The Synergistic effect of nanoclay/nanosilica on mechanical properties and swelling resistance of ternary rubber (NR/SBR/NBR) blends nanocomposites

doi:10.21203/rs.3.rs-3440559/v1

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The Synergistic effect of nanoclay/nanosilica on mechanical properties and swelling resistance of ternary rubber (NR/SBR/NBR) blends nanocomposites

https://doi.org/10.21203/rs.3.rs-3440559/v1

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In this research, we delve into the influence of hybrid filler loading, comprising both nanoclay and nanosilica (NC/NS), on the mechanical properties and resistance to swelling in ternary rubber nanocomposites composed of natural rubber (NR), styrene-butadiene rubber (SBR), and nitrile butadiene rubber (NBR). In this investigation, the composition of NR/SBR/NBR was kept constant at 30/30/40 phr, while the nanoclay was maintained at 6 phr and the proportion of nanosilica was varied from 0 to 6 phr. The nanocomposites were meticulously prepared through a melt mixing technique. Subsequently, we conducted an exhaustive assessment of their mechanical properties, encompassing parameters such as tensile strength, stress at 100% elongation (M100), elongation at break (EB), tear strength, compression set, hardness, rebound resilience, and abrasion resistance. Our findings reveal a discernible trend: as the nanosilica loading increased in the presence of the SCA, several key characteristics exhibited significant improvements. Specifically, the minimum torque, maximum torque, cure rate index, tensile strength, M100, tear strength, compression set, hardness, and crosslink density all showcased notable increases. In contrast, scorch time, cure time, EB %, and mole percent uptake (MPU) demonstrated a consistent decrease.

NC/NS

NR/SBR/NBR

Mechanical

Swelling resistance

Today, researchers are increasingly exploring the creation of innovative polymeric materials by blending together two or more distinct polymers. Polymer blending emerges as an appealing approach due to its straightforwardness and affordability, providing a pathway to create adaptable polymeric materials suitable for a broad spectrum of commercial uses [1]. Among the various polymeric blends, NR/SBR/NBR ternary rubber blends represent a noteworthy category. These blends involve the combination of three different types of rubber: natural rubber (NR), styrene-butadiene rubber (SBR), and nitrile rubber (NBR). Each of these rubber types possesses its unique properties, and their amalgamation allows manufacturers to achieve a balanced blend of desirable characteristics. Natural rubber (NR), derived from the latex of rubber trees, is celebrated for its exceptional attributes, including high resilience, elasticity, and exceptional tear strength. It is particularly adept at providing flexibility at low temperatures, although it exhibits relatively limited resistance to high temperatures, ozone, and chemical exposure [2]. SBR stands out as a synthetic rubber manufactured through the copolymerization of styrene and butadiene. This synthesis imparts SBR with a diverse set of advantageous properties, rendering it a versatile material suitable for numerous applications. SBR is recognized for its notable attributes, such as impressive resistance to abrasion, exceptional resilience, and remarkable stability against aging. Furthermore, SBR boasts enhanced resistance to ozone, heat, and various chemical agents when compared to NR [3–4]. NBR is a synthetic rubber made by copolymerizing acrylonitrile and butadiene. It exhibits excellent oil and fuel resistance, making it suitable for applications in automotive seals, gaskets, and hoses. NBR also provides good resistance to chemicals and has better heat and ozone resistance than natural rubber [5–6]. By combining these three rubbers in different proportions, manufacturers can tailor the properties of the ternary rubber blend to meet specific requirements. For example, increasing the percentage of NR in the blend can enhance its elasticity and tear strength, while incorporating more SBR can improve its abrasion resistance and aging stability. The addition of NBR can further enhance oil and fuel resistance. Ternary rubber blends are commonly used in various industries, including automotive, construction, and manufacturing, where a combination of different rubber properties is required to achieve optimal performance in specific applications.

The practice of enhancing the performance of rubber products by integrating fillers like carbon blacks (CBs), silica (SiO₂), talc, and clays has been a well-established and crucial tradition in the industrial sector [7–10]. The level of improvement achieved with these fillers is contingent upon a multitude of influential factors, including the filler's composition, particle size, shape, the ratio of filler to rubber, and the interactions between the rubber/elastomer matrix and the filler material [11]. The incorporation of fillers into rubber formulations consistently leads to modifications in physical properties while simultaneously contributing to cost-effectiveness. As a result, the use of rubber materials infused with particulate additives has garnered increasing attention in both industrial and research circles [12]. The primary mechanism driving reinforcement in these materials revolves around interactions between the filler particles, particularly when the filler loading is high. Several factors affect these interactions, including chemical bonds formed on the surfaces of filler particles, the structural arrangement of the filler network, physical forces at play, and the proportion of the rubber matrix occupied by the filler material [8].

Composite materials serve a wide array of applications, spanning from everyday household appliances to cutting-edge aerospace technologies. The fundamental strength and stiffness of these composites are primarily governed by their reinforcing elements, which can take various forms, including particles, whiskers, or fibers. Incorporating fillers such as SiO₂, clay, wood flour, fly ash, lignocellulosic residues, glass flakes, and others can lead to a substantial improvement in the mechanical characteristics of plastics. While cost savings frequently serve as a primary incentive for the inclusion of fillers, they can also be introduced with the intent of imparting particular attributes to the plastic, such as flame resistance, coloration, or opacity (resulting in the prevention of light transmission). Typically, fillers are mixed into the liquid or molten plastic material to ensure uniform distribution and incorporation. The effectiveness of fillers in reinforcing the material depends on numerous factors, including their surface properties, purity, particle size, and overall structure [13–15].

In their extensive research, Jovanovic´ et al. [16] conducted a comprehensive analysis of the rheological and mechanical characteristics of ternary rubber blends. These blends consisted of three key components: NR, Butadiene Rubber (BR), and SBR, with a fixed composition ratio of NR/BR/SBR = 25/25/50. The pivotal focus of their study revolved around the incorporation of varying quantities of SiO₂ nanoparticles, spanning from 0 to 100 phr, into these rubber blends. The outcomes of their investigations revealed several noteworthy trends. First and foremost, as the SiO₂ content increased within the NR/BR/SBR ternary rubber blend, there was a substantial reduction in both the t₉₀ (time required to achieve 90% cure) and the t_s2 (scorch time). This reduction in curing times can be directly linked to a decrease in cure rate index (CRI) within the NR/BR/SBR = 25/25/50 ternary rubber blend. This effect was particularly pronounced when a 60 phr filler loading was introduced. Moreover, the Tensile Strength (TS) values of the nanocomposites derived from the NR/BR/SBR/SiO₂ ternary rubber blend displayed an upward trajectory as the filler content increased. The peak TS was attained in the sample containing 60 phr of filler, showcasing the highest mechanical strength. Nevertheless, as the filler loading exceeded this threshold, there was a gradual decline in TS. This observation suggests the existence of an optimal balance between filler content and mechanical strength at the 60 phr level. Many researches have been focused on the replacement of a significant amount of CB by filler, such as silica [17–18], nano-Al₂O₃ [19], carbon nanotubes [20], and nanoclay [21–29].

Over the past two decades, significant attention from both the academic and industrial sectors has been directed toward polymer/clay nanocomposites [30–32]. The primary goal in creating these nanocomposites has been to achieve a high level of intercalation, or even exfoliation, along with maximum dispersion of clay platelets within the polymer matrix. This pursuit aims to bring about substantial enhancements in the physical, mechanical, and thermal properties compared to the base polymer [33]. While rubber nanocomposites containing a single type of nanofiller are prevalent and well-established in research and industry [34–38], it's important to recognize that individual reinforcing fillers may not universally improve all performance aspects, especially in rubber materials. In other words, each filler may have limitations that prevent it from delivering synergistic improvements across all desired properties. This consideration prompted us to investigate the utilization and impact of using dual fillers in rubber nanocomposites [39].

Rice husk (RH), often considered a byproduct of agriculture, holds significant value as a source of amorphous SiO₂ [40]. The composition of RH is diverse, consisting of cellulose (25–35%), lignin (26–31%), hemicelluloses (18–21%), SiO₂ (15–17%), resolvable fractions (2–5%), and a moisture content of approximately 7.5% [41]. However, our study places a strong emphasis on exploring the potential of SiO₂ extracted from RH. This focus is motivated by the remarkably high SiO₂ content found in RH, its widespread availability, and its cost-effectiveness as a raw material. The utilization of rice husk silica not only imparts aesthetic appeal to composites but also aligns with environmentally responsible practices, repurposing an agricultural byproduct and potentially mitigating greenhouse gas emissions and environmental pollution. The process of burning rice husk at temperatures below 700^oC yields amorphous rice husk ash. However, this ash may contain various metallic impurities such as manganese (Mn), iron (Fe), potassium (K), sodium (Na), calcium (Ca), etc., which can diminish its surface area and overall purity. Various techniques have been developed to synthesize pure RH SiO₂ by effectively eradicating these metallic impurities. Usually, these techniques entail the acid or alkaline extraction of RH, followed by a process of high-temperature calcination [42–44]. Impressively, highly pure SiO₂ with a 99.9% purity level has been successfully synthesized through treatments such as potassium permanganate treatment of rice husk or a combination of treatments involving Aquaregia followed by piranha solution.

Silica is a widely employed filler in rubber and plastics, owing to its advantageous characteristics. Its hydrophilic properties primarily stem from the occurrence of surface silanol groups. These -OH groups on the surface of silica tend to encourage particle aggregation and reaggregation. On the other hand, smaller silica particles present a larger specific surface area, enabling enhanced interactions with the polymer matrix. Since most polymers are inherently hydrophobic, they struggle to effectively coat the filler's surface, leading to less than optimal mechanical properties in the composite material. To address this issue, SCAs are typically employed to enhance adhesion between silica and hydrophobic matrices, such as rubber.

Research on the impact of fillers on the mechanical properties and resistance to swelling in ternary rubber blends, crucial for diverse industrial applications, is conspicuously lacking. As a result, the primary goal of this research study is to develop a nanocomposite formulation by introducing a hybrid filler, comprising nanoclay and nanosilica, into a ternary rubber matrix composed of NR/SBR/NBR. This study extensively investigates the effects of varying nanoclay/nanosilica content, precisely in ratios of 6/0, 6/1, 6/2, 6/3, 6/4, 6/5, and 6/6 phr/phr, on a comprehensive set of crucial mechanical properties and swelling resistance of the ternary rubber blend. The examined properties encompass TS, hardness, M100, RR, EB %, AR, tear strength, compression set, and swelling resistance. Furthermore, this research critically evaluates how the presence of hybrid fillers, both in the presence and absence of a SCA, influences the aforementioned properties. A comparative analysis is meticulously conducted to assess the outcomes obtained with and without the SCA, providing valuable insights into its potential impact.

2.1 Materials

In the present study, all rubber compounds and curing chemicals used were generously supplied by Vignesh Chemicals Pvt. Ltd. and Asian Rubbers Pvt. Ltd., both located in Chennai, India. These materials were utilized in their original, as-received condition, without any further modifications or treatments. Here is a description of the characteristics and properties of each of these materials:

NR: The NR used in this study was Ribbed Smoked Sheet (RSS-1). It possesses a plasticity value of 40, an ash content of 0.58%, a plasticity retention index of 40%, a nitrogen content of 0.63%, Mooney viscosity (MV) (ML_{(1 + 4)} 100^oC) measuring 57 M, and a density of 0.9125 g/cm³.

SBR-1502: SBR-1502 comprises 23.5% styrene and has a specific gravity of 0.94. Its MV (ML_{(1 + 4)} 100^oC) is measured at 55.

NBR: The NBR utilized in this study has an acrylonitrile content of 33%. It exhibits a specific gravity of 0.97 and a MV (ML_{(1 + 4)} 100^oC) of 50.

Accelerator − 2,2'-Dibenzothiazyl Disulphide (MBTS): MBTS was employed as an accelerator in the study to facilitate the vulcanization process.

Secondary Vulcanization Accelerator - Diphenyl Guanidine (DPG): DPG served as a secondary vulcanization accelerator, contributing to the rubber curing process.

Homogenizing Agent (Compatibilizer) - Ultra-Blend 4000 (UB4000): UB4000 played the role of a homogenizing agent or compatibilizer, aiding in achieving uniform dispersion of components within the composite.

RH Source: RH was sourced from a local mill in Puducherry, providing the raw material for the study.

Nanoclay (Cloisite 30B): Cloisite 30B, a nanoclay, was utilized, featuring an organic modifier of MT2EtOH, modifier 90 (mgequiv/100 g), and a % weight loss on ignition of 30%.

Standard Rubber Compounding Ingredients: The formulation was completed by adding other conventional rubber compounding ingredients, such as SCA, zinc oxide (ZnO), stearic acid, and sulphur, all of which were used at commercially accepted grades.

Solvents: In this study, various solvents are employed to investigate swelling resistance, quantified in terms of mole percent uptake. Three distinct categories of solvents were utilized, namely aromatic, aliphatic, and chlorinated solvents. These solvents were carefully selected to encompass a broad spectrum of chemical properties and interactions. a) Aromatic Solvents: The aromatic solvents used include benzene, toluene, xylene, and mesitylene. b) Aliphatic Solvents: The aliphatic solvents in this study consist of n-pentane, n-hexane, n-heptane, and n-octane. c) Chlorinated Solvents: The chlorinated solvents employed are dichloromethane, chloroform, and carbon tetrachloride. All of these solvent chemicals were sourced from reputable suppliers, including Sigma-Aldrich, Subra Chemicals Agency in Puducherry, India, and they all meet the high-quality standards of Merck grade.

2.2 Synthesis of nanosilica

The process of obtaining nanosilica from rice husk (RH) involved a meticulous series of steps. At the outset, a meticulous cleaning process was employed to eliminate any soil or dirt adhering to the rice husk. Following the cleaning process, precisely 100g of thoroughly cleaned rice husk was subjected to a reflux procedure with 1L of 1N HCl at 90^oC for a duration of one hour, successfully eradicating metallic impurities. After the acid treatment, a meticulous rinsing with distilled water was carried out to remove any residual acid, followed by overnight drying at 110^oC. The subsequent calcination process, conducted in a muffle furnace at 650^oC for a duration of six hours, resulted in the production of high-purity silica, manifesting as a white ash. This silica was then used to create a sodium silicate solution by refluxing 10g of the white ash with a 2.5N sodium hydroxide (NaOH) solution for four hours. After filtering to achieve a clear Na₂SiO₃ solution, silica gel formation occurred through the gradual addition of concentrated HCl until the solution's pH reached 9, with subsequent aging for 24 hours and thorough washing with double-distilled water. The silica gel, having been thoroughly washed, was then subjected to a 24-hour drying process in an oven set at 80^oC. Subsequently, it was finely ground to attain the targeted nanosilica particle size. This intricate procedure ensured the production of high-quality nanosilica from rice husk, suitable for various applications.

2.3 Preparation of NR/SBR/NBR blends nanocomposites

Meticulously prepared, each of the ternary rubber compounds was labeled as S₁, S₂, S₃, S₄, S₅, S₆, and S₇ when prepared without a silane coupling agent (SCA), and as S₈, S₉, S₁₀, S₁₁, S₁₂, and S₁₃ when prepared with an SCA, as indicated in Table 1. The preparation of these compounds followed a precise and systematic procedure. This involved using a two-roll mill with dimensions of 11 cm in diameter, 28 cm in length, 22 cm in working length, and a friction ratio of 1:1.4, in accordance with ASTM D 3184 [45]. The compounding process commenced with the introduction of NR into the open mill mixer, where it was masticated for 5 minutes. Consequently, SBR and NBR were successively incorporated and mixed for a duration of 10 minutes each. Subsequent to this, the nanoclay and nanosilica were introduced and mixed for an additional 10 minutes. The addition of ZnO, stearic acid, and UB4000 followed, with mixing continuing for another 10 minutes. Finally, during the last stage of compounding, MBTS, DPG, and sulfur were introduced. The rheological properties of each blend were comprehensively assessed using a ODR 100 in accordance with ASTM D 2084 [46], with testing carried out at a temperature of 150^oC. Subsequently, the uncured compounds were subjected to compression molding using stainless steel molds, at a temperature of 150^oC, and with specific t₉₀, all under a pressure of 60 MPa.

Table 1

Compounding formulations for NR/SBR/NBR-NC/NS nanocomposites.
Ingredients	Sample
Ingredients	S₁	S₂	S₃	S₄	S₅	S₆	S₇	S₈	S₉	S₁₀	S₁₁	S₁₂	S₁₃
NR	30	30	30	30	30	30	30	30	30	30	30	30	30
SBR	30	30	30	30	30	30	30	30	30	30	30	30	30
NBR	40	40	40	40	40	40	40	40	40	40	40	40	40
Nanoclay	6	6	6	6	6	6	6	6	6	6	6	6	6
Nanosilica	0	1	2	3	4	5	6	1	2	3	4	5	6
SCA	-	-	-	-	-	-	-	3	3	3	3	3	3
ZnO	5	5	5	5	5	5	5	5	5	5	5	5	5
Stearic acid	2	2	2	2	2	2	2	2	2	2	2	2	2
UB4000	5	5	5	5	5	5	5	5	5	5	5	5	5
MBTS	2	2	2	2	2	2	2	2	2	2	2	2	2
DPG	0.5	0.5	0.5	0.5	0.5	0.5	0.5	0.5	0.5	0.5	0.5	0.5	0.5
Sulphur	2	2	2	2	2	2	2	2	2	2	2	2	2

2.4 Characterizations

To assess the cure behaviors of the unvulcanized rubber compounds, we utilized an oscillating disc rheometer (ODR), following the prescribed standards detailed in ASTM D2084. The assessment was conducted at a temperature of 150^oC.

Mechanical performance evaluations of the NR/SBR/NBR nanocomposites, which included tensile and tear tests, were conducted using a universal tensile testing machine. For the tensile evaluations, dumb-bell-shaped specimens were cut from a compression-molded vulcanized lap sheet, adhering to the standards outlined in ASTM D 412 [47]. This allowed for the determination of essential parameters such as tensile strength (TS), stress at 100% elongation (M100), and elongation at break (EB). Tear strength analyses were carried out on unaltered specimens in accordance with the protocols outlined in ASTM D 624 [48]. These assessments included both tensile and tear tests, all conducted under standard room temperature conditions, with a consistent cross-head speed of 500 millimeters per minute upheld throughout the testing process. It is noteworthy that the data presented in all tests represent the average values obtained from five replicate measurements, ensuring statistical reliability.

Compression set, in the context of elastomers, quantifies the extent to which an elastomer's thickness decreases after undergoing a 30-minute testing period. It serves as an important indicator of the elastomer's ability to maintain its elastic properties even under extended exposure to compressive loads. This measurement is expressed as a percentage of the original thickness and is determined through the following procedure:

$$Compression set= \frac{{t}_{o}- {t}_{1}}{{t}_{o}- {t}_{a}} X 100$$

In accordance with ASTM D 395 [49–50], the compression set is determined by measuring the original thickness (t_o), the final thicknesses (t₁), and the spacer bar thickness (t_s). To perform this test, a specimen with a diameter of 29 ± 0.5 mm and a thickness of 12.5 ± 0.5 mm is positioned between two plates within a compression device, with spacers placed on both sides. The compression process is carried out until it reaches 25% of the original thickness.

FESEM (Field Emission Scanning Electron Microscopy) was employed to delve deeper into the internal structures of the compounds. The analysis of fractured surfaces resulting from tensile testing was conducted to gain a better understanding of the interfaces within the rubber and to identify potential micro defects. To prepare these surfaces for examination, they underwent an initial coating with a conductive layer of platinum. This coating effectively mitigated any possible accumulation of electrostatic charges that could have occurred during the analysis process.

Rubber hardness is typically determined using a durometer, which measures the material's resistance to indentation. The result is expressed in Shore hardness units. The assessment of hardness was carried out in accordance with ASTM D2240 guidelines, employing a Shore A durometer hardness tester.

The determination of abrasion resistance in rubber involves assessing the material's ability to withstand wear and tear. One common method is to measure volume loss, which quantifies the amount of material removed during abrasion testing. The volume loss is calculated by measuring the change in the specimen's dimensions before and after testing. A lower volume loss indicates higher abrasion resistance, making this an essential test in evaluating the durability and longevity of rubber materials in various applications, such as tires, conveyor belts, and seals. To gauge the material's abrasion resistance, we executed an abrasion resistance test with a Zwick abrasion tester, meticulously adhering to the standardized procedures outlined in ASTM D5963.

Furthermore, we determined the rebound resilience by employing a vertical rebound resilience tester, and our testing methodology was in strict accordance with the guidelines outlined in ASTM D2632. A higher rebound percentage indicates better resilience. This test provides valuable insights into the energy-absorbing and elastic properties of rubber materials, helping manufacturers ensure optimal performance and durability in their products.

The crosslink density of the rubber specimens was assessed through immersion techniques, involving the measurement of toluene uptake over a 72-hour duration at 23°C. This analysis utilized the Flory–Rehner equation [51] to determine crosslink density.

$${M}_{c}= \frac{-{\rho }_{p}{V}_{s}{V}_{r}^{1/3}}{\text{ln}\left(1-{V}_{r}\right)+{V}_{r}+\chi {V}_{r}^{2}}$$

The equation employs several key parameters: M_c represents the molar mass of the polymer between crosslinks. V_s denotes the molar volume of the solvent. ρ_p stands for the density of the polymer, with values of ρ (NR) – 9125 g cm^-3, ρ (SBR) – 0.94 g cm^-3, and ρ (NBR) -0.97 g cm^-3. V_r signifies the volume fraction of polymer in the solvent-swollen filled compound. χ represents the interaction parameter of the polymer, with χ (NR) – 0.393, χ (SBR) – 0.446, and χ (NBR) – 0.390 as respective values. V_r is determined using the following equation [52–54]:

$${V}_{r}= \frac{1}{1+{Q}_{m}}$$

The equation can be expanded as follows: where: Q_m represents the weight swell of the NR/SBR/NBR nanocomposites in toluene. The degree of crosslink density (v) is determined using Eq. (4):

$$v= \frac{1}{2{M}_{c}}$$

Swelling tests were conducted using various solvents in adherence to ASTM D471 [55]. The method employed for assessing swelling resistance was the equilibrium immersion swelling technique, which quantifies the extent of swelling as mole percent uptake in the swollen state. Rubber specimens were immersed in a range of chemical solvents (including mesitylene, toluene, xylene, benzene, n-pentane, n-hexane, carbon tetrachloride, n-octane, n-heptane, dichloromethane, and chloroform) at room temperature (23°C) for a duration of 3 days. Afterward, they were extracted from the solvent, and any residual solvent on the rubber surface was promptly blotted dry using tissue paper. The rubber samples were immediately weighed on an analytical balance to calculate the mole percent uptake (MPU) (Q_t) following a specific formula.

$${Q}_{t}\left(mol\%\right)= \frac{({M}_{t}- {M}_{0})/MW}{{M}_{0}} X 100$$

Here, the variables in the formula are defined as follows: M_t represents the mass of the specimen after 3 days of immersion. M₀ is the initial mass of the specimen. MW signifies the molecular weight of the solvent.

3.1 Cure features

3.1.1 Scorch time

In Fig. 1, the scorch time (t_s2) of NR/SBR/NBR nanocomposites filled with a hybrid of nanoclay and nanosilica is illustrated. Noticeably, with an escalation in nanosilica content, the t_s2 of these nanocomposites diminishes, a trend observed consistently regardless of the presence or absence of a SCA. As the nanosilica content increases, there is an increase in the number of active filler sites that can interact with the rubber molecules. This enhanced interaction promotes cross-linking and accelerates the vulcanization process, leading to a decrease in t_s2. Nanosilica particles can absorb and dissipate heat more effectively than larger filler particles. This enhanced heat absorption can raise the temperature of the rubber compound locally, leading to accelerated chemical reactions and a reduction in t_s2. Nanosilica particles have the capacity to function as catalysts in the vulcanization process, facilitating the creation of cross-links among rubber chains. As the nanosilica content rises, this catalytic influence becomes more pronounced, resulting in accelerated vulcanization and a consequent reduction in t_s2. The increase in t_s2 of NR/SBR/NBR nanocomposites filled with nanosilica in the presence of a SCA can be owing to the interactions and chemical bonding that occur at the filler-rubber interface. A SCA is often used to increase the compatibility between the inorganic filler (nanosilica) and the rubber matrix. The SCA can promote better dispersion of the nanosilica within the NR/SBR/NBR matrix. When the nanofiller is uniformly dispersed, it provides a larger number of sites for cross-linking and vulcanization reactions to occur. This can delay the initiation of vulcanization and extend the t_s2.

3.1.2 Optimum cure time

In Fig. 2, the optimal cure time (t₉₀) of NR/SBR/NBR nanocomposites filled with a hybrid of nanoclay and nanosilica is presented. It is evident that as the nanosilica content increases, the t₉₀ of these nanocomposites decreases, regardless of the presence or absence of a SCA. As the nanosilica content increases, the overall surface area of the nanofiller within the NR/SBR/NBR matrix also increases. This enhanced surface area provides more sites for interaction with polymer chains and facilitates more efficient cross-linking during the curing process. Because of their diminutive size, nanosilica particles have a propensity to achieve more homogeneous dispersion within the NR/SBR/NBR matrix in contrast to larger fillers. This uniform dispersion leads to better interaction between the NR/SBR/NBR chains and the nanofiller particles. The strong interfacial interactions can facilitate cross-linking and curing, reducing the time required for the curing process to reach completion. The rise in the t₉₀ of NR/SBR/NBR nanocomposites filled with nanosilica in the presence of a SCA, as opposed to its absence, can be elucidated by examining the interplays among the nanosilica filler, the polymer matrix, and the SCA. When a SCA is employed, it establishes a chemical bridge connecting the surface of the nanosilica particles and the polymer matrix. While this is intended to improve filler-polymer interactions and compatibility, it can also lead to the formation of an intermediate interaction layer around the particles. This layer might restrict the mobility of polymer chains and hinder the cross-linking process, causing a delay in the curing reaction. The introduction of a SCA adds an additional chemical component to the system. The reactions involving the SCA, the nanosilica surface, and the NR/SBR/NBR matrix can introduce more complex reaction pathways. These additional reactions may compete with or divert resources from the main curing reactions, resulting in a longer overall curing time. The presence of a SCA might affect the diffusion of curing agents and other reactants within the polymer matrix. While it aims to improve compatibility, it can also create additional diffusion barriers due to the formation of the interaction layer. This can slow down the movement of reactants and extend the curing time. The SCA itself can interact with the curing agents present in the formulation. Depending on the nature of these interactions, they could either accelerate or inhibit the curing process. If the interactions slow down the activation of curing agents, it could lead to an extended curing time. The presence of a SCA can alter the reaction kinetics of the curing process. It can affect the rate at which cross-links form and modify the overall curing behavior. This alteration in kinetics might lead to a longer time required for the reaction to reach completion.

3.1.3 Cure rate index

Figure 3 illustrates the variation in the cure rate index (CRI) for nanoclay/nanosilica-filled NR/SBR/NBR nanocomposites. Notably, the CRI shows an upward trend with increased nanosilica content, whether a SCA is present or not. As the nanosilica content increases, there are more filler particles dispersed within the NR/SBR/NBR matrix. These nanoparticles provide additional nucleation sites for the cross-linking reactions, leading to a higher rate of cross-link formation and consequently an increased cure rate. Additionally, when a SCA is present, the CRI of these nanocomposites is higher compared to when the SCA is absent. The SCA forms a chemical bond between the nanosilica surface and the polymer matrix, promoting better filler dispersion. This improved dispersion leads to increased interactions between the NR/SBR/NBR chains and the nanosilica particles, facilitating faster cross-linking. The presence of a SCA can modify the reaction pathways by providing pathways for more efficient cross-linking. The coupling agent's interaction with both the nanofiller and the NR/SBR/NBR matrix can create conditions that favor faster curing reactions. The coupling agent's role in improving filler-polymer compatibility can also lead to reduced diffusion barriers for reactants, allowing curing agents to more effectively reach their targets and participate in cross-linking reactions.

3.1.4 Minimum torque

Figure 4 illustrates how the minimum torque (M_L) of hybrid filler (nanoclay/nanosilica)-loaded NR/SBR/NBR nanocomposites behaves. The M_L in these composites rises as the nanosilica content increases, whether a SCA is present or not. This increase in M_L correlates with a higher concentration of nanofiller particles within the polymer matrix. The interactions between these nanoparticles can lead to the formation of filler-filler networks or agglomerates. These networks resist the deformation during the curing process, requiring more torque to overcome the resistance and promote proper mixing. This results in an increase in the M_L value. Furthermore, when a SCA is introduced, the minimum torque of these nanocomposites is higher compared to when the SCA is not present. The SCA enhances the compatibility between the nanofiller particles and the NR/SBR/NBR matrix. This improved dispersion leads to more uniform distribution of nanofiller throughout the matrix. However, in some cases, improved dispersion can lead to increased filler-filler interactions, forming stronger networks or agglomerates. These enhanced filler-filler interactions can increase the resistance to mixing, requiring higher torque to achieve proper dispersion. The SCA can also chemically interact with both the nanofiller surface and the polymer matrix. These interactions can lead to the formation of additional cross-linking sites or junction points, which can contribute to stronger filler networks within the nanocomposite. These networks can result in increased torque during mixing.

3.1.5 Maximum torque

Figure 5 depicts the maximum torque (M_H) of hybrid filler (nanoclay/nanosilica)-filled NR/SBR/NBR nanocomposites. The M_H in nanoclay/nanosilica-filled NR/SBR/NBR nanocomposites exhibits a rise with an increase in nanosilica content, regardless of the presence or absence of a SCA. As the nanosilica content increases, the total surface area of the nanofiller within the NR/SBR/NBR matrix also increases. This can lead to enhanced interactions between the nanofiller particles, resulting in the formation of nanofiller-nanofiller networks or agglomerates. These networks can resist deformation and movement during the mixing process, requiring more torque to break them apart and achieve proper dispersion. Furthermore, the maximum torque of these nanocomposites is higher when a SCA is present compared to when the SCA is absent. The SCA enhances the compatibility between the filler particles and the NR/SBR/NBR matrix. While improved dispersion is generally desired, it can also result in increased filler-filler interactions and the formation of stronger networks. These interactions can lead to more resistance during mixing, requiring higher torque to overcome and disperse the nanofiller particles evenly. The presence of a SCA promotes stronger chemical interactions between the filler, the SCA, and the polymer matrix. These interactions have the potential to generate additional and robust cross-linking sites. This elevated cross-linking density can result in the development of more rigid filler networks, necessitating greater torque to disrupt them during the mixing process [56].

3.1.6 Delta torque

Figure 6 depicts the delta torque (ΔM) of nanoclay/nanosilica hybrid filler-filled NR/SBR/NBR nanocomposites. The ΔM of nanoclay/nanosilica-filled NR/SBR/NBR nanocomposites increases with an increase in nanosilica content, both in the presence and absence of a SCA. As the nanosilica content increases, the number of nanofiller particles within the polymer matrix also increases. This results in more interactions between the filler particles. These interactions can lead to the formation of filler-filler networks or agglomerates, which require more energy (torque) to break apart and disperse during the mixing process. Consequently, the ΔM, which represents the difference in torque during mixing, increases. Furthermore, when a SCA is present, the ΔM of these nanocomposites is higher compared to when the SCA is absent. The SCA enhances the compatibility between the filler particles and the NR/SBR/NBR matrix. While improved dispersion is intended, it can also lead to stronger interactions between the filler particles, forming more rigid networks or agglomerates. Disrupting these stronger networks requires more energy, leading to an increase in the ΔM. The presence of a SCA promotes chemical interactions between the filler, the SCA, and the polymer matrix. This can lead to the formation of stronger cross-linking sites and enhanced interactions within the nanocomposite. These interactions contribute to the formation of more robust filler networks, which require higher energy to break during mixing.

3.2 Mechanical properties

3.2.1 Tensile strength

Figure 7 depicts the tensile strength (TS) of nanoclay/nanosilica-filled NR/SBR/NBR nanocomposites. The TS of hybrid filler-filled NR/SBR/NBR nanocomposites increases with an increase in nanosilica content up to 4 phr; thereafter, it begins to decline, both in the presence and absence of a SCA. At lower nanosilica content, the nanoparticles can reinforce the polymer matrix by acting as physical reinforcement and stress transfer agents. This leads to an increase in tensile strength as the nanosilica content increases. As nanosilica content continues to increase, there's a point where filler agglomeration might occur. This agglomeration can lead to uneven dispersion and weak points in the composite, resulting in a decline in TS. At higher nanosilica content, excessive filler loading might lead to a higher concentration of filler-polymer interfaces. These interfaces can act as stress concentrators, reducing the overall strength of the material and causing a decline in TS. Furthermore, when a SCA is present, the TS of these nanocomposites is higher compared to when the SCA is absent. The SCA enhances the interaction between the nanofiller and the polymer matrix. This improved compatibility leads to better stress transfer between the two components, resulting in higher TS. The presence of a SCA can mitigate filler agglomeration by improving filler dispersion and reducing the likelihood of weak points in the composite. This contributes to maintaining higher tensile strength. The chemical bonding introduced by the SCA not only improves filler dispersion but also leads to stronger filler-matrix bonding. This effect further enhances the stress transfer and overall TS of the nanocomposites. The addition of a SCA to the NR/SBR/NBR nanocomposites, consisting of 6 phr nanoclay and 4 phr nanosilica, resulted in a 116% increase in TS, surpassing the performance of the nanocomposites containing 6 phr of nanoclay filler alone.

3.2.2 Elongation at break

Figure 8 illustrates the elongation at break (EB) behavior in nanoclay/nanosilica hybrid filler-incorporated NR/SBR/NBR nanocomposites. The EB of these composites consistently decreases as the nanosilica content increases, regardless of the presence or absence of a SCA. As the nanosilica content increases, the stiffness of the nanocomposite also increases due to the rigid nature of the filler. This stiffness can limit the ability of the polymer chains to deform and elongate, resulting in reduced elongation at break. Higher nanosilica content can lead to increased filler-filler interactions and agglomeration. These agglomerates can create weak points in the material, leading to premature failure and reduced elongation at break. The introduction of nanoparticles, such as nanosilica, into the material can impede the mobility of polymer chains, restricting their capacity to undergo deformation. This interference with chain movement contributes to decreased EB. Furthermore, when a SCA is present, the EB of these nanocomposites is lower compared to when the SCA is absent. The presence of a SCA can enhance the chemical bonding between the nanofiller and the polymer matrix. While this improves compatibility and tensile strength, it can also make the material more rigid and less able to undergo significant deformation, resulting in lower elongation at break. The improved filler dispersion achieved by the SCA might lead to increased filler-filler interactions and agglomeration. This can reduce the material's ability to undergo plastic deformation before breaking, leading to lower EB. The introduction of strong filler-matrix bonds due to the SCA can increase the likelihood of brittle fracture. Brittle materials tend to exhibit limited EB compared to ductile materials. The EB of the hybrid filler (nanoclay/nanosilica)-filled NR/SBR/NBR nanocomposites demonstrated a 34% decrease with the inclusion of a SCA, in contrast to nanocomposites containing 6 phr of nanoclay filler.

3.2.3 Stress at 100% elongation

Figure 9 illustrates the Stress at 100% elongation (M100) behavior in nanoclay/nanosilica hybrid filler-loaded NR/SBR/NBR nanocomposites. The M100 of these composites demonstrates an uptrend with the escalating nanosilica content up to 4 phr. Beyond this point, it initiates a descent, irrespective of the presence or absence of a SCA. At lower nanosilica content, the nanoparticles can act as reinforcement and stress transfer agents, enhancing the stress-carrying capacity of the material. This leads to an increase in M100 as the nanosilica content increases. As nanosilica content continues to increase, filler-filler interactions can lead to the formation of filler networks or agglomerates. These networks can create stress concentrations and weak points, causing a decline in stress at 100% elongation. At higher nanosilica content, the concentration of filler-polymer interfaces can increase. These interfaces can act as stress concentrators, leading to a decline in stress at 100% elongation due to the potential for early failure. Furthermore, when a SCA is present, the M100 of these nanocomposites is higher compared to when the SCA is absent. The SCA enhances the compatibility between the nanofiller and the NR/SBR/NBR matrix, resulting in better stress transfer between the two components. This improved transfer of stress contributes to higher M100. The presence of a SCA can help disperse the filler more uniformly and reduce the likelihood of agglomeration. This results in a more consistent stress distribution throughout the material and contributes to maintaining higher stress at 100% elongation. The chemical bonding introduced by the SCA not only improves compatibility but also leads to stronger filler-matrix bonding. This increased bonding contributes to better stress transmission and overall higher M100. The addition of a SCA to the NR/SBR/NBR nanocomposites, consisting of 6 phr nanoclay and 4 phr nanosilica, resulted in a 111% increase in M100, surpassing the performance of the NR/SBR/NBR nanocomposites containing 6 phr of nanoclay filler alone.

3.2.4 Tear strength

Figure 10 illustrates the tear strength performance in nanoclay/nanosilica hybrid filler-incorporated NR/SBR/NBR nanocomposites. The tear strength of these composites consistently rises with an increasing nanosilica content, regardless of the presence or absence of a SCA. The increased nanosilica content contributes to better filler dispersion within the polymer matrix. Uniform distribution of nanosilica particles reduces stress concentrations and enhances tear resistance by providing a more continuous network of reinforcement. Nanosilica particles can interact more effectively with the polymer matrix as their content increases. This improved interaction leads to better stress transfer and increased resistance to tear propagation [57]. Furthermore, when a SCA is present, the tear strength of these nanocomposites is higher compared to when the SCA is absent. A SCA facilitates the formation of chemical bonds between the nanosilica filler and the polymer matrix. This stronger bonding improves the load transfer between the two components, enhancing tear resistance. The SCA helps disperse the nanosilica more uniformly within the matrix, reducing the likelihood of filler agglomeration. This even distribution results in a more uniform stress distribution and higher tear strength. The compatibility introduced by the SCA affects polymer chain mobility and entanglement. This impact on polymer structure can contribute to increased tear resistance, as chains may be less prone to disentanglement. Compared to nanocomposites containing 6 phr of nanoclay filler, the inclusion of a SCA led to a notable 65% increase in the tear strength of the hybrid filler-loaded NR/SBR/NBR nanocomposites.

3.2.5 Hardness

Figure 11 portrays the hardness behavior in nanoclay/nanosilica hybrid filler-incorporated NR/SBR/NBR nanocomposites. The hardness of these composites steadily rises with increasing nanosilica content, regardless of the presence or absence of a SCA. Incorporating nanosilica filler reinforces the polymer matrix, and with higher nanosilica content, the material becomes progressively denser owing to the increased concentration of nanofiller particles, resulting in heightened hardness. The increasing nanosilica content improves the interaction between the filler particles and the NR/SBR/NBR matrix. This improved compatibility results in better stress transfer and load-bearing capabilities, contributing to increased hardness. Nanosilica particles can fill the void spaces between polymer chains, resulting in a more compact structure. This filling effect reduces the deformability of the material, leading to higher hardness. Moreover, the presence of a SCA results in higher hardness in these nanocomposites compared to instances where the SCA is absent. A SCA enhances the formation of robust chemical bonds between the nanosilica particles and the NR/SBR/NBR matrix. This enhanced bonding leads to improved load transfer and mechanical integrity, resulting in higher hardness. The SCA promotes better filler dispersion, minimizing filler agglomeration. Improved filler dispersion results in a more uniform distribution of stress and contributes to higher hardness. The chemical interactions introduced by the SCA can influence the cross-linking density of the material. Higher cross-linking can lead to increased hardness by creating a more rigid network structure.

3.2.6 Rebound resilience

Figure 12 illustrates the rebound resilience (RR) in nanoclay/nanosilica hybrid filler-infused NR/SBR/NBR nanocomposites. The RR of these composites consistently diminishes with an escalating nanosilica content, regardless of whether a SCA is present or not. The addition of nanosilica filler leads to increased stiffness of the nanocomposite. A stiffer material is less capable of absorbing and storing energy during deformation, resulting in lower RR. Nanosilica particles, when present, have the potential to restrict the mobility of polymer chains, thereby diminishing their capacity for elastic deformation and energy recovery following an impact. This reduced chain mobility contributes to lower RR. The rigidity introduced by the nanosilica particles can lead to higher energy dissipation during deformation, reducing the amount of energy available for rebound. This results in decreased RR. Moreover, in the presence of a SCA, the RR of these nanocomposites is diminished compared to cases where the SCA is not present. The presence of a SCA can lead to stronger chemical bonding between the nanofiller and the polymer matrix. While this bonding can improve other mechanical properties, it can also result in reduced rebound resilience by limiting the material's ability to elastically deform and recover energy. The improved compatibility and stronger bonding introduced by the SCA can increase the overall rigidity of the material. A more rigid material has lower RR due to reduced elastic deformation and energy storage capabilities. The interaction between the SCA and the NR/SBR/NBR matrix might modify the behavior of the polymer chains during deformation, affecting their ability to store and release energy. This alteration can lead to decreased RR. The RR of the hybrid filler-filled NR/SBR/NBR nanocomposites demonstrated a 35% decrease with the inclusion of a SCA, in contrast to nanocomposites containing 6 phr of nanoclay filler.

3.2.7 Compression set

Figure 13 illustrates the compression set behavior in nanoclay/nanosilica hybrid filler-infused NR/SBR/NBR nanocomposites. The compression set of these nanocomposites displays a progressive rise as the nanosilica content increases, a trend observed in both the presence and absence of a SCA. As the nanosilica content rises, the density of nanofiller particles within the polymer matrix also increases. This higher filler concentration can lead to increased particle-particle interactions, hindering the ability of the polymer chains to return to their original positions after deformation. This results in a reduced capacity of the nanocomposite to recover its original shape and size after being compressed, thus leading to a higher compression set. This increase in compression set implies a reduced capacity for retaining memory in the nanocomposites due to heightened crosslinking density. The increase in compression set is likely attributed to the heightened crosslinking density within the nanocomposite, which can be attributed to the interactions between the NR/SBR/NBR matrix and the nanosilica particles. Crosslinking refers to the formation of covalent bonds between NR/SBR/NBR chains, creating a network-like structure. While crosslinking can enhance mechanical properties, an excess of crosslinks can also limit the polymer's ability to return to its original shape upon deformation, thereby increasing compression set. Additionally, when a SCA is introduced, the compression set of these nanocomposites becomes even higher compared to its absence. SCAs are used to improve the compatibility between the nanofiller particles and the polymer matrix. However, in some cases, the presence of a SCA can lead to unintended consequences. It's possible that an excessive amount of coupling agent or improper bonding between the silane and the nanosilica could lead to increased agglomeration of filler particles. This agglomeration can contribute to the observed higher compression set, as these agglomerates create regions of increased stiffness within the nanocomposite, reducing its ability to recover from compression. Lower compression set values indicate the suitability of nanocomposite materials for various applications. Lower compression set values are desirable for many applications involving materials that need to maintain their shape and properties after being subjected to deformation or compression. For instance, in industries like automotive, aerospace, and consumer goods, materials with low compression set are preferred because they can better withstand repeated mechanical stresses without suffering from permanent deformation. Therefore, understanding the factors influencing compression set in nanocomposite materials is crucial for tailoring their performance to specific applications.

3.2.8. Abrasion resistance

Figure 14 depicts the abrasion loss of nanoclay/nanosilica hybrid filler-filled NR/SBR/NBR nanocomposites. The abrasion loss of nanocomposites, comprising nanoclay/nanosilica fillers within NR/SBR/NBR blends, exhibits a consistent decrease with rising nanosilica content. This trend is observed regardless of the presence or absence of a SCA. The decrease in abrasion loss with higher nanosilica content can be attributed to the strengthening effect of the nanosilica particles within the NR/SBR/NBR matrix. Nanosilica, being a hard and rigid material, can act as a filler that strengthens the polymer composite. As the nanosilica content increases, it creates a more robust and wear-resistant network within the polymer matrix. This added reinforcement leads to reduced wear and abrasion loss when subjected to friction and mechanical stresses, thus improving the overall abrasion resistance of the nanocomposites. The reduction in abrasion loss signifies an enhancement in abrasion resistance within the nanocomposites. The increased crosslinking density is accountable for augmenting the abrasion resistance. The increased crosslinking density is a result of stronger chemical bonds forming between polymer chains within the nanocomposite. This higher crosslinking density contributes to improved mechanical properties, including abrasion resistance. The crosslinked structure provides better integrity to the material, making it more resistant to abrasion and wear. The polymer chains are less prone to break or move during friction, leading to a reduction in the loss of material due to abrasion. Moreover, the introduction of a SCA further elevates the abrasion resistance of these nanocomposites compared to instances where the SCA is not used. SCAs are used to enhance the compatibility between nanofiller particles and the polymer matrix. When a SCA is present, it can promote better adhesion and interaction between the nanosilica particles and the polymer chains. This improved interaction leads to a more homogeneous dispersion of the nanosilica throughout the matrix, which in turn enhances the overall reinforcement and mechanical properties of the nanocomposite. As a result, the presence of a SCA contributes to a higher level of abrasion resistance due to the improved interfacial interactions between components.

3.3 Crosslinking density

Figure 15 portrays the crosslinking density behavior in nanoclay/nanosilica hybrid filler-infused NR/SBR/NBR nanocomposites. The crosslinking density of these nanocomposites consistently rises with increasing nanosilica content, regardless of whether a SCA is present or not. The increase in crosslinking density with higher nanosilica content is tied to the interaction between the silica nanoparticles and the NR/SBR/NBR matrix. Nanosilica particles can facilitate additional crosslinking sites within the polymer network, thereby promoting the formation of more crosslinks. As the nanosilica content rises, the availability of these sites for crosslinking increases, leading to a denser and more interconnected polymer structure. This enhanced crosslinking density contributes to improved mechanical properties, such as stiffness and strength. The crosslinking density is influenced by the molecular weight of the polymer between crosslinks (M_c), with M_c being inversely proportional to the crosslinking density of the nanocomposites. The M_c is a measure of the average length of polymer chains between crosslinking points. A higher M_c indicates longer polymer chains between crosslinks, implying lower crosslinking density [58]. Conversely, a lower M_c signifies shorter polymer chains between crosslinks, leading to higher crosslinking density. This inverse relationship means that as the molecular weight between crosslinks decreases, the crosslinking density increases. Moreover, when a SCA is introduced, the crosslinking density of these nanocomposites is higher compared to instances where the SCA is not used. The presence of a SCA can influence the degree of crosslinking in the nanocomposite. SCAs are designed to improve the adhesion and compatibility between nanofiller particles and the polymer matrix. This improved interaction can promote more efficient crosslinking between polymer chains and the nanosilica particles. The coupling agent might provide functional groups that facilitate the formation of stronger covalent bonds, leading to a higher crosslinking density in the presence of the agent.

3.4 Morphology

Figure 16(a-d) presents the field emission scanning electron microscope (FESEM) images of the tensile fractured specimens for various composite formulations. These FESEM micrographs offer valuable insights into the dispersion and distribution of different filler components within the composite matrix. In particular, when examining the composite with 6 phr of nanoclay (as depicted in Fig. 16a), we observe an excellent dispersion of nanoclay particles. A closer inspection of the FESEM micrograph representing the blend of 6 phr nanoclay and 1 phr nanosilica reveals an equally impressive dispersion of both nanoclay and nanosilica within the NR/SBR/NBR matrix, as clearly evident in Fig. 16b. A striking observation comes to light when comparing the tensile-fractured surfaces of nanoclay filled nanocomposites containing 1 phr and 4 phr nanosilica, as depicted in Fig. 16b and Fig. 16c respectively. The former presents a noticeably smoother and more even surface, which stands in contrast to the latter exhibiting a comparatively rougher appearance. This disparity is substantiated by the micrographs, indicating a homogenous phase with fewer discernible tear lines. Moreover, a substantial portion of the nanosilica entities appear to be enveloped within the NR/SBR/NBR matrix, underscoring the proficient interaction between the nanosilica and the rubber, as highlighted in Fig. 16c. These visual analyses collectively emphasize the effective dispersion of nanosilica at lower loading levels. Upon progressing to the FESEM micrograph corresponding to the mixture of 6 phr nanoclay and 4 phr nanosilica, illustrated in Fig. 16c, a commendable dispersion of both filler types within the rubber matrix is once again manifest. This dispersion is juxtaposed against the micrograph of the nanocomposite with elevated nanosilica content (Fig. 16d), wherein a distinct contrast in filler distribution is discernible. Notably, the nanocomposites incorporating 6 phr nanoclay and 4 phr nanosilica not only exhibit superior dispersion but also achieve an enhanced distribution of nanosilica particles within the NR/SBR/NBR matrix. Consequently, this optimized distribution contributes to the observed heightened tensile strength exhibited by the composite. However, the FESEM micrograph depicting the nanocomposite imbued with 6 phr of nanoclay and an equal quantity of 6 phr nanosilica, as represented in Fig. 16d, uncovers a different scenario altogether. In this case, the dispersion of silica nanoparticles within the NR/SBR/NBR matrix is conspicuously subpar. Agglomeration of nanosilica particles dominates the micrograph, attesting to the potent interaction between individual silica compounds. This aggregation is attributed to the strong intermolecular forces between silica entities.

3.5 Swelling resistance

Table 2 displays the mole percent uptake (MPU) of both the pristine vulcanized NR/SBR/NBR ternary blend and the nanocomposites containing nanoclay/nanosilica fillers. The absorption of benzene by NR/SBR/NBR nanocomposites diminishes as the nanosilica content in the hybrid filler increases, regardless of whether a SCA is present or not. This phenomenon can be ascribed to the nanosilica particles forming a denser and more compact structure within the polymer matrix. This structural configuration serves as an obstacle, increasing the difficulty for benzene molecules to permeate and diffuse into the nanocomposite material. As a result, higher nanosilica content leads to a reduction in the amount of benzene that can be absorbed by the nanocomposites. Toluene, xylene, and mesitylene also exhibit similar trends. One of the main reasons is the improved dispersion and distribution of the silica nanoparticles within the NR/SBR/NBR matrix. With the incremental rise in nanosilica loading, these particles serve as impediments, obstructing the diffusion of permeating substances into the polymer matrix. This barrier effect becomes more pronounced as the nanosilica particles create a denser network, reducing the available pathways for molecules to permeate. Furthermore, the inclusion of nanosilica fillers has the potential to augment the mechanical properties and rigidity of the nanocomposites. This increased stiffness can lead to reduced polymer chain mobility, further limiting the diffusion of molecules and decreasing the uptake of substances. As the nanosilica content increases, the structure of the nanocomposites becomes denser and more compact [59]. This structural change creates a barrier that hinders the penetration and diffusion of not only benzene but also toluene, xylene, and mesitylene molecules into the nanocomposite material. Consequently, the increased nanosilica content leads to reduced uptake of these aromatic compounds across all cases, regardless of the presence or absence of a SCA. The order of uptake for the aromatic solvents is as follows: benzene > toluene > xylene > mesitylene. The sequence of aromatic solvent uptake, from highest to lowest, follows the trend of increasing molecular weight: benzene > toluene > xylene > mesitylene. This pattern can be ascribed to the molecular size and mass of these compounds. Benzene, the simplest aromatic hydrocarbon with the lowest molecular weight, has the smallest molecular size. It can more easily fit and diffuse within the polymer matrix, resulting in relatively higher uptake. As we move along the series to larger and heavier molecules like toluene, xylene, and mesitylene, the molecular sizes increase. The larger molecules encounter more resistance when trying to diffuse through the polymer matrix, which leads to reduced uptake compared to benzene.

The mole percent absorption of n-pentane by NR/SBR/NBR nanocomposites decreases with an escalation in the nanosilica content in the hybrid filler, regardless of the presence or absence of a SCA. This behavior can be linked to the surface properties of nanosilica and its interactions within the composite matrix. Nanosilica, due to its highly developed surface area and strong affinity for polar compounds, tends to create a more tortuous pathway for the diffusion of solvent like n-pentane. As the nanosilica content increases, the number of these tortuous pathways also increases, impeding the easy penetration of solvent molecules. This effect is especially pronounced in the presence of a SCA, which enhances the interaction between nanosilica and the NR/SBR/NBR matrix. The strong coupling between nanosilica and the NR/SBR/NBR matrix leads to a denser and more compact structure, further limiting the diffusion of solvent molecules. Additionally, the incorporation of nanosilica can alter the overall matrix morphology, leading to a reduction in free volume and increased crosslinking density. This altered morphology creates a less permeable structure, making it challenging for solvent molecules to permeate through the composite material. The increased rigidity and decreased chain mobility in the presence of nanosilica also contribute to the reduced solvent uptake. A similar trend is observed with other aliphatic solvents, such as n-hexane, n-heptane, and n-octane. The order of solvent uptake, from highest to lowest, is as follows: n-pentane > n-hexane > n-heptane > n-octane. This order of uptake is in line with the size and molecular weight of the solvents. Generally, smaller molecules with lower molecular weights, such as n-pentane, have higher diffusion rates and can penetrate more easily through the composite matrix. As the molecular size and weight of the aliphatic solvents increase (from n-hexane to n-octane), their diffusion rates and permeation capabilities become progressively reduced due to greater resistance encountered while traversing the composite structure. Nanosilica's influence on this trend can be attributed to its ability to modify the matrix structure. As the nanosilica content increases, the composite matrix becomes more compact and resistant to the diffusion of larger molecules, thus enhancing the selective permeation of smaller molecules. This effect leads to the observed order of uptake for the aliphatic solvents. The intricate pathways formed by nanosilica are instrumental in constraining the mobility of larger molecules, such as n-octane, while exerting a relatively lesser influence on the diffusion of smaller molecules, such as n-pentane. The presence of a SCA further strengthens the interactions between nanosilica and the NR/SBR/NBR matrix, thereby augmenting the composite's resistance to solvent permeation.

The percentage of dichloromethane absorption by NR/SBR/NBR nanocomposites decreases as the nanosilica content in the hybrid filler increases, irrespective of whether a SCA is present or not. Nanosilica is known to have a higher surface area and increased affinity for polar solvents due to its hydrophilic nature. With an escalation in the nanosilica content within the hybrid filler, the surface area available for solvent interaction likewise expands. This leads to a more pronounced adsorption of the polar solvent molecules (like dichloromethane) onto the surface of the nanosilica particles. As a result, fewer solvent molecules are available to be absorbed by the polymer matrix, causing a decrease in the overall molar percentage uptake of dichloromethane. The presence or absence of a SCA doesn't significantly alter this trend. This pattern holds true across other chlorinated solvents, including chloroform and carbon tetrachloride. Chlorinated solvents like dichloromethane, chloroform, and carbon tetrachloride share similar chemical properties and have comparable polarities. They also interact with the filler and polymer matrix in a similar manner [60–61]. Thus, the trend observed with dichloromethane—where higher nanosilica content reduces solvent uptake—is consistent with the behavior of other chlorinated solvents. The interaction between the nanosilica and these solvents remains relatively constant due to their similar chemical nature. The sequence of solvent uptake, arranged from highest to lowest, is as follows: dichloromethane > chloroform > carbon tetrachloride. The sequence of solvent uptake is primarily influenced by the polarity of the solvents. Dichloromethane is the most polar among the three solvents, featuring a dipole moment that promotes strong interactions with polar functional groups present on the nanosilica surface and within the polymer matrix. Chloroform is less polar compared to dichloromethane but still possesses some polar characteristics, leading to intermediate solvent uptake. Carbon tetrachloride, on the other hand, is the least polar among the three solvents, resulting in weaker interactions with both the nanosilica and the polymer matrix [62].

Furthermore, the presence of a SCA can promote better interaction between the nanosilica particles and the polymer matrix. This interaction can reinforce the polymer-filler interface, which in turn contributes to the reduction in permeability and uptake of substances.

Table 2

Mole percent uptake of hybrid filler filled NR/SBR/NBR nanocomposites
Sample code	Mole percent uptake (MPU) (mol%)
	Aromatic				Aliphatic				Chlorinated
	Benzene	Toluene	Xylene	Mesitylene	n-pentane	n-hexane	n-heptane	n-octane	Dichloromethane	Chloroform	Carbon tetrachloride
S₁	2.94	2.72	2.58	2.38	1.55	1.46	1.28	1.15	1.24	1.02	0.87
S₂	2.89	2.67	2.55	2.35	1.51	1.43	1.24	1.12	1.2	1	0.84
S₃	2.85	2.62	2.51	2.31	1.48	1.4	1.2	1.08	1.17	0.97	0.83
S₄	2.81	2.55	2.48	2.25	1.45	1.35	1.17	1.05	1.13	0.95	0.8
S₅	2.74	2.46	2.41	2.17	1.38	1.28	1.11	1	1.08	0.9	0.76
S₆	2.72	2.42	2.39	2.15	1.35	1.25	1.08	0.98	1.05	0.88	0.74
S₇	2.7	2.4	2.37	2.11	1.33	1.22	1.05	0.95	1.03	0.86	0.73
S₈	2.85	2.63	2.5	2.31	1.48	1.41	1.21	1.08	1.17	0.96	0.82
S₉	2.78	2.57	2.44	2.21	1.41	1.38	1.16	1.03	1.12	0.91	0.8
S₁₀	2.75	2.51	2.4	2.12	1.35	1.31	1.08	0.98	1.08	0.86	0.75
S₁₁	2.68	2.41	2.32	2	1.24	1.22	1	0.91	1	0.8	0.7
S₁₂	2.65	2.38	2.28	1.97	1.22	1.19	0.98	0.9	0.96	0.78	0.68
S₁₃	2.63	2.35	2.27	1.94	1.2	1.17	0.96	0.88	0.94	0.75	0.66

The central aim of this research endeavor was to scrutinize the influence of a hybrid filler on the curing behavior, mechanical attributes, and swelling response of rubber composites. These composite materials maintained a fixed 6 phr nanoclay content, while the concentration of nanosilica was systematically adjusted from 0 to 6 phr within the ternary NR/SBR/NBR matrices. The experimental exploration of these NR/SBR/NBR ternary rubber nanocomposites, incorporating varying levels of the hybrid filler, yielded the subsequent revelations.

The increase in torques signifies enhanced composite processability, while the decrease in t_s2 and t₉₀ with rising nanosilica content in NR/SBR/NBR rubber composites is attributed to the catalytic effect of nanosilica particles on the curing reactions, accelerating the crosslinking process.

The initial increase in TS and M100 of the NR/SBR/NBR composites with a rise of 6 phr filler can be attributed to the effective dispersion and reinforcement provided by the added filler. However, as the filler content further increases, excessive filler aggregation might occur, disrupting the polymer-filler interaction and leading to a subsequent decrease in above-mentioned properties due to compromised composite integrity.

The improved tear strength, hardness, and abrasion resistance in ternary composites with elevated hybrid filler contents can be due to the reinforcing effect of the fillers, enhancing intermolecular interactions and strengthening the composite's structural integrity. Additionally, the increased filler loading leads to a higher degree of filler-polymer interactions, resulting in enhanced mechanical properties.

The increased compression set observed with increasing nanosilica loading can be attributed to the rigid nature of nanosilica particles, which could hinder polymer chain mobility and resilience, leading to decreased elastic recovery. Simultaneously, the reduction in % elongation of break and rebound resilience is likely due to the stiffer filler hindering polymer chain movement and flexibility, diminishing the material's ability to absorb energy and elongate before fracture.

The increase in crosslink density and filler-rubber interaction with rising nanosilica content is a result of enhanced physical entanglements between the polymer chains and the rigid nanosilica particles, fostering stronger network formation. Conversely, the decrease in mole percent uptake can be attributed to the escalating filler content occupying available polymer-filler interaction sites, reducing the capacity for solvent absorption and resulting in lowered uptake.

The improved performance of the hybrid filler with a SCA in the NR/SBR/NBR composites can be due to the enhanced interfacial bonding achieved through the SCA. This agent promotes stronger adhesion between the nanosilica and the NR/SBR/NBR matrix, reducing filler aggregation and enhancing load transfer, which collectively leads to superior mechanical properties and overall composite performance compared to the absence of the coupling agent.

i. Ethics approval

This research study received formal approval and ethical clearance from the Institutional Review Boards at several prominent educational institutions in Chennai, Tamil Nadu, India. The participating institutions include Panimalar Engineering College, SRM Easwari Engineering College, and SA Engineering College. These approvals were sought to ensure that the research adheres to the highest ethical standards and complies with all relevant guidelines and regulations.

ii. Consent to participate

This research study was conducted without the involvement of any human subjects or animals. Ethical considerations were carefully addressed, and the study design and methodologies ensured that no human participants or animals were utilized in any aspect of the research process.

iii. Consent for publication

The contents of this manuscript are entirely original and have not been previously published in any form, nor are they subject to any existing copyright restrictions. Furthermore, no part of this manuscript is currently under consideration for publication in any other journals, books, or publications. We affirm that this work is the result of our independent research efforts, and all due diligence has been undertaken to ensure its originality and adherence to ethical publishing standards.

iv. Availability of data and materials

Not Applicable

v. Funding

Not applicable

vi. Authors' contributions

Mr. J. Srinivas – Experimental work and prepared all figures was done

Dr. M.S. Jagatheeshwaran,– Supervise the overall work and Removal of plagiarism from the main manuscript.

Dr. S. Vishvanathperumal – Wrote the main manuscript text.

Mr. K. A. V. Roossvelt Prabhu - Plot the graph and assist in conducting the experiment.

And then all authors reviewed the main manuscript.

vii. Acknowledgements

Not applicable

viii. Conflict of Interest

The authors declare that they have no conflict of interest.

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No competing interests reported.

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The Synergistic effect of nanoclay/nanosilica on mechanical properties and swelling resistance of ternary rubber (NR/SBR/NBR) blends nanocomposites

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Abstract

Figures

1. Introduction

2. Experimental

2.1 Materials

2.2 Synthesis of nanosilica

2.3 Preparation of NR/SBR/NBR blends nanocomposites

2.4 Characterizations

3. Results and discussions

3.1 Cure features

3.1.1 Scorch time

3.1.2 Optimum cure time

3.1.3 Cure rate index

3.1.4 Minimum torque

3.1.5 Maximum torque

3.1.6 Delta torque

3.2 Mechanical properties

3.2.1 Tensile strength

3.2.2 Elongation at break

3.2.3 Stress at 100% elongation

3.2.4 Tear strength

3.2.5 Hardness

3.2.6 Rebound resilience

3.2.7 Compression set

3.2.8. Abrasion resistance

3.3 Crosslinking density

3.4 Morphology

3.5 Swelling resistance

4. Conclusions

Declarations

References

Additional Declarations

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