Aluminum hydroxide (ATH, aluminum trihydrate) is a widely used halogen‑free flame retardant filler for polyolefin cables, plastics, rubber and composite materials. Raw unmodified ATH powder has abundant hydrophilic hydroxyl groups (-OH) on its particle surface. These polar inorganic surfaces are incompatible with non‑polar polymer matrices such as EVA, PE and PP. Surface modification is essential to solve this interfacial mismatch and unlock the full potential of ATH in polymer composites.
1. Improve interfacial compatibility between ATH and polymer resin
Untreated ATH is hydrophilic, while most polymer matrices are hydrophobic. When directly blended, ATH particles cannot form strong bonding with resin. Weak interfaces create voids inside the composite. This leads to poor mechanical performance, including sharp drops in tensile strength and elongation at break, especially at high filler loading.
Surface modification uses silane coupling agents, stearic acid or other modifiers. The modifier reacts with surface hydroxyl groups of ATH, covering particles with an organic hydrophobic layer. This organic outer layer can entangle or chemically bond with polymer chains, forming stable interfacial adhesion. It allows manufacturers to load much higher ATH content while retaining acceptable mechanical properties.
2. Reduce powder agglomeration and improve dispersion
Fine ATH powder tends to cluster into micro‑agglomerates due to hydrogen bonding between surface hydroxyl groups. During compounding, these agglomerates remain intact and become defects in finished plastic or cable products. Poor dispersion also increases melt viscosity and makes extrusion difficult.
After surface activation, the hydroxyl groups on ATH are capped. Inter‑particle hydrogen attraction is greatly reduced. Modified ATH disperses uniformly in the resin melt, without hard agglomerates. This stabilizes product quality and reduces surface defects on extruded parts.
3. Lower melt viscosity and optimize processing performance
Unmodified fine ATH significantly increases melt viscosity when added into polymers. High viscosity raises extrusion pressure, increases energy consumption, and limits production speed.
The hydrophobic coating from surface modification acts as a lubricant between ATH particles and polymer chains. It reduces internal friction of the melt. The compound flows better during extrusion and injection molding, supporting high‑speed continuous production for wire and cable, plastic sheets and profiles.
4. Maintain critical flame retardant and smoke suppression performance at high loading
ATH releases crystal water upon heating to absorb heat and suppress smoke. If unmodified ATH is loaded at high percentages, the composite becomes brittle and cannot be used practically. Surface modification enables high ATH loading, which is required to achieve low‑smoke, zero‑halogen flame retardancy. The modification process is strictly controlled below 110 °C, so ATH crystal water remains intact and flame retardant capacity is preserved.
5. Enhance electrical insulation properties for cable-grade materials
Low‑smoke halogen‑free cable compounds require stable dielectric performance. Unmodified ATH’s hydrophilic surface readily absorbs moisture, which reduces insulation resistance and impairs electrical properties. Surface‑coated ATH is hydrophobic, so moisture absorption is minimized. This ensures consistent electrical performance for power and communication cables.
6. Improve powder handling characteristics
Raw fine ATH is prone to dusting and absorbs ambient moisture easily, causing flow problems during conveying and weighing. Modified ATH powder has better free‑flowing behavior, less dust emission and lower tendency to pick up moisture in storage. This stabilizes feeding consistency in compounding factories.
What happens if you skip surface modification?
- Severe particle agglomeration in polymer blends
- Dramatic loss of tensile strength and elongation
- High melt viscosity, difficult extrusion processing
- Poor electrical insulation for cable grades
- Unstable product batches and higher reject rates
Surface modification is necessary for ATH filler mainly to solve the inherent incompatibility between hydrophilic inorganic ATH and hydrophobic polymer matrices. It improves interfacial bonding, particle dispersion and melt flow. It enables high ATH loading to achieve flame retardancy, while protecting mechanical and electrical properties of finished products. Without surface activation, ultrafine ATH cannot be effectively applied in high‑end low‑smoke halogen‑free cables and plastic composites.
