Rheology, the science of flow and deformation, is of paramount importance to understanding the use, application and quality control of coatings. Viscosity, resistance to flow, is an important rheological property of liquids and coatings and inks. Even more important is the way the viscosity changes during coating and printing. Newtonian fluids, like solvents, have an absolute viscosity that is constant under mechanical shear. However, almost all coatings exhibit significant viscosity changes under different applied forces. We will study the apparent viscosity of coatings and inks and discover that these force-induced in-process changes are a desired part of the application process.
Viscosity, the resistance of a liquid to flow, is an important property that describes how a liquid behaves under effects such as mixing forces. Other important forces are gravity, surface tension, and methods of shearing and applying materials. Viscosity is simply the ratio of shear stress to shear rate (Equation 1.3). High viscosity liquids require considerable force (work) to change shape. For example, high viscosity paints are not as easy to pump as low viscosity paints. High viscosity paints also require longer run-off times when applied.
As shown in the figure above, shear stress is the force exerted on the liquid per unit area, usually in dyne per square centimeter, the force per unit area. Shear rate is the amount of mechanical energy exerted on the liquid in reciprocal seconds (sec - 1). Using Equation 1.3, the units of viscosity are dyne-seconds per square centimeter or equilibrium (P). For low-viscosity liquids such as water (≈0.01 P), the poise unit is small, and the more common centipoise (0.01 P) is used. Since 100 centipoise 1 poise, water has a viscosity of 1 centipoise (CP). Screen inks are more viscous, ranging from 1,000 to 10,000 CP for graphics and 50,000 CP for some high-load high polymer thick film (PTF) inks and adhesives. Viscosity is expressed in Pascal seconds (Pascal seconds) in the International System of Units (SI: 1 Pascal second 1000 cp). The viscosity values of common industrial liquids are shown in Table 1.1.
Viscosity is a fairly simple concept. Thin or low-viscosity liquids flow easily, while high-viscosity liquids have a lot of resistance. The desirable, or Newtonian case has been assumed. In Newtonian fluids, the shear force is constant in any shear region. Very few liquids are truly Newtonian. More typically, liquids lose viscosity when sheared or worked. The above phenomenon is considered to be a shear thinning phenomenon. Therefore, conditions for accurately measuring viscosity values are necessary. In addition to shear stress, time also needs to be considered. The amount of time the liquid is affected by the force. Shear-thinning liquids tend to return to their initial tack over time. Therefore, time under shear and rest time are necessary quantifiers if viscosity is to be accurately reflected.
It's clear that we're actually dealing with a viscosity curve, not a fixed point. The need to deal with viscosity curves is even more evident in plastic trim. A particular material will experience various shear stresses. For example, coatings can be mixed at relatively low shear stresses of 10 to 20 CP, drawn through the gun line at 1000 CP, sprayed through airless orifices at extreme pressures in excess of 106 CP, and finally allowed to Flows over the substrate under gravity (slight) and surface tension. It is likely that the material will have a different viscosity at each stage. In fact, a good product should change in viscosity during application.
