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32 FLEXOGRAPHY: PRINCIPLES & PRACTICES

• and ambient conditions, e.g., tempera-

ture or atmospheric pressure.

To prevent imbalance, any solvent blend

added as a replacement should be of a simi-

lar nature to the escaping solvents. Failure

to use a compatible replacement may also

result in resin kick-out or ink souring, loss of

gloss, increasing ink viscosity or lack of

adhesion. Such problems are more notice-

able in jobs where ink usage is limited, such

as process work or small spot colors and it is

the responsibility of the ink formulator to

identify a suitable “balanced solvent” to pre-

vent this from occurring.

The importance of solvent balance is

demonstrated in Figure

. The horizontal

axis represents the percentage of n-propyl

acetate in an ink fountain that contains a

blend of n-propyl acetate and n-propyl alco-

hol. The vertical axis represents the percent

of n-propyl acetate in the solvent evaporating

from the fountain. The solid curve represents

the composition of the solvent vapors from

the various mixtures. For example, if you

start with a fountain blend of 10% n-propyl

acetate, it follows that the solvent vapors

contain 35% n-propyl acetate. This excessive

loss of n-propyl acetate will shift the solvent

balance and result in a leaner, less acetate

rich solvent resulting in print problems. In

this case, the problem can be avoided by

using a 35:65 blend of n-propyl acetate/n-

propyl alcohol to replace the solvents lost by

evaporation from the ink fountain.

In summary, the original ink and any fresh

ink added to the fountain should be diluted

to target viscosity with the 10:90 blend of n-

propyl acetate/n-propyl alcohol. Any subse-

quent manual viscosity reduction, while on

press, should be carried out with the 35:65

blend of n-propyl acetate/n-propyl alcohol to

maintain solvent balance.

Similarly, Figure

shows a plot of a gly-

col ether solvent and n-propyl alcohol. In

this case, the percent of glycol ether evapo-

rating is significantly lower than the level of

glycol ether in the fountain. It would be

extremely dangerous to replace any evapo-

rating solvent with the same blend used to

make the initial cut. The fountain would

become increasingly richer in glycol ether,

leading to poor drying, blocking in the

rewind, retained odor, and lamination prob-

lems such as blistering, tunneling and poor-

bond strengths.

Most solvents present a fire hazard, and it

is important to take note of flash points and

explosive limits. In addition, some solvents

are considered hazardous to health or the

environment above certain concentrations.

The properties of a number of common sol-

vents used in flexographic inks are detailed

in Table 5.

The importance of

solvent balance is

shown in this graph,

where the solid curve

represents the

composition of the

solvent vapors from

the various mixtures.

A comparison of

glycol ether solvent and

n-propyl alcohol levels

shows the percent of

glycol ether evaporating

is significantly lower

than the level of glycol

ether in the fountain.

010 9080706050403020 100

100

Percent Glycol Ether in Fountain

Percent Glycol Ether in

Evaporating Solvent

010 9080706050403020 100

100

Percent Normal Propyl Acetate in Fountain Ink

Percent Propyl Acetate in

Evaporating Solvent

Additives

Although the pigments, resins and solvents

chosen provide the ink formula “skeleton,” it

is often necessary to enhance or modify cer-

tain ink characteristics to achieve the neces-

sary performance. While various additives

are used to modify the performance of the

ink, it is important to recognize that with a

correctly formulated vehicle, the use of addi-

tives will be minimized. Poorly formulated

vehicles will require material additions that

may cause secondary problems, which in

turn require the use of further additives. The

additives typically used in modifying flexo-

graphic ink fall into many categories. The

most common, briefly detailed below:

• Plasticizers. The main function of a plasti-

cizer in an ink film is to make the dried print

more flexible and elastic. They do this by

acting as non-volatile solvents for the film,

forming resins. Certain plasticizers are used

to promote specific properties in the printed

ink film, such as increased gloss, improved

flexibility or increased adhesion on difficult

substrates.

• Waxes. Different chemical types of wax can

be incorporated into flexographic inks (1–3%

dry weight) to achieve mar resistance, reduce

blocking or set-off, and improve slip and

water repellency. Keep usage as low as possi-

ble as excess wax levels may lead to reduced

gloss, poor ink rheology and reduced transfer

characteristics.

• Silicones. This class of materials find use at

low levels (0.1–1.0%) as substrate- or pig-

ment-wetting agents, additives to improve

mar/slip, anti-foams and release agents.

While it appears they have wide utility, they

do have drawbacks. Excessive use can lead

to print defects such as pinholing or crawl-

ing. Also the presence of low-molecular-

weight silicones (<50 centiStokes) in inks

can lead to the poisoning of catalytic beds

within incineration units.

• Surfactants. These additives are used to

improve wetting and spreading. Surfactants

are regarded as high performance deter-

gents, finding use as dispersion aids, flow

promoters and wetting aids on difficult sub-

strates. Care has to be exercised in surfac-

INK 33

Table 5

DRYING BOILING POINT SPECIFIC FLASH POINT FLAMMABILITY LIMITS

SOLVENT RATE

°F °C GRAVITY °F °C LOWER% UPPER%

Ethyl alcohol 4.4 173° 78° 0.79 55° 13° 4.3% 19.0%

n-Propyl Alcohol 2.39 208° 98° 0.80 77° 25° 2.1% 13.5%

n-Propyl Acetate 5.78 215° 102° 0.89 58° 14° 2.1% 8.0%

Isopropyl Acetate 9.5 191° 88° 0.87 40° 4° 1.8% 8.0%

Heptane 13.5 209° 98° 0.68 39° 4° 1.8% 6.7%

VM&P Naptha 4.1 212°–320° 100°–160° 0.67 20°–50° -7°–10° 1.1% 6.7%

Dowanol PM 2.3 250° 121° 0.92 97° 36° 0.9% 13.1%

Propylene Glycol 0.03 370° 188° 1.04 210° 99° 1.1% 9.2%

Acetone 15.7 134° 57° 0.79 0° 15° 0.9% 12.8%

Methyl Ethyl Ketone 10.6 176° 80° 0.80 20° 7° 2.6% 10.0%

Toluene 5.7 232° 111° 0.87 40° 4° 1.8% 7.1%

Water 1.0 212° 100° 1.0 ——1.2% —

Ammonia — 232° 111° 0.9 ————

Water=1

SOLVENT PROPERTIES

34 FLEXOGRAPHY: PRINCIPLES & PRACTICES

tant choice. Watch for problems with foam-

ing, adhesion or reduced water resistance.

• Defoamers. Foaming is a problem that most

commonly occurs in water-based inks and is

evident under conditions of high agitation.

Obviously, prevention is better than cure,

but where foaming is present it can be dealt

with by the addition of defoaming agents.

Such materials work by dramatically reduc-

ing surface tension in the system, causing

existing bubbles to burst, and preventing

unstable foams from forming. These materi-

als are usually derived from mineral oils or

silicones.

• Pure Chemicals. This category of additives

covers a diverse group of materials including

acids, alkalis, metal chelates, polyols, metal

salts and anti-oxidants. Such materials can

function as adhesion promoters, fixatives,

drying aids, stabilizers and cure agents, as

determined by the ink chemistry and formu-

lation.

INK CHARACTERISTICS

There are a number of fundamental prop-

erties required of flexographic inks that are

determined by the nature and demands of

the printing process and the final application

of the print:

• rheology;

• transfer;

• color and strength;

• print appearance;

• adhesion; and

• functional properties.

The fundamental property required of an

ink is that it prints well. To do this, it must

possess the rheology or fluidity to be trans-

ported through the inking system onto the

anilox. It has to have sufficient wet tack to

transfer from the anilox onto the plate and

then finally to the substrate (Figure

). The

drying speed of the ink needs to be such that

it remains fluid while on the press, but dries

rapidly after application to the substrate.

The ink must be of suitable shade and

strength. It must print cleanly, adhere to the

chosen substrate and provide the properties

necessary to meet customer specifications.

Rheology. Fluidity and low viscosity are cru-

cial to the flexographic process. While on

press, the ink is required to be pumped and

transported through various pieces of equip-

ment and subjected to extreme shear forces.

Maintaining a fluid ink at a low viscosity

reduces the wear and tear on press compo-

nents and can be achieved by the incorpora-

tion of suitable solvents. The actual viscosi-

ty chosen for printing is determined by a

variety of factors including the metering sys-

tem, substrate characteristics, press speed

and print requirements.

Transfer. Of all the ingredients present in the

ink, the binder system impacts most heavily

on transfer. In general, as the molecular

weight of the chosen resin and the resin

solids present in the ink increase, so does

the transfer. Transfer properties of differing

formulations or resin systems can be easily

compared in the laboratory by applying the

test inks – after ensuring the strength and

viscosities are equal – side by side using a

hand anilox proofer.

Color and Strength. The color and strength of

a flexographic print are largely determined

In order for an ink

to print well, it must

possess the rheology to

be transported through

the inking system onto

the anilox. It has to have

sufficient wet tack to

transfer from the anilox

onto the plate and then

finally to the substrate.

Impression

Cylinder

Plate

Cylinder

Anilox

Roll

Web

Doctor

Blade

Assembly

Ink Drying on Plate

Ink Drying

into Anilox Cells

Resolubility

Points

Ink

Film

Split

by the type and concentration of the col-

orant used within the ink and the thickness

of the wet film applied. Other factors such as

substrate and converting processes can also

affect appearance. The thickness of the wet

film laid down is determined largely by the

metering system employed and can vary in

different applications from 2–12 microns.

Since this variation is so large, it is difficult

to formulate an ink consistently with the

precise amount of colorant present to meet

requirements. To combat this, the ink maker

commonly supplies the ink at a higher color

strength and viscosity to allow adjustments

to be made on press.

Print Appearance. In addition to basic color

requirements, the ink may have to be glossy,

matte, transparent or opaque as determined

by the design and substrate. The ink should

also print clean, smooth solids, fine type and

process screens. To achieve these properties,

it is desirable that the ink display good flow,

wetting and resolubility. Most inks today are

formulated with pigments, due to required

end-use application. Care has to be taken

with pigment selection to prevent poor “ink

working” and flow properties. This caution

also extends to higher performance binders

like high molecular-weight polyamides that

exhibit low solvent solubility, which can lead

to dirty printing.

After printing, it is critical that the ink

dries quickly. This need is particularly

noticeable when printing a multiple “trap”

job on a common impression press. In such

cases, the minimal gap between subsequent

printing units and the speed of the press

necessitate the rapid drying of the ink.

Drying rate should not be so rapid that it

causes the ink to dry on the plate or anilox.

A balance must be struck. Any ink that dries

on either the anilox or plate should be easily

resolubilized on the next revolution, mini-

mizing the risk of poor image reproduction

or dirty working. Care has to be exercised

when “trapping” or printing color-on-color to

prevent solvents from the overprinting color

resolubilizing the first layer down. This

rewetting can lead to solvent retention, loss

of print quality or print blocking.

Adhesion. This property is particularly

important with non-absorbent substrates

such as polyethylene or polypropylene. With

paper or board printing, adhesion problems

with an ink seldom arise.

The binder is the most important primary

constituent in determining ink adhesion. It is

common for an ink to require a blend of

resins to achieve the desired level of adhe-

sion and other performance properties. Each

substrate presents different challenges, and

in general the more inert the substrate, the

more difficult it is to obtain adhesion. In

practice, this means that resins conferring

INK 35

In this figure, common,

everyday cellophane

tape is used for the

adhesion test by

placing it on the

printed substrate.

To test for adhesion,

the tape is peeled off

and examined for the

effect on both tape and

substrate.

36 FLEXOGRAPHY: PRINCIPLES & PRACTICES

adhesion on the myriad of available sub-

strates must be identified on a case-by-case

basis (Figures

3! 3@

End-use Application. This requirement has a

particular bearing on the formulation of an

ink. There may be requirements during pro-

cessing, as outlined in the previous chapter,

for heat resistance, the ability to be laminat-

ed and certain slip or scuff characteristics.

The package may have to show product

resistance to fats, oils, detergents, solvents

acids or alkalis. Consideration of the printed

product’s end use can be used to determine

the gross performance needs. However, all

technical requirements for a particular appli-

cation should be fully specified by the cus-

tomer prior to formulation. Table 6 shows a

few flexographic end-use markets and the

general properties required.

The enormity of formulating an ink capa-

ble of complying with all these requirements

should not be underestimated. One also

needs to keep in mind the added complica-

tions of converting needs, regulatory con-

cerns and quality performance objectives.

Converting Needs. The converting processes

in operation have a dramatic influence on

the formulation of an ink. Changes in the ink

may require changes in the process. For

example, while solvent-based inks may not

require a corona treatment unit to print suc-

cessfully on polyolefinic films, it is almost

certainly required with water-based inks.

Some of these process parameters are out-

lined in Table 7.

Regulatory Controls. The composition of an

ink has to account for local, state and gov-

ernment regulations covering:

• air remissions;

• metal content;

• material toxicity; and

• food and drug standards.

Sweeping environmental legislation within

the last 20 years has necessitated major

reformulation efforts to remove, replace or

reduce harmful materials, and require even

tighter controls over incoming raw materials.

This situation is becoming ever more com-

plex since federal legislation is increasingly

enacted differently on a regional basis,

depending on local conditions and needs.

The changes ultimately require the ink man-

ufacturer to supply different ink formula-

tions for identical applications, based on geo-

graphic environmental need. This topic is

expanded upon in a following section.

Table 6

END USE SUBSTRATE CONSTRUCTION INK PERFORMANCE REQUIREMENTS

■Bakery Ink-LDPE High performance, low COF, low retained solvent,

ice water crinkle resistance

■Milk Carton Ink-LDPE-Board Product resistance, alkali resistance,

wet/dry rub resistance

■Snack Package OPP/Polyester-Ink Low retained solvent

Adhesive polyethylene High bond strength

■Confectionery Coex. OPP-Ink Low odor, low solvent retention

Cold seal adhesive Cold-seal release

■Display, Corrugated Ink-bleached Kraft High gloss, rub/scruff resistance

END-USE APPLICATIONS

INK FORMULATION

AND SELECTION

Having an understanding of the properties

of the materials used within inks, the print

and conversion processes utilized, and the

end-use application, allows inks to be for-

mulated to meet all required specifications.

As mentioned previously, flexographic inks

consist of four main ingredients: colorant,

binder, solvent and additives. The composi-

tion varies widely depending on the choice

of substrate, press conditions, and final use.

Figures

illustrates the ingredients which

make up a typical ink.

It is normal to classify inks into product

types according to formulation, substrate

used and performance properties, rather

than have discrete inks for individual situa-

tions. This helps avoid product proliferation

and inventory problems both at the ink man-

ufacturer and converter.

Product categorization eases ink selection

and benefits manufacturing and product

quality through improved inventory control

and material planning. The system has to

maintain flexibility since flexographic indus-

try is subject to increasingly rapid changes

that may require ink modification. A selec-

tion of characteristic formulations is out-

lined with respect to the substrate printed

and end-use needs (Table 8).

Water-based Inks

Water-reducible flexo inks, based on

casein, were first used in the 1940s on corru-

gated cartons. During the 1950s and 1960s, as

printability improved, their use in paper/

paperboard printing increased. The imple-

mentation of the Clean Air Acts during the

early 1980s spurred the development of

water-based technology. Today, water-based

inks are used successfully in all areas of flex-

ographic printing. Conversion to water-based

inks, particularly in demanding applications,

has been a success where this change has

been accompanied by an increase in press-

drying capacity, improved ink-metering sys-

tems, and the replacement of press compo-

INK 37

Four primary

ingredients constitute

flexographic inks:

colorant, binder,

solvent and additives.

Table 7

PRINTING

1. Corona Treaters.

2. BC and OH Dryer Capacities.

3. Line Speeds.

4. Tension Controls.

5. Cooling Rolls.

6. Ink Pumps/Viscosity Controls.

7. Run Times.

8. Design Parameters: % Coverage, Traps.

9. Press Room Temperature and Humidity.

LAMINATIONS

1. Ink-to-Substrate Adhesion.

2. Ink-to-Laminate Adhesion.

3. Corona Treaters.

4. Use of Primers.

5. Wetting of Ink by Adhesive.

6. Extrudate Temperature.

7. Extrudate Compositions.

CONVERTING NEEDS –

PERFORMANCE ISSUES

Resin

10%–30

Solvents

40%–60%

Additives

10%–25%

Colorant

4%–20%

38 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Table 9

HIGH-GLOSS INK FOR LDPE (low density polyethylene).

HEAT-RESISTANT, HIGH-ABRASION INK FOR POLYPROPYLENE.

SOLVENT INK FOR FOLDING CARTON

Color strength is a primary concern along with

good print quality, rapid drying, good flexibility

(folding) and scuff for sheeting equipment. Wax

and plasticizers improve scuff and flexibility.

Isopropyl acetate and ethanol speed up drying for

less absorbent coated stock

MAKE UP %

calcium 2B pigment 14.0

titanium dioxide 6.0

maleic resin (dry) 8.0

nitrocellulose (dry) 11.5

n-propyl alcohol 25.0

ethanol 18.0

isopropyl acetate 10.0

plasticizer 5.0

wax compound 3.5

100.0

Inert nature of polypropylene film requires use of

adhesion promoter. Nitrocellulose has necessary

heat reistance and hardness. Urethane provides

adhesion and flexibility. PTFE and silicone provide

scuff resistance. For use on VFFS and HFFS filling

machinery. Low odor needed if used for food pack-

aging.

MAKE UP %

organic pigment 12.0

nitrocellulose (dry) 10.0

polyurethane 14.0

n-propyl alcohol 36.0

ethanol 10.0

n-propyl acetate 15.8

adhesion promoter 1.0

PTFE wax 1.0

silicone 0 . 2

100.0

Used for bakery and deep-freeze applications.

Requires fast drying because of press speeds, low

odor for food packaging. Deep freeze and water

resistance require careful selection of pigments.

Waxes required to prevent blocking and sticking in

roll and wickets.

The traditional ink for polyethylene is based on

polyamide, although at one time it was shellac. To

achieve acceptable adhesion, the film must have a

surface energy of 38 to 42 dynes/cm. Below this

level, poor ink adhesion will be observed. Above

this energy level, the water resistance of the printed

film and its heat sealability may be compromised.

MAKE UP %

organic pigment 12.0

alcohol-soluble polyamide 22.0

nitrocellulose (dry) 4.0

n-propyl alcohol 34.0

ethanol 13.0

n-propyl acetate 12.0

polyethylene wax 2.0

amide wax 1.0

100.0

INK FORMULATIONS

Table 8

nents based on “electrically” different metals

(e.g., copper, iron) that come into contact

with the ink. Installing corona-discharge

equipment for film printing, and retraining

press operators also must be part of the pic-

ture. While water-based inks bear some simi-

larity to their solvent-based counterparts,

there are some differences, particularly in

the chemistry of the resins used and their

handling.

Colorants. The colorants available for use are

largely identical to those used in solvent

inks. Many have been specially treated or

prepared to make them suitable for use in

water systems where alkali or water solubil-

ity are concerns.

Vehicles. The primary vehicles used can be

classified either as alkali-soluble vehicles,

emulsions or colloidal-dispersion vehicles.

The fundamental requisite in all these is that

they retain water solubility while being print-

ed but become water insoluble after printing

and drying. These apparently contradictory

requirements are largely achieved by using

water-insoluble carboxylated (acid-contain-

ing) resins and converting them to their

water-soluble salts, using volatile alkaline

materials such as ammonia or organic

amines. Organic amines such as monoethan-

olamine evaporate more slowly than ammo-

nia, so resins solubilized with an organic

amine dry more slowly and take more time to

achieve water and product resistance.

Just as solvent balance in solvent inks gov-

erns print performance, the alkalinity or

amount of alkali present in the water-based

ink determines performance. The amount of

alkali present can be measured using a pH

meter, which measures acidity or alkalinity

and is associated with water-based inks. The

pH value can vary in range from 1.0 (battery

acid) to 14.0 (household ammonia solution),

where 7.0 is neutral – equivalent to distilled

water. A pH higher than 7.0 indicates

increased alkalinity. Maintaining a level of

alkalinity, typically between pH 8.6–9.4, is

vital to controlling the performance of an ink

on press. Such control can be a problem

with long runs of water-based inks, particu-

larly with printing a low coverage design in

hot weather.

A comparison of the properties of these

vehicles is given in Table 9.

Auxiliary Solvents. A variety of alcohols, gly-

cols and glycol ethers are used to control

drying speed, improve resolubility or aid

film formation. Caution should be exercised

when using these materials to prevent fire

and health hazards.

Additives. The additives used within water-

based inks include waxes, plasticizers and

defoamers and are largely similar to those

used in solvent inks, with the exception that

the chemistries used have been modified to

make them suitable for an aqueous environ-

ment.

INK 39

Table 9

SOLVENT SOLUTION EMULSION COLLOIDAL

POLYMER POLYMER DISPERSION

Printability E F P

Drying Speed P G VG

Product P G E

Resistance

Resolubility E P VP

Applied Solids L H VH

Freeze Stability G F P

Foaming G P P

Pigment E P NR

Dispersion

KEY:

E=Excellent

G=Good

VG=Very Good

P=Poor

L=Low

H=High

VH=Very High

NR=Not Recommended

VEHICLE PROPERTIES

40 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Using Water-based Inks

The problems encountered in handling

water-based inks typically stem both from

the physical properties of water and the

chemistry of the ink and include:

• pH control;

• volatility of water;

• specific heat capacity of water;

• conductivity; and

• surface energy.

Water-based inks are commonly regarded

as easier to handle than solvent-based inks

because of their lower volatility. However,

this is a misconception since pH control in

water-based inks is just as vital as maintain-

ing solvent balance in solvent-based inks.

Water-based inks are most stable within cer-

tain pH limits. After prolonged use, the pH

can drop below the lower limit leading to

false heavy body and resin kick-out (similar

to solvent-based ink souring). A pH reading

higher than the suggested upper limit can

lead to both drying, odor and product resis-

tance problems (similar to solvent retention).

Caution has to be exercised in reducing

the viscosity of water-based inks as they

tend to lose viscosity faster than solvent-

based inks. Over-reduction often leads to

poor print quality, e.g., crawling and poorer

performance properties.

Water dries much more slowly than typical

flexographic solvents like propyl alcohol or

propyl acetate (See Table 5, Solvent Proper-

ties). To compensate for this, water-based

inks are often formulated to have higher

resin and pigment solids. The higher solids

allow a thinner wet-ink film to be printed,

which speeds up drying – especially on non-

absorbent substrates – while maintaining

performance properties (Table 10). Since

inks on non-absorbent substrates dry solely

by evaporation, the drying capacity of the

press is very important. It is critical that the

inter-unit and overhead drying units operate

under negative pressure with an air velocity

of approximately 3,500 cu.ft/min. The higher

specific heat capacity of water also requires

the ovens to be set at slightly higher settings

to achieve the same web temperature as

with solvent-based inks.

In the case of absorbent substrates, the

need for efficient driers is lessened. On high-

ly absorbent stocks, the free water is drawn

into the surface, often by capillary action,

leaving the pigment and resin solids deposit-

ed on the surface. This can occur in as quick-

ly as 0.1 second. Drying can also be en-

hanced on acidic paper stocks where the

acid neutralizes the solubilizing amines.

Although no longer a major problem with

ceramic aniloxes, water-based inks are often

associated with increased chrome anilox

wear for two reasons. The first is mechani-

cal: water-based inks have less lubricity and

cause mechanical breakdown of the chrome

anilox from frictional wear. The second is

chemical: the inks aid the generation of a

galvanic cell by functioning as an electrical

conductor between dissimilar metals.

Ink will only transfer or “wet out” on a sub-

strate when the surface tension of the ink is

lower than that of the substrate to be print-

Table 10

Typical water-based flexographic formulations

are as follows:

For For

Nonabsorbent Absorbent

Substrate Substrate

35% pigment dispersion 50.0 40.0

acrylic solution polymer 10.0 30.0

acrylic emulsion 30.0 12.5

water 5.0 13.0

organic amine 1.0 1.0

polyethylene wax compound 3.0 3.0

surfactant 0.5 —

organic anti-foam 0.5 0.5

TOTALS 100 100

WATER-BASED INK FORMULATION

ed. Pure water has a surface tension of 72

dynes/cm, while the surface energy of most

untreated polyolefin films lies in a range of

31 to 35 dynes/cm. To facilitate the printing

of water-based inks on film substrates, it is

necessary to reduce the surface tension of

the inks by incorporating wetting agents and

boosting the surface energy of the film using

corona discharge or flame treatment. Film

treatment, while aiding ink adhesion, also

helps in “burning off” the migratory fatty

acid amides which are used as a slip agent in

the film. Such slip agents commonly migrate

to the surface of the film, and at high levels

can cause wetting problems like pinholing or

reticulation.

Catalytic Inks. The resistance properties of

prints produced from conventional inks that

dry by evaporation are dictated by the prop-

erties of the resins employed. With the flex-

ographic process, the choice of resins is lim-

ited by the available range of solvents, which

in turn places limits on resistance properties

achievable with regular flexographic inks.

This problem can be overcome by utilizing

ink systems that undergo specific chemical

reactions upon drying. The principle types

are usually based on epoxy amine or aziri-

dine acid chemistry and rely on a reaction to

crosslink the individual reactive elements

together to generate a composite polymer

with improved heat, solvent and product

resistance and more gloss and adhesion than

a conventional ink. The inks do have some

disadvantages:

• the need to mix two components;

• limited pot life after mixing; and

• specific cure conditions.

Such inks typically have discrete curing

conditions requiring a high temperature and

long-drying time to complete reaction. Care

has to be taken to ensure that the materials

used to produce the ink and those inks or

materials in contact with the printed catalyt-

ic material do not act as cure inhibitors.

UV- and Electron Beam-Cured Inks

Drying vs. Curing. In simplest terms, drying of

an ink film occurs with conventional inks

when the ink vehicle (solvent or water) evap-

orates or is absorbed, leaving behind the

solids (pigments, resins, waxes, etc.) to form

a film on the substrate surface. In radiation

curing, on the other hand, all of the compo-

nents in the ink or coating remain on the sur-

face of the substrate, but are chemically

transformed into a hard film through expo-

sure to ultraviolet (UV) lights or a concentrat-

ed beam of highly energized electrons (EB).

The difference lies in the chemistry of the

materials in the inks and coatings and in the

pressroom equipment needed to “energize”

the curing process.

Rudimentary Ink Chemistry. The materials

used in radiation-curable ink formulas are

considerably more “user friendly” today than

ever before. Advancements in raw materials

will continue to make radiation-curable inks

and coatings more commonplace in the years

ahead. The major components of UV and EB

inks and the function of each of these chem-

icals are as follows:

• Reactive Diluent (Monomer). A reactive dilu-

ent monomer is a simple, lightweight chemi-

cal similar to a solvent in its ability to thin

down the ink. Monomers help determine the

characteristics of the ink, such as gloss,

hardness and flexibility. These low-viscosity

monomers, which can be readily absorbed

by unprotected skin, are also the chemicals

which give uncured UV and EB inks their

most hazardous characteristics. The poten-

tial of a monomer to cause skin irritancy or

sensitization problems can be gauged from a

given Draize value. The Draize value is a 1 to

4 numeric measure of its irritation or sensiti-

zation potential. The higher the Draize value,

the more hazardous the material. Most com-

mercially available UV or EB flexographic

products utilize materials with a Draize val-

ues of 2 or less and do not pose any major

health and safety concerns, providing of

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