Flame Retardancy and Hydrophobicity Enhancement for HDPE Containing an Organic–Inorganic Hybrid P–N–Si–Cu Cooperative Flame-Retardant System

High-density polyethylene (HDPE) has been widely used as packaging and protection materials owing to its electrical insulation and mechanical strength. However, HDPE is flammable and requires flame-retardant modification. The addition of flame retardant leads to deteriorated compatibility of blends, reducing hydrophobic performance. Balancing the flame retardancy and hydrophobicity of the blend is challenging. Ammonium polyphosphate (APP) was modified by sequential loading of Cu-MOF followed by silane coupling agent treatment to prepare Si-Cu-APP, and lignin (Lig) was modified via piperazine-formaldehyde to obtain lignin-based organic skeleton (Pip-Lig). The Si-Cu-APP/Pip-Lig system achieves P–N–Si–Cu quaternary cooperative flame-retardant effect. while this system reduces the intramolecular hydroxyl content and weakens the polarity of the system. It makes the surface energy of the blend lower and improves its internal dispersion, forming an efficient hydrophobic protective layer. At a total additive loading of 30 wt%, the blend exhibits an LOI of 28.5%. The peak heat release rate (PHRR) decreases by 68.45%, and the total smoke production (TSP) decreases by 26.52%. Its water contact angle rises from 91.58° to 124.24°, water absorption decreases by 72%, and flame-retardant leaching is only 0.11%. This work provides an effective strategy for fabricating HDPE with simultaneously enhanced flame retardancy and hydrophobicity.