Emulsion polymerisation cold rubber

Nitrile rubbers are copolymers of butadiene and acrylonitrile which are produced by emulsion polymerisation hot and cold polymerised types are available. The hot polymerised types generally have higher green strength and are slightly harder to process than cold copolymers. 

When the USA and Germany were cut off from the supplies of natural rubber during the Second World War both countries sought to produce a synthetic alternative SBR was the result, and at one stage it was the most commonly used synthetic rubber. It can be produced by both emulsion and solution polymerisation techniques, with the emulsion grades being the most widely used. Emulsion polymerisation yields a random copolymer, but the temperature of the polymerisation reaction also controls the resultant properties obtained. Cold polymerisation yields polymers with superior properties to the hot polymerised types. 

The resultant latex contains about lO particles/cm and each particle is about 60 nm in diameter. The content of solids is typically between 20 and 25 %. The basic technology of emulsion polymerisation has remained more or less unchanged since the 1940s, when the introduction of the redox catalyst system saw the production of the so-called cold SBR . The redox system allows the production of free radicals at a low temperature of 5 °C instead of 50 C (hot SBR), resulting in a better controlled reaction and a rubber with improved mixing characteristics and better final properties. 

Until the early 1950s, the major method of emulsion polymerisation involved water-soluble initiators, such as potassium persulphate, being used to initiate polymerisation in an emulsion system stabilised by a fatty acid soap. Molecular weight was controlled by the use of a mercaptan and polymerisation proceeded at about 50 °C until approximately 72% of the monomer had been converted into polymer. This process yielded the so-called hot rubbers. Today, the bulk of SBR materials are prepared using so-called redox initiators which comprise a reductant such as ferrous sulphate with sodium formaldehyde sulphoxylate in combination with an oxidant such as /7-menthane hydroperoxide. In this case, the polymerisation temperatures are as low as 5 °C and conversion of monomer to polymer is only about 60%. Both the hot and cold rubbers are taken to number average molecular masses (molecular weights) of about 100 000, unless they are being used for oil extension (see later). 

Branching is usually greater with emulsion polymers than is really desirable. Solution polymers may be either completely linear or contain a controlled degree of branching. The two polymer types are also different in the amount of gel that is present. Emulsion polymers contain micro-gel (extensively in hot rubbers and moderately in cold rubbers) as a result of the very nature of the free-radical emulsion polymerisation process, whereas the gel content of solution polymers is very small. Rather interestingly, there have been deliberate attempts recently to incorporate controlled amounts of gel into the SBR in order to improve polymer green strength as will also be discussed later (see Section 5.2). 

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