# Investigation of X-Rays, e-Beam, and Gamma Irradiation on Chemical, Thermal, and Mechanical Properties of Polypropylene-Derivatives

Blanche Krieguer, Sylvain R A Marque, Fabien Girard, Nathalie Dupuy, Nicolas Ludwig, Florent Kuntz, Samuel Dorey

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Blanche Krieguer, Sylvain R A Marque, Fabien Girard, Nathalie Dupuy, Nicolas Ludwig, et al. Investigation of X-Rays, e-Beam, and Gamma Irradiation on Chemical, Thermal, and Mechanical Properties of Polypropylene-Derivatives. Macromolecular Chemistry and Physics, 2025, 226 (20), pp.e00130. 10.1002/macp.202500130. hal-05451089

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RESEARCH ARTICLE

OPEN ACCESS

# Investigation of X-Rays, e-Beam, and Gamma Irradiation on Chemical, Thermal, and Mechanical Properties of Polypropylene-Derivatives

Blanche Krieguer¹ ID | Sylvain R. A. Marque² | Fabien Girard¹ ID | Nathalie Dupuy¹ ID | Nicolas Ludwig⁴ |

Florent Kuntz⁴ | Samuel Dorey³ ID

¹CNRS, IRD, Aix Marseille Univ, Avignon Univ, IMBE, Marseille, France | ²CNRS, ICR, Aix Marseille Univ, Marseille, France | ³Sartorius Stedim FMT S.A.S, Aubagne, France | ⁴Aerial, Illkirch, France

**Correspondence:** Blanche Krieguer (blanche.krieguer@univ-amu.fr) | Sylvain R. A. Marque (sylvain.marque@univ-amu.fr) | Nathalie Dupuy (nathalie.dupuy@univ-amu.fr) | Samuel Dorey (samuel.dorey@sartorius.com)

**Received:** 12 March 2025 | **Revised:** 8 July 2025 | **Accepted:** 8 July 2025

**Funding:** The authors received no specific funding for this work.

**Keywords:** DSC | ESR | FTIR | HPLC | irradiation technologies | mechanical properties | polypropylene

## ABSTRACT

This research investigated the impact of gamma, e-beam, and X-ray irradiation on medical-grade polypropylene (PP). A comprehensive array of analytical methods was employed to assess the chemical and physical changes in irradiated PP. The main findings revealed that irradiation did not significantly affect the mechanical properties, as tensile strength and elongation at break remained unaltered across all irradiation types and doses. Thermal analysis using DSC indicated a slight reduction in melting temperature at higher doses. ESR identified peroxyl radicals, which decayed similarly regardless of dose or irradiation type. Colorimetric analysis revealed yellowing in PP samples, particularly with gamma and X-ray irradiation, likely associated with specific additives. HPLC analysis showed that the oxidation potential was more influenced by the PP formulation than by the irradiation technology.

## 1 | Introduction

Polymers are essential in the pharmaceutical industry, particularly in various single-use medical devices such as syringes, gloves, and disposable plastic bags. The type of polymer used can differ based on the application and required properties. Polypropylene, known for its excellent performance, chemical stability, attractive mechanical and thermal properties, and cost-effectiveness [1–3], is widely used in the production of plastic components (filters, connectors, clamps, etc.).

Among sterilization technologies for single-use devices made of polymer, the usage of radiation is one of the most common.

Currently, gamma radiation, provided by the isotope cobalt-60, is the most commonly used radiation technology for sterilizing medical devices and biopharmaceutical products [4]. The current healthcare context has led to a strong growth in the manufacture of biopharmaceutical products. In addition, there are concerns about the future capacity of gamma radiation. Two alternative technologies, e-beam and X-ray, are being considered as additional options to gamma sterilization [4–7].

Electron beam irradiation is produced when electrons are accelerated by an electromagnetic field in an accelerator. X-rays are photons generated using an electron beam accelerator [8]. The electron beam is converted into photons by accelerating the

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2025 The Author(s). Macromolecular Chemistry and Physics published by Wiley-VCH GmbH
electrons into a high-density material (tungsten, tantalum). X-rays and gamma rays are both photons, but gamma rays originate from radioactive sources and are emitted during a radioactive decay process [9]. E-beam exhibits a different dose profile and a much more limited penetration, which requires reconsidering the packaging or irradiation geometry when transitioning from photon-based irradiation to e-beam. Among these irradiation technologies, the main difference lies in average dose rates delivered, which vary significantly (in the order a few kGy/h for gamma rays, kGy/min for X-rays, and kGy/s for e-beam). High-energy radiation exhibits significant penetration capabilities, with photon radiation penetrating approximately 1 m, and a 10 MeV e-beam penetrating about 5 cm in materials with a density of 1. As radiation travels through matter, it deposits energy along its path, leading to the generation of secondary electrons and the breaking of chemical bonds. This process creates localized concentrations of radicals, known as spurs. These radicals serve as the precursors for radio-induced modifications such as crosslinking and chain scission. The density of energy deposition is related to the concept of Linear Energy Transfer (LET) [10]. The LET of a 10 MeV electron beam can be considered equivalent to that of photon radiation, as the secondary electrons generated possess similar energy levels.

The exposure of polypropylene (PP) to ionizing radiation alters its performance characteristics [11, 12]. Various physical and physicochemical methods are available to assess the effects of such radiation doses [13]. The ultimate radiative transformations that lead to irreversible changes in the properties of irradiated PP are influenced by the irradiation conditions and the input dose. Previous studies on the radiolysis of PP have exemplified chain scission, accompanied by chain crosslinking, increased chain unsaturation, and a reduction in crystallinity [14]. Crosslinking of polymer chains leads to an increase in molecular weight and enhances the polymer's thermal stability and mechanical properties. Conversely, chain scission results in a decrease in molecular weight, thermal stability, melting temperature, and mechanical strength. In the case of PP, the coexistence of both chain scission and crosslinking generally results in an overall loss of matrix properties. Medical grades (particularly stabilized ones) are known to withstand typical sterilization doses [15], but the maximum acceptable dose must be carefully assessed for each grade and application.

PP is a widely used polymer, yet there is limited literature comparing the effects of the three irradiation technologies (e-beam, X-ray, and gamma) on its material quality. This study investigates the impact of these technologies through a multi-level approach (molecular, macromolecular, and material levels) [4], and employs a multi-technique strategy focusing on oxidation processes, radical generation, and mechanical properties.

In this study, irradiation doses of 50 and 100 kGy were used, and the primary objective was to compare the effects of the three different irradiation technologies. This research investigates the modifications in polymer structures resulting from these irradiation methods, aiming to provide insights into their general suitability for use in biopharmaceutical applications. Changes in transition temperatures of PP and heat capacity were measured using Differential Scanning Calorimetry (DSC), as indicators of material degradation. The generation of reactive species

such as peroxides and peracids, due to post-irradiation radical reactivity, was assessed through the oxidation of methionine (a proxy for protein oxidation) monitored by High-Performance Liquid Chromatography (HPLC). The yellowing of polypropylene was analyzed using colorimetry. A spectroscopic study using (Fourier Transform Infrared Spectroscopy) FTIR was performed to identify chemical modifications in the polymer structure. The mechanical properties of the polymers were evaluated through tensile tests, which measure the strength and elasticity of materials under tensile stress. Finally, long-lived radicals were monitored using Electron Spin Resonance (ESR).

<figure><img src="image_3.png" /><figcaption>**FIGURE 1** | Chemical structure of Polypropylene.</figcaption></figure>

## 2 | Materials and Methods

### 2.1 | Samples

The samples investigated (Figure 1) consist of polypropylene (PP) and a thermoplastic elastomer (TPE). Table 1 summarizes the typical physical properties provided by resin suppliers.

### 2.2 | Samples Packaging

The samples were packaged in cardboard boxes at room temperature at Sartorius Stedim FMT S.A.S., Aubagne. This method, combined with the proper arrangement of plastic components in thin cardboard boxes, ensured uniform irradiation. Dose uniformity was monitored using dosimetry.

### 2.3 | Samples Irradiation

Gamma ray irradiation, sourced from $^{60}$Co, was performed at Ionisos in Dagneux, France, with an average dose rate of 2 kGy/h. The irradiation process spanned several days, with the dose accumulated over multiple runs within the irradiation bunker. Samples were storing a random time between the irradiation tours.

X-ray irradiation was carried out at Aerial-CRT using the Feerix facility, which is based on a Rhodotron (TT300 – IBA). A 7 MV X-ray field was generated by converting a 7 MeV electron beam using a tantalum target. The irradiation was performed horizontally using a pallet conveyor, and multiple passes were conducted in the X-ray field to reach the target dose. This process was completed in one day with an average dose rate of approximately 12 kGy/h.

E-beam irradiation was performed at Aerial-CRT using the same Feerix facility as the X-ray treatment. A 10 MeV vertical electron beam was used, with an average dose rate of 18 MGy/h (5 kGy/s).
<table><caption>TABLE 1 | Non-irradiated PP characteristics.</caption><thead><tr><th rowspan="2">Polymers</th><th colspan="5">Polypropylene (PP)</th><th rowspan="2">TPE</th></tr><tr><th>PP-1</th><th>PP-2</th><th>PP-3</th><th>PP-4</th><th>PP-5</th><th>TPE-6</th></tr></thead><tbody><tr><td>Identification number</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Typical industrial application</td><td>Slide clamp</td><td>Cartridge filters</td><td>Connector</td><td>Filter housing</td><td>Connector</td><td>TPE tubing</td></tr><tr><td>Density (g/cm³)</td><td>0.9</td><td>0.9</td><td>0.9</td><td>0.905</td><td>0.905</td><td>0.9</td></tr><tr><td>MFI (g/cm³)</td><td>2.1</td><td>18</td><td>35</td><td>0.3</td><td>25</td><td>/</td></tr><tr><td>Polymer type</td><td>Homopolymer</td><td>Homopolymer</td><td>Copolymer<br>(ethylene-propylene)</td><td>Homopolymer</td><td>Homopolymer</td><td>Copolymer<br>(ethylene-propylene)</td></tr></tbody></table>

<table><caption>TABLE 2 | Effective doses on samples irradiated by gamma-rays, X-rays, and e-beam.</caption><thead><tr><th rowspan="2">Target doses(kGy)</th><th colspan="3">Effective doses (kGy; ± 10%)</th></tr><tr><th>Gamma</th><th>X-rays</th><th>e-beam</th></tr></thead><tbody><tr><td>50</td><td>51</td><td>50</td><td>55</td></tr><tr><td>100</td><td>90</td><td>99</td><td>109</td></tr></tbody></table>

Irradiation was applied in 50 kGy increments to prevent excessive temperature increases in the samples.

## 2.4 | Dosimetry

The targeted delivered doses were approximately 50 and 100 kGy, with a dose uniformity of ± 10%. Table 2 summarizes the effective doses. To accurately measure the absorbed dose, alanine pellets combined with ESR spectroscopy (Magnettech MS5000 ESR, Bruker) were used, along with AerEDE dosimetry software (Aerial, France). Dosimetry readings were taken in a manner traceable to international standards.

Three dosimeters were placed within the volume of the box containing the samples to assess dose uniformity throughout the sample box.

## 2.5 | Electron Spin Resonance

ESR measurements were performed using a Magnettech MS5000X ESR spectrometer. Samples were prepared from either irradiated dogbone specimens or sections of irradiated materials. Small pieces were cut to fit into a glass probe with an inner diameter of 4.2 mm. The samples, approximately 4 cm in height and weighing between 100 to 200 mg, were then ready for analysis.

First, the signal position was verified over a wide magnetic field range (150–400 mT). Then, the magnetic field range was narrowed to 320–340 mT around the sample, where the signal indicative of organic radicals was detected. Spectra were recorded using a modulation amplitude of 0.2 mT, a sweep time of 60 s, and a single scan. Measurements were recorded from 1 to 60 days after sample irradiation, on the following days (D): D9, D15, D30,

and D60. During this period, samples were kept in the dark in controlled temperature and humidity (25°C and 45% RH).

Signal amplitude (peak-to-peak) was measured on ESR spectra and calibrated using TEMPO (2,2,6,6-Tetramethylpiperidin-1-oxyl) standard samples with spin concentrations between 1•10¹⁶ and 1•10¹⁹ spins/g. TEMPO and standard samples were prepared according to published procedures [16]. TEMPO standards are highly stable over time, and the calibration curve was measured eight times over several months to assess measurement variability. Therefore, ESR quantification in this study should be considered as TEMPO equivalent, since the amplitude between the TEMPO radical and the radical species probed in the polymer may differ.

## 2.6 | Differential Scanning Calorimetry

Differential Scanning Calorimetry (DSC) was used to analyze the thermal properties of the polymers. In this method, a thermocouple inside the furnace measures the temperature of the sample, while another thermocouple measures the temperature of a reference (an empty crucible). Heat flow variations observed during the melting transition were recorded, resulting in the generation of a thermogram. These DSC tests were carried out using a Sensys Evo131 thermal analyzer from Setaram.

For the DSC analysis, each sample, weighing approximately 20 mg, underwent three heating and cooling cycles. For PP samples, the heating range was set between –50°C and 220°C, with a heating rate of 3°C/min. For TPE, the temperature range was set between room temperature and 220°C, with the same heating rate of 10°C/min. A stabilization period was included before and after each cycle to ensure measurement accuracy. Only the data from the second and third cycles were used for analysis, as the first cycle served to eliminate any prior thermal history of the material. The melting peak temperature and the onset of melting were both calculated and integrated using Calisto Processing software.

## 2.7 | Tensile Test

Elongation, ultimate tensile strength (UTS), toughness, and yield strength were measured on the test samples. The mechanical properties of irradiated and nonirradiated samples were measured using a universal testing machine (Zwick Z005). The
crosshead speed used was 50 mm/min (PP-1 and PP-2) and 500 mm/min. The initial gauge length was 100 mm. Five to eight specimens were tested for each sample type. Tensile strength tests were measured at 23 ± 3°C and 1 atm. UTS was defined as the maximum stress the material can withstand, calculated by dividing the maximum load by the original cross-sectional area of the specimen [17].

## 2.8 | Colorimetry

Color measurements of the polymeric materials were outsourced to AERIAL.

Color analysis was performed using the CIELAB color space, also referred to as L*a*b*, with the Konica Minolta CR 400 chromameter. Verification measurements were conducted using a Konica Minolta CR-A43 white calibration plaque. L*, a*, and b* values were recorded for each sample. Measurements were only considered valid if the sample surface was flat; therefore, between two and five measurements were taken at different locations on each sample, depending on its shape. L*, a*, and b* were measured for PP-1 and PP-2 samples prior to irradiation and after gamma and X-ray irradiation at 50 kGy.

## 2.9 | Fourier Transform InfraRed

Samples were directly placed onto the Bruker "Golden Gate" Attenuated Total Reflectance (ATR) accessory, which features a diamond crystal, without any prior preparation. ATR-FTIR spectra of the polymer samples were collected in the range of 4000 to 650/cm, with a resolution of 4/cm and 100 scans, using a Thermo Nicolet IS10 spectrometer. This spectrometer was equipped with an MCT/A detector, an Ever-Glo source, and a KBr/germanium beam splitter. The Mid-InfraRed (MIR) spectrometer was located in an air-conditioned room maintained at 21°C.

To mitigate the potential effects of polymer inhomogeneity, three spectra were recorded for each sample. A background scan without samples was recorded before each set of three acquisitions, using the same resolution and scan parameters. The ATR crystal was thoroughly cleaned with ethanol between each measurement to remove any residual traces from previous samples, and cleanliness was confirmed by recording a background spectrum. The penetration depth into the sample was only a few microns.

## 2.10 | Oxidation Assay by High Performance Liquid Chromatography

Each sample was cut into small pieces (2 mm) and placed in a vial. The vials were filled with a 50 µM methionine solution prepared in a buffer (10 mM NaH₂PO₄, 10 mM Na₂B₄O₇·10H₂O, 5 mM NaN₃, pH 8.2). To determine the concentrations of methionine and methionine sulfoxide, an Agilent 1260 HPLC system (Waghaeusel-Wiesental, Germany) equipped with a quaternary pump (G1311C), an autosampler (G1329B), and a fluorescence detector (G1321B) was used. The automated online derivatization

(performed in the autosampler) using an injection program is detailed in Figure 2. Ortho-phtalaldehyde (OPA) was used as the derivatization reagent. The gradient program was as follows: 0–13.4 min, 2% phase B; 13.4 min, 57% phase B; 13.5–15.8 min, 100% phase B and at18 min, 2% phase B. Separation was carried out on an Agilent Poroshell HPH-C18 column (4.6 mm x 100 mm, 2.7 µm particle size—Waghaeusel-Wiesental, Germany) used with a pre-column, UHPLC guard Poroshell HPH-C18, 4.6 mm. The column was maintained at 40°C ± 0.8°C in a thermostated column compartment (G1316A) throughout the analyses. The fluorescence detector was set to an excitation wavelength of 340 nm and an emission wavelength of 450 nm. The total runtime of the method was 20 min. Chromatographic data were acquired and processed with the HPLC 1260 OpenLAB software (Waghaeusel-Wiesental, Germany). Internal calibration was performed by spiking 20 µL of L-Norvaline in each sample and standard [18].

## 2.11 | Equivalency Analysis

Regression models were performed using the Minitab software to assess the effects of doses (50 and 100 kGy, post-irradiation ageing, and irradiation technology (gamma, X-ray, and e-beam) on the spin/g values measured by ESR and on methionine sulfoxide concentration measured by HPLC. These models were used to determine the statistical significance of each independent variable, with a 95% confidence interval ($α = 0.05$). The Pareto chart shows the absolute values of the standardized effects from the largest to the smallest effect. The chart also plots a reference dotted line indicating if the effects are statistically significant. The reference line for statistical significance depends on the significance level (denoted by $α$), the number of samples, and the degrees of freedom.

To assess the equivalence of mechanical properties between gamma, X-rays, and e-beam irradiated samples, equivalence tests (TOST, two one-sided t-test [19]) were performed using Minitab software. For the tensile tests, equivalence was concluded when the ratio [Radicals]$_{X-ray}$ or e-beam/[Radicals]$_{gamma}$ or [Radicals]$_{e-beam}$/[Radicals]$_{X-ray}$ was within the defined acceptance range. For elongation, the acceptance criterion was set at 25% and for yield strength, it was 15%.

## 2.12 | Principal Component Analysis (PCA)

Principal component analysis (PCA) is a widely utilized technique in chemometrics [20, 21] and has been detailed in a prior study [16]. Each principal component (PC) captures the maximum variance remaining in the data. By projecting the scores onto the space defined by the PCs, similarities and differences among samples can be visualized. The loading reveals which variables contribute most to each PC. In this study, we focused on PCs that accounted for more than 15% of the variance. Consequently, sometimes only the first PC was significant, allowing scores to be displayed as a bar chart, while in other cases, the first two PCs were significant, and the scores were represented as a scatter plot.

PCA was applied on FTIR on non-irradiated and gamma, e-beam, and X-ray irradiated PP and TPE samples at doses of 50 and 100 kGy to highlight the impact of the irradiation
<figure><img src="image_4.png" /><figcaption>**FIGURE 2** | Automatic derivatization reaction for detecting methionine with a fluorescence detector.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**FIGURE 3** | ESR signal of PP-2, PP-3, and TPE-6 nine days after irradiation at 50 kGy (solid lines) and 100 kGy (dashed lines) by gamma irradiation (red line), X-ray (blue line), and e-beam (green line).</figcaption></figure>

technology. The spectra were baseline-corrected and pretreated using Standard Normal Variate (SNV) to address additive and multiplicative effects. PCA models were developed using CAMO (Computer Aided Modelling, Trondheim, Norway) software, UNSCRAMBLER X.1.

# 3 | Results and Discussion

## 3.1 | Electron Spin Resonance

Preliminary results on 50 kGy for X-ray and gamma irradiation for PP [3] have been extended to include e-beam, X-ray, and gamma at doses of 50 and 100 kGy. This study provides additional data at higher irradiation doses for both X-ray and gamma, as well as additional results for e-beam. Figure 3 displays the ESR Signals of PP-2, PP-3, and TPE-6 after irradiation with gamma, e-beam, and X-ray irradiations at doses 50 and 100 kGy, nine days post-irradiation. No ESR signals were detected in non-irradiated samples. Similar signals to those of PP-2 were also observed for PP-1, PP-4, and PP-5 (Figure S1).

The radicals detected in irradiated PP have a g-factor of 2.015, which is attributed to a peroxyl radical (RO₂•; g = 2.015) as reported in the literature [22–24]. An additional unidentified radical was observed in TPE-6, and its signal overlaps with that of the peroxyl radical. A significant signal from a side product is observed alongside the peroxyl signals in PP-3. This signal persists and becomes dominant from 60 days of aging post-irradiation. The g-value of 2.004 is attributed to a nitroxide radical (stable side-product). For industrial raison, the formulation of PP-3 cannot be disclosed. Nevertheless, it can be mentioned that the formulation involves a nitrogen-based component able to generate a stable radical exhibiting a g-factor = 2.004.

Figure 4 shows the concentration of radicals for each sample. Radical levels in samples exposed to gamma, X-ray, and e-beam irradiation at 50 and 100 kGy were monitored for up to 60 days post-irradiation. The ESR signal and the quantity of radicals generated decreased over time, the three irradiation technologies displaying thoroughly the same trends with the same amount of radicals generated.
<figure><img src="image_6.png" /><figcaption>**FIGURE 4** | Monitoring of the concentration of radicals for PPs and TPE samples by ESR as concentration (spin/g) vs post-irradiation ageing (days). Solid squares correspond to samples irradiated at 50 kGy and open triangles correspond to samples irradiated at 100 kGy, red for gamma irradiation, blue for X-ray, and green for e-beam.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**FIGURE 5** | Pareto analysis of significant effects on radical concentration on PP-3 and TPE-6: Ageing after irradiation (A in blue), Dose (B in red), and Irradiation technologies (C in green).</figcaption></figure>

Both post-irradiations ageing and irradiation technologies significantly affect the radical concentration of PP-3 (Figure 5a). Fewer radicals are produced in samples irradiated by X-ray and e-beam than in those exposed to gamma irradiation (see the model equation and coefficient in Supporting Information).

For TPE-6 (Figure 5b), only post-irradiation ageing significantly affects the radical concentration, which decreases after irradiation. Similar trends are observed for PP-1, PP-2, PP-4, and PP-5 (see Figure S2). The equations and coefficients of the regression models obtained for each sample are reported in the Supporting Information.

## 3.2 | Thermal Behaviors

The DSC curves for PP-1, PP-2, and TPE-6 are shown in Figure 6a-c, while the DSC curves for PP-3, PP-4, and PP-5 are presented in Figure S3. The melting peak temperatures are shown in Figure S4. The DSC results reveal three distinct behaviors: some samples develop a shoulder peak (Figure 6a), PP-2 develops a shoulder peak only after gamma irradiation (Figure 6b), and others exhibit a single peak after irradiation (Figure 6c).

The shape of the DSC curves (Figure 6) provides valuable insights into the structural characteristics. Initially, all PP samples displayed a single melting peak. After irradiation, the melting peak shifts to a lower temperature, and a shoulder peak appears in PP-1 (for PP-5 in Supporting Information) (Figure 6a). Interestingly, PP-2 develops a shoulder peak after gamma irradiation, suggesting that gamma rays have a more pronounced effect on this sample (Figure 6b). For TPE-6, PP-3, and PP-4, the DSC curves of both non-irradiated and irradiated samples display only a broad single melting peak (Figure 6c and Figure S3). All PPs tend to show a decrease in melting temperature and onset with increasing dose (up to 100 kGy) compared to the non-irradiated reference samples (Figures 6d,e, and Figure S4). The decrease in melting peak temperature with increasing irradiation dose indicates a greater degree of amorphization and structural changes in the polymer, such as crosslinking and oxidation [25, 26]. The literature [27–29] reports that irradiation on PP leads to molecular changes that shorten the polymer chains, thereby reducing the melting peak values as a function of the absorbed dose.

The decrease in melting peak temperature with increasing irradiation dose indicates a greater degree of amorphization and
<figure><img src="image_8.png" /><figcaption>FIGURE 6 | DSC thermograms for (a) PP-1, (b) PP-2, and (c) TPE-6 (d) materials irradiated at 50 kGy, (e) materials irradiated at 100 kGy. Black for non-irradiated, red for gamma irradiation, blue for X-rays, and green for e-beam. Solid lines for samples irradiated at 50 kGy are represented and dashed lines for samples irradiated at 100 kGy.</figcaption></figure>

structural changes in the polymer, such as crosslinking and oxidation [25, 26]. The literature [27–29] reports that irradiation on PP leads to molecular changes that shorten the polymer chains, thereby reducing the Tm values as a function of the absorbed dose.

## 3.3 | Mechanical Properties

Tensile tests were performed on four polypropylene grades (PP-1 to PP-4) with varying melt flow indices (MFI), ranging from 0.3 g/10 min (PP-4) to 35 g/10 min (PP-3) (Table 1). MFI, an indirect measure of molecular weight, correlates inversely with melt viscosity. Low-MFI grades typically exhibit higher stiffness and lower elongation at break, while high-MFI grades show enhanced deformability due to reduced molecular weight and lower entanglement density [30].

Tensile strength and elongation at break remained unchanged across all irradiation types and doses (Figure 7 and Table S1). A TOST-equivalence test was performed, and irradiated samples are equivalent to the non-irradiated sample. These findings align with previous studies showing that significant degradation of polypropylene under ionizing radiation is primarily driven by oxidative chain scission, which becomes prominent at higher doses or in oxygen-rich environments [22, 28, 31].

## 3.4 | Colorimetry

A PCA was performed using the results from the color analysis (Figure 8). The PCA revealed three distinct groups of samples: Group 1 for non-irradiated samples, group 2 for irradiated PP-2 and PP-1 irradiated by e-beam, and group 3 for gamma

and X-ray irradiated PP-1. The groups corresponding to PP-1 irradiated is positively projected on the first component (100%). The examination of the corresponding loading shows that these samples are characterized by a high value of L* and b* and a low value of the component a*.

The b* values increase when PP-1 is irradiated with gamma rays and X-rays, resulting in a more yellow appearance compared to non-irradiated and e-beam irradiated samples. In contrast, PP-2 showed a difference when irradiated but no difference between the three irradiation technologies, becoming less yellow than PP-1 after irradiation. For both PP-1 and PP-2, the L* values increase upon irradiation. Consequently, irradiated samples appear brighter than non-irradiated samples.

The yellowing observed in PP (polypropylene) after irradiation can be attributed to the presence of certain additives commonly found the polymer formulation [27, 32, 33].

## 3.5 | Infrared Spectroscopy

The infrared spectra of non-irradiated and irradiated by gamma, e-beam, and X-ray at 50 and 100 kGy of PP-1 are displayed in Figure 9, while those of PP-2 to PP-5 are presented in Figure S5. According to the literature [12, 28, 34, 35], the main signal changes are observed in the carbonyl absorption region (1680–1680/cm). The IR band assignments for the PP spectra are in consistent with those reported in literature [36] and are represented in Figure 9. For all PPs, no broad absorption band centered at 1745/cm was detected, which is characteristic of the stretching vibration of the carbonyl group C=O. With this technique, oxidation of PP during the degradation
<figure><img src="image_9.png" /><figcaption>**FIGURE 7** | Elongation for PP-1, PP-2, PP-3, and PP-4 as a function of the irradiation technology and the dose. The mean is plotted with a confidence interval at 95%.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**FIGURE 8** | PCA of L*, a*, and b* values obtained by color analysis on PP-1 and PP-2 non-irradiated and irradiated by gamma, X-ray, and e-beam at 50 kGy. (a) Score plot of PCA with irradiation technology (black: Non-irradiated, red: gamma, blue: X-ray, green: e-beam). (b) Loading plot of PC-1.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**FIGURE 9** | FTIR spectra of PP-1 non-irradiated (black line), irradiated by gamma (red line), by X-ray (blue line), and by e-beam (green line) at 50 kGy (solid line) and 100 kGy (dashed line).</figcaption></figure>

induced by gamma, e-beam, and X-ray irradiation could not be observed.

PCA was performed on the 105 infrared spectra, representing the effects of all irradiation doses and technologies for all PP samples (Figure S6). For the FTIR fingerprint spectral region [1800—700/cm], the PP spectra were baseline corrected and normalized.

The sample separation observed in the PCA plot suggests that the different additives used in the PP formulations result in distinct spectral characteristics. Due to industrial confidentiality, the exact formulations of the PPs cannot be disclosed. The presence of additives in the PPs is consistent with the results obtained from ESR and colorimetry analyses.

It is important to note that no significant effects of irradiation dose or technology were observed, as the samples do not cluster according on these variables.
<figure><img src="image_12.png" /><figcaption>**FIGURE 10** | Methionine sulfoxide concentration (µM) in stored solution after 1, 10, 21, 42, and 63 days, analyzed by HPLC for all PP samples. Irradiation doses were 50 and 100 kGy for gamma, X-ray and e-beam.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**FIGURE 11** | Pareto analysis of significant effects on methionine sulfoxide concentration: Methionine contact day (days) (red), Dose (kGy) (blue), and Irradiation technologies (green). (a) PP-1. (b) PP-2. (c) PP-3. (d) PP-4. (e) PP-5. (f) TPE-6.</figcaption></figure>

**3.6 | Oxidation Assay by High Performance Liquid Chromatography**

Figure 10 shows the oxidation potential of PPs irradiated by X-ray, e-beam, and gamma irradiation, as measured by the HPLC-detection of methionine sulfoxide in the solution stored for up to 63 days. Two groups were identified: PP-1, PP-2, and PP-5, which exhibit higher oxidation levels, and PP-3, PP-4, and TPE6, which have oxidation levels comparable to the non-irradiated

sample. Their differing formulations account for these variations in oxidation.

For all PPs, the irradiation dose had no significant effect on methionine sulfoxide concentration, whereas the contact time with the solution significantly increased it (Figure 11). Statistically significant differences in methionine sulfoxide concentration were observed between irradiated and non-irradiated samples for PP-1, PP-5, and TPE-6. However, no
significant differences were found among the three irradiation technologies.

PP oxidation is attributed to the generation of peracids [37, 38]. The variations among the PPs are explained by their specific formulations and additive content, indicating that methionine solution oxidation depends more on the polymer formulation than on the irradiation technology.

## 4 | Conclusion

The study provides an analysis of the effects of X-rays, e-beam, and gamma irradiation on the chemical and physical properties of polypropylene and thermoplastic elastomer samples.

ESR analysis revealed that the generation of peroxyl radicals in irradiated samples is not significantly influenced by the type of irradiation or dose, with these radicals gradually decreasing over time.

Thermal analysis through DSC shows that the thermal properties of the samples remained equivalent across all irradiation technologies, except for the PP-2 sample. Although a slight reduction in melting temperature was observed at higher doses, the overall thermal stability was largely maintained. Mechanical testing further confirmed that tensile strength and elongation at break were unaffected by irradiation, thus preserving the mechanical integrity of PP. Colorimetric analysis indicated minor changes in brightness and color balance, with PP-1 exhibiting a notable yellowing effect post-irradiation. Infrared spectroscopy showed that the chemical structure of PP remained largely unaltered. The study also assessed oxidation potential, with methionine sulfoxide concentrations revealing differences primarily attributed to the PP formulation rather than the irradiation technology. This underscores the critical role of polymer formulation in determining the material's response to irradiation. Therefore, irradiation technology does not significantly impact the properties of PP; the polymer formulation is a key factor influencing its behavior post-irradiation.

### Acknowledgements

N.D., F.G., and S.R.A.M. are thankful to Aix-Marseille University (AMU), Institut de Recherche pour le Développement (IRD), and Centre National pour la Recherche Scientifique (CNRS) A.M.U., I.R.D. and C.N.R.S. for support.

### Conflicts of Interest

The authors declare no conflicts of interest.

### Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

### References

1. BPSA Technical Guide. X-Rays Sterilization of Single-Use BioProcess Equipment, Part 1: Industry Need 2021.

2. Polypropylene Handbook: Morphology, Blends and Composites, Springer International Publishing, Cham, 2019.

3. B. Krieguer, S. R. A. Marque, and S. Dorey, "Radical Detection And Electron-Spin Resonance (Esr) Monitoring In Polymer Materials Irradiated With Gamma And X-Rays: Polyethylene And Polypropylene," *Journal of Applied Polymer Science* 141 (2024): 55098.

4. N. Dupuy, S. R. Marque, L. S. Fifield, et al., "Supplementing Gamma Sterilization With X-Ray And E-Beam Technologies," *Bioprocess Technical* 20 (2022): 24–28.

5. P. M. Armenante and O. Akiti, "Sterilization Processes in the Pharmaceutical Industry," in *Chemical Engineering in the Pharmaceutical Industry*, eds. D. J. Ende and M. T. Ende (John Wiley & Sons, Inc., 2019), 311–379.

6. H. De Brouwer, "Comparison of the Effects of X-ray and Gamma Irradiation on Engineering Thermoplastics," *Radiation Physics and Chemistry* 193 (2022): 109999.

7. D. Darwis, A. Erizal, B. Nurlidar, and D. P. Putra, "Radiation Processing of Polymers for Medical and Pharmaceutical Applications," *Macromolecular Symposia* 353 (2015): 15–23.

8. W. D. Loveland, D. J. Morrissey, and G. T. Seaborg, *Modern Nuclear Chemistry* (Wiley, 2017), https://doi.org/10.1002/9781119348450.

9. K. Makuuchi and S. Cheng, *Radiation Processing of Polymer Materials and Its Industrial Applications* (Wiley, 2012).

10. M. Ferry and Y. Ngono, "Energy Transfer in Polymers Submitted to Ionizing Radiation: A Review," *Radiation Physics and Chemistry* 180 (2021): 109320.

11. S. R. Allayarov, S. V. Demidov, A. T. Kapasharov, et al., "The Impact of Post-irradiation High-temperature Shear Grinding on the Thermal and Mechanical Properties of Polypropylene, Radiation," *Physics and Chemistry* 221 (2024): 111718.

12. A. Rivaton, D. Lalande, and J.-L. Gardette, "Influence of the Structure on the γ-irradiation of Polypropylene and on the Post-irradiation Effects," *Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms* 222 (2004): 187–200.

13. I. Krupa and A. S. Luyt, "Thermal Properties of Isotactic Polypropylene Degraded With Gamma Irradiation," *Polymer Degradation and Stability* 72 (2001): 505–508.

14. C. Vasile and E. Butnaru, *Applications of Ionizing Radiation in Materials Processing* (2017): 117–142.

15. AAMI TIR., *Compatibility of Materials Subject to Sterilization* (2017).

16. G. Audran, S. Dorey, N. Dupuy, F. Gaston, and S. R. A. Marque, "Degradation of γ-irradiated Polyethylene-ethylene Vinyl Alcohol-polyethylene Multilayer Films: An ESR Study," *Polymer Degradation and Stability* 122 (2015): 169–179.

17. S. Dorey, F. Gaston, N. Girard-Perier, et al., "Generation of O₂ -Permeation Barrier During the Gamma-Irradiation of Polyethylene/Ethylene-Vinyl Alcohol/Polyethylene Multilayer Film," *Industrial & Engineering Chemistry Research* 58 (2019): 14115–14123.

18. N. Girard-Perier, M. Claeys-Bruno, S. R. A. Marque, N. Dupuy, F. Gaston, and S. Dorey, "Monitoring of Peroxide in Gamma Irradiated EVA Multilayer Film Using Methionine Probe," *Polymers* 12 (2020): 3024.

19. Y. Ni, T. T. Bisel, M. K. Hasan, et al., "Compatibility of Ethylene Vinyl Acetate (EVA)/Ethylene Vinyl Alcohol (EVOH)/EVA Films With Gamma, Electron-beam, and X-ray Irradiation," *Mater Degrad* 7 (2023): 93.

20. N. Kumar, A. Bansal, G. S. Sarma, and R. K. Rawal, "Chemometrics Tools Used in Analytical Chemistry: An Overview," *Talanta* 123 (2014): 186–199.

21. H. Martens and T. Naes, Multivariate Calibration. (1989).

22. A. C. Abraham, M. A. Czayka, and M. R. Fisch, "Electron Beam Irradiations of Polypropylene Syringe Barrels and the Resulting Physical
and Chemical Property Changes,” *Radiation Physics and Chemistry* 79 (2010): 83–92.

23. G. Burlińska, J. Bojarski, and J. Michalik, “Studies on Irradiated Polypropylene, its Copolymers and Blends—i. Esr Studies, Radiation,” *Physics and Chemistry* 47 (1996): 449–451.

24. P. Silva, C. Albano, R. Perera, J. González, and M. Ichazo, “An Electron Paramagnetic Resonance Study of PP and PP/SBS Blends Irradiated With Gamma Rays,” *Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms* 226 (2004): 320–326.

25. C. Aymes-Chodur, N. Betz, B. Legendre, and N. Yagoubi, “Structural and Physico-chemical Studies on Modification of Polypropylene and Its Polyphenolic Antioxidant by Electron Beam Irradiation,” *Polymer Degradation and Stability* 91 (2006): 649–662.

26. A. T. Fintzou, A. V. Badea, M. G. Kontominas, and K. A. Riganakos, “Changes in Physicochemical and Mechanical Properties of γ-irradiated Polypropylene Syringes as a Function of Irradiation Dose,” *Radiation Physics and Chemistry* 75 (2006): 87–97.

27. M. M. Hassan, N. A. El-kelesh, and A. M. Dessouki, “The Effect Of Gamma And Electron Beam Irradiation On The Thermal And Mechanical Properties Of Injection-Moulded High Crystallinity Poly(Propylene),” *Polymer Composites* 29 (2008): 883–889.

28. H.-T. Wang, H.-Q. Jiang, and R.-F. Shen, “Electron-beam Radiation Effects on the Structure and Properties of Polypropylene at Low Dose Rates,” *Nuclear Science and Techniques* 29 (2018): 87.

29. N. Yagoubi, R. Peron, B. Legendre, J. L. Grossiord, and D. Ferrier, “Gamma and Electron Beam Radiation Induced Physico-chemical Modifications of Poly(propylene),” *Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms* 151 (1999): 247–254.

30. H. C. Lau, S. N. Bhattacharya, and G. J. Field, “Melt Strength of Polypropylene: Its relevance to thermoforming,” *Polymer Engineering & Sci* 38 (1998): 1915–1923.

31. K. A. Riganakos, W. D. Koller, D. A. E. Ehlermann, B. Bauer, and M. G. Kontominas, “Effects of Ionizing Radiation on Properties of Monolayer and Multilayer Exible Food Packaging Materials,” *Radiation Physics and Chemistry* 54 (1999): 527–540.

32. F. Gaston, N. Dupuy, S. R. A. Marque, D. Gigmes, and S. Dorey, “Monitoring of the discoloration on γ-irradiated PE and EVA films to evaluate antioxidant stability,” *Journal of Applied Polymer Science* 135 (2018): 46114.

33. H. B. Thorat, C. S. Prabhu, S. Kumar, and M. V. Pandya, “?-Ray-Induced Degradation in Ethylene?Propylene Copolymer,” *Journal of Applied Polymer Science* 59 (1996): 1769–1773.

34. S. Mouaci, M. Saidi, and N. Saidi-Amroun, “Oxidative Degradation and Morphological Properties of Gamma-irradiated Isotactic Polypropylene Films,” *Micro & Nano Letters* 12 (2017): 478–481.

35. E. Bormashenko, R. Pogreb, and A. Sheshnev, “Resonance Absorption of Coherent Infrared Radiation by Thin Polypropylene Films and Its Technological Applications,” *Applied Surface Science* 220 (2003): 125–135.

36. J. Fang, L. Zhang, D. Sutton, X. Wang, and T. Lin, “Needleless Melt-Electrospinning of Polypropylene Nanofibres,” *Journal of Nanomaterials* 2012 (2012): 1–9.

37. P. Gijsman and R. Fiorio, “Long Term Thermo-oxidative Degradation and Stabilization of Polypropylene (PP) and the Implications for Its Recyclability,” *Polymer Degradation and Stability* 208 (2023): 110260.

38. N. Girard-Perier, M. Claeys-Bruno, S. R. A. Marque, N. Dupuy, F. Gaston, and S. Dorey, “Effects of X-ray, Electron Beam and Gamma Irradiation on PE/EVOH/PE Multilayer Film Properties,” *Chemical Communications* 57 (2021): 11049–11051.

**Supporting Information**

Additional supporting information can be found online in the Supporting Information section.

**Supporting file:** macp70069-sup-0001-SuppMat.docx.