Pre-carbonized PBO / BNNs composite heat-conducting film, preparation method thereof and heat-not-burn tobacco product

Abstract

The invention relates to the technical field of heating cigarettes, in particular to a pre-carbonized PBO / BNNs composite heat conduction film, a preparation method thereof and a heating non-combustion tobacco product. The preparation method comprises the following steps: mixing hexagonal boron nitride powder with anhydrous dextrose, carrying out ball milling, and then adding into a mixed solvent of isopropanol and water to obtain hydroxylated boron nitride nanosheets; the preparation method comprises the following steps: adding PBO fibers and hydroxylated boron nitride nanosheets into a strong acid mixed solvent to obtain uniform PBO / BNNs acid sol; adding a gel initiator into the acid sol to obtain a PBO / BNNs acid gel film; carrying out solvent replacement on the acid gel film by using an alcohol solvent and deionized water in sequence, and then carrying out pre-pressing treatment and drying treatment; and placing the composite film in an inert atmosphere, and carrying out pre-carbonization treatment at a temperature higher than 350 DEG C to obtain the pre-carbonized PBO / BNNs composite heat-conducting film. The prepared composite heat conduction film has high heat conductivity, the heat conduction capacity of the heat-not-burn tobacco matrix can be remarkably improved, and product degradation caused by local heat concentration is avoided.

Description

Pre-carbonized PBO / BNNs composite thermally conductive film, its preparation method, and heated non-combustible tobacco products Technical Field

[0001] This invention relates to the field of heated cigarette technology, and more particularly to a pre-carbonized PBO / BNNs composite thermally conductive film, its preparation method, and heated non-combustible tobacco products. Background Technology

[0002] With the increasing health awareness and evolving consumption concepts among global tobacco consumers, heated tobacco products (HNB), as a harm-reduction innovative product, have become an important development direction for the tobacco industry. Current mainstream technologies are mainly divided into two categories based on the location and direction of the heat source: one is internal center heating, where sheet-like or needle-like resistance elements are directly inserted into the tobacco cartridge, generating heat through the Joule heating effect, with radial heat conduction from the center of the tobacco column outwards; the other is external periphery heating, including using cylindrical resistance heating tubes to wrap the tobacco cartridge for contact heat transfer, or employing electromagnetic induction technology to generate eddy currents in the outer metal foil layer of the cartridge for self-heating, thus forming a heat conduction path from the outside in. All of these technologies aim to control the tobacco material to a low temperature range below 350°C during baking to release aerosols containing nicotine and flavor substances, while avoiding high-temperature combustion.

[0003] However, regardless of whether the heat conduction mode is from the inside out or from the outside in, existing technologies face a common physical bottleneck: heat must be transferred through the low thermal conductivity tobacco matrix via heat conduction, thus creating a significant temperature gradient between the heat source and the far end of the tobacco. This fundamental defect leads to a series of problems: First, uneven heating and low efficiency, with some tobacco materials failing to reach the effective volatilization temperature, resulting in insufficient aerosol generation, incomplete release of effective components, and material waste; Second, local overheating and product degradation, as tobacco near the heat source is prone to local overheating due to continuous heating, leading to an increase in carbonization or pyrolysis byproducts, affecting aerosol purity and sensory quality; Third, unstable performance, as the existence of the temperature gradient makes the heating process heavily dependent on variables such as the bulk density, humidity, and contact state of the tobacco material, making it difficult to guarantee the consistency of product performance between batches.

[0004] To improve the overall thermal conductivity of tobacco and reduce the internal temperature gradient, a common industrial solution is to add high thermal conductivity filler materials, such as aluminum or boron nitride powder, to the heated tobacco matrix. However, this powder addition method has significant limitations in practical applications. Firstly, it is constrained by the uniform dispersion of inorganic thermally conductive fillers in a tobacco matrix containing organic matter and moisture; the powder is prone to agglomeration and is physically isolated by the low thermal conductivity of the tobacco matrix. Secondly, to ensure the mechanical stability and processing performance of the tobacco matrix, the upper limit of the inorganic filler content must be strictly controlled, and high filler content severely affects aerosol release and inhalation sensations. These two factors combined make it difficult for traditional powder fillers to form a continuous and efficient phonon transport network within the matrix, limiting the actual improvement in thermal conductivity and thus restricting the effect of reducing the internal temperature gradient during the heating process. The industry urgently needs a new thermal conductivity solution or material to optimize the efficiency and uniformity of heat transfer within the matrix. Summary of the Invention

[0005] To address or partially address the problems existing in related technologies, this invention provides a pre-carbonized PBO / BNNs composite thermally conductive film, its preparation method, and a heated non-combustible tobacco product.

[0006] This invention provides a method for preparing a pre-carbonized PBO / BNNs composite thermally conductive film, comprising:

[0007] Step 1) Hexagonal boron nitride powder is mixed with anhydrous glucose and ball-milled. After washing, it is added to a mixed solvent of isopropanol and water, and ultrasonically exfoliated and centrifuged to obtain hydroxylated boron nitride nanosheets. Step 2) PBO fibers and the hydroxylated boron nitride nanosheets are added to a strong acid mixed solvent, and PBO is exfoliated into nanofibers by mechanical stirring to obtain a uniform PBO / BNNs acid sol. Step 3) A gel initiator is added to the acid sol to deprotonate the system and crosslink the exfoliated PBO nanofibers to obtain PB. Step 4) The acid gel membrane is replaced with an alcohol solvent to obtain a PBO / BNNs alcohol gel membrane; then, the acid gel membrane is replaced with deionized water to obtain a PBO / BNNs hydrogel membrane; Step 5) The hydrogel membrane is pre-pressed and dried to obtain an uncarbonized PBO / BNNs composite film; Step 6) The composite film is placed in an inert atmosphere and pre-carbonized at a temperature above 350°C to obtain the pre-carbonized PBO / BNNs composite thermally conductive film.

[0008] Furthermore, in step 1), the ratio of hexagonal boron nitride powder to anhydrous glucose is 1:(0.1~1).

[0009] Furthermore, in step 1), the ball milling speed is 500-1500 rpm and the processing time is 720-2880 min.

[0010] Furthermore, the temperature of the pre-carbonization treatment in step 6) is 350-450℃.

[0011] Further, in step 2), the mass concentration of PBO fibers in the acid sol is 1-10 mg / mL, and the mass concentration of BNNs nanosheets is 0.3-3 mg / mL.

[0012] Further, the alcohol solvent in step 4) is isopropanol; and / or, the pre-compression treatment in step 5) is performed at a pressure of 0.1-0.5 MPa for 8-12 h.

[0013] The present invention also provides a pre-carbonized PBO / BNNs composite thermally conductive film, which is prepared according to any one of the methods described above.

[0014] Furthermore, its thickness is 10-100 μm, and its thermal conductivity is 15-50 W / m². -1 K -1 .

[0015] The present invention also provides a heated non-combustible tobacco product, comprising a tobacco matrix and the aforementioned pre-carbonized PBO / BNNs composite thermally conductive film.

[0016] Furthermore, the amount of the composite thermally conductive film added is less than 10% of the mass of the tobacco matrix.

[0017] The method for preparing the pre-carbonized PBO / BNNs composite thermally conductive film provided by this invention can include the following beneficial effects: This method utilizes polymer fiber exfoliation and gel phase transition processes to successfully construct a layered heterogeneous continuous network film based on organic nanofibers and inorganic nanosheets. The intrinsically high thermal conductivity of PBO rigid molecular chains and BNNs forms a heterogeneous composite phonon transport network. The resulting composite thermally conductive film exhibits high thermal conductivity. Adding it to a heated tobacco matrix can significantly improve the thermal conductivity of the heated tobacco matrix, preventing product degradation caused by localized heat concentration. Furthermore, the high thermal stability of PBO fibers and the pre-carbonization pretreatment ensure that the pre-carbonized PBO / BNNs composite film has no risk of thermal decomposition within the conventional heating temperature range of heated tobacco products up to 350°C, thus ensuring that it does not affect the sensory characteristics of heated tobacco products.

[0018] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and are not intended to limit the invention. Attached Figure Description

[0019] Figure 1 is a scanning tunneling electron microscope (STM) image of the exfoliated hydroxylated boron nitride nanosheets prepared in Example 1 of the present invention; Figure 2 is an atomic force microscope (AFM) image of the exfoliated hydroxylated boron nitride nanosheets prepared in Example 1 of the present invention; Figure 3 is a STM image of the PBO fibers before and after exfoliation in Example 1 of the present invention, where a is before exfoliation and b is after exfoliation; Figure 4 is a cross-sectional STM image of the pre-carbonized PBO / BNNs composite thermally conductive film prepared in Example 1 of the present invention; Figure 5 is a bar chart comparing the thermal conductivity of the pre-carbonized PBO / BNNs composite films prepared in Examples 1-6 of the present invention and the PBO film prepared in Comparative Example 1; Figure 6 is a thermogravimetric curve comparing the thermal loss of the pre-carbonized PBO / BNNs composite thermally conductive film provided in Example 1 of the present invention and the PBO film provided in Comparative Example 1; Figure 7 is a bar chart comparing the thermal conductivity of the cigarette cartridges prepared in Example 7 of the present invention and Comparative Example 2. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The singular forms “a,” “the,” and “the” used in this invention and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used herein refers to and includes any or all possible combinations of one or more of the associated listed items.

[0022] It should be understood that although the terms "first," "second," "third," etc., may be used in this invention to describe various information, this information should not be limited to these terms. These terms are only used to distinguish information of the same type from one another. For example, without departing from the scope of this invention, first information may also be referred to as second information, and similarly, second information may also be referred to as first information. Thus, features defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0023] To address the aforementioned issues, the inventors of this application first considered abandoning the traditional approach of powder filling and instead using an ultra-thin thermally conductive film with a large specific surface area to dope the tobacco sheet. The aim was to prepare a film that could provide a larger continuous thermally conductive network with extremely low addition amounts, while having minimal impact on the overall performance of the tobacco sheet.

[0024] Based on the above considerations, the inventors of this application initially considered using poly(p-phenylenebenzodioxazole) (PBO), which has excellent thermal conductivity, to form a nanofilm. However, the inventors of this application found through research that although PBO fibers alone can form a continuous network and have basic thermal conductivity, their thermal conductivity is limited by the phonon scattering limit of the polymer and cannot meet the transient high heat transfer requirements of HNB (Heated Tobacco Products) devices.

[0025] To further improve the thermal conductivity of the thin film, the inventors of this application considered preparing a composite thin film by introducing inorganic materials, leveraging the higher thermal conductivity of inorganic materials to increase the overall thermal conductivity limit of the material. Based on this, this application further considers introducing boron nitride nanosheets (BNNs) to prepare an organic / inorganic composite thin film. The extremely high in-plane thermal conductivity of BNNs serves as a high-speed phonon channel, and a PBO network acts as a continuous physical bridge connecting the dispersed BNNs, significantly reducing the interfacial thermal resistance between the inorganic phases. The synergistic effect of these two methods allows the composite thin film to overcome the thermal conductivity limitations of a single material at an extremely thin thickness.

[0026] However, to successfully prepare PBO / BNNs composite thermally conductive films and apply them in HNBs, the following technical obstacles need to be overcome: Obstacle 1: Boron nitride (BN) is an inert inorganic material, while PBO is a rigid organic polymer with only amide groups in its side chains, lacking active groups such as hydroxyl and carboxyl groups. There is no interaction between the two, making it difficult to achieve uniform composite formation.

[0027] Technical Obstacle Two: Although PBO is a heat-resistant material, trace amounts of volatile components or residual solvents may still be released within the heating range of room temperature to 400°C. If directly heated in HNB (operating temperature approximately 350°C), these trace volatiles may severely damage the pure taste of tobacco and even produce harmful impurities, making it difficult to pass the stringent testing of tobacco additives.

[0028] To address the first technical obstacle, the inventors of this application employed two methods: firstly, hydroxylating boron nitride; and secondly, protonating the PBO molecular chains to impart a positive charge. This allows the hydroxylated boron nitride to improve its compatibility with the exfoliated PBO nanofibers through electrostatic and hydrogen bonding interactions. However, due to the high inertness of boron nitride (BN), hydroxylation is challenging. Through repeated research and exploration, the inventors discovered that anhydrous glucose ball milling yields better results. Furthermore, by adding a dispersant composed of isopropanol and water, the recombination and aggregation of BNNs can be prevented, thus increasing the yield of hydroxylated BNNs.

[0029] To address the second technical obstacle, the inventors of this application considered pre-carbonizing the composite film at temperatures higher than those used in HNB smoking devices. This not only releases all potential volatile impurities in advance, ensuring safe vaping and sensory purity, but also improves the heat resistance, mechanical strength, and final thermal conductivity of the PBO material.

[0030] Based on the above inventive concept, this invention provides a method for preparing a pre-carbonized PBO / BNNs composite thermally conductive film, comprising the following steps: Step 1) Hexagonal boron nitride powder is mixed with anhydrous glucose and ball-milled, then washed and added to a mixed solvent of isopropanol and water, and subjected to ultrasonic exfoliation and centrifugation to obtain hydroxylated boron nitride nanosheets; Step 2) PBO fibers and the hydroxylated boron nitride nanosheets are added to a strong acid mixed solvent, and PBO is exfoliated into nanofibers by mechanical stirring to obtain a uniform PBO / BNNs acid sol; Step 3) A gel initiator is added to the acid sol to degrade the system. Step 4) The PBO nanofibers are cross-linked after being exfoliated to obtain a PBO / BNNs acid gel film; Step 5) The acid gel film is solvent-displaced using an alcohol solvent to obtain a PBO / BNNs alcohol gel film; then, the acid gel film is solvent-displaced using deionized water to obtain a PBO / BNNs hydrogel film; Step 6) The hydrogel film is pre-pressed and dried, and then dried again to obtain an uncarbonized PBO / BNNs composite film; Step 7) The composite film is placed in an inert atmosphere and pre-carbonized at a temperature above 350°C to obtain the pre-carbonized PBO / BNNs composite thermally conductive film.

[0031] In the above preparation method, step 1) serves to modify the surface of the inert inorganic boron nitride material and perform nanoscale exfoliation to obtain a thermally conductive reinforcing phase with good interfacial compatibility. During implementation, conventional hexagonal boron nitride lacks active groups and is prone to repulsion and aggregation when directly compounded with organic materials. To address this interfacial incompatibility issue, this embodiment uses anhydrous glucose for ball milling, utilizing mechanochemical action to effectively graft hydroxyl groups onto the boron nitride surface, thereby significantly improving its bonding strength with the subsequent polymer matrix.

[0032] The inventors of this application, through extensive research, discovered that the following factors have the greatest impact on the hydroxylation process: the mass ratio of hexagonal boron nitride powder to glucose; and the ball milling speed. Based on this, the optimized process conditions are as follows: the preferred mass ratio of hexagonal boron nitride powder to anhydrous glucose is 1:(0.1~1). The preferred ball milling speed is 500-1500 rpm, and the preferred processing time is 720-2880 min. More preferably, the mass ratio of hexagonal boron nitride powder to anhydrous glucose is 1:(2-3), and most preferably 1:2.8; the ball milling speed is 500-1000 rpm, and the processing time is 1440-2880 min; most preferably, the preferred ball milling speed is 800 rpm, and the preferred processing time is 1440 min.

[0033] After ball milling, the material is washed to remove impurities. The added isopropanol and water mixture acts as an intercalating agent, allowing isopropanol molecules to penetrate the interlayer of hexagonal boron nitride, weakening the van der Waals forces. Water molecules synergistically promote interlayer expansion, thus achieving efficient nanosheet exfoliation under ultrasonication and effectively preventing the exfoliated nanosheets from re-stacking. The volume ratio of isopropanol to water is 1:(0.5-1.5). The ultrasonic exfoliation power can be 100-150 W, and the ultrasonication time can be 20-40 min. After ultrasonication, the mixture is preferably subjected to oscillation treatment, with an oscillation amplitude of 3-6 mm, a rotation speed of 1000-5000 rpm, and a power of 10-20 W. Finally, centrifugation is performed, the supernatant is collected and dried to obtain hydroxylated BNNs. The aforementioned process of ultrasonication, oscillation, and centrifugation is preferably repeated 3-5 times. The lateral length of the BNNs prepared in this step is preferably 1-5 μm. The thickness of the BNNs is preferably less than 5 nm.

[0034] Step 2) above is used to break the inherently strong intermolecular interactions of PBO, preparing a uniformly dispersed system suitable for subsequent film formation. PBO is an extremely rigid polymer with strong π-π conjugation between its molecular chains, making it difficult to process in conventional solvents. Faced with this material dissolution barrier, this application employs a strong acid mixed solvent combined with mechanical stirring. This protonation forces the PBO molecular chains to acquire a positive charge, and the electrostatic repulsion of like charges forces them to be exfoliated into PBO nanofibers. The exfoliated nanofibers have a very large specific surface area. The positively charged PBO nanofibers and the hydroxyl-containing boron nitride nanosheets (BNNs) obtained in step 1) are well bonded through electrostatic and hydrogen bonding, thus overcoming the physical repulsion between the organic matrix and the inorganic phase, forming a stable and dispersed acid sol.

[0035] Preferably, the PBO fiber concentration in the acid sol is 1-10 mg / mL, and the BNNs concentration is 0.3-3 mg / mL. Controlling the concentration within this range imparts suitable rheological properties to the sol, which is beneficial for the subsequent construction of the three-dimensional framework. More preferably, the PBO fiber concentration is 10 mg / mL, and the BNNs concentration is 3 mg / mL. Optionally, the strong acid mixed solvent can be composed of methanesulfonic acid and trifluoroacetic acid, with a mass ratio of 1:1. The mechanical stirring speed is preferably 500-1500 rpm, and the stirring time is preferably 24-72 h. In the PBO / BNNs acid sol obtained in this step, the diameter of the PNFs (PBO nanofibers) is preferably 9-30 nm. The average length of the PNFs is preferably 3-20 μm.

[0036] Step 3) involves adding a gel initiator to the acid sol obtained in the previous step to deprotonate the system and crosslink the exfoliated PBO nanofibers, resulting in a PBO / BNNs acid gel film. The purpose of this step is to lock the liquid sol into a solid gel framework through a thermodynamic phase change mechanism. If the strong acid sol is directly heated and evaporated using conventional methods, the capillary contraction force generated during evaporation can easily destroy the three-dimensional nano-network established by the polymer and nanosheets. To address the risk of network collapse, a gel initiator is added to gently reduce the system's polarity, using water in the initiator as a proton acceptor to slowly deplete the surface charge of the PBO nanofibers. The gradual decrease in electrostatic repulsion allows the polymer chains to crosslink and lock together, thus completing the curing and shaping without damaging the microscopic thermally conductive network. Preferably, the gel initiator can be a mixture of methanesulfonic acid, ethyl acetate, and water, with a preferred mass ratio of 76.6:23.3:3. Specifically, the gel initiator is added dropwise to the acid sol while stirring. The preferred stirring speed is 900-1000 rpm.

[0037] Step 4) is used to remove residual highly corrosive acid from the gel pores. Although the acid gel is formed, it is filled with strong acid. Direct drying will lead to water loss and rapid concentration of acid, which in turn will cause damage to the internal polymer network and cracking of the film. This step uses a gradient solvent extraction and displacement method to replace the destructive acid with water without damage, thus preserving the microscopically fragile phonon transport network. Preferably, the alcohol solvent is isopropanol. Specifically, the acid gel membrane can be extracted in isopropanol for 20-30 h, with the isopropanol solvent being replaced every 6-8 h to obtain an alcohol gel membrane; then, the alcohol gel membrane can be extracted in deionized water for 20-30 h, with the deionized water solvent being replaced every 6-8 h to obtain a PBO / BNNs hydrogel membrane. This preferred operation ensures that the acid is thoroughly washed away while avoiding structural stress caused by excessively rapid displacement.

[0038] Step 5) involves pre-compressing and drying the hydrogel membrane to obtain an uncarbonized PBO / BNNs composite film. This step aims to remove moisture and achieve physical densification of the micro-interface. The displacement-obtained hydrogel is rich in moisture and has a loose structure with large physical spacing between the thermally conductive components. Pre-compressing allows for mechanical compaction of the network, forcing the inorganic thermally conductive phase and the organic framework to adhere tightly at the microscale, thereby significantly reducing interfacial thermal resistance. Preferably, the pre-compressing pressure is 0.1-0.5 MPa, and the time is 8-12 h. This parameter range effectively drains water while preventing macroscopic fracture of the film. More preferably, the pre-compressing pressure is 0.5 MPa, and the time is 12 h, under which the film exhibits better densification and thermal conductivity. Optionally, the drying temperature can be 20-50°C, and the ambient humidity can be 10-30%. For example, gentle drying can be carried out at 25°C and 10% humidity.

[0039] Step 6) is a pre-carbonization treatment of the composite film. Although PBO itself is heat-resistant, trace amounts of solvent or unclosed oligomers may remain during film formation. When exposed to the baking environment of approximately 350°C in a heated non-combustible smoke device, these trace impurities will volatilize upon heating and interfere with the aroma of the aerosol. This step, by performing pre-carbonization in an inert atmosphere as a controlled thermal quenching, releases potential volatile components in advance, thereby transforming the composite film into a high-purity thermally inert matrix, greatly reducing the risk of odor during subsequent inhalation; simultaneously, moderate carbonization of local structures can further increase the overall thermal conductivity of the material. Preferably, the pre-carbonization temperature is 350-450°C, and most preferably, the pre-carbonization temperature is 400°C. The pre-carbonization time is preferably 8-12 h, and most preferably 12 h. The pre-carbonization heating rate is preferably 4-8 °C / min, and most preferably 5 °C / min.

[0040] The preparation method of the pre-carbonized PBO / BNNs composite thermally conductive film provided in this embodiment has the following advantages: Utilizing polymer fiber exfoliation and gel phase transition processes, a layered heterogeneous continuous network film based on organic nanofibers and inorganic nanosheets was successfully constructed. Through the intrinsically high thermal conductivity of the rigid PBO molecular chains and BNNs, a heterogeneous composite phonon transport network is formed. The resulting composite thermally conductive film exhibits high thermal conductivity. Adding it to a heated tobacco matrix can significantly improve the thermal conductivity of the heated tobacco matrix, avoiding product degradation caused by localized heat concentration. Furthermore, the high thermal stability of the PBO fibers themselves and the pre-carbonization pretreatment ensure that the pre-carbonized PBO / BNNs composite film has no risk of thermal decomposition within the conventional heating temperature range of heated tobacco products up to 350°C, thus ensuring that it does not affect the sensory characteristics of heated tobacco products.

[0041] This invention also provides a pre-carbonized PBO / BNNs composite thermally conductive film, which is prepared according to the preparation method described in the foregoing embodiments. The specific preparation method, preferred scheme, and beneficial effects of the composite thermally conductive film are the same as described in the foregoing method embodiments, and will not be repeated here. Preferably, the thickness of the composite thermally conductive film is 10-100 μm, and the thermal conductivity is 15-50 W / m². -1 K -1 The thinner thickness allows for flexible distribution within complex matrices, while the high thermal conductivity ensures efficient operation of the phonon transport network. More preferably, its thermal conductivity is 18.3–28.0 W / m². -1 K -1 ).

[0042] This invention also provides a heated tobacco product, comprising a tobacco matrix and the aforementioned pre-carbonized PBO / BNNs composite thermally conductive film. In this product, the tobacco matrix serves as the main component for aroma generation and aerosol release, while the composite thermally conductive film acts as an internal thermal conductivity enhancement component. The synergistic mechanism between the two lies in the fact that the composite film exists as a macroscopically continuous two-dimensional material within the low thermal conductivity tobacco matrix, providing a smooth, high-speed heat network, thereby effectively mitigating temperature gradients caused by uneven local heating. Preferably, the amount of the composite thermally conductive film added is less than 10% of the mass of the tobacco matrix in the tobacco product. Controlling the amount within this range allows for improved thermal conductivity without significantly altering the proportion of real tobacco components or compromising the mechanical flexibility and smoking experience of the tobacco sheet itself.

[0043] Optionally, the composite thermally conductive film is alternately stacked with the tobacco sheets before cutting, and then cut. After being stacked into bundles, the bundles are directly inserted into the tobacco cartridges. After being inserted into the smoking device, the structure will not shift or be damaged, and the already formed uniformly dispersed system will not be disrupted. This method is simple to operate.

[0044] Heated tobacco products have the following advantages: By introducing a composite film with high thermal conductivity and a continuous structure, the heat generated by the heat source can be conducted more quickly and evenly to the deep and distant parts of the tobacco structure, significantly reducing the internal temperature gradient. This effectively avoids the problems of near-end matrix carbonization and insufficient heating at the far end caused by heat accumulation, ensuring more uniform tobacco roasting and greatly improving the purity of the aerosol and the consistency of product performance between batches.

[0045] The technical solution of the present invention will be further described below with reference to specific embodiments: Embodiment 1 This embodiment provides a method for preparing a pre-carbonized PBO / BNNs composite thermally conductive film, specifically including the following steps: 1. Preparation of hydroxylated BNNs: 0.9 g of hexagonal boron nitride microparticles and 2.5 g of anhydrous glucose are added to a ball mill jar and ground at 800 rpm for 1440 min. The resulting mixture is then washed with deionized water to remove glucose by dissolution, and vacuum filtration is performed. This washing process is repeated three times. The washed filter cake is dissolved in deionized water and centrifuged at 2654×g for 10 min, and the precipitate is retained. This process is repeated three times. Subsequently, the precipitate is dissolved in a mixture of 15 ml isopropanol and 15 ml deionized water and treated with ultrasound (120 W, 40 kHz) for 30 min. Afterward, the mixture is oscillated on a vortex shaker (amplitude 4.5 mm, 3000 rpm, 12 W) for 30 min. Finally, the shaken mixture was centrifuged at 1300×g for 5 min, and the supernatant was collected. This sonication-shaking-centrifugation collection process was repeated five times to obtain hydroxylated BNNs.

[0046] The microstructure of the prepared hydroxylated BNNs was characterized by scanning tunneling electron microscopy, and the scanning tunneling electron microscopy image is shown in Figure 1. Figure 1 shows that the average lateral diameter of the BNNs exfoliated after the above treatment ranges from 1 to 3 μm.

[0047] The atomic force microscopy image of the prepared hydroxylated BNNs is shown in Figure 2. As can be seen from Figure 2, the thickness of the BNNs exfoliated after the above treatment is less than 5 nm.

[0048] 2. Preparation of PNFs / BNNs acid sol: 1 g of PBO fiber and 0.3 g of BNNs were added to a mixture of methanesulfonic acid (50 g) and trifluoroacetic acid (50 g) and stirred at room temperature for 72 h to obtain a uniform PNFs / BNNs acid sol.

[0049] Figure 3 shows the scanning tunneling electron microscopy (STEM) images of the PBO fibers before and after stripping. As can be seen from Figure 3, the diameter of the PBO fibers is significantly reduced, decreasing from a macroscopic scale of 9–30 μm to PNFs with an average diameter of approximately 16 nm. STEM observation revealed that the average length of the prepared PNFs was 3–20 μm.

[0050] 3. Gelation treatment: Take 35 g of the above acid sol and dilute it with a mixture of 32.5 g methanesulfonic acid and 32.5 g trifluoroacetic acid. Pour the diluted acid sol into a glass dish, and then add a mixture of 76.6 g methanesulfonic acid, 23.3 g ethyl acetate and 3 g deionized water dropwise to the diluted PBO acid sol over 5 min while stirring at 950 rpm. This breaks the thermodynamic equilibrium of the acid sol system, causing deprotonation and initiating crosslinking of PNFs in the acid sol to form a PBO / BNNs acid gel.

[0051] 4. Solvent extraction and displacement: The acid gel was immersed in isopropanol for 24 h to extract and displace the solvent, with the isopropanol solvent being replaced every 8 h during this period, to obtain PBO / BNNs alcohol gel. Then, the alcohol gel was immersed in deionized water for 24 h to extract and displace the solvent, with the deionized water solvent being replaced every 8 h during this period, to obtain PBO / BNNs hydrogel.

[0052] 5. Pre-compression and drying: The PBO / BNNs hydrogel was pre-compressed at 0.5 MPa for 12 h, and then dried in an environment with a temperature of 25℃ and a humidity of 10% to obtain an uncarbonized PBO / BNNs composite film.

[0053] 6. Pre-carbonization treatment: Finally, the dried PBO / BNNs composite film is placed in a tube furnace and heated to 400℃ at a heating rate of 5℃ / min under the protection of nitrogen atmosphere, and held for 12 h to obtain the pre-carbonized PBO / BNNs composite film.

[0054] The cross-section of the pre-carbonized PBO / BNNs composite film finally prepared in this embodiment was characterized by scanning tunneling electron microscopy, as shown in Figure 4. A tightly bonded heterogeneous layered composite network was formed between the BNNs and PNFs, with a thickness of approximately 20 μm. The PNFs, whose thermal conductivity was further improved after pre-carbonization, and the intrinsically high thermal conductivity of the BNNs synergistically cooperated in the macroscopic film to successfully construct a fast phonon transport network.

[0055] The preparation process of Example 2 is basically the same as that of Example 1, except that in step 2, 0.3 g BNNs is replaced with 0.1 g BNNs.

[0056] The preparation process of Example 3 is basically the same as that of Example 1, except that in step 2, 0.3 g BNNs is replaced with 0.5 g BNNs.

[0057] The preparation process of Example 4 is basically the same as that of Example 1, except that in step 2, 0.3 g BNNs is replaced with 0.7 g BNNs.

[0058] The preparation process of Example 5 is basically the same as that of Example 1, except that in step 2, 0.3 g BNNs is replaced with 0.9 g BNNs.

[0059] The preparation process of Example 6 is basically the same as that of Example 1, except that in step 2, 0.3 g BNNs is replaced with 1.0 g BNNs.

[0060] The difference between Comparative Example 1 and Example 1 is that BNNs are not added (i.e., step 1 is not included, and BNNs are not added in step 2); the pre-carbonization step is not performed (i.e., step 6 is not included), and the PBO film obtained in step 5 is the final product.

[0061] The thermal conductivity of the pre-carbonized PBO / BNNs composite films prepared in Examples 1-6 and the PBO film prepared in Comparative Example 1 was measured using a flash thermal conductivity meter. The test results are shown in Figure 5: the introduction of BNNs significantly improved the thermal conductivity of the film; due to the tightly bonded heterogeneous composite layered thermal conductive network formed by the pre-carbonized PNFs and BNNs, the thermal conductivity of the PBO / BNNs composite film in Example 1 reached 18.3 W / m². -1 K -1 The thermal conductivity of the composite film increases with the increase of BNNs filling amount, and the highest thermal conductivity of the sample in Example 6 reaches 28.0 W / m. -1 K -1 .

[0062] The thermal stability of the pre-carbonized PBO / BNNs composite film of Example 1 and the PBO film prepared in Comparative Example 1 were tested using a thermogravimetric analyzer. The test results are shown in Figure 6: The pre-carbonized PBO / BNNs composite film showed no significant mass loss in the temperature range of 25–550 °C, indicating that it did not undergo thermal decomposition during heating. This ensures that when used as a thermally conductive reinforcing phase in heated non-combustible tobacco, it will not produce volatile substances affecting sensory quality due to the material's own pyrolysis. In contrast, the pure PBO film in Comparative Example 1, without pre-carbonization treatment, showed significant thermal decomposition within the same temperature range.

[0063] Example 7: The pre-carbonized PBO / BNNs composite film prepared in Example 1 was alternately stacked with tobacco sheets before cutting, then cut, and the stacked bundles were directly loaded into tobacco cartridges to prepare heated cigarettes. The amount of pre-carbonized PBO / BNNs composite film added was approximately 10 wt% of the tobacco sheets.

[0064] The difference between Comparative Example 2 and Example 7 is that no pre-carbonized PBO / BNNs composite film was added. Instead, tobacco sheets were directly cut, stacked into bundles, and directly loaded into tobacco cartridges to prepare heated cigarettes.

[0065] The thermal conductivity of the e-cigarette cartridges prepared in Example 7 and Comparative Example 2 was measured using a flash thermal conductivity meter. The test results are shown in Figure 7. As can be seen from Figure 7, the overall thermal conductivity of the e-cigarette cartridge of Comparative Example 2 (before lamination) without the addition of a thermally conductive film is only 0.531 W / m. -1 K -1 The overall thermal conductivity of the e-cigarette cartridge in Example 7 (after composite) with the addition of the pre-carbonized PBO / BNNs composite film of the present invention was significantly improved to 3.561 W / m. -1 K -1 This fully demonstrates that alternating the composite thermally conductive film prepared by this invention with tobacco sheets can successfully construct a continuous and efficient phonon transport network within the tobacco matrix. This structure greatly enhances the heat transfer capability of the tobacco cartridge at the macroscopic level, thereby effectively mitigating the internal temperature gradient of heated tobacco products during the baking process, verifying the excellent effect of this invention in practical end-user applications.

[0066] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or improvement of the technology in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A method for preparing a pre-carbonized PBO / BNNs composite thermally conductive thin film, characterized in that, include: Step 1) Hexagonal boron nitride powder is mixed with anhydrous glucose and ball-milled. After washing, it is added to a mixed solvent of isopropanol and water, and ultrasonically exfoliated and centrifuged to obtain hydroxylated boron nitride nanosheets. Step 2) PBO fibers and the hydroxylated boron nitride nanosheets are added to a strong acid mixed solvent, and PBO is exfoliated into nanofibers by mechanical stirring to obtain a uniform PBO / BNNs acid sol. Step 3) A gel initiator is added to the acid sol to deprotonate the system and crosslink the exfoliated PBO nanofibers to obtain PB. Step 4) The acid gel membrane is replaced with an alcohol solvent to obtain a PBO / BNNs alcohol gel membrane; then, the acid gel membrane is replaced with deionized water to obtain a PBO / BNNs hydrogel membrane; Step 5) The hydrogel membrane is pre-pressed and dried to obtain an uncarbonized PBO / BNNs composite film; Step 6) The composite film is placed in an inert atmosphere and pre-carbonized at a temperature above 350°C to obtain the pre-carbonized PBO / BNNs composite thermally conductive film.

2. The preparation method according to claim 1, characterized in that, In step 1), the ratio of hexagonal boron nitride powder to anhydrous glucose is 1:(0.1~1).

3. The preparation method according to claim 1, characterized in that, In step 1), the ball milling speed is 500-1500 rpm and the processing time is 720-2880 min.

4. The preparation method according to claim 1, characterized in that, The temperature for the pre-carbonization treatment in step 6) is 350-450℃.

5. The preparation method according to claim 1, characterized in that, In step 2), the mass concentration of PBO fibers in the acid sol is 1-10 mg / mL, and the mass concentration of BNNs nanosheets is 0.3-3 mg / mL.

6. The preparation method according to claim 1, characterized in that, The alcohol solvent in step 4) is isopropanol; and / or, the pre-compression treatment in step 5) is performed at a pressure of 0.1-0.5 MPa for 8-12 h.

7. A pre-carbonized PBO / BNNs composite thermally conductive film, characterized in that, It is prepared according to the method described in any one of claims 1-6.

8. The composite thermally conductive film according to claim 7, characterized in that, Its thickness is 10-100 μm, and its thermal conductivity is 15-50 W / m. -1 K -1 .

9. A heat-not-burn tobacco product, comprising a tobacco matrix, characterized in that, It also includes the pre-carbonized PBO / BNNs composite thermally conductive film as described in claim 7 or 8.

10. The heated non-combustible tobacco product according to claim 9, characterized in that, The amount of the composite thermally conductive film added is less than 10% of the mass of the tobacco matrix.

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