Polyurethane foams are made from polyfunctional isocyanates and hydroxyl-containing polymers, along with the catalysts necessary to control the rate and type of reaction, foaming agents, and other additives necessary to control the surface chemistry of the process. The methods of production of polyurethane foams are well known. The Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley and Sons, Third Edition and Saunders and Frisch, Polyurethanes, Krieger Publishing, are excellent references for the chemistry of polyurethane foams. The uses of polyurethane foams are also well known. For example, foamed polyurethane compositions have found widespread use in the fields of insulation and structural reinforcement. Flexible polyurethane foams are used as cushioning in furniture and automobiles and as acoustical deadening layers in cabinets such as computer housings Melamine crystal is frequently used in these polyurethane foams to make the foams fire retardant U.S. Pat. Nos. 4,385,131 and 4,221,875 describe the incorporation of melamine into foams for resistance to both smoldering combustion and flaming combustion.
One commercial process for formulation of a fire resistant polyurethane foam consists of blending a first steam combining polyol and melamine with a second stream combining isocyanate and foaming agent with a third stream combining water, surfactant and reaction catalyst. The first, second and third streams are mixed in a multi-stream "mixing head". On exiting from the mixing head the polyurethane forms in a foamed state. In this process, other known materials, such as extenders, fillers and pigments are commonly employed as well.
Several major difficulties are encountered when combining polyol and melamine in a mixing tank to prepare the first stream in this commercial process. When a blend having a 1 to 1 weight ratio of polyether polyol to melamine is prepared the viscosity levels are excessively high for conventional commercial equipment and the uniformity of the blend is only temporary, the melamine separates out and settles to the bottom of the mixing tank. In the mixing tank, the viscosity of the melamine-polyol blends, measured at typical shear levels of 10 to 20 seconds.sup.-1, ranges from 7,000 to 15,000 centipoise, while in the pipe network system the viscosity measured at typical shear levels of 50.sup.-1 to 100 seconds.sup.-1 ranges from 6000 to 9000 centipoise. These viscosities are too high for the equipment in existing foam manufacturing facilities where high viscosities lead to excessive pressure drops that cannot be handled by conventional pumping systems and also lead to excessively slow flow rates for production. It is preferred to have viscosities less than 6000 centipoise at shear levels of 50.sup.-1 seconds. If agitation of the blend is interrupted, as for example, by equipment malfunction, the melamine settles out forming a compact dense sediment which cannot be resuspended by mixing. Attempts to use an impeller to resuspend the sediment can result in damage to the impeller motor when the sediment is deep enough to cover the blades. If the compacted dense sediment layer of melamine forms in a tank, its removal is difficult and may even require workers to enter the tank to remove it manually. The settling out of melamine also creates a severe specific gravity gradient from the top to the bottom of the mixing tank which leads to non-uniform distribution of melamine in the final flexible foam product. Major problems are thus encountered in production which are directly caused by the inability to create and maintain a stable suspension of melamine and polyol.