Flame retardancy and mechanical properties of epoxy thermosets modified with a novel DOPO-based oligomer

https://doi.org/10.1016/j.polymdegradstab.2016.04.005Get rights and content

Abstract

A novel 9, 10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO)-based oligomer indicated as PDAP was synthesized via the nucleophilic addition reaction between DOPO and imine obtained from the condensation reaction of terephthalaldehyde and p-phenylenediamine. The chemical structure of PDAP was characterized by FTIR, 1H-NMR and 31P-NMR. The PDAP serving as co-curing agent of 4, 4′-diaminodiphenylmethane was employed to develop epoxy resins with highly improved flame retardancy. The thermal stabilities, flame-retardant and mechanical properties of epoxy thermosets were studied by thermogravimetric analysis (TGA), limiting oxygen index (LOI) measurement, UL-94 test, dynamic mechanical analysis (DMA) and tensile test. The flame-retardant mechanism of epoxy thermoset modified with PDAP was investigated by FTIR, Py-GC/MS, SEM and laser Raman spectroscopy. The results showed that with the incorporation of 7 wt% PDAP, the modified epoxy thermoset achieved a LOI value of 35.3% and V-0 rating in UL-94 test. Interestingly, all the modified thermosets showed blowing-out effects during UL-94 test. The flame-retardant mechanism was attributed to the quenching effect of phosphorus-containing free radicals and diluting effect of nonflammable gases in the gas phase, and the formation of phosphorus-rich char layers in the condensed phase. Furthermore, the results of DMA and tensile test revealed that the incorporation of PDAP decreased the glass transition temperature slightly, and meanwhile improved the tensile strength of epoxy thermoset.

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).

References (51)

  • S. Yang et al.

    Preparation and flame retardancy of an intumescent flame-retardant epoxy resin system constructed by multiple flame-retardant compositions containing phosphorus and nitrogen heterocycle

    Polym. Degrad. Stab.

    (2015)
  • L.J. Qian et al.

    Thermal degradation behavior of the compound containing phosphaphenanthrene and phosphazene groups and its flame retardant mechanism on epoxy resin

    Polymer

    (2011)
  • M.J. Xu et al.

    Synthesis of a novel flame retardant based on cyclotriphosphazene and DOPO groups and its application in epoxy resins

    Polym. Degrad. Stab.

    (2016)
  • W.C. Zhang et al.

    Novel flame retardancy effects of DOPO-POSS on epoxy resins

    Polym. Degrad. Stab.

    (2011)
  • Z. Wang et al.

    Flame-retardant materials based on phosphorus-containing polyhedral oligomeric silsesquioxane and bismaleimide/diallylbisphenol a with improved thermal resistance and dielectric properties

    J. Appl. Polym. Sci.

    (2015)
  • Z.Q. Li et al.

    Study of the synergistic effect of polyhedral oligomeric octadiphenylsulfonylsilsesquioxane and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide on flame-retarded epoxy resins

    Polym. Degrad. Stab.

    (2014)
  • L.J. Qian et al.

    Pyrolysis route of a novel, flame retardant constructed by phosphaphenanthrene and triazine-trione groups and its flame-retardant effect on epoxy resin

    Polym. Degrad. Stab.

    (2014)
  • B. Perret et al.

    Novel DOPO-based flame retardants in high-performance carbon fibre epoxy composites for aviation

    Eur. Polym. J.

    (2011)
  • X. Wang et al.

    Flame retardancy and thermal degradation mechanism of epoxy resin composites based on a DOPO substituted organophosphorus oligomer

    Polymer

    (2010)
  • X. Wang et al.

    Synthesis and characterization of a DOPO-substitued organophosphorus oligomer and its application in flame retardant epoxy resins

    Prog. Org. Coat.

    (2011)
  • X.D. Qian et al.

    Thermal degradation and flammability of novel organic/inorganic epoxy hybrids containing organophosphorus-modified oligosiloxane

    Thermochim. Acta

    (2013)
  • W.C. Zhang et al.

    Blowing-out effect in epoxy composites flame retarded by DOPO-POSS and its correlation with amide curing agents

    Polym. Degrad. Stab.

    (2012)
  • A.I. Balabanovich

    Thermal decomposition study of intumescent additives: pentaerythritol phosphate and its blend with melamine phosphate

    Thermochim. Acta

    (2005)
  • K. Li et al.

    The change of thermal–mechanical properties and chemical structure of ambient cured DGEBA/TEPA under accelerated thermo-oxidative aging

    Polym. Degrad. Stab.

    (2013)
  • Y. Zahra et al.

    Thermo-oxidative aging of epoxy coating systems

    Prog. Org. Coat.

    (2014)
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