Biermann Ch. Handbook of Pulping and Papermaking

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164

7. PAPER AND ITS PROPERTIES

"60"

—1—

80 35 40 45 50 55

Relative Humidity, %

Fig. 7-3. The EMC of four types of paper with increasing or decreasing RH. The lower line for

a given paper is increasing RH. The data are from Jarrell (1927).

conditioned when you perform routine quality

control at a paper mill? Clearly, if the paper is

not conditioned to a known moisture content, then

the results may be misleading. Fig. 7-4 shows the

moisture content of paper preconditioned at 35%

RH as a function of time the paper is at 65% RH

and 21°C (70°F). Under these conditions paper

(the average of several types) has gained 36% of

the weight in 0.5 h, 52% in 1 h, 86% in 4 h, and

92%

in 6 h. When paper is conditioned at 35%

RH from 65% RH, on the average 62% of the

water is lost after 2 h, 72% is lost after 4 h, 86%

is lost after 24 h, and 94% is lost after 48 h.

The most recent extensive publication giving

paper properties as a ftmction of moisture content

is from the staff of the Institute of Paper

Chemistry (IPC) from 1937. Data from this study

for changes in structural properties of four types

of paper as a ftmction of RH at

21 °C

(70°F) are

presented in Table 7-3. (The original data were

presented with 65% RH as the baseline, the

standard for testing paper at that time.) Data on

sizing and linear expansion are also available in

this reference. While some of the data in this

table are inconsistent, the trends are valid and are

a good approximation for certain grades of paper

if one has no other information available. The

results of the study by Carson (1944), which

investigated folding endurance as a function of

relative humidity, are presented in Table 7-4. As

indicated in these studies, strength properties may

vary by as much as 40% with relative humidity

changes that are within the range of normal

conditions. Indeed, fold values may change by a

factor of five.

Considering the importance of moisture

content on paper properties, one would assume

this would be well studied so producers would be

able to help their customers to know how their

materials will behave under various conditions.

Manufactured paper immediately off the reel

that is not conditioned before testing may have a

moisture content that is several percent below

paper that has been conditioned in a standard

BASIC PAPER PROPERTIES 165

C^ 2.5'

-a

.2 1.5-

W 0.5-

0.1

NEWSPRINT

KRAFT WRAP

-^-

BOND,

FILTER, RAG

1—I—I

I I I I

1—I—t

I I I I—

-1—I—I I I I

100

Time, Hours

Fig. 7-4. Moisture gain of paper conditioned at 35% RH vs. time at 65% RH at 21**C (TO^'F).

From Jarrell (1927). The x-axis is a logarithmic scale.

room for an hour. Since paper properties may

differ significantly over such a moisture content

range, one might expect considerable problems in

routine mill measurement and quality control of

paper properties. The importance of controlling

the relative humidity during paper and textile

processing has been known for some time. Willis

Carrier installed several of his early air

conditioning units in textile and printing shops at

the turn of this century for this very reason.

These were air conditioning units and not air

cooling units. The three papers cited in this

section are excellent, thorough works and should

be consulted for more information on this topic.

7.5 PHYSICAL PROPERTIES OF PAPER

Ream

A ream is a specified area of paper or

paperboard, most frequently expressed in terms of

a specified nimiber of sheets (usually 500) of a

certain dimension. For example, a 24 X 36 x

500 ream means the combined area of 500 sheets.

each 24 in. by 36 in., for a total of 3000 ftl

Other ream sizes are expressed in area, such as

1000

ft^

for paperboard. The Tappi Standard ream

is 1 square meter of paper or board. For other

standard paper ream sizes see Table 12-8.

Basis weight, grammage

The weight of paper per ream, normally

expressed on an air-dry basis (about 6-10%

moisture, with 9% the standard) is known as the

basis weight. For example, a printing paper is

reported at 20 pounds per (24 in. X 36 in. x 500

sheet) ream; newsprint is 49 gsm (g/m^ is the

Tappi Standard

unit);

and one grade of lightweight

linerboard is 26 pounds per 1000 ft^ The basis

weight is measured either directly during paper

manufacturing with the j3-ray gauge or off-machine

by weighing a precisely cut piece of paper with a

balance (TAPPI Standard T 410, which includes a

table of ream sizes with conversion factors.)

Electronic balances (in preference to mechanical

balances) should always be used to increase speed,

accuracy, and precision.

166 7. PAPER AND ITS PROPERTIES

Table

7-3.

Paper properties as

function of

70**F

(IPC, 1937). Paper samples were allowed

to reach equilibrium at each relative humidity.

1 Base property values at 50% RH increasing

from

30% RH at 70°F |

Sample

Bond, rag

Bond, sulfite

glassine, sulfite

newsprint

Basis weight

g/w?

92.5

74.8

37.7

54.7

Burst, psi

97.3

35.7

16.2

8.3

Tensile, lbs

35.7

21.4

10.4

6.0

20.7

9.8

4.3

1.7

Stretch, %

5.8

2.5

1.8

1.6

9.8

4.6

3.2

2.1

Tearing g/16

sheets

100

Relative

humidity

40%,

increasing

65%,

increasing

75%,

increasing

75%,

decreasing

65%,

decreasing

50%,

decreasing

40%,

decreasing

Bond, 100% rag

Bond, sulfite

Glassine, sulfite

Newsprint

% Change in bursting strength from 50% RH (preconditioned at 30% RH)

-0.9

+ 1.9

-10.3

-12.4

-8.1

-4.6

-10.1

+ 1.5

-4.7

-10.1

-11.2

-3.4

-1.6

1 -1.1

+ 1.1

-2.0

-3.9

-11.1

-1.4

+0.7

+3.7

+3.6

+3.1

-15.3

-29.4

-27.1

-40.7

-18.6 1

PHYSICAL PROPERTffiS OF PAPER 167

Table 7-3 Cont'd. Paper properties as a function of

at 70''F (IPC, 1937).

Relative

humidity

40%,

increasing

65%,

increasing

75%,

increasing

75%,

decreasing

65%,

decreasing

50%,

decreasing

40%,

decreasing

40%,

increasing

65%,

increasing

75%,

increasing

75%,

decreasing

65%,

decreasing

50%,

decreasing

40%,

decreasing

40%,

increasing

65%,

increasing

75%,

increasing

75%,

decreasing

65%,

decreasing

50%,

decreasing

40%,

decreasing

Bond, 100% rag

Bond, sulfite

Glassine, sulfite

Newsprint

%Cliange in tensile, MD

from

50% RH (conditioned from 30% RH) ||

+4.5

-8.1

-10.3

-17.1

-11.1

-1.1

-1.4

-2.6

-3.8

-22.1

-20.2

-9.5

-6.0

-4.1

+7.1

-6.5

-14.4

-21.1

-11.0

-4.7

-3.4

-0.6

-7.4

+4.1

-16.8

-7.8

1.0

+8.1

%Change in tensile, CD

from

50% RH (conditioned from 30% RH) |

H-2.5

-3.4

-13.4

-15.5

-7.3

-3.4

-0.7

+4.9

-8.3

-22.1

-23.5

-18.0

-8.0

-2.2

+0.9

-10.7

-18.1

-21.0

+5.5

-5.8

-2.8

-0.7

-5.7

-15.6

-19.3

-11.6

-3.7

% Change in sttetch, MD

from

50% RH (conditioned from 30% RH) ||

+3.4

+8.6

+22.1

+24.1

18.9

+7.7

+8.6

-10.5

10.4

14.6

+24.9

+29.0

16.7

+6.2

-8.8

-2.9

+23.6

17.7

+8.8

+ 11.8

-0.7

-5.7

-15.6

-19.3

-11.6

-3.7

-3^

168 7. PAPER AND ITS PROPERTIES

Table 7-3 Cont'd. Paper properties as a function of

at 70°F (IPC, 1937).

Relative

humidity

40%,

increasing

65%,

increasing

75%,

increasing

75%,

decreasing

65%,

decreasing

50%,

decreasing

40%,

decreasing

40%,

increasing

65%,

increasing

75%,

increasing

75%,

decreasing

65%,

decreasing

50%,

decreasing

40%,

decreasing

40%,

increasing

65%,

increasing

75%,

increasing

75%,

decreasing

65%,

decreasing

50%,

decreasing

40%,

decreasing

Bond, 100% rag Bond, sulfite

Glassine, sulfite

Newsprint

% Change in stretch, in CD from

50%

RH (conditioned from 30% RH) ||

+ 1.6

+20.5

+29.2

+37.3

+35.3

+8.2

+6.7

-3.3

+30.4

+76.0

+41.2

+8.6

+5.3

-15.3

+4.0

+72.8

+50.4

+56.8

18.4

10.4

-9.5 1

+7.2 1

+23.9 1

+21.4 1

-7.1 1

-11.9 1

%Change in tear, MD

from

50% RH (conditioned from 30% RH) ||

-2.4

+ 16.4

+20.5

+28.8

+20.5

+ 17.2

10.2

+39.3

+79.5

+78.1

+81.0

+78.1

+45.8

+76.8

-6.5

11.7

+29.3

+55.3

+20.2

+3.9

+9.8

-6.8

13.1

12.8

+22.6 1

+ 18.6

+9.8 1

+7.8 1

% Change in tear, CD from 50% RH (conditioned from 30% RH) ||

-6.5

15.5

16.7

+33.0

17.6

[ +15.0

+11.4

-2.9

+9.9

+ 15.8

+24.6

12.9

1 +6.9

+1.0

-7.9

+ 11.9

12.6

1 +41.2

19.0

12.6

+12.6

-5.6 1

+ 14.8

+24.0 1

+31.3 1

+20.1 1

+^^-'^

+11.4 1

PHYSICAL PROPERTIES OF PAPER 169

Table 7-4. Fold endurance of paper from Carson (1944).

Relative

Humidity

1 15%

25%

1 35%

"^^^^

1 55%

65%

1 75%

Cotton writing

59^

115

219

276

468

697

160

171

229

366

444

Index bristol,

50:50

cotton:bleached

chemical

253

612

870

1070

1262

1139

1257

111

235

253

247

270

827

Softwood kraft,

unbleached

132

152

582

933

140 1

227

358

1252

1378

25:75 softwood:

unbleached

manila and jute

260

. 498

914

2126

1 2486

1 5007

10^+

163

280 1

504

835

1156

1427

1596

' Schopper units, no. of double folds required to reduce the breaking strength of 15 mm strips to 1 kg^.

Fig. 7-5 shows a mechanical and an electronic

balance for basis weight measurements.

Caliper

The nominal thickness of paper is known as

its caliper. Caliper is measured in mils for paper

or points for paperboard, both of which are 0.001

in. The metric unit is mm. The caliper of paper

is measured by using a micrometer with circular

contact surfaces of 16 mm (0.63 in.) diameter as

described in TAPPI Standard T 411. Fig. 7-6

shows two examples of micrometers for measuring

the caliper of paper.

Density, bulk

The density of a sheet of paper is its mass

per unit volume, which is also called the apparent

density. Normally density is expressed as

glove?.

The density of papers is typically

0.5-0.8

glove?.

For comparison, cell wall material has a density of

1.5 g/cm^ which means most paper has a high

volume of air space. The density is an indication

of the relative amount of air in the paper which, in

turn, affects optical and strength properties of

paper. Density is calculated from the basis weight

and caliper of paper as shown below. Bulk is the

reciprocal of density and is expressed as cmVg.

Fig, 7-5. Mechanical (left) and electronic basis weight scales (right).

170 7. PAPER AND ITS PROPERTIES

Fig. 7-6. Mechanical Oeft) and electronic calipers (right) for paper.

density

basis weight g/cm^

caliper

tliickness

(7-1)

0.001

mmm^ g/m^ basis weight

cm"

thickness

Smoothness

Smoothness is a measure of the surface

contour of paper. Smoothness is measured by the

Bekk method, TAPPI Standard T 479, by

measuring the time to draw 10 ml of air radially

over the surface of paper clamped in an anvil.

Fig. 7-7 shows the apparatus. The insert

shows the anvil in the reflection of the glass. Air

is introduced through four small holes inside of the

inner ring. Air escaping through the first ring is

trapped within the second ring and diverted to one

of the flow meters. The less smooth the paper

surface, the faster the air will escape through the

inner ring.

Gloss finish

Paper with a gloss finish has a very smooth

surface (see Fig. 9-62) giving a high degree of

specular light reflectance. TAPPI Standard T 480

describes specular gloss at 75°. For lacquered,

highly varnished, or waxed papers the gloss at 20°

should be measured (TAPPI Standard T 480.)

Transport properties (air resistance and water

vapor transmission)

The air resistance of paper is measured by

the Gurley densitometer (TAPPI Standard T 460)

as shown in Fig. 7-8. It depends on the level of

pulp refining, the paper's absorbency, specific

gravity, and other variables. TAPPI Standard T

536 describes a high pressure method for papers

with high levels of air resistance. The Sheffield

test of air porosity is best suited for smooth,

reasonably incompressible papers (TAPPI Standard

T 547 which also includes a conversion table for

estimating Gurley-Hill values from Sheffield

PHYSICAL PROPERTffiS OF PAPER 171

Fig. 7-7. Paper smoothness by the Bekk

method. The msert shows the paper test area.

units).

In this test, the specimen is exposed to air

pressure of 10.3 kPa (1.5 psi).

The rate of water vapor transport through

paper is measured with 90% RH on one side of

the paper and desiccant on the other side (TAPPI

Standards T 464 at 37.8°C or 100°F and T523.)

It is significant in packaging papers that must

retain moisture of their contents.

Wire side and felt side (air side)

The wire side is the side of the paper formed

against the moving wire of the paper machine.

Paper formed on twin wire machines has two wire

sides.

The felt side is the opposite side of the

paper from the wire side (for paper from single

wire machines). TAPPI Standard T 455 helps

with the identification of the wire side using direct

observation, carbon smudge, wetting, wetting and

charring, tearing, and marking with a soft pencil.

It may be difficult to ascertain the wire side of

coated and certain specialty grades.

Trim

Trim is the width of the paper on the

machine. The web is usually trimmed twice, once

at the couch roll by water squirts and once at the

rewinder with circular knives.

Formation

Formation is a term used to describe the

uniformity of the sheet structure and orientation of

the fibers. Poor or wild formation means irregular

distribution of fibers in the plane of the sheet,

resulting in many thick and thin spots. Good

formation indicates uniform fiber and filler

distribution in the sheet.

Watermark

A watermark is a pattern that is deliberately

put on some bond papers. It becomes visible by

looking through the sheet as it is held up to light.

It is imparted by a raised pattern on a dandy roll

near the end of the fourdrinier wire.

Deckle

The two edges of the paper web on the

forming wire are called the deckle. Deckle edge

means untrimmed paper with its original rough

textured edges on the side.

Finish and cockle

Finish refers to the appearance or feel of the

paper surface. Cockles are irregularities in the

sheet conformation leading to a rough, pock-

marked surface.

Curl

Papers tend to curl, especially when each side

of the paper is at a different moisture content. A

paper that has different properties or compositions

on the top and bottom sides (two-sided papers)

also tend to curl, acting much like a bimetallic

strip does in a thermostat coil. The edges usually

tend to contract in the cross machine direction.

The inclination of paper to curl is measured by

TAPPI Standard T 466 which involves treating

one side of paper with water and measuring the

curl as a function of time.

Sizing (or lack thereof)

Most printing papers and other papers used in

contact with liquids must be able to resist water

172 7. PAPER AND ITS PROPERTIES

Fig. 7-8. Mechanical Qeft) and electronic Gurley densitometers.

penetration. (See sizing agents in Section 8.4.)

Sizing, however, tends to reduce paper strength

and brightness, and increases the cost if ASA and

AKD are used. The more commonly used sizing

tests are reported here, but there are many more

as well. The surface wetability of paper is

determined by measuring the contact angle of a

drop of water on the paper. It is used to measure

sizing and printability of glassines, printing

papers, and film laminates (TAPPI Standard T

458.) Holdout is related to sizing and is the

ability of paper to resist penetration by liquids.

Non-bibulous or hard-sized papers have a

high degree of resistance to water penetration.

They are characterized by the Cobb size test

(TAPPI Standard T 441) or the Hercules Size Test

(TAPPI Standard T 530). In the Cobb size test

(Plate 25), a 1 cm thick pool of water is placed

over 100 cm^ of weighed paper restrained by a

ring collar (11.28 cm in diameter.) After a period

of time (usually 2 minutes), the water is drained,

and the sheet is treated with blotting paper and

then weighed. The absorbed water is reported in

mg/w?.

Low Cobb size values indicate good

PHYSICAL PROPERTffiS OF PAPER I73

sizing. This method does not distinguish between

hard sized and very hard sized paper.

With the Hercules size test (HST, Plate 26),

10 ml of green dye (with 1% formic acid, 10%

formic acid or other solution depending on how

well sized the paper) is added to a reservoir

contained by a metal collar with paper on the

bottom. A timer is started and the reflectance of

the bottom surface of the paper is monitored.

Once the reflectance decreases to a pre-determined

point, such as 80%, the timer stops and the time

in seconds is recorded. High HST values indicate

good sizing. Its reproducibility and sensitivity

makes it very useful for determining subtle

differences in hard sized papers that the Cobb test

would not be able to distinguish.

Slack-sized (moderately sized) papers can be

characterized by the so-called sugar-boat or dry

indicator test (Fig. 7-9, TAPPI Standard T 433).

A sample of paper is formed into a "boat" (with

the inside of the bag outward) that is filled with

powdered sugar containing a dye. The boat is

placed on the surface of water and the time is

measured for water to travel through the paper

until the sugar absorbs some water, which causes

the dye to appear abruptly. Although there is

variability from operator to operator, this is a U.S.

government specification for some grades of

paper. Slack sized papers may also be char-

acterized by the water inmiersion test (TAPPI

Standard T 491) that involves 10 minute

immersion into water, blotting, and weighing.

Water absorbency of

bibulous

papers (unsized

Fig. 7-9. The sugar-boat test. The sample on

the left has not passed the point of water

absorption, but the sample on the right has

absorbed water.

papers such as blotting, tissue, or towel) is

described in TAPPI Standard T 432. The

absorptive quality of blotting paper can be

quantified by ink absorbency according to TAPPI

Standard T 431.

Printability, Bristow test

Often the traditional sizing tests have been

used to indicate how well paper will respond to

ink. However, ink jet printers print on the surface

of paper and so conventional sizing techniques,

which measure liquid penetration, are not suitable

indicators of print quality for these papers. The

Bristow test gets around this limitation. The test

measures the track length produced when a liquid

is spread over paper at different speeds

corresponding to 0-2 seconds distribution time to

give a roughness index and absorption coefficient.

Fig. 7-10. The Bristow printabUity test. The

insert shows the "headbox" where ink

applied.