How to Process ATH for Use in Adhesives and Sealants

Alumina Trihydrate (ATH) is widely used in adhesives and sealants as a halogen‑free flame retardant, low‑smoke filler, and functional modifier. It boosts dimensional stability, improves sag resistance, and reduces formulation cost for silicone sealants, epoxy adhesives, PU sealants, and construction joint compounds. However, raw ATH cannot be directly added into adhesive systems. Adhesive and sealant formulations require tightly controlled particle size, ultra‑low moisture, low oil absorption, and tailored surface activation. This article describes the full processing workflow for ATH for adhesive and sealant grades, based on ATH deep processing technology from JACAN Powder Equipment.

1. Raw Material Selection & Pre‑Treatment

The starting ATH feedstock must meet strict impurity controls for adhesive applications. High iron, free alkali, or coarse grit will cause discoloration, poor curing, and degradation of adhesive bonding strength.

• Select high‑purity ATH crystals with low Fe₂O₃ and Na₂O content. Iron impurities lead to yellowing and weaken electrical properties for electronic adhesives.
• Pre‑drying and de‑agglomeration: Remove surface free moisture before milling. Residual moisture causes bubbles, pinholes, and blistering during sealant curing, especially for moisture‑curing silicone systems.
• Pre‑screening: Remove oversized hard agglomerates and foreign grit that would scratch mixing equipment and create surface defects in cured sealant beads.

The critical rule at this stage: keep material temperature low. ATH begins to lose crystal water near 150 °C; once decomposed, it loses flame‑retardant performance and cannot be used in sealant formulations.

2. Low‑Temperature Ultrafine Grinding & Precision Air Classification

Adhesives and sealants require balanced fineness and oil absorption. Too fine ATH sharply increases oil absorption and raises compound viscosity, limiting high filler loading. Too coarse particles reduce tensile strength and create rough sealant surfaces. Typical target D50 ranges from 2 μm to 8 μm for most silicone and epoxy sealant grades.

• Use closed‑circuit low‑temperature ultrafine milling. Ceramic lining is preferred to avoid iron contamination. JACAN’s milling system controls peak material temperature ≤110 °C to protect crystal water.
• Integrate high‑precision turbine air classification inside the closed loop. The classifier cuts oversize particles and delivers narrow particle‑size distribution. Narrow PSD helps stabilize viscosity, improve sag control, and maintain consistent extrusion performance for cartridge‑packaged sealants.
• Recirculate coarse fractions back to the mill; only qualified fine powder proceeds to surface modification.

3. Surface Modification (The Most Critical Step for Adhesive & Sealant Grade ATH)

Raw ATH is hydrophilic. When mixed into organic adhesive resins (silicone, epoxy, PU), untreated ATH agglomerates, increases viscosity, and weakens interfacial bonding. Surface modification converts the hydrophilic mineral surface to organophilic, enabling high loading while preserving mechanical and bonding properties.

Two main modifier systems are selected based on adhesive resin type:

1. Silane coupling agents (preferred for silicone sealants, electronic adhesives, epoxy adhesives)
   Vinyl or amino silane coupling agents form chemical bonds between ATH surface hydroxyl groups and polymer matrix. Silane‑treated ATH reduces viscosity build‑up, improves water resistance after curing, and preserves insulation properties for electronic sealants.
2. Stearic acid / fatty acid coating (common for construction sealants and low‑cost adhesive systems)
   Lower cost than silane treatment; improves dispersion and reduces oil absorption, though less effective for high‑performance electronic and long‑term waterproof grades.

Modification process:
Feed classified fine ATH into high‑shear continuous surface coating reactor. Add metered modifier at controlled temperature (80–120 °C) under high turbulence. Maintain high coating uniformity and high surface coverage rate. Poor coating leads to inconsistent batch viscosity, poor storage stability, and filler sedimentation in sealant paste.

4. Post‑Processing, Quality Inspection & Packaging

After modification, the powder goes through de‑agglomeration and cooling, then full quality testing before packaging. Key inspection indexes for adhesive/sealant ATH:

• Moisture content ≤0.3–0.5%
• Whiteness, residual coarse particles, oil absorption value
• PSD (D50, D97)
• Impurity levels (Fe, Na)
• Surface modifier coverage and dispersion test in target resin

Sealant manufacturers are highly sensitive to batch variation. Consistent oil absorption is especially important: small shifts will change paste viscosity, extrusion force, sag resistance and final Shore hardness of cured sealants.

Packaging must use moisture‑proof sealed bags to prevent re‑absorption of ambient moisture during storage and transport.

5. Key Application‑Specific Processing Adjustments

• Silicone flame‑retardant sealants: Require silane‑modified ATH, low moisture, low iron, narrow PSD. Filler loading can reach 40–60 wt% in formulation, for construction firestop and electronic potting sealants.
• Epoxy structural adhesives: Medium fineness, low alkali ATH, controlled oil absorption to avoid excessive viscosity rise.
• PU construction sealants: Stearic‑acid coated ATH is widely adopted for cost balance, good sag resistance and smooth surface finish.

6. Common Processing Pitfalls to Avoid

1. Overheating during grinding → ATH dehydration, permanent loss of flame retardancy.
2. Insufficient surface coating → severe agglomeration, high viscosity, poor bonding strength.
3. High residual moisture → bubbles, pinholes and curing defects in sealant.
4. Wide particle distribution → inconsistent rheology, poor extrusion and uneven cured mechanical performance.

Processing ATH for adhesives and sealants is a controlled low‑temperature chain: raw material purification → low‑temperature ultrafine grinding + air classification → high‑shear surface modification → quality control and moisture‑proof packaging. The core target is to produce low‑moisture, low‑oil‑absorption, uniformly surface‑activated ATH powder. This allows adhesive and sealant formulators to achieve high filler loading, stable rheology, good mechanical strength, and halogen‑free flame retardancy.

For equipment suppliers like JACAN, the goal is to deliver integrated closed‑loop milling, classification and surface modification systems that reliably produce consistent ATH grades tailored for silicone, epoxy and PU adhesive & sealant manufacturers.