Processing of polymer and plastics (2)

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Information about Processing of polymer and plastics (2)

Published on February 19, 2014

Author: raghvmech



Processing of polymer

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 18-Feb-14 1

Contents Title 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 18-Feb-14 Page number 17 2

Introduction  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 18-Feb-14 3

Classification:Thermoplastic Plastics Thermosetting plastic Polymers Elastomers 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.

 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 18-Feb-14 5

Polymer synthesis polymerization Addition polymerization condensation polymerization • Processing of polymers primarily limits to synthesis followed by forming. • Polymers are synthesized by process known as polymerization. • Polymerization is process in which multi-functional monomers are attached to form linear/3-D macro molecular chains. • 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 • 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 18-Feb-14 6

 Polythene:- Combination Disproportionation 18-Feb-14 7

 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. 18-Feb-14 8

Mechanical behavior of polymers 1) 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. 18-Feb-14 9

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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 18-Feb-14 11

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. 18-Feb-14 12

Injection moulding 1. 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. 18-Feb-14 13

extrusion 1. 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. 18-Feb-14 14

Blow moulding 1. 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. 18-Feb-14 15

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 18-Feb-14 16

Characteristics and typical application 1. Thermoplastic Material Characteristics Applications • 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 • • • • 1. Acrylonitrile-butadiene-styrene (ABS) 18-Feb-14 Magnetic recording tapes Clothing Automotive tire chords Beverages container 17

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 • • • • • Material 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 Flexible bottle Toys Bags Tumblers Ice trays Thermosetting plastics 18-Feb-14 18

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. 18-Feb-14 19

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