Evaluation of the effect of graphene addition in fluoroelastomer compositesABSTRACT
Fluoroelastomers are widely used in demanding applications due to their excellent thermal and chemical resistance, however, their intrinsically low electrical conductivity limits their application in multifunctional systems, especially in the oil and gas industry. The incorporation of graphene has emerged as a promising strategy to improve conductivity, although its influence on peroxide-cured fluoroelastomer compounds processed by internal mixing is still not fully understood, particularly regarding the balance between curing behavior, mechanical properties, and electrical performance. In this study, graphene was incorporated into fluoroelastomer compounds at concentrations of 0.5, 1.5, and 5.0 phr. Cure characteristics, hardness, tensile strength, and electrical conductivity were evaluated. All formulations exhibited similar torque profiles and processing times, indicating good processability even with graphene addition. The curing behavior showed a tendency toward reduced maximum torque and shorter optimum cure time for the formulation containing 5.0 phr graphene. Tensile strength gradually decreased as graphene content increased. In contrast, the compound with 5.0 phr graphene exhibited an increase of approximately 4.7 orders of magnitude in electrical conductivity compared with the unfilled formulation. Overall, graphene contents between 0.5 and 1.5 phr provided the best balance among processability, mechanical performance, and electrical conductivity, minimizing detrimental effects on mechanical properties.
Keywords:
Composite; Fluoroelastomer; Graphene; Mechanical Properties
1. INTRODUCTION
The optimization of materials to meet specific performance requirements is a central objective in both academic research and industrial applications. In this context, advanced materials such as graphene and fluoropolymers have received increasing attention due to their superior properties compared to conventional fillers and elastomers, especially in critical applications where high mechanical, thermal, conductive, and resistance requirements to fluids derived from the oil and gas industries are demanded [1].
Fluoroelastomers (FKM) are widely recognized for their excellent resistance to aggressive chemical environments, including hydrocarbons, solvents, and acids, which is attributed to their high fluorine content and molecular structure. These characteristics make them suitable for the manufacture of sealing components, such as O-rings, gaskets, and dynamic seals, used in drilling equipment, valves, and oil transportation systems [2,3,4]. However, despite their excellent chemical and thermal stability, their intrinsic electrical conductivity is limited, restricting their application in multifunctional systems.
Graphene, a two-dimensional material composed of a single layer of carbon atoms arranged in a hexagonal lattice, exhibits exceptional mechanical, thermal, and electrical properties. It presents thermal conductivity in the range of 1500–5300 W·m−1·K−1, tensile strength of approximately 130 GPa, and a Young’s modulus close to 1 TPa, making it one of the most promising nanomaterials for advanced composite applications [5]. Additionally, its low density, high flexibility, chemical stability, and electrical conductivity enable its use as a multifunctional reinforcement [6].
Despite these advantages, the incorporation of graphene into elastomeric matrices remains a challenge. In fluoroelastomers, this difficulty is intensified due to their low polarity, which limits interfacial interactions with graphene. Furthermore, strong π–π stacking interactions and van der Waals forces promote agglomeration, impairing dispersion and reducing the efficiency of stress transfer and functional properties [7,8,9]. Therefore, the development of effective processing strategies is essential to achieve adequate dispersion and fully exploit the potential of graphene in these systems.
Previous studies have demonstrated that the addition of graphene-based nanofillers, such as graphene oxide (GO) and reduced graphene oxide (rGO), can significantly improve the mechanical, thermal, and electrical properties of fluoroelastomer composites. MONI et al. [2] investigated the effect of incorporating mRGO (microwave-reduced graphene oxide) in the production of fluororubber nanocomposites. The authors tested compositions of 0, 0.25, 0.5, 0.75, 1.25, and 2 phr of mRGO mixed with FKM using a two-roll mill. The nanocomposite with 0.75 phr loading exhibited the best thermal, electrical, and mechanical properties.
However, the effect of different graphene concentrations in peroxide-cured fluoroelastomer systems, particularly processed under controlled internal mixing conditions, remains insufficiently explored, especially regarding the balance between mechanical performance, curing behavior, and electrical conductivity.
In this context, the present study investigates the effect of incorporating graphene at different concentrations (0.5, 1.5, and 5.0 phr) on the rheological, curing, mechanical, and electrical properties of peroxide-cured fluoroelastomer composites. This research aims to enhance the properties of FKM rubber, making it more versatile for applications that require conductivity.
2. MATERIALS AND METHODS
2.1. Materials
The materials used in the preparation of the composites are summarized in Table 1. A medium-fluorine-content fluoroelastomer (FKM) was used as the polymer matrix. Graphene was employed as the conductive nanofiller, while carbon black, mineral filler, curing agents, and auxiliary additives were incorporated according to conventional elastomer compounding practices [10,11,12,13,14,15,16].
2.2. Composite preparation
Graphene was incorporated directly into a closed mixer, a crucial step to promote its proper dispersion and homogenization with the polymer matrix and other fillers. Subsequently, the composites were homogenized and laminated in an open roller mixer under controlled conditions [17].
The production of the composites was carried out using a Haake Polylab OS torque rheometer, coupled to a mixing chamber module with tangential rotors (Banbury type), a capacity of 78 cm3, an initial temperature of 60 °C, a rotation speed of 80 rpm, and a filling factor of 65%, in two stages, where:
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Step 1 - Production of the composite without a curing system, with the aim of avoiding pre-curing due to the high shear and time required for incorporation and dispersion of the ingredients, mainly carbon black and graphene,
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Step 2 - After the composite produced in Step 1 cooled, it was processed again in Haake for the addition of the curing system.
Table 2 presents the formulations to be used.
The theoretical processing times and order of addition were the same for all formulations, and are therefore described generically in Table 3.
After the final processing in a closed mixer, the resulting composites were processed in an open mixer using a temperature of 80 °C, cylinder speeds of 15 and 21 rpm (front and rear, respectively), and an opening of 1.5 mm. To improve the homogenization of the composites, 3 cuts were made on each side, and subsequently, the sheet was formed into a roll and passed through the cylinder perpendicular to the rollers 6 times, followed by 4 passes in the same direction to guide the flow. Each composite was laminated into a single sheet of approximately 2.5 mm.
2.3. Preparation of test specimens
Test specimens were prepared according to ASTM D3182-21a [18]. The compounds were cured in a hydraulic press at 180 °C for a time corresponding to t90 + 2 min, under a specific pressure of 100 kgf·cm−2. The cured plates were subsequently post-cured at 200 °C for 4 h. Specimens with appropriate dimensions were then die-cut from the plates for subsequent testing.
2.4. Cure characteristics
Cure characteristics of the uncured compounds were evaluated using an oscillating disc rheometer (RPA 2000, Alpha Technologies) in accordance with ASTM D5289-19a [19]. Measurements were performed at 180 °C for 5 min, with an oscillation amplitude of 0.5° and a frequency of 100 cycles per minute. From the torque–time curves, minimum torque (ML), maximum torque (MH), scorch time (ts1), optimum cure time (t90), torque difference (MH − ML), cure rate, and cure rate index were determined.
2.5. Hardness
Shore A hardness was measured in accordance with ASTM D2240-15 (reapproved 2021) using a digital Shore A durometer [20]. Specimens were stacked in three layers, and measurements were taken before and after curing with a dwell time of 1 s. The results represent average values of multiple measurements.
2.6. Electrical conductivity
Volume electrical resistivity was measured according to ASTM D257-14 [21] using a Keithley 6517B electrometer equipped with a Keithley 8009 test fixture and Keithley 6524 software. A concentric electrode with an internal diameter of 57.2 mm was used. Measurements were conducted at an applied voltage of 500 V with a stabilization time of 60 s. Electrical conductivity was calculated as the inverse of the measured resistivity and expressed in S·m−1.
2.7. Tensile properties
The tensile properties test is a widely used method to evaluate the mechanical behavior of materials, including elastomers, under the application of forces that tend to elongate them until rupture. In the case of elastomers, this test is essential to determine properties such as maximum tensile strength, elongation at break, and modulus of elasticity, which are fundamental to predicting the performance of these materials in applications requiring flexibility and mechanical strength [22].
Tensile properties were determined in accordance with ISO 37:2024 using type 2 dumbbell specimens. Tests were performed on an EMIC DL 2000 universal testing machine at a crosshead speed of 500 mm·min−1, using a 1 kN load cell. Five specimens were tested for each formulation, and the reported values correspond to the average results.
3. RESULTS AND DISCUSSION
3.1. Composite processing behavior
All formulations containing different graphene concentrations exhibited good processability during internal mixing. The stable torque profiles indicate that the incorporation of graphene, even at the highest loading investigated, did not significantly impair the processing conditions. The selected order of ingredient addition proved effective, promoting progressive torque increases and suggesting satisfactory dispersion and homogenization of the composites [23].
The sequential addition of components, aiming at the progressive increase of shear during mixing, is discussed by SHARAF and KLOCZKOWSKI [24]. According to the authors, efficient graphene dispersion requires the exfoliation of its layers, a process hampered by strong interlayer interactions, which demands a greater input of energy and shear forces. Due to the high viscosity of elastomers, intensive mixing conditions are necessary to ensure homogeneous dispersion, in which larger particles are fragmented by erosion and rupture, resulting in smaller particles distributed in the polymer matrix.
Figure 1 presents the evolution of torque as a function of processing time for the base compound and graphene-modified composites. Similar torque development trends were observed for all formulations, confirming that graphene addition did not cause abnormal increases in viscosity or processing instability.
Development of torque as a function of processing time of composites modified with different graphene contents and of the graphene-free composite (Base).
3.2. Cure characteristics
The curing behavior of the compounds was evaluated by rheometric analysis. Table 4 summarizes the minimum torque (ML), maximum torque (MH), scorch time (ts1), and optimum curing time (t90) values obtained from the torque–time curves, while Figure 2 shows the corresponding rheometric profiles.
The typical behavior of peroxide curing is observed, evidenced by the abrupt increase in torque, mainly in the first minute, where peroxide decomposition and system activation (curing system) occur, indicating the advancement of crosslinking reactions. The differences between the compound profiles reflect the impact of the formulation on the curing kinetics, maximum torque achieved, and scorch time.
In general, the composites containing low graphene contents (0.5 and 1.5 phr) showed behavior similar to the base compound, with only minor variations in ML and ts1, indicating that, at these concentrations, graphene does not significantly affect processability or the initial thermal stability of the system. However, for the formulation containing 5.0 phr of graphene, a significant reduction in maximum torque (MH), of approximately 21.7%, was observed, accompanied by an increase in optimum cure time (t90), suggesting a decrease in curing efficiency.
The reduction in MH can be attributed to a combination of microstructural and chemical factors. At higher loadings, graphene tends to agglomerate due to strong van der Waals interactions, resulting in heterogeneous dispersion within the elastomeric matrix [25]. These agglomerates act as stress concentration regions and reduce the effectiveness of polymer–filler interactions, impairing the formation of a homogeneous crosslinked network [26]. Additionally, the graphene surface may interact with radicals generated during peroxide decomposition, reducing their availability for crosslinking reactions and consequently decreasing the crosslink density. This behavior is consistent with the observed reduction in MH, ΔTorque, and kinetic parameters.
The increase observed in optimum cure time (t90) with increasing graphene content further supports this interpretation, indicating a reduction in the rate of network formation. This behavior may be associated with restricted polymer chain mobility [25]. Similar trends have been reported in elastomeric composites containing carbon-based nanofillers, where increasing filler content alters the microstructure and curing kinetics [25, 27].
The scorch time (ts1) remained relatively constant across all formulations, with insignificant variations, indicating that graphene, at the concentrations studied, did not compromise the initial thermal stability of the compounds.
To better understand the rheological behavior of the analyzed samples, the curing speed (VR), curing rate index (CRI), and bond density (Δ Torque) parameters were calculated, and their values are presented in Table 5 and Figure 3.
(a) Rheological behavior Δ Torque of elastomeric composites obtained using an RPA. (b) Rheological behavior of cure rate (VR) and cure rate index (CRI) parameters of elastomeric composites obtained using a Rubber Process Analyzer (RPA), according to ASTM D5289 standard [19].
A progressive reduction in these parameters is observed with increasing graphene content, with the most pronounced effect for the composite containing 5.0 phr of graphene, which exhibited decreases of 45.2% in VR and 22.1% in CRI compared to the base compound. These results confirm a slowdown in curing kinetics and reduced efficiency in crosslink network formation.
This interpretation is reinforced by the reduction of approximately 29.6% in ΔTorque for the composite with the highest graphene content, since this parameter is directly related to the crosslink density developed during curing. In contrast, studies such as MONI et al. [2] and LI et al. [27] report an increase in ΔTorque at low nanofiller concentrations, indicating enhanced polymer–filler interaction and improved crosslinking efficiency. This difference highlights that the effect of graphene is strongly dependent on its dispersion and concentration. Therefore, while low graphene contents do not significantly affect curing behavior, higher loadings compromise the formation of an efficient crosslinked network, mainly due to agglomeration and interference with the curing mechanism.
3.3. Mechanical properties of graphene-modified composites and the base composite
3.3.1. Tensile strength
The tensile strength results for the different composites modified with graphene and Base are presented in Table 6 and Figure 4, which illustrates the tensile strength results.
Results of tensile strength analyses of graphene-modified composites and the base composite.
From the graph in Figure 4, a decrease in tensile strength is observed with increasing graphene content. The base formulation showed 17.29 MPa, while the composite with 5.0 phr of graphene showed a decrease to 15.80 MPa (a reduction of 8.6%). This behavior may be associated with the formation of graphene agglomerates at higher contents, resulting in stress concentration points that decrease the material’s resistance [28]. Studies such as those by GARG et al. [29] showed improvements in tensile strength of 22.9% only for 0.25 phr, at higher concentrations, the gains are reduced or reversed, corroborating the results found, and confirming that, although low filler contents can become advantageous, high contents impair the integrity of the structure. The authors attributed the higher concentrations to the formation of clusters.
3.3.2. Stretching
As with the tensile strength results, elongation also decreases with increasing graphene content. Figure 5 shows a decrease from 270% to 250% as the amount of graphene increases, evidencing a progressive loss of this property, which is one of the main characteristics of elastomers. This suggests that the material becomes less elastic and more rigid, which is typical when reinforcing fillers are added that restrict the mobility of the polymer chains. This is in line with many reports demonstrating that excessive additions of graphene stiffen the matrix and limit its mobility [30].
Convergent results are described in the literature. LI et al. [27] and CHEN et al. [31] reported similar behavior in elastomeric composites containing carbon nanoparticles, in which contents of 5.0 phr and 2.0 phr promoted reductions in elongation at break of approximately 54% and 13.8%, respectively. These effects were attributed to the lower mobility and deformation of the molecular chains, concomitantly with increases in tensile strength of approximately 25.0% and 20.5%.
3.3.3. Module
Figure 6 shows the modulus of the graphene-modified composites and the Base composite, with modulus at 100% (Figure 6a) and modulus at 200% (Figure 6b).
Modulus of the graphene-modified composites and the base composite, being modulus at 100% (a) and modulus at 200% (b).
The increase in modulus, both by 100% and 200%, confirms the effectiveness of graphene as a stiffening agent, even in small proportions.
The same behavior was observed by CHEN et al. [31], with the addition of 5 phr of CNTs, and MONI et al. [2], with the addition of 1.25 phr of rGO, associating it with increased system stiffness and improved interactions between the matrix and loads.
Observing Figure 6a, it is possible to see that the increase is more pronounced in the modulus at 100%, suggesting that the reinforcement is more effective in the early stages of deformation. This behavior was observed in the studies by LEE et al. [32], in which, for example, PP nanocomposites with 1.5% graphene increased the modulus by up to 66%.
Figure 7 shows the superposition of the stress-strain curves. A different curve behavior is observed only for the sample with 5 phr of graphene, characteristic of yielding, which may be associated with rupture or lack of interaction between the nanofiller and the elastomer, corroborating the reduction in the rupture stress results of this study.
The change in the behavior of the composite sample curve with 5 phr confirms that the addition of graphene to the elastomer promoted changes in the material’s mechanical behavior. The base sample, composed only of the elastomer without the addition of fillers, showed greater tensile strength and a lower elastic modulus compared to the samples with graphene. This behavior can be attributed to the reduction of structural reinforcement in the polymer matrix. With the incorporation of graphene at different concentrations, a progressive increase in the modulus was observed, indicating an improvement in stiffness. This reinforcement is a result of the high specific surface area and high elastic modulus of graphene, which favor efficient load transfer between the matrix and the nanomaterial, in addition to restricting the mobility of the polymer chains. Thus, the modification of the base sample curve reflects the action of graphene as a reinforcing agent in fluorinated elastomeric composites [5].
3.4. Hardness
There are several scales for measuring hardness, with Shore A being the most suitable for measuring soft materials such as elastomers. Rubber hardness is a key property for predicting resistance to permanent deformation and penetration. In industry, various formulations can be used depending on the application required, which also applies to the hardness value of the composite. The most significant factor in hardness is the fillers in the formulation [33].
The results of the hardness test before and after the post-curing process are summarized in Figure 8.
Shore A hardness of graphene-modified composites and the base composite before and after the post-curing process.
Due to the high cost of graphene, one option used is the partial replacement of the carbon black filler or the complementary addition of graphene in small proportions, as a way to improve the properties of the rubber, enabling a multifunctional composite [34]. In this work, the complementary addition of graphene was used, and no major variations in hardness were observed in the samples with graphene compared to those without graphene (Base).
To evaluate the influence of post-curing, the hardness measurements of the samples were performed before and after post-curing for comparative purposes. The composites did not show significant variations in hardness after the 4-hour post-curing process at 200°C. Among the results obtained, the 5 phr sample showed a slightly higher result after post-curing when compared to the other samples (3.28%), which had averages within the standard deviation. This behavior can be attributed to the rigidity of graphene, which is also related to the restriction of mobility and elasticity of the rubber chains, culminating in a decrease in elongation at break [35], which consolidates the results obtained in this work, with the 5 phr sample showing the greatest variations in the evaluated properties.
3.5. Electrical conductivity
Post-curing is indicated for the removal of volatile substances and the improvement of certain properties. In the studies by VRYONIS et al. [36], the behavior of commercial rubbers was evaluated before and after post-curing at 200 °C for 4 hours. The authors observed the removal of volatiles and pronounced structural changes, indicating that post-curing promoted the crosslinking of long chains that had not reacted in the initial curing, promoting better electrical performance in the elastomers. Therefore, electrical conductivity tests were performed with the samples after post-curing, and the results illustrated in Figure 9 were obtained.
The result found for the Base formulation (without graphene) shows the lowest conductivity by approximately 4.7 orders of magnitude, similar to that obtained by CHEN et al. [31]. Due to the excellent conductive properties of graphene, with increasing concentration there was a progressive increase in the electrical conductivity of the samples with increasing graphene content. According to MONI et al. [2], the addition of graphene increases the conductivity of nanocomposites due to the incorporation of conductive filler and interfacial polarization, which form microcapacitor structures that, with increasing filler charge, form more conductive pathways, thus paving the way for higher conductivity values.
XIONG et al. [8], report a similar behavior trend using the rapid evaporation method, but with the use of lower concentrations of nanofillers.
It is also observed that the use of higher concentrations of graphene does not indicate a proportional increase in conductivity, since the composites have an addition limit, as there is an influence on the rheological and mechanical properties, as observed in this work. In other words, the choice of the best graphene formulation or concentration should consider other properties, especially regarding rheological and mechanical behavior.
4. CONCLUSIONS
Based on the production and characterization of the developed materials, it was possible to systematically analyze the influence of incorporating graphene into fluoroelastomeric matrices and its effects on the evaluated properties. The results demonstrate that the presence of this nanomaterial promotes relevant changes in the performance of the systems, which are strongly dependent on the amount used.
During processing, no significant variations in torque and temperature were identified, a behavior that can be attributed to the low concentrations used. On the other hand, it was found that the addition of graphene significantly influences the rheological, mechanical, and electrical properties, with optimized performance observed in the range between 0.5 and 1.5 phr. For concentrations above this range, a more pronounced drop in the investigated properties occurred.
The results also indicate that graphene has a direct impact on the curing process of elastomers, especially at higher concentrations, where the formation of agglomerates and possible interaction with the crosslinking system can compromise curing efficiency. In this context, controlling the dispersion and the amount incorporated is essential for obtaining materials with superior performance.
In terms of reinforcement, graphene proved particularly efficient when used in low concentrations (0.5 to 1.5 phr), promoting a significant increase in elastic moduli and, consequently, greater system stiffness. Conversely, its use in higher concentrations (above 1.5 phr) presented limitations, resulting in more pronounced reductions in tensile strength and elongation at break. Regarding electrical properties, an increase of approximately 3.8 orders of magnitude in conductivity was noted with the incorporation of 1.5 phr of the nanomaterial.
Therefore, considering the set of results obtained, the use of graphene in the range of 0.5 to 1.5 phr is recommended, as it provides an adequate balance between mechanical, rheological and electrical performance, constituting the most favorable condition for the optimization of reinforced fluoroelastomeric composites.
5. ACKNOWLEDGEMENTS
This work was carried out with the support of CNPq, a Brazilian government entity focused on human resource development. The authors also acknowledge the financial support of the Brazilian agencies CAPES, FAPERGS, and FINEP.
The authors acknowledge the Instituto SENAI de Tecnologia em Química e Meio Ambiente for providing the equipment used in the characterization of the composites.
6. DATA AVAILABILITY
All the data supporting the results of this study were published in the article itself.
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