Viscosity and pH!

From a presentation by Stephen Olney
at the 1992 FTA Forum

Viscosity and pH Control

Here's a puzzling scenario. A press operator measures the viscosity of the ink and detects an increase of I Zahn cup second. Adding water to thin out the ink, his new reading does not correct the situation. Finally he adds so much water that the ink "kicks out" and the resins in the ink become unstable, gum-like masses, an irreversible condition.

What was the operator's error? The answer is that the increase in viscosity was not due to evaporation, it was due to a drop in pH from 9.0 to 8.0. This caused a viscosity increase of one complete Zahn cup second which the operator interpreted as a thickening of the ink. The sad result in this case was a complete loss of materials and a major problem in the print run.

This is not an uncommon occurrence, although the result in this one case was devastating. In most cases, the results would be problems in the print run and quality or consistency shifts that make the printing job unacceptable.

This scenario points out the interrelation between viscosity and pH control. Both these parameters are too frequently neglected and all too often misunderstood. This article will explain these two parameters, provide insight into how to measure and control them, and provide information on how to evaluate some of the equipment available to monitor and control them.

What is viscosity?

Figures I and 2 show the non-linearity of viscosity. Assume that motor oil has been poured on the floor and a desk placed upside down on the oil (Fig. 1). One person pushing the desk will result in the desk moving at some velocity.

If one more joins in the pushing, the desk will move faster - in theory it will move twice as fast because the force is doubling and because the oil is NEWTONIAN. Newtonian means it has a shear response linear with shear force. If still two more join the pushing of the desk it will move 4 times as fast as when one was pushing alone.

If the oil were replaced with ink and then one person begins pushing, again a certain velocity will be attained (Fig. 2). Then if one more joins in to push, the velocity will increase. Instead of twice as fast, as with oil, the velocity might increase three fold. If four push, the additional velocity might only increase by another 20% (Fig. 3).

The results are non-linear because inks are thixotropic. They are thixotropic because they are a mixture of pigments, suspenders, polymers, solvents, etc. Each of these items by itself may or may not have Newtonian characteristics, but when combined they shear in unique, non-linear ways. Even changing a pigment color can change the thixotropic qualities of ink.

A good incentive for understanding viscosity is to understand how improper viscosity can cost money. Keep in mind that a shift of I second on Zahn Cup can result in 50% excess ink laydown.

Knowing what viscosity is may be important, but more important is learning where, how and when to measure it. If fresh unmixed or poorly mixed ink is measured with a Shell or Zahn cup and then again after 10-20 minutes of pumping the result is two different readings. Both are correct but only one should be used. The latter reading more correctly relates to what is needed for correct printing. Ink must be conditioned to remove its "false body". And this conditioning usually results from the shearing action of the pumps, anilox rolls, metering rolls and/or doctor blades.

Not only can one get different viscosities from the ink, different viscometers can measure the same ink, give different viscosity values and yet all could be correct. This is because different amounts of shear force are applied to the ink by the different measuring principles. Since the ink undergoes a fair amount of shear as it passes the anilox roll and onto the web, it only follows that one should choose a viscometer that applies a fair amount of shear during measurement. In this way the nature of the ink will be measured under more realistic printing conditions. If a viscometer indicates no difference between fresh and conditioned ink, or between different colors of conditioned ink, then its effectiveness should be questioned.

Another consideration is that water-based products are much more thixotropic than solvent based products. Hence, the selection of viscometers must be made with more care when working with water-based products.

Understanding that viscosity and how it is measured are important, a review of the measuring devices available is in order.

Efflux cups are used for manual control. These high speed photos show the difference between a Shell Cup and Zahn Cup as they finish draining (Fig. 4). The Shell Cup cuts much more cleanly at the end of it's draining, allowing those measuring ink viscosity to obtain much more consistent stopwatch readings. This is due to a longer snout and recessed tip. The recessed tip also provides protection if the cup is dropped or tossed into solvent cans. The long snout also increases sensitivity so that a change in viscosity of 16-17 #2 Zahn would read as a change from 18-25 for a #2 Shell. This means that with the Shell Cup you can measure a viscosity change equivalent to .2 Zahn Cup Seconds.

Automated viscosity control systems are impacted by the sensor technology being used and for those in the printing field there are several key points to remember in evaluating viscometers. The measuring element should be simple, rugged and accurate. Can an operator easily determine whether or not it is working correctly? Can it stand the rough operating environment of a printing plant? Is it sensitive to changes in viscosity? The chart here shows the viscosity of water and was obtained with a falling piston viscometer (Fig. 5).

The water was heated to 212'F and then allowed to cool to room temperature. During this cooling process the viscosity was measured once every 3 minutes as the recording was made. No other printing ink viscometer has been able to approach this kind of sensitivity and repeatability.

This sensitivity on water is unique to the falling piston process viscometer. While inks are not water, today's printing processes are becoming ever more sensitive to ink shifts, and in some cases are running at viscosities approaching those of solvent and water. If the viscometer you are evaluating cannot reproduce this chart, it may be lacking the sensitivity and reliability needed to achieve the efficiencies, quality and savings sought.

A final point to keep in mind in selecting a measuring element is drying resistance. How well can the sensor handle ink buildup? Does buildup effect the viscosity readings?

When considering automated viscosity measurement there are several types of systems available. One system is the automated efflux cup. Ink is pumped into the cup and then allowed to drain. By means of photo optic sensor, the drain time is determined.

These photo optic sensors are very susceptible to material build up, are difficult to keep clean, and lack sensitivity. Furthermore, they require extra ink lines, resulting in more congestion around the press and excess solvent generation during cleaning.

Another system is built around a rotational device. There are a number of versions of this principle on the market. In these devices a member is rotated in the ink and the viscous drag on the member is measured. The greater the viscosity, the greater the drag.

Some of these sensors can be affected by turbulence in the ink tank and therefore require separate sampling chambers. Overall, they lack good sensitivity to shifts in viscosity, which limits their ability to control. Also, the design of these sensors makes visual diagnosis of the problem difficult.

The most appropriate system for printing plants utilizes the falling piston viscometer (Fig. 6). Shown here is an illustration of the interesting relationship between the falling piston and the basic definition of viscosity. You can see that the falling piston acts like the illustration of the desk on the floor, thus providing some shearing to the ink so as to be measuring it under more realistic printing conditions.

The first phase of the falling piston Measuring cycle is the filling phase (Fig. 7). Here the piston is raised and a sample of ink is pulled into the space formed beneath the piston. The second part of the measuring cycle is the measuring phase. Here the piston is allowed to fall, by gravity, and the time of fall is a function of viscosity.

The falling piston cycle can be easily observed by any operator who can then take corrective action should a problem arise. The falling piston offers tremendous sensitivity to shifts in viscosity and at the same time features a simple and rugged design. This insures that the equipment can be easily maintained and kept operational for many years.

Of all three methods for measuring viscosity this is the most sensitive, simple and rugged way to measure viscosity.

Where to place the viscometer is another important consideration. Viscosity can be measured in one of three locations. Some viscometers measure in sampling chambers. In this case the ink is pumped to the chamber, measured and returned to the ink tank. Others measure directly in closed lines, where again the ink is pumped through the unit and either back to the ink tank or on to the fountain. Most common is the measurement of viscosity directly in the ink tank (Fig. 8).

While some suppliers offer their units in one or more of these locations, the ink tank location is preferred because it is much more easy to clean and maintain. This location also will result in less solvent being used because of the absence of extra ink lines or equipment to clean and there is less clutter around the ink pails. This is where the main mixing activity takes place. This is where the operator takes his measurements. And this enables a sensor to be cleaned right along with the pump without generating extra waste solvent or cleaning fluid.

Only Norcross can supply a viscosity sensor for use in any of these three locations: in a tank, in a sampling chamber, or in-line.

Once a sensor is chosen, a viscosity controller can then be interfaced. There are controllers that feature single-station microprocessor control as well as advanced PC based systems providing a single control display for multiple viscosity control stations, SQC data for quality control, as well as for EPA/OSHA and IS09000 requirements.

Controlling viscosity can improve product quality and can cut production costs, waste, and material costs. But controlling viscosity means controlling any variable that can cause a shift in viscosity. And that means controlling pH.

What is pH?

pH is a measure of the acidity or alkalinity of an ink or coating. This is determined by measuring the presence or absence of charged Hydrogen ions in the coating or ink (Fig. 9).

If there are a large number of negative ions then the pH is alkaline, greater than 7.0. If there are very few, the pH is acidic, less than 7.0. Water typically has a pH of 7.0 and is considered neutral, straight ammonia has a pH of 11, Baking Soda, 8.2, and Coffee or Wine, 5.0.

Maintaining pH is important because it is negative ions that keep the ink in suspension. The ink manufacturers coat the pigment molecules with negative charges. These provide a repulsive force that keeps the ink in suspension. When there are too few negative ions, the ink molecules begin to move closer to each other. This closeness causes an increase in viscosity and can cause the packing of anilox rolls and printing plates.

pH shifts in water-based inks can impact your printing in several ways. A high pH can cause slower drying, blocking in the web, poor water resistance of the finished piece, odor problems, pigment burnout and low viscosity. A low pH can cause dirty printing due to anilox roll and plate packing, irretrievable settling of the ink and it can increase viscosity .

To measure pH, pH probes are constructed with thin glass membranes which contain very small holes (Fig. 10). These holes are so small that only ions can pass through them. On one side of the membrane is ink. On the other side is a fill solution. The fill solution is capable of picking ions off the ink molecules and then passing their charge to a wire. As ink brushes against the membrane some of its ions are transferred through the membrane. If a large number of negative ions pass through there is a high pH. As the number of ions decrease, the current flow decreases and the pH is lower.

Figure 10 illustrates the basic process. At Step I the ion is transferred from the ink to the fill solution. At Step 2 the ink molecule is moving away while the ion is moving towards the wire. At Step 3 the ion charge is passing to the wire as the ink moves out of the chamber.

All pH probes work in this manner though the shape of the glass membrane may vary. All probes are susceptible to damage from ink drying on the glass. When ink dries, it covers the holes (Fig. 11) and, since water-based inks resist rewetting, the holes remain permanently blocked. This limits the flow of ions so the probe will require recalibration or replacement.

Even recalibration can be fraught with peril. Calibration solutions can change pH over time, and once exposed to air, the solution begins to change its pH. Aside from the need to clean and calibrate probes, temperature differences between the fill solution and the ink being measured must be considered. When you insert a hand probe into ink the fill solution will be at a different temperature than that of the ink. This temperature difference can cause errors in reading pH by as much as 1 whole point. Probes must be given sufficient time to come to ink temperature. Unfortunately a press operator is not likely to wait for the 15 minutes to get a truly reliable measurement. To resolve this, Norcross Corporation introduced an on-line pH systems that eliminates this problem as well as provide continuous pH indication and/or control (Fig. 12).

To succeed, an automatic pH measuring system must be designed to deal with these temperature and drying problems. The system shown here is a patented design which is able to overcome the problems encountered with handheld probes.

This sensor assembly mounts in the ink line, typically on the discharge of the ink pump. The ink flows in through the lower left connector and on to the fountain through the upper right connector. A flushing valve is provided for clean up.

A cross section view provides an inside view of the device (Fig. 13). The ink flows through the chamber, is diverted across the surface of the pH sensor and on to the ink fountain. The sensor is always submersed in fluid and remains wet at all times. When the ink lines and doctor blade chambers are flushed, the pH sensor is simultaneously flushed.

When the pump is turned off a small pool of cleaning solution remains trapped in the chamber - this keeps the probe wet and in good operating condition, minimizing calibration and/or probe replacement.

In an integrated, automated system (Fig. 14), the pH sensor is connected to a digital indicator that has both high and low alarm settings with relay outputs. If the pH drops below the set point, the relay can turn on a metering pump to add correcting solution to the ink or sound an alarm. Alternately, a gravity feed correction solution can be used with its flow controlled by a two-way solenoid valve.

In summary, the most important point to remember is that pH and viscosity are both important parameters and they are both unique, non linear and interdependent. Thus, the viscosity control system should be able to determine if pH is out of range and not take corrective action until the pH system has restored the pH to its correct value.

While temperature, pressure and flow are quite straightforward, viscosity and pH are not. We cannot see or feel them, they are totally dependent upon other devices to measure them. Perhaps the best advice is to ask lots of questions about the equipment being evaluated, and to work with companies that are dedicated and proficient in supporting and understanding these parameters.