What are the applications of high-silica fiberglass in aerospace?
1. What is High-Silica Fiber?
High-silica fiber is short for high-purity silica amorphous continuous fiber. Its silica content is 96-98%, with continuous temperature resistance up to 1000°C and short-term temperature resistance up to 1400°C. Finished products mainly include continuous yarn, ropes, sheaths, mesh fabrics, and woven products. It is primarily used for ultra-high temperature fireproofing and heat insulation up to 1000°C. The single fiber diameter is greater than 5 micrometers, and it contains no asbestos or ceramic wool, making it completely harmless to health.
2. Production Process High-silica glass fiber is produced by using suitable raw glass components and following the ordinary glass fiber production process to create yarns, fabrics, and other products. After acid leaching and hot sintering, high-silica products with high-temperature resistance close to quartz fiber are obtained. The raw glass components mainly include E-glass and binary systems of SiO2-B2O3-Na2O and SiO2-B2O3. In my country, three-component sodium borosilicate glass is mainly used.
In production, high-silica products are acid-leached, utilizing the phase separation of their structure to leach out B2O3 and Na2O components into a solution, resulting in a microporous silica skeleton enriched with SiO2 exceeding 96%. This skeleton is then sintered at 600-800℃ to close the micropores and create a denser skeleton structure, thus producing high-performance high-silica glass fiber products.
3. Performance Characteristics
(1) Softening point close to 1700℃, can be used long-term at 900℃, and can withstand instantaneous airflow of several thousand degrees.
(2) Maintains good performance stability in organic and inorganic acids (except hydrofluoric acid, phosphoric acid, and hydrochloric acid), even at high temperatures, and in weak alkalis.
(3) High stability against thermal shock and ultra-high radiation, excellent insulation performance under high temperature and high humidity conditions, and good adhesion to high-temperature adhesives.
(4) Resistant to moisture and sunlight radiation, and shock-resistant, suitable for various sealing materials, producing robust products that remain flexible under many high-temperature conditions. (5) Stable structure and performance, harmless to the human body, and can replace ceramic fibers and asbestos fibers (which undergo a phase transformation at high temperatures).
(6) High fire resistance temperature; does not oxidize or discolor when burning in gasoline; high strength; and does not cause secondary pollution.
(7) Good wear resistance and insulation properties; competitively priced in most cases.
(8) Its skeleton can be appropriately controlled to manufacture various membrane components for liquid filtration, gas separation, or as a catalyst or enzyme carrier.
Due to these numerous advantages and characteristics, high-silica fiberglass products have received high attention and extensive research, and have been widely used in many demanding working environments. Developing and researching high-performance high-silica fiberglass products holds great market potential. 4. Process Theory Research
Due to the lower cost and safer use of high-silica glass fiber compared to high-temperature resistant inorganic fibers such as quartz, several companies in Europe and America have developed a series of products based on its high performance. Notable companies include ACIT High-Temperature Composite Materials (USA), Dakota Southern (USA), Kelevers (Germany), and H.I. Thompson (USA). However, my country's research and development has largely followed a follow-up approach, generally lagging behind international levels. Only Tonghua New Materials, Shaanxi Huate, and Nanjing Glass Fiber Research and Design Institute have conducted sustained research and achieved large-scale production and application technology development.
High-Silica Glass Structure Analysis
The process principle of high-silica glass fiber utilizes the phase separation of two or more immiscible liquid phases during the melting or cooling process of glass, creating a slight inhomogeneity. The high-silica glass composition falls within the phase separation region. When the glass cools or is reheated, it separates into two phases: one phase is almost entirely SiO2, and the other is rich in B2O3 and Na2O, which are easily dissolved by acids. From a glass structure perspective, when the SiO2 content is high and the molecular ratio Na2O : B2O3 < 1, the glass simultaneously contains [SiO4] tetrahedra, [BO4] tetrahedra, and [BO3] trigonometric bodies. Some of the [BO4] tetrahedra and [SiO4] tetrahedra form a uniform, unified, and continuous network structure, while others form an independent layered network structure, thus exhibiting a certain degree of phase separation within the glass. SiO2 is relatively stable in acids, while B2O3-Na2O is readily soluble in acids. The production of high-silica glass fibers utilizes this principle. Yarn or fabric containing the basic glass components is acid-leached under certain conditions, causing the B2O3-Na2O phase to transfer into the solution, leaving behind a microporous silica skeleton. This is then subjected to high-temperature treatment at 600-800℃ to close the micropores and compact the skeleton structure, resulting in a highly stable fiber material. In this way, the three-component sodium borosilicate glass is transformed into high-silica glass fiber.
5. Analysis of High-Silica Glass Fiber Production Process
The production processes for high-silica glass fibers are largely the same both domestically and internationally, involving suitable raw material drawing, spinning, and acid asphalt treatment.
The raw materials for high-silica glass fiber production consist of two components. Some manufacturers, in order to reduce costs, also use E-glass for subsequent processing to produce high-silica glass fiber products. In my country, a ternary system is generally used to produce high-quality fiber products. However, during melting, the significant volatilization of B2O3 and Na2O can lead to compositional instability and severe erosion of refractory materials. Compared to traditional flame furnaces, using an all-electric melting furnace at around 1450℃ to melt high-silica spheres can greatly reduce the volatilization rate and minimize refractory corrosion.
High-Silica Glass Fiber Molding Process
High-silica glass has a low hardening rate, with a drawing temperature of around 1150℃ and a crystallization upper limit of only 983℃, so crystallization rarely occurs during the drawing process. However, the wetting angle between the molten glass and the platinum stencil is small, so the stencil design must consider the influence of various factors, and high requirements are placed on the control of forced cooling airflow and the use of special wetting agents during drawing.
High-Silica Glass Fiber Acid Leaching Process
Acid leaching precipitates the non-silica phase in the structure under the action of acid, so that the SiO2 content in the structure reaches more than 96%. The phase migration rate of ions is affected by factors such as acid concentration, leaching time, temperature, and the amount and rate of H3BO3 in the acid solution. The setting of each process and the reasonable control of each parameter are key to the production of high-silica glass fiber products, especially the H3BO3 in the solution. The content of these components has a significant impact on the tensile strength of the product. Hot sintering process: In high-silica glass fiber products that have undergone acid leaching, the easily soluble components such as B₂O₃ and Na₂O in the phase separation are filtered out, leaving a continuous porous skeleton rich in SiO₂. Hot sintering at 600-800℃ or higher is required to close the micropores and restore some strength. However, when the temperature is too high, the microcracks on the glass surface will expand under internal stress, thus reducing the strength of the product. Similar to acid leaching, this process also has a significant impact on the physical and mechanical properties of the product, as shown in Table 2. Therefore, during production, it is necessary to reasonably control the temperature regime and sintering time of hot sintering to ensure maximum strength recovery while ensuring that the linear shrinkage rate of the product under application conditions does not exceed 3%.