Sustainable Approaches to Polymer-Modified Asphalt Binder: Comparison
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Due to periodic variations in temperature and heavy traffic loading, hot-mix asphalt (HMA) pavements undergo considerable distress during their service life. The rheological properties of asphalt binder, when subjected to complex physical and chemical processes, make it stiff and sometimes brittle, which ultimately plays a huge part in pavement deterioration. This phenomenon is commonly known as asphalt aging. Incorporating polymer modifiers with virgin asphalt can work as an effective means to change the binder properties and alleviate the issues related to asphalt aging. Different types of polymers, including elastomers, plastomers, and reactive polymers, can mixed in different combinations with the virgin asphalt to create polymer-modified binders (PMBs). In general, polymers are typically added to the virgin asphalt binder in PMB manufacturing at weight percentages ranging from 3% to 7%.

  • hot-mix asphalt
  • polymer-modified asphalt binder
  • waste polymers
  • binder modification
  • biopolymers

1. Introduction

Hot-mix asphalt (HMA) pavements are prone to three main types of distress (i.e., permanent deformation, fatigue cracking, and low-temperature cracking) during their service life, which are responsible for their poor performance. These types of distresses occur in flexible pavements because of traffic load and temperature differences that are associated with the rheological properties of asphalt. The alteration of the virgin asphalt binder’s properties through the incorporation of polymer modifiers is an effective means to mitigate distress-related issues. The incorporated polymer should transfer its intended properties to the polymer–asphalt system and alter the rheological and failure properties of the virgin asphalt, thereby mitigating the problems associated with rutting, fatigue, and thermal cracking. Rigidity, brittleness, elasticity, durability, resistance to accumulated damage, and storage stability are the fundamental properties of asphalt. The magnitude of these properties can be improved through the modification of asphalt binders [1][2].
Polymers are very large molecules with many atoms (copolymers) that include successive linking of one or more types of small molecules (monomers) into a chain or network structure (straight, linked, or crosslinked) [3]. The configuration and conformation of the monomers are responsible for the viscoelastic properties of the polymers. While the monomer configuration is changed when the chemical bonds are broken, the conformation is altered due to the rotation of molecules about the single bonds. The crosslinking phenomenon, which is the formation of weak chemical bonds between main chains through side chains, also affects the viscoelastic properties of the polymer [3]. A high degree of crosslinking induces “memory” within the polymer structure, which restricts the sliding of the main chains when elongated, which retain their initial shape after the stress is removed. Various polymer types can be added to virgin asphalt to modify its properties, including thermoplastic elastomers, plastomers, and reactive polymers, and polymer-modified binders (PMBs) are defined as mixtures of asphalt with one or more polymers. Polymers are usually added in percentages ranging from 3–7% by weight of the virgin asphalt during PMB production [4].

2. Incorporation of Waste Polymers in Binder Modification

In just 10 years, the consumption of polymer products has increased by 60%, from 230 million tons in 2009 to 368 million tons in 2019. This propensity results in a substantial waste stream that must be properly managed to safeguard the environment in order to encourage a circular economy, which would reduce production costs [5][6][7]. The incorporation of waste polymers with virgin asphalt may improve environmental sustainability by minimizing the amount of plastic and rubber waste, energy use, and the extraction of virgin asphalt, which is generated from crude oil. Almost all waste plastics (such as polyethylene tetrapthalate (PET), polystyrene (PS), polypropelene (PE), etc.) can be processed using the wet technique for binder modification; however, polyvinyl chloride (PVC) cannot be mixed with virgin asphalt, due to the risk of deleterious chloride emissions [8]. In a study by Pradhan et al. [9], waste polystyrene (PS) was utilized to modify the asphalt binder. The longevity of the asphalt could potentially be increased by adding PS to the asphalt binder, but there were issues with the paving process, and particularly with miscibility and phase separation. Researchers investigated these issues by modifying the virgin binder by including both waste polystyrene and trans-polyoctenamer (TOR) polymer. Sulfur was also utilized as a crosslinking agent throughout this modification process. Waste polystyrene (PS) and trans-polyoctenamer (TOR) were combined with the virgin binder at a mass ratio of 2:1, creating a modified binder known as polyoctenamer-modified polystyrene binder (PS-1). Then, the reseauthorchers utilized 0.1%, 0.5%, and 1% sulfur as the crosslinking agent to produce four different bitumen-modified mixture samples. According to the outcomes of the penetration and softening point tests, the addition of additives led to a slight increase in the virgin binder’s stiffness. The addition of sulfur substantially enhanced the modified binders’ elastic recovery and storage stability capabilities, allowing for the efficient anchoring of polymeric additives inside the asphalt matrix. As shown by the lower Jnr values and better average% recovery in the MSCR tests, the additives also showed enhanced high- and moderate-temperature characteristics. In the BBR tests, the modified binders showed less stiffness and higher m-values than the virgin binders. The best additive content for the crosslinking agent was 0.1% in PS-2, which outperformed the other evaluated binders in terms of rheological performance [8]. A very recent study conducted by Vargas et al. [10] aimed to develop a suitable asphalt additive by pyrolyzing plastic wastes such as HDPE and PP. It was discovered that only HDPE formed a wax that had a negligible effect on the bitumen’s flashpoint when PP and HDPE polymers were pyrolyzed. Then, both virgin asphalt and pyrolyzing plastic-waste-modified bitumen with 5% polypropylene were used to assess the wax formulation. According to the softening point and consistency measurements, the inclusion of wax enhanced the bitumen’s tolerance to higher temperatures. Furthermore, even though the wax made the bitumen more rigid, its impact was not as great as the polymer’s. As a result, the wax-modified binder outperformed the polymer-modified binder in terms of fatigue life and resistance to breaking at low temperatures. When the wax concentration reached 7%, segregation in the PMB decreased by 17% and did not negatively affect the segregation in the virgin bitumen. Additionally, it was discovered that the pyrolytic wax made from HDPE reduced the bitumen’s viscosity, requiring less energy to compact and mix the asphalt mixes. These results led to the conclusion that 7% is the ideal wax content for virgin bitumen and PMB. Segregation and increased viscosity were two major issues with the use of plastic waste as a bitumen component that were addressed using pyrolytic wax. Additionally, it was shown that pyrolysis pretreatment, as opposed to dry mixing, could permit better plastic utilization. Utilizing the pyrolyzing wax to reduce the temperature for mixing and compaction is a novel finding that has not been observed before. This technique has the potential to reduce the environmental effect of producing asphalt by reducing plastic waste and energy [10]. Another study conducted by Hu et al. [11] investigated the feasibility of using polyethyrene waste material tape as a bitumen mixture modifier. Researchers found that increasing the waste tape content in the bitumen mixture led to improved performance against rutting. Although the fatigue performance of the additive showed a positive trend at lower strain levels, increasing the waste tape content by 6% weight showed a downward trend in the fatigue performance. Hu et al. also found that waste-tape-modified bitumen showed poor performance against low-temperature cracking, and this modification can only be applicable where the temperature in general does not go below 32 degrees Fahrenheit [10]. Rahman et al. [12] conducted a review of the use of recycling waste in asphalt concrete and bitumen mixtures. In this revisewarch, the authoresearchers summarized how recent research has effectively incorporated plastics into binders, improving their rutting and fatigue performance. For example, using tire rubber powder can effectively improve the high-temperature performance of the asphalt binder, whereas the combination of waste cooking oil with palm oil fuel has the potential to replace the traditional bitumen binder by 5% [12][13]. The use of polymer-modified binders (PMBs) is limited to the surface layer of high-volume roads due to their higher market cost [12]. Reclaimed polymers can therefore be an alternative to reduce the production cost of polymer-modified asphalt binders. Brasileiro et al. conducted a review study to compile the latest enhancements of producing “Reclaimed-Polymer modified binder (RPMB)”. The reseauthorchers of the paper summarized how the most researched reclaimed polymers that are being used are polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), ethyl vinyl acetate (EVA), and ground tire rubber (GTR). It was found that polyethylene (PE) and ground tire rubber (GTR) are the most frequently used in producing RPMBs, as they are more available, stable, and have lower melting points than the mixing temperature [14].

3. Incorporation of Biopolymers in Binder Modifications

Due to their potential to improve the functionality and sustainability of asphalt pavements, the use of biopolymers in asphalt binder modification is essential. Biopolymers, which are made from renewable resources, are essential for creating environmentally acceptable and highly effective asphalt mixtures for creating sustainable road infrastructure. The two categories of biodegradable polymers are separated based on the method of degradation, i.e., synthetic and natural biopolymers [15]. Synthetic polymers are generally obtained from petroleum resources. Often these are referred to as biodegradable polymers. Synthetic polymers can be separated into oil- and bio-based polymers and are not very widely used in asphalt binder modification [15][16][17]. On the other hand, natural biopolymers, often referred to as “Bio-plastics”, can be mixed with the virgin asphalt and reduce the % of bitumen in the mixture [18]. The biodegradability of natural biopolymers varies depending on the source, i.e., plants/animals/microorganisms or commercially synthesized biological materials [15]. As the scope of this revisewarch is to discuss different polymers that are being used in asphalt binder modification, this section only discusses the “Biopolymers” that are currently being used in the asphalt industry. The most abundantly used biopolymers are called polysaccharides. Polysaccharides are formed by the linking of multiple monosaccharide molecules through glycosidic connections. Using polysaccharides to modify the virgin binder’s properties in warm-mix asphalt (WMA) have already shown great promise by enhancing the mixture’s durability and resistance to temperature; also, algae-derived polysaccharides (alginates) have shown the ability to rejuvenate aged asphalt in reclaimed pavement (RAP) [19][20][21]. Table 1 presents some recent knowledge about the incorporation of biopolymers in bitumen modification.
Table 1.
Recent literature about the incorporation of biopolymers in bitumen modification.
Major Class Biopolymers Recent Literature Mentioning the Use of Biopolymers
Polysaccharides Chitosan

Derived from chitin, which is found in different insects and shells [21][22]. Chitosan is an FDA-approved biopolymer used as a food additive [22].		 Mallawarachchi et al. used chitosan to partially replace the emulsifier by 10%−20% by weight. The chitosan–emulsifier mixture was then used to create an asphalt emulsion, and the results showed that the positively charged chitosan–amines and negatively charged asphaltenes interacted and increased the viscosity and stability of the mixture [20].
Natural Fibers

Generally derived from animals or plant bodies. Previous research shows that fibers reinforce the composite materials by incorporating their fibrous features in the structure of the composite materials [22][23].		 Kundal et al. added 0.4% sisal fiber with 5% bitumen content to asphalt mixtures. They found that the stability of the mixture increased until 0.4% but started to decrease after that [24]. Oyedepo et al. conducted similar research in 2021 and found that the stability of the mixture was highest when 0.2% sisal fiber was used [25]. Bonica et al. concluded that fiber derived from different sources of cellulose increased the rutting resistance in the asphalt mixture [26].
Starch

One of the most commonly used biopolymers from the polysaccharide group. Two types of glucose, namely, amylose and amylopectin, make up this biopolymer, and the flexibility and strength of materials made from starch increase with the amount of amylose present [27][28].		 Research conducted by Porto et al. showed that starch, when mixed with bitumen at 4.8% by weight, enhanced the mixtures rigidity and increased the resistance to high temperatures [18]. A recent study by Komba et al. verified the previous research and concluded that bitumen treated with 5%, 10%, and 16% starch improved the elasticity and rheological properties of the mixture [29]. Both studies concluded that starch-modified mixtures can be produced at lower temperatures and in less time [14][18][28].
Natural rubber (NR) Most of the natural rubber used in bitumen modification is derived from a very specific tree called Hevea brasiliensis. NR is an elastomer that resembles milky, runny latex from tree sap [30]. In a research study conducted by Krishnapriya et al., the reseauthorchers used 2% natural rubber by weight in bitumen modification, where the results showed improved resistance to rutting and the fatigue performance of the modified bitumen enhanced significantly [31]. Shaffie et al. conducted similar research, and the researchers found that incorporating 8% natural rubber in the bitumen modification reduced the stripping phenomena [32]. In general, utilizing natural rubber in bitumen modification has been proven to be economical and environmentally friendly [33][34][35][36]

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