Magnesium Hydroxide (MDH) Flame Retardant: Natural (Brucite) and Synthetic Grades

Magnesium hydroxide (MDH, Mg(OH)2, CAS 1309-42-8) is a mineral flame retardant that absorbs 1,316 joules per gram as it decomposes to magnesium oxide and water, which is why compounders load it into polymers that are processed too hot for aluminium trihydrate. Because the mineral is sold both as a synthetic precipitate and as ground natural brucite, the first question a buyer faces is which of the two a data sheet is describing.

Magnesium hydroxide is registered under REACH (Regulation (EC) No 1907/2006, EC 215-170-3), is absent from the REACH Candidate List of Substances of Very High Concern, and is listed in Annex I of Regulation (EU) No 10/2011 as food-contact substance 396 without a specific migration limit. MDH is one of 87 flame-retardant pages in this directory of plastic additives, each carrying the same identity, dosage and regulatory fields.

This page covers magnesium hydroxide as a plastics additive; the same compound sold as an antacid or laxative is a pharmaceutical use and is out of scope here. It sets out MDH's identity and the two CAS numbers behind it, its endothermic mechanism and smoke suppression, its physical properties, the loading it takes in each polymer, its 6 applications in plastics, its LOI, cone calorimeter and CPR cable-class performance, its behaviour beside other flame retardants, its dated regulatory matrix, its health and environmental profile, the verdict against ATH, boehmite and huntite-hydromagnesite, and the producers and grades on the market.

Table T1. Identity of magnesium hydroxide (MDH).

What Is Magnesium Hydroxide (MDH)?#

MDH is the metal hydroxide of magnesium, Mg(OH)2, a white powder of hexagonal platelets with a molecular weight of 58.32 g/mol that works as a flame retardant because of what it does when it decomposes, not because of what it does chemically in the flame. Its function, endothermic flame retardant and smoke suppressant for polymers processed above 200 °C (392 °F), is a physical one: the mineral cools the polymer surface and dilutes the flame gases by giving up its own mass as water.

Which material does the abbreviation MDH actually cover? It covers a single chemical entity sold under several names and two CAS numbers: magnesium hydroxide, magnesium dihydroxide and, for the mined form, brucite. As a mineral additive, MDH belongs to the highest-loading class of flame retardants for plastics; the hub compares all classes side by side.

MDH sits inside the metal hydroxide subclass of mineral flame retardants, the subclass it shares with aluminium trihydrate, boehmite and huntite-hydromagnesite, and Table T1 above already carries every identifier a buyer needs to match a data sheet to this record: two CAS numbers, two EC numbers, one formula and a molecular weight of 58.32 g/mol. The hexagonal platelet morphology recorded for the powder is also what gives the mineral its name outside chemistry, since the same platelet structure defines brucite as a mineral before it is ever loaded into a compound.

What does MDH stand for?#

MDH stands for magnesium dihydroxide, the flame-retardant industry's name for magnesium hydroxide; the same material appears in data sheets as MH and as Mg(OH)2. All three abbreviations refer to one compound and one chemical record, and a specification that uses any of them is buying the same mineral. Data sheets from different producers mix the three abbreviations freely, so MDH on one sheet, MH on another and the bare formula Mg(OH)2 on a third can all describe the identical grade.

Is brucite the same as magnesium hydroxide?#

Brucite is the natural mineral form of magnesium hydroxide, so the chemistry is identical, but the trade treats the two as different products: brucite grades are mined and ground and carry CAS 1317-43-7, while synthetic grades carry CAS 1309-42-8. Our source library records both numbers for this substance, and a fibrous variety of the mineral is called nemalite.

The distinction matters at the point of purchase rather than in the laboratory: a certificate of analysis, a REACH registration dossier or a customs declaration filed against the wrong CAS number creates a paperwork mismatch even though the mineral inside the bag is chemically identical Mg(OH)2. A grade sold as ground brucite and a grade sold as synthetic MDH can end up in the same compound formulation, and neither the endothermic mechanism nor the 1,316 J/g heat of decomposition changes with the CAS number printed on the drum.

Our source library holds no verified purity, particle-size or price difference between natural and synthetic grades, so the distinction described here is a naming and registration question, not a performance ranking. Table T2 sets out what each CAS number covers.

Table T2. Synthetic MDH vs natural brucite MDH.

Why magnesium hydroxide carries two CAS numbers#

Both CAS numbers describe Mg(OH)2: 1309-42-8 is the registry entry for magnesium hydroxide as a substance, and 1317-43-7 is the entry for brucite, the mineral, which is what ground natural grades are registered under. Their EC numbers follow the same split, 215-170-3 and 215-274-9, so a REACH dossier or a safety data sheet can legitimately show either one.

A formulator who receives a certificate of analysis citing 1317-43-7 rather than 1309-42-8 has not received a different flame retardant, only a different registry pathway for the same Mg(OH)2 chemistry, and both numbers are valid to cite on a technical data sheet or an EU customs declaration.

Is MDH the same as MH, Mg(OH)2 and magnesium dihydroxide?#

Yes: MDH, MH, Mg(OH)2 and magnesium dihydroxide are four names for one substance, and our source library treats them as synonyms of the same record. A specification written against any of the four buys the same mineral, subject to the synthetic-versus-brucite distinction above. The only situation where the distinction matters is a regulatory filing that requires the exact CAS number, since REACH, EU 10/2011 and customs declarations are keyed to CAS rather than to trade abbreviation.

How Does MDH Work as a Flame Retardant?#

MDH retards flame by decomposing endothermically: Mg(OH)2 breaks down to magnesium oxide and water, and the Huber cable-industry brochure puts the heat absorbed at 1,316 joules per gram, or about 328 calories per gram. That decomposition begins once the compound's local temperature exceeds the mineral's stability limit, and everything downstream, cooling, dilution and residue, follows from that single reaction.

Where does that energy come from, and where does it go? The mechanism runs in three steps.

1. Endothermic decomposition draws heat out of the polymer surface as the mineral breaks down.
2. Water release dilutes the combustible gases in the flame; the water is 31.0 % of the mineral's mass at full decomposition.
3. Magnesium oxide residue stays on the surface as a barrier layer once the water has left.

The 1,316 J/g figure only tells half the story unless it is compared with a baseline: most engineering polymers pyrolyse and release far more energy than that as they burn, so a flame retardant that removes 1,316 J/g for every gram of mineral in the compound needs a high loading to make a measurable difference to the fire's heat-release rate. That arithmetic is the reason the loading tables later on this page run in the tens of percent rather than in the single digits typical of brominated or phosphorus chemistry.

Because the mechanism is physical rather than chemical, its effect scales with the mass of mineral in the compound, which is the reason MDH is used at loadings no organic flame retardant would need. Cooling and dilution are two of the routes described on how flame retardants work, alongside gas-phase radical scavenging, condensed-phase charring and intumescence.

Why does MDH also suppress smoke?#

The magnesium oxide left behind adsorbs soot, so the same loading that cools the surface also cuts smoke: Hallstar's plasticizer literature notes that Mg(OH)2 turns the black smoke of aryl phosphate systems white. No smoke-density figure under EN 61034 is recorded for an MDH compound in this reference, so the class effect is named here without a number attached to it. Smoke suppression this way is a side effect of the same particle chemistry that gives MDH its flame-retardant function, not a separate additive mechanism, which is why a formulator does not have to add a dedicated smoke suppressant on top of the mineral to see some benefit. Molybdates, zinc stannate and zinc borate are compared on smoke suppressants.

What Are the Physical and Chemical Properties of MDH?#

MDH is a white powder of hexagonal platelets with a density of 2.36 g/cm3, a refractive index of 1.58 and a Mohs hardness of 2 to 3, and it is alkaline: a water slurry sits at pH 10 to 11. Table T3 lists the values a supplier data sheet carries.

Table T3. Physical and chemical properties of MDH.

Every value in Table T3 comes from the same Huber cable-industry data set used throughout this page, with two exceptions: the molecular weight and formula, which come from PubChem's record for the compound, and the water-release temperature, where PubChem's own decomposition figure of 350 °C runs higher than Huber's process-oriented 330 °C. A formulator reading both sheets side by side is not looking at a contradiction so much as two different definitions of the same event, the point where the mineral is judged to have finished losing water versus the point where measurable weight loss begins.

A refractive index of 1.58 sits close enough to most polyolefins that heavily filled MDH compounds stay translucent rather than chalk-white, and a Mohs hardness of 2 to 3 keeps screw and die wear low compared with harder mineral fillers.

At what temperature does magnesium hydroxide decompose?#

Magnesium hydroxide is stable to about 320 °C (608 °F) and releases its water around 330 °C (626 °F), which is roughly 110 °C above the point where aluminium trihydrate starts to release water at about 200 °C (392 °F). Huber's cable brochure and product literature give this 320 to 330 °C window; PubChem records the decomposition at 350 °C (662 °F), so a formulator should compare data sheets before setting a barrel profile.

That 110 °C gap is the entire selection rule between the two minerals, because it decides which polymers can be compounded with MDH at all, up to processing temperatures of about 330 °C. Below that ceiling the compound normally uses aluminum trihydrate (ATH, alumina trihydrate), which releases its water from about 200 °C.

Which Polymers Use MDH, and at What Loading?#

MDH is a high-loading additive: compounders run it at 160 to 180 phr in halogen-free cable compounds and up to 65 wt% in polypropylene, because a physical mechanism only works in proportion to its mass. No organic flame retardant needs comparable loading, which is the trade a compounder accepts for a mineral with no harmonised hazard classification.

How do the phr values in a cable recipe convert to the weight percent on a data sheet? wt% equals the phr of the ingredient divided by the total phr of the formulation, multiplied by 100. In the EVA/LLDPE HFFR architecture, 160 phr of mineral in a compound totalling 260 phr gives 160/260 x 100 = 61.5 wt%, which matches the Huber reference figure; in the Huber HFFR PP reference, 185.7 phr of coated MDH in a total of 285.7 phr gives 185.7/285.7 x 100 = 65.0 wt%, exactly the "up to 65 wt%" ceiling quoted for PP. Real compounds also contain coupling agents and antioxidants, so the conversion has to use the full formulation total, and both worked examples round to one decimal place, matching the precision Huber's own brochure uses for the PP reference formulation. Cable recipes state MDH loading in PHR (parts per hundred resin), which converts to weight percent only once the full formulation total is known.

Table T4. MDH loading by polymer.

Loadings above 65 wt% are a Huber statement, not an independently verified value.

MDH in polypropylene compounds#

Polypropylene is the polymer where MDH earns its processing advantage: Huber's halogen-free PP reference formulation uses 185.7 phr of coated MDH, which is 65.0 wt% of the compound, and reaches a limiting oxygen index of 30.2 vol % O2 against about 17.5 vol % O2 for unmodified PP. That LOI gain is the largest single performance figure this reference holds for the substance.

Polypropylene is compounded at melt temperatures of roughly 220 to 260 °C, comfortably inside MDH's stability window of about 320 °C but well above the roughly 200 °C onset that rules aluminium trihydrate out of the same process, which is the processing reason PP and MDH pair well together. The 30.2 vol % O2 result also shows how far a mineral loading has to climb before it moves the needle: unmodified PP's approximately 17.5 vol % O2 sits only slightly above the roughly 21 vol % O2 oxygen content of ordinary air, so the coated MDH loading nearly doubles the oxygen concentration PP needs to sustain a flame.

A compound that is two thirds mineral by weight behaves like a filled compound first and a polymer second, which is why coated grades and coupling agents exist for high-filling PP systems. MDH competes with intumescent and brominated systems across flame retardants for polypropylene, and the rest of the PP package, nucleator, antioxidant and acid scavenger, is on additives for polypropylene (PP).

MDH in EVA and polyethylene HFFR cable compounds#

Halogen-free cable compounds are the largest use of MDH: Huber's EVA/LLDPE and silane-crosslinked EVA formulations run 160 to 170 phr of mineral, and at 160 phr that is 61.5 wt% of the finished compound. The same compound family travels under nine abbreviations.

• LSZH, low smoke zero halogen
• LS0H, low smoke zero halogen, written with a zero
• LSOH, low smoke zero halogen, written with the letter O
• LSF, low smoke and fume
• OHLS, oxygen and halogen low smoke
• ZHLS, zero halogen low smoke
• ZHFR, zero halogen flame retardant
• NHFR, non-halogen flame retardant
• HFT, halogen-free thermoplastic

Compound architectures for each CPR class are set out on flame retardants for wire and cable. Table T5 sets out the Huber reference architecture that this loading comes from.

Table T5. Two halogen-free reference architectures (Huber), in phr.

The HFFR PP reference reaches LOI 30.2 vol % O2 with coated MDH at 185.7 phr.

EVA's vinyl acetate content in these cable compounds runs 12 to 70 %, and above 40 % VA the polymer behaves like a rubber and needs curing, which changes how much mineral the polymer will accept before viscosity becomes unworkable. Silane crosslinking changes what the mineral has to survive during processing: after extrusion, a moisture-cure step crosslinks the polyethylene backbone, which is why Huber recommends a vinyl-silane coating on the MDH particles for this compound family rather than the amino-silane or fatty-acid coatings used in thermoplastic, non-crosslinked systems. Coated and uncoated grades are therefore not interchangeable across the silane-crosslinked and standard thermoplastic branches of the same HFFR product line. The polyolefin side of the compound is covered on flame retardants for polyethylene.

MDH in polyamide and high-temperature engineering plastics#

Polyamide is the second polymer family our source library records for MDH, and it is there for one reason: PA is compounded well above the 200 °C ceiling that rules aluminium trihydrate out. Huber records polyolefins, PA and cable compounds among the polymers that take coated MDH grades above 65 wt%, and MDH itself stays stable to about 320 °C (608 °F), inside the processing range PA6 and PA66 require.

Our source library records no MDH loading, UL 94 rating or LOI value for a polyamide compound, so this page names the polymer without importing dosage figures from other nylon flame-retardant chemistries, which run on different chemistry entirely. The phosphinate and nitrogen systems that dominate PA are compared on flame retardants for nylon, and processing windows for PA6, PA66 and the high-temperature grades are on additives for nylon (polyamide).

MDH in PVC and roofing membranes#

PVC is already chlorinated and therefore already flame retardant, so MDH enters PVC and TPO roofing membranes as a smoke suppressant and a cost-carrying mineral rather than as the primary flame retardant. In this role the mineral works beside the chlorine already in the polymer backbone rather than replacing it.

PVC cable insulation is specified to a minimum limiting oxygen index of 26 vol % O2, and in PVC the mineral hydroxides work alongside antimony trioxide, zinc borate at 3 to 6 phr, zinc stannate and molybdates. None of these partner additives changes the chlorine content of the PVC resin itself; they manage the smoke and dripping behaviour of a system that is already flame retardant by composition. Our source library gives no MDH loading for PVC or for TPO roofing membranes, so this page names the role without a dosage figure. Antimony trioxide, zinc borate and the molybdates are compared on flame retardants and smoke suppressants for PVC, and the stabilizer and lubricant side of the recipe is on rigid PVC (uPVC) formulations.

What Is MDH Used For? 6 Applications in Plastics#

MDH is used in 6 plastics application areas: halogen-free cable and wire, polypropylene and polyamide compounds, TPO roofing membranes, rigid PVC, thermosets and electrical components, and construction polymers. Three of the six areas carry enough sourced detail for their own subsection below.

• Halogen-free cable, wire and connectors (LSZH/HFFR)
• Polypropylene and polyamide compounds
• TPO roofing membranes
• Rigid PVC
• Thermosets and electrical components
• Construction polymers

Halogen-free low-smoke (LSZH/HFFR) cable and wire#

Halogen-free cable is where the regulation does the choosing: the EN 13501-6 class B2ca-s1a,d0,a1 combines a smoke class and an acidity class that a halogenated compound cannot meet, which leaves mineral-filled polyolefin as the only architecture. Under EN 50399 at a 20.5 kW heat source, class B2ca requires flame spread of 1.5 m or less, total heat release over 1,200 s of 15 MJ or less, peak heat release rate of 30 kW or less and a FIGRA of 150 W/s or less.

The a1 acidity class, tested to EN 60754-2, requires a conductivity below 2.5 µS/mm and a pH above 4.3, a combination a chlorinated or brominated compound cannot meet because it releases acid gas by design. A compound that fails either the smoke class or the acidity class fails the whole B2ca-s1a,d0,a1 rating regardless of how well it performs on flame spread alone, which is why cable manufacturers treat the three criteria as a single pass-or-fail decision rather than as three separate scores to optimise independently. Outside Europe the closest equivalents are UL 1581 VW-1, UL 1685, CSA FT4/FT6, IEC 60332-3, IEC 61034 for smoke and IEC 60754 for acid gas. Compound packages for cable are set out on additives for wire and cable compounds.

Roofing membranes and construction plastics#

TPO roofing membranes and construction polymers are the second recorded use of MDH, where the mineral carries fire performance into a product that sits outdoors for decades. Our source library records the application but no loading, fire class or weathering data for TPO membranes, so this section describes the use without quantifying it.

Roofing membranes face a different exposure profile from cable insulation: years of UV exposure, thermal cycling and weathering rather than a single fire event, so the mineral's role there is as much about maintaining fire performance over the service life of the membrane as about the initial flame-retardant rating. This reference records the application without a service-life or weathering figure, so the description stops at naming the use. Fire classes for building plastics are set out on additives for building and construction.

Electrical components and thermosets#

Thermosets and electrical components complete the list: both are cured or moulded in windows where a mineral that holds its water to about 320 °C (608 °F) keeps its flame-retardant payload intact. No loading, resin system or electrical test value such as GWFI, GWIT or CTI is recorded for MDH in these applications, so this section stays brief rather than filling the gap. A resin system cured at 150 to 180 °C, typical of many thermoset moulding compounds, never approaches MDH's roughly 320 °C stability ceiling, which is a processing-level reason the mineral is compatible with the application even though this reference records no loading figure for it.

How Does MDH Perform? LOI, Cone Calorimeter and CPR Cable Classes#

The clearest performance number our source library holds for MDH is a limiting oxygen index of 30.2 vol % O2 in Huber's halogen-free polypropylene reference formulation at 185.7 phr of coated mineral, against about 17.5 vol % O2 for unmodified polypropylene. The limiting oxygen index (LOI) method, standardised as ISO 4589-2 and ASTM D2863-23e1, measures the oxygen concentration that just sustains flaming, and vol % O2 is the unit every figure on this page is reported in.

In the cone calorimeter, tested to ISO 5660-1 and ASTM E1354-26 at 35 or 50 kW/m2, an unfilled EVA/LLDPE reference peaks above 550 kW/m2 at a 35 kW/m2 heat flux, and the Huber data set records that ATH or MDH delays ignition of that compound by 120 to 160 seconds. An ignition delay of 120 to 160 seconds is significant in a cable fire scenario because most reaction-to-fire test protocols, including the one behind EN 13501-6, score a cable on how far flame spreads and how much heat it releases within a fixed test window, so a compound that starts later has less time inside that window to accumulate heat release and flame spread before the test ends. No UL 94 rating for an MDH compound is recorded in this reference; the UL 94 flammability ratings page explains the V-0 criteria, single afterflame 10 s or less, total 50 s or less over 10 applications, afterglow 30 s or less, no flaming drips igniting cotton, and the thickness rule that any future rating would have to meet.

What the page cannot state matters as much as what it can: our source library holds no UL 94 rating, no smoke-density value and no mechanical-property data for an MDH compound, so this page reports the gap rather than filling it. Time to ignition and peak heat release rate for MDH compounds come from cone calorimeter testing at 35 or 50 kW/m2, the same cone method behind the ignition-delay figure above.

Table T6. Performance indicators and test methods.

How Does MDH Interact with Other Flame Retardants and Additives?#

MDH is never used alone at these loadings: it travels with a synergist, a coupling agent and a surface treatment, and each of the three solves a different problem. MDH is combined with 3 groups of additives, listed below.

• Synergists. Zinc borate: 10 wt% zinc borate with 10 wt% MDH in a PP/CaCO3 composite reached a limiting oxygen index of 29.4 vol % O2 in a 2024 study published in Materials; at 3 to 6 phr in mineral-filled PVC, zinc borate cuts smoke and dripping.
• Coupling agents. Maleated and acrylate polyolefins in the HFFR cable architecture, and silane coatings applied directly to the mineral surface.
• Antioxidants. The phenolic and phosphite pair used in the HFFR reference formulation.

The 2024 study, published as Materials 17(18) 4553, is the strongest independent synergy result this reference holds for MDH outside the Huber data set. None of the three roles substitutes for another: a synergist without a coupling agent raises fire performance but not mechanical properties, and a coupling agent without a synergist restores elongation without moving the oxygen index. Zinc borate is the synergist our source library records with MDH, at 10 wt% alongside 10 wt% MDH in a PP composite.

Surface treatment and coupling: why coated MDH grades exist#

A compound that is 61 to 65 wt% mineral loses elongation at break unless the particle surface is treated, which is why the grade lists of every MDH producer split into untreated and coated families. Vinyl and amino grades of silane coupling agents are what make a 65 wt% mineral loading processable at all.

Three treatment types cover the grade range: vinyl silane for silane-crosslinked systems, amino silane for thermoplastics, and fatty acid coatings, all aimed at restoring elongation at break at high mineral loadings. Selecting the wrong treatment for the process, an amino silane grade in a silane-crosslinked recipe, for example, does not necessarily block processing, but it forfeits part of the elongation recovery the coating is meant to deliver. Coating chemistries are compared on filler surface treatment.

What Is the Regulatory Status of Magnesium Hydroxide?#

Magnesium hydroxide is registered under REACH, is not a Substance of Very High Concern, is not restricted under REACH Annex XVII and is listed in Annex I of Regulation (EU) No 10/2011 as food-contact substance 396 without a specific migration limit (status 23 September 2026). Table T7 sets out each instrument, the status and its date.

Table T7. Regulatory status of MDH, as of 23 September 2026.

Registration, evaluation and the Candidate List process are explained on REACH and plastic additives. None of the eleven rows in Table T7 has changed since the previous review of this page, and the three rows marked as being verified are the only open items the regulatory matrix currently carries.

Is MDH REACH registered, and is it an SVHC?#

Yes, magnesium hydroxide is registered under REACH (Regulation (EC) No 1907/2006) as EC 215-170-3, and no, it is not a Substance of Very High Concern. Five brominated and chlorinated flame retardants carry Candidate List entries and the mineral hydroxides carry none: HBCD (28 October 2008), decaBDE (19 December 2012), Dechlorane Plus (15 January 2018), TBBPA (17 January 2023) and DBDPE (5 November 2025). Magnesium hydroxide has never appeared on the SVHC Candidate List; those five brominated and chlorinated flame retardants have.

Registration under REACH means the substance has cleared the tonnage-based data requirements the regulation sets for a chemical placed on the EU market; it is a data and hazard-communication obligation, not a safety endorsement, and this page does not treat REACH registration as a substitute for a hazard assessment. The absence from the Candidate List is periodically re-checked as ECHA adds new entries twice a year, in January and June, so a buyer relying on this page for a compliance filing should confirm the current list date against the Candidate List before citing it in a customs or REACH declaration.

Is magnesium hydroxide allowed in food-contact plastics?#

Yes, in the EU: magnesium hydroxide is listed in Annex I of Regulation (EU) No 10/2011 as food-contact substance 396 (Ref 64640) with no substance-specific migration limit, so only the overall migration limit of 10 mg/dm2 applies. The sibling minerals share that status: aluminium trihydrate is FCM 629, huntite is FCM 627 and hydromagnesite is FCM 600, all listed without an SML, while antimony trioxide, the synergist the mineral route replaces, carries an SML of 0.04 mg/kg expressed as antimony.

The overall migration limit rather than a substance-specific limit is the norm for mineral fillers in Regulation (EU) No 10/2011, because an inert, insoluble mineral has little capacity to migrate into food in the first place; the regulation reserves substance-specific migration limits for additives that are soluble or reactive enough to move out of the polymer matrix, which is why antimony trioxide, a soluble oxide, carries a limit while the four mineral hydroxides and carbonates named above do not.

US food-contact status: status being verified. This page states the 21 CFR section or FCN number only once that verification closes, and never writes "FDA approved". The Annex I entries and the overall migration limit are explained on EU 10/2011.

Is magnesium hydroxide listed under California Proposition 65?#

The California Proposition 65 status of magnesium hydroxide is not established in this reference: no OEHHA listing entry is recorded for either CAS 1309-42-8 or CAS 1317-43-7, and our source library's Proposition 65 dates were checked only against an archived list. Two other flame-retardant substances used in the same compounds are listed for cancer: antimony trioxide since 1 October 1990 and molybdenum trioxide since 19 March 2021. Listing dates for every flame retardant are on California Proposition 65.

Is MDH Safe? Health, Safety and Environmental Profile#

Magnesium hydroxide is not classified in the aggregated GHS notification data that PubChem holds for the substance, which is the strongest statement this page makes about its hazard profile. No harmonised CLP entry is recorded either way in our source library, so this page reports the aggregated-notification wording rather than a harmonised classification.

The EU FP7 project ENFIRO (grant 226563, concluded 2012) screened halogen-free flame retardants and reported good environmental and health profiles for MDH alongside ammonium polyphosphate, aluminium diethylphosphinate, ATH, melamine polyphosphate, DOPO, zinc stannate and zinc hydroxystannate, while RDP and BDP in styrenics produced more smoke. Three items make up the profile.

• Classification. Not classified in aggregated notifications; harmonised entry being verified.
• Screening result. ENFIRO, EU FP7 grant 226563, concluded 2012.
• Handling note. The powder is alkaline, pH 10 to 11 in slurry; a supplier safety data sheet governs handling.

None of the three items above is a toxicological endpoint in the strict sense: an aggregated notification status describes what companies reported to ECHA, a screening project result describes a comparative assessment against other halogen-free options, and a handling note describes practical dust and alkalinity management rather than a quantified hazard. This page reports all three as they stand in our source library and adds no LD50, NOAEL or occupational exposure limit, because none is recorded there.

The health debate summarised on flame retardants and human health concerns brominated and organophosphate chemistries, not mineral hydroxides.

What Are the Alternatives to MDH?#

MDH has 3 mineral alternatives and 1 chemical one: aluminium trihydrate, boehmite and huntite-hydromagnesite compete on decomposition temperature, while brominated systems with antimony trioxide compete on loading. The comparison in Table T8 below is built entirely from values this reference can source; where a competing mineral's figure is not established, the cell says so rather than repeating an unsourced number from a supplier blog. Table T8 sets the four minerals side by side.

Table T8. Mineral flame retardant comparison.

Boehmite's decomposition onset, mass loss and heat absorbed are marked for verification in our source library, so the cells are left open rather than estimated.

Minerals, phosphorus and nitrogen chemistries together make up the halogen-free flame retardants.

MDH vs ATH: which mineral flame retardant?#

MDH is the choice whenever the compound is processed above about 200 °C (392 °F), because it holds its water to about 320 °C (608 °F) and absorbs 1,316 J/g when it releases it, against ATH's roughly 200 °C onset and 1,051 J/g. The temperature ceiling is the whole selection rule for mineral flame retardants (ATH and MDH).

ATH releases more of its mass as water, 34.6 % against 31.0 %, and stays the default wherever the processing window allows it, which is EVA, PE, PVC and thermosets. Both minerals reach comparable loadings in the same HFFR architecture, 160 phr of ATH against 160 to 170 phr of MDH, and ATH remains the lower-density option at 2.42 g/cm3 against 2.36 g/cm3 for MDH. This reference states no price or cost difference between the two minerals, although every ranking competitor asserts one.

The practical selection rule reduces to one question: what temperature does the compound see during extrusion or injection moulding? Below about 200 °C, ATH is the incumbent mineral in the largest-volume application, EVA and polyethylene cable compounds, and it needs a comparable loading for a comparable mass-loss contribution. Above that temperature, MDH is not a preference but a requirement, because ATH would decompose in the barrel before the compound ever reached a fire test. Both minerals carry the same regulatory profile, REACH registered, not an SVHC, listed in Regulation (EU) No 10/2011 without a substance-specific migration limit, so the choice between them is never a compliance decision, only a processing one.

MDH vs boehmite#

Boehmite (AlO(OH), CAS 1318-23-6) is the aluminium mineral that competes with MDH rather than with ATH: it dehydrates at a higher temperature than aluminium trihydrate, which puts it in the same processing bracket as magnesium hydroxide. The two diverge in application: boehmite's recorded uses are epoxy PCB laminates and engineering thermoplastics, while MDH's are cable, PP, PA and roofing.

Boehmite's exact decomposition onset is marked for verification in the our sources and is not printed here as a number. That gap matters because it is the only differentiator this reference can offer between the two aluminium-bracket minerals above ATH's temperature ceiling: no numeric onset, mass-loss or heat-absorption figure lets a formulator rank boehmite against MDH directly, so the choice in practice follows the application precedent, laminates for boehmite, cable and moulding compounds for MDH, rather than a data sheet comparison. Boehmite dehydrates above aluminium trihydrate's ceiling, which puts it in MDH's processing bracket.

MDH vs huntite-hydromagnesite#

Huntite-hydromagnesite is the mineral blend that does in three stages what MDH does in one: it releases water around 220 °C (428 °F), carbon dioxide around 330 °C (626 °F) and forms a cement-like char around 560 °C (1,040 °F). LKAB Minerals states that its UltraCarb grade of huntite-hydromagnesite allows lower loadings than ATH or MDH in some systems; no loading figure is published for that claim.

The staged release also means huntite-hydromagnesite reaches its final protective stage later in a fire than MDH does, since the char recorded at about 560 °C (1,040 °F) only forms well after MDH has already finished releasing its water at around 330 °C (626 °F). Which behaviour suits a given fire scenario better is a question this reference does not resolve, because no comparative fire-test result between the two minerals is recorded. Huntite hydromagnesite releases water, then carbon dioxide, then forms a char, a staged release that MDH's single-step decomposition does not offer.

MDH vs halogenated flame retardants and antimony trioxide#

The trade-off against a brominated system with antimony trioxide is mass against hazard: a halogenated compound needs a fraction of MDH's loading, but it generates acid gas and its synergist, antimony trioxide, carries a harmonised Carc. 2 classification and a California Proposition 65 cancer listing from 1 October 1990. Halogenated flame retardants work in the gas phase, scavenging H· and OH· radicals as HX, while antimony trioxide forms SbOX and volatile SbX3 alongside them; MDH's mineral route works in the condensed and gas phases without either mechanism.

The regulatory direction favours the mineral route: the Ecodesign Regulation (EU) 2019/2021 bans halogenated flame retardants in the enclosures and stands of electronic displays from 1 March 2021, and the CPR acidity class a1 pushes whole cable categories toward mineral-filled compounds. Regulatory certainty is the other half of the trade: none of the entries in Table T7 carries an SVHC date, a POPs listing or a harmonised carcinogen classification, a record no brominated flame retardant on this site can match. Antimony trioxide, the synergist a brominated system needs, carries a harmonised Carc. 2 classification; MDH carries none. Bans, furniture standards and the Ecodesign display rules are compared on flame retardant regulations.

Who Manufactures MDH? Grades and Suppliers#

Our source library records two producers of MDH for plastics: Huber Advanced Materials, which sells Magnifin, Vertex and Zerogen, and Kyowa Chemical, which sells Kisuma. Huber Advanced Materials is J.M. Huber Corporation, headquartered in Atlanta and founded in 1883 as a privately held company.

The Magnifin MDH line came to J.M. Huber with the Martinswerk business, acquired from Albemarle in a transaction completed on 1 February 2016. Buyers should ask for the grade's surface treatment, particle size and the CAS number on the certificate of analysis, because natural and synthetic grades are registered differently. This reference records no manufacturing location, plant capacity or regional distribution detail for either producer, so buyers sourcing MDH by region should use the supplier finder or the manufacturer directory rather than this page. This page prints no price figure; the dated guides sit under plastic additive prices. More producers and their locations are in the directory of flame retardant manufacturers and suppliers.

Table T9. MDH producers and brands.

Other producers rank for this query; they are not listed here because our source library holds no verified company record for them.

Synthetic MDH vs natural ground brucite grades#

The grade lists of MDH producers split three ways, and the split is what the H1 of this page calls natural and synthetic: Huber sells Magnifin and Zerogen as synthetic lines and Vertex as the natural one. Three grade families cover the market.

• Synthetic precipitated grades. Magnifin, Zerogen, Kisuma; CAS 1309-42-8.
• Natural ground brucite grades. Vertex; CAS 1317-43-7.
• Surface-coated grades. Any of the above with a silane or fatty-acid treatment; Huber states these allow filling levels above 65 %.

A purchase order that simply specifies MDH without stating synthetic or natural risks a substitution the buyer did not intend, since the two grade families carry different CAS numbers on the certificate of analysis even when the particle size and treatment are otherwise comparable. Coated grades add a third variable on top of the synthetic-natural split, so a complete specification states all three: origin, CAS number and coating. Our source library records no purity, particle-size, brightness or price difference between natural and synthetic grades, so this page treats the difference as a data-sheet question rather than a performance ranking. The full Magnifin, Vertex and Zerogen ranges are listed on the Huber profile.

How Does MDH Fit into the Mineral Flame Retardant Family?#

MDH is the high-temperature member of the mineral flame retardant class, the group that also holds aluminium trihydrate, boehmite and huntite-hydromagnesite and that works by cooling and dilution rather than by chemistry in the flame. Mineral flame retardants sit beside gas-phase radical scavenging, condensed-phase charring and intumescence in the flame-retardant mechanism taxonomy, and the four minerals differ mainly in the temperature at which decomposition begins.

At 61 to 65 wt%, these minerals are the largest ingredient in the compound by mass, so they behave like every other mineral filler in dispersion, wear and mechanical terms, not only as flame retardants. That crossover is also why a mineral flame retardant page on this site reads differently from a chemical flame retardant page: the properties that matter, particle size, surface treatment and loading, are filler properties first and flame-retardant properties second. At those loadings, MDH behaves like every other mineral in the guide to fillers for plastics.

Brucite as a mineral: why the search results are mostly geology#

Brucite is a mineral before it is an additive, and most of what is written about it is mineralogy: a soft layered magnesium hydroxide with a Mohs hardness of 2 to 3, of which the fibrous variety is called nemalite. Brucite carries its own CAS number, 1317-43-7, and EC number, 215-274-9, distinct from the synthetic substance record.

For a compounder, the only questions that matter are the CAS number on the certificate, the particle size and the surface treatment; the mineralogical questions that dominate the search results, hardness, colour, lustre and crystal habit, belong to a different audience. None of those mineralogical facts changes how the mineral performs in a compound, which is why this reference treats them as background rather than as part of the flame-retardant record above.

Non-plastics uses of magnesium hydroxide (outside this site's scope)#

The same compound is sold far outside plastics, and those uses are not covered on this site: magnesium hydroxide as an antacid and laxative is a pharmaceutical product, and its water-treatment and flue-gas uses belong to process chemistry. This site covers additives used in plastics, and the pharmaceutical, water-treatment and flue-gas-desulfurisation uses each carry their own dosing, regulatory and safety framework that this reference does not cover.

Water-treatment applications use magnesium hydroxide to neutralise acidic wastewater, and flue-gas desulfurisation uses it to neutralise acid gases from combustion; both are process-chemistry uses with their own dosing and regulatory frameworks that sit outside the additive identity, dosage and regulatory fields this reference tracks for plastics. Nothing on this page is medical or pharmaceutical advice.

Is magnesium hydroxide hazardous?#

Magnesium hydroxide is not classified in the aggregated GHS notification data held for the substance, and the EU FP7 project ENFIRO reported a good environmental and health profile for it in 2012. The practical caveat is that the powder is alkaline, pH 10 to 11 in slurry, and is handled as a dust, so the supplier's safety data sheet governs handling. This answer intentionally stops short of the word safe: an aggregated notification status and a decade-old screening project describe what has been reported and studied, not a guarantee, and the supplier safety data sheet remains the controlling document for handling, storage and disposal decisions.

Can MDH alone give a UL 94 V-0 rating?#

Our source library records no UL 94 rating for an MDH compound, so this page does not claim one: the only recorded fire-performance value is a limiting oxygen index of 30.2 vol % O2 in Huber's polypropylene reference at 185.7 phr. What that leaves the reader is that any rating depends on the polymer, the thickness and the synergist used alongside the mineral. A supplier claim of V-0 performance for a specific commercial compound is not the same statement as a substance-level rating, and this page deliberately keeps the two apart. Match polymer, thickness and rating in the flame retardant selector.

Why does a magnesium compound retard flame?#

MDH is magnesium hydroxide, Mg(OH)2, not magnesium metal: the magnesium in it is already bound to oxygen and hydrogen, and breaking that bond absorbs energy rather than releasing it. Decomposition takes in 1,316 joules per gram and leaves magnesium oxide plus water, which is the opposite of a fuel. The confusion is understandable given how search engines pool queries about magnesium hydroxide together with queries about magnesium metal, but the two entities have separate chemistries, separate safety profiles and separate literatures, and our sources for this page covers only the hydroxide used as a plastics additive, not the metal.