Aluminum hydroxide (ATH) filled polymer composites are widely used for low-smoke zero-halogen (LSZH) cables, plastic profiles and artificial stone. Unmodified ATH introduces a sharp drop in mechanical strength, especially elongation at break, when loaded at high concentrations. Surface modification changes the filler-polymer interface, directly improving tensile strength, elongation, impact resistance and other mechanical performance indicators.
The core reason: filler-polymer interfacial bonding
Raw ATH has hydrophilic hydroxyl groups on its surface. It is incompatible with non-polar polyolefin resins. During melt blending, voids easily form at the interface between ATH particles and polymer matrix. Under external stress, cracks start from these voids and expand rapidly, leading to early fracture.
Surface modifiers build a bridge between inorganic ATH and organic polymer.
- Silane coupling agent: Forms chemical bonds with ATH surface hydroxyl groups and creates entanglement or chemical linkage with polymer chains. Stress can transfer smoothly from the resin to ATH particles.
- Stearic acid: Forms a physical lubricating layer on ATH. It improves powder dispersion and reduces melt viscosity, but cannot form strong chemical interfacial bonding.
Mechanical property changes after modification
1. Tensile strength
- Unmodified ATH: Tensile strength declines significantly as loading increases. Poor interfacial adhesion creates voids and stress concentration points.
- Silane-modified ATH: Tensile strength is well retained or slightly improved even at high filler loading (60–65 wt%). Strong interfacial bonding suppresses crack initiation.
- Stearic-acid-modified ATH: Tensile strength is better than unmodified ATH, but lower than silane-treated ATH. The lubricating layer does not support efficient stress transfer.
2. Elongation at break
Elongation is the most sensitive indicator for LSZH cable materials.
- Unmodified ATH composite becomes brittle, elongation drops sharply at high loading.
- Silane modification greatly preserves elongation. The interface can deform together with the polymer matrix without debonding immediately. This is the key reason silane is used for flexible cable compounds.
- Stearic acid improves dispersion and gives moderate elongation gain, but the gain is limited compared with silane.
3. Impact toughness
- Unmodified ATH agglomerates act as crack initiators, reducing impact resistance.
- Modified ATH achieves better dispersion. Silane treatment enhances interfacial adhesion, so cracks need more energy to propagate. Impact toughness rises notably.
- Stearic acid reduces agglomeration and improves impact performance to a certain degree, yet the weak interface limits further improvement.
4. Modulus (stiffness)
ATH itself is rigid inorganic filler. Surface modification mainly optimizes interface rather than changing ATH intrinsic rigidity.
- After modification, the composite’s Young’s modulus remains high. The material still maintains good dimensional stability, while the brittleness caused by unmodified ATH is reduced.
Side effects from improper modification
- Over-dosing modifier: Excess free modifier acts as an internal plasticizer. It reduces tensile strength and modulus, and may cause blooming on product surfaces.
- Too high modification temperature: ATH partial dehydration degrades flame retardancy and also creates defects, lowering mechanical performance.
- Poor coating uniformity: Partially unmodified ATH particles form local agglomerates, leading to inconsistent mechanical values across batches.
Silane vs Stearic acid mechanical performance summary
| Modifier | Tensile Strength | Elongation | Impact Toughness | Best For |
|---|---|---|---|---|
| Silane coupling agent | High retention | Excellent | Good | LSZH cable, high-performance composites |
| Stearic acid | Moderate improvement | Limited gain | Moderate | Artificial stone, general plastic |
Surface modification improves composite mechanical properties primarily by optimizing the interface between ATH filler and polymer matrix. Silane modification delivers strong interfacial bonding, effectively retaining tensile strength and elongation at high ATH loading, which is essential for flexible LSZH cable materials. Stearic acid improves dispersion and reduces brittleness to a lesser extent, suitable for cost-sensitive non-flexible products. Without proper surface modification, high loading of ATH will severely weaken composite mechanical performance.
