Rheology, the science of flow and deformation, is key to understanding the use, application and quality control of coatings. Viscosity, the resistance to flow, is an important rheological property of liquids and therefore of coatings and inks. One more thing to keep in mind: the way viscosity changes during the coating and printing process. Newtonian fluids, like solvents, have an absolute viscosity that is not affected by mechanical shear. However, almost all coatings show significant changes in viscosity under different forces. We will look at the apparent viscosity of coatings and inks and discover that these force-induced changes during processing are a necessary part of the application process.
Viscosity, or a liquid's resistance to flow, is a key property that describes how a liquid behaves under forces such as mixing. Other important forces are gravity, surface tension, and shear depending on the method of applying the material. Viscosity is the ratio of shear stress to shear rate (Equation 1.3). High viscosity liquids require considerable force (work) to produce a change in shape. For example, high viscosity paints are not as easy to pump as low viscosity paints. High viscosity paints also take longer to flow when applied.
As mentioned above, shear stress, the force per unit area acting on a liquid, is usually expressed in dynes per square centimeter, or force per unit area. The shear rate is measured in reciprocal seconds (s), the mechanical energy applied to the liquid. Applying Equation 1.3, the units of viscosity become dyne-seconds per square centimeter or poise (P). For low-viscosity liquids such as water (=0.01 P), the unit of poise is rather small, and the more common centipoise (0.01 P) is used. Since 100 centipoise = 1 centipoise, the viscosity of water is about 1 centipoise (cP). Screen inks are much more viscous, graphic inks range in viscosity from 1,000 to 10,000 cP, and some highly loaded polymer thick film (PTF) inks and adhesives have viscosities as high as 50,000 cP. Viscosity is expressed in Pascal seconds (Pa-sec) in the International System of Units (SI: 1 Pa-sec= 1000 cP).
Viscosity is a fairly simple concept. Thin, or low-viscosity liquids flow easily, while high-viscosity ones have a lot of resistance to movement. The desirable case, or Newton's case, has been assumed. In Newtonian fluids, viscosity is constant over any shear region. Very few liquids are Newtonian. More typically, the viscosity of a liquid decreases with shear or work. The above phenomenon was identified as shear thinning. Therefore, it is necessary to accurately specify the conditions for measuring viscosity values. In addition to shear stress, time also needs to be considered. A liquid is affected by how long a force is applied. Shear-thinning fluids tend to return to their original viscosity over time. Therefore, time under shear and time to recovery are necessary quantifications if viscosity is to be accurately reported.
Clearly we are dealing with a viscosity curve and not a fixed point. In plastic trim, the need to deal with viscosity curves is even more pronounced. A given material experiences various shear stresses. For example, coatings can be mixed at relatively low shear stress (10 to 20 cP), pumped through a Spray Gun tube at a pressure of 1000 cP, sprayed through an airless Spray Gun hole at extreme pressures in excess of 106 cP, and finally Flows over the substrate under slight gravity (minor) and surface tension. It is likely that this material has a different viscosity at each stage. In fact, a good product should change viscosity during application processing.
