Apr 7, 2023 1

Processing of polymer and plastics

Department Of Mechanical Engineering

National Institute Of Technology, Durgapur

Raghvendra Kumar P andey

Under the supervision of

Prof . A.B. Puri

Prof. J. Dey

Apr 7, 2023 2

Title Page number

Introduction 1

Classification on polymers 2

Polymers synthesis and types of polymerization

6

Mechanical behavior of polymers 9

Polymer forming 11

Types o polymer forming 12

Characteristics and typical application 17

Contents

Apr 7, 2023 3

The word polymer is originated from Greek word meros which means unit.

Monomer = single unit and polymer =many units joined together

Polymers are basically organic compounds but few are inorganic too.

Characteristics of polymers :-

- Low temperature stability

- Low hardness

- Low mechanical strength - High elongation under application of stress

- Low thermal and electrical conductivities

- High sensitivity of properties to their morphology

Introduction

Apr 7, 2023 4

Classification:-

Thermoplasts

• Plastics which softens up on heating and hardens up on cooling where the softening and hardening are totally reversible

processes. Hence, Thermoplasts can be recycled.

• They consist of linear molecular chains bonded together by weak secondary bonds or by inter-winding.

• Cross-linking between molecular chains is absent.

• Thermoplasts have the property of increasing plasticity, i.e., increasing ability to deform plastically with increasing

temperature.

• E.g.: Acrylics, PVC, Nylons, Perspex glass, etc.

Polymers

Plastics

Thermoplastic

Thermosetting plastic

Elastomers

Apr 7, 2023 5

Thermosetting plastics:-

• Plastics which are ‘set’ under the application of heat and/or pressure. This process is not reversible hence,

thermosets can not be recycled.

• They consist of 3-D network structures based on strong covalent bonds to form rigid solids. Linear

molecular chains bonded together by weak secondary bonds or by inter-winding.

• Characterized by high modulus / rigidity / dimensional stability when compared with Thermoplasts.

• E.g.: Epoxies, Amino resins, some polyester resins, etc.

Elastomers • These polymers are known for their high elongations, which are reversible upon release of applied loads.

• They consist of coil-like molecular chains, which straighten up on application of load.

• Characterized by low modulus / rigidity / strength, but high toughness.

• E.g.: natural and synthetic rubber

Apr 7, 2023 6

Polymer

synthesis

• Processing of polymers primarily limits to synthesis followed by forming.• Polymers are synthesized by process known as polymerization.

polymerizati

on

• Polymerization is process in which multi-functional monomers are attached to form linear/3-D macro molecular chains.

Addition

polymerizati

on

• Involves single kind of monomer• Three stages i.e. initiation (e.g., benzoyl peroxide) , propagation and termination• As molecular chain grows longer, reaction rate decreases. However the growth process is terminated either by

the combination or disproportionation process.• E.g., polyethylene

condensatio

n polymerizati

on

• Involves more than one monomer• Also known as step growth polymerization• Unpredicted chemical formula• Reaction time is longer as compare to addition polymerization• E.g., Formation of a polyester from Ethylene glycol and Adipic acid

Apr 7, 2023 7

Polythene:-

Combination

Disproportionation

Apr 7, 2023 8

Formation of a polyester from Ethylene glycol and Adipic acid:-

Degree of polymerization • Extant of polymerization is characterized in terms of ‘degree of polymerization’.

• It is defined as number of mer units in molecular chain or ratio of average molecular weight of polymer

to molecular weight of repeat unit.

• Average molecular weight is defined in two ways: Weight average molecular weight (based on weight

fraction) and Number average molecular weight (based on the number fraction).

• Number average molecular weight is always smaller than the weight average molecular weight.

Apr 7, 2023 9

Mechanical behavior of polymers1) Parameters like temperature, strain rate and morphology of polymers has strong influence on

mechanical behavior.

2) Mechanical property changes dramatically with temperature, going from glass like brittle at very low temperature to rubber like behavior at high temperature.

3) Due to cross linkage, recoverable deformation up to high strain also observed in polymers(elastomers).

4) Highly crystalline polymers behaves like brittle whereas amorphous polymers exhibit plastic deformation.

5) Tensile modules and strength is very lower than those of metals but elongation can be up to 1000% in some cases.

6) Unlike metals, plastics typically possesses glass transition temperature, exhibits prolonged creep, and tend to degrade ( due to heat) rather than corrode.

7) Plastics impregnated with organic can be subjected to bacterial infestation.

Apr 7, 2023 10

Apr 7, 2023 11

Polymer forming

Thermoplasts usually formed above their glass transition temperature.

Thermosets are formed in two stages – making liquid polymer and then moulding it.

Melting of pallets Injection into the mould.

Crystallization temperature polymer ∝ molecular weight

Glass transition temperature range 0.5 to 0.75 of absolute melting temperature.

Polymer melting and TG depends upon following factors:-

- chain stiffness (e.g., single vs. double bonds) - size, shape of side groups - size of molecule - side branches - cross-linking

Apr 7, 2023 12

Compression moulding

1) Place the appropriate amount of heated polymer (say pallet) in between the male and female part of mould parts.

2) Punch presses the molten pallets causing the viscous plastic flow to attend the predetermined shape.

3) If flash present, it is trimmed.

Apr 7, 2023 13

Injection moulding1. Palletized material is fed

with use of hopper into a cylinder where material melts due to heating coils.

2. Molten metal is impelled through nozzle into the enclosed cavity.

3. Edges are trimmed if present

4. Outstanding characteristic of this mounding process is cycle time is very less. i.e. rate of production is very high.

Apr 7, 2023 14

extrusion1. This technique is adapted

to produce continuous length with constant cross section.

2. Thermoplastic material is forced through a die orifice.

3. One of the most famous products of extrusion moulding is the optical fibre cable.

Apr 7, 2023 15

Blow moulding1. Using this manufacturing

process hollow parts are made like bottle or sphere etc.

2. Air is blown into a thin walled plastic cylinder called the parison. The parison is formed by melted plastic material being pushed through an extruder.

3. When the parison reaches a certain length, the two halves of the mould close around the parison sealing it at the bottom.

4. Compressed air is then used to inflate the parison to form the shape of the cavity inside the mould.

Apr 7, 2023 16

Transfer moulding

1. The molten resin is transferred by pressure from a melt pot into a mould which is at a temperature above the melt point of the resin.

2. Durable and dimensionally stable parts

3. Minimal release of airborne emissions during production

4. Uniform thickness of parts

5. Prevents corrode or rust

Apr 7, 2023 17

Material Characteristics Applications

1. Acrylonitrile-butadiene-styrene

(ABS)

• Outstanding toughness and strength

• Resistance to heat distortion• Soluble in some organic

solvents

• Refrigerator lining• Highway safety devices

2. Acrylics • Outstanding light transmission• Resistance to weathering

• Lenses • Drafting equipment's• Transparent aircraft enclosures

3. Polyester (PET or PETE)

• One of the toughest plastic film• Excellent fatigue and tear

resistance• Resistance to grease, acids and

oils

• Magnetic recording tapes• Clothing• Automotive tire chords• Beverages container

Characteristics and typical application

1. Thermoplastic

Apr 7, 2023 18

Material Characteristics Application 4. Polystyrene • Good thermal and dimensional

stability• Relatively inexpensive• Electrical resistance

• Wall tile• Battery cases• Indoor lighting panels

5. Polyethylene • Chemically resistance and electrically insulating

• Tough and low coefficient of friction

• Poor resistance to weathering

• Flexible bottle• Toys• Bags• Tumblers• Ice trays

Thermosetting plasticsMaterial Characteristics Application

1. Epoxies • Good adhesion• Corrosion resistance

• Protecting coating• With fiberglass laminates

2. Phenolic • Excellent thermal stability over 150 degree C

• Motor housing• telephones

Apr 7, 2023 19

conclusion

"Better things for better living... through chemistry“

Polymers are being used now smartly in electrical and electronic, packaging, automobile and construction areas. This sector has potential to increase exponentially through proper planning of recycling.

Any shaped product is now possible to manufacture through various moulding processes to manufacture.

The global market for engineering plastics in terms of revenue was estimated to be worth $45.2 billion in 2011 and is expected to reach $76.8 billion by 2017.

Apr 7, 2023 20