Biermann Ch. Handbook of Pulping and Papermaking

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184 7. PAPER AND ITS PROPERTIES

< 0.0

VOLTS

10;240>

Fig. 7-22.

SEM

/ elemental analysis of three areas on a 3M Post-it™. Courtesy of R. L. Krahmer.

Starch retention of fines, fillers, and other materials. In

The use of various types of starch is the latter case, cationic starches are commonly

becoming increasingly important in paper in order used. Starch may be measured qualitatively by an

to increase its strength and to increase the iodine solution or quantitatively by HCl extraction

CHEMICAL ANALYSIS OF PAPER 185

followed by colorimetric determination with iodine

(TAPPI Standard T 419). Starch consumption in

corrugated board is measured by enzymatic

hydrolysis followed by either gravimetric

determination of the dissolved solids (TAPPI

Standard T 531) or colorimetric assay of the

liberated glucose (TAPPI Standard T 532).

Other chemical analyses of paper

There are numerous other tests of paper. For

example, TAPPI Standard Tests are used to

measure rosin (T 408 by gravimetric analysis of an

ethanol extract), dirt (T 437), melamine resin (T

493),

and other components of paper. The pH is

measured at the surface (T 529), in hot water ex-

tracts (T 435), or in cold water extracts (T 509).

Fiber analysis of paper and paperboard is de-

scribed in T 401. Qualitative analysis of mineral

fillers and coatings by optical microscopy and

chemical analysis is described in TAPPI Standard

421,

which includes several photomicrographs.

7.8 BASIC OPTICAL TESTS OF PAPER

Introduction

The optical properties of paper can be

extremely complex. Some of the rudimentary

concepts are presented here, but the reader should

be aware that this field is complex. Dirt is

considered in Section 34.18.

Color

Color is the measure of the hue or chroma of

light reflected from the surface of

paper.

It cannot

be easily put in numbers, and it is fre-quently

expressed descriptively, as "red" or "blue". Color

of paper can be expressed by a series of three

numbers in the International scale (CIE) system,

JC,

y, and z. Details of this system are given in

Chapter 24. The spectral reflectance,

transmittance and color of paper are mea3ured as

a function of wavelength (poly-chromatic) in T

442.

Related tests are T 524 and T 527. The

perception of color is dependent upon the light

source used to view the object.

Brightness

Brightness is a measure of

the

"whiteness" of

paper. These methods are not applicable to

colored papers that are characterized with the tests

described above. Precisely, brightness is the

percentage of diffuse reflected light from a thick

pad of paper to visible light at a wavelength of

457 nm. This brightness is designated as/^oo- 457

nm is in the blue range of light, where yellowish

paper would absorb this light instead of reflecting

it. Therefore, brightness is a good indicator of the

yellowness of paper or the degree of

bleaching.

a black body is used behind a single sheet of paper

while the diffuse reflectance is measured, a lower

value is obtained (unless the paper is opaque),

which is designated/?o- If the diffuse reflectance of

a single sheet of paper is measured backed by a

surface with reflectance of 0.89 (or 89%), the

result is designated as /?o.89-

Two main types of instruments are used. In

the G.E. brightness (TAPPI Standard T 452),

shown in Fig. 7-23, the light is illuminated on the

paper at a 45° angle and the reflected light at 0°

is measured. (If an ultraviolet source is used, the

effect of fluorescent dyes, the optical brighteners,

may also be measured.) This technique uses a

reference cell to compensate for slight changes in

the light output of the source. A simple orifice

directs some of the light from the source to a

reference cell, but this is less desirable than having

an identical light path for the reference beam as in

the Elrepho test.

In the Elrepho test (TAPPI Standard T 525),

the light source is diffuse and the reflected light is

measured at 90° from the surface of the paper

(Fig. 7-24, Plate 29). The diffuse light source is

a sphere coated with titanium dioxide from which

light from two light bulbs reflects. A reference

photocell measures the light on the diffuse white

surface near the sample. Because the reference

photocell detects changes in the light level, large

changes in the light source have almost no effect

on the reading of paper brightness; it is an

extremely robust method compared to the G.E.

brightness. (I have taken out one of the light

bulbs with a test in progress and observed less

than a 0.2% difference in brightness.) A variety

of filters can be used with this instrument, but the

457 nm filter is the most commonly used filter.

Opacity

Opacity is the ability of paper to hide or

mask a color or object in back of the sheet. A

186 7. PAPER AND ITS PROPERTIES

PAPER SPECIMEN

.<3^

.IGHT

SOURCE

IRIS

REFERENCE

DETECTOR

/45

r*:^

»—1

cTi-n

jzr-

-nm

-H

FILTER

PHOTOCELL

I COLOR

GREY WEDGE

LIGHT

REFERENCE

SPOT

^^SAMPLE

DETECTDR

Fig. 7-23. Brightness measurement by Fig. 7-24. Brightness by diffuse reflectance

directional reflectance (G.E. brightness). using the Elrepho brightness meter.

high opacity in printed paper allows one to read

the front side of the page without being distracted

by print images on the back side. Printing and

TAPPI opacity are defined below; the higher the

opacity, the better the hiding power, with opaque

paper having an opacity of 100%.

printing opacity

TAPPI opacity

xlOO%

TAPPI Standard T 425 describes the

measurement of opacity

Ro.89,

which is sometimes

called printer's opacity. TAPPI Standard T 519 is

used for diffuse opacity (paper backing).

Opacity is a very important property in

printing papers. It is important to consider what

properties of paper contribute to its opacity.

Opacity is the result of light scattering in many

directions when the path of light is bent. This

occurs as light goes from material of one index of

refraction to a material with a different index of

refraction. The classic example is putting a

wooden stick into clear water at an angle; the

stick appears to bend at the water-air interface.

Rough, non-flat surfaces help scatter the light m

different directions.

Cellophane is almost identical in chemical

structure to paper, since it is fairly pure cellulose.

Cellophane, unlike paper, is transparent since it is

solid and has two uniform, parallel surfaces. The

air spaces in paper provide interfaces for light to

scatter. Thus, bulky sheets have much higher

opacity than dense sheets. Unfortunately, bulky

sheets are less strong than dense sheets since fiber

to fiber bonding is poorer in bulky sheets. Since

refining increases fiber to fiber bonding and leads

to a denser sheet, increased refining lowers

opacity. The highly refined glassine papers are

very dense and translucent (partially transparent in

that they let a large amount of light through, but

images are not as clear as they are in transparent

materials).

Pulp fines have very high surface areas and

irregular shapes that contribute to high opacity.

Stone groundwood pulps are of especially high

opacity. Fillers (Table 8-1) tend to have high

indices of refraction and provide interfaces for

light scattering as well. The very high index of

refraction of titanium dioxide allows thin papers to

have high opacity as in the case of bible papers.

Gloss

Gloss is a measure of the sheen or polish of

paper. It is measured by illuminating the paper at

a very low angle and reading the reflectance at a

similar low angle.

BASIC OPTICAL TESTS OF PAPER 187

TAPE

Fig. 7-25. Beloit sheet splitter.

7.9 SHEET SPLITTING OF PAPER

Introduction

Sheet splitting refers to the separation of a

sheet of paper in the z-axis. In this delamination

process, the top of the sheet is separated from the

bottom of the sheet. This has always been

difficult to do without altering the sheet to the

point that many of

its

aspects could not be studied.

Past methods were time consuming, complex,

and/or only applicable to small areas. It has been

used as an important tool for analysis of fillers

across the thickness of the sheet, although SEM

might be an easier, less time consuming, and more

direct tool for this.

The first method of sheet splitting involved

the use of razor blades. One other method of

doing this is to use a precision grinder to remove

the portion of paper that is not of interest. A

vacuum table can hold the sheet down so that it is

not altered. Still another method is to use double -

sided adhesive tape on both sides of the paper.

The exposed area of tape is attached to some

surface and the paper is pulled apart. Toluene can

then be used to dissolve the tape and leave the

paper (to some degree) intact. This is often used,

with repeated applications, to measure the filler

location across the thickness of the paper.

TAPE

CRANK

Fig. 7-26. Minter's method of sheet splitting.

Beloit sheet splitter

In 1963, the Liberty Engineering Company

invented what is now the Beloit sheet splitter.

This method (TAPPI UM 576, Fig. 7-25) works

by soaking the paper in water and then putting it

through a nip made by two metal rolls that are

chilled below the freezing point of water. Each

side of the paper is frozen to one of the rolls, so

it is effectively split. The paper is doctored from

the chilled roll. Unfortunately, the paper is

altered to a large degree by this method, but it is

useful for mineral contents by ashing.

Minter's method

Minter developed a method (Fig. 7-26) where

two pieces of double-sided adhesive tape are

applied to the leading edge (the edge first off the

paper machine). The paper is then put in the nip

formed between two rubber press rolls. Each side

of the paper is anchored to the roll at the leading

edge so that the split is started. Turning the roll

propagates the split indefinitely. The width of

rolls determines the CD length that is split, and

the circumference of the roll determines the MD

length that can be split. The nip pressure is not

high, but should be adjustable to give the best

split. Two large pieces of unaltered paper are

obtained. Strength, sizing, ash contents, and most

any other test can be applied to the individual

"sides"

of the paper. This should prove to be an

invaluable tool for determining the two-sidedness

of paper and formation characteristics on the paper

machine. The one disadvantage of the method is

that only a single split may be made.

188

7. PAPER AND ITS PROPERTIES

7.10 ANNOTATED BIBLIOGRAPHY

Paper

properties—physical

and mechanical

Jarrell, T.

D.,

Effect of atmospheric humidity

on the moisture content of paper, Paper

Trade J. 85(3):47-51(1927).

Institute of Paper Chemistry, Instrumentation

studies. XII, Effect of relative humidity on

physical properties with respect to the

hysteresis effect in changes from one

humidity to another. Paper Trade J.

104(15):45-48(1937).

Carson, F.T., Effect of humidity on physical

properties of paper, U.S. Dept. Commerce,

Circular National Bureau of Standards C445

(1944).

Moody, R.C. and J.W. Konig, Jr., Effect of

loading rate on the edgewise compressive

strength of corrugated fiberboard, U.S.

Forest Service Research Note FPL-0121

(April, 1966), 11 p.

Page, D.H., A theory for the tensile strength

of paper,

TappiJ,

52(4):674-681(1969). The

tensile strength of paper is considered in

terms of fiber strength and fiber bonding.

Paper made from pulps of high freeness have

a tensile strength limited by fiber to fiber

bonding. Paper made from highly refined

pulps have a tensile strength limited by the

strength of the individual fibers.

6. Sachs, I.B. and T.A. Kuster, Edgewise

compression failure mechanism of linerboard

observed in dynamic mode,

Tappi

63(10):69-

73(1980). This is a fascinating visual picture

of paper compression failure. The authors

state that failure occurs by delamination at

the S1/S2 cell wall interface. Buckling of the

fibers follows failure. There are 13 electron

micrograph photographs.

Three consecutive articles in Tappi J. are of

interest in regards to Item 222/Rule 41 and

follow:

Batelka, J.J., Compliance statistics for the

edge-crush specifications of Item 222/Rule

41,

TappiJ,

75(l):75-78 (1992).

Kroeschell, W.O., The edge crush test,

TappiJ,

75(1):79-82(1992).

10.

McNown, W.J., Short-span compressive

strength testing: procedures and tools for

improved tester accuracy, Tappi J. 75(1)83-

86(1992).

Bristow test

11.

Bristow, J.A., Liquid absorption into paper

during short time intervals, Svensk

Papperstid.

70(19):623-629(1967).

12.

Lyne, M.B. and J.S. Aspler, Wetting and the

sorption of water by paper under dynamic

conditions, Tappi J. 65(12):98-101(1982).

This article presents some modifications to

the Bristow test.

13.

Bares, S.J. and K.D. Rennels, Paper

compatibility with next generation ink-jet

printers, TappiJ

73(1):

123-125(1990).

General chemical analysis

14.

B.L. Browning, Analysis of

Paper,

2nd ed.,

Marcel Dekker, Inc., New York, New York,

1977,

366 pp. This volume covers many

spectrometric, colorimetric, and other

methods for the analysis of paper constituents

including fillers, additives, coatings,

polymers, etc.

SEM with elemental analysis

15.

Gibbon, D.L., G.C. Simon, and R.C.

Cornelius, New electron and light optical

techniques for examining papermaking, Tappi

J, 72(10):87-91(1989).

Item 222/Rule 41

Kroeschell, W.O., New carrier regulations

for corrugated shipping containers are now in

effect, Tappi J, 74(7):63-65(1991).

16.

Ormerod, D.L., G.C. Simon, and D.L.

Gibbon, Paper mill additives and con-

taminant distribution mapping using energy

dispersive X-ray analysis, Tappi J,

ANNOTATED BffiLIOGRAPHY 189

71(4):211-214(1988). Both of these papers

show color diagrams that indicate the

importance and usefulness of this method.

Neither paper has references. This method

has widespread applicability to corrosion and

scale analysis.

Optical

properties 5.

17.

TAPPI Technical Information Sheet TIS

0804-02 1986 gives an extensive glossary of

optical measurements terminology. 6.

Sheet splitting

18.

Minter, S., Delamination as a sheet splitting 7.

method, TAPPI 1992 Papermaking

Conference, pp 81-84.

General interest

19.

Mark, R.E., Ed., Handbook of Physical and

Mechanical Testing of Paper and

Paperboard, Marcel Dekker, Inc., New

York, 1983, 2 volumes, 640 p., 508 p.,

respectively. This is useful for background

knowledge of a variety of paper and fiber

tests of mechanical, optical, wetting and

penetration, electrical, thermal, structural,

and bonding properties.

20.

Dahl, H. H. Holik, and E. Weisshuhn, The

influence of headbox flow conditions on

paper properties and their constancy, Tappi

J, 71(2):93-98(1988). (As shown in Fig. 7-

15) the maximum strength in paper is not

always exactly in the machine direction, due

to headbox factors explained by a simple

fluid model in this work.

EXERCISES

Give four specific types of paper and their

properties and uses.

Why is it important to put paper in a hot, dry

room before putting it in the TAPPI Standard

room?

What are the problems associated with

measuring paper strength properties of paper

immediately off the reel during quality

control at a paper mill?

Which property of paper can change by a

factor over ten depending on the EMC of

paper?

Define these

terms:

formation, gloss, caliper,

density, bulk, and sizing.

Describe how sizing is measured by the Cobb

and Hercules size tests.

Give two possible reasons why tensile

strength and other paper properties vary

depending upon the direction of paper?

8. The basis weight variation in 12 in. by 12.

square sheets might be on the order of 0.2%

while the fold endurance of these sheets

might vary by 40%. Give two contributing

reasons to explain this phenomenon.

9. Give two general methods whereby the

amount or type of metal atoms/ions can be

determined.

10.

What is paper brightness? (Be precise in

your answer.)

11.

Describe two ways of measuring brightness.

12.

What is paper opacity? Describe how the

following factors influence opacity:

a) bulk

b) presence of fines

c) presence of fillers

d) level of refining

e) thickness of paper

13.

Use equation 7-1 (page 170) to determine the

density of corrugating medium that has a

basis weight of 26 lb/1000 ft^ and caliper of

0.009 in. If the cell wall material has a

specific gravity of 1.50, what percent air is

this sheet?

STOCK PREPARATION AND ADDITIVES FOR PAPERMAKING

8.1 INTRODUCTION

This chapter discusses preparation

of the

stock (Fig.

8-1)

prior

to the

headbox

the paper

machine. Included is

discussion of the numerous

additives and fillers used

make paper. Wet end

chemistry,

the

chemistry

of the

dilute aqueous

solution

fibers, fillers,

and

additives,

also

discussed. Knowledge

wet end chemistry helps

predict retention

additives, drainage during

•s

Stuff Box

Keeps constant head

of stock pressure

consistency

to the

primary

fan

pump

•^

S ^

V V

-I

stock

Proportioner

•^

PM Whitewater

for dilution

Primary

Fan Pump

Machine

Chest

Consistency u

Sensor "^

Dilution

Water

Secondary

Fan Pump

To headbox

at 0.5%

consistency

Accepts

To:

^Primary Secondary

inlet

,r^

inlet

Cleaners: Primary Secondary

Fig.

8-1. Stock preparation system to the headbox feed.

190

Tertiary

INTRODUCTION 191

papermaking and the level of sizing of the final

paper. The operation of the size press and coating

processes are discussed in Chapter 9.

8.2 FIBER PREPARATION AND AP-

PROACH

Fig. 8-1 shows a typical stock preparation

system for paper machines. The system includes

stock proportioning and stock cleaning. Some

auxiliary equipment, such as pumps between the

primary and secondary and secondary and tertiary

clarifiers, is not shown.

Consistency

Consistency is a measure of the solids con-

tent; it is the dry weight of fibers and other solids

divided by the total wet weight of stock weight

and expressed as a percentage. The consistency of

stock at the headbox may be as low as 0.3%.

weight of dry material ^^^^^

consistency = ^ x 100%

weight of suspension

Hydrapulper

Hydrapulpers are large mixing vessels used

to disintegrate purchased pulp (either wet lap at

50%

solids or dry sheets at 80-85% solids), broke

(paper that must be reprocessed within the mill),

and recycled paper into a relatively dilute slurry

which can be processed within the mill. Pulps and

papers with high lignin contents and low moisture

contents require relatively high energy to be

repulped.

Machine chest

The machine chest is an agitated storage test

that holds stock prior to being sent to the paper-

making process. If the pulper or other equipment

must be shut down temporarily, the paper machine

can continue running by drawing pulp from the

machine chest. Being forced to shut down the

paper machine is something of a disaster since it

may take several tons of production before high

quality paper is again formed, not to mention lost

production time.

Stuff box

The stuff box is a small box that pulp enters

prior to the fan pump for use in the paper ma-

CLEAN STOCK

OUTLET

INLET

chine. Pulp enters at a regulated 3-4% consisten-

cy through a stock or stuff valve and maintains a

constant head to precisely control the flow rate to

the fan pump.

Fan

pump

Stock and recirculated white water are mixed

together at the primary fan pump, a very large

pump. For example, a paper machine producing

200 tons per day of paper requiring the equivalent

of a 65 foot head of water uses a 10,000 gal/min

pump powered by a 200 hp motor.

Vortex cleaners (also centrifugal, hydrocyclone,

centri or cyclone cleaners)

Introduced around 1950, vortex cleaners are

plastic or stainless steel cylinders 0.5-1.5 m (20-60

in.) long tapered from 10-20 cm (4-8 in.) at the

top to less than 1 cm

(0.4 in.) at the bottom.

Stock enters the side

near the top tangen-

tially to the cylinder

causing a vortex to

form (Fig. 8-2). Clean

stock is removed from

the top and stock con-

taining dense contami-

nants is removed from

the bottom. Centrifu-

gal action causes the

dense materials to lose

their momentum on the

inside walls of the

cleaner. This allows

the dense material to

settle much more

quickly than fibers.

The vortex clean-

ers used in the paper

machine approach are

operated at low consis-

tency (0.5-1.5%) and

remove contaminants

of high specific gravi-

ty. Rejects are re-

moved continuously REJECTS

with high percentages OUTLET

(10-30%) of usable Fig. 8-2. Operation of

fiber. a centrifugal cleaner.

192 8. STOCK PREPARATION AND ADDITIVES FOR PAPERMAKING

Fig. 8-3. Two methods of arranging vortex cleaners. The left setup is prior to a fine paper

machine and the right is at a secondary fiber plant that employs deinking.

To recover the fiber lost with the rejects, the

rejects from the first bank of cleaners {primary

cleaners) are treated in a bank of secondary

cleaners',

the secondary rejects are treated in the

tertiary cleaners', and sometimes the tertiary

rejects are treated in a set of

quaternary

cleaners.

Fig. 8-1 shows a system where the secondary

accepts are returned to the primary cleaners and

the tertiary accepts go to the secondary inlet. In

practice, many diverse configurations are possible.

See Section 17.6 for more detail on calculating

optimal configurations.

Flow in the vortex cleaners is controlled by

pressure drops. For example, by decreasing the

pressure in the rejects outlet, thereby increasing

the pressure drop from the input to the reject

output, a higher flow to the reject output will

resuU. This may lead to better removal of the

rejects, but with additional fiber loss as well.

Vortex cleaners can be grouped together in

several fashions. Fig. 8-3 shows two common

configurations for arranging vortex cleaners.

Medium (1-3%) or high consistency (2-5%)

vortex cleaners are used as junk removers in

secondary fiber plants (Section 10.3). In second-

ary fiber plants there are lightweight contaminants

such as plastics that must be removed. This is

effected by using the same type of cleaners except

that the rejects are removed at the top while the

cleaned fibers are removed at the bottom. These

are called

reverse cleaners

or through-flow and are

discussed in more detail in Section 10.3.

Deculator

The deculator is a device for subjecting the

stock going to a paper machine with a partial

vacuum. This causes small air bubbles that are

entrained in the stock to expand, separate from the

stock, and come to the surface of the stock.

Dissolved gases will also be removed to some

extent. This reduces the formation of foam and

pits in the papermaking process. De-aeration

increases the drainage rate and decreases fiber

flotation and flocculation.

Screens, pressure screens

Pulp is screened through small holes or slots

to remove shives, dirt, and other large particles.

The reject material is usually refined and screened,

and the final rejects discarded to the sewer system.

Vibratory flat

screens

and rotary screens are open,

gravity flow systems, which may have high fre-

quency vibration to help the screening action.

Usually they have round holes and are often used

in knotter screens at the pulp mill or course

screens in the paper machine. They are now

obsolete because they are expensive to operate and

maintain, although they are quite useful in the

laboratory. Centrifugal screens are enclosed,

FIBER PREPARATION AND APPROACH 193

FLDV

i _* _i

LEHMAN SLOTTED BASKET

TOP VIEW

SIDE VIEW

OF BASKET

rREJECTS DUT

Fig. 8-4. Radial outward flow pressure screen.

The arrows show the direction of pulp flow.

non-pressurized screens with a horizontal cylinder

containing round

holes.

Centrifugal rotation of the

stock by a rotor with large paddles facilitates the

screening action by keeping the screen clean;

these are used as fine screens in the pulp mill.

Pressure screens, introduced in the 1950s for

headbox approach systems, are enclosed, pressur-

ized screens of high capacity. Fig. 8-4 demon-

strates the principle of operation as do Plates 30 to

32.

Improvements in designs, such as the use of

slots instead of holes in the early 1970s, allow

them to be used in any fine screening application

throughout the pulp and paper mill. One impor-

tant improvement was the reduction in the screen

slot width from 0.50 mm (0.020 in.) prior to

1970,

which is still used for corrugating medium,

to as low as 0.20-0.30 mm (0.008-0.012 in.), or

less,

now used for fine papers. The holes or slots

are tapered through the thickness of the screen so

that the wider portion is on the exit side (the

accepts) to limit the trapping of dirt.

The operation of pressure screens is similar

to that of centrifugal screens where the screen is

an enclosed, horizontal cylinder. Fiber flow may

be outward, inward, or both if two concentric

screens are used. Screening action and screen

cleaning is most often carried out by non-contact-

ing, rotating hydrofoil surfaces as shown in Fig.

8-4, a major advance in pressure screens. The

hydrofoil surface causes the debris, which con-

stantly plugs the holes of the screen, to be lifted

away by the back-flushing action of

the

liquid flow

around the hydrofoil surfaces. The frequency and

magnitude of the pulse generated by the hydrofoil

surface are two of the primary design criteria.

The efficiency of pressure screens is a function of

the basket (hole size and hole shape), while the

fiber reject rate is a fimction of the rotor (configu-

ration and speed). Pressure screens require about

20-40 kWh/t (0.5-2 hp-day/ton) of

pulp

for slotted

screens and less for screens with holes, and they

use an inlet consistency of

2-5 %.

Black Clawson

uses a roughened screen surface to induce micro-

turbulence, which is called a Lehman slotted plate

(Fig 8-4), while other manufactures use other

profiles to do the same thing. In the future,

screen baskets will be made in sections that are

bolted together so that individual sections can be

replaced without replacing the entire screen.

8.3 RAW MATERIALS

Stock,

furnish, broke

Stock is the mixture (slurry) of pulp, fillers,

other papermaking materials, and water. Furnish

is the combination of all of the materials used to

make paper. Broke consists of paper trimmings

and waste, paper not up to specification, and other

paper that.is often reused in the paper mill. A

mill producing more than 10-15% broke is not

operating efficiently. (Broke is generally not

considered recycled paper.)

Additives

Additives are materials used to improve the

finished paper itself or aid in the process of