The Role of Graphite-like Materials in Modifying the Technological Properties of Rubber Composites

Abstract

In this work, general purpose rubber composites were created based on a mixture of non-polar cis-1,4-isoprene rubber and cis-1,4-divinyl rubber as components. The main filler used was carbon black, while various graphite-like materials (graphite oxide, reduced graphite oxide, expanded graphite, and graphite nanoplatelets) served as additives. It was determined that the addition of these graphite-like materials resulted in a reduction in Mooney viscosity, with the introduction of graphene nanoplatelets having the most significant effect, contributing to a viscosity decrease of 8.5%. The relaxation rate increased, positively impacting elastic recovery and consequently reducing shrinkage. The introduction of graphite oxide, graphite nanoplatelets, and expanded graphite also increased the time to the onset of the vulcanization reaction; moreover, these additives lengthened the time needed to reach the optimum level of vulcanization. The addition of various graphite-like materials significantly affected the elongation at break, with the highest increase attributable to the addition of expanded graphite and reduced graphite oxide. It was found that the conditional tensile strength of these additives had little effect. Upon assessing the elastic-strength properties after aging, it was found that the inclusion of graphite-like materials reduced the elongation at break.

1. Introduction

Reinforcement of rubbers with carbon nanomaterials, such as graphene, carbon nanotubes, carbon nanofibers, expanded graphite, and graphite oxide, has been widely studied [1,2,3]. The main application of these materials is their ability to improve the mechanical characteristics of rubber matrix composites due to the unique morphology, structure, and other properties of carbon nanomaterials [4].

In ref. [5], reclaimed rubber/natural rubber composites with the addition of single-walled carbon nanotubes (SWCNTs) were investigated. SWCNTs enabled an increase in tensile strength, hardness, and modulus of the composites, although a decrease in elongation at break was observed. In ref. [6], it was found that the sub-nanolevel free volumes and nano-level structure of the composites play significant roles in regulating the mechanical properties of rubber/multi-walled carbon nanotubes (MWCNTs). In ref. [7], the authors reported a general approach to obtain elastomeric composites with high conductivity based on the Hansen solubility parameters of SWCNTs. Additionally, the problem of dispersing carbon nanotubes in the rubber matrix is crucial, as the role of interfacial interaction is important [2]. A novel mixing approach for better dispersion of multi-walled carbon nanotubes in a styrene–butadiene rubber composite was reported in [8]. In this method, CNTs were dispersed in ethanol, followed by the suspension of CNTs in alcohol and mixing with a 50:50 composite of butadiene rubber and a solution of styrene–butadiene rubber.

Graphene-based materials are also used for creating rubber composites. Rubber/graphene nanocomposites were obtained in [9], where it was found that graphene acted as an accelerating agent during vulcanization, while graphene oxide served a function related to curing. Graphene oxide (0.5 parts per hundred parts of rubber (phr)) was also incorporated into natural rubber using melt mixing and latex precompounding [10]. The improvement in the properties of the nanocomposites was attributed to better dispersion of graphene oxide after precompounding compared to the melt mixing of block natural rubber. Le et al. [11] investigated self-healable materials based on butyl imidazole–modified bromobutyl rubber/natural rubber composites filled with carbon nanotubes, demonstrating an enhancement in the tensile strength of the composites. Cambraia et al. [12] investigated the effect of carbon nanofillers (CNTs, reduced graphene oxide) with high levels of carbon black and silica on the mechanical properties of polybutadiene (60 phr) and natural rubber (40 phr) composites. Pirityi et al. [13] proposed the hybrid reinforcement of styrene–butadiene rubber nanocomposites with carbon black and graphene, achieving almost a 100% increase in tensile strength and elongation at break using 10 phr of graphene nanoplatelets and 10 phr of carbon black.

The problem of interfacial interaction between rubbers and carbon nanomaterials is addressed in many articles, facilitated by the application of various modifications. Xie et al. improved the interfacial interaction between isoprene rubber and graphene by using a coupling agent (bis-[γ-(triethoxysilyl)propyl]-etrasulfide) for grafting onto graphene oxide [14]. The important role of co-coagulation and in situ interfacial modification in forming the characteristics of alkylamine-modified styrene–butadiene rubber/graphene oxide composites was also reported in [15].

The majority of articles are devoted to the creation of graphene-based and CNT-based rubber composites, whereas the class of graphite-related materials is rarely considered for rubber composites, except for papers devoted to expanded graphite/silicone rubber [16,17] and graphite platelets/silicone rubber [18]. Materials such as graphite oxide [19], graphite nanoplatelets [20], and expanded graphite [21] can be easily synthesized and scaled-up. The two last factors make these graphite-like materials attractive for application in rubber composites.

Despite the numerous works dedicated to the creation of rubber composites with carbon nanomaterials, the precise role of each type of material is not clear. A comparison of the impact of four types of graphite-like materials, e.g., graphite oxide, reduced graphite oxide, expanded graphite, and graphite nanoplatelets, with different morphology, porosity, and concentration of surface functional groups has been analyzed.

2. Experiment

2.1. Graphite-like Materials

The objects of study were general purpose rubber mixtures based on a combination of non-polar rubbers: CIR (cis-1,4-isoprene rubber) and BR (cis-1,4-polybutadiene rubber). The following graphite-like materials (carbon additives) were incorporated: graphite oxide (GO), reduced graphite oxide (rGO), expanded graphite, and graphite nanoplatelets (GNPs). The additives included the following materials:

• A graphite oxide (GO) sample was obtained using the modified Hummers method [22]. High-purity artificial graphite (99.99%), typically used for the preparation of nipples for metallurgical graphitized electrodes, with a mass of 7.5 g and a particle size of less than 250 μm, was used for synthesis. This graphite powder was placed into a flask with 3.0 g of NaNO₃ and 115 mL of concentrated H₂SO₄ (with a sulfuric acid content of 96.0%). The obtained mixture was stirred magnetically for 10 min at 0 °C (in an ice bath). Then, 15 g of anhydrous KMnO₄ was added, and the mixture was kept at 0 °C for 20 min. After that, the mixture was heated to 35 °C and stored for 35 min. The mixture was poured into a flask containing ice (230 g) and allowed to sit for 15 min at room temperature (25 ± 2 °C) without any heating or cooling. Then, 210 mL of H₂O₂ (with a hydrogen peroxide mass fraction of 32%) was added to the reaction mixture and kept for 15 min at room temperature. The mixture was then filtered and thoroughly washed with deionized water. The filter cake (graphite oxide powder) was dried in air at 80 °C for 48 h.
• Sample rGO#1 was obtained using the programmable heating of GO described above. The method for obtaining rGO was first reported in [23,24], where the heating of the GO sample at a rate of 15 °C/min was utilized. A GO sample (3 g) was placed into a stainless-steel barrel (250 mL) and then inserted into a laboratory furnace. The sample was heated from room temperature to 350 °C at a heating rate of 15 K/min. After reaching 350 °C, the sample was exposed for 55 min, and then the furnace was cooled down.
• Sample rGO#2 was obtained using the programmable heating of GO described above (the rGO#1 sample), but a different type of graphite oxide was used. Briefly, this sample of graphite oxide was synthesized using the same modified Hummers’ method described for the rGO#1 sample, but with a reduced amount of H₂O₂ (21 mL). The volume of hydrogen peroxide was 10 times lower compared to that used for the synthesis of rGO#1, which led to a decrease in the oxygen content of the material.
• Thermally expanded graphite (TEG) was obtained by programmable heating of commercial intercalated graphite (EG-350-50, Khimicheskie Sistemy, Moscow, Russia) in a muffle furnace. The sample was placed into an Al₂O₃ crucible and heated from 25 °C to 400 °C at a rate of 20 °C/min. After reaching 400 °C, the furnace was switched off, and the sample was allowed to cool inside.
• Graphite nanoplatelets (GNPs) were obtained by sonication (22 kHz) of a reduced graphite oxide (rGO#1) sample in isopropanol. The ultrasonic bath operated at a specific power of 1.6 W/cm³ for 6 h.

2.2. Rubber Composites and Their Characteristics

General purpose synthetic rubbers are the most common and widely available in the rubber industry. The composite investigated included two rubbers and other additives (

Table 1. Composition of a rubber composite based on a combination of general purpose rubbers.

No.	Component	Content, phr
1	CIR (cis-1,4-isoprene rubber)	50.00
2	BR (cis-1,4-polybutadiene rubber)	50.00
3	Carbon black N339	44.00
4	Oil I-40	2.50
5	Durez 29,095 resin	3.00
6	Pine rosin	5.00
7	Zinol	2.60
9	ZnO	3.00
10	Stearic acid	5.00
11	Okerin wax	2.50
12	Dusatntox 6PPD (N-1,3 dimethylbutyl-N’-phenyl-p-phenylenediamine)	2.50
13	Acetonanil N (poly(1,2-dihydro-2,2,4-trimethylquinoline))	2.00
14	Santokur CBS (N-Cyclohexyl-2-benzothiazolesulfenamide)	1.10
15	Sulfur Cristex OT 33	1.80
Total		175.0

).

The different graphite-like materials were added in the amount of 0.5 phr.

The rubber composites were manufactured on laboratory mixing rollers (320 × 160 × 160 mm) at a rotational speed of 17 rpm on a slow roll and a friction of 1:1.2; the temperature of the rolls was maintained at 50–60 °C.

Determination of viscosity: The viscosity of the rubber composites was measured using a Mooney viscometer (Alpha Technologies MV2000 Viscometer, Bellingham, WA, USA) in accordance with ASTM D1646-19a. Before testing, the sample was placed in the mold, which was then closed and maintained under pressure at the specified temperature (100 ± 1 °C) for 1 min. The rotor was then activated, and Mooney viscosity readings were recorded 4 min after startup. Upon test completion, the mold halves automatically opened. A detailed description of the technique is given in Supplementary Materials.

Determination of relaxation: Relaxation properties were evaluated according to ASTM D1646-19a using an MV2000 rotary viscometer. Immediately following the Mooney viscosity test (ML(1 + 4)), residual torque values were recorded at short intervals for 1 min after rotor stoppage.

Determination of vulcanization kinetics: The essence of the test is to obtain kinetic curves of the vulcanization process on a rheometer at a constant temperature (150 °C). From the obtained rheograms, the parameters characterizing the rheological and vulcanization properties of the mixtures were determined:

• − the time necessary for increase in minimum torque to 2 units ts2, min;
• − the time to reach the optimum vulcanization t₉₀, min;
• − the rate of vulcanization Rh, dNm/min.

The tests were carried out on an ODR 2000 rheometer manufactured by Alpha Technologies (USA) according to ASTM D2084.

Cross-linking density (ν, mol/m³) was calculated using Equation (1). A detailed description of the technique used for determination is presented in Supplementary Materials.

υ = ρ_r/M_c

(1)

where ρ_r—density of rubber, kg/m³; M_c—average molecular weight of a chain segment enclosed between two cross-links, g/mol.

Determination of elastic-strength properties: The mechanical properties of vulcanized rubbers under tension were evaluated according to ASTM D412, including conditional tensile strength, conditional stresses at specified elongations, and elongation at break. The samples were produced in a hydraulic press using the compression method under the following conditions: at a temperature of 150 °C for 15 min, under a pressure of 24 MPa. Testing was performed no earlier than 6 h after vulcanization. Sample preparation involved cutting blade specimens from rubber plates using a manual punching press; numbering of each specimen; marking the working area and reference lines with contrasting paint; and measuring thickness at three points in the working area (using the minimum value for calculations).

The tests were carried out on a Tensometer T 2020 DC10 by Alpha Technologies (Bellingham, WA, USA). Only specimens with thickness variations within ±10% of the arithmetic mean were accepted for testing.

Conditional tensile strength (f_p, MPa) was determined according to Equation (2):

f_p = P_p/S₀

(2)

where P_p is the breaking load, N; S₀ is the cross-section area of sample, m².

Relative elongation at break (ε_p, m) was determined according to Equation (3):

ε_p  = (l_p − l₀)/l₀,

(3)

where l_p is the distance between marks on the sample at the moment of break, mm; l₀ is the distance between marks on the sample after testing, mm.

Conditional stress at a given elongation (F_ɛ, MPa) was determined according to Equation (4):

F_ɛ = P_ɛ/S₀,

(4)

where P_ɛ is conditional stress at a given elongation, N.

Rubber specimens were tested for strength properties using tensile testing machines under strictly controlled conditions: a constant elongation rate of 500 ± 25 mm/min and a maintained temperature of 22 ± 2 °C.

Heat aging resistance: The heat aging resistance of the rubber composites was determined according to ISO 188:2013. This test involves evaluating physical and mechanical properties after accelerated aging. The aging duration was selected based on the rubber’s application temperature range, set to 72 h in our case, at a test temperature of 100 ± 1 °C. Following aging, samples were removed from the thermostat and conditioned at room temperature for 16 h to 6 days before tensile testing.

Results from pre- and post-aging tests were recorded, with percentage changes in measured properties calculated when necessary using Equation (5):

Δ = (x_a − x₀)/x₀

(5)

where x_a—physical and mechanical index of rubber after aging; x₀—physical and mechanical index of rubber before aging.

The above-mentioned characteristics of the rubber composites were compared with composites without graphite-like materials (named as “composites without carbon additives”). The relative errors calculated when testing the samples are given in Table S1 (Supplementary Materials).

2.3. Characterization of Graphite-like Materials

Scanning electron microscopy (SEM) was used to control the surface morphology using an S-3400N (Hitachi, Tokyo, Japan) microscope equipped with an add-on for energy dispersive X-ray spectroscopy (EDX) (Oxford Instruments Co., Oxfordshire, UK). The phase composition of the samples was investigated using X-ray diffraction (XRD) with a DRON-3 diffractometer (Burevestnik, Russia) (Cu Kα radiation, λ = 1.54 Å). Fourier transform infrared spectroscopy (FTIR) was used to analyze the functional groups in the graphite-like materials using an FT-801 spectrometer (Simex, Novosibirsk, Russia). Determination of the specific surface area of the samples by the BET method was carried out by using a Nova 1200e Quantachrome Nova (Boynton Beach, FL, USA).

3. Results and Discussion

3.1. Characterization of Graphite-like Materials

SEM images of graphite-like materials are presented in Figure 1. Graphite oxide (Figure 1a,b) appeared as a bulk 3D material composed of large micron-sized particles. The reduced graphite oxide materials exhibited exfoliated structures, where temperature treatment induced graphite domain delamination and increased the distance between graphite domains, which is typical for thermally reduced graphite oxides [25,26,27] (Figure 1c–f). However, the key difference lies in using programmable heating [24] for GO reduction, which enables longer reduction durations under mild conditions compared to the commonly applied thermal shock technique [28,29].

The TEG sample exhibited curved particles with large interdomain space (Figure 1g,h). GNPs, produced via sonication of the rGO#1 sample, displayed platelet-coated particles. The relatively low sonication power yielded a material containing both particles and graphite nanoplatelets (Figure 1i,g). The formation of nanoplatelets is clearly seen in the STEM images (Figure S1, Supplementary Materials).

The GO sample exhibited the highest concentration of functional groups according to EDX analysis (C:O = 3.1 at.). Among the reduced graphite oxides, the rGO#2 sample showed oxygen content closer to graphite oxide (C:O = 4.7 at.). The reduction dynamics differed between rGO#1 and rGO#2, resulting in lower functional group concentration in the former compared to the latter. TEG demonstrated a high degree of graphitization, as evidenced by its 95.14% carbon content along with low oxygen content compared to other samples. GNPs displayed oxygen content similar to rGO#1, confirming the mild sonication conditions. The slight difference in C:O ratios between these materials arises from the additional removal of functional groups from the rGO surface during ultrasonic treatment.

All samples contained sulfur as a principal impurity residue, originating from both the GO synthesis via Hummers method (which uses sulfuric acid) and the sulfuric acid treatment during the synthesis of their precursor, intercalated graphite, used to produce thermally expanded graphite (

Table 2. EDX data of graphite-like materials.

Sample	Concentration of Elements, at. %
	C	O	S	Other Elements	C:O (at.)
GO	70.44	22.46	7.14	-	3.1
rGO#1	85.56	10.95	1.76	K—0.3; Mn—0.43	7.8
rGO#2	80.68	17.31	0.84	Si—0.14; K—0.17; Mn—0.68	4.7
TEG	95.14	4.33	0.29	Si—0.13; Cu—0.12	22.0
GNPs	87.79	10.95	0.72	K—0.31; Mn—0.24	8.0

).

FTIR spectra of graphite-like materials are presented in Figure 2.

GO sample showed four main vibrations attributed to O-H bonds in water (588 cm⁻¹) [27], C-H out-of-plane vibrations (776 cm⁻¹) [28], O-H vibrations (926 cm⁻¹) [29,30], C-O bonds in carboxyls (1094 cm⁻¹), C-O bonds in Ar-OR compounds (1225 cm⁻¹), stretching vibrations in carboxylate groups (1525 cm⁻¹) [31], skeletal C=C vibrations (1649 cm⁻¹), and ester C=O groups (1765 cm⁻¹). Samples of rGO#1 and rGO#2 showed almost the same vibrations of bonds but with different relative intensities. The TEG sample showed the presence of a strong peak at 1225 cm⁻¹ related to C-O bonds and skeletal C=C vibrations (1649 cm⁻¹) caused by the low oxidation degree of the material. GNPs had a strong peak related to O-H bending vibrations in the carboxyl group (940 cm⁻¹) [30] and the same groups presented in the spectra of rGO samples. The strong peak at 3300–3400 cm⁻¹ was related to O-H vibrations in the water [32] (with almost no peak in TEG, which is a product of thermal shock in intercalated graphite, avoiding the stage of hydrolysis of graphite intercalation compounds).

The GO sample exhibited a specific surface area (by low-temperature nitrogen adsorption) of only 6 m²/g. The rGO#1 and rGO#2 samples showed significantly higher values of 41 m²/g and 40 m²/g, respectively. GNPs demonstrated a surface area of 35 m²/g, while TEG displayed the highest value at 428 m²/g, indicating superior porosity among all the graphite-like materials studied.

3.2. Viscosity

The introduction of various additives into the elastomeric matrix can affect the intermolecular interactions, for the estimation of which, the Mooney viscosity index was used. Additives, in relation to the matrix, can be both compatible and incompatible. With the introduction of modifiers incompatible with the polymer, its molecules penetrate only into the interstructural space. In this case, the modifying additive cannot penetrate between the polymer molecules, since the interaction of the additive molecules with the polymer is much weaker than the intermolecular interaction of polymer chains. The data on determination of the initial viscosity of rubber composites (60 s after test beginning) carried out on a Mooney viscometer are presented in

Table 3. Initial viscosity of rubber composites.

Rubber Composite	Initial Viscosity of Rubber Mixture, MU
Composite without carbon additives	86.9
Composite + GO	86.5
Composite + rGO#1	85.1
Composite + rGO#2	88.6
Composite + TEG	86.5
Composite + GNPs	87.6

.

The addition of these additives practically had no effect on the change in initial viscosity of rubber compounds; however, when determining the Mooney viscosity (according to ASTM D1646-19a) for mixtures containing these additives, a change in this characteristic was found (Figure 3).

Figure 3 shows that the Mooney viscosity decreased with the addition of any carbon material used. The introduction of rGO of different grades reduced this value by 5%. The addition of GO affected the decrease in the Mooney viscosity of these rubber composites to a lesser extent, and the introduction of GNPs had the greatest effect. The Mooney viscosity of rubber composite with the addition of GNPs decreased by 8.5% (5 MU decrease), which can be associated with both differences in the specific surface area of these additives, and with the structural peculiarities of the additive itself (the lamellar structure facilitates the movement of macromolecules in the direction of deformation without affecting the interaction itself with its molecules). The shape of nanoplatelets had a positive effect on the decrease in Mooney viscosity. This may be due to the fact that these additives are interstructural plasticizers that promote better sliding of rubber macromolecules.

It can be noted that the Mooney viscosity decrease has a positive effect on the processing processes; that is, less energy is spent due to the lower shear stresses in the mixture, and there will be less heat generation, which means that this makes it possible to reduce the amount of wastewater produced during processing. The decrease in the Mooney viscosity of rubber filled with CNTs was also reported in [4]. The authors noted a decrease in Mooney viscosity exceeding 10 MU for rubber/CNT and rubber/rGO composites when compared to rubber without carbon additives; however, the concentration of these fillers was higher (2 phr) than that reported in this paper [12].

3.3. Relaxation Characteristics

Relaxation can be influenced by the type of polymer and the peculiarities of its synthesis (polymerization process, molecular weight distribution, branching, average molecular weight, microstructure), as well as the mixing mode and equipment used, preliminary plasticization, filler content, and particle size, among others. These factors can lead to significant differences in the properties of rubber composites, which are useful for predicting workability characteristics. The results on relaxation characteristics are presented in

Table 4. Mooney viscosity of the studied rubber composites 1 s after the rotor stopped.

Rubber Composite	The Mooney Viscosity of the Rubber Mixture Measured 1 s After Stopping the Rotor, MU
Composite without carbon additives	23.6
Composite + GO	22.3
Composite + rGO#1	21.8
Composite + rGO#2	21.6
Composite + TEG	21.7
Composite + GNPs	21.1

.

Based on the results of studies to determine relaxation characteristics, the relaxation coefficients for each rubber compound were calculated (Figure 4).

The somewhat higher relaxation rate of the composites containing the graphite materials rGO#2 and TEG, estimated by the relaxation coefficient (Figure 4), demonstrates the advantages of these additives over others in terms of the workability of the composites. In addition, the data obtained regarding the relaxation coefficients indicate a better distribution of these additives within the volume of the elastomeric composites. Perhaps this is due to the easier orientation and movement of macromolecules in space and peculiarities of the interaction of particles with macromolecules and the particles’ shape.

3.4. Vulcanization Kinetics

The essence of the test method for determining the kinetics of vulcanization of rubber mixtures is to obtain the kinetic curves of vulcanization using a rheometer at a constant temperature. From the obtained rheograms, indicators characterizing the rheological and vulcanization properties of the mixtures are determined. Tests were carried out in accordance with ASTM D2084.

Table 5. Results of rheometry.

Rubber Composite	ts2, min 1	t90, min 2	Rate of Vulcanization, dNm/min (Rh)
Composite without carbon additives	2.67	5.51	17.43
Composite + GO	3.23	8.18	11.16
Composite + rGO#1	3.29	8.44	10.25
Composite + rGO#2	3.62	8.69	10.24
Composite + TEG	3.11	8.00	8.56
Composite + GNPs	3.54	8.81	9.62

¹ Time, t_s2, necessary to increase minimum torque to 2 units; ² Time to reach the optimum vulcanization degree.

shows the rheometry results.

It was observed that the graphite-like materials increased the scorch time by 16.5% to 35.6%, which is equivalent to an increase in this characteristic by 30 to 60 s. Additionally, the introduction of additives significantly influenced the time to reach optimal vulcanization, increasing it by a minimum of 45.2% and a maximum of 59.9%. The vulcanization rate was also greatly reduced by 36.0% to 50.9% with the introduction of carbon additives, indicating the role of these additives in the processes occurring during the formation of a spatial network. It is possible that intermediate complexes are formed with the involvement of the studied additives, which increases the amount of energy and time required for the reaction to proceed. The time t_s2 reached its maximum value for rGO#2 and GNPs was almost the same as for t₉₀. At the same time the rate of vulcanization decreased compared to rubber without additives (17.43 dNm/min).

Reaching the growth in t_s2 has a positive impact on the better flow of composites into molds of complex configuration; however, the rate of vulcanization in the main period decreases, which leads to an increase in the time to achieve the vulcanization optimum. The shortest time to reach the optimum t₉₀ is characterized by a mixture without additives and a mixture with TEG. It can be noted that all the additives were relatively chemically active and interact with sulfur, forming the vulcanization agent.

It should be noted that as a result of the vulcanization process, a spatial network was formed, the density of which can be indirectly estimated using the maximum torque, as illustrated in Figure 5.

Figure 5 shows that when modifying additives are introduced into the rubber mixture based on a combination of general purpose rubbers, the torque can both increase and decrease. The latter was observed in the TEG-based composite, which was apparently related to the highest degree of graphitization and the highest C:O ratio.

3.5. Crosslinking Density

The structure of the vulcanization mesh is one of the determining factors on which the physicomechanical parameters and the resistance of rubbers to aging under the influence of heat depend, as this process is associated with the destruction of the polymer and the rupture of crosslinks. When elastomers are exposed to elevated temperatures, crosslinking, destruction of macromolecules, depolymerization, a change in the degree of saturation, and the release of volatile products occur; in air, oxidation and the formation of carbonyl and other oxygen-containing groups also take place.

Table 6. Density of crosslinking of rubbers prior to aging.

Rubber Composite	Mc 1, kg/mol	ν·105, mol/cm3	n·10−19, cm−3
Composite without carbon additives	12,894.10	7.10	4.27
Composite + GO	11,722.25	8.07	4.86
Composite + rGO#1	12,285.37	7.45	4.49
Composite + rGO#2	12,433.88	7.37	4.43
Composite + TEG	13,471.77	6.79	4.09
Composite + GNPs	13,108.57	6.98	4.20

¹ Where M_c is the average molecular weight of the chain section between two cross-links, kg/mol; $\mathrm{\nu }$ is the density of nodes in the spatial network, mol/cm³; n is the number of cross-links, cm³.

shows the results of density of crosslinking of rubbers prior to aging.

It can be seen from Table 6 that the largest number of bonds is formed with the introduction of graphite oxide, rGO#1, and rGO#2, by 14%, 5%, and 4%, respectively. Conversely, the smallest number of bonds is found in the samples with the addition of TEG, which is again attributed to its high degree of graphitization and low concentration of functional groups capable of interacting with rubber. The crosslinking density is directly related to the torque, as the same elastomeric mixture exhibits the highest torque and the maximum number of crosslinks, while other samples lend themselves to similar comparisons. The number of crosslinks in the rubber composites was higher when GO was used (C:O = 3.1 at.), due to the higher concentration of functional groups on its surface compared to other graphite-like materials. This value decreased for composites based on TEG and GNPs, which possess high C:O ratios, e.g., 22.0 and 8.0, respectively.

3.6. Elastic Strength

The resistance of rubbers to the destructive action of mechanical stresses characterizes their strength. Depending on the nature of the deformation, a distinction is made between the static and dynamic strength of rubbers. The static strength of rubbers can be determined through stress relaxation, creep, durability, and tensile tests at a specific strain rate. The most dangerous deformations are tensile deformations, which can cause rupture.

To assess the elastic strength of the rubbers, the conditional tensile strength and elongation at break were determined. The practical determination of the strength properties was conducted under simple stretching conditions performed at a constant rate. The results of the study on the effect of modifying additives on the physical and mechanical properties of unfilled rubber compounds under normal conditions are presented in

Table 7. Strength properties of rubbers before aging.

Rubber Composite	Elongation at Break, %	Tensile Strength, MPa
Composite without carbon additives	418.1	19.39
Composite + GO	520.3	18.44
Composite + rGO#1	523.6	17.38
Composite + rGO#2	585.3	20.71
Composite + TEG	581.4	18.20
Composite + GNPs	484.2	19.96

.

According to Table 7, it can be concluded that the addition of these graphite-like modifying additives increased the elongation at break. The greatest effect was detected for rGO#2 and TEG, the elongation of which were 585.3 and 581.4%, respectively. This is due to the easier movement of macromolecules in matrices containing these additives (which was revealed during the study of stress relaxation).

These additives have an ambiguous effect on the conventional tensile strength: the maximum effect was shown by rGO#1, which reduced this value by 1.32 MPa, and GNPs, which increased strength by 0.57 MPa. Usually, the tensile strength of rubber composites must increase when adding graphite-like materials [14,15]; however, it could, in fact, decrease (when adding GO, rGO#1, TEG). Interestingly, materials with various oxidation degrees such as GO, rGO, and TEG showed a similar decrease in tensile strength. Apparently, stress concentrators are formed, which leads to the destruction of the material despite the difference in the structure of both additives and rubbers. These effects are usually observed in graphene- or carbon-filler composites [9,33,34]. The appearance of stress concentrators may be caused by graphite-like materials with low bulk density, such as the materials we used, especially TEG, rGO#1.

3.7. Heat Aging Resistance of Rubbers

Resistance to heat aging is the ability of rubbers to retain their strength, high elasticity, and other properties when exposed to a short-term increase in temperature. The change in rubber properties during heat aging is irreversible.

When elastomers are exposed to elevated temperatures, crosslinking and destruction of macromolecules, depolymerization, a change in the degree of saturation, the release of volatile products occur; in air, oxidation and the formation of carbonyl and other oxygen-containing groups also occur. The nature and speed of these processes depend on the type of rubber, the composition of the rubber compound, and temperature. Exposure to temperature and oxygen in the air leads to the decomposition of polysulfide bonds, while this process occurs disproportionately faster than the oxidative decomposition of rubber macromolecules. The thermal decomposition of polysulfide bonds is accompanied by a decrease in the degree of their sulfidicity and the release of sulfur, which can further participate in the formation of new bonds.

The results of studying the effect of modifying additives on the physical and mechanical properties of rubber composites based on a combination of general purpose rubbers after aging are presented in

Table 8. Elastic strength of rubbers after aging.

Rubber Composite	Elongation at Break, %	Tensile Strength, MPa	Change
			Elongation at Break, %	Tensile Strength, %
Composite without carbon additives	425.0	18.39	+1.65	−5.16
Composite + GO	390.1	17.33	−25.03	−6.02
Composite + rGO#1	366.6	16.78	−29.98	−3.45
Composite + rGO#2	327.0	15.84	−44.13	−23.52
Composite + TEG	338.0	15.66	−41.86	−13.96
Composite + GNPs	323.0	15.36	−33.29	−23.05

.

From Table 8 it can be seen that when additives are added to a rubber compound during aging, the elongation at break and the conventional tensile strength decrease, which indicates the process of destruction during the aging process. It can be said that the oxygen contained in graphite-like materials takes part in the oxidation processes. The destruction of the materials probably occurs not only from the surface regions, where there is contact with air, but partially in the bulk as well. There is not so much oxygen in the graphite-like materials in the rubber composites, but it is enough to have an impact.

Overall, it can be concluded that there are two types of interactions between rubber and graphite-like additives at various stages:

• Before vulcanization. These are physical interactions of different types, such as simple interlocking and surface area changes, as well as Van der Waals interactions.
• After vulcanization. Since vulcanization involves changes in the crosslinking density and reaction kinetics, it appears that additives also participate in these reactions, leading to the formation of new bonds or just their reorganization/modification. This, in turn, affects the change in mechanical properties and the aging process.

4. Conclusions

The general purpose rubber composites were successfully developed using a mixture of non-polar cis-1,4-isoprene rubber and cis-1,4-polybutadiene rubber and various graphite-like materials. It has been found that the addition of graphite-like materials leads to a significant reduction in Mooney viscosity (from 86.9 MU to 85.1 rGO#1 additive) correlating with an increased relaxation rate, enhancing elastic recovery and subsequently minimizing shrinkage. These additives were found to prolong the vulcanization process, increasing both the onset time and the achievement of optimal vulcanization levels. This increases the processability of rubber compounds. It was found that these additives allow for an increase in the residence time of the mixture in a viscous state and slow down the vulcanization process, reaching rates of vulcanization ranging from 8.56 to 11.16 dNm/min. At the same time, the cross-linking density increases from 7.1·10⁻⁵ mol/cm³ to 8.07·10⁻⁵ mol/cm³, which affects the elastic strength properties. Thus, graphite-like additives have been shown to significantly increase elongation at break, with expanded graphite (581.4%) and reduced graphite oxide (585.3%) making the most significant contributions to this. However, the conditional tensile strength of the composites is practically independent of these additives. The elastic strength properties of rubber composites after aging were investigated, revealing that the introduction of graphite-like materials slightly reduces the resistance of rubbers to elevated temperatures. The results obtained highlight the potential of graphite-like additives in improving the properties of rubber composites for various applications, particularly in enhancing technological and mechanical characteristics. However, the operating conditions should be taken into account.