Mineral Flame Retardants (ATH and MDH): 4 Types, How They Work, Loading and Selection

Mineral flame retardants are metal hydroxides and hydroxycarbonates, mainly aluminum trihydrate (ATH) and magnesium hydroxide (MDH), that absorb heat and release water when a plastic heats up, starting at about 200 °C for ATH and about 320 °C for MDH. Because they work physically rather than chemically, they have to be added at about 60 wt% of the compound; so which mineral suits which plastic, and how is such a highly filled compound made to pass UL 94 or a cable fire class?

Flame retardants make up about 13 % by weight of all plastic additives, and the mineral group, aluminium hydroxide, magnesium hydroxide, boehmite and huntite-hydromagnesite, is the halogen-free option that also suppresses smoke.

This guide covers the physical mechanism behind the 3 modes of action, the 4 mineral types and how they differ, the ATH vs MDH selection rule by processing temperature, the loading levels in phr and wt% for cable, PVC, polypropylene and thermoset compounds, the effect of surface treatment and particle size, the synergists that reduce the loading burden, the test methods that verify performance, the REACH, CLP and EU food-contact status of each mineral, and the manufacturers that supply them.

Key figures

• About 200 °C ATH water-release onset vs about 320 °C MDH stability
• 1051 J/g heat absorbed by ATH vs 1316 J/g by MDH
• 160-180 phr (61.5 wt% at 160 phr) typical loading in HFFR cable compounds
• 4 mineral flame retardant types used in plastics

What Are Mineral Flame Retardants?#

Mineral flame retardants are inorganic additive flame retardants, metal hydroxides and hydroxycarbonate minerals such as ATH, MDH, boehmite and huntite-hydromagnesite, that contain no halogen and act as functional fillers at loadings of about 60 wt%. They belong to the mineral or inorganic branch of the flame retardant family tree, alongside the synthetic-versus-natural split within MDH itself (synthetic, precipitated grades and natural ground brucite). Because they act as additive flame retardants, they are physically blended into the polymer melt rather than chemically bound to the polymer chain, the same incorporation route used for other functional fillers chosen for a specific property.

The hub on flame retardants for plastics places the mineral group next to the brominated, phosphorus and intumescent classes, the other 3 chemistries that reach the same fire ratings by different mechanisms. Cable compounders most often meet mineral flame retardants under their processing name: low smoke zero halogen (LSZH, also written LSF, LS0H, LSOH or LSFH) and halogen-free flame-retardant (HFFR, also ZHFR, NHFR, OHLS or HFT) compounds.

Mineral flame retardants share 3 traits that set them apart from other flame retardant classes.

• Additive incorporation: the mineral is dispersed through the polymer melt during compounding rather than reacted into the polymer backbone, so it can be removed or replaced without changing the base resin.
• Halogen-free composition: none of the 4 types contains bromine, chlorine or antimony, which keeps the smoke free of the acidic and corrosive gases that halogenated systems can release.
• Filler-level loading: because the mechanism is physical, not chemical, mineral flame retardants need about 60 wt% of the compound, an order of magnitude above the 1-5 wt% typical of an additive that works by chemical action.

How Do Mineral Flame Retardants Work?#

Mineral flame retardants work in 3 physical ways: their endothermic decomposition cools the polymer, the water or CO2 they release dilutes the flammable gases, and their oxide residue shields the surface and adsorbs soot. Gas-phase radical trapping and charring, the other modes of how flame retardants work, are covered separately, because mineral flame retardants use none of them.

Endothermic decomposition: cooling the polymer#

ATH absorbs 1051 J/g (about 280 cal/g) as it decomposes to alumina and water from about 200 °C, and MDH absorbs 1316 J/g (about 328 cal/g) as it decomposes to magnesium oxide and water at about 330 °C. Both reactions are endothermic, so the mineral pulls heat out of the burning surface instead of feeding it back into the polymer, and the reaction only proceeds once the compound reaches the mineral's onset temperature.

The two reactions follow simple stoichiometry:

2 Al(OH)3 -> Al2O3 + 3 H2O   (ATH, onset about 200 °C, 1051 J/g)
Mg(OH)2 -> MgO + H2O          (MDH, about 330 °C, 1316 J/g)

MDH absorbs about 25 % more heat per gram than ATH (1316 vs 1051 J/g, roughly 328 vs 280 cal/g), and it does so at a higher temperature, which is why the choice of mineral tracks the polymer's own processing and service temperature rather than the fire rating alone. Huntite-hydromagnesite releases its heat in 2 stages instead of one: water leaves from about 220 °C and CO2 from about 330 °C, spreading the cooling effect across a wider temperature band than either single-stage hydroxide.

Water and CO2 release: diluting the flame#

The water vapour and CO2 released by mineral flame retardants dilute the flammable decomposition gases and the oxygen above the plastic: ATH gives off 34.6 % of its mass as water and MDH 31.0 %. This dilution lowers the fuel concentration and the local oxygen partial pressure at the flame front, starving the combustion reaction of both ingredients it needs, and it works independently of the endothermic cooling described above.

Huntite-hydromagnesite gives up the largest share of its mass as gas because it carries both a hydroxide and a carbonate structure in one mineral blend, which is also why LKAB Minerals markets UltraCarb specifically on its combined water and CO2 dilution effect.

Oxide residue: barrier and smoke suppression#

After decomposition, ATH leaves alumina and MDH leaves magnesium oxide on the burning surface, and this oxide residue insulates the polymer beneath and adsorbs soot, which makes mineral flame retardants smoke suppressants as well. Huntite-hydromagnesite goes a step further and forms a cement-like char at about 560 °C, a third barrier stage that neither single-stage hydroxide reaches on its own.

The soot-adsorbing property of the oxide layer is why mineral-filled cable compounds pass the smoke-density classes of the cable fire test described later on this page. Molybdates and stannates, the other smoke suppressants, work by char chemistry rather than by an oxide residue, so they are usually paired with a different base flame retardant, most often halogenated systems in PVC, rather than with ATH or MDH.

What Are the 4 Types of Mineral Flame Retardants?#

The 4 types of mineral flame retardants used in plastics are aluminum trihydrate (ATH), magnesium hydroxide (MDH), boehmite and huntite-hydromagnesite, with ATH and MDH covering most applications. All 4 are inorganic minerals that release water, or water and CO2, as they decompose, and they are ordered here, as throughout this page, by market importance: ATH first, then MDH, then the 2 smaller-volume minerals used for specific temperature or performance windows.

Table T1. The 4 mineral flame retardants

Onset values from Huber (ATH, MDH) and LKAB (HMH) technical literature. Boehmite values are qualitative until verified.

1. Aluminum trihydrate (ATH)#

Aluminum trihydrate (ATH, alumina trihydrate, Al(OH)3, CAS 21645-51-2) is the mineral flame retardant for plastics processed below about 200 °C, such as EVA and PE cable compounds, flexible PVC and thermoset composites. Identified by its EC number 244-492-7 and a molecular weight of 78.00 g/mol, it also carries the synonyms aluminium hydroxide and gibbsite for its natural mineral form. It appears as a white, free-flowing powder of hexagonal platelets, with a density of 2.42 g/cm3, Mohs hardness of 2.5-3.5, a refractive index of 1.57, a pH of 9-10 in aqueous suspension, and particle sizes spanning 0.25-80 µm across a single supplier's range, a spread wide enough to cover both fine, high-surface-area grades and coarse, low-cost fillers.

ATH's processing ceiling sits at about 200 °C, so melt temperatures should only briefly exceed that point, which is why it dominates low-smoke zero halogen (LSZH) and halogen-free flame-retardant (HFFR) cable insulation and sheathing, PVC cable and flooring, unsaturated polyester and epoxy composites, solid surface material, and other building and construction plastics that process below its decomposition onset. Grades, particle sizes and the full regulatory matrix sit on the aluminum trihydrate (ATH) page.

2. Magnesium hydroxide (MDH): synthetic grades and natural brucite#

Magnesium hydroxide (MDH, Mg(OH)2, CAS 1309-42-8) is the mineral flame retardant for plastics processed above 200 °C, because it stays stable to about 320 °C, about 110 °C higher than ATH. It carries EC number 215-170-3, a molecular weight of 58.32 g/mol, a density of 2.36 g/cm3, Mohs hardness of 2-3, a refractive index of 1.58 and a pH of 10-11, and its higher heat absorption (1316 vs 1051 J/g for ATH) combines with that processing headroom to make it the default hydroxide for polypropylene, polyamide compounds, TPO roofing membranes, rigid PVC and thermosets.

MDH comes in 2 forms: synthetic MDH, precipitated from seawater or brine, and natural brucite (CAS 1317-43-7, EC 215-274-9), ground from mined rock. Synthetic grades give finer, more uniform particles and higher purity, while natural brucite is coarser and lower purity, used where cost matters more than particle-size control. Coated MDH grades reach up to 65 wt% in polypropylene without losing processability. Synthetic and natural grades are compared on the magnesium hydroxide (MDH) flame retardant page.

3. Boehmite (AlOOH)#

Boehmite (aluminium oxyhydroxide, AlO(OH), CAS 1318-23-6) is the higher-temperature aluminium mineral flame retardant, used where ATH would decompose too early, for example in epoxy PCB laminates and engineering thermoplastics. It carries EC number 215-284-3, a molecular weight of 59.99 g/mol, a mineral specific gravity of 3.00-3.07 and a Mohs hardness of 3-3.5, and it dehydrates endothermically at a higher temperature than ATH, leaving an alumina residue behind on the burning surface. Its exact decomposition temperature and water content are not established in our source library and are left out of Table T1 rather than estimated.

Nabaltec supplies boehmite for plastics under its Apyral AOH and Actilox lines. Identity and grade data are on the boehmite page.

4. Huntite-hydromagnesite (HMH)#

Huntite-hydromagnesite is a natural blend of two carbonate minerals that releases water from about 220 °C and CO2 from about 330 °C, and then forms a cement-like char at about 560 °C. The blend combines huntite, Mg3Ca(CO3)4 (CAS 19569-21-2), with hydromagnesite, Mg5(CO3)4(OH)2 x 4H2O (CAS 12072-90-1); huntite alone decomposes across a broader 450-800 °C range. LKAB Minerals markets the mineral as UltraCarb, with a composition of 36-39 % MgO, 6-9 % CaO and 0.2-1 % SiO2 and an LOI of 51-54 % at 1,000 °C from its combined water and CO2 loss.

Huntite-hydromagnesite goes into low-smoke halogen-free cable compounds (EVA, PE, PP) and PVC. LKAB states that the mineral needs lower loadings than ATH or MDH in some systems, although no numeric loading figure has been published to verify that claim. Deposits, grades and regulatory identifiers of huntite hydromagnesite are on its own page.

ATH vs MDH: Which Mineral Flame Retardant Works Better?#

ATH works better in plastics processed below about 200 °C, such as EVA, PE and flexible PVC cable compounds, while MDH works better in plastics processed above that point, such as polypropylene and polyamides, because MDH stays stable to about 320 °C and absorbs more heat (1316 vs 1051 J/g). The processing temperature of the polymer decides between them; the fire rating decides the loading. Between them, boehmite covers the aluminium-mineral gap above ATH's ceiling, and huntite-hydromagnesite offers a staged, lower-temperature-onset alternative with an added carbonate dilution effect that neither single-stage hydroxide provides.

Table T2. ATH vs MDH

Supplier technical data (Huber); final loading depends on the fire class and part thickness.

Boehmite and huntite-hydromagnesite fill the gaps that ATH and MDH leave open. Boehmite dehydrates at a higher temperature than ATH, so it survives the cure and lamination temperatures of epoxy PCB boards where ATH would decompose too early, while remaining an aluminium mineral chemically related to ATH. Huntite-hydromagnesite, by contrast, starts releasing water even earlier than ATH, at about 220 °C, then adds a second CO2-release stage from about 330 °C, spreading dilution and cooling across a wider window than a single hydroxide reaction can. The interactive flame retardant selector filters mineral, phosphorus and brominated systems by polymer and UL 94 rating, for compounders comparing all 4 minerals against non-mineral alternatives in one pass.

How Much Mineral Flame Retardant Does a Plastic Need?#

A plastic needs about 60 wt% of mineral flame retardant to pass common fire tests, which is 160-180 phr of ATH or MDH in an EVA/LLDPE cable compound and up to 65 wt% of MDH in polypropylene. The exact figure is set by the target fire class, the part thickness and whether the grade is surface treated for higher filling.

Table T3. Master loading table

Loadings are supplier reference formulations and single studies; trials decide the final level. phr = parts per hundred parts of polymer.

Why mineral flame retardants need loadings of about 60 wt%#

Mineral flame retardants need about 60 wt% because they act only physically, by absorbing heat and diluting gases, so the effect grows with the amount of mineral rather than with a chemical chain reaction. An unfilled EVA/LLDPE compound shows a peak heat release rate above 550 kW/m2, and ATH or MDH delays ignition by 120-160 s while cutting that peak roughly in proportion to the loading added, which is why compounders push the loading as high as the process allows rather than adding a fixed small percentage.

High mineral loadings have 3 consequences for the compound.

• Density rises in step with the loading, since ATH and MDH (2.42 and 2.36 g/cm3) are both denser than the base polymer.
• Elongation at break drops at these loadings unless the mineral carries a surface treatment that restores polymer-filler adhesion.
• Melt viscosity rises, which pushes processing equipment and screw design toward higher torque and, in some cases, lower line speed.

For comparison, an intumescent flame retardant system in polypropylene can reach UL 94 V-0 at 22-30 wt%, roughly half the mineral loading, because it builds a protective char instead of relying on dilution and cooling alone.

Converting phr to wt% for highly filled compounds#

A loading in phr converts to wt% by dividing the mineral phr by the total phr of the recipe, so 160 phr of ATH in 100 phr of polymer equals 61.5 wt% of the compound. The formula is:

wt% = phr_mineral / total phr x 100

Calculation example: 180 phr of ATH in 100 phr of polymer gives 180 / 280 x 100 = 64.3 wt% of the compound.

In the Huber reference recipe, the total formulation also contains coupling agents and antioxidants in addition to polymer and mineral, so the real wt% of ATH is slightly lower than a two-component calculation using only polymer and mineral would suggest. Recipes with plasticizer, coupling agents and antioxidants follow the same PHR (parts per hundred resin) arithmetic, adding each ingredient's phr to the denominator before converting.

Which Mineral Flame Retardant Suits Each Polymer and Application?#

The right mineral flame retardant follows the polymer's processing temperature: ATH for cable compounds, flexible PVC and thermosets processed below about 200 °C, MDH for polypropylene and polyamides, and boehmite where an aluminium mineral must survive higher temperatures.

Wire and cable: LSZH and HFFR compounds (EVA, PE, XLPE)#

Low smoke zero halogen (LSZH) and halogen-free flame-retardant (HFFR) cable compounds are EVA or PE filled with 160-180 phr of ATH or MDH, which is the usual way to meet the smoke and acid-gas classes of the EU cable fire classification. Huber's HFFR reference formulation runs 67 phr EVA, 17 phr LLDPE, 16 phr of coupling agents (8 phr Fusabond 226D plus 8 phr Lotader 3210), 160 phr ATH and 1.0 phr of antioxidants (0.75 phr Ethanox 310 plus 0.25 phr Ethaphos 368). Silane-crosslinked PE (XLPE) cable uses 180 phr of vinyl-silane coated ATH in the equivalent Huber example.

EVA grades used in cable compounds carry 12-70 % vinyl acetate content, and a higher VA content raises the LOI of the filled compound, giving formulators a second lever alongside mineral loading. Brominated and intumescent cable systems are compared on flame retardants for wire and cable.

In a 2025 study titled "Fire Retardancy of Polyethylene-EVA Cable Sheaths", Alsayed D.A. and co-authors (Polymers 17, 2679) can report that adding 10 wt% of a phosphate low-melting glass to a PE-EVA/ATH sheath cut the peak heat release rate to 142 kW/m2, a 52 % reduction versus the ATH-only formulation. Crosslinking, antioxidant and metal-deactivator packages that complete an EVA or PE cable formulation are on additives for wire and cable compounds.

Polypropylene and TPO#

Polypropylene uses MDH rather than ATH because PP is processed above ATH's 200 °C onset, and coated MDH at 185.7 phr lifts the oxygen index of PP from about 17.5 to 30.2 vol % O2. Coated MDH grades reach up to 65 wt% loading in PP without losing processability, and the same mineral serves TPO roofing membranes, which share PP's processing temperature range. In PP filled with calcium carbonate, a combined 10 wt% MDH plus 10 wt% zinc borate on top of 50 wt% CaCO3 raises the LOI to 29.4 vol % O2, showing that MDH still contributes flame retardancy even in an already heavily filled system.

Intumescent APP-based systems reach UL 94 V-0 in PP at only 22-30 wt%, so compounders choose MDH specifically where a halogen-free, low-smoke profile outweighs the higher loading it demands. Intumescent and brominated alternatives are compared on flame retardants for polypropylene.

Flexible PVC cable and flooring#

Flexible PVC uses ATH at 45-100 phr, often with 3-6 phr of zinc borate, because plasticizers such as DIDP dilute the chlorine that makes rigid PVC self-extinguishing. In Huber's reference cable insulation (PVC K70 100 phr, DIDP 55 phr, lead-free stabilizer 2.7 phr, chalk 10 phr), 45, 50 or 100 phr of ATH, with 5 phr of zinc borate in the highest-performing formulation, reaches an LOI of 26-27 vol % O2 and a UL 94 V-0 rating at 3 mm thickness, against a target LOI of at least 26 % for PVC cable insulation. Zinc borate at 3-6 phr next to ATH also reduces smoke and dripping during combustion, and even at 150 phr of ATH, flexible PVC still smokes more than the halogen-free HFFR reference compound.

Antimony, stannate and molybdate systems for PVC are covered under flame retardants and smoke suppressants for PVC.

Thermosets: unsaturated polyester, epoxy and PCB laminates#

Thermosets such as unsaturated polyester, epoxy and FR-4 PCB laminates use ATH, or boehmite where the laminate sees higher temperatures. Unsaturated polyester composites for transport parts carry ATH at high loadings, usually alongside ammonium polyphosphate, zinc borate or stannate synergists, and solid surface material shares the same ATH-filled approach. FR-4 epoxy laminates combine ATH or boehmite fillers with reactive TBBPA or DOPO flame retardant chemistry rather than relying on the mineral alone.

Reactive TBBPA and DOPO systems are covered under flame retardants for epoxy resins.

Nylon and other engineering thermoplastics: why ATH is excluded#

Nylon and other engineering thermoplastics exclude ATH because they are processed at 240-320 °C, far above its 200 °C onset, so their mineral options are MDH and boehmite. Typical E&E targets for these compounds include UL 94 V-0 at 0.4-1.6 mm, a glow-wire ignition temperature (GWIT) of 775 °C and a comparative tracking index (CTI) of 600 V, and our source library gives no MDH or boehmite loading figure for polyamide compounds at this time. Phosphinate systems such as aluminium diethylphosphinate (DEPAL) dominate glass-filled PA formulations instead, mentioned here for context rather than as a mineral option. Phosphinate and melamine systems are covered under flame retardants for nylon.

How Do Surface Treatment and Particle Size Affect Mineral Flame Retardants?#

Surface treatment lets a compound carry 60 wt% or more of ATH or MDH without losing its elongation at break, and particle size sets how the mineral disperses and how smooth the extruded surface is. Coated MDH grades push past 65 wt% loading in polypropylene, a level that an uncoated grade cannot reach without a sharp drop in mechanical properties. Coating chemistry for every mineral is explained under filler surface treatment.

Silane, fatty-acid and coupling-agent treatments#

Mineral flame retardants are coated with vinyl-silane for silane-crosslinked cable compounds, with amino-silane for thermoplastics and with fatty acids for general compounding, and the compound often adds a coupling agent such as Fusabond 226D or Lotader 3210. Huber's silane-XLPE example uses vinyl-silane coated Martinal ATH grades so the mineral bonds into the crosslinked polyethylene network during the silane cure, rather than remaining a passive filler.

Table T4. Surface treatments

Vinyl- and amino-functional silane coupling agents are named here by the trade designations used in the Huber recipe rather than by chemical structure, since that structure is not part of our source library entry.

Particle size, filler loading and mechanical properties#

ATH is sold in particle sizes from 0.25 to 80 µm, and finer, treated grades are used where a highly filled cable compound must keep its elongation at break and a smooth extruded surface. Both ATH and MDH form hexagonal platelets, and a narrower particle-size distribution generally packs to a higher loading before viscosity becomes unworkable, while a coarser distribution processes more easily at a given loading but leaves a rougher extruded surface. Mineral flame retardants follow the same particle-size logic as other fillers for plastics such as talc and calcium carbonate, where grade selection balances loading, dispersion and surface finish.

Which Synergists Improve Mineral Flame Retardants?#

The 3 synergists documented with mineral flame retardants are zinc borate, phosphate low-melting glass and nanoclays, and zinc borate is the most common, at 3-6 phr next to ATH in flexible PVC.

• Zinc borate (Firebrake ZB, 2ZnO x 3B2O3 x 3.5H2O, CAS 138265-88-0) releases water above 290 °C and forms a glassy borate layer that supports the mineral's char and oxide residue; in PP with 50 wt% calcium carbonate, 10 wt% zinc borate combined with 10 wt% MDH raises the LOI to 29.4 vol % O2.
• Phosphate low-melting glass cuts the peak heat release rate of a PE-EVA/ATH cable sheath by 52 %, to 142 kW/m2, at a 10 wt% addition, in the Alsayed et al. (2025) study cited earlier on this page.
• Nanoclays are documented as synergists with mineral flame retardants, though our source library does not establish a specific dosage for this use.

Zinc borate forms a glassy borate layer that supports the mineral's oxide residue, and suppliers self-classify it as Repr. 2 (H361d) rather than under a harmonised CLP entry, since no harmonised classification for the substance has been identified. It is not an SVHC. Antimony trioxide is never classed as a mineral flame retardant synergist here; it belongs to halogenated systems and is covered separately. Stannates and anti-drip agents are covered with the other flame retardant synergists.

How Is the Performance of Mineral Flame Retardants Tested?#

Mineral-filled plastics are tested by limiting oxygen index and UL 94 for material ratings, by the cone calorimeter for heat release, by EN 50399 and related methods for cable classes, and by ash or TGA for the mineral content itself.

Limiting oxygen index (LOI) and UL 94#

LOI measures the minimum oxygen concentration that keeps a sample burning, and mineral flame retardants raise it from about 17.5 vol % O2 for unmodified polypropylene to 30.2 vol % O2 with 185.7 phr of coated MDH. The test follows ASTM D2863-23e1 or ISO 4589-2, reporting the result as vol % O2 against ordinary air's roughly 21 % oxygen content; the method and typical values are on limiting oxygen index (LOI).

UL 94 rates flammability by class, and mineral loading is set to reach the target class at a stated thickness, since a rating without a thickness is not meaningful.

Table T2a. UL 94 classes

Specimens measure 125 x 13 mm and are exposed to a 20 mm, 50 W flame in two 10 s applications. All classes and specimen rules are on UL 94 flammability ratings.

Cone calorimeter: heat release and time to ignition#

In the cone calorimeter at 35 kW/m2, ATH or MDH delays the ignition of an EVA/LLDPE cable compound by 120-160 s and cuts its peak heat release rate, which exceeds 550 kW/m2 when the compound is unfilled and ignites at about 80 s. The peak heat release rate falls roughly in proportion to the mineral loading added, which is why the loading tables earlier on this page report both a phr figure and its measured fire performance rather than the phr figure alone.

The cone calorimeter itself follows ISO 5660-1 or ASTM E1354-26, most often run at a 35 or 50 kW/m2 heat flux, and was built by Vytenis Babrauskas and colleagues at the US National Bureau of Standards (now NIST) in 1982. Interpretation of the resulting heat release curves follows Schartel B. and Hull T.R., "Development of Fire-Retarded Materials: Interpretation of Cone Calorimeter Data", Fire and Materials 31 (2007) 327-354. Heat flux, specimen and data interpretation are on cone calorimeter testing.

Cable fire tests and CPR classes: flame spread, smoke and acidity#

EU cable fire classes under EN 13501-6 rate flame spread and heat release in EN 50399 and add smoke (s1-s3) and acidity (a1-a3) classes, and the s1 and a1 classes are the reason halogen-free cables use mineral flame retardants. The EU Construction Products Regulation covers cables under EN 50575, and the new CPR Regulation (EU) 2024/3110 applies from 8 January 2026, placing cables in product family 31.

Table T5. EN 50399 / EN 13501-6 class limits

Beyond flame spread and heat release, the s1 smoke class requires a total smoke production (TSP1200) of 50 m2 or less and a peak smoke production rate of 0.25 m2/s or less, with s1a adding a light transmittance of at least 80 % under EN 61034-2; the a1 acidity class requires a gas conductivity below 2.5 µS/mm and a pH above 4.3 under EN 60754-2. Test set-ups for all of these methods are described on cable fire tests and CPR classes.

Checking the mineral content: ash and TGA#

Ash and TGA tests confirm how much mineral flame retardant a compound contains, but the result has to be corrected for water loss, because ATH leaves only about 65 % and MDH about 69 % of its mass as oxide residue. Ash content follows ASTM D5630-22 or ISO 3451, and thermogravimetric analysis follows ISO 11358; both figures are consistent with the 34.6 % and 31.0 % theoretical mass losses given earlier for ATH and MDH. Muffle and rapid-ash procedures are on ash content and filler content testing.

What Is the Regulatory Status of Mineral Flame Retardants?#

ATH, MDH and boehmite are REACH-registered substances that are not SVHCs, and ATH, MDH, huntite and hydromagnesite are listed as additives in the EU plastics food-contact regulation without a specific migration limit. ATH and MDH carry no harmonised CLP classification; boehmite is REACH-registered and not an SVHC, though its CLP status is not established in our source library.

Table T6. Regulatory matrix

FCM numbers from the consolidated Regulation (EU) No 10/2011 (version 2026-07-14), pending spot-check. The overall migration limit of 10 mg/dm2 applies to all.

REACH registration, CLP classification and SVHC status#

ATH and MDH are REACH-registered, are not on the SVHC Candidate List and have no harmonised CLP classification, and the EU-funded ENFIRO project (FP7 226563, concluded 2012) rated both among the halogen-free flame retardants with good environmental and health profiles, next to ammonium polyphosphate, aluminium diethylphosphinate, melamine polyphosphate, DOPO, zinc stannate and zinc hydroxystannate. Neither ATH nor MDH appears on the Stockholm Convention's POPs list or under RoHS. Registration duties are explained under REACH and plastic additives.

EU 10/2011 food-contact listing#

Regulation (EU) No 10/2011 lists ATH (FCM 629), MDH (FCM 396), huntite (FCM 627) and hydromagnesite (FCM 600) as authorised additives without a specific migration limit, although aluminium from ATH counts toward the 1 mg/kg aluminium limit set in Annex II. That Annex II metal limit, amended by Regulation (EU) 2020/1245, applies alongside the standard overall migration limit of 10 mg/dm2 (60 mg/kg for infant articles) that covers every additive on the Union list. SML, OML and Annex II metal limits are explained on EU 10/2011.

Who Makes Mineral Flame Retardants?#

Mineral flame retardants come mainly from Huber Advanced Materials (Martinal and Micral ATH, Magnifin and Vertex MDH), Nabaltec (Apyral ATH and Apyral AOH boehmite), Kyowa Chemical (Kisuma MDH) and LKAB Minerals (UltraCarb huntite-hydromagnesite). Plants and grades by company are in the directory of flame retardant manufacturers and suppliers, which lists the same producers by facility and product line.

Table T7. Producers and brand lines

Huber built its current mineral flame retardant portfolio through acquisition: it completed the purchase of Albemarle's Martinswerk business, the source of the Martinal ATH and Magnifin MDH lines, on 1 February 2016. This table is not a performance-equivalence claim between brands; buyers should compare grades by mineral, particle size and surface treatment, not by trade name.

How Do Mineral Flame Retardants Compare with Other Flame Retardant Classes?#

Mineral flame retardants trade a high loading of about 60 wt% for a halogen-free, low-smoke profile, while halogenated, phosphorus and intumescent systems reach the same fire ratings at far lower loadings through chemical action. The flame retardant family also includes nitrogen-based and nanocomposite systems, both covered in their own sections of the halogen-free flame retardants page.

Mineral vs halogenated flame retardants#

Halogenated flame retardants stop flames chemically by trapping radicals in the gas phase at low loadings, whereas mineral flame retardants work physically at high loadings but release no corrosive or acidic smoke. Halogenated systems usually pair a bromine or chlorine donor with antimony trioxide as a synergist and release HBr or HCl gas as they act, gas that a mineral-filled compound never produces because it carries neither halogen. The EU cable fire classes that carry the s1a smoke and a1 acidity ratings, described earlier on this page, are the practical reason cable compounders choose a halogen-free mineral system over a halogenated one. Regulatory status of every bromine system is on brominated flame retardants, including the Ecodesign Regulation (EU) 2019/2021 ban on halogenated flame retardants in electronic display enclosures and stands, in force since 1 March 2021.

Mineral vs phosphorus and intumescent flame retardants#

Phosphorus and intumescent flame retardants give polypropylene UL 94 V-0 at 21-30 wt%, about half the loading of MDH, because they build a protective char instead of diluting and cooling. Phosphorus-based systems act in both the condensed phase, through charring and intumescence, and the gas phase, a dual mechanism described by Schartel B. at the Bundesanstalt für Materialforschung und -prüfung (BAM, 2010). The reported V-0 rating for these PP systems does not come with a stated test thickness in our source library, so it should be read alongside the thickness rule applied to every UL 94 class on this page. APP, phosphinates and red phosphorus are covered under phosphorus flame retardants; intumescent systems specifically are covered on the flame retardant hub's intumescent section.

Is aluminum hydroxide a flame retardant?#

Yes: aluminum hydroxide (ATH) is the most widely used mineral flame retardant in plastics, absorbing 1051 J/g and releasing water from about 200 °C. It reaches that role as a halogen-free additive filler rather than through any chemical reaction with the polymer.

What is a natural flame retardant?#

In plastics, natural flame retardants are mined minerals such as natural brucite (ground magnesium hydroxide) and huntite-hydromagnesite, used in the same way as synthetic ATH and MDH. Mineral wool is a building insulation material, not a plastic additive, and is outside the scope of this page even though search engines sometimes group it with these minerals.

Are mineral flame retardants toxic?#

ATH and MDH carry no harmonised hazard classification in the EU and are not SVHCs, although some suppliers notify ATH dust as a skin, eye and respiratory irritant (H315, H319, H335). Exposure studies across all flame retardant classes are summarised in flame retardants and human health, which covers the wider comparison between mineral, halogenated and phosphorus systems.

What is the difference between ATH and ATO?#

ATH is aluminium trihydrate, a halogen-free mineral flame retardant, while ATO is antimony trioxide (CAS 1309-64-4), a synergist for halogenated flame retardants with a harmonised Carc. 2 classification. Its Carc. 2 and Prop 65 status is on antimony trioxide, a substance that never appears alongside ATH or MDH in a mineral-only formulation.

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