Integrating Recycled Acrylonitrile–Butadiene–Styrene Plastics from Electronic Waste with Carbon Black for Sustainable Asphalt Production

Abstract

As the global demand for electronic equipment continues to grow, many devices are being replaced more frequently, resulting in a rapid rise in electronic waste (e-waste), now the fastest growing waste stream worldwide. Motivated by this, the objective of this study is to present an environmentally friendly method to recycle acrylonitrile–butadiene–styrene (ABS), one of the most common e-waste plastics, by using it for asphalt production. In contrast to earlier methods of plastic-modified asphalt production involving complex pretreatments or complimentary additives unsuitable for plant-scale use, this study aims to demonstrate a practical, low-cost solution through the use of carbon black. This approach included physically pretreating ABS plastics for size reduction and incorporating waste tire-derived carbon black to promote effective dispersion in asphalt during wet modification. The rheological properties of the e-waste-modified asphalt were subsequently assessed. The test results indicated that recycling ABS plastics with a blending content of 5% alongside 5% carbon black can enhance cold-weather cracking resistance and high-temperature anti-rutting performance of asphalt. The enhancement can be attributed to the proper preparation procedures of ABS plastics and the addition of carbon black, which can further improve the performance by promoting the proper dispersion of plastic particles in asphalt. The outcome of this study indicates that recycling e-waste plastics through asphalt production can lead to more green and sustainable asphalt construction, reduce total construction costs, and most importantly enhance performance.

1. Introduction

With widespread global economic development and technological advancements, new electrical or electronic equipment products are being continuously introduced to the current global market, while the previous products are becoming obsolete at a fast pace. In recent years, the growing urbanization and mobility, further industrialization, and higher levels of disposable incomes have led to a rapid increase in the consumption of electronic equipment worldwide. While the global market for electronic equipment continues to grow, many devices are being replaced more frequently. After the usage and disposal of electronic equipment, a waste stream, which contains both hazardous and valuable materials, is generated. This waste stream, which includes common electronic products such as computers, televisions, monitors, VCRs, copiers, fax machines, hard drives, and other similar products is referred to as electronic waste (e-waste). The volume of e-waste grows rapidly every year, and today, e-waste is experiencing the most rapid growth among all types of waste on Earth [1,2,3,4,5].

The disposal and management of e-waste is an extremely challenging problem worldwide owing to its high consumption rate, large volume, short life span, and non-biodegradable structures. Moreover, the rapid growth of technology is resulting in the obsolescence of many electronic products each and every day. According to the “Global E-waste Monitor 2020” report, a striking 59 million tons of e-waste was generated worldwide only in 2019. The global e-waste monitor projected these numbers to increase to 67.5 million tons in 2023. In addition, data indicate that global e-waste generation has increased by 10.1 million tons since 2014 and is expected to reach 82.3 million tons by 2030. On the other hand, according to the reports, the formal documented collection and recycling of e-waste in 2019 was only 10.25 million tons, which is only 17.4% of the total e-waste generated. The statistics indicate that the recycling efforts of e-waste are not keeping pace with its global growth. The increasing volume of e-waste poses serious environmental and health risks due to the presence of hazardous substances, comprising toxic metals and long-lasting organic pollutants like flame retardants and dioxins [2,6]. On the other hand, e-waste also contains a variety of valuable materials including plastic that can be recovered and reutilized; however, recycling plastics from e-waste, which can make up as much as 20% of total e-waste, is often more difficult than recycling plastics from conventional sources, primarily because of the presence of flame-retardant additives [4,7]. Therefore, it is vital to not only increase the recycling rate and environmentally friendly management of these waste streams, but also to explore alternative applications that allow for the more sustainable utilization of e-waste [2,8].

Concurrently, the rapid expansion of transportation networks and highway infrastructure in today’s world is closely linked to the increased use of bituminous materials [9]. As a result, recycling of waste plastics is considered as one of the most ideal approaches that can have numerous economic and environmental advantages for asphalt industry [10]. Plastics can be integrated into asphalt pavements using two primary methods: the wet process and the dry process. In the wet process, plastic waste is blended directly into the asphalt binder as a modifier. In contrast, the dry process involves adding plastic materials to the asphalt mix itself, either as a partial substitute for aggregates or as a performance-enhancing additive. Selection of a suitable mixing temperature is an important factor in the mixing process of plastics and the asphalt binder since the melting temperature of various plastics can exhibit significant variation, influenced by factors such as the source of the plastic and its thermal history. Overall, waste plastics with melting points lower than the production temperatures used in hot mix asphalt plants are generally considered more suitable for incorporation through the wet process. Conversely, the dry method, involving the incorporation of waste plastic particles before adding the asphalt binder, is better suited for hard and rigid plastic types characterized by higher melting points [8,11].

Generally, in order to have a successful wet modification procedure, several key stages such as physical or chemical pretreatment methods should be carefully planned based on each plastic type. These methods can potentially play a vital role in the compatibility of the two materials and the performance of the final e-waste plastic-modified asphalt binder [12]. However, various limitations, notably a lack of compatibility between asphalt and plastics, phase separation and low storage stability, and poor cold-weather cracking resistance have been impeding the successful application of plastics in asphalt [7,11]. The differences in the properties in terms of density, molecular weight, polarity, and solubility can directly play a role on the compatibility and overall performance. Thus, the application of pretreatment methods or the incorporation of other materials that can potentially improve compatibility are vital factors in the successful application of plastics [11,13]. Alongside the reutilization of plastics, another area of interest has been the application of carbon-based materials, such as carbon black, which can enhance the compatibility between asphalt and plastic particles by minimizing density differences [11,14,15].

Utilizing plastics recovered from electronic waste in roadway infrastructure is one of the topics that has been investigated by multiple researchers. The incorporation of these persistent and hazardous waste materials into pavement construction applications can not only relieve the pressure on landfills and decrease their environmental impacts, but also lead to substantial savings in resources [8,16]. As mentioned earlier, in order to achieve a satisfactory performance, recycling of plastics into asphalt needs to be accompanied by suitable pretreatment methods or the incorporation of other materials that can potentially improve compatibility. As a result, methods such as functionalization with the aid of irradiation, grafting, and the addition of radical initiators and crosslinking agents such as sulfur and reactive polymers such as trans-polyoctenamer (TPOR) have been explored by different researchers [17,18,19,20,21,22,23,24].

Overall, studies have indicated that incorporating e-waste plastics like ABS and high-impact polystyrene (HIPS) can improve high-temperature properties, particularly by enhancing its resistance to rutting, though the major concern of cold-weather cracking resistance still remains. For example, Mohd Hasan et al. studied the cold-weather cracking performance of untreated and treated e-waste-modified asphalt binders by the addition of radical initiators and reported that the treated e-waste plastics do not necessarily improve the thermal cracking resistance of the conventional asphalt binders and exceed their low-temperature performance [8,25]. At the same time, some promising results have been reported for the application of carbon black in asphalt. In a study conducted by Cong et al., incorporating carbon black into styrene–butadiene–styrene (SBS) polymers has been shown to improve both the anti-aging and high-temperature performance, while also enhancing its thermal and electrical conductivity [15]. In general, it has been reported that carbon black reduces the difference between conventional asphalt and plastic particles in terms of density, which also leads to better compatibility and storage stability [11].

In summary, while many of the incorporation methods of plastics into asphalt show promising results, there is a lack of a simple, cost-effective, and practical approach that can promote the recycling of plastics in pavement construction by enhancing the compatibility and different aspects of the performance. In essence, while many pretreatment methods have demonstrated promising results, their practical implementation remains limited. For example, techniques such as irradiation or the use of radical initiators often require trained personnel and strict safety protocols, making them unsuitable for asphalt plants or construction sites. Accordingly, carbon black offers a promising alternative that could help overcome the limitations of existing pretreatment techniques. Motivated by the aforementioned fact, this study aims to present a novel approach to recycle acrylonitrile–butadiene–styrene (ABS), one of the most common types of e-waste plastics, into the asphalt binder by focusing on two major steps in the modification process: physical pretreatment methods for the preparation of ABS plastics prior to mixing with asphalt and the incorporation of carbon black to improve the compatibility and performance.

2. Materials and Methods

2.1. Production of Plastic-Modified Asphalt with Carbon Black

The bulk ABS plastics from e-waste that were recycled from electronic products such as computer circuit boards are shown in Figure 1. These plastics were acquired from a recycling facility in Grand Rapids, Michigan (Grand Rapids, MI, United States). PG 58-28 asphalt binder, which is a common type of asphalt binder used in the Upper Peninsula of Michigan, was selected as the base binder for the studies. Incorporation of plastics into asphalt includes several key stages that can potentially play a vital role in the compatibility of the two materials and the performance of the produced plastic-modified asphalt. As mentioned earlier, the first stage includes the crushing, shredding, and pulverizing of the bulk plastics to reduce their size. Increasing the brittleness of the plastics in the grinding process can also result in smaller particles. This can be performed by adding liquid nitrogen to reduce the temperature below the glass transition temperature of plastics and freeze the particles. Glass transition temperature (Tg), which is a property of amorphous plastics or the amorphous portion of semicrystalline plastics, is the temperature below which the plastics behave like rigid materials. In other words, when the ambient temperature of the plastics is below Tg, the molecular chains of the plastics are frozen in place and behave like solid glass. As a result, grinding plastics, which are in a solid glassy state, can yield smaller particles [8,25]. After reducing the size of the bulk plastics, the smaller particles can be sieved to obtain a plastic powder for the modification of the asphalt binders.

After the acquisition of bulk e-waste plastics, the particles were pretreated with liquid nitrogen to rapidly reduce the thermal condition of the plastics well below the glass transition temperature (Tg, approximately 105 °C), thereby inducing a brittle, glassy state that was sustained during the grinding process. This pretreatment enabled the efficient shredding of ABS plastics into finer particles during pulverization using an industrial grinder. After the size reduction, the shredded plastics were sieved, and the ABS particles passing through sieve No. 60 (250 µm) were obtained for the modification process. The size reduction process of ABS particles from bulk e-waste plastics into particles smaller than 250 µm is demonstrated in Figure 2.

ABS is an amorphous polymer, thus lacking a true melting point. In general, ABS plastics exhibit a broad melting range and undergo melting between 190 and 270 °C [26], which is higher than the preparation temperature of modified asphalt in the wet process (140 to 180 °C). As a result, to fully melt and disperse ABS particles in a hot asphalt binder, the mixing temperature for blending of the asphalt binder and ABS particles should be selected to be in the melting range of the ABS particles. However, considering the effect of excessive mixing temperature and prolonged blending procedures on the further aging of asphalt, the mixing time was controlled to be around 15 min for ABS particles. On the other hand, to reassure the proper dispersing of the melted particles in the asphalt matrix, the blending speed was selected as 5000 revolutions per minute (RPM).

For the production of ABS plastic asphalt, after heating the neat PG 58-28 binder to 180 °C, the sieved ABS particles were added and manually premixed with the neat binder in a container. Next, the asphalt/plastic blend was transferred into a controlled temperature chamber with a high shear mixer and the temperature was increased to 200 °C. As the temperature of the blend increased to 200 °C, it was observed that ABS particles started to melt and disperse in asphalt. The full melting and dispersion of ABS particles took place in approximately 7 min. Moreover, to further improve the performance of ABS-modified asphalt by ensuring a uniform and proper dispersion of plastics, carbon black additives were added to the asphalt and plastic mix. The same procedures were repeated, and carbon black and ABS particles were simultaneously added to the heated asphalt.

Carbon black, used as a complementary additive in this study, was sourced from Bolder Industries (Boulder, CO, USA), and produced through the processing of post-consumer and post-industrial tires and rubber scrap. Figure 3 shows the initial form of carbon black particles used in this study as agglomerated granules. Although initially agglomerated, the particles can be readily broken down into fine components smaller than 150 µm, corresponding to material passing a No. 100 sieve. Figure 4 illustrates the dispersed form of carbon black particles prior to their incorporation into plastic-modified asphalt. As shown in

Table 1. Summary of blending contents of ABS plastic-modified asphalt.

Asphalt Sample	ABS Content	Carbon Black Content
2%ABS	2%	0%
2%ABS + 2%CB	2%	2%
5%ABS	5%	0%
5%ABS + 5%CB	5%	5%

, the blending content of the ABS plastics was 2% and 5% by the total weight of the asphalt binder, and the dosage of carbon black was 100% of the blending content of the ABS particles (e.g., 5% ABS and 5% carbon black by the total weight of asphalt).

2.2. Aging Procedures

The short-term and the long-term aging of asphalt binder samples were replicated using standard procedures outlined in AASHTO T240 and AASHTO R28, involving the Rolling Thin Film Oven (RTFO) and Pressure Aging Vessel (PAV), respectively [27,28]. To simulate short-term aging, approximately 35 g of liquid binder was placed into RTFO bottles and exposed to hot air circulation at a rate of 4000 ± 200 mL/min while rotating at 15 ± 0.2 rpm for 85 min at 163 °C. These RTFO-aged samples were subsequently analyzed using the Dynamic Shear Rheometer (DSR) to evaluate their resistance to rutting. For long-term aging, around 50 g of RTFO-aged binder was placed in PAV pans and subjected to elevated pressure (2070 kPa) and temperature (100 °C) for 20 h. The resulting PAV-aged binders were then tested using DSR and the Linear Amplitude Sweep (LAS) method to assess their fatigue performance.

2.3. Asphalt Binder Cracking Device (ABCD)

Cold-weather cracking performance of the asphalt binders was assessed using the Asphalt Binder Cracking Device (ABCD) following AASHTO TP 92. This test identifies the temperature at which thermally induced cracking occurs. To begin, binder samples were poured into invar rings equipped with strain gauges and temperature sensors, then placed inside an air-cooled environmental chamber. The chamber first stabilizes at 25 °C before initiating a controlled cooling process that lowers the temperature from +25 °C to −60 °C at a uniform rate of 20 °C per hour. Throughout the test, both temperature and strain data are continuously recorded. Cracking is identified by a sharp drop in measured strain, indicating stress release within the sample. The corresponding temperature at this point is reported as the binder’s cracking temperature.

2.4. Dynamic Shear Rheometer (DSR)

The Dynamic Shear Rheometer (DSR) test, following AASHTO T315 [29], was employed to evaluate the viscoelastic properties of asphalt binders across a range of temperatures and loading frequencies. This test was performed on unaged, RTFO-aged, and PAV-aged samples. For unaged and RTFO-aged specimens, a 25 mm spindle with a 1 mm gap was used, whereas PAV-aged samples were tested using an 8 mm spindle with a 2 mm gap. The data obtained from these tests will be used to analyze the binders’ resistance to deformations under various service conditions.

2.5. Linear Amplitude Sweep (LAS)

To analyze how the asphalt binder responds to repeated loading, the Linear Amplitude Sweep (LAS) test was performed using the same Dynamic Shear Rheometer (DSR) setup, following the procedures outlined in AASHTO T 391-20 [30]. Testing was carried out at an intermediate temperature of 25 °C, utilizing an 8 mm diameter plate with a 2 mm testing gap. The LAS test results were then analyzed using the viscoelastic continuum damage (VECD) model to quantify the fatigue life of the plastic-modified binders by calculating the rate of damage accumulation.

3. Results and Discussion

3.1. Low-Temperature Performance

The ABCD test results including the cracking temperature for base PG 58-28, solo ABS-modified, and ABS–carbon black-modified samples are presented in Figure 5. In general, lower cracking temperatures are an indication of better cold-weather cracking resistance. Overall, it was observed that the addition of ABS plastics leads to a better low-temperature cracking resistance performance than the base PG 58-28 binder. On the other hand, the addition of carbon black further enhanced the cold-weather cracking performance of the plastic-modified binder, indicating the positive role of this additive. For example, the cracking temperature of base binder reduced (improved) by −3.8 and −7 °C after the addition of ABS plastics and the blending content increased from 2 to 5%. Additionally, these values were −5.2 and −9.1 °C for asphalt samples containing both ABS and carbon black, with the sample containing 5% ABS and 5% carbon black demonstrating the best crack resistance performance among the tested samples.

To further explain the positive effect of carbon black on cold-weather cracking resistance, it has been reported that the incorporation of materials with elastomeric characteristics, such as rubber, can compensate for the brittleness introduced by plastics [11,31]. In this study, carbon black was sourced from rubber scrap, and its residual elastomeric characteristics contributed to enhanced cracking resistance in plastic-modified asphalt.

3.2. High-Temperature Performance

The DSR results for the unaged and RTFO-aged rutting factor (G*/sinδ) values of the base PG 58-28 and ABS-modified asphalt samples in the temperature range from 52 to 70 °C are summarized in Figure 6 and Figure 7, respectively. Generally, higher values of rutting factor are desirable in terms of high-temperature anti-rutting performance. For the unaged asphalt samples, the addition ABS plastics and an increase in their blending content leads to an increase in the rutting factor values of the asphalt when compared to the base PG 58-28 asphalt binder. When the usage dosage of both ABS and carbon black were low (e.g., 2% ABS and 2% CB), the influence of carbon black was relatively low. As the ABS content increases to 5%, there is a significant enhancement in the rutting factor values of the samples.

Furthermore, it was observed that the addition of carbon black can further enhance the rutting resistance performance, with samples containing 5% ABS and 5% carbon black showing the best rutting resistance performance. As shown in Figure 7, the positive effects of carbon black after RTFO aging of the asphalt samples is also notable, with the sample containing 5% of ABS plastics and carbon black demonstrating the best anti-rutting performance. At 58 °C, the high service temperature of the base binder, the incorporation of 2% and 5% ABS increased the rutting factor of the base binder by approximately 77% and 108%, respectively. On the other hand, the rutting factor further increased by approximately 28% for the 2% ABS binder and by 21% for the 5% ABS binder when carbon black was added to plastic-modified asphalt, highlighting the beneficial effect of carbon black when used in combination with waste plastics. In general, it was concluded that both samples containing carbon black (e.g., 2% ABS + CB and 5%ABS + CB) show better rutting resistance performance than the samples containing only ABS plastics, with the sample containing 5% ABS and 5% carbon black showing the best rutting resistance performance. Finally, according to the superpave criteria, the asphalt binder should have a rutting factor higher than 1 and 2.2 kPa under unaged and RTFO-aged conditions, respectively. In contrast to the base PG 58-28 binder failing these criteria at 64 °C after RTFO aging, all of the samples containing ABS or carbon black passed these criteria.

3.3. Intermediate-Temperature Performance

Table 2. DSR results for fatigue factor values (kPa) vs. temperature at 10 rad/s.

Temperature	25 °C	22 °C	19 °C	16 °C	13 °C
PG58-28	4602	6509	8909	11,965	15,941
PG58-28 + 2% ABS	3599	5322	7679	10,788	14,756
PG58-28 + 5% ABS	4579	6417	8976	12,419	16,841
PG58-28 + 2% ABS + 2% CB	4193	5884	8088	10,954	14,646
PG58-28 + 5% ABS + 5% CB	5847	8303	11,373	15,313	20,370

presents the fatigue-related results from the Dynamic Shear Rheometer (DSR) tests conducted on PAV-aged samples across a temperature range of 16 °C to 25 °C. In general, lower fatigue factor values are preferable as they indicate increased resistance to cracks occurring at intermediate temperatures. The results indicate that the introduction of 2% ABS enhanced the fatigue characteristics of the base asphalt, leading to a reduction in the fatigue factor value. When the ABS content was further increased to 5%, although the fatigue factor values for the samples rose, the performance of ABS-containing samples remained superior relative to the base binder between 22 °C and 25 °C. At 19 °C, the fatigue factor values of samples with 5% ABS were marginally higher than those of the base asphalt, suggesting a comparable performance. Additionally, the inclusion of carbon black resulted in elevated fatigue factor values, indicating its adverse impact.

To further analyze the fatigue behavior of the long-term-aged base and e-waste-modified samples, the LAS test was conducted on the PAV-aged binder samples. The fatigue life of the samples was calculated using the viscoelastic continuum damage (VECD) model to quantitatively assess the effect of aging. Additionally, the number of cycles to failure under repeated fatigue loading (Nf) was determined. Generally, higher values for Nf are desirable in terms of resistance against fatigue cracking. The VECD analysis was conducted based on utilizing the 2.5% and 5% strain levels. The final results for ABS samples are tabulated in Figure 8 and Figure 9. As shown, the fatigue life of all samples was much higher under the 2.5% strain level when compared to the 5% stress level, which indicates that asphalt binder samples can exhibit better fatigue resistance performance under a lower strain level. Additionally, the initial addition of ABS plastics resulted in a lower fatigue life compared to the base asphalt. Most importantly, with introduction of carbon black at a blending content of 2% into ABS-modified samples, there was a significant enhancement in fatigue performance under both strain levels, with the improvement being more significant under the 2.5% strain level. On the other hand, further increasing the content of carbon black to 5% did not show a notable improvement in samples with 5% ABS.

4. Conclusions and Recommendations for Future Research

This study aimed to present a novel and sustainable approach to further increase the recycling rate of e-waste plastics in asphalt pavements. Furthermore, the role of carbon black in the compatibility and rheological performance of the plastic-modified asphalt with one of the most common e-waste plastics, which was ABS, was investigated. Based on the rheological tests performed across low, intermediate, and high temperatures and the subsequent analysis of the results, the conclusions reached were as follows:

• Preparation procedure and size reduction of e-waste plastics led to improvements in the cracking resistance of the base binder by resulting in lower (better) cracking temperatures; however, the incorporation of carbon black further enhanced the low-temperature performance, with the sample containing 5% ABS and 5% carbon black exhibiting the best low-temperature performance. This indicates that proper mixing conditions as well as the incorporation of carbon black can eliminate one of the major drawbacks in the application of plastics, which is poor low-temperature performance.
• The addition of e-waste ABS plastics improved the high-temperature rutting resistance performance by leading to higher rutting factor values when compared to the base binder. Additionally, samples containing carbon black demonstrated similar performance under unaged conditions; additionally, these samples performed better after RTFO aging, resulting in higher rutting factor values.
• For fatigue behavior, LAS results showed that the initial incorporation of ABS led to a lower fatigue life compared to virgin asphalt. However, when carbon black was introduced at a blending content of 2% into the ABS-modified samples, there were notable positive effects on fatigue performance under both strain levels. The improvement was particularly notable under the 2.5% strain level. Conversely, increasing the blending content of carbon black to 5% did not result in a significant improvement in samples with 5% ABS.
• The inclusion of carbon black, as well as the careful planning of the size reduction of the plastics and modification process of asphalt and plastics, proved to enhance the compatibility between these two types of materials, by leading to improved performance in terms of both high- and low-temperature behavior. Previous research also reported that carbon black contributes to better workability, compatibility, and mechanical behavior due to its favorable interaction with asphalt. Additionally, carbon-based additives have also been recognized for their inherent compatibility with bituminous materials, and it has been indicated that carbon black can reduce the density differences between polymers and asphalt [11,32,33]. Higher incorporation dosages of carbon black are recommended to lead to more effective results, particularly in regions susceptible to cold-weather distresses such as Michigan, as well as areas experiencing rutting-related distresses during warmer seasons.

The findings of this study demonstrated that the modification of asphalt binder with ABS plastics from e-waste can be a simple and practical approach that can increase the sustainability in asphalt construction. When compared to traditional modifiers such as styrene–butadiene–styrene (SBS), recycled plastics can offer a more cost-effective alternative. As an example, the high price of SBS significantly increases the production cost of modified asphalt, limiting its use in large-scale or budget-constrained projects. Moreover, the incorporation of recycled plastics has been associated with reductions in both energy demand and greenhouse gas emissions, further supporting their viability as a sustainable and economical substitute for conventional polymer modifiers [34,35,36]. Thus, the combination of plastics with carbon black derived from end-of-life tires presents an even more economical option compared to traditional asphalt modification.

Additionally, substituting 10% of the asphalt binder with plastics and carbon black could significantly cut down production costs for asphalt, particularly given the recent rise in prices for conventional and modified binders containing commercially available polymers. Potential future directions include the morphological evaluation of e-waste-modified asphalt containing carbon black and ABS plastics, as well as investigating the influence of carbon black on other types of e-waste plastics such as high-impact polystyrene (HIPS). Other potential directions include conducting life cycle assessment (LCA) for background processes of e-waste-modified asphalt and studying the leaching behavior of different waste plastic-modified asphalt mixtures and binders.