Biermann Ch. Handbook of Pulping and Papermaking

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464 23. PRINTING AND THE GRAPHIC ARTS

discovery, offset printing (printing to a blanket and

then to the paper) was developed. Now most offset

printing is practiced as lithography so the term

"offset" is now svnonvmous with "lithography".

Not every possible variation of printing that

has ever been used (especially in regard to sheet

handling) will be considered here so that the

common methods of printing used today may be

emphasized. Many subtle aspects of printing that

are not critical to paper use are not presented

either. Some practices that are no longer in

common usage are presented because they are

necessary for comparison or historical develop-

ment. For example, letterpress printing has

negligible conmiercial use now but must be con-

sidered in any chapter on printing.

23.2 LETTERPRESS

Introduction

Letterpress uses raised surfaces (re/zV/print-

ing) upon which ink is transferred. An even layer

of ink is applied to the raised surfaces by inked

rollers. When paper is pressed against the plate,

the raised surfaces contact the paper and leave the

ink impression. This is the only method whereby

replaceable type can be used; it is useful for text

information that may be changed often. Later,

most letterpress plates were made by photographic

methods (with plates coated with egg albumin and

ammonium dichromate). Since the ink may exude

around the characters during the printing opera-

tion, a ring of ink may form at the edges of

printed areas (visible under magnification).

Variations

letterpress

Flexography

uses rotary web letterpress with

flexible rubber plates and fast—drying inks. The

rubber plates are wound around a printing cylin-

der, Flexography has widespread use in packag-

ing and has grown rapidly with the packaging

industry. For example, it is widely used to print

corrugated boxes, plastic films, and foils.

Photopolymers are used to photographically

prepare the plates. In modern flexography, the

ink is applied to the plate by anilox rolls, which

are discussed in the appendix of this chapter.

Thermography is used for printing raised

letters

{embossed)

for wedding invitations, greeting

cards,

and especially high—value printing jobs.

An adhesive is applied (which remains on the sur-

face),

a powder is applied which is retained on the

areas with adhesive, and the excess powder is

removed by an air stream or vacuum.

23.3 LITHOGRAPHY

Introduction

Lithography (stone printing) was invented by

Alois Senefelder in 1798. The original method

used sandstone rock upon which an image was

made using a thick, greasy material. During

printing, the surface was treated with a water

solution of gum arable wetting only the nonimage

areas.

The surface was then treated with a non-

polar ink (oil, wax, soap, and carbon soot) that

soaked into the image area, but avoided the water-

treated, nonimage area. The image was then

transferred to paper. The same basic technique is

Key: Ink:

Plate: Water:

Letterpress printing

Lithography (offset) printing

^S^^^^^^i^^.

Gravure (intagho) printing Screen (mask) printing

Fig. 23-1. The four major printing processes as seen through a side view of a plate cross section.

LITHOGRAPHY 465

Still used today. The water solution used to

prevent printing in the nonprint areas is called the

fountain solution. This method is sometimes

called planographic printing since the image and

nonimage areas are in the same plane.

The system used to apply the water solution

to the plate surface is called the dampening sys-

tem. It makes the nonimage area hydrophilic and

leaves the image area hydrophobic [water hating,

but nonpolar ink loving (oleophilic)]. The damp-

ening solution usually contains some isopropanol

(though it is used less for environmental and health

reasons) which decreases its surface tension and

increases its viscosity.

Lithographic

plates

The plate surface is often a thin aluminum

plate wrapped tightly around a cylinder. Every

time the cylinder rotates, a copy can be made.

Bimetallic plates are made using two metals of

different properties. Oleophilic materials include

certain resins, copper, and brass (a copper alloy

with up to 40% zinc). Hydrophilic metals include

aluminum, chromium, and stainless steel. The

hydrophilic surface is kept hydrophilic by treat-

ment with materials such as gum arable (a water-

soluble polysaccharide). Platemaking can also be

done by use of computer—controlled laser light on

plates with a Mylar (polyester) coating. This

method has become very popular as new Mylar

coatings allow 100,000 or more impressions to be

made from the plate.

Offset

printing

Most lithography is carried out as offset

printing; the image on the ink plate is transferred

(offset) to a rubber sheet (the blanket) and from

the blanket to the paper (as opposed to direct

printing). In this way the original plate is not a

mirror image of the desired print. Offset printing

gives smooth edges on printed areas (as observed

with magnification). [Offset printing is possible

with letterpress or gravure printing; however,

most lithography is done with offset, and as

already mentioned the term offset printing is

essentially synonymous with lithography.] Mod-

ern lithography prints a thin layer, about 3—5 /*m,

so complete opacity is very difficult to obtain.

The rubber blanket usually contains air

pockets (Fig. 23-2), if it is the common compress-

ible type and not the conventional type, in a layer

below the surface layer to give it good compress-

ibility so that printing on relatively rough surfaces

is improved. This makes paper compression a

relatively unimportant factor, but has the disadvan-

tage that some print distortions (such as dot gain

or paper distortion) may be realized. In most

cases,

compressible blankets give higher print

quality, better wear, and are more forgiving of

paper wrinkles than conventional blankets; they

are used for over 90% of offset printing on paper.

Offset printing has several advantages. The

rubber blanket gives a smooth image on relatively

rough pieces of paper. Since the plate only

contacts a rubber offset blanket, plate wear is

minimized for long plate life. (Paper can be a

very abrasive material and a material over which

the printer does not have direct control.) Since

there are two transfers, the original plate is a

positive of the image.

Offset lithography is the most widely used

printing method for commercial printing, newspa-

pers,

books, and magazines. There are three

major types of this: sheet—fed, non—heat—set

web,

and heat—set web. The nonheat—setting

process requires an absorbent paper and is used

for newsprint and book papers. Heat—set presses

are used for higher quality publications, often on

coated papers.

Sheet—fed presses run as fast as 12,000

copies per hour, while web presses may be double

Fig. 23-2. Cross section of a compressible

blanket (left, 30 x) and air pockets (200x).

466 23. PRINTING AND THE GRAPHIC ARTS

this.

However, the highest quality papers are

often available in the sheet form only.

Offset duplicators (such as A.B. Dick Co.

products) are very similar to presses, but there are

some important differences including their lower

cost. Duplicators use a higher pressure on the

paper to the blanket that results in a higher dot

gain and possible smearing on some coated papers;

however, duplicators can often use paper with

lower smoothness. Press blankets are said to just

kiss,

i.e., lightly contact, the paper.

Offset

press configurations

Figure 23-3 shows a schematic diagram of

the dampening and ink rollers in relation to the

plate, blanket, and impression rolls or cylinders;

it includes many of the features common to most

offset presses, but presses have many more rolls

and many configurations possible. Many two-

color presses operating in a single unit will use a

double—sized impression cylinder with two blan-

ket rolls. (Each blanket roll has its own plate roll

with separate ink and dampening systems.) Here,

the registration is very good since the paper is

held by a single gripper for both impressions.

Waterless

printing

The ink/water balance in offset lithographic

printing is critical. Also, the use of the dampen-

ing system complicates the press and its operation

and adds to its initial cost. The 3M company

developed and introduced waterless printing in the

late 1960s; the product was soon abandoned.

Toray Industries further developed the process in

the 1970s. The process was not widely used until

the mid—1990s. It offers the potential to greatly

affect the printing industry. This method is a bit

of a competitor to stochastic printing (below).

The Toray plate uses a silicone rubber layer

iivd)

on top of a aluminum plate coated with a

light—sensitive photopolymer. The silicone rubber

is removed from the print areas on the plates by

exposure to light. In fact, the printing area of the

plate is slightly recessed, but the ink rises above

the surrounding silicon rubber area.

Inks must be designed especially for this

system to be effective; the principal difference is

that the ink vehicle that is selected must give

suitable rheological properties such as a fairly high

viscosity. The temperature of the plate during

Adjustment

Key

Rear Ink

Roll

Former

Plate

Impression \ Gap

Cylinder

Fig. 23-3. General diagram of an offset press.

LITHOGRAPHY 467

operation is extremely critical and should be kept

within a 2°C (5°F) tolerance. This is done using

a cooling system to control the temperature of the

ink vibrator rollers. The exact temperature used

is variable, depending on the ink that is used. If

the ambient temperature and relative humidity of

the press area are high, water may condense onto

the ink with an unsatisfactory result.

Most offset presses can be converted to a

waterless printing by adding an ink cooling system

and using the appropriate plates and inks. Dot

gain in the midtones is said to be about 30% lower

than with conventional lithography.

Waterless inks should be used with papers

having a high surface strength (picking resistance)

because waterless inks tend to have higher tack

when pulling away from the paper {releasefactor);

these inks have no dampening solution with which

to emulsify, which would otherwise tend to de-

crease their tack at this point. Press rooms should

be as clean as possible, and paper that does not

lint should be used since the dampening solution

that usually lubricates the system is absent.

23.4 GRAVURE

Introduction

Gravure printing (also called intaglio, pro-

nounced with a silent g) uses an indented area of

the plate from which to print. An excess of fluid

is applied to the plate, and the excess is removed.

This leaves the raised areas free of ink, and the

ink is transferred from the depressed areas. The

edges of printed areas have a serrated appearance

(under magnification). Web printing with the

gravure process is called rotogravure.

Gravure printing is usually carried out as a

web process on lightweight (below 40 lb/ream)

coated or uncoated groundwood paper with mini-

mal pressure on the plate. Therefore, paper

smoothness

very important. Compressible paper

and good ink receptivity help print quality.

The major components of a rotogravure press

include the engraved cylinder (the plate), the ink-

fountain, a doctor blade to remove the ink from

the nonprinting area, and the impression roller.

Gravure plates

The ink is held in the plate by a series of

cells or wells etched in a metal surface (often

copper). The cells are filled by rolling the plate

cylinder in a bath of

ink.

Ink is removed from the

nonprinting areas by a doctor blade (usually made

of flexible steel).

Gravure is the preferred method for high-

quality pictures (especially color), but the high

cost of plate preparation limits its use to medium

to long press runs. The resolution of gravure is

150 or more lines per inch.

Conventional gravure uses fixed area and

fixed depth cells; it is used for short—run, high-

quality reproductions of photographs. Variable

area gravure is used for packaging, and variable

area and depth is used for newspaper supplements

and catalogs. The latter method allows some

direct reproduction of shades of gray by control-

ling the amount of ink that is transferred.

Plate

preparation

The printing plate

prepared (Blasche, 1993)

on a copper plated [25—75 /^m (1—3 mil) thick]

steel roll. The surface is engraved by one of

several methods: the majority are engraved by

electromechanical methods, but carbon tissue and

direct transfer methods are also employed.

Electromechanical methods use a computer-

controlled oscillating (in and out) diamond that

penetrates variable distances (in 256 steps) to cut

the copper

plate,

which results

pyramid—shaped

cells of variable depth and variable area. Carbon

tissue (gelatin light—cured surface followed by

ferric chloride etching) methods give variable

depth cells. Direct transfer (photoresist surface

followed by chemically etching with ferric chlo-

ride) results in variable area cells. After engrav-

ing, a chrome surface is applied by electroplating

to give a hard, long—lasting surface that can with-

stand millions of impressions on paper. Ideally,

plates can be reused by removal of the engraving

and application of a new layer of copper.

Steel—die engraving

This method uses a mechanically engraved or

chemically etched copper (for one—time use) or

steel plate. Ink is applied to the plate and doc-

tored off. Slightly moistened paper is pressed

with high pressure against the plate. The result is

a characteristic embossed printed surface and a

slightly indented impression on the backside of the

468 23. PRINTING AND THE GRAPHIC ARTS

paper. This method is capable of very sharp

images and is used for printing currency and

certificates with fine details.

23.5 SCREEN PRINTING

Introduction

Screen printing uses a mask {stencil or

porous plate) to control the location of the ink.

(Perhaps you have used pieces of heavy paper with

the area of a letter cut away and a can of spray

paint to "print" a sign.) Silk screen printing is the

only commonly used stencil printing method used.

It uses a fine—mesh screen (silk, hard plastic, or

stainless steel) mounted to a frame. The screen

supports the prepared stencil (the plate). The

stencil is held against the surface to be printed,

and ink is forced through the stencil (and support-

ing screen) with use of a squeegee. A thick layer

of ink that gives a good impression on virtually

any material is obtained. Mimeographing is the

other common method of stencil printing, although

it is seldom used anymore.

23.6 OTHER PRINTING METHODS

Introduction

Other types of printing methods should be

mentioned because one often encounters them on

a daily basis and they account for an appreciable

use of paper because so many people use them,

although individually they are not high—volume

reproduction methods. The geometry can usually

be compared to one of the four main methods in

most cases. (Some people may not wish to call

these processes printing, but they still consume

lots of paper and make lots of images.) Many of

these methods are used with computers or other

electronic instrumentation to produce the output.

Electrophotocopy, laser printing

The electrophotocopy (photocopy) process

was introduced by the Xerox company in 1960.

An image is produced on a charged drum (a flat

surface, making it a planographic method) by

means of a light source that scans the surface of

the document and, by means of optics, reflects the

light to the charged drum. Dark areas on the

original correspond to dark areas on the reflected

image. The light neutralizes the charge on the

drum where it strikes. The drum then picks up

dry toner particles on the charged surfaces. These

particles are then transferred to the paper and

fused onto the paper surface by heat or other

mechanisms (TIS 108-03 has four pages of defini-

tions relevant to this process).

The original drums used a surface of seleni-

um (Xerography) or cadmium sulfide. Both of

these materials are toxic and most manufacturers

have used organic polymer coatings on the drum

surfaces since the early 1980s.

Laser printers use this same process, except

the light source is a laser that is controlled by a

computer to give the computer—generated output

without an original. Lasers allow a more precise

(higher resolution) image to be formed. Photo-

copy machines are available that include scanners

and image—processing computers and use a laser

to generate the output; these have 600 dots per

inch resolution or higher.

Color copiers or color laser computer printers

use the same process as black and white copiers,

but use four different color toners to produce a

wide range of colors.

Photocopy methods are efficient for some-

where around

5,000

copies or less. The printed

product is less durable, does not fold well due to

toner separation from the paper, and has poor

halftone properties. However, photocopiers are

fast (no intermediate steps from the original to the

product), inexpensive for

small

jobs,

and allow the

operator flexibility with little training.

Inkjet and related printers

Ink—jet printers spray the ink directly

through a series of holes onto the surface of paper

as the printhead scans back and forth across the

paper. Several mechanisms are available to effect

the transfer. Piezoelectric crystals surrounded by

ink vibrate as electric current activates them,

causing a droplet to form and pass through a fine

nozzle; very small electric heaters boil the ink to

transfer it to the paper. Four printheads (or

portions of) are used for color printing.

Dye sublimation printers use a dye from a

page—size ribbon that vaporizes and condenses on

the paper. (Four ribbons are used for color

printing.) This method allows a continuous tone

of ink intensities to be produced; it produces

OTHER PRINTING METHODS 469

high—quality outputs for modest capital cost. It is

the only computer printer (at this time) suitable for

color printing of photographs. Thermal wax

printers use an ink that is a solid wax at room

temperature. The wax is purchased as an inexpen-

sive stick that is easy to load into the printer. The

wax is melted by heating the entire wax delivery

system and applied to the paper. Four waxes are

used for color printing.

Thermal

printers

Thermal printers are generally reserved for

instrumentation. For example, EKG machines and

other "chart recorders," many FAX receivers,

some computer printers, and other devices use this

method. A special ink is in the paper that, when

heated, turns dark (such as black or blue). Print-

ing occurs by a scanning needle or printhead that

heats appropriately as it scans. This method has

the advantage that ink will not clog in equipment

that gets intermittent use so that the reliability is

greatly improved.

23.7 HALFTONE PRINTING

Halftone printing

A particular area of paper is printed either as

black or white by most printing methods. This is

useful for text and line art, artwork characterized

by relatively large areas of black or white, but not

gray. A photograph is the opposite of line art and

is continuous tone with various shades of gray.

However, areas of various shades of gray cannot

be printed directly as gray. In 1886, Frederick

Ives developed tonal reproduction by use of the

halftone screen; it is based on the optical illusion

that areas of very small black dots appear as gray,

since the eye cannot distinguish the individual dots

without magnification. The sizes of the dots are

such that on the order of 50 (newspapers) to 200

(high quality reproduction of photographs) occur

per linear inch. The first edition of

this

book used

150 dots per inch (22,500 dots per square inch).

The screen is aligned so that the rows and columns

are at a 45° angle in the printed product.

Fig. 23-4. CorelDraw shades of gray for 40

dots per inch.

These patterns of dots are prepared with

half—tone screens, special photographic tech-

niques, and other methods. Figure 23-4 shows a

very low resolution halftone that gives continuous

shades of gray. Figure 23-5 shows several tech-

niques to reproduce a photograph; the area shown

is the pupil of an eye. For additional examples it

is recommended that the reader secure a dissecting

microscope and examine several books with

reproduced photographs (for example, 50% tones

are often produced by a checkerboard pattern of

black and white squares). Perhaps some original

photographs can also be examined for contrast.

The overall tone of a halftone is defined as

the relative area that is inked or the overall area

that is inked multiplied by the transmittance of the

printed area. Stochastic printing is a modification

of angled, conventional halftone printing and is

discussed in the next section.

Fig. 23-5. A portion of a printed photograph

that demonstrates halftones (top two) and

stochastic screening (bottom two) with fixed size

variable spaced dots at two resolutions each.

470 23. PRINTING AND THE GRAPHIC ARTS

23.8 COLOR PRINTING

Introduction

Multicolor presses use printing units in

tandem to apply several colors (Fig. 23-6), often

two (a single color and black) or four colors (three

colors and black for "full color" printing). Four-

color printing will be considered here. (For

relatively small runs, a single color press can be

used by a skilled craftsman to produce multicolor

prints by using several passes through the press

with a change of ink between passes. It is also

common

carry out four—color printing with two

passes through a two—color press.)

Most of the colors can be adequately repro-

duced with translucent yellow, magenta, and cyan

inks.

Since these inks do not block (absorb) all

light, black ink is used to give the overall shade to

a particular color. The four process colors are

known as the CYMK color system, where K

indicates black (if it were designated B it may be

confused with blue) or key. These inks produce

various colors by subtractive mixing to obtain red,

blue and green (Plate 41). Color theory is pre-

sented in Chapter 24.

The four process colors have the same names

globally. The shades are uniform in North Ameri-

ca, but a different set of shades is used in Europe,

and other sets are used in other parts of

the

world;

therefore, be careful when printing outside your

own country (Beach, 1993). The 1992 worldwide

color printing market was about $150 billion.

Color

separation

Color separation is the process of making

four halftone negatives for color pictures. Until

the mid—1980s, it was accomplished by taking

consecutive pictures of the colored object with red,

green, and blue filters, respectively. Color is

usually evaluated with a light source corresponding

to blackbody radiation at a temperature of 5000 K.

If a colored object is photographed through a

red lens, red light is transmitted, while blue and

green colors are blocked. Since this photograph is

a negative, it best represents the blue and green

areas of the object. The red negative will be

printed with cyan (a combination of blue and

green).

In an analogous fashion, the green nega-

tive is printed with magenta and the blue with

yellow. Color corrections must usually be ap-

plied, and this is accomplished in any of a variety

of methods. The entire process can now be

accomplished with the use of computer—controlled

scanners, but the method of collecting the informa-

tion does not change the theory of the color

printing process.

While color separation is now always carried

out with scanners, the principle is still the same.

When four—color printing is used, trapping is

often employed in platemaking. This is the pro-

cess of overlapping color ink (on the order of 0.01

in.) to insure faithful color reproduction and paper

choked,

while one that is made bigger is

spread.

Fig. 23-6. A five color offset printing press.

COLOR PRINTING 471

Fig. 23-7. A densitometer

darkness of a film.

measuring the

alignment of the color negatives during color

separation or alignment of the paper between

printings of various colors. Misregister indicates

poor alignment of the colors. Register of color

negatives is accomplished by register pin systems.

Color printing on the press

the degree to which the paper remains in position

between the printing operations for the various

colors. Press manufactures have systems that

maintain register. One method is by the use of

Misregister indicates poor alignment of the

colors. The tolerance may be from 0.005 in. to

0.05 in., depending on the nature of the print,

i.e., a folding box print versus a high—quality

photographic reproduction. Commercial register

is within one dot, while hairline (tight or close)

can often see the color register marks in obscure

areas of packaging papers or boxes. These marks

are printed across the entire width of the press so

that the color balance across the press can be

adjusted to make it uniform.

The darkness of

the

printed areas is measured

by a densitometer (as in Fig. 23-7), and the ink

flow can be adjusted by the ink keys across the

width of the printing press where the ink is me-

tered (much like the slice adjustment on a

headbox). Figure 23-8 shows a densitometer

checking the print density across the width of a

printed page where the numbers correspond to the

ink keys of the ink fountain. Transmission densi-

tometers are used for photographic films, and

reflectance densitometers are used for printed

surfaces. Color filters may be used to evaluate

each of

the

four (or more) colors used in the print

process. Polarized light may be used to mitigate

the effect of gloss on shiny paper or wet ink.

The halftone patterns are usually printed at

30° angles to each other (Plate 42), except yellow,

since it is a light color, which is printed at a 15°

relative to two of the colors, to avoid what are

called moire patterns. A common set of angles is

black, 45°; magenta, 75°; yellow, 90°; and cyan,

105°.

(This is the same as yellow, 0°; cyan,

15°;

black, 45°; and magenta, 75°.) Figure 23-9

demonstrates moire patterns when two dot grids

are offset by 4° (left) and the lack of these pat-

terns when they are offset by 30°.

The first ink alters the surface properties of

paper, so succeeding inks in multicolor printing

usually have lower tack and/or surface tension in

order to achieve uniform results. The usual order

for color printing is yellow, magenta, cyan, and

black. The yellow, magenta, and cyan inks are

usually transparent to show the color of inks

underneath, though opaque inks are occasionally

used for specialized purposes. Paper color, of

course, strongly affects the product color.

Stochastic screening

The traditional method of color printing uses

a grid of halftone dots for each color (angled to

avoid moire patterns). A new method of color

printing has the generic term stochastic printing

(Fig. 23-5). It is also called frequency modulation

or random dot. Two systems are possible: fixed

dot sizes with variable dot spacing or variable dot

sizes with variable dot spacing. This system is

now possible because computers can make the

calculations required to set up the dot patterns.

Agfa uses the trademark CristalRaster. It has

a different structure for gain than conventional

halftone dot printing. Another trade name is

Diamond Screen of Linotype—Hell.

472 23. PRINTING AND THE GRAPHIC ARTS

S:.f ^H "^.'"T^S densitometer on a press console that is used to check the print intensity

Other aspects of color printing

It is not uncommon to have a five or six

color press. The extra press may be used for a

variety of purposes. A protecting topcoat (perhaps

containing a UV absorbing material to limit color

fading) may be used in one press. A special color

of ink may be used that is impossible to achieve

by mixing the primaries, such as vibrant red

colors. If this color is used only in certain parts

of the print without mixing with other colors, it is

•••J

»•#•••

Fig. 23-9. Two dot grids offset by 4» Oeft) showing the moire pattern and offset by 30» (rieht)

showmg a more uniform dot distribution. u ""sei oy jo (right)

COLOR PRINTING 473

termed a spot color or a flat color. When flat

colors are combined with other colors, one is said

to be building colors.

A conmion name and brand of spot color is

Pantone Matching System or preferably Pantone

Color. Other manufacturers of spot colors are

Focoltone and Trumatch. Swatch books contain-

ing samples of colors identify the color by number

(not name), the industry practice.

Eight color presses are being used increasing-

ly to produce a wider color range; this is some-

times called hi—fi (high—fidelity) color. When

used with other printing techniques such as

stochastic printing or variable dot size, photo-

graph—like quality can be achieved and certain

effects not available in photography are possible.

The use of eight colors on each side of a page puts

extra demands on the paper substrate.

23.9 COMMON ASPECTS OF PRBVTEVG

There are some other important consider-

ations in printing. Dot gain represents the in-

crease in dot size from the plate to the printed

image. For example, a dot that is 50 /xm on the

plate may print as 75 /xm on the paper. Dot gain

is a function of the size of the dot to be printed

and often is greatest in the midtones; this function

is called a dot

gain

curve. Dot

distortion

refers to

a round dot being printed unevenly. These are a

function of the amount of ink used to print, paper

characteristics, and other factors.

Other terms

perfecting press prints on both sides of the

sheet with one pass through the printing press.

Printing of individual sheets (sheet^rfed press)

used to be quite common, but the use of a continu-

ous web

(roll—fed

press) is much faster and has

replaced individual sheet printing in many applica-

tions.

The paper is then sheeted and folded at the

end of the press.

23.10 INK AND INK APPLICATION

Introduction

Early inks were all pretty similar and consist-

ed primarily of boiled linseed oil (from flax) and

carbon black (soot or lampblack). Some mile-

stones are as follow: in the middle 1700s, ink

becomes available in more colors than just black;

1850s, coal tar dyes developed; 1920s, introduc-

tion of synthetic resins; 1978, acrylic resins used

in inks for coated papers.

In 1990 the U.S. printing ink industry had

about $3 billion in sales. Gravure inks are pro-

duced in the highest mass, but lithographic inks

are produced with the highest sales (value). About

200 companies produce ink at nearly 500 plants.

Inks cost about $5 to $40 per pound.

Ink components

Ink consists of pigments or dyes, a vehicle

(binder) to attach the pigment to the paper, and

other components. Other components include

solvents for faster drying and ink additives.

Ink pigments

Ink consists of pigments, such as carbon

black or titanium dioxide, to supply color and

opacity and a vehicle to carry the pigment and

bind it to the paper. Pigments should be insoluble

in water and the ink formulation to prevent run-

ning. Some pigments are inorganic materials such

as alumina, chrome yellow, or iron blues. Many

pigments are insoluble variants of organic dyes.

Heat transfer pigments are solids that sublime

upon heating (to fabrics).

Ink

additives

Ink additives have numerous functions; some

of the more common ones are considered here.

Ink driers are often heavy metal salts of

(unsaturated fatty acid) soaps that promote oxida-

tion and/or polymerization. The metals include

cobalt, manganese, cerium, and others (these are

often used as free radical initiators in polymer

chemistry). Wax makes ink more slippery, in-

creases abrasion resistance, decreases

setoff,

and

decreases tackiness with minimal effect on other

properties. Small amounts of antioxidants prevent

ink from drying in bulk but allow it to dry on the

substrate.

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vehicles and ink drying or setting

The type of resin depends on the type of ink

and how it attaches to the paper. Inks may "set"

on the paper by one of several mechanisms.

Glossy inks use high molecular weight resins that

decrease the absorption of ink into the paper.

Absorption of the hydrocarbon vehicle into

the paper substrate occurs by capillary action.