Concentration-Dependent Thermo-Oxidative Degradation Behavior and Filler Loading Effects in Talc- and Whisker-Reinforced PP/EOR Composites

Abstract

This study examines the concentration-dependent thermo-oxidative behavior of polypropylene/ethylene–octene rubber (PP/EOR, 80/20) composites reinforced with talc, whisker, and hybrid talc/whisker fillers (5–15 phr) for automotive interior applications. Accelerated thermal aging was conducted at 140 °C for 300 h to evaluate mechanical retention, oxidation behavior, crystallinity evolution, and residual thermal stability. Low talc loadings (≤10 phr) provided the most favorable aging response: T5 and T10 exhibited impact retention values of 165.46% and 139.37%, respectively, together with crystallinity retention values of 142.86% and 124.49%. These improvements were associated with crystallinity enhancement, limited carbonyl formation, and the barrier-related effect of platelet-like talc fillers. In contrast, high-whisker-loading and hybrid systems showed severe deterioration after aging, with W15 and TW7.5 retaining only 33.56% and 25.50% of their initial impact strength, respectively. These samples also showed pronounced early stage thermal instability, as indicated by reductions in the 5% weight-loss temperature (T _5%) from 364.57 to 289.65 °C for W15 and from 374.78 to 305.32 °C for TW7.5. FT-IR, DSC, TGA/DTG, SEM, and ANOVA results indicate that excessive filler loading, particularly in whisker-rich and hybrid systems, promotes filler–matrix interfacial defects, stress concentration, oxidation, and impact embrittlement. These findings provide a quantitative structure–property–degradation framework linking filler morphology, crystallinity evolution, oxidation behavior, thermal stability, and long-term mechanical durability of PP/EOR composites under high-temperature aging conditions.

1. Introduction

Polypropylene (PP) is widely used in automotive interior components due to its low density, chemical resistance, and cost-effectiveness, offering a strong balance between manufacturability and performance. − However, its inherent brittleness and susceptibility to thermo-oxidative degradation at elevated temperatures restrict its long-term reliability in enclosed cabin environments that are often subjected to cyclic heating. To address this limitation, elastomeric toughening using ethylene–octene rubber (EOR) has been widely adopted, enabling PP/EOR composites to exhibit enhanced impact resistance through energy dissipation mechanisms. − Despite these improvements, the heat sensitivity introduced by rubber modification often exacerbates thermo-oxidative aging, thereby complicating composite design for automotive applications such as dashboards, pillar trims, and airbag housings.

The incorporation of inorganic fillers is a common strategy for improving the stiffness, dimensional stability, and thermal resistance of PP-based composites. Talc, a platelet-like magnesium silicate, can act as a nucleating agent and may reduce oxygen transport by increasing the tortuosity of diffusion pathways, thereby contributing to improved thermal stability and dimensional retention. − In contrast, whisker-type fillers, characterized by their high aspect ratio and rod-like morphology, excel in load transfer efficiency and tensile stiffness but may introduce stress concentration sites and interfacial defects under thermal and mechanical fatigue conditions. − Although talc and whisker fillers are widely used in industrial PP formulations, most previous studies have focused on their reinforcing and crystallization effects, whereas their influence on thermo-oxidative degradation under accelerated thermal aging conditions remains less clearly understood. In this study, aging at 140 °C was used not to represent typical service temperature, but to accelerate degradation within a practical laboratory time frame and to evaluate filler-dependent oxidation sensitivity and degradation behavior. −

Thermo-oxidative degradation of PP generally proceeds through radical-mediated oxidation processes involving chain scission, hydroperoxide formation, and the development of carbonyl-containing oxidation products, which can be monitored by FT-IR spectroscopy. − The carbonyl index (CI) is commonly used as an indicator of oxidative degradation, while thermal stability changes can be evaluated using TGA parameters such as the 5% weight-loss temperature (T _5%) and the maximum decomposition temperature obtained from TGA/DTG analysis. − Previous studies have reported degradation behavior, activation energy, and lifetime prediction models for PP and PP/EOR-based systems. − However, the combined effects of filler type, filler morphology, and filler loading on the thermo-oxidative behavior of PP/EOR composites remain insufficiently clarified. Understanding these relationships is important for establishing structure–property–degradation correlations that are relevant to automotive polymer composite design.

This study investigates the concentration-dependent thermo-oxidative behavior of PP/EOR composites reinforced with talc, whisker, and hybrid talc/whisker fillers. A controlled thermal aging protocol at 140 °C for 300 h in air was applied to evaluate filler-loading-dependent changes in mechanical retention, oxidation behavior, crystallinity evolution, and thermal stability. The degradation behavior was assessed using FT-IR carbonyl index, DSC crystallinity, TGA/DTG parameters, and mechanical property retention, with particular emphasis on impact strength because of its relevance to safety-related automotive interior components. Statistical analysis using ANOVA and post hoc comparison was employed to evaluate formulation-dependent differences in mechanical performance. −

To connect microstructural changes with property evolution, SEM was used to examine fracture surface morphology before and after thermal aging. Morphological features such as void formation, filler pull-out, interfacial debonding, talc-rich regions, and whisker-related defects were discussed in relation to the mechanical, FT-IR, DSC, and TGA/DTG results. − Because the SEM images were obtained from fractured impact surfaces, these observations were used as qualitative morphological evidence rather than quantitative microstructural descriptors. This approach allows the distinct roles of platelet-like talc and rod-like whisker fillers to be interpreted in terms of barrier-related effects, interfacial stability, stress concentration, and aging-induced embrittlement. In addition, the hybrid filler system was examined to determine whether the combined use of talc and whisker provides improved durability or instead promotes degradation under high total filler loading.

The results are integrated into a structure–property–degradation framework that links filler morphology, crystallinity evolution, oxidation behavior, thermal stability, and mechanical retention. Rather than defining a critical filler threshold, this study identifies filler-loading-dependent trends within the investigated composition range and provides guidance for selecting favorable filler systems for durable PP/EOR composites. By emphasizing thermo-oxidative stability in addition to conventional reinforcement, this work contributes to the design of lightweight and reliable PP-based materials for high-temperature automotive interior environments.

2. Experimental Section

2.1. Materials and Sample Preparation

PP/EOR composites were prepared by melt blending a polypropylene (PP) base polymer with an ethylene–octene rubber (EOR) toughener, followed by the incorporation of inorganic fillers. Talc and whisker fillers were selected as representative platelet-like and rod-like reinforcements, respectively, and were introduced either individually or in combination to examine the effects of filler morphology and loading on thermo-oxidative behavior.

Detailed commercial identifiers, including supplier names, grade names, production origin, and proprietary formulation details, are not disclosed due to a nondisclosure agreement (NDA) with the industrial collaborator. However, nonconfidential material descriptors were provided to improve reproducibility and comparability. The PP matrix was a high-crystallinity impact polypropylene with a melt index of 10 g 10 min^–1, measured at 230 °C under a load of 2.16 kg according to ASTM D1238. The EOR toughener had a melt index of 5.0 g 10 min^–1, measured at 190 °C under a load of 2.16 kg according to ASTM D1238, and a density of 0.870 g cm^–3. These melt-index values were provided as nonconfidential processing descriptors of the PP/EOR materials used in this study.

The talc filler was a platelet-like inorganic filler with an average particle size of 3.5 ± 0.5 μm. The whisker filler was a rod-like magnesium oxysulfate-type inorganic filler with a reported length of 10–20 μm, average diameter of 0.5 μm, aspect ratio of 20–40, and bulk density of 0.25 g cm^–3.

Compounding was conducted using a corotating twin-screw extruder (Coperion STS MC series). The barrel temperature profile was controlled between 170 and 230 °C, and the screw rotation speed was maintained at 200–400 rpm to ensure homogeneous dispersion. The extrudates were cooled in a water bath, pelletized, and conditioned at room temperature (23 ± 2 °C) for 48 h prior to molding. Test specimens were injection molded using an LS Mtron LGH-80D system under standard processing conditions. Tensile specimens were prepared in accordance with ISO 527–1 using Type A geometry, while impact specimens were fabricated following the notched Izod method specified in ISO 180.

The selected filler contents were designed to examine concentration-dependent changes in thermo-oxidative behavior across the composition range investigated in this study. The 5, 10, and 15 phr loadings were used to represent different filler levels and to compare how increasing filler content affects the balance between reinforcement, interfacial stability, and oxidation sensitivity. In addition, a hybrid formulation containing 7.5 phr talc and 7.5 phr whisker was included to evaluate the combined effects of platelet- and rod-like filler morphologies at the same total filler loading of 15 phr. Details of each formulation are summarized in Table .

1. Formulations of PP/EOR Composites Reinforced with Talc and Whisker Fillers, and Hybrid Talc/Whisker Fillers.

Sample	PP (wt %)	EOR (wt %)	Talc (phr)	Whisker (phr)
PP5E5	50	50	0	0
PP8E2	80	20	0	0
T5	80	20	5	0
T10	80	20	10	0
T15	80	20	15	0
W5	80	20	0	5
W10	80	20	0	10
W15	80	20	0	15
TW7.5	80	20	7.5	7.5

2.2. Thermal Aging Procedure

Thermal aging was conducted to evaluate the thermo-oxidative aging response of PP/EOR composites and to determine how different fillers influence filler-dependent degradation behavior. Preliminary tests were first performed on unfilled PP/EOR systems with two matrix ratios, PP/EOR 80/20 and PP/EOR 50/50 by weight, aged at 140 °C for 500 h. Impact testing before and after aging showed that the EOR-rich 50/50 formulation exhibited greater susceptibility to thermal aging, with pronounced softening and mechanical strength loss compared with the more PP-rich 80/20 composition. Based on these findings, the PP/EOR 80/20 blend was selected as the baseline matrix for subsequent filler-modified systems.

The thermal aging condition was selected based on reported Arrhenius-type degradation trends and existing reliability models for PP composites. − Under accelerated aging at 140 °C, measurable thermo-oxidative changes can be induced within a practical laboratory time frame, whereas lower temperatures such as 120–130 °C generally require longer exposure times to produce comparable oxidation-related changes. Therefore, an aging condition of 140 °C for 300 h was adopted for the main filler-aging study to induce measurable degradation while maintaining sufficient specimen integrity for mechanical, thermal, and morphological evaluation. This condition is consistent with accelerated thermal aging approaches used for assessing long-term durability of automotive polymer materials. −

To distinguish the preliminary matrix screening from the main filler-aging study, the thermal aging conditions and corresponding analyses are summarized in Table .

2. Summary of Thermal Aging Conditions and Analyses Used in This Study.

Stage	Samples	Aging temperature	Aging time	Purpose	Main analysis
Preliminary matrix screening	PP/EOR = 5:5 and 8:2	140 °C in air	500 h	Selection of the PP/EOR matrix composition for the filled composite study	SEM and Mechanical testing
Main filler-aging study	PP/EOR (80/20) composites containing talc, whisker, or talc/whisker hybrid fillers	140 °C in air	300 h	Evaluation of filler-type- and filler-loading-dependent thermo-oxidative behavior	SEM, mechanical testing, FT-IR, DSC, TGA, and ANOVA

Composite samples, including unfilled, talc-filled, whisker-filled, and hybrid talc/whisker systems, were aged in a forced-air convection oven under atmospheric conditions. After aging, the samples were cooled to room temperature in a desiccator to minimize moisture uptake and uncontrolled postaging exposure, followed by characterization using mechanical testing, FT-IR spectroscopy, DSC, TGA, SEM, and statistical analysis. This aging design provides a consistent comparative framework for assessing the filler-dependent influence of talc, whisker, and hybrid fillers on the thermo-oxidative durability of PP/EOR composites.

2.3. Measurement

Mechanical, spectroscopic, thermal, and morphological analyses were conducted to evaluate the effects of thermal aging on the filler-modified PP/EOR composites.

Tensile and impact properties were measured in accordance with international standards. Tensile tests were performed according to ISO 527–1 using Type A specimens with a gauge length of 50 mm, width of 10 mm, and thickness of 2 mm after conditioning at room temperature for 48 h. The crosshead speed was maintained at 50 mm min^–1, corresponding to a nominal strain rate of approximately 1.7 × 10^–2 s^–1. Strain was recorded using an extensometer until yielding, after which crosshead displacement was used to determine elongation at break. Impact strength was measured using the notched Izod method according to ISO 180 with specimen dimensions of 80 × 10 × 4 mm³ and a V-notch depth of 2 mm. Each measurement was repeated using at least ten specimens for each composition and aging condition, and the results are reported as mean ± standard deviation.

Fourier Transform Infrared (FT-IR) spectroscopy in attenuated total reflection (ATR) mode (Bruker VERTEX 80v) was used to detect oxidation-related chemical changes induced by thermal aging. Spectra were recorded in the range of 4000–650 cm^–1 after background correction. Because ATR-FTIR probes the near-surface region of the specimen, the carbonyl index obtained in this study should be interpreted as a surface-sensitive indicator of thermo-oxidative degradation rather than a bulk-average oxidation value. , For carbonyl index calculation, baseline correction was applied to the selected absorption regions before peak-area integration. The integrated carbonyl peak area was obtained from the CO stretching region of (1680–1850 cm^–1), while the integrated reference peak area was obtained from the C–H stretching region of (2750–3050 cm^–1). The same baseline correction and integration procedure were applied consistently to all samples before and after thermal aging.

Differential scanning calorimetry (DSC, Q200, TA Instruments) was conducted under a nitrogen atmosphere to evaluate the melting behavior and crystallinity of the composites before and after thermal aging. Samples were heated from 25 to 200 °C at 10 °C min^–1. The first heating scan was used to determine the melting temperature (T _m), melting enthalpy (ΔH _m), and crystallinity (X _c), because it preserves the thermal history associated with the thermal aging treatment. The crystallinity was calculated from the melting enthalpy after correction for the PP weight fraction in each formulation, following previously reported methods for PP-based composites. The DSC results were used to examine aging-induced crystallinity changes, possible lamellar reorganization, and filler-related nucleation effects. ,

Thermogravimetric analysis (TGA, Q500, TA Instruments) was performed to assess the residual thermal stability of the composites before and after thermal aging. Measurements were conducted from room temperature to 600 °C at 10 °C min^–1 under nitrogen. Although thermal aging was performed in air, TGA was carried out under nitrogen to compare the thermal stability of the aged and unaged materials under a controlled inert atmosphere. The 5% weight-loss temperature (T _5%) was obtained from the TGA curve, while the maximum decomposition temperature (T _max) was determined from the peak temperature of the derivative thermogravimetry (DTG) curve. These parameters were compared among different filler systems to evaluate the effects of filler type and loading on residual thermal stability after thermo-oxidative aging.

Field-emission scanning electron microscopy (FE-SEM, SU8600, Hitachi) was used to examine the fracture-surface morphology of the composites before and after thermal aging. Prior to observation, the fractured surfaces were sputter-coated with a thin layer of gold to enhance surface conductivity and reduce charging effects. SEM observations were performed at an accelerating voltage of 10 kV. The images were used to identify morphological features such as filler distribution, void formation, interfacial debonding, talc-rich regions, whisker pull-out traces, and filler-related cavities. Because the SEM images were obtained from fractured impact surfaces, they were interpreted as qualitative morphological evidence supporting the mechanical, FT-IR, DSC, and TGA/DTG results rather than as quantitative descriptors of the bulk microstructure.

2.4. Statistical Analysis

Statistical analysis was performed using Minitab software to evaluate formulation-dependent differences in impact strength before and after thermal aging. Because the experimental design consisted of selected composite formulations rather than a full factorial combination of filler type and filler loading, one-way analysis of variance (ANOVA) was applied to compare the impact strength among the investigated formulations.

Impact strength was selected as the primary response variable because it showed the highest sensitivity to thermal aging-induced degradation among the measured mechanical properties. Separate one-way ANOVA analyses were performed for the unaged and aged specimens to determine whether the differences among formulations were statistically significant under each condition. When significant differences were detected, Tukey’s honestly significant difference (HSD) test was used for post hoc multiple comparisons.

All statistical analyses were conducted at a 95% confidence level, with statistical significance defined as p < 0.05. Results are reported as mean ± standard deviation based on at least ten specimens per formulation and aging condition. The adjusted coefficient of determination (R ²adj) was used to evaluate the explanatory strength of formulation-dependent effects on impact strength.

3. Results and Discussion

3.1. Morphological Observation before and after Thermal Aging

Figure presents SEM images of PP/EOR composites before and after thermal aging at 140 °C for 500 h, showing the effect of EOR content on fracture-surface morphology. Yellow circles indicate voids or cavities, while yellow arrows indicate directionally oriented fracture or shear features. Because the SEM images were obtained from fractured impact surfaces, the annotated features were used for qualitative comparison of fracture morphology rather than quantitative microstructural analysis.

1.

SEM images of PP/EOR composites before and after thermal aging at 140 °C for 500 h, showing the effect of EOR content on fracture surface morphology (main: 500×; inset: 2000×). (a) PP/EOR 80/20 before aging, (b) PP/EOR 80/20 after aging, (c) PP/EOR 50/50 before aging, and (d) PP/EOR 50/50 after aging.

For the PP/EOR 80/20 composite, the unaged sample showed a rough and discontinuous fracture surface with widely distributed voids, as indicated by the yellow circles in Figure a. These voids are direct morphological features associated with cavity formation during fracture, while the rough and stepped surface morphology suggests interfacial debonding between the PP matrix and dispersed EOR domains. After thermal aging, the PP/EOR 80/20 composite exhibited a more pronounced void-rich morphology with larger and more frequently observed cavities (Figure b), indicating enhanced interfacial separation during fracture. This change is interpreted as a consequence of thermo-oxidative damage and reduced interfacial integrity in the PP/EOR matrix.

In contrast, the PP/EOR 50/50 composite showed a smoother and more continuous fracture surface with fewer distinguishable voids before aging (Figure c). The directional features marked by yellow arrows indicate shear-oriented deformation or tearing during fracture. After aging, the PP/EOR 50/50 composite exhibited a smooth, undulated, and directionally aligned fracture surface with very few visible voids (Figure d). This smoother appearance should not be interpreted as improved interfacial adhesion. Rather, it is attributed to aging-induced softening and relaxation of the EOR-rich continuous phase, which reduced the distinction between PP/EOR phase boundaries and produced a smeared fracture morphology. Based on the comparison between the 80/20 and 50/50 composites, the PP/EOR 80/20 formulation was selected as the reference matrix for subsequent talc and whisker reinforcement because it provides a more PP-dominant and structurally stable baseline for evaluating filler-induced effects on thermo-oxidative degradation. −

Figure shows the SEM fracture surfaces of PP/EOR (80/20) composites containing talc, whisker, and hybrid talc/whisker fillers before and after thermal aging at 140 °C for 300 h. The marked regions highlight directly observed filler-related features, including talc-rich lamellar regions, whisker pull-out traces, and voids or cavities associated with interfacial separation. To reduce repetition and clarify the morphology–degradation relationship, the SEM observations are reorganized by comparing low-talc-loading systems, high-whisker-loading systems, and the hybrid talc/whisker system, while W5 is briefly discussed as a lower-loading whisker reference.

2.

SEM images of PP/EOR (80/20) composites with talc, whisker, and hybrid fillers before and after thermal aging at 140 °C for 300 h. (a, b) Talc 5 phr, (c, d) Talc 15 phr, (e, f) Whisker 5 phr, (g, h) Whisker 15 phr, (i, j) Talc 7.5 phr + Whisker 7.5 phr (main: 500×; inset: 2000×).

For the low-talc-loading system, the T5 composite exhibited embedded platelet-like talc features and a relatively stable fracture morphology before and after thermal aging (Figure a,b). The direct SEM observations include broadly distributed talc-rich lamellar regions and the absence of extensive filler pull-out, large cavities, or severe matrix discontinuity. These features indicate that low talc loading was accommodated within the PP/EOR matrix without strongly disrupting matrix continuity. The stabilizing role of talc is interpreted from the combination of this morphology with the mechanical and thermal results, rather than from SEM alone. Specifically, the embedded platelet-like talc morphology is consistent with a barrier-related effect, in which platelet fillers increase diffusion tortuosity, help limit oxidation, and contribute to the improved impact retention observed after aging. ,

At higher talc loading, the morphology became more defect-sensitive. The T15 composite showed more irregular fracture surfaces with talc-rich regions and cavity-like defects before and after aging (Figure c,d). These directly observed features indicate that excessive talc increased the filler–matrix interfacial area and introduced more sites for interfacial separation. Although talc may still contribute to stiffness and barrier behavior through its platelet structure, this benefit is interpreted to be partially offset by filler-related defects and reduced matrix continuity at high loading.

The whisker-filled composites showed a degradation pattern distinct from the talc-filled systems because of the rod-like geometry of the whisker filler. At low whisker loading, the W5 composite exhibited limited whisker-related pull-out traces before aging and shorter, more embedded whisker features after aging (Figure e,f). These direct observations suggest that the matrix retained a relatively continuous fracture morphology at low whisker loading, although localized whisker–matrix slippage was still present. The smoother appearance after aging should not be interpreted as improved intrinsic adhesion, but rather as an aging-induced change in matrix deformation around the whisker fillers.

The high-whisker-loading system showed a more pronounced degradation pattern. In the W15 composite, directly observed features included rod-like pull-out traces, exposed or partially embedded whiskers, and localized filler-related defects (Figure g,h). These features indicate interfacial slippage and pull-out during fracture.

Compared with W5 and low-talc-loading systems, the increased number of whiskers at high loading enlarged the filler–matrix interfacial area and introduced more stress concentration sites. , After aging, the smoother and more compact appearance of the W15 fracture surface should not be regarded as evidence of improved adhesion. Instead, it is interpreted as an aging-induced change in matrix deformation and fracture behavior, where thermo-oxidative degradation and softening of EOR-rich regions promoted brittle fracture through a weakened matrix. ,− High-magnification images in Figure S1 further show laterally oriented and partially pulled-out whiskers within the cracked interior, supporting the interpretation that whisker pull-out and interfacial weakening contribute to the brittle fracture behavior of the aged whisker-rich composite.

The hybrid talc/whisker composite exhibited combined features of both filler types. Direct SEM observations for TW7.5 included platelet-related talc features, rod-like whisker pull-out traces, interfacial cavities, and disrupted fracture morphology before and after aging (Figure i,j). Because the total filler loading reached 15 phr, the matrix contained a high density of filler–matrix interfaces. The simultaneous presence of talc platelets and whiskers is therefore interpreted to increase the probability of interfacial defects rather than producing a synergistic reinforcement effect. Although talc can provide a barrier-related contribution at low loading, this effect appears to be counterbalanced in the hybrid system by high total filler loading, whisker-induced stress concentration, and reduced matrix continuity. ,

Overall, the SEM results reveal three morphology-dependent aging patterns. Low-talc-loading systems are characterized by embedded platelet-like features and relatively stable fracture morphology after aging, whereas high-whisker-loading systems promote pull-out features, stress concentration, and interfacial defects owing to the rod-like filler geometry. The hybrid system combines the drawbacks of platelet-like talc and rod-like whisker fillers under high total filler loading, resulting in disrupted matrix continuity and limited synergistic improvement. These morphology-based observations are consistent with the mechanical and thermal results, where T5 and T10 exhibit improved aging resistance, whereas W15 and TW7.5 show severe degradation after thermal aging.

3.2. Integrated Thermo-Oxidative Degradation Mechanism and Property Retention

Figure compares the FT-IR spectra, DSC thermograms, TGA curves, and DTG curves of PP/EOR (80/20) composites before and after thermal aging at 140 °C for 300 h. To improve the logical flow of the discussion, the degradation behavior is interpreted in relation to the morphology-based observations described in Section , followed by oxidation behavior, crystallinity evolution, mechanical retention, and residual thermal stability. FT-IR spectroscopy was used to evaluate oxidation-related chemical changes, particularly the formation of carbonyl-containing groups, while DSC analysis was used to examine changes in melting behavior and crystallinity after thermal aging. TGA and DTG analyses were further used to evaluate changes in early stage mass loss and the main decomposition event after aging. Rather than assigning a single degradation pathway, these complementary analyses provide a basis for interpreting how filler morphology and loading affect the balance among oxidation behavior, crystallinity evolution, filler–matrix interfacial stability, mechanical retention, and residual thermal stability. The corresponding quantitative parameters are summarized in Tables and S4–S5.

3.

FT-IR spectra, DSC thermograms, TGA curves, and DTG curves of PP/EOR (80/20) composites containing talc, whisker, and hybrid fillers before and after thermal aging at 140 °C for 300 h: (a, b) FT-IR spectra, (c, d) DSC thermograms, (e, f) TGA curves, and (g, h) DTG curves. Panels (a), (c), (e), and (g) compare the talc-filled composites with the hybrid composite, whereas panels (b), (d), (f), and (h) compare the whisker-filled composites with the hybrid composite.

3. Carbonyl Peak Area (A _C_O), C–H Reference Peak Area (A _C–H), and Calculated Carbonyl Index (CI) of PP/EOR (80/20) Composites before and after Thermal Aging at 140 °C for 300 h.

	Before aging			After aging
Sample	A C–H	A CO	CI	A C–H	A CO	CI	ΔCI
T5	3027.50	70.10	0.02	3365.90	66.50	0.02	0.00
T10	3408.88	41.65	0.01	3092.96	48.74	0.02	0.00
T15	3189.54	47.04	0.01	2889.61	42.64	0.01	0.00
W5	3215.61	50.81	0.02	3059.61	56.42	0.02	0.00
W10	3062.14	58.44	0.02	3175.19	56.93	0.02	0.00
W15	2888.28	57.18	0.02	908.14	125.21	0.14	0.12
TW7.5	3258.56	79.19	0.02	1419.43	209.10	0.15	0.12

3.2.1. Thermo-Oxidative Degradation

Thermo-oxidative degradation of the PP/EOR composites was evaluated using ATR-FTIR spectroscopy. Figure a,b show the infrared spectra of the talc-filled, whisker-filled, and hybrid composites before and after aging at 140 °C for 300 h. Because ATR-FTIR probes the near-surface region of the specimen, the carbonyl index values should be interpreted as surface-sensitive indicators of oxidation rather than bulk-average oxidation levels. This distinction is important because long-term aging in air at 140 °C can produce diffusion-limited oxidation, in which the surface region may become more oxidized than the specimen core. ,

After thermal aging, changes in the carbonyl stretching region were observed in the range of 1680–1850 cm^–1. The development of this absorption region is associated with carbonyl-containing oxidation products formed during thermo-oxidative degradation of polyolefin-based materials. To quantify this oxidation behavior, the carbonyl index (CI) was calculated as a ratio of carbonyl to reference C–H peak area according to the following relationship −

$$\mathrm{C}\mathrm{I}=\frac{A_{1680-1850{\mathrm{c}\mathrm{m}}^{-1}}(\mathrm{C}=\mathrm{O})}{A_{2750-3050{\mathrm{c}\mathrm{m}}^{-1}}(\mathrm{C-H})}$$ 1

Table summarizes the CI values for all formulations.

The talc-filled composites showed only small changes in CI after aging, maintaining values close to their initial levels. This result suggests that talc-filled systems, particularly at low to moderate loading, underwent limited near-surface oxidation during the aging treatment. The platelet-like morphology of talc can increase the tortuosity of oxygen diffusion pathways and may therefore contribute to limiting oxidation during thermal exposure. This interpretation is consistent with the relatively high impact retention and stable TGA parameters observed for T5 and T10.

The W5 and W10 composites also showed relatively limited CI changes after aging. In contrast, W15 and TW7.5 exhibited a marked increase in CI after aging, with values increasing from approximately 0.02 to 0.14–0.15. This increase indicates more pronounced formation of carbonyl-containing oxidation products in the high-whisker-loading and hybrid systems. The higher CI values are consistent with the severe decrease in impact retention observed for W15 and TW7.5, suggesting that oxidation-induced embrittlement contributed to the loss of impact performance. In these systems, the high filler loading and increased filler–matrix interfacial area may promote localized oxidation and interfacial weakening under long-term thermal exposure.

3.2.2. Crystallinity Evolution and Melting Behavior

Figure c,d show the DSC thermograms of talc-filled, whisker-filled, and hybrid PP/EOR composites before and after thermal aging, and the corresponding parameters are summarized in Table S4. The composites showed a main melting transition in the PP melting region, indicating that the overall melting behavior of the PP phase was largely maintained after filler incorporation and thermal aging. Most formulations showed only modest changes in melting temperature after aging, although W15 and TW7.5 exhibited a larger downward shift compared with the low-talc-loading systems. These results suggest that the main melting behavior was relatively stable in low-talc-loading and moderately filled systems, whereas high-whisker-loading and hybrid systems were more affected by thermal aging.

More pronounced changes were observed in melting enthalpy and crystallinity after aging. T5 and T10 showed substantial increases in melting enthalpy and crystallinity retention, with crystallinity retention values of 142.86% and 124.49%, respectively. These increases suggest that thermal exposure at 140 °C promoted annealing-induced crystallinity enhancement, which may involve secondary crystallization, lamellar reorganization, and relaxation of molding-induced residual stresses. Therefore, the postaging increase in impact strength for T5 and T10 should not be attributed to secondary crystallization alone. Rather, it is more appropriately interpreted as the combined effect of crystallinity enhancement, possible lamellar reorganization, residual stress relaxation, maintained matrix integrity, and limited oxidation.

The whisker-filled and hybrid systems showed different behavior. Although some formulations exhibited increased crystallinity after aging, this did not necessarily lead to improved mechanical retention. In particular, W15 and TW7.5 showed severe impact deterioration despite the presence of measurable crystallinity after aging. This indicates that crystallinity evolution alone cannot explain the postaging mechanical behavior. In high-whisker-loading and hybrid systems, the beneficial effect of crystallinity enhancement appears to be outweighed by oxidation, interfacial debonding, filler pull-out, and reduced matrix continuity. These results support the interpretation that postaging mechanical retention is governed by the combined balance among crystallinity evolution, oxidation behavior, and filler–matrix interfacial integrity. ,

3.2.3. Mechanical Retention and Statistical Validation

The mechanical responses of the PP/EOR composites before and after thermal aging are shown in Figures and , and the corresponding retention values are summarized in Tables S1 and S2. These results show that the mechanical stability of PP/EOR composites after thermal aging depends strongly on EOR content, filler morphology, and filler loading.

4.

Tensile properties of (a, b) PP/EOR composites with different EOR ratios before and after thermal aging at 140 °C for 500 h and (c, d) PP/EOR (80/20) composites with various talc, whisker, and hybrid fillers before and after thermal aging at 140 °C for 300 h.

5.

Impact strength of (a) PP/EOR composites with different EOR ratios before and after thermal aging at 140 °C for 500 h and (b) PP/EOR (80/20) composites with various talc, whisker, and hybrid fillers before and after thermal aging at 140 °C for 300 h.

Figure a,b compare the tensile properties of the unfilled PP/EOR composites with different EOR contents. The PP5E5 sample showed severe deterioration after thermal aging, with elongation at break decreasing from 646.24% to 9.35%, corresponding to a retention of only 1.45% (Table S1). In the corresponding impact results, the impact strength decreased from 82.09 to 1.05 kJ m^–2, corresponding to a retention of 1.28% (Figure a and Table S1). This pronounced loss of ductility and toughness indicates that the EOR-rich 50/50 composition is highly vulnerable to thermal aging at 140 °C. The deterioration is consistent with thermal softening, oxidative degradation, and loss of rubber-phase integrity in the EOR-rich matrix. In contrast, the PP8E2 composition retained 97.94% of its tensile strength and showed improved elongation and impact retention after aging, indicating that the PP-rich 80/20 matrix provides a more stable baseline for evaluating filler-dependent thermo-oxidative behavior. ,

Figure c,d show the tensile properties of the filler-modified PP/EOR (80/20) composites before and after aging. Low-talc-loading systems, particularly T5 and T10, maintained tensile strength after thermal aging, with retention values of 94.96% and 100.58%, respectively (Table S2). Their elongation at break was also retained or slightly increased after aging, suggesting that low talc loading did not severely disrupt matrix continuity. This behavior is consistent with the relatively stable fracture morphology observed in the SEM images and the crystallinity enhancement observed in the DSC results.

At higher filler contents, the tensile response became more sensitive to filler morphology and interfacial defects. T15 showed reduced tensile strength retention and a sharp decrease in elongation at break, indicating that excessive talc loading can introduce filler-related defects and reduce matrix deformability. The decrease was more pronounced in high-whisker-loading and hybrid systems. W15 and TW7.5 retained only 5.29% and 3.96% of their tensile strength, respectively, and their elongation at break decreased to below 7% retention after aging (Table S2). These results suggest that high whisker loading and hybrid filler loading promote stress concentration, interfacial debonding, and crack initiation under thermal aging conditions. Possible whisker attrition during compounding and injection molding may also reduce the effective aspect ratio of the whisker fillers, thereby limiting reinforcement efficiency and weakening load transfer after aging. ,

Figure compares the impact strength before and after thermal aging. The unfilled PP5E5 sample nearly lost its impact resistance after aging, whereas PP8E2 retained its impact strength, further supporting the selection of the PP/EOR 80/20 matrix for the filler-aging study. For the talc-filled composites, T5 and T10 showed postaging increases in impact strength from 6.89 to 11.40 kJ m^–2 and from 6.30 to 8.78 kJ m^–2, corresponding to impact retention values of 165.46% and 139.37%, respectively (Table S2). This increase should be interpreted as the combined effect of annealing-induced crystallinity enhancement, possible lamellar reorganization, residual stress relaxation, and limited oxidation during aging, rather than secondary crystallization alone. The DSC results support this interpretation, as T5 and T10 showed increased crystallinity after aging, while the FT-IR results indicate comparatively limited oxidation in these low-talc-loading systems. The residual thermal stability of these samples is further discussed in Section .

In contrast, high-whisker-loading and hybrid systems showed substantial impact deterioration after aging. W15 and TW7.5 exhibited impact retention values of only 33.56% and 25.50%, respectively (Table S2). These decreases are consistent with the SEM observations showing whisker pull-out, interfacial cavities, and disrupted fracture surfaces. The higher carbonyl index and reduced 5% weight-loss temperature (T _5%) values observed for W15 and TW7.5 further indicate that oxidation and early stage thermal instability contributed to the loss of impact performance. Therefore, the mechanical degradation of high-whisker-loading and hybrid composites is interpreted as resulting from the combined effects of interfacial weakening, stress concentration, oxidation-induced embrittlement, and reduced matrix continuity.

Figure presents the main-effect and interaction plots of impact strength generated using Minitab to visualize formulation-dependent mechanical trends before and after thermal aging. These plots were used as descriptive graphical summaries of the impact response, while statistical significance among formulations was evaluated separately using one-way ANOVA and Tukey HSD testing.

6.

Main effect plots (a, b) and interaction plot (c) of impact strength for PP/EOR (80/20) composites with various talc, whisker, and hybrid fillers before and after thermal aging at 140 °C for 300 h. Note: Impact trend lines represent mean impact strength values for each formulation before and after thermal aging. Statistical comparisons among formulations were performed using one-way ANOVA followed by Tukey HSD testing.

The main-effect plots (Figure a,b) show that low-talc-loading systems maintained relatively high impact strength before and after thermal aging, whereas whisker-rich formulations showed lower impact strength, particularly after aging. This trend indicates that the impact response is strongly affected by filler morphology and filler loading.

The interaction plot in Figure c further illustrates that the impact response before and after aging varied depending on the formulation. T5 and T10 showed increased impact strength after aging, whereas W15 and TW7.5 showed marked deterioration. The postaging increase in T5 and T10 is interpreted as the combined effect of annealing-induced crystallinity enhancement, possible lamellar reorganization, residual stress relaxation, and limited oxidation. In contrast, the decrease in W15 and TW7.5 is consistent with interfacial weakening, whisker pull-out, interfacial cavities, and oxidation-induced embrittlement. Therefore, Figure supports the interpretation that impact strength is a sensitive mechanical indicator of filler-loading-dependent thermo-oxidative stability in PP/EOR composites.

Because impact strength showed the largest formulation-dependent variation after thermal aging, one-way ANOVA was conducted to evaluate the statistical significance of impact strength differences among the investigated formulations. As summarized in Table S3, formulation effects were statistically significant both before and after thermal aging, with p < 0.001 in both cases. The adjusted R ² values were 95.30% before aging and 93.13% after aging, indicating that the selected formulations explain most of the variation in impact strength. These statistical results support the interpretation that impact strength is a sensitive indicator of filler-loading-dependent thermo-oxidative stability in PP/EOR composites.

Table presents the Tukey HSD grouping of impact strength before and after thermal aging. T5 was assigned to the highest statistical group after aging, while T10 remained in an upper-performance group. In contrast, W15 and TW7.5 were classified in the lowest statistical groups after aging. This grouping is consistent with the mechanical retention results and supports the interpretation that low-talc-loading systems improve impact resistance, whereas high-whisker-loading and hybrid systems reduce impact resistance after thermal aging.

4. Tukey Grouping of Impact Strength for PP/EOR (80:20) Composites with Various Talc and Whisker Fillers before and after Thermal Aging at 140 °C for 300 h.

	Impact strength (Relative ranking)		Tukey group
Sample	Before thermal aging	After thermal aging	Before thermal aging	After thermal aging
T5	High	Very High	A	A
T10	High	High	B	B
T15	Medium	Medium	C	C
W5	Medium	Medium	C	C
W10	Low	Low	D	D
W15	Very Low	Very Low	E	E
TW7.5	Low	Very Low	D	E

3.2.4. Thermal Decomposition Stability

Figure e,f show the TGA curves, while Figure g,h present the corresponding DTG curves of the composites before and after thermal aging. The TGA and DTG parameters, including the 5% weight-loss temperature (T _5%), maximum decomposition temperature (T _max), and residual mass at 600 °C (W ₆₀₀), are summarized in Table S5. All formulations exhibited a major mass-loss step typical of PP-based composites, indicating that the overall decomposition behavior was dominated by the polymer matrix. The DTG curves provide additional information on the temperature at which the mass-loss rate reaches its maximum and therefore allow a clearer comparison of the main decomposition event after aging. In Figure g, the talc-filled composites show relatively small changes in the position and shape of the main DTG peak after aging, consistent with their stable T _max values. In contrast, Figure h shows more pronounced changes in the whisker-filled and hybrid systems, particularly for W15 and TW7.5, where the main DTG peak shifts to a higher temperature after aging despite the decrease in T _5%.

The 5% weight-loss temperature (T _5%) reflects the early stage of mass loss and is sensitive to thermally weakened polymer fractions, low-molecular-weight degradation products, and early decomposition events. , For T5 and T10, T _5% remained nearly unchanged after aging, with values of 426.68 and 420.97 °C, respectively. These stable T _5% values indicate that low-talc-loading systems did not promote early stage thermal instability after aging. This result is consistent with the low CI values, the stable DTG peak positions in Figure g, and the improved impact retention observed for T5 and T10.

In contrast, W15 and TW7.5 showed a pronounced decrease in T _5% after thermal aging. For W15, T _5% decreased from 364.57 to 289.65 °C, while TW7.5 decreased from 374.78 to 305.32 °C. This decrease indicates that thermal aging generated chemically weakened polymer fractions that decomposed or volatilized at lower temperatures during subsequent TGA measurement. In PP/EOR systems, thermo-oxidative aging can induce chain scission and the formation of carbonyl-containing oxidation products. Therefore, the lower T _5% values of W15 and TW7.5 are consistent with their increased CI values and severe impact deterioration.

The maximum decomposition temperature (T _max) should be interpreted separately from T _5%. T _max is obtained from the maximum of the DTG curve and represents the temperature at which the main mass-loss rate reaches its maximum. As shown in Figure g,h, the DTG peak position provides direct evidence for changes in the main decomposition event, which are not fully captured by the early stage TGA parameter alone. For W15 and TW7.5, T _max increased after aging from 443.71 to 463.49 °C and from 433.55 to 465.11 °C, respectively. This apparent opposite trend between T _5% and T _max suggests a two-stage degradation response. The decrease in T _5% reflects the early decomposition of oxidized or weakened polymer segments. After these unstable fractions are removed during the initial stage of TGA, the main decomposition event is dominated by the remaining polymer matrix, which is influenced by the high filler loading and filler-constrained morphology. The dense whisker or hybrid filler structure may restrict polymer chain mobility and alter heat and mass transfer during the main decomposition stage, resulting in an upward shift of the DTG peak temperature. Thus, the simultaneous decrease in T _5% and increase in Tmax for W15 and TW7.5 does not indicate improved overall thermal stability. Instead, it reflects the early formation of thermally unstable oxidized fractions together with a shifted main decomposition event of the remaining filler-constrained matrix. The DTG curves in Figure g,h support this interpretation by distinguishing the main decomposition peak from the early mass-loss behavior and by showing the aging-induced shift of the dominant decomposition peak in W15 and TW7.5.

The residual mass at 600 °C (W ₆₀₀) broadly follows the inorganic filler content of the composites, but it should not be interpreted as an exact quantitative measure of filler loading. The residual mass can also be influenced by sample-scale compositional variation, inorganic residue, and carbonaceous char formation during TGA. Nevertheless, the W ₆₀₀ values indicate the presence of inorganic filler residues and support the classification of W15 and TW7.5 as high-filler-loading systems. The severe decrease in T _5% for these samples is therefore more reasonably associated with high filler loading, increased filler–matrix interfacial area, and oxidation-prone microdomains around densely packed whisker or hybrid filler regions than with minor variations in residual mass.

4. Conclusions

This study investigated the concentration-dependent thermo-oxidative behavior of PP/EOR composites reinforced with talc, whisker, and hybrid talc/whisker fillers for automotive interior applications. The results show that the thermal aging response of PP/EOR composites is strongly influenced by filler morphology and loading. Low-talc-loading systems, particularly those containing 5–10 phr talc, provided a favorable balance between mechanical retention and thermo-oxidative stability. This behavior was associated with the barrier-related effect of platelet-like talc fillers, increased crystallinity after aging, limited carbonyl formation, and stable thermal decomposition behavior.

In contrast, excessive filler loading reduced mechanical durability, especially in high-whisker-loading and hybrid systems. W15 and TW7.5 exhibited severe losses in tensile and impact properties after aging, together with increased carbonyl index values and a pronounced decrease in the 5% weight-loss temperature (T _5%). SEM observations indicated that these reductions were associated with interfacial defects, whisker pull-out, interfacial cavities, and disrupted matrix continuity. These results suggest that the reinforcing effect of whisker fillers can be outweighed by stress concentration, interfacial weakening, and oxidation-induced embrittlement when the filler loading becomes excessive.

The thermal analysis results further indicate that postaging property evolution is governed by a combined balance among crystallinity evolution, oxidation behavior, residual thermal stability, and filler–matrix interfacial integrity. The improved impact retention of T5 and T10 is therefore attributed not to secondary crystallization alone, but to annealing-induced crystallinity enhancement, possible lamellar reorganization, residual stress relaxation, and limited oxidation. In contrast, the simultaneous decrease in T _5% and increase in T _max for W15 and TW7.5 suggests a two-stage degradation response involving early decomposition of oxidized or weakened polymer fractions, followed by a shifted main decomposition event of the remaining filler-constrained matrix.

Statistical analysis verified that formulation-dependent differences in impact strength were significant before and after thermal aging. The one-way ANOVA results showed p < 0.001, with adjusted R ² values exceeding 90%, supporting the use of impact strength as a sensitive mechanical indicator of thermo-oxidative stability in filled PP/EOR composites.

These results provide a structure–property–degradation framework linking filler morphology, crystallinity evolution, oxidation behavior, thermal stability, and mechanical retention. Rather than defining a critical filler threshold, this study identifies a favorable talc loading range within the investigated compositions and shows that excessive whisker or hybrid filler loading can accelerate degradation through interfacial and oxidative damage. These findings provide practical guidance for selecting filler systems and loading ranges for lightweight and durable PP/EOR composites under high-temperature automotive aging conditions.

The data underlying this study are available in the article and its Supporting Information. Additional raw data and proprietary material or formulation information are not publicly available due to confidentiality restrictions under a nondisclosure agreement with the industrial collaborator. Nonconfidential additional data may be available from the corresponding author upon reasonable request with permission of the industrial collaborator.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c01337.

• Mechanical property retention data for PP/EOR composites with different EOR ratios and filler systems; one-way ANOVA summary of impact strength; DSC thermal properties; TGA parameters; additional SEM images of aged PP/EOR (80/20)/W15 composite showing whisker orientation and interfacial pull-out (PDF)

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D.K. and I.S.H. contributed equally to this work.

The authors declare no competing financial interest.