ATH Aluminum Trihydrate and MDH Magnesium Hydroxide

A typical example of active flame retardants is represented by ATH, that is, aluminum trihydrate. When ATH is heated with a heating rate of 10°C/min, it starts releasing water at about 225°C (437°F). At 300°C, ATH releases 12% water by weight. At 334°C (633°F), the rate of water reaches its maximum, with about 28% of water release, and the process slows down, reaching 35% of water release at 900°C. Water release reduces the heat from the oxidized plastic and quenches flames. 

Generally, ATH is considered to be thermally stable at around 180-200°C (360-400°F), and—with some reservations—until 216°C (420°F). Some data indicate the decomposition of ATH in the temperature range of 180-240°C (360-460°F). ATH is usually cream-colored, free-flowing powder with specific gravity of 2.42 g/cm loose bulk density of 70 Ib/ft (1.12 g/cm ), and the so-called packed bulk density of 55 Ib/ft (1.36 g/cm ). A typical commercial grade of ATH (for example, product of Alcan Chemicals, Cleveland, OH), contains up to 15% of powder with mesh size of 100 and larger particles, 67-87% with mesh size 200-200, and between 1 and 12% of mesh size 325. 

This is also an endothermic decomposition, with AH = 380 kJ/mol (90.8 kcal/mol). 

Obviously, ATH cannot be used in thermoplastics other than polyethylene and PVC, and in the respective plastic-based composite materials. A common disadvantage of ATH and MDH (magnesium hydroxide) is that in order to provide a sufficient level of flame retardancy, they have to be used in large amounts, such as 50-65% and not below 40%. 

As it could have been expected, the decomposition of ATH to AI2O3 during the heating of the polymer resulted in an increase of the ignition time [12]. 

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