Thermoplastic polyurethane elastomer (TPU) is a linear block copolymer composed of oligomer glycol soft segments and diisocyanate chain extender hard segments. TPU can be plasticized after heating, with excellent processing performance; At the same time, TPU has excellent strength, elasticity, and wear resistance, and has been widely used in industries such as automobiles, coatings, national defense, building materials, clothing, wires and cables. The limiting oxygen index (LOI) of ordinary TPU is only about 19%, which affects its application in areas with high fire safety requirements such as automobiles and cables. Flame retardant modification of TPU is very important.

1● Phosphorus based flame retardant

Phosphorus based flame retardants decompose into phosphoric acid or its anhydride at high temperatures, which can promote the formation of carbon. The formed carbon layer can provide insulation and oxygen insulation, making it a solid phase flame retardant; Phosphorus based flame retardants generate PO · free radicals during combustion to capture combustion free radicals and interrupt combustion chain reactions to improve the flame retardancy of materials, belonging to gas-phase flame retardancy. Commonly used phosphorus based flame retardants include red phosphorus, ammonium polyphosphate (APP), phosphate esters, phosphates, and phosphorus nitrogen compounds. Phosphorus based flame retardants have low toxicity but high prices.

2 ● Nitrogen based flame retardant

Nitrogen based flame retardant materials absorb a large amount of heat and decompose into non combustible gases such as NH3 and N2 during combustion, diluting flammable gases and belonging to gas-phase flame retardancy. Currently, the commonly used nitrogen based flame retardants are melamine and its derivatives.

Non toxic and pollution-free, but the flame retardant efficiency is not high when used as a single component, and it is often used in combination with other flame retardants.

3 ● Silicone based flame retardant

Silicon based flame retardants are mainly divided into two categories: organic silicon and inorganic silicon. The commonly used inorganic silicon flame retardants include SiO2, silica gel, and layered silicates. Their flame retardant mechanism is that inorganic silicon compounds act as barriers on the surface of the material, isolating oxygen and improving the carbonization rate; Organosilicon flame retardants include silicone resin and silicone rubber, which have good anti droplet effects and can decompose to form a dense silicon carbon layer. Silicon based flame retardants often synergistically flame retardant with other flame retardants.

4 ● Nano inorganic flame retardant

There are various types of nano inorganic flame retardants, mainly including some metal oxides, metal hydroxides, and nano layered silicates such as montmorillonite, hydrotalcite, and attapulgite. Their biggest characteristics are green, pollution-free, and low toxicity. By introducing inorganic flame retardants in the form of nanoscale particles, layers, or tubes, the thermal stability and mechanical properties of materials can be improved.

Nanoscale inorganic flame retardants can serve as barrier layers to effectively isolate oxygen and heat, belonging to the category of condensed phase flame retardancy. However, inorganic flame retardants have low flame retardancy efficiency, large addition amounts, and are prone to damage the mechanical properties of materials. Therefore, they are often used as synergistic flame retardants in combination with other flame retardants.

5 ● Intumescent flame retardant

Intumescent flame retardant (IFR) is generally a composite flame retardant composed of carbon source, acid source, and gas source. The carbon source can form a carbon layer during heating, mainly consisting of high carbon content polyhydroxy compounds, such as pentaerythritol (PER); The acid source generates inorganic acids during the material combustion process to dehydrate carbon containing polyols, usually inorganic acids or compounds that can generate acids in situ during combustion, such as ammonium polyphosphate; The gas source can generate a large amount of incombustible gas during material combustion, which dilutes the oxygen concentration, mostly nitrogen containing compounds such as melamine. The commonly used IFR is generally composed of APP, PER, melamine, etc. IFR has the advantages of high flame retardant efficiency, low toxicity, and smoke suppression, and is currently widely concerned.

Intumescent flame retardants have high flame retardancy efficiency, and bio based intumescent flame retardants are harmless to the environment and human body.