Zinc Borate in PVC Cable Compounds: Dosing, Synergy and the Processing Window

PVC cable compounds are where zinc borate earns most of its money, and they are also where it is most often dosed badly. The reason is that a cable compound is a system – plasticiser, filler, stabiliser, flame retardant and smoke suppressant all interacting – and zinc borate changes several of those interactions at once. This article covers what it does in a PVC sheath or insulation, how much to use, and the two processing details that decide whether it helps or hurts.

1. What a flame-retardant PVC cable compound contains

Component	Typical role	Typical loading
PVC resin	Matrix	100 phr (basis)
Plasticiser (often chlorinated or phosphate ester in FR grades)	Flexibility; in FR grades the plasticiser itself is part of the flame-retardant package	40-60 phr
Filler (calcium carbonate, calcined clay, talc)	Cost, hardness, smoke reduction	20-40 phr
Heat stabiliser (lead-based historically, calcium-zinc now)	Prevents HCl elimination during processing	3-6 phr
Halogenated flame retardant or the chlorinated plasticiser itself	Gas-phase flame inhibition	System-dependent
Antimony trioxide	Synergist for the halogen	3-5 phr
Zinc borate	Partial antimony replacement, smoke suppression, char promotion, afterglow reduction	2-4 phr typical
Lubricants, antioxidant, colour	Processing and service life	1-3 phr

The numbers above are a structure, not a recipe – the actual loading depends on the fire class, the plasticiser system and the wall thickness. What matters is where zinc borate sits: in the small-dose additive group, alongside antimony, not in the filler group.

2. The three things it does in a PVC compound

Function	Mechanism	What you measure
Partial antimony replacement	Borate/zinc shifts the halogen chemistry towards char formation, so less antimony is needed to hold the class	Oxygen index, UL-94 or the applicable cable fire test
Smoke suppression	Vitreous borate layer seals the surface; zinc promotes char rather than soot precursors	Smoke density (IEC 61034 / ASTM E662)
Afterglow reduction	Borate residue on the char interrupts the glowing combustion that continues after the flame goes out	Glow time in the relevant cable standard – often the specific reason zinc borate is added

Of the three, afterglow is the one most often written into a specification and least often explained. In mining, tunnel and rail cable standards an afterglow limit exists because a glowing residue can ignite methane or dust after the fire is out. Zinc borate is one of the few additives that addresses it directly.

3. The processing window is the whole point

Temperature	What happens in a PVC compound
Dry blend, 80-110 °C	Plasticiser absorption; surface moisture on the fillers causes pre-gelling and voids – this is why the 0.5% surface-water limit matters
Extrusion, 160-200 °C	Gelation and shaping. Zinc borate is stable here: its dehydration starts above 320 °C, so it does nothing in the extruder
Fire, 320 °C and above	Dehydration, glass formation, char promotion – the additive finally acts

This is the design elegance of zinc borate in PVC: it is inert through the entire processing window and active only in the fire. An additive that started dehydrating at 180 °C would foam the compound in the extruder; one that started at 600 °C would act too late. The 320 °C onset sits in the right gap for PVC, and the same property is what makes it work in polyamide and PBT.

4. Dosing, and what changes when you substitute

Change	Expected effect	What to re-test
Replace part of the Sb₂O₃ with zinc borate at similar total loading	Fire class usually held; smoke and afterglow improve	Oxygen index, smoke density, glow time, tensile and elongation
Add zinc borate on top without removing antimony	Best smoke result; total additive cost rises	Smoke density, physical properties, compound density
Push the substitution too far	Fire class drops – the gas-phase mechanism has been removed faster than the condensed-phase one compensates	Everything; this is the failure mode to avoid
Change the antimony supplier or grade at the same time	Results become uninterpretable	Nothing – change one variable at a time

Substitution work on a cable compound should change one thing per trial. Zinc borate is cheap enough that formulators are tempted to over-dose it; the cost is not the additive, it is the failed qualification run.

5. Two specification items that affect the compound, not the fire

Surface moisture below 0.5%. In a dry-blend process, moisture on a fine powder competes with plasticiser absorption and produces pre-gel and porosity in the extrudate. It is not a fire-retardant parameter at all, which is why it is easy to overlook.
Particle size 3-5 µm with a low 45 µm sieve residue. Coarse grit shows as surface defects on a thin insulation wall, and agglomerates act as stress concentrators. For thin-wall insulation, ask for a finer grade.

6. What to put on the purchase order

Hydrate ratio and the three assays (ZnO, B₂O₃, water) – the fingerprint of the grade.
Particle size and 45 µm residue, matched to your wall thickness.
Surface moisture, especially for dry-blend lines.
Whiteness if the sheath is coloured or light.
Halogen-free and RoHS-type declarations where the cable standard or the customer requires them.

7. Packaging and supply

Zinc borate for cable compounding is supplied in 25 kg paper or PP-woven bags with an inner liner, and in big bags; it is a light powder at 0.4-0.5 g/cm³ bulk density, so silo and feeder sizing should be checked before switching from a denser filler. Keep bags sealed and dry – surface moisture is the failure mode. Every lot ships with a COA and SDS, FOB or CIF against destination, with halogen-free and RoHS declarations available with the first order when the final article is regulated.