The Influence of Glass Fiber Content on The Flame Retardant Performance of Halogen-Free Flame Retardant Glass Fiber Reinforced Polypropylene

The Influence of Glass Fiber Content on the Flame Retardant Performance of Halogen-Free Flame Retardant Glass Fiber Reinforced Polypropylene

Glass fiber-reinforced polypropylene (PP/GF) is characterized by its low density, excellent heat resistance and creep resistance, and high cost-performance ratio. It is widely used in industries such as electronics, aerospace, and automotive to manufacture lightweight and thin-walled components as a substitute for steel and engineering plastics.

The limiting oxygen index (LOI) of PP is about 17.0%, classifying it as a flammable material. When it burns, it produces a large number of flaming melt drops and releases a significant amount of heat. Although the addition of GF significantly suppresses the melt drop phenomenon, the "wick effect" of GF leads to longer combustion duration and greater heat release. In demanding application fields, it is necessary to treat PP/GF with flame retardants. In recent years, some bromine-antimony flame retardant systems have produced toxic fumes upon combustion, leading to the prohibition of certain bromine-based flame retardants, such as decabromodiphenyl ether, by relevant domestic and international laws and regulations.

The phosphorus-nitrogen intumescent, environmentally friendly, halogen-free flame retardant system has the advantages of being eco-friendly and cost-effective, and has been practically applied in the field of polyolefin materials. For example, piperazine diphosphoric acid (PAPP) contains phosphorus and nitrogen elements and has a relatively high number of hydroxyl groups, which can serve as both the "acid source" and "char source" in the intumescent flame retardant system. This paper investigates the impact of GF content on the properties of PP/GF materials by formulating an intumescent flame retardant with PAPP and MPP, while keeping the amount of flame retardant constant.

I. The Impact of Glass Fiber (GF) Content on Flame Retardancy

From Figures 1 and 2, it can be observed that the samples' surfaces formed an expanded carbon layer during both the LOI (Limiting Oxygen Index) test and the vertical burning test, indicating that the flame retardant functions by solid-phase expansion and char formation.

When the content of the flame retardant is constant, an increase in GF content enhances the flame retardancy of the PP/GF material (when GF is less than 30%).

This is because, on one hand, as the GF content increases, the relative amount of the matrix PP decreases, leading to a reduction in the amount of combustible fragments produced by the pyrolysis of the matrix during the LOI test. Additionally, with the increase in GF content, the melt flow rate of the material decreases, which is beneficial for improving the dripping phenomenon in thinner samples, making it easier for them to pass the vertical burning test. On the other hand, it is due to the flame retardant's "solid-phase carbonization" mechanism, where the carbon layer formed effectively encapsulates the surface of the sample and is not "punctured" by the high-temperature residues of the GF. The resulting insulating and oxygen-isolating protective layer reduces the escape of combustible materials, providing better flame retardant effects.

II. The Impact of Glass Fiber (GF) Content on Thermal Stability

The incorporation of GF into polymer materials enhances various physical properties such as dimensional stability and heat deflection temperature. TG (Thermogravimetric) analysis can provide thermal decomposition parameters of polymer materials under both oxygen-free and oxygen-containing conditions, which is crucial for studying the flame retardant mechanisms of materials.

Upon the addition of GF, the char yield at high temperatures increases with the increase in GF content. When the GF content is increased to 25%, the char yield of sample 4 at 700°C increases to 39.4%, indicating that the sample produces less combustible gas at high temperatures, primarily consisting of non-combustible solid carbonaceous residues. From Table 3 and Figure 4, it can be observed that under air atmosphere, the material undergoes thermo-oxidative degradation under the influence of oxygen. The initial decomposition temperature of the comparative sample 1 under air is reduced to 306.4°C compared to the nitrogen atmosphere, and the initial decomposition temperatures of samples 2 to 4 are also reduced to around 298°C under air, with the main weight loss interval being between 300 and 500°C. The early stage involves the thermo-oxidative decomposition of the matrix PP along with the decomposition and weight loss of the flame retardant. At high temperatures, the carbon layer further undergoes oxidative decomposition in the presence of oxygen. At the high-temperature stage, the char yield of samples with different GF contents is higher than that of sample 1, mainly because GF is not easily decomposed at high temperatures.

In summary, the addition of GF lowers the initial thermal decomposition temperature of the flame-retardant PP/GF material and enhances its thermal stability at high temperatures.

III. The Impact of Glass Fiber (GF) Content on Combustion Performance

Under the influence of external heat radiation, the flame retardant forms an insulating protective layer by expanding into char on the surface of the samples. After the test, the expanded char layer thickness of the comparative sample 1 was approximately 2.5 cm. In comparison, the sample 2 with 15% GF had an expanded char layer thickness increased to about 6.2 cm.

As the GF content increases, the thickness of the expanded char layer decreases, with the sample 4 showing a reduced expanded char layer thickness to about 5.0 cm. This is because, on one hand, GF is relatively stable at high temperatures, and the high-temperature residues of GF act as a "char skeleton" during the expansion of the char layer, thus promoting an increase in the thickness of the expanded char layer upon the addition of GF. On the other hand, when the GF content is high, the greater amount of high-temperature GF residues also suppresses the expansion of the char layer, leading to a gradual decrease in the thickness of the expanded char layer.

The addition of GF did not affect various combustion parameters of the material, such as the peak heat release rate (PHRR), indicating that the materials have good fire safety. Moreover, with the addition of GF as an inert fiber, the relative amount of the matrix PP decreases, leading to a reduction in gas-phase combustibles during combustion. At high temperatures, the solid phase consists of non-combustible residues, and the residue-time curve indicates that with the addition of GF, the residue at high temperatures is higher, resulting in lower combustible gas emissions, lower heat release during combustion, and lower smoke production. At the same time, the addition of GF did not affect various fire safety indices of the material, such as the fire growth rate index (FIGRA), which is the ratio of PHRR to the time to reach PHRR peak, and the maximum average heat radiation rate (MAHRE), none of which showed significant changes.

IV. Conclusion

In response to the market demand for halogen-free flame retardants for polypropylene (PP), Guangzhou YINSU Flame Retardant Company has actively invested in research and development, successfully launching a variety of halogen-free flame retardant products, which can achieve V0 and V2 ratings. These include intumescent flame retardants based on piperazine diphosphoric acid, red phosphorus flame retardants, and environmentally friendly low-halogen flame retardants. These products not only meet the market's demand for environmentally friendly and highly effective flame retardants but also demonstrate the company's technical strength and innovation capabilities in the field of flame retardants.

For instance, the company has developed the PPAP-15 series of halogen-free flame retardants, specifically designed for polyolefin products such as polypropylene (PP) and polyethylene (PE). These products exhibit excellent carbonization and flame retardant properties, capable of meeting the UL-94 V-0 standard. Additionally, the company's red phosphorus masterbatch PPV2-8H has been developed to address the challenging issue of flame retardancy in recycled PP. These products are characterized by their low addition, good dispersion effects, and compatibility with various materials. They can effectively enhance the flame retardancy of materials while maintaining their original physical and chemical properties.