Predecorated Gypsum Board

These and other objects and advantages of the invention will be more readily apparent when considered in relation to the preferred embodiments of the invention as set forth in the following specification.

In accordance with the invention, gypsum wallboard paper, of about 0.005 to 0.020 inch thickness, is first manufactured using known Fourdrinier or cylinder type paper machines. The quality of the predecorated gypsum board made by the invention will be, in part, dependent on the smoothness of the front surface of the face paper, with a smoothness of from about 60 to about 400 Sheffield units being satisfactory for most designs, and about 60 to about 200 Sheffield units being essential for high fidelity designs, such as wood grain patterns. The lower the Sheffield units for any paper, and thus the smoother the paper, the better it will be for use in the invention.

The porosity of the paper to be predecorated preferably has a porosity of about 25 to 70 seconds, when tested using a Gurley Densometer, in accordance with TAPPI Standards T460m-49, however less porous paper, up to 100 seconds or more, can still be used to make predecorated front paper in accordance with the invention. More porous paper, as fast as about 10 seconds, can also be successfully used. Considering porosity alone, the more porous the paper is the better for making gypsum board.

The Cobb value of the paper, tested on the front surface, in accordance with the general test outlined by TAPPI, should be no greater than 1.8 grams. The consistent brightness and color of the paper can be of importance in maintaining a consistent product, with each predecorated board of a given design matching all other boards of the same given design.

The front surface of the face paper is, first, printed, by the rotogravure process, with a plurality of closely spaced minute dots or extremely narrow, closely spaced parallel lines, when viewed through a microscope, forming a coat of a thermosetting aqueous latex emulsion having substantially throughout minutely closely spaced, minute openings or voids, which open areas may be continuous with discontinuous minute areas of base coat, or discontinuous minute areas surrounded by continuous or adjoined areas of base coat, or a combination of the two. This thermosetting aqueous latex emulsion coat may be either clear or tinted. It is preferably disposed uniformly throughout substantially all of the face paper front surface. This coat of separated minute dots or lines or the like of thermosetting aqueous latex emulsion, immediately after application, normally flows to a limited extent prior to its drying, forming a base coat, which when dried may be in the form of minute separated dots or lines or in the form of minute interconnected dots with a great plurality of openings resulting from the emulsion of the dots or lines being insufficient to completely coat and close any substantial area. These openings are spaced apart at average distances of between 0.1 inch and 0.001 inch, and preferably about 1/32 inch to about 1/320 inch, relatively uniformly throughout the base coat.

The viscosity of the thermosetting aqueous latex emulsion is important in order to be able to deposit a proper amount by a rotogravure process onto the face paper front surface. Viscosity determination, by a General Electric Zahn viscometer method, should indicate a viscosity of about 15 to about 30 seconds, when measuring the time for a measured amount of emulsion, contained in a #2 Zahn cup, to flow out through the orifice in the bottom, a test method commonly used in the ink and paint industry. Plain water, tested in a #2 Zahn cup, has a viscosity of about 15 seconds, and, thus, about 15 seconds is the minimum viscosity of a suitable thermosetting aqueous latex emulsion. The maximum permissible viscosity is that viscosity at which the emulsion can still be deposited by a rotogravure process.

Preferably the base coat is a self-reactive, cross-linking copolymer, activated by heat after having a second design coat printed over the base coat. This heat activation, to cure the base coat, can be delayed until after the paper has been used to form gypsum wallboard, at which time curing is carried out in the wallboard dryer, as the wallboard is dried, and excess water in the set gypsum core is removed. The curing forms a tough, hard, durable, non-blocking coating, from a coating which had no durability before curing.

The thermosetting latex base coat may be a self-reactive acrylic, or an acrylic-vinyl copolymer. Examples of suitable thermosetting acrylic latex emulsions include Amsco RES 3112 sold by Union Chemicals Division of Union Oil Company of California as number 8262, and a Clear Gloss Aqualure sold by Glidden Coatings and Resins Division of SCM Corporation as number 847-C-02109. Prior to use, the Clear Gloss Aqualure must be catalyzed in the ratio of one part by weight of Catalyst Converter 297-C-12128 to thirteen parts by weight of Clear Gloss Aqualure.

The base coat is applied at a rate of about one to three pounds of 30% solids emulsion per thousand square feet of paper, or about 1/4 to one pound of solids per thousand square feet of paper. This emulsion is applied to a continuously moving web, about four feet wide, of face paper, by the rotogravure process, using a chromed steel roll rotary press, printing the emulsion in the form of minute shapes minutely spaced apart substantially throughout the whole front surface of the face paper, with the exception of an uncoated edge portion at each side, of about 5/8" to 3/4" width.

If minute dots are employed, they are of a size such that there are between about 1000 and 100,000 spaced apart dots per square inch, preferably from about 3000 to 40,000 spaced apart dots per square inch, such as about 55 to 200 dots per lineal inch in each of two perpendicular directions, for example. The dots are formed by the emulsion being placed in minute holes extending into the rotogravure printing roll and deposited on the paper surface in a manner similar to rotogravure printing of ink solutions. The holes, and the resultant dots may be of any shape, circular, square, oblong, etc., so long as the holes will retain the emulsion until the paper is contacted and the emulsion will then deposit on the paper, at the desire rate of application. Typically the holes may average about 0.005 inch in diameter and between about 0.001 and 0.002 inch deep. If a pattern of narrow lines are employed, there should be about 30 to about 300 lines per inch.

As an example, the rotary press cylinder may have holes or depressions of a generally semi-spherical shape, arranged in diagonal rows, 45.degree. in each direction from a line circumscribing the circumference of the cylinder, with 120 holes per inch in each diagonal direction, and thus 14,400 holes or depressions per square inch. The holes or depressions are separated by lands between depressions which extend in a generally zig-zag manner circumferentially around the cylinder, resulting from the diagonal arrangement of the rows of depressions, and from the narrow shallow groove adjoining depressions lying adjacent to each other in circumferential directions. This gravure print cylinder is referred to as having a QCH cell configuration in the rotogravure printing industry and is a preferred cell configuration in the present invention.

The base coat is dried, but not cured, immediately and the base-coated paper is fed to a plurality of, from about 2 to about 8, printing rolls, each of which prints a design onto the base-coated, but not cured, paper using what are referred to as high-binder durable inks. The base coat, once cured, protects the paper, and the durability of the inks protects the inks.

Each printing roll applies a portion of a design, each in different colors or different shades of a color, preferably in small blotches, i.e. small, irregular spots or marks using the high-binder inks. The high-binder inks are a mixture of pigment, a thermoplastic resin, and a solvent, all of which preferably are relatively non-reactive with the base coat.

The base-coated and printed front paper is then either fed directly to a machine for forming gypsum wallboard, or wound into a roll temporarily, to subsequently be unwound and fed to a wallboard machine. When fed to the wallboard machine, the face paper is conveyed, front surface down, and a settable gypsum aqueous slurry is disposed on the face paper back surface. A continuous web of back paper is then disposed over the gypsum slurry and the edges of the face paper are wrapped up and around the edge of the slurry, as the composite is formed into a flat thin board form.

The back paper may be of any known sultable type of gypsum board paper, preferably generally similar to the basic uncoated face paper, particularly in weight and porosity, but of lower cost paper fiber raw material, less brightness and less smoothness, these characteristics being of less importance on the gypsum board back surface.

After forming, the gypsum boards of the invention are cut into suitable lengths and conveyed through a high temperature board dryer. The base coat is cured in the board dryer, forming the very durable coating essential in predecorated wallboards. A period of about 5 minutes at 300.degree. F. or a period of about 30 minutes at 200.degree. F. are typical of the amount of heat required for a satisfactory cure of a preferred thermosetting base coat.

Having completed a detailed disclosure of the preferred embodiments of our invention so that those skilled in the art may practice the same, we contemplate that variations may be made without departing from the essence of the invention or the scope of the appended claims.