Shape and Structure
While conventional filler shape properties such as aspect ratio have some importance, a property referred to as structure has particular importance in fillers used in elastomer applications. The term structure is widely used in elastomer filler technology and can cause considerable confusion. There are two distinctly different types of structure which need to be differentiated and are generally referred to as transient and permanent structure.
Both silicas and carbon blacks exhibit a phenomenon known as transient structure when dispersed in liquids or polymers. This is the formation, at rest, of a network of interacting filler particles. This network increases the viscosity of the system, but is readily broken down by shear. This is most noticeable in the thixotropy of some silica dispersions in liquids. It is mentioned here because, as we shall see later, it is of considerable importance in tire technology.
Permanent (Persistent) Structure
This refers to the particles themselves and has a very distinct meaning and importance in elastomer technology, where it is related strongly to aggregation and shape. In essence, the important factor is the ability of a particle shape to trap or shield polymer. This trapped polymer does not experience the full stress field in the matrix and in some ways this increases the effective volume fraction of the filler. This is a property of many types of filler, but is usually used with reference to carbon black, where it is particularly marked. Permanent structure is usually measured by a simple oil absorption test, with a high value indicating high structure.
A simple schematic view of the concept of structure is presented in Fig. 4. As it is related to the shape of aggregates, structure can be much affected (reduced) by aggregate break down during processing. This is illustrated by the data presented in Fig. 5, where carbon black was recovered from compounds before vulcanization and the oil absorption compared with that of the original black. It can be seen that the high structured black (HAF-HS) suffers a significant loss in oil absorption (i.e., structure) while the low structured one (HAF-LS) shows no loss. Various techniques have been developed to take account of this loss during processing; one such is to compress or crush the carbon black before determining the oil absorption (Dollinger et al. 1967). This is sometimes referred to as the 24 M4 value.