Quick answer: With rigid PVC you are usually not buying flame retardancy at all — roughly 57% chlorine in the backbone already gives it an LOI near 45 and self-extinguishing behaviour. What a fire retardant PVC compound actually buys you is smoke suppression, antimony reduction and thermal stability during processing. If a supplier is selling you flame retardant for rigid PVC without asking about smoke, they are selling you something the resin already does.
That makes PVC the odd one out. In every other polymer the flame rating is the hard part; here it comes free and the difficult parts are elsewhere.
What You Are Actually Paying For
| Requirement | Does Rigid PVC Need Help? | What Goes In |
|---|---|---|
| Flame rating (UL94, B1) | Usually not. Rigid PVC self-extinguishes on its own chlorine | Nothing, in most formulations |
| Smoke density | Yes — this is the real job. PVC burns dirty even when it barely burns | Zinc borate, zinc stannate, ATH as a partial filler |
| Antimony reduction | Where cost or policy demands it | Zinc borate for partial, zinc stannate for full — see substitution ratios |
| Thermal stability | Always. PVC degrades and releases HCl near processing temperature | Heat stabiliser system — and your flame additives must not fight it |
| Acid gas (LSZH markets) | Cannot be fixed. The chlorine is the polymer | Change material — see LSZH vs PVC |
That last row is worth stating plainly because it saves people a trial: no additive package makes PVC halogen-free. If a specification names acid gas limits or IEC 60754, PVC is out of the running and the conversation is about a different polymer, not a different additive.
Rigid vs Flexible: Different Problems Entirely
These get quoted as one product category and behave nothing alike.
Rigid PVC — profiles, pipe, sheet, fittings. High chlorine, low plasticiser, already self-extinguishing. Your additive work is smoke, stabiliser compatibility and impact modifier balance. Filler content is often calcium carbonate for cost, which changes what a flame package can do.
Flexible PVC — cable, tape, membrane, coated fabric. The plasticiser is the problem: it is organic, it is often 30% or more of the compound, and it burns. Flexible PVC genuinely needs flame retardant help, and the higher the plasticiser loading the more it needs. That is a different formulation conversation, covered in flame retardant PVC for cable, sheet and tape.
If someone quotes you a single “PVC flame retardant” without asking which one you run, the quote is not based on your formulation.
Where Rigid PVC Compounds Actually Fail
Four failure modes, none of which is the flame rating.
Smoke, in enclosed installations. Building codes for interior applications frequently cap smoke, and rigid PVC passes flame comfortably then loses on the smoke index. This is where zinc borate earns its place — it promotes char and suppresses smoke at 2–10 phr, which is far cheaper than reformulating.
The calcium carbonate conflict. Rigid PVC formulations often carry substantial CaCO₃ for cost and stiffness. If you are also running an APP-based intumescent system for a higher fire class, the carbonate neutralises the acid source and quietly destroys it. The two do not coexist without deliberate formulation.
Stabiliser interaction. PVC always carries a heat stabiliser, and some flame additives interfere with it. The symptom is discolouration or early degradation at normal processing temperature — read as a stabiliser problem when it is actually an additive conflict. This is the same class of issue covered in additive interactions.
Impact modifier as fuel. Rigid PVC needs impact modifier to be usable, and impact modifiers are organic and combustible. Raising impact resistance quietly lowers fire performance — one of the few places where a mechanical decision moves the flame result.
Compound, Masterbatch or Additive?
| Your Setup | Buy This |
|---|---|
| You dry-blend your own PVC formulation (most rigid processors) | Powder additives. You already have the mixer and the recipe — see PVC additives |
| You buy ready-made dry blend and want to modify it | PVC masterbatch, matched to your base compound |
| You extrude bought compound and do not formulate | Finished compound to your specification |
Rigid PVC processors are unusual in that most of them already dry-blend — which means powder additives are normally the right answer here, and paying for a masterbatch carrier you do not need is the more common mistake than the reverse.
What We Would Actually Advise
Read your specification for the word “smoke” before anything else. If it is not there and you are running rigid PVC, you may not need a flame retardant package at all — check what your unmodified compound already achieves before buying anything.
If smoke is the problem, start with zinc borate. It is the cheapest step that moves the smoke number, and it reduces antimony at the same time. Full reformulation is for when the gap is large, not marginal.
If the clause mentions acid gas or halogen-free, stop. No package fixes that in PVC. Either the specification is negotiable or the material is — and it is worth finding out which before spending anything on trials.
FAQ
Does rigid PVC need a flame retardant?
Usually not for the flame rating itself. Around 57% chlorine in the backbone gives rigid PVC an LOI near 45 and self-extinguishing behaviour, so it passes most flame requirements untreated. What it commonly needs is smoke suppression, and in some markets antimony reduction. Check what your current compound already achieves before buying a package.
What is a fire retardant PVC compound?
In practice, a PVC compound formulated for a fire class where smoke, and sometimes a higher building classification, matter — not one where flame ignition resistance had to be added. The additives doing the work are typically zinc borate or zinc stannate for smoke and char, with antimony trioxide where a halogenated synergist is still used.
Can PVC be made halogen-free?
No. The chlorine is part of the polymer, not an additive. If your specification requires halogen-free or limits acid gas under IEC 60754, you are changing material rather than formulation.
Why does my rigid PVC pass the flame test but fail on smoke?
Because those are different mechanisms. Chlorine self-extinguishes the flame efficiently and produces dense smoke doing it. Zinc borate and zinc stannate are the usual answers, and reducing antimony often helps as well since it contributes to smoke.
Does calcium carbonate affect flame retardancy in PVC?
It can, significantly, if you are running an intumescent system — carbonate neutralises the acid source that APP-based systems depend on. In a straightforward chlorine-plus-synergist formulation it is largely inert to the flame chemistry, but it does dilute everything else and changes your loading arithmetic.
Do you supply rigid PVC compound or only additives?
Both routes are available. Most rigid PVC processors dry-blend their own formulation and buy powder additives, which is usually the lower cost per part. Where you would rather run a single material stream, finished compound can be specified to your fire class and colour. Tell us which you run and what your smoke clause says.
Final Thoughts
Rigid PVC is the one polymer where the flame rating is rarely the problem, and treating it like the others leads to buying performance you already have. Work out whether your clause is about smoke, about antimony or about acid gas — those need three different answers, and only the first two have one. Send us your formulation type, the fire class you must meet and whether a smoke limit applies, and we will say what actually needs to change, including when the answer is nothing. Start from PVC additives or the compound range.
—— Rectivas Materials 团队 技术工程师老陈