Flame Retardancy, Hydrolysis Resistance, and Mechanical Properties of Cluster‐Like Cage‐Shaped Phosphate Ester in Thermoplastic Polyurethane Elastomer

Most phosphorus‐containing flame retardants currently used in thermoplastic polyurethane (TPU) suffer from poor compatibility with the TPU matrix and the release of hydrolytic acids, significantly limiting their application in high‐end industrial fields. To address these limitations, a novel cluster‐like cage‐shaped phosphate ester (CCPE) was synthesized via an addition reaction. A TPU composite incorporating 15 wt.% CCPE achieved a UL‐94 V‐0 rating at both 3.2 and 1.6 mm thicknesses and exhibited a limiting oxygen index (LOI) of 27.6%. Furthermore, the 15CCPE/TPU composite demonstrated lower total heat release and peak heat release rate compared to TPU containing a non‐cluster‐like flame retardant (PEPA). Remarkably, the water contact angle of 15CCPE/TPU reached 106.8°, significantly higher than that of PEPA/TPU (63.7°), indicating the excellent hydrophobicity of CCPE. Even after 192 h of immersion in hot water at 85°C, 15CCPE/TPU maintained high flame retardancy and satisfactory mechanical properties, demonstrating superior water resistance. In terms of mechanical performance, the tensile stress at 300% strain for 15CCPE/TPU increased by 35.1% compared to PEPA/TPU. Additionally, CCPE imparted excellent optical transparency to the composite. Overall, the unique cluster‐like cage‐shaped phosphate ester structure endows TPU with outstanding flame retardancy, hydrolytic stability, mechanical strength, and optical clarity.