Flame-retardant plastics molding composition of improved stability

A process for the preparation of a flame-retardant plastics molding composition comprising

• 60 to 85% by weight, based on the total amount of the molding composition, of an olefin polymer and
• 15 to 40% by weight, based on the total amount of the moulding composition, of a flameproofing system,
• by mixing the constituents, which comprises admixing, at a temperature exceeding 250.degree. C.,
• to the olefin polymer a flameproofing system comprising
• 80to 50% by weight of an ammonium polyphosphate of the formula (NH.sub.4 PO.sub.3).sub.n, in which n is a number from 200 to 1000 (=component A), and
• 50 to 20% by weight of a compound (=component B) comprising
• a) units of the formula I ##STR12## b) units of the formula II ##STR13## c) units of the formula III ##STR14## and d) units of the formula IV
• Ar(COO--).sub.m (IV)
• in which Ar is a mono-,di- or trinuclear aromatic six-membered ring system and m is 2, 3 or 4.

A fire-retardant moldable masterbatch suitable for incorporation into a flame-retardant, plastics molding composition, said masterbatch comprising:

• 30 to 90% by weight, based on the total masterbatch composition, of a flameproofing system comprising
• A. 80 to 50% by weight of an ammonium polyphosphate of the formula (NH.sub.4 PO.sub.3).sub.n, in which n is a number from 200 to 1000, and
• B. 50 to 20% by weight of an aromatic carboxylic acid ester of tris(2-hydroxyethyl) isocyanurate containing mono-, di-, and tri-esterified units, the aromatic carboxylic acid residues of said units having the formula
• Ar(COO--).sub.m
• in which Ar is a mono-, di- or tri-nuclear aromatic six-membered ring system and m is 2, 3 or 4,
• the balance of said masterbatch composition consisting essentially of an olefin polymer.