Elif Temiz, 030070195, 11th Week Definition, Bonus Words

GLASS

New Definition (Materail)

Glass is one of the oldest and most versatile materials known to mari. The term 'glass' in its widest sense is used to describe a particular state of matter, known as glassy or vitreous state, which is obtained when a liquid cools without crystallisation taking place. Therefore, glass is defined as a super-cooled liquid. Glasses do not show changes like metals, ceramics or even plastics on cooling from the molten condition. Therefore, glass is considered as a very viscous liquid when it is a solid. It has apparent physical properties of a solid, such as, brittleness, hardness, transparency and chemical inertness.

Glass is manufactured by mixing the raw materials in proper proportions. The glass raw materials include sand, soda ash, limestone, dolomite, feldspar, sodium sulphate, broken glass, etc. The mixture is melted by heating upto 1500 °C. Some modifiers and refining agents are added to the melt. The melt can be cast, drawn or rolled to produce different shapes. Tubing is made by extrusion; bottles are made by a technique similar to the blow-molding techniques used to make plastic containers. Glasses are cast and ground to make precision shapes such as lenses. Sheet glass is made by casting in flat plate-type molds. The flat window glass is made by drawing a sheet out of a molten pool.

Types of Glasses

 Glass may be considered as a complex form of silicate. The basic ingredient for most glasses is sand (silica, SiO₂). Silica or quartz glass is pure silicon dioxide. Ordinary window glass is usually made from a mixture of sand ( SiO₂), limestone (CaCO₃), and soda ash (Na₂CO₃). When fused, glass has a composition consisting of a mixture of oxides. Other glasses are made by varying the amounts of SiO₂, CaO, Na₂O, B₂O₃, MgO, PbO, K₂O, BaO, etc. Coloured glasses are made by adding the oxides of specific metals; cobalt oxide yields blue glass, iron oxide yields green, and pure selenium yields red.

 Depending upon the chemical composition, glasses can be classified into the following types.

(a) Silica Glass (Fused Silica): Silica or quartz glass is a single-oxide glass and is made from commercially pure SiO₂. It is characterised by high melting point, low coefficient of thermal expansion and high chemical resistance. This type of glass is suitable for use as laboratory ware. It contains 96 to 99.9 percent silica depending upon the cost.

(b) Soda-lime-Silica-Glasses: These glasses are characterised by the presence of 15-25 percent Na₂O and CaO. This type of glass is widely used in the manufacture of glass containers, flat and plate glass, domestic ware, electrical lamps bulbs, etc.

(c) Borosilicate Glasses: Presence of boric oxide (B₂O₃) and AI₂O₃ results in better thermal shock resistance and chemical durability. This is called 'Pyrex' glass and greatly used in laboratory, medical ware, telescope lenses and chemical piping.

 (d) Aluminosilicate Glasses: This glass contains about 10-15 per cent Al₂O₃ which improves strength and chemical durability of the glass. One such type, known as E-glass (lime aluminoborosilicate), is used in the manufacture of glass fibres. It has high strength and thermal shock characteristics.

(e) Lead Glasses : Addition of lead oxide (PbO) to glass improves its refractive index and lowers the melting point. This type of glass is used where high refractive index, dispersion and radiation shielding are required. Combinations of K₂O, PbO, BaO and ZnO produce glass suitable for optical applications. They are called flint glasses. Chemical compositions of some commercial glasses are given in Table 13-2.

Properties of Glass

Glass is an important engineering material. It may be rolled, drawn, blown, moulded, pressed, or built up out of large sections by fusing, welding or soldering two halves. Many of the physical properties of glass may be controlled by the addition of chemicals and heat treatment.

 Glass is a super cooled liquid and considered as a brittle amorphous solid. Its optical properties show an index of refraction of about 1.5, which can be varied to accommodate special needs. It becomes ductile at elevated Temperatures. Its importance lies in the fact that it is chemically stable. It is insoluble in water. It has greater resistance to wear and abrasion than steel.

The tensile strength of ordinary glass increases inversely with its size. Thus, 12mm glass rods have ultimate tensile strengths of about 7 kg/mm², whereas fibres, which have diameters of a micron, may have ultimate tensile strengths of 400 kg/mm².

 The specific gravity of glass averages about 2.5. The coefficient of thermal expansion is very low. Leaded glass softens at a temperature as low as 600 °C and fused silicate glass as high as 1550 °C. 

Applications

Glasses are widely used in packaging, construction and engineering applications. Glass containers and bottles are extensively used. They are sufficiently strong, chemically inert and resistant to thermal shock. Glasses are extensively used in food industry. Almost all beer is processed in glass-lined tanks. Such tanks are also used to handle aggressive chemicals.

Probably, the largest use of glass is in the building construction. Various types of glasses used in buildings are transparent, translucent, decorative and roughened glasses, glass blocks, mirror glasses and glass claddings. Glass is used as vitreous coating on ceramic and metal surfaces.

A recent development in glass is in the fibre form. Glass fibres arc used for insulation purposes (thermal, sound and electrical) and as reinforcing materials. A- and E- glass fibres are used for reinforcing plastics, plasters and cements. Fibre glass is produced when molten glass, under steam pres-sure, is forced through very small holes. When cooled, it forms a network of thin glass fibres and air.

Safety glass is wired or laminated. Laminated glass is a sandwich of a layer of tough transparent material, such as plastic, between two sheets of glass The sandwich is rolled into a single unit. Safety glass is used in automobiles due to its transparency, resistance to ultraviolet light, and resistance to shattering.

(B. K. Agrawal, Introduction to Engineering Materials, pp.283,285)


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