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Article

Enhancement of Ethylene-Butene Terpolymer Performance via Carbon Nanotube-Induced Nanodispersion of Montmorillonite Layers

by
Li Zhang
1,*,
Jianming Liu
2,
Duanjiao Li
2,
Wenxing Sun
2,
Zhi Li
1,
Yongchao Liang
2,
Qiang Fu
1,
Nian Tang
1,
Bo Zhang
3,
Fei Huang
3,
Xuelian Fan
3,
Yuansi Wei
3,
Pengxiang Bai
3 and
Yuqi Wang
3,*
1
Guangdong Provincial Key Laboratory of Electric Power Equipment Reliability, Electric Power Research Institute of Guangdong Power Grid Co., Ltd., Guangzhou 510080, China
2
Guangdong Power Grid Co., Ltd., Guangzhou 510080, China
3
Advanced Sealing Technology Innovation Center, Guangzhou Mechanical Engineering Research Institute Co., Ltd., Guangzhou 510700, China
*
Authors to whom correspondence should be addressed.
Crystals 2025, 15(7), 612; https://doi.org/10.3390/cryst15070612
Submission received: 2 June 2025 / Revised: 28 June 2025 / Accepted: 29 June 2025 / Published: 30 June 2025
Browse Figures
Versions Notes

Abstract

In this study, the enhancement mechanism of the nano-dispersion of stearic acid-modified montmorillonite (SMMT) induced by carbon nanotubes (CNTs) in ethylene-butene terpolymer (EBT) was comprehensively investigated, and the regulation effect of composite fillers on EBT properties was revealed. Scanning electron microscopy (SEM) confirmed that SMMT achieved homogeneous nanoscale dispersion after CNT addition, and the size of aggregates was greatly reduced. Four-cycle strain-scanning analysis revealed a 200% increase in rubber–filler (R-F) interaction strength due to CNT incorporation. At the optimal CNT/SMMT ratio of 1:5, the EBT composites exhibited a 40.4% increase in Young’s modulus, 71.4% enhancement in tensile strength, and maintained 250% elongation at break, effectively addressing the strength–toughness trade-off of traditional rigid fillers. Thermogravimetric analysis (TGA) showed near 20 °C elevation in EBT composites’ maximum decomposition temperature, while water contact angle measurements indicated a hydrophobicity increase to 117.5° and water absorption rate below 0.2%. The quantitative improvement in thermal oxidation stability and hydrophobic barrier performance was achieved simultaneously.

1. Introduction

With the increasingly complex service environments of electrical equipment, the performance requirements for rubber seals have become more demanding. For example, rubber seals need to simultaneously meet mechanical stability and creep resistance requirements across a wide temperature range. While traditional ethylene propylene diene monomer (EPDM) rubber can improve low-temperature performance by adding low-temperature modifiers (such as plasticizers), the introduction of these additives can reduce the mechanical and heat-resistance properties of EPDM, creating a performance contradiction between low-temperature toughness and high-temperature strength [1,2,3].
Ethylene-butene-terpolymer (EBT) is a novel terpolymer developed by Mitsui Chemicals using metallocene catalyst technology. By introducing butene monomers into the molecular backbone, it effectively enhances the flexibility of the molecular chains. Research by Xie and others has shown that, compared to EPDM, EBT exhibits superior processing performance, low-temperature resistance, and adhesion properties [4]. A ternary composite material of EPDM/polybutadiene/EBT developed by Xiao [5] further achieves excellent cold resistance (Tg = −68.9 °C), with low- and high-temperature compression set values of 14% and 17%, respectively. However, the relatively low content of the third monomer (ENB) in the EBT molecular chain results in a lower crosslinking density compared to traditional EPDM, leading to a reduction in mechanical properties. At the same time, its highly flexible molecular chains are prone to irreversible sliding under stress, further diminishing mechanical properties. This makes it challenging for EBT to meet the application requirements in fields such as electrical equipment seals, thus becoming a key bottleneck limiting its engineering application.
Nanoreinforcement technology offers a promising new pathway to overcome the mechanical bottlenecks of EBT. Among various nanomaterials, silicate and carbon-based nanomaterials, including montmorillonite (MMT) and carbon nanotubes (CNTs), remain the most widely studied polymer reinforcement materials [6,7,8,9,10,11]. CNTs, as representative 1D nanomaterials, possess an axial tensile strength exceeding 100 GPa and an elastic modulus of 1 TPa, theoretically enabling the construction of an efficient stress transfer network [12,13]. Both theoretical and experimental studies have demonstrated that CNTs are ideal nanoreinforcements, significantly improving the mechanical and functional properties of polymer composites [14,15,16,17]. However, the strong van der Waals forces between CNTs lead to agglomeration within the matrix, and the high specific surface area induces interface friction losses, which limit their reinforcing effectiveness [18,19]. Montmorillonite, as a typical 2D layered silicate, has an intrinsic modulus of 175 GPa and a high aspect ratio (1 nm thickness/1–2 μm diameter), but its hydrophilic nature makes it difficult to disperse in non-polar rubbers [20,21,22,23,24,25]. Recent studies suggest that constructing a 0D (e.g., carbon black, titanium dioxide, nanodiamonds, alumina microspheres…)/1D (e.g., carbon nanotubes, carbon fibers, silicon nanowires…)/2D (e.g., graphene, boron nitride, molybdenum disulfide, MXene…) multi-scale hybrid filler system can leverage synergistic effects to achieve better dispersion and enhance filler–matrix interactions. This forms a three-dimensional network structure with a high contact area with the polymer matrix, maximizing stress transfer efficiency and overcoming the performance limitations of single fillers [26,27,28,29,30]. Sun [31,32,33] and others have studied the interactions between multi-walled carbon nanotubes (MWCNTs) and various clay minerals. Research by Ivanoska-Dacikj [34,35] et al. shows that the presence of MMT significantly enhances the dispersion of CNTs in water. Rooj [36] et al. reported that the dispersion of MWCNTs in NR was significantly improved with the help of organic montmorillonite.
In our previous work, MWCNT and silicon carbide were employed to improve the modulus, wear resistance, and heat resistance of EBT rubber [37]. However, this modification led to a loss of elongation at break, accompanied by limitations such as non-uniform crosslinking networks and poor medium resistance. This study investigates the modification effects of stearic acid-modified montmorillonite (SMMT)/MWCNT hybrids on EBT rubber. The reinforcing mechanism of nano-fillers relies on three core mechanisms: effective dispersion, interfacial load transfer, and network construction. Specifically, the dispersion determines the effective specific surface area of fillers and stress transfer paths; the crosslink density directly affects the network stiffness and restriction of molecular chain movement; the interfacial bonding strength dominates the load transfer efficiency and energy dissipation. These factors constitute key variables for enhancing macroscopic properties. Based on this, the study focuses on examining the influences of filler dispersion, filler–rubber interfacial interaction, and crosslinking network on EBT properties. It aims to break through technical bottlenecks such as strength–toughness imbalance, poor interfacial compatibility, and insufficient environmental resistance in EBT reinforcement, providing technical support for functional materials in industrial equipment, infrastructure sealing and protection, and extreme environment applications.

2. Methods and Materials

2.1. Materials

Ethylene-butene rubber (grade K9330M, with 50% by mass ethylene and 7.1% by mass third monomer) was purchased from Mitsui Chemicals, Japan. Multi-walled carbon nanotubes (MWCNTs), with an outer diameter of 5~15 nm, length of 10~30 μm, specific surface area of 220~300 m2/g, and ash content less than 2 wt%, were purchased from Zhongke Shidai Nano Co., Ltd. (Chengdu, China). Montmorillonite (MMT) was obtained from Jiangsu Xianfeng Nano Materials Technology Co., Ltd. (Nanjing, China). The other reagents, including zinc oxide (ZnO, purity ≥ 99.7%), stearic acid (SA, purity ≥ 90%), sulfur (purity ≥ 99.5%), and tetramethylthiuram disulfide (TMTD, purity ≥ 97%), were purchased from Xinji Metal Co., Ltd. (Changshu, China), Jiangsu Ruiba New Material Technology Co., Ltd. (Lianyungang, China), and Kemaiwei New Material Co., Ltd. (Zhuhai, China), respectively. In this paper, a MWCNT is referred to as a CNT.

2.2. Preparation of Modified SMMT

MMT was expanded by intercalating SA into its interlayer space. Specifically, 2 g SA was placed in a beaker and heated until melting into liquid phase on a heating plate at 75 °C. Then, 10 g MMT was added to a mortar, and the molten SA was slowly added to the MMT while grinding continuously for approximately 30 min until a uniform mixture was achieved.

2.3. Preparation of EBT Composites

A measure of 100 phr of EBT was placed in an open mixer with a roll speed set at 16 r/min and mixed for 5 min. According to the formulations in Table 1, the corresponding amounts of CNT and SMMT, 5 phr ZnO, and 2 phr SA were added, followed by 1.5 phr sulfur and 2 phr TMTD, and mixed for 20–25 min. The initial roll gap was set at 0.3 mm to facilitate heat dissipation and adjusted to 3 mm when adding fillers and vulcanizing agent. After that, the component dispersion was optimized through alternating cutting and two cycles of triangle-bag treatment. Finally, the composite was passed through the roller four times at a nip gap of 0.2 mm, and then the nip gap was set to 3 mm to form the sheet. Vulcanization was carried out in a hydraulic press at 160 °C and 10 MPa.
The vulcanization characteristics of the EBT composites were studied using a rotorless rheometer (MDR), where the scorch time (Tc10), optimal vulcanization time (Tc90), minimum torque (ML), and maximum torque (MH) were determined. The rubber mixture was then vulcanized into samples using a hydraulic press (Jiaxin Electronic Equipment Technology (Shenzhen) Co., Ltd.) for subsequent testing.

2.4. Characterization

In accordance with the requirements of GB/T 16584-1996 [38], vulcanization characteristics were tested using a rotorless rheometer (MM4310C, Huanfeng Chemical, Beijing, China) at 160 °C for a duration of 25 min. Shore hardness was measured on the vulcanized rubber at room temperature using a Shore hardness meter (Yizhong Precision Instruments Co., Shanghai, China) following GB/T 531.1-2008 [39]. Five measurements were taken at different positions on each sample, and the average value was used to minimize errors. Tensile strength and elongation at break were tested using a universal materials testing machine (Zwick Roell Z010, Zwick, Ulm, Germany) in accordance with GB/T 528-2009 [40]. Testing was conducted at a constant speed of 500 mm/min, with each test repeated three times (n = 3), and the instrument accuracy was within ±2%. The median value of the three replicates was used to construct the stress–strain curve for the material.
To investigate the effects of filler dispersion, filler–rubber interfacial interaction, and crosslinking network on material properties, X-ray diffraction (XRD, Bruker D8 Advance, Karlsruhe, Germany) was used to characterize the interlayer spacing changes of fillers before and after modification. Scanning electron microscopy (SEM, JEOL 7600F, Tokyo, Japan) was employed to observe the micro-dispersion morphology of fillers in the EBT matrix. Dynamic rubber processing analysis (D-RPA 3000, Tianjin Mengtai Trading Co., Ltd., Tianjin, China) was conducted to quantify the interaction strength between fillers. The strain scanning conditions were set at 1 Hz with a strain range of 0.56% to 100%, and four-cycle consecutive scans were performed for each sample. Dynamic mechanical analysis (DMA) was performed using a dynamic analyzer (NEXTA DMA200, Hitachi Analytical Instruments Co., Shanghai, China), following GB/T 40396-2021 [41], to analyze the dynamic mechanical properties of the blend. The sample dimensions were 25 × 12 × 2 mm3, and the analysis was conducted at a constant frequency of 10 Hz with a strain amplitude of 30 μm over a temperature range of −60 to 40 °C at a heating rate of 3 °C/min in a single cantilever mode. The storage modulus, loss modulus, glass transition temperature (Tg), and dynamic loss factor (tanδ) were determined from the analysis.
The equilibrium solvent swelling measurements were conducted in cyclohexane at 25 °C. Samples were cut into disk shapes with a diameter of 25 mm and a thickness of 2 mm, with three disks used for each sample. The samples were dried at 60 °C for 24 h and then weighed (m0). After drying, the samples were immersed in a dark-colored bottle containing cyclohexane. The immersion time was 72 h, after which the samples reached equilibrium and were weighed again (m1). The samples were then dried to constant weight, denoted as m2. The crosslinking density (νe) was determined using the Flory–Rehner equation [42]:
v e = − ln 1 − V r + V r + χ V r 2 V s ( V r 3 − V r 2 )
where χ describes the Flory–Huggins polymer–solvent interaction parameter; Vs is the molar volume of the solvent, and the molar volume of cyclohexane is 82.89 g/mol; Vr is the volume fraction of the sample at swelling equilibrium, with the unit of g/mol, which is calculated by Equation (2):
V r = m 2 ρ r m 2 ρ r + m 1 − m 2 ρ s
where ρs is the solvent density and ρr is the rubber density, with the unit of g/cm3; m1 denotes the mass of the swollen vulcanizate, and m2 denotes the mass after deswelling, with the unit of g.

3. Results and Discussion

3.1. Structure and Morphology

Figure 1 shows the XRD pattern of stearic acid-modified MMT in the 2θ range of 2° to 10°. It can be observed that MMT exhibits a diffraction peak at 2θ = 7.14°. According to Bragg’s diffraction equation, 2dsinθ = nλ (where d is the interlayer spacing of the MMT; θ is the incident angle; λ is the wavelength of the X-ray; n is the diffraction order), the interlayer spacing of the MMT is approximately 1.21 nm. After stearic acid treatment, the diffraction angle of SMMT shifts, indicating an increase in the interlayer spacing. This shift is attributed to the presence of stearic acid between the layers of SMMT.
In Figure 2a, the surface of the pure EBT composite material is dense and smooth, with no obvious defects. Figure 2b shows that after the addition of SMMT, agglomerates of SMMT with uneven size and distribution appear in the composite material, and the interface of these agglomerates is clearly visible, indicating weak interfacial interactions between SMMT and the EBT matrix. As shown in Figure 2c, after introducing CNTs, the agglomerates disappear, and SMMT is evenly embedded within the EBT matrix, with the SMMT-EBT interface becoming blurred. However, when the SMMT content is increased to 20 phr, some agglomerates reappear in the composite material (Figure 2d). These results suggest that the strong interfacial interaction between SMMT and EBT is due to the expansion of the interlayer spacing of SMMT caused by stearic acid intercalation, and the synergistic effect of CNTs significantly improves the dispersion and interfacial compatibility of SMMT in the rubber matrix. However, when the SMMT content is too high, the dispersion ability of CNTs is insufficient, leading to partial aggregation of the filler. EDS mapping of Si elements (Figure 2e–g) was used for quantitative visual analysis to assess the uniformity of the SMMT dispersion in the rubber. The analysis results show that the distribution of Si elements is consistent with the microstructural features observed in SEM, proving that CNTs enhance the reliability of SMMT dispersion.

3.2. Vulcanization Performances

MMT, as a layered nanoclay, has a large specific surface area and surface activity due to its sheet structure, which can adsorb sulfur and reduce the activation energy of the crosslinking reaction. Therefore, understanding the effect of SMMT on crosslinking and curing in EBT composites is crucial. Figure 3a and Table 2 present the vulcanization curves and parameters obtained for EBT composites at 160 °C. The results show that the addition of SMMT and CNT/SMMT increases the minimum torque (ML), maximum torque (MH), and torque difference (MH-ML) of the EBT composites. The change in ML is related to viscosity. CNT and SMMT dispersed in the rubber matrix occupy some of the free volume, reducing the available space for rubber molecular chains, which leads to a decrease in flowability. After vulcanization, the crosslinking density and network strength of the rubber increase, resulting in an increase in MH and thereby expanding the MH-ML difference.
Furthermore, after the addition of SMMT, the vulcanization time of the EBT composite is shortened. This is attributed to the long-chain alkyl groups of stearic acid inserted between the MMT layers, which expand the interlayer spacing, reduce surface polarity, and significantly enhance the compatibility of MMT with EBT rubber under the synergistic effect of CNTs. This improves the uniform dispersion of MMT in the rubber matrix, providing more efficient transmission channels and enrichment sites for the vulcanizing agents (sulfur, accelerators, etc.), which accelerates the diffusion of the vulcanizing agents into the rubber molecular chains, shortening the induction period and increasing the reaction rate. However, the filler also increases the resistance to molecular chain movement, as the agglomerates hinder the diffusion of the rubber chains and reduce the effective vulcanization sites, which ultimately results in a longer time to reach the optimal vulcanization state (an increase in the optimum vulcanization time).
Figure 3b illustrates the variation trend in the crosslink density of EBT composites. The crosslink density of EBT composites increases with the addition of CNT, SMMT, and CNT/SMMT fillers. This is because the intercalation of stearic acid in SMMT expands the interlayer spacing, making the surface active sites more accessible and thereby promoting the vulcanization reaction and enhancing the crosslink density of EBT rubber. Meanwhile, the oxygen-containing groups (such as carboxyl and hydroxyl groups) on the surface of CNTs can enrich the vulcanization accelerators (such as TMTD) and sulfur radicals, acting as catalytic sites to accelerate the vulcanization reaction. The high aspect ratio of CNTs can also enhance the entanglement and constraint between molecular chains, further increasing the crosslink density. When CNTs are combined with SMMT, they facilitate the uniform dispersion of SMMT in the rubber matrix and enrich the vulcanizing agents, generating a synergistic effect with the catalytic crosslinking reaction of SMMT and significantly improving the vulcanization efficiency of the composite system. In summary, the EBT/CNT/SMMT composite exhibits a higher crosslink density and stronger interfacial bonding force (as shown in Figure 2c,d).

3.3. R-F and F-F Interaction Analysis

The interaction between fillers (F-F) plays a crucial role in determining the aggregation behavior of fillers, directly impacting the mechanical strength and toughness of the composite material. The interaction between fillers and rubber (F-R) enhances the adhesion between the fillers and the matrix, thereby improving the mechanical properties. Achieving an optimal balance between F-F and F-R interactions is essential for optimizing the performance of the composite material. In this study, rubber processing analysis was conducted on different formulations of EBT composites using four-cycle strain scans. At low-strain levels, the modulus difference between the first and subsequent scans is primarily attributed to the destruction of filler agglomerates. The filler network is disrupted during the first scan and does not fully recover, resulting in a decrease in modulus between the first and second scans. The term Δ(G′(1)–G′(4)) is introduced to represent the F-F interaction in the EBT composite, where the numbers in parentheses refer to the scan count [43]. Similarly, Δ(G′(4)–G′(0)) describes the F-R interaction in the EBT composite, where G′(0) represents the modulus of the EBT without fillers at the initial strain amplitude [43].
As shown in Figure 4, with the increase in filler volume fraction in EBT composites, the F-F interaction gradually strengthens. The introduction of CNT significantly enhances the R-F interaction, reaching a peak in the EBT/CNT/SMMT-10 sample. This is because the organic modification of montmorillonite (SMMT) with stearic acid improves compatibility with the EBT matrix, but the limited interaction between the single filler and the rubber matrix results in a weaker R-F interaction. When CNTs are added to EBT/SMMT composites, the high aspect ratio structure of CNTs forms a filler contact network with SMMT layers through physical entanglement, enhancing the F-F interaction. Simultaneously, the incorporation of CNTs improves the dispersion of SMMT in the rubber matrix. Moreover, the carboxyl and hydroxyl groups on the CNT surface interact with the organic moieties of SMMT (e.g., long-chain alkyl groups and carboxyl groups) via hydrophobic interactions and van der Waals forces, thereby enhancing the interfacial compatibility with the nonpolar EBT rubber and significantly strengthening the F-R interactions.
Further increasing the SMMT content to 20 phr leads to a higher filler volume concentration, which increases the layer-stacking density. This results in a denser three-dimensional filler network formed by CNTs and SMMT. While the F-F interaction continues to strengthen, the excessive SMMT causes the formation of local agglomerates, reducing the effective interface area and slightly diminishing the R-F interaction. This suggests that filler overload may weaken the reinforcement efficiency of the rubber matrix due to agglomeration effects. In conclusion, CNT adjusts the F-R interaction strength by improving filler dispersion and interface interaction, and the rational control of the CNT:SMMT ratio can effectively balance the F-F and R-F interactions, leading to the optimized design of the composite material’s performance.

3.4. Mechanical Properties

The influence of different CNT/SMMT composite filler ratios on the mechanical properties of EBT composites was investigated under conditions where no additional fillers were introduced into the base formulation, as shown in Figure 5. For the EPDM/SMMT, EPDM/CNT/SMMT-10, and EPDM/CNT/SMMT-20 systems, the Young’s modulus exhibited an increasing trend (Figure 5b), due to the enhanced F-F interactions with increasing filler volume fraction, which form a denser, more rigid network, effectively improving the material’s stiffness. Both the tensile strength and elongation at break also increased gradually, peaking in the EPDM/CNT/SMMT-10 system (Figure 5c,d). In this system, the tensile strength increased by 71.4%, and the elongation at break improved by 57.0%. CNTs promote the dispersion of SMMT and improve the interfacial bonding, while the interpenetrating network structure formed by CNTs (1D) and SMMT (2D) enhances the modulus and strength through a bridging-restriction effect, a mechanism widely validated in polymer composites [44,45,46]. Additionally, the CNT/SMMT composite system promotes a more uniform vulcanization process, forming a more stable crosslinked network, which ensures the material can withstand greater deformation without fracturing during stretching. The stearic acid intercalation modification of SMMT also optimizes the interface, allowing the layers to slide and enhancing toughness. However, CNTs restrict chain segment sliding and orientation, reducing material ductility, and at higher CNT concentrations, aggregation can lead to early crack formation. Therefore, the CNT content was limited to 2 phr.
In the EPDM/CNT/SMMT-20 system, the limited dispersion ability of the small amount of CNTs and the excessive amount of SMMT led to aggregation, forming stress concentration points. Additionally, excessive adsorption of the curing agents reduced the crosslink density, resulting in a decline in tensile strength. Furthermore, the aggregated fillers restricted the chain segments, making them difficult to stretch, leading to a decrease in the flexibility of EPDM/CNT/SMMT-20. The results indicate that the filler content and dispersion state significantly affect the material’s stiffness, toughness, strength, and ductility through their impact on interfacial interactions and crosslinking structure. Filler synergy can optimize the comprehensive performance, while excessive filler-induced aggregation can compromise the material’s uniformity and degrade its performance.

3.5. Dynamic Mechanical Analysis (DMA)

Dynamic mechanical analysis (DMA) helps to understand the reinforcement effect of hybrid fillers on the EBT rubber matrix. Figure 6a,b show the variation in the storage modulus (E′) and tan δ value of the EBT composites with temperature. In the rubber plateau region, compared to the unfilled EBT, the EBT/CNT/SMMT composites exhibit a significant increase in E′, with the EBT/CNT/SMMT composites showing higher storage moduli than EBT/SMMT composites. This indicates that in the EBT/CNT/SMMT system, CNTs and SMMT are well dispersed throughout the rubber matrix, generating a strong R-F interaction, and the filler network structure is also enhanced, as previously confirmed in Figure 4. This increasing trend continues with the rising SMMT content.
Furthermore, the study of the tan δ-temperature dependence shows that the tan δ peak values of the EBT/CNT/SMMT composites are lower compared to the unfilled EBT rubber. This reduction can be attributed to several factors, including the enhanced rigidity of the fillers, their uniform dispersion, strengthened interface bonding, and optimized crosslinking network. These factors effectively restrict the molecular chain movement, thereby reducing energy dissipation. Additionally, after the incorporation of EBT/CNT/SMMT, the glass transition temperature (Tg) of the EBT composites slightly increases. This is because the improved interface interactions reduce the energy required for molecular chain relaxation, thus limiting segmental movement.

3.6. Thermogravimetric Analysis (TGA)

Thermogravimetric analysis (TGA) was used to investigate the effect of CNTs and SMMT on the thermal stability of EBT composites. As shown in Figure 7, the introduction of CNTs and SMMT enhances the thermal decomposition temperature of the EBT composites. Specifically, in the EBT/CNT/SMMT composite system, the maximum mass decomposition temperature of EBT rubber increased from 486.2 °C of pure EBT rubber to 499.6 °C (EBT/CNT/SMMT-10) and 505.8 °C (EBT/CNT/SMMT-20), and the maximum mass decomposition rate slowed down. The improvement is attributed to the synergistic barrier effect created by CNT and SMMT within the rubber matrix, which effectively hinders the transfer of heat and degradation products, thus delaying the thermal decomposition process. Additionally, the increased crosslinking density and interface interactions, along with improved filler dispersion within the rubber matrix, further suppress high-temperature degradation behavior, collectively enhancing the thermal stability of the composite material.

3.7. Surface Wettability and Water Absorption Performance Tests

Figure 8 shows the water contact angle test results of EBT composites, revealing the regulation law of SMMT/CNT composite fillers on the surface hydrophobicity of EBT composites. The introduction of single-component SMMT increases the contact angle of EBT/SMMT composites by 6.5° compared with the blank vulcanizate, indicating that the long-chain fatty acid-modified layer of SMMT preliminarily improves the surface hydrophobic performance through a physical barrier effect. When CNT/SMMT composite fillers are introduced, the contact angle shows a significant increasing trend, among which the contact angle of the EBT/CNT/SMMT-10 sample increases by 29.4°, showing the most remarkable hydrophobic optimization effect. This phenomenon is attributed to the dispersion-enhancing effect of CNTs on SMMT: the nanobridging network formed by CNTs with high aspect ratio effectively inhibits the agglomeration tendency of SMMT, enabling its nanoscale uniform dispersion in the rubber matrix (as observed by SEM in Figure 2), thereby maximizing the exposure of hydrophobic alkyl chains on the surface of SMMT. However, when the CNT/SMMT ratio exceeds the critical value, the interaction between excessive fillers leads to a decrease in dispersion and an increase in interfacial defects, which instead cause the contact angle to decline and the hydrophobic performance to weaken.
As shown in Figure 9, the dynamic water absorption curves of EBT composites after immersion in boiling water for 12 h are presented. Pure EBT exhibits water absorption behavior in boiling water, reflecting the permeability of water molecules due to the relaxation of its molecular chains at high temperatures. The equilibrium water absorption rate of the single-component EBT/SMMT composite is lower than that of pure EBT, which can be attributed to the physical barrier effect of SMMT, but obvious water absorption still occurs. In contrast, the EBT/CNT/SMMT-10 sample shows significantly reduced water absorption, with a gentle growth rate throughout the immersion period. For the EBT/CNT/SMMT-20 sample, the excessive filler ratio leads to aggravated agglomeration, causing the water absorption rate to rise again. In summary, the EBT/CNT/SMMT-10 composite significantly enhances the barrier performance in high-temperature liquid environments by optimizing filler dispersion and interfacial bonding.

4. Conclusions

  • This study investigates the performance regulation of EBT rubber-based composites with CNT/SMMT hybrid fillers. Experiments show that when the CNT/SMMT mass ratio is optimized to 1:5, the comprehensive properties of EBT composites are improved in multiple dimensions: Young’s modulus increases by 40.4%, tensile strength by 71.4%, and elongation at break remains at 250%, addressing the strength–toughness trade-off of traditional rigid fillers. In terms of thermal stability, the maximum decomposition temperature rises from 486.2 °C to 505.8 °C, while environmental resistance is significantly enhanced, with the water contact angle increasing from 88.1° to 117.5° and water absorption rate dropping below 0.2%. The study also indicates that an excessive SMMT:CNT ratio leads to filler aggregation, deteriorating the properties of EBT composites.
  • SEM observations and four-cycle strain-scanning analysis reveal that the introduction of CNTs significantly improves the dispersion uniformity and interfacial compatibility of SMMT in the EBT matrix, enhancing F-F and F-R interactions. This promotes the formation of a denser rigid network and a 1D CNT/2D SMMT interpenetrating network, boosting the modulus and strength of EBT composites. Meanwhile, CNT/SMMT hybrid fillers facilitate uniform vulcanization reactions, forming a more stable crosslinked network and further optimizing network load-bearing capacity. Additionally, the “molecular spring” effect from stearic acid intercalation endows the material with unique strength-ductility synergy, maintaining elongation at break through lamellar sliding while increasing modulus.
  • This material system demonstrates clear application values in fields such as building waterproof sealing materials (adapting to structural deformation and resisting rainwater penetration), automotive component seals (withstanding high temperatures in engine compartments and reducing costs), protective casings for outdoor electronic devices (resisting performance degradation in humid-heat environments), and moisture-proof encapsulation for transformers (combining moisture resistance and heat tolerance).

Author Contributions

Conceptualization, L.Z. and Y.W. (Yuqi Wang); data curation, L.Z.; formal analysis, J.L., D.L., P.B. and Y.W. (Yuqi Wang); funding acquisition, L.Z.; investigation, W.S.; methodology, W.S. and Z.L.; project administration, Y.L.; resources, Q.F. and N.T.; software, B.Z. and F.H.; supervision, P.B.; visualization, X.F. and Y.W. (Yuansi Wei); writing—original draft, L.Z.; writing—review and editing, Y.W. (Yuqi Wang). All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by the Science and Technology Project of China Southern Power Grid Co., Ltd. (Development of Key Materials for Highly Sealed Outdoor Equipment Boxes for Substations and Demonstration Project Application Research, GDKJXM20230771).

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
EPDMEthylene propylene diene monomer
EBTEthylene-butene-terpolymer
CNTsCarbon nanotubes
MWCNTMulti-walled carbon nanotube
MMTMontmorillonite
SMMTStearic acid-modified montmorillonite
0D/1D/2DZero-dimensional/one-dimensional/three-dimensional
ZnOZinc oxide
SAStearic acid
TMTDTetramethylthiuram disulfide
MDRRotorless rheometer
Tc10Scorch time
Tc90Optimal vulcanization time
MLMinimum torque
MHMaximum torque
XRDX-ray diffraction
SEMScanning electron microscopy
D-RPADynamic rubber processing analysis
DMADynamic mechanical analysis
TGAThermogravimetric analysis
F-FFillers and Fillers
F-RFillers and Rubber

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Crystals 15 00612 g001
Figure 1. XRD curves of MMT and SMMT.
Figure 1. XRD curves of MMT and SMMT.
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Figure 2. SEM image of (a) pure EBT, (b) EBT/SMMT, (c) EBT/CNT/SMMT-10 and (d) EBT/CNT/SMMT-20, the red arrow points to the SMMT distributed in the matrix; Si mapping of (e) EBT/SMMT, (f) EBT/CNT/SMMT-10 and (g) EBT/CNT/SMMT-20.
Figure 2. SEM image of (a) pure EBT, (b) EBT/SMMT, (c) EBT/CNT/SMMT-10 and (d) EBT/CNT/SMMT-20, the red arrow points to the SMMT distributed in the matrix; Si mapping of (e) EBT/SMMT, (f) EBT/CNT/SMMT-10 and (g) EBT/CNT/SMMT-20.
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Figure 3. (a) Vulcanization curves and (b) crosslinking density of EBT composites.
Figure 3. (a) Vulcanization curves and (b) crosslinking density of EBT composites.
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Figure 4. The interactions between fillers in EBT composites: (a) EBT/SMMT, (b) EBT/CNT/SMMT-10, and (c) EBT/CNT/SMMT-20 during four-cycle consecutive strain scans (the black curve represents the sample without fillers). (d) The variation in F-F and F-R interactions for different formulations.
Figure 4. The interactions between fillers in EBT composites: (a) EBT/SMMT, (b) EBT/CNT/SMMT-10, and (c) EBT/CNT/SMMT-20 during four-cycle consecutive strain scans (the black curve represents the sample without fillers). (d) The variation in F-F and F-R interactions for different formulations.
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Figure 5. Mechanical properties of the composite: (a) stress–strain curves, (b) Young’s modulus, (c) tensile strength and (d) elongation at break.
Figure 5. Mechanical properties of the composite: (a) stress–strain curves, (b) Young’s modulus, (c) tensile strength and (d) elongation at break.
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Figure 6. (a) Storage modulus versus temperature curves and (b) Tan δ versus temperature curves obtained in DMA measurements for EBT composites.
Figure 6. (a) Storage modulus versus temperature curves and (b) Tan δ versus temperature curves obtained in DMA measurements for EBT composites.
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Figure 7. Thermal stability of different EBT composites (a) TGA and (b) DTG.
Figure 7. Thermal stability of different EBT composites (a) TGA and (b) DTG.
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Figure 8. Water contact angle: (a) pure EBT (b) EBT/SMMT (c) EBT/CNT/SMMT-10 (d) EBT/CNT/SMMT-20.
Figure 8. Water contact angle: (a) pure EBT (b) EBT/SMMT (c) EBT/CNT/SMMT-10 (d) EBT/CNT/SMMT-20.
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Figure 9. The water absorption rate of EBT composites in boiling water.
Figure 9. The water absorption rate of EBT composites in boiling water.
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Table 1. CNT/SMMT ratio in EBT composites.
Table 1. CNT/SMMT ratio in EBT composites.
Pure EBTEBT/SMMTEBT/CNT/SMMA-10EBT/CNT/SMMA-20
CNT/phr0022
SMMT/phr0101020
Table 2. Vulcanization parameters of EBT composites.
Table 2. Vulcanization parameters of EBT composites.
ML (dNm)MH (dNm)MH-ML (dNm)Tc10 (min)Tc90 (min)
Pure EBT0.1212.3012.184.0212.66
EBT/SMMT0.1613.5513.391.369.90
EBT/CNT/SMMT-100.2116.9116.701.2011.41
EBT/CNT/SMMT-200.1915.9215.730.9912.45
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MDPI and ACS Style

Zhang, L.; Liu, J.; Li, D.; Sun, W.; Li, Z.; Liang, Y.; Fu, Q.; Tang, N.; Zhang, B.; Huang, F.; et al. Enhancement of Ethylene-Butene Terpolymer Performance via Carbon Nanotube-Induced Nanodispersion of Montmorillonite Layers. Crystals 2025, 15, 612. https://doi.org/10.3390/cryst15070612

AMA Style

Zhang L, Liu J, Li D, Sun W, Li Z, Liang Y, Fu Q, Tang N, Zhang B, Huang F, et al. Enhancement of Ethylene-Butene Terpolymer Performance via Carbon Nanotube-Induced Nanodispersion of Montmorillonite Layers. Crystals. 2025; 15(7):612. https://doi.org/10.3390/cryst15070612

Chicago/Turabian Style

Zhang, Li, Jianming Liu, Duanjiao Li, Wenxing Sun, Zhi Li, Yongchao Liang, Qiang Fu, Nian Tang, Bo Zhang, Fei Huang, and et al. 2025. "Enhancement of Ethylene-Butene Terpolymer Performance via Carbon Nanotube-Induced Nanodispersion of Montmorillonite Layers" Crystals 15, no. 7: 612. https://doi.org/10.3390/cryst15070612

APA Style

Zhang, L., Liu, J., Li, D., Sun, W., Li, Z., Liang, Y., Fu, Q., Tang, N., Zhang, B., Huang, F., Fan, X., Wei, Y., Bai, P., & Wang, Y. (2025). Enhancement of Ethylene-Butene Terpolymer Performance via Carbon Nanotube-Induced Nanodispersion of Montmorillonite Layers. Crystals, 15(7), 612. https://doi.org/10.3390/cryst15070612

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