# Thermal ageing of PP-EPDM thermoplastic vulcanizate: Multiscale study and kinetic modeling

Ibou Ndour, Quentin Mougne, Marie France Falzon, Emmanuel Richaud

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Ibou Ndour, Quentin Mougne, Marie France Falzon, Emmanuel Richaud. Thermal ageing of PP-EPDM thermoplastic vulcanizate: Multiscale study and kinetic modeling. Polymer Degradation and Stability, 2025, 240, pp.111478. 10.1016/j.polymdegradstab.2025.111478. hal-05466285

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# Thermal ageing of PP-EPDM thermoplastic vulcanizate: Multiscale study and kinetic modeling

Ibou Ndour<sup>a,b</sup>, Quentin Mougne<sup>a</sup>, Marie France Falzon<sup>a</sup>, Emmanuel Richaud<sup>b,*</sup> <sup>D</sup>

## ARTICLE INFO

**Keywords:**

Thermoplastic vulcanizate
Thermal oxidation
Plasticizer
Kinetic model
PP
EPDM

## ABSTRACT

This work presents a multi-scale study of the impact of thermal ageing of thermoplastic vulcanizate (TPV) obtained by blending a thermoplastic matrix (polypropylene), plasticizer, and dispersed crosslinked EPDM rubber particles. This study focuses on the case of TPV without pigments or stabilizers. The thermal ageing was mainly monitored by Fourier transform infrared and thermal analysis, to identify oxidation products and their kinetics of appearance, and GPC analysis performed on extracted PP fraction of the TPV. Data indicated that EPDM phase oxidizes first which drives degradation of the TPV. Appearance changes were characterized by colorimetry and gloss. Oxidation leads to strong changes of mechanical properties and aspect properties observed from the early hours of ageing, yellowing (change of Δb*) being also induced by the plasticizer. Yellowing reached unacceptable level (regarding the requirements for exterior automotive parts) before mechanical properties. The oxidation of EPDM and TPV was thus simulated by a kinetic model based on kinetic parameters already available in the literature. Its validity was verified by its capability to simulate the appearance of primary (hydroperoxides) and secondary (carbonyls) oxidation products.

## 1. Introduction

Thermoplastic vulcanizates (TPV) constitute a family of polymers in which rubbery crosslinked particles (typically in EPDM) are mixed with a thermoplastic matrix (typically polypropylene) [1,2]. Thermoplastic polymer allows the recyclability of materials after their end of life, since PP generates a continuous matrix [3] meanwhile the dispersed elastomeric phase allows a moderate stiffness. TPV's are envisaged for many outdoor applications (automotive, buildings...) which makes necessary to understand the ageing mechanisms involved in the loss of their aspect and mechanical properties, and establish a kinetic model for predicting the degradation.

The oxidation mechanisms of polypropylene and EPDM have been extensively studied for many years, together with their consequences in terms of mechanical properties. The main chemical trackers have been determined by FTIR [4-7], NMR [8,9], iodometry [4,10,11] and chemiluminescence [12] so that the possible oxidation mechanism has been discussed [4,7,8,12-16]. For PP, it was in particular shown that tertiary carbons were the major oxidation sites [16]. The resulting products were mainly methyl ketones, carboxylic acids, accompanied with a high level of chain scissions [17]. Were successfully compared

with kinetic models tests for thermal oxidation [10,18-20] and able to simulate the effect the effect of temperature, oxygen pressure, and UV light on the rate of appearance of carbonyls and hydroperoxides. For polypropylene, the consequences of chemical changes have been investigated in terms of molar mass changes [21], crystalline microstructure which allowed to propose embrittlement criteria based on the chemi-crystallization mechanism: embrittlement comes from molar mass decrease together with the increase of crystallization due to segments released from chain scission [21,22]. For EPDM, FTIR coupled with gaseous treatment and iodometry were used to identify the nature of oxidation products. Rheometry analysis performed on virgin linear uncrosslinked EPDM and its aged analogues revealed the predominant crosslinking [23-26] For samples crosslinked during the manufacturing process, ageing was monitored by hardness [25], thermoporometry [25], DMA [27], tensile tests [28]. Classical theories of rubber elasticity were used for predicting effect of chain scission or crosslinking on elastic parameters, meanwhile the effect on ultimate properties remains to be fully understood.

Despite this impressive understanding of the long-term behaviour of its single components, the prediction and the improvement of TPV lifetime under thermal oxidative ageing remains an open issue for

<footnote><sup>a</sup> CRI Hutchinson RUE Gustave Nourry, 45120 Chalette-Sur-Loing</footnote>

<footnote><sup>b</sup> Laboratoire Pimm, Arts et Métiers Sciences et Technologies, CNRS CNAM, 151 boulevard de l'Hopital, Paris 75013, France</footnote>

<footnote>* Corresponding author.
E-mail address: emmanuel.richaud@ensam.eu (E. Richaud).</footnote>
several reasons. It is first needed to identify the root causes and the level of degradation responsible for the loss of properties (embrittlement, yellowing). It is thus needed to describe the oxidation of this biphasic material, keeping in mind that the formulation of TPV also contains a plasticizer needed to make processing easier. It is last needed to propose a kinetic model where the key steps of oxidation are considered but capable to represent the overall degradation and adaptable to the case of stabilized materials.

For that purpose, a multiscale approach will be performed including the study of chemical changes occurring in TPV and in its single components (PP, EPDM, plasticizer). This will be completed by a study of ageing consequences on mechanical and aspect properties. Finally, a preliminary kinetic model will be proposed in order to prepare future studies on stabilized TPV photo-thermal ageing.

# 2. Experimental

## 2.1. Materials

Materials under investigation are detailed in Table 1.

Polypropylene (PPH3060, Total Energies) used in this study is a grade for injection purpose application with a melt flow index MFI = 1.8 g/10 min at 230 °C/2.16 kg according to ISO 1133 standard test, with a density of 0.905 g/cm³ and with an initial OIT equal to 113 min (Fig. SI-1) before processing. M_n = 159 kg/mol, M_w = 480 kg/mol in PS equivalent were measured by GPC. EPDM (Keltan 9950 from Arlanxeo) contains 44 % by weight ethylene, 47 % by weight propylene and 9 % by weight 5-ethylidene-2-norbornene (ENB). The plasticizer (Nypar 330 from Nynas) used is a paraffinic oil with a density of 0.890 g/cm³.

The TPV blend (EPDM/PP/plasticizer in equal weight ratio) was obtained by dynamic vulcanization (using an organic peroxide) in an internal mixer. After extrusion of a 2 mm thick strip, the resulting material is made of co-continuous phases of cross-linked EPDM with PP (see Fig. SI-2).

Some TPV plates were reprocessed by compression molding at 180 °C during 60 s using a Gibrite laboratory press under a 24 MPa pressure in order to obtain 70–100 µm thin films for which oxidation is expected to be not controlled by oxygen diffusion since the thickness of oxidized layers of PP and EPDM are respectively about 10 µm and 1 mm [29,30] and, as it will be seen later, oxidation is driven by EPDM phase. In such thin films, FTIR in transmission mode are possible.

## 2.2. Ageing

Samples were subjected to thermal oxidative ageing at 120 °C in ventilated ovens (AP60 System Climatic Service).

## 2.3. Testing procedures

### 2.3.1. Infra-red spectrophotometry IR

Structural characterization was carried out using a Nicolet 560 Fourier Transform Infrared Spectrometer (FTIR). Both transmission-mode and ATR-mode FTIR were used.

TPV films with thicknesses of between 70 and 100 µm were analyzed in transmission mode in the 4000–400 cm⁻¹ wavelength range. The oxidation was followed determined by monitoring the appearance of

<table><caption>**Table 1**
Samples geometry, corresponding testing methods and figures.</caption><thead><tr><th>sample</th><th>Thickness</th><th>testing method</th><th>Figure</th></tr></thead><tbody><tr><td>TPV</td><td>2 mm plate</td><td>FTIR-ATR, Tensile test</td><td>1, 3, 9, 10</td></tr><tr><td>TPV</td><td>70–100 µm film</td><td>FTIR, GPC</td><td>3, 4, 6</td></tr><tr><td>EPDM</td><td>2 mm plate</td><td>FTIR-ATR, DSC</td><td>2a, 3, 5</td></tr><tr><td>PP</td><td>2 mm plate</td><td>FTIR-ATR</td><td>2b, 3a</td></tr><tr><td>Plasticizer</td><td>Liquid</td><td>FTIR, UV</td><td>8a, b</td></tr></tbody></table>

oxidation products such carbonyls and hydroperoxides. Absorbances were converted to concentrations using the Beer-Lambert law with molar absorption values ε₁₇₁₅ = 300 l/mol/cm for carbonyls and ε₃₄₁₀ = 70 l/mol/cm for hydroperoxides [31].

TPV and EPDM plates were analyzed by FTIR-ATR using a ZnSe crystal with a 45° incident angle in the 4000 to 650 cm⁻¹ spectral range. All FTIR spectra were recorded by signal of 8 scans at 4 cm⁻¹ resolution.

Analyses of plasticizer were performed by placing a drop of plasticizer directly on the diamond ATR crystal.

### 2.3.2. UV-visible spectrophotometer

UV-visible spectra of unaged and aged plasticizer were recorded on a PERKIN-ELMER Lambda 35 UV-visible. About 1 g of plasticizer was added to 25 ml of cyclohexane.

The spectra were recorded between 200 and 800 nm, with a slit width of 2 nm and a scanning speed of 60 nm/min.

### 2.3.3. Hydroperoxides titration by SO₂ treatment coupled with FTIR

The hydroperoxides (POOH in the following) were dosed by treatment with sulphur dioxide (SO₂) basing on the following reaction [10]:

$$POOH + SO_2 \rightarrow PO - SO_2 - OH$$

Treatments were performed on about 100 mg TPV films. SO₂ was generated by dissolving approximately 100 mg of Na₂SO₃ in 40 ml of HCl (analysis grade 35 % - VWR). The treatment was carried out at room temperature for 24 h.

### 2.3.4. DSC analysis

DSC was used to characterize the exotherm due to POOH decomposition [32]. The concentration of hydroperoxides was determined from Eq. (1):

$$[POOH] = \frac{\Delta H_{POOH \text{ experimental}}}{\Delta H_{POOH \text{ theory}}} \quad (1)$$

With ΔH experimental corresponding to the enthalpy of the exotherm between 130 °C - 200 °C on the irreversible flow (in J/g). The theoretical molar enthalpy (325 kJ/mol) was obtained from literature values [33].

In EPDM, experiments were conducted by heating samples from room temperature to 300 °C at a constant 10 °C/min rate under nitrogen atmosphere supplied by a constant 50 ml/min flux.

In TPV, POOH decomposition exotherm is overlapped with melting of PP phase. This is the reason why modulated DSC was preferred. Modulation allows the overall signal to be decomposed into a reversible component corresponding to melting and a non-reversible component which is attributed to the heat of decomposition of hydroperoxides [10]. The tests were carried out with an MDSC Q2500 (TA Instruments) under a nitrogen flow of 50 ml/min. 5–10 mg TPV samples were heated from 0 to 300 °C with a 3 °C/min ramp using a 50 s period and a 0.265 °C amplitude. Those conditions correspond to "heat only" mode with at least 5 cycles of temperature modulation carried out in the temperature range where the phenomenon takes place.

### 2.3.5. Spectrocolorimeter analysis

The study of color and gloss was carried out using a Spectro 2 Guide gloss portable spectrophotometer from BYK-Gardner. This instrument consists of a spectrophotometer with a fluorimeter, which allows both color and gloss to be measured.

Dimensionless colorimetric parameters L* a* b* of the CIELAB reference system were measured at a 45° observation angle under D65 illuminant corresponding to daylight. Measurements were done in triplicate. The average colorimetric values (ΔL*, Δa* and Δb*) were used to determine the color changes (ΔE) using Eq. (2):

$$\Delta E = \sqrt{(\Delta L^*)^2 + (\Delta a^*)^2 + (\Delta b^*)^2} \quad (2)$$
ΔL*, Δa* and Δb* are the total changes in L*, a*, b* during ageing. An increase in L* means that the color of the sample lightens. A positive Δa* means a color change towards red and a negative Δa* a color change towards green. A positive Δb* means a shift towards yellow and a negative Δb* a shift towards blue.

Gloss was measured in triplicate for a 60° test angle under D65 illuminant. ΔG positive means a more glossy surface and ΔG negative means a rougher aspect.

### 2.3.6. High temperature gel permeation chromatography (HT-GPC)

Molecular weight analysis of the TPV films was performed using an Agilent-PL GPC HT220 chromatograph. Approximately 15 mg of TPV samples were first immerged in 10 ml of 1,2,4-trichlorobenzene during 2 h. PP chains were extracted from TPV at 135 °C during 5 min under vigorous shaking. Extraction time was chosen to be short enough to avoid PP degradation. The resulting solutions were subsequently filtered through 5 µm stainless steel filters to separate the polypropylene (PP) phase from the insoluble EPDM phase. Filtered solutions were analyzed at 135 °C using a 50 µl injection volume. Trichlorobenzene was used as mobile phase (1 ml/min). The analysis was conducted using a refractive index detector. Calibration curves were established using four polystyrene standards with molecular weights of 3.07, 46.5, 257 and 1470 kg/mol. Values are given as PS equivalent.

### 2.3.7. Tensile tests

Tensile tests were performed on TPV aged up to 100 days according to ISO 527-2 at a constant cross-head speed of 500 mm/min using tensile testing machine Instron 5544 equipped with a 1 kN load cell. Strain was calculated from the displacement of clamps. Dumb-bell shaped specimens (type 2) were punched from 2 mm extruded plates before ageing. Three samples were tested for each ageing duration.

## 3. Results

### 3.1. FTIR analysis of TPV oxidation

Fig. 1a shows the FTIR spectra in the 2700 - 3100 cm⁻¹ region corresponding to methyl and methylene groups stretching. A strong band at 2919 cm⁻¹ is attributed to the tertiary (>CH-) and >CH₂ stretching in PP and EPDM. Two other bands present at 2851 and 2950 cm⁻¹ correspond respectively to the (>CH-) stretching of the methyl and methylene groups in EPDM, and to the >CH- of the methyl groups in PP. Despite the difficulty to normalize spectra (since almost every function is likely to react), it seems however that the intensity of band at 2850 cm⁻¹ faster than 2919 cm⁻¹ one, meanwhile the band at about 2950 cm⁻¹ (-CH₃) remained unchanged throughout the ageing process.

In the hydroxyl region (Fig. 1b), a broad band appears with a maximum around 3400 cm⁻¹ which progressively shifts to around 3500 cm⁻¹. It corresponds to alcohols, hydroperoxides and carboxylic acids [4].

Finally, in the carbonyl region (Fig. 1c), one can also see the formation of a broad band with several maxima or shoulders at 1715, 1735 and 1775 cm⁻¹ corresponding respectively to carboxylic acids, ketones, esters, peresters and lactones [6]. The maximum peak appears at 1715 cm⁻¹ during the early stages of ageing, and is shifted to 1735 cm⁻¹ at longer times.

As shown in Fig. 1c, the oxidation of TPV takes place at early exposure times, which means that TPV is very sensitive to oxidation.

To better understand the origin of TPV oxidation, a comparison of TPV, PP and EPDM ageing was carried out. The IR spectra of thermally aged EPDM and PP are shown in Fig. 2. They call for the following comments:

- The oxidation of EPDM occurs at early ageing times (typically 1 day at 120 °C), in the same manner than TPV (Fig. 2a). The shape and evolution of the peaks of the oxidized EPDM are similar to those

<figure><img src="image_4.png" /><figcaption>Fig. 1. ATR-FTIR spectra of TPV aged 120 °C for various durations (expressed in days) in the C—H stretching region (a), hydroxyl stretching region (b) and carbonyl region (c).</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 2. ATR-FTIR spectra of EPDM (a), PP (b) as a function of ageing time at 120 °C.</figcaption></figure>

observed in the case of TPV aged under the same conditions. A clear maximum characteristic of esters is detected at 1735 cm⁻¹. It is accompanied by a shoulder at 1715 cm⁻¹ (ketones and associated carboxylic acids) and 1770 cm⁻¹ (lactones) [14]. It is noteworthy that after peroxide crosslinking, no traces of residual double bonds were observed neither in EPDM (Fig. SI-3) nor in TPV.

- At the same time, PP aged at 120 °C under air gets oxidized, but at much longer times than TPV and EPDM (Fig. 2b).

This difference is clearly seen by plotting the evolution of the band at 1715 cm⁻¹ with the ageing time of TPV, EPDM and PP (Figs. 3a and 3b). The formation of oxidation products can be seen at early ageing times (about 1 day) in the case of TPV and EPDM with a very low oxidation period. However, for PP, this latter is much longer which was expected since PP oxidation induction time (OIT) at 180 °C was about 113 min (see Fig. SI-2) guaranteeing its stability during TPV process and early ageing times at 120 °C. These results lead to the conclusion that EPDM part is the most susceptible to rapid degradation during ageing which will be tentatively confirmed later.

In order to quantify oxidation, the concentration of oxidized species was estimated. Despite molar absorption values are still a subject of discussion, as pointed out by Celina et al. in a recent study [34], ε₁₇₁₅ = 300 l/mol/cm for carbonyls and ε₃₄₁₀ = 70 l/mol/cm for

<figure><img src="image_6.png" /><figcaption>Fig. 3. a) Variation of absorbance at 1715 cm⁻¹ versus ageing time at 120 °C for EPDM (■), TPV (◆) and PP (●), and b) carbonyl concentration estimated from FTIR-ATR in EPDM (■), in TPV plates (▶), in TPV plates from EPDM data using Eq (17) (□), using FTIR-Transmission in TPV films (◆) and simulations from best kinetic model (see “Discussion”).</figcaption></figure>

hydroperoxides were used.

For thin TPV aged films, the carbonyl concentration in the TPV was determined directly using the Beer-Lambert law. In the case of EPDM and TPV plates analyzed by FTIR-ATR, the depth of analysis (dₐ) was estimated from Harrick's laws (Eqs. (3)–6) and then used in Beer-Lambert's law:

$$d_{e\perp} = \frac{\lambda n_{21} \cos\theta}{n_1 \pi (1 - n_{21}^2) (\sin^2\theta - n_{21}^2)^{1/2}} \quad (3)$$

$$d_{\parallel} = \frac{\lambda n_{21} \cos\theta (\sin^2\theta - n_{21}^2)}{n_1 \pi (1 - n_{21}^2) [(1 - n_{21}^2) \sin^2\theta - n_{21}^2] (\sin^2\theta - n_{21}^2)^{1/2}} \quad (4)$$

$$d_e = \frac{d_{e\perp} + d_{\parallel}}{2} \quad (5)$$

$$n_{21} = \frac{n_2}{n_1} \quad (6)$$

Where dₐ⊥ and dₐ‖ respectively are the effective thicknesses of the sample in the s and p planes of polarization, λ is the wavelength, θ is the
angle of incidence of the light beam ($\theta = 45^{\circ}$), $n_1$ and $n_2$ respectively are the crystal refractive index of diamond ($n_1 = 2.4$) and of the sample ($n_2 = 1.5$). Finally, $d_e$ is estimated to approximately 1.16 µm at 1715 cm$^{-1}$.

Corresponding results given in Fig. 3b show the concentrations of carbonyls in EPDM and TPV obtained by ATR spectroscopy. The carbonyl concentration in TPV is very close to the value obtained by multiplying the concentration of carbonyl in EPDM by weight fraction of EPDM in TPV (0.37).

### 3.2. Hydroperoxides titration

Changes of FTIR spectra in the region between 3100 and 3600 cm$^{-1}$ are due to the overlapping of several hydroxylated species among which hydroperoxides. Those later are of key importance since they are linked to initiation and propagation reaction, and the shape of POOH curves vs times offers a reliable criterion to validate kinetic models.

However, their detection using simple FTIR measurements are somewhat difficult due to the overlap of various hydroxyl groups bands. To overcome this limitation, an alternative is to submit aged samples to a SO$_2$ vapor treatment [31]. This one leads to the formation of new absorption bands characteristic of sulphates (O=S=O located at 1177 cm$^{-1}$) and others at 1050 cm$^{-1}$ (S=O), 910 cm$^{-1}$ (S-O-C) and 593 cm$^{-1}$ (C-S) [35]. In the hydroxyl region, the generated sulphates are expected to be unstable, so that a noticeable decrease of the band located at 3410 cm$^{-1}$ (Fig. 4) is observed and can be used to estimate the concentration of hydroperoxides [33].

Besides SO$_2$ treatments, hydroperoxides can be quantified by DSC analysis since their decomposition induces a broad exotherm in the 150–250 °C region [33]. Corresponding titrations performed on EPDM are given in Fig. 5.

Fig. 5a shows thermograms of oxidized EPDM samples with the increase of an exotherm centered around 150 °C where the corresponding enthalpy is attributed to exothermic processes associated with the decomposition of hydroperoxides [33,36].

Data for EPDM (from MDSC) and TPV (MDSC and SO$_2$) are superimposed in Fig. 5b. It can be seen that curves display a slight induction period (about 5 h), followed by a rapid increase to reach a value of 0.1 mol/l at 15 h ageing. After 30 h of ageing, a decrease in concentration is observed, probably due to consumption of the substrate and the progressive decomposition of the hydroperoxides.

Interestingly, the induction times are relatively close in EPDM and TPV and the maximal level in TPV (about 0.15 mol/l) is relatively close to the concentration observed in EPDM “normalized” by the weight fraction of EPDM in TPV which is a supplementary argument in favor of oxidation predominating in EPDM phase.

<figure><img src="image_7.png" /><figcaption>Fig. 4. Effect of SO$_2$ treatment on FTIR spectra of TPV.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 5. DSC thermograms of EPDM exposed to 120 °C in air at different ageing times (a) and changes in POOH concentration in EPDM (■), in TPV using Eq. (17) (□), in TPV films using SO$_2$ treatment (●) (b). Data are overlapped with best kinetic model (see “Discussion”).</figcaption></figure>

### 3.3. Macromolecular analysis of PP phase

Fig. 6a displays molecular weight distributions obtained by Gel Permeation Chromatography for the PP phase extracted from TPV. Molar mass value of PP extracted from TPV is significantly lower than initial molar mass of PP, due to possible degradation of PP chains during processing at high temperatures with possible presence of residual peroxides (compounding and extrusion of TPV plate, and compression molding of this later into film). To highlight the changes induced by thermal ageing, the evolution of the weight-average molecular weight ($M_w$) of the PP fraction extracted from TPV with exposure time at 120 °C is shown in Fig. 6b. In contrast to previous studies performed on PP [11] where molar mass decreases simultaneously with chemical changes, the results obtained in TPV show that the average molar mass of extracted PP fraction remains almost constant during the first hours of ageing, and starts to drop significantly after 25 h of ageing whereas carbonyls are clearly visible (Fig. 1). These observations confirm that TPV oxidation is mainly driven by the EPDM phase.

### 3.4. Change in appearance properties

Pictures of thermally aged TPV are given in Fig. SI-4. Samples turn
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Molar mass distributions of PP extracted in TPV (a) and changes in the average molar mass by weight with ageing time at 120 °C for PP extracted from TPV films (b).</figcaption></figure>

yellow after 1 day of exposure. Yellowing increases up to 7 days. After 10 days, color changes from yellow to yellow-orange, and finally turns brown after 20 days of ageing.

These observations were quantified by color measurements from changes of ΔL*, Δa*, Δb* and ΔE parameters corresponding respectively to the bleaching, reddening, yellowing and total color difference (according to Eq. (1)) as illustrated in Fig. 7a.

Before exposure, the surface of the TPV is characterized by a luminosity of 68 units. After 5 days of ageing, ΔL* decreased significantly until 50 days of ageing, and then reached a plateau. Contrary to ΔL* values, Δa* and Δb* values increase from the first days of ageing. In particular, Δb* reaches 30 units after 10 days, expressing the yellowing of the TPV surface. This increase in Δb* is due to the accumulation of chromophore groups at early ageing stage, as confirmed by FTIR spectroscopy (see Fig. 1), and also to the plasticizer used in the TPV formulation, as it will be seen later. Then, after 20 days of ageing, the surface begins to redden, resulting in the predominance of Δa* parameter (compared to Δb*), which increases significantly between 30 and 60 days of ageing.

ΔE changed almost in the same way as Δb* and for the early times of ageing. It increased sharply in the first 10 days, and then slightly during

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Effect of ageing time on the surface color (a) and gloss (b) at 120 °C.</figcaption></figure>

the following days before plateauing after 30 days of exposure.

Gloss is another important parameter for describing the surface appearance of a material. Fig. 7b shows the variation in gloss with ageing time. It can be seen that the surface of the TPV increased with ageing time, indicating a smooth surface. After 100 days of ageing, the gloss begins to decrease but remains higher than its initial value.

### 3.5. Structural changes of plasticizer

The paraffinic oil used here as plasticizer has an important role during TPV processing since it lowers the viscosity of compound, favors homogenization, decreases the processing temperature and avoids premature crosslinking of EPDM part. To better understand its effect on thermal ageing, pure plasticizer was aged at 120 °C. Plasticizer turns yellow within the first hours of exposure which contributes to TPV yellowing (Fig. SI-5 and Fig. 7). FTIR spectra (Fig. 8a) display typical signals of paraffinic structure i.e. symmetrical and asymmetrical stretching vibration bands in the 2800 to 3000 cm⁻¹ wavenumber range (methyl and methylene groups), at 1377 cm⁻¹ and 1460 cm⁻¹ (bending vibrations of the methyl and methylene units). After 192 h ageing, no significant spectral change due to oxidation can be detected whereas in the case of TPV, changes are observed within first hours of ageing, particularly in the carbonyl zone. These observations seem to confirm
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 8.** Changes of FTIR (a) and UV spectra (b) of virgin and aged paraffinic plasticizer used in TPV during thermal oxidation.</figcaption></figure>

the oxidative stability of the plasticizer.

UV analysis were thus conducted (Fig. 8b) on plasticizer before and after different ageing times at 120 °C. Unaged plasticizer displays two bands around 250–260 nm, which could be due to the presence of chromophore or a trace of an antioxidant present in low concentration in the plasticizer [37]. These bands gradually disappear meanwhile sample absorb more and more at longer wavelengths. The presence of unsaturations seems unlikely since it would oxidize very fast into α-β-unsaturated carbonyls, which is contrary to experimental results. The presence of a stabilizer added to the plasticizer seems hence the most plausible explanation of the yellowing observed from the first hours of ageing.

### 3.6. *Changes in mechanical properties*

Fig. 9a shows the stress-strain curves of unaged and aged TPV. Unaged TPV has a high strain at break, which is characteristic of elastomers. It decreases during ageing with the same fracture envelope than unaged samples.

Fig. 9b illustrates the evolution of the mechanical properties of vulcanized thermoplastics (TPV) during ageing at 120 °C for different

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 9.** Stress-Strain curves of aged and virgin TPV (a) and corresponding changes of elongation at break and stress at break with ageing time (b).</figcaption></figure>

exposure times. There is a noticeable decrease in elongation at break and stress at break. Indeed, after 48 h of ageing at 120 °C, these two parameters undergo a reduction of >50 %, resulting in significant oxidation of the material.

This simultaneous reduction in elongation at break and stress at break can be directly attributed to the oxidation of EPDM as confirmed “Discussion”. The changes in elastic modulus can be considered as relatively weak, so that it was difficult to conclude if chain scission or crosslinking predominates in the EPDM phase. To go further, two complementary analyses were performed: sol-gel analysis (see Fig. SI-6) and micro-hardness measurements (Fig. SI-7). The increase of soluble fraction content and the decrease of micro-hardness indicate that at early times, chain scission should predominate. However, it seems also that at longer times, some crosslinking should co-exist and partially compensate chain scissions.

### 4. Discussion

The aims of this section are to discuss the most possible mechanisms of ageing in TPV, propose an associated kinetic model (and likely to be used in future studies on photooxidation of TPV at lower temperature), and discuss on the possibility to correlate the model outputs (namely concentration intermediate and stable species, such as hydroperoxides and carbonyls) with changes of engineering properties (yellowing,
mechanical properties). We stress on the fact that both aspect (Fig. 7) and mechanical properties (Fig. 9b) display strong changes in the earlier ageing times, so that kinetic model will be aimed at describing oxidation curves at short ageing times.

## 4.1. Origin of oxidation

According to present experimental results, the oxidation of TPV occurs in the EPDM part. This later contains secondary and tertiary sites, which are more affected by thermal oxidation than primary ones (-CH₃). Considering that there are two possible oxidation sites: secondary sites S (>CH₂ type) and tertiary sites T (>CH-) type), 4 propagation reactions can be envisaged for the C—H abstraction reaction by peroxy radicals:

$$P_{T}OO^{\circ} + P_{T}H \rightarrow P_{T}OOH + P_{T}^{\circ} \qquad (7)$$

$$P_{S}OO^{\circ} + P_{S}H \rightarrow P_{S}OOH + P_{S}^{\circ} \qquad (8)$$

$$P_{T}OO^{\circ} + P_{S}H \rightarrow P_{T}OOH + P_{S}^{\circ} \qquad (9)$$

$$P_{S}OO^{\circ} + P_{T}H \rightarrow P_{T}OOH + P_{S}^{\circ} \qquad (10)$$

Their rate constants are predicted by the semi-empirical laws proposed by Korcek et al. [38]:

$$\log_{10} k_{3}^{PSOO^{\circ}} (30^{\circ}\text{C}) = 16.4 - 0.2 \times \text{BDE(C-H)} \qquad (11)$$

$$E_{3}^{PSOO^{\circ}} = 0.55 \times (\text{BDE(C-H)} - 62.5) \qquad (12)$$

$$\log_{10} k_{3}^{PTOO^{\circ}} (30^{\circ}\text{C}) = 15.4 - 0.2 \times \text{BDE(C-H)} \qquad (13)$$

$$E_{3}^{PTOO^{\circ}} = 0.55 \times (\text{BDE(C-H)} - 65) \qquad (14)$$

The use of these equations leads to values summarized in Table 2.

According to these results, the ratio giving the probability of an attack by an sPOO° radical on a tertiary or secondary site is given by:

<table><caption>**Table 2**
Kinetic parameters for propagation from literature [38].</caption><thead><tr><th>Reaction</th><th>PSOO<sup>°</sup>+PSH</th><th>PTOO<sup>°</sup>+PSH</th><th>PSOO<sup>°</sup>+PTH</th><th>PTOO<sup>°</sup>+PTH</th></tr></thead><tbody><tr><td>Abbreviated notation</td><td>k<sub>3SS</sub></td><td>k<sub>3TS</sub></td><td>k<sub>3ST</sub></td><td>k<sub>3TT</sub></td></tr><tr><td>BDE (kJ/mol)</td><td>398</td><td>398</td><td>385</td><td>385</td></tr><tr><td>log<sub>10</sub> k<sub>3</sub>(30 °C)</td><td>-2.6</td><td>-3.6</td><td>-2</td><td>-3</td></tr><tr><td>E<sub>3</sub> (kJ/mol)</td><td>74.8</td><td>74.8</td><td>62.1</td><td>62.1</td></tr><tr><td>k<sub>0</sub> (l/mol/s)</td><td>1.95 × 10<sup>10</sup></td><td>1.95 × 10<sup>9</sup></td><td>5.1 × 10<sup>8</sup></td><td>5.1 × 10<sup>7</sup></td></tr><tr><td>k<sub>3</sub> (120 °C) (l/mol/s)</td><td>2.227</td><td>0.222</td><td>2.8</td><td>0.28</td></tr></tbody></table>

$$r_{1} = \frac{k_{3ST}}{k_{3SS}} \frac{[PTH]}{[PSH]} \qquad (15)$$

$$r_{2} = \frac{k_{3TT}}{k_{3TS}} \frac{[PTH]}{[PSH]} \qquad (16)$$

According to Eqs (15)–16, secondary hydroperoxides would predominate over tertiary ones in PP where [sC–H] = [tC–H], whereas tertiary hydroperoxides actually predominate [16]. However, tertiary peroxy radicals decompose much faster than secondary radicals, which explains why the carbonyl region of TPV, FTIR spectra shows very similar characteristics to those of oxidized PP. This suggests that oxidation starts mainly from the tertiary (>CH-) fraction of the propylene in EPDM and continues at longer time by propagating on secondary >CH₂. Those observations are in line with hardness tests (Fig. SI-7) assuming that chain scissions are for us associated to tertiary >CH-reactivity (beta scission of tertiary alkoxyl) and crosslinking to secondary groups reactivity (>CH° coupling). It allows us, as a first approach, to strongly simplify the analysis of the mechanisms by considering that the oxidation mainly concerns the tertiary sites (Scheme 1).

At longer ageing times, the PP phase (protected by some antioxidants – see Fig. SI-1) and poorly miscible with EPDM, certainly will get oxidized, but this is not simulated by the present kinetic model presented here, aimed at describing the early ageing times. In the same time, plasticizer evaporation and direct polymer thermolysis seems negligible (see Fig. SI-8), which simplifies the kinetic modeling given in the following.

## 4.2. Kinetic modeling

Reconsidering the possible complexity of propagation reaction (see above), two kinds of kinetic model can be envisaged:

- a complete co-oxidation kinetic model, with the main drawback of unneeded complexity and possible adjustable parameters [39,40].

- a simplified kinetic model for EPDM considering only the potential oxidation of secondary >CH₂ groups and tertiary >CH- groups, and comparable of models used for PP [10,18] or PE [20]. For that purpose, we will check the capability of kinetic models to simulate data in EPDM using rate constants previously found either for PP or for PE. In a second time, kinetic model used for EPDM will be exploited to simulate data obtained in TPV using the relationship:

$$[\text{Oxidation product}]_{\text{TPV}} = [\text{Oxidation product}]_{\text{EPDM}} \times w_{\text{EPDM}} \qquad (17)$$

<figure><img src="image_15.png" /><figcaption>**Scheme 1.** Summary of the mechanisms of tertiary methyl involved in TPV thermal oxidation.</figcaption></figure>
Where $w_{EPDM}$ is the EPDM weight fraction in TPV.

The kinetic model is based on the following scheme [10]:

$$(Iu) POOH \rightarrow 2P^{\cdot} + P = O \quad k_{1u}$$

$$(Ib) POOH + POOH \rightarrow P^{\cdot} + POO^{\cdot} + P = O \quad k_{1b}$$

$$(2) P^{\cdot} + O_2 \rightarrow POO^{\cdot} \quad k_2$$

$$(3) POO^{\cdot} + PH \rightarrow POOH + P^{\cdot} \quad k_3$$

$$(4) P^{\cdot} + P^{\cdot} \rightarrow inactive product \quad k_4$$

$$(5) P^{\cdot} + POO^{\cdot} \rightarrow inactive products \quad k_5$$

$$(6) POO^{\cdot} + POO^{\cdot} \rightarrow inactive products + O_2 \quad k_6$$

Where $P^{\cdot}$, $POO^{\cdot}$, $POOH$, $PH$, $P=O$ respectively correspond to alkyl radicals, peroxy radicals, hydroperoxides, unreacted repetitive units, and carbonyl groups. Basing on these reactions, the following set of differential equations is derived:

$$\frac{d[POOH]}{dt} = -k_{1u}[POOH] - 2k_{1b}[POOH]^2 + k_3[POO^{\cdot}][PH] \quad (18)$$

$$\frac{d[P^{\cdot}]}{dt} = 2k_{1u}[POOH] + k_{1b}[POOH]^2 - k_2[P^{\cdot}][O_2] + k_3[POO^{\cdot}][PH] - 2k_4[P^{\cdot}]^2 - k_5[P^{\cdot}][POO^{\cdot}] \quad (19)$$

$$\frac{d[POO^{\cdot}]}{dt} = k_{1b}[POOH]^2 + k_2[P^{\cdot}][O_2] - k_3[POO^{\cdot}][PH] - k_5[P^{\cdot}][POO^{\cdot}] - 2k_6[POO^{\cdot}]^2 \quad (20)$$

$$\frac{d[PH]}{dt} = -k_{1u}[POOH] - k_3[POO^{\cdot}][PH] \quad (21)$$

$$\frac{\partial[O_2]}{\partial t} = -k_2[P^{\cdot}][O_2] + k_6[POO^{\cdot}]^2 \quad (22)$$

The system was solved numerically using Matlab® ode23s solver with the following boundary conditions:

* At $t = 0: [P^{\cdot}]_0 = [POO^{\cdot}]_0 = 0$

* $[POOH]_0 = 10^{-3} \text{mol/l}$

* $[PH] = 12 \text{ mol/l}$ determined from the monomer units of the oxidizable tertiary >CH- sites in EPDM. In the case of an EPDM matrix containing 47 % w/w propylene units and 9 % w/w ENB ([PH] = 40 mol/l in the of oxidizable secondary site).

* $[O_2] = S_{O_2} \times P_{O_2}$; $S_{O_2}$ being the oxygen solubility in the matrix (mol/l/Pa) and $P_{O_2}$ the external oxygen pressure of atmosphere. According to the literature, S is generally between $10^{-6}$ et $10^{-8}$ mol/l/Pa [18, 41]. In the case of this study, we have taken $S_{O_2} = 1.4 \times 10^{-8}$ mol/l/Pa.

In this work, the TPV films are thin enough (70–100 µm) to neglect the oxygen concentration gradient through the thickness and neglect the diffusion term associated to Fick’s law in Eq. (22) [29,30].

The kinetic curves for the carbonyl build up were generated using the following equation:

$$\frac{d[P=O]}{dt} = \gamma_{CO_u} k_{1u}[POOH] + \gamma_{CO_b} k_{1b}[POOH]^2 \quad (23)$$

<table><caption>**Table 3**

Kinetic parameters of PP, PE, EPDM and TPV by best fitting (see Figs. 3b and 5b).</caption><thead><tr><th>Rate constant</th><th>k<sub>1u</sub></th><th>k<sub>1b</sub></th><th>k<sub>2</sub></th><th>k<sub>3</sub></th><th>k<sub>4</sub></th><th>k<sub>5</sub></th><th>k<sub>6</sub></th></tr></thead><tbody><tr><td>unit</td><td>s<sup>-1</sup></td><td>1/mol/s</td><td>1/mol/s</td><td>1/mol/s</td><td>1/mol/s</td><td>1/mol/s</td><td>1/mol/s</td></tr><tr><td>PP</td><td>0.9 &times; 10<sup>-6</sup></td><td>2.5 &times; 10<sup>-4</sup></td><td>1.4 &times; 10<sup>7</sup></td><td>2.1 &times; 10<sup>-1</sup></td><td>10<sup>10</sup></td><td>2.6 &times; 10<sup>9</sup></td><td>2.1 &times; 10<sup>4</sup></td></tr><tr><td>PE</td><td>3.0 &times; 10<sup>-6</sup></td><td>3.1 &times; 10<sup>-5</sup></td><td>10<sup>8</sup></td><td>3</td><td>8 &times; 10<sup>11</sup></td><td>2.3 &times; 10<sup>11</sup></td><td>1.1 &times; 10<sup>9</sup></td></tr><tr><td>TPV and EPDM<br>(best fitting)</td><td>0.9 &times; 10<sup>-6</sup></td><td>2.5 &times; 10<sup>-4</sup></td><td>1.4 &times; 10<sup>7</sup></td><td>2.1 &times; 10<sup>-1</sup></td><td>10<sup>9</sup></td><td>2.6 &times; 10<sup>8</sup></td><td>2.1 &times; 10<sup>4</sup></td></tr></tbody></table>

$\gamma_{CO_u}$, $\gamma_{CO_b}$ are the carbonyl yields of the uni-and bimolecular POOH decomposition respectively chosen here equal to 1.

Table 3 shows the kinetic constants for PP [42] PE [20], and those used to simulate the hydroperoxides and carbonyls concentrations in EPDM. Simulations are directly superimposed with data in Figs. 3 and Fig. 5. As expected, neither PE kinetic parameters of PE nor PP ones allowed a satisfying simulation of EPDM data (see Fig. SI-9). However, it seems that the values of the rate constants providing the best simulations of the experimental results for the simulation of EPDM ageing at 120 °C are very close to those used for PP which seems logical since EPDM is rich in propylene groups. Comparable simulations can be obtained using $S_{O_2}$ increased to $10^{-7}$ mol/l using the rate constants values are even exactly equal to PP ones.

These results confirm that the oxidation of tertiary >CH- (propylene units in the EPDM phase) predominates. It validates the proposed model and is consistent with the matter that the shape of carbonyl spectra that rather look like to PP spectra [43] (i.e. oxidation of tertiary >CH-, with a $1715 \text{ cm}^{-1}$ band slightly higher than $1735 \text{ cm}^{-1}$ one) than PE (oxidation of secondary >CH$_2$ leading to a $1715 \text{ cm}^{-1}$ band much higher than $1735 \text{ cm}^{-1}$ one) [44].

### 4.3. Correlations between structural changes and macroscopic properties

In literature, the lifetime of polymers is generally defined according to a mechanical criterion, most often the time required for the elongation at break to reach 50 % of the value of unaged polymer. Fig. 10a illustrates the evolution of the elongation at break and the yellowing index, represented by the $\Delta b^*$ with ageing time. Here, the elongation at break decreases to 50 % of its initial value after 2 days of ageing, meanwhile the yellowing index reaches the limit of unacceptable changes much sooner (0.5 for exterior parts in automotive industry).

Fig. 10b shows that the decrease in elongation at break correlates well with the formation of oxidation products in the TPV matrix. Indeed, from the very early stages of exposure, we observe a decrease in elongation at break and a sharp increase in carbonyl products.

Fig. 10c shows the correlation between oxidation products and the yellowing index ($\Delta b^*$) of plasticized TPV and of a TPV for which the plasticizer was extracted. For unplasticized TPV, yellowing almost linearly changes with carbonyl accumulation of. On the contrary, for plasticized TPV, the linear correlation is not visible anymore particularly during the first few hours of ageing, which is explained by the yellowing of plasticizer evidenced in Fig. 7a and Fig. 8.

## 5. Conclusion

This study improved understanding of the thermo-oxidation mechanisms of thermoplastic vulcanizates (TPV) at 120 °C. It was found that the EPDM phase of TPV is more sensitive to oxidation than the PP phase. These hypotheses were confirmed by the facts that GPC analyses show the stability of the PP phase at early ageing stages, and the ratio of hydroperoxides concentration in EPDM (DSC measurements) to TPV ($SO_2$-FTIR) was close to the weight fraction of EPDM in TPV. Similarly, the carbonyl concentration obtained in transmission on TPV films was comparable to that obtained in ATR by estimating the depth of analysis using Harrick’s laws.

Moreover, yellowing was proved to be a relevant indicator of the
<figure><img src="image_16.png" /><figcaption>**Fig. 10.** Comparison of elongation at break and yellow index changes with ageing time (a), correlation between ultimate elongation and carbonyl appearance for TPV 2 mm plates (b) and correlation between yellow index and oxidation carbonyl appearance for TPV 2 mm plates (c). Dashed lines are guides for the eyes.</figcaption></figure>

material state of degradation. It occurs faster than the loss of mechanical properties. It is therefore necessary to correlate changes in aspect with outputs of kinetic model in order to reliably estimate the service life of the TPV.

In parallel, a kinetic model derived from a classical radical chain scheme was used to simulate and predict the chemical changes occurring in EPDM during ageing and then deduce those occurring in TPV. It was found that the PP kinetic parameters could be adapted to simulate the hydroperoxide and carbonyls accumulation during EPDM and TPV ageing. Since carbonyls build up is linked to mechanical properties drop, and yellowing, the model can thus predict the change in those properties, apart in the case where other molecules (e.g. plasticizer) contribute to yellowing.

In conclusion, this study has enabled us to gain a better understanding of the TPV oxidation mechanism submitted to thermal ageing, to propose a predictive model and to establish structure-property relationships. Further studies are needed to improve choice of stabilizers, and address exposures to UV light to get closer to real cases.

### CRediT authorship contribution statement

**Ibou Ndour:** Writing – original draft, Investigation, Formal analysis, Data curation. **Quentin Mougne:** Validation, Supervision, Resources, Conceptualization. **Marie France Falzon:** Validation, Supervision, Methodology, Conceptualization. **Emmanuel Richaud:** Writing – review & editing, Validation, Methodology, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The author is an Editorial Board Member/Editor-in-Chief/Associate Editor/Guest Editor for this journal and was not involved in the editorial review or the decision to publish this article.

### Acknowledgements

Authors are thankful to the Hutchinson thermoplastic polymers laboratory and the Agence Nationale de la Recherche Technologique (convention 2022/1770) for the financial support. Hutchinson staff (Sebastien Benoit) is gratefully acknowledged for providing the materials.

Gel Permeation Chromatography was bought thanks to a Carnot ARTS grant ('CEQUAM' project).

### Data availability

Data will be made available on request.

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