Module 1 Polymer EC (Manipal University) PDF

B.TECH FIRST YEAR ACADEMIC YEAR: 2024-2025 Course name: Engineering Chemistry Course code : CHY1001 lecture series no : 1 Credits : 4 Mode of delivery : Hybrid Faculty : Email-id : Text Books Engineering Chemistry...

B.TECH FIRST YEAR ACADEMIC YEAR: 2024-2025 Course name: Engineering Chemistry Course code : CHY1001 lecture series no : 1 Credits : 4 Mode of delivery : Hybrid Faculty : Email-id : Text Books Engineering Chemistry, Jain P.C.; Jain M., Dhanpat Rai and Sons, Delhi, 17th Edn. 2015. Engineering chemistry, Wiley India Pvt. Ltd., 2nd Edn. 2014. Reference Books Chemistry in Engineering and Technology, Kuriacose J.C.; Raja R. J., Vol. I/II McGraw Hill Education, 2001. Materials Science for Engineering Students, Fischer T., Academic Press, London, 2009. Fuel Science &Technology, James G Speight; Marcel Dekker, New York, 1990 Syllabus Module: Advanced materials and polymers: Advanced materials and polymers: Introduction, Definition, classification of polymers – based on origin, thermal behaviour, Polymerization reactions and applications, Tacticity. Functionality, Degree of polymerization, Co-polymerization – alternating, random, block and graft polymers. Mechanism of free radical polymerization and ionic polymerization. Mechanism of coordination polymerization, Condensation polymerization reactions, Glass transition temperature & factors affecting it. Molecular weight of polymers, Number average and weight average molecular weights, Numerical problems. Preparation, properties and applications of Polythene (LDPE and HDPE), Nylon(6:6, 6, 6:10, 11), PF resins and Polyester. Natural rubber, Processing of Natural Rubber, Vulcanization, Compounding of rubber; Synthetic Rubber: Buna-N, Buna-S. Module – biomaterials, Composites, Advanced Materials andand their properties Polymers applications.. Module 1: Advanced materials and polymers: Introduction, Definition, classification of polymers – based on origin, thermal behaviour, Polymerization reactions and applications, Tacticity. Functionality, Degree of polymerization, Co-polymerization – alternating, random, block and graft polymers Mechanism of free radical polymerization and ionic polymerization. Mechanism of coordination polymerization, Glass transition temperature & factors affecting it. Molecular weight of polymers, Number average and weight average molecular weights, Numerical problems. Preparation, properties and applications of Polythene (LDPE and HDPE), Nylon (6:6, 6, 6:10, 11), PF resins and Polyester. Natural rubber, Processing of Natural Rubber, Vulcanization, Compounding of rubber; Synthetic Rubber: Buna-N, Buna-S. Composites and biomaterials: properties and applications  Development of civilization Stone age → Bronze age → Iron age → Polymer age A world of polymer  What are polymers? A polymer is a high molar mass molecular compound made up of many repeating chemical units (Monomers). Imagine the necklace you get when you string together lots of beads-that’s a polymer. The word ‘polymer’ comes from the Greek words poly (meaning ‘many’) and meros (meaning ‘parts’). The number of repeating units in chain formed in a polymer, is known as the degree of polymerization. It may be hundreds/thousands/ or many more  Few Queries??? What are some of the polymers that you encounter every day? Sandwich bags, carpets, nylon stockings, stackable chains, milk cartons, etc. Why do different polymers have different properties? They have different chemical compositions (different monomer units), different structures, different ways of being fabricated, etc. Imagine samples of LDPE (sandwich bag, squeeze bottle) and HDPE (milk jug, grocery bag). What are some of the differences in the physical properties of these substances? LDPE – more transparent, flexible, waxy HDPE – more opaque, rigid, non-waxy DEFINITION: The word polymer is derived from the two greek words poly and mers many parts or units e.g. mer mer mer H H H H H H H H H H H H H H H H H H C C C C C C C C C C C C C C C C C C H H H H H H H Cl H Cl H Cl H CH 3 H CH 3 H CH 3 Polyethylene (PE) Polyvinyl chloride (PVC) Polypropylene (PP) Polymers are macro molecules formed by linking smaller molecules repeatedly, called monomers. Examples: Polyethylene is formed by linking a large number of ethylene molecules H H H H n C C Polymerisation C C H H n H H Ethylene polyethylene polystyrene is formed by linking styrene molecules H H H H Polymerisation n C C C C H n H styrene polystyrene  Polymerization Polymerization is the process in which monomers (small, single molecules) join together to form polymer (a large molecule).  The process by which  This process can be happen in monomers are transformed into various way such as adding polymers is called monomer one by one or by polymerization. combining them all at once. e.g. Formation of polyethylene, a widely used plastic, made by polymerizing ethylene monomers. Degree of Polymerization (DP): is the number of repeating monomer units contained in a polymer chain. To calculate degree of polymerization, you can use the formula DP = Let us consider a polymer made from monomer ethylene monomer (C2H4). a. Molecular weight of ethylene (C2H4) molecules is = 28 g/mol b. Molecular weight of polyethylene is = 560,000 g/mol DP = DP = = 20,000 Drawing polymers – shorthand formulae Polymers contain thousands of molecules, so how can their structures be easily drawn? Part of the polymer molecule can be drawn: A better way is to show a shorthand formula: The ‘n’ means that the polymer contains a very large number of the repeating unit shown in the brackets. Classification of Polymers: 1. Based on origin: A. Natural Polymers These occur in nature in plants and animals and are very essential for life. For e.g., proteins, Starch, cellulose, silk, and natural rubber are some natural polymers. B. Synthetic Polymers Synthetic polymers are man-made polymers which is synthesized in laboratory, synthetic rubbers, Polyethylene, Polystyrene, polyvinylchloride, nylon, polyester, Teflon etc. 2. Based on monomer composition Homopolymer is a polymer made up of only one type of monomer ( CH2 CH )n ( CF2 CF2 )n ( CH2 CH2 )n Cl Teflon Polyethylene PVC Copolymer is a polymer made up of two or more monomers ( CH CH2 CH2 CH CH CH2 )n Styrene-butadiene rubber Copolymers two or more monomers polymerized together A B Alternating A and B alternate in polymer chain Block large blocks of A units alternate with large blocks of B units Random A and B randomly positioned along chain Graft chains of B units grafted onto A backbone 3. based on polymerization reactions: Based on the mode of polymerization, polymers are classified as: 1. Addition polymers 2. condensation polymers Addition polymers Addition polymers are formed when monomer units are separately added to form long chains without elimination of any by-product molecules. These polymers are formed by reactions between monomer molecules possessing multiple bonds. Example: Ethylene undergoes polymerization to form polythene. The empirical formula of the monomer and polymer are the same. Other examples: Polypropylene is an addition polymer of propylene. Styrene-butadiene rubber is an addition polymer formed by addition reactions between butadiene and styrene. Condensation polymers Condensation polymers are formed when the monomers containing active functional groups (generally two), which react together with the elimination of a small molecule like water, ammonia, alcohol, carbon dioxide etc. Examples: Nylon-66, polyester, Bakelite etc.Nylon-66 is formed by condensation between hexamethylene diamine and adipic acid as shown below: 4. Based on thermal behavior The response of a polymer to mechanical forces at elevated temperature is related to its dominant molecular structure. Thermoplastics and Thermosets are the 2 categories. 1. A thermoplastic is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently. Thus thermoplastics can be moulded on heating. Most thermoplastics are high-molecular-weight polymers whose chains associate through weak Van der Waals forces (polyethylene); stronger dipole-dipole interactions and hydrogen bonding (nylon). These polymers have no cross-linking between chains. Examples: Polyethylene, polystyrene etc. 2. Thermosetting polymers Thermosetting polymers made from relatively low molecular mass semi-fluid polymers which when heated in a mould forms an insoluble hard mass which is infusible. This is due to extensive crosslinks between the different polymer chains forming three dimensional network of bonds. Example: Bakelite and melamine 19  weak intermolecular forces – these let the chains slide past each other Plastics made of these polymers are stretchy and have a low melting point. They are called thermoplastic polymer.  strong intermolecular forces (cross-links) – these hold the chains firmly in place Plastics made of these polymers cannot be stretched, are rigid and have a high melting point. They are called thermosetting plastics (or ‘thermosets’). 5. Classification of polymers based on applications: 1. Elastomers: a natural or synthetic polymer having elastic properties, e.g. rubber. 2. Resins: Resin is a natural or synthetic compound that begins in a highly viscous state and hardens with treatment. Eg. PF resins , epoxy resins 3. Fibers: a thread like, can be easily woven, intermolecular forces may be H-bod or dipole –dipole interaction. Eg. Nylon 4. Plastics : Organic materials of high molecular weight, which can be moulded into any desired form, when subjected to heat and pressure in the presence of a catalyst eg. Polyethylene, polystyrene, pvc etc. Tacticity : the orientation of monomeric units in a polymer molecule It affects the physical properties of polymers. R groups on same side of chain Isotactic R groups alternate from side to side Syndiotactic R groups disposed at random Atactic Functionality The number of bonding sites in a monomer is referred as its functionality Bifunctional: eg. alkene Trifunctional: eg. Monomethylsilicon trichloride (MeSiCl3) Polyfunctional: eg. Silicon tetrachloride (SiCl4)  Functionality the number of reactive sites or bonding sites Some mono functional hydrocarbons Alcohols Methyl alcohol Ethers Dimethyl Ether Aromatic hydrocarbons Phenol Some bi functional hydrocarbons Terephthalic acid adipic acid (hexanedioic acid) ethylene glycol 1,6-hexanediamine Ethylene is considered to be bifunctional H H H H C C C C H H H H H H C O C O H H General method of polymerization Two major methods generally used for preparing polymers: 1. Addition polymerization 2. Condensation polymerization Addition Polymerization It is a reaction that yields a product, which is an exact multiple of the original monomeric molecule. Such a monomeric molecule usually contains one or more double bonds, which by intermolecular rearrangement may make the molecule bifunctional. The addition polymerization reaction must be instigated by the application of heat, light, pressure or a catalyst for breaking down the double covalent bonds of monomers. Condensation polymerization May be defined as a reaction occurring between simple polar-group-containing monomers with the formation of polymer and elimination of small molecules like water, HCl etc. Examples: Nylon-66, polyester, Bakelite etc.Nylon-66 is formed by condensation between hexamethylene diamine and adipic acid as shown below: Mechanism of addition polymerization: 1. Free radical polymerization 2. Cationic polymerization 3. Anionic polymerization 4. Coordination polymerization or Ziegler-Natta polymerization  Free redical polymerization mechanism: (i) Initiation step is considered to involve two reactions. The first step is the production of free radicals by homolytic dissociation of an initiator to yield pair of radicals.(benzoylperoxide, tert-butylperoxide, AIBN) I 2R (Initiator) Free radicals Azobisisobutyronitrile The second part of the initiation steps involves the reaction of this radical with first monomer unit (M) to produce chain initiating species O R + M M1 O monomer O O (ii) Propagation step consists of the growth of M1 free radical by successive reaction of large numbers of monomers molecules M1 + M M2 monomer M2 + M M3 monomer M3 + M M4 monomer In general Mn + M Mn+1 monomer Termination steps a)Termination by coupling b) Termination by disproportionation Termination by coupling Termination by disproportionation  IONIC CHAIN POLYMERIZATION Ionic polymerization is more complex than free-radical polymerization Whereas free radical polymerization is non-specific, the type of ionic polymerization procedure and catalysts depend on the nature of the substituent (R) on the vinyl (ethenyl) monomer. Cationic initiation is therefore usually limited to the polymerization of monomers where the R group is electron-donating This helps stabilize the delocalization of the positive charge through the p orbitals of the double bond Anionic initiation, requires the R group to be electron withdrawing in order to promote the formation of a stable carbanion Using catalyst, not initiator Highest reaction rate Termination step is just disproportionation Environment must be pure Ionic Polymerization A). Anionic Polymerization Involves the polymerization of monomers that have strong electron-withdrawing groups, eg, acrylonitrile, vinyl chloride, methyl methacrylate, etc. typical catalyst include KNH2, n-BuLi, and Grignard reagents such as alkyl magnesium bromides If the monomer has only a weak electron-withdrawing group then a strong base initiator is required, eg, butyllithium; for strong electron-withdrawing groups only a weak base initiator is required, eg, a Grignard reagent. Steps of polymerization i) Initiation: KNH2 initiates the reaction and carbanion is formed ii) Propagation iii) Termination  B) Cationic Polymerization Involves the polymerization of monomers that have strong electron-donating groups, eg, propylene etc. typical initiators (catalysts) are compounds with electron acceptor properties eg. AlCl3, SnCl4, TiCl4, and BF3 Steps of polymerization i) Initiation : AlCl3 initiates the reaction and carbocation is formed ii) Propagation iii) Termination Coordination polymerization (Ziegler-Natta catalysts) Coordination polymerisation is a form of addition polymerization in which monomer adds to a growing macromolecule through an organometallic active center. The development of this polymerization technique started in the 1950s with heterogeneous Ziegler-Natta catalysts based on titanium tetrachloride and an aluminium co-catalyst (TiCl4 + R3Al) Coordination polymerization has a great impact on the physical properties of vinyl polymers such as polyethylene and polypropylene compared to the same polymers prepared by other techniques such as free radical polymerization. The polymers tend to be linear and not branched and have much higher molar mass. Coordination type polymers are also stereoregular and can be isotactic or syndiotactic instead of just atactic. This tacticity introduces crystallinity in otherwise amorphous polymers. From these differences in polymerization type the distinction originates between low-density polyethylene (LDPE), high-density polyethylene (HDPE). Mechanism: Coordination polymerization Initiation Propagation Termination  Cl C H 2 5 Cl C2H5 TiCl4 + Al(C2H5)3 Ti Al Cl Cl C2H5 C2H5 CH2 H Cl C H Cl C2 2 5 Cl C2H5 Cl C2H5 Ti Al Ti Al Cl Cl C2H5 Cl Cl C2H5 H2C CH2 CH2 (CH2-CH2)n C2H5 H Cl C2 Cl H Cl C2H5 Cl C2H5 Ti Al Ti Al Cl Cl Cl C2H5 Cl C2H5 H2C CH (CH2-CH2)n C2H5  CONDENSATION POLYMERIZATION Minumum two functional groups required Usually linear Molecular weight increases slowly at low conversion High extents of reaction are required to obtain high chain length Step-growth polymerization refers to a type of polymerization mechanism in which bi-functional or multifunctional monomers react to form first dimers, then trimers, longer oligomers and eventually long chain polymers. Many naturally occurring and some synthetic polymers are produced by step-growth polymerization, e.g. polyesters polyamides, polyurethanes, etc. Due to the nature of the polymerization mechanism, a high extent of reaction is required to achieve high molecular weight. The easiest way to visualize the mechanism of a step-growth polymerization is a group of people reaching out to hold their hands to form a human chain — each person has two hands (= reactive sites). There also is the possibility to have more than two reactive sites on a monomer: In this case branched polymers are produced. A generic representation of a step-growth polymerization. (Single white dots represent monomers and black chains represent oligomers and polymers)  Differences between step-growth polymerization and chain- growth polymerization Step-growth polymerization Chain-growth polymerization Growth throughout matrix Growth by addition of monomer only at one end of chain Rapid loss of monomer early in Some monomer remains even at the reaction long reaction times Same mechanism throughout Different mechanisms operate at different stages of reaction (i.e. Initiation, propagation and termination) Average molecular weight Molar mass of backbone chain increases slowly at low increases rapidly at early stage conversion and high extents of and remains approximately the reaction are required to obtain same throughout the high chain length polymerization Ends remain active (no Chains not active after termination) termination No initiator necessary Initiator required Glass transition temperature: (Tg) At low temperatures, all amorphous polymers are stiff and glassy. On Warming, polymers soften in a characteristic temperature range known as the glass-rubber transition region. The glass transition temperature (Tg), is the temperature at which the amorphous phase of the polymer is converted between rubbery and glassy states. Tg constitutes the most important mechanical property for all polymers. In fact, upon synthesis of a new polymer, the glass transition temperature is among the first properties measured. So, the glass transition temperature can be defined as the temperature below which an amorphous polymer is brittle, hard and glassy and above the temperature it becomes flexible, soft and rubbery. Glassy state rubber state (Hard brittle plastic) (soft flexible) In the glassy state of the polymer, there is neither molecular motion nor segmental motion. To become more quantitative about the characterization of the liquid-glass transition phenomenon and Tg, we note that on cooling an amorphous material from the liquid state, there is no abrupt change in volume such as occurs in the case of cooling of a crystalline material through its freezing point, Tf. Instead, at the glass transition temperature, Tg, there is a change in slope of the curve of specific volume vs. temperature, moving from a low value in the glassy state to a higher value in the rubbery state over a range of temperatures. This comparison between a crystalline material (1) and an amorphous material (2) is illustrated in the figure below. Note that the intersections of the two straight line segments of curve (2) defines the quantity T g. Factors affecting Tg the bulky groups on chain, increases the Tg of the polymer Polyethylene Tg = -110 0C Polypropylene Tg = R Polystyrene Tg = 100 0C The presence of H-bonds between the polymer molecules increases the Tg e.g., the Tg of nylon 6,6 (Tg = 50 0C) is higher than PE (Tg = -110 0C) H  O H  O H H nylon 6,6   |  || |    || polyethylene  N  C   N  C  C   N  C   C C     | |  | |  | H H H H  6 H  H  4 H + + + + + + Hydrogen Van der Waals + +  + +  bonds + bonds + H H H  O H  O   || |  ||  C C |      N  C   N  C  C   N  C  H H     | |  | |  | H H  6 H  H  4 H   With H-bonds vs vdW bonds, nylon is expected to have (and does) higher Tg. The presence of a plasticizer reduces the Tg of a polymer The plasticizers are usually dialkyl phthalate esters, such as dibutyl phthalate, a high boiling liquid. O C O CH2CH2CH2CH3 O CH2CH2CH2CH3 C O dibutyl phthalate The plasticizer separates the individual polymer chains from one another. It acts as a lubricant which reduces the attractions between the polymer chains. The Tg of a polymer is influenced by its molecular weight With increase in molecular mass, the Tg increases e.g., PE (low Mw) -110 0C PE (high Mw) - 90 0C Thus, 1) The presence of bulky side groups increase Tg. 2) Polar side atoms or groups of atoms increase Tg. 3) Double chain bonds and aromatic chain groups which tend to stiffen the molecular backbone increases Tg. 4) Increase in the molecular weight also tends to raise the glass transition temperature. 5) A small amount of branching will tend to lower Tg; on the other hand , a high density of branches reduces chain mobility, and elevates glass transition temperature.  Which polymer more likely to crystallize? Can it be decided? alternating random Poly(styrene-Ethylene) poly(styrene - ethylene) copolymer Copolymer Alternating co-polymer more likely to crystallize than random ones, as they are always more easily crystallized as the chains can align more easily.  Molecular Weight of Polymers The molecular weight of a polymer can only be viewed statistically and expressed as some average of the Mol. Wt.s contributed by the individual molecules that make the sample the molecular weight of a polymer can be expressed by two most and experimentally verifiable methods of averaging (i) Number – average (ii) weight – average Number average molecular mass of a polymer can be defined as the total mass of all the molecules in a polymer sample divided by the total number of molecules present The molecular mass of a polymer can use either number fractions or the weight fractions of the molecules present in the polymer In computing the number average molecular mass of a polymer, we consider the number fractions Polymers: Molecular Weight Ni: no. of molecules with degree of polymerization of i Mi: molecular weight of i number average, Mn weight average, Mw The dispersity index, or formerly polydispersity index (PDI)= Mw Mn The number-average molecular mass (Mn) is determined by the measurement of colligative properties such as lowering of vapour pressure osmotic pressure depression in freezing point elevation in boiling point The weight-average molecular mass (Mw) is determined by light scattering ultra-centrifugal techniques  Numerical Consider a polymer sample containing 50 molecules of molecular weight 10000, 35 molecules of molecular weight 12000 and 15 molecules of molecular weight 14000. Determine Number Average Molecular Weight and Weight Average Molecular Weight. If the polymer is polypropylene, what will be the degree of polymerization?  (50x10000) + (35x 12000) + (15x14000) Mn = (50+35+15) Ans. =11,300 (Number Average Molecular Weight ) 50(10000) 2 + 35(12000) 2 + 15(14000) 2 Mw = (50x10000)+(35x12000)+(15x14000) Ans. =11,486.7 (Weight Average Molecular Weight ) Mn DP= Molecular weight of monomer Ans. = 269 (DP) olytetrafluoroethylene (TEFLON) or FLUON (PTFE) F F F F F F benzoyl peroxide n C C n C C C C F F F F F F n Tetrafluoro ethene Polytetrafluoroethene (PTFE) Properties of TEFLON: They are extreme tough, high softening point, exceptionally high chemical-resistances towards all chemicals, high density, waxy touch. It can be punched, machined and drilled Uses As an insulating materials (for motors, transformers, cables, wires, fitting) For making gaskets, packing, pump parts, tank, chemical-carrying pipe lines, tubing For coating In non-lubricating bearing and non-sticking stop-cocks.  Nylon:A man-made polymer Nylon is used in clothes, shoes, jackets, belts, Tires, ropes and various accessories. Polyamides are synthetic polymers, which have recurring amide groups. Nylons, used mostly for making fibers, belong to polyamides. Nylon 6:6 Nylon 6 Nylon 11 Nylon 6:10 Nylon 6:6 Nylon 6:6 is obtained by the polymerization of adipic acid with hexamethylene diamine. H H O O N CH2 N C CH2 C n H 6 H OH 4 OH Hexamethylene Adipic acid diamine Polymerization O O N CH2 N C CH2 C + 2n H2O H 6 H 4 n Nylon 6:6 Nylon 6: Nylon-6 is produced by the following two methods: 1. Self-condensation of 6-aminohexanoic acid (ε-aminocaproic acid) 2. Ring opening polymerization of a caprolactum: Heat Nylon-11 Nylon-11 is made by self condensation of 11-aminoundecanoic acid. Nylon 6:10 Nylon 6:10 is obtained by the polymerization of Decanedioyl dichloride with hexamethylene diamine. H H O O N CH2 N C CH2 C n H 6 H Cl 8 Cl Hexamethylene Decanedioyl dichloride diamine Polymerization O O N CH2 N C CH2 C + 2n HCl H 6 H 8 Nylon 6:10 n Properties of Nylon: 1. One of the very strong fibers. Extremely resistant to abrasion and flexing. 2. The specific gravity is 1.14 Very light, i.e., 80% of that of silk fibers, and 70% of that of cotton fibers. 3. Since nylon fibers absorb little water even though they are wetted., they dry fast and simple in laundering. 4. Excellent in elasticity and resistant to wrinkle. 5. Resistant to chemicals and oil. Non-attackable by sea water. 7. Non-attackable by molds and insects. Uses: 1. Nylon-6:6 is primarily used for fibers, which find use in making socks, under-garments, dresses, carpets etc. 2. Nylon -6 and nylon-11 are mainly used for moulding purposes for gears, bearings etc. Nylon bearing and gears work quietly without any lubrication. 3. They are also used for making filaments for ropes, bristles for tooth brushes and films, tyre-cords etc. Polyethylene (PE) Polyethylene (PE)is obtained by polymerization of ethylene. Properties: 1) Polyethylene is a rigid , waxy, white translucent, non polar material. 2) Polyethylene exhibits considerable chemical resistance to strong acids, alkalis and salt solutions at room temperature. 3) It is good insulator of electricity. 4) It is swollen and permeable to most oils and solvents. Low-density polyethylene (LDPE) Low-density polyethylene (LDPE) is a linear polymer with branching. It is manufactured under high pressure (1000-3000 atm) and in the temperature range of 80-350oC using initiators such as benzoyl peroxide or oxygen. Method: free radical polymerization. Properties of LDPE 1)LDPE is defined by a density range of 0.910–0.925 g/cm3. 2) It is not reactive at room temperatures, except by strong oxidizing agents, and some solvents cause swelling. 3)It can withstand temperatures of 80 °C continuously and 95 °C for a short time. 4)Made in translucent or opaque variations, it is quite flexible, and tough but breakable. 5) LDPE has more branching than HDPE. 6)Since its molecules are less tightly packed and less crystalline because of the side branches, its density is lower. LDPE contains the chemical elements carbon and hydrogen. Uses of LDPE LDPE is widely used for manufacturing various containers, dispensing bottles, wash bottles, tubing, plastic bags for computer components, and various molded laboratory equipment. Its most common use is in plastic bags. Other products made from it include: 1)Trays and general purpose containers. 2)Corrosion-resistant work surfaces. 3)Parts that require flexibility, for which it serves very well 4)Juice and milk cartons are made of liquid packaging board, a laminate of paperboard and LDPE (as the water-proof inner and outer layer), and often with of a layer of aluminum foil (thus becoming aseptic packaging) 5) Parts of computer hardware, such as hard disk drives, screen cards, and optical disc drives High-density polyethylene (HDPE) High-density polyethylene (HDPE) is a linear polymer with little or no branching. It is manufactured under low pressure (1000-3000 atm) and below 100oC using Ziegler Natta catalyst (Et3Al and TiCl4) by coordination polymerization or using metal oxide catalyst by Phillips process. Properties of HDPE 1)HDPE is known for its large strength to density ratio. 2)The mass density of high-density polyethylene can range from 0.941 to 0.97 g/cm3. 3) Although the density of HDPE is only marginally higher than that of low-density polyethylene, HDPE has little branching, giving it stronger intermolecular forces and tensile strength than LDPE.The difference in strength exceeds the difference in density, giving HDPE a higher specific strength. 4) It is also harder and more opaque and can withstand somewhat higher temperatures (120 °C/ 248 °F for short periods, 110 °C /230 °F continuously). High-density polyethylene, unlike polypropylene, cannot withstand normally required autoclaving conditions. The lack of branching is ensured by an appropriate choice of catalyst (e.g., Ziegler-Natta catalysts) and reaction conditions. HDPE is resistant to many different solvents and has a wide variety of applications, including: 3-D printer filament Banners Bottle caps Chemical resistant piping systems Food storage containers Fuel tanks for vehicles Electrical and plumbing boxes Folding chairs and tables  P – F Resins These are formed by condensation polymerization and are thermosetting polymers There are two important commercial PF resins: Novolacs and Resoles Both novolacs and resoles are linear, low molecular weight, soluble and fusible prepolymers The nature of the product formed depends largely on the molar ratio of phenol to formaldehyde and also on the nature of the catalyst During moulding operations, these two undergo extensive branching leading to the formation of highly cross linked, insoluble, hard, rigid and infusible products The phenol ring has three potential reactive sites while the formaldehyde has two reactive sites The polycondensation reaction between these two are catalyzed by either acids or alkalies  Novolacs When P/F molar ratio is > 1 and the catalyst used is an acid, low mol. wt. polymers formed are called Novolacs The first step in the reaction is the addition of formaldehyde to phenol to form ortho or para methylol phenols OH H + C=O H Phenol (excess) H+ formaldehyde OH OH CH2OH and o-methylol phenol CH2OH p-methylol phenol These methylol phenols condense rapidly to form Novolacs OH OH CH2OH or o-methylol phenol CH2OH OH p-methylol phenol OH H2 H2 OH H2 H2 OH C C C C HO OH Novolacs These novolacs are linear and low mol. wt. polymers. They are soluble and fusible. About 5 – 6 phenol rings per molecule are linked through methylene bridges Since they contain no active methylol groups, they themselves do not undergo cross linking. However, when heated with formaldehyde or hexamine, they undergo extensive cross linking, resulting in the formation of infusible, insoluble, hard and rigid thermosetting product OH H2 H OH H H OH C 2 2 2 C C C OH HO Novolacs (prepolymer) Curing with Formaldehyde or hexamine Resoles When the molar ratio of P/F is < 1 and the catalyst used is a base, the polymer formed are called Resoles The first step in the reaction is the formation of mono, di and trimethylol phenols. They undergo condensation to form resoles The resoles in which phenols are linked through methylene bridges are soluble and fusible Since they contain alcoholic groups, further reaction during curing leads to cross linking, resulting in a network, infusible and insoluble product OH H + C=O H Phenol Formaldehyde OH-- (excess)  OH OH OH OH CH2OH + CH2OH HOH2C CH2OH + + CH2OH CH2OH CH2OH o-methylol phenol p-methylol di methylol tri methylol phenol phenol phenol Curing Properties of Bakelite: They are rigid, hard, scratch-resistant, infusible, water resistant, insoluble solid. Resistant to non-oxidizaing acids, salts and many organic solvents. They are attacked by alkalis They possess excellent electrical insulting character Uses For making electric insulators parts like switches, plugs, switch board, heater-handles etc. For making moulded articles like telephone parts, cabinets for radio and television. For impregnating fabrics, woods and paper As adhesives (binder) for grinding wheels. In paints and varnish As hydrogen exchanger resins in water softening  ELASTOMERS Elastomer is defined as a long chain polymer which under stress undergoes elongation by several times and regains its original shape when the stress is fully released Stretched Returned to randomization Natural rubber is obtained from the exudates of the rubber trees in the form of a milky colloidal solution called latex containing 25-45% of rubber dispersed in a watery medium with small amounts of protein and other materials. The latex is tapped by cutting through the bark of the tree and the liquid that comes out is collected in containers. The latex is processed either by i) coagulation by acid or by heat and processed further or ii) mixed with compounding materials and precipitated directly from the solution. Natural rubber is an addition polymer formed from the monomer called isoprene i.e., 2- methyl-1,3-butadiene The average D.P. (n) of rubber is around 5000 Addition between molecules of isoprene takes place by 1,4 addition and one double bond shifts between 2nd and 3rd positions Drawback of raw rubber  It is plastic in nature: It becomes soft at high temperature and is too brittle at low temperature. So it can be used in temperature range between 10 to 60˚C only  It is weak: tensile strength is only 200kg/cm2  It has large water absorption capacity  It is non-resistant to non-polar solvents like vegetable and mineral oil  It is attacked by oxidizing agent  It swells in organic solvent and gradually degrade  It has little durability Vulcanization Vulcanization is a process, through which elasticity of the rubbers increases and reduces plasticity by the formation of a crosslinked molecular network. Vulcanization is done by heating the rubber with sulfur or other vulcanizing agents under pressure Charles Goodyear (December 29, 1800 – July 1, 1860) was an American inventor who developed a process to vulcanize rubber in 1839—a method that he perfected while living and working in Springfield, Massachusetts in 1844, and for which he received patent number 3633 from the United States Patent Office. e.g., a tyre rubber may contain 3 to 5% sulphur, but a battery case rubber may contain as much as 30% sulphur H H H H H C H H H C H H H C H H H H H H H C C C C C C C C C C C C H H H H H H +S+ H H H H H H C C C C C C C C C C C C H H H H H H H H H H H H C H C H C H H H H  H H H C HH HH HH C H H C H H H H H H H C C C C C C C C C C C C H H H H H H S S H H H H H H C C C C C C C C C C C C H H H H H H H C HH H C HH H C HH H H H  Compounding of Rubber Compounding is mixing of the raw rubber (synthetic or natural) with other substances so as to impart the product specific properties suitable for particular job. (1) Softener and plasticizers are added to give the rubber greater tenacity and adhesion. For e.g. vegetable oil, waxes, stearic acid. (2) Vulcanizing agents: sulfur, sulfur monochloride, benzoyl chloride etc. (3) Accelerators: These materials drastically shorten the time required for vulcanization. 2-mercaptol, Benzothiazole etc. (4) Antioxidants: Natural rubbers has a tendency to perish due to the oxidation. For this reason antioxidants are required, such as complex amine, phosphites etc. (5) Reinforcing fillers: theses materials gives strength and rigidity to the rubber products. For e.g. CaCO3, ZnO etc. (6) Colouring agents  Styrene rubber (GR-S or Buna-S or SBR) Preparation This is produced by copolymerization of butadiene (about 75% by wt.) and styrene (about 25% by wt.) H2C CH n x H2C CH CH CH2 + n H2C CH CH CH2 H2C CH x n  Styrene-butadiene copolymer Styrene domains act as anchors or junctions Butadienes provide flexible linkages The desire to maximize the ways you can arrange the flexible links is what causes rubbers to return to given shapes  Buna-N( Nitrile rubber) Buna –N is copolymer of butadiene and acrylonitrile rubber. H H H H H H m C C C C +n C C H H H CN 1, 3-Butadiene Acrylonitrile Polymerization H H H H H H C C C C C C H H H CN m n Nitrile rubber or Buna-N Properties of Buna-N It possesses excellent resistance to heat, sunlight, oils, acids and salts, but less resistant to alkalis than natural rubber, because of presence of cyano (-CN) groups. As the proportion of acrylonitrile is increased, the resistance to acids, salts, oils, solvents increases but the low temperature resilience suffers. Vulcanized nitrile rubber is more resistant to heat and ageing than natural rubber. Uses: For making conveyer belts, high altitude aircraft components, tank lining, hoses, gaskets, printing rollers, adhesives, oil resistant foams and automobile parts. Properties: 1. PET can exist both in amorphous and semicrystalline form 2. It has high mechanical strength and dimension stability and is stable over temperature range of -40 oC to 100 oC. 3. It shows creep and abrasion resistance and good electrical insulating properties. 4. At room temperature, it is resistant to water, dilute acids, salts, aromatic and aliphatic hydrocarbons and alcohols. Applications: 5. It is used for making video and audio tapes. 6. It is commonly used in making clear bottles for foods and beverages. 7. It is used to make films for shrink packaging. 8. As an electrical insulator, it is used for making molds for electrical appliances. 9. The synthetic fibers can be woven to make fabrics, upholstery, artificial grass etc. 10. The fiber may be blended with wool and other natural fibers. 11. The films may be combined with paper or other polymeric films to form multilayer materials. 12. It can be used to produce injection molded articles, such as switches, valves etc.