CA1048869A - Treatment of wallboard edges to control rate of water evaporation - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

This invention relates to wallboard and a method that is useful in protecting the edges of wallboard, such as gypsum wallboard, from burning while being passed through the drier on the wallboard production line. The method involves coating the edges of the wallboard with a coating that is at least semi-permeable to the transmission of water vapour.

Description

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 - 1- ~

:
1~)4~ 9 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 , .: :

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 :.- '' ~ ; ' ' 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 , - ' ~ '' :' .':
': . . , ~ ,, :' ' ~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:

~ - 5 -C

.

161 ~81369 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, ~ - 5a -, ' , 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;

- 5b -'.' .

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.

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 . .

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 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.

- . .:

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 , . . , , - . .
: .
- . ' ' 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~

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.

.. ~ . .
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 , , ' ~ ~

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.

Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a method of producing gypsum wallboard which comprises the step of drying the wallboard by the application of heat, the improvement which comprises:
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.

2. The method of claim 1 wherein the coating is 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, cellulosic coatings, and aqueous dispersions of ethylene-acrylic copolymers.

3. The method of claim 1 wherein the coating is an aqueous dispersion of ethlene-acrylic copolymers.

4. The method of claim 1 wherein the coating is an aqueous dispersion of ethylene-acrylic copolymers available under the trade mark ADCOTE 37F1.

5. The method of claim 1 wherein the wallboard has tapered edges and at least a portion of the tapered face, the perpendicular edge and the underside of the wallboard opposite the tapered face are coated with the coating.

6. The method of claim 5 wherein only at least a - Page 1 of Claims -portion of the tapered face and the perpendicular edge are coated with the coating.

7. The method of claim 1 wherein the coating is applied to the edges of the paper before the gypsum is encased in the paper.

8. 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.

9. The wallboard of claim 8 wherein the coating is 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, cellulosic coatings, and aqueous dispersions of ethylene-acrylic copolymers.

10. The wallboard of claim 8 wherein the coating is an aqueous dispersion of ethylene-acrylic copolymers.

11. The wallboard of claim 8 wherein the coating is an aqueous dispersion of ethylene-acrylic copolymers available under the trade mark ADCOTE 37F1.

12. The wallboard of claim 11 wherein the wallboard has tapered edges and at least a portion of the tapered face, the perpendicular edge and the underside of the wallboard opposite the tapered face are coated with the coating.

13. The wallboard of claim 12 wherein only at least a portion of the tapered face and the perpendicular edge are coated with the coating.

- Page 2 of Claim -