|Chemical name||Aluminium monohydrate (Boehmite)|
|H phrases according to GHS Labelling||none|
Suppliers / trade names (pinfa member companies)
|Nabaltec||Actilox u.Apyral AOH|
|Solid Thermoplastics||Polyethylene (PE)||Applicable|
|Polycarbonate (PC)||Could be applied|
|Foams||PUR flexible foam||Applicable|
|PUR Rigid foam||Applicable|
|Polyethylene foam (PE)||Applicable|
|Plypropylene foam (PP)||Applicable|
|Wire and cables||PE/EVA||Applicable|
Aluminium monohydrate (AOH) – also called Boehmite – is hydrated aluminium. It is a fine, white, highly pure, inorganic crystalline powder. Compared to Aluminium Tri Hydrate (ATH), which starts to decompose at 200°C, the decomposition of Boehmite starts only from 340 °C upward. With 17 wt.-% AOH also has a significantly lower loss on ignition than ATH (34.6 wt.-%).
Likewise ATH, the working principle as a flame retardant is based on the thermal decomposition of Boehmite into aluminium oxide and water. The generated water vapour cools the polymer surface, dilutes the concentration of burnable gases in the surroundings and the oxide layer acts as a barrier protecting the polymer against further decomposition. The remaining aluminium oxide residue has a high internal surface where sooty particles, respectively polycyclic aromatic hydrocarbons, are absorbed, making AOH also a smoke suppressant. But due to the lower amount of crystal bound water the endothermic reaction in case AOH consumes less energy from the ignition source compared to ATH. In summary, AOH has a much wider processing window caused by its high decomposition temperature, but has a lower flame retardant efficiency per mass. Hence, AOH is rarely used as a sole flame retardant (FR). But combined with other FR, e.g. phosphorous and nitrogen based FR, remarkable synergistic effects have been reported in polyamide and polyesters.
AOH is commercially available in grain sizes ranging from 0.3 – 3 µm in median particle size (D50). AOH has an extremely low electrolyte content. This low electrolyte content is also an important influencing factor regarding resin viscosity and resins hardening and makes AOH ideal for utilization in electrically insulating plastic products, e.g. PCB laminates or cast resins.
Beside this, AOH acts as an acid scavenger. It can capture aggressive by-products formed during processing of phosphorous containing FR, to avoid corrosion of e.g. compounding and injection moulded screws.
By a comprehensive set of toxicological data performed under the REACH regulation (ECHA, EU) Boehmite has been proven as a non-hazardous and non-classified substance. Due to this favourable environmental and health profile Boehmite is a very good alternative for a wide range of application.
1. Alexander B. et al. “Non-halogenated Flame Retardant Handbook”, Scrivener Publishing LLC., USA
(2014), ISBN 978-1-118-68624-9.
2. PINFA Brochure: Innovative Flame Retardants in E&E Applications,
“Non-halogenated phosphorus, inorganic and nitrogen flame retardants” (2010, 3nd edition).
3. www-entry https://echa.europa.eu
4. Nabaltec AG, Brochure: “Mineral based flame retardancy with metal hydrates” (03/2019).
5. Nabaltec AG, Brochure: “Metal hydrates for thermosets” (03/2019).
6. Wikipedia entry https://en.wikipedia.org/wiki/Flame_retardant (6-26-2019).
7. Wikipedia entry https://en.wikipedia.org/wiki/Aluminium_hydroxide (6-26-2019).