An Investigation on the Effect of the Surface Modifications and HNTs loading on the Cure behaviours, Abrasion Resistance, Mechanical and Morphological Properties of NR/EPDM

doi:10.21203/rs.3.rs-3335231/v1

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Research Article

An Investigation on the Effect of the Surface Modifications and HNTs loading on the Cure behaviours, Abrasion Resistance, Mechanical and Morphological Properties of NR/EPDM

https://doi.org/10.21203/rs.3.rs-3335231/v1

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published 28 Dec, 2023

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Research was conducted to investigate the modification of natural halloysite nanotubes (HNTs) using amino silanes, namely (γ-aminopropyl) triethoxysilane (APTES) and Diethoxydimethyl silane (DMS). The impact of these modified HNTs, referred to as APTES-HNTs and DMS-HNTs, on various properties of nanocomposites made from natural rubber (NR) and ethylene-propylene-diene monomer (EPDM) was studied. The properties examined included cure characteristics, tensile properties, hardness, rebound resilience, and compression set qualities of the NR/EPDM nanocomposites. Swelling and cross-link density measurements, abrasion resistance analysis, and fracture morphology examination using FESEM were also performed to validate the experimental outcomes. Utilizing APTES-HNTs as fillers resulted in noteworthy improvements in multiple attributes such as tensile strength, stress at 100% elongation, abrasion resistance, hardness, tear strength, and curing behavior. Notably, the APTES-HNTs filled nanocomposites demonstrated superior resistance to swelling when compared to both HNTs and DMS-HNTs. The enhanced interaction between the filler and rubber matrix, owing to the increased contact surface area, contributed to advancements in mechanical properties and resistance to swelling. Comparing nanocomposites with a base vulcanizate, the incorporation of 10 parts per hundred rubber (phr) APTES-HNTs led to a 36% reduction in abrasion loss. The research findings indicated that the addition of 8 phr APTES-HNTs could potentially increase tensile strength, elongation at break, and stress at 100% elongation by approximately 112%, 31%, and 66%, respectively.

NR/EPDM

HNTs

APTES-HNTs

DMS-HNTs

Nanocomposites

Mechanical

Swelling resistance

The process of blending two rubbers presents a highly effective approach for enhancing characteristics that may not be naturally present within a single rubber compound. The resulting qualities of the blend are intricately tied to the degree of adhesion achieved between its constituent components. Although a significant portion of these blends may exhibit thermodynamic incompatibility, the realm of technological significance has yielded the discovery of numerous blends that offer substantial benefits. This underscores the intricate interplay between material properties and the potential for engineered synergies, as highlighted in various studies [1–3]. Stress-induced crystallisation of natural rubber (NR) serves as a fascinating phenomenon with significant implications. When subjected to stretching forces, NR exhibits the ability to crystallise, a behavior that can be harnessed to enhance its mechanical properties. This unique attribute enables the augmentation of both modulus and deformation resistance, effectively impeding the propagation of defects within the material. On a divergent note, ethylene propylene diene monomer (EPDM) rubber distinguishes itself with a composition rich in saturated hydrocarbon backbones. This intrinsic molecular structure translates into remarkable resilience against a range of environmental challenges encompassing weathering, oxidation, and chemical onslaught [4–5]. An innovative avenue arises through the strategic amalgamation of NR with EPDM and other diene rubbers, fostering compounds that exhibit an amalgamation of virtues. Specifically, the resultant concoction attains a commendable equilibrium of ozone and chemical resistance while concurrently exhibiting reduced compression set tendencies. One intriguing facet of this synergy resides in the integration of NR into EPDM matrices to bolster the adhesiveness of the composite material [6].

Nevertheless, this amalgamation is not devoid of intricacies. Distinct disparities in the unsaturated bonds characterizing EPDM and NR chains render the curing properties of the composite somewhat suboptimal. These two rubber variants, NR and EPDM, stand as representatives of the nonpolar realm and, perhaps unsurprisingly, manifest a certain degree of incompatibility [7]. It is within the delicate interplay of these inherent dissimilarities that an opportunity for innovation lies. Each rubber species brings forth a distinct array of beneficial physical and chemical attributes, poised to coalesce into a harmonious synergy capable of engendering commercially valuable products. In contemplating the union of NR and EPDM, an intriguing realm of possibilities materializes—where the inherent strengths of each constituent rubber may counterbalance the weaknesses of the other, giving rise to a new horizon of versatile and robust materials with manifold applications. In order to address the inherent weakness of immiscible polymer blends, the incorporation of ethylene propylene diene monomer-grafted-maleic anhydride (EPDM-g-MA) as a compatibilizer has emerged as a pivotal strategy [8–10]. The success of this approach extends beyond mere enhancement of mechanical properties; it hinges on the imperative improvement of interfacial adhesion between the distinct phases within the blend. The utilization of the compatibilizer assumes a critical role in tailoring the desired properties of the composite materials, effectively bridging the compatibility gap between polymers that would otherwise remain incompatible [11–13].

Several experimental techniques have been employed to bolster interfacial adhesion, thereby ameliorating the overall performance of the polymer blend. Among these techniques, one involves the modification of the EPDM phase through exposure to reactive chemicals such as maleic anhydride during the peroxide initiation process. Another approach entails the halogenation of the EPDM phase within a solution. Additionally, the incorporation of accelerators with heightened solubility in the EPDM phase has been explored [14–16]. A notable facet of this endeavor lies in the dearth of research focused on immiscible polymer blends featuring solid particle-stabilized interfaces. An intriguing advancement in this arena is the economic compatibility achieved through the inclusion of inorganic nanoparticles. These nanoparticles have demonstrated an exceptional capacity to adsorb at the interface of the blend, conferring stabilization and facilitating compatibility [17–18].

Overall, the adoption of ethylene propylene diene monomer-grafted-maleic anhydride as a compatibilizer underscores the vital role of tailored interfacial adhesion in rendering immiscible polymer blends mechanically robust and functionally versatile. The amalgamation of innovative strategies, such as EPDM modification and nanoparticle incorporation, propels the field forward by offering promising avenues to address the long-standing challenge of enhancing the compatibility and mechanical performance of polymer blends. As research continues to delve into the intricate dynamics of these systems, the pursuit of multifaceted solutions holds the potential to revolutionize the realm of polymer science and engineering, unlocking new horizons of material design and application.

Researchers [19–20] are actively incorporating fillers into polymeric materials to enhance their properties, catering to the demands of diverse end-use applications. Particularly, nanometer-scale inorganic fillers have emerged as pivotal reinforcing agents within polymers. Capitalizing on their remarkable attributes, such as elevated surface area, distinctive aspect ratio (comprising the length/thickness or length/diameter ratio), diminished density, and the presence of functional moieties on their surfaces, these nanofillers confer enhanced reinforcement even at relatively lower concentrations than their micron-sized counterparts. This advancement holds great significance, given the relatively modest strength and modulus of gum rubber, compelling the augmentation of elastomers through synergistic combinations of rigid and pliable substances, thus paving the way for the pragmatic utilization of rubber-based products [21–22]. Noteworthy players in this endeavor within the rubber industry encompass carbon black, silica, and fibers, serving as commonplace fillers to fortify elastomers [23]. On the contrary, conventional fillers necessitate substantial quantities—ranging from 30–60%—to impart optimal qualities to the ultimate applications. Consequently, the spotlight has shifted onto nano-sized inorganic particles, capturing considerable attention within the realm of rubber production [24–25].

In parallel, the arena of organic/inorganic hybrid nanocomposites is a fertile ground for innovation, predominantly hinging on naturally occurring clays and minerals boasting at least one dimension within the nanoscale ambit (1–100 nm). Particularly prominent within elastomeric formulations, nanoclays emerge as stalwart candidates, their surface chemistry easily modifiable through the introduction of functional groups [26–27]. Meanwhile, an exhilarating trajectory unfolds in the rubber sector, catalyzed by the advent of nanoscale graphene and carbon nanotubes, encompassing both functionalized and nonfunctionalized variants. This burgeoning class of advanced materials tantalizes with the promise of amplifying electrical conductivity alongside mechanical prowess [28–30].

Halloysite nanotubes (HNTs) are remarkable aluminosilicates characterized by their unique hollow micro and nanotubular structure, bearing a resemblance to naturally occurring carbon nanotubes. Noteworthy for their exceptional mechanical strength and Young's modulus (approximately 1 TPa), HNTs have emerged as a versatile and valuable material for the formulation of polymer nanocomposites. These nanotubes exist in two primary forms: the hydrated form, denoted by an interlayer spacing of 10Å, with the chemical composition Al₂Si₂O₅(OH)₄.2H₂O, and the anhydrous form, featuring an interlayer spacing of 7Å, described by the formula Al₂Si₂O₅(OH)₄. The distinctive structure of halloysite nanotubes, coupled with the presence of functional groups on their surfaces, facilitates their extensive applicability in both biological and nonbiological domains. Notably, HNTs find utility as agents for corrosion prevention, exhibit impressive thermal resistance, serve as potential carriers for biomaterials, hold promise as vehicles for drug delivery, function as matrices for immobilization, and play a role in polymerization processes. These diverse applications underscore the versatility and potential of HNTs within the realm of polymeric materials [31–34]. According to the findings of Bates et al. [35], the intriguing curvature of the nanotube wall results from an imbalance in the alignment of the silica tetrahedral sheet with the alumina octahedral sheet, inducing a cylindrical shape. The repeated two-layered sheets of the spiral wall may contain intercalated water, although this water is typically removed irreversibly upon drying, yielding a commercially viable halloysite mineral substance. The physical dimensions of halloysite particles vary, with lengths ranging from 1 to 15 µm and inner diameters spanning 10 to 100 nm. Distinguishing itself from other types of nanofillers, HNTs possess a high aspect ratio (length-to-diameter ratio), which facilitates a greater degree of interaction between the filler and polymer phases. As demonstrated in recent research, HNTs have emerged as a novel class of fillers, effectively enhancing the mechanical and thermal properties of polymer composites. These composites, encompassing polymers like epoxy, polypropylene, and polyvinyl alcohol, have exhibited tangible improvements through the incorporation of HNTs [36–39].

Halloysite nanotubes (HNTs), akin to other clay materials, exhibit hydroxyl groups on their tube surfaces. However, these hydroxyl groups pose a challenge when attempting to disperse the material within a polymer matrix. Due to their tendency to agglomerate, achieving a uniform distribution within a nonpolar rubber matrix becomes a formidable task. This phenomenon bears resemblance to the well-documented behavior of silica fillers, which also demonstrate a propensity to aggregate. Interestingly, HNTs, being naturally occurring, offer an alternative to silica in rubber compounds. Their remarkably high aspect ratio positions HNTs as potential rivals to silica fillers. Notably, the expansive diameter of the halloysite lumen raises the possibility of accommodating polymer molecules. This accommodation opens avenues for polymeric composites endowed with unique attributes, such as elevated degradation temperatures [40–41]. In the realm of chemical modification, bifunctional organosilanes play a pivotal role. One such example is bis(triethoxysilylpropyl)-tetrasulfide (TESPT), frequently employed for the alteration of silica surfaces. The endeavor to impart novel properties led Yuan et al. to explore the modification of HNTs using amino silane (3-aminopropyl) triethoxysilane (APTES). Concurrently, the incorporation of silane-bearing glycidyl group-treated HNTs into a cyanate ester-cured epoxy resin was pursued by Liu et al. [42]. Further insights were gleaned by Raman et al. [43], who investigated the modification of tubular halloysite via two distinct silane coupling agents: diethoxydimethyl silane (DMS) and bis[3-(triethoxysilyl)-propyl]tetrasulfide (TESPT). Their goal was to ensure optimal dispersion of these nanoparticles within a hydrophobic solution styrene butadiene rubber matrix. An intriguing dichotomy emerged: the interaction of halloysite with DMS lacked the introduction of novel elements, precluding labeling as grafting organic molecules. Conversely, successful interaction with TESPT was evidenced by sulfur's presence in the surface region of TESPT-HNTs, discernible through XPS survey scans. Comparisons between original HNTs and DMS-treated variants revealed a slight increase in carbon content for DMS-HNTs. Notably, DMS introduction correlated with augmented silicon levels within the surface region [43].

Recent efforts have been focused on utilizing nanoscale reinforcements to develop a rubber compound based on NR/EPDM blends with the desired mechanical properties. A study conducted by Alipour et al. [44] highlighted the impact of adding organoclay to NR/EPDM blends with varying ratios. The inclusion of 7 parts per hundred rubber (phr) of organoclay led to a noteworthy 40% increase in tensile modulus for NR/EPDM (75/25 phr/phr). This current research aims to delve into the cure characteristics, mechanical strength, abrasion resistance, and structure of NR/EPDM/EPDM-g-MA blends. These blends have been reinforced with different types of Halloysite Nanotubes (HNTs), including APTES-functionalized and DMS-functionalized HNTs. Of particular interest is the impact of varying HNTs content on these properties. Additionally, the behavior of these composite materials in various solvent environments is thoroughly examined, providing valuable insights into their versatility and performance. Overall, this study contributes to the advancement of innovative rubber composites and their potential applications.

2.1. Materials

The mixing components were employed in their original, as-received state.

Rubber materials were sourced from reputable suppliers: Natural rubber (NR) in the form of Ribbed Smoked Sheets (RSS)-1 with specific properties, including a Mooney viscosity (ML 1 + 4) at 100°C of 57 M, a density of 0.9125 g/cm³, ash content of 0.58%, and nitrogen content of 0.63%. Ethylene-propylene-diene monomer (EPDM) rubber, denoted as KEP-270, possessed a Mooney viscosity (ML 1 + 4) at 125°C of 60 M, comprising an ethylene content of 68 wt.%, a termonomer content (ethylidene norbornene) of 4.5%, and a density of 0.86 g/cm³. The acquisition of these rubber types was facilitated by Asian Rubber Pvt. Ltd., situated in Ambattur, Chennai, India, and Supple Rubber Chemicals Pvt. Ltd., located in New Delhi, India.

Compatibilizer: Procured from Songhan Plastic Technology Co. Ltd. in Shanghai City, China, the EPDM-g-MA compound named Bondyram® 7001 is a maleic anhydride-grafted ethylene-propylene-diene rubber. This particular EPDM-g-MA variant exhibits a melt flow rate (MFR) of 7 g/10 min (conforming to ASTM D1238 standards), a mass density of 0.87 g/cm³ (in accordance with ASTM D792), and contains 0.7% maleic anhydride by content.

Reinforcement: Halloysite nanotubes, belonging to the kaolin clay family, have a linear chemical composition represented as Al₂Si₂O₅(OH)₄ • 2 H₂O, with a molecular weight of 294.19 g/mol. These nanotubes exhibit dimensions ranging from 30 to 70 nanometers in diameter and 1 to 3 micrometers in length. The source of these Halloysite nanotubes was Sigma-Aldrich, located in Puducherry.

Reinforcement modifier: Diethoxydimethyl silane (DMS, (CH₃)₂Si(OC₂H₅)₂), as well as (γ-aminopropyl) triethoxysilane (APTES), were procured from Sigma-Aldrich, located in Puducherry, India.

Rubber chemicals: The activator components, namely stearic acid and zinc oxide, along with accelerators such as mercaptobenzothiazyl disulphide (MBTS) and tetramethylthiuram disulphide (TMTD), as well as zinc diethyldithiocarbamate (ZDC) and 2-mercaptobenzothiazole (MBT), were procured from Vignesh Chemicals Pvt. Ltd., situated in Ambattur, Chennai, India, while the essential vulcanizing agent, sulphur, was also sourced from the same supplier.

Swelling chemicals: The array of solvents investigated in this study encompassed aromatic solvents, namely benzene, mesitylene, toluene, and xylene, along with aliphatic solvents such as n-pentane, n-heptane, n-hexane, and n-octane. Additionally, chlorinated solvents including chloroform, dichloromethane, and carbon tetrachloride were part of the investigation. These solvents were procured from Sigma-Aldrich, Puducherry, ensuring the reliability and quality of the experimental materials.

2.2 Preparation of DMS-HNTs

Due to their hygroscopic properties, HNT nanotubes underwent a drying process at 105°C for one hour prior to utilization. For bulk modification of the HNT, a quantity of 60 grams of the material was employed. Within a flask featuring a round bottom, 300 ml of dry toluene was introduced to the desiccated HNT. Subsequently, this mixture was combined with 4 ml of DMS before sealing the flask promptly. Employing ultrasonication at room temperature for a duration of 30 minutes, the contents of the flask were homogenized. This was followed by a reflux procedure lasting 72 hours at 70°C, carefully maintaining a dry environment using guard tubes containing calcium chloride and Teflon tapes. Cooling the system to room temperature took less than an hour upon completing the reaction phase. To eliminate any remaining unreacted DMS, the modified HNT (referred to as DMS-HNT) was filtered through a sintered funnel and underwent thrice-washing with toluene. Subsequent to drying at 50°C for two hours, the DMS-HNT was further pulverized to a powdered state with a mortar and pestle. Lastly, the DMS-HNT was placed within desiccators to establish a dry atmosphere, as outlined in reference [26].

2.3 Preparation of APTES-HNTs

In preparation for alteration, the inherent hygroscopic nature of the pristine nanotubes necessitated a pre-drying step at 105°C for a duration of one hour. Within a volume of 70 mL of desiccated toluene, a concoction of 2 g of the silane coupling agent (γ-aminopropyl) triethoxysilane, commonly referred to as APTES, was meticulously blended with 1 g of meticulously dried nanotubes. The initial phase of generating surface-functionalized HNTs involved subjecting the amalgamation to ultrasonic energy for a span of 30 minutes, thereby achieving a homogeneous dispersion. Subsequently, this amalgam underwent reflux at a consistent reaction temperature of 80°C for a continuous duration of 48 hours, all the while being subjected to constant agitation. To eliminate any excess organosilane, the resultant modified nanotubes underwent an extensive washing process utilizing toluene, this procedure being iterated a total of five times. The filtered product was obtained using a Buchner funnel, and for further refinement, the aminosilane-modified HNTs were subjected to an overnight drying regimen at 100°C within a vacuum oven. The culmination of these processes led to the emergence of grafted organosilane nanotubes, christened as APTES modified HNTs.or APTES-HNTs

2.4 Characterization of the modified HNTs Fillers

By means of an immersion test, alterations in surface polarity were meticulously examined. Employing a minute quantity of filler, the surface of a solvent, specifically water in this instance, was endowed for the experimental analysis. The unaltered Halloysite Nanotubes (HNTs), due to their hydrophilic nature, swiftly descended upon contact, while the chemically modified counterparts demonstrated prolonged buoyancy. A water penetration assessment was then conducted to attain more precise insights. This evaluation encompassed the introduction of 0.02 g of filler into a glass column, subsequently sealing the aperture with a dual layer of a hydrophobic filter featuring a pore dimension of 0.2 µm. The assembled column was submersed within a water medium, and alterations in weight resulting from water absorption were diligently monitored and recorded over a specified temporal interval.

2.5. Preparation of Nanocomposites

Utilizing an open-mill mixer, a series of NR/EPDM blends incorporating varying filler loadings was meticulously produced. The process involved distinct stages of mastication and blending for each polymer, independently conducted between precision mill rolls at ambient conditions, following the formulations outlined in Table 1. The initial phase encompassed a mastication duration of approximately 5 minutes for the NR rubber on a two-roll mill. Subsequently, EPDM-g-MA and EPDM were introduced to the mix, undergoing an additional 5-minute amalgamation on the same mill, succeeded by the incorporation of vulcanization constituents. Introducing either HNTs or modified HNTs transpired five minutes subsequent to the component amalgamation, after which the resulting nanocomposite underwent stirring for a period spanning five to fifteen minutes. Depending on the specific filler loading, the compounding duration for each composite was meticulously maintained within the range of 20 to 30 minutes. The ultimate vulcanization of the composites transpired within a hydraulic press, boasting platen dimensions of 15 cm x 15 cm x 2 cm. This process was executed at a pressure of 60 MPa and a temperature of 160°C, adhering to an optimal curing period.

Table 1

Formulation of NR/EPDM-HNTs Compounds
Sample code	Compounds (phr)
Sample code	NR	EPDM	EPDM-g-MA	HNTs	DMS-HNTs	APTES-HNTs	Zinc oxide	Stearic acid	MBTS	TMTD	Sulphur
H₀	75	25	5	0	-	-	4	1.5	1.2	1	2.5
H₂	75	25	5	2	-	-	4	1.5	1.2	1	2.5
H₄	75	25	5	4	-	-	4	1.5	1.2	1	2.5
H₆	75	25	5	6	-	-	4	1.5	1.2	1	2.5
H₈	75	25	5	8	-	-	4	1.5	1.2	1	2.5
H₁₀	75	25	5	10	-	-	4	1.5	1.2	1	2.5
DH₂	75	25	5	-	2	-	4	1.5	1.2	1	2.5
DH₄	75	25	5	-	4	-	4	1.5	1.2	1	2.5
DH₆	75	25	5	-	6	-	4	1.5	1.2	1	2.5
DH₈	75	25	5	-	8	-	4	1.5	1.2	1	2.5
DH₁₀	75	25	5	-	10	-	4	1.5	1.2	1	2.5
AH₂	75	25	5	-	-	2	4	1.5	1.2	1	2.5
AH₄	75	25	5	-	-	4	4	1.5	1.2	1	2.5
AH₆	75	25	5	-	-	6	4	1.5	1.2	1	2.5
AH₈	75	25	5	-	-	8	4	1.5	1.2	1	2.5
AH₁₀	75	25	5	-	-	10	4	1.5	1.2	1	2.5

2.6. Characterization

Cure characteristics: The evaluation of the cure characteristics of the unvulcanized rubber samples was conducted by subjecting them to analysis using an oscillating disc rheometer at a temperature of 160°C, following the standardized protocol outlined in ASTM D2084 (ODR). Tensile strength, elongation at break and stress at 100% elongation: Dumbbell-shaped and un-nicked 90-degree test samples were precision-cut from the molded sheets using a die cutter. Tensile testing of the composites followed the ASTM D412-C standard and was conducted using a Universal Testing Machine (UTM) (series: 7200, make: Dak System Inc., model: T-72102) at a cross-head speed of 500 mm/min and a temperature of 23°C, as outlined in reference [45]. Tear characteristics were determined using the same equipment and testing conditions, following the ASTM D624-B standard. To ensure robust results, a minimum of five samples were analyzed for each set of experiments, thereby establishing a high level of confidence in the obtained data.

Hardness and abrasion resistance: The assessment of hardness was conducted using the shore A durometer hardness tester, following the guidelines outlined in ASTM D2240. In accordance with ASTM D5963, an abrasion resistance test was carried out utilizing the Zwick abrasion tester.

Rebound resilience: The measurement of rebound resilience was carried out using a vertical rebound resilience tester, following the guidelines outlined in ASTM D2632 [46–47]. This method involves quantifying the ratio between the height of the rebound and the initial deformation resulting from impact. High-quality rubber demonstrates excellent rebound resilience, indicating its proficiency in efficiently absorbing and subsequently releasing energy. This attribute renders such rubber particularly well-suited for applications demanding effective shock absorption capabilities.

Crosslink density: The determination of the crosslink density in rubber samples was accomplished using immersion techniques, specifically through equilibrium solvent-rubber swelling measurements. This involved subjecting the specimens to toluene uptake over a 72-hour period at 23°C. The Flory–Rehner equation, as outlined in references [48–51], was employed to analyze and quantify the obtained data.

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

Here, the symbols hold specific significance: M_c represents the molar mass of the polymer within crosslinks, V_s corresponds to the molar volume of the solvent (measuring 106.3 mL/gmol), ρ_p stands for the polymer's density, V_r denotes the volume fraction of polymer within the solvent-swollen filled compound, and χ signifies the polymer's interaction parameter (notably set at 0.25) [52]. Furthermore, the calculation of V_r is determined by the subsequent equation [53].

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

In this context, the variable "Q_m" represents the extent of weight swell exhibited by the NR/EPDM-HNTs nanocomposites when exposed to toluene..

The degree of crosslink density ($v$) is given by Eq. (3):

$$v \left(\frac{mol}{{cm}^{3}}\right)= \frac{1}{2{M}_{c}}$$

FESEM microscopy: The investigation of the tensile fracture surface involved the utilization of a Hitachi S-3400 field emission-scanning electron microscope (FE-SEM) operating at 30 kV voltage. The primary objective was to gain insight into the fracture mechanism. To mitigate potential electrical charge interference during examination, the fractured ends of the tensile specimens were meticulously affixed onto stubs and subsequently coated with a fine layer of gold using a sputter coating process.

3.1 Filler characterization

The water immersion test was done to determine whether the hydrophobicity has changed: As anticipated, the untreated HNTs powder instantly sank in water; while the surface treated HNTs remained afloat on the water surface for an extended duration. This change in hydrophobicity can be ascribed to the reduced polarity of the HNTs resulting from the surface modifications. Furthermore, this phenomenon showcases an irregular coating, likely stemming from the incorporation of filler aggregates during the surface modification procedure.

The experimental observations confirmed the initial expectations, revealing a distinct contrast in behavior between untreated and surface-treated HNTs powder when introduced to water. In the aqueous environment, the untreated HNTs sank instantly, confirming the inherently hydrophilic nature of the untreated particles, as water molecules readily interacted with their surfaces, compelling them to submerge. Conversely, a markedly dissimilar scenario unfolded with the surface-treated HNTs which stayed afloat the water's surface for a significantly prolonged period. This phenomenon was indicative of a noteworthy shift in hydrophobicity, a property that emerged due to the alterations introduced to the surface of the HNTs. The surface modifications induced a decrease in the inherent polarity of the particles, thereby diminishing their propensity to readily engage with water molecules. This behavior of the surface modified HNTs can also be caused by the uneven distribution of the surface treatment as well as the dilution of the surface coating on prolonged exposure to water.

In Fig. 1, a comprehensive depiction of water absorption results is presented, shedding light on the behavior of unmodified, DMS, and APTES surface-modified HNTs. The investigation showcases a gradual water absorption pattern exhibited by all three variants, ultimately culminating in an equilibrium state. Intriguingly, a discernible distinction emerges as pure HNTs exhibit a higher water absorption capacity in comparison to both DMS and APTES-modified HNTs. This disparity in water absorption underscores the enhanced hydrophobic nature of the surface-modified HNTs. A noteworthy inference drawn from this exploratory endeavor is the varying degrees of hydrophobicity among the HNTs. Specifically, the APTES-modified HNTs exhibit a remarkable reduction in polarity, rendering them significantly more hydrophobic than both the unmodified counterparts and the DMS-modified HNTs. This particular observation opens up intriguing possibilities, suggesting that APTES-modified HNTs could potentially manifest as a more compatible and efficacious candidate when incorporated into a non-polar rubber matrix.

3.2 Cure characteristics

Rubber cure characteristics refer to the changes in mechanical and chemical properties that occur during the vulcanization or curing process of rubber. Vulcanization is a process in which raw rubber is transformed into a more durable and elastic material by cross-linking polymer chains using various curing agents or additives. The following are key terms used to describe rubber cure characteristics: (a) Minimum Torque (M_H): This is the initial torque value observed when the rubber compound begins to cure. It represents the resistance to deformation and the initiation of cross-linking between polymer chains. (b) Maximum Torque (M_L): The maximum torque value reached during the curing process. It indicates the point at which cross-linking is nearly complete and the rubber has achieved its maximum strength and stability. (c) Delta Torque (M_H - M_L): This is the difference between the maximum torque and the minimum torque. It gives an idea of the overall torque change during curing and is related to the degree of cross-linking that occurs in the rubber. (d) Scorch Time (t_s2): Scorch time is the time it takes for the torque to reach a specific value (often 2 units) above the minimum torque. It is an indication of the rubber's processing safety, i.e., the time available for molding or shaping the rubber before it begins to cure and lose its flow properties. (e) Optimum Cure Time (t_c90): This is the time at which the torque reaches 90% of the maximum torque. It indicates the point at which vulcanization is mostly complete and the rubber has achieved its desired properties. (f) Cure Rate Index (CRI): The CRI is calculated from the scorch time and the optimum cure time. It provides insight into how rapidly the rubber compound cures. A higher CRI suggests a faster curing process [54].

3.2.1 Minimum torque

The influence of surface treatment on the minimum torque of HNTs filler filled NR/EPDM nanocomposites at various loadings is shown in Fig. 2. The results obtained from Fig. 2 indicate that the minimum torque values generally increased with increasing HNTs loading for all three cases: unmodified HNTs, DMS modified HNTs, and APTES modified HNTs. Unmodified surfaces showed relatively constant minimum torque values across all loading conditions, ranging from 3.45 to 4.52 d.Nm for HNTs loading of 0 to 10 phr, respectively. The increase in minimum torque of NR/EPDM nanocomposites with rising HNTs content is attributed to intensified filler-filler interactions, elevated viscosity hindering matrix flow, stronger interfacial bonding, potential agglomeration, altered curing behavior, and enhanced nanotube alignment, all demanding higher energy input during compounding. In contrast, both DMS and APTES surface modifications exhibited a gradual increase in minimum torque values with increasing HNTs loading. This is due to improved dispersion, increased surface area, and enhanced load-bearing capacity. Chemical functional groups on the modified surface interact more effectively with HNTs, leading to stronger adhesion and resistance to rotational motion. This combination of factors, including synergistic effects and physical interlocking, contributes to the observed torque increase. With DMS modification, the minimum torque increased from 3.45 d.Nm at 0 phr HNTs loading to 5.17 d.Nm at 10 phr loading, indicating improved minimum torque. Similarly, APTES modification showed consistent increases, with minimum torque values ranging from 3.45 d.Nm at 0 phr loading to 5.26 d.Nm at 10 phr loading. APTES-modified surfaces displayed higher minimum torque values compared to DMS modification, highlighting the superior performance of APTES-functionalized surfaces. It can be attributed to the stronger and more versatile chemical bonding of APTES functional groups with HNTs, resulting in enhanced adhesion and load-bearing capacity. This stronger interaction leads to greater resistance against rotational motion, translating into higher torque values.

3.2.2 Maximum torque

The effect of surface treatment on maximum torque of HNTs filler filled nanocomposites at different loadings is illustrated in Fig. 3. Both unmodified and surface-treated nanocomposites exhibited a gradual increase in maximum torque as the HNTs loading increased. The maximum torque values for the unmodified HNTs nanocomposites ranged from 45.03 d.Nm at 0 phr loading to 54.97 d.Nm at 10 phr loading, consistent with the trends observed in minimum torque in Fig. 3. This can be attributed to improved dispersion and interfacial interactions. As HNTs loading rises, more filler material becomes available, enhancing the reinforcement effect. This leads to stronger filler-polymer interactions, resulting in increased resistance to deformation and greater torque values during testing. The DMS and APTES surface modifications also demonstrated a gradual increase in maximum torque with increasing HNTs loading. In case of DMS surface treatment, the maximum torque improved from 45.03 d.Nm at 0 phr HNTs loading to 56.85 d.Nm at 10 phr loading. Similarly, APTES surface treatment exhibited consistent increases, with maximum torque values ranging from 45.03 d.Nm at 0 phr loading to 59.25 d.Nm at 10 phr loading. Notably, APTES-modified surfaces displayed higher maximum torque values compared to DMS modification, indicating the superior performance of APTES-functionalized surfaces. This is attributed to the stronger and more effective bonding of APTES functional groups with both HNTs and the polymer matrix. This enhanced interaction results in improved dispersion, better load transfer, and increased resistance to deformation, leading to higher torque values during testing.

3.2.3 Delta torque

Figure 4 demonstrates the impact of surface treatment on the delta torque of HNTs nanocomposites at different loadings. The data reveals a consistent trend of increased delta torque with higher HNTs loading for all three cases. In the case of Unmodified HNTs, the delta torque values start at 41.58 for 0 phr loading and steadily rise as the HNTs loading increases is a result of improved dispersion and stronger interaction between the nanotubes and the NR/EPDM rubber matrix. Higher HNTs loading enhances the physical entanglement and interlocking of the nanotubes within the NR/EPDM, leading to increased resistance to deformation and rotational motion. This effect contributes to the observed rise in delta torque, indicating improved mechanical properties and reinforcing behavior in the NR/EPDM nanocomposite. The highest loading of 10 phr corresponds to a significant enhancement, with a delta torque of 50.45 d.Nm compared to the initial value. Similarly, the DMS surface modification exhibits a comparable increasing trend in delta torque. Starting at 41.58 d.Nm for 0 phr HNTs loading, the delta torque steadily increases, reaching 51.68 d.Nm at the maximum loading of 10 phr. Notably, the APTES surface modification has the most substantial effect on delta torque. Beginning at 41.58 d.Nm for 0 phr HNTs loading, the delta torque experiences a significant increase with higher HNTs loading. At the maximum loading of 10 phr, the delta torque reaches 53.99 d.Nm, surpassing the other two cases. This can be attributed to improved dispersion, increased surface area, and the formation of reinforcing networks. As HNTs loading increases, stronger interactions between surface modification agents and HNTs lead to enhanced mechanical properties and stability. Additionally, synergistic effects and the inherent mechanical strength of HNTs contribute to the observed rise in torque values. The notably higher delta torque values exhibited by APTES-modified surfaces, in contrast to DMS modification, highlight the superior performance of APTES-functionalized surfaces. This suggests that APTES surface modification results in stronger interactions and enhanced mechanical properties, underscoring its effectiveness in improving surface characteristics compared to DMS modification.

3.2.4 Scorch and optimum cure time

The impact of surface modification on the scorch time and optimum cure time with respect Nanotubes loading are depicted in Fig. 5 and Fig. 6, respectively. Both scorch time and cure time followed a declining trend with an increase in HNTs loading for the all three cases. Reduced scorch time can be attributed to the enhanced nucleation effect of HNTs. As the loading of HNTs increases, their presence provides more nucleation sites for the polymer matrix, promoting the formation of a higher number of cross-links and polymer chains at an earlier stage during the curing process. This accelerated cross-linking leads to a reduction in scorch time, the time it takes for premature curing to begin, resulting in improved processing and overall material properties. Reduced optimum cure time can be attributed to improved filler dispersion and interaction. At higher HNTs loadings, the nanotubes provide more nucleation sites for crosslinking, facilitating a more efficient curing process. Additionally, enhanced filler-filler and filler-polymer interactions at higher loadings promote polymer chain mobility and crosslinking, leading to faster curing kinetics and a reduced optimum cure time. APTES-modified HNTs had the lowest scorch time of 1.28 minutes at 10 phr loading, followed by DMS-modified HNTs at 1.3 minutes and unmodified HNTs at 1.35 minutes at the same HNTs loading, can be attributed to the stronger and more stable interaction between APTES-HNTs and the NR/EPDM rubber matrix.. The optimum cure time also followed a similar trajectory, reducing with an increase in HNTs loading. The surface modifications also had a similar impact on optimum cure time, where in APTES-modified had the lowest optimum cure time of 3.46 minutes at 10 phr loading followed by DMS-modified HNTs and Unmodified HNTs with curing times of 3.76 and 4.92 minutes respectively both at 10 phr HNT loading. This can be attributed to the stronger and more effective chemical interaction between APTES-HNTs and the NR/EPDM rubber matrix. APTES functionalization introduces amino groups that can form covalent bonds with the polymer chains, enhancing filler dispersion and interfacial adhesion. This improved compatibility leads to faster and more efficient curing kinetics, resulting in a reduced optimum cure time for APTES-modified HNTs filled composites compared to those modified with DMS, where the interaction might be weaker.

3.2.5 Cure rate index

The cure rate index, which is quantifies the speed of vulcanization in rubber compounding, indicating how quickly a rubber compound reaches its maximum crosslinking density during curing, is showed an increasing trend with an increase in HNTs loading as evident from Fig. 7. This can be attributed to enhanced crosslinking efficiency and improved interaction between the polymer matrix and the HNTs. As the HNTs loading increases, more nanotubes are dispersed within the polymer matrix, providing additional sites for crosslinking reactions. This results in a higher degree of network formation, leading to increased stiffness and improved curing kinetics. Additionally, the high aspect ratio and unique structure of HNTs facilitate a stronger physical and chemical bond with the polymer chains, further promoting efficient curing and enhancing the overall performance of the nanocomposites. APTES-modified HNTs had the highest cure rate index of 45.87 at 10 phr HNT loading, followed by DMS-modified HNTs with a cure rate index of 40.65 and unmodified HNTs which had the lowest cure-rate index of the three cases with 28.01 at the same HNT loading. This can be attributed to the stronger and more versatile chemical interaction between APTES-modified HNTs and the polymer matrix. APTES functionalization introduces amino groups that facilitate enhanced crosslinking and bonding with the rubber matrix, promoting efficient curing kinetics. In contrast, DMS modification might offer weaker interactions, limiting effective crosslinking and resulting in a comparatively lower cure rate index in the nanocomposites.

3.3 Mechanical properties

3.3.1 Tensile strength

Tensile strength is the maximum amount of tensile (pulling) stress a rubber material can withstand without breaking. Rubber with higher tensile strength is more resistant to tearing and stretching. The impact of surface modification on the tensile strength of the nanocomposites at different loadings is portrayed in Fig. 8. The tensile strength of the nanocomposites exhibited a proportional escalation corresponding to the increase in loading across all three examined scenarios. As the loading of HNTs increases, more nanotubes become dispersed within the polymer, creating a network-like structure. This network enhances the load-bearing capability of the composite, leading to improved tensile strength. It is worth mentioning that the unmodified, DMS, and APTES surface-modified HNTs displayed maximum tensile strengths of 31.53 MPa, 33.14 MPa, and 35.17 MPa, respectively, at 8 phr. Initially, as the loading of HNTs increases up to 8 phr, they contribute to reinforcing the polymer matrix. The high aspect ratio and strong interaction of HNTs with the polymer chains provide effective stress transfer and load distribution, resulting in improved tensile strength. The HNTs act as reinforcing agents, enhancing the composite's mechanical properties and stiffness. In addition, at lower loadings of HNTs, there is a higher likelihood of achieving good dispersion within the polymer matrix. This uniform dispersion ensures that the reinforcing effects of HNTs are evenly distributed, resulting in improved mechanical properties like tensile strength. Furthermore, any subsequent augmentation in HNTs loading led to a discernible decrement in the overall tensile strength across all three cases, a phenomenon that can be ascribed to the agglomeration of HNTs molecules in the rubber matrix [55]. This agglomeration can create regions of weakness within the composite, reducing stress transfer and leading to decreased tensile strength. Difficulty in dispersing a higher amount of HNTs uniformly within the polymer matrix can result in non-uniform reinforcement and reduced tensile strength. In addition, excessive HNTs loading can hinder the mobility of polymer chains during deformation, leading to a more brittle behavior and reduced overall tensile strength. Beyond a 8 phr HNTs loading, the strong interaction between HNTs and the NR/EPDM rubber matrix can lead to a less favorable polymer-filler interface, potentially causing stress concentrations and weakening the composite.

The HNTs subjected to DMS surface treatment exhibited a superior tensile strength surpassing that of the unmodified HNTs devoid of any surface treatment across the entire spectrum of HNT loadings. This enhanced affinity leads to better dispersion and distribution of treated HNTs within the polymer, resulting in a more uniform and efficient stress transfer. As a result, the composite with DMS-treated HNTs experiences improved load-bearing capabilities and higher tensile strength. DMS treatment introduces functional groups on the surface of HNTs, which can form stronger chemical bonds with the polymer matrix. This creates a robust interface between the treated HNTs and the polymer, enhancing the overall interfacial adhesion. The improved bonding at the interface contributes to better load transfer and increased tensile strength. APTES surface treatment exhibited the most pronounced influence on the tensile strength of the nanocomposites, consistently surpassing the performance of both the DMS-surface treated HNTs and the HNTs with no surface modifications across all loadings up to 8 phr. APTES treatment introduces amino groups onto the surface of the HNTs, which can form strong chemical bonds with the polymer matrix. This can lead to a highly effective interface between the treated HNTs and the polymer, resulting in excellent stress transfer and improved tensile strength. In addition, APTES treatment can improve the wettability of the HNTs by the polymer matrix, facilitating better wetting and distribution of the filler within the polymer. This, in turn, leads to a more homogeneous composite structure and improved mechanical properties. Furthermore, the combination of strong chemical bonding, improved wettability, reduced agglomeration, and tailored interactions can create synergistic effects that amplify the overall mechanical properties of the composite.

3.3.2 Elongation at break and stress at 100% elongation

Elongation at break is the percentage increase in length a rubber material can undergo before breaking. It indicates the material's flexibility and ductility. Rubber with high elongation at break can deform significantly before rupturing, making it suitable for applications requiring stretching and flexibility. Stress at 100% elongation property represents the amount of stress required to stretch the rubber material by 100% of its original length. It provides an indication of the material's initial elasticity and resistance to deformation [56].

The impact of surface modification on the elongation at break and stress at 100% elongation of the nanocomposites at different loadings is portrayed in Figs. 9 and 10, respectively. Both the elongation at break and the stress at 100% elongation exhibited a progressively increasing pattern with an increase in HNT loading across all three scenarios, until reaching 8 phr HNT loading. At lower HNT loadings, the nanotubes are more effectively dispersed within the polymer matrix. This uniform dispersion allows the HNTs to act as reinforcement throughout the composite material, contributing to improved load-bearing capacity and mechanical properties. The increased interaction between the HNTs and the polymer chains enhances the ability of the material to deform and distribute stress before failure. The presence of HNTs can facilitate stress dissipation and energy absorption during deformation. As the nanotubes are subjected to stress, they can help absorb and distribute the applied force, delaying the onset of failure. This characteristic contributes to the observed increase in elongation at break and stress at 100% elongation. The addition of HNTs, especially in surface-treated or modified forms, can lead to improved bonding at the filler-polymer interface. This enhanced interfacial interaction allows for more effective stress transfer between the HNTs and the polymer matrix, leading to higher mechanical performance. There was a decline in both elongation at break and stress at 100% elongation when the loading was increased further to 10 phr for all three cases. At higher HNTs loadings, there is an increased likelihood of agglomeration or clustering of nanotubes within the polymer matrix. Agglomerated filler particles create regions of stress concentration, weakening the composite material and leading to premature failure. Poor dispersion also reduces the effectiveness of stress transfer and load distribution, negatively impacting elongation at break and stress at 100% elongation values. This excessive filler content can hinder the mobility of the polymer chains, making it harder for the material to undergo deformation. As a result, the composite becomes more brittle and less capable of elongation before breaking. In addition, higher HNT loadings can significantly increase the overall stiffness of the composite. While some stiffness is desirable for mechanical reinforcement, an overly stiff material can lead to reduced flexibility and elongation at break. This increased stiffness can prevent the material from absorbing and dissipating energy, resulting in decreased elongation and stress at 100% elongation. Furthermore, the presence of a large number of HNTs can impede the movement of polymer chains during deformation. This reduced chain mobility limits the material's ability to elongate and absorb energy, causing a decrease in both elongation at break and stress at 100% elongation. It can be noted that, higher filler loadings can lead to interfacial disruption between the HNTs and the polymer matrix. This disruption can result in reduced stress transfer efficiency and weaker bonding, contributing to the decline in mechanical properties.

The HNTs subjected to APTES surface modification exhibited a substantially superior magnitude of elongation at break compared to both the HNTs modified with DMS as well as the unmodified nanocomposites for a nanotube loading up to 8 phr. The stress at 100% elongation for APTES-HNTs also followed a similar trend of outperforming the DMS and unmodified HNTs, up to a nanocomposite loading of 8 phr. Despite the DMS-modified HNTs exhibiting inferior elongation at break and stress at 100% elongation compared to the APTES-modified HNTs, it showcased superior performance in relation to the unmodified HNTs. APTES treatment introduces amino groups onto the surface of HNTs, which can react with functional groups in the polymer matrix. This chemical interaction promotes better dispersion and adhesion between the treated HNTs and the polymer, allowing for effective stress transfer and load distribution. The APTES surface treatment can create synergistic effects with the polymer matrix, enhancing the above mentioned performance of the composite beyond what can be achieved with untreated HNTs [58].

3.3.3 Hardness, Tear Strength & Rebound Resilience

Hardness measures a material's resistance to indentation or penetration. Higher durometer values indicate harder rubber with less deformation under load, while lower values indicate softer rubber with greater flexibility. Rebound resilience, also known as resilience or rebound elasticity, is the ability of rubber to regain its original shape after deformation. Rebound resilience is the ability of the rubber to bounce back to its original shape after being deformed. Tear strength is the resistance of rubber to propagating a tear once an initial cut or nick is made. It measures the material's ability to resist tearing forces and is important for applications subjected to mechanical stress or sharp objects [59].

The impact of surface modification on the tear strength, hardness, and resilience of the nanocomposites with respect to the HNTs loadings are illustrated in Figs. 11, 12 and 13 respectively. It is evident that the increase in both hardness and tear strength coincides with an elevation in the HNTs loading, while the trajectory of rebound resilience exhibits a declining pattern with an increase in HNTs loading. This suggests that the incorporation of HNTs into the rubber matrix is leading to a reinforcement effect. HNTs likely enhance the material's resistance to deformation and tearing, resulting in higher hardness and tear strength values. Higher concentrations of HNTs are causing a reduction in the rebound resilience of the rubber material. A decrease in rebound resilience might indicate that the addition of HNTs is affecting the material's ability to efficiently absorb and release energy during deformation.

It is evident from the figures that APTES-HNTs exhibit superior hardness and tear strength compared to DMS-HNTs and unmodified HNTs across all loadings. Notably, APTES-HNTs reach a maximum of 80 Shore A hardness and 57.98 N/mm tear strength at 10 and 8 phr loadings, respectively. When APTES is used to modify the HNTs before adding them to the NR/EPDM composites, the resulting composites show enhanced hardness and tear strength. APTES modification likely improves the bonding between the HNTs and the rubber matrix, leading to increased resistance to deformation and tearing forces. The nanocomposites containing APTES-modified HNTs exhibit better hardness and tear strength compared to those with DMS modified HNTs and unmodified HNTs. This suggests that APTES modification is more effective in reinforcing the rubber matrix in terms of hardness and tear resistance. It is worth noting that APTES-HNTs demonstrate the lowest rebound resilience among DMS-HNTs and unmodified HNTs at all loadings, reaching a minimum of 43% at 10 phr loading. Furthermore, DMS-HNTs, despite displaying inferior hardness and tear strength in comparison to APTES-HNTs, consistently outperforming the unmodified HNTs at all loadings, peaking at 79 Shore A hardness and 56.84 N/mm at 10 phr HNT loading. Regarding rebound resilience, DMS-HNTs exhibit superior rebound resilience compared to APTES-HNTs but remain inferior to unmodified HNTs. In contrast, the unmodified HNTs consistently exhibit lower hardness and tear strength, inferior to both APTES and DMS-HNTs at all loadings, while displaying superior rebound resilience across all loadings. When HNTs are added without any surface modification, the resulting composites have a higher ability to return to their original shape after deformation. The modifications (DMS and APTES) might be affecting the material's ability to efficiently absorb and release energy during deformation, leading to reduced rebound resilience. APTES and DMS modification might lead to stiffer interactions between the modified HNTs and the rubber matrix. While increased stiffness can contribute to improved hardness, it can also lead to decreased energy dissipation during deformation. Rebound resilience relies on the material's ability to store and release energy upon deformation. If the modified HNTs result in a stiffer composite, more energy might be stored during deformation, but this energy might not be effectively released during the recovery phase, leading to reduced rebound resilience. APTES and DMS modification could potentially promote cross-linking or network formation within the rubber matrix. While this can enhance certain mechanical properties, it might also affect the rubber's ability to recover its original shape. Cross-linking can restrict the movement of polymer chains, leading to decreased elasticity and rebound resilience [60].

3.4 Abrasion resistance

Abrasion resistance is the ability of rubber to withstand wear and erosion caused by frictional forces. It is crucial for materials used in applications where rubbing, sliding, or scraping against surfaces occurs, such as tires, conveyor belts, and seals. The impact of surface modification on the abrasion loss (mm³) of the nanocomposites with respect to the HNTs loadings is illustrated in Fig. 14. Abrasion resistance is evaluated by measuring the extent of abrasion loss incurred by the material. All three, APTES-HNTs, DMS-HNTs and unmodified HNTs displayed a decreasing trend with an increase in HNTs loading. The unique structure of HNTs, with their hollow and tubular morphology, can create a physical barrier against abrasion. As the HNTs disperse within the rubber matrix, they can help hinder the propagation of cracks and reduce the material's susceptibility to abrasive wear [61]. This effect becomes more prominent as the concentration of HNTs increases. The presence of HNTs within the rubber matrix creates a reinforced network that resists the wear and tear caused by abrasive forces. It is to be noted that APTES-HNTs had the least abrasion loss among the three cases studied for all loading cases, specifically 23.98 mm³ at 10 phr loading, indicating its superior performance over DMS-HNTs and unmodified HNTs. APTES modification involves functionalizing the HNTs with amino groups, which can promote stronger chemical bonding or interactions between the filler and the rubber matrix. This improved interface leads to efficient load transfer and stress distribution during abrasive forces, resulting in enhanced abrasion resistance. APTES modification can enhance the dispersibility of HNTs within the rubber matrix. This prevents the formation of agglomerates or clusters, ensuring a more uniform distribution of the filler. A uniform dispersion of modified HNTs contributes to consistent and improved resistance to abrasion throughout the composite. It can also be noted that DMS-HNTs though inferior to APTES-HNTs performed superior to unmodified HNTs for all loading cases. Harder and stronger materials tend to better withstand the abrasive forces generated during contact with rough surfaces. This improved mechanical integrity translates to higher resistance against material loss due to abrasion. In addition, the chemical modifications introduced by APTES can enhance the adhesion between the modified HNTs and the rubber matrix. This optimized adhesion ensures that the filler remains well-bound within the matrix even under abrasive conditions, reducing the likelihood of material loss due to abrasion.

3.5 FESEM morphology

Figure 15 presents FESEM micrographs of the tensile-broken surfaces of NR/EPDM nanocomposites: (a) NR/EPDM blends, (b) NR/EPDM-8 phr HNTs nanocomposites, (c) NR/EPDM-10 phr HNTs nanocomposites, (d) NR/EPDM-8 phr DMS modified HNTs nanocomposites and (e) NR/EPDM-10 phr DMS modified HNTs nanocomposites (f) NR/EPDM-8 phr APTES modified HNTs nanocomposites and (g) NR/EPDM-10 phr APTES modified HNTs nanocomposites. It is evident from Fig. 15(a) that the NR and EPDM are homogeneously blended owing to their good compatibility achieved with the help of EPDM-g-MAH compatibilizer. The Fig. 15 (b) shows the rugged surface obtained after breaking the samples, which indicates a high interaction between HNTs and the rubber matrix as well as homogeneous blending with less agglomeration. The absence of tear lines in 10 phr of HNTs filler filled NR/EPDM composites shown in Fig. 15 (c) when compared with 8 phr filled samples shown in Fig. 15 (b) can be attributed to the cluster formation. The surface of the DMS-modified HNTs, as depicted in Fig. 15(d), exhibits a more rugged contour in contrast to the unmodified HNTs displayed in Fig. 15(b). This distinction highlights the enhanced interaction between the DMS-modified HNTs and the rubber matrix. The improved interaction results from reduced agglomeration in the DMS-modified HNTs compared to their unmodified counterparts. The decline of tear lines in 10 phr of DMS-modified HNTs shown in Fig. 15 (e) when compared with 8 phr filled samples shown in Fig. 15 (d) can be attributed to agglomeration. The surface of the APTES-modified HNTs, as illustrated in Fig. 15(f), manifests a slightly uneven topography when compared with the DMS-modified HNTs showcased in Fig. 15(d). This increased roughness in topography can be attributed to a lower agglomeration in APTES modified HNTs when compared to both unmodified and DMS modified HNTs. The sample with 10 phr of APTES-modified HNTs, shown in Fig. 15 (g), exhibits a smoother surface in comparison to the 8 phr sample showcased in Fig. 15 (f), this can be attributed to weaker interaction between the APTES-HNTs and the rubber matrix owing to the presence of larger-sized aggregates.

3.6 Crosslinking density

Crosslinking density refers to the number of chemical bonds, known as crosslinks, that are present per unit volume of a polymer material. In rubber, crosslinking density is a critical parameter that influences the material's mechanical properties, such as elasticity, strength, and hardness, as well as its abrasion resistance and chemical stability.

The observed trend of increasing crosslinking density with the addition of HNTs up to 8 phr, followed by a decrease, can be explained through a combination of factors related to the interaction between the nanotubes and the rubber matrix, as well as the influence of the filler content on the overall network structure. At lower HNTs loadings, the addition of nanotubes can promote better dispersion within the rubber matrix. This improved dispersion enhances the interaction between the HNTs and the polymer chains, facilitating crosslinking. The presence of well-dispersed HNTs can act as effective crosslinking sites, leading to an increase in crosslinking density. Up to an 8 phr loading, the HNTs can reinforce the rubber matrix and improve stress distribution. This reinforcement can lead to a more efficient load transfer between polymer chains, thereby promoting the formation of additional crosslinks and an increase in crosslinking density. As the HNTs loading continues to increase beyond the optimal range, aggregation or clustering of the nanotubes might occur. This aggregation can hinder the formation of an interconnected network structure, limiting the ability of polymer chains to form crosslinks with the nanotubes. This aggregation-induced hindrance can result in a decrease in overall crosslinking density. A higher crosslinking density leads to increased mechanical strength and toughness. While higher crosslinking generally enhances strength and stiffness, a balance needs to be struck to maintain elasticity and flexibility. Proper crosslinking can provide elasticity while preventing excessive deformation. A densely crosslinked network between modified HNTs and NR/EPDM vulcanizates enhances the NR/EPDM blend material's resistance to wear and abrasion. The interconnected structure prevents the blend from breaking apart or undergoing significant surface damage when subjected to friction and abrasive forces.

In conclusion, this study aimed to evaluate the suitability of APTES-modified, DMS-modified HNTs, and unmodified HNTs as fillers in NR/EPDM nanocomposites. The investigation unveiled that the incorporation of APTES modified HNTs yields significant enhancements in curing behaviors, mechanical characteristics, and abrasion resistance of NR/EPDM nanocomposites. The findings of this study have led to the following key conclusions:

The minimum torque, maximum torque, delta torque, and cure rate index exhibit an ascending trend with the increase of HNTs loading. However, scorch and optimum cure time demonstrate a declining pattern. This behavior can be attributed to the improved cross-linking efficiency facilitated by the presence of HNTs, which accelerates curing kinetics and enhances mechanical properties.

Tensile strength, elongation at break, and stress at 100% elongation increase with the augmentation of HNTs loading up to 8 phr, after which they start to decrease. This phenomenon is attributed to the optimal reinforcement effect up to 8 phr, beyond which the excessive filler content may lead to aggregation or hindered polymer-filler interaction. Notably, at 8 phr of APTES-HNTs, tensile strength, elongation at break, and stress at 100% elongation could potentially experience remarkable improvements of approximately 112%, 31%, and 66%, respectively.

Tear strength and hardness demonstrate an upward trajectory with increasing HNTs loading due to the fortifying influence of the HNTs within the rubber matrix. Notably, at 8 phr of APTES-HNTs, both tear strength and hardness could experience significant enhancements of around 48% and 16%, respectively.

Rebound resilience displays a diminishing trend as HNTs loading increases. This behavior can be attributed to the alterations in the viscoelastic properties of the nanocomposites introduced by the HNTs, resulting in reduced energy dissipation during deformation.

Abrasion resistance displays an improvement with escalating HNTs loading owing to the reinforcement provided by the HNTs against abrasive forces. Notably, the addition of 10 phr of APTES-HNTs leads to a substantial reduction of 36% in abrasion loss when compared to base vulcanizates.

Overall, APTES surface treatment emerges as the most influential factor in shaping the cure behaviors, mechanical properties, and abrasion resistance of NR/EPDM nanocomposites. APTES-modified HNTs consistently outperform DMS-modified HNTs and unmodified HNTs across all loading levels, affirming its superior effectiveness in enhancing the performance of the NR/EPDM nanocomposites.

i. Ethics approval

This study was approved by the Department of Mechanical Engineering, SA Engineering College, Chennai, Tamilnadu, India.

ii. Consent to participate

No human subjects or animals were used in this study.

iii. Consent for publication

The information within this manuscript is entirely original and has not been previously published or copyrighted. Furthermore, the content presented in this document is not currently being reviewed for publication in any other source.

iv. Availability of data and materials

Not Applicable

v. Competing interests

All authors have not declared any conflict of interest.

vi. Funding

Not applicable

vii. Authors' contributions

Dr. S. Vishvanathperumal – Experimental work and prepared all figures was done

Mr. J. Vasanthe Roy – Supervise the overall work and Removal of plagiarism from the main manuscript.

Dr. G. Anand, Dr. S. Praveen Kumar & Mr. Ramu K.N – Wrote the main manuscript text.

And then all authors reviewed the main manuscript.

viii. Acknowledgements

Not applicable

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No competing interests reported.

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An Investigation on the Effect of the Surface Modifications and HNTs loading on the Cure behaviours, Abrasion Resistance, Mechanical and Morphological Properties of NR/EPDM

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Journal Publication

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Abstract

Figures

1. Introduction

2. Experimental

2.1. Materials

2.2 Preparation of DMS-HNTs

2.3 Preparation of APTES-HNTs

2.4 Characterization of the modified HNTs Fillers

2.5. Preparation of Nanocomposites

2.6. Characterization

3. Results and discussions

3.1 Filler characterization

3.2 Cure characteristics

3.2.1 Minimum torque

3.2.2 Maximum torque

3.2.3 Delta torque

3.2.4 Scorch and optimum cure time

3.2.5 Cure rate index

3.3 Mechanical properties

3.3.1 Tensile strength

3.3.2 Elongation at break and stress at 100% elongation

3.3.3 Hardness, Tear Strength & Rebound Resilience

3.4 Abrasion resistance

3.5 FESEM morphology

3.6 Crosslinking density

4. Conclusions

Declarations

References

Additional Declarations

Status:

Journal Publication

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