1. Introduction
Hyperelastic materials such as rubbers and elastomers constitute a crucial category of polymeric materials extensively utilized across various industries, including automotive, construction, aerospace, and medical device manufacturing. Their widespread adoption is attributed to their distinctive properties, such as exceptional elasticity, abrasion resistance, thermal and electrical insulation, and their capability to perform under extreme environmental conditions. These attributes make such materials indispensable for producing seals, shock absorbers, tires, protective coatings, and other structural components requiring durability and reliability under diverse operating conditions [
1].
However, despite their numerous advantages, the performance of hyperelastic materials is highly influenced by environmental factors and the duration of use. Variables such as temperature, humidity, UV radiation, and exposure to aggressive chemicals can instigate material degradation, leading to diminished mechanical properties and a shorter service life. These degradation mechanisms—categorized as thermal, mechanical, or chemical ageing—are particularly critical for materials deployed in demanding industrial applications, where even minor reductions in strength or durability can result in component failure [
2]. For this reason, it is essential to know exactly how the properties of these types of materials change over the course of their lifetime as they age.
Several studies have elucidated the impact of thermal ageing on elastomers. Liu et al. (2015) investigated the thermal ageing of EPDM at elevated temperatures, reporting an increase in cross-link density and a corresponding reduction in elongation at break [
3]. Bouaziz et al. (2020) examined polychloroprene rubber and proposed predicting the lifetime of elastomers based on an elongation at break criterion [
4]. Mohammadi and Dargazany (2019) developed a micromechanical model that captures the thermal-induced degradation of elastomeric networks by comparing experimental stiffness evolution and damage accumulation [
5]. The work of Le Gac et al. (2012) provides a comprehensive comparison between accelerated and natural marine ageing of polychloroprene rubber, noting pronounced increases in modulus and loss of flexibility [
6]. Zaghdoudi et al. (2024) demonstrated that EPDM and HNBR undergo significantly different degradation paths under air and hydrogen environments, revealing material-specific responses to ageing agents [
7]. Wei et al. (2004) quantified reductions in elasticity in rubbers post-thermal and UV exposure using dynamic mechanical analysis [
8].
The research presented in the article focuses on two commonly used polymers: ethylene propylene diene rubber (EPDM) and chloroprene rubber (CR). EPDM exhibits high thermal and oxidative stability due to the absence of reactive double bonds in its backbone while CR contains chlorine-substituted monomers that confer chemical resistance but simultaneously make it more susceptible to thermal degradation and crosslinking instabilities at higher temperatures [
9,
10]. By selecting these two polymers with divergent ageing resistances, our study captures a broad spectrum of degradation mechanisms and enables validation of the proposed modeling approach under varied conditions. Additionally the extensive industrial use of both materials supports the practical relevance of our findings for applications requiring long-term mechanical performance.
Ethylene propylene diene rubber (EPDM) is a terpolymer, which is a type of polymer produced by polymerizing three different monomers. This process, known as copolymerization, integrates the properties of each monomer into a single material with specific characteristics. EPDM rubber is synthesized from ethylene, propylene, and dienes, providing the material with resistance to high temperatures and weathering. The vulcanization process, using sulfur or dicumyl peroxide, results in the formation of cross-linked structures within the material, enhancing its durability [
10].
A key advantage of EPDM is its exceptional resistance to weathering, including weathering caused by ozone, UV radiation, and high temperatures. The glass transition temperature for EPDM is −60 °C. The glass transition temperature is a key parameter from the point of view of elastic and hyperelastic materials because it defines the point at which a significant change in the mechanical properties of the material under consideration occurs. In other words, up to this temperature, the material retains its elastic properties and does not become rigid. EPDM maintains its properties over an extensive temperature range, from −50 °C to +120 °C and sometimes even up to +150 °C, although it should be noted that this is a peak value that only allows short-term operation. The wide range of temperature tolerances means that the material is often used in products requiring resistance to large temperature changes, such as gaskets, hoses, diaphragms, or heating and cooling systems [
10].
EPDM also has some disadvantages. One of these is its limited fire resistance, which limits its use in certain environments. Although it is resistant to many substances, it should not be used in contact with mineral oil-based products such as greases, oils, fuels or solvents, which can damage it. In addition, EPDM has difficulty forming permanent rubber–metal joints. Despite its good tear resistance, there are other rubber materials that can offer better durability in this regard [
10].
The second material selected for study was chloroprene rubber. It is characterized by a high tensile strength, which makes it resistant to damage even under intense loads. Such a characteristic makes it an ideal material for use in areas where it is regularly subjected to continuous stress, such as seals and hoses [
11,
12]. The permissible operating temperature of chloroprene rubber ranges from −40 °C to +120 °C, making it very practical in a variety of environmental conditions. It can briefly withstand temperatures of up to +130 °C. Long-term storage of CR at temperatures below 0 °C can lead to crystallization and irreversible stiffening. The glass transition temperature of neoprene, as reported in the literature, is approximately −45 °C [
13].
Chloroprene rubber (known also as neoprene) is highly regarded for its general resistance to oils and other chemicals. For this reason, it is often used in industry, where components made with it are exposed to corrosive substances. Due to this chemical resistance and tensile strength, it is preferred for the manufacture of hoses and gaskets.
The main disadvantage of chloroprene is the fact that it is not as resistant to oils and fuels as other hyperelastic materials, such as nitrile rubber. Neoprene exhibits sensitivity to extended exposure to ultraviolet radiation and ozone, which accelerates its degradation and reduces its longevity. Additionally, it tends to rapidly lose its mechanical and chemical properties when subjected to extreme temperature conditions. At lower temperatures, neoprene experiences a marked reduction in flexibility, whereas at elevated temperatures, it softens excessively, compromising its structural integrity.
Modeling hyperelastic materials using the finite element method (FEM) presents a substantial challenge, primarily due to their inherent non-linearity and complex mechanical behavior. A key difficulty arises from the material’s non-linear characteristics, rendering traditional linear constitutive models inadequate for accurately describing their response. To address this, advanced models such as the Ogden or polynomial models must be employed. These models, however, are often complex to calibrate and demand comprehensive experimental data for proper parameterization and reliable implementation [
14,
15].
Another significant challenge in modeling rubber materials is their propensity for large deformations and strains, which are characteristic of these materials. Accurately capturing changes in the material’s geometry under such conditions often leads to intricate numerical problems. Complex shapes and substantial deformations can induce numerical instabilities, necessitating advanced computational techniques, such as load path tracking or geometry monitoring. Dividing the loading process into smaller increments can facilitate more precise tracking of the material’s behavior at each stage of deformation.
Additionally, extensive deformations can profoundly alter the material’s shape and size, which is critical for accurate analysis. Hyperelastic materials also exhibit varying responses depending on factors such as temperature extremes or prior deformation history, requiring sophisticated constitutive models to account for these effects. Furthermore, when subjected to critical loads, these materials may exhibit unexpected phenomena, including localized densification of the material structure or bifurcations, which are challenging to predict and demand detailed investigation [
15,
16].
For this reason, accurately representing the changes in material behavior due to ageing within numerical models is of paramount importance. This requires determining the material constants of the selected constitutive model to ensure precise characterization of the material’s response under various conditions. In the literature we can find several constitutive material models which can be used in order to describe the behavior of hyperelastic materials such as the Mooney–Rivlin, neo-Hookean, Gent–Thomas, Yeoh, polynomial, and Arruda–Boyce models [
17]. In the present study, we focused on the polynomial material model. This model was selected for its exceptional versatility. By incorporating a range of polynomial terms, it effectively captures the non-linear stress–strain behavior characteristic of various materials. Moreover, this type of model is well-suited for handling diverse stress states, including uniaxial, biaxial, and pure shear, ensuring robust and accurate representation across a wide array of loading conditions.
The primary objective of the article is to present the effect of accelerated thermal ageing on the mechanical properties of two selected hyperelastic materials, ethylene-propylene-diene rubber (EPDM) and chloroprene rubber (CR), along with the process of determining constitutive material model parameters for use in numerical simulations using finite element method.
4. Constitutive Model Parameter Determination
One of the most commonly used constitutive models to describe the behavior of hyperelastic materials is the 6-term polynomial model. This model extends simpler models such as the Mooney–Rivlin or neo-Hookean models, adapting them to more complex scenarios. In this approach, the material’s free energy is expressed as a polynomial, where the individual coefficients represent different aspects of elastic energy. The free energy is formulated as a function of the invariants of the strain tensor, with
I1,
I2, and
I3 denoting the first, second, and third principal invariants of the Cauchy–Green strain tensor, respectively. The 6-term polynomial model was selected due to its superior ability to capture the complex non-linear stress–strain behavior of hyperelastic materials across diverse loading conditions, including uniaxial, biaxial, and shear states, compared to simpler 2-term models like the Mooney–Rivlin model [
8]. The inclusion of higher-order terms (
C11,
C20,
C02,
C30) enables accurate representation of strain hardening and large deformations, which are critical for aged elastomers exhibiting increased stiffness [
24]. Polynomial models outperform simpler models in describing the behavior of aged rubbers, enhancing the accuracy of numerical simulations for real-world applications [
24]. Such models are better suited for materials that deviate from ideal elasticity due to ageing-induced structural changes [
25]. Costa et al. (2015) also recommended polynomial models with ≥5 terms when dealing with mechanically and thermally altered elastomeric materials [
26].
The basic equation for this model is [
27]
where
W is the material’s free energy,
Cpq is the material’s constants, and
I1 and
I2 are first and second Couchy–Green strain tensor invariants [
28].
For compressible materials, the volume dependence is added as follows:
where
,
,
J is the determinant of the deformation gradient tensor, and
D1 is the material constant controling the volumetric compressibility [
15].
If
D1 is a negative value then the volume modulus should be used:
where
K—is the linear volume modulus determined from the corresponding linear shear modulus
and Poisson’s ratio [
28].
To determine the values of the 6-term polynomial material model, the curve-fitting technique was used. In this way, all material constants (
C10,
C01,
C11,
C20,
C02, and
C30) were determined for chloroprene rubber and EPDM. The values obtained are shown in
Table 3 and
Table 4. The R
2 coefficient, a measure of the fit of the curve to the dataset, was at least 0.99 for all cases considered.
5. Discussion
The first investigated parameter was density, in which a clear change is visible. The observed increase in density for both materials, with a 3.4% rise for CR and 2.6% for EPDM, can be attributed to enhanced cross-link density during thermal ageing [
29,
30]. The higher density increase in CR compared to EPDM can be attributed to the chlorine-substituted monomers in CR, which facilitate dehydrochlorination and subsequent cross-linking [
6].
The Shore A hardness measurements revealed a pronounced increase, particularly for CR, which exhibited a 32% rise after 35 days of ageing compared to a 20% increase for EPDM. The increase in Shore A hardness indicates reduced chain mobility, attributed primarily to thermally activated oxidative cross-linking and chain scission phenomena. For CR, the presence of chlorine enhances susceptibility to dehydrochlorination, leading to additional cross-linking and network densification, hence a more pronounced hardness increase. EPDM, though more thermally stable due to its saturated backbone and ethylene-propylene structure, still undergoes oxidative cross-linking over prolonged exposure, albeit at a slower rate [
31,
32].
The tensile test results (
Figure 6 and
Figure 7) demonstrate a gradual transition in the mechanical response of the investigated elastomers, evolving from a predominantly hyperelastic character toward increasingly brittle behavior. For chloroprene rubber (CR), the extension ratio declined by nearly 40% after 35 days of accelerated ageing, while the maximum stress exhibited a 37% increase. This inverse correlation between ductility and strength is characteristic of oxidative degradation, where chain scission combined with secondary cross-link formation diminishes extensibility and plastic relaxation, thereby elevating the stress required for deformation.
Ethylene-propylene-diene rubber (EPDM) exhibited a comparable trend, albeit less pronounced. After 35 days, the extension ratio decreased by approximately 28%, accompanied by only a 20% increase in maximum stress. These results suggest that EPDM preserves a more favorable balance between stiffness and extensibility, which contributes to its enhanced resistance against severe embrittlement under prolonged thermal exposure.
The thermal ageing process, for both chloroprene rubber and EPDM, resulted in the rupture of polymer chains at a much faster rate than under normal conditions, leading to the formation of new cross-links that exhibited significantly reduced elasticity and increased hardness.
Young’s modulus measurements further highlight the divergent ageing responses of CR and EPDM. In the case of the EPDM, the increase was more gradual and linear compared to CR. This suggests that EPDM is less susceptible to degradation processes over a comparable period, exhibiting greater stability and potentially better retention of its elastic properties. In contrast, although chloroprene rubber (CR) showed a Young’s modulus similar to that of EPDM in the early stages of ageing, the modulus increased to significantly higher values in the later stages of the ageing process. This increase in stiffness directly impacted the material’s effective use, particularly in applications subjected to high temperatures.
The Mooney–Rivlin plots provide additional insight into the materials’ mechanical behavior. For CR, the transition from a linear stress response in unaged samples to a steep rise in reduced stress after 35 days reflects a loss of hyperelasticity [
6]. For EPDM, the plots show a plateau phase in early ageing, which disappears after 21 days, indicating progressive stiffening. The fracture energy and upturn strain results further confirm these trends, with CR exhibiting a steady decline in fracture energy and upturn strain, indicative of embrittlement, while EPDM shows a more stable fracture energy between 7 and 21 days [
33]. By analyzing the curves, we can also observe a change in their nature. In the case of material that has not undergone ageing, their course can be approximated using a linear function. However, as the ageing period increases and the degradation processes intensify, the Mooney–Rivlin curves become highly non-linear, which is particularly evident for EPDM. Therefore the use of the two-parameter Mooney–Rivlin models would not be appropriate to model the behavior of thermal aged materials. For this reason, a 6-term polynomial material model was selected for further consideration.
The reduction in upturn strain (
Figure 12) reflects the decreasing capacity of polymer chains to reorient under applied load, with CR once again demonstrating a more rapid deterioration compared to EPDM.
The 6-term polynomial constitutive model provides a quantitative framework for linking microstructural degradation to macroscopic mechanical behavior.
For CR, the C01 coefficient increased markedly from 4.39 in the unaged state to 15.95 after 35 days, reflecting the material’s increasing resistance to deformation due to cross-link densification. Simultaneously, coefficients such as C10 and C11 diminished to negligible levels, suggesting that the contributions of extensibility-related mechanisms were largely suppressed. The dominance of C01 at later ageing stages underscores that the response of CR becomes increasingly governed by stiffening rather than elastic recovery.
In EPDM, the C01 coefficient also rose, albeit less sharply, from 1.31 to 3.91 over 35 days. Other coefficients, including C11, C20, and C02, retained finite values throughout ageing, indicating that multiple deformation mechanisms remained active. This observation correlates with the experimental evidence that EPDM preserves some degree of ductility and toughness even after extended thermal exposure. The persistence of higher-order coefficients such as C30 reflects EPDM’s ability to accommodate strain-hardening behavior, whereas CR’s coefficients converged towards values indicative of brittle response.
The numerical validation of the 6-term polynomial constitutive model demonstrates its robustness in capturing the non-linear behavior of aged elastomers (
Figure 13,
Figure 14 and
Figure 15). In the case of chloroprene rubber aged for 7 days, a high degree of consistency was observed between the obtained results (
Figure 13). The experimental data show relatively high stiffness during the initial ageing phase. The numerical model accurately reproduces this behavior across the entire deformation range, indicating that the model parameters were appropriately calibrated for samples with short ageing times. An almost perfect match between the experimental and numerical curves is evident, particularly in the low strain range (ε < 0.2). After 35 days of ageing, the numerical model still shows strong agreement with the experimental results, although there is a slight overestimation of the true stress values across nearly the entire strain range.
For specimens aged for 21 days (
Figure 14), good agreement between the numerical model and the experimental data was achieved in the low and medium strain ranges (ε < 0.2). However, larger deviations were observed in the higher strain range (ε > 0.2), particularly at the maximum true strain value. This is likely due to the limitations of the polynomial model, which is especially sensitive to non-linear changes in material properties at higher strain levels.
Table 5 presents a comparison of the maximum true stress values for both experimental and numerical results. The largest discrepancy occurred for the 21-day ageing period, in which the difference was less than 8%. For ageing periods of 7 and 35 days, the difference was much smaller, not exceeding 2%.
In the case of EPDM, an even greater consistency of results was observed (
Figure 16,
Figure 17 and
Figure 18). Unlike CR, there was no significant discrepancy in the results for the 21-day ageing (
Figure 17), which is further supported by the true stress values presented in
Table 6. The maximum difference was 1.38% for the 21-day ageing, while for the 7- and 35-day ageing periods, the difference was less than 1%.
6. Conclusions
The conducted research provides a comprehensive analysis of the mechanical and physical properties of various technical rubbers commonly used in industrial applications. The primary aim of the study was to evaluate the impact of accelerated thermal ageing on the mechanical behavior and durability of the materials under investigation. Additionally, all material constants for the polynomial constitutive model were determined for both chloroprene rubber and EPDM.
The findings of the study clearly demonstrate that the mechanical properties of technical rubbers, including elastic modulus, tensile strength, and elastic deformation, are highly influenced by environmental conditions. Elevated temperatures notably reduce the strength of these materials, particularly over extended periods of use. After 35 days of ageing, the properties of both rubbers deteriorated significantly, resulting in increased stiffness and a substantial loss of deformability. Chloroprene rubber exhibited more pronounced changes in mechanical properties, indicating a higher sensitivity to thermal ageing compared to EPDM, which displayed greater resistance to high-temperature conditions. This is further supported by the higher deformation observed for EPDM across all ageing periods. At the prolonged ageing period of 35 days, both rubbers showed a significant reduction in elasticity, which can be likely attributed to structural-level changes. These changes may involve the formation of new cross-links or the degradation of polymer chains, potentially altering the material’s crystalline structure.
The determined material constants effectively represent the properties of both tested materials, as validated by numerical analyses. The true stress–true strain curves for all considered scenarios closely align with the experimental data. The largest deviation was observed for chloroprene rubber aged for 21 days, yet even in this instance, the maximum difference between the experimental and numerical maximum true stress values did not exceed 8%.
Despite the robust findings, this study has several limitations. The investigation focused solely on thermal ageing at 100 °C, excluding other environmental factors such as UV radiation or chemical exposure, which significantly affect elastomer performance in real-world conditions [
2]. Additionally, testing at a single temperature limits insights into temperature-dependent degradation rates. Furthermore, only uniaxial tensile testing was employed, whereas real-life applications often involve multi-axial loading, fatigue, or creep. In future work, we plan to investigate combined ageing mechanisms with complex loading paths such as biaxial deformation or cyclic fatigue.
We believe that determined parameters would be a great help for anybody interested in numerical simulations of components made from hyperelastic materials. The determined material constants can directly support design and durability assessments of components such as engine gaskets, thermal insulation bushings, or vibration isolators subjected to prolonged thermal loads. Using updated hyperelastic models that reflect aged properties in simulations improves predictive accuracy for deformation, failure, and seal efficiency [
34]. The data presented in this article enable such predictive simulations, reducing the need for conservative over-design and minimizing the risk of premature component failure. Furthermore, accurate constitutive models enhance failure analysis under cyclic loading, enabling better design of elastomer-based components in dynamic environments. These applications underscore the practical value of the study’s findings for enhancing material performance and durability [
34].