materials

Article VOC Emission Analysis of Bitumen Using Proton-Transfer Reaction Time-Of-Flight Mass Spectrometry

Jaffer Bressan Borinelli 1,* , Johan Blom 1, Miguel Portillo-Estrada 2 , Patricia Kara De Maeijer 1 , Wim Van den bergh 1 and Cedric Vuye 1 1 Road Engineering Research Section (RERS), EMIB, Faculty of Applied Engineering, University of Antwerp, 2020 Antwerp, Belgium; [email protected] (J.B.); [email protected] (P.K.D.M.); [email protected] (W.V.d.b.); [email protected] (C.V.) 2 Research Group PLECO (Plants and Ecosystems), Faculty of Science, University of Antwerp, 2610 Wilrijk, Belgium; [email protected] * Correspondence: jaff[email protected]; Tel.: +32-3-265-9676



Received: 17 July 2020; Accepted: 17 August 2020; Published: 19 August 2020


Abstract: Bitumen is one of the most important materials used in roads. During asphalt pavement construction, workers can be affected by emissions, such as volatile organic compounds (VOCs), when bitumen is heated. Therefore, it is crucial to correctly identify and measure VOCs. This paper presents a novel, promising method to determine VOC emissions. The proposed method offers a way to standardize routine measurements on a lab scale, enabling reliable comparison across bitumen types and their modifications or additives. A proton-transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) was used to monitor VOC emissions from commercial unmodified bitumen and crumb rubber modified bitumen (CRMB) with heating of up to 180 ◦C. Results confirmed that the temperature range of 160–180 ◦C is a highly influential factor for VOC emissions from heated commercial bitumen and particularly CRMB. A significant increase in alkane and aromatic emission was detected when the binders were heated to 180 ◦C. Sulfur-containing VOCs were almost nonexistent for the base bitumen fumes, while a significant increase was detected in the fumes when two different types of CR were added to the bitumen, even at 120 ◦C. The additional CR in the bituminous binder contributed to the potentially harmful VOC emission of benzothiazole, which belongs to the class of sulfur-containing compounds. The concentration of benzothiazole was 65%, 38%, and 35% higher for CR1 in comparison to CR2 at 140, 160, and 180 ◦C, respectively. It is clear from the results that this method allows different bitumen sources or modifications to be quickly analyzed and their VOC emissions cross-compared. If adopted and confirmed further, the method could offer the asphalt industry a viable solution to monitor VOC emissions by analyzing samples in real time at different steps of the production process.

Keywords: volatile organic compounds (VOCs); crumb rubber modified bitumen (CRMB); bitumen fumes; proton-transfer reaction time-of-flight mass spectrometry (PTR-TOF-MS)

1. Introduction Bitumen is a residue of crude oil refinery, which passes through a refinery process to be used as a binder. Although most lightweight compounds in crude oil are removed during this process, there is still some remaining in the final bitumen residue, which can be released as emissions during the production process of asphalt mixture at a high temperature (>150 ◦C) [1,2]. The most significant components of bitumen fumes, released at higher or lower concentrations, are volatile organic

Materials 2020, 13, 3659; doi:10.3390/ma13173659 www.mdpi.com/journal/materials Materials 2020, 13, 3659 2 of 14 compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs), sulfur, nitrogen oxides, particulates, and carbon monoxide [3–5]. These components can also be found in crumb rubber modified bitumen (CRMB). In recent years, new-end market solutions have been developed to find an alternative solution for end-of-life tires (ELTs) than sending them to landfills or burning them [6]. The rubber employed for the fabrication of tires is of high quality and resilient, leading to great potential for high-end recycling. The application of rubberized bitumen for road construction using CR from ELTs has potential in Europe due to governmental support for recycling and increasing concern about waste accumulation. CR blended into bitumen can improve asphalt pavement performance, such as high-temperature stability, fatigue life, and cracking resistance, and can be considered as a possible solution for waste tire recycling [6–10]. During asphalt pavement construction, the bitumen is heated to 160–180 ◦C. The surrounding air quality is affected by these fumes, which potentially causes harmful health effects to on-site workers [11] due to their toxic and carcinogenic effects [5]. The most common symptoms experienced by workers during short-term exposure (15 min) to high levels of bitumen fumes are irritation of the nose, upper respiratory tract, skin, and eyes. However, these symptoms are reversible once the exposure has stopped [3,11]. VOC concentrations while paving are highly dependent on air movement. Therefore, higher concentrations are registered in the absence of wind. Despite that, the local concentrations of harmful VOCs are usually kept below the exposure limits according to several regulatory agencies [2,11,12]. In Europe, the VOC Solvents Emissions Directive [13] is considered the main policy instrument to reduce VOC emissions. The Chemical Agents Directive (98/24/EC) [14] and the Directive on Carcinogens and Mutagens at Work (2004/37/EC) [15] aim to protect workers from chemical risks at the workplace. The National Institute for Occupational Safety and Health (NIOSH) has evaluated scientific evidence concerning the potential health effects of occupational exposure to bitumen [16]. NIOSH recommended bitumen fumes to be considered as a potential occupational carcinogen based on the results from animal studies. Although carcinogenic PAHs have been identified in bitumen fumes at various worksites, the measured concentrations and the frequency of their occurrence have been low. In order to minimize possible acute or chronic health effects from exposure to bitumen, bitumen fumes, and vapors, a maximum emission of 5 mg/m3 during any 15 min period was recommended as a limit [17]. In the study by Lin et al. [5], the concentrations of total VOC and the 11 detected VOC compounds were extremely low and barely exceeded 3 parts per billion (ppb) by volume when tested by a gas chromatograph/mass spectrometer (GC/MS). The VOC concentration was shown to vary in accordance with the type of bituminous mixture and temperature [5]. The temperature is an essential factor when it comes to the amount of VOC emissions, following an exponentially rising curve with the increase in temperature. The bitumen source significantly affects VOC due to the presence of additives added by bitumen suppliers [2]. Moreover, the addition of CR to bitumen contributes to the total VOC emissions and especially the emission of pollutants such as xylene, benzothiazole, and toluene [2]. Due to the significant effect of pollutants in fumes to human health and the complexity of emission dynamics, which depends on the initial composition and the additives used, more insight into the relationship between temperature and VOC emissions is needed, especially if CR is added. Warm mixture asphalt (WMA) plays an important role in asphalt technology as this technique allows the production of healthier working conditions and environmentally friendly bituminous mixtures by reducing the mixing, production, and compaction temperatures using additives or foamed bitumen. The reduction in temperature for WMA can reach up to 90 ◦C below that used for hot mixture asphalt [18]. Comparing the emissions from recycled tire rubber modified bitumen in hot and warm mix conditions, the temperature is an important factor when it comes to the emission of VOCs [2]. Using zeolite materials through bitumen foaming can reduce the production and compaction temperatures by 30 ◦C[19]. Another recent technology is the use of geopolymer additives in WMA. Due to the porous structure of geopolymers, a high capability of absorbing bitumen VOCs is achieved during asphalt production, thereby reducing VOC emissions [20]. Materials 2020, 13, 3659 3 of 14

Previous studies have included tests performed to simulate the fume generation process using bitumen fume generators in a laboratory [21] and to collect the fumes on-site [2,11,12]. As shown in previous studies [4,22,23], the environment in the field is always different from one site to another. Differences in air temperature, wind speed, sun exposure, materials, mixture and compaction temperatures, sampling methods, or even the combination of all these factors are the reasons for the high discrepancies in the chemical composition and toxicological properties of bitumen fumes that are found between samples collected in the field and those produced in laboratories [11]. Current research methods on laboratory scale include flame ionization detection (FID) and photo ionization detection (PID), which are commonly used to test the total mixed VOCs [24]. In addition, GC/MS and ambient volatile organic canister sampling (AVOCS) are used to determine the VOC components [25]. However, these available measurement methods are selective in what they can accurately measure and quantify [26]. Proton-transfer reaction time-of-flight mass spectrometry (PTR-TOF-MS) represents a useful and innovative solution in the monitoring of VOC emissions. The technique has been successfully applied to environmental research such as measurement of ecosystem VOC fluxes by coupling the instrument to a flux tower [27], to plant VOC emissions by coupling it to a leaf chamber [28], and to air pollution cleaning potential of bacteria by measuring the headspace of the cultures [29]. Other applications include the measurement of VOCs that characterize food flavor [30], indoor air quality studies [31], and breath analysis in medical research [32]. Accordingly, PTR-TOF-MS has potential application for bitumen research and the asphalt sector in monitoring VOC emissions at different steps of the production process [33]. The current study was carried out on a laboratory scale and presents a promising method to identify VOC emission for bituminous samples. This way, different bitumen modifications can be quickly analyzed, and their VOC emissions crossed-compared. In the present study, this method was not used to characterize airborne emissions during on-site work. In fact, it targeted the emission source in controlled conditions to improve the identification of VOCs and, consequently, identify potential health risks for workers before or during pavement construction. Using PTR-TOF-MS, the VOC blend in the fumes of commercial unmodified bitumen was analyzed qualitatively and quantitatively along a gradient of temperatures while being heated. The tests were repeated with the same bitumen containing CR additions of different particle sizes. The rest of the paper is structured as follows: introduction of the research materials and methods, presentation of the VOC measurements, reporting of the results using a semiquantitative approach, and presentation of the conclusions.

2. Materials and Methods

2.1. Materials The base bitumen used in the current study was a standard 50/70 unmodified binder. The physical properties of the bitumen are given in Table1. The penetration and softening points were obtained in the laboratory following the standards EN 1426 and EN1427, while the density at 25 ◦C was reported by the supplier. Two different commercially available crumb rubber types (named CR1 and CR2) were used in the current study as modifications of the base bitumen. Both CR were produced by ambient grinding (i.e., passenger car tires milled into small particles at ambient temperature). This type of grinding generates irregularly shaped particles with relatively large surface areas, which improves the interaction between bitumen and CR [10]. This type of grinding is also the most used and the most cost-effective way to process ELTs. CR1 particles have an approximate length of up to 0.8 mm (Figure1a), while CR2 is composed of smaller-sized rubber granulates with length of up to 0.5 mm (Figure1b). Materials 2020, 03, x FOR PEER REVIEW 4 of 13

MaterialsThe2020 CRMB, 13, 3659 was prepared by blending for 30 min the base bitumen with CR particles with a4 ratio of 14 of 20% mass (80% bitumen and 20% CR). A high shear mixer with a speed of 3000 r.p.m. at a temperature of 180 °C was used to prepare the samples. A heating plate and a thermocouple were used to ensure the temperatureTable 1. Rheological was maintained properties throughout of the reference the process bitumen. (180 °C), and the proper digestion of CR was guaranteed [34]. The CRMB1 samples were produced with the CR1 and CRMB2 Property Unit Results Test Standard samples with CR2. The REF samples were produced as the reference bitumen samples without any Penetration (at 25 C) dmm 51 EN 1426 CR addition. For each mixture, 500 g ◦of CRMB was prepared, and 5 g (±0.01 g) of the mixture was Softening point ◦C 50.2 EN 1427 transferred into individual containers with a circular3 area of 707 mm2 (30 mm of inner diameter). The Density (at 25 ◦C) g/cm 1.025 EN 15326 samples were prepared 24 h before the VOC measurements.

(a) (b)

Figure 1.1. ScanningScanning electronelectron microscopy microscopy images images of of the the crumb crumb rubber rubber (CR) (CR) particles: particles: (a) CR1 (a) andCR1 (band) CR2. (b) CR2. The CRMB was prepared by blending for 30 min the base bitumen with CR particles with a ratio of 20%2.2. VOC mass Sampling (80% bitumen Method and 20% CR). A high shear mixer with a speed of 3000 r.p.m. at a temperature of 180 C was used to prepare the samples. A heating plate and a thermocouple were used to ensure The◦ experimental setup to measure VOCs is shown in Figure 2a. This setup allows heating of the temperature was maintained throughout the process (180 C), and the proper digestion of CR was the bitumen samples while continuously measuring the fumes◦ emitted at different temperatures. The guaranteed [34]. The CRMB1 samples were produced with the CR1 and CRMB2 samples with CR2. VOC emission rates are referred to as the units of surface area (µmol m−2 s−1). Measurements of the The REF samples were produced as the reference bitumen samples without any CR addition. For each background air in the empty chamber were performed to correct the values measured during the mixture, 500 g of CRMB was prepared, and 5 g ( 0.01 g) of the mixture was transferred into individual tests for a possible influence of the chamber materials± and tubing. containers with a circular area of 707 mm2 (30 mm of inner diameter). The samples were prepared 24 h The containers with 5 g (±0.01 g) of bitumen were placed in a 120 mL glass chamber on top of a before the VOC measurements. hot plate T1 (Cimarec + HP88857105, ThermoFisher Scientific, Waltham, MA, USA) equipped with a 2.2.thermostat. VOC Sampling A thermocouple Method sensor monitored the chamber headspace temperature T3, and a second sensor T2 was inserted into the bitumen sample to monitor the actual temperature of the bitumen (see TheFigure experimental 2b). The fumes setup were to measuresampled VOCsat 100 issccm shown (0.682 in µmol Figure s2−1a.) through This setup a 1/16 allows inch heating(outside ofdiameter) the bitumen PEEK samples (polyether while ether continuously ketone) capillary measuring placed the fumes5 cm above emitted the at bitumen different container. temperatures. The µ 2 1 Thecapillary VOC emissionwas directly rates connected are referred to to a as PTR-TOF- the unitsMS of surface 8000 (Ionicon area ( mol Analytik m− s− ).GmbH, Measurements Innsbruck, of theAustria) background for real-time air in themonitoring empty chamber of VOC wereconcentrations performed and to correctfurther the analysis. values measured during the tests for a possible influence of the chamber materials and tubing. Materials 2020The, 03, PTR-TOF-MSx FOR PEER REVIEW was operated using H3O+ as the primary ion. The instrumental conditions5 of 13 of the drift tube were 600 V electric potential, 80 °C temperature, and 2.3 mbar pressure, affording a field density ratio of ≈140 Td (1 Townsend = 10−17 cm2 V−1 s−1). A multicomponent gas standard calibration mixture (Apel Riemer, Broomfield, CO, USA) containing 10 VOCs (methanol, acetaldehyde, isoprene, acetone, methyl vinyl ketone, benzene, toluene, t-2-hexen-1-al, c-3-hexen-1- ol, and α-pinene) with masses ranging from m +/z 33 to 137 was used to assess the transmission factor of VOCs of different masses. The gas mixture was diluted through a commercially available gas calibration unit (Ionicon, Innsbruck, Austria) by adding VOC-free air with the same relative humidity as room air. The transmission factors of other compounds were calculated via interpolation using the transmission curve. (a) (b) The coefficients of reaction between each VOC and H3O+ were calculated via direct calibration FigureFigure 2. Measurements 2. Measurements of volatile of volatile organic organic compounds compounds (VOCs) (VOCs) from froma bitumen a bitumen sample: sample: (a) the (a ) the for acetone, benzene, toluene, and hexenol using the abovementioned gas standard calibration. For experimentalexperimental setup setupand (b and) its (simplifiedb) its simplified scheme scheme (T1: hot (T1: plate hot thermometer, plate thermometer, T2: thermocouple T2: thermocouple sensor sensor monitoringthe othermonitoring VOCs,bitumen bitumen approximations temperature, temperature, T3: thermocoupwere T3: thermocouplemadele using sensor sensorva monitoringlues monitoring previously chamber chamber described temperature). temperature). by Cappellin et al. [35], or otherwise, 2 × 10−9 cm³ s−1 was assumed. The VOCs identified in the bitumen fumes were grouped into three classes: alkanes, aromatics, and sulfur-containing. The hazard level of each VOC was classified according to the Globally Harmonized System of Classification and Labeling of Chemicals (GHS), which is based on the PubChem database [36].

2.3. Procedure to Measure and Identify VOCs To evaluate the influence of temperature on VOC emission rates, six samples (two samples per CR type and two reference samples) were tested. Two series of tests were performed, as shown in Figure 3. The step-by-step experimental procedure was applied in the first series, where the samples were heated in steps of 20 °C from 120 to 180 °C. During each stage, the same sample was first heated to the required temperature (e.g., 120 °C). Then, VOCs were measured, and the sample was cooled until room temperature (20 ± 5 °C). As soon as the sample was cooled enough, it was heated to the next temperature (140 °C), and VOCs were measured again; the same test procedure was repeated for 160 and 180 °C. As two samples of each mixture were used for this series, an average between the two results was used to represent the results.

200 180 Step-by-step 160 Incremental 140 120 100 80

Temperature(°C) 60 40 20 0 0 30 60 90 120 150 180 210 240 Time (minutes) Figure 3. Schematic representation of the heating process for test series 1 (step-by-step) and series 2 (incremental).

An incremental increase of temperature was adopted in the second series, where the samples were tested with a continuously increasing temperature from 120 to 180 °C. The VOCs were measured in steps of 10 °C. These two series were adopted in order to evaluate the influence of the heating process on VOC measurements. Because the PTR-TOF-MS is a sensitive equipment, the intention behind the stages in series 1 was to guarantee the temperature was stable during measurements and to avoid interferences from the previous stages on the measurement as fumes from the previous heating might be trapped inside the chamber. In series 2 (with an incremental increase in temperature), the stabilization of temperature and quantity of emission during the test was observed starting from 100 °C. However, the results for analysis were considered starting from 120 °C in order to compare both series.

Materials 2020, 13, 3659 5 of 14

The containers with 5 g ( 0.01 g) of bitumen were placed in a 120 mL glass chamber on top of a ± hot plate T1 (Cimarec + HP88857105, ThermoFisher Scientific, Waltham, MA, USA) equipped with a thermostat. A thermocouple sensor monitored the chamber headspace temperature T3, and a second sensor T2 was inserted into the bitumen sample to monitor the actual temperature of the bitumen (see 1 Figure2b). The fumes were sampled at 100 sccm (0.682 µmol s− ) through a 1/16 inch (outside diameter) PEEK (polyether ether ketone) capillary placed 5 cm above the bitumen container. The capillary was directly connected to a PTR-TOF-MS 8000 (Ionicon Analytik GmbH, Innsbruck, Austria) for real-time monitoring of VOC concentrations and further analysis. + MaterialsThe 2020 PTR-TOF-MS, 03, x FOR PEER was REVIEW operated using H3O as the primary ion. The instrumental conditions5 of 13 of the drift tube were 600 V electric potential, 80 ◦C temperature, and 2.3 mbar pressure, affording a field density ratio of 140 Td (1 Townsend = 10 17 cm2 V 1 s 1). A multicomponent gas standard ≈ − − − calibration mixture (Apel Riemer, Broomfield, CO, USA) containing 10 VOCs (methanol, acetaldehyde, isoprene, acetone, methyl vinyl ketone, benzene, toluene, t-2-hexen-1-al, c-3-hexen-1-ol, and α-pinene) with masses ranging from m +/z 33 to 137 was used to assess the transmission factor of VOCs of different masses. The gas mixture was diluted through a commercially available gas calibration unit (Ionicon, Innsbruck, Austria) by adding VOC-free air with the same relative humidity as room air. The transmission factors of other compounds were calculated via interpolation using the transmission curve. (a) (b) + FigureThe coe 2.ffi Measurementscients of reaction of volatile between organic each compounds VOC and H (VOCs)3O were from calculated a bitumen via sample: direct ( calibrationa) the for acetone,experimental benzene, setup toluene, and (b) its and simplified hexenol scheme using the(T1: abovementioned hot plate thermometer, gas standard T2: thermocouple calibration. sensor For the othermonitoring VOCs, approximations bitumen temperature, were madeT3: thermocoup using valuesle sensor previously monitoring described chamber bytemperature). Cappellin et al. [35], or otherwise, 2 10 9 cm3 s 1 was assumed. × − − TheThe VOCs VOCs identified identified in in the the bitumen bitumen fumes fumes were were gr groupedouped into into three three classes: alkanes, alkanes, aromatics, aromatics, andand sulfur-containing. The The hazard hazard level level of of each each VOC VOC was was classified classified according according to to the the Globally HarmonizedHarmonized System System of of Classification Classification and and Labeling Labeling of of Chemicals Chemicals (GHS), (GHS), which which is is based based on on the PubChemPubChem database [36]. [36].

2.3.2.3. Procedure Procedure to to Measure Measure and and Identify Identify VOCs ToTo evaluate the influence influence of temperature on VO VOCC emission rates, six samples (two samples per CRCR type and two reference samples) were tested. Tw Twoo series series of of tests were performed, as as shown in FigureFigure 33.. The step-by-step experimentalexperimental procedure waswas applied in thethe firstfirst series,series, wherewhere thethe samplessamples were heated in steps of 20 °C◦C from from 120 120 to to 180 180 °C.◦C. Du Duringring each each stage, stage, the the same same sample sample was was first first heated heated toto the the required temperature (e.g., (e.g., 120 120 °C).◦C). Then Then,, VOCs were measured, and the sample was cooled until room temperature (20 5 C). As soon as the sample was cooled enough, it was heated to the until room temperature (20 ± 5 °C).◦ As soon as the sample was cooled enough, it was heated to the nextnext temperature (140(140 ◦°C),C), andand VOCsVOCs were were measured measured again; again; the the same same test test procedure procedure was was repeated repeated for for160 160 and and 180 180◦C. °C. As As two two samples samples of each of each mixture mixture were were used used for thisfor this series, series, an average an average between between the twothe tworesults results was was used used to represent to represent theresults. the results.

200 180 Step-by-step 160 Incremental 140 120 100 80

Temperature(°C) 60 40 20 0 0 30 60 90 120 150 180 210 240 Time (minutes) FigureFigure 3. 3. SchematicSchematic representation representation of the of heating the heating process process for test for series test 1 series (step-by-step) 1 (step-by-step) and series and 2 (incremental).series 2 (incremental).

An incremental increase of temperature was adopted in the second series, where the samples were tested with a continuously increasing temperature from 120 to 180 °C. The VOCs were measured in steps of 10 °C. These two series were adopted in order to evaluate the influence of the heating process on VOC measurements. Because the PTR-TOF-MS is a sensitive equipment, the intention behind the stages in series 1 was to guarantee the temperature was stable during measurements and to avoid interferences from the previous stages on the measurement as fumes from the previous heating might be trapped inside the chamber. In series 2 (with an incremental increase in temperature), the stabilization of temperature and quantity of emission during the test was observed starting from 100 °C. However, the results for analysis were considered starting from 120 °C in order to compare both series.

Materials 2020, 13, 3659 6 of 14

An incremental increase of temperature was adopted in the second series, where the samples were tested with a continuously increasing temperature from 120 to 180 ◦C. The VOCs were measured in steps of 10 ◦C. These two series were adopted in order to evaluate the influence of the heating process on VOC measurements. Because the PTR-TOF-MS is a sensitive equipment, the intention behind the stages in series 1 was to guarantee the temperature was stable during measurements and to avoid interferences from the previous stages on the measurement as fumes from the previous heating might be trapped inside the chamber. In series 2 (with an incremental increase in temperature), theMaterials stabilization 2020, 03, x FOR of temperature PEER REVIEW and quantity of emission during the test was observed starting6 fromof 13 100 ◦C. However, the results for analysis were considered starting from 120 ◦C in order to compare bothOne series. specimen of each bitumen (CRMB1, CRMB2, and REF) from the series with the step-by-step increaseOne in specimen temperature of each was bitumen used (CRMB1,in the series CRMB2, with and an REF)incremental from the increase series with in thetemperature step-by-step to increaseguarantee in temperaturethat the same was mixture used in and the the series values with from an incremental each series increase could be in temperaturecompared pairwise to guarantee with thateach the other. same Before mixture the and start the valuesof the fromsecond each se seriesries, each could sample be compared was cooled pairwise down with until each other.room Beforetemperature the start (20 of ± the5 °C). second series, each sample was cooled down until room temperature (20 5 C). ± ◦ 3.3. Results and Discussion InIn the current study, study, a a total total of of 35 35 VOCs, VOCs, based based on on literature, literature, was was selected selected to be to measured be measured by the by thePTR-TOF-MS. PTR-TOF-MS. These These VOCs VOCs were were emitted emitted by by the the he heatedated bitumen bitumen samples samples and and classified classified for theirtheir hazardoushazardous potentialpotential accordingaccording toto GHSGHS [[36]36] (see(see TableTable2 ).2). TheThe VOCVOC concentrationconcentration rate rate was was increased increased in allin all bitumen bitumen samples samples with with a rise a inrise temperature, in temperature, with anwith exponential an exponential increase increase detected detected between between 160 and 160 180 an◦C,d 180 as can °C, beas seencan be in Figuresseen in 4Figures and5. The4 and highest 5. The ratehighest of VOCs rate of was VOCs found was in found alkanes in (lighteralkanes VOCs),(lighter whileVOCs), the while lowest the rate lowest was rate found was for found sulfur for (heavier sulfur VOCs).(heavier This VOCs). can beThis explained can be byexplained the fact thatby the lighter fact compounds that lighter are compounds released faster; are released therefore, faster; their emissiontherefore, rate their is higher.emission In generalrate is terms,higher. it canIn gene be observedral terms, that it thecan higher be observed concentration that the rate higher for all samplesconcentration occurred rate at for a temperature all samples of occurred 180 ◦C in theat a current temperature study. Theof 180 temperature °C in the is current a highly study. influential The factortemperature for bitumen is a highly emissions influential as it is relatedfactor tofor molecule bitumen stimulation, emissions thatas it is,is arelated higher temperatureto molecule stimulatesstimulation, molecular that is, a movementhigher temperature and, consequently, stimulates more molecular compounds movement escape and, and consequently, are detected by more the equipmentcompounds [ 2escape]. The and diff erenceare detected in methodology by the equipm betweenent [2]. the The two difference series did in notmethodology show a significant between dithefference two series in emission did not rates.show a significant difference in emission rates.

600 600 600 Alkanes Alkanes Alkanes ] ] ]

Aromatics -1 -1

-1 Aromatics s s 500 Aromatics 500 500 s Sulfur-containing VOCs -2 -2 Sulfur-containing VOCs -2 Sulfur-containing VOCs 400 400 400

300 300 300

200 200 200 Emission rate [µmol m [µmol rate Emission Emission rate [µmol m [µmol rate Emission 100 m Emissionrate[µmol 100 100

0 0 0 110 120 130 140 150 160 170 180 190 110 120 130 140 150 160 170 180 190 110 120 130 140 150 160 170 180 190 Temperature (°C) Temperature (°C) Temperature (°C) (a) (b) (c)

FigureFigure 4.4. VOC concentration rate of alkanes; aromatics, andand sulfursulfur compoundscompounds forfor eacheach samplesample withwith aa step-by-stepstep-by-step increaseincrease inin temperature:temperature: (a) REF, ((bb)) crumbcrumb rubberrubber modifiedmodified bitumenbitumen 11 (CRMB1)(CRMB1) andand ((cc)) CRMB2.CRMB2.

600 600 600 Alkanes Alkanes Alkanes ] ] ] -1 -1

Aromatics Aromatics -1 Aromatics 500 s 500 500 s -2

Sulfur-containing VOCs Sulfur-containing VOCs -2 Sulfur-containing VOCs 400 400 400

300 300 300

200 200 200 Emission rate [µmol m [µmol rate Emission Emission rate [µmol m [µmol rate Emission Emissionrate [µmol m-2s 100 100 100

0 0 0 110 120 130 140 150 160 170 180 190 110 120 130 140 150 160 170 180 190 110 120 130 140 150 160 170 180 190 Temperature [°C] Temperature [°C] Te mpe r at ur e [° C] (a) (b) (c)

Figure 5. VOC concentration rate of alkanes; aromatics, and sulfur compounds for each sample with an incremental increase in temperature: (a) REF, (b) CRMB1, and (c) CRMB2.

Materials 2020, 13, 3659 7 of 14 MaterialsMaterials 2020Materials 2020, Materials03, x, 202003, FORMaterials x ,2020 FOR 03,PEERMaterials x, 03,2020PEERFOR REVIEW x , FOR PEER 03, 2020REVIEW x PEER ,FOR REVIEW03, x PEER REVIEWFOR PEERREVIEW REVIEW 7 of 713 of 137 of 713 of 137 of 137 of 13

TableTable 2. TableList 2. ListTableofTable 2.VOCs ofListTable 2. VOCs ListofListidentifiedTable VOCs2. of identifiedofList VOCs VOCs 2. identifiedof Listby VOCs identified identifiedproton-transfe ofby VOCs proton-transfeidentified by proton-transfe byidentified by proton-transfe proton-transfer rby reaction proton-transfer reactionby rproton-transfe reactiontime-of-flightr time-of-flightreaction reactionr time-of-flight reaction time-of-flight time-of-flightrmass reaction time-of-flightmass spectrometer mass time-of-flightspectrometer mass mass spectrometer massspectrometer spectrometer(PTR-TOF-MS) (PmassspectrometerTR-TOF-MS) (P spectrometerTR-TOF-MS) (P (PTR-TOF-MS)TR-TOF-MS) in (Pbitumen inTR-TOF-MS) bitumen (P inTR-TOF-MS) bitumensamples. in in bitumensamples. bitumen in samples. bitumenThe in samples. samples.TheVOCs bitumen VOCsThesamples. are TheVOCs The samples. classifiedare VOCs VOCsTheclassified are VOCs classifiedTheare for are classifiedVOCs theirclassified forare their classifiedfor are their for forclassified their their for their for their hazardoushazardoushazardous potentialhazardoushazardous potentialhazardous potential accordinghazardous potential potentialaccording potentialaccording to accordingthe accordingpotential to Glthe according toobally Gl the oballytoaccording to GlHarmonizedthe theobally to Gl GloballyHarmonized theobally to HarmonizedGl theobally Harmonized System Globally System Harmonized of System HarmonizedClassifi of System Classifi of cationSystem Classifi of cation ClassifiSystem Classification and ofcation ClassifiandLabelingcation of Labelingand Classification and Labelingof Chemicalscation Labeling ofand Chemicals ofLabeling and Chemicals of (GHS)Labeling Chemicals (GHS)of [36]Chemicals (GHS) of [36]. Chemicals(GHS) .[36] (GHS) . [[36] 36]. (GHS). [36]. [36].

CompoundCompoundCompoundCompound ClassCompound Class Compound Class Compound ClassCompound Compound Class Compound Class Compound Molecular Compound Molecular Molecular Compound MolecularFormula MolecularFormula Formula Molecular Formula Molecular Formula Formula ReferencesReferencesReferences References References References Compound References Class Formula FlammableFlammableFlammable IrritantFlammable Irritant Flammable Corrosive Irritant Flammable Corrosive Irritant Corrosive IrritantHealth Corrosive Health Irritant Hazard Corrosive Health Hazard Health Corrosive AcuteHazard Health Acute Hazard Toxic Acute Health ToxicHazard Environ. Acute Toxic Hazard Environ. AcuteToxic Hazard Environ. AcuteToxicHazard Environ. HazardToxic Environ. Hazard Environ. Hazard Hazard Flammable Irritant Corrosive Health Hazard Acute Toxic Environ. Hazard AceticAcetic acidAcetic acidAcetic acid Acetic acid C 2AceticacidH4 CO 22H acid4O C22 H 4O C2 H4OX C2 2H4XO C2 2HX4O - 2 X- X-X X-X X- -X - - X - X ------[2,12] - [2,12] - [2,12] - [2,12] [2,12] [2,12] AceticAcetone acidAcetone Acetone Acetone Acetone C C2H3HAcetone46 OC3 2H6OC 3H6OC 3H6OX-X---[XC 3H6XO C 3HX6X O XX XX- X-X X- -- X ------[2,37] - [2,37] - [2,37] - [2,37] [2,37] [2,37]2 ,12] ButenoneButenone Butenone Butenone Butenone C4ButenoneH6CO 4 H6OC 4H6OC 4H6OXC 4H6XO C 4HX6X O XX XXX XX XX -X X - X - X X- X- X - X X X X X X[37] [37] X [37] X [37] [37] [37] Acetone C3H6OXX----[2,37] ButenalButenal Butenal Butenal Butenal C4HButenal6 CO4 H6OC 4H6OC 4H6OXC 4H6XO C 4HX6X O XX XXX XX XX -X X - X - X X- X- X - X X X X X X[37] [37] X [37] X [37] [37] [37] ButenoneCyclohexanoneCyclohexanoneCyclohexanone Cyclohexanone Cyclohexanone CCyclohexanoneC46H 106COOXXX-XX[6H 10CO6 H10 CO6 H10OXC 6H10XO C 6HX10X O XX XX- X-X X- -- X ------[2,37] - [2,37] - [2,37] - [2,37] [2,37] [2,37] 37] DecaneDecane Decane Decane Decane C10HDecane C22 10H22C 10H22C 10H22XC 10HX22 C10HX -22 X- X-- --X -- X- - X - X - X- -X - X - - - - - [2,27] - [2,27] - [2,27] - [2,27] [2,27] [2,27] Butenal C4H6OXXX-XX[37] EthylbutenalEthylbutenalEthylbutenal Ethylbutenal Ethylbutenal CEthylbutenal 6H10CO6H 10CO6 H 10CO6 H10OXC 6H10XO C 6HX10X O XX XX- X-X X- -- X - - - - X- X- X - X - X - - X [2,37] - [2,37] - [2,37] - [2,37] [2,37] [2,37] CyclohexanoneHexadienolHexadienolHexadienol Hexadienol Hexadienol CC6 Hexadienol6HH1010COOXX----[6 H 10CO6 H 10CO6 H10OXC 6H10XO C 6HX10X O XX XX- X-X X- -- X ------[2,37] - [2,37] - [2,37] - [2,37] [2,37] [2,37]2 ,37] HeptanalHeptanal Heptanal Heptanal Heptanal C7HHeptanal14C O7H 14CO7 H14CO7 H14OXC 7H14XO C 7HX14X O XX XX- X-X X- -- X ------[37]- [37] - [37] - [37] [37] [37] AlkanesAlkanes Alkanes Alkanes DecaneAlkanes Alkanes C10H22 X--X--[2,27] HexanalHexanal Hexanal Hexanal Hexanal C6HHexanal12C O6H 12CO 6 H12CO6 H12OXC 6H12XO C 6HX12X O XX XX- X-X X- -- X ------[2,37] - [2,37] - [2,37] - [2,37] [2,37] [2,37] EthylbutenalHexanolHexanol Hexanol Hexanol Hexanol C C66HHHexanol1014C OOXX--X-[6H 14CO 6 H14CO6 H14OC- 6H14-O C 6H-14 XO -X -X - X-- X- -- X ------[2,37] - [2,37] - [2,37] - [2,37] [2,37] [2,37]2 ,37] Methyl-isobutyl-ketoneMethyl-isobutyl-ketoneMethyl-isobutyl-ketoneMethyl-isobutyl-ketoneMethyl-isobutyl-ketone Methyl-isobutyl-ketone C 6H12CO6 H 12CO6 H12CO6 H 12OXC 6H12XO C 6HX12X O XX XX- X-X X- -- X ------[2,37] - [2,37] - [2,37] - [2,37] [2,37] [2,37] Hexadienol C H OXX----[2,37] 2-Methyl-furan2-Methyl-furan2-Methyl-furan 2-Methyl-furan 2-Methyl-furan 2-Methyl-furanC6 5 H106CO5 H 6OC 5H6 OC 5H6OXC 5H6XO C 5HX6X O XX XX- X-X X- -- X - - - - X- X- X - X X X X X X[37] [37] X [37] X [37] [37] [37] Methyl-heptaneHeptanalMethyl-heptaneMethyl-heptaneMethyl-heptane Methyl-heptane CMethyl-heptaneCH7 H16COXX----[ 7H 16C 7H 16C 7H16X C 7HX16 C7HXX 16 XX XX- X-X X- X- X X - X - X- -X - X - X X- X- X[37] [37] X [37] X [37] [37] [37] 37] Alkanes 7 14 Methyl-heptyneMethyl-heptyneMethyl-heptyneMethyl-heptyne Methyl-heptyne Methyl-heptyneC7 H12C 7H 12C 7H 12C 7H12X C 7HX12 C7HXX 12 XX XX- X-X X- X- X X - X - X- -X - X - - - - - [37] - [37] - [37] - [37] [37] [37] Hexanal C H OXX----[2,37] PentadienePentadienePentadiene Pentadiene Pentadiene 6CPentadiene5H128C 5H8 C 5H8 C5H8X C 5HX8 C5HX -8 X- X-- --X ------[37] - [37] - [37] - [37] [37] [37]

PentanalPentanal 5 10 5 10C5H10CO5 H10O 5 10 5 X10 X X X ------[2,37][2,37] HexanolPentanalPentanal Pentanal CC6HHPentanal14C OO-X----[H O XC H XO C H XO X X- -X X - X ------[2,37][2,37] - - [2,37] [2,37]2 ,37] UndecaneUndecane Undecane Undecane Undecane C11UndecaneHC24 11H24C 11H24C 11H24-C 11H-24 C11H- -24 ------X- - X - X - X- -X - X - - - - - [27,37] - [27,37] - [27,37] - [27,37] [27,37] [27,37] Methyl-isobutyl-ketone C H OXX----[2,37] BenzaldehydeBenzaldehydeBenzaldehyde Benzaldehyde Benzaldehyde BenzaldehydeC6 7H126CO7 H 6OC 7H 6OC 7H6O-C 7H6-O C 7H-6 XO -X -X - X-- X- -- X ------[37] - [37] - [37] - [37] [37] [37]

BenzeneBenzene 6Benzene6 6 6 C6H6 C6H6 6 6 C6HX 6 X X XX - - X X - X - X - - - - [12,37] - [12,37] [12,37] 2-Methyl-furanBenzeneBenzene Benzene C5CHH6 OXX--XX[C H XC HX X X X- - X X X - - -X - - [12,37][12,37] - [12,37] 37] Bromo-benzeneBromo-benzeneBromo-benzeneBromo-benzene Bromo-benzene Bromo-benzeneC6H 5CBr6 H 5BrC6 H5 BrC6 H5BrXC 6H5XBr C 6HX5XBr XX XX- X-X X- -- X ------X X - X - X[12] [12] X [12] X [12] [12] [12] Methyl-heptane C H XX-X-X[37] Butyl-benzeneButyl-benzeneButyl-benzene Butyl-benzene Butyl-benzene Butyl-benzeneC7 10H16C14 10 H14C 10H 14C 10H14XC 10HX14 C10HXX 14 XX XX- X-X X- -- X ------X X- X- X[12] [12] X [12] X [12] [12] [12]

DiethylthiopheneDiethylthiopheneDiethylthiophene Diethylthiophene8 12C8H12CS 8H12C S 8H12S 8 12- C8H-12 S -X X X-- - - X ------[2] [2] - [2] [2] Methyl-heptyneDiethylthiophene Diethylthiophene CC7H 12S XX-X--[-C H S X - - X ------[2] - [2] 37] EthylbenzeneEthylbenzeneEthylbenzene Ethylbenzene Ethylbenzene EthylbenzeneC 8H10C 8 H10C 8H 10C 8H10X C 8HX10 C8HXX 10 XX XX- X-X X- X- X X - X - X- -X - X - - - - - [2,12,37] - [2,12,37] - [2,12,37] -[2,12,37] [2,12,37] [2,12,37] Pentadiene C H X-----[37] XyleneXylene Xylene Xylene Xylene C58HXylene 108C 8H10 C 8H10C 8H10X C 8HX10 C8HXX 10 XX XX- X-X X- X- X X - X - X- -X - X - - - - - [2,12,37] - [2,12,37] - [2,12,37] -[2,12,37] [2,12,37] [2,12,37]

AromaticsAromaticsAromatics AromaticsEthylphenyl-ethanone Ethylphenyl-ethanone Ethylphenyl-ethanoneAromaticsEthylphenyl-ethanone Ethylphenyl-ethanoneC 10HC12O 10H 12CO10 H12CO10 H 12O- 10 12- C10H-12 XO -X X- X-- - -- X ------[37]- [37] [37] - [37] [37] PentanalAromatics Ethylphenyl-ethanone C5H10OXX----[ C H O - X - - - - [37] 2,37] EthyltolueneEthyltolueneEthyltoluene Ethyltoluene Ethyltoluene CEthyltoluene 9H12C 9H12C 9H 12C 9H12X C 9HX12 C9HXX 12 XX XX- X-X X- X- X X - X - X- -X - X - X X- X- X[12] [12] X [12] X [12] [12] [12] Undecane C H ---X--[27,37] TrimethylbenzeneTrimethylbenzeneTrimethylbenzeneTrimethylbenzene Trimethylbenzene Trimethylbenzene C119H 1224C 9H12 C 9H12C 9H12X C 9HX12 C9HXX 12 XX XX- X-X X- -- X ------[12] - [12] - [12] - [12] [12] [12] IsopropyltolueneIsopropyltolueneIsopropyltolueneIsopropyltoluene Isopropyltoluene IsopropyltolueneC10 HC14 10H 14C 10H14 C 10H14XC 10HX14 C10HX -14 X- X-- --X -- X- - X - X - X- -X - X - X X- X- X[12] [12] X [12] X [12] [12] [12] Methyl-benzaldehydeMethyl-benzaldehydeMethyl-benzaldehydeMethyl-benzaldehydeMethyl-benzaldehyde Methyl-benzaldehydeC 7H6CO 7 H6OC 7H6OC 7H 6O-C 7H6-O C 7H-6 XO -X -X - X-- X- -- X ------[37] - [37] - [37] - [37] [37] [37] StyreneStyrene Styrene Styrene Styrene C8HStyrene 8C 8H8 C8H8 C8H8X C 8HX8 C8HXX 8 XX XX- X-X X- X- X X - X - X- -X - X - - - - - [12] - [12] - [12] - [12] [12] [12] TolueneToluene Toluene Toluene Toluene C7TolueneH 8C 7H8 C7H8 C7H8X C 7HX8 C7HXX 8 XX XX- X-X X- X- X X - X - X- -X - X - - - - - [2,12,37] - [2,12,37] - [2,12,37] -[2,12,37] [2,12,37] [2,12,37] Trichloro-benzeneTrichloro-benzeneTrichloro-benzeneTrichloro-benzene Trichloro-benzene Trichloro-benzene C6H 3CCl63H 3Cl C63 H3ClC 63H 3ClC-3 6 H3Cl- C3 6H-3Cl X 3 -X -X - X-- X- -- X ------X X - X - X[12] [12] X [12] X [12] [12] [12] BenzothiazoleBenzothiazoleBenzothiazole Benzothiazole Benzothiazole CBenzothiazole7 H5CNS7H 5NSC7H 5 NSC7H 5NSC- 7 H5NS- C 7H5-NS X -X -X - X-- X- -- X - - - - X- X- X - X - X - - X[2,27,37–41] -[2,27,37–41] -[2,27,37–41] [2,27,37–41] - [2,27,37–41] [2,27,37–41] SulfurSulfur Sulfur Sulfur Sulfur Sulfur Sulfur-diozideSulfur-diozideSulfur-diozide Sulfur-diozide Sulfur-diozide Sulfur-diozide S8 S 8 S8 S8 - S8 - S8- X -X -X - X-- X- -- X ------[37]- [37]- [37]- [37] [37] [37]

MaterialsMaterials 2020Materials 2020, Materials03, x, 202003, FORMaterials x ,2020 FOR 03,PEERMaterials x, 03,2020PEERFOR REVIEW x , FOR PEER 03, 2020REVIEW x PEER ,FOR REVIEW03, x PEER REVIEWFOR PEERREVIEW REVIEW 7 of 713 of 137 of 713 of 137 of 137 of 13 Materials 2020, 13, 3659 8 of 14

TableTable 2. TableList 2. ListTableof 2.VOCs ofListTable 2.VOCs ListofidentifiedTable VOCs2. of identifiedList VOCs 2. identifiedof Listby VOCs identifiedproton-transfe ofby VOCs proton-transfeidentified by proton-transfe byidentified proton-transfe rby reaction proton-transfer reactionby rproton-transfe reactiontime-of-flightr time-of-flightreactionr time-of-flight reaction time-of-flightr mass reaction time-of-flightmass spectrometer mass time-of-flightspectrometer mass spectrometer massspectrometer (PTR-TOF-MS) (PmassspectrometerTR-TOF-MS) (P spectrometerTR-TOF-MS) (PTR-TOF-MS) in (Pbitumen inTR-TOF-MS) bitumen (P inTR-TOF-MS) bitumensamples. in bitumensamples. in samples. bitumenThe in samples. TheVOCs bitumen VOCsThesamples. are TheVOCs samples. classifiedare VOCs The classified are VOCs classifiedTheare for classifiedVOCs their forare their classifiedfor are their for classified their for their for their hazardoushazardoushazardous potentialhazardous potentialhazardous potential accordinghazardous potential according potentialaccording to accordingthe potential to Glthe according toobally Gl the oballytoaccording GlHarmonizedtheobally to GlHarmonized theobally to HarmonizedGl theobally HarmonizedSystem Globally System Harmonized of System HarmonizedClassifi of System Classifi of cationSystem Classifi ofcation ClassifiSystem and ofcation Table ClassifiandLabelingcation of Labelingand Classifi 2.cation Cont.andLabeling of Chemicalscation Labeling ofand Chemicals ofLabeling and Chemicals of (GHS)Labeling Chemicals (GHS)of [36]Chemicals (GHS) of [36]. Chemicals(GHS) .[36] (GHS) .[36] (GHS). [36]. [36].

CompoundCompoundCompoundCompound ClassCompound Class Compound Class Compound ClassCompound Compound Class Compound Class Compound Molecular Compound Molecular Molecular Compound MolecularFormula MolecularFormula Formula Molecular Formula Molecular Formula Formula ReferencesReferencesReferences References References References Compound References Class Formula FlammableFlammableFlammable IrritantFlammable Irritant Flammable Corrosive Irritant Flammable Corrosive Irritant Corrosive IrritantHealth Corrosive Health Irritant Hazard Corrosive Health Hazard Health Corrosive AcuteHazard Health Acute Hazard Toxic Acute Health ToxicHazard Environ. Acute Toxic Hazard Environ. AcuteToxic Hazard Environ. AcuteToxicHazard Environ. HazardToxic Environ. Hazard Environ. Hazard Hazard Flammable Irritant Corrosive Health Hazard Acute Toxic Environ. Hazard AceticAcetic acidAcetic acidAcetic acid Acetic acid C 2AceticacidH4 CO 22H acid4O C22 H 4O C2 H4OX C2 2H4XO C2 2HX4O - 2 X- X-X X-X X- -X - - X - X ------[2,12] - [2,12] - [2,12] - [2,12] [2,12] [2,12] BenzaldehydeAcetoneAcetone Acetone Acetone Acetone CC73HHAcetone6 COO-X----[3 H6OC 3H6OC 3H6OXC 3H6XO C 3HX6X O XX XX- X-X X- -- X ------[2,37] - [2,37] - [2,37] - [2,37] [2,37] [2,37] 37] ButenoneButenone Butenone Butenone Butenone C4ButenoneH6CO 4 H6OC 4H6OC 4H6OXC 4H6XO C 4HX6X O XX XXX XX XX -X X - X - X X- X- X - X X X X X X[37] [37] X [37] X [37] [37] [37] Benzene C6H6 XX-X--[12,37] ButenalButenal Butenal Butenal Butenal C4HButenal6 CO4 H6OC 4H6OC 4H6OXC 4H6XO C 4HX6X O XX XXX XX XX -X X - X - X X- X- X - X X X X X X[37] [37] X [37] X [37] [37] [37] Bromo-benzeneCyclohexanoneCyclohexanoneCyclohexanone Cyclohexanone Cyclohexanone CCyclohexanoneC66HH 105CBrO6H 10CO6 H10 CO6 H10O XC 6H10XO C 6HX10X O X X XX- X-X X - -- X ------[2,37] - [2,37] - [2,37] X - [2,37] [2,37] [2,37] [12] DecaneDecane Decane Decane Decane C10HDecane C22 10H22C 10H22C 10H22XC 10HX22 C10HX -22 X- X-- --X -- X- - X - X - X- -X - X - - - - - [2,27] - [2,27] - [2,27] - [2,27] [2,27] [2,27] Butyl-benzene C10H14 XX---X[12] EthylbutenalEthylbutenalEthylbutenal Ethylbutenal Ethylbutenal CEthylbutenal 6H10CO6H 10CO6 H 10CO6 H10OXC 6H10XO C 6HX10X O XX XX- X-X X- -- X - - - - X- X- X - X - X - - X [2,37] - [2,37] - [2,37] - [2,37] [2,37] [2,37] DiethylthiopheneHexadienolHexadienolHexadienol Hexadienol Hexadienol CC 8Hexadienol6H1012CO6S-X----[ H 10CO6 H 10CO6 H10OXC 6H10XO C 6HX10X O XX XX- X-X X- -- X ------[2,37] - [2,37] - [2,37] - [2,37] [2,37] [2,37] 2] HeptanalHeptanal Heptanal Heptanal Heptanal C7HHeptanal14C O7H 14CO7 H14CO7 H14OXC 7H14XO C 7HX14X O XX XX- X-X X- -- X ------[37]- [37] - [37] - [37] [37] [37] AlkanesAlkanes Alkanes AlkanesEthylbenzene Alkanes Alkanes C8H10 XX-X--[2,12,37] HexanalHexanal Hexanal Hexanal Hexanal C6HHexanal12C O6H 12CO 6 H12CO6 H12OXC 6H12XO C 6HX12X O XX XX- X-X X- -- X ------[2,37] - [2,37] - [2,37] - [2,37] [2,37] [2,37] XyleneHexanolHexanol Hexanol Hexanol Hexanol CC6HHHexanol14C O6H 14CO 6 H14CO6 H14OXX-X--[C- 6H14-O C 6H-14 XO -X -X - X-- X- -- X ------[2,37] - [2,37] - [2,37] - [2,37] [2,37] [2,37]2,12 ,37] Aromatics 8 10 Methyl-isobutyl-ketoneMethyl-isobutyl-ketoneMethyl-isobutyl-ketoneMethyl-isobutyl-ketoneMethyl-isobutyl-ketone Methyl-isobutyl-ketone C 6H12CO6 H 12CO6 H12CO6 H 12OXC 6H12XO C 6HX12X O XX XX- X-X X- -- X ------[2,37] - [2,37] - [2,37] - [2,37] [2,37] [2,37] Ethylphenyl-ethanone C H O-X----[37] 2-Methyl-furan2-Methyl-furan2-Methyl-furan 2-Methyl-furan 2-Methyl-furan 2-Methyl-furan10C5 H126CO5 H 6OC 5H6 OC 5H6OXC 5H6XO C 5HX6X O XX XX- X-X X- -- X - - - - X- X- X - X X X X X X[37] [37] X [37] X [37] [37] [37] EthyltolueneMethyl-heptaneMethyl-heptaneMethyl-heptaneMethyl-heptane Methyl-heptane Methyl-heptane CC97 HH1216C 7H 16C 7H 16C 7H16XX-X-X[X C 7HX16 C7HXX 16 XX XX- X-X X- X- X X - X - X- -X - X - X X- X- X[37] [37] X [37] X [37] [37] [37] 12] Methyl-heptyneMethyl-heptyneMethyl-heptyneMethyl-heptyne Methyl-heptyne Methyl-heptyneC7 H12C 7H 12C 7H 12C 7H12X C 7HX12 C7HXX 12 XX XX- X-X X- X- X X - X - X- -X - X - - - - - [37] - [37] - [37] - [37] [37] [37] Trimethylbenzene C H XX----[12] PentadienePentadienePentadiene Pentadiene Pentadiene C9Pentadiene5H128C 5H8 C 5H8 C5H8X C 5HX8 C5HX -8 X- X-- --X ------[37] - [37] - [37] - [37] [37] [37]

PentanalPentanal 5 10 5 10C5H10CO5 H10O 5 10 5 X10 X X X ------[2,37][2,37] IsopropyltoluenePentanalPentanal Pentanal CC10HPentanalHC O14H O X--X-X[XC H XO C H XO X X- -X X - X ------[2,37][2,37] - - [2,37] [2,37] 12] UndecaneUndecane Undecane Undecane Undecane C11UndecaneHC24 11H24C 11H24C 11H24-C 11H-24 C11H- -24 ------X- - X - X - X- -X - X - - - - - [27,37] - [27,37] - [27,37] - [27,37] [27,37] [27,37] Methyl-benzaldehyde C H O-X----[37] BenzaldehydeBenzaldehydeBenzaldehyde Benzaldehyde Benzaldehyde BenzaldehydeC7 7H6CO7 H 6OC 7H 6OC 7H6O-C 7H6-O C 7H-6 XO -X -X - X-- X- -- X ------[37] - [37] - [37] - [37] [37] [37]

BenzeneBenzene 6Benzene6 6 6 C6H6 C6H6 6 6 C6HX 6 X X XX - - X X - X - X - - - - [12,37] - [12,37] [12,37] StyreneBenzeneBenzene Benzene CC8H 8C H XX-X--[XC HX X X X- - X X X - - -X - - [12,37][12,37] - [12,37] 12] Bromo-benzeneBromo-benzeneBromo-benzeneBromo-benzene Bromo-benzene Bromo-benzeneC6H 5CBr6 H 5BrC6 H5 BrC6 H5BrXC 6H5XBr C 6HX5XBr XX XX- X-X X- -- X ------X X - X - X[12] [12] X [12] X [12] [12] [12] Toluene C H XX-X--[2,12,37] Butyl-benzeneButyl-benzeneButyl-benzene Butyl-benzene Butyl-benzene Butyl-benzeneC 710HC148 10 H14C 10H 14C 10H14XC 10HX14 C10HXX 14 XX XX- X-X X- -- X ------X X- X- X[12] [12] X [12] X [12] [12] [12]

DiethylthiopheneDiethylthiopheneDiethylthiophene Diethylthiophene8 12C8H12CS 8H12C S 8H12S 8 12- C8H-12 S -X X X-- - - X ------[2] [2] - [2] [2] Trichloro-benzeneDiethylthiophene Diethylthiophene C6CHH3ClS 3 --X---X[C H S X - - X ------[2] - [2] 12] EthylbenzeneEthylbenzeneEthylbenzene Ethylbenzene Ethylbenzene EthylbenzeneC 8H10C 8 H10C 8H 10C 8H10X C 8HX10 C8HXX 10 XX XX- X-X X- X- X X - X - X- -X - X - - - - - [2,12,37] - [2,12,37] - [2,12,37] -[2,12,37] [2,12,37] [2,12,37] Benzothiazole C7H5NS - X - - X - [2,27,37–41] Sulfur XyleneXylene Xylene Xylene Xylene C8HXylene 10C 8H10 C 8H10C 8H10X C 8HX10 C8HXX 10 XX XX- X-X X- X- X X - X - X- -X - X - - - - - [2,12,37] - [2,12,37] - [2,12,37] -[2,12,37] [2,12,37] [2,12,37] AromaticsAromaticsAromatics AromaticsEthylphenyl-ethanone Ethylphenyl-ethanone Ethylphenyl-ethanoneAromaticsEthylphenyl-ethanone Ethylphenyl-ethanoneC 10HC12O 10H 12CO10 H12CO10 H 12O- 10 12- C10H-12 XO -X X- X-- - -- X ------[37]- [37] [37] - [37] [37] Sulfur-diozideAromatics Ethylphenyl-ethanone S8 C-X----[H O - X - - - - [37] 37] EthyltolueneEthyltolueneEthyltoluene Ethyltoluene Ethyltoluene CEthyltoluene 9H12C 9H12C 9H 12C 9H12X C 9HX12 C9HXX 12 XX XX- X-X X- X- X X - X - X- -X - X - X X- X- X[12] [12] X [12] X [12] [12] [12] TrimethylbenzeneTrimethylbenzeneTrimethylbenzeneTrimethylbenzene Trimethylbenzene Trimethylbenzene C9H 12C 9H12 C 9H12C 9H12X C 9HX12 C9HXX 12 XX XX- X-X X- -- X ------[12] - [12] - [12] - [12] [12] [12] IsopropyltolueneIsopropyltolueneIsopropyltolueneIsopropyltoluene Isopropyltoluene IsopropyltolueneC10 HC14 10H 14C 10H14 C 10H14XC 10HX14 C10HX -14 X- X-- --X -- X- - X - X - X- -X - X - X X- X- X[12] [12] X [12] X [12] [12] [12] Methyl-benzaldehydeMethyl-benzaldehydeMethyl-benzaldehydeMethyl-benzaldehydeMethyl-benzaldehyde Methyl-benzaldehydeC 7H6CO 7 H6OC 7H6OC 7H 6O-C 7H6-O C 7H-6 XO -X -X - X-- X- -- X ------[37] - [37] - [37] - [37] [37] [37] StyreneStyrene Styrene Styrene Styrene C8HStyrene 8C 8H8 C8H8 C8H8X C 8HX8 C8HXX 8 XX XX- X-X X- X- X X - X - X- -X - X - - - - - [12] - [12] - [12] - [12] [12] [12] TolueneToluene Toluene Toluene Toluene C7TolueneH 8C 7H8 C7H8 C7H8X C 7HX8 C7HXX 8 XX XX- X-X X- X- X X - X - X- -X - X - - - - - [2,12,37] - [2,12,37] - [2,12,37] -[2,12,37] [2,12,37] [2,12,37] Trichloro-benzeneTrichloro-benzeneTrichloro-benzeneTrichloro-benzene Trichloro-benzene Trichloro-benzene C6H 3CCl63H 3Cl C63 H3ClC 63H 3ClC-3 6 H3Cl- C3 6H-3Cl X 3 -X -X - X-- X- -- X ------X X - X - X[12] [12] X [12] X [12] [12] [12] BenzothiazoleBenzothiazoleBenzothiazole Benzothiazole Benzothiazole CBenzothiazole7 H5CNS7H 5NSC7H 5 NSC7H 5NSC- 7 H5NS- C 7H5-NS X -X -X - X-- X- -- X - - - - X- X- X - X - X - - X[2,27,37–41] -[2,27,37–41] -[2,27,37–41] [2,27,37–41] - [2,27,37–41] [2,27,37–41] SulfurSulfur Sulfur Sulfur Sulfur Sulfur Sulfur-diozideSulfur-diozideSulfur-diozide Sulfur-diozide Sulfur-diozide Sulfur-diozide S8 S 8 S8 S8 - S8 - S8- X -X -X - X-- X- -- X ------[37]- [37]- [37]- [37] [37] [37]

Materials 2020, 03, x FOR PEER REVIEW 6 of 13

One specimen of each bitumen (CRMB1, CRMB2, and REF) from the series with the step-by-step increase in temperature was used in the series with an incremental increase in temperature to guarantee that the same mixture and the values from each series could be compared pairwise with each other. Before the start of the second series, each sample was cooled down until room temperature (20 ± 5 °C).

3. Results and Discussion In the current study, a total of 35 VOCs, based on literature, was selected to be measured by the PTR-TOF-MS. These VOCs were emitted by the heated bitumen samples and classified for their hazardous potential according to GHS [36] (see Table 2). The VOC concentration rate was increased in all bitumen samples with a rise in temperature, with an exponential increase detected between 160 and 180 °C, as can be seen in Figures 4 and 5. The highest rate of VOCs was found in alkanes (lighter VOCs), while the lowest rate was found for sulfur (heavier VOCs). This can be explained by the fact that lighter compounds are released faster; therefore, their emission rate is higher. In general terms, it can be observed that the higher concentration rate for all samples occurred at a temperature of 180 °C in the current study. The temperature is a highly influential factor for bitumen emissions as it is related to molecule stimulation, that is, a higher temperature stimulates molecular movement and, consequently, more compounds escape and are detected by the equipment [2]. The difference in methodology between the two series did not show a significant difference in emission rates.

600 600 600 Alkanes Alkanes Alkanes ] ] ]

Aromatics -1 -1

-1 Aromatics s s 500 Aromatics 500 500 s Sulfur-containing VOCs -2 -2 Sulfur-containing VOCs -2 Sulfur-containing VOCs 400 400 400

300 300 300

200 200 200 Emission rate [µmol m [µmol rate Emission Emission rate [µmol m [µmol rate Emission 100 m Emissionrate[µmol 100 100

0 0 0 110 120 130 140 150 160 170 180 190 110 120 130 140 150 160 170 180 190 110 120 130 140 150 160 170 180 190 Temperature (°C) Temperature (°C) Temperature (°C) (a) (b) (c)

Figure 4. VOC concentration rate of alkanes; aromatics, and sulfur compounds for each sample with Materialsa step-by-step2020, 13, 3659 increase in temperature: (a) REF, (b) crumb rubber modified bitumen 1 (CRMB1) and9 of 14 (c) CRMB2.

600 600 600 Alkanes Alkanes Alkanes ] ] ] -1 -1

Aromatics Aromatics -1 Aromatics 500 s 500 500 s -2

Sulfur-containing VOCs Sulfur-containing VOCs -2 Sulfur-containing VOCs 400 400 400

300 300 300

200 200 200 Emission rate [µmol m [µmol rate Emission Emission rate [µmol m [µmol rate Emission Emissionrate [µmol m-2s 100 100 100

0 0 0 110 120 130 140 150 160 170 180 190 110 120 130 140 150 160 170 180 190 110 120 130 140 150 160 170 180 190 Temperature [°C] Temperature [°C] Te mpe r at ur e [° C] (a) (b) (c)

Figure 5. VOC concentration rate of alkanes; aromatics, and sulfur compounds for each sample with an incremental increase in temperature: (a) REF, (b) CRMB1, and (cc) CRMB2. an incremental increase in temperature: ( ) REF, ( ) CRMB1, and ( ) CRMB2. TheThe addition addition of of CR CR into into bitumen bitumen influenced influenced the the percentage percentage of of alkanes, alkanes, aromatics, aromatics, and and sulfur sulfur The addition of CR into bitumen influenced the percentage of alkanes, aromatics, and sulfur releasedreleased when when the the mixture mixture was was heated heated in in both both series series of of tests tests (Figures (Figures 6 and6 and 7). 7). In In these these figures, figures, the the released when the mixture was heated in both series of tests (Figures6 and7). In these figures, the relative relativerelative presence presence of of the the three three VOC VOC classes classes is isdisplayed displayed in in relation relation to to the the test test temperature. temperature. Sulfur Sulfur presence of the three VOC classes is displayed in relation to the test temperature. Sulfur compounds compoundscompounds were were almost almost nonexistent nonexistent for for base base bitu bitumenmen fumes. fumes. A Asignificant significant increase increase of of sulfur- sulfur- were almost nonexistent for base bitumen fumes. A significant increase of sulfur-containing compounds containingcontaining compounds compounds were were observed observed in in the the fumes fumes for for bitumen bitumen with with CR1 CR1 and and CR2 CR2 addition, addition, even even at at were observed in the fumes for bitumen with CR1 and CR2 addition, even at 120 C. CRMB1 exhibited 120120 °C. °C. CRMB1 CRMB1 exhibited exhibited more more emission emission of of sulf sulfurur compounds compounds in in comparison comparison◦ to to CRMB2. CRMB2. This This more emission of sulfur compounds in comparison to CRMB2. This difference in emission can differencedifference in in emission emission can can be be explained explained by by the the different different origins origins and and mechanical/chemical mechanical/chemical treatment treatment be explained by the different origins and mechanical/chemical treatment of the CR. The alkanes ofof the the CR. CR. The The alkanes alkanes became became more more evident evident in in the the VOC VOC emissions emissions for for bitumen bitumen as as the the temperature temperature became more evident in the VOC emissions for bitumen as the temperature increased from 160 to increasedincreased from from 160 160 to to 180 180 °C. °C. The The highest highest percentage percentage of of aromatics aromatics was was noticed noticed for for bitumen bitumen within within 180 C. The highest percentage of aromatics was noticed for bitumen within the temperature range thethe temperature◦ temperature range range of of 120–170 120–170 °C. °C. of 120–170 ◦C.

100100 100100 100100

80 80 80 80 80 80

60 60 60 60 60 60

40 40 40 40 40 40

20 20 20 20 20 20 Percent of total VOCs [%] VOCs total of Percent [%] VOCs total of Percent Percent of total VOCs [%] VOCs total of Percent Percent of total VOCs [%] VOCs total of Percent [%] VOCs total of Percent Percent of total VOCs [%] VOCs total of Percent 0 0 0 0 0 0 120120 140 140 160 160 180 180 120120 140 140 160 160 180 180 120120 140 140 160 160 180 180 TemperatureTemperature [°C] [°C] TemperatureTemperature [°C] [°C] TemperatureTemperature [°C] [°C] Sulfur-containing VOCs Aromatics Alkanes Sulfur-containing VOCs Aromatics Alkanes Sulfur-containingSulfur-containing VOCs VOCsAromaticsAromaticsAlkanesAlkanes Sulfur-containingSulfur-containing VOCs VOCsAromaticsAromaticsAlkanesAlkanes (a()a ) (b()b ) (c)( c)

FigureFigure 6. 6.CR CR influence influence influence on on the the percentage percentage of of alkanes, alkanes, aromatics aromatics and and sulfur-containingsulfur-con sulfur-containingtaining VOC VOC emission emission withwith a step-by-stepa step-by-step increase increase in in temperature: temperature: (a )( a REF,) REF, ((bb )()b CRMB1,CRMB1,) CRMB1, andand and ((cc) )( CRMB2.cCRMB2.) CRMB2.

100100 100100 100100 80 80 80 80 80 80 60 60 60 60 60 60 40 40 40 40 40 40

20 20 20 20 20 20 Percent of total VOCs[%] Percent of total VOCs[%] Percent of total VOCs [%] VOCs total of Percent [%] VOCstotal of Percent Percent of total VOCs [%] VOCs total of Percent [%] VOCstotal of Percent 0 0 0 0 0 0 120120 130 130 140 140 150 150 160 160 170 170 180 180 120120 130 130 140 140 150 150 160 160 170 170 180 180 120120 130 130 140 140 150 150 160 160 170 170 180 180 TemperatureTemperature [°C] [°C] TemperatureTemperature [°C] [°C] TemperatureTemperature [°C] [°C] Sulfur-containing VOCs Aromatics Alkanes Sulfur-containing VOCs Aromatics Alkanes Sulfur-containingSulfur-containing VOCs VOCsAromaticsAromaticsAlkanesAlkanes Sulfur-containingSulfur-containing VOCs VOCsAromaticsAromaticsAlkanesAlkanes (a()a ) (b()b ) (c)( c)

FigureFigure 7.7. Influence7.Influence Influence of of CR ofCR onCR on the on the percentage the percentage percentage of alkanes, of ofalka alka aromatics,nes,nes, aromatics, aromatics, and sulfur-containing and and sulfur-containing sulfur-containing VOC emission VOC VOC withemissionemission an incremental with with an an incremental incremental increase in increase temperature:increase in intemperature: temperature: (a) REF, ( b(a) )( CRMB1, aREF,) REF, (b () andb CRMB1,) CRMB1, (c) CRMB2. and and (c )( cCRMB2.) CRMB2.

ToTo evaluate evaluate the the profile profile of of VOC VOC emissions emissions at at differ differentent temperatures, temperatures, 12 12 VOCs VOCs classified classified as as irritant irritant and/orand/or health health hazard hazard and/or and/or acute acute toxic toxic and/or and/or environmental environmental hazard hazard according according to to the the overview overview shownshown in in Table Table 2 and2 and with with the the highest highest emission emission rates rates were were chosen, chosen, as as depicted depicted in in Figures Figures 8 and8 and 9. 9.In In thesethese figures, figures, the the relative relative presence presence of of the the 12 12 VO VOC Ccompounds compounds is isdisplayed displayed in in relation relation to to the the test test temperature.temperature. CR CR used used for for the the modification modification of of bitu bitumenmen can can be be the the source source of of a avariety variety of of hazardous hazardous substancessubstances released released into into the the environment. environment. This This co contributionntribution can can be be clearl clearly ynoticed noticed for for the the CRMB1 CRMB1 andand CRMB2 CRMB2 samples. samples. AsAs shown shown in in Figure Figure 8, 8,the the analysis analysis confirmed confirmed th the presencee presence of of benzothiazole. benzothiazole. The The addition addition of of CR CR inin the the bituminous bituminous binder binder contributed contributed to to the the highest highest emission emission rate rate of of benzothiazole, benzothiazole, which which was was not not observedobserved for for the the reference reference bitumen. bitumen. The The concentr concentrationation of of benzothiazole benzothiazole was was 65%, 65%, 38%, 38%, and and 35% 35% higherhigher for for CR1 CR1 in in comparison comparison to to CR2 CR2 at at 140, 140, 160, 160, and and 180 180 °C, °C, respectively. respectively. The The toxicity toxicity level level values values derivedderived for for benzothiazole benzothiazole for for acute acute and and chronic chronic expo exposuresure are are intended intended to to eliminate eliminate risk risk factors factors that that impairimpair health health (health-protective) (health-protective) [38,39]. [38,39]. Considerin Considering theg the acute, acute, chronic, chronic, mutagenic, mutagenic, and and carcinogenic carcinogenic effectseffects of of benzothiazole, benzothiazole, it itwas was included included in in the the assessment assessment of of VOCs VOCs with with risk risk to to human human health health [38]. [38]. TheThe emission emission of of benzothiazole benzothiazole can can be be detrimental detrimental to to the the health health of of workers workers and and have have an an effect effect on on the the MaterialsMaterials 2020 2020, 03, ,03 x;, doi:x; doi: FOR FOR PEER PEER REVIEW REVIEW www.mdpi.com/journwww.mdpi.com/journal/materialsal/materials Materials 2020, 03, x FOR PEER REVIEW 9 of 13 Materials 2020, 13, 3659 10 of 14 local flora. Moreover, the use of CR1 in the process of bitumen modification represented a higher emission of benzothiazole compared to CR2. Figure 9 shows that the addition of CR1 and CR2 into bitumenTo evaluate influenced the profile the emission of VOC emissions percentage at diofff beerentnzothiazole temperatures, when 12 the VOCs mixtures classified were as heated. irritant In andCRMB1/or health and hazard CRMB2, and the/or acutehighest toxic emission and/or environmentalrate of benzothiazole hazard accordingoccurred between to the overview 170 and shown 180 °C, inthe Table temperature2 and with range the highest generally emission used ratesfor the were producti chosen,on asof depictedhot mix asphalt in Figures mixture.8 and9 In. In both these cases, figures,this emission the relative represented presence ofmore the 12than VOC 35% compounds of the total is emission displayed of in compounds relation to the classified test temperature. as irritant, CRcorrosive, used for thehealth modification hazardous, of bitumenor toxic. canThe beincrease the source in temperature of a variety ofresulted hazardous in a significant substances increase released in intobenzothiazole the environment. emission This due contribution to the digestion can be process clearly noticedof CR. for the CRMB1 and CRMB2 samples.

350 350

300 300 ] ] -1 -1 s s -2 -2

-2 250 250

200 200

150 150

100 100 Emission rate [µmol m [µmol rate Emission Emission rate [µmol m [µmol rate Emission

50 50

0 0 120 140 160 180 120 140 160 180 Temperature [°C] Temperature [°C] (a) (b) 350

300 ] -1 s

-2 -2 250

200

150

100 Emission rate [µmol m [µmol rate Emission

50

0 120 140 160 180 Temperature [°C] (c) (d)

FigureFigure 8. 8.Distribution Distribution of of VOCVOC emissionsemissions withwith aa step-by-stepstep-by-step increase in in temperature: temperature: (a ()a )REF, REF, (b) (b)CRMB1, CRMB1, ( (cc)) CRMB2, CRMB2, and and ( (dd)) legend. legend.

AsIn shown both test in Figure series,8 a, theclear analysis presence confirmed at 180 °C the of pentadiene, presence of benzothiazole.ethyltoluene + trimethylbenzene, The addition of CR and inbutanone the bituminous + butenal binder was contributedidentified for to the the CR-modified highest emission binders. rate From of benzothiazole, 160 to 180 °C, the which concentration was not observedof pendatiene for the referenceincreased bitumen. by more Thethan concentration 67% for both ofCRMBs benzothiazole in series was1 and 65%, more 38%, than and 54% 35% in higher series 2. forAs CR1 for in butanone comparison + butenal, to CR2 an at 140,increase 160, of and more 180 ◦thanC, respectively. 80% was found The toxicityfor both leveltypes values of CRMBs derived in series for benzothiazole1 and more than for acute 46% andin series chronic 2. The exposure same trend are intended occurred to for eliminate the other risk 11 factors compounds, that impair showing health that (health-protective)the increase in temperature [38,39]. Considering from 160 to the 180 acute, °C is a chronic, crucial factor mutagenic, for VOC and emissions. carcinogenic effects of benzothiazole,The analysis it was in included the present in the study assessment showed of VOCsa high with increase risk to of human VOCs, health especially [38]. Thebenzothiazole, emission ofwith benzothiazole the addition can of be CR1 detrimental and CR2 toto thebitumen, health potentially of workers leading and have to anmore eff ecthealth on therisks local for flora.on-site Moreover,workers, theas confirmed use of CR1 previously in the process in the of literature bitumen [2,27,37–39]. modification Results represented confirmed a higher that the emission 160–180 of °C benzothiazoletemperature comparedrange is toa highly CR2. Figureinfluential9 shows factor that for the VOC addition emissions of CR1 andfrom CR2 heated into commercial bitumen influencedunmodified the bitumen emission and percentage particularly of benzothiazole CRMB. Reduci whenng the the temperature mixtures were of heated.the mixture In CRMB1 can decrease and CRMB2,the emission the highest of VOCs, emission especially rate of benzothiazole. benzothiazole occurred between 170 and 180 ◦C, the temperature range generally used for the production of hot mix asphalt mixture. In both cases, this emission represented more than 35% of the total emission of compounds classified as irritant, corrosive, health

Materials 2020, 13, 3659 11 of 14 hazardous, or toxic. The increase in temperature resulted in a significant increase in benzothiazole emissionMaterials due 2020 to, 03, the x FOR digestion PEER REVIEW process of CR. 10 of 13

350 350

300 300 ] ] -1 -1 s s -2 250 -2 250

200 200

150 150

100 100 Emission rate [µmol m [µmol rate Emission Emission rate [µmol m [µmol rate Emission

50 50

0 0 120 130 140 150 160 170 180 120 130 140 150 160 170 180 Temperature [°C] Temperature [°C] (a) (b) 350

300 ] -1 s

-2 250

200

150

100 Emission rate [µmol m [µmolrate Emission

50

0 120 130 140 150 160 170 180 Temperature [°C] (c) (d)

FigureFigure 9. 9.Distribution Distribution of of VOCs VOCs with with an an incremental incremental increase increase inin temperature:temperature: ( a) REF, ( b)) CRMB1, CRMB1, (c) (c)CRMB2, and (d) legend.

In bothThe method test series, introduced a clear presence in the present at 180 ◦studyC of pentadiene, can be implemented ethyltoluene as a+ standardizedtrimethylbenzene, and reliable and butanonemethod+ tobutenal cross-compare was identified the VOC for the emission CR-modified data from binders. different From bitumen 160 to 180 samples,◦C, the concentrationincluding with of pendatienemodifications increased or additives. by more The than characterization 67% for both of CRMBs the composition in series 1 andof bitumen more than fumes 54% by in PTR-TOF- series 2. AsMS for butanoneallows the+ identificationbutenal, an increase of several of more VOCs, than including 80% was those found grouped for both according types of CRMBs to the compound in series 1 andclasses more alkanes, than 46% aromatics, in series 2.and The sulfur-containing same trend occurred components. for the other It 11is also compounds, possibleshowing to highlight that a thehierarchy increase inof temperaturetheir appearance from in 160 relation to 180 ◦toC the is a temperature. crucial factor The for VOCPTR-TOF-MS emissions. 8000 achieves a limit of Thedetection analysis of <50 in thepptv present (benzene) study at 1 showed s average a highand a increase sensitivity of VOCs,of >100 especiallycounts per benzothiazole,second per ppbv with(benzene) the addition at 40 kHz of CR1 sampling and CR2 rate. to The bitumen, extreme potentially sensitivity leading of thisto instrument more health and risks its sampling for on-site time workers,resolution as confirmed (up to 10 previouslyHz) can bein a theuseful literature and innovative [2,27,37–39 solution]. Results for confirmed the asphalt that industry the 160–180 to monitor◦C temperaturethe emissions range of is VOCs a highly from influential hot bitumen. factor Furthermore, for VOC emissions it can from be coupled heated commercialto a flux mast unmodified to monitor bitumenVOC fluxes and particularly in situ during CRMB. asphalt Reducing pavement the temperatureconstruction—an of the interesting mixture can topic decrease to explore the emission further. of VOCs,Although especially the benzothiazole. proposed method clearly shows its potential to compare the emissions of samples withThe and method without introduced CR, it is instill the challenging present study to correlate can be implemented these results as to a the standardized particular andcircumstances reliable methodoccurring to cross-compare during the production the VOC emissionof rubberized data asphalt. from di ffFurthererent bitumen research samples, is needed including to establish with the modificationsrelationship or between additives. VOC The emission characterization concentrations of the composition in the laboratory of bitumen and at fumes road byconstruction PTR-TOF-MS sites. allowsNevertheless, the identification this innovative of several VOCs,method including can be thoseuseful grouped in order according to develop to the compoundand test additives, classes alkanes,modification aromatics, technologies, and sulfur-containing or methods components.to control and It isreduce also possible emissions to highlightduring the a hierarchyproduction of of theircrumb appearance rubber inmodified relation bitumen. to the temperature. The PTR-TOF-MS 8000 achieves a limit of detection of

4. Conclusions VOCs generated on-site during asphalt pavement construction can have a negative effect on the health of workers. The present paper introduces an innovative method to identify VOC emissions for

Materials 2020, 13, 3659 12 of 14

<50 pptv (benzene) at 1 s average and a sensitivity of >100 counts per second per ppbv (benzene) at 40 kHz sampling rate. The extreme sensitivity of this instrument and its sampling time resolution (up to 10 Hz) can be a useful and innovative solution for the asphalt industry to monitor the emissions of VOCs from hot bitumen. Furthermore, it can be coupled to a flux mast to monitor VOC fluxes in situ during asphalt pavement construction—an interesting topic to explore further. Although the proposed method clearly shows its potential to compare the emissions of samples with and without CR, it is still challenging to correlate these results to the particular circumstances occurring during the production of rubberized asphalt. Further research is needed to establish the relationship between VOC emission concentrations in the laboratory and at road construction sites. Nevertheless, this innovative method can be useful in order to develop and test additives, modification technologies, or methods to control and reduce emissions during the production of crumb rubber modified bitumen.

4. Conclusions VOCs generated on-site during asphalt pavement construction can have a negative effect on the health of workers. The present paper introduces an innovative method to identify VOC emissions for heated bitumen samples under laboratory conditions. This method intends to standardize routine measurements to enable a reliable comparison across bitumen types and their modifications by means of PTR-TOF-MS. The following observations, conclusions, and recommendations are suggested:

1. PTR-TOF-MS can be utilized to analyze VOC emission compounds from heated bitumen samples at temperatures ranging from 120 to 180 ◦C. Clear differences in VOC emissions were found between commercial unmodified bitumen and bitumen samples after its modification with crumb rubber, which have to be mixed at higher temperatures or in combination with WMA technology. 2. Temperature is a highly influential factor in bitumen VOC emission. A higher temperature stimulates molecular movement and, consequently, lighter compounds (alkanes and aromatics) are released faster. The emission rate exhibits an exponential relationship with temperature increase at the 160–180 ◦C range. 3. The addition of CR in a bituminous binder contributes to the potentially harmful emission of benzothiazole, which belongs to the sulfur-containing compound class. 4. The CR source affects the amount of VOC emission. The difference in VOCs released in CRMB1 and CRMB2 is due to the different materials and sizes used (CR1 and CR2). 5. The experimental setup developed for this study can be used as a standard method to characterize VOC emissions from different bitumen sources, modifications, or additives as it can be replicated easily and at different temperatures. 6. Very low concentrations of VOCs can be measured due to the extreme sensitivity of PTR-TOF-MS 8000 (>100 cps/ppbv) and the low limit of detection (<50 pptv). Isobaric species can be identified in a split second, and the whole spectrum of VOCs is monitored and recorded in real time.

Author Contributions: Data curation and methodology, M.P.-E. and J.B.; investigation, M.P.-E., J.B., and J.B.B.; formal analysis, P.K.D.M. and J.B.B.; SEM image processing, J.B.; project administration, J.B., C.V., and W.V.d.b.; writing—original draft preparation, J.B.B. and P.K.D.M.; writing—review and editing, J.B., M.P.-E., C.V., and W.V.d.b. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Green.er funds managed by the King Baudouin Foundation, grant number KBS/2019-E1170430-212694. Acknowledgments: Ankersmid M&C Bvba (www.ankersmid.eu) is acknowledged for providing the SEM device. Conflicts of Interest: The authors declare no conflict of interests. Materials 2020, 13, 3659 13 of 14

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