Back to Basics: Ink Viscosity – The Importance of Mixing and Agitation

In my recent travels I have encountered a trend that affects inks and coatings resulting in problems that appear to be viscosity-related, when, in fact, they are caused by mixing issues.

The non-Newtonian and thixotropic properties of inks and coatings require mixing or some form of agitation to remain homogeneous to ensure proper measurement and application.

Understanding Non-Newtonian & Thixotropic Fluids

I know what you are thinking, “what is non-Newtonian & thixotropic”? I am glad you asked. 

The difference between Newtonian or Non-Newtonian

A Newtonian fluid is any fluid that exhibits a viscosity that remains constant regardless of any external stress (shear) that is placed upon it, such as mixing or a sudden application of force. One example is water since it flows the same way regardless of whether it is left alone or agitated vigorously. 

This can be contrasted with non-Newtonian fluids, which become thicker or thinner when stress is applied. It is possible for the viscosity of a Newtonian fluid to change if it is exposed to different temperatures or pressures instead of external applications of force. Many fluids become thicker as they are cooled, for example, though they still react to shear forces without a change in viscosity.

The defining factor of any Newtonian fluid is that it will flow the same when a great deal of force is applied as when it is left alone. This means that it can be mixed vigorously without changing its viscosity. Another way to describe these fluids is that they have a linear relationship between viscosity and shear stress.

A non-Newtonian fluid is a fluid whose flow properties differ in any way from those of Newtonian fluids. Most commonly, the viscosity (the measure of a fluid’s ability to resist gradual deformation by shear or tensile stresses) of non-Newtonian fluids is dependent on shear rate or shear rate history.

Inks and coatings are good examples. Each ink has different properties due to the distinctive chemical make-up of the inks. They may have more pigment, paraffin’s, solvents, and such. The inks are non-Newtonian in nature, displaying thixotropic properties; as such, they react differently to various “shear forces’’ being applied.

The Thixotropic Behavior of Inks and Ketchup

Thixotropic refers to the property exhibited by certain gels that become fluid when stirred or shaken and return to a semisolid state upon standing.

The easiest, most effective example of a thixotropic material and the effect of mixing on it is ketchup! Ketchup is a product used throughout the world and everybody has the same problem with it, getting it out of the bottle. When the bottle has been sitting for a period of time, the components that make up the ketchup settle out and separate. So, when it’s time to use it, the ketchup does not flow out of the bottle, but shake up the bottle and voila! The ketchup pours. The reason is the components that make up the ketchup are now mixed, or bonded and in an “excited” state which exhibits flow properties.

Ketchup is thixotropic, inks and coatings are thixotropic, the difference being ketchup is a much better choice for a hot dog condiment. 

Shear Stress and Its Effects on Inks and Coatings

Shear stress is the fundamental action that is applied to inks viscosity and coatings in their respective processes. Fluid running in process is exposed to shear stress as it moves through the pump, in the line, at point of application and in the return; resistance to the fluid flow is a shear stress.  Non-Newtonian and thixotropic fluids exhibit certain behavior: they can thin out, or appear to thicken, when shear stress is applied.  

Paint is a great example and brings us to the reason for this blog. When you purchase a can of paint from your local home improvement center, the person at the paint counter, takes the paint you picked, adds the tint, seals the can, places it on a high speed agitator, removes it, checks the color, and hands you the now correctly tinted paint can, with one or two wooden mixing sticks. 

You may not use that paint for the next few days, so when the paint is opened it is very thick. If you were to paint without first mixing the paint, it would go on very thick, be difficult to spread, would take a long time to dry, and you would end up using more paint than necessary.

Were you to mix the paint prior to use, the paint spreads easier, dries faster and you can cover a larger area with the same volume of paint.

The Importance of Mixing in Ink and Coating Processes

Now that we have some of the terminology and some of the behaviors associated with inks, coatings and yes, ketchup, I can now take a minute to talk about the importance of mixing.

In the past, most of the printers and coaters used centrifugal pumps to move the ink through the process. Centrifugal pumps by their design agitated and mixed the inks to a homogenous state (A mixture which has uniform composition and properties throughout), so no additional mixer was required. Unfortunately, the pumps are heavy and take up valuable real estate on the ink pail.

In an effort to free up space and lighten the load, diaphragm and peristaltic pumps are being incorporated to replace the centrifugal pump. Their advantage is that they are light and relatively easy to work with. The disadvantage being the pumps do not mix the inks. Unfortunately, mixers are not always incorporated to ensure proper ink or coating blending.

Based on the behaviors discussed earlier, left unagitated, an ink or coating material may build up to a false viscosity and appear to be “heavier” than it really is. This can lead to a variety of problems, including poor print quality, poor yield, and out-of-spec coating weights, which can result in material waste, lost revenue, and wasted time.

Conclusion 

In ink viscosity measurement and control, proper mixing of fluids is crucial to achieving a quality finished product. If ink or coating materials are not properly agitated, viscosity-related issues may arise. To avoid these problems, it’s important to ensure that the materials are mixed thoroughly before use.

For more information, please contact INKSPEC.  Sales@inkspec.com  +14504415005  www.inkspec.com