A flame retardant additive comprises (A) a gassing agent which is a
compound containing both phosphorus and nitrogen, typically ammonium
polyphosphate, which is surface coated with (B) a silicone-base water
repellent treating agent comprising a co-hydrolytic condensate obtained
through co-hydrolytic condensation of (i) an organosilicon compound and
(ii) an amino group-containing alkoxysilane or a partial hydrolyzate
thereof in the presence of an organic or inorganic acid. The additive is
improved in moisture absorption resistance and dispersion and suited for
use in resins or elastomers.