Читать книгу The Rheology Handbook - Thomas Mezger - Страница 213

When sticky, tacky, stringy and ropy foodstuffs or pharmaceuticals containing thickening agents show a too high elastic portion, this may lead to unpleasant mouth sensation and might even cause problems when swallowing [5.9].

For “Mr. and Ms. Cleverly“

Note 1: Explanation of the Weissenberg effect

For rotational systems, the circumferential velocity v increases with increasing distance from the axis of rotation: v [m/s] = ω ⋅ r, with the angular velocity ω [s-1] = (2π ⋅ n)/60, the radius r [m], and the rotational speed n [min-1]. Since many polymer molecules cannot follow the rapid motion, e. g. at the edge of the stirrer vessel, they gather in the middle. This causes the “mountain of liquid” in the center of the container. As a simple illustrative example you can imagine: On a roundabout (merry-go-round) rotating at a constant speed, it is better to sit closer to the center than more outside. On the latter position, the “stress by motion” is increased due to the higher circumferential velocity compared to a place which is closer to the rotational axis. Polymer molecules under shear and extensional stresses are trying permanently to recoil against the external forces, and this counter action leads to three-dimensional effects producing normal forces (see Chapter 5.3).

Note 2: Tack and stringiness

Stringiness and tack can be tested and scientifically analyzed, adjusted and controlled via the damping factor tanδ obtained when performing oscillatory tests. See also the Notes and Examples in Chapters 8.2.4a (Note 2) and 8.3.4.5 (Example 2 about construction adhesives in automotive industry). A further testing mode are tack tests, see Chapter 10.8.4.2.

Note 3: Normal stresses

Extrudate swell, the Weissenberg effect, as well as all other viscoelastic effects are resulting from normal forces or normal stresses, respectively (see Chapter 5.3).

End of the Cleverly section

Summary: Behavior of viscoelastic liquids

In order to get an optimal behavior for practical use, the viscous and elastic portions of viscoelastic materials should show a well-balanced ratio.