Facile and green preparation of biobased graphene oxide/furan resin nanocomposites with enhanced thermal and mechanical properties

Abstract

A facile and green approach was developed to prepare biobased graphene oxide (GO)/furan resin nanocomposites by directly transferring GO from water dispersion into furan resin. The structure of nanocomposites was characterized by Fourier infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). A good dispersion of GO within furan resin was observed. Performance investigation revealed there was a significant improvement in thermal, mechanical and chemical resistance properties of biobased GO/furan resin nanocomposites when a low content of GO was introduced into neat furan resin. The nanocomposite with a low content of 0.05 wt% GO showed better improvement in tensile strength and Young's modulus of 72% and 282%, respectively. Thermal analysis of the nanocomposites also illustrated an enhancement in thermal stability. The obtained biobased GO/furan resin showed good chemical resistance.

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1. Introduction

Recently, biobased polymers and nanocomposites from renewable resources have gained considerable attention because they could potentially replace or partially replace environmentally and energy unfavorable petroleum-based products.^1–6 Furan resin is a typical biobased thermosetting resin. The main raw materials in the preparation of furan resin are furfural and its derivatives, which are biobased monomers obtained from hydrolysis of pentosan-rich biomass such as agricultural residues of corn, sugarcane, wheat, oat, cottonseed hulls, rice hulls, birch wood, and hazelnut shells.^7–10 Furan resin has been widely used in the elaboration of foundry moulds, fire resistant and corrosive resistant materials employed in the building due to high carbon content, chemical inertness and thermal stability, etc. However, with the expanding use of furan resin, the continuous improvement of engineering quality requirements, its poor mechanical properties and some shortcomings gradually expose, thus limit its development. In order to meet the application requirements, it urgently needs to improve its comprehensive performance.^11–15

Graphene, a one-atom-thick 2D layer of sp²-bonded carbon, has been intensively studied since its discovery.¹⁶ Because of its unique two-dimensional crystal structure and excellent crystalline quality, graphene has shown many excellent properties, for example, high tensile strength, high thermal conductivity, high electron mobility at room temperature and large specific surface area.^17–19 Many researchers have incorporated graphene into the polymer to get graphene based polymer composites with excellent properties.^20–30 To enhance the properties of the graphene based polymer composites, graphene must be exfoliated and dispersed in polymers well.^21,22 Various graphene based polymer composites have been developed using GO as a precursor because GO can be dispersed in water as individual sheets easily. Furthermore, GO have a strong mechanical strength (fracture stress about 63 GPa)¹⁶ and can be modified by functionality. Bora et al.²³ reported preparation of nanocomposites based on GO and unsaturated polyester resin using tetrahydrofuran as a solvent, a significant enhancement in mechanical properties of the PE/GO composites was obtained at low graphene loading. Liu et al.²⁴ reported a convenient one-pot method integrating a novel solvent-exchange method into in situ melt polycondensation to fabricate unsaturated polyester nanocomposites containing functionalized graphene sheets. Wang et al.²⁵ prepared GO/epoxy nanocomposites using a blend of GO and epoxy resin in acetone, the incorporation of GO also enhanced the stiffness and thermal stability of the epoxy. Yang et al.²⁶ developed a novel method to prepare GO reinforced epoxy resin nanocomposites through two-phase extraction. They found that the compressive failure strength and toughness of 0.0375 wt% GO/epoxy resin increased by 48.3% and 1185.2%, respectively. However, in summary, these methods are either not green (require a large amount of organic solvents) or relatively complicated. Additionally, most of literatures focus on the GO/epoxy resin nanocomposites, few reports are on GO/furan resin nanocomposites.

Since furan resin is consisted of polyfurfuryl alcohol oligomers, which have many hydrophilic hydroxyl groups.¹⁵ The structure of GO also has many hydrophilic oxygenated functional groups such as hydroxyl, epoxy, carbonyl and carboxyl groups.¹⁶ This makes it possible for GO to be disperse in hydrophilic furan resin easily. Thus, here, we report a facile and green method to prepare biobased GO/furan resin nanocomposites. GO was dispersed into biobased furan resin by directly blending and evaporating of GO water dispersion in furan resin. Subsequently, a step curing procedure of the GO/furan resin was performed with p-toluenesulfonic acid (PTSA) as a curing agent (Fig. 1). The thermal, mechanical and chemical resistance properties of biobased GO/furan resin nanocomposites were also investigated.

Fig. 1 Schematic representation of green preparation of biobased GO/furan resin nanocomposites.

2. Experimental

2.1. Materials

Graphite was purchased from Huadong Graphite Co., China. Sodium nitrate, potassium permanganate, hydrogen peroxide and PTSA were purchased from Shanghai Reagent Co. (Shanghai, China). Furan resin containing polyfurfuryl alcohol oligomers with a viscosity value of about 340 mPa s (25 °C) was a gift from Huangshi Fybo Material Technology Co., China. All the reagents were analytical grade and used as received without any further purification.

2.2. Preparation of GO water dispersions

GO was prepared according to the modified Hummers' method.³¹ Briefly, 1.25 g of sodium nitrate was stirred with 2.5 g of natural graphite in an ice-water bath for 10 minutes. Once the mixture was well-mixed, 60 mL of sulfuric acid was added gradually into the mixture. The mixture was kept between the temperature range of 0 °C to 5 °C in the ice-water bath as a safety precaution. While maintaining vigorous agitation, 7.5 g of potassium permanganate was added into the suspension gradually. The rate of addition was controlled to prevent the temperature of the mixture from exceeding 20 °C. The mixture was kept mixing for a total of 2 hours with the temperature of the ice-water bath kept at ≤5 °C. The ice-water bath was then removed and the mixture was stirred overnight at room temperature. As the reaction progressed, the mixture gradually thickened. After a night of stirring, the mixture became pasty and it became brownish grey in color. Thereafter, 135 mL of deionized water was slowly stirred into the paste and this resulted in violent effervescence and a temperature hike to 98 °C. A watch glass was used to minimize evaporation and the level of the mixture was maintained at 250 mL with periodical addition of deionized water for 1 hour. Subsequently, the heater was turned off and the mixture was then left to cool for 1 hour to room temperature. Once completed, 25 mL of hydrogen peroxide was added dropwise to reduce the residual permanganate and manganese dioxide to colorless soluble manganese sulfate. After the filtration of the mixture, the residue was washed by HCl solution four times and ultrapure water five times. The product was dialyzed by deionized water for one week. Then, it was dried at 60 °C and stored for further use. To prepare GO water dispersion, GO powder was dispersed into 100 mL deionized water, followed by an ultrasonic for 30 min to form a yellow stable GO water dispersion.

2.3. Preparation of biobased GO/furan resin nanocomposites

As shown in Fig. 1, in order to prepare biobased GO/furan resin nanocomposites, firstly, 20 mL GO water dispersion with different concentration was added into 30 mL furan resin. Then, the mixture was sonicated for 10 min and became homogenized brown without stratification. After most of water was gradually evaporated under stirring on a heating plate at 60 °C for 12 h, stoichiometric amount of curing agent (PTSA) was added into the mixture. Finally, the mixture was casted into a standard mold and placed in an oven for curing. The curing cycle was 2 h at 80 °C, 2 h at 100 °C, 2 h at 120 °C, and 2 h at 160 °C, respectively. After curing, the cured samples were cooled to room temperature for further properties evaluation. The cured samples with different contents of GO (0, 0.05, 0.1 and 0.3 wt%) were designed as FGO0, FGO1, FGO2 and FGO3, respectively.

2.4. Characterization

FTIR spectra were recorded on a Nicolet Nexus 670 FTIR spectrometer in the region of 400–4000 cm⁻¹ by using the KBr wafer technique. The phase structure was examined by XRD using a D8 Advance X-ray diffractometer with Kα radiation (25 mA and 40 kV) at a scan rate of 2° min⁻¹ with a step size of 0.02°. SEM of the fractured surfaces was performed using SH-1500 SEM. The fractured surfaces of the specimens were covered with gold vapor. The acceleration voltage was 20 kV. The T_g of the sample was measured with a dynamic mechanical analyzer (DMA, Thermal Analysis DMA-Q800). The applied static force and dynamic force were 0.11 and 0.10 N, respectively. The heating rate was 5 °C min⁻¹ and the frequency was 1 Hz. The peak temperature of the tan δ plot was taken as T_g of the sample. Thermogravimetric analysis (TGA) was performed on STA449F3 DSC/TG Thermal Analyzer, from room temperature to 800 °C with a heating rate of 10 °C min⁻¹ under the nitrogen flow rate of 30 mL min⁻¹. Tensile tests were carried out with a CMT4000 material testing machines at room temperature with crosshead speeds of 2 mm min⁻¹. The chemical resistance of samples was studied using a modified ASTMD 543-87 method. In each case, the samples were dipped in the respective chemicals for 15 days, and the changes in appearance were monitored.

3. Results and discussion

3.1. Green dispersion of GO in biobased furan resin

In general, most of GO based polymer resin nanocomposites are prepared using a method of solvent mixing or in situ polymerization, which often requires a large amount of organic solvents.^20–30 With the assistance of organic solvents, GO can be dispersed into the polymeric matrix well. Finally, the organic solvents require removal. The removed organic solvents volatiles will do harm to human beings. However, in our preparation process, furan resin was consisted of polyfurfuryl alcohol oligomers, which had many hydrophilic hydroxyl groups and oxygen atoms on the molecule chain. The oxygenated functional groups i.e. hydroxyl, epoxy, carbonyl and carboxyl groups attached on GO was also hydrophilic. The same hydrophilic characteristics of GO and furan resins made it possible for GO to be dispersed into hydrophilic furan resin easily. In other words, the compatibility of GO with furan resin was good enough to get a good dispersion without any further surface treatment. So, at first, GO was prepared by the modified Hummers' method.³¹ Then, the as-purified GO powder was distributed in water to create a homogenous yellow water dispersion ready for use (Fig. 2a). The resulting GO sheets had been characterized by TEM and XRD (Fig. 2b and c, the range of 2θ was from 4° to 50°). It can be seen that the sizes of as-prepared GO nanosheets were about several micrometer or several hundred nanometers. They exhibited a characteristic diffraction peak at 2θ = 9.8° (in dry state), corresponding to an inter-planar spacing of 7.76 Å.³¹

Fig. 2 Photograph of GO water dispersion (a), TEM image of GO (b) and XRD pattern of GO (c), respectively.

In the next stage, GO water dispersion was mixed with furan resin under stirring. Then, the water was gradually evaporated from the GO/furan resin mixture on a heating plate under mild conditions. In this process, water not only acted as solvents but also acted as dispersion agents. Upon the water was evaporated completely, a stable brown black sticky dispersion of GO/furan resin can be observed (Fig. 3a). By this simple and environmentally-friendly process, GO can be directly dispersed into furan resin from water dispersion through blending and evaporating methods. Therefore, it avoided using a large amount of organic solvents as dispersion agents for GO.

Fig. 3 Photographs of freshly prepared furan resin and GO/furan resin before (a) and after (b) 3 days, and the viscosities of GO/furan resin with different contents of GO (c).

Fig. 3a and b show photographs of freshly prepared furan resin and GO/furan resin before and after 3 days, respectively. Both neat furan resin and GO/furan resin showed brown black. The viscosity of GO/furan resin was slightly higher than that of neat furan resin. Both of them were homogeneous. There was no gel or delamination after 3 days when the content of GO was very low. The content of GO on the viscosities of GO/furan resin was further investigated. As shown in Fig. 3c, with higher GO contents, substantial increases in apparent viscosities were observed. When the content of GO exceeded 0.3 wt%, a drastic increase in viscosity of GO/furan resin made it very difficult to flow. This result suggested interactions of dispersed GO with furan resin molecules, which would certainly restrict the free flow of furan resin.^32–36

3.2. Characterization of biobased GO/furan resin nanocomposites

The brown black sticky dispersion of GO/furan resin was subsequently cured with PTSA as a curing agent to obtain biobased GO/furan resin nanocomposite. All cured samples were black films (Fig. S1†). FTIR was used to investigate the structure of the nanocomposites and the interaction or reaction between GO and furan resin. Fig. 4 shows the FTIR spectra of GO, neat furan resin and its GO nanocomposites, respectively. For GO, several of characteristic adsorption peaks of GO can be seen (the C=O carbonyl stretching vibration at 1732 cm⁻¹, the O–H deformation vibration at 1390 cm⁻¹, and the C–O stretching vibration at 1225 and 1053 cm⁻¹).^16–18 For GO/furan nanocomposites, the absorption band at about 3400 cm⁻¹ was due to O–H stretching vibration. The C=O stretching was observed around 1714 cm⁻¹. The peak near 1055 cm⁻¹ and 1609 cm⁻¹ was responded to C–O–C stretching vibration and C=C deformation vibration respectively.^11–15

Fig. 4 FTIR spectra of GO, neat furan resin and its GO nanocomposites, respectively.

Comparing the FTIR spectra of FGO0 with FGO1, FGO2 and FGO3, it can be found that the absorption band at 3400 cm⁻¹ became broader, while the intensity of C=O stretching vibration peak at 1714 cm⁻¹ became weaker. This revealed that the hydrogen bond existed between the carboxyl group of GO and hydroxyl group of furan resin.^37–40 In addition, the enhancement of intensity of C–O–C stretching vibration peak at 1714 cm⁻¹ was mainly because that the cross-linking reaction between GO and furan resin had produced new C–O–C bonds. The schematic reaction mechanism is shown in Fig. 5.

Fig. 5 The schematic reaction between GO and furan resin.

Fig. 6 shows the XRD patterns of neat furan resin (FGO0) and its GO nanocomposites, respectively. From Fig. 6, it can be seen that FGO0 showed a broad and weak peak at about 20.3°, which illustrated that the neat furan resin was amorphous in nature. Furthermore, the characteristic diffraction peak of GO (9.8°) became negligible in all of the GO/furan resin nanocomposites, which can be owing to the exfoliation and homogeneous dispersion of GO in the furan resin. Similar results were reported by Liu and Bora et al.^23,24

Fig. 6 XRD patterns of neat furan resin and its GO nanocomposites, respectively.

3.3. Thermal properties of biobased GO/furan resin nanocomposites

The loss factor tan δ is defined as the ratio of the loss modulus to the storage modulus, which is very sensitive to solid structural transformation in materials. T_g can be determined from the peaks of the loss factor tan δ.⁴¹ DMA was conducted to study the T_g of biobased GO/furan resin nanocomposites. Fig. 7 shows the temperature dependence of tan δ for neat furan resin and its GO nanocomposites, respectively. As shown in Fig. 7, the T_g of FGO0 was only 93 °C. However, with the addition of GO, the T_gs of the nanocomposites gradually increased. When the content of GO was 0.3 wt%, the T_g of the nanocomposite can reach 129 °C. This meant the T_g of the cured GO/furan resin nanocomposite could be improved 36 °C by the addition of GO. This improvement in T_g may be attributed to the fact that the hydrogen bond interaction and high degree of cross-linking reaction between GO and furan resin restricted the macromolecular motion.⁴²

Fig. 7 The dissipation factor (tan δ) of neat furan resin and its GO nanocomposites, respectively.

Thermal stability of GO, neat furan resin and its GO nanocomposites was also evaluated by TGA (Fig. 8), and the results of the temperature for 5 wt% and 15 wt% weight loss (T₅ and T₁₅), the char yield at 800 °C are listed in Table 1. It was observed that T₅ and T₁₅ of GO was 107 °C and 136 °C, respectively. This indicated that GO was unstable and easily decompose at high temperature because of the organic functional groups were introduced into the graphene nanosheets. If these groups are present in the furan resin, the stability of furan resin will decrease. However, in fact, all decomposition temperature of the GO furan nanocomposites was higher than those of neat furan resin. T₅ and T₁₅ of the FGO0 were 191 °C, 242 °C, while those for FGO1, FGO2 and FGO3 were 204 °C, 203 °C, 207 °C; 275 °C, 278 °C, 280 °C, respectively. The char yields of GO and neat furan resin at 800 °C were 37.6% and 50.7%, respectively. However, for FGO1, FGO2 and FGO3, the char yields at 800 °C were 52.2%, 53.9% and 52.5%, respectively, this meant the char yields of all GO/furan resin nanocomposites at 800 °C were higher than that of neat furan resin. The main reason may be the interaction and cross-linking reaction between GO and furan resin could increase the cross-linking density and restrict thermal motion of the polymer chains. The results were in accordance with the previous work.¹⁶

Fig. 8 TGA curves of GO, neat furan resin and its GO nanocomposites, respectively.

Table 1 Thermal properties of GO, neat furan resin and its GO nanocomposites, respectively

Sample	T5 (°C)	T15 (°C)	Char yield at 800 °C (%)
GO	107	136	37.6
FGO0	191	242	50.7
FGO1	204	275	52.2
FGO2	203	278	53.9
FGO3	207	280	52.5

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3.4. Mechanical properties of biobased GO/furan resin nanocomposites

GO was expected to have good reinforcement effect for tensile properties of the furan resin due to its large aspect ratio and excellent mechanical strength.^14–16 Therefore, the mechanical properties of neat furan resin and its GO nanocomposites were tested with a CMT4000 material testing machines. The stress–strain curves, Young's modulus, elongation at break and the tensile strength of neat furan resin and its GO nanocomposites are showed in Fig. 9, respectively. It was found that the addition of GO into the furan resin matrix had a significant influence on the mechanical properties of the GO/furan resin nanocomposites. The Young's modulus of all cured GO/furan resin was higher than that of the neat furan resin. Compared with that of the neat furan resin, the Young's modulus of FGO1 increased from 121.4 MPa to 462 MPa. FGO1 showed a maximum increase of 72.3% in tensile strength. Similar results about the significant mechanical effect at a very low GO loading had been found in GO/polybenzimidazole⁴³ and FGS/epoxy⁴⁴ nanocomposites. The enhanced tensile properties can be attributed to the homogeneous dispersion of GO in furan resin and the hydrogen bond and cross-linking reaction between the GO and furan resin. But the elongation at break of FGO1 decreased from 4.6% to 2.3%, compared with FGO0. The reason may be attributed to a large aspect ratio and the interaction and crosslinking reaction between GO and the furan resin, which confined the movement of the polymer chains. The same results were obtained for other GO based polymer composites.^20–25 Moreover, the decrements of tensile strength, elongation at break and Young's modulus with higher GO loadings (the GO loading content was above 0.05 wt%) can be observed. This may be caused by the possible agglomeration of GO sheets at high content. Similar results were reported in the previous research.^45,46 Liu et al.⁴⁵ prepared polyester/reduced GO composites. They found the strengthening and toughening effect was reduced when the GO loading content increased to 0.5 wt%. They believed this was attributed to the possible agglomeration of GO at high content. Liu et al.⁴⁶ fabricated unsaturated polyester nanocomposites containing functionalized graphene sheets (FGS). They also found the improvement in stiffness reduced when the content of FGS exceeded 0.08%. They concluded it resulted from the gradual FGS agglomeration at higher loading.

Fig. 9 Stress–strain curves (a), Young's modulus (b), elongation at break and the tensile strength (c) of neat furan resin and its GO nanocomposites, respectively.

To understand the effect of GO on the mechanical properties of the furan resin, the fracture surfaces of the cured samples are characterized with SEM, as shown in Fig. 10. Neat furan resin showed a smooth and flat fracture surface (Fig. 10a and A). For its GO nanocomposites, with the content of GO increased, the fracture surfaces of the cured samples became more rougher and fluctuant (Fig. 10b–d and B–D), indicating higher energy was required.⁴ These rough fracture surfaces can be attributed to the good compatibility, interaction and cross-linking reaction between the GO and furan resin. Such interaction and cross-linking reaction favored the stress transfer from the polymer matrix to GO nanosheets, therefore improving the tensile strength and Young's modulus of the composite than those of the neat furan resin samples.

Fig. 10 SEM images of the fracture surfaces of FGO0 (a, A), FGO1 (b, B), FGO2 (c, C) and FGO3 (d, D) (magnification scales: up, ×200, down, ×2000).

3.5. Chemical resistance of biobased GO/furan resin nanocomposites

The chemical resistance of neat furan resin and its GO nanocomposites was investigated. All samples were dipped in the respective chemicals for 15 days, and the changes in appearance were evaluated.^47,48 As shown in Table 2, all cured samples showed good water resistance. FGO0 showed a slight blush when it was immersed in sulfuric acid (25%) and sodium hydroxide (25%). However, FGO1, FGO2 and FGO3 remained unaffected under the same conditions. This suggested that the addition of GO could improve the chemical resistance of the furan resin. This improvement may be attributed to improving the cross-linking density resulted from the cross-linking reaction between GO and the furan resin, which prevented the permeation of solvents. In addition, poor acetone resistance for neat furan resin can be observed. With the increase of GO content, acetone resistance of GO nanocomposites were enhanced gradually due to more GO involving in the cross-linking reaction.

Table 2 The chemical resistance of neat furan resin and its GO nanocomposites in different chemical environment, respectively^a

Sample	Water	Sulfuric acid (25%)	Sodium hydroxide (25%)	Acetone
a “+” = “Unaffected”.
FGO0	+	Slight blush	Slight blush	Blush
FGO1	+	+	+	Slight blush
FGO2	+	+	+	+
FGO3	+	+	+	+

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4. Conclusions

In summary, we have successfully developed a simple and green process to prepare biobased GO/furan resin nanocomposites by directly transferring GO from water dispersion into furan resin through blending and evaporating methods. A stable dispersion of GO in furan resin can be obtained. The thermal, mechanical and chemical resistance properties of the nanocomposites were significantly improved due to hydrogen bond and cross-linking reaction between GO and furan resin. Addition of GO into furan resin led to significant increases of 72% and 282% in tensile strength and Young's modulus of GO/furan resin nanocomposites, respectively. These sustainable biobased GO/furan resin nanocomposites have a potential application in various industry areas.

Acknowledgements

The project sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, National Technology Support Program (2015BAB07B04), China.

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Footnote

† Electronic supplementary information (ESI) available: The photographs of cured samples of neat furan resin and its GO nanocomposites (Fig. S1). See DOI: 10.1039/c6ra11247a

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