Unique characteristics of compounds

The first part of this chapter concerned the viscoelastic properties of gum rubber and how they affect those of the compound. The second part discussed the use of strain amplification as a quantitative measure of the compound characteristics, where the effect of the formulation, i.e., rubber and carbon black variation, were investigated. 

There are other qualitative aspects which are unique to rubber compounds and different from gum rubber behaviour. The increase of modulus as a result of physical ageing has already been discussed in section 7.3. 

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]. 

Compounds tend to slip more easily at the metal-rubber interface compared with gum rubbers. This is observable in extrusion with use of a colour-marker on the specimen [25]. 

Yielding is observed in elongation of compounds. This occurs at large deformations. The decrease of modulus with increase of the strain amplitude occurs at very small deformations (Payne effect) [9]. This may also be regarded as a yielding of the carbon black network. When the steady state flow measurement is carried to very low shear 

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Viscoelastic methods for the characterisation of gum rubbers are extended to rubber compounds, and the ways in which the viscoelastic properties of gum rubbers are manifested in the properties of the corresponding compounds are examined. The development of a method for evaluating strain amplification and strain rate amplification is described. Examples are presented of the characterisation of compounds with respect to variations in gum rubbers and carbon black grades, and consideration is given to the unique characteristics of compounds which are not observed in gum rubbers. Quality control tests for gum rubbers and compounds based on viscoelasticity are reviewed. 32 refs. 

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