Aerosol Formation and Transfer in Open- and Closed-Ended Heated Tobacco Products
Pubblicato online: 10 dic 2022
Pagine: 162 - 174
Ricevuto: 17 gen 2022
Accettato: 30 giu 2022
DOI: https://doi.org/10.2478/cttr-2022-0017
© 2022 Bin Li et al., published by Sciendo
This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Smoke from a combustible cigarette is formed through a series of physical and chemical processes such as distillation, pyrolysis, combustion and filtration from the burning cone to the unburned tobacco rod and its filter (1,2,3). The formation of mainstream smoke has two essential steps, smoke generation in the burning cone and downstream transfer in the porous tobacco media. The smoke generation consists of a series of complex chemical reactions, while the smoke delivery is mainly governed by the Darcy flow of the puffing air in the porous tobacco rod. In the literature (4,5,6,7,8), the relationship between pressure drop, filterventilation and flow rate in a cigarette rod has been extensively studied. For example, increased pressure drop at the forefront of the burning cone through tobacco carbonization and air expansion has been discussed to understand why the puffing air flows into the tobacco rod at the region behind the paper burn line and through the semi-charred paper wrapper (1,2,3); meanwhile, the effect of temperature on the viscosity of the incoming air in the porous tobacco has also been calculated in this respect.
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The tobacco rods used in the experiments were made with flue-cured blended reconstituted material, which had three segments (Figure 1), including a tobacco section (12 mm in length), a hollow acetate tube (25 mm in length) and an acetate filter plug (8 mm in length) at the mouth end. The outer diameter of the tobacco rod was 7.2 mm with an overall length of 45 mm. The main blend components of the tobacco plug are shown in Table 1. Figure 1A shows the exterior of the tobacco rod, and Figure 1B displays its 3-segment construction. The difference between the HNB and NSC rod constructions was that the NSC rods had a single row of 10 holes evenly distributed along its circumference to allow the puffing airflow; the position of the air inlet holes was laser-perforated at the hollow acetate tube at 21 mm from the filter tip, and each hole had a diameter of 0.05 mm.
The main blend components of the tobacco plug on each rod.
| Components | Nicotine | Propylene glycol | Glycerol | H2O | Tobacco plug |
|---|---|---|---|---|---|
| Weight (mg/plug) | 3.79 ± 0.22 | 4.77 ± 0.12 | 13.79 ± 0.25 | 35.23 ± 1.75 | 179.40 ± 3.73 |
Figure 1
Schematic diagram of the tobacco rod structure for HNB and NSC: the NSC structure is shown with the ventilation holes. HNB is identical but without the ventilation holes.
Two types of airflow pathways are illustrated in Figure 2 in conjunction with the heating device: The HNB airflow pathway (Figure 2A) was from the tobacco end and the NSC airflow pathway (Figure 2B) was from the perforation on the hollow acetate tube because the tobacco end of the heating device was completely sealed. Photos of the HNB and NSC systems are shown in Figure 3A and Figure 3B, respectively.
Figure 2
Airflow pathways of HNB (A) and NSC (B) systems.
Figure 3
Photos of HNB (A) and NSC (B) systems.
In this work, the heating devices were self-made to supply the heat source required for smoke formation and to create two different airflow pathways in conjunction with the tobacco rod. The devices used resistive heating, simply to compare the differences between the two systems. Other heating methods could be equally applied but this was not been evaluated. The working parameters of the heating devices are shown in Table 2. A simple ramped to constant working temperature of 240 °C was applied which is on the lower end for heating temperatures in current commercial HNB products, again for the purpose of demonstrating the differences between the two systems. Other more complex temperature programs could also be explored. Prior to smoking experiments, all the tobacco rods were removed from their packaging and conditioned at 22 ± 2 °C and a relative humidity of 60 ± 3%. Devices were fully charged and cleaned between different smoking cycles.
Working parameters of the two heating devices.
| Airflow pathway | Heating mode | Heating temperature | Preheating time | Working time |
|---|---|---|---|---|
| HNB | Peripheral resistive heating | 240 °C | 45 s | 4 min 12 s |
| NSC | Peripheral resistive heating | 240 °C | 45 s | 4 min 12 s |
Nicotine (> 99.5%) and 2,3-butanedione (≥ 99%) were purchased from Sigma Corporation (New York, NY, USA); methanol (chromatographic grade), 1,2-propanediol and propanetriol were purchased from Thermo Fisher Scientific Co., Ltd. (Beijing, China); 2-methylquinoline (> 99%) and 1,3-butanediol (> 99.5%) were supplied by TRC (Toronto, Canada); isopropanol (chromatographic grade) was supplied by Fisher (New Jersey, NJ, USA); ultra-pure water was generated by ELGA/MILLTI-Q generator from Millipore (Burlington, MA, USA); sodium hydroxide and perchloric acid (analytical grade) were purchased from Sinopharm Chemical Reagent Co., Ltd (Beijing, China); acetaldehyde-2,4-dinitrophenylhydrazone (> 98%), acrolein-2,4-dinitrophenylhydrazone (> 95%), crotonaldehyde-2,4-dinitrophenylhydrazone (> 98%) were supplied by Tokyo Kasei Kogyo Co. (Tokyo, Japan). The chromatography-grade acetonitrile was purchased from Dikma Company (Foothill Ranch, CA, USA); 2,4-dinitrophenylhydrazine (DNPH, 98.0%, 50% water mixture) was purchased from CNW Company (Boston, MS, USA); 2,3-pentanedione (≥ 98 %) was purchased from Maya Reagent (Zhejiang Jiaixng, China); chromatography grade ethanol was purchased from Toshi Ai Shanghai (Shanghai, China). TRACE 1310 gas chromatograph (Thermo Technology Co., Ltd., Spartanburg, SC, USA) with hydrogen flame detector and thermal conductivity detector was used in combination with an Agilent1200 high performance liquid chromatograph and 7890B/5977A gas chromatography/mass spectrometer (Agilent Technologies, Inc., Santa Clara, CA, USA).
The mainstream aerosol was generated on a Puffman X500E-A smoking machine and captured by a Cambridge glass-fiber filter pad at 22 ± 2 °C and 60 ± 5% RH. After the heating device was preheated, 12 replicated cigarettes were smoked under the Health Canada (HCI) machine-smoking parameters, which yielded 9 puffs for the two test devices. The total and puff-by-puff aerosol collected mass (ACM) were obtained by measuring the mass differences of the Cambridge filter pads before and after smoking. The puff-by-puff deliveries of selected aerosol components (PG, VG and nicotine) were analyzed by the CORESTA recommended method N° 30. PG and VG in the tobacco material were also measured. Nicotine and water in the tobacco material and aerosol were analyzed by CORESTA recommended methods (31–32).
The same applied to the above process to generate aerosols for ACM and PG/VG/nicotine analyses, formaldehyde, acetaldehyde, acrolein and crotonaldehyde, which were detected by the CORESTA recommended method (33). In addition, 2,3-butanedione and 2,3-glutaraldehyde were analyzed according to the methods of previous studies (34–35).
A bespoke temperature detection system for using fine thermocouples for cigarette combustion temperature measurements was adapted for the heated tobacco systems (11). Specifically, the heated tobacco product was connected to the smoking machine (Figure 4), a high-speed analog-to-digital recorder read the temperature outputs from the inserted thermocouples at the center of the tobacco plug and the hollow acetate tube.
Figure 4
1: Heating device; 2: Temperature detection point of tobacco plug; 3: Tobacco rod; 4: Temperature detection point of hollow acetate tube; 5: Thermocouple; 6: Thermocouple compensation wire; 7: Data collector; 8: Data acquisition software; 9: Data connection cable; 10: Single-channel smoking machine.
The point of measuring temperature of the hollow acetate tube is outside of the heating device and thus the temperature could be measured directly; while the tobacco plug of the tobacco rod was inside the smoking device during heating. To overcome this, a small hole was made through the wall of the heating device to allow the thermocouple to be inserted at a target location. The hole was then sealed with a hot-melt adhesive.
Aerosol particles generated from the two heated tobacco systems were measured by a smoking cycle simulator and differential mobility spectrometry (SCS-DMS, London, UK). The measurements were characterized by aerosol particle number concentrations (APNC) and count median diameter (CMD) under the HCI smoking regime. The SCS-DMS system was set up following experimental parameters described by A
In order to compare the aerosol generation and retention within the three main sections of the tobacco rod under the two air flow modes, this work measured ACM, nicotine, PG, VG, H2O, selected aldehydes and ketones in the mainstream aerosol. In addition, residual nicotine, PG, VG and H2O in the used tobacco plug and their retention by the filter rod (including hollow section and CA section) after heating were also measured. These results were supported by the temperature profiles in the different locations (Figure 4) during the puffing process and the aerosol particle distribution. The purpose was to provide a comparison as comprehensive as possible between the aerosol formation and transfer processes between the HNB and NSC systems.
Figure 5 shows the comparison of ACM in mainstream aerosol on a puff-by-puff basis for the two airflow pathways. The delivery of ACM showed an initial rapid increase for the first 1–3 puffs and it then declined gradually, and this trend was similar for the two airflow pathways.
Figure 5
Mainstream aerosol collected mass (ACM) as a function of puff number under the two airflow pathways.
For the first puff, the ACM of HNB was greater than that of NSC, but the ACM in NSC quickly overtook HNB in subsequent puffs and stayed significantly higher for the remaining puffs. The maximum difference for the third puff was more than doubled.
The comparison of nicotine, PG, VG and H2O in the mainstream aerosols for the two systems as a function of puff number is shown in Figure 6. Broadly speaking, the deliveries of nicotine, PG and VG followed the same trend as that of ACM: an initial increase to about 2–4 puffs and then a gradual decrease for the remaining puffs. Apart of the first puff, the contents of nicotine and VG were significantly higher for the NSC system. For PG, the delivery was very similar for the two systems (Figure 6B). Figure 6D displays the variation of H2O release during the puff-by-puff process: water measurements using Cambridge filter pads were reported to be less stable than the other aerosol constituents for all HNBs (38); this has been attributed to a higher percentage of water being produced in the form of steam and therefore it is more challenging to quantify gravimetrically. Giving the uncertainty regarding the water determination, Figure 6D could be interpreted as that the NSC and HNB systems investigated in this work had a similar H2O delivery, unless a more accurate method shows otherwise. Overall, the NSC system's ability to deliver a much higher nicotine and VG is worth noting.
Figure 6
The deliveries of nicotine (A), PG (B), VG (C) and H2O (D) in mainstream aerosol on a puff-by-puff basis for the two different airflow pathways.
Figure 7 plots the proportions of nicotine, PG, VG and H2O in the ACM on a puff-by-puff basis for the HNB and NSC systems, respectively. For both systems, water made up the largest portion of the ACM, a fact that has been reported before. For the HNB (Figure 7A), the proportions of nicotine, PG and VG in the ACM showed a gentle increase first with puff number to about 2–4 puffs and then leveling off, with the percentage order as nicotine < PG < VG. For the NSC system (Figure 7B), different trends could be observed for the proportions of nicotine and PG in the ACM, as compared to that for VG. The proportion of nicotine and PG increased first, reaching a maximum at the second puff, and then fell slightly before increasing again. The proportion of VG in ACM increased first and then decreased with the puff number, reaching a maximum of 35.54% in the fifth puff.
Figure 7
Proportions of nicotine, PG, VG and H2O in the aerosol collected mass (ACM) as a function of puff number for the two airflow pathways: HNB (A) and NSC (B).
Figures 7A and 7B show that airflow path is a critical parameter in heated tobacco product design, which could influence either the absolute and/or the relative yield of substances in the mainstream aerosol depending on their own thermophysical properties and their interaction with the heating and airflow conditions.
The total release of main components in the mainstream aerosol are listed in Table 3 for the HNB and NSC systems. The relative increase of the NSC over the HNB is clearly seen, especially for ACM, nicotine and VG, which were 1.5, 1.4 and 2.6 times higher than those under the HNB, respectively. The remaining tobacco components were 0.33 mg and 3.49 mg under the HNB and NSC conditions, which were calculated by subtracting the values of nicotine, PG, VG and H2O from that of ACM. These components, although not yet characterized for their full chemical compositions were largely made up of the releases from the tobacco substrates, thus demonstrating that the NSC system potentially could deliver significantly more tobacco-related substances, which would be useful to explore in terms of improving the tobacco aroma components under heated tobacco conditions.
Total releases of main components in the mainstream aerosol.
| Specimens | ACM | PG | VG | Nicotine | H2O | Other components a |
|---|---|---|---|---|---|---|
| (mg/stick) | ||||||
| HNB | 15.48 ± 1.30 | 1.65 ± 0.09 | 2.35 ± 0.17 | 0.87 ± 0.04 | 10.28 ± 0.26 | 0.33 ± 0.03 |
| NSC | 23.18 ± 1.35 | 1.69 ± 0.05 | 6.15 ± 0.35 | 1.22 ± 0.06 | 10.63 ± 0.53 | 3.49 ± 0.29 |
| NSC / HNB | 150% | 102% | 262% | 140% | 103% | 1058% |
The other components were obtained by subtracting nicotine, PG, VG and H2O from ACM.
Most toxicants in mainstream cigarette smoke are generated during tobacco combustion (2), while heated tobacco products are designed to prevent combustion (39). In our study, we measured selected volatile toxicants of regulatory interests, such as formaldehyde, acetaldehyde, acrolein, crotonaldehyde, 2,3-butanedione and 2,3-pentanedione. Their levels in the mainstream smoke aerosol for the two systems are shown in Table 4. It showed that the release of formaldehyde, acetaldehyde and 2,3-pentanedione under the NSC condition were significantly lower than those under the HNB, with a reduction of 15.0%, 14.4% and 7.4%, respectively. Acrolein was not detected under the two airflow pathways, which was consistent with previous studies (40). The release of 2,3-butanedione was basically the same under the both airflow pathways. Crotonaldehyde level was significantly increased for the NSC system.
Aldehydes and ketones in mainstream aerosol of HNB and NSC systems.
| Specimens | ACM | PG | VG | Nicotine | H2O | Other components a |
|---|---|---|---|---|---|---|
| (mg/stick) | ||||||
| HNB | 15.48 ± 1.30 | 1.65 ± 0.09 | 2.35 ± 0.17 | 0.87 ± 0.04 | 10.28 ± 0.26 | 0.33 ± 0.03 |
| NSC | 23.18 ± 1.35 | 1.69 ± 0.05 | 6.15 ± 0.35 | 1.22 ± 0.06 | 10.63 ± 0.53 | 3.49 ± 0.29 |
| NSC / HNB | 150% | 102% | 262% | 140% | 103% | 1058% |
The air flow for the HNB passed through the heated tobacco plug. From the perspective of heat transfer, the incoming room-temperature air would cause a convective heat transfer from the heated tobacco (solid phase) to the air flow (gas phase), this would result in temporary cooling of the tobacco bed, while the airflow of the NSC system was designed to bypass the entire heated tobacco bed with minimum forced heat transfer from the tobacco. The measured temperature variation patterns inside the center of the tobacco plug (Figure 8A) confirmed the different heat transfer phenomena. The temperature in the HNB condition showed a clear puff-induced temperature drop corresponding to each puff, an observation seen in previous literature (28–29), while the temperature in the NSC condition was not affected by the puffing and remained relatively stable. In addition, the temperature variations in the center of the hollow acetate tube were also measured on a puff-by-puff basis (Figure 8B). As can be seen, the HNB temperature was significantly above that in the NSC condition during each puff; again, in agreement with the fact that air flow pathway in the NSC system did not undergo convective heating, which may have produced a less hot aerosol stream.
Figure 8
Variation of temperature at the center of tobacco rod (A) and inside the filter rod (B) during puffing for the HNB and NSC systems.
The physical properties of the aerosols generated by the two heating systems under the HCI puffing regime indicated that they both produced inhalable aerosols. The measured aerosol's APNCs and CMDs under the two airflow pathways are shown in Figure 9 and the puff-by-puff particle size distribution of aerosols showed a log-normal distribution as described in previous studies (37). The APNCs under the NSC and HNB systems showed a trend to increase during the puffing process, and the NSC's was significantly higher than that of HNB. The CMDs increased for the first three puffs and then decreased steadily for the two systems, however the CMDs under the NSC were generally between 28–58 nm, while that of the HNB was between 36–104 nm, so the CMD of the aerosol under the NSC system was significantly lower than that of the HNB until the last two puffs.
Figure 9
Aerosol particle number concentration (APNC) and count median diameter (CMD) of mainstream aerosols for HNB and NSC systems.
By combining the analysis of the initial contents, residual contents and those delivered in the mainstream aerosol for the main components before and after puffing, it is possible to compare the differences in the aerosol generation, migration and retention of the key aerosol substances between the two different airflow pathways. This is shown in Table 5 for the HNB and NSC systems. We defined η
Transfer ratio of main components in the tobacco substrate, filter rods, and mainstream aerosol under the HNB and NSC system.
| Specimens | PG | VG | Nicotine | H2O | PG | VG | Nicotine | H2O | ||
|---|---|---|---|---|---|---|---|---|---|---|
| mg/cig | % | |||||||||
| Tobacco | HNB | 0.16 ± 0.14 | 0.00 | 0.03 ± 0.04 | 3.92 ± 0.61 | η |
3.30 | 0.00 | 0.67 | 11.13 |
| NSC | 0.24 ± 0.00 | 0.00 | 0.08 ± 0.00 | 2.28 ± 0.57 | 4.99 | 0.00 | 2.08 | 6.46 | ||
| Filter rod | HNB | 2.33 ± 0.15 | 9.77 ± 0.79 | 1.26 ± 0.10 | 8.53 ± 0.94 | η |
48.93 | 70.91 | 33.11 | 24.22 |
| NSC | 2.59 ± 0.08 | 4.97 ± 0.25 | 1.07 ± 0.05 | 9.28 ± 0.26 | 54.39 | 36.09 | 28.09 | 26.35 | ||
| Aerosol | HNB | 1.65 ± 0.09 | 2.35 ± 0.17 | 0.87 ± 0.04 | 10.28 ± 0.26 | η |
34.51 | 17.08 | 22.95 | 29.17 |
| NSC | 1.69 ± 0.05 | 6.15 ± 0.35 | 1.22 ± 0.06 | 10.63 ± 0.53 | 35.40 | 44.62 | 32.10 | 30.16 | ||
To explain the source of our novel NSC design and the phenomenal differences between the two heated aerosol generation and transfer processes, it is necessary to review thermophysical details of a burning cigarette (16). Figure 10 (replicated from that study for easy reference here) showed the momentary measurement of four key combustion parameters (1-s into a 2-s puffing at 35 mL). As B
Figure 10
As described, the NSC system removed the convective heat transfer of the tobacco bed as compared with existing openended HNB. The NSC's mainstream aerosol was thus formed without the repeated interruption of incoming air and its cooling effects. The positive air pressure in the NSC system is through the entrained air expansion and aerosol formation by the thermochemical reactions of the heating tobacco plug. Both the positive and negative pressure systems in the NSC worked simultaneously when a puff was taken to deliver the mainstream aerosol, it was just that the introduction of the airflow bypassed the main tobacco section and instead flowed in the hollow filter section. The two phenomena combined to form a much larger pressure drop in the tobacco rod. The peak gas-phase temperature inside the filter rod was below 90 °C under the NSC (Figure 8B), which was much lower than the 150 °C detected under the HNB. These temperatures may be too high for human puffing, but they were prototypes for comparative purposes and without consideration of the aerosol cooling. Nevertheless, they showed that the hollow tube section in the filter could still influence the aerosol particle growth and distribution as the initial aerosol condensed and travelled through the section. This was thought to be the main reason behind the significant differences for the particle size parameters between the NSC and HNB systems (Figure 9). Meanwhile, it seemed reasonable to assume that those distillation and evaporation reactions for the heated tobacco would be more productive for releasing tobacco components if there was no disturbance or cooling effect by the incoming puff, and the measurements of the ACM and its main components largely demonstrated this effect.
From the transfer of PG, VG, nicotine and H2O in the aerosol and their retentions in the filter rods (Table 5), the NSC system offered much higher transfer rates. The opposite was that the retention rates of glycerol and nicotine were significantly lower, whereas the retention rates of PG and H2O were significantly higher. Two factors are behind the above observations: the temperature of the airflow and the particle size of aerosol. Cooling-related aerosol condensation and the filtration-related particle retainment both worked to a certain degree in the two systems. Generally, the higher the filter section temperature, the less favorable for the retention of low boiling point substances, and the smaller the particles, the less favorable for the improvement of the retention efficiency. At this moment the results of the NSC system could not be fully explained based on this understanding. However, there are also most likely some other factors in operation that affected the NSC system. These should be further investigated.
The reduced aldehydes and ketones for the NSC system could largely have been caused by the formation processes: by reducing the incoming oxygen level in the NSC the small degree of hydrocarbon pyrolysis was surpressed (43), and this was supported by the fact that the formation of formaldehyde and acetaldehyde from tobacco was sensitive to the presence of oxygen (28). In the NSC system, the oxygen content of the original tobacco rod would be consumed under the heating and no further oxygen could be supplied as the tobacco plug became a positive-pressured system. Therefore, the NSC system represented a gradual oxygen depletion system for the heated tobacco; the effect of this on the release of inherent tobacco flavour substances is yet to be fully understood.
In this study, we constructed two types of different airflow pathways in order to investigate their effects on heated tobacco product designs. One system representing current commercial HTPs was coded as HNB system, and in contrast a novel airflow system was designed which was coded as NSC. The results showed some profound differences in the delivery behaviours of ACM, nicotine, PG, VG and H2O release on a puff-by-puff basis. Under the same heating conditions, the NSC system obtained a greater transfer rate for ACM, nicotine, PG, and VG as compared with those of the HNB condition. The temperature dynamics in the tobacco bed and the hollow filter tube section were also significantly modified by the NSC design during a puff, which collectively caused the observed differences in the particle characteristics of the aerosols, and also the differences behind the release of ACM, nicotine, PG, VG, H2O, aldehydes, and ketones in the mainstream aerosol of the two airflow pathways. This study provided some novel foundational insights into aerosol generation and transfer processes governing the heated tobacco product performance and could be used to guide the design of future heated tobacco products based on the NSC principle.