Because of the natural flammability of organic polymer resins, it is common practice to incorporate a flame retardant into the formulation of a resin based composition or system in order to improve the safety of the final product.
A common approach to flame resistance is to incorporate into the resin certain flame inhibiting hydrated minerals, such as alumina trihydrate (ATH). When compounded into resins at sufficient levels, these hydrated minerals impart both flame and smoke retardancy by evolving non-toxic gases, such as water, to dilute the combustion products and to promote char formation. Although these hydrated minerals have met with much success as flame retardants, certain problems exist.
Because of the technique used in the production of crude alumina trihydrate, particulate fillers made from this material contains varying amounts of basic alkali metal salts. These alkali metal salts form sites of basic salt concentrations on the surface which are conventionally known as "hot spots". These hot spots, or active sites, are well known to cause problems in various resin matrix systems. Surface activity promotes the absorption of water by filled resins such that the electrical properties of the resins are adversely effected. Another problem attributed to surface activity, and discussed in more detail below, is the adverse effect on polyurethane foam manufacture as, for example, in carpet backing manufacture.
In urethane foam, these "hot spots" or active sites are known to interfere with the rate of reaction of the urethane precursors. These reaction rates vary from time to time making precise prediction of urethane cure rate very difficult. Second, the presence of these active sites on the filler surface can promote excessive foaming during the urethane reaction.
One way to combat surface activity is to vigorously water wash the ground ATH until the amount of basic salts, expressed as percent titratable soluble soda (Na.sub.2 O), is greatly reduced. This technique, though effective, is very expensive and is not economically feasible.
A second technique is to carefully monitor the soluble soda levels on the filler and use only low soda crude. However, this approach severely limits sources of feedstock and may result in production of large quantities of unacceptable flame retardant. A third technique is aging the ground flame retardant. Some believe that aging the filler for up to a week prior to use can reduce surface activity. This approach requires excessive inventory storage demands and is often ineffective.
In the case of urethane foam manufacture, one of the more preferred techniques for combating surface activity is to use a less active catalyst in the urethane reaction to slow down the process to the point that variation is more easily handled. Although slowing down the reaction rate makes the problem easier to handle, it does not solve the problem itself and result in significant variations in production working time. Because of this variation, work must be done in smaller or less efficient batches, there is greater scrap resulting from premature setup, lumping can occur because of different reaction rates throughout the batch, the process is resistant to automation, and machines must run slower to permit constant adjustment for the less predictable reaction time.
Coupled with all of the above techniques is the practice of using a filler flame retardant having as large a particle size, and hence the least surface area, as practical. However, high loading level of relatively large filler in a matrix significantly degrades certain physical properties of the composite and thus fine particle size hydrated fillers are preferred.
These and other disadvantages of the prior art hydrated mineral flame retardants are overcome by the novel composition and method disclosed herein.