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12 February 2026

Mechanical Property Evolution and Mechanisms of Polyolefins Under Thermo-Oxidative Aging

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School of Transportation and Vehicle Engineering, Shandong University of Technology, Zibo 255000, China
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Hongyuan Waterproof Technology Group Co., Ltd., Taitou Industry, Weifang 261000, China
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Shandong Key Laboratory of Integrated Design and Intelligence of New Energy, Vehicles, Zibo 255000, China
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School of Aerospace Engineering and Applied Mechanics, Tongji University, Shanghai 200092, China
Polymers2026, 18(4), 462;https://doi.org/10.3390/polym18040462
This article belongs to the Special Issue Technological Innovations for Polymer Circularity: Upcycling, Biodegradation, and Processing
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Abstract

Polyolefin materials are widely used due to their excellent properties and low cost. However, in high-temperature oxygen environments, they are susceptible to thermo-oxidative aging, which reduces mechanical properties and durability. This study systematically analyzed the aging behavior and mechanisms of polyolefins at varying temperatures and exposure durations through accelerated thermal oxidation experiments. The results indicate that the thermo-oxidative aging behavior of polyolefins can be divided into three stages. In Stage I, elevated temperature promotes segmental mobility and chain rearrangement, which increases crystallinity and temporarily improves mechanical properties. In Stage II, antioxidants are progressively consumed and oxygen-containing groups begin to accumulate, resulting in reduced crystallinity, a decline in mechanical performance, and the onset of slight surface yellowing. In Stage III, the antioxidant system is largely depleted and oxidative reactions are intensified, leading to mainchain scission and molecular weight reduction. This causes a further decrease in crystallinity, a significant deterioration in both strength and toughness, accompanied by aggravated yellowing. This study elucidates the thermo-oxidative aging mechanism of polyolefins, providing a theoretical basis for assessing their service life and evaluating the stability of waste plastics.

1. Introduction

Polymers play a crucial role in modern society owing to their excellent performance, low density, and cost-effectiveness. Among them, polyolefins are particularly significant, characterized by low production cost, high output, and favorable mechanical and chemical stability. This category of materials accounts for nearly half of the world’s total plastic production and holds over 60% of the plastic market share [1,2]. Currently, polyolefins are extensively employed in packaging, automotive components, construction materials, piping systems, and electrical insulation, serving applications that range from everyday consumer products to industrial manufacturing [3,4,5].
Polymers are susceptible to oxidative degradation in service, predominantly via thermo-oxidation. As the temperature rises, the mobility of polyolefin segments increases, making it easier for them to overcome the energy barrier of oxidation reactions and generate free radicals. These free radicals trigger chain reactions, leading to main-chain scission and cross-linking as well as the accumulation of oxygen-containing functional groups such as carbonyls and peroxides [6]. Meanwhile, oxygen molecules in the environment continuously diffuse into the bulk, sustaining the reaction and accelerating oxidation [7]. These microstructural and compositional changes collectively diminish the material’s mechanical properties and durability, thereby reducing its long-term service reliability.
Extensive research has investigated the oxidation mechanisms of polyethylene (PE) and polypropylene (PP). As reported by Zhu et al. [8] and Gardette et al. [9], thermo-oxidative aging of PE is initiated by hydrogen abstraction at primary/secondary C-H sites. The resulting alkyl radicals (R•) add O2 to form peroxy radicals (ROO•), which subsequently abstract hydrogen to generate hydroperoxides (ROOH). The ROOH species then decompose thermally or catalytically to yield alkoxy radicals (RO•) and hydroxyl radicals (•OH). Oxygenated products such as ketones accumulate, and the oxidation rate is governed by the initial level of unsaturation. Gijsman et al. [10] and Aboulkas et al. [11] noted that thermo-oxidative aging of PP initiates at tertiary C-H sites with formation of ROO•/ROOH; the ROOH produced can undergo low-activation-energy bifunctional decomposition via in-chain adjacent hydroperoxides or metal-residue-catalyzed cleavage to yield RO•, thereby triggering a self-accelerating, two-stage process with β-scission that leads to backbone cleavage. Despite the overall similarity of PE and PP within the framework of free-radical autoxidation, substantial differences persist in initiation sites, hydroperoxide decomposition pathways, and product distributions; accordingly, a systematic investigation of their synergistic co-oxidation mechanism is warranted.
In order to inhibit the above-mentioned thermo-oxidative aging reactions, the thermal stability of polyolefins is often improved by the introduction of antioxidants, such as hindered phenolics and phosphites [12,13,14,15,16]. Antioxidants can effectively scavenge free radicals or decompose peroxides, thereby slowing the aging process. Zhu et al. [8] constructed PE-HP copolymers by introducing hindered phenol groups onto polyethylene chains. During the initial oxidation stage, the hindered phenol acts as a hydrogen donor to scavenge polymer radicals. Following oxidation, it undergoes dimerization to form linking units, thereby achieving in situ crosslinking between the two polyethylene chains. A reanalysis of the TGA results indicated that the PE-HP copolymers exhibited outstanding thermo-oxidative stability. Zhang et al. [17] successfully synthesized a novel organic-inorganic hindered phenol antioxidant, AO-POSS. When incorporated into polypropylene, AO-POSS significantly extended the oxidation induction time from 38.1 to 50.7 min. This improvement highlights AO-POSS’s superior thermal oxidative stability and its effectiveness in enhancing the anti-aging performance of polypropylene. Chung et al. [18] achieved uniform distribution of antioxidant groups within the material by chemically bonding hindered phenolic antioxidant groups to the main chains of polyethylene and polypropylene. Wang et al. [19] found that N, N′-di-2-naphthyl-p-phenylenediamine (DNP) exhibits excellent radiation resistance and strong resistance to migration in ethylene–propylene copolymers. Under thermo-oxidative conditions, these groups underwent in situ coupling reactions to form a network structure, thereby significantly inhibiting radical-chain oxidation, delaying mass loss, and maintaining mechanical properties.
However, under prolonged high-temperature conditions, antioxidants are progressively consumed and their protective effect diminishes. Although antioxidants can delay the thermo-oxidative aging of polyolefins to some extent, their long-term stability and effectiveness remain uncertain. Therefore, a systematic investigation of the aging behavior is still required to provide a scientific basis for improving the service life of these materials.
Accordingly, this study focuses on polyolefins, specifically ethylene–propylene copolymers. Specimens were subjected to accelerated thermo-oxidative aging at elevated temperature in a forced-air oven. Mechanical performance was quantified by tensile testing (tensile strength and elongation at break). Color evolution was analyzed to probe chromophore formation, and scanning electron microscopy (SEM) was used to examine the microstructure of tensile-fracture surfaces before and after aging. Fourier-transform infra-red spectroscopy (FTIR) tracked the evolution of functional groups, while differential scanning calorimetry (DSC) assessed thermal transitions and crystallinity under thermo-oxidative conditions. In addition, the weight-average molecular weight of the polyolefins during thermo-oxidative aging was analyzed using high-temperature gel permeation chromatography (GPC). These measurements were used to elucidate the thermo-oxidative aging behavior of polyolefins and to provide a basis for future studies on polymer thermo-oxidative aging.

2. Materials and Methods

2.1. Materials and Sample Preparation

The manufacturing process flow for polyolefin sheets is shown in Figure 1. The polyolefin material was prepared by compounding polyolefin pellets, antioxidant B225, and fillers at predetermined proportions. The polyolefin pellets were ethylene–propylene copolymers and comprised two grades supplied by LyondellBasell, namely CA10A and CA60A. The melt flow index of CA10A was 0.6 g/10 min, whereas that of CA60A was 14 g/10 min. These two grades were blended at a mass ratio of 10:1. Antioxidant B225 is a 1:1 (by mass) blend of the hindered phenolic antioxidant 1010 and the phosphite antioxidant 168. The content of antioxidants is 0.5 wt%. It was uniformly mixed with the polyolefin particles and then simultaneously fed into a twin-screw extruder. The filler used was ground calcium carbonate. The CaCO3 filler content was 30 wt% of the total mass.
Figure 1. Manufacturing route of the polyolefins.
The extrusion process was performed using an STS 96 MC11 twin-screw extruder (Kobelco Nanjing Machinery Co., Ltd., Nanjing, China) operating at a screw speed of 190 rpm. The barrel temperature profile was set to 180 °C (zone 1), 185 °C (zones 2–3), and 195 °C (zones 4–11). The connecting body was maintained at 200 °C, and the die head temperature was set to 230 °C. All other sections were kept at 200 °C. The extruded molten sheet was then molded through the die head and transferred into a three-roll calendering machine with rolls of 500 mm in diameter and 2400 mm in length (Changzhou Wujin Guangyu Flower Stick Machinery Co., Ltd., Changzhou, China) for cooling and shaping. The three-roll calendering machine is composed of an upper roll, a middle roll, and a lower roll. The roll temperatures were set at 45 °C (upper roll), 60 °C (middle roll), and 45 °C (lower roll), with a calendering speed of 4.5 m/min. After calendering and cooling, the final thickness of the sheet was 1.5 mm. The resulting sheets exhibited a good surface finish and were used for subsequent mechanical and aging tests.

2.2. Ageing Procedure

Thermo-oxidative aging of polyolefin samples was carried out in a forced-air oven (101A-ES, Beiyi Ruixing Instrument Co., Beijing, China) at 115 °C, 125 °C, and 135 °C for durations of 14, 29, 44, and 98 days. During exposure, specimens were arranged with a minimum spacing of 10 mm between specimens and at least 50 mm from the oven walls to promote uniform airflow and temperature uniformity. Unaged specimens were used as the control group. After aging, all specimens were conditioned at standard laboratory conditions (23 ± 2 °C and 60 ± 15% relative humidity) for 24 h prior to tensile testing. This procedure allowed a systematic assessment of the temperature- and time-dependent aging behavior under accelerated thermo-oxidative conditions.

2.3. Characterization

2.3.1. Mechanical Testing

Tensile properties were measured using a microcomputer-controlled electronic universal testing machine (LD23.202, Shenzhen Lambor Sansi Materials Testing Co., Ltd., Shenzhen, China). Tensile testing was conducted according to GB/T 328.9-2007 (Method B) [20], using type I dumbbell specimens prepared in accordance with GB/T 528 [21]. Specimens were firmly clamped in the grips, with the specimen midline aligned to the grip centerline. A preload of no more than 5 N was applied to remove slack and minimize relaxation. The initial grip separation was set to 80 ± 5 mm, and the crosshead speed was 250 ± 50 mm/min. Five replicates were tested for each condition and the results were averaged and reported.

2.3.2. Scanning Electron Microscopy (SEM) Analysis

The tensile fracture surfaces of aged polyolefin samples were examined using a field emission scanning electron microscope (FEI Sirion 200, FEI Company, Hillsboro, OR, USA). Fracture-surface specimens (5 mm × 5 mm × 2 mm) were mounted on aluminum stubs using conductive adhesive. Prior to observation, the specimens were sputter-coated with a thin layer of gold to improve surface conductivity. The fracture morphologies were analyzed by SEM to evaluate microstructural changes induced by thermo-oxidative aging. Energy-dispersive X-ray spectroscopy (EDS) was also performed to determine the elemental composition of the fracture surfaces.

2.3.3. Fourier Transform Infrared (FT-IR) Analysis

Changes in the functional groups of polyolefin samples before and after thermo-oxidative aging were analyzed using a Fourier-transform infrared (FTIR) spectrometer (Nicolet iS50, Thermo Fisher Scientific, Shanghai, China). Spectra were acquired in ATR mode using a diamond crystal, with an air background, 32 scans, and a spectral resolution of 4 cm−1. This approach allows direct analysis of solid specimens and was used to track chemical changes during aging, providing insight into the oxidation chemistry of polyolefins.

2.3.4. Thermal Analysis

Differential scanning calorimetry (DSC) was conducted using a DSC Q2000 instrument (Serial No. 2000–2755, TA Instruments, New Castle, DE, USA) to assess the thermal properties of the polyolefin samples. Approximately 4–5 mg of each sample was encapsulated in a standard aluminum pan and analyzed under a nitrogen flow rate of 50 mL/min. The temperature program consisted of heating from 40 °C to 180 °C at a rate of 10 °C·min−1, holding isothermally for 5 min, cooling to 40 °C, and then reheating to 180 °C at the same rate. Thermal transitions, such as melting and crystallization, were monitored to assess the effect of thermo-oxidative aging on the physical properties of the polyolefins.

2.3.5. Gel Permeation Chromatography (GPC) Analysis

The molecular-weight evolution of the polyolefin samples during thermo-oxidative aging was determined by high-temperature gel permeation chromatography (HT-GPC) using an Agilent PL-GPC 220 system (Agilent Technologies, Santa Clara, CA, USA). The molecular weight and molecular-weight distribution were measured on the Agilent PL-GPC 220 system equipped with PLgel 10 μm MIXED-B LS columns (300 × 7.5 mm) connected in series. The experiment employed 1,2,4-trichlorobenzene as the mobile phase, with isocratic elution at a column temperature of 150 °C and a flow rate of 1.0 mL/min. Detection was performed using a differential refractive index detector. A calibration curve was established using narrow-distribution polystyrene standards, and molecular weights were reported as polystyrene-equivalent values.

3. Results

3.1. Mechanical Behavior

In practical applications, mechanical testing is not only essential for predicting material service performance and evaluating material safety, but also serves as a critical link between aging-mechanism studies and engineering practice. It provides core data support for product durability design and safety evaluation. Different materials can exhibit different aging patterns. Ning et al. [22] and Li et al. [23] found that different types of antioxidants can lead to several aging patterns in ethylene–propylene polymer materials, with the tensile strength remaining unchanged, decreasing, or increasing initially and then decreasing. Zhang et al. [24] reported a similar observation when incorporating different contents of carbon nanotubes into ethylene–propylene polymers. Accordingly, the effects of thermo-oxidative aging on the mechanical properties of the polyolefin samples were investigated. Figure 2 shows the variations in tensile strength and elongation at break as a function of aging time.
Figure 2. Effect of thermal ageing on mechanical properties for thermally exposed Polyolefin samples: (a) tensile strength, (b) elongation at break.
As shown in Figure 2a, at all aging temperatures, the tensile strength of the polyolefin specimens first increased and then decreased with aging time. At 115 °C, the tensile strength increased significantly after the early stage of aging and then remained relatively stable. At 125 °C, tensile strength increased significantly with aging time at first (peaking at 29 d, with 44 d close to the peak), and then gradually decreased. By 84–98 d, tensile strength declined significantly, with the 98 d group being the lowest and significantly lower than the unaged group (0 d). At 135 °C, tensile strength exhibited an overall trend of remaining at a comparable level initially, followed by a pronounced decrease at later stages. Specifically, tensile strength remained similar during early aging, but deteriorated significantly as aging progressed to 44 d. This is similar to the findings of Hu et al. [25], who investigated the aging behavior of an ethylene–propylene copolymer aged for 300 h at 120 °C. However, an increase was observed at both 115 °C and 125 °C, indicating that at relatively lower temperatures, the mechanical properties tend to increase initially and then decrease with prolonged aging time. These results suggest that tensile strength can increase during the early stage of thermo-oxidative aging. With increasing aging time, the tensile strength shows a decreasing trend. Evidently, at the same aging time, the tensile strength decreased as the aging temperature increased, indicating that more severe thermo-oxidative conditions accelerated chain scission and structural degradation, thereby impairing mechanical integrity.
As shown in Figure 2b, the effects of aging time and temperature on elongation at break were similar to those on tensile strength. Specifically, elongation at break also followed a trend of first increasing and then decreasing with aging time, but its variation was smaller than that of tensile strength. At the same aging time, the elongation at break also decreases as the aging temperature increases. Since elongation at break is closely associated with ductility, these results indicate that prolonged thermo-oxidative aging ultimately reduces the ductility of polyolefins. It is noteworthy that the specimens aged at 115 °C maintained relatively high mechanical performance within the time window investigated in this study, which does not imply that degradation will not occur during long-term service. According to the time temperature equivalence principle, increasing temperature markedly accelerates thermo-oxidative structural evolution; therefore, the earlier onset of property deterioration observed at higher temperatures can be regarded as the state that may be reached at lower temperatures after longer aging times. In other words, with further extension of the aging duration, the mechanical performance at 115 °C is expected to progressively enter a declining regime similar to that observed at 125 °C and 135 °C, albeit on a longer time scale.

3.2. The Effect of Thermal Aging on Macrostructure and Microscopic Morphology

With increasing aging temperature and prolonged aging time, the material exhibited pronounced yellowing, with the surface color changing from off-white to light yellow and eventually to dark yellow. These changes are illustrated in Figure 3.
Figure 3. Changes in the appearance of materials.
The observed yellowing can be attributed to multiple factors, primarily involving chemical degradation and physical changes. During thermo-oxidative aging, the ethylene-propylene copolymer undergoes bond scission and subsequent crosslinking reactions. In the presence of oxygen, new chromophoric groups such as conjugated double bonds (C=C) and carbonyl groups (-C=O), together with auxochromic groups such as hydroxyl (-OH) and ether (-OR), were generated [26]. With increasing aging time, these groups continuously accumulate, leading to progressive yellowing of the polymer surface.
In addition, degradation of additives or impurities can further contribute to discoloration. Besides polyolefin elastomers, the system contains antioxidants, titanium dioxide, and calcium carbonate. Antioxidant B225, a 1:1 blend of hindered phenolic antioxidant 1010 and phosphite antioxidant 168, may form quinone-methide derivatives upon depletion of the phenolic component, leading to discoloration [27]. Phenolic antioxidants may also interact with titanium ions to form aromatic complexes, further intensifying yellowing [28]. Apart from chemical degradation, changes in surface morphology also affect the optical properties of the material. When the characteristic surface roughness becomes comparable to the wavelength of visible light, the specular reflection regime breaks down. As a result, incident light undergoes pronounced scattering and diffraction [29,30]. Consequently, the yellowing of polyolefin materials is the combined outcome of chemical transformations and optical mechanisms.
Figure 4 illustrates the SEM micrographs of the fracture surfaces of polyolefins subjected to different aging temperatures and aging durations. The fracture surface of the unaged specimens exhibits pronounced features characteristic of extrusion processing. Specifically, discrete elastomeric phases are forcibly compacted together during processing, resulting in well-defined phase domains with sharp interfacial boundaries. This morphology is likely attributed to insufficient melt mixing and limited interfacial adhesion, reflecting poor phase compatibility within the system. Overall, the fracture surface is rough and densely populated with fine particles and irregular protrusions, which can be ascribed to the surface enrichment of additives and the exposure or pull-out of calcium carbonate particles. These microstructural heterogeneities readily act as preferential sites for void nucleation and subsequent defect evolution during aging.
Figure 4. SEM surface images of Polyolefin.
At the early stage of thermo-oxidative aging, the fracture surface gradually evolves from distinct phase-separated domains to a morphology with blurred interfacial boundaries, and no obvious voids or interfacial debonding are observed. This indicates that interfacial compatibility has improved at this stage, resulting in a more uniform fracture surface. Accordingly, the fracture process is dominated by overall plastic deformation. As aging progresses, the fracture morphology changes markedly, and a large number of irregular voids and localized interfacial cavities are observed in the SEM images. These voids exhibit pronounced non-uniformity in both size and spatial distribution. They are frequently accompanied by structural discontinuities near the interfaces, indicating significant accumulation of microstructural defects within the material. At this stage, the fracture surface transitions from an initially uniform and continuous morphology to one dominated by defects. Voids then serve as preferential sites for crack initiation and propagation, reflecting a deterioration in microstructural integrity.
In the later stage of aging, the fracture surface remains overall continuous but becomes markedly loosened, and the fracture interfacial boundaries become increasingly indistinct. At this stage, β-scission reactions dominate, leading to the formation of carboxyl and other oxidation products. As a result, the oxidative chain reactions are accelerated. This suggests that the fracture behavior is no longer primarily governed by discrete voids, but is instead characterized by overall structural softening and cooperative failure mechanisms.

3.3. FT-IR Analysis

The FTIR spectra of the polyolefin samples before and after aging are shown in Figure 5. Two absorption peaks are observed near 2850 cm−1 and 2920 cm−1 in the unaged samples, corresponding to the symmetric and asymmetric stretching modes of methylene (-CH2-), respectively. In addition, the absorption peaks near 1370 cm−1 and 1460 cm−1 are attributed to the bending mode of the C-H bond and the scissoring mode of methylene (-CH2-), respectively. Comparative analysis shows that the C–H stretching bands at 2920 cm−1 and 2850 cm−1 (asymmetric/symmetric stretching of –CH2–) remain clearly visible at all aging times, with essentially no peak shift. This indicates that the dominant chemical backbone of the samples retains the typical polyolefin structure. Accordingly, no chemical transformation capable of altering the main-chain type is observed during aging. Even when oxidation takes place, it primarily involves the introduction of a limited amount of oxygen-containing groups and/or chain scission on the polyolefin backbone, rather than a fundamental change in the chemical composition of the matrix.
Figure 5. FTIR spectra of Polyolefin.
The hydroxyl stretching vibration is typically observed in the range of 3000–3500 cm−1. At 125 °C, as a representative case, the absorption spectrum in this interval was magnified (Figure 6a). With increasing aging time, the hydroxyl absorption band in the 3000–3500 cm−1 region shows a continuous attenuation. At the initial stage, this band is primarily attributed to the presence of hindered phenolic antioxidant (ArOH), which are progressively consumed as they participate in oxidation reactions during aging. Although the polyolefin matrix and antioxidants can generate limited amounts of hydroxyl-containing intermediates, such as alcohols, hydroperoxides (ROOH), and carboxylic acids, during the oxidation process [31,32]. These species are relatively unstable under thermal conditions and tend to volatilize or undergo further oxidation. This process leads to the formation of more thermodynamically stable products, such as carbonyls and esters [33]. Therefore, the attenuation of the hydroxyl absorption band is mainly attributed to the consumption of the antioxidants.
Figure 6. Localized magnified FTIR images of polyolefin materials at different wavelengths: (a) 3000–3500 cm−1, (b) 1571 cm−1, (c) 1734 cm−1 and (d) 2920 cm−1.
Additionally, as aging progresses, two new absorption peaks with relatively low intensity emerge at approximately 1571 cm−1 and 1734 cm−1. The magnified FTIR spectra in the corresponding regions (Figure 6b,c) clearly reveal the evolution of these peaks. The band at 1734 cm−1 is attributed to the stretching vibration of carbonyl groups (-C=O), while the peak at around 1571 cm−1 corresponds to the asymmetric stretching vibration of carboxylate groups (-COO−). With increasing aging time, the absorption intensities of the peaks at 1734 cm−1 and 1571 cm−1 gradually increase, indicating the continuous formation and accumulation of oxidation products, such as ketones, aldehydes, carboxylic acids, and their corresponding salts, even when the antioxidant system remains partially effective. However, after 44 days of aging, a noticeable decrease in the intensity of these absorption peaks is observed. This behavior may be attributed to the further oxidation and transformation of certain carbonyl-containing species into more stable products, such as carboxylic acids (-COOH), esters (-COOR), or carboxylate salts (-COO−). In addition, possible volatilization of low-molecular-weight oxidation products may also contribute to the decrease.
Concurrently, the absorption peak near 2920 cm−1 exhibits a partial attenuation, suggesting a decrease in the relative content of methylene groups (-CH2-) and a disturbance of the main-chain hydrocarbon structure. Combined with the enhanced absorption at 1734 cm−1, these spectral changes provide clear evidence for the occurrence of thermo-oxidative degradation in the polyolefin system.

3.4. Thermal Behavior

Under thermo-oxidative aging conditions, the melting peak of the sample gradually decreases in intensity and becomes broader with increasing aging time, while the crystallization peak shifts toward lower temperatures, as shown in Figure 7. During aging, the melting temperature decreases and the melting endotherm broadens. This is consistent with the findings reported by Li et al. [34]. During aging, β-scission preferentially occurs at relatively weak sites along the polymer backbone, cleaving long chains that originally traversed crystalline regions into shorter segments and generating carbonyl and other oxidized structures at the scission sites. As these short chains and carbonyl-rich segments progressively accumulate, the crystalline lamellae become thinner and more heterogeneous in thickness, leading to a lower melting temperature and a broader melting peak [35]. In addition, oxidation-induced branching and limited crosslinking, together with chain scission and the loss of tie molecules, disrupt chain regularity and hinder segmental rearrangement during crystallization [36,37]. Consequently, a larger effective undercooling is required for crystallization, leading to a gradual shift of the crystallization peak toward lower temperatures.
Figure 7. The melting curve of DSC spectra for thermally aged Polyolefin, (a) 115 °C, (b) 125 °C, (c) 135 °C.
As shown in Figure 8, the crystallinity initially exhibits a slight increase, followed by a gradual decrease with prolonged aging time. This is consistent with studies on polyolefins containing antioxidants [38,39,40]. In the early stage of aging, a transient enhancement in crystallinity is observed, which is mainly attributed to the rearrangement of molecular chains in initially disordered regions and partial recrystallization. Limited chain scission relaxes the original entanglements and constraints between polymer chains, allowing previously disordered segments to reorganize and participate in crystallization. Consequently, new crystalline lamellae can grow at the surfaces or edges of pre-existing lamellae [41], leading to a temporary improvement in the mechanical properties of the material.
Figure 8. The variation in crystallinity at different aging temperatures.
During the subsequent aging stage, after the mechanical properties reach their maximum, the antioxidants remain effective and continue to suppress thermo-oxidative degradation. At this stage, the limited extent of chain scission exerts only a minor influence on the integrity of the crystalline framework, thereby allowing the mechanical performance to be maintained at a relatively high level.
As aging proceeds and the antioxidants are progressively depleted, thermo-oxidative reactions gradually become dominant. Continuous chain scission, together with the formation of aldehyde groups and other oxidized structures along the polymer backbone, progressively undermines the stability of the crystalline lamellae. Meanwhile, oxidation-induced chain branching and even partial crosslinking further reduce the number of chain segments capable of participating in crystallization. As a result, the crystallinity decreases, ultimately leading to the deterioration of the material’s mechanical properties.

3.5. EDS and GPC Analysis

Cross-sectional energy-dispersive spectroscopy (EDS) was employed to evaluate the evolution and spatial distribution of oxygen-containing species by quantifying the atomic ratios of oxygen to carbon (O/C) and calcium to carbon (Ca/C), as shown in Figure 9. The O/C ratio was used to reflect variations in oxygen-bearing functional groups within the polymer matrix, while the Ca/C ratio was monitored to track the content of the calcium carbonate (CaCO3) additive and to assess possible additive loss or transformation during aging. With increasing thermo-oxidative aging time, the O/C ratio decreased from 0.195 at the initial stage to 0.135 after 29 days and further declined to 0.128 after 98 days. In contrast, the Ca/C ratio initially decreased from 0.187 to 0.163 and subsequently rebounded to 0.188. These results indicate that the CaCO3 additive did not undergo systematic loss or significant compositional changes during aging, whereas the reduction in the O/C ratio reflects a gradual decrease in the relative content of oxygen-containing species within the cross-sectional region of the polymer matrix. This trend can be attributed to two main factors. Firstly, the chain breakage will produce low-molecular-weight oxygen-containing oxidation products, such as aldehydes, etc. These volatile components will lead to a decrease in the content of oxygen-containing substances in the polyolefin [42]. Second, due to the intrinsic heterogeneity during polyolefin aging, diffusion-limited oxidation (DLO) can occur when the rate of oxygen consumption within the material exceeds the rate of oxygen diffusion [43]. Under such diffusion constraints, thermo-oxidative aging is typically concentrated near the surface, and the degree of oxidation decreases progressively toward the interior. As aging proceeds, once sufficient polymer chain scission has occurred, the increased chain mobility alleviates the heterogeneous oxidation behavior, leading to enhanced oxidation in the interior. Consequently, oxygen-containing species are further depleted. As a result, the O/C ratio measured in the cross-section continuously decreases, whereas the Ca/C ratio remains nearly constant.
Figure 9. Elemental composition and atomic ratios of the polyolefin surface as a function of aging time. The atomic percentages of C, O, and Ca are shown as bars corresponding to the left y-axis, while the O/C and Ca/C ratios are plotted as lines corresponding to the right y-axis (As indicated by the arrow.).
Meanwhile, GPC measurements conducted at 125 °C reveal a gradual decrease in the weight-average molecular weight (Mw) with increasing aging time (Table 1). The reduction in molecular weight reflects the formation of shorter chain segments, which results in a broader molecular weight distribution and alters the aggregation state of the polymer [44]. During this process, the generation of short chains not only introduces additional reactive sites for oxygen uptake but also facilitates the diffusion and migration of low-molecular-weight oxidation products, thereby promoting further oxidation. Moreover, the increased number of chain-end functional groups, such as carboxyl and aldehyde moieties, provides new kinetic pathways for subsequent oxidation and limited crosslinking reactions. Together, these effects accelerate the overall thermo-oxidative degradation process.
Table 1. Changes in the Weight-Average Molecular Weight of Polyolefins During Thermal-Oxygen Aging.
Therefore, the decrease in Mw and the pronounced reduction in the O/C ratio, together with the nearly constant Ca/C ratio, corroborate each other and collectively indicate that, once the antioxidants are depleted, the material enters an accelerated aging stage dominated by oxidative chain scission and the diffusion and migration of low-molecular-weight oxidation products.

3.6. Mechanism Analysis

Based on the above analysis, the thermo-oxidative aging process of the polyolefin can be divided into three stages. During Stage I, an enhancement in mechanical properties is observed. This enhancement is primarily attributed to the early-stage thermal effect, whereby elevated temperature accelerates segmental mobility and promotes chain rearrangement in amorphous regions, inducing secondary crystallization and thus increasing the overall crystallinity. Meanwhile, because the antioxidant system remains effective in suppressing radical chain oxidation, main-chain scission is still limited and the molecular weight is largely preserved. In addition, chromophoric species have not yet accumulated to a significant extent; therefore, no obvious color change is observed (I in Figure 10).
Figure 10. Thermal Degradation Mechanism of Polyolefins.
As aging proceeds into Stage II, the gradual consumption of antioxidants reduces their effectiveness in inhibiting free radicals and hydroperoxide intermediates, leading to increasingly pronounced thermo-oxidative degradation. As depicted in Figure 10, chain scission preferentially occurs at labile sites along the polymer backbone ((i) in Figure 10), particularly at tertiary C-H bonds in propylene units, generating alkyl radicals (R•). These radicals rapidly react with molecular oxygen to form peroxyl radicals (ROO•, ii), which abstract hydrogen atoms to produce hydroperoxides (ROOH, iii) while regenerating R•, thereby sustaining the oxidation chain process. Subsequently, ROOH decomposes under thermal conditions to yield highly reactive alkoxy radicals (RO•) and related species; these radicals trigger β-scission (iv) and promote radical multiplication, driving the system from a relatively slow regime toward accelerated oxidation. Concomitantly, the progressive accumulation of oxygen-containing functionalities (notably carbonyl groups) and the emergence of microstructural defects compromise lamellar stability and chain regularity. As a result, crystallinity transitions from the initial increase to a decline, mechanical properties begin to deteriorate, and slight yellowing becomes apparent (II in Figure 10).
In Stage III, the antioxidant system approaches exhaustion and the oxidation process enters a markedly accelerated regime. Hydroperoxides (ROOH) decompose more readily via thermal and/or catalytic pathways, continuously generating RO• and other highly reactive radical species. The intensified β-scission reactions, together with radical multiplication, substantially increase the rate of main-chain scission. Consequently, a pronounced decrease in molecular weight is observed, indicating extensive chain fragmentation. In parallel, the integrity and stability of crystalline lamellae are further undermined, leading to a sustained reduction in crystallinity and a rapid deterioration in strength and toughness. Moreover, oxygen-containing groups and oxidation-related defects accumulate swiftly, and chromophoric species become enriched, resulting in markedly intensified yellowing (III in Figure 10).
Overall, the antioxidant package plays a central role in regulating the above three-stage evolution. Hindered phenol 1010 (ArOH) quenches alkyl (R•) and peroxyl (ROO•) radicals via hydrogen donation, forming a comparatively stable phenoxy radical (ArO•) and lowering the steady-state radical concentration. Phosphite 168 ((ArO)3P) both reduces ROOH to relatively inert alcohols (ROH)—thereby attenuating the autocatalytic acceleration associated with ROOH decomposition—and regenerates active phenolic species, enabling antioxidant turnover. In addition, ArO• can further terminate residual radicals through coupling or addition reactions. The presence of these antioxidants therefore preserves molecular weight and crystalline structure during the early aging period, sustaining mechanical performance and limiting discoloration. Once 168 and 1010 are progressively depleted to the point that radical reactions and ROOH decomposition can no longer be effectively controlled, the system transitions into an accelerated oxidation regime, ultimately giving rise to concurrent decreases in molecular weight and crystallinity, accompanied by mechanical degradation and aggravated yellowing.

4. Conclusions

In this study, polyolefin materials containing antioxidants were investigated by accelerated high-temperature aging tests, revealing their thermal oxidative aging behavior and mechanisms. In the early stage of aging, the mechanical properties of polyolefins improved with increased crystallinity, and no significant changes in color appearance were observed. As aging progressed, the hydroxyl absorption band weakened gradually, while the carbonyl absorption band intensified continuously, accompanied by an overall decrease in -CH2- absorption. These results indicate that antioxidants effectively inhibit oxidative degradation, allowing the mechanical properties of aged polyolefins to be maintained or to decrease slowly. In the later stage of aging, the mechanical properties of polyolefins deteriorated quickly and severe yellowing occurred, which was mainly attributed to rapid main-chain scission after substantial depletion of the antioxidant system. The aging mechanism of antioxidant-containing polyolefins involves the interaction of different components, consumption of antioxidants, polymer main-chain scission, changes in crystallinity, and breaking of molecular chains. This study provides a scientific basis for developing methods and standards related to polyolefin aging and offers valuable guidance for optimizing antioxidant systems and formulations.

Author Contributions

Conceptualization, N.J.; methodology, R.L. and Y.X.; validation, C.L. and X.D. and Z.L.; formal analysis, R.L. and C.L.; investigation, C.L. and Y.X.; resources, N.J. and Z.Z.; data curation, Y.X. and X.D.; writing—original draft preparation, N.J., R.L. and Y.X.; visualization, R.L. and Z.L.; supervision, N.J. and Z.Z.; project administration, N.J. and Z.Z.; funding acquisition, N.J. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (Grant No. 12302182) and Young Talent of Lifting engineering for Science and Technology in Shandong, China (Grant No. SDAST2024QTA082).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

The authors acknowledge the National Natural Science Foundation of China (Grant No. 12302182) and Young Talent of Lifting engineering for Science and Technology in Shandong, China (Grant No. SDAST2024QTA082).

Conflicts of Interest

Authors Chao Li, Xuewei Duan, and Zhenyang Liu were employed by the company Hongyuan Waterproof Technology Group Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
PEPolyethylene
PPPolypropylene
R•Alkyl radicals
ROO•Peroxy radicals
ROOHHydroperoxides
RO•Alkoxy radicals
•OHHydroxyl radical
FTIRFourier-transform infrared spectroscopy
DSCDifferential scanning calorimetry
SEMScanning electron microscopy
GPCGel permeation chromatography
C=CConjugated double bonds
-C=OCarbonyl groups
-OHHydroxyl
-OREther
PVCPolyvinyl chloride
0 d0 days
14 d14 days
29 d29 days
44 d44 days
98 d98 days
-CH2-Methylene
C-HHydrocarbon bond
-COOHCarboxylic acid
-COOREsters
-COO−Carboxylate salt
O/CCarbon-to-oxygen atom ratio
Ca/Ccalcium-to-carbon atom ratio
CaCO3Calcium carbonate
RHChain segments
B225Antioxidant package
ArOHHindered phenol 1010
ArO•Phenoxy radical
(ArO)3PPhosphite 168
(ArO)3P=Ophosphate

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