Supermolecular flow units

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]. 

Figure 8.14 shows the shear rate dependence and L/D dependence of extrudate swell of compound [2]. Both dependencies are there even though they are rather small compared to those of gum rubbers. The trend is to imply a relaxation mechanism, the higher swell at the higher shear rates and the lower swell with the larger L/D. The deformation of the supermolecular flow units and their relaxation may be the explanation. 

The previous sections dealt with mixing mechanisms down to the order of 0.1 pm in the rubber domains, which were also interpreted to be the ultimate size of the supermolecular flow units. 

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