A mechanism for reducing the specific surface area of polycrystalline magnesium hydroxide

Magnesium hydroxide powder is used as a flame retardant in resin cable and wire sheathing. To be considered effective as a flame retardant, magnesium hydroxide needs to consist of primary particles having a diameter of 0.5-1.5 μm and has to be less cohesive with a small specific surface area of 5 m2·g-1. However, magnesium hydroxide obtained from seawater and slaked lime slurry consists of polycrystalline secondary particles with an average diameter of about 3 μm that result form scale-shaped primary particles and have a high specific surface area of over 10 m2·g-1, making magnesium hydroxide highly hygroscopic and thus unsuitable for application as a flame-retardant. In this study, a mechanism for reducing the specific surface area of polycrystalline magnesium hydroxide powder was investigated to reduce its hygroscopicity. The Brunauer-Emmett-Teller (BET) specific surface area was found to change depending on the drying rate and could be reduced upon dehydration at a low drying rate. A mechanism for reducing the BET specific surface area of magnesium hydroxide was identified: when magnesium hydroxide was dehydrated at a low drying rate, the decreasing drying period increased and the supersaturation of magnesium hydroxide by the moisture remaining in the pores was prolonged. Consequently, the primary particles of magnesium hydroxide grew larger, and the number of 3-10 nm sized pores, particularly 3-5 nm pores, in the polycrystalline secondary particles significantly decreased, thereby reducing the BET specific surface area.