ATH Filler for Flame-Retardant FRP: Loading Levels and Processing Tips

Aluminum trihydrate (ATH, Al(OH)₃) is the workhorse halogen-free flame retardant for fiberglass-reinforced plastics (FRP). It decomposes endothermically above 200°C, releasing water vapor that cools the substrate and dilutes combustible gases. For composite manufacturers building rail interiors, mass-transit panels, electrical enclosures, and construction profiles, ATH is the most cost-effective route to UL 94 V-0, EN 45545, and ASTM E84 Class A ratings.

Loading Levels by Application

The minimum ATH loading to reach meaningful flame retardancy is around 100 phr (parts per hundred resin). Practical ranges by application:

• Pultruded profiles & gratings (UL 94 V-0): 150–200 phr in unsaturated polyester (UPR) or vinyl ester. Combine with 5–10 phr Sb₂O₃ if halogenated UPR is used.
• Hand lay-up panels (Class 1 surface burn): 100–150 phr in iso-UPR.
• Mass-transit interior (EN 45545 HL2/HL3): 180–220 phr ATH plus 10–15 phr zinc borate or APP synergist in modified vinyl ester.
• Electrical SMC/BMC: 250–300 phr is common because thermoset compression molding tolerates very high filler loading.

Above 200 phr, expect viscosity, wet-out, and mechanical penalties. Tensile strength typically drops 15–25% at 150 phr versus unfilled UPR; flexural modulus rises.

Processing Tips

Particle size matters more than loading. Coarse ATH (D50 ~20–40 μm) wets out faster and keeps mix viscosity manageable for hand lay-up and pultrusion. Fine precipitated ATH (D50 1–3 μm) gives better surface finish and higher flame performance per phr but spikes viscosity — reserve it for SMC, casting, and gel coats.

Control viscosity with thixotropes. When ATH loading exceeds 120 phr, fumed silica at 1.0–1.5 phr (e.g. SEMISIL 200) restores sag resistance without further raising base viscosity. Hydrophobic grades (SEMISIL R202) are preferred when humidity exposure is a concern.

Use silane-treated ATH for wet electrical performance. Untreated ATH absorbs moisture, degrading CTI and dielectric strength. Vinyl- or methacryl-silane surface treatment improves resin coupling and long-term wet-aging performance.

Mix order: Disperse ATH into resin under high-shear before adding catalyst, then add fumed silica last to set thixotropy. Avoid trapping air — pull vacuum if possible.

Synergists and Smoke Suppression

ATH alone reduces heat release but does little for smoke. For EN 45545 R1 smoke targets, blend with:

• Zinc borate (5–15 phr) — char promoter, smoke suppressor.
• Ammonium polyphosphate (APP, 20–40 phr) — intumescent synergist.
• Magnesium hydroxide (MDH) — partial replacement when processing temperatures approach 220°C (ATH starts decomposing at 200°C; MDH at 330°C).

Get the Right Grade for Your Resin System

Resinspot supplies ATH in coarse, fine, and silane-treated grades, plus zinc borate, APP, and SEMISIL fumed silica from a single source. Send your resin system, target standard, and target loading — our team will recommend a grade and ship samples.

Contact Resinspot: [email protected] · WhatsApp +86 156 3910 0440