Flame Retardancy and Smoke Suppression of Rigid Polyurethane Foam Based on a Synergistic Expansion Composite System with Modified Aluminum Hydroxide

To develop an eco-friendly and efficient flame-retardant system for rigid polyurethane foam (RPUF), this study utilizes a silane coupling agent (KH-550) to modify the aluminum hydroxide (ATH), synthesizing modified aluminum hydroxide (MATH). MATH is combined with an intumescent flame retardant system (CME), consisting of melamine salt of 3-hydroxyphenylphosphinylpropionic acid (CMA), modified ammonium polyphosphate (MAPP), and expandable graphite (EG), to enhance the flame retardancy of RPUF. The impacts of MATH and CME on the structure, mechanical properties, limiting oxygen index (LOI), smoke density, thermal stability, and flame-retardant performance of RPUF were systematically investigated, and the synergistic mechanism of MATH with CME was elucidated. Results demonstrated that the addition of MATH improved the foaming process of RPUF, enhancing cell structure, size distribution, and physical properties. The LOI of RPUF-(2%MATH, 18%CME) reached 34.1%, increasing by 14.9 units compared to pure RPUF, and the char residue at 700 degrees C was 30.2%, 1.59 times higher than pure RPUF. The synergistic flame-retardant mechanism involves the physical barrier effect of EG, which expands upon heating to form a "worm-like" char layer, and the gas-phase and condensed-phase flame-retardant effects of MAPP decomposition. This study offers a feasible approach for the development of eco-friendly and high-performance flame-retardant RPUF technologies.