Reference

NBS

Kererence

PUBLICATI0NS

NBS1R 85-3216

llntagiio Ink Considerations

/

iffllfflliliii nil ill iiiiot o:

ill I iiii i BMDfll

Brian Dickens

U.S. DEPARTMENT CF COMMERCE

National Bureau of Standards

Institute for Materials Science and Engineering

Polymers Division

Gaithersburg, MD 20899

August 1 985

Prepared for: Bureau of Engraving and Printing

‘\S. Department of the Treasury

-GC- -Washington, DC 20228

100

NBSIR 85-3216

# * INTAGLIO INK CONSIDERATIONS

NATIONAL BUREAU

OF STANDARDS

LIBRARY

Brian Dickens

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Institute for Materials Science and Engineering

Polymers Division

Gaithersburg, MD 20899

August 1 985

Prepared for: Bureau of Engraving and Printing

U.S. Department of the Treasury

Washington, DC 20228

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Intaglio Ink Considerations July 7 1985

B Dickens

Polymers Division

National Bureau of Standards

Gaithersburg, Md 20899

Table of Contents

1 Introduction . 2

2 Film-forming resins 3

3 Intaglio printing ink vehicles 6

4 Initiation systems for intaglio printing inks 7

5 Filler considerations 10

6 Pigment considerations 12

7 Mechanism of cure for drying oils and alkyds 13

8 Resin considerations 16

9 Solvent/dispersing phase considerations .... 17

10 Drying agent considerations 20

11 Recommendations 21

12 References 25

Abstract

Alkyds are recommended as the most practical resin system to

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

weight distribution and be dispersible in a water-containin

solvent system. The solubility of oxygen in the ink formulation

and solvent mixtures should be determined and the compositions

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

UV irradiation of the newly printed 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. The silica filler may be acting as a dessicant as

well as a thickening agent. If this dessicant action is not

important, thixotropy of the ink should be adjusted by using

sodium carboxy methyl cellulose instead. Other recommendations

are given and the formulation of intaglio printing inks is

reviewed with special reference to BEP requirements.

Page 1

(om
1 Introduction

This report provides information on ways in which improved

intaglio printing inks can be developed. Particular emphasis has

been 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 are

to help in the selection of a new ink vehicle 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. The ink setting reaction is a type of

polymerization. Hence, polymerization reactions will be

discussed first.

Many of the desirable properties of polymeric materials are

conferred by the entanglement of the long polymer molecules with

one another. Polymers are made by joining small molecules

together to make long molecules.

In free radical polymerization, free radicals are generated and

add to carbon-carbon double bonds in the small molecules

(monomers). This effectively opens one of the two bonds in the

carbon-carbon double bond in the small molecule. One of the

bonds links the small molecule to the attacking radical, and the

other becomes a new radical center which continues the linking

process to other small molecules. When this process is repeated

many times, a polymer is formed. The polymerization takes place

at reasonably rapid rates at ordinary temperature because the

free radicals used are extremely reactive and their reactions are

very rapid.

Another way to make long chain molecules is to use reactants

which each have two groups which enter into chemical reactions at

moderature temperatures. This process is referred to as

step-polymerization. Two different types of reactants may be

used, such as a di-alcohol and a di-acid. Such bifunctionality

would give long linear chains with the component molecules

alternating. This class of reactions may be subdivided into

those in which the molecules combine by rearrangement of atoms

with no net loss of atoms from the molecular structure, and those

in which the molecular rearrangement leads to the formation of

two or more new molecules, one of which is the polymer or

pre-polymer, and the other of which is some small molecule such

as water.

Linear chains are expected to be soluble in a suitable solvent,

and to melt if they are stable enough to withstand the necessary

heat. A two-component reaction mixture in which one of the

reactants is present in excess will give short chains, because

the chain-forming process will end when one of the chain formers

is used up. A reaction mixture of components with functionality

greater than two will give chains with bonds to other chains.

Page 2
i.e., a three-dimensional network or cross-linked system.

Cross-linked systems cannot be melted and are insoluble unless

degraded into small molecules or broken at the interchain links.

A third polymerization process is to use ions in either anionic

or cationic polymerization. To obtain good yields of polymeric

material, low temperatures must generally be used, and the

reactions are very easily stopped by traces of oxygen and water.

The method is somewhat analagous to free radical polymerization,

but the required conditions are so special that ionic

polymerization will not even be considered here as a possibility

for intaglio printing inks curing on large presses running in the

open air

.

The first two polymerization methods (i.e., free radicals and

"normal" chemical reactions) of making polymers are routinely

used to produce polymeric compositions for organic films and

coatings. The resin supplier can draw on a wide range of

possibilities when choosing the composition of the prepolymer

molecules and the conditions of their preparation. Hence, resins

can be designed and made in large variety. The problem is to

design a method of polymerization and a resin which will give

polymerization of the ink under the conditions imposed by the

printing conditions.

Later sections of this report will discuss the constraints

imposed on the formulation of intaglio inks by the necessity to

polymerize the ink in the open air, and a recommendation for a polymerizable system will be given. In order to make clear which

kinds of polymerization systems are in common use, a review will

first be given of the simple resins on which more complicated

formulations and mixtures are based.

2 Film-forming Resins

The earliest polymeric system used in films and coatings was

based on naturally available and slightly processed oils from

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

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

oils are usually heat-treated prior 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.

Page 3
Alkyds are modified drying oils and dry by the same mechanism.

Different alkyds have different numbers of functional groups per

molecule because they are made by modifying drying oil molecules

or by joining two or more drying oil molecules together into more

extended molecules. 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.

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

autoca taly t ic , as will be explained in the sections on Initiation

Systems and on Mechanism of Cure, and that the rate of drying

increases with time, at least in the early stages. 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 (Mayo et al .

,

1958a, 1958b, 1958c). 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 (see the section on

Mechanism of Cure).

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

Page 4
candidates for use in intaglio ink3, 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 are discussed in Section 4.

Polybutadiene and its derivatives are used to coat the inside of

can 3 because they have good adhesion, good chemical resistance,

and are without taste. These films form in free radical

reactions in which carbon-carbon double bonds are opened to make

new bonds which join previously separate molecules together. The

carbon-carbon double bonds are not next to each other and the

adjacent hydrogen atoms are relatively unreactive. Therefore,

heat and a special catalyst are required to accelerate the

particular chemical reaction of interest, and the reactions are

easily poisoned by oxygen.

Unsaturated polyesters polymerize by the opening of double bonds

in free radical reactions to give new bonds which become the

backbone of the polymer chain. Often, the materials are used in

a partially preformed state in a solvent which is itself

polymerizable because it contains suitable double bonds, and the

systems are known as "high solid" or "sol ventless" systems. The

free radical reactions are strongly inhibited by oxygen.

The following polymerization reactions proceed by rearrangement

of atoms, usually by moving a loosely bonded, or "active"

hydrogen atom. Because free radicals are not involved, the

reactions are relatively slow.

Epoxy resins are based on the cyclic epoxy group -(CH-0-CH2),

which can be opened in the presence of catalyst (both acids and

bases have suitable catalytic activity). The epoxy group can

combine with itself or extract labile hydrogen atoms from groups

such as -OH (alcohol) and =NH (amine). Most epoxy resin systems

consist of two components which are mixed just before use.

One-component systems are available in which the activating

species is chemically combined (or "blocked") to give an inactive

form which can be reactivated by decomposition of the blocked

group, either thermally or by hydrolysis by moisture in the

surrounding air. The catalyst can also be blocked - the addition

complex of a tertiary amine and boron trifluoride is stable at

25C but dissociates rapidly at HOC (Solomon, 1977, pl95).

Polyurethanes are formed by addition of labile hydrogens (in -OH

or -NH- groups as mentioned above) to the isocyanate group

-(N=C=0) to give -(NH-CO-O)- groups. One-component systems

require chemical blocking of the active groups. Thirty minutes

at 150C is typical of the reaction time required to unblock the

system

.

Page 5
Formaldehyde reacts with active hydrogen atoms in other molecules

to give larger molecules. Its reactions with amine (-NH-) groups

and -OH groups on phenols are extensively used to produce films.

Melamine has six active hydrogen atoms per molecule and with

formaldehyde gives stable, hard, and fast-drying films.

Films can also be formed without polymerization taking place in

the film-forming step. Cellulosic derivatives and acrylates and

other vinyl polymers are made prior to film formation, usually by

free radical reactions in the absence of oxygen in the case of

the vinyl type polymers, and are used as heat-softened,

dispersed, or soluble materials which on cooling or evaporation

of solvent give thin films. These materials are of relatively

low molecular weight, and the film is in principle soluble.

Silicone groups also enter into reactions with various types of

groups such as active hydrogens to give new chemical bonds which

can be used to loin molecules together, but these formulations

are very expensive and are only used where the superior

environmental resistance obtained is worth the extra expense (a

factor of about 10 over the simple drying oils)

.

3 Intaglio Printing Ink Vehicles

Film-forming materials can be designed to contain reactive groups

more or less as desired. The procedure is to design small

molecules which contain groups necessary to confer the required

properties in the finished film and groups which will allow the

film to be polymerized. Resistance to exposure can be built in.

Solubility or dispersibility in an appropriate solvent can

usually be built in in the same way.

The polymerizable chemical groups in the small molecules are then

activated or attacked and lead to chemical reactions which link

the small molecules together to give larger molecules.

The first consideration for most formulations is that of cost. A

very important second consideration for intaglio printing inks is

environmental protection. Thus, the small molecules used should

not be particularly combustible, volatile, or harmful, or even

too unpleasant in odor.

For water-wipe systems, the ink should be water-dispersible from

the wiping cylinder which removes excess ink from the engraved

cylinder. This means that the small molecules should contain

hydroxyl and/or carboxylic acid groups which are sufficiently

polar to interact with water or with an aqueous solution of a base . The next question is whether the ink should be cross-linked into

a permanent film or deposited as an entanglement of more or less

linear molecules. Intaglio printing is used for high quality

Page 6
printing, usually of valuable documents such as currency and

financial papers, and a permanent film is therefore usually

required. This means that some polymerization must take place

after the ink has been applied.

The conditions of the polymerization are dictated by the nature

of the substrate (rag-based paper for US currency at the present

time) and the required speed of printing, which for currency is

essentially to be as high as feasible. The only realistic

possibility is to use free radicals. Free radicals are extremely

reactive. They react so quickly that the fact that the

temperature coefficients of free radical reactions are small is

of no consequence. However, hydrocarbon free radicals combine

very readily with oxygen when it is available and the resulting

peroxyl radicals are much less reactive than the original

hydrocarbon radical. Thus the high reactivity of hydrocarbon

radicals is greatly reduced when they combine with oxygen.

The role of oxygen has to be established. Either the reaction is

carried out so quickly that oxygen does not have time to

interfere once the small quantity originally present in the ink

components has been used up, or oxygen is used as a reactant.

For oxygen not to interfere with a free radical polymerization,

the initiation process must be very rapid and intense. When

oxygen is a reactant, the initiation process must take place over

a long period so that the needed oxygen can diffuse into the

film. The appropriate rate of initiation depends on whether

oxygen enters into the crosslinking (setting) reaction, in which

case the rate of initiation should be slow so that enough oxygen

will be available by diffusion into the ink layer from the

surrounding air, or whether oxygen merely helps in the production

of free radicals, which will combine with oxygen if it is

available or cause setting of the ink by cros3linking if it is

not. In this second case, an intermediate rate of initiation i3

required

.

4 Initiation systems for intaglio printing inks

Free radicals can be produced at enormous rates by the

interaction of high energy radiation with the film-forming

material. The rate of radical production is much greater than

the rate of oxygen diffusion. Therefore, polymerizations which

are poisoned by oxygen can be contemplated as candidates.

Electron beams tear organic materials apart to form free

radicals. Because electron beams produce ozone from room air,

they also require an inert atmosphere. Initiation by electron

beam is therefore an option which is not particularly simple to

implement. Gamma rays are readily absorbed by some chemical

groups, which could be incorporated into the film-forming

material, but they are so damaging and penetrating that very

thick shielding is required to protect operating personnel

.

Page 7
Microwaves (billions of hertz) and radiofrequencies (millions of

hertz) act on polar and dipolar molecules to tumble them rapidly.

This, then, is a method of selectively heating a component in the

reaction mixture, in principle without limit on the amount of

heat absorbed. Microwaves have been used to selectively

decompose peroxyesters in prepolymer mixtures to effect

polymerization (Menges et al . , 1980) and to polymerize acrylates

by irradiating the monomers (Teffal et al . , 1983). One of the

problems in the use of microwave and radiofrequencies in 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 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. Since

printing inks are necessarily colored, and hence absorb visible

and UV radiation very strongly, neither UV initiation nor visible

light initiation is an attractive option for polymerization of

these inks.

Cure of 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. Using UV radiation to

decompose the surface hydroperoxide groups would give two free

radicals per hydroperoxide group. Hydroperoxide groups absorb

very weakly at about 320nm, which is in the spectral range of

medium pressure mercury lamps. In principle, the UV wavelength

could also be chosen to break up the double bonds in the resin.

However, these groups absorb at about 265nm for three conjugated

double bonds, and 225 nm for two conjugated double bonds.

Radiation in this spectral region requires special care in

handling. Cure of the surface using UV or visible light could

also be effected by including a suitable photo- ini tiator in the

ink formulation. This may be the most promising route to

reducing spoilage from blocking.

The system used for initiation of polymerization should allow a storage time of at least two days (the real average storage time

is more like eight days) at room temperature (which is often 300

and yet perform effectively at the temperature of cure (in the

region of 70C to 80C for paper products) in the time allowed.

Table 1 shows the Arrhenius parameters which such an initiation

would need to have to be successful even under the rather

favorable conditions detailed in Table 1. The necessary

equations have been given by Pappas and Hill (1981). There is

probably no feasible simple chemical reaction suitable for use as

a printing system even under such idealized conditions because

Page 8
chemical reactions do not have sufficiently large pre-exponential

coefficients. Additional work by Pappas et al . <1983, 1984) has

concentrated on reactions which may be expected to have higher

reaction rates, but sufficiently high rates are probably

unattainable, especially in condensed phases. These

considerations rule out systems which use simple combination of

functional groups on the small molecules. For the same reason,

unblocking of chemically blocked groups and generation of free

radicals from an added initiator will not work.

Table 1 Arrhenius parameters needed for initiation system

for two sets of cure requirements (after Pappas and Hill, 1981)

case 1 case 2

storage press storage press

y* cure 10 50 20 35

temperature 30 80 25 80

time, days

time, secs

3

30

1.5

60

E (kcal/mole) 47 32

A (pre-exponential) 1.5E27 3E17

The most 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 and 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 a 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 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 possibly using visible light,

although the ink would then be slightly light-sensitive.

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

Page 9
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.

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 been shown to have a very significant effect on

the rate of formation and final properties of a film (Bobalek et

al . , 1964a, 1964b, Fletcher et al . , 1964). As the molecules in

the resin become larger, fewer bonds are needed to set the ink.

Thus, inks containing large molecules set more quickly. At some

point, the ink will become unstable. A resin system which allows

fine-tuning of the molecular weight distribution should be used,

and the influence of molecular weight distribution on the resin

properties should be studied. A mixture of alkyds of the same

chemical composition but with well-chosen molecular weight

distributions is a good candidate.

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 drying. Polar groups and

durability can be added to drying oils by making them into

alkyds. Therefore an alkyd is the material of choice as an

intaglio ink vehicle. Because alkyds are used in the paint

industry, there seems no danger of shortages in supply. Also,

since wel 1 -control led changes can be made in the functionality

(number of potential polymerizable links per molecule) of the

drying oils as they are made into alkyds, the fatty acid

components can be adjusted more or less at will to control drying

and the likelihood of further oxidative degradation. Thus,

molecules which are larger than modified drying oils can be

prepared with consistent properties. Fewer bonds are needed to

link them into a three-dimensional network. Hence, an alkyd can

easily be made which will dry more quickly than a drying oil.

5 Filler Considerations

Fillers, or extenders, were originally added to coatings such as

paints to add volume to the dried film. It is now recognized

that they also contribute to such properties of the final film as

sheen, film reinforcement, and hiding background details which

would otherwise been visible through the film. Finally, they

reduce the cost of the film. In inks, solids are added to

control viscosity, improve the ability to wipe the ink off the

engraved cylinder, and provide the desired color.

Page 10
Amorphous silica, calcium carbonate, and barium sulfate are the

main fillers and extenders used in BEP intaglio inks.

(Definitions of filler, extender and inert pigments and a short

survey of their characteristics are given in Stoy and Brown

(1982); pigments will be discussed in the next section). One

form of amorphous silica is pyrogenic (fumed) silica, which is

made from volatilized sand. The material is relatively free from

impurities, such as metal ions, which would affect the setting

reaction of the resin. It readily takes up water, which affects

its adhesion to the resinous component of the ink. The hydration

of the surface produces silonol groups, which combine with

alcohols and amines. Removal of this water by heating causes

condensation of the silica particles with one another to give

larger particles. A little-recognised property of the silica is

that in non-aqueous formulations it keeps the other components of

the ink dry.

Another form of amorphous silica is dehydrated gel. The gels

formed by acidification of aqueous silicate solutions. Sil

gels made in this way contain large quantities of trapped wa

and metal ions, and are far from being pure or very reproducib

The viscosity of ink or paint thickened with silica changes w time (Lippe, 1969, 1976). The viscosity- thickening effect

silica and the rate of change of th

found (Wason and Maisel, 1981,

size of the particles, the part

nature of the surface of the si

average size of 1.8 microns with

7 microns was desirable.

this effect with time has b

1983) to depend on the aver

icle si ze distribution and

1 ica

.

The y suggested that

a range of sizes between 0.5

are

ica

ter

le .

i th

of

een

age

the

an

and

From the discussion given above, it is seen that silicas are a possible source of trouble, especially in water-based

formulations. It may be appropriate to find a substitute

material. Silica is added to liquids to make them thixotropic - viscous when flowing and free-standing when stationary. This

means that they flow easily enough for use on the printing

presses yet are able to form thick films after they have been

deposited on the paper. This behavior is related to the

extension of order through the liquid phase resulting from

polarization and attraction of molecules in the liquid phase by

the silica. When the mixture is at rest, the order builds up.

When the mixture is disturbed, the order is destroyed, and the

material flows easily with a viscosity somewhat related to the

rate of shear. Thixotropy is a property of colloidal

suspensions

.

Various gelling agents can be used to confer thixotropic

properties. When thixotropy is produced by including filler in

the formulation, the more finely divided filler is more effective

in conferring thixotropy but the solutions are more unstable. In

addition to silica, there are amine-treated clays and

hydrogenated castor oil. Alkyds may be gelled by the

Page 11
incorporation of polyamides. Aqueous solutions and suspensions

may be gelled by the addition of cellulose derivatives. One of

the best is sodium carboxy methyl cellulose, although ethyl

hydroxy ethyl cellulose has also been used. The cellulose

derivatives do not confer thixotropic properties, but their

solutions have a highly ordered structure which behaves somewhat

similarly

.

Barium sulfate and calcium carbonate are widely used as extenders

in paints (Boxall et al., 1981), and are also used in BEP ink

formulations. They are added to modify the flow and leveling

properties of the liquid, and the mechanical, permeability and

gloss properties of the resulting solid. Barium sulfate is more

or less inert , but , because of its high density, tends to settle

out of thin formulations. Calcium carbonate is more unstable,

and combines readily with fatty acids to give water- insoluble

salts. The surface of calcium carbonate is appreciably changed

by the absorption of atmospheric water (Solomon et al . , 1983,

p96). Water-dispersible resins contain carboxyl groups, which

with calcium carbonate in an aqueous environment would give a scum of insoluble calcium salts if the pH is not kept high by the

presence of amine. This may lead to printing problems and it may

be necessary to replace the calcium carbonate with a more inert

material in a water-dispersible ink.

One possibility for a suitably inert material is a glassy organic

polymer which is not swelled or hydrolysed by water. From this

point of view, polystyrene is a good candidate, while

polymethylmethacrylate is not. An extension of this idea is to

use a mixture of resin which dries by polymerization and a material which simply dries by loss of solvent to give a film

with a relatively high glass transition temperature. The

advantages of this kind of approach were described by Ellis and

Bakule < 1982)

.

6 Pigment Considerations

Several BEP formulations contain pigments based on iron oxides.

These inorganic solid materials are surface-hydrolyzed by

atmospheric water. They act as drying agents (in some proportion

to their surface area) in catalyzing

hydroperoxides formed in the drying

alkyds (Solomon et al . , 1983, plOO).

little effect at room temperature,

baking temperatures (Whitaker, 1966).

the decomposition of the

step of drying oils and

Iron-containing soaps have

but are very reactive at

The ink must be milled to disperse the pigments throughout the

liquid phase, and must be cooled throughout the milling to reduce

the effect of the curing reactions. Both the pigments and the

fillers, especially carbon black, adsorb the drying agents, which

must be added in considerable excess of the amounts needed to

cure the neat resin. Most grades of carbon black seem to repress

Page 12
oxidative crosslinking reactions by functioning as antioxidants

(Gruver and Rollmann, 1964). One of the few exceptions is

graphitized channel black.

Some BEP inks currently contain organic dyes as well as inorganic

pigments. It may be that inks which cure more reproducibly would

be produced if the pigments were completely replaced by organic

dyes. Certainly, the drying stage would then not be influenced

by adsorption of drying agent, catalytic action of the pigments

or antioxidant activity. The extensive milling required to

disperse solid inorganic pigments would not be needed. This

would remove or reduce variability in the initiation period of

the cure of the ink because fewer hydroperoxide groups would be

produced during the mixing process.

The color of the dye depends on the interaction between the

polarity of the dye and that of the matrix on the one hand, and

the pH of the environment on the other. The dye must not change

color in the weak acidic or alkaline environments conceivably met

by currency during service. Leaching out of the dyes should not

be a serious problem, because they can be attached to polymer

backbones. For example, it may be possible to use as pigment an

organic glass containing dissolved or grafted dyes. This is

similar to the earlier suggestion of replacing the extenders with

a chemically inert organic glass or with materials, such as the

water-dispersed acrylics described by Ellis and Bakule (1982),

which merely dry by evaporation but have a high glass transition

temperature and therefore toughen the film, especially in the

early stages.

7 Mechanism of Cure for Drying Oils and Alkyds

Because an alkyd system is the most viable ink resin material for

BEP intaglio inks at the present time, a brief review of the cure

mechanism will now be given. In one of the earliest systematic

studies, some 24 aspects of the drying of drying oils were

considered by the Research Committee of the Federation of Paint

and Varnish Production Clubs (Bragdon, 1958), and most of these

were characterized in considerable detail. In their publication,

one can find details of the changes in drying oils during curing,

for example, expansion, weight gain, spectrophotometr ic changes,

oxygen penetration, oxygen uptake, and so on. Oils containing

various fatty acids were synthesized.

The chemical mechanisms of drying have been summarized in some

detail in the Encyclopedia of Polymer Science and Technology

(Wiley, 1967) and in the Encyclopedia of Chemical Technology (3rd

edition, Wiley, 1977). Other reviews include those by Sims and

Hoffman (1962), Rheineck and Austin (1968), Solomon (1977), and

Wexler (1964). In addition, there is a large body of pertinent

knowledge available from countless studies of oxidations of

organic compounds.

Page 13
Most vegetable oils are glycerol esters of CIS fatty acids.

There are three fatty acids per oil molecule. Those oils which

contain unsaturated fatty acids, i.e., with carbon-carbon double

bonds in the C18 backbone, react with free radicals to form

inter-fatty acid bonds. The double bonds activate the hydrogen

atoms on the neighboring (allylic) CH2 group. Those CH2 groups

which are between two double bonds are the most reactive. The

double bonds themselves also undergo reaction, those in the cis

configuration being more reactive than those in the trans

configuration. Because the more easily extracted hydrogen atoms

are next to carbon-carbon double bonds, the radical resulting

from their extraction is stabilized by resonance over the

neighboring double bond. During the later reaction of the

radical, the most stable configuration is adapted, and the double

bond is regenerated where possible so that it is conjugated with

other double bonds.

The polymerization scheme for the setting reaction is essentially

as follows. Hydroperoxide groups are inevitably formed during

oxidation of the oil whenever the oil is in contact with air.

These hydroperoxide groups are broken up by photolysis, heat, or

the action of the drying agent, which is typically a transition

metal ion in soluble organic form.

R-O-O-H + Co ( 2 + ) = R-0* + 0-H < - ) + Co(3+)

The drying agent has been reported (Girard, Beispiel and Bricker,

1965) to be regenerated by the reaction

R-O-O-H + Co(3+> = R-0-0 0 + Co(2+) + H<+)

The hydrogen and hydroxyl ions form water, i.e.,

H( + ) + O-H(-) = H20

.

The radicals formed (shown with dots in the above equations)

abstract a hydrogen atom from the allylic CH2 group or add to the

double bond.

R-0» -CH=CH-CH2-CH=CH- = R-O-H + -CH=CH-C- H-CH=CHR-0 • + Q-CH=CH-P = R-0- (Q)CH-C* H-P (P and Q are ends of the

drying oil molecule).

The hydrocarbon radicals are peroxidized by rapid combination

with molecular oxygen, which diffuses into the film from the air.

-C H- 02 = -CH-0-0"

The peroxyl radicals undergo several reactions, including

hydrogen abstraction from the allylic group to form more

hydroperoxide groups. The hydrogen on allylic groups is one of

the few types of hydrogen that these relatively unreactive

Page 14
radicals can abstract. This is the importance of the activated

-CH2- groups in the fatty acid side chains. In this way, more

initiating groups (R-O-O-H) are produced in a chain reaction,

i.e., in a series of steps which is repeated several times, and

the curing reaction accelerates. Cross-linking is accomplished

by a radical on one fatty acid side chain adding to a double bond

in a fatty acid in another oil molecule, viz..

-CH=CH-C- H-CH=CH- + P-CH=CH-Q = P-CHR-OH-Q,

where R = -CH = CH-0 H-CH = CH- , and P and Q are ends of the drying

oil molecule. The second reaction involving the RQ' radical,

given above, is another example of cross-linking.

When two peroxyl radicals meet, they interact. If there is a hydrogen atom bonded to the carbon atom on which the peroxide

group is located, this interaction is usually terminating for the

radicals. Both radicals cease to exist because a hydrogen atom

is transferred from one to the other and a molecule of oxygen is

produced (this is the mechanism of the Russell termination

reaction). If there is no hydrogen atom available on the carbon

atom to which the peroxide radical is attached, the interaction

is non-terminating , and produces two highly reactive alkoxyl

radicals and a molecule of oxygen. Although oxygen is produced

in these interactions, it should be noted that two molecules of

oxygen were originally consumed to produce the two peroxyl

radicals. Whether alkoxyl radicals abstract hydrogen from their

environment or undergo beta-scission to cut up the molecule

depends mostly on their lifetime, which is almost always very

short. The products of both possibilities are more likely to

crosslink than the alkoxyl radicals are, and this may be an

important source of the crosslinks which set the ink.

Termination, or removal of radicals, also occurs by combination

of two radicals to form a bond between the two parent species.

This is more likely for hydrocarbon radicals than for peroxidized

radicals, and therefore will occur mainly in the interior of the

film, which is relatively sheltered from oxygen during the curing

for fast-curing systems.

Thus, the film slowly oxidizes, cross-links, shrinks (beca

intermolecular distances of about 2.8A are being replaced

covalent bond distances of about 1.5A), and hardens. Van

small molecules escape from the film and, in the case of rapi

drying oils such as tung oil, puff up the film. These sm

molecules are water from the combination of hydrogen ions

hydroxyl ions produced from the hydroperoxide groups by

driers, and scission products from one of the reactions ( beta-scission reaction) undergone by alkoxyl radicals. The

bonds in the setting film are both oxygen-carbon bonds

carbon-carbon bonds, but not all these bonds contribute

crosslinking of the molecules in the ink. The nature of

crosslinks is not known. The range of reactions which occur

large. The importance of each reaction path in the over

setting of the film is governed by activation energies, react

use

by

OU3

d 1 y

all

and

the

the

new

and

to

the

is

all

ion

Page 15
rates, temperature of cure, concentrat ions of reactive species,

group mobilities, and the catalytic effects of driers, pigments,

fillers and impurities. It is not worthwhile to attempt to give

every possible reaction in this report.

8 Resin Considerations

In the earlier sections of this report, we have shown that the

requirements of the intaglio printing system limit the resin

system to an air-oxidation drying oil. The need for water

dispersibility and, if feasible, use of water as solvent,

strongly suggests the use of an water-dispersible alkyd as the

resin component. The resin can be combined with other

non-polymerizing materials, which are actually polymers

themselves, to give the partially dried film more toughness and

less tackiness than it would otherwise have. Ellis and Bakule

(1982) described the addition of water-soluble acrylic resin to

water-reducible alkyds to give a faster-drying tougher film. The

acrylic component acts as an organic filler.

An interesting combination of allylic methylene (-CH2-) groups

and acrylate groups in the same monomer molecule was reported by

Larson and Emmons (1984), who described the polymerization of

dicyclo pentenyl oxyethyl methacrylate. The compound polymerizes

with alkyds, drying oils, and methacrylates, with or without air.

In the drying oil-air oxidation mode, the molecular weight

achieved was only 1000 to 2000, whereas when the material was

polymerized in the methacrylate mode and initiated by UV

irradiation, molecular weights over the range 10,000 to 1,000,000

were obtained. This difference was attributed by the authors to

the higher radical density in the case of UV initiation, but is

more likely to be the effect of the methacrylate radicals being

far more reactive than the oxygen-containing radicals which cause

the polymerization in the drying oil mode. Steric hindrance of

the constrained dicyclo pentenyl moeity probably also slows down

the drying oil reactions. Availability of oxygen is known to

greatly retard the polymerization of methyl methacrylate (Mayo

and Miller, 1958). It seems that the exterior of the material

cured oxidatively and the oxygen -starved interior cured by methyl

methacrylate polymerization reactions, which were initiated by

free radicals diffusing in from the exterior layers. This effect

begins to be important at thicknesses of greater than 50 microns

(2 mils). It should also be noted that there was some problem

with tackiness during the early stages of drying. The shelf life

of the system was controlled by adding the volatile antioxidant

methyl ethyl ketone oxime. The drying time of long oil alkyds

was shortened from 5.5 hours to 3.5 hours by adding 10 to 30%

content of the monomer to the alkyd.

It may be feasible to UV cure this material to give a solid skin

by adding photoinitiators. This would dry the outer surface of

the ink to a depth dependent on the levels of irradiation and

Page 16
added photoinitiator and the

interior would then polymerize

(because oxygen would diffuse

than through a fluid skin).

UV absorbance of the

under oxygen starvation

more slowly through a

ink. The

conditions

solid skin

Holmberg and Johansson (1984) studied the effects of molecular

weight distribution, hydroxyl group content, and double bond

content on the viscosity and film properties of air-drying alkyd

resins which they synthesized. The alkyd with the widest

molecular weight distribution gave the hardest films. It also

gave the most viscous solutions for a given weight of polymer, as

expected, since it had the highest molecular weight material.

Hydroxyl groups increased the viscosity (in xylene solution) by

intermolecular hydrogen bonding, and double bond content

decreased it.

Kumanotani et al . (1984) studied the properties of colloidal

alkyds. They had previously reported on the properties of alkyds

with microgel content. These alkyds have fairly high molecular

weights. The colloidal alkyds are of very high molecular weight

(of the order of 1 million). Although these solutions are very

viscous, the highest molecular weight resin gave the best

film-forming results in all of the several tests they conducted.

Nakayama, Watanabe, and Toyomoto (1984) described a high

performance coating formulated from water-based emulsions and

microgels stablized by a water-soluble air-drying resin.

Gooch et al . , (1984) have studied the effect of auto-oxidative

cross-linking, defined from the swelling behavior of the

crosslinked polymer, of alkyds derived from vegetable oils. The

alkyds were polymerized as emulsions in air and in pure oxygen.

Crosslinking in air gave gas-filled voids which had deleterious

effects on the mechanical properties. The emulsion materials

dried much more rapidly in pure oxygen and had about 75-85% of

the tensile strength of the film produced using metallic driers

and at most 60% of the hardness. According to Gooch et al . , the

crosslinks include ester and ether bonds. The study was carried

out to provide information on natural products, specifically soya

bean oil. Commercial emulsions dried as well as the experimental

materials and had much better properties.

9 Solvent/Dispersing Phase Considerations

Although water can probably be used as a dispersing agent for

some alkyd formulations, it is probable that on the printing

press water would not consistently evaporate at the right rate.

The precise details are heavily dependent on the relative

humidity of the air surrounding the press and the temperature of

the press (Culver, 1982). A mixture of water and a "cosol vent

such as a glycol is an often-used compromise, but evaporation

from cosolvent systems in which one component is water is also

greatly affected by the relative humidity of the surrounding air

( Culver , 1982)

.

Page 17
Since a resin used in new BEP intaglio inks would be

water-compatible or at least water-dispersible, an appreciable

part of the glycol content of the solvent would probably remain

in the film and of course the glycol would not cure. Any glycol

exuded from the film would probably make the film tacky.

Film-forming compositions which require a cosolvent for

solubility crack as the solvent mixture becomes richer in water.

This arises when the organic cosolvent evaporates at its usual

rate but the high humidity hinders the evaporation of water.

Moderately high molecular weight ethylene glycol monoethers are

used as so-called "tail solvents" which are slow to evaporate.

This ensures that the solvent mixture retains some organic

solvent in situations where the humidity is so high that water

evaporates only slowly.

A constant boiling mixture of water and one or two other

materials where water forms a high percentage of the liquid would

be an better candidate. However, most of the azeotropic mixtures

have boiling points less than those of the components.

The following are water-containing azeotropes which have boiling

points above that of water

ethylene diamine 81.6%: water 18.4%, b. pt . = 119C

formic acid 77.5%: water 22.5%, b. pt . = 107C

hydrochloric acid 20.2%: water 79 . 8%

,

b. pt . = 108. 6C

hydrobromic acid 47 . 5%

;

water 52.5%, b. pt . = 1 26C

(Other acids also form azeotropic mixlures with water - the

constant boiling mixture between hydrochloric acid and water was

the first to be discovered and is used to provide analytical

solutions of the acid in water . ) The acids would be incompatible with many fillers and pigments

and dyes, and would degrade the paper currently in use. The

diamine mixture contains so little water and so much organic

material that it is not a worthwhile candidate.

An attractive choice to consider is to run the printing press in

an environment with some appropriate combination of temperature

and relative humidity so that water can be used undiluted. Once

applied, the ink rapidly cools off, regardless of the temperature

of application. As was pointed out above, the ink can probably

be skinned by passing the sheets under a medium pressure mercury

lamp. Another possibility is to heat and dry the

water-containing ink using radio- or microwaves.

While water is very attractive as a solvent, there are many

complications. Engel (1983) discussed problems arising from

retention of the amine needed to solubilize or disperse the resin

in water, high acid values from the acid groups added to the

resin to confer solubility in water, solvent-resin interactions.

Page 18
viscosity-temperature behavior, and hydrolysis. The formation of

resinous aggregates was suggested to occur at high water contents

(Overton and Herb, 1983; Wicks, Anderson and Culhane, 1982). The

appropriate temperature to use for water-containing formulations

is not immediately obvious. Popping in the form of bubble

formation in the setting film is a serious problem of water-based

formulations of paints and baking enamels ( Imai and Tsubouchi,

1980, Watson and Wicks, 1983) . The cause seems to be retained

water, which is harder to drive off than many organic solvents

because of its higher heat of vaporization. There is an inverse

relationship between the glass transition temperature of the

resin system and the maximum film thickness obtainable

blemish-free (Watson and Wicks, 1983) . Our own work has shown

that water complexes the driers.

The questions of surface tension, viscosity control and

evaporation of solvents from water-reducible, high solids

coatings were investigated by Hansen (1984). Water-reducible

means that although the polymer or pre-polymer is not itself

soluble in water, it can be dispersed in water in the form of

micelles, probably with the help of some other solvent or a complexing agent such as an amine. Water itself and its

dispersions have surface tensions much higher than the optimum

for wetting the substrate. The surface tension can be reduced by

adding suractants. Added polymer has a perturbing effect on the

usual agents added to reduce surface tension in that it tends to

absorb them (Hansen, 1977). Once the substrate has been wet, the

surface tension of the ink serves to drag the ink into the pores

of the substrate by capillary action and hence promotes adhesion.

The viscosity of the ink must be kept in the appropriate range

(about 5000 to 20,000 cp) while the fraction of the liquid which

ends up as solid film should be as high as possible. This

requires a solvent or, more usually, a solvent mixture that

dissolves as much low and high molecular weight material as

possible. Therefore, attention must be paid to the compatibility

between the polymer, water, and any additives (Hansen, 1984) by

considering their solubility parameters, which can be measured,

but can also be estimated from activity coef f icients . A

considerable quantity of water would be absorbed from the ink by

the paper.

In high sol ids formulations, or in the later stages of

evaporation of solvent from most films, the evaporation of

solvent is controlled by diffusion through the film. Changes in

rates of evaporation have been quantified by Hansen (1968) and

Ellis (1983). The rate of evaporation of solvent (Grant, 1981)

has a complex effect on the properties of the drying film.

Hansen (1984) points out that the effect of evaporation of the

solubilizing amine from water dispersions of air-drying alkyds .s

a neglected or unexplored area, and that retention of the amine

lengthens the drying time (Dhein et al . , 1978). Perera and

Vanden Eynde (1983) have developed a method to measure th-'- relationship between the stress in a coating and the evaporat: :n of solvent. The stress is affected mostly by the rite of

Page 19
evaporation of the slowest evaporating solvent, and is least for

the slowest evaporating solvent because the film then has time to

undergo stress relaxation. Accumulated stress adversely affects

cohesion of the film and adhesion to the substrate.

10 Drying Agent (Drier) Considerations

The generally accepted view of the action of the metal ion drying

agents is that the metal ion changes valence to produce free

radicals and ions from hydroperoxides. These reactions have been

mentioned above in the section

effective, the metal ion must

drying oil (or alkyd) . This

with an anion which is soluble

usually used. Both cobalt and

component. Various other salt

function seems to be to neutrali

and those formed during the ox

keep the driers in solution as

they may confer some crossli

oxygenated groups formed in the

Howvever, this would immobilize

inhibit further setting. There

humidity on the action of the

used (Wexler 1964). As noted

also functions as a drier, espec

on Mechanism of Drying. To be

be in intimate contact with the

means that it must be associated

in the resin. Naphthalates are

manganese are used for the metal

s are sometimes added - their

ze acid groups initially present

idative cure reaction, and thus

the naphthalates. In addition,

nking by ionic coordination to

ink by the air -oxidation process,

the active part of the drier and

is supposedly little effect of

cobal t-manganese driers usually

earlier, the iron oxide pigment

ially in hot ink.

Amines have been suggested as activators for the driers (Solomon,

1977, p56, and references therein) . Amines are known to

decompose hydroperoxides (Hiatt, 1971, and references therein),

and this may be their mode of action. Thus, an alkyd solution

containing an amine may cure more quickly than expected, and this

kind of solution may also be advantageous from the point of

retarding the evaporation of water. It should be noted that the

metal salt driers give only one free radical per hydroperoxide

group, whereas routes exist, as in UV photolysis for example, to

break the hydroperoxide group into two radicals. The amine can

be recovered unchanged when the reactions are carried out in

dilute solution, yet the decomposition of the hydroperoxide is a free radical reaction. This suggests that the hydroperoxide has

been decomposed catalytically into two free radicals and that

amines may be very effective drying agents, even more so than

metal salts, which produce only one free radical per

hydroperoxide group. Hurley and Buono (1982) report that amines

are known to complex cobalt driers (Morral, 1979) and to affect

the reversibility of the trivalent to divalent cobalt transition

(Laitmen and Kivalo, 1953). For the cobalt to act as a catalytic drier, it must cycle between these two valence states

(Girard et al . , 1965).

Page 20
Girard et al. (1965) also showed that divalent cobalt aids the

isomerism of double bonds in the oil molecules from the cis to

the trans form, which they thought oxidized more quickly than the

cis form. The action of drying agents is at best only partially

understood

.

Recommendations

1. The ink vehicle should be an alkyd because only molecules

with allylic hydrogens will polymerize successfully in air. One

possible candidate is the water-dispersible "Lustrasol 93-996"

resin solution from Reichold Chemicals.

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

containing methacrylate groups, based on that 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

resi n/ 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 less fine-tuned (and hence less prone to

variation) to rely exclusively on the drying oil mechanism and to

select a candidate from the modified drying oil field.

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. 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.

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.

Page 21
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 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 is to act as a dessicant 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.

7. A replacement should be sought for the calcium carbonate

extender. An organic extender such as polystyrene or sulfonated

polystyrene should be considered, provided it can be safely and

successfully mixed into the resin and the effect of the

difference in refractive index of polystyrene from that of

calcium carbonate can be tolerated.

8. A water-soluble acrylic or equivalent resin should be added

to ink. This will impart a reasonably high glass transition

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

a tougher and more tack-free film.

9. 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 by 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

aspect may not be worthwhile. Although a powerful fluorescer

such as anthracene could maybe overpower most other fluorescers,

one could conceivably change the apparent denomination of a bill

(as read by a fluorimeter) by soaking it in an appropriate

solution

.

Page 22
11.

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

the interior. UV initiation will probably be too slow beyond

depths of about 10 microns in inks. The engavings 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.

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

added. As the molecular weight is inceased, fewer bonds will be

needed to set the ink and the ink may become unstable.

14.

Oxygen is needed for the cure

ink is stable in the absence of oxyg

in solvents as the temperature is

soluble in water than in many organi

and diffusion rate of oxygen in the

agent formulations should be determin

solubility on ink stability and cure

of alkyds. Convers

en . Oxygen is more

increased. Oxygen

c solvents. The so

various solvent/di

ed, and the effect o should be establish

ely, the

soluble

is less

lubility

sper si ng

f oxygen

ed .

15.

The UV absorption 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 "skin" it using a photoinitiator and possibly allowing

immediate printing on the other side.

16.

irrad

deter

hard

The effect on

iating to acti

mined. Oxygen

skin on the ink.

the

vate

diff

cure

an

usion

rate of skinning of the ink by

added photo- ini tiator should be

will probably be slowed down by a

17. The action of 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.

IS. The influence of

water dispersion of the

viscosity-reducing agents

ink on the cure rate should

added to the

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 tran3

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 23
12 References

E. G. Bobalek, E. R. Moore, S. S. Levy, and C. C. Lee, "Some

implications of the gel point concept to the chemistry of alkyd

resins", J. Appl . Polym. Sci . , 8, 625-657, 1964.

E. G. Bobalek and M. T. Chiang, "Synthesis and properties of

some alkyds of more complex carboxyl functionality", J. Appl.

Polym. Sci., 8, 1147-1168, 1964.

J. Boxall and J. A. von Fraunhofer "Paint Formulation,

Principles and Practice", Industrial Press, New York, 1981.

C. R. Bragdon, Technical Editor, "Film Formation, Film

Properties, and Film Deterioration, A Study by the Research

Committee of the Federation of Paint and Varnish Production

Clubs", Wiley-Interscience, 1958.

L. J. Culver, "Organic solvent evaporation from aqueous

compositions". Modern Paint and Coatings, October, 133, 1982.

R. Dhein, H. U. Pohl, and J. Schoeps, Farbe Lack, 84, 680, 1978.

W. H. Ellis, "Comparative solvent evaporative mechanisms for

conventional and high solids coatings", J. Coat. Tech ., 55, 63,

1983.

M. J. Ellis and R. D. Bakule, "Industrial waterborne acrylic

polymers come of age". Mod. Paint and Coat., October, 156, 1982.

J. J. Engel, "Problems of air-drying water borne alkyds", J.

Water Borne Coat., August, 3, 1983.

J. R. Fletcher, L. Polgar, and D. H. Solomon, "Molecular

weight distribution in alkyd resins. Part 1", J. Appl. Polym.

Sci., 8, 659-671, 1964.

T. A. Girard, M. Beispiel, and C. E. Bricker, "The mechanism

of cobalt drier action", J. Am. Oil Chemists Soc . , 42, 82,

1965.

J. W. Gooch, G. C. Wildman, and B. G. Bufkin, "Auto-oxidative

crosslinking of vegetable oils and alkyd emulsions", J. Coat.

Tech., 56, 33, 1984.

P. M. Grant, J. Coatings Techn. , 53, 33, 1981.

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.

C. M. Hansen, J. Oil Colour Chem . Assoc., 51, 27, 1968.

C. M. Hansen, Ind. Eng. Chem. Prod. Res. Dev., 16, 266, 1977.

Page 24
• / C. M . Hansen, "Solvents in coatings", in "Organic Coatings

Science and Technology", G. D. Parfitt and A. V. Patsis, Eds

Vol . 6, Marcel Dekker, New York, 1984, pages 125-150.

R. Hiatt, "Hydroperoxides", Chapter 1 in "Organic Peroxides",

Vol. II, D. Swern, Ed., Wi ley- Interscience , New York, 1971.

K. Holmberg and J.-A. Johansson "High solids alkyd resins", in

"Organic Coatings - Science and Technology", G. D. Parfitt and A.

V. Patsis, Eds., Vol. 6, Marcel Dekker, New York, 1984, pages

23-34.

R. Hurley and F. Buono, "Practical considerations in the drying

of water-reducible alkyds", J. Org. Coat. Tech., 54, 55, 1982.

T. Imai and K. Tsubouchi, "Evaporation of solvents from

water-borne systems during baking", J. Coat> Tech., 52, 71,

1980.

J. Kumanotani, H. Hata and H. Masuda, "Formation and

properties of colloidal alkyds", in "Organic Coatings - Science

and Technology", G. D. Parfitt and A. V. Patsis, Eds., Vol.

6, Marcel Dekker, New York, 1984, pages 35-54.

D. B. Larson and W. D. Emmons, "The Chemistry of high solids

alkyd/reacti ve diluent coatings", in "Organic Coatings - Science

and Technology", G. D. Parfitt and A. V. Patsis, Eds., Vol. 6,

Marcel Dekker, New York, 1984, pages 1-34.

H. A. Laitinen and P. Kivalo "Polarographic reduction of

hexami ne cobal t ( III) ion", J. Amer. Chem. Soc. , 75, 2198, 1953

R . J. Lippe, "A method of improving the stabi 1 i ty

thixotropic epoxy resin compositions", Soc . Plastics Eng. Tech.

Papers, Vol. XV, 66, 1969.

R. J. Lippe, "Some factors affecting thixotropy recovery

states", Proc. 31st Annual Conference, Reinforced

Plastics/Compsi tes Institute, Soc. Plast. Eng., Section 14-C,

1, 1976.

F. R. Mayo and A. A. Miller, "The oxidation of unsaturated

compounds. VII. The oxidation of methacrylic esters"' J. ftmer

.

Chem. Soc., 80, 2493-2496, 1958a.

F. R. Mayo, "The oxidation of unsaturated compounds. VIII. The

oxidation of aliphatic unsaturated compounds", J. Am. Chem.

Soc., SO, 2497-2500, 1958b.

F. R. Mayo, A. A. Miller, and G. A. Russell, "The oxidation of

unsaturated compounds. IX. The effects of structure on the

rates and products of oxidation of unsaturated compounds", J.

Amer. Chem. Soc., 80, 2500-2507, 1958c.

Page 25
G. Menges, K. Kircher, and B. Franzkoch, "Crosslinkage of

polyethylene in the UHF field, using t-butyl perbenzoate as

energy absorber and radical former", Kunststoffe, 70, 45-48,

1980.

F. R. Morral, "Cobalt compounds", Kirk-Othmer encyclopedia of

chemical technology, Vol. 6, Wiley, New York, 1979.

Y. Nakayama, T. Watanabe, and I. Toyomoyo, "High performance

coatings from blends of emulsions and water-soluble resins", J.

Coat. Tech., 56, 73, 1984.

J. R. Overton and C. A. Herb, "Colloid chemistry of

water-reducible coatings, part l: viscosity behavior", J. Org

.

Coat. Tech., 55, 33, 1983.

S. P. Pappas and L. W. Hill,

considerations for maximizing stability

temperature of thermosetting coatings",

53,43-51, 1981.

"Kinetic parameter

and minimizing cure

J . Coat . Techn .

,

S. P. Pappas, "On the selection of latent catalysts and

reactants for optimizing ambient stability and cure schedules of

high solids coatings". High Solids Coatings, 8, 2-5, 1983.

S. P. Pappas and H-B. Feng, "Latent curatives for

thermally-induced cationic polymerization". Cationic

Polymerization and Related Processes, ISBN 12.287470.6, 1984.

D. Y. Perera and D. Vanden Eynde, "Solvent influence on the

development of internal stress in a thermoplastic coating", J.

Coat. Tech., 55, 37, 1983.

A. E. Rheineck and R. 0. Austin, "Drying oils - modification

and use". Chapter 4 in "Film Forming Compositions", Vol. 1, Part

II, Marcel Dekker, New York, 1968.

R. P. A. Sims and W. H. Hoffman, "Oxidative polymerization".

Chapter 15 in "Autoxidat ion and Antioxidants", W. 0. Lundberg,

Ed., Vol. 2, Wi ley- Interscience, New York, 1962.

D. H. Solomon, "The Chemistry of Organic Film Formers", Wiley,

New York, 1967, and 2nd ed . , Krieger, Huntingdon, NY, 1977.

D. H. Solomon and D. G. Hawthorne, "Chemistry of Pigments and

Fillers", Wiley-Interscience, New York, 1983.

W. S. Stoy and T. G. Brown, "Extenders: coatings'' inorganic

backbone". Mod. Paint Coatings, October, 162, 1982.

M. Teffal and A. Gourdenne, "Activation of radical

polymerization by microwaves. I. Polymerization of

2-hydroxyethy 1 methacrylate", Eur. Polym. J., 19, 543-9, 1983.

Page 26
S. K. Wason and J. W. Maisel , "Thixotropic properties of new

precipitated silica in gel coats", Proc. 36th Annual Conference,

Soc . Plast. Industry, Section 13-C, 1, 1981.

S. K. Wason and J. W. Maisel, "Evaluation of new precipitated

silica thixotrope in plastics", Proc. 38th Annual Conference,

Soc. Plast. Industry, Section 3-F, 1, 1983.

B. C. Watson and Z. W. Wicks, "Popping of water-soluble acrylic

coatings", J. Coat. Tech., 55, 59, 1983.

H. Wexler, "Polymerization of drying oils", Chem. Rev., 64,

591, 1964, and references therein.

G. C. Whitaker, "Driers and metallic soaps". Encyclopedia of

polymer science and technology", volume 5, Wiley, New York, 1966.

Z. W. Wicks, E. A. Anderson, and W. I. Culhane, "Morphology of

water-soluble acrylic copolymer solutions", J. Org . Coat. Tech,

54, 57, 1982.

Page 27

»OAM NBS-1 14A BEV I184i

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 85-3216

2. Performing Organ. Report No. 3. Publication Date

SEPTEMBER 1985

4. TITLE AND SUBTITLE

Intaglio Ink Considerations

5. AUTHOR(S)

B. Dickens

6. PERFORMING ORGAN I ZATION (If joint or other than N BS, see in struct ion s)

NATIONAL BUREAU OF STANDARDS

U S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

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

10.

SUPPLEMENTARY NOTES

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

11.

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

bi bl iography or literature survey, mention it here)

Alkyds are recommended as the most practical 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. The solubility of oxygen in the ink formulations

and solvent mixtures should be determined and the compositions adjusted to provide a level of oxygen which gives optimum cure. UV irradiation of the newly printed

currenc / 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. The silica filler may be acting as a dessicant as well as a thickening agent. If this dessicant action is not important, thixotropy of the ink

should be adjusted by using sodium carboxy methyl cellulose instead. Other

recommendations are given and the formulation of intaglio printing inks is reviewed

with special reference to BEP requirements.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key wo'ds :-~i

13.

AVAILABILITY

Uni imited

| | For Official Distribution. Do Not Release to NTIS

! J Order From Superintendent of Documents, U.S. Government Printing Office, Washington, DC

20402.

Order From National Technical Information Service (NTIS), Springfield, VA 2216 1

14. NO. OF

PRlNTE

15. P r