4. Plastics, Rubber and Fibers

Outline Introduction to polymersNomenclature of polymers Addition and condensation polymerization Methods of PolymerisationEffect of polymer structure on propertiesPlastics-Properties and classification Moulding constituents of plastics Moulding of plastics into articlesPreparation, properties and uses of PE, PVC and Bakelite Rubber – propertiesNatural and synthetic rubberNatural and synthetic fibers

Generally In this chapter we will discuss man made materials that are based on organic chemistry.

The entire man made organic materials industry depends on the process of polymerization.

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4.1. Introduction to polymers

• A polymer is a giant molecule made up of a large number of repeating units joined together by covalent bonds.

• The simple compounds from which polymers are made are called monomers.

• The word polymer is derived from the Greek words poly (many) and meros (parts).

• Polymer molecules have molecular weight in the range of several thousands or more, and therefore, are also referred to as macro molecules.

Example :The formation of the polymer polystyrene.

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The styrene molecule is the monomer, and the resulting structure, enclosed in square brackets, is the polymer polystyrene

•The unit in square brackets is called the repeating unit.Example :Poly( Hexamethylene adipamide) or nylon-6, 6 is made from the reaction of hexamethylenediamine and adipic acid, as shown in the equation below :

The number of repeating units in the polymer structure is indicated by the index n. This implies the degree of polymerization (DP).

(DP) specifies the length of the polymer chain & one way of quantifying the molecular weight or size of a polymer.

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4.2 Nomenclature of Polymers• Polymer nomenclature usually applies to idealized representations; minor structural irregularities are ignored.

• A polymer can be named in one of two ways: Source –based Structure-basedWhatever method is used, all polymer names have the prefix poly, followed by enclosing marks around the rest of the name. The marks are used in the order:[( )]

Source-Based Nomenclature• Source-based nomenclature used when the monomer be identified.

1. Homo Polymers• Named using the name of the real or assumed monomer (the ‘source’) from which it is derived

• For example, the source-based name poly(vinyloxirane)4

.2. Copolymers• The structure of a copolymer can be described using the most appropriate of the connectives. connectives are written in italic font.

3. Non-linear polymers• Non-linear polymers and copolymers, and polymer assemblies are named using the italicized qualifiers.

Copolymer

Qualifier

Example

Random ran poly[(methyl methacrylate)-ran-(butyl acrylate)]

Alternative

alt poly[styrene-alt-(maleic anhydride)]

Block block poly(buta-1,3-diene)-block-poly(ethene-co-propene)

Graft graft polystyrene-graft-poly(ethylene oxide)

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The qualifier, such as branch, is used as a prefix when naming a (co)polymer, or as a connective

Structure-Based Nomenclature• Nomenclature can be used when the polymer structure is proven

• Structure based nomenclature uses that of the preferred constitutional repeating unit (CRU).

polymer qualifier

Example

blend blend poly(3-hexylthiophene)-blend -polystyrene

branch branch branch-poly[(1,4-divinyl benzene)-stat-styrene]

cyclic cyclo cyclo-polystyrene-graft-polyethylenenetwork net net-poly(phenol-co-formaldehyde)

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.

IUPAC Name :Poly[oxy(1-bromoethane-1,2-diyl)]• Polymers that are not made up of regular repetitions of a single CRU are called irregular polymers. For these, each constitutional unit (CU) is separated by a slash and end groups are connected using additional dashes outside of the enclosing marks

• Example:

IUPAC Name: α-methyl-ω-hydroxy-poly[oxirane-co-(methyloxirane)]

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Representations of divalent groups in polymers

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Traditional names are also acceptableVinyl polymers are among the easiest polymers to name when following the common name method

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Nonvinyl polymers generally have some atom other than carbon as a part of the backbone of the polymer.

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4.3. Addition and condensation polymerization

1. Condensation or Step-Reaction Polymerization polymer formation through the condensation b/n

two complementary functional groups with elimination of a small molecule such as water or HCl.

Example: The synthesis of nylon-66

2. Addition or Chain-Reaction Polymerization• Involves joining monomers together without the splitting of small molecules.

• Polymer formation involves three distinct kinetic steps: initiation, propagation, and termination. 11

The initiation step• The start of the reaction and requires an initiator to begin polymerization of a monomer.

• The initiator might be an anion, a cation, or a free radical (R*).

Propagation (Growth of the polymer chain )step Occurs through continuous addition of monomer to the reactive chain end.

Monomer molecules are added rapidly & the free radical moves to the end of the chain.

Termination step• In the termination reaction, two free radicals react with each other.

• Termination is either by combination or by disproportionation

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Example : Free radical polymerization

• The synthesis of poly (ethylacrylate),using benzoyl peroxide as a common initiator.

• Initiation Step: benzoyl peroxide decomposes to give two free radicals:

The radical R. react with the monomer ethyl acrylate

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Propagation step • Many monomer molecules are added to the generated radical

• Termination step Two free radicals react with each other

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Copolymerization• Copolymer is a macromolecule consisting of two or more different types of repeat units.

• Copolymerization provides a means of producing polymers with new and desirable properties by linking two or three different monomers or repeat units.

• Example:

1. Random copolymer monomer placement may be totally random or may be

perfectly alternating and sometimes unevenly. If repeating units are represented by A and B, then

the random copolymer might have the structure shown as:

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AABBABABBAAABAABBAB

• Free radical copolymerization of vinyl chloride and vinyl acetate:

2. Alternating copolymer: Each monomer of one type is joined to a monomer of a second type.

• Arrangement of the two repeating units along the polymer chain as shown in the following sequence: ABABABABABABABABABABABABABABABABABAB

• Example: free radical polymerization of equimolar quantities of styrene and maleic anhydride 16

3. Block copolymer:• Copolymers that contain a long block of one monomer (A) followed by a block of another monomer (B)

• AAAAAAAAABBBBBBBBBBBAAAAAAAAAAABBBBBBBBBB• Example : Thermoplastic elastomers.

4. Graft copolymers: Graft copolymers can be prepared, in which sequences of one monomer are grafted onto a backbone of another monomer type:

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. Graft copolymers are important as elastomeric (e.g., styrene-butadiene rubber (SBR))

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4.4 Methods of Polymerisation• Polymers can be prepared by many different processes.

• Free radical polymerization can be accomplished in bulk, suspension, solution, or emulsion.

• Ionic & other nonradical polymerizations are usually produced in solution polymerizations.

1. Bulk polymerization:• The simplest and most direct method for converting monomer to polymer

• It requires only monomer (and possibly monomer-soluble initiator or catalyst)

• Poly(methyl methacrylate), polystyrene, or low-density (high pressure) polyethylene produced from heating the respected monomer in the presence of an initiator and the absence of oxygen.

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Polyamides through the direct interaction between a dicarboxylic acid and a diamine &Nylon 66 from reaction between hexamethylene diamine and adipic acid.2. Solution polymerization:• Involves polymerization of a monomer in a solvent in which both the monomer (reactant) and polymer (product) are soluble.

• Many free radical polymerizations are conducted in solution

• Some water-soluble polymers are poly(acrylic acid), poly acrylamide, poly(vinyl alcohol), Poly (methyl methacrylate),polystyrene, polybutadiene &poly (vinyl chloride).

The use of solvent over bulk polymerization• solvent acts as diluent that reduces the tendency toward autoacceleration

• The viscosity of the solution continues to increase until the polymerization is complete.

• the solvent aids in the transfer of heat of the bulk process 20

3. Suspension polymerization: A polymerization process in which the monomer, or mixture of monomers, is dispersed by mechanical agitation in a liquid phase, usually water.

Advantages:• Low viscosity due to the suspension• Easy heat removal due to the high heat capacity of water

• Polymerization yields finely divided, stable latexesand dispersions to be used directly in coatings, paints, and adhesives.

Disadvantages:• Cannot be used for polymers whose glass transition temperature is less than the polymerization temperature, or else aggregation will occur.

• Must separate and purify polymer, or acceptcontaminated product.

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4. Emulsion Polymerization:• It is a type of radical polymerization that usually starts with an emulsion incorporating water, monomer, and surfactant.

• The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer (the oil) are emulsified (with surfactants) in a continuous phase of water.

• Water-soluble polymers, such as certain polyvinyl alcohols or hydroxyethyl celluloses, can also be used to act emulsifiers/stabilizers.

Advantages of emulsion polymerization include:• High molecular weight polymers can be made at fast polymerization rates. By contrast, in bulk and solution free radical polymerization

• The continuous water phase is an excellent conductor of heat and allows the heat to be removed from the system. 22

Disadvantages of emulsion polymerization include:

• For dry (isolated) polymers, water removal is an energy-intensive process

• Emulsion polymerizations are usually designed to operate at high conversion of monomer to polymer. This can result in significant chain transfer to polymer.

4.5. Effect of polymer structure on properties• The ultimate properties and the final uses of a polymer are intimately related to the chemical composition and structure of the polymer.

• The physical properties of polymers are related to:The strength of the covalent bondsthe stiffness of the segments in the polymer backbone, and

the strength of the intermolecular forces between the polymer molecules

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A. Thermal Properties

• A polymer to be thermally stable or heat resistant, it should not decompose below 400oC, and should retain its useful properties at temperatures near the decomposition temperatures.

• Aromatic polymers have found uses in applications that involve exposure to high temperatures and aerospace industries (for example as fabrics for astronauts' clothing) because of being resistant to high temperatures.

• ladder or semi ladder polymers usually have higher thermal stabilities than open-chain polymers

B. Mechanical Properties• Mechanical properties of a polymer depend on many variables molecular weight , molecular weight distribution, morphology, additives, temperature, time, and so on.

• Mechanical properties of polymers are much more dependent on molecular weight over a very broad rang.

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Some fundamental Mechanical Properties 1. Tensile strength- A measure of resistance to

stretching• The presence of bulky side groups, cyclic units, cross-linking, and crystallinity, will increase the tensile strength.

2. Compressive strength:• It is the extent to which a sample can be compressed before it fails.

3. Flexural strength: • It is a measure of resistance to breaking, or snapping, when a sample is bent (flexed).

4. Creep:• It is the cold flow of a polymer. It is a measure of the change in strain when a polymer sample is subjected to a constant stress (such as gravity).

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C. Solubility• Solvents are frequently used during the polymerization processes, during fabrication (i.e., film casting, fiber formation, and coatings), and for the determination of molecular weight and molecular weight distribution

A number of structural parameters of the polymer affect the ease and speed of the polymer solution

1. Chemical structure: Based on like dissolves like principle Polar polymers tend to dissolve in polar solvents

• For example, ethanol can dissolve poly(vinyl alcohol) but not polystyrene, whereas toluene can dissolve polystyrene but not poly (vinyl alcohol).

• 2. Molecular weight: In a given solvent at a particular temperature, the solubility of a polymer decreases as its molecular weight increases.

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3. Cross-linking: • If a polymer is cross-linked to a significant degree, it will only swell as a result of taking liquid into the network, but complete dissolution does not occur.

4. Branching: In general, solubility increases with increasing polymer branching.

D. Viscosity• Viscosity of a material is a measure of its resistance to flow. As a material's resistance to flow increases, its viscosity increases.

• The flow of polymer is impeded by chain entanglement, high intermolecular forces, and other factors such as the presence of reinforcing agents and cross-links.

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4.6. Plastics-Properties and classification

• plastic is usually considered to be an amorphous or crystalline polymer which is hard and brittle at ordinary temperatures

• It is a material that contains as an essential ingredient, an organic substance of a large molecular weight, is solid in its finished state.

There are two type of polymer: that are 1.Thermoplastics:• A thermoplastic material is one which can be softened and molded on heating. They are elastic and flexible above a certain glass transition temperature.

• Example: Nylon ,it was the first commercial polymer to be made as a substitute for silk for making parachutes, vehicle tires, garments, fabrics, footwear ,etc.

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Uses of some thermoplastic materials

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2. Thermosetting plastics• Thermosetting resins do not become softy on heating & they never melt once set.

• Example: Bakelite, polyester resin, vulcanized rubber, epoxy resin

Some use of thermosetting

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Difference between Thermoplastics & Thermosetting plasticsNo Thermoplastics Thermosetting

1 Formed by addition Polymn.

Formed by condensation Poly

2 Long chain linear polymers

Three dimensional networkstructure joined by strong covalentbonds

3 Soften on heating and stiffen oncooling

Do not soften on heating

4 Can be remoulded Cannot be remoulded

5 Can be reclaimed from wastes

Cannot be reclaimed from wastes

6 Soluble in organic solvents

Insoluble in organic solvents

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4.7. Moulding constituents of plastics

• Higher polymer material is mixed with 4 to 10 ingredients each of which imparts some useful property to the moulded products. Some are:

• Resins: Holds the different constituents togetherExample: Linear polymers with low molecular weights. Resins form 40-50%of moulding mixture

• Plasticisers: To increase the plasticity and flexibilityExample: Castor oil, Dibutylphthalate, Alkyl & Aryl Phosphate etc.,• Fillers: To impart valuable service properties such asstrength, resistance, finish and workability .Reducing cost, shrinkage on setting and brittleness.

Example: Wood flour, asbestos, china clay, talc, gypsum, saw dust. Fillers form 60%of plastics

• Lubricants: To facilitate the moulding operations. Gives glossy surface

Example: Waxes, soaps, Oleates, stearates .• Catalyst/accelerators: To accelerate the polymerization (added only in thermosetting resins)

Example: Hydrogen peroxide, Benzoyl peroxide, Metals (Ag, Cu & Pb)

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4.8. Moulding of plastics into articles

• Because of the properties of polymers it is possible to mould them and change their shape using a number of different manufacturing processes.

• The most important are: Extrusion ,injection moulding, blow moulding, rotational moulding, and compression moulding.

• Extrusion: For producing common plastic materials

eg: films, filaments, sheets & coating wires and cables with PVC

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Injection moulding• It is one of the most common of all plastics manufacturing processes. The polymer, in granule form, is heated until fused and forced into a closed mould cavity.

• Blow moulding: plastic material is first extruded as a tube shape into an open mould . The mould is then closed to seal the ends of the tube and air is blown in forcing the plastic tube to take up the shape of the mould cavity

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produce soft drink bottles, containers using Polyethylene, PVC, Polystyrene

4.9. Preparation, properties and uses of PE, PVC and Bakelite

1. Polyethylene: It is the most extensively used thermoplastic

• The two most widely used grades of polyethylene are low-density polyethylene(LDPE) and high-density polyethylene (HDPE

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Polyethylene was made by a high pressure, high temperature &radical process Polyethylene low density (LDPE) , –(CH2-CH2)n–None linear polyethylene & reduce to pack closely together and therefore lowers the density

Properties: soft, waxy solid Use : film wrap, plastic bagsPolyethylene high density (HDPE), –(CH2-CH2)n–A linear ethylene homopolymer can pack together more tightly and has a higher density

Properties: rigid, translucent solid Use : Electrical insulation, bottles, toys

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Preparation:

• LDPE

• HDPE

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poly(vinyl chloride) PVC –(CH2-CHCl)n–• It is made by a free radical polymerization.

• It is strong ,rigid solid• PVC is used in a variety of applications such as pipe , electrical tape, & tubing

• Preparation: PVC is produced by the polymerization of vinyl chloride monomer (VCM) in the presence of free radical initiators.

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PHENOLIC RESINS (Bakelite)• copolymers of phenol and formaldehyde, It was the first fully made synthetic polymers.

• The use an acid catalyst and excess phenol(Novolac) give a linear polymer that has no free methylol groups for cross linking and a cross-linking is further reaction with Hexamethylenetetramine (source of aldehyde)to form Bakelite

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.

• Bakelites are hard, rigid and withstand very high temperature.

• They have very good electrical insulating character. Because of their good dielectric properties, they are widely used in electrical, automotive, radio and TV parts. They are also used in paints and varnishes as hydrogen exchanger resins in water softening.

• Production of phenolic resin

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4.10. Rubber – properties• An elastomer (or rubber) is a word having its origin from two words: “elastic” which means the ability to return to original shape when a force or stress is removed and “mero”which means “parts“ implying many parts or monomers.

• Elastomers are polymeric materials with irregular structure and weak intermolecular attractive forces.

• Elastomers are capable of high extension (up to 1000%) under ambient conditions. That is, they have the particular kind of elasticity characteristic of rubber. The elasticity is attributed to the presence of chemical and/or physical cross-links in these materials.

• In their normal state, elastomers are amorphous, and as the material is stretched, the random chains are forced to occupy more ordered positions.

• Releasing the applied force allows the elongated chains to return to a more random state

• Rubber has the properties of flexibility, strength and elasticity 41

4.11 Natural and synthetic rubber• Natural Rubber:• It is a polymer found in the sap of the rubber tree, Hevea brasilensis, originally native to Brazil & now it is cultivated in Malaysia & Indonesia.

• The sap is collected, and on exposure to air and mild heat gives natural rubber .

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Chemically, this is a polymer of 2-methyl-1,3-butadiene (isoprene),CH2=C(CH3)-CH=CH2

Natural rubber has long been known, but became valuable only with the development of vulcanizing (heating in the presence of sulfur) which gives a much more rubbery and coherent substance• Natural rubber has a high tensile strength and is resistant to fatigue from wear such as chipping, cutting or tearing

• It has only moderate resistance to damage from exposure to heat, light and the ozone in the air.

• Polymerization of isoprene

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Synthetic Rubber• Elastomers of diene (e.g., polybutadiene,, polychloroprene, and so on,), nondiene elastomers (e.g., polyisobutylene, polysiloxanes, polyurethanes), nitrile & butyl rubber.

• These materials have chemical stability, high abrasion resistance, strength, and good dimensional stability.

• Many of these properties are imparted to the original polymer through crosslinking agents and additives

• The Buna-N rubber, which is soil-resistant, is a copolymer of 1,3-butadiene and acrylonitrile

nCH2=CHCH=CH2 + nCH2=CHCN -[CH2CH=CHCH2CHC=N]n-.• synthetic rubber offers better resistance to abrasion than natural rubber, as well as superior resistance to heat and the effects of aging.

• It is flame-resistant, insulation for electrical devices, flexible at low temperatures & is resistant to grease and oil.

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4.12. Natural and synthetic fibers

• Fibers are long strands of molecules interwoven to form a linear, string like structure.

• They may be natural or made by humans and are essential to clothing, industry

Natural Fibers:• Natural fibers are cellulose, the primary structural component of plants and animal fibers such as wool and silk.

• Cellulose, the most widespread organic molecule on Earth, is the major component of plant cell walls.

• A linear polymer made up of 10,000 to 15,000 glucose molecules

bonded in a 1-4 glycoside linkage. 45

Cellulose fiber portion

These fibers are structurally strong and resistant to chemical attack, so wood products are widely used in construction and production of paper.• Cotton, a vegetable fiber that is almost pure cellulose, grew to become an important industrial commodity

• Wool fibers are made of animal hair, principally from sheep.

• Each hair is a complex structure made up of layers of protein molecules

• Synthetic Fibers:• Synthetic fibers are usually polyesters or polyamides. The most important synthetic fibers are dacron, nylon, orlon, and polypropylene.

• Acrylic fibers are vinyl polymers valued for resistance to chemical and biological degradation

• Acrylonitrile (or vinyl cyanide) forms a homopolymer that is used in filters and artificial wool, which is widely used in sweaters. 46