Zinc Stannate vs Zinc Hydroxystannate: Choosing Between FL-ZS and FL-ZHS

FL-ZS and FL-ZHS are both inorganic zinc-tin flame-retardant synergists and smoke suppressants that Flamisol supplies as alternatives to antimony trioxide (ATO). They are closely related chemistries that differ in one defining respect: water of hydration. Zinc stannate (FL-ZS) is anhydrous, while zinc hydroxystannate (FL-ZHS) carries bound hydroxyl groups. That single structural difference drives most of the practical selection decision between them, particularly around processing temperature and smoke performance. Neither is a universal drop-in replacement for ATO; both act as partial, formulation-dependent substitutes, and substitution always requires formulation-specific testing against your target fire standard.

FL-ZS vs FL-ZHS at a Glance

Values reflect literature and commercial formulation experience. Performance is system-specific and requires formulation testing.

When to Use FL-ZS (Zinc Stannate)

FL-ZS is the anhydrous option. Because it has higher thermal stability than FL-ZHS, it tolerates higher processing temperatures without dehydration, which makes it the more flexible choice across rigid and flexible PVC, polyolefin masterbatch, halogenated rubber, engineering thermoplastics, and thermoset systems. It functions as both a gas-phase and solid-phase synergist and contributes strong smoke suppression. FL-ZS is a sensible starting point where the compound is processed at elevated temperatures, where broad substrate compatibility matters, or where partial ATO reduction is the goal rather than maximum smoke performance.

When to Use FL-ZHS (Zinc Hydroxystannate)

FL-ZHS is the hydrated option, and its bound water is the source of its advantages. The endothermic loss of that water during a fire event provides additional cooling and is associated with measurably stronger smoke suppression and lower CO evolution than FL-ZS or ATO, particularly in flexible PVC. This makes FL-ZHS the preferred starting point for flexible PVC wire and cable insulation where NBS smoke density or IEC 61034 smoke-emission limits apply. The trade-off is thermal: because bound-water release begins at elevated processing temperatures, FL-ZHS is best suited to lower-temperature processing, and for higher-temperature processing FL-ZS is generally preferred over FL-ZHS; processing suitability should be confirmed by application-specific testing.

Processing and Temperature Considerations

The defining practical difference is dehydration. FL-ZS remains stable far above normal polymer processing temperatures, so it carries essentially no processing-temperature constraint for typical PVC and polyolefin compounding. FL-ZHS, by contrast, begins to lose its bound water through endothermic dehydration at elevated processing temperatures. In lower-temperature flexible PVC processing, that endotherm is a benefit. In systems processed at substantially higher temperatures, premature dehydration can occur, which is why FL-ZHS is generally reserved for lower-temperature systems and FL-ZS is selected for higher-temperature ones. Confirm the dehydration behavior against your specific compound and processing window before scaling.

Applications by Polymer System

How This Relates to ATO Reduction

Both FL-ZS and FL-ZHS are evaluated by formulators looking to reduce or replace antimony trioxide as ATO pricing and supply tighten. Neither carries an IARC classification, and both add smoke suppression that ATO does not provide. They are best understood as partial, formulation-dependent ATO synergists rather than guaranteed one-for-one replacements: the achievable level of ATO reduction depends on the primary flame retardant, loading, substrate, and target fire standard. For a broader view of zinc-based ATO strategy, see the related reading below. As always, validate any substitution with formulation-specific testing.