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Phenol-rich bio-oils as free-radical scavengers to hinder oxidative aging in asphalt binder

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Abstract

This study examined the capability of phenol-rich bio-oils to improve the durability of bituminous composites by hindering oxidative aging in asphalt binder. Bio-oils derived from six wood-based bio-sources were introduced to asphalt binder, each specimen was then exposed to thermal aging. A chemical aging index was measured using Fourier-transform infrared spectroscopy, and a rheological aging index was measured using a dynamic shear rheometer. The results of the study showed that all phenol-rich bio-oils reduced the extent of aging in bio-modified binder, however, the bio-oils’ efficacy against aging varied, partly based on the content of their phenolic compounds. Computational modeling showed that the efficacy of each bio-oil is also affected by the molecular structures of its phenolic compounds, electron-donor substituents in the phenolic compound structure promote its antioxidant activity. The study outcomes show that using phenol-rich bio-oils can enhance the durability of asphalt binder while promoting a carbon-negative built environment.

Introduction

Asphalt binder (called “binder” in this paper) is used as an adhesive to hold aggregate materials together, to form bituminous composites used in roads, roofs, airports, and bridge decks. The organic nature of binder makes it susceptible to chemical aging during its service life in outdoor settings. The chemical aging of binder is generally thought to be a result of irreversible polymerization, oxidation reactions, and the evaporation of lighter components. (Traxler, 1961), (Bell, 1989) Oxidative aging is one of the critical factors contributing to the deterioration of a binder's mechanical properties and performance. (Lu and Isacsson, 2002), (Qin et al., 2014) Oxidation results in embrittlement of asphalt under thermal and mechanical stress, promoting pavement fracturing or cracking and eventually leading to pavement failure. (Wright, 1965), (Isacsson and Zeng, 1998, Sirin et al., 2018) Aged asphalt is less durable due to its susceptibility to moisture and wear. The asphalt binder durability depends on the intermolecular interactions between asphalt components and the potential of the asphalt components to resist oxidation. (Petersen, 2000)

Changes in the relative quantities of the SARA (saturate, aromatic, resin, and asphaltene) fractions of binder show how its chemical composition changes during oxidative aging. The ratio of polar components (such as asphaltenes) to nonpolar components is higher in oxidized binder . (M. Mousavi et al., 2016), (Le Guern et al., 2010) Oxidative aging is believed to cause further molecular agglomeration of asphaltene components due to increasing aromatization of reactive compounds in the binder and the introduction of new polar functional groups on components of the binder. (Le Guern et al., 2010) Thus, increases in both the asphaltene content and the asphaltene agglomerates are significant characteristics of oxidized binder. (Le Guern et al., 2010, Siddiqui and Ali, 1999, Lemarchand et al., 2013) Changes in the arrangement of a binder's components during aging are reflected in changes in its physicochemical and rheological properties, giving rise to deterioration of the pavement's mechanical properties and performance. (Lu and Isacsson, 2002), (Qin et al., 2014), (Cavalli et al., 2018), (Pahlavan et al., 2018)

In the United States, annual expenditures for asphalt pavement are approximately $11.7 billion. With the depletion of non-renewable petroleum-based sources of asphalt binder and advances in refining that leave less binder at the end of the refining process, there is increased demand for renewable alternative asphalt binder resources. Recent studies have promoted the use of bio-oils derived from biomass resources such as animal waste, wood-based biomass, algae, and waste cooking oils as partial or full replacements of binder. (Yang et al., 2013, You et al., 2011, M.A. Raouf and Williams, 2010, Fini et al., 2011, Podolsky et al., 2016) In addition to several applications for bio-oils proposed in the literature, such as catalytic cracking and hydrodeoxygenation to stabilize, condition, and upgrade bio-oils to fuels and raw chemicals, (B. Valle et al., 2019) another plausible beneficial use is the application of bio-oils as a partial replacement of asphalt binder to increase the durability of asphalt binder, while reducing the cost and frequency of asphalt pavements’ repair and rehabilitation. Cost estimation for the similar conversion of biomass to bio-oil has shown that the overall production cost of bio-oil is $0.54/gal, which is economically competitive with the cost of conventional asphalt binder from petroleum refining. (Fini et al., 2011) Economic analysis reported in the literature showed that bio-oil production from plant-based biomass can be commercially practicable, and the scale-up of the conversion process is favorable from an economic point of view. (Patel et al., 2019, Xin et al., 2016, Inayat et al., 2022) However, bio-oil production cost can also be influenced by parameters such as feedstock, processing methods, and labor. (Oudenhoven et al., 2016) Most of the facilities documented for the production of bio-oils are at pilot scales, so the cost estimate is rough, and it is believed that economies of scale can further reduce the cost to be more competitive with that of petroleum-based counterparts. Federal and state incentives, mandates, and strategies in support of a carbon-neutral economy can further increase the attractiveness of bio-based alternatives.

Our research team conducted a comprehensive study of the role of bio-oils from plant or animal sources in the treatment of oxidized binder; we evaluated from both rheological and morphological viewpoints the efficacy of the bio-oils to restore the desirable physical performance characteristics of binder and increase its resistance to cracking. (Pahlavan et al., 2018), (Pahlavan et al., 2020), (F. Pahlavan et al., 2019) The efficacy of a bio-oil as a binder rejuvenator was evaluated based on the bio-oil's capability to replenish lost compounds and deagglomerate large molecular assemblies such as the asphaltene stacks that are intensified during oxidative aging. (F. Pahlavan et al., 2019, M. Mousavi et al., 2016, F. Pahlavan et al., 2019) Studies show that the efficacy of bio-oils at deagglomerating oxidized fragments and dispersing them in the binder is greatly dependent on the bio-oil's molecular composition, which is affected by the nutrient composition of the initial feedstock. (Pahlavan et al., 2020), (F. Pahlavan et al., 2019), (Fini et al., 2017) Such evaluation can help develop bio-rejuvenator recipes that effectively target aging-related aggregation and disrupt the strong interactions of polar functionalities in heavily aged binder.

Aside from a bio-oil's capacity to rejuvenate aged binder and restore its morphological and rheological properties, the chemical composition of a bio-oil modifier affects the long-term performance of bio-modified binder by affecting the aging mechanism. (S. Hosseinnezhad et al., 2019), (Hosseinnezhad et al., 2020) So, the resistance of a bio-modified binder against oxidative aging is greatly affected by the biomass resource used to make the bio-oil modifier. (S. Hosseinnezhad et al., 2019) For example, some bio-oils that show promise in retrieving the desirable physicochemical properties of aged binder also contain molecular species that are susceptible to oxidative damage and reduce the mixture's resistance. (S. Hosseinnezhad et al., 2019, Hosseinnezhad et al., 2020, Zhang et al., 2018, M.A. Raouf and Williams, 2010) Previous studies focusing on the mechanism of resistance to oxidative aging confirm the source-dependence of a bio-rejuvenator's durability against oxidation agents. (S. Hosseinnezhad et al., 2019), (Hosseinnezhad et al., 2020) A high polarizability of bio-oil fragments in a bio-rejuvenated binder leads to a higher propensity of the bio-rejuvenated binder to oxidative aging. Thus, to hinder oxidative aging, it is important to identify the molecular fragments in bio-oils that are responsible for preventing the progress of oxidation reactions. Understanding molecular mechanisms through which antioxidant molecules interact with oxygen-centered radicals would aid in optimizing bio-oil formulations to hinder oxidative aging in the binder and increase dosage effectiveness. Therefore, the long-term effect of bio-oils as modifiers on the durability of asphalt pavement in terms of its susceptibility to oxidative aging warrants further investigation.

Our previous findings showed that bio-oil components can defer oxidative aging of the vulnerable components of binder through a dual-protection mechanism: first, the bio-oil components that are less reactive to oxidation act as a protective shield against oxidation; second, the bio-oil components that are highly reactive to oxidation become the primary targets for oxidation. (M. Mousavi et al., 2016) Designing a bio-oil formulation made from biomass that contains components with “antioxidant” activity enhances the bio-modified binder's resistance against oxidative aging while promoting resource conservation and a biomass value chain. Antioxidants that trap free radicals can quench active radicals generated during aging, terminating the reaction sequences for asphalt oxidation. (Wright et al., 2001, Jongberg et al., 2012, Tejero et al., 2007, Pfaendtner and Broadbelt, 2007, Dizhbite et al., 2004) Petroleum-derived carbon black has been found to delay photodegradation in binder via free-radical scavenging. (Ghasemi-Kahrizsangi et al., 2015) Therefore, it is logical to use bio-oils containing compounds with the ability to scavenge the free radicals produced during oxidative aging.

Modifying high-sulfur binder with phenol-rich bio-oil decreases the rate of stiffening of binder during curing time. This shows that phenolic compounds can trap polysulfide radicals in the binder and consequently hinder the crystallization of sulfur. It has been shown that phenolic compounds can boost the effect of sulfur to improve the thermo-mechanical properties of bituminous composites through activating sulfur radical interactions in the bulk. (Mousavi and Fini, 2021) Polymerization of phenolic compounds present in the raw bio-oil might contribute to the deactivation of a catalyst used in a reaction process such as steam reforming of bio-oil for sustainable hydrogen production. (B. Valle et al., 2019) So, separation of phenolic compounds from bio-oil can increase catalyst stability due to mitigation of phenolic polymerization and coke deposition over a catalyst. (B. Valle et al., 2019) In addition, due to the antioxidant nature of phenolic compounds, isolated phenolic compounds could be more effective than bio-oils as antioxidants for asphalt binder. The phenolic family is a major group of antioxidants that can deactivate free radicals and thereby terminate radical chain reactions causing oxidation. Phenolic compounds have the potential to be oxidized to phenoxyl radicals either by oxygen or by other reactive oxygen species generated during oxidative aging. Phenoxyl radicals (or their quinone derivatives) can trap more oxygen-centered radicals, including peroxyl radicals, ROO•. Phenolic compounds have a good record of antioxidant activity against photo-oxidation and dark oxidation of binder, (Martin, 1968) and they also improve the resistance to thermo-oxidation of sulfur-containing materials such as the rubber matrix. Studies of the physical and rheological properties of modified binder showed that adding phenols to bitumen improves its aging resistance; (Wu et al., 2017) this was evidenced by a lower gain in stiffness when exposed to aging. In addition, adding phenols to bitumen led to longer service life, improved rutting resistance, and improved resistance to moisture in asphalt mixtures containing co-polymers. (Dessouky et al., 2013) Pyrolysing of the lignin-rich content of plant-based biomasses results in a high contribution of phenolic compounds in the extracted bio-oil. Binder modified with bio-oils made from plant-based biomass such as miscanthus showed higher resistance to UV aging, probably due to the presence of components that absorb ultraviolet light and scavenge free radicals formed during aging. (Hosseinnezhad et al., 2020)

This study investigates the efficacy of phenol-rich bio-oils made from six plant-based resources on the performance and durability of bio-modified binder against oxidative aging, by examining the extent of change in the physicochemical and rheological properties of bio-modified binder samples. The durability comparison is performed after simulating long-term oxidative aging on bio-modified specimens by placing them in a pressure aging vessel (PAV). A PAV uses elevated temperature and air pressure to simulate in-service oxidative aging of binder, where exposure to atmospheric oxygen causes chemical changes that control the hardening of binder. (Sirin et al., 2018) The study further relates a binder's resistance to oxidative aging to the structure and antioxidant activity of the predominant phenolic compounds in the bio-oils from woody biomass, by modeling the hydrogen-transfer mechanism and O‒H bond dissociation using quantum-based computations in the framework of density functional theory.

Section snippets

Materials

PG64–22 with the properties reported in Table 1 was used as the binder in this study. The plant-based bio-oils were produced using pyrolysis by researchers at the University of Seoul in Seoul, South Korea. The bio-oils were from six plant sources: pine bark (PB), walnut shell (WS), peanut shell (PS), coconut husk (CH), birch (BR), and fir (FR). To prepare bio-modified binder samples, 10% of each bio-oil by weight of binder was mixed with neat binder at 135 °C for 30 min using a benchtop shear

Results and discussion

It should be noted that the aging mechanism involves not only oxidation but also chain scission, aromatization, carbonation, and agglomeration of asphaltene molecules. (Hung and Fini, 2019) The extent of oxidation (which is the main focus of this study) is commonly tracked based on the amount of increase in carbon-oxygen bonds and sulfur-oxygen bonds formed as a result of aging. From these two indicators, we chose the carbon-oxygen bond, which is indicative of carbonyl functional groups.

Conclusion

To promote a carbon-negative built environment for roads, this study focused on extending the life of asphalt roads by using phenol-rich high-carbon bio-oils; bio-oils made from biomass are up to 77% carbon. Using experiments and computational techniques, this study had two phases: first, it evaluated the efficacy of six bio-oils made from plant-based biomasses to delay long-term oxidative aging in bio-modified asphalt binders; second, it related the durability of a bio-modified binder to the

Authors contribution statement

Below please see the statement of contribution from co-authors.

  • Farideh Pahlavan performed the literature review and conducted molecular modeling and analysis and developed the write-up.

  • Anthony Lamanna provided review and editing as well as guidance with analyzing the industry implications of the research.

  • Ki-Bum Park performed data curation and data analysis.

  • Sk Faisal Kabir performed laboratory experiments including rheological study and FTIR characterization.

  • Joo-Sik Kim: supervising experiments

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Science Foundation (Award Number 1935723). The authors acknowledge the valuable assistance of Amirul Rajib and Albert Hung with Arizona State University.

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