A Method of Preparing a Microcapsule with High Thermal Conductivity and Its Application

Microcapsules are widely used to add aroma to cigarettes and electronic cigarettes. However, at present, many cigarettes containing aroma microcapsules release insufficient aroma and the active ingredients cannot be fully released, which is due to the fact that the wall materials for preparing such microcapsules are mainly organic molecules, resulting in low thermal conductivity of such microcapsules. Moreover, these materials suffer from uneven distribution, low loading rate, uncontrollable preparation, etc., and has a limited effect on the thermal conductivity of microcapsules. In this study, aromatic and highly thermally conductive microcapsules were obtained by reaction using β-cyclodextrin as wall material and expanded graphite and menthol as core material. It was found that the thermal conductivity of the microcapsules increased exponentially with the increase in the amount of expandable graphite added, and this also indicates a gradual increase in the loading of the core material in the microcapsules. Further, the microcapsules have a high decomposition weight loss temperature, and there is no effect of using them in cigarettes and electronic cigarettes. Moreover, the microcapsules contain menthol, which can enhance the aroma of cigarettes.


Introduction
Currently, microcapsules [1] containing aroma components are mostly used in the market to add aroma to cigarettes and electronic cigarettes.However, at present, many cigarettes containing aroma microcapsules release insufficient aroma and the active ingredients cannot be fully released, which is due to the fact that the wall materials [2] for preparing such microcapsules are mainly organic molecules, resulting in low thermal conductivity of such microcapsules [3] .For this reason, it has been considered to introduce materials with high thermal conductivity (e.g., graphene, expandable graphite [4] , etc.) into the microcapsules to improve the degree of thermal conductivity of the microcapsules and the tobacco.However, these materials are loaded into the microcapsules by physical mixing, which suffers from uneven distribution, low loading rate, uncontrollable preparation, etc., and has a limited effect on the thermal conductivity of microcapsules.
Therefore, in this paper, in order to improve the thermal conductivity of the microcapsules, modified expandable graphite was introduced into the microcapsules, and through its reaction with the microcapsule wall materials, the microcapsules with high embedding rate, high thermal conductivity, and homogeneous distribution of carbon nanomaterials were finally obtained.

Materials
DL-Menthol and β-Cyclodextrin were purchased from Shanghai Macklin Biochemical Co., Ltd.Expandable Graphite was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.Glutaraldehyde was purchased from Tianjin Hiens Biochemical Technology Co., Ltd.

Preparation of modified expanded graphite
In the first step, dissolve 1 g of expanded graphene and 4 g of urea in 120 ml and 30 ml of ethanol solution, respectively, and mix them, ultrasonic after mixing, then grind them into powder and put them into an inert gas-protected tube furnace for high-temperature carbonization at 150-300°C for 2-4 h at a temperature increase rate of 1-3°C/min; and continue to increase the rate to 300-550°C for high-temperature carbonization at a temperature increase rate of 1-3°C/min; then continue to increase the rate to 300-550°C to obtain a black solid powder of 1.73 g of graphene nitride.After that, the temperature was further increased to 300-350 °C at a rate of 1-3 °C/min to carry out high-temperature carbonization, and 1.73 g of graphene nitride, a black solid powder was obtained.

Preparation of nanomaterial intermediates
500 mg of graphene nitride and 3 g of β-cyclodextrin were dispersed in 50 ml, 1 mol/L HCl, and reacted at 85 °C.Add 0.749g of glutaraldehyde in the above reaction solution, and low 1 mol/L NaOH solution to adjust the PH to 7-8, reaction 1h, to obtain carbon nanomaterial intermediates.

Preparation of microcapsules containing different ratios of nanomaterial intermediates
Carbon nanomaterial intermediates (100 mg, 300 mg,500 mg), DL-menthol (1 g), and pure β-cyclodextrin (3 g) were weighed and dissolved in 50 ml of distilled water, and the reaction was carried out at 50 °C for 4 h.At the end of the reaction, the reaction solution was centrifuged, washed with water, and freeze-dried, and the final product of the microcapsules was obtained (Figure 1).

Property Tests
Field emission scanning electron microscopy (SEM) images were taken on a scanning electron microscope (Hitachi 88020) with an operating voltage of 5kv.The microencapsulated powder is pressed at 20mpa and the sample and probe are formed into a sandwich pattern and tested with a thermal conductivity meter (Hot Disk TPS 2500).The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis were determined by the thermogravimetric analyzer (Netzsch STA 449 F3) in the 20℃ to 600℃ range with a heating rate of 10 °C /min under nitrogen atmosphere.The difference between the microscopic surface morphology of the wall material β-cyclodextrin and the encapsulated material was observed by SEM to determine whether microcapsules were formed.β-cyclodextrin particles have a rough surface with irregularly shaped edges and are amorphous, whereas, as can be seen in Fig. 2, the shape and size of the microcapsule samples enriched with expandable graphite are distinctly different: they appear to be compact, flat, and regular crystals.Therefore, it was shown that this core material did form a new stable encapsulation state with β-cyclodextrin, formed a new stable state of encapsulation, and the encapsulation effect was relatively satisfactory.

Thermal Conductivity Properties Analysis
The thermal conductivity was tested using the transient flat plate heat source method.The microcapsules contained three different molar ratios, carbon nanomaterial intermediate: DL-menthol: β-cyclodextrin in molar ratios of 0.1/0.3/0.5:1:3.Thermal conductivity tests were performed separately, and the thermal conductivity of the three microcapsules at 25℃ was 1.474 W/m• k, 2.214 W/m• k, and 3.923 W/m• k respectively; and at 100 ℃, the coefficients of thermal conductivity were 1.668 W/m• k, 2.468 W/m•k, and 4.134 W/m•k, while the thermal conductivity of the microcapsules of carbon nanomaterials intermediates modified without the addition of expandable graphite was 0.1615 W/m•k and 0.1473 W/m•k at 25℃ and 100 ℃.It can be illustrated that the thermal conductivity of the microcapsules with the addition of expandable graphite is much higher than that of the simple microcapsules, and with the increase in the content of the added content and the temperature, the thermal conductivity has been greatly improved.As can be seen in Figure 3, the weight loss of β-cyclodextrin has two main stages: the first stage, 25℃ ~125℃, is mainly caused by the volatilization of water loss; 125℃~300℃ is the stable area, almost no weight loss; 300 ℃ ~600 ℃ is the most important decomposition loss area, the fastest decomposition rate at about 330℃, reaching 18.81%/min.Expandable graphite microcapsules lose weight slowly from 25℃ to 300℃, and 300℃~600℃ is also the most important decomposition always area, with the fastest decomposition rate at about 325℃, reaching 28.53%/min.And from Figure 3, it can be seen that the microcapsules are more weight loss than β-cyclodextrin, and the maximum decomposition rate is fast, which is probably due to the inclusion of the volatile flavoring agent menthol in the microcapsules, at the same time, these differences also indicate that the expandable graphite interacted with β-cyclodextrin to form microcapsules.

Conclusions
In this study, aromatic and highly thermally conductive microcapsules were obtained by reaction using β-cyclodextrin as wall material and expanded graphite and menthol as core material.It was found that the thermal conductivity of the microcapsules increased exponentially with the increase in the amount of expandable graphite added, and this also indicates a gradual increase in the loading of the core material in the microcapsules.Further, the microcapsules have a high decomposition weight loss temperature, and there is no effect of using them in cigarettes and e-cigarettes.Moreover, the microcapsules contain menthol, which can enhance the aroma of cigarettes and is also a special feature of the microcapsules.

Figure 1 .
Figure 1.Schematic diagram for the preparation of microcapsules.

Figure 3 .
Figure 3. TG-DSC for microencapsulation.As can be seen in Figure3, the weight loss of β-cyclodextrin has two main stages: the first stage, 25℃ ~125℃, is mainly caused by the volatilization of water loss; 125℃~300℃ is the stable area, almost no weight loss; 300 ℃ ~600 ℃ is the most important decomposition loss area, the fastest decomposition rate at about 330℃, reaching 18.81%/min.Expandable graphite microcapsules lose weight slowly from 25℃ to 300℃, and 300℃~600℃ is also the most important decomposition always area, with the fastest decomposition rate at about 325℃, reaching 28.53%/min.And from Figure3, it can be seen that the microcapsules are more weight loss than β-cyclodextrin, and the maximum decomposition rate is fast, which is probably due to the inclusion of the volatile flavoring agent menthol in the microcapsules, at the same time, these differences also indicate that the expandable graphite interacted with β-cyclodextrin to form microcapsules.