Unsaturated polyester resins modified with phosphorus-containing groups: Effects on thermal properties and flammability Kang Dai a, b, c , Lei Song a , Saihua Jiang a, b, c , Bin Yu a, b, c , Wei Yang a , Richard K.K. Yuen b, ** , Yuan Hu a, c, * a State Key Laboratory of Fire Science, University of Science and Technology of China and USTC-CityU Joint Advanced Research Centre, Suzhou, PR China b Department of Civil and Architectural Engineering, City University of Hong Kong and USTC-CityU Joint Advanced Research Centre, Suzhou, PR China c Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute of University of Science and Technology of China, Suzhou, PR China article info Article history: Received 21 March 2013 Received in revised form 2 July 2013 Accepted 8 July 2013 Available online 17 July 2013 Keywords: Unsaturated polyester resin Thermal properties Flame retardancy Degradation mechanism abstract A novel reactive phosphorus-containing monomer [1-oxo-2,6,7-trioxa-1- phosphabicyclo-[2.2.2]octane- methyl diallyl phosphate, PDAP] was synthesized, and various amounts of PDAP were combined with unsaturated polyester by radical bulk polymerization. The resulting flame-retardant unsaturated poly- ester resin (FR-UPR) samples were investigated by thermogravimetric analysis (TGA), microscale com- bustion calorimetry (MCC), and limiting oxygen index (LOI) tests. Due to the relatively high phosphorus content of PDAP (18.2 wt%), incorporation of this monomer into unsaturated polyester resin (UPR) led to a marked decrease in the heat release capacity (HRC), the total heat release (THR), an increase in the LOI and the char yield upon combustion. In order to elaborate the interactions between the UPR and PDAP in degradation, differences between the experimental and theoretical mass losses of a FR-UPR sample were evaluated. Furthermore, thermogravimetry-Fourier transform infrared (TG-FTIR) and real-time Fourier transform infrared (RTIR) spectroscopy were employed to investigate the degradation behavior of UPRs, providing insight into the degradation mechanism. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction As representative thermosetting materials with prominent pro- cessability and low cost, unsaturated polyester resins (UPRs) have been extensively employed as the polymer matrices to produce fiber- reinforced composites and substitute for traditional materials in many modern engineering applications [1,2]. Generally, UPRs processing are based on a radical polymerization between unsaturated polyester chains and styrene. During the reaction, styrene plays a versatile role both as a diluent and a cross-linking agent. The rather high intrinsic flammability of styrene as well as the high content (ca. 35e40 wt %), however, gives rise to high fire risks in UPRs. Consequently, the drawback limits the application of UPRs to a great extent and must be overcome to reach these polymer materials’ full potential. To meet this challenge, amounts of flame retardants were utilized to enhance the fire resistance of UPRs, and substantial progress has been achieved by a variety of additive or reactive approaches over the past decades [3e 8]. While adding flame retardants into a polymer is a facile way, the unfavorable matrix-additive interfacial relationship will deteriorate the other properties of the polymer, in particular inducing a severe degradation of the mechanical properties [9]. In contrast, reactive flame retardant molecules demonstrate desirable characteristics of tailoring the properties of the material and optimizing its overall performance. Based on these conceptions, improved fire behavior as well as other properties in polystyrene [10,11], polyurethane [12], and epoxy resin [13e16] was reported in the literatures. From a sustain- ably developmental perspective, developing halogen-free flame re- tardants is a promising trend in both academic and industrial fields. Among numerous candidates, phosphorus-containing compounds have been the subject of intense research in recent years [17e22], owing to the environmental friendliness and efficacy in flame retardancy. Particularly, reactive phosphorus-containing flame re- tardants exhibit little or no compromise of the matrices’ intrinsic properties in addition to a low required composition to achieve prominent nonflammability. Furthermore, for oxygen-containing polymers, the chemically incorporated phosphorus-containing moi- eties can convert matrices fuel into carbonaceous char rather than * Corresponding author. State Key Laboratory of Fire Science, University of Sci- ence and Technology of China and USTC-CityU Joint Advanced Research Centre, Suzhou, PR China. Tel./fax: þ86 551 3601664. ** Corresponding author. Tel.: þ86 852 2788 7621. E-mail addresses: Richard.Yuen@cityu.edu.hk (R.K.K. Yuen), yuanhu@ ustc.edu.cn (Y. Hu). Contents lists available at ScienceDirect Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab 0141-3910/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2013.07.008 Polymer Degradation and Stability 98 (2013) 2033e2040