A boron nitride composite powder for epoxy potting adhesive and a preparation method thereof

Abstract

The application discloses a boron nitride composite powder for epoxy pouring sealant and a preparation method thereof, and belongs to the technical field of electronic packaging materials and preparation thereof. The application solves the problem that the rheological behavior and multifunctional performance of the existing epoxy resin pouring sealant cannot be simultaneously compatible at high filler content. In the application, thermoplastic polyurethane (TPU) is used as an adhesive, h-BN layers are assembled into spherical hierarchical particles BN@TPU, and the BN@TPU is added to an epoxy system as a filler, so that the interface energy between the filler and the epoxy matrix is significantly reduced, excellent process flowability and operating viscosity can be maintained at high filler content, and the heat conduction performance is not lost. Experimental results show that the initial viscosity of the pouring sealant with a BN@TPU addition amount of 30wt% is obviously lower than that of a pure BN addition system with the same addition amount, the tensile strength is also significantly improved, and the processing window range is significantly widened.

Description

Technical Field

[0001] This invention relates to a boron nitride composite powder for epoxy potting compound and its preparation method, belonging to the field of electronic packaging materials and their preparation technology. Background Technology

[0002] As dry-type transformers develop towards higher power density, traditional epoxy resin potting compounds, due to their low thermal conductivity, are prone to causing localized overheating in the windings, threatening insulation safety and equipment lifespan. Furthermore, their inherent brittleness and mismatch with the thermal expansion of metallic materials can lead to cracking during thermal cycling, resulting in insulation degradation.

[0003] Currently, the method of adding large amounts of rigid fillers (such as alumina and boron nitride) to improve the thermal conductivity of epoxy resin potting compounds drastically increases the viscosity of the system, resulting in poor flowability and easy formation of bubbles during potting, further deteriorating the toughness of the material. Existing technologies cannot synergistically resolve the contradiction between the high thermal conductivity, high toughness, and good processability of epoxy resin potting compounds, thus hindering the improvement of the reliability of next-generation high-performance dry-type transformers.

[0004] In composite materials, thermally conductive fillers such as h-BN are favored due to their good insulation, low dielectric constant, and high in-plane thermal conductivity. However, they are prone to agglomeration due to van der Waals forces, resulting in poor compatibility with epoxy resins and difficulty in uniform dispersion within the system. This inevitably reduces the toughness of the epoxy resin. Furthermore, in practical dry-type transformer encapsulation processes, achieving high thermal conductivity and crack resistance usually requires high addition amounts, which further leads to a sharp increase in material viscosity, thereby affecting processing performance and narrowing the processing window. Therefore, how to simultaneously achieve excellent thermal conductivity, good toughness, and controllable rheological properties in epoxy resin encapsulation systems with high filler content remains a major challenge in both basic research and industrial applications. Summary of the Invention

[0005] To address the problem that existing epoxy potting compounds cannot simultaneously possess both rheological behavior and multifunctional properties at high filler contents, this invention provides a boron nitride composite powder for epoxy potting compounds and its preparation method.

[0006] The technical solution of the present invention: One objective of this invention is to provide a method for preparing boron nitride composite powder, the method comprising the following steps: Step 1: Weigh the following raw materials by mass: 60-62 parts polyether polyol, 15-17 parts isocyanate, 10-12 parts chain extender, 80-84 parts butyl acetate, 100-102 parts h-BN, and 0.3 parts catalyst. Step 2: After mixing polyether polyol, a portion of butyl acetate and isocyanate, heat the mixture in an oil bath and then add a mixture of chain extender and another portion of butyl acetate. After 20 minutes of adding the chain extender, add the catalyst and continue stirring to react, thus obtaining a transparent TPU solution. Step 3: h-BN is gradually added to the TPU solution obtained in Step 1, dispersed using a high-speed shear emulsifier, and then ultrasonically treated to obtain a uniform milky white suspension. Finally, the suspension is spray-dried and the resulting composite powder is sieved to obtain boron nitride composite powder, named BN@TPU.

[0007] Further specified, the chain extender is 1,4-butanediol, the catalyst is triethylamine, and the h-BN particle size is 10-1 μm.

[0008] Further specifying, the isocyanate is MDI, TDI, or IPDI.

[0009] Furthermore, the molar ratio of polyether polyol, isocyanate and 1,4-butanediol is 1:(0.95~1):(0.05~0.07).

[0010] Further specifying, the oil bath heating reaction process in step 2 is as follows: stirring at 60°C for 0.5 hours, then raising the temperature to 80°C and stirring for 2 hours.

[0011] Further specifying, in step 2, a peristaltic pump with a rotation speed of 20 rpm is used for dripping.

[0012] Further specified, in step 3, the dispersion treatment speed is 10000 rpm and the time is 60 min; the ultrasonic treatment time is 30 min.

[0013] Further specifying, the solid content of the suspension slurry obtained in step 3 is 30~40wt%.

[0014] Further specifying, the boron nitride composite powder obtained in step 3 has a particle size of 200-400 μm.

[0015] The second objective of this invention is to provide a boron nitride composite powder obtained by the above preparation method.

[0016] The third objective of this invention is to provide an application of the above-mentioned boron nitride composite powder, specifically as a filler in the preparation of epoxy potting compound.

[0017] The fourth objective of this invention is to provide an epoxy potting compound, specifically comprising component A of an epoxy resin containing the aforementioned BN@TPU and component B of an anhydride curing agent.

[0018] Furthermore, the mass percentage of BN@TPU shall not exceed 35 wt% of epoxy resin.

[0019] Beneficial effects: This invention uses thermoplastic polyurethane (TPU) as a binder to assemble h-BN sheets into spherical, graded BN@TPU particles, which are then added as fillers to an epoxy system. This solves the problems of filler agglomeration, increased system viscosity, and decreased toughness at high filler contents. Compared with existing technologies, it has at least the following advantages: (1) TPU is used as a pre-coating layer to pre-assemble h-BN sheets into spherical particles BN@TPU. During the initial mixing stage of BN@TPU with epoxy resin, TPU acts as an outer shell to effectively block direct contact between BN and epoxy resin, thereby significantly reducing the viscosity of the system and delaying viscosity growth during processing. As the mixing and curing process proceeds, TPU segments swell in the epoxy resin and partially release the internal BN sheets, causing the BN originally wrapped in the spherical particles to gradually expose and rearrange. The BN sheets released by adjacent particles can overlap each other, thereby constructing a highly efficient thermally conductive network with a low overall filler content.

[0020] (2) In addition to promoting the uniform dispersion of h-BN in the epoxy matrix and avoiding agglomeration and stress concentration caused by direct contact of inorganic fillers, the TPU pre-coating layer, as a stress buffer medium, can effectively transfer and relax the thermal and mechanical stress at the interface, and inhibit the initiation and propagation of microcracks. (3) The epoxy potting compound made from BN@TPU-containing epoxy resin achieved a balanced improvement in thermal conductivity, mechanical strength, and electrical insulation properties after curing. Compared with traditional filler-filled epoxy resin systems, BN@TPU / EP exhibited superior tensile and flexural properties, indicating enhanced interfacial stress transfer capability and crack resistance. This superior mechanical property helps to effectively resist the effects of thermal stress and vibration during transformer operation. This invention also demonstrates that interfacial rheological modulation can be used as an effective strategy to simultaneously optimize the processing and performance of high-filler epoxy encapsulation systems. Attached Figure Description

[0021] Figure 1 The FT-IR spectrum of the TPU prepared in Example 1; Figure 2 SEM images of BN@TPU and h-BN prepared in Example 1; Figure 3 The tensile and flexural strengths of composite materials with different h-BN and BN@TPU ratios were determined. Figure 4 The thermal conductivity and thermal conductivity enhancement of composite materials with different BN@TPU ratios were studied. Figure 5DSC curing behavior diagrams of the prepared EP, BN / EP and BN@TPU / EP composites; Figure 6 Viscosity variation curves of composite materials with different BN@TPI ratios were prepared. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0023] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, the materials, reagents, methods, and instruments used are all conventional materials, reagents, methods, and instruments in the art, and can be obtained commercially by those skilled in the art.

[0024] The polyether polyol used in the following examples is PPG230, purchased from Wanhua Chemical Group Co., Ltd.; h-BN is BN-AT, purchased from Shandong Pengcheng Ceramic New Material Technology Co., Ltd.; bisphenol A epoxy resin is E51, purchased from Jiangsu Xingsheng Chemical Co., Ltd.; and the acid anhydride curing agent is methylhexahydrophthalic anhydride, purchased from Jinan Qingtian Chemical Technology Co., Ltd.

[0025] Example 1 The preparation of the epoxy potting compound in this embodiment includes the following steps: Step 1: Add 60g of polyether polyol and 100g of butyl acetate to a three-necked flask equipped with a mechanical stirrer, and stir until homogeneous. Under continuous stirring, slowly add 18g of MDI dropwise to obtain the reaction system. Place the reaction system in a 60℃ constant temperature oil bath and react for 0.5h, then increase the temperature by 5℃·min. -1 The temperature was increased to 80℃ at a controlled heating rate, and the reaction was continued at this temperature with stirring for 2 hours to complete the prepolymerization reaction and obtain the prepolymer system. 0.25 g of 1,4-butanediol was dissolved in 5.5 g of butyl acetate to prepare a chain extender solution. The chain extender solution was slowly added dropwise to the prepolymer system at a rate of 20 rpm using a peristaltic pump. After the addition was complete, the reaction was continued at 80℃ for 20 minutes to allow the chain extender to fully react with the prepolymer, resulting in a homogeneous and transparent TPU prepolymer solution.

[0026] Step 2: Using the TPU prepolymer solution obtained in Step 1 as the matrix, h-BN filler is gradually added at a solid content of 35wt%, and the mixture is continuously stirred during the addition process to avoid local agglomeration; then, it is dispersed at 10,000 rpm for 60 min using a high-speed shear emulsifier, and then subjected to ultrasonic treatment for 30 min to obtain a stable and uniform milky white suspension.

[0027] The obtained slurry is transferred to the feeding system of a centrifugal spray dryer, where it is atomized into tiny droplets and rapidly dried under the action of hot air. The solvent evaporates quickly, accompanied by resin curing, forming spherical or near-spherical BN@TPU composite powder primary particles. These particles are then graded and sieved using a 100-500 mesh standard sieve, selecting particles with a diameter range of 200-400 μm to remove large particle agglomerates and fine powder impurities, resulting in BN@TPU composite powder with uniform particle size distribution and good dispersibility.

[0028] Step 3: The prepared BN@TPU composite powder is mixed with bisphenol A epoxy resin and methylhexahydrophthalic anhydride curing agent at a ratio of 5 wt% of the total mass of BN@TPU composite powder to 5 wt% of the total mass of bisphenol A epoxy resin and methylhexahydrophthalic anhydride curing agent (mass ratio of bisphenol A epoxy resin to methylhexahydrophthalic anhydride curing agent is 1:1). The mixture is placed in an oil bath and heated at 60°C with mechanical stirring for 30 min to obtain a uniform suspension. Subsequently, it is degassed in a vacuum oven to remove entrained air bubbles. The degassed suspension is injected into a mold pre-coated with a release agent and cured in three steps: 90°C / 2 h, 130°C / 2 h, and 160°C / 3 h to ensure complete cross-linking. After curing, the sample is allowed to cool naturally to room temperature and then demolded to obtain the BN@TPU / EP encapsulated composite material.

[0029] Example 2 The difference between this embodiment and Embodiment 1 is that in step three, the proportion of BN@TPU composite powder is 10wt% of the sum of the mass of bisphenol A epoxy resin and acid anhydride curing agent. The remaining process steps and parameter settings are the same as in Embodiment 1.

[0030] Example 3 The difference between this embodiment and Embodiment 1 is that in step three, the proportion of BN@TPU composite powder is 15wt% of the sum of the mass of bisphenol A epoxy resin and acid anhydride curing agent. The remaining process steps and parameter settings are the same as in Embodiment 1.

[0031] Example 4 The difference between this embodiment and Embodiment 1 is that in step three, the proportion of BN@TPU composite powder is 20wt% of the sum of the mass of bisphenol A epoxy resin and acid anhydride curing agent. The remaining process steps and parameter settings are the same as in Embodiment 1.

[0032] Example 5 The difference between this embodiment and Embodiment 1 is that in step three, the proportion of BN@TPU composite powder is 25wt% of the sum of the mass of bisphenol A epoxy resin and acid anhydride curing agent. The remaining process steps and parameter settings are the same as in Embodiment 1.

[0033] Example 6 The difference between this embodiment and Embodiment 1 is that in step three, the proportion of BN@TPU composite powder is 30wt% of the sum of the mass of bisphenol A epoxy resin and acid anhydride curing agent. The remaining process steps and parameter settings are the same as in Embodiment 1.

[0034] Example 7 The difference between this embodiment and Embodiment 1 is that in step three, the proportion of BN@TPU composite powder is 35wt% of the sum of the mass of bisphenol A epoxy resin and acid anhydride curing agent. The remaining process steps and parameter settings are the same as in Embodiment 1.

[0035] Comparative Example 1 The difference between this comparative example and Example 1 is that in step three, the proportion of BN@TPU composite powder is 0 wt% of the sum of the mass of bisphenol A epoxy resin and acid anhydride curing agent. The remaining process steps and parameter settings are the same as in Example 1.

[0036] Example 8 The difference between this embodiment and Embodiment 1 is that in step one, TDI is used instead of MDI, while the remaining process steps and parameter settings are the same as in Embodiment 1.

[0037] Example 9 The difference between this embodiment and Embodiment 1 is that in step one, IPDI is used instead of MDI, while the remaining process steps and parameter settings are the same as in Embodiment 1.

[0038] Comparative Example 2 The difference between this comparative example and Example 1 is that in step three, h-BN is used instead of BN@TPU composite powder. The remaining process steps and parameter settings are the same as in Example 1, resulting in BN / EP encapsulation composite material.

[0039] Comparative Example 3 The difference between this comparative example and Example 2 is that in step three, h-BN is used instead of BN@TPU composite powder, while the remaining process steps and parameter settings are the same as in Example 2.

[0040] Comparative Example 4 The difference between this comparative example and Example 3 is that in step three, h-BN is used instead of BN@TPU composite powder, while the remaining process steps and parameter settings are the same as in Example 3.

[0041] Comparative Example 5 The difference between this comparative example and Example 4 is that in step three, h-BN is used instead of BN@TPU composite powder, while the remaining process steps and parameter settings are the same as in Example 4.

[0042] Comparative Example 6 The difference between this comparative example and Example 5 is that in step three, h-BN is used instead of BN@TPU composite powder, while the remaining process steps and parameter settings are the same as in Example 5.

[0043] Comparative Example 7 The difference between this comparative example and Example 6 is that h-BN is used instead of BN@TPU composite powder in step three, while the remaining process steps and parameter settings are the same as in Example 6.

[0044] Comparative Example 8 The difference between this comparative example and Example 7 is that in step three, h-BN is used instead of BN@TPU composite powder, while the remaining process steps and parameter settings are the same as in Example 7.

[0045] Example of effect The morphology and properties of the BN@TPU composite powders and encapsulated composite materials prepared in Examples 1-9 and Comparative Examples 1-8 were characterized, and the test results are as follows: (1) Figure 1 The image shows the FT-IR spectrum of the TPU transparent adhesive prepared in step 1 of Example 1. As can be seen from the image, for MDI, the characteristic absorption peak corresponding to -NCO is clearly observed at 2249 cm⁻¹. -1 The peak completely disappears in the TPU spectrum, indicating that the isocyanate groups were completely consumed during polymerization. In the TPU spectrum, the peak is located at 3320–3400 cm⁻¹. -1 The broad absorption band at 1739 cm⁻¹ is attributed to the NH stretching vibration of the polyurethane groups; simultaneously, the absorption band at 1739 cm⁻¹ is also significant. -1 The strong peak at 1530 cm⁻¹ is attributed to the C=O stretching vibration of the polyurethane bond, confirming the formation of the urethane structure. Furthermore, the peak at 1530 cm⁻¹... -1 (NH bending vibration) and 1226cm -1 The characteristic peak at the (CN stretching vibration) further confirms the successful formation of the polyurethane chain. Compared with the spectra of PPG and BDO, the -OH stretching vibration observed in BDO significantly weakened or disappeared after the reaction, indicating that the hydroxyl group participated in the polymerization process; the COC stretching vibration of the polyether segment was retained at approximately 1108 cm⁻¹. -1 The presence of these features indicates that the soft segment structure remains intact. These results collectively confirm the successful synthesis of TPU via the reaction of MDI, PPG, and BDO, resulting in a typical segmented polyurethane structure.

[0046] (2) The microstructures of BN@TPU / EP, BN@TPI, and the original h-BN prepared in Example 1 were characterized, and the results are as follows: Figure 2As shown in Figure (a), the original h-BN material exhibits a typical two-dimensional sheet-like morphology with a smooth surface and a significant tendency to aggregate due to strong interparticle interactions. After spray-assisted granulation, as shown in Figure (b), the h-BN sheets successfully assembled into spherical or near-spherical BN@TPU particles with a particle size range of 200~400μm. In this structure, TPU acts as the interfacial bonding phase, connecting adjacent BN sheets and stabilizing the particle structure. Furthermore, as shown in Figure (c), spherical BN@TPU structural regions can still be clearly observed on the fracture surface of BN@TPU / EP, indicating that the particle structure remained largely intact during mixing and curing.

[0047] (3) Figure 3 The graph compares the tensile and flexural strengths of BN@TPU / EP composites with different h-BN and BN@TPU addition amounts. As can be seen from the graph, at the same addition amount, the tensile and flexural strengths of the BN@TPU / EP composite are higher than those of the BN / EP composite. When the filler content is 30 wt%, the tensile strength of the BN@TPU system increases from 25.65 MPa to 33.46 MPa, and the flexural strength increases from 51.43 MPa to 80.88 MPa. This improvement is attributed to the enhanced interfacial compatibility effect introduced by TPU, which effectively alleviates stress concentration and promotes stress transfer between the BN filler and the epoxy matrix. Under high filler content conditions, traditional h-BN filled systems often exhibit severe particle agglomeration and interfacial defects, leading to a decline in mechanical properties; while the BN@TPU structure can maintain a high degree of integration between the filler and the matrix interface and inhibit crack initiation at the filler boundary.

[0048] (4) Figure 4 The figure shows the thermal conductivity of composites with different BN@TPU content. As can be seen from the figure, the thermal conductivity of the composites significantly increases with increasing BN@TPU filler content, exhibiting typical filler-reinforced thermal conductivity characteristics. When the filler content is 30-35 wt%, the thermal conductivity reaches approximately 0.74 W·m. -1 ·K -1 The value is significantly higher than that of pure epoxy resin (0.15 W·m). -1 ·K -1At low filler contents (≤15wt%), performance improvement is relatively limited because the BN filler dispersed in the epoxy resin matrix fails to form an effective heat transfer path; at this point, interfacial thermal resistance becomes the main limiting factor in the heat transfer process. When the filler content exceeds 20wt%, the thermal conductivity increases more rapidly, indicating that a continuous thermally conductive network is gradually forming. The inherent high thermal conductivity of the BN sheets and their enhanced interconnectivity promote efficient phonon transmission and reduce interfacial scattering, thereby improving overall thermal conductivity. The TPU interfacial layer enhances the interaction between BN and the epoxy resin matrix, reduces interfacial thermal resistance, and suppresses filler agglomeration at high filler contents, forming a more uniform and stable thermally conductive network.

[0049] (5) Figure 5 The figures show the DSC curing behavior of the composite materials prepared in Example 6, Comparative Example 2, and Comparative Example 1. As can be seen from the figures, pure EP exhibits a typical exothermic curing peak with a central temperature of approximately 148.09 °C. After the introduction of BN, this exothermic peak shifts significantly to a higher temperature (approximately 151.34 °C), indicating a slower curing reaction rate. This phenomenon can be attributed to the formation of a dense BN particle network, which increases the system viscosity and restricts molecular motion, thus hindering the diffusion of the active material. In contrast, the BN@TPU / EP system exhibits a curing peak at approximately 152.24 °C, accompanied by a broader peak shape. Although its peak temperature is slightly higher than that of the BN / EP system, the overall curing process is more gradual. This behavior indicates that the introduction of TPU creates an interfacial buffering effect, which can regulate the local reaction environment and alter the curing kinetics. The broad exothermic peak implies a wider curing window, which has significant advantages for practical processing and encapsulation applications.

[0050] (6) Figure 6The viscosity of composites with different BN@TPU addition amounts varies with temperature. The figures show that the addition of BN@TPU particles effectively modulates the rheological behavior of epoxy resin systems with high filler content. Composites with 5 wt% and 10 wt% filler exhibited extremely low viscosity throughout the testing period, indicating insufficient structural reinforcement and weak interaction between particles and the resin matrix. Low viscosity is generally unfavorable for practical encapsulation and coating applications, potentially leading to filler sedimentation, poor dimensional stability, resin leakage, and unsatisfactory coating thickness control during processing. Therefore, while low filler content provides excellent flowability, it cannot meet the rheological requirements of engineering-grade packaging applications. With increasing filler content, viscosity gradually increases due to enhanced interparticle interactions and restricted molecular motion. However, compared to traditional BN-filled systems (Table 1 below), BN@TPU composites still maintain significantly lower initial viscosity and a milder viscosity increase rate during processing. This characteristic is attributed to the unique hierarchical structure of BN@TPU particles. When the initial viscosity of the BN@TPU / EP system with an addition of 30wt% is 3511mPa·s, and the viscosity remains relatively stable during processing, efficient filling and molding can be achieved while avoiding premature viscosity increase. This extended processing window is particularly advantageous for practical packaging applications.

[0051] Table 1 In summary, by adding a small amount of thermoplastic resin as a binder, BN sheets can be assembled into spherical graded particles, effectively suppressing viscosity increase during processing while maintaining a continuous heat conduction path under high filler content.

[0052] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Anyone skilled in the art can make various modifications and alterations without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be determined by the claims.

Claims

1. A method for producing a boron nitride composite powder, characterized by comprising: include: ​ Step 1: Weigh the following raw materials by mass: 60-62 parts polyether polyol, 15-17 parts isocyanate, 10-12 parts chain extender, 80-84 parts butyl acetate, 100-102 parts h-BN, and 0.3 parts catalyst. Step 2: After mixing polyether polyol, a portion of butyl acetate and isocyanate, heat the mixture in an oil bath and then add a mixture of chain extender and another portion of butyl acetate. After 20 minutes of adding the chain extender, add the catalyst and continue stirring to react, thus obtaining a transparent TPU solution. Step 3: h-BN is gradually added to the TPU solution obtained in Step 1, dispersed using a high-speed shear emulsifier, and then ultrasonically treated to obtain a uniform milky white suspension. Finally, the suspension is spray-dried and the resulting composite powder is sieved to obtain boron nitride composite powder, named BN@TPU.

2. The method for producing a boron nitride composite powder according to claim 1, wherein The chain extender is 1,4-butanediol, the catalyst is triethylamine, and the h-BN particle size is 10-1 μm.

3. The method for preparing boron nitride composite powder according to claim 2, characterized in that, The molar ratio of polyether polyol, isocyanate and 1,4-butanediol is 1:(0.95~1):(0.05~0.07).

4. The method for preparing boron nitride composite powder according to claim 1, characterized in that, The oil bath heating reaction process in step 2 is as follows: stir at 60℃ for 0.5h, then raise the temperature to 80℃ and keep stirring for 2h; a peristaltic pump with a rotation speed of 20rpm is used for dripping in step 2.

5. The method for preparing boron nitride composite powder according to claim 1, characterized in that, In step 3, the dispersion treatment speed is 10,000 rpm and the time is 60 min; the ultrasonic treatment time is 30 min.

6. The method for preparing boron nitride composite powder according to claim 1, characterized in that, The solid content of the suspension slurry obtained in step 3 is 30~40wt%; the particle size of the boron nitride composite powder obtained in step 3 is 200-400μm.

7. A boron nitride composite powder obtained by the preparation method according to any one of claims 1 to 6.

8. An application of the boron nitride composite powder according to claim 7, characterized in that, As a filler, it is used to prepare epoxy potting compound.

9. An epoxy potting compound, characterized in that, It includes component A of the epoxy resin containing the BN@TPU of claim 7, and component B of the anhydride curing agent.

10. The epoxy potting compound according to claim 9, characterized in that, The mass percentage of BN@TPU shall not exceed 35 wt% of epoxy resin.

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