Aluminum hydroxide (ATH) is a common halogen-free flame retardant filler for polyolefin polymers such as EVA, PE and PP. Raw ATH particles carry abundant polar hydroxyl (-OH) groups on their surface, making them hydrophilic. Polymer matrices like LSZH cable compounds are non-polar. This fundamental surface mismatch causes fine ATH powder to form tight micro-agglomerates, which do not separate easily during melt compounding. Surface modification changes the surface chemistry of ATH particles and dramatically improves dispersion within polymer melts.
1. Eliminates hydrogen bonding between ATH particles
Unmodified ATH particles attract each other through hydrogen bonds between surface hydroxyl groups. Ultrafine ATH tends to clump into stable micro-clusters. During extrusion, shear forces often cannot fully break these agglomerates. These clusters remain as defects in the finished composite.
When modified with silane or stearic acid, the modifier reacts or adsorbs onto the ATH surface and caps most hydroxyl groups. The inter-particle hydrogen attraction is greatly reduced. Individual ATH particles no longer stick together, so agglomerates are much easier to separate under compounding shear.
2. Converts ATH surface from hydrophilic to hydrophobic
Untreated ATH prefers to interact with water and other polar substances, not non-polar polymer chains. Modified ATH gains a hydrophobic organic outer layer. This organic shell is compatible with polyolefin resins. The particles are wetted quickly by the molten polymer during mixing, instead of repelling the resin. Good wetting is the prerequisite for uniform dispersion.
3. Improves interfacial lubrication and reduces melt viscosity
Stearic acid modification adds a lubricating organic layer on ATH. Silane modification also reduces surface friction between particles and polymer chains. Lower internal friction helps polymer melt flow around individual ATH particles. The molten resin can penetrate into small gaps between particles, separating them fully. This prevents re-agglomeration during high-temperature extrusion.
4. Prevents re‑agglomeration after dispersion
Even if agglomerates are temporarily broken during mixing, unmodified ATH can re-cluster when shear reduces. The organic coating layer acts as a steric barrier around each ATH particle. The steric hindrance keeps separated particles from touching and re-forming agglomerates in the polymer melt. This maintains stable dispersion throughout extrusion.
5. Strengthens interfacial bonding (silane modification)
Silane coupling agents form chemical bridges between ATH and polymer chains. This not only improves dispersion but also transfers stress across the filler-resin interface. Well-dispersed, strongly bonded ATH preserves mechanical properties such as elongation and tensile strength at high loading levels. Without silane treatment, voids form around poorly dispersed ATH particles, which weaken the composite.
Practical benefits of good ATH dispersion
- No visible particle clusters, pinholes or surface defects on extruded cable or plastic parts
- Consistent flame retardancy and smoke suppression across the whole product
- Stable mechanical properties, even at high ATH loading (up to 60–65 wt% in LSZH compounds)
- Lower extrusion pressure and more stable production
- Uniform dielectric performance for cable-grade materials
What happens without surface modification
Unmodified ATH agglomerates survive compounding. These clusters create voids in the polymer matrix. The composite becomes brittle, melt viscosity rises sharply, extrusion becomes unstable, and local weak points reduce flame and electrical performance.
Surface modification improves ATH dispersion in polymers mainly by capping surface hydroxyl groups to reduce inter-particle hydrogen bonding, changing particle surface from hydrophilic to hydrophobic for better resin wetting, and providing steric barriers to prevent re-agglomeration. Silane additionally creates chemical bonding between ATH and polymer matrix. Uniformly dispersed modified ATH enables high filler loading while maintaining stable mechanical, flame-retardant and electrical properties for LSZH cables and plastic composites.
