A furfural-based DOPO-containing flame retardant, 6,6′-(((methylenebis(4,1-phenylene))bis(azanediyl))bis(furan-2-ylmethylene))bis(dibenzo[c,e][1,2]oxaphosphinine 6-oxide) (MBF-DOPO), was synthesized and utilized as a co-curing agent of 4,4′-diaminodiphenyl methane (DDM) for fire-safe epoxy thermosets. For the cured epoxy resin containing 4.0% MBF-DOPO, the limiting oxygen index (LOI) reached 32.9% (with the V-0 rating in UL-94 test), and the peak heat release rate and total smoke production values were respectively decreased by 29.3% and 33.6%, compared to pure epoxy resin. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) results confirmed that the furfural-based flame retardant MBF-DOPO promoted the charring formation of the epoxy matrix, which effectively isolated the gas and heat transfer during combustion and thus enhanced the fire-safety performance of the epoxy thermosets. This work provides an effective route for synthesizing a furfural-based flame retardant, which possesses great potential for application in fire-safe epoxy thermosets.

This paper introduces an effective approach to fabricate novel graphene/polyimide nanocomposite films with a low dielectric constant. Graphene fluoroxide (GFO) nanosheets were produced through the exfoliation of graphite fluoroxide (GiFO) in N-methyl-2-pyrrolidone (NMP). GFO nanosheets reacted with 4,4′-diaminodiphenyl ether (ODA) to become ODA-functioned GFO (GFO-ODA) with amine end groups, which provided reactive sites for covalent bonding with PI chains. GFO-ODA/PI composite films were prepared via the thermal imidization of a GFO-ODA/PAA solution. In this composite, GFO-ODA nanosheets were chemically bonded to the PI matrix. The dielectric constant of the films depended on the loading of GFO-ODA and its minimum value reached 2.75 (at 10⁶ Hz) when the content of GFO-ODA was 1.0 wt%. Strong GFO-ODA nanosheets facilitate the load transfer and enhance the mechanical properties of PI films. GFO-ODA/PI film with 1.0 wt% loading had a Young's modulus ∼40% larger than pure PI film. In addition, the incorporation of GFO-ODA increased the glass transition temperature of the films and had no significant effect on the thermal stability.

5,10-Dihydro-phenophosphazine-10-oxide (DPPA) served as a co-curing agent for 4,4′-diaminodiphenylmethane in the curing reaction of bisphenol A diglycidyl ether (DGEBA) epoxy resin (EP). ¹H NMR tracking the reaction of DPPA with DGEBA revealed that the P–H bond in DPPA had a higher reactivity with the epoxy group than the N–H bond. DPPA could promote the curing reaction of DDM with DGEBA. DPPA endowed the epoxy resin with a high flame-retardant efficiency due to the unique combination of phosphorus and nitrogen in the phenophosphazine ring. The cured epoxy resin could achieve a V-0 rating in the UL-94 test with a limiting oxygen index (LOI) of 33.6% at only 2.5 wt% DPPA. The reduced peak heat release rate and total heat release, and increased char yield further verified the excellent flame retardancy of EP. The flame-retardant mechanism of the epoxy resin was investigated by thermogravimetry-Fourier transform infrared spectrometry (TG-FTIR), scanning electron microscopy, elemental analysis and FTIR spectrometry. The results indicated that DPPA catalyzed the formation of the rigid intumescent char and the generation of the blowing-out effect in the epoxy resin matrix.