POLYMER CHEMISTRY SEM-6, DSE-B3 PART-3, PPT-3 Dr. Kalyan Kumar Mandal Associate Professor St. Paul’s C. M. College Kolkata Polymer Chemistry Part-3 Contents • Styrene Based Copolymers • Poly(Vinyl Chloride): A Thermoplastic Polymer Styrene Based Copolymers Styrene-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) and acrylonitrile (CH2=CH-CN). It is also known as SAN. It is widely used in place of polystyrene owing to its greater thermal resistance. • The chains of between 70 and 80% by weight styrene and 20 to 30% acrylonitrile. Larger acrylonitrile content improves mechanical properties and chemical resistance, but also adds a yellow tint to the normally transparent plastic. This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Styrene-Acrylonitrile (SAN) Copolymers • Styrene-acrylonitrile copolymer (SAN), a rigid, transparent plastic produced by the copolymerization of styrene and acrylonitrile. SAN combines the clarity and rigidity of polystyrene 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. The two 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 resultant plastic 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 of butadiene rubber to produce a more shatter-proof product known as ABS, or acrylonitrile- butadiene-styrene copolymer. This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Acrylonitrile-Butadiene-Styrene (ABS) Copolymer • Acrylonitrile-butadiene-styrene (ABS) (chemical formula ​(C3H3N)x·​(C4H6)y·(C8H8)z) is a common 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. This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata 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 a shiny, impervious surface, as well as hardness, rigidity, and improved processing ease. The polybutadiene, a rubbery substance, provides toughness and ductility at low temperatures, at the cost of heat resistance and rigidity. • For the majority of applications, ABS can be used between -20 °C and -80 °C (-4 °F and 176 °F), as its mechanical properties vary with temperature. The properties are created by rubber toughening, where fine particles of elastomer are distributed throughout the rigid matrix. • 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 ABS deriving from the initials of' the three monomers. Again, development of' the best properties requires grafting between the glassy and rubbery phases. This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata 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. This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata 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. This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata 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. This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata 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. This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata 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.