(1) Field of Invention
The present invention relates to the process of setting a calcined gypsum slurry, is more particularly concerned with a composition for the delayed accelerating of the setting of an aqueous calcined gypsum slurry to form gypsum wallboard or plasterboard.
(2) Prior Art
Gypsum board or plasterboard has long been a large volume commercial article of commerce. Generally, gypsum board is manufactured by dispersing calcined gypsum in water and adding thereto a lightweight pregenerated foam to control the finished density of the slurry and the subsequent board. Additives conventionally used in minor amounts include accelerators, bond protecting agents, fibrous reinforcements, and consistency reducers. Typical of accelerators are calcium sulfate dihydrate, potassium sulfate, ammonium sulfate, and aluminum sulfate. Bond protecting agents are usually cereal flours or starches. The fibrous reinforcements may be either cellulosic or glass. Consistency reducing agents are typified by the lignosulfonates, of which ammonium lignosulfonate is particularly advantageous. These additives are used in minor quantities in relation to the total weight of the board core, and represent, in total, less than 5%, usually less than 2%, of the weight of the finished core.
The calcined gypsum slurry containing the desired added ingredients is prepared in continuous mixers, as for example a primary-secondary mixer combination described in Lane et al., U.S. Pat. No. 3,359,146, or a multi-pass mixer described in Camp U.S. Pat. No. 2,660,416. The mixed slurry is continuously deposited on a paper cover sheet moving beneath the mixer. A second paper cover sheet is applied thereover and the board is passed under a roll or rolls to determine the thickness. The continuous strip thus formed is conveyed on a belt until the calcined gypsum has set, and the strip is thereafter cut to form boards of desired length, which boards are conveyed through a drying kiln to remove excess moisture.
The most common form of accelerator used for setting a calcined gypsum slurry is calcium sulfate dihydrate. This material appears in many forms many of which can be used when properly prepared by grinding to a high degree of fineness. This material has been found to be an excellent accelerator. However, it has been found that when the material is stored prior to use, it is highly susceptible to becoming calcined, particularly in the presence of calcined gypsum which has been bagged while still warm or in the presence of moisture or heat generally. U.S. Pat. Nos. 2,078,199 and 3,813,312 have disclosed that when sugar or soluble dextrin are ball-milled together with the calcium sulfate dihydrate, the material functions to prevent dehydration of the accelerator, even when subjected to various atmospheric conditions on storage. Additionally, U.S. Pat. No. 3,870,538 has disclosed that starch may be finely ground together with calcium sulfate dihydrate to serve as a preservative of the accelerator. However these materials, particularly sugar, have become very expensive to a degree almost prohibitive of their use.