Abstract
The utilization of recycled plastics in asphalt pavements has become increasingly popular because of its potential benefits for the environment and economy. Prior studies have concentrated on incorporating high- and low-density polyethylene into asphalt mixtures as an additive to enhance their rheological and mechanical properties. Nevertheless, new techniques have surfaced for utilizing post-consumer plastics (PCP) that cannot otherwise be recycled. One example of PCP is toner, an electronic waste that is challenging to recycle but contains polymeric components such as styrene-acrylate, styrene-butadiene, and polyester, making it a promising candidate for modifying asphalt binders. This research explores the feasibility and potential of modified post-consumer plastic (MPCP), predominately composed of toner, as an additive to improve asphalt binder performance and address issues such as rutting and low-temperature cracking. The research employs Superpave performance grading, phase separation, frequency sweep, and multiple stress creep and recovery testing to assess the effectiveness of different MPCP dosages. The results indicate that MPCP-modified asphalt binders exhibit increased stiffness and viscosity proportional to the MPCP content, resulting in improved resistance to permanent deformation. However, a 20% MPCP dosage level significantly increases phase separation and adversely affects the low-temperature performance grade (PG), while a 10% MPCP dosage does not affect PG. The research recommends limiting MPCP dosage to 10% for optimal storage stability, increased stiffness, improved rutting resistance, and minimal changes in cracking resistance. Overall, this study provides insights on the effective use of MPCP in asphalt binders and the potential influence on the performance of asphalt concrete mixtures.
Keywords
The Federal Highway Administration advocates the use of recycled materials in asphalt concrete pavements, highlighting the advantages of engineering, economic, and environmental benefits ( 1 ). This viewpoint is based on the asphalt industry’s 6-decade history of using recycled materials. Although some recycled materials have proven their effectiveness, research on the reuse of plastics, despite expanding, still experiences knowledge gaps that limit its widespread implementation.
There are several types of plastic, differing by chemical composition and classified by resin identification codes. Currently, industry and academia have focused on testing type 2, high-density polyethylene (HDPE), and type 4, low-density polyethylene (LDPE) plastics for potential performance. In general, previous research suggests polyethylene (PE) is the primary plastic suitable for incorporating into asphalt mixtures and has demonstrated effectiveness in improving rutting resistance, although few long-term performance data are available ( 2 , 3 ).
In 2020, the National Asphalt Pavement Association (NAPA) released a comprehensive literature review on using recycled plastics in asphalt binders and mixtures ( 4 ). The review analyzed over 110 research reports, journal articles, trade publications, and magazine articles, with approximately 70% of the literature published within the past 10 years. The United States, India, Malaysia, Canada, and China were the leading countries with the most literature documents. Of the 110 documents, HDPE and LDPE were the most studied type of recycled plastics for use in asphalt concrete pavements, followed by polyethylene terephthalate and polypropylene. Interestingly, LDPE and HDPE have high recycling rates, according to the Environmental Protection Agency ( 5 ). These plastics can be used, recycled, and reused in a PE loop. In contrast, other plastics contribute to the growing 18.5% of all municipal solid waste that is landfilled ( 6 ).
Background
New solutions have emerged for utilizing post-consumer plastics (PCPs) that cannot otherwise be recycled, such as waste toner that is polymeric in nature. Toner is the dry ink used in printing operations in the form of fine powder. Toner powder and cartridges are typically classified as electronic waste, making them difficult to recycle and contributing an estimated 350 million cartridges or 75,000 tons of waste each year in the U.S. ( 7 ). This waste is typically added to landfills as there is no application for better utilization. However, some researchers have attempted to find ways to reuse waste toner, as approximately 6,000 tons of unused carbon powder is released into the environment every year, in the U.S. alone. One approach is to blend it with asphalt binders as an additive, as the toner includes some polymeric components such as styrene-acrylate, styrene-butadiene, and polyester ( 8 ). The toner-modified asphalt binder has shown improved stiffness and viscosity, making it an appropriate option to modify asphalt binder and improve its rheological and mechanical characteristics, especially in areas where permanent deformation is a significant concern ( 7 , 9 , 10 ).
Solaimanian et al. employed a technique to determine the degree of modification of asphalt binders with waste toner containing styrene acrylic copolymers ( 9 ). Their approach considered factors such as blending time, performance grading, storage stability, and mixing and compaction temperature calculations to design the binder. Their research demonstrated that the blend’s stiffness and viscosity increase proportionally with the toner content. The mixture analysis also indicated higher strength and stability for the toner-modified asphalt concrete mix than for unmodified mixtures. The increase in binder stiffness at high temperatures is positive, since resistance to permanent deformation is increased. However, increased stiffness at low temperatures is unfavorable because of the increased potential for low-temperature cracking. However, results from toner-modified test sections demonstrated that these sections showed no significant distresses and high resistance to rutting.
Evaluations on fatigue resistance have been conducted to address the cracking potential. Notani et al. found that using up to 12% waste toner to modify neat asphalt binders enhances the number of loading cycles to fatigue failure, demonstrating the positive effects of waste toner modification on fatigue resistance ( 11 ). It was suggested that the optimum amount of waste toner is 12% by weight of asphalt binder, representing better fatigue behavior than higher or lower amounts of waste toner usage.
In the case of construction, Diamond reported problems with rolling, flaking, and poor adhesion when waste toner was added to the aggregate for a resurfacing project on I-15 in Nevada ( 12 ). In this study, the waste toner was simply added to the aggregate and may have been the cause of the overall dissatisfaction. Solaimanian et al. reported that blending dry toner into the aggregate is not recommended because of the challenges posed by the fine size of the toner particles and the health hazards associated with creating black dust ( 7 ). Additionally, adding waste toner directly may not engage the polymeric properties of the toner but rather behave as filler. Instead, it is recommended to incorporate the toner powder into the asphalt binder and stir for at least 2 h above the toner melting point to obtain a homogeneous material. In the case of high-shear blending, a shorter stirring period of 20–30 min can be used. Each toner-asphalt combination should be tested separately, and the material needs to be agitated before mixing with aggregates.
While the application of toner powder as an additive for asphalt binder is considered an environmentally friendly alternative to landfilling waste toner, its overall recycling process remains challenging. Despite the challenges in the recycling process, toner-modified asphalt binder has shown potential enhancements in stiffness and viscosity. However, concerns about its low-temperature response and potential for cracking still need to be addressed through further research and testing. Most studies have focused on the use of raw waste toner rather than engineered toner polymer, which adds further uncertainty to the effectiveness of using toner powder as an asphalt binder additive. Given the current lack of research and data on the long-term performance of toner-modified asphalt binders, there is a need for more research to support its widespread use. As such, efforts should focus on exploring the use of engineered toner polymer formulated to enhance the viscoelastic properties of asphalt binder while also providing an alternative to landfilling.
Study Objectives
The primary goal of this research effort is to investigate the impact of incorporating engineered toner polymer, a PCP, on the rheological properties of asphalt binder. The aim is to understand how the addition of this material will influence the binder’s stiffness and viscosity. The paper presents the experimental design employed to achieve this research goal and the analysis of the produced data.
Experimental Plan
Materials
In this research, performance grade (PG) 58-28 and PG 64-22 unmodified asphalt binders were acquired from SeaPort Sound Terminal, located in Tacoma, Washington, U.S. The asphalt binders were modified with engineered toner polymer, regarded as modified post-consumer plastic (MPCP) in this study, designed to recycle and repurpose hard-to-recycle and heavy plastic-laden materials. The resulting asphalt additive is obtained from recycling and reusing toner cartridges as well as small amounts of other PCPs, producing a non-hazardous powder containing styrene acrylate polymers, PE, carbon black, and colorants. The additive was processed into a solid material, 1–3 mm in diameter, as illustrated in Figure 1. Three different dosage levels of MPCP (0%, 10%, and 20% by weight of virgin binder) were added to the two asphalt binders to create binder blends for the evaluations.
Picture of the modified post-consumer plastic sample used for asphalt binder modification.
Asphalt Binder Modification Procedure
Each dosage level of MPCP was carefully weighed out and blended with the unmodified asphalt binder using a high-shear radial flow impeller at a speed of 6,000 rpm for 30 min at a constant temperature of 163°C. To conduct the mixing process, an aluminum container filled with 500 g of the unmodified asphalt binder was preheated in an oven at 163°C for 2 h. The preheated aluminum container was then transferred to a thermo-electric heating plate and wrapped with a thermo-electric heating pad (shown in Figure 2) to ensure a constant temperature of 163°C during the blending period. Two original samples and four MPCP binder samples were fabricated as summarized in Table 1.
Photo of the asphalt binder modification procedure setup.
Summary of Dosage Levels of Plastic Modification
Note: MPCP = modified post-consumer plastic; PG = performance grade.
Aging Procedure
Both the unmodified asphalt binders and the blended MPCP asphalt binders were subjected to short- and long-term laboratory aging. The short-term aging procedure was carried out according to AASHTO T 240-22 using the rolling thin-film oven to simulate the condition of the asphalt binder immediately after construction. The long-term aging process was conducted by placing the short-term aged asphalt binders in a pressure aging vessel (PAV) at a temperature of 100°C for 20 h, as specified in AASHTO R 28-22, to simulate in-service oxidative aging of the asphalt binders.
Test Methods
The experimental plan for this study included evaluating the asphalt binders by the Superpave performance grading system, multiple stress creep and recovery (MSCR) test, frequency sweep, and polymer separation test. The Superpave performance grading was conducted according to AASHTO M 320-22, to determine the three different service temperatures including high, intermediate, and low temperature. Superpave binder performance tests, including rotational viscometer, Cleveland open cup, and dynamic shear rheometer (DSR) for high and intermediate temperatures, and bending beam rheometer (BBR) for low temperatures were used to evaluate binder properties for different dosage levels of MPCP.
Performance grading was also determined as per AASHTO M 332 using the MSCR test to characterize the rutting resistance of the asphalt binders under high-temperature conditions. The MSCR test produces two primary parameters that characterize the permanent deformation of the asphalt binder: average nonrecoverable creep compliance (Jnr) and average percentage of recoverable strain. In essence, Jnr serves as a measure of the ability of the asphalt binder to resist permanent deformation in the mixture, with lower Jnr values indicating greater resistance to rutting. The average percent of recoverable strain measures the ability of the asphalt binder to recover after deformation, with a higher percentage indicating less permanent deformation.
The DSR equipment was used to conduct a frequency sweep test on unaged unmodified and MPCP asphalt binders, with 25 mm diameter specimens and 1 mm gap opening, at various test temperatures (60°C, 70°C, and 80°C) and loading frequencies (ranging from 0.01 to 100 rad/s) to determine their viscoelastic properties.
To ensure storage stability of the blended MPCP asphalt binders, ASTM D7173 was conducted to determine the separation tendency of the plastic under static heated storage conditions. An MPCP asphalt binder sample, directly after being blended, was poured into a sealed aluminum tube and held in a vertical position at 163°C for 48 h. At the end of the conditioning period, the sample was immediately placed in a freezer at –10°C for a minimum of 4 h. Then the tube was cut into three pieces, with the top and bottom pieces placed in separate containers and the middle discarded. The resulting top and bottom portions were subsequently tested using the DSR at the high-temperature grade of the unmodified binder.
Results and Discussion
Effect of Plastic Modification on Separation
Storage stability was determined based on top and bottom phase separation per ASTM D7173 tested using the DSR. The G* values were compared at both MPCP dosage levels (as illustrated in Figure 3). Separation was calculated by the difference between the top and bottom G* divided by the average G*. MPCP blends with base PG 64-22 binder were tested at 64°C, and blends with base PG 58-22 binder were tested at 58°C. For both binder types, the inclusion of 10% MPCP resulted in a significantly lower separation than the addition of 20% MPCP. Generally, a greater than 10% difference is an indication of separation. The addition of 10% MPCP resulted in a 6%–14% separation, whereas by doubling the MPCP dosage, the separation increased to 70%–85%. These results indicate that asphalt binders with 20% MPCP experience separation and have limited storage stability. During construction, this may pose a problem if the modified asphalt binder is prepared in advance and stored for an extended period. Previous research has concluded similar results when using toner powder in dosages over 10% and have recommended that the modified asphalt binder be agitated before mixing with aggregates because of the insufficient storage stability ( 9 ).
Phase separation test (ASTM D7173) values for the modified post-consumer plastic asphalt binders on top and bottom portions: (a) summarizes the testing on base PG 64-22 binder and (b) summarizes the testing on base PG 58-28 binder.
Effect of Plastic Modification on Performance Grading and MSCR
Superpave performance grading protocols were used to assess the MPCP effect on the rheological characteristics of the two asphalt binders. Table 2 provides a summary of the Superpave PGs. The unmodified asphalt binders were graded as PG 64-22 and PG 58-28, with a true grade of PG 66.4-25.9 and PG 59.7-31.3, respectively. Overall, the measured properties showed that the addition of 10% MPCP did not affect the final PGs but increased the traffic level from standard to heavy based on MSCR testing, while the addition of 20% MPCP affected both the high and low PGs and maintained the traffic level as standard at the new high PG. It is also important to note that the M-value controlled the low PG in all cases.
Summary of the Superpave Performance Grade (PG) Results by Modified Post-Consumer Plastic (MPCP) Dosage and Binder Type
Note: H = heavy traffic; S = standard traffic.
Figure 4 illustrates how the different levels of MPCP dosage affect the high, intermediate, and low temperatures for each binder type. Although a 10% MPCP dosage did not affect the PG, an increase in stiffness can be observed at all temperature levels through the change in true grade. Similarly, the stiffening effect is apparent with the 20% MPCP dosage, but it is more significant.
True grade for high, intermediate, and low service temperatures at varying modified post-consumer plastic dosages: (a) summarizes the true grade for the base PG 64-22 binder and (b) summarizes the true grade for the base PG 58-28 binder.
Previous studies have established a parabolic trend between the stiffening effect and toner level, indicating that higher percentages of toner lead to a more pronounced stiffening effect ( 9 ). This trend is also observed in this study for both binder types, demonstrating that the relative stiffening effect is not influenced by the base asphalt binder.
Delta Tc (ΔTc) is an asphalt binder parameter that indicates asphalt binder relaxation properties and can be determined by calculating the difference in critical low-temperature PG limiting temperatures using results from the BBR test, as outlined in Table 2. ΔTc can serve as an indicator of the impact of additives on asphalt binder response to aging. As asphalt binder ages, the ΔTc value becomes increasingly negative, signifying a loss of relaxation properties and greater control by the M-component. As noted previously, the low-temperature PG was controlled by the M-value in all cases in this study, leading to negative ΔTc values. Figure 5 illustrates the ΔTc values for each binder type and MPCP dosage level. The results show that as the MPCP dosage level increases, the ΔTc differential also increases, with a notable increase observed at the 20% dosage level. Research has shown that more-negative ΔTc values are linked to fatigue cracking and other distresses related to poor relaxation properties. Some researchers recommend a warning limit of –2.5°C and a failure limit of –5°C ( 13 ). Therefore, the large negative ΔTc values observed at the 20% MPCP dosage level suggest a greater potential for fatigue cracking, as they approach or surpass the proposed failure limit. This, along with the adverse impact on the low-temperature PG, suggests that high MPCP dosages above 10% may not be suitable for asphalt binder modification. Furthermore, as shown in Figure 5, a stiffer asphalt binder results in more negative ΔTc values compared with a softer asphalt binder, owing to its reduced relaxation properties.
Delta Tc at 20 h pressure aging vessel (PAV) aging by plastic modification dosage.
AASHTO M 332 was used to determine performance grading, utilizing the MSCR test, to evaluate the rutting resistance of modified asphalt binders at high temperatures. Figure 6 displays a comparison of the percent recovery at two stress levels for the two asphalt binders with different MPCP dosage levels. The data suggest that the use of MPCP enhances the elastic portion of the asphalt binder, leading to better recovery from deformation. There is a direct correlation between an increase in MPCP dosage and an increase in elastic recovery. For instance, the 20% MPCP asphalt binder exhibited a 2x improvement compared with the 10% modified asphalt binder. Notani et al. reported opposing observations when 12% toner modification resulted in better enhancement of the elastic portion of the asphalt binder than 16% toner modification ( 14 ). The difference may have been a result of using raw waste toner, rather than MPCP which is a manufactured plastic compound. Because of the raw waste toner’s composition, the remaining components characterized by higher melting points can dominate the rheological behavior when higher dosages are used. As a result, those components may not have dissolved in the asphalt blend, rendering the higher dosage blend with poor performance compared with the lower dosage blend. Similar behavior was found with modified asphalt using crumb rubber and PE ( 15 ). Additionally, Figure 6 indicates that increasing the test temperature results in a reduction in the recovery percentage for the two MPCP dosage levels because of the dominant role of the viscous portion of the asphalt binder at higher temperatures.
Percent recovery for asphalt binders with varying modified post-consumer plastic dosage levels: (a) summarizes the testing on base PG 64-22 binder and (b) summarizes the testing on base PG 58-28 binder.
The results in Figure 7 compare Jnr at two stress levels for the two asphalt binders with different MPCP dosage levels. The data suggests that the modified asphalt binders have minor improvement on resistance to rutting than the unmodified asphalt binders. Similar to the percent recovery, an increase in MPCP dosage correlates to a direct decrease in creep compliance. Therefore, a marginal enhancement of rutting resistance is observed at both MPCP dosage levels. Notably, the non-recoverable creep compliance showed no sensitivity to an increase in test temperature. The MSCR data demonstrated that the MPCP asphalt binders exhibited increased elastic response and rutting resistance compared with the unmodified asphalt binders, as evidenced by higher percent recovery and lower Jnr results. Furthermore, the MSCR data in combination with the ΔTc parameter revealed that the 10% MPCP dosage level leads to an asphalt binder with higher stiffness, improved rutting resistance, and minimal change in cracking resistance.
Non-recoverable creep compliance (Jnr) for asphalt binders with varying modified post-consumer plastic dosage levels: (a) illustrates the testing on base PG 64-22 binder and (b) illustrates the testing on base PG 58-28 binder.
As discussed earlier, the increase in percent recovery mirrored the direct decrease in non-recoverable creep compliance as MPCP dosage increased. These correlations demonstrate the observed increase in percent recovery is a result of the increase in stiffness caused by the MPCP, rather than a mere increase in elasticity. To verify this point, AASHTO R92 criteria were employed to evaluate the elastic behavior of asphalt binders through MSCR testing. If a significant elastic response is observed for the relevant non-recoverable creep compliance value, it would indicate that the asphalt binder has been modified by an elastomeric polymer, thereby exhibiting improved performance. However, as depicted in Figure 8, the percent recovery at 3.2 kPa falls below the minimum requirement of 15% for any given Jnr. Although the MPCP asphalt binders exhibited an increase in percent recovery, it cannot be concluded that the modification enhanced elasticity. To gain further insights into the effects of the modification, additional testing of elasticity using a ductilometer may be necessary.
Comparison of multiple stress creep and recovery (MSCR) non-recoverable creep compliance (Jnr) and percent recovery to assess elastic response.
Effect of Plastic Modification on Complex Modulus and Phase Angle
Complex modulus (G*) and phase angle (δ) were used to understand the effect of MPCP on the rheological properties and confirm observations made from the PG and MSCR testing. Testing was conducted using a frequency sweep test at 60°C, 70°C, and 80°C. Figures 9 and 10 show the complex modulus of the two unmodified asphalt binders and MPCP asphalt binders with the variation of loading frequency at a reference temperature of 60°C. Both figures indicate that an increase in MPCP dosage results in higher G*, with 20% MPCP dosage showing the highest G*. These results coincide with the increase in stiffness determined by the PG and MSCR testing and confirm the strong potential of improving the permanent deformation resistance using MPCP.
Base PG 64-22 and modified post-consumer plastic asphalt binders: (a) master curve and (b) black space diagram at a reference temperature of 60°C.
Base PG 58-28 and modified post-consumer plastic asphalt binders: (a) master curve and (b) black space diagram at a reference temperature of 60°C.
Figures 9 and 10 also include the black space diagram, which represents rheological data of asphalt binder in the form of complex modulus versus phase angle and has been used to interpret material behavior. The black space diagrams show a minor shift in the phase angle values for the modified asphalt binders. The effect of adding MPCP may be identified as an increase in the elasticity of the binder at higher temperatures; however, the shift is not significant and may be because of the variability in the DSR measurements. It is also possible that the phase separation of the modified asphalt binder hinders the test from fully capturing the elasticity of the modified binder. The minimal discrepancies in stiffness observed on the master curve could be attributed to this as well. Therefore, no significant conclusions can be drawn from the black space diagram.
Findings and Conclusions
The main purpose of this research was to investigate the impact of MPCP on the rheological properties of two asphalt binders and evaluate its potential influence on the performance of asphalt concrete mixtures. The effectiveness of two dosage levels of the plastic modification were evaluated using Superpave performance grading, phase separation testing, and MSCR testing. Based on the analyses of the data generated in this study, the following findings and conclusions were made:
The 10% MPCP dosage level generated minor phase separation using a base PG 64-22 binder but did not generate significant phase separation for base PG 58-28 binder. The 20% MPCP dosage level resulted in a significant increase in phase separation that indicates limited storage stability. Therefore, it is recommended that the modified asphalt binder be agitated well before mixing and the storage time of the asphalt binder is limited.
The addition of 10% MPCP did not affect PG enough for a full grade bump; however, the addition of 20% MPCP increased the high-temperature PG and adversely affected the low-temperature PG.
Higher percentages of MPCP lead to a more pronounced stiffening effect.
Increasing the MPCP dosage level increases the ΔTc differential, with a significant increase observed at the 20% dosage level, signifying a loss in relaxation properties.
Increasing the amount of MPCP in the asphalt binder increased the binder’s elastic response at high temperatures but did not meet the minimum value to be considered a significant response for the associated value of non-recoverable creep compliance.
The minor increase in elastic response was also evident in the phase angle but, similarly, was not significant to elicit greater elasticity. It is recommended that elastic recovery testing using a ductilometer be completed to properly determine the elastic effect of the modification.
The MPCP asphalt binders have lower nonrecoverable creep compliance compared with the unmodified asphalt binder. Enhanced rutting resistance is observed for both MPCP dosage levels.
The study suggests limiting the MPCP dosage to 10% by weight of binder for optimal rheological properties and storage stability, while cautioning against higher MPCP dosages to prevent poor relaxation properties and related distresses.
Overall, this study provides insights on the effective use of MPCP in asphalt binders, providing a promising approach to improve the rheological properties of asphalt binders while diminishing environmental risks associated with landfilling toner. However, there are insufficient data on the impact of MPCP on asphalt mixtures and their long-term performance. Therefore, it is recommended that additional research be completed on the effects of MPCP on mixtures’ rutting, fatigue, and thermal cracking characteristics, as well as comprehensive mechanistic-empirical pavement analysis.
Footnotes
Author Contributions
The authors confirm contribution to the paper as follows: study conception and design: P. Sebaaly, E. Hitti, E. Hajj, A. Hand; data collection: J. Rodriguez; analysis and interpretation of results: J. Rodriguez, P. Sebaaly; draft manuscript preparation: P. Sebaaly, E. Hitti, E. Hajj, A. Hand, J. Rodriguez. All authors reviewed the results and approved the final version of the manuscript.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was funded by Granite Construction Inc.
