Aluminium-hydroxides are used as non halogen flame retardant fillers in plastics and rubber in order to retard the expansion of fire in environmentally friendly way.
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The flame retardant effect of ATH is based on the endothermic decomposition into aluminium oxide and water: 2Al(OH)3→Al2O3 + 3H2O
ATH decomposes at about 220 oC.
While burning the polymer is protected against the quick decomposition i.e. burning is retarded.
Aluminium hydroxides are used for the production of low-smoke and halogen free flame retarders.
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Advantages of aluminium hydroxides:
Dry hydrates are used as raw material for the production of below products: synthetic zeolites, aluminum compounds, glass, ceramics, refractory materials, carpet backings, artificial marble, roofing
Ground and viscosity optimized hydrates are mainly used as environmentally friendly filler for the production of the following products: artificial marble, roofing, carpet backing, coatings of PUR foam, polyester resins, cross linked elastomers, PVC, dispersions, polyurethans, adhesives
The raw material (aluminium-hydroxide, precalcined alumina) is calcined in rotary kilns at a temperature corresponding to the degree of calcination of the desired final product.
Above 900 oC the formation of α-Al2O3 (corundum structure) begins.
Above 1300 oC the so-called formation and growth of primary crystals starts.
On the basis of Na2O content we can distinguish between normal, medium and low-sodium alumina.
According to the degree of calcination, we can produce weak, medium or heavily calcined alumina.
The particle size can vary from coarse, medium, fine or reactive.
Important characteristics of aluminium-oxides: