Estimation of Melt Shear Modulus from Extrudate Swell Ratio and Exit-pressure Drop Data

ABSTRACT: Obvious viscoelastic phenomena, such as die-swell and exit-pressure losses, appear when a viscoelastic fluid leaves a die. It is generally believed that the die-swell ratio (B), the melt shear modulus (G), and the exit-pressure drop (Pex) are important parameters for the characterization of the elastic behavior of viscoelastic fluids during die flow. There may be, therefore, some interrelations between them. In the present article, relationships among G, B, and Pex are discussed. On the basis of previous work, some expressions for describing relationships between B and G,Pex, and B, as well as Pex and G during flow of viscoelastic fluids in a long circular die are proposed. The values of G are estimated from the measured B during extrusion of a carbon black and calcium carbonate-filled rubber compound, and predictions of G by using the measured data of Pex are compared with its calculations from the B data during capillary extrusion of a high-density polyethylene (HDPE) melt. The results showed that G increases linearly with increasing shear stress. Furthermore, a good agreement was obtained among these theoretical calculations of G at high shear stress level, also being consistent with predictions by using the Bagley–Duffey equation. KEY WORDS: polymer melt, extrusion, shear modulus, mathematic model