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With the wide application of polymer materials and consumer electronic products, the incidence of fire is also on the rise, and the harm it brings is alarming.
The flame retardant properties of polymer materials are becoming more and more important, and flame retardants play an indispensable role in it. The development direction of flame retardants in the future will be high efficiency, green and environmental protection.
How do flame retardants work
Flame retardants are used to improve the flammability of molecular materials. By increasing the ignition point of molecular materials or reducing the burning rate of materials, rescue time is increased, lives are saved and losses are reduced.
According to the structure, flame retardants can be divided into macromolecular polymerization type and small fraction flame retardants.
According to the use method, it can be divided into additive flame retardant and reactive flame retardant.
According to whether the material contains halogen, it can be divided into halogen flame retardant and halogen-free flame retardant.
The additive flame retardant is mostly used in thermoplastic polymer materials, and other components in the material does not react, only exists in the polymer materials in a physical way.
Reactive flame retardants are mostly used in thermosetting polymer materials, participate in the chemical reaction of synthetic polymer materials, and become the structural unit of polymer materials.
When the polymer material is ignited by open fire, violent oxidation reaction will occur and a large number of hydroxyl groups will be released.
Because hydroxyl groups are very reactive, they combine easily with other substances to form water and other organic compounds.
Other organic matter is further combined with oxygen and decomposes to form new organic matter.
In this series of cyclic reactions, the combustion continues.
How do flame retardants play a role in this process?
According to the three elements of combustion -- combustibles, combustibles and ignition sources, the principle of flame retardant can be summarized as:
1. By absorbing heat to achieve flame retardant, such as the inorganic flame retardant aluminum hydroxide decomposition at high temperature, the release of crystal water, crystal water evaporates into water vapor.
This process absorbs a lot of heat, lowering the surface temperature of the material and making it flame retardant.
2. By producing non-combustible gases, such as aluminum hydroxide flame retardants, to dilute oxygen and slow the burning rate, the resulting water vapor can reduce the concentration of oxygen around the material and inhibit the spread of the fire.
3. A dense covering layer is formed on the surface of the material to isolate the contact between the material and oxygen, such as phosphorus series flame retardant, which will turn into a more stable structure of cross-linked solid substance or a carbonized layer to wrap the material under high temperature conditions, preventing the continued combustion.
4. Capture in combustion reaction of free radicals, inhibit free radical chain reaction, such as bromine flame retardants, under the condition of high temperature, high polymer material thermal decomposition, the bromine flame retardant and thermal decomposition products into the gas phase combustion zone at the same time, to capture the gas phase combustion zone of free radicals, inhibit free radical chain reaction, so as to prevent the spread of flames.
What kinds of flame retardants are there
According to the categories, flame retardants can be divided into halogenated flame retardants and halogen-free flame retardants.
Literally, the biggest difference between the two is that the former contains halogens while the latter does not.
However, halogen-free is not absolutely halogen-free, but is defined according to its content.
Halogen free is defined as bromochloride less than 900ppm, with a total bromochloride content less than 1500ppm.
Halogen flame retardants mainly refer to brominated flame retardants and chlorinated flame retardants, among which brominated flame retardants are the most widely used flame retardants at present.
Most flame retardant materials contain halogens in the field of daily life and transportation.
The halogenated flame retardant has the characteristics of small amount of addition, remarkable flame retardant effect and low price.
In the addition of flame retardant, the halogen element has good compatibility with the polymer material, and will not affect the physical and chemical properties of the material itself.
At the same time, however, we should not ignore that some brominated flame retardants, such as hexabromocyclododecane, emit large amounts of smoke and toxic gases during combustion, making it difficult to extinguish, escape and recover.
As halogenated flame retardants face a lot of controversy, we need to know the following facts:
(1) Brominated flame retardants are still widely used in the world, and only 2 ~ 3 halogenated flame retardants including hexabromocyclododecane are included in the authorized list and candidate list of the European Union and many countries, and there are more than 70 kinds of brominated flame retardants at present.
② Most of the brominated flame retardants have been proved to be harmless to human body and the environment through rigorous evaluation, and some brominated flame retardants, such as tetrabromobisphenol A, have lower toxicity than table salt.
(3) Under the correct operating conditions, the use and recycling of brominated flame retardants will not produce new harmful gases, and their recoverability is better than that of other flame retardants;
④ Bromine flame retardants have a wide range of applicability, almost applicable to all the need for flame retardant materials, has never stopped the research and application of bromine flame retardants.
Halogen-free flame retardants include phosphorus series flame retardants, expansion flame retardants, inorganic flame retardants, silicon-based flame retardants and bio-based flame retardants.
Most halogen-free flame retardants contain phosphorus and nitrogen elements, which are mainly divided into:
Phosphorus flame retardant
Phosphorus flame retardants can be divided into inorganic phosphorus flame retardants and organophosphorus flame retardants.
Phosphorus flame retardants mainly play a role in the decomposition of materials in the early stage of fire, forming a protective film to isolate the outside heat and air.
Inflatable flame retardant
Expansive flame retardants can effectively protect materials exposed to flame for a long time.
Inorganic flame retardant
Inorganic flame retardant has good thermal stability, flame retardant, non-volatile, no corrosive gas, small smoke and other characteristics, is the main raw materials of low halogen-free flame retardant system.
Flame retardants containing silicon
In the flame retardants containing silicon, Si -- O -- Si structure is stable, and has the characteristics of low toxicity, anti-dropping, promoting carbon formation and smoke suppression during combustion.
Bio-based flame retardant
Although biobased flame retardants are only in the preliminary stage of research, they have gradually become the focus of attention and research due to their advantages of low price, non-toxicity and wide resources.
There are many kinds of halogen-free flame retardants, but they face a common problem -- it is difficult to achieve good flame retardant effect, and it has an impact on the mechanics and processing properties of materials.
Therefore, halogenated flame retardants and halogen-free flame retardants have their own advantages and disadvantages. When choosing flame retardants, people should fully consider the structure, use environment and recycling of the flame retardant substrate.
Scientific understanding of the environmental protection of flame retardants
In recent years, halogen-free flame retardants are becoming more and more popular.
For a long time, even now, the wrong view of "halogen flame retardants are toxic and harmful, and halogen-free flame retardants are environmental protection, which is the development direction of flame retardants in the future" has been deeply popularized.
Flame retardant "halogen-free = environmental protection" view has been popular.
First, after the adverse effects of individual flame retardants, people overgeneralize the adverse effects to the whole halogenated flame retardants.
Second, some research results are misinterpreted and taken out of context.
Thirdly, since it is more difficult to detect the structure of a compound than to detect the elements, people only judge whether the compound contains halogens for convenience.
Fourth, it is used by some for-profit individuals, enterprises and research institutes to publicize that halogen is harmful and halogen-free, and force downstream manufacturers to give up using all halogen-containing flame retardants, so as to carry out blue ocean marketing.
In fact, out of more than 70 commercial brominated flame retardants, only two products, TBBA(tetrabromobisphenol A) and HBCD(hexabromocyclododecane), have been explicitly defined as toxic or hazardous under the international classification and labelling system for chemicals.
Some halogen-free flame retardants, such as most phosphates and some phosphates and hypophosphates, carry environmental hazardous substance labels.
Therefore, there is no scientific basis for judging environmental friendliness by the presence of a chemical element!
There is no regulation that restricts or exempts the entire halogen family in favor of being halogen-free.
The one-sided emphasis on halogen-free will only bring many harms to the society. First, it confuses the concept and misleads the public.
Second, it destroys the science-based chemical assessment and management system and misleads the regulatory authorities to adopt one-sided policy measures.
Third, forcing manufacturers to adopt immature halogen-free alternatives, or reducing the flame retardant level, thereby amplifying the risk of fire and threatening the safety of life and property;
Fourth, increase unnecessary inspection and testing, increase costs for manufacturers, and ultimately increase the burden on consumers;
Fifth, break the original market pattern of auxiliary agent supply, reduce the number of auxiliary agent suppliers, supply concentration, increase the risk of monopoly;
Sixth, widening the gap between domestic electronic product manufacturers and international first-class manufacturers, weakening the competitiveness of domestic brands.
Flame retardant industry should blossom, different flame retardants have different application fields, should be in compliance with the regulatory framework of the market to choose, rather than one-sided advocacy and emphasis on the halogen-free because of individual problems.
Admittedly, the pursuit of environmental protection is a necessary condition for the sustainable development of flame retardants, but we should not go too far. We must first understand the nature of environmental protection.
Environmental protection means environmental friendliness. As a kind of chemical, flame retardant needs to meet three conditions to achieve environmental friendliness.
First, from the chemical structure itself, it should achieve two properties: the environment is non-PBT and human non-toxic harmless.
Specifically, environmental non-PBT refers to chemicals that have a low impact in terms of persistent pollution, bioaccumulation, migration, and ecotoxicity, and are non-toxic to humans and have no impact on human health.
Second, it has a small impact on the environment in the whole life cycle, which is mainly manifested in the low carbon footprint, small environmental release and low comprehensive energy consumption in the whole process from the production, manufacturing, transportation, use to waste of the product. In addition, the waste in the process should be properly treated, and it is better to recycle it.
Third, the product has the necessary physical and chemical properties, can achieve the performance, make the best use of the material, and meet the regulatory requirements of the relevant laws and regulations.
When the flame retardant meets these three conditions, it can comply with the principle of green chemistry and find the direction of environmental friendly development in the future.
As is well known, in the case of the product itself, chemicals can only cause harm if they enter the organism.
Macromolecular compounds, on the other hand, are not likely to produce bioaccumulation because they are too large to penetrate biological cell membranes and participate in systemic circulation and metabolism.
So macromolecular polymerized flame retardant is one of the green development directions.
From the traditional high polymer bromine flame retardants and high polymer phosphorus flame retardants to the new organosilicone flame retardants and polymerized nitrogen-phosphorus flame retardants, the environmental friendly direction of flame retardants is explored and pursued.
To sum up, for the understanding of flame retardants, we must hold a scientific, rational attitude, must not hearsay.
With the enhancement of people's awareness of environmental protection, more and more attention has been paid to reducing the harm to people's health. The development of flame retardants towards environmental protection is bound to be the future trend.
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