1. Field of the Invention
The present invention pertains to a modified lignosulfonate composition; its method of preparation and its use as a dispersant, particularly in the manufacture of gypsum wallboard.
2. Description of Related Art
The use of lignosulfonate as a dispersing agent in the preparation of gypsum wallboard is disclosed, for example, in U.S. Pat. No. 2,856,304. In the manufacture of gypsum products such as wallboard, lath, plaster board, sheathing or other products, calcined gypsum is formed into a slurry with a suitable amount of water and, where customary, with other additives such as paper fiber, wood fiber, starch, rosin, etc.
In the preparation of wallboard or similar products, in particular, the slurry then is deposited between paper liners, pressed to the desired thickness by forming rolls, allowed to set and harden, cut to desired lengths, and passed through a dryer to remove excess moisture. A portion of the water used in making the slurry combines with the calcined gypsum, as water of crystallization, in forming the final set mass of interlaced crystals, but a large portion of the water must be removed in the dryer. Obviously, the drying process is more costly as the proportion of water to be removed from the formed board is higher.
It is known that using lignosulfonate as a dispersing agent reduces the amount of water required to provide a flowable gypsum slurry during the deposition and forming steps. Consequently, by using lignosulfonate, the amount of water to be removed during the final drying step can be reduced, resulting in a significant economy of operation, particularly as regards lower energy costs. An ancillary benefit also often observed is higher board strength. It also is known that lignosulfonate exhibiting improved dispersing ability is prepared by base exchange of the lignosulfonate with various metals, including iron, aluminum, chromium and copper, by alkaline treatment, by oxidation and the like. See, for example U.S. Pat. Nos. 2,935,504; 3,007,910; and 3,108,008.
Unfortunately, lignosulfonates also tend to retard the hardening or cure rate of the gypsum board, referred in the art as set retardation. While this does not present a problem with slower forming operations characteristic of the prior art, with the advent of faster processes, set retardation has matured into a significant concern. In fact, while hydrolysis and oxidation reactions tend to enhance the dispersing behavior of the lignosulfonate, such treatments tend to exacerbate set retarding characteristics.
In the prior art, the problem of set retardation has been dealt with primarily by adding various accelerating agents to the aqueous lignosulfonate-gypsum slurry. The prior art indicates that materials such as sodium chloride, aluminum sulfate, potassium sulfate, calcium sulfate dihydrate, and uncalcined or raw gypsum help to ameliorate set retardation.
Finally, in U.S. Pat. No. 2,856,304 it is taught that calcining raw gypsum containing a small amount of lignosulfonate helps reduce the setting time of the calcined gypsum product.
In accordance with the present invention, lignosulfonate is treated in a way which enhances its capability to disperse gypsum, while at the same time, avoids imparting to the lignosulfonate an undesirable increase in its set retardation characteristics. By using the modified lignosulfonate composition of the present invention, the amount of water needed to form a calcined gypsum slurry having the necessary plasticity is considerably reduced, and the production of gypsum shapes is simultaneously accelerated.