated gypsum board to a structural support.
[0024] An embodiment of a coated gypsum board 100 is shown in Figure 1. A
gypsum core 105 is disposed between a backing sheet 110 and a facing sheet
115.
A coating 120 is disposed on the facing sheet 115. An alternative embodiment
of
a coated gypsum board 200 is shown in Figure 2, in which a coating 205 is
directly disposed on a gypsum board 210. It should be clear that any
combination
of facing sheet and backing sheet may be utilized in practicing the invention.
In
cases where one or more sheets have been removed, the coating may be applied
directly to the gypsum core with attendant penetration of the coating into the
gypsum core.
[0025] The coating is evenly applied onto the surface of the gypsum board
(e.g.,
applied to the facing sheet, if used, or directly to the gypsum core) to a
uniform
thickness t that is preferably not sensitive to surface irregularities. A
typical
thickness t for the coating is up to 30 mils, preferably from 3-20 mils, and
more
preferably from 5-15 mils. The preferred thickness can be dependent on the
ultimate application. For example, for wallboards, the thickness t is
preferably 10
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mils; for ceiling tiles it is 20 mils. Furthermore, thicknesses outside of
these
ranges may also be contemplated by the present invention. However, a coating
of
approximately 30 mils or greater may result in cracks and grazing of the
surface
finish when dried. Accordingly, thicker coatings should be used carefully.
[0026] In embodiments in which the gypsum board has a facing sheet, the
coating
may penetrate into at least a portion of the facing sheet and, in some cases,
all the
way through the facing sheet and into sections of the gypsum core, over at
least a
portion of the gypsum board. In one embodiment, the coating may penetrate into
the gypsum board over substantially the whole of the area to which it is
applied,
i.e, the whole gypsum board, or predetermined portions if selected regions of
the
gypsum board have been coated.
[0027] The depth of penetration of the coating can be influenced by the
relative
moisture level and/or degree of set of the gypsum board, although the coating
preferably penetrates into the gypsum board to a substantially uniform depth.
In
the cross-sectional view as shown in Figure 1, the coating 120 is shown to
have
penetrated into the facing sheet 115 and into the gypsum core 110 to a depth
d. In
other cases, i.e., where the moisture content is lower or the gypsum is in a
state of
more advanced set, the coating only penetrates a certain depth into the facing
sheet. In one embodiment of a gypsum board with a facing sheet, the depth of
penetration into the gypsum core is from about 5 mils to 20 mils, preferably
from
mils to 15 mils. In embodiments of a gypsum board without a facing sheet, the
depth of penetration is from about 5 mils to 30 mils, preferably from 10 mils
to 20
mils.
[0028] A method of producing a coated gypsum board is shown in Figure 3, in
which a flow chart of a coated gypsum board manufacturing line is depicted.
The
general manufacturing steps 300 include mixing 305 the gypsum slurry and
forming 310 the gypsum into a desired shape, cutting 315 the gypsum into the
desired dimensions, followed by coating 320 to form a gypsum board product and
then drying 325. Setting of the gypsum occurs primarily between the forming
step
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310 and the cutting step 315. The manufacturing steps from forming 310 to
cutting, inclusive, are herein described as the forming and setting line; the
steps
following cutting 315 are herein described as the coating and drying line.
[0029] Starting at the forming and setting line, a formed, coated gypsum board
is
manufactured. In a first embodiment, a gypsum core is coated on a first
surface
with a coating while the gypsum core is wet. In alternative embodiments, the
gypsum board has a facing sheet and/or a backing sheet and is coated on a
first
surface, e. g. , on the facing sheet surface, with a coating while the gypsum
core is
wet. For purposes of this application, a gypsum core is considered wet at any
time during the manufacturing process before the gypsum board is passed
through
the drying oven.
[0030] To control the setting time of the gypsum core, the components of the
gypsum board can be adjusted, by, for example, controlling the quantity of
accelerator and the retarder used in the composition of the gypsum slurry. The
accelerator includes small crystal-like objects which are coated with a starch
or
other dissolvable substances. As the coating on the starch or other
dissolvable
substances is dissolved, the accelerator crystals form a starting point or
seed for
crystal growth of the gypsum. Retarder is preferably added to the gypsum board
components in order to delay the set time of the gypsum core. The retarder has
no
long term effect on the strength or other characteristics of the final board
product.
[0031] It is desirable to delay the set time under certain circumstances so
that the
gypsum slurry does not set earlier than desired. For example, if the gypsum
slurry begins to set in the mixer, inefficiencies result, such as blocked
and/or
clogged apparatus with attendant down time for maintenance and/or repairs. In
addition, if the slurry sets beyond the mixer but prior to forming, forming of
the
slurry may cause the crystal structure to be disrupted during the setting
process,
which can result in a weaker final board product. Accordingly, accelerator and
retarder may be used singularly or in combination to adjust the set time of
the
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gypsum to achieve a desired set or hardness at a desired time in the
manufacturing
process.
[0032] After forming, the gypsum board is conveyed by a conveying system along
the forming and setting line to a cutting position. The conveying speed and
distance are chosen in combination with the set time of the gypsum to result
in
gypsum having a desired set or hardness for subsequent cutting and
manipulation.
Cutting is accomplished by a cutting machine, as is conventionally known in
the
art. After being cut, the gypsum board is turned over and placed on a
conveying
surface and transported to a coating and drying line. Where the gypsum core
has a
facing and/or backing sheet, the manipulation of the gypsum board results in
placing the backing sheet in contact with conveying surface and exposing the
facing sheet.
[0033] On the coating and drying line, the gypsum board, with or without the
facing and/or backing sheet, is conveyed by the conveying means through
coating
machinery, which is located prior to the drying apparatus. Figure 4
schematically
depicts an in-line coating process 400. The coating machinery 405 is
positioned
over the conveying path 410 of the gypsum board 415. The conveying path
typically includes a conveying belt 420 on which the gypsum board to be coated
has been placed, although other suitable conveying paths can be used, such as
driven and nondriven rollers. An uncoated and wet gypsum board 425 is passed
under the coating machinery 405 and a coating is applied via a coating
applicator
430. The coated gypsum board 435 is then conveyed to the next manufacturing
steps, if any, and is then dried in, for example, an oven.
[0034] The coating can be applied using traditional coating machinery, such as
a
curtain coater. An example of a curtain coater is that provided by Hymmen
International, which can be custom designed to adapt to a particular
production
line or manufacturing facility. The curtain coater provides a continuous
curtain of
the coating through which the wet gypsum board is passed. Alternative coating
machinery may include sprayers, such as nozzle or shower sprayers, drip lines,
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and atomization techniques. An example of a spray technology is that provided
by
Graco, Inc. using spray tips suspended over the wallboard line. Another
alternative method is a blade technology that scrapes the coating mix on to
the
surface of the gypsum board, i.e., on the surface of the gypsum core or the
facing
sheet.
[0035] Coatings may be applied to the gypsum board under any condition of set
of
the gypsum. For example, the coating can be applied before oven drying, thus
allowing the coating to penetrate into the wet facing sheet and/or the wet
gypsum
core during the conveying period. After coating, the coated gypsum board is
dried
and then prepared for storage and shipping. Alternatively, the coating can be
applied after drying where, for example, the coating is heat sensitive or
penetration into the facing sheet is to be minimized.
[0036] Coatings can be applied to the whole surface area of the gypsum board ,
or
to predetermined portions thereof. In the latter, predetermined portions can
be so
designated by, for example, masking the gypsum board to prevent coating of the
designated sections or by adjusting the coating machinery to only apply a
coating
to predetermined portions.
[0037] Several alternative embodiments of compositions of coating are
envisioned,
each affecting the end product. One embodiment of a coating, referred to
herein
as the "rapid coating", is a coating that hardens by evaporation of the water
content. The rapid coating comprises water; calcium carbonate; fillers such as
limestone, mica, talc, and/or clay; binder; latex emulsion; and other
additives such
as preservative, and thickener. The rapid coating is provided with a water
content
of 35-45 wt. % and is diluted in the mixture to between 10% and 60% dilution,
e.g., to between 25-70 wt. %, preferably between 39 wt. % and 56 wt. % water,
by
the addition of water. Table 1 presents an exemplary formulation of an
embodiment of the rapid coating.
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[0038] Table 1
Com onent Amount (wt. % )
water 25-75
calcium carbonate30-70
mica 0-10
talc 0-10
cla 0-10
latex emulsion2-10
other additives0-10
[0039] In a lightweight embodiment of the rapid coating, the filler can
include
from 2% to 8% by weight perlite. An additional embodiment of the rapid coating
incorporates a pigment added in an amount effective to provide a desired tint
to
the coating. Additionally, the rapid coating may be made so as to
substantially
match a property of the joint compound, such as the absorbency, the
reflectance,
and so forth. The substantial matching of one or more properties may improve
the
aesthetic appearance of the gypsum board in the finished condition after a
decorative coating has been applied.
[0040] Thus, according to one aspect of the invention, a wall can be assembled
by
producing a gypsum board and coating the gypsum board before the drying step
with a joint compound or a diluted joint compound. And, when the gypsum board
is fastened to a supporting structure to form the wall, the seams between
adjacent
boards can be taped and finished using a joint compound having a substantially
similar composition to the coating.
[0041] Another embodiment of the present invention uses a coating referred to
hereinafter as the "setting coating". The setting coating forms a hard gypsum-
based coating by a setting process similar to that of the gypsum in the gypsum
board. The setting coating comprises is a dry product comprising calcined
gypsum; binder; fillers such as limestone, mica, talc, and/or clay; and other
additives such as preservative, accelerator, and thickener. The calcined
gypsum
may be either alpha or beta type or a blend of these types. Prior to use the
setting
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coating is mixed with 30-60 vol. % water to form a mixture. Table 2 presents
an
exemplary formulation of a setting coating.
[0042] Table 2
Com onent Amount (vol. %
)
Water 10-60
Calcined sum 50-90
Binder 0.1-10
Limestone 0-50
cla 0-10
Other fillers 0-30
Other additives0-10
~
[0043] In a lightweight embodiment of the setting coating, the filler can
include
from 2% to 15% by volume perlite. An additional embodiment of the setting
coating incorporates a pigment added in an amount effective to provide a
desired
tint to the coating.
[0044] The setting coating is generally more resilient to impact, i.e.,
tougher, than
the rapid coating, and may be utilized to provide an abuse resistant surface
on the
gypsum board, for example, for use in high traffic areas or public spaces.
Additionally, the rapid coating may be made so as to substantially match a
property of the joint compound, such as the absorbency, the reflectance, and
so
forth. The substantial matching of one or more properties may improve the
aesthetic appearance of the gypsum board in the finished condition after a
decorative coating has been applied.
[0045] Binders suitable for use in the above rapid coating and setting coating
include starches, which assist the binding properties. Examples of such
binders
are wheat starch which can be preset between 0.01 and 5 % .
[0046] A preservative suitable for use in the above rapid coating and setting
coating can be present between 0.01 and 5 % . This additive prevents the
product
from spoiling prematurely, prior to application. The coatings can spoil
because
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there can exist inside the formulations water and/or a number of organic raw
materials. The preservative is also commonly known as the biocide.
[0047] Accelerators may be used to adjust the set time in the above setting
coating. An example of an accelerator is potassium sulfate. The accelerator
can
be present in a suitable amount to achieve the desired set time. Typically,
the
accelerator is present from 0.01 and 15 % by solids weight.
[0048] Thickeners, such as generic cellulose-based thickeners, may be used to
adjust the rheological properties of the coatings and can be present in
suitable
amounts. Typical values are between 0.01 and 35 % by solids weight.
[0049] The penetration of the coating provided by the application of coatings
to a
wet gypsum board results in improved coated gypsum board properties, such as
better adhesion of the facing sheet to the gypsum core. In one example, a
coating
improves board strength, as measured by nail hold. Table 3 includes nail hold
values for both coated gypsum board and uncoated gypsum board. From Table 3,
it is shown that the coating improves nail hold by up to 25 % of nail hold
values
for traditional uncoated gypsum board products.
[0050] Table 3
sam 1e nail hold
value
coated 80 ounds
uncoated ~ 63 pounds
[0051] Additionally, a coated gypsum board displays a mechanical or strength
advantage over uncoated gypsum board products that may allow for utilization
of
facing sheets of lower weight with attendant cost reductions. Typically,
facing
sheets are anywhere between 40 to 60 lbs. weight. In some applications, the
facing sheet is a paper blended with mineral or synthetic fibers. If the
facing sheet
is to be coated, the improved board strength imparted by the coating allows
the
paper weight to be reduced. This advantage of the invention is realized while
still
obtaining the required handling characteristics to allow manipulation without
breakage during the manufacturing process.
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[0052] According to one preferred embodiment, a gypsum board is produced by
forming a gypsum slurry and depositing the slurry onto a facing sheet laid out
on
an endless conveyor belt. A backing sheet is then laid on top of the gypsum
slurry. The facing sheet, the gypsum slurry, and the backing sheet are then
formed into a wet gypsum board by passing the gypsum and optional facing and
backing sheet through a forming roller or rollers to produce the desired
thickness.
The wet gypsum board is carried by a conveyor belt until a set time expires,
after
which the gypsum board is cut into desired lengths with a blade cutter, and
then
passed through a mechanical coater. A subsequent application of heat, such as
in
a dryer or an oven, removes excess water. At any stage during the process,
either
the facing sheet or the backing sheet or both may be removed.
[0053] In an alternative embodiment of the present invention, the gypsum board
product can be formed in a manufacturing line as generally known for making
ceiling tiles. In such a process, the desired components are blended with a
high
shear mixer in a batch. The batch is then extruded through an opening of an
extruder onto an endless conveyor belt. After a predetermined set time, the
mixture is then cut into desired lengths. The cut lengths are then passed
through a
mechanical coater followed by a dryer to evaporate the excess water.
[0054] According to an embodiment of the present invention, lightweight gypsum
boards can be made. The density of such boards can range from as low as 1,000
lbs. per thousand square feet to about 1,700 lbs. per thousand square feet, or
greater. After cutting the gypsum and while the gypsum is still not dry, the
wet
gypsum board is transported to the coating and drying line. A coating of
thickness
of up to 30 mils is applied which is subsequently dried in a drying process,
e.g.,
dried in an oven.
[0055] According to another embodiment of the present invention, a gypsum
board
product of a desired thickness can be made as conventionally known. For
example, a'/z inch or 1/4 inch gypsum board can be made. After cutting the
gypsum and while the board is still not dry, the wet gypsum board is
transported
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to the coating and drying line. A coating of thickness of up to 30 mils is
applied
which is subsequently dried in a drying process, e.g., dried in an oven.
[0056] Although the present invention has been described in connection with
preferred embodiments thereof, it will be appreciated by those skilled in the
art
that additions, deletions, modifications, and substitutions not specifically
described
may be made without department from the spirit and scope of the invention as
defined in the appended claims.
