Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов
fluorspar (CaF2), dolomite [(Mg, Ca)(CO3)2], spodumene (LiAl(SiO3)2), and soda ash (Na2CO3). For C‐glass fiber, albite (NaAlSi3O8) is commonly used to reduce the need for soda ash. For simplicity, primary phases of the above natural raw materials are used; all of them can contain other mineral phases and/or impurities, for example, iron from ore and grinding process.
Batch reaction chemistry and kinetics in glass melting have been widely studied with various experimental techniques, including differential thermal analysis (DTA), differential scanning calorimetry (DSC), X‐ray diffraction, electrical resistance and temperature distributions from molten glass to the batch pile, for laboratory batch sizes from as little as 0.1 g up to 10 kg in mass [12–14]. Models have also been developed from a thermodynamic approach [15].
As illustrated in Figure 3, the batch‐to‐melt conversion process of representative E‐glass and ECR‐glass batches can be characterized in the following steps:
1 De‐hydroxylation of batch ingredients, i.e. removal of both moisture water (120–150 oC) and crystalline water in the minerals, (colemanite, 350–450 oC; kaolinite or pyrophyllite, 500–600 oC).Figure 3 Differential scanning calorimetry output of E‐glass batches with and without boron from room temperature to 1400 oC (10 K/min heating rate), illustrating both endothermic and exothermic reaction events as batches are converted into glass through initial steps of dehydration and decarbonation. The intermediate silicates formed are anorthite (CaAl2Si2O8) and/or wollastonite (CaSiO3).Source: PPG, 2014.
2 Decarbonization, i.e. removal of carbonates (limestone, 700–800 oC).
3 Formation of intermediate crystalline phases (~1000 oC).
4 Liquid phase formation (1000–1400 oC).
5 Final dissolution of residual phases (sand, 1250 oC) [12].
The majority of the batch reactions are endothermic, the energy consumption of these steps constituting a significant portion of the total energy spent in the glass melting process. Finally, the glass fining step, which varies as a function of furnace throughput, consumes even more energy. The fiberglass industry is continuously searching for new raw materials to lower the energy consumption in batch melting [12] and improved technology to increase the furnace throughput without sacrificing glass quality. This in turn reduces the unit energy consumption of glass melted and glass fiber produced. In practical terms, successful implementation of these approaches can provide savings of up to 20% compared to typical E‐glass melting energy demands of 7000–8000 kJ/kg of glass [12].
One generally carries out glass melting and fining by controlling the melt temperature of the furnace hot spot near or higher than TM at which the melt viscosity is 10 Pa∙s (Figure 4), which will ensure sufficient fluidity of the molten glass to promote both mixing of the batch components and removal of seeds (or bubbles). The latter is also aided by the use of fining agents such as sodium sulfate (Na2SO4), which is commonly used in making E‐glass fibers. However, because much higher melting temperatures are often required for specialty glass fibers, other fining agents such as manganese dioxide (MnO2) or cerium dioxide (CeO2) are also used with or without added sodium sulfate. Fining agents may also serve as oxidizing agents to control the iron oxidation states for both glass melting and fiber drawing process as will be discussed in more detail in a later section.
Figure 4 Viscosity curves of E‐glass, E‐CR‐glass, AR‐glass, D‐glass (derivate), S‐glass, and C‐glass, illustrating composition effects on reference temperatures of glass melting/fining and fiber drawing.
Source: PPG, 2014.
Fining action from the decomposition of Na2SO4 dissolved in the melt is created as the molten glass stream moves toward the higher temperature fining zone, or hot spot, of the furnace. The melt with dissolved sulfate becomes oversaturated at higher temperatures (>1350 oC) and sulfate decomposition and bubble formation begins to occur. The onset temperature of sulfate decomposition depends on many factors – water vapor pressure in the furnace combustion space, water dissolved in the glass, organic species in the batch materials, any addition of carbon to the batch, partial pressure of oxygen above the melt, and others [16]. Driven by buoyancy action, bubbles containing SO2 and O2 travel to the melt surface; along the way the sulfate bubbles expand, stripping smaller bubbles of CO2 (primarily from decomposition of carbonates) and air (primarily N2) trapped in the melt. The sulfate fining process can be described by the reaction
(1)
where O2− is the free oxygen, which depends on glass composition. With other fining agents, for example CeO2, oxygen fining gas is generated according to the reaction
(2)
Adequate use of fining agents is critical. The quantity needs to be optimized based on the furnace firing setup, types of fuel and air or oxygen used, glass composition, and types of fiber products to be made. Excessive use of fining agents can result in uncontrolled melt foaming. The foam layer reduces energy transfer from the combustion space to the glass melt beneath the foam blanket, resulting in energy waste because of excessive firing required to maintain the underglass temperature and in poor glass quality caused by inhomogeneous melting. Excessive foam can also lead to potential overheating of the furnace crown, shortening the furnace service life.
3.3 Fiber Forming
The bushing is the device that controls the fiber‐drawing process. It is the interface between the glass melting and fiber formation processes. Bushings are made of precious metal alloys of platinum (Pt) and rhodium (Rh), typically in alloys of 90Pt/10Rh or 80Pt/20Rh. The tip plate of the bushing contains an appropriate number of tips or nozzles, typically ranging in number from 100 up to 8000 and in diameter from 0.75 to 2.00 mm. Proper selection of these parameters then enables production of the desired number and diameter of glass fiber filaments in a strand of fiberglass (Figure 2b). The bushing is electrically heated to provide very precise control of the temperature and, thus, of the viscosity of the glass flowing through the nozzle. The combination of tip size and glass viscosity coupled with the controlled speed of the take up winder allows for very good control of finished‐product filament diameters and linear‐strand density.
The rate of melt flow (F) through the bushing nozzle or bushing tip follows Poiseuille's relationship, i.e. flow is proportional to r 4 h/lη where r is the fiber radius, h is the depth of molten glass above the bushing tip plate, l is the length of nozzle, and η is the melt viscosity at the tip plate [3]. In practice, the actual interior geometry of the nozzle may be varied, which affects the melt flow rate for a given winder speed; modified Poiseuille equations can be found in the literature [3].
The fiber attenuation process is usually completed in less than a second, during which the diameter of the melt stream through the tip changes by three orders of magnitude from millimeters at the tip to micrometers in the finished filament. At a fiber attenuation speed between 3 104 and 3 105 m/s, depending on bushing type and specified fiber product, the estimated