Waste Polypropylene in Asphalt Pavements: A State-of-the-Art Review Toward Circular Economy

Abstract

With the rapid increase in plastic consumption, waste polypropylene (WPP) has become one of the major components of municipal solid waste, posing significant environmental and resource challenges. According to statistics, polypropylene accounts for approximately 19.1% of the total global plastic waste, posing significant environmental challenges. In recent years, the recycling and reuse of WPP in asphalt pavement materials have received increasing attention due to its excellent mechanical properties, thermal stability, and low cost. This review systematically summarizes the physicochemical properties and recycling technologies of WPP, including mechanical, chemical, and energy recovery routes. Furthermore, the modification mechanisms, preparation methods, and performance characteristics of WPP-modified asphalt binders and mixtures are comprehensively discussed, focusing on their high-temperature stability, compatibility, low-temperature cracking resistance, and anti-moisture damage. Research indicates that WPP modification significantly enhances high-temperature rutting resistance, and thermo-chemical modifiers have successfully enabled the application of WPP in warm-mix asphalt. This review uniquely integrates recent advances in thermo-mechanochemical upcycling with mixture-level performance, bridging molecular design and field application. However, critical challenges, including poor compatibility, insufficient storage stability, and the lack of a unified assessment for the high variability of WPP raw materials, still need to be addressed. Finally, this review primarily focuses on the recycling technologies of WPP, its modification mechanisms in asphalt binders, and the resulting impact on the pavement performance of WPP-modified mixtures.

1. Introduction

With the acceleration of global urbanization, road construction, as an essential component of infrastructure, has developed rapidly. Asphalt pavements have been widely used in various types of roads due to their advantages such as driving comfort, wear resistance, low noise, and ease of construction and maintenance [1]. However, asphalt pavements inevitably undergo aging during long-term service, influenced by factors such as ultraviolet radiation, oxidation, temperature fluctuations, and water damage. Moreover, the increasing traffic load, frequent passage of heavy vehicles, and overloading can easily lead to rutting, potholes, cracks, and stripping, resulting in a decline in pavement performance and a shortened service life [2].

Plastics, as typical polymer materials synthesized from basic monomers through addition or condensation reactions, have been widely used in daily life and industrial production due to their high specific strength, chemical resistance, and processability [3]. However, with the continuous increase in plastic production and consumption, a large amount of waste plastic has been discarded, leading to severe ecological and environmental problems [4].

Among various plastic types, polypropylene (PP) is one of the most widely used materials due to its excellent comprehensive properties and low production cost [5]. PP is a thermoplastic polyolefin polymer characterized by low density, good chemical resistance, favorable mechanical strength, and easy processability. It is commonly used in packaging materials, automotive components, appliance housings, fiber products, and disposable medical devices [6,7,8,9]. According to global plastic production statistics, PP ranks second only to polyethylene among general-purpose plastics, occupying a significant market share [10]. However, the absence of degradable functional groups in its molecular structure renders PP highly stable in natural environments, making its waste disposal a major challenge in solid waste management [11]. Currently, the main disposal methods for WPP include mechanical recycling, incineration, and landfilling. However, the efficiency of global plastic waste treatment is low; according to OECD data from 2022, globally only about 9% of plastic waste is recycled, approximately 19% is incinerated, and nearly 50% ultimately ends up in landfills [12]. This grim reality underscores the urgency of finding high-value utilization pathways for WPP in the field of sustainable materials. Therefore, achieving efficient recycling and reuse of waste PP not only helps mitigate environmental pressure but also provides a practical pathway for resource circularity and sustainable development.

At present, WPP is mainly treated by incineration, landfilling, and recycling [13]. Although incineration can significantly reduce waste volume and recover part of the thermal energy in a short time, it generates harmful substances such as dioxins, hydrogen chloride, and particulate matter, which cause secondary pollution to the atmosphere, soil, and water [14]; landfilling, on the other hand, consumes land resources and may lead to groundwater contamination due to leachate leakage, while the high molecular stability of PP results in long-term environmental persistence [15]; and in contrast, recycling can effectively reduce the environmental impact of WPP and convert it into valuable secondary materials, achieving both environmental and economic benefits, and is therefore considered a sustainable and feasible approach [16].

In recent years, the reuse of WPP in pavement engineering materials has attracted increasing attention [17,18,19]. This trend benefits from the intrinsic properties of PP, such as high mechanical strength, thermal stability, and chemical resistance, which are also advantageous in asphalt modification [20,21]. Incorporating WPP into asphalt binders not only enhances high-temperature stability, rutting resistance, and durability but also improves anti-aging and moisture damage resistance to some extent [22]. Compared with incineration or landfilling, this recycling pathway combines environmental and economic advantages, alleviating plastic waste pressure while broadening the applications of recycled materials in road engineering [23].

While previous review studies have offered valuable contributions to waste plastic modified asphalt, their focus has largely been restricted to the compilation of physical properties of various waste plastic modified asphalts. Consequently, a deficiency persists regarding the systematic linking of WPP’s upstream functionalization design to its downstream engineering performance. To comprehensively understand the WPP recycling technologies and their modification mechanisms in asphalt pavement engineering, this review systematically collected a vast body of literature spanning the period 2009–2024, utilizing research databases such as Scopus, Web of Science, and Google Scholar, and details the discussion across three interconnected themes: (a) physicochemical properties and recycling methods of WPP plastics; (b) high-temperature performance, compatibility, low-temperature properties, and blending behavior of WPP-modified asphalt binders; and (c) performance evaluation of WPP-modified asphalt mixtures, including resistance to moisture damage, high–low temperature behavior, and fatigue performance. This review aims to offer theoretical support and technical guidance for the efficient utilization of WPP in asphalt pavements and to provide valuable insights for future research and practical applications.

2. Characteristics and Recycling Methods of WPP

2.1. Main Characteristics of Original PP

Polypropylene (PP) is a thermoplastic polyolefin material primarily composed of isotactic macromolecular chains, synthesized via the polymerization of propylene monomers. Its physical appearance and molecular structure are illustrated in Figure 1 and Figure 2. Due to the high proportion of crystalline regions within its molecular chains, PP exhibits excellent mechanical and thermal properties among commonly used general-purpose plastics [24]. The main characteristics of PP are summarized as follows: (1) Low density and light weight: PP has a density of 0.90–0.92 g/cm³, making it one of the lightest general-purpose plastics. (2) Excellent mechanical properties and strong chemical resistance: PP possesses high specific strength, good stiffness, and outstanding resistance to chemical corrosion and moisture. It remains stable in most inorganic salt, acid–base, and organic solvent environments [25]. Its glass transition temperature (Tg) is approximately −10 °C, imparting flexibility at room temperature, while its high melting point (160–170 °C) endows it with excellent thermal deformation resistance and heat stability [26]. (3) Good moisture resistance and electrical insulation: PP has very low water absorption, excellent electrical insulation properties, and is easy to process by injection molding, extrusion, or blow molding. Owing to its comprehensive performance, PP has become one of the most widely used plastics in packaging, automotive components, household appliances, and building materials [27].

Compared with other common plastics, PP exhibits a range of distinctive advantages. Relative to polyethylene (PE), PP demonstrates higher rigidity and heat resistance, with significantly higher melting and heat deformation temperatures, making it more suitable for applications requiring load-bearing and elevated operating temperatures [30]. Compared with polyethylene terephthalate (PET), PP has a lower density, reduced processing energy consumption, and superior chemical resistance, although it is slightly inferior to PET in ultraviolet aging resistance and gas barrier properties [31]. When compared with biodegradable plastics such as polylactic acid (PLA), PP offers higher mechanical strength, thermal stability, lower cost, and a mature industrial supply chain, giving it a clear advantage for large-scale applications [32]. Other detailed comparisons are shown in

Table 1. Comparison of the physical properties of PP, PE, PET, and PLA.

Property	PP (Virgin) [34,35,36]	PE [34,35,36]	PET [34,35,36]	PLA [34,35,36]
Density (g/cm3)	0.90–0.92	0.91–0.96	1.34–1.39	1.20–1.25
Melting point (°C)	160–170	110–135	250–260	150–170
Glass transition temperature (°C)	−10	−120	70–80	55–65
Tensile strength (MPa)	25–40	10–30	50–80	50–70
Elastic modulus (GPa)	1.2–1.8	0.2–1.0	2.0–2.7	2.7–3.5
Chemical resistance	Excellent	Excellent	Good	Moderate (hydrolyzable)
Biodegradability	Non-degradable	Non-degradable	Non-degradable	Degradable
Cost level	Low	Low	Medium	High

Note: The actual performance of WPP (such as melt flow index, molecular weight) is influenced by its service history, degree of degradation, and contamination level, exhibiting significant variability compared to virgin PP.

. However, due to its resistance to biodegradation, PP poses environmental challenges compared with biodegradable materials such as PLA. Therefore, research on the modification and recycling of waste polypropylene (WPP) has become a focus in the context of sustainable development and green materials research [33].

2.2. Main Recycling Methods for WPP

With the continuous increase in plastic consumption, waste polypropylene (WPP) has become a major component of solid waste. Recycling WPP not only helps mitigate environmental pollution, but also reduces raw material consumption. The recycling of WPP is primarily categorized into three general routes: mechanical recycling, chemical recycling, and energy recovery. This section will provide a review of these macroscopic recycling methods, serving as the fundamental background for understanding the subsequent modifier preparation (Section 3.1).

2.2.1. Mechanical Recycling

Mechanical recycling of WPP typically involves sorting, cleaning, crushing, melting extrusion, and pelletizing to convert waste materials into reusable granules for new product manufacturing [37]. Figure 3 illustrates the schematic of the single-screw extrusion process, which is the core step in mechanical recycling. Mechanical recycling is the dominant and technologically mature industrial route for converting WPP into granules or powder suitable for asphalt modification. Therefore, this figure visually explains the source material and the starting point of the physical pretreatment for the WPP modifier used in this study. This approach is technologically mature, cost-effective, and suitable for large-scale industrial applications, making it the dominant route in the plastic recycling industry [38].

Taking examples, Lamtai et al. [39] found that particle size, cutting conditions, and pre-sorting strategies directly affect the homogeneity and impurity level of WPP during extrusion, influencing failure initiation and mechanical stability of reprocessed products. Main et al. [40] demonstrated that repeated extrusion cycles lead to reduced crystallinity and elongation at break due to cumulative thermal–mechanical stresses. It is crucial to acknowledge that WPP exhibits significant variability compared to virgin PP. Specifically, post-industrial WPP generally presents lower contamination and less degradation, whereas post-consumer WPP is subject to high contamination, severe aging, and thermal degradation. This difference in source and history leads to substantial variations in the molecular weight, impurity content, and thermal stability of WPP modifiers, consequently imposing a significant impact on the performance consistency and stability of the final modified asphalt. Vacano et al. [41] clarified the molecular pathways of chain scission in PP during multiple reprocessing cycles, highlighting the synergistic effects of oxidation and thermal stress. Polachova et al. [42] quantified the release of low-molecular-weight compounds and degradation of mechanical properties after repeated processing, emphasizing the role of stabilizers and process optimization. Studies showed that multiple recycling cycles cause molecular chain breakage, changes in crystallinity, and a loss of mechanical properties in plastics. Optimizing processing conditions and adding stabilizers can help reduce this degradation.

However, mechanical recycling of WPP still faces limitations such as property deterioration, contamination control difficulties, and limited high-value applications [34,35,36]. Mantia et al. [43] reported that multiple reprocessing cycles lead to a decrease in molecular weight and toughness, while Dawoud et al. [44] pointed out that mixing PP of different grades or additive systems causes phase separation and poor interfacial compatibility. Tratzi et al. [45] further observed that residual inorganic fillers and pigments in waste PP may accelerate oxidative degradation during extrusion.

In summary, while mechanical recycling is mature and scalable, challenges such as performance degradation, purity control, and structural uniformity hinder its high-end applications. Future research should focus on process optimization, additive-assisted modification, and composite reinforcement strategies to enhance WPP quality and broaden its use in engineering and high-performance products.

Figure 3. Schematic of a single-screw extrusion process [46].

Figure 3. Schematic of a single-screw extrusion process [46].

2.2.2. Chemical Recycling

WPP, as a commonly used plastic material, has attracted increasing attention for its resource recovery and reuse. Chemical recycling technology converts WPP into monomers or low–molecular-weight hydrocarbons, providing high–value-added recycled products. Figure 4 illustrates the schematic diagram of typical chemical recycling methods. The most common chemical recycling approaches include pyrolysis, catalytic cracking, and dissolution–reprecipitation [47,48,49]. Parku et al. [50] found that reaction temperature, heating rate, and pressure significantly affect the yield distribution of WPP pyrolysis products—higher temperatures favor the formation of light liquid and gaseous hydrocarbons. Fukumasa et al. [51] demonstrated that the pore structure and acidity of H-MFI zeolite catalysts strongly influence the ratio of light hydrocarbons to aromatics. Palmay et al. [52] reported that catalytic pyrolysis using regenerated FCC catalysts enhances valuable hydrocarbon yield at lower temperatures compared to thermal pyrolysis. Xuan et al. [53] showed that catalytic cracking of pyrolysis oil using treated ZSM-5 catalysts can achieve light olefin yields up to 44.1 wt%, higher than those from naphtha feedstocks (34.6 wt%). This demonstrates that subjecting recycled polypropylene (WPP) to pyrolysis followed by catalytic cracking is a crucial strategy for enhancing the yield of high-value hydrocarbons. Additionally, Gravgaard et al. [54] demonstrated an efficient solvent-based separation process for PP/glass-fiber composites, achieving high-purity recovery of both PP and glass fibers at 130 °C. The yield of high-value chemicals from recycled polypropylene can be significantly increased by optimizing cracking conditions and catalysts, while advanced solvent separation techniques enable the recycling of composites.

Although laboratory-scale results are promising, large-scale application of chemical recycling is limited by high energy consumption, complex product distribution, and economic constraints. Continued research is needed to develop more efficient catalysts and intensified reaction processes.

2.2.3. Energy Recovery

Given its high calorific value (approximately 46 mJ/kg), WPP can also be used for energy recovery through combustion to produce heat or electricity [50]. This waste-to-energy approach can be applied in industrial boilers, power plants, or district heating systems, offering a potential alternative energy source [56].

However, combustion of WPP emits pollutants such as NOx, CO₂, CO, and particulate matter (PM), posing risks to air quality and human health [56,57]. During the combustion process, toxic by-products containing chlorine- or bromine-based additives may also be released, increasing the difficulty of flue gas treatment and solid residue disposal [57,58]. Brooke et al. [59] conducted an experiment using a laboratory-scale drop-tube furnace at temperatures ranging from 1300 to 1500 K to combust various plastics, including PP, in order to study particulate (soot) emissions. The results showed that under oxygen-rich or well-ventilated conditions, the emission of fine particulates from PP combustion was significantly reduced; however, under oxygen-deficient or incomplete combustion conditions, the particulate matter (PM) emissions were considerably higher. Wang et al. [60] found that thermogravimetric analysis (TGA) combined with a cone calorimeter and online FTIR detection of IFR–OMMT composite materials revealed that flame-retardant PP released higher peak concentrations of NOx and CO than pure PP during thermal decomposition and combustion. Moreover, the emission levels varied significantly with combustion temperature and oxygen supply conditions. Mentes et al. [61] investigated the co-combustion of waste plastics such as PP, HDPE, and PET with wood in residential stoves and measured the concentrations of particulate matter, CO, NOx, and polycyclic aromatic hydrocarbons (PAHs) in the air.

Figure 5 shows the particle mass emissions per unit of fuel and the mass distribution of different PM size fractions in this experiment. The results indicated that, although co-combustion improved thermal efficiency, increasing the proportion of waste plastics led to a significant rise in PAH concentrations, as well as elevated CO and NOx emissions, thereby exerting substantial pressure on air quality.

In summary, although energy recovery from WPP combustion offers clear short-term advantages in waste reduction and energy substitution, its practical application requires stringent combustion control and exhaust gas treatment measures to reduce the emissions of air pollutants such as NOx, CO, and PM, as well as to suppress the formation of toxic by-products originating from halogenated flame retardants.

3. Research Progress on WPP-Modified Asphalt Binders

3.1. Pretreatment of WPP-Based Asphalt Modifiers

As one of the key application fields for plastic recycling, road engineering—particularly in the use of asphalt modifiers—places high demands on the rheological properties, dispersibility, and thermal stability of materials. Therefore, pretreatment of WPP before its incorporation into the asphalt system is of critical importance. Pretreatment not only involves size control and impurity removal but can also improve the dispersibility and compatibility of WPP with asphalt through processes such as melting, blending, or reactions with additives, thereby laying a solid foundation for the subsequent optimization of modified asphalt performance.

The most common method for using WPP plastics in asphalt modification is to first mechanically crush them into small granules, and then incorporate them into asphalt under mixing conditions that are typically more than 20 °C higher than the conventional asphalt blending temperature [62]. This modification method generally requires the mixing temperature to reach at least the melting point of WPP (170–190 °C) to ensure the effective modification of asphalt by the WPP plastic [63]. However, the excessively high mixing temperatures (typically 170–190 °C) required for traditional wet-process modification lead to two main issues. First, it can cause significant emissions of asphalt fumes, as evidenced in Figure 6, which illustrates the harmful volatile organic compounds released during high-temperature asphalt heating, highlighting an environmental concern. Second, even at these high temperatures, the method often fails to achieve uniform dispersion of non-functionalized WPP within the asphalt matrix [64,65]. These combined environmental and technical limitations have driven research toward developing innovative WPP pre-treatment methods, such as thermochemical degradation, that can lower the required mixing temperature and promote cleaner production.

Dalhat et al. [62] confirmed that good compatibility between WPP modifiers and asphalt can only be achieved when the mixing temperature exceeds 180 °C, thereby significantly improving the high-temperature performance and adhesion of base asphalt. Xu et al. [66] modified asphalt with WPP plastics and found that excessively high modification temperatures led to the release of toxic and harmful gases, while the non-uniform dispersion of WPP particles within the asphalt resulted in inconsistent overall performance of the modified binder. Saikrishnan et al. [67] pointed out that during the processing of WPP-modified asphalt, excessively high modification temperatures can cause thermal cracking of WPP, producing asphalt fumes, carbon monoxide, sulfur dioxide, and other harmful gases, which contribute to air pollution. Therefore, reducing the melting temperature of WPP and improving the homogeneity of modified asphalt performance are key priorities for its high-quality application.

Figure 6. Harmful substances released during the heating of various types of asphalt [68].

Figure 6. Harmful substances released during the heating of various types of asphalt [68].

At present, research focusing on employing appropriate physicochemical methods to degrade WPP for cleaner utilization in asphalt materials has proven to be highly effective. For instance, Zhou et al. [69] used WPP as a raw material and prepared a warm-mix asphalt modifier through pyrolysis, which is potentially lower-cost, depending on pyrolysis scale and energy source. The physical and rheological properties of the modified asphalt were evaluated through penetration, softening point, ductility, rotational viscosity, and dynamic shear rheometer (DSR) tests, while the adhesion characteristics were analyzed using surface energy measurements. The results indicated that the mixing temperature of asphalt modified with the pyrolyzed WPP could be reduced to 120 °C, and the resulting modified asphalt still exhibited excellent high-temperature performance, elasticity, rutting resistance, and adhesion. Li et al. [70] thermally decomposed WPP under high temperature and pressure within a specific pyrolysis duration to obtain a solid pyrolytic wax (PW) as a warm-mix asphalt modifier. When incorporated into base asphalt, the PW acted as a colloidal dispersion medium, reducing the viscosity of the binder and enhancing its viscoelasticity, high-temperature stability, resistance to permanent deformation, short- and long-term aging resistance, and fatigue performance. Moreover, PW-modified asphalt exhibited good storage stability without any significant chemical changes. Chu et al. [71] reported that through a thermo-mechanochemical approach, WPP was synergistically treated with dicumyl peroxide (DCP), maleic anhydride (MAH), and epoxidized soybean oil (ESO), successfully upcycling it into a maleic anhydride–epoxidized degradation product, Figure 7 shows the reaction mechanisms of thermochemical and mechanochemical degradation and graft modification. This product can serve as a warm-mix asphalt modifier that not only reduces the asphalt mixing temperature by 20–40 °C, enabling energy-efficient and cleaner production, but also significantly improves the high-temperature performance, storage stability, and compatibility of asphalt. The multi-step reaction mechanism shown in Figure 7 strategically addresses the dual challenges of high viscosity and poor polarity. Step (i) involves the thermo-mechanochemical degradation of the high-molecular-weight WPP (PPM0). Dicumyl peroxide (DCP) acts as an initiator, generating free radicals that fracture the long PP chains into lower-molecular-weight oligomers (PPM3). This reduction in molecular weight is the fundamental reason for the lowered viscosity and the subsequent 20–40 °C reduction in mixing temperature. Steps (ii) and (iii) detail the graft modification using maleic anhydride (MAH). MAH is grafted onto the PP chain through free radical reactions, forming an interim ring-closed structure (PPM0-g-MAH), which is then converted into the final ring-opened structure (PPM3-g-MAH). This process introduces polar carbonyl functional groups onto the non-polar PP backbone. Finally, step (iv) introduces epoxidized soybean oil (ESO) for further functionalization, resulting in the highly functionalized product (PPM3-g-MAH/ESO). The newly introduced polar groups significantly improve compatibility by facilitating stronger chemical interactions between the modified WPP and the polar components (asphaltenes and resins) of the asphalt matrix, thereby enhancing storage stability and high-temperature performance.

These research findings indicate that through appropriate pretreatment and modification, WPP can achieve more uniform and efficient dispersion within the asphalt matrix, thereby providing a solid foundation for optimizing the performance of modified asphalt. In the following section, further discussion will focus on the rheological properties, storage stability, and practical application performance of WPP-modified asphalt in pavement engineering.

3.2. Performance and Application of WPP in Asphalt Binders

3.2.1. Softening Point and Penetration of WPP-Modified Asphalt

The softening point is a key indicator for evaluating the high-temperature stability of asphalt materials, reflecting the temperature at which asphalt gradually softens and loses its load-bearing capacity during heating. The penetration of asphalt is a measure of the hardness of asphalt under standard conditions; a higher value indicates a softer asphalt. Numerous studies have shown that incorporating WPP into the asphalt system can significantly increase the softening point of base asphalt, thereby enhancing its rutting resistance and high-temperature stability. Li et al. [72] systematically investigated the effects of plastic dosage and preparation process on the thermal properties of waste plastic–modified asphalt. The results revealed a significant dose–response relationship between WPP content and softening point improvement: as the WPP content increased from 0 wt% to 8 wt%, the softening point of asphalt rose from approximately 45 °C to over 70 °C, accompanied by an increase in viscosity, indicating a simultaneous enhancement in stiffness and thermal stability of the system. Figure 8 illustrates the variation curves of softening point, penetration, and ductility at different polypropylene dosages. The prevalent research findings indicate that the incorporation of Waste Polypropylene (WPP) significantly reduces the penetration of the base asphalt, which signifies an increase in system hardness and a rise in viscosity at ambient temperatures. This change directly improves the asphalt’s resistance to deformation, thus effectively enhancing its high-temperature rutting resistance [73]. Research universally confirms a negative correlation between WPP dosage and penetration: as the WPP content increases, the penetration continuously decreases [74]. For instance, in the research on polypropylene-based multilayer plastic waste, the increase in dosage significantly altered the physical properties of the asphalt, leading to a decrease in penetration, which in turn increased the softening point and stiffness [75]. Cheng et al. [73] further found that when waste packaging WPP was combined with organo-montmorillonite (OMMT) for asphalt modification, a more uniform polymer–asphalt network structure was formed, resulting in significantly higher softening point and thermal stability compared with the base asphalt. This research proves that modifying asphalt with either controlled amounts of waste plastic or layered silicate composites significantly boosts its high-temperature performance and thermal stability, validating a key approach for high-value plastic recycling.

In addition to dosage, the type and pretreatment method of polypropylene also have a decisive influence on the improvement of the softening point. Zhang et al. [76] found that modifying asphalt with maleic anhydride–grafted polypropylene (PP-g-MAH) significantly increased both the softening point and the complex modulus, and the degree of improvement was positively correlated with the grafting content. This enhancement primarily resulted from the improved compatibility between polypropylene and the asphalt phase induced by the grafted structure. Chu et al. [71] discovered that through a thermo-mechanochemical approach using dicumyl peroxide (DCP), maleic anhydride (MAH), and epoxidized soybean oil (ESO), WPP was synergistically modified to prepare maleic anhydride–epoxidized degradation products. These products can be blended with asphalt at relatively low temperatures while still significantly enhancing the softening point and viscosity, indicating that the pretreatment process improves the dispersion and chemical compatibility of WPP within the asphalt matrix, thereby enhancing its high-temperature performance. Hu et al. [77] verified a similar trend in an analogous upgrading–recycling strategy, finding that chemically modified WPP derivatives not only increased the softening point under warm-mix conditions but also improved storage stability.

In summary, the incorporation of WPP can significantly enhance the softening point and high-temperature stability of asphalt. The strengthening mechanisms can be attributed to two main aspects: (1) Physical effect—the high crystallinity and elevated melting temperature of WPP increase the overall stiffness of the modified system, making the asphalt less prone to softening or flowing at high temperatures; (2) Chemical or interfacial effect—especially after grafting, oxidation, or copolymer modification, stronger interfacial interactions and spatial network structures are formed between WPP and the asphalt matrix, further improving thermal stability and rutting resistance. However, excessive dosage or poor dispersion of WPP may lead to phase separation, reduced storage stability, and increased brittleness at low temperatures. Therefore, future research should focus on modifier structure optimization, interfacial compatibility regulation, and precise control of mixing parameters to achieve an optimal balance between the high-temperature performance and overall pavement properties of WPP-modified asphalt.

3.2.2. Ductility of WPP-Modified Asphalt

Ductility is an important indicator for evaluating asphalt’s resistance to cracking at low temperatures. Although WPP exhibits a significant positive effect on the high-temperature performance of asphalt, its influence on low-temperature performance is more complex. Multiple studies have shown that the addition of WPP generally reduces the ductility of asphalt, especially under low-temperature conditions (e.g., 5 °C or below). Zhang et al. [76] experimentally found that the ductility of unmodified base asphalt at 5 °C could exceed 20 cm, whereas WPP-modified asphalt exhibited a reduced ductility of 8–12 cm, indicating that the inclusion of WPP decreased low-temperature toughness. While a ductility value of 8–12 cm might marginally meet the minimum threshold for certain modified asphalt binder grades (the minimum 10 cm ductility requirement at 4 °C in AASHTO), this value is generally considered unacceptable for practical high-performance pavements. Consequently, high dosages of WPP are typically not considered in engineering practice. This phenomenon is mainly attributed to the relatively high glass transition temperature of WPP (approximately −10 °C); at low temperatures, WPP behaves in a brittle manner, limiting the overall deformability of the asphalt system and thus reducing its low-temperature crack resistance. To overcome this issue, researchers typically adopt composite modification strategies to mitigate the negative effects of WPP on low-temperature performance. For example, Cheng et al. [73] found that by blending WPP with styrene–butadiene–styrene (SBS), the modified asphalt could still maintain a relatively high m-value (above 0.45) at −18 °C, indicating that the low-temperature crack resistance of the WPP/SBS composite system was effectively improved. Similar composite modification approaches are often employed to optimize asphalt ductility at low temperatures while simultaneously enhancing high-temperature performance. Figure 9 illustrates the effect of PP-g-MAH content on the ductility of modified asphalt. Additionally, Zhang et al. [76] employed PP-g-MAH as a polar grafting compatibilizer in an rPE/WPP ternary modified asphalt system. In BBR tests (−18 °C), an appropriate amount of PP-g-MAH (9%) increased the m-value above 0.45 while maintaining stiffness S below 300 MPa, demonstrating that chemical grafting improved the system’s compatibility and low-temperature performance. Li et al. [72] studied the performance changes in WPP-modified asphalt at different dosages, showing that as the WPP content increased from 0 to 12 wt%, the ductility at 5 °C gradually decreased, demonstrating a clear dosage effect. Although high dosages of WPP (such as exceeding 8 wt%) can lead to significant improvements in mechanical properties, in practical engineering applications, this high addition level usually results in a sharp increase in the viscosity of the modified asphalt, deterioration of workability, and serious phase separation and storage stability issues, thereby greatly limiting its economic and practical application value. Most successful application cases control the addition amount between 2 and 6 wt%. Xu et al. [66] summarized that multiple studies confirmed a general reduction in ductility of 10–30% after WPP incorporation, primarily due to the relatively high glass transition temperature of WPP (around −10 °C), which causes brittleness at low temperatures and limits the deformability of asphalt.

In addition, composite modification strategies have also been shown to improve the low-temperature performance of WPP-modified asphalt. Guo et al. [78] found that WPP/SBS blended systems exhibited better ductility and m-values at low temperatures compared to systems modified solely with WPP. Buruiana et al. [79] further demonstrated that, at the level of hot-mix asphalt mixtures, WPP combined with crumb rubber could enhance both freeze–thaw resistance and low-temperature crack resistance.

Overall, the low-temperature performance of WPP-modified asphalt is significantly influenced by the intrinsic brittleness and dispersion state of WPP. However, methods such as chemical grafting (e.g., PP-g-MAH) or elastomer-based composite modification (e.g., SBS, crumb rubber) can effectively improve the system’s low-temperature ductility and crack resistance, achieving a balance between high-temperature and low-temperature performance.

3.2.3. Storage Stability and Compatibility of WPP-Modified Asphalt

Storage stability and compatibility are key indicators for evaluating whether WPP-modified asphalt is suitable for practical engineering applications. [80] Cheng et al. [73] demonstrated that using waste packaging PP in combination with organically modified reduced exfoliated clay (OREC) to modify asphalt resulted in a low softening point difference after high-temperature storage, with no obvious phase separation. This indicates that the layered structure of the nanoclay and the physical intercalation of WPP molecules enhanced the system’s compatibility and stability. Figure 10 shows the storage stability and microscopic images of the asphalt. Melekhina et al. [81] indicated that asphaltenes disperse well in the polypropylene (PP) matrix up to 30 wt% content. This favorable dispersion leads to the formation of stable composites characterized by good interfacial adhesion and the absence of coarse aggregates. At the same time, the reduction in polypropylene viscosity caused by UV radiation is suppressed by a factor of 6, and the loss of strength is decreased by a factor of 2, when 20 wt% asphaltenes are present. Previously, Kong et al. [82] reported in similar rubber powder systems that surface activation or chemical treatment can enhance the microstructural uniformity of polymer–asphalt systems, providing a methodological reference for WPP modification studies. Wang et al. [83] studying PE/CTR (recycled rubber) systems, found that hot extrusion and compatibilizer treatment help reduce density differences and improve microdispersion between materials, thereby significantly enhancing storage stability. This approach can be analogously applied to composite systems of WPP and crumb rubber.

In summary, introducing polar grafting agents (e.g., PP-g-MAH), nanofillers (e.g., OREC), surface treatments, or thermo-mechanical processing methods into WPP-modified asphalt systems can systematically enhance their microscopic compatibility and storage stability. However, existing mechanistic studies predominantly rely on static micro-testing (such as AFM or TEM images). These models are insufficient to fully explain the long-term, dynamic phase separation process of WPP particles within asphalt, nor have they quantitatively established a direct relationship between the molecular weight distribution and grafting rate of WPP degradation products and the macroscopic rheological performance of the modified asphalt. Therefore, future efforts necessitate the development of more sophisticated molecular dynamics simulations or multi-scale characterization techniques to accurately predict the long-term service performance under various WPP pre-treatment conditions.

3.2.4. Viscosity and Rheological Properties of WPP-Modified Asphalt

Viscosity, as a key parameter of workability, is critical for evaluating the feasibility of construction and the mixing uniformity of modified asphalt. [85] Due to the regular molecular chain structure and relatively high melting point of WPP (approximately 160–170 °C), its incorporation often leads to an increase in asphalt viscosity, which can affect construction flowability. Most studies suggested that properly controlling the WPP dosage and mixing temperature is essential to balance workability and modification performance. For example, Guo et al. [78] investigated the rheological behavior of WPP/SBS composite-modified asphalt using a dynamic shear rheometer (DSR). They found that as the WPP content increased, the asphalt viscosity showed a noticeable upward trend. However, at a dosage of 2–3 wt%, the high-temperature workability remained acceptable, allowing adequate mixing within the range of 160–170 °C. This indicates that small amounts of WPP do not significantly impair mixing flowability while enhancing the viscoelastic characteristics of the system and improving rutting resistance. Buruiana et al. [79] reported that incorporating finely WPP into hot-mix asphalt reduced penetration, increased the softening point, and significantly raised mixture viscosity, thereby improving high-temperature stability. Xu et al. [66] reviewed studies on waste-plastic-modified asphalt, noting that the addition of polyolefins such as WPP generally increases viscosity and stiffness, with the magnitude depending closely on the type of plastic, molecular structure, and pretreatment method. Ma et al. [86] summarized that the application of waste plastics in asphalt pavements, similarly indicated that powder or granular WPP can effectively increase viscosity and improve rutting resistance, but excessive dosages may lead to higher mixing temperatures and difficulty in handling. Hu et al. [87] prepared functionalized WPP derivatives via a mechanochemical method for asphalt modification. The results showed that the functionalized WPP could enhance high-temperature viscosity while significantly improving flowability during mixing.

Figure 11(I) presents the viscosity–temperature characteristics of PPMAs under (a) NHPI catalyst and (b) BPO catalyst systems. Hu et al. [87] found that the temperature dependence of G* and δ for PPMA cement. As the temperature increases from low to high, with the increase in DCP content, the G* value of the modified cement first increases and then decreases, while δ exhibits the opposite trend, as shown in Figure 11(II). Liu et al. [88] further demonstrated through rheological testing that the synergistic effects of PP with other additives can effectively regulate the high-temperature viscosity of asphalt, allowing tunable viscoelastic transition behavior at different temperatures. Additionally, Wang et al. [89] emphasized from the perspective of high-viscosity modification mechanisms that introducing thermoplastic polymers to control the temperature sensitivity of asphalt viscosity is an important approach to balancing high-temperature stability and workability.

In summary, the viscosity of WPP-modified asphalt exhibits significant temperature dependence and dosage sensitivity. Properly controlling the WPP content (≤3 wt%) and mixing temperature (160–170 °C), combined with chemical grafting or pretreatment techniques, can effectively improve both workability and thermal stability, providing a feasible approach for low-energy, low-emission green road construction.

4. Performance Study of WPP-Modified Asphalt Mixtures

4.1. Preparation Methods and Process of WPP-Modified Asphalt Mixtures

In recent years, research on the mix design of WPP-modified asphalt mixtures has primarily focused on two approaches: the dry process and the wet/melt-blending process, each with its own advantages and limitations in terms of material dispersion, equipment requirements, and construction parameters.

The dry process involves directly incorporating powdered or granular WPP into aggregates or mixtures. The dry process, by eliminating the need for additional pre-mixing equipment, in principle possesses the potential for implementation in existing asphalt mixing plants (or: conventional mixing plants). However, in order to ensure the uniform dispersion of WPP powder and to prevent risks such as agglomeration, clogging, or segregation with aggregates during the mixing process, it still requires the integration of specialized feeding equipment and the optimization of the mixing procedure. Studies at the mixture scale have shown that finely ground WPP can act like an “elastic aggregate,” enhancing high-temperature stability and rutting resistance. However, it also alters particle distribution and void characteristics. Therefore, key sieve pass rates (e.g., 2.36 mm, 4.75 mm) and volumetric parameters (VMA, VA) need to be adjusted in the mix design to prevent plastic particle aggregation from compromising the aggregate structure or causing storage instability [78,90].

The wet process involves melting WPP under high-temperature, high-shear conditions and fully blending it with the asphalt matrix. This approach significantly improves polymer dispersion and storage stability, thereby enhancing modification efficiency and material uniformity [73]. However, due to the high melting point and melt viscosity of WPP, conventional wet processes typically require elevated mixing temperatures (160–190 °C), which increases energy consumption and may accelerate the volatilization of light asphalt fractions and thermal oxidation.

To address this, recent studies have focused on reducing the melt viscosity of WPP through pretreatments—such as pyrolysis, thermo-mechanochemical degradation, or chemical grafting—to enable warm-mix or low-temperature wet blending. For example, Zhou et al. [69] prepared wax-based warm-mix additives via pyrolysis, reporting that the mixing temperature could be reduced to around 120 °C while significantly lowering rotational viscosity at 135 °C. Xu et al. [71] produced WPP derivatives with improved flowability via thermo-mechanochemical or graft–epoxidation treatments, achieving good dispersion and modification effects at lower temperatures.

Overall, the dry process is more suitable when operational convenience and material recovery are the primary considerations in mix design, whereas the wet process offers advantages in achieving modification uniformity and storage stability. A promising approach is the “pretreatment + blending” strategy, in which WPP undergoes chemical or thermo-mechanochemical pretreatment to reduce melt viscosity and is then compounded with elastomers, allowing a balance of high-temperature stability and low-temperature toughness at lower processing temperatures.

4.2. Performance Evaluation of WPP-Modified Asphalt Mixtures

4.2.1. Moisture Damage Resistance

Moisture damage resistance is an important indicator for evaluating asphalt mixtures, reflecting the material’s ability to resist stripping, strength reduction, and structural deterioration under water action. WPP possesses strong hydrophobicity and low water absorption; its appropriate incorporation can reduce water penetration into the mixture, thereby enhancing water stability.

Abdulghafour et al. [91] studied hot-mix asphalt mixtures modified with WPP at 1%, 3%, and 5% by weight, systematically testing the indirect tensile strength (ITS) and tensile strength ratio (TSR). The results showed that at 3% WPP content, the mixture TSR reached 86.45%, approximately 7.7% higher than that of the base mixture (80.25%), indicating that an appropriate amount of WPP helps improve moisture damage resistance, Figure 12 shows the influence of the recycled polypropylene polymer on the TSR. However, when the content was further increased to 5%, the TSR improvement diminished or slightly decreased, suggesting that the enhancement effect has an optimal dosage range. Meanwhile, the incorporation of WPP slightly increased mixture stability, while flow values and air voids increased slightly, indicating some adjustment in the material structure. Behera et al. [92] found that asphalt mixtures containing 2% WPP fibers maintained a relatively high TSR even after freeze–thaw cycles, significantly outperforming the control group. This indicates that the reinforcing and water-resistant effects of WPP fibers can effectively delay moisture-induced damage. Buruiana et al. [79] reported that incorporating 0.1–0.6% micron-sized polypropylene particles into hot-mix asphalt showed that a moderate dosage (0.3%) effectively reduced water absorption and improved impermeability, further confirming the contribution of WPP’s hydrophobicity to moisture damage resistance. Lim et al. [93] in studying the recyclability of different waste-plastic-modified systems, also noted that polypropylene-modified mixtures outperformed polyethylene-modified systems in water stability and chemical resistance, providing theoretical support for the application of WPP under wet–hot conditions.

Overall, the incorporation of an appropriate and well-dispersed amount of WPP can significantly improve the moisture damage resistance of asphalt mixtures through mechanisms such as hydrophobic protection, void optimization, and interfacial reinforcement. However, the enhancement effect is jointly influenced by the dosage, dispersion, and environmental conditions. Future research could further explore the use of chemical coupling agents, nanofillers, or polymer-modified systems such as SBS to achieve synergistic effects of WPP and optimize long-term durability.

4.2.2. Low-Temperature Performance

Resistance to low-temperature cracking is a core indicator for evaluating the long-term reliability of asphalt mixtures in cold climates. Studies have shown that when the ambient temperature drops sharply, the viscoelastic properties of asphalt binders deteriorate rapidly, and thermal shrinkage stresses cannot be sufficiently relieved through rheological deformation, leading to thermal or fatigue cracking in pavements. Therefore, enhancing the low-temperature flexibility and stress-relaxation capacity of asphalt mixtures is a key research focus for ensuring pavement performance in cold regions.

For example, Buruiana et al. [79] experimentally incorporated micro-sized polypropylene (micro-PP) into hot-mix asphalt mixtures and found that while micro-PP improved mechanical strength and rutting resistance, it also increased stiffness and brittleness under freeze–thaw cycles and low-temperature conditions. Li et al. [94] systematically compared WPP from different sources and forms, showing that unmodified WPP, while enhancing high-temperature modulus, reduces low-temperature ductility; however, blending with rubber or elastomers can partially restore low-temperature flexibility. Soliman et al. [95] evaluated the low-temperature performance of recycled-plastic-modified (including PP) asphalt binders using BBR and DSR tests. They reported that binder stiffness increased and low-temperature sensitivity worsened with higher plastic content, but reasonable dosages still met Superpave low-temperature specifications. Qabur et al. [96] further studied the low-temperature fracture behavior of micro-PP (MPP)-modified binders and mixtures, finding that the morphology and dispersion of MPP significantly affect low-temperature cracking resistance, with well-dispersed systems maintaining higher fracture strain. Hu et al. [77] proposed surface functionalization of WPP via mechanochemical methods to improve compatibility with asphalt. Experimental results showed that the modified systems exhibited significantly enhanced low-temperature ductility and stress-relaxation capacity compared to unmodified WPP, where the macroscopic fracture patterns of RCAAMs containing PPMs at low temperatures were illustrated in Figure 13. Dalhat et al. [62] reported that WPP/low-density polyethylene (LDPE) blends, while improving high-temperature stability, could still maintain good low-temperature ductility. Guo et al. [78] found that compounding WPP with elastomeric modifiers, such as SBS or waste rubber powder, significantly increased fracture strain and flexibility at low temperatures, effectively mitigating brittleness.

In summary, most studies indicate that while the incorporation of WPP enhances the high-temperature stability of asphalt mixtures, it may increase low-temperature brittleness. However, chemical modification, mechanochemical activation, or blending with elastomers can effectively improve low-temperature cracking resistance, achieving a balanced performance profile.

4.2.3. High-Temperature Performance

High-temperature performance is a critical indicator for evaluating the serviceability of asphalt mixtures under hot conditions. It reflects not only the material’s ability to resist permanent deformation and rutting but also directly impacts the smoothness and service life of the pavement surface. Guo et al. [78] analyzed the rheology and microstructure of WPP/SBS co-modified asphalt and found that the blended system significantly increased the high-temperature modulus and rutting resistance of the binder, with the DSR parameter G*/sinδ showing a marked improvement. Li et al. [94] studied binders modified with recycled polyolefins from different sources and reported that WPP effectively enhances high-temperature stiffness and resistance to permanent deformation, although the effect is strongly influenced by polymer morphology and incorporation method. Zhang et al. [76] investigated the high-temperature performance of asphalt modified with RPE, WPP, and PP-g-MAH. They found that increasing the PP-g-MAH content significantly improved the high-temperature modulus (|G*|) of the composite system, with the most pronounced effect occurring in the 3–7% dosage range. Oyelere et al. [97] systematically evaluated the high-temperature performance of asphalt binders modified with recycled HDPE (rHDPE) and WPP. The results showed that the addition of these recycled plastics markedly enhanced rutting resistance, as evidenced by improvements in non-recoverable creep compliance (G*/sinδ) and high-temperature rheological modulus (|G*|). Qabur et al. [96] studied micro/recycled polypropylene (MPP)-modified binders and mixtures, observing that the melt/dispersion state and dosage of MPP significantly affect low-temperature performance, highlighting the need for modification or blending strategies to control low-temperature stiffness.

In summary, existing studies indicate that waste polypropylene (WPP) and its blended modifiers can significantly enhance the high-temperature performance of asphalt mixtures, including increasing high-temperature modulus, rutting resistance, and resistance to permanent deformation. The source of WPP, type of co-modifiers, and incorporation methods all influence the modification effect, and proper blend design and dosage control remain crucial for balancing high- and low-temperature performance.

4.2.4. Fatigue Performance

Fatigue performance is an important indicator for evaluating the long-term service behavior of pavement materials. It reflects not only the material’s ability to resist the initiation and propagation of microcracks under repeated loading but also directly impacts the structural integrity and service life of the pavement. During actual service, asphalt mixtures are often subjected to the coupled effects of temperature variations and vehicular loads, leading to complex viscoelastic deformations and micro-damage evolution. Therefore, improving the fatigue resistance of asphalt systems is crucial for extending pavement life.

Figure 14 illustrates the effect of PPMS on the fatigue performance of RCAAM. Guo et al. [78] found that incorporating WPP into base asphalt enhanced high-temperature stability while simultaneously improving low-temperature cracking resistance and fatigue-related properties. Vamegh et al. [98] experimentally investigated the fatigue behavior of asphalt mixtures modified with PP/SBR polymer blends and found that the addition of 5 wt% WPP–SBR composite extended the fatigue life by approximately 50% compared with conventional SBS-modified asphalt, highlighting the synergistic effect of WPP and SBR in delaying crack propagation and improving overall durability. Xue et al. [99] showed through fatigue testing that waste plastics (mainly PP) can effectively improve both fatigue and rutting resistance of asphalt mixtures. Zhou et al. [100] observed that PP-modified asphalt exhibits higher ductility and energy absorption under cyclic loading, thereby delaying fatigue failure. Additionally, Cai et al. [101] used rheological studies to indicate that the incorporation of recycled polypropylene reduces the relaxation rate of asphalt, enhancing structural stability under fatigue conditions.

Overall, these studies consistently indicate that the incorporation of WPP not only enhances the high-temperature stability of asphalt materials but also significantly improves their fatigue performance. However, the mechanisms by which different WPP sources, particle sizes, and surface modification methods affect fatigue behavior remain insufficiently understood. Future research could integrate molecular dynamics simulations, digital image correlation (DIC) techniques, and fatigue life prediction models to further elucidate the damage evolution and microscopic mechanisms of WPP-modified asphalt.

5. Summary and Recommendations

This review provides a comprehensive overview of the recycling and utilization of waste polypropylene (WPP) in asphalt pavement engineering. The main conclusions can be summarized as follows:

• From the viewpoints of material properties and recycling, WPP demonstrates outstanding mechanical strength, heat resistance, and chemical stability; however, its high crystallinity and hydrophobic nature restrict its natural degradability. While mechanical recycling remains the primary method due to its established efficiency and scalability, emerging techniques such as chemical recycling and thermo-mechanochemical upcycling offer promising potential for converting WPP into valuable functional additives for asphalt modification.
• From modification and performance enhancement, the addition of WPP to asphalt binders improves high-temperature performance, softening point, and rutting resistance but also increases viscosity and reduces low-temperature ductility. These adverse effects can be mitigated by incorporating compatibilizers like PP-g-MAH, reactive additives, or elastomeric polymers including SBS or crumb rubber to achieve balanced rheological and mechanical properties.
• The mixture performance indicated that the incorporation of a moderate and well-dispersed amount of WPP in asphalt mixtures can effectively enhance high-temperature stability and moisture resistance by improving hydrophobicity and interfacial bonding. However, excessive WPP content or poor dispersion may lead to increased stiffness and reduced crack resistance at low temperatures, emphasizing the importance of optimizing both dosage and dispersion for balanced performance.
• New emerging technologies such as thermo-mechanochemical and catalytic degradation present promising approaches for transforming waste PP into warm-mix additives with lower viscosity, reduced emissions, and improved compatibility. These innovations support the advancement of cleaner production practices and promote the realization of a circular economy within the pavement industry.
• Future research should focus on developing multi-scale models that integrate molecular dynamics simulations with experimental validation to better understand WPP–asphalt interaction mechanisms, optimizing pretreatment and blending techniques to lower energy consumption and improve storage stability, conducting comprehensive life cycle assessments (LCA) to evaluate environmental and economic benefits, and advancing standardization and field validation to facilitate large-scale engineering applications.

Overall, WPP shows great potential as a sustainable asphalt modifier, offering both environmental and technical benefits. However, critical challenges persist, such as the deterioration of asphalt performance at high WPP dosages and the complexity of using degradation treatment methods. Future research should prioritize the development of high-dosage WPP-modified asphalt and focus on low-energy, solvent-free WPP activation technologies to convert WPP into efficient warm-mix asphalt (WMA) modifiers. This is essential for realizing industrial-scale implementation of WPP.