Aluminum hydroxide (ATH, alumina trihydrate) is a mainstream halogen‑free flame‑retardant filler widely applied in wire‑and‑cable compounds, plastics, rubber, coatings and artificial stone. Its flame‑retardant effect mainly comes from endothermic crystal‑water release upon heating, diluting combustible gas and forming an insulating alumina barrier layer. Simply adding more ATH cannot guarantee better flame‑retardant performance; improper particle characteristics, processing conditions and formulation design will greatly weaken its fire‑suppression capability. This article describes practical technical routes to boost the flame‑retardant performance of ATH filler.
1. Optimize Particle‑Size Distribution and Fineness
Particle size directly determines thermal‑decomposition efficiency and barrier‑layer compactness.
- Too‑coarse ATH: Low specific surface area, slow water‑release rate, forms loose char layers, poor flame‑retardant effect and worsens mechanical properties of polymer matrix.
- Ultra‑fine ATH: Higher specific surface area enables faster endothermic reaction under fire exposure, generating denser alumina barrier film to block heat and volatile fuel gas.
For typical low‑smoke zero‑halogen cable materials, D97 is commonly controlled between 2‑8 μm, with narrow particle‑size distribution and minimal coarse oversize particles. Avoid excessive ultrafine particles that cause severe agglomeration; agglomerated ATH cannot fully participate in endothermic decomposition and reduces actual flame‑retardant efficiency. Low‑temperature ultrafine grinding with precise air classification is essential to produce qualified narrow‑span ATH powder without triggering premature crystal‑water loss.
2. Strictly Prevent Thermal Decomposition During Powder Processing
ATH begins to lose crystal water near 150 °C. Pre‑decomposed ATH loses part of crystal water before compounding, which permanently reduces its endothermic capacity and directly cuts flame‑retardant performance.
Key processing controls:
- Keep material temperature below 110 °C during grinding, drying and classification. Adopt water‑jacket cooling for grinding mill bodies.
- Avoid high‑temperature drying processes that exceed safe temperature thresholds.
- Prevent local hot spots inside grinding equipment. Retain complete crystal‑water content to guarantee maximum heat‑absorbing effect under combustion conditions.
3. Improve Surface Modification to Enhance Dispersion in Polymer Matrix
Unmodified ATH has high surface polarity and poor compatibility with non‑polar resins. Poor dispersion creates voids and defects inside composites. Flames spread easily along these gaps, even with high ATH loading.
Efficient surface treatment measures:
- Use silane coupling agents or fatty‑acid modifiers suitable for ATH. Reach coating coverage ≥95 % and high modifier utilization rate.
- Modified ATH disperses evenly inside plastics or rubber, eliminating internal voids. When burning, closely‑packed filler particles quickly form continuous dense alumina barrier layers, blocking heat transfer and volatile‑gas escape.
- Insufficient modification leads to agglomeration, material defects and deteriorated flame‑retardant rating, despite high filler addition amount.
4. Optimize Filler Loading and Compound Formulation Matching
Flame retardancy does not rise linearly with ATH loading. Too‑high loading sharply damages tensile strength, elongation and processing fluidity of polymer products, bringing processing difficulties.
Practical formulation strategies:
- Reach reasonable critical loading according to resin types and product requirements, balancing mechanical performance and fire‑safety requirements.
- Adopt particle‑size gradation: mix fine and medium ATH fractions to increase packing density of filler system. Higher packing density helps build a tighter protective char layer during combustion.
- Synergistic compounding with other halogen‑free flame‑retardant additives: small‑dose addition of boehmite, magnesium hydroxide, silicone flame retardants or nano‑metal oxides can produce obvious synergistic effects. It reduces required ATH loading while improving UL94 or oxygen‑index performance.
5. Control Impurity Content
Impurities negatively affect flame‑retardant performance and thermal stability:
- Iron‑containing impurities will catalyze thermal‑oxidative decomposition of polymer matrix, accelerating material burning. Strictly control iron contamination in grinding system by adopting ceramic‑contact‑part protection.
- Low‑purity ATH with soluble salt impurities worsens electrical properties for cable‑grade products and may also accelerate thermal aging under high‑temperature conditions. Select high‑purity ATH raw ore and anti‑pollution milling technology.
6. Control Composite‑Material Processing Temperature
Even high‑quality ATH filler will degrade if composite processing temperature is too high. Keep extrusion and granulation temperature well below 150 ~ 160 °C to avoid crystal‑water loss during plastic compounding. Partial dehydration during compounding will permanently reduce the endothermic potential of finished products under fire test.
To improve the flame‑retardant performance of aluminum hydroxide filler, focus on six core dimensions: appropriate particle‑size and narrow distribution, full retention of crystal water through low‑temperature processing, high‑efficiency surface modification, reasonable loading and particle gradation, impurity control, and low‑temperature compounding technology. Only when ATH maintains intact crystal‑water structure, good dispersion and dense packing inside polymer matrix can its halogen‑free flame‑retardant potential be fully realized.
