plasticity
Rheologically speaking, plastic fluids behave more like plastic solids up to a specific minimum force applied to overcome the yield point. Gels, sols, and ketchup are extreme examples. Once the yield point increases, the liquid begins to approach Newtonian behavior as the shear rate increases. Figure 1.1
The shear stress-shear rate curve and yield point are shown. Although plastic behavior is questionable. Valuable to ketchup, it is somewhat good for inks and paints. In fact, it is a yield point phenomenon. Has practical value. Non-drip paint is an excellent example of where yield points are useful. After brush force has been removed, the viscosity of the paint builds rapidly until flow ceases. Trickle flow is arrested due to the yield point exceeding gravity.
The ink flows in the printing ink, and its flow tendency exceeds the printing boundary, which is controlled by the yield. point. Inks with high yield points will not bleed, but may flow poorly. A very low yield point will provide excellent outflow but may bleed excessively. It is the Yield Point that provides what is needed. The flow is even and there is no overcurrent. Both polymeric binders and fillers can explain the yields. point phenomenon. At rest, the polymer chains are randomly oriented and provide more resistance to flow. Apply shear force straight to the chain in the direction of flow, reducing drag. Solid fillers can form a loose molecular attraction structure, which quickly decomposes under the action of shear. For materials that are pseudoplastic like plastic, the viscosity of the pseudoplastic liquid is used to decrease the force. No yield point however. The more energy, the greater the thinning. When the shear rate decreases, the viscosity increases with decreasing force. No lag, clipping.
The stress-shear rate curve is the same in both directions, as shown in Figure 1.1. Figure 1.2 compares the pseudoplastic behavior of shear rate curves using viscosity – . Many coatings exhibit this behavior, but over time. There is a noticeable delay. The viscosity increases after the force is removed. This form of sex and hysteresis loop is called thixotropy. Pseudoplasticity is generally used for the usefulness of coatings and inks. However, thixotropy is more useful.
Thixotropy
Thixotropy is a special case of pseudoplasticity. The material undergoes "shear thinning", but as shear. As the force decreases, the viscosity increases at a smaller rate, creating a hysteresis loop. Thixotropy is very common and very useful. Dripless paint houses owe their driplessness to thixotropy. Paint starts off as a medium viscous material that stays on the brush. It falls very quickly under shear. Bristle stress for easy handling. When the shearing action ceases, the viscosity returns to a higher viscosity, preventing drips from sagging. Screen printing inks also benefit from thixotropy. Relatively high viscosity screen ink drops suddenly. Viscosity under high shear stress is related to pressure through a fine mesh screen. The momentary low viscosity allows the printed ink dots to coalesce into a continuous solid 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 the viscosity up to a certain point. Higher forces produce no further changes. As the energy input to the liquid decreases, the viscosity begins to drop. Rebuild, but at a slower rate than the initial decline. It is not necessary to know the shape of the viscosity. cycle, but just realized that this reaction is common in decorative inks, paints and coatings. Matting agents in paints, vehicle trim, and other solid fillers often create or increase thixotropic behavior. Higher loading materials, such as inks, tend to be highly thixotropic.
Thixotropic agents, consisting of flattened platelet structures, can be added to fluids to adjust thixotropy. A loose interconnected network is formed between platelets to produce an increase in viscosity. Shear breaks down the network, resulting in a drop in viscosity. Agitation and other high shear forces rapidly reduce viscosity. However, as the thixotropic ink continues to shear, it will thin down even if the shear stress is constant. This can be measured with a Brookfield viscometer as the viscosity drops continuously while rotating at constant spindle RPM. When the ink is still, the viscosity returns to its original value. This can happen slowly or quickly. 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 important.
The characteristics of the ink are checked after the ink printing is carried out step by step. Thixotropy is important for correct ink behavior, and the property of changing viscosity makes screen printing possible. printing possible.
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Liquids that increase in viscosity upon application of shear are known as dilatants. There are very few liquids with this property. Properties should not be confused with ordinary viscosity build-up that occurs when inks and paints lose solvent. For example, a solvent coated coating roll coater will show an increase in viscosity during operation. The rotating drum acts as a solvent evaporator, increasing. Coating solids content and viscosity. True volume expansion is independent of solvent loss.
Rheology
Sounding more like a disease than a property, rheoplexy is the exact opposite of thixotropy. It's time to mix the relevant dilatational forms that lead to shear thickening. Figure 1.3 shows the hysteresis loop. rheoplexy is fortunately rare, as it is completely useless as a feature of screen printing inks.
