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Application and research progress of magnesium hydroxide flame retardant in polyethylene

Polyethylene (PE) is a thermoplastic resin obtained by polymerization of monomer ethylene, with good cold resistance, good mechanical strength, dielectric properties, widely used in cables, films, pipes, packaging, containers, medical appliances and other products. However, PE has an oxygen index of 17.4%, which is a flammable material. PE materials burn quickly, generate heat/smoke, and melt easily when burning, which poses a great threat to the safety of life and property, limiting the use and development of PE, so it is imperative to modify its flame retardant.

Metal hydroxide flame retardants are mainly aluminum hydroxide and magnesium hydroxide. Magnesium and aluminum flame retardants are stable, non-toxic, small smoke generation, and will release water vapor during combustion to dilute combustible gases, take away some of the heat, inhibit combustion, and produce flame retardant effect. Aluminum and magnesium flame retardants can extend the ignition time and reduce the rate of heat release, magnesium hydroxide and PE compatibility is poor and low flame retardant efficiency, need to add a lot to improve the flame retardant properties, a large number of additions will reduce the processability and mechanical properties of composite materials.

Sodium stearate and polyethylene glycol were used as modifiers to modify the surface of magnesium hydroxide and to prepare high density polyethylene flame retardant composites. It was shown that when the modified magnesium hydroxide was added at 30%, the tensile strength of HDPE/magnesium hydroxide composites was 12.3 MPa, the magnesium hydroxide had good compatibility with HDPE, the ultimate oxygen index was increased to 24.6%, and the flame retardant properties were improved less.

The laminated double hydroxide will release CO2 and H2O when decomposing, diluting and blocking oxygen, making it a good flame retardant, which can replace halogen and phosphorus-containing flame retardants.

Aluminum hydroxide/Mg-Fe-LDH/HDPE flame retardant composites were prepared by using aluminum hydroxide and homemade magnesium-iron double hydroxide (Mg-FeLDH) as flame retardants. It was found that aluminum hydroxide and Mg-Fe-LDH could effectively inhibit CO release and heat release during combustion of the composites (HDPE1, HDPE2, HDPE3), making it difficult for HDPE to ignite. When the total amount of flame retardant is 40% (Mg-Fe-LDH dosage is 2%, HDPE2), HDPE composites have good flame retardant performance.

Aluminum hydroxide, expanded vermiculite and antimony trioxide were used as flame retardants to prepare HDPE composites. It was found that when the ratio of aluminum hydroxide/expanded vermiculite was 3:2, the mechanical properties of the composites were better and the smoke suppression and flame retardant properties reached FV-0 level. When the total amount of aluminum hydroxide and expanded vermiculite was 50%, the limiting oxygen index increased and then decreased with the increase of aluminum hydroxide, and reached the best at the ratio of 3:2.

The effects of magnesium hydroxide and zinc borate on the flame retardant properties of linear low density polyethylene and ethylene acrylate copolymers were investigated. It was found that the flame retardant performance of the composites improved as the ratio of magnesium hydroxide and zinc borate increased, and the best flame retardant performance was achieved when magnesium hydroxide was added at 65%, reaching UL94V-0 level.

The effect of magnesium hydroxide on the flame retardant properties of linear low density polyethylene was studied. The ultimate oxygen index reached 31.4% when the amount of magnesium hydroxide reached 70%, which was about 71% higher than that of pure material, and the vertical


burning test reached V-0 grade.

Metal hydroxide flame retardants are safe, environmentally friendly and inexpensive. When used alone, flame retardant effect is not good, need to add a large amount to improve the flame retardant properties of the material, but a large number of additions will reduce the mechanical properties. Therefore, the study of surface modification and compounding with nitrogen and phosphorus flame retardants to improve the flame retardant properties and reduce the amount added is the direction of research on hydroxide flame retardants.

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