Relationships Between Mechanical

Properties and Performance of

Inks As the Basis for Quality

Control Techniques

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Polymer Science and Standards Division

Washington, DC 20234

May 1983

Issued July 1 983

Prepared for U.S. Department of Treasury

r u of Engraving and Printing

100

\

.1156

tment of Research and Technical Services

ngton, DC 20226

33-2691

1983

C.2

NATIONAL BUREAU

OF STANDARDS

LIBRARY AUG 2 5 l&u

NBSIR 83-2691

7

RELATIONSHIPS BETWEEN MECHANICAL

PROPERTIES AND PERFORMANCE OF

INKS AS THE BASIS FOR QUALITY

CONTROL TECHNIQUES

Donald L. Hunston

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Polymer Science and Standards Division

Washington, DC 20234

May 1983

Issued July 1 983

Prepared for U.S. Department of Treasury

Bureau of Engraving and Printing

Department of Research and Technical Services

Washington, DC 20226

2

o

(

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director

Relationships between Mechanical Properties and Performance

of Inks as the Basis for Quality Control Techniques

Background

For the last two years the National Bureau of Standards has been working

with the Bureau of Engraving and Printing in a program designed to gain a better

understanding of the factors that control ink performance in the manufacture

of paper currency and through this knowledge to develop improved quality

control (QC) tests for these inks. The purpose of this report is to summarize

the results of this program to date and to recommend the most promising areas

for continued research. Much of the detailed information developed during this

study is contained in Appendix I and so the main test focuses more on

recommendations and conclusions.

The approach taken in this program can be roughly divided into six steps.

First, a small number of ink samples were to be rheologically characterized

in a very complete way to develop the necessary test technology and equipment.

Second, this technology was to be extended to develop the cabability for

measuring the rheological changes that occur when the ink is cured under simulated

press conditions. Third, a few ink samples were to be thoroughly characterized

before and during cure so that those parameters most directly related to

performance could be identified. This minimizes the number of tests that must

be performed in subsequent part of the program and thus a larger number of

ink samples can be examined. Fourth, the parameters identified in this way

were to be determined for a larger number of selected ink samples with known

performance so that property/performance relationships could be established.
- 2 -

Fifth, based on this information, specialized measurement techniques

were to be designed and/or adapted for QC applications. Sixth, the validity

of these tests was then to be established by again examining inks with

known performance. This requires both the reexamination of inks previously

tested and the characterization of new ink samples. It was felt that this

approach provides not only insight into the fundamental properties that

control ink performance on the press but also a basis for selecting the best

QC techniques that can be developed.

* During the past two years, the joint NBS/BEP program has completed

steps 1, 2, and 3 and has made a significant start on step 4. (Details of

this work are covered in Appendix I). Although future plans will be

considered in more detail later in this report, it is worthwhile to point

out here that the future work would involve the completion of step 4 and a

major effort to address steps 5 and 6.

Experiments

In presenting the conclusions and recommendations of the work to date

it is useful to discuss them in terms of the types of experiments performed.

All of the techniques employed thus far looked at the shear properties of the

inks. Three basic types of shear experiments were examined: Steady Shear,

Oscillatory Shear, and a combination of the two. All of the experiments

utilized the convensional cone and plate geometry similar to that used in the

Haake viscometer at BEP but with a much more versatile instrument. As

indicated in Appendix I the oscillatory behavior and thus the combined

oscillatory and steady shear behavior were very complex. In general, however,

the results lead to the same conclusions as the steady shear experiments.

Consequently, only the latter tests will be discussed here but details of the

other tests can be found in Appendix I and previous progress reports.
- 3

-

Steady Shear. In the steady shear tests the sample is initially at

rest and then suddenly subjected to a constant rate of shear flow. The

stress level associated with the initiation and maintenance of this shear

is measured as a function of time. When the experiment first begins a

high stress level is required to initiate the flow but then over a period of

less than 10 seconds, the stress level falls and approaches a limiting value

corresponding to steady-state flow (Figure 1 in Appendix I). A similar

experiment can be performed in which a steady flow is suddenly stopped and the

fall off of the stress is recorded as a function of time but the discussion

here will be restricted to the former experiment. The other variable that can be

examined in this test is the rate of shear. For the experiments performed here,

-? -1 -1

the shear rate was varied from 5x10 " sec to 500 sec . The experiments

were conducted by allowing the ink to rest in the viscometer for ten minutes or

more before each test with a new test for each shear rate.

The results for specific experiments are given in Appendix I but the

general behavior can be illustrated by a 3-D plot showing the viscosity, n,

as a function of time and shear rate. This is shown schematically in Figure 1.

For a simple material information such as that in Figure 1 would provide a

complete characterization of the shear properties, i.e., based on this data any

flow behavior could be predicted. In this case however the material is not simple

since the high values of n at short time are the result of a need to break-up

a structure which forms when the ink is allowed to rest. If the experiment is

stopped and then restarted without the rest time, the high values of r> at short

times are not obtained; i.e., the short time values are more similar to the

steady state values. The magnitude of the short time viscosity therefore depends

on the previous shear history of the ink. Only if the ink is allowed to rest

for a significant period of time can the original behavior again be obtained.
- 4 -

In terms of quality control tests both the short time properties (yield

behavior) and the steady state values may be of interest. In the test now

performed by BEP the viscosity is measured for a specific time/shear rate

history (i.e., both time and shear rate are changing simultaneously during the

experiment). This means the properties are determined along a specific path

such as that illustrated in Figure 1 by the dotted line. Consequently, this

test provides some information about both the short time and steady state

behavior but only to a very limited extent. In considering what aspects of the

behavior shown in Figure 1 might be most appropriate for a QC technique, it must

be noted that the short time (yield) behavior is not only complex but also

dependent on how the experiments are performed (i.e. the shear history of the

sample). Consequently, it is felt that a simple QC test is not well suited to

examining such complex behavior in detail and thus we would conclude, for the

moment at least, that the qualitative test of yield behavior now performed at BEP

is as good as anything. Moreover, one can argue that since the ink in the fountair

is in constant motion (no rest time), the yield behavior may not be as important ir

determining performance as the steady flow viscosity. (One exception to this

general conclusion will be mentioned later.) As a result, the efforts in this

program were channeled away from yield behavior into a detailed study of the

steady-state flow viscosity for the inks.

The results of this aspect of the program are covered in Appendix I and

previous Progress Reports. The general conclusions, however, can be illustrated

with one particular example. Figure 2 shows a plot of the steady-state viscosity

vs. shear rate for three ink formulations (BK-60, Bk-62, and Bk-62 Mod 3).

At high and low shear rates all three inks have similar viscosities but at

intermediate shear rates the viscosities are quite different. It is known that

two of these inks give reasonable, although not ideal, press performance while the
- 5

-

other (BK-62) is unacceptable. Based on these data, if we ask how it is

possible for two of the inks to work while the other does not, the only

conclusion we can reach is that steady-state viscosity is not a very sensitive

parameter for assessing performance. Obviously, there are limits on how much

the viscosity can change before unacceptable performance is obtained and

certainly some viscosity QC is needed to be sure that these limits are not

exceeded. The results suggest, however, that the range of useable values is

relatively broad. One explanation for this result is that the shear rates

seen on the press are very high during ink transfer and very low in the

fountain. For these ranges all the inks look quite similar. Our conclusion

then is that steady-state viscosity is a useful but not sufficient QC parameter.

It is worth noting, however, that the fabrication of the inks involves shear

rate in the intermediate range where the viscosities of different ink formulations

vary substantially. Viscosity information may therefore be quite important in

ink fabrication.

Aging . One other aspect of the behavior of the uncured inks was measured,

i.e. aging effects. A sample of BK-62 was stored at room temperature and during

the first few days a hard crust formed on the ink surface that was in contact

with air. After more than 6 months, the measured mechanical properties were

examined for a sample extracted from a region that was about 1 cm below the

crust. Di spite the aging time, the viscosity was quite similar to that obtained

for the ink on the same day it was made (Figure 3). A similar test was performed

using a sample of BK-62 Mod 3 that had been aged for 5 days in an oven at 60°C.

In this case there was a measurable increase in the viscosity but this increase

was quite small (less than 40%). The conclusion from these tests is that, at

least after the first day, aging in the absence of oxygen does not change the

mechanical properties of the ink very much. When oxygen is present, however,

very large changes occur. These changes can be accelerated by heat but heat

alone has only a small effect.
- 6 -

Curi nq . In addition to examining the behavior of the uncured ink it was

felt that the curing behavior should be studied. To accomplish this, ink samples

were placed on an apparatus with two rollers one of which could be heated to

80°C (roughly press temperature) and driven to produce rotation of the two

rollers. In this way the ink could be cured under conditions that provide a

crude simulation of what might be seen on the press: exposure to oxygen and

heat in a thin film and a constant mixing and renewal of the surface to prevent

the formation of an outer crust. At selected time intervals, samples were

removed and cooled rapidly to room temperature. This cooling did not stop the

cure reactions but did slow them sufficiently so that short time experiments

could be conducted to characterize the mechanical properties at various stages

of cure.

Details of the results for these tests are contained in Appendix I, but

the conclusions can be briefly summarized as follows: (1) the cure behavior

of the inks can be characterized, (2) different formulations have quite

different curing behavior, and (3) the curing behavior of an ink can have an

important influence on the performance on the press. These tentative conclusions

have some important implications since it means that anything that advances the

cure of the ink too soon can affect the press performance. There are a number

of areas where this might happen but the two major situations involve over

(or under) exposure to oxygen and temperature either during fabrication

(particularly on the mill) or during printing while the ink is in the fountain.

If these exposures could be better controlled and made more reproducible from

batch to batch, we believe the variations in ink performance characteristics

could be reduced.
- 7 -

Concl usions

Ink Problems . Before addressing the question of quality control

techniques directly, it is useful to say a few words about ink problems.

These problems can be divided into three categories: those unrelated to

ink quality, those marginally or indirectly related to ink quality, and

those directly related to ink quality. The first category can be

illustrated by the results from split lots tested during the BEP effort last

year to monitor inks. In some cases a single barrel of ink was used with

several lots of paper. The spoilage figures for the lots printed with the

same ink should be very similar but the results show the spoilage can be

quite different. Regardless of why the results were different (press set up,

paper, etc.), the important point is that there are major uncontrolled factors

other than ink quality that influence performance. Consequently, some of the

variations in spoilage have little to do with ink quality. This makes the

establishment of QC relationships difficult although not impossible.

The second category of problems involve ink quality in only a marginal or

indirect way. For example, the ink in the fountain is exposed to oxygen at

the upper surface and as a result, some curing will take place. Since the ink

in the fountain is rotating, however, on average the exposure time is short and thus

curing is slow. In the corners of the fountain, however, it is oossible for reaions

of ink to become stagnant and receive large exposures to oxygen. Curing may

then produce a semi-solid chunk of ink which can get transferred to the rollers

as a lump and lead to problems. There are also other places in the ink supply

chain where similar things can happen. This problem is not attributable to the

ink but rather to the design of the ink supply system of the press. Consequently,

better quality inks will not solve this problem. Nevertheless, there are

features of the ink which can aggravate the problem. For example, a high yield

stress may make the ink more likely to develop stagnation regions in the
- 8 -

fountain. Thus ink quality may play an indirect role in the problem. The

final category of problems are those that can be directly related to ink

quality. Perhaps the most obvious example of this is a non-turning ink.

Improved ink quality could greatly reduce spoilage associated with these

problems.

Quality Control Tests . In terms of QC techniques, the methods presently

used at BEP can be described as follows. The measurement of viscosity at 25 rpm

provides information on steady-state viscosity at high shear rates. This

is important for broad control of properties but is not very sensitive to the

type of problems seen with the present inks. The yield viscosity may provide

direct information with regard to the tendency for stagnation regions to

develop in the fountain or el sewhere, but the yield measurement is in other

respects more of an indirect measure of other properties which influence

performance since on the press the ink is always in motion. Finally, the TIC

and the tack-o-scope measurements provide a useful measure of cure which we

have concluded is an important factor in determining press performance. The

information obtained, however, is only a single point in each case. This

provides a very limited picture of the cure behavior and in this sense may

not be the information of most value in predicting performance. This suggests

that the future work should seek to obtain methods for determining more

comprehensive information on the cure behavior of the inks. Consequently,

this area will receive a major effort next year.

Future Work . In future work on this program, 4 different cure monitoring

techniques will be studied. First, viscometry will be investigated to determine

what aspects of the behavior are most closely related to performance and the

degree of sophistication in the measurement that must be used to obtain the

desired information. Second, a close look at the tack-o-scope will be made to
- 9 -

determine how it might best be used as a QC technique. Third, a new measurement

method that utilizes ultrasonic shear waves to examine the mechanical properties of

thin films will be tested. Preliminary work during the past year has shown that

this experiment can be used successfully to follow the cure of an ink sample

(see Appendix II). Finally, an effort will be made to study the film splitting

behavior of these inks. All of the work in this program to date has studied

shear flow. For a simple material this is sufficient to completely characterize

the flow behavior. With more complex materials, however, it is also useful to

examine elongational flow particularly when the application involves film

splitting since this has a substantial elongational component. During the last

year, some initial studies have examined the elongational flow behavior of the

inks in a qualitative way and found that batch-to-batch variations could be

detected. Future studies will investigate this measurement approach in more

detail.

In studying the relationship between QC measurements and ink performance,

it is important to have ink samples whose press performance is known. A large

number of samples from last years ink evaluation program are still available,

however, a number of extraneous uncontrolled variables were present in that

program. A new evaluation project at BEP is planned for this year and by utilizina

what was learned during the previous effort, the new samples should provide a much

clearer picture of the relationship between performance and properties. To the

extent possible, these samples will be included in future work on this project.

Figure Caption

Figure 1: Schematic diagram of start-up viscosity plotted against

time and shear rate. Figures 2 and 3 here and Figure 3

in Appendix I represent slices of this surface along

planes of constant time while Figure 2 (and Figure 1 if

converted to viscosity rather than shear stress) in

Appendix I represent slices at constant shear rates.

Figure 2: Steady flow viscosities as a function of shear rate for

X BK-60; A BK-62; and BK-62 Mod 3.

The steady flow viscosity for samples of BK-62 unaged

and aged X and of BK-62 Mod 3 unaged /\ and aged . Figure 3:

Lo

g(Viscosity)

Figure 1 0) 4-* 03

cr

L.

03

<D

JC

c/)

O)o

_]

fi

me
VISCOSITY:

BK-60,

BK-62,
AGING

STUDY

:

BK-62

Appendix I

Rheology of Cure for Intagl

Printing Inks

Rlujology of Cure for Intaglio Printing Inks

D. L. Hunston and J. L. Rushford

National Bureau of Standards

Polymer Division

Washington, D.C. 20234

W. Rc Newitt and B. A. Vaudreuil

Bureau of Engraving and Printing

Research Division

Washington, D.C. 20228

ABSTRACT

The intaglio inks used to print currency in the United States contain

relatively little solvent and dry primarily by chain extension and crosslinking reactions in the vehicles. To obthin good performance, the initially

fluid ink must change Theologically in the manner required to give proper

f transfer to the plate and then to the paper and to obtain sufficient hardness

at the end of the process so that the printed sheets do not smear when stacked.

To study these changes,. the rheology of two ink formulations wT ith very differen

press performances was examined, first in t'ne-uncured state and then during

curing. The uncured inks exhibited complex rheological properties including

time dependence, yield behavior, elasticity, and non-linearity. Curing of the

inks produced an increase in both viscosity and elasticity. The viscosity chan

» could be roughly fitted to a first order type equation. Comparisons

between these results and the performances of the inks on the press show that

if the rata at which properties change during cure falls outside a certain rang

acceptable print quality cannot be achieved.

INTRODUCTION

The manufacture of paper currency in the United States is the responsibiii

of the Bureau of Engraving and Printing (BE?) which is the world's largest

securities manufacturing establishment. BE? produces, on average, 16 million

notes per day which represents an annual face value of 43 billion dollars. An
- 2 -

intaglio printing process is used because it is the cost difficult process

to perform and thus to counterfeit.^ Other processes lack the fidelity of

fine lines and the distinctive third-dimensional effect of raised line on paper

inherent in intaglio printing.^ The plates are hand-tooled by highly skilled

engravers who engrave the designs using grooves of varying depths. The

currency is then printed on high-speed sheet-fed rotary presses that employ

two or four plates containing 32 notes each. The presses are capable of

printing 9,000 sheets per hour. The ink is supplied to the roller containing

the plates from the ink trough, or fountain, by a series of feed rollers. The

ink in the fountain is heated and kept in slow but usually constant notion.

After the hot ink is transferred by the feed rollers to the plate, ink is

removed from the elevated portions of the plate by a series of three wiping

cloths. A sheet of paper is then forced under heavy pressure against the plate

so that it picks up most of the ink remaining in the fine engraved lines. The

sheets are immediately stacked by the press and on the following day the reverse

side of the sheets are printed in the same way

To efficiently produce a high quality product requires the use of special

formula, fast-drying inks, developed in BEP's laboratories. The rapid drying

eliminates the time-consuming need for tissuing or interleaving between sheets to

prevent smearing when stacked. Rapid drying on the sheets alone however is not

sufficient to guarantee good results. The proper transfer of the initially

liquid ink from the fountain to the plate via the feed rollers, the complete

removal of the ink from the high portions of the plate by the wiping cloths,

and the necessary transfer of the remaining ink to the paper require that the

ink be in the proper rheological states at the appropriate times. As a result

the rheological changes that occur during drying must proceed both rapidly and

correctly if proper print quality is to be achieved.
- 3 -

To minimize the problems associated with poorly performing inks, a

number of quality control (QC) tests are presently being used. Unfortunately,

the complexity of the printing process is such that undetected variations in

ink batches are still present and this leads to a higher than desirable

percentage of ink batches with unacceptable performance. To help minimize the

problem, a cooperative study was initiated between the National Bureau of

Standards and BEP with the objective of obtaining a more complete picture of

the basic rheological properties of the black ink and how these properties

change as the ink cures. This information combined with the results of other

2—6

studies will provide both the basis for more reliable Q.C. techniques and

the data required to help design improved ink formulations in the future.

BACKGROUND .

The inks used in currency printing are complex formulations based on

natural drying oils such as tung oil and linseed oil. These oils contain

multiple double bonds which act as polymerization sites when exposed to

oxygen. Although the details of the chemistry are not fully understood,

some important observations can be made. If a bulk ink sample is exposed to

air for several days a hard skin forms on the surface. Underneath this skin,

however, the mechanical properties of the ink remain virtually unchanged for

many months. Thus the polymerization requires direct exposure to oxygen and

so the cure can be accurately studied only with experiments using films.

The reactions are also greatly accelerated by increased temperature. Changes

which require many hours at room temperature will occur in minutes or seconds

at press temperatures.
It is the black currency ink which has been the most troublesome and is

therefore the subject of this study. This ink contains very little solvent

drying experiments show a weight loss of less than 1% to full cure—and thus the

hardening must proceed by polymerization and cross-linking reactions. In a

recent study of this ink^ it was demonstrated that cure reactions are a critical

factor in the performance of the ink on the press. Consequently, an examination

of curing behavior is necessary to the development of an understanding of ink

performance.

In the present study two different ink formulations were examined. They

are designated as BK-62 and BK-60 modification 17 (hereafter designated

simply as BK-60) . Formulations BK-60 was used for a number of years and gave

acceptable performance although there was room for improvement. Formulation

* BK-62 was developed more recently as a trial material with a much simpler

composition. Experiments with this ink on the press showed that it had

unacceptable performance and this has led to a modification in the BK-62

formulation. This new version which shows marked improvement^ is now being

utilized in production and studied in this program. For the present discussion,

however, it is useful to examine the original BK-62 and BK-60 as examples of

an unacceptable ink and an acceptable, although not ideal, ink. It is also

helpful to note that although a number of problems were observed with BK-62, a

2 major difficulty was "excessive drying on the plate".

EXPERIMENTAL SECTION

Various samples of the two different ink formulations were evaluated for

mechanical properties prior to and during cure. Both formulations are based on
- 5 -

natural vehicles and are highly filled with pigments and additives.

There are a number of differences between the two formulations but the

most important is that the samples designated BK-60 contain a linseed oil

vehicle system while those designated BK-62 contain a tung oil system.

Details of the ink’s composition are given in Table I.

The inks were examined by using transient, steady flew, and

oscillatory tests performed at room temperature , 22±0 . 5° C. These

experiments were performed on a specially modified and computerized cone

and plate viscometer. The cone utilized was 2.5 cm in diameter and 0.5

degrees in angle. Details of the data acquisition system for this

device have been described previously^ and will only be summarized here.

The plate can be driven in steady rotation or oscillatory rotation. The

motion of the cone, which is attached to a torsion bar and held in line by

an air bearing, is monitored to provide a .measure of the stress. In the

oscillatory experiments, the drive shaft is attached to a shaft angle

encoder that generates 360 equally spaced pulses per cycle. Each pulse

triggers the acquisition of position data from displacement transducers

attached to the cone and the plate. This information is transferred to a

minicomputer for storage and analysis. From this data the stress and strain

in the sample can be calculated and thus the mechanical properties can be

determined. In some cases it is useful to examine the stress curve itself

and for this purpose the digitized data can be accessed.’ These data are

in arbitrary units but for the test conditions used here they can be

2 converted to stress in dynes/cm“ by multiplying by 8.120. The experiments

-4

were performed at frequencies from 10 Hz to 6x10 Hz and amplitudes between

-4 -2

4x10 radians and 4x10 radians.
- 6 -

In the initiation of steady shear flow and the steady shear flow

experiments, the shaft angle encorder was attached to a timing motor

and data acquisition proceeds in the same way as in the oscillatory

_2 experiments. These tests were performed at shear rates between 5x10

sec ^ and 2000 sec .

After characterizing the basic shear properties of the inks, their

cure behavior was investigated by placing samples on a specially designed 2

roller apparatus where one of the rolls was heated to 80°C (roughly equivalent

to the press temperature) . This setup provides some of the important

conditions seen on the press, i.e. the application of heat, the formation

of a thin film for maximum exposure to oxygen, and the constant renewal

of the free surface to prevent a hard crust from forming. After various

. > cure times a small portion of the ink was removed from the mill and

characterized for shear properties using the same techniques described

previously. Once curing had begun on the mill, it continued even after the

ink was removed and cooled to room temperature. Nevertheless, the cooling

slowed the changes in mechanical properties due to curing sufficiently so

that short term characterization tests could be performed. On the mill

complete curing of the- ink film required more than half an hour. This is

much longer thnn required on the press. The difference can be attributed, at

least in part, to the fact that the film thickness on the mill is much

larger than that on the press. Dispite the -difference in cure rate, however,

this technique was throught to be a useful method for comparing the cure

of different ink formulations and for determining the rheological changes

associated with cure.
- 7 -

RESULTS AND DISCUSSION

Uncured Ink

The behavior of both ink formulations can be roughly divided into

three regions: initiation behavior, short term steady flow, and long

term steady flow. -This can be illustrated by experiments on the start-up

of steady shear. When the ink is allowed to rest in the instrument for

30 minutes or more and then steady shear is initiated, there is a

significant stress overshoot (Figure 1). Subsequently, the stress level

shows a significant time dependence for a period of time that depends on

the experimental conditions but is generally less than 10 seconds. After

this initial period the stress appears to level-off at what will be

termed the short term steady flow value. If the steady shear is maintained

for long periods of time, however, it is found that the stress is not

constant but shows a small and very slow decrease. For the range of

conditions tested here, the stress, and therefore the viscosity, drops by

about 15% in one hour (Figure 2). The decrease is approximately linear in

a log (q) vs log (time) plot.

# If the shear flow is stopped and then reinitiated within a few minutes,

the stress will return to the level achieved just prior to stopping the

flow with little or no overshoot. Only if the ink is allowed to recover

for a significant period of time, say 30 minutes, will the response be

similar to that observed initially.

All three aspects of the ink behavior — initiation, short term steady

flow, and long term response — may have importance in ink performance and

are being studied in this program. This paper, however, focuses only on

the short term steady flow regime and small amplitude oscillatory experiments.
- 8 -

Steady Shear:

The short terra steady flow stress levels (average values achieved

between 30 and 120 seconds) were determined as a function of shear rate and

used to calculate viscosities. . Values of these viscosities for samples

of the two ink formulations are plotted against shear rate in Figure 3.

Above shear rates of 500 sec ^ the data are unreliable because there is

often a loss of adhesion between the sample and the cone or plate;

however, the indications are that the viscosity may drop significantly

at high shear rates. In the' region between 80 and 500 sec ^ as well as

-2 -1

the region between 6x10 and 0.6 sec , the viscosities for the two ink

formulations are similar. Only at the intermediate shear rates, 0.6 to

80 sec \ is the behavior substantially different. In this region the

. \ viscosity of BK-60 reaches values more than 3 times as large as those for

BK-62.

This difference is important because a number of the mixing steps

required to prepare these inks involve shear rates in this intermediate

range. On the press, however, the ink sees dnly very low* shear rates in

the fountain and very high shear rates during printing. For these conditions

the behavior of both -inks is similar in Figure 3. Since the results in this

Figure are for room temperature while the ink on the press is hot (50°C

2 -1

to 80°C) , additional tests were performed at high shear rates (475 sec )

and elevated temperatures (60°C) . Here again the behavior of the twT o ink

formulations was similar. Consequently, although the differences in short

tern steady flow* viscosities may have an effect on ink preparation, they

do not appear to be a likely cause of the poor press performance for

BK-62
- 9 -

Oscillatory Shear ;

Because the viscosities of these inks are highly non-linear,

the oscillatory behavior is quite complex. For example. Figure 4

shows the stress curves associated with a pure sine wave strain imposed on

samples of BK-60 at 3 different frequencies. For the lowest frequency

the. stress curve is approximately sinusoidal although shifted in phase,

but the curves become increasingly non-sinusoidal as the frequency is

increased. Moreover, even at the lowest frequency, if the sample is

subjected to steady shear for a few minutes before the oscillatory test,

a non-linear response is obtained (Figure 5) . A comparison between the

behavior of the two ink formulations indicates a general similarity in

the affects of changing the amplitude or frequency or shearing the sample

before the test. In most cases, however, the stress levels are greater

in BK-60 while the non-linearity is greater in BK-62 (Figure 6).

In light of this complexity only a simple analysis of the oscillatory

results was performed. Nevertheless, this analysis provides several

interesting general conclusions. First, the .behavior of the inks is

viscoelastic in that the stress leads the strain (Figure 4-6) by an angle

between 0° (elastic) and 90° (viscous). Second, a simple Fourier analysis

indicates that the stress curves can be fit quite well with, a response

involving the fundamental frequency and the first two odd harmonics.

For example, Figure 7 shows the measured and calculated curves for the

case that gave the poorest fit of all the experimental conditions

analyzed. Even for this case the agreement is reasonably good. An

examination of the results also indicates that in the calculated curves

the total amplitude of the harmonics is in all cases less than 14% of that

for the f undamen tal and in many cases less than 7%. Thus to a large extent

the fundamental frequency dominates the response.
- 10 -

. As a result it is of interest to examine some of the general

characteristics of this component of the total response. If the

real part of the dynamic shear viscosity is calculated for this

component, it is found to be a strong function of the strain

amplitude and frequency. Data were obtained for BK-60 at 6 different

amplitudes and 9 different frequencies. In an effort to systematize

t this information, the dependence of on the shear rate was examined

by plotting log (n^) against the time average shear rate during the

oscillatory cycle (Figure 8)

time average shear rate =

2 u A

tt tan 8

( 1 )

where w is the angular velocity, A is the amplitude of oscillation

*

"

. \

(radians), and 9 is the cone angle. Although this does not collapse the

data to a single curve, it does compress the variations to a narrow band

which is quite narrow at low shear rates and broadens somewhat as the

shear rate is increased. • If the short term steady flow data are added

to the graph, they fall roughly within this same band. A much less

extensive study using formulation BK-62 gave similar results. Consequently,

although the oscillatory behavior is quite complex, there is an interesting

correlation between the oscillatory data and the short term steady shear

results.

Curing Study

Although the data on the mechanical properties of the uncured inks

provide information that is useful in fabricating the inks, no explanation

for the poor performance of BK-62 was found. As a result, experiments aimed

at examining the cure behavior of the inks were conducted. Samples of the inks

were cured on a heated 2 roller apparatus, and aftor various curing times

small portions of ink were removed and characterized for oscillatory and
- 11 -

short term steady flow viscosity. In view of the complexity of the

oscillatory behavior, most of the emphasis is on the steady flow tests;

however, it is useful to examine the general trends exhibited in the

oscillatory data.

Steady Shear:

Figures 9 and 10 illustrate the types of changes that are observed

in the short term steady flow viscosity as the ink cures. Over the

entire range of shear rates tested, the curing produces an increase in

viscosity. For a given curing time the data for BK-60 show an approximately

equal increase in log (n) at all but the highest shear rates. For BK-62

the longer cure times show the same trend but the 5 minute cure produces

larger changes at the low shear rates than at the intermediate and high

shear rates. One factor that could contribute to this difference in

behavior at short cure times is the possibility of an induction period.

2 Thermal experiments have demonstrated that the cure reactions do not begin

immediately but only after the sample has been held at the cure temperature

for a few minutes. In the present experiment the cure temperature is

reached very rapidly but as will be seen later, there is evidence for an

induction period.

Before curing begins, BK-60 has a higher viscosity over most of the

shear rate range than BK-62. Once curing starts however, the rate of

increase in n is much greater for BK-62. Consequently, within a short time

the viscosity for BK-62 is larger than that for BK-60. It is clear therefore

that the two ink formulations have very different curing behaviors.

Oscillatory Shear:

Very similar trends are seen in the oscillatory data. Figures 11 and

12 show the stress curves associated with a sine wave strain for ink

samples cured various lengths of time. As shown previously, the initial
12 -

stress levels in BK-62 are much lower than in BK-60. The changes

associated with curing, however, are substantially larger for BK-62

and within a short time the stress levels in BK-62 are larger than those

in BK-60. In view of the correlation that was found between the

oscillatory and short term steady flow experiments on the uncured inks,

the similar trends in the curing tests are a logical result. With both

inks there is a decrease in non-linearity as the inks cure; i.e., the

stress curves become more sinusoidal. The angle by which the stress leads

the strain (sine wave) also decreases with curing indicating that although

both the elastic and viscous components of the response increase, the

elasticity increases more rapidly at least in the initial phases of cure.

Analysis of Cure

A simple analysis of the cure results for short term steady flow

can be performed by noting that for a number of polymerization reactions,

the early stages of cure can be described by a first order type equation.

In the simplest case this would mean that log (n) would vary linearly with

time. To examine this possibility the data for various shear rates ware

analyzed by plotting log (n) vs. time (Figures 13 and 14). If the initial

points (zero cure time data) are excluded, the data for each shear rate

can be fit, to a first approximation, with a straight line. The fact that

the zero cure time points do not fall near the lines suggests that the

mechanical property results show an initiation time just as was found

2 previously in thermal experiments.

The slopes, k, of the lines in Figures 13 and 14 provide a measure of

the rate of change of viscosity with time during curing. The initiation

period can also be characterized by determining the time, t Q

, corresnondin

to the point on cadi straight line where the viscosity equals the zero cur

tine viscosity of the ink at that shear rate. Values of k and t for the
13 -

various shear rates and inks are given in Table II. Over most of the

measured shear rate range, the slopes are quite similar. At the two

highest shear rates the slopes are less as would be suggested by the

data in Figures 9 and 10. It should be noted, however, that the initiation

times are greater at these shear rates, and as a result the data points at

the shortest non-zero cure time may be in the initiation region. If

straight lines are fit to the data for the shear rate of 55.6 sec

^

without including the results for either zero cure time or the shortest

non-zero cure time, the slopes are greater although still not as large as

the values obtained at lower shear rates (Table II). Consequently,

although the data here are limited, the changes do appear to be somewhat

slower at the higher shear rates.

The results in Table II help quantify the differences in cure

2 behavior between BK-60 and BK-62. Previous experiments using thermal

analysis 'techniques have found that the initiation period for BK-62 is

shorter than that for BK-60. This same trend is seen in the mechanical

properties data. Moreover, the rate at which the properties change once

curing has begun is approximately 50% greater for BK-62 than for BK-60.

When these results are combined with the observation that a major problem

with the performance of BK-62 on the press is excessive drying on the plate,

the inescapable conclusion is that the differences in curing behavior are

a major source of the problems with BK-62.

It is easy to see how these differences in curing could lead to

excessive drying on the plate. Even under conditions where excessive

drying on the plate is not observed, however, there will still be

significant differences in the mechanical properties of the two inks

formulations due to the curing behavior. This would affect transfer on the
- 14 -

feed rollers, wiping- of the plate, and pick-up of ink by the paper and

thus other problems with BK-62 may also be related to the cure behavior.

In general it would appear that the changes in ink properties during

cure must fall within a given range if proper performance on the press

is to be achieved. Unfortunately, the original BK-62 ink fails to meet

this criterion.

CONCLUSIONS

The rheology of two intaglio ink formulations with very different

performance on the press was examined prior to and during the early phases

of cure. The uncured inks exhibit complex viscoelastic properties with

stress overshoot, time dependence, and non-linearity. A major aspect of

the non-linearity is a large shear rate dependence of the viscosity. Curing

. \

of the inks produces changes in both the viscosity and elasticity of the

inks. After a short initiation period, the increase in the logarithm of the

short term steady flow viscosity is linear with respect to tine so the

slopes of the best-fit straight lines at various shear rates can be used to

characterize the rate of change in properties. Over a wide range of shear

rates the slopes of these lines show only a small variation. A comparison

between the two formulations, however, reveals significant differences:

BK-62 has a shorter initiation period than BK-60 and a rate of increase in

viscosity during curing that is 50% greater than BK-60. These differences

in curing behavior play a major role in the poor performance of BK-62.

ACKNOWLEDGEMENT

The equipment utilized in this work was developed and used in

cooperation with the Naval Research Laboratory. Without their assistance

. this work would not have been possible.
References

1. "Production of Government Securities", Public Brochure, P.15 - REV 9-80, Bureau of Engraving and Printing. •

2. W. R. Newitt, B. Lipman, and B. A. Vaudreuil, Bureau of Engraving

and Printing Research Report BEP-81-013, February, 1932.

3. K. Hirayama, Research Bulletin Printing Bureau , Ministry of Finance

Japan, 47_, 1 (1979).

A.- J. Shurz and T. Kashnoula, Papier , 30 , V52 (1976).

5. P. Oittinen, Acto Poly technics Scandihavica , No. 131, 1 (1976).

6. T. Amari, et al., Prog . Polym . Phys . in Japan , 22, 101 & 105 (1979)

7. D. Hunston, et al. Adhesion and Adsorption of Polymers , Vol . 12A , Ed. L. H. Lee, p.321 (Plenum Press, New York, 1930).

Table I Composition of Ink Formulations

Ingredients BK-60 BK-62

Pigments 51.4 36

Barytes - 30

Calcium Carbonate 17.4 17

Linseed Oil Vehicle 16.5 - Varnish 6.5 9

Bodied Tung Oil - . 8

Amorphous Silica 5.7 - Driers 1.9 - Petroleum Solvent 0.6 - Total 100 ioo
Table II Cure Parameters

Shear Rate

(sec“l)

BK-60 BK-62

kxl04 t Q (sec) kxlO^ t Q (sec)

0.56 6.1 330 9.4 150

1.76 6.4 360 9.6 180

5.56 6.1 410 9.8 270

17.60

‘ 5.6 410 8.8 280

55.60 4.7 420 6.4 340

55. 60a 5.0 500 • 7.2 530

a Does not include data point for shortest non-zero cure time.

. \
Figure Captions

Figure 1: The stress associated with the initiation of steady shear

flow in BK-60 at shear rates of 55.7 sec"'^ (A) and 5.6 sec“^

(b)-

Figure 2: Viscosity of BK-62 as a function of tine for steady shear

flow at shear rates of 1.76 sec”^- (A), 5.57 sec“l (X), and

17.6 sec”-*- ()

.

Figure 3: Short term steady flow viscosity vs. shear rate for two

samples of BK-62: X & <0 and two samples of BK-60: Q & + .

Figure 4: The stress curves for BK-60 associated with a sine wave strain

at an amplitude of 1.5 x 10~3 radians and frequencies of

6.0 HZ (A), 0.6 Hz (B) , and 0.06 Hz (C)

.

Figure 5: The stress curves for BK-60 associated with a sine wave strain

of amplitude 1.5 x 10

-

^ radians and frequency of 0.06 Hz

before (A) and after (B) shearing with steady shear flow

(shear rate 60 sec

- -1-) for 1 minute.

Figure 6: The stress curves for BK-60 (A) and 3K-62^(3) associated with

a sine wave strain of amplitude 1.5 x lO

-

^ radians and

frequency 0.06 Hz.

.*v A Figure 7:

J Comparison between the measured stress curve (overlapping data

points appear as heavy line) and calculated curve (thin line)

for BK-60 at frequency of 6 Hz and sine wave strain of

amplitude 1.5xl0“3 radians.

Figure 8:

i The dependence of log (r^) on time average shear rate for BK-60

at amplitudes of 4.5xl0“4 rad (A), 2.2x10“-^ rad (£7) , 6.4x10“^

rad. (<», 9.8x10- 3 rad. (+) , 2.2x10“^ rad. (A), and 4.5x10“^

rad. (X). Short term steady flow viscosity vs. shear rate (Cj -)

.

Figure 9: Short term steady flow viscosity vs. shear rate data for samples

of BK-62 cured for 0 min. (X), 7.5 min. (), 20 min. (O’) » ^-nd

30 min. (+)

.

Figure 10: Short term steady floxj viscosity vs. shear rate data for samples

of BK-60 cured for 0 min. (X), 10 min. (*^)>) , 20 min. (lj) , and

30 min. (+)

.

Figure 11: The stress curves for BK-62 associated with a sine wave strain

of amplitude 1.5 x 10

-

^ radians and frequency 0.06 Hz cured for

0 min. (A), 5 min. (B) , 10 min. (C) , 20 min. (D) , and 30 min. (E)

Figure 12: The stress curves for BK-60 associated with a sine wave strain

of amplitude 1.5xl0- -^ radians and frequency 0.06 HZ cured for

0 min. (A), 10 min. (B) , 20 min. (C)

,

and 30 min. (D)

.
Figure 13: Short term steady flow viscosity of BK-62 vs. cure time- at

shear rates of 0.56 sec

-

^ (X), 1.76 sec“^(Q), 5.57 scc~^

(<£» , 17.6 sec”^ (+) , and 55.6 sec"^ (A).

Figure 14: Short term steady flow viscosity of BK-60 vs. cure time at

shear rates of 0.56 sec

- '1

' (X), 1.76 sec

-1

- (Q), 5.57 sec

- -1" (<>). 17 -& sec

- ^- (+) , and 55.6 sec”^ (A).
“5 2

Stress X 10 (dynes cm )

( LUD/S9UAp)

g

_oi X ss0j;s.

T

ime

(sec)
0’9

(/4 iscosi a) 5o~i

Log(Time)

(sec)
cr Figure 3

« »

(O0S- BdUJ)

Log

(Shear

Rate)

(sec
INK

BK-60,

FREQUENCY

DEPENDENCE

Figure 4

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CDEG:
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Figure 6

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CURE

Figure 7 •

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Figure 8

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STUDY”

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Figure 9

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CURING

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Figure 10
CURING

STUDY

BK

-

62

Figure 11

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s s s s s s

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i i i csiiNn Aavanaavu ssaais

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.STUDY

BK-60

Figure 12

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S3 S3 S3 S3 S3 S3

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i i i .tsiiNn Aavanaav: ssaais

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to

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(Dss-eduJ) (A;is o ds_ia)Bo“i
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Figure 14

(D9S • BdLU) (Ai!SODS!A)6oi
Appendix II

Non-Destructive Characterization of Polymer Films via Ultrasonics

. \

Non-Destructive Characterization of Polymer Films via Ultrasonics

D. L. Hunston

Polymer Science and Standards Division

Center for Material Science

In applications such as adhesives, fiber-reinforced composites, paints,

coatings, etc. the polymer must be fluid in the early stages of fabrication

so that the proper flow, leveling, wetting and spreading at interfaces, etc. can be achieved. The polymer must then harden by cooling, solvent loss, or

chemical reactions to form a rigid material. In most cases the successful

fabrication of a final product requires that associated changes in mechanical

properties take place at the appropriate times and rates. It is desirable

therefore to have nondestructive techniques that can monitor these changes as they occur. Unfortunately, this is generally very difficult, particularly

since many of the applications involve thin films, whose mechanical properties

are difficult- to measure and can not usually be predicted from experiments on bulk samples.

To address this problem, an effort is being made to develop new methods

for nondestructively characterizing thin films. The work to date has focused

on the investigation of an ultrasonic shear wave propagation technique. A

shear deformation was chosen because the shear properties of liquids and solids

are very different. Ultrasonic frequencies were selected because in this range

the attenuation of shear waves in many polymers is so high that a thin film can

be treated as if it were infinitely thick and this greatly simplifies the

analysis. The devise consists of a thin strip of aluminum or quartz in which

a shear wave is propagated. When the surface is coated with the'

1 polymer to be

tested, the wave in the strip generates a shear wave in the coating. Although

the high attenuation in the polymer makes it impossible to observe this wave

directly, its generation causes changes in the wave in the strip, i.e., an increase in attenuation and a decrease in velocity measured along the strip.

By examining the wave in the strip before and after the coating is applied, the

shear mechanical properties of the coating can be determined. Although a quantitative evaluation requires the measurement of changes in both velocity

and attenuation, it is sometimes useful . simply to follow changes by monitoring

attenuation.

During the last year the apparatus necessary to implement this technique

has been set uo and tested by examining several model polymer systems. The

measurements are made using the pulse-echo technique, and to assist in the data

acquisition and analysis, the apparatus has been interfaced to a minicomputer.

The first samples tested were based on an epoxy-type polymer that cures by free

radical initiation. Figure 1 shows a plot of attenuation vs. curing time for

two formulations containing the same epoxy but with different concentrations of

accelerator. The experiments clearly show that there is an initiation period

prior to the onset of cure. The rate of cure influences the width of the

transition while the ultimate attenuation level depends cn the structure of

the cured film among other factors. When the accelerator concentration is

changed both the reaction rate and the induction period are altered but the

ultimate attenuation level remains the same. This type of information can be

crucial in designing epoxy-based adhesives and composites.
The second model system studied this year involved natural -product

based drying oils that are used in some printing inks (l). Thin films of

these liquids harden by polymerization when exposed to oxygen. The rate

of hardening must be correct if proper performance is to be achieved.

Figure shows the results of a very simple test in which a thin film of the

sample was placed on the quartz strip and the attenuation was monitored as

a function of time. The initial increase in attenuation is rapid but

eventually the attenuation approaches a limit when the film becomes fully

cured. The curve in Figure 2. represents the best fit with a simple first

order rate equation. Data of this type can be very helpful in optimizing

the design of polymer systems.

The results to date indicate that this technique can be used to

qualitatively monitor changes that occur during the 1 iquid-to-sol id

transition in important polymer systems. During the next year an effort

will be made to quantify this test. As part of this effort the measurement

capability will be expanded to permit the determination of velocity change

as well as attenuation. In addition the program will investigate the

possibilities for coupling this technique with other methods such as dielectric tests so that both can be used simultaneously. The information

provided by this coupling would make both measurement techniques more useful.
(I) D. L. Hunston, J. L. Rushford, W. R. Newitt, and B. A. Vaudreuil,

“Rheology of Cure for Intaglio Printing Inks", Organic Coatings

and Plastics Preprints, in press.

' Figure Captions

Figure /: Cure data for the epoxy/accelerator system: das hed line for

uncoated strip, X for 0.25% accelerator, and 1 | for 0.35%

accelerator.

Figure 2; Attenuation vs. time plot for the curing of a drying oil based

polymer system. The curve represents the best fit with a first

order rate equation.

Figure 1

S

LO CO

S (SI (SI

o

o

(!)

X

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(WO /d q u

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10
NBS-T14A (rev. 2 -eo) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3.

SHEET (See instructions)

4. TITLE AND SUBTITLE

5. AUTHOR(S)

1 ati onshi ps Between Mechanical

Properties and Performance of Inks As

the Basis for Quality Control

Tec hni ques

Publication Date

Donald L. Hunston

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No. T-0283-02

8. Type of Report & Period Covered

Progress Report

Oct. 1, 1981 to mSept 30,

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP

)

Bureau of Engraving and Printing, Treasury Department,

Office of Research and Technical Services

14th and C St., S.W.

Washington, D.C. 20226

10.

SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a si gnificant

bibliography or literature survey, mention it here)

Three different intaglio ink formulations have been examined to determine their

mechanical properties. In all cases the properties share a strong shear rate and time

dependence. In addition, the behaviors of the different formulations vary substantially

at intermediate shear rates but are similar at the very high and very low shear rates

that are seen on the printing press. Consequently, the shear behavior has importance

for ink fabrication but does not explain the differences in printing performance that

are seen for these formulations. The ink samples were then examined for the changes

in mechanical properties that occur during drying (curing) . Distinct differences in

curing behavior are observed for the various formulations and it was determined that

these differences can influence performance. It was also found that direct exposure

to air (oxygen) will produce curing. The major implications of these results in terms

of fabrication, quality control, and printing procedures are discussed.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) cure; drying; intaglio ink; linseed oil; printing; rheology; tung oil; viscoelasticity;

viscosity

13. AVAILABILITY

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j For Official Distribution. Do Not Release to NTIS

| Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

1 X Order From National Technical Information Service (NTIS), Springfield, VA. 22161

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