Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 142

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 fiber cooling rate is approximately 0.5 10⁶ K/s for a typical commercial fiber‐forming process. Stable drawing processes require adequate control of the fiber cooling rate, which is managed through a combination of process controls, including appropriate bushing design, cooling manifolds (commonly known as fin coolers), cooling air flow, and water spray. In addition, one optimizes the fiber drawing and cooling rates by tuning the oxidation state of iron (either from raw material impurities or intentionally added) in the glass, specifically the concentration of ferrous ion (Fe²⁺).

The oxidation–reduction of iron in the melt is affected by glass chemistry and oxygen partial pressure ([16, 17], Chapter 5.6)

(3)

and by the presence of other multivalent species either from additives or impurities,

(4)

In general, drawing processes for making finer or smaller diameter fibers (and typically smaller bushings also) require glasses with a relatively lower concentration of Fe²⁺ (hence, slower cooling rate) than coarse fibers made from larger bushings (hence, higher cooling rate). The control of the iron oxidation state (or ferrous ion concentration in glass) is directly related to the amount of fining agent in the batch and the total iron in the batch, both of which are adjustable variables to maintain process stability. The ferrous ion concentration in glass is routinely monitored in production with a colorimetric method through acid digestion of a glass powder sample or with a calibrated UV‐VIS spectroscopic method on an optically polished glass disk.

In commercial production, glass fibers are drawn at a temperature T_F where melt viscosity is near 100 Pa∙s (Figure 4). Stable melt viscosity at the tip plate is critical for an efficient fiber‐forming process. If melt viscosity at the tip plate is too low (i.e. too high temperature at the tip plate), spreading of the glass melt across multiple tips on the tip plate results in flooding and disruption. If melt viscosity is too high (i.e. too low temperature at the tip plate), fiber drawing tension increases and the stable forming cone at the tip exit begins to fail, causing fiber breakage [3]. In addition, the actual forming temperature must be greater than the glass liquidus temperature, T_liq, by at least 50 ^oC to reduce the risk of devitrification. Microcrystals formed in colder spots along a primary canal or forehearth (Figure 2a), no matter how small, can lead to significant fiber breakage in the drawing process and hence, adversely impact productivity.

In making commercial glass fiber products for reinforcements, various processes are used to provide the desired end form. Direct draw or single end winding, direct chopped fibers, and numerous downstream secondary processes may be employed. Depending on the product needs, the types of bushings vary in dimension, number of tips, and tip diameters. In addition to the glass and process elements necessary to produce commercial glass fibers, the surface of the fiber must be treated to provide optimum compatibility of the inorganic glass fiber with the organic resins used in the reinforcements industry. This leads naturally into a discussion on the role of sizing chemistry.