urethane reaction to effectively isolate the active site with a thin multimolecular layer of the highly cross linked urethane at its locus because of the catalytic activity of the active site the more surface active the filler the more chemically active the site to promote the cross linking reaction such that all sites on the surface become completely covered after surface treatment of the hydrated filler is completed the polyurethane coating is chemically bound to the surface of the particle it is believed that a small excess of unreacted isocyanate groups are then available to chemically attach to the matrix into which the filler is placed the end result is a hydrated filler in a composite having a chemically bound surface layer which is chemically bound to the matrix to provide a very strong interface this interfacial bonding can improve physical properties such as tensile strength of the final composite it is another benefit that moisture normally found on the surface of the filler also reacts with the polyfunctional isocyanate and is removed to render the filler hydrophobic in order to be effective the urethane coating on the surface of the filler should be greater than a monolayer but not so great as to promote agglomeration of particles which would decrease the dispersion of the hydrated filler in the desired matrix application it is believed that a premix coating between 0 25 and 5 by weight is acceptable in the following examples samples of ath particulate flame retardant were made from.