1. Plasticity
Rheologically, plastic fluids behave more like plastic solids until a specific minimum force is applied to overcome the yield point. Gels, sols, and ketchup are extreme examples. Once the yield point is reached, the liquid begins to behave close to Newtonian as the shear rate increases. Figure 1.1
Figure 1.1 Shear stress-shear rate curve.
is the shear stress-shear rate curve and the yield point. While the behavior of plastic is of questionable value to ketchup, it has some benefits in inks and paints. In fact, it is the yield point phenomenon that has practical value. Dripless paint is a great example of the usefulness of yield points. When the force of the brush stroke is removed, the viscosity of the paint increases rapidly until the liquid stops flowing. Prevents dripping due to yield point over gravity. The tendency of ink to bleed in printing inks is controlled by the yield point. Inks with a high yield point will not bleed, but their flow may be poor. A very low yield point will provide good outflow, but leakage may be excessive. A suitable yield point can provide the desired outflow and leveling without excessive overflow. Both polymeric binders and fillers can explain the yield point phenomenon. At rest, the orientation of the polymer chains is random and provides more resistance to flow. Applying a shear force straightens the chain in the direction of flow, reducing drag. Solid fillers can form a loose molecular attraction structure that quickly decomposes under shear.
2. Pseudoplasticity
Like plastic materials, the viscosity of pseudoplastic liquids decreases with force. However, there is no yield point. The more energy used, the greater the thinning. As the shear rate decreases, the viscosity increases at the same rate as the force decreases. There is no hysteresis; the shear stress-shear rate curve is the same in both directions, as shown in Figure 1.1. Figure 1.2 compares pseudoplastic behavior with visco-shear rate curves.
Many coatings exhibit this behavior, but in a time-dependent manner. After the force was removed, there was a noticeable delay in viscosity increase. This form of pseudoplasticity with hysteresis loops is known as thixotropy. Pseudoplasticity is generally a useful property of coatings and inks. However, thixotropy is more useful.
1) Thixotropy
Thixotropy is a special case of pseudoplasticity. The material undergoes "shear thinning"; but when the shear decreases, the viscosity increases at a smaller rate, creating a hysteresis loop. Thixotropy is very common and very useful. The non-drip properties of non-drip house paints are due to thixotropy. Paint starts out as a medium viscous material that stays on the brush. Under the shear stress of brushing, the viscosity drops rapidly, and it is easy to use and smooth. When shear ceases, a higher viscosity is restored, preventing dripping and sagging. Screen printing inks also benefit from thixotropy. Screen inks with relatively high viscosity will suddenly drop in viscosity under high shear stress.
Figure 1.2 Viscosity shear rate curve.
The momentary low viscosity allows the printed ink dots to coalesce into a solid continuous film. The viscosity returns to a higher range before the ink "bleeds" beyond a predetermined boundary.
Thixotropic materials produce individual hysteresis loops. Shear stress reduces viscosity to a point where higher forces produce no further change. As the energy input into the liquid decreases, the viscosity begins to increase again, but at a slower rate than the initial decrease. It is not necessary to know the shape of the viscosity ring, only that such reactions are common in decorative inks, paints and coatings. The presence of pigments, leveling agents and other solid fillers often creates or increases thixotropic behavior. Higher-loaded materials, such as inks, tend to be highly thixotropic.
Thixotropic agents consist of flattened platelet structures that can be added to liquids to adjust thixotropy. Platelets form a loose, interconnected network that produces an increase in viscosity. Shear breaks down the network, causing a drop in viscosity. Stirring and other high shear forces rapidly reduce viscosity. However, thixotropic inks continue to thin during shearing, even when the shear stress is constant. This can be seen with a Brookfield viscometer, where the measured viscosity continues to drop while the spindle turns at a constant speed.
When the ink is left still, the viscosity returns to its original value. This can be slow or fast. Curves of various shapes are possible, but they will all show a hysteresis loop. In fact, this hysteresis curve is used to detect thixotropy (see Figure 1.3). The rate of viscosity change is an important characteristic of inks, which is revealed when we test the ink step by step by screen printing. Thixotropy is very important to the performance of inks, and changes in viscosity properties make screen printing possible.
2) Expansion
Liquids that increase in viscosity when shear is applied are called bulking agents. Very few liquids have this property. The behavior of swelling agents should not be confused with the common viscosity build-up that occurs when inks and coatings lose solvent. For example, a solvent-based coating applied by a roll coater will have a viscosity that increases as the coating progresses. The rotating drum acts as a solvent evaporator, increasing the coating's solids content and, therefore, viscosity. True swelling occurs independently of solvent loss.
3. Rheology
Sounds more like a disease than a trait, and it's the exact opposite of thixotropy. Dilatation is the time-dependent form when mixing causes shear thickening. Figure 1.3 is the hysteresis loop. Fortunately, rheology is rare, as it is a completely useless property for screen printing inks.
Figure 1.3 Shear stress-shear rate curve: hysteresis loop
