While MDH and ATH are different flame retardants, the idea behind metal hydroxide's flame resistance is essentially the same and heavily depends on the crystal water content of the formulation at above 30%. The release of the crystal water will cool the combustion by absorbing heat and diluting the combustible gas. Second, the metal oxide that is created will combine to form a heat-insulating layer that will cover the area where the fire is to separate the oxygen and stop the flame from spreading, similar to how phosphorus works as a flame retardant.
MDH and ATH will both produce metal oxide after combustion, but MDH performs better than ATH at neutralizing the acidic gases produced during the combustion of plastics, such as SO2, NO2, and CO2.
Heat will be absorbed by MDH decomposition at a rate of 1.316 J/g, 17% more than ATH at 1.051 J/g.
Flame Retardant | Reaction Formulation | Decomposition Heat |
ATH(Aluminium Hydroxide) | 2AI(OH) 3→AI2O3 + 3H2O | 1.051 J/g |
MDH(Magnesium Hydroxide) | Mg (OH)2→MgO + H2O | 1.316 J/g |
MDH has greater ability than ATH to promote char during burning, and higher char will result in shorter smoke and lower smoke density.
The MDH and ATH smoke densities are different, as seen in the table below.
Smoke Density | ATH | MDH |
Flame | 150-180 | 20-60 |
Non-Flame | 260-320 | 40-80 |
MDH offers a far wider range of uses than ATH since its decomposition temperature is 330°C, which is over 130°C higher than ATH's decomposition temperature of 200°C. Additionally, a greater process temperature might speed up the extrusion or molding process.
Although MDH performs slightly better than ATH, ATH has several uses where these advantages are beneficial. The market for ATH is larger than MDH, similar to Low Smoke Halogen Free Cable.
ATH benefits:
easier distribution, improved compatibility with plastics materials;
improved mechanical properties;
greater plasticity and surface polish due to improved processability;
higher color requirements-satisfying tinting strength;
Applications requiring over 99% high purity grade MDH and ATH, which are all produced by chemical reactions, include wire or cable, copper-clad plate, etc. However, in some coarse applications, such as rubber belt, board, insulation material, etc., 90% purity is sufficient. Because the MDH ore grade in China is low, milled MDH performs slightly worse than milled ATH, which requires a higher dosage.
