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

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22 FLEXOGRAPHY: PRINCIPLES & PRACTICES

sodium, fluorescent, halogen and mercury

vapor, emit visible light with different wave-

length compositions. The lack of certain

wavelengths or parts of the spectrum means

that these light sources can display color.

Consider a sodium lamp: light from this

lamp is pure yellow and contains no blue

component. Therefore articles which appear

blue (absorbing red and green wavelengths)

in normal daylight appear almost black

under a sodium lamp. Although this is an

extreme example, it illustrates the need to

view all colors under identical, specified

light sources for color matching purposes.

The visual characteristics of an ink are

recognized in terms of its color and can be

defined by three independent variables: hue,

saturation, and lightness. Hue (H) actually

describes the color, for example, yellow, red,

or green. These “hues” can be arranged in a

“color circle” (Figure

). Saturation or

chroma (C) refers to the intensity and

strength of the color, with the strongest,

most saturated colors being on the periphery

of the circle.

Lightness (L) represents purity, or how

light/dark the color is, and is indicated on

the z axis. This “map” or color space pro-

vides the ability to specify colors in numeri-

cal terms (L,C,H) which can be accurately

measured using a spectrophotometer. This

device is much more sensitive than the

human eye and can be used to measure the

absorption spectrum of an object by illumi-

nating it with a standard light source of

known intensity and measuring the intensi-

ties of the various wavelengths reflected.

The equipment can then plot a graph of

wavelength vs. percent reflectance to give a

spectral or color curve (Figure

) that rep-

resents the color of the object. This curve

can then be used as a standard when trying

to match the color.

Colorants

Pigments together with dyestuffs, provide

the color or visual identity of an ink and rep-

resent the largest share of the total cost. They

are present to provide both decorative and

functional properties: for example, lightfast-

ness, opacity/transparency and product resis-

tance. Both types of colorants are chemical

compounds that alter appearance by the

selective absorption and/or reflection of light

energy. For organic pigments and dyes, this is

determined by specific groups of atoms,

called chromophores (C=C, C=O, C=N,

N=N), present within the molecules which

absorb light energy. Different combinations

of chromophores absorb different levels of

energy, thereby producing different observed

colors. Other chemical groups, known as aux-

ochromes (OH, Cl, Br, NH2 , CH3), while not

responsible for selective absorption, help

Hues can be arranged in

a “color circle”. This

“map” or color space

provides the ability to

specify colors in

numerical terms (L,C,h),

which can be accurately

measured using a

spectrophotometer.

A graph can be plotted

of wavelength vs. per-

cent reflectance to give

a spectral or color curve

that represents the color

of the object. This curve

can then be used as a

standard when trying to

match that color.

Red

Yellow

L=100

White

L=0

Black

Hue

-a

Green

-b

Blue

400 500 600 700

100

Wavelength (nm)

Reflectance(%)

enhance the effects. Processing conditions

during pigment manufacture and subsequent

chemical treatments also affect the observed

colors. The color of inorganic pigments is

also a function of chemical composition and

oxidation state, and is influenced by the crys-

tal form of the substance.

A pigment is largely insoluble in the ink

media, requiring it to be dispersed, while

dyestuffs are soluble in the vehicle system.

Along with this solubility difference, there

are other basic variations between pigments

and dyes as listed in Table 2. Clearly, the

properties required of the ink and finished

product dictate the colorant used.

Dyes

The dyes used in flexographic printing fall

into four categories:

Solvent Soluble. Solubility in a range of

organic solvent is a typical physical charac-

teristic of solvent-soluble dies. These dyes

often contain the heavy metals Chromium

and Cobalt, which lead to environmental,

health and safety concerns. Used for their

purity of shade and transparency on foil

coatings, they have better lightfastness than

the basic dyes.

Basic or Cationic Dyes. Although they show

high color intensity, brilliancy and solubility

in blends of alcohols and water, the use of

cationic dyes in the flexo industry has

decreased because of toxicity concerns.

These dyes are often used in conjunction

with mordants or fixative agents, such as tan-

nic acid, to improve physical properties like

water resistance and lightfastness. These

dies are suitable only for short-term use on

paper with minimal-resistance requirements.

They still remain the primary components

for the triarylmethane pigment range.

Disperse Dyes. The primary use of disperse

dyes in the printing industry is in heat-trans-

fer inks for printing on textiles. After disper-

sal in a flexographic vehicle, the dye-based

ink is printed on paper. The printed image is

brought into contact with the fabric under

conditions of high heat and pressure. The

dye sublimes, penetrating the fabric where it

condenses, giving bright saturated colors.

Acid Dyes. Acid dyes have a strong affinity for

cellulosic materials and are used for water-

based fugitive check inks, invisible inks in

painting books, and in dyeing paper. Primarily

soluble in water, they give bright hues with

light-fastness ranging from very poor to good.

Pigments

There are numerous different types of pig-

ments. Some are available naturally, primari-

ly as minerals, but the majority are synthetic,

meaning that they are generated from petro-

INK 23

Table 2

PROPERTIES DYES PIGMENTS

■Color Strong, vivid and clean Weak to strong, dirty to clean

■Lightfastness Poor Fair to excellent

■Bleed Resistance Poor Fair to excellent

■Chemical Resistance Poor Fair to excellent

■Heat Resistance Poor to fair Fair to excellent

■Optics Very transparent Opaque to transparent

■Rheology Excellent Poor to good

■Toxicity Fair (except FC&D dyes) Fair to very good

PIGMENT AND DYE VARIATIONS

24 FLEXOGRAPHY: PRINCIPLES & PRACTICES

leum feedstocks. A simple, though imperfect,

way to classify them is as organic and inor-

ganic pigments.

Organic pigments are those derived from

carbon-based materials, while inorganic pig-

ments are compounds of various metals

which contain no carbon atoms with the

exception of carbon black. Although there

are numerous types of pigments, few find

their way into ink formulas. Many are uneco-

nomical, do not provide the necessary resis-

tance or performance properties, or have

associated environmental or toxicity hazards

which preclude their use in flexographic inks.

The following section is a detailed descrip-

tion of the most commonly used pigments

within the industry. Each pigment can be

identified by names in common use, togeth-

er with the appropriate color index number.

(The Color Index is a method devised by the

Society of Dyeists and Colorists for classify-

ing pigments based on their chemical type

and structure). Miscellaneous materials in-

cluding metallic powders, pearlescents, fluo-

rescents and specialty pigments are covered

separately.

Inorganic Pigments

With a few minor exceptions, inorganic

pigments have certain notable features.

These include: high lightfastness, economy,

high opacity, weak tinctorial strength, high

specific gravity and a lack of cleanliness of

hue. Toxicity is also a common feature asso-

ciated with inorganic pigments that contain

harmful metals such as cadmium, lead,

chrome and molybdenum. Inorganic pig-

ments commonly used in flexographic inks

include: titanium dioxide, carbon blacks,

iron blues, iron oxides and extenders such as

calcium carbonate, silica, lithopone and clay.

Titanium Dioxide. This is the most important

white pigment (PW 6) in use today due to its

chemical inertness. A variety of grades are

available. The different grades are surface

treated with coatings of silicon oxides, zirco-

nium, aluminum, zinc, or organic chemicals

to aid dispersion, maximize opacity or gloss

and improve durability. There are two major

crystal forms: anatase and rutile. The rutile

grade is more opaque, but slightly more abra-

sive and yellow than the anatase grades.

Most grades are produced using the chlo-

ride process, rather than the environmental-

ly unfriendly sulfate process. The chloride

process generates a harder crystal structure

with higher dry brightness. The anatase

grade is preferred in situations where doctor

blade, cylinder or die blade wear is a prob-

lem. A dispersed particle size of approxi-

mately 0.2 microns is necessary to achieve

optimal light scattering.

Carbon Blacks. These pigments have an

extremely fine particle size with a high sur-

face area, which can cause body and flow

problems. Like titanium dioxide, they show

outstanding chemical inertness, being

extremely resistant to acids, alkalis, light,

heat and solvents. Almost all grades of carbon

black (PBk. 7) available are produced by the

furnace process. Such furnace grades often

undergo further chemical processing with

oxygen and surfactants to mimic the superior

wetting and flow of the now virtually defunct

channel blacks.

Iron Blues. Also known as Milori, Bronze,

Chinese, or Prussian Blue (PB 27), iron blues

range in shade from a dirty reddish tone to a

cleaner green shade and can show consider-

able bronzing. These pigments show excel-

lent resistance to solvents, fats and light

(except tints with titanium dioxide), but are

difficult to grind. They have poor alkali resis-

tance and are unsuitable for use in water-

based systems or for use on soap wrappers.

These pigments should not be used in oxida-

tively sensitive ink formulas.

Iron Oxides. Typically opaque and tinctorially

weak, iron oxides vary in shade from dirty

yellow (PY 42), through dull red brown (PR

101, PR 102, PBr. 6, PBr. 7), to black (PBk. 7).

They exhibit exceptional chemical and

weather resistance, are strong UV

absorbers, economical and suitable for

direct food contact. They can be extremely

difficult to disperse, and use of micronized

grades is advised to prevent mill- and press-

wear problems.

Extenders

Extender pigments have a myriad of prop-

erties. They are used to reduce costs without

affecting printing properties, e.g., calcium

carbonate (PW 18). They are used to reduce

abrasion and provide opacity, e.g., lithopone

(PW 5), or prevent settling and aid printabil-

ity, e.g., clay (PW 19). Certain extenders are

used to aid flatting or reduce tack, e.g., silica

(PW 27).

Organic Pigments

Organic pigments can be subdivided into

three categories pigmentary colors, high per-

formance pigments and metal salt pigments.

Pigmentary Colors are “true” pigments – very

insoluble compounds that happen to be col-

ored. Many important groups of pigments,

particularly the yellow and blues, are repre-

sented in this area. Pigmentary colors are

generally water-resistant and relatively unaf-

fected by reagents, such as acids and alkalis.

However, the absence of salt groups does

make them prone to solvent solubility and

fat or wax sensitivity. The resistance proper-

ties of pigments are improved by increasing

their molecular complexity and molecular

weight.

• Diarylide Yellows. Characterized by high

strength and good resistance to acids and

alkalis, this group of pigments ranges in

transparency. AAOT (PY 14) and AAA yellow

(PY 12) are inherently opaque pigments that

can be coated with resin during manufacture

to improve their transparency. In contrast,

AAOA (PY 17) and HR Yellows (PY 83) are

intrinsically transparent. AAOT Yellow, a

greenish-yellow pigment, is the most com-

monly used yellow pigment within the

United States for flexo packaging. It displays

good flow and working characteristics, good

print strength, reasonable gloss and accept-

able lightfastness. HR Yellow (PY 83), an

extremely red-shade pigment, has a higher

level of chemical complexity and molecular

weight than PY 14, giving it excellent light-

fastness, transparency, heat and solvent

resistance. It is suitable for many demanding

applications.

• Hansa Yellows. Typically greener in shade

than the Diarylide pigments, they have signif-

icantly greater lightfastness at the expense of

tinctorial strength and heatfastness. These

lower-molecular weight pigments are also

prone to bleed in fats, oils, plasticizers and

aromatic hydrocarbons. Common pigments

include Hansa 10G (PY 3) and Hansa 5GX

(PY 74).

• Naphthol Reds. A very wide range of pig-

ments based on the b-oxynaphthyl (BON)

unit. These red pigments are commonly

durable soap-fast reds with good lightfast-

ness. However, as with any pigment, their

properties vary with chemical structure.

Care should be exercised in selecting the

correct Naphthol red for a given application.

Naphthol reds lack the cleanliness, gloss, fat

resistance and cost effectiveness of the

metal salt pigments. Care has to be taken in

formulation to maintain flow properties. Six

of the most common Naphthol pigments –

Red 112, Red 2, Red 5, Red 7, Red 23, and

Red 12 – are arranged in shade order with

PR 112 being the yellowest shade and PR 12

offering the bluest shade of red.

• Phthalocyanines. In most respects, these are

ideal pigments in that they provide strong,

clean shades together with outstanding

resistance properties at a reasonable cost.

With the Phthalo blues there are three crys-

tal forms available: alpha (PB 15 and PB

15:1, red shade), beta (PB 15:3 and PB 15:4,

green shade), and epsilon (PB 15:6, redder

than alpha). The most important in terms of

volume are the beta forms. Another special-

INK 25

26 FLEXOGRAPHY: PRINCIPLES & PRACTICES

ized Phthalo blue is the metal-free variety

(PB 16) used in situations where copper can-

not be tolerated even if it is “locked up” in

the pigment. There are two Phthalo greens

available: PG 7 and PG 36. They are consid-

erably more expensive than the phthalo

blues and only used where mixtures of

phthalo blue and yellow are inadequate. The

two grades vary in shade with PG 36 being

yellower and weaker than the PG 7.

High Performance Pigments. There are a great

number of specialty high performance pig-

ments available including: isoindolines,

perylenes, diketopolypyrolidones and indan-

thrones. When cost allows, indanthrones are

being used to a greater extent in flexograph-

ic inks. Two, special and costly, red and vio-

let pigments are used when extreme resis-

tance properties are required. These two are

Quinacridone Red (PR 122), which is similar

in color to Rhodamine Red, and Carbazole or

Dioxazine Violet (PV 23). Their properties

are similar to those of the Phthalocyanines

pigment, but unfortunately, the same cannot

be said of their costs.

Metal-salt Pigments are water-soluble “dyes”

that have been converted into water-insolu-

ble salts. Most notable in this area are a spe-

cific group of red pigments and the Fanals,

e.g., methyl violet (Fanal is an early trade

name given to triarylmethane class of pig-

ments). Metal-salt pigments show excellent

resistance to fats, oils and waxes. Except for

the fanals, they remain relatively unaffected

in the presence of solvents; however, all are

extremely sensitive to aqueous reagents

(acids, alkalis, soaps).

• 2B Reds. Calcium 2B Red (PR 48:2) is a ver-

satile blue-red shade with good working

properties and reasonable lightfastness. The

Barium 2B Red (PR 48:1) is yellower than

the calcium salt and preferred for its opacity

and better flow, though environmental con-

cerns sometimes preclude its use. The

Manganese 2B Red (PR 48:4), which is a

clean medium-scarlet shade, is notable for

its lightfastness and is used in applications

requiring outdoor exposure. All these pig-

ments are prone to “hydration,” in that when

exposed to water for prolonged periods,

they tend to change shade, becoming more

yellow.

• Lithol Reds. Like the 2B reds, the hues of

these pigments vary with the salt. Eco-

nomical pigments, like the calcium lithol

(PR 49:2) can be used successfully in both

water-based and solvent-based inks.

• Lake Red C. This low cost pigment (PR 53:1)

has good working characteristics: it is a

clean, bright yellow-red that has good fat or

oil resistance. Drawbacks include poor resis-

tance properties to light, even at full strength,

and reactivity with acids and alkalis. Use has

been diminishing because of barium content.

• Clarion Red or Ethyl Lake Red C. Similar

chemically to Lake Red C (PR 53:1) in that it

is a barium salt that can limit its utility,

Clarion Red (PO 46) is a highly transparent

orange-red shade with good gloss.

• Lithol Rubine. Commercially available as

the calcium salt, (PR 57:1), lithol rubine is

often referred to as a 4B pigment. Although

many grades are available, it typically has

good transparency, prints well, and is com-

monly used because of its shade, trans-

parency and cleanliness as a process magen-

ta. Red 2G (PR 52:1) offers a similar if slight-

ly bluer shade, but has a slight advantage

with gloss and flow.

Triphenylmethane Salts. This group includes

the pigments more commonly known as

Methyl Violet (PV 3, PV 27), Rhodamine Red

(PR 81, PR 169), Alkali Blue (PB 56), and

Brilliant Green (PG 1). They are expensive

due to the high cost of raw materials, howev-

er, their brightness and cleanliness of shade

cannot be achieved in any other way at a

competitive cost. Resistance properties on

the whole are poor. They all bleed into vari-

ous organic solvents, soaps, fats, oils and

plasticizers. Extreme care has to be taken

when formulating with inks based on these

pigments, including reviewing the process

and end use, to prevent any problems.

Chemical structure dictates that the prop-

erties shown by the pigments in the different

classes listed above vary significantly. For a

more detailed look at the generic properties

of various pigments consult Table 3. In this

table, the numbers for lightfastness are pre-

sented on a scale of 1 to 5, where 5 is best.

The scale for chemical resistance is from 1 to

8, where 8 is best.

Laked Pigments. A laked pigment is pro-

duced when a water-soluble salt is precipi-

tated onto an inorganic carrier such as alu-

mina hydrate or barium sulfate. Such pig-

ments have minimal use in the ink industry,

but are used as food colorants or artists col-

ors, e.g., Tartrazine Yellow lake.

Miscellaneous Pigments

Fluorescent Pigments. These pigments are

comprised of weak solutions of specialty

dyes in a resin matrix. The chemical compo-

sition of the dye gives them the unusual

property of fluorescence. This phenomenon

occurs when a substance absorbs light of a

shorter wavelength (UV light, which is not

visible to the human eye, in this case) and re-

emits it as visible light. Therefore, these sub-

stances emit more visible light than they

absorb, which multiplies their brilliance to

the eye. Because the resin matrix is variable,

these pigments are offered with a variety of

functional properties including: specific sol-

vent resistance, water resistance, and limit-

ed heat resistance. However, there still

remain some severe functional and applica-

tion limitations:

• Light-fastness. Because dyes are used to

achieve the color in fluorescent pigments,

the light-fastness is poor.

• Color Strength. Fluorescent pigments have

low pigment-tinctorial strength due to large

particle size. This low tinctorial strength

requires a heavy film-weight application

either from multiple passes or by using large,

deep anilox cells. Heavy application weights

may also give rise to drying problems.

• Particle Size. Maximizing fluorescence

requires a large pigment particle size. This

larger size can result in the pigment settling,

or in plugged anilox cells. Attempts to reduce

particle size severely curtail fluorescence.

• Contamination. Excessive toning or incor-

poration of other pigment types into the flu-

orescent ink will reduce or remove the fluo-

rescent effect.

• Stability. Other components used within the

ink must be selected to prevent attack upon

the resin matrix, which destabilizes and

destroys the pigment.

An alternative to using pigments is to use

soluble fluorescent toners. These materials

can be easily incorporated into vehicles to

produce useful flexographic inks. The only

drawback is that these toners are dyes, and

therefore subject to the same resistance

properties.

Metallic Pigments. These pigments are used

to impart a metallic appearance on the print-

ed substrate and mimic silvers and golds.

Metallic pigments are derived from

micronized flakes of aluminum (Al) metal

and alloys of copper (Cu) and zinc (Zn),

respectively. Unfortunately there are intrin-

sic problems with metallic inks:

• Reactivity. The metals used to make these

pigments, particularly aluminum and copper,

are reactive. The vehicles chosen to disperse

these pigments have to be inert, e.g.,

polyamides or solvent acrylics. Both acidic

and alkaline vehicles can react to different

degrees with both metals. For example, cop-

per in combination with nitrocellulose caus-

es a reaction that releases nitrogen oxide

(No

) gases and significant amounts of heat,

resulting in a dangerous fire hazard.

Similarly, although more controllable, alu-

minum reacts with the water and amines pre-

sent in a water-based ink to generate hydro-

gen gas. Both examples illustrate the care

INK 27

28 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Table 3

LIGHTFASTNESS/

CHEMICAL RESISTANCE

COLOR

PIGMENT INDEX SHADE FLOW OPACITY COMMENTS

KEY: E=Excellent G=Good M=Medium P=Poor Opaq=Opaque Semi=Semitransparent Trans=Transparent

PIGMENT PROPERTIES

FULL

TINT

ALKALI

ALCOHOL

FATS

SOAPS

Alkali Blue PB 56 Strong R/S blue P Semi 2 1 2 4 5 3 Very alkali sensitive

Alumina Hydrate PW 24 Extender M Trans N/A N/A 2 5 5 5 Poor ucid resistance

Barium 2B Red PR 48.1 Bright Y/S red G Semi 4 2 2 5 4 2 Contains barium

Barium Lithol PR 49.1 Bright B/S red M Semi 2 2 2 4 3 2 Contains barium

Calcium 2B Red PR 48.2 Very blue shade M Semi 6 4 2 5 5 1

Calcium Carbonate PW 18 Extender G Trans N/A N/A 5 5 5 5 Poor acid resistance

Calcium Lithol PR 49.2 Strong B/S red M Semi 2 2 3 4 3 3

Carbazole Violet PV 23 Dull R/S purple M Semi 7 6 5 5 5 5 Expensive

Carbon Black PB 7 Black M Opaq 8 8 5 5 5 5

China Clay PW 19 Extender M Semi N/A N/A 5 5 5 5 Low-cost extender

Clarion Red PO 46 Y/S red G Trans 3 2 2 5 5 2 Contains barium

Cu-free Phthalo Blue PB 16 Dirty G/S blue M Trans 8 7 5 5 5 5 S1 less heat stable

CuFe Rhodamine PR 169 Strong rose pink M Trans 5 3 2 2 4 1 Darkens in light

Dianisidine Orange PO 13 Y/S orange M Semi 5 4 5 4 4 5

Diarylide Orange PO 34 Bright M Trans 6 4 5 5 4 5

Diarylide Yellow PY 14 Warm yellow G Semi 5 3 5 5 5 5

Diarylide Yellow PY 17 Lemon yellow P Trans 6 4 5 5 4 5

DNA Orange PO 5 Dirty red-orange G Opaq 6 4 4 4 2 2

Hansa Yellow PY 74 G/S yellow G Opaq 6–7 5 5 4 2 5 Sublimes on heating

HR Yellow PY 83 Red shade M Trans 6 4 5 5 5 5

Iron Blue PB 27 Dirty blue violet M Trans 4 3 2 3 4 2 Dirtier than PV 3

Iron Blue PB 27 Dirty R/S blue M Opaq 6 4 1 4 4 2 Hard pigment

Iron Oxide Yellow PY 42 Dirty yellow G Opaq 8 8 5 5 5 5 FDA suitable

Lake Red C PR 53.1 Warm Y/S red M Trans 3 2 2 5 4 2 Contains barium

Lithol Rubine PR 57.1 Strong B/S red M Trans 3 2 2 5 4 1 Std. process color

Lithopone PW 5 White E Opaq 8 8 5 5 5 5 Poor acid resistance

Mac BON Red PR 52.1 Strong B/S red G Trans 4 3 2 5 4 1

Manganese 2B Red PR 48.4 Med. scarlet M Semi 7 5 2 5 5 2

Naphlhol Dark Red PR 23 Dark B/S red P Trans 6 3 4 2 5 5 Good in NC chip

Naphtbol Carmine FB PR 5 Strong B/S red M Semi 7 5 5 4 4 5 Very high cost

Naphthol Bordeaux PR12 Very blue shade P Semi 6 4 5 5 4 5 Dull, very soapfast

Naphthol F5RK Red PR 170 Bright B/S red P Semi 7 4 5 4 5 5 Very high cost

Naphthol FRR Red PR 2 Bright Y/S red M Semi 6 4 5 4 2 5

Naphthol Red FGR PR 112 Clean med. red M Trans 6 6 5 5 4 5 Soap-fast scarlet

Phthalo PB 15.4 G/S blue M Trans 8 7 5 5 5 5 Process

Phthalo Blue PB 15.1 R/S blue P Trans 8 7 5 5 5 5

Phthalo Green PG 36 Y/S green P Trans 8 7 5 5 5 5

Phthalo Green PG 7 Bright green P Trans 8 7 5 5 5 5 Expensive, poor flow

PTMA Methyl Violet PV 3 Bluish violet G Trans 5 3 4 1 4 2 Darkens in light

PTMA Rhodamine B PV I Clean magenta G Trans 4 3 4 1 5 2

PTMA Rhodamine Y PR 81 Rose pink shade G Trans 4 3 2 3 3 2 Bleed prone

Quinacridone Red PR 122 Bright B/S red M Semi 8 7 5 5 5 5 Duller than PR 81

Silica PW 27 Extender P Trans N/A N/A 5 5 5 5 Matting agents

Titanium Dioxide PW 6 White G Opaq 8 8 5 5 5 5 Can be abrasive

required to formulate a safe metallic ink.

• Rub and Cohesion. One of the goals with

metallic inks is to provide a printed surface

that can mimic a true metal surface, by hav-

ing a similarly high reflectivity. To achieve

this reflectivity, the pigments are specially

treated and size-graded to help determine

their orientation in the printed ink film. For

high reflectivity or brightness, it is important

that the metallic flakes are flat or plate-like

and that they stack on top of each other in an

ordered fashion. Unfortunately, this stacking

results in poor cohesion in the metallic ink

itself and consequently poor rub. These

properties can be improved, but always at

the expense of brightness.

• Specific Gravity. Because of their high spe-

cific gravity, metallic inks are prone to set-

tling, especially in low-viscosity systems,

reduced inks and ink fountains with minimal

agitation.

Pearlescents. These titanium-treated mica-

based (silicon oxide) pigments are used for

their optical properties, which impart a

pearlescent effect. Plate-like in structure,

these inert pigments can be incorporated into

a wide range of vehicle systems. Minimal dis-

persion should be used to avoid destroying

platelets and care taken to avoid settling.

High binder levels are required to “fix”

pearlescents. They can be used with iron

oxide dispersions to achieve simulated gold

and silver, and are useful where aluminum or

bronze cannot be used.

Thermochromic Pigments. These expensive

pigments consist of thermally sensitive liquid

crystals encapsulated in a transparent poly-

mer shell. The fragile nature of this polymer

shell means that the pigments are usable only

in water-based systems with minimal organic

solvent content and that high-shear process-

es must be avoided. To get a noticeable

response, a heavy film weight must be

applied, requiring multiple passes.

INK VEHICLE

The “transparent” part of the ink is the

vehicle. The purpose of the vehicle is to act

as carrier for the colorant, to bind this col-

orant to the substrate being printed, and to

contribute the functional properties required

by the finished print. This ink vehicle is a

composite of resins, solvents and additives.

Table 4 outlines these elements and their

functional properties.

Resins

There are a large number of resins avail-

able to the ink formulator; following are the

most common.

• Nitrocellulose. The most common resin used

in solvent flexographic inks is nitrocellulose.

It offers good pigment wetting, low odor,

good solvent release, high heat resistance,

economy and wide compatibility. This com-

patibility allows nitrocellulose to be modified

with other resins that have complementary

properties to offer the possibility of produc-

ing inks for nearly all substrates. Available in

a variety of viscosity grades, the degree of

nitration and hydrolysis achieved during resin

production determines the solubility and

physical properties, such as viscosity and

heat resistance.

• Rosin Esters. These esters include maleic

and fumaric modified rosins. Medium acid-

INK 29

Table 4

INGREDIENT RESPONSIBLE FOR

Resin Pigment dispersibility and carrier

Ink transfer and printability

Adhesion and functional properties

Solvent Viscosity control

Drying speed

Additives Defoaming

Rub and slip modification

Anti-oxidants Flexiblity modifiers

INK VEHICLES

30 FLEXOGRAPHY: PRINCIPLES & PRACTICES

value maleics have utility in alcohol-based

flexographic inks and are used in combina-

tion with nitrocellulose and polyamides for

papers, films, and foils. High acid-value

resins have utility in water-based flexo inks

as grinding and modifying vehicles. They

typically improve printability and gloss by

aiding ink flow and transfer. They also can

be used to help increase the heat resistance

of softer resins.

• Polyamide Resins. These resins can be

broadly categorized into three types: alcohol

soluble, co-solvent soluble and hot melt.

Those with a molecular weight (MW) below

4,000 have good alcohol solubility and nitro-

cellulose compatibility. When incorporated

into flexographic inks, they impart excellent

adhesion to a variety of corona-treated and

coated-polyolefin films. They are also noted

for their excellent printability, transfer, high

gloss and solvent release properties. They are

also compatible with shellac, rosin esters,

phenolics and polyketones. Although they

exhibit outstanding pigment wetting, they are

ill suited for use as sole grinding vehicles and

commonly require modification with “harder”

resins. Co-solvent polyamides require a blend

of an alcohol and aliphatic/aromatic hydrocar-

bon for solubilization. Noted for their wide-

ranging adhesion, co-solvent polyamides are

slightly harder than the alcohol-soluble types

and are commonly used for their release prop-

erties in cold-seal release packaging. The

polyamide resins derived from hot melt adhe-

sive-grade materials display adhesion to most

substrates and are widely used in “universal”

laminating inks. Their higher molecular

weight means that they show lower resin

compatibility than other polyamides, poorer

pigment wetting, and lower solubility, which

impacts on color strength and print perfor-

mance (Figure

Care should be taken with any polyamides

to avoid incorporation of heavy metals from

drying agents or pigments. These metals cat-

alyze the oxidation of the dimerized fatty

acids used in the production of the resin to

produce extremely rancid odors and a dis-

tinct possibility of print blocking.

• Acrylic Resins. These resins have found

wider use in flexographic inks in recent

years. In solvent-based inks, acrylics are

used primarily for their adhesion characteris-

tics in combination with nitrocellulose or

other cellulose esters. Careful formulation is

needed to ensure the sufficient presence of

“active” solvent that will maintain the solu-

bility of the resin without significantly affect-

ing the printing plate and to minimize solvent

retention. Acrylic or acrylate resins also form

the basis of most water-based and UV/EB-

curable resin technology currently on the

market. These technologies are discussed in

greater depth in following sections.

• Polyketone Resins. These resins are used as

modifiers to assist gloss and adhesion.

Polyketones are inert, hard, non-film-form-

ing resins that dry rapidly, but show tenden-

cies to “skin over,” potentially leading to

increased solvent retention.

• Polyvinylbutyral Resins. These resins,

derived from polyvinyl alcohol and aldehyde

butyra, are used for their flexibility and adhe-

sion properties in lamination and heat-seal-

ing inks. Specially purified grades are avail-

able; however, care should still be exercised

when selecting these materials for food-use

Polyamide gloss and

transfer test.

applications since rancid odors can arise

from one of the constituent components.

• Polyurethane Resins. For flexographic inks,

these resins typically function as high mole-

cular-weight plasticizers, conferring in-

creased adhesion, flexibility and toughness.

Used widely in Europe for high performance

surface print and lamination inks, their use

in the United States is limited because of

high cost.

• Epoxides. These resins find utility in cross-

linking systems as reactive diluents and

resins in UV cationic inks, and in combina-

tion with polyamine resins in high perfor-

mance catalytic lacquers.

Solvents

The colorant and resin constituents of an

ink are both solids. Therefore, the primary

function of the solvent is to convert these

ingredients into a fluid form capable of being

printed. Solvent selection is critical in deter-

mining the performance of the printing ink

and is governed by a number of factors. The

solvents used should:

• solubilize the resin or resins chosen to

produce a fluid, rheologically suitable

vehicle during all phases of the print

process;

• be easily removed by evaporation or

absorption.

• impart minimal odor in printed ink film;

• aid substrate wetting and adhesion;

• not affect the printing plate or fountain

roll;

• interact minimally with other ink ingre-

dients, thereby preventing ink instabili-

ty; and

• comply with customer specifications,

and with local, state and federal legisla-

tion governing environmental, health

and safety issues.

Solvency power is the most important fac-

tor in considering the utility of a solvent. In

polymer solutions of high concentration, liq-

uids with high solvency power produce solu-

tions with the lowest viscosity. This proper-

ty can be used as a guide to judge the relative

merits of a solvent for a particular resin.

Such solvents commonly fall into three cate-

gories: active, diluent and latent. Solvents

which can solely dissolve a resin are denot-

ed as active. Liquids which are non-solvents

for the primary resin, but which can be

added to an existing resin solution without

increasing viscosity or causing precipitation

are called diluents. Occasionally, it is found

for certain principal resins that blends of liq-

uids considered to be diluents or non-sol-

vents for this resin interact to form a co-sol-

vent blend capable of solubilizing the mater-

ial. These solvents are regarded as latent sol-

vents for this resin.

Just as an ink may contain a variety of

resins to achieve the required physical prop-

erties, most inks are formulated to contain a

mixture of solvents offering a satisfactory

balance of solubility, rheology and drying

speed. If a solvent blend is used, it is crucial

that the slowest evaporating solvent be a

good solvent for the ink system. Problems

with loss of gloss, adhesion, poor printabili-

ty, film integrity and product resistance will

result if the last solvent to evaporate is a

non-solvent for the resin system used. This

behavior is commonly referred to as ink

souring or kick-out. Conversely, the solvent

blend chosen should not have such an affin-

ity for the resin system that there is difficul-

ty in removing the solvent.

Prior to printing, the viscosity of the ink is

reduced by the addition of an appropriate

solvent blend. During a print run, the solvent

or solvents will evaporate from the ink foun-

tain. The composition of this escaping sol-

vent blend is determined by a number of

well-known factors, including:

• the vapor pressure of each constituent

solvent;

• molecular solvent and resin interac-

tions;

INK 31