Study on high-temperature hydrophobic modification and failure mechanism of silica aerogel thermal insulation composites

SiO2 aerogel has broad application prospects in the field of thermal insulation due to its high porosity and low thermal conductivity. However, its surface hydroxyl groups make it easy to absorb water in a humid environment, reducing the thermal insulation performance. To this end, researchers used in-situ methods and surface post-treatment methods to prepare hydrophobic SiO2 aerogels and their composites. However, the temperature resistance of the hydrophobic aerogels prepared at present needs to be improved, especially for the use requirements of aerospace and petrochemical fields in higher temperature environments.

In this paper, polysiloxane-modified silicate coatings were used to perform high-temperature hydrophobic modification on SiO2 aerogel thermal insulation composites, and a high-temperature resistant hydrophobic coating was prepared by a simple brushing process. The study found that the coating thickness has an effect on the expansion of surface cracks, and the number of cracks increases with the increase of coating thickness. After hydrophobic modification, the contact angle of the composite material can reach (113±2)°. When the heat treatment temperature increases, the contact angle decreases, but it is still as high as 105° when it is maintained for at least 1800s at 450℃ for a short time. The attenuation of hydrophobic performance is caused by the oxidative decomposition of hydrophobic groups in the coating at high temperatures. In addition, the hydrophobic coating simultaneously improves the wear resistance of the composite material.

Introduction

SiO2 aerogel is a highly dispersed solid material formed by the interaction of nano-scale particles. Its extremely high porosity and specific surface area give it extremely low thermal conductivity and are widely used in the field of thermal insulation. However, a large number of hydroxyl groups on the surface of SiO2 aerogel are prone to absorb moisture in a humid environment, resulting in increased thermal conductivity and decreased thermal insulation performance. To this end, researchers used in-situ methods and surface post-treatment methods to prepare hydrophobic SiO2 aerogel and its thermal insulation composite materials to improve their temperature resistance. In this paper, polysiloxane-modified silicate coatings were used to perform high-temperature hydrophobic modification on SiO2 aerogel thermal insulation composite materials, and the effect of coating thickness on the expansion of surface cracks, as well as the hydrophobic failure mechanism of the coating at high temperatures and the wear resistance of the composite materials before and after brushing were studied.

1 Experiment

1. 1 Materials

SiO2 aerogel insulation composite material (self-made), polysiloxane modified silicate coating (Changsha Yiheng Instrument Technology Co., Ltd.).

1. 2 Sample preparation

The SiO2 aerogel insulation composite material was cut into small flat plates, and then the polysiloxane modified silicate coating was brushed onto the sample surface. After curing at room temperature for 30 minutes, it was placed in a muffle furnace and dried at 280℃ for 30 minutes to allow the coating to cure on the surface of the composite material to form a hydrophobic coating.

1. 3 Analysis and testing

The samples were tested using a contact angle tester, scanning electron microscope, synchronous thermal analyzer, Fourier transform infrared spectrometer, etc.

2 Results and discussion

2. 1 Analysis of coating composition

The infrared absorption spectrum of the coating showed that there were functional groups such as Si—O—Si bonds, —OH and —CH3 in the coating.

2. 2 Effect of coating thickness on surface morphology

With the increase of brushing times, the coating thickness increases, the thermal expansion coefficient of the coating and the substrate does not match, and thermal stress is generated, resulting in crack growth and expansion.

2. 3 Microstructure

After hydrophobic modification, the surface of the composite material becomes smooth and dense, and the coating consists of a double composite structure of large particles and small particles, similar to the micro-nano structure of the lotus leaf surface.

2. 4 Hydrophobic properties

After modification, the surface of the composite material has good hydrophobic properties, and the contact angle can reach (113±2)°.

2. 5 Thermal stability

When the heat treatment temperature increases, the contact angle decreases, but it is still as high as 105° when it is maintained at 450℃ for at least 1800s.

2. 6 Wear resistance

The hydrophobic coating improves the wear resistance of the composite material.

3 Conclusion

In this paper, polysiloxane-modified silicate coating is used to perform high-temperature hydrophobic modification on SiO2 aerogel thermal insulation composite materials, and a high-temperature resistant hydrophobic coating is prepared. The coating thickness has an effect on the surface crack expansion. After hydrophobic modification, the contact angle of the composite material can reach (113±2)°. When the heat treatment temperature increases, the contact angle decreases, but it can be maintained for at least 1800s at 450℃.