Unit V

Polymers and Rubber

Polymers Union of many simple organic molecules to form a

giant molecule with large molecular mass in the order of 15000 to 20000 with or without elimination of simple molecules like H2O, NH3etc.

eg. Ethylene Polyethylene

Monomer Polymer (Single unit) (Many units)

Types of polymerization

polymer

Addition polymerization Condensation

Polymerization

Addition polymerization

Addition of simple molecules to form a giant molecule

n(CH2=CH2) ( -CH2-CH2-)n

Ethylene Polyethylene

Condensation Polymerization

These types of polymers are formed by reaction between small molecules with elimination of molecules like H2O,NH3 , etc.

Eg. Formation of Phenol-formaldehyde resin. It is formed by the reaction between phenol and formaldehyde.

OH C 6H5 OH + HCHO - (-C 6H4 –CH2 -) n

Phenol Formaldehyde Phenol-formaldehyde (Bakelite)

Types of Plastics

Thermoplastics They are the resins which soften on heating and set on

cooling. Therefore, they can be remoulded any number of times and used.

Eg: Polythene, PVC

and Thermosetting plastics

They are the resins which set on heating and cannot be resoftened. Hence, their scrap cannot be reused.

Eg: Phenol-formaldehyde resin (Bakelite)

Distinction between thermoplastics and thermoset plasticsThermoplastics Thermoset plastics

They soften on heating and set on

cooling every time

They hardened on heating

The polymer chains are held together

by weak force called vander Waal’s

force of attraction.

The polymers chains are linked by

strong chemical bonds.

(Covalent bonds)

They are soluble in organic solvents. They are insoluble in organic solvents.

They expand very much on heating. Their expansion is only marginal due to

heat.

They are formed by addition

polymerization

They are formed by condensation

polymerization

They are processed by injection

moulding.

They are processed by compression

moulding.

Scarp can be reused. Scarp cannot be reused.

Polythene, PVC, Nylon Bakelite, Plaskon

Mechanical properties of Plastics1. Creep or Cold flow: Plastics undergo deformation when a load is

applied continuously.

2. Strength to weight ratio

Plastics have good strength when compared to their lightweight. Therefore, they replace lightweight metals like magnesium, aluminium in many fields

3. Impact strength: Plastics have better impact strength when compared to glass. Hence, they are replacing glasses in many places.

4. Tear resistance Plastics have poor tearing resistance

5. Thermal stability: Plastics either degrade or soften at high temperatures

6. Hardness: Thermosetting plastics are hard in nature when compared to thermoplastics.

7. Softening temperature

The temperature below which a polymer is hard and above which it is soft is known as glass transition temperature (Tg).

Advantages of plastics over other traditional materials (like wood and Metals)

1. Plastics are available in attractive colours.

2. They do not undergo corrosion

3. They are not affected by insects

4. They are light in weight

5. They are cheap.

6. They can be moulded into any shape easily

7. They are chemically inert.

8. They have good abrasion resistance

Specific uses of some plastics Bakelite (Phenol-formaldehyde):

TV cabinets, excellent adhesive., housing laminates, telephone components, electrical goods,

P.V.C (Polyvinyl chloride):

table clothes, rain coats, toys, tool handles, radio components, pipes, hoses, helmets, refrigerator components, cycle and automobile parts

Nylon (Nylon 6:6): Ropes , gears and bearings, household articles, as fibre in textile industry.

Urea-formaldehyde (Beetle ware):

making radio cabinets, switches, buttons, cups, plates cation exchanger in water treatment,

SPECIFIC USES OF PLASTICS

Polymers in Medicine and Surgery

Polymer Applications

Polyurethane Heart valves, blood filters, artificial

hearts, vascular tubes, etc.

Polyvinyl chloride

(PVC)

Disposable syringes, etc.

Polypropylene Heart valves, blood filters, etc.

Polyethylene Disposable syringes, etc.

Medical application of polymers

RUBBER

Rubber

Rubber is a natural elastic polymer of isoprene. It is obtained from the milk of rubber called ‘Latex’. The structure of natural rubber is as follows.

-(-CH2-C=CH-CH2-)n-

| CH3

Preparation of Natural Rubber from Latex

• Latex is rubber milk containing about 30 to 45% of rubber.• The rubber milk is diluted with water and allowed to stand

for sometime. • To the clear liquid from the top is treated with acetic acid or

formic acid to precipitate rubber.• The precipitated rubber is collected and passed through

rollers to get sheets of rubber. • Rubber sheets are finally dried by smoking. This rubber is

called ‘Smoked rubber’.• During the coagulation of rubber milk with acetic or formic

acid, retardant like sodium bisulphite (NaHSO3) is added to prevent oxidation of rubber. This is called ‘Creep rubber’.

Defects of natural rubber

• It becomes soft and sticky during summer.• It becomes brittle during winter.• It swells up in oils.• It flows plastically due to prolonged stress.• Chemicals easily affect rubber.

Compounding of rubberReinforcing agents or Hardeners

They are compounded with natural rubber to get soft spongy rubber.Example: vegetable oils, Stearic acid and paraffin oil.

Softeners They are compounded with natural rubber to get soft spongy rubber.Example: vegetable oils, Stearic acid and paraffin oil.

Anti-Oxidants They are added to prevent aerial oxidation of rubber.Example: Beta-naphthol.

Vulcanization Vulcanization is compounding of rubber with sulphur. By vulcanization, we get rubber of different hardness.

Colouring matter Zinc oxide – White ,Lead chromate – YellowChromium oxide – Green,Carbon black – Black

Accelarators They are added to speed up the vulcanization reaction of rubber.Example: Lime-magnesia, White lead, etc.

. Fillers Fillers are added to i) reduce the cost, ii) increase the bulk and i) introduce new characters.Example: Textile wastes, Asbestos, Mica, Gypsum, Talc, etc.

Tyres

Thank you