Biermann Ch. Handbook of Pulping and Papermaking

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154 ^. REFINING AND PULP CHARACTERIZATION

Fig. 6-21. Britt jar test.

having high tear resistance. The authors cite the

work of Runkel who says that if the thickness of

the cell wall is less than the radius of the lumen of

fibers in wood, the wood makes a good paper.

The relative thickness of cell walls is reflected in

the fiber coarseness (mg/100 m of fiber). These

workers minimized the importance of the fiber

length/diameter ratio that many workers feel is

CCA.-26 CCA:20

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230 T- 230

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Viscosity Viscosity Viscosity Viscosity Viscosity

OP 03%eenc llconc. IXcone. kteoctoMn In CEO

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1300-1

1200-

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1000-

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300-

400-

2000-1

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1400-

1200-

1000-

000-

200O-

1000-

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1400-

1200-

1000-

800-

000-

30-

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30-

20-

10-

12-

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300-

400-

300-

200-

100-

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Fig. 6-22. Conversion nomograph for pulp

al, (1963) with permission.

more important than the cell wall thickness;

however, all of these fiber properties are interre-

lated, and no single variable means much alone.

Many studies (for example, Kellogg and

Gonzalez, 1976) show that mature boles, i.e.,

outer growth rings that are 10-50 rings from the

center, have fibers with the longest length, the

largest radial and tangential diameters, and the

smallest fibril angles. Once again, these facts

indicate that short rotation, plantation grown wood

may have decreased strength properties.

6.5 ANNOTATED BIBLIOGRAPHY

General aspects of refining (Also see references

14,

15 for the effect of pH on refining)

Clark, J. d'A., Pulp Technology and Treat-

ment for Paper, Miller Freeman Pub. Co.,

San Francisco, 1978, 751 p. (The 2nd ed.

1985,

878 p., is updated for some of the

introductory, basic material, but is otherwise

very similar).

Numerous chapters in this book relate to

refining including Chapter 8, Fibrillation

and fiber bonding, pp 160-180, not a look at

the refining process, but a fundamental look

at fibrillation of fibers induced by various

treatments and how this promotes fiber bond-

ing; Chapter 7, Bonding of cellulose surfac-

(/)

li.

(/)

\ 1

/KRAFT /

/ /SULFITE

1 1 1 1

800 700

600

500

CSF

400 300 200

Fig. 6-23. Specific surface area versus CSF for

laboratory pulp. After Robertson and Mason

(1949).

ANNOTATED BIBLIOGRAPHY 155

es,

pp 145-159 and early refiners, pp 205-

215 are of related interest; Chapter 12,

Nature and effects of beating, pp 257-280;

Chapter 13, Mill beating and refining, pp

281-316; Chapter 14, Laboratory beating, pp

317-362;

and Chapter 25, Control of beating

and refining, pp 560-568.

Many chapters from the first edition relate to

pulp and fiber quality and testing. Chapter

9, Properties of pulps, pp 181-200; Chapter

11,

Moisture content of bales, pp 230-256;

Chapter 15, Test sheet making, pp 363-379;

Chapter 16, Hand sheet testing, pp 380-400;

Chapter 17, Fiber length, pp 402-437; Chap-

ter 18,

fiber

coarseness,

438-449; Chapter

19,

Intrinsic fiber strength, pp 450-464;

Chapter 20, Cohesiveness, pp 465-487;

Chapter 21, Wet fiber compactability, pp

488-503;

Chapter

22,

Drainage of

water

from

pulp,

pp 504-532, Chapter 23, Surface mea-

surements,

533-540;

Chapter

24,

Wet-web

strength, pp 541-559; Chapter 27, Character-

ization and control of mechanical pulps, pp

579-602;

Chapter 28, Chemical and micro-

scopical analysis, pp 603-613; Chapter 29,

Optical characteristics, dirt, and shives, pp

614-638; and Chapter 32, Formulas for pulp

properties, pp 680-698.

Clark describes the properties of fibers (dis-

cussed in chapters 17-21) for papermaking in

terms of five of their principal properties:

the length-average fiber length, the coarse-

ness (mg per 100 m of fiber, measured by

Tappi Standard T 234, with coarse fibers

giving lower strength

due to

poorer bonding),

wet compactability (measured by the apparent

sheet density, for example, TAPPI Standards

T 205 and T 220), the intrinsic fiber

strength, and the fiber cohesiveness.

Calderon, P., P.E. Sharpe, and J.L.

Rodarmel, Low consistency refiner plate

design and selection. Spectrum (Sprout-

Bauer), Summer, 1987, 7 p.

Root, E.M., Stock preparation, in Pulp and

Paper Science and Technology, Vol. 2,

Libby, C.E., Ed. McGraw-Hill, New York,

1962,

pp 1-39. A lot of useful information

on early refining methods such as beaters and

the Jordan refiner. This also has a good

overview

on the

physical properties of fillers.

Spencer, H.S., N.G.M. Tuck, and R.W.

Gordon, Beating and refining, in Pulp and

Paper

Manufacture,

Vol 5, MacDonald, R.

G., Ed., McGraw-Hill, New York, 1969, pp

131-185. Scanning electron micrographs,

fundamental aspects of refining, equipment,

and refiner plates are included in this article.

Pulp

surface

area

5. Casey, J.P., Ed., Pulp and Paper

Chemistry

and Chemical

Technology,

2nd ed. (1960),

volume 2, p 621 has a discussion on the

surface area of

pulps.

The more sophisticat-

ed types of studies mentioned use silver

deposition or gas adsorption to measure the

total available surface area of pulps. These

studies give reasonably close values to each

other (within a factor of two) for surface

area. Fig. 6-23 is an approximation of the

results of Robertson and Mason (1949) cited

in this work.

6. Robertson, A.A. and S.G. Mason, Specific

surface area of cellulose fibres by the liquid

penetration method. Pulp Paper Mag. Can,

50(13):

103-110(1949).

Fiber morphology and performance

7. Dadswell, H.E. and A.J. Watson, Influence

of the morphology of woodpulp fibres on

paper properties. Transactions of the "For-

mation and Structure of

Paper"

Symposium,

25-29 Sept., 1961, pp 537-572 (1962). (See

also Appita

14(5):

168-176(1961) and ibid.

17(6):

146-156(1964).)

8. R.B. Brown, Paper Trade J. 95(13):27-

29(1932).

156 <5- REFINING AND PULP CHARACTERIZATION

9. Kellogg, R.M., and J.S. Gonzalez, Relation-

ship between anatomical and sheet properties

in western hemlock kraft pulps. Part I. Ana-

tomical relationships. Transactions Tech,

Soc, 2(3):69-72(1976).

10.

Smith, W.E. and Von L. Byrd, Fiber bond-

ing and tensile stress-strain properties of

earlywood and latewood handsheets, USDA,

For. Res. Ser. Res. Pap. FPL 193(1972), 9

11.

Horn, R. A., Morphology of wood pulp fiber

from softwoods and influence on paper

strength, USDA, For. Res. Ser. Res. Pap.

FPL 242(1974), 11 p. This study concluded

that the fibril angle of fibers was the most

important factor in

stretch properties

of paper

made from unrefined, softwood pulp and an

appreciable factor in stretch properties of

paper made from refined, softwood pulp.

12.

Dinwoodie, J.M., The relationship between

fiber morphology and paper properties: A

review of the literature,

Tappi

J, 48(8):440-

447(1965). This includes 115 references.

13.

Megraw, R.A., Wood Quality Factors in

Loblolly

Pine,

Tappi Press, 1985, 88 p. This

book is subtitled "The influence of tree age,

position in tree, and cultural practice on

wood specific gravity, fiber length, and fibril

angle".

Fiber

chemical

properties

and performance

14.

Scallan, A.M., The effect of acidic groups

on the swelling of

pulps:

a review,

Tappi

66(11):73-75(1983). This has 31 references.

Acidic groups of

pulp,

i.e., carboxylic acids

from hemicelluloses and possibly sulfonate

groups in sulfite-pretreated mechanical pulps

or unbleached or semi-bleached sulfite chem-

ical pulps, contribute to fiber swelling in

water. Fiber swelling contributes to ease of

refining and development of paper strength.

At pH below 7 the carboxylate groups are

protonated and no longer dissociated, de-

creasing fiber swelling. At higher pH, the

metal counter ion plays a big role in fiber

swelling with M+ > M^"^ > M^+. Also, it

is recognized that swelling increases with the

series H+ < Ca^"' < Mg^^ < NH4+ <

Na+.

Pulps bleached with hydrogen peroxide are

often treated magnesium. It may be usefiil to

wash these pulps with some dilute alkali to

improve their papermaking

properties.

Alum

would have a detrimental effect on pulp

swelling and should be kept out of the pro-

cess until after refining.

15.

Lindstrom, T. and G. Carlsson, The effect of

chemical environment on fiber swelling,

Svensk Papperstidn

85(3):R14-20(1982).

Swelling of pulps under a variety of pH and

metal ion conditions was determined by the

water retention values (WRV). Bleached

softwood sulfate

pulp

was fairly insensitive to

changes

in WRV

under these conditions since

it had only 2.5 meq/100 g (acidic groups on

pulp).

An unbleached softwood sulfate pulp

(47%

yield with 38 kappa number and 7.4

meq/100 g acidic groups on pulp) had larger

changes in WRV going from a low of 200 to

a high of 260. The maximum WRV oc-

curred at pH 8-9.5 with NaCl concentration

below about 0.01 M. 0.05 M NaCl de-

creased the WRV to 220 at pH 9, and a

NaCl concentration of 0.5 M decreased it to

200,

the base value. The tensile index was

directly proportional to WRV. These results

indicate that one should keep the ionic

strength of

pulp

low during pulping and pulp

storage before and during papermaking.

Pulp viscosity

16.

Siholta, H.,B. Kyrklund, L. Laamanen, and

L Palenius, Comparison and conversion of

viscosity

and

DP-values determined

differ-

ent methods. This is the most extensive and

about the only publication on the subject.

Paperija

Puu

(4a):225-232(1963).

EXERCISES 157

EXERCISES

Refining

Plot representative tensile strength, tear

strength, CSF, and specific surface area of

paper made from pulp as a function of refin-

ing energy (time) applied to the pulp.

Describe how refining helps fiber to fiber

bonding in the final sheet. Give three mech-

anisms for its action.

Describe the effect of pulp consistency on the

refining process in terms of bar-fiber con-

tacts.

Use Fig. 6-24 to help you.

What is the major drawback of any laborato-

ry refining process?

Give two methods whereby the level and

effectiveness of refining is measured.

6. One is visiting a mill using redwood fiber

and sees a Jordan refiner. Is this unusual

considering very few mills use Jordan refin-

ers anymore?

Using the discussion following references 14

and 15, describe why refining and paper-

making at elevated pH (say pH 8 versus pH

4) often leads to a significant increase in the

strength of the paper. When is this especial-

ly true?

As the upper bar passes the lower, low consistency

will favor either trapping single fibers and cutting

them, as in B, or stretching and breaking them, as

in Views C and D.

At higher consistencies, the fibers will tend to

form a blanket or cushion on the bars, leading to

crushing rather than a cutting action on the fibers.

Fig. 6-24. Effects of consistency on refining.

®1991 James E. Kline. Reprinted from Paper

and Paperboard with permission.

Pulp characterization

8. How are fiber size distributions measured?

9. What is the significance of fiber size distribu-

tions?

10.

What is the purpose of making laboratory

handsheets?

PAPER AND ITS PROPERTIES

7.1 INTRODUCTION

Paper

Paper is a pliable material used for writing,

packaging, and a variety of specialized purposes.

Paper consists of a web of pulp fibers (normally

from wood or other vegetable fibers), usually

formed from an aqueous slurry on a wire or

screen, and held together by hydrogen bonding.

Paper may also contain a variety of additives and

fillers (Section 8.4).

The first paper is credited to a Chinese man

named Ts'ai Lun in the early part of the second

century A.D. and was made from a mixture of

mulberry bark and hemp. The art of papermaking

was a closely guarded secret and only very slowly

moved westward. The first American paper mill

was built in 1690 in the northeastern U.S. The

process was very slow, because it was performed

entirely by hand. In the late 18th century, a

Frenchman named Louis Robert invented a

papermaking machine that used a continuous,

moving wire screen belt. He sold the invention

rights to the Fourdrinier brothers, who made

improvements in the machine's design. It came

into widespread use from 1830-1850.

Fiber

A fiber is a tubular or cylindrical element,

obtained from plant matter, containing cellulose as

the principal constituent. Fibers are the basic

components of paper. The properties of pulp

fibers determine many of the characteristics of the

final paper and have been described (Chapter 6).

7.2 GENERAL GRADES OF PAPER

Paper materials are classified as paper

(newsprint, stationary, tissue, bags, towels,

napkins, etc.) or paperboard (linerboard,

corrugating media, tubes, drums, milk cartons,

recycled board used in shoe and cereal boxes,

roofing felt, fiberboard, etc.) The industry

typically divides paper into broad categories based

on tiie types of fibers used in paper and the weight

of the paper. Table 7-1 shows the production

quantities of some grades of paper and their

approximate cost.

Tissues

Tissue papers are a class of lightweight

papers of 15-60 g/ml They are made prhnarily of

bleached chemical softwood pulps and may contain

bleached softwood sawdust or hardwood pulps to

impart smoothness. Sanitary tissues are the most

important type by production amount.

Increasingly, tissue paper is made with some or all

deinked secondary fiber. Some mechanical pulp

may be used as well.

Uncoated groundwood

This grade is made from mechanical pulps,

although small amounts of chemical pulps are used

to increase the strength. TMP pulps, being

stronger than groundwood, have decreased the

chemical pulp requirements of these grades.

Newsprint accounts for about 80% of this grade,

but it also includes directory, computer, catalog,

and similar grades. The term uncoated

ground-

wood is a misnomer, since other mechanical

pulping processes have largely replaced ground-

wood. (Fig. 9-62 contains figures of uncoated

groundwood, coated groundwood, and super-

calendered, coated groundwood papers.)

Coated groundwood

Coated groundwood by definition includes at

least 10% mechanical pulps, although 50% is

more typical. This grade is used in magazines,

catalogs, some No. 4 and all No. 5 enamel grades,

letterpress, offset, and LWC. Coated groundwood

was used in almost 70% of the 1.6 million tons of

paper used in U.S. magazines produced in 1990.

Uncoated wood-free

paper

(printing

or writing)

Uncoated wood-free paper is predominantly

made from kraft or sulfite softwood pulps and may

contain limited amounts of mechanical pulp or

158

GENERAL GRADES OF PAPER 159

Table 7-1. Capacities (thousands of tons) and price of paper by grade.^*^

1 Paper type |

1 Newsprint |

II Coated groundwood |

1 Coated free-sheet |

II Uncoated free-sheet |

II Bristols 1

II Industr. packaging |

1 Tissue 1

II TOTAL 1

Paperboard type

II Unbleach. kraft liner |

1 Solid bleached j

II Semi-chemical j

1 TOTAL

U.S. 1988

6116

1 4348

1 3437

1 12 062

1170

1 5404

1 5667

1 40 205

1 20 269

1 4732

1 5789

326

U.S.

1976

4015

2298

1903

7465

1165

6199

4500

29 219

14 568

3998

4758

32 082

1988 US$/MT

$540

$1100

$880-1600

$650-860

$1000

unbleach. $550

$420

$730

$360

Global,

1989 J

35 000

12 500

88 000

*The

principal sources are the 1990

North American Factbook

and

Pulp

and Paper

Week,

Nov

21,

1988.

Paper costs are approximate. Price ranges indicate a wide range of possible products and grades.

^The global production of printing and writing papers in 1988 was 60,358 tons with percentages follow:

North America,

37.3%;

EEC,

21.5%;

Asia,

17.8%;

Scandinavia, 10.6%; the rest of Europe, 7.3%;

Latin America,

4.1%;

Australia, 0.52%, and Africa, 0.85%. The per capita consumption of all paper

and board in the same year in pounds was: North America, 669; Scandinavia, 537; EEC,

321;

Australia,

279;

rest of Europe, 96; Latin America, 55; Asia, 39; and Africa, 12. PPI, July 1989,

recycled fiber. (Wood-free implies less than 10%

mechanical pulp.) It is used for envelopes,

photocopy, bond, and tablet papers.

Coated

wood-free

paper

The base sheet of coated, wood-free paper is

made from kraft or sulfite softwood pulps.

Coating is applied on one or both sides. Coated

paper is supercalendered to produce smooth,

glossy surfaces for good printing. This paper is

used for high grade (No. 1-3 and some 4) enamel

papers for magazines, books, and printing.

Kraft wrapping or bag

Kraft bag or wrapping papers are made from

bleached or unbleached kraft softwood pulp of

southern pine or Douglas-fir. They are made in

various weights from 50 to 134 g/m^.

Cast-coated paper, machine glaze, MG kraft

Cast-coated paper is a very high gloss paper

made by allowing the coating on paper to dry on

a large, chrome plated dryer with a polished

surface. These papers are used for wrapping,

carbonizing, wax base, and specialty papers.

Specialty

papers

Specialty papers are made for specific uses.

They are produced in small volumes, but have the

potential for high profit margins. These papers

are used in packaging, printing, manufacturing,

electronics, etc. They include capacitor, cigarette,

bible, glassine, and greaseproof papers.

160

7. PAPER AND ITS PROPERTIES

Kraft paperboards

Paperboards are any of the heavyweight

papers, generally above 134 g/m^ used in

packaging. Bleached paperboards, about half of

which are coated, are made from bleached kraft

pulp and are used in folded milk cartons, index

cards,

cups, and plates. Unbleached paperboards

are used in linerboard and corrugating medium for

the production of corrugated boxes. Recycled

fiber is being used in larger amounts in the

production of unbleached paperboards.

Chipboard, recycled paperboard

Chipboard is a thick paper of low density and

quality used for making solid fiber boxes and

papers that require low strength. It has multi-ply

construction and is often made on cylinder ma-

chines. The furnish is often recycled newspapers

and other inexpensive pulps, which is known as

recycled paperboard. Grades include gypsum

liner, core (tube) stock, and clay coated folding

boxboard. These grades have a characteristic

grayish cast because the furnish is not deinked.

Market pulp

Some mills produce pulp but do not have

paper machines. Their pulp is sold on the open

market as wet lap (about 50% solids) or dry lap,

which, is carefiilly dried to no more than about

80-85%

solids to avoid irreversible loss of

hydrogen bonding sites that occurs when pulp is

over-dried. Other mills produce excess pulp that

is also sold on the market. Newsprint makers that

use bleached kraft pulp usually purchase it on the

open market. Dissolving pulp is also produced as

market pulp since a handful of mills provide pulp

to the many users of dissolving pulp. Fig. 7-1 and

Plate 16 show the production of market pulp.

7.3 SPECIFIC TYPES OF PAPER

TAPPI TIS 0404-36 lists the general grades

of paper that are used to make specific types of

papers with designated end uses. Over 400 types

of paper are included in this reference, which is an

indicator of the diversity of the pulp and paper

industry. Specific descriptions of a few high

production, commodity types of paper are listed

here.

Electron micrographs of some of these

papers may be found in Fig. 3-2.

Newsprint

Newsprint is paper used for printing

newspapers and other low cost, short lived pub-

Fig. 7-1. Wet lap production for market pulp or for use within the same miU at times when the

pulp mill may shut down but it is important to keep the paper machines running.

SPECmC TYPES OF PAPER 161

lications. Newsprint contains a high percentage of

mechanical pulp and a small percentage of full

chemical pulp to increase the strength. Newsprint

must have moderate strength, good printability,

and low cost. Today, the basis weight of

newsprint is 48.8 g/m^ (30 lb/3000 ft^), down

from 52.2 g/m^ (32 lb/3000 ft^) that was typical

before 1974. This decrease in basis weight has

been made possible by improved manufacturing

techniques that produce a stronger, more opaque

paper. Over 70% of newsprint in North America

is made on twin-wire machines at speeds up to

3500 ft/min. The caliper of newsprint is

nominally 0.085 mm (3.3 mil or 0.0033 in.)

Bond paper

Bond papers are a broad category of high

quality printing or writing papers. They are made

from bleached chemical pulps and cotton fibers

and may be watermarked. Rag bonds are made

with a minimum of 25 percent rag fiber from

cotton. They are high quality, long lasting,

strong, expensive papers. Nominal weights of

bond are 48, 60, 75, or 90 g/m^ (corresponding to

13,

16, 20, or 24 pounds per 17 in. by 22 in. by

500 sheet ream.)

Fine

papers

Fine papers are intended for writing, typing,

and printing purposes. They may be white or

colored, are made from bleached kraft or sulfite

softwood pulps, and may contain hardwood pulp

for smoothness and opacity.

Tissue

Sanitary tissues are made from bleached

sulfite or kraft pulps and are soft, bulky,

absorbent, and moderately strong. Wrapping

tissues are used for wrapping clothes, flowers, etc.

and are made of bleached kraft or sulfite softwood

pulps that impart high strength. Tracing tissue is

a dense, transparent grade of paper that is used for

tracing.

Glassine and greaseproof paper

Glassine papers are made from highly refined

chemical pulps, resulting in a very dense,

translucent sheet. Often a special grade of un-

bleached sulfite pulp with a high hemicellulose

content is used to make a nonporous sheet. These

papers are used in laboratory weighing papers,

greaseproof

papers,

and food handling papers like

the fluted paper cups for chocolates. The nominal

caliper of glassine is 0.03 mm (1.2 mil or 0.0012

in.)

Linerboard

Linerboard is an unbleached kraft softwood

sheet of southern pine or Douglas-fir (when made

in the U.S.), made in various weights.

Frequently, linerboard is a two ply (duplex) (or

even triplex) sheet, made on a flat wire paper

machine with two headboxes. It is used in making

corrugated boxboard; the two outer visible layers

of corrugated boxes are linerboard. The

compression strength and burst strength are

important. Some common structural linerboards

have basis weights of

205,

229, and 337 g/m^ (42,

47,

and 69 lb/1000 ft^), with calipers of 12, 13,

and 19 thousandths of an inch (0.001 in.) and

burst strength of 80, 90, and 130 psi (1990 Pulp

and Paper Factbook). Other basis weights are

127,

161, 186, and 439 g/m^ (26, 33, 38, and 90

lb/1000 ft2) with 9-24 point caliper (0.23-0.64

mm).

Linerboard is assembled to corrugating

medium with the machine direction perpendicular

to the flutes of corrugating medium since the flutes

contribute strength and stiffness in the weaker

cross machine direction.

Corrugating

medium

Corrugating medium is made from

unbleached, semi-chemical pulp, especially NSSC

hardwood pulp, and recycled fiber from corrugat-

ed boxes. It is formed into a fluted (wavy)

structure and sandwiched between two plies of

linerboard to form the corrugated structure for

boxes.

Occasionally the green liquor process is

used as are softwoods to make the pulp furnish.

This paper must be stiff and inexpensive. This

board normally has a basis weight of 127 g/m^ (26

lb/1000 ft2) and a caliper of 0.23 mm or 0.009 in.,

so-called 9 point board. There are four standard

flute shapes commonly used in the U.S. Letting

F denote the number of flutes (thickness does not

include the linerboards), they are:

162 7. PAPER AND ITS PROPERTIES

Flute A with 118 F/m (36 F/ft) with a

thickness of 4.8 mm (3/16 in.)

flute

Flute B with 168 F/m (51 F/ft) with a flute

thickness of 2.4 mm (3/32 in.)

Flute C with 130 F/m (40 F/ft) with a flute

thickness of 3.6 mm (9/64 in.)

Flute E with 316 F/m (96 F/ft) with a flute

thickness of 1.2 mm (3/64 in.)

Corrugating medium must have relatively

high stiffness and resistance to crush. Also,

corrugating medium should be made from

adequately refined pulp, since shives will cause

paper breaks during its conversion into boxes.

Plasticity of corrugating medium also contributes

to good performance during conversion without

paper breaks and is enhanced by drying it under

only moderate restraint.

No carbon required (NCR)

Carbonless paper was developed by the 3M

company in the 1950s and is manufactured by

several companies today. This paper uses

encapsulated chemicals on the backside of one

sheet that react with a chemical on the front of the

adjacent sheet to give color. The capsules and

second chemical may be on the same sheet so that

any paper can be used as the original.

The chemicals are encapsulated using

emulsion polymerization techniques. When the

capsules containing the chemicals are crushed, the

chemicals react to form a colored substance and

leave a mark. Fig. 7-2 shows a scanning electron

micrograph of carbonless paper where pressure

from a pen has crushed the chemical containing

capsules.

Miscellaneous

paper types

Construction

boards are thick boards used in

building construction (insulation board, etc.).

Molded pulp products include egg cartons, flower

pots,

and spacers for packaging. These products

are molded primarily from recycled newspapers,

although other papers can be used.

Forms

papers

are fine papers normally in roll form for

Fig. 7-2. SEM of NCR carbonless paper showing destruction of the encapsulated pigments.

SPECIFIC TYPES OF PAPER 163

conversion into continuous, multiple-copy,

business forms. These are sometimes called

bond.

Construction paper is a heavy,

colored paper that consists primarily of mechanical

pulps and that is used for drawing and art work.

Bristols are heavy, stiff papers of high quality

made from bleached kraft pulp, and they are used

for postcards, greeting cards, invitations, and

announcements.

7.4 BASIC PAPER PROPERTIES

Paper may be characterized by moisture

content, physical characteristics, strength

properties, optical properties, and other criteria

depending on its end use. It is very important to

obtain a sufficient number of representative

samples by using a sampling procedure such as

that recommended in TAPPI Standard T 400.

Chapter 15 discusses the detailed calculations

for paper tests, the interconversion of metric and

English units, and other aspects of paper test

calculations. Table 15-2 shows some typical

values for paper tested by OSU pulp and paper

students in a laboratory exercise. These values are

included as "typical" values of a variety of papers

and are used for examples of calculations.

When measuring strength properties MD

refers to the test force applied in the machine

direction and CD is force applied in the cross

machine direction. Many paper properties depend

on the orientation of the paper since the MD and

CD properties are not identical.

Effect of moisture content

The moisture content of paper has an

important effect on paper quality; therefore, paper

properties must be measured under standard

conditions of temperature and relative humidity

[generally 23 °C (73 °F) and 50% relative

humidity] since these affect the equilibrium

moisture content (EMC). As stated above, the

EMC affects strength and other properties of

paper. According to TAPPI T 402, paper should

be placed in a hot, dry room (20-40°C or 68-

105

°F,

at 10-35% relative humidity) before

placement in the standard room so that the

moisture content of paper approaches its EMC by

adsorbing water from the atmosphere. Due to the

Table 7-2. EMC values of various papers at

50%

RH and 72**F.

Paper type

1 Filter, 100% rag

II Bond, 100% rag

Bond, 100%

softwood

1 Wrapping, kraft

Newsprint

EMC,

%, at50%RH

(based on dry weight

of paper)

From

35%

6.6

7.4

7.8

9.0

From

80%

7.2

7.5

8.4

9.3

10.9

hysteresis effect, a slightly higher EMC would

result if paper approached the EMC for a given

relative humidity and temperature, by giving off

water. (Laboratory prepared handsheets are

usually not treated in this way as they are air dried

without ever going into a hot, dry room.)

Table 7-2 gives some EMC values at 50%

relative humidity for several paper samples

approaching from below 50% and above 50%

relative humidity to demonstrate the hysteresis

effect. Fig. 7-3 shows EMC with increasing and

decreasing relative

humidities.

In these figures the

softwood bond is paper made from acid sulfite

pulp;

thus, much of the hemicellulose has been

removed by acid hydrolysis, making the paper

similar to rag paper. The bond paper of today,

made from bleached kraft pulp, probably behaves

somewhere between kraft wrapping and sulfite-

derived bond paper. There is no such thing as an

average paper when it comes to moisture content

and the changes in properties as a fimction of

moisture content. Throughout this subsection four

or five papers of widely differing qualities are

used to demonstrate the differences one expects to

encounter among all of the possible papers.

It is important to know the rate of moisture

gain or loss of paper to achieve equilibrium in

order to understand how long to condition

samples. For example, how long should paper be