Читать книгу The Rheology Handbook - Thomas Mezger - Страница 137

Inorganic primary particles of clay exhibit the shape of thin platelets. A “house of cards structure” with a network of secondary forces is built up between the surfaces and edges of the platelets when at rest; see Figure 3.32. This is due to the fact that the large flat surfaces of the platelets show negative electrical charge, whereas the narrow sides and edges are positively charged. Therefore, at rest a superstructure in the form of a gel structure is occurring if the additive is incorporated in an appropriate way. In this case, in the low-shear range there is a yield point or an “infinitely high” viscosity, respectively.

The network of forces breaks if the yield point is exceeded under sufficiently high shear force. With increasing shear rates the viscosity values are decreasing continuously since finally, the “house of cards structure” will be completely destroyed. Increasingly, the individual sheet-like and very flat particles of the additive are oriented into shear direction. At high shear rates, and at a typical concentration of around 0.1 to 0.3 % as it is used in practice, the clay particles in the flowing state display no longer a significant thickening effect.

Figure 3.32: Nano- and microstructures in water-based coating systems (the black bar indicates the

dimension of 100 nm.). Left side: at rest, and right side: in a shearing state. 1) clay (as an inorganic

gellant), 2) polymer molecules in solution, 3a) dispersed polymer particles, without an additive,

3b) polymer dispersion with a polymeric associative thickener, here, also surfactant molecules are integrated in the bridge-like clusters

Dimensions: Primary particles of bentonite are around 800 ⋅ 800 ⋅ 1 (in nm) and those of hectorite are around 800 ⋅ 80 ⋅ 1; with an organic surface modification the distance between individual platelets is around 4 nm.

Figure 3.33 presents a typical viscosity function of a pigmented water-based coating with clay as the thickener. On the one hand there is a very pronounced thickening effect in the low-shear range at γ ̇ < 1 s-1 which for example may cause problems when starting to pump the dispersion from a state of rest. On the other hand, there is no more thickening effect at high shear rates, e. g. at γ ̇ > 1000 s-1, and this may cause uncontrolled splashing and spattering of droplets when the coating is applied using a brush or a roller. Therefore, this coating shows pronounced shear-

thinning flow behavior throughout the entire shear rate range.

Figure 3.33: Principal shape of viscosity curves of pigmented water-based coatings containing

different additives as rheology thickeners: 1) clay, as an inorganic gellant, 2) dissolved polymer,

3a) polymer dispersion without an additive, 3b) polymer dispersion including an associative thickener