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Molybdenum Oxides

Molybdenum was one of the first elements used to retard the flames of cellulosics. More recently it has been used to impart flame resistance and smoke suppression to plastics. Molybdic oxideammonium octamolybdate, and zinc molybdate are the most widely used molybdenum flame retardants. Properties are given in Table. These materials are recommended almost exclusively for poly(vinyl chloride), its alloys, and unsaturated polyesters.

Properties of Molybdenum Flame Retarders

PropertyMolybdic oxide aAmmonium octamolybdate bZinc molybdate

CAS Registry Number[1313-27-5][13767-32-3]
particle size, µm2521.88
bulk density, kg/m338480
H2O solubility, g/mL0.6840.04
specific gravity4.673.183.0
oil adsorption per 100 g oil, g3520
loss on ignition, wt %8.29

a The refractive index of MoO3 is .
b The decomposition temperature is 250°C.

Molybdenum trioxide is a condensed-phase flame retardant. Its decomposition products are nonvolatile and tend to increase char yields. Two parts of molybdic oxide added to flexible poly(vinyl chloride) that contains 30 parts of plasticizer have been shown to increase the char yield from 9.9 to 23.5%. Ninety percent of the molybdenum was recovered from the char after the sample was burned. A reaction between the flame retardant and the chlorine to form MoO2·Cl2·H2O, a nonvolatile compound, was assumed. This compound was assumed to promote char formation.

Molybdenum is also a smoke suppressant for poly(vinyl chloride). It promotes the formation of cis- rather than the trans-polymeric decomposition products which are the precursors for smoke. The sources for molybdates are Climax Performance Material Corp. and Sherwin Williams.