When discussing ATH (aluminum hydroxide, alumina trihydrate) powder moisture, it is critical to distinguish free surface moisture (free water) from crystal‑bound water. Crystal water (~34.5 % LOI) is an inherent chemical component of ATH particles and delivers its flame‑retardant function; it must be fully retained. Free moisture refers to unwanted adsorbed surface water, which is the controlled index in industrial specifications. Excess free moisture creates serious troubles for polymer compounding, cable manufacturing and surface‑modification processes.
Typical Free‑Moisture Specifications by ATH Grade
Free moisture is measured at 105 °C drying test, expressed as weight percentage.
- LSZH cable‑grade ultrafine ATH (silane‑modified, D97:2‑8 μm)
- Standard requirement: ≤0.20 %‑0.30 %, target optimal value 0.10‑0.20 %.
- Ultra‑high‑end cable formulation: ≤0.15 %.
- Excessive moisture causes bubbles, voids, surface blistering during extrusion, degrades volume resistivity and electrical aging performance of wire‑and‑cable insulation.
- General‑purpose flame‑retardant ATH for plastics & rubber
- Acceptable range: ≤0.30‑0.40 %.
- Minor moisture tolerance compared with cable grade, yet moisture above 0.5 % will still produce foaming defects in injection‑molded parts.
- Artificial stone / solid‑surface ATH filler
- Requirement: ≤0.30‑0.50 %.
- Higher moisture leads to pinholes, pores and poor surface gloss in cast solid‑surface panels.
Critical warning: Free moisture shall never be confused with crystal water (loss‑on‑ignition, LOI ≈ 34.5 ± 0.5 %). A drop in LOI indicates thermal decomposition inside grinding/drying equipment, which permanently damages flame‑retardant performance, even if free‑moisture test looks good.
Problems Caused by High Free Moisture in ATH Powder
- Polymer processing defects: Water vaporizes under extrusion temperature, generating bubbles, voids, pinholes, rough product surfaces.
- Surface‑modification failure: For dry‑modified ATH, surplus surface water consumes silane‑coupling‑agent dosage. Modifier hydrolyzes prematurely, resulting in incomplete particle coating, poor dispersion and higher melt viscosity.
- Storage agglomeration: Fine ATH particles absorb ambient humidity; high‑moisture powder tends to cake and form hard lumps during storage.
- Degraded electrical performance: For cable‑grade products, adsorbed moisture reduces insulation resistance and accelerates material aging.
Moisture‑Related Process Control in ATH Grinding Line (reference for ath‑mill.com technical content)
- Filter‑cake raw‑material drying: Use low‑temperature flash drying, material temperature strictly kept below 110 °C. Remove free moisture without triggering crystal‑water decomposition.
- Closed‑circuit grinding & air classification: Keep the whole system under negative‑pressure dry atmosphere; avoid humid ambient air ingress.
- Post‑modification temperature control: Dry surface‑modification units run below 120 °C. High humidity input will ruin coupling‑agent reaction effect.
- Finished‑product protection: Sealed moisture‑proof packaging; avoid open‑air storage for ultrafine ATH powder, as fine particles easily re‑absorb moisture from air.
Common Misunderstandings
- “Lower moisture is always better”: Pursuing ultra‑low free‑moisture below 0.05 % needs over‑aggressive high‑temperature drying, which carries risk of partial ATH dehydration and crystal‑water loss.
- “Low free‑moisture guarantees good ATH quality”: Always cross‑check LOI (loss‑on‑ignition). Good ATH must satisfy both low free‑moisture and standard 34.5 % crystal‑water content.
ATH powder moisture requirement mainly targets free adsorbed moisture. LSZH cable‑grade ATH demands ≤0.2‑0.3 % free moisture; general‑purpose grades allow up to 0.4‑0.5 %. Crystal water must remain intact at ~34.5 % LOI. Proper drying, low‑temperature grinding‑classification and sealed packaging are essential to meet moisture specifications in industrial ATH production.
