In modern industrial fields, glass fiber reinforced polypropylene (PP/GF), with its low density, excellent heat and creep resistance, and high cost-performance ratio, has become a “rising star” in industries such as electronics, aerospace, and automotive manufacturing. This material is often used to produce lightweight and thin-walled components as a substitute for steel and conventional engineering plastics.

However, polypropylene itself is a flammable material, with a limiting oxygen index (LOI) of only about 17.0%. During combustion, it generates a large amount of flaming drips and releases significant heat. Although the addition of glass fiber (GF) alleviates the dripping phenomenon to some extent, the so-called “wick effect” of GF extends the burning time and increases the heat release. Therefore, in safety-critical applications, flame retardant treatment of PP/GF is indispensable.

It is worth noting that the once widely used bromine–antimony flame retardant system has been restricted by regulations both domestically and internationally, due to the toxic smoke released during combustion. For example, decabromodiphenyl ether and other brominated flame retardants have already been banned.

As an alternative, halogen-free phosphorus–nitrogen intumescent flame retardant systems are gaining attention in the polyolefin field, thanks to their environmental friendliness and cost advantages. Piperazine pyrophosphate (PAPP), for instance, contains phosphorus and nitrogen elements as well as abundant hydroxyl groups, allowing it to act simultaneously as the “acid source” and “carbon source” in an intumescent flame retardant system. In this study, PAPP was compounded with melamine polyphosphate (MPP) to form a PAPP-based intumescent flame retardant. Under a constant loading of flame retardant, the influence of glass fiber content on the performance of PP/GF composites was systematically investigated.

How Does GF Content Influence Material Performance?

1. Significant Improvement in Flame Retardancy (GF < 30%)
As the glass fiber (GF) content increases, the flame retardant performance of PP/GF composites improves. On the one hand, a higher GF content means a lower proportion of PP matrix, which reduces the generation of flammable fragments during thermal decomposition. At the same time, GF lowers the melt flow rate, effectively alleviating dripping issues in thin samples and enabling the material to more easily pass vertical burning tests. On the other hand, the carbon layer formed by the flame retardant through the “solid-phase char formation” mechanism can tightly cover the specimen surface without being disrupted by high-temperature GF residues, thereby isolating heat and oxygen and reducing the release of combustible volatiles.

2. Changes in Thermal Stability
The incorporation of GF into polymer materials can effectively optimize multiple physical properties. On the one hand, GF significantly enhances the dimensional stability of the composite, making it less prone to deformation under varying environmental conditions. On the other hand, the heat distortion temperature (HDT) of the material is notably increased, thereby greatly improving its high-temperature resistance. The incorporation of GF alters the thermal stability of the material. Although it lowers the initial thermal decomposition temperature of flame-retardant PP/GF composites, it significantly enhances their stability at high temperatures. Experimental data show that when the GF content increases to 25%, Sample #4 achieves a char residue rate of 39.4% at 700 °C. This indicates that at elevated temperatures, the release of combustible gases is greatly reduced, while more non-combustible solid carbides are formed. In an air atmosphere, due to thermo-oxidative degradation, the initial decomposition temperature is lower than that under a nitrogen atmosphere. However, at high temperatures, the char residue of samples with varying GF contents remains higher than that of GF-free samples, which can be attributed to the inherent high-temperature stability of GF, as it is less prone to decomposition.

3. Dual Effect on Combustion Performance
Under external heat radiation, the flame retardant FR-1420 forms an insulating char layer on the specimen surface through intumescent charring. Experimental results show that for Sample #1 without GF, the char layer expanded to a thickness of about 2.5 cm, while the char layer thickness of Sample #2 with 15% GF increased to approximately 6.2 cm. However, when the GF content was further increased, the char layer thickness decreased to about 5.0 cm (Sample #4). This phenomenon can be explained by the high thermal stability of GF: it can serve as a “char skeleton” that supports char expansion, but excessive GF residue at high temperatures hinders further char growth.

It is noteworthy that the incorporation of GF did not affect the key combustion parameters of the material, such as the peak heat release rate (PHRR), indicating that the overall fire safety performance remained satisfactory. Furthermore, due to the inert nature of GF and the reduced proportion of PP matrix, the release of flammable volatiles during combustion was diminished, while more non-combustible solid residues were retained at elevated temperatures. As shown by the mass–time curve, samples with GF exhibited higher residual mass at high temperatures, with lower heat release and smoke generation. Fire safety indices such as the Fire Growth Rate Index (FIGRA) and the Maximum Average Heat Release Rate (MAHRE) showed no significant change.

Conclusions

The halogen-free flame retardant FR-1420 demonstrates remarkable flame-retardant effectiveness in PP/GF composites. At the same flame retardant loading, higher GF content leads to better flame retardancy.

While GF decreases the initial thermal decomposition temperature, it enhances thermal stability at high temperatures.

In cone calorimetry tests, GF acts as a “char skeleton,” increasing char expansion thickness while reducing the total heat release (THR) and total smoke production (TSP), thereby significantly improving the fire safety performance of PP/GF composites.