UNIT IV IG POLYMERS 2015-16 IG POLYMERS Syllabus: Types of polymerization Stereo specificity of polymers properties of polymers Plastics Thermoplastics and thermo setting plastics ompounding and Fabrication of plastics Preparation and properties of Polyethylene, PV and Bakelite Elastomers Rubber and Vulcanization Synthetic rubbers Styrene butadiene rubber Thiokol applications. Fibre reinforced plastics Biodegradable polymers onducting polymers and their applications. Objectives: Plastics are widely used engineering materials and understanding their properties helps in selecting suitable materials for various purposes. Engineers should also be aware some of the advanced polymer materials that have specific significance. Outcomes: Students gain the knowledge on structure, synthesis properties and applications of polymers, additives to be mixed with polymers to obtain desired plastics and moulding techniques, advanced topics on plastics like conducting polymers and biodegradable polymers, fibre reinforced plastics and bullet proof plastics, synthetic plastics that are essential to latest technology. OUTLINES Introduction Methods of polymerization Stereo specific polymers Properties of polymers, PE, PV and Bakelite Plastics ompounding of a plastic Selected individual polymers Rubbers or elastomers Vulcanization Synthetic rubbers Fabrication of plastic articles Biodegradable polymers onducting polymers Engineering hemistry Page 79
UNIT IV IG POLYMERS 2015-16 1. Introduction Polymers are macromolecules built up by the linking together of a large number of small molecules or units. Thus, small molecules which combine with each other to form polymer molecules are termed monomers; and the repeat unit in a polymer is called as -mer. For example, polythene is a polymer formed by linking together of a large number of ethene ( 2 4 ) molecules. Similarly polystyrene is formed by the linking of styrene monomer molecules. n Mer Styrene (Monomer) Polystyrene, PS (Polymer) The number of repeating units (n) in chain formed in a polymer is known as the degree of polymerization (DP). The process of formation of a polymer from its monomer units is termed as polymerization. Many polymers are naturally occurring like starch, cellulose etc., and as many are synthetically made such as polystyrene, PV, etc. The organic polymers like starch, polyethene have carbon backbone, while the inorganic polymers have atoms other than carbon, which have catenation property like silicon, sulphur, phosphorous. E.g: silicates. 1.1.Nomenclature of polymers Polymers consisting of identical monomer units are called homo-polymers and monomers of different chemical unit structures are called hetero-polymers or co-polymers. -M-M-M-M-M-M-M-M- -M 1 -M 2 -M 1 -M 2 -M 1 -M 2 -M 1 -M 2 - omopolymer etero or copolymer Based on the arrangement of monomeric units (structural units), copolymers can be classified as: Engineering hemistry Page 80
UNIT IV IG POLYMERS 2015-16 Alternating copolymers: These polymers are formed by regularly altering the two different monomeric units. -M 1 M 2 M 1 M 2 M 1 M 2 Statistical copolymer (Random copolymer): These are copolymers in which the sequence of monomer units follows a statistical rule. The probability of finding a given type of monomer unit, at a particular point in the chain is equal to the mole fraction of that monomer unit in a chain. -M 1 M 2 M 2 M 2 M 1 M 2 M 1 M 1 Block copolymer: The copolymer consisting of two or more homopolymer subunits linked through covalent bonds is called a block copolymer. -M 1 M 1 M 1 M 1 M 2 M 2 M 2 M 2 M 3 M 3 M 3 M 3-1.2 Functionality: In the process of polymerization, for any molecule or unit to act as a monomer, it must have at least two reactive sites or bonding sites for the extension of a monomer to a dimer, trimer and ultimately a polymer. The number of such reactive sites in the monomer is termed as its functionality. Ex: In ethylene the double bond can be considered as site for two free valancies. Thus, ethylene is considered to be bifunctional. If the monomer has bifunctionality, it can only form a linear polymer. If the functionality is more than two, the monomer has a chance to form cross linked polymers having 2D or 3D structures. Based on functionality and the process of polymerization, the polymer may be present in linear, branched or cross-linked (three-dimensional) structure as illustrated below: -M 1 M 1 M 1 M 1 M 1 M 1 M 1 M 1 M 1 M 1 M 1 M 1 - Linear homopolymer Backbone M M M M M M Branching M 2 M 1 M M 1 M 2 M 1 M 2 M 1 M 2 M Branching M 1 Branched hain homopolymer M 2 Branched hain eteropolymer Engineering hemistry Page 81
UNIT IV IG POLYMERS 2015-16 M M M M M M M 1 M 2 M 1 M 2 M 1 M 2 M ross Linkage M M 1 ross Linkage M 2 M M M 2 M 1 M M M M M M rosslinked omopolymer M 2 M 1 M 2 M 1 M 2 M 1 rosslinked eteropolymer 2. Methods of polymerization The process of polymerization reaction involves union of two or more small, same or different monomer molecules to form a single large macro-molecule, called polymer. The conversion of a monomer into a polymer is an exothermic process and if heat is not dissipated or properly controlled, explosion may result. This is basically due to the difference in the mechanisms of the two different types of polymerization processes i) Addition or chain polymerization and ii) Step or condensation polymerization. 2.1 Addition or chain polymerization: The reaction that yields a product, which is an exact multiple of the original monomeric molecule. Such a monomeric molecule, contains one or more double bonds, which by intermolecular rearrangement, may make the molecule bi-functional. The monomer molecules simply add themselves at the double bonds (π bond) by self addition and form a chain of a macro-molecule, leaving the ends open for further addition, if any. Since the process takes place by a chain reaction, it is also termed as chain polymerization. The length of the chain is controlled by external factors. The addition polymerization reaction must be instigated by the application of heat, light, pressure or a catalyst for breaking down the double bonds of monomers. For this, unsaturation in the monomer units is a necessary factor. The polymer will have the same chemical composition as the monomer. Addition polymers will have their molecular weights as integral multiple of their monomer unit. i.e., M = n. m where M and m are the molecular weights of the polymer and the monomer respectively and n is the degree of polymerization. Engineering hemistry Page 82
UNIT IV IG POLYMERS 2015-16 2.2 ondensation or step polymerization: ondensation or step-polymerization may be defined as a reaction occurring between monomers having simple polar functional groups (like -O, OO etc.,) forming a polymer by the elimination of small molecules like water, l, ammonia etc. For example, hexamethylene diamine and adipic acid condense to form a polymer, nylon 6:6. n N N examethylene diamine O + n O O O 6 4 Adipic acid ondensation Polymerization - 2n 2 O N N O O 6 4 n Polyhexamtheylene Adipate (Nylon 6,6) (Polyamide) The molecular weight of a condensation polymer is always less than the integral multiple of their monomer units. ondensation polymerization is an intermolecular combination, and it takes place through different functional groups (in the monomers) having affinity for each other in a step-wise process. When monomers contain three such functional groups, they may give rise to a cross-linked polymer. opolymerization: opolymerization is the joint polymerization of two or more different monomer species. igh molecular weight compounds obtained by copolymerization are called copolymers. For example, butadiene and styrene copolymerize to yield SBR (Styrene butadiene rubber). Engineering hemistry Page 83
UNIT IV IG POLYMERS 2015-16 2.3 Differences between addition and condensation polymerization processes: Addition polymerization ondensation polymerization 1. The functionality of a monomer is the 1. The functionality of a monomer 2, 3 or Π bond which is bifunctional any 2. The polymerization is by self addition 2. The polymerization is by condensation And is by chain mechanism which is slow step wise 3. No by products are produced. 3. By products of small molecules like 2 O, N 3, 3 O & l are formed. 4. The molecular weight of the polymer is 4. The molecular weight of the polymer is less sum of molecular weights of monomer. Than the sum of molecular weights of monomers 5. The process is highly exothermic. 5. The process not exothermic. 6. An initiator is required for the reaction. 6. A catalyst is required for the reaction. 3 Stereo-specific polymers 3.1. Tacticity: The stereo chemical placement of the asymmetric carbons in a polymer chain is called tacticity. The differences in configuration or arrangement of functional groups on the carbon backbone of the polymer (tacticity) affects the physical properties. Based on the stereo chemical orientation of the atoms or groups at asymmetric carbons, the polymers can be classified as 1. In a head-to-tail configuration, if the arrangement of functional groups are all on the same side of the chain, it is called as an isotactic polymer. e.g., PV R R R R R ISOTATI or R R R R R 2. If the arrangement of functional groups is in an alternating fashion in the chain, it is called syndiotactic polymer. e.g., gutta-percha. R R R R R R SYNDIOTATI or R R R R R 3. If the arrangement of functional groups is at random around the main chain without any regularity, it is called atactic polymer. e.g., polypropylene. Engineering hemistry Page 84
UNIT IV IG POLYMERS 2015-16 R R R R R or ATATI 3.2. o-ordination polymerization (Ziegler-Natta catalysts): R R R R R Ziegler (1953) and Natta (1955) suggested that in the presence of a combination of a transition metal halide (like Til 4 or Til 3, ZrBr 3, Til 2, halides of V, Zr, r, Mo and W) with organo metallic compounds like triethyl aluminium or trimethyl aluminium, stereospecific polymerization can be carried out. A combination of such metal halides and organo-metallic compounds is called as Zeigler- Natta catalysts. Mechanism of co-ordination polymerization can be illustrated as : Initiation: at-r' + 2 = R at- 2 (R)R' omplex catalyst Monomer Propagation : at- 2 --R' + n 2 = R' R Termination (with acive hydrogen compound) : at- 2 -- 2 --R' R n R at- 2 -- 2 --R' + X at-x + 3 -- 2 --R' R R n R R n Ziegler- Natta polymerization is useful in the preparation of polypropylene, poly ethylene, etc. The importance of this method lies in the fact that stereospecific polymers of desired configuration are obtained. For example, during the polymerization of propylene, using conventional catalysts, normally random or atactic polymer is obtained. But by using suitable Zeigler-Natta catalyst, solvent and temperature, it is possible to make a desired type (atactic or isotactic or syndiotactic) of polypropelyne. 4. Properties of polymers for engineering applications 4.1. Structure and chemical properties a) hemical Reactivity: The polymer is prepared by linking small monomeric units. So their properties depend upon number and chemical nature of chemical groups present in the monomers. The thermal stability and mechanical strength of different polymers are related to difference in bonding and structure of the monomer. Polymers containing high electronegative atoms in their Engineering hemistry Page 85
UNIT IV IG POLYMERS 2015-16 backbone chain undergo hydrolysis. E.g. Nylon and polyester. Polymers containing double bonds undergo ozonalysis. E.g., Rubbers like isoprene, neoprene. b) Solubility and swelling nature: Polar polymers such as PVA, PV, and polyamide are soluble in polar solvent like water, alcohol, phenol etc,, while non-polar polymers like PE, PP, PS can be dissolved in non-polar solvents like benzene, toluene, xylene, n - hexane etc. Polymers of aliphatic character are more soluble in aliphatic solvents, whereas aromatic polymers are soluble in aromatic solvents. Polymers dissolve in solvents and swell in size. c) Ageing and weathering: The reason for the stability of the polymer is bond strength between the atoms in the polymer chain. The stability of polymer can be enhanced by increasing bond strengths. eat, ultraviolet light, high energy radiation, atmospheric effect and chemical environments are the main agencies to affect the properties of polymers. PTFE, PE and PV have good stability towards light and heat due to the fact that the bond energies of these are greater than light energy. The heat stability of these polymers is in the order PTFE > PV > PE d) Permeability and diffusion: Diffusion occurs in polymers through vacant gaps between adjacent polymer molecules. rystalline polymers resist in diffusion because of greater degree of molecular packing. Amorphous polymers above T g have appreciable permeability The crystalline polymers have high resistance to permeability than amorphous polymers. 4.2. Physical Properties a) rystallinity: The degree of structural order arrangement of polymeric molecules is known as crystallinity. rystallinity favours denser packing of molecules, thereby increasing the intermolecular forces of attractions. This accounts for a sharp and higher softening point, greater rigidity and strength. The polymers with low degree of symmetry and with long repeating units are partially crystalline and are amorphous in structure. The crystalline polymer units have packing close to each other through intermolecular forces. ompletely crystalline polymers are brittle. The crystallinity influences properties like solubility, diffusion, hardness, toughness, density and transparency of polymers. b) Amorphous state: Random arrangement of molecules, less intermolecular forces lead to amorphous nature of a polymer. So they can be moulded into a desired shape. Both thermosetting and thermoplastic polymers can exist in amorphous state. 4.3. Mechanical properties: a) Strength: The polymer chains adjacent to each are held together by weak intermolecular forces. The strength of intermolecular forces can be increased by either increasing chain length or molecular Engineering hemistry Page 86
Stress UNIT IV IG POLYMERS 2015-16 weight or the presence of polar groups (-O, -OO, -OMe, -OOR, -X). The lower molecular weight polymers are quite soft and gummy. igh molecular weight polymers are tough and heat resistant. The cross linked polymer chains are strongly linked to each other by strong covalent bonds, which cause greater strength, toughness, brittleness and low extensibilities. The strength of the polymer is characterized by the stress and strain curve. Strength of the polymers also depends on the shape of the polymer. Eg: In PV, large size chlorine atoms are present. The strong attractive forces restrict the movement of molecules and so PV is tough and strong. ard, Brittle Polymers Rigid high Impact thermoplastics Polymer fibres Rubbery polymers Strain b) Elastic character:. Elasticity is the relaxation to original shape after removal of applied stress. Polymers like nylon, having this stretching nature are called elastomers. Elastomers are slightly cross linked, amorphous and rubber like polymers. In the absence of deforming forces these polymers have peculiar chain configuration of irregularly coiled snarls. So the polymer is amorphous due to random arrangement. When they are stretched cross-links begin to disentangle and straighten out. c) Plastic deformation: This is found in thermoplastics; These polymers have structure which is deformed under heat or pressure. This property is used to process them into desired shape. Due to weak inter molecular forces, these polymers show permanent deformation at high temperature and pressure. The Vander wall forces are weak in a linear polymer at high temperature and result in slippage. The plasticity of a polymer decreases with temperature. Engineering hemistry Page 87
UNIT IV IG POLYMERS 2015-16 d) Structure and electric properties: Most of the polymers are electrical insulators and the insulating property can be removed by application of a strong field. The electronic polarization is responsible for dielectric constant in non-polar polymers. Water has high dielectric constant and conducting property so the absorbed water molecules enhance the conductivity of a polar polymer. 5. PLASTIS Plastics are polymers which can be moulded into any desired shape or form, when subjected to heat and pressure in the presence of a catalyst. They undergo permanent deformation under stress termed as plasticity. The term plastic and resin are synonymous. Plastics are obtained by mixing a resin with other ingredients to impart special engineering properties. These are characterized by light weight, good thermal and electrical insulation, corrosion resistance, chemical resistance, adhesive nature, low cost, high abrasion resistance, dimensional stability, strength, toughness and impermeability to water. A plastic material should have sufficient rigidity, dimensional stability and mechanical system at room temperature to serve as a useful article. It may be moulded to shape by application of reasonable temperature and pressure. Types of plastics 5.1.Thermoplastics These are linear, long chain polymers, which can be softened on heating and hardened on cooling reversibly. Their hardness is a temporary property and it changes with the raise or fall of temperature. They can be reprocessed. Examples: Polyethylene (PE), Polypropylene (PP), Polyvinyl chloride (PV), polystyrene (PS), Nylons, Poly tetra fluoro ethylene (PTFE) etc. 5.2.Thermosets These polymers, during moulding get hardened and once they are solidified, cannot be softened i.e, they are permanently set polymers. During moulding, these polymers acquire three dimensional crosslinked structure, with strong covalent bonds. Thermosets once moulded cannot be reprocessed. Examples: Polyester (terylene), Bakelite, epoxy- resin (araldite), Melamine, urea- formaldehyde resin etc. Engineering hemistry Page 88
UNIT IV IG POLYMERS 2015-16 Thermoplastics Thermoplastics Vs Thermosets Thermosets 1. They soften on heating readily 1. They do not soften on heating. On prolonged heating, they get charred. 2. They consist of long chain linear 2. Their set molecules have threemolecules. dimensional network structure, joined by strong covalent bonds 3. They are mostly formed by addition 3. They are formed by condensation polymerization polymerization 4. They can be softened, reshaped and reused by heating. (recycled) 4. They cannot be softened, reshaped and reused. 5. They are usually soft, weak and less brittle 5. They are usually hard, strong and brittle. 6. They can be reclaimed from wastes. 6. They cannot be reclaimed from wastes. 7. They are usually soluble in some organic solvents 6. ompounding of a plastic 7. They are insoluble in almost all organic solvents, because of their structures. A high polymeric material is mixed with 4 to 10 ingredients during fabrication, each of which these ingredients either discharge a useful function during moulding or impart some useful property to the finished article. This is called a mix. Some of the main types of compounding ingredients are: (1) Resin or a binder; (2) Plasticizers; (3) Fillers; (4) Lubricants; (5) atalysts or accelerators; (6) Stabilizers. 6.1. Resin or a binder: The product of polymerization is a resin, which forms the major portion of the body of the plastic. It also holds the different constituents together. The binders used may be natural or synthetic resin or cellulose derivatives. Resin forms the major part of the plastic and determines the types of treatment needed in the moulding operations. 6.2. Plasticizers: These are the materials that are added to resins to increase their plasticity and flexibility. Their action is considered to be the result of the neutralization of part of the intermolecular forces of attraction between macro molecules. They decrease the strength and chemical resistance. They impart greater freedom of movement between the polymeric macro molecules of resins. Most commonly used plasticizers are vegetable oils (non-drying type), camphor, esters (of Stearic, Oleic or phthalic acids) and some phosphates ( tricresyl phosphate, tributyl phosphate, tetra butyl phosphate and triphenyl phosphate) 6.3. Fillers: Fillers are added to give to the plastic better hardness, tensile strength, opacity, finish and workability. They reduce the cost, shrinkage on setting and brittleness. They are also added to impart special characters to the product. The percentage of fillers is up to 50% of the total moulding mixture. Engineering hemistry Page 89
UNIT IV IG POLYMERS 2015-16 Eg:- a). arborundum and mica are added to provide extra hardness b). Barium salts are added to make plastic impervious to X- rays. c ). Addition of asbestos provides heat and corrosion resistance. Most commonly used fillers are wood flour, asbestos, china clay, talc, gypsum, metallic oxides like ZnO, PbO and metal powders like Al, u, Pb etc. The fillers which enhance mechanical strength are reinforcing fillers. Eg:- Addition of carbon black to natural rubber, increase its strength to 40% and also enhances its abrasion resistance. 6.4. Lubricants: Lubricants like waxes, oils, soaps are employed to make the moulding of plastic easier. They impart a glossy finish to the products. They also prevent the plastic material from sticking to the fabricating equipment. They make moulding easier and impart glossy flawless finish to the product. ommonly used lubricants are waxes, oils, stearates, oleates and soaps. 6.5. atalyst or promoters: These are added to thermosetting plastic, during moulding operation, to accelerate the polymerization of fusible resin, into cross-linked infusible form. Eg: atalysts used for compounding include 2 O 2, benzoyl peroxide, acetyl sulphuric acid, metals like Ag, u, and Pb; metallic oxides like ZnO, N 3 and its salts. 6.6. Stabilizers: They improve the thermal stability during polymerization and further processing. Vinyl chloride shows a tendency to undergo decomposition and discoloration at moulding temperature. ence, during moulding, heat stabilizers are used. ommonly used stabilizers; a) Opaque moulding compounds like salts of lead (viz. white lead, litharge, lead chromate, red lead etc.) b) Transparent moulding compounds like stearates of lead, d and Ba. 6.7. olouring materials: olor and appeal are very important for commercial high polymer goods. ommonly used coloring materials are organic dye stuffs and opaque inorganic pigments. Eg: arbon black, anthra quinones (yellow), azodyes (yellow, orange, red), phthalocyanins (green) 7. Fabrication of plastics into articles The fabrication of plastic into commercial goods is done by five common methods 1. asting; 2. Blowing; 3. Extrusion; 4. Lamination; 5. Moulding 7.1. asting: This method of moulding is used to mould both thermoplastic and thermosetting resins. ere, the molten resin is poured into a suitable mould and heated up to 70 for several hours at atmospheric pressure. The products formed are free from internal stress. 7.2. Blowing: In this process, the softened thermoplastic resin is blown by air or steam into a close mould. Engineering hemistry Page 90
UNIT IV IG POLYMERS 2015-16 7.3. Extrusion: In this method, the material of the required composition is forced by a screw conveyer into a heated chamber, where it softens and then is forced through a die, having the desired shape. The finished product that extrudes out is cooled by atmospheric exposure or by blowing air or by spraying water. This method is only used for thermoplastic. It is used for the manufacture of articles like sheets, tubes, rods etc. 7.4. Lamination: Sheets of cloth, wood or paper are impregnated with a resin solvent solution. These are then piled up one over the other until the desired thickness is obtained and heated to remove the excess of the solvent, pressed together between two highly polished steel surfaces to get the laminated product. Phenolic and urea type resins are commonly used. Laminated plastic have high tensile strengths and impact resistance. 7.5. Moulding: Moulding is an important method of fabrication of plastic. The moulding of the plastic is done around a metal insert so that the finished product has a metal part firmly bonded to the plastic. ommonly used moulding methods are A. ompression moulding B. Injection moulding. Transfer moulding D. Extrusion moulding 7.5.1. ompression moulding: This method is applied to both thermoplastic and thermosetting resins. A predetermined quantity of ingredients required for the plastic are filled between the two half- pieces of the mould. eat and pressure are then applied as per required standards. The cavity gets filled with fluidized plastic. The halves of the mould are closed slowly. Final curing (the time required for the plastic to set in the shape) is done either by heating (for thermosetting) or cooling (for thermoplastic). These moulded articles are then taken out by opening the parts of the mould. Engineering hemistry Page 91
UNIT IV IG POLYMERS 2015-16 7.5.2. Injection moulding: This method is applicable for thermoplastic resins. The moulding plastic powder is fed into a heated cylinder through a hopper and is injected at a controlled rate, into the tightly locked mould, by means of a screw arrangement or by a piston plunger. The mould is kept cold to allow the hot plastic to cure and become rigid. When the material has been cured sufficiently, half of the mould is opened to allow the removal of the finished article without any deformation. eating is done by oil or electricity. Advantages: This is most widely used method because of its high speed of production, low mould cost, very low loss of material and low cost. There is a limitation of design of articles to be moulded, because large number of cavities cannot be filled simultaneously. 7.5.3. Transfer moulding: This method is useful for moulding of thermosetting plastics.the powdered compounding material to be moulded is placed in a heated chamber, maintained at a minimum temperature, where powder just begins to become plastic. This is then injected through an orifice into the mould by a plunger, working at high pressure. Due to the friction developed at the orifice, the temperature rises to the extent that the moulding powder becomes liquid and flows quickly into the mould. This is then heated up to curing temperature for setting. This is then heated up to curing temperature for setting. Engineering hemistry Page 92
UNIT IV IG POLYMERS 2015-16 Advantages: The plasticized mix flows into cavity in highly plasticized condition and hence very delicate articles can be handled without distortion or displacement. Thick pieces can also be cured completely and uniformly. Non attainable shapes by compression moulding can be obtained. The article produced is free from flow marks. Finishing cost of fabricated article is almost low and blistering of the goods is almost eliminated.. 7.5.4. Extrusion moulding: This method is mainly used for continuous moulding of thermo plastics into materials of uniform crosssection like tubes, rods, sheets, wires, cables etc.. The thermoplastic ingredients are heated to plastic state ( a semi solid condition) and then pushed by means of screw conveyor into a die, having the shape of the article to be fabricated. The plastic mass gets cooled due to atmosphere exposure or artificially by air jets or a spray of water. 8. Some individual polymers 8.1. Polyethylene or PE Polyethylene is most commonly used polymer, produced by the polymerization of ethylene in presence of a catalyst. By using free radical initiator (benzoyl peroxide) at 80-125 0 low density polythene (LDPE) with density of 0.92g/ccis produced, while by using an ionic catalyst like tri ethyl aluminium,, highdensity polythene (DPE) with density 0.965g/cc is obtained. 8.1.1. Preparation: n n-3 Ethylene (Monomer) Mer Polyethylene, PE (Polymer) Engineering hemistry Page 93
UNIT IV IG POLYMERS 2015-16 8.1.2. Properties: 1. Polyethylene is a rigid, waxy, white, translucent, non polar solid material with good electrical insulation property. It is a soft flexible polymer. 2. It exhibits chemical resistance to strong acids, alkali and salt solutions at room temperature but attacked by oils, organic solvents especially kerosene. 3. Polyethylene crystallizes very easily due its highly symmetrical structure. The degree of crystallization varies from40-95% depending on the number of branching in the polymeric chain. 4. ommercially polyethylenes are sub divide in to three groups based on its density. i) Low Density Polyethylene LDPE ; ii) Medium density Polyethylene; iii) igh Density Polyethylene(DPE) 5. It is resistant to atmospheric gases, moisture and UV light. 8.1.3. Engineering applications: PE is used for making high frequency insulator parts, bottle caps, packing materials, tubes, coated wires, tank linings in chemical plants and domestic appliances. 8.2. Poly vinyl chloride or PV It is a thermoplastic polymer and is obtained by the free radical addition polymerization of vinyl chloride in the presence of benzyl peroxide or hydrogen peroxide. In PV the mass of chlorine is 57% of the total mass of the polymer. Vinyl chloride is obtained by treating acetylene with l at 60-80 0 in the presence of metal oxide catalyst. + l 2 l Vinyl chloride n l benzoyl peroxide l n Vinyl chloride Poly Vinyl hloride 8.2.1. Properties: 1. PV is colorless, non inflammable and chemically inert powder. It is strong but brittle. 2. It is resistant to ordinary light, atmospheric gases, moisture, inorganic acids and alkalis, but undergoes degradation in heat or UV light. 3. It is soluble in hot chlorinated hydrocarbons like ethyl chloride 4. Pure resin possesses a high softening point. 5. It has greater stiffness and rigidity compared to polyethylene. Engineering hemistry Page 94
UNIT IV IG POLYMERS 2015-16 8.2.3. Engineering applications: 1. It is widely used as a synthetic plastic. 2. Rigid PV is used for making sheets, light fittings, safety helmets, refrigerator components, tyres, and cycle and motor cycle mudguards. 3. Plasticized PV is used in making continuous sheets viz., table cloths, raincoats, curtains etc., 4. Used in injection moulding of articles like toys, tool handles, radio components, chemical containers, conveyor belts etc. 8.3. Bakelite It is prepared by condensing phenol with formaldehyde in presence of acidic/alkaline catalyst. The initial reaction results in the formation of non polymeric mono, di and tri methylol phenols depending on the reactant ratio. These compounds in the first stage react to form a linear polymer, Novolac. Novolac in the second stage undergoes further reaction with these linear polymers to form cross linking and bakelite plastic resin is produced. All these stages in a step wise manner are shown in the reaction below, ultimately giving the cross linking polymer, bakelite. O O O 2 O Phenol + O o-ydroxy methyl phenol and 2 O p-ydroxy methyl phenol O O O O 2 - O + + O - 2 + O - 2 Monomethyl phenol Phenol Monomethyl phenol Monomethyl phenol O 2 O 2 O 2 O 2 Novolac Engineering hemistry Page 95
UNIT IV IG POLYMERS 2015-16 O O O 2 2 2 2 2 8.3.1. Properties: O 2 O 2 ross-linked polymer bakelite Phenolic resins ( like bakelite) set to rigid, hard, strong, scratch-resistant, infusible, water-resistant, insoluble solids, which are resistant to non-oxidizing acids, salts and many organic solvents. But these are attacked by alkalis, because of the presence of free hydroxyl group in their structures. They possess excellent electrical insulating character. They are good anion exchange resins capable of replacing anions with O groups. They are good adhesives, corrosion resistant and resistant to atmospheric gases, moisture and UV light. 8.3.2. Engineering applications: The phenol-formaldehyde resins are extensively used 1. for making electric insulator parts like switches, plugs, switch-boards, heater handles, etc. 2. for making moulded articles like telephone accessories, cabinets for radio and television. 3. for impregnating fabrics, wood and paper. 4. as adhesives (e.g., binder) for grinding wheels. 5. in paints and varnishes. 6. as hydroxyl group exchanger resins in water softening 7. for making bearings, used in propeller shafts for paper industry and rolling mills. 9. Rubbers or elastomers Rubbers, also known as elastomers are high polymers, having elastic property i.e.; the ability to regain their original shape after releasing the stress. They have temporary deformation in their physical structure on application of stress of more than 600 elastic units. Thus, a rubber can be stretched to 4 to 10 times its original length. The elasticity of rubber is due to its coiled structure. Elastomers are expected to have the following characteristics. O 2 Engineering hemistry Page 96
UNIT IV IG POLYMERS 2015-16 1. They have elasticity i.e.; it can be stretched by applying stress and can regain original shape and dimension by releasing the stress. 2. They have very low inter chain attraction forces. 3. They have coiled structure. 4. They can absorb water. 5. They has low chemical sensitivity 6. At high temperature they become sticky. Elastomers are classified into two types 1) Natural rubber; 2) Synthetic rubber 9.1. Natural rubber: Natural rubber consists of basic material latex (cell sap), which is a dispersion of isoprene. During the treatment, these isoprene molecules polymerize to form, long-coiled chains of cispolyisoprene. The main source of natural rubber is the latex of the evea brasiliensis. More than 95% of the rubber is obtained from evea brasiliensis. Natural rubber obtained from evea brasiliensis is a cis- polymer of isoprene (2-methyl. 1, 3 butadiene). The polyisoprene in natural rubber is in long coiled chain form, responsible for its elasticity. n Isoprene is-polyisoprene (Natural rubber) n 9.1.1. Processing of latex: Latex obtained from tapping of the tree is diluted to contain between 15 to 20% of rubber and filtered to eliminate any impurity like bark or leaves present in it. Then natural rubber is coagulated to soft white mass by addition of water /acetic acid or formic acid. The coagulated white mass is washed. The coagulum is treated as below: (a) repe rubber: The coagulum is allowed to drain for about 2 hours. It is then passed through a creping machine and the spongy coagulum is converted into a sheet, dried in air for 5 to 10 days at about 50 o. Theses sheets posses an uneven rough surface resembling a crepe paper. (b) Smoked rubber: oagulation is carried out in long rectangular tanks fitted with metal plates. Diluted latex is poured into these tanks to which dilute acetic acid or formic acid is added and the mixture is stirred thoroughly. The tanks are kept undisturbed for about 16 hrs. After inserting the partition plates into the grooves, the coagulum forms into tough slabs between the plates. The slabs are Engineering hemistry Page 97
UNIT IV IG POLYMERS 2015-16 passed through a series of rollers, so as to give ribbed pattern to the final rubber sheet. The sheets are then hung for about 4 days in a smoke chamber, at a temperature between 40-50 o. The rubber thus obtained is amber in coloured and translucent. 9.2. Gutta-Percha This is another type of natural rubber obtained from the mature leaves of dichopsis gutta and palgum gutta trees. The mature leaves are ground carefully; treated with water at about 70 o for half an hour and then poured into cold water, when gutta perch floats on water surface and is removed by extraction with l 4. After the evaporation of the solvent, it is extruded in a sheet form by passing between two rollers. Gutta percha Structure of gutta percha Properties a. Gutta-percha is tough and horny at room temperature but turns soft at about 100 o. b. It is soluble in chlorinated and aromatic hydrocarbons, but not in aliphatic hydrocarbons. c. Gutta percha is used in the manufacture of submarine cables, golf ball covers, tissue for adhesive and surgical purposed. Engineering applications 1) Dentists use it to make temporary fillings. 2) It is used in conjunction with Balata resin, in conveyor belts. 9.3. Draw backs of natural rubber 1) It is soft at high temperature, brittle at low temperatures, weak and has poor tensile strength. 2) It has a high water absorption capacity, swells in water. 3) It dissolves in mineral oils, acids, bases and non-polar organic solvents like benzene. 4) It is attacked by oxidizing agents including atmospheric oxygen and becomes sticky. 5) It undergoes permanent deformation when stretched. These properties make rubber limited in use and compounding of rubber solves the problems. 10. Vulcanization Rubber is compounded with some chemicals like sulphur, hydrogen sulphide, benzoyl chloride, zinc oxide etc., to improve the properties of rubber. The process is called vulcanization, which makes rubber Engineering hemistry Page 98
UNIT IV IG POLYMERS 2015-16 stable and more useful. The most important vulcanizer is sulphur. When rubber is heated with sulphur and lead oxide at temperature of 100-140 o, sulphur combines chemically at the double bonds of the different chains of rubber and produces three dimensional crossed linked rubber, which over comes all the drawbacks of natural rubber. This vulcanized rubber does not melt on heating. This is the fundamental difference between a thermoplastic and rubber. The extent of stiffness of vulcanized rubber depends on the amount of sulphur added. For example, a tyre rubber may contain 3-5 % of sulphur, but a battery case rubber may contain as much as 30% sulphur. Vulcanization provides cross linking of sulphur atoms between the adjacent chains of rubber. The reaction is: 3 3 2 2 + 2 2 Vulcanization 2 2 2 2 ( + Sulfur) 3 3 Raw or unvulcanized rubber springs 3 3 2 2 2 2 S S S S Sulphur cross link 2 2 2 2 3 3 Vulcanization of raw rubber with sulphur as vulcanizing agent The temperature used is 100 to 140 o. The curing time may vary and over curing temperature decreases stretch and tensile strength, under the curing makes it too soft. So proper curing is required. The amount of sulphur used for ordinary soft rubber is 1 to 5% where as for hard rubber it is 40 to 45% of the rubber. The other vulcanizing agents used include Se, Te, benzoyl chloride, tri nitro benzene, alkyl phenol sulphides, 2 S, MgO, benzoyl peroxide etc. 10.1. Advantages of vulcanization Vulcanization transforms the weak, thermoplastic rubber into a strong and tough rubber. 1. The working temperature range is 10 o to 100 o. 2. The tensile strength increases. (2000kg/cm 3 ) 3. The water absorptivity decreases. Engineering hemistry Page 99
UNIT IV IG POLYMERS 2015-16 4. The article made from vulcanized rubber returns to the original shape when the deforming load is removed, i.e., the resilience power is increased. 5. The vulcanized rubber becomes resistant to organic solvents like l 4, benzene, fats and oils; however it swells in these solvents 6. It becomes resistant to abrasion, ageing and reactivity with oxygen & ozone. 7. It becomes better electrical insulator. 8. It can be easily manipulated into desired shape. 10.2. The other ingredients in the compounding of rubber 1) Accelerators: These are meant for catalyzing the vulcanization process, thus reducing the time required for vulcanization and maintain the vulcanization temperature. The inorganic accelerators include lime, magnesia, litharge and white lead, where as the organic accelerators are complex organic compounds such as aldehydes and amines. Sometimes, ZnO can acts as an accelerator activator. 2) Antioxidants: These substances retard the deterioration of rubber by light and air. These are complex organic amines like phenyl naphthyl amine, phenolic substance and phosphates. 3) Reinforcing agents: These are usually added to give strength, rigidity and toughness to the rubber and may form as much as 35% of the rubber. The commonly used reinforcing agents are carbon black, ZnO, Mgo 3, BaSO 4, ao 3 and some clays. 4) Fillers: The function of the fillers is to alter the physical properties of the mix to achieve simplification of the subsequent manufacturing operations, or to lower the cost of the product. 5) Plasticizers (or) softeners: These are added to impart great tenacity and adhesion to the rubber. The most commonly used plasticizers are vegetable oils, waxes, stearic acid, rosin etc. 6) oloring agents: These are added to impart desired colour to the rubber. TiO 2, lithophane - White Ferric oxide - Red Lead chromate - Yellow arbon black - Black hromium trioxide - Green Ultra marine - Blue 6) Miscellaneous agents: These include baking soda for sponge rubber, abrasives (eg: silica and pumice), 10.3. Engineering applications of rubber: The major application of rubber is in making tyres and tubes. It is also used in making belts for transport, material handling, tank inner lining in chemical plants where corrosive materials are stored. Rubber sandwiches are used in machine parts as gaskets to reduce vibrations. Foamed rubber is used in making cushions, mattresses and paddings. Engineering hemistry Page 100
UNIT IV IG POLYMERS 2015-16 11. Synthetic rubber The natural rubber sources are not sufficient and could not supplement the needs of automobile industry. An attempt was made to synthesize rubber, but rubber like materials were synthesized to supplement the needs of various industries. These materials synthesized by various processes are called elastomers. The artificially prepared polymer, which has elastomeric property, is known as synthetic rubber. There are several types of synthetic rubbers available and used on commercial grade. 11.1. SBR (Styrene Butadiene Rubber) or BUNA -S It is a copolymer of about 75% butadiene and 25% styrene. ence it is called as styrene rubber. Preparation It is produced by the copolymerization of butadiene, 2 = = 2 (about 25% by weight) and styrene, 6 5 = 2 (75% by weight), in presence of sodium as catalyst. n + n Butadiene Styrene opolymeri zation x n Properties 1) It has excellent abrasion resistance and high load bearing capacity. 2) A reinforcing filler (carbon black) is essential to achieve good physical properties. 3) It is a good electrical insulator Uses: It is used for lighter duty tyres, hose pipes, belts, moulded goods, unvulcanized sheet, gum, floorings, rubber shoe soles and electrical insulation cables, chemical plant inner linings etc. 11.2. BUNA-N or Nitrile Rubber (NBR) Nitrile rubber is the copolymer of butadiene and acrylonitrile. Bu stands for Butadiene, N- stands for acrylonitrile. m + n N Butadiene Acrylonitrile Polymerization m Polybutadine-co-acrylonitrile (Nitrile rubber) N n Engineering hemistry Page 101
UNIT IV IG POLYMERS 2015-16 Properties 1) Because of the presence of N group in the structure BUNA-N possess excellent resistance to heat, sunlight, oils, acids and salts and less resistant to alkalis than natural rubber. 2) It is a strong and tough polymer with light weight 3) BUNA-N is also vulcanized with sulphur Engineering Applications 1) It is used for making conveyor belts, aircraft components. 2) BUNA-N is extensively used for fuel tanks, gasoline hoses, creamery equipment, and automobile parts. 11.3. Polyurethane foam (PUF) Preparation: Polyurethanes is produced by the reaction of polyalcohols with di-isocyanates. O O O O n O O + N N Polymerization O O N N 2 2 2 2 n Ethylene glycol Ethylene diisocyante Polyurethane rubber (or isocyanate rubber) Properties 1. It has high strength, good resistance to ozone and aromatic hydrocarbons and weather proof. 2. It is highly resistant to oxidation, because of the saturated character. It have good resistance to many organic solvents. Engineering Applications 1. It used for surface coatings, manufacture of foams & spandex fibres. 2. PU flexible foams are employed as furniture material, insulation & crash pads. 3. It is used for insulating wires, the PU coated wires can be soldered directly. 11.4. Polysulphide rubber (or) Thiokol rubber (GR-P) This is synthesized by the copolymerization of sodium polysulphide (Na 2 S 4 ) and ethylene dichloride and during the reaction Nal gets eliminated. n l l S S 2 Na S S Na 1,2 Dichloroethane Sodium polysulphide S S S S 2 Thiokol rubber ( or thiokol) Engineering hemistry Page 102
UNIT IV IG POLYMERS 2015-16 Properties 1. The properties of the material depend upon the length of aliphatic group and number of sulphur atoms. It possess strength and impermeability to gases and low abrasion resistance. 2. Thiokol is resistant to swelling, oils, solvents and fuels. 3. Thiokol is inert to fuels, lubricating oils, gasoline and kerosene. Uses 1. It is used for coating fabrics, for making life rafts and jackets. 2. It is used for making gaskets, diaphragms and seals in contact with solvent and for printing rolls. 3. It is used for lining hoses for gasoline and other transport pipes 4. Liquid Thiokol can be used to make tough solvent resistant temperature liquid compounds which are used as liners for aircraft. 12. FIBRE RIENFORED PLASTIS Fibre Reinforced plastic (FRP) is one of a composite material. An FRP composite is defined as a polymer that is reinforced with a fibre. A composite is an artificially prepared multiphase material.. The primary function of fibre reinforcement is to carry load along the length of the fibre and to provide the strength and stiffness. omposite materials consists of two phases, one is called the matrix which is continuous and surrounds the other phase called the dispersed phase (reinforcement). FRP is produced by reinforcing a plastic matrix with a high strength fibre material such as glass, graphite, alumina, carbon, boron, beryllium and aromatic polyamides. Glass fibre is most widely used reinforced fibre, because of its durability, acid/water/fire proof nature of glass. The polymer is usually an epoxy, vinyl ester or polyester thermosetting plastic. The composite materials are prepared by binding two or more homogeneous materials with different material properties to derive a final product with certain desired material and mechanical properties. 12.1. omposite omponents i) Fibres: The composite s properties are mainly influenced by the choice of fibers. These have generally higher stress capacity and linearly elastic until failure. In civil engineering materials, three types of fibres viz. carbon, glass and aramid fibres are used. They have different properties. ii) Matrix: Matrix should transfer the forces between the fibers and protect the fibers from the environment. ommonly used matrices are thermo-sets viz. vinyl ester or epoxy. Epoxys are employed mostly as they have good strength, bond, creep properties, chemical resistances and low cost. 12.2. Methods for producing FRP: The Fibre reinforced plastic are produced by suitably bonding a fiber material with a resin matrix and curing the same under pressure and heat. The common resin Engineering hemistry Page 103
UNIT IV IG POLYMERS 2015-16 matrices used in FRP are polyesters, epoxy, phenolics, silicones, melamine, vinyl derivatives and polyamides. The following common methods are employed as processing techniques for producing FRP. a) Matched metal die moulding: This is the most efficient and economical method for mass production of high strength parts. The parts are press moulded in matched male and female moulds at a pressure of 200-300 psi and at a temperature of 235 260 o. The upper mould containing the resin and reinforced fibres is pressed on to the lower mould. b) Injection moulding : A mix of short fibres and resin is forced by a screw or plunger through an orifice into the heated cavity of a closed matched metal mould and allowed to curve. This is suitable for reinforced thermoplastics. c) and lay- up: In this method, the reinforcing mat or fabric is cut to fit, laid in the female mould and saturated with resin by hand, using a brush, roller or a spray gun. Layers are built up to their desired thickness and then the laminate is cured to render it hard, generally at room temperature. This is the simplest method for thermosetting composites. d) Spray up: This method is well suited for complex thermosetting moulds and its portable equipment is amenable for onsite fabrication and repair. Short lengths of reinforcement and resin are projected by a specially designed spray gun so that they are deposited simultaneously on the surface of the mould. uring is done with a catalyst in the resin at room temperature. e) ontinuous lamination: This is the most economical method of producing flat and corrugated panels, glazings etc. In this method, reinforcing mats or fabrics are impregnated with resin, run through and resin content. They are then cured in a heating chamber. f) entrifugal casting: In this method, chopped fibres and resins are placed inside a mandrel and are uniformly distributed as the mandrel is rotated inside an oven. This method is suitable for providing round, oval, tapered or rectangular parts. g) Pultrusion: In this method, continuous fibre strands combined with mat or woven fibres for crossstrength, are impregnated with resin and pulled through long heated steel die. The die shapes the product and controls the resin content. This method is suitable for providing shapes with high unidirectional strength. h) Filament windings: In this method, continuous fibre strands are wound on a suitably shaped mandrel or core and positioned in a predetermined pattern. The strands may be pre-impregnated or the resin may be applied during or after winding. Final curing is done by heating. 12.3. Types of Fiber Reinforced Plastics (a) Glass Fiber Reinforced Plastics (GFRP): Glass fibres are basically made by mixing silica sand, limestone, folic acid and other minor ingredients. The mix is heated up to 1260 o and allowed Engineering hemistry Page 104