ATH for Phenolic Resin (PF) in Brake Pads: Why Loading > 60% Wins Fade Tests (SAE J661 Data Inside)

Why ATH Loading Above 60% Is the Fade-Test Winner

Aluminum trihydroxide (ATH, Al(OH)3) is the dominant inorganic filler in a phenolic resin brake pad, occupying 50 to 75 percent of the friction material by mass. The two jobs ATH does in the pad are (1) absorb the heat of friction by endothermic dehydration (220 to 480 C, +739 kJ per kg of ATH), which keeps the surface temperature low and prevents the resin from charring and losing mu; and (2) provide the third-body abrasion that maintains a stable friction coefficient as the pad and rotor wear. The first job is the fade-resistance mechanism; the second job is the mu-stability mechanism. Both jobs are mass-fraction dependent, and the loading must be tuned to the application: 60 to 65 percent for organic passenger pads, 45 to 55 percent for heavy-truck pads, 30 to 40 percent for low-metallic pads, and 5 to 10 percent for ceramic pads.

The endothermic heat absorption is the single most important fade-resistance mechanism in an organic brake pad. The thermal decomposition of ATH in the pad follows a two-step dehydration: Al(OH)3 to AlOOH (boehmite) at 220 to 280 C, +298 kJ per kg; then AlOOH to gamma-Al2O3 at 380 to 480 C, +441 kJ per kg. The total +739 kJ per kg of ATH, when multiplied by the 60 percent loading and the 0.5 to 1.0 kg of ATH in a typical passenger-car pad, gives a total heat sink of 220 to 440 kJ per pad. This is enough to absorb the friction-induced heat during a 10-stop fade cycle (the SAE J661 schedule), keeping the surface temperature at 280 to 340 C instead of the 400 to 480 C that an unloaded pad would reach. The temperature difference is the difference between a pad that maintains mu 0.38 to 0.42 and a pad that fades to mu 0.20 to 0.25 (a fade that fails ECE R90 effectiveness and triggers a recall).

The third-body abrasion mechanism is the second key function. As the pad and rotor wear, ATH particles (now partially or fully converted to gamma-Al2O3 by the friction heat) become the third-body abrasive that maintains the contact between the pad and rotor. The third-body layer stabilizes mu by providing a controlled, conformal contact. Without the third-body layer, the pad would either glaze (polish the rotor to a mirror finish, mu drops to 0.10 to 0.20) or pick up (transfer pad material to the rotor, mu spikes to 0.50 to 0.60). The third-body layer needs a minimum of 30 to 40 percent ATH to be stable, and the optimum is 50 to 60 percent. Above 70 percent, the third-body layer becomes too thick and the mu becomes erratic (high variability from stop to stop).

The 60 to 65 percent loading is the SAE J661 Chase machine sweet spot for organic passenger pads. Below 50 percent, the fade Delta mu (max mu minus min mu during the 10-stop fade) rises to 0.18 to 0.25, which is a fade that fails the ECE R90 second-effectiveness-stop test (minimum mu 0.35) and the GB 7258 China brake-pad standard. Above 70 percent, the pad becomes brittle (the resin content falls below 15 percent and the pad cannot be hot-pressed into a coherent mass), the wear rate rises, and the noise propensity jumps. The 60 to 65 percent range is the industry standard for organic pads in the European, North American, and Chinese markets, and the Aluminaworld ATH-25F and ATH-25C grades are formulated to deliver this loading in the standard 8-step pad recipe.

The Fade Mechanism: How ATH Absorbs Heat and Resin Pyrolyzes

The brake-pad fade mechanism is a balance between two competing thermal events: the endothermic dehydration of ATH and the exothermic pyrolysis of the phenolic resin. The two events occur in the same temperature range (300 to 600 C), and the ratio of endothermic to exothermic heat determines whether the pad temperature rises (fade) or stabilizes (fade resistance). At 60 percent ATH loading, the ratio favors the endothermic side and the pad stabilizes; at 50 percent, the ratio favors the exothermic side and the pad fades.

The table shows the four thermal events and the net heat flux. The first endothermic event (220 to 280 C) is a strong heat sink that cools the pad during the early fade. The second endothermic event (380 to 480 C) is a moderate heat sink that arrests the pad temperature during the mid-fade. The post-480 C exothermic resin pyrolysis is unopposed because ATH has finished its decomposition, and the pad temperature rises to 500 to 600 C during the late fade. The pad survives this late-fade heat because the resin has already charred to a stable carbon residue that maintains mu at 0.20 to 0.25. The pad cannot recover from a complete resin burn-out (above 650 C), and the fade Delta mu becomes catastrophic.

The standard 60 percent ATH loading is calibrated to match the resin pyrolysis curve so that the two endothermic events at 220 to 280 C and 380 to 480 C coincide with the two resin char events at 280 to 380 C and 480 to 600 C. The matching produces a "double-buffer" effect that holds the pad temperature below 500 C for the full 10-stop fade cycle. At 50 percent ATH, the two endothermic events are too small to buffer the resin exotherm, and the pad temperature climbs to 550 to 600 C by stop 6, triggering catastrophic fade. At 70 percent ATH, the two endothermic events are larger than needed and the pad temperature stays below 300 C, but the resin content is too low to maintain pad integrity and the pad disintegrates by stop 4.

The phenolic resin pyrolysis enthalpy is the other side of the balance. The standard straight phenolic resin (PF-2230, a cashew-modified phenolic) has a pyrolysis enthalpy of +1200 to +1800 kJ per kg of resin, with a mass loss of 35 to 45 percent at 350 to 600 C. The exothermic heat is what makes the pad hot, and the resin content must be high enough to maintain the mu (resin char is the load-bearing phase at high temperature) but low enough to limit the exothermic heat. The standard 18 to 22 parts resin in a 100-part pad recipe gives a resin content of 18 to 22 percent by mass, which is the optimum for organic pads. Below 15 percent resin, the pad cannot be hot-pressed; above 25 percent resin, the exothermic heat dominates and the pad fades.

ATH Grades for Brake Pads: ATH-25F, ATH-25C, ATH-75, and ATH-75HV

Aluminaworld supplies four ATH grades engineered for the four friction-material positions in a brake pad: ATH-25F (fine, D50 2.5 micron, organic passenger pad), ATH-25C (coarse, D50 12 micron, bimodal blend partner), ATH-75 (coarse, D50 18 micron, heavy-truck pad and drum brake lining), and ATH-75HV (high-viscosity, D50 25 micron, railway brake block). The four grades share the same Bayer-process alumina hydrate source (Al(OH)3 purity 99.6 percent, Fe2O3 0.015 percent, SiO2 0.012 percent, Na2O 0.18 percent), but they differ in the air-classifier mill cut point, the surface treatment, and the moisture specification. The choice of grade depends on the pad position, the mu target, and the noise constraint.

The grade selection logic is straightforward. For a passenger-car organic pad with mu target 0.38 to 0.45 and ECE R90 compliance, ATH-25F is the standard. The fine D50 (2.5 micron) gives a smooth, conformal contact with the rotor, and the stearic acid treatment gives moisture resistance and internal mold release. For a pad that needs higher mu (0.42 to 0.50), ATH-25F is blended with ATH-25C in a 70:30 mass ratio to create a bimodal particle size distribution; the fine fraction stabilizes mu and the medium fraction raises mu. For a heavy-truck pad with mu target 0.50 to 0.60 and SAE J661 fade requirement of 0.35 minimum, ATH-75 is the standard; the coarse D50 (18 micron) gives a higher mu through third-body abrasion, and the vinyl silane treatment couples the ATH to the resin matrix for higher pad shear strength. For a railway brake block with mu target 0.45 to 0.55 and high thermal mass requirement, ATH-75HV is the standard; the very coarse D50 (25 micron) gives a high mu and a low resin demand.

The surface treatment is the most important differentiator. Stearic acid (0.3 to 0.5 percent) is used on ATH-25F to give moisture resistance and internal mold release. The stearic acid coats the ATH surface with a hydrophobic monolayer, which prevents the ATH from absorbing moisture during storage and which gives a clean release from the hot-press mold. The stearic acid also reduces the bond strength to the phenolic resin by 5 to 10 percent, but this is acceptable for organic pads because the pad is not structural (the back-plate carries the mechanical load). Vinyl silane (0.1 to 0.3 percent, 3-methacryloxypropyltrimethoxysilane KH-570) is used on ATH-75 to give a chemical bond between the ATH and the phenolic resin. The silane couples the ATH surface hydroxyl groups to the resin methylol groups during the 160 to 180 C hot-press cure, raising the pad shear strength by 25 to 40 percent. The silane is essential for heavy-truck pads because the pad carries a higher mechanical load and the bond must survive the 500 to 600 C fade peak.

The whiteness specification (L* > 96 for ATH-25F, L* > 93 for ATH-75) is a proxy for Fe2O3 and TiO2 impurity content. The Fe2O3 comes from the bauxite ore and the steel-mill grinding media; the TiO2 comes from the bauxite. The whiteness target is set by the pad color requirement: light-colored pads (white, beige, cream) need ATH with L* > 95; dark-colored pads (black, gray) can tolerate ATH with L* > 85. The Aluminaworld ATH-25F at L* 96 to 97 is suitable for light-colored pads; the ATH-75 at L* 93 to 95 is suitable for dark-colored pads and heavy-truck pads. The whiteness is measured by ISO 2470 (paper and board whiteness) modified for powders, with the sample pressed into a 30 mm diameter pellet at 5 MPa.

ATH Particle Size: How D50 Controls mu, Wear, and Noise

ATH mean particle size D50 controls the friction coefficient mu, the wear rate, and the brake squeal propensity of an organic pad, all independently of the loading. The D50 effect is mediated by the contact pressure at the pad-rotor interface: a fine ATH gives a smooth, conformal contact (low contact pressure, low mu, low wear, low noise); a coarse ATH gives a hard-particle contact (high contact pressure, high mu, high wear, high noise). The industry optimum is a bimodal blend of 70 percent fine (D50 2 to 4 micron) plus 30 percent coarse (D50 10 to 15 micron), which gives mu 0.40 to 0.45, wear rate 1.0 to 1.3 g per application, and AK-Master squeal index 2.

The D50 effect is driven by the contact mechanics. A fine ATH particle (D50 2.5 micron) has a small contact area with the rotor, and the local contact pressure is high (because the total load is divided by a small area). The high contact pressure increases the third-body abrasion rate, but the abrasion is spread over many small particles, so the wear rate is low and the mu is moderate. A coarse ATH particle (D50 18 micron) has a larger contact area and a lower local contact pressure, but the third-body abrasion is concentrated in fewer, larger particles, so the wear rate is higher and the mu is higher. The transition from fine to coarse is also the transition from conformal contact to hard-particle contact, and the noise propensity rises because the hard particles excite the rotor-disc modal density.

The bimodal blend (70 percent fine + 30 percent coarse) is the engineering solution to the trade-off. The fine fraction fills the interstitial space between the coarse particles and provides a continuous third-body layer that stabilizes mu. The coarse fraction provides the mu boost through hard-particle abrasion. The blend is formulated by dry-blending the two grades in a high-intensity mixer (Eirich or Lodige) for 8 to 12 minutes, then proceeding to the hot-press step. The blend ratio is 70:30 to 60:40 by mass, with the higher fine fraction used for low-noise pads and the higher coarse fraction used for high-mu pads. The Aluminaworld ATH-25F and ATH-25C grades are the standard blend components for organic passenger pads.

The D99 top cut is the second important particle size parameter. D99 is the particle size at which 99 percent of the ATH is finer; the top 1 percent is the oversize that can scratch the rotor or generate visible dust on the wheel. The industry standard is D99 below 45 micron for organic pads (to prevent rotor scoring) and D99 below 75 micron for heavy-truck pads (where rotor scoring is less critical). The Aluminaworld ATH-25F has D99 below 45 micron (typical 30 to 40 micron) and the ATH-25C has D99 below 75 micron (typical 55 to 70 micron). The D99 is controlled by the air-classifier mill cut point, with a higher cut point giving a coarser product.

SAE J661 Chase Machine: The Fade and Recovery Test

SAE J661 is the standard test method for measuring brake-pad friction and wear on the Chase machine. The Chase machine is a sub-scale dynamometer that mounts a 30.5 mm x 25.4 mm friction material sample against a rotating cast-iron rotor of 279.4 mm diameter. The standard test schedule is a 30-minute burnish at 308 C, then a fade cycle of 10 incremental temperature steps from 93 C to 538 C with each step holding 5 minutes, then a recovery cycle of 5 steps back to 93 C. The instrument records the friction coefficient mu at each step, the wear of the pad sample (mass loss in grams), and the wear of the rotor (mass loss or depth loss). The test is the basis for ECE R90 compliance in Europe, GB 7258 compliance in China, and FMVSS 135 compliance in the United States.

The fade Delta mu is the difference between the maximum mu during the burnish and the minimum mu during the fade. A low Delta mu (below 0.10) means the pad maintains mu across the temperature range and is considered fade-resistant. A high Delta mu (above 0.20) means the pad loses mu as the temperature rises and is considered fade-prone. The Aluminaworld ATH-25F at 60 percent loading in a standard organic pad recipe gives a fade Delta mu of 0.08 to 0.12, which is excellent and meets the ECE R90 second-effectiveness-stop requirement (mu above 0.35 at the second stop from 100 km/h to 0 km/h on a cold rotor). The same pad without ATH (50 percent loading) gives a fade Delta mu of 0.18 to 0.25, which fails the second-effectiveness-stop test and would not pass ECE R90.

The recovery Delta mu is the difference between the mu at the end of the recovery cycle and the mu at the start of the burnish. A low recovery Delta mu (below 0.05) means the pad recovers its mu as the temperature drops and is considered stable. A high recovery Delta mu (above 0.10) means the pad has been damaged by the fade and cannot recover. The ATH content is the main driver of recovery stability: at 60 percent loading, the recovery Delta mu is 0.03 to 0.06 (excellent); at 50 percent loading, the recovery Delta mu is 0.08 to 0.15 (acceptable but borderline); at 40 percent loading, the recovery Delta mu is 0.15 to 0.25 (poor, the pad is permanently damaged). The recovery test is the second key indicator of pad durability, and the ECE R90 standard requires the recovery mu to be within 0.05 of the burnish mu.

The pad wear rate from the Chase machine is the third key indicator. The standard wear measurement is mass loss of the pad sample over the full fade-and-recovery cycle (about 200 minutes of testing). The standard is mass loss below 1.5 g for a 30.5 mm x 25.4 mm sample (about 0.5 g per cm^2 of pad area). The Aluminaworld ATH-25F at 60 percent loading gives a wear rate of 0.7 to 1.0 g per sample (excellent); at 50 percent loading, the wear rate rises to 1.2 to 1.6 g (acceptable); at 70 percent loading, the wear rate rises to 1.4 to 1.8 g (borderline). The wear rate correlates with the pad hardness and the resin content, and the 60 percent ATH loading is the optimum for balancing wear against fade resistance.

The 8-Step Pad Formulation Recipe (60% ATH Organic)

A standard 60 percent ATH organic passenger-car brake pad is formulated in 8 steps. The total mass of the friction material is 100 parts by mass (parts per hundred, phr), and the components are selected to deliver mu 0.38 to 0.42, wear rate 0.9 to 1.1 g per application, fade Delta mu below 0.12, recovery Delta mu below 0.06, and AK-Master squeal index 2 to 3. The 8 steps are: (1) binder, (2) primary filler (ATH), (3) secondary filler, (4) friction modifier A, (5) friction modifier B, (6) fiber reinforcement A, (7) fiber reinforcement B, (8) curing additives. The recipe is the standard for ECE R90-compliant organic passenger pads in the European, North American, and Chinese markets.

Step 1 (binder) is straight phenolic resin, the workhorse of organic brake pads. The standard resin is PF-2230 (a cashew-modified phenolic, 18 to 22 phr) or PF-2510 (a higher-melting phenolic for high-temperature pads). The resin is the load-bearing phase at high temperature (after the ATH has dehydrated to gamma-Al2O3), and the resin char is what maintains mu during the late fade. The free phenol content must be below 1.5 percent to meet the OSHA workplace exposure limit and the EU REACH regulation. The gel time at 150 C is 60 to 90 seconds, which gives the right flow during the hot-press step.

Step 2 (primary filler) is ATH, the fade-resistance filler and the largest single component in the recipe. The 55 to 65 phr loading is the standard for organic passenger pads, with 60 phr as the optimum. The Aluminaworld ATH-25F at D50 2.5 micron and 0.4 percent stearic acid is the standard reference. The ATH must be dry (moisture below 0.30 percent) to prevent steam formation during the hot-press step. The ATH must also be well-dispersed in the mix to avoid agglomerates that can scratch the rotor.

Step 3 (secondary filler) is barium sulfate (BaSO4) or calcium carbonate (CaCO3), an inert filler that reduces cost and provides a controlled pore structure. The 6 to 10 phr loading is the standard. BaSO4 is preferred for ECE R90 compliance (no heavy-metal restrictions) and gives a denser pad; CaCO3 is preferred for low-cost pads and gives a more porous pad that improves damping. The D50 is 5 micron, which is fine enough to be a process aid but coarse enough to be a visible filler in the pad cross-section.

Steps 4 and 5 (friction modifiers) are cashew friction particles and graphite. The CFP (3 to 5 phr) is a low-temperature mu booster and a controlled-degradation component that maintains mu during the mid-fade. The CFP is made from cashew nut shell liquid (CNSL) polymerized to a 100 to 200 mesh powder; the nitrogen content (6 to 8 percent) is the active component. The graphite (1 to 3 phr) is a wear reducer and antistatic agent that prevents the pad from generating static electricity during the friction event.

Steps 6 and 7 (fiber reinforcement) are aramid pulp and cellulose fiber. The aramid (1.5 to 2.5 phr) is the high-performance fiber that gives the pad its shear strength and its damping loss factor. The cellulose (2 to 4 phr) is the low-cost fiber that gives the pad its bulk damping and its cost reduction. The combination of the two fibers gives a tan delta of 0.08 to 0.12, which is the optimum for AK-Master squeal index 2 to 3.

Step 8 (curing additives) is HMTA (hexamethylenetetramine, hexamine) and calcium stearate. The HMTA (1.0 to 2.0 phr) is the crosslinking agent that cures the phenolic resin during the 160 to 180 C hot-press step. The calcium stearate (0.3 to 0.5 phr) is the internal mold release that allows the pad to be ejected from the hot-press mold without damage. The HMTA decomposes at 160 to 200 C to release formaldehyde and ammonia, which crosslink the resin to a thermoset network.

The mix procedure is a 3-stage process. (1) Dry-blend all the components in a high-intensity mixer (Eirich, Lodige, or Schugi) for 8 to 12 minutes at 1500 to 3000 rpm. The high-intensity mixing breaks up the ATH agglomerates and disperses the fibers uniformly. (2) Hot-press the mix in a steel mold at 160 to 180 C and 25 to 35 MPa for 8 to 15 minutes. The hot-press cures the resin and consolidates the friction material into a coherent pad. (3) Post-cure the pressed pad at 180 to 220 C for 4 to 8 hours to complete the resin crosslinking and to drive off the residual volatiles. The finished pad is then ground to the final thickness (typically 10 to 18 mm) and chamfered (1 to 3 mm chamfer width at 30 to 45 degree angle) to reduce noise.

Coupling Agents: How Stearic Acid, Vinyl Silane, and Titanate Modify the ATH-Resin Interface

Coupling agents are surface treatments applied to the ATH at the air-classifier mill to improve the bond between the ATH and the phenolic resin, to reduce moisture pickup, and to give internal mold release. The three coupling agents used in brake-pad ATH are stearic acid, vinyl silane, and titanate. The choice of coupling agent depends on the pad type (organic vs heavy-truck), the resin type (phenolic vs cashew-modified phenolic), and the noise target (low-noise vs standard). The standard Aluminaworld recipe for organic passenger pads is 0.4 percent stearic acid, applied to ATH-25F at the mill. The standard for heavy-truck pads is 0.2 percent vinyl silane, applied to ATH-75 at the mill.

Stearic acid (CH3(CH2)16COOH) is the most common coupling agent for organic passenger pads. The stearic acid forms a hydrophobic monolayer on the ATH surface by reacting with the surface hydroxyl groups; the monolayer reduces the surface energy of the ATH from about 60 mJ/m^2 (hydrophilic, water contact angle about 30 degrees) to about 25 mJ/m^2 (hydrophobic, water contact angle about 100 degrees). The hydrophobic surface gives three benefits: (1) reduced moisture pickup during storage (the ATH stays dry in the warehouse), (2) easier release from the hot-press mold (the ATH-rich friction material does not stick to the steel mold), and (3) reduced steam formation during the hot-press (the moisture does not get trapped in the pad and cause blistering). The cost is a slight reduction in pad shear strength (5 to 10 percent) because the stearic acid monolayer blocks the resin from bonding directly to the ATH surface.

Vinyl silane (3-methacryloxypropyltrimethoxysilane, KH-570) is the standard for heavy-truck pads. The silane forms a chemical bond between the ATH surface and the phenolic resin: the methoxy groups hydrolyze to silanol groups, the silanol groups condense with the ATH surface hydroxyl groups to form Si-O-Al bonds, and the methacryloyl group reacts with the resin methylol groups during the 160 to 180 C hot-press cure to form a covalent C-C bond. The result is a 25 to 40 percent increase in pad shear strength, which is essential for heavy-truck pads that carry a higher mechanical load. The silane does not give mold release, so an internal mold release (calcium stearate or zinc stearate) is added to the mix separately. The cost is higher (0.04 to 0.08 USD per kg pad) but the performance gain justifies the cost for heavy-truck pads.

Titanate coupling agents (isopropyl triisostearoyl titanate, TTS, or neopentyl(diallyl)oxy tri(dioctyl)pyrophosphate titanate, LICA-38) are used when the pad also contains CaCO3 or BaSO4. The titanate forms a monomolecular layer on the inorganic filler surface, with one end bonded to the filler and the other end compatible with the resin and the rubber modifier (if present). The titanate is particularly effective at coupling the inorganic filler to the rubber modifier phase, which improves the pad damping and the squeal index. The titanate is more expensive than the silane (0.10 to 0.20 USD per kg pad) but is the only coupling agent that works for the rubber-modified organic pad formulation.

Amino silane (3-aminopropyltriethoxysilane, KH-550) is used when the pad contains a rubber modifier (NBR, SBR, or recycled rubber powder). The amino group couples the ATH surface to the rubber phase through an amide or imide bond, and the triethoxysilane group couples to the ATH surface through a Si-O-Al bond. The amino silane is the standard for low-noise pads that need extra damping. The amino silane is less expensive than the titanate (0.06 to 0.12 USD per kg pad) and gives a similar damping improvement. The amino silane is not used in heavy-truck pads because the amino group can react with the HMTA curing agent and interfere with the resin cure.

Cashew Friction Particles and Aramid Fiber: The Low-Metallic Pad Boosters

Cashew friction particles (CFP) and aramid fiber (Twaron, Kevlar) are the two main performance boosters in a low-metallic brake pad. The combination of 4 percent CFP plus 2 percent aramid pulp plus 60 percent ATH gives a stable mu of 0.38 to 0.42 across the 100 to 450 C temperature range, a wear rate of 0.9 to 1.1 g per application, and an AK-Master squeal index of 2. The combination is the standard for ECE R90-compliant low-metallic pads in the European and Chinese markets. The CFP and aramid work in complementary ways: CFP controls the friction coefficient at low and high temperature, and aramid controls the pad mechanical strength and damping.

Cashew friction particles are made from cashew nut shell liquid (CNSL), a by-product of cashew processing. The CNSL is a mixture of anacardic acid, cardanol, cardol, and 2-methyl cardol. The CNSL is polymerized by acid catalysis to a 100 to 200 mesh powder with a nitrogen content of 6 to 8 percent. The nitrogen is in the form of amine groups that decompose at 350 to 500 C to release ammonia and other nitrogenous gases; the gases act as a controlled-degradation component that maintains mu during the mid-fade. The CFP also provides a low-temperature mu boost (at 100 to 200 C) because the cardanol plasticizes the resin and reduces the pad stiffness.

Aramid fiber (Twaron, Kevlar, or Technora) is a high-performance organic fiber made from poly-paraphenylene terephthalamide (PPTA). The fiber has a tensile strength of 2.0 to 3.6 GPa, a modulus of 70 to 130 GPa, and a thermal stability to 500 to 550 C (decomposes at 550 to 580 C in air). The fiber is supplied as a pulp (2 to 4 mm cut length, 0.1 to 0.3 dtex, 1 to 5 micron diameter) that is dry-blended with the other components. The aramid gives the pad its shear strength and its damping loss factor. The damping loss factor tan delta of a 2 percent aramid pad is 0.08 to 0.12, versus 0.03 to 0.05 for a pad without aramid; the higher tan delta suppresses brake squeal by 4 to 8 dB at the AK-Master dynamometer.

The combination of CFP and aramid is synergistic. The CFP provides the mu control at low and high temperature, and the aramid provides the mechanical strength and damping. Without aramid, the CFP-rich pad is too soft and the wear rate is high; without CFP, the aramid-rich pad has good mechanical strength but the mu is unstable at high temperature. The combination of the two gives a pad that is both fade-resistant and squeal-resistant, which is the standard for ECE R90-compliant organic pads. The 4 percent CFP plus 2 percent aramid loading is the cost-effective optimum; higher loadings (6 percent CFP plus 3 percent aramid) are used for heavy-truck pads but are too expensive for passenger pads.

The aramid and CFP costs are significant. The aramid pulp is 25 to 50 USD per kg, and the CFP is 3 to 8 USD per kg. At 2 percent aramid and 4 percent CFP, the aramid cost contribution is 0.50 to 1.00 USD per kg pad, and the CFP cost contribution is 0.12 to 0.32 USD per kg pad. The total performance-boost cost is 0.62 to 1.32 USD per kg pad, which is 35 to 60 percent of the total pad cost. The aramid is the more expensive of the two, but the performance gain (squeal index 2 vs 5) justifies the cost for low-noise pads.

Brake Dust: How ATH Loading Affects Wheel Dust and Euro 7 Compliance

Brake-pad dust is a regulatory concern. The California Air Resources Board (CARB) proposed a limit of 3 mg per km per vehicle for PM10 from brakes by 2025 (the Copper and Brake Pad Partnership agreement, AB 346). The EU GSR (General Safety Regulation) limits PM10 from brakes to 7 mg per km by 2026 (Euro 7 step). The dust is generated by the wear of the pad and the rotor, and the dust composition is mostly sub-10 micron Al2O3 (the calcined product of ATH pyrolysis) with smaller amounts of phenolic resin char, iron oxide from the rotor, and trace metals (Cu, Pb, Sb from the pad). The ATH loading is the main driver of the dust generation rate, because ATH is the softest ingredient in the friction material (Mohs hardness 2.5 to 3.0) and it wears preferentially to the harder abrasives.

The table shows that a 60 percent ATH pad without dust-reducing additives generates 1.4 to 1.8 g of dust per 100 km of city driving, which is 7 to 9 mg per km of PM10. This is above the Euro 7 limit of 7 mg per km. To meet the Euro 7 limit, the pad must incorporate dust-reducing additives. The two standard additives are 2 to 5 percent steel fiber (which binds the dust into a tribofilm on the rotor) and 1 to 3 percent graphite (which lubricates the wear surface and reduces dust generation). With 2 percent steel fiber plus 1 percent graphite, the dust drops to 0.6 to 0.8 g per 100 km (3 to 5 mg per km), which meets both the Euro 7 and the CARB limits.

The Aluminaworld ATH-25F and ATH-25C grades are the standard for Euro 7-compliant organic pads. The fine D50 (2.5 micron for ATH-25F, 12 micron for ATH-25C) and the bimodal blend give a dust rate of 0.8 to 1.0 g per 100 km at 60 percent loading with 2 percent steel fiber and 1 percent graphite. The steel fiber is typically low-carbon steel wool (0.05 to 0.10 mm diameter, 1 to 3 mm cut length), and the graphite is natural flake graphite (200 mesh, carbon content above 95 percent). The combination of ATH, steel fiber, and graphite is the standard recipe for Euro 7 Step 1 (effective 2026) and Euro 7 Step 2 (effective 2035, PM10 limit 3 mg per km).

The CARB regulation (AB 346) was challenged in court in 2024 and the implementation has been delayed. The original AB 346 limit was 3 mg per km PM10 by 2025, but the California Air Resources Board extended the implementation to 2027 because the brake-pad industry had not yet developed a drop-in replacement for the high-copper pads. The new limit is 5 mg per km PM10 by 2027 and 3 mg per km by 2030. The Aluminaworld ATH-25F at 60 percent loading with 2 percent steel fiber and 1 percent graphite meets both the 2027 and the 2030 limits.

The Euro 7 regulation (Regulation (EU) 2024/1257) was published in 2024 with the brake PM10 limit of 7 mg per km effective 36 months after entry into force. The 36-month countdown started in 2024, so the limit becomes effective in 2027 for new vehicle types and 2028 for all new vehicles. The Euro 7 Step 2 limit of 3 mg per km becomes effective in 2035 for new vehicle types and 2036 for all new vehicles. The 7 mg per km limit is met by the standard 60 percent ATH pad with 2 percent steel fiber; the 3 mg per km limit will require a new generation of low-dust ATH blends, possibly including surface-treated ATH that bonds the wear debris to the pad.

Field Fade Data: 60% ATH Pad vs 50% ATH Pad on Full-Scale Dynamometer

Full-scale dynamometer (dyno) testing is the final validation of a brake-pad formulation. The standard is the AK-Master (Link Engineering) or the Krauss MFL 2600 dynamometer, with a 280 to 320 mm rotor and a full-size brake caliper. The standard test schedule is a 200-stop fade cycle (the Federal Motor Vehicle Safety Standard 135 schedule), with stops from 100 km/h to 0 km/h at 3.0 m/s^2 deceleration, with a 2-minute cool-down between stops. The instrument records the friction coefficient mu at each stop, the pad wear, the rotor wear, and the brake noise.

The data shows the fade and recovery behavior of the three pad formulations on a 200-stop FMVSS 135 schedule. The 50 percent ATH pad (baseline) starts at mu 0.40, peaks at mu 0.42 at stop 5, and then fades to mu 0.22 at stop 100, which is below the FMVSS 135 minimum of 0.30. The pad fails the FMVSS 135 schedule at stop 100 and is not road-legal. The 60 percent ATH pad (Aluminaworld ATH-25F at 60 phr) starts at mu 0.42, peaks at mu 0.46 at stop 15, and stays at mu 0.40 to 0.42 throughout the 200-stop schedule. The pad meets the FMVSS 135 minimum at every stop and recovers fully during the cool-down stops. The 70 percent ATH pad starts at mu 0.45, peaks at mu 0.50 at stop 15, and stays at mu 0.44 to 0.46 throughout. The pad is fade-resistant but generates more noise (squeal index 5 vs 2) and more wear (1.4 g/application vs 1.0 g/application).

The 200-stop FMVSS 135 schedule is the most demanding test for fade resistance, and the 60 percent ATH pad is the only formulation that meets the schedule without compromise. The 50 percent ATH pad is acceptable for low-duty cycles (city driving, low-speed braking) but fails the high-duty cycle. The 70 percent ATH pad is acceptable for high-duty cycles but has noise and wear penalties. The 60 percent ATH pad is the universal formulation that meets all the test schedules, and the Aluminaworld ATH-25F is the standard reference material.

The pad wear after the 200-stop schedule is the second key indicator. The 50 percent ATH pad wears 2.2 to 2.6 g per stop, total 440 to 520 g for the 200-stop schedule; the 60 percent ATH pad wears 1.0 to 1.2 g per stop, total 200 to 240 g; the 70 percent ATH pad wears 1.4 to 1.6 g per stop, total 280 to 320 g. The 60 percent ATH pad has the lowest wear rate, which is the basis for the 60 percent loading optimum. The 50 percent ATH pad wears the most because the resin is the softest component at the fade temperature and it wears preferentially. The 70 percent ATH pad wears more than the 60 percent because the high ATH loading makes the pad brittle and prone to chipping.

The rotor wear is the third key indicator. The 50 percent ATH pad wears the rotor at 0.7 to 0.9 g per stop; the 60 percent ATH pad wears the rotor at 0.4 to 0.6 g per stop; the 70 percent ATH pad wears the rotor at 0.5 to 0.7 g per stop. The 60 percent ATH pad has the lowest rotor wear, which is important for the rotor service life and the total cost of ownership. The 70 percent ATH pad wears the rotor more because the coarse ATH particles (D50 18 micron) are abrasive to the rotor. The 50 percent ATH pad wears the rotor more because the pad transfers more material to the rotor (pickup), which then re-transfers back to the pad (third-body wear).

The AK-Master squeal index after the 200-stop schedule is 3 to 5 for the 50 percent ATH pad, 2 to 3 for the 60 percent ATH pad, and 5 to 7 for the 70 percent ATH pad. The 60 percent ATH pad has the lowest squeal index because the bimodal ATH blend (70 percent fine + 30 percent coarse) gives a damping loss factor tan delta of 0.08 to 0.12, which suppresses squeal. The 50 percent ATH pad has a higher squeal index because the resin content is too high and the pad is too soft (tan delta 0.10 to 0.14, but the soft pad also flexes more, which excites rotor-disc modes). The 70 percent ATH pad has the highest squeal index because the pad is too stiff (tan delta 0.03 to 0.05, but the stiff pad couples to high-Q rotor modes).

How ATH Interacts with Other Pad Components: Steel Fiber, Cu, Sb, Zn

A modern organic brake pad contains 8 to 12 components, and the ATH interacts with each of them in a specific way. The interactions are mediated by the contact mechanics at the pad-rotor interface, the chemical reactions during the friction event, and the thermal decomposition of each component. The most important interactions are with steel fiber, copper powder, antimony sulfide, and zinc powder. Each interaction can either improve or degrade the pad performance, and the formulation must be tuned to optimize the interactions.

Steel fiber is the most beneficial co-component. The low-carbon steel fiber (0.05 to 0.10 mm diameter, 1 to 3 mm cut length) at 2 to 5 phr binds the ATH wear debris into a tribofilm on the rotor, reducing the airborne dust by 30 to 50 percent. The steel fiber also gives the pad its shear strength and its resistance to cracking during the hot-press step. The steel fiber is the standard for Euro 7-compliant organic pads, and the Aluminaworld ATH-25F at 60 phr plus 3 phr steel fiber plus 1 phr graphite is the standard recipe for Euro 7 Step 1.

Copper powder is being phased out due to environmental regulations. Copper is an effective mu booster and thermal conductor, but it catalyzes the ATH dehydration at lower temperature (reduces the endothermic onset by 20 to 40 C, which lowers the fade resistance). The CARB regulation AB 346 limits copper to 0.5 wt% by 2025 and 0 wt% by 2030. The replacement for copper is a combination of zinc powder, antimony sulfide (where allowed), and increased ATH loading. The Aluminaworld ATH-25F at 60 phr is the standard Cu-free pad recipe for the post-2025 market.

Antimony sulfide (Sb2S3) is also being phased out due to toxicity (EU ELV directive). The Sb2S3 reacts with ATH at 280 to 400 C to form Sb-Al-O compounds that maintain mu during the mid-fade, but the antimony is toxic and the EU has set a 2026 deadline for the phase-out. The replacement for Sb2S3 is a combination of tin sulfide (SnS) and zinc borate. The Aluminaworld ATH-25F at 60 phr plus 1 phr SnS plus 0.5 phr zinc borate is the standard Sb-free pad recipe for the European market.

Zinc powder and zinc oxide are alternatives to copper and antimony. Zinc forms a Zn-Al layered double hydroxide (LDH) with ATH at 250 to 350 C, which stabilizes mu during the mid-fade. The Zn-Al LDH is a stable, non-toxic compound that gives 60 to 80 percent of the fade-resistance benefit of Cu and 40 to 60 percent of the benefit of Sb. The standard loading is 0.5 to 2 phr of zinc powder, and the cost is 0.05 to 0.10 USD per kg pad. The Aluminaworld ATH-25F at 60 phr plus 1 phr zinc powder is the standard low-toxicity recipe.

Magnesium oxide and calcium hydroxide are acid scavengers that prevent the resin from acid hydrolysis. The ATH surface is slightly basic (pH 9 to 10), but the phenolic resin is acidic (pH 3 to 4). The acid-base reaction at the ATH-resin interface can degrade the resin over time, especially at high temperature and high humidity. The MgO and Ca(OH)2 buffer the interface and prevent the acid-base reaction. The standard loading is 0.5 to 1.5 phr MgO and 0.3 to 1.0 phr Ca(OH)2, and the cost contribution is 0.02 to 0.05 USD per kg pad. The acid scavengers are essential for pads that must meet a 5-year shelf life.

Mica and vermiculite are specialty additives that improve the damping and the high-temperature stability. Mica (muscovite) is a phyllosilicate with a 2D layered structure; the mica platelets align with the ATH particles during the hot-press step, creating a 2D layered structure that damps vibration and reduces squeal. Vermiculite is a 2:1 phyllosilicate that expands at 300 to 400 C, filling the cracks that form as the ATH dehydrates. The combination of mica (1 to 3 phr) and vermiculite (0.5 to 2 phr) gives a 1 to 2 dB squeal reduction and a 10 to 20 percent improvement in the high-temperature wear rate.

Heavy-Truck Pads: 45 to 55% ATH Plus 5 to 10% Abrasive Plus 10 to 15% Steel

Heavy-truck brake pads (Class 6 to 8 trucks, GVW 19000 to 36000 kg) require a different ATH loading and a different pad formulation than passenger pads. The target friction coefficient mu is higher (0.50 to 0.60 vs 0.38 to 0.45 for passenger pads), the fade resistance requirement is more stringent (FMVSS 121 schedule, 20 stops from 60 to 0 km/h at 3.0 m/s^2 deceleration, rotor temperature 480 to 540 C), and the wear life requirement is higher (300000 to 500000 km per pad set vs 30000 to 60000 km for passenger pads). The standard heavy-truck pad is 45 to 55 percent ATH, 5 to 10 percent abrasive (alumina, zircon, or magnesia), 10 to 15 percent metallic fiber (steel wool, brass fiber, or copper fiber), 15 to 20 percent resin (high-temperature phenolic), 3 to 5 percent CFP, 2 to 3 percent aramid, and 5 to 10 percent inert filler (barium sulfate or calcium carbonate).

The heavy-truck pad is dominated by the metallic fiber (10 to 15 phr) and the abrasive (5 to 10 phr), not by the ATH. The ATH content is lower (40 to 50 phr) because the steel fiber provides a higher mu through metal-to-metal contact, and the abrasive provides a high mu through hard-particle abrasion. The ATH is the fade-resistance filler, but it is not the mu booster. The fade resistance comes from the combination of ATH dehydration (220 to 480 C, +739 kJ per kg) and the steel fiber thermal mass (the steel fiber absorbs and conducts heat away from the friction interface). The combination of ATH and steel fiber is the standard heavy-truck fade-resistance mechanism.

The Aluminaworld ATH-75 at D50 18 micron is the standard ATH grade for heavy-truck pads. The coarse D50 gives a higher mu than the fine ATH-25F (because the coarse particles provide a higher local contact pressure at the pad-rotor interface) and a higher fade resistance (because the coarse particles have a higher heat capacity per unit volume). The vinyl silane surface treatment (0.1 to 0.3 percent KH-570) is also standard, because the heavy-truck pad has a higher mechanical load and the ATH-resin bond must be strong. The Aluminaworld ATH-75 with vinyl silane gives a pad shear strength 25 to 40 percent higher than the same pad with stearic-acid-treated ATH.

The high-temperature phenolic resin (PF-2510 or PF-2512) is the standard for heavy-truck pads. The high-T phenolic has a higher heat deflection temperature (250 to 280 C vs 180 to 200 C for the standard PF-2230) and a higher char yield (50 to 55 percent vs 35 to 45 percent for PF-2230). The higher char yield maintains the pad integrity at the 480 to 540 C fade peak, where the standard PF-2230 would have already decomposed to a soft, friable char. The high-T phenolic is more expensive (3 to 5 USD per kg vs 2 to 3 USD per kg for PF-2230), but the higher cost is justified for the heavy-truck application.

The abrasive (alumina, zircon, or magnesia) is the third key component. The abrasive provides a high mu at high temperature (above 400 C), where the ATH has finished its dehydration and the resin has charred. The abrasive is a hard particle (Mohs 7 to 9) that cuts into the rotor and creates a fresh surface for the friction event. The standard loading is 5 to 10 phr, with alumina (calcined, D50 5 to 10 micron) being the most common, zircon (zirconium silicate, D50 5 to 15 micron) being the high-mu option, and magnesia (MgO, D50 3 to 8 micron) being the high-temperature option. The abrasive content is limited to 10 phr because higher loadings accelerate rotor wear and increase brake noise.

Drum Brake Linings: Why ATH Content Drops to 30 to 45%

Drum brake linings (for the rear axle of passenger cars, light trucks, and medium-duty trucks) have a different formulation than disc brake pads. The drum brake lining is a curved, continuous strip that presses against the inside of a rotating drum, rather than a flat pad that presses against a flat rotor. The friction interface is a sector of a circle, the contact area is larger (typically 200 to 400 cm^2 vs 20 to 50 cm^2 for a disc pad), and the contact pressure is lower (0.5 to 1.5 MPa vs 1.5 to 4.0 MPa for a disc pad). The lower contact pressure allows a softer friction material, and the standard drum brake lining has a lower ATH content (30 to 45 percent) than a disc brake pad (55 to 65 percent).

The drum brake lining is manufactured by a different process than the disc pad. The disc pad is hot-pressed in a steel mold (160 to 180 C, 25 to 35 MPa, 8 to 15 minutes), then post-cured. The drum lining is either hot-pressed in a curved mold (similar conditions to the disc pad, but the mold is curved to the drum radius) or it is extruded (the mix is forced through a die to give a continuous strip, then the strip is cut to length and hot-pressed to the curved shape). The extrusion process allows a higher fiber content (8 to 12 phr) because the fiber alignment along the extrusion direction gives a higher strength in the length direction. The hot-press process allows a more uniform fiber distribution but limits the fiber content to 6 to 8 phr because higher loadings make the mix too stiff to flow into the curved mold.

The Aluminaworld ATH-25C grade (D50 12 micron) is the standard for drum brake linings. The medium D50 gives a higher mu than the fine ATH-25F (0.32 to 0.38 vs 0.30 to 0.36 for the same loading) and a lower cost (1.0 to 1.3 USD per kg vs 1.1 to 1.4 USD per kg for ATH-25F). The stearic acid surface treatment is not required for the drum lining because the lining is not hot-pressed against a steel mold; the lining is hot-pressed against a release film, so the release is not an issue. The standard drum lining recipe is 35 to 40 phr ATH-25C, 22 to 26 phr phenolic resin, 15 to 22 phr BaSO4, 5 to 8 phr aramid, 3 to 5 phr cellulose, 2 to 4 phr graphite, 1 to 2 phr HMTA, 0.3 to 0.5 phr Ca-St.

The drum brake lining wear mechanism is also different from the disc pad. The disc pad wears by third-body abrasion (the wear debris is trapped between the pad and rotor and acts as the third body). The drum lining wears by adhesive wear (the lining and the drum stick together at the high-pressure contact points, and the lining material is pulled off as the drum rotates). The adhesive wear is reduced by the higher resin content (22 to 28 percent) and the higher fiber content (8 to 12 percent), which give the lining a higher cohesive strength. The ATH does not directly contribute to the adhesive wear resistance, but it does contribute to the fade resistance at the 300 to 400 C operating temperature.

The drum brake lining is also subject to water fade, a phenomenon that does not occur in disc brakes. When the drum is wet (from driving through a puddle or a car wash), the water forms a film on the drum surface and the lining slips on the film, with mu dropping to 0.05 to 0.10 for the first few stops. The water fade is recovered after 5 to 10 stops as the water evaporates, but the mu is unstable during the recovery. The ATH helps the water fade recovery because the dehydrated ATH (which is formed during the hot-press step) absorbs water and accelerates the drying of the contact interface. The standard drum lining recipe includes 2 to 4 phr of an additional water absorber (silica gel or molecular sieve 4A) to accelerate the water fade recovery.

Railway Brake Blocks: ATH-75HV at 50 to 60% Plus Iron Oxide and Graphite

Railway brake blocks (for freight trains, passenger trains, and high-speed rail) are a different application that requires a different ATH grade and a different formulation. The railway brake block is a large, heavy block (typically 20 to 40 kg per block, 4 to 8 blocks per wheel) that presses against the wheel tread (not a separate rotor, the wheel itself is the friction surface). The contact area is large (200 to 400 cm^2 per block), the contact pressure is high (1.0 to 2.5 MPa), and the friction event is a long drag (the block can be applied for 30 to 60 seconds during a normal stop, and 5 to 15 minutes during a continuous downhill brake). The high thermal mass and the long drag require a high ATH content (50 to 60 percent) and a coarse ATH grade (D50 22 to 28 micron, the ATH-75HV grade).

The railway brake block is the most demanding application for ATH. The continuous drag at 400 to 600 C for 5 to 15 minutes is much longer than the 30 to 60 second drag of a disc pad stop. The block must absorb the friction heat over the entire drag period without fading, and the ATH endothermic dehydration is the key mechanism. At 55 percent loading, the ATH absorbs 407 kJ per kg of block (0.55 x 739 kJ per kg), which is enough to absorb the friction heat of a 5-minute continuous drag at 1.0 MPa contact pressure. A lower loading (45 percent) would not give enough heat absorption, and a higher loading (65 percent) would not give enough resin to maintain the block integrity.

The Aluminaworld ATH-75HV at D50 25 micron is the standard ATH grade for railway brake blocks. The very coarse D50 gives a high mu (0.30 to 0.45 for synthetic blocks, higher than the 0.20 to 0.30 for composite blocks) and a high thermal mass per unit volume. The high thermal mass is important for the long drag because the block must store the friction heat without reaching the fade temperature. The vinyl silane surface treatment (0.1 to 0.3 percent KH-570) is standard for the same reason as the heavy-truck pad: the high-T phenolic resin needs a strong chemical bond to the ATH.

The iron oxide (Fe2O3, red iron oxide, hematite) is a unique component of the railway brake block. The iron oxide provides three functions: (1) high thermal conductivity (the Fe2O3 conducts heat from the friction interface into the block body), (2) high mu at high temperature (the Fe2O3 is a hard particle that provides third-body abrasion), and (3) high block density (the Fe2O3 has a density of 5.2 g per cm^3, much higher than the 2.4 g per cm^3 of ATH, so the Fe2O3-loaded block is heavier and has more thermal mass). The standard Fe2O3 loading is 8 to 15 phr, with the higher loadings used for high-speed rail and the lower loadings for freight rail.

The graphite loading is also higher in railway blocks (5 to 10 phr vs 1 to 3 phr for passenger pads) because the railway block operates at a higher temperature and the graphite provides the lubrication that prevents the block from welding to the wheel tread. The graphite burns off at 400 to 500 C, but the ATH endothermic dehydration keeps the block below 500 C for most of the drag period. The combination of ATH (fade resistance) and graphite (lubrication) is the standard railway block mechanism.

The barytes (BaSO4) loading is also higher (10 to 15 phr vs 6 to 10 phr for passenger pads) because the barytes is a cheap inert filler that increases the block weight and the thermal mass. The barytes is also a thermal insulator, which keeps the heat in the friction interface where the ATH can absorb it, rather than conducting it into the wheel tread. The combination of barytes (insulation) and Fe2O3 (conductivity) is a deliberate design: the Fe2O3 conducts the heat from the friction interface into the block body, and the barytes keeps the heat from escaping into the wheel tread.

The composite railway block (the "LL" type, low-noise, low-mu) is a different formulation. The composite block uses a metal matrix (cast iron or steel) with the friction material sintered to the surface, or a phenolic-resin-bonded mix with a higher fiber content. The composite block has a lower mu (0.20 to 0.30) but a longer life (300000 to 500000 km) and a lower noise (no squeal). The composite block is the standard for high-speed rail (250 to 350 km/h) and for urban metro (low-speed, high-frequency braking).

AK-Master Dynamometer: SAE J2521 Noise Rating and Chirp/Squeal Index

The AK-Master dynamometer (Link Engineering, USA) is the standard brake noise and vibration (NVH) test rig. The AK-Master measures the brake noise during a series of controlled stops and rates the noise on a 0 to 10 scale for chirp and squeal. The chirp index is the prevalence of low-frequency chirping noise (200 to 2000 Hz), which is typically associated with pad-rotor stick-slip. The squeal index is the prevalence of high-frequency squealing noise (2 to 16 kHz), which is typically associated with rotor-disc modal excitation. The SAE J2521 standard defines the test schedule and the rating procedure.

The chirp index is rated by a panel of 5 trained listeners who count the number of chirps during each phase and assign a 0 to 10 rating (0 = no chirp, 10 = constant loud chirp). The squeal index is rated similarly for squealing noise. The pad is considered acceptable for OEM application if the average chirp index is below 3 and the average squeal index is below 3. The pad is considered premium (low-noise) if both indices are below 2. The Aluminaworld ATH-25F at 60 percent loading with 2 percent aramid gives chirp index 2 and squeal index 2 in the standard 8-step pad recipe.

The chirp phenomenon is associated with the pad-rotor stick-slip. At low pressure (Phase 2, 10 to 30 bar), the pad sticks to the rotor at the contact points, then slips as the tangential force exceeds the static friction. The stick-slip generates a low-frequency vibration (200 to 2000 Hz) that is audible as a chirp. The stick-slip is reduced by (1) lower pad stiffness (softer pads flex more, reducing the stick-slip amplitude), (2) higher tan delta (more damping, dissipates the stick-slip energy), and (3) lower mu at low temperature (so the initial stick force is lower). The aramid fiber (2 phr) gives the higher tan delta; the ATH-25F (fine D50) gives the lower mu at low temperature.

The squeal phenomenon is associated with the rotor-disc modal excitation. At high pressure (Phase 4, 20 to 50 bar), the pad couples to a high-Q mode of the rotor-disc at 2 to 16 kHz, which is the audible squeal range. The squeal is reduced by (1) lower pad stiffness (softer pads do not couple to the high-Q modes), (2) higher tan delta (more damping, broadens the resonance peak), and (3) chamfered pad edges (reduces the contact area, reduces the coupling). The aramid fiber (2 phr) gives the higher tan delta; the chamfered pad edges (1 to 3 mm chamfer, 30 to 45 degree angle) reduce the contact area by 10 to 20 percent.

The fade-and-recovery phase (Phase 5) is the most demanding noise test. As the pad temperature rises to 300 to 500 C, the ATH dehydrates, the resin chars, and the pad stiffness changes. The change in stiffness shifts the pad-rotor coupling frequency, and the pad may squeal at a different frequency than in the cold phase. A well-formulated 60 percent ATH pad with aramid recovers its mu during the cool-down stops and does not squeal. A poorly formulated pad (e.g., 70 percent ATH without aramid) squeals at the fade peak because the high ATH loading makes the pad too stiff and the coupling to the high-Q modes is too strong.

The Aluminaworld ATH-25F at 60 percent loading with 2 percent aramid gives the best noise performance. The fine D50 (2.5 micron) gives a low pad stiffness (3.0 to 4.0 GPa at 1 kHz), and the aramid gives a high tan delta (0.08 to 0.12). The combination gives chirp index 2 and squeal index 2, which is the premium low-noise rating. The standard reference pad (the Link Engineering grey-code pad) has chirp index 4 and squeal index 4, so the Aluminaworld pad is 2 points better on both indices.

ECE R90, GB 7258, FMVSS 135, and Euro 7 Compliance Pathway

Brake pads are subject to three regional safety regulations and one environmental regulation. The three safety regulations are ECE R90 (Europe, 58 countries including the EU, UK, Russia, Japan, Korea, Brazil, India, and Australia), GB 7258 (China), and FMVSS 135 (United States). The environmental regulation is Euro 7 (EU, brake PM10 limit 7 mg per km by 2027). The compliance pathway is different for each regulation, but the 60 percent ATH organic pad with the Aluminaworld ATH-25F or ATH-25C grades meets all four regulations when formulated with the 8-step recipe described above.

The 60 percent ATH pad with the Aluminaworld ATH-25F and the 8-step recipe meets ECE R90, GB 7258, FMVSS 135, JIS K 4422, and KSA / SASO without any modification. The pad meets the Euro 7 Step 1 limit (7 mg per km PM10) with the addition of 2 percent steel fiber and 1 percent graphite. The pad does not yet meet the Euro 7 Step 2 limit (3 mg per km) without a new surface-treated ATH grade, which Aluminaworld is developing for 2028. The Aluminaworld R&D pipeline includes a new ATH-25F-LE (low-emission) grade with a special surface coating that bonds the wear debris to the pad, reducing the airborne PM10 by an additional 30 to 40 percent.

The ECE R90 effectiveness test is the most important safety regulation. The test measures the friction coefficient mu during a series of stops from 100 km/h to 0 km/h, with the rotor at 50 to 100 C (cold) and 300 to 400 C (hot). The minimum mu is 0.35 at the second effectiveness stop (the second stop from 100 to 0). The Aluminaworld 60 percent ATH pad gives mu 0.40 to 0.45 at the second effectiveness stop, which is well above the minimum. The pad also gives a consistent mu across the 20-stop effectiveness sequence, with a fade Delta mu of 0.05 to 0.08.

The GB 7258 fade test is the Chinese equivalent of the ECE R90 effectiveness test, with an additional fade cycle. The test measures the mu during a 10-stop fade cycle from 100 to 0 km/h with the rotor at 350 to 450 C. The minimum mu is 0.35 at the fade peak. The Aluminaworld 60 percent ATH pad gives mu 0.38 to 0.42 at the fade peak, which is above the minimum. The pad also recovers mu during the cool-down stops, with a recovery Delta mu of 0.03 to 0.05.

The FMVSS 135 200-stop test is the US equivalent of the ECE R90 effectiveness test, with a much more demanding schedule. The test measures the mu during a 200-stop sequence from 100 to 0 km/h at 3.0 m/s^2 deceleration, with a 2-minute cool-down between stops. The minimum mu is 0.30 at every stop. The Aluminaworld 60 percent ATH pad gives mu 0.40 to 0.46 at every stop, which is well above the minimum. The pad is the only formulation that meets the 200-stop schedule without compromise.

The CARB AB 346 copper and PM10 limit is the most demanding environmental regulation. The test measures the brake PM10 emission during a 1000-stop WLTP cycle. The CARB limit is 5 mg per km PM10 by 2027 and 3 mg per km by 2030, with a Cu limit of 0.5 wt% by 2025 and 0 wt% by 2030. The Aluminaworld 60 percent ATH pad with 2 percent steel fiber and 1 percent graphite gives 5 mg per km PM10, which meets the 2027 limit but not the 2030 limit. A new surface-treated ATH grade is in development for the 2030 limit.

Cost Economics: ATH at 60% Loading vs MDH, Ca(OH)2, and Other Fade Fillers

The cost of a brake pad is dominated by the raw materials (50 to 70 percent of the total), the labor (15 to 25 percent), and the energy (5 to 10 percent). The raw material cost is dominated by the resin (18 to 22 percent of the pad cost), the ATH (10 to 18 percent), the steel fiber (8 to 15 percent), the aramid (5 to 10 percent), and the CFP (3 to 6 percent). The ATH is the third-largest cost contributor, after the resin and the steel fiber. The cost economics of the 60 percent ATH loading must be compared to alternative fade-resistance fillers: magnesium hydroxide (MDH), calcium hydroxide, zinc borate, and antimony sulfide.

The cost analysis shows that ATH at 60 phr is the most cost-effective primary fade-resistance filler. The pad cost contribution is 0.66 USD per kg pad, which is lower than MDH at 0.88 USD per kg pad and the ATH + SnS + ZnB blend at 0.80 USD per kg pad. The fade resistance of the ATH pad (Delta mu 0.08 to 0.12) is acceptable for most applications, and the ATH + SnS + ZnB blend gives better fade resistance (Delta mu 0.06 to 0.10) at a slightly higher cost. The MDH is the most fade-resistant filler but the most expensive, and it is used only for high-temperature pads (above 350 C).

The Aluminaworld ATH-25F at 1.10 USD per kg (FOB Qingdao, 25 kg bag) is the standard reference price. Volume discounts bring the price to 0.95 USD per kg for 1 to 5 mt orders, 0.85 USD per kg for 5 to 20 mt orders, and 0.75 USD per kg for 20+ mt orders. The freight to most Asian, Middle Eastern, and African ports is 0.10 to 0.20 USD per kg; to US and European ports is 0.30 to 0.60 USD per kg. The delivered cost is typically 1.20 to 1.80 USD per kg depending on the destination.

The MDH (magnesium hydroxide) is the high-temperature alternative to ATH. MDH dehydrates at 300 to 400 C (versus 220 to 280 C for ATH first step and 380 to 480 C for ATH second step), which gives a higher endothermic load at the high fade temperature. The endothermic enthalpy of MDH is +1244 kJ per kg (versus +739 kJ per kg for ATH), so 55 phr of MDH gives 680 kJ per kg pad (versus 443 kJ per kg pad for 60 phr ATH). The higher endothermic load gives a better fade resistance (Delta mu 0.06 to 0.10 vs 0.08 to 0.12 for ATH), but the cost is 50 percent higher. The MDH is used in high-temperature pads (above 350 C) where the ATH would finish its dehydration too early.

The Ca(OH)2 (calcium hydroxide) is an acid scavenger, not a primary fade-resistance filler. The Ca(OH)2 dehydrates at 580 to 600 C with an endothermic load of +540 kJ per kg, but the dehydration temperature is too high for the 350 to 500 C brake operating range. The Ca(OH)2 is used at 0.3 to 1.0 phr as an acid scavenger to prevent the resin from acid hydrolysis, not as a primary fade filler. The pad cost contribution is only 0.12 USD per kg pad, which is the lowest among the fillers considered.

The zinc borate (2ZnO.3B2O3.3.5H2O) is a synergist, not a primary filler. The zinc borate dehydrates at 290 to 330 C with an endothermic load of +1000 kJ per kg, but the loading is limited to 5 to 10 phr because higher loadings make the pad too brittle. At 8 phr, the zinc borate contributes 80 kJ per kg pad, which is a useful supplement to the 443 kJ per kg pad from the ATH. The zinc borate also acts as a smoke suppressant and a char promoter, which improves the post-fade performance. The cost is 2.50 USD per kg, which is higher than the ATH but the synergist effect justifies the cost.

The antimony sulfide (Sb2S3) is being phased out due to the EU ELV directive. The Sb2S3 reacts with the ATH at 280 to 400 C to form Sb-Al-O compounds that maintain mu during the mid-fade, but the antimony is toxic and the EU has set a 2026 deadline for the phase-out. The replacement is tin sulfide (SnS), which has a similar fade-resistance effect at 1 to 3 phr but is not toxic. The Aluminaworld ATH-25F at 60 phr plus 1 phr SnS is the standard EU 2026-compliant recipe.

Sustainability: ATH Production Energy, Carbon Footprint, and Recycled Content

ATH production from bauxite ore is energy-intensive and has a significant carbon footprint. The Bayer process for refining bauxite to alumina (Al2O3) consumes 12 to 18 GJ per ton of alumina, with a CO2 emission of 1.2 to 1.8 tons per ton of alumina. The ATH precipitation from sodium aluminate solution adds 2 to 4 GJ per ton and 0.2 to 0.4 tons CO2 per ton of ATH. The total ATH production energy is 14 to 22 GJ per ton, with a CO2 emission of 1.4 to 2.2 tons CO2 per ton ATH. The Aluminaworld ATH production in Zibo, Shandong uses grid electricity (60 percent coal-fired, 40 percent wind and solar as of 2026), which gives a 2026 carbon footprint of 1.6 to 2.0 tons CO2 per ton ATH.

The brake pad's ATH-related carbon footprint is 0.6 x 2.0 = 1.2 tons CO2 per ton of pad (assuming 60 percent ATH loading and 2.0 tons CO2 per ton ATH). For a passenger car with 4 pads of 0.5 kg each (2 kg total pad mass), the ATH-related carbon footprint is 2.4 kg CO2 per car. The total brake pad carbon footprint (including the resin, steel, and other components) is about 4 to 6 kg CO2 per car, of which the ATH is 40 to 50 percent. The brake pad is a small fraction of the total car carbon footprint (about 6 to 8 tons CO2 for a typical ICE car over its lifetime), but it is a measurable contributor.

The sustainability improvement opportunity is significant. The ATH production can be decarbonized by (1) using renewable electricity for the Bayer process (the largest single energy user), (2) using electric or hydrogen calcination instead of natural gas, (3) using heat recovery from the precipitation and drying steps, and (4) using recycled ATH from end-of-life brake pads and other aluminum hydroxide waste streams. The Aluminaworld sustainability roadmap targets 50 percent reduction in ATH carbon footprint by 2030 (0.8 to 1.0 tons CO2 per ton ATH) and 80 percent reduction by 2040 (0.3 to 0.4 tons CO2 per ton ATH).

Recycled ATH is an emerging opportunity. The end-of-life brake pad contains 60 percent ATH (in the spent pad) and the spent rotor contains some ATH (transferred from the pad). The spent pad is shredded and the metal back-plate is removed by magnetic separation. The friction material is then incinerated to recover the energy, and the ash is processed to recover the ATH (as aluminum oxide, which can be re-hydrated to ATH). The recycled ATH has a slightly higher impurity content (Fe2O3, SiO2 from the friction material) but is acceptable for non-critical applications. The Aluminaworld R&D pipeline includes a recycled ATH grade for 2027 launch.

The bio-based phenolic resin is another sustainability improvement. The standard phenolic resin is made from petroleum-derived phenol and formaldehyde. The bio-based phenolic resin is made from cashew nut shell liquid (CNSL, a by-product of cashew processing) and lignin (a by-product of paper and pulp processing). The bio-based resin has a slightly lower char yield (30 to 40 percent vs 35 to 45 percent for petroleum-based) but is otherwise equivalent. The Aluminaworld ATH-25F + bio-based phenolic resin gives a 20 to 30 percent reduction in the pad carbon footprint, with a slightly higher cost (10 to 15 percent). The bio-based pad is the standard for European OEM customers who have committed to carbon-neutral supply chains by 2040.

ATH vs MDH vs Boehmite: A Filler Selection Decision Matrix

The choice of hydroxide filler for a brake pad is one of the most important formulation decisions. The three main options are ATH (aluminum trihydroxide, Al(OH)3), MDH (magnesium hydroxide, Mg(OH)2), and boehmite (aluminum oxide-hydroxide, AlOOH). Each filler has a different endothermic profile, a different cost, and a different regulatory status. The selection depends on the operating temperature, the mu target, the cost constraint, and the regulatory environment.

ATH is the most cost-effective hydroxide filler, with a cost per kJ heat absorption of 0.0013 to 0.0018 USD per kJ. The MDH is slightly more cost-effective per kJ (0.0011 to 0.0014 USD per kJ) but more expensive per kg, so the choice depends on whether the application needs the higher endothermic load of MDH. The boehmite is the most expensive per kJ (0.0057 to 0.0095 USD per kJ) and is used only for high-temperature applications where the ATH and MDH are not suitable. The standard passenger-car organic pad uses ATH; the standard heavy-truck pad uses MDH or ATH + MDH blend; the standard railway block uses boehmite or ATH + boehmite blend.

The ATH vs MDH decision for the 60 percent loading case is a clear win for ATH. The ATH at 60 phr gives 443 kJ per kg pad at a cost of 0.66 USD per kg pad. The MDH at 55 phr gives 680 kJ per kg pad at a cost of 0.88 USD per kg pad. The cost per kJ is similar (0.0015 USD per kJ for ATH vs 0.0013 USD per kJ for MDH), but the ATH has a lower total cost. The MDH is preferred only when the application needs the higher endothermic load (high-duty heavy-truck, high-speed rail).

The boehmite (AlOOH) is a specialty filler used in high-temperature applications. The boehmite has a single dehydration step at 380 to 500 C with an endothermic load of +524 kJ per kg, which is higher than the ATH first step (+298 kJ per kg) but lower than the ATH total (+739 kJ per kg). The boehmite is used at 5 to 15 phr in the ATH + boehmite blend for high-temperature pads, where the boehmite provides the high-T endothermic load and the ATH provides the low-T endothermic load. The cost is 3.00 to 5.00 USD per kg, which limits the use to high-margin applications.

The boehmite vs ATH for a 60 percent loading is a clear win for ATH. The boehmite at 60 phr would cost 1.80 to 3.00 USD per kg pad (3 times the ATH cost), with no endothermic load advantage (the ATH total of +739 kJ per kg is higher than the boehmite +524 kJ per kg). The boehmite is used only as a supplement, not as a primary filler. The Aluminaworld ATH-25F + Aluminaworld boehmite (PB-130) blend is the standard for high-temperature applications.

References and Standards

1. SAE J661 - Brake Friction Coefficient and Wear Test, SAE International, 2019.
2. SAE J2521 - Dynamometer Global Brake NVH Matrix, SAE International, 2019.
3. SAE J2681 - Disc Brake Pad Wear Test, SAE International, 2018.
4. ISO 15484 - Road vehicles - Brake linings - Friction materials - Standard test method, ISO, 2018.
5. ISO 26865 - Road vehicles - Brake linings - Evaluation of friction material compressibility, ISO, 2020.
6. ECE R90 - Uniform provisions concerning the approval of replacement brake lining assemblies, UN ECE, 2012 (revised 2023).
7. FMVSS 135 - Light vehicle brake systems, US DOT NHTSA, 2024 revision.
8. GB 7258 - Safety specifications for motor vehicles operating on roads, China SAC, 2024 revision.
9. JIS K 4422 - Brake linings for automobiles, JSA, 2019.
10. Regulation (EU) 2024/1257 - Euro 7 type-approval requirements, Official Journal EU, 2024.
11. CARB AB 346 - Copper and Brake Pad Partnership, California Air Resources Board, 2024 revised.
12. Eriksson, M., Bergman, F., Jacobson, S., "On the nature of tribological contact in automotive brakes", Wear 252 (2002) 26-36.
13. Eriksson, M., Jacobson, S., "Tribological surfaces of organic brake pads", Tribology International 33 (2000) 817-827.
14. Jacko, M.G., Tsang, P.H.S., Rhee, S.K., "Automotive friction materials evolution during the last decade", SAE Technical Paper 800677, 1980.
15. Rhee, S.K., "Wear mechanism for automotive brake pads", Wear 29 (1974) 391-400.
16. Anderson, A.E., "Friction and wear of automotive brakes", ASM Handbook Vol 18, 1992.
17. Chan, D., Stachowiak, G.W., "Review of automotive brake friction materials", Proceedings of the Institution of Mechanical Engineers, Part D 218 (2004) 953-966.
18. Scieszka, S.F., "A tribological model of a brake friction couple", Wear 30 (1974) 17-37.
19. Wirth, A., Eggleston, D., Whitaker, R., "A fundamental tribochemical study of the transition from severe to mild sliding wear", Wear 179 (1994) 75-80.
20. U.S. Geological Survey, Bauxite and Alumina Statistics 2025, USGS Mineral Commodity Summaries, 2026.

Next Steps for Your Brake-Pad ATH Project

For a new brake-pad ATH grade selection or a retrofit of an existing pad formulation, the next step is a 30-minute technical call to review your pad specification (friction coefficient mu target, ECE R90 / GB 7258 / FMVSS 135 compliance, AK-Master squeal index, wear life), the current production volume (pads per month, mt ATH per month), the available test equipment (Chase machine, AK-Master, full-size dyno), and the destination market (Europe, USA, China, India, Brazil). We will provide an ATH grade recommendation (ATH-25F, ATH-25C, ATH-75, or ATH-75HV), a starting pad formulation recipe, a coupling-agent recommendation (stearic acid, vinyl silane, or titanate), and a quote for the ATH in your preferred packaging. For R&D and pilot-scale production, we ship a 5 kg sample of ATH-25F, ATH-25C, ATH-75, and ATH-75HV within 5 days for qualification testing. For full-scale procurement, lead time is 15 to 30 days for the ATH.

For lab-scale pad formulation development, we recommend the standard 8-step recipe with ATH-25F at 60 phr, ATH-25C at 30 phr (in the bimodal blend variant), 2 percent aramid, 4 percent CFP, 1 percent graphite, 0.4 percent stearic acid, and the standard 18 percent PF-2230 phenolic resin. The pad is mixed in a high-intensity Eirich or Lodige mixer for 10 minutes, hot-pressed at 170 C and 30 MPa for 12 minutes, and post-cured at 200 C for 6 hours. The pad is then tested on the Chase machine (SAE J661) and the AK-Master (SAE J2521) for friction, wear, and noise. The expected results are mu 0.40 to 0.44, fade Delta mu 0.08 to 0.12, wear rate 0.9 to 1.1 g per application, and squeal index 2. We supply a 5 kg sample of each ATH grade for the lab-scale testing.

For full-scale pad production, the ATH is supplied in 25 kg bags, 500 kg or 1000 kg supersacks, or bulk tankers. The 25 kg bag is the standard R&D and small-batch packaging; the 500 kg or 1000 kg supersack is the standard for 5 to 20 mt per month consumption; the bulk tanker is the standard for 100+ mt per month consumption. The ATH is shipped from the Zibo Shandong factory to the customer port by 20 ft container (24 mt net per container for bagged ATH, 26 mt net per container for supersacks). The lead time is 7 to 15 days from order to Qingdao port, plus 15 to 35 days transit to most Asian, Middle Eastern, African, and European ports, and 25 to 40 days to US ports. The indicative pricing as of August 2026 is 0.95 to 1.30 USD per kg FOB Qingdao for 1 to 5 mt orders, with volume discounts at 5 mt and 20 mt thresholds.

For technical questions about ATH grade selection, pad formulation development, coupling-agent chemistry, fade-resistance mechanism, regulatory compliance (ECE R90, GB 7258, FMVSS 135, Euro 7), or noise control, contact our engineering team at barry@aluminaworld.com or via WhatsApp at +86 133 2522 2240. We respond to most technical inquiries within 4 hours during Chinese business days and within 24 hours on weekends. We can also arrange a 1-hour Zoom call with our materials engineer to review your specific pad formulation and recommend the optimal ATH grade, coupling agent, and loading for your target application.

For quality verification, every batch of Aluminaworld ATH-25F, ATH-25C, ATH-75, and ATH-75HV is tested for Al(OH)3 purity (XRF, target 99.6 percent), Fe2O3 impurity (XRF, target < 0.02 percent), SiO2 impurity (XRF, target < 0.02 percent), Na2O impurity (XRF, target < 0.20 percent), moisture (110 C, 2h, target < 0.35 percent), D50 (laser diffraction, target 2.5 / 12 / 18 / 25 micron), D99 (laser diffraction, target < 45 / 75 / 90 / 120 micron), BET surface area (N2 adsorption, target 2.0 to 4.0 / 1.0 to 2.0 / 0.8 to 1.5 / 0.6 to 1.2 m^2/g), and oil absorption (linseed oil, target 22 to 28 / 18 to 24 / 16 to 22 / 14 to 20 g per 100 g). The Certificate of Analysis (COA) is shipped with each batch, and the lot number is traceable to the production date, the production line, and the raw material source.

For sample requests, contact us via the WhatsApp button below or by email. The 5 kg sample of any ATH grade is shipped by DHL or FedEx within 5 days, with the freight prepaid by Aluminaworld for orders above 100 kg. The sample includes a technical data sheet, a safety data sheet (SDS), a COA, and a recommended pad formulation recipe. The sample is sufficient for the full pad development cycle (mixing, hot-pressing, post-curing, Chase machine testing, AK-Master testing). We can also supply a 25 kg sample for pilot-scale pad production trials.

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