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Abstract

A composite paper particularly adapted for use as cover sheets in the production of gypsum wallboard, the paper being sufficiently porous to permit better drainage and more rapid drying in the production of the paper, and when applied to the surfaces of a gypsum slurry for forming wallboard, permits less heat to be utilized in the wallboard conversion, thereby saving energy in the board production required for drying the board. The paper comprises in weight percent: (A) cellulosic fibers in an amount of from about 65% to about 90% and preferably having a fiber freeness of from about 300 ml to about 550 ml Canadian Standard Freeness, (B) calcium sulfate as a filler in an amount of from about 10% to about 35%, (C) a binder in an amount from about 1% to about 31/2%, (D) a flocculant in an amount of from about 0.1% to about 0.2%, (E) a buffering agent in an amount from about 0.25% to about 10%, (F) a neutral sizing agent in an effective amount to prevent water penetration, (G) an anionic polymer in an amount suitable for retaining said filler in said paper, and (H) a cationic starch when a succinic anhydride is used as the neutral sizing agent. In a preferred embodiment, after the paper is treated with a neutral internal sizing agent during its formation, it is subsequently treated with a surface sizing agent after formation of the paper, in order to provide certain properties including better adhesion to the gypsum core when used to make gypsum wallboard.

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