Enhancing the compatibility of low-value multilayer plastic waste in bitumen mixtures using atmospheric cold plasma and thermal oxidation

Abstract

Multilayer plastic (MP) commonly used in food and beverage packaging is difficult to recycle due to its layered structure, resulting in its accumulation over time; the consequent environmental harm is further exacerbated by its short lifespan. This study investigates recycled low-value MP as a modifier for polymer-modified bitumen (PMB). However, the difference in polarity between MP and PMB mixtures is a challenge, resulting in their poor compatibility and reduced mechanical properties. To overcome this, low-value MP was treated with atmospheric cold plasma and thermal oxidation to enhance its compatibility with PMB. The results indicate that plasma and thermal treatments increase the hydrophilicity of low-value MP through the formation of low-molecular-weight oxidized molecules containing hydrophilic hydroxyl (–OH) and carbonyl (C = O) groups that act as an intermediary boundary layer between the low-value MP and asphaltene-rich bitumen. Further, the optimal oxidation conditions for MP are revealed as 60 s of plasma treatment followed by heating at 150 °C for 60 min. Mixtures of PMB and optimally oxidized MP have optimal compositions of 1 wt.%, with ductility and penetration values of 87.7 cm and 57.4 mm, respectively.

Keywords:

• Multilayer plastic
• compatibility
• interfacial adhesion
• atmospheric cold plasma
• polymer-modified bitumen
• critical surface tension
• thermal oxidation
• radical formation

1. Introduction

The demand for multilayer plastic (MP) has exponentially increased over the years, with studies estimating that the global packaging industry alone produces over 100 million tons of multilayer thermoplastic each year [1, 2]. MP is a polymer product composed of two or more types of layered plastic and is widely used for packaging in the food and beverage industry, as it accommodates the differences in environmental conditions between the inner and outer packaging layers [3]. The inherent heterogeneity of MP ensures the quality of the packaged product but makes MP recycling difficult owing to the different properties of its constituent components [4]. This issue is further exacerbated by its short lifespan, as the MPs used especially in packaging are single-use. Accordingly, MP is labeled as low-value plastic waste and ends up in landfills as non-recyclable waste [5–7]. A 2018 study across European countries found that 28% of MP packaging is discarded in landfills, and 42.6% of it is incinerated for energy recovery [8]. In addition to releasing substances that harm the environment, these processes contribute to the carbon footprint, with landfill disposal and incineration for energy recovery emitting CO₂ amounting to 253 and 4605 g kg⁻¹, respectively [9]. Considering the difficulties in MP recycling, one promising application of low-value MP waste is its use as a composite material, particularly as a modifier in plastic-modified bitumen (PMB) [10–15].

Bitumen is an asphalt binder made from petroleum and demonstrates viscoelastic properties, where its mechanical properties heavily depend on temperature. Furthermore, the polar bonds that dominate its structure make bitumen hydrophilic, leading to an affinity to water and is susceptible to deformation [16–19]. To address these issues, studies have investigated the addition of polymers into bitumen to form PMB. The incorporation of thermoplastics in bitumen increases its hardness, thermal resistance, and water-resistant properties [12, 20–22]. However, the drastic difference in polarity between the polar asphaltenes present in bitumen and the non-polar polymer backbone of MP results in high interfacial tension between the components, leading to their low compatibility, which poses challenges for homogenous mixing [12, 23]. This low compatibility has detrimental effects on the mechanical properties of PMB, particularly its ductility and penetration [24, 25]. Several methods can be employed to improve the compatibility between plastics and bitumen. One method is to utilize a coupling agent or compatibilizer that modifies the surface of the plastic to increase its hydrophilicity. The addition of a compatibilizer that has both polar and non-polar bonds (amphiphilic) is expected to reduce the interfacial tension between plastic and bitumen, thus increasing their compatibility [26, 27].

The direct modification of the polymer surface through ionizing radiation such as gamma rays, electron beams, and plasma discharge has recently captured research attention as an effective method for enhancing compatibility and adhesion relative to conventional procedures such as flame and chemical treatments [28–30]. Ionizing radiation initiates chain-scission reactions that break the covalent bonds in the bulk polymer, resulting in the formation of radicals and hydrophilic groups that serve as cross-linking sites [31–35]. Among the several ionizing radiation treatments available, plasma-based treatment has gained popularity, as it is a physical procedure that does not generate any chemical waste and effectively increases adhesion while being scalable and cost-effective [36–39]. Extensive research has investigated the influence of plasma treatment and corona discharge on the oxidative scission of polypropylene (PP) and the subsequent formation of water-soluble low-molecular-weight oxidized material (LMWOM) [40–42]. LMWOM may act as a weak boundary layer that serves as a chemical bridge, enhancing the adhesion between the polymer’s surface and another constituent of opposite polarity [43].

In this study, cold plasma and thermal oxidation were utilized to modify the surface chemistry of low-value MP and enhance its adhesion with bitumen as well as the overall mechanical properties of PMB. Owing to atmospheric cold plasma treatment, hydrophilic polar groups such as –COOH, C = O, and –OH may be generated on the surface of the plastic through a combination of radical formation and oxidation reactions [43, 44]. Furthermore, oxidation through thermal treatment increases compatibility owing to the increased formation of polar hydroxyl groups such as ROOH, ROOR, ROR, and ROH. Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and sessile drop contact angle tests were employed in this study to investigate the increase in the hydrophilicity of low-value MP. The improvements in the mechanical properties of the PMB mixture were assessed by measuring its ductility and penetration, whereas the homogeneity of the mixture was evaluated via optical microscopy.

2. Materials and methods

2.1. Materials

In this study, PMB is prepared using 60/70 penetration grade bitumen obtained from PT Pertamina (Indonesia). The MP is recycled food packaging made of layers of oriented polypropylene and MB Haimaster; it was collected from various municipal sources and cleaned thoroughly before atmospheric cold plasma and thermal treatments.

2.2. Multilayer plastic and polymer-modified bitumen preparation

MP samples are treated with cold plasma for 60 s using a self-assembled double barrier discharge (DBD) cold plasma reactor (15 W/10 V/15 mA); Figure 1 presents the general schematic. Next, the plasma treated multilayer plastic (MP-P) samples are placed in an oven (VOV-50, B-ONE, China), and thermal oxidation treatment is performed at 100 °C, 110 °C, 120 °C, 130 °C, 140 °C, and 150 °C for 15, 30, 45, and 60 min under standard atmospheric conditions. Table 1 summarizes the MP sample designations. After both treatments, PMB is prepared by homogenously mixing 300 g of bitumen with 1, 3, and 5 wt% of low-value MP using a self-assembled hot-melt mixer for 30 min at 180 °C. The oxidized plastic films are crushed into 3–5 mm particles using a shredder (SS 01B, Sumpah Sampah, Bogor, Indonesia) before being fed into the hot-melt mixer.

Figure 1. Schematic of the self-assembled DBD cold-plasma reactor.

Table 1. Sample treatment and code names for MP and PMB, where ‘xxx’and ‘yy’ represent the temperature and duration, respectively, of the thermal oxidation treatment.

Name	Cold plasma treatment (s)	Thermal oxidation treatment
		Temperature (°C)	Duration (min)
MP	–	–	–
MP-P	60	–	–
MP-P–Txxx–yy	60	100, 110, 120, 130, 140, 150	15, 30, 45, 60

2.3. Characterization methods

The contact angle of a liquid with known surface tension was determined via sessile drop wettability testing. This method was employed to directly characterize and evaluate the plasma interaction with the surface energy of the polymer film specimen. Distilled water (DW) and ethylene glycol (EG) were carefully dropped onto the surface of the MP using a pipette; the contact angle between the surface and liquid was imaged with a camera and measured via ImageJ software analysis. Surface tension values were determined based on the average contact angles of DW and EG—test liquids with established surface tensions of 72 and 48.5 mN/m, respectively—using the Zisman method. On the Zisman plot, the x-axis represents surface tensions, and the y-axis represents the cosine of each liquid’s contact angle. Each sample’s critical surface tension was ascertained by applying the Zisman equations at complete wetting (θ = 0).

Further, the physical properties of the treated MP were explored. Chemical characterization was performed using FTIR (Thermo Scientific, Nicolet iS50 FTIR & NIR Spectrometer, Massachusetts, USA). The thermal characteristics of the PMB were analyzed via DSC (Simultaneous Thermal Analyzer 6000, PerkinElmer, Massachusetts, USA), and its melting temperature was recorded in the second heating cycle under N₂ atmosphere (20 mL/min) and at a heating rate of 10 °C/min. In addition, the oxidation induction time (OIT) test was performed on the MP-P samples at 110, 130, 150, 170, and 190 °C to further analyze the oxidation time. The morphological and mechanical properties of PMB were also investigated in this study. The surface morphology, distribution, and homogeneity of PMB were determined via optical microscopy (Zeiss, Primotech, Oberkochen, Germany). A ductility test was performed to determine the maximum strain before breakage and adjust it to the ASTM D-113-99 standard, and a penetration test determined the strength of PMB using a penetration detector with standardized parameters as outlined in SNI 06-2456-1996.

3. Results and discussion

3.1. Cold plasma and thermal oxidation treatments

3.1.1. Contact angle and surface tension analysis

The interfacial adhesion between the MP and PMB was determined using the sessile drop wettability test; Figure 2(a) presents the general schematic. Surface tension values of the samples were calculated based on the average contact angle measurements of DW and EG known tensions of 72 and 48.5 mN/m, respectively, using the Zisman method. On the Zisman plot, the x-axis represents surface tension, and the y-axis plots the cosine of each liquid’s contact angle. Subsequently, the critical surface tension of each sample was determined by applying the Zisman equations during complete wetting (θ = 0) or y = cos (0) = 1.

Figure 2. (a) Schematic of the contact angle measurement of MP samples. The effects of thermal oxidation treatment duration on the critical surface tension of MP (b) and the contact angles of (c) DW and (d) EG.

The results displayed in Figure 2(b–d) indicate that higher temperatures and longer durations of thermal oxidation reduce the contact angle between the surface and liquid, increasing the surface tension of the MP. This observed phenomenon is attributed to an increased rate of radical formation via thermal oxidation at elevated temperatures and longer treatment durations. Thermal oxidation is heavily influenced by oxygen uptake, where an increase in temperature and heating duration is proportional to the rate of oxygen diffusion and absorption [45, 46]. It thus increases the number of polymer chains being oxidized, thereby increasing the number of polar groups such as R–OOH, R–OO–R′, R–O–R′, and R–OH [47, 48].

The increased surface tension of the MP achieved through a combination of cold-plasma and thermal oxidation treatments reduces the interfacial tension between the MP and bitumen. The highest critical surface tension of 30.91 mN/m was observed in MP-P–T150–60. The optimal interfacial tension was noted between bitumen and MP-P–T150–60 (1.08 mN/m), which was significantly lower than that between bitumen and MP (7.01 mN/m) as well as MP-P (3.94 mN/m). Thus, MP-P–T150–60 samples are considered to have optimal treatment variables and are further characterized and analyzed for comparison with MP and MP-P only.

The increased wettability could be attributed to the formation of LMWOM through chain scission mechanisms. Strobel and Lyons first investigated the influence of LMWOM produced through the corona-treated oxidative scission of PP films and determined that their presence affects adhesion properties of the films [40]. As LMWOM are more oxidized (possess a higher O/C ratio) than the underlying insoluble surface, the resulting surface tends to be more hydrophilic [42]. Strobel and Lyons’s results align with those of other studies on atmospheric cold plasma-treated PP, indicating the accumulation of water-soluble LMWOM alongside the creation of a fine nanostructure with a mean roughness ranging from 4.9 to 8.2 nm, etched on the film’s surface [33, 43]. In this study, the combination of cold plasma and thermal oxidation treatments effectively increased the critical surface tension of MP by modifying its surface chemistry through the formation of LMWOM. The presence of LMWOM as a boundary layer could facilitate improved compatibility with the hydrophilic asphaltene-rich bitumen.

3.1.2. Chemical analysis

In addition to the sessile drop test, FTIR spectroscopy was employed for the chemical analysis of the MP samples (Figure 3). The chemical properties of MP-P–T150–60 were further investigated owing to its lowest contact angle and optimum surface tension, as previously determined during surface tension analysis. Atmospheric cold plasma treatment led to the generation of C = O bonds that correspond to COOH functional groups, as indicated by the stretching signal at 1742 cm⁻¹. Dorai and Kushner performed extensive research on the reaction mechanism model on plasma-treated PP and determined that OH_(g), O_(g), O_3(g), and O_2(g) serve as the primary reactive species that initiate radical formation and functionalization on the polymer’s surface [32]. Reactive oxygen species generated through cold plasma treatment led to the formation of polar functional groups such as alcohols (C–OH), hydroperoxides (C–O–O–H), aldehydes (H–C = O), carbonyls (C = O), esters (COC = O), and acids (H–O–C = O) on the surface of treated MP. Du et al. [49] further investigated the influence of oxygen radicals on PP plasma surface treatment; they confirmed that >60% of the LMWOM identified on the surface of the treated PP corresponded to oxygen-containing functional groups such as hydroxyls (–OH) and carbonyls (C = O). These findings are in line with those reported by several other similar studies [30, 50]. Residual radical molecules such as alkyl radicals, peroxy radicals, alkoxy radicals, and inactive products are also formed owing to oxidation [41]. This observation corresponds with the findings of other studies that report on the generation of LMWOM with oxygen-containing hydrophilic functional groups on the polymer surface [31, 33, 43, 47, 48, 51].

Figure 3. The influence of cold plasma and thermal oxidation on the FTIR spectrum of MP.

A subsequent thermal oxidation treatment led to a decline in the number of C = O groups and conversely to the formation of the –OH group, as indicated by the 3413 cm⁻¹ stretching signal. Thermal oxidation of PP involves four key stages: initiation, where initial radicals form; propagation, which involves the reaction with oxygen; chain branching, where hydroperoxides decompose and radicals accumulate; and termination, where radicals recombine [52]. Notably, the tertiary hydrogen present in PP is most labile and preferably abstracted from the polymer chain. During initiation, the fragmentation of alkoxy radicals (RO•) was considered the leading degradation caused by chain-scission mechanisms, ultimately reducing the molecular weight of the polymer and forming LMWOM products, with ketones being formed in tertiary carbon atoms and aldehydes being formed in secondary carbon atoms [45, 46, 53]. This phenomenon occurs as the carbonyl group experiences oxidation reactions, forming carbonyl oxide (H₂COO) that reacts with hydroxyl radicals (•OH), ultimately producing hydroxyl groups from peroxide (R–OOH), alcohol (R–OH), and hydroxyl (–OH) [51]. A thermal oxidation reaction also occurs in the PP chain, which produces hydroperoxide (ROOH) and alcohol (ROH) containing –OH bonds from hydroxyl groups. Figure 4 schematizes the atmospheric cold plasma and thermal oxidation treatments on the MP surface.

Figure 4. Schematic of the chain-scission and oxidation reaction resulting in the formation of LMWOM and a boundary layer on the surface of the polymer.

3.1.3. Thermal analysis

Changes in the thermal properties of the MP samples treated with cold plasma and thermal oxidation were obtained through DSC analysis. Figure 5 shows the respective thermograms of the samples. The performed surface treatments led to a negative deviation in the MP melting temperature (T_m), which further supports the results of the previous chemical analysis that indicate changes in the MP’s chemical structure. Closer inspection of the endothermic peak revealed that a combination of cold plasma and thermal oxidation treatments decreased the melting temperature of MP (T_m = 158.97 °C) compared with its cold plasma treated (T_m = 160.58 °C) and untreated counterparts (T_m = 163.19 °C).

Figure 5. DSC thermogram of the influence of cold plasma and thermal oxidation on MP samples.

To obtain further insights into the DSC characterization of the MP-P samples, the OIT test was performed at 110 °C, 130 °C, 150 °C, 170 °C, and 190 °C. The results (Figure 6) show that MP oxidizes after more than an hour when heated to 110 °C and 130 °C. In contrast, MP heated at 150 °C, 170 °C, and 190 °C requires 24, 12, and 9 min, respectively, to oxidize. These results highlight that an increase in temperature can accelerate the oxidation time of MP. However, elevated temperatures can potentially degrade the MP samples.

Figure 6. OIT analysis results for MP samples heated to 110 °C, 130 °C, 150 °C, 170 °C, and 190 °C.

Cross-linking, oxidation, and degradation through chain-scission mechanisms are competing processes that occur during both cold plasma and thermal treatment [31, 43, 53]. In this study, both chain-scission and oxidation dominated as the primary mechanisms. Infrared analysis revealed the presence of OH stretching signals, indicating the occurrence of oxidation mechanisms in the MP-P–T150–60 sample. Interestingly, OIT analysis revealed that the oxidation time for MP-P–T150 was approximately 24 min. However, contact angle and surface tension analyses revealed that the critical surface tension for the MP-P–T150 sample continued to increase with increasing heating durations, reaching a maximum at 60 min. This discrepancy between the determined optimal treatment of MP-P–T150 and the OIT analysis results at 150 °C suggests that the remaining time after the initial 24 min of thermal oxidation contributed to degradation, particularly through chain scission mechanisms.

Cold plasma and thermal oxidation treatments are thought to be accompanied by the breaking of bonds through chain scission on the surface of the MP, which ultimately contribute to a reduced molecular weight (M_w) and lower melting temperature (T_m). This phenomenon is attributable to the changes in the polymer structure and reduced crystallinity owing to degradation during cold plasma and thermal oxidation treatments [44]. Further, the chemical analysis results suggest that thermal oxidation led to the modification and functionalization of the polymer surface. Moreover, the observed enhancement in wettability could be attributed to LMWOM produced through chain-scission mechanisms, as indicated by the OIT analysis results. The formation of LMWOM on the surface of the MP may be attributed to the decreases both T_m and M_w and the simultaneous increase in the interfacial adhesion between the MP and bitumen constituents.

3.2. Polymer-modified bitumen performance

3.2.1. Morphology analysis

Figure 7 shows the morphology of the PMB mixtures observed through optical microscopy along with the particle size analysis and distribution. Table 2 summarizes the average particle size. The average particle size of MP in bitumen mixtures decreased with physical treatment. The average particle size of the PMB–MP mixture, determined to be 76.54 (±7.24) μm, decreased to 59.66 (±5.45) μm after cold plasma treatment for 60 s. Moreover, thermal oxidation treatment at 150 °C for 60 min on plasma-treated MP further decreased its particle size in bitumen mixtures to 37.73 (±3.88) μm. In addition, cold plasma and thermal treatments resulted in the formation of a larger number of MP particles. This observation was confirmed by the dispersion or the value of the number of particles/μm² in bitumen mixtures, which was determined to be 0.018, 0.186, and 0.256 for PMB–MP, PMB–MP-P, and PMB–MP-P–T150–60, respectively.

Figure 7. Micrographs of (a) PMB–MP, (b) PMB–MP-P, and (c) PMB–MP-P–T150–60 mixtures; particle size analysis and distribution for (d) PMB–MP, (e) PMB–MP-P, and (f) PMB–MP-P–T150–60 mixtures.

Table 2. Average particle size for the designated PMB mixtures.

Sample	Average particle size (μm)
PMB–MP	76.54 ± 7.24
PMB–MP-P	59.66 ± 5.45
PMB–MP-P–T150–60	37.73 ± 3.88

A clear-cut boundary between the dispersed plastic and bitumen matrix indicates a phase boundary observed in all samples, with that in PMB-MP being the most prominent. The addition of plastic content into bitumen leads to the formation of an unstable phase that is partially miscible [24, 54]. This results in the accumulation of an aromatic bitumen phase that encapsulates the MP filler, leading to the formation and agglomeration of a polymer-rich phase [26]. Consequently, the concentration of asphaltenes increases, affording bitumen mixtures with enhanced hardness and deformation resistance. After atmospheric cold plasma and thermal treatments, the MP filler in the PMB–MP-P and PMB–MP-P–T150–60 samples was considerably incorporated and dispersed into the bitumen matrix. The inverse relationship between average MP particle size and dispersion value (number of particles/μm²) reflects the enhanced interfacial adhesion owing to cold plasma and thermal treatments; oxidation and chain-scission degradation mechanisms produce a weak LMWOM boundary layer that ultimately enhances interfacial adhesion and the compatibility of the PMB constituents.

3.2.2. Mechanical performance

Figure 8(a) and (b) show the average results of the ductility and penetration tests, respectively. We observe that plastic content and ductility are inversely proportional. This finding is in line with that of Yan et al. [55] regarding the addition of reclaimed low-density polyethylene in asphalts, where increasing the plastic content reduces the ductility of the mixture. This phenomenon may be attributed to the low adhesiveness between bitumen and MP, which tends to bond cohesively and agglomerate, making the resulting PMB mixture inhomogeneous. Untreated MP exhibits hydrophobic behavior and has poor compatibility with the hydrophilic asphaltene-rich bitumen. The clumping and agglomeration of MP results in the local anisotropic hardening of the PMB mixture, ultimately resulting in a stiff PMB mixture [19, 56].

Figure 8. The influence of cold plasma and thermal oxidation treatments on the mechanical properties of the PMB mixture: (a) average ductility and (b) average penetration.

The combined cold plasma and thermal oxidation treatments were observed to significantly lower the average ductility of the PMB mixtures. At a fixed plastic content of 1 wt.%, cold plasma and thermal treatments reduced the average ductility by 37.4% from the base bitumen value of 140 to 87.7 cm. The decrease in the average penetration is attributed to the difference in the bonds formed between the treated MP and bitumen relative to those between untreated MP and bitumen. In plasma-treated MP, the carbonyl groups (C = O) react with the aromatic and alkyl compounds in bitumen, resulting in stiff PMB mixtures [18, 51]. On the other hand, the combination of both cold plasma and thermal treatments leads to the oxidation of the remaining C = O bonds in MP, producing hydroxyl radicals that form hydroxyl groups (–OH). These –OH groups in MP further bond with the hydroxyl groups in the asphaltene-rich bitumen and form ethers [23]. Notably, Han et al. [57] and Li et al. [58] demonstrate that thermal treatment could lead to a rougher surface of the polymer filler and enhance mechanical interlocking with the surrounding matrix; this is reflected by the PMB mixture’s enhanced mechanical properties. Therefore, a combination of surface roughening and generation of hydrophilic bonds increases the adhesiveness between bitumen and MP, resulting in the increased homogeneity of the PMB mixture; this enhances the recovery and subsequently lowers the average ductility of bitumen.

The average penetration is inversely proportional to the plastic content of the PMB mixture; this finding corresponds with past studies on the influence of plastic content in asphalt mixtures [55, 59]. The inverse relationship may be explained by the higher viscosity of PP relative to bitumen, which causes the surface of the PMB mixture to harden with increasing plastic content. The results indicate that cold plasma and thermal treatments enhance average penetration. Notably, the penetration value of PMB mixed with 1 wt.% of MP treated with cold plasma and thermal oxidation (57.4 mm) lies within the range of the standard specifications outlined by the Indonesian Ministry of Public Works for hard asphalt that is used for concrete, suitable for hot climates and applications demanding high deformation resistance. This highlights the improved compatibility between bitumen and treated MP, which leads to the optimum dispersion, distribution, and homogeneity of the PMB mixture, thus increasing average penetration.

3.3. Challenges and industrial implications

Research has investigated several pretreatment approaches for enhancing the adhesion between PMB constituents for potential industrial applications. Coupling agents such as maleic anhydride act as a bridge between the PMB constituents without altering the physical and chemical nature of the waste plastic [60, 61]. Mousavi and Fini [62] demonstrated that oil treatment utilizing waste cooking oil enhanced the compatibility of waste polyethylene terephthalate while improving the ability of the bitumen mixture to retain volatile compounds after being subjected to accelerated aging. Studies have also suggested the synergistic effect of using chemical and physical treatments; Kabir et al. [63, 64] and Zhou et al. [65] have suggested the use of microwave irradiation along with the oil treatment of various thermoplastics and elastomers to enhance polarity through grafting the OH groups of oil onto the plastic surface exposed to radiation. Microwave irradiation, a chemical-free and environmentally friendly method, allows the rapid processing of plastics by directly interacting with their molecules, enhancing both surface and bulk properties [66, 67]. However, careful control of exposure time is crucial to prevent hot spots and excessive degradation, necessitating the use of specialized reactors that add to the upfront operational costs [68, 69]. Similarly, UV irradiation enhances thermoset surface roughness for better matrix compatibility, but its initial cost and potential for excessive degradation owing to long treatment durations and high-power requirements must be considered [70–72].

This study explores the combination of cold plasma and thermal oxidation treatments. Compared with other irradiation methods, cold plasma treatments have risen in popularity, as they rapidly and effectively increase adhesion, do not generate any chemical waste, and are scalable and cost-effective [36–39]. Past studies have shown that plasma treatment on MP leads to the formation of LMWOM, creating a weak boundary layer that acts as a chemical bridge to enhance adhesion through constituents with opposite polarities [40–43]. Plasma treatment also improves surface roughness, promoting better interlocking with the matrix [73]. Likewise, thermal treatment increases the roughness of the polymer’s surface, thus promoting mechanical interlocking between the polymer and the surrounding matrix and enhancing its mechanical properties [57, 58]. Moreover, thermal treatment results in oxidation, which can be considered a means of surface activation that breaks crosslinks and leads to a rougher surface while grafting polar functional groups such as R–OOH, R–OO–R′, R–O–R′, and R–OH [74, 75].

The utilization of cold plasma and thermal oxidation treatments proposed in this study offers a promising step forward for increasing the compatibility of MP waste with bitumen, resulting in enhanced dispersion and mechanical performance of the PMB mixture. However, several key aspects should be considered. The proposed method offers the advantage of valorizing plastic waste, thereby addressing environmental concerns associated with plastic disposal. In particular, the method addresses issues related to MP that is commonly used for single-use packaging; it has a very short lifespan and difficult to recycle, making it end up in landfills or being incinerated [4, 7]. In addition, the combined cold plasma and thermal oxidation treatments for the surface activation or functionalization for MP were performed under standard atmospheric conditions, which prevents the necessity of additional mediums such as those in oil-microwave treatment [63–65] as well as costly or specialized reactors.

Notably, the treated MP may cause the properties of the PMB to vary, which could affect its performance under certain conditions. The oxidation and degradation mechanisms present in both treatments require further investigation to determine their influence on PMB mixtures with MP. Concerns also remain regarding the rutting, fatigue life, and moisture resistance of the modified bitumen, as the interaction between the treated plastics and bitumen constituents may influence these parameters. Future studies may further investigate these limitations. Moreover, although the proposed method holds promise for sustainable waste management and road construction, its economic viability and scalability in industrial settings warrant further evaluations. The costs associated with implementing cold plasma and subsequent thermal oxidation treatments, as well as the potential need for specialized scaled-up equipment, may pose challenges to the widespread adoption of this method in the bitumen production industry. Moreover, the adaptation of modified MP into existing bitumen mixing instruments and infrastructure should be explored. Therefore, although the proposed method presents innovative solutions to plastic waste management and bitumen enhancement, careful consideration of its practical implications and economic feasibility is essential for its successful integration into the industry.

4. Conclusions

This study revealed that atmospheric cold plasma and thermal treatments successfully enhanced the adhesion and wettability of recycled low-value MP waste. Analyses of contact angle and surface tension indicated a decrease in the interfacial tension of MP from 7.01 to 3.94 and further to 1.08 mN/m following a 60-s cold plasma treatment and subsequent thermal oxidation at 150 °C for an hour. This observed decrease in critical surface tension was supported by chemical and physical analyses, which suggests the occurrence of oxidation mechanisms indicated by the formation of hydrophilic functional groups with peaks corresponding to hydroxyls (–OH) and carbonyls (C = O). Thermal characterization through DSC and OIT analyses further suggests that the oxidation mechanism was most likely accompanied by degradation through chain-scission mechanisms, which is reflected by the decrease in the T_m of MP and MP-P–T150–60 from 163.19 °C to 158.97 °C, respectively. A combination of both oxidation and degradation mechanisms was thought to form LMWOM on the surface of the MP owing to the atmospheric cold plasma and thermal treatments; LMWOM subsequently acted as an intermediary boundary layer, which increased the compatibility of the MP with the bitumen matrix.

Moreover, the treated MP samples had enhanced compatibility with bitumen. The observed inverse correlation between MP particle size and dispersion value confirms the improved interfacial adhesion owing to cold plasma and thermal treatments. This phenomenon was attributable to oxidation and chain-scission degradation mechanisms that formed a weak LMWOM boundary layer; this layer enhances PMB constituents’ interfacial adhesion and compatibility, as evidenced by the dispersion values of 0.018, 0.186, and 0.256 (number of particles/μm²) for PMB–MP, PMB–MP-P, and PMB–MP-P–T150–60, respectively. This finding was confirmed by the dispersion values in the bitumen mixtures, which were determined to be 0.018, 0.186, and 0.256 for PMB–MP, PMB–MP-P, and PMB–MP-P–T150–60, respectively. Finally, the analysis of mechanical performance indicates that the values of penetration and ductility for PMB mixtures were inversely proportional to the plastic content. An optimal PMB mixture containing 1 wt.% MP demonstrated favorable ductility (87.7 cm) and penetration (57.4 mm) values.

The results presented in this study highlight the potential of combining atmospheric cold plasma and thermal treatments to enhance the wettability of MP. This innovative approach to producing PMB offers a sustainable and environmentally friendly solution, as it involves a chemical-free physical process of recycling and valorizing low-value MP waste into a value-added PMB mixture product. This promising technique addresses the challenge of recycling low-value MP waste while contributing to the development of greener and more efficient methods for polymer utilization and waste management.

Author contributions

Adam F. Nugraha: Conceptualization, resources, writing, review, and editing. Calvin Simon Andreas Lumban Gaol: conceptualization, investigation, writing. Mochamad Chalid: resources, supervision. Gusaimas Matahachiro Hanggoro Himawan Akbar: writing, review, and editing. Havid Aqoma: review and editing, resources.

Funding

This study was financially supported by the PUTI 2022 program, Contract No.NKB1317/UN2.RST/HKP.05.00/2022, allowing for the facilitation and completion of the research and associated activities.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The data that support the findings in this investigation can be made available from the corresponding author upon reasonable request.

Additional information

Funding

This study was financially supported by the PUTI 2022 program, Contract No.NKB1317/UN2.RST/HKP.05.00/2022, allowing for the facilitation and completion of the research and associated activities.