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INTRODUCTION TO POLYMERS: Polymers are complex and giant molecules and are different from the low molecular weight compounds. These big molecules or 'macro-molecules' are made up of much smaller molecules called monomers which combine to form a big molecule, this process is called 'polymerization'. Polymerization is possible with molecules of the same or of different monomeric compounds. When molecules just add on to form the polymer, the process is called 'addition' polymerization but when molecules do not just add on but also undergo some reaction in forming the polymer, the process is called 'condensation' polymerization.
CLASSIFICATION OF POLYMERS:
Polymer is a generic name given to a vast number of materials of high molecular weight. These materials exist in countless forms and numbers because of very large number and types of atoms present in their molecules. Polymers can have different chemical structures, physical properties, mechanical behavior, thermal characteristics, etc., and can be classified as follows:
Natural and Synthetic Polymers :
Depending on the origin, polymers can be classified as natural or synthetic.
Those isolated from natural materials are called natural polymers, e.g., cotton, silk, wool and rubber.
Polymers synthesized from low molecular weight compounds are called synthetic polymers, e.g., polyethylene, PVC, nylon and Terylene.
Organic and Inorganic Polymers:
A polymer whose backbone chain is essentially made of carbon atoms is termed as an organic polymer. The majority of synthetic polymers are organic.
The molecules of inorganic polymers generally contains no carbon atom in their chain backbone, e.g., glass and silicone rubber.
Thermoplastic and Thermosetting Polymers:
Polymers which soften on heating and can be converted into any shape that they can retain on cooling are termed as thermoplastics. This process of heating, reshaping and retaining the same on cooling can be repeated several times, e.g., polyethylene, PVC, nylon and sealing wax.
Polymers which undergo some chemical change on heating and covert themselves into infusible mass which cannot be reshaped are known as thermosetting polymers, e.g., yolk of the egg.
Plastics, Elastomers, Fibers and Liquid Resins:
When a polymer is shaped into hard and tough utility articles by the application of heat and pressure, it is used as plastic, e.g., polystyrene, PVC and polymethyl methacrylate.
When vulcanized into rubbery products exhibiting good strength and elongation, polymers are used as elastomers, e.g., natural rubber, synthetic rubber, silicone rubber.
If drawn into long filament like materials, whose length is at least 100 times its diameter, polymers are said to have converted into fibers, e.g., nylon and terylene.
Polymers used as adhesives, potting compounds, sealants, etc., in a liquid are described as liquid resins, e.g., epoxy adhesive, polysulphide sealants.
Various types of polymers, their properties and applications are given below:
Polychloroprene is the polymer name for the synthetic rubber known as neoprene. Polychloroprene is obtained from the monomer chloroprene. It is prepared by the catalytic addition of HCl to vinyl acetylene. It is synthesized by the emulsion polymerization technique. Trans 1,4 is the preferred configuration produced during the polymerization. It is therefore an easily crystallisable elastomer. It can be readily vulcanized by the help of zinc oxide or magnesium oxide.
Polychloroprene is not characterised by one outstanding property, but its balance of properties is unique among the synthetic elastomers. It has:
Good mechanical strength
High ozone and weather resistance
Good aging resistance
Good resistance toward chemicals
Moderate oil and fuel resistance
Adhesion to many substrates
It is used as a raw material for adhesives, both solvent and water based.
It is widely used in rubber industry.
It also has different latex applications such as dipped articles, moulded foam and improvement of bitumen.
It is also used as moulded goods, cables, transmission belts, conveyor belts, profiles etc.
Slow crystallizing grade elastomer of rubber is principally used for providing oil-resistant insulation coatings to wires and cables and for producing shoe soles, solid tyres, wetsuits, gloves, hoses, radiator hoses and industrial hoses.
Precrosslinked grades are particularly suitable for the extrusion of profiled parts.
Sulfur-modified grades are used in particular for parts exposed to dynamic stress, such as driving belts, timing belts or conveyor belts because of their excellent mechanical properties.
Crystallization resistant grades are used to produce rubber articles, which have to retain their rubbery properties at very low temperatures.
Polyacrylonitrile (PAN), also known as polyvinyl cyanide. It is produced from acrylonitrile by the radical polymerization technique using peroxide initiators or by ammonoxidation of propylene.
PAN is soluble in dimethyl formamide, dimethyl sulphoxide, adipo nitrile, and so on.
It has a remarkable resistance to heat upto around 220áµ’ C and exhibits very good mechanical properties.
Polyacrylonitrile (PAN)-based polymer electrolytes have obtained considerable attention due to their fascinating characteristics such as appreciable ionic conductivity at ambient temperatures and mechanical stability.
It is used to produce what are known as PAN fibres.
The copolymer of acrylonitrile with butadiene is a material of great industrial importance.
Copolymers containing PAN are used as fibers to make knitted clothing like socks and sweaters.
It is also used to manufacture outdoor products, such as tents.
It is used to make civil and military aircraft primary and secondary structures.
Other applications of PAN contain production of carbon fibers, missiles, solid propellant rocket motors, pressure vessels, fishing rods, tennis rackets, badminton rackets & high-tech bicycles.
POLYMETHYL METHACRYLATE (PMMA)
It is a clear, colorless transparent plastic with a higher softening point, better impact strength, and better weatherability. It is available3 in moulding and extrusion compositions, syrups, and cast sheets, rods and tubes.
It is a linear thermoplastic, about 70-75% syndiotactic.
It is amorphous due to lack of complete stereoregularity and its bulky sized groups.
It is resistant to many aqueous inorganic reagents including dilute alkalis and acids.
It undergoes pyrolysis almost completely to monomer by a chain reaction.
It has poor abrasion resistance as compared to glass.
In Medical Science:
It is used in cosmetic surgery to reduce the wrinkles and marks permanently.
PMMA bone cement is used to remodel lost bone and to affix implants in orthopedic surgery.
It is also used for replacing intraocular lenses in the eye.
It can be also used as an alternative to the glass and steel in bioprocess chromatography columns.
It can be used in dental filling along with other compounds.
PMMA is used to create microfluidic lab-on-a-chip devices used in biomedical researches.
Sunlight can be redirected from a pipe to a room with the help of Laser Cut acrylic panels.
It is used as visors in motorcycle helmets.
Regular glass of riot control vehicles is replaced with acrylic to protect the occupants from thrown objects.
Polycast acrylic sheet is widely used material in aircraft transparencies.
PMMA Acrylic glass is commonly used in residential and commercial aquariums.
Copolymers of methyl methacrylate, ethyl acrylate, and monomers containing reactive functional groups are widely used as thermosetting resins in baked enamel applications.
It is used for protective coating and for the manufacture of safety glass.
It is used for making long-lasting lenses for automobile tail lights, windscreens, TV/Computer screen guards, fibre optics.
It can take up beautiful colours and tints and hence is used in the manufacture of light shades, transparent domes, adhesives, bomber noses, cockpit canopies, artificial eyes, plastic jewelry, etc.
Acrylic sheets are used for signs, glazing, furniture, petitions, and lighting-fixture diffusers.
Other various applications of PMMA are injection-molded articles including automotive lenses, reflective devices, instrument and appliance covers, optical equipment, and home furnishings.
POLYVINYL ACETATE (PVA)
The most widely used polymer of a vinyl ester is polyvinyl acetate. It is prepared by the free radical addition polymerization of vinyl acetate in emulsion in the presence of little benzoyl peroxide.
It is clear, colorless and transparent material and is fairly soluble in organic solvents.
The polymer is atactic and hence amorphous. Its glass transition temperature is only 28áµ’C, so it becomes sticky and gummy at temperatures higher than 28áµ’ C.
The articles formed from it are distorted even at room temperature under the influence of compressive and tensile forces.
Polymers with lower molecular weights are brittle but gum - like when masticated and so they are used for making chewing gums.
It can be easily co-polymerized with other hard polymers to give copolymers of varying degree of softness.
Emulsions of PVA are used as adhesives, both of the emulsion type and of the hot-melt type.
It is used for making chewing gums, since it is harmless when taken orally.
A copolymer of vinyl chloride and vinyl acetate commercially named as vinylate is especially useful as a surface coating material for metals and as finishing material for textiles and other fabrics.
It is also used for bonding paper, leather and textiles, etc.
The copolymer of vinyl acetate with maleic acid is one of the important soil conditioners.
It is used for the manufacture of polyvinyl alcohol; polymer by hydrolysis, which is used as a water soluble adhesive.
It is also used in the production of water-based emulsion paints and latex paints.
It is used for binding books, as wood glue and in hand crafted works.
It is one of the most common addition polymers and is synthesized from ethylene, a byproduct of petrochemical industries, via free radical polymerization of ethylene at high temperature and pressure. The monomer is a gas, which boils at -103.2áµ’C and solidifies at -169.2áµ’C. depending upon the manufacturing conditions, two types of polyethylenes are obtained.
Low Density Polyethylene (LDPE)
It is manufactured by the free radical polymerization of ethylene. The gas is first liquefied at 1500 atm and then pumped into autoclaves maintained between 180-200áµ’C, in the presence of oxygen, a free radical initiator. The chain transfer steps result in the formation of branched polymers. The polyethylene obtained so possesses a molecular mass of about 20,000 and low density and therefore it is also termed as high pressure polyethylene or lo density polyethylene.
It has a branched structure containing both short and long chain branches. The side chain consists only upto 4-5 carbon atoms. The presence of branches in the polymer does not permit it to pack close together and hence its density is low.
LDPE is white, rigid, waxy, translucent, non-polar insoluble in common organic solvents.
Strong acids, strong bases and the salt solutions don't have appreciable action on it at ordinary temperatures.
It undergoes swelling when kept in kerosene.
The wide spread use of polyethylene is due to the low cost of both the starting monomer and the final polymer. It is widely used for a variety of products such as:
Polyethylene-bottles/ bags/ pipes/ sheets/ dishes.
Coating on milk cartoons.
As a packing and wrapping material, for frozen food, textile products, and so on, in the form of thin plastic films, bags, etc.
In high frequency insulation for electric wires and cables.
It is also used for making kitchen and domestic appliances toys, etc.
LDPE's inertness to chemicals and resistance to breakage is made use of in 'squeeze bottles' and in many attractive containers.
Due to its low density and low crystallinity, it has low rigidity and is not suitable for load bearing application. For instance, domestic water pipes are not made from LDPE because they may creep at any time.
It is permeable to gas molecules because of its partial amorphous nature. So LDPE is not suitable for the manufacture of pipes for gas distribution.
High Density Polyethylene (HDPE)
HDPE is manufactured in two ways. First one is by using Zeigler Natta catalyst, in which ethylene is polymerized under 6-7 atmospheric pressure at 60-70áµ’C in presence of Zeigler Natta catalyst dispersed in an inert solvent. Second method is by using mixed oxide catalyst, in which ethylene is polymerized under 35 atmospheric pressure at a temperature of 60-200áµ’C in the presence of
aluminium-based metal oxide catalyst.
HDPE is highly crystalline and melts at 144-150áµ’.
It has a linear structure containing a lower amount of short chain branching.
These polymer molecules are packed well and hence polyethylene of this type has higher density.
It is comparatively more chemically inert than LDPE.
It also exhibits better tensile strength than LDPE.
HDPE film is used as wrapping material for food products instead of paper, because of its crisp feel and greases proof nature.
It is used for the manufacture of crates, food tubs, industrial containers and overhead tanks, because of its better oxidation and u.v. resistance.
Bottles for milk, household chemicals and drug packaging are also made from it.
It can also be used for domestic water and gas piping.
POLYVINYL CHLORIDE (PVC)
It is prepared from its monomer vinyl chloride, which is a gas and is commercially prepared by catalytic addition of dry HCl to acetylene. The two gases are passed together over heated charcoal catalysts containing mercuric or other heavy metal salts at 100 to 250áµ’C. it is of two types rigid and plasticized PVC.
It is a colorless, odorless, amorphous and non-inflammable plastic.
Owing to high intermolecular attraction forces present, it is a hard and stiff polymer.
Its softening point is quite high i.e. 148áµ’C.
It has a glass transition temperature of 81áµ’C and is soluble in cyclohexanone and tetrahydrofuran.
It is a polar polymer due to C-Cl dipole, but its power factor is comparatively low due to immobility of the dipole at room temperature.
It has outstanding strength, lightness and chemical resistance.
It has excellent oil resistance and weathering resistance.
Unplasticised PVC can be extruded, calendered or press laminated.
Due to its high chemical resistance, it is used in acid recovery plants and in plants for handling hydrocarbons.
It is used for making pipes for drainage and guttering. Due to its excellent resistance to weather, it is replacing wood for making window frames which do not corrode.
PVC is less brittle, lighter and more flexible as compared to glass. It has therefore, replaced glass for making bottles for storing consumable liquids such as edible oils, fruit squashes, vinegar, cosmetics, detergents, etc.
The bottles made of PVC are being used for storing mineral waters.
Rigid PVC sheets are used for tanks lining, light fittings, safety helmets, refrigerator components, trays, cycle and motorcycle mudguards etc.
Plasticized PVC is very good insulator for current. It has completely replaced rubber for insulating electric wires.
Plasticized PVC is used for making leather clothes, which has greater abrasion resistance, flex resistance and wash ability as compared to leather. Therefore, it is used for ladies handbags, bathroom curtaining, and car and kitchen upholstery.
It is also used for making plastic rain wears, baby pants, PVC shoes and garden hoses.
The major plastics application for polyvinylidene chloride is saran film.
Other significant markets of PVC are automotive uses, meat and food packaging, bottles, footwear, outerwear, phonograph records, sporting goods and toys.
Bakelite also called phenol formaldehyde is prepared by the condensation polymerization between phenol and formaldehyde in presence of acid/ alkali.
It is hard, dense material made by applying heat and pressure to layers of paper or glass cloth impregnated with synthetic resin.
It is brittle in nature.
Resoles and novolacs are used as coating materials.
The lamination sheets are prepared from resoles.
Bakelite, compounded with fillers like asbestos powder or saw dust finds uses for moulding electrical items like telephone instruments, electric insulators parts such as switches, plugs, switch boards, heater, pressure cookers handle.
It is also used as distributor leads of cars, adhesive for grinding wheels and brake linings.
It is used for varnishes, electrical insulation and protective coatings.
It is also used for impregnating paper, wood and other fillers, for producing decorative laminates and wall coverings and industrial laminates for electrical parts including printed circuits.
It is used for production of ion-exchange resins.
It is widely used for the production of moulded disc brake cylinders and parts for electrical irons.
UREA FORMALDEHYDE RESINS
Urea reacts with formaldehyde by addition to form methylol compounds depending upon the quality of the reactants. A 1:1 ratio of the two reactants gives mono methylol urea, whereas 1:2 ratio give dimethylol urea. These may then react with more urea with the elimination of water.
Urea formaldehyde resins have a distinct advantage over the phenolic resins in that they are clear and colorless; and thus can be synthesized in all desired colors by adding the proper pigment.
The hardness and tensile strength of these resins is better than those of phenolics but their impact strength and heat moisture resistance are low.
They have excellent abrasion resistance and good adhesive characteristics.
These resins are widely used in the manufacture of buttons, bottle caps, surgical items, cosmetic closures, house appliances, colored toilet seats and several other purposes.
They are also used as adhesives in the plywood industry and furniture.
They impart stiffness, crease resistance, fire retardation and shrinkage control to cotton fabrics and so are used for the finishing of cotton textiles.
These resins are prepared by the condensation polymerization of epichlorohydrin and bis-phenol A.
The resin is a viscous liquid with the molecular weight ~340 i.e., contains one bisphenol and two epoxy moieties.
The polymer becomes a brittle solid when its molecular weight is as high 8000.
The resin melts at 145-155áµ’C
Due to the presence of stable ether linkage, the polymer shows remarkable resistance towards water and other solvents.
Being polar, they have excellent adhesive characteristics and are used as adhesives, for glass, metals, etc. and are popularly known as araldite.
They have excellent resistance to wear; they are also tough and heat resistant. So they are used for surface coatings particularly for making skid resistant surfaces for highways, road junctions and round abouts.
Good heat and electrical resistance, low water absorption tendency, dimensional stability make them very good materials for electronic applications particularly in moldings containing inerts and encapsulations.
RESULT AND DISCUSSION:
Polymers as we saw above are widely used in each and every aspect of modern life. Hence polymers are the backbone of the modern civilization and are the chief products of the modern chemical industries. An important attribute of the polymers is the ability to modify their inherent physical properties by the addition of certain additives while retaining their characteristics. Most of the polymers are of recent origin but they have made an impact on our daily life, starting from our kitchen/ drawing room to space.
From this term paper, we have come across the various uses and applications of polymers which are affected by the properties which they possess individually. Hence the impact of properties of even the smallest unit i.e. monomer results in the production of wide range of applications of polymers without which survival in today's era will become almost impossible.