Conventional methods of characterisation

For a given type of gum rubber, if the compositional aspects such as the comonomer ratio, microstructure and their distributions are known, characterisation becomes centered around the determination of MW, its distribution, and the degree of branching. 

Items (iii), (iv), and (v) give much uncertainty in interpreting results, because of the presence or absence of molecular association, ultra-high MW and gel usually remains unknown. The high MW fraction and gel often have significant effects on processability even though the amount is small. 

In production control, GPC data are sometimes collected routinely along with measurements of the Mooney index [14]. When these two sets of data are compared for hundreds of production lots, in only about 50% of cases may a correlation be found. This is partly because the Mooney index is a very crude measure of rubber property, but the uncertainties described above on GPC data are also responsible. 

The Mooney index is often called Mooney viscosity hut there is no rationale that the index represents viscosity. It is an expression of some aspect of viscoelasticity in that there may be some relationship to the processability. The method uses an instrument with a particular design, which provides a simple operation and a quick result. Compared to the dilute solution method, the sample size required is much larger so that it is more representative of the commercial lot. The Mooney index has been used worldwide as a specification for gum rubbers. 

However, there is no firm theoretical basis for this index. It is only a very crude method of differentiation between samples for the same value of the index, the processability of two gum rubbers may be different. In other cases, with rather wide differences in the value of the Mooney index, two rubbers may process almost equally well. 

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