Phosphorus flame retardant application in polyurethane

Text/Polymer

Polyurethane material (PU) has good mechanical properties, high wear resistance and high tear resistance, and is widely used in real life. The oxygen index of polyurethane is low and its LOI is only 14% to 16%. Flammability limits the use of polyurethane in some applications with high fire resistance.

1. Phosphor flame retardant mechanism

Phosphorus-based fire retardants work together in condensed phase and gas phase during combustion of polyurethane. The flame retardant mechanism in polyurethane can be explained using four elements of combustion: temperature, combustibles, oxygen, and free radical chain reaction. Thermal decomposition of phosphorus compounds can promote production of carbon from polyurethane to increase coke yield; thermal decomposition of phosphorus compounds gives water and dilutes combustible gases; thermal decomposition of phosphorus compounds into polymetaphosphoric acid has a strong dehydrating and carbonizing effect; phosphorus compound thermally decomposes, it releases active substances that prevent combustion of polyurethane.

2. Classification of phosphorus flame retardants

Phosphorus-based flame retardants can be divided into reactive flame retardants and additive flame retardants according to the way of use, they can be divided into inorganic flame retardants and organic flame retardants according to their properties and composition. shown in Table 2-1.

Table 2-1 Classification and characteristics of phosphorus flame retardants

2.1 Application of phosphorus-based flame retardant in polyurethane

Phosphorus-based flame retardant additive uses mechanical equipment such as a high speed mixer to mix flame retardant with material and introduce it into polyurethane matrix in a specific way. The advantage of additive type is that total cost is small, operation method is simple, and in practical applications, operation is faster. The disadvantages are mainly manifested in poor compatibility between flame retardant and matrix, which is prone to problems such as small molecule migration and agglomeration of additives, which affects activity of PU group; it will also cause problem of incompatibility of flame retardant with molecular weight, which will affect activity of material and function.

The addition of flame retardant will reduce strength, modulus of elasticity, compatibility and stability of material. To comprehensively improve compatibility of flame retardants and polyurethanes and to ensure thermal stability of products, it is necessary to modify flame retardant additives. Currently, there are two main methods: microencapsulation and surface modification.

Microencapsulation. Microencapsulation technology is relatively mature as well as a widely used method for modifying flame retardants. Microencapsulation of red phosphorus (Figure 2-1) is most common technology in industrial applications, comprehensively improving compatibility and stability of acidic sources and ensuring activity of substances.

Figure 2-1 Microencapsulated red phosphorus

Surface modification: coating surface of particles through surface adsorption or chemical reaction to make surface of particles organic or change polarity, and further improve flame retardant effect. On one hand, this method can reduce amount of flame retardant added, and on other hand, reduce production costs.

2.2 Application of reactive phosphorus-based flame retardant in polyurethane

The distribution of reactive flame retardants is more uniform and has certain advantages. Through reaction, compatibility of flame retardants and polyurethane can be guaranteed, which solves problem of incompatibility of substances in reaction process and ensures higher efficiency. Longer flame retardant. The disadvantage is that cost is high, manufacturing process is complicated, and market application is not widespread.

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