Flame retardant is a special additive designed to reduce the flammability of plastics, textiles, wood, etc. This review will consider flame retardants used to modify polymer materials. Its main task is to prevent the spread of fire or slow it down so that there is the possibility of evacuating people in case of fire.
Types of flame retardants
Flame retardants for polymers can be subdivided according to the mechanism of action and the nature of the substance. As a rule, they are closely related: the mechanism is a consequence of the nature of the chemical.
Endothermic (heat absorbing) - mineral flame retardants. Basically, this class includes minerals of natural and artificial origin, such as magnesium hydroxide. For effective performance, the dosage of mineral flame retardants exceeds 50-60%.
Halogen-containing organic compounds. Chlorine- and bromine-containing substances fall into this category. Fluorine-containing additives help combat dripping but cannot work as a basic flame retardant because they are too stable and reluctant to enter into radical absorption reactions. Iodine-containing compounds are often not stable enough concerning heat and light and are too expensive, so they are not used in this capacity.
Nitrogen and phosphorus compounds. This class includes substances whose decomposition produces phosphoric acids, which help the formation of char in the presence of the necessary functional groups in the polymer.
Synergists. For almost every kind of flame retardant, there are substances that enhance its effect. Such additives are called synergists. Antimony oxide and zinc borate are the most common. Synergists for halogen-containing flame retardants.
How flame retardants work
However, flame retardants of different nature work on different principles. The basic principles of action of the listed flame retardants are discussed below.
For combustion to occur, a contact between combustible substance, an oxidant and an energy source has to happen. A stable flame occurs when the temperature is high, and there is enough oxygen. So to stop the material from burning, the temperature has to be reduced and oxygen has to be cut off. That's precisely the job that flame retardants do. Depending on the chemical composition, they inhibit different processes that accompany combustion in the solid or gaseous phase. Typically, simultaneous action in both phases is necessary to suppress combustion.Processes
Mineral flame retardants decompose when heated, releasing water vapor or carbon dioxide, which reduce oxygen concentration when entering the gas phase. At the same time, the solid residue forms a layer on the surface of the burning polymer - char, which acts as a barrier, preventing air and heat from reaching the unaffected polymer. Finally, as they decompose, they absorb heat, which lowers the temperature of the combustion zone and slows down the process.
Halogenated flame retardants act in the gas phase: they "catch" radicals formed during polymer chain destruction, preventing further chain reaction with the polymer and removing them from the reaction zone - inhibiting the radical chain destruction reaction. Therefore, they are used in polymers prone to char formation or together with synergists that act in the solid phase or strengthen their action in the gas phase.
Phosphorus-containing flame retardants release phosphoric acids during decomposition, which attack oxygen in the polymer chain, forming char. Such flame retardants are ineffective in the absence of oxygen atoms. Therefore, they are most often used as a component of "intumescent" systems: ones responsible for acid generation.
Application of flame retardants
Flame retardants are necessary for working with different materials: for fire protection of cable lines, wood, metal structures, fabrics, and polymeric materials. Wood and plastic remain among the most flammable materials, so building and other standards for their use are tightening from year to year.
Flame retardants for polymers
Plastics remain one of the most flammable materials: most of them ignite easily and spread flame quickly. The fire hazard of polymers is determined by the rate of heat and smoke release and the ability to retain properties when exposed to flame.
Polymers burn because of their chemical composition: hydrocarbon chains are an excellent fuel. When heated, macromolecules are degraded with the formation of unsaturated bonds, which are most susceptible to oxidation. This process has an exothermic effect and when occurs quickly is called combustion. The task of flame retardants is to reduce the ability of polymeric materials to ignite and spread the fire. For this purpose, they are introduced at the production stage of the polymer compound, which is the raw material for the manufacture of the final product: cable, siding, the housing of the electrical installation product, etc. There are also compositions for the treatment of the plastic surface.
Mineral flame retardants are added to the polymer in large quantities – more than 50% by mass. This amount of filler embrittles the polymer, so elastic grades, copolymers of ethylene-vinyl acetate, ethylene-hexene, ethylene-octene, and ethylene-propylene, are used as a base. They can retain elasticity at a fill rate of 70-80%.
Halogenated flame retardants are always combined with synergists. Otherwise, their dosage would be too high. Depending on the polymer matrix, they are introduced from 15 to 30%, including 3-5% synergists - antimony oxide and zinc borate. It is also possible to combine them with mineral flame retardants to reduce the content of halogens, which are considered toxic and can be harmful to human health.
Phosphorus-containing flame retardants are used in polymers containing oxygen functionality or as part of "intumescent" systems. Ammonium polyphosphate-based systems are the most common. Their dosage is relatively small - from 10 to 25%. Therefore it is possible to preserve the mechanical properties of the polymer.
Characteristics of flame retardants. What to pay attention to when choosing.
The choice of flame retardant is a responsible decision that has a direct impact on the safety of human life. It should be approached with a complete understanding of the goals.
When choosing a flame retardant, you should pay attention to:
- Polymer matrix;
Processing temperature and operating temperature;
Resistance to water or other media;
Requirements for the degree of fire resistance and toxicity of combustion products;
Requirements for physical and mechanical properties of the product.