Classification of halogen-free flame retardants

In addition, phosphorus and nitrogen halogen-free flame retardants also include intumescent halogen-free flame retardants, which mainly function through condensed phase. At low temperature, the acid source generates esterifiable polyol (carbon source) and acid which can be used as dehydrating agent; At a slightly higher temperature, the acid reacts with polyol (carbon source), and the amine in the system acts as a catalyst to accelerate the esterification reaction. The system melts before or during esterification; The water vapor generated in the reaction process and the nonflammable gas generated by the gas source make the system in the molten state expand and foam, at the same time, polyol and ester dehydrate and carbonize to form inorganic substances and carbon residue, and the system further foams; When the reaction was near completion, the system gelled and solidified, and finally a porous carbon foam layer was formed. Such as ammonium polyphosphate, melamine, expanded graphite, melamine phosphate, zinc borate, TGIC.

Inorganic flame retardant

Aluminum hydroxide AL(OH)3 accounts for more than 40% of the total flame retardant. Aluminum hydroxide itself has three functions: flame retardant, smoke suppression and filling. Because it is non-volatile, non-toxic and can produce synergistic flame retardant effect with many substances, it is known as a pollution-free inorganic flame retardant. However, aluminum hydroxide has the disadvantage of large dosage, and it usually needs to be added more than 50% to have good flame retardant effect. In order to overcome this shortcoming, granulation technology can be used to develop in the direction of ultra-fine and narrow the particle size distribution; Improve the wrapping technology and improve its dispersibility in polymer; Macromolecular bonding treatment and other methods are adopted.

Magnesium hydroxide is a rapidly developing additive flame retardant. It is low-smoke, non-toxic, can neutralize acidic and corrosive gases in the combustion process, and is an environmentally friendly and green flame retardant. Its flame retardant mechanism is similar to that of aluminum hydroxide. Compared with AL(OH)3, the decomposition temperature of Mg(OH)2 is higher than that of AL(OH)3 100- 150C, which can be used for flame retardant of engineering plastics with processing temperature higher than 250C, and can also promote char formation of polymers. However, in order to achieve a certain flame retardant effect, the addition amount needs to be above 50%, which has a great influence on the properties of the material. In order to reduce the addition of Mg(OH)2 in polymer, one method is to refine the particles of Mg(OH)2, and the other method is to modify the surface of Mg(OH)2 by coating technology to improve its compatibility with polymer.

Red phosphorus is an excellent flame retardant with high efficiency, smoke suppression and low toxicity. However, it is easy to absorb moisture, oxidize and release toxic gases, and the dust is easy to explode and is dark red, so its use is greatly limited. In order to solve these shortcomings, surface treatment of red phosphorus is the main research direction, and microencapsulation is the most effective method. There are many kinds of microencapsulated red phosphorus products in the international market, and a lot of research has been carried out in China. Aluminum hydroxide, metal sulfate and synthetic resin are generally used as capsule wall materials, but not many of them are introduced to the market. In the future, the development direction of surface treatment of red phosphorus is as follows: first, by modifying the coating capsule material, it has the functions of thermal stability, plasticization and flame retardant at the same time, and develops a multifunctional microencapsulated red phosphorus flame retardant; The second is to study the effective compound relationship between various flame retardants and red phosphorus flame retardants, and microencapsulate them to increase the flame retardant effect and improve the mechanical properties of the materials; Third, red phosphorus has the function of smoke suppression, and suitable smoke suppressants can be found to be compounded with it. Smoke suppression in fire is more important than fire prevention, which promotes the development of smoke suppression technology.

Expandable graphite is a new type of halogen-free flame retardant, which is made of natural graphite acidified by concentrated sulfuric acid, then washed, filtered and dried, and expanded at 900- 1000℃. The initial expansion temperature of expandable graphite is about 220℃. Generally, it begins to expand slightly at 220℃ and expands rapidly at 230-280℃, and then its volume can reach more than 100 times or even 280 times of the original volume. Expandable graphite mainly plays the following roles in the flame retardant process: (1) forms a tough carbon layer on the polymer surface to separate combustible materials from heat sources; (2) absorb a lot of heat in the expansion process and reduce the system temperature; (3) During the expansion process, acid ions in the interlayer are released, which promotes dehydration and carbonization, and can combine with free radicals generated by combustion to interrupt the chain reaction. expandable

When graphite is combined with phosphorus compounds and metal oxides, it can produce synergistic effect, and a small amount of graphite can achieve the purpose of flame retardant.

Ammonium polyphosphate is an inorganic flame retardant with excellent performance and an active research field of phosphorus flame retardant. Its appearance is white powder, decomposition temperature >: 256C, polymerization degree 10-20, soluble in water, and insoluble in water if the polymerization degree is greater than 20. APP is cheaper than organic flame retardants, with low toxicity and good thermal stability, and can be used for flame retardant of plastics alone or in combination with other flame retardants. At high temperature, APP is rapidly decomposed into ammonia and polyphosphoric acid, and ammonia can dilute the oxygen concentration in the gas phase, thus preventing combustion. Polyphosphoric acid is a strong dehydrating agent, which can dehydrate and carbonize the polymer to form a carbon layer, isolate the contact between the polymer and oxygen, and prevent solid phase combustion.

2. Organic flame retardant

2. 1 halogen-containing traditional flame retardant materials

Traditional flame retardant materials widely use halogen-containing polymers or halogen-containing flame retardants as flame retardant mixtures. Halogen flame retardant has the advantages of low dosage, high flame retardant efficiency and wide adaptability, but its serious disadvantage is that it will produce a lot of smoke and toxic and corrosive gases when burning, which is very harmful. In case of fire, due to thermal decomposition and combustion, a large amount of smoke and toxic corrosive gases will be generated, which will hinder fire fighting and personnel evacuation and corrode instruments and equipment. In particular, it is found that more than 80% of the deaths in the fire are caused by thick smoke and toxic gases produced by materials. Therefore, the research and development of halogen-free flame retardant will surely attract people's attention.

Although the development of bromine flame retardant produces a lot of smoke, it will remain the main flame retardant for a long time to come because of its good flame retardant performance, small dosage and little influence on product performance. With the development of technology, the new feature of the development of bromine-based flame retardants in the world is the continuous improvement of bromine content and molecular weight. For example, PB-68 of American F erro Company is mainly composed of brominated polystyrene, with molecular weight of 15000 and bromine content of 68%. Poly (pentabromophenol acrylate) developed by Fast Bromine Chemical Company and Ameribrom Company respectively has a bromine content of 70.5% and a molecular weight of 30,000 ~ 80,000. These flame retardants are especially suitable for all kinds of engineering plastics, which are much better than many small molecular flame retardants in fluidity, compatibility, thermal stability and flame retardancy, and may become the updated products in the future.

2.2 Halogen-free organic flame retardant

There are many kinds of organic flame retardants, which are developing rapidly. They can be divided into halogen flame retardants and halogen-free flame retardants. Halogen flame retardant is the earliest type of flame retardant, but its use is greatly limited because of its decomposition and release of toxic gases. Halogen-free flame retardant does not contain halogen cable, which has good flame retardant effect, and the gas produced by thermal decomposition is low in smoke and toxicity, so it is very popular. Halogen-free flame retardants can be divided into phosphorus flame retardants, nitrogen flame retardants and intumescent flame retardants.

Organophosphorus flame retardant is one of the most important flame retardants, which has dual functions of flame retardant and plasticization. It can make the flame retardant completely halogen-free, improve the fluidity in plastic molding and inhibit the residue after combustion. It produces less toxic and corrosive gases than halogen flame retardants. The flame retardant mechanism is as follows: on the one hand, the flame retardant is decomposed by heat to produce phosphoric acid, metaphosphoric acid and polymetaphosphoric acid. These phosphoric acids have strong dehydration, which can dehydrate and carbonize the polymer surface, while elemental carbon cannot produce flame evaporation, combustion and decomposition. On the other hand, when the flame retardant is heated, it produces PO free radicals, which can absorb a large number of H HO free radicals, thus interrupting the combustion reaction. Organophosphorus flame retardants mainly include phosphate, phosphonate, phosphine oxide and heterocyclic compounds.

Phosphate flame retardant belongs to additive flame retardant. Because of its rich resources, low price and wide application. Phosphate ester is prepared by the reaction of corresponding alcohol or phenol with phosphorus trichloride and then hydrolysis. Phosphate flame retardants that have been successfully developed and widely used in the market include triphenyl phosphate, triphenyl phosphate, triisopropyl phosphate, tributyl phosphate, trioctyl phosphate and methyl phosphate.

Phenyl diphenyl phosphate, etc. There are many kinds of phosphates, which have a wide range of uses. However, phosphate is mostly liquid, which has poor heat resistance and high volatility, and its compatibility with polymers is not ideal. Therefore, a number of new phosphate flame retardants have been developed at home and abroad, such as tri (1- oxo-1- phosphorus-2,6,7-trioxane bicyclo [2,2,2] octane -4- methylene) phosphate (trimer) and 65438 developed by Grent Lake Company in the United States. The trimer is symmetrical in structure, with phosphorus content of 2 1. 1% and PEPA of 17.2%. Both phosphate flame retardants are white powder. Excellent thermal stability and good compatibility with polymers.

Phosphonate flame retardant is a kind of flame retardant with great development prospect. Because of the existence of C-P bond in phosphonate molecules, it has very good stability, water resistance and solvent resistance. The foreign phosphonate products include Pyrovatex developed by Giba-Geigy Company, namely N- hydroxymethyl propionamide methyl phosphonate, and Antiblaze developed by Mobil Company, namely cyclic phosphonate. Phosphonates have also been studied in China. The synthesized phosphonates include tetraethyl N, N- p-phenylenediamine (2- hydroxy) dibenzyl phosphonate and dimethyl methylphosphonate (DMMP), among which DMMP is an additive flame retardant. DMMP is prepared by isomerization of trimethyl phosphite under the action of catalyst and molecular rearrangement. The most remarkable feature of DMMP is that the phosphorus content is as high as 25%, and the flame retardant effect is very good. The same effect can be achieved when the addition amount is half of that of ordinary flame retardant.

The hydrolytic stability of phosphine oxide is better than that of phosphate ester, and it is a highly stable organic phosphine compound. It can be used as a flame retardant for polyester and has good color and mechanical properties. This kind of flame retardant can be divided into two categories, one is additive and the other is reactive. It has become a research hotspot to introduce triarylphosphine oxide monomer into homopolymer with high relative molecular weight to prepare flame retardant engineering plastics. Flame-retardant polyester, polycarbonate, epoxy resin, polyurethane, etc. can be made by doping phosphine oxide monomer with active functional groups into poly, and the phosphorus-containing monomer is bonded to the molecular chain of the synthetic material through reaction, giving the material permanent flame retardancy without exudation.

Organophosphorus heterocyclic compounds are one of the most active fields in flame retardant research, including five-membered, six-membered and spiro compounds. Among them, five-membered phosphorus heterocyclic flame retardants are few in variety and are generally used for flame retardant of polyester, polyamide and polyolefin; Six-membered heterocyclic ring plays a leading role in phosphorus heterocyclic flame retardant, mainly including phosphine oxide, phosphate, cage phosphate, phosphonate and phosphite, which can be used for flame retardant treatment of polyester, epoxy resin and polyurethane. Phosphospiro flame retardant is mainly prepared by the reaction of pentaerythritol with phosphorus compounds. Generally, molecules contain a lot of carbon and two phosphorus atoms, which have high phosphorus content and good flame retardant effect. They can be used as intumescent flame retardants, which play the roles of plasticization, thermal stability and flame retardancy in materials.

Organic nitrogen-based flame retardants have the advantages of low volatility, non-toxicity, good compatibility with polymers, high decomposition temperature and suitability for processing, and become a very popular type of flame retardants. The flame retardant mechanism is as follows: (1) CO2, NH3, N2 and H2O are released when heated, which reduces the concentration of oxygen in the air and combustible gas generated when polymer is decomposed by heating; (2) The generated uneven gas takes away a part of heat and reduces the temperature of the polymer surface; (3) The generated N2 can capture free radicals and inhibit the chain reaction of polymers, thus preventing combustion. Melamine is the most commonly used nitrogen-based organic flame retardant, and its effect is not very good when used alone. It needs to be used in combination with other flame retardants such as polyphosphate and pentaerythritol.

Intumescent flame retardant is a kind of flame retardant with carbon, nitrogen and phosphorus as its core components. IFR is mainly composed of three parts:? Carbon source (char-forming agent): it is generally a multifunctional substance rich in carbon, such as starch, pentaerythritol and diethylene glycol; ? Acid source (dehydrating agent): generally inorganic acid or salts that can generate acid in situ when heated, such as phosphoric acid and ammonium polyphosphate. ? Gas source (foaming agent): a nitrogen-containing multi-carbon compound, such as urea, melamine, dicyandiamide and its derivatives. The flame retardant mechanism of IFR is that when heated, the char-forming agent is dehydrated into char under the action of acid source, and under the action of gas decomposed by foaming agent, a fluffy, porous and closed-cell carbon layer is formed, which can weaken the heat transfer between polymer and heat source and prevent gas diffusion. The polymer stopped burning because there was not enough fuel and oxygen. The internationally commercial intumescent flame retardant is CN-329 developed by GreatLake Company of the United States, and Borg-Warner developed by Chemical Company is suitable for PP, which is relatively stable at the processing temperature of PP and has good electrical properties. When the content is 30%, the oxygen index of the material can reach 34, which shows that CN-329 is a good PP flame retardant. From the molecular point of view, Melabis is rich in acid source and carbon source, and the ratio of acid source, carbon source and gas source increases, which makes Melabis hygroscopic far lower than CN-329, and it is an excellent flame retardant.

With the rapid development of flame retardant, flame retardant technology is also making breakthroughs. Among them, surface modification technology, composite synergy technology, microcapsule technology, superfine technology, crosslinking technology and polymer technology have all developed rapidly and played a very important role in flame retardant and fire extinguishing.