Aluminium Trihydroxide Alumina Trihydrate, ATH

Metal hydroxides provide an important alternative to halogenated flame retardants. Aluminium trihydroxide, sometimes known as alumina trihydrate, is the most widely used of all FRs in plastics. Magnesium hydroxide is also finding increasing acceptance, and calcium hydroxide is being marketed as an additive for different reasons. 

Aluminium trihydroxide decomposes on heating at about 190 °C to form alumina and water, according to the eqnation  

The most important mechanism by which ATH moderates combnstion is the removal of heat from the system the decomposition reaction is endothermic to the extent of 1 kJ/g. It also prodnces water vaponr which, being nonvolatile, dilutes any flammable gases prodnced by the degradation of the polymer. A third contributory mechanism is the formation of alumina, which adsorbs some of the volatile combustible gases, reducing smoke. Char formation can also contribute to flame retardancy and smoke suppression. 

ATH is only effective as a flame retardant and smoke suppressant when used in large doses (typically 50 parts per hundred, but sometimes over 100 parts by weight at least one manufacturer makes grades that can be used at 400 parts per hundred). It has been used 

ATH works well with polar polymers but does not adhere easily to PP, so it needs surface modification with silanes or titanates. Another approach is to functionalise the PP by grafting an acrylic monomer onto it. Market acceptability is restricted for three main reasons. First, many polymers cannot be protected by ATH because they have processing temperatures near to or above its decomposition temperature. Special high temperature grades of ATH are now marketed that have lost some of their water of hydration, although this reduces the effectiveness as a FR to some extent. 

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