LECTURE NO. 45 (Handout) GLASS AND PLASTICS Objectives: To explain the engineering properties and usages of glass and plastics.

LECTURE NO. 45 (Handout)GLASS AND PLASTICS

Objectives:• To explain the engineering properties and usages of

glass and plastics

GLASS:Introduction

• Glass has been used as an engineering material sine ancient times. As a result of the rapid progress made in the glass industry in recent times, glass has come out as the most versatile engineering material of the modern times.

• Glass is extensively used in the following ways:– A modern Boeing 707 jet plane contains more than 5000

components of glass.– Fibre glass reinforced with plastics can be used in the

construction of furniture, lampshades, bathroom fittings, navy boats, aeroplanes, cars, trucks, etc.

– In the construction of modern homes, walls and ceilings of hollow glass blocks can be made.

– The body of guided missile contains thousands of glass items.

GLASS:Types of glass

Following are various types of glass:•Soda-lime glass•Potash-lime glass•Potash-lead glass•Common glass

GLASS:Soda-lime glass

• Soda-lime glass is also known as soda-glass or soft-glass.

• It is mainly a mixture of sodium silicate and calcium silicate.

Properties:• It is available in clean and clear state• It is cheap• It is easily fusible at comparatively low temperatures• It is possible to blow or to weld articles made from this

glass with the help of simple sources of heat.Uses:• It is used in the manufacture of glass tubes and other

laboratory apparatus, plate glass, window glass, etc

GLASS:Potash-lime glass

• Potash-lime glass is also known as Bohemian-glass or hard-glass

• It is mainly a mixture of potassium silicate and calcium silicate.

Properties:• It fuses at high temperatures.• It is not easily affected by water and other solvents.• It does not melt so easily.

Uses:• This glass is used in the manufacture of glass articles

which have to withstand high temperatures such as combustion tubes, etc.

GLASS:Potash-lead glass

• Potash-lead glass is also known as flint glass. It is mainly a mixture of potassium silicate and lead silicate.

Properties:• It fuses very easily.• It is easily attacked by aqueous solutions.• It possesses bright luster and great refractive power.• Its specific gravity is about 3 to 3.30.• It turns black and opaque, if it comes into contact with

reducing gases of the furnace during heating.

Uses:• It is used in the manufacture of artificial gems, electric

bulbs, lenses, prisms etc.

GLASS:Common glass

• Common glass is also known as bottle glass. It is prepared from cheap raw materials. It is mainly a mixture of sodium silicate, calcium silicate and iron silicate.

Properties:• It fuses with difficulty.• It is brown, green or yellow in colour.• It is easily attacked by acids.

Uses:• It is mainly used in the manufacture of medicine

bottles.

GLASS:Special varieties of glass

• It is possible to alter the chemical, electrical, mechanical and optical properties of glass by suitably changing the basic composition of the glass.

• This way, glass has emerged as a versatile engineering material which can be tailor-made to meet with the requirements of different industries in the most effective and economic way.

• Some of the important special varieties of glass are:– Bullet-proof glass– Fibre glass– Foam glass– Glass blocks– Heat-excluding glass– Obscured glass– Perforated glass– Safety glass– Shielding glass– Soluble glass– Structural glass– Ultra-violet glass– Wired glass

PLASTICS

Introduction:• Plastic is one of the recent engineering materials, which has

appeared in the market all over the world.

• Plastic is an organic substance and it consists of natural or synthetic binders or resins with or without moulding compounds.

• In general, it may be stated that plastics are compounds of carbon with other elements such as hydrogen, nitrogen and oxygen. Carbon combines with itself and other elements and it forms more complicated compounds.

Polymerization:• Polymerization is the process of forming the polymers by

combining or synthesizing the monomers or monoliths (i.e., simplest substances consisting of one primary chemical).

• The properties such as strength, rigidity, and elasticity are considerably improved by polymerization and it further leads to the manufacture of plastics in an economic way.

PLASTICS:Classification of plastics

• The classification of plastics may be made on the basis of following:– Behavior with respect to heating– Structure– Physical and mechanical properties

PLASTICS:Classification of plastics on the basis of their behavior

with respect to heating(a) Thermo-plastic:• The thermo-plastic or heat non-convertible group is the general

term applied to plastics, which become soft when heated and hard when cooled.

• One important advantage of this variety of plastics is that the scrap obtained from old and warn-out articles can be effectively used again.

(b) Thermo-setting:• The thermo-setting or heat convertible group is the general term

applied to plastics, which become rigid when moulded at suitable pressure and temperature.

• This type of plastics passes originally through thermo-plastic stage.

• The thermo-setting plastics are durable, strong and hard.

• They are available in a variety of beautiful colors.

• They are mainly used in engineering application of plastics.

PLASTICS:Classification of plastics on the basis of their structure

(a) Homogenous plastic:• This variety of plastic contains carbon chain, i.e., the plastics of

this group are composed only of carbon atoms and they exhibit homogeneous structure.

(b) Heterogeneous plastic:• This variety of plastic is composed of the chain containing

carbon and oxygen, nitrogen and other elements and they exhibit heterogeneous structure.

PLASTICS:Classification of plastics on the basis of their physical

and mechanical properties(a) Rigid plastics:• These plastics have a high modulus of elasticity and they retain

their shape under exterior stresses applied at normal or moderately increased temperatures.

(b) Semi-rigid plastics:• These plastics have a medium modulus of elasticity and the

elongation under pressure completely disappears, when pressure is removed.

(c) Soft plastics:• These plastics have a low modulus of elasticity and the

elongation under pressure disappears slowly when pressure is removed.

(d) Elastomers: • These plastics are soft and elastic materials with a low modulus

of elasticity.• They deform considerably in tension and the deformation

disappears rapidly at room temperatures.

PLASTICS:General properties of plastics

1.Appearance:•Some plastics are completely transparent in appearance.•With the addition of suitable pigments, plastics can be made to have appearance of variety of attractive, opaque, stable and translucent colors.

2.Chemical resistance:•Plastics offer great resistance to moisture, chemicals and solvents.

•The degree of chemical resistance depends on the chemical composition of plastics.

•Many plastics are found to possess excellent corrosion resistance. Hence they are used to convey chemicals.

3.Dimensional stability:•This property of plastic favors quite satisfactorily with that of other common engineering materials.

PLASTICS:General properties of plastics-------contd.

4. Ductility:•Plastic lacks ductility. Hence its members may fail without warning.

5. Durability:•Plastics are quite durable, if they possess sufficient surface hardness.

•Plastics, especially thermo-plastic varieties, are likely to be attacked by termites and rodents. But the danger of such an attack is not very serious due to the fact that plastics have no nutritional value.

6. Electric insulation:•Plastics possess excellent electric insulation property. They are far superior to ordinary electric insulators.

7. Finishing: •Any surface treatment may be given to plastics. It is also easy to have technical control during its manufacture. It results in mass production of plastic articles with uniformity of surface finish.

PLASTICS:General properties of plastics-------contd.

8. Fire-resistance:• Plastics are organic in nature and hence, all plastics are combustible.

But, depending upon the structure, the resistance to high temperature and fire varies considerably among various varieties of plastics.

• Cellulose acetate plastics burn slowly.• Ployvinyl chloride plastics are non-inflammable. Phenol formaldehyde

and urea formaldehyde resist fire and they are used as fire proofing materials.

9. Fixing:• Plastics can be easily fixed in position.• They can be bolted, clamped, drilled glued, screw-threaded or simply

push-fitted in position.

10. Humidity effect on plastics:• The properties of plastics are governed to some extent by humidity.• The strength of plastics containing water-attracting groups such as

cellulosic plastics is considerably affected by the presence of moisture.• On the other hand, plastics, which do not contain water-attracting

groups such as polyvinyl chloride plastics, offer great resistance to moisture.

PLASTICS:General properties of plastics-------contd.

11. Maintenance:•It is easy to maintain plastic surface. They do not require any protective coat of paints.

12. Melting Point: •Most of the plastics have low melting point and melting point of some plastics is only about 50 oC therefore they cannot be used in position having high temperatures or to convey boiling water.

13. Optical property:•Several types of plastics are transparent and translucent.

14. Sound absorption:•Acoustical boards are prepared by impregnating fibre-glass with phenolic resins. This material has absorption coefficient of about 0.67.

PLASTICS:General properties of plastics-------contd.

15. Strength:• Plastics are reasonably strong.• The strength of plastics may be increased by reinforcing with

various fibrous materials.• Plastic members can be used as tensile members as their

strength to weight ratio in tension very nearly approaches to that of metals.

• However, the following considerations are responsible to discourage the use of plastics as structural material:

– Plastics are costly.

– Plastics are subject to creep under constant heavy load.

– The behavior of plastics is very sensitive to changes in temperature.

– The stiffness of plastics is very poor.

PLASTICS:General properties of plastics-------contd.

16. Thermal property:• The thermal conductivity of plastics is low and it can be compared

with that of wood.• Foamed or expanded plastics are among the leading thermal

insulators.

17. Weather resistance:• Only limited varieties of plastics can be exposed to weather.• The important group of plastics, which can resist weather effects, is

one prepared from phenolic resins.• Certain plastics are seriously affected by ultraviolet light in the

presence of sunlight. The resistance to sunlight of such plastics can be improved by incorporating fillers and pigments, which absorb or reflect the ultraviolet light at the surface protecting the interior of plastics.

18. Weight:• Plastics, whether thermo-plastic or thermo-setting, have low specific

gravity, the average being 1.30 to 1.40• The lightweight of plastics reduces the transport costs and

facilitates fixing

PLASTICS:Uses of plastics

Following are the typical uses of plastics in building: Bath and sink units, Cistern ball floats, Corrugated and plain sheets, Decorative laminates and mouldings Electrical conduits, Electrical insulators, Films for water-proofing, damp-proofing and concrete curing, Floor tiles, Foams for thermal insulation, Joint less flooring, Lighting fixtures, Overhead water tanks, Paints and varnishes, Pipes to carry cold water Roof lights, Safety glass, Wall tiles, Water-resistant adhesives, etc.

PLASTICS:PVC pipes in buildings

• PVC or polyvinyl chloride is the most versatile plastic and the use of PVC pipes in buildings is becoming popular day by day.

• The advantages of PVC pipes can be summarized as follows:

– They have good insulating properties and hence, the temperature of water passing through such pipes is not affected by the outside temperatures.

– They permit high, smooth and undiminished flow of water.– They have no problems of incrustation.– They possess high Hazen Williams Constant and it results

into adoption of smaller sizes PVC pipes as compared to the sizes of pipes of other conventional materials under similar conditions.

– They prove to be economical as compared to other pipes of conventional materials, such as asbestos cement, cast-iron and galvanized iron.

– They provide resistance to a variety of chemicals.

PLASTICS:PVC pipes in buildings----contd.

Following are some of the disadvantages of PVC pipes:• They are liable to creep phenomena requiring closer

spacing when installed above ground level.• They cannot be used at high temperatures, as they are

basically thermo-plastics. The recommended range of temperature for pressure applications is -1°C to +49°C and for non-pressure applications; higher temperature up to 80°C can be used.

• They don not have the same strength as cast-iron or galvanized iron pipes.

• They possess higher coefficient of expansion as compared to cast-iron or galvanized iron pipes.

• It may however be noted that that above drawbacks of PVC pipes are not very serious and they are now widely used as pressure pipes for rural water supply, electrical conduits, telephone ducts, tube well castings, etc.