Preparation and Performance Evaluation of a Low-Fume Asphalt Binder

Abstract

Asphalt fume emissions cause significant environmental hazards during the preparation of hot-mix asphalt. In this study, experimental investigations were conducted employing a reactor vessel to simulate asphalt fumes under controlled conditions. Asphalt fumes were obtained through an integrated system comprising glass fiber filter cartridges and an impinger absorption bottle. Quantitative analysis was then conducted using gravimetric analysis and UV-Vis spectrophotometry. Through systematic monitoring of compositional changes in asphalt binder fractions, the fume emission characteristics during in-plant mixing operations were quantitatively correlated with the following processing parameters: temperature, airflow rate, and mixing duration. Comparative evaluation revealed optimal performance from a ternary compound inhibitor containing cuprous chloride, ditert-butylhydroquinone, and ferric chloride in mass proportions of 4:4:2. At a critical dosage of 0.6 wt%, this compound inhibitor demonstrated significant reduction in total particulate matter emissions without compromising asphalt binder properties. In addition, comprehensive performance characterization through rheological testing and thin-film oven aging (TFOT) showed that the modified low-fume asphalt binder maintained equivalent or improved performances compared to a conventional asphalt binder.

1. Introduction

As a primary raw material for pavement construction, asphalt binder is an extremely complex mixture composed of hydrocarbons and their derivatives with different molecular weights. During road paving at high-temperature conditions, a large number of volatile substances, i.e., asphalt fumes, are generated [1]. Baird et al. [2] and Thives et al. [3] found that asphalt fumes contain high levels of volatile organic compounds (VOCs), which are harmful to human health and environment. Previous research regarding the influence of asphalt fume revealed the following: (1) accelerated health deterioration among aged workers (>20 years exposure) in coal tar asphalt operations [4]; (2) 23–41% elevated cancer risk in petroleum asphalt handlers across seven nations [5]; (3) dose–response correlations between occupational exposure limits (OELs > 0.35 mg/m³ 8 h TWA) and respiratory morbidity rates, particularly in roofing asphalt applicators [6].

Asphalt binder, the principal bonding material used in road engineering, is a highly complex colloidal system consisting of hydrocarbon derivatives with heterogeneous molecular weight distributions [7]. In its applications, particularly at high-temperature conditions (140–180 °C) during hot-mix asphalt mixing and paving procedures, substantial emissions of hazardous fumes are generated through volatilization processes [8]. These emissions, classified as occupational aerosols, predominantly contain respirable particulate matter and gaseous polycyclic aromatic hydrocarbons (PAHs) [9]. Notably, benzo[a]pyrene and other PAH congeners exhibit confirmed carcinogenic potential through inhalation exposure pathways, establishing fume generation as the primary vector for toxic component migration from asphalt matrices [10].

Asphalt fumes are primarily composed of particulate and gaseous components. The former can be enriched using fiber filtration materials, while the latter can be absorbed by organic solvents. Previous studies have shown that particulate components account for more than 99% of the total mass of asphalt fumes, making them the primary factor influencing the variation patterns of asphalt fumes [11]. In this study, the collection of asphalt fumes was achieved using a combined method of glass fiber filter cartridges and solvent absorption, aiming to capture all the components in asphalt fumes as comprehensively as possible.

Extensive investigations have been undertaken regarding the primary factors responsible for asphalt fume emissions. Chen et al. [11] investigated the influencing factors of asphalt fume generation and their respective impact levels; their study showed that the fume emission quantity increases with prolonged heating time. The fastest fume generation rate occurs during the first 30–60 min; a decelerated increase is observed between 60 and 120 min, and regenerated acceleration emerges after 120 min. This phenomenon originates from alternating oxygen-rich/oxygen-deficient conditions within the experimental apparatus, leading to cyclic oxidation and dehydrogenation reactions.

Yang et al. [12] employed TG-FTIR-coupled analysis and revealed that asphalt begins significant VOC emissions at 135 °C, while PAHs demonstrate a step growth at 175 °C. Li et al. [13] verified through a closed-loop fume generation system that 160 °C serves as the inflection point for SBS-modified asphalt fuming, and beyond this temperature, benzene homolog concentrations in fumes increase exponentially.

Wang et al. [14] employed a GC-MS analysis and demonstrated that when temperature increased from 155 °C to 185 °C, the content of saturated hydrocarbons decreases by 28.6%, the proportion of aromatic hydrocarbons increases from 42% to 67%, and the concentration of benzo[a]pyrene shows a 15-fold increase.

Research on suppress asphalt fume generation primarily focuses on two approaches. The first method involves warm mix technology, which aims to reduce fume generation by lowering mixing temperatures. Zhao et al. [15] demonstrated that incorporating Sasobit warm-mix additive could delay the optimal fuming temperature of PG64-22 asphalt by 12 °C and reduce PAHs emissions by 39.8% at 190 °C. The second approach utilizes fume inhibitors, which mainly fall into two categories: adsorbents and free radical inhibitors. Almeida-Costa et al. [16] reported that adding 3% nano-montmorillonite (functioning through adsorption) could decrease fume production by 54% in 70# asphalt binder (Pen grade) at 180 °C.

Substantial studies have been carried out and found that reducing the asphalt fume emissions from the source is the most effective method. Among them, adding fume-suppressing modifiers to asphalt is a commonly used method [17]. Some studies have used adsorption materials to capture the generated asphalt fumes, such as tourmaline composite and activated carbon [18]. Some have employed adsorbents to capture the generated asphalt fumes, achieving odor reduction [19], and others have used inhibitors to suppress the free radical reactions in asphalt [20]. Based on the asphalt fume generation mechanism, it has been found that the most effective approach to suppress asphalt fumes is to target the source of their production.

Although laboratory simulation has become a primary method for studying asphalt fumes [21], existing research has largely been limited to single inhibitors, with a lack of systematic exploration of compound inhibitor formulations based on synergistic mechanisms and their source-reduction efficiency. This study aims to address this research gap by innovatively proposing and developing a novel composite asphalt fume inhibitor based on a synergistic mechanism. The formulation is designed to simultaneously suppress the polymerization of volatile organic compounds while utilizing radical scavengers to block oxidation pathways. The core objective of this research is to validate the synergistic enhancement effect of this composite formulation in achieving source reduction in asphalt fumes through laboratory simulation, thereby providing a more efficient technical solution to address practical fume pollution issues at paving sites.

2. Material and Methods

2.1. Base Asphalt Binder

The experimental program employed a penetration-grade 70# asphalt sourced from intermediate-sulfur crude oil. This binder demonstrates representativeness in physicochemical characteristics matching Chinese specifications. The basic performance indexes were tested and the results are listed in

Table 1. Properties of asphalt binder.

Item	Test Result	Test Method
Penetration (25 °C, 100 g, 5 s), 1/10 mm	65	ASTM D5/D5M-20
Penetration index	−1.34	ASTM D5/D5M-20
Ductility (15 °C, 5 cm/min), cm	150+	ASTM D113/D113M-17
Ductility (10 °C, 5 cm/min), cm	31.4	ASTM D113/D113M-17
Soft point, °C	48.6	ASTM D36/D36M-14
Flash point, °C	329	ASTM D92-18
Wax content, %	1.8	SH/T 0425-2003
Solubility, %	99.81	ASTM D2042-22
Density (15 °C), g/cm3	1.039	ASTM D70/D70M-21
Viscosity (60 °C), Pa·s	261	ASTM D4402/D4402M-22
Thin-film oven test (163 °C, 5 h)		ASTM D1754/D1754M-20
Mass loss, %	−0.107	ASTM D6/D6M-95
Retained penetration, %	67	ASTM D5/D5M-20
Ductility (10 °C, 5 cm/min), cm	7.3	ASTM D113/D113M-17

.

This study selected penetration-grade 70# asphalt as the experimental binder based on the following three considerations: Firstly, 70# asphalt is one of the most commonly used grades in road construction in China, making the research findings directly valuable for engineering guidance. Secondly, the medium-penetration 70# asphalt exhibits typical characteristics in terms of volatile component content (e.g., saturate fractions), colloidal stability (asphaltene/resin ratio), and high-temperature viscosity. It effectively represents fume emission properties while avoiding extreme interference from excessively soft asphalt (with overly high volatility) or excessively hard asphalt (with overly low reactivity), thereby facilitating a clear evaluation of the inhibitor’s true effectiveness. Finally, from an experimental controllability perspective, using a single grade of asphalt eliminates variability in the base material that could interfere with the assessment of the composite inhibitor’s efficacy, providing a benchmark system for mechanistic research.

2.2. Inhibitors

The selected cuprous chloride and ferric chloride are produced by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), with purities exceeding 98.5%. DTBHQ is produced by Hubei Coward Chemical Co., Ltd. (Wuhan, China), with a purity exceeding 99%.

2.3. Asphalt Fume Preparation and Collection Equipment

A self-designed asphalt fume enrichment apparatus was developed to simulate the generation of asphalt fumes. The system primarily consists of a reaction vessel, integrated with a flow rate control system and asphalt fume capture equipment, forming a comprehensive setup for the preparation and collection of asphalt fumes. A schematic diagram of the apparatus is illustrated in Figure 1. The fume collection system was designed similar with previous research [22,23].

In the asphalt fume capture equipment, a glass fiber filter cartridge combined with two absorption bottles was used, which was determined to be the most effective method based on the authors’ preliminary research.

2.4. Experimental Methods

Methods for Generating and Enriching Asphalt Fume

The experimental procedure for asphalt fume generation and component enrichment was systematically conducted as follows: A specified mass of asphalt (500 ± 5 g) was introduced into the reaction vessel and subjected to thermal decomposition using a programmable temperature-controlled heating system. For effective component separation and collection, a dual-stage enrichment apparatus was employed, comprising one glass fiber filter cartridge (Model 3#, dimensions: Φ28 mm × 70 mm) connected in series with two absorption bottles. The filtration system demonstrated size-dependent separation characteristics, with the glass fiber cartridge effectively capturing liquid particulate matter exceeding 3 μm in diameter, while the subsequent absorption bottles retained both gaseous constituents and finer liquid particulates (<3 μm). To optimize light component recovery, each absorption bottle contained 200 mL of cyclohexane as absorption medium. This serial configuration ensured comprehensive retention of volatile organic compounds throughout the asphalt fume generation process.

2.5. Calculation Methods for Asphalt Fume Quality

2.5.1. Gravimetric Method

The gravimetric method quantifies asphalt fume by analyzing particulate matter captured on a glass fiber filter cartridge. The experimental procedure commenced with labeling the cartridge and dehydrating it in a 105 °C oven for 2 h to eliminate moisture. The dried cartridge was subsequently transferred to a desiccator for temperature equilibration prior to initial weighing (W₁), and performed with a precision of ±0.001 g. Following mass calibration, the cartridge was installed at the asphalt fume outlet to collect particulate matter during thermal decomposition experiments. Upon completion of fume exposure, the loaded cartridge underwent 72 h desiccation in the desiccator to remove the adsorbed water vapor before final mass determination (W₂). The heavy component content was calculated as the mass difference, which was normalized to the asphalt mass and expressed in mg·kg⁻¹ according to Equation (1).

M₁ = (W₂ − W₁) × 10⁶/M

(1)

where

• W₁—Mass of glass fiber filter cartridge, g;
• W₂—Total mass of glass fiber filter cartridge and asphalt fume particles, g;
• M—Mass of asphalt used in the experiment, g.

2.5.2. UV Spectrophotometry

UV spectrophotometry was primarily employed to analyze asphalt fume components captured in cyclohexane [24]. The prepared asphalt fume samples were diluted to achieve cyclohexane solutions with the following concentrations: 45.8320, 9.1664, 2.9332, 1.1733, 0.4693, and 0.1877 μg·mL⁻¹. For instance, full-spectrum scanning (200–300 nm) of the 0.4693 μg·mL⁻¹ asphalt fume solution revealed a maximum absorption peak at 227 nm.

Subsequently, the absorbance of the aforementioned six distinct concentrations of asphalt fume cyclohexane solutions was measured at 227 nm, establishing a concentration-absorbance calibration curve. Regression analysis yielded the following relationship (Equation (2)) between solution concentration (X) and absorbance (Y):

Y = 0.02242 + 0.03012X

(2)

M₂ = Y × 50 × 100 × 1000/(M × 1000)

(3)

where

• X—UV absorbance of asphalt fume cyclohexane solution;
• Y—The concentration of asphalt fume;
• M—Mass of asphalt used in the experiment, g, and the total amount of asphalt fume M_A = M₁ + M₂.

2.6. Experiments

2.6.1. Research on the Mechanism of Asphalt Fume Production

The effects of nitrogen and air as carrier gases on asphalt fume emissions were investigated under controlled conditions (temperature: 163 °C, duration: 2 h, airflow rate: 300 L h⁻¹, stirring rate: 180 rpm). The correlation between oxygen content in the carrier gas and fume release was established. Primary causative factors of asphalt fume generation were elucidated through combined analysis of infrared spectroscopy and oxygen content measurements.

2.6.2. Simulation of the Mixing Aging Conditions

To improve the efficiency of identifying simulated mixing conditions, experiments were initially conducted to investigate the effects of temperature (duration: 2 h, airflow rate: 100 L·h⁻¹, stirring rate: 180 rpm) and air velocity (temperature: 163 °C, duration: 2 h, stirring rate: 180 rpm) on asphalt fume emissions.

This experiment employed four-component analysis of asphalt as the foundational methodology. By systematically adjusting temperature, airflow rate, and duration, the four-component composition (saturates, aromatics, resins, asphaltenes) of asphalt fumes generated under varying conditions was characterized. Experimental parameters yielding four-component distributions analogous to those observed in thin-film oven test (TFOT)-aged asphalt were identified as representative of simulated mixing conditions. Therefore, the asphalt fume produced under these optimized conditions exhibited compositional similarity to emissions generated during practical mixing processes.

2.6.3. Preparation of the Low-Fume Asphalt Binder

Based on the mechanism of asphalt fume generation, suppression strategies were developed through incorporation of functional additives in asphalt binder. Preliminary evaluation focused on asphalt binder A (base asphalt binder), wherein three candidate inhibitors were assessed for efficacy and optimal dosage. The most effective inhibitor formulation was subsequently validated for fume suppression in asphalt binder B (SBS-modified asphalt binder) and asphalt binder C (oxidized asphalt binder), ensuring broad applicability across asphalt types.

2.6.4. Performance Evaluation of the Low-Fume Asphalt

To assess inhibitor impacts on asphalt performance, three key parameters—penetration, softening point, and ductility—were monitored during fume generation. Subsequent analysis evaluated inhibitor effects on aged asphalt properties via TFOT, including mass loss, post-aging penetration, softening point, and ductility retention. These metrics collectively verified the compatibility of inhibitors with asphalt’s mechanical performance requirements.

3. Results and Discussion

3.1. Research on the Mechanism of Asphalt Fume Generation

To suppress asphalt fume production, understanding its generation mechanism is important. The prevailing literature [25] indicated that the asphalt fume arises from oxidative degradation of asphalt during high-temperature mixing processes. This study validates this hypothesis using asphalt binder A as a model system, providing a theoretical foundation for developing fume suppression strategies.

If oxidative degradation of asphalt binder under thermal conditions drives fume generation, oxygen availability should directly modulate oxidation severity and subsequent fume emissions. To test this hypothesis, experiments were conducted under controlled conditions (temperature: 163 °C, duration: 2 h, airflow rate: 300 L·h⁻¹, stirring rate: 180 rpm) using nitrogen and air as carrier gases. Asphalt fume release was quantified, with the results illustrated in Figure 2.

Figure 2 demonstrates significant disparities in fume emissions between carrier gases. Under nitrogen, approximately 25 g of fumes per kilogram of asphalt (25 g·kg⁻¹) was emitted. In contrast, when air was used as the carrier gas, this produced over 250 g·kg⁻¹ of fume—exceeding emissions under nitrogen by an order of magnitude. This remarkable contrast confirms that oxygen presence in the carrier gas markedly amplifies fume release, unequivocally linking asphalt fume generation to oxidative mechanisms.

The influence of carrier gas composition on asphalt fume release demonstrates that oxygen-containing carrier gases significantly enhance fume emissions, establishing a direct correlation between oxygen availability and fume generation. To understand more deeply about the mechanism of this phenomenon, infrared spectroscopy was employed to analyze matrix asphalt and asphalt samples treated with nitrogen or air under controlled conditions (temperature: 163 °C, duration: 2 h, space velocity: 300 L·h⁻¹, stirring rate: 180 rpm). The three infrared spectra—matrix asphalt, nitrogen-treated asphalt, and air-treated asphalt—were superimposed for comparative analysis (Figure 3).

Figure 3 reveals minimal spectral differences between nitrogen-treated and matrix asphalt under experimental conditions. However, a distinct enhancement of the 1700 cm⁻¹ absorption peak was observed in air-treated asphalt. This peak, attributed to carbonyl group vibrations, indicates a marked increase in carbonyl-containing components following oxidative treatment with air. These findings confirm that oxygen-rich carrier gases promote oxidation-driven structural modifications in asphalt, corroborating the role of oxidative degradation in fume generation.

Following exposure to air as the carrier gas, a significant increase in carbonyl-containing components within asphalt was observed. To further quantify oxidative degradation induced by atmospheric oxygen, elemental composition (C, H, S, N) analyses were performed on base asphalt, nitrogen-treated asphalt, and air-treated asphalt. Oxygen content was calculated via the subtractive method (Figure 4).

As illustrated in Figure 4, oxygen content in nitrogen-treated asphalt marginally exceeded that of the base asphalt, though the difference was negligible. In contrast, air-treated asphalt exhibited a substantial increase in oxygen content relative to both base and nitrogen-treated samples. This confirms that atmospheric oxygen reacts with asphalt molecules under the experimental conditions (163 °C, 2 h, 300 L·h⁻¹ airflow, 180 rpm), inducing oxidative incorporation of oxygen into the asphalt matrix.

The above findings demonstrate that using air as a carrier gas promotes carbonyl group formation and elevates oxygen content within asphalt. In comparison, nitrogen-treated asphalt exhibited minimal structural or compositional alterations. Coupled with fume emission data (Figure 2), these results establish a direct relationship between oxidative degradation and asphalt fume generation: elevated oxidation severity correlates with heightened fume release, confirming an oxidation-driven mechanism.

The generation of asphalt fumes involves complex chemical reactions, primarily including the thermal decomposition and oxidation of asphalt. Figure 5 is a simplified chemical model diagram, and the main processes of asphalt fume generation can be described as follows:

(1)

Thermal decomposition of asphalt

At elevated temperatures, asphalt undergoes thermal decomposition, leading to the generation of free radicals through the cleavage of chemical bonds.

(2)

Formation of peroxy radicals and intermediate products

The generated free radicals react with atmospheric oxygen, resulting in the formation of peroxy radicals (ROO•). These reactive intermediates further decompose or react, yielding various oxygenated compounds such as aldehydes, ketones, and carboxylic acids.

(3)

Secondary reactions and byproduct formation

The intermediate products undergo subsequent oxidation or polymerization reactions, leading to the formation of more complex chemical species. This process produces volatile organic compounds (VOCs) and particulate matter (PM), which are significant contributors to environmental emissions. At the same time, new free radicals are generated, perpetuating the chain reaction and sustaining the degradation process.

From the above research, it can be concluded that the generation of asphalt fumes is influenced by temperature and oxygen. Therefore, the quantity and composition of asphalt fumes produced will vary under different temperature and oxygen conditions. This outcome indicates that identifying experimental parameters in laboratory that can simulate the generation of asphalt fumes during the mixing process is essential.

3.2. Study on Simulated Mixing Conditions

According to the research by previous research [11], exposed area and heating temperature are the key factors affecting the fume production of asphalt mixtures. Therefore, by adjusting the temperature and the exposure time of the asphalt to air, it is feasible to discover the release of asphalt fumes under simulate asphalt aging during mixing.

The effects of temperature and airflow velocity on asphalt fume emissions were studied; the results are detailed in Figure 6 and Figure 7. From Figure 6, it can be observed that when the temperature is below 140 °C, the emission of asphalt fumes shows minimal variation with temperature. However, within the range of 140 °C to 200 °C, the fume emission exhibits an approximately linear relationship with temperature, increasing significantly as the temperature rises. From Figure 7, it is evident that the asphalt fume emission also follows an approximately linear trend with changes in space velocity. These findings indicate that temperature and space velocity are the key parameters in simulating mixing conditions.

This study employed four-component analysis (SARA: saturates, aromatics, resins, asphaltenes) to simulate asphalt aging during mixing. Given that asphalt fume generation is oxidation-dependent—a process analogous to aging—the thin-film oven test (TFOT), a globally recognized protocol for simulating mixing-induced aging, was adopted. Experimental conditions yielding four-component distributions comparable to TFOT-aged asphalt were identified as representative of field mixing. By systematically varying temperature, airflow rate (300 L·h⁻¹), and duration, asphalt fumes were generated under controlled laboratory conditions. As shown in

Table 2. SARA fractions of asphalt binder.

Experimental Conditions	Saturates, %	Aromatics, %	Resin, %	Asphaltene, %	Asphalt Fume, mg·kg−1
TFOT	17.89	31.86	42.41	7.84	/
163 °C; 100 L/h; 6 h; 180 rpm	19.59	39.26	33.18	7.97	76
200 °C; 200 L/h; 6 h; 180 rpm	20.25	41.97	32.31	7.90	858
200 °C; 300 L/h; 6 h; 180 rpm	20.85	40.19	32.89	8.24	1280
200 °C; 300 L/h; 12 h; 180 rpm	17.89	33.43	40.25	7.79	1425

, parameters of 200 °C, 300 L·h⁻¹ airflow, 12 h duration, and 180 rpm stirring produced four-component ratios closely aligned with TFOT-aged asphalt. Consequently, these conditions were designated as simulated mixing conditions, with resultant fume emissions (1425 mg·kg⁻¹) deemed representative of practical mixing processes (Table 2).

To validate the simulated aging degree, key performance metrics—softening point, 25 °C penetration, and 15 °C ductility—were analyzed (

Table 3. Basic properties of asphalt binder after aging.

Experimental Conditions	Softening Point, °C	Penetration (25 °C), 0.1 mm	Ductility (15 °C), cm
TFOT	52.4	62	>150
200 °C; 300 L/h; 12 h; 180 rpm	51.6	66	>150

). The results demonstrated close agreement between laboratory-aged (200 °C, 300 L·h⁻¹, 12 h, 180 rpm) and TFOT-aged asphalt, confirming the validity of the simulated conditions. Table 2 shows that under simulated mixing conditions, 1425 mg of asphalt fume can be collected per kilogram of asphalt binder. The increase in temperature and stirring accelerates the movement of asphalt molecules, resulting in a multiplicative effect on the volume of gas produced.

3.3. Influence of Different Types of Inhibitors

In recent years, numerous studies [11] have been conducted by researchers to explore various methods for suppressing asphalt fumes, with the majority utilizing adsorption techniques. The study investigates the generation mechanism of asphalt fumes, with the objective of mitigating their production at the source.

This study selected CuCl, DTBHQ, and FeCl₃ to formulate a composite inhibitor based on their unique and complementary mechanisms of action. Firstly, DTBHQ, as an efficient phenolic antioxidant, can scavenge free radicals by providing hydrogen atoms, effectively interrupting the chain reaction during thermal-oxidative aging of asphalt, thereby reducing the generation of oxidative fumes at the source [26,27]. Secondly, CuCl and FeCl₃ act as Lewis acid catalysts, promoting the polymerization of light volatile components in asphalt, converting them into high-molecular-weight, low-volatility substances, thus directly minimizing the release of volatiles [28,29,30]. Although the existing literature primarily focuses on single inhibitors, we hypothesize that combining the antioxidant defense mechanism of DTBHQ with the catalytic polymerization treatment mechanism of metal chlorides could yield a synergistic effect, enabling multiple pathways and more efficient source suppression of asphalt fumes. Furthermore, these three compounds exhibit good thermal stability and relatively suitable costs, meeting basic requirements for engineering applications.

The inhibitory efficacy of three asphalt fume inhibitors was evaluated under controlled conditions (temperature: 163 °C, airflow rate: 100 L·h⁻¹, stirring speed: 180 rpm, duration: 5 h). A base asphalt binder was heated to 163 °C, followed by the addition of 0.6 wt% inhibitors. The formulations and constituent ratios of the three inhibitors are presented in

Table 4. The composition of inhibitors.

Inhibitor	Component Name	Chemical Composition/Formulation
Inhibitor 1	cuprous chloride	CuCl
Inhibitor 2	2,5-di-tert-butyl-hydroquinone	C14H22O2(DTBHQ)
Inhibitor 3	Ferric chloride	FeCl3
Inhibitor 4	Mixed Inhibitor	CuCl:DTBHQ = 1:1
Inhibitor 5	Mixed Inhibitor	FeCl3:DTBHQ = 1:1
Inhibitor 6	Mixed Inhibitor	CuCl:FeCl3 = 1:1
Inhibitor 7	Mixed Inhibitor	CuCl:DTBHQ:FeCl3 = 1:1:1
Inhibitor 8	Mixed Inhibitor	CuCl:DTBHQ:FeCl3 = 4:4:2

. Asphalt fume emissions were quantified (

Table 5. Release of asphalt fume under different proportions of inhibitors.

Inhibitor	Asphalt Fume, mg/kg
	M2	M1	MA
Inhibitor 1	0.2753	117.6324	117.9074
Inhibitor 2	0.3390	110.2171	110.5561
Inhibitor 3	0.3472	106.3918	106.7390
Inhibitor 4	0.4766	96.3966	96.8732
Inhibitor 5	0.3529	94.2802	94.6331
Inhibitor 6	0.5871	111.6963	112.2834
Inhibitor 7	0.3110	74.7271	75.0381
Inhibitor 8	0.2444	67.4961	67.7405

).

Based on the data presented in Table 4 and Table 5, the following analytical observations can be made regarding the effectiveness of various inhibitors and their combinations in reducing asphalt fume release:

The experimental results demonstrate significant variations in asphalt fume suppression across different inhibitor formulations. Among the single-component inhibitors, Inhibitor 3 (Ferric Chloride, FeCl₃) exhibited the best performance with a total fume release (M_A) of 106.7390 mg/kg, followed closely by Inhibitor 2 (DTBHQ) at 110.5561 mg/kg, while Inhibitor 1 (Cuprous Chloride, CuCl) showed the highest emission value at 117.9074 mg/kg among single inhibitors. Notably, the mixed inhibitors consistently outperformed single-component formulations, indicating clear synergistic effects. The ternary mixture Inhibitor 7 (CuCl:DTBHQ:FeCl₃ = 1:1:1) achieved a substantial reduction in total fume release (75.0381 mg/kg), which was further enhanced in Inhibitor 8 (CuCl:DTBHQ:FeCl₃ = 4:4:2) with the lowest overall emission of 67.7405 mg/kg. This represents approximately 42.5% reduction compared to the least effective single inhibitor (CuCl). Among binary mixtures, Inhibitor 5 (FeCl₃:DTBHQ = 1:1) demonstrated the best performance (94.6331 mg/kg), suggesting particularly effective synergy between ferric chloride and the organic antioxidant DTBHQ. The combination of CuCl with FeCl₃ (Inhibitor 6) showed less pronounced improvement, indicating that metal chloride combinations without the antioxidant component are less effective. These results strongly support the strategy of using compound inhibitors for enhanced fume suppression, with the optimal performance achieved through the balanced combination of catalytic metal chlorides and radical-scavenging antioxidant components.

3.4. Influence of Inhibitor Content

The effects of different dosages of Inhibitor 8 were assessed by simulating indoor asphalt mixing aging (163 °C, 5 h). Asphalt blends containing 0, 0.4, 0.6, and 0.8 wt% Inhibitor 8 were prepared, respectively. Fume emissions were collected over 5 h and analyzed (

Table 6. Asphalt fume under different conditions.

Content of Inhibition 3, %	Asphalt Fume, mg/kg			Reduction, wt%
	M2	M1	MA
0	0.2406	137.9852	138.2258	— —
0.2	0.2460	120.8674	121.1134	12.4
0.4	0.2396	88.4523	88.6919	35.8
0.6	0.2444	67.4961	67.7405	50.1
0.8	0.2899	64.8647	65.1546	52.9

).

It is clear that the fume reduction increases with larger content of Inhibitor 8. At 0.6 wt% dosage, fume reduction reached 50.1% (67.74 mg·kg⁻¹ vs. 138.23 mg·kg⁻¹ control). Increasing the dosage to 0.8 wt% yielded marginal gains (52.9% reduction), suggesting 0.6 wt% as the cost-effective threshold for optimal inhibition.

To validate the effectiveness of Inhibitor 8, comparative tests were conducted using both SBS-modified asphalt and oxidized asphalt under identical experimental conditions to evaluate their respective asphalt fume emissions. The results, presented in Figure 8, demonstrate that the addition of 0.6% Inhibitor 8 significantly reduced fume emissions in both asphalt binders—achieving reduction rates of 52.8% and 74.3%, respectively.

The data reveals that the inhibitor exhibits superior performance in suppressing fume emissions from oxidized asphalt. This enhanced efficacy suggests that Inhibitor 8 effectively mitigates the asphalt oxidation process, demonstrating excellent performance in controlling fume generation. The different efficiency between the two asphalt binders further highlights the compound’s targeted action on oxidative degradation pathways.

3.5. Mechanisms of Asphalt Fume Inhibitors

As evidenced by the comparative data in Table 5, the synthesized low-emission asphalt manifests a 52.9% reduction in fume generation. These synergistic improvements stem from the mechanism that targets the free radical-mediated oxidation pathways through the incorporation of the radical scavengers and antioxidants, effectively disrupting the chain propagation reactions responsible for fume formation and age hardening.

3.5.1. DTBHQ

It provides a broad background knowledge for studying the relationship between DTBHQ and free radicals, helping to understand the properties and effects of free radicals in depth [31]. DTBHQ serves as an effective radical scavenger [32], capable of capturing free radicals (e.g., peroxyl radicals (ROO•), hydroxyl radicals (OH•)) generated during oxidation processes (DTBHQ+ROO•→DTBHQ•+ROOH). This action interrupts radical chain propagation reactions, thereby inhibiting further oxidative degradation. The mechanism arises from the two phenolic hydroxyl groups (-OH) in DTBHQ’s molecular structure, which donate hydrogen atoms to highly reactive radicals (e.g., ROO•, OH•) via a hydrogen atom transfer (HAT) pathway. This hydrogen donation effectively quenches radical reactivity, converting them into stable compounds such as hydroperoxides or terminated organic species.

3.5.2. CuCl

Zubanova et al. [33] pointed out that the interaction of copper chlorides with radicals is a key step in single-electron-transfer reactions. The cuprous ion (Cu⁺) in cuprous chloride (CuCl) exhibits reducing properties towards radicals (OH•) through a single-electron transfer (SET) mechanism. This redox interaction reduces highly reactive radicals into stable non-radical species. For instance, Cu⁺ can donate an electron to a peroxyl radical (ROO•), oxidizing itself to Cu²⁺ while converting ROO• into a less reactive anion (ROO⁻). This process (Cu⁺+ROO•→Cu²⁺+ROO⁻) effectively terminates radical chain propagation, thereby suppressing oxidative degradation pathways.

Moreover, Cu²⁺ can be regenerated to Cu⁺ via electron donation from reducing agents (Cu²⁺+O₂•→Cu⁺+O₂). This sustained redox cycling facilitates the catalytic depletion of reactive oxygen species (ROS) such as superoxide anion (O₂•) within radical-mediated reactions (Cu⁺+O₂•→Cu²⁺+O₂²⁻). By continuously scavenging ROS intermediates, the Cu⁺/Cu²⁺ redox pair disrupts radical chain propagation, effectively suppressing oxidative cascade reactions.

3.5.3. FeCl₃

Ferric chloride (FeCl₃) can inhibit oxidative radical reactions through mechanisms distinct from classical antioxidants like DTBHQ. Its inhibitory action primarily stems from two synergistic properties [34], which are redox cycling and Lewis acidity.

(1)

Redox Cycling

The Fe³⁺ ion in FeCl₃ undergoes reversible reduction to Fe²⁺ (Fe³⁺+O₂•−→Fe²⁺+O₂), which subsequently oxidizes back to Fe³⁺ (Fe²⁺→Fe³⁺+e⁻). This cyclic electron transfer catalytically scavenges reactive oxygen species (ROS) such as superoxide anion (O₂•), effectively disrupting radical chain propagation.

(2)

Lewis Acidity

Fe³⁺ acts as a strong Lewis acid, coordinating with radical intermediates (e.g., hydroxyl radicals •OH) to form stable metal–radical complexes (Fe³⁺+R•→Fe²⁺+R⁺), thereby deactivating their reactivity.

Due to the extremely complex molecular composition of asphalt, the oxidation free radical reactions generated by asphalt fumes are also very intricate. Therefore, in this study, three products with different functions and principles are selected and combined in a certain proportion. This combination can specifically capture R•, O₂•, and ROO• during the generation process of asphalt fumes, thereby achieving a remarkable effect of suppressing the generation of asphalt fumes.

3.6. Performance Evaluation of the Low-Fume Asphalt Binder

Low-fume asphalt (0.6 wt% Inhibitor 3) was compared to base asphalt across key performance metrics (

Table 7. Properties of different asphalt binders.

Item	Low-Fume Asphalt Binder	Base Asphalt Binder	Test Method
Penetration (25 °C, 100 g, 5 s), 1/10 mm	67	65	ASTM D5/D5M-20
Penetration index	−1.39	−1.34	ASTM D5/D5M-20
Ductility (15 °C, 5 cm/min), cm	150+	150+	ASTM D113/D113M-17
Ductility (10 °C, 5 cm/min), cm	32.6	31.4	ASTM D113/D113M-17
Soft point, °C	48.5	48.6	ASTM D36/D36M-14
Flash point, °C	338	329	ASTM D92-18
Wax content, %	1.6	1.8	SH/T 0425-2003
Solubility, %	99.89	99.81	ASTM D2042-22
Density (15 °C), g/cm3	1.038	1.039	ASTM D70/D70M-21
Viscosity (60 °C), Pa·s	260	261	ASTM D4402/D4402M-22
Thin-film oven test (163 °C,5h)			ASTM D1754/D1754M-20
Mass loss, %	−0.115	−0.107	ASTM D6/D6M-95
Retained penetration, %	73	67	ASTM D5/D5M-20
Ductility (10 °C, 5 cm/min), cm	8.9	7.3	ASTM D113/D113M-17

). The results are shown in Table 6. From the data of various parameters in Table 6, it can be seen that the addition of Inhibitor 3 did not deteriorate the performance of asphalt, but rather improved its performance after aging, increasing retained penetration and ductility after the thin-film oven test.

4. Conclusions and Findings

By using the asphalt fume generation and collection device described in this study, the weight of asphalt fumes was determined using gravimetric and ultraviolet spectrophotometry methods. The following findings can be obtained from this study.

(1)

The oxidation of the asphalt binder is the main reason for asphalt fume generation.

(2)

To simulate the on-site mixing conditions, preparation parameters for asphalt fumes were determined to be as follows: temperature 200 °C, air velocity 300 L/h, time 12 h, and mixing rate 180 rpm.

(3)

The best inhibition effect on asphalt fume was achieved with the following inhibitor recipe: mass percentages of cuprous chloride, ditert butylhydroquinone, and ferric chloride at a ratio of 40%, 40%, and 20%. When the inhibitor was added at a dosage of 0.6%, it could effectively reduce 50% of the production of asphalt fumes.

(4)

Inhibitors can enhance the antioxidant capacity of the asphalt binder and improve the properties after short-term laboratory aging.

This study provides a preliminary exploration of the effect of different inhibitors on the asphalt fume emissions. Future research should be conducted to further evaluate the influence of these inhibitors on the overall performance of asphalt binder, considering rutting, fatigue, and thermal cracking resistance.