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Dickens. Brian/Study o( Intaglio Ink mix QC100 U56 NO. 86-3303 V1986 C.1 NBS-PUB- Study of Intaglio Ink Mixing and

Cure and Ingredients

Reference

NBS

PUBLICATIONS

3rian Dickens, Donald L. Hunston, Joseph C. Phillips and

George W. Bullman

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Polymers Division

Gaithersburg, MD 20899

September 1985

Issued January 1986

-QC

100

U56

86-3303

1986

Prepared *or U.S. Department of Treasury " reau of Engraving and Printing

jartment of Research and Technical Services

shington, DC 20226

NRS RESEARCH INFORMATION CENTER

NBSIR 86-3303

* * » STUDY OF INTAGLIO INK MIXING AND

CURE AND INGREDIENTS

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Ub(?

ft-33

Brian Dickens, Donald L. Hunston, Joseph C. Phillips and

George W. Bullman

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Polymers Division

Gaithersburg, MD 20899

September 1985

Issued January 1986

Prepared for U.S. Department of Treasury

Bureau of Engraving and Printing

Department of Research and Technical Services

Washington, DC 20226

V

\ of

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Study of intaglio ink mixing and cure and ingredients

Brian Dickens, Donald L. Hunston, Joseph C. Phillips,

George W. Bullman

Polymer Division

National Bureau of Standards

Gaithersburg, Md 20899

Annual report September 30, 1985

Prepared for

U. S. Department of Treasury

Bureau of Engraving and Printing

Department of Research and Technical Services

Washington D.C., 20226

and

page 1
Executive summary

This report covers the period October 1, 1984 to September 30,

1985. During this period, a report discussing principles

important in formulating intaglio inks and giving several

recommendations was provided to BEP. Other reports on

evaporation from intaglio inks, on a short thermogravimetr ic

study of correctness of weighing out the ingredients m the

inks, and on a thermomechanical study of gummed paper were aiao

provided to BEP. Reviews of these reports are included here.

In the report which discussed the formulation of intaglio inks,

alkyds were recommended as the most practical new resin system to

polymerize in air. The alkyd should have a well-chosen molecular

weight distribution and be dispersible in a water-containing

solvent system. UV irradiation of the newly printed currency

paper can probably be used to skin the ink rapidly and prevent

blocking, especially if a photoinitiator system is added to the

formulation. This may allow printing of both sides of the

currency paper m one pass. Replacement of the various pigments

and fillers currently in use by dyes attached to the backbone of

an organic polymer, either in water-soluble form or in the form

of a glass should be investigated.

We have developed several methods to characterize mtaqlio inks

and have used these methods to examine whether change take place

in inks during mixing and milling. Two intaglio inks were sampled

during mixing and milling at BEP and characterized in terms of

cure times. ash residues, ultraviolet absorbance spectra

measurements, and gel permeation chromatographic measurements.

The results show that even when mixing to homogeneity on the

scale of fractions of milligrams has been accomplished, the state

of dispersion of the solid components of the ink in the liquid

components of the ink may be critical on a scale much finer than

this.

Cure times, ultraviolet absorbance measurements. and gel

permeation chromatography measurements have been used to follow

ink cure. Refinement and application of the techniques developed

in these studies are expected to lead to data which will help

greatly in understanding the variables which influence the

air-cure of intaglio inks. Application of these techniques and

the development of new techniques as the needs arise will help in

the design of improved inks.

A rapid method of analysis using thermogravimetry to determine

solid residues left after an ink has been ashed has been

developed. Analysis time is 15 minutes. The reproducibility of

one determination is roughly 0.3X. The method snowed that

specimens from one ink sample had on the average 0.3x more liquid

than specimens from the other (this average value was obtained

from about 25 separate determinations and hence has an

page 2
uncertainty of about 0.0650. UV spectroscopy showed that the

second ink sample contained more tung oil. Cure time studies

showed that it cured more quickly.

Several interesting results arise from the study of cure time

variation with extent of mixing and milling. First, both the

variations in properties within a batch and between the two

batches were leas than our earlier work on these inks had led us

to expect. These two ink samples clearly do not show the width

of variation that exists in intaglio inks. When we examine more

inks in the future we may find other differences. Even so, our

results are very informative. Four points are worth noting:

1) The inks were fairly homogeneous on a scale of about 0.1

milligrams yet showed significant differences in curing behavior.

2) The time of cure increased with increased grinding.

3) The addition of the small scale mix, which contains the

driers. had little or no effect on the cure time for tne second

ink until the ink had been milled.

4)

The intensity of the blue color m the liauid-extracted

portion of the ink changed with extent of cure.

Dispersion of the solids in the ink seems to be critical. An

explanation for the variation of cure time with extent of mixing

is that the solids remain aggregated, or at least poorly

dispersed, and therefore absorb more liquid Inside the aggregate

by capillary action than they would if the aggregates were well

dispersed and the liquid were merely absorbed on the separated

particles. The ink then acts as though it were more highly

filled than it really is. With a highly filled ink such as BK62

m»od 3, this may make the ink unstable. The phenomenon is well

known in the manufacture of paints.

Carbon black acts as an antioxidant in most of its forms.

Dispersion of carbon black may be an important factor in slowing

the cure rate of the ink. Thought should be given to omitting

the furnace black from the ink formulation and using a volatile

antioxidant instead. This will give good storage properties and

perhaps better control of cure since furnace black is Known to

aggregate easily and to be difficult to disperse. A volatile

antioxidant would evaporate from the ink on the press and allow

the setting reaction to proceed from that point on.

The technique of gel permeation chromatography was developed to

provide distributions and averages of these distributions in

terms of log hydrodynamic volume. This is a new way of

classifying molecular size for systems which are mixtures of

materials. The average log hydrodynamic volume of the soluble

part of the vehicle is a useful index with which to follow the

cure of the ink.

page 3
Table of contents

Executive summary 2

Table of contents 4

1: Introduction 5

2: Review of intaglio ink formulation and cure 5

2.1: Formulation of intaglio printing inks 5

2.2: Film-forming resins 6

2.3: Effect of oxygen 7

2.4: Initiation systems for intaglio inks 7

2.5: Stabilization of the inks 9

2.6: Effect of the initial molecular size distribution . 9

2.7: Recommendations 10

3: Report on test for solvent evaporation from green ink . 14

4: Report on test for inhomogeneity of mixing 14

4.1: Introduction 14

4.2: Experimental 15

4.3: Results 15

4.4: Discussion 15

5: Report on test of thermomechanical stability of gummed

paper 17

6: Further tests for mixing homogeneity 18

6.1: Experimental Techniques 13

6.1.1: Introduction 19

6.1.2: Experimental details 19

6.1.3: Rapid ashing using thermogravimetry 19

6.1.4: Molecular sizes from gel permeation

chromatography 20

6.1.5: Cure tiiKds 21

6.2: Results of study of mixing and milling 21

6.2.1: Introduction 21

6.2.2: Thermogravimetr ic measurements 21

6.2.3: Determination of hydrodynamic volume averages 22

6.2.4: UV absorbance measurements 22

6.2.5: Differences in cure times 22

6.2.6: Dispersion of solids in the inks 23

6.2.6: Dispersion of furnace black 24

6.2.7: Critical pigment volume considerations .... 24

6.2.3: Drying (dehydration) of the ink 25

7: Cure measurements 25

7.1: Introduction 25

7.2: Ink cure on the rubber mill . 26

7.3: Film reflectance 26

7.4: Mill resistance as a measure of tack 27

7.5: Clumping of the ink in solvents 27

7.6: Color intensity of drier 27

7.7: UV absorbances 28

7.8: GPC measurements 29

7.9: Infrared absorbances 30

8: Conclusions 32

9: Figure legends 34

page 4
1 : Introduction

Our work during the report period has covered three main areas.

First, the fundamental chemical and physical principles involved

in the formulation of a water-dispersible intaglio ink usable on

the printing presses at BEP were developed. Based on the review,

19 recommendations for further study were presented. Second,

efforts were made in close cooperation with BEP to develop

measurement techniques and to supply data to BEP to aid in

diagnosing the sources of problems with inks currently in use.

Third, two BEP intaglio inks were studied in considerable detail.

These BK62 black intaglio inks were followed through the

processes of mixing and milling. Cure studies on these inks were

also carried out.

2: Review of intaglio ink formulation and cure

2.1: Formulation of intaglio printing inks

The report “Intaglio Ink Considerations" by B. Dickens has been

accepted as NBSIR 85-3216. The areas covered are:

Film-forming resins

Intaglio printing ink vehicles

Initiation systems for intaglio printing inks

Filler considerations

Pigment considerations

Mechanism of cure for drying oils and alkyds

Resin considerations

Solvent/dispersing phase considerations

Drying agent considerations, and

Recommendations

This report provides information on ways in which improved

intaglio printing inks can be developed. Particular emphasis has

bean placed on the part of the ink which sets, i.e., the

so-called vehicle or resin, on the ways in which setting of an

ink can be induced, on ways in which the formulation of the ink

may be expected to influence the setting reaction, and on the

constraints imposed by the BEP printing presses. The goals were

to help in the selection of a new ink yehicle which will perform

satisfactorily on the presses at BEP. and to provide insight into

other aspects of ink formulation and cure to reduce the spoilage

in production runs.

Alkyds were recommended as the most practical ream system to

polymerize in air. The alkyd should have a well-chosen molecular

weight distribution and be dispersible in a water-containing

solvent system. The solubility of oxygen in the ink formulations

and solvent mixtures should be determined and the composition*

adjusted to provide a level of oxygen which gives optimum cure.

UV irradiation of the ink immediately after it has been Dlaced on

page 5
the currency paper can probably be used to skin the ink rapidly

and prevent blocking. This may allow printing of both sides of

the currency paper in one pass. Replacement of the various

pigments and fillers currently in use by dyes attached to the

backbone of an organic polymer. either in water-soluble form or

in the form of a glass should be investigated. Thixotropy of the

dispersed ink should be adjusted by using sodium carboxy methyl

cellulose instead of the silica currently in use. Reasons for

the selection of alkyds and a brief discussion of the

possibilities for initiation of cure in the ink are given below.

2.2: Film-forming resins

The earliest polymeric system used in films and coatinas was

based on naturally available and slightly processed oils from

various seeds. fruits, and nuts. Some ot these oils "dry", that

is, polymerize and become non-tacky, and some do not. Drying

oils are usually heat-treated onor to application to increase

their degree of polymerization. This increases the viscosity and

decreases the drying time. The rate at which they dry depends on

the number of active groups per molecule and on the rate of

attack on these groups. The physical and chemical changes

accompanying the drying of drying oils have been studied

extensively. The reaction responsible for the drying process is

a free radical reaction. In the normal mode of use, the free

radicals are produced by the attack of the drying agents (certain

metal salts) on hydroperoxide groups, which are only formed in

the presence of oxygen. - Drying oils are one of the few systems

which polymerize satisfactorily in the presence of air.

An alkyd is basically a long chain molecule with several kinds of

pendant groups. Some of these groups are derived from drying oil

molecules. In this sense, alkyds are modified drying oils and

dry by the same mechanism. The length of the alkyd molecule is a matter of choice. Long chains require fewer bonds to polymerize

than short chains do. Also, chemical groups are incorporated

into the alkyd molecule to confer such useful properties as

dispersibility in water and resistance to weathering. Alkyds

are extensively used in paints, for which there is an enormous

market. Alkyd paint films do not survive severe weathering

conditions, although they are appreciably more resistant than

films based on simple drying oils. Acrylic and other vinyl

components have been incorporated into alkyds to give better

environmental resistance and to give a faster dry by producing a material with a glass transition temperature near room

temperature so that the material quickly becomes a glass rather

than a sticky rubber. The various monomers may be chemically

combined together to give an extended molecule or the alkyd can

be physically mixed with an acrylic or similar polymer which is

deposited unchanged on evaporation of the solvent.

page 6
2.3: Effect of oxygen

Oxygen is needed in the drying of drying oils to produce

hydroperoxide groups. In the drying oil polymerization

mechanism, the hydroperoxide groups formed during the oxidation

are decomposed by the driers to give more free radicals which

continue the drying. This means that the drying reaction is

autocatalytic and that the rate of drying increases with time, at

least in the early 3tages. The usual situation in free radical

polymerization is that the free radicals are initially generated

from an added initiator, so that both the concentration of

initiator and the rate of polymerization decrease with time. In

general, oxygen inhibits free radical polymerizations because

oxygen, when available, adds very quickly to the hydrocarbon free

radicals on the polymerizing molecules <20,000 times more rapidly

than the monomer does for methyl methacrylate) to give peroxyl

radicals which are not as reactive as the original hydrocarbon

radicals. Polymerization of drying oils using the relatively

unreactive peroxyl radicals is possible only because drying oils

contain very easily abstracted hydrogen atoms in the -CH2- groups

next to the double bonds.

Where very easily abstracted hydrogen atoms are not available,

which is the case for most materials, the peroxyl radicals attack

the monomer molecules only very slowly. Peroxyl radicals on

polymethyl methacrylate add to methyl methacrylate monomer at

1/60 the rate of the parent hydrocarbon radicals. In most

polymerizations, transformation of the original reactive

hydrocarbon radicals into peroxyl radicals causes the

polymerization reaction- to slow down or stop. Most classes of

free radical polymerization are significantly inhibited by

oxygen, so oxygen must' therefore be excluded as much as possible

from the polymerization reaction. For these materials to be good

candidates for use in intaglio inks, the rates of initiation

would have to be high enough to consume all the oxygen already

dissolved in the film and to carry the polymerization reaction to

completion before much more oxygen could diffuse into the film

and interfere. Initiation mechanisms will now be discussed

briefly

.

2.4: Initiation systems for intaglio inks

Electromagnetic radiation in the form of microwaves (billions of

hertz) or radiofrequencies (millions of hertz) acts on polar and

dipolar molecules to tumble them rapidly. This is a method of

selectively heating a component in the reaction mixture.

Microwaves have been used to selectively decompose peroxyesters

in prepolymer mixtures to effect polymerization and to polymerize

acrylates by irradiating the monomers. One of the problems in

the use of microwave and radiofrequencies m intaglio printing on

paper is not to heat the water in the paper to unacceptable

limits. Exploratory work will be necessary to find whether a

page 7
suitable frequency domain exists to accomplish this. It is also

uncertain whether thin films can be heated sufficiently by these

frequencies to generate free radicals at the appropriate rate.

In the case of films which are somewhat transparent in the

ultra-violet (UV) part of the electromagnetic spectrum, UV

radiation can be used to break apart the more sensitive chemical

groups and form free radicals, especially in thin films.

Printing inks are necessarily colored and are highly filled with

mineral phases. Since they absorb and scatter visible and UV

radiation very strongly, neither UV initiation nor visible light

initiation is an attractive option for the main method for

initiating polymerization in these inks.

Cure of the top 10 microns on the surface of ink already in place

on the currency paper using UV radiation could reduce or prevent

sticking together of adjacent sheets of currency paper

(••blocking** in the printing sense), which is a significant

problem. Cure of the surface using UV or visible light could be

effected by including a suitable photo-initiator system in the

ink formulation. This is probably the most promising route to

reducing spoilage from blocking.

The mo3t promising approach for the general in-depth

polymerization of intaglio inks at the present time is to take

advantage of the fact that free radicals can be produced over a long period of time in an auto-catalytic fashion without the use

of high energy radiation by using drying oil technology. The

first necessity in designing the ink system is to overcome the

limitations of conventional chemical systems in performing

adequately in both storage and on the press. Using as the

initiating species €' group which is produced in the

polymerization reaction means that the system will be

auto-catalytic. It is this property which has made drying oils a viable system for film formation and in particular for intaglio

inks. In the drying of drying oils, the hydroperoxides produced

by the action of oxygen on hydrocarbon free radicals are

decomposed by the driers into one free radical and one ion per

hydroperoxide group. The hydroperoxide groups are formed in a repetitive "propagation** step of the oxidation reaction. Hence,

one free radical produces several hydroperoxide groups. Because

these hydroperoxide groups later produce free radicals under the

influence of the driers, the rate of polymerization accelerates

with time. The newly applied ink can probably then be skinned

using a medium pressure mercury lamp, which requires much less

shielding than electron beams, or by visible light, although in

this case the ink would be slightly light-sensitive.

page 8
2.5: Stabilization of the inks

The second necessity in designing the ink system is to control

the early effect of this free radical oxidation reaction, and

hence stabilize the ink during pre-use storage. This may be done

by including in the formulation a small quantity of antioxidant,

which combines with free radicals containing oxygen, and which

will soon be used up at the temperature of the printing press but

which will last for days at room temperature. If an antioxidant

which is volatile at the printing temperature is used, then its

concentration will be depleted by evaporation as well as

reaction. Methyl ethyl ketone oxime and butanal oxime are

feasible candidates. Considerations presented later in this

report suggest that the carbon black in current BEP ink

formulations may be acting as an antioxidant which is not only

nonvolatile but also is difficult to disperse.

2.6: Effect of the initial molecular size distribution

Drying oils are not water-soluble because they do not contain

groups which are sufficiently polar. Also, drying oils are known

to continue oxidizing after they have become non-tacky, and this

further oxidation eventually causes deterioration in the film.

Therefore, the number of oxidizable groups should be just

sufficient to give an acceptable rate of drying and acceptable

mechanical properties of the cured film. Alkyds can be made with

polar groups, air-drying cure behavior, and form more durable

films than drying oils do. Therefore an alkyd is the material of

choice as an intaglio ink vehicle. Because alkyds are used in the

paint industry, there sSems no danger of shortages in supply.

Fewer bonds are needed to "cure** molecules which are already

large. A film made from large molecules will dry more rapidly

than a film made from small molecules. If the larger molecules

contain more reactive groups than the smaller molecules, then

drying is even faster. Wei 1 -control led changes can be made in

the functionality (number of potential polymerizable links per

molecule) drying oil type of alkyd, and the fatty acid

components can be adjusted to influence the rate of drying

and the likelihood of further oxidative degradation. Thus,

molecules which are larger than either drying oil3 or modified

drying oils can be prepared with consistent properties. Hence,

an alkyd which will dry more quickly than a drying oil can easily

be made.

Polymers made by free radical processes or simple addition

processes contain molecules with a range of molecular weights.

The population of polymer molecules is known as the molecular

weight distribution of the polymer. The molecular weight

distribution has a very significant effect on the rate of

formation and final properties of a film. Thus, a mixture of

alkyds of the same chemical composition but with well-chosen

page 9
molecular weight distributions should be used. The influence of

molecular weight distribution on the resin used should be

studied

.

2.7: Recommendations

1. The ink vehicle

allylic hydrogens

water-dispersible

Water-dispersible

companies

.

should

will

alkyd

alkyds

be an alkyd because only

polymerize successfully

should be strongly

are available from at

molecules with

in air. A

considered

.

least 10 US

2. A second candidate would be a water-dispersible alkyd resin

containing methacrylate groups, as described by Larson and

Emmons. The polymerization of the methacrylate part will be

poisoned by air, so a formulation based on this kind of resin

could remain tacky. This depends on how much oxygen is available

in the interior of the resin during setting. The oxygen content

of the resin depends on the solubility of oxygen in the

resin/water /organic solvent mixture and on the rate of the

setting reaction, because the drying oil part of the molecule

consumes oxygen as it sets. On the other hand, the drying oil

part will not set without some oxygen being available, because

oxygen is needed to produce more hydroperoxide groups which will

then be decomposed by the drying agents to give the free radicals

which drive the setting process. It is probably safer, more

reasonable, and requires less fine-tuning (and hence is less

prone to variation) "to rely exclusively on the drying oil

mechanism and to select a drying oil type of alkyd.

3. From the points of view of reducing environment pollution,

costs, and maintenance problems, the resins should be dispersible

in water using the amine or ammonium salt as necessary. There

are conflicting reports as to whether these amines retard

setting. Ammonia is more volatile, and would retard the setting

reaction less than the less volatile amines. However, films

based on ammonium salts are known to discolor more rapidly than

films based on higher amines. Fortunately, BEP's currency inks

are dark in color, so this is probably not important in BEP's

application. If water is to be used as a cosolvent for the ink,

efforts should be made to decrease the press temperature during

printing. This will decrease the rate of evaporation of water,

which will make the behavior of the ink on the roller more

stable. The humidity in the printing room should also be

control led

.

page 10
4.

Microwave and radiofrequency radiation could play an

important part in evaporating the water from an ink dispersed in

water. This would simultaneously heat the ink, dry it and

therefore make it more able to dissolve oxygen, and help it set.

Dielectric measurements on the ink formulations and currency

paper should be made to provide estimates of the efficacy of the

process. Test cures should be carried out.

5.

The effect of water as solvent on the cure rate must be

established. From the point of view of retarding the evaporation

of water, decreasing the temperature will keep more water in the

ink. This will also decrease the amount of oxygen able to

dissolve in the ink because oxygen is less soluble in water than

in most organic materials. From the point of view of oxygen

solubility, decreasing the temperature will also decrease the

amount of dissolved oxygen, since oxygen solubility in organic

materials generally increases with increasing temperature.

6.

Investigations should be carried out to determine whether the

solution thixotropy can be adjusted using sodium carboxy methyl

cellulose instead of silica. The intended function of silica is

to act as a thickening agent. Our very recent work suggests that

a second function of silica may be to act as a desiccant which

dehydrates the ink by combining with any water present. The

final effect of this is not yet known, although some

considerations were given in paragraph 5. A recent paper (K.

O'Hara and W. P. Gordon, "Thixotropy in industrial coatings",

Xlth International Conference in Organic Coatings Science and

Technology, Athens, Greece, July 8-12, 1985) discusses the use of

reains which themselves are thixotropic.

7.

If water is to be considered as a possible solvent or

component of the solvent system, the ink vehicle will have to

contain acid or other polar groups to make it soluble. Since the

calcium carbonate would react with these groups to form a scum, a replacement should be sought. An organic extender such as

polystyrene, sulfonated polystyrene, acrylate, or polyester

should be considered, provided it can be safely and successfully

mixed into the resin and the effect of tne difference in

refractive index of the polymeric material from that of calcium

carbonate can be tolerated. A recent paper (S. Konishi . T.

Nakaya, and T. Ueda, "Rheological properties and application

performances of the new water-borne paint", Xlth International

Conference in Organic Coatings Science and Technology, Athena,

Greece, July 8-12, 1985) documents the superior performance of a paint containing small spherical polyester particles as a filler.

page 11
8.

A water-soluble acrylic or equivalent resin should be added to

the ink. This will impart a reasonably hiqh qlass transition

temperature to the film in the early stages of drying to produce

9.a tougher and more tack-free film.

The replacement of pigments by stable (and soluble) dyes

should be investigated. This will reduce the necessity to mill

the ink, and will make the cure more reproducible because the

ingredients will be more easily dispersed.

10.

The use of organic glasses containing the appropriate dyes

grafted onto the polymer backbone should be explored. An

extension of this scheme would be to consider currency note

validation or even recognition schemes using automated equipment

by including a fluorescent species on the polymer backbone.

Unfortunately, practically all salts fluoresce strongly by

emission from defects in the crystals and crystallites, and this

would complicate the situation. A powerful fluorescer such as

anthracene might be able to overpower most other fluorescers, but

the apparent denomination of a bill (as read by a fluorimeter)

could conceivably be changed by soaking it in an appropriate

solution

.

11.

Ink layers thicker than 2 mils (50 microns) cure slowly in

the interior. UV initiation of cure would probably not be useful

in assisting curing beyond depths of about 10 microns in black

inks. The engravings should be designed so that they are as

shallow as feasible, with 50 microns as the maximum depth.

12.

A volatile oxime anti-oxidant such as methyl ethyl ketone

oxime or butanal oxime should be added to the ink. This will

retard the setting reaction until the ink is heated on the press.

Since carbon black acts as a nonvolatile antioxidant and is

difficult to disperse, its use in BEP inks should be minimized.

13.

The effect of molecular weight distribution of the resin in

the uncured ink on the cure rate should be investigated. The

molecular weight distribution can be adjusted by mixing two or

more resins with different molecular weight distributions. It

was shown in one investigation that adding 20* of a higher

molecular weight material greatly decreases the drying time, in

agreement with crosslinking theory. There is presumably a limit

to the amount of high molecular weight material which can be

page 12
Also, as the molecular

needed to set the ink

added. The viscosity will be increased,

weight is increased, fewer bonds will be

and the ink may become unstable.

14.

Oxygen is necessary for the cure of alkyds. Conversely, the

ink is stable in the absence of oxygen. Oxygen is more soluble in

organic solvents as the temperature is increased and is less

soluble in water than in many organic solvents. The solubility

and diffusion rate of oxygen in the various solvent/dispersing

agent formulations should be determined, and the effect of oxygen

solubility on ink stability and cure should be established.

15.

The UV absorbance spectra of the curing resin and the mixed

inks should be measured and used to determine the optimum

wavelength at which to irradiate the newly printed ink in order

to "akin" it using a photoinitiator. The procedure may allow

immediate printing on the other side.

16.

The effect on the cure rate of skinning of the ink by

irradiating to activate an added photo- ini ti ator should be

determined. Oxygen diffusion will probably be slowed down by a hard skin on the ink, and will presumably be slower m materials

with components such as acrylics added to raise the glass

transition temperature.

17.

If water is to be used as a cosolvent, amines must be added

to disperse the resin. The action of such amines on the cure

must be clarified. There are conflicting reports on whether they

accelerate the reaction or inhibit it. They seem to inactivate

the cobalt drier.

18.

The influence on the cure rate of viscosity-reducing agents

added to the water dispersion of the ink should be clarified.

19.

There is need for a better understanding of the action of

the driers, in particular whether they "fix'* oxygen as well as

decompose hydroperoxide groups, whether they influence the

reactivity of the carbon-carbon double bonds by producing trans

configurations from cis configurations in the alkyd side chain,

whether they are “consumed** by coordination to oxygenated groups,

and whether they are poisoned by amines.

page 13
3: Report on test for solvent evaporation from green ink

A report "Thermogravimetric Study of Low Temperature Weight Loss

from Three Intaglio Printing Inks", by J. C. Phillips and B.

Dickens was provided to BEP. Thermogravimetry consists of

monitoring the weight of a specimen continuously while it is

being heated. Dr. E. Graminski of BEP provided for weight-loss

measurements the three intaglio orintinq inks: BEP black ink BK

ASB 85-074; SICPA black ink ASB 85-073; and SICPA green ink ASB

85-072

.

The inks were heated at 2 degrees Celsius Der minute in mtroqen

and their weights monitored continuously using the

thermogravimetric apparatus. This is the heating rate used m

the flashpoint test for highly viscous liquids. The results are

3hown in graphical form in Figure 1.

The BEP black ink lost less weight than the two SICPA inks. The

BEP black ink lost 0

.

9X of its initial weight by the time the

temperature reached 50C. At the same temperature, the two SICPA

inks had each lost 1 . 3* of their initial weights. This difference

in weight loss behavior took place over the range 30 to 60C, at

which temperature the two black inks began to follow very similar

weight-loss trends with increasing temperature. At 85C, the

green ink began to lose weight much more rapidly than the two

black inks. The runs were ended at 130C because the original

request was to characterize their behavior from room temperature

to a temperature sufficiently above the temperature of interest,

50C (roughly 120F) . By the time the specimens had reached 130C,

the total weight losses were 10. 4X for the BEP black ink, 11. IX

for the SICPA black ink, and 14. 8X for the SICPA green ink.

4: Report on test for inhomogeneity of mixing

4.1: Introduction

A report on a prototype study employing thermogravimetry was

supplied to BEP by J. C. Phillips and B. Dickens. Intaglio

printing inks currently in use at BEP give small but significant

amounts of spoilage. During one of our early visits to BEP, it

was pointed out to us that a possible source of variability

leading to abnormal behavior of BEP inks is that the ingredients

have not been correctly weighed out. Since thermogravimetry is a rapid method of analysis in favorable cases, we decided to apply

the method in an exploratory study to several BEP intaglio

printing inks for which spoilage records were available. Such

inks were already on hand at NBS.

page 14
4.2: Experimental

Both black and green lnk3 were used. Between 40 and 70 mg of ink

were weighed into the thermogravimetry cup. Runs were made in

nitrogen or in mixtures of oxygen and nitrogen. A heating rate

of 4C/min was used. The specimen weight was recorded

continuously . In the case of the black inks, the color of the

residue was noted.

4.3: Results

Figure 2 shows duplicate runs of weight loss from specimens of

black ink BK62 mod 3 heated in nitrogen. The differences in the

final residues are about IX . This is an indication of the

reproducibility of the process.

Figure 3 shows weight-loss data for several black inks for which

spoilage data are also given. The ink with the highest spoilage

rate gave the smallest residue. There is some correlation

between these quantities for the four inks.

Figure 4 shows the effect of increasing the oxygen content of the

atmosphere surrounding the black ink specimen during the

thermogravimetr ic experiments. Two effects are obvious. The

first is that, for weight-loss in oxygen, the non-volatile

residue is a greater fraction of the initial weight of the

specimen. The second effect is that weight-loss occurs at lower

temperatures in oxygen-containing atmospheres.

Figure 5 shows weight-loss data for green inks, which are simpler

than black inks to examine by thermogravimetry because they do

not contain carbon black. Again, the lowest residue and hence

the lowest filler content is associated with the worst ink.

4.4: Discussion

The weight-los3 curves in Figures 2 to 5 show several stages,

which are associated with the components in the inks. Thus,

thermogravimetry is a possible method of rapid analysis to aid in

quality control of amounts of ingredients in newiy mixed inks.

The runs in this short study were all made at heating rates of 4

degrees Celsius per minute. Using slower heating rates and

smaller sample will probably increase the separation of the

weight loss stages for each of the components. Such refinements

were beyond the range intended for the study.

A residue of about 55X was found for degradation in nitrogen, and

67x for degradation in oxygen. The value of 67x for degradation

in oxygen is numerically consistent with the various iron

pigments being oxidized to their highest valence states in

page 15
oxygen, the water of hydration in yellow iron oxide being lost,

the furnace black being oxidized to carbon dioxide, and the

calcium carbonate being decomposed to calcium oxide, with the

barium sulfate remaining unchanged. The value of 55k for

degradation in nitrogen is consistent with the furnace black

reducing the other materials, so that the iron oxides go to their

lowest valence state, the other iron pigments are decomposed to

either oxides or carbides, the calcium carbonate is decomposed to

calcium oxide, and the barium 3ulfate is reduced to barium oxide.

Differences in solid residues from specimens taken from different

parts of a newly mixed batch of ink can test uniformity of mixing

to a level of mixing of about lk based on specimens weights of

several milligrams. A filler particle of 5 microns radius weighs

about 0.1 nanograms. The thermogravimetric ashing procedure

obviously only checks the grosser features of the mixing of the

ink

.

The inks which had higher failure rates had lower residues in our

study, but it is not to be expected that all failures would

correlate in the same way. An aspect of ink formulation that may

be relevant is that more drier (an accelerating agent for the ink

cure process) must be added to the ink formulation when fillers

are used than is needed in the absence of fillers. The

explanation for the add it ion a 1 amount of dr ler is that the mai or

part of the drier is absorbed on the fillers. Thus, a lower

residue in the thermogravimetric experiments corresponds to an

ink which contains less filler and hence effectively contains

more free drier. Such an ink will cure more auickly than

expected. Faster cure would also be expected in those cases

where the extra liquid is tung oil or 3mall scale mix. In an

earlier study conducted' for BEP, Hunston in the Polymers Division

at NBS showed that spoilage rates could be correlated with speed

of cure, the worst inks being those which cured the fastest.

The weight-loss data in the presence of nitrogen are expected to

correspond to simple evaporation of the components, with perhaps

some reaction between such components of the residue as carbon

black and the pigments as the temperature approaches the upper

limit used. Runs in nitrogen and oxygen show differences which

are associated with the oxidation of the organic components to

small volatile molecules and addition of oxygen to the

non-volatile ink components. Analysis of these stages and

differences in weight loss would aid in differentiating between

the various non-volatile components but are too time consuming to

be useful to BEP.

Similarly, the thermogravimetric stages of weight loss could be

associated with specific ingredients by conducting experiments

where ink samples were prepared by mixing a little of each of the

various ingredients in turn into otherwise normal pre-mixed inks.

The details of the thermogravimetric curves depend on the sample

page 16
size and heating rate. An investigation of the effects of these

variables on the intermediate stages of weight loss was too

extensive for this kind of initial study.

The reactions taking place in the thermogravimetr ic experiments

in the presence of oxygen are complex. No effort has been made

here to characterize these reactions because they take place at

temperatures which are very high compared with the temperatures

of ink cure and are therefore of limited interest.

The results obtained suggest that useful information on weighing

of ingredients. and possibly on spoilage, especially where the

residue is seen to be abnormal, can be obtained fairly rapidly (3

hours) using thermogravimetry as a method of relatively rapid

analysis. (Note: Later in this report, we give thermogravimetr ic

determinations of ink residues which were carried out at 200C per

minute. A total time of only 15 minutes was required for heating

up and cooling down.)

5: Report on test of thermomechanical stability of gummed papers

Several samples of gummed paper were received from Mrs. B.

Vaudreuil of BEP for study of the thermal mechanical properties

of the attached gum. The samples, with their BEP codes, were:

LP-52-33 (PVA-dextran gum combination), which gave a build-up of

gum on the impression roller of the printing press when used in

the temperature range 50C to 70C.

LP-52-29 (gum as above)

,

the impression roller.

LP-30-PC (dextran gum)

LP-30-EF (dextran gum)

LP-53-SMCL (dextran gum)

which did not give a build-up of gum on

no gum build-up.

caused gum build-up.

no gum build-up.

Thermomechanical measurements were made with a Perkln-Elmer

thermal analysis module. The specimens were heated from 25C to

100C at 4C per minute while their thicknesses were monitored

continuously using a weighted probe pressing on the gummed side.

Both round ended and flat ended probes were tried, with various

weights on the probe. The results indicate that these

differences did not affect the experimental data.

There was 3ome initial problem with specimen preparation.

Specimens cut out of the supplied sheets using a corn borer qave

unreliable measurements of the initial thicknesses of the

specimens and of the change of thickness with temperature because

the twisting action of the cork borer buckled the specimens.

Specimens cut out of the supplied paper sheets with scissors qave

page 17
good measures of the initial specimen thicknesses. The specimens

were about 120 microns thick <0.12 mm) and the thickness

decreased about 5 microns during the measurements. The spread of

the measurements was about 1 micron. In spite of this fairly

good precision, we could not differentiate between the gummed

papers on the basis of our measurements (Figures 6, 7, and 8).

Gel permeation measurements of the molecular weight distribution

of these gums proved not to be feasible. The gum on specimens

LP-29 and LP-33 did not dissolve m the solvent < tetrahydrofur an

)

for which the GPC columns are calibrated. The remaining

specimens produced cloudy suspensions which, if used unfiltered,

would block the filters and columns, and, if filtered, would no

longer be representative of the original specimens. It appeared

that water or an aqueous-compatible solvent would be needed to

examine the molecular weight distribution of the gum materials.

Changing solvents requires a major effort to recalibrate, even

when the appropriate molecular weight standards are available.

We did not have the calibration standards and chromatographic

columns needed to use water as the solvent.

6: Further tests for mixing homogeneity

6.1: Experimental Techniques

6.1.1: Introduction

Intaglio inks are stiff pastes containing about ten ingredients

which must be weighed out, mixed, and then milled to disperse the

various solid aggregates of particles in the liquid phase. We

proposed to answer fhe questions:

1) how well can the weighing/mixing processes be checked in a quick test?

2) does the ink cure significantly during the mixing and milling

processes?

3) does the cure behavior of the ink change during mixing and

milling?

In response to the first question, we refined our

thermogravimetr ic technique of determining ink residues to

develop a rapid method of analysis by ashing small specimens of

ink. The analysis time is 15 minutes per specimen and the method

now has a precision of about 0.3X for a single determination.

In response to the second question, we devised a method to follow

changes in molecular size of the soluble part of the ink using

gel permeation chromatography.

page 18
In response to the third question, we used the tine required by

the curing ink to attain a pre-determined viscosity as a rapid

estimate of cure time. The cure time was then used to follow any

changes in cure behavior with mixing and milling.

For three specimens of mixed and milled ink, we also followed the

cure of the ink in more detail using "molecular" parameters such

as the ultraviolet absorbance of the triple bond moiety in the

tung oil and the hydrodynamic volume of that part of the curing

ink which was soluble in tetrahydrofuran

.

6.1.2: Experimental details

Two successive batches of black intaglio inks (BEP BK62 mod3)

were sampled during mixing and milling, as indicated in the left

hand column of Table 1. Specimens were taken from various parts

of the mixing vessel before and after the small scale mix was

added. The specimens were taken from the top and from the middle

of the mixing vessel, and from locations near the side of the

vessel where the ink appeared to be in a thick layer and in a thin layer. Specimens were also taken of the ink being pumped

onto the three roller grinding mill, and before and after each of

the two grinds in the milling process. The specimens taken after

each grind were from three positions (designated left, middle and

right in Table 1) on the apron over which the milled ink falls

into the storage drum, and after the ink had sat in air for

several minutes on the apron. Left, middle and right refer to

directions seen when looking at the roller mill over the apron.

After the ink had been pumped to the second grind, a sample was

taken from the bottom of the mixing vessel. The temperature of

the ink was measured at each of these stages. The inks attained

temperatures of roughly 50C during mixing, and roughly 60C during

milling. The identification of the various specimens is given in

Table 1. The specimens were characterized in three ways: ash

content (determined in air); average size of soluble molecules;

and cure time.

6.1.3: Rapid ashing using thermogravimetry

In the ashing experiments, a Perkin-Elmer system

thermogravimetr ic analyzer was used to provide quick and precise

estimates of the uniformity of mixing from the residue left after

combustion of the inks. (Manufacturers" names and products are

mentioned where necessary in this report to define the procedures

and materials.) Ink specimens of 10 to 20mg were heated in air

from room temperature to 800C at 200C/min and held at the higher

temperature for about 3 minutes. Weight loss was essentially

complete after the first 3.5 minutes of the total time of seven

page 19
minutes, but the additional time provided a plateau from which

the weight of the residue could easily be estimated. Reddish

residues which were roughly 70x of the initial weights of the

specimens were obtained. The iron oxide in the ink was presumably

being oxidized to the trivalent state from the divalent state. In

the cases of the larger specimens, the residue gained weight

slightly during the last 3.5 minutes because sufficient oxygen

had not been available for all the oxidation to take place during

the time the specimen was being heated to 800C. The weight

percentages of residue (or ash) found for the specimens are given

in Table 1

.

In most runs, sample sizes of about 15mq were used because we

wanted to minimize effects from oxidation of the specimen as it

was being placed in the sample holder and weighed. Later, we ran

several specimens at much smaller sample size, and showed that

appreciably better resolution is obtained at specimen sizes of 2

to 4mg (Figure 9)

.

6.1.4: Molecular sizes from gel permeation chromatography

For the estimation of molecular size using gel permeation

chromatography, about 40mg of ink were dissolved in 10ml of

tetrahydrofuran and the solutions were centrifuged for 2 hours at

about 3500rpm . Trials showed that these loncj centrifuae times

produced solutions freer of particulate matter (which would have

eventually blocked the filters and perhaps the columns in the

GPC) but that there was no loss of soluble material compared with

solutions which had been centrifuged for l/2h or lh or had been

filtered twice. Specimens which had been milled extensively gave

blue-black solutions. The other solutions were clear. The

chromatograms were measured using the refractive index detector

on' a Waters 150C chromatograph equipped with 10,000, 1000, and

two 100 angstrom micro-styragel columns. The columns were

calibrated in terms of hydrodynamic volume using narrow molecular

weight standard samples of polystyrene. The measurements were

taken and the results processed using computer programs written

by Barnes, Dickens and McCrackin (1985). Some aspects of these

computer programs were specially written for this work. In

particular, the chromatograms can be transformed into terms of

hydrodynamic volume. This is a quantitative way to use the GPC

technique to characterize materials for which the Mark-Houwink

parameters, which relate hydrodynamic volume to molecular weight,

are unknown, as in the case of bodied tung oil and the other ink

ingredients, or inapplicable, as in the case of mixtures of

materials such as an ink. Averages calculated from the

distribution of hydrodynamic volumes m the mixed and milled ink

specimens are given in Table 1.

page 20
6.1.5: Cure times

"Cure" times for exposure of ink on a rubber mill at 70C in air

were determined, and are given in Table 1. The cure was followed

using a squeeze flow technique in which specimens of ink were

taken from the roller of the rubber mill every 2 minutes and

placed between two parallel plates to give a layer of ink at

least 1/8 of an inch thick. A load was immediately applied to

the top plate and the thickness of the ink layer was measured

after the squeezing flow of the ink out from between the plates

had ceased (about 30 seconds). In the early stages of cure of

the ink, the final thickness of ink layer obtained was quite

small. As the ink became more cured, the final thickness of the

ink layer increased considerably. The cure time is defined as

the time required for the minimum thickness to exceed 30

thousands of an inch. This occurs within 2 minutes of the point

at which the viscosity becomes so high that it can no longer be

measured with standard equipment. and the material is near the

gel point.

6.2: Results of study of mixing and milling

6.2.1: Introduction

In the manufacture of an intaglio ink at BEP, some ten

ingredients are weighed out and mixed. The basic problem with

relating analyses of homogeneity of mixing to ink performance is

that the desired rapid methods of analysis rarely measure

individual components.

6.2.2: Thermogravimetr ic measurements

The thermogravimetrically determined ash contents show that the

inks were satisfactorily mixed, even on a scale of milligrams.

This means that a specimen of 10 milligrams is indistinguishable

from another specimen of 10 milligrams taken from either a completely different part of the mixing process or, in the case

of the two inks studied here, from a specimen of the other ink.

Mo special care was taken when the ingredients were weighed and

mixed at BEP. The first ink was mixed 10 minutes before and 10

minutes after the adding of the small scale mix. The second ink

was mixed 26 minutes before the small scale mix was added and 5

minutes after. The averages of the residues from the two inks

indicate (Table 1) that the second ink had 0 . 3* more liquid than

the first.

page 21
6.2.3: Determination of hydrodynamic volume averages

The size of the molecules in the liquid has been estimated from

their hydrodynamic volume. Since the ink contains a mixture of

liquids, including bodied tung oil and varnish, which themselves

contain molecules with a range of sizes, a distribution of

hydrodynamic volumes is obtained. The distribution has been

averaged in various ways to search for a numerically stable

quantity to characterize this aspect of ink cure. The averages

of the logarithm of the hydrodynamic volume are the most stable,

as expected. The hydrodynamic volume averages obtained for the

various specimens from the different stages of mixing and milling

are all very similar. When these values are compared with those

given for curing ink in the next section of the report, it will

be obvious that the ink does not cure measurably during the

mixing and milling processes.

6.2.4: UV absorbance measurements

In work reported later in this report, the process of cure was

monitored using UV absorbance to follow the remaining abundance

of the tung oil triple bond sequence. The preliminary

measurements we made to check the feasibility of this approach

were also useful to show (Figure 10) that the specimen of the

second ink had slightly more tung oil (more UV absorbance at

271nm> than the two specimens of the first ink. This is

consistent with the specimen of the second ink curing in 4

minutes less time than the two specimens from the first ink. The

thermograv imetr ic measurements had shown that the second ink

contained slightly more' liquid.

6.2.5: Differences in cure times

The most striking result from these studies of mixing and milling

is that the cure times for the specimens of the second ink taken

during the mixing process are much lower than those for the first

ink. although the cure times become comparable for the two inks

for those specimens which have been milled. Intermediate cure

times were found for the specimens of the second ink which had

stood at 50C or so for the 3.25 hours which elapsed between

mixing and milling. The first ink stood for 4.2 hours at about

50C before being milled. Both inks were within specif ication at

BEP when the milling had been completed. The first ink contains

slightly more (0.3*) solid than the second ink. Only the second

ink was sampled extensively before the small scale mix was added.

The initial viscosity of this ink was higher before the small

scale mix was added, as expected because at that point there was

leas liquid in the mix. The specimen of the first ink taken

before the small scale mix was added was unusually small and

therefore was unusually accessible to the air. This specimen

page 22
cured rapidly in the container and could not be used in the cure

time measurements. The other specimens were much larger than this

specimen. These larger specimens may be kept uncured for months,

especially if kept in a freezer.

The actual cure times for the specimens of the second ink are

(Table 1): 20 to 21 minutes before the small scale mix was added;

20 to 22 minutes after the small scale mix was added; about 26

minutes after the ink had stood 3.25 hours at 45 to 50C and then

had been mixed further by being forced through the delivery pipe

to the roller mill; and about 30 minutes after milling. The cure

times are similar for the specimens of the second ink taken

before and after the small scale mix was added and mixed into the

ink for five minutes. The thermogravimetrlcal ly determined

residues of the mixed ink specimens are very similar to those of

the milled ink specimens, so the ink is obviously well mixed.

The variables which remain for consideration are the dispersion

of the ingredients in the ink and the absorption of the liquid

components by the 3olid fillers.

6.2.6: Dispersion of 3olids in the inks

Formulations of BEP inks are given in terms of weight and weight

percent in Table 2. The small scale mix contains the drying agent

which is supposed to increase the rate at which the ink cures.

It also contains some surf actant/ viscosity thickener and is about

50* solvent. The net result of adding the small scale mix is to

decrease the viscosity of the ink. The thermograv imetr ical ly

determined residues show that even after only 5 minutes of

mixing after the small scale mix had been added, the specimens of

ink appear to be well mixed on the scale of milligrams. (Since

even a coarse filler particle of 10 microns in diameter weighs

only about 0.1 nanograms, the thermogravimetr ic method can give

no idea of the amount of dispersion of small aggregates of

materials .

)

Although it may turn out that poor dispersion of the solids is

not the real reason why the second ink cured more quickly, it is

otherwise difficult to explain why 5 minutes of mixing to a level

of homogeneity better than 0.5* according to the

thermogravimetric results and several hours of standing were not

sufficient to change the cure behavior of the ink. We presume

that the 25 minutes of mixing before the small scale mix was

added were not sufficient to disperse the solids, either because

the viscosity of the ink was too high or because the solids were

not dispersed until they had absorbed the surfactant and/or

solvent in the small scale mix.

Thus, the dispersion of the solid ingredients in the ink seems to

be critical. 10 minutes of mixing seems to be enough to disperse

the ingredients sufficiently well in the ink. 5 minutes seems to

be too little time. Delivery to the roller mill and the milling

page 23
process itself will increase the dispersion considerably. It may

be worthwhile to install timers on the mixing machines if they

are not already in place, and to specify a longer mixing time,

perhaps 15 minutes after adding the small scale mix, to be more

certain that enough mixing has taken place.

6.2.6: Dispersion of furnace black

One possible explanation of the behavior we have observed is that

the furnace black, which in most commercially available grades

acts as an antioxidant, became less aggregated on soaking in the

lower viscosity ink (as the solvents in the small scale mix

permeated throughout the ink and were absorbed by the furnace

black) . With the furnace black acting as an antioxidant and in

more intimate contact with the ink, the peroxyl radicals which

lead to ink curing would be trapDed and hence the ink would cure

more slowly. A study documenting this effect was published by J.

T. Gruver and K. W. Rollmann < ’'Antioxidant properties of

carbon black in unsaturated elastomers. Studies with

cis-polybutadiene”. J. Appl . Polym. Sci . , 8, 1169-1183, 1964).

Lyon and Stoy showed that furnace black aggregates easily and is

difficult to disperse <F. Lyon and W. S. Stoy, "Properties and

ink applications of carbon black". Paint Manuf . , 36., 36, 1966).

Estimations of the degree of dispersion of the carbon black could

be based on rate of sedimentation of the solid filler from a solution of the ink or determination of turbidity by UV-visible

spectrometry (see K. C. Yang and R. Hogg, "Estimation of particle

size distributions from turbidimetr ic measurements". Anal.

Chem., 51, 758-763, 1979).

6.2.7: Critical pigment volume considerations

An alternative explanation to the antioxidant nature of furnace

black being the determining factor may be derived from the

well-known absorption of ink and paint vehicles by filler. The

critical pigment volume concentration is a very important concept

in paint formulation and manufacture (see, for example, G. P.

Bierwagen and T. K. Hay, "The reduced pigment volume

concentration as an important parameter in interpreting and

predicting the properties of organic coatings". Progress in

organic coatings, 3, 281-303, 1975).

Poorly dispersed filler contains more space between the

components of the aggregates than the same filler would if it

were broken up into smaller particles. Small particles which are

completely solid can pack with reasonably high space-filling

efficiency. The poorly dispersed material is in effect

aggregated into honeycombed structures contain an appreciable

amount of interior space and which can not rearrange themselves

to pack efficiently. Liquid is drawn into the interior of the

honeycomb structures by capillary action. More liquid is

page 24
consumed in this way than would be if the particles in the

aggregate were separated. Therefore, the mixture acts as though

it is more highly filled with solid material than it actually is.

This means that the viscosity of the mixture will be higher and

that less setting of the liquid phase will be necessary to give

such an increase in viscosity that the material will seem to be

cured. In an ink containing less than the usual amount o±' liquid,

and especially in the case of a very highly filled ink such as

8K62 (see Table 2), this may be enouah to make the ink unstable.

6.2.8: Dryinq (dehydration* of the ink

The development of the blue color during mixing is at least in

part due to drying of the driers - the cobalt salt solutions

which in theory decompose hydroperoxide groups in the setting ink

to give more free radicals and hence a quicker setting action.

The drier as delivered by the supplier and added to the ink is

purple. Figure 12 shows the visible spectrum of drier as received

and after drying. The absorbance shifts to longer wavelength as

the cobalt ions in the cobalt drier dehydrate. The appearance to

the eye is a shift from purple to blue because the red is

increasingly absorbed from the transmitted light. This is

visually the same blue as that seen in solutions of the ink. The

3ilica filler is obviously acting as a desiccant, as it does in

many domestic products which have to pour, as well as in factory

packaging of electronic components, and is keeping the ink dry.

Wet inks are expected to dissolve less oxygen. The role of

oxygen in the drying of drying oils is only partially understood.

Certainly, it is needed to form the hydroperoxide groups on which

the driers act. Therefore, insufficient oxygen will reduce the

rate of formation of the groups necessary for the generation of

the free radicals which start the setting of the ink. However,

it is well known that oxygen adds to hydrocarbon free radicals

arid thus greatly deactivates them, so too much oxygen may slow

down the ink setting reaction. Variations in the oxygen content

of inks because of variations in the drying ability of different

silicas could be a source of significant variation in the

performance of the ink on the presses.

7 : Cure measurements

7.1: Introduction

The cure behavior of an ink is one of its most important

properties, and depends on the interplay of many variables which

are not well understood. As steps are made to introduce water,

either as a wiping medium, or, more seriously, as a component of

the solvent system, the interplay between variables which

complicate the study of ink cure becomes the interplay between

requirements which are almost mutually exclusive. Previous work

page 25
in this program has examined the changes in mechanical properties

such as viscosity which occur during cure. It is now desirable

to complement these measurements with data from techniques which

focus on chemical and other molecular level changes which take

place during cure. To gain experience in the analysis of inks

and the selection of the important variables in ink cure and the

characterization of these variables, we have followed ink cure in

several ways. The approach was to quickly use each technique to

see how useful and feasible it was, and to gather information

which, although incomplete because of the survey-type approach

and the constraints of limited time and resources, would be

pertinent to interpreting the course of ink cure on a fundamental

level

.

7.2: Ink cure on the rubber mill

"Cure” times were measured as previously described

specimens exposed to air on a rubber mill at 70C and the

have been given in Table 1. The cure behavior measured

way follows changes in the viscosity of the material,

well-known that the viscosity increases very sharply

material approaches the gel point. We have shown in

work that the cure time measured in this way is quite

the gel time and is a good method of monitoring the cure

for ink

results

in this

It is

as the

previous

close to

time

.

To carry out tests based on gel permeation chromatography and UV

absorbance. we had to dissolve ink specimens in suitable

solvents. Three specimens of ink (numbers 38, 44, and 59 in

Table 1) were selected from those which had essentially completed

the milling process. These specimens of ink were cured on a rubber mill at 70C. Th'e two rollers on the rubber mill rotate at

different speeds, with the net result that the ink is kept as a single film on one of the rollers. The mill was set to produce a film * 0.012 inches thick (i.e., 0.3mm = 300 microns).

Small specimens of the ink were taken every minute during the

curing of the inks and introduced into preweighed vials which

were immediately reweighed. One of three solvents was added:

cyclohexane, to allow measurements of the ultraviolet spectrum

down to 190nm; methylene chloride, as a backup solvent; and

tetrahydrof uran , to allow gel permeation estimates of the

molecular size of the soluble part of the inks. Thus, the ink

was sampled every three minutes for each of the techniques.

7.3: Film reflectance

In the initial stages of cure, the ink is fluid enough to be able

to flow at the temperature <700 of the rollers of the rubber

mill. A lightly or insignificantly cured ink containing most of

the original solvent very quickly reforms a disturbed surface and

thus maintains a high reflectance. The ink appears glossy. As

page 26
the solvent evaporates and the ink cures, the viscosity of the

ink increases and the ink responds more slowly to the

disturbances its surface suffers when it passes through the

rollers. Thus, the glossiness of the ink decreases. Easily

noticeable changes take place as soon as 15 minutes on the

rollers. We have not pursued this approach, but note that it is

a quick test which could be used to assay rates of drying of the

ink.

An indication of the evaporation rate for an undisturbed film is

given by the thermogravimetr ic weight loss curves in Figure 13,

where there is an appreciable rate of weight loss from the

surface of the ink in the f irst 12 to 15 minutes . This trend i

s

then overpowered by a weight gain, presumably from the oxidation

of the ink as it sets. The fact that the effect is independent

of sample size shows that it is a surface effect, as expected for

an undisturbed film.

7.4: Mill resistance as a measure of tack

A meter was installed to measure the current drawn by the mill as

the ink cured. The current decreases to a steady value after

about 2056 of the cure time. This corresponds to about 7 minutes

in Figure 13, i.e., where the rapid rate of weight loss from

solvent evaporation falls off sharply. As solvent is lost from

the ink, the tackiness of the ink decreases and the ink remains

as a film attached to only one of the rollers rather than gluing

the two rollers together.

7.5: Clumping of the ink in solvents

When the ink is introduced into a solvent, it dissolves with more

and more difficulty as cure proceeds and eventually remains

undispersed in the "solvent". The time of such "clumping"

depends to some extent on the solvent used. Clumping took place

after the ink had cured for about 20 minutes on the rubber mill

when cyclohexane was the solvent. We have not followed this

index more quantitatively.

7.6: Color intensity of drier

After the specimens of cured ink had been placed in the solvents

and centrifuged, we noticed that the intense blue coloration

character i st ic of the early stages of cure was much less intense

in the specimens of well-cured ink. This provides another index

with which to follow the extent of cure. The mechanism is not

clear. Presumably, the cobalt ions are being bound up in the

cured ink, rather than existing in soluble form by being

coordinated to an anion which is itself soluble in the solvents

page 27
used. As cure proceeds, oxygen-containing and hence polar groups

are formed in the ink. Therefore, the cobalt ions have many more

possibilities for bonding, and the metal ions would be expected

to be attached to polar groups, such as carboxylic acid groups,

in the cured ink.

The relationship between removal of cobalt ions from solution and

crosslinking in the ink is not known. It is reasonable to assume

that crosslinking via ionic bonds to the cobalt ions occurs. If

so, it may be possible to make the cobalt ions more available to

the ink setting reaction by including some small fraction (say

about IX) of free fatty acid in the ink formulation. Presumably

there are other sources of crosslinks than ionic bonds to the

cobalt ions, so loss of such bonds will not be detrimental to the

setting of the ink.

7.7: UV absorbances

The most obvious use of UV absorbance measurements is to follow

the disappearance of the unsaturation in the tung oil as the ink

sets. Tung oil has a characteristic set of three absorbances at

263, 271, and 283nm. The liquid components of ink are petroleum

solvent and various natural and modified oils. The oils consist

of fatty acids esterified with the trihydric alcohol glycerol.

The ester groups in the oils absorb at 204nm in the UV spectrum.

Therefore, the quantity of unreacted tung oil can be estimated

and the total amount of oil can also be found. UV spectra of the

soluble components of the ink are given in Figure 14.

Cyclohexane was used as the solvent because it does not absorb

much in the range of interest, and matched cells were used so the

solvent spectrum is —not included in the spectra shown in the

F igures

.

Figures 15 and 16 contain graphical representations of the

consumption of tung oil as the ink sets. The ratio of tung oil

to total oil la shown in Figures 17 and 18. After the 24 to 27

minutes of curing at which the viscosity of the curing ink

increases sharply, about 3/4 of the tung oil has been consumed,

as shown by both Figure 15 and Figure 16. The spectra were

normalized to equal sample weights.

The quantity of drier in the solution was determined from its

absorbance in the visible region at 660nm . The amount in

solution increases slowly up to a cure time of about 17 minutes.

To some extent, this is a consequence of evaporation of solvent

from the ink. At the time at which the ink begins to lose the

capability to reform the surface after it has been disturbed by

the rollers of the rubber mill, the quantity of soluble drier

begins to fall off sharply. After 23 minutes, the quantity of

soluble drier has fallen to a very low value.

page 28
The setting reaction of the ink comes from the unsaturation in

the ink. The three neighboring double bonds m tung oil are

especially reactive. It is convenient to follow the degree of

cure by following the absorbance of UV radiation at 27lnm, the

center of the tung oil absorbance. Figure 19 shows the UV

absorbance spectrum of a thin film of tung oil. Almost all the

detail is missing. Addition of a small quantity of a solvent

such as cyclohexane brings back the characteristic tung oil

absorbance. We tentatively ascribe this phenomenon to the

lowering of the high viscosity of the neat tuna oil by the added

solvent. We currently do not know whether cure of ink films can

be followed using UV spectra taken on films of the ink without

using a solvent. although we note that there is solvent in the

ink. Certainly, the viscosity will change greatly in the final

stages of cure. The safest route will be to dissolve small

quantities of a large film in a solvent. These results also

suggest that in principle it should be possible to follow changes

in viscosity using UV absorbance spectra of suitable probes, and,

conversely, that the UV absorbance spectra of probes and

photoinitiators may be affected by the viscosity of the medium.

7.8: GPC measurements

When a polymeric molecule is dissolved in a solvent, it assumes a size which is governed by how large the molecule is and how well

the molecule is solvated by the solvent. The average size is

called the hydrodynamic volume of the molecule. Hydrodynamic

volume is related to molecular weight through the Mark-Houwmk

parameters, which must be determined for each type of polymer and

each solvent. Gel permeation chromatography is a technique which

determines the distribution of hydrodynamic volumes in a material. Polymeric materials are injected in very dilute

solution into a stream of solvent which flows through a set of

columns containing a distribution of holes. The largest

molecules are able to enter few of the holes, and the smallest

molecules are able to enter most of the holes. Therefore, the

largest molecules come out of the columns before the smallest

molecules. The columns are calibrated in terms of narrow

molecular weight fractions of well-known polymers such as

polystyrene and poly methyl methacrylate. Tetrahydrofuran is used

as solvent.

Figure 20A contains a chromatogram of an ink specimen which had

cured for 24 minutes at 70C on a rubber mill. The chromatogram

for the same ink after only 30 seconds of cure is shown in Figure

20B . We are currently developing methods of quantifying and

comparing such chromatograms. Figures 20C and D show essentially

pictorial ways of adjusting the relative heights of two

chromatograms and performing a subtraction to determine the

difference chromatogram. The abscissa units in Figure 20 are

elution volume, which are dependent on the set of columns used in

the chromatograph. Only the elution volume range in Figures 20 C

and D was characterized in terms of hydrodynamic volume. We

page 29
calculate indices to quantify the misfit in the chromatograms,

but these approachs are still in the early stages, and more

experience in using the results will be necessary before we can

have confidence that the development is reasonably complete.

Chromatograms of the liquid ingredients of BEP BK62 mod 3 are

given in Figure 21. Figure 22 contains chromatograms of

specimens taken at the cure times shown. Hiqh molecular weight

material appears at elution volumes between 24 and 28mls as cure

proceeds

.

We have developed our own computer programs for the analysis of

gel permeation chromatograms. The classical approach is to

relate the time at which the material comes out of the columns to

the molecular weight of the material. To do this, however, one

needs the Mark-Houwink parameters. If the Mark-Houwink

parameters are not known, or if the injected material is a mixture, the molecular weight distribution can not be obtained.

We have therefore introduced the concept of characterizing

materials through their distribution of hydrodynamic volumes, and

have provided parameters, analogous to the various molecular

weight averages, which give some idea of the shape of this

distribution. Number average log hydrodynamic volumes and

weighted average hydrodynamic volumes are given for the curing

inks in Table 3. The changes in hydrodynamic volume as a function of cure time are shown in the chromatograms in Figures

23 and 24. The averages of log hydrodynamic volume attain a maximum value after 24 to 27 minutes. The setting times,

measured in the same experiment, were all about 28 minutes, but

are not precisely known. The trend of average log hydrodynamic

volume with time is shown in Figures 25 for the three inks and in

Figure 28 for specfimen 38. From these figures, it is

qualitatively clear that the amount of large molecular weight

material in the curing ink grows perceptibly for about 27

minutes, the time at which the viscosity measurements indicate

that the inks had cured. The size of the soluble material then

decreases to a constant value at a cure time of about 40 minutes.

From crosslinking theory, the decrease is a consequence of the

larger molecules being linked together to form insoluble gel,

leaving only the smaller molecules m solution.

7.9: Infrared absorbances

Infrared spectra of the liquid ingredients of the ink are shown

in Figure 27. The spectra are fairly simple, and those of tung

and linseed oil are very similar. The major difference is

between cis <735 wavenumbers) and trans <965 wavenumbers) C-H out

of plane deformations next to the double bonds. In addition,

tung oil has an intense absorbance at 995 wavenumbers. This may

be from the C-H groups in the double bonds. Interestingly, the

special intaglio vehicle contains both the cis and the trans C-H

absorbances but not the 995 wavenumber absorbance. The soya

lecithin contains many 0-H bonds. IR spectroscopy is known to be

page 30
fairly insensitive. In the near Infrared, water can be determined

at 5160 wavenumbers as a combination band, but our specimens were

too thin for us to be able to see this absorbance. A large

quantity of work has already been published on IR studies of the

oxidation of the isolated pure fatty acids and esters which make

up the drying oils. It remains to be seen to what extent the

oxidation of pure fatty acid esters parallels the oxidative

curing of an ink contains metal drier salts and furnace black

antioxidant

.

A specimen of ink placed on a sodium chloride plate was allowed

to cure at room temperature, and the infrared spectrum taken at

intervals, as shown in Figure 28. The difference spectra show

that little change is seen up to 42 hours. Some of the OH

absorbance at 3500 wavenumbers is at first lost <m the down

direction in Figure 28) and then regained. After 5 days, the ink

cracked on its support, as shown by the overall smaller signal it

gave and the less intense absorbance of the background. It is

obvious that extremely precise spectra will be required to show

details in which one can have confidence. The IR spectra in

Figure 28 were taken on the ink itself, and little energy was

transmitted through the ink to the detector. In addition, the

tung oil vehicle was only a small fraction of the total absorbing

species. Features such as the C-H stretch next to the double

bonds can be seen clearly at 3010 wavenumbers, and the trans and

cis C-H bends can be seen at 965 and 735 wavenumbers,

respectively. The carbonyl region at 1735 wavenumbers contains

only the ester group absorbance. We will examine the feasibility

of taking IR spectra on specimens of ink which we have dissolved

in some solvent, probably cyclohexane, and relating the results

to those found earlier on pure compounds by other workers.

page 31
8 : Conclusions

Alkyds were recommended as the most practical new resin system to

polymerize in air. The alkyd should have a well-chosen molecular

weight distribution and be dispersible in a water-containing

solvent system. UV irradiation of the newly printed currency

paper can probably be used to skin the ink raDidlv and orevent

blocking, especially if a photoinitiator system is added to the

formulation. This may allow printmcr of both sides of the

currency paper in one pass. Replacement of the various pigments

and fillers currently in use by dyes attached to the backbone of

an organic polymer, either in water-soluble form or in the form

of a glass should be investigated.

A rapid method of analysis using thermogravimetry to determine

solid residues left after an ink has been ashed has been

developed. Analysis time is 15 minutes. The reproducibility of

one determination is roughly 0 . 3X . The method showed that

specimens from one ink sample had on the average 0.3* more liquid

than specimens from the other (this average value was obtained

from 25 determinations and hence has an uncertainty of about

0.06JO. UV spectroscopy showed that the second ink sample

contained more tung oil. Cure time studies showed that it cured

more quickly.

Several interesting results arise from the study of the variation

of cure time variation with extent of mixing and milling. First,

both the variations in properties within a batch and between the

two batches were less than our earlier work on these inks had led

us to expect. These two ink samples clearly do not show the width

of variation that exists in intaglio inks. When we examine more

inks in the future we' may find other differences. Even so, our

results are very informative. Four points are worth noting:

1) The inks were fairly homogeneous on a scale of about 0.1

milligrams yet showed significant differences in curing behavior.

2) The time of cure increased with grinding, rather than

decreasing as would be expected if the ink cured somewhat during

the grinding process.

3) The addition of the small scale mix, which contains the

driers, had little or no effect on the cure time for the second

ink until the ink had been milled.

4) The intensity of the blue color in the i iquid-extracted

portion of the ink changed with extent of cure.

Dispersion of the solid3 in the ink seems to be critical. An

explanation for the variation of cure time with extent of mixing

is that the solids remain aggregated, or at least poorly

dispersed, and therefore absorb more liquid inside the aggregate

by capillary action than they would if the aggregates were well

dispersed and the liquid were merely absorbed on the separated

paqe 32
particles. The ink then acts as though it were more highly

filled than it really is. With a highly filled ink such as BK62

mod 3, this may make the ink unstable. The phenomenon is well

known in the manufacture of paints.

Carbon black acts as an antioxidant in most of its forms.

Thought should be given to omitting the furnace black from the

ink formulation and using a volatile antioxidant instead. This

will give good storage properties and perhaps better control of

cure since furnace black is known to aggregate easily and to be

difficult to disperse. A volatile antioxidant would evaporate

from the ink on the press and allow the setting reaction to

proceed from that point on.

The technique of gel permeation chromatography was developed to

provide distributions and averages of these distributions in

terms of log hydrodynamic volume. This is a new way of

classifying molecular size for systems which are mixtures of

materials. The average log hydrodynamic volume of the soluble

part of the vehicle is a useful index with which to follow the

cure of the ink.

page 33
S: Figure legends

Figure 1. Weight loss of BEP inks as marked, showing the

influence of evaporation of solvent.

Figure 2. Comparison of thermogravimetry curves of specimen

weight versus temperature for two runs of a specimen of the same

sample of a BEP BK62 mod 3 black ink.

Figure 3. Thermogravimetry curves and spoilage records for

several BEP BK62 mod 3 black inks heated in nitrogen.

Figure 4. Comparison of thermogravimetry curves for specimens of

BEP BK62 mod 3 black inks heated in oxvqen and nitrogen, as

indicated

.

Figure 5. Thermogravimetry curves and Qualitative sooilage

records for BEP green inks 4717 RCA Mill #5 heated in oxygen.

Figure 6. Decrease in

specimens of LP-52-33.

Figure 7. Decrease in

specimens of LP-52-29.

thickness with

thickness with

heating

heating

to

to

100C for various

100C for various

Figure 8. Decrease in thickness with heating to 100C for various

specimens of LP-30-PC, LP-30-EF, and LP-53-SMCL.

Figure 9. Increase in detail in thermogravimetry curves obtained

by heating ink at 200C per minute as the sample size is

decreased

.

Figure 10. UV absorbance of three specimens of BK62. normalized

to the absorbance at 204nm . Specimens 38 and 44 are from the

f^irst ink, specimen 59 is from the second ink. Specimen 59

contains slightly more tung oil, as shown by its higher

absorbance at 271nm.

Figure 11. Experiments to show the loss of carbon black from

BK62 ink using thermogravimetry at 200C per minute.

Figure 12. Ultraviolet absorbance spectra of the cobalt drier at

various stages of dehydration in methylene chloride. As the

absorbance shifts to longer wavelength, the color becomes more

blue because the red is being absorbed from light transmitted

through the specimen.

Figure 13. Initial loss of solvent and later weight gain from

oxidation, as shown for BEP BK62 mod 3 using thermogravimetry.

Figure 14. UV absorbance spectra of the liquid components of BEP

BK62 mod 3 intaglio ink.

page 34
Figure 15. Changes in UV absorbance of the soluble components of

ink as the ink cures. Specimens were taken from an ink curing at

70C at the times (in minutes) noted in the figure. Solutions are

in cyclohexane and were centrifuqed before the spectra were

taken . Figure 16. Amounts of tung oil (absorbance at 271nm) and cobalt

drier (absorbance at 600nm) remaining as a function of time of

cure for BK62 mod 3 curing on a rubber mill at 70C.

Figure 17. Consumption of tung oil (271nm> and cobalt drier (600

to 700nm> as a BEP BK62 mod 3 ink cures at 70C on a rubber mill.

The spectra have been normalized to the ester group absorbance at

204nm. The solvent was cyclohexane.

Figure 18. The ratio of tung oil concentration to that of total

oil /varnish content as a BEP BK62 mod 3 ink cured at 70C on a rubber mill.

Figure 19. Comparison of the UV absorbance spectra of tung oil

in cyclohexane solution (well defined peaks) and as a smear on a quartz plate (very poorly resolved spectrum, shown as a dotted

line)

.

Figure 20. Gel permeation chromatograms for two ink specimens

cured for 24 minutes (20A) and 30 seconds (20B) . Figures C and D

show two possible ways of subtracting the chromatograms from each

other to show the effect of cure. In Figure 20C. the two

chromatograms have been equalized at the peak corresponding to

unpolymerized oi 1 / varni sh/sol vent . In Figure 20D, they have been

equalized at a point between the non-polymerizing component and

the high molecular weight material.

Figure 21. Chromatograms of the liquid components of BEP BK62 mod

3 intaglio ink in tetrahydrofuran

.

Figure 22. Chromatograms of specimens of a BEP BK62 mod 3

intaglio ink curing at 70C on a rubber mill. The specimens were

dissolved in tetrahydrofuran and the solutions then centrifuged.

The units of the abscissa are in elution volume. The higher

molecular weight material elutes at the smaller volumes. The cure

times are as shown. Newly formed hiqh molecular material appears

at 24 to 28mls as cure proceeds.

Figure 23. Chromatograms of a curing ink transposed into terms

of log hydrodynamic volume. The high molecular weight material

formed during curing appears at log hydrodynamic volumes higher

than 4. Cure times as shown.

Figure 24. Superposition of selected chromatograms of a curing

ink. The times cover the time at which the viscosity of the

curing ink suddenly increases. The curve for 27 minutes la

page 35
lowest near 2 In log hydrodynamic volume and highest near 5. The

curves for 21 minutes and 33 minutes are highest near 2 and

lowest near 5. The other two curves are intermediate.

Figure 25. Change in average log hydrodynamic volume with cure

as determined from GPC chromatograms. The log hydrodynamic

volume passes through a maximum }ust before the viscosity of the

curing ink suddenly increases. (Only the soluble portion of the

ink is characterized in this way.)

Figure 26. Log hydrodynamic volume versus time of cure for a specimen of BEP BK62 mod 3. The maximum in log hydrodynamic

volume is shown clearly. "Cure time" for this ink specimen was

32 minutes, based on increase of viscosity.

Figure 27. Infrared spectra of the liquid components of BEP BK62

mod 3 spread on a sodium chloride plate. The spectral range is

from 4000 wavenumbers (left) to 800 wavenumbers (right).

Figure 28. IR spectra of an ink curing on a sodium chloride plate

taken at several long cure times at room temperature. The

spectral range is from 4000 wavenumbers (left) to 800 wavenumbers

(right)

.

page 36
Table

1:

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log

hydrodynamic

volumes,

sample

numbers,

cure

times

and

TG

residues

for

specimens

of

BEP

BK62

mod

3

selected

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Table

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hydrodynamic

volume

as

a

function

of

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three

specimens

of

BEP

BK62

mod 3

intaglio

inks

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at

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mill.

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3

Figure 1

Figure

1.

Weight

loss

of

BEP

inks

as

marked,

Figure

I

showing

the

influence

of

evaporation

of

solvent.
jqSjaM jeujSjJO 9^

Figure 2

Figure

2.

Comparison

of

thermogravimetry

curves

of

specimen

weight

versus

temperature

for

two

runs

of

a

specimen

of

the

same

sample

of

a

BEP

BK62

mod

3

black
Black

ink

BK62

mod

3

in

2

jqSraM praiSuo

Figure 3

Figure

3.

Tlierinogravimetry

curves

and

spoilage

records

for

several

BEP

BK62

mod

3

black

Inks

heated

in

Figure

3

nitrogen.
Black

ink

BK62

mod

3

in

N,

and N

}

/0

8 S

;qSl3M 9^

Figure 4
Green

Ink

in03

/N

Figure 5

5.

Thermogravimetry

curves

and

qualitative *

records

for

BEP

green

inks

4717

RCA

Mill

#5

Figure 5

in

oxygen.
0.010

tkkpehatore

(°c)

Figure 6

Figure

6.

Decrease

in

thickness

with

heating

to

100C

for

various

specimens

of

LP—

52—

33.
SAMPLE

G

BA

PH

SHBET

LOAD

PROBg

+ * + I

o

‘soisiw*i

TEMPKBATUBB

(°c)

Figure

7.

Decrease

in

thickness

with

heating

to

100C

for

various

specimens

of

LP—

52—

29.
OlO'O

TEMPBHATUHJB

(°C)

Figure

8.

Decrease

in

thickness

with

heating

to

100C

for

various

specimens

of

LP—

30— PC,

LP—

30— EF,

and

LP—

53—

SMCL.
Figure

9.

Increase

in

detail

in

thermogravimetry

curves

obtained

by

heating

ink

at

200C

per

minute

as

the

sample

size

is

decreased.
a & r a a

- wt W td <d

cn

*—

O -j >

i c c

>

ZJ S g

0 13

(JZ

lu

LU>

<*

33Nvayosav
c». n

igure

11.

ExperimeDt8

to

show

the

loss

of

carbon

black

from

K62

ink

using

therraogravimetry

at

200C

per

minute.
2 <->

w « " ! e > X>—

CDX

LU

_l

LU><

l°||I P “ - “ . (U if 3 .» £ f-s -2 s’ 3 .2 C 0) O .&n

2p i 3 £ > rt j3 -»j

£s tc

oi

R

ad

Pij ad 23NVSa039V —(-

S3

e»
100%

Figure 13. Initial loss of solvent and later weight gain from

oxidation, as shown for BEP BK62 mod 3 using thermogravimetry.
Bodied tung oil special intaglio

vehicle

cobalt drier

2.9

QJi

X>

UiC

cn -O>

L 13

l a

in

solar varnish soya lecithin high gloss

varnish

Figure 14. UV absorbance spectra of the liquid components of BEP

BK62 mod 3 intaglio ink.
o

cyclohexane

and

were

centrifuged

before

the

spectra

were

i
tung

oil

<3 *C

T3

_ CD

B

31

am

i B-i r QO m fNJ CS

ca

CO

cam

_ css co .3

CO

CM

_ CD

(S

eo

aoireqjosq* i\j|
8.76

U1>

I >

ZD

LU>

<

-+

toa

i

33Nvaaosav

Figure

17.

Consumption

of

tung

oil

(271nra)

and

cobalt

drier

(60

to

700nm)

asa

BEP

BK62

mod

3

ink

cures

at

70C

on

a

rubber

mill.

The

spectra

have

been

normalized

to

the

ester

group

absorbance

at

204nm.
ca

LO

C55 m

ca

co

cs>

CVJ

152

sajqnyos i^o:»/[to Sun*

Figure

18.

The

ratio

of

tung

oil

concentration

to

that

of

total

oil/varnish

content

asa

BEP

BK62

mod

3

ink

cured

at

70C

on

a

rubber

mill.
O rf’J

5P c 5 S c o

J3u ^3 Ba « . u rt

8. 

a 75 v 5 “ a

I!aso

-=

SB

J2uC~»

i.8. mm TP) > 3 -

«S > # 5< *S

•+3

g

aO - u * a 5

sj l

S'1 a ®

v

v > d : - ^ *o c. £ &.S . .3) " I s

fa .5

NVSMOSSV
Figure 20. Gel permeation chromatograms for two ink specimens

cured for 24 minutes (20A) and 30 seconds (20B). Figures C and D

show two possible ways of subtracting the chromatograms from each

other to show the effect of cure. In Figure 20C, the two

chromatograms have been equalized at the peak corresponding to

unpolymerized oil/varniBh/solvent. In Figure 20D, they have been

equalized at a point between the non- polymerizing component and

the high molecular weight material.
Bodied

tung

oil IW8b81?

33

special

intaglio

petroleum

varnish

vehicle

solvent

Figure

21.

Chromatograms

of

the

liquid

components

of

BEP

BK62

mod 3

intaglio

ink

in

THF.
lunoure aunoore aAi^pj o
c

iunoure aAi^pi innoure aAi^-epj

Figure

23.

Chromatograms

of

a

curing

ink

transposed

into

terras

of

log

hydrodynamic

volume.

The

high

molecular

weight

material

formed

during

curing

appears

at

log

hydrodynaimc

volumes

higher

than

4.

Cure

times

as

shown.
REL QUANTITY OF MATERIAL VS. LOG HYDRODYNAMIC VOLUME

BEP BK82 ML4G2—44—8 21 MINS CURE

BEP BK62 ML4G2—44—9 24 MINS CURE

BEP BK62 ML4G2—44— 10 27 MINS CURE

BEP BK82 ML4G2—44— 11 30 MINS CURE

BEP BK82 ML4G2—44— 12 33 MINS CURE

04—JUN—85 04: 21: 28

04-JUN—85 05-22:22

04—JUN—85 10'55-05

04-JUN—86 08-23-30

04—JUN—85 12:00 = 20

log hydrodynamic volume

Figure 24. Superposition of selected chromatograins of a curing

ink. The times cover the time at which the viscosity of the

curing ink suddenly increases. The curve for 27 minutes is

lowest near 2 in log hydrodynamic volume and highest near 5. The

curves for 21 minutes and 33 minutes are highest near 2 and

lowest near 5. The other two curves are intermediate.
hydrodynamic volume vs time during curing

specimen 38

9 10 29 38 40

specimen 44

X specimen 59

curing time, min

Figure 25. Change in average log hydrodynamic volume with

cure as determined from GPC chromatograms. The log

hydrodynamic volume passes through a maximum just before the

viscosity of the curing ink suddenly increases. (Only the soluble

portion of the ink is characterized in this way.)
hydrodynamic volume vs time during curing

<V

210

1

E

J3

O

>

208

190-

'U

180-

a

>> 170-

'S

168

"

>> -a 150 -

D Oo

D 0

° D

D Dd

0

t

1 1 r 6 18 20 33 48 50 60

curing time, minutes

Figure 26. Log hydrodynamic volume versus time of cure for a

specimen of BEP BK62 mod 3. The maximum in log

hydrodynamic volume is shown clearly. "Cure time* for this ink

specimen was 32 minutes, based on increase of viscosity.
woeqiosq* HI

figure

27.

Infrarod

spectra

of

the

liquid

component#

of

BEP

BK62

mod

3

spread

on

a

sodium

chloride

plate.

The

spectral

range

is

from

4000

wavenumbers

(left)

to

800

wavenumbers

(right).
IR

absorbance

spectra

of

BK62

mod3

ink

1-4 -uO

8.

03

OJoa

0>

u>0) 53

"O

4^

00a- T3

iO

00>Epo

-aEpo

-a

cs

tj*

03>Po

-ao

wtreqjosq* >u Figure 28

Figure

28.

IR

spectra

of

an

ink

curing

ona

sodium

chloride

plate

taken

at

several

long

cure

times

at

room

temperature.

The

spectral

range

is

from

4000

wavenumbers

(left)

to

800

wavenumbers

(right).
NBS-1 MA irev. 2 -ac) U.5. OEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 86-3303 (/£)

2. Performing Organ. Report No.| 3. Publication Date JANUARY 1986

4. TITLE AND SUBTITLE

Study of Intaglio Ink Mixing and Cure and Ingredients

5. AUTHOR(S)

Brian Dickens, Donald L. Hunston, Joseph C. Phillips and George W. Bullman

S. 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.

8. Type of Report & Period Covered

9.

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

10.

SUPPLEMENTARY NOTES

| 1 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 significant

bibliography or literature survey, mention it here)

This report covers the period October 1, 1984 to September 30, 1985. A report

discussing principles important in formulating intaglio inks and giving several

recommendations was provided to BEP and is reviewed here. Other reports on evaporation from intaglio inks, on a short thermogravimetric study of correctness of

weighing out the ingredients in the inks, and on a thermomechanical study of gummed

paper were provided to BEP but are also included here.

Two intaglio inks were sampled during mixing and milling at BEP and

characterized in terms of cure times, ash residues, ultraviolet absorbance spectra

measurements, and gel permeation chromatographic measurements. The results show

that even when mixing to homogeneity on the scale of fractions of milligrams has

been accomplished, the state of dispersion on a scale much finer than this may be

critical. Cure times, ultraviolet absorbance measurements and gel permeation

chromatography measurements have been used to follow ink cure. Refinement and

application of the techniques developed in these studies are expected to lead to

data which will help greatly in understanding the variables which influence the

air-cure of intaglio inks. Application of these techniques and the development of

new techniques as the needs arise will help in the design of improved inks.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by .emicoion

Alkyds; analysis; formulation; gel permeation chromatography; intaglio

printing inks; milling; mixing; thermo gravimet ry ; UV absorbance.

13.

AVAILABILITY

1 | Unlimited

[X] For Official Distribution. Do Not Release to NTIS

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

20402.

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

14. NO. OF

PRINTED PAGES

15. Price