Flame retardant mechanism of hyperbranched polyurethane acrylates used for UV curable flame retardant coatings

A hyperbranched polyurethane acrylate containing phosphorus was found to display flame retardance with a limiting oxygen index of 27.0 after being UV cured. The retardant mechanism was investigated by observing the site where retardance take places and the synergistic effect of phosphorus and nitrogen. It is shown that the best synergistic effect happens when the phosphorus content is around 0.7 wt.%. The thermo-degradation process was monitored by in-situ FTIR. The P-O-C bonds break more easily than C-O-C bonds, forming P-O-P bonds. The morphological characteristics of the charred crust of the product were studied by scanning electron microscopy based on comparison with that of the urethane acrylate without phosphorus. The presence of phosphorus promotes the formation of compact char, which protects the underlying polymeric materials from further attack from flame or heating. (C) 2002 Elsevier Science Ltd. All rights reserved.