Processability and viscoelastic characteristics

Bearing the cost-benefit relationship in mind, an important question is what is the minimum activity required for obtaining sufficient information on the characteristics of a given gum rubber. Section 6.6 showed that at least two types of measurement are required one is the dynamic mechanical measurements over the temperature and the frequency region covering processing conditions. The other is the tensile stress-strain measurements at room temperature with at least three and preferably four strain rates. In this section examples are presented to show how this characterisation scheme is used to determine structure-processability relationships. 

The first example uses c/s-l,4-BR. A major application of cis-l,4-BR is for tyres. The rubber improves wear resistance, rolling resistance, and flexibility at low temperature. It is usually blended with other rubbers, such as NR or SBR because it does not have a good processability, flex-crack resistance and green strength. Because the rubber has a very low of -112 C, it tends to fall in Region HI or IV of the mill processability. 

In order to overcome the problems associated with milling, long branching is considered to be one of the variables to manipulate. Questions are how much and how long the long 

The mechanism of branch formation is discussed in Section 1.1.3. In the free radical polymerisation, the chain-transfer mechanism is responsible and the branched molecule becomes more branched leading to gel-formation. In the metal-coordinated catalyst systems used for C25-1,4-BR polymerisation, branches are formed by insertion of macromers. Once insertion occurs, the probability of another insertion is considerably decreased because of the steric effect. Therefore, no gel is formed. Not only are fewer branches present but also the branch pattern is different from those in the free-radical polymerised rubbers. 

The samples were four commercial polybutadienes produced by Bayer AG, see Table 6.3. 

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