CA2070489A1

CA2070489A1 - Gypsum board coating

Info

Publication number: CA2070489A1
Application number: CA 2070489
Authority: CA
Inventors: Ian Norris Davie; John Gilmour Ewen Mcewen; Gregory Lazenka
Original assignee: IGI INTERNATIONAL WAXES DIVISION OF THE INTERNATIONAL GROUP INC.
Current assignee: IGI INTERNATIONAL WAXES DIVISION OF INTERNATIONAL GROUP Inc
Priority date: 1992-06-04
Filing date: 1992-06-04
Publication date: 1993-12-05

Abstract

ABSTRACT
A gypsum board having an integrated moisture vapour barrier layer of an improved moisture vapour impermeability is disclosed. The layer is applied to the surface of the board by curtain coating a hot melt of a material comprising a natural or synthetic wax, such as a paraffin or polyethylene wax and, for example, an ethylene-vinyl acetate copolymer or a microcrystalline wax, and permitting the melt to cool. To improve the integrity of the moisture vapour barrier layer by reducing the formation of pinholes and blisters during the curtain coating, a layer of a paraffin or microcrystalline wax may be applied to the surface of the board, for example by brushing or spraying, prior to curtain coating.

Description

~7~ 9 2796~-2 GYPSUM BOARD COATING
This lnvention relates to gypsum board provided with an integrated molsture vapour barrie:r and to a method of preparing such gypsum board. The invention also relates to a method of treating gypsum board prior to cuxtain coating to improve the moisture vapour impermeability of the coating.
Panels of gypsum board are a well known building material used mostly to form walls and ceilings of buildings and also, to some extent, as an exterior material. While gypsum boards provide excellent fire resistance, they also have a dis-advantage of being susceptible to moisture, which easily pene-- trates into and through the boards. When put in contact with liquid water, gypsum board absorbs in a short time a considerable amount of water. This usually damages the board in that it causes ; deformation, decreases mechanical strength and may even destroy the structural integrity of the board.
Numerous attempts to enhance the water resistance of ~- gypsum boards are known from the prior art. The most common practice is the incorporation of water-dispersible hydrophobic materials, such as paraffin wax or asphalt, into gypsum slurry prior to setting (see, for example, U.S. 2,604,411). Another approach is a surface post-treatment of gypsum boards, by applying a water impermeable or repellant coating to the surface (see, for example, U.S. 4,350,736, U.S. 3,839,141 and U.S. 3,850,680).
The moisture resistance of a surface coating may be seriously impaired if pinholes are formed in the process of coating. This happens, for example, when the surface of a gypsum ~ 1 .
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2796~-2 board is being covered with a hot material. Such a process usual-ly results in blistering o~ the coating and in disr~lpting of the integrity of th~ produced coating.
The effect of blistering was noted, for example, in U.S.
4,879,173, where hot asphalt was applied to gypsum boards used in roof construc-tion. In this process gypsum boards faced on both sides with a glass filament mat were used. Blister formation was attributed to the vaporization of water contained in the gypsum itself, caused by contact with the hot asphalt ~400-450F). Pre-treatment of the gypsum board with little amounts of a reinforcingresin binder prevented the formation of blisters without affecting the drying characteristics of the board, i.e. without providing a barrier to the passage o water vapour.
The known waterproofing treatments, even though impart-ing resistance to the entry of liquid water, usually do not pro-vide an efficient moisture vapour barrier. The inclusion of such a barrier, in particular in an exterior wall or a ceiling structure of a building is a standard practice in the fabrication of buildings and can be accomplished by various methods, such as extending large sheets of plastic film, for example polyethylene, - throughout the vapour protected surface.
The prior art of producing gypsum boards having moisture vapour barrier properties, for use in constructions, is limited to laminating aluminum foil to the paper backing of the gypsum board (see, for example, CA 1,091,566). The aluminum foil must be glued manually to each sheet of drywall, which makes the process both time consuming and expensive.

. ' ' ~7~3~3 2796~-2 According to one aspect, the invention provides a method for coating a gypsum board to provide a gypsum board having a moisture vapour barrier layer on a surface thereof, which method comprises applying to the surface by curtain coating a hot melt of a material that will cool to form a moisture vapour barrier layer and permitting the melt to cool to form the required moisture vapour barrier layer.
According to another aspect, the invention provides a gypsum board having a moisture vapour barrier layer applied to a surface of the board by curtain coating a hot melt of a material that form -the moisture vapour barrier layer when permitted to ` cool.
Since the application of a vapour barriers by curtain coating is carried out mechanically, labour requirements and costs are reduced and the overall speed of the production line may be increased. The curtain coating material is also less expensive, leading to further reduction in costs.
In ~he curtain coating process, the coating composition is delivered in a falling sheet or curtain onto the substrate, which is moved through the curtain at a controlled rate. The coating composition must be sufficiently fluid to fall freely and sufficiently cohesive to form a continuous film on the substrate.
In one embodiment of the invention, the coating composition is first melted and heated in a tank to the required temperature.
The composition is then pumped from the tank to a reservoir which has a wide, narrow slit opening in its bottom. The slit allows a : ~:
wide, thin flow of molten coating, referred to as a curtain, to :~ 3 ~. . . ~ :......... ' ~ . ' : - . , : . ' , ' . :,, . ' ' ' fall upon the substrate as it passes underneath the reservoir.
The thickness of the resulting coating can be controlled by vary-ing the slit opening, the rate of pumping of the molten composi-tion and/or the speed at which the substrate is passed under the slit.
In the conventional process of producing gypsum boards, the wet gypsum board is placed in an oven to remove excess water.
According to the invention, the gypsum board may be curtain coated directly upon leaving the oven or, alternatively, after cooling or being cooled to required temperature, for example the ambient temperature. If coating takes place immediately after the board leaves the oven, the melting point of the hot melt coating material must be higher than the temperature of the gypsum board - as it leaves the oven. In general, application temperatures of the hot melt may range from about 180 to about 350F. Application temperatures from about 250 to about 300F are preferred.
The thickness of the coating may be adjusted as re-quired. A thickness of from about one-half to about ten thousandths of an inch has been found satisfactory for most applications, a thickness of from about two to about four thousandths of an inch being preferred. The board may be coated - in one or several passes through a curtain coating machine. A
single or double pass is preferred. Depending on the intended application of the board, the coating may be applied to one or to both faces of the gypsum board.
The preferred gypsum board to receive the coating according to the invention is of the standard type, having a core :
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2 ~ r~ 8 ~l 279~4-2 of set yypsum sandwiched between two sheets of facing paper.
However, other types of gypsum boards, for example those haviny facing sheets of glass fibers, as described in U.S. patent 4,879,173, or even those having th~e core waterproofed by admixing water-resistant additives, such as wax emulsions, may also be used.
A preferred hot melt applied by curtain coating com-prises a paraffin wax admixed with one or more of the following:
ethylene-vinyl acetate copolymer, tackifying resins, styrene-rubber block copolymers, an N,N'-ethylene bis-amide and micro-crystalline wax.
Synthetic or natural waxes other than a paxaffin wax may be used as components in the curtain coating blend. Examples of ; synthetic waxes may be polyethylene waxes, oxidized polyethylene waxes, chlorinated paraffin waxes and Fisher-Tropsch waxes.
Examples of natural waxes may be carnauba and candellila waxes.
;~ Similarly, other polymers and copolymers, such as ethylene-maleic acid anhydride copolymer, ethylene-acrylic acid copolymer, ethylene-acrylic ester copolymers, ethylene-alkyl acrylic acid and ester copolymers, as well as ionically crosslinked variations thereof can also be used in combination with or in place of the ethylene-vinyl acetate and styrene-rubber block copolymers.
Ethylene-vinyl acetate copolymer having acetate content of from about 5% to about 50%, preferably from about 18~ to about 28%, with softening points up to about 400F may be used.
Tackifying resins suitable for the practice of the invention include C5 aliphatic and Cg aromatic hydrocarbon resins .
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27g6~-2 and copolymers, resin acids and derivati~es thereof, ~tyrenic and polyterpene resins and copolymers and phenolic resins, with softening points up to about 140C. The choice of resin is made according to softening point, colour and compatibility require-ment, well known to those skilled in the art.
Styrene-rubber block copolymers are thermoplastic rubbers which are block copolymers composed of styrene or methyl styrene in combination with one or more of ethylene, butylene, isoprene, and isobutylene. The styrene content may range from about 10 to about 50%.
N,N'-Ethylene bis-amides are di-amide waxes, containing either bis-stearami~e or bis-oleamide with melting points of 110-120C and 130 150C, respectively.
In general, the coating composition may comprise 54-90 paraffin wax, 0-20% microcrystalline wax, 0-25% ethylene-vinyl acetate copolymer, 0-10% tackifying resin, 0-10% low molecular weight polyethylene wax, and 0-6% styrene-rubber block copolymer.
A composition comprising from about 65~ to about 90% paraffin wax, from about 5% to about 20% ethylene-vinyl acetate copolymer, from about 5% to about 10% tackifying resin and from 0% to about 5% low molecular weight polyethylene wax is preferred.
The hot melt used for curtain coating may contain further additives, such as antioxidants and/or flame retardants.
1,3,5-Trimethyl-~,4,6-tris-(3,5-di-tert-4-hydroxybenzyl)benzene is preferred as antioxidant. The preferred flame retardant composi-tion is obtained by combining 100 parts by weight of the hot melt -,. ~ .

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~ 796~-2 coating with 5 parts by welght of chlorinated paraffin and 2.5 parts by weight of antimony trioxide.
The gypsum board coated according to the invention is characterized by an improved moisture vapour impermeability.
However, as discussed earlier in connection with U.S. 4,879,173, the application of a hot melt coating to the surface of a gypsum board may result in formation of pinholes and blisters in the coating as it solidifies. This, in turn, may impair the moisture vapour properties of the coating. It is believed that the pin-holes and blisters form as a result of water present in the paperfacing and/or the core of the gypsum board, which water vaporizes upon contact with hot melt coating, which may be applied at temperatures as high as 350F. It has now been found that this adverse effect can be substantially reduced by covering the surface of the gypsum board with a layer of a paraffin or micro-crystalline wax, or a mixture thereof, prior to curtain coating.
~ It is believed that this pre-treatment of the gypsum board serves - the purpose of both absorbing heat from the molten hot melt and providing an additional barrier to the vaporized water, thus preserving the integrity of the subsequently applied hot melt coating as the hot melt solidifies.
Waxes suitable for the pre-treatment are those capable of forming a surface layer at the facing of the gypsum board.
These include paraffin and microcrystalline waxes, polyethylene waxes, Fischer-Tropsch waxes, and natural waxes, such as carnauba and candellila waxes. For the purpose of pre-treatment, the hot -~ melt curtain coating material may also be applied.
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Penetration of wax into the paper facing is not desir-able, in that it increases the amount of the material used without significantly increasing the beneficial effects of the treatment.
Factors which determine the degree of penetration of the wax into the paper facing include the method of application, the melting point of the wax and the temperature of the gypsum board, as dis-cussed below.
For the pre-treatment, various methods of applying the molten wax to the surface of the gypsum board, such as brushing or spraying, may be used. Of these, spraying molten wax is pre-ferred. It was found that spraying allows control over the degree of penetration of wax into the paper facing, by adjusting -the wax temperature, spraying intensity, and spraying distance.
Paraffin waxes with melting points in a range of from about 120 to about 165F and microcrystalline waxes with melting points of from about 150 to about 1930F have been found to be suitable for application at room temperature to gypsum boards.
; The applied wax blend may consist entirely of paraffin or micro-crystalline wax, or it may be a mixture of the two.
Pr -treatment of the gypsum board may take place immedi-ately after the board leaves the drying oven. In this case, the melting point of the wax or wax blend should be preferably greater than the temperature of the gypsum board. If the melting point of the wax is lower than the temperature o~ the gypsum board to be coated, excessive penetration of wax into the paper facing will occur. Consequently, proper selection of the wax or wax blend '~' ' : ' .
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with which to spray the gypsum board is important. If the temperature of the gypsum board is 1400F, then paraffin waxes with melting polnts in the range of from about 140 to about 165F or microcrystalline waxes with melting points in the range of from about 150 to about 193F may be used alone or in combination wi~h each other. Paraffin waxes with melting points lower than 140F
may be used in combination with waxes of higher melting points to produce a blend of sufficiently high melting point.
The thickness of the wax pre-treatment layer may be varied as required, but is normally in the range of from about one-half to about 10 thousandths of an inch. A thickness of from ; about 2 to about 4 thousandths of an inch is preferred. Depending on the intended application of the board, the wax layer may be applied to one or to both sides of the gypsum board. Curtain coating may take place immediately after applying the wax coating,:
or after allowing the wax coating to cool to ambient temperature.
When the surface of the board was pre-treated according to the invention, it could be observed that in some cases bubbles were absent from the subsequently applied coating, whereas in other cases bubbles trapped in the coating were noted, but the coating surface integrity was not affected. Thus, the wax layer effectively absorbs heat from the hot melt in the former cases, whereas in the latter cases it impedes expulsion of water vapour through the coating surface.
The invention will be further described by way of a ~ preferred embodiment and non-limiting examples.

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2796~-2 ~L~ 1 An 111- X 11" piece of gypsum board was warmed in an oven to 140F, then removed from the oven and i.mmediately passed twice through a curtain coater. From the curtain coater was applied a hot melt coating having temperature of 300F and comprising a blend of 17.5% ethylene-vinyl acetate copolymer and 82.5% of paraffin having melting point o~ 140F. The coating thickness of each pass was 2 thousandths of an inch for a final thickness of ~ thousandths of an inch.
10 EXAMI?LE 2 Gypsum board was cut into 11" x 11" pieces and warmed in an oven to 140F. The boards were removed from the oven one at a time and immediately sprayed on one side with a paraffin wax having melting point of 160F, using a hand held spray gun driven by compressed nitrogen gas. The final coating thickness of the wax layer was 4 thousandths of an inch. The wax coated board was immediately curtain coated with a blend of 17.5~ ethylene-vinyl acetate copolymer and 82.5% of paraffin having melting point of 140F, at an application temperature of 300F. The thickness of the hot melt coating was 2 thousandths of an inch.
EXAMPL~ 3 A 11" x 11" piece of gypsum board having room tempera-ture was sprayed on one side with a paraffin having melting poin~
of 120F using a hand held spray gun, as in Example 2. The thick-ness of this coating was 4 thousandths of an inch. Curtain coating was carried out as in Example 1. In this case, some .

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bubbles were no~ed in the coating, but they were retained in the coating and did not disrupt the surface integrity.

The coated gypsum boards from Examples 1, 2 and 3 were tested in a controlled environment cabinet with a relative humidity of 92~ at a temperature of 100F. To prevent the entry of water through the untreated surfaces of the board the uncoated sides of the gypsum boards were covered with a double layer of a polyethylene plastic sheet covered with a sheet of aluminium foil.
A control sample consisted of an 11" x 11" piece of gypsum board completely covered with polyethylene plastic and aluminium foil.
A reference sample was identical with samples of examples 1, 2 and 3 except that it was not pretreated with wax nor curtain coated.
The edges of the gypsum boards were sealed by dipping in a 60/40 mixture of microcrystalline wax/paraffin wax. After the samples were covered and sealed, the surface area exposed to moisture vapour was in each case nine square inches. The samples were weighed, placed in the controlled environment cabinet for twenty-four hours and weighed again. The difference between the weight after the exposure in the cabinet and the original weight constituted the water uptake. The values repor~ed in the follow-ing table are averages of at least two determinations.

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~ , ~7~89 27g6~-2 5~MPL~ ~T~ ~PTAX~ (GR~M5 Control o Reference 7.3 Example 1 0.42 Example 2 0.06 Example 3 0.12 The ~ater uptake of the control demonstrates that the wrapping with polyethylene plastic and aluminum foil was effective in preventing passage of moisture. Consequently, any water uptake will have occurred through untreated surfaces, in the reference sample, or through surfaces treated in accordance with the inven-tion, in Examples 1, 2 and 3.
The results clearly demonstrate the moisture vapour ~; barrier properties of the curtain coated gypsum board (Example 1) compared with the uncoated board ~Reference), as measured by the amount of moisture penetrating into the board in the form of vapour. Examples 2 and 3 also show the effectiveness of the wax ~ pre-treatment in improving the moisture vapour barrier properties : of the curtain coating.

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Claims

1. A method for coating a gypsum board to provide a gypsum board having a moisture vapour barrier layer on a surface thereof, which method comprises applying to the surface by curtain coating a hot melt of a material that will cool to form a moisture vapour barrier layer and permitting the melt to cool to form the required moisture vapour barrier layer.

2. A method according to claim 1, wherein the thickness of the barrier layer is from about one-half to about 10 thousandths of an inch.

3. A method according to claim 2, wherein the thickness of the barrier layer is from about 2 to about 4 thousandths of an inch.

4. A method according to claim 1, wherein the gypsum board consists of a core of gypsum sandwiched between two layers of paper.

5. A method according to claim 1, wherein the gypsum board consists of a core of gypsum sandwiched between two layers of glass fibre mat.

6. A gypsum board according to claim 1, wherein the barrier layer is adhered to one face of the gypsum board.

7. A gypsum board according to claim 1, wherein the barrier layer is adhered to both faces of the gypsum board.

8. A method according to claim 1, wherein the hot melt has a temperature of from about 180 to about 350°F.

9. A method according to claim 1, wherein the hot melt is applied to the gypsum board having a room temperature.

10. A method according to claim 1, wherein the hot melt is applied to the gypsum board having a temperature of about 140°F.

11. A method according to claim 1, wherein the curtain coating is effected in a single pass through a curtain coater.

12. A method according to claim 1, wherein the curtain coating is effected in a double pass through a curtain coater.

13. A method according to claim 1, wherein the moisture vapour barrier layer comprises from about 54 to about 90% paraffin wax, from 0 to about 20% microcrystalline wax, from 0 to about 25%
ethylene-vinyl acetate copolymer, from 0 to about 10% low molecu-lar weight polyethylene wax, from 0 to about 10% tackifying resins, and from 0 to about 6% styrene-rubber block copolymer.

14. A method according to claim 13, wherein the moisture vapour barrier layer comprises from about 65 to about 90% paraffin wax, from about 5 to about 20% ethylene-vinyl acetate wax, from about 5 to about 10% tackifying resin, and from 0 to about 5% low molecular weight polyethylene wax.

15. A method according to claim 13, wherein the moisture vapour barrier layer further comprises an antioxidant and/or a flame retardant.

16. A method according to claim 15, wherein the antioxidant is 1,3,5-trimethyl-2,4,6-tris-(3,5-di-tert-4-hydroxybenzyl)ben-zene.

17. A method according to claim 16, wherein the antioxidant is added in an amount of from about 0.001 to about 0.1% by weight.

18. A method according to claim 17, wherein the antioxidant is added in the amount of 0.01% by weight.

19. A method according to claim 15, wherein the flame re-tardant consists of 5 parts by weight of chlorinated paraffin and 2.5 parts by weight of antimony trioxide per 100 parts by weight of the moisture vapour impermeable material.

20. A method according to claim 1, further comprising applying a wax layer to the surface of the gypsum board prior to applying the hot melt, the wax layer comprising a paraffin wax, a microcrystalline wax or a blend thereof.

21. A method according to claim 20, wherein the paraffin wax has a melting point of from about 120 to about 165°F and the microcrystalline wax has a melting point of from about 150 to about 193°F.

22. A method according to claim 20, wherein the wax layer is applied by spraying the molten wax onto the surface of the gypsum board.

23. A method according to claim 20, wherein the thickness of the wax layer is from about one-half to about 10 thousandths of an inch.

24. A method according to claim 23, wherein the thickness of the wax layer is from about 2 to about 4 thousandths of an inch.

25. A method according to claim 20, wherein the wax layer is applied to one face of the gypsum board.

26. A method according to claim 19, wherein the wax layer is applied to both faces of the gypsum board.

27. A method according to claim 20, wherein the wax layer is applied to the gypsum board having a room temperature.

28. A method according to claim 20, wherein the wax layer is applied to the gypsum board having a temperature of about 140°F.

29. A gypsum board having a moisture vapour barrier layer applied to a surface of the board by curtain coating a hot melt of a material that form the moisture vapour barrier layer when per mitted to cool.

30. A gypsum board according to claim 29 further comprising a wax layer applied to the surface of the board prior to curtain coating, the wax layer comprising a paraffin wax, a micro-crystalline wax or a blend thereof.