- Flow instability and stick-slip transition
With the increase of flow velocity, polymer melt extruded in the tube will become
unstable due to the stick-slip transition near the tube wall, which makes the
extrudate shows wave-like, bamboo-like, or spiral-like distortions. All these phe-
nomena are known as melt-broken phenomena. In these cases, the shear rate
suddenly rises, as illustrated in Fig. 7.17, thus this behavior is also called
capillary-jet phenomenon. The string-like shark-skin phenomenon upon the extru-
sion of polyethylene melt can be attributed to the intermittent stick-slip transition
near the tube wall of the exit (Wang 1999 ).
Question Sets
- What are the advantages of the shear-thinning phenomenon in polymer melt
processing? - Why are some polymers able to spin into fibers and the others not?
- Why are the branched polymers “magic” additives for the processings of linear
polymers? - Why are the viscous polymer fluids viable to climb up along the stirring shaft?
References
Bent J, Hutchings LR, Richards RW, Cough T, Spares R, Coates PD, Grillo I, Harlen OG, Read DJ,
Graham RS, Likhtman AE, Groves DJ, Nicholson TM, McLeish TCB (2003) Neutron-mapping
polymer flow: scattering, flow visualization, and molecular theory. Science 301:1691–1695
Bingham EC (1916) An investigation of the laws of plastic flow. US Bur Stand Bull 13:309–353
Bird RB, Curtiss CF, Armstrong RC, Hassager O (1987) Dynamics of polymeric fluids, vol 2.
Wiley, New York
De Gennes PG (1979) Scaling concepts in polymer physics. Cornell University Press, Ithaca
De Waele A (1923) Viscometry and plastometry. Oil Color Chem Assoc J 6:33–88
Fig. 7.17 Illustration of conformation change in relation with the stick-slip transition near the
tube wall of the exit, with the increase of shear rates of polymer coils
References 143