Shear viscosity of particle filled and rubber modified thermoplastics at very low shear stresses

The shear viscosity of carbon black filled and calcium carbonate filled polypropylene, PP EPDM dynamically vulcanized thermoplastic elastomers (TPE) and ABS resions were investigated over a very wide shear rate and shear stress range. This was accomplished using (i) a constant shear stress creep instrument, (ii) a cone-plate rotational rheometer, (iii) a capillary extrusion rheometer. It was found that stresses exist for some of the-se compounds below which there are only finite deformations and no steady flow, These stresses define yield values. Much attention was given to measurements of creep at low stresses, especially in the neighborhood of the yield value. The magnitudes of yield stresses obtained from such creep measurements are significantly lower than those obtained from using standard extrapolation to zero shear rate of higher stresses from rotational instruments. The shear viscosity behavior near the yield value differs considerably depending upon the type of particle filler in the compound and the rubber content in TPE and ABS. We also observed a high shear viscosity (ca 10(9) Pas) plateau in many of our systems.