Study on the effects of incremental curing on the thermal conductivity, insulation, and mechanical properties of thermal conductive silicone rubber

With the development of fields such as electronics and telecommunications, electronic devices are becoming more integrated and powerful. Therefore, there is an increasing demand for high thermal conductive and insulating flexible materials. Silicone rubber (SR), as an excellent flexible substrate, is often combined with various thermal conductive fillers to enhance its thermal conductivity (TC). Carbon materials are commonly used as thermally conductive fillers. To improve the insulation performance while maintaining the TC of the material, uncured SR filled with boron nitride (BN) is used as an insulating layer on the same substrate. The TC of the once-cured BN/SR composite and the incremental cured BN/SR composite as a coating are 0.492 W/(mK) and 0.484 W/(mK), respectively, with a BN content of 10 vol%. The TC of carbon fiber (CF)/SR composites before and after surface treatment with BN/SR are 1.760 W/(mK) and 1.682 W/(mK), respectively, with a CF content of 20 vol%. The volume resistivity of the former is less than 104 Ω cm, while the latter is greater than 1014 Ω cm.


Introduction
With rapid advances in fields such as electronic communications, electronic devices are becoming more powerful, leading to an increasing demand for high TC and electrical insulation flexible materials [1][2].Due to its excellent mechanical properties, silicone rubber has been widely used in various applications.One common method to enhance the TC while maintaining the insulation performance of SR is by incorporating insulating thermal fillers such as BN [3][4][5] and aluminum oxide (Al 2 O 3 ) [6] into the SR matrix.Li et al. [6] prepared thermal conductive SR composites using BN, Al 2 O 3 , and aluminum nitride (AlN) as fillers and liquid SR as the base material.The TC of the composites was significantly improved compared to the SR matrix, with values of 1.04, 0.47, and 0.56 W/(mK) at a loading of 120 Phr, corresponding to a 316%, 88%, and 124% increase, respectively.
In addition to insulating thermal fillers, carbon-based materials [7][8][9][10][11][12][13] are also commonly used as conductive thermal fillers.These materials often exhibit high TC and low cost due to the involvement of free electrons in the heat conduction process.However, the challenge lies in forming a continuous conductive network with such fillers.Hu et al. [7] utilized a combination of silicon carbide (SiC) and carbon fiber powders to disrupt the conductive network formed by carbon fiber powder while maintaining excellent TC in the network composed of both materials.The prepared composite exhibited a TC of 12.8 W/(mK) and a volume resistivity of 3.4×10 10 Ω cm.
In this study, a method was employed to achieve incremental curing of SR on the surface of cured SR.A thin layer of insulating thermally conductive SR was applied to the surface of the conductive thermally conductive SR composite.This approach significantly reduced the electrical conductivity of the material while maintaining high TC.Additionally, the impact of one-time curing and sequential curing on the mechanical properties and TC of thermally conductive SR was investigated.

Preparation of BN/SR.
We mix the A and B components of the SR along with the silicone oil in a ratio of 3.5:3.5:3and add 10 vol% of BN.After mechanical stirring for 30 minutes, the mixture is placed in a vacuum chamber to remove bubbles.Then SR is poured into the mold and placed in a 50℃ oven for complete curing.We cut the cured BN/SR into square blocks measuring 1 cm × 1 cm × 3 mm for testing.Additionally, for the uncured BN/Liquid silicone rubber (LSR), we pour a portion into a rectangular mold measuring 6 cm × 1 cm × 2.5 mm for curing.The remaining portion is poured into the same mold with a central partition then the first 3 cm × 1 cm × 2.5 mm part is cured.After curing, we remove the partition and continue to cure the remaining part.The test pieces used for compression testing are cylindrical with a size of ø22.5 mm × 12.5 mm.Both one-time cured and sequentially cured samples were prepared.

Preparation of CF/BN/SR.
We mix components A and B of the SR with silicone oil in a ratio of 3.5:3.5:3and add 10 vol% of BN.The mixture is stirred for 30 minutes, then it is placed in the vacuum chamber to remove bubbles without undergoing curing.We directly immerse the CF in BN/LSR, then wind them on a uniformly rotating shaft.After winding, the CF-wound shaft is placed into BN/LSR, the air is expelled, and it is cured thoroughly at 50℃.After curing, we remove the CF/BN/SR from the shaft and cut it into square pieces measuring 1 cm×1 cm×3 mm.

Surface coating of BN/SR and CF/BN/SR composites.
We prepare a 10 vol% BN/LSR mixture and pour it onto the surfaces of the BN/SR and CF/BN/SR composite blocks.Sandwiching the BN/SR and CF/BN/SR blocks with BN/LSR in between using two polypropylene sheets, then we apply a weight of approximately 10 g and apply pressure for about 30 seconds.After removing the weight, we place the CF/BN/SR back into a 50℃ oven for further curing.

The effect of incremental curing on mechanical properties of SR
The compression strength of the BN/SR composite is not significantly affected by the incremental curing.For the one-time cured and incremental cured BN/SR composite, the corresponding compression stress at 30% strain is 149 kPa and 146 kPa, respectively (Figure 1b).However, there is a slight decrease in the tensile fracture stress.In the tensile tests, fracture occurs at the interface of the cured samples.The fracture stresses are 396 kPa and 371 kPa, respectively.Compared to the one-time cured samples, the sequentially cured BN/SR composite exhibits a decrease in strength of approximately 6.3% (Figure 1a).Coating the BN/SR composite with BN/SR did not show a significant impact on the TC (Figure 2a), and there was no clear boundary between the coating and the composite (Figure 3a).However, when BN/SR was used as a coating for the CF/BN/SR composite, a slight decrease in TC was observed, with values of 1.760 W/(mK) and 1.682 W/(mK), respectively (Figure 2b).The thickness of the coating was approximately 360 μm (Figure 3b).The coating did not have an apparent effect on the TC of the composite.The volume resistivity of the CF/BN/SR composite before and after coating with BN/SR was measured using a high-impedance analyzer (MCP-HT450).Before coating treatment, the volume resistivity of the CF/BN/SR composite is less than 10 4 Ω/cm.However, the volume resistivity of the CF/BN/SR composite increased to greater than 10 14 Ω/cm with BN/SR coating.The coating effectively reduced the material's conductivity.

Conclusion
This study provides a method to reduce the electrical conductivity of composites by using the same SR substrate as a coating.By using the BN/SR coating, the TC of the BN/SR and CF/BN/SR composites is maximally maintained.The TC of the former is reduced from 0.492 W/(mK) to 0.484 W/(mK), and the latter is reduced from 1.760 W/(mK) to 1.682 W/(mK).At the same time, the volume resistivity of the CF/BN/SR composite is significantly increased (>10 14 Ω cm).By optimizing the coating preparation method, the thickness of the coating can also be further controlled to explore the most suitable thickness, thereby further reducing its impact on the TC of the composite.In addition, a method for preparing continuously long fiber-filled thermal conductive SR using a simple mechanical winding process is provided.By optimizing the preparation process, continuous, efficient, and controllable fabrication of long fiber-filled thermal conductive SR can be achieved.

Figure 1 .
Figure 1.Tensile fracture stress (a) and compression stress at 30% strain (b) of one-time cured and incremental cured BN/SR composites.3.2The effect of incremental curing on TC of BN/SR and CF/BN/SR composite.

Figure 2 .
Figure 2. TC of BN/SR (a) and CF/BN/SR (b) composite before and after BN/SR coating.

Figure 3 .
Figure 3. Optical microscopy image of the cross-section of (a) BN/SR; (b) CF/BN/SR composites with BN/SR coating.