Flame retardancy and mechanical properties of epoxy thermosets modified with a novel DOPO-based oligomer
Introduction
Epoxy resin, as one of the most prominent thermosetting polymers, possesses outstanding advantages such as low shrinkage, high tensile strength, good adhesion and insulation property, and excellent chemical corrosion resistance, which make it widely used in laminates, adhesives, surface coating materials, molding compounds, microelectronic materials, printed circuit boards and matrices for advanced fiber-reinforced composites [1]. However, just like other polymer materials, its highly flammable nature has severely restricted its further applications [2]. Therefore, it is imperative to develop flame retarded epoxy resin to broaden its applications in the fields requiring remarkable flame retardancy.
Over the past decades, halogenated compounds, either as reactive co-reactants or additives, have been widely utilized to develop epoxy resins with superior flame retardancy. Unfortunately, the use of such system is often accompanied by the release of corrosive or toxic gases during combustion, which could do harm to the environment [3], [4]. Therefore, many efforts have been dedicated to the flame-retardant modification of epoxy resins with halogen-free flame retardants such as boron- [3], [5], aluminum- [6], [7], silicon- [8], [9], phosphorus-containing compounds [10], [11] and nano-scaled fillers including clays [12], [13], layered double hydroxides [2], [14], graphene [15], [16] and MoS2 [17]. Thereinto, the organophosphorus-based system has attracted great attention from both industrial and academic research studies owning to its low toxicity during combustion and outstanding flame-retardant efficiency.
Recently, 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), as a phosphorus-based flame retardant, has received notable attention due to the multiple structural diversification by functionalization [4]. The active hydrogen of DOPO can react with a variety of electron-deficient compounds containing imine [18], [19], [20], [21], maleimide [22], [23], phosphazene [24], [25], silsesquioxane [26], [27], triazine [28], [29], triazine-trione [30], [31] and phosphate [32], [33] structures, leading to various DOPO-based derivatives. These derivatives, either as reactive hardeners or non-reactive additives incorporated into epoxy matrices, have endowed epoxy resins with improved flame retardancy due to the synergistic effect.
Amongst the aforementioned compounds, DOPO-based derivatives by covalently bonding DOPO and imine (DPI), with the secondary amine in the molecular structure, have captured tremendous interest toward researchers. They could serve as co-curing agents for epoxy resins, which not only remarkably improve the flame retardancy due to phosphorus-nitrogen synergism, but also endow modified resins with other improved properties. Many efforts have been made on developing flame retarded epoxy resins based upon DPI. Yao et al. synthesized a series of DPIs, which noticeably improved the flame retardancy of epoxy thermoset at a low phosphorus content (1.0 wt%) [20], [21]. Xu et al. reported a highly effective flame retarded epoxy resin cured by a DPI co-curing agent, and the glass transition temperature (Tg) of modified thermoset decreased slightly compared to that of pure epoxy thermoset [18]. Notwithstanding the above researches which have proven that epoxy resin with improved flame retardancy can be achieved by DPI, the flame-retardant efficiency could be further enhanced by developing novel DPI oligomers with richer aryl group structure and higher phosphorus content. Moreover, further studies are needed to understand the flame-retardant mechanism of DPI in epoxy resin intensively.
In this work, a novel DPI oligomer indicated as PDAP was synthesized by nucleophilic addition of DOPO on the imine linkage. The as-prepared PDAP serving as co-curing agent was utilized to improve the flame retardancy of epoxy resin. The performances of corresponding thermosets in terms of thermal stability, burning behavior have been studied by thermogravimetric analysis (TGA), limiting oxygen index (LOI) measurement and UL-94 test. The flame-retardant mechanism of modified epoxy thermoset was investigated by FTIR, Py-GC/MS, scanning electron microscopy (SEM) and laser Raman spectroscopy. Moreover, the mechanical properties of epoxy thermosets were evaluated by dynamic mechanical analysis (DMA) and tensile test.
Section snippets
Materials
9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) was kindly supplied by Jiangyin Hangfeng Technology Co., Ltd. (Jiangsu, China), and recrystallized from ethanol before use. Terephthalaldehyde, p-phenylenediamine, 4, 4′-diaminodiphenylmethane (DDM), ethanol and N, N-dimethylformamide (DMF) were all reagent grade and purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Epoxy resin (DGEBA, commercial name: E-44, with an epoxy value of 0.44 mol/100 g) used herein was
Synthesis and characterization of PDAP
The synthetic route of PDAP is illustrated in Scheme 1. As an intermediate, PSB was first prepared by the condensation reaction between terephthalaldehyde and p-phenylenediamine. Then, the final product PDAP was synthesized through the nucleophilic addition reaction between DOPO and PSB. The chemical structure of PDAP was characterized by FTIR, 1H-NMR and 31P-NMR.
Fig. 1 shows the FTIR spectra of DOPO, PSB and PDAP. In the spectrum of DOPO, the peak at 2434 cm−1 belongs to the stretching
Conclusions
In this work, a novel DOPO-based oligomer indicated as PDAP was synthesized through a two-step reaction, and its chemical structure was confirmed by FTIR, 1H-NMR, 31P-NMR. The as-prepared PDAP, serving as co-curing agent of DDM incorporated into epoxy matrix, endowed the epoxy thermoset with highly improved flame retardancy. With the incorporation of 7 wt% PDAP, the epoxy thermoset achieved a LOI value of 35.3% and V-0 rating in UL-94 test. All the modified epoxy thermosets showed blowing-out
Acknowledgements
This work was financially supported by Program for Specialized Research Fund for the Doctoral Program of Higher Education in China (Grant No. 20130075130002) and the National Natural Science Foundation of China (Grant No. 51303022 and 51203018).
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