Reactive vs Additive Flame Retardants: Which One Can You Actually Use?

Quick answer: Additive flame retardants are physically dispersed into the polymer and can, over time, migrate out of it. Reactive flame retardants are chemically bonded into the polymer backbone during polymerisation and can never leave. Additives dominate commercial volume because they are cheaper and can be added by anyone with a compounding line; reactive types cost more, must be built in at the resin plant, and are chosen when migration, transparency or decades-long permanence make anything else unacceptable. The practical dividing line is not performance — it is which part of the supply chain you sit in.

This is the most fundamental split in flame retardant chemistry, and it is also the one most likely to send a buyer down a road they cannot travel. We regularly get enquiries asking for a reactive flame retardant from companies that mould finished parts — a request that cannot be fulfilled by anyone, because by the time you are holding pellets, the moment for reactive chemistry has passed. Understanding why takes five minutes and saves weeks.

The Chemical Difference

Q: What is the difference between reactive and additive flame retardants? Whether the flame retardant is chemically bonded to the polymer or merely mixed into it.

Additive flame retardants are separate substances dispersed through the polymer matrix. Nothing bonds them in place — they are held by dispersion, compatibility and viscosity alone, like sand in concrete. Mineral hydroxides, ammonium polyphosphate, most brominated products and phosphate esters all work this way. You can add them at any point where the polymer is molten, which means a compounder, a masterbatch producer or a moulder can all introduce them.

Reactive flame retardants carry a functional group that participates in the polymerisation reaction itself. A DOPO molecule with its reactive P–H bond opens an epoxy ring and becomes a permanent link in the cured network; tetrabromobisphenol-A reacts into epoxy resin as a monomer. Once cured, the flame retardant is not in the polymer — it is part of the polymer. There is nothing left to migrate, bloom, wash out or evaporate.

Side by Side

The Row That Decides Everything

Q: Can I use a reactive flame retardant in my process? Only if you polymerise or cure the resin yourself.

Look again at the second row, because it settles the question before any of the others matter. Reactive chemistry happens when the polymer is being made. If you buy resin and mould it, that moment happened in someone else’s factory, months before the material reached you. No supplier can sell you a reactive flame retardant that works in your injection machine — not because they won’t, but because unreacted reactive chemistry in a thermoplastic melt behaves like a mediocre, expensive additive.

Where does that leave you? Two options, depending on your position:

• You buy resin and process it — injection moulder, extruder, compounder. Your route is additive chemistry, in powder, masterbatch or ready-made compound form. If your specification genuinely demands reactive-grade permanence, you buy a resin grade that already contains it, from the resin producer.
• You formulate thermosets — epoxy systems, polyurethane, coatings, laminates. You cure the polymer yourself, so both doors are open. This is the only segment where the reactive-versus-additive question is a live decision rather than a fact of your position.

The clearest illustration is copper-clad laminate. Halogen-free FR-4 gets its rating from DOPO-type chemistry reacted into the epoxy at the resin plant — which is why a board buyer specifies “halogen-free per IEC 61249-2-21” and never touches the flame retardant. We unpack that supply chain in our articles on DOPO flame retardant and halogen-free PCB material.

The Third Category Everyone Forgets: Polymeric Additives

Framing this as a binary misses the most interesting development of the last two decades. Polymeric flame retardants are additives — physically dispersed, no chemical bond — but with molecular weights so high that they cannot migrate. They capture most of the permanence benefit without needing access to the polymerisation step.

The clearest case is PolyFR, the brominated styrene-butadiene copolymer that replaced HBCD in polystyrene foam insulation. HBCD was a small molecule that migrated and bioaccumulated, and was listed as a persistent organic pollutant. PolyFR works on the same bromine chemistry but is a polymer chain — too large to leach, too large to bioaccumulate, still added at the compounding stage. That story is in our article on flame retardant polystyrene.

The same logic explains why DBDPE displaced decaBDE, and why the industry has moved from TPP toward oligomeric BDP in PC/ABS. The trend is not “additive to reactive” — it is “small molecules to large ones.” If your concern is migration, you can usually solve it by choosing a higher-molecular-weight additive rather than by re-architecting your supply chain. Our comparison of BDP, RDP and TPP is exactly this progression within one chemical family.

When Reactive Is Genuinely Necessary

Four situations where nothing else will do, all of which happen to be thermoset applications where the choice is available anyway:

1. Electrical performance over decades. Laminates must hold dielectric properties for twenty years; a migrating additive changes surface chemistry and undermines them.
2. Optical clarity with flame retardancy. Particles scatter light. If a part must be transparent and rated, the flame retardant cannot exist as a discrete phase.
3. Extraction and contact restrictions. Where leaching is regulated or unacceptable, a bonded atom is the only defensible answer.
4. Very thin sections where loading has nowhere to go. Reactive chemistry reaches its rating at 2–3% phosphorus; an additive route might need ten times that, which a thin laminate cannot physically accommodate.

Outside these, additive chemistry is not a compromise — it is the correct engineering answer, and it is why it holds the overwhelming majority of commercial volume. It lets you change formulation between batches, tune loading to thickness, switch chemistry when a regulation moves, and buy from more than one supplier. Reactive chemistry locks all of that into a resin grade.

How to Decide in Practice

• Identify your position first. Do you make the polymer or buy it? This answers the question more often than any technical criterion.
• If migration is the worry, try molecular weight before rewriting the supply chain. A polymeric or oligomeric additive usually solves blooming and extraction at a fraction of the disruption — the mechanisms are in our guide to flame retardant ageing.
• If you need reactive performance but buy resin, specify the performance to your resin supplier — halogen-free, low extractables, transparent, rated at your thickness — and let them deliver the grade. Do not specify the molecule; you will narrow the field for no benefit.
• If you formulate thermosets, run the real comparison: reactive gives permanence and low loading, additive gives cost and flexibility. Our article on flame retardant epoxy works through that trade-off in the system where it matters most.

Where we fit, stated plainly: Rectivas produces additive flame retardants — powders, masterbatch and ready-to-mould compounds. We do not produce reactive intermediates like DOPO or reactive polyols; those come from fine-chemical and resin producers. What we do regularly is help buyers work out which category their problem actually falls into, because a meaningful share of “we need a reactive flame retardant” enquiries turn out to be solvable with a high-molecular-weight additive. Send the polymer, the process you run and the requirement driving the question — we will tell you which route applies, including when the answer is that we are not the right supplier.

FAQ

Are reactive flame retardants better than additive ones?

Not better — different. Reactive chemistry gives permanence, low loading and preserved transparency; additive chemistry gives cost, flexibility and universal availability. Additives hold the overwhelming majority of commercial volume because for most parts they are the correct answer, not a compromise.

Can a reactive flame retardant be added during compounding?

No, not meaningfully. Without a polymerisation reaction to join, the molecule behaves as an ordinary — and usually poor, expensive — additive. Reactive chemistry has to be introduced where the polymer is formed.

Is PolyFR reactive or additive?

Additive, and it is the best illustration of why the binary is too simple. PolyFR is physically dispersed with no chemical bond to the polystyrene, but its polymeric molecular weight makes migration and bioaccumulation impossible — delivering reactive-like permanence through an additive route.

Which flame retardants are reactive?

The main commercial ones are DOPO and its derivatives in epoxy, tetrabromobisphenol-A reacted into epoxy resin, reactive phosphorus polyols in polyurethane, and phosphorus comonomers in copolyesters. All of them are used at the resin or system manufacturing stage, not at compounding.

Does reactive mean halogen-free?

No — the two are independent. TBBPA is reactive and brominated; DOPO is reactive and halogen-free; ATH is additive and halogen-free; DBDPE is additive and brominated. Reactive describes how the flame retardant joins the polymer, not what it is made of.

Final Thoughts

The reactive route is only open to you if you control the polymerisation – buy finished resin and additives are the whole conversation. What is genuinely worth watching is the shift from small molecules to polymeric and reactive chemistry, because that is where the regulatory pressure is heading. Our chemical families overview maps which of our additive systems sit on which side of that line.

—— Rectivas Materials 团队 老陈