Flexible Polyurethane Foam with Durable Flame Retardancy and Mechanical Properties Based on Stable Hydrophobic Protection and Cross-Linking

Flexible polyurethane foam (FPUF), with low cross-linking density and open-cell structure, is intrinsically hydrophilic and prone to structural collapse. This inherent vulnerability is exacerbated by the incorporation of hydrophilic flame retardants like melamine (MEL), rendering FPUF vulnerable to water erosion and chemical attack, with a rapid loss of both flame retardancy and mechanical integrity. To address these challenges, a surfactant (F4) with both hydrophobicity and flame retardancy was prepared, which not only could be used as a foam stabilizer in FPUF but also can serve as a durable hydrophobic shield to the foam surface, owing to the low surface energy of its fluorinated groups and hydrogen bonds formed by phosphorus in surfactant F4. Furthermore, blocked isocyanate (BI), as a post-cross-linker, enhances chemical cross-linking density to improve water resistance, effectively reinforcing the FPUF structure while preserving its open-cell architecture. Therefore, with the combined effect of stable hydrophobic protection and enhanced chemical cross-linking, 1F4–30MEL-3BI-FPUF, with just 0.53 wt % of F4, demonstrates optimal comprehensive performance in flame retardancy (limiting oxygen index = 26.5%), hydrophobicity (water contact angle = 122.5°), and mechanical properties (compressive strength = 14.8 kPa), as well as good absorption capability and excellent oil–water separation. Notably, the retention of these properties after 24 h of immersion in water, strong acid, strong alkali, saturated NaCl, and EtOH confirms its exceptional short-term stability under harsh conditions. This study proposes a straightforward and scalable strategy for fabricating high-performance FPUF.