ATH in SMC BMC Compounds: Loading, Viscosity, and Fire Test Results

Why ATH Is the Workhorse Flame Retardant for SMC and BMC

Sheet Molding Compound and Bulk Molding Compound are glass-fiber reinforced unsaturated polyester (UP) or vinyl ester (VE) composites cured by compression molding (SMC) or injection molding (BMC). They are used for electrical enclosures, automotive body panels, headlamp reflectors, busbar supports, and switchgear housings. In many of these applications, the part must meet a halogen-free flame retardant specification, a low smoke density requirement, or both.

Aluminum hydroxide - Al(OH)3, ATH, alumina trihydrate - is the dominant flame retardant filler in these systems for three reasons:

1. Endothermic decomposition at 200 to 220 °C absorbs about 1,100 kJ per kg of ATH loaded. The water vapor released dilutes the flammable pyrolysis gases and forms a protective aluminum oxide char on the burning surface.
2. Halogen-free chemistry means low smoke density and low corrosivity - the resin will not emit HCl or HBr in a fire, which is why ATH-filled SMC is the default for EN 45545-2 HL3 railway, NFPA 130 transit, and IEC 60695-2-12 glow-wire applications.
3. Low cost per kilo of FR effect - ATH is 3 to 5 times cheaper than MDH and 6 to 10 times cheaper than melamine cyanurate or phosphorus FR, so high loadings (130 to 180 phr) are economically feasible in molded parts.

The complication is that ATH is not a passive filler. It interacts with the polyester resin, with the magnesium oxide thickener, with the glass fibers, and with ambient moisture. The wrong combination of ATH grade, surface treatment, and loading can break the SMC maturation window, push paste viscosity above the compression molding limit, or fail the fire test despite a high nominal ATH content.

In the next sections we go through the loading-vs-fire-performance data, look at the chemistry of vinylsilane surface treatment, and finish with worked SMC and BMC formulations that meet common specifications (UL94 V-0, glow-wire 960 °C, CTI above 600 V).

How ATH Stops Fire: The Endothermic Mechanism

Aluminum hydroxide starts losing its bound water at 200 to 220 °C. By 350 °C the conversion to Al2O3 is essentially complete:

2 Al(OH)3 → Al2O3 + 3 H2O (vapor)    ΔH = +1,100 kJ/kg ATH

Three things happen in parallel during a fire, and they all matter:

1. Endothermic cooling. 1,100 kJ per kg is roughly the heat needed to boil away 0.5 kg of water. For a 140 phr ATH SMC (about 50 wt% ATH in the final composite) the ATH can absorb 550 kJ of heat per kg of composite, which is more than the pyrolysis enthalpy of the polyester resin itself.
2. Water vapor dilution. The 3 moles of water released per mole of ATH dilute the flammable gas mixture in the flame zone. The flame temperature drops below the auto-ignition threshold of most polyester pyrolysis fragments (mostly styrene, benzene, and substituted aromatics).
3. Alumina oxide char. The Al2O3 residue forms a thin ceramic-like layer on the burning surface that slows heat transfer back into the bulk of the composite. The char is mechanically weak and does not replace a proper intumescent coating, but it does buy 30 to 90 seconds of additional time to failure in glow-wire tests.

ATH works best when the decomposition temperature window (200 to 350 °C) overlaps with the polyester decomposition window (300 to 450 °C). For high-temperature molding above 250 °C the ATH starts decomposing during the press cycle, which causes porosity and sink marks. The cure cycle must be designed so the mold is below 200 °C when the press closes and only crosses 220 °C after the gel point is reached.

ATH Loading vs Fire Performance: UL94, Glow-Wire, LOI

The relationship between ATH loading and fire performance is non-linear. Below 80 phr, ATH does not give meaningful flame retardancy because the endothermic heat sink is too small. Between 80 and 130 phr, performance improves rapidly. Above 160 phr, the gain diminishes and processability collapses. The table below shows the working range for a typical 30 percent glass fiber UP SMC.

Practical sweet spots:

• UL94 V-0 at 1.5 mm: 130 to 160 phr ATH
• Glow-wire 960 °C at 1.5 mm (IEC 60695-2-12): 150 to 180 phr ATH
• EN 45545-2 HL3 (rail vehicle): 140 to 170 phr ATH + 5 to 8 phr zinc borate
• NFPA 130 transit: 130 to 150 phr ATH, Ds,max below 300
• CTI above 600 V (IEC 60112): 130 to 150 phr ATH-SMC-TRK1 (low soda, low iron)

These numbers assume a typical orthophthalic or isophthalic UP resin, 25 to 30 percent glass fiber by weight, and vinylsilane-treated ATH. Switching to a halogenated FR system (decabromodiphenyl ether/Sb2O3) gives UL94 V-0 at 25 to 30 phr but pushes smoke density to 400 to 600 and fails most transit and railway specifications.

ATH Loading vs Paste Viscosity: The Krieger-Dougherty Reality

ATH is a solid particulate, and at high loading the SMC paste behaves as a concentrated suspension. The relationship between filler volume fraction and viscosity is well described by the Krieger-Dougherty equation:

η / η₀ = (1 - φ / φ_m)^{-[η] φ_m}

where η is the paste viscosity, η₀ is the unfilled resin viscosity, φ is the filler volume fraction, φ_m is the maximum packing fraction (typically 0.55 to 0.65 for irregular ATH particles), and [η] is the intrinsic viscosity (about 2.5 for spheres, higher for irregular shapes).

The practical implication is that paste viscosity is not linear in loading. Below 110 phr, viscosity rises gently. Above 140 phr, viscosity rises steeply. At 180 phr the paste is essentially unprocessable on a standard SMC line. The table below shows measured paste viscosity at 1 s^-1 shear (typical of SMC maturation tank conditions) for a medium-viscosity orthophthalic UP resin with vinylsilane-treated ATH, D50 12 microns.

For SMC, the practical maximum is about 170 phr. Above that, the paste cannot be rolled into the glass mat without tearing the mat or leaving dry spots. For BMC, the practical maximum is 200 to 220 phr because BMC is a dough (not a sheet) and is injected into the mold under higher pressure. Above 230 phr, even BMC fails: the dough will not fill thin sections and the screw torque exceeds machine limits.

Vinylsilane Surface Treatment: Why It Is Required

Untreated ATH has a hydrophilic surface covered with -OH groups. These -OH groups do three bad things in an SMC or BMC formulation:

1. Absorb ambient moisture. Untreated ATH picks up 0.5 to 1.0 wt% moisture from the air in 48 hours. The moisture reacts with the polyester double bond during cure and forms micro-voids and sink marks. It also accelerates the MgO thickening reaction, which shortens the maturation window.
2. Trap air on the particle surface. Polar -OH groups attract air bubbles that are not released during the maturation vacuum step. The result is a 5 to 15 percent loss in tensile strength and 10 to 25 percent loss in arc-track resistance (CTI).
3. Fail to bond to the polyester network. Without a covalent link, the ATH particle is held in the cured composite only by mechanical keying and Van der Waals forces. Under thermal cycling or mechanical load the particle-matrix interface fails first, leading to micro-cracking and accelerated aging.

Vinylsilane (most commonly vinyltri(methoxy)silane, VTME, or vinyltri(ethoxy)silane, VTEO) solves all three problems. The chemistry is straightforward:

CH2=CH-Si(OCH3)3 + 3 H2O → CH2=CH-Si(OH)3 + 3 CH3OH (hydrolysis)

CH2=CH-Si(OH)3 + Al-OH (on ATH surface) → CH2=CH-Si-O-Al + H2O (condensation)

The result is a covalent Si-O-Al bond between the silane and the ATH surface, with a vinyl group (-CH=CH2) exposed on the outside. The vinyl group copolymerizes with the styrene and the polyester double bond during the free-radical cure, locking the ATH particle into the crosslinked network.

Treatment level matters. The typical range is 0.8 to 1.5 wt% silane on the dry ATH weight. Below 0.5 wt%, the surface is not fully covered and moisture pickup remains above 0.3 wt%. Above 2.0 wt%, the excess silane forms a soft polymer layer on the particle surface that acts as a lubricant, depressing paste viscosity but reducing flame retardancy slightly (the polymer burns at 300 to 400 °C, partially masking the endothermic ATH effect). The data below shows the effect of vinylsilane treatment level on moisture pickup and CTI for ATH-SMC1 grade.

The sweet spot is 1.0 to 1.2 wt% vinylsilane. Below 1.0 wt% the moisture pickup starts to rise and CTI drops. Above 1.5 wt% there is no meaningful gain and the cost goes up linearly with silane content.

MgO Thickening and Its Interaction with ATH

SMC is matured (thickened) before molding. The paste is rolled into a glass mat and stored at 30 to 40 °C for 1 to 5 days, during which the viscosity rises from about 20 Pa.s to 80 to 200 Pa.s. The thickening is driven by magnesium oxide (or magnesium hydroxide), which reacts with the polyester carboxyl end groups and any residual maleic anhydride to form a magnesium carboxylate network:

MgO + 2 R-COOH (UP resin) → (R-COO)₂Mg + H2O

ATH interacts with this reaction in two ways:

1. ATH surface provides Mg2+ nucleation sites. Each ATH particle has many surface -OH groups that coordinate Mg2+ ions. The higher the ATH surface area (i.e. the finer the ATH), the more nucleation sites, the faster the thickening. Going from D50 of 15 microns to D50 of 4 microns roughly doubles the maturation rate.
2. ATH picks up trace moisture. Any moisture in or on the ATH drives the hydrolysis of MgO to Mg(OH)2, which is less reactive. Over-dried ATH accelerates thickening; damp ATH slows it down. This is why incoming ATH moisture should be specified at below 0.10 wt% and why SMC production lines keep the ATH hopper sealed and dry.

The table below shows the effect of ATH loading on maturation time to 100 Pa.s at 32 °C for a typical orthophthalic UP with 2.0 wt% MgO:

Practical rule: at 170 phr ATH, reduce MgO from 2.0 to 1.0 wt% on resin. Otherwise the maturation window shrinks below 24 hours and you risk over-thickened, unprocessable sheets.

Aluminaworld ATH Grades for SMC and BMC

Aluminaworld supplies three ATH grades optimized for SMC and BMC. All three are produced from Chinese bauxite via the Bayer process, ground to spec, and surface-treated in a continuous fluidized-bed reactor. Each grade ships with a lot-level Certificate of Analysis covering 16 properties.

For each grade we provide a 16-property CoA including Al(OH)3 content (≥ 99.5 wt%), loss on ignition (34.0 to 35.0 wt%, theoretical 34.6%), Fe2O3, Na2O, SiO2, TiO2, CaO, MgO, moisture, D10/D50/D90/D97, oil absorption, specific surface area, whiteness L*, bulk density, and treatment level. SGS on-site audit reports and REACH SVHC declarations are available on request.

Worked Formulation 1: UL94 V-0 SMC for Electrical Enclosure

Target specification: UL94 V-0 at 1.5 mm wall, UL 746C 150 °C RTI, IEC 60112 CTI above 400 V, glow-wire ignition above 750 °C, tensile above 45 MPa. This is a typical industrial electrical enclosure used in motor control centers and PV combiner boxes.

Process: Mix resin, styrene, ATH, CaCO3, zinc stearate, and pigment in a high-shear dissolver at 800 rpm for 15 minutes. Add Trigonox C and mix for 5 more minutes. Add MgO and mix 3 minutes. Transfer to the SMC machine, wet out two glass mats (25 mm chopped roving, 450 g/m2), mature 36 to 48 hours at 32 °C. Compression mold at 150 °C, 80 to 100 bar, 90 seconds per mm of wall thickness. Post-cure 4 hours at 150 °C.

Resulting properties:

• UL94 (1.5 mm): V-0 (no burn to clamp, total afterflame below 5 seconds)
• Glow-wire ignition (IEC 60695-2-12, 1.5 mm): 825 to 860 °C
• CTI (IEC 60112, 140 phr ATH-SMC1): 540 to 580 V
• Tensile strength (ISO 527, 5 mm): 48 to 55 MPa
• Flexural strength (ISO 178): 110 to 130 MPa
• Heat deflection temperature (ISO 75, 1.8 MPa): 215 to 235 °C
• Smoke density (ISO 5659-2, 50 kW/m2, flaming): Ds,max 230 to 270
• Mold shrinkage: 0.05 to 0.15%

Worked Formulation 2: Glow-Wire 960 °C SMC for White Goods

Target specification: IEC 60695-2-12 glow-wire 960 °C at 1.5 mm, IEC 60112 CTI above 500 V, tensile above 40 MPa, no halogen. This is a typical appliance enclosure or HVAC housing for the European market.

The vinyl ester base (instead of polyester) gives higher HDT and better chemical resistance, which suits appliance housings that may see hot air or detergent exposure. The 6 phr zinc borate synergistically raises the glow-wire ignition temperature by 30 to 50 °C and improves char integrity. MgO is dropped to 1.0 phr to compensate for the high surface area of the 170 phr ATH.

Resulting properties:

• Glow-wire ignition (IEC 60695-2-12, 1.5 mm): 925 to 960 °C
• UL94 (1.5 mm): V-0
• CTI (IEC 60112): 480 to 520 V
• Tensile strength: 42 to 48 MPa
• HDT (1.8 MPa): 235 to 260 °C

Worked Formulation 3: BMC for CTI above 600 V (Busbar / PV Combiner Box)

Target specification: IEC 60112 CTI above 600 V, UL94 V-0 at 1.5 mm, EN 45545-2 HL3 smoke density, no halogen. This is a busbar support, PV combiner box base, or switchgear housing where high tracking resistance is essential.

The combination of low-iron ATH-TRK1 + 25 phr calcined alumina pushes CTI above 600 V. The 4N (99.99% purity) alumina contributes no iron or sodium, both of which accelerate tracking. The shorter 6 mm glass fiber is appropriate for BMC injection molding (longer fibers jam the screw).

Resulting properties:

• CTI (IEC 60112): 610 to 650 V
• UL94 (1.5 mm): V-0
• Arc resistance (ASTM D495): 180+ seconds
• Tensile strength: 35 to 42 MPa
• HDT (1.8 MPa): 220 to 240 °C
• Smoke density (ISO 5659-2, flaming): Ds,max 200 to 240

Worked Formulation 4: Low-Smoke BMC for EN 45545-2 HL3 Rail Interior

Target specification: EN 45545-2 HL3 (R1 hazard level), Ds,max below 300, CIT below 0.75, no halogen, glow-wire 960 °C, tensile above 35 MPa. This is a railway interior trim, seat back, or cable trough cover used in metros, high-speed rail, and tram interiors.

The 190 phr ATH + 30 phr MDH blend gives a wide decomposition window (200 to 350 °C ATH, 300 to 420 °C MDH) that produces a more uniform char and lower smoke than either filler alone. The 8 phr zinc borate catalyzes char formation in the 400 to 700 °C range where the polyester backbone is breaking down, dropping Ds,max by another 30 to 50 points.

Resulting properties:

• EN 45545-2 HL3 (R1): Pass (Ds,max 200-260, CIT 0.55-0.70)
• NFPA 130: Pass (Ds,max 200-260)
• Glow-wire 960 °C (IEC 60695-2-12): Pass
• UL94 (1.5 mm): V-0
• Tensile strength: 36 to 42 MPa
• Specific optical density at 4 min: below 200

ATH Effect on Cure Kinetics and Exotherm

Aluminum hydroxide is chemically inert in polyester resin at room temperature, but at cure temperature the ATH surface does interact with the cure chemistry in two important ways.

First, the ATH surface is mildly basic. The Al-OH groups on the surface can act as weak Lewis bases, accelerating the peroxide decomposition that initiates the free-radical cure. In a typical 1.0 phr t-butyl peroxybenzoate system, the addition of 140 phr ATH shortens the gel time at 140 °C by 10 to 20 percent (from 35 to 40 seconds down to 28 to 32 seconds). For a fast-cure SMC line that runs at 90 to 120 second cycles, this is helpful. For a thick part where the exotherm needs to be controlled, the accelerated gel time can cause thermal cracking.

Second, the high specific heat of ATH (1.30 kJ/(kg.K) versus 1.55 kJ/(kg.K) for the resin) means the filled composite absorbs more heat during the cure exotherm. The peak exotherm temperature in a 6 mm thick SMC part drops from 195 to 210 °C (no ATH) to 165 to 180 °C (140 phr ATH). For a 2 mm thick part, the drop is smaller (15 to 20 °C). This is a real benefit for thick parts: less thermal stress, less micro-cracking on cool-down, and lower scrap.

Vinylsilane-treated ATH slightly moderates the cure acceleration effect because the silane layer covers the surface Al-OH groups. The treated ATH shows gel time within 5 percent of unfilled resin, while untreated ATH shows 10 to 20 percent acceleration. If you are switching from untreated to vinylsilane-treated ATH at the same loading, expect the SMC cure cycle to extend by 5 to 8 percent and adjust the press timer or initiator level accordingly.

How to design the mold heating cycle around ATH decomposition

ATH starts losing bound water at 200 °C. If the mold surface temperature at the moment of press closure is above 200 °C, the ATH at the part surface will start decomposing before the resin gels. The water vapor released at this point gets trapped in the part as porosity and sink marks. The fix is to bring the mold to 140 to 150 °C, close the press at low pressure to allow the paste to flow, then ramp the mold to 150 to 160 °C over 30 to 60 seconds for cure. The total cycle for a 3 mm SMC part is 90 to 120 seconds at 150 to 160 °C mold temperature, 80 to 120 bar pressure.

For BMC injection, the screw plasticizing zone must stay below 70 °C to avoid premature decomposition of the ATH. Barrel zones: 40 to 50 °C feed, 50 to 60 °C compression, 60 to 70 °C metering, 145 to 155 °C mold. Higher barrel temperature causes ATH to lose bound water inside the screw and the resulting steam blows the part surface.

ATH and Glass Fiber: Wetting, Sizing, and Mechanical Retention

The mechanical performance of ATH-filled SMC and BMC depends on how well the ATH and the glass fiber transfer stress to the polyester matrix. The two fillers interact in two ways.

First, ATH particles at the glass fiber surface act as stress concentrators. A 12-micron ATH particle bonded to a 12-micron glass filament creates a notch at the interface. Under tensile load, micro-cracks initiate at this notch. The crack then propagates through the polyester matrix. The net effect is that tensile strength drops by 1.0 to 1.5 percent for every 10 phr of ATH added, all else being equal. Vinylsilane treatment reduces the drop to 0.5 to 0.8 percent per 10 phr because the silane bonds the ATH to the matrix and blunts the crack initiation.

Second, the ATH surface competes with the glass fiber surface for resin wetting. Untreated ATH is polar and preferentially wets the (also polar) polyester resin. The glass fiber, which is sized with a silane coupling agent, also wants to be wetted. In a poorly designed system, the ATH wins the wetting contest and the glass fiber ends up with dry spots, breaking the load path. Vinylsilane treatment on the ATH (which is non-polar) shifts the wetting balance back toward the glass fiber, so the polyester resin wets both surfaces evenly.

The table below shows the effect of ATH surface treatment on glass fiber wet-out and mechanical properties for a 30 percent glass / 140 phr ATH SMC:

Vinylsilane treatment gives 25 to 30 percent higher tensile and 35 to 45 percent higher impact strength, at a silane cost of about 0.10 to 0.15 USD per kg of ATH (or 8 to 12 USD per metric ton of SMC compound). The economics are overwhelmingly positive.

ATH vs MDH: When to Use Which, and When to Blend

Magnesium hydroxide (MDH, Mg(OH)2) is the other common metal hydroxide flame retardant. MDH decomposes at 300 to 350 °C - significantly higher than ATH. This temperature difference matters in three situations.

Higher processing temperature. MDH lets you mold at 180 to 200 °C without premature decomposition. Useful for thick parts or fast cure cycles where the mold runs hot. ATH is limited to 160 to 170 °C for the same reason.

Higher endothermic heat. MDH absorbs 1,300 kJ per kg versus 1,100 for ATH. About 18 percent more heat sink per kg loaded. The trade-off is that you need more MDH by weight to get the same fire performance because MDH is heavier (molecular weight 58 versus 78 for ATH) and contains less hydroxyl per unit mass.

Higher smoke density. MDH produces about 10 to 15 percent more smoke than ATH at the same fire performance level, because the magnesium oxide residue is less effective at forming a continuous char. MDH is not preferred for EN 45545-2 or NFPA 130.

The table below compares ATH and MDH on the key selection criteria:

For most applications ATH is the better choice. MDH wins when you need high processing temperature (above 180 °C) or very high CTI (above 650 V, achievable with 200+ phr MDH without the iron/sodium penalty). A 70/30 ATH/MDH blend combines the wide decomposition window of MDH with the low smoke and lower cost of ATH, and is the preferred solution for EN 45545-2 HL3 rail interior (see Formulation 4 above).

Incoming QC: 16-Property ATH Specification for SMC and BMC

Every batch of ATH you receive should be tested against a 16-property specification. The QC data below corresponds to the Aluminaworld ATH-SMC1, ATH-BMC1, and ATH-TRK1 grades. For your own spec, copy the test methods, accept/reject criteria, and sampling plan.

The sampling plan we recommend: pull 1 in 25 bags from each 20 MT container, blend, and reduce to a 1 kg lab sample via a rotary splitter. Test moisture first (most variable parameter, fastest indicator of storage problems). Then run the full 16-property panel on the reduced sample. If any property fails, segregate that lot and call the supplier immediately. Vinylsilane-treated ATH has a shelf life of 12 months in unopened bags at 20 to 30 °C. Once opened, use within 30 days or re-seal with desiccant.

Storage, Packaging, and Logistics

ATH is hygroscopic. The hydroscopicity is mild (a 25 kg bag stored at 60 percent relative humidity picks up 0.3 to 0.5 wt% moisture in 30 days) but enough to cause fire test failures and surface defects. The table below gives practical storage guidance.

Packaging options:

• 25 kg multi-wall paper bag with PE liner. Standard. Palletized 40 bags per pallet (1 MT). 20 MT per 20 ft container.
• 500 kg or 1000 kg polypropylene big bag (FIBC). Cost-effective for high-volume users. 20 MT per 20 ft container with 20 to 40 big bags.
• Super sack with PE liner and desiccant. For tropical climates or extended storage.
• Bulk tanker (20 MT silo truck). For the largest users. The tanker must be dry and sealed; we recommend a 1 to 2 hour dry nitrogen blanket on the load.

Aluminaworld ships from Qingdao Port (80 km from our factory) and Shanghai Port. FOB, CIF, and CFR terms are all available. Standard 20 ft container fits 20 MT of ATH in 25 kg bags or 20 to 22 MT in 1000 kg big bags. Lead time: 7 to 10 days for trial orders, 15 to 20 days for bulk 100+ MT orders.

Total Cost of Ownership: ATH-Filled SMC and BMC

A common procurement mistake is to compare ATH price per kilo without accounting for the system-level economics. The table below shows the total cost of ownership for a 140 phr ATH SMC for an electrical enclosure, comparing Aluminaworld ATH-SMC1 (China), European ATH, and North American ATH.

For a compounder consuming 1000 MT of SMC per year, switching from European to Chinese ATH saves about 600,000 USD per year. The trade-off is longer lead time (15 to 20 days vs 3 to 6 weeks), potential customs issues, and the need to qualify a new supplier. Most compounders we work with run a dual-source strategy: 70 to 80 percent from a low-cost Chinese supplier, 20 to 30 percent from a regional supplier as backup.

Rheology Models for ATH-Filled Polyester

For compounders who want to model paste viscosity and predict processability from ATH grade and loading, the Krieger-Dougherty equation (shown earlier) is the workhorse. The other useful model is the Mooney equation for highly filled systems, which gives a more accurate prediction at volume fractions above 0.40:

ln(η / η₀) = K_E φ / (1 - φ / φ_m)

where K_E is the Einstein coefficient (2.5 for spheres, 3.5 to 4.5 for irregular particles like ATH) and φ_m is the maximum packing fraction (0.60 to 0.65 for vinylsilane-treated ATH with broad PSD, 0.55 to 0.60 for untreated ATH with narrow PSD).

The practical use of the Mooney model is to compare candidate ATH grades before doing physical trials. A 130 phr loading of a vinylsilane-treated ATH with D50 12 microns (broad PSD, φ_m = 0.64) gives predicted paste viscosity about 30 percent lower than the same loading of a narrow-PSD untreated ATH with D50 8 microns (φ_m = 0.57). The difference is large enough to determine whether the compound can be processed on the SMC machine.

For more sophisticated modeling, the Cross-Williamson model captures the shear-thinning behavior of filled polyesters:

η = η_∞ + (η₀ - η_∞) / (1 + (K · γ)^m)

where η_∞ is the high-shear viscosity (about 0.5 to 2 Pa.s for filled SMC), η₀ is the low-shear viscosity (the Krieger-Dougherty result), K is a time constant, m is the shear-thinning exponent (about 0.5 to 0.7 for filled polyesters), and γ is the shear rate.

The implication: at the low shear rates in the maturation tank (1 to 10 s^-1), the paste is much more viscous than at the high shear rates in the SMC machine nip (100 to 1000 s^-1). A 100 Pa.s paste at 1 s^-1 drops to 10 to 20 Pa.s at 100 s^-1. This is why an SMC paste that looks "thick" in the maturation tank can still be rolled into the glass mat without tearing it.

The Future of ATH in SMC and BMC

Three trends are worth watching.

First, ultra-fine surface-treated ATH grades (D50 1.0 to 2.0 microns) with sub-micron top-cuts are entering the market. These allow higher loadings (200 to 220 phr in BMC) without catastrophic viscosity rise, because the PSD is engineered to pack efficiently. Aluminaworld is developing a nano-dispersed ATH for a Tier-1 European switchgear compounder that targets 220 phr ATH + 30 phr MDH in a 200 mm diameter busbar support.

Second, ATH is being combined with phosphorus-based FR synergists (aluminum diethyl phosphinate, AlPi) for hybrid systems that hit UL94 V-0 at 80 to 100 phr ATH + 15 to 20 phr AlPi. The advantage is lower loading and lower smoke. The disadvantage is AlPi cost (12 to 18 USD/kg) and some tracking penalty. The combination works for low-voltage electrical enclosures where the customer can pay for the premium.

Third, recycled ATH from end-of-life SMC/BMC parts is starting to be commercialized. The recyclate is calcined to remove organic contamination, re-ground, and re-treated with vinylsilane. Performance is 5 to 10 percent below virgin ATH but cost is 30 to 50 percent lower. For non-critical applications (cable tray, junction box base) the recycled ATH is a viable option. Aluminaworld does not currently supply recycled ATH but is evaluating a 5000 MT/year line for 2027.

Troubleshooting Flowchart: From Symptom to Root Cause

The most common processing problems with ATH-filled SMC and BMC have well-understood root causes. The following flowchart maps the symptom to the most likely cause and the corrective action.

Symptom: Part has surface porosity or pinholes.

1. Check ATH moisture with a 105 °C / 2 hour loss-on-drying test. If above 0.15 wt%, re-dry the ATH at 110 °C for 4 hours or reject the lot.
2. Check the mold surface temperature at the moment of press closure. If above 200 °C, the ATH at the part surface is decomposing prematurely. Lower the mold to 145 to 155 °C and let the exotherm carry the temperature to 160 to 170 °C after gel.
3. Check the resin moisture. Unsaturated polyester resin picks up 0.1 to 0.3 wt% moisture during storage. A vacuum-degas step (50 mbar, 30 minutes) before compounding removes this moisture.
4. Check the SMC maturation history. Under-matured SMC (less than 24 hours) has higher styrene content and is more prone to void formation during the high-temperature press cycle.

Symptom: Paste viscosity too high in the maturation tank, SMC sheets too stiff to roll.

1. Reduce MgO from 2.0 to 1.0 wt% on resin.
2. Switch from active MgO (40 m2/g) to standard MgO (10 m2/g).
3. Check ATH moisture: above 0.15 wt% moisture accelerates thickening via MgO hydrolysis to Mg(OH)2 and free-radical acceleration of the resin-magnesium gel.
4. Reduce ATH loading by 10 to 15 phr if the fire spec allows.
5. Switch from D50 10 to 15 micron ATH to a coarser grade (D50 15 to 20 micron) to reduce surface area.

Symptom: Part fails UL94 V-0 (afterflame exceeds 10 seconds).

1. Confirm ATH loading in the compound. A 5 percent loss (e.g. 140 phr instead of 150 phr) is enough to drop V-0 to V-1. Check the ATH weight in the formulation against the actual batch sheet.
2. Confirm ATH grade. A switch from vinylsilane-treated to untreated ATH drops fire performance by 10 to 15 percent at the same loading due to the porosity and micro-void issues.
3. Check the ATH moisture. Above 0.20 wt% moisture means 1 to 2 percent of the ATH is already converted to Al2O3, which does not deliver flame retardancy. Re-dry the ATH or reject.
4. Check the resin type. A dicyclopentadiene (DCPD) UP resin has lower heat resistance and produces more flammable pyrolysis gases than isophthalic. Switch to isophthalic or vinyl ester.
5. Increase ATH loading by 10 to 20 phr.

Symptom: Part fails glow-wire ignition (char cracks or ignites before 750 °C).

1. ATH alone rarely achieves glow-wire above 850 °C. Add 5 to 8 phr zinc borate to synergize char formation.
2. Check that the part wall thickness is at the test spec. A 1.5 mm wall has different glow-wire performance than a 3.0 mm wall.
3. Check that the test laboratory used the correct IEC 60695-2-12 procedure. The 1 N applied force and the 30 second exposure time are common source of inter-lab variation.

Symptom: CTI drops below the target (e.g. below 400 V for a 400 V spec).

1. Check ATH Fe2O3. Iron is the dominant tracking accelerator. Switch to ATH-TRK1 (Fe2O3 below 50 ppm) or ATH-BMC1 (Fe2O3 below 60 ppm).
2. Check that the resin is isophthalic or vinyl ester. Orthophthalic with high acid value degrades CTI.
3. Check the surface treatment. Untreated ATH gives 50 to 100 V lower CTI than vinylsilane-treated ATH at the same loading.
4. Add 5 to 10 phr of sub-micron calcined alumina to displace ATH at the high-electric-field surface.

Symptom: Mold sticking, parts tear on ejection.

1. Increase zinc stearate from 3.0 to 4.0 to 5.0 phr.
2. Check the mold surface for residual ATH dust. A mold release wax applied at the start of each shift is standard practice.
3. Check the part cure state. Under-cured parts stick more than fully cured parts. Verify that the exotherm peak is reaching 160 to 170 °C.
4. Reduce ATH loading by 5 to 10 phr (high loading increases resin-rich surface at the mold interface).

Abbreviations and Standards Reference

For reference, the standards and abbreviations used throughout this article:

• ATH: Aluminum hydroxide, Al(OH)3, alumina trihydrate, CAS 21645-51-2
• MDH: Magnesium hydroxide, Mg(OH)2, CAS 1309-42-8
• SMC: Sheet Molding Compound, compression-molded glass-fiber reinforced UP
• BMC: Bulk Molding Compound, injection-molded glass-fiber reinforced UP
• UP: Unsaturated polyester resin
• VE: Vinyl ester resin (epoxy-based backbone)
• VTME: Vinyl trimethoxy silane, CAS 2768-02-7
• VTEO: Vinyl triethoxy silane, CAS 78-08-0
• LOI: Limiting Oxygen Index, ASTM D2863, percent oxygen in O2/N2 mixture to sustain burning
• UL94: Underwriters Laboratories vertical burn test, V-0/V-1/V-2 ratings
• CTI: Comparative Tracking Index, IEC 60112, voltage at which tracking failure occurs
• Dm: Maximum smoke density, ISO 5659-2, 50 kW/m2 flaming
• CIT: Conventional Index of Toxicity, EN 45545-2 Annex C
• RTI: Relative Thermal Index, UL 746C, long-term aging temperature
• PSD: Particle Size Distribution, laser diffraction (Malvern, Sympatec, Beckman Coulter)
• ICP-OES: Inductively Coupled Plasma Optical Emission Spectroscopy (for trace element analysis)
• XRF: X-ray Fluorescence (for major oxide analysis)
• TGA: Thermogravimetric Analysis (for LOI, moisture, and surface treatment level)
• BET: Brunauer-Emmett-Teller surface area, N2 adsorption at 77 K

The full text of these standards is available from ANSI (US), BSI (UK), DIN/VDE (Germany), AFNOR (France), and GB (China). For EN 45545-2 the railway certification bodies (ERA, EBA, RMQS) provide interpretation guidance.

3 Production Case Studies from Real Compounders

Case A: Electrical enclosure, Italian compounder, UL94 V-0 + glow-wire 850 °C

A Tier-1 electrical enclosure manufacturer in northern Italy was running an orthophthalic SMC line for motor control center panels. They needed to pass UL94 V-0 at 2.0 mm and IEC glow-wire 850 °C at 1.5 mm while keeping a Class A paintable surface. Their previous formulation used 110 phr untreated ATH and 18 phr decabromodiphenyl ether / 4 phr Sb2O3 halogen package. Smoke density failed the customer spec at Ds,max 480.

Aluminaworld recommended ATH-SMC1 at 145 phr + 4 phr zinc borate, with vinylsilane treatment at 1.0 wt%. The first trial at the customer plant hit UL94 V-0 at 2.0 mm and glow-wire 875 °C at 1.5 mm on the first run. Smoke density (ISO 5659-2) dropped to 240. The customer paid a slight premium per kilo of ATH but saved by dropping the Sb2O3 cost entirely. Annual contract: 240 metric tons ATH-SMC1, FOB Genoa.

Case B: Automotive headlamp reflector, Mexican compounder, BMC injection

An automotive Tier-1 headlamp reflector manufacturer in Puebla was using a BMC formulation with 160 phr untreated ATH and was experiencing high scrap rates (8 to 12 percent) due to surface defects that ruined the metallization step. The defects were traced to moisture pickup in the untreated ATH (0.5 to 0.8 wt%), which vaporized during the BMC injection cycle and left micro-voids at the surface.

Aluminaworld ATH-BMC1 (D50 5 microns, vinylsilane 1.2 wt%, moisture ≤ 0.08 wt%) was qualified after three trials. The moisture-driven void defects disappeared. The finer D50 also improved the surface smoothness of the molded reflector and reduced the metallization rework rate. The customer reported a 4 percent drop in scrap and a 15 percent increase in metallization line throughput. Annual contract: 80 metric tons ATH-BMC1, FOB Veracruz.

Case C: Busbar support, Korean switchgear manufacturer, CTI above 600 V

A switchgear manufacturer in Korea needed a BMC for low-voltage busbar supports that would pass CTI above 600 V per IEC 60112, plus UL94 V-0 at 3.0 mm. Their previous formulation with commodity ATH gave CTI of 380 to 410 V, well below the requirement. The compounder tried blending in 20 phr calcined alumina but the CTI only reached 520 V.

Aluminaworld supplied ATH-TRK1 (Fe2O3 ≤ 50 ppm, Na2O ≤ 0.05 wt%) at 170 phr + 25 phr 4N calcined alumina. CTI reached 620 to 650 V. The customer was able to drop the wall thickness of the busbar support from 4.0 to 2.5 mm, saving 38 percent in material per part. Annual contract: 120 metric tons ATH-TRK1, FOB Busan.

The Standards Ladder: From UL94 to EN 45545

SMC and BMC parts in transit, rail, EV battery, and switchgear applications must satisfy a stack of standards. The most common ones:

Each step up the ladder costs roughly 10 to 20 phr more ATH. Plan the compound budget accordingly: ATH alone is 3 to 4 USD/kg, so going from UL94 V-0 (140 phr) to EN 45545-2 HL3 (180 phr) costs about 1.5 to 2.0 USD per kg of compound extra in ATH alone. This is still far cheaper than the alternative (phosphorus FR at 12 to 20 USD/kg).

Controlling MgO Thickening at High ATH Loading

Two practical patterns for high-ATH SMC:

1. Use less MgO. At 170 phr ATH, drop MgO from 2.0 to 1.0 wt% on resin. The high surface area of the ATH provides enough Mg2+ nucleation to drive thickening in 24 to 36 hours. Using active MgO (high surface area, 30 to 50 m2/g) is more controllable than standard MgO (10 to 15 m2/g).
2. Use pre-thickened resin. Buy a pre-thickened UP resin from the supplier with viscosity already at 50 to 80 Pa.s. This eliminates the maturation variability caused by inconsistent ATH moisture. The downside is shorter pot life of the SMC paste (8 to 12 hours vs 24 to 48 hours for fresh mix).

For BMC, the MgO level is usually lower (0.3 to 0.8 wt% on resin) because BMC does not need the same maturation window. BMC is mixed and injected within 1 to 4 hours. Over-thickening in BMC causes screw jam and short shots.

10 Common Mistakes When Formulating ATH-Filled SMC and BMC

1. Using untreated ATH and expecting UL94 V-0. Untreated ATH picks up moisture and fails the fire test by porosity and sink marks. Always use vinylsilane-treated ATH at 0.8 to 1.5 wt%.
2. Specifying D50 too fine. Sub-2-micron ATH is not a cure-all. It drives paste viscosity to unprocessable levels. Stick to D50 5 to 15 microns for SMC and BMC.
3. Ignoring Fe2O3. Iron above 100 ppm drops arc-track resistance. For CTI above 600 V specify Fe2O3 below 50 ppm (ATH-TRK1).
4. Over-thickening with MgO at high loading. 2.0 wt% MgO at 170 phr ATH gives a 12-hour maturation window. Use 1.0 wt% or pre-thickened resin instead.
5. Forgetting the zinc stearate. Zinc stearate at 2 to 4 phr is the mold release and also reduces ATH-resin friction. Skipping it makes parts stick to the mold and tear on ejection.
6. Compounding at high temperature. ATH starts to decompose at 200 °C. If the dissolver or extruder heats the paste above 70 °C, you lose active ATH to premature dehydration. Keep paste temperature below 50 °C during compounding.
7. Storing ATH in an open hopper. ATH picks up 0.5 wt% moisture in 24 hours. Keep the hopper sealed and dry, or use big-bag unload directly into the dissolver.
8. Mixing ATH with Sb2O3 + decabromodiphenyl ether. This was a common legacy combination. Sb2O3 + bromine gives high smoke and fails EN 45545-2. Use ATH + zinc borate for the halogen-free system.
9. Using regular polyol or dicyclopentadiene (DCPD) UP resin. These resins have lower HDT and higher smoke than isophthalic or vinyl ester. For high-temperature or rail applications, spend the extra 0.5 to 1.0 USD/kg on isophthalic or vinyl ester resin.
10. Ignoring ATH storage temperature. ATH stored above 40 °C for weeks can lose 0.3 to 0.5 wt% bound water. Check incoming moisture with a loss-on-drying test at 105 °C for 2 hours. If it is above 0.15 wt%, condition the bag in a cool dry area before use.

Regional Sourcing: Chinese vs European ATH for SMC and BMC

ATH for SMC and BMC is a global commodity. Three regional price-and-quality brackets exist:

For most SMC and BMC compounders the Chinese value-priced grade (Aluminaworld ATH-SMC1 at 0.85 to 1.05 USD/kg) is the right economic choice. For CTI-above-600 V busbar and PV applications, the European or North American low-Fe2O3 grades command a 30 to 50 percent premium that is justified by the failure-cost avoidance.

Aluminaworld SMC/BMC ATH Capability

Aluminaworld has supplied ATH to SMC and BMC compounders for 15 years. Our 28,000 m2 facility in Zibo, Shandong runs three continuous surface-treatment lines (vinylsilane, methacrylic silane, stearic acid) with a combined capacity of 20,000 metric tons per year. We have supplied:

• ATH-SMC1 to a Tier-1 Italian electrical enclosure maker (240 MT/yr since 2022)
• ATH-BMC1 to a Mexican automotive headlamp supplier (80 MT/yr since 2023)
• ATH-TRK1 to a Korean switchgear compounder (120 MT/yr since 2024)
• Custom ATH/MDH/zinc borate pre-blends to three European rail interior compounders (60 to 150 MT/yr each)

All shipments are backed by SGS on-site audit reports, REACH SVHC declarations, and 16-property lot-level CoA. We can ship 1 MT trial quantities within 7 to 10 days and full 20 MT bulk orders within 15 to 20 days FOB Qingdao (80 km from our factory).

Next Steps for Your SMC or BMC Project

If you are designing or sourcing an ATH-filled SMC or BMC compound, the right grade and loading depend on three decisions: what fire standard you must hit (UL94, glow-wire, EN 45545), what electrical performance you need (CTI, arc resistance), and what processability you can tolerate (paste viscosity, maturation window). The data and formulations above should let you converge on a starting point without expensive trial-and-error.

For samples, technical data sheets, MSDS, or a quote on ATH-SMC1, ATH-BMC1, or ATH-TRK1, contact us via:

• WhatsApp: +86 133 2522 2240 (fastest, 12-hour reply)
• Email: barry@aluminaworld.com
• Sample request: 25 kg bag (1 MT MOQ for trial), 7-10 day lead time, full CoA included
• Bulk orders: 20 MT MOQ, 15-20 day production, FOB/CIF/CFR from Qingdao Port (80 km from our factory)

Aluminaworld has supplied ATH to SMC and BMC compounders in 60+ countries for 15 years. Our ATH is manufactured under ISO 9001 quality control with SGS on-site audits, full Alibaba Trade Assurance, and a 16-property lot-level CoA on every shipment. Tell us the fire standard you are targeting, the resin system, and the ATH loading range. We will send back a recommended grade, a sample, and a quote within 12 hours.