POSS-functionalized graphene oxide hybrids with improved dispersive and smoke-suppressive properties for epoxy flame-retardant application

Journal Pre-proofs POSS-functionalized graphene oxide hybrids with improved dispersive and smoke-suppressive properties for epoxy flame-retardant application Lijie Qu, Yanlong Sui, Chunling Zhang, Peihong Li, Xueyan Dai, Baosheng Xu, Daining Fang PII: DOI: Reference:

S0014-3057(19)32083-X https://doi.org/10.1016/j.eurpolymj.2019.109383 EPJ 109383

To appear in:

European Polymer Journal

Received Date: Revised Date: Accepted Date:

11 October 2019 7 November 2019 20 November 2019

Please cite this article as: Qu, L., Sui, Y., Zhang, C., Li, P., Dai, X., Xu, B., Fang, D., POSS-functionalized graphene oxide hybrids with improved dispersive and smoke-suppressive properties for epoxy flame-retardant application, European Polymer Journal (2019), doi: https://doi.org/10.1016/j.eurpolymj.2019.109383

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POSS-functionalized graphene oxide hybrids with improved dispersive and smoke-suppressive properties for epoxy flame-retardant application

Lijie Qu a,b,c, Yanlong Sui a, Chunling Zhang a,*, Peihong Li a, Xueyan Dai a, Baosheng Xu b,c,*, and Daining Fang b,c a

Key Laboratory of Automobile Materials, Ministry of Education, College of Materials

Science and Engineering, Jilin University, Changchun 130025, P. R. China. b

Institute of Advanced Structure Technology, Beijing Institute of Technology, Beijing

100081, P. R. China. c

Beijing Key Laboratory of Lightweight Multi-functional Composite Materials and

Structures, Beijing Institute of Technology, Beijing 100081, PR China

* Corresponding author. E-mail: [email protected]; [email protected]

Declarations of interest: none.

1

POSS-functionalized graphene oxide hybrids with improved dispersive and smoke-suppressive properties for epoxy flame-retardant application

Lijie Qu a,b,c, Yanlong Sui a, Chunling Zhang a,*, Peihong Li a, Xueyan Dai a, Baosheng Xu b,c,*, and Daining Fang b,c a

Key Laboratory of Automobile Materials, Ministry of Education, College of Materials

Science and Engineering, Jilin University, Changchun 130025, P. R. China. b

Institute of Advanced Structure Technology, Beijing Institute of Technology, Beijing

100081, P. R. China. c

Beijing Key Laboratory of Lightweight Multi-functional Composite Materials and

Structures, Beijing Institute of Technology, Beijing 100081, PR China

Abstract Functionalized graphene oxide sheets (FGO) with improved dispersive and smokesuppressive properties were synthesized by covalently grafting octa (propyl glycidyl ether) polyhedral oligomeric silsesquioxane on graphene oxide sheets (GO) with γaminopropyl triethoxysilane as a chemical bridge. The good dispersion of FGO sheets in the epoxy resin (EP) matrix endowed EP composites with stable thermal resistance, enhanced tensile and smoke-suppressive properties. Cone calorimeter tests indicated that the addition of 0.7 wt.% FGO sheets to EP composites reduced the peak of heat release rate, total heat release, and total smoke release by 49.7%, 34.3%, and 41.5%, respectively. Two important effects that originated from FGO promoted this improvement; that is, well-dispersed FGO sheets exhibited a tortuous effect by lengthening the heat path and inhibiting heat diffusion in the EP matrix, and the increase in char residue and the reduction in gas volatiles confirmed the barrier effect of FGO sheets by forming protective char structures on the surface of the matrix, which 2

restrained smoke release. This method provided a feasible concept for effectively enhancing the flame retardancy of EP composites by combining the characteristics of graphene and polyhedral oligosilsesquioxane through the construction of effective interfacial interactions. Keywords Flame retardancy; Graphene; Epoxy resin; Thermal properties

1. Introduction Epoxy resin (EP), a prominent thermosetting polymer material with high adhesion, chemical and corrosion resistance, excellent electrical insulation, and high strength and modulus, has been widely applied in adhesives, coatings, electrical materials, and aerospace domains.[1-3] However, the inherent flammability of EP restricts its further application in some fields with special requirements. Considering environmental health, many studies have incorporated nonhalogen flame-retardant additives into EP networks. One of the revolutionary investigations on the fabrication of advanced materials is the development of nanotechnology and nanofillers with polymeric fire retardancy.[4] Many nanomaterials, such as layered graphene,[5] fibrous carbon nanotubes,[6] and particulate polyhedral oligosilsesquioxane (POSS),[7] can play an important role in the flame-retardant modification of EP composites. The appropriate dispersion of nanomaterials within a substrate severely affects the enhancement of several properties, such as flame retardancy and mechanical properties.[8] Given the excellent heat, electrical, optic, and mechanical properties of graphene, numerous studies have reported its massive potential as a reinforcement filler to the enhance the performance of polymer composites comprehensively.[9] Graphene can restrain gas volatiles to improve the flame retardancy of polymers.[10, 11] However, difficulty in achieving uniform dispersion and the weakness of smoke suppression are the two main problems in graphene application.[12] Therefore, the surface modification of graphene should be further explored. Graphene oxide (GO) has been widely investigated because of its similar layered structure, which is rich in oxygen groups (hydroxyl, carbonyl, and epoxy groups) that offer numerous active sites for covalent 3

functionalization. Functional molecules on GO can enhance the compatibility between GO sheets and an EP matrix by providing interfacial interactions and further improving GO dispersion.[13] POSS compounds possess a typical structure of cube-octameric frameworks with eight organic vertex groups, and one or more of these groups are reactive or polymerizable.[14] The Si–O–Si structures of POSS stabilize char layers during combustion by forming a firm protective screen over the matrix. POSS has conspicuous flame-retardant and smoke-suppressive properties. Some studies have also been conducted on GO–POSS hybrids and their enhancement in polymer composites.[15] Octa (propyl glycidyl ether) POSS (OGPOSS) is a hybrid molecule with organic glycidyl groups attached at the corners of the cage. Given their special structure that is similar to EP chains, the abundant epoxy groups of OGPOSS provide great possibilities for organic combination between nanofillers and EP networks, therefore improving compatibility and dispersion. Nevertheless, research on the combination of OGPOSS with GO sheets for flame-retardant application remains limited. To effectively link the OGPOSS molecules to the surface of GO sheets, we noticed that γ-aminopropyl triethoxysilane (KH550), as a common coupling agent with active amino groups, yields great potential competence. KH550 can build chemical bonds between hydrophilic GO and hydrophobic OGPOSS through siloxane structure hydrolyzation and epoxide ring opening. In this work, we prepared organosilane-functionalized GO (FGO) sheets by grafting KH550 as a bridge to connect GO and OGPOSS. The FGO sheets were subsequently utilized to modify EP composites. The FGO sheets exhibited outstanding dispersibility in EP matrix and smoke suppression during combustion. The flame-retardant mechanism of FGO was investigated intensively.

2. Experimental 2.1.

Materials

4

Natural graphite powders (2000 mesh) were purchased from Aladdin Reagent Co., Ltd., China. Sodium nitrate (NaNO3), sulfuric acid (H2SO4), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 30%), and hydrochloric acid (HCl, 36%) were provided by Beijing Chemical Works (China). KH550 and the curing agent 4,4ʹdiamino diphenylmethane (DDM) were supplied by Sinopharm Chemical Reagent Co., Ltd., China. Ethanol and N,N-dimethylformamide (DMF) were procured from Tianjin Fuchen Chemical Reagents Co., Ltd., China. OGPOSS was supplied by Hybrid Plastics Incorporation, USA. The diglycidyl ether of bisphenol A (DGEBA; E-44) was supplied by Guangzhou Suixin Chemical Industrial Co., Ltd., China. All the reagents were used without further purification. 2.2.

Synthesis of GO

GO was synthesized in accordance with the improved methods of Hummer.[16] A typical procedure was as follows: natural graphite powders (2.5 g) and NaNO3 (1.0 g) were added to concentrated H2SO4 (60 mL) in a 500 mL flask equipped with mechanical stirring in an ice bath to keep the mixture cooled down to approximately 0 °C. After the mixture was stirred for 30 min, KMnO4 (10 g) was slowly added in parts for 60 min of reaction. The reaction temperature was maintained below 10 °C. Then, the whole system was transferred to a warm water bath and further stirred at 40 °C for 4 h. Afterward, the temperature was maintained at 95 °C, and 115 mL of deionized water was slowly added for 5 min of stirring. Approximately 30% H2O2 was added to the mixture until no bubbles escaped. Subsequently, a large amount of water and 173 mL of 5% HCl solution were used to wash the mixture. The mixture was concentrated and dialyzed until the dispersion was neutral. Aqueous GO dispersion was obtained through ultrasonication. Scheme 1 illustrates the synthesis of GO. 2.3.

Preparation of KH550-functionalized GO

KH550-functionalized GO (GO-NH2) was prepared through hydrolysis between siloxane groups on KH550 and carboxyl or hydroxy groups on GO sheets. First, the obtained GO dispersion (50 mL) was diluted with deionized water to 400 mL in a 500 mL flask under sonication for 30 min. Subsequently, 7.0 g of KH550 was dropped into 5

the suspension equipped with magnetic stirring and condensing installations. The mixture was heated to 60 °C and refluxed for 12 h. Afterward, the mixture was separated through filtration and washed with large quantities of ethanol and deionized water to remove excess KH550. The final brown black solid was further dried in a lyophilizer. The preparation of GO-NH2 is presented in Scheme 1. 2.4.

Preparation of FGO

FGO was prepared via a ring-opening reaction between the amido groups of KH550 grafted on GO sheets and the epoxy groups of OGPOSS molecules. First, 0.5 g of the obtained GO-NH2 was dispersed into DMF (200 mL) in a 500 mL flask under sonication for 30 min. Then, 50 mL of DMF with dissolved OGPOSS (7.0 g) was slowly added into the system. The whole reaction was controlled at 130 °C and refluxed for 12 h under protective N2 installations. The resulting solution was filtered and washed with excess DMF and deionized water to remove residual OGPOSS. The final black solid was further dried in a lyophilizer. The preparation of FGO is also presented in Scheme 1. 2.5.

Fabrications of FGO-reinforced epoxy nanocomposites

FGO-reinforced epoxy (FGO/EP) nanocomposites were fabricated with a series of FGO nanofillers via a thermal curing procedure. First, FGO nanofillers (0.1, 0.3, 0.5, 0.7, and 0.9 wt.%) were added to preheated neat DGEBA monomers in a 250 mL flask with vigorous mechanical stirring at 80 °C. After the entire mixture transformed into a homogeneous system without evident particles, a stoichiometric amount of the curing agent (DDM) was slowly added with continuous stirring until a uniform solution was obtained. Afterward, the resulting mixture was transferred and cast into preheated Teflon molds for curing. The whole process was set as follows: 105 °C for 2 h, 155 °C for 2 h, and 199 °C for 2 h. GO/EP composites at 0.1 wt.% additive and pure EP thermosets were also fabricated for comparison. The whole preparation process is also illustrated in Scheme 1.

6

Scheme 1 Preparation of GO, GO-NH2, FGO, and FGO/EP nanocomposites. 2.6.

Characterization

The chemical functional groups of GO, GO-NH2, and FGO were detected through Fourier transform infrared (FTIR) spectroscopy (Bruker Vertex 70, Germany) at room temperature. The samples were mixed with potassium bromide powder and pressed into flasks. The interlayer spacings of the prepared materials were determined by using a Xray diffraction spectrometer (XRD; PW 1800, Netherlands) with a Cu–Ka filament. The 7

2θ range was taken between 3° and 40° with a scan rate of 4°/min. The graphitization degree of the samples was obtained by applying a LabRAM-HR confocal Raman microprobe (JobinYvon, France). The detected wavenumber was between 800 and 2000 cm–1. The thermal stability of the samples was investigated with a thermogravimetric analyzer (TGA; Perkin-Elmer, USA) under a N2 atmosphere. The chemical compositions of the samples were detected with an X-ray photoelectron spectroscope (XPS; Thermo ESCALAB 250, USA) with an Al–Ka anode. C, O, N, and Si of the samples were tested. The surface characteristics and layer morphologies of the samples were observed with a scanning electron microscope (SEM; FEI XL30 ESEM FEG, USA) and a transmission electron microscope (TEM; FEI Tecnai F30 G2, USA). The elemental composition of the samples was investigated using an energy-dispersive spectrometer (EDS; EDAX Genesis XM2, USA) connected to the SEM. The thermal properties and thermal stabilities of the FGO/EP nanocomposites were detected with a differential scanning calorimeter (DSC; TA Q20, USA) and a TGA. In all cases, the samples were heated under a N2 atmosphere from ambient temperature to 250 °C or 800 °C at a heating rate of 10 °C/min. The thermal conductivity of the samples was measured on a thermal constant analyzer (TPS 2000 hot-disk, Sweden) in accordance with a transient plane heat source method at 20 ± 2 °C. The samples were prepared in the form of two flat discs specimen with dimensions of 20 mm × 20 mm × 3 mm. The probe was clipped between the two solid disks, therefore, the surface of samples should be smooth and flat. This measurement was used to detect the through-plane thermal conductivity of samples. The mechanical properties of FGO/EP nanocomposites were measured by using an electronic universal testing machine (WSM-5KN, China) with a dumbbell-shaped specimen in accordance with GBT 2567-2008. The fracture surface of EP nanocomposites was detected through SEM. Cone calorimetric measurements were conducted on a cone calorimeter (Fire Testing Technology FTT0007, UK) to detect the flame retardancy of FGO/EP nanocomposites. The samples were prepared in the form of flat discs with dimensions of 100 mm × 100 8

mm × 3 mm in accordance with ISO 5660. The residues of FGO/EP composites were observed and studied via SEM, EDS, and Raman spectroscopy. The released gas was also detected through TGA-FTIR spectroscopy (Nicolet 6700, USA) under a N2 atmosphere at a heating rate of 20 °C/min from ambient temperature to 800 °C.

3. Results and discussion 3.1.

Characterization and morphological analysis of FGO

Figure 1 Characterization of GO, GO-NH2, and FGO: a FTIR analysis, b XRD analysis, c Raman spectrum analysis, and d TGA analysis. Figure 1a shows the FTIR spectroscopy results for GO, GO-NH2, and FGO. The broad peak located at 3436 cm−1 in the spectrum of GO was caused by the –OH bonds of hydroxyl groups on the surface of GO. The peaks at 1741 and 1386 cm−1 were attributed to the C=O bonds of carbonyl and the C–O bonds of epoxy groups on the surface of GO. The strong peak at 1631 cm−1 corresponded to the C=C stretching vibration of sp2hybrid-carbon in the plane of the graphitic backbone of GO.[13, 17, 18] The doublet at 9

2926 and 2865 cm−1 that appeared after GO was modified with KH550 corresponded to the symmetric and asymmetric vibrations of alkyl groups from siloxane. The weakening of the peak at 1741 cm−1 suggested that the carbonyl groups of GO reacted with siloxane moieties. In addition, the new peaks at 1106, 1030, and 754 cm−1 in the GO-NH2 spectrum indicated the characteristics of Si–O–Si, Si–O–C, and Si–C.[19] After further modification with OGPOSS, FGO exhibited new peaks at 1656 and 1590 cm−1 that represented the C–N and N–H bonds of amido bonds originating from the epoxy groups of OGPOSS and the amino groups of GO–NH2.[20] The consistent presence of the peak at 1256 cm−1 indicated that epoxy groups remained on FGO sheets after modification. Moreover, the peak at 1106 cm−1 of Si–O–Si bonds was stronger than that of Si–O–C at 1030 cm−1 and indicated that OGPOSS was chemically grafted onto the GO-NH2 surface.[15, 20-22] XRD analysis (Figure 1b) revealed that GO had a strong peak at 2θ = 10.8° (dspacing of 8.080 Å) that corresponded to the (001) diffraction peak of graphitic lattice.[23] This result indicated that GO was exfoliated into stacked layered sheets.[24] After functionalization with KH550, the diffraction peak of GO-NH2 shifted to a small angle at 2θ=5.4° and had a weakened intensity. The corresponding interlayer spacing enlarged to 16.395 Å, suggesting that the silane and alkyl chains of KH550 were chemically grafted onto the surfaces of the GO sheets by reacting with oxygencontaining groups.[13] Moreover, the sharp peak disappeared in the FGO profile because of the abundant OGPOSS molecules grafted on GO-NH2 that restrained the restacking of GO sheets. The molecules were inserted between the sheets and increased the nonuniformity of the GO structure. These results indicated that GO nanosheets were functionalized successfully by KH550 and OGPOSS, and disordered structures further increased.[25] Raman spectroscopy was also performed to study the structures of GO, GO-NH2, and FGO and could partially support the chemical modification of GO.[21] Figure 1c presents typical broad D (1348–1354 cm−1) and G bands (1596–1607 cm−1), which were ascribed to the sp3 carbons in the nanoscale-oxidized domain (defect- or disorder10

induced modes) and sp2 carbons in the aromatic domain (in-plane graphene vibrations).[26] After functionalization with KH550 and OGPOSS, the intensity of the D band of the modified GO was stronger than that of the raw GO. The intensity ratio of the D band and G band (ID/IG) is an important metric of irregularity in graphene and can be used to evaluate the extent of the reduction of GO to functionalized GO.[27] ID/IG (1.912 and 2.031) of GO-NH2 and FGO enhanced relative to those of GO (1.232) because of the structural distortions induced by the KH550 chains and the bulky POSS cages.[21, 28] The TGA profiles of GO, GO-NH2, and FGO as a function of temperature under N2 are shown in Figure 1d. After GO was grafted by KH550 and OGPOSS, the first weight loss stage of the modified nanosheets was postponed at temperatures above 100 °C because of the reduction of volatile water molecules on the surfaces of GO, GO-NH2, and FGO. The main mass loss stage of GO was caused by the decomposition of oxygencontaining groups on GO and occurred between 162 °C and 251 °C. This stage was signally weakened, indicating that the oxygen-containing groups of GO were replaced by KH550 and OGPOSS. The third stage was attributed to the decomposition of the carbon skeleton.[29] The char residues of GO at 800 °C were equivalent to 46.8 wt.%. However, the GO-NH2 and FGO curves indicated that decomposition terminated before 600 °C, indicating that the carbon skeleton may be partially protected by siliconcontaining products from KH550 and OGPOSS. The char residues of GO-NH2 and FGO at 800 °C reached 57.2 wt.% and 60.4 wt.%, respectively. The above TGA results verified the successful functionalization of the samples, and the increase in char residue implied the potential flame retardancy of FGO. XPS analysis was employed to further evaluate the chemical bonds that formed on the surfaces of GO, GO-NH2, and FGO. The XPS survey spectrum and the highresolution C1s, O1s, N1s, and Si2p spectra are given in Figure 2 and Figures S1 and S2 in the supporting information. In Figure 2a, the main signals observed in the XPS spectra of GO-NH2 and FGO were C1s, O1s, N1s, Si2s, and Si2p. The comparison with GO revealed that the extra N and Si elements in GO-NH2 and FGO confirmed covalent 11

functionalization. Figures 2b to e further verify the chemical bonds of FGO. Figure 2b shows that the C1s spectrum of FGO could be fitted by five Gaussian–Lorentzian peaks related to C–Si (283.7 eV), C=C/C–C (284.7 eV), C–O/C–O–Si (285.6 eV), C–N (286.3 eV), and C–O–C (286.8 eV).[13] This result was the same as that of the C1s spectrum of GO-NH2 in Figure S2a, indicating the successful grafting of KH550 on the GO surface. Figure 2c presents that the O1s spectrum of FGO could be separated into five peaks, including COO (530.4 eV), C=O (531.5 eV), C–O/C–O–Si (532.5 eV), Si–O–Si (532.8 eV), and C–O–C (533.5 eV).[13, 30] The comparison with the spectrum of GO-NH2 in Figure S2b revealed the appearance of new Si–O–Si bonds that confirmed the introduction of OGPOSS. In Figure 2d, the N1s spectrum of FGO was split into C–NH–C (399.5 eV) and C–N (401.2 eV).[31] In comparison with the intensity of peaks in Figure S2c, the increased ratio of C–NH–C relative to that of C– N indicated that a covalent reaction occurred between the amino groups of KH550 and the epoxy groups of OGPOSS. Moreover, as shown in Figure 2e, the Si2p spectrum of FGO was divided into Si–O–C (102.1 eV), Si–C (102.6 eV), and Si–O–Si (103.5 eV). The presence of the Si–O–Si peak further indicated the cages of OGPOSS in FGO structures. All these results demonstrated the successful covalent functionalization of GO by KH550 and OGPOSS.

12

Figure 2 XPS results: a XPS survey spectra of GO, GO-NH2, and FGO, b C1s spectrum, c O1s spectrum, d N1s spectrum, and e Si2p spectrum of FGO.

13

Figure 3 SEM images of a GO, b GO-NH2, and c FGO; TEM images of d GO, e GONH2, and f FGO.

SEM, EDS, and TEM results were employed to determine the morphological characteristics and elements distribution of GO, GO-NH2, and FGO (Figure 3 and Table S1). In Figure 3a, GO displayed a smooth and flat layer structure of individual sheets with waves. The transparent single-layer and black multilayer structure of GO are shown in Figure 3d. The structure was caused by defects and the formation of functional groups, such as hydroxy, carboxyl, and epoxy groups, on the surface of GO.[32] The uneven GO stack would induce agglomeration and heterogeneous dispersion in composites. GO-NH2 sheets were fuzzier and more flexible and wrinkled than GO (Figures 3b and e) because KH550 molecules were covalently grafted on the GO surface.[33] Moreover, FGO was rough and intumescent with more distinct wrinkles without multilayer structure (Figures 3c and f). OGPOSS cages further increased the structural distortions of graphene sheets, thus restraining the restacking of GO sheets and increasing disorder structures. In Table S1, compared with the elements of GO, the obvious N and Si elements of GO-NH2 implied successfully grafted silane molecules of KH550. Moreover, the content of Si element of FGO was distinctly increased to 22.32 wt.%, which indicated the further reaction and graft of Siabundant OGPOSS. The SEM, EDS, and TEM results further verified the structure and latent good dispersion ability of the FGO sheets. 3.2.

Thermal behaviors 14

Figure 4 Thermal properties of EP, 0.1GO/EP, and FGO/EP composites: a DSC analysis, b and c TGA results, and d thermal conductivity values.

FGO and GO were utilized as nanofillers to prepare EP composites with specific contents. The thermal properties of pure EP, EP composite reinforced with 0.1 wt.% GO (0.1GO/EP), and EP composites modified with 0.1, 0.3, 0.5, 0.7, and 0.9 wt.% FGO (0.1FGO/EP,

0.3FGO/EP,

0.5FGO/EP,

0.7FGO/EP,

and

0.9FGO/EP)

were

systematically investigated through DSC, TGA, and thermal conductivity analyses. Figure 4a presents the DSC curves of the EP composites. The glass transition temperature (Tg) of the samples is an important parameter that reflects the highest usage temperature of epoxy matrix composites. This parameter is marked on the graph and summarized in Table 1. All the EP samples displayed one Tg step without any curing peak in the experimental temperature range. This characteristic indicated a fully cured status. The pure EP exhibited Tg of 157.0 °C. After 0.1 wt.% GO was added, 0.1GO/EP composite yielded a slightly increased Tg of 158.1 °C, and this increase was attributed 15

to the wrinkled structure and large specific surface area of GO sheets that likely constrained the segmental movement of polymer chains.[34, 35] With the same amount of addition as that of 0.1GO/EP, 0.1FGO/EP exhibited a slightly decreased Tg of 156.0 °C. When the filler content gradually increased from 0.1 wt.% to 0.7 wt.%, the corresponding composites presented step-up Tg from 156.0 °C to 159.6 °C. However, when the filler content exceeded 0.7 wt.%, Tg of 0.9FGO/EP decreased to 156.9 °C. Various factors affected Tg possibly because of the following. On the negative side, FGO with POSS cages and enlarged sheet sizes might replace EP and amplify the free volume of EP networks; therefore, the effective number of cross-linking points per volume decreased to some extent and resulted in the incomplete curing of EP.[36, 37] At the same time, the localized clustering of FGO sheets at a high content can facilitate the movement of polymer chains because of the reduction in cross-linking density.[38] On the positive side, the covalent bonding between FGO and the matrix improved the dispersion of FGO sheets, thus enhancing the interfacial interaction between the sheets and the matrix. Thus, the mobility of the polymer chains was constrained, and thermal resistance was improved.[35, 38] All these factors affected the glass transition behavior of FGO/EP composites and achieved a balance, so changes in Tg in this work were negligible. Therefore, the thermal resistance of FGO/EP composites was maintained. Figures 4b and c present the thermal stability of pure EP, 0.1GO/EP, and FGO/EP composites as examined via TGA tests under a N2 atmosphere. Table 1 also summarizes the TGA and derivative TGA (DTG) parameters. As shown in Figures 4b and c, all the EP samples displayed one sharp weight loss stage that corresponded to the breakdown of the primary chains of the EP cross-linking networks. The weight loss temperatures that corresponded to samples containing 10 wt.% (T 10 wt.%) almost did not differ from those of the pure EP except the 0.7FGO/EP data that slightly increased. The DTG curves indicated that the incorporation of GO and FGO obviously decreased the maximum weight loss rate of the composites as reflected by DWmax shown in Table 1. Moreover, the residues at 800 °C were higher than those of pure EP as the FGO contents increased. In particular, 0.7FGO/EP yielded the highest char residue, indicating that the thermal 16

stability of EP composites improved at high temperatures. These results were attributed to the improved dispersion and interfacial interactions between FGO and EP chains that acted as obstacles by reducing matrix mobility.[35, 39, 40] Figure 4d shows the thermal conductivities of the EP samples with different loadings of GO and FGO fillers. Table 1 also presents the results. The thermal conductivity values in this study decreased with the introduction of nanosheets and were less than the value of pure EP (0.2534 W/mK) in contrast to those in other studies.[32, 41] Graphene-based nanomaterials have an ultrahigh in-plane thermal conductivity of ∼5300 W/mK. However, the vertical testing pattern of the flat samples in this work might not yield the desired results in comparison with the horizontal method. This discrepancy could be attributed to the horizontally aligned nanosheets in cured networks, which are not an efficient thermal conduction path along the through-plane direction.[42] The large surface area of FGO with cube-octameric frameworks from POSS branches might lengthen the thermal path

and cause a labyrinth effect[43]

(tortuous effect) that could inhibit heat diffusion and potentially improve the flame retardancy of EP composites. Table 1 Thermal parameters from DSC, TGA, DTG, and thermal conductivity analyses. Tg

T10 wt.%

DWmax

Residue

Thermal conductivity

(°C)

(°C)

(%/min)

at 800 °C (wt%)

(W/mK)

EP

157.0

375.0

-2.7

14.4

0.2534

0.1GO/EP

158.1

375.9

-1.9

13.8

0.2513

0.1FGO/EP

156.0

375.2

-1.9

14.2

0.2481

0.3FGO/EP

157.2

375.0

-2.1

15.1

0.2454

0.5FGO/EP

158.0

375.5

-1.8

15.9

0.2427

0.7FGO/EP

159.6

376.6

-1.9

17.2

0.2377

0.9FGO/EP

156.9

375.1

-1.8

16.3

0.2324

Samples

3.3.

Mechanical properties

17

Figure 5 Mechanical properties of EP, 0.1GO/EP, and FGO/EP composites: a tensile stress–strain curves, b tensile strength

The mechanical properties of pure EP, 0.1GO/EP, and FGO/EP composites were investigated through tensile tests. The stress–strain curves and tensile strength values are presented in Figures 5a and b. The fracture surfaces of the samples after tensile tests are shown in Figure 6. As illustrated in Figure 5a, EP and its composites exhibited a typical brittle fracture behavior. In Figure 5b, the 0.1GO/EP composite yielded the lowest tensile strength among all the samples. The FGO/EP composites with different filler loadings strengthened as their filler content increased to 0.7 wt.%. The 0.7FGO/EP composite showed the best tensile property with a strength of 41.43 MPa, which was approximately 27% higher than the strength of pure EP (32.59 MPa). The increment in strength could be attributed to the epoxy groups on FGO sheets that might be involved in the curing reaction and to the wrinkled structure of FGO, which could enhance mechanical interlocking with EP chains.[41] However, the strength distinctly decreased when the filler content was 0.9 wt.%. Figures 6a and b show that the pure EP exhibited smooth and river-like lines on its surface (Figure 6a) and mirror-like features in the magnified image (Figure 6b) that are typical characteristics of a brittle fracture. The agglomerated GO sheets in 0.1GO/EP composites (Figure 6d) could induce crack initiation and propagation under tension.[44] However, the dispersion of FGO was more uniform than that of GO in the matrix. The interface between the matrix and FGO sheets was cohesive, providing good interfacial adhesion (Figure 6f) likely because of 18

the good interaction stemming from FGO with functional groups that could react with the EP matrix during curing. The surface roughness of the samples increased as the filler concentration increased (Figures 6e, g, and i), suggesting that the rigid fillers might hinder crack fronts from propagating. The deterioration of the tensile property of the 0.9FGO/EP composite was likely a result of the degradation in the dispersion of agglomerated FGO sheet clusters as shown in Figure 6j.[35, 41] Overall, the FGO sheets that were well dispersed in the matrix provided the EP composites with an improved tensile property. This improvement implied that the modification of GO sheets by OGPOSS was effective.

Figure 6 SEM images of fracture surfaces after tensile tests: a and b EP, c and d 0.1GO/EP, e and f 0.1FGO/EP, g and h 0.7FGO/EP, and i and j 0.9FGO/EP composites.

3.4.

Flame-retardant properties

Cone calorimetry tests were conducted to detect the flame retardancy of FGO/EP 19

nanocomposites. The representative graphs and detailed data are presented in Figure 7 and Table 2. The time to ignition (TTI) of 0.7FGO/EP nanocomposites was postponed from 40 s to 49 s compared with that of pure EP, indicating the improvement of thermal stability in an actual flame environment (Table 2). The peak of heat release rate (PHRR), total heat release (THR), and total smoke release (TSR) of 0.7FGO/EP nanocomposites decreased by 49.7%, 34.3%, and 41.5%, respectively, indicating a reduced fire hazard and a distinct smoke suppression effect. The PHRR reduced because of the inhibition of heat diffusion and release due to the tortuous effect of FGO sheets as confirmed by the thermal conductivity results in Figure 4d. The interaction between FGO sheets and EP hindered the movement of segments and EP chains. Thus, the THR decreased because of the good dispersion of FGO. In addition, the decreased TSR and the increased char residue of 0.7GO/EP (from 6.7 wt.% to 17.3 wt.%) suggested that the EP composites showed restrained mass loss and improved thermal stability at high temperatures. The char residue result was more obvious than TGA results. As shown in the discussion of a previous work,[9] FGO fillers can exhibit different effects on TGA and cone calorimetry tests with different sample scales, boundary conditions, and heating conditions. Gas volatiles from the samples of cone calorimetry tests can only move in the vertical direction of the samples with a large planar size, drastically reducing pyrolysis gas because of the tortuous effect of FGO nanosheets on gas products and heat transfer. This phenomenon was in accordance with the results of thermal conductivity tests shown in Figure 4d. By contrast, gas volatiles from samples in TGA could rapidly escape from all directions because TGA samples had comparable dimensions and were uniformly heated in all directions. In this case, the tortuous effect of FGO nanosheets was negligible. Furthermore, the trends of CO production curves was similar to those of HRR curves. The average of effective heat of combustion (avEHC) of FGO was reduced to 26.1 MJ/kg, indicating the less burning rate of volatile gaseous phase.[45] The reduced release of volatile gases indicated that a possible barrier effect developed in the condensed phase. All these results verified that FGO could provide EP with an excellent smoke-suppressive property. The fire growth rate 20

(FIGRA) and fire performance index (FPI) are two important parameters for evaluating fire safety. FIGRA is defined as PHRR divided by TPHRR, and FPI is defined as TTI divided by PHRR. Generally, material with superior fire safety performance should possess a low FIGRA value and a large value of FPI.[46] As shown in Table 2, the FIGRA value of 0.7FGO/EP nanocomposite was lower, and the FPI value was higher, compared with those of pure EP, thereby indicating the apparent flame retardancy and remarkable effect on minimizing fire hazard. Therefore, FGO sheets played an important part in restraining the heat and smoke release during combustion and contributed to the improvement of the flame retardancy of EP composites.

Figure 7 Cone calorimetry test results: a heat release rate curves, b total heat release curves, c total smoke release curves, d mass loss curves, and e CO production curves of pure EP and 0.7FGO/EP nanocomposites.

Table 2 Cone calorimetry parameters of pure EP and 0.7FGO/EP nanocomposites. TTI

TPHRR

PHRR

THR

TSR

Residue

av-COY

av-EHC

Samples

FPI FIGRA

(s)

(s)

(kW/m2)

(MJ/m2)

(m2/m2)

(wt.%)

(kg/kg)

EP

40

135

1677.9

148.0

6009.4

6.7

0.13

33.7

12.4

2.4

0.7FGO/EP

49

120

844.7

97.3

3518.5

17.3

0.09

26.1

7.0

5.8

21

(×10-2)

(MJ/kg)

3.5.

Char residue analysis

Figure 8 Char residue structures after cone calorimetry tests: a and b SEM images of EP, c and d SEM images of 0.7FGO/EP nanocomposite, and e–i EDS results of 0.7FGO/EP nanocomposite.

The char residue structures of pure EP and 0.7FGO/EP nanocomposites after cone calorimetry tests were investigated through SEM, EDS, and Raman spectroscopy to analyze the internal flame-retardant and smoke-suppressive mechanisms of FGO nanosheets in the solid phase. As shown in Figures 8a and b, numerous through-holes were present in the char residue of pure EP. These holes would provide considerable channels for the combustible volatiles to the gas phase and increase the combustion intensity. By contrast, the char of 0.7FGO/EP nanocomposite modified with FGO sheets considerably differed from that of the aforementioned sample. As shown in Figures 8c and d, a firm and compact ceramic layer was formed on the char surface. This morphology suggested that FGO could promote EP to form an effective char layer and work as a protective barrier that could avoid the feedback of heat, oxygen diffusion between the condensed and gas phases, and subsequent decomposition, thus improving 22

flame retardancy and smoke suppression. In addition to C and O in the char of EP, elemental Si element was detected in the EDS results (Figure 8e) of the 0.7FGO/EP nanocomposite, suggesting the successful formation of a stable Si-containing structure after combustion. In addition, the distribution patterns of elements in the basement layer and surface layer of the char structure were different. The ratios of Si/C or O/C in the surface layer were higher than those in the basement layer (Figure 8e–i), indicating that the loading of FGO into the EP matrix promoted the formation of Si–O structures over that of char residues. ID/IG values of the char in Raman spectra were also detected, which represents the degree of graphitization of chars after combustion. Lower ID/IG value means higher graphitization degree of carbonaceous materials. 0.7FGO/EP nanocomposite in the Raman spectra was lower than that of pure EP (Figure 9). The high graphitization degree of carbonaceous materials was related to a high thermal stability. Therefore, the improvement in flame retardancy as a result of the barrier effect of Si-containing char layers in the condensed phase was further confirmed.[47]

Figure 9 Raman spectra of the char residues obtained from cone calorimetry tests: a pure EP, b 0.7FGO/EP nanocomposite.

3.6.

Gas volatile analysis

23

Figure 10 Absorbance of the main gas volatiles of pure EP and 0.7FGO/EP nanocomposites: a and b FTIR spectra at different temperatures, c water/phenol derivatives, d aliphatic compounds, e aromatics compounds, and f ether compounds.

TGA–FTIR tests were conducted to obtain the detailed information of gas volatiles of pure EP and its nanocomposites. The absorbance results of different products are presented in Figure 10. As shown in Figures 10a and b, the pyrolysis gases of pure EP and the 0.7FGO/EP nanocomposite exhibited similar characteristic peaks during the whole thermal decomposition. This result indicated that the improvement in flame retardancy would be difficult to interpret on the basis of the gas phase mechanism.[48] Several specific volatiles were studied (Figures 10c–f) to further understand the change in the intensity of gas products during thermal decomposition. The intensities of water/phenol derivatives, aliphatic compounds, aromatic compounds, and ether compounds in the 0.7FGO/EP nanocomposite were distinctly lower than those in pure EP. The decrement in these organic pyrolysis products could effectively inhibit the release of smoke and mass loss during combustion, and this observation was consistent with the results of char residue in cone calorimetry tests; as a result, flame retardancy and fire safety enhanced.[49] These results could be attributed to the barrier effects from FGO fillers that acted as a protective layer to isolate combustible gases from

24

transferring between the inner chamber and the air full of oxygen and heat. Thus, the smoke suppression of FGO sheets was confirmed to be efficient and obvious. 3.7.

Flame-retardant mechanism

A possible flame-retardant mechanism of FGO in the EP matrix was proposed in accordance with the above analysis (Figure 11). The tortuous and barrier effects of well-dispersed FGO sheets endowed EP composites with improved smoke-suppressive property and flame retardancy. After modification, the FGO sheets with OGPOSS branches and functional epoxy groups could disperse well in EP networks (Figure 6h) as expected. The FGO with a large surface could construct a long thermal path in the condensed phase and therefore exhibited a tortuous effect on the EP matrix and inhibited the transfer of gas volatile products and heat during combustion (Figures 4d and 7d). The Si-containing groups of OGPOSS on FGO decomposed into stable and compact char residue layers on the surface of the EP matrix (Figure 8d). These layers could work as an effective protective layer and isolate the pyrolysis products and heat or oxygen. Therefore, combustion would be weakened by the barrier effect. Both of these two effects, which were attributed to the FGO sheets, drastically reduced fuel and smoke release and maintained a high char yield. Thus, the addition of FGO into EP networks is a viable approach to the effective flame-retardant modification of EP composites.

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Figure 11 Proposed flame-retardant mechanism of FGO sheets in EP composites.

Conclusions In this work, FGO sheets were synthesized by using KH550 and OGPOSS through two graft processes to obtain nanofillers with good dispersion and smoke-suppressive properties for flame-retardant EP composites. The obtained FGO sheets were confirmed to exhibit structural distortions and objective functional groups through systematic characterizations, indicating that they were successfully prepared. FGO/EP nanocomposites with various contents of FGO sheets were subsequently manufactured. Given the good dispersion and improved interfacial interaction between the FGO sheets and EP matrix, the nanocomposites maintained their thermal resistance, exhibited an increased char yield, and had an improved tensile property in DSC, TGA, and tensile tests. The improvements of 0.7FGO/EP nanocomposites were more prominent than those of other composites. Cone calorimetry indicated that the modification of EP composites with 0.7 wt.% FGO sheets could reduce PHRR, THR, and TSR values by 49.7%, 34.3%, and 41.5%, respectively. The reduction in thermal conductivity was 26

attributed to the tortuous effect of FGO sheets that lengthened the heat path in the EP matrix. Enhanced char residue and reduced gas volatiles provided evidence that the protective char structure that formed on the surface of the matrix originating from the FGO sheets exerted a barrier effect that restrained mass loss through smoke release. The flame-retardant mechanism was ascribed to the tortuous and barrier effects of FGO sheets in the condensed phase. The approach described herein is a feasible concept for enhancing the flame retardancy of EP composites by using well-dispersed modified GO sheets, which effectively constructed an interfacial interaction between nanofillers and matrixes and simultaneously presented outstanding smoke suppression in combustion. Conflicts of interest The authors have no conflicts of interest to declare. Data availability statement The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Acknowledgments This work was supported by the Natural Science Foundation of Jilin Province (No. 20180101197jc), the International Science and Technology Cooperation Programme of China (20190701001GH), the National Natural Science Foundation of China (No. 11602125), and Beijing Institute of Technology Research Fund Program for Young Scholars.

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31

GA

32

Highlights: 1. Epoxy resin composites with improved flame retardancy were fabricated based on functionalized graphene oxide (FGO) with improved dispersive and smokesuppressive properties. 2. The flame-retardant mechanism was ascribed to the tortuous and barrier effects of FGO sheets in the condensed phase. 3. This method provided a feasible concept for effectively enhancing the flame retardancy of EP composites by combining the characteristics of graphene and polyhedral oligosilsesquioxane through the construction of effective interfacial interactions.

33

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

34