The synthesis and properties of a reactive flame‐retardant unsaturated polyester resin from a phosphorus‐containing diacid

A transparent flame‐retardant unsaturated polyester resin (FR‐UPR) was obtained by reacting propylene glycol (PG) with maleic anhydride (MA), phthalic anhydride (PA), and 9,10‐dihydro‐10[2,3‐di(hydroxy carbonyl)propyl]‐10‐phosphaphenanthrene‐10‐oxide (DDP) synthesized from 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene 10‐oxide (DOPO) and itaconic acid (ITA). The chemical structure of the resulting FR‐UPR was confirmed by FTIR, 1H‐NMR and 31P‐NMR. The average molecular weight and viscosity of the FR‐UPR were determined by gel permeation chromatography (GPC) and viscometer, respectively. Thermal stability was studied by thermogravimetric analysis (TGA) both in air and nitrogen to determine the thermal decomposition mechanism, and the apparent activation energy (Ea) was calculated by both the Kissinger and Ozawa methods. Compared to unsaturated polyester resin (UPR), the higher Ea of FR‐UPR3 implied an improved thermal stability. According to variations of the limited oxygen index (LOI) values, the UL 94 rating of vertical burning test and scanning electron microscopy (SEM) photographs of char residues, the flame retardance of cured FR‐UPR was enhanced with increasing DDP content. The study of fire reaction tests, using a cone calorimeter, suggested that there was a significant reduction of flammability in the FR‐UPR. Copyright © 2010 John Wiley & Sons, Ltd.