Aluminum hydroxide (ATH, Aluminum Trihydrate) is a widely used halogen‑free flame retardant filler for polymers, rubber, coatings and artificial stone. Raw ATH powder has hydrophilic hydroxyl groups (-OH) on its particle surface. When directly mixed with non‑polar polymer matrices such as PE, EVA and PP, the inorganic ATH particles show poor compatibility with organic resins. This leads to serious agglomeration, high melt viscosity, reduced mechanical strength of composite parts and limited filling loading.
Surface activation modification (also known as surface coating or surface treatment) is a process to chemically or physically graft organic modifiers onto ATH particle surfaces. It converts the hydrophilic surface of ATH into hydrophobic, improves dispersion in polymer systems, enhances interfacial bonding between inorganic filler and resin, and maintains the flame‑retardant performance of ATH. This is an essential post‑processing step for high‑end cable‑grade, plastic‑grade ATH powder.
Main modifiers for ATH surface activation
- Silane coupling agents
Silane is the most common choice for high‑performance low‑smoke halogen‑free cable ATH. One end of the silane molecule reacts with hydroxyl groups on ATH surface; the other organic functional group can form good compatibility and bonding with polyolefin resins. Silane‑treated ATH can achieve high filling ratios while retaining tensile strength and elongation of cable compounds. - Stearic acid / stearate
Stearic acid and metal stearates are cost‑effective modifiers. They coat ATH particles to reduce agglomeration and lower melt viscosity during plastic extrusion. Stearic acid modified ATH is widely used in general plastics and artificial stone, though interfacial bonding strength is lower compared with silane coupling agents. - Other optional modifiers
Titanate coupling agents, aluminate coupling agents and specific polymer waxes are selected for special formulations such as rubber or coating applications.
Working principle of high‑shear activation modification system
The surface activation reaction is usually carried out in a high‑speed vortex mixer after ATH ultrafine grinding and air classification.
- Qualified fine ATH powder is fed into the high‑shear modification chamber.
- ATH particles are fully dispersed by high‑speed rotor impact and turbulent airflow to break micro‑agglomerates.
- Liquid or molten modifier is atomized and evenly sprayed into the powder stream.
- Intense particle collision ensures uniform coating of modifier on every ATH particle surface. The modifier reacts with surface hydroxyl groups under controlled temperature and residence time.
- Surface‑activated ATH powder is discharged, cooled and packaged.
The whole modification temperature must stay below 110 °C to avoid decomposition of ATH crystal water and loss of flame retardancy.
Key quality indicators of modified ATH
- Hydrophobicity: treated ATH powder floats on water surface instead of wetting and sinking.
- Coating coverage rate: good process reaches ≥95 % surface coverage.
- Modifier dosage: typically 0.3 – 1.2 wt% based on ATH weight. Too much modifier increases cost and may degrade flame resistance; insufficient dosage causes poor dispersion.
- Oil absorption value: surface activation reduces oil absorption of ATH, which helps increase filler loading in polymers.
Core benefits of ATH surface activation
- Improve powder dispersion in polymer matrix, eliminate particle agglomerates.
- Reduce melt viscosity during extrusion and injection molding, improve processing flowability.
- Enhance interfacial adhesion between ATH and resin, so composite materials maintain mechanical properties even at high filler loading.
- Lower dusting and improve powder handling characteristics.
- Keep original flame retardant and smoke suppression properties of ATH.
Typical full process flow
ATH raw material → Low‑temperature flash drying → Closed‑circuit ultrafine grinding → Air classification → High‑shear surface activation modification → Cooling → Finished modified ATH powder.
Common pitfalls in ATH surface modification
- Overheating during modification operation triggers ATH dehydration, destroying flame retardant function.
- Uneven powder dispersion before coating leads to partial over‑coating and partial un‑coated particles.
- Incorrect modifier selection: stearic acid for cable grade will reduce electrical insulation performance of cable materials.
- Residual free modifier may migrate to product surface and cause blooming.
Surface activation modification for aluminum hydroxide is a controlled high‑shear coating process that changes ATH surface from hydrophilic to hydrophobic. By using silane, stearic acid or other coupling agents, treated ATH achieves excellent compatibility with organic polymers. It enables high filling loading while maintaining mechanical, flame‑retardant and insulation properties, which makes surface‑activated ATH the preferred flame retardant filler for wire & cable, plastic and composite industries.
