TYPES OF GLASS:
(1) Soda-lime glass
(2) Potash-lime glass
(3) Potash-lead glass
(4) Common glass
(1) Soda-lime glass:
This is also known as soda-glass or soft glass. It is mainly a mixture of sodium silicate and calcium silicate.
Properties of Soda-lime glass:
(i) It is available in clean and clear state.
(ii) It is cheap.
(iii) It is easily fusible at comparatively low temperature.
Uses of Soda-lime glass:
It is used in the manufacture of glass tubes and laboratory apparatus, plate glass, window glass, etc.
(2) Potash-lime glass:
Also known as bohemian-glass or hard glass.It is mainly a mixture of potassium silicate and calcium silicate.
Properties of Potash-lime glass:
(i) it fuses at high temperature.
(ii) it is not easily affected by water and other solvents.
(iii) it does not melt so easily.
Uses of Potash-lime glass:
It is used in manufacture of glass articles.
(3) Potash-lead glass:
Also known as flint glass.It is a mixture of potassium silicate and lead sillacate.
Properties of Potash-lead glass:
(i) Fuses very easily.
(ii) Easily attacked by aqueous solution.
(iii) Posses great refractive power.
(iv) Specific gravity is about 3 to 3.50.
(v) Turns black and opaque.
Uses of Potash-lead glass:
It is used in the manufacture of artificial gems, electric bulbs, lences, prisms etc.
(4) Common glass
Also known as bottle glass. Manufacture of sodium silicate, calcium silicate and iron silicate.
Properties of Common glass:
(i) Fuses with difficulty.
(ii) It is brown, grey or yellow in colour.
(iii) easily attacked by acids.
Uses of Common glass:
It is mainly used for medicine bottles.
MANUFACTURING OF GLASS:
1. Batch processing system (batch house):
Batch processing is one of the initial steps of the glass-making process. The batch house simply houses the raw materials in large silos (fed by truck or railcar) and holds anywhere from 1–5 days of material. Some batch systems include material processing such as raw material screening/sieve, drying, or pre-heating (i.e. cullet). Whether automated or manual, the batch house measures, assembles, mixes, and delivers the glass raw material recipe (batch) via an array of chutes, conveyors, and scales to the furnace. The batch enters the furnace at the ‘dog house’ or ‘batch charger’. Different glass types, colors, desired quality, raw material purity / availability, and furnace design will affect the batch recipe.
The hot end of a glassworks is where the molten glass is formed into glass products, beginning when the batch is fed into the furnace at a slow, controlled rate by the batch processing system (batch house). The furnaces are natural gas- or fuel oil-fired, and operate at temperatures up to 1,575°C. The temperature is limited only by the quality of the furnace’s superstructure material and by the glass composition. Types of furnaces used in container glass making include ‘end-port’ (end-fired), ‘side-port’, and ‘oxy-fuel’. Typically, furnace “size” is classified by metric tons per day (MTPD) production capability.
There are, currently, two primary methods of making a glass container: the blow and blow method, used for narrow-neck containers only, and the press and blow method used for jars and tapered narrow-neck containers.
In both methods, a stream of molten glass, at its plastic temperature (1050°C-1200°C), is cut with a shearing blade to form a solid cylinder of glass, called a gob. Both processes start with the gob falling, by gravity, and guided, through troughs and chutes, into the blank moulds, two halves of which are clamped shut and then sealed by the “baffle” from above.
In the blow and blow process the glass is first blown through a valve in the baffle, forcing it down into the three piece “ring mould” which is held in the “neckring arm” below the blanks, to form the “finish”, [The term “finish” describes the details (such as cap sealing surface, screw threads, retaining rib for a tamper-proof cap, etc.) at the open end of the container.]
Containers are made in two major stages. The first stage moulds all the details (“finish”) around the opening, but the body of the container is initially made much smaller than its final size. These partly manufactured containers are called parisons, and quite quickly, they are blow-molded into final shape.
Referring to the mechanism, the “rings” are sealed from below by a short plunger. After the “settleblow” finishes, the plunger retracts slightly, to allow the skin that’s formed to soften. “Counterblow” air then comes up through the plunger, to create the parison. The baffle rises and the blanks open. The parisonis inverted in an arc to the “mould side” by the “neckring arm”, which holds the parison by the “finish”.
As the neckring arm reaches the end of its arc, two mould halves close around the parison. The neckring arm opens slightly to release its grip on the “finish”, then reverts to the blank side. Final blow, applied through the “blowhead”, blows the glass out, expanding into the mould, to make the final container shape.
In the press and blow process, the parison is formed by a long metal plunger which rises up and presses the glass out, in order to fill the ring and blank moulds. The process then continues as before, with the parisonbeing transferred to the final-shape mould, and the glass being blown out into the mould.
The container is then picked up from the mould by the “take-out” mechanism, and held over the “deadplate”, where air cooling helps cool down the still-soft glass. Finally, the bottles are swept onto a conveyor by the “push out paddles” that have air pockets to keep the bottles standing after landing on the “deadplate”; they’re now ready for annealing.
The forming machines hold and move the parts that form the container. The machine consist of basic 19 mechanisms in operation to form a bottle and generally powered by compressed air (high pressure – 3.2 bar and low pressure – 2.8 bar), the mechanisms are electronically timed to coordinate all movements of the mechanisms. The most widely used forming machine arrangement is the individual sectionmachine (or IS machine). This machine has a bank of 5–20 identical sections, each of which contains one complete set of mechanisms to make containers. The sections are in a row, and the gobs feed into each section via a moving chute, called the gob distributor. Sections make either one, two, three or four containers simultaneously. (Referred to as single, double, triple and quad gob). In the case of multiple gobs, the shears cut the gobs simultaneously, and they fall into the blank moulds in parallel.
COMPOSITION OF GLASS
The following is a list of the more common types of silicate glasses, and their ingredients, properties, and applications:
1. Fused quartz, also called fused silica glass, vitreous silica glass, is silica (SiO2) in vitreous or glass form (i.e., its molecules are disordered and random, without crystalline structure). It has very low thermal expansion, is very hard, and resists high temperatures (1000–1500 °C). It is also the most resistant against weathering (caused in other glasses by alkali ions leaching out of the glass, while staining it). Fused quartz is used for high temperature applications such as furnace tubes, lighting tubes, melting crucibles, etc.
2. Soda-lime-silica glass, window glass: silica 72% + sodium oxide (Na2O) 14.2% + lime (CaO) 10.0% + magnesia (MgO) 2.5% + alumina (Al2O3) 0.6%. Is transparent, easily formed and most suitable for window glass (see flat glass). It has a high thermal expansion and poor resistance to heat (500–600 °C). It is used for windows, some low temperature incandescent light bulbs, and tableware. Container glass is a soda-lime glass that is a slight variation on flat glass, which uses more alumina and calcium, and less sodium and magnesium which are more water-soluble. This makes it less susceptible to water erosion.
3. Sodium borosilicate glass, Pyrex: silica 81% + boric oxide (B2O3) 12% + soda (Na2O) 4.5% + alumina (Al2O3) 2.0%. Stands heat expansion much better than window glass. Used for chemical glassware, cooking glass, car head lamps, etc. Borosilicate glasses (e.g. Pyrex) have as main constituents silica and boron oxide. They have fairly low coefficients of thermal expansion (7740 Pyrex CTE is 3.25×10–6/°C as compared to about 9×10−6/°C for a typical soda-lime glass), making them more dimensionally stable. The lower CTE also makes them less subject to stress caused by thermal expansion, thus less vulnerable to cracking from thermal shock. They are commonly used for reagent bottles, optical components and household cookware.
4. Lead-oxide glass, crystal glass: silica 59% + lead oxide (PbO) 25% + potassium oxide (K2O) 12% + soda (Na2O) 2.0% + zinc oxide (ZnO) 1.5% + alumina 0.4%. Because of its high density (resulting in a high electron density) it has a high refractive index, making the look of glassware more brilliant (called “crystal”, though of course it is a glass and not a crystal). It also has a high elasticity, making glassware ‘ring’. It is also more workable in the factory, but cannot stand heating very well.
5. Aluminosilicate glass: silica 57% + alumina 16% + lime 10% + magnesia 7.0% + barium oxide (BaO) 6.0% + boric oxide (B2O3) 4.0%. Extensively used for fiberglass, used for making glass-reinforced plastics (boats, fishing rods, etc.) and for halogen bulb glass.
6. Oxide glass: alumina 90% + germanium oxide (GeO2) 10%. Extremely clear glass, used for fiber-optic waveguides in communication networks. Light loses only 5% of its intensity through 1 km of glass fiber. However, most optical fiber is based on silica, as are all the glasses above.
PROPERTIES OF GLASS
The properties of glass are mainly governed by factors like composition of the constituents, state of surface, thermal treatment conditions, dimension of specimen etc. Following are the properties of glass which have made the glass popular and useful:
I. It absorbs, refracts or transmits light.
II. It can take up a high polish and may be used as substitute for every costly gems.
III. It has no definite crystalline structure.
IV. It has no sharp melting point.
V. It is affected by alkalis.
VI. It is an excellent electrical insulator at elevated temperatures due to the fact that glass can be considered as an ionic liquid. The ions are not easily moved at room temperature because of the high viscosity. But when the temperature rises, the ions are permitted to flow and thus they will sustain an electric current.
VII. It is available in beautiful colours.
VIII. It behaves more as a solid than most solids in the sense that it is elastic. but when the elastic limit is exceeded, it fractures instead of deforming.
IX. It is capable of being worked in many ways. it can be blown,drawn,or pressed. But it is strange to note that it is difficult to cast in large pieces.
X. It is extremely brittle.
XI. It is not usually affected by air and water.
XII. It is not attacked by ordinary chemical reagents.
XIII. It is possible to intentionally alter some of its properties such as fusability,hardness,refractive power etc. To suit different purposes.
XIV. It is possible to obtain glasses with diversified properties. The glass may be clear, colourless, diffused and stained.
XV. It is possible to weld pieces of glass by fusion.
XVI. It is transparent and translucent. Transparency is the most used characteristic of glass and it is due to the absence of the free electron. For the same reason it also works as a good insulator.
XVII. When it is heated , it becomes soft and soft with rise in temperature . it is ultimately transformed into a mobile liquid. The liquid when allowed to cool , passes to all deegres of viscosity. The property of glass has made its manufacturing process easy. It can also be formed into articles of desired shape. Thus the amorphousness of glass permits it to be blown, drawn from furnaces and continuously worked.
XVIII. Due to advancement made in the science of the glass production , it is possible to make glass lighter than cork or softer than cotton or stronger than steel. The presence of glass however is considerably affected by foreign inclusions , internal defects and cords or chemically heterogeneous areas.
XIX. The glass panes can be cleaned easily by anyone of the following methods-
[i]By applying methylated spirit [ii]painting the glass panes with lime wash and leaving it to dry and then washing with clean water. [iii]rubbing damp salt for cleaning paint spots and;
[iv]rubbing finely powdered chalk