In this article we will discuss about:- 1. Introduction to Glasses 2. Glass Fabrication and Properties 3. Forming 4. Types.
Introduction to Glasses:
Glass is an amorphous solid with the structure of liquid or we can say it is a supercooled liquid that is cooled at a rate too high to allow crystal to form.
It does not have distinct melting and freezing point. All glass contain atleast 50% silicate defined as glass former and their strength can be modified by adding the oxide of aluminium, Ca, Ba, B, Mg and these oxide defined as intermediate or modifiers, e.g. – soda lime glass, borosilicate glass etc.
Normally more than 500 type of glasses which are available today.
Type of glass depends on input means if we modify the input we can make different type of glass.
Final properties of glass would be depend upon the constituent and processing techniques.
Most inorganic glasses can be transformed to crystalline state by a proper high temperature heat treatment called “Devitrification”.
The product is a fine grained polycrystalline material called glass ceramic devitrification process.
Glass ceramics have low coefficient of thermal expansion, high thermal shock resistance and relatively high mechanical strengths and thermal conductivities. Also, conventional glass forming technique may be used in the maximum production of glass ceramics.
Glass Fabrication and Properties:
i. Glass Fibres:
Continuous glass fibre produced by drawing molten glass through multiple orifices. Fibre of small diameter (2µm) can be produced.
ii. Short Glass Fibre or Glass Wool Process:
In this molten glass ejected from a rotating head by centrifugal spraying process. Rotating head contains more than 2000 holes. In this process fiber diameter can be achieved from 20.30 µm air is supplied from the top to direct the fiber down ward and reduce the temperature. As the fiber descend, the binder is mixed to achieve required wool criteria the amount of bidder decide mechanical properties.
For glassy materials specific volume decreases continuously with temperature reduction. At glass transition temperature, there is a slight decrease in the slope of cooling curve. Below this temperature the material is considered to be glass and above it first a supercooled liquid, and finally a liquid.
ii. Melting Point:
At viscosity of 103 Pa-S glass is fluid enough to be considered a liquid.
iv. Working Point:
Corresponding to viscosity of 103 Pa-S, the glass is easily deformed at this viscosity.
v. Softening Point:
At viscosity of 4 × 106 Pa-S, this is max. Temperature at which glass may be handled without causing significant dimensional alterations.
vi. Annealing Point:
At viscosity 1012 Pa-S. At this temperature the atomic diffusion is sufficient rapid that any residual stress may be removed within about 15 minutes.
vii. Strain Point:
At viscosity level (4 × 1013) Pa-S. For temperature below strain point, fracture will occur before the onset of plastic deformation. The glass transition temperature will be above strain point temperature.
Various glass forming methods are as follow:
i. Glass Manufacturing Stages:
Here a glass piece is formed by pressure application in a graphite coated cast mold having shape. The mold is ordinarily heated to ensure an even surface. This method is used for producing thick walled pieces such as plates and dishes.
Done for art objects, the process is completely automated for production of glass bottles and light bulbs.
First a temporary shape is given to the gob of glass by pressing it in a mold and then it is inserted in a blow mold and forced to conform to mold contour by blowing a blast of air.
It is used to form long glass pieces such as sheet, rod, tube and fibres which have constants cross section flatness and surface finish can be greatly improved by floating the sheet in a bath of molten tin at elevated temperature.
The pieces is then slowly cooled and heat treated by annealing.
d. Fibre Forming:
Here the molten glass contained in a platinum heating chamber fibres are formed by drawing molten glass through small orifices at the base of the chamber. Glass viscosity is controlled by chamber and orifice temperatures. By this glass wool wound be made.
Float process in which flat glass sheet will be made. Glass can be processed in the form of fibers, tubing, tempered glass and laminated glass.
For tubing we require glass as flat sheet.
ii. Glass-Forming Constituents:
a. Glass formers,
b. Intermediates and
a. Glass Formers:
Glass formers and network formers include oxide such as SiO2, B2O3, P2O5, V2O6 and GeO2. They form the basis of random three-dimensional network of glass.
Intermediates include AI2O3, ZrO2, PbO, BeO, TiO3 and ZnO. These oxides are added in high proportions for linking-up with the basic glass network to retain structural continuity.
Modifiers include MgO, LiO2, BaO, CaO, SrO, Na2O and K2O. These oxides are added to modify the properties of glasses.
Types of Glasses:
i. Devitrified Glass:
It is made by mixing of glass with fluxes which lower-down the fusion temperature of glass, and render the molten glass workable at reasonable temperatures.
But fluxes may reduce the resistance of glass against chemical attack.
Devitrified glass is undesirable since the crystalline areas are extremely weak and brittle. Stabilizers are therefore added to this type of glass to overcome these problem.
ii. Heat Treating Glasses:
When glass is cooled from an elevated temperature thermal stresses are introduced as a result of difference in cooling rates and thermal contraction between the surface and interior regions. These stresses weaken the material and in extreme cases leads to fracture which is called ‘thermal shock’. Annealing is done by heating glass to annealing point and then slowly cooling to room temperature.
b. Glass Tempering:
In this process compressive residual stresses are induced at the surface of the glass. Here, the glass is heated to a temperature above the glass transition temperature but below the softening temperature. It is then cooled to room temperature by a jet of air or in an oil both. The residual stresses than are induced due to difference in cooling rates for surface and interior regions.
As the cooling advanced, the surfaces cools more rapidly and becomes rigid, but slowly, it tries to contract but the rigid outer surface prevents it when the cooling is complete.
There are net tensile stresses in the interior and net compressive stresses on the surfaces.
Applications of tempered glass are making of large doors, automobile windshields, eyeglass lenses.
iii. Soda Lime Glasses:
Mainly contain oxides of sodium and calcium, and silica. Its formula is Na2OCaO.6SiO2. Small amount of alumina and magnesium oxides are added to it to improve the chemical resistance and durability of the glass.
Soda lime glasses are cheap, resistance to water and devitrification.
They are widely used as window glass, in electric bulbs, bottles and tableware where high temperature resistance and chemical stability are not essentially required.
iv. Lead Glasses:
It contain 15 to 40% lead oxide. They are also known as flint glasses.
They are used to make high quality tableware, optical devices, neon sign tubing etc.
Glasses having a high lead content, upto 80%, have relatively low melting points, high electrical resistivity and high refractive index.
They are used for extra dense optical glasses, windows and protective shields to protect against X-ray radiations.
v. Borosilicate Glasses:
It contain mainly silica and boron oxide. Small amounts of alumina and alkaline oxide are also added to it.
It has low coefficient of thermal expansion and high chemical resistance. They are used in scientific piping, gauge glasses, laboratory ware, electrical insulators and domestic items. This glass is known as Pyrex glass by trade name.
vi. High Silica Glasses:
High silica glasses contains upto 96% silica are made by removing alkalis from a borosilicate glass. They are much more expensive than other types of glasses.
They have very low thermal expansion and high resistance to thermal shock, High- silica glasses are mainly used where high temperature resistance is required.
vii. Photo-Chromatic and Zena Glasses:
Photochromatic and zena glasses are also in common application. Photo-chromatic glasses are used in making lens for goggles. Silver chloride is mixed in ordinary glass to make them. Zena glass is used to make chemical containers.
The world’s telescope situated on Mount Semivodrike in USSR, which utilizes 6 metre diameter mirror of 1 metre thickness and 70 tones weight is example of glass.
viii. Wired Glass:
Made by meshing steel wire into molten glass by rolling process. It is used for low cost fire resistant glass which automatically break at high temperature
ix. Patterned Glass:
It is made by passing the glass through rollers of 1050°C. Mode in single pass. Gap between rollers are adjusted to get the desired thickness of flat glass. Molten metal and then moves through the temperature controlled kiln (also known as lehr) and solidifies.