Introduction of flame retardant mechanism of several flame retardants and Prospect of industry
- time:
- 2019-05-11
In the 1950s, with the explosive development of material science and technology, organic polymer materials began to be widely used in various fields of production and life. Today, plastics, rubber, textiles and coatings, which are the main components of macromolecule materials, have become indispensable production elements. Electrical appliances, tableware, transportation and other articles made of macromolecule materials bring great convenience to our lives, but also bring many potential safety hazards.
Because of the low oxygen index of polymer materials, air containing 21% oxygen is enough to meet the combustion conditions of most polymer materials. In addition, the release of smoke and toxic gases during the combustion of polymer materials will not only endanger human health, but also cause environmental pollution. In order to reduce the flammability of polymer materials and prevent fire, flame retardants emerge as the times require. Flame retardant is an assistant that can be used to prevent the flammability of materials, that is, to prevent materials from being ignited and to inhibit flame propagation.
There are many kinds of flame retardants, which can be divided into organic flame retardants and inorganic flame retardants according to their composition. Organic flame retardants include halogen flame retardants, phosphorus-nitrogen flame retardants, organophosphorus flame retardants, organosilicon flame retardants and so on. In this paper, several types of flame retardant mechanism are introduced.
Halogen-based flame retardants are an important member of organic flame retardants. The main component of halogen-based flame retardants is halogen-based organic compounds. Free radicals produced by polymer degradation at high temperature are the factors to maintain the combustion of organic compounds. Hydrogen halide (HX) gas produced by halogenated flame retardants during thermal decomposition can capture free radicals, thus preventing flame diffusion. In addition, hydrogen halide itself can not burn and its density is higher than that of air. A gas barrier is formed on the surface of polymer materials to prevent the contact between polymer materials and oxygen.
Phosphorus-nitrogen flame retardant is mainly composed of phosphorus and nitrogen. These flame retardants are also called intumescent flame retardants because the surface of these flame retardants can produce a uniform layer of carbon foam when heated, which can isolate heat and oxygen. Intumescent flame retardant system generally consists of three parts: acid source (dehydrant), carbon source (charring agent) and gas source (nitrogen source, foaming source). After heating, the inorganic acids which can esterify polyols and can be used as dehydrating agents are released from the acid source. As the temperature increases further, the inorganic acids react with polyols (carbon sources) to form esters, which are in a viscous melting state. The water vapor generated in the reaction process and the non combustible gas generated by the gas source make the system in the molten state expand and foam. At the same time, polyols and ester dehydration carbonization, forming inorganic and carbon residues. With the increase of temperature and reaction, the system is gelled and solidified, and finally the porous carbon foam layer is formed.
Inorganic flame retardants mainly include expansible graphite, hydroxide, red phosphorus or polyphosphate, etc. Generally, they are mixed with several different inorganic substances. Expanded graphite expands rapidly at high temperatures, forming a tough carbon layer on the surface of the polymer, which can separate the combustible from the heat source; hydroxides absorb a lot of heat and release steam, dilute the combustible gas, and produce refractory metal oxides; red phosphorus or polyphosphate combustion forms polymetaphosphate glass, which can cover the surface of the combustor to form a protective film, and form at the same time. The phosphoric acid formed has strong dehydration property and can carbonize the polymer to form a carbon isolation layer.
In summary, it can be found that the functions of flame retardants can be summarized as follows: (1) absorbing the heat released by combustion, (2) releasing non-combustible gases, diluting and isolating combustible and combustion-supporting gases, (3) producing the components reacting with organic free radicals, (4) forming solid isolation layer.
Impact on human body and environment
The application of flame retardants is regarded as an effective way to prevent fire, but the harm of flame retardants to the environment and human beings can not be ignored. Articles containing certain flame retardants will gradually release toxic chemicals into water and air. When these toxic chemicals are absorbed by human body, they can accumulate in human body for a long time and disturb human nervous, endocrine and immune systems. It is especially noteworthy that if pregnant women are overexposed to these toxic chemicals, they will lead to low birth weight, mental retardation and behavioral impairment of newborns. In addition, when some flame retardants are burned, a lot of smoke and poisonous, harmful and corrosive gases will be produced, resulting in "secondary disasters".
Prospect
The harmfulness of flame retardants to the environment has led to the introduction of relevant laws and regulations to restrict the use of flame retardants in various countries. At present, China has promulgated the "Plastic Furniture Hazardous Substances Limit Standard" (GB28481-2012), which stipulates that the use of PBBs and PBDEs should not exceed 1 gram per kilogram.
In recent years, with people's attention to fire safety and increasing requirements for flame retardant, the flame retardant industry has developed rapidly. Increasing living environment requirements and stricter flame retardant standards and environmental regulations will be great challenges for the flame retardant industry. Therefore, developing and producing environmentally friendly flame retardants and improving the carbonization of flame retardants are new ideas for the development of flame retardants industry while improving their flame retardancy. At the same time, promoting the legalization and standardization of the production and use of flame retardants is also a necessary way to standardize the flame retardant industry.
