Thermal degradation behavior of the compound containing phosphaphenanthrene and phosphazene groups and its flame retardant mechanism on epoxy resin Lijun Qian * , Longjian Ye, Yong Qiu, Shuren Qu Department of Materials Science & Engineering, Beijing Technology and Business University, Beijing 100048, PR China article info Article history: Received 3 August 2011 Received in revised form 6 September 2011 Accepted 29 September 2011 Available online 6 October 2011 Keywords: Flame retardant Structure-effect DOPO abstract The flame retardant epoxy resin has been prepared by mixing the flame retardant additive hexa- (phosphaphenanthrene-hydroxyl-methyl-phenoxyl)-cyclotriphosphazene (HAP-DOPO) into diglycidyl ether of bisphenol-A (DGEBA). After cured by 4,4 0 -Diamino-diphenyl sulfone (DDS), the flame retardant properties of thermosets were characterized by the limited oxygen index (LOI), UL-94 test and cone calorimeter. The results show the lower peak of heat release rate (pk-HRR), the higher flammability rating than that of flame retardant epoxy resin by 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10- oxide (DOPO), hexa-phenoxyl-cyclotriphosphazene (HPCP) and their mixture cloning the ratio of group component of HAP-DOPO. The degradation route of HAP-DOPO was disclosed by thermogravi- metric analysis (TGA), the real time Fourier transform infrared spectra (FTIR), thermogravimetric anal- ysis/infrared spectrometry (TGA-FTIR), pyrolysis gas chromatography mass/spectrometry (Py-GC/MS). During combustion, HAP-DOPO continues to release the PO radicals and o-phenylphenoxyl radical during two degradation stages from 200  C for its unstable trisubstituted methyl structure of HAP-DOPO, inhibits the chain reaction of decomposition and exerts the flame retardant effect in gas phase. The phosphazene groups link with the residual phosphate from degraded phosphaphenanthrene, which increases the crosslink density of residue, effectively promotes the formation of high-strength, high-yield and phosphorus-rich char layer. The structure of HAP-DOPO shows a remarkable flame retardant molecular structure-effect on enhancing the flame retardant efficiency on thermosets. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Epoxy resins have been widely used as matrix resin for struc- tural compositions and electronic parts due to its ease of handling and processability, low shrinkage on cure, superior electrical and mechanical properties, and remarkable adhesion to many substrates [1e5]. However, the matrix resin is difficult to meet those high heat resistance and flame retardant fields and applica- tions. Many methods have been developed to improve its thermal stability and flame retardancy [6e10]. In recent years, 9,10-dihydro-9-oxa-10-phosphaphenanthrene- 10-oxide (DOPO) and its derivatives have received outstanding attention because of their high reactivity and applicability on epoxy resin [8]. The works reported for halogen-free flame retardant epoxy resins based on DOPO mainly include the multi-flame- retardant-group epoxy resins [11e 14], the novel structure epoxy resins [15,16], the high T g epoxy resin [17,18], epoxy resin compo- sition [19,20] and DOPO-modified curing agents [2,21,22]. Although a lot of researches have been carried out, some considerable question still need to be explored, such as how to increase flame retardant efficiency of the functional groups and that what kind of molecular structure of flame materials can contribute more to the flame retardant efficiency of materials. Therefore, it is necessary to explore the degradation route and the pyrolysis behavior of flame retardant materials [23,24]. Fortunately, a few works has been reported about the pyrolysis mechanism of DOPO-containing polymers [1,12,25,26]. Recently, we attempt to construct the additive molecule with double functional groups for obtaining flame retardant materials with high efficiency. We integrated phosphaphenanthrene and cyclotriphosphazene groups into one molecule as flame retardant additive instead of the reactive DOPO, phosphazene compounds and their derivatives in thermosets. It directly leads to a higher T g and perfect flame retardancy of thermosets with a lower addition [14]. However, the molecular degradation and flame retardant mechanisms are still undisclosed in the former research. * Corresponding author. Tel.: þ86 15810812389; fax: þ86 (0)10 68985531. E-mail address: qianbtbu@163.com (L. Qian). Contents lists available at SciVerse ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.09.053 Polymer 52 (2011) 5486e5493