Study on the Volatile Organic Compound Emission Characteristics of Crumb Rubber-Modified Asphalt

Abstract

Crumb rubber used in asphalt modification can generally improve the road performance of asphalt mixture pavement while offering substantial environmental and economic benefits. This study investigates the volatile organic compound emissions from crumb rubber-modified asphalt binders via gas chromatography–mass spectrometry, focusing on the effects of crumb rubber types (e.g., activated crumb rubber, non-activated crumb rubber), contents, and additives (warm-mix agents, deodorants, styrene–butadiene–styrene (SBS)). The analysis encompasses total volatile organic compound emissions, compositional variations, secondary organic aerosol and ozone formation potentials, and carcinogenic risks. Results indicate that non-activated crumb rubber increases volatile organic compound emissions initially, peaking at a 15% content (3.99 times higher than base asphalt), dominated by trichloroethylene. The surfactant-based warm-mix additive significantly reduces emissions by 73%, whereas deodorants exhibited limited efficacy. At equivalent contents, activated crumb rubber-modified asphalt emits more volatile organic compounds than non-activated crumb rubber-modified asphalt and leads to a higher ozone formation potential. Activated crumb rubber/SBS-modified asphalt blends reduce emissions by 69%–81% due to synergistic effects. In contrast, non-activated crumb rubber/SBS blends increase emissions, likely due to phase separation. All samples contain carcinogens, primarily trichloroethylene (20%–79%) and benzene (0.1%–9%). These findings underscore the critical importance of crumb rubber activation status and SBS addition in controlling volatile organic compound diffusion. The activated crumb rubber/SBS combination achieves a synergistic reduction exceeding the sum of individual effects (“1 + 1 > 2”). These findings provide valuable insights for designing eco-friendly asphalt.

1. Introduction

The increased demand for vehicles generates growing volumes of end-of-life tires [1]. The waste rubber tires are difficult to dispose of owing to their chemical inertness and excellent resistance to thermal, mechanical, and biological degradation [2], resulting in black pollution. Accumulated end-of-life tires pose serious risks, including environmental pollution, land occupation, auto-ignition hazards, and resource wastage [3]. Numerous studies demonstrate that end-of-life tires can be processed into crumb rubber (CR) powder and incorporated into asphalt binder at optimal quantities. This enhances overall performance through improved elasticity, recoverability, and reduced glass transition temperature [4,5,6,7,8,9,10,11,12,13,14]. Studies internationally confirm that CR modified asphalt binder (CRMA) outperforms conventional asphalt by (1) enhancing resistance to high-temperature rutting, low-temperature cracking, fatigue, and aging; (2) improving pavement skid resistance and noise reduction; and (3) reducing distresses, delaying maintenance procedures, and extending service life [5,15,16,17,18,19,20,21,22,23,24,25,26,27,28]. Utilizing CR in asphalt provides an environmentally sustainable recycling solution for end-of-life tires, mitigating black pollution [29]. Evolving CRMA technology addresses environmental issues from end-of-life tire accumulation through resource-efficient recycling [6,27,30,31,32,33,34]. These demonstrated outstanding environmental and socio-economic benefits promote the sustainable utilization of end-of-life rubber resources, offering comprehensive economic and environmental advantages [11,19,35,36,37].

However, CRMA technology faces significant challenges limiting its widespread utilization and potential applications [14,27,38,39,40,41,42,43]. During production, mixing, transportation, and paving, CRMA generates substantial odorous and hazardous gases—primarily volatile organic compounds (VOCs) and volatile sulfur compounds—resulting from partial swelling and degradation of CR particles at high temperature [44,45,46,47,48,49,50,51,52,53,54]. CRMA’s higher viscosity [55] requires elevated construction temperature to achieve workability, significantly increasing odorous and hazardous gases [45,46,56,57]. These emitted toxic odor gases pose greater environmental and occupational health risks than base asphalt [58,59,60,61]. Studies report that the CRMA mixture produces offensive odors and carcinogens [43,46,62,63,64,65] during production and paving. Growing demand for cleaner asphalt paving and environmental concerns have intensified focus on controlling CMRA’s toxic odor emissions [60,66,67]. Significant research focuses on environmental hazards from CR-modified asphalt [68,69,70].

Pretreatment, warm-mix additive, and deodorant are internationally recognized as the most effective solutions to inhibit the total released VOC emissions [22,46,53,56,71,72,73,74,75,76,77]. Specific pretreatment methods include microwave activation [78,79,80,81], surface active treatment and chemical modification [82,83,84] of CR, plasma treatment [85], microbial desulfurization [86], supercritical carbon dioxide (CO₂) treatment [87], pre-swelling, pre-degradation, pre-oxidation, graft activation, polymer coating, solution immersion, and radiation [88,89,90,91,92]. These methods enhance CR–asphalt compatibility by modifying physical properties (e.g., altering cross-link network structures) and chemical structures (e.g., activating surfaces of CR particles). CR pretreatment significantly reduces polycyclic aromatic hydrocarbons (PAHs) and sulfur-containing compounds, thereby minimizing odor during CRMA production [80,81]. Asphalt exhibits temperature-sensitive properties. Warm-mix additives improve fluidity and modify viscosity characteristics, reducing VOC generation through lower construction temperature [46,74,77,93,94,95,96,97,98,99,100,101,102]. Chemical reactants may also neutralize toxic gases emitted from CRMA [103,104]. Deodorants reduce fume emissions through (a) high internal specific surface area, (b) porous structure, and (c) strong intermolecular forces on pore walls [8,76,105,106,107,108]. These properties enable physical absorption of lightweight fractions and fumes via lattice energy and van der Waals forces [27,38,53,62,71,80,109,110,111,112,113,114]. Alternatively, deodorants chemically capture emitted fumes by reacting with small molecules and volatile components in CRMA [20]. Researchers have incorporated absorption materials—activated carbon, organic mon–montmorillonite, nano-calcium carbonate, and metal–organic frameworks (MOFs) materials, and zeolite—into CRMA to surpass volatilization of lightweight fractions (particularly hydrocarbons and benzene) [38,95,111,112,113,115,116,117,118,119,120,121,122,123,124].

Previous studies have revealed that various factors influence the emission behavior of CRMA. Refs. [125,126,127,128,129] identified that base asphalt binder composition critically determines CRMA emission behavior. Substantial research [125,129] demonstrated that temperature dramatically affects the composition and concentration of VOC emissions from CRMA. Furthermore, other factors significantly influence CRMA emission behavior and must be considered during fume assessment, including CR particle size [125], CR types and activated method [130,131], CR content, warm-mix additive, preparation process [125], deodorant type, and content [28,38,95,111,112,113,115,116,117,118,119,120,121,122,123,124]. However, previous studies typically focus on one or a few factors influencing CRMA emission behavior. Comparing results and conducting in-depth exploration across studies is challenging due to variations in base asphalt binders, test temperatures, CR sources, contents, or additives. Therefore, this study employs a consistent preparation process to fabricate CRMA with various compositions. It aims to investigate the influence of CR types, CR content, warm-mix additive type, deodorant content, and SBS polymer on CRMA emission release behavior, providing insights into the VOCs release mechanism. Using an indoor fume collection system, GC–MS was employed to identify and quantify compositions within the fume emissions. A hazardous evaluation of the CRMA asphalt fumes was also performed. The findings enhance understanding of the multifaceted impacts of CRMA asphalt fumes and propose optimal, targeted measures to mitigate VOC emissions.

2. Materials and Methods

2.1. Raw Materials

The crumb rubber particles were recycled and activated through the combined action of heat, chemical agents, and controlled mechanical shearing using a twin-screw extruder. The activated process breaks the S–S and C–S bonds to the greatest possible extent while preserving the C–C main chain, ensuring the high strength of activated CR materials. Two types of rubber particles were used to modify the base asphalt. The first type is non-activated crumb rubber (NCR) particles produced through mechanical grinding of end-of-life tires at ambient temperature. The second type is activated reclaimed rubber (ACR) particles produced using twin-screw extruder at high temperatures (200–220 °C). The characteristics and particle size distributions of ACR and NCR are presented in

Table 1. The characteristics of activated and non-activated CR particles.

NO.	Test Item	Test Result
		ACR	NCR
1	Relative density	1.18	1.17
2	Ash content/%	5.40	6
3	Acetone extract content/%	13.40	11
4	Rubber hydrocarbon content/%	53	51
5	Carbon black content/%	30.60	30

and

Table 2. The screening results of the activated and non-activated CR particles.

Mesh	30	40	50	60	70	80	>80
ACR (%)	3.1	47.6	16.1	5.7	12.4	5.1	8.7
NCR (%)	10.3	42.2	15.9	5.4	11.5	4.3	8.5

, respectively.

The base asphalt binder used was 70#, supplied by Qingdao Luhuitong Industrial Co., Ltd., Qingdao, China. Two warm-mix additives were used: (1) liquid warm-mix additive (W1) (recommended content of 0.5%) from Beijing Road Animal Husbandry Technology Co., Ltd., Beijing, China; and (2) liquid warm-mix additive (W2) (recommended content of 0.5%) from Meidewei Shiwek Company. The deodorant additive was developed in solid form by Qingdao University of Science and Technology. The CR particles, warm-mix additives, and deodorant are shown in Figure 1.

2.2. Preparation of CR Modified Asphalt

This study investigated the influence of multiple factors, including CR type, CR content, warm-mix additive type, deodorant content, and SBS modifier. The experimental design (

Table 3. The descriptions of all tested groups.

NO.	Label	Description
1	BA	70# base asphalt
2	SBS	70# base asphalt + 4 wt.% SBS
3	SA10	70# base asphalt + 2 wt.% SBS + 10 wt.% ACR
4	SA15	70# base asphalt + 2 wt.% SBS + 15 wt.% ACR
5	SA20	70# base asphalt + 2 wt.% SBS + 20 wt.% ACR
6	SN15	70# base asphalt + 2 wt.% SBS + 15 wt.% NCR
7	SN10	70# base asphalt + 2 wt.% SBS + 10 wt.% NCR
8	N10	70# base asphalt + 10 wt.% NCR
9	N15	70# base asphalt + 15 wt.% NCR
10	N20	70# base asphalt + 20 wt.% NCR
11	N15W1	70# base asphalt + 15 wt.% NCR + 0.5 wt.% W1
12	N15W2	70# base asphalt + 15 wt.% NCR + 0.5 wt.% W2
13	N15D1	70# base asphalt + 15 wt.% NCR + 1 wt.% deodorizer
14	N15D2	70# base asphalt + 15 wt.% NCR + 2 wt.% deodorizer
15	N15D3	70# base asphalt + 15 wt.% NCR + 3 wt.% deodorizer
16	A10	70# base asphalt + 10 wt.% ACR
17	A15	70# base asphalt + 15 wt.% ACR
18	A20	70# base asphalt + 20 wt.% ACR

) comprised 18 sample groups, with BA and SBS-modified asphalt samples serving as controls. Additive content is expressed as a percentage relative to the mass of the asphalt binder.

Samples were prepared according to the following procedure:

(a) Base asphalt heating

Base asphalt was heated in an oven until flowable, then transferred to a reaction vessel maintained at 175 °C with continuous stirring (500 rpm).

(b) SBS modifier addition

Control Group: At 175 °C, 4 wt% SBS modifier was added and mixed at 500 rpm for 60 min while maintaining temperature. The blend was then processed through a colloid mill five times, followed by additional mixing (500 rpm, 120 min).

Test Group: At 175 °C, 2% wt SBS modifier was added and stirred at 500 rpm for 60 min.

(c) Crumb rubber addition

Following SBS modification, the set temperature was increased to 190 °C. Pre-weighed CR was added incrementally over 60 min with continuous stirring (500 rpm), while maintaining the temperature at 190 ± 5 °C.

(d) Final processing and additive incorporation

After CR incorporation, blends were processed through a colloid mill for five cycles, then mixed at 500 rpm and 190 °C for 120 min to ensure complete dispersion and the desired microstructure. Other additives were incorporated during this final mixing stage.

2.3. Generation and Collection Process for VOC Emissions

A custom-designed gas generation and collection apparatus (Figure 2) was employed to simulate high-temperature VOC release behavior from asphalt binder under laboratory conditions while preventing fugitive emissions. The collecting apparatus is composed of (a) a temperature-controlled fume generation platform and (b) a VOC collection/absorption module using a ZR–3922 environmental air particulate matter sampler with a mercury suction intake, which captures both direct and indirect asphalt VOC emissions. A Teflon tube connects the fume generation platform to the collection module. During heating/stirring, fresh air was continuously pumped into the flask to (1) maintain constant internal pressure and (2) simulate the VOC–air reaction occurring during actual mixing/construction.

The VOC emission generation and collection procedure of all samples, including N15W1 and N15W2, comprised the following:

(a) Asphalt samples were melted to a fluid state. Next, 600 g±5 g was transferred into a glass flask sealed with a three-neck glass lid.

(b) The flask was placed in a temperature-controlled oil bath (180 °C) with a magnetic stirrer. The asphalt binder was stirred at 500 rpm to ensure uniform heating and facilitate VOC fume generation.

In this study, all samples, including those with and without the warm-mix additive, were tested under identical temperature conditions. This approach was taken to control for the temperature variable and to isolate the effect of the additive itself on viscosity and VOC reduction.

(c) The VOC collection system (0.2 L/min) was connected, using pre-aged Tenax TA tubes (6 mm× 150 mm). After temperature stabilization, absorption was processed for 30 min, collecting 6 L of VOC-laden air.

2.4. VOC Emission Composition Analysis

Collected VOC emissions were analyzed using thermal desorption gas chromatography–mass spectrometry (GC–MS; Trace GC Ultra DSQII, Thermo Fisher Scientific, Waltham, MA, USA), which identifies compound types and relative proportions. VOC samples underwent thermal desorption using Tenax TA tubes, with separation on an HP–5 MS ultra inert capillary column (30 m × 0.25 mm × 0.25 μm) and detection via quadrupole mass spectrometer (PerkinElmer, Waltham, MA, USA). Thermal desorption conditions were as follows: initial adsorbent tube temperature: room temperature; initial focusing trap temperature: room temperature; dry purge flow rate: 40 mL/min; dry purge time: 2 min; desorption temperature of adsorbent tube: 300 °C; desorption time of adsorbent sampling tube: 3 min; desorption flow rate: 40 mL/min; focusing trap temperature: −10 °C; desorption temperature of focusing trap: 300 °C; trap desorption time: 5 min; and transfer line temperature: 150 °C.

Compounds were identified by matching mass spectra against the NIST database (match threshold >85%), with quantification based on characteristic ion peak areas. Concentrations were quantified against liquid standards (2000 mg/L) using external calibration. Calibration employed 34 compounds per HJ 644-2013 standard, predominantly benzene derivatives and chlorinated organics.

3. Results

3.1. Total Concentration

VOCs, the primary asphalt fume emissions, are defined as organic compounds with >70 Pa vapor pressure at 25 °C and a boiling point < 260 °C. VOCs comprise complex mixtures classified by functional groups and molecular weight: oxygenated hydrocarbons (OXHs), halogenated alkenes (HKE), halogenated alkanes (HKA), aromatic hydrocarbons (ARHs), aromatic hydrocarbon derivatives (ARHD), and sulfides (SCs) [22,23,38,76,125,132,133,134,135]. Using 34 standard solutions, compounds in fume samples were categorized into four groups: HKE, HKA, ARHs, and ARHD. Analysis of all 18 samples detected only 21 compounds. All detected compounds fell into these four groups. Group concentrations per sample are shown in Figure 3 and

Table 4. The total and composition concentration of all test groups (μg/m³).

Group	Total Concentration (mg/m3)	HKE (mg/m3)	HKA (μg/m3)	ARHs (mg/m3)	ARHD (μg/m3)
BA	1.4	0.5	20.2	0.8	0.0
SBS	2.9	1.4	0.0	1.5	0.0
SA10	1.3	0.5	3.0	0.8	0.0
SA15	2.0	0.4	2.5	1.6	0.0
SA20	3.1	2.7	7.2	0.4	0.0
SN15	8.1	3.0	1.1	5.1	0.0
SN10	7.3	2.4	1.9	4.9	0.0
N10	3.2	1.5	0.0	1.7	0.0
N15	5.5	3.2	7.2	2.3	7.2
N20	3.9	2.2	6.9	1.7	3.9
N15W1	1.5	0.7	16.4	0.7	0.0
N15W2	5.2	1.8	11.8	3.5	0.0
N15D1	3.6	1.9	0.0	1.7	0.0
N15D2	6.4	2.6	15.8	3.7	0.0
N15D3	5.8	2.0	2.9	3.8	0.0
A10	6.9	2.6	0.0	4.2	2.6
A15	6.7	5.4	13.9	1.4	0.0
A20	4.6	3.5	0.0	1.1	0.0

. Total VOC concentrations (sum of all groups) appear above each bar.

3.2. Composition Concentration Variation

Figure 4 and

Table 5. Total and proportion of compounds in VOC emissions of all test groups.

Group	>10% Percent (%)	>10% Number	1%–10% Number	<1% Number
BA	83.9	3	5	1
SBS	71.6	3	4	1
SA10	83.1	3	5	1
SA15	88.8	3	3	4
SA20	82.3	1	6	7
SN15	87.6	3	3	5
SN10	78.7	3	4	4
N10	55.6	2	8	0
N15	77.5	2	6	2
N20	75.5	2	5	4
N15W1	86.7	3	2	5
N15W2	81.8	3	6	3
N15D1	67.5	2	7	2
N15D2	82.3	3	4	5
N15D3	81.3	3	4	5
A10	67.9	3	7	2
A15	79.4	1	5	2
A20	91.2	2	3	2

show, for each sample group, the number of VOC compounds categorized by concentration proportion: >10%, 1%–10%, and <1%. It also displays the cumulative concentration proportion from compounds exceeding 10%.

3.3. Potential of Secondary Organic Aerosols (SOA) and Ozone (O₃) Formation

VOC emissions harm humans and the environment through photochemical reactions that generate ozone (O₃) and secondary organic aerosols (SOA) [136]. Two indices—ozone (O₃) formation potential (OFP) and secondary organic aerosol formation potential (SOAFP)—identified key VOC species contributing to O₃/SOA formation and qualified environmental burden across all samples. OFP was calculated using the maximum incremental reactivity (MIR) coefficient (Equation (1)) [137,138,139,140,141,142,143,144].

$${\mathrm{O}\mathrm{F}\mathrm{P}}_{\mathrm{i}}={\mathrm{V}\mathrm{O}\mathrm{C}\mathrm{s}}_{\mathrm{i}}\times {\mathrm{M}\mathrm{I}\mathrm{R}}_{\mathrm{i}}$$

(1)

where OFP_i is the ozone formation potential of certain compound i in VOCs (μg/m³); VOC_si is the corresponding compound concentration (μg/m³); and MIR_i is the maximum incremental reactivity coefficient of certain substance i in VOCs from William P.L. Carter [140].

The fractional aerosol coefficient (FAC) method estimates SOA formation using reported FAC values (the ratio of the aerosol yield from a specific compound to its initial concentration [138,139]) and the measured VOC concentrations [145,146,147]. The potential of SOA formation (or SOA concentration, μg/m³) of a species i can be calculated by the following equation: [148].

$${\mathrm{S}\mathrm{O}\mathrm{A}\mathrm{F}\mathrm{P}}_{\mathrm{i}}={\mathrm{V}\mathrm{O}\mathrm{C}\mathrm{s}}_{\mathrm{i}}\times {\mathrm{F}\mathrm{A}\mathrm{C}}_{\mathrm{i}}$$

(2)

where SOAFP_i is the SOA formation potential of certain compound i in VOCs (μg/m³); VOCs_i is the compound concentration (μg/m³), and FAC_i is the aerosol formation coefficient of certain compound i in VOCs from Derwent et al. [138,148,149,150].

To assess the environmental burden of compounds in VOC emissions,

Table 6. The contribution ratios of detected substances in VOC emissions.

Serial No.	Compound	Contribution Value		Serial No.	Compound	Contribution Value
		SOA/%	O3			SOA	O3
1	1,1-dichloroethylene	N	1.7	11	tetrachloroethene	N	3.1 × 10−2
2	chloropropene	N	12.22	12	chlorobenzene	N	0.32
3	methylene chloride	N	1.87 × 10−2	13	ethylbenzene	5.4	3.04
4	cis-1,2-dichloroethene	N	1.79	14	p,m-xylene	3.15	9.75
5	chloroform	N	0.03	15	styrene	5.7	1.73
6	1,2-dichloroethane	N	0.21	16	o-toluene	5	7.64
7	benzene	2.6	0.72	17	4-ethyltoluene	2.5	4.44
8	carbon tetrachloride	N	0	18	1,3,5-triethyltoluene	2.9	11.76
9	toluene	5.4	4	19	1,2,4-triethyltoluene	2	8.87
10	1,1,2-trichloroethane	N	8.6 × 10−2	20	1,2,4-trichlorobenzene	N	N

summarizes the contribution ratios of VOC species to SOA and O₃ formation across all sample groups. “N” in Table 6 denotes a negligible contribution to SOA/O₃ formation. Consistent with U.S. Environmental Protection Agency (EPA) stationary sources AP–42 and California Air Resources Board (CARB) VOC exemptions, the HKE compounds exhibit negligible SOA formation potential. The ozone (O₃) and SOA formation potential of the VOC emissions are calculated using Equations (1) and (2), based on the contribution ratios for all tested compounds (Table 6) and their concentrations (Figure 3 and Table 4). All calculated results are presented in Figure 5.

3.4. Human Carcinogen Risk Analysis

According to the International Agency for Research on Cancer (IARC) carcinogen list of the World Health Organization, substances classified as group 1 carcinogens are confirmed anthropogenically determined human carcinogens with unequivocal evidence of carcinogenicity from extensive scientific investigation. Group 2A carcinogens are probable human carcinogens, with limited human evidence but sufficient evidence in experimental animals. Group 2B carcinogens are possible human carcinogens with inadequate human evidence and insufficient animal data. Carcinogen risk indices for detected compounds in VOCs with potential human carcinogenic risk are presented in

Table 7. Human carcinogen risk analysis results.

No.	Name	Carcinogen Risk
1	Benzene	IARC 1
2	1,2-dichloroethane	IARC 1
3	Trichloroethylene	IARC 1
4	Tetrachloroethylene	IARC 2A
5	Methylene chloride	IARC 2A
6	Styrene	IARC 2A
7	Carbon tetrachloride	IARC 2B
8	1,1–Dichloroethylene	IARC 2B
9	Chloroform	IARC 2B
10	Ethylbenzene	IARC 2B

. Figure 6 presents human carcinogenic and non-carcinogenic risk analysis results of VOC emissions across all sample groups.

4. Discussion

4.1. Total Concentration Analysis

Figure 3 shows that not all four compound groups appear in every sample; some contain only two or three groups. ARHD and HKA concentrations fell below the 20 μg/m³ detection threshold and thus are not displayed. Consequently, HKA and ARHD did not constitute primary sample components. Since HKA and HKE are halogenated hydrocarbons (HHs) and ARHD derives from ARHs, compounds were regrouped as HHs and ARHs. VOC results are therefore categorized as HHs and ARHs, with their proportions showing a consistent trend across samples. Asphalt fume emissions constitute a complex system influenced by multiple factors.

Total VOC concentrations in Figure 3 represent the sum of four compound groups. SBS-modified asphalt emitted 2.9 mg/m³ total VOC emissions, 2.1 times higher than the BA sample (BA, 1.4 mg/m³). This result could be explained by the fact that the resin and asphaltene in SBS-modified asphalt binder began to volatilize at a higher temperature (180 °C) [151], while its spatial network structure only constrains the evaporation of certain compounds ≈ 180 g/mol. Previous work [23] confirms higher VOC emissions from SBS-modified versus base asphalt mixture. However, other studies [102,134,152,153] reported that SBS could mitigate the VOC emissions and exhibit a fume suppression effect; the analyzed VOC profiles in these prior studies differed substantially from those in this paper.

Non-activated CR-modified sample groups (N10, N15, and N20) showed total VOC emissions 2.3 times, 4.0 times, and 2.9 times higher than BA, respectively. This aligns with the existing literature [28,125,129,154,155,156]. Prolonged high-temperature exposure causes non-activated CR particles to swell and degrade, resulting in significant VOCs. At 10%–15% CR content, increased non-activated CR particle loading required greater absorption of base asphalt’s lightweight fractions for swelling. Consequently, the molecular chain (main chain/crosslink bond) approached maximum tensile strength, increasing susceptibility to disruption [157]. Elevated temperature ruptured certain main chain and crosslink bonds within non-activated CR particles, releasing additional VOCs [44]. Above 15% non-activated CR content, excessive non-activated CR particles continued to absorb more lightweight fractions, inhibiting VOCs evaporation [80,130,158]. Depleted lightweight fractions prevent full non-activated CR particle swelling and degradation, also suspending volatilization of small/medium asphalt components. Similar non-activated CR-dependent emission behavior at higher temperatures is documented [154,159].

N15W1 and N15W2 samples contained 0.5% liquid (W1) and solid (W2) warm-mix additives, respectively, added to the N15 group. At identical temperatures, the N15W1 sample emitted significantly fewer VOCs than N15 and N15W2, while remaining slightly above BA sample levels. N15W2 sample emissions were marginally lower than the N15 sample but 3.58 times that of N15W1. It can be calculated that the doping of W1 warm-mix additive reduced the total VOC emissions by 73%. The W1 warm-mix additive is a surfactant-based additive containing both lipophilic and hydrophilic groups. When blended with asphalt, surfactant gel groups envelop smaller asphalt molecules via lipophilic interactions. This encapsulation inhibits the emission of smaller molecular or lightweight fractions in the asphalt binder [151]. Simultaneously, the nitrogenous organic compounds (Lewis bases) in the W1 warm-mix additive react with asphalt sulfides (Lewis acids), further prompting the decline of VOC emissions. The W2 warm-mix additive’s stable structure limits reactivity with asphalt. Consequently, the N15W2 sample showed minimal VOC emission reduction versus the N15 sample. In this paper, samples containing warm-mix additives were tested with the same temperature as those without additives. Consequently, the isolated effect of the warm-mix additives was analyzed. Future studies should investigate the combined influence of temperature and warm-mix additives.

Solid deodorant was added to the N15 sample at 0.1% (N15D1), 0.2% (N15D2), and 0.3% (N15D3). The N15D1 sample showed 34% lower VOC emissions than the N15 sample at 0.1% deodorant loading. However, the VOC emissions increased by 18% and 6% at 0.2% and 0.3% deodorant loading. Without a micropore/layered structure, the deodorant’s VOC absorption ability would not be enhanced regardless of content.

Activated CR particle was incorporated at 10% (A10), 15% (A15), and 20% (A20). The A10 sample showed peak VOC emissions, with the A15 and A20 samples decreasing by 2% and 33%, respectively, versus the A10 sample. Higher activated CR content absorbs more lightweight fractions in the asphalt binder, increasing asphalt viscosity. The increased asphalt viscosity restricts molecular mobility, limiting VOC emissions. Activated CR samples emitted 2.15 times (A10), 1.24 times (A15), and 1.17 times (A20) more VOC emissions than non-activated CR counterparts at equivalent CR content conditions. This result can be explained by the fact that the activation procedure used in this paper breaks certain chemical bonds of the CR particle; the fume would more easily escape, which contributes to the increased VOC emissions. This outcome was inconsistent with the findings in reference [130], in which CR pre-treated with microwave activation, waste cooked oil (WCO) swelling, microwave/WCO combined, HCl solution soaking, and NaOH solution soaking reduced VOC emissions in the corresponding modified asphalt. Microbial pretreatment (7–14 days) for CR helped in reducing VOC emissions of the corresponding CR-modified asphalt [131]. The outcome suggested that solution immersion removes VOC precursor substances from CR particles but minimally weakens the chemical bond of CR particles. The chemical bond, like the S–S bond, prefers to rupture at high temperature due to lower dissociation energies [160].

Adding 2% SBS modifier to non-activated CR-modified asphalt increased VOC emissions by 128% (SN10) and 49% (SN15), respectively. SN15 and SN10 samples showed the highest and the second-highest VOC emissions overall. The SBS modifier absorbs the lightweight fractions in the asphalt binder. Leaving insufficient lightweight fractions for non-activated CR particles swelling, this prolongs the degradation of untreated CR particles under thermal conditions and increases VOCs release. The CR particles undergo simultaneous swelling/degradation during prolonged thermal conditions [161]. Degradation weakens chemical bonds and ruptures the net structures of the CR particles, releasing additional fumes.

With activated CR, SBS addition reduced VOC emissions by 81% (SA10), 69% (SA15), and 33% (SA20), respectively. Gel permeation chromatography (GPC) analysis [151] showed VOC emission molecular weight ranging from 0 to 1000 across binders. According to the analysis results, most VOC emissions (83%) comprised 100–299 g/mol compounds, with 15% at 300 to 399 g/mol. Thus, low molecular weight species dominated the VOC profiles. Consequently, the additive or the structure of modified asphalt that immobilizes lightweight asphalt fractions effectively suppresses VOC emissions. Activated CR’s broken chemical bonds enhance reactivity with SBS modifiers, forming a stable, easily spatial network structure that traps saturates/aromatics as less volatile compounds [162]. The improved net-shaped structure encapsulates lightweight fractions or smaller molecular substances, preventing leakage. It further prompts small molecule conversion to macromolecular, enhancing the cohesive force of molecules and increasing the viscosity of the combined modified asphalt [153]. This mechanism inhibits VOCs volatilization, aligning with [151], where absorption of lightweight components reduced 200–400 g/mol volatiles.

4.2. Composition Concentration Variation Analysis

As evident from Figure 4, the number of VOC compounds emitted from all tested samples ranged from 7 to 13. In samples SA10, SA15, and SA20, the number of emitted VOC compounds increased with the proportion of activated CR. A similar trend occurred in groups of SN10, SN15, and N10, N15, and N20. An increased proportion of CR particles enhanced interaction with the asphalt binder, consequently prompting the release of VOCs compounds.

Compositional analysis revealed that only one to three kinds of compounds per sample exceeded 10% concentration. However, these dominant compounds collectively accounted for 56% to 91% of total VOC emissions. In three samples (N10, N15D1, and A10), dominant compounds (>10% concentration) accounted for less than 70% of total VOC emissions. This indicates that higher-concentration compounds contribute over half of VOC emissions in all samples, while other species are trace substances. Targeting these dominant compounds is therefore the most effective strategy for reducing total VOC emissions.

Six VOC compounds consistently exceeded 10% concentration: trichloroethylene, toluene, p,m-xylene, styrene, o-xylene, and 4-ethyltoluene. Trichloroethylene was detected in all samples, with concentrations ranging from 20% to 79%, consistent with reference [133]. Therefore, based on convergent evidence for its definitive identification, trichloroethylene—which exhibited the highest concentration in most samples—is the primary target for mitigation, with the exception of samples SA15 and N15W2. However, numerous previous studies have not detected the trichloroethylene in VOCs of tested samples. It is widely accepted that trichloroethylene should not be present in asphalt binder, SBS, or CR. This consensus contrasts sharply with the findings of this study. It is hypothesized that the trichloroethylene may originate from the base asphalt, which was contaminated with trichloroethylene. Further studies should focus on the original base asphalt resource to eliminate the influence of unusual composition. Toluene was detected in 13 groups (excluding SA20, N10, N15, A15, and A20), with concentrations ranging from 12% to 55%, making it the second mitigation priority. Styrene and o-xylene were detected in five samples, 4-ethyltoluene in three groups, and p-xylene/m-xylene solely in sample A20. Trichloroethylene and toluene were the most prevalent compounds across all tested samples.

4.3. Potential of Secondary Organic Aerosols (SOA) and Ozone (O₃) Formation Analysis

According to the analysis in Section 4.2, although trichloroethylene is a compound detected at relatively high concentrations, the previous literature suggests that its presence may result from contamination of the base asphalt. In order to more accurately assess the potential risks of VOCs in asphalt, trichloroethylene will be excluded from the discussion in Section 4.3 and Section 4.4. Figure 5a shows that ARH compounds were the main contributors to OFP. Aromatic hydrocarbons (ARHs) contribute to OFP by participating in tropospheric photochemical reactions with nitrogen oxides (NO_x = NO + NO₂) and radicals. This process involves sunlight-driven chain reactions, where ARHs act as key precursors. The OFP contribution of VOC emissions did not correlate with their total concentration of distribution across all samples. Sample N15W1 exhibited lower OFP values, indicating that the W1 warm-mix additive inhibits ozone formation. Samples SN10 and SN15 contributed most to OFP, whereas groups N10, N15, and N20 contributed less. This difference arises because the SBS modifier prompts emissions of toluene and o-xylene, compounds exhibiting high concentrations and high ozone formation potential. The W2 warm-mix additive had an adverse effect on ozone formation inhibition. OFP results for samples N15D1, N15D2, and N15D3 indicate that deodorant content did not significantly influence ozone formation inhibition. Activated CR-modified asphalt sample (A10) showed higher OFP values than non-activated samples, while samples of A15 and A20 showed smaller OFP values. Samples SA10, SA15, and SA20 exhibited lower OFP values than the corresponding activated CR-modified asphalt samples (A10, A15, and A20). This demonstrates that the combined CR and SBS modification enhanced ozone formation inhibition, thereby reducing the environmental impact of CR-modified asphalt application.

Since not all compounds contribute to SOA formation, the SOAFP distribution differed from both OFP and total VOCs concentration distribution across samples. HH compounds contributed most significantly to SOA formation. Samples N10, N15, and N20 exhibited higher SOAFP values than the BA and SBS samples. Similarly, activated CR-modified samples (A10, A15, and A20) exhibited higher SOAFP, indicating that CR particles comparably promote SOA formation in the asphalt binder. The W1 warm-mix additive significantly inhibited SOA formation, reducing its harmful environmental effects. Conversely, the W2 warm-mix additive and deodorant showed a moderate inhibitory effect on SOA formation. CR/SBS-modified asphalt samples (SA20, SN10, and SN15) exhibited elevated SOAFP values due to high trichloroethylene concentration. Trichloroethylene, with an aerosol formation coefficient (AFC) of 5%, emerged as the primary SOA formation contributor, heightening the risk of haze pollution (e.g., PM2.5). Therefore, a VOC species’ contribution to SOA depends not only on its AFC value but also on its concentration. Consequently, trichloroethylene is the primary target for SOA pollution control; reducing its emissions would effectively mitigate SOA pollution.

4.4. Human Carcinogen Risk Analysis

According to the carcinogen risk indices of compounds in VOCs detected in all samples, Table 5 identifies three class 1 carcinogens in VOC emissions: benzene and 1,2-dichloroethane. As previously noted, benzene and trichloroethylene were detected in all samples. Trichloroethylene concentration varied significantly across samples, ranging from 0.4 mg/m³ (SA15) to 5.3 mg/m³ (A15). The percentage of concentration of trichloroethylene in total VOC emissions exhibited an enlarged range, accounting for 20% (SA15) to 79% (A15). Only the SA15 sample contained <30% trichloroethylene, while five samples exceeded 50%. Thus, trichloroethylene represents the highest-priority carcinogen due to its dominant concentration and risk. Therefore, it is crucial to verify the origin of trichloroethylene. Consequently, trichloroethylene will be excluded from the discussion in this section to enable a more precise analysis. Benzene (also a class 1 carcinogenic substance) was detected in all samples, with concentrations ranging from 1.9 μg/m³ (N15W1) to 0.6 mg/m³ (N15D1). Benzene concentrations were consistently lower than trichloroethylene’s concentration, constituting 0.1% to 9% of total VOC emissions. Given its lower but non-negligible concentration, benzene reduction measures are warranted. 1,2-Dichloroethane appeared in six samples (2.5–11.8 μg/m³), contributing <0.3% of total VOC emissions—insignificant for carcinogen risk.

Tetrachloroethane, methylene chloride, and styrene are class 2A carcinogens (probable human carcinogens). Methylene chloride appeared only in the N15 sample (1.1 μg/m³) and the N15W1 sample (13 μg/m³). Tetrachloroethane occurred in six samples (0.8–11.7 μg/m³), exceeding 2 μg/m³ only in the N15W1 sample. Dichloromethane and tetrachloroethane (class 2A carcinogens) showed lower concentrations and limited occurrence compared to class 1 carcinogens. Thus, tetrachloroethane and methylene chloride (class 2A carcinogens) pose minimal carcinogen risk. Styrene was the predominant class 2A carcinogen, detected in 13 samples at 12.8 μg/m³–1.1 mg/m³ (0.2%–26% of total VOC emissions).

Four class 2B carcinogens were identified: carbon tetrachloride, 1,1-dichloroethene, chloroform, and ethylbenzene, showing variable occurrence and concentrations. Carbon tetrachloride appeared solely in N15W1 (3.3 μg/m³). 1,1-Dichloroethene occurred in the SA20 sample (4.6 μg/m³) and the N15W2 sample (21.9 μg/m³). Chloroform appeared in six samples (0.8 μg/m³), contributing <1% of total VOC emissions. Ethylbenzene occurred in ten samples (7–474 μg/m³, 0.1%–7%), making it the most significant class 2B carcinogen.

HHs constituted a significantly larger proportion of Table 5 compounds than ARHs. Thus, HH contributed most to the human carcinogen risk for all VOC emissions. Mitigation strategies should target HHs volatilization and concentration to reduce human carcinogen risk in CR-modified asphalt.

According to previous research, not all VOC compounds pose a human health risk. Non-carcinogenic risks encompass VOCs’ health effects excluding cancer and genetic mutations [133,163]. Compounds of VOC emissions should be classified via the globally harmonized system of classification and labeling of chemicals (GHS) for health hazards, acute toxicity, or environmental hazards. Figure 6 shows varying proportions of non-carcinogenic risk substances across samples, with all samples exceeding 70% (72%–98%). Therefore, non-carcinogens dominate VOC emissions in most samples.

4.5. VOC Emission Mechanism Analysis

VOC emissions in non-activated CR-modified asphalt arise from physical volatility and chemical decomposition. Non-activated CR particles contain networks of stable S–S bonds and C–S bonds, resulting in a smooth surface and inertia characteristic that exhibits poor compatibility with asphalt binder [130,154]. Non-activated CR particles (crosslink density >1.5 × 10⁴ mol/cm³) [154] illustrate poor interfacial interaction with asphalt binder and limited swelling (<120%), absorbing minimal lightweight asphalt fractions. Non-activated CR modification relies on physical filling and limited swelling, exhibiting poor absorption of lightweight asphalt fractions. Most lightweight fractions remain suspended in the asphalt binder and volatilize at elevated temperatures. However, unevenly distributed non-activated CR particles partially inhibit the volatilization of lightweight fractions through the physical barrier [76]. The volatilized substances in processing additives (vulcanization accelerators, antioxidants, plasticizers) remain trapped within non-activated CR’s dense crosslink net structure due to limited swelling rate (<120%), inhibiting volatilization [154]. Elevated temperatures and shear degrade swollen non-activated CR particles, breaking part of S–S/C–S bonds and releasing low molecular weight additive decomposition products that increase VOC emissions [134,154,155].

Activated CR modification employs absorption inhibition and structural stability to control VOC emissions. Activation disrupts CR particles’ crosslink networks and dense structure, therefore increasing specific surface area (30%–50%) while reducing crosslink density (<0.3 × 10⁻⁴ mol/cm³) [154]. This enhances asphalt compatibility and swelling capacity (from <120% to 200%–300%) [130,154,155]. Activated CR modification involves multi-level swelling and interface enhancement, where VOC adsorption occurs when the pore diameter of porous materials exceeds molecular size [164]. Activated CR’s spongy-like micropore structure and swelling ability absorb lightweight asphalt components (saturated and aromatic components) via van der Waals force and capillary effect, preventing high-temperature volatilization and reducing VOC emissions [131,155]. Excessive activation (too many crosslink bonds were disrupted) or high-temperature cleavage of unstable bonds (e.g., single sulfur bond) overwhelms the absorption ability of lightweight asphalt fractions, increasing VOC emissions through accelerated molecule mobility at high temperature [154,155].

Activated CR particles exhibit significantly enhanced swelling, absorbing substantial lightweight asphalt fractions during the early modification stage. Over-enhanced compatibility between activated CR particles and asphalt binder causes (1) excessive swelling loosens activated CR structure, weakening van der Waals force/capillary absorption forces/lightweight asphalt fraction absorption ability. Absorbed lightweight fractions by activated CR particles volatilize under increased high-temperature molecular motion [154]. (2) Active surface groups (–SH, C=C) on the surface of activated CR particles generated during the activation effectively bond with polar asphalt components (e.g., resin), improving activated CR/asphalt compatibility, but compatibility remains limited. Small gaps, existing in the interface of activated CR particles and asphalt binder, became the evaporating tunnel for VOC substances [76]. The activated CR particles aggregated due to limited compatibility, which displaces the lightweight asphalt fraction within the activated CR particles aggregation area to free phases [61,154], thus, high-temperature VOC volatilization was enhanced [76,130].

SBS modifier forms a continuous 3D elastic network: styrene hard segments in SBS create physical crosslinking while butadiene soft segments establish an elastic continuous phase. This network encapsulates lightweight asphalt fractions (saturated and aromatic components) and volatile substances evaporated from the swelling of CR particles [76,134]. SBS reduces total VOC emissions by 47%. The core mechanism of VOC decline was that the network formed by the SBS modifier significantly reduces the diffusion rate of lightweight fractions and inhibits high-temperature volatilization [134].

The emission suppression of non-activated CR/SBS combined modified asphalt binder relies on limited physical absorption. The match between non-polar rubber and polar segments of SBS is low; therefore, non-activated CR particles and SBS modifier exhibit poor compatibility and cannot form a collaborative network. The modification mechanism of non-activated CR/SBS combined modified asphalt binder is an independent network and a limited hybrid. The inert surface of non-activated CR particles is difficult to combine with the chain segment of the SBS modifier; the SBS modifier constructs a network alone, and non-activated CR particles would be embedded into the network in the form of an isolated island. The network of SBS cannot effectively package the non-activated CR particles; the effect of the SBS network is influenced by the reunion of non-activated CR particles, and the synergistic effect of the SBS network and non-activated CR particles is difficult to exhibit. The gap that existed in the network due to the interface separation phenomenon in the combined modification system became a volatilizing tunnel of VOCs and cannot hinder the VOCs dispersion [76].

The introduction of the SBS modifier increases swelling in non-activated CR particles but inadvertently promotes VOC emissions. In the combined modification system, the SBS acts as an interface compatibilizer: its butadiene segments bond to the rubber chain segment in non-activated CR particles, increasing swelling from <120% to 150%–180%. Enhanced swelling disrupts the dense crosslinking structure in non-activated CR particles, releasing part of the encapsulated volatile substances (vulcanization accelerators, antioxidants, plasticizers) [154]. Improved compatibility of asphalt/non-activated CR particles enables more uniform dispersion of non-activated CR particles, partially disrupting agglomeration-induced physical barriers. Lightweight asphalt fraction volatilizes through the phase gap between asphalt and non-activated CR particles [61,76]. At 2% concentration, SBS forms a discontinuous elastic network that inadequately encapsulates volatile substances. The compatibility effect of SBS outweighs the inhibition effect; therefore, SBS may turn out to be the evaporating tunnel for VOCs, increasing the total VOC emissions concentration. Low SBS content thus reduces the inhibition effect on VOC emissions in non-activated CR/SBS combined modified asphalt binder.

Activated CR/SBS systems inhibit VOC emissions through three mechanisms: physical absorption, chemical control, and network collaboration. Enhanced physical absorption/chemical control of activated CR particles was previously described. The effect of SBS addition will be discussed. Activated CR particles exhibit a higher swelling rate. The high temperatures, excessive swelling rate, and broken micropore structure of non-activated CR particles cause the non-activated CR structure to become loose, leading to the desorption and volatilization of absorbed lightweight asphalt fractions. The SBS spatial network restricts excessive swelling of activated CR particles through steric hindrance. SBS chain segments interpenetrate pores in activated CR particles, forming mechanical constraints and interpenetrating structures between SBS and activated CR particles that maintain swelling at 180%–220% and prevent the activated CR structure from loosening [76]. SBS acts as an interface compatibilizer, enhancing asphalt-activated CR particle compatibility. Its butadiene segments (non-polar) bind surface-activated groups (–SH, C=C, –OH, –COOH) on activated CR particles [165], immobilizing absorbed VOC precursors at the interface of CR–SBS and reducing desorption probability [61,76]. Its styrene segments in SBS (highly polar) bond polar asphalt fractions (asphaltenes, resin) via hydrogen bonding/dipole interactions, strengthening the interface bond. SBS butadiene segments (non-polar) combined with a rubber chain (polyisoprene, butadiene) [61,76] to form a dense, three-phase continuous structure by the interpenetration of the swelling phase of the activated CR particles and the SBS spatial network [61,154]. This structure prompts uniform activated CR particle dispersion and minimized phase gaps that serve as VOC migration pathways. VOC diffusion coefficient emissions decreased by 40%–60% due to restricted volatilization through phase interfaces in the dense three-phase structure [61,76]. The network physically blocks VOC emissions, particularly small-molecule alkanes and aromatics. Uniform activated CR particle dispersion prevents localized high-temperature areas that concentrate lightweight fractions evaporation [130,154]. Activated CR particles exhibit enhanced micropore thermal stability, and the swelling–shrinking process of activated CR particles is more gentle. The interpenetration networks formed by SBS distribute thermal stress, maintaining structural integrity. Thus, this network cannot be easily destroyed, preventing sudden VOC release at high temperatures. Consequently, activated CR/SBS systems maintain stable, low-concentration VOC emissions [61,154].

Table 8. The inhibition mechanism of different CR-modified asphalt.

System Type	Structural Characteristics	VOC Emission Mechanism	Key Difference Symbols
Non-activated CR-modified asphalt	Dense crosslink network (intact S–S/C–S bonds)	1. Physical volatility: Light fractions escape through gaps (★★→↑↑)	★★: Floating light fractions ╳╳: Volatilized VOCs ↑↑: Emission direction
	Limited swelling (<120%)	2. Chemical decomposition: Additive release (╳╳)
	Gaps exist in the asphalt matrix	3. High volatility (gaps act as emission channels)
Activated CR-modified asphalt	Porous sponge-like structure (broken crosslinks)	1. Adsorption: Porous structure traps light fractions (▒▒▒)	▒▒▒: Adsorbed fractions ║║: Capillary retention └┘: Structural collapse
	High swelling (200%–300%)	2. Capillary retention: Restricts light fraction escape (║║)
	Capillary adsorption effect	3. Over-activation: Structural collapse leads to desorption (└┘→↑↑)
Non-activated CR/SBS combined modified asphalt	CR agglomerates into “islands”	1. Gaps act as VOC tunnels (╱╱╱→╳╳)	╱╱╱: Gap channels ╳╳: VOC emissions
	Fragmented SBS network	2. SBS-enhanced emission (additive release)
	Obvious phase separation with gaps	3. Low SBS (2%) causes network discontinuity, failing to contain emissions
Activated CR/SBS combined modified asphalt	Activated CR and SBS form an interpenetrating continuous network	1. Synergistic suppression: Physical adsorption + chemical bonding (–SH/SBS binding) + network encapsulation
	Triple suppression mechanism (adsorption, bonding, encapsulation)	2. VOCs diffusion reduced by 40%–60%
	Good high-temperature stability	3. Uniform heat distribution and stable structure at high temperatures

summarizes VOC emission inhibition mechanisms across all CR-modified asphalt samples.

5. Conclusions

This study analyzed VOC emission characteristics from CR-modified asphalt binder with varying CR types, CR contents, and additives using GC–MS. How CR-modified asphalt binder parameters influence total VOC emissions, composition concentration variation, potential of secondary organic aerosols (SOA) and ozone (O₃) formation, and human carcinogen risk analysis was evaluated. The VOC emission mechanisms across CR modification types were analyzed, with conclusions summarized below.

(1). Increasing non-activated CR content initially elevates total VOC emissions. Peak VOC emissions occur at 15% non-activated CR content (3.99 times that of base asphalt). At 20% CR content, VOC emissions decrease due to the lightweight asphalt fraction. Higher non-activated CR contents increase compound diversity and shift the composition of VOC emissions toward the HHs group, dominated by trichloroethylene (20%–79%). These also elevate SOA formation potential while showing lower OFP values than SBS-modified asphalt.

(2). Surfactant warm-mix additive (W1) reduces the total VOC emission concentration by 73% in 15% non-activated CR content asphalt through lightweight fractions encapsulation and sulfide reaction. Warm-mix additive (W2) shows a negligible reduction. W1 significantly suppresses OFP, while W2 marginally reduces SOAFP, demonstrating differential efficacy. W1 minimizes carcinogenic trichloroethylene and benzene.

(3). Additionally, 0.1% deodorant reduces VOC emissions by 34% in 15% non-activated CR-modified asphalt, but higher doses (0.2%–0.3%) increase emissions by 6%–18% due to the non-porous structure. Deodorants show limited VOC emission inhibition efficacy and negligible OFP/SOAFP impact.

(4). Activated CR-modified asphalt shows peak VOC emissions at 10% CR content (A10); A15 and A20 show 2% and 33% reductions versus A10. However, activated CR-modified asphalt VOC emissions exceed non-activated counterparts at equal contents. Trichloroethylene dominates, with diversity increasing with CR content. OFP increases due to ARHs, while SOAFP remains high (HHs-driven).

(5). Non-activated CR/SBS compound blends increase VOC emissions by 128% (10% CR content) and 49% (15% CR content) due to phase gaps and additive release. OFP peaks are due to toluene/o–xylene. Activated CR/SBS compound blends reduce VOC emissions by 81% (10% CR content) and 69% (15% CR content) through triple synergy: adsorption, SBS–CR bonding, and network encapsulation. OFP decreases, while SOAFP correlates with trichloroethylene.

(6). Class 1 carcinogen benzene (0.1%–9%) was ubiquitous. HHs contribute >70% of the carcinogenic risk. Non-carcinogenic risks exceed 70% in all samples. Activated CR/SBS combinations reduce carcinogen proportions.

(7). Non-activated CR particles feature dense crosslinks and low swelling (<120%), releasing VOCs through bond breakage and poor lightweight asphalt fraction absorption. Activated CR exhibits a porous structure and high swelling (200%–300%), absorbing VOCs but risking structure collapse at high temperatures, which prompts VOCs. SBS addition creates interpenetrating networks with activated CR, reducing VOCs by 40%–60%. Non-activated CR/SBS blends suffer phase separation, creating VOCs diffusion pathways.

Non-activated CR’s dense crosslink structure and poor compatibility create barriers that inhibit inner compound release, lowering total VOC emissions. SBS improves compatibility with asphalt and prompts non-activated CR swelling (150%–180%), disrupting physical barriers and creating an incomplete network. Then, the diffusion tunnels and releasing efficiency of VOCs will be enhanced, and the volatilizing composition in non-activated CR particles and lightweight asphalt fractions will evaporate by the phase gap between non-activated CR and asphalt. SBS/non-activated CR interactions decompose part of the stable fractions (heavy aromatic hydrocarbons) into small molecular VOC compounds, increasing VOC emissions. Thus, SBS dose and CR activation status jointly influence VOC emissions, with low SBS dose in a non-activated CR system weakening VOC emission inhibitions.

Activated CR particles rely only on porous structures to absorb part of the VOC precursors. Without physical network restraint, excessive swelling loosens the structure of activated CR particles, increasing VOC emissions. In an activated CR/SBS system, porous structure, network isolation, optimized compatibility, inhibition of excessive swelling, and reduced temperature sensitivity collectively inhibit VOC volatilization. SBS forms a higher effective collaborative inhibiting mechanism with activated CR particles, yielding lower VOC emissions than the non-activated counterparts at equal CR content. This highlights CR activation’s critical role in VOC emission inhibition. Activation maximizes SBS network inhibition and CR absorption, demonstrating a synergistic (“1 + 1 > 2”) emission reduction effect.

While this study provides a comprehensive profile of VOC emissions, certain limitations should be acknowledged. Firstly, although GC–MS is the standard technique for such analyses, the quantitative accuracy of MS can be influenced by factors such as ionization efficiency and matrix effects. Although we employed internal standards to calibrate and improve quantitative reliability, these inherent limitations of the method may affect the absolute concentrations reported. Secondly, the unexpected presence of compounds like trichloroethylene, while strongly indicated by convergent evidence (including characteristic isotope patterns), could be further confirmed in future work using high-resolution mass spectrometry to achieve unambiguous identification. These points highlight avenues for more precise methodological development in subsequent studies.