REVIEW: Capillary rheometry for molten polymers: A versatile rheological tool

This paper is a Review Article on capillary rheometry for molten polymers as a versatile rheological tool. The capabilities of this technique to measure a number of fundamental rheological and other properties of molten polymers are reviewed. Specific examples are used to make the article illustrative, pedagogical, and effective to be used by rheologists working in the area of rheology and processing of molten polymers. These include (i) determination of flow curves to be compared with linear viscoelastic properties (Cox–Merz rule); (ii) effects of viscous heating on rheological measurements; (iii) study of wall slip of molten polymers; (iv) thermodynamic properties (density and isothermal compressibility); (v) the effects of pressure and temperature on the viscosity of molten polymers at high temperatures, pressures, and shear rates not accessible by conventional rotational rheometry; (vi) approximate methods to evaluate the extensional properties of polymers; (vii) exit pressure, hole pressure, and extrudate swell that are related to normal stress differences; (viii) processing properties and instabilities such as melt fracture and evaluation of effectiveness of processing aids to eliminate melt fracture; (ix) flow-induced crystallization under real processing conditions; and (x) mimicking polymer processing operations such as sheet extrusion or film casting (slit rheometry), profile extrusion (dies of any profile), tubing and wire coating extrusion (annular rheometry). The determination of this plethora of properties makes capillary rheometry an invaluable rheological tool that can complement rheological studies using rotational rheometry and, thus, bridge the gap between fundamental polymer rheology and polymer processing.