LECTURE NO. 45 (Handout) GLASS AND PLASTICS

1
LECTURE NO. 45 (Handout)GLASS AND PLASTICS

• Objectives
• To explain the engineering properties and usages
  of glass and plastics

2
GLASSIntroduction

• 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.

3
GLASSTypes of glass

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

4
GLASSSoda-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

5
GLASSPotash-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.

6
GLASSPotash-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.

7
GLASSCommon 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.

8
GLASSSpecial 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

9
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.

10
PLASTICSClassification of plastics

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

11
PLASTICSClassification 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.

12
PLASTICSClassification 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.

13
PLASTICSClassification 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.

14
PLASTICSGeneral properties of plastics

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

15
PLASTICSGeneral 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.

16
PLASTICSGeneral 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.

17
PLASTICSGeneral 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.

18
PLASTICSGeneral 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.

19
PLASTICSGeneral 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

20
PLASTICSUses 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.

21
PLASTICSPVC 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.

22
PLASTICSPVC 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
  -1C to 49C and for non-pressure applications
  higher temperature up to 80C 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.