Читать книгу The Rheology Handbook - Thomas Mezger - Страница 139

Associative thickeners consist of molecules which are able to connect to one another. This occurs in water-based systems via hydrophobic molecular groups, whose connections and association takes place via non-permanent secondary bonds. Connections of these surface-active groups of the thickener molecules can take place between one another or via other hydrophobic components such as particle surfaces (see also Chapter 9.1.3: Surfactant-like polymers). At rest a loose, non-permanent network is built up which is only held together by the physical-chemical secondary forces of the hydrophobic interactions; see Figure 3.32. This Figure also displays small surfactant molecules which by self-organization can integrate themselves into these clusters. The thickener molecules are not remaining permanently in one place, they are continuously changing positions; they show fluctuation. The result is a weaker network, if compared as well to the network of entangled polymers as well as to the gel structure mentioned above; see Figure 3.33. In the range of very low shear rates, viscosity can be adjusted to the required value if the concentration of the dispersion components remains within the corresponding limits. Therefore, flow behavior can be controlled and optimized as desired for any requirement. A higher proportion of hydrophobic groups or longer hydrophobic molecule segments will lead to a higher structural strength at rest. In a sheared state, an increased number of hydrophobic molecule clusters and associative bridges are breaking, and the thickener molecules are oriented more and more into shear direction. The viscosity values may still decrease but only to a limited extent since also here, the particles of the polymer dispersion are not destroyed, even at high shear rates. Due to their size they are showing also here considerably more friction between one another compared to dissolved polymer molecules, and therefore, correspondingly higher viscosity values.

Dimensions: Surface-active polymers exhibit a length of around 50 to 100 nm when stretched. Comparison: Many surfactant molecules are merely 0.5 to 5 nm long.

Figure 3.33 presents a typical viscosity function of a pigmented water-based coating (at Φ < 50 %) including an associative thickener (with a concentration of around 0.1 to 0.2 % of the active components). In the low-shear range of γ ̇ < 1 s-1 there is a limited, not excessive thickening effect. But also, in the range of high shear rates (at γ ̇ > 1000 s-1) the shear-thinning effect is limited. Thus, the viscosity values are changing only comparatively little also in the sheared state. As the viscosity here is not very dependent on the shear rate, therefore showing only moderate shear-thinning flow behavior, some users talk about “quasi-Newtonian behavior”. Even when this term is not scientific, sometimes for practical users it is useful to a certain degree. Associative thickener molecules with a comparatively higher proportion of hydrophobic groups are causing a more pronounced shear-thinning effect, whereas associative thickener molecules with a comparatively higher hydrophilic proportion are leading to more ideal-viscous (Newtonian) flow behavior.

The great advantage can be found in both: On the one hand, in the range of low shear rates the viscosity values are not too high but are usually sufficient as required for sedimentation stability. On the other hand, in the range of high shear rates the viscosity values are usually significantly higher compared to the other two thickener types mentioned above (clay and dissolved polymers). This may help, for example, to prevent spattering when performing fast application processes.

End of the Cleverly section