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A P/N‐containing flame retardant (PPT) constructed by phosphaphenanthrene, phosphonate, and triazole groups was successfully synthesized and used as a reactive co‐curing agent for epoxy resin (EP). The curing behavior, thermal property, combustion behavior, and flame retardant mechanism of EP thermosets were comprehensively investigated. According to the analysis of DSC and TGA, PPT accelerated the crosslinking reaction and enhanced the charring ability for EP thermosets at high temperature. The results of combustion test indicated that PPT endowed epoxy thermoset with outstanding flame retardancy. When the phosphorus content was 0.71 wt%, EP/DDS/PPT‐2 achieved a LOI value of 33.2% and passed V‐0 rating in UL‐94 test, and its peak heat release rate and total heat release were decreased by 63.7 and 30.5%, respectively, relative to EP/DDS. Moreover, the FIGRA of EP/DDS/PPT‐2 was reduced from 9.7 to 3.5 kW m−2 s−1, manifesting the significantly improved fire safety of EP thermoset. The flame retardant mechanism was summarized as two parts: (a) the barrier effect of continuous phosphorus‐rich char layers in condensed phase, (b) the quenching effect of phosphorous radicals and diluting effect of nonflammable gases in gaseous phase. [ABSTRACT FROM AUTHOR]

A high-efficiency flame retardant composed of phosphaphenanthrene, benzothiazole and imidazole groups (PBI) was synthesized and served as flame retardant co-curing agent to reduce the fire hazard of epoxy resin (EP). The chemical structure of PBI was characterized by Fourier transform infrared spectroscopy (FTIR), high-resolution mass spectroscopy (HR-MS), 1H and 31P nuclear magnetic resonance (NMR). The curing behavior, thermal stability and flame retardant properties of the prepared EP systems were investigated. The curing behavior study disclosed that PBI accelerated the crosslinking reaction of EP and was chemically bonded with EP matrix to obtain intrinsic flame retardant thermoset. The resulting EP thermosets showed only slight decrease in glass transition temperature (T g). The thermogravimetric analysis (TGA) results indicated both the catalytic decomposition and catalytic charring effects of PBI demonstrated by decreased thermal stability and enhanced charring capability of EP/DDS/PBI thermosets. The combustion test results showed remarkable improvement in the flame retardant properties of EP/DDS/PBI thermosets. When the phosphorus content was only 0.75 wt%, EP/DDS/PBI-0.75 thermoset achieved a limiting oxygen index (LOI) value of 34.6% and passed UL94 V-0 rating. Moreover, the peak of heat release rate (pk-HRR), average of heat release rate (av-HRR) and total heat release (THR) values were decreased by 48.7%, 31.1% and 28.3%, respectively, in comparison with those of the EP/DDS thermoset. The flame retardant effect of PBI on EP was attributed to the catalytic charring effect to form protective char layer in condensed phase and release of free radicals with quenching effect and nonflammable gases with diluting effect in gaseous phase. • A DOPO based reactive flame retardant (PBI) constructed by multiple heteroaromatic groups was successfully synthesized. • PBI accelerated the crosslinking reaction of EP. • Blowing-out effect was observed for EP/DDS/PBI thermosets. • PBI exhibited bi-phase flame retardant effect on EP. [ABSTRACT FROM AUTHOR]

In order to meet the requirement in the fields with high flame resistance, the flame retardancy of epoxy resin (EP) must be ameliorated. Herein, a P/N/S-based flame retardant (PAD) was synthesized and used to prepare flame retardant EP. The combustion behavior, thermal property and flame retardant mechanism of the prepared EP thermosets were investigated. PAD induced the decomposition of EP thermoset in advance. Meanwhile, PAD enhanced the flame retardance of EP thermosets and reduced the fire risk of EP matrix. For example, with only 0.5 wt% phosphorus, EP/DDS/PAD-0.5 easily passed UL-94 V-0 rating and achieved a LOI value of 33.6%, together with the declined heat release rate (HRR). Impressively, the glass transition temperature (T g) of EP/DDS/PAD thermosets dropped by only 3–13 °C, compared with that of EP/DDS. During combustion, PAD prompted the formation of an intumescent char layer in condensed phase, and exerted blowing-out effect in gaseous phase. [ABSTRACT FROM AUTHOR]

Aminobenzothiazole-substituted cyclotriphosphazene derivative (ABCP) was successfully synthesized and used as reactive flame retardant to improve the fire safety of epoxy resin (EP). The chemical structure of ABCP was confirmed by Fourier transform infrared spectroscopy (FTIR), 1H and 31P nuclear magnetic resonance (NMR), high-resolution mass spectroscopy (HR-MS) and elemental analysis (EA). The curing behavior, thermal stability, thermo-mechanical property and flame retardancy of the prepared EP systems were studied. The differential scanning calorimeter (DSC) results indicated that ABCP induced the anionic polymerization of EP and facilitated the addition reaction between EP and 4,4′-diamino-diphenyl sulfone (DDS). The thermogravimetric analysis (TGA) results indicated that ABCP accelerated the thermal degradation of EP matrix and contributed to the formation of more residual chars with better thermo-oxidative stability. The dynamic mechanical analysis (DMA) results indicated that EP/DDS/ABCP thermosets exhibited higher storage modulus at 50 °C and their glass transition temperature (T g) values were slightly reduced by 6–10 °C, in comparison with EP/DDS thermoset. The combustion test results revealed the fire hazards of EP/DDS/ABCP thermosets were effectively reduced. Compared with EP/DDS thermoset, the limiting oxygen index (LOI) value of EP/DDS/ABCP-1.2 thermoset was increased to 31.2% and the sample passed UL94 V -0 rating; the average of heat release rate (av-HRR), peak of heat release rate (pk-HRR) and total heat release (THR) values of EP/DDS/ABCP-1.2 thermoset were reduced by 36.5%, 53.7% and 34.5%, respectively; particularly, the fire growth rate (FIGRA) of EP/DDS/ABCP-0.6 thermoset was reduced by half. The research on flame retardant mechanism disclosed that ABCP played dominated flame retardant effect in condensed phase. [ABSTRACT FROM AUTHOR]

Latent curing epoxy resin (EP) system with satisfactory flame retardancy has attracted rather considerable interest both in academic and industrial communities over the years. Herein, imidazole-blocked bismaleimide compounds were synthesized through the Michael addition reaction of N,N′-bismaleimide-4,4′-diphenylmethane (BDM) with imidazole (IM), 2-methylimidazole (2MI), and 2-ethyl-4-methylimidazole (EMI), respectively, and were simultaneously used as latent curing agent and flame retardant synergist for 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) based EP (DOPO-EP). The storage stability study disclosed that imidazole-blocked bismaleimide compounds kept chemically inert towards epoxy at room temperature, resulting in long pot life over 40 days. When heated, imidazole-blocked bismaleimide compounds underwent retro-Michael addition reaction to regenerate BDM and the corresponding imidazoles. The fast curing ability was thus achieved via chain polymerization of epoxy initiated by thermally generated free imidazoles. Moreover, polybismaleimide component derived from the homopolymerization of thermally generated BDM was incorporated into the cross-linked EP network, resulted in enhanced thermal and charring performance of epoxy matrix. Compared with the contrast samples EP/2MI-BDM and DOPO-EP/2MI thermosets, DOPO-EP/2MI-BDM thermoset exhibited better flame retardancy with remarkably higher limiting oxygen index (LOI) value and UL94 rating as well as lower peak of heat release rate (pk-HRR), average of heat release rate (av-HRR) and total heat release (THR) from cone calorimeter test. The flame-retardant mechanism study revealed the synergistic flame-retardant effect of phosphaphenanthrene and bismaleimide in condensed phase. Image 1 • Imidazole-blocked bismaleimide were used as latent curing agent for DOPO-EP. • Imidazole-blocked bismaleimide underwent retro-Michael addition reaction when heated. • The epoxy systems containing imidazole-blocked bismaleimide exhibited long shelf life and fast curing ability. • DOPO-EP/2MI-BDM thermoset exhibited excellent flame retardancy. [ABSTRACT FROM AUTHOR]

Epoxy resin (EP) is inherently inflammable and most flame retardants used now unavoidably deteriorate other properties of EP. Moreover, the components of EP system tend to be formulated as one-component system for large-scale industrial production instead of being blended just before use. In order to overcome these issues, benzimidazolyl-substituted cyclotriphosphazene (BICP) was synthesized and utilized as latent flame-retardant curing agent for EP. According to the curing and rheological behavior studies, BICP maintained chemically inert towards EP at room temperature and could rapidly cure EP when heated. The combustion test results indicated the outstanding flame retardancy of EP/BICP thermosets. With 10.7 wt% addition of BICP, the limited oxygen index (LOI) of EP/BICP-12 thermoset was as high as 33.5% and UL94 V-0 rating was achieved; the peak of heat release rate (pk-HRR), average of heat release rate (av-HRR), total heat release (THR) and fire growth rate (FIGRA) of EP/BICP-12 thermoset were decreased by 69.8%, 33.2%, 34.7% and 71.8%, respectively, compared with those of the neat EP thermoset. Most attractively, other properties of the EP/BICP thermosets were not impaired despite of the enhancement of flame retardancy. Compared with the neat EP thermoset, the EP/BICP thermosets showed comparable mechanical properties and improved dielectric properties. Especially, the T g values of EP/BICP thermosets (except for EP/BICP-8 sample) were around 190 °C, 34 °C higher than that of the neat EP thermoset. All these results demonstrated that EP/BICP system with remarkable comprehensive performance has the potential to achieve wider application in key fields. [ABSTRACT FROM AUTHOR]