A Pumping Method for Assessing Airtightness of Packs - Application to Heated Tobacco Products *

The experimental setup for the assessment of airtightness for HTP packs used in this work is shown in Figure 1. The setup includes a program control system (controlling the procedure of assessment, including the punching position, pumping flow rate, sucking the pack and reading the differential pressure gauge), a punching device, a testing chamber (cylinder shape: 130 mm diameter and 90 mm height) as well as a sucker (20 mm diameter, with two holes connecting the ventilation channel and the vacuum channel, respectively) connected to the bottom of the chamber’s lid. The chamber’s lid was used to seal the chamber with an O-ring. The diameter of punching device was set between 2 mm and 3 mm and the inner diameter of the ventilation channel (3 mm diameter and 25 mm length) should be not smaller than the diameter of the punching device. Moreover, the flow rate of deflation pump was set to be 200 mL/min with a tolerance of 5 mL/min. The range of differential pressure gauge was (0–10) kPa with a resolution of 0.01 kPa and an accuracy of 0.02 kPa. The range could be adjusted based on the real test situation. For clarification, all assessments were conducted in the environment specified in ISO 3402 (14).

Figure 1.

The HTP pack was punched to create a ventilation hole and sealed tightly by a sucker mounted on the inside of the testing chamber lid connected to a vacuum generator. The inside of the pack was connected to the outside atmosphere via the punched hole and the ventilation channel. Then the chamber was sealed and continuously deflated at a stable flow rate of 200 mL/min, until the pressure difference between the inside of the chamber and atmosphere maintained steady for a certain period of time. The final pressure difference was recorded as a reflection of the airtightness of the HTP pack. Higher pressure difference indicates a better airtightness of the HTP pack.

The accuracy of this method was assessed using the transfer standard of permeable disks with a series of pressure differences from 0.1 kPa to 8.1 kPa, which was similar to the one used for air permeability measurement according to ISO 2965 (15). Each disk was measured 10 times by a single operator with the same equipment. All procedures were included except for the punching step. The results are given in Table 1. As can be observed, the highest standard deviation of 0.025 kPa and repeatability of 0.07 kPa was obtained for the disk of lowest permeability (corresponding to a pressure difference of 8.08 kPa on average). Furthermore, three different HTP pack samples were tested, using two randomly selected packs per sample. Each individual pack was continuously tested 10 times, and the results are shown in Table 2. All procedures were applied for the repeatability tests, including the punching step. It can be seen that the standard deviation ranges from 0.019 kPa to 0.085 kPa, while the repeatability is from 0.06 kPa to 0.28 kPa. These results demonstrate that the developed method is valid to assess the airtightness of the pack for HTPs products.

The effect of airtightness on the conditioning process was also investigated by monitoring the mass change during a conditioning process. The same HTP samples (Sample C, Sample M and Sample T) were selected for testing, and the conditioning duration was set to 10 days. The test atmosphere was specified as in ISO 3402 (14). The mass of each individual pack was recorded every 4 min, and the airtightness of each pack was assessed after the conditioning process. The airtightness and the mass increase of samples C, M and T are shown in Figure 2. The sample mass increase for the tested samples after 10 days of conditioning is shown in Figure 3. As shown in Figure 2, a continuous increase of sample mass was observed for all tested samples during the 10 days of conditioning. This suggests that, no matter how well the HTP packs are sealed, the transmission of moisture between HTPs and atmosphere cannot be entirely prevented by the packaging under such conditions. This might be attributed to the presence of considerable amounts of glycerol and propylene glycol in the tobacco substrate, which are highly hydrophilic. Furthermore, as can be seen from Figure 2a, when the airtightness was higher than 2.48 kPa, similar gradients of mass increase were observed. After 10 days of conditioning, a mass increase of approximately 0.023 g was found for the sample C with airtightness of 2.48 kPa, 3.65 kPa, 6.20 kPa and 7.37 kPa (Figure 2a). On the other hand, when the airtightness was at a relatively low level (i.e., 0.12 kPa, 0.63kPa and 1.24 kPa), a severe raise was shown. The mass increase of 0.035 g, 0.041 g and 0.081 g was observed for the sample with the airtightness of 1.23 kPa, 0.63 kPa and 0.12 kPa, respectively (Figure 2a). Moreover, similar results were observed for Sample M and Sample T, respectively (Figures 1b, 1c and Figures 2b, 2c). As long as the pressure difference during airtightness assessment was higher than 2 kPa, no obvious difference of mass increase was detected when improving the airtightness of the pack. While the airtightness was lower than 1.5 kPa, poorer airtightness led to more rapid mass increase during the 10 days of conditioning. It is important to mention that the increase of the sample mass was mainly attributed to the adsorption of water, which could influence the result of the determined aerosol collected mass (ACM). Therefore, it is deemed that the airtightness of HTPs’ pack plays a significant role during the conditioning process, for which certain level of quality, i.e., no lower than 2 kPa, is recommended under the conditions of the test.

Figure 2.

Figure 3.