Synergistic Flame Retardancy of Fullerene‑Anchored Reduced Graphene Oxide Hybrids in Epoxy Resin

Background

Polymeric materials dominate construction, electronics, and coatings due to their lightweight and versatility, yet their inherent flammability poses safety risks. Nanofillers have emerged as a powerful class of flame retardants, delivering high efficiency at low loadings while preserving base material properties. Their flame‑retarding action is evident in reduced peak heat release rate (PHRR), total heat release (THR), and increased limiting oxygen index (LOI).

While nanofillers markedly improve flame resistance in thermoplastics, their effect on thermosetting resins is limited. For instance, graphene oxide (GO) added to epoxy (EP) reduced PHRR by only 16 % at 1 wt % loading, and graphene achieved a 23 % PHRR drop under similar conditions. This muted response is attributed to the high cross‑link density of thermosets, which hampers significant structural alteration, and the limited barrier efficacy of nanofillers alone.

To enhance flame retardancy in thermosets, researchers have functionalized graphene with organic moieties or combined it with other additives. A notable example is the grafting of octa‑aminophenyl polyhedral oligomeric silsesquioxane (OapPOSS) onto graphene, producing OapPOSS‑rGO, which improved EP flame resistance. However, key metrics such as ignition time and peak heat release remain underreported, and the underlying synergistic mechanisms warrant deeper investigation.

Fullerene (C₆₀) has attracted attention for its ability to quench free radicals, thereby delaying thermo‑oxidative degradation. Yet, its tendency to aggregate reduces its efficacy. Combining C₆₀ with carbon nanofillers—particularly layered graphene—offers dual benefits: C₆₀ provides radical scavenging, while graphene offers barrier protection and facilitates better dispersion. Prior studies have applied C₆₀–graphene hybrids to thermoplastics, but no work has explored their impact on thermosetting resins.

In this study, we designed a C₆₀-PEI‑rGO hybrid via a three‑step reaction and incorporated it into EP. The hybrid’s amino‑rich, loosely layered architecture promotes uniform dispersion, radical absorption, barrier action, and cross‑linking density enhancement, thereby aiming to simultaneously elevate flame retardancy, thermal stability, and mechanical performance.

Methods

Materials

Graphite (3000 mesh), sulfuric acid (H₂SO₄ 98 %), sodium nitrate, potassium permanganate, hydrogen peroxide, ethanol, dimethyl sulfoxide (DMSO), toluene, acetone, and distilled water were analytical grade. Fullerene (C₆₀, >99 % purity) was sourced from Henan Puyang Co. Ltd. Branched polyethyleneimine (PEI, 50 % aq., Mn = 70 000) was purchased from Sigma‑Adrich. Epoxy monomer diglycidyl ether of bisphenol A (DGEBA) and curing agent diethyltoluenediamine (DETDA) were obtained from Shanghai Resin Factory Co. Ltd. and Chongshun Chemical Co. Ltd., respectively.

Preparation of C₆₀-PEI‑rGO

GO was synthesized via a modified Hummer’s method (see Additional file 1). PEI‑modified rGO (PEI‑rGO) was prepared by reacting PEI with GO. 150 mg of PEI‑rGO was dispersed in 300 mL DMSO using ultrasonication for 30 min. This dispersion, together with 300 mg C₆₀, was added to a 350 mL DMSO‑toluene (4:3 v/v) mixture and sonicated for 30 min at room temperature, then stirred at 90 °C for 24 h. The product was washed with toluene and ethanol three times, then dried at 60 °C under vacuum for 12 h, yielding C₆₀-PEI‑rGO (Scheme 1).

Schematic illustration of the preparation of C₆₀-PEI‑rGO

Preparation of EP Resin and Nanocomposites

DGEBA and DETDA were blended at a weight ratio of 1:0.234, heated to 100 °C for 15 min, and vigorously stirred to form a light yellow prepolymer. The mixture was degassed at 110 °C for 30 min and poured into a preheated (100 °C) U‑type mold. Curing followed a two‑step schedule: 120 °C/1 h + 180 °C/2.5 h, then post‑curing at 190 °C/2 h, producing cured EP resin. For nanocomposites, PEI‑rGO, C₆₀, or C₆₀-PEI‑rGO (0.4–1.0 wt %) were dispersed in the EP prepolymer/ethanol mixture by sonication for 30 min, degassed at 60 °C, and cured with the same thermal cycle. The resulting samples were labeled PEI‑rGO1.0/EP, C₆₀1.0/EP, and C₆₀-PEI‑rGO_n/EP (n = 0.4, 0.6, 0.8, 1.0).

Apparatus and Experimental Method

Morphology and microstructure were examined by AFM (Veeco Instruments), TEM (JEOL JEM‑2010), SEM (HITACHI SU8010/EDX), and FTIR (AVATAR360N). TGA of nanofillers (TA Instruments STA449C) spanned 25–800 °C in N₂ at 10 °C/min. EP and composites were thermally analyzed from 25–800 °C in air at 10–40 °C/min. DMA (TA DMA Q800) measured storage modulus from 25–250 °C at 1 Hz. Tensile tests followed ASTM D638 (5 mm/min). LOI values were determined per ASTM D2863/77. Cone calorimetry (FTT device) followed ISO 5660 with 35 kW/m² flux.

Results and Discussion

Characterization of GO, PEI‑GO, and C₆₀-PEI‑rGO

PEI‑rGO and C₆₀-PEI‑rGO dispersed readily in ethanol, forming stable colloids—a sign of successful PEI functionalization and C₆₀ anchoring. Color change from yellow GO to black PEI‑rGO and C₆₀-PEI‑rGO confirms reduction of GO.

FTIR spectra (Fig. 1) revealed diminished O–H and C=O bands in PEI‑rGO, indicating partial reduction and amide bond formation. In C₆₀-PEI‑rGO, characteristic C₆₀ peaks (1426, 1180, 574, 525 cm⁻¹) and a new C₆₀–H band at 2973 cm⁻¹ confirm covalent attachment.

Raman XPS data (Fig. 2) showed C1s peaks at 286.7 and 532.6 eV for all samples. N1s peaks at 399.7 eV (PEI‑rGO) and 400.1 eV (C₆₀-PEI‑rGO) confirm amide formation. Elemental analysis indicated C content rising to 86.55 at.% in C₆₀-PEI‑rGO, implying a ~45 wt % C₆₀ loading.

AFM and TEM images (Figs. 3–4) showed GO sheets (~0.9 nm thick) transformed into ~1.5 nm PEI‑rGO and further into a loosely layered C₆₀-PEI‑rGO with ~20 nm C₆₀ aggregates uniformly distributed. This architecture prevents restacking and enhances interfacial interaction with EP.

TGA (Fig. 5) demonstrated superior thermal stability of C₆₀-PEI‑rGO: a 79.4 % residue at 600 °C, compared to 42.9 % for GO. Weight loss analysis suggested ~50 wt % C₆₀ loading, consistent with XPS.

Microstructure of C₆₀-PEI‑rGO/EP Nanocomposites

SEM of fractured surfaces (Fig. 6) confirmed homogeneous dispersion of PEI‑rGO and C₆₀-PEI‑rGO without significant agglomeration, indicating strong interfacial adhesion. DMA curves (Fig. 7) revealed increased rubber plateau modulus for all hybrids, with C₆₀-PEI‑rGO1.0/EP exhibiting the highest modulus—attributable to PEI cross‑linking and the rough, layered surface enhancing physical bonding.

Flame Retardancy and Mechanism

Cone calorimetry (Fig. 8, Table 1) showed that C₆₀-PEI‑rGO1.0/EP extended t_ign by 21 s and delayed peak heat release by 28 s relative to neat EP. PHRR dropped by 40.0 % and THR by 15.6 %. LOI rose to 30.1 % for C₆₀-PEI‑rGO0.8/EP, 1.18× that of neat EP. These improvements surpass those of PEI‑rGO or C₆₀ alone, underscoring synergistic effects.

Thermal‑oxidation kinetics (Fig. 9–10, Tables 2–3) revealed increased activation energy (E_a) during the first decomposition stage for C₆₀-PEI‑rGO/EP, confirming delayed chain scission. The presence of C₆₀ scavenges radicals, while the layered rGO hinders volatile transport, collectively raising E_a and suppressing combustion.

Proposed mechanism (Fig. 11): (1) C₆₀ aggregates trap radicals, raising decomposition energy; (2) the layered structure forms a physical barrier, limiting heat and mass transfer; (3) PEI‑induced cross‑linking densifies the matrix, further enhancing resistance.

Mechanical and Thermal Properties

Tensile testing (Fig. 12) demonstrated that all C₆₀-PEI‑rGO/EP composites surpassed neat EP in strength and modulus. C₆₀-PEI‑rGO1.0/EP achieved a Young’s modulus of 2810 MPa (1.35× EP) and tensile strength of 77.4 MPa (1.22× EP). Rough fractured surfaces indicate efficient stress transfer between filler and matrix.

DMA results (Fig. 13) showed a 53.7 % increase in storage modulus at 30 °C and a Tg rise of 11.3 °C for C₆₀-PEI‑rGO1.0/EP, reflecting enhanced cross‑linking and restricted chain mobility.

TGA/DTG profiles (Fig. 14) revealed higher onset temperatures (ΔT_onset = 28 °C) and maximum degradation temperatures (ΔT_max = 16 °C) for C₆₀-PEI‑rGO1.0/EP, confirming superior thermal stability. The improved char microstructure and delayed oxidation further support the synergistic flame‑retardant action.

In summary, the C₆₀-PEI‑rGO hybrid delivers multi‑faceted benefits—flame retardancy, mechanical reinforcement, and thermal resilience—through radical trapping, barrier formation, and cross‑linking enhancement.

Conclusions

The covalently anchored C₆₀-PEI‑rGO hybrid exhibits a loosely layered, amino‑rich structure with uniformly dispersed ~20 nm C₆₀ aggregates. Incorporation of 1.0 wt % of this hybrid into EP reduced PHRR by 40.0 % and THR by 15.6 %, while increasing t_ign and peak‑time by 21 s and 28 s, respectively. The hybrid’s efficacy originates from enhanced cross‑link density via PEI, barrier protection from the layered rGO, and radical scavenging by C₆₀. Consequently, it represents a potent, multifunctional nanofiller for advanced fire‑retardant thermosetting resins.

Abbreviations

AFM:

Atomic force microscope

C₆₀:

Fullerene

DETDA:

Diethyltoluenediamine

DGEBA:

Diglycidyl ether of bisphenol A

DMA:

Dynamic mechanical analysis

DMSO:

Dimethyl sulfoxide

EP:

Epoxy

FTIR:

Fourier transform infrared spectrometer

GO:

Graphene oxide

LOI:

Limiting oxygen index

PEI:

Branched polyethyleneimine

PHRR:

Peak heat release rate

rGO:

Reduced graphene oxide

SEM:

Scanning electron microscope

TEM:

Transmission electron microscopy

TGA:

Thermogravimetric analysis

THR:

Total heat release

TSR:

Total smoke release