ABSTRACT
Observed steady-state extensional viscosity behavior is compared with shear viscosity in Fig. 7.2. Three general types of tensile behavior have been reported in the past as a function of tensile stress (or rate of elongation). Tensile viscosity may be nearly independent of tensile stress for some polymers [6.7]. In other cases. extensional viscosity starts to increase with tensile stress at a value of stress about the same as where the shear viscosity starts to decrease 17]. In
sti II other cases. particularly those involving suspensions. the elongational viscosity starts to decrease at about the same value of stress as where the shear viscosity decreases from the Newtonian value 17.81. Note though that stress levels or stretch rates in industrial polymer processing operations are usually significantly higher than those indicated in Fig. 7.2: however. these flows are typically of short duration. and steady states in the stress are rarely. if ever, attained. Furthermore, tensile viscosity increases with decreasing temperature. and this increase often overshadows the change in tensile stress due to changes in the rate of defonnation. At present. there is no single theory capable of quantitatively predicting this wide diversity of behavior found for extensional viscosity.
