Preparation of Electrospun Composite PEG/PVP Phase-Change Nanofibers and their Application in Cigarette Cooling Filters

Polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP, K30, average molecular weight - 40000); chloroform; dimethylacrylamide were purchased from Sinopharm chemical reagent Co. Ltd. (Shanghai, China). All reagents were used directly without further purification.

Firstly, 5 g of PVP was dissolved in ultra- pure water to form a PVP solution with a mass fraction of 50%, then PEG with different relative molecular weights and contents was added and stirred for 12 hours.

The PEG/PVP solutions were loaded into a syringe and the injection needle replaced with a metal needle. The speed of the automatic syringe pump was set to 1ml/h, and an aluminum foil was placed at a distance of 15 cm to the metal needle to collect the PEG/PVP electrospun fibers. The ambient temperature was 27 °C and the relative humidity was 55%. The PEG/PVP solution was injected into the metal needle, and the nanofibers formed were collected on the aluminum foil under a voltage of 18 kV. All the PEG/ PVP electrospun fibers were collected and dried in a vacuum oven at 60 °C for 24 hours to remove the residual organic solvent.

PCMs are functional materials that can release or absorb heat through reversible phase transitions to maintain the temperature of the system [4, 5]. They have excellent heat storage and temperature adjustment properties and can be used in clothing, construction, military, and many other fields [6-9]. However, PCMs are unstable and prone to leakage during the phase transition process, which severely limits their application in the field of energy storage. Electrospinning technology [10, 11] is a feasible method to encapsulate PCMs in support materials to prepare PCM@ polymer fibers [12, 13].

PEG is a kind of phase change material frequently used in HnB cigarettes. Low, medium and high molecular weight PEG are mixed and sprayed on the filter tow in a certain proportion; PEG can also be mixed with hexanol. The mixture can be made into a granular state, round strip shape or coating state heat-absorbing gel and added to the filter rod, which can reduce the flue gas temperature by 1-8 ℃ [14].

PEG/PVP electrospun nanofibers were collected under a voltage of 18kV, and the receiving distance was 15 cm. PEG with different relative molecular mass was used to prepare the phase-change nanofibers. When the relative molecular weights of PEG were 8000, 10000 and 20000, the corresponding diameters of the composite fibers obtained were 100 nm, 180 nm and 600 nm, respectively (Fig.1). As the relative molecular weight of PEG in the mixed solution increased, the viscosity of the solution and the average diameter of the composite fiber increased correspondingly.

Fig. 1.

To test the cooling effect of the phase-change nanofibers, PEG/PVA nanofibers were added in a sandwich rod with a thickness of about 2 cm (Fig. 2). A fast miniature temperature-measuring thermocouple was introduced at a distance of 2 mm from the front and back of the interlayer and at the center of the cooling material, and the temperature measurement point of the thermocouple was located at the center of the filter tip. The temperature changes of manually lighted cigarette smoke were recorded under standard conditions at the temperature measurement points throughout the process. The purpose of short-distance temperature measurement was to reduce the influence of the environment and individual differences in filters on temperature measurement.

Fig. 2.

From Fig. 1, we can see that the average diameter of electrospun composite nanofibers clearly increased with an increase in the relative molecular mass of PEG, and the cooling effect of 3 kinds of PEG/PVP phase-change nanofibers on the flue gas temperature were tested. The results are shown in Fig. 3, When the fiber diameter increased from 100 nm to 180 nm, the temperature decreased slightly by 3 °C, with 180 nm nanofibers showing a better cooling effect. But when the fiber diameter increased from 180 nm to 600 nm, only a 0.5 ℃ degree difference was observed, indicating that the diameter of nanofibers showed little effect on the cooling performance of the phase-change nanofiber filter.

Fig. 3.

We investigated the cooling effect of the PCM content of phase-change nanofiber on the smoke temperature at the outlet of heated cigarettes, the results of which are shown in Fig. 4. It can be seen that when the PEG content increased from 30% to 50%, PEG/PVP=50:50, the maximum smoke temperature at the outlet of the heated cigarette dropped from 60 °C to 45 °C, and the temperature drop was as high as 15 °C, indicating that the amount of PCM used can significantly reduce the temperature of the flue gas. However, when the PEG content increased from 50% to 70%, the maximum outlet flue gas temperature only decreases by 2 °C. These results indicated that PCM endows PEG/PVP nanofibers with heat storage and temperature regulation capabilities. The larger the amount of PCM fed, the greater the latent heat value of PEG/PVP fibers, and the more obvious the heat storage and temperature regulation effect.

Fig. 4.

In order to investigate the effect of the length of the cooling section on the cooling performance of the filter rod, the same PEG/PVP phase-change nanofiber section and cigarette section were used, and only the cooling section and the empty tube section (total length of 26 mm) of an ordinary cigarette were replaced with 30, 25 and 20 mm nanofibers, respectively. The results are shown in Fig. 5. It can be seen that the longer the cooling section is, the lower the outlet flue gas temperature will be. The outlet smoke temperature was 15 and 12 ℃ lower than that of the ordinary cigarette, respectively. Even with the cigarette with the shortest cooling section (20 mm), the outlet smoke temperature was 8 °C lower than that of the ordinary cigarette. The contact area of the flue gas increased for the longer nanofiber filter to improve the cooling effect.

Fig. 5.

During the 9 puff smoking experiment, two thermocouples on both sides of the middle layer added along with a cigarette cooling agent were used to measure the temperature. The temperature at the outlet of the filter rod is shown in Fig. 6.

Fig. 6.

It can be seen that the mainstream smoke temperature of the commercial filter increased greatly, reached a maximum of 86 ℃ at the 8th puff, and then decreased to 84 ℃ at the 9th puff. While for the filter rod containing PEG/PVP phase-change nanofibers smoked to the first 6 puffs, the smoke temperature at the outlet of the filter was below 40 ℃, which then increased to 43 ℃ at the 7th puff, but next decreased to 41 ℃ at the 8th puff and to 42 ℃ at the 9th puff. The mainstream smoke temperature during the whole smoking process was much lower than that of the commercial cigarettes, and the highest mainstream smoke temperature did not exceed 45 ℃. The mainstream smoke temperature of the control group cigarettes was as high as 84 °C, while the oral cavity, esophagus and gastric mucosa generally tolerated temperatures of 50-60 °C, which would make smokers feel burning and irritation. Whereas the composite PEG/PVP showed an excellent cooling effect, avoiding burning harm and improving the sensory quality of cigarettes.