Halogen-free flame retardants now cover most new polymer compounds for cable, electrical, automotive and building parts. The two main families, phosphorus-based systems and magnesium hydroxide (MDH), stop a fire in very different ways, and that decides the loading, the mechanical properties and the tests each one passes. As a formulation development partner, SYCHEM TECH helps compounders choose and combine them.
This comparison covers how both flame retardants work, how they differ in PP, which one suits which application, how to combine them, and how to process and test the compound.
Why halogen-free flame retardants
Brominated flame retardants with antimony trioxide are efficient, but they release dense smoke and corrosive hydrogen bromide in a fire, and several of them are restricted. Cable, transit, building and electronics customers increasingly specify halogen-free, low-smoke compounds. Phosphorus systems and metal hydroxides are the two families that meet that demand; the flame retardant overview on Wikipedia lists the others.
How phosphorus flame retardants work
Phosphorus flame retardants are a broad family. In polyolefins, the most common type is the intumescent phosphorus-nitrogen (P-N) system, based on ammonium polyphosphate (APP) or piperazine pyrophosphate (PPAP). In a fire, the phosphorus forms phosphoric acid, which chars the polymer, while the nitrogen part releases gas that blows the char into a thick foam. The foam insulates the polymer below and keeps oxygen out.
Other phosphorus types serve other polymers: metal phosphinates for glass-fiber polyamide, liquid phosphate esters such as BDP and RDP for PPE/HIPS blends, and red phosphorus for some nylon grades. Our phosphorus flame retardant range covers the P-N intumescent systems for polyolefins and elastomers: the HF-80 series (PPAP) and the HF-90 series (APP).
How MDH works
Magnesium hydroxide starts to decompose at about 340 °C. The reaction absorbs a large amount of heat, cools the polymer and releases water vapour that dilutes the flammable gases. The magnesium oxide left behind forms a protective ash layer that also suppresses smoke. Because the effect is physical, MDH needs high loadings, typically 50–65 wt%, and a fatty-acid surface coating so the polymer can take that much filler.
Aluminium trihydroxide (ATH) works the same way but decomposes from about 200–220 °C, so it is limited to low-temperature polymers such as EVA and PE. Our magnesium hydroxide (MDH) page compares the two in detail.
Phosphorus vs MDH flame retardants at a glance
| Property | Phosphorus (P-N intumescent: APP, PPAP) | Magnesium hydroxide (MDH) |
|---|---|---|
| Main mechanism | Condensed phase: swells into an insulating char foam | Endothermic decomposition, water release, MgO ash |
| Loading for UL94 V-0 in PP at 1.5 mm | About 20–30 wt% | About 50–60 wt% |
| Compound density (PP base) | Stays close to unfilled PP, about 1.0–1.1 g/cm³ | About 1.4–1.5 g/cm³ at 60% loading |
| Mechanical properties and flow | Moderate change; good melt flow | Large loss of elongation and impact; high melt viscosity unless coupled |
| Decomposition onset | About 250 °C (APP) to 260–270 °C (PPAP) | About 340 °C |
| Smoke and gases | Low smoke, no halogen acids | Very low smoke, only water vapour |
| Water resistance | APP is moisture sensitive; PPAP grades resist migration | Good; fatty-acid coating limits moisture pickup |
| Typical fit | PP and PE molded parts, glass-fiber PP, TPE, electrical housings | Low-smoke zero-halogen cable, EVA compounds, fire-rated rubber |
Loadings and temperatures in this table are typical starting values from our grade guides; the final dose depends on the resin grade, colour, part thickness and the test you need to pass.
Which one for your application
| Application | Usual choice | Why |
|---|---|---|
| Low-smoke zero-halogen (LSZH) cable, EVA/PE | MDH, often with a coupling agent | Cable tests measure smoke and flame spread along bundled cables, where water release and MgO ash matter; EVA tolerates 55–65% filler |
| PP appliance housings and electrical boxes | APP-based P-N system (HF-90) | UL94 V-0 at 24–30%, keeps density low and the part easy to mold |
| Glass-fiber PP, EV battery parts | PPAP-based P-N system (HF-80) | V-0 at 20–26% with a higher decomposition onset; no anti-drip agent needed in glass-fiber PP |
| TPE and soft parts | PPAP fine-particle grade (HF-802B) | Good dispersion and surface; V-0 at 30–40% in TPE |
| PE sheet and pipe | P-N system or MDH | PE needs more phosphorus than PP (about 28–37%); MDH suits thick parts where density is acceptable |
| Polyamide (PA6, PA66) | Phosphinate systems or MDH | Phosphinates are the mainstream for glass-fiber nylon; MDH survives PA compounding thanks to its 340 °C onset |
| PPE/HIPS blends | Phosphate esters (BDP, RDP) | Liquid aryl phosphates are the classic route for PPO/PPE blends; ask us about the base resin |
Combining phosphorus and MDH
The two families complement each other. MDH cools the polymer and dilutes the flame; the phosphorus system builds the char barrier. In PP and PE, co-loading about 8–12% of an APP-based system with MDH lets you lower the total filler while keeping the smoke advantage. Other synergists that help reduce MDH loading include zinc borate and silicone masterbatch, which also improves surface finish and extrusion.
In high-loading MDH cable compounds, a maleic-anhydride-grafted coupling agent is almost always used. It bonds the polymer to the filler surface and recovers much of the elongation and tensile strength lost by the high filler content; see our compatibilizer range.
Processing tips for halogen-free flame retardants
- Watch the melt temperature. Keep it below the decomposition onset of the flame retardant, about 250 °C for APP, 260–270 °C for PPAP and 340 °C for MDH, and avoid long residence times.
- Keep them dry. APP picks up moisture, and wet MDH causes splay and voids. Store bags sealed and dry the material if it has been exposed.
- Feed high loadings properly. 60% MDH needs a twin-screw extruder with side feeding and good venting. For P-N systems, a ready masterbatch (for example a 70% PPAP masterbatch in PP) simplifies dosing.
- Add an anti-drip agent where needed. Unreinforced PP with a P-N system usually needs 0.2–0.4%; glass-fiber PP does not.
- Avoid ingredients that fight intumescence. Calcium carbonate, high carbon black loadings and phthalocyanine green can weaken the char. For copolymer PP, add about 2 wt% more flame retardant than for homopolymer.
- Stabilize for heat and light. Keep a full antioxidant package. For outdoor flame-retardant parts, acidic species call for low-basicity HALS; our HALS guide explains why.
How flame-retardant compounds are tested
- UL94: vertical burn ratings V-0, V-1 and V-2, and 5VA/5VB for enclosures, always quoted at a thickness.
- Limiting oxygen index (ISO 4589): the oxygen level needed to sustain burning; useful for screening.
- Cone calorimeter (ISO 5660): heat release rate and smoke production, closest to a real fire.
- Glow wire (IEC 60695-2): GWIT and GWFI for parts near live electrical contacts.
- Cable tests: flame spread (IEC 60332), smoke density (IEC 61034) and, in Europe, the construction-product classes of EN 50575.
- Water immersion: for example UL 746C, to show that the flame retardant does not leach out.
Frequently asked questions
Can I replace MDH with a phosphorus flame retardant in a cable compound?
Partly. A P-N system reaches V-0 at a much lower loading, but cable standards also measure smoke, heat release and flame spread, where MDH’s water release and ash matter. Many low-smoke formulas combine both. Test against the actual cable standard before switching.
Do phosphorus flame retardants work in PE and PP?
Yes. Intumescent APP and PPAP systems are designed for polyolefins. PP reaches UL94 V-0 at about 20–30%, while PE needs more, about 28–37% depending on colour and grade.
Should I use MDH or ATH?
Use ATH only when the processing temperature stays below about 200 °C, as in many EVA and PE cable compounds. MDH, with its 340 °C onset, is needed for PP, polyamide and other polymers processed hotter.
Do halogen-free flame retardants corrode equipment?
Far less than brominated systems, because they release no halogen acids. APP can give off ammonia and acidic phosphorus species if it is overheated, so keep the melt below its decomposition onset and purge with unfilled resin at the end of a run.
Can I get the TDS and samples?
Yes. We send the TDS for each grade by email together with the quotation, and arrange samples for trials.
Get a flame retardant recommendation
Tell us your polymer, part thickness, colour, the fire test and rating you need, and any limits on density or mechanical properties. We will suggest a phosphorus, MDH or combined system with a starting dosage, and email you the TDS and arrange samples. Contact our technical team or write to info@sychemtech.com.
Related products: phosphorus-based flame retardants (HF-80 and HF-90), magnesium hydroxide (MDH), our full range of halogen-free flame retardants, compatibilizers for filled compounds and silicone masterbatch.
