ATH Surface Coatings for Polypropylene: Stearic Acid vs Vinyl Silane vs Titanate

Why Surface Coating Decides PP Compound Performance

Aluminum hydroxide is the workhorse halogen-free flame retardant for polypropylene and polyethylene, used at 55 to 70 wt% loading in cable insulation, appliance housings, and automotive interior parts to reach UL94 V-0 at 1.6 to 3.2 mm. ATH works by decomposing endothermically above 200 degrees C, releasing about 1.2 kJ/g of water and cooling the pyrolysis zone. The catch is that ATH is a hydrophilic inorganic filler with a polar -OH-rich surface, and polypropylene is a non-polar polymer. Without a surface coating, the two materials are thermodynamically incompatible, and the compound suffers from poor dispersion, high viscosity, low tensile strength, and brittle failure.

Uncoated ATH in PP at 60 wt% loading gives tensile strength below 18 MPa (compared to 33 MPa for unfilled PP), elongation at break below 5 percent (compared to over 200 percent for unfilled PP), and MFI below 5 g/10 min (compared to 12 g/10 min for unfilled PP at 230 degrees C / 2.16 kg). The compound is visually heterogeneous with visible particle agglomerates 30 to 100 micrometers across. Adding a surface coating - stearic acid, vinyl silane, or titanate coupling agent - replaces the surface hydroxyls with an organophilic layer, reduces the ATH surface energy from 65 to 75 mN/m down to 25 to 40 mN/m, and creates either a chemical bond or a polarity match between filler and polymer. Mechanical properties recover to within 70 to 85 percent of unfilled PP values, and the compound processes with reasonable MFI.

This guide walks through the three common surface treatments used on ATH for PP, compares their performance in 60 to 65 wt% filled compounds, explains the coating chemistry, gives the QC tests that catch under- or over-coated ATH, and ends with selection guidance for cable, appliance, and automotive applications.

What Happens When You Compound Uncoated ATH into PP

Drop uncoated ATH into a polypropylene melt and the first thing you notice is the melt viscosity spike. A standard homopolymer PP (MFI 12 g/10 min at 230 degrees C) filled with 60 wt% uncoated ATH has MFI below 5 g/10 min. The torque on the extruder screws rises, the die pressure climbs, and the throughput drops. Worse, the extruded strand becomes rough because the large ATH agglomerates (30 to 100 micrometers across) do not disperse into the melt.

Mechanical properties degrade even faster than processability. Tensile strength at yield drops from about 33 MPa for unfilled PP to 15 to 18 MPa for 60 wt% uncoated ATH, a 50 percent loss. Elongation at break collapses from over 200 percent to under 5 percent because the filler agglomerates act as stress concentrators. Notched Izod impact drops from 30 to 50 J/m for unfilled PP to 15 to 20 J/m. None of these values are acceptable for cable insulation or appliance housing. A second problem: moisture pickup. Uncoated ATH picks up 0.5 to 1.5 wt% moisture from ambient air within hours, and 2.5 to 4.0 wt% after a week at 70 percent relative humidity. This moisture flashes to steam in the extruder, causing die lip build-up, surface streaks, and steam marks on molded parts. All three problems are fixed by a surface coating that changes the ATH from hydrophilic to organophilic.

Stearic Acid Coating: The Low-Cost Default

Stearic acid (CH3-(CH2)16-COOH) is the most common and least expensive surface treatment for ATH. It is a saturated C18 fatty acid that binds to ATH surface hydroxyls through its carboxylic acid head, leaving a 16-carbon aliphatic tail pointing outward. The result is a hydrophobic surface layer about 2.5 nanometers thick that lowers the surface energy from 70 mN/m to about 30 to 35 mN/m and matches the non-polar character of the PP melt.

Stearic acid is typically applied to ATH at 1.0 to 2.0 wt% based on dry ATH weight using either the dry method (stearic acid melted at 70 to 80 degrees C and sprayed onto ATH in a high-speed mixer for 5 to 10 minutes) or the wet method (stearic acid dissolved in isopropanol and sprayed onto ATH, then dried). Most Chinese ATH suppliers use the dry method at 1.5 wt% stearic acid as the default grade.

Performance of stearic acid-coated ATH in 60 wt% PP compound: tensile strength at yield 22 to 25 MPa (versus 15 to 18 MPa uncoated), elongation at break 8 to 12 percent, notched Izod impact 22 to 28 J/m, MFI at 230 degrees C / 2.16 kg 7 to 10 g/10 min, and LOI 27 to 29 percent. Stearic acid delivers about 70 to 80 percent of the property recovery achievable with silane or titanate coupling agents, at roughly half the coating cost. For general-purpose flame-retardant PP at moderate loading (40 to 55 wt% ATH), stearic acid is the right choice. For demanding cable insulation at 65 to 70 wt% ATH, silane or titanate is preferred.

Vinyl Silane Coating: Best Mechanical Performance

Vinyltrimethoxysilane (VTMO, CH2=CH-Si(OCH3)3) is the workhorse silane coupling agent for polypropylene. The chemistry has two parts. First, the methoxy groups hydrolyze in the presence of trace water to form silanols (Si-OH). The silanols then condense with the surface hydroxyls of ATH to form covalent Si-O-Al bonds, releasing water. Second, the vinyl group on the other end of the silane molecule is a polymerizable C=C double bond that can graft onto polypropylene chains under free-radical initiation (typically 0.1 to 0.3 wt% dicumyl peroxide added during compounding). The result is a true chemical bridge between the inorganic filler and the polymer matrix, qualitatively different from stearic acid which only provides a non-polar monolayer with no chemical bonding to the polymer.

Vinyl silane is applied at 0.8 to 1.5 wt% based on dry ATH, almost always by the wet method because the silane needs to hydrolyze before bonding. A typical procedure: dissolve 1.0 to 1.5 parts vinyl silane in 5 to 10 parts ethanol/water (90:10), adjust pH to 4 to 5 with acetic acid, stir for 30 to 60 minutes at 25 to 40 degrees C, then spray the solution onto ATH in a high-speed mixer. The silane reacts with the ATH surface during the spray step and during subsequent drying at 100 to 120 degrees C for 2 to 4 hours.

Performance of vinyl silane-coated ATH in 60 wt% PP compound with 0.2 wt% DCP initiator: tensile strength at yield 25 to 28 MPa (10 to 15 percent higher than stearic acid), elongation at break 12 to 18 percent (50 percent higher), notched Izod impact 28 to 35 J/m (25 percent higher), MFI at 230 degrees C / 2.16 kg 5 to 8 g/10 min (slightly lower because of chemical grafting), LOI 28 to 30 percent, and UL94 at 1.6 mm V-0 with cleaner surface and lower smoke density in cone calorimeter. The downside is the peroxide requirement. Without peroxide initiation during compounding, the vinyl group does not graft to PP, and the silane acts more like a long-chain stearic acid. Vinyl silane is the right choice for high-performance cable insulation and appliance housings that need high impact strength.

Titanate Coupling Agent: Best Melt Flow at High Loading

Titanate coupling agents are organotitanate molecules with the general structure (RO)4-Ti or (RO)3-Ti-O-X-R'. Common examples for ATH are isopropyl triisostearoyl titanate (TTS), isopropyl trioctyl titanate, and the neoalkoxy titanate family (LICA 12, LICA 38 from Kenrich Petrochemicals). The titanate molecule has two functions: the alkoxy groups (OR) bind to the ATH surface through Ti-O-Al bonds, and the long alkyl or isostearoyl chains create a thick non-polar monolayer that acts as an internal lubricant in the polymer melt.

Titanate coating is applied at 0.5 to 1.5 wt% based on dry ATH, lower than stearic acid because the titanate molecule is much larger and reaches monolayer coverage at lower mass. Application is typically by wet spray from isopropanol or by direct addition to the high-speed mixer at 80 to 100 degrees C. Titanate is more tolerant of ATH moisture content than silane and does not require a separate hydrolysis step.

Performance of titanate-coated ATH in 65 wt% PP compound with no peroxide required: tensile strength at yield 22 to 25 MPa (similar to stearic acid), elongation at break 10 to 14 percent (slightly higher), MFI at 230 degrees C / 2.16 kg 12 to 18 g/10 min (60 to 100 percent higher than stearic acid, almost matching unfilled PP), LOI 27 to 29 percent, and UL94 at 1.6 mm V-0 at 65 wt% ATH loading. The killer feature of titanate is the MFI improvement. At 65 wt% ATH loading in PP, stearic acid-coated ATH typically drops MFI below 5 g/10 min, making the compound hard to injection mold into thin-walled parts. Titanate-coated ATH at the same loading keeps MFI above 12 g/10 min, well within the processable range. The mechanism is that the long isostearoyl chains create a true lubricant layer between the filler and the polymer, reducing melt viscosity at the filler-polymer interface. Titanate is the right choice for high-loading flame-retardant PP that needs to flow into thin sections: cable insulation with 0.5 to 0.8 mm wall thickness, complex appliance housings, and thin-film extrusion.

Amino Silane: The Wrong Choice for Polypropylene

3-aminopropyltriethoxysilane (APTES) is the second-most-common silane coupling agent after vinyl silane, but it is the wrong choice for polypropylene. The chemistry is the same - the silanols bond to ATH surface hydroxyls - but the organofunctional group is a primary amine (-NH2) that is highly polar and reactive. In a non-polar PP matrix, the amine groups hydrogen-bond with each other rather than with the polymer, causing three problems.

First, the amine-functionalized ATH agglomerates during storage because the surface is no longer hydrophobic - the amine is hygroscopic and absorbs water from air, just like uncoated ATH. Second, the compound yellows during processing and during aging because the amine catalyzes PP oxidation. Third, the amine does not graft to PP, so the compound shows lower mechanical properties than even stearic acid-coated ATH despite the higher coating cost. Amino silane is the right choice for polyamide (PA6, PA66), where the amine reacts with the amide end-groups, and for epoxy systems, where the amine is a co-reactant in the cure. For PP, stearic acid, vinyl silane, or titanate should always be specified.

Side-by-Side Performance Comparison

Putting all three coatings into a single comparison table for a 60 wt% ATH, 40 wt% homopolymer PP compound with the same base ATH (1.5 micrometer D50, BET 4 m2/g) and the same compounding recipe:

The table tells the selection story. Vinyl silane wins on mechanical performance and long-term aging. Titanate wins on melt flow at high filler loading. Stearic acid wins on cost for general-purpose compounds. Uncoated ATH loses on every metric except price, and even on price it loses once you account for the higher scrap rate and re-tooling cost from steam marks and poor dispersion.

Flame Retardancy vs Mechanical Properties: The Trade-Off

A point that surprises newcomers: surface coating does not significantly affect the flame-retardant mechanism of ATH. ATH works by endothermic decomposition releasing water, and that reaction depends on the chemistry of the AlOOH, not on the surface coating. The coating burns off cleanly during compounding or at the very start of the pyrolysis zone in the fire test. The result is that LOI values for 60 wt% ATH in PP are within experimental error (27 to 30 percent) regardless of coating type.

Where coating does matter for fire performance is in smoke density and char integrity. Vinyl silane coating produces a slightly more coherent ceramic char because the silane residue acts as a weak silica binder between ATH particles, holding the char together longer during combustion. In cone calorimeter tests at 50 kW/m2 heat flux, vinyl silane-coated ATH typically shows 10 to 20 percent lower smoke density than stearic acid-coated ATH. Both are small effects but matter for railway (EN 45545-2) and aircraft (FAR 25.853) applications where smoke and toxicity are regulated. The dominant variable for flame retardancy is ATH loading, not coating type. Going from 60 to 65 wt% ATH raises LOI by about 1.5 points and is the difference between UL94 V-2 and V-0 at 1.6 mm.

Three QC Tests for Surface Coating Quality

How do you tell whether the ATH you received has the right coating level and complete coverage? Three quick lab tests.

Float test for hydrophobicity. Place 5 g of coated ATH gently on the surface of 200 mL of deionized water in a 250 mL beaker. Do not stir. Time how long the powder floats. A properly coated ATH floats for 30 seconds to several minutes before partial wetting and sinking. Uncoated ATH sinks within 2 seconds. The float time is a qualitative measure of coating completeness: longer floats mean better surface coverage. ATH with degraded or excessive coating may float briefly then sink in chunks.

Methanol wet-out test. Place 5 g of coated ATH into 50 mL of methanol in a 100 mL beaker and stir gently with a glass rod. A properly coated ATH wets out in methanol within 5 to 10 seconds and forms a milky suspension. Under-coated ATH clumps at the bottom because the polar ATH surface does not wet in methanol. Over-coated ATH (above 2.5 wt% treatment) wets out too fast and forms a clear solution because excess coating dissolves in methanol.

TGA weight loss for coating level. Weigh 10 g of coated ATH into a TGA crucible, ramp 10 degrees C/min from 25 to 900 degrees C in air or nitrogen. The mass loss between 200 and 400 degrees C is the coating level (stearic acid decomposes around 250 degrees C, silane around 280 to 320 degrees C, titanate around 280 to 350 degrees C). Compare to the supplier-quoted coating level. A properly coated ATH shows 0.8 to 2.0 wt% loss. Below 0.5 wt% means incomplete coating; above 2.5 wt% means excess coating that will migrate to the extruder die and cause plate-out. For more detailed analysis, FTIR of the coated ATH shows C-H stretch peaks at 2850 and 2920 cm-1 from stearic acid or silane alkyl chains, and the Si-O-Al peak at about 1000 cm-1 for silane coatings. XPS gives the surface C/Al ratio: properly coated ATH shows C/Al above 0.5, while under-coated ATH shows C/Al below 0.2.

Compounding Guidelines for Surface-Coated ATH in PP

Use a co-rotating intermeshing twin-screw extruder with L/D ratio of at least 40:1. Feed PP pellets through the main hopper at the throat. Feed coated ATH through a side-feeder located 30 to 50 percent down the barrel length, after the PP has fully melted. Melt temperature 180 to 210 degrees C. Screw speed 200 to 400 rpm. Throughput 20 to 40 kg/hr per liter of screw volume. A vacuum vent at 70 to 80 percent barrel length removes residual moisture from the coating burn-off and prevents die lip build-up. Direct ATH feed at the main hopper works for low loadings (under 40 wt%) but causes melt-temperature spikes and poor dispersion at 60+ wt% because the high-surface-area ATH absorbs heat from the melt as it warms up. Side-feeding is mandatory for high-loading ATH compounds.

A typical compounding recipe: PP homopolymer (MFI 10 to 15) 38 to 42 parts, surface-coated ATH (60 wt% target) 58 to 62 parts, DCP peroxide initiator 0.1 to 0.3 parts (for vinyl silane-coated ATH), optional processing aid (PE wax, EBS) 0.5 to 1.0 parts, optional heat stabilizer (Irganox 1010 + 168) 0.2 to 0.4 parts. Dry the compound before injection molding: 80 degrees C for 2 hours in a desiccator dryer. Even coated ATH picks up some moisture during storage, and a desiccant drying step prevents steam marks on the molded parts.

Which Coating Should You Specify?

Use this quick decision tree when you talk to your ATH supplier or to us:

• General-purpose flame-retardant PP at 40 to 55 wt% ATH - stearic acid-coated ATH (1.5 wt% coating). Lowest cost, good enough mechanical performance, no peroxide required.
• Cable insulation at 60 to 65 wt% ATH, V-0 at 1.6 mm, 90 or 105 degrees C aging - vinyl silane-coated ATH with peroxide initiation. Best mechanical properties and long-term aging.
• Thin-wall appliance housing at 55 to 65 wt% ATH - titanate-coated ATH. Best melt flow at high loading for thin-wall injection molding.
• Polyamide or epoxy matrix (NOT PP) - amino silane-coated ATH. The amine group reacts with amide or epoxy groups.
• PE-copolymer (EVA, EPDM) cable - vinyl silane-coated ATH (the vinyl group grafts to PE backbone under peroxide initiation).
• Storage in humid climate (Southeast Asia, Brazil, Gulf) - titanate-coated ATH (lowest moisture pickup) or vinyl silane-coated ATH.

Aluminaworld Surface-Coated ATH Specifications

Aluminaworld supplies three commercial surface-coated ATH grades for PP and PE compounds:

Free 5 kg samples of all three grades are available for laboratory evaluation. Each shipment includes a lot-level CoA with coating level, D50, BET, moisture, and TGA loss data, plus a recommended compounding recipe for your specific PP grade.

Cost Economics: Why Surface-Coated ATH Pays Back

Surface-coated ATH costs USD 0.05 to 0.60 per kg more than uncoated ATH, depending on coating type. For a compounder running 1000 ton/year of flame-retardant PP at 60 wt% ATH, the coating cost adds USD 30,000 to 180,000 per year to the ATH bill. The savings come from three sources that more than pay back the coating premium.

First, scrap rate reduction. Uncoated ATH causes steam marks and surface streaks on molded parts; scrap rates of 3 to 8 percent are typical. Coated ATH reduces scrap to under 1 percent. For a 1000 ton/year compounder producing parts at USD 3 to 8 per kg, scrap reduction of 5 percent saves USD 150,000 to 400,000 per year. Second, higher throughput. Titanate-coated ATH at 65 wt% loading flows 30 to 50 percent faster than stearic-acid-coated ATH at the same loading because of the lower melt viscosity. Higher throughput directly improves compounder productivity without capital investment. Third, faster color changeover. Uncoated ATH leaves white residue in the extruder that requires purging with 50 to 100 kg of next-resin before the color clears. Coated ATH residue is much easier to purge, reducing changeover time by 30 to 60 minutes per changeover. For most compounders the payback period on the coating premium is 3 to 6 months.

Frequently Asked Questions

Next Steps for Your FR-PP Project

If you are compounding ATH into polypropylene or polyethylene for cable insulation, appliance housings, or automotive parts, the surface coating on the ATH decides whether your compound processes cleanly, meets mechanical specs, and passes UL94. The data above should let you pick the right coating grade for your application and your processing window. When you are ready to evaluate coated ATH samples, request a free 5 kg pack of all three grades (ATH-S, ATH-V, ATH-T) and run the float test, methanol wet-out test, and TGA loss test in your own QC lab before scaling up to a 1-ton pilot batch.

For surface-coated ATH, regular ATH grades, custom coating levels, or matched ATH + coupling agent + peroxide starter kits, contact us via:

• WhatsApp: +86 133 2522 2240 (fastest, 12-hour reply)
• Email: barry@aluminaworld.com
• Sample request: 5 kg R&D pack of ATH-S, ATH-V, or ATH-T, 5-7 day lead time, full CoA with coating level, D50, BET, and TGA curve
• Bulk orders: 20 ton MOQ, 15-20 day production, FOB/CIF/CFR from Qingdao Port (80 km from our factory)

Aluminaworld has supplied surface-treated ATH to PP and PE compounders in 60+ countries for 15 years. Our coated ATH grades are manufactured under ISO 9001 quality control with SGS on-site audits and full Alibaba Trade Assurance. We also supply matching coupling agents, peroxide initiators, and processing aids as a single kit so you can avoid the compatibility issues that come from sourcing from three different vendors. Let us put our coating experience to work on your next flame-retardant compound.