polymer chemistry sem-6, dse-b3 part-3, ppt-3

POLYMER CHEMISTRY

SEM-6, DSE-B3 PART-3, PPT-3

Dr. Kalyan Kumar Mandal

Associate Professor

St. Paul’s C. M. College

Kolkata

Polymer ChemistryPart-3

Contents

• Styrene Based Copolymers

• Poly(Vinyl Chloride): A Thermoplastic Polymer

Styrene Based CopolymersStyrene-Acrylonitrile (SAN) Copolymers and ABS Resins

• To obtain a styrene-based polymer of higher impact strength and higher heat distortion

temperature at the same time, styrene is copolymerized with 20-30% acrylonitrile. Such

copolymers have better chemical and solvent resistance, and much better resistance to stress

cracking and crazing while retaining the transparency of the homopolymer at the same time.

In many respects SAN copolymers are also better than poly(methyl methacrylate) and

cellulose acetate, two other transparent thermoplastics.

• ABS resins are terpolymers of acrylonitrile, butadiene and styrene, prepared by

interpolymerization (grafting) of styrene and acrylonitrile on polybutadiene or through

blending of SAN copolymers with butadiene–acrylonitrile (Nitrile) rubber. Impact

improvement is far better if the rubber in the blend is lightly cross-linked. The impact

resistance of ABS resins may be as high as 6-7 ft lb. per inch of notch.

This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata

Styrene-Acrylonitrile (SAN) Copolymers

• Styrene acrylonitrile resin is a copolymer plastic consisting of styrene (Ph-CH=CH2) andacrylonitrile (CH2=CH-CN). It is also known as SAN. It is widely used in place ofpolystyrene owing to its greater thermal resistance.

• The chains of between 70 and 80% by weight styrene and 20 to 30% acrylonitrile. Largeracrylonitrile content improves mechanical properties and chemical resistance, but also adds ayellow tint to the normally transparent plastic.


Styrene-Acrylonitrile (SAN) Copolymers

• Styrene-acrylonitrile copolymer (SAN), a rigid, transparent plastic produced by thecopolymerization of styrene and acrylonitrile. SAN combines the clarity and rigidity ofpolystyrene with the hardness, strength, and heat and solvent resistance of polyacrylonitrile.It was introduced in the 1950s and is employed in automotive parts, battery cases,kitchenware, appliances, furniture, and medical supplies.

• SAN consists of styrene units and acrylonitrile units in a ratio of approximately 70 to 30. Thetwo compounds are mixed in bulk-liquid form or in a water-based emulsion or suspension,and polymerization is conducted under the action of free-radical initiators. The resultantplastic material displays better resistance to heat and solvents than does polystyrene alone.

• The impact resistance of the copolymer is not satisfactory for many engineering applications,however, and styrene and acrylonitrile are therefore often copolymerized with admixtures ofbutadiene rubber to produce a more shatter-proof product known as ABS, or acrylonitrile-butadiene-styrene copolymer.


Acrylonitrile-Butadiene-Styrene (ABS) Copolymer

• Acrylonitrile-butadiene-styrene (ABS) (chemical formula (C3H3N)x·(C4H6)y·(C8H8)z) is acommon thermoplastic polymer. Its glass transition temperature is approximately 105 °C(221 °F). ABS is amorphous and therefore has no true melting point.

• ABS is a terpolymer made by polymerizing

styrene and acrylonitrile in the presence of

polybutadiene. The proportions can vary

from 15% to 35% acrylonitrile, 5% to 30%

butadiene and 40% to 60% styrene. The

result is a long chain of polybutadiene criss-

crossed with shorter chains of poly(styrene-

co-acrylonitrile). The nitrile groups from

neighboring chains, being polar, attract each

other and bind the chains together, making

ABS stronger than pure polystyrene.


https://en.wikipedia.org/wiki/Acrylonitrile_butadiene_styrene#cite_note-Prospector-4

Acrylonitrile-Butadiene-Styrene (ABS) Copolymer• The acrylonitrile also contributes chemical resistance, fatigue resistance, hardness, and

rigidity, while increasing the heat deflection temperature. The styrene gives the plastic ashiny, impervious surface, as well as hardness, rigidity, and improved processing ease. Thepolybutadiene, a rubbery substance, provides toughness and ductility at low temperatures, atthe cost of heat resistance and rigidity.

• For the majority of applications, ABS can be used between -20 °C and -80 °C (-4 °F and176 °F), as its mechanical properties vary with temperature. The properties are created byrubber toughening, where fine particles of elastomer are distributed throughout the rigidmatrix.

• Like the rubber-modified polystyrenes, ABS resins are two-phase systems consisting of'inclusions of' rubber in a continuous glassy matrix. In this case the matrix is a styrene-acrylonitrile copolymer, and the rubber a styrene-butadiene copolymer, the name ABSderiving from the initials of' the three monomers. Again, development of' the best propertiesrequires grafting between the glassy and rubbery phases.


Acrylonitrile-Butadiene-Styrene (ABS) Copolymer

• The ABS resins have higher temperature resistance and better solvent resistance than the

high-impact polystyrenes and are true engineering plastics, particularly suitable for high-

abuse applications. They can easily be decorated by painting, vacuum metalizing, and

electroplating. ABS is flammable when it is exposed to high temperatures, such as those of a

wood fire. It will melt and then boil, at which point the vapors burst into intense, hot flames.

• Since pure ABS contains no halogens, its combustion does not typically produce any

persistent organic pollutants, and the most toxic products of its combustion or pyrolysis are

carbon monoxide and hydrogen cyanide.

• Key Properties of ABS Plastic: (i) High rigidity; (ii) Good impact resistance, even at low

temperatures; (iii) Good insulating properties; (iv) Good weldability; (v) Good abrasion and

strain resistance; (vi) High dimensional stability (Mechanically strong and stable over time);

(vii) High surface brightness and excellent surface aspect; (viii) Shows excellent mechanical

properties i.e. it is hard and tough in nature and thus delivers good impact strength.


Styrene-Butadiene Rubber (SBR)

• SBR (Buna-S rubber) is a copolymer obtained by the addition of butadiene and styrene at a

ratio 3:1 in an emulsion system in presence of free radical initiator like benzoyl peroxide or

cumene hydro peroxide with support of dextrose. The rubber was made by emulsion

polymerization at 50°C. The product quality was improved by carrying out the

polymerization at 5 °C (41 °F) with some being made at temperatures as low as -10 °C or

-18 °C. These changes were brought about by the use of more active initiators, such as

cumene hydroperoxide and p-menthane hydroperoxide, and the addition of antifreeze

components to the mixture The product is known as cold rubber.

• Anionic solution copolymerization of butadiene and styrene with alkyl lithium catalysts is

used to produce so-called solution SBR. This product has a narrower molecular-weight

distribution, higher molecular weight, and higher cis-l,4-polybutadiene content than

emulsion SBR. Tread wear and crack resistance are improved, as is economy because oil

extension and carbon-black loading can be increased.


Structure of SBR• SBR is a random copolymer, by virtue of its free radical polymerization. The butadiene units

are found to be about 20% in the 1,2 configuration, 20% in the cis-1,4, and 60% in the trans-

1,4 for polymer made at 50°C, with the percentage of trans-1,4 becoming higher for polymer

made at lower temperatures. In consequence of its irregular structure, SBR does not

crystallize.


• Branching reactions due to chain transfer to

polymer and to polymerization of both

double bonds of a diene unit become

extensive if conversion is allowed to become

too high or a chain transfer agent is not used

in SBR polymerization. However, SBR has

been shown to have exactly one double bond

per butadiene unit. Thus no extensive side

reactions occur during its formation, at least

up to about 75% conversion.

Properties and Use of Styrene-Butadiene Rubber (SBR)

• The produced rubber is called cold rubber as the polymerization carried at temperature

-15 °C to 5 °C. At this temperature the chain length can be controlled. If the reaction

temperature is 50 °C then the rubber is called hot rubber and in this case the chain can not be

controlled. Such types of synthetic rubbers are more efficient than natural rubber.

• Tire tread stocks made from regular SBR are inferior in tensile strength to those from natural

rubber (3000 versus 4500 psi), whereas those from “cold rubber” are almost equivalent to

Hevea (3800 psi). At elevated temperatures, however, regular and “cold” SBR lose almost

two-thirds of their tensile strength whereas natural rubber loses only 25%. The ozone

resistance of' SBR is superior to that of natural rubber, but when cracks or cuts start in SBR

they grow much more rapidly. These rubber have high tensile strength, low abrasion

oxidation and resistance to weather oil and acid base.

• The material was initially marketed with the brand name Buna S. Its name derives Bu for

butadiene and Na for sodium (natrium), and S for styrene. Buna S is an addition copolymer.


Poly(Vinyl Chloride)

• Poly(vinyl chloride), commonly named as PVC, is the most important of the vinyl

thermoplastics considering volume of production and fields of application, the commercial

products ranging from very rigid to very flexible items. The polymer is highly unstable when

thermally treated at the processing temperatures. However, the prospect of PVC technology

became very bright due to the discovery of a variety of heat stabilizers.

• PVC is one of the three most abundantly produced synthetic polymers. PVC is one of the

earliest produced polymers. In 1835, Justus von Liebig and his research student, Victor

Regnault, reacted ethylene dichloride with alcoholic potash forming the monomer vinyl

chloride.

• Today, PVC is made from the polymerization of vinyl chloride as shown in the following

equation:


Preparation of Vinyl Chloride

• Vinyl chloride monomer (CH2=CH-Cl) is being synthesized according to the following

processes employing (1) acetylene, and (2) ethylene as the immediate organic raw materials.

Main processes developed and in practice are:

a) hydrochlorination of acetylene,

b) chlorination of ethylene to ethylene dichloride (EDC) and thermal cracking of the latter to

vinyl chloride and hydrogen chloride,

c) the byproduct hydrogen chloride in process (b) can be utilized in: (i) hydrochlorinating

acetylene to produce more vinyl chloride straight away, or (ii) in oxychlorinating more

ethylene to produce EDC, and

d) mixed gas process starting with a dilute mixed stream of acetylene and ethylene



• Vinyl chloride is an acetylene derivative and as late as 1965, more than 45% of all vinyl

chloride produced was based on acetylene derived from coal via calcium carbide or from

petroleum sources. However, the technology later shifted in favour of ethylene.

• The earliest route to vinyl chloride (VC) was from acetylene and HCl. In a typical synthesis

dry hydrogen chloride free from chlorine is mixed with an equimolar proportion of dry

acetylene and the mixture is then passed through a multitubular reactor packed with

mercuric chloride catalyst on an activated carbon support. Temperature is maintained at

90-100 °C. Vinyl chloride monomer formed is then purified and stored under nitrogen in

stainless steel tanks.



• As ethylene became more abundant, technology based on chlorination of ethylene to

ethylene dichloride (EDC) and then cracking of EDC to vinyl chloride was developed. The

byproduct HCl was recovered and utilized to make more vinyl chloride by reaction with

acetylene.


• The next development was the so-called ‘oxychlorination’ process involving reaction of HCl

with ethylene in presence of air (oxygen) to produce EDC.

In a subsequent step, the EDC is cracked to yield vinyl chloride and HCl. The byproduct HCl

is recycled in the oxychlorination step.

Preparation of Vinyl Chloride• Mixed gas processes based on cracking of naphtha to equimolar proportions of ethylene and

acetylene have also been developed with the objective of total chlorine utilization. The

mixture of the two is first reacted with HCl to form vinyl chloride, the reaction taking place

between the acid and acetylene contained in the cracked gases.

• Vinyl chloride formed is separated from ethylene which is then hydrochlorinated to EDC and

then cracked to vinyl chloride and HCl. The mixed gas processes are competitive with the

oxychlorination process.

Polymerization of Vinyl Chloride

• Commercial polymerization is done by using free radical catalysts and employing bulk,

suspension and emulsion techniques. Suspension and emulsion techniques are, however,

most commonly employed.

• Bulk polymerization is heterogeneous in view of insolubility of the polymer in the monomer.

Peroxydicarbonates are conveniently used as initiators in the bulk or suspension

polymerization. Bulk polymerization may be done in rotating cylindrical reactors with

tumbling steel balls inside to facilitate removal of heat of polymerization from the monomer-

polymer heterogeneous system.

• The bulk polymerization may also be done in two stages as in the Pechiney-St. Gobain

process to overcome the difficulties of heat removal and to have better control on molecular

weight and particle size of the polymer.


Polymerization of Vinyl Chloride

• Suspension and emulsion polymerizations are done in stirred tank jacketed pressure vessels.

For injection molding, extrusion and calendering purposes, particularly for clear objects and

for electrical insulation purposes, bulk and suspension grade PVC resins are used.

• Emulsion grades are utilized in organosols and plastisols and in some other areas, but they

are unsuitable in insulation (wire and cable) industry because of the presence of traces of

detergent or soap and other ionic components of the polymerization recipe in the isolated

polymer (PVC).


Structure and Properties of PVC

• Poly(vinyl chloride), PVC is by and large a linear polymer, colourless and thermoplastic in

nature, and having a chlorine content of about 56.8%. The polymer is thermally unstable and

extensive heating transforms it into a dark coloured residue resembling polyacetylene and

liberating HCl as the volatile.

• PVC is insoluble in all hydrocarbon solvents. Two of its important solvents are

cyclohexanone and tetrahydrofuran. It is also soluble in ethylene dichloride and

nitrobenzene.

• It possesses flame retardation and self-extinguishing characteristics. The polymer as

produced commercially, is substantially amorphous in nature. Structural irregularities in the

polymer arise due to occasional branching effect during polymerization and due to chain-end

unsaturation consequent to termination by disproportionation.


Structure and Properties of PVC

• Technically, the polymer is graded on the basis of a solution viscosity parameter known as

the K-value but not by melt viscosity or melt flow index, in view of its poor thermal

stability.

• Commercial polymers differ not only in molecular weight and molecular structure (degree of

branching) but also in their particle characteristics such as porosity, shape, size and size

distribution. The processing behaviour of the polymer is largely linked with these particle

characteristics. In its massive form, PVC is a hard, horny, rigid material with a characteristic

tendency to stick to metallic surfaces at elevated temperatures.

• Because of its versatility, some unique performance characteristics, ready availability, and

low cost PVC is now the third largest produced synthetic polymer behind polyethylene and

polypropylene.


Compounding and Processing of PVC

• PVC can be conveniently processed to rigid items if only it is compounded with stabilizers

and lubricants. It is variously compounded with many other compounding ingredients such

as fillers, plasticizers, extenders, and other process aids, impact improvers, colouring matters,

etc.

• Stabilizers of PVC protect it from measurable degradation at processing temperatures.

Among the common stabilizers are basic lead salts such as basic lead carbonate, tribasic lead

sulphate, dibasic lead phosphate, etc. Organo compounds of other metals used as stabilizers

include those of cadmium, barium, calcium, zinc and tin, mostly in the form of phenates,

octoates, benzoates and laurates. Some of them produce much improved effects in presence

of phosphite antioxidants, such as tris(nonylphenyl) phosphite. Organo-tin compounds are

specially useful for producing (crystal) clear compounds and products.



• All flexible applications of PVC depend on significant use of plasticizers; notable among

them are the high boiling phthalates such as, dibutyl phthalate, dioctyl or diisooctyl

phthalate, etc., and the phosphate plasticizers, such as trioctyl phosphate and tricresyl

phosphate.

• Chlorinated paraffin and the phosphate plasticizers are used in fire retardant compounds.

Aliphatic esters such as dibutyl sebacate and dioctyl sebacate or adipate are specially useful

for having compounds with high resilience and a low cold flex temperature.

• Impact modification of rigid PVC is variously accomplished by blending with it different

proportions of such polymers as nitrile rubber, ABS graft terpolymers, chlorinated

polyethylene, selected polyacrylates, etc.



• Compounded PVC is converted into molded or formed objects by melt processing or by

processing of PVC pastes and latices. Widespread processing techniques include injection

molding, extrusion, calendering, blow molding and thermoforming.

• The processing of unplasticized (rigid) PVC is much more difficult and critical than that of

plasticized PVC, primarily because of much higher temperature needed for the processing of

the former at which measurable decomposition of the polymer occurs.

• However, trouble-free processing of rigid PVC can be done through judicious blending and

compounding of lubricants, stabilizers (organotin compounds) and other process aids.

Blending or compounding of different ingredients is conveniently accomplished by dry

blending of powders, thus avoiding unnecessary heating in mills and mixers.


Applications of PVC

• Both rigid and flexible applications of PVC have been developed. Rigid applications include

chemical plants and equipments, storage tanks, building items, pipes, sheets, specific molded

objects and containers.

• PVC guttering and rain water piping window frames and transparent roof sheeting are some

of its building applications.

• Floor tiles and wall linings from plasticized PVC are also worthy of mention.

• Other flexible or semi-rigid applications include toys, packaging items, tubes, pipes and

hoses, leather cloths, molded objects, sheets, films, containers, footwear, belting, wire

insulation and cables.


polymer chemistry sem-6, dse-b3 part-3, ppt-3

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