Study of the Effect of Accelerated Ageing on the Properties of Selected Hyperelastic Materials

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The determined material constants effectively represent the properties of both tested materials. They can be used in a variety of numerical simulations using finite element method.

Abstract

Hyperelastic materials, which include various types of rubber, are widely used in industry (such as the automotive industry). Their main disadvantage is the loss of their original properties over time due to environmental factors (called ageing). The ageing process is long-lasting, which is why so-called accelerated ageing is used when studying the effect of ageing on material properties. Accelerated ageing is realized with a higher intensity of the ageing agent, e.g., by irradiating specimens with UV radiation or by holding them at elevated temperatures. In the literature, there is a lack of parameters for constitutive models that take into account the effect of ageing on material properties. This paper presents the process of determining the parameters for a selected constitutive model using two commonly used rubbers in industry: chloroprene (CR) and ethylene-propylene-diene (EPDM). Before determining the material parameters, the samples were subjected to accelerated ageing at 100 °C for periods of 7, 21, and 35 days. Stress–strain curves were then determined from a tensile test and the parameters of the constitutive model were determined using the non-linear least squares method. Finally, numerical validation of the obtained values was also carried out.

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.

2. Materials and Methods

2.1. Specimen Preparation

As previously mentioned, the materials under investigation are chloroprene rubber and ethylene-propylene-diene rubber (EPDM). Specimens were cut from commercially available sheets with dimensions of 2000 × 1000 mm and a thickness of 2 mm, following the shape specified in ISO 37:2024 [18] (Figure 1). All specimens were extracted from a single sheet of each specific material to avoid variations in mechanical properties or composition differences. The samples were consistently cut in one fixed orientation, specifically along the longer edge of the sheet. The specimens were cut using a 415 MPa high-pressure water jet system with garnet abrasive (mesh 80). To minimize deformation of the cut material, and consequently to reduce geometric inaccuracies of the extracted specimens, rubber sheets were trimmed to dimensions of 300 × 300 mm and subsequently placed between two aluminum plates, which were then fastened together. This procedure allowed us to achieve dimensional accuracy within ±0.05 mm (Figure 2).

An accelerated thermal ageing process was conducted using an air-circulating oven (made by Zalmed, Warsaw, Poland) at a temperature of 100 °C for varying exposure times. The temperature was controlled within ±2 °C. The specimens were kept in the oven for 7, 21, and 35 days, after which they were promptly removed, in accordance with ISO 188:2023 [19]. Subsequent to their removal, the specimens were further marked with additional indicators applied within the measuring section of each specimen. The placement of these markers was consistent across all specimens, allowing for the subsequent measurement of deformation during the static tensile test using optical system.

Following the thermal ageing process, the density and hardness of all samples were measured to assess any potential changes in these parameters. The results were then compared to baseline measurements obtained from non-aged samples. Density measurements were conducted using an Hildebrand H-300DS densitometer (Wendlingen am Neckar, Germany), which operates based on the principle of Archimedes’ law.

The hardness of the specimens was assessed using a Sauter Shore A scale hardness tester (Basel, Switzerland), which is specifically recommended for evaluating soft materials. This method involves measuring the resistance generated when a needle of predetermined shape and dimensions is pressed into the sample under examination.

2.2. Tensile Test

For the static tensile test, a KAPPA 50 DS tensile machine with an electromechanical drive and a maximum speed of 100 mm/min was employed. The test was conducted in accordance with the ISO 37:2024 standard, with a crosshead speed of 5 mm/min [18]. Displacement and force were recorded at a frequency of 50 Hz. To assess the deformation of the unaged specimens, a motion tracking method was additionally utilized. As previously mentioned, a set of markers was applied to each specimen, and the entire testing process was recorded using a high-resolution camera (1920 × 1080 pixels). The marker displacement over time was then analyzed using TEMA ver. 3.0-024 (Image Systems AB) image analysis software. For the aged specimens, which exhibited a lower extension rate, an extensometer was employed for measurement. Deformation in the transverse direction was not considered during the tests.

To ensure statistical significance, five specimens per each condition (unaged, 7, 21, and 35 days aged) were tested, as recommended in ISO 37:2024 [18]. Similar sample sizes are adopted in comparative studies on thermally aged elastomers [7]. The results in Section 3 represent averaged values.

In the case of hyperelastic materials, stress–extension ratio curves are usually given instead of traditional stress–strain curves. We also used this approach in our article. The extension ratio is defined as follows:

$$\mathsf{\lambda }=\epsilon +1$$

(1)

where $\epsilon$ is an engineering strain.

In the analysis of hyperelastic materials, so-called Mooney–Rivlin plots are also used. These plots are based on the Mooney–Rivlin equation [20,21,22]:

$$\mathsf{\sigma }=2\left(C_1+{\displaystyle \frac{C_2}{\lambda }}\right)\left(\lambda -{\displaystyle \frac{1}{{\lambda }^2}}\right)$$

(2)

where C₁ and C₂ are material constants and λ is the extension ratio. These constants are associated with the intermolecular forces between the polymer chains and C₁ is related to the crosslink density for elastomers without fillers [23]. Equation (2) can be rewritten using the term of reduced stress ${\mathsf{\sigma }}^{*}$:

$${\mathsf{\sigma }}^{*}={\displaystyle \frac{\sigma }{\lambda -\frac{1}{{\lambda }^2}}}$$

(3)

2.3. Numerical Modeling

Following the experimental test, constitutive material model parameters were determined and numerically validated. For this purpose, a geometric model representing the sample used in the tests was created and subsequently discretized using fully integrated eight-node hexahedron 3D elements (Figure 3). A total of 6080 elements of this type were used for the analysis. The boundary conditions applied were also fully aligned with the experimental conditions. On one side of the specimen model, a full displacement constraint was applied, i.e., all degrees of freedom were fixed to prevent any movement, which corresponds to the lower grip of the testing machine. On the opposite side, one translational degree of freedom along the X-axis was released, and a prescribed motion was applied. This setup allowed the specimen to move exclusively in the loading direction, thereby reproducing the conditions of a uniaxial tensile test. Releasing a single degree of freedom enabled the specimen to deform in response to the applied load, while simultaneously preventing displacements in the remaining directions and eliminating any rigid-body rotations.

The analyses were performed under a quasi-static implicit scheme with displacement control implemented in the LS-Dyna solver. Absolute tolerance for convergence value used during the research was equal to 1 × 10⁹, while relative tolerance was equal to 0.001. Constant time step size was used.

Due to the fact that the results obtained from numerical analysis were the true stress and true strain, the experimental results, being the engineering stress and strain, were converted using the following formulae:

$${\epsilon }_{true}=\mathrm{l}\mathrm{n}(1+{\epsilon }_e)$$

(4)

$${\sigma }_{true}={\sigma }_e\times (1+{\epsilon }_e)$$

(5)

where ${\epsilon }_e$ and ${\sigma }_e$ are the engineering strain and stress, respectively.

3. Results

3.1. Density

The results of the density measurements for CR and EPDM are presented in Figure 4. A slight increase is observed for both materials, with a change of 3.4% for chloroprene rubber and 2.6% for EPDM.

As mentioned in Section 2.2, five samples from each ageing period were tested. Very good repeatability of results was obtained.

Table 1. Standard deviation of the density measurements.

	CR	EPDM
0 days	0.007	0.004
7 days	0.011	0.001
21 days	0.003	0.002
35 days	0.001	0.001

shows the standard deviations for density measurements of both materials.

3.2. Hardness

The results of the hardness measurements are presented in Figure 5. The initial hardness of the tested materials was approximately 70 (68.2 for CR and 69.7 for EPDM). With increasing exposure to accelerated thermal ageing, a marked change in hardness was observed for both materials, especially in the case of chloroprene rubber. After 7 days of ageing, the hardness of CR increased to 80.3, representing a 17% increase, while EPDM exhibited an increase to 75.6, corresponding to a rise of approximately 8.4%. As the ageing process continued, these changes became even more pronounced, with the hardness reaching 92.9 for CR (a 32% increase from the initial value) and 82.9 for EPDM (a 20% increase) after 35 days.

Analogous as in density investigation five samples from each ageing period were tested. Very good repeatability of results was obtained.

Table 2. Standard deviation of the hardness measurements.

	CR	EPDM
0 days	0.294	0.287
7 days	0.455	0.125
21 days	0.665	0.125
35 days	0.236	0.163

shows the standard deviation for density measurements of both materials.

3.3. Tensile Test

The stress–extension ratio curves for the averaged results from the chloroprene rubber and EPDM tensile tests are shown in Figure 6 and Figure 7.

For chloroprene rubber, a reduction in the extension ratio to approximately 1.78 was observed after an accelerated ageing period of 7 days. For EPDM, the extension ratio for the same duration was 1.84. The maximum stress value for chloroprene rubber was 3.64 MPa, while for EPDM it was 3.51 MPa.

An increase in ageing time leads to an intensification of the degradation processes within the structures of both materials. This is particularly evident in chloroprene rubber, where ageing for 21 days resulted in a reduction in the extension ratio to 1.32, a decrease of approximately 28% (Figure 6). The maximum stress, however, increased to 3.83 MPa, representing a decrease of only 9%. Extending the ageing period to 35 days caused a further, more rapid reduction in the extension ratio to 1.07, a decrease of about 40% compared to 7 days and 19% compared to 21 days. The maximum stress value rose to 4.98 MPa, marking an increase of 37% relative to the 7-day ageing period.

For EPDM, the trend of decreasing extension ratio coupled with increasing maximum stress as the ageing period lengthened was observed (Figure 7). The extension ratio decreased to values of 1.67 and 1.33 after 21 and 35 days of ageing, respectively. Similarly to chloroprene rubber, the stress values also increased, but to a much smaller extent. The maximum stress after 21 days of ageing was 3.69 MPa, reflecting a 5% increase compared to the 7-day ageing period, and after 35 days, it rose to 4.22 MPa, which represented a 20% increase relative to 7 days.

The change in stiffness of both materials is also clearly reflected in the variation in Young’s modulus (Figure 8). Analysis of the CR graph reveals that, during the initial ageing period, Young’s modulus underwent significant changes. After just 7 days, it increased to 7.943 MPa, a rise of approximately 70% compared to the initial value of 4.685 MPa. This trend continued through to 21 days, when the modulus reached 11.858 MPa, more than doubling. Beyond this point, the degradation processes accelerated considerably, with the value of Young’s modulus reaching 77.935 MPa after 35 days.

For EPDM, a consistent increase in Young’s modulus was also observed, rising from a value of 3.652 MPa for the unaged material to 8.23 MPa for the rubber aged for 21 days, which, as with CR, represented a more than twofold increase. Similarly, the degradation processes accelerated after 21 days of ageing, but at a much slower rate, resulting in an increase in Young’s modulus to 20.803 MPa.

Figure 9 and Figure 10 show the Mooney–Rivlin plots. In the case of chloroprene rubber (Figure 9), the Mooney–Rivlin curve for the unaged material is nearly linear across the entire range from low strain to high strain, with a gradual increase in reduced stress from approximately 0.6 MPa to 1.2 MPa. After 7 days of ageing, only the intermediate and low strain regions were observed, with a linear increase in reduced stress up to an extension rate of 0.75, followed by a more pronounced increase in stress. After 21 and 35 days of ageing, only the low strain region remained, with a very steep rise in reduced stress observed over the 35-day ageing period.

When analyzing the curves for aged EPDM (Figure 10), a clear change can be observed. After 7 days of ageing, the diagram initially followed a linear course (up to a value of approximately 0.65 1/λ), after which there was a gradual increase in reduced stress. However, for the ageing periods of 21 and 35 days, the plateau phase is no longer present. Additionally, a significant increase in reduced stress was observed after 35 days of ageing, with the curve shifting to only the low-strain range. Additionally, a change in the course of the curves, from a linear course for aged materials to a strongly non-linear course for materials aged for 35 days, can be observed.

The change in fracture energy for both the materials and ageing periods considered is presented in Figure 11. It was determined as the area under the stress–strain curve using a numerical integration algorithm. For chloroprene rubber, a steady decrease in fracture energy was observed throughout the ageing process. In contrast, for EPDM, a nearly constant fracture energy was observed between 7 and 21 days of ageing, followed by a rapid decrease.

Figure 12 shows the upturn strain values. The upturn strain is defined as the strain corresponding to the minimum reduced stress. In all cases considered, a clear decrease in values with increasing ageing time is observed, with a particularly pronounced decrease for chloroprene rubber.

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]

$$W=\sum _{p,q=0}^n{C}_{pq}{(I_1-3)}^p{\left(I_2-3\right)}^q$$

(6)

where W is the material’s free energy, C_pq 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:

$$W=\sum _{p,q=0}^n{C}_{pq}{({\overline{I}}_1-3)}^p{\left({\overline{I}}_2-3\right)}^q+\sum _{k=1}^3{\displaystyle \frac{1}{D_k}}{\left(J-1\right)}^{2k}$$

(7)

where ${\overline{I}}_1=J^{-{\displaystyle \frac{2}{3}}}{I}_1$, ${\overline{I}}_2=J^{-{\displaystyle \frac{4}{3}}}{I}_2$, 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:

$$W=\sum _{p,q=0}^n{C}_{pq}{({\overline{I}}_1-3)}^p{\left({\overline{I}}_2-3\right)}^q+{\displaystyle \frac{K}{2}}{\left(J-1\right)}^2$$

(8)

where K—is the linear volume modulus determined from the corresponding linear shear modulus $\mathrm{G}=(C_{10}+C_{01})$ 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. Polynomial material model constants for chloroprene rubber.

	Polynomial Material Model Constants
	C10	C01	C11	C20	C02	C30
Unaged CR	−2.6724	4.3870	−7.9789	2.8989	6.8326	−0.0732
CR 7 days	−9.9512	13.6042	−9.0913	2.7847	11.3484	−0.0506
CR 21 days	1.789 × 10−6	3.6507	2.225 × 10−8	5.943 × 10−7	2.636 × 10−7	1.973 × 10−6
CR 35 days	0.0252	15.9529	0.0023937	1.316 × 10−4	0.0137	6.514 × 10−4

and

Table 4. Polynomial material model constants for EPDM.

	Polynomial Material Model Constants
	C10	C01	C11	C20	C02	C30
Unaged EPDM	2.342 × 10−5	1.3084	1.077 × 10−4	1.080 × 10−4	5.736 × 10−8	0.0093
EPDM 7 days	1.312 × 10−7	1.6954	3.089 × 10−4	2.835 × 10−4	3.234 × 10−4	0.03644
EPDM 21 days	1.247 × 10−5	2.26512	4.217 × 10−7	2.880 × 10−7	2.977 × 10−7	1.123 × 10−4
EPDM 35 days	5.392 × 10−6	3.90569	1.708 × 10−6	5.455 × 10−7	2.331 × 10−6	4.780 × 10−6

. The R² 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. True stress obtained for constant true strain in every considered case for chloroprene rubber.

		True Stress [MPa]
	Ageing Time [Days]	Exp. Avg.	Polynomial	Polynomial Diff. [%]
CR	7	6.41	6.31	1.56
	21	5.01	5.40	7.78
	35	4.93	4.87	1.22

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. True stress obtained for constant true strain in every considered case for EPDM rubber.

		True Stress [MPa]
	Ageing Time [Days]	Exp. Avg.	Polynomial	Polynomial Diff. [%]
EPDM	7	6.49	6.48	0.16
	21	5.06	4.99	1.38
	35	5.22	5.23	0.19

. 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].