Knowledge of rheology and surface chemistry, the two primary sciences of liquid flow and solid-liquid interactions, is necessary to understand coating and printing processes and materials. A comprehensive qualitative treatment of these topics will suffice to provide the insight needed to use and apply paints and inks and help to resolve issues related to their use.
Broadly speaking, rheology is the study of the physical behavior of all materials under pressure. Four broad categories are recognized: elasticity, plasticity, stiffness, and viscosity. We are concerned with liquids and pastes. The scope of fluid rheology includes the change and removal of a liquid's shape under the action of physical forces. Viscosity is an important rheological property of coatings and inks. Viscosity is simply the ink's resistance to flow - the ratio of shear stress to shear rate.
Throughout the coating and printing process, various types and amounts of mechanical forces are applied. The size of the shear force directly affects the viscosity value of the non-Newtonian fluid. Most coatings exhibit some degree of "shear thinning" as they are mixed or run on the coater. Heavy inks are especially prone to shear thinning. As the shear rate increases, the viscosity decreases, and in some cases, sharply.
This seems pretty straightforward except for two effects. One is called the yield point. This is the shear rate required to induce flow. Ketchup will often be rejected until a little extra shear is applied. Then it often flows too freely. Once the yield point has been exceeded the solid behavior disappears. The loose network structure is broken. Inks also exhibit this yield point property, but to a lesser extent. Yield point is one of the important ink properties.
Yield value, an important but often overlooked property of liquids, will also be discussed. We need to study rheology as a dynamic variable and explore how it changes throughout the coating process. The interaction during the coating process changes the viscosity and rheology, which will be a key concept in our discussion of coating technology.
The second factor is time dependence. Some inks change viscosity over time, even when a constant shear rate is applied. This means that the viscosity can depend on the amount and length of time the mechanical force is applied. When the shear force is removed, the ink will return to its original viscosity. This rate of return is another important ink property. It can vary from seconds to hours.
Rheology goes far beyond the common snapshot view of viscosity at a single shear rate, which is commonly reported by ink suppliers. When the temperature changes, it exerts different viscosity changes with different viscosity when solvents and additives come into play. Brookfield viscometer readings, while valuable, do not give a full picture of non-Newtonian liquids.
Surface chemistry describes the phenomenon of wetting (dewetting) resulting from the mutual attraction between ink molecules, as well as the intramolecular attractions between the ink and the substrate surface. The relative strength of these molecular interactions determines some ink performance parameters. Good print definition, adhesion and a smooth ink surface all require the correct surface chemistry. Bubble formation and associated film-forming defects are also fundamental to surface chemistry.
Surface chemistry, for our purposes, deals with the attractive forces liquid molecules exhibit to each other and as substrates. We will focus on wetting phenomena and relate them to coating processes and problems. It can be seen that an understanding of wetting and its behavior will help to elucidate many anomalies in coatings, inks, etc.
Together, the two sciences of rheology and surface tension provide the necessary tools to handle increasingly complex coating technologies. For a better understanding of inks and screen printing processes, rheology and surface chemistry need to be combined into a unified theme. We combine this unification in a simple and semi-qualitative manner. One benefit has been the discovery that printing and coating problems, often attributed to rheology, have a basis in surface chemistry. We will also find that coating planarity is affected by rheology and surface chemistry.
