How much influence do compatibilizer, content and process conditions have on the mechanical properties of PP materials?

Polypropylene (PP), one of the top five general-purpose resins globally, is valued for its low density, abundant availability, affordability, excellent mechanical properties, and chemical stability. However, its poor notch impact performance limits its application.

Blending modification is an effective way to enhance the mechanical strength of PP, with material performance largely influenced by interfacial bonding and the crystallinity of the polymer matrix.

Existing research

1. The effects of maleic anhydride grafted polypropylene (PP-g-MAH), acrylic acid grafted polypropylene (PP-g-AA), and organosilane coupling agents on the interface and mechanical properties of polypropylene/short glass fiber (PP/SGF) composites were investigated. Results showed that PP-g-AA reduced PP grain size but had limited impact on mechanical properties, while PP-g-MAH provided better toughening for the PP matrix.

2. The influence of PP molecular weight in PP-g-MAH on interfacial bonding strength with glass fiber (GF) was examined. Higher molecular weight in the compatibilizer improved bonding strength due to better entanglement of longer molecular chains with the PP matrix.

3. Hydrogenated copolymer (SEBS) was used to toughen PP. To enhance phase compatibility, PP-g-MAH or maleic anhydride grafted SEBS (SEBS-g-MAH) were added. MAH effectively improved bonding strength between SGF, PP, and SEBS, resulting in high-impact strength for mPP/SEBS/SGF composites.

These studies highlight that PP-g-MAH outperforms PP-g-AA in toughening PP, mainly due to better bonding with glass fibers. A higher MAH grafting rate enhances the compatibilizer's toughening effect. However, further evaluation of different PP-g-MAH brands from various manufacturers is necessary to optimize formulations and mechanical properties through systematic analysis.

Research Results and Conclusions

This study examines the effects of compatibilizer types, content, and process conditions on the mechanical properties of PP materials.

1.Compatibilizer Analysis

The interfacial bonding effect of the compatibilizer is critical for glass fiber-reinforced resins. PP-g-MAH combines with PP through physical entanglement, which improves with higher molecular weight due to longer polymer chains. The maleic anhydride grafting rate directly affects its reaction with glass fibers, enhancing bonding strength.

System fluidity, measured via melt flow rate (MFR), also influences bonding. Poor fluidity reduces wetting between resin, fibers, and compatibilizer, weakening mechanical properties. In this study, MFR serves as an indicator of molecular weight, with details provided in Table 2.

2.Effect of compatibilizer type on overall mechanical properties

Figure 1 is a bar chart of the comprehensive mechanical properties of GF/PP materials without compatibilizer and modified with different compatibilizers. It can be seen from the figure that compared with the GF/PP materials modified with compatibilizer and without compatibilizer, the overall mechanical properties of the GF/PP materials with compatibilizer are greatly improved, especially the notched impact strength.

The PC-3 compatibilizer significantly enhances mechanical properties, with tensile strength increasing by 26.33%, flexural strength by 42.41% (to 135 MPa), and notched impact strength nearly doubling to 16.1 KJ/m² (compared to 8 KJ/m² for unmodified GF/PP). PC-1 also improves tensile and flexural strength to 90 MPa and 122 MPa, respectively, but performs less effectively than PC-3.

While PC-3 and PC-1 have similar melt flow rates, PC-3 has a higher anhydride grafting rate, likely explaining its superior interfacial bonding and mechanical performance. AD-105 and 1001CN, with grafting rates close to PC-1 but lower melt flow rates (higher viscosity), struggle with wetting the high-viscosity PP matrix, affecting bonding.

Overall, selecting a compatibilizer requires balancing molecular entanglement, grafting rate, and system wetting ability.

3.SEM structural characterization

In order to further reveal the influence of different compatibilizers on the macroscopic mechanical properties of GF/PP modified materials, the surface morphology of GF/PP modified materials was observed by scanning electron microscopy (Figure 2), and the number of glass fiber pullouts was counted. The results are shown in Table 3.

The stronger the bond between glass fibers and PP, the better the material transmits force to the fibers, causing them to break or pull out. A weaker bond results in easier fiber pull-out. Table 2 shows that compatibilizers significantly reduce the number of pull-out holes, indicating improved bonding. Among them, PC-3 performs best, with only 24 pull-out holes, reflecting superior fiber-PP bonding and enhanced reinforcement.

SEM images (Figure 2) further confirm this, showing that PC-3 forms the best interface between PP and fibers, achieving complete coating and leaving more residual PP on pulled-out fibers. This demonstrates PC-3’s strong binding effect, optimizing fiber reinforcement.

These findings confirm that grafting rate significantly influences bonding and mechanical properties. Higher grafting rates enhance bonding, while molecular weight affects both physical entanglement and fluidity, requiring system-specific analysis.

4.Effect of compatibilizer content on overall mechanical properties

Using PC-3 as a compatibilizer, the effect of its content on GF/PP mechanical properties was studied. Without a compatibilizer, the material showed a flexural strength of 94.8 MPa, tensile strength of 76.78 MPa, and notched impact strength of 8 KJ/m². As shown in Figure 3, increasing the compatibilizer content initially improves mechanical properties, peaking at 3%, where it fully interacts with PP and glass fibers, before declining at higher concentrations.

When the content of compatibilizer continues to increase, the bending strength and tensile strength of the compatibilizer itself are not as good as those of PP, and too many anhydride groups cannot effectively react with related molecular structures to produce enrichment, which leads to the reduction of the mechanical properties of the overall material. Therefore, through the above experiments, it can be concluded that the most appropriate proportion of adding compatibilizer is 3%.

5.Effect of process parameters on mechanical properties of materials

In the co-rotating twin-screw extrusion process, shear rate and residence time significantly impact the mechanical properties of the material. Using PC-3 as the compatibilizer, the extrusion temperature profile was set to 80/240/240/240/230/210/200/200/200/220℃.

After optimizing the formula, the study examined how process parameters affect mechanical properties, focusing on glass fiber retention length. Longer retention improves reinforcement. Table 4 shows that at a fixed feed of 80 kg/h, increasing the screw speed initially improves tensile, impact, and flexural strength, peaking at 350 rpm, where shear rate is balanced with residence time.

At a fixed speed of 350 rpm, increasing the feed rate shows a similar trend, with optimal performance at 100 kg/h. This is due to the balance between shear rate, material filling, and residence time, which maximizes fiber retention length and enhances mechanical properties.

Conclusion

1. Among the four compatibilizers, PC-3 achieves the best mechanical properties for GF/PP, with tensile strength at 97 MPa, flexural strength at 135 MPa, and notched impact strength at 16.1 KJ/m².

2. All compatibilizers enhance GF/PP properties, with overall performance ranked as PC-3 > PC-1 > 1001CN > AD105. The optimal PC-3 content is 3%, achieving the best mechanical results.

3. For process conditions, the optimal settings are a rotation speed of 350 rpm and a feed rate of 100 kg/h, yielding the best mechanical properties and the longest fiber retention length of 524 μm.