A New Type of Expanded Graphite-Based Heat-Conducting Glue and Its Application in Heat-Non-Burning Cigarette

With the popularity of heat non-combustible cigarettes, a new type of tobacco product, higher requirements have been put forward for the thermal conductivity of tobacco materials compared to traditional tobacco. In order to make the tobacco uniformly heated, adding high thermal conductivity materials to the tobacco to improve the thermal conductivity of the tobacco in the heat-non-burning smoke is a conventional practice. Therefore, this article introduces expanded graphite with high specific surface area and high thermal conductivity as the main material of thermal conductivity, and the polymer obtained by esterification reaction of biomass material polylactic acid, citric acid and diol are utilized as the adhesive glue. Research shows that this material enhances the heating performance of tobacco in heat non-combustible cigarettes. The thermal conductivity measured by adding 3 ml, 4 ml, and 6 ml of the expanded graphite viscous mixture to the filter paper spacer increased sequentially both at room temperature and at 100 degrees Celsius, and all of them were higher than that of the blank filter paper. This method effectively realized the fixation of the expanded graphite thermal conductive material on the surface of the tobacco, which greatly improved the thermal conductivity performance of the tobacco and made this new type of heat cigarettes more uniformly heated.


Introduction
Heat-non-burning cigarette 1 is a new type of tobacco product, in which heating instead of combustion reduces the harmful components, so heat-non-burning tobacco products are also known as new cigarettes or low-temperature cigarettes.The tobacco substance in cigarettes is heated but not burned, and its heating temperature (250-350°C) is much lower than the burning temperature of traditional cigarettes (800-900°C) Therefore, the thermal conductivity of the tobacco segment is required to be higher compared to traditional tobacco.In order to make the tobacco uniformly heated, adding high thermal conductivity materials 2 to the tobacco to improve the thermal conductivity of the tobacco in the heat-non-burning smoke is a conventional practice, for example, the commonly used high thermal conductivity materials are carbon nanotubes, graphene, m alumina, silicon carbide and so on.In addition to these main thermal conductive materials, it is also necessary to add some auxiliary materials to improve the dispersion of thermally conductive materials in the tobacco and the ability to adhere, such as the addition of a certain viscosity of the organic material, and the main thermal conductive materials to form a thermally conductive adhesive, the main thermal conductive materials can be well attached to the surface of the tobacco, to maximize the thermal conductivity of these thermal conductive materials.
Expanded graphite 3 is prepared by high-temperature expansion of natural flake graphite.After expansion, the graphite is in the state of worm curling, and the surface is in the form of net-like pore structure, which has a larger specific surface area compared with graphene or carbon nanotubes.Expanded graphite combines the high thermal conductivity and non-toxicity of natural flake graphite, so it can be used as a thermal conductive material for tobacco.As cigarette smoking process will inhale a lot of smoke generated after heating, which also includes the gas generated by the decomposition of the heat-conducting adhesive 4 , so the safety requirements are higher, if the natural biomass can be used to prepare a certain viscosity of the heat-conducting adhesive, it will be heat non-burning cigarettes in the tobacco to obtain uniform and effective heat, but also will make the smoking safer.
Therefore, this manuscript introduces expanded graphite with high specific surface area and high thermal conductivity as the main material of thermal conductivity, and the polymer obtained by esterification reaction of biomass material polylactic acid, citric acid and diol are utilized as the adhesive glue.The uniform mixing of the two realises the effective adsorption and fixation of the expanded graphite heat-conducting material on the surface of the tobacco, which can effectively improve the heating of the tobacco in the heating of the non-burning smoke.

Preparation of Adsorbable Graphite Thermally Conductive Nanosheets
In the first step, PLA (10 mmol, 10 g), citric acid (10 mmol, 1.921 g) and hexanediol (10 mmol, 1.187 g) with molecular weight of 2000 g/mol were added to a reaction flask, and the catalyst p-toluenesulfonic acid hydrate (0.15 mmol, 0.028 g) was added, and the reaction was carried out under a nitrogen atmosphere at a reaction temperature of 120°C and a reaction time of 12h.A light yellow viscous liquid was obtained.Next, weighed 3g of yellow viscous liquid was dissolved in 40ml of ethanol, then 250mg of expanded graphene was taken and ultrasonically dispersed in the above ethanol solution.At last, a syringe was used to aspirate 3 ml, 4 ml and 6 ml of the above mixed solution on a filter paper sheet with a diameter of 5 cm, and it was coated uniformly and dried to obtain nanomaterial sheets adhering to different amounts of graphene (Figure1).

Property Tests
Differential scanning calorimetry (DSC) analyses were determined by differential scanning calorimetry (DSC 250, TA, USA) over a temperature range of 25℃ to 600 ℃in a nitrogen atmosphere at a ramping rate of 10℃/min.Field emission scanning electron microscopy (SEM) images were taken on a scanning electron microscope (Zeiss Sigma 300) with an operating voltage of 3 kv.Thermal conductivity was tested by a thermal conductivity meter (Hot Disk TPS 2500S) after pressing two samples at a pressure of 20mpa.

Fourier transforms infrared (FT-IR) spectra of the synthesized Heat-Conducting Glue
. FTIR spectra of the prepared of the synthesized Heat-Conducting Glue were collected in Figure 2. As displayed, the peak at 2939 cm -1 and 2861.5 cm -1 is assigned to the CH3 and CH2 stretch, the sharp peak at 1721.0 cm -1 originates from C=O stretching, and the peaks at 1174 cm -1 and 729 cm -1 could be ascribed to the C-O group, demonstrating the successful synthesis of the Heat-Conducting glue.In the digital photographs in Figure .3, 3 ml, 4 ml and 6 ml of the expanded graphite viscous mixture were added to the filter paper sheet in order from left to right.As can be seen from the photographs, the biomass viscous adhesive adhered the expanded graphite uniformly to the filter paper and became darker in colour as the amount added increased.This Figure illustrates that it is feasible to increase thermal conductivity by adhering highly thermally conductive expanded graphite to a roll of tobacco.

Figure 4. SEM micrographs of microcapsule samples
Expanded graphite is prepared by high-temperature expansion of natural flake graphite, expanded graphite is a worm curled state, the surface is a network of pore-shaped structure.From the scanning electron microscope in Figure 4, we can see the worm-like and reticular structure of the expanded graphite, and the adhesive glue did not affect the structure and properties of the expanded graphite, which belongs to the physical mixing, and it just adheres the expanded graphite to the paper sheet.This shows that it is feasible to use bioadhesive glue to stick high thermal conductivity materials in cigarette leaves.

Thermal Conductivity Properties Analysis
The viscous polymer as thermal conductive adhesive and expandable graphite were sonicated to form a viscous mixture.Using a syringe, 3 ml, 4 ml,6 ml of the mixture were uniformly spread on a filter paper sheet with a diameter of 5 cm, after which a thermal conductivity test was carried out.be seen that the thermal conductivity of the heat-conducting sheet is gradually improved with the increase of temperature and the increase of the amount of expandable graphite, and this kind of heat-conducting material is applied to tobacco products to greatly improve the thermal conductivity of the tobacco.

Conclusions
This paper enhances the heating of tobacco in heat non-combustible cigarettes by applying expanded graphite with high thermal conductivity to a roll of tobacco with over-biomass viscosity adhesive.The thermal conductivity measured by adding 3 ml, 4 ml, and 6 ml of the expanded graphite viscous mixture to the filter paper spacer increased sequentially both at room temperature and at 100 degrees Celsius, and all of them were higher than that of the blank filter paper.This method effectively realized the fixation of the expanded graphite thermal conductive material on the surface of the tobacco, which greatly improved the thermal conductivity performance of the tobacco and made this new type of heat cigarettes more uniformly heated.

Figure 1 .
Figure 1.Schematic diagram for the preparation of adsorbable graphite thermally conductive nanosheets.

Figure 3 .
Figure 3. Digital photo of thermal conductor