Quick answer: “Halogen-free” is not one chemistry — it is four families that fail in completely different ways. Mineral hydroxides need 55–65% loading. Phosphorus systems work at 15–25% but are resin-specific. Nitrogen systems only work in some polymers. Synergists do almost nothing alone. Picking the wrong family for your resin wastes a trial batch, and the resin rules out more options than price ever will.
This is the counterpart to our list of brominated flame retardants: what the halogen-free options actually are, what each one needs, and where each stops working.
The Four Families
| Family | How It Works | Typical Loading |
|---|---|---|
| Mineral hydroxides | Absorb heat and release water. Purely physical — the effect scales with quantity | 55–65% |
| Phosphorus | Forms a protective char layer in the condensed phase | 15–25% |
| Nitrogen | Releases inert gas, dilutes fuel; often promotes dripping that carries heat away | 10–20% |
| Synergists | Improve char quality, suppress smoke, replace antimony. Not flame retardants on their own | 1–10 phr |
The Products, and Where Each One Stops Working
The limits matter more than the strengths — every one of these is a good product inside its window and a wasted trial outside it.
| Product | Suits | Where It Stops Working |
|---|---|---|
| ATH aluminium trihydroxide | LSZH cable, polyolefin, low-temperature processing | Decomposes around 180–200 °C — it foams in anything processed hotter |
| MDH magnesium hydroxide | Same jobs at higher temperature — PP, PA, hot extrusion | Stable to roughly 300–330 °C, but costs more and still needs 55–65% |
| APP ammonium polyphosphate | Intumescent systems, coatings, PP, thermosets | ⚠️ Calcium carbonate in the same formulation neutralises it. Also moisture-sensitive |
| Phosphinate aluminium diethylphosphinate | Glass-filled PA and PBT; supports high CTI. The E&E workhorse | Expensive. Usually needs MPP alongside it in reinforced grades |
| MCA melamine cyanurate | Unfilled polyamide, at low cost and low loading | ⚠️ Close to useless once glass fibre is added — it relies on dripping, and reinforcement prevents it |
| MPP melamine polyphosphate | Partner to phosphinate in glass-filled PA66 and PBT | Rarely used alone; it is a system component |
| Expandable graphite | Foams, coatings, seals — expands into an insulating layer | Works mechanically, so over-shearing in a twin-screw destroys the expansion. Black only |
| KSS potassium sulfonate | Transparent polycarbonate, at fractions of a percent | PC-specific. Does nothing in most other resins |
| Zinc borate · zinc stannate | Smoke suppression, char reinforcement, antimony replacement | Synergists — they will not carry a rating on their own |
Start From Your Resin, Not From the List
The fastest way to use the table above is backwards — find your resin first, and most of the options disappear.
| Your Resin | Practical Halogen-Free Route |
|---|---|
| PE / EVA, LSZH cable | ATH or MDH at 55–65%, surface treated, with a coupling agent |
| PP | MDH for hot processing, or an APP intumescent system at lower loading |
| Unfilled PA6 / PA66 | MCA — cheapest route that works here |
| Glass-filled PA / PBT | Phosphinate + MPP. Do not raise MCA dosage — the mechanism is gone, not weak |
| PC, transparent | KSS at very low dose, plus anti-drip |
| PC / ABS blends | Phosphate esters (BDP, RDP) with anti-drip |
| POM | Genuinely difficult. The oxygen is in the backbone — expect compromises whatever you choose |
What Halogen-Free Actually Costs You
Three trade-offs that are physics rather than formulation skill, so no supplier can promise them away.
Loading, and therefore mechanicals. A mineral system at 60% filler is more mineral than polymer. Elongation and impact drop, and surface treatment recovers a large share of that but not all of it. Anyone promising no property loss at that loading is overselling.
Cost per part, not per kilo. Mineral hydroxides are cheap per kilo and you need a lot; phosphinate is expensive per kilo and you need much less. The ranking on price per kilo tells you almost nothing about the ranking on cost per finished part.
Processing window. This is the constraint people discover last: ATH decomposing around 180–200 °C rules it out of any polymer needing a hotter barrel. That single number eliminates more candidates than budget does.
And one thing halogen-free buys you in return, which is often the actual reason for the specification: no acid gas. Burning PVC releases hydrogen chloride that corrodes switchgear and electronics the fire never touched. In data centres and rail, that is an asset-protection argument as much as a life-safety one.
What We Would Actually Advise
Check whether you need halogen-free at all. RoHS restricts only PBB and PBDE — most brominated products are fully RoHS compliant. If your customer’s clause says RoHS, it is not asking for halogen-free, and switching is spending money on a requirement you do not have.
If the driver is antimony cost rather than halogen policy, you can stay with brominated chemistry and cut the antimony instead. That is a far cheaper project than reformulating — set out in cutting or replacing antimony trioxide.
Start from the resin and the processing temperature, then the standard, then cost. Reversing that order is how trial batches get wasted on a chemistry the polymer was never going to accept.
FAQ
What are the main halogen-free flame retardants?
Four families: mineral hydroxides (ATH, MDH), phosphorus systems (APP, phosphinates, phosphate esters, red phosphorus), nitrogen systems (MCA, MPP), and synergists (zinc borate, zinc stannate, expandable graphite, KSS). They are not interchangeable — each is tied to particular resins and processing temperatures.
Which halogen-free flame retardant is best?
The question does not have an answer independent of your resin. Mineral hydroxides dominate LSZH cable, phosphinate dominates glass-filled polyamide, KSS is the only practical route in transparent PC. Start from the polymer and the processing temperature and the list shortens to one or two options.
Is halogen-free safer?
It avoids acid gas in a fire and the persistence concerns attached to some brominated molecules. It is not automatically safer in every sense — phosphorus chemistry is itself under review at ECHA, and high mineral loading brings its own mechanical and processing costs. Safer against which failure mode is the question worth asking.
Why does halogen-free need so much more additive?
Because most halogen-free systems work physically or in the condensed phase rather than by interrupting flame chemistry. Mineral hydroxides absorb heat and release water, so the effect is proportional to how much is present. There is no low-loading version of that mechanism.
Does halogen-free mean RoHS compliant?
They are different requirements and the confusion is expensive. RoHS restricts ten substance groups, only two of them brominated. Halogen-free in electronics means IEC 61249-2-21: bromine ≤900 ppm, chlorine ≤900 ppm, total halogen ≤1500 ppm. A material can be fully RoHS compliant and nowhere near halogen-free.
Can I get halogen-free V-0 without losing mechanical properties?
Partly. Surface-treated fine-particle grades, coupling agents and correct dispersion recover a large share of what loading costs. But some trade-off at 60% mineral is physics — and in reinforced grades a phosphinate system at 15–25% will preserve far more than a mineral system will.
Final Thoughts
Halogen-free is a category, not a product, and most wasted trials come from treating it as one. Your resin and your processing temperature eliminate most of the list before cost is even discussed — so send those first. Tell us the polymer, glass content if any, target rating at your real wall thickness and your barrel temperature, and we will name two candidates with honest trade-offs rather than a catalogue. Start from the halogen-free additive range, or the chemistry overview if you want the mechanisms first. The mineral family is also the hardest to process at full loading; what that means on a twin-screw is in LSZH compound production at 60% filler. Within the mineral family the choice goes further than ATH against MDH — boehmite and huntite change the decomposition window again, compared in flame retardant fillers. Once the family is chosen, the next question is commercial: what “halogen-free” has to say on the purchase order, and why three quotes come back 20 % apart — halogen free compound works through that.
—— Rectivas Materials 团队 技术工程师老陈