Material Science : Chapter 6: Polymers

Environmental Impact Assessment
June 30, 2017
Environment – Concepts
July 3, 2017

1. Introduction

2. Understanding Basic words

Organic substance

Polymer

       Polymeric material

       Mer

Monomer

Polymerization

Degree of polymerization

       Plastic material

      Vulcanization

3. Molecular structure of polymers

a) Linear

b)Branched

c) Cross linked

4. Polymers classification

4.1 Plastics

a) Thermoplastics

b) Thermosets

4.2 Elastomers

       4.3 Plastics Recycling

5.Mechanical behaviour of polymers
6.Properties of polymers
7. Compounding materials
8. Typical applications of few plastic materials
9. Reinforcement of polymers

10. Other important materials
11. FAQs

 

1. Introduction

What Are Polymers?

What do DNA, a plastic bottle, and wood all have in common? Give up? They are all polymers!

Polymers are very large molecules that are made up of thousands – even millions – of atoms that are bonded together in a repeating pattern. The structure of a polymer is easily visualized by imagining a chain. The chain has many links that are connected together. In the same way the atoms within the polymer are bonded to each other to form links in the polymer chain.

  • Polymers play a very important role in human life. In fact, our body is made of lot of polymers, e.g. Proteins, enzymes, etc.
  • Other naturally occurring polymers like wood, rubber, leather and silk are serving the humankind for many centuries now.
  • Modern scientific tools revolutionized the processing of polymers thus available are the synthetic polymers like useful plastics, rubbers and fiber materials.
  • Most of the polymers are basically organic compounds, however they can be inorganic (e.g. silicones based on Si-O network) also.

2. Understanding basic words

Organic substance

  • Substance containing carbon

Polymer

  • The word Polymer comes from the Greek ”poly” meaning many, and ”meros”, parts or units.
  • A polymer is a group of many units.
  • You combine many monomers (one unit) to create a polymer

Many many many MONOmers make a POLYmer! You got it!

  • These repeating monomers are linked to one another by covalent bonding
  • Properties of polymer is entirely different from constituent monomer properties
  • Polymers are usually, but not always, carbon based

Polymeric material

  • Material containing polymer as a main substance. Often containing different kinds of additives.
  • Ex: Plastics, Rubber, Glue, Paint
  • In the above example, paint, coloring additive is added to the polymeric(repeating units) material, which is a basic constituent.

Mer

  • The repeating unit in a polymer chain
  • A group of atoms or molecules treated as a unit that defines characteristic arrangement for a polymer

Monomer

  • Monomers are small molecules which may be joined together in a repeating fashion to form more complex molecules called polymers.
  • Bonded by covalent bonds  
  • Need to remember that all simple molecules cannot behave as monomers.
  • Eg: molecules like water and ammonia etc. are not monomers
  • Molecules with two or more bonding sites can act as monomers
  • In the polyethylene molecule CH2 == CH2 , — CH2 — CH2 is repeating unit, which is having two bonding sites
  • Other examples are vinyl chloride, glucose, and amino acids.

Polymerization

  • It is the process of synthesis of polymers
  • Multi-functional monomers are attached to form linear/3-D macro molecular chains
  • Polymers are often formed at elevated temperatures under pressure

Two types of polymerization

  • Additional or homopolymer synthesis
    • Polymerization process involves single kind of monomer

    • Resultant macro-molecule’s composition is an exact multiplication of composition of individual monomer

    • General initiator to start the polymerization process is benzoyl peroxide

  • Condensation or Copolymer synthesis
    • Polymerization process involves more than one monomer species

Degree of Polymerization

  • Polymers, unlike organic/inorganic compounds, do not have a fixed molecular weight. It is specified in terms of degree of polymerization
  • The average number of mer-units in a chain

Degree of polymerization =Molecular weight of polymer/Molecular weight of monomer

Plastic Material

  • Polymeric materials which turns stiff after forming in a soft plastic state.

 Plastic = Polymer+Additives

Vulcanization

  • The crosslinking process in elastomers is called vulcanization.
  • This can be achieved by a nonreversible chemical reaction, ordinarily carried out at an elevated temperature.
  • In most vulcanizing reactions, sulfur compounds are added to the heated elastomer

Characteristics:

  • Unvulcanized rubber, which contains very few cross links, is soft and tacky and has poor resistance to abrasion.
  • Vulcanization increases
    • Modulus of elasticity
    • Tensile strength
    • Resistance to degradation by oxidation.
  • The magnitude of the modulus of elasticity is directly proportional to the density of the crosslinks

3. Molecular structure of polymers

  • Polymer structures are categorized as below
    • Linear chain structure
    • Branched chain structure
    • Cross-linked structure

a)Linear chain structure

  • This is formed by joining of simple molecules or mers with their end to end

Eg: Polyethylene

b)Branched chain structure

  • If there are branches to simple linear chain then it is branched chain structure.
  • Because of branches, its elongation, i.e ductility would reduce

Eg: Bakelite

c) Cross-linked structure

  • Adjacent linear chains are connected with interlinked chains.
  • It would increase the strength and reduce plasticity

Eg: Natural rubber

Generally cross linked chain polymers are thermosets and linear, branched chain polymers are thermoplastics

4. Polymers classification

 

Basis of Classification Polymer type Examples

Origin

(a) Natural

(as available in nature)

Natural rubber, natural silk, cellulose, proteins, starch etc.

(b) Semisynthetic

(Man – made)

Hydrogenated, halogenated and cyclo (natural) rubber; cellulosics (cellulose esters/ethers), etc

(c) Synthetic

(Man – made)

Polyethylene, polypropylene, poly- styrene, polycarbonates, phenolics, amino resins, epoxy resins etc

Thermal response

(a) Thermoplastics (they soften or melt on heating and harden on cooling)

Acrylics, PVC, Nylons, Perspex glass, etc.

(b) Thermosetting

(which are ‘set’ under the application of heat and/or pressure. This process is not reversible)

Epoxies, Amino resins, some polyester resins, etc.

Mode of formation

(a) Chain – growth or addition or homopolymer

Polyethylene and other polyolefins, Polystyrene and related vinyl polymers etc

(b) Step – growth or condensation or copolymer

Polyesters, phenol (urea, melamine) – formaldehyde resins, epoxy resins etc

Line Structure

(a) Linear
(having no branches)

High density polyethylene (HDPE), polyvinyls, bifunctional (polyesters and polyamides) etc

(b) Branched
(having branches)

Low density polyethylene (LDPE), higher poly (α-olefins), phenolic resoles  etc.

(c) Cross linked or network (having a complex network structure)

Phenolic C-stage resin, C- stage amino (urea / melamine-formaldehyde) resins, cured epoxy resin and unsaturated polyester resin etc.

Application and Physical properties

(a) Rubbers
(showing long – range elasticity)

Natural rubber, synthetic rubbers, nitrile rubber, polychloroprene rubber, silicone rubbers etc.

(b) Plastics
(shapeable under pressure, aided by heat)

Polyethylenes, polystyrene, poly (vinyl chloride), polyamides, polycarbonates, acetal resins etc

(c) Fibres
(available in fibrillar or filamentous form)

Cotton (cellulose), natural silk, artificial silk (rayons), poly (ethylene terephthalate) fibre, nylon polyamide fibres etc

Crystallinity

(a) Crystalline (crystallinity,> 50%)

Polyethylene (HDPE and LDPE), polypropylene (isotactic), stretched nylon polyamides, polyoxymethylene etc. cellulose (cotton) fibre

(b) Semi– crystalline (crystallinity, 30 – 50%)

Polybutene, cellulosics (cellulose esters (rayons) particularly if stretched)

(c) Amorphous or noncrystalline

(crystallinity < 25%)

Natural rubber and most synthetic rubbers,N-alkylated(>15% alkylation) nylon polyamides, polystyrene etc.

 

4.1 Plastics

Plastics are important among all polymer types. Plastics are further divided into different groups. Among these, classification depending on their mechanical and thermal behavior as thermoplastics (thermoplastic polymers) and thermosets (thermosetting polymers) is important

a) Thermoplastic

  • Soften when heated and harden when cooled
  • These processes are totally reversible and may be repeated.
  • They are linear polymers without any cross-linking in structure
  • Plasticity increases with temperature by breaking the secondary bonds between individual chains.
  • Eg: Acrylics, PVC, nylons, polypropylene, polystyrene etc.

b) Thermosets

  • They are formed into a permanent shape with heating
  • This is due to extensive cross-linking.
  • Require heat and pressure to mould them into shape
  • Cannot be remelted or reformed into another shape but decompose at too high temperature.
  • Cannot be recycled, whereas thermoplastics can be recycled.
  • Thermosets are generally stronger, but more brittle than thermoplasts.

Eg: Epoxies, vulcanized rubbers, phenolics, unsaturated polyester resins, and amino resins (ureas and melamines) etc.

4.2 Elastomers

  • Also known as rubbers
  • Can undergo large elongations under load, at room temperature,
  • Return to their original shape when the load is released.
  • Consist of coil-like polymer chains those can reversibly stretch by applying a force.

Eg: Natural Rubber (NR), Synthetic Polyisoprene (IR)

 

S.No Thermoplastics Thermosets
1

Softens up on heating and hardens up on cooling

Set under the application of heat

 2

Can be recycled

Can not be recycled.

 3

Linear molecular chains without cross linking

 Linear molecular chains with cross linking
 4

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

 

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

 5 Applications: in making of  carry bags, pipes and buckets etc. Applications: involving hot temperature works like socket and plug, utensil handles and automobile parts

4.3 Plastic Recycling

What is Plastic Recycling?

Plastic recycling is the process of recovering different types of plastic material in order to reprocess them into varied other products, unlike their original form. An item made out of plastic is recycled into a different product, which usually cannot be recycled again.

Recycled plastics are substituting products otherwise produced from virgin materials, and the quality of the recyclates governs the substitution ratio. The lower the quality of the recycled plastics is, the lower will be the substitution ratio, and the smaller will be the benefits from their recycling

Stages in Plastic Recycling

  1. Sorting: It is necessary that every plastic item is separated according to its make and type so that it can be processed accordingly in the shredding machine.
  2. Washing: Once the sorting has been done, the plastic waste needs to be washed properly to remove impurities such as labels and adhesives. This enhances the quality of the finished product.
  3. Shredding: After washing, the plastic waste is loaded into different conveyer belts that run the waste through the different shredders. These shredders tear up the plastic into small pellets, preparing them for recycling into other products.
  4. Identification and Classification of Plastic: After shredding, a proper testing of the plastic pellets is conducted in order to ascertain their quality and class.
  5. Extruding: This involves melting the shredded plastic so that it can be extruded into pellets, which are then used for making different types of plastic products.

DIfferent stages in Plastic recycling

 

Benefits of Plastic Recycling

  • Provision of a Sustainable Source of Raw Materials
  • Reduces Environmental Problems
  • Reduces Landfill Problems
  • Consumes Less Energy

Challenges in plastic recycling

Dyed and pigmented plastics, for example, can be troubling for materials recovery facilities (MRFs) as they have a much lower market value.

Clear plastics are always preferred in the recycled materials market, and have the highest material value. This is because transparent plastic can typically be dyed with greater flexibility. The next best is white, as its only limit is that it cannot become clear, but can be made into any other colour

Primarily influencing quality of plastics recycling are

  • Polymer cross contamination
  • Presence of additives
  • Non-polymer impurities
  • Polymer degradation.

Deprivation of plastics quality, with respect to recycling, is shown to happen throughout the plastics value chain, but steps where improvements may happen have been preliminarily identified.

Another issues is economic viability of recycled plastics compares to new(virgin) plastics

 

5. Mechanical behaviour of plastics

  • Mechanical behaviour of polymers is similar to metals and ceramics to a large extent.
  • Parameters that have strong influence on mechanical behaviour of polymers are
    • temperature
    • strain rate and
    • morphology of polymers
  • Influence of temperature on mechanical properties
    • Glass-like brittle behaviour at low temperatures
    • Rubber-like behaviour at high temperatures
  • Highly crystalline polymers behave in a brittle manner, whereas amorphous polymers can exhibit plastic deformation.
  • Recoverable deformations up to very high strains / point of rupture are also observed due to unique structures of cross-linked polymers.
  • Tensile modulus (modulus) and tensile strengths are orders of magnitude smaller than those of metals, but elongation can be up to 1000 % in some cases.

A simple stress- strain curve can describe different mechanical behaviour of various polymers. As shown in figure, the stress-strain behaviour can be brittle, plastic and highly elastic (elastomeric or rubber-like)

image source: NPTEL by Satish Kailash

  • Curve A illustrates the stress–strain character for a brittle polymer.
  • The behaviour for a plastic material, curve B, is similar to that for many metallic materials
    • the initial deformation is elastic, which is followed by yielding and a region of plastic deformation.
  • Finally, the deformation displayed by curve C is totally elastic; this rubber-like elasticity is displayed by a class of polymers termed the elastomers.

6. Properties of Polymers

  • Light weight
  • Wide range of colours
  • Low thermal and electrical conductivity
  • Less brittle
  • Good toughness
  • Good resistance to acids, bases and moisture
  • High dielectric strength (use in electrical insulation)

7. Compounding materials

Most of the polymer properties are characteristic of a specific polymer.

Foreign substances called additives are intentionally introduced to enhance or modify these properties.

7.1 Fillers

  • Fillers are used to improve tensile and compressive strength, abrasion resistance, dimensional stability etc
  • Eg: wood flour, sand, clay, talc etc are example for fillers

7.2 Plasticizers

  • Plasticizers aid in improving flexibility, ductility and toughness of polymers by lowering glass transition temperature[FAQ] of a polymer
  • These are generally liquids of low molecular weight
  • Eg: Polyesters and Epoxies

7.3 Stabilizers

  • Stabilizers are additives which counteract deteriorative processes such as oxidation, radiation, and environmental deterioration due to polymers(plastics).
  • Eg: Barium, white lead and cadmium laurate

7.4 Colorants

  • Colorants impart a specific colour to a polymer, added in form of either dyes (dissolves) or pigments (remains as a separate phase).
  • Eg: Zinc oxide titanium oxide and white lead

7.5 Flame retardants

  • Flame retardants are used to enhance flammability resistance of combustible polymers.
  • They serve the purpose by interfering with the combustion through the gas phase or chemical reaction

8. Characteristics and typical applications of few plastic materials

a) Thermo plastics

1.Acrylonitrile-butadiene-styrene (ABS)

Trade name: Abson, Kralastic

It is made by dissolving polybutadiene in liquid acrylonitrile and styrene monomers and then polymerizing the monomers by the introduction of free-radical initiators.

ABS can also be made in an emulsion process, in which polybutadiene is prepared as a watery latex into which styrene and acrylonitrile are introduced and copolymerized. 

Characteristics:

  • Outstanding strength and toughness
  • Resistance to heat distortion;
  • Good electrical properties;
  • Flammable and soluble in some organic solvents.

Application:

  • Refrigerator lining
  • Lawn and garden equipment, toys
  • Highway safety devices.

2.Acrylics (poly-methyl-methacrylate)

Trade name: Acrylite, Plexiglas

Polymethyl methacrylate (PMMA), a synthetic resin produced from the polymerization of methyl methacrylate. 

In modern production it is obtained principally from propylene, a compound refined from the lighter fractions of crude oil.

Characteristics:

  • Outstanding light transmission
  • Resistance to weathering

Application:

  • Lenses
  • Transparent aircraft enclosures
  • Drafting equipment
  • Outdoor signs

3.Fluorocarbons (PTFE or TFE)

Trade name: Teflon, Neoflon

Teflon or polytetrafluoroethylene is obtained by addition polymerization technique from tetrafluoroethylene(C2F4)

Characteristics:

  • Chemically inert in almost all environments, excellent electrical properties
  • Low coefficient of friction
  • Relatively weak and poor cold-flow properties.

Application:

  • Anticorrosive seals
  • Chemical pipes and valves
  • Bearings, anti adhesive coatings
  • High temperature electronic parts.

4.Polyamides (nylons)

Trade name: Nylon, Baylon, Nomex 

Adipic acid + Hexamethylene diamine  →  Nylons + water

Characteristics:

  • Good mechanical strength, abrasion resistance, and toughness
  • Low coefficient of friction
  • Absorbs water and some other liquids.

Application:

  • Bearings, gears, cams, bushings, handles
  • Jacketing for wires and cables

5.Polyethylene(polythene)

Trade name: Alathon, Alkathene, Petrothene, Zendel

polyethylene can be prepared by purifying a quantity of ethylene, known as the feedstock, before adding a catalyst. This will begin a reaction that causes the ethylene molecules to form the polymer polyethylene.

Characteristics:

  • Chemically resistant and electrically insulating
  • Tough and relatively low coefficient of friction
  • Low strength and poor resistance to weathering.

Application:

  • Flexible bottles, toys, tumblers
  • Battery parts, ice trays
  • Film wrapping materials.

6.Polyester (PET or PETE)

Trade name: Celanar, Dacron, Mylar 

Polyester is a synthetic polymer made of purified terephthalic acid (PTA) or its dimethyl ester dimethyl terephthalate (DMT) and monoethylene glycol (MEG). 

Characteristics:

  • One of the toughest of plastic films
  • Excellent fatigue and tear strength
  • Resistance to humidity acids, greases, oils and solvents

Application:

  • Magnetic recording tapes
  • Clothing,
  • Automotive tire cords,
  • Beverage containers.

7. Vinyls

Trade name: Darvic, Exon, Vista

It is a type of plastic that is made from ethylene (found in crude oil) and chlorine (found in regular salt). When processed, both the substances are combined to form Polyvinyl Chloride (PVC) resin, or as is commonly referred to – Vinyl.

Characteristics:

  • Good low-cost, general-purpose materials
  • Ordinarily rigid, but may be made flexible with plasticizers
  • Often copolymerized
  • Susceptible to heat distortion

Applications:

  • Floor coverings.
  • Electrical wire insulation.
  • Pipes, garden hose
  • Phonograph records

b) Thermosetting polymers

1.Epoxies

Trade name: Araldite, Epikote, Lytex

These are obtained by condensation polymerization of epichlorohydrin and poly hydroxyl compounds

Characteristics:

  • Excellent combination of mechanical properties and corrosion resistance
  • Dimensionally stable
  • Good adhesion
  • Good electrical properties.

Application:

  • Electrical moldings, sinks, adhesives, protective coatings
  • Used with fiberglass laminates.

2.Phenolics(Phenol formaldehyde)

Trade name: Bakelite, Arofene, Resinox

Condensation polymerization between phenol and formaldehyde

Phenol + Formaldehyde → Phenolics(Bakelite)

Characteristics:

  • Excellent thermal stability to over 150o C
  • May be compounded with a large number of resins, fillers, etc.

Application:

  • Motor housing
  • Auto distributors
  • Electrical fixtures.

3.Polyester

Trade name: Aropol, Laminac, Selectron

It is formed by condensation polymerisation of dicarboxylic acid and dihydric alcohol

                       Dicarboxylic acid + Dihydric alcohol → Polyester

Characteristics:

  • Excellent electrical properties and low cost
  • Can be formulated for room- or high-temperature use
  • Often fiber reinforced

Applications:

  • Helmets
  • Fiberglass boats
  • Auto body components
  • Chairs, fans

9. Reinforcement of polymers

  • Fibers used as reinforcements are made of either glass, carbon or aramid.
  • Glass fibers are two varieties
    • E-glass
      • This is cheaper
    • S-glass.
      • This offers more strength
  • Aramid (aromatic polyamide) fibers – also known as Kevlar
    • These are commercially highly successful fibers.

Application: including protection from ballasts, ropes, aerospace, marine, and many other industrial applications.

10. Other important materials

Shrink-Wrap Polymer Films

An interesting application of heat treatment in polymers is the shrink-wrap used in packaging. Shrink-wrap is a polymer film, usually made of poly(vinyl chloride), polyethylene, or polyolefin (a multilayer sheet with alternating layers of polyethylene and polypropylene).

It is initially plastically deformed (cold drawn) by about 20-300% to provide a prestretched (aligned) film. The film is wrapped around an object to be packaged and sealed at the edges. When heated to about 100 to 150 C, this pre stretched material shrinks to recover 80-90% of its initial deformation, which gives a tightly stretched, wrinkle-free, transparent polymer film.

For example, water bottle packaging and many other objects that you purchase are packaged in shrink- wrap.

Phenolic Billiard Balls

billiard balls relatively expensive to manufacture; shellac was were made of ivory that came only from the tusks of elephants. Consequently, substitutes for ivory were sought for billiard balls

The most suitable replacement (which is still being used for billiard balls today) is one of the first man-made polymers—phenol-formaldehyde, sometimes also called “phenolic”.

The discoverer of the process for synthesizing phenol-formaldehyde was Leo Baekeland Baekeland named his new material “Bakelite”.

Phenol-formaldehyde is a thermosetting polymer, and has a number of desirable properties:

  • For a polymer it is very heat resistant and hard
  • Less brittle than many of the ceramic materials
  • Very stable and unreactive with most common solutions and solvents
  • Doesn’t easily chip, fade, or discolor

10. FAQs

1. Explain about Kevlar and its uses?

Kevlar is a material formed by combining para-phenylenediamine and terephthaloyl chloride. Aromatic polyamide (aramid) threads are the result. They are further refined, by dissolving the threads and spinning them into regular fibres.

When woven, Kevlar forms a strong and flexible material. If layers of the woven Kevlar are combined with layers of resin, the resulting ‘rigid’ material is light and has twenty times the strength of steel. It is also superior to specialist metal alloys.

Uses:

  • Kevlar is used as a reinforcement material for some car tyres and bicycle tyres
  • Kevlar 29 is used in the manufacture of body armour (panels) for lightweight military vehicles.
  • Kevlar 49 is used for specialist boat hulls and in the aerospace industry

Disadvantages of kevlar:

  • Kevlar textiles tend to absorb moisture.
  • Kevlar reacts well under a tensile force (stretching force) but badly under a compressive force
  • It is difficult to cut and shape, unless through the use of special tools and equipment.
  • Kevlar reacts badly to UV light (sunlight) unless it is protected / hidden from direct sunlight. 
  • Kevlar suffers some corrosion if exposed to chlorine.

References

  1. William D. Callister, Jr.David G. Rethwisch: Materials Science and Engineering: An Introduction, Wiley publication, 2014
  2. NPTEL material science material by Satish Vasu Kailas (IISc)

9 Comments

  1. click here says:

    I’ve been exploring for a little for any high-quality articles or blog posts in this sort of area . Exploring in Yahoo I eventually stumbled upon this web site. Reading this info So i’m satisfied to show that I have a very good uncanny feeling I came upon exactly what I needed. I such a lot indisputably will make sure to don?t put out of your mind this web site and provides it a look regularly.

  2. ig says:

    I think that is among the most important information for me.
    And i’m glad studying your article. However want to observation on some normal
    things, The site taste is great, the articles is really
    great : D. Excellent activity, cheers

  3. krati says:

    Thank you, sir, excellent work. huge respect for you, finally someone is so dedicated for the student.
    sir, i am also a mechanical engineer and I want to join your platform to share information. please reply .

  4. Ganesh more says:

    Great to content

  5. Pramod kumar mahato says:

    Sir, you have given the example of branched chain polymer as- BAKALITE but NCERT books say they are crossed link polymer

  6. IMDAD says:

    sir is it necessary to memorise all commercial names of polymers

Leave a Reply

Your email address will not be published. Required fields are marked *