Influence of graphene/ferriferrous oxide hybrid particles on the properties of nitrile rubber

Abstract

The reinforcement of rubber by graphene has generated great interest in recent years. The dispersion of graphene in a rubber matrix, however, remains a challenge. In this study, ferroferric oxide (Fe₃O₄) was used as a carrier to prepare graphene/Fe₃O₄ hybrid particles, in order to improve the dispersion of graphene in nitrile rubber (NBR). The graphene hybrid particles prepared by co-precipitation synthesis was added into NBR solution to prepare composites. Results show that there was about 90% improvement in tensile strength when 4 phr of graphene hybrid particles (equally only 0.5 phr of graphene in NBR) were added to the matrix. In contrast, the tensile strength of NBR decreased when 4 phr of Fe₃O₄ were added. SEM and XRD both explained the mechanism for the reinforcement of graphene hybrid particles. The DMA results show higher T_g and a higher value of tan δ for the modified NBR, further proving the reinforcement mechanism, i.e. the hybridization of graphene and Fe₃O₄ improves the dispersion of graphene in NBR and the interfacial action of graphene with NBR. Moreover, addition of graphene hybrid particles gives NBR extra properties such as surface wettability and magnetism, providing a useful guide for the preparation of functional graphene/rubber composites.

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1 Introduction

NBR (nitrile rubber) is an important bulk rubber often used to prepare materials for seal and other industries requiring high gas-barrier performance. The more and more complex usage, however, requires NBR composites with higher performance.

Graphene has been widely used as a functional filler to improve thermal, mechanical and electrical properties of polymers, especially polymeric rubbers.^1–9 Ruoff,^1–3 Wu,⁴ Xia,^5–7 Guo^8,9 et al. successively reported that the mechanical properties of natural rubbers or butadiene styrene rubber are apparently improved by introducing 3% or less content of graphene. In these studies, the blending technologies and dispersing performances have been considered as important factors to the properties of the composites. Blending technologies includes mechanical, solution and latex blending methods. Mechanical blending is the simplest method to operate but usually accompanies with the aggregation during processing.¹⁰ When solution and latex blending is applied, the aggregation is not significant but the solvent or emulsifier in composites cannot be completely remove, which changes the dynamics properties of rubber.^11–13 A novel and simple method that has been reported by Prasad,¹⁴ Ramesh¹⁵ and Tang¹⁶ et al. uses more than one types of particles with different dimensionality to improve the mechanical properties of rubbers. So, is it possible to utilize the hybridization of graphene to improve the dispersion of graphene in rubber matrix?

In our previous study, Fe₃O₄ (ferriferrous oxide) was found to have strong interaction with EVA (ethylene vinyl acetate, which has polar VA segments).^17–19 Theoretically, Fe₃O₄ should also have strong interaction with NBR, because of its polar side group –CN. So after hybridization with Fe₃O₄, the hybrid particles will have interaction with NBR. While the Fe₃O₄ with three-dimensional sphere shape prevent graphene with two-dimensional layer to aggregate, which improves the dispersion of graphene in rubber matrix. Meanwhile, due to the magnetism of graphene,²⁰ it is easier for Fe₃O₄ to load graphene as the core, which might be helpful to improve the dispersion of graphene in rubber matrix.

The paper is structured such that in the first two parts the synthesis of graphene hybrid particles and the preparation of graphene hybrid particles/NBR composites are presented. The structures and morphology of graphene hybrid particles are then followed by analysis of XRD (X-ray diffraction) and SEM (scanning electron microscope). Last, properties of graphene hybrid particles/NBR composites have been investigated through tensile testing, DMA (dynamic mechanical analysis), magnetic testing and contact angle measurement.

2 Experimental

2.1 Materials

Pristine graphene (number of layers, 1–10) was supplied by Deyang Carbonene co., LTD, having platelet morphology with average thickness 3 nm, specific surface area 40–60 m² g⁻¹. NBR used was Perbunan 2907E with acrylonitrile content 29%, supplied by Lanzhou Petrochemical Company. Ferrous sulfate, ferric trichloride, polyvinyl alcohol and sodium hydroxide were all purchased from Chengdu Kelong Chemical Reagent. The reagents used for the synthesis of Fe₃O₄, including ferrous sulfate, ferric trichloride, polyvinyl alcohol and sodium hydroxide, were analytical grade.

2.2 Preparation of graphene/Fe₃O₄ hybrid particles

First, 0.12 g of pristine graphene was added into a NaOH (sodium hydroxide) aqueous solution (0.1 mol L⁻¹) and heated to 80 °C with vigorous stirring. Then 200 mL of PVA aqueous solution (including 0.05 mol of Fe³⁺ and 0.025 mol of Fe²⁺) was added into the NaOH aqueous solution. The reaction continued for 40 min, and the precipitates were washed with alcohol and deionized water for at least five times, followed by freeze drying at −50 °C for 96 h.

2.3 Preparation of graphene hybrid particles/nitrile rubber composites via solution blending

First, a certain amount of graphene hybrid particles were dispersed into DMF (dimethyl formamide) through ultrasound. NBR was then added into the DMF suspension with vigorous stirring. Finally, the resulting graphene hybrid particles/NBR compounds were filtered and dried in vacuum at 50 °C for 48 h.

The dried compounds were compounded with the ingredients using a two-roll mill and subjected to compression at 150 °C for 30 min. All the processing of preparation was shown in Scheme 1 and the basic formulation of the composite was listed in Table 1. It should be noted that the actual graphene content in graphene hybrid particles is much lower than the content of the graphene hybrid particles in NBR. In fact, the actual graphene contents in composites are only 0.05/0.1/0.15/0.2 phr (parts per hundred rubber).

Scheme 1 Schematic diagram for the preparation of graphene hybrid particles and NBR composites.

Table 1 Formulations of graphene hybrid particles/NBR nanocomposites

Material	Weight (phr)
NBR	100
Zinc oxide	5
Stearic acid	1
Sulphur	1.5
2,2′-Dibenzothiazole disulfide (DM)	0.5
Graphene hybrid particles	0/1/2/3/4

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2.4 Instrumentation

All the mechanical properties such as the notch impact strength, the stretch property and the bending property were tested on ZBC-25 charpy impact, CMT6104 universal tester, respectively and the finial value of the test was taken as the average of at least 5 results. X-ray diffraction (XRD, X'Pert PRO, Netherlands) was used to test what Fe₃O₄ influence on structure of graphene. During the experiment, Cu target was selected and K_α ray, whose wave length was 0.154 nm, was chosen to sweep continuously at step length of 4° min⁻¹. The scanning angle was 5–70°. The morphology of the hybrid particles and composites were observed using a JSM-7500F field-emission (SEM). DMA was performed on a TA Q800 DMA in double cantilever beam mode from −60 to 20 °C at a heating rate of 3 °C min⁻¹ and frequency of 1 Hz.

3 Results and discussion

3.1 Mechanical performance

To clearly visualize the reinforcement effect of the graphene/Fe₃O₄ hybrid particles on the mechanical performance of the composites, the tensile strength and elongation at break of neat NBR, Fe₃O₄/NBR and graphene hybrid particles/NBR composites prepared by mechanical and solution blending were compared, as shown in Fig. 1 and Table 2. Fig. 1(a) is the stress–strain curves of selected composites with neat NBR. It is obvious that all the fillers improve the tensile performance of NBR except for the pristine Fe₃O₄, and composites with 4 phr of graphene hybrid particles own more balanced mechanical properties. As shown in Fig. 1(a), it is evident that the graphene hybrid particles give rise to a significant increase in the tensile strength and elongation of the composites, especially when the content of graphene hybrid particles is 4 phr, a 90% of increase in tensile strength and a 20% in elongation at break are achieved. It is worth to note that the 4 phr of graphene hybrid particles only contains 5% of graphene, i.e., there is merely about 0.2 phr of graphene in the composites. On the contrary, however, the addition of 4 phr of Fe₃O₄ decreases the tensile strength and elongation, as shown in Fig. 1(b), which can be explained by the fact that the Fe₃O₄ without modification has poor compatibility with NBR matrix, causing an increasing number of internal defects. Similarly, Wang et al.²¹ also have reported that pure Fe₃O₄ will decrease the tensile strength of NBR because of its agglomeration and poor compatibility with NBR. In Fig. 1(c), when adding 3 phr of graphene via solution blending and 4 phr graphene hybrid particles, respectively, the two samples have similar tensile strength and elongation, which indicates that 0.2 phr of graphene after hybridization can supply similar reinforcement of 4 phr of pristine graphene. In order to further demonstrate that it was the hybridization of graphene improved the dispersion of graphene, 1 phr, 3 phr of graphene and 4 phr of graphene hybrid particles were added via mechanical blending, respectively. The results of Fig. 1(d) show that NBR with 4 phr of graphene hybrid particles owns best mechanical properties in the four samples, which indicates that it is easier for hybrid particles than pristine graphene to disperse in rubber matrix during mechanical blending. The shape of graphene hybrid particles may be significant for the dispersion of graphene hybrid particles during the mechanical blending, and the magnetism of Fe₃O₄ may lead a magnetic action between Fe₃O₄ and metal device during the blending, which amount to adding an extra perturbation field, except for the basic shear force effect, which is helpful for better dispersion of Fe₃O₄ in matrix.

Fig. 1 (a) Stress–strain curves of selected graphene/NBR, Fe₃O₄/NBR and graphene hybrid particles/NBR with neat NBR; (b) tensile strength and elongation at break of NBR with different content of hybrid particles prepared by solution blending; contrast of mechanical properties of (c) Fe₃O₄/NBR and hybrid particles/NBR prepared by solution blending, (d) graphene/NBR and hybrid particles/NBR prepared by solution blending and (e) graphene/NBR and hybrid particles/NBR prepared by mechanical blending.

Table 2 Mechanical properties of various NBR composites^a

Composite	Weight (phr)	Tensile strength (MPa)	Elongation at break (%)
a sb: solution blending; mb: mechanical blending.
NBR	0	2.36 ± 0.2	1346 ± 110
NBR–GHP (sb)	1	3.45 ± 0.3	1478 ± 100
NBR–GHP (sb)	2	3.82 ± 0.5	1521 ± 130
NBR–GHP (sb)	3	4.19 ± 0.6	1599 ± 130
NBR–GHP (sb)	4	4.69 ± 0.5	1672 ± 150
NBR–Fe3O4 (sb)	4	1.96 ± 0.4	947 ± 100
NBR–G (mb)	1	2.50 ± 0.2	1193 ± 90
NBR–G (mb)	3	3.62 ± 0.4	1259 ± 100
NBR–Fe3O4 (mb)	4	1.77 ± 0.2	900 ± 80

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As shown in Table 3, the hardness of the graphene hybrid particles/NBR and Fe₃O₄/NBR composites are larger than that of neat NBR. Shore A hardness of NBR is 50, while shore A hardness of the graphene hybrid particles/NBR and Fe₃O₄/NBR composites are 56 and 57, respectively, when the content of fillers is 4 phr. The reason for the increase of hardness, on one hand, is that the hardness of rigid Fe₃O₄ is greater than that of NBR. On the other hand, graphene hybrid particles filled the cavity of NBR, which increased the hardness as well. In addition, due to the low content of graphene in hybrid particles, the shore A hardness of hybrid particles/NBR and Fe₃O₄/NBR composites is very close to each other.

Table 3 Shore A hardness of various NBR composites

Weight (phr)	Hardness of hybrid particles/NBR	Hardness of Fe3O4/NBR
0	50	50
1	53	53
2	54	53
3	54	55
4	56	57

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3.2 Morphologies of graphene hybrid particles and graphene hybrid particles/NBR composites

In graphene/rubber composites, the dispersion of graphene and their interfacial interactions with rubber matrix are critical to the properties of the composites. The morphology of hybrid particles and the dispersion of the fillers were observed by SEM measurements on the brittle and snapped sample surface. As shown in Fig. 2(a), the diameter of graphene hybrid particles was about 5 μm, which agrees with the results of laser particle size distribution analysis. Graphene hybrid particles are near-spherical. Most of the particle surface is surrounded by aggregations of graphene. However, despite the existence of aggregations to some extent, the loading of Fe₃O₄ changes the dispersion mode of graphene, which has influence on the dispersion of graphene in NBR matrix. Comparing the images of Fig. 2(b) and (c), we observed a better dispersion of graphene hybrid particles in NBR matrix and phase separation in Fe₃O₄/NBR composites, which indicates that graphene/Fe₃O₄ hybrid particles have better compatibility with NBR matrix than Fe₃O₄. The results also verify that Fe₃O₄ decreases the mechanical properties of NBR. It is worth to mention that, although SEM sees only the surface of the composites, the morphologies observed nevertheless reflect the entire bulk sample and can provide a comprehensive comparison between the hybrid particles/NBR and Fe₃O₄/NBR. In Fig. 2(c), graphene hybrid particles are nearly wrapped with NBR molecule and embedded into NBR matrix, showing good compatibility between NBR and graphene hybrid particles. Furthermore, as shown in Fig. 2(e) the image of tensile fractured surface of graphene hybrid particles/NBR composites, graphene hybrid particles are not only pulled out, but also connected with NBR molecule. Both Fig. 2(d) and (e) proved that the graphene hybrid particles have strong interfacial interaction and adhesion with NBR molecular, which is why graphene hybrid particles have higher reinforcing efficiency than pristine graphene. When external stress is applied onto the composites, the interfacial interaction is strong enough to transfer the load from the matrix to the graphene hybrid particles, keeping the graphene hybrid particles and the NBR matrix bonded.

Fig. 2 SEM images of (a) hybrid particles, (b) Fe₃O₄/NBR composites and graphene hybrid particles/NBR composites before (c and d, differ in magnification) and after (e) tensile test.

3.3 XRD patterns

The dispersion of hybrid particles in NBR was further characterized by XRD. Fig. 3 shows the XRD patterns of graphene (curve a), Fe₃O₄ (curve b), graphene particles (curve c), NBR and the graphene hybrid particles/NBR composites (curve d), respectively. As shown in curve c, the typical diffraction peak of graphene at 2θ = 26.7° disappeared and the typical diffraction peak of Fe₃O₄ shifted partly after hybridization, indicating the stack order of graphene broke down and the periodic structure of Fe₃O₄ was partly intercalated by graphene. Similar phenomenon was also observed in the study of Jiang.²² A broad diffraction peak at about 20° was observed for NBR, representing its amorphous phase (curve d of Fig. 3). For the graphene hybrid particles/NBR composites with 4 phr of hybrid particles (curve d), only the diffraction peak of NBR and Fe₃O₄ were observed. Moreover, the typical diffraction peaks of Fe₃O₄ in composites became about the same with those of pure Fe₃O₄, which could be explained by the exfoliation of graphene from the surface of Fe₃O₄ and dispersion in NBR matrix during the compounding process. Bai et al.²³ observed the absence of characteristic diffraction peak of graphene in the composites and attributed it to the single-sheet level dispersion of graphene in the HNBR matrix.

Fig. 3 XRD diffract gram of (a) graphene, (b) Fe₃O₄, (c) graphene/Fe₃O₄ hybrid particles and (d) hybrid particles/NBR composites.

3.4 Dynamic mechanical properties

It is believed that the improvement of tensile strength is contributed to good dispersion of graphene and the strong interfacial action between NBR and graphene hybrid particles, which is considered to affect the transfer of external force around the interface between NBR and graphene hybrid particles.²⁴ The results of mechanical performance test, SEM and XRD have shown some evidences. To prove it further, DMA was used to investigate the effects of graphene hybrid particles on the dynamic mechanical properties of the composites. The results of DMA of the graphene hybrid particles/NBR composites are shown in Fig. 4 and Table 4.

Fig. 4 Dynamic mechanical analysis curves of graphene hybrid particles/NBR composites with different loading of graphene hybrid particles.

Table 4 Characteristic value of tan δ vs. temperature curves of graphene hybrid particles/NBR composites

	0 phr	1 phr	2 phr	3 phr	4 phr
Tg (°C)	−19	−15.5	−14.5	−14.5	−14.5
Value of tan δ peak	1.37	1.45	1.45	1.43	1.52

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As shown in Fig. 4(a), the storage modulus (G′) of NBR composites increases with the content of graphene hybrid particles. However, after adding 4 phr of Fe₃O₄, G′ of NBR decreases, which verify further the results of mechanical property.

tan δ vs. temperature curves are displayed in Fig. 4(b). Compared with neat NBR (T_g = −19 °C), the graphene hybrid particles/NBR composites shows higher T_g, especially when adding 4 phr of graphene hybrid particles, where the T_g of composites shifts to −14.5 °C. However, pure Fe₃O₄ has no obvious effect on the T_g of NBR because of its poor compatibility with NBR matrix which could hinder the motion of rubber molecular chain. Clearly it is the graphene hybrid particles influences the T_g of NBR, which could be explained by that the graphene hybrid particles intensify the physical entanglement network of NBR²⁵ and the strong interfacial action restrict the motion of the NBR molecular chain around the interface. This is also known as the entanglement effect and the interface effect. Araby et al.^26,27 believe that graphene limits the motion of SBR molecular chain around the interface between SBR and graphene, which will cause the T_g of SBR to shift to higher temperature. The free volume theory could also explain the increase of T_g. Below T_g, addition of nano-sized graphene decrease the free volume of NBR molecule to some extent, so more free volume is needed to provide space for the movement of chain segments, causing the T_g shifts to higher temperature. Meanwhile, the graphene hybrid particles/NBR composites have higher tan δ peak value than neat NBR, which might be due to that the surface area of graphene was extremely high and the interface between NBR and the hybrid particles was as rough as what is seen in the SEM images, so the friction between NBR molecular chains will increase so as the energy loss.

3.5 Surface wettability and magnetism

Due to the hybridization with Fe₃O₄, graphene hybrid particles possess extra functions more than merely giving a reinforcement of mechanical properties. For example, due to the polarity of graphene hybrid particles, NBR composites have better surface wettability. As shown in Fig. 5(a)–(e) and Table 5, the water contact angle of NBR tends to decrease after addition of graphene hybrid particles. Especially when adding 4 phr of graphene hybrid particles, the contact angle of NBR composites is 51.50°, which shows that NBR composites have better hydrophilia. In other words, the oil resistivity of NBR becomes better. Meanwhile, the composites show better affinity with NaOH aqueous solution (40 wt%) and worse affinity with H₂SO₄ aqueous solution (40 wt%), which broaden their range of application.

Fig. 5 Measurement of the contact angle of (a)–(e) graphene hybrid particles/NBR composites and (f) magnetic hysteresis loops of graphene hybrid particles/NBR composites.

Table 5 Contact angle of NBR composites with various graphene hybrid particles content

Solution	Contact angle^a (°)
	0 phr	1 phr	2 phr	3 phr	4 phr
a Measured for samples with different filler content shown below.
Pure water	76.66	77.12	65.44	65.00	51.50
NaOH (40 wt%)	104.33	84.86	77.66	71.36	65.54
H2SO4 (40 wt%)	66.62	67.02	83.22	85.31	86.71

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Fig. 5(f) shows the magnetic hysteresis loops of graphene hybrid particles/NBR composites. Obviously, by increasing the content of graphene hybrid particles, magnetic properties of the graphene hybrid particles/NBR composites increase monotonically, whereas saturation magnetization strength of the composites increases constantly. The composites show typical characteristic of super-paramagnetism in which there is nearly no magnetic hysteresis. When the content of graphene hybrid particles increases, the magnetic density also increases, so does the saturation magnetization strength of the graphene hybrid particles/NBR. When the magnetic strength increased to 5000 Oe, the value of saturation magnetization strength basically turns to be steady. It is noted that in general, magnetic fillers tend to aggregate due to the magnetic interaction between them. However, hybridization with graphene affects the magnetism of Fe₃O₄, which allows for better dispersion of graphene hybrid particles in NBR matrix.

4 Conclusion

Graphene/Fe₃O₄ hybrid particles have been synthesized using co-precipitation method. The as prepared graphene hybrid particles can be finely dispersed in NBR as fewer-layered sheets at low loading of graphene and have strong interfacial interaction with the NBR matrix. The mechanical properties of NBR are significantly improved at very low filler loading. A 90% of increase in tensile strength of NBR is achieved with the 4 phr of graphene hybrid particles which contain only 0.5 phr of graphene. The reinforcement of 4 phr of graphene hybrid particles (equally 0.2 phr of graphene) on NBR is similar to that of 3 phr of graphene when NBR composites are prepared via mechanical blending. This result suggests that graphene/Fe₃O₄ hybrid particles have promising applications in improving the mechanical of various polar nanocomposites. Meanwhile, graphene hybrid particles/NBR composites show higher T_g and higher value of tan δ than neat NBR, especially when the content of graphene hybrid particles was 4 phr, where the T_g and value of tan δ was −14.5 °C and 1.52, respectively. Moreover, the better hydrophilia and paramagnetism of graphene hybrid particles/NBR composites can also broaden the application of NBR.

Acknowledgements

The authors acknowledge the financial support from the Science & Technology Department of Sichuan province (No. 2015JY0052) and from the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (No. X151516KCL32) of China. The authors also thank the Deyang Carbonene co., Ltd for the supply of graphene used in this study.

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