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Fire Performance Synergies of Metal Hydroxides and Metal Molybdates in Antimony-Free Flexible PVC

1. Introduction
Viability of continued use of antimony oxide is among the topics of broad interest to PVC compound
suppliers as well as their end users. Existing and emerging regulatory mandates have continued to drive
an industry-wide effort to minimize use of antimony oxide in PVC, while being able to maintain adequate
fire performance when formulating the low-smoke compounds for a variety of applications such as
construction materials and wire and cable products.
Metal hydroxides such as aluminum hydroxide (ATH) and magnesium hydroxide (MDH) are among
several alternative “green” fire retardants used to reformulate fire-rated PVC compounds in order to be
regulatory compliant. ATH and more recently MDH are widely used to achieve low-smoke flame retardant
compounds. Both ATH and MDH function as a flame retardant by decomposing endothermically into
water and metal oxide upon heating. The released water vapor, in an amount of about a third of total
metal hydroxide used, dilutes the combustible ambience while removing the combustion heat. The oxide
by-product formed from the hydroxide decomposition chars on the surface of polymeric materials to
prevent heat and oxygen from approaching the polymer.
Molybdate based additives are also used in PVC to both lower smoke generation during combustion and
to improve overall FR properties. Molybates are known to chemically influence the formation of organic
char, which effectively removes fuel from the gas phase and thereby lowers smoke and heat release.
MoO3 has been shown to react with the HCl produced by the pyrolysis of PVC to form MoO2Cl2, a potent
Lewis acid. This can then promote crosslinking in a number of ways. One likely mechanism is the
Friedel-Crafts alkylation of alkene linkages that are formed during PVC thermolysis [1-3].
Kemgard® structured metal molybdates are common smoke suppressants used to achieve low-smoke
standards. Kemgard® products utilize a patented process in which moybdates are precipitated on an
inert mineral core. This “coated core” approach makes more efficient use of the molybdate species by
maximizing the active surface area, and at much lower cost than pure molybdate chemicals.
This paper discusses the results from a recent study on the synergistic use of metal hydroxides and metal
molybdates to enable the low-smoke performance for select flexible PVC compositions. Efforts were
made to compare the overall FR performance characteristics between an antimony oxide-free compound
system and the systems that contain varying amount of antimony oxide. We also examined the
interactions between metal hydroxides (both ATH and MDH) and metal molybdates both in presence or
absence of the antimony oxide. The performance synergy between a hydroxide and a molybdate was
closely investigated in two PVC compositions that are of commercial interest in terms of the type of the
plasticizer used. The inclusion of zinc borate for added FR benefits was also explored.
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