Properties of Glass Fiber Reinforced Flame Retardant PBT

Polybutylene terephthalate (PBT) is a thermoplastic polyester and one of the top five engineering plastics. PBT has excellent comprehensive properties and is one of the toughest engineering plastics. It has high dimensional stability, good chemical resistance, excellent electrical insulation, good mechanical properties and elasticity, and low water absorption. However, its heat resistance is not as good as that of polyethylene terephthalate (PET). PBT is extremely flammable, melts and drips during combustion and the flame spreads quickly. Therefore, the research on the flame retardant properties of PBT is particularly important.

In the process of synthesizing melamine cyanurate, aluminum hypophosphite was coated with cyanuric acid and melamine, and surface treated with aluminum hydroxide and coupling agent (KH-560) to obtain melamine cyanurate coated aluminum hypophosphite flame retardant. In the process of preparing flame retardant PBT products, the dosage of polyethylene (PE) resin, calcium stearate and anti-dripping agent is adjusted to improve processing flow properties and mechanical properties. Finally, under the condition of less flame retardant added, a 30% glass fiber (GF) reinforced PBT flame retardant material with flame retardant and excellent mechanical properties is obtained.

Sample preparation

(1) add 150 parts of water, 85 parts of inorganic aluminum hypophosphite, 7.5 parts of cyanuric acid successively in the reactor, open steam heating, open stirring, after the temperature of the system rises to 95°C, 7.3 parts of melamine are added, and after the constant temperature reaction is completed, the stirring is stopped; Cool down, settle, remove the supernatant, wash and dry, put it into a high-speed mixer, add 0.3 parts of aluminum hydroxide, 0.06 parts of KH-560, start the low-speed gear, after 3 minutes, the high-speed gear was started, and the material was discharged after 6 minutes to obtain a melamine cyanurate-coated aluminum hypophosphite flame retardant, that is, a halogen-free flame retardant for PBT.

(2) Add 10 parts of decabromodiphenylethane and 5 parts of antimony trioxide into the high-speed mixer, start the low-speed gear, start the high-speed gear after 3 minutes, and discharge after 6 minutes to obtain a bromine-antimony flame retardant.

(3) Blow dry the PBT pellets at (105±5) ℃ for 2 h, and mix them with the special flame retardant and bromine-antimony flame retardant respectively according to the proportions. The mixed materials are extruded and granulated by an extruder, and chopped GF is added from the side feeding port. The temperature of each zone of the extruder is: 225°C for the first zone, 228°C for the second zone, 230°C for the third zone, 230°C for the fourth zone, 228°C for the fifth zone, and 225°C for the sixth zone. The head temperature was 225℃, the screw speed was 260 r/min, the side feeding speed was 23 r/min, and the mass fraction of chopped GF was controlled to be 30%.

Comparison of flame retardant properties of special flame retardant and brominated antimony flame retardant PBT

The formulas of 30% GF reinforced special flame retardant PBT and bromine antimony flame retardant PBT are shown in Table 1, and the corresponding performance comparison is shown in Table 2. It can be seen from Table 2 that when the mass fraction of special flame retardants added is 13%, or the mass fraction of bromine-antimony flame retardants is 15%, the vertical combustion grade of the prepared flame retardant PBT reaches UL94 V-0 level, which can pass the 960 ℃ glow wire ignition temperature test, so the special flame retardant can completely replace the bromine antimony flame retardant to achieve the flame retardant effect. The tensile strength, flexural strength and flexural elastic modulus of the flame retardant PBT with the special flame retardant are almost the same as the test data of the flame retardant PBT with the bromine antimony flame retardant. When the amount of flame retardant is reduced to 13%, the mechanical properties of the special flame retardant PBT can completely reach the mechanical properties of the bromine-antimony flame retardant PBT. In addition, the CTI of the flame-retardant PBT with the special flame retardant can reach 600 V, while the CTI of the flame-retardant PBT with the bromine-antimony flame retardant is less than 200 V. The thermal deformation temperature of the two flame-retardant PBT is not much different. Therefore, the electrical and thermal properties of PBT using special flame retardants can fully meet the requirements of traditional bromine-antimony flame-retardant products.

Effect of PE Resin Addition on the Properties of Flame Retardant PBT

The flame retardant PBT formulations with different PE additions are shown in Table 3, and the corresponding properties are shown in Table 4. The flame retardant added in the GF-enhanced PBT formulation and the poor encapsulation between GF and PBT pellets have a certain impact on its processing. Therefore, PE is added to the formulation to improve the processing fluidity. It can be seen from Table 4 that adding a small amount of PE (0.5%, 1.5%) has no effect on the vertical combustion grade of PBT, and has little effect on the mechanical properties. However, when the mass fraction of PE added is 3%, the vertical combustion level of flame retardant PBT can only reach UL94 V-2 level. This is mainly because PE can improve the fluidity of the substrate. Too much addition will cause the sample to drip during combustion, and the molten droplets that are not extinguished will ignite the absorbent cotton. Therefore, considering the flame retardant performance, the amount of PE added should not be too much, with a mass fraction of 1.5% being appropriate.

Effect of calcium stearate addition on the properties of flame retardant PBT

The flame retardant PBT formulations with different additions of calcium stearate are shown in Table 5, and the corresponding properties are shown in Table 6. The flame retardant added in the formula of GF reinforced PBT and the compatibility between GF and PBT are poor, and the friction force is large during the processing, which has a certain impact on the equipment. Calcium stearate is an excellent lubricant, so calcium stearate is added to the formula to improve processing fluidity. It can be seen from Table 6 that the addition of a small amount of calcium stearate (0.2%, 0.4%) has no effect on the vertical combustion grade of PBT, and has little effect on the mechanical properties.

However, when the mass fraction of calcium stearate is 0.6%, the vertical combustion grade of flame retardant PBT can only reach UL94 V-2 grade. This is mainly because calcium stearate improves the fluidity of the substrate. Too much addition will cause the sample to form dripping phenomenon during combustion, and the molten droplets that are not extinguished will ignite the absorbent cotton. Therefore, considering the vertical combustion performance, the addition amount of calcium stearate should not be too much, and the appropriate mass fraction is 0.4%.

Effect of Anti-dripping Agent Addition on the Properties of Flame Retardant PBT

The flame retardant PBT formulations with different anti-drip agent additions are shown in Table 7, and the corresponding properties are shown in Table 8. GF-enhanced PBT is easy to drip and ignite absorbent cotton during the combustion process and reduce the vertical combustion level. Therefore, a small amount of anti-drip agent is often added to PBT to suppress the phenomenon of molten droplets igniting absorbent cotton during PBT combustion. It can be seen from Table 8 that in the formula without anti-drip agent, the mass fraction of special flame retardant added is 15%, When the mass fraction of anti-drip agent is 0.3% and 0.5%, the added mass fraction of special flame retardant is 13%, 12%, Adding a small amount of anti-drip agent can reduce the amount of flame retardant added, and adding a small amount of anti-drip agent has no effect on the vertical combustion grade of flame retardant PBT, and has little effect on the mechanical properties. According to the comprehensive formula ratio, adding an anti-drip agent with a mass fraction of 0.3% to the flame retardant PBT can reduce the added mass fraction of the flame retardant by 2%.

Conclusion

Using cyanuric acid and melamine to coat aluminum hypophosphite in the process of synthesizing melamine cyanurate, and use aluminum hydroxide and coupling agent for surface treatment to obtain melamine cyanurate-coated aluminum hypophosphite flame retardant . The flame retardancy of PBT can reach UL94 V–0 (1.6 mm) at lower dosages by applying flame retardants to PBT substrates. In the process of preparing PBT, adding 1.5% PE resin and 0.4% calcium stearate can improve the processing fluidity of PBT, and adding 0.3% anti-drip agent can reduce the 2% flame retardant. Finally, under the condition of less flame retardant added, a 30% GF reinforced PBT flame retardant material with excellent comprehensive properties such as flame retardant and mechanical properties was obtained.

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