This paper has been prepared by Robert Norcross, President of Norcross Corporation. March 11, 2006.
Lessons on Viscosity and Viscosity Control
Why worry about viscosity?
What is viscosity?
How to measure viscosity manually?
How to measure viscosity automatically?
What makes a viscometer measure well?
How sensitive is the viscometer?
How to decide whose viscometer to use?
1. Why worry about viscosity?
Before we begin to study what viscosity is and how it can be measured let us first clarify why we should be interested in viscosity.
When a machine is applying a printing ink, adhesive or other coating to a web it is the intention of the machine operator that the machine apply only what is needed to insure a proper film thickness.
As the % solids varies so also will the film thickness and this variance in film thickness will manifest itself in color shifts, adhesive problems, and/or excess consumption. There can also be problems with solvent retention, poor drying and excess tack.
It is well known that if the % solids content of a fluid changes then there will be a change in that fluids viscosity. Since there are no simple devices to measure % solids it has been long understood that if the viscosity is controlled then the % solids is controlled.
Also well known is the fact that solvents evaporate from inks and coatings and thus the % solids tends to increase. There is also the problem of new additions, to the supply tank, which may or may not be of the correct % solids and thus when added to the existing ink/coating will distort the % solids.
Actual data on the problems associated with incorrect % solids are hard to obtain but here are a few benchmark pieces of data.
1. If one is using the original two roll flexographic coating/printing process and if the viscosity increases by 1 Zahn Cup Second then the printing process will begin to apply 50% more ink than is required. This excess ink will not improve color but simply results in a very large increase in ink consumption, an unnecessary expense.
2. A flexographic printer in Canada compared the amount of ink consumed to print 750,000 pouches. They found that with manual control 81kg of ink were consumed but with automatic control they consumed only 22kg of ink.
3. Another printer in Canada was applying white ink to 50,000 pounds of plastic film. Normally they would consume $5,300 of ink with manual control. When automatic viscosity control was used, the ink consumption was reduced to $2,700.
4. A printer in England did a test of manual vs automatic control ( see our Reprint RP155 ) and found that besides better quality they reduced their ink consumption by 22%.
Whether one is using a flexographic or rotogravure printing/coating machine the benefits of viscosity control are better quality and less material consumption.
Now that we have answered the question about why one should worry about viscosity let us proceed to just what is viscosity.
2. What is viscosity?
Viscosity is unlike temperature and pressure, which can be measured with static devices such as thermometers or pressure gauges. Viscosity is a dynamic number, ratio of shear stress to shear rate. This dynamic is impacted by the molecular bonding within and between pigments, binders and other materials in the ink or coating. The viscosity value obtained from a viscometer will not only be a function of the coating but also a function of how the coating’s viscosity was measured.
The basic unit of viscosity is the Poise whose unit of measure is gm/(sec-cm). This unit is determined by imagining a small rectangular block on a large flat surface ( parallel plates ) with the fluid to be measured between them. A force is then applied to the block and it’s resultant velocity is measured. Viscosity is the ratio of these two factors.
The units of measure for a Poise are derived from the following:
Shear Stress Force / Area Dynes / cm2 gm
––––––––– = ––––––––––– = –––––––––– = –––––
Shear Rate Velocity / Gap (cm/sec)/cm sec-cm
Admittedly, all of us can feel temperature and pressure yet who of us can feel gm/(sec-cm). This is a combination of measuring units that are hard for us to touch, feel or see yet at the same time viscosity is very important to many processes.
There are some fluids, such as motor oils, which have a consistent response to shear stress and fluids such as these are called Newtonian. Fluids such as inks are Non-Newtonian and are in fact a special type of Non-Newtonian, called Thixotropic, meaning they are shear sensitive.
Since different components are used to make different types and colors of ink you will thus have different shear rate responses to given shear stresses.
There are different methods of measuring viscosity being utilized in today’s equipment and because of the forgoing you can often have several viscometers indicating different viscosities for the same sample.
An interesting illustration of this phenomenon is to calibrate several viscometers on an oil sample and then have them measure the viscosity of an ink or coating. They would all give you different values of viscosity and they would all be correct. The differences arise because different viscometers use different principles of operation and thus apply different amounts of shear stress and/or monitor the shear rate to a greater or lesser degree.
It is for this reason that when selecting a viscometer you should focus on one that is most similar to the method used to define viscosity and to one that is sensitive to changes in viscosity as well as being simple to maintain and understand. Those that are most sensitive will adhere most closely to the parallel plates used to define viscosity.
Now that we know why it is important to worry about viscosity and what viscosity is let us now discuss different ways to measure viscosity.
3. How to measure viscosity manually?
There are several different ways of measuring viscosity and it is logical to compare those methods to the parallel plates used to define viscosity.
The most common and simplest is a ‘dip’ or ‘efflux’ cup. There are many of these on the market and they are used by dipping the cup into the ink, lifting it out of the ink and timing how long it takes for the ink to drain out of the cup. The drain time is a function of the inks viscosity.
This technique measures viscosity as Centistokes and not Centipoise but you can convert Centistokes to Centipoise by simply multiplying by specific gravity.
4. How to measure viscosity automatically?
There are five primary techniques being used, by today’s manufacturers of process controls, to measure viscosity.
The Falling Ball or Dart.
These devices have a glass tube approx 1 cm ID within which is a ball or dart. Outside the glass tube are two or more proximity switches. Fluid flow is used to lift the ball/dart and then stopped. When the fluid flow stops the ball/dart will drop and the proximity switches can measure the drop time. This drop time is a measure of viscosity.
The Moving Piston.
A small piston, held in the magnetic field of two electric coils, can be moved back and forth a small distance. The ink/coating will flow in and around this moving piston and by measuring how long it takes the piston to move from one coil’s magnetic field into the other’s field one can monitor changes in viscosity.
The Falling Piston.
A piston of a known size falling into a fixed bushing, under the constant force of gravity, will fall in a time that is a function of viscosity. The sample is measured as the piston falls and the sample is pressed out of the bushing, through the gap between the outside of the piston and the inside of the bushing. The time the piston takes to fall (TOF) is directly proportional to the viscosity in Centipoise.
The Rotating Cylinder
A cylinder rotating in the fluid will experience viscous drag on it’s surface. This viscous drag is a function of viscosity. The amount of drag can be measured through a number of means.
The Vibrating Rod
This concept relies upon the dampening nature of fluids. The viscosity measurement is taken at the surface of the rod, where vibrations interact with a layer of fluid. These vibrations do not penetrate more than a fraction of a mm into the fluid. Thus there is very little measurement of shear rate, however, one can see changes in viscosity by measuring the dampening from the fluid.
The Double Diaphragm Pump.
If a double diaphragm pump is supplied with a constant pressure air supply and the fluid viscosity changes then the pump rate will shift. This shift in pump rate can be measured and used to determine changes in viscosity.
5. What makes a viscometer measure well?
A. A consistent and well defined movement between two surfaces.
B. Able to function well under various flow, pressure and density shifts.
6. How sensitive is the viscometer?
The viscosity of water changes by a factor of three as it cools from 100C to 25C.
In order to detect this viscosity shift the sensor must have two surfaces with clearly defined movement between them.
To the best of our knowledge the only off the shelf viscometer that can make this chart is the Falling Piston type.
7. How to decide whose viscometer to use?
In the world of machine control it is important that when a device like a viscometer is purchased that it will operate for many years and provide consistent results during that time.
Some points to consider before purchasing:
Is the sensor simple and easy for both operators and maintenance staff to understand?
How rugged is the sensor? Will it survive years of (abusive) use?
When there is an operational question can the sensor problem be diagnosed with simple tools and minimal training ?
Can the sensor be calibrated in your plant or will you need to send it back to the
factory for repair/calibration ?
How sensitive is the sensor to changes in viscosity?
Can it measure the difference in the viscosity of hot and cold water?
Can the sensor operate on both water and solvent based inks?
Are there sensors available for either In-Tank or In-Line use?
For In-Line sensors, how many coating or solvent lines are required to operate
the sensor ?
Do the controllers provide easy to use screens and trend plots?
Do the controllers easily reprogram for use on solvent or water based products?
What technical support can you expect from the supplier?
What is the level of experience and/or expertise which the supplier brings with their equipment ?