Halogen-free flame retardants are widely used in polystyrene (PS), mainly through the addition of flame retardants or chemical modification. The following is an introduction from three aspects: common types, mechanism of action, and application effects:
Common types
Phosphorus-based flame retardants: They can be classified into organic phosphorus-based flame retardants and inorganic phosphorus-based flame retardants. The phosphorus-based flame retardants mainly used in PS are inorganic phosphorus-based flame retardants such as red phosphorus and phosphate esters. Red phosphorus is a pure flame-retardant element with good flame-retardant effect, but it has bright color, high water absorption and poor interfacial adhesion with PS resin, which limits its application to some extent. Microcapsule red phosphorus (MRP) coated with a protective film on the surface has the advantages of weather resistance, thermal stability and good interfacial adhesion with polymer substrates, and is widely used. In addition, phosphoronitrile polymer flame retardants also have excellent flame-retardant effects.
Nitrogen-based flame retardants: represented by triazine compounds, they mainly exert flame-retardant effects by absorbing heat through thermal decomposition and generating non-flammable gases that can dilute combustibles. They feature halogen-free, low smoke, and low toxicity. Compared with inorganic filled flame retardants, nitrogen-containing flame retardants require less addition and do not cause severe deterioration of the physical and mechanical properties of materials. However, their flame retardant efficiency is poor, and they have poor compatibility with polymers, which is not conducive to dispersion in the flame-retardant polymers and can easily lead to an increase in the viscosity of the flame-retardant polymers, etc., thus restricting their wide application. When expandable graphite is compounded with nitrogen-containing flame retardants, it shows a good synergistic flame retardant effect. For example, expandable graphite and melamine cyanurate show a good synergistic flame retardant effect when flame-retardant styrene-based polymers, and the flame retardant materials also have good mechanical properties.
Silicon-based flame retardants: They can be classified into inorganic silicon-based flame retardants and organosilicon-based flame retardants. These flame retardants are not only environmentally friendly but also can improve other properties of the substrate while endowing it with excellent flame retardant performance. They have developed rapidly in recent years.
Metal hydroxide flame retardants: Aluminum hydroxide (ATH) and magnesium hydroxide (MH) are the main metal hydroxide flame retardants, which have the advantages of being non-toxic, non-corrosive, having good stability, not volatile, and not generating toxic gases at high temperatures. They are flame retardants that integrate the three functions of flame retardancy, smoke suppression, and filling. However, ATH and MH usually require a relatively large addition amount to achieve an appropriate flame retardant effect, which will lead to a decline in the mechanical properties of the material and a deterioration in its processing performance. Therefore, before use, they need to undergo appropriate surface treatment and be used in combination with other flame retardants to reduce their filling amount.
Mechanism of action
Phosphorus-based flame retardants: During combustion, they decompose to form phosphoric acid or polyphosphoric acid, which then further forms a highly viscous molten glassy substance or a dense carbon layer, isolating the polymer matrix from the heat generated by combustion and the oxygen from the outside in a solid form. Capture free radicals, decompose to generate free radicals such as PO or HPO· during combustion, and capture active H· free radicals or OH· free radicals in the gas phase state. Expansion foaming can promote the formation of a fluffy porous carbon layer during the combustion process, protecting the base material.
Nitrogen-based flame retardants: They mainly exert flame-retardant effects by absorbing heat through thermal decomposition and generating non-flammable gases that can dilute combustibles.
Expellable graphite: When heated, it expands to form an insulating expansion layer on the material surface, thereby exerting a flame-retardant effect. It can effectively increase the oxygen index of the material and reduce the heat release rate.
Metal hydroxide flame retardants: They absorb heat and dehydrate at high temperatures, thereby removing the heat generated during combustion. The water vapor produced can then dilute oxygen. The metal oxides generated by dehydration contribute to catalytic carbonization. Meanwhile, ATH and MH form an active metal oxide layer with a large surface area that can absorb smoke dust, combustible particles, and even free radicals, thereby endowing the composite material with excellent smoke suppression performance.
Application effect
Improving flame retardancy: The addition of halogen-free flame retardants can significantly enhance the flame retardancy of polystyrene materials and reduce the risk of fire. For example, when PS-HI is flame-retardant with MRP and phenolic epoxy resin (NR), the combined use of the two has a synergistic effect, making the oxygen index of PS-HI reach 28.8% and the UL94 vertical combustion level reach V-0 grade. The green and efficient formaldehyde-free phosphorus-containing polysiloxane coating developed by the team led by Academician Wang Yuzhong from Sichuan University, through the hydrolysis condensation of organosilane and the construction of a flame-retardant and smoke-suppressing coating (DG/EG) with expandable graphite, endows EPS with excellent flame-retardant and smoke-suppressing performance.
Environmental protection and safety: Halogen-free flame retardants do not contain halogen elements and do not produce toxic and harmful hydrogen halide gases when burning, making them safer for the environment and human health.
Improving material properties: Some halogen-free flame retardants can not only endow the substrate with excellent flame retardant properties but also improve other properties of the substrate. For instance, silicon-based flame retardants have developed rapidly in recent years because they can not only endow the substrate with excellent flame retardant performance but also improve other properties of the substrate.
