Polyethylene (PE) is a thermoplastic resin obtained by polymerization of monomer ethylene. It has good cold resistance, good mechanical strength and dielectric properties. It is widely used in products such as cables, films, pipes, packaging, containers, and medical appliances. However, the PE oxygen index is 17.4%, which is a flammable material. PE materials burn quickly, generate a lot of heat/smoke, and are easy to melt and drip when burning, posing a great threat to the safety of life and property, limiting the use and development of polyethylene. Therefore, it is imperative to modify it to be flame retardant.
Metal hydroxide flame retardants are mainly aluminum hydroxide and magnesium hydroxide. Magnesium-aluminum flame retardants have good stability, are non-toxic, and have low smoke production. During the combustion process, water vapor will be released to dilute the combustible gas, take away some heat, inhibit combustion, and produce a flame retardant effect. Aluminum-magnesium flame retardants can extend the ignition time and reduce the heat release rate. Magnesium hydroxide has poor compatibility with PE and low flame retardant efficiency. It needs to be added in large quantities to improve the flame retardant performance. Large amounts of addition will reduce the processability and mechanical properties of the composite material.
Zhang Hongxia et al. used sodium stearate and polyethylene glycol as modifiers to modify the surface of magnesium hydroxide and prepared high-density polyethylene flame-retardant composite materials. Studies have shown that when the addition amount of modified magnesium hydroxide is 30%, the tensile strength of the HDPE/magnesium hydroxide composite material is 12.3MPa, magnesium hydroxide has good compatibility with HDPE, the limiting oxygen index is increased to 24.6%, and the flame retardant performance is improved less.
Layered double hydroxides release CO2 and H2O when decomposed, dilute and block oxygen, so that it has a good flame retardant effect and can replace halogen and phosphorus-containing flame retardants.
Gao Jie et al. used aluminum hydroxide and homemade magnesium iron double hydroxide (Mg-FeLDH) as flame retardants to prepare aluminum hydroxide/Mg-Fe-LDH/HDPE flame retardant composite materials. Studies have found that aluminum hydroxide and Mg-Fe-LDH can effectively inhibit the release of CO and heat during the combustion of composite materials (HDPE1, HDPE2, HDPE3), making HDPE difficult to ignite. When the total amount of flame retardant is 40% (Mg-Fe-LDH dosage is 2%, HDPE2), HDPE composite material has good flame retardant properties.
Chen Xiaosong et al. prepared HDPE composite material with aluminum hydroxide, expanded vermiculite and antimony trioxide as flame retardants. The study found that when the ratio of aluminum hydroxide/expanded vermiculite is 3:2, the mechanical properties of the composite material are good, and the smoke suppression and flame retardant properties reach FV-0 level. When the total amount of aluminum hydroxide and expanded vermiculite is 50%, the limiting oxygen index increases first and then decreases with the increase of aluminum hydroxide, and reaches the best when the ratio is 3:2.
Zhao Song et al. studied the effects of magnesium hydroxide and zinc borate on the flame retardant properties of linear low-density polyethylene and ethylene ethyl acrylate copolymer. It was found that with the increase of the ratio of magnesium hydroxide and zinc borate, the flame retardant properties of the composite material improved. When the addition amount of magnesium hydroxide was 65%, the flame retardant properties were the best, reaching UL94V-0 level.
Wang Weiling studied the effect of magnesium hydroxide on the flame retardant properties of linear low-density polyethylene. When the dosage of magnesium hydroxide reaches 70%, the limiting oxygen index reaches 31.4%, which is about 71% higher than that of pure material, and the vertical combustion test reaches V-0 level.
Metal hydroxide flame retardants are safe, environmentally friendly and inexpensive. When used alone, the flame retardant effect is not good, and a large amount of addition is required to improve the flame retardant properties of the material, but a large amount of addition will reduce the mechanical properties. Therefore, the research direction of hydroxide flame retardants is to study surface modification and use it in combination with nitrogen-phosphorus flame retardants to improve flame retardant properties and reduce the amount of addition.