Insightful! In-Depth Analysis of Heating Technology and Thermal Insulation Materials for Heat-Not-Bu

Most of the new smoking equipment currently on the market use electric heating to generate heat to evaporate and heat the smoke bomb. The evaporation heating temperature of e-cigarette oil type e-cigarette is about 200℃, and gas-liquid transformation and heat absorption occur during the atomization process. Therefore, this type of e-cigarette transfers less heat from the atomization chamber to the outside.

However, the heating temperature inside a heated non-burning cigarette is as high as 300-350 ° C, and it is mainly heated in the form of baking. The heat is not easily carried away by the smoke. If these heat is not blocked, it will easily cause the surface temperature of the product to be too high, and ultimately affect the consumer's sense of experience and even the risk of burns, so the heat insulation technology for heating non-burning cigarettes is particularly important.

In terms of market conditions, global sales of new cigarettes in 2018 reached US$27.74 billion, of which approximately US$11.9 billion was heated without burning, and approximately US$15.8 billion was used for atomized e-cigarettes. How to design heat barrier structures and which materials have thermal insulation functions are all innovative technologies that can be explored at present.

Below, we will focus on several aspects such as heating methods, surface temperature of smoking equipment, blocking and heat insulation methods, and further in-depth analysis based on relevant products currently on the market.

1. Heating method

At present, the main heating body substrates on the market are: stainless steel, zirconia, alumina, and zirconia toughened alumina ceramics. There are three main heating methods: internal heating, external heating, and internal and external mixed heating.
 
Figure Heating method schematic from the network

1. internal heating

Internal heating, the heating needle or sheet is at a distance from the wall of the cigarette bin, and the packaging and structural design of the cigarette bomb can reduce the conduction of some heat to the product surface.

The heating area is short, the heat is concentrated in the cigarette bin, and the temperature of the middle and lower areas where the heat is transferred to the product is relatively reduced.

The product is small in size and the surface temperature exceeds the standard. IQOS uses thermal convection insulation. When pumping, the air enters from the upper circumferential ring groove and then flows to the middle smoke cavity, taking away the hot air through the cold air, as shown in the following figure.
  Figure IQOS Smoke cavity part comes from patent

Figure IQOS Smoking Set Schematic Diagram from the Internet
2. external heating

By external heating, heat can be directly transferred to the product surface, causing the product surface to be too hot and hot. Thermal insulation treatment must be carried out, otherwise the experience will be seriously affected. It is understood that glo uses vacuum tube insulation method.
  Picture glo smoking set from root-nation
3. mixed heating

Independent internal and external heating uses two heat sources, which is complex in structure, increases difficulty in control, allows for more complete baking, and has a high degree of internal and external heat conduction. Shenzhen Lvxinfeng Technology Co., Ltd. has launched the world's first non-burning electronic cigarette MOX-IOH, which is independently heated internally and externally, using a combination of needle internal heating and 360-degree annular external heating. Internal and external mixing heating requires higher temperature control requirements.
  Tulu Xinfeng MOX Smoking Set
2. Some temperature requirements for e-cigarette smoking equipment

External parts and accessible surfaces of the device that are continuously held during normal use (including the nozzle)
 
External parts and accessible surfaces of the device are held for a short period of time during normal use.# p #pagination title #e #
 
How to control these temperatures so that their heat is blocked and lost? Let's first take a look at the heat blocking method for heating higher temperatures HNB type e-cigarettes.

Heat blocking generally includes direct blocking and indirect blocking. Indirect blocking conducts heat to places that are not in contact with the hand, and sets up high-emissivity materials to achieve rapid heat dissipation.

However, it is more commonly used to directly block heat. By arranging an insulation layer on the periphery of the heating element, this blocking method can not only reduce the external temperature of the product, but also reduce the heat loss of the heater, thereby improving the utilization rate of heat energy and reducing costs.

The following focuses on the heat insulation materials that are commonly used in direct barrier methods.

3. Thermal insulation materials

The temperature of the core part of the new cigarette when heated is about 220℃~300℃. After a certain amount of heat is lost, the surface of the heating area will reach more than 45℃, which exceeds the comfortable temperature of the human body and feels hot.

In order to ensure that consumers are not scalded or burned when using it, an insulation layer is generally provided between the new cigarette shell and the heating chamber.

Heat conduction, heat convection and heat radiation are three methods of heat transfer. The IQOS smoke cavity mentioned above uses thermal convection insulation method, which will not be described here. Next, we will focus on the two insulation technologies of heat conduction blocking and heat radiation blocking. Common insulation layer structures mainly involve: vacuum pipes, insulation cotton, PEEK, aerogel, etc.

1. Heat conduction blocking technology

Heat conduction blocking technology can be divided into two categories: porous materials and vacuum insulation pipes. Among them, porous materials use the pores contained in the material itself to insulate themselves, because the thermal conductivity of the air or inert gas in the pores is very low, such as insulation cotton, foam materials, fiber materials, etc.; vacuum insulation pipes reduce the impact of molecules through the vacuum environment, thereby blocking heat transfer. Several insulation materials are introduced in detail below.

1.1 heat insulation cotton

The insulation cotton material is generally asbestos, which is used as a material or lining for transmission, insulation, heat insulation, insulation, sealing and other parts, and the cost is low. Thermal insulation cotton is widely used in atomized electronic cigarettes, but its thermal insulation effect is poor and its high temperature stability is poor, and it cannot meet the requirements for heating and non-burning types.
  Figure Steam e-cigarettes come from the Internet
1.2 Aerogel insulation

Aerogel is a highly dispersed solid material composed of ultrafine particles agglomerated with each other and using air as the dispersion medium. It has a high-permeability three-dimensional nano-network structure. The nano-porous structure of aerogels has excellent thermal insulation properties. It is currently the solid material with the lowest thermal conductivity in the world. SiO2 and Al2O3 systems are more common, and their highest temperatures are 800℃ and 1200℃ respectively.
  Figure Aerogel insulation diagram of heating smoking set comes from Yunnan China Tobacco Patent No. CN208798697U
The aerogel phase change material will absorb and store heat by using its own phase state or structure changes, so that the temperature outside the heat insulation layer will not rise too high; when the pumping is over, when the temperature drops below the phase change temperature of the phase change material, the phase change material will release the stored heat and return to the state before the endothermic phase change, and it will be recycled in this way to regulate the temperature.

The aerogel insulation layer can well isolate the heat from the heater, reduce the touch temperature, and ensure that the temperature of the shell is 45℃.

The insulation layer can slow down heat loss, save energy, and improve utilization.

Phase-change heat absorbing materials are filled or adsorbed in the pores of the aerogel, which can enhance its thermal insulation ability, making the aerogel layer thinner with the same thermal insulation ability, which is conducive to its use in precision devices such as new cigarettes.

1.3, vacuum insulation pipe

Vacuum insulation pipes are double-layered structures, including inner pipes and outer pipes. The space between the inner and outer pipes is evacuated. The gas absorber is located between the inner tube and the outer tube. The gas absorber has a getter capable of absorbing gas, a housing for keeping the getter in a vacuum sealed state, and a breaking element for deforming and damaging the housing at a predetermined temperature. The inner and outer walls of the vacuum tube are connected with stainless steel, and the heat from the inner wall is directly transferred to the outer wall through the edges. #p#Pagination Title #e# Figure Vacuum insulation pipe
The limitations of heat insulation pipes are that the vacuum degree of the vacuum pipe cannot be effectively tested, the heat insulation effect is unstable, leakage of the vacuum pipe is inevitable, and the heat insulation effect will be lost, and the cost is high, and the quantity of different size is small and the cost is high.

1.4, thermal insulation engineering plastics

Generally, insulation engineering plastics are generally used for insulation around the heating parts and between the suction nozzle and the top cover. At present, PEEK material is obviously used for heat insulation when heating but not burning. However, PEEK has a high cost and is mainly used in smoking casings, heating plate bases, atomization bins and other parts in the field of heating non-burning electronic cigarettes. During the heating process of non-burning smoking equipment, the surrounding temperature can reach about 300 ° C. The surrounding materials generally need to meet the characteristics of high temperature resistance and heat insulation. The melting point of PEEK is as high as 343 ° C, and the continuous use temperature is 260 C, which can adapt to heating and non-burning smoking equipment. The high temperature characteristics of smoking equipment.
  Figure PEEK heating non-burning smoking parts (source QOQ)
2. Thermal radiation blocking technology

Reflecting materials have a high reflection coefficient and can reflect heat, such as gold, silver, nickel, aluminum foil or metallized polyester, polyimide films, etc.

The following figure shows a typical reflective material structure, in which the reflective and thermal insulation material adopts the form of a multi-layer thin film wound. The thin film structure: Al/PI/nano thermal insulation composite coating, with a polyimide film as the substrate (sheet-like polyimide is used at temperature-250~400℃, and the thermal conductivity can reach 30mW /K.m). An aluminum foil layer (with a thickness of about 80 - 100nm) is deposited on one side of the film, and a nano thermal insulation coating is applied on the other side of the polyimide film.

Among them, aluminum foil has the effect of reflecting heat. The nano thermal insulation coating adopts nano porous particles + hollow ceramic microspheres + silicone heat-resistant resin components. This junction can be wound in multiple layers around the periphery of the heating chamber.
 
The following figure shows a patent on the application of reflective materials. Conductive traces are deposited or coated on an electrically insulating substrate (in the form of a thin layer), and multiple layers of reflective materials are wrapped around the periphery of the conductive heater. The material uses high-reflectivity gold, silver, and aluminum foil materials. The radiation heat is reflected back into the heating chamber, and the form of a honeycomb is used to further reduce heat conduction (porous metal, which can be metal foam).

By arranging a multi-layer reflective screen, the heat can be fully reflected back into the heating cavity, and good thermal insulation can be achieved. On the other hand, the honeycomb porous form can reduce the heat conduction process, thereby further reducing the heat transfer.
 
  Figure Application source patent of reflective materials
At present, new cigarettes are still under research and development, and the design of thermal insulation layers is a major problem in the research and development of new cigarettes. Therefore, selecting the appropriate heat insulation method and appropriate heat insulation layer materials is of great significance to the development of heating appliances with excellent heat insulation effect.