Aluminum hydroxide (ATH) acts as a halogen‑free flame retardant in coatings by releasing crystal water at ~200 °C. This endothermic reaction absorbs heat, dilutes flammable gases, and forms a protective alumina char layer. ATH particle size directly impacts flame retardant efficiency, coating dispersion, viscosity, surface finish and char structure.
1. Fine ATH (small particle size)
Benefits for flame retardancy
- Higher specific surface area. More ATH particles are exposed to heat, so crystal water releases faster and more uniformly under fire conditions.
- Forms a denser, more continuous alumina char barrier. This char layer blocks heat transfer and suppresses smoke and volatile fuel gas escape.
- Better dispersion inside the coating matrix. Fine particles distribute evenly, avoiding local low‑ATH weak zones where flame can penetrate.
Limitations
- Higher oil absorption value. Finer ATH sharply increases coating viscosity, limiting maximum filler loading. If loading cannot be raised, total flame retardant effect may drop despite high surface area.
- Greater tendency to agglomerate. Without surface modification, fine ATH clusters create defects in the coating film, reducing film integrity and flame resistance.
- Higher powder cost.
2. Coarse ATH (larger particle size)
Benefits for flame retardancy
- Lower oil absorption, so higher loading can be added into coatings without excessive viscosity rise. More total ATH mass means more crystal water available for heat absorption.
- Lower agglomeration risk and easier handling.
Limitations
- Lower specific surface area. Water release is slower and less uniform.
- Forms a discontinuous, porous char layer. Gaps in the char allow heat and flammable gases to pass through, weakening barrier protection.
- Large particles create rough coating surface, poor film smoothness, and may reduce film mechanical strength.
3. Particle size distribution (PSD) effect
A well‑graded (broad, matched) PSD blends fine and coarse ATH fractions. Coarse particles fill bulk volume for high loading; fine particles fill gaps between coarse particles. This balances viscosity, high filler loading, and dense char formation. This is a common optimization strategy for industrial flame retardant coatings.
4. Interaction with surface modification
Fine ATH must be surface modified (silane or stearic acid) to reduce oil absorption and agglomeration. Modified fine ATH can maintain good dispersion and relatively low viscosity, unlocking its flame retardant potential. Unmodified fine ATH is difficult to incorporate at high loading and often causes film defects.
5. Practical selection guidelines for coatings
- Thin film coatings: Prefer medium‑fine ATH. Film thickness is limited; fine particles ensure uniform distribution and smooth surface.
- Thick intumescent / fire protective coatings: Graded PSD with a portion of coarser ATH allows higher loading for greater total heat absorption.
- Topcoat / decorative flame retardant coating: Use finer, surface‑modified ATH for smooth appearance without sacrificing flame performance.
Smaller ATH particle size improves char quality and the uniformity of water release, boosting barrier flame retardancy, but raises oil absorption and limits filler loading. Coarser ATH enables higher loading for greater total endothermic capacity, yet forms a weaker char layer. Optimized PSD grading combined with surface‑modified ATH achieves the best balance of coating processability, film quality and flame retardant performance.
