Glass Properties Affecting Glass Fiber Forming
The glass properties that affect glass fiber forming are crucial. They directly determine whether the molten glass can be smoothly drawn into continuous, uniform, high-strength fibers, as well as the efficiency and stability of the production process. The following are the key glass properties and their impacts:
1. Viscosity
Core impact: This is the "most critical" property. Glass fiber forming (mainly the bushing drawing of continuous fibers) occurs in a specific viscosity range (usually considered to be 10^2 to 10^3 Pa·s (100 - 1000 poise).
Requirements:
Suitable viscosity at forming temperature: The molten glass must reach this target viscosity range at the bushing temperature and drawing speed. If the viscosity is too low, the glass liquid will drip like water and cannot form stable filaments and continuous fibers; if the viscosity is too high, it will be difficult to be drawn, the filaments will break easily, and energy consumption will increase.
Viscosity-temperature characteristics: The rate at which the viscosity of the glass changes with temperature (the slope of the viscosity-temperature curve) is very important.
Short material: The ideal glass fiber composition is usually "short material" glass. This means that near the forming temperature, its viscosity changes with temperature very "dramatically" (that is, the slope of the viscosity-temperature curve is steep). This:
Allows melting at relatively high temperatures (when the viscosity is low and it is easy to clarify and homogenize).
Only a "slight cooling" at the leak plate is needed to achieve the high viscosity (10^2-10^3 Pa·s) required for forming, which is conducive to precise temperature control (±1°C) and stable wire drawing.
During the forming process (from the leak to cooling and solidification), the viscosity can increase rapidly, so that the fiber can be quickly solidified and shaped, reducing wire breakage.
Influencing factors: Mainly determined by the chemical composition of the glass (such as the content ratio of SiO₂, Al₂O₃, CaO, MgO, B₂O₃, Na₂O, K₂O, etc.). For example, increasing SiO₂ and Al₂O₃ will increase the viscosity, and increasing alkali metal oxides (Na₂O, K₂O) will reduce the viscosity.
2. Surface Tension
Influence: Surface tension acts on the surface of molten glass and tends to minimize its surface area (forming a spherical shape).
Role in forming:
Fiber thinning: During the drawing process, surface tension resists the tensile force and affects the final diameter of the fiber. Lower surface tension helps to obtain thinner fibers under the same pulling force.
Fiber root stability: The cone-shaped fiber root formed at the outlet of the nozzle (transitioning from the nozzle diameter to the fiber diameter), its shape and stability are affected by the combined effect of viscosity and surface tension (usually the **Bond number** is used to measure the relative importance of the two). Excessive surface tension may cause unstable fiber roots and increase the risk of broken wires.
Wettability: Affects the wettability of the glass liquid to the nozzle material (usually platinum-rhodium alloy). Moderate wettability helps the glass liquid to flow out of the nozzle stably, but excessive wettability may cause the glass to climb onto the outer wall of the nozzle.
Requirement: Need to match the viscosity to maintain a stable fiber root morphology. For fiber forming, it is usually desirable that the surface tension is "relatively low".
Influencing factors: Chemical composition (alkali metal oxides usually reduce surface tension), temperature (temperature increases, surface tension decreases).
3. Crystallization Tendency / Liquidus Temperature Impact: refers to the tendency of molten glass to precipitate crystals during the cooling process.
Hazards in forming:
Clogged nozzles: The formation of crystals will clog the tiny nozzles on the platinum-rhodium alloy leak plate (usually 1-2mm in diameter), resulting in broken wires or uneven wire output.
Destroy fiber integrity: The crystals present in the fiber will become stress concentration points, greatly reducing the strength and flexibility of the fiber, and even directly causing the fiber to break during forming or subsequent processing.
Increase in wire breakage rate: It is one of the main reasons for production interruptions.
Requirement: It must have "extremely low crystallization tendency", or "sufficiently low liquidus temperature".
Key indicators:
Liquidus temperature: The highest temperature at which glass begins to precipitate crystals. In order to ensure that there is no crystallization during the forming process, the "forming temperature" (the temperature corresponding to the viscosity of 10^2-10^3 Pa·s) must be significantly higher than the liquidus temperature (usually the forming temperature is required to be > liquidus temperature + 50°C Even more). The larger this difference is, the wider the process window is and the more stable the production is.
Crystallization temperature range: The temperature range with the maximum crystallization rate should be far away from the forming temperature range.
Influencing factors: Chemical composition is the key. Some components (such as CaO, MgO) are prone to crystallization, and it is necessary to add components that inhibit crystallization (such as Al₂O₃, B₂O₃, Fe₂O₃, etc.) and accurately control the ratio. Poor homogenization will also increase the risk of crystallization.
4. Thermal Stability / Coefficient of Thermal Expansion (CTE)
Impact:
Stress under temperature gradient: The forming process involves rapid cooling from the molten state (about 1300°C) to room temperature. The temperature difference between the inside and outside of the glass will produce thermal stress.
Thermal shock fracture: If the thermal expansion coefficient of the glass is too high or the thermal conductivity is too low, the thermal stress generated may exceed the strength limit of the glass, causing the fiber to "spontaneously break" during the forming process or after cooling.
Requirement: It needs to have a "lower coefficient of thermal expansion" to reduce thermal stress during cooling, improve the fiber's ability to resist thermal shock, and reduce the risk of broken wires and spontaneous fiber powdering during forming.
Influencing factors: Chemical composition (SiO₂, B₂O₃ generally reduce CTE, alkali metal oxides increase CTE). Common E glass CTE is relatively low (~5x10^-6 /°C).
5. Mechanical Strength (Mechanical Strength - although it is mainly the performance of the final fiber, the forming process affects its potential)
Impact:
Stretch resistance: During high-speed drawing (up to 20-25 m/s), the molten glass fiber needs to withstand considerable tension to be stretched and thinned. The glass melt itself must have sufficient "cohesive strength" (related to viscosity) to resist the tensile force and avoid being pulled apart in the root area or before solidification.
Solidified fiber strength: The forming process (cooling rate, thermal history) affects the microstructure and number of defects (such as microcracks) of the final solidified fiber, thereby affecting its theoretical strength. Rapid cooling is usually beneficial to obtain higher strength.
Requirement: Sufficient resistance to tensile fracture at forming temperature (closely related to viscosity). The final fiber needs high strength and low defect density.
Influencing factors: Chemical composition (high SiO₂, Al₂O₃ content is usually beneficial), forming process parameters (cooling rate), avoidance of crystallization and impurities.