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Research and development of organic boron flame retardants

at present, the most common elements with flame retardancy are: Boron of the third main group, titanium/zirconium of the fourth B group, nitrogen/phosphorus/antimony of the fifth a group, bromine/chlorine of the seventh main group, etc. Among them, nitrogen and phosphorus are used for cellulose fibers, and halogen and antimony are used for durability and flame retardancy of synthetic fibers

in addition to the flame retardant performance, the standard to measure the quality of flame retardants should consider whether they are toxic, whether toxic gases are produced during finishing, and whether a large amount of smoke and harmful gases are produced during combustion. Many flame retardants have been banned because of their toxicity, such as the early Tri (azacyclopropyl) phosphine oxide (APO, also known as TEPA), and the recent decabromodiphenyl ether (debde) and hexabromocyclododecane (HBCD)

phosphorus flame retardants have good flame retardancy and good washing resistance and durable flame retardancy when used in fabric finishing, but they are easy to make the fabric change color, produce odor and the internal crystal structure of the sample material has regained its dispensing toxicity due to plastic deformation; Halogen containing flame retardants have good flame retardancy, but secondary pollution occurs in the process of use and combustion

with the continuous introduction of new environmental protection regulations, boron flame retardants are attracting attention because of their excellent flame retardancy, low toxicity and smoke suppression, which is in line with the development trend of halogen-free, non-toxic and smoke suppression of flame retardants

boron is rich in resources. Borax was used as a flame retardant as early as the 18th century, and it is the earliest inorganic boron flame retardant. However, the application of organic boron compounds is limited to a certain extent due to the factors of water washing resistance and high price due to the instability of hydrolysis. However, compared with organophosphorus flame retardants, organic boron has far less toxicity than the latter, and has good smoke suppression

if organic boron compounds are introduced into the same molecular structure with nitrogen, phosphorus, halogen, silicon, etc. to synthesize boron containing composite flame retardants, and the content of boron in the molecule is increased as much as possible, on the one hand, the hydrolysis stability of pure boron flame retardants can be improved and the price can be reduced, and at the same time, the toxicity of bromine and phosphorus flame retardants can be reduced

boron nitrogen composite flame retardants play a synergistic effect

when nitrogen flame retardants are used alone, the flame retardant effect is poor, but when combined with phosphorus compounds, the flame retardant effect is enhanced, such as inorganic ammonium phosphate and ammonium polyphosphate, phosphoramides, phosphite esters, phosphonate esters, etc., organic N-hydroxymethyl dimethylphosphite acrylamide (nmppa), tetrahydroxymethyl phosphine chloride and urea precontracts (proban), etc

the compound of boron and nitrogen flame retardants, or the compound flame retardant formed by the coordination chain of boron and nitrogen, can show good flame retardancy. Their synergistic flame retardancy is carried out separately. When nitrogen-containing additives are heated to produce non combustible gases such as ammonia and nitrogen, plus carbon dioxide produced by cellulose due to the presence of flame retardants, these non combustible gases dilute oxygen in the air and combustible gases produced by polymers when heated, and when non combustible gases are generated, they take away part of the heat due to heat absorption reaction; At the same time, nitrogen can trap free radicals, inhibit the chain reaction of polymers, and play the role of scavenging free radicals

boron nitrogen composite flame retardant is to introduce boron and nitrogen into the same molecular structure, and the two elements interact during combustion. In 2012, a 10000 ton/year thermoplastic vulcanized rubber complete industrial plant was built to produce better synergistic flame retardancy

boric acid is the basic raw material for the synthesis of boron heteroatom flame retardant. It is a monobasic weak acid. The main intermediate for the synthesis of this kind of flame retardant is 2-allyl-1,3-oxyboron heterocyclopentane, which is obtained by the reaction of potassium borohydride with allyl alcohol under the action of acetic acid. The compound is a boron oxide five member heterocycle, which is very stable and will not hydrolyze. Monocyclic and bicyclic compounds of B ← n ligand were obtained by reacting with ethanolamine and diethanolamine, respectively

for the flame retardant with B ← n coordination chain, the conventional rolling and baking finishing process is used to finish the cotton fabric with good flame retardant effect, and the LOI values are 24.2 and 25.2 respectively

heat the intermediate to 150~160 °, partially hydrolyze, and then often press distillation to remove allyl alcohol; Or reacting with ethylene glycol and heating to remove allyl alcohol, two products are obtained respectively, both of which have flame retardant properties, with LOI of 25.2 and 22.4 respectively. The former has higher boron content than the latter, which shows better flame retardancy

boron phosphorus composite flame retardancy occurs in the solidified phase

phosphorus containing flame retardants will decompose at a lower temperature to produce phosphoric acid, which will become metaphosphoric acid with the increase of temperature, and then shrink to polymetaphosphoric acid. Metaphosphoric acid is a strong dehydrating agent, which promotes the carbonization of cellulose and inhibits the formation of combustible cellulose lysates, resulting in flame retardation. In addition, the decomposed phosphoric acid forms a non-volatile protective layer to isolate the air

however, phosphoric acid is the catalyst for cellulose to oxidize carbon into carbon monoxide during combustion, which reduces the continuous oxidation of carbon dioxide and prevents the fire retardation of cellulose due to exothermic reaction. The flame retardancy of phosphorus containing flame retardants mainly occurs in the solidified phase

boron based flame retardants produce boric anhydride or boric acid during combustion, and form a glass like molten fabric during thermal cracking to cover the fabric, isolating the propagation of oxygen and heat when the small angle reaches the preset value. In the solidification phase, boric acid reacts with cellulose hydroxyl to form borate ester, which inhibits the formation of L-glucose, directly oxidizes cellulose to carbon dioxide, and reduces the generation of combustible gas carbon monoxide. This is different from the combustion process of phosphorus containing flame retardants, which mainly produces carbon monoxide due to the catalysis of phosphoric acid, and its flame retardancy also occurs in the solidified phase, and the resulting glass body can prevent the outward diffusion of combustible gases. The above factors can achieve the purpose of flame retardation and good smoke suppression

the synergistic effect of boron phosphorus composite flame retardants improves the hydrolysis stability of organic boron containing flame retardants and reduces the toxicity of phosphorus containing flame retardants

organophosphorus flame retardants have washing resistance, but they are toxic. If organoboron compounds are combined in the molecule to synthesize boron phosphorus flame retardants, a variety of products can be obtained. Chloroethyl borate was obtained by reacting boric acid with chloroethanol, and then reacted with ethylene glycol amine; In addition, chloroethyl phosphonate was obtained from phosphorus oxychloride and chloroethanol, and the two reacted to obtain the same boron phosphorus composite flame retardant. Because the molecules contain boron, phosphorus and nitrogen, the flame retardant performance is very good, and the LOI value can reach 37~38

for example, diethylene glycol borate is obtained by reacting boric acid with glycerol, and diethylene glycol phosphoryl chloride is obtained by reacting ethylene glycol with phosphorus oxychloride. The residual hydroxyl phosphate of the former is used to esterificate, and boron and phosphorus are introduced into the same molecule as a flame retardant. Because there are many hydroxyl groups in the molecule, it can be covalently bonded with cotton fiber under the catalysis of Lewis acid, so it has good washing resistance. Using water-soluble polyurethane as crosslinking agent, it can be used for flame retardant of polyester fabric, with good flame retardant effect. The LOI value is 38.5 (the LOI value of unfinished polyester fabric is 29.0), and it is reduced to 30.8 after washing for 5 times

boron halogen composite flame retardant is suitable for polyester materials. When halogen compounds are used as flame retardants, halogen free radicals are first released during combustion, which act with the high-energy free radicals produced by polyester during combustion, inhibiting chain growth. At the same time, due to the formation of hydrogen halide with combustible gas, hydrogen halide also acts with free radicals generated by chain growth disproportionation reaction during polyester combustion, resulting in flame retardancy. In addition, hydrogen halide gas dilutes the concentration of combustible gas generated by polyester cracking, delaying combustion

The effect of halogenated flame retardants mainly occurs in the gas phase. In the gas phase, the boron halogen composite flame retardant produces gaseous boron trihalide during combustion, and then releases hydrogen halide, which also prevents the chain reaction between highly active free radicals. In the solid phase, the boron flame retardant melts, seals the surface of the combustor, and the formed glass covering layer acts as a barrier, which also controls the outward escape of corrosive toxic gas hydrogen halide. In addition, the endothermic effect produced by the release of bound water from boron compounds changes the thermal decomposition path of combustibles and inhibits the generation of combustible gases

brominated flame retardants are the main varieties of halogen flame retardants, which are widely used in the flame retardant finishing of synthetic fibers such as polyester. However, decabromodiphenyl ether, hexabromocyclododecane, polybrominated biphenyls and others have been banned by the European Union because of their serious harm to human health and the environment. Therefore, the development of boron bromine composite flame retardant can be a substitute for bromine flame retardant

FR-B currently produced in China is a liquid additive flame retardant containing bromine and boron, called tris (2,3-dibromo) propyl borate, which is obtained by esterification and dehydration of 2,3-dibromopropanol and boron trioxide

the LOI value of the flame retardant is 28.5, and it also has smoke suppression effect, which has less impact on the physical and mechanical properties of the products and is easy to process and shape. It is mainly used for polyurethane, unsaturated polyester resin and phenolic epoxy resin. It can also produce good flame retardant effect on cotton fabrics. However, due to the presence of bromine in the molecules, hydrogen bromide gas is formed during combustion, which has an adverse impact on the environment

boron halogen composite flame retardants can be obtained by 2-allyl-1,3-oxyboron heterocyclopentane. One boron halogen composite flame retardant can be obtained by the double chain bromination of allyl vinyl, and another boron halogen composite flame retardant can be obtained by mixing with 3-chloro-1,2-dihydroxypropane. The LOI value can reach 28.5, which has good flame retardancy

applying these boron halogen composite flame retardants to cotton fabrics can produce good flame retardant effect. The flame retardancy can be greatly improved by introducing halogen atoms into boron molecules, which has the same synergistic effect as organophosphorus compounds

the main raw materials for the synthesis of the above three boron, nitrogen, phosphorus and halogen composite flame retardants are basically boric acid and 2-allyl-1,3-oxyboron heterocyclopentane. The former is boric acid ester, which has lower hydrolysis resistance than the latter, affecting its washing resistance. After washing, the flame retardancy decreases by 20%, but it is still a semi permanent flame retardant. The latter has high hydrolysis resistance due to the existence of B ← n ligand in the molecule, and its flame retardancy decreases by 4% after washing

there is a large space for the development of boron silicon composite.

the research and development of the flame retardant mechanism of boron silicon flame retardants are in the embryonic stage. Some people believe that boron will catalyse the formation of carbon during the combustion of boron silicon flame retardant, while silicon will increase the stability of the formed carbon layer; The layered silica formed by the degradation of siloxane prevents the oxidation of the carbon layer and improves the stability of the carbon layer, so as to achieve the flame retardant effect

organosilicon flame retardant materials have the characteristics of high efficiency, non-toxic, low smoke, no droplet, and no pollution. Among many non halogen flame retardants, they are very popular, but they have the problem of high price. Introducing silicon and boron into the same molecular structure can not only reduce the price, but also improve the hydrolysis stability of boron flame retardant. The structure composed of boron oxygen chain and silicon oxygen chain, as a flame retardant, the flame retardant resin polymer mixed with PC, pet, PBT, ABS and other resins has good flame retardant effect, which is very valuable to the resin industry. However, if used alone on cotton fabrics, the flame retardant effect is not good enough

some researchers reacted Phenyltrichlorosilane with boric acid to obtain an oligomer with ring structure containing silicon and boron as a flame retardant, which can be used on cotton fabrics. It has good flame retardancy and can improve the hydrolysis stability of borate esters. (end)

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