Introduction

The issue of plastic waste management represents a significant environmental problem, with post-consumer recycled (PCR) plastics playing a pivotal role in finding solutions to this issue. Among commodity polyolefins, polypropylene (PP) and high-density polyethylene (HDPE) dominate in packaging, industrial and transport applications, and yet PCR variants often underperform relative to virgin grades in mechanical properties and thermal stability1. A primary limitation arises from the intrinsic immiscibility of PP/HDPE blends, which promotes phase separation and weak interfacial adhesion, thereby degrading stress transfer and bulk performance2. It is important to note that both polymers are classified as hydrocarbons. However, linear and highly crystalline HDPE and methyl-substituted, more amorphous PP exhibit differences in their chain structure and crystallization behavior. These differences have been shown to weaken intermolecular interactions and reduce interfacial adhesion. These characteristics result in the phenomenon of immiscibility, leading to restricted compatibility between the components3. Superimposed thermomechanical and oxidative aging during collection, sorting, and repeated reprocessing accelerates chain scission and structural degradation, thereby shortening service life and impairing processability and heat resistance4.

Maleic anhydride (MA), a reactive coupling agent, has been employed extensively to circumvent the disadvantages. In the presence of a radical initiator such as dicumyl peroxide (DCP), MA is grafted onto PP backbones to form PP-g-MA, introducing polar functionalities that strengthen interfacial adhesion with HDPE, enhance stress transfer, and stabilize long-term performance5. Numerous studies have demonstrated MA-mediated improvements in mechanical and thermal responses across diverse polymer systems, including bio- and engineering-polymer composites6. While other coupling agents such as silanes and glycidyl methacrylate (GMA) have also been explored for polymer blends, MA grafting remains the preferred method for polyolefins due to its high reactivity and cost-effectiveness7,8. However, the optimal MA loading is intrinsically composition- and history-dependent: insufficient amounts fail to compatibilize effectively, whereas excessive MA and/or initiator can trigger chain scission, crosslinking imbalance, or thermal degradation, ultimately compromising properties9. Optimization of coupling agents, with consideration for the composition of PCR plastics, has emerged as a pivotal aspect.

Although numerous studies have focused on compatibilizing virgin polyolefin blends, research specifically targeting PCR plastics remains limited. Unlike virgin polymers, PCR materials possess complex thermal histories and contain degradation byproducts that can significantly alter their interaction with coupling agents. Furthermore, while most existing literature focuses primarily on immediate improvements in mechanical or rheological properties, there is a critical lack of comprehensive studies that link the optimization of coupling agent composition directly to the long-term thermal reliability and service life of the material. Therefore, a systematic approach that not only optimizes the mechanical performance of PCR blends but also quantitatively predicts their lifetime using kinetic models is essential for expanding their industrial applicability.

The present study investigated the role of a coupling agent in improving the properties of film-based PCR plastics, as a function of the PP/HDPE composition ratio. The enhanced interfacial bonding strength and the resulting tensile strength and melt index (MI) were quantitatively evaluated. Thermal decomposition kinetics were analyzed using thermogravimetric analysis (TGA), and activation energy (Ea) and lifetime were estimated by applying a non-isothermal model. The findings suggest that the optimal MA/DCP composition range has the potential to mitigate immiscibility. These findings represent essential conditions for producing high-quality recycled PCR products suitable for industrial applications.

Materials and methods

Materials

PP and HDPE were obtained from Sarom ENG (Hwaseong, Republic of Korea), and film-based PCR plastics were supplied by Cheongsol Co., Ltd. (Goyang, Republic of Korea) as received without further processing. MA and DCP were purchased from Sigma-Aldrich (USA). The preparation of MA-treated blends is schematically illustrated in Fig. 1. MA and DCP were dissolved in acetone to provide a homogeneous reaction medium, and the solution was magnetically stirred to ensure complete interaction. The solvent was removed on a hot plate at 60 °C, slightly above the boiling point of acetone (56 °C), to accelerate evaporation and promote uniform MA adhesion to PP. The PP/HDPE mass ratios applied in this study are listed in Table S1, and the MA and DCP mixing ratios are presented in Table S2. The material was milled into particles of 2 mm diameter using liquid nitrogen. The residual moisture was then removed using a vacuum oven at 80 °C for two hours. The dried material was then injection molded at 230 °C using a laboratory-scale automatic injection molding machine (Model AD-30, QNES, South Korea) to produce standard test specimens. A comparison of the untreated blend and the MA-modified blend in Fig. 1 reveals enhanced interfacial adhesion and phase stability through MA bonding, which is crucial for improving the mechanical properties of the recycled composite.

Fig. 1
figure 1

Schematic diagram of the PCR plastic specimen preparation and coupling agent application procedure.

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Methods

Thermal decomposition was characterized by TGA with derivative thermogravimetry (DTG) analysis, using a TA Instruments SDT 650 (New Castle, DE, USA). Approximately 5–10 mg was heated from 40 to 600 °C at 10 °C/min to obtain stability profiles, and additional scans were collected at 5, 10, 15, and 20 °C/min to compute Ea by the Flynn-Wall-Ozawa (FWO) method under non-isothermal conditions. The resulting Ea values were used with Toop’s equation to estimate service lifetime using 5% mass loss as the failure criterion. Chemical structure and grafting signatures were examined by Fourier-transform infrared spectroscopy (FT-IR) (Nicolet iS50, Thermo Fisher Scientific, Waltham, MA, USA) in ATR mode over 550–4000 cm− 1 with 8 cm− 1 resolution and 16 scans. Relative band intensities including the carbonyl region were used to assess grafting level and compatibility changes in the blends. Melt flow behavior was measured by MI on a Dynisco LMI 5000 (Franklin, MA, USA) at 230 °C and 2.16 kg. Samples were vacuum-dried at 80 °C for 2 h before testing. At least five replicates per condition were measured and the mean MI was reported. Tensile properties were obtained on an Instron 8801 (USA) with a 50 mm gauge length, a 5 mm/min crosshead speed, and 25 °C test temperature. A minimum of five specimens per condition was tested and the mean tensile strength was calculated. Morphology and interfacial features were observed by field emission scanning electron microscopy (FE-SEM) (JSM-7100 F, JEOL Ltd., Japan). To observe the cross-sectional morphology, the specimens were cryo-fractured in liquid nitrogen. Specimens were then sputter-coated with a thin platinum layer to prevent charging and to enhance image clarity. Imaging focused on phase dispersion and interfacial adhesion in PP/HDPE-based PCR blends with and without MA coupling.

Results and discussion

Material characterization

Prior to optimizing the coupling agent, a comprehensive analysis was conducted to establish the baseline thermal, chemical, mechanical, and rheological characteristics of the constituent materials: PP, HDPE, and PCR plastics.

The thermal stability of the materials was investigated using TGA, with the results presented in Fig. 2. The TGA curve in Fig. 2 (a) shows that the PCR plastic left a significantly higher residue of 5.07 wt%, indicative of non-polymeric impurities, compared to virgin PP (2.07 wt%) and HDPE (1.46 wt%). The DTG curve in Fig. 2 (b) further reveals that the onset decomposition temperature of the PCR plastic (208.80 °C) is substantially lower than those of PP (365.71 °C) and HDPE (395.43 °C). This premature degradation is a direct consequence of polymer chain scission and oxidation resulting from thermo-mechanical stresses during the recycling history10. Residual additives and oxidation byproducts are known to further accelerate this thermal degradation11.

The chemical composition was further analyzed using FT-IR (Fig. S1) to elucidate the structural factors influencing this thermal behavior. The spectrum of the PCR plastic exhibited characteristic absorption bands of both PP and HDPE. Specifically, the peaks at 2950 cm− 1 and 1375 cm− 1 correspond to the C-H stretching and symmetric methyl (-CH3) deformation of PP, respectively. The peaks observed at 2847 cm− 1 and 718 cm− 1 are attributed to the symmetric C-H stretching and methylene (-CH2-) rocking vibration of HDPE, respectively12. A quantitative analysis based on the characteristic peak areas yielded a composition ratio of approximately 28.95% PP to 71.05% HDPE. This spectroscopic identification of the blend composition directly correlates with the intermediate thermal stability observed in TGA13. While the presence of distinct PP and HDPE functional groups confirms the blend nature, the lower degradation temperature noted in Fig. 2 implies that these polymer chains have undergone prior oxidative damage, which is consistent with the characteristics of recycled materials identified by the spectral analysis.

Fig. 2
figure 2

Results of thermal characterization for PP, HDPE and PCR plastic. (a) TGA (b) DTG.

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​The mechanical and rheological performance of the materials was evaluated and compared against theoretical values calculated from the rule of mixtures. Figure 3 (a) illustrates the tensile strength results. While virgin PP and HDPE exhibited tensile strengths of 16.61 MPa and 17.60 MPa, respectively, their blends demonstrated inferior performance. For example, the PP: HDPE (3:7) blend yielded a tensile strength of 14.99 MPa, significantly below the theoretical prediction of 17.30 MPa. This deviation is attributed to the inherent immiscibility between the nonpolar polyolefins, which leads to poor interfacial adhesion and creates weak points for stress concentration14. Consequently, the PCR plastic, which also suffers from recycling-induced degradation, recorded a much lower tensile strength of 8.22 MPa.

A similar trend was observed for the melt flow properties, as depicted in Fig. 3 (b). The MI of the PP: HDPE (3:7) blend was 2.95 g/10min, again falling short of the theoretical 4.60 g/10min. The phase-separated morphology of the immiscible blend disrupts laminar melt flow, thereby increasing viscosity and reducing the MI value15. As a result, the PCR plastic exhibited an even lower MI of 2.28 g/10min, indicating poor processability.

In summary, this initial characterization quantitatively confirms that the PCR plastic’s properties are severely compromised by the combined effects of polymer immiscibility and degradation. This establishes a clear rationale for the necessity of an effective compatibilizer, such as MA, to enhance interfacial properties and enable the upcycling of this material16.

Fig. 3
figure 3

Comparison of measured and theoretical values for PP, HDPE, PP: HDPE (3:7, 5:5, 7:3) blends and PCR plastics. (a) Tensile strength, (b) MI.

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MA coupling agent optimization based on PP and HDPE

The inherent immiscibility between PP and HDPE, arising from their distinct molecular architectures and crystallinities, necessitates the use of a compatibilizer to improve the performance of their blends. In this study, MA was employed as a coupling agent, which was grafted onto the PP backbone via a radical reaction initiated by DCP to form MA-grafted polypropylene (PP-g-MA). In order to derive the optimal composition, reaction conditions were evaluated by systematically varying the concentrations of MA and DCP for PP: HDPE blend ratios (3:7, 5:5, 7:3).

The influence of the MA coupling agent on the tensile strength of the blends is presented in Fig. 4. The results indicate that the tensile strength was significantly enhanced across all blend ratios, with the optimal performance achieved at a DCP concentration of 0.4 wt%. Specifically, at this optimized DCP content, the tensile strengths of the PP: HDPE (3:7, 5:5, and 7:3) blends reached 18.06, 16.93, and 19.05 MPa, respectively, representing improvements of 20.48%, 22.50%, and 46.31% compared to the uncompatibilized blends. This enhancement is attributed to the increased crosslinking density facilitated by DCP, which strengthens the interfacial bonding between the PP and HDPE phases and improves the overall mechanical integrity of the material17.

Fig. 4
figure 4

Results of tensile strength measurements for PP: HDPE (3:7, 5:5, 7:3) blends with varying MA and DCP content. (a) PP: HDPE (3:7), (b) PP: HDPE (5:5), (c) PP: HDPE (7:3).

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In contrast to the tensile strength, the MI showed an inverse relationship with DCP concentration, as depicted in Fig. 5. Higher DCP content (0.4 wt%) led to increased crosslinking, which restricted polymer chain mobility and consequently reduced melt flowability17. The optimal MI values were observed at a lower DCP concentration of 0.2 wt%. This condition strikes a balance between improving interfacial compatibility and minimizing viscosity increases, thereby enhancing overall processability, which is crucial for subsequent manufacturing steps such as pelletizing and injection molding.

Fig. 5
figure 5

Results of MI measurements for PP: HDPE (3:7, 5:5, 7:3) blends with varying MA and DCP content. (a) PP: HDPE (3:7), (b) PP: HDPE (5:5), (c) PP: HDPE (7:3).

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The successful grafting of MA onto the PP chains was confirmed by FT-IR analysis, as presented in Fig. 6. The stacked spectra in Fig. 6 (a) clearly display the emergence of a distinct carbonyl (C = O) stretching peak at 1720 cm− 118 for the MA-treated samples. To quantitatively evaluate the grafting efficiency, the integrated area of this peak was calculated for the PP: HDPE (3:7) blends with various MA and DCP contents, as shown in Fig. 6 (b). This figure demonstrates that the peak area, indicative of the grafting degree, varies significantly with the additive concentration. Notably, the sample with 5 wt% MA and 0.4 wt% DCP exhibits the largest peak area, and this trend is directly consistent with the mechanical performance results, confirming that optimized grafting efficiency strengthens interfacial adhesion and maximizes tensile strength.

Fig. 6
figure 6

Results of FT-IR characterization of PP: HDPE (3:7) blends with varying MA and DCP contents, showing (a) carbonyl-region spectra and (b) integrated 1720 cm− 1 peak areas.

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The morphological changes induced by the compatibilizer were visualized using FE-SEM, with the results shown in Fig. 7. The images of the uncompatibilized polymer blends (Fig. 7 (a)) and PCR plastic (Fig. 7 (c)) reveal a coarse, rough surface morphology characteristic of pronounced phase separation between immiscible polymers. In stark contrast, the addition of the optimized MA coupling agent resulted in a remarkably more continuous and homogeneous surface structure for both the blend (Fig. 7 (b)) and the PCR plastic (Fig. 7 (d)). This morphological transformation provides direct evidence that the coupling agent effectively suppresses phase separation by enhancing interfacial adhesion, which corroborates the observed improvements in mechanical and rheological properties19,20.

Fig. 7
figure 7

Results of FE-SEM analysis showing the cryo-fractured surface morphology with MA coupling agent. (a) PP: HDPE (3:7) blend, (b) PP: HDPE (3:7) blend with MA coupling agent, (c) PCR plastic, (d) PCR plastic with MA coupling agent.

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Improvement of PCR plastics properties

Following the optimization studies on virgin blends, the most effective coupling agent formulation was applied to the target PCR plastic to validate its performance-enhancing capabilities in a real-world recycled material. The compositional analysis results in Sect. 3.1 confirmed that the PCR plastic consists of approximately 28.95% PP and 71.05% HDPE. Accordingly, the optimized composition for a 3:7 (PP: HDPE) blend (5 wt% MA and 0.4 wt% DCP) was selected as the representative composition for subsequent experiments and process applications in this study.

The successful grafting of the coupling agent onto the PCR material was confirmed by FT-IR analysis, as shown in Fig. 8 (a). A distinct carbonyl peak appeared at 1720 cm− 1 in the compatibilized PCR plastic, which was absent in the original material, providing clear evidence of successful MA grafting. This spectroscopic confirmation establishes the chemical basis for the observed property improvements. Figure 8 (b) quantifies the resulting enhancements in mechanical and rheological properties. The tensile strength of the PCR plastic increased from 8.22 MPa to 10.86 MPa, a significant improvement of 32.12%. This increase is a direct result of the enhanced interfacial adhesion between the PP and HDPE phases, facilitated by the polar functional groups of the grafted MA, which allow for more efficient stress transfer across the blend matrix. This result is consistent with findings by Graziano et al.5, who reported that MA-grafted compatibilizers significantly enhance tensile properties by strengthening interfacial adhesion in polyolefin blends. Even more dramatically, the MI surged from 2.28 g/10min to 10.63 g/10min, a remarkable 366.23% increase. This substantial improvement in flowability is attributed to the compatibilizer’s role in reducing interfacial tension between the immiscible phases, leading to a more uniform and stable melt. Similar improvements in flowability were observed by Mofokeng et al.15, where compatibilization reduced the domain size of the dispersed phase, thereby minimizing flow resistance. This enhanced processability is widely recognized as a key factor determining the industrial applicability of recycled plastics.

An optimized MA-based coupling agent has been demonstrated to effectively mitigate the negative effects inherent in PCR plastics, namely polymer non-homogeneity and thermo-mechanical degradation. The substantial enhancements in mechanical strength and processability underscore the viability of this methodology for the upcycling of low-value plastic waste into higher-performance materials that are conducive to a circular economy21.

Fig. 8
figure 8

Results of MA coupling agent application to PCR plastic (a) FT-IR analysis, (b) mechanical properties.

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Long-term lifetime analysis to improve material reliability

To evaluate the long-term reliability of the compatibilized PCR plastic, its thermal degradation kinetics and service lifetime were investigated. The stability of recycled plastics can vary significantly, but the incorporation of effective coupling agents can mitigate these issues and enhance durability over repeated processing cycles22. The analysis was based on a non-isothermal kinetic model using TGA data collected at multiple heating rates (5, 10, 15, and 20 °C/min), as shown in Fig. 9.

The Ea for thermal degradation was determined using the FWO method, an integral approach for calculating Ea under non-isothermal conditions23. The FWO method is described by Eq. (1):

$$\:\text{F}\text{l}\text{y}\text{n}\text{n}-\text{W}\text{a}\text{l}\text{l}-\text{O}\text{z}\text{a}\text{w}\text{a}::\:lnq=ln({AE}_{a}/RF\left(\alpha\:\right))-5.331-1.052({E}_{a}/R)(1/T)$$
(1)

Where q is the heating rate (°C/min), A is the pre-exponential factor (min− 1), Ea is the activation energy (kJ/mol), R is the universal gas constant (8.314 J/mol·K), T is the absolute temperature (K) and F(α) is the integral conversion function depending on the reaction mechanism.

As demonstrated in Fig. 9, the compatibilized PCR plastic consistently exhibited higher decomposition temperatures at 5% conversion across all heating rates, indicating enhanced thermal stability. From the data obtained, the calculated Ea for the original PCR plastic was found to be 105.00 kJ/mol, whereas the value for the compatibilized PCR plastic increased to 134.53 kJ/mol. The linear FWO plots used to derive these Ea are presented in Fig. S2. This 28.12% increase in Ea indicates that a higher energy barrier must be overcome to initiate degradation. The enhancement in performance is attributed to the improved interfacial adhesion and stronger polymer-polymer interactions that are facilitated by the MA coupling agent, which results in a more thermally stable material structure. This increase in Ea aligns with previous studies suggesting that improved interfacial interaction acts as a barrier to thermal degradation, effectively retarding the decomposition process6.

Subsequently, the service lifetime (tf) was predicted using Toop’s equation, which utilizes the previously calculated Ea to estimate the time to failure at a given temperature23. Toop’s equation is presented as Eq. (2):

$$\:\text{Toop equation}::{\:ln}{t}_{f}={E}_{a}/R{T}_{f}+ln\left[\right({E}_{a}/qR)\times\:P({x}_{f}\left)\right]$$
(2)

Where tf is the estimated lifetime to failure, Tf is the service temperature (K), and P(xf) is the temperature integral function evaluated at the TGA decomposition temperature Tc (temperature at 5% conversion), defined as P(xf) ≈ e− xf/xf2 where xf = Ea/RTc.

Fig. 9
figure 9

Results of temperature variation depending on heating rate and the presence of a coupling agent in PCR plastic at 5% conversion.

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The predicted lifetimes for both materials at various temperatures are plotted in Fig. 10. The results show a dramatic extension in the predicted lifetime for the compatibilized PCR plastic. Notably, at 200 °C, a temperature relevant to polyolefin processing, the lifetime increased from 61.48 h to 388.96 h, a remarkable 532.66% improvement24. Across the entire temperature range studied, the average lifetime extension was 1,667.83%.

Optimized MA coupling agents have been proven to enhance not only the mechanical and rheological properties of materials but also the long-term thermal reliability of PCR plastics. These findings are of crucial importance to producing durable and recyclable materials for high-value-added applications.

Fig. 10
figure 10

Results of predicted lifetime analysis of PCR plastic depending on the presence of a coupling agent at various temperatures at 5% conversion.

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Conclusions

This study confirmed that an optimized MA coupling agent can dramatically improve the key properties of PCR plastic. After coupling agent treatment, tensile strength increased by 32.12% and MI by 366.23%, simultaneously enhancing mechanical durability and processability. Moreover, the long-term thermal stability of the material was significantly enhanced, with the predicted service life increasing by 532.66% at 200 °C. This enhancement in overall performance is attributed to the MA coupling agent strengthening the interfacial bonding between immiscible PP and HDPE, which consequently increases the Ea required for material decomposition. In conclusion, the MA coupling agent can be regarded as a highly effective solution for enhancing the recyclability of low-value PCR plastics. This suggests that it has considerable technological potential within the sustainable polymer industry.