Biermann Ch. Handbook of Pulping and Papermaking

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484 23. PRINTING AND THE GRAPHIC ARTS

perfect

lie

flat

perfect

burst

perfect

case

side stitch

saddle stitch

screw and

post

plastic grip

comb spiral double-loop

wire

y ^i^a

Fig. 23-15. Common methods of binding. From Getting It

Printed,

revised edition ®1986,1993 by

Mark Beach. Used by permission of North Light Books.

International Paper Co., Pocket Pal, A

Graphic Arts

Production

Handbook,

13th ed.,

International Paper, New York, 1983 (216 p.

with several color plates). The first edition

was published in 1934. It is called pocket

pal due to its 4.2 by 7.2 in. size. This is a

concise, easy to read book on the topic.

Kline, J.E., Paper and

Paperboard,

Manu-

facturing and

Converting

Fundamentals, 2nd

ANNOTATED BffiLIOGRAPHY 485

Square back

Text

Endsheet

Cover

Board

Open

Detatched spine

Headband

Round back

Text

Cover

Open I Headband

Detatched spine

Board

Fig. 23-16. Case binding. ®1994 Lincoln and Allen Bindery, Portland, Oregon (with permission).

ed., Miller Freeman Pub. Inc., San Francis-

co,

1991. One chapter (pp. 162-183) con-

tains information on printing and printing

grades of paper.

U.S.

Dept. of Commerce, Statistical Abstract

of the

United

States, 112th ed., 1992. There

is a section on the printing industry.

Williams, R.L., Paper & Ink Relationships,

Mennonite Press, Inc., Newton, Kansas,

1985,

150 p. a) p. 44. This is an easy to

read, perhaps more practical reference on

printing methods than the ones listed above;

it is a useful addition to one's library.

Tappi Technical Information Sheets

Information sheets exist for the properties of

aqueous slurries of various pigments and fillers

including calcium carbonate (TIS 106-01), hydrous

clay (106-02), and anatase (106-03). Pigment

indexes for these materials are 106-05 to 106-07,

respectively. Graphic art terms (somewhat dated

in terms of twin-wire formers since the last ver-

sion is 1964) are defined in

108-01.

TIS 0305- (14-19, 38) and 0306-01 to 0306-

09 contain methods for printing corrugated con-

tainers. These, like many of the TIS sheets for

corrugated containers, are fairly generic (of

widespread use) in their treatment of the subject

matter. Examples include 0305-16 (Flexographic

486 23. PRINTING

AND THE

GRAPHIC ARTS

ink),

0305-38 (Printing section, troubleshooting),

0306-05 (Glossary of flexographic terms), and

0306-03 (Photopolymer plate update).

EXERCISES

Describe the geometries of the four major

types of printing processes.

What is offset printing? To what process

does this usually refer?

What is halftone printing? Why is it used?

What is stochastic printing? Give two major

advantages of this process over traditional

screen printing of halftones. One advantage

is especially applicable to color printing.

What is four—color process printing? What

is its theoretical basis?

6. What is an important measurement for quali-

ty control on a four—color printing press?

What printing process requires a paper sur-

face that is very resistant to surface picking?

APFElSfDIX

Anilox, ink

transfer rolls

The ink is applied to the plate by a series of ink

rollers. Traditionally, soft roll applicators were used,

but many flexographic applications now use metal

rolls having a mechanically engraved surface. These

are termed

anilox

rolls,

which were developed in the

1930s (the term is related to the fact that flexography

was initially called aniline printing, since aniline-

based coal—tar dyes were used as inks). The en-

graved surface allows metering of the ink (which is

otherwise difficult to control in

flexography)

and can

carry more ink that flat—surfaced rolls. The en-

165-Pyramid cell. 42 um high. 300.000 um ink/cell

i40-Quadragravure cell, 64 um cell depth

Fig. 23-17. Engraving of anilox rolls.

graved surface is chemically and physically protected

by chrome plating or ceramic (AI2O3) spray. A

ceramic—coated roll may be engraved with a laser.

TIS 0305-17 (1982) states that for flexographic

printing on corrugated boxes, the industry standard

for mechanical surface engraving of anilox rolls is the

165—pyramid cell (42 fim deep) when the ink-

fountain contains a doctor blade or the 140—

quadragravure cell (64 /xm deep) when presses use a

doctor blade wiper (Fig. 23-17). (Cells are listed as

number per linear inch.) Laser engraved ceram-

ic—covered rolls can be engraved with much finer

patters (250 is common, 550 is specialty grade) to

give better printing resolution TIS 0306-1 (from

1992).

Engraving angles are typically 45° for corru-

gated box applications, though other angles are

available for more viscous ink. Since most of

the

ink

is carried at the surface of the roll, it is very impor-

tant to maintain ink capacity by decreasing wear.

This is done by running the system with ink (a

lubricant) and keeping the rolls clean.

Various types and sizes

of fonts (Beach, 1993)

Point sizes vary

ft-om

6 to 72 and refer to the

height of the line of type in 1/72 in. increments (in

North America and the United Kingdom) from the

bottom of

letter witii a

descender

(like y or p) to the

top of a letter like T or W; the common 10 point

type with 2 point spacing is 6 lines per inch.

pica

is 12 points, and 6 picas make an inch.

Fonts of

the

serif

type

have bases on letters such

as A, F, H, and f. Examples include Times Roman

and Garamond Antiqua, which are used for use of

text and captions. Since

sans

means without, sans

serif fonts are solid and include Antique Ollve

and Arial. These fonts are used in headlines,

posters, callouts, and a short index.

This book is set in CG times proportional

spaced font. Some examples of other fonts are

presented here. This is fixed spaced

courier, like a typewriter. Notice

the width of the i and w are the

same. This is the same typeface as

italics and bold italics. This is

letter gothic. and with Italics. This is

new times roman.

Gamma function

A gamma function is a function used to alter

the shades of darkness of a printed surface. For

example, it is used to enhance darkness (since the

eye is more sensitive to dark areas) so that the eye

can more easily recognize images.

OPTICAL PROPERTIES OF PAPER

24.1 INTRODUCTION

Some aspects of optics and dyes have been

included in Chapter 8 on additives. The optical

properties of many additives are of central impor-

tance to their use. Optical properties of paper

have been briefly discussed on pages 185 and 186.

Electromagnetic waves and light

Some of the basic properties of hght will be

presented in this chapter. The theoretical devel-

opment is fairly complicated, but is of little use to

papermakers. Most introductory physics textbooks

at the college level cover these topics in various

levels of mathematical detail.

Light is an electromagnetic wave and has

properties of waves and particles. It consists of

individual photons that have properties of waves.

The properties and applications of electromagnetic

waves depend on their frequency (hertz, Hz,

"cycles per second") or wavelength. Both units

have common uses depending on the area in which

they are used; for example, light is often reported

in wavelengths while radio waves are reported as

either depending on the band. Frequency and

wavelength are related to the speed of light, c,

(which is 2.9979246 x 10*m-s-^ in a vacuum) as:

c=/X

(24-1)

EXAMPLE 1. Shortwave radio bands are often

described in terms of their wavelengths.

What is the approximate frequency of the 40

meter band?

SOLUTION: Rearrangement of Eq. 24-1 gives

/= 3.00 X 10«m-s-V40m = 7.50 MHz

PROBLEM: The wavelength of light used in the

TAPPI standard brightness test is 457 nm.

What is its frequency?

Answer: 6.56 X 10^^ Hz.

Electromagnetic waves

(in

increasing order of

energy and frequency) include radio waves.

microwaves (used in satellite dish conmiunications

and microwave ovens), infrared light, visible light,

ultraviolet light, x—rays, and gamma rays. Table

24-1 shows the electromagnetic spectrum with

landmarks inserted in nearly correct positions.

The energy of a light particle, E, is usually

given as a ftmction of frequency, /, and Planck's

constant, h, as:

= hf (24-2)

Planck's constant has the value of 6.62 x 10'^

J-s.

Since photons are involved in chemical

reactions, such as color reversion in mechanical

pulps,

it is useftil to consider the energy of a mole

of light particles. The energy of light at 400 nm

is approximately 300 kJ/mol (72 kcal/mol), which

is below that of most chemical bonds (see page

315).

UV light at 200 nm has 600 kJ/mol, which

is sufficient to break many chemical bonds.

Table 24-1. The electromagnetic spectrum.

Wavelength Landmarks

higher energy!

0.01 nm

0.1 nm

1 nm

10 nm

100 nm

1 iim

10 /xm

100

fim

1 mm

10 mm

100 mm

1 m

10 m

100 m

1 km

10 km

100 km

- gamma ray; cosmic ray

- X-ray; gamma ray

- X-ray

- far UV; X-ray

- near UV

- visible light 400-700 nm

- near infrared light

- far infrared light

- radar, microwave

- radar, UHF TV

- TV Chan. 7-13

- FM 88-108 MHz

- shortwave radio

- AM 550-1600 kHz

- long radio waves; VLF

lower energy 1

Frequency

(Hz)

- 3x10^9

- 3x10^^

- 3xlO^«

- 3x10^^

- 3x10^5

- 3x10^^

- 3x10^3

- 3x10^2

- 3x10^1

- 3x10^0

- 3x10^

- 3x10'

- 3x10^

487

488 24. OPTICAL PROPERTffiS OF PAPER

The overall relative sensitivity to light by the

human eye (i.e., the standard luminosity function

of the standard observer as will be given in jT of

Fig. 24-10) is about 20% at 470 nm, 40% at 490

nm, 60% at 500 nm, 80% at 510 nm, 100% at

550 nm, 80% at 590 nm, 60% at 610 nm, 40% at

630 nm, and 20% at 650 nm.

However, the brain applies compensation in

that the appearance of brightness is a logarithmic

function of the light level (as is the perception of

sound level by our ears). For example, a surface

that reflects 10% of its light contrasted to one that

reflects 20% has an equal apparent brightness

difference when compared (at the same time to

avoid dilation of the pupil as a variable) to the

difference in a surface that reflects 20% of

its

light

contrasted to one that reflects 40%.

Index of

refraction,

reflection, and absorption

The speed of light in matter is always smaller

than that in a vacuum. The ratio of the speed of

light in a vacuum to that in a given material is

known as its index of

refraction,

n. The index of

refraction of a material is usually a function of the

wavelength of light. Values in the literature for

various materials correspond to n determined with

the yellow light of a sodium light (589 nm) near

the center of the visible spectrum.

In practice the index of refraction is impor-

tant not because the speed of light in a material is

of consequence, but because light "bends" (unless

it is normal to the surface) when it travels across

\^ a

Incident ray \

/ Reflected ray

Refracted ray

an interface of two materials of differing indexes

of refraction. This is what causes the image over

a hot road or heater to appear to ripple; differenc-

es in the density of air cause differences in its

index of refraction. The index of refraction of air

is about

1.0003

and is usually treated as unity.

24.2 BEHAVIOR OF LIGHT RAYS

If a beam of light traveling in a vacuum (air

is close enough) strikes a flat surface of a trans-

parent material at an angle of incidence

(f)^

relative

to normal (Fig. 24-1), some of the light will be

reflected at angle

4>,

and some of the light will be

transmitted to the object at angle 0b. It is known

that the angle of reflection from normal is equal to

the angle of the incident light from normal (0^ =

0r).

It is also known that the sin ^J^m

01,

the transparent material. The incident, reflected,

and refracted rays and the normal line are all in

the same plane. If the refracted light were to

travel the same path backward the same relation-

ship would exist, that is sin ^^sin

(t>^

= n.

Refraction means that when light travels from

a material of low index of refraction to a material

of higher index of refraction, the light is "bent"

toward the normal. The converse is also true.

Fig. 24-1 can be generalized for any two materials

as «a sin 0^ = ^b sin 0^.

With total internal reflection, it is possible

that a light ray will be reflected back into a mate-

rial when striking at a high angle. It is this

phenomenon that keeps light inside glass fiber

cables so that it can be transmitted large distances

with relatively little loss of intensity.

Fig. 24-1. Reflection and refraction of light.

prism

Light source

Fig. 24-2. Use of a prism and slit to produce

monochromatic light.

BEHAVIOR OF LIGHT RAYS 489

Dispersion uses the fact that the index of

refraction of a material is a function of the wave-

length of the light ray. The index of refraction of

a material is often higher for blue light than for

red light. White light can be separated into its

component colors by the use of a prism as demon-

strated in Fig. 24-2 or as observed in a rainbow.

Light may also be absorbed as it is transmit-

ted through a material (see Section 14.6). Reflec-

tion, refraction, and absorption of light are all

very important in determining and predicting the

optical properties of paper, as we will see.

Absorption, transmittance, and

reflection

in matter

Of course the interaction of light with paper

and other materials is much more complex than

the simple example just considered. In fact, the

surfaces of

the

particles are in random orientations

and a beam of light is usually scattered numerous

times (multiple scattering), as shown in Fig. 24-3,

and the scattered light goes in haphazard directions

in all three dimensions. We refer to diffuse

reflectance (as opposed to specular reflection as in

a mirror and Fig. 24-1) and transmittance.

It is interesting to consider some overall

generalizations. Imagine a sheet of paper or

powder that is infinitely thick in terms of light

penetration (> 1 cm). If no light is absorbed,

then all light would be scattered out the top sur-

face;

the light will continue to scatter indefinitely

until it reaches the top surface where nothing can

send it back into the sheet. (See the average

end—to—end distance of a polymer for the reason-

ing on page 437.) Reference standards such as

MgO and BaS04 have low absorption coefficients

with high scattering coefficients and, therefore, are

Diffuse

reflectance

Diffuse

transmittance

Fig. 24-3. Diffuse reflection and transmission.

nearly perfectly white (because they reflect nearly

all visible light that shines on them).

Black paper has very high absorbance across

the spectrum and so little light is reflected or

transmitted. Gray paper has some absorbance

which is approximately constant across the visible

light spectrum. Gray or white is said to be achro-

matic (without color) and has a flat spectral re-

flectance curve.

Glassine paper scatters light very little and

has relatively low absorbance so that most light is

transmitted through it, unless it is very thick. An

extreme example of this case is a clear, plastic,

solid, film, such as cellophane (cellulose), which

behaves as in Fig. 24-1, mostly transmitting light

through the material with essentially no scattering.

Fillers and scattering coefficients

It is known that the maximum amount of

scattering results when the minimum particle

dimension is about one—half

the

wavelength of the

light being scattered. This corresponds to about

0.25 jLtm, which is a common size for certain

fillers.

While some fillers have smaller particle

dimensions than this, they often tend to agglomer-

ate into particles of larger effective size. More

information is contained in the chapters on colloid

and wet end chemistry.

24.3 COLOR

Color

Visible light consists of electromagnetic

radiation from about 380 nm to 700 nm, though

some people may be able to detect light below 300

nm and above 800 nm. Monochromatic light

ideally consists of photons all having exactly the

same wavelength. Lasers approach this very

closely and give pure colors within 0.1 nm.

High—quality color filters typically have 90% of

the light within about ±25 nm. The colors (the

order is ROY G. BIV) one perceives have the

following approximate wavelengths in vacuum:

violet

indigo

blue

green

yellow

orange

red

380—430 nm

430-450 nm

450—500 nm

500-560 nm

560-590 nm

590-630 nm

630-720 nm

490 24. OPTICAL PROPERTIES OF PAPER

Human response to color, three—color

theory

The light—sensing components of the human

eye consist of rods and cones, named on the basis

of their shapes. Rods respond to all light (with an

absorption peak at 500 nm) and confer black and

white vision, the primary source of visual informa-

tion. Because rods are much more sensitive to

light than cones, night vision is based largely on

rods,

but does not allow for color perception.

The three—color theory of light perception

was proposed by the English physician Thomas

Young in 1801. The German physicist H.L.F.

Helmholtz published his hypotheses in 1852 and

1856 regarding the three channel nature of color

vision. He hypothesized that three types of sen-

sors in the eye have their own spectral response

curves and send signals to the brain when trig-

gered. The brain takes the three signals for a

given area and converts them into a single color.

All combinations of light consist of various levels

of response from these three types of sensors in

the eye. Furthermore, it should be possible to

define color in terms of the equivalent amount of

the three monochromatic components of red, blue,

and green. We now know these three channels

correspond to the three types of cones in the eye.

Three types of cones differ in their photo-

receptive pigments. (Light must be absorbed in

order to cause the chemical reactions that lead to

its perception.) Each of these pigments has a

different absorption spectrum (absorption spectra

will be described below and are demonstrated in

Fig. 24-8 for various color papers) as a function

of wavelength (color).

Fig. 24-4 shows the approximate relative

response of the three types of cones (see Levine,

1985,

page 338 or Judd and Wyszecki, 1975, page

84).

The three cones have absorption spectra with

peaks at about 440 nm (perceived as indigo), 535

nm (perceived as green), and 565 nm (perceived

as green—yellow) and are distinguished as the

short—, medium—, and long—wave (5, M, andL,

referring to wavelength of light) absorbing cones,

respectively (Levine, 1985). There is a high

degree of overlap in the absorption spectra of the

latter two pigments. It is important to note that

the long—wave type cones are best stimulated

(with a minimum level of stimulation of the medi-

um wave absorbing cones) by using red light. All

shades of color correspond to the relative intensi-

ties of these three types of cones. Conversely, it

should be possible to simulate any color by appro-

priately stimulating the three types of cones (by

using three suitable colors). Much of color theory

is logical if one understands these concepts.

When one speaks of

color,

one is referring to

the way the eye responds to the light spectra that

are emitted from objects. Hunter and Harold

(1987) point out three visual conditions: the

illuminant mode, where the stimulus one sees is

the source of light

itself,

such as a light bulb; the

object mode, where one see the stimulus as being

illuminated by a light source; and the aperture

mode, where one sees the stimulus simply such as

light, as light coming through a door that is

opened just a crack.

(It is estimated that most people can distin-

guish well over 100,000 different shades of color.)

Therefore, it is relevant to consider how the

human eye works. With this background, color

measurement methodologies become logical.

Additive colors

A practical demonstration of the

three—component nature of color lies in CRT

S-cones

M-cones

L-cones

400 700

500 600

Wavelength, nm

Fig. 24-4. The spectral response of the three

cones of the human eye.

COLOR 491

(computer or television) screens that have separate

red,

blue, and green light sources that are reason-

ably monochromatic. If each of the three light

sources selectively stimulated one type of cone,

then all possible colors that are perceived could be

achieved by adding the light outputs in various

proportions. In practice, the system is somewhat

limited in response due to the overlap in response

of the three types of cones. Nevertheless, blue,

green, and red are called the

additive

primaries.

Red, blue, and green are the primary colors.

Black (what do you see in a "pitch black" room?)

is the absence of any color (light). As shown in

Fig. 24-5 and depicted in Plate 43 (although the

shades are not quite correct due to color reproduc-

tion in the printing process), equal parts of green

and red light added together give yellow; equal

parts of red and blue give magenta; and equal

parts of green and blue give cyan. Approximately

equal parts of red, blue, and green give white

light. A complementary color of a color is the

color that is added such that the overall product is

white. From the argument above yellow and blue

equal white, so blue is the complementary color of

yellow (and the converse is also true).

Suhtractive mixing

Dyes

and pigments behave

differently because

they absorb light that is not part of the given

color. This is called

suhtractive

mixing.

Color printing uses yellow, magenta, and

cyan inks to produce various colors by suhtractive

mixing to give red, blue, and green. (Each color

ink should be thought of

transmitting two of the

additive primaries, which is the same as absorbing

one of the additive primaries.) These inks are

transparent so where they overlap they each

subtract one of the main additive primary colors.

As Fig. 24-6 shows, yellow ink transmits red and

green; it is yellow by preferentially absorbing

blue light (leaving the green

and

red components).

Cyan ink selectively absorbs red light. Where

cyan

and

yellow are printed together, blue and red

are selectively absorbed to leave green light to be

reflected. Thus the suhtractive mixing of

cyan

and

yellow ink gives the appearance of green. By

using the inks in various combination, most shades

of color can be achieved.

Suhtractive mixing of these colors is shown

Fig.

24-5 and is depicted in Plate 41 (again with

poor color reproduction due to printing limita-

tions).

Imagine the three color filters in front of

a white light source where these filters partially

overlap. Theoretically, where the three filters

overlap, all light is absorbed. This diagram only

shows mixing of equal parts; the entire rainbow

is more or less available by mixing (printing

together) various proportions of each dye.

Mixing green and red dyes or pigments gives

brown or a mud color because most light is ab-

Additive colors Suhtractive colors (for printing)

Fig. 24-5. Additive mixture of light and suhtractive mixing of inks used in printing.

492 24. OPTICAL PROPERTffiS OF PAPER

Wavelength, nm

Fig. 24-6. Reflectance of the three process colors and green produced by prmting cyan and yellow

together. (Using printed color from Pocket Pal, 1983, page 94.)

sorbed and little light is reflected. (Brown is the

perceived color of dark yellow or dark orange.)

A similar example is the use of two filters of

different colors (in series or partially overlapping)

between a source of white light and the observer.

Paper whiteness

Since yellow light and blue light give white

light, it is arguable that white light minus blue

light should equal yellow light. Thus, a material

that absorbs blue light leaves yellow light. Paper

that is somewhat yellow appears to be dingy

(drab).

That is why we measure paper brightness

in the blue region—to insure that we are not taking

any of the blue light away. (If blue is being

selectively removed, we add a blue dye which

selectively removes all colors except blue. This

balances the reflectance spectrum of the paper.)

On the other hand, paper that we want to be

yellow will absorb blue light from the spectrum,

but very little of other colors.

Laboratory measurement of

color

Spectrophotometers, A reliable way to

measure color involves the recording spectropho-

tometer. Traditionally spectrophotometers consist

of a light source that [via a prism (Fig. 24-2) or

diffraction grating] produces reasonably mono-

chromatic light (the bandwidth is often as low as

2-4 nm at any given point) as it scans across the

visible spectrum and measures the intensity of that

light as it reflects off the surface of paper (or

other object) for each point in the spectrum rela-

tive to the level of lighting which is measured by

a reference photocell. Any type or model of

spectrophotometer should give the same result for

a given sample.

In the so—called reverse spectrophotometer,

one shines a white light on paper and then applies

a diffraction grating (or filters) to analyze the rela-

tive reflectance across the visible spectrum.

Differences may result in these two methods if

fluorescent dyes or other agents are used or are

present (most discussions of color in this chapter

COLOR 493

100%-

90%H

80%H

70%H

0%-

Kraft Linerboard

400

450

—I—

500

550

600

650

700 750

800

Wavelength, nm

Fig. 24-7. Spectral responses of several types of undyed papers relative to a white standard.

ignore this possibility). The reverse spectropho-

tometer was used to generate the curves shown in

Fig. 24-6 and Fig. 24-7 for various papers. The

small shoulder in the blue region of the 25% rag

bond curve in Fig. 24-7 (which could theoretically

go above 100% with a suitable light source) is

most likely due to fluorescent brighteners. If a

conventional spectrophotometer is used, one would

expect this shoulder to disappear.

This latter point demonstrates how the source

of the light affects light reflected from an object.

Another example comes from photography. When

film designed for outdoor use is used indoors

under incandescent (tungsten) light without correc-

tive filters, the pictures appear to be yellowish

since tungsten light bulbs emit much more light in

the yellow region of the spectrum than the blue

region, while sunlight is fairly uniform in intensity

across the visible light spectrum.

(One's eye automatically compensates for the

source of light, which is why the yellowishness of

incandescent lights is not as noticeable to the eye

as to the camera. Color

constancy

is the ability of

the eye (and brain) to see a color as (essentially)

constant, even when the light source changes

considerably. However, this also makes the

human eye an unreliable judge in some cases. For

example, gray against a green background appears

slightly reddish, while gray against a red back-

ground appears slightly greenish.)

In summary, color perception depends on

the reflectance curve and fluorescence of an

object, the nature of the Ught shining on the

object, and the spectral response of the human

eye.

A consequence of this statement is that two

objects may appear to be the same color under one

set of lighting conditions, but diflerent colors

under a second set of lighting conditions; this

phenomenon is called metamerism. Metamerism

is avoided (or at least determined in the laboratory

before it becomes a problem) by viewing the two

objects under two light sources of widely differing

characteristics. [Metamerism could not happen to

two objects with the same spectral reflectance

curve; a color will always appear the same as

itself under the same light source. It occurs when