FTA (изд-во). Flexography: Principles And Practices. Vol.1-6

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92 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Regardless of which instrument is used to

measure viscosity, calibration must be done

on a regular basis. Slight variations in read-

ings can result in significant print problems.

Calibration should be done when new equip-

ment is received, and a log should be estab-

lished to maintain a regular calibration pro-

gram.

Color Adjustment at Press

The basics of ink viscosity – how it is mea-

sured and some of the factors that affect it

have been discussed. The next topic is how

to use ink viscosity and ink metering to

adjust and control color strength. In any

printing configuration, the actual metering

system will determine the ink strength need-

ed to achieve the desired color. The color

strength achieved when ink is added to the

press may be acceptable. If it is too strong,

however, at least three options exist:

• Add solvent or water depending on the

system, thereby reducing viscosity and

strength. This is the easiest way to

adjust down color strength. It may,

however, lead to drying problems in

water-based inks or even drying prob-

lems in solvent systems.

• Add extender varnish; this second way

to reduce color strength is often best as

it provides color strength reduction

with the least effect on drying or block-

ing and optimum color value.

• Change the anilox to reduce color vol-

ume. This approach provides the least

effect on the ink itself and probably the

best long-term balance of good printing

and high color intensity.

On the other hand, if the ink is too weak,

different options exist:

• If the ink is pre-reduced on press start-

up, uncut ink should be added to the

fountain or reservoir. This will build

strength and viscosity. This is certainly

the easiest cure if the higher viscosity

has not caused printing quality prob-

lems. Increasing viscosity and increas-

ing strength often go together with dirty

printing.

• If an ink based on a dispersion and let-

down varnish is being used, additional

dispersion may be added. This can

eventually lead, however, to poor adhe-

sion or a loss of other properties as the

dispersion is not a complete ink vehicle.

• If other factors are equal, changing the

anilox to a higher volume is certainly

preferred. Here more ink is carried to

the substrate at the lowest viscosity pos-

sible to provide optimum strength with

the cleanest colors.

Printing the same color ink at excessive

strength or viscosity makes the color itself

muddy looking. At the same time, dirty print-

ing often occurs. A more transparent, finer

dispersion will negate this effect somewhat.

However, it often does so with a loss of color

intensity and an increase in cost. Sometimes

black is added to an ink to “fake” higher

strength. While this trick often will help, it

does so at the expense of color sharpness,

and the result is somewhat muddy.

Ideally, the ink supplier will provide an ink

that will yield the proper color values and

intensities with only minor metering or vis-

Another method of

checking viscosity is

using a Shell Cup,

which is more

commonly used in

gravure applications

than in flexo. However,

because it is more

accurate than a Zahn,

printers favor this

method for on-press

checks.

cosity adjustments. If, however, the ink is

already on press and color changes are nec-

essary, there are several areas of concern:

• A good, well-lighted area is needed for

color viewing. A standard light box is

recommended, since color varies under

different lights. A well-lighted area in the

pressroom is the last chance to ensure a

good color match to a color standard. A

board of retained prints from previous

rolls is also important for comparison so

that no color drift occurs. Whatever

standard light is used, it should be com-

parable to the light that the product will

be viewed by.

• Metamerism is defined as two colors

that match under one light source, but

are different under another light source.

A standard light source in the press-

room will help prevent this effect.

However, metamerism is not due only to

light effects on the color; different pig-

ments used in a color match will also

create the effect. Therefore, if matching

at press-side, try to use the same pig-

ments as originally used in the color

match. If this is not possible, be alert to

metamerism problems that may occur.

If metamerism does occur, the only solu-

tion is to change pigments.

• If color is altered at press side, be sure

that end-use problems aren’t created. All

colors have different fastness proper-

ties, and the end use for a given print job

may have very specific needs.

• The substrate is also of great concern. If

it has a color of its own, this may well

affect the outcome of the print job. It

may be necessary to use opaque pig-

ments to hide the substrate. These

opaque pigments will also, of course,

have an effect on other colors on the

job. If inks are used to hide the sub-

strate, metering effects and viscosity

control become critical. Any changes

during the run will create color shifts.

• Finally, another area of concern with on-

press color adjusting is the combination

of ink trap and ink transparency. If the

job being printed has multiple traps

(whether these be color-over-white or

color-over-color), the final color

achieved is a combination of the colors

involved. Any changes in the color

matches of the involved single colors

will alter the trapped colors as well.

These changes may involve revising a

particular color match on press or alter-

ing the transparency of the ink involved

with the actual color staying the same.

Both of these changes can affect the

final color printed.

Trapping. The trapping of one color over

another is very dependent on ink metering,

viscosity and drying. All of these factors

must be at their optimum for good trapping.

First-down inks that dry too slow or are

too low in viscosity will allow second-down

inks to dive into them, creating muddy col-

ors and dirty print. For good trapping, first-

down inks must dry faster than subsequent

inks. Thinner inks usually dry faster, but in

water systems this may not be true because

there is more water to dry. Excessively low

viscosity can cause dirty, sloppy trapping. A

balance has to be maintained as high viscos-

ity and fast dry can cause dirty colors in the

trap, as well as poor color fidelity.

MANAGING pH WITH

WATER-SOLUBLE INK SYSTEMS

Perhaps the most essential element of

water-soluble inks on press is the proper

control of pH. This is not an issue for sol-

vent-based or UV inks. Water-soluble inks,

however, can become virtually useless if the

pH is not maintained correctly.

What Is pH?

The pH value is the degree of acidity or

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94 FLEXOGRAPHY: PRINCIPLES & PRACTICES

alkalinity of a substance measured on a scale

of 0 to 14. From 0 to 7 is acid, and from 7 to

14 is alkaline. The neutral point is 7. Although

many believe water to be neutral, it is impor-

tant to remember that water is usually, but

not necessarily, approximately 7 pH. The pH

of water is determined by the pH of the soil

in the surrounding source area. In some

areas, this pH is also affected by the pH of the

rain water (acid or otherwise).

Resins used in the manufacture of water-

based inks are both of the solution and emul-

sion types, which can be carefully formulat-

ed for tailor-made performance relative to

specific press speeds, drying conditions,

application volumes and the like. The resins

used are generally alkali-soluble, acrylic

polymers. Simply, that means the resins –

when synthesized into high molecular

weight polymers – have numerous active

acid sites. In this slightly acidic condition,

the resins are not suitable for printing and

are coiled. The result is that the body of the

polymers is heavy, and the viscosity is very

high, rendering ink neither pourable nor

pumpable in a press-and-ink pan loop. When

the polymers are adjusted with an amine or

other alkali to an alkaline pH range of 8.0 to

9.5, the resins perform optimally and have

the best characteristics for dispersing and

wetting-out pigments, for transferring and

laying out on the substrate, and imparting

the product resistance requirements.

While printing with water-based inks, the

heat produced from running the press and

heat from the outside environment can lower

the pH of the ink by evaporating the ammo-

nia and/or amines in the ink. As the amine

evaporates, the pH of the ink falls, and the

resin begins to revert back to the heavy-body,

higher viscosity ink. At that point, adjusting

the viscosity with water will not quickly or

effectively lower the viscosity or heavy body

because it is a chemical problem and not a

physical one. It is very important that the pH

of the ink be raised. One 6-ounce cup of

ammonia or an appropriate amine aded to 20

gallons of ink will raise the pH of the ink

from 8.0 to 8.9. A heavy, high viscosity ink

can occur at a pH around 8.1 to 8.3. Table 13

outlines what happens with an ink and pH.

In summer months, it is advisable to mon-

itor and adjust pH on an hourly schedule. In

the cooler months, every two to three hours

should be adequate.

When the pH of an ink becomes too low,

the ink will begin to body up or get higher in

viscosity. The resins in the ink begin to fall out

of solution. The lids on drums or buckets of

ink can sometimes be seen to have ink strings

coming from them as a result of poor resin

solubility. At low pH, the ink will also begin to

transfer improperly from the anilox to the

plate and from the plate to the substrate,

causing a decrease in color strength. Also, the

inks will start to build up on the plates, caus-

ing dirty printing. These are some of the

noticeable signs of low pH. Other problems

could occur with the printed material as well,

which are harder to detect. If the pH of the

ink is too high, the printed material will usu-

ally have poorer water resistance than nor-

mal. This may not be an issue if the ink is not

designed for water resistance in the first

place. The job may be run without any notice-

able print problems, but there is a chance for

a potential claim when the print rubs off

under wet conditions. In addition, if the inks

are too high in pH, the amine odor can also

become a problem.

How pH is Measured

There are several different ways to mea-

sure pH. The best way is to use a reliable pH

meter, which can be purchased from any

scientific equipment facility .and can cost

from $100 to several thousand dollars. For

the purpose of ink-pH control in the press-

room, a pH meter costing several hundred

dollars is usually sufficient.

The least-expensive version is a pocket

model, which has too high a variance range

for these purposes. These only read to one

decimal place and have a variance of ±0.2.

With this type of instrument, a pH reading of

8.5 might appear to be up to specification.

However, with a variance range of ±0.2 the

ink could actually have a pH of 8.3, which is

too low and could create printing problems.

A pH meter which reads to two decimal

places and has a variance range of only ±0.01

is recommended. Using this type of meter,

the same ink that reads between 8.3 and 8.7

would show a pH of 8.49 ±0.01.

This doesn’t mean that the pocket model

doesn’t have its place in the pressroom. For

example, it is feasible to have a pocket

model at every press, but impractical to have

a $200 model at every press. It is important

that press personnel be trained to check the

pH of water-based inks at least every four

hours. If they have a pocket model, which is

also much easier to use, they can check the

pH of the inks often and easily. If the ink has

a pH of 9.0, even with a ±0.2 variance, the ink

would still be in spec. If they run into situa-

tions where the pH of the ink is question-

able, a sample of the ink could be brought to

the location of the more accurate instru-

ment, usually the quality control laboratory.

If the only pH meter in the facility is the one

in the quality control laboratory, the likeli-

ness of press personnel bringing samples of

all the inks to be checked every four hours is

slim to none. Purchasing anything more than

an instrument which reads to two decimal

places and a variance range of ±0.01 is

overkill for many purposes. Some of the

more expensive units can read to four or

even six decimal places with variance ranges

of 0.000001. This type of accuracy would

never be needed with water-based inks.

Adjusting pH

When running press-return inks (inks

which have previously been used) or run-

ning for a long period of time, the pH of a

water-based ink can fall. When the pH of the

ink has slipped below its specified level, an

adjustment is usually necessary.

The ink supplier will specify how and with

what amine to adjust the pH. Different

water-based inks will need adjustments dif-

ferently. If a new water-based ink system is

being used in the pressroom, the first ques-

tions that should be asked are: How often

should the pH be checked? What amine

should be used to adjust the pH? How

should it be added? What is the proper pH

range for this ink system?

The proper way to adjust the pH of most

water-based inks is to make up a mixture of

water and ammonia, or whatever amine is

recommended. This mixture should be

approximately nine parts water to one part

amine. Undiluted amine should never be

added to the ink, because it can cause a

shocking effect to the ink, and make it kick

out, or have suspended particles form. Even

if the ink doesn’t kick out, it is very difficult

to adjust the pH properly with a full strength

amine. An ink that is at pH 8.0 requires only

a slight increase to reach the prper level. It

INK 95

Table 13

6.0–7.0 PRECIPITATE:

Ink and resin separate. Result is a

high viscosity and a poor print

7.0–8.0 INK UNSTABLE:

Dirty, fuzzy print; high viscosity,

heavy body and some buildup on

anilox and plate.

8.0–9.5 GOOD FLOW CHARACTERISTICS:

Good print, good adhesion and

excellent wet-out properties.

If viscosity is high, adjust by adding

2%–5% water. High viscosity does-

n’t necessarily mean a low pH, but a

low pH means a high viscosity.

9.5–11.0 POTENTIAL PIGMENT BURN-OUT:

Excess foam, corrosive to steel

and iron. Lack of water resistance is

possible.

PROPER pH CONDITIONS FOR

WATER-BASED INKS

96 FLEXOGRAPHY: PRINCIPLES & PRACTICES

would be very easy to add too much amine,

and the result could be an ink with a pH of

10, 11, or higher.

The alkaline mixture should be added

slowly while agitating the ink. The viscosity

of the ink should decrease as more alkaline

mixture is added. This is partly because of

the additional water, but more importantly,

the resultant lower viscosity is due to the pH

rising to the desired range. Add only a little

at a time, stopping to check the pH, making

sure not to overshoot the target range.

Unfortunately, there is nothing that can be

added to inks which are too high in pH. It

might seem that adding an acid would

reduce the pH level. In theory, it might give

the desired pH, but the resins and additives

in the inks are intolerant to acids and the

result would be wasted ink. The only possi-

ble addition to an ink with too high a pH

would be more virgin ink, and this is only

effective if the pH of the virgin ink is lower.

When adding amines, stop once the ink is in

the desired pH range. A pH level of 9.5 is not

better than 9.0; anywhere in the range is

ideal. Adding more amine once desired

range is achieved only increases the chances

of overshooting the range.

After the pH is in the desired range, the

viscosity should be checked to ensure it is

still where it needs to be. If the viscosity is

too high, a little plain water can be added to

get to the desired viscosity. Always adjust

the ph before adjusting the viscosity!

For almost any problem encountered

while running water-based inks the pH level

is the first thing to check. While pH is not the

only problem that will be encountered, and

pH is not the root of all water-based ink

problems, it is a good first step.

WATER- VS. SOLVENT-BASED INKS

Water dries more slowly than most sol-

vents. With many printers moving to water-

based inks, how do they deal with slower-

drying inks? Ironically, one of the largest

water-based problems we see is that the inks

dry too fast, and solvents need to be added

to remedy this problem. The following will

hopefully explain how this actually works.

Most water-based inks are formulated to

be stronger in color strength. Therefore, less

ink is needed to achieve the desired color

strength than solvent-based inks. By apply-

ing less ink, usually by using finer line

aniloxes, less water is also applied. The less

water applied, the less water there is to dry.

In addition, many printers run water-based

inks at slightly higher viscosities than sol-

vent-based inks. This allows for more color

strength with less water present in the ink.

Finally, resins unique to water-based sys-

tems are incorporated in the form of emul-

sions. The resin is not dissolved in the water

the way it is in a solution varnish but

remains suspended as a particulate. What

this does for drying is that it allows virgin

inks to be formulated at lower viscosities.

The lower the viscosity of a virgin ink, the

less water it takes to reduce it to press print-

ing viscosity. Once again, the less water, the

easier the drying.

With all these methods of reducing the

amount of water in the final ink film, a point

is reached where in many cases, slow sol-

vents (often glycol ethers) are added to

water-based inks to slow them down, so they

print cleaner. This has created problems for

many printers over the years, where the use

of these glycol ethers has sometimes

become rampant. Many press operators see

that a little glycol helps them to print clean-

er causing less downtime to clean plates,

and assume that if a little is good, more is

better. This assumption is wrong! Glycol

ethers need to be used only when absolutely

necessary and only in the recommended

amounts. Too much glycol ethers added to

inks will cause long-term problems that the

press operator won’t see.

First, glycol ethers do not always dry com-

pletely even with the best of dryers. The

retained glycol ether in the print can cause

blocking problems in the rewind. Sometimes

the retained glycol ether causes only slight

blocking, but when unwound from the roll,

the gloss of the ink is severely diminished.

Also, water resistance can be affected by

retained glycol ethers. Using glycol ethers can

be an asset to the pressroom, but only when

used correctly. A good general rule for the

addition of glycol is to add 2% to the sump

while agitating. If more is needed, add 1% at a

time until a maximum of 5% is reached.

Consult with your ink representative for

recommendations based on your ink system.

Drying speed of an ink is dependent on the

water and solvents in that ink, as well as on

the pH of the ink and the amine or ammonia

used in it. The most common amine used for

fast drying in water-based inks is ammonia,

but, many other amines exist and are widely

used due to drying speed considerations or

performance issues. Ammonia is used more

widely because its odor is considered less

offensive than many other amines, and it is

not a volatile organic compound (VOC). If an

ink is too high in pH, the smell of the amine

or ammonia can sometimes fill the press-

room causing eye irritations or even rashes.

Although this usually occurs only in severe

cases, many people have allergies to amines.

Care must be taken to avoid excess amine

and to keep press fountains well covered.

CLIMATIC EFFECTS

How well an ink performs on press is

affected by the pressroom conditions.

Temperature and humidity will affect how

well an ink will dry and therefore how fast a

press can be run. This is especially true for

water-borne inks. In addition, the pressroom

conditions can also affect resin solubility,

ink viscosity and overall ink performance.

Whether running water-borne or solvent-

based inks, climate is an important variable

that must be understood.

Humidity

Since water-borne inks dry by evaporation

of water from the print, conditions which

are optimal for this removal will assist in

drying. Dry air can accept more moisture

than humid air. Higher humidity can result in

poorer press speeds.

Solvent-based inks can also be affected by

climatic conditions. High humidity can

result in the alcohol in the ink absorbing

moisture from the air. This additional water

in the ink can cause certain resins to have

poorer solubility and result in dirty printing.

Keeping fountains covered and reducing the

amount of ink in the sump can minimize this

absorption of moisture by the ink.

Temperature

Warmer air can hold more moisture than

colder air. Therefore, water inks will dry best

under warm conditions with low humidity. To

a lesser degree, solvent inks are affected by

climatic temperature conditions. Solvent will

evaporate more quickly under higher temper-

atures. Due to the relative evaporation speeds

of common solvents used in flexo inks, the

window is much greater for acceptable tem-

perature conditions with solvent-based inks

then it is with water-based inks.

Temperature and humidity are closely

related to water-borne-ink performance. If

humidity is high, additional heat may be

required. If the temperature is high it may

increase the level of humidity that will result

in acceptable results.

Air Circulation

For water-based inks, dryers on the press

should also be designed to allow for maxi-

mum air passage. This will allow the saturat-

ed air to be replaced with lower humidity air

and assist in better drying. Care should be

used to prevent unbalanced dryers. Unbal-

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98 FLEXOGRAPHY: PRINCIPLES & PRACTICES

anced dryers, which blow air onto the plates

and/or anilox rollers, can result in ink drying

on the plates or drying in the anilox roller

causing dirty printing. Increasing tempera-

ture alone on a press running water-borne

inks may not be enough to attain acceptable

drying results. Optimum conditions for water-

borne inks are high temperature, low humidi-

ty and maximum air circulation.

Air circulation in the dryers is also impor-

tant for solvent-based inks. Since the air

coming into the ovens from the outside is

low in solvent content, it can readily accept

the solvents from the inks. With the move to

solvent incineration and solvent recovery,

dryers are being designed to permit higher

levels of solvent to accumulate before the air

is removed. This optimizes the incineration

or recovery aspect of the dryers. Under

these conditions, solvent removal from the

printed web may be lessened, due to the

higher solvent content in the dryers. Where

there is no solvent recovery or solvent incin-

eration, concern over solvent levels in the

dryers is not a major issue if the dryers are

performing correctly.

Climatic Effects on Ink Blocking

High humidity or high temperatures can

cause conditions in the rewind that will

result in ink blocking. High humidity can

cause condensation on the chill rollers, and

this moisture can transfer to the substrate.

Moisture in the rewind can cause certain

substrates to block. Chill rollers should be

checked on a regular basis to ensure they

are not forming condensation on the sur-

face. High humidity can also prevent com-

plete drying of water-borne inks. This resid-

ual moisture in the inks can increase poten-

tial blocking problems.

Blocking of the web is affected by temper-

ature, pressure and time. If high temperature

conditions persist, the printed web may not

be cooled properly before rewinding. This,

coupled with too high a rewind tension, can

promote blocking. To control this situation,

the use of chill rollers on the press is need-

ed. These chill rollers must be monitored to

spot condensation and correct the problem

immediately.

Climatic Effects on Ink Solubility

One of the most serious concerns with ink

solubility involves the absorption of water

into the alcohol in flexo inks. If these inks

also contain certain resins, such as nitrocel-

lulose, the presence of the water will signifi-

cantly affect the solubility of the nitrocellu-

lose resin. The result is a blushing of the ink,

giving it a flat appearance or causing a kick-

out. The solution usually involves switching

to a higher-molecular-weight alcohol, adding

additional acetate or adding glycol ether sol-

vents. Care must be taken not to add too

much acetate to avoid potential plate-swell

problems. If slower solvents are used, block-

ing may become a concern.

Water-based inks may show poorer solu-

bility in high heat conditions due to a loss of

amine in the ink. Most water-borne inks for-

mulated for printing nonporous substrates,

use a very fast-drying amine, which tends to

evaporate out of the ink under high temper-

atures. Loss of the amine results in poorer

resin solubility. By covering the sumps and

minimizing the amount of ink in each sump,

the evaporation of amine can be reduced.

Climatic Effects on Dirty Printing

Dirty printing can be the result of poor ink

resolubility. It can result from the ink drying

too fast or too slow. In either case, the ink

builds up on the plate. With both water-

borne and solvent-based inks, there is an

optimum temperature at which the ink will

print best. One of the major causes of ink

drying on the plate is the presence of stray

air from the in-between dryers. This air

blowing on the plates causes the ink to dry

and eventually build up until it prints in the

nonimage area. Checking the in-between

dryers to ensure they are balanced will

address this concern. The key to avoiding

ink resolubility problems is to provide prop-

er drying conditions.

Climatic Effects on

Retained Solvents

Poor drying of either water- or solvent-

based inks will result in higher levels of

retained solvents. This becomes a bigger

problem as the number of ink traps increas-

es. These increased traps cause the first-

down inks to hold onto their solvent in the

in-between dryers and hamper the removal

of the trapped solvents in the final tunnel

dryer. This can be minimized with proper

solvent selection. The key is to have the

slowest solvent in the solvent blend be a true

solvent for the ink.

It is not unusual to have retained solvent

levels higher than desirable, due to skinning

on the ink surface because the web tempera-

ture is too high. Reducing web temperature

is the first step to correct this problem. If not

successful, the solvent blend in the inks

should be adjusted. This would include a

review of true solvents for the resin system.

Water-based inks printed in high humidity

conditions tend to also show higher retained-

solvent levels, due to poor drying. One suc-

cessful approach is to treat the air going into

the ovens by removing some of the moisture.

Climatic Effects on Press Speeds

Water-borne inks generally will print at

optimum press speeds with higher tempera-

tures and lower humidity. At a certain point,

excessive temperatures can cause either

skinning over on the web, or ink drying and

building up on the printing plates. Optimum

temperature and humidity should be cou-

pled with maximum air circulation to

achieve optimum drying results. Solvent-

based inks can also experience ink buildup

on the printing plates if the air temperature

is too high. The window of acceptable tem-

perature and humidity, however, is greater

with solvent-based inks than with water-

based inks.

UV FLEXO INKS

Ultraviolet flexographic inks differ signifi-

cantly in composition from solvent-based

inks (Table 14). Most inks contain pigments

and special additives, but what separates UV

inks from conventional solvent- or water-

based inks is the use of oligomers,

monomers and photoinitiators. The oligo-

mer is the resin, or vehicle, of the UV ink.

The functional properties are dictated by the

choice of oligomers and monomers.

Unlike conventional inks, the press proper-

ties of UV inks are not sacrificed by attempt-

ing to achieve superior end use or perfor-

mance characteristics. This is due to the ink’s

capacity for intermolecular bonding or

crosslinking. Another unique trait of UV inks

is that their superior functional properties

can be modified by the addition of mono-

mers. Monomers, like solvents in convention-

al inks, are used to adjust the viscosity of an

ink. Unlike their solvent counterparts,

monomers do not evaporate, but rather cross-

link and become part of the cured ink film.

Photoinitiators are molecules that absorb

UV energy and then use that energy to initi-

ate a polymer chain reaction. The two most

common photochemical mechanisms used in

UV flexo inks are free radical and cationic.

They each have their advantages and disad-

vantages, so it is important to look into both

chemistries when choosing a UV ink.

UV Curing

After the ink is printed on the substrate, it

is passed under a source of UV energy, typi-

cally a UV-curing system. The UV-curing sys-

tem is composed of a UV lamp, a reflector, a

power supply and a control unit. The lamp is

a transparent quartz tube filled with an inert

gas, typically argon and a small amount of

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100 FLEXOGRAPHY: PRINCIPLES & PRACTICES

mercury (Figure

). Quartz is used because

it is transparent to UV, whereas normal glass

is not. Mercury is used because of its strong

emissions in the ultraviolet range. The sys-

tems are rated in watts per linear inch.

Newer systems are normally 400 to 600

watts per linear inch. The reflectors for UV-

curing units typically employ the elliptical or

focused geometry to optimize the energy

delivered to the chemistry (Figure

). This

focused UV energy is then absorbed by the

photoinitiators. The photoinitiators trans-

form into free radicals for the acrylate chem-

istry or Bronsted acids for the cationic

chemistry (Table 15).

Free-radical and cationic inks utilize a

chemically different set of oligomers and

monomers, so the end properties are also

affected. Free-radical acrylate chemistry is

based on acrylate-modified epoxies, ure-

thanes, polyesters and other materials. Co-

initiators are added to prevent oxygen inhi-

bition of the free-radical curing. Cationic

chemistry is based on epoxies, which cross-

link when reacting to acids. The initiator in

cationic curing is a blocked-acid catalyst

which is released by UV energy. There are

advantages and disadvantages to each chem-

istry (Table 16).

Despite many concerns about embracing

what they consider as experimental new

technology, printers should be aware that UV

is not new and not experimental. Ultraviolet

inks have been commercially available for

more than 25 years. The flexo market – espe-

cially the domestic wide-web flexo market –

is just the next printing method to find a use

for the technology. UV is the next step in the

natural progression of flexographic printing

technology, just as it was in offset lithogra-

phy, and perhaps just as it will be in gravure.

In the simplest terms, UV-curing is nothing

more than a different way to dry inks on a

printing press.

UV- vs. Solvent-based Inks

Solvent-based inks certainly have their

advantages. They are typically easy to clean

up, and wet out most printing substrates

well. The biggest advantage of solvent flexo

technology, however, is the comfort level it

A standard UV lamp is a

transparent quartz tube

filled with an inert gas,

typically argon and

a small amount of

mercury.

Reflectors for UV-curing

units optimize the

energy delivered to the

chemistry. This focused

UV energy is then

absorbed by the pho-

toinitiators. The pho-

toinitiators evolve into

free radicals for the

acrylate chemistry or

Bronsted acids for the

cationic chemistry.

Polished

Metal

Reflector

Mercury Vapor Lamp

(Quartz Tube)

7& 7*

Table 14

CONVENTIONAL INK UV INK

Pigments Pigments

Solvents Monomers

Resins Oligomers

Additives Additives

Photoinitiators

INK COMPOSITION

inspires for both press operators and press-

room managers. Ink suppliers have been

fine-tuning these formulations for 40 years.

This is the tried-and-true technology. It has

worked in the past, so it will surely work in

the future. On the downside are VOCs. You

can burn them, you can collect them, but

you can’t ignore the environmental reper-

cussions of solvents. Using this technology

is only going to be more and more difficult

as we proceed into the 21st century.

On porous substrates like paper, water-

based inks are hard to beat. Overall equip-

ment costs can be lower since explosion-

proof wiring, solvent incineration or, for that

matter, UV lamps are not required. And

water inks are at least perceived by the lay-

man to be the most benign. On nonporous

substrates, ink formulators continue to

explore better ways to wetout the stock.

Compared to the solvent technology, water-

based ink technology is relatively new.

Unless the laws of nature can be fooled and

water evaporates as if it is solvent, then

water-based inks may be limited for this

application.

What are the advantages of UV inks? The

most obvious are the environmental bene-

fits. By eliminating solvents, printers can

enjoy two bonuses: removal of a potential

fire hazard and EPA compliance. Local,

state, and federal EPAs have wholeheartedly

embraced the UV-curing concept. Several

printers have commented that the EPA

seemed positively overjoyed at the prospect

that the printers were considering a conver-

sion from solvent to UV. Since UV inks don’t

dry until they are passed under a UV lamp,

the ink stays completely open. Unsightly

INK 101

Table 15

UV POLYMERIZATION MECHANISMS

ACRYLATE & CATIONIC

UV POLYMERIZATION MECHANISMS

Ultraviolet Energy Photoinitiation

Free Radicals Bronsted Acids

Acrylate Chemistry Cationic Chemistry

Monomer/Oligomer Monomer/Oligomer

Cured Polymer Cured Polymer

Table18

DISADVANTAGES

■ Cationics are relatively new to flexo and

require differenent handling

■ Amines will poison the cure of cationics

■ Amine functional substrates are prob-

lematic

■ Amine funcitonal pigments cannot be

used (fluorescents)

■ Perceived higher pricing

■ For CI press, speeds of white ink limited

by cure time after UV lamps

ADVANTAGES

■ Low potiential skin irritaion

■ Not oxygen inhibited

■ True metallic pigments can be used

■ Static dissipation

■ Reduced shrinkage – improved adhesion

to films

■ Post cure

■ Low odor

■ Food package: LD

’s are known

■ Product shelf live

■ Inherent low viscosity

CATIONIC WITH ACRYLATE CHEMISTRY