Polymers PDF

# Polymers ## 7.1 Introduction - The word 'polymer' literally means "many parts", i.e., the basic make up of polymers consists of smaller units (mers) joined together either naturally or synthetically to produce polymers or macromolecules. - A polymeric solid material may be considered to be one that contains many chemically bounded parts or units that themselves are bonded together to form a solid. - Polymers are materials consisting of giant or macromolecules, chain-like molecules having average molecular weights from 10,000 to more than 10,00,000 g/mole built by joining many mers or units through chemical bonding. - Molecular weights is defined as the sum of atomic masses in each molecule. - Nature has given us a number of polymers like proteins, carbohydrates, silk, wool, cotton, rubber, leather, etc. - Man has made a variety of polymers with wide ranging properties such as softness like silk and wool and strong like steel. - Most polymers, solids or liquids, are carbon based organic; however, they can be inorganic (e.g., silicones based on Si-O network. ## 7.2 Basic Definitions ### 7.2.1 Polymers - Polymers are materials made from giant (or macromolecular), chain-like molecules having average molecular weights from 10000 to more than 1000000 g/mol built by the joining of many mers or units by chemical bonds, i.e., a polymer is a large molecule built by the repetition of small, simple chemical unit called monomer. - The repeating chemical units are covalently linked to each other in a macromolecule. Polymers are usually, but not always, carbon based. - The properties of a polymer are entirely different from those of the chemicals (or monomers) from which it is formed. ### 7.2.2 Mer - A unit group of atoms and molecules that defines a characteristic arrangement for a polymer. ### 7.2.3 Monomer - Monomer is defined as a simple molecule with two or more binding sites through which it forms covalent linkages with other monomer molecules to form the macromolecule. - Obviously, monomers are building blocks of polymers. Interstingly, all simple molecules cannot behave as monomers, e.g., molecules like ammonia, water, ethanol, etc. are not monomers. - Only molecules with two or more bonding sites can act as monomers, e.g., alkenes, vinyl chloride adipic acid, glycol with two bonding sites act as monomers. | Polymer | Monomer | Repeat Unit | |---|---|---| | Polyethylene | $CH_2 = CH_2$ | $ -CH_2 - CH_2 -$ | | Poly(vinyl chloride) | $CH_2 = CHCI$ | $ -CH_2 - CHCI -$ | | Polyisobutylene | $CH_3$ $CH_2 = C -$ $CH_3$ | $CH_3$ $-CH_2 - C -$ $CH_3$ | | Polystyrene | $CH_2 = CH -$ $\phi$ | $CH_2 - CH -$ $\phi$ | | Poly caprolactan (6-nylon) | $H - N(CH_2)_5C - OH$ $O$ | $H - N(CH_2)_5C - $ $O$ | | Polyisoprene (natural rubber) | $CH_2 = CH - C = CH_2$ $CH_3$ | $CH_2CH = C - CH_2 -$ $CH_3$ | 1. represents benezene ring, double bonds being omitted. ### 7.2.4 Polymerisation - This is defined as the chemical reaction in which a monomer is converted to the polymer under specific condition. However, monomer alone cannot undergo polymerisation, but requires the presence of chemical called initiator. - $Monomer + Initiator required -$ $\longrightarrow$ $Polymer$ - Pressure - n $[ CH_2 = CH ]_n$ $\longrightarrow$ $[ CH_2 - CH ]_n$ - Peroxide - Initiator - $Cl$ $Cl$ - $PVC (Polymer)$ ### 7.2.5 Degree of Polymerisation - The size of polymer depends on the number of repeating monomers constituting it. The degree of polymerisation is determined by dividing the molecular weight of polymer by the mer weights. - Degree of polymerisation = $\frac{Molecular weight of polymer }{Molecular weight of monomer}$ ## 7.3 General Characteristics of Polymer - General characteristics of polymers can be summarized as follows: - $(\iota)$ Polymers have long chain structures. The individual molecule of a polymer is very large i.e. may consists of thousands of similar small molecules, all bonded together covalently. - $(\iota\iota)$ All polymers have one thing common, i.e., carbon, which further bonds with hydrogen, nitrogen, halogens or other organic or inorganic substances. ## 7.4 Molecular Structure of Polymers - The polymers can have structures which includes: - $(\iota)$ linear chain structure - $(\iota\iota)$ branched chain structure and - $(\iota\iota\iota)$ cross-linked structure. ### 7.4.1 Linear Chain Structure - The mers or simple molecules join together end to end in single chains to form linear chain structure or linear polymers. The linear polymer is shown in figure 7.1. The structure is simple and uniform and units are held together by relatively weak secondary bonds. - Example is polyethylene which is a thermoplastic. Thermoplastics soften at high temperatures and they cannot be used at high temperatures. ### 7.4.2 Branched Chain Structure - The polymer has also branched chains besides linear chains as shown in figure 7.2. The polymer is more stronger and less ductile due to the inlocking of the chains with each other. Branching is generally formed by removing, a side atom from the main chain and replacing it by another C-C bonding. The chain packing efficiency is reduced due to the branching chains, thereby lowering the polymer density. ### 7.4.3 Crosslinked Structure - The polymer has interlinking chains connecting adjacent linear chains as shown in figure 7.3. Due to the interlinking of individual molecular chains, the movement of individual chain is restricted as the interlocking anchors, the adjacent chains together. Cross-linking provides increased strength and reduces plasticity to the polymer. Thermosetting polymers have cross-linking between chains Once hardened and set, thermosetting plastics do not soften with the application of heat. Hence, they can be used at high temperatures. ## 7.5 Classification of Plastics ### 7.5.1 Thermoplastics - A plastic which can be melted repeatedly by heating and which can be moulded again and again into different shapes with repeatedly melting is called thermoplastic. Hence, a thermoplastic becomes soft on heating and this property allows it to set in a desired shape again and again which it retains on cooling. - The examples of thermoplastics are: - $(\iota)$ polyethylene - $(\iota\iota)$ polystyrene - $(\iota\iota\iota)$ nylon - $(\iota\nu)$ polyvinyl chloride (PVC) - $(\nu)$ polycarbonate and ### 7.5.2 Thermosetting Plastics - It is a plastic substance which once set or moulded does not become soft on further heating. Instead of melting, it degrades on further heating. Thermosetting plastics are irreversible plastics which cannot be melted or softened once set of moulded into a given shape. - The example of thermosetting plastics are: - $(\iota)$ bakelite - $(\iota\iota)$ urea formaldehyde resin - $(\iota\iota\iota)$ melamine and | Thermoplastics | Thermosetting Plastics | |---|---| | 1. They have long chains of polymers which are not cross-linked. | 1. They have long chains of polymers which are cross-linked. | | 2. They become soft on heating. They can be moulded on heating and under pressure. | 2. They become soft on heating. They can be moulded on heating and under pressure similar to thermoplastics. | | 3. They can be remoulded on reheating. | 3. They cannot be remoulded on heating. | | 4. Polythelene, polycarbonate and polyvinyl chloride are examples of thermoplastics. | 4. Bakelite and melamine are examples of thermosetting plastics. | | 5. They can be reused or recycled. | 5. They cannot be reused or recycled. | | 6. They are used for making (i) buckets, (ii) pipes, (iii) carry bags, (iv) toilet goods and (v) toys. | 6. They are used for hot temperature applications such as (i) utensil handles, (ii) bodies of socket and plug, (iii) furniture, (iv) telephone bodies, (v) T-V cabinets and (vi) automobile parts. | ## 7.6 Thermoplastic Materials ### 7.6.1 Polythelene - It can be: - high density polyethelene (HDPE) produced at low pressure - High density polyethelene (LDPE) produced at high pressure. - LDPE has low density levels and only a small amount of branching. It is very flexible and easy to clean. It is often used to make plastic film wrap and plastic bags. It can be easily moulded. - HDPE has higher density levels. It has linear structure with no branching. It is more stronger and resistant to chemicals. It is suitable for blow moulding. - The properties of polyethylene include: - $(\iota)$ lighter plastic - $(\iota\iota)$ chemically resistant - $(\iota\iota\iota)$ does not absorb moisture and - $(\iota\nu)$ relatively low cost - It is used for making: - $(\iota)$ pipes - $(\iota\iota)$ fan and blower casing - $(\iota\iota\iota)$ packaging and - $(\iota\nu)$ buckets - $2 \times C_2H_4$ $\longrightarrow$ $H-H-H-H-H-H$ $C-C-C-C-C-C$ $H-H-H-H-H-H$ - Ethylene - Polyethylene ### 7.6.2 Polyvinyl Chloride (PVC) - PVC plastic is produced by the addition polymerisation of vinyl chloride. It is different from polyethylene as one of four hydrogen atoms attached to carbon is replaced by a heavier chlorine atom which provides rigidity. - $CH_2 = CH_2$ **+** $Cl$ $\longrightarrow$ $H-H-H-H-H-C$ $C-C-C-C-C-C$ $H - Cl - Cl - Cl$ - The plastic is a self-extinguishing type and it is sued widely for - $(\iota)$ cable jackets - $(\iota\iota)$ lead wire insulation - $(\iota\iota\iota)$ fibre coating - $(\iota\nu)$ pipes and fitting and - $(\nu)$ flooring and ceiling panelling ### 7.6.3 Teflon - Teflon is polytetrafluoroethylene (PTEE) and it is obtained by addition polymerisation technique from tetrafluoroethylene ($C_2F_4$). - C C Polymetisation F2 F2 C-C C=C C-C-C- F F F F Tetrafluoroethylene - Polytetrafluoroethylene - PTEE is completely inert chemically due to the presence of strong carbon fluorine covalent bond. It is non-flammable and a good electrical insulator. It has high corrosion resistance and low coefficient of friction. - It is used for: - $(\iota)$ bearing bushes - $(\iota\iota)$ piston rings - $(\iota\iota\iota)$ anticorrosion seals - $(\iota\nu)$ gaskets for IC-engines and - $(\nu)$ chemical pipes ### 7.6.4 Polystyrene - It is obtained from addition polymerisation of monomers styrene, a liquid petrochemical as shown below: - Polymerisation CH = CH2 Styrene H -c-c- 1 1 HH n - The polymer is: - $(\iota)$ hard - $(\iota\iota)$ brittle - $(\iota\iota\iota)$ transparent - $(\iota\nu)$ free from taste and harm to foodstuff - $(\nu)$ corrosion resistant and - $(\nu\iota)$ low dielectric constant. - It is used for: - $(\iota)$ electrical insulation - $(\iota\iota)$ food containers - $(\iota\iota\iota)$ house wares - $(\iota\nu)$ packaging and - $(\nu)$ lenses ## 7.7 Thermosetting Materials ### 7.7.1 Phenol Formaldehyde (PF) or Bakelite - It is made by condensation polymerisation of phenol and formaldehyde as shown below: - $OH$ - $HO$ - $CH₂$ - $HO$ - $CH_2$ - $nx$ - + - $nx$ - $CH_2$ - $CH_2$ - $H$ - $H$ - + - $X.420$ - $HO$ - $Phenol formadehyde$ - $CH_2$ - $CH$ - $X=1,2,3,$ - $CH_2$ - Bakelite is strong, rigid and dimensionally stable. It is resistant to heat, chemicals and solvents. - It is used for making: - $(\iota)$ electrical parts - $(\iota\iota)$ handles and knobs for utensils and appliances - $(\iota\iota\iota)$ paints and adhesives - $(\iota\nu)$ lavatory seats and - $(\nu)$ thermal insulators ### 7.7.2 Polyester - It is made by condensation polymerisation of dicarboxylic acid and dihydric alcohol as shown below: - $(n+1). R(OH)_2 + n. (R' (COOH)_2)$ $\longrightarrow$ $HO[ROOCR' COOL, ROH + 2n. H_2O$ - Dihydric alcohol - Dicarboxylic acid - Polyesters have good resistance to heat and chemicals. - They are used for: - $(\iota)$ components of automobiles - $(\iota\iota)$ helmets - $(\iota\iota\iota)$ paints - $(\iota\nu)$ borders for glass fibres and - $(\nu)$ jointing and repair work ### 7.7.3 Melamine - It is obtained by condensation of ureas (melamine) with formaldehyde as shown below: - $HN<$ - $N$ - $N$ - $Polarisation$ - $N$ - $N$ - $HN$ - $A$ - $N$ - $HN$ - $A$ - $>N$ - $N$ - $N$ - $-CH-N-CH_2-$ - Melamine has high resistance to chemicals, heat and moisture. It is also hard and scratch free. It has good electrical properties. - It is used for: - $(\iota)$ surface coating - $(\iota\iota)$ paints - $(\iota\iota\iota)$ crockery items - $(\iota\nu)$ doors and plugs, switches and buttons ### 7.7.4 Epoxies - They are obtained by condensation polymerisation of epichlorohydrin and polyhydroxyl compound as shown below: - $OH$ - $CH₂$ - $HO$ - $ + 2CICH_2CH-CH_2$ - $Polymerisation$ - $CH₂$ - $0$ - $CH₂$ - $CH_2$ - $CHCH_2O$ - $OCH CHCH_2O$ - $C$ - $-OCH CH-CHO$ - $C$ - $CH_2$ - $CH_2$ - Epoxies are transparent, tough material and chemical resistant: They are used as insulating material in cable box jointing for cable and transformers. They are also used as adhesive for jointing of various types of components. ## 7.8 Mechanical Behaviour of Plastics - Melting point: the plastics have low melting point and they are unsuitable for high temperature applications above 100°C. - Dimensional stability and water effect: The plastics have poor dimensional stability. They are susceptible to weather conditions, i.e., they lose colour and rigidity, thereby distorting when exposed to continuous sunlight. - Impact strength: They have low impact strength which further reduces when material softens at high temperatures. - Hardness: They have lower hardness than metals and ceramics. - Fatigue strength: They have poor resistance to fatigue failure. - Tear strength: Tear strength is the ability to resist tearing. Plastic films have moderate tear strength and commonly used for packaging. - Elasticity: Plastics have low elasticity but they display linear elasticity. - Service behaviour: Plastics develop cracks in the regions of localized stress concentrations. They fail due to formation and propagation of cracks. - Fracture mode: The thermosetting plastics are hard and rigid. They fail in the brittle mode. However, thermoplastics fail in ductile-brittle transition mode. - Thermal and electrical conductivity: They are poor conductors of heat and electricity. They are used as insulators. ## 7.9 Compounding Materials - The compounding materials are added in polymers to reduce the cost and to enhance the mechanical properties of the material. - The compounding materials which are generally added include: - $(\iota)$ Binders: The main purpose of binder is to hold other constituents of the plastic together. The binders may be synthetic or natural resions. On the basis of the type of resins used in preparation, the plastic itself can be called thermoplastic or thermosetting plastic. - $(\iota\iota)$ Fillers: Fillers are added to reduce the cost and enhance the strength and hardness of plastics. These may be (i) fibrous such as glass and asbestos fibre or (ii) non-fibrous type such as clay and carbon black. The proportion of filler added can be as high as 50% of plastic. - $(\iota\iota\iota)$ Plasticisers: The materials with low molecular weight (approx 300) are blended with polymers as plasticisers. These plasticisers are added to improve flexibility and processing capacity of the plastics by helping to reduce the temperature and pressure needed for moulding. Plasticisers should be added in small quantity as it also reduce the chemical resistance and tensile strength of the plastics. Polysters and epoxies are generally added as plasticisers. The plasticiser percentage in a plastic may be as high as 60% of the plastic. - $(\iota\nu)$ Blending: It is combining of two or more distinct polymer granules to form a new product with different characteristics. - $(\nu)$ Stabilisers: These are added to minimize the harmful effects of heat, sunlight and ozone. Barium. cadmium laurate and white lead are added as stabilisers - $(\nu\iota)$ Colouring agents: Pigments are added to give desired colour to plastics. Zinc oxide, white lead and titanium dioxide are commonly used pigments.

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