Rheology of Gum Rubber and Compound

Rheology is a study of deformation and flow. In a simplistic way the deformation may be associated with elasticity and the flow with viscosity. Then, rheology is synonymous to viscoelasticity. Indeed, sometimes these terms are used interchangeably. An extensive treatment has already been given for the viscoelastic behaviour of gum rubbers and compounds in Chapter 6 and 7. 

The reasons why rheology is selected as a separate chapter is as follows the capillary rheometer and the rotational rheometer, which had originally been designed for the rheological measurements of liquids have been used for the observation of gum rubber and compound behaviour. The question is what these measurements really mean, because gum rubbers as well as compounds are not liquids but they are in the rubbery state. However, in this chapter, the conventional practice of treating the material as if it were liquid is followed. Not only is the viscosity-shear rate relationship discussed but also the melt fracture, extrudate swell and slip. Shown in Figure 8.1 are flow curves of NBR samples. A, B, C, and D at 100 °C [1]. 

Rubber compounds tend to give a smooth extrudate without melt fracture. Also, the magnitude of the extrudate swell is smaller than that of the gum ruhher [5]. Compounds consist of supermolecular flow units, which form during compounding. No fracture occurs at the entrance of the capillary because it is a fluid material. Consequently, the memory of deformation at the entrance is smaller than that of gmn rubber, which is an elastic material. This explains the low extrudate swell. The supermolecular flow units are carbon black with adsorbed rubber and comminuted particles of the matrix rubber. There is some resemblance between the rheology of rubber compoimds and that of PVC both are in a particulate state during flow [5]. 

---