Halogen-free flame retardants are widely used in the modification of polypropylene, mainly achieved through chemical modification and physical filling. The following is a detailed analysis of their modification methods, principles and effects:
Modification method
Chemical modification: Introducing elements or groups with flame-retardant functions into the molecular chain structure to increase the carbon-hydrogen ratio, introduce aromatic or conjugated structures, enter rigid structures or polar groups, and enhance the degree of crosslinking, etc.
Physical filling: Halogen-free flame retardants are added to the polypropylene matrix through melt blending. Mainly, flame retardants are compounded in a certain proportion, and extrusion granulation is carried out according to different application scenarios and mechanical and other performance requirements to prepare composite materials, which are suitable for the needs of large-scale industrial production.
Common types of halogen-free flame retardants and their flame retardant mechanisms
Phosphorus-based flame retardants
Types: including inorganic phosphorus-based flame retardants (such as red phosphorus, ammonium polyphosphate, phosphate, etc.) and organic phosphorus-based flame retardants (such as phosphate esters, phosphonate esters, etc.).
Flame retardant mechanism: When phosphorus-based flame retardants are heated, they decompose into phosphorus-containing oxyacids. These phosphorus-containing oxyacids can promote the dehydration and carbonization of polymers to form a graphite-like coke layer, which can prevent the internal polymer from coming into contact with oxygen, slow down thermal decomposition, and thus play a flame-retardant role. For polypropylene, the flame-retardant effect is not good when phosphorus-based flame retardants are used alone. Usually, they are compounded with substances such as aluminum hydroxide and magnesium hydroxide to produce a synergistic effect.
Nitrogen-based flame retardants
Types: Mainly include triazine compounds (such as melamine), dicyandiamide, guanidine salts and their derivatives, especially phosphate derivatives.
Flame retardant mechanism: When nitrogen-based flame retardants are heated, they decompose to produce non-combustible gases such as NO, NO2, NH3, etc. These gases have the functions of cooling, absorbing heat and diluting oxygen, thereby preventing the material from further burning and spreading flames.
Silicon-based flame retardant
Type: It is divided into two major categories: organosilicon series and inorganic silicon series. The organosilicon series mainly consists of polysiloxanes, including silicone oil, silicone rubber, various siloxane copolymers and silicone resins, etc. The inorganic silicon-based series mainly include silicates (such as montmorillonite), silica gel, talcum powder, etc.
Flame retardant mechanism: When organosilicon-based flame retardants burn, they form a dense and stable silicon-containing carbon layer, which prevents the escape of flammable substances decomposed during combustion and also serves to insulate heat and oxygen. The flame-retardant effect of inorganic silicon-based flame retardants belongs to the condensed phase flame-retardant mechanism. It is generally believed that the flame-retardant purpose is achieved through the barrier and shielding effect of the amorphous silicon or silicide protective layer formed during combustion.
Aluminum-magnesium flame retardants
Type: Mainly aluminium hydroxide and magnesium hydroxide.
Flame retardant mechanism: The thermal decomposition process of aluminium hydroxide is an endothermic reaction, which can carry away a large amount of heat generated by combustion and reduce the temperature at the combustion interface. Meanwhile, one of the decomposition products is water vapor, which can play a role in cooling and diluting the concentration of oxygen and flammable gases. Another decomposition product, alumina, is a dense inorganic oxide powder that can cover the surface of polypropylene flame-retardant materials, forming a protective carbon layer with heat insulation and oxygen isolation functions. The flame retardant mechanism of magnesium hydroxide is similar to that of aluminium hydroxide, but it has a higher decomposition temperature and relatively better thermal stability.
Intumescent flame retardant (IFR) :
Type: It is a flame-retardant system that forms a porous and expanded carbon layer during the heating process, thereby exerting flame-retardant effects through heat insulation, oxygen isolation, and prevention of the volatilization of cracking products.
Flame retardant mechanism: When polypropylene containing IFR is burned and thermally cracked, it exerts its flame retardant effect through the carbonization mechanism that occurs in the condensed phase. The oxygen index of some polymers has a good correlation with the amount of carbon formed during combustion. In recent years, a series of phosphorus-nitrogen mixed IFRs suitable for polypropylene have been developed. When each component in the mixed IFRs is used alone, the flame retardant effect on polypropylene is not good. However, when they are used together, the flame retardancy to polypropylene is significantly improved due to the increase in carbon formation rate.
Modification effect
Environmental friendliness: Halogen-free flame retardants do not contain halogens. When burning, they do not release toxic halides and dioxins, making them safer for the environment and human health.
Flame retardant performance: It can achieve a relatively high flame retardant grade (such as UL94 V-0), and can quickly extinguish after being removed from the fire source, reducing the risk of fire.
Mechanical properties: It retains the high strength, toughness and impact resistance of polypropylene, and is suitable for applications that require high strength and durability. Meanwhile, by adding compatibilizers and other methods, the compatibility and adhesion between flame retardants and polypropylene can also be enhanced, which is conducive to improving the mechanical properties of flame-retardant materials.
Processing performance: Halogen-free flame-retardant polypropylene can be processed into products of various shapes and sizes through multiple techniques such as injection molding, extrusion, and blow molding.
