Acrylonitrile-butadiene rubber copolymerisation

An important class of copolymers made by chain copolymerisation is graft copolymers, synthesized in order to toughen brittle materials through inclusion of a rubber phase. Examples are the cases of styrenic copolymers called "HIPS" for High-Impact Polystyrene and ABS for Acrylonitrile-Butadiene-Styrene. Both are synthesized in two steps. 

The generic term applied to all elastomers resulting from the copolymerisation of butadiene and acrylonitrile. Nitrile rubbers are available with different butadiene/acrylonitrile ratios ranging from 18% to 50%, a high acrylonitrile content giving rubbers excellent oil resistance, a lower acrylonitrile content giving improved low-temperature flexibility. 

Chlorine-containing rubbers based on the polymerisation of chloro-prene [reaction (1.4), R = Cl] are speciality polymers with good resistance to organic solvents and chemical reagents. Nitrile-butadiene rubber (NBR) has similar properties and is obtained by randomly copolymerising acrylonitrile with butadiene ... 

Natural polystyrene has a strong tendency to crack, and can only be used in most potential applications when modified to form high-impact or toughened polystyrene grades or ABS. It can be toughened by polybutadiene, butadiene-acrylonitrile copolymer rubber, or SBS or SEBS block copolymers. The rubber can be added before or after polymerisation of the styrene monomer. In the case of polybutadiene, two reactions take place styrene polymerisation, and graft copolymerisation of the styrene with the polybutadiene. A brief outline of this operation is available on the Dynasol website (www.dynasolesastomers.com). 

Several other elastic materials may be made by copolymerising one of the above monomers with lesser amounts of one or more monomers. Notable amongst these are SBR, a copolymer of butadiene and styrene, and nitrile rubber (NBR), a copolymer of butadiene and acrylonitrile. The natural rubber molecule is structurally a c/i -1,4-polyisoprene so that it is convenient to consider natural rubber in this chapter. Some idea of the relative importance of these materials may be gauged from the data in Table 11.14. 

The common feature of these materials was that all contained a high proportion of acrylonitrile or methacrylonitrile. The Vistron product, Barex 210, for example was said to be produced by radical graft copolymerisation of 73-77 parts acrylonitrile and 23-27 parts by weight of methyl acrylate in the presence of a 8-10 parts of a butadiene-acrylonitrile rubber (Nitrile rubber). The Du Pont product NR-16 was prepared by graft polymerisation of styrene and acrylonitrile in the presence of styrene-butadiene copolymer. The Monsanto polymer Lopac was a copolymer of 28-34 parts styrene and 66-72 parts of a second monomer variously reported as acrylonitrile and methacrylonitrile. This polymer contained no rubbery component. 

The material therefore readily absorbs moisture and must be effectively dried prior to moulding, otherwise a hazy or milky appearance may result. Polyacrylonitriles have a relatively low heat distortion temperature (around 75°C), hence they have limitations for products requiring a hot fill. The material is suitable for thermoforming operations but its relatively high moisture permeation is likely to restrict ultimate usage. The presence of acrylonitrile residues also placed the material under a cloud with the FDA, but use is now accepted provided acrylonitrile monomer does not exceed 0.1 ppm. Nitrile copolymers are usually copolymerised with methyl acrylate together with small quantities of butadiene/acrylonitrile rubbers. 

This is the trade name of a product made by copolymerising acrylonitrile and methyl acrylate in the presence of butadiene - acrylonitrile rubber. It is clear, has good barrier properties and impact strength. This polymer may also, have future applications in the bottle blowing field for carbonated soft drinks bottles. 

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