Sulfidic crosslinks in EPM: a strategy for advanced flexible EPM composites

Ethylene–propylene rubber (EPM) is a fully saturated elastomer, which prevents its crosslinking by conventional accelerated sulfur curing that requires unsaturation. In this study, a hybrid curing approach is introduced to generate sulfidic crosslinks in EPM. The method combines organic peroxide curing with sulfur-based vulcanization. Peroxide generates radicals on the EPM backbone and forms macro radicals, which can be captured by sulfur species, forming sulfur macroradicals that couple either with each other or with other EPM macro radicals to create alkyl-alkyl sulfidic crosslinks in EPM. Alongside these, the system also contains conventional alkyl–alkyl carbon crosslinks generated by peroxide. While carbon–carbon crosslinks impart excellent thermal stability and compression set resistance in EPM, they often limit flexibility and tear strength. The incorporation of sulfidic crosslinks alongside peroxide-derived linkages enhances flexibility and stress strain performance without sacrificing high temperature capability. This hybrid curing route thus provides a promising strategy for developing advanced flexible EPM composites for applications demanding contradictory property requirements and higher thermal ratings than EPDM.

1 Introduction

Ethylene-propylene rubber (EPM) possesses several intrinsic advantages over ethylene-propylene-diene monomer (EPDM), primarily owing to its fully saturated molecular backbone (Chatterjee and Naskar, 2007). Unlike EPDM, which contains diene units responsible for residual unsaturation, EPM features remarkable resistance to heat, steam, and oxidative environments traits that stem from its chemical stability and absence of vulnerable diene unit (Thomas and Groeninckx, 1999). This enhanced resistance not only ensures prolonged durability in demanding applications but also results in superior resistance to ozone, atmospheric pollutants, and weathering, allowing EPM to maintain its physical properties and appearance during extended outdoor exposure. Furthermore, EPM’s saturated structure yields better color stability, lower density for lightweight product designs, and advantageous foaming properties that support advanced flexible composite manufacturing (Brown et al., 1998). Although EPDM is favored for its ease of sulfur vulcanization and preferable processing characteristics, EPM remains the material of choice in applications where chemical inertness, long-term stability, and robust performance under high-temperature or oxidizing conditions are paramount, such as electrical insulation, steam-resistant environments, and food-grade applications (Easterbrook et al., 1987).

EPM offers unique application advantages over EPDM in environments demanding extreme chemical stability, such as the nuclear industry. The defining absence of unsaturation in EPM’s structure grants it exceptional resistance to ozone, steam, ionizing radiation, and aggressive oxidizing agents, which are crucial requirements for elastomeric components exposed to radiolytic and high-temperature conditions typical in nuclear plants and other critical infrastructure (Datta, 2004). This inherent inertness allows EPM-based materials to maintain their physical integrity and insulating properties despite prolonged exposure to radiation and corrosive media, minimizing risk of premature aging, material embrittlement, or degradation. As a result, EPM is widely adopted in nuclear sealing, gasket, and cable insulation applications where reliability and service life are paramount and chemical resistance cannot be compromised (Plaček et al., 2009). For these high-demand sectors, crosslinking of EPM is predominantly accomplished via peroxide curing due to the polymer’s lack of pendant diene functionality, which excludes effective sulfur vulcanization routes. Peroxide curing produces a saturated, thermally robust network offering superior resistance to heat, oxidation, and ionizing radiation. However, this method also introduces significant trade-offs compared to sulfur-cured elastomers. Peroxide-cured EPM compounds generally exhibit lower elasticity, flexibility, and inferior dynamic mechanical properties including reduced resilience under cyclic loading and diminished tear strength. Such compromises arise because peroxide crosslinking forms shorter, stiffer carbon-carbon and carbon-oxygen crosslinks rather than the long, flexible polysulfidic bridges provided by sulfur systems. Although the crosslinks produced are stable and withstand harsh chemical and thermal attack, they do not enable the same level of energy dissipation and mechanical flexibility desired in dynamic and highly deformable applications (Endstra and Wreesmann, 1993; Abtahi et al., 2006; Para et al., 2025; Baldwin, 1970).

During peroxide curing of EPM, the process typically initiates with hydrogen abstraction from the propylene unit of the polymer chain, generating macro-radicals on the polymer backbone. These radicals subsequently recombine to form direct carbon–carbon crosslinks, resulting in a thermally stable, oxidation-resistant network (Akiba, 1997; van Duin and Machado, 2005). However, alongside that process, when sulfur is present in the compound alongside peroxide, the cure chemistry undergoes a notable modification. The macro-radical formed on the EPM chain can transfer its reactive site to sulfur, effectively linking the sulfur atom to the polymer backbone. This mechanism enables the sulfur atom to act as a bridge either by coupling two polymer-bound sulfur radicals or by integrating with another EPM macro-radical ultimately forming sulfur-inserted, alkyl–alkyl sulfidic crosslinks between the EPM chains. This paper describes such crosslinking process as sulfur-peroxide hybrid crosslinking. Figure 1 shows various reaction (both crosslinking and scission) in EPM in presence of Peroxide and sulfur at curing temperature (Para et al., 2025; Beek et al., 2017; Parathodika et al., 2023; Parathodika and Naskar, 2024).

Figure 1

Figure 1. Schematic representation of various reactions in EPM rubber in presence of peroxide and sulfur.

This hybrid crosslinking pathway introduces unique structural and property enhancements compared to only carbon–carbon crosslinking. Indeed this pathway creates a co-existence of both carbon-carbon linkages and sulfidic linkages. The inclusion of sulfur within the crosslink structure imparts increased flexibility and improved dynamic mechanical properties, as the presence of sulfidic linkages within the network can absorb and dissipate energy more efficiently under cyclic and dynamic loads. Such synergy between peroxide and sulfur in the crosslinking process not only enhances the elasticity and toughness of EPM, but also enables tailored compound design for demanding applications where a balance between high thermal/oxidative resistance and dynamic performance is required. The resulting materials thus leverage the advantages of both curing methods, achieving properties not attainable by either peroxide or sulfur cure alone. This peroxide-induced sulfidic crosslinking strategy represents an advanced avenue for engineering next-generation, high-performance flexible EPM composites (Beek et al., 2017; Kruželák et al., 2020; Rahman Parathodika et al., 2022).

Previously studies have reported the use of sulfur–peroxide hybrid curing in rubbers such as EPDM, NBR, and SBR. In many cases, sulfur has been employed as a co-agent in peroxide curing to suppress chain scission and thereby improve the mechanical properties (Parathodika et al., 2023; Loan, 1965; Chakraborty et al., 1981). Conversely, Manik et al. investigated the use of peroxide within sulfur curing systems to gain insights into the underlying mechanisms of sulfur vulcanization (Manik, 1970; Manik and Banerjee, 1970). Our previous work has also demonstrated that hybrid curing can enhance the mechanical performance of EPDM compounds (Para et al., 2025; Parathodika et al., 2023; Parathodika and Naskar, 2024; Rahman Parathodika et al., 2022). However, very limited research exists in hybrid curing of EPM; to date, only one detailed study by Beek et al. has reported hybrid curing of EPM, showing that such systems can improve tear strength and tensile properties. Despite these findings, the dynamic mechanical behavior of hybrid-cured EPM remains largely unexplored (Beek et al., 2017).

In this work, we address this gap by formulating EPM-based compounds cured through different systems: (i) peroxide, (ii) co-agent-assisted peroxide, (iii) activated accelerated sulfur–co-agent-assisted peroxide, (iv) activated accelerated sulfur–peroxide, (v) activated sulfur–peroxide, and (vi) sulfur–peroxide hybrid curing. The properties of these compounds are systematically investigated, with a particular focus on detailed dynamic mechanical analysis, to elucidate the roles of accelerators, co-agents, and activators in sulfur–peroxide hybrid curing of EPM rubber.

In this work, this gap is addressed by formulating EPM-based compounds cured using different curing systems: (i) peroxide, (ii) co-agent-assisted peroxide, (iii) activated accelerated sulfur–co-agent-assisted peroxide, (iv) activated accelerated sulfur–peroxide, (v) activated sulfur–peroxide, and (vi) sulfur–peroxide hybrid curing. The properties of these compounds are systematically investigated to clarify the effects of hybrid curing on the rheological behavior, mechanical performance, thermal stability, and viscoelastic response of EPM rubber. In comparison to conventional sulfur vulcanization, which primarily yields flexible but thermally less stable polysulfidic networks, and peroxide vulcanization, which produces thermally stable yet relatively rigid carbon–carbon crosslinks, sulfur–peroxide hybrid curing enables the formation of a mixed crosslink network combining flexible sulfur bridges with thermally robust C–C linkages. This hybrid network is expected to offer improved processing safety and controlled cure behavior, enhanced tensile and dynamic mechanical properties, and superior heat-aging resistance relative to single-system curing. Particular emphasis is placed on detailed dynamic mechanical analysis to elucidate the roles of accelerators, co-agents, and activators in tailoring the crosslink architecture and achieving a balanced combination of mechanical resilience, thermal stability, and dynamic performance in sulfur–peroxide hybrid-cured EPM.

2 Experimental

2.1 Materials

This study utilized the ethylene–propylene rubber grade Dutral CO 38, procured from Vesalis Elastomers, San Donato Milanese (MI), Italy. Dutral CO 38 is an EPM elastomer produced via suspension polymerization using a Ziegler–Natta catalyst. It contains 72 wt% ethylene and exhibits a Mooney viscosity of 60 MU (ML (1 + 4) at 120 °C). This grade is also supplied with a premixed non-staining antioxidant. Zinc oxide (ZnO) and sulfur were obtained from Merck India Limited (Mumbai, India). Other compounding ingredients, including stearic acid and vulcanization accelerators—2-mercaptobenzothiazole (MBT) and tetrabenzylthiuram disulfide (TBzTD)—were supplied by NOCIL India Limited (Mumbai, India). Triallyl cyanurate (TAIC-70, 70% active) was sourced from Kettlitz Chemie Rennertshof (Germany). The primary crosslinking agent employed was Perkadox 14-40B-PD-S, a 40% active organic peroxide, specifically di (tert-butylperoxyisopropyl)benzene (DTBPIP), supplied by Nouryon (Amsterdam, Netherlands). N-550 grade fast extrusion furnace (FEF) carbon black was obtained from PCBL Limited (Gujarat, India). All ingredients were used as received unless susceptible to moisture absorption, in which case they were oven-dried before use. For swelling studies, cyclohexane solvent was procured from Merck India Limited (Mumbai, India). In addition to that paraffinic oil used in this study was procured from Witprol Lubricants Mumbai, India.

2.2 Methodology

The experimental work commenced with the preparation of a masterbatch comprising the elastomer, filler, and vulcanization activators, as summarized in Table 1. Mixing was carried out in a Brabender S-300 measuring mixer at 140 °C and 60 rpm. The obtained masterbatch was allowed to mature for 24 h before incorporating the crosslinking agents and curing additives. The final compounds were subjected to rheological testing to determine the optimum cure time and to analyze crosslinking kinetics. Test specimens were subsequently prepared from the compounded materials through compression molding at 170 °C. All mixing and specimen preparation steps were performed in strict accordance with ASTM D3182-21.

Table 1

Table 1. Composition of samples present in this study in parts per hundred rubber (phr).

2.3 Characterization methods

2.3.1 Crosslink network

The crosslink density of the samples was determined by equilibrium swelling experiments in cyclohexane, conducted for 72 h at 25 °C ± 2 °C using disc-shaped specimens of 2 mm thickness. The degree of swelling was evaluated by measuring the specimen mass before swelling, after solvent immersion, and after thorough drying. The polymer volume fraction in the swollen network (V_r) was calculated from the weights of the dried (m1) and swollen (m2) samples, together with the densities of the polymer (ρ1) and solvent (ρ2), according to Equation 1. The crosslink density (ν) was then obtained using the Flory–Rehner equation (Equation 2), with the molar volume of cyclohexane taken as 107.7 cm³ mol^-1 and the Flory–Huggins interaction parameter (χ) for the EPDM–cyclohexane system set to 0.38. For comparative studies of compounds with identical filler content, relative crosslink density values were evaluated without applying the Kraus correction.

${\mathrm{V}}_{\mathrm{r}}=\frac{\text{Volume}\text{of}\text{sample}}{\text{Volume}\text{of}\text{sample}+\text{Volume}\text{of}\text{solvent}}=\frac{\mathrm{m}1/\mathrm{\rho }1}{\mathrm{m}1/\mathrm{\rho }1+\mathrm{m}2/\mathrm{\rho }2}(1)$

$\mathrm{\nu }=-\frac{1}{2{\mathrm{V}}_{\mathrm{s}}}\times \frac{\ln \left(1-{\mathrm{V}}_{\mathrm{r}}\right)+{\mathrm{V}}_{\mathrm{r}}+\mathrm{\chi }{\mathrm{V}}_{\mathrm{r}}^2}{{\mathrm{V}}_{\mathrm{r}}^{1/3}-{\mathrm{V}}_{\mathrm{r}}/2}(2)$

2.3.2 Mechanical properties

Tensile properties, including tensile strength, elongation at break, and uniaxial modulus, were measured for all samples. Dumbbell-shaped specimens were punched from 2 mm thick compression-molded sheets and tested in accordance with ASTM D412 at a crosshead speed of 500 mm/min using a Hioks-Hounsfield universal testing machine (Test Equipment Ltd, Surrey, England). All measurements described in this section were carried out at ambient temperature in multiple specimens and the mean of the specimens were reported.

2.3.3 Compression set

The compression set of the samples was determined in accordance with ASTM D395-18, Method B, using molded specimens of approximately 13 mm thickness (Type 2). A compressive strain of ∼25% was applied using spacers, and the test was conducted for 70 h at various temperatures as mentioned in the results in multiple specimens and the mean of the observation is reported in the results. Compression set is calculated then using Equation 3.

$\text{Compression\hspace{0.17em}set\hspace{0.17em}}\left(\%\right)=\frac{\mathit{I}\mathit{n}\mathit{i}\mathit{t}\mathit{i}\mathit{a}\mathit{l}\mathit{t}\mathit{h}\mathit{i}\mathit{c}\mathit{k}\mathit{n}\mathit{e}\mathit{s}\mathit{s}-\mathit{F}\mathit{i}\mathit{n}\mathit{a}\mathit{l}\mathit{t}\mathit{h}\mathit{i}\mathit{c}\mathit{k}\mathit{n}\mathit{e}\mathit{s}\mathit{s}}{\mathit{I}\mathit{n}\mathit{i}\mathit{t}\mathit{i}\mathit{a}\mathit{l}\mathit{t}\mathit{h}\mathit{i}\mathit{c}\mathit{k}\mathit{n}\mathit{e}\mathit{s}\mathit{s}-\mathit{G}\mathit{a}\mathit{p}\mathit{o}\mathit{f}\mathit{s}\mathit{p}\mathit{a}\mathit{c}\mathit{e}\mathit{r}}\times 100(3)$

2.3.4 Thermogravimetric analysis (TGA)

The thermal degradation behavior of crosslinked samples (10–12 mg) was analyzed using a TGA-50 thermogravimetric analyzer (Shimadzu, Kyoto, Japan). The measurements were conducted under a nitrogen purge to maintain an inert atmosphere. Samples were placed in a platinum pan and heated from 30 °C to 600 °C at a rate of 5 °C/min, while the mass loss was continuously recorded. The procedure and subsequent calculations followed ASTM D6370.

2.3.5 Rheometric studies

Disc-shaped specimens with a volume of approximately 5 cm³ were prepared using a volumetric sample cutter. Isothermal cure studies were performed in a Monsanto Moving Die Rheometer (MDR 2000; Akron, United States) at 170 °C for 60 min with an oscillation angle of ±0.5°, in accordance with ASTM D5289. From the rheometer data, crosslinking characteristics such as optimal cure time (TC₉₀), cure rate index (CRI), and torque difference (ΔTorque) were determined.

2.3.6 DeMattia flex fatigue

The flex-fatigue resistance of the samples was evaluated using the DeMattia flexing test in accordance with ASTM D813. Rectangular specimens were prepared from compression-molded sheets and subjected to repeated flexing under a specified strain using a DeMattia flex fatigue tester (Goodrich-type). The test was carried out at ambient temperature, and the crack growth behavior was monitored at regular intervals. The resistance to flex-fatigue was quantified in terms of the number of cycles for crack initiation.

3 Results and discussion

3.1 Rheological studies

The curing profiles of the seven EPM formulations were systematically studied using a Monsanto Moving Die Rheometer, and the resulting torque–time data elucidate significant differences in crosslink development and network architecture and the same is represented in Figure 2. EPM 1, formulated with the conventional peroxide loading (6.3 phr), exhibited robust torque evolution, indicative of efficient radical-induced C–C crosslink formation and a well-developed elastomeric network. Reducing the peroxide content to 5 phr in EPM 2 resulted in a noticeable decrease in maximum torque (M_H), suggesting a lower crosslink density due to diminished radical formation; however, the optimum cure time (Tc₉₀) remained practically unchanged, implying that initiation kinetics and propagation steps were unaffected by this moderate reduction in peroxide. The incorporation of the multifunctional co-agent TAIC 70 in EPM 3 led to a marked increase in M_H (by 14 dNm relative to EPM 2), reflecting enhanced crosslink efficiency. Co-agents such as TAIC are known to facilitate radical stabilization, participate directly in network formation, and mitigate inefficient chain scission events, thereby enabling a denser and more uniform crosslinked structure. This finding underscores the influence of co-agent chemistry on both cure dynamics and the mechanical resilience of the cured network.

Figure 2

Figure 2. Rheo-plots from MDR 2000 at 170 °C.

Introduction of an activated accelerated sulfur system alongside the co-agent and peroxide in EPM 4 produced further improvement, as evidenced by increased M_H and a shortened Tc90. This behavior is characteristic of hybrid curing systems, where sulfur supplies an additional crosslinking pathway forming sulfidic bridges complementary to the carbon–carbon bonds generated by peroxide. Hybrid formulations thus exhibit accelerated cure kinetics and increased crosslink density, synergistically combining the attributes of both curing chemistries. For EPM 5, the omission of the co-agent (but retention of sulfur, accelerators and activators) maintained similar M_H values but further decreased Tc90, signifying that the accelerator package alone can greatly promote hybrid crosslinking efficiency, even without a co-agent.

EPM 6, lacking both co-agent and accelerators, displayed a modest increase in M_H, which suggests that direct interactions between peroxide and sulfur is sufficient for a dense crosslinked network, albeit with moderately retarded cure kinetics. EPM 7, containing only peroxide and sulfur, achieved the highest MH among all samples, demonstrating that sulfur can act in a co-agent–like capacity, directly engaging in peroxide-mediated crosslinking. This formulation yields the densest and stiffest network, likely due to the formation of complex sulfidic and carbon–carbon crosslinks.

Collectively, these results demonstrate that co-agents play a crucial role in peroxide-only systems by boosting crosslink density and torque development; however, in hybrid curing systems, the presence of sulfur especially in combination with peroxide substantially dominates network formation, accelerates cure, and delivers superior torque through the synergistic formation of both carbon–carbon and sulfidic crosslinks. This highlights the critical interplay between formulation components and their impact on the crosslinked structure and ultimate performance of EPM vulcanizates (Brown et al., 1998; Beek et al., 2017).

3.2 Crosslink density

The crosslink density results for the seven EPM formulations are presented in Figure 3, highlighting pronounced differences in network structure resulting from compositional variations. EPM 1, cured solely with the standard peroxide dose, shows a moderate crosslink density (88.1 mol/m³), establishing a baseline for radical-mediated C–C bond formation. Reducing the peroxide content in EPM 2 leads to a further decline in crosslink density (79.6 mol/m³), confirming that lower radical availability restricts network formation. The introduction of TAIC as a co-agent in EPM 3 substantially increases crosslink density to 114 mol/m³. This enhancement is attributable to the co-agent’s capacity to stabilize active radicals and participate in crosslink formation, which suppresses chain scission and increases network uniformity (Akiba, 1997; Babu et al., 2010; Bandzierz et al., 2019).

Figure 3

Figure 3. Crosslink density of samples through swelling.

EPM 4, he complete hybrid system, featuring both co-agent assisted peroxide and accelerated sulfur, achieves an even higher crosslink density (126 mol/m³). Here, the hybrid curing mechanism capitalizes on both sulfidic and carbon–carbon bridging, contributing synergistically to network growth. Formulations EPM 5 and EPM 6 further demonstrate the critical roles of sulfur and accelerators. EPM 5 with sulfur and accelerators but without a co-agent exhibits a crosslink density of 129 mol/m³, showing that accelerators efficiently promote network development alongside peroxide and sulfur. EPM 6, devoid of both co-agent and accelerators, registers 137 mol/m³, indicating that peroxide–sulfur hybrid alone is capable of forming a densely crosslinked matrix when allowed to proceed unhindered. EPM 7 stands out with the highest crosslink density (142 mol/m³), revealing that the presence of both peroxide and sulfur, without any additional co-agent or accelerator, maximizes crosslink formation. This likely reflects an optimal radical–sulfur interaction, yielding the stiffest and most densely crosslinked network among all formulations. It could be attributed the less peroxy radical destruction in the absence of activators and accelerators (Manik, 1970; Manik and Banerjee, 1969).

In summary, the addition of sulfur to peroxide-cured EPM systems progressively enhances the crosslink density more effectively than conventional co-agents such as TAIC 70. While co-agents improve radical efficiency and mitigate chain scission in peroxide-only systems, sulfur not only participates in hybrid curing reactions but also acts synergistically with peroxide to form additional C–C and sulfidic crosslinks. This demonstrates that sulfur plays a dual role, functioning both as a crosslinking agent and as a co-agent, thereby surpassing the effect of traditional co-agents in increasing final network strength. Although crosslink density is a critical factor in determining the mechanical and performance characteristics of EPM vulcanizates, it is not the sole determinant. The overall properties are also influenced by the spatial distribution of the crosslinks in elastomer matrix, the type of the crosslinks formed such as carbon–carbon, polysulfidic, disulfidic, or monosulfidic linkages and the presence of network imperfections or heterogeneities. These factors can significantly affect properties such as elasticity, strength, toughness, and fatigue resistance, as well as dynamic and thermal behavior. Therefore, subsequent sections of this paper explore how these various network features contribute to the development of other mechanical and performance attributes arising from the distinct crosslinking architectures established in the different EPM formulations (Prasertsri et al., 2016).

3.3 Tensile properties

Figure 4 presents a comprehensive evaluation of the mechanical performance of the seven EPM formulations, highlighting the tensile strength, elongation at break, and modulus at 300% strain. These parameters collectively provide valuable insight into how different curing chemistries and network structures govern the macroscopic properties of EPM vulcanizates. The stress–strain behavior shown in Figure 4a clearly distinguishes between the peroxide-only cured samples (EPM1 and EPM2) and the formulations incorporating co-agents and hybrid peroxide–sulfur systems (EPM3–EPM7). The peroxide-only systems exhibit comparatively lower tensile strengths and limited extensibility, reflecting the dominance of carbon–carbon crosslinks that, while thermally stable, often lack the ability to redistribute stress efficiently under deformation. In contrast, the hybrid systems demonstrate enhanced tensile strength and higher strain tolerance, suggesting that the coexistence of sulfidic linkages with carbon–carbon bonds introduces a more adaptable and energy-dissipative network (Plaček et al., 2009).

Figure 4

Figure 4. Tensile properties of the samples (a) stress-strain curve, (b) Tensile strength of the samples, (c) Elongation at break of samples, (d) Modulus at 300% elongation at ambient temperature.

Quantitative comparisons of tensile strength are provided in Figure 4b. Among all formulations, EPM6 (15.7 MPa), EPM3 (15.4 MPa), and EPM5 (15.0 MPa) achieve the highest tensile strength values. This outcome highlights the synergistic role of co-agents and accelerators, which optimize crosslink density while also tailoring the chemical architecture of the network. The strong performance of these samples suggests that tensile strength is not merely a function of the number of crosslinks formed, but also of their spatial distribution and chemical composition. It also demonstrates activated sulfur-peroxide hybrid cured samples can achieve a higher tensile strength than co-agent assisted peroxide cured samples, indicating the superiority of hybrid systems in reducing chain scission and there by improving mechanical properties (Chen et al., 2023; Wang et al., 2022).

The elongation at break data in Figure 4c further underscores the importance of network architecture. The highest elongations are observed in EPM4 (1058%) and EPM5 (1041%), both of which rely on hybrid curing strategies. The exceptional extensibility of these systems indicates that hybrid networks containing both sulfidic and carbon–carbon crosslinks are capable of sustaining large deformations without undergoing catastrophic failure. Such a balance arises because sulfidic linkages impart flexibility and chain mobility, while the carbon–carbon bonds provide stability and load-bearing capability. Conversely, EPM3 exhibits the lowest elongation at break (719%), a result consistent with its high modulus. The relatively stiff, compact network generated by the TAIC co-agent promotes efficient load transfer at small strains but restricts large-scale chain extensibility.

The modulus at 300% strain (Figure 4d) provides additional insight into stiffness and load-bearing capacity. EPM3 displays the highest modulus (5.81 MPa), reflecting the rigid and tightly packed network formed in the presence of TAIC, which is known to promote efficient crosslink formation and shorter average chain lengths between crosslinks. In contrast, hybrid formulations such as EPM5–EPM7 exhibit intermediate modulus values (3.87–4.04 MPa), representing a balance between stiffness and elasticity. This balance can be attributed to the spatial arrangement and chemical diversity of the crosslinks, which together modulate the overall deformation response of the elastomer. Taken together, these findings reinforce that the mechanical performance of EPM vulcanizates cannot be rationalized solely on the basis of crosslink density. Instead, it is the interplay between crosslink density, crosslink type (carbon–carbon, polysulfidic, disulfidic, monosulfidic), and spatial distribution that governs macroscopic outcomes such as strength, elongation, and modulus. Furthermore, the presence of network defects, heterogeneities, and variations in crosslink efficiency can significantly influence stress dissipation and failure behavior. The ability to fine-tune these structural attributes through formulation design such as the selection of co-agents and hybrid curing strategies enables the development of EPM materials with tailored property profiles suitable for diverse application requirements. The subsequent sections of this work will expand upon these observations by correlating the network structure with additional functional and dynamic mechanical properties, thereby elucidating how formulation-driven modifications to crosslink architecture translate into performance advantages in application-specific contexts (Wang et al., 2022; Lu et al., 1996).

3.4 Other mechanical properties

In addition to tensile performance, hardness and compression set are critical indices of elastomer reliability in service conditions. Figure 5 summarizes the hardness (Shore A) and compression set values of the seven EPM formulations at 70 °C, 100 °C, and 130 °C. These data provide valuable insight into how different crosslink chemistries influence dimensional stability and elastic recovery under static load and thermal stress. The hardness values (Figure 5a) range from 68 to 72 Shore A, with only marginal differences across formulations. Peroxide-only vulcanizates (EPM1 and EPM2) exhibit slightly higher hardness compared to hybrid formulations, consistent with the predominance of rigid carbon–carbon crosslinks. Hybrid cured systems (EPM3–EPM7) maintain comparable hardness levels, indicating that the incorporation of sulfidic linkages does not compromise bulk rigidity. These results suggest that hardness is governed primarily by filler content rather than the specific chemical type of crosslinks. Compression set behavior provides a more sensitive probe of network architecture. At 70 °C (Figure 5b), all types of crosslinks including polysulfidic, disulfidic, monosulfidic, and carbon–carbon bonds remain stable, and this stability is reflected in the relatively low and well-differentiated compression set values across samples. Peroxide-only cured EPM1 (35%) and EPM2 (41%) display higher compression set values, contradicting with their rigid, less energy-dissipative networks. In contrast, hybrid systems show markedly improved resistance, with EPM7 (18%) and EPM6 (25%) achieving the lowest compression set. The coexistence of flexible sulfidic linkages with carbon–carbon bonds appears to facilitate more efficient elastic recovery under stress, reducing permanent deformation at 70 °C.

Figure 5

Figure 5. (a) Shore A hardness of the samples, (b) Compression set of samples at 70 °C, (c) Compression set of samples at 100 °C, (d) Compression set of samples at 120 °C.

At 100 °C (Figure 5c), significant differences emerge owing to the thermal instability of polysulfidic crosslinks, which begin to cleave or rearrange at this temperature. This degradation alters the effective crosslink structure, thereby impacting elastic recovery. For example, hybrid systems such as EPM7 (25%) and EPM6 (33%) still demonstrate superior recovery due to the stabilizing influence of monosulfidic and carbon–carbon crosslinks, which maintain the network integrity. Conversely, peroxide-only EPM2 (40%) shows the poorest performance, while hybrid systems containing higher fractions of polysulfidic linkages (e.g., EPM4, 36%) also exhibit relatively high compression set values. These results highlight that the stability of crosslink types, not merely their density, dictates thermal resilience. At 130 °C (Figure 5d), the challenge of maintaining elastic recovery becomes more pronounced for all formulations. The progressive breakdown of polysulfidic bonds and relaxation of shorter crosslinks leads to elevated compression set values. While peroxide-only systems (EPM1, 39%; EPM2, 37%) maintain moderate recovery due to the inherent stability of carbon–carbon crosslinks, hybrid formulations vary in performance depending on their network composition. EPM3 (35%) and EPM7 (36%) retain relatively good resistence, indicating that optimized hybrid networks combining stable C–C and monosulfidic bonds can counteract the degradation of polysulfidic linkages. In contrast, EPM4 records the highest compression set (42%), suggesting that its network relies heavily on thermally labile crosslinks, which compromise recovery under severe conditions.

Overall, these findings demonstrate that while hardness remains relatively insensitive to curing chemistry, compression set behavior is strongly governed by the thermal stability of the crosslink network. At lower temperatures (70 °C), all crosslink types are stable, and hybrid curing strategies provide a clear advantage in minimizing permanent deformation. At elevated temperatures (100 °C–130 °C), the instability of polysulfidic linkages becomes apparent, leading to higher compression set values unless counterbalanced by more stable monosulfidic or carbon–carbon bonds. These results reinforce the importance of carefully designing crosslink structures to balance flexibility, resilience, and thermal stability in EPM vulcanizates intended for demanding service environments.

3.5 Strain dependence of viscoelastic properties

The strain dependence of viscoelastic properties of the EPM vulcanizates is illustrated in Figure 6, which presents the evolution of storage modulus (E′), loss modulus (E″), and loss tangent (tan δ) with increasing strain amplitude. These results provide insight into the network integrity and filler–polymer interactions, as well as the extent of the linear viscoelastic region (LVR). The storage modulus (E′, Figure 6a) remains essentially constant at low strain amplitudes (below ∼0.1%), indicating that all samples exhibit a well-defined LVR where the network responds elastically and without structural rearrangements. Beyond this region, E′ progressively decreases with increasing strain, reflecting the onset of non-linear viscoelastic behavior commonly associated with the Payne effect, where disruption of filler–filler and polymer–filler interactions reduces stiffness. Among the samples, EPM4 exhibits the highest initial E′, followed by EPM5 and EPM6, consistent with their co-existing hybrid network. In contrast, EPM2 displays the lowest storage modulus, reflecting less tightly crosslinked structure.

Figure 6

Figure 6. (a) Strain dependence of E′ of samples, (b) Strain dependence of E″ of samples, (c) Strain dependence of Tan δ; of samples.

The loss modulus (E″, Figure 6b) shows a more complex strain dependence. At low strains, E″ remains nearly constant, followed by a distinct peak at intermediate strains before decreasing at higher strain levels. This peak corresponds to the energy dissipation associated with structural rearrangements and interfacial slippage between polymer chains and crosslink junctions. Notably, hybrid cured EPM4 shows the most pronounced peak, indicating greater internal friction and stronger filler–matrix interactions that are progressively broken with strain. In contrast, peroxide-only systems such as EPM2 exhibit relatively lower E″ values across the strain range, consistent with their simpler network topology. The damping factor (tan δ, Figure 6c), defined as the ratio of E″/E′, further highlights the balance between energy storage and dissipation. At small strains, tan δ values are low (∼0.1), indicating predominantly elastic behavior across all formulations. With increasing strain, tan δ rises gradually and shows a broad maximum at intermediate strains, reflecting enhanced viscous contributions due to microstructural rearrangements. EPM4 and EPM5 demonstrate higher tan δ maxima, suggesting greater energy dissipation capacity because of co-existence of both sulfur and carbon-carbon linkages, whereas peroxide-only systems (EPM1 and EPM2) maintain lower tan δ values, consistent with their rigid and less dissipative networks.

A key outcome from these measurements is the identification of the linear viscoelastic region (LVR). For all samples, the LVR extends up to approximately 0.1% strain, where both E′ and E″ remain strain-independent. This strain amplitude was therefore selected for subsequent frequency- and temperature-dependent dynamic mechanical analyses, ensuring that the measured viscoelastic properties truly reflect the intrinsic network structure rather than strain-induced non-linearities. In summary, strain sweep results emphasize that hybrid curing provides a more complex and dissipative viscoelastic response compared to peroxide-only systems, due to the coexistence of multiple crosslink types and stronger filler–polymer interactions. These features govern both the onset and the magnitude of non-linear effects under large strain conditions, while in the LVR all samples can be reliably compared at 0.1% strain for further dynamic mechanical studies.

3.6 Temperature dependence of viscoelastic properties

The temperature sweep analysis of the EPM samples was performed to evaluate the temperature dependence of the storage modulus (E′), loss modulus (E″), and damping factor (tan δ). The results are summarized in Figure 7, which shows E′, E″, and tan δ plotted as a function of temperature over the range from −80 °C to 100 °C for all seven EPM samples (EPM1 to EPM7). The storage modulus (E′) exhibits a characteristic decrease with increasing temperature (Figure 7a), indicative of the thermally activated softening behavior typical of elastomeric materials. At low temperatures (below −50 °C), the high modulus levels correspond to the glassy state, where polymer chain mobility is significantly restricted, and the amorphous segments are essentially frozen. As the temperature increases toward the glass transition region (∼−40 °C to −20 °C), a pronounced drop in E′ is observed, reflecting enhanced segmental mobility as the polymer transitions from the glassy to the rubbery state. Beyond this region, the modulus stabilizes into a rubbery plateau, which signifies the elastic network integrity maintained by the crosslinked structure. Among the samples, EPM4 consistently exhibits higher E′ values across the entire temperature range, suggesting a relatively higher crosslink density, and co-existence of sulfidic and peroxide linkages in EPM4.

Figure 7

Figure 7. (a) Temperature dependence of E′ of samples, (b) Temperature dependence of E″ of samples, (c) Temperature dependence of Tan δ of samples.

The loss modulus (E″), representing the viscous response of the system, shows a distinct peak around −40 °C to −30 °C (Figure 7b), which corresponds to the glass transition temperature (Tg) of the EPM samples. This peak arises from the maximum energy dissipation associated with cooperative molecular motions in the amorphous phase. The position and intensity of the E″ peak are comparable among all formulations, indicating similar Tg values and comparable polymer backbone flexibility. However, the slightly higher E″ values of EPM4 in the rubbery region suggest an enhanced ability of the network to dissipate mechanical energy through molecular friction, which may be attributed to its denser or more heterogeneous crosslink structure. The breadth of the E″ peak also provides insight into the distribution of relaxation processes, with broader peaks implying a wider distribution of molecular environments due to filler–matrix interactions or compositional heterogeneity.

The damping factor (tan δ), defined as the ratio of loss to storage modulus, provides a measure of the energy dissipation capability and viscoelastic balance of the material. Figure 7c shows the typical tan δ peak near the glass transition region, confirming Tg values in the range of −35 °C to −30 °C for all EPM samples. The magnitude of the tan δ peak is closely related to molecular mobility and internal friction; the comparable peak heights among the samples suggest similar main-chain segmental motion characteristics. At higher temperatures (>50 °C), the tan δ values show moderate fluctuations, with EPM1 and EPM2 exhibiting slightly higher damping levels, possibly due to crosslink cluster developed in peroxide that facilitate internal friction. Such variations in tan δ at elevated temperatures can influence the vibration damping and dynamic fatigue performance of the materials.

Overall, the temperature-dependent dynamic mechanical behavior of the EPM samples reveals typical viscoelastic characteristics of saturated elastomers. All formulations exhibit similar glass transition temperatures and transition behavior, confirming comparable polymer microstructures. However, the distinct differences in modulus values and damping trends among the samples indicate variations in crosslink network structure polymer–filler interfacial interactions. These differences directly affect the thermomechanical stability and energy dissipation capacity, thereby influencing the performance suitability of each EPM formulation for applications exposed to broad temperature ranges or dynamic loading environments.

3.7 Frequency dependence of viscoelastic properties

The frequency sweep response of the EPM vulcanizates is presented in Figures 8a–c. The storage modulus (E′) (Figure 8a) shows a progressive increase with frequency for all formulations, which is typical of elastomeric networks due to restricted chain mobility at shorter timescales. Among the samples, the hybrid cured systems (EPM4–EPM7) exhibit consistently higher E′ values across the entire frequency window compared to the peroxide-only and co-agent systems (EPM1–EPM3). This reflects their higher effective crosslink density and heterogeneous network structure, which impart greater stiffness. Notably, EPM5, containing sulfur in combination with peroxide, demonstrates the highest E′, indicating the dominance of sulfidic linkages in enhancing network elasticity.

Figure 8

Figure 8. (a) Frequency dependence of E′ of samples, (b) Frequency dependence of E″ of samples, (c) Frequency dependence of Tan δ of samples.

The loss modulus (E″) (Figure 8b) also increases with frequency, highlighting the enhanced viscous energy dissipation at higher deformation rates. Hybrid formulations again show superior E″ values, with EPM5 and EPM7 standing out. This suggests that sulfidic linkages not only increase elastic contributions but also facilitate localized energy dissipation through bond slippage and dynamic rearrangements, which are absent in peroxide-only networks (Chonkaew et al., 2011; Dijkhuis et al., 2009).

The tan δ response Figure 8c provides further insights into damping behavior. All samples display a peak in tan δ at intermediate frequencies, corresponding to a balance between viscous and elastic contributions. Peroxide-only systems (EPM1–EPM3) exhibit relatively higher tan δ values, reflecting a more dissipative character due to their lower crosslink density and weaker filler networking. In contrast, hybrid systems (especially EPM5 and EPM6) exhibit lower tan δ values across the frequency range, indicating reduced hysteresis and improved elastic recovery. This behavior is particularly advantageous for dynamic sealing and vibration damping applications, where low heat build-up is desirable.

Overall, the frequency dependence underscores that hybrid curing imparts a dual advantage of higher stiffness and lower damping, arising from the coexistence of sulfidic and carbon–carbon crosslinks. This unique combination is unattainable in conventional peroxide or co-agent systems.

3.8 Tear strength

The tear resistance of the EPM vulcanizates is shown in Figure 9a. A clear distinction is observed between the peroxide-only/co-agent cured systems (EPM1–EPM3) and the hybrid cured systems (EPM4–EPM7). The peroxide–co-agent formulation (EPM3) achieves the highest tear strength (64.2 N/mm), followed closely by peroxide-only EPM1 (59.8 N/mm). This behavior can be attributed to the relatively lower crosslink density and greater chain extensibility of these systems, which allow elastomer chains to orient in the direction of the applied stress during crack initiation and propagation. In addition, the co-agent in EPM3 promotes multifunctional bonding and filler–rubber adhesion, which further strengthens resistance to tearing.

Figure 9

Figure 9. (a) Tear strength of samples, (b) Crack initiation resistance of samples, (c) Abrasion resistance index of samples.

On the other hand, the hybrid systems show slightly lower tear strengths (51–54 N/mm). The decline arises from the increased network rigidity imposed by the incorporation of sulfidic crosslinks. These flexible but more numerous linkages elevate the effective crosslink density and suppress chain mobility, thereby restricting the ability of the material to redistribute stresses at a propagating crack tip. As a result, crack growth proceeds more readily in hybrids despite their overall stronger network architecture. Subtle differences among hybrids are also evident: EPM5 and EPM6 show marginally higher tear strength than EPM4 and EPM7, suggesting that their optimized co-agent–sulfur ratios provide a better balance of extensibility and stiffness. These findings highlight a well-known trade-off in elastomer networks: higher crosslink density improves durability but may compromise resistance to crack initiation.

3.9 Flex fatigue resistance (DeMattia test)

The results of the DeMattia flex fatigue test (Figure 9b) reveal the most striking benefits of hybrid curing. While peroxide-only (EPM1) and co-agent formulations (EPM2, EPM3) fail after relatively short lifetimes (123–191 kilo cycles), the hybrid cured samples demonstrate much longer flex lives, ranging from 311 kilo cycles (EPM4) to 354 kilo cycles (EPM7). The improvement is nearly two-to three-fold compared to conventional systems.

The superior fatigue resistance of hybrid systems can be explained by the nature of hybrid crosslinking, i.e.,; co-existing network. Unlike carbon–carbon bonds formed in peroxide-only cures, sulfidic bridges are relatively flexible and possess limited dynamic adaptability under cyclic loading. These features allow localized bond rearrangements, which relieve stress concentrations around crack tips and slow down the rate of crack propagation. Within the hybrid set, EPM5 and EPM7 emerge as the best performers, with lifetimes exceeding 340 kilo cycles. This suggests that their sulfur–peroxide ratio produces an optimally clustered network architecture where flexible bridges are evenly distributed, thereby maximizing energy dissipation and delaying fatigue failure. EPM6 also exhibits excellent performance, slightly lower than EPM7, whereas EPM4, although superior to peroxide-only systems, appears less optimized. This comparison underlines the importance of tailoring the sulfur content and co-agent concentration in achieving maximum fatigue resistance (Bhattacharya et al., 2020).

3.10 Abrasion resistance

The abrasion resistance index (Figure 9c) provides further insights into the durability of the networks. Peroxide-only EPM1 exhibits the lowest resistance to wear (103), and EPM2 and EPM3 show modest improvements (120 and 125, respectively), primarily due to the incorporation of co-agents that promote filler–rubber interactions. However, none of these formulations approach the performance of the hybrid systems.

The hybrids (EPM4–EPM7) display significantly higher abrasion resistance, with indices ranging from 144 to 163. This enhancement arises from the coexistence of sulfidic and carbon–carbon linkages, which produces a tougher and more energy-dissipative network. Under repeated frictional contact, sulfidic linkages allow partial relaxation of stress concentrations and distribute energy more effectively across the network, thus resisting micro-cutting and surface damage. Among them, EPM7 once again demonstrates the best performance (163), correlating well with its highest fatigue life. EPM6 also performs strongly (152), suggesting a balanced co-agent–sulfur contribution to the abrasion mechanism. EPM4 and EPM5, although improved compared to conventional systems, show slightly lower indices (144–151), reflecting less optimized network architecture.

3.11 Thermogravimetric analysis

Thermogravimetric analysis was conducted to investigate the thermal stability and decomposition behavior of the different EPM formulations (EPM 1 to EPM 7). The TGA curves (Figure 10a) reveal that all samples maintain near 100% mass up to approximately 400 °C, indicating excellent thermal stability without significant degradation within this temperature range. Beyond 400 °C, a pronounced mass loss occurs between 400 °C and 500 °C, corresponding to the primary thermal degradation of the elastomer network. This sharp weight reduction is attributed to the main chain scission and decomposition of polymer backbones and associated crosslinks.

Figure 10

Figure 10. (a) TGA plot of samples, (b) DTG plot of samples.

The residual mass at temperatures above 600 °C suggests the presence of thermally stable char or inorganic fillers remaining in the network post-decomposition. Notably, the mass loss profiles of all formulations are closely similar, reflecting consistent thermal degradation behavior across the various curing systems, whether peroxide-only or sulfur-peroxide hybrid cured.

The DTG curves (Figure 10b) complement these observations by depicting the rate of mass loss with temperature. Each sample exhibits a pronounced peak centered around 480 °C–500 °C, marking the temperature where the degradation rate is greatest. The similarity in peak shapes and positions indicates that the degradation mechanism remains analogous among all EPM variants irrespective of the crosslinking chemistry involved.

This thermal stability across diverse curing approaches demonstrates that the hybrid sulfur-peroxide crosslinking strategy maintains robust heat resistance comparable to conventional peroxide-only curing. The presence of carbon-carbon bonds from peroxide curing principally governs the high-temperature performance, while sulfidic crosslinks introduced in hybrid systems do not compromise thermal decomposition characteristics. Overall, the TGA-DTG analysis confirms that the tailored crosslink networks, combining peroxide and sulfur chemistries, afford elastomer formulations suitable for applications demanding elevated thermal endurance without sacrificing the balance of mechanical properties enhanced by hybrid curing (Parathodika and Naskar, 2024).

4 Conclusion

This study demonstrates the successful formulation and characterization of ethylene-propylene rubber (EPM) compounds cured via hybrid sulfur-peroxide system alongside traditional peroxide curing. The hybrid curing approach effectively integrates sulfidic crosslinks with peroxide-generated carbon-carbon bonds, producing a unique network architecture that synergistically balances thermal stability, elasticity, and mechanical strength. Rheological analysis revealed accelerated cure kinetics and increased crosslink densities in hybrid formulations, while tensile and dynamic mechanical properties confirmed enhanced flexibility, tensile strength, and improved stress dissipation compared to peroxide-only systems. The thermogravimetric analysis supported the robust thermal stability of all EPM variants, with decomposition onset around 400 °C and no compromise in heat resistance by incorporating sulfur chemistry. The hybrid networks exhibited superior compression set resistance at moderate temperatures and maintained good dynamic mechanical behavior across frequency and temperature ranges relevant to practical applications. These results highlight that sulfidic crosslinks contribute notably to network adaptability and energy dissipation under cyclic loading without detracting from high-temperature performance.

Overall, the sulfur-peroxide hybrid curing system represents a promising strategy for engineering advanced flexible EPM composites with tailored properties. This approach allows overcoming limitations inherent to peroxide-only curing, offering enhanced tear strength, elasticity, and fatigue resistance while preserving excellent thermal resilience. The findings provide a valuable foundation for expanding the application scope of EPM elastomers in demanding environments such as automotive, electrical insulation, and chemical-resistant seals, where combined mechanical durability and thermal stability are essential. Future work may explore further optimization of accelerator packages, the role of co-agents, and long-term aging resistance to fully harness hybrid curing potential in commercial elastomer formulations.

Data availability statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author contributions

AP: Software, Writing – review and editing, Methodology, Investigation, Writing – original draft, Supervision, Data curation, Funding acquisition, Visualization, Formal Analysis, Validation, Resources, Project administration, Conceptualization. KN: Supervision, Investigation, Funding acquisition, Software, Writing – original draft, Formal Analysis, Resources, Writing – review and editing, Visualization, Methodology, Data curation, Validation, Project administration, Conceptualization.

Funding

The author(s) declared that financial support was not received for this work and/or its publication.

Acknowledgements

The authors express their gratitude to the Indian Institute of Technology Kharagpur for providing assistance for this research endeavor.

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The author KN declared that they were an editorial board member of Frontiers at the time of submission. This had no impact on the peer review process and the final decision.

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The author(s) declared that generative AI was not used in the creation of this manuscript.

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