Note: Descriptions are shown in the official language in which they were submitted.
10~8869
WALLBOARD PROTECTED AGAINST EDGE BURNING
AND A METHOD OF PRODUCING SUCH WALLBOARD
This invention relates to wallboard and a method of
producing the wallboard that is commonly used in dry wall
systems, and in particular relates to an improved method of
manufacturing gypsum wallboard. More particularly this invention
relates to a treatment that is used on the edges of the gypsum
wallboard to render the edges of the wallboard resistant to
burning, which tends to occur while the wallboard is being
dried in the drier on the production line.
BACKGROUND OF THE INVENTION
The wallboard that is used in erecting dry wall
systems conventionally comprises a core material, such as
gypsum, and an outer covering of paper, e.g. ivory paper,
which envelops the core material and is fixedly bonded thereto.
Gypsum wallboard is produced on the production line
by introducing a slurry of gypsum between two continuously
moving webs of paper disposed one above the other and drawn
from separate supply rolls. The gypsum slurry and the two
paper webs positioned respectively above and below the slurry
pass through a device which forms the slurry-paper webs
combination into a broad flat wallboard. The gypsum wallboard
has flat front and back faces and a predetermined thickness
(normally in the range 1/4" to 1" thickness) that is
considerably less in dimension than the width of the flat faces
(normally in the range of four feet).
Wallboard is usually constructed so that the thickness
of the wallboard is generally tapered at the edges of the
wallboard, (used in this sense, the "edges" are determined in
the direction of the width of the wallboard). The "edge" of
the wallboard in this description is intended to include the
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tapered face on the front face, the edge face existing between
the front and back faces formed by the predetermined thickness
of the wallboard, and the area on the back face corresponding
to and opposite the tapered portion of the front face. Several
types of wallboard constructions are common in the trade. These
are known as (1) tapered edge, (2) round edge, (3) dual
edge and (4) square edge.
In the tapered edge, the wallboard edge when viewed
along a section cut through the thickness of the wallboard, and
commencing with the front face and continuing around the edge
of the wallboard to the back face, consists of a flat horizontal
front face, a flat tapered face extending at an angle with the
front face and sloping downwardly from the front face to the
edge face, the flat edge face extending vertically and meeting
at a reasonably sharp angle with the tapered face, and finally
the back face extending horizontally and meeting at right angles
with the vertical edge face.
In the round edge, when considered in the same manner
as the tapered edge above, the flat horizontal front face first
passes through a flat tapered face sloping downwardly at an
angle with the front face, and then continues smoothly along a
rounded convex curve into the vertical edge face.
In the dual edge, when considered in the same manner
as the tapered edge above, the flat horizontal front face
continues into a first flat tapered face sloping downwardly
at an angle with the front face. The first tapered face is
then followed by a second flat tapered face which slopes downwardly
at a greater angle than the first tapered face before meeting
the vertical edge face.
In the square edge, no tapered face(s) or rounded
edge is present. The flat horizontal front face meets directly
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10~8869
at right angles with the vertically extending flat edge face
which meets directly at right angles with the horizontal back
face.
It will be recognized that the presence of a tapered
or a rounded edge necessarily means that the thickness of the
wallboard at the edge face is appreciably less than the thickness
of the wallboard in the central area.
Once the wallboard is formed with one of the foregoing
edges, it is carried along the production line for a period of
time in order to permit the gypsum to set to a point where the
board is reasonably stiff and can be handled. Once the gypsum
has set sufficiently, the wallboard is cut into lengths. The
two edge faces of the wallboard are not affected by the cutting
of the wallboard into lengths. The cut wallboard then proceeds
into a drier. The function of the drier is to accelerate the
drying of the gypsum in the wallboard, thereby reducing the
overall length of the production line, and permitting increased
production of wallboard. The drier is usually operated at a
temperature of about 450F at the point of entry of the wallboard,
and at gradually reduced temperatures at successive points as
the wallboard travels through the drier toward the drier exit.
It would be advantageous if the drier could be
operated at temperatures higher than about 450F because the
wallboard could be dried more quickly and the speed of the
production line could thereby be increased. However, experience
has shown that if the drier temperature is too high, the edges
of the wallboard tend to "burn". This "burning" occurs when
the gypsum in the wallboard recalcines by being exposed to
high temperatures. Wallboard that has recalcined edges is an
undesirable product that has poor market acceptance because
the recalcined edges tend to be soft and crumbly and do not
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1048869
stand up to present wallboard handling and installation
techniques.
The maximum temperature at which the drier can be
operated, and the tendency for the wallboard to "burn" at
the edges, is determined to a major degree by the rate of drying
at the edges of the wallboard. The edges of the wallboard dry
faster than any other part of the board because firstly, there
are at least three drying surfaces at each of the two edges of
the wallboard, and secondly, the water content of wallboard
formed with either tapered, round or dual edges is less in the
edge regions because the thickness is reduced thereby reducing
the quantity of gypsum.
SUMMARY OF THE INVENTION
The applicants have discovered that the incidence of
"edge burning" can be reduced, and the rate of production of
wallboard can be increased to a significant degree, by coating
one or more of the faces of the edges of the wallboard with a
water evaporation control agent before the board is passed
through the drier. The coating can be applied to the edge face,
the tapered, round or dual face, and, if desired, the region
of the back face that is adjacent to the edge face of the
wallboard. It is believed that the coating decreases the rate
of water evaporation from the edges of the wallboard and thus
water is retained in the critical edge portions of the wallboard
for a longer period of time than in untreated wallboard. It
is believed that by applying the coating to the edges of the
wallboard, the drying rates throughout the wallboard are equalized
to a large degree. It has been found that coating the edges of
the wallboard with a water control agent permits the optimum
temperature of operation of the driers to be increased. This
enables the rate of wallboard travel through the drier to be
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~48869
increased, thereby increasing the rate of wallboard production.
It is important that the water evaporation control
agent be at least partially permeable to the transmission
therethrough of water vapour. If the water evaporation control
agent is impervious to the transmission of water vapour,
water vapour pressure can build up behind the impervious
agent and force the paper at the wallboard edges to part form
the underlying gypsum, thereby destroying the commercial
acceptability of the wallboard. To be effective, the water
evaporation control agent must be of a type that is capable
of controlling by means of its permeability to water but not
preventing the rate of water loss from the edge of the wallboard
during the drying operation. It is contemplated that a coating
should have a water vapour permeability rate of at least about
20 percent to be acceptable. The coating should not, of course,
be completely permeable to water because it will then have
no control over the rate of water loss.
Suitable coatings, for example, are silicone based
coatings, hot melt adhesives, paints (both aqueous and oil based),
thermoset resins, thermoplastic resins and cellulosic coatings.
Aqueous dispersions of ethylene-acrylic copolymers have been
found to be especially suitable. Among the latter family of
coatings, it has been found that an aqueous dispersion of
ethylene-acrylic copolymers sold by Morton Chemical Company,
Division of Morton-Norwich Products, Inc. under the trade mark
ADCOTE 37Fl is particularly suitable.
The invention is directed to an improvement ln a
method of producing gypsum wallboard which comprises the step
of drying the wallboard by the application of heat. The
improvement comprises:
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providing at least a portion of at least one of
the edges of the wallboard with a coating which is partially
but not completely permeable to the transmission of water
vapour, before drying the wallboard by the application of -
heat, to protect the edges of the wallboard against edge
burning during drying.
In the method described, the coating may be
selected from the group of substances comprising silicone
based coatings, hot melt adhesives, aqueous based paints, oil
based paints, thermoset resins excluding phenol formaldehyde
and urea formaldehyde, cellulose coatings, and aqueous dis-
persions of ethylene-acrylic copolymers.
The coating may be an aqueous dispersion of
ethylene-acrylic copolymers available under the trade mark
ADCOTE 37Fl.
When the wallboard has tapered edges, at least a
portion of the tapered face, the perpendicular edge and the
underside of the wallboard opposite the tapered face may be
coated with the coating. Alternatively, only at least a
portion of the tapered face and the perpendicular edge may be
coated with the coating. In the method described, the coating
may be applied to the edges of the paper before the gypsum is
encased in the paper.
The invention is also directed to a wallboard
comprising a gypsum core encased in paper having at least a
portion of one or more of its edges treated with a water
evaporation control agent that is at least partially but not
completely permeable to the transmission of water vapour.
The coating may be selected from the group of sub-
stances comprising silicone based coatings, hot melt adhesives,
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1~48869
aqueous based paints, oil based paints, thermoset resins
excluding phenol formaldehyde and urea formaldehyde,
cellulosic coatings, and aqueous dispersions of ethylene-
acrylic copolymers.
The coating may be an aqueous dispersion of
ethylene-acrylic copolymers available under the trade mark
ADCOTE 37Fl.
When the wallboard has tapered edges, at least a
portion of the tapered face, the perpendicular edge and the
underside of the wallboard opposite the tapered face may be
coated with the coating, or only at least a portion of the
tapered face and the perpendicular edge may be coated with
the coating.
DRAWINGS
FIGURE 1 illustrates a cross-sectional view of a
wallboard constructed with a tapered edge;
FIGURE:2 illustrates a cross-sectional view of a
wallboard constructed with a rounded edge;
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1~4l~869
FIGURE 3 illustrates a cross-sectional view of a
wallboard constructed with a dual edge;
FIGURE 4 illustrates a cross-sectional view of a
wallboard constructed with a square edge;
FIGURE 5 illustrates the relative positions of two
wallboards and a drier used to conduct tests on wallboard
treated to protect the edges thereof against burning.
DESCRIPTION OF THE INVENTION
In wallboard of the tapered edge type shown in
Figure 1, the gypsum 1 is encased in a top paper layer 2
and bottom paper layer 3. The top paper layer 2 is folded
around the edge 4 meets with bottom paper layer 3 to form a
joint 5. A tapered face 6 is present between top paper layer
2 and edge 4. The coating 7 is applied to the tapered surface
6 and edge 4. Although not shown, the coating 7 can also,
when required to control the rate of water evaporation from
the edge of the wallboard, be applied to the bottom paper
layer 3 in the location of joint 5.
As can be seen in the round edge wallboard type
shown in Figure 2, the wallboard edge 4 meets smoothly in a
convex rounded curve 8 with top paper edge 2, rather than
according to a tapered face 6 as shown in Figure 1, The
coating 7 is applied to edge 4 and a substantial portion of
curve 8, as required to control the rate of water evaporation
from the edge. The coating 8 can also be applied over joint 5
if required to provide proper control over the water evaporation
rate.
As shown in Figure 3, the dual edge type of wallboard
construction includes a second tapered face 9 in addition to
first tapered face 6. The coating 7 is normally applied to
edge 4, and tapered faces 6 and 9, but can also be applied over
joint 5, in order to control the water evaporation rate at the
edge of the wallboard.
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1~8869
Figure 4 illustrates wallboard with a square edge.
No rounded or tapered faces are present. The coating 7 is
normally applied to edge 4 and the outer region of top paper
layer 2. However, as with the other constructions, the coating
7 can also be applied over the joint 5 if required to control
the rate of water evaporation to prevent or reduce edge
burning.
SUITABLE WATER EVAPORATION CONTROL AGENT TESTING PROGRAM
The applicant has tested a substantial number of
coating agents that appeared to have promise as satisfactory
water evaporation control agents in the production of wall-
board. These coating agents were tested for water repellency.
Other characteristics such as joint cement adhesion were also
noted. A table of the results of this testing program are
tabulated below.
COATING CHARACTERISTICS
Dow Corning lD4* some repellency
Dow Corning 772* some repellency
Cyanamid Paramul 115* some repellency
20 Dow Corning CP701566* some repellency
Esso Polymer CTLA* some repellency
Wesco microfil some repellency
Parawax* good repellency
Alwax 2536* some repellency
Alwax 204A* good repellency - poor joint
cement adhesion.
Atlac* some repellency
Parez* some repellency
Swift 2560* some repellency
30 Shellac good repellency
Dow Catalyse 21* some repellency
Cypel 221* some repellency
Swift Strippable Coating some~repellency
Urea formaldehyde good repellency
Oil based paint fair repellency
Latex paint some repellency
Stein Hall hot melt good repellency - poor joint
cement adhesion
Industrial adhesive hot melt good repellency - poor joint
cement adhesion
Swift hot melt 625* good repellency - good joint
cement. Expensive.
Eastman hot melt good repellency - poor joint
cement adhesion.
National - Resyn 25-2813* some repellency - poor joint
cement adhesion.
*Trade marks
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1~48869
National - Durclok 100* some repellency - poor joint
cement adhesion
Industrial paint C-70-57A some repellency
Industrial paint C-70-57B good repellency - good joint
cement adhesion
Urea formaldehyde fair repellency - good joint
cement adhesion
Scripset 500* some repellency
Scripset 520* good repellency - poor application
10 Melamine formaldehyde some repellency
Suitable coatings can be applied in two strips at
the edges of the front face of the wallboard extending inwardly
up to about 6 inches from the two edges of the wallboard, on
the edges of the wallboard, and if required, in two strips at
the edges of the back face. Usually, it is only necessary,
in order to properly control the water evaporation rate, to
apply the coating at the two edges of the front face of the
wallboard in two strips extending about 1/2 to 1 inch from
each of the two edges,and the two sides.
It has been found that water vapour permeability of
the coating is a factor to be considered in achieving protection
against edge burning. While it is believed that acceptable -
coatings should have a water vapour permeability of at least
about 20 percent, water vapour permeabilities in the range
; of about 50 to 85 percent are usually preferred. Coating
thickness is also a factor to be considered in controlling
water evaporation rate because the thicker the coating, the
less the water vapour permeability. The coatings can be used
on any shape of wallboard edge, including tapered, round,
dual or square. It has been found that the different shapes
of wallboard vary as to evaporation rate and the type, area and
thickness of coating must be adjusted accordingly.
DESCRIPTION OF PREFERRED EMBODIMENT
The applicant, after considerable experimentation,
has found that a coating of an aqueous dispersion of an
*Trade marks
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1~48~369
ethylene-acrylic copolymer, sold under the trade mark ADCOTE
37Fl by Morton Chemical Company, Division of Morton-Norwich
Products, Inc., Chicago, Illinois, is particularly effective
in controlling the rate of water vapour evaporation from the
edges of the wallboard and thus reducing the incidence of
edge burning in gypsum wallboard production.
The ADCOTE 37Fl coating is preferred to other
coatings tested for a number of reasons which are listed below:
(1) The ADCOTE 37Fl coating does not provide an
environment pollution problem;
(2) The drying rate of the ADCOTE 37Fl coating is
relatively uniform over a wide range of conditions and thus
does not provide a drying time problem;
(3) The ADCOTE 37Fl coating has good shelf life;
(4) The ADCOTE 37Fl coating is unexpectedly easy
to apply to the edges of the wallboard on the production line;
(5) The ADCOTE 37Fl coating is relatively non- -
toxic;and
(6) The ADCOTE 37Fl coating is semi-water permeable
and provides good control over the rate of water evaporation
from the edges of the wallboard.
The applicant has performed tests under production
conditions on untreated 1/2 inch thick Standard wallboard
(tapered edge) and 1/2 inch thick Standard wallboard (tapered
edge) treated with ADCOTE 37Fl that demonstrate the ADCOTE 37Fl
coating protects the edges of the wallboard from burning. The
incidence of edge burning can be related to the percentage of
hemihydrate at the edges of the wallboard and this was one
criteria used to demonstrate reduced edge burning.
Moisture absorbency measurements on the edges of treated
and untreated wallboard were used to demonstrate the protective
effect of the coatings.
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1048869
A series of untreated boards and boards treated with
ADCOTE 37Fl were produced at the applicant's Clarkson plant
and analysed for:
1. Moisture absorbency in a 1/2" strip
adjacent to the edge of the wallboard.
2. Hemihydrate content of the gypsum
core in the same 1/2" strip adjacent
the edges of the wallboard.
The details of the methods used to test moisture
absorbency and hemihydrate content are given as follows:
Moisture Absorbency Test Method
Samples of wallboard approximately 30 cm long and
approximately 2.5 cm wide were cut from the edges of untreated
board and board treated with ADCOTE 37Fl.
Masking tape was used to seal off the edge faces
leaving exposed a rectangular test area of treated or untreated
face paper.
The samples were then weighed.
Joint cement paste was then placed over the exposed
test area, covered with a protective sheet to prevent drying
and left overnight (16 hours).
The joint cement was then scraped off the test area
and the samples were quickly reweighed.
The increase in weight of each sample, due to
absorption of moisture from the joint cement, was obtained
by the weight difference.
The change in moisture absorbency of treated
wallboard can be calculated according to the following equation: ~ ;
WMAU - WMAT x 100 = DMA
WMAU
where: WMAU is weight of moisture absorbed
by untreated sample;
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1~114~869
WMAT is weight of moisture absorbed
by treated sample; and
DMA is percent decrease in moisture
absorbency.
Hemihydrate Content of Gypsum Core - Test Method
Samples 30 cm long x 10 cm wide were cut from the
edges of the test boards according to the positions detailed
in Figure 5.
A strip of the front face and edge face paper
approximately 1" wide was detached from the edge and the face
of the test wallboard. The surface layer, approximately
1/16" deep,of the exposed gypsum core was then removed and
dried to constant weight at 40C + 2 for 24 hours.
The dried sample was ground to allow passage through
a 50 mesh sieve and approximately 2 g weighed accurately
in a weighing bottle. An excess of water (5 ml) was added ~-
and after one hour the sample was again dried at 40C + 2
for 24 hours followed by one hour over silica gel in a
dessicator to remove all free moisture before reweighing.
From the chemical equation
CaSO4 1/2 H2O + 1-1~2 H2O ~ CaSO4-2H2O
Hemihydrate Gypsum
the ~ hemihydrate in the test samples can be calculated as
follows:
FSW - OSW x 145 x 100 = H
OSW x 27
where: FSW is final sample weight;
OSW is original sample weight; and
H is percent hemihydrate.
EXAMPLE
Product: 1/2" Standard Board (tapered edge)
Coating: ADCOTE 37 Fl
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1~48869
Area Treated: 1/2" on tapered face, 1/4" to 1/2" on edge.
Coating Rate: 0.2 lbs/MSF.
Film Thickness: less than 0.001"
The results of the experiments conducted are shown
in the following tables.
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1~4886~
TABLE 2
Edge Sample ~ Hemihy( ~rate
(See Position Untreated Treated Hemihydrate Average
FIG. 5) (See Reduction Hemihydrate
FIG. 5) _ % Reduction
_ _
1 19.4 16.0 17 )
A 52l96 96 12o6 91 l2l ) 18%
7 24.5 20.716 )
1 14.4 5.4 63 )
B 3 13.6 3.2 76 ) 78%
19.8 2.5 87 )
7 16.2 2.4 85 )
1 34.1 8.8 74 )
3 19.1 4.4 77 ) 7~%
C 5 17.0 4.9 71 )
7 21.7 5.6 74 )
1 16.3 13.921 )
D 53 47 5 33 2527 ) 35%
7 8.9 5 5 38 )
The ADCOTE 37Fl coating can be applied at the edges
of the wallboard in the form of two strips along the front face
of the wallboard extending up to about 6 inches from each of
the two edges of the wallboard. The width of each strip is
directly dependent upon the width of edge that it is found
necessary to treat in order to eliminate or reduce edge
burning under different conditions. Usually, the width of
the two strips of ADCOTE 37Fl coating necessary to eliminate
or reduce edge burning will fall in the range 1/2 to 1 inch
from each of ~he two edges.
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The ADCOTE 37Fl can also be applied to the perpendicular
edges of the wallboard to reduce the rate of water loss through
the edges of the wallboard. The ADCOTE 37Fl can also be applied
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1~8869
to the edge regions of the back (bottom) face of the wallboard
opposite the treated areas on the front face, if it is found
to be necessary to reduce the rate of water evaporation more
than is possible merely by coating the edges of the front
face and the perpendicular edges.
The ADCOTE 37Fl coating can be applied in various
thicknesses and amounts to the edges of the wallboard in
order to control the rate of water vapour transmission. Ideally,
the rate of water vapour evaporation over the whole board
should be reasonably uniform and the thickness of coating
and the area of the coating in the form of strip width are
adjusted experimentally to bring this about.
The coating can be applied at any location along
the wallboard production line between the point where the
gypc-um in the wallboard has set sufficiently to provide a
measure of stability to the wallboard, and the entry point
of the drier. -
Alternatively, the coating can be applied to the ;
edges of the paper and dried before the paper is wound onto
rolls for use in the production of wallboard. The coating
can be applied by the paper manufacturer as part of the paper
specification requirements of the wallboard manufacturer.
While a number of substances, and a preferred
substance, have been discussed as being of use in the invention,
nevertheless it should be understood that the scope of the
invention is not limited to the specific substances disclosed.
Other suitable substances no doubt exist that can be tested
and found suitable by any person skilled in this art working
according to the principles of the invention. Any substances
that therefore fall within the spirit and scope of the
following claims are to be considered as comprising part of
the invention.
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