PP and PE can be blended, but adding PE increases the difficulty of flame retardancy. Currently, antimony bromide-type flame retardants are commonly used to improve the LOI (Low Impact Index). Below -18℃, the improvement trend of the three types of PE on PP toughness is consistent with that at room temperature, but LLDPE has a better toughening effect on PP. To achieve the same impact strength, a lower LLDPE dosage means more PP rigidity can be maintained; and at the same dosage, LLDPE-modified PP has better impact strength, which in turn gives the material superior toughness. As the PE content increases, the two-phase interface gradually becomes blurred. When the material fractures, the fracture surface exhibits fibrosis and microporous structures, indicating that the fracture mode changes from brittle fracture to ductile fracture that can absorb more energy.

1. PP & LLDPE Blends

When the mass fraction of LDPE is greater than 40%, its impact strength is significantly improved. When the LLDPE mass fraction reaches 70%, the impact strength of the blend is 37.5 kJ/m², which is 20 times that of pure PP, and 10 times and 4 times that of PP/HDPE and PP&LDPE blends with the same amount, respectively.

2. PP&HDPE Blending: When the HDPE mass fraction is below 60%, the blend strength remains essentially unchanged; the impact strength only increases when the HDPE mass fraction is above 60%.

3. PP&LDPE Blending: A significant increase in impact strength only occurs when the LDPE mass fraction is above 60%.

4. To address the incompatibility issue between PP and LDPE, compatibilizers can be added to improve interfacial bonding, a key method for enhancing impact performance. Adding EEBE compatibilizer reduces the size of the PP dispersed phase from 4.24 μm to 1.62 μm, resulting in a finer and more stable phase structure, thus significantly improving impact strength, especially low-temperature impact strength, which increases dramatically near -40℃.

When the PP & LLDPE mass ratio is 30/70, the impact strength of the blend system is 23.2 kJ/m², which is 20 times that of pure PP. Under the same conditions, the impact strength of PP & HDPE and PP & LDPE blend systems is only about 5 kJ/m². Generally, when HDPE content is below 60%, the blend strength does not change significantly; only when it exceeds 60% does the impact strength increase significantly. LLDPE has moderate branching and good dispersibility, resulting in optimal toughening. The impact strength remains essentially unchanged at low PE content; it increases significantly above the threshold. PP and PE are only partially compatible; a high PE content is needed to form an effective toughening network. Strong shear (twin-screw extruder) facilitates the fine dispersion of PE, improving toughness. Adding PP-g-MA, POE, etc., can lower the toughening threshold and improve interfacial bonding.