Magnesium hydroxide (Mg(OH)₂) is a highly efficient halogen-free inorganic flame retardant and smoke suppressant widely used in polyvinyl chloride (PVC) composites. Its flame retardant mechanism relies on endothermic thermal decomposition at about 330–350 °C: it releases crystal water to absorb massive heat, lower the surface temperature of PVC substrates, and dilute flammable pyrolysis gases generated from PVC thermal degradation. The decomposition product magnesium oxide (MgO) forms a dense, stable protective char layer on the material surface, isolating oxygen and heat transfer, thereby inhibiting further combustion of PVC.
Compared with traditional halogenated flame retardants, magnesium hydroxide produces zero toxic halide fumes during combustion, featuring low smoke density, low corrosiveness, and environmental friendliness. It also avoids the secondary pollution risks of antimony trioxide synergists. In addition, Mg(OH)₂ boasts stable chemical properties, non-toxicity, low volatility, and excellent weather resistance, matching the long-service-life requirements of PVC products.
PVC itself releases a large volume of black toxic smoke when burning. Adding modified magnesium hydroxide significantly reduces smoke emission and eliminates corrosive hydrogen halide gas. It meets strict fire safety standards for building PVC materials, wire & cable PVC insulation, and automotive PVC parts without adding halogen flame retardants.
Magnesium hydroxide acts as a functional inorganic filler. Partial replacement of calcium carbonate fillers lowers raw material costs. Surface-modified magnesium hydroxide (silane or stearic acid coated) achieves good compatibility with PVC resin and plasticizers, improving tensile strength, impact resistance, and dimensional stability of PVC products, while effectively reducing material shrinkage during processing.
Magnesium hydroxide exhibits weak alkalinity, which can neutralize trace hydrogen chloride released from PVC thermal aging during extrusion or injection molding. It alleviates the self-catalytic degradation of PVC, assists calcium-zinc composite heat stabilizers, and delays yellowing and carbonization of PVC under high-temperature processing.
The compact MgO char layer formed after combustion prevents molten PVC droplets from falling, avoiding secondary fire hazards, which is critical for flame-retardant PVC cables, window profiles, and architectural decorative panels.
