Effects of Carbon Black on Rheological Properties

Generally the (shear) viscosity of a filled polymer increases with filler content, as easily understood through simple hydrodynamic considerations. However the flow behavior of filled polymers remains relatively simple only at very low shear rate, with low structure materials of large particle size, for instance glass micro-spheres, or thermal blacks with particle size larger than 500 nm. 

Effects of carbon black on the shear viscosity function. (Data from S. Montes, J.L. White, N. Nakajima, /. Non-Newtonian Fluid Mech., 28,183-212,1988.) 

The effect of filler level is shown in the left graph. At 100°C, gum natural rubber (NR) exhibits the expected behavior of a pseudo-Newtonian plateau at low shear stress (and hence low shear rate), then a shear thinning behavior. As filler content increases, the Newtonian plateau progressively disappears and above 20 phr, the viscosity variation with decreasing shear stress is such that a yield stress behavior is suggested. 

The effect of the structure is shown in the right graph. As can be seen, as the filler structure is more and more complex (from N990 to N326, then NllO), the Newtonian plateau disappears, and again a yield stress behavior is suggested for the reinforcing carbon blacks. 

Note that there are many (complex) fluids that do not exhibit a Newtonian plateau at low shear rate (stress) and whose shear viscosity function feeds the controversy on the existence of a yield stress. As noted by Barnes, when the flow is so slow than ages are necessary to detect it, at least one could consider that the yield stress is an engineering reality (H. Barnes. The yield stress—a review or jtotvra pei —everything flows /. Non-Newtonian Fluid Mech., 81,133-178,1999.) 

---

Effect of Carbon Black on Rheological Properties of Thermoplastics... 

---