Zinc borate (2ZnO·3B2O3·3.5H2O, CAS 138265-88-0) is a white, halogen-free flame-retardant synergist. It works by releasing water of crystallisation above 320 °C, forming a glassy borate char, and co-operating with halogenated or phosphorus systems – which is why it displaces part of the antimony trioxide in PVC and rubber compounds. PolyBlueChem supplies it as a white powder, whiteness 95-98, with per-batch COA and SDS.
Specifications
| Property | Specification |
|---|---|
| Appearance / whiteness | White powder; whiteness 95 – 98 |
| Surface water | 0.5% max |
| Particle size | 3 – 5 µm |
| ZnO content | 37.0 – 40.0% |
| B2O3 content | 47.0 – 49.0% |
| Water of crystallisation | 13.0 – 15.5% |
| Fineness (residue on 45 µm sieve) | 0.5% max |
| Bulk density | 0.4 – 0.5 g/cm3 (400 – 500 g/L) |
| Dehydration temperature | Above 320 °C |
| Melting point | 980 °C |
| Refractive index | 1.58 |
| CAS | 138265-88-0 |
Why the three assay figures are the specification
Zinc borate is sold on its ZnO, B2O3 and water-of-crystallisation content, and those three figures are not independent – they are a fingerprint of the hydrate ratio. For the 2:3 hydrate:
| Component | Contribution to molecular weight | Theoretical share | Typical specification |
|---|---|---|---|
| ZnO (2 mol, 81.38 each) | 162.76 | 37.4% | 37.0 – 40.0% |
| B2O3 (3 mol, 69.62 each) | 208.86 | 48.0% | 47.0 – 49.0% |
| H2O (3.5 mol, 18.02 each) | 63.05 | 14.5% | 13.0 – 15.5% |
| Total | 434.67 g/mol | 100% | – |
So a lot that assays ZnO at the top of the range must show B2O3 and water at correspondingly different levels – they cannot all sit at the top. If a certificate shows high ZnO AND high B2O3 AND low water, the hydrate ratio is not 2:3, and the material is a different zinc borate grade with different flame-retardant behaviour. This is the single most useful check on an incoming certificate: the three numbers should sum to about 100% and should sit in the proportions above.
Other zinc borate hydrates exist commercially (1:2, 4:1, anhydrous), and they are not interchangeable – the hydrate ratio sets how much water is released, and at what temperature, which is the whole basis of the flame-retardant action.
Basic Information
| CAS number | 138265-88-0 |
| Chemical name | Zinc borate; 2ZnO·3B2O3·3.5H2O (zinc borate hydrate, ZB-2335 type) |
| Formula / MW | 2ZnO·3B2O3·3.5H2O / about 434.7 g/mol |
| Appearance | White crystalline powder |
| Refractive index | 1.58 – close to most polymers, so it does not scatter light as strongly as mineral fillers with higher indices |
| Dehydration onset | Above 320 °C |
| Melting point | 980 °C |
How it retards flame
Zinc borate is almost never used as the sole flame retardant. It is a synergist and smoke suppressant, typically dosed at a few phr alongside a primary system. Three mechanisms act together:
| Step | What happens | Consequence |
|---|---|---|
| 1. Endothermic dehydration | Above 320 °C the water of crystallisation is driven off, absorbing heat | Cools the condensed phase and dilutes combustible gases with steam |
| 2. Glassy char formation | The borate forms a vitreous layer on the burning surface, and the zinc promotes charring | Barrier against heat and oxygen; reduces dripping and smoke |
| 3. Halogen synergy | With a halogenated flame retardant, zinc borate promotes the formation of the char and suppresses afterglow | Lets you reduce the halogen and antimony loading while keeping the class |
The 320 °C dehydration onset is the reason zinc borate fits engineering plastics where aluminium trihydroxide (which decomposes around 180-220 °C) would be destroyed during processing. It also explains why zinc borate cannot replace ATH as a bulk flame retardant: it releases far less water per unit mass, and it is dosed in single-figure phr rather than tens of percent.
Applications
- PVC cable compounds – partial replacement of antimony trioxide, with better smoke performance and lower toxicity.
- Halogenated rubber and wire-and-cable sheathing – smoke suppression and afterglow reduction.
- Engineering thermoplastics – polyamide, PBT, PET and their glass-filled grades, where the high dehydration temperature survives processing.
- Coatings and intumescent systems – borate char promotion in fire-retardant paints.
- Flame-retardant masterbatches – with the particle size chosen for dispersion in the target polymer.
Buying notes
- Check the hydrate ratio via the three assays – ZnO, B2O3 and water should be in the 37 / 48 / 14.5 proportions. A mismatch means a different grade.
- Whiteness matters in light-coloured compounds: 95-98 is a good commercial range; a falling whiteness figure points to iron contamination.
- Particle size and sieve residue are dispersion parameters, not marketing ones: 3-5 µm with a low 45 µm residue is what keeps a cable sheath smooth.
- Surface water below 0.5% matters in polymers that hydrolyse, and it matters in PVC dry blending – moisture causes pre-gelling and voids.
- Bulk density affects feeding: at 0.4-0.5 g/cm3 it is a light powder, so check that your feeder and silo are sized for it.
Packaging and supply
Supplied as a white powder in 25 kg paper or PP-woven bags with an inner liner, and in big bags for bulk users. Store dry and sealed – the material is not hygroscopic in use but surface moisture affects dispersion and dry-blend behaviour, so bags should be kept closed and off damp floors. Each lot ships with a COA and SDS, with FOB or CIF terms quoted against destination. Where the application is a regulated cable or construction product, state it at enquiry stage so that the documentation package (and, where relevant, a halogen-content or RoHS statement) is supplied with the first lot.
Further reading
- Zinc borate: flame-retardant mechanism, grades and where each one fits
- Zinc borate vs antimony trioxide vs ATH vs MDH: four flame retardants compared
- Zinc borate in PVC cable compounds: dosing, synergy and the processing window
PolyBlueChem supplies zinc borate (CAS 138265-88-0), 2ZnO·3B2O3·3.5H2O, whiteness 95-98, 3-5 µm, with per-batch COA and SDS and FOB/CIF delivery. Tell us the polymer system and the flame-retardant package you are using, and we will confirm the grade and the dose.
