By Liu Li, Suzhou Antefei’er New Materials Co., Ltd.
Flame retardants are mainly used in fields such as electronic appliances, wire and cable, construction, textiles, transportation tools, and glue coatings. Among them, transportation tools, electrical cables and architectural textiles are the main application sectors. The global demand for flame retardants has increased by 4% to 5% in the past five years, while the Asia-Pacific region (especially China) has achieved a compound growth rate of more than 10%.
Requirements on the use of halogenated products are becoming more stringent internationally. HBCD, as a brominated flame retardant, has been restricted by the Stockholm Convention; the Directive on Restriction of the Hazardous Substances in Electrical and Electronic Equipment (ROHS) has limited the use of polybrominated biphenyl ethers and polybrominated diphenyl ethers; REACH has also sorted out HBCD and decabromodiphenyl ether. Vigorously developing environmental-friendly new flame retardants is the only way to develop flame retardants in the future and being halogen-free and low-smoke is an effective way to solve this problem.
The halogen-free flame retardant can be generally classified into phosphorus and nitrogen containing type, silicon-containing type, and inorganic magnesium and aluminum type according to the composition. At present, the phosphorus and nitrogen containing type has better comprehensive performance, higher flame-retardant efficiency and relatively lower cost, while the inorganic magnesium and aluminum product and silicon-containing products’ consumption is usually larger.
1. Phosphorus containing product
Phosphorus-containing flame retardants are products with relatively high flame-retardant efficiency among halogen-free flame retardants, including coated red phosphorus, hypophosphites, DOPO series, phosphates, phosphazenes, and phosphorus-containing polymers.
The coated red phosphorus has been rapidly accepted by plastic manufacturers due to its high efficiency and low flame retardancy. Its raw material supply is sufficient, and the price is relatively stable, hence a better substitute for bromine products. In the past three years, the price of brominated flame retardants, taking decabromodiphenylethane for instance, has risen steadily from RMB30 000 /t at the beginning of 2015 to RMB60 000 /t now. The stricter environmental protection policies and shortage of raw materials greatly restricted the capacity expansion of bromine-based flame retardants. However, red phosphorus product has its own shortcomings that limit its application. For example, its color problem hinders its use in light-colored products.
A typical representative product of phosphorus-based flame retardants is hypophosphite, which has high heat resistance and can be used for processing engineering plastics in higher temperature. In addition, as hypophosphite is white powder, it can be used in white and light-colored products. Its thermal stability and electrical properties are also good. The disadvantage is that the price is relatively high, and the consumption is large. With the expansion of domestic production, the cost of phosphorus-based flame-retardant products will be reduced, which is favorable to the promotion of this type of products.
DOPO is a monomer of a phosphorus phenanthrene-based reactive flame retardant. It has a reactive hydrogen element and can be reacted with an epoxy resin to generate phosphorus-containing epoxy resins. It is widely applied in copper clad laminates. Compared with the traditional bromine-containing epoxy resin, it has the advantages of environmental protection, low toxicity, solder resistance, low dielectric constant, etc., and thus is widely used in electronic products with high environmental protection requirements. Meanwhile, DOPO can also be polymerized with polyester monomers and polyurethane monomers to form phosphorus-containing flame-retardant resins. In the future, DOPO will develop in the direction of macromolecularization and intrinsic flame retardancy and will be more widely used.
Phosphate esters are mature flame retardants, and halogen-free phosphate ester flame retardants include BDP, RDP, TPP, IPPP, TCP, TXP, BDEP and Trimer. Most of them are liquid and have a significant flame-retardant effect on polyurethane foaming materials. Therefore, it is mainly used for the flame retardant of rigid foam and soft foam of polyurethane, and is applied in sectors like home decorative, automobile interiors, seating and building insulation.
The phosphorus-containing polymer mainly includes phosphorus-containing epoxy, phosphorus-containing polyester, phosphorus-containing polyurethane and phosphate polymer. These flame-retardant polymers generally have better affinity with the matrix resin, less consumed, higher flame retardancy, less influence on mechanical properties, smooth surface, and good thermal stability. It has been rapidly promoted in some resins with high performance requirements, especially in the fields of flame-retardant fibers and copper clad laminates.
2. Nitrogen containing product
The nitrogen-containing flame retardant is mainly composed of melamine and melamine cyanurate (MCA) products. At present, the large-scale applications include the use of melamine on fireproof panels and the use of MCA in nylons, silicone resins and elastomers. Due to its low price and good thermal stability, such products enjoy a stable capacity of 20 000 to 30 000 t/a in China, which has been able to meet domestic demand.
3. Phosphorus-nitrogen containing product
Phosphorus-nitrogen flame retardant contains both phosphorus and nitrogen. It has synergistic flame-retardant function and has better flame-retardant effect, including ammonium polyphosphate, melamine polyphosphate (MPP) derivatives, poly-piperazine pyrophosphate, polyphosphazenes, etc.
The typical representative is ammonium polyphosphate, which is the earliest and most widely used product. It exhibits high flame-retardant properties for a variety of resins and rubber systems. The price is low, only over RMB10 per kilogram. Its disadvantage is that it has a certain water solubility, especially under high temperature, so the performance of the product will be effected during high temperature processing, causing problems such as black shadows and whitening, leaving it hard to use this product when processing nylon and polyester under high temperature. Its precipitation at room temperature also limits its application in some areas.
The MPP series of derivatives have high heat resistance. The typical products are BASF's MELAPUR, whose decomposition temperature can reach above 360°C. It can be applied to nylon and polyester processing under high temperatures. However, when it is used alone, the flame-retardant efficiency is not high, so it needs to be used together with other flame retardants.
The poly-piperazine pyrophosphate is a polycondensate of phosphoric acid and piperazine. At present, it is applied to polyolefins with remarkable flame-retardant effect, and its decomposition temperature exceeds 300°C.
The polyphosphazene flame retardant is typically represented by hexaoxane triphosphazene and Japan's SPB series. Their phosphorus content is higher than that of common phosphate esters. For example, the phosphorus content of SPB is 13%, and it can be dissolved in methyl ethyl ketone. It is able to meet the needs of different fields. In addition, their hydrolysis resistance and heat resistance are higher than those of general phosphate esters, and therefore, they have been recognized by customers in electronic automotive components, packages, and printed circuits where performance requirements are relatively stable.
The market of flame retardants is expanding rapidly, and the advantages of halogen-free products have been widely recognized. With the continuous reduction of costs and the continuous improvement of technology, halogen-free flame retardant will embrace fast development, and there will be huge market space in the future.
