Plastic materials that are manufactured in the industries are widely used for various day to day applications in our life. These materials have both advantages and disadvantages in our lives.

1. POLYMER-PLASTIC
By
CH. SAI SRI RAMA
CHANDRA MURTHY

2. Plastics as Engineering materials: Advantages and limitations
Plastic materials that are manufactured in the industries are widely used for
various day to day applications in our life. These materials have both advantages
and disadvantages in our lives.
Advantages of plastics:
1. No Corrosion: Articles prepared from metals get corroded after an interval of
time while the articles prepared from plastics do not undergo corrosion.
2. No damage by insects or pests: If articles are prepared from wood or plant
materials they are damaged from fungus and insects. When plastics are used
instead of wood they will not be damaged.
3. High strength to weight ratio: Plastics are light than traditional materials like
wood and metals, so they found great use both in domestic and industries.
4. Low cost: Metals and wood are very costly but cost of production of plastics is
low.

3. 5. No maintenance: The maintenance of plastic articles are very low when
compared to metals and wood.
6. Easy cutting and drilling: The process of cutting and drilling of plastics is easy
when compared to traditional materials.
Disadvantages of Plastics:
1. Plastics when dumped into the earth, they cause lot of damage to the
environment, viz. the prevention of production of nutrients in the soil causing
fertility change in the soil.
2. Plastics also cause harm and damage to the marine aquatic life.
3. Covering the plant parts with polymer materials for longer intervals of time
cause damage to plants.
4. Colours applied to the plastics for beautification and attraction cause allergies
to man.

4. Types of plastics:
They are of two general kinds.
1. Thermoplastics
2. Thermosetting plastics.
1. Thermoplastics:
They have linear or branched structures. They can be amorphous or semi-
crystalline materials. neighboring polymer chains are held together by weak
VandeWaal‟s forces or dipole-dipole forces or hydrogen bonding. There are no
cross-links. On heating, they soften very readily and can be moulded to any shape.
Their softness and hardness are temporary phenomena and are reversible. So they
can be remoulded, reshaped and reused. Hence they can be reclaimed from wastes.
These plastics are usually soluble in suitable solvents.
Eg: PE, PS, PVC, PP, nylons, polyesters, cellulose derivatives and acrylic.

5. 2. Thermosetting plastics:
They have three dimensional cross-linked, net worked structures. Cross-
links between neighboring polymeric chains are by covalent bonds. These can set
upon heating but cannot be reformed when once they are set. Condensation
polymers are thermosetting. Because of cross-links, these cannot be remoulded,
reused and reclaimed. These are hard, strong, brittle and are generally insoluble in
organic solvents.
Eg:
1. Bakelite
2. Silicones
3. Urea-formaldehyde resin
4. Vulcanized rubber
5. Epoxy etc.

6. Differences between thermoplastics and thermosetting plastics
Thermoplastics Thermosetting plastics
1. They are the products of addition
polymerizations.
1. These are the products of
condensation polymerization.
2. They have either linear or branched
structures.
2. They have three dimensional cross
linked network structures.
3. They soften on heating and stiffen
on cooling.
3. They do not soften on heating.
4. Adjacent polymer chains are held by
either Vander Waal‟s forces or by
dipole-dipole forces or H- bonding.
4. Adjacent polymer chains are held
together by strong covalent bonds
called cross-links.
5. Soluble in organic solvents. 5. Insoluble in organic solvents.
6. They are soft, weak and less brittle. 6. They are hard, strong and more
brittle.

7. Compounding of plastics
To produce plastics it is essential to have polymers blended with some
external materials called moulding constituents of plastics and are called as
moulding ingredients. These ingredients give required property to plastics such as
sturdiness. The process of mechanical mixing of various additives with polymers is
called compounding of plastics.The functions of these additives are
1. to modify the properties of the polymer
2. to introduce new properties into the fabricated articles
3. to assist polymer processing
4. to make the polymer products more pleasing and colourful.
7. They can be remoulded, reshaped
and reused.
7. They cannot be remoulded and hence
cannot be reused.
8. They can be reclaimed from waste
i.e., they can be recycled.
8. They cannot be reclaimed from waste
and hence cannot be recycled.

8. Depending on the purpose for which additives are used they are classified as
(a) Binders/ Resins
(b) Plasticizers
(c) Fillers
(d) Lubricants
(e) Colourants
(f ) Stabilizers
(g) Antioxidants
(h) Accelerators or Catalysts.

9. (a) Binders:
The main purpose of the binder is to hold the other constituents of the plastic
together. The binder used determines the type of treatment needed to mould the
articles from the plastic materials. The binder may be composed of 30 to 100% of
plastic. The binders may be natural or synthetic resins or cellulose derivatives which
have high molecular weight. So resins are basic binding materials in the plastic.
Eg: PE, PP, PVC, polyphenols, polyurethanes, polycarbonates, cellulose derivatives.
(b) Plasticizers:
These are added to polymers to increase plasticity and flexibility so as to
reduce temperature and pressure required for moulding. Further they improve flow
and process ability of the plastics and reduce the brittleness of the products. These
plasticizers possess high boiling points. Cellulose acetate has a tendency to
discolour.

10. Eg: Non-drying vegetable oil, triacetin, camphor, triphenyl phosphate, phthalic
esters, fatty acid esters, tetra butyl phosphate etc.
(c) Fillers or Extenders:
These are added to give the final plastic better hardness, tensile strength,
opacity, finish and workability. It reduces cost, shrinkage and brittleness. These are
two types
• Particulate fillers
• Fibrous fillers
Particulate fillers: These are used mainly to reduce the cost, help increase
hardness, tensile strength and retention of shape.
Eg: Asbestos powder, clay, silica, talc, limestone, wood flour or pulp etc.

11. Fibrous fillers:
Used mainly to increase the tensile strength, wear and tear resistance. They
are like steel in reinforced concrete.
Eg: Cotton thread
Nylon
Polyester fibers etc.
(d) Lubricants:
Used for good finishing, added to improve flow within processing
machineries and to reduce friction.
Eg: Waxes, oils, stearates, oleates and soaps.

12. (e) Colourants:
To get attractive colour to plastics. These are inorganic or organic
pigments.
Eg: Organic pigments: carbon black for black colour, Anthraquinone for yellow
colour, Azodyes for yellow, orange, red colours, Phthalocyanines for green colour
etc. Inorganic pigments: They give less bright and opaque colours. BaSO4 and
TiO2 give white colour, Lead chromate and iron oxide give yellow colour,
Antimony sulphide gives crimson red colour etc.
(f) Stabilizers:
They are used for improving the thermal stability during processing or
when they are put to use.
Eg: alkali earth oxides, organometallic salts, epoxy compounds, amines, naphthol,
phenyl salicylate, etc.

13. (g) Antioxidants:
They are used to protect against oxidative degradation, antioxidants are
incorporated into polymers. These oppose oxidation and suppress reactions
promoted by either O2 or peroxides.
Eg: Di-t-butyl p-cresol, phenyl β-naphthyl amine, diphenyl β-phenylene diamine
etc.
(h) Accelerators or Catalysts:
They are used to accelerate polymerization of fusible resin during
moulding into cross-linked infusible form (in thermosetting resins).
Eg: H2O2, benzoyl peroxide, ZnO, ammonia etc.

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