9-13 GLASS MANUFACTURE

9-13 GLASS MANUFACTURE

Glass is manufactured by melting suitable materials in required proportions and fabricating the molten glass into desired articles. The melting is carried out in a glass- tank furnace. This process is always used for the mass production of glasses that can tolerate direct contact of the reacting mixture with the flame. The raw materials, together with cullet (broken glass), are fed at one end of the furnace, while the molten glass is continuously withdrawn at the other end so that the level of glass in the furnace remains constant. The flow is controlled, so that sufficient time is allowed for the complete melting and refining of the mass. The furnace temperature required to secure melting at a desirable rate is about 1500°C (2732°F). This corresponds to the viscosity of molten glass of about 10 Pas (100 P). From the refining section the molten glass travels slowly to the working pit, from which it is drawn for fabrication.

The temperature at this section is only about 1000°C (1852°F), giving the glass a viscosity of about i0 Pa’s (10e P). For small amounts or for special glasses, the melting is done in pots that are placed in the furnace. A pot is a one-piece refractory container for molten glass. The pots may be open or closed: closed pots are used for glasses that cannot be exposed directly to the flame.

Forming and Shaping. Formation and shaping of glass articles are usually accomplished by various casting techniques. Flat glass is produced by rolling a continuous stream of glass from a tank furnace passing between water-cooled rolls. Rods and tubes are made by a drawing process, while various containers and specific articles can be made by pressing, blowing, and similar operations. During the shaping of glass, internal stresses are produced due to temperature gradients developed within the glass during cooling. The most recent process involves casting on molten tin, which results in a nearly perfect surface of a plate. Most glass articles are now formed by highly complicated machines although. in certain cases, the old method of hand blowing has survived. The molten glass must possess an adequate range of working plasticity so as to be easily formed into articles of various shapes. The working plasticity is determined by the viscosity of glass, which varies with the temperature, as shown in Fig. 9-7. In the working range the viscosity of glass is from 10 to 1066 Pa’s (1076 P), which is suitable for shaping and forming operations. During working operations, the temperature decreases and the glass viscosity increases, making it stiff enough to support its own weight without deformation. At room temperature, the viscosity of glass is about 1O’ Pa’s (1020 P).

Annealing. The cooling of glass from its working range to room temperature is relatively rapid in practice and results in thermal stresses in the glass, which adversely affects its strength and physical properties. This adverse effect of rapid cooling can be eliminated by a proper heat treatment that consists of heating glass for a sufficiently long period in the annealing temperature range and cooling it slowly to room temperature Experience has shown that to prevent stresses in glass, cooling should be very slow during a short interval in the neighborhood of the glass transition temperature; after that. it may proceed at a more rapid rate. A proper annealing treatment produces a glass free from internal stress or strain and results in its higher density and higher refractive index. At the annealing temperature, the viscosity of glass is sufficiently low to permit a slight viscous flow in the mass. which results in relaxation of stress according to the Maxwell relation, as given by Equation 7-55. It is estimated that the relaxation time in the annealing range is about 100 s. although it may vary for different types of glass. In practice annealing schedules are based on experience, and an optimum cooling rate depends on the required properties of glass and the size of the specimen. Optical glasses are annealed for a longer period and are cooled very slowly in the neighborhood of the glass transition temperature (1/2 to 1°C/h). since any internal stress in the glass wall cause double refraction, which cannot be tolerated in optical glasses. Ordinary glassware, however, can be cooled at a Annealing. The cooling of glass from its working range to room temperature is relatively rapid in practice and results in thermal stresses in the glass, which adversely affects its strength and physical properties. This adverse effect of rapid cooling can be eliminated by a proper heat treatment that consists of heating glass for a sufficiently long period in the annealing temperature range and cooling it slowly to room temperature Experience has shown that to prevent stresses in glass, cooling should be very slow during a short interval in the neighborhood of the glass transition temperature; after that. it may proceed at a more rapid rate. A proper annealing treatment produces a glass free from internal stress or strain and results in its higher density and higher refractive index. At the annealing temperature, the viscosity of glass is sufficiently low to permit a slight viscous flow in the mass. which results in relaxation of stress according to the Maxwell relation, as given by Equation 7-55. It is estimated that the relaxation time in the annealing range is about 100 s. although it may vary for different types of glass. In practice annealing schedules are based on experience, and an optimum cooling rate depends on the required properties of glass and the size of the specimen. Optical glasses are annealed for a longer period and are cooled very slowly in the neighborhood of the glass transition temperature (1/2 to 1°C/h). since any internal stress in the glass wall cause double refraction, which cannot be tolerated in optical glasses. Ordinary glassware, however, can be cooled at a

Strengthening. Since the strength of the glass is determined by its surface conditions, it can be greatly increased by eliminating the larger surface flaws or introducing residual compressive stresses in the surface to counteract any present internal or applied tensile stresses. This process, called prestressing. can be accom plished by tempering or by chemical strengthening. Tempering or thermal strengthening involves heating the glass uniformly to the annealing temperature range to induce a slight viscous flow and then chilling the two outside glass surfaces very rapidly by blasts of air below the glass transition temper ature. This causes the glass skin to become rigid, while its interior is still in a viscous state. On further cooling the interior contracts, causing the compressive stress in the outside surfaces, while the glass interior is in tension (Fig. 9-8). The introduced compressive stress will counteract any tensile stress that may develop on loading the specimen, thereby considerably increasing the strength of glass. Tempered glass exhibits a strength up to 140 MPa (20 ksi) and an impact resistance from three to five times greater than that of annealed glass, but it retains the same appearance, clarity, hardness, and coefficient of expansion as the original glass. Chill-tempered glass cannot

be cut, machined, or ground. since this would disturb the system of prestresses, resulting in disintegration of the glass into small, but fairly harmless, fragments. For this reason all machining operations must be done before the glass is tempered.

FIGURE 9-7 Viscosity—temperature curves for glasses. (Properties of Glasses

and Glass-Ceramics, Cornrng , 1973)

Chemical strengthening involves changes in the composition of the surface layer of the glass. which results in a material with a very low or sometimes zero coefficient of thermal expansion. The glass interior, however, maintains its high coefficient of thermal expansion. Thus, on cooling, the interior contracts much more than outside surfaces, causing compressive stresses in the glass surface (Fig. 9-8).

FIGURE 9-8 Distribution of residual stresses across the sections of glasses, tempered and chemically strengthened. (Engineering Glass, Modern Materials. Vol.6, edited by B. W. Gonser, Copyright Academic Press Inc., New York. 1968.)

Chemical strengthening can be accomplished by surface crystallization, ion exchange, or surface glazing processes. Surface crystallization involves nucleation of crystals of lithium—aluminum—silicate glass, using titanium oxide as a nucleating agent. The resultant 13-eucryptite crystals have a negative expansion coefficient. On cooling, these crystals expand, introducing compressive stresses in the surface. The ion exchange process consists of heating a soda—alumina—titania—silica glass in a bath of molten lithium sulfate at 600°C (1110°F). Small lithium ions diffuse into the glass, replacing the larger sodium ions and forming -eucryptite, as above. When the same glass is immersed in a molten potassium salt, sodium ions are replaced by larger potassium ions, causing the glass surface to be in compression.

Finally, in surface glazing, the glass surface is coated with a finely powdered glass or crystalline material of the composition mentioned above and baked in an oven to produce

a hard enamellike glazing. Chemically strengthened glass may attain a strength as high as 690 MPa (100 ksi).