Flame Retardants for Polypropylene: 5 Classes, Dosage and UL 94 Selection

Flame retardants for polypropylene are additives, mainly intumescent phosphorus-nitrogen systems, brominated compounds with antimony trioxide and magnesium hydroxide, that are compounded into PP at about 21 wt% (intumescent systems) to 65 wt% (magnesium hydroxide) so that parts pass UL 94 V-2 or V-0. PP needs them because it burns with a limiting oxygen index of only about 17.5% and drips without forming char, so which system suits which PP part?

Flame retardants make up about 13% by weight of all plastic additives, and PP is one of the hardest polymers to protect because it leaves no char behind as it burns. This page compares the 5 classes of flame retardant used in polypropylene, their loading, and their fit by application: why PP burns and drips, which class reaches UL 94 V-2 or V-0 in moldings, cable compounds, fibres and recycled PP, how halogen-free and brominated routes compare, how much additive each rating needs, how synergists and antagonists interact with the flame retardant, how FR-PP is compounded and tested, and which substances face regulatory limits in 2026.

Key figures for flame-retardant PP

• Neat PP limiting oxygen index: about 17.5%
• PAPP:MPP (2:1) intumescent system for UL 94 V-0, virgin and recycled PP: 21 wt%
• APP-based intumescent system for UL 94 V-0: 22-30 wt%
• Magnesium hydroxide in PP compounds: up to 65 wt%
• Peak heat release rate at 35 kW/m2: 1148-1332 kW/m2 for neat PP versus 220-255 kW/m2 for intumescent PP

Why Does Polypropylene Need Flame Retardants?#

Polypropylene needs flame retardants because it is a pure hydrocarbon with a limiting oxygen index of only about 17.5%, so it burns steadily in air, drips burning melt and leaves almost no protective char. NBS researchers demonstrated the practical value of this protection in 1988: room-fire tests showed that flame-retarded products gave occupants more than 15 times longer escape time and released about one quarter of the heat of their non-retarded equivalents. The hub on flame retardants for plastics compares the same 5 classes across every polymer, from PVC to nylon, using the same mechanism categories applied here to PP.

What happens when polypropylene burns?#

When polypropylene burns, heat breaks its chains into flammable hydrocarbon volatiles, these mix with air and ignite, and the flame feeds heat back to the surface, which pyrolyses more polymer and melts it into burning drips. The combustion cycle of PP runs in 5 steps.

1. Heating raises the surface of the PP part until the polymer backbone starts to break down.
2. Pyrolysis splits the polypropylene chains into flammable hydrocarbon volatiles that migrate to the surface.
3. Mixing brings these volatiles into contact with atmospheric oxygen above the melting surface.
4. Ignition starts an exothermic gas-phase oxidation chain (H· and O2 react to OH· and O·) once the mixture reaches its ignition temperature.
5. Heat feedback radiates from the flame back to the polymer surface, pyrolysing more material and sustaining the cycle.

B. Schartel at BAM Berlin (2010, Materials 3, 4710) showed that flame retardants attack this cycle either in the gas phase, by interrupting the radical chain reaction, or in the condensed phase, by forming a protective barrier on the polymer surface. Cone calorimeter tests quantify how severe unprotected PP combustion is: neat PP plaques (100 x 100 x 3 mm) reach a peak heat release rate of 1148-1332 kW/m2 under a 35 kW/m2 external heat flux (ISO 5660-1), the same test used later on this page to compare flame-retarded formulations. Neat PP reaches a limiting oxygen index (LOI) of only about 17.5%, below the 21% oxygen present in ordinary air.

Is polypropylene flammable?#

Yes, polypropylene is flammable, and unmodified PP is not fire resistant; it becomes self-extinguishing only when flame retardants such as intumescent phosphorus-nitrogen systems or brominated additives are compounded into it. Its limiting oxygen index of about 17.5% sits well below the 21% oxygen concentration in normal air, so a flame sustains itself in PP without any additive support. No UL 94 flammability class exists for neat, unmodified polypropylene, because the material burns and drips continuously once ignited rather than self-extinguishing.

Which fire ratings do polypropylene parts need?#

Polypropylene parts are rated to 5 main fire standards, depending on the end use: UL 94 for electrical parts, the glow-wire tests of IEC 60695-2 for appliances, FMVSS 302 for car interiors, EN 13501-6 for building cables and the limiting oxygen index as a laboratory screen.

• UL 94 V-0 requires each specimen's afterflame to stop within 10 seconds, the total afterflame time across 10 applications on 5 specimens to stay under 50 seconds, afterglow after the second flame application to stop within 30 seconds, and no flaming drip that ignites the cotton indicator below the specimen.
• UL 94 V-2 follows the same total-time limits as V-1 (each afterflame ≤30 s, total ≤250 s) but allows flaming drips to ignite the cotton, which makes it the easier class to reach for thin parts.
• IEC 60335-1 sets the glow-wire requirement for unattended household appliances: a glow-wire flammability index (GWFI) of at least 850°C and a glow-wire ignition temperature (GWIT) of at least 775°C, or an end-product test at 750°C with flaming that stops within 2 seconds.
• FMVSS 302 governs automotive interior trim in the United States: a horizontal burn rate no faster than 102 mm/min.
• EN 13501-6, applied through the EN 50399 test, classifies building and installation cables (B2ca, Cca and related classes) by flame spread, heat release and smoke.

How each class is tested is explained under UL 94 flammability ratings, which covers vertical and horizontal burning in full.

What Are the 5 Classes of Flame Retardants for Polypropylene?#

The 5 classes of flame retardants used in polypropylene are intumescent phosphorus-nitrogen systems, brominated flame retardants with antimony trioxide, mineral hydroxides such as magnesium hydroxide, expandable graphite and NOR HALS radical generators, with the first two covering most UL 94 V-0 and V-2 grades.

1. Intumescent phosphorus-nitrogen flame retardants (APP, PAPP, MPP)#

Intumescent flame retardants are phosphorus-nitrogen systems, based on ammonium polyphosphate or piperazine pyrophosphate, that swell into an insulating char on the surface of burning PP and give UL 94 V-0 at 21-30 wt%. Ammonium polyphosphate (APP) exists in two crystalline phases; phase II grades, with a polymerization degree above 1000, decompose from about 240°C into ammonia and polyphosphoric acid, and commercial phase II grades such as Clariant Exolit AP 422 contain under 0.1% free melamine for hydrolytic stability. Formulated APP systems such as Exolit AP 750 and AP 766 reach UL 94 V-0 in polypropylene at 22-30 wt%, with processing possible up to about 220°C. An independent, lower-loading route exists in piperazine pyrophosphate combined with melamine polyphosphate at a 2:1 ratio, which reaches UL 94 V-0 at 21 wt% total. The ENFIRO project (EU FP7 grant 226563, concluded 2012) rated APP and melamine polyphosphate among the flame retardants with favourable environmental and health profiles when it compared alternatives to halogenated systems.

How intumescent flame retardants protect polypropylene#

Intumescent flame retardants protect polypropylene in 3 coordinated reactions: phosphoric acid from APP dehydrates a carbon source such as pentaerythritol into char, and gases from melamine blow that char into a thick foam that shields the melt below. An intumescent system for PP contains 3 components.

• Acid source, typically ammonium polyphosphate, decomposes under heat to release phosphoric acid.
• Carbon source, usually pentaerythritol (never abbreviated "PE" in PP formulation notes, to avoid confusion with polyethylene, and written as PENTA or PER), reacts with the phosphoric acid to dehydrate into a carbon-rich char.
• Blowing agent, commonly melamine, decomposes to release gases that expand the forming char into an insulating foam layer.

These 3 reactions occur together in the melt during a fire, producing the swollen char layer that separates the flame from the unburned polymer beneath it. Identity and grades of the acid-source component are detailed on the page for ammonium polyphosphate.

Piperazine pyrophosphate and melamine polyphosphate one-pack systems#

Piperazine pyrophosphate (PAPP) combined with melamine polyphosphate (MPP) in a 2:1 ratio is the leanest intumescent route for PP: 21 wt% of the blend gives UL 94 V-0 in both virgin and recycled polypropylene. This combination was reported in a study published in ACS Applied Polymer Materials (2026, doi 10.1021/acsapm.6c00885), which tested the 2:1 PAPP:MPP blend as a one-pack intumescent system and confirmed the V-0 result held after mechanical recycling of the compound. Adeka's FP series supplies commercial PAPP-based blends for this route. Identity and grades are on piperazine pyrophosphate, CAS 66034-17-1. Melamine polyphosphate, CAS 218768-84-4 and sold by BASF as Melapur 200, supplies the blowing-agent function in the same system.

2. Brominated flame retardants with antimony trioxide#

Brominated flame retardants protect polypropylene in the gas phase: the hydrogen bromide they release traps the H and OH radicals that carry the flame, and antimony trioxide multiplies the effect by forming volatile antimony halides. Antimony trioxide has no flame-retardant effect when used alone; it reacts with the hydrogen halide released by the brominated additive to form antimony oxyhalide and then volatile antimony trihalide, which carries the halogen into the gas-phase flame zone where it continues trapping radicals. Every brominated system used in PP is paired with antimony trioxide or another halogen donor for this reason. All brominated flame retardants are compared by bromine content and regulatory status on the family page, which covers their use across every polymer, not only PP.

UL 94 V-0 systems: DBDPE and EBTBP with antimony trioxide#

UL 94 V-0 polypropylene with bromine uses DBDPE or EBTBP together with antimony trioxide, the thermally stable decaBDE replacements that do not bloom. DBDPE (CAS 84852-53-9, sold as Saytex 8010 by Albemarle) contains at least 82% bromine, has a melting point above 345°C, and was developed as a decaBDE replacement in polyolefins with listed uses that include polypropylene. The synergist antimony trioxide has no flame-retardant effect on its own. EBTBP (CAS 32588-76-4, sold as Saytex BT-93 and BT-93W by Albemarle) is UV-stable and non-blooming, and is used in polyolefin wire and cable compounds. The loading for either system in PP depends on part thickness and target rating and is set from the supplier technical data sheet; no PP-specific figure is published in current literature, and figures from other polymers, such as high-impact polystyrene, do not transfer to PP formulations. DBDPE has been a Substance of Very High Concern (vPvB) since 5 November 2025 and falls within the scope of the EU restriction of aromatic brominated flame retardants now in preparation, covered in full under the regulations section below. DBDPE (decabromodiphenyl ethane) replaced decaBDE in polyolefins after decaBDE itself became restricted.

UL 94 V-2 systems: FR-370 and TBBPA-DBPE#

UL 94 V-2 polypropylene uses melt-blendable brominated flame retardants such as FR-370 or TBBPA-DBPE, which let burning drips carry heat away from the part.

FR-370 is melt-blendable, UV- and light-stable, and ICL markets it as a solution to the blooming that affects some brominated systems in PP; patents describe masterbatches containing 25-80% FR-370 with a radical initiator. TBBPA-DBPE stays a plain mention here because it lacks the same processing data in our source library, though it serves the same V-2 role in PP formulations.

3. Mineral flame retardants: magnesium hydroxide and huntite-hydromagnesite#

Magnesium hydroxide is the mineral flame retardant of choice for polypropylene because it stays stable to about 320°C, above PP compounding temperatures, and then absorbs 1316 J/g while releasing water that cools and dilutes the flame. Magnesium hydroxide (Mg(OH)2, CAS 1309-42-8, natural brucite CAS 1317-43-7) decomposes to magnesium oxide and water, with a theoretical loss on ignition of 31.0%, and processes up to about 330°C, roughly 110°C higher than aluminum trihydrate. All mineral flame retardants share this endothermic mechanism, though ATH and MDH differ sharply in their processing windows. PP compounds accept magnesium hydroxide up to 65 wt% when coated grades from suppliers such as Huber Advanced Materials are used to maintain dispersion and mechanical properties; the oxide residue that remains after decomposition also adsorbs soot, which suppresses smoke. A Huber-formulated halogen-free flame-retardant (HFFR) PP compound containing 185.7 phr of coated magnesium hydroxide reached a limiting oxygen index of 30.2%. Huntite-hydromagnesite releases water from about 220°C and carbon dioxide from about 330°C, and it is used in low-smoke halogen-free cable compounds alongside or instead of magnesium hydroxide, though no numeric PP loading for this mineral is published. Coated, natural and synthetic grades are compared on magnesium hydroxide (MDH) flame retardant.

Why aluminum trihydrate (ATH) rarely suits polypropylene#

Aluminum trihydrate rarely suits polypropylene because it starts releasing water at about 200°C, below the temperatures at which PP is compounded, so it would dehydrate during processing before it could act in a fire. ATH decomposes with a loss on ignition of 34.6% and an enthalpy of 1051 J/g, figures close to but lower than magnesium hydroxide's. This lower onset temperature confines ATH to polymers processed at or below about 200°C, including EVA, polyethylene, PVC and several thermosets. Aluminum trihydrate (ATH) remains the mineral of choice for EVA and PE cables precisely because those polymers process within its stability window.

4. Expandable graphite#

Expandable graphite is a physical intumescent that expands 30 to 400 cm3/g from about 200°C into an insulating layer of carbon worms, which suits black polyolefin profiles but rules out light-coloured PP. Expandable graphite (CAS 12777-87-6) is sulfuric-acid-intercalated graphite; when heated, the intercalant compound decomposes and the resulting gas pressure exfoliates the graphite layers into millimeter-scale carbon "worms" that form a low-density insulating barrier, with an expansion onset around 200°C across a 140-230°C range and processing typically kept below 230°C. Expansion ratios by grade are on the page for expandable graphite. No UL 94 rating or loading level for expandable graphite in polypropylene is established in current literature, and its black colour rules it out wherever a light-coloured or transparent PP part is required.

5. NOR HALS radical generators#

NOR HALS are low-basicity hindered amines that BASF markets for flame-retarded polyolefins as well as for light stabilization, and they tolerate the acidic by-products of halogenated flame retardants that deactivate ordinary HALS. Tinuvin NOR 371, a BASF NOR HALS with a molecular weight between 2800 and 4000 g/mol, is marketed for flame-retarded polyolefins and agricultural film applications, though its exact CAS and EC identity are not established in this source and are not published here. Basicity and grades are on NOR HALS. The radical-generator mechanism that some NOR HALS grades use as a standalone flame-retardant route, and any associated loading level, is not established for polypropylene and is not stated as fact on this page; what is established is that NOR HALS, with a pKb around 8 to 10, keep working in the acidic environment created by halogenated flame retardants, where standard basic HALS lose effectiveness.

Halogen-Free vs Brominated Flame Retardants in Polypropylene: Which Works Better?#

Halogen-free flame retardants work better in polypropylene for cables, building products and recycled PP because they avoid corrosive smoke and restricted substances, while brominated systems remain the choice for thin UL 94 V-2 parts.

The 21-65 wt% span that halogen-free systems require sits well above what brominated grades typically need to reach the same effect, which is why thin UL 94 V-2 parts and fibres, where every gram of loading affects wall thickness and mechanical properties, still favour the brominated route. Every class of halogen-free flame retardants is compared across polymers on the family page, which covers the same intumescent, mineral and nitrogen-based chemistries used here for PP.

Which Flame Retardant Is Best for Each Polypropylene Application?#

The best flame retardant for polypropylene depends on the required rating, wall thickness, colour and end use: melt-blendable brominated grades for thin UL 94 V-2 parts, intumescent phosphorus-nitrogen systems for halogen-free V-0, and coated magnesium hydroxide for low-smoke cable compounds.

Loadings are literature and supplier values; trials decide the final level. 185.7 phr MDH corresponds to 65.0 wt% in a two-component PP/MDH blend, as shown in the phr-to-weight-percent conversion below.

Filter systems by polymer and rating with the flame retardant selector, which applies the same rules used to build the table above.

Request quotes for flame retardants for polypropylene: send CAS or grade, target UL 94 rating and thickness, volume, and country to the plastic additive supplier finder.

Download the Flame Retardant Selector Matrix (PDF): FR systems by polymer and UL 94 rating. Provide an email, role and company to receive the guide.

Thin-wall PP parts and UL 94 V-2#

Thin-wall PP parts that need only UL 94 V-2 use melt-blendable brominated flame retardants such as FR-370, because V-2 allows flaming drips and these additives let the dripping melt carry heat out of the flame. This dripping behaviour is precisely what separates the V-2 class from V-0: cotton indicator ignition is permitted under V-2 but disqualifying under V-0, so a formulation that relies on melt dripping to remove heat is only viable when the target rating tolerates it.

PP housings and appliance parts for UL 94 V-0 and glow-wire#

PP housings and appliance parts that need UL 94 V-0 use either an intumescent phosphorus-nitrogen system at 21-30 wt% or a brominated flame retardant with antimony trioxide, and unattended appliances add the glow-wire requirement of GWFI 850°C and GWIT 775°C under IEC 60335-1. The intumescent route spans 21 wt% for the PAPP:MPP blend up to 30 wt% for formulated APP grades, while the brominated route uses DBDPE or EBTBP with antimony trioxide at a loading set from the supplier's technical data sheet rather than a fixed PP figure.

PP wire and cable compounds (HFFR)#

Halogen-free PP wire and cable compounds use coated magnesium hydroxide at loadings around 185.7 phr, which gave an LOI of 30.2 in Huber's HFFR PP formulation, because only mineral systems meet the low-smoke and low-acidity cable classes. High filling levels of this kind rely on coated grades to preserve dispersion, and huntite-hydromagnesite serves the same low-smoke halogen-free cable compounds without a published numeric loading for PP. EBTBP remains an option for polyolefin wire and cable where some halogen content is acceptable in the specification. Flame retardants for wire and cable compares these HFFR routes across PVC, PE and EVA cable systems as well.

Automotive PP and FMVSS 302#

Automotive PP interior parts must burn no faster than 102 mm/min in the horizontal FMVSS 302 test, a requirement set by 49 CFR 571.302 and mirrored by ISO 3795 and GB 8410. No PP-specific flame-retardant loading for this test is published in current literature, so automotive formulators size the system from the part specification rather than a standard reference loading. Rail and aviation requirements are on flame retardants for transportation, which covers FMVSS 302 automotive rules alongside EN 45545-2 rail standards.

PP fibres, films and nonwovens#

PP fibres, films and nonwovens rarely carry the 20-65 wt% loadings of intumescent or mineral systems, so they rely on low-loading, melt-blendable additives such as FR-370, which also resists UV light. FR-370 reaches the UL 94 V-2 class in PP fibre applications while retaining the fibre-drawing characteristics that higher-loading intumescent or mineral systems tend to disrupt. DecaBDE was historically used as a flame retardant in PP textiles before its restriction; this is noted here only as historical context, not as a current option. Spin-finish and fibre additives are covered under additives for synthetic fibres.

Recycled and filled PP#

Recycled PP can reach UL 94 V-0 with the same 21 wt% PAPP:MPP (2:1) intumescent system as virgin PP, but recyclate from electronics must also be checked for legacy PBDEs, which the EU limits to 350 mg/kg in recovered material from 30 December 2025. Filled PP compounds respond differently: a study in Materials (2024, doi 10.3390/ma17184553) found that adding 10 wt% zinc borate alongside 10 wt% magnesium hydroxide to a PP compound already containing 50 wt% calcium carbonate raised the limiting oxygen index to 29.4%. Restabilization and further additive needs of recyclate are covered under additives for recycled plastics.

How Much Flame Retardant Does Polypropylene Need?#

Polypropylene needs about 21-30 wt% of an intumescent flame retardant or up to 65 wt% of magnesium hydroxide for UL 94 V-0 or low-smoke cable ratings, while brominated V-2 grades are dosed from the supplier technical data sheet rather than a fixed PP figure.

Four factors set where a formulation lands in this range.

• Required rating determines the floor: V-2 tolerates lower loadings than V-0 because burning drips are allowed to remove heat.
• Wall thickness changes the loading needed for the same rating, since thinner sections have less material available to char or absorb heat.
• Flame-retardant class matters directly, since brominated systems act in the gas phase at far lower loadings than the condensed-phase char or endothermic mechanisms of intumescent and mineral systems.
• Fillers and glass fibre raise the loading required, because glass fibre reinforcement produces a candle-wick effect that carries the flame along exposed fibre ends.

Across plastic products generally, flame retardants range from 2 to 28 wt% of the formulation, according to Hahladakis et al. (2018), cited via Chea et al. (2025); PP's 21-65 wt% span for the ratings covered on this page sits at the upper half of that industry-wide range because PP's lack of char makes it a harder polymer to protect than average. Converters that do not compound often add the system as a flame retardant masterbatch rather than dosing raw powder into the extruder.

Converting phr to weight percent for filled PP#

Mineral flame retardants are often given in phr (parts per hundred parts of polymer), and the weight percent follows from wt% = phr of the additive divided by total phr, multiplied by 100, so 185.7 phr magnesium hydroxide in 100 phr PP equals 65.0 wt%. This two-component calculation illustrates the formula: 185.7 divided by 285.7 (100 phr PP plus 185.7 phr MDH), multiplied by 100, equals 65.0 wt%. The Huber reference formulation contains further components beyond PP and MDH, so this worked example serves as a calculation illustration rather than the exact composition of that product. Check the conversion in the PHR to weight percent calculator for formulations with more than two components.

How Do Synergists and Other Additives Interact with Flame Retardants in Polypropylene?#

Flame retardants in PP interact with 2 groups of additives: synergists such as zinc oxide, zinc borate and antimony trioxide multiply their effect, while antioxidants, basic HALS and amine antistats can lose activity or react with them.

Flame retardant synergists in PP: metal oxides, zinc borate and antimony trioxide#

Small amounts of metal oxides are the strongest synergists for intumescent PP: 0.25 wt% zinc oxide or manganese oxide lifts an APP-pentaerythritol system to an LOI of 30% and UL 94 V-0, and 1.5 wt% zinc oxide raises the LOI to 43.7%. These figures come from a 2025 study published in Polymers (doi 10.3390/polym17202734) that tested metal oxide addition to an APP-pentaerythritol intumescent PP system.

Stannates and anti-drip agents are covered under flame retardant synergists, which extends this synergist table across every polymer that uses brominated or intumescent chemistry. Zinc borate suppliers self-classify the substance as Repr. 2 (H361d); its harmonised CLP status has not been confirmed against the ECHA classification and labelling inventory.

Antagonisms: additive interactions with antioxidants, HALS and antistats#

3 additive interactions weaken flame-retardant PP: flame retardants reduce the effectiveness of antioxidants, acidic by-products of halogenated flame retardants deactivate basic HALS, and amine or amide antistats react with halogenated flame retardants.

• Flame retardants reduce antioxidant effectiveness in the finished compound, which is why FR-PP formulations typically need a stronger antioxidant package than unmodified PP.
• Acidic species released by halogenated flame retardants deactivate basic HALS; NOR HALS, with a pKb around 8 to 10, tolerate this acidic environment where standard HALS lose activity.
• Amine and amide antistats react with acidic additives and with halogenated flame retardants, which limits the antistat choices available in halogen-containing FR-PP formulations.

Glass fibre reinforcement compounds this challenge through the candle-wick effect, in which exposed fibre ends at the surface of a burning part carry the flame along the fibre rather than allowing it to self-extinguish. Synergy and antagonism across every additive family are mapped under additive interactions, which explains the mechanism behind each of the 3 interactions listed above.

How Is Flame-Retardant Polypropylene Compounded?#

Flame-retardant polypropylene is compounded by melt-mixing the flame retardant into PP on an extruder or by letting down a masterbatch, and the decomposition temperature of the flame retardant sets the ceiling for every processing step.

Compounding temperature limits for flame retardants in PP#

Each flame retardant sets its own compounding ceiling in PP: aluminum trihydrate and expandable graphite react from about 200°C, APP-based intumescent systems are processed up to about 220°C, and magnesium hydroxide stays stable to about 320°C.

Formulators sequence these limits against PP's own processing window: extrusion and injection molding of unmodified PP typically run at temperatures compatible with all but the lowest-onset minerals, so ATH and expandable graphite need careful profile control while APP, FR-370 and MDH tolerate standard PP processing conditions with more margin.

Blooming and dispersion of flame retardants in PP#

Blooming, the migration of a flame retardant to the part surface, is the main appearance defect of FR polypropylene, and melt-blendable grades such as FR-370 and non-blooming EBTBP were developed to prevent it. Surface treatments, including vinyl-silane, amino-silane and fatty acid coatings, restore elongation at break in compounds carrying high mineral loadings, where the filler level would otherwise embrittle the part. Coated magnesium hydroxide grades depend on these same surface treatments to achieve the high filling levels, up to 65 wt%, that PP compounds require for adequate flame retardancy. Solubility-based causes of blooming are explained for every additive family, not only flame retardants.

How Is the Flammability of Polypropylene Tested?#

The flammability of polypropylene is tested with 4 laboratory methods: the UL 94 burning test for classification, the limiting oxygen index for screening, the cone calorimeter for heat release and the glow-wire test for electrical parts.

UL 94 vertical and horizontal burning tests#

UL 94 V-0 is the strictest small-flame class for PP: after two 10-second flame applications, no specimen may burn longer than 10 seconds, the 5 specimens may not exceed 50 seconds in total, and no drip may ignite the cotton below. V-1 relaxes these limits to an afterflame of 30 seconds per application, a total of 250 seconds, and afterflame plus afterglow after the second application capped at 60 seconds, still with no cotton ignition allowed. V-2 uses the same time limits as V-1 but permits flaming drips to ignite the cotton, which is the classification that most thin-wall brominated PP formulations target. Higher-energy classes, 5VA and 5VB, apply a 500 W flame to a standard 125 x 13 mm specimen with a 20 mm flame in two 10-second applications, a more severe test reserved for parts with stricter fire-safety requirements. Glass fibre reinforcement raises the risk of failing any of these classes through the candle-wick effect described earlier in the synergists section.

Limiting oxygen index (LOI)#

The limiting oxygen index of polypropylene rises from about 17.5% for neat PP to about 30% with an intumescent or magnesium hydroxide system, and to 43.7% with 1.5 wt% zinc oxide in an intumescent formulation. LOI is measured under ISO 4589-2 or ASTM D2863-23e1 as the minimum oxygen concentration, by volume, in a flowing oxygen and nitrogen mixture that just sustains candle-like flaming combustion of the test specimen. The index functions as a fast laboratory screen for formulation development, not as a substitute for UL 94 classification, and this page does not state an LOI threshold at which a PP compound becomes self-extinguishing, because no such threshold is established in current literature.

Cone calorimeter: heat release of FR polypropylene#

The cone calorimeter shows the effect of flame retardants on polypropylene most clearly: at 35 kW/m2, neat PP plaques reach a peak heat release rate of 1148-1332 kW/m2, while intumescent PP stays at 220-255 kW/m2. The test works on the oxygen-consumption principle established by W. J. Huggett, using a conversion factor of about 13.1 MJ released per kilogram of oxygen consumed, and the instrument itself was developed by Vytenis Babrauskas and colleagues at the US National Bureau of Standards (now NIST) in 1982. B. Schartel and T. R. Hull explained how to interpret the resulting data in "Development of fire-retarded materials: Interpretation of cone calorimeter data" (Fire and Materials 31, 2007, pages 327-354). Replacing 1 wt% of an intumescent system with 1 wt% layered cerium oxide cut total heat release by 38.9% and total smoke release by 74.3% in one tested formulation. How to read pHRR and THR curves from this test is covered under cone calorimeter testing.

Glow-wire tests (GWFI, GWIT)#

Glow-wire tests press a wire heated to between 550 and 960°C onto a PP specimen for 30 seconds, and household appliances under IEC 60335-1 require a glow-wire flammability index of at least 850°C and a glow-wire ignition temperature of at least 775°C. Clause 30.2.3 of IEC 60335-1 applies this requirement to unattended appliances with connections carrying more than 0.2 A, and allows an alternative end-product test at 750°C with flaming limited to 2 seconds as an alternative route to compliance. IEC 60695-2-13:2021 governs the current GWIT test method. No GWFI or GWIT values specific to flame-retardant PP are established in current literature; figures from other polymers such as glass-filled PA66 do not transfer to PP formulations. Specimen preparation and the glow-wire end-point test (GWEPT) are explained on the glow wire test page.

Which Regulations Apply to Flame Retardants in Polypropylene?#

Flame retardants in polypropylene fall under 4 regulatory regimes: REACH (SVHC listing and the planned restriction of aromatic brominated flame retardants), the POPs rules for legacy brominated and chlorinated substances, CLP hazard classification, and product rules such as Ecodesign, RoHS and US TSCA.

Furniture and consumer-product rules are on flame retardant regulations, which covers bans and flammability standards for furniture and consumer goods outside the polymer-specific scope of this page.

DBDPE, SVHC status and the EU restriction of aromatic brominated flame retardants#

DBDPE, the main brominated flame retardant for V-0 polypropylene, has been a Substance of Very High Concern since 5 November 2025 and is one of 3 mandated substances in the EU restriction of non-polymeric aromatic brominated flame retardants now being prepared by ECHA. DBDPE is registered under REACH in the 10,000 to 100,000 tonnes per year band. Scope and the timeline are tracked under EU restriction of aromatic brominated flame retardants: ECHA published its aromatic brominated flame retardant investigation report on 18 December 2024, identifying about 60 aromatic brominated flame retardants potentially on the EU market with 25 already registered; the European Commission issued its restriction mandate to ECHA on 11 November 2025; a call for evidence ran from 21 January to 18 March 2026; and a draft Annex XV restriction dossier is planned for December 2026. The restriction, as currently scoped, covers non-polymeric aromatic brominated flame retardants, with DBDPE, TBPH and BTBPE mandated as the core substances and 19 further substances under PBT or vPvB assessment in a broader option covering up to 24 substances; polymeric brominated flame retardants sit outside this scope. DBDPE joined the SVHC Candidate List in the November 2025 update. TBBPA-DBPE may fall within this wider screening group, according to our source library; EBTBP's membership of the group is a possible candidate status only and is not confirmed. No part of this restriction has entered into force, and none of these substances is currently banned.

POPs limits: decaBDE and Dechlorane Plus in new and recycled PP#

New polypropylene may contain no more than 10 mg/kg of polybrominated diphenyl ethers such as decaBDE in the EU, and PP recycled from older products may contain up to 350 mg/kg from 30 December 2025 and 200 mg/kg from 30 December 2027, under Delegated Regulation (EU) 2025/1482. DecaBDE was added to the Stockholm Convention's Annex A at the eighth Conference of the Parties in 2017, and the EU implements the resulting restriction through Regulation (EU) 2019/1021 on persistent organic pollutants, with the 2025 delegated regulation published in the Official Journal on 28 October 2025 setting the general unintentional trace contaminant limit at 10 mg/kg for the sum of tetra- through decaBDE congeners in mixtures and articles. A further concession applies to toys and childcare articles made from recovered material: 10 mg/kg from 17 May 2027. Dechlorane Plus, a chlorinated flame retardant historically used in nylon, wire and cable and PP, followed the same restriction path: listed on Stockholm Annex A at the eleventh Conference of the Parties in 2023, and limited in the EU under Delegated Regulation (EU) 2025/1930, in force since 15 October 2025, at 1,000 mg/kg until 15 April 2028 and 1 mg/kg thereafter. Every listed substance is tracked under POPs in plastics. DecaBDE remains a legacy concern in recyclate rather than a substance used in new formulations.

Antimony trioxide: classification and supply#

Antimony trioxide, the standard synergist for brominated PP, carries a harmonised Carc. 2 (H351) classification in the EU and an IARC Group 2A rating (2022), but it is not a Substance of Very High Concern. The substance also carries a California Proposition 65 cancer listing dating from 1 October 1990, and the EU sets a specific migration limit of 0.04 mg/kg as antimony under FCM entry 398 of Regulation (EU) No 10/2011. Flame retardants accounted for 39% of US antimony end use in 2024 data reported by the US Geological Survey, and USGS reported antimony metal prices of USD 25 per pound in 2025, peaking at USD 27.50 in June 2025 and falling to USD 20.30 in November 2025; these are USGS metal prices, not antimony trioxide product prices. About 66% of US antimony oxide imports came from China between 2021 and 2024. Zinc borate, zinc stannate and zinc hydroxystannate serve as the main substitutes where antimony trioxide's classification or supply concentration is a formulation concern.

Ecodesign, RoHS and US rules#

Halogenated flame retardants may not be used in the enclosures and stands of electronic displays sold in the EU since 1 March 2021, under Ecodesign Regulation (EU) 2019/2021, a restriction that the EU General Court has upheld. RoHS limits polybrominated diphenyl ethers to 0.1% by weight in each homogeneous material of electrical and electronic equipment; a proposed 2024 addition of TBBPA and medium-chain chlorinated paraffins to RoHS Annex II was dropped in 2024 rather than adopted. In the United States, a revised TSCA rule covering decaBDE and the PIP (3:1) flame retardant was published on 19 November 2024 and took effect on 21 January 2025, setting a 0.1 wt% unintentional-presence threshold for decaBDE. New York State separately banned organohalogen flame retardants in the enclosures and stands of electronic displays, effective 1 January 2024.

Who Supplies Flame Retardants for Polypropylene?#

Flame retardants for polypropylene are made by named producers, grouped by chemistry: Clariant (Exolit AP intumescent grades), Adeka (piperazine pyrophosphate blends), BASF (Melapur MPP), ICL (FR-370), Albemarle (Saytex DBDPE and EBTBP), Huber and Kyowa (magnesium hydroxide), LKAB (huntite-hydromagnesite) and U.S. Borax (zinc borate).

Buyers should compare FR systems by CAS number, loading for the target rating and SVHC status, not by trade name. Clariant announced an expansion of Exolit OP production at Daya Bay on 5 June 2026, and Huber Advanced Materials completed its acquisition of Martinswerk from Albemarle on 1 February 2016, adding mineral flame-retardant capacity to its portfolio. Plants and certifications by company are in the directory of flame retardant manufacturers and suppliers; the global flame-retardant market was valued at USD 8.1 to 9.3 billion in 2025 across analyst estimates.

Request quotes for flame retardants for polypropylene: send CAS or grade, target UL 94 rating and thickness, volume, and country to the plastic additive supplier finder.

What Other Additives Does Flame-Retardant Polypropylene Need?#

Flame-retardant polypropylene still needs the full PP additive package, in which the flame retardant is one layer next to antioxidants, an acid scavenger and, depending on the part, UV stabilizers, fillers or impact modifiers. A complete PP formulation typically combines a primary phenolic antioxidant with a phosphite or thioester secondary antioxidant, an acid scavenger, HALS and a UV absorber for outdoor parts, nucleating or clarifying agents, antistats, slip and antiblock additives for film, fillers such as talc or calcium carbonate at 20-40 wt%, and impact modifiers such as EPR or POE, with the flame retardant added on top of this base package. The full PP package is on additives for polypropylene, which sets out every layer of this formulation in detail.

Antioxidants and UV stabilizers for flame-retardant polypropylene#

Flame-retardant PP needs a stronger antioxidant package than standard PP, because flame retardants reduce antioxidant effectiveness, and outdoor grades with halogenated flame retardants need NOR HALS instead of basic HALS. Standard polyolefin antioxidant packages combine a phenolic primary antioxidant such as Irganox 1010 at 0.05-0.4 wt% with a phosphite secondary antioxidant such as Irgafos 168 at 0.05-0.2 wt%, and flame-retardant formulations typically sit toward the upper end of these ranges to compensate for the flame retardant's interference. Dosage by grade is on antioxidants for polypropylene. Outdoor FR-PP parts, such as automotive underhood components or exterior housings, need UV stabilizer selection that accounts for the acidic environment created by any halogenated flame retardant present; HALS choice for outdoor FR-PP is covered under UV stabilizers for polypropylene.

Flame retardants for polyethylene and other polymers compared#

Polyethylene, like polypropylene, is not flame retardant by itself, but its lower processing temperature lets it use aluminum trihydrate at 160-180 phr in halogen-free cable compounds, which PP cannot. PE and EVA cable compounds also use silane crosslinking alongside ATH or MDH, and expandable graphite in profile extrusion, options that overlap with PP's mineral and physical-intumescent routes but differ in loading because of PE's lower processing temperature. Polyethylene, in common with polypropylene, has a low limiting oxygen index and is not flame retardant without additives. HFFR PE and EVA systems are on flame retardants for polyethylene.

What is flame-retardant PP (PP-FR) made of?#

Flame-retardant PP (PP-FR) is polypropylene compounded with a flame-retardant system, typically 21-30 wt% of an intumescent phosphorus-nitrogen additive, up to 65 wt% of magnesium hydroxide, or a brominated additive with antimony trioxide. FR-PP sheet and other finished grades are compared on flame-retardant plastics, which covers material-level products rather than the additive systems detailed on this page.

Is there a natural flame retardant for polypropylene?#

The closest to a natural flame retardant for polypropylene is ground natural brucite, a magnesium hydroxide mineral (CAS 1317-43-7) that is cheaper and coarser than synthetic MDH, together with the mineral blend huntite-hydromagnesite. Both minerals are used in cable and PP compounds in the same endothermic role as synthetic magnesium hydroxide, though natural brucite's coarser particle size and lower purity generally limit it to less demanding applications than coated synthetic grades.

Can a flame-retardant spray be used on polypropylene?#

Flame retardants for polypropylene are compounded into the melt before moulding; sprays and surface coatings are coating products outside the scope of this page. This page covers additives compounded into the polymer during processing, not post-application surface treatments, which fall under coatings rather than plastic additives.

Are flame retardants in polypropylene toxic?#

Toxicity depends on the substance: magnesium hydroxide and ammonium polyphosphate carry no harmonised hazard classification, antimony trioxide is classified as a suspected carcinogen (Carc. 2), and DBDPE is regulated as very persistent and very bioaccumulative. The ENFIRO project rated ammonium polyphosphate, aluminum trihydrate, magnesium hydroxide and melamine polyphosphate among the flame retardants with favourable environmental and health profiles when it compared alternatives to halogenated systems in 2012. Exposure studies are summarised under flame retardants and human health.

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