Common Plastic Flame Retardants

Halogen-based flame retardants are divided into chlorine-based flame retardants and bromine-based flame retardants. Chlorine-based flame retardants are cheap, but have poor thermal stability, and are only suitable for products with processing temperatures below 200°C. Brominated flame retardants have high flame retardant efficiency, twice that of chlorine-based flame retardants, relatively less dosage, good compatibility with matrix resins, little influence on the mechanical properties of materials, and good weather resistance and thermal stability. It has an important position in the field of flame retardant

Halogen-based flame retardants mainly play a flame retardant role in the gas phase. They decompose and release flame-retardant hydrogen halide gas when heated, which can dilute the concentration of oxygen and flammable gases in the air, and can eliminate or reduce the polymer combustion process. The generated active free radicals can terminate or slow down the chain reaction in the gas phase combustion process and play a role in flame retardant.

At the same time, in the condensed phase, the halogen-based flame retardant can also form a carbonized layer through dehydration reaction, covering the surface of the polymer to isolate the air, so as to play a flame retardant role in the condensed phase.

Halogen-based flame retardants are used in conjunction with synergists such as metal oxides and phosphorus-containing compounds, and the flame retardant effect is better. Therefore, in practical applications, halogen-based flame retardants are often used in conjunction with antimony trioxide (Sb₂O₃).

However, halogen-based flame retardants will generate thick smoke and corrosive gases during the combustion process, which will bring serious secondary disasters to human health and the environment. Therefore, low-halogen or halogen-free environment-friendly flame retardants have attracted much attention.

Phosphorus-based flame retardants have formed wide varieties, including inorganic phosphates, phosphate esters, phosphaphenanthrenes, phosphazenes, and red phosphorus. The high-boiling oxyacid formed by the decomposition of phosphorus-based flame retardants during the combustion process can promote the dehydration and carbonization of the polymer, and form a protective layer of carbonized residues to isolate the polymer from the air. At the same time, the released water absorbs a lot of heat, which can also reduce the surface temperature of the polymer, so as to achieve the effect of flame retardant.

The advantages of phosphorus-based flame retardants are low toxicity and corrosiveness, good thermal stability and long-lasting effect. The EU has banned the use of halogenated flame retardants such as polybrominated biphenyls and polybrominated diphenyl ethers in the manufacture of electronic equipment. Phosphate-based phosphorus-based flame retardant bisphenol A-diphenyl phosphate (BDP) has good compatibility with engineering plastics such as PC/ABS, and has good environmental protection performance, high thermal stability, and little impact on material properties. Therefore, it is widely used in electronic and electrical equipment such as mobile phones, notebook computers and TV sets.

PC has the advantages of high impact strength, creep resistance, good dimensional stability, heat resistance, transparency and excellent dielectric properties, but it also has disadvantages such as poor processing fluidity, prone to stress cracking, sensitivity to notch, and poor wear resistance; ABS has good chemical resistance and molding processability, but its heat resistance and weather resistance are relatively poor. The PC/ABS alloy obtained by blending and modifying the two can complement each other in performance and are widely used in the automotive industry, communication equipment, electronic appliances, lighting equipment, etc.

In order to meet the fire safety requirements in some application fields, PC/ABS alloys must have good flame retardancy. For example, in new energy vehicle charging piles and some electronic and electrical flame retardant grades to reach V-0. For PC/ABS blending system, the best ones are bisphosphonates and phosphate oligomers with good thermal stability. Among them, BDP shows good flame retardant properties to PC/ABS alloys.

PC has the characteristics of good heat resistance, excellent optical performance, good dimensional stability, high mechanical strength and excellent impact strength. It is widely used in home appliances, electronic appliances, auto parts, mechanical equipment and optical instrument parts, etc. The global PC production capacity exceeds 10 million tons.

The limiting oxygen index (LOI) of PC can reach 21% to 24%, and the material itself can reach UL94 V-2 flame retardant, but it still cannot meet the requirements of flame retardant performance in special fields, such as electrical and electronic and automotive fields. The flame retardant grade reaches V-0.

Among the flame retardant PCs, sulfonate flame retardants represented by potassium perfluoroalkyl sulfonate (PPFBS) have the best effect, and adding a small amount (below 0.5%) of sulfonate flame retardants can significantly improve the performance of PC. The flame retardant performance reaches the flame retardant grade of 1.6 ~ 3.0mm V-0, and the physical and mechanical properties of the modified PC are equivalent to the general PC, and it can remain transparent.

The most used phosphorus-based flame retardants in industrial PC products are triphenyl phosphate (TPP), resorcinol bis (diphenyl phosphate) (RDP) and BDP, and the addition amount of the flame retardant is 10% (mass fraction), the thickness of 1.6mm PC can reach V-0 level, in which TPP is volatile at high temperature and only plays a flame retardant effect in the gas phase; RDP and BDP can play a flame retardant effect in the gas phase and solid phase at the same time. TPP is solid at room temperature, with relatively poor thermal stability, and is easy to volatilize at PC processing temperature; RDP and BDP are liquid at room temperature and have better thermal stability; BDP has better hydrolysis resistance than RDP.

ABS has high gloss, good toughness, good heat and low temperature resistance, excellent processing performance, and excellent comprehensive mechanical properties; ABS electrical properties do not change greatly with changes in temperature and humidity. It is a good insulating material and is widely used. In the fields of household appliances, audio-visual equipment, office supplies and electrical appliances, the global production capacity exceeds 10 million tons.

ABS resin is flammable, LOI is 18.3% to 18.8%, and it belongs to HB level according to UL94 standard. When ABS is on fire, the burning rate is fast, and a lot of poisonous gas and black smoke are released, which is not conducive to practical application. At present, halogen flame retardants are mainly used in combination with Sb₂O₃, and ABS is modified by the synergistic effect of halogen and antimony. The main brominated flame retardants used are TBBA, DBDPE and so on. Flame retardant ABS is mainly used in the casings of electrical and electronic equipment such as telephones, computers, liquid crystal displays (LCDs), interior and exterior components of automobiles, small kitchen appliances, casings of tool boxes and vacuum cleaners, etc.

HIPS is prepared by copolymerization of polystyrene (PS) and polybutadiene (PB). It has the common advantages of both, such as easy processing, good dimensional stability, excellent electrical insulation, high impact resistance, and low cost. , is widely used in food packaging, household appliances, electronic products and shell packaging of military supplies and other fields.

However, the LOI of HIPS is only 18%, which is easy to burn. In order to improve the safety of use, it must be flame-retardant modified. Brominated flame retardants such as decabromodiphenyl ethane (DBDPE) are mainly used for HIPS flame retardant modification, and most of them cooperate with Sb₂O₃ and have excellent flame retardant effect.

PBT is a crystalline saturated polyester with excellent mechanical properties, good dimensional stability, good electrical insulation and chemical resistance, and is widely used in automobiles, electronic appliances, machinery and other fields. However, PBT has poor flame retardant performance (UL94HB grade), it is easy to burn in the air, it is difficult to form charcoal, it is easy to drip when burning, and a large amount of smoke and toxic gases are released, which is easy to produce relatively large dangers, so it must be modified with flame retardant.

The flame retardants most commonly used for PBT are mainly halogen-based and brominated ones such as DBDPE and brominated epoxy resin with the co-function of Sb₂O₃. Other flame retardant used in PBT also includes aluminum diethylphosphinate (ADP) or its compound products with magnesium hydroxide, aluminum hydroxide, etc.

PA has good mechanical properties, electrical properties and relatively prominent lubricating properties, and is widely used in electronic components and auto parts. The unmodified PA flame retardant grade is UL94 V-2, and the LOI is 20% to 22%. In the case of contact with an open flame, it will burn quickly, release a lot of heat, and there will be dripping due to melting. This creates conditions for the further spread of open flames. In order to prevent fire hazards, the industry has clear flame retardant requirements for PA composites used in automobiles, electrical appliances and other products, generally requiring them to reach the UL94 V-0 level.

Halogen-based flame retardants are widely used in PA due to their good compatibility with PA and high flame retardant efficiency. The flame retardants used in halogen-free flame retardant PA are different. Generally, pure PA flame retardant can be added with 12-15% MCA. However, due to the "candle wick effect", the PA/GF system requires a higher flame retardant efficiency. Aluminum ethyl phosphinate (ADP), supplemented with MPP, etc. can play a synergistic and cost-reducing role, and there are also compound flame-retardant PA products with magnesium hydroxide and aluminum hydroxide.

To sum up, in the current flame retardant system of engineering plastics, flame retardant PC/ABS and flame retardant PC mainly use phosphorus BDP; flame retardant ABS, flame retardant HIPS, flame retardant PBT mainly use bromine; flame retardant PA has both bromine and phosphorus nitrogen.

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