Stock material or miscellaneous articles – Composite – Of carbohydrate
Reexamination Certificate
2000-02-15
2002-12-03
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Composite
Of carbohydrate
C428S536000, C428S535000, C428S537100, C428S537500, C428S537700, C428S703000, C428S479600
Reexamination Certificate
active
06489040
ABSTRACT:
The present invention relates to an improved form of gypsum wall board, in which the paper cover sheets have improved resistance to delamination and to longitudinal splitting of the board, particularly, when the board is subjected to handling in post-manufacturing operations, particularly in high humidity environments. The preferred embodiments of the present invention provide a gypsum wall board with improved resistance to “Roll-up” wherein at least one of the paper cover sheets is made with neutral size and a small amount of a cationic polyamide resin is added to the stock used to make the cover sheet.
BACKGROUND
As is well known, paper covered gypsum wall boards are made by enclosing a mass of plastic water-gauged gypsum composition between paper sheets and allowing the resulting product to setup; whereafter it is dried and cut into sheets of the desired size. One of the essential features of such a board is a firm adherence of the central gypsum core to the cover sheets, so that the board will not delaminate at the interface between the core and the cover sheets.
This has been quite satisfactorily accomplished in the past by the addition of farinaceous materials to the core. These farinaceous materials contain a certain percentage of a water-soluble starch which, during the drying of the gypsum board, will migrate to the interface between the core and the cover sheets, and will hence be closely associated with the gypsum crystals which interlock with the fibers of the cover sheets, thereby preventing their release. The presence of the farinaceous material also serves to protect the fine gypsum crystals from becoming calcined during the drying of the board, for if they did so, they would lose their strength, with resulting destruction of the bond between the core and the cover sheets. When such loosening of the bond occurs, the resulting boards are known as “peelers.” The gypsum core is thus exposed, and the board, at least to the degree that the peeling extends into the board, is rendered useless, and must be trimmed down to obtain pieces of board of a useful size.
Another type of defect, “rollers”, can make its appearance, in the general nature of “peelers”, is the result of the delamination of the cover sheets themselves, which split between the plies of paper.
A defect similar to a “roller” and in the general nature of a “peeler” is called a “Roll-Up”. A “roll-up” is a post-manufacturing problem, similar to the “peeler” problem, which occurs when paper covered gypsum board is subjected to certain post-manufacture operations. The problem frequently appears in the manufactured housing area, wherein a vinyl layer is adhered to the face side of a gypsum board using a laminating process. Because of the manipulation of the gypsum board in conditions encountered in the lamination process (high humidity and temperature), one or more plies of the 6 or 7 ply paper on the back of the board can become damaged. The “Roll-Up” damage occurs when portions of one or more plies of the backing paper separate from the underlying plies of the backing paper and create various defects including the formation of small, tightly packed rolls, not unlike a cigar, on the back surface of the gypsum board. The cigar-like rolls and other defects on the back of the gypsum board can damage the vinyl surface when the laminated boards are stacked after the laminating operation. The “Roll-Up” problem involves the delamination of paper plies, not the separation of the paper backing from the core of the gypsum board.
The cover sheets usually employed in the production of gypsum core boards consist, in the case of wallboard that is intended to form the internal walls of rooms, of two types. The cover sheet used on the face side of the board is in the form of a paper built up of a plurality of plies, the two outer plies being usually of a somewhat better grade of recycled paper, such as fly-leaf and newsprint paper. The furnish is a mixture of ground wood, thermal mechanical and sulfite pulps for the outer plies. The inner plies, including the one which will be in contact with the gypsum core of the board is usually made of repulped newsprint and pre-consumer corrugated boxes. The paper used to line the backside of the board is usually made of a plurality of plies of repulped newsprint and post-consumer corrugated boxes. In any event, the papers are made on what is known as a cylinder machine, the pulp being picked up on rotating screens, so that a plurality of layers or plies will be obtained, which, by being superimposed upon each other immediately after their formation will bond to each other and produce a substantially uniform piece of paper. Under certain circumstances, such paper is subject to separation of the plies from each other, i. e. it may delaminate.
When gypsum board is covered with multiple ply paper is dried, there appears to be set up in the paper rather severe strains. Water vapor is absorbed and relaxes the severe strain when the gypsum board is shipped into areas of high humidity and temperature. By relaxing the strain, in severe cases, even the longitudinal splitting of the ply which is nearest to the core may occur. This may occur on either the front or the back of the gypsum board, and is probably attributable to the development of undue strain in the plies, of the paper during the drying of the board.
A factor of great importance is the strength of the bond between the gypsum core and the paper cover sheets, so that the paper cover sheets will not “peel” or de-laminate from the gypsum core. A weakness of the interface bond will produce a totally unacceptable wall finish. Additionally, it is necessary that the paper cover sheets, which are conventionally manufactured, do not delaminate between their adjacent plies.
With reference to the paper cover sheets themselves, they must be of uniform strength throughout with strong bonds between the plies. Additional wet strength in the paper cover sheets make it possible ship gypsum board panels into high humidity and temperature regions without post manufacturing “roller” defect problems.
THE PRIOR ART
The prior art describes a wallboard in which at least one ply of the paper cover sheet incorporates a cationic polyamide type resin to supplement the conventional rosin and alum size in order to address the “peeler” problem for “high suction” gypsum board panels. These high suction panels are used as a base for the application of conventional plaster. Gypsum board panels used as the substrate for the application of plaster are required to absorb high amounts of water from the plaster in order to allow the plaster to set.
However, the addition of polyamide type resin, which is compatible with the rosin and alum size, does not change the high suction properties of the gypsum board. The alkaline salt that is added to the paper plies does prevent the migration of the size into the cover sheet to maintain the high suction properties of the paper. At times the presence of the size at the surface of the gypsum board produces a slick bond, which means the crystals of gypsum plaster at the interface do not penetrate the “high suction” paper and the bond between the paper and the plaster is greatly impaired. This problem has been addressed in the prior art by adding an alkaline salt to the central plies of the paper to prevent the alum from acting as a mordant. The rosin size can not properly impart water resistant to the paper so the high suction properties of the paper are maintained.—See U.S. Pat. No. 3,300,371 to Hart.
U.S. Pat. No. 2,806,811 to von Hazmburg describes an improved gypsum-core plasterboard, having paper cover sheet containing a strength-imparting resin, such as a melamine-aldehyde condensation product or a urea-aldehyde resin to impart a greater degree of wet strength to the plies. The resins described by von Hazmburg, such as the melamine-aldehyde condensation products or the urea-aldehyde resins, are compatible with the acid paper sizing systems based on rosin and alum that were used in the 1960s, but that those resins do not work fo
Cochran Charles W.
Rohlf Evan V.
Egan Donald E.
Janci David F.
Kiliman Leszek
Lorenzen John M.
United States Gypsium Company
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