Alumina trihydrate (ATH) for flame‑retardant composites falls into two main commercial categories: precipitated ATH (synthetic precipitated crystals) and ground ATH (milled natural bauxite‑derived gibbsite). Their crystal morphology, purity, particle size distribution, impurity profile and thermal behavior differ substantially, which directly changes flame retardancy, smoke suppression, mechanical performance and formulation suitability. This article references ATH processing technology from ath‑mill.com and JACAN powder engineering.
1. Raw origin & crystal morphology
Precipitated ATH
Produced via chemical precipitation from sodium aluminate liquor in the Bayer process. Crystals grow under controlled crystallization conditions.
- Crystal shape: uniform blocky/equant hexagonal crystals, low aspect ratio, smooth surfaces, minimal sharp fracture edges.
- PSD: narrow particle size distribution, controllable primary particle size.
- Agglomeration: soft agglomerates only; no hard mineral grit.
Ground ATH
Made by crushing and ultrafine grinding of natural gibbsite ore.
- Crystal shape: irregular fractured particles, sharp edges, uneven surface texture, broad aspect ratio range.
- PSD: inherently wider particle distribution, even after air classification. Fine fractions and residual coarse fragments coexist.
- Agglomeration: may contain hard aggregates and trace native mineral impurities locked inside particles.
2. Key flame‑retardant related properties
Thermal decomposition behavior
- Precipitated ATH: Consistent crystal structure, sharp and stable dehydration onset around 180–220 °C. Crystal water release is predictable and uniform. This is critical for intumescent coatings, cable compounds and sealants where timing of water release must match polymer thermal breakdown.
- Ground ATH: Natural gibbsite has minor crystal lattice variations. Dehydration temperature range is broader. Some surface‑damaged fine fractions may start releasing water slightly earlier. In intumescent systems, uneven water release can rupture the forming char foam and reduce fire protection efficiency.
Smoke suppression
- Precipitated ATH: Higher purity and uniform crystal structure deliver superior low‑smoke performance. Water vapor evolves evenly to dilute flammable volatiles; the resulting alumina residue forms a continuous, dense barrier layer. Preferred for low‑smoke zero‑halogen (LSZH) cable and public building fire materials.
- Ground ATH: Natural mineral impurities (iron, silica, trace clay) increase smoke density and may introduce catalytic effects during combustion. Smoke suppression is generally poorer than precipitated grades.
Char layer formation
- Precipitated ATH: Uniform fine particles disperse well in polymer matrix. Upon heating, decomposed alumina particles distribute evenly inside the char, forming a continuous, reinforcing skeleton that prevents char cracking or peeling.
- Ground ATH: Irregular fractured particles create non‑uniform alumina residue. Coarse fragments act as defects within char, creating gaps and heat transfer shortcuts. The char is weaker and more prone to spallation under flame exposure.
3. Impurity profile and secondary fire side effects
- Precipitated ATH: Low Fe₂O₃, low Na₂O, minimal heavy metals. Less discoloration, no catalytic polymer degradation. Suitable for transparent/light‑colored silicone sealants, electronic potting and high‑end LSZH materials.
- Ground ATH: Naturally contains iron oxide, quartz, kaolin and alkali impurities. Iron impurities cause yellowing; alkali residues accelerate polymer aging. These impurities can also catalyze polymer decomposition during fire, partially offsetting flame‑retardant benefits.
4. Processing and formulation trade‑offs
Precipitated ATH
- Pros: narrow PSD, low oil absorption after surface modification, good dispersion, consistent batch‑to‑batch flame performance.
- Cons: higher production cost.
Ground ATH
- Pros: lower raw material cost.
- Cons: wider PSD, higher oil absorption, harder to disperse fully. Hard grits may create micro defects in composites, lowering mechanical properties and creating weak points under fire. Even after ultrafine grinding and classification, residual mineral impurities remain.
5. Typical application selection
- Precipitated ATH: LSZH cable compounds, silicone flame‑retardant sealants, intumescent coatings, electronic adhesives, high‑performance rubber. Used where consistent flame retardancy, low smoke and light color are mandatory.
- Ground ATH: General‑purpose low‑cost flame‑retardant fillers, construction composite boards, low‑grade cast polymers, non‑critical fire applications, where cost is prioritized and smoke/color requirements are relaxed.
6. Summary of flame retardancy difference
Precipitated ATH provides predictable, stable endothermic dehydration, better smoke suppression and more uniform char reinforcement, making it the premium choice for high‑performance halogen‑free flame retardant composites. Ground ATH is cost‑effective, but its variable crystal structure and inherent mineral impurities lead to inconsistent dehydration, higher smoke and weaker char integrity. For demanding flame‑retardant formulations, precipitated ATH is normally selected, often further processed by low‑temperature ultrafine grinding and surface modification to optimize dispersion.
