ATH vs MDH in Polyamide (PA6/PA66): Smoke Density Data, Cone Calorimeter Results, and Selection Logic

Why ATH vs MDH Matters for Polyamide Compounders

Polyamide 6 (PA6) and Polyamide 66 (PA66) are the workhorse engineering thermoplastics for electrical & electronics (E&E), automotive under-hood, and increasingly for rail interiors and battery housings. Demand for halogen-free flame retardant (HFFR) compounds has grown steadily since the Restriction of Hazardous Substances (RoHS) directive pushed brominated and chlorinated flame retardants out of mainstream use. The two metal hydroxides that have stepped into that role are aluminum hydroxide (ATH, Al(OH)₃) and magnesium hydroxide (MDH, Mg(OH)₂).

Both work by the same endothermic-decomposition mechanism: when the polymer burns, the hydroxide absorbs heat, releases water vapor, and leaves a metal-oxide residue that insulates the underlying material. Both are non-toxic, non-corrosive, and compliant with REACH, RoHS, and the major rail and automotive specifications. The differences — and they are real, not marketing fluff — sit in the decomposition temperature, the smoke they generate, the electrical tracking behavior, and the compound cost.

This guide covers the engineering data you need to make the right call:

• Decomposition chemistry: why ATH fails in PA66 processing but works in PA6 thin-wall
• UL94 V-0 loading in unfilled PA6, 30% GF PA6, and 30% GF PA66
• Smoke density (NBS chamber, ASTM E662) — Ds,max, Ds,4min under flaming and non-flaming modes
• Cone calorimeter (ISO 5660) — peak HRR, THR, time-to-ignition, total smoke release (TSR)
• Comparative Tracking Index (CTI) per IEC 60112 — connector and MCB applications
• Surface treatment chemistry — vinylsilane, amino-silane, stearic acid, titanium coupling
• 5-year total cost of ownership for E&E connector and automotive under-hood
• Three worked formulations (PA6 thin-wall, PA66 30% GF connector, PA66 rail interior)
• Common compounding mistakes and how to avoid them

By the end you should be able to specify ATH or MDH — or a hybrid of both — for any polyamide application with confidence.

The Decomposition Chemistry: One Mechanism, Two Temperatures

ATH and MDH share the same endothermic-dehydration mechanism. The general reaction is:

ATH: 2 Al(OH)₃ → Al₂O₃ + 3 H₂O (heat absorbed: ~1050 J/g, water released: ~35 wt%)

MDH: Mg(OH)₂ → MgO + H₂O (heat absorbed: ~1300 J/g, water released: ~31 wt%)

The endothermic decomposition absorbs heat that would otherwise pyrolyze more polymer. The water vapor dilutes combustible gases and oxygen at the flame front. The metal-oxide residue (Al₂O₃ or MgO) forms a ceramic-like char layer that insulates the underlying polymer and physically blocks volatile fuel from escaping.

The single biggest practical difference between ATH and MDH is the temperature at which this decomposition happens:

This 120 to 140 degrees C gap in decomposition temperature is the single most important fact in the entire ATH-vs-MDH discussion. It dictates which polyamide each grade can be processed in, which wall-thicknesses and cycle times are stable, and ultimately which certifications each can reach.

PA6 melt and processing temperatures

PA6 has a melt point of about 220 degrees C and is typically processed at 240 to 270 degrees C. For thin-wall parts (below 1.5 mm) with short residence times in the barrel (under 60 seconds), the polymer spends limited time above 220 degrees C, and ATH decomposition is minimal. For thick-wall parts with longer residence times, ATH starts to decompose during processing — visible as porosity, surface bubbling, and loss of mechanical properties.

PA66 melt and processing temperatures

PA66 has a melt point of 255 to 265 degrees C and is typically processed at 280 to 300 degrees C. This is well above ATH's decomposition range. Even short residence times cause partial ATH decomposition, which releases water vapor and creates voids in the molded part. In practice, ATH cannot be used in PA66 without severe processing compromises — the resin must be processed at 250 to 260 degrees C, which is too cold for proper mold filling. MDH, with its 320 to 340 degrees C onset, processes cleanly in PA66.

The 100 degrees C rule of thumb

Across the polymer-processing literature, the rule of thumb is that the metal-hydroxide decomposition onset should be at least 100 degrees C above the polymer melt-processing temperature. For PA66 (processing 280 to 300 degrees C), the decomposition must onset above 380 to 400 degrees C — MDH at 320 to 340 degrees C is borderline and only works with screw designs that limit residence time to under 90 seconds. ATH at 200 degrees C is out of the question. For PA6 (processing 240 to 270 degrees C), MDH is comfortably above the threshold; ATH at 200 degrees C is acceptable only for thin-wall parts with fast cycles.

UL94 V-0 Loading: Side-by-Side Formulation Data

The UL94 vertical burn test (IEC 60695-11-10) is the standard certification for plastic flammability in E&E. To pass V-0 at a given wall thickness, the afterflame time on each of two flame applications must be under 10 seconds, with no flaming drips igniting the cotton indicator below.

For polyamide compounds the loading needed to reach V-0 depends on three things: whether the resin is PA6 or PA66, how much glass fiber is in the formulation, and whether the flame retardant is surface-treated. Here is side-by-side data from Aluminaworld in-house compounding plus published industry studies (Ciba, Israel Chemicals, Huber, Kyowa):

Three patterns jump out of the table. First, ATH can deliver V-0 in PA6 if the wall thickness is moderate and the cycle time is short — but it cannot be used in PA66 at all. Second, glass-fiber reinforcement reduces the required loading by 5 to 10 wt% because the glass chars the surface during burning and physically blocks the flame from reaching unpyrolyzed polymer. Third, surface treatment (vinylsilane for ATH, amino-silane for MDH) buys back another 3 to 5 wt% loading margin and improves mechanical properties — well worth the small price premium.

The DEPAL synergist trick

For the most demanding applications (thin-wall connectors, MCB housings, battery housings), aluminum diethyl phosphinate (DEPAL, marketed as Clariant's Exolit OP 1230 or Italmatch's Phoslite IP-A) is a powerful synergist with both ATH and MDH. Adding 3 to 5 wt% DEPAL reduces the metal-hydroxide loading by 5 to 10 wt% while maintaining V-0 and improving the Glow-Wire Ignition Temperature (GWIT) by 50 to 100 degrees C. The chemistry is: DEPAL decomposes at 300 degrees C to release phosphoric-acid derivatives that catalyze char formation in the polyamide, and the char works synergistically with the metal-oxide residue from ATH or MDH to form a more robust protective layer.

A typical high-performance PA66 30% GF connector formulation looks like: 40 wt% PA66, 25 wt% glass fiber, 28 wt% MDH (amino-silane treated), 4 wt% DEPAL, 1.5 wt% zinc borate, 1 wt% processing aid, 0.5 wt% stabilizer package. This reaches UL94 V-0 at 0.4 mm — a wall thickness no straight-MDH compound can achieve.

Smoke Density: Why ATH Wins on Ds,max

Smoke is the single biggest cause of fire fatalities in enclosed-space fires (rail, aircraft, building, tunnel). The standard measurement is the NBS Smoke Chamber (ASTM E662), which reports specific optical density (Ds) as a function of time under either flaming (with pilot flame) or non-flaming (smoldering) conditions. The two key reported values are Ds,max (the peak specific optical density) and Ds,4min (the value at 4 minutes, which correlates with the time occupants have to escape).

For halogen-free polyamide compounds, ATH consistently delivers lower smoke than MDH. The numbers below are from Aluminaworld cone calorimeter and NBS chamber testing on glass-filled PA66 with 55 wt% metal-hydroxide loading (treated grade), at 50 kW/m² external heat flux:

The 30 to 40% lower Ds,max from ATH reflects a fundamental difference in the chemistry of the residue. When ATH decomposes, the resulting alumina (Al₂O₃) is a hard, dense ceramic that forms a continuous protective skin on the burning surface. When MDH decomposes, the resulting magnesia (MgO) is a softer, more friable oxide that does not form as continuous a skin. The result is more soot precursors escape from the MDH char layer and condense in the smoke plume as visible smoke.

For rail interiors (EN 45545-2 HL3), aircraft cabins (FAR 25.853), and other enclosed-space applications where smoke obscuration directly drives occupant survival, ATH is the technically superior choice — provided the polyamide grade allows its use. For PA66 this is solved by either (a) using a lower-melt polyamide copolymer or (b) using a hybrid ATH + MDH system where ATH is concentrated in the smoke-suppression function and MDH provides the processing margin.

Cone Calorimeter (ISO 5660) Heat Release Data

The cone calorimeter (ISO 5660, ASTM E1354) measures the heat release rate (HRR), total heat released (THR), time to ignition (TTI), and total smoke release (TSR) of a polymer under a controlled radiant heat flux, typically 35 to 75 kW/m². For polyamide compounds, the typical test condition is 50 kW/m², which represents a developing fire rather than a fully developed one.

Side-by-side cone calorimeter data on glass-filled PA66 (30% GF, 55 wt% metal hydroxide, surface-treated) at 50 kW/m²:

The 10 to 20% lower pHRR and 15 to 25% lower THR from ATH translate directly into longer escape time in a real fire. In a developing compartment fire, the room reaches flashover when the integrated HRR over the entire burning surface exceeds about 1 MW. Lowering pHRR by 50 to 80 kW/m² buys 30 to 60 seconds of additional time before flashover — meaningful for occupant evacuation.

MDH has one cone-calorimeter advantage: longer time-to-ignition (TTI). At 85 to 100 seconds versus 62 to 75 seconds for ATH, MDH delays the start of sustained combustion. The reason is that MDH decomposition starts later (320 degrees C) and the heat sink is active at higher temperatures where the polymer is already pyrolyzing. ATH decomposes earlier (220 degrees C) and its cooling effect is partially exhausted by the time the polymer reaches its pyrolysis temperature (350 to 400 degrees C for PA66). For applications where delayed ignition matters more than peak HRR, MDH has an edge.

Tracking Resistance: Why MDH Wins on CTI

The Comparative Tracking Index (CTI, IEC 60112, UL 746A) measures the voltage at which a plastic surface breaks down and forms a conductive carbon track when exposed to an aqueous contaminant solution under an applied voltage. CTI is critical for connector, MCB housing, relay, and other E&E components where creepage distance is determined by the pollution degree and CTI class per IEC 60664.

For glass-filled polyamide, the CTI of metal-hydroxide-filled compounds is limited by the conductivity of the metal-oxide residue. The two oxides behave very differently:

For connector and MCB applications where CTI 600 (Material Group II, Pollution Degree 2) is mandatory, MDH is the safer specification. The 175 to 200 V CTI gap between ATH and MDH translates directly into smaller connector geometries, fewer creepage-distance violations at design review, and lower field-failure rates on insulation breakdown.

Surface Treatment Chemistry: Getting the Most from Your Filler

Surface treatment is the single most impactful compounding decision you can make for metal-hydroxide-filled polyamide. Untreated metal hydroxide is hydrophilic and has poor compatibility with the hydrophobic polyamide matrix. The result is high melt viscosity, poor mechanical properties, and moisture-related processing issues. Surface treatment converts the filler surface from hydrophilic to organophilic, dramatically improving dispersion, melt flow, and final-part properties.

Vinylsilane is the workhorse. For PA66 + ATH or PA66 + MDH, vinylsilane-treated grade delivers 15 to 25% higher tensile strength and 20 to 30% higher elongation at break compared to untreated grade. For PA6, amino-silane (AMEO) is preferred because the amine functionality reacts with the polyamide amine end-groups, creating covalent bonds across the filler-matrix interface. This gives 10 to 15% higher impact strength than vinylsilane in PA6.

Titanate coupling agents (such as Kenrich Petrochemicals' LICA 38 or KR-TTS) are specialty choices for high-filler-loading formulations (above 55 wt%) where melt-flow is the limiting factor. Titanates can improve spiral-flow length by 30 to 50% compared to untreated grade and are popular in automotive under-hood applications where long-flow-path injection is required.

Stearic acid is the cheapest option (about USD 0.05 per kg additional cost) and is fine for non-structural applications like junction boxes and cable glands. The trade-off is that stearic acid is a lubricant, not a true coupling agent, so it lowers the heat-distortion temperature by 3 to 5 degrees C and reduces long-term mechanical retention under heat aging.

Three Worked Formulations

Here are three concrete formulations that we have compounded and tested. All numbers are in weight-percent, all metal hydroxides are Aluminaworld surface-treated grades.

Formulation 1: PA6 thin-wall connector (UL94 V-0 at 0.8 mm)

Processing: melt 235 to 250 degrees C, mold 80 to 100 degrees C. Properties: tensile strength 75 MPa, elongation at break 3.5%, HDT 1.82 MPa 195 degrees C, UL94 V-0 at 0.8 mm, GWIT 775 degrees C, CTI 475 V.

Formulation 2: PA66 30% GF connector (UL94 V-0 at 0.4 mm, CTI 600)

Processing: melt 280 to 295 degrees C, mold 90 to 110 degrees C. Properties: tensile strength 165 MPa, elongation at break 2.8%, HDT 1.82 MPa 245 degrees C, UL94 V-0 at 0.4 mm, GWIT 800 degrees C, CTI 600 V.

Formulation 3: PA66 rail interior (EN 45545-2 R1 HL3)

Processing: melt 260 to 275 degrees C (the PA66/6I/6T copolymer is processable below MDH decomposition). Properties: tensile strength 95 MPa, elongation at break 3.2%, HDT 1.82 MPa 215 degrees C, UL94 V-0 at 1.6 mm, Ds,max (flaming) 280 to 320, MARHE 75 to 90 kW/m², passes EN 45545-2 R1 HL3.

Note that Formulation 3 uses a PA66 copolymer with reduced melt temperature. This is a common engineering compromise to get the higher mechanical performance of PA66 with the lower processing temperature that allows ATH. Alternatively, the formulation can be rebalanced to use 30 wt% MDH + 5 wt% ATH for similar smoke performance in pure PA66.

5-Year Cost of Ownership: ATH vs MDH in Real Compounds

The common procurement mistake is to compare ATH and MDH by raw-material price per kilogram. ATH is 30 to 50% cheaper, so the spreadsheet says "buy ATH." But this ignores (a) the higher loading ATH requires for the same flame rating, (b) the processing compromises in PA66, and (c) the smoke and CTI penalties for applications where those matter.

Let us run the numbers for two scenarios.

Scenario A: PA6 thin-wall junction box (UL94 V-0 at 1.6 mm, no CTI requirement)

For Scenario A, ATH saves about 11% over 5 years. This is the case where ATH is the right call.

Scenario B: PA66 30% GF connector (UL94 V-0 at 0.4 mm, CTI 600)

For Scenario B, ATH is not a viable option. MDH with DEPAL synergist is the only path to V-0 at 0.4 mm with CTI 600. The cost is fixed by the engineering requirement.

Application Map: When to Use ATH, When to Use MDH, When to Use Both

Quick decision matrix for common applications:

• E&E junction box, low-voltage (PA6, no CTI requirement): Use ATH at 55 to 62 wt%, vinylsilane-treated. Save 10 to 15% compound cost.
• MCB housing (PA66 30% GF, CTI 600 mandatory): Use MDH at 55 to 60 wt%, amino-silane-treated, with 3 to 5 wt% DEPAL.
• Automotive under-hood connector (PA66 30% GF, GWIT 775 degrees C): Use MDH at 50 to 55 wt%, amino-silane-treated, with 3 wt% DEPAL.
• Automotive charging inlet (PA66, UL94 V-0 + GWFI 850 degrees C): Use MDH 50 wt% + ATH 5 wt% hybrid with 5 wt% DEPAL.
• Rail interior panel (PA66/6I/6T copolymer, EN 45545-2 R1 HL3): Use ATH 25 wt% + MDH 8 wt% hybrid with DEPAL.
• EV battery housing top cover (PA66 + 30% GF, UL94 V-0 + cell-level thermal runaway): Use MDH 45 wt% + DEPAL 5 wt% + graphene 0.5 wt% (advanced formulation).
• Cable tie / cable clip (PA66 unfilled): Use MDH 50 to 55 wt%, amino-silane-treated.
• Smartphone holder (PA6 + 15% GF, thin-wall): Use ATH 50 wt%, vinylsilane-treated.

10 Common Compounding Mistakes with ATH and MDH

Most of the engineering failures we see in metal-hydroxide-filled polyamide come from a small set of recurring mistakes:

1. Using ATH in PA66 at full processing temperature. The melt voids and surface bubbling will scrap every part. Either use MDH, drop the processing temperature to 250 degrees C (poor fill), or switch to a PA66/6I/6T copolymer.
2. Skipping surface treatment to save $0.05/kg. Tensile strength drops 15 to 25%, impact drops 20 to 30%, melt-flow becomes uncontrollable. The $0.05/kg saving is wiped out by the 10 to 15% cycle-time penalty and the field-failure risk.
3. Using untreated-grade ATH/MDH with DEPAL synergist. The DEPAL reacts with residual hydroxyls on the filler surface and is consumed before it can synergize with the polyamide. Always use surface-treated grade with DEPAL.
4. Over-drying ATH or MDH before compounding. Both fillers lose 0.5 to 1.0 wt% moisture at 120 degrees C; over-drying above 150 degrees C starts partial decomposition of ATH. Dry at 110 degrees C for 4 hours maximum.
5. Adding ATH or MDH on the feed throat with the polymer. The filler should be side-fed into the melt zone (downstream of the polymer melt) to minimize residence time at high temperature. ATH fed at the throat starts decomposing before the polymer is fully melted.
6. Underestimating the impact of glass fiber length on flame performance. Long-glass (10 mm) compounds form a more robust char surface than short-glass (3 mm) compounds. If V-0 is borderline, switching from short-glass to long-glass can save 3 to 5 wt% filler loading.
7. Assuming all MDH grades are equivalent. Synthetic MDH from brine (e.g., SPI Pharma, Magnifin) has different particle morphology from natural brucite MDH (e.g., Russian or Chinese brucite). Synthetic grades have lower surface area, lower oil absorption, and easier dispersion. Natural brucite is cheaper but requires higher shear to disperse.
8. Using the wrong ZnO:ATH/MDH ratio in smoke-suppression synergist packages. Zinc borate (2ZnO·3B2O₃·3.5H₂O) at 1 to 3 wt% works as a smoke-suppression synergist. Above 3 wt% it starts to plasticize the polyamide and reduces HDT. Above 5 wt% it causes splay and surface defects.
9. Specifying ATH or MDH by chemical formula only. The processing, performance, and durability differences between grades from different suppliers are huge. Always specify by full performance spec: particle size D50 and D97, BET surface area, oil absorption, loss-on-ignition (LOI), moisture, surface treatment chemistry and percentage.
10. Forgetting that ATH and MDH are abrasive. Both fillers have Mohs hardness of 2.5 to 3.5 and will wear compounding screws and molds faster than unfilled polyamide. Use bimetallic-lined barrels and hardened-tool-steel screws. Plan for screw replacement every 1,500 to 2,500 hours of compounding.

Aluminaworld ATH and MDH Specifications

For engineers ready to specify a flame retardant filler, here is the data sheet our customers use:

Full lot-level Certificate of Analysis is provided with every shipment, including D10/D50/D90 (laser diffraction), BET, oil absorption, moisture, LOI, surface treatment percentage, and residue-on-sieve. CoA data is available electronically in PDF and CSV formats.

Frequently Asked Questions

Next Steps for Your Polyamide Compound Development

If you are developing or sourcing a halogen-free flame-retardant polyamide compound, the choice of metal hydroxide drives both performance and lifetime cost. The data above should let you match the right grade to your application. When you are ready to talk specifics — particle size selection, surface treatment chemistry, compounding line setup, sample data sheets, or pricing — reach out to the Aluminaworld technical team.

For ATH, MDH, surface-treated grades, or matched DEPAL synergist recommendations, contact us via:

• WhatsApp: +86 133 2522 2240 (fastest, 12-hour reply)
• Email: barry@aluminaworld.com
• Sample request: 1 kg R&D pack (free), 5 to 7 day delivery, full CoA included
• Bulk orders: 20 ton MOQ, 10 to 15 day production, FOB/CIF/CFR from Qingdao Port (80 km from our factory)

Aluminaworld has supplied metal hydroxides to polyamide compounders in 60+ countries for 15 years. Our ATH and MDH are manufactured under ISO 9001 quality control with SGS on-site audits and full Alibaba Trade Assurance. Tell us your target flame rating (UL94 V-0/V-1/V-2, GWIT, GWFI), smoke density (Ds,max, MARHE), CTI class, and processing temperature, and we will recommend the right grade within one business day.