Effects of Varying Tobacco Rod Circumference on Cigarette's Dynamic Ventilation Rate and Combustion State During Machine Smoking

Three cigarettes with different circumferences were designed for the study, which were identical in regard to their cigarette paper, tobacco composition (100% flue-cured tobacco), tobacco strip width (0.9 mm), filter ventilation rate and tobacco density, while they differed with respect to their length and rod ventilation, which led to different weight and draw resistance. Therefore, the combustion and smoke yield of cigarettes with different circumferences are not synchronous. In this work, the change of ventilation with puff during the smoking process was more important. The physical parameters of the cigarettes are shown in Table 1. For smoking tests, the length of the standard cigarette butt was marked as the length of overwrap plus 3 mm. The samples were conditioned at (22 ± 1)°C and (60 ± 3)% relative humidity for at least 48 h prior to testing. The cigarettes used in the experiments were selected based on their average weight of ± 5 mg, and an average pressure drop of ± 50 Pa.

Cigarettes were smoked by a SML100 single-channel smoking machine (Hefei Zhongwo Instrument Technology Co., Ltd., Hefei, China). A VUY6002-12V speed-regulating vacuum pump (Chengdu Qihai Electromechanical Manufacturing Co., Ltd., Chengdu, China) was used during the process of measuring the ventilation of the cigarettes. A CP224S electronic balance (0.0001 g, Sartorius AG, Göttingen, Germany) was used to weigh cigarettes. Eight K-type micro thermocouples (∅ 0.254 mm, Omega, Norwalk, CT, USA) and their clamping and insertion module have been described before (22). To verify the accuracy of the self-made ventilation measuring device, a KT-DC Cigarette test station with ventilation test module (Körber Technologies Instruments GmbH, Hamburg, Germany) was also used to test the ventilation rate of cigarettes.

In order to obtain ventilation and temperature of a lit cigarette during each puff, and to ensure the repeatability of the experiment, it was necessary to record the puff number and the position of the burning line during the puff. Specifically, the cigarette was placed in a SML100 single-channel smoking machine, and puffed under the ISO 3308:2012 smoking regime (2 s puff duration, 35 mL puff volume, once every 60 s, bell-shaped puff profile) (24). During smoking, the burning line was measured and its position was recorded in real time with the scale as shown in Figure 1. For each cigarette sample, the test was repeated 8 times to obtain the average burn line position on a puff-by-puff basis. The puff numbers for the 17-, 20- and 24-mm circumference cigarettes were 5, 7 and 5, respectively. In order to facilitate comparison, only the first five puffs of the three cigarettes were used for analysis. The burning line position at the start of each puff is shown in Table 2. As the burning line was usually curved, we recorded the burning line position of cigarettes whose burning line positions were almost in the same radial direction.

Figure 1.

The cigarette's coal temperature measurement method used in this study has been described before (20, 21, 25). In the thermocouple temperature measurement module, the 6^th thermocouple was used for temperature control startup (20). The positions shown in Table 2 are also the insertion positions of the 6^th thermocouple. The combustion temperature at different puffs was measured individually to relate it to the puff number. After the cigarette was lighted, the burning line was pushed to the front end of the thermocouple insertion module under ISO smoking regime, and then the puffing mode was adjusted to temperature control. In addition, because the measurement position of the first puff was close to the front end of the cigarette rod, the thermocouple module could not be inserted to the cigarette and therefore the characterization of the combustion state started from the second puff.

In order to describe the state of the combustion coal in detail, the volume (V₀), maximum temperature (T_max), characteristic temperature (T_0.5) and average volume temperature (T_m) of the combustion coal were selected where V₀ is the total cumulative volume of combustion charcoal at a temperature of 200°C or above. This is a reasonable assumption, given that cellulosic materials or tobacco typically start to experience a measurable weight loss above this temperature. T_0.5 refers to the temperature when the cumulative volume above a certain temperature in the combustion coal accounts for 50% of the volume of the combustion coal, and it indicates the change of the high-temperature region inside the combustion coal (20). The temperature T_m refers to the volume weighted average of the temperature in the test area (26).

As shown in Figure 2, in the process of cigarette smoking, the volume of mainstream smoke includes three parts: a mixture of the gas that is being produced in the combustion coal and air that is entering with a puff (the gas volume is given as V₁), gas entering through the cigarette paper (the gas volume is given as V₂), and gas entering into the cigarette filter through the ventilation holes (the gas volume is given as V₃). To accurately measure these airflows during a puff, a device was designed to measure these three airflows as a function of the of puff number. As shown in Figure 3, the device included a movable but air-tight glass tube, the tightness of the air seal mechanism was monitored by a soap film flow tube; in addition, a vacuum pump and an airflow speed controller were used to produce a soap bubble in the soap film flow tube during cigarette smoldering.

Figure 2.

Figure 3.

In an experiment to determine the total ventilation of a test cigarette, the main cigarette rod was put inside the movable sealing tube and only its burning coal was exposed to air. The total ventilation of the tobacco rod and the filter ventilation of the test cigarette in unburned and burning conditions (smoldering and puffing) could be measured by this device.

Specifically, to measure the total ventilation of a cigarette during puffing, the entire cigarette rod behind the paper burn line was placed in the movable sealing tube with the burning coal exposed to air. Before starting the smoking machine to initiate a puff, as shown in Figure 3, the valve 9 should be closed and the valve 7 should be opened. When the cigarette was puffed, air entered through the cigarette paper (V₂) and some through the filter ventilation holes (V₃), and the sum of volumes V₂ and V₃ was measured by the soap film flow tube. During smoking, due to the movement of the paper burn line, the labyrinth seal 5 was placed 5 mm downstream of the measuring position, and the puff at the smoking machine was started. For this study, the puff position was adjusted to be consistent with values given in Table 2. If only the filter was placed in the movable sealing tube, the measured value of the soap film flow tube was the air entering the cigarette through the filter ventilation (V₃). When measuring the ventilation of the unlit cigarette, the length of the tobacco rod was cut according to Table 2, so that the residual rod length was the same as that of the unburned part during smoking. The total ventilation rate can be calculated by equation [1]. The filter ventilation rate can be calculated by equation [2]. Each sample was tested at least 5 times in parallel.

[1] Total ventilation rate=100%×(V2+V3)/V {\rm{Total}}\;{\rm{ventilation}}\;{\rm{rate = 100\% }} \times \left( {{V_2} + {V_3}} \right)/V

[2] Filter ventilation=100%×V3/V {\rm{Filterl}}\;{\rm{ventilation = 100\% }} \times {V_3}/V

Selected mainstream smoke chemical components were measured. For this purpose, the cigarettes were conditioned under (22 ± 1)°C and (60 ± 3)% relative humidity for at least 48 h, and then smoked under the ISO smoking regime (ISO 3308:2012). Total particulate matter (TPM) and moisture (H₂O) were determined according to ISO 4387:2000 and ISO 10362-2:2013, respectively (27, 28).

In order to reduce the flow resistance of the designed device itself, a soap film flow tube with low resistance was chosen when selecting the flow volume measurement components of the device. While the resistance of common mass flow meters on the market is 100 Pa to 200 Pa, the resistance of the soap film flow tube was only 20 Pa to 30 Pa, which means that the systematic error of the device was not significant. In order to check the reliability of the device during the experiment, the air-tightness of the device and the accuracy of the experimental data were verified. The filter ventilation and the total ventilation of an unlit cigarette were measured using this device and a cigarette test station. As shown in Table 3, the absolute difference between the results of this device and the cigarette test station was below 1.3%, less than the standard deviation (SD) of multiple tests (N = 20) of the cigarette test station. This proved that the device shown in Figure 3 had a satisfactory accuracy and could be used to measure the ventilation rate of unlit cigarettes and during smoking.

By reconstructing and analyzing the temperature data of the combustion coal measured by thermocouples during the smoking process, the parameters combustion coal volume, maximum temperature, characteristic temperature and average volume temperature could be obtained (19). Figure 7 shows the mean values of each parameter in the smoking process of the three different cigarettes. With the increase of cigarette circumference, the combustion coal volume increased, and the characteristic temperature decreased. The results indicate that the proportion of higher temperature volume of the combustion coal at 17 mm circumference was higher than that of the other two cigarettes, which is consistent with the results of Deng et al. (22). To statistically test whether these temperature parameters differed significantly between puffs, one-way ANOVA was used, in which the temperature parameters were the dependent variables and the puff number was the independent variable. The results are shown in Table 5. When the probability value (p) was higher than 0.05, it was considered that there were no statistically significant differences. The results indicate that there was no significant effect of the puff number on the combustion coal volume for the 17-mm and 20-mm cigarettes, while the combustion coal volume of the 24-mm cigarette increased first and then decreased and showed a statistically significant effect. There was no significant effect on the maximum temperature for any of the three cigarettes. This indicates that the maximum temperature of the cigarettes remained relatively stable during the smoking process. The characteristic temperature and volume-averaged temperature of the three cigarettes both showed a fluctuating and partially decreasing trend during smoking. The decrease in temperature reduced the resistance of the cigarette combustion coal to the air flow, which was one of the reasons for the decrease in the total ventilation of the cigarette. But the effect was smaller than the effect of the decrease in total ventilation caused by the reducion of rod length.

Figure 7.

Figure 8 shows TPM and H₂O contents of the cigarettes measured using the ISO 3308:2012 puffing protocol. The results per puff indicate that the cigarette circumference was positively correlated with the release of TPM, and negatively correlated with the release of H₂O. The TPM release of the three cigarettes increased gradually from puff to puff. As the number of puffs for the cigarette with 17-mm-circumference was less than five, the TPM value for the fifth puff was lower than that for the other cigarettes. The results confirm the observation by LI et al. (29), stating that as total ventilation and filter ventilation decrease during smoking, the dilution of the mainstream smoke also decreases and thus the TPM content increases during the last few puffs.

Figure 8.

Figure 9 shows that the tobacco density of the three cigarettes along the cigarette axis was very similar between the cigarettes. According to Figure 7 at the second puff, the characteristic temperature and the volume-averaged temperature of the cigarette were relatively high compared to the other puffs. With the progress of smoking, the ventilation of the cigarettes decreased and the amount of air entering through the combustion coal increased, which contributed to the combustion of cigarette and the combustion coal increase. However, moisture and “tar” were gradually deposited or accumulated inside the tobacco rod (30, 31), which could lead to a decreased efficiency of the tobacco burning and being adverse to combustion. The characteristic temperature for the 17-mm-circumference cigarettes had a slightly upward trend at the fourth puff, which was due to the effect of the incoming air volume from the combustion coal on the combustion at this position. The relative decrease of the total ventilation at the fifth puff compared to the second puff was 32.9%, 24.1%, 15.7% for the cigarettes with 17 mm, 20 mm, and 24 mm circumference, respectively.

Figure 9.

This indicates that in cigarettes with 17 mm and 20 mm circumference, the amount of air entering through the combustion coal increased more than in the cigarettes with 24 mm circumference. The total ventilation of the cigarettes with 24 mm circumference decreased comparably less. Therefore, for 24-mm circumference cigarettes, the accumulation of “tar” and moisture dominated, resulting in a decreased trend in T_0.5 and T_m during the puff-by-puff smoking process.