The effect of thermal aging on the mechanical properties and thermal stability of modified polypropylene insulating materials

The effect of thermal aging on the mechanical properties and thermal stability of modified polypropylene insulating materials | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article The effect of thermal aging on the mechanical properties and thermal stability of modified polypropylene insulating materials Guobin MA, Guowen DAI, Xiufeng LI, Xingzhen WANG, Chuanke DING, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8861746/v1 This work is licensed under a CC BY 4.0 License Status: Under Revision Version 1 posted 20 You are reading this latest preprint version Abstract As a novel cable insulation material, thermoplastic polypropylene (PP) offers advantages such as excellent electrical properties, good chemical corrosion resistance, and a relatively high temperature rating, aligning with China's current "dual-carbon" goals. Two types of modified PP insulation materials are investigated: blend-modified and copolymer-modified. Accelerated thermo-oxidative aging tests were conducted at different temperatures and durations. Variations in mechanical and thermal properties before and after aging were characterized using tensile testing, dynamic thermomechanical analysis (DMA), and thermogravimetric analysis (TGA). The performance differences between the two modified PP materials and XLPE were systematically analyzed. Results indicate that both XLPE and PP undergo oxidative degradation and a decline in mechanical performance during thermo-oxidative aging. However, PP demonstrates overall superior aging resistance. XLPE degrades relatively slowly in the initial aging stage but suffers rapid performance deterioration later due to the breakdown of its cross-linked network structure. In contrast, while PP exhibits a faster oxidation rate, it maintains better mechanical property retention and thermal stability than XLPE. Specifically, the blend-modified PP, toughened by elastomers, shows the best toughness and ductility, with the smallest decrease in mechanical properties. The copolymer-modified PP, owing to its high crystallinity, possesses more excellent thermal stability. Both modified polypropylene materials demonstrate potential for replacing XLPE as cable insulation. Thermal aging Cross-linked polyethylene Polypropylene Mechanical properties Thermal stability Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1 INTRODUCTION With the proposal and steady advancement of China's national strategies of "Carbon Peak" and "Carbon Neutrality," the construction of a clean and low-carbon new power system presents unprecedented challenges and opportunities for the power cable industry [ 1 ] . As the deployment scale of cross-linked polyethylene (XLPE) cables expands, their shortcomings, such as complex production processes [ 2 ] , high energy consumption [ 3 ] , and low efficiency [ 4 ] , have become increasingly apparent. Furthermore, as a thermosetting material, XLPE can only be disposed of through incineration or landfilling after decommissioning, which contradicts the "Dual Carbon" philosophy. Consequently, low-carbon environmental friendliness, energy efficiency, and recyclability have become new development directions for cable insulation materials [ 5 ] . Thermoplastic polypropylene (PP) material, with its advantages of melt recyclability [ 6 ] , excellent electrical properties [ 7 ] , and relatively high-temperature resistance [ 8 ] , has emerged as a significant direction for developing environmentally friendly high-voltage cable insulation. However, due to its inherent high crystallinity and high modulus, polypropylene material exhibits high rigidity, poor toughness, and insufficient low-temperature impact resistance [ 9 ] . This leads to noticeable stress whitening during cable installation, preventing its direct use as the primary cable insulation. Many researchers have conducted modification studies using polypropylene as the base matrix. Hu Shixun et al. [ 10 ] compared the thermal, mechanical, and electrical properties of three materials: copolymerized polypropylene, nano-doped modified polypropylene, and graft-modified polypropylene. They found that compared to XLPE, all three materials demonstrated higher thermal stability and comparable mechanical properties. Moreover, the modified polypropylene materials showed significantly improved electrical insulation performance under DC conditions. Chen Hong et al. [ 11 ] compared the structures and properties of different blend-modified PP materials and copolymer-modified PP materials. They observed that modified PP materials exhibited significantly enhanced toughness, tensile strength, and elongation at break, but experienced reduced thermal performance and increased dielectric loss. Copolymer-modified PP materials demonstrated superior thermal properties and higher breakdown field strength, while blend-modified PP materials showed better mechanical performance and lower dielectric loss. Zhao Peng et al. [ 12 ] investigated the thermal-oxidative aging characteristics of different modified polypropylene materials. They found that during the aging process, modified polypropylene insulation materials exhibited macroscopic change trends similar to XLPE and maintained high stability during long-term aging, demonstrating the feasibility of replacing XLPE. Therefore, PP materials subjected to different modification methods exhibit variations in structure, properties, and thermal-oxidative aging characteristics. Studying the thermal-oxidative aging behavior of modified polypropylene materials under various conditions is thus beneficial for predicting their long-term service performance. This paper focuses on two materials: blend-modified polypropylene (BPP) and copolymer-modified polypropylene (CPP). Through tensile property testing, dynamic thermomechanical analysis (DMA), and ther-mogravimetric analysis (TGA), the dynamic evolution and macroscopic aging performance of modified PP materials under different aging temperatures and durations are thoroughly investigated. The aging mechanisms are analyzed to provide theoretical support for the application of polypropylene in cable insulation materials. 2 TEST METHODS 2.1 Primary Raw Materials 35 kV cross-linked polyethylene (XLPE) insulating material, produced by Nanjing Zhongchao New Material Co., Ltd. Polypropylene (PP), produced by Sinopec Yanshan Petrochemical Company.Polyolefin Elastomer (POE), manufactured by LG Chem Ltd. Polyolefin Plastomer (POP), produced by Dow Chemical Company, USA. Copolymerized PP insulating material, produced by Sinopec Yanshan Petrochemical Company. 2.2 Sample Preparation (1) XLPE: The cross-linkable polyethylene insulating pellets were preheated without pressure at 120°C on a flat-plate vulcanizing press for 5 minutes. Subsequently, the specimens were subjected to pressure for cross-linking at 175°C and 15 MPa for 15 minutes. After pressurized cooling to room temperature, multiple XLPE specimens with thicknesses of 1 mm and 4 mm were prepared. (2) Blended Polypropylene (BPP): PP was respectively mixed with POE, POP, and antioxidants according to specific ratios. The mixture was melt-blended for 15 minutes using a precision open mill at 190°C and 40 rad/min. After discharging, the material was pressed at 200°C and 15 MPa for 8 minutes to prepare multiple blended polypropylene specimens with thicknesses of 1 mm and 4 mm. (3) Copolymerized Polypropylene (CPP): The copolymerized PP insulating material was preheated without pressure on a flat-plate vulcanizing press, followed by pressing at 200°C and 15 MPa for 8 minutes. Multiple copolymerized polypropylene specimens with thicknesses of 1 mm and 4 mm were thus prepared. 2.3 Performance Testing 2.3.1 Thermal Aging In accordance with IEC 60811-1-2:1985 "Common test methods for insulating and sheathing materials of electric and optical cables - Part 12: Methods for general application - Thermal aging methods", specimens of XLPE, BPP, and CPP were subjected to thermal aging at two different temperatures, 135°C and 150°C, for durations of 7, 14, and 21 days, respectively. To ensure the accuracy of the test data, all aged specimens were removed and allowed to condition under standard ambient conditions (23 ± 2°C, relative humidity 50 ± 5%) for 24 hours prior to conducting the various performance tests. 2.3.2 Tensile Properties In accordance with ISO 527-2:2012 "Plastics - Determination of tensile properties - Part 2: Test conditions for moulding and extrusion plastics", tensile tests were conducted using a UTM2103 electronic universal testing machine manufactured by Shenzhen Sansi Zongheng Technology Co., Ltd. The tests were performed at a crosshead speed of 250 ± 50 mm/min and a temperature of 23 ± 2°C. Five specimens of each type were tested, and the average value was calculated. 2.3.3 Dynamic Thermomechanical Analysis (DMA) A Dynamic Mechanical Analyzer (model SDT961, manufactured by METTLER TOLEDO, Switzerland) was employed to investigate the dynamic mechanical behavior of the specimens. This was achieved by applying a time-varying alternating force while subjecting the samples to a programmed temperature control. The instrument recorded the variations of storage modulus, loss modulus, and loss factor (tan δ ) as functions of temperature. The tests were conducted on specimens with a thickness of 4 mm over a temperature range of -50°C to 150°C, with a heating rate of 2°C/min, a vibration frequency of 1 Hz, and an applied force of 0.1 N. 2.3.4 Thermogravimetric Analysis (TGA) The thermal stability of the samples was investigated using a Thermogravimetric Analyzer (TGA 2, Mettler Toledo, Switzerland). The tests were conducted under a nitrogen atmosphere with a flow rate of 50 mL/min. The temperature was increased from 40°C to 800°C at a constant heating rate of 10°C/min. 3 RESULTS AND DISCUSSION 3.1 Effect of Thermal Aging on the Mechanical Properties of PP Insulation 3.1.1 Tensile Properties The macroscopic mechanical properties are essentially an external manifestation of the material's micr-ostructure and can directly reflect the aging degree of cable insulation. Tensile testing clearly reveals the typical characteristics of the material during the three stages of deformation [ 13 ] : elastic deformation, plastic deformation, and fracture. In the elastic deformation stage, the strain distribution follows Hooke's Law, meaning stress is proportional to strain. Upon entering the plastic deformation stage, localized "necking" occurs in the specimen. As stretching continues, the necked region elongates until the entire gauge section thins uniformly, ultimately leading to tensile fracture. The tensile stress-strain curves for all specimens are shown in Fig. 2 . The apparent morphology of XLPE and PP specimens under different aging temperatures and durations is presented in Table 1 . The variation trends of elastic modulus, tensile strength, and elongation at break for the different specimens are illustrated in Fig. 3. As shown in Table 1 , with the intensification of thermal aging, the surface color of XLPE specimen-s undergoes a progressive change, shifting from an initial translucent state to a deep yellow and finally to a brownish hue. This color change serves as a direct visual indicator of severe oxidative degradation within the material. The underlying cause is attributed to the chain scission of XLPE molecular chains under thermo-oxidative conditions, which generates a significant amount of carbonyl-containing compounds [ 14 ] , such as ketones, aldehydes, and carboxylic acids. The continued deepening of the color corresponds to the accumulation of the carbonyl index. In contrast, both types of PP specimens exhibit an opaque, milky-white appearance prior to aging. Even after prolonged thermal aging, only very slight yellowing is observed on their surfaces. The color stability of the PP materials indicates the high structural integrity of their molecular chains. The minimal yellowing demonstrates that the incorporated antioxidants effectively retard the oxidation process, suppress the formation of carbonyl compounds, and prevent significant macromolecular chain scission. These differences fundamentally confirm that the modified PP materials possess more stable and superior resistance to thermo-oxidative aging. The evolution of mechanical properties for each specimen can be observed from Fig. 3(a). The elastic modulus of XLPE specimens shows an overall declining trend after aging, exhibiting clear temperature dependence with aging time. Under aging at 135°C, the elastic modulus of the specimens displays a non-monotonic trend, initially increasing slightly and then decreasing significantly with prolonged aging time. This is because, during the initial aging stage, peroxides within the XLPE initiate short-chain extensions under thermal effects, leading to a denser cross-linked network structure [ 15 ] that restricts molecular chain movement and thus increases the elastic modulus. Additionally, thermal effects promote the rearrangement of molecular segments in the amorphous regions and improve the crystallinity in some crystalline areas [ 16 ] , resulting in increased rigidity of the specimens. However, with extended aging time, oxidative chain scission occurs, gradually reducing the elastic modulus. In contrast, the elastic modulus of the two modified PP materials are significantly higher than that of XLPE, indicating that even after modification, PP retains relatively high rigidity and hardness. Furthermore, the elastic modulus of modified PP exhibits a trend of initially decreasing and then increasing with prolonged aging time. During the early stages of aging, thermo-oxidative effects cause breakage of the PP polymer main chains, generating small molecular fragments [ 17 ] and reducing entanglement density. This leads to structural relaxation within the specimen, resulting in a decrease in elastic modulus. In the later stages of aging, prolonged aging induces oxidation reactions in PP, forming polar groups such as carbonyl groups. This promotes the reorientation and rearrangement of molecular segments, increases the crystallinity of the specimens [ 18 ] , and consequently raises the elastic modulus. With increasing aging temperature, the elastic modulus of XLPE demonstrates a monotonic decreasing trend during the aging process at 150°C. This is primarily attributed to the sustained high-temperature thermo-oxidative aging, which induces oxidation of C-C and C-H bonds in XLPE, generating functional groups such as carbonyls. As a result, the cross-linked network structure is severely disrupted, and the molecular weight decreases, leading to a reduction in the material's rigidity. In contrast, the elastic modulus of BPP is significantly lower than that of CPP under both aging temperatures. This difference is mainly due to the incorporation of toughening modifiers such as elastomers in the BPP formulation, which endows BPP with superior toughness. Tensile strength represents the stress at which a material withstands the maximum load before fracture, while elongation at break measures the maximum deformation the material can achieve prior to failure, reflecting its ductility and resistance to fracture [ 19 ] . As shown in Figs. 3(b) and (c), after aging, the tensile strength and elongation at break of XLPE specimens decrease significantly with prolonged aging time. After 21 days of aging at 135°C, the tensile strength decreases by 12.5%, and the elongation at break drops by 23.8%, indicating substantial degradation in specimen performance. Evidently, high temperatures induce molecular chain scission in XLPE, leading to reduced molecular weight and weakened intermolecular forces [ 20 ] , resulting in the formation of micro-defects and stress concentration points, thereby deteriorating its mechanical properties. In contrast, both modified PP materials exhibit significantly higher tensile strength and elongation at break compared to XLPE. This is because PP, as a typical thermoplastic polymer, possesses linear or branched molecular chains [ 21 ] without the cross-linked network structure present in XLPE. Free from the spatial constraints of chemical cross-linking points, PP more readily undergoes orientation-induced crystallization during stretching [ 22 ] . Macroscopically, this results in excellent plastic deformation capability while reducing the tendency for brittle fracture caused by stress concentration. Consequently, the molecular chains of PP demonstrate greater flexibility and ductility. Under aging at 135°C, the tensile strength of all specimens shows a trend of "initial increase followed by decrease." During the early aging stage, enhanced molecular chain mobility promotes recrystallization [ 12 ] , leading to increased crystallinity and a more ordered structure. The lower the initial crystallinity, the more pronounced the recrystallization effect, resulting in a slight increase in tensile strength. In the middle to late stages of aging, however, oxidation causes molecular chain scission [ 23 ] , and the exponential growth of carbonyl and other functional groups intensifies the aging degree, leading to a noticeable decline in mechanical properties. With an increase in aging temperature, under the condition of 150°C, high temperature directly induces molecular chain scission, causing a substantial reduction in tensile strength. Although increased crystallinity can partially offset this effect, the overall trend remains monotonically decreasing. Comparing the mechanical properties of the two modified PP materials reveals that the BPP specimen, modified with elastomers, exhibits significantly superior tensile strength and elongation at break relative to CPP. This is attributed to the uniform dispersion of elastomers (POE and POP) within the BPP matrix. Under external stress, these elastomeric domains act as stress-concentration points, inducing substantial shear deformation that dissipates energy. Consequently, the yield stress of the composite system is progressively reduced [ 24 ] , resulting in an effective toughening effect. In contrast, CPP possesses a higher degree of crystallinity, with its molecular chains arranged in a more ordered structure. The well-defined crystalline regions restrict molecular chain slippage, thereby rendering the material less tough than BPP. 3.1.2 Dynamic Thermomechanical Properties Dynamic thermomechanical analysis (DMA) me-asures the relationship between a material's dynamic modulus, mechanical loss, and temperature under alternating stress [ 19 ] . Specimens aged at 150°C for 21 days were selected as representative aged samples for DMA testing. The results are presented in Fig. 4 , and the corresponding storage modulus data at different temperatures are listed in Table 2 . Table 2 Storage modulus of each sample at different Temperatures Specimen 25℃ 90℃ 110℃ 150℃ XLPE-Before Aging 377.46 46.88 16.93 14.73 XLPE- After Aging 436.02 159.48 118.73 20.80 BPP- Before Aging 595.24 132.12 69.29 64.77 BPP- After Aging 626.96 168.86 166.04 86.06 CPP- Before Aging 1173.03 312.72 189.12 65.88 CPP- After Aging 1205.45 343.65 199.01 80.64 The storage modulus of a material reflects the elastic component in its viscoelastic behavior, indicating the material's stiffness; The loss modulus characterizes the energy dissipated due to viscous deformation within the material; The ratio of the loss modulus to the storage modulus is defined as the loss factor tan δ . A higher tan δ value signifies greater energy loss in the material under alternating stress [ 21 ] . As shown in Fig. 4 (a) and Table 2 , compared to the values at 25°C, the storage modulus of all specimens exhibits a decreasing trend with rising temperature. This is because as temperature increases, the crystallinity of the specimens decreases, and larger structural units within the molecular chains, such as side groups and chain segments, gradually acquire sufficient activation energy [ 25 ] . Their mobility is significantly enhanced, the entanglement degree of molecular chains is reduced, and the specimens become softer, leading to a decline in storage modulus. However, under the same temperature condition (e.g., 90°C), the storage modulus of XLPE is notably lower than that of the PP specimens. This difference is primarily attributed to the distinct molecular structural characteristics of the two materials. Although the unique cross-linked network structure in XLPE restricts molecular chain motion, its relatively high amorphous content results in a lower storage modulus. In contrast, PP is a highly crystalline polymer, and the introduction of specific molecular groups during modification enhances intermolecular interactions, thereby endowing it with higher rigidity. When the temperature is further increased to 110°C, the branching degree of molecular chains rises, intermolecular bonding weakens, and microcrystal melting occurs under high temperature [ 26 ] , leading to a pronounced decrease in storage modulus. In the high-temperature region (around 150°C), the crystal melting process dominates [ 26 ] , and the difference in storage modulus between XLPE and modified PP specimens diminishes. This phenomenon indicates that after melting, the destruction of the crystalline structure substantially reduces the influence of the cross-linked network in XLPE and the chain regularity in PP on their respective rigidity, causing their storage modulus to converge. Following aging, the storage modulus of all specimens increased. This rise is attributed to the closer packing of molecular chains within the material during thermal aging, which enhances its mechanical strength. However, this process is accompanied by chain scission, particularly damaging the flexible -CH 2 -CH 2 - long-chain backbone [ 21 ] , leading to a significant reduction in material flexibility and an increase in rigidity, thereby elevating the storage modulus. The change in storage modulus is particularly pronounced for the XLPE specimen after aging. At 90°C, its storage modulus becomes 3.4 times that of the unaged state, and at 110°C, it reaches 7.1 times the pre-aged value. In contrast, the storage moduli of BPP and CPP at 90°C are only 1.3 times and 1.1 times their respective pre-aged values, far lower than the change observed for XLPE. This significant discrepancy is attributed to two main factors: on one hand, XLPE undergoes short-chain extension and forms new cross-linking points between molecular chains during the early stage of thermo-oxidative aging, enhancing its rigidity; on the other hand, polar groups such as carbonyls generated from oxidation participate in intermolecular interactions, leading to a substantial increase in storage modulus. Figure 4 (b) also shows that the loss modulus of aged XLPE exhibits only a single loss peak. This indicates the molecular relaxation transitions of the material in different temperature zones: the low-temperature region corresponds to β relaxation, and the high-temperature region is associated with α relaxation [ 19 ] . In the tan δ temperature spectrum of XLPE before and after aging (Fig. 4 (c)), the loss peak of the aged sample shifts to a lower temperature, decreasing by approximately 30°C compared to the unaged state. This corresponds to the melting of smaller crystallites and the process where the degree of intermolecular entanglement decreases, transitioning into a rubbery state [ 21 ] . The difference in elastic potential energy is dissipated as mechanical loss, leading to reduced crystal integrity after thermo-oxidative aging and consequently a lower loss peak temperature. In contrast, the loss modulus and tan δ curves of the two modified PP specimens after aging show only minor changes in intensity and peak value. Their overall shapes remain relatively stable, with no new loss peaks emerging, and the extent of change is significantly smaller than that of XLPE. This is because the aging process of PP is dominated by molecular chain scission, and the formation of oxidation products has a relatively limited impact on the segmental motion of the molecular chains, resulting in smaller changes in its dynamic mechanical properties. Comparing the dynamic thermomechanical properties of the two modified PP specimens reveals that the storage modulus of both BPP and CPP remain almost unchanged before and after thermo-oxidative aging, showing an increase only in the high-temperature region compared to the unaged state. Furthermore, the storage modulus of CPP is significantly higher than that of BPP. This difference primarily stems from their distinct microstructures: the introduction of elastomers in BPP makes it prone to phase separation from the PP matrix [ 27 ] , which hinders the orderly packing of molecular chains and results in the formation of extensive amorphous regions, thereby yielding a lower storage modulus. In contrast, CPP possesses higher crystallinity and a more regular molecular chain arrangement, leading to a more perfected crystalline structure [ 28 ] . Its ordered crystalline regions restrict molecular chain slippage, resulting in greater stiffness and a higher storage modulus. Although the peak intensity of the loss modulus curve for BPP changes after aging, its position remains nearly unchanged, exhibiting a double-plateau feature. The loss peak near − 35°C corresponds to β relaxation. After aging, the intensified phase separation behavior restricts molecular chain motion at the interface regions [ 11 ] , and the compatibility between the elastomer and the matrix decreases, leading to more significant changes in peak intensity. The loss peak near 8°C is associated with α relaxation. For CPP, the introduction of ethylene units disrupts the regularity of the PP molecular chains, reducing the material's crystallinity [ 28 ] and increasing its amorphous content. Consequently, CPP primarily exhibits the α relaxation of PP chain segments, while the β relaxation introduced by ethylene copolymerization is not prominent. After aging, oxidative scission of the PP main chain broadens the molecular weight distribution in the amorphous regions. Simultaneously, the generation of polar groups such as carbonyls through oxidation restricts segmental motion, resulting in a noticeably broader peak and increased peak intensity. 3.2 Effect of Thermal Aging on the Thermal Stability of PP Insulation The TGA curves illustrating the mass variation of XLPE, BPP, and CPP specimens with temperature are presented in Fig. 5 (a). Their first derivatives, yielding the DTG curves, are shown in Fig. 5 (b). Corresponding thermal decomposition parameters are summarized in Table 3 . Here, T 5% denotes the initial decomposition temperature (temperature at 5% mass loss), and T 50% represents the temperature at 50% mass loss. Furthermore, V max is the maximum mass loss rate during decomposition, and T max is the temperature at which this maximum rate occurs. Table 3 Thermal decomposition parameters of XLPE, BPP and CPP samples Aging State Specimen T 5% T 50% T max V max Before Aging XLPE 413.83 456.17 460.17 3.08 After Aging XLPE-135℃-7d 415.83 462.33 470.67 2.75 XLPE-135℃-14d 431.50 471.17 476.17 2.94 XLPE-135℃-21d 443.67 477.50 480.00 3.32 XLPE-150℃-7d 440.33 475.50 480.67 3.23 XLPE-150℃-14d 443.67 477.17 479.67 4.12 XLPE-150℃-21d 441.00 475.17 477.83 3.63 Before Aging BPP 412.50 454.50 456.83 3.30 After Aging BPP-135℃-7d 423.33 458.00 459.33 3.32 BPP-135℃-14d 423.67 457.83 460.67 3.09 BPP-135℃-21d 424.45 458.21 461.35 3.14 BPP-150℃-7d 405.17 455.00 460.67 2.66 BPP-150℃-14d 409.17 455.67 461.33 2.78 BPP-150℃-21d 406.00 455.00 462.00 2.81 Before Aging CPP 423.50 457.67 460.67 3.04 After Aging CPP-135℃-7d 423.00 458.17 461.33 3.13 CPP-135℃-14d 422.83 458.17 461.83 3.16 CPP-135℃-21d 423.83 458.17 458.17 3.22 CPP-150℃-7d 423.33 458.00 461.67 3.12 CPP-150℃-14d 422.67 457.17 456.50 3.39 CPP-150℃-21d 423.83 458.17 462.67 3.19 As shown in Fig. 5 , the TGA curves of all specimens clearly exhibit three typical stages: the weight stabilization region, the primary thermal degradation region, and the completed weight loss region. The DTG curves, representing the relationship between the rate of weight change and temperature, can more precisely reflect the characteristic thermal decomposition parameters of the specimens, namely the onset reaction temperature, the temperature corresponding to the maximum reaction rate, and the reaction termination temperature. A higher initial decomposition temperature indicates better thermal stability [ 21 ] . Combining with Table 3 , it can be observed that the initial decomposition temperatures of aged XLPE specimens are elevated to varying degrees compared to their unaged counterparts. This is attributed to the short-chain extensions occurring within XLPE during the initial aging stage, resulting in a denser cross-linked network structure, thereby raising the specimen's initial decomposition temperature. However, with prolonged aging time, molecular chains undergo scission under extended thermo-oxidative action, the cross-linked network becomes disrupted, the degradation rate of the specimen accelerates, and its thermal stability declines. In contrast, the thermal degradation of both modified PP specimens exhibits a single-step characteristic, with their DTG curves showing a single sharp peak. This corresponds to a homogeneous chain scission degradation process of the main molecular backbone, indicating that degradation is dominated by the complete breakage of primary chains. Furthermore, under different aging temperatures, the thermal decomposition temperature of PP specimens shows no significant change with increasing thermal aging duration. This demonstrates that the PP specimens possess relatively high thermal resistance. As the aging temperature increases, under the 150°C aging condition, the sustained high-temperature exposure elevates the maximum decomposition rates of both XLPE and PP specimens. This finding further confirms that elevated temperatures accelerate chain scission and structural breakdown, leading to a reduction in the thermal stability of the specimens. However, significant differences exist in the thermal decomposition characteristics and degradation mechanisms between XLPE and PP. The TGA curves of XLPE aged at both 135°C and 150°C are relatively broad, and the corresponding DTG curves exhibit bimodal or multimodal features. This is primarily due to the competitive mechanism within XLPE involving cross-linked network extension and oxidative decomposition. During the initial aging stage, the increased cross-linking density restricts molecular chain movement, hinders oxygen diffusion, and retards the oxidation reaction. In contrast, during the later stage, polar groups such as carbonyls generated from the oxidation reaction degrade the cross-linked structure. Conversely, PP specimens undergo primarily oxidative degradation. Their linear molecular chain structure facilitates oxygen diffusion along intermolecular pathways into the specimen interior, resulting in the distinct characteristics observed in their respective TGA curves. A comparison between the two modified PP specimens reveals distinct aging characteristics. The BPP exhibits minor signs of aging: its main decomposition peak broadens, and its intensity weakens somewhat. In contrast, the initial decomposition temperature of CPP remains virtually unchanged before and after thermo-oxidative aging. Its DTG curves show a high degree of overlap, with the peak width remaining largely consistent, demonstrating superior thermal stability. This advantage is primarily attributed to the higher crystallinity of CPP. Within its crystalline regions, the molecular chains are packed more densely. Its uniform molecular chain structure effectively avoids the interfacial issues associated with physical blending [ 29 ] . Furthermore, the dense crystalline regions formed by the high crystallinity significantly retard oxygen permeation and the process of molecular chain scission. This helps maintain a stable molecular weight distribution in CPP during thermo-oxidative aging [ 30 ] and ensures a highly consistent main-chain scission mode. Consequently, CPP exhibits a higher initial thermal decomposition temperature. 4 CONCLUSIONS Thermo-oxidative aging leads to a decline in the mechanical properties of both XLPE and PP. However, the overall mechanical performance of the two modified PP materials remains superior to that of XLPE, demonstrating better aging resistance and toughness. Among them, BPP exhibits the most outstanding toughness and ductility due to the toughening effect of elastomers. After aging, it retains the highest levels of both elongation at break and tensile strength, showing the smallest reduction in mechanical performance. During thermo-oxidative aging, the dynamic mechanical properties of XLPE degrade significantly due to structural damage, whereas both modified PP materials exhibit minimal performance changes, demonstrating superior aging resistance and structural integrity. During thermo-oxidative aging, the thermal stability of XLPE demonstrates an initial increase followed by a subsequent decline, attributable to the initial extension of its cross-linked network and subsequent oxidative degradation. In contrast, both types of PP exhibit superior thermal stability compared to XLPE. CPP, owing to its dense and well-ordered crystalline structure, maintains a stable molecular weight distribution and a uniform degradation mode throughout the aging process, thereby achieving the most exceptional thermal stability. Declarations Conflict of interest The authors declare no conflicts of interest. Author Contribution Guobin MA: involving in investigation, experiments,analysis. Xiufeng LI: writing – review and editing, investigation, project ad-ministration, resources. Guowen DAI: data curation. Xingzhen Wang: data curation. Chuanke Ding: data curation. Yaqi ZHANG: writing – original draft Chenyang JIAO: writing – review and editing. Acknowledgement The authors are grateful for the financial support of the National Natural Science Foundation of China (52477147) and the Shandong Provincial Natural Sci-ence Foundation (ZR2023ME129). References Ge W, Wang Z, Wang X .Optimized allocation of energy storage for integrated energy systems with coordinated source-load-storage interaction[J].Electric Power Systems Research, 2025, 248. 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Breakdown voltage of polypropylene film during C and thermal ageing[C]//2017 18th International Scientific Conference on Electric Power Engineering. Kouty nad Desnou, Czech Republic: IEEE,2017. Wang W, Min D, Li S .Understanding the conduction and breakdown properties of polyethylene nanodielectrics: effect of deep traps[J].IEEE Transactions on Dielectrics & Electrical Insulation, 2016, 23(1):564–572. Yu Qinxue, Ren Wen'e. Electrical Insulation Testing and Analysis[M]. Xi'an: Xi'an Jiaotong University Press, 2013: 93–96. ((in Chinese)). Zhan W, Chu X, Shen Z ,et al.Study on Aggregation Structure and Dielectric Strength of XLPE Cable Insulation in Accelerated Thermal-oxidative Aging[J].Proceedings of the CSEE, 2016, 36(17). Xie Darong, Wu Songzhen. Polymer Physics for Electrical Engineering[M]. Xi'an: Xi'an Jiaotong University Press, 1990: 114–120. ((in Chinese)). Ouyang Benhong, Zhao Peng, Huang Kaiwen, et al. Review on Research Progress of Thermoplastic Polypropylene Cable Materials[J]. High Voltage Engineering, 2023, 49(03): 907–919. Garton A, Bamji S, Bulinski A ,et al.Oxidation and Water Tree Formation in Service-Aged XLPE Cable Insulation[J].IEEE Transactions on Electrical Insulation, 1987, EI-22(4):405–412. Chang S M, Mun B C, Lee S H ,et al.22.9 kV Polypropylene Insulated Power Cable with Soft Polypropylene[C]//CIGRE Session.2018. Huang Xingyi, Zhang Jun, Jiang Pingkai, et al. Material progress toward recyclable insulation of power cables part 2: polypropylene-based thermoplastic materials[J]. IEEE Electrical Insulation Magazine, 2020, 36(1): 8–18. Li X, Gao Y, Ma G ,et al.Correlation Between Relative Water Diffusion Coefficient and Water Trees in Crosslinked Polyethylene/Organic Montmorillonite Nanocomposites[J].Polymer Engineering & Science, 2025, 65(11). Xu Hang, Du Boxue, Li Jin, et al. Mechanical and Space Charge Characteristics of Polypropylene/Elastomer Composites[J]. High Voltage Engineering, 2019, 45(10): 3214–3220. ((in Chinese)). Yang K, Ren Y, Wu K ,et al.Enhancing electrical properties of impact polypropylene copolymer for eco-friendly power cable insulation by manipulating the multiphase structure through molten-state annealing[J].Composites science and technology, 2022(May 26):223. Wu K, Sui H, Yang Z ,et al.Largely Improved Creep Resistance and Thermal-Aging Stability of Eco-Friendly Polypropylene High-Voltage Insulation by Long-Chain Branch-Induced Interfacial Constraints[J]. [2025-11-03]. Sui H, Wu K, Zhao G ,et al.Greatly enhanced temperature stability of eco-friendly polypropylene for cable insulation by multifold long-chain branched structures[J].Chemical Engineering Journal, 2024, 485(000):12. Additional Declarations No competing interests reported. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8861746","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":598619838,"identity":"40a4c57b-2391-4b52-bc37-25bafb7d0705","order_by":0,"name":"Guobin MA","email":"","orcid":"","institution":"Shandong University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Guobin","middleName":"","lastName":"MA","suffix":""},{"id":598619839,"identity":"c56999bc-6168-4761-ba95-9cf8834d8579","order_by":1,"name":"Guowen DAI","email":"","orcid":"","institution":"Shandong University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Guowen","middleName":"","lastName":"DAI","suffix":""},{"id":598619840,"identity":"9f67bc93-aaf3-48af-a843-5c5e78d5f2a0","order_by":2,"name":"Xiufeng LI","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA+ElEQVRIiWNgGAWjYBACPmYGhgMMNkAWM/OBAwkVEnL8hLSwgbWkAVnsbYkHPpyxMJZsIKQFTIK08JwxPjizrSJxA0Et7DyGh3kSbPLkI3IMDvPOk2DcwMD88NENvA5jSwBqSSs2vJFWcJh3mwSzOQObsXEOXi3MBw7z/jicuHFG8gaQFjbLBh42afxaGBuAtvwHakkAOmyOBI/BAYJagLbwJBxInM9zxODgzAYJCSK0sCUcnJOQnLiBvS3hwIdjEgaSzQT8ws9/xvjDmwS7xPnNzIc/JNTU1fezNz98jE8LHBgcgLGYiVEOAvINxKocBaNgFIyCEQcA8ZNNZQD+0h4AAAAASUVORK5CYII=","orcid":"","institution":"Shandong University of Technology","correspondingAuthor":true,"prefix":"","firstName":"Xiufeng","middleName":"","lastName":"LI","suffix":""},{"id":598619841,"identity":"c9193f93-adf8-4248-b237-718b044893ab","order_by":3,"name":"Xingzhen WANG","email":"","orcid":"","institution":"Shandong University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Xingzhen","middleName":"","lastName":"WANG","suffix":""},{"id":598619842,"identity":"52a6611f-0096-4bc7-8a97-7694bfd8110b","order_by":4,"name":"Chuanke DING","email":"","orcid":"","institution":"Shandong University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Chuanke","middleName":"","lastName":"DING","suffix":""},{"id":598619843,"identity":"f43f52ba-ef4d-4269-895b-832620c81152","order_by":5,"name":"Yaqi ZHANG","email":"","orcid":"","institution":"Shandong University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Yaqi","middleName":"","lastName":"ZHANG","suffix":""},{"id":598619844,"identity":"29558852-7149-4e93-aff9-216cec5a23d2","order_by":6,"name":"Chenyang JIAO","email":"","orcid":"","institution":"Shandong University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Chenyang","middleName":"","lastName":"JIAO","suffix":""}],"badges":[],"createdAt":"2026-02-12 11:53:33","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8861746/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8861746/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":103746767,"identity":"358dc8d2-1221-4ac2-9f26-6038b76b3fd8","added_by":"auto","created_at":"2026-03-02 12:12:19","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":87119,"visible":true,"origin":"","legend":"\u003cp\u003eSample preparation and testing methods\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8861746/v1/f7238158eb896edbe921a45c.jpg"},{"id":103746733,"identity":"828b964f-a1c7-4cc5-8196-d7627d94eefa","added_by":"auto","created_at":"2026-03-02 12:12:12","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":48745,"visible":true,"origin":"","legend":"\u003cp\u003eStress-strain curves of each sample\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8861746/v1/9f22c50f60b31c099db5af53.jpg"},{"id":103746717,"identity":"5c85a608-8da3-474c-bb9a-c5fd018f79f7","added_by":"auto","created_at":"2026-03-02 12:12:05","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":104694,"visible":true,"origin":"","legend":"\u003cp\u003eMechanical Property Variation of Each Specimen\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8861746/v1/58f4165efb1808dae6a4bac6.jpg"},{"id":103746771,"identity":"60dfed55-4f76-4dfb-927a-83f85b4b7ec0","added_by":"auto","created_at":"2026-03-02 12:12:20","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":142391,"visible":true,"origin":"","legend":"\u003cp\u003eDMA spectra of each sample before and after thermal oxygen aging\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8861746/v1/51282cb16c5cabe537d93c8f.jpg"},{"id":103746766,"identity":"6054aedc-5586-4022-9a55-20c0b9c41418","added_by":"auto","created_at":"2026-03-02 12:12:19","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":181777,"visible":true,"origin":"","legend":"\u003cp\u003eTGA and DTG curves of each sample\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8861746/v1/8a1f8b23c9d7d73a24177b53.jpg"},{"id":103746775,"identity":"4f24b079-c24a-4a6d-a5ee-013bc4fc60e0","added_by":"auto","created_at":"2026-03-02 12:12:25","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1297705,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8861746/v1/1033ad7b-7fe4-4eb8-b6a2-70401952827f.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"The effect of thermal aging on the mechanical properties and thermal stability of modified polypropylene insulating materials","fulltext":[{"header":"1 INTRODUCTION","content":"\u003cp\u003eWith the proposal and steady advancement of China's national strategies of \"Carbon Peak\" and \"Carbon Neutrality,\" the construction of a clean and low-carbon new power system presents unprecedented challenges and opportunities for the power cable industry\u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]\u003c/sup\u003e. As the deployment scale of cross-linked polyethylene (XLPE) cables expands, their shortcomings, such as complex production processes\u003csup\u003e[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]\u003c/sup\u003e, high energy consumption\u003csup\u003e[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]\u003c/sup\u003e, and low efficiency\u003csup\u003e[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/sup\u003e, have become increasingly apparent. Furthermore, as a thermosetting material, XLPE can only be disposed of through incineration or landfilling after decommissioning, which contradicts the \"Dual Carbon\" philosophy. Consequently, \u003cem\u003elow-carbon environmental friendliness, energy efficiency, and recyclability\u003c/em\u003e have become new development directions for cable insulation materials\u003csup\u003e[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]\u003c/sup\u003e. Thermoplastic polypropylene (PP) material, with its advantages of melt recyclability\u003csup\u003e[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/sup\u003e, excellent electrical properties\u003csup\u003e[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/sup\u003e, and relatively high-temperature resistance\u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e, has emerged as a significant direction for developing environmentally friendly high-voltage cable insulation.\u003c/p\u003e \u003cp\u003eHowever, due to its inherent high crystallinity and high modulus, polypropylene material exhibits high rigidity, poor toughness, and insufficient low-temperature impact resistance\u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e. This leads to noticeable stress whitening during cable installation, preventing its direct use as the primary cable insulation. Many researchers have conducted modification studies using polypropylene as the base matrix. Hu Shixun et al.\u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e compared the thermal, mechanical, and electrical properties of three materials: copolymerized polypropylene, nano-doped modified polypropylene, and graft-modified polypropylene. They found that compared to XLPE, all three materials demonstrated higher thermal stability and comparable mechanical properties. Moreover, the modified polypropylene materials showed significantly improved electrical insulation performance under DC conditions. Chen Hong et al.\u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e compared the structures and properties of different blend-modified PP materials and copolymer-modified PP materials. They observed that modified PP materials exhibited significantly enhanced toughness, tensile strength, and elongation at break, but experienced reduced thermal performance and increased dielectric loss. Copolymer-modified PP materials demonstrated superior thermal properties and higher breakdown field strength, while blend-modified PP materials showed better mechanical performance and lower dielectric loss. Zhao Peng et al.\u003csup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e investigated the thermal-oxidative aging characteristics of different modified polypropylene materials. They found that during the aging process, modified polypropylene insulation materials exhibited macroscopic change trends similar to XLPE and maintained high stability during long-term aging, demonstrating the feasibility of replacing XLPE. Therefore, PP materials subjected to different modification methods exhibit variations in structure, properties, and thermal-oxidative aging characteristics. Studying the thermal-oxidative aging behavior of modified polypropylene materials under various conditions is thus beneficial for predicting their long-term service performance.\u003c/p\u003e \u003cp\u003eThis paper focuses on two materials: blend-modified polypropylene (BPP) and copolymer-modified polypropylene (CPP). Through tensile property testing, dynamic thermomechanical analysis (DMA), and ther-mogravimetric analysis (TGA), the dynamic evolution and macroscopic aging performance of modified PP materials under different aging temperatures and durations are thoroughly investigated. The aging mechanisms are analyzed to provide theoretical support for the application of polypropylene in cable insulation materials.\u003c/p\u003e"},{"header":"2 TEST METHODS","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Primary Raw Materials\u003c/h2\u003e \u003cp\u003e35 kV cross-linked polyethylene (XLPE) insulating material, produced by Nanjing Zhongchao New Material Co., Ltd. Polypropylene (PP), produced by Sinopec Yanshan Petrochemical Company.Polyolefin Elastomer (POE), manufactured by LG Chem Ltd. Polyolefin Plastomer (POP), produced by Dow Chemical Company, USA. Copolymerized PP insulating material, produced by Sinopec Yanshan Petrochemical Company.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Sample Preparation\u003c/h2\u003e \u003cp\u003e(1) XLPE: The cross-linkable polyethylene insulating pellets were preheated without pressure at 120\u0026deg;C on a flat-plate vulcanizing press for 5 minutes. Subsequently, the specimens were subjected to pressure for cross-linking at 175\u0026deg;C and 15 MPa for 15 minutes. After pressurized cooling to room temperature, multiple XLPE specimens with thicknesses of 1 mm and 4 mm were prepared.\u003c/p\u003e \u003cp\u003e(2) Blended Polypropylene (BPP): PP was respectively mixed with POE, POP, and antioxidants according to specific ratios. The mixture was melt-blended for 15 minutes using a precision open mill at 190\u0026deg;C and 40 rad/min. After discharging, the material was pressed at 200\u0026deg;C and 15 MPa for 8 minutes to prepare multiple blended polypropylene specimens with thicknesses of 1 mm and 4 mm.\u003c/p\u003e \u003cp\u003e(3) Copolymerized Polypropylene (CPP): The copolymerized PP insulating material was preheated without pressure on a flat-plate vulcanizing press, followed by pressing at 200\u0026deg;C and 15 MPa for 8 minutes. Multiple copolymerized polypropylene specimens with thicknesses of 1 mm and 4 mm were thus prepared.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Performance Testing\u003c/h2\u003e \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e \u003ch2\u003e2.3.1 Thermal Aging\u003c/h2\u003e \u003cp\u003eIn accordance with IEC 60811-1-2:1985 \"Common test methods for insulating and sheathing materials of electric and optical cables - Part 12: Methods for general application - Thermal aging methods\", specimens of XLPE, BPP, and CPP were subjected to thermal aging at two different temperatures, 135\u0026deg;C and 150\u0026deg;C, for durations of 7, 14, and 21 days, respectively. To ensure the accuracy of the test data, all aged specimens were removed and allowed to condition under standard ambient conditions (23\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u0026deg;C, relative humidity 50\u0026thinsp;\u0026plusmn;\u0026thinsp;5%) for 24 hours prior to conducting the various performance tests.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section3\"\u003e \u003ch2\u003e2.3.2 Tensile Properties\u003c/h2\u003e \u003cp\u003eIn accordance with ISO 527-2:2012 \"Plastics - Determination of tensile properties - Part 2: Test conditions for moulding and extrusion plastics\", tensile tests were conducted using a UTM2103 electronic universal testing machine manufactured by Shenzhen Sansi Zongheng Technology Co., Ltd. The tests were performed at a crosshead speed of 250\u0026thinsp;\u0026plusmn;\u0026thinsp;50 mm/min and a temperature of 23\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u0026deg;C. Five specimens of each type were tested, and the average value was calculated.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section3\"\u003e \u003ch2\u003e2.3.3 Dynamic Thermomechanical Analysis (DMA)\u003c/h2\u003e \u003cp\u003eA Dynamic Mechanical Analyzer (model SDT961, manufactured by METTLER TOLEDO, Switzerland) was employed to investigate the dynamic mechanical behavior of the specimens. This was achieved by applying a time-varying alternating force while subjecting the samples to a programmed temperature control. The instrument recorded the variations of storage modulus, loss modulus, and loss factor (tan\u003cem\u003eδ\u003c/em\u003e) as functions of temperature. The tests were conducted on specimens with a thickness of 4 mm over a temperature range of -50\u0026deg;C to 150\u0026deg;C, with a heating rate of 2\u0026deg;C/min, a vibration frequency of 1 Hz, and an applied force of 0.1 N.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section3\"\u003e \u003ch2\u003e2.3.4 Thermogravimetric Analysis (TGA)\u003c/h2\u003e \u003cp\u003eThe thermal stability of the samples was investigated using a Thermogravimetric Analyzer (TGA 2, Mettler Toledo, Switzerland). The tests were conducted under a nitrogen atmosphere with a flow rate of 50 mL/min. The temperature was increased from 40\u0026deg;C to 800\u0026deg;C at a constant heating rate of 10\u0026deg;C/min.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"3 RESULTS AND DISCUSSION","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Effect of Thermal Aging on the Mechanical Properties of PP Insulation\u003c/h2\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e3.1.1 Tensile Properties\u003c/h2\u003e \u003cp\u003eThe macroscopic mechanical properties are essentially an external manifestation of the material's micr-ostructure and can directly reflect the aging degree of cable insulation. Tensile testing clearly reveals the typical characteristics of the material during the three stages of deformation\u003csup\u003e[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e: elastic deformation, plastic deformation, and fracture. In the elastic deformation stage, the strain distribution follows Hooke's Law, meaning stress is proportional to strain. Upon entering the plastic deformation stage, localized \"necking\" occurs in the specimen. As stretching continues, the necked region elongates until the entire gauge section thins uniformly, ultimately leading to tensile fracture. The tensile stress-strain curves for all specimens are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The apparent morphology of XLPE and PP specimens under different aging temperatures and durations is presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The variation trends of elastic modulus, tensile strength, and elongation at break for the different specimens are illustrated in Fig.\u0026nbsp;3.\u003c/p\u003e \u003cp\u003eAs shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, with the intensification of thermal aging, the surface color of XLPE specimen-s undergoes a progressive change, shifting from an initial translucent state to a deep yellow and finally to a brownish hue. This color change serves as a direct visual indicator of severe oxidative degradation within the material. The underlying cause is attributed to the chain scission of XLPE molecular chains under thermo-oxidative conditions, which generates a significant amount of carbonyl-containing compounds\u003csup\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e, such as ketones, aldehydes, and carboxylic acids. The continued deepening of the color corresponds to the accumulation of the carbonyl index. In contrast, both types of PP specimens exhibit an opaque, milky-white appearance prior to aging. Even after prolonged thermal aging, only very slight yellowing is observed on their surfaces. The color stability of the PP materials indicates the high structural integrity of their molecular chains. The minimal yellowing demonstrates that the incorporated antioxidants effectively retard the oxidation process, suppress the formation of carbonyl compounds, and prevent significant macromolecular chain scission. These differences fundamentally confirm that the modified PP materials possess more stable and superior resistance to thermo-oxidative aging.\u003c/p\u003e \u003cp\u003eThe evolution of mechanical properties for each specimen can be observed from Fig.\u0026nbsp;3(a). The elastic modulus of XLPE specimens shows an overall declining trend after aging, exhibiting clear temperature dependence with aging time. Under aging at 135\u0026deg;C, the elastic modulus of the specimens displays a non-monotonic trend, initially increasing slightly and then decreasing significantly with prolonged aging time. This is because, during the initial aging stage, peroxides within the XLPE initiate short-chain extensions under thermal effects, leading to a denser cross-linked network structure\u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e that restricts molecular chain movement and thus increases the elastic modulus. Additionally, thermal effects promote the rearrangement of molecular segments in the amorphous regions and improve the crystallinity in some crystalline areas\u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e, resulting in increased rigidity of the specimens. However, with extended aging time, oxidative chain scission occurs, gradually reducing the elastic modulus. In contrast, the elastic modulus of the two modified PP materials are significantly higher than that of XLPE, indicating that even after modification, PP retains relatively high rigidity and hardness. Furthermore, the elastic modulus of modified PP exhibits a trend of initially decreasing and then increasing with prolonged aging time. During the early stages of aging, thermo-oxidative effects cause breakage of the PP polymer main chains, generating small molecular fragments\u003csup\u003e[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e and reducing entanglement density. This leads to structural relaxation within the specimen, resulting in a decrease in elastic modulus. In the later stages of aging, prolonged aging induces oxidation reactions in PP, forming polar groups such as carbonyl groups. This promotes the reorientation and rearrangement of molecular segments, increases the crystallinity of the specimens\u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e, and consequently raises the elastic modulus.\u003c/p\u003e \u003cp\u003eWith increasing aging temperature, the elastic modulus of XLPE demonstrates a monotonic decreasing trend during the aging process at 150\u0026deg;C. This is primarily attributed to the sustained high-temperature thermo-oxidative aging, which induces oxidation of C-C and C-H bonds in XLPE, generating functional groups such as carbonyls. As a result, the cross-linked network structure is severely disrupted, and the molecular weight decreases, leading to a reduction in the material's rigidity. In contrast, the elastic modulus of BPP is significantly lower than that of CPP under both aging temperatures. This difference is mainly due to the incorporation of toughening modifiers such as elastomers in the BPP formulation, which endows BPP with superior toughness.\u003c/p\u003e \u003cp\u003eTensile strength represents the stress at which a material withstands the maximum load before fracture, while elongation at break measures the maximum deformation the material can achieve prior to failure, reflecting its ductility and resistance to fracture\u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e. As shown in Figs.\u0026nbsp;3(b) and (c), after aging, the tensile strength and elongation at break of XLPE specimens decrease significantly with prolonged aging time. After 21 days of aging at 135\u0026deg;C, the tensile strength decreases by 12.5%, and the elongation at break drops by 23.8%, indicating substantial degradation in specimen performance. Evidently, high temperatures induce molecular chain scission in XLPE, leading to reduced molecular weight and weakened intermolecular forces\u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e, resulting in the formation of micro-defects and stress concentration points, thereby deteriorating its mechanical properties. In contrast, both modified PP materials exhibit significantly higher tensile strength and elongation at break compared to XLPE. This is because PP, as a typical thermoplastic polymer, possesses linear or branched molecular chains\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e without the cross-linked network structure present in XLPE. Free from the spatial constraints of chemical cross-linking points, PP more readily undergoes orientation-induced crystallization during stretching\u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e. Macroscopically, this results in excellent plastic deformation capability while reducing the tendency for brittle fracture caused by stress concentration. Consequently, the molecular chains of PP demonstrate greater flexibility and ductility. Under aging at 135\u0026deg;C, the tensile strength of all specimens shows a trend of \"initial increase followed by decrease.\" During the early aging stage, enhanced molecular chain mobility promotes recrystallization\u003csup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e, leading to increased crystallinity and a more ordered structure. The lower the initial crystallinity, the more pronounced the recrystallization effect, resulting in a slight increase in tensile strength. In the middle to late stages of aging, however, oxidation causes molecular chain scission\u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e, and the exponential growth of carbonyl and other functional groups intensifies the aging degree, leading to a noticeable decline in mechanical properties. With an increase in aging temperature, under the condition of 150\u0026deg;C, high temperature directly induces molecular chain scission, causing a substantial reduction in tensile strength. Although increased crystallinity can partially offset this effect, the overall trend remains monotonically decreasing.\u003c/p\u003e \u003cp\u003eComparing the mechanical properties of the two modified PP materials reveals that the BPP specimen, modified with elastomers, exhibits significantly superior tensile strength and elongation at break relative to CPP. This is attributed to the uniform dispersion of elastomers (POE and POP) within the BPP matrix. Under external stress, these elastomeric domains act as stress-concentration points, inducing substantial shear deformation that dissipates energy. Consequently, the yield stress of the composite system is progressively reduced\u003csup\u003e[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e, resulting in an effective toughening effect. In contrast, CPP possesses a higher degree of crystallinity, with its molecular chains arranged in a more ordered structure. The well-defined crystalline regions restrict molecular chain slippage, thereby rendering the material less tough than BPP.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section3\"\u003e \u003ch2\u003e3.1.2 Dynamic Thermomechanical Properties\u003c/h2\u003e \u003cp\u003eDynamic thermomechanical analysis (DMA) me-asures the relationship between a material's dynamic modulus, mechanical loss, and temperature under alternating stress\u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e. Specimens aged at 150\u0026deg;C for 21 days were selected as representative aged samples for DMA testing. The results are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e, and the corresponding storage modulus data at different temperatures are listed in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eStorage modulus of each sample at different Temperatures\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSpecimen\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e25℃\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e90℃\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e110℃\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e150℃\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eXLPE-Before Aging\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e377.46\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e46.88\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e16.93\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e14.73\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eXLPE- After Aging\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e436.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e159.48\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e118.73\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e20.80\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBPP- Before Aging\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e595.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e132.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e69.29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e64.77\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBPP- After Aging\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e626.96\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e168.86\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e166.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e86.06\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCPP- Before Aging\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1173.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e312.72\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e189.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e65.88\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCPP- After Aging\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1205.45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e343.65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e199.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e80.64\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe storage modulus of a material reflects the elastic component in its viscoelastic behavior, indicating the material's stiffness; The loss modulus characterizes the energy dissipated due to viscous deformation within the material; The ratio of the loss modulus to the storage modulus is defined as the loss factor tan\u003cem\u003eδ\u003c/em\u003e. A higher tan\u003cem\u003eδ\u003c/em\u003e value signifies greater energy loss in the material under alternating stress\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a) and Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, compared to the values at 25\u0026deg;C, the storage modulus of all specimens exhibits a decreasing trend with rising temperature. This is because as temperature increases, the crystallinity of the specimens decreases, and larger structural units within the molecular chains, such as side groups and chain segments, gradually acquire sufficient activation energy\u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e. Their mobility is significantly enhanced, the entanglement degree of molecular chains is reduced, and the specimens become softer, leading to a decline in storage modulus. However, under the same temperature condition (e.g., 90\u0026deg;C), the storage modulus of XLPE is notably lower than that of the PP specimens. This difference is primarily attributed to the distinct molecular structural characteristics of the two materials. Although the unique cross-linked network structure in XLPE restricts molecular chain motion, its relatively high amorphous content results in a lower storage modulus. In contrast, PP is a highly crystalline polymer, and the introduction of specific molecular groups during modification enhances intermolecular interactions, thereby endowing it with higher rigidity. When the temperature is further increased to 110\u0026deg;C, the branching degree of molecular chains rises, intermolecular bonding weakens, and microcrystal melting occurs under high temperature\u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e, leading to a pronounced decrease in storage modulus. In the high-temperature region (around 150\u0026deg;C), the crystal melting process dominates\u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e, and the difference in storage modulus between XLPE and modified PP specimens diminishes. This phenomenon indicates that after melting, the destruction of the crystalline structure substantially reduces the influence of the cross-linked network in XLPE and the chain regularity in PP on their respective rigidity, causing their storage modulus to converge.\u003c/p\u003e \u003cp\u003eFollowing aging, the storage modulus of all specimens increased. This rise is attributed to the closer packing of molecular chains within the material during thermal aging, which enhances its mechanical strength. However, this process is accompanied by chain scission, particularly damaging the flexible -CH\u003csub\u003e2\u003c/sub\u003e-CH\u003csub\u003e2\u003c/sub\u003e- long-chain backbone\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e, leading to a significant reduction in material flexibility and an increase in rigidity, thereby elevating the storage modulus. The change in storage modulus is particularly pronounced for the XLPE specimen after aging. At 90\u0026deg;C, its storage modulus becomes 3.4 times that of the unaged state, and at 110\u0026deg;C, it reaches 7.1 times the pre-aged value. In contrast, the storage moduli of BPP and CPP at 90\u0026deg;C are only 1.3 times and 1.1 times their respective pre-aged values, far lower than the change observed for XLPE. This significant discrepancy is attributed to two main factors: on one hand, XLPE undergoes short-chain extension and forms new cross-linking points between molecular chains during the early stage of thermo-oxidative aging, enhancing its rigidity; on the other hand, polar groups such as carbonyls generated from oxidation participate in intermolecular interactions, leading to a substantial increase in storage modulus.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b) also shows that the loss modulus of aged XLPE exhibits only a single loss peak. This indicates the molecular relaxation transitions of the material in different temperature zones: the low-temperature region corresponds to \u003cem\u003eβ\u003c/em\u003e relaxation, and the high-temperature region is associated with \u003cem\u003eα\u003c/em\u003e relaxation\u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e. In the tan\u003cem\u003eδ\u003c/em\u003e temperature spectrum of XLPE before and after aging (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e(c)), the loss peak of the aged sample shifts to a lower temperature, decreasing by approximately 30\u0026deg;C compared to the unaged state. This corresponds to the melting of smaller crystallites and the process where the degree of intermolecular entanglement decreases, transitioning into a rubbery state\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e. The difference in elastic potential energy is dissipated as mechanical loss, leading to reduced crystal integrity after thermo-oxidative aging and consequently a lower loss peak temperature. In contrast, the loss modulus and tan\u003cem\u003eδ\u003c/em\u003e curves of the two modified PP specimens after aging show only minor changes in intensity and peak value. Their overall shapes remain relatively stable, with no new loss peaks emerging, and the extent of change is significantly smaller than that of XLPE. This is because the aging process of PP is dominated by molecular chain scission, and the formation of oxidation products has a relatively limited impact on the segmental motion of the molecular chains, resulting in smaller changes in its dynamic mechanical properties.\u003c/p\u003e \u003cp\u003eComparing the dynamic thermomechanical properties of the two modified PP specimens reveals that the storage modulus of both BPP and CPP remain almost unchanged before and after thermo-oxidative aging, showing an increase only in the high-temperature region compared to the unaged state. Furthermore, the storage modulus of CPP is significantly higher than that of BPP. This difference primarily stems from their distinct microstructures: the introduction of elastomers in BPP makes it prone to phase separation from the PP matrix\u003csup\u003e[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e, which hinders the orderly packing of molecular chains and results in the formation of extensive amorphous regions, thereby yielding a lower storage modulus. In contrast, CPP possesses higher crystallinity and a more regular molecular chain arrangement, leading to a more perfected crystalline structure\u003csup\u003e[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e. Its ordered crystalline regions restrict molecular chain slippage, resulting in greater stiffness and a higher storage modulus. Although the peak intensity of the loss modulus curve for BPP changes after aging, its position remains nearly unchanged, exhibiting a double-plateau feature. The loss peak near \u0026minus;\u0026thinsp;35\u0026deg;C corresponds to \u003cem\u003eβ\u003c/em\u003e relaxation. After aging, the intensified phase separation behavior restricts molecular chain motion at the interface regions\u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e, and the compatibility between the elastomer and the matrix decreases, leading to more significant changes in peak intensity. The loss peak near 8\u0026deg;C is associated with \u003cem\u003eα\u003c/em\u003e relaxation. For CPP, the introduction of ethylene units disrupts the regularity of the PP molecular chains, reducing the material's crystallinity\u003csup\u003e[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e and increasing its amorphous content. Consequently, CPP primarily exhibits the \u003cem\u003eα\u003c/em\u003e relaxation of PP chain segments, while the \u003cem\u003eβ\u003c/em\u003e relaxation introduced by ethylene copolymerization is not prominent. After aging, oxidative scission of the PP main chain broadens the molecular weight distribution in the amorphous regions. Simultaneously, the generation of polar groups such as carbonyls through oxidation restricts segmental motion, resulting in a noticeably broader peak and increased peak intensity.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Effect of Thermal Aging on the Thermal Stability of PP Insulation\u003c/h2\u003e \u003cp\u003eThe TGA curves illustrating the mass variation of XLPE, BPP, and CPP specimens with temperature are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a). Their first derivatives, yielding the DTG curves, are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b). Corresponding thermal decomposition parameters are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. Here, \u003cem\u003eT\u003c/em\u003e\u003csub\u003e5%\u003c/sub\u003e denotes the initial decomposition temperature (temperature at 5% mass loss), and \u003cem\u003eT\u003c/em\u003e\u003csub\u003e50%\u003c/sub\u003e represents the temperature at 50% mass loss. Furthermore, \u003cem\u003eV\u003c/em\u003e\u003csub\u003emax\u003c/sub\u003e is the maximum mass loss rate during decomposition, and \u003cem\u003eT\u003c/em\u003e\u003csub\u003emax\u003c/sub\u003e is the temperature at which this maximum rate occurs.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eThermal decomposition parameters of XLPE, BPP and CPP samples\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAging State\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSpecimen\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003eT\u003c/em\u003e\u003csub\u003e5%\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003eT\u003c/em\u003e\u003csub\u003e50%\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003eT\u003c/em\u003e\u003csub\u003emax\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003cem\u003eV\u003c/em\u003e\u003csub\u003emax\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBefore Aging\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eXLPE\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e413.83\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e456.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e460.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.08\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"5\" rowspan=\"6\"\u003e \u003cp\u003eAfter Aging\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eXLPE-135℃-7d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e415.83\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e462.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e470.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2.75\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eXLPE-135℃-14d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e431.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e471.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e476.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2.94\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eXLPE-135℃-21d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e443.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e477.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e480.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.32\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eXLPE-150℃-7d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e440.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e475.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e480.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.23\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eXLPE-150℃-14d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e443.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e477.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e479.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e4.12\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eXLPE-150℃-21d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e441.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e475.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e477.83\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.63\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBefore Aging\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBPP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e412.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e454.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e456.83\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.30\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"5\" rowspan=\"6\"\u003e \u003cp\u003eAfter Aging\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBPP-135℃-7d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e423.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e458.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e459.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.32\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBPP-135℃-14d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e423.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e457.83\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e460.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.09\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBPP-135℃-21d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e424.45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e458.21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e461.35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.14\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBPP-150℃-7d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e405.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e455.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e460.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2.66\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBPP-150℃-14d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e409.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e455.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e461.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2.78\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBPP-150℃-21d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e406.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e455.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e462.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2.81\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBefore Aging\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCPP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e423.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e457.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e460.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.04\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"5\" rowspan=\"6\"\u003e \u003cp\u003eAfter Aging\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCPP-135℃-7d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e423.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e458.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e461.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.13\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCPP-135℃-14d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e422.83\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e458.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e461.83\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.16\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCPP-135℃-21d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e423.83\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e458.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e458.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.22\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCPP-150℃-7d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e423.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e458.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e461.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.12\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCPP-150℃-14d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e422.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e457.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e456.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.39\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCPP-150℃-21d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e423.83\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e458.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e462.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.19\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003e, the TGA curves of all specimens clearly exhibit three typical stages: the weight stabilization region, the primary thermal degradation region, and the completed weight loss region. The DTG curves, representing the relationship between the rate of weight change and temperature, can more precisely reflect the characteristic thermal decomposition parameters of the specimens, namely the onset reaction temperature, the temperature corresponding to the maximum reaction rate, and the reaction termination temperature. A higher initial decomposition temperature indicates better thermal stability\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eCombining with Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, it can be observed that the initial decomposition temperatures of aged XLPE specimens are elevated to varying degrees compared to their unaged counterparts. This is attributed to the short-chain extensions occurring within XLPE during the initial aging stage, resulting in a denser cross-linked network structure, thereby raising the specimen's initial decomposition temperature. However, with prolonged aging time, molecular chains undergo scission under extended thermo-oxidative action, the cross-linked network becomes disrupted, the degradation rate of the specimen accelerates, and its thermal stability declines. In contrast, the thermal degradation of both modified PP specimens exhibits a single-step characteristic, with their DTG curves showing a single sharp peak. This corresponds to a homogeneous chain scission degradation process of the main molecular backbone, indicating that degradation is dominated by the complete breakage of primary chains. Furthermore, under different aging temperatures, the thermal decomposition temperature of PP specimens shows no significant change with increasing thermal aging duration. This demonstrates that the PP specimens possess relatively high thermal resistance.\u003c/p\u003e \u003cp\u003eAs the aging temperature increases, under the 150\u0026deg;C aging condition, the sustained high-temperature exposure elevates the maximum decomposition rates of both XLPE and PP specimens. This finding further confirms that elevated temperatures accelerate chain scission and structural breakdown, leading to a reduction in the thermal stability of the specimens. However, significant differences exist in the thermal decomposition characteristics and degradation mechanisms between XLPE and PP. The TGA curves of XLPE aged at both 135\u0026deg;C and 150\u0026deg;C are relatively broad, and the corresponding DTG curves exhibit bimodal or multimodal features. This is primarily due to the competitive mechanism within XLPE involving cross-linked network extension and oxidative decomposition. During the initial aging stage, the increased cross-linking density restricts molecular chain movement, hinders oxygen diffusion, and retards the oxidation reaction. In contrast, during the later stage, polar groups such as carbonyls generated from the oxidation reaction degrade the cross-linked structure. Conversely, PP specimens undergo primarily oxidative degradation. Their linear molecular chain structure facilitates oxygen diffusion along intermolecular pathways into the specimen interior, resulting in the distinct characteristics observed in their respective TGA curves.\u003c/p\u003e \u003cp\u003eA comparison between the two modified PP specimens reveals distinct aging characteristics. The BPP exhibits minor signs of aging: its main decomposition peak broadens, and its intensity weakens somewhat. In contrast, the initial decomposition temperature of CPP remains virtually unchanged before and after thermo-oxidative aging. Its DTG curves show a high degree of overlap, with the peak width remaining largely consistent, demonstrating superior thermal stability. This advantage is primarily attributed to the higher crystallinity of CPP. Within its crystalline regions, the molecular chains are packed more densely. Its uniform molecular chain structure effectively avoids the interfacial issues associated with physical blending\u003csup\u003e[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e. Furthermore, the dense crystalline regions formed by the high crystallinity significantly retard oxygen permeation and the process of molecular chain scission. This helps maintain a stable molecular weight distribution in CPP during thermo-oxidative aging\u003csup\u003e[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/sup\u003e and ensures a highly consistent main-chain scission mode. Consequently, CPP exhibits a higher initial thermal decomposition temperature.\u003c/p\u003e \u003c/div\u003e"},{"header":"4 CONCLUSIONS","content":"\u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eThermo-oxidative aging leads to a decline in the mechanical properties of both XLPE and PP. However, the overall mechanical performance of the two modified PP materials remains superior to that of XLPE, demonstrating better aging resistance and toughness. Among them, BPP exhibits the most outstanding toughness and ductility due to the toughening effect of elastomers. After aging, it retains the highest levels of both elongation at break and tensile strength, showing the smallest reduction in mechanical performance.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eDuring thermo-oxidative aging, the dynamic mechanical properties of XLPE degrade significantly due to structural damage, whereas both modified PP materials exhibit minimal performance changes, demonstrating superior aging resistance and structural integrity.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eDuring thermo-oxidative aging, the thermal stability of XLPE demonstrates an initial increase followed by a subsequent decline, attributable to the initial extension of its cross-linked network and subsequent oxidative degradation. In contrast, both types of PP exhibit superior thermal stability compared to XLPE. CPP, owing to its dense and well-ordered crystalline structure, maintains a stable molecular weight distribution and a uniform degradation mode throughout the aging process, thereby achieving the most exceptional thermal stability.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eConflict of interest\u003c/h2\u003e \u003cp\u003eThe authors declare no conflicts of interest.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eGuobin MA: involving in investigation, experiments,analysis. Xiufeng LI: writing \u0026ndash; review and editing, investigation, project ad-ministration, resources. Guowen DAI: data curation. Xingzhen Wang: data curation. Chuanke Ding: data curation. Yaqi ZHANG: writing \u0026ndash; original draft Chenyang JIAO: writing \u0026ndash; review and editing.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThe authors are grateful for the financial support of the National Natural Science Foundation of China (52477147) and the Shandong Provincial Natural Sci-ence Foundation (ZR2023ME129).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eGe W, Wang Z, Wang X .Optimized allocation of energy storage for integrated energy systems with coordinated source-load-storage interaction[J].Electric Power Systems Research, 2025, 248.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAngalane S K, Kasinathan E .Influence of nanofiller concentration on polypropylene nanocomposites for high voltage cables[J].Journal of Electrical Engineering, 2022, 73(3):174\u0026ndash;181.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHosier I L, Vaughan A S, Swingler S G .An investigation of the potential of polypropylene and its blends for use in recyclable high voltage cable insulation systems[J].Journal of Materials Science, 2011, 46(11):4058\u0026ndash;4070.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBarber K, Alexander G .Insulation of electrical cables over the past 50 years[J].IEEE Electrical Insulation Magazine, 2013, 29(3):27\u0026ndash;32.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWei Y, Liu L, Liu T ,et al.Electrical and mechanical properties of POP-toughening semi-conductive shielding layer for PP cables[J].Electrical Engineering, 2024, 106(1).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHuang X, Fan Y, Zhang J ,et al.Polypropylene based thermoplastic polymers for potential recyclable HVDC cable insulation applications[J].IEEE Transactions on Dielectrics and Electrical Insulation, 2017, 24(3):1446\u0026ndash;1456.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYang K, Liu Y, Yan Z ,et al.Enhanced Morphology-Dependent Tensile Property and Breakdown Strength of Impact Polypropylene Copolymer for Cable Insulation[J].Materials, 2020, 13(18).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eXie D, Min D, Huang Y ,et al.Classified effects of nanofillers on DC breakdown and partial discharge resistance of polypropylene/alumina nanocomposites[J].IEEE Transactions on Dielectrics and Electrical Insulation, 2019, 26(3):698\u0026ndash;705.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKurahashi K, Matsuda Y, Ueda A ,et al.The application of novel polypropylene to the insulation of electric power cable[J].Electrical Engineering in Japan, 2010, 155(3):1\u0026ndash;8.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHu Shixun, Zhang Yaru, Shao Qing, et al. 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((in Chinese)).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYang K, Ren Y, Wu K ,et al.Enhancing electrical properties of impact polypropylene copolymer for eco-friendly power cable insulation by manipulating the multiphase structure through molten-state annealing[J].Composites science and technology, 2022(May 26):223.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWu K, Sui H, Yang Z ,et al.Largely Improved Creep Resistance and Thermal-Aging Stability of Eco-Friendly Polypropylene High-Voltage Insulation by Long-Chain Branch-Induced Interfacial Constraints[J]. [2025-11-03].\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSui H, Wu K, Zhao G ,et al.Greatly enhanced temperature stability of eco-friendly polypropylene for cable insulation by multifold long-chain branched structures[J].Chemical Engineering Journal, 2024, 485(000):12.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"electrical-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"elen","sideBox":"Learn more about [Electrical Engineering](http://link.springer.com/journal/202)","snPcode":"202","submissionUrl":"https://submission.nature.com/new-submission/202/3","title":"Electrical Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Thermal aging, Cross-linked polyethylene, Polypropylene, Mechanical properties, Thermal stability","lastPublishedDoi":"10.21203/rs.3.rs-8861746/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8861746/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAs a novel cable insulation material, thermoplastic polypropylene (PP) offers advantages such as excellent electrical properties, good chemical corrosion resistance, and a relatively high temperature rating, aligning with China's current \"dual-carbon\" goals. Two types of modified PP insulation materials are investigated: blend-modified and copolymer-modified. Accelerated thermo-oxidative aging tests were conducted at different temperatures and durations. Variations in mechanical and thermal properties before and after aging were characterized using tensile testing, dynamic thermomechanical analysis (DMA), and thermogravimetric analysis (TGA). The performance differences between the two modified PP materials and XLPE were systematically analyzed. Results indicate that both XLPE and PP undergo oxidative degradation and a decline in mechanical performance during thermo-oxidative aging. However, PP demonstrates overall superior aging resistance. XLPE degrades relatively slowly in the initial aging stage but suffers rapid performance deterioration later due to the breakdown of its cross-linked network structure. In contrast, while PP exhibits a faster oxidation rate, it maintains better mechanical property retention and thermal stability than XLPE. Specifically, the blend-modified PP, toughened by elastomers, shows the best toughness and ductility, with the smallest decrease in mechanical properties. The copolymer-modified PP, owing to its high crystallinity, possesses more excellent thermal stability. Both modified polypropylene materials demonstrate potential for replacing XLPE as cable insulation.\u003c/p\u003e","manuscriptTitle":"The effect of thermal aging on the mechanical properties and thermal stability of modified polypropylene insulating materials","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-03-02 12:10:08","doi":"10.21203/rs.3.rs-8861746/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-04-28T18:36:27+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-03T17:56:20+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-31T07:15:24+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-30T10:09:07+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-28T22:36:43+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-28T07:12:27+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-23T14:57:38+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-08T13:35:22+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"75684018094624329627168573818014227163","date":"2026-03-04T05:59:50+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"158506280918842994877229704592489280718","date":"2026-03-03T03:45:30+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"50007489780483594809697967558789188028","date":"2026-02-28T18:35:41+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"250033196223530922394921291215764103619","date":"2026-02-27T21:06:13+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"94199842699682183172933611693597795352","date":"2026-02-27T13:59:37+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"208621254017150239146971437692815178893","date":"2026-02-26T13:13:35+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"319790270660642007162099085760976702296","date":"2026-02-26T10:10:06+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"222626790009090647076230371799062374416","date":"2026-02-25T17:21:37+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-02-25T13:44:29+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-02-18T11:33:45+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-02-14T05:00:51+00:00","index":"","fulltext":""},{"type":"submitted","content":"Electrical Engineering","date":"2026-02-12T11:37:16+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"electrical-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"elen","sideBox":"Learn more about [Electrical Engineering](http://link.springer.com/journal/202)","snPcode":"202","submissionUrl":"https://submission.nature.com/new-submission/202/3","title":"Electrical Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"e41ea50d-54ca-4f5e-b2f5-b7d118ae6eeb","owner":[],"postedDate":"March 2nd, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"in-revision","subjectAreas":[],"tags":[],"updatedAt":"2026-04-28T18:39:09+00:00","versionOfRecord":[],"versionCreatedAt":"2026-03-02 12:10:08","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8861746","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8861746","identity":"rs-8861746","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}