Method for the Production of a Decorative Gypsum Board

This invention relates to an improved method for the production of decorative gypsum board and a product obtained thereby.

Gypsum board has generally industrially been produced by mixing gypsum hemihydrate, a light weight additives such as of pealite, a setting-time adjusting agent and a suitable amount of water by means of a mixer to give a slurry, continuously injecting the mixture between gypsum board card papers, pressing the mixture-injected boards between rollers to form the boards in a predetermined thickness, and then subjecting the boards to a drying step.

While in the production of decorative gypsum boards, gypsum board card paper on one surface of which a suitable pattern such as of a wood grain type has been preliminarily printed is generally used. Decorative gypsum board card paper is generally required to have suitable water permeability since the bonding between gypsum and the card paper, which is one of important properties of decorative gypsum board as well as of gypsum board, is derived from permeation of a gypsum hemihydrate slurry into card paper and hardening of the gypsum hemihydrate into gypsum. Moreover, the card paper is also required to have suitable gas permeability so as to allow water vapor, which is generated in a drying step, to escape without causing any trouble of separation between layers of card paper.

In order to meet the requirements mentioned above, there is generally used as decorative gypsum board card paper on the surface of which a suitable pattern is merely directly printed, so that a pattern of the decorative gypsum board finally obtained lacks tone and depth in color, when compared with that of decorative plywood, due to poor printability of the gypsum board card paper used.

Moreover, where a decorative surface is discolored or stained, in use, it is difficult to cleanse the surface with washing.

It has been once proposed to use as a decorative material a synthetic resin film, synthetic resin-coated printing paper, or gypsum board card paper which is laminated with the above-mentioned synthetic resin film or the synthetic resin-coated printing paper because the synthetic resin or its film is excellent in properties such as water repellency, weatherability and resistance to stains. However, such materials are considered unsuitable for use as a gypsum board card paper due to their low gas permeability.

There has been further proposed a method for the production of a decorative gypsum board wherein a gypsum board is first produced by a conventional method and then a decorative material having excellent surface properties is laminated with or pressed on the gypsum board. However, this method is not necessarily industrially advantageous in that the gypsum board originally has a change in thickness ranging 0.3 - 1.0 mm, so that it is necessary to subject the substrate to a sanding or other suitable treatment before the pressing or laminating step for making the thickness thereof uniform. Moreover, it is disadvantageous to additionally require a pressing or laminating step from the economical point of view.

Furthermore, a method for the production of a decorative gypsum board has been known wherein a conventional printed gypsum board card paper is used and then the decorative gypsum board is coated with a synthetic resin by means of a roll coater or a flow coater. However, this is also disadvantageous in that the resin coated is absorbed in the gypsum board card paper, that the coated resin film is not made uniform due to a considerable change in thickness of the gypsum board substrate, and that it is difficult to uniformly coat edge portions of the gypsum board substrate with a resin, thus it being difficult to produce decorative gypsum boards having a predetermined quality level.

It is therefore an object of the present invention to provide a method for the production of a decorative gypsum board which overcomes the aforementioned disadvantages.

It is a further object of the invention to provide an improved method for the production of a decorative gypsum board which will improve surface properties of the board.

It is another object of the invention to provide an improved method for the production of a decorative gypsum board wherein the board is obtained using a laminated gypsum board card paper of a paper sheet having excellent printability and a conventionally used gypsum board card paper.

The present inventors have found through intensive studies on decorative gypsum board having excellent surface properties that when gypsum board card paper laminated with an urethane resin-coated paper sheet is used as decorative gypsum boards which have excellent surface properties can be produced in an industrial scale. That is, there is provided a method for the production of a decorative gypsum board wherein the decorative gypsum board comprises two sheets of card paper and gypsum which is sandwich between decorative card paper and conventional card paper, characterized in that the decorative gypsum board card paper is obtained by coating low-weight basis decorative paper with an urethane resin solution, drying the solution-coated paper, and bonding the resultant paper to a gypsym board card paper.

In this connection, a suitable pattern may be printed on the paper sheet prior to the urethane coating step for enhancing decorative effects of gypsum boards by printing. Examples of the low-weight basis and good quality paper which should have excellent printability include tissue paper, machine glazed white paper, etc.

As described hereinbefore, a gypsum board card paper should have excellent gas- and water-permeable properties so as not to cause any trouble concerning separation between layers of the board per se in a forming or drying step. In general, air resistance of the gypsum board card paper is required to be within 30 min when determined in accordance with "Test on Permeability" prescribed in Japanese Industrial Standard P-8117 (wherein a time period required for allowing 100 ml of air to be passed through a test specimen having an area of 645.16 cm.sup.2 is determined). In this connection, when an urethane resin is applied onto a substrate board in an amount as small as 1 - 5 g/m.sup.2, weather resistance, resistance to stains, etc., are remarkably improved and the air resistance of the board is maintained within the range.

The decorative gypsum boards obtained by the method of the present invention which use gypsum board card paper laminated with an urethane resin-coated paper sheet are excellent in depth and tone in color, as well as in weatherbility, resistance to stains, and surface strength since the printed paper is coated with an urethane resin.

The present invention is particularly illustrated in the following examples and comparative examples employing an aminoalkyd resin as a coating material.

A teak wood grain-like pattern was printed on tissue paper having weight basis of 23 g/m.sup.2. Then, the printed tissue paper was coated with an urethane resin solution in an amount of 2 - 4 g/m.sup.2 based upon a solid component by means of a roll coater and the coated paper was dried. The thus dried tissue paper was laminated by means of a laminator with gypsum board card paper with use of an vinyl acetate emulsion adhesive to obtain decorative card paper. Decorative gypsum board was produced in a conventional industrial manner by the use of the decorative gypsum board card paper and a hemihydrate gypsum slurry at a forming speed of 17 m/min. The resultant decorative gypsum board was cut into three test specimens, which were then subjected to an adhesion test (in accordance with "Test for adhesion between Gypsum and Gypsum board card paper" which is prescribed in Japanese Industrial Standard A 6904. 6.4). The three specimens passed the test without separation between the gypsum board card paper and gypsum.

As for surface properties of the decorative gypsum board, various kinds of tests were conducted in a manner as summarized in Table 1. The test results are shown in Table 2. As is apparent from Table 2, the decorative gypsum board of the present invention is superior in the surface properties to a prior-art decorative gypsum board using directly printed gypsum board card paper.

A zelcova wood grain-like pattern was printed on machine glazed white paper having a weight basis of 35 g/m.sup.2. Then, the printed machine glazed paper was coated with an urethane resin solution in an amount of 3 - 5 g/m.sup.2 based on a solid component by means of a roll coater, and the coated paper was dried. The thus dried paper was laminated to gypsum board card paper with use of a vinyl acetate emulsion adhesive by means of a laminator.

Decorative gypsum board was produced in a conventional industrial manner by the use of the decorative card paper and a gypsum slurry at a forming speed of 20 m/min.

The resultant decorative gypsum board was cut into three test specimens, which were then subjected to an adhesion test (which is prescribed in Japanese Industrial Standard A 6904. 6.4 as "Test for Adhesion between Gypsum and Gypsum board card paper"). The three specimens passed the test without separation between the gypsum board card paper and gypsum.

As for surface properties of the decorative gypsum board, the same tests as in Example 1 were conducted. The test results are shown in Table 3 below. It is apparent from the Table that the decorative gypsum board of the present invention is far superior in the surface properties to a prior-art decorative gypsum board.

The following two comparative examples were shown for the production of decorative gypsum boards having aminoalkyd resin-coated gypsum board card paper, for comparison with the decorative gypsum board of the present invention as obtained in the foregoing examples.

A teak wood grain-like pattern was printed directly on a gypsum board card paper. Then, the printed board was coated with 8 - 11 g/m.sup.2 of an aminoalkyd resin by means of a roll coater, and the thus coated board was dried at 70.degree. - 115.degree.C.

The thus obtained decorative gypsum board card paper had air resistance of 65 min when determined by the method described hereinbefore. Then, decorative gypsum board was produced in a conventional industrial manner by the use of the decorative gypsum board card paper and a gypsum slurry at a forming speed of 17 m/min.

The resultant decorative gypsum board had blisters on the surface thereof. The decorative gypsum board was cut into three test specimens, which were then subjected to a test for adhesion between gypsum and the decorative gypsum board card paper in the same manner as described in Example 1. As a result, two test specimens did not pass the test, thus the aminoalkyd resin-coated decorative gypsum board being unsatisfactory.

A zelkova wood grain-line pattern was printed on white machine glazed paper having a weight basis of 35 g/m.sup.2. The printed paper was coated with 2 - 4 g/m.sup.2 of an aminoalkyd resin by means of a roll coater, and the thus coated paper was dried. The dried paper was laminated on a gypsum board card paper with use of a vinyl acetate emulsion adhesive. The thus laminated decorative gypsum board card paper had an air resistance of 23 min when determined in accordance with a method of Japanese Industrial Standard P 8117.

Then, a decorative gypsum board was produced in a conventional industrial manner by the use of the decorative card paper and a gypsum slurry at a forming speed of 20 m/min.

The resultant decorative gypsum board was cut into three test specimens for conducting an adhesion test in the same manner as in Comparative Example 1. As a result, the three test specimens passed the test with separation between the board and gypsum. Moreover, the decorative gypsum board was subjected to tests for surface properties in a manner similar to those of Example 1. Test results are shown in Table 4. It is apparent from the Table that the aminoalkyd resin-coated decorative gypsum board is inferior in surface properties to the decorative gypsum board of the invention.