A halogen-free, flame retardant, waterborne polyurethane coating based on the synergistic effect of phosphorus and silicon

Abstract In this study, p-hydroxybenzaldehyde, 3-aminopropanol and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) were used as raw materials, and a novel flame retardant with dihydroxy groups, namely 4-DOPO-((3-hydroxypropyl) imino) methyl) phenol (PHID), was synthesized. Then, a flame retardant, water-based polyurethane with phosphorus (P) and silicon (Si) units (FR/Si-WPU) was successfully prepared. Limited oxygen index (LOI), vertical burning tests, and cone calorimetry were employed to investigate the flammability characterizations of the WPU. In contrast to pure WPU, the LOI value of FR/Si-WPU was found to be increased from 18.2% to 28.6%, the UL 94 rating was upgraded to V-0. The peak heat release rate (pk-HRR) and peak smoke production rate (pk-SPR) were found to be decreased by 80.5% and 70.5%, respectively. In addition, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), thermogravimetric analysis-infrared spectrometry (TG-IR), laser Raman spectroscopy (LRS), and X-ray photoelectron spectroscopy (XPS) were employed to evaluate the synergistic mechanisms of P and Si. The results demonstrated that the synergistic effect of PHID with hydroxyl silicone oil (HO-Si) can generate a more compact char to protect the underlying polyurethane, the decomposition of PHID can produce PO· radicals that combine with H· and OH· radicals to interrupt the combustion reaction, and HO-Si can produce a special insulating Si O layer to improve the thermo oxidative stability. The synergistic effects of these factors contribute to the designable flammability of the WPU coating. Further investigation revealed that Si units can efficiently balance the other properties of the WPU, and the cold resistance and mechanical properties of FR/Si-WPU were found to increase instead of decrease. In this work, a flame retardant WPU coating with good comprehensive performance was fabricated via a simple and efficient method.