Sustainable Modification of Bitumen Using Waste Toner and Lignin

Abstract

Integrating waste materials into road infrastructure is essential for environmental sustainability and resource efficiency. This study addresses the modification of short-term-aged 50/70-penetration-grade bitumen using two sustainable additives: waste toner powder and lignin. Waste toner was added at weight percentages of 4%, 8%, 12%, and 16%, while lignin was added at 15% and 20%. Since these modifiers have individual uses, this study examines how they may strengthen the oxidized binder. It focuses on extending the lifespan of the mixture by combining industrial and bio-based polymers. The main aim was to delineate the impact of these modifiers on the physical consistency, low-temperature flexibility, and microstructural morphology of the binder. The results show that both modifiers increase binder stiffness by reducing penetration at all modification rates. The resins in the waste toner enhance the polymer matrix, and the lignin’s aromatic structure increases the elastic components, improving high-temperature stability. However, ductility tests showed a reduction in elongation capability, suggesting a brittle state at lower temperatures. Also, waste toner additive is identified as the ideal modifier for high-temperature applications. SEM analysis illuminated the mechanisms underlying these performance modifications. Both additives had homogeneous distribution and good bitumen matrix interfacial bonding at lower concentrations.

1. Introduction

Environmental sustainability conserves resources, reduces pollution and waste, and safeguards ecosystems. Carbon dioxide emissions have historically been dominated by the Global North, particularly the US and EU [1]. Although climate change accountability may differ, the former Center for Global Development Senior Fellow believes decarbonization is a communal task [2,3]. High-quality aggregates, bitumen, and industrial fuels are unsustainable and harm the environment, requiring efforts to reduce environmental consequences and ensure sustainability [4,5,6]. Demand for electronics rises with the population. Worldwide, 8% of the one million cartridges discarded every day contain unused toner. Toner powder contains carbon, heavy metals, polymers, and resins [7,8]. Air pollution and respiratory illnesses are connected [9].

Waste toner mostly comprises residual toner particles that fail to attach to the paper during the printing process. It comprises a blend of plastic polymers, pigments, and various additives. The appropriate disposal of spent toner is crucial. Waste toner canisters are generally sealed and discarded in accordance with municipal waste management regulations to avert environmental contamination. Initiatives for recycling and proper waste management designed to reduce the environmental impact of toner waste have intensified. Approximately 18 million copiers and printers are in use worldwide, and 7200 tons of cartridges are consumed each year [10,11,12]. To solve the problem, waste toner must be collected and safely treated through recycling and heat treatment [13,14,15]. Toner waste production has increased worldwide as the printing industry has grown rapidly [16,17,18].

Bitumen, a non-renewable resource obtained from oil, is the primary binder in asphalt road construction. Although bitumen is a crucial construction material across various industries, oil refining primarily focuses on the production of fuels, lubricants, and chemical industry raw materials. Consequently, as oil supplies diminish and fuels and lubricants remain essential to industry and society, the trend of declining bitumen use is steadily increasing. A total rejection of bitumen, especially road asphalt, is presently unfeasible due to the absence of a quality replacement [19].

Research conducted by the Department of Transportation (DoT) in Texas, USA, evaluated the potential of using waste toner to improve the performance of asphalt binders and confirmed that it is possible to change the properties of asphalt mixtures by increasing viscosity. It has been observed that using approximately 2% to 10% waste toner relative to the binder weight in bituminous mixtures increases Hweem stability by reducing temperature sensitivity [20]. The addition of waste toner to asphalt binder decreased the penetration and ductility, while increasing the specific gravity, softening point, and rotational viscosity, regardless of the dosage rate [21]. Follow-up studies have shown that bituminous mixtures modified with waste toner improve low-temperature performance and resistance to rutting [22,23]. Studies conducted in recent years have confirmed an increase in performance rating (PR) and anti-rutting performance at high temperatures through the modification with waste toner. A review of the literature indicates that waste toner additives for asphalt binders and mixtures are likely to improve high-temperature performance. This effect is observed at high and low temperatures. Different printer models use different toner formulations, which are less useful in bitumen. For bitumen use, the waste toner composition must be controlled and maintained consistently. It is being investigated whether waste toner could be used as a bitumen improver or additive, but it is important to keep in mind that this use has not yet been widely accepted or standardized. More studies and tests are needed to fully understand its effects and ensure that its use in road-building materials is safe and feasible.

Also, the viscosity of the binder increases as the percentage of waste toner in the mixtures increases [24]. At the same time, the addition of waste toner to asphalt binder reduces penetration and ductility values, while the specific gravity, softening point, and rotational viscosity increase with increasing dosage rate. The rheological and creep recovery effects of waste toner addition to the asphalt binder were examined by adding it at 7%, 14% and 21% by weight. Accordingly, it has been observed that the rutting resistance rate of the binder increases at high temperatures [25]. Undoubtedly, adding waste toner to asphalt materials is likely to improve high-temperature performance, as shown by previous studies [23,25]. Properties related to rutting performance should be examined at high temperatures, as rutting is more pronounced at high service temperatures, when the binder exhibits highly viscous behavior [26,27].

The polymer reinforcement process is what makes this research show better performance. Waste toner is a complex combination of plastic polymers and resins. When mixed at high temperatures, it softens and mixes with the bitumen, making it thicker and heavier. The situation strengthens the internal network, leading to less rutting at high service temperatures.

Lignin is the main component of woody biomass and is well-suited for energy production. It is also suitable for many higher-value applications, including asphalt for road construction. Asphalt is traditionally made using bitumen obtained from crude oil. We demonstrate that plant-based lignin effectively enables a significant reduction in the carbon footprint of road construction. The second most prevalent plant polymer on the planet is lignin [28]. It is an aromatic biopolymer formed via C-C and C-O bonds from three 4-hydroxyphenylproanoid monolignols [29,30]. Recent research has focused on lignin, an abundant material known for its potential to synthesize aromatic chemical products. The goal is to improve lignin extraction and utilization to convert it into aromatic platform chemicals and products. This approach aims to achieve a more efficient, environmentally sustainable use of naturally abundant plant polymers. Lignin is biocompatible, environmentally sustainable, low in toxicity, and susceptible to enzymatic breakdown [31]. Approximately 70 million tons of lignin is produced annually worldwide as a by-product of the paper industry. However, less than 5% of this by-product is used as a binders or additives, while the remaining portion is burned [32].

Inclusion of lignin helps lower temperature sensitivity, increase the hardness, reduce ductility, reduce low-temperature cracking performance, and increase the viscosity of base asphalt [33]. It is also indicated that increasing lignin content enhances the cohesiveness and hardenability of the glue. The low-temperature flow properties of the binder diminished as the wood content increased. The rutting resistance of lignin-modified asphalt increased at elevated temperatures. Lignin substantially elevated the viscosity of the asphalt binder. The bond between lignin-modified asphalt and aggregate was superior [34]. Modified asphalt exhibits enhanced deformation resistance due to lignin’s augmentation of the asphalt’s elastic components, resulting in a more rigid base-asphalt binder. The incorporation of lignin into asphalt can delay its aging. Nonetheless, incorporating lignin may reduce the fatigue life of the asphalt [35]. Eight types of asphalt binders and two types of lignin commonly used in Kansas were selected and blended at different concentrations. The aging viscosity index was used to examine the antioxidant effect of lignin, and the results showed that the effect depended on the binder type. Therefore, the anti-aging effect of lignin should be investigated using more accurate observations of changes in the chemical structures of the asphalt binder. There are very few studies on the use of lignin as an antioxidant in bituminous binders [36].

The evolution toward a circular economy has accelerated research into the high-value uses of lignin, surpassing its conventional role as a low-grade fuel in paper mills. Recent breakthroughs have underscored lignin’s potential for developing innovative functional materials and high-performance alloys, utilizing its distinctive aromatic structure for chemical synthesis and material reinforcement. Incorporating lignin into bitumen modification is a strategic approach that leverages its biocompatibility and antioxidant properties to improve infrastructure durability and reduce the carbon footprint of road construction [37,38,39,40].

Recent comprehensive reviews have documented the broader landscape of bitumen modification, including both physical and chemical pathways [41,42,43,44,45]. However, there is a significant deficiency in the micro-morphological and rheological characterization of binders specifically modified with the combination of waste toner and lignin. This study fills this gap by examining how to use these materials as sustainable modifiers for short-term-aged 50/70-penetration-grade bitumen. The study examines the twin methods of stiffness augmentation and chemical–structural reinforcement in the binder matrix using waste toner at 4%, 8%, 12%, and 16%, and lignin at 15% and 20%. The main goal is to show how these modifiers affect physical consistency and flexibility at low temperatures by using standardized penetration and ductility tests. Scanning Electron Microscopy (SEM) will also be used to show how the particles are distributed and how they link to each other. The authors assert that this work is a significant contribution to literature, especially from a national standpoint, by offering novel insights into the synergy between polymer-rich industrial waste and bio-based aromatic polymers to improve infrastructure durability.

The beneficial aspect of lignin as a modifier is based on its ability to work with other aromatic compounds and its ability to act as an antioxidant. Lignin is an aromatic biopolymer made up of C-C and C-O bonds. It is chemically comparable to asphaltenes, which helps it bind more effectively at the interface. Mechanistically, lignin enhances the elastic components of the binder, making it harder, thicker, and less temperature sensitive. Also, lignin’s polyphenolic structure makes it a radical material, which helps to slow the aging process.

To the authors’ knowledge, although the contemporary literature documents bitumen modification options, some major research gaps remain. First, discarded toner can serve as a bitumen modifier, but its use is not yet widespread or standardized. Second, few studies have examined the rheological and micro-morphological alterations of bitumen treated with lignin. Finally, exploratory national data on waste-modified binders is unavailable. This study tested how waste toner and lignin affect the physical consistency and microstructural morphology of aged 50/70-penetration-grade bitumen to address these issues. This study evaluates waste toner (4%, 8%, 12%, and 16%) and lignin (15% and 20%) dosages to understand the interfacial bonding mechanisms and internal structural reinforcement needed for sustainable pavement infrastructure.

2. Materials and Methods

The bitumen used in this investigation was 50/70-penetration-grade. The bitumen was initially aged for a brief period to simulate the conditions expected in recycled or repaired pavement. In these situations, modifiers are frequently added to binders that have already undergone severe oxidation. The experiment was carried out in a laboratory at 163 degrees Celsius for 5 h. The bitumen’s physical characteristics can be seen in Table 1.

Table 1. Properties of bitumen.

Test Parameter	Value	Test Standard
Penetration	52.4	ASTM D5
Softening Point	52	ASTM D36
Viscosity (135 °C)	300	ASTM D4402
Specific Gravity	1.037	ASTM D70
Flash and Fire Point	305 °C	ASTM D92

Waste toner was collected from traditional office printers and copiers for recycling. Styrene–acrylate resins, polyesters, and pigments are the components that make up this substance, which is a complicated composition. The alkali (Kraft) lignin by-product was obtained from Ataman Chemicals in Istanbul, Turkey; its main properties are shown in Table 2.

Table 2. Properties of the used lignin (supplied by the provider).

Property	Value
Passing through 100 mesh (%)	98.7
Passing through 200 mesh (%)	97.1
Density (g/cm3)	1.2
Moisture Content (% mass)	1
Ash Content (% mass)	14
Purity (%)	305 °C

In this study, waste toner from conventional office printers and kraft lignin from paper mills were used. The selection of lignin type was based on its elevated aromatic content and its chemical compatibility with asphaltenes, thereby enhancing interfacial bonding within the matrix. Because solid waste particles must be transformed into a stable, homogeneous dispersion within a highly viscous binder, the selection of parameters for combining bitumen, toner, and lignin was motivated by the need to achieve this transformation. Mixing conditions must overcome significant internal friction while allowing the softening of the toner resins. This was necessary because the base material had already oxidized after a short period. Before bitumen modification, both materials were sieved, and pan-sized materials were used. To examine the effect of both materials, the bitumen was first subjected to short-term-aging. For this purpose, it was aged at 163 °C for 5 h (with a penetration value of 50/70). To ensure homogeneous mixing of both waste toner (Figure 1a) and lignin (Figure 1b) in the short-term aged bitumen, high-speed mixing was performed on the heating table. The composition of the mixtures by weight is shown in Table 3.

Figure 1. Preparation of modified bitumen with (a) waste toner, (b) lignin.

Table 3. Composition of mixtures.

Sample	Base Bitumen 50/70 (gr)	Waste Toner (gr)	Lignin (gr)	Total Mixture
Neat	100.00	0.00	0.00	100.00
4% Waste Toner	96.15	3.85	0.00	100.00
8% Waste Toner	92.59	7.41	0.00	100.00
12% Waste Toner	89.29	10.71	0.00	100.00
16% Waste Toner	86.21	13.79	0.00	100.00
15% Lignin	86.96	0.00	13.04	100.00
20% Lignin	83.33	0.00	16.67	100.00

The methodology combines the optimization of waste integration for environmental sustainability with the technical need to achieve suitable binder quality for pavement repair. By establishing these benchmarks—namely, high-temperature stability, aging resistance, and microstructural homogeneity—we elucidate the rationale for the chosen dosage levels of waste toner (4–16%) and lignin (15–20%) in the Introduction.

Waste toner was added to the aged bitumen at rates of 4%, 8%, 12%, 16% of bitumen weight, while lignin was added at rates of 15% and 20% of bitumen weight. A high-speed shear mixer with a 40 mm impeller was used in a 500 mL beaker to achieve uniform dispersion of waste toner and lignin. The mixing temperature was maintained at 150 °C to promote the softening of the toner resins and ensure sufficient fluidity of the aged bitumen. A rotating velocity of 3000 rpm, equating to a tip speed of roughly 6.28 m/s, was maintained for 30 min. This high-shear environment is essential for disaggregating lignin and toner clusters, hence averting agglomeration.

The detailed flowchart of the experimental program is shown in Figure 2. Also, the experiments applied to bitumen are shown in Table 4. Accordingly, the aged bitumen was modified at the above-mentioned ratios based on weight. After mixing homogeneously in a high-speed rotational mixer, penetration, ductility, softening-point and SEM analyses were performed for each mixing ratio.

Figure 2. Flowchart of the experimental program.

Table 4. Experimental Procedures.

Test Method	Specification/Standard	Testing Conditions
Short-term Aging	Laboratory Protocol	163 °C for 5 h
Penetration Test	ASTM D5	25 °C, constant load, and defined time
Ductility Test	ASTM D113	Standardized elongation rate
Softening Point Test	ASTM D36	Heating 5 ± 0.5 °C
Scanning Electron Microscopy (SEM)	Microscopic Analysis	High-resolution imaging

A penetration test is a standard laboratory test to determine the consistency of bitumen. This test aims to measure depth in millimeters. This is done by releasing a needle with a specified weight (defined in the specifications) at a constant temperature, load, and time into the test sample. The test can also indicate the behavior of bitumen under different environmental conditions. Lower penetration values generally indicate harder specimens that may perform better at higher temperatures, while higher penetration values indicate softer samples, that are useful at lower temperatures. The test was conducted in accordance with ASTM D5 (Standard Test Method for Penetration of Bituminous Materials).

A ductility test is a standard laboratory test for measuring the elongation of a bitumen specimen. Ductility refers to a sample’s capacity to lengthen at low temperatures without breaking or cracking. The main aim of this test is to figure out the elongation properties of bitumen under certain conditions. This test makes it easy to see how bitumen specimens behave at low temperatures. Since the road’s non-deformation in cold weather depends on the material’s flexibility, important information about road performance is obtained through ductility testing. Various organizations standardize bitumen ductility testing to ensure consistent, accurate test results across laboratories. The experiment was conducted according to ASTM D113 (Standard Test Method for Ductility of Asphalt Materials).

The ASTM D36 softening-point test measures bituminous binders’ thermal sensitivity and high-temperature stability. Pre-melted bitumen is poured into smooth-surfaced brass rings and placed in distilled water or glycerin for the test. Each binder ring has a 3.5 g steel ball in the center. The bath temperature is adjusted by 5 ± 0.5 °C per minute using a magnetic stirrer to ensure homogeneity. As the temperature rises, bitumen becomes more fluid and sags under the ball. The material’s “softening point” is the temperature at which the binder on the ball contacts the metal plate 25 mm below it.

Scanning Electron Microscopy (SEM) is used to analyze bitumen’s microstructure and morphology at the microscopic level. SEM is a powerful tool that provides high-resolution images and detailed information about the surface properties, particle distribution, and internal structure of bitumen samples. The main purpose of the bitumen SEM Test is to determine the microstructure of bitumen. SEM allows researchers to visualize the surface morphology of bitumen samples, providing information about texture, roughness, and surface properties. This information is valuable in understanding the properties that affect the adhesion, cohesion, and durability of bitumen in construction applications. Additionally, SEM analysis for modified bitumen helps evaluate the distribution and interaction of additives or modifiers within the bitumen matrix. Understanding how these additives affect the overall structure and performance of modified bitumen is helpful.

It should be noted that the decision to apply bitumen after short-term aging was made to mimic real-world conditions and pavement repair. In the field, modifiers are frequently added to binders that have experienced considerable oxidation. This study aims to demonstrate the capacity of waste toner and lignin to improve the mechanical properties of an oxidized matrix by employing short-term-aged 50/70-penetration-grade bitumen as the base for modification. This method is quite similar to what is done in industry, like Hot-In-Place Recycling, where modifiers are applied to existing pavement to make it structurally sound again. This sequence also allows a close look at lignin’s role as an antioxidant and its ability to stabilize the internal network of a binder that is already having problems.

3. Results and Discussion

This section presents the results of laboratory tests. It should be noted that the present work illustrates the efficacy of 15% and 20% lignin addition in enhancing binder stiffness and aging resistance; nevertheless, subsequent research using reduced dosage levels, such as 5% and 10%, would be advantageous. Such data would help create a more complete quantitative “content–property” link that could be used to determine the optimal quantity of additive for diverse environmental service conditions.

Figure 3a,b shows the changes in the penetration of bitumen—neat, modified with waste toner added, and with lignin added—after the short-term-aging test. The results show reduced penetration of unmodified-bitumen and modified-bitumen binders with both waste materials. Additionally, higher waste toner and lignin contents indicate a lower penetration value. In this case, while the modification rate increased with the addition of waste toner, it decreased at the same rate.

Figure 3. Penetration test results for modified bitumen with (a) waste toner, (b) lignin.

The ductility test results for waste toner- and lignin-modified bitumen are presented in Figure 4a,b. The addition of waste toner decreases the ductility of short-term-aged bitumen (Figure 4a). The results are compatible with the penetration test, which means that, with the modification of bitumen, it becomes more brittle. The same result is observed for lignin-modified bitumen.

Figure 4. Ductility test results for modified bitumen with (a) waste toner, (b) lignin.

Figure 5 shows variations in softening points by material type and percentage. The results for the modification with waste toner indicate a progressive improvement in the binder’s high-temperature resistance. Compared with the pure binder, the incorporation of toner at 4%, 8%, and 12% elevated the softening point to 52, 54, and 55 °C, respectively. The polymeric components and carbon black in the toner enhance the binder’s stiffness and thermal stability. A minor reduction in the softening point to 54 °C was noted as the toner level increased to 16%. This indicates that a 12% content is the “optimal dosage” for this mixture, and that levels above this threshold may result in performance degradation due to issues of homogeneity or binder matrix over-saturation. Figure 5b reveals a notable reduction in softening point as lignin content increases. The softening point of neat bitumen, initially at 50 °C, diminished to 45 °C with the incorporation of 15% lignin, and subsequently plummeted to 30 °C with the inclusion of 20% lignin. This suggests that lignin functions as a “plasticizer” in the binder matrix, reducing the material’s viscosity and adversely affecting stability at elevated temperatures. This change may enhance flexibility against low-temperature cracking but is deemed to elevate the risk of rutting in high-temperature areas.

Figure 5. Softening point test results for modified bitumen with (a) waste toner, (b) lignin.

Figure 6 shows the effect of waste toner addition on aged bitumen. SEM analysis shows a large difference in the fracture surface at the same magnification. Accordingly, the low amount of toner waste used indicates that there is a flat fracture surface, a strong bond at the bitumen interface, and that the waste toner is distributed homogeneously in the bitumen. On the other hand, increasing the amount of toner waste used causes irregular breakage and creates agglomeration. This indicates that the toner particles are not at the desired level of adhesion in the bitumen, indicating weak interface strength.

Figure 6. SEM analysis results for waste toner modified bitumen with (a) 4%, (b) 8%, (c) 12%, (d) 16%.

The noted reduction in penetration and ductility can be ascribed to two synergistic mechanisms: physical stiffening and microstructural reinforcing. Lignin, an aromatic bio-polymer akin to asphaltenes, fortifies the stiff component of the bitumen matrix, thus enhancing its resistance to deformation. As the resins and polymers included in waste toner mix with the bitumen’s polymer matrix, enhancing its viscosity and overall density. The increase in high-temperature stiffness compromises low-temperature flexibility. The solid particles of toner and lignin act as fillers that augment internal friction and diminish the mobility of the liquid phase of bitumen. At elevated concentrations, SEM images indicate agglomeration, resulting in highly concentrated stress sites that promote brittle fracture, as evidenced by the markedly reduced ductility values.

Figure 7 shows the effect of lignin on aged bitumen. SEM analysis images show a significant difference in the fracture surface at the same magnification. As the lignin percentage increases in aged bitumen, there is a flat fracture surface, a strong bond at the bitumen interface, and the lignin is distributed homogeneously in the bitumen. On the other hand, increasing the amount of waste results in irregular fractures and agglomeration areas. This shows that the lignin particles are not at the desired level of adhesion in the bitumen, indicating weak interface strength. The chemical compatibility between lignin and the bitumen matrix—due to their common aromatic characteristics and asphaltene-like structures—could be enhanced by choosing lignin from various botanical sources. By-products from annual plants such as miscanthus have been recognized as promising modifiers owing to their distinctive molecular compositions [45].

Figure 7. SEM analysis results for lignin-modified bitumen with (a) 15%, (b) 20%.

4. Conclusions

In this study, waste toner and lignin powder were added to aged asphalt binder to investigate their effects on the rheological properties and aging process of bitumen. According to laboratory testing results, the findings can be summarized as follows:

• Both modifiers significantly increase binder stiffness, as evidenced by a consistent reduction in penetration values at all modification rates. This effect is attributed to the reinforcement of the bitumen’s internal network.
• The reduction in elongation capability observed during ductility testing is an important result. As the proportion of additives in the binder increases, the binder becomes more brittle. The conclusion is that although performance at high temperatures is enhanced, lower concentrations are desirable to preserve flexibility at low temperatures.
• Scanning Electron Microscopy (SEM) demonstrated that both modifiers, added at lower concentrations, achieve homogeneous distribution and robust interfacial bonding. On the other hand, higher modification rates lead to particle aggregation and irregular fractures, indicative of localized stress concentrations that favor brittle behavior.
• The process of adapting bitumen has reached maturity, creating the possibility of implementing recycled asphalt pavement (RAP) applications. By utilizing waste toner and lignin as sustainable, bio-based antioxidants, it is possible to drastically minimize the carbon footprint associated with road maintenance and rehabilitation.

The results demonstrated that lignin and waste toner additions exerted contrasting mechanisms on the thermal stability of the bitumen; the incorporation of lignin produced a plasticizing effect within the matrix, decreasing the softening point by as much as 40% and compromising the material’s high-temperature resistance. Conversely, the alteration of waste toner progressively increased the binder’s rigidity, improving heat resistance, with the peak softening point reaching 55 °C at a 12% weight concentration. In conclusion, waste toner additive is identified as the ideal modifier for high-temperature applications requiring rutting resistance. In contrast, lignin additives soften the binder, limiting its high-temperature performance, and should be considered primarily for specific scenarios requiring low-temperature flexibility.