Ceramic and glass technology

Size: px

Start display at page:

Download "Ceramic and glass technology"

Jodie Phillips
6 years ago
Views:

1 29 Glass Properties Glass is an inorganic, nonmetallic material which cools to a rigid solid without crystallization. Glassy, or noncrystalline, materials do not solidify in the same sense as do those that are crystalline. Upon cooling, a glass becomes more and more viscous in a continuous manner with decreasing temperature; there is no definite temperature at which the liquid transforms to a solid as with crystalline materials. In fact, one of the distinctions between crystalline and noncrystalline materials lies in the dependence of specific volume (or volume per unit mass, the reciprocal of density) on temperature, as illustrated in Fig (22) Fig (22): Contrast of specific volume versus- temperature behavior of crystalline and noncrystalline materials. For crystalline materials, there is a discontinuous decrease in volume at the melting temperature Tm. However, for glassy materials, volume decreases continuously with temperature reduction; a slight decrease in slope of the curve occurs at what is called the glass transition temperature, or fictive temperature, Tg. Below this temperature, the material is considered to be a glass; above, it is first a super cooled liquid, and finally a liquid. Glass noncrystalline silicates containing other oxides, notably CaO, Na2O, K2O, and Al2O3, which influence the glass properties. A typical soda lime glass consists of approximately 70 wt% SiO2, the balance being mainly Na2O (soda) and CaO (lime). Plastic deformation does not occur by dislocation motion for noncrystalline ceramics because there is no regular atomic structure. Rather, these materials deform by viscous flow, the same manner in which liquids deform; the rate of deformation is proportional to the applied stress. In response to an applied shear stress, atoms or ions slide past one another by the breaking and reforming of interatomic bonds.

2 30 The characteristic property for viscous flow, viscosity, is a measure of a noncrystalline material s resistance to deformation. For viscous flow in a liquid that originates from shear stresses imposed by two flat and parallel plates, the viscosity is the ratio of the applied shear stress and the change in velocity dv with distance dy in a direction perpendicular to and away from the plates, or This scheme is represented in Fig (23). The units for viscosity are poise (P) and pascal-seconds (Pa.s); 1 P =1 dyne s/cm2, and 1 Pa.s = 1 N.s/m 2 So 1Pa.s =10P Fig (23) glasses have extremely large viscosities at ambient temperatures, which is accounted for by strong interatomic bonding. As the temperature is raised, the magnitude of the bonding is diminished, the sliding motion or flow of the atoms or ions is facilitated, and subsequently there is an attendant decrease in viscosity. Fluidity is the reciprocal of the viscosity. A melt with a large fluidity will flow readily, whereas a melt with a large viscosity has a large resistance to flow. While fluidity is often used in dealing with ordinary liquids, virtually all literature dealing with glass forming melts discusses flow behavior in terms of the viscosity. As with many transport properties, the viscosity fits an Arrhenius-type equation over some ranges of temperature: where Q and 0 are temperature-independent coefficients called the activation energy and the pre-exponential factor, respectively. So the viscosity temperature characteristics is very important in glassforming operations

3 31 Fig (24) plots the logarithm of viscosity versus the temperature for fused silica, high silica, borosilicate, and soda lime glasses. On the viscosity scale, several specific points that are important in the fabrication and processing of glasses are labeled: 1. The melting point corresponds to the temperature at which the viscosity is 10 Pa.s (100 P); the glass is fluid enough to be considered a liquid. 2. The working point represents the temperature at which the viscosity is 10 3 Pa.s (104 P); the glass is easily deformed at this viscosity. 3. The softening point, the temperature at which the viscosity is 4* 10 6 Pa.s (4 * 10 7 P), is the maximum temperature at which a glass piece may be handled without causing significant dimensional alterations. 4. The annealing point is the temperature at which the viscosity is Pa.s (10 13 P); at this temperature, atomic diffusion is sufficiently rapid that any residual stresses may be removed within about 15 min. 5. The strain point corresponds to the temperature at which the viscosity becomes 3* Pa.s (3* P); for temperatures below the strain point, fracture will occur before the onset of plastic deformation. The glass transition temperature will be above the strain point. Most glass-forming operations are carried out within the working range between the working and softening temperatures. Of course, the temperature at which each of these points occurs depends on glass composition. For example, the softening points for soda lime and 96% silica glasses from Figure 24 are about 700 and 1550 C (1300 and 2825F), respectively. That is, forming operations may be carried out at significantly lower temperatures for the soda lime glass. The formability of a glass is tailored to a large degree by its composition.

4 32 Fig (24) Glass Forming Five different forming methods are used to fabricate glass products: pressing, blowing, drawing, and sheet and fiber forming. Container Process Containers are formed on a large scale using different processes listed in Table below, among which we shall focus on pressing, press-and-blow and blow-and blow processes. Processes and associated products. Process Articles Pressing Culinary and table vessels, pots, lenses, bricks Centrifugal casting (or spinning) Plates, cathodic tubes Blow-blow Bottles Press-blow Goblets, light bottles Press-blow-spin Goblets, laboratory All processes start up with batching and melting, and after conditioning to the desired temperature, the hot gob (molten glass) is transferred from the furnace to the forming operations through gob feeders (Fig 25). The gob feeder controls the weight, temperature and shape of the gob, all of which are critical to container quality.

5 33 Gob feeder rates can approach 300 gobs per minute. Several forming machines are distributed by one gob feeder. The gob feeder comprises a plunger that drives the glass melt to the orifice and shears that cut the gob at the desired volume The gob formed, next enters the forming operations. While forming, heat is extracted from the glass in a controlled manner. The moulds have to be made of good thermal conductors with a smooth surface aspect and lubricated. Fig (25) Pressing Pressing is used for widened containers (plates, cups and glasses) and also for producing thicker cross-sections for automotive lenses. The gob of molten glass is loaded in a mould, the plunger is lowered and forces the glass to spread and fill the mould (Fig. 26). Pressing can be performed with free sides (without a ring) when dimension tolerance is acceptable. The size of the plate or glass is then determined by the parison temperature and the pressing force. Patterns on the mould surface allow for imprinting the glass object and make it look more attractive and more expensive For more dimensional control, the mould is closed by a ring allowing for production of automotive headlight lenses with high-precision shape

6 34 Fig (26) Press-and-Blow, Blow-and-Blow Processes These processes offer the possibility of producing narrow necks for bottles Three moulds are used: (i) the parison mould, (ii) the finishing mould and (iii) the necking (Fig.27). The latter is used throughout the process and allows for forming the neck and transferring the parison from the parison mould to the finishing one. Moulds are made of two parts which open to release the parison and the glass object. As a matter of fact a slight line is marked (so-called mould mark) on the containers. There are two important processes to fabricate bottles named after the way used to produce the parison and to finish the container, namely pressing and then blowing or both steps blowing Both processes start with gob feeding (Fig 27a). In the press-and-blow process a plunger (mandrel) is used to press the parison (Fig. 27b) against the parison mould. The plunger progressively enters the gob while it is pressed at the other extremity. Next the parison is transferred into the blowing section (Fig. 27c and d). The final shape is obtained after blowing the parison into the forming mould (Fig. 27e and f). The container is then transferred on a chain belt (which may mark the container bottom), annealed and treated (Fig.27g;). The blow-and-blow process fig (28) mostly differs by the way the parison is formed. Instead of being pressed, the gob is blown in the parison mould. Next, as in the press-and-blow process, the parison is then transferred and blown into a finishing mould to achieve the final shape. Temperature control is essential to control the glass flowand distribution. It should be noted that the glass deformation under its own load (sagging) is to be considered to achieve optimized distribution of the glass. This has become a technological

7 35 challenge when fabricating light bottles. Also, the control of the adhesion between the metal tools and the hot glass requires careful control of the working temperature. The press-and-blow process allows for a better control of the shape of the parison and hence of the glass distribution as compared to the blow-andblow process. Light bottles are produced with the press-and-blow process while heavier bottles are produced with the blow-and-blow process. Notably, in the pressand blow process internal defects are introduced by the plunger when it comes in contact with the glass parison These deleteriously affect the mechanical performance of the bottles. Also, glass manufacturers lubricate the external surface to limit contact damage in the production lines and in use.

8 36 Fig 27 Fig 28

9 37 Float Process In this process firstly, the raw materials (sand, soda, lime) are continuously introduced into the furnace, melted at 1500 C, homogenized by convection and fined to eliminate bubbles. The furnace contains typically 2000 tons of glass and produces every day 500 tons of glass. The viscous liquid travels onto the float (fig 29) at a temperature of 1100 C under a nitrogen atmosphere in order to prevent corrosion of the tin bath, where it floats because of the density difference. The floated glass melt spreads to form a ribbon with a defined thickness. Under these conditions the equilibrium thickness of the glass sheet is about 6 mm so that the sheet has to be expanded or contracted by top rolls (or rollers working from the top of glass) to produce thinner or thicker glass sheets respectively. The equilibrium equation is written : where g : is the gravitational acceleration, t H the sheet thickness, and the density of glass and bath, the surface energy and subscripts g, b and a indicate glass, bath and atmosphere respectively. The range of commercial thickness is between 2 and 19mm. Glass with lower thickness (<2mm) is difficult to produce and fusion draw is preferred The glass sheet is extracted from the float at 600 C and transferred to the lehr. At such temperature the glass ribbon is viscous enough to be drawn upward out of the tin bath. This process has been a revolution for the glass industry because it offers very good optical quality without requiring any further operation, Therefore, most flat glass is produced through this process. generating less glass waste (these wastes are recycled however). The float technology is now used for the production of flat LCD- and plasma- TV screens and flat glass used for encapsulation of electronic devices. The float methods allow for the continuous production of very smooth flat glass sheets of a width of more than 3m

10 38 Fig 29 Flat sheet and plate Rolling process Plate glass was originally formed by casting molten glass on a metal table and rolling it into a sheet, or by continuous rolling between water cooled rollers. Since the surface of the sheet reproduced the flaws of the rollers and table surfaces, the glass was ground and polished to a high degree of parallelism and to an optical finish. The resulting glass plates were optically superior to those produced by sheet methods. Patterned glasses are produced by rolling the cooling melt between a roller engraved with the negative of the desired pattern and a smooth roller. Glasses containing wire mesh are formed by a similar process, where the mesh in introduced into the molten glass just before it passes through the rollers.

11 39 Fusion Draw With this process (Fig. 30), the glass sheet is formed from a continuous glass flow delivered by a refractory piece called an isopipe. Glass flows out laterally along the two sides of the pipe. These two flows later join downwards to form the glass sheet. The control of the thickness is carried out with rollers and pull down rate. In fact, the two glass surfaces that were in contact with the refractory are fused together when they join. This process allows production of sheets with an excellent control of thickness down to 0.5mm. This is an important point to reduce flat display panel weight (display applications). Moreover, no polishing of the glass is necessary since the process delivers excellent glass surface quality. Fig (30) Drawing of Tubes and Rods Glass tubing is manufactured on the principles patented by danner ; where glass flows continuously onto a rotating mandrel and the tube is drown mechanically while compressed air is blown into the mandrel. The drawing speed varies between 10 and 200 mm/min according to the tube diameter. the process suitable for thin walled tubes of smaller diameter. Thick walled tubes of larger diameter is manufactured by vertical drawing based on principle similar to fusion drawing, where the tube formed by blowing air through a nozzle below the molten glass surface. For making thin walled tubes of both smaller and larger diameter the vello system based on drawing in downward direction is used the formed tube is then deflected and pulled in horizontal direction.

12 40

Haseeb Ullah Khan Jatoi Department of Chemical Engineering UET Lahore

Haseeb Ullah Khan Jatoi Department of Chemical Engineering UET Lahore Greek word Keramikos which means Burnt Stuff indicating that desired properties of these materials are normally achieved through a