Extensional rheometry

The lessons learnt from the Ml program has a bearing on food products testing. Hence, a brief discussion of the Ml program is given before discussing the applications of extensional rheometry to food products. 

However, recent publications are indicating a switch to describing dough quality by dough rheology, including extensional rheometry. Some of these rheology-based methods are summarized below. 

Extensional rheometry represents an increasingly important research tool fc - our understanding of dough quality. Results are of interest to both the food and agricultural industries. 

The comparatively less elastic nature of many food products makes the analyses of extensional rheology data easier than has been possible with polymeric fluids. This relative ease of data interpretation provides an opportunity both for learning the extensional behavior of materials and for effective application of extensional rheometry in the food industry. 

It is clear from the apparatus description in Fig. 2.49 that carrying out tests to measure extensional rheometry is a very difficult task. One of the major problems arises because of the fact that, unlike shear tests, it is not possible to achieve steady state condition with elongational rheometry tests. This is simply because the cross-sectional area of the test... 

Example 11.2 Investigation of the Effects of Interfacial Cross-linking, Diffusion, and Area Generation Rates on Multilayer Miscible (PE-8% GMA)/PE-4% MAH) Films on the Extensional Rheometry of Such Films. Saito and Macosko (54) prepared multilayer films of two low density polyethylene (LDPE) miscible copolymers a random copolymer of... 

Fig. E11.2b Interfacial stress versus Neo-Hookean strain for three samples of different total residence times at 140°C, showing, Si thickening. [Reproduced by permission from T. Saito and C. W. Macosko, Interfacial Cross-linking and Diffusion via Extensional Rheometry, Polym. Eng. Sci., 42, 1-9 (2002).]...

Levitt L and Macosko CW (1997) Extensional rheometry of polymer multilayers a sensitive probe of interfaces. J Rheol 41 671-85. 

Secor R.B. 1988. Operability of Extensional Rheometry by Stagnation, Squeezing, and Fiber-Drawing Flows Computer-Aided-Analysis, Viscoelastic Characterization, and Experimental Analysis. [CWM, LES]... 

V. S. Au-Yeung and C. W. Macosko, Extensional rheometry of several blow molding polyethylenes, Proc. Vlllth Int. Congress on Rheology, Naples, Italy, 3, pp. 717-722 (1980). 

J. Meissner, T. Raible, and S. E. Stephenson, Rotary clamp in uniaxial and biaxial extensional rheometry of polymer melts, J. Rheol. 25, 1-28 (1981). 

The rotary clamp consisting of a pair of gears is a basic construction element for the design of various types of extensional rheometer described earlier. The fact that the design is amenable for use in vmiaxial and biaxial extensional rheometry has been shown by Meissner et al. [92]. Other biaxial extensiometers have also been described [93,94] by other researchers. 

Meissner, J., Raible, T. and St henson, S.E. (1981) Rotary clamp in uniaxial and biaxial extensional rheometry of pol)uner melts, /. Kheol, 25,1-28. 

The onset of ductile failure in sohds is determined by the Considire construction, in which a maximum in the stress-strain curve causes an instability that manifests itself as a neck. This concept is unhkely to be apphcable to the onset of necking in polymer melts. All constitutive equations, including the Maxwell model. Equation 9.16, predict a maximum in the stress-strain curve for stretching at a constant stretch rate, and this maximum normally occurs prior to the attainment of steady state. Hence, hteral interpretation of the construction as a sufficient condition for failure would imply that uniform uniaxial extensional experiments could never be carried out past the force maximum, which often corresponds to a relatively low strain such an interpretation is clearly contrary to substantial experimental experience in extensional rheometry, and several experimental studies focusing specifically on the Considere construction have shown that it does not predict the experimental onset of necking in melts. 

Steady state shear conditions are relatively easy to establish with appropriate instruments in laboratory conditions but steady state extensional conditions require that an initial reference length (of the material) is growing exponentially with time. There are thus inherent complexities in assessing extensional viscosity, whose detailed discussion is outside the scope of this chapter but is found in many textbooks [6, 7]. Practically extensional rheometry is limited to very delicate and tedious laboratory practices, on a limited number of polymer systems. 

In addition to shear rheometry, polymers can also be investigated by extensional rheometry. Here the specimen is subjected to extension or compression. This is particularly useful for the investigation of polymer films and fibres. Polymer fibres are often strengthened by drawing, i.e. by being stretched out. This forces the polymer chains to become more oriented, and thus makes the fibre tougher. The drawing process and a... 

In addition to the rotational shear tests with the rheometer, oscillation tests, like amplitude and frequency sweeps, are done. Viscoelastic behavior of the solutions, meaning a higher storage modulus, is only measurable at a mass fraction of 15 % in the solution. Extensional rheometry has also been done by project partners with different K90 solution and a CABER by Haake, but no reliable results have been generated. 

The extensional viscosity functions of emulsions were characterized using a Capillary Breakup Extensional Rheometry (CaBER). The apparent extensional viscosities ) of SE-la emulsions as a function of strain rate e are given in Fig. 23.5 for the different mean emulsion drop sizes of 2,4, and 10 pm. From this, an extensional viscosity characteristics for the emulsion with a drop size of 10 pm was derived close to constant and rather low (ca. 0.2 Pas, Newtonian-like). For emulsion drop sizes of 2 and 4 pm, pronounced extensional thinning was observed in the low elongation rate domain, whereas some pronounced extensional thickening behavior was monitored in the higher elongation rate domain. 

In the following sections, in addition to giving the work ing equations for determining these material functions, we discuss corrections, applicability, and limitations for each of the methods depicted in Figure 7.1.2. Further information on extensional rheometry can be found in the references at the end of this chapter. Reviews by Meissner (1985,1987), a monograph by Petrie (1979), and Dealy s book (1982) are also recommend. ... 

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