Zinc Borate (CAS 138265-88-0): Flame-Retardant Mechanism, Grades and Where Each One Fits

Zinc borate is the most misunderstood product in the flame-retardant toolbox. It is not a flame retardant in the way aluminium trihydroxide is – you cannot load a compound with 50% zinc borate and expect it to work. It is a synergist and smoke suppressant, dosed at a few parts per hundred, whose value is that it makes the primary flame-retardant system work better while cutting smoke and afterglow. Understanding that difference decides whether you use it well or waste it.

1. What the product actually is

Property	Value
CAS	138265-88-0
Formula	2ZnO·3B₂O₃·3.5H₂O
Molecular weight (from the hydrate formula)	About 434.7 g/mol
Appearance	White crystalline powder
Dehydration onset	Above 320 °C
Melting point	980 °C
Refractive index	1.58
Typical dose as a synergist	A few phr, not tens of percent

2. The three assays are a fingerprint, not three independent numbers

Zinc borate is bought on ZnO, B₂O₃ and water of crystallisation. Those three are linked by the hydrate ratio, and the arithmetic is worth doing because it is the cheapest incoming-goods check available.

Component	Theoretical share (2:3 hydrate)	Commercial specification
ZnO	37.4%	37.0 – 40.0%
B₂O₃	48.0%	47.0 – 49.0%
H₂O (hydrate)	14.5%	13.0 – 15.5%

Two consequences follow. First, the numbers must total about 100% – if they do not, something else is in the powder. Second, they cannot all sit at the top of their ranges at once – if the hydrate ratio has changed, the three figures shift out of the proportions below. A certificate showing 40% ZnO and 49% B₂O₃ and 13% water cannot describe a 2:3 hydrate; it describes a different zinc borate, and a different zinc borate releases its water at a different temperature.

Several hydrates are commercial – 1:2, 2:3, 4:1, and anhydrous – and they behave differently. The 2:3 hydrate is the workhorse for polymer flame retardancy because its dehydration temperature sits above the processing window of most thermoplastics and below the ignition temperatures that matter.

3. The mechanism, in the order it happens

Stage	Physical event	Effect on the fire
Heating to about 320 °C	No chemical change – the hydrate is stable through normal processing	This is why it can be compounded into PVC, polyamide and PBT without decomposing in the extruder
Dehydration	Water of crystallisation is released endothermically	Cooling of the condensed phase; dilution of flammable volatiles by steam
Borate glass formation	The oxide residue fuses into a vitreous layer	Barrier to heat and oxygen; the surface is sealed, so pyrolysis products are retained
Zinc-assisted charring	Zinc species promote crosslinking and carbonisation of the polymer	More char, less fuel; reduced dripping
Halogen synergy	With a halogenated flame retardant, borate and zinc shift the halogen chemistry towards char rather than towards volatile antimony or halogen species	Smoke and afterglow fall; part of the antimony can be removed

Notice what is not in that list: a large endotherm per unit mass. Zinc borate releases about 14.5% of its own weight as water, against roughly 34% for aluminium trihydroxide. That is exactly why it is a synergist and not a bulk retardant – the physics does not support high-loading use.

4. Where it earns its place

PVC cable compounds with a halogenated plasticiser or FR – the classic application: partial antimony replacement with better smoke numbers.
Halogenated rubber and cable sheathing – afterglow suppression is often the specific requirement, particularly in mining and rail cable standards.
Engineering thermoplastics – polyamide, PBT, PET and glass-filled grades, where ATH would decompose during processing.
Intumescent coatings – borate contributes to the char structure.
Where antimony is a commercial or regulatory problem – supply concentration, price volatility or the toxicological profile of antimony trioxide all push formulators to look for partial replacement.

5. Choosing a grade

Parameter	Why it matters	What to ask for
Hydrate ratio	Sets the dehydration temperature and the water yield	ZnO, B₂O₃ and water figures that fit the intended ratio
Particle size	Dispersion, surface finish, and how much you need for a given surface area	3-5 µm is a general-purpose range; finer for thin-wall or fibre applications
Sieve residue	Grit that shows as surface defects in extruded or moulded parts	A low 45 µm residue limit
Whiteness	Visible in light-coloured compounds	95-98; a falling figure suggests iron
Surface moisture	Pre-gelling in PVC dry blend, voids, hydrolysis in condensation polymers	Below 0.5%
Bulk density	Feeder and silo sizing; a light powder at 0.4-0.5 g/cm³ behaves nothing like a mineral filler	Check your feeding equipment before the first order

6. What to put on the purchase order

The hydrate ratio, not just the CAS – the CAS covers a family; the ratio defines the product.
The three assay figures, with the specification limits you will test against.
Particle size distribution and sieve residue, not only a D50.
Surface moisture, particularly if the compound is processed dry.
A halogen-free and RoHS-type statement where the finished article is regulated – zinc borate itself is halogen-free, but the certificate should say so in the form your customer’s auditor expects.

7. Packaging and supply

Zinc borate ships as a white powder in 25 kg paper or PP-woven bags with an inner liner, and in big bags for bulk consumption. Store sealed, dry and off damp floors: the product is not strongly hygroscopic, but surface moisture is a dispersion and dry-blend problem rather than a chemical one, so the packaging exists to keep the surface dry. Each lot arrives with a COA and SDS; FOB and CIF terms are quoted against destination. For a regulated cable or construction application, say so at enquiry stage and the halogen-free declaration and any RoHS statement are supplied with the first lot rather than requested later.

8. The honest summary

If you are looking for a drop-in replacement for a bulk flame retardant, zinc borate is the wrong product and no amount of loading will fix it. If you already have a halogenated, phosphorus or intumescent system and you want less smoke, less afterglow, more char and a route to reducing antimony, then zinc borate at a few phr is one of the most cost-effective changes available – provided the certificate’s three assays actually describe the hydrate you think you bought.