chem third

7/29/2019 Chem Third

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Third Session:

Preparation, properties and uses of Teflon

Preparation: By polymerization of water, emulsion of tetraflouroethylene under pressure

in presence of benzoyl peroxide as catalyst.

n(CF2=CF2)

-(CF2 CF2)- nTetraflouroethylene Teflon (PTFE)

Properties:

High Chemical resistance towards all common chemicals. The presence of themost electronegative element F atoms and the regular configuration results in a

very strong attractive force between the different chains. So Teflon is resistant to

chemicals making it an inert polymer.

Low co efficient of friction, waxy touch (not wetted by water/oils). The moleculesof Teflon are among the largest ones and its surface is so smooth and polished

that no substance would stick on it.

High thermal stability, good mechanical strength High density (2.3g/cc), high MP(~350C) High degree of crystillanity (93%); good electrical insulating properties.

Uses:

As insulating material insulation of motors, generators, transformers, cablewires, fitting.

Autistic applications Non-sticking stop cocks for burettes, bakery trays, fryingpans, food processing equipment.

Anticorrosive coatings for making army weapons, pump parts, tank linings,chemical carrying pipes and tubings.

Non Lubricating bearings Good material for bearings and artificial joints coatedwith PTFE.

Nylon 6,6

Preparation Sequential reaction of two dysfunctional molecules

nNH2-(CH2)6-NH2 + NCOOH-(CH2)4-COOH[-NH-(CH2)6-NH-CO-(CH2)4-CO-]n + nH2o

Hexamethylene diamine plus Adipic acid, when heated to 280C gives polyhexamethylene

adipate or Nylon 6,6

Properties:- High temperature stability and high MP(~250C) Good abrasion resistance Have good mechanical strength depending on the degree of crystallinity Flexible and retain original shape after use Insoluble in common organic solvents, but soluble in phenol and formic acid It has got a long life


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Uses:

For fibers to manufacture textiles, socks, shoelaces, swimsuits, raincoats, carpets) High tensile strength, water resistant non absorbent, semi elastic and dryquickly. Nylon melts on high temperature so iron from the wrong side with a

press cloth. Synthetic fabrics burn faster and then melt and cling to the body in the

event of a fire. For molding purposes: gears and bearings, combs, dishware and machinery parts.

High impact strength and resistance to abrasion, nylon bearings and gears perform

quietly and need no lubrication.

Filaments for mountaineering ropes, bristles for toothbrushes, parachutes, insectscreening, strings for tennis rackets etc.

Molded nylon is also used for wire insulation, tyre-cods of automobiles, applianceparts.

PF resin (bakelite)

Preparation:

By condensing phenol with formaldehyde in the presence of acidic/alkalinecatalyst, the initial reaction results in the formation of ortho and para hydroxylmethyl phenol, which reacts to form linear polymer, novolac {Phenol (Xs) +

formaldehyde novolacs ==.HeatTS plastic. Phenol +

CH2)(X5)ResolesHeat, curing agentTS plastic}

During molding, hexamethylene tetraamine [(CH2)6N4] is added that provides HCHO.This converts the soluble fusible novolac to hard, infusible and insoluble solid ofcross

linked structure

Cross linked polymer bakelite (turned resole resins)

Properties

It is rigid, hard, scratch-resistant, infusible, water-resistant, insoluble solid. Resistant to non-oxidizing acids, salts, organic solvents. Excellent electrical insulating properties

Uses

For making molded articles like telephone parts, cabinets for TVs and radios Electrical insulator parts, switches, plugs, switch boards, heater handles. As an adhesive for grinding wheels In paints and varnishes

Epoxy Resins [Araldite]Preparation: Polycondensation of bisphenol and epichlorohydrin with elimination of HCl

to give a 3D cross linked structure. Molecular weight reanges from 350 (a mobile liquid)

to 8000 (high melting solid)


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Properties:-

High flexibility It is tough and heat resistant Excellent adhesive quality

Uses

As surface coatings, adhesives like araldite For skid resistant surfaces on highways For molding components of aircrafts, automobiles Applied over cotton, rayon for shrinkage control As laminating material

Elastomers

ElastomersPolymeric material strength (large elongation 500 1000%) when force

is applied. Return to their original shape upon removal of distorting force [exhibit elastic

behavior when subject to stretching or bending within elastic limit] eg rubber.

Relaxed state of rubber band, coiled polysoprene chains held together by cross links.

Stretched state of rubber band: Polymer chains straighten out to the extent the cross links

allows.

Returns spontaneously to its original shape due to inverse in entropy.

Characteristics of Elastomers

Have highly coiled structure unstretched rubber is amorphous Have high dergree of elasticity [>300%] can be stretched like a spring

marcromolecules getpartially aligned with respect to another causing

crystallization.

Rubber band can be stretched to 4-10 times its original length Can retract rapidly chains get reverted back to their original coiled state and the

material becomes amorphous again

Cross links (inter-chain cohesive forces) connect the chain in elastomers (rubber)upon stretching. The (cross links prevent any further motion of one chain withrespect to another.

Natural RubberA resilient, elastic substance obtained from a variety of unrelated, latex producing, and

tropical trees. Raw rubber is obtained from the milky latex extracted from the trees as aresponse to injury. Latex is collected by making series of spiral cuts halfway around the

circumference of the trunk, allowing the fluid to collect in cups. Latex during treatment

polymerises to form long coiled chains of cis-polyisoprene (natural rubber).


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Gutta percha obtained from the mature leaves of gutta trees. (Malaya and Sumatra)

Used in the manufacture of golf balls, submarine cables, adhesives, issures for surgicalpurposes

Malaysia worlds largest produces of rubber

Drawbacks of natural rubber

Hard and brittle at low temp where as soft and sticky at high temp Low TS (200kg/cm^2) and sticky, which is difficult to handle Sensitive to oxidative degradation by agents like HNO3 and chromic acid Swells considerably and gradually and disintegrates in organic solvent High water absorption capacity It undergoes permanent deformation if stretched to a great extent because of

sliding and slippage of certain molecule chains over each other

Virgin rubber is as useless as pure gold Both are soft and have very low strength

Vulcanization

Process of heating raw rubber mixed with sulfur to 100 150C for 1-4hrs to promote acertain degree of cross linking between chains. The sulfur cross links are introduced

between polymer molecules of rubber. The vulcanized rubber is stiff and the extent of

stiffness depends on the amount of sulfur added. The rubber would be harder and lessflexible as the percentage of sulfur increases due to increased degree of cross linking.

Which covalently bond the polymer chains together. Thus vulcanization is the

modification of the properties of rubber by chemical treatment mainly with sulfur. Egtyre rubber3-5%sulfur, Ebonite hard and rigid rubber. (black material) high

proportion.

Vulcanization process

Application of rubber

Insulating coatings on wires Rubber bands, golg balls, automobile/aero plane/bicycle tubes and tyres Shoe heals and soles, conveyer belts Rubber lined metal tanks, vessels used in chemical industry Rubber hoses for transmission of petrol, oil, water Hard rubber storage of battery cases, switch boards, panels Sponge rubbers, for shock absorption, thermal insulation Foam rubber bed pillows, mattresses, cushions Rubber gasket for sealing cookers and refrigerator doors Heart valves, transmission tuber


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Advantages of vulcanized rubber

The cross linking gives good TS Have excellent resilience (elasticity) Possess low water absorption tendency Higher resistance to oxidation Higher resistance to wear and tear/ abrasion resistant Better electrical insulator and low thermal conductivity Ease of fabrication to produce desired shape Because chains are linked together, vulcanized rubber does not soften as much

as normal rubber as the temp is raised useful temp range -40 to 100C

Low elasticity depends on the cross linking. (High percentage of sulfur resultsin hardest and strongest form of rubber much more resistant to deformation

when stretched, because the cross links pull it back)

Resistance to the action of H2O, bases and dilute acids and organic solvents.It is more durable when exposed to the weather. (Concentrated acid reacts

with rubber and destroys it)

Compounding of rubberMixing of raw rubber with other substances to impart product specific properties for

particular applications

1. softeners and plasticizers: to provide greater resilience and adhesion for easyprocessing eg vegetable oils waxes, staric acid, ester plasticizers

2. vulcanizing agents: To give mechanical strength. Sulfur percentage variesbetween 0.15 to 32. other agents sulfur monochloride, h2S benzoyl chloride,trinitrobenzene, benoyl peroxide

3. Accelerators to shorten the time required for vulcanization and of cure of thecompounded rubber -2- metacaprel, thiocarbonates, thiozoles

4. Antioxidants to retard the oxidation of rubber and thus check deterioration phenyl napthalamine, phosphates.

5. Reinforcing pigments: To give strength and rigidity and to lower the cost. Carbonblack, zinc oxide, calcium carbonate, titanium dioxide is used

6. Coloring matter to impart the desired color. White products TiO2, coloredproducts calcium oxide (green), ferric oxide (red), lead chromate (yellow)

7. Process: Properly weighted amounts of ingredients are mixed and then kneaded tomake the final rubber uniform, soft and plastic using roll mill. The roll mill

consists of parallel / horizontal hollow rolls rotating in opposite direction. Hot /

cold water can be circulated in rolls to maintain the desired temp. During thisprocess, the material is softened by continuous break down. After several passes

through the mill, rubber forms a sufficient coherent, plastic and soft mass suitablefor use for specific purpose.

Synthetic Rubber SBR (styrene butadiene rubber)

Preparation: By copolymerization of butadiene (75 parts) and styrene (25 parts). Themonomers are emulsifies in water using soap or detergent. The retain is initiated by

peroxide initiators like Cr2O3 + Al2O3


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n(CH2=CH-CH=CH2) + n(CH2=CH(C6H5))-[CH2-CH=CH-CH2-CH(C6H5)]-n

ie 1,3 Butadiene plus styrene yields Polybutadie coherene

Properties of SBR

High abrasion resistance

High load bearing capacity and resilience Heat fluid up under heavy loading Oxidized in the presence of traces of ozone in the atmosphere Swells in oil and organic solvents

Uses:

Manufacture of car, scooter, motorcycle tires and tuber either by itself or as highpercentage of blend with natural rubber

Mechanical goods, floor tiles, foot ware components, gaskets, electricalinsulation(wire and cable insulators) adhesives

Used in chewing gumWhat is silicon rubber [ SR polymer containing alternating oxygen and silicon atoms]

SR is a unique synthetic elastomers made from a cross linked polymer which isreinforced with silica. It is a polymer that has a back bone of silicon oxygen required to

vulcanize the silicone rubber. This is normally carried out in a two stage process at the

point of manufacture into the desired shape and then in a prolonged post cure process

Preparation: dimethyl silicon hydrozides polymerize to give silicon rubbers, long chains

polymers arising from intermolecular condensations. They can be vulacanized by organicperoxides. (1-3%) benzoyl peroxide. To from cross links between two neighboring

chains. Peroxide causes the formation of dimethyl bridges between methyl groups ofadjacent chains. Free radical capture h atom from methyl group of adjacent linear

molecules. Thus formation of ethylenic bridges. It is not vulcanized by normal sulfur asthe reaction is extremely slow even in the presence of a catalyst.

Properties

High and low temp stability (thermal stability), inertness (oxidative stability),their strength and inertness are related to two factors. 1) Their stable silica like

skeleton of Si-O-Si-O-Si; the Si-O bond energy is very high (502 KJ/mole) 2) The

high strength of the Si-O bond

Wide hardness range, chemical resistance (oils, hot water, oxygen, dilute acidsand alkaline), exceptional resistance to weathering (weatherability), prolongedexposure to sunlight (UV radiation), excellent electrical insulators.

Silicone rubber flows very easily and as a result can be molded using relativelylow amount of energy. Extremely easy to fabricate, particularly when compared

to conventional organic elastomers

Complete physiological inertness (highly biocompatible polymers) and excellentwater repellency


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Depending on the length of the chains and the degree of cross linking betweenchains, silicones can be either oils or rubber like materials. (Silicone elastomers)

Silicone is non toxic. No taste or smell, translucent and easy to color.Silicone rubber as organic rubber

Silicones have many unique proper uses which most organic elastomers areunable to copy. One excellent example is the wide operating temp of -100 to500C. In such conditions, its tensile strength, elongation and compression can befar superior to organic rubber. This makes silicones excellent for many

automotive and industrial applications

Unlike many organic rubbers, which may only be available in black or pastelshades, silicones can be transparent or brightly colored enabling them to properly

match many applications

Silicones have no taste or smell, they are nitrosamine free and many grades aresuitable for food contact

Organic rubber has carbon to carbon backbone. This can leave them susceptible toozone, UV heat and other ageing factors. Silicone rubber is the best material inextreme industries

Applications

Once milled and coated, silicone rubber can be extruded into tubers, strips, soniccords or custom profiles within the size restrictions.

For insulating electric lining in strips and washing machines Making artificial heat valves and transfusion tubing and surgical implants. The

silicones are biologically intact when exposed to body fluids, they dont coagulate

blood and do not stick to body tissues

Widely used as industrial sealants and greases, where their water repellentproperties are useful, as adhesives in electronic industry, burette tap, car polishes

Their water repellent property brings them to be used in fabrics used in rain water.Silicones are used to waterproof fabrics because their O atoms attach to fabric

surface, leaving hydrophobic methyl groups to repel water

Anti rubber

Prepared from polymer foam compressed and heated to slightly above its softeningpoint, then cooled to room temp and released

Reclaimed rubberIts the recovered rubber in a unstable condition from worn out articles and rubber waste

from factories

Process

Waste rubber material is sent into small pieces and powdered by using a cracker Iron containing impurities are removed by electromagnetic method The purified waste powder rubber is heated at about 200C in a closed iron vessel

with aqueous solution of Na ion to remove the fabric and the rubber gets de

vulcanized due to reversal of S as alkali sulphide.


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Rubber is washed in water and dried Mixed with small quantity of reinforcing agents, like clay, C black incorporated

with a compound of fresh rubber

Properties

Possesses lower TS and elasticity, lesser wear resistance Much cheaper uniform composition and better ageing properties Easily fabricated

Uses

Manufacture of tires and tuber, floor mats, belts, shoes etc

Corrosion

Is spontaneous destruction/ deterioration of a metal or alloy by the surrounding

environment through chemical or electrochemical interaction.

chem third

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