NBSIR 80-2190 (R)

JUN 2 4 1981 Characterizing the Interfiber

Bonding of Currency Paper Pulps

J. C. Smith and E. E. Toth

Polymer Science and Standards Division

Center for Materials Science

U.S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

Progress Report Covering the Period

October 1, 1979 through September 30, 1980

November 1980

Note: This document has been prepared for the use of the

Bureau of Engraving and Printing, Responsibility for its further use rests with that agency.

Prepared for Bureau of Engraving and Printing

U.S. Department of Treasury

Washington, DC 20401

NBSIR 80-2190

CHARACTERIZING THE INTERFIBER

BONDING OF CURRENCY PAPER PULPS

J. C. Smith and E. E. Toth

Polymer Science and Standards Division

Center for Materials Science

U.S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

Progress Report Covering the Period

October 1, 1979 through September 30, 1980

November 1980

Note: This document has been prepared for the use of the

Bureau of Engraving and Printing. Responsibility for its further use rests with that agency.

Prepared for Bureau of Engraving and Printing

U.S. Department of Treasury

Washington, DC 20401

U.S. DEPARTMENT OF COMMERCE. Philip M. Kiutznick, Secretary

Jordan J. Baruch. Assistant Secretary for Productivity, Technology, and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director

Table of Contents

Page

Summary and Conclusions i

Introduction 1 Materials Studied 2 Results From Scanning Electron Microscopy 4

Interaction of Water and Cellulose 5

Density of Bonding 7 Characterization of the Force-Elongation Curve 8

A Mechanism for Force-Elongation Behavior 11

Characteristic Lengths for Low-Density Handsheets 13

Made From Various Pulps

References 15

i.

Summary and Conclusions

The nature of the adhesion between fibers in a currency pulp is different

from that between fibers in kraft woodpulps. These differences are revealed by

visual inspection using scanning electron microscopy, by studies of the effect of

the beating process in the cellulose-water interaction in pulps and by the

tensile behavior of low-density open-web handsheet specimens made from the

various pulps.

A beaten currency stock was obtained, and pulp from this stock was

2 fractionated by fiber length to obtain four fractions. Handsheets of 2.5 g/m

mass per unit area were prepared from the unfractionated stock, from each of the

first four fractions and from a batch of stock from which fines passing through a

150 mesh screen were removed. Similar handsheets, for comparison purposes, were

prepared from beaten and unbeaten Northern and Southern kraft woodpulps.

Morphologies of the handsheets and constituent pulp fibers were examined by

scanning electron microscopy. These studies showed that the unfractionated

currency pulp contained a large proportion of fines that collected at the fiber

crossovers and appeared to act as a glue holding the fibers together. Fibers of

the long-fiber-length fraction had thick cross-sections, but also were highly

fibrillated, each fiber having many fine fibrils that formed bonds with adjacent

fibers. This was in contrast to the morphologies of the woodpulps. Southern kraft

pulp has coarse fibers that become ribbon-like after beating, but do not

fibrillate. Bonding between Southern pulp fibers depends upon intimate contact

between the fibers. Northern kraft pulp fibers have a finer texture and are more

ribbon-like by nature, and do not require as much beating to induce flexibility

and promote intimate interfiber contacts.

The morphological behavior of these pulps brought about by beating is in

agreement with results obtained in a separate study of the cellulose-water

interaction in the two types of pulps. During the preparation process pulp is

subjected to mechanical action such as beating or refining. This treatment breaks

down the structure of the fibers and permits them to imbibe water. In places

where the crystalline microstructure is broken water will be strongly bound to

the freshly formed crystalline surfaces, but where only the macrostructure is

broken forming fissures or holes in the cell walls, the entering water is not

strongly bound and may be regarded as free water. This free water is believed to cause swelling thereby imparting flexibility to the fiber and facilitating

interfiber conf ormability . It was found that beating causes increasing amounts of

water to be imbibed by both cotton and woodpulp fibers. However in the cotton

more of this additional imbibed water becomes bound water than is the case in

woodpulp. This suggests that beating of woodpulp produces flexible conformable

fibers, but causes fibrillation and fines formation in cotton fibers. Similar

measurements have recently been carried out on microcrystalline cellulose, which

simulates the fines produced by beating currency stock. Results are inconclusive

but will be supplemented by additional research planned for next year.

Specimens 1 cm wide by 2 cm long from the handsheets were extended in a tensile tester. The force-elongation curves obtained had numerous force drops,

each drop caused by the breakage of a bond between fibers constituting the

handsheet network. The relative number of bonds per unit area ordinarily can be

characterized by determining the average elongation between bond breaks.

Measurements of this type were made without difficulty on specimens from the

Southern pulp handsheets. However force-elongation curves for specimens from the

currency pulp handsheets all had numerous force drops, mostly small and

undifferentiated. It was not possible to obtain reliable representative

measurements for these materials. This force-elongation behavior, though, is in

accord with the behavior that might be expected of a highly fibrillated material

held together by numerous small bonds.

Originally it was intended to characterize bond strength in terms of an

energy parameter, obtained by averaging the energy dissipated by the fibrous

network for each of a number of selected bond breaks, or by an alternative

parameter obtained by averaging the force drops. These parameters could be useful

for characterizing woodpulps, but would be difficult to determine for currency

pulps. A more appropriate parameter might be related to the average force level

at which bonds break. Work toward the development of this parameter is now in

progress. As part of this work a considerable effort has been devoted to the

problem of characterizing the force-elongation curve, up to the first bond break,

of a fibrous network.

It has been found experimentally that when the slope F' of the forceelongation curve for a specimen from a low-density open-web handsheet is plotted

versus the force F, the resultant plot can be closely fit by a straight line

having the equation

dF

dx

+ F)
'
111..

where x is a distance variable measured in the direction of extension, and x and ’ c

C

2 are constants. Thus, by integration, an equation for the force-elongation

curve can be obtained in the form

(x-x )/x

F = C

2 [e ° ° - 1]

where x is the value of the distance variable at which the force just increases

o from zero, so x-x

q

is the elongation of the network. The parameters x^ and C

2 are found by fitting a straight line to the plot of F' versus F, and the

parameter x

q is found by fitting the equation of the curve (with x^ and C

2 already evaluated) to the force-elongation data.

If at intervals during a test on a handsheet specimen the extension is

stopped, reversed until the tensile force in the specimen is zero, and then

resumed, a series of force-elongation curves is obtained characterizing the

specimen at various stages of deterioration during the test. It is found that x

q

increases with each successive curve, indicating that the "unstrained" length

increases during a test, x^ decreases slightly as the test proceeds, and C

2 decreases fairly rapidly.

x^ and C

2 are expected to be strongly dependent on the density or mass per

unit area of the specimen. This was verified for x by measuring values of x for

c

2 C

a series of specimens having densities varying between 1.25 and 2.50 g/m . x^ was

found to decrease as the density increased. C

2 is expected to increase with

density but this expectation has not been adequately checked experimentally. If

x^ and C

2 are sensitive to specimen density changes they should vary from

specimen to specimen because of fluctuations in the local density of the

handsheet from which the specimens are taken. This suggests the possibility of

using x and C

2 to reduce adhesion parameters determined on different specimens

to a standard value, and thus improve the precision of measurements on a given

handsheet.

The parameter x^ has the dimensions of length and can be referred to as a characteristic length. This characteristic length appears to increase or decrease

with the average size of the meshes in a specimen fiber network. For instance, x^

was measured on specimens from Northern and Southern kraft pulp handsheets, both

2

of 2.5 g/m density, x^ for the Southern pulp was larger than the x^ for Northern

pulp. This is because fibers of Southern pulp are coarser than fibers of Northern

pulp; hence a Southern pulp handsheet has fewer fibers per unit area and larger

IV.

meshes than does a Northern pulp handsheet. In another instance it was shown by

measurements of x^ that beating decreases the mesh size of Northern and southern

pulp handsheets, presumably by increasing the proportion of fiber crossovers that

are bonded. Measurements of x^ on handsheets from currency pulp fractions showed

that the mesh size of the handsheets made from fraction III (short fibers) was

appreciably less than that for the others, x^ appears to be an especially useful

and easily measured parameter for use in these studies, and further applications

of it will be investigated.

1 .

Introduction

Paper used to make currency should have a good balance between durability

and cost, as the replacement of worn out currency is an expensive operation.

Therefore it would be desirable to identify and characterize those aspects of

the morphology of paper networks that affect the durability and other desirable

properties, and to obtain a better understanding of how these properties are controlled by the manufacturing process. Researches of this nature might

contribute to the development of durable paper made from blended woodpulps

instead of the more expensive cotton and linen pulps presently used.

During the past year these researches have concentrated on studying the

properties of very thin low-density open-web handsheets made from currency pulp

and from Northern and Southern kraft woodpulps. Samples from the handsheets were

extended in a tensile tester to obtain force-elongation curves from which data on interfiber adhesion could be obtained. Mathematical relationships describing

force-elongation behavior were derived, and a parallel-spring model for

elucidating this behavior was proposed. Preliminary values were obtained for some parameters of use in characterizing interfiber bonding. Morphologies of the

handsheets and constituent fibers were examined by scanning electron microscopy.

Some research on the cellulose-water interaction was also carried out. This

progress of research conducted this year is reported in the following text.

V

2 .

Materials Studied

Currency stock was obtained from Crane and Co. , Inc. The stock was collected

after the beater, but before passing through the refiners and jordan. A portion

of this stock was fractionated in a Bauer-McNett classifier using screens of

mesh sizes 14, 35, 65 and 150. Four fractions were obtained. A fifth fraction

consisting of fines passing through the 150 mesh screen was not collected.

Classification data are given in table 1. Open-web handsheets were prepared from

the unfractionated material, from each of the first three fractions, and from a pulp obtained by a separate classification in which only the fines were removed.

Two types of woodpulp, Northern and Southern softwood kraft pulps, were also

used in these studies. Batches of these pulps at a consistency of ten percent

were subjected to beating in a PFI laboratory beater for the desired number of

revolutions. There was no clearance between the bedplate and roll, force was

33 N (3.4 kg), and the relative velocity of roll to bedplate was 6 m/s.

Handsheets were formed from the woodpulps as follows: A 33 x 33 cm sheet of

filter paper (Whatman No. 541) was placed on top of the support wire of the

deckle box and wetted. The deckle box was closed, clamping the sheet firmly at

the bottom. Water was then added to the box. An appropriate amount of pulp

was placed in approximately one liter of water and disintegrated in a British

disintegrator for 7500 revolutions for beaten pulp, or 25,000 revolutions for

unbeaten pulp. Whenever unbeaten pulp was used it was first soaked overnight in a liter of water prior to disintegration. The disintegrated pulp was added to the

water in the deckle box, agitated carefully, and then drained through the filter

paper.

The filter paper and deposited pulp fibers were placed on a cushion of

pulpsheet. A 33 x 33 cm ferrotype plate was placed carefully over the top of the

filter paper and layer of pulp fibers. Another pulpsheet cushion was placed over

the top of the ferrotype plate, and the entire sandwich was placed in a hydraulic

2 press for five minutes at a pressure usually near 0.34 MPa (50 lb/in ). During

the pressing the layer of fibers transferred to the ferrotype plate. It was

allowed to dry and was then carefully removed. In some cases the handsheet did

not transfer to the ferrotype plate, but pretreating the filter paper with a fluorocarbon spray lubricant usually eliminated this difficulty.

2 Densities of most of the handsheets manufactured in this way were 2.5 g/m

mass per unit area, but for some special purposes other handsheets of densities

3.

varying between 1.0 and 5.0 g/m were also prepared. Handsheets in this density

range have open webs, and are only two or three fibers thick in most places.

The process just described was used to prepare handsheets from the

unfractionated currency pulp, from each of the first three fractions and from a reconstituted pulp made from the first four fractions recombined in their

original proportions. It was more difficult to prepare these handsheets because

the webs did not transfer readily to the ferrotype plate, and those that did

transfer could not be easily removed subsequently. This was particularly true of

the webs made from the unfractionated pulp. However it was found that if the

filter paper was given a heavy coating of Scotchgard transfer was achieved, and

sufficiently large pieces of handsheet could later be removed from the ferrotype

plate

.

Proprietary water repellant containing f luoroaliphatic resin, manufactured by

3 M company.
'
4.

Results From Scanning Electron Microscopy

Specimens from the currency pulp handsheets were photographed in a scanning

electron microscope. For comparison purposes SEM photographs were also obtained

for similar handsheet specimens of Southern kraft pulp, unbeaten and beaten

5000 revolutions in a laboratory beater. Figure 1 is a micrograph of an

unfractionated currency pulp specimen, and figure 2 is a micrograph of a specimen

reconstituted from the first four fractions. Most of the fines material in figure 1

is not present in figure 2. The appearance of these micrographs suggests that the

fines act as a glue filling the interstices between pulp fibers and helping

cement them together. Figure 2 shows that the pulp fibers have rounded crosssections that do not always make intimate contact, but that fibrillation provides

many minor adhesions between fibers. The unbeaten Southern pulp specimen is shown

in figure 3. The fibers in this specimen have a rounded cross-section and do not

make intimate contact, so that fiber cross-overs are not strongly bonded. The

effect of beating the Southern pulp is shown in figure 4. The fibers here have

been flattened into ribbons that make better contact and provide stronger bonds,

but the beating has not caused fibrillation, and the frequent minor adhesions

shown in figure 2 are not present. The appearance of Northern pulp fibers is

similar except that the unbeaten fibers are flatter, and subsequent beating

enhances this ribbon-like structure.

It should be recalled that when beaten Southern pulp handsheets were

prepared, the webs transferred readily to the ferrotype plate, and were easily

removed subsequently. When handsheets were prepared from unfractionated currency

pulp the webs would not transfer unless the filter paper bed had previously been

heavily coated with a water repellent. This reinforces the idea that bonding in

currency pulps is facilitated by the hydrophylic fines acting as a glue, and is

also enhanced by a large number of bonds to the thin fibrils produced by a

beating treatment. Thus the bonding mechanism in currency paper is different

from that in wood pulps where bond strength depends on intimate contacts between

pliable ribbon-like fibers relatively free of small fibrils 0

5.

Interaction of Water and Cellulose

During the preparation process, pulp is subjected to mechanical action

such as beating or refining. This treatment breaks down the structure of the

fibers and permits them to imbibe water. In places where the crystalline microstructure of the fiber is broken water will be strongly bound to the freshly

formed crystalline surfaces, but where only the macrostructure is broken to form

fissures or holes in the fiber cell walls the entering water is not strongly

bound and may be regarded as free water. This free water is believed to cause

swelling thereby imparting flexibility to the fibers and facilitating interfiber

conf ormability . The way in which different pulps respond to a beating treatment

can be studied by measuring the proportions of bound and free water in different

pulp samples.

The water imbibed by a pulp specimen in the swollen state is measured by

solute exclusion. A pulp specimen is added to a solution of dextran in water, and

the excess water in the specimen dilutes the solution. The amount of dilution is

measured by polarimetry, and the imbibed water, or fiber saturation point, of the

specimen is calculated. Part of this imbibed water is tightly bound to the

cellulose and does not freeze at temperatures down to 233 K. This bound water

fraction is measured by differential scanning calorimetry. The difference between

the fiber saturation point and bound water is considered to be free water.

In some previously reported work [1] it was found that beating causes

increasing amounts of water to be imbibed by both cotton and woodpulp fibers.

However beating causes an increasing fraction of the water imbibed to be bound

in cotton. This does not happen to as great an extent in woodpulp. Thus beating

of woodpulp produces flexible conformable woodpulp fibers, but causes fibrillation

and fines formation in cotton fibers.

Some more recent work has been concerned with achieving a better

understanding of what the fiber saturation point of a sample actually measures.

In determining the total amount of water within pulp fibers in the swollen state

(fiber saturation point) it is assumed that water bound to the surface of the

fibers and in contact with the dextran solution behaves as free water, and will

take part in the dilution of the dextran solution. If water bound to the surface

does not dilute the. dextran solution the calculated value for the fiber

saturation point will be misleading. The magnitude of the discrepancy involved

will depend upon the extent of the surface area.

6 .

In an effort to determine whether water bound to the surface does dilute the

dextran solution, bound water and the fiber saturation point were determined on a commercial sample of microcrystalline cellulose a Presumably the crystalline

cellulose is nonporous, and no water would be expected to enter the crystalline

lattice. If water bound to the surface of the crystalline cellulose does not

enter into the dilution of the dextran solution, the bound water content and the

fiber saturation point would by nominally equal. If the surface bound water does

dilute the dextran solution there would be no fiber saturation point.

For the commercial sample of microcrystalline cellulose the bound water

content was 0.25 g l^O/g cellulose, while the fiber saturation point was

0.56 g H^O/g cellulose. These results indicate that not only the bound water but

additional water, such as perhaps loosely bound water, does not enter into the

dilution of the dextran solution, or that the microcrystalline cellulose is

porous and is penetrated by water.

7.

Density of Bonding

Specimens from the handsheets were tested in a tensile tester to obtain

recordings of force versus elongation. Figure 5 shows the result of a test run

on a 2-cm specimen of unfractionated currency pulp. Full scale force is

49 mN (5 g), and 1 cm of chart travel corresponds to a specimen elongation of

0.01 cm. The numerous "jags" or force drops indicated on the curve correspond

to interfiber bond breaks; thus the average elongation between breaks should

provide a measure of the density of bonding per unit area of the test specimen.

In this case there are frequent jags of small magnitude suggesting the presence

of numerous minor adhesions between the constituent fibers. Force-elongation

recordings for specimens from the other currency pulp handsheets were similar.

Figure 6 shows the result of a test run on a 2-cm specimen of Southern

kraft pulp beaten 5000 revolutions in a laboratory beater. The jags in this case

are not so frequent or so small in magnitude, suggesting the relative absence

of minor adhesions.

For handsheets of the same mass per unit area the density of bonding per

unit area should depend upon the coarseness (mass per unit length) of the fibers

forming a web, and may also be slightly dependent upon the average length of the

fibers. In addition, the average elongation between bonds, which gives an indication of bonding density, should be larger for narrow specimens than for

wide ones. The average elongation per break was measured on the currency pulp

specimens to see if differences in the density of bonding could be detected. The

results of these measurements are given in table 2. No obvious differences in

density of bonding are revealed by these measurements. However it should be noted

that the values given for specimens of different width are essentially the same.

A possible interpretation of this is that the numerous breaks due to minor

adhesions are swamping the data. Under these conditions it will be necessary to

devise a consistent means of screening out the jags due to minor adhesions in

order to obtain more meaningful results 0 Measurements on specimens from the Southern kraft pulp handsheets are given

in table 3. These results show that beating greatly increases the number of bonds

at fiber cross-overs. Also, the elongation per break is seen to increase as the

specimen width decreases, in accordance with expectations but in contrast with

the behavior observed in the tests on currency pulps. This difference in behavior

may be attributed to the fact that beating Southern pulp fiber does not produce

the kind of fibrillation observed in the beaten currency pulp.
..

-
8 .

Characterization of the Force-Elongation curve A previous report [21 has discussed various bonding parameters that can be

derived from tensile test data. The bond strength could be characterized in

terms of an energy parameter, obtained by averaging the energy dissipated by the

fibrous network for each of a number of selected bond breaks, or in an alternative characterization the drops in force could be averaged. An examination

of figure 5 however shows that the force drops resulting from breakage of bonds

between the pulp fibers are obscured by the many small drops resulting from bond

breaks in which tiny fibrils are involved. For this reason a different bonding

parameter such as the force level at which the principal bonds break might be

more appropriate. In order to develop this and other parameters and to increase

our knowledge of the mechanical behavior of fiber networks, much of this year's

research effort was devoted to the problem of characterizing the force-elongation

curve of a fibrous network.

Consider the force-elongation curve, figure 7, obtained by testing a

2

specimen from a 2.5 g/m handsheet of Northern kraft pulp. The specimen was 2 cm

long and 1 cm wide. 1 cm of chart travel corresponds to a specimen extension of

0.01 cm. Full scale force for the specimen was 98 mN (10 g) . At intervals the

test was stopped, the extension was reversed until the tensile force in the

specimen was zero, and the specimen then reextended to obtain a new forceelongation curve. Recording traces during reversals of the cross-head travel are

not shown.

The general condition of the specimen at various stages is noted on the

figure. For the first five reextensions the specimen did not develop any large

holes or tears, and the state of the specimen at the beginning of the reextension

is described as "intact". At later stages of the test the specimen developed

holes, and tore at the edges until in the final stages the tensile force was

sustained by only a few parallel fiber paths collapsed together into filaments.

In figure 8 are shown plots of slope F', or force per unit elongation,

versus force F for each of the reextended force-elongation curves of figure 1.

The origin of the F' scale has been shifted vertically for each of these plots,

and arbitrary units have been selected to display the plots to best advantage.

The curve corresponding to each plot is identified by its region of chart travel.

The plots are seen to be linear and to have positive intercepts on the F' axis.

Data from the initial force-elongation curve obtained before any reextension

does not plot satisfactorily and is not shown.

9.

|

j

j

Similar behavior has been observed in tests on handsheets of various masses

"per unit area formed from the different pulps. The linearity of the plots

| obtained from these tests is not affected by the amount of deterioration incurred

by the specimen during extension. In many instances the slopes corresponding to

initial and final forces do not plot on a straight line, but the intermediate

data are linear to a good approximation. The intercepts on the F

1 axis are

usually positive, and tend toward zero as the specimen deteriorates. In a few

instances the intercepts become negative. This latter condition is probably an

artifact. It can be caused, for instance, by a specimen with a large tear

extending inward from one side. Such a specimen requires a large initial

extension before the network structure becomes appreciably strained.

It has been found from many tests that F

1 , F data from the initial forceelongation curve in a series often does not plot on a straight line. In some

instances a straight-line plot is obtained but the slope of the plot is much

steeper than the slopes of F

1 , F plots for subsequent reextension curves. It is

only after the specimen has been "broken in" that data can be obtained from

subsequent curves. In the test of figure 7 the first reextended curve was

obtained before the specimen was completely broken in, and the curve is similar

to one that would be obtained by an additional force acting in parallel with the

tensile forces sustained by the specimen network. This strong parallel force may

involve bending forces and other forces required for an initial alining and

shaping of the specimen.

Force-elongation curves of these paper network specimens exhibit viscoelastic behavior. Thus the shapes of the reextension curves will depend on the

waiting time between halting of an extension and the starting of reextension. In

order that experimental results be as consistent as possible the waiting times

should be approximately the same for all reextension curves. The effects of

visco-elasticity are not considered in the theory presented here, but their

possible influence upon experimental results should be kept in mind.

Let the equation of a straight line faired through a plot such as those

shown in figure 8 be

S ' I

(C

2

+ F)

c

( 1 )

where x is a distance variable measured in the direction of extension, x is the

c reciprocal of the slope of the straight line, and C 9 /x is the intercept on the

L.
.
10 .

F

1 axis. Eq (1) has the solution

x/x

F = C^e ° - C

2 (2)

where is a constant to be evaluated from data such as figure 7.

Table 4 illustrates how data from the curve between 7.0 and 13.0 cm chart

reading on figure 7 can be fitted by a curve of the type, eq (2). The first

column lists cross-head displacements, and column 2 lists corresponding values

of force obtained from the force-elongation curve. Intermediate values of force

obtained by interpolation are listed in column 3. The slopes in column 4 are obtained by dividing successive force differences by differences in cross-head

displacement. The data of columns 3 and 4 are plotted, as in figure 8, and a least-squares fit of eq (1) yields the values: C^/x^ = ^50. 48 mN/cm,

x^ = 0.033434 cm. Eq (2) can then be fit by the least-squares technique to data

in columns 1 and 2 to yield the value = 1.0636 mN. The fitted equation is used

to calculate the forces in column 5. In this instance a good fit is apparent.

According to eq (2), F becomes zero when

x = x = x la(C /C ) (3)

o c 2 I Thus eq (2) can be put in the form

(x-x )/x

F = C

2

[e ° ° - 1] . (4) x is the value of the distance variable at which the force just increases from

zero, so x - x^ is the elongation of the network. As the specimen fibers become

better alined, and as the specimen deteriorates with increasing stretch, the

unstrained length of the specimen increases. Thus for each successive reextension

curve of figure 7, successively increasing values of x

q

should be expected.

Ideally the successive values of x

q

could be calculated using eq (3).

Table 5 provides an example. Here are tabulated the values of C^, C

2

, x^ and x

q

calculated from each of the reextension curves of figure 7. The agreement between

values of x^ in cm of chart with the apparent values obtained by visual

inspection of figure 7 is generally good. However the value of x^ calculated for

the curves in the 27.0-35.0 cm range is too low. In this case the curve, eq (2),

that best fits the data over most of the range does not fit well initially. This

is probably an effect due to deterioration of the specimen.
.
11 .

A Mechanism for Force-Elongation Behavior

When a fibrous network is stretched segments of the fibers composing the

network tend to orient themselves in the direction of the extension, and to

resist the extension through the combined action of forces along their lengths.

As the extension proceeds more and more of these segments between bonds become

oriented and bear load. As a result of this process the force-elongation curve

has a slope that increases with increasing elongation. Thus the reaction of the

network to extension could be modeled by a system of parallel filaments of

unequal length, each filament adding to the resistive force of the system as the

extension is increased. The stiffness and the degree of orientation of the

network fibers also contribute to the network's reaction to stretching, but

these and other effects will be considered as secondary in importance and usually

be disregarded.

Consider a system of parallel filaments. Let the length of the shortest

filament, or unstrained length of the system be J , and let the length of any

other filament be 2 = \ . Let the number of filaments having lengths between

H and 2L + d^i be f(7’)d?1 . Let the filaments all have the same spring constant k,

neglecting the decrease in k with increase in unstrained length. If this system

of parallel filaments is extended from a length to a length l

Q + S the sum S

of the tensile forces in the filaments is

s = j kfOjHf-pd->i

If it is assumed that for ^>0

2 ?/x

ftf?) = ( c

2

/ kx

c ) e eq (5 ) becomes

(5)

( 6 )

S = (C

2

/x

c*”

) (f-?)d^ = C

2

[< ih

C

- 1] - C

2

(f/x

c

) fJ

(7)

2

:S 7/x s /x

or (C

2

/x

c ) j

e + C

2

(^/X

c

) = C

2 [e ° “ 1]

If j is set equal to x - x^ the expression on the right hand side of this equation

is the same as the expression, eq (4), for the force in a fibrous network. This

can be interpreted to mean that the force in a fibrous network can be simulated

by a system of parallel filaments each of spring constant k and having a length

distribution function given by eq (6), acting in parallel with a spring of spring

constant C n /x and unstrained length t . 2 c o

12

This spring may be considered as composed of n filaments each of length L

and spring constant k. That is,

C 0 /x = nk (8)

L c Thus when a fibrous network specimen is elongated by an amount 5, the number of

extended filaments in the model representing tne network is

1

\ c ?/x //x C //x

N = ntl + ± e

C

d^] = ne

0 = ^ ec (9)

4> c The values of the parameters and x^ are dependent upon the density of

distribution of the parallel filaments modeling the handsheet test specimen.

Suppose, for instance, that there are two specimens A and B taken from two

handsheets of different masses per unit area made from the same pulp furnish.

Let specimen B have the greater density. The parameters x _ and C 0 _ might be

expected to have the following relationships to the corresponding parameters for

specimen A:

x

cB

_1cl X

cA

'2B = pc 2A

( 10 ) (ID

where CL and p are functions of the density ratio D^/D^ and have values greater

than unity.

According to the assumptions, eqs (10) and (11), the distribution functions,

eq (6), for the two specimen models are related as

F ('h

)

= b

n y

'2B ^

/X

cB 2

C

2A

e =ap

xx ... kx

5? a/X

cA

( 12 )

'cB TM cA

Thus to obtain the distribution function for the specimen B from that for

specimen A, the scaling factor x should be multiplied by 1/cc, and the

CA. coefficient C„ by p. This effectively raises the exponential term by the

factor CL and the constant multiplier by the factor CL p. The scaling factor has

the dimensions of length, and will sometimes be refered to as a "characteristic'

length.

13.

Characteristic Lengths for Low-Density Handsheets Made From Various Pulps

Characteristic lengths x^ were obtained by measuring the slopes of the

F

1 versus F plots for a number of test specimens. The value of x^ for a given

specimen was taken as the average of the x^ values for the first few reextension

curveSo Only reextension curves taken when the specimen was "intact" were used.

The value of x^ tended to decrease slightly for each of the successive

reextension curves, as may be seen in the data of table 5, but this was not

believed to affect significantly the average values of x

c that were obtained.

Table 6 lists the characteristic lengths obtained from tests on a series

/ 2

of Northern kraft pulp handsheets of densities from 1.25 to 2.50 g/m . According to eq (10) values of x^ should decrease as the density of the

handsheet increases. The experimental values are in agreement with eq (10) for

2 densities of 2.0 g/m or less. This is more easily seen in the plot of x^ versus

handsheet density, figure 9. More tests are needed to check eq (10) at densities

2

greater than 2.5 g/m . Tables 7 and 8 list characteristic lengths x for Northern and Southern

2 c kraft pulp handsheets of 2.5 g/m density made from pulps subjected to different

amounts of beating. The Northern pulp handsheets made from pulp beaten 1000,

2000 and 5000 revolutions in a laboratory beater had approximately the same values for x^, but the characteristic length for the handsheet made from pulp

beaten 10,000 revolutions was slightly lower in value. Although all the

S, handsheets had the same density, or mass per unit area, more fiber cross-overs

were bonded in the handsheet made from the highly beaten pulp, resulting in a network with a shorter average mesh length, and smaller values of x^ for the

specimens tested 0 The results for Southern pulp handsheets were similar, as may

be seen from table 8.

2 According to tables 7 and 8 the characteristic length x^ for a 2.5 g/m

handsheet specimen of Northern pulp is appreciably less than the value of x

2

C

obtained for a 2.5 g/m specimen from a handsheet of Southern pulp. Southern

pulp is coarser than Northern pulp; hence a Southern pulp handsheet of given mass

per unit area will consist of thicker but fewer fibers, and its average mesh

length and the associated characteristic length for the specimen will be larger.

Table 9 gives the characteristic lengths x for handsheet specimens made from

the currency pulp fractions listed in table 1. The values of x^ vary from

handsheet to handsheet in a way that might have been expected, but the precision

of measurement is usually too low to interpret these variations with much

14 .

certainty. The characteristic length for the fraction III handsheet is noticeably

lower than that for any of the others. The average length of the fibers in this

fraction is very short; thus there are a large number of fibers per unit mass

which might be expected to form a network of small average mesh size.

With less certainty one is tempted to note that for the unfractionated

handsheet is the same as that for the handsheet made from fractions I, II, III

and IV c Fines is a significant part of the mass in the handsheet of

unfractionated pulp, so one might expect the fibers to form a network of larger

mesh size than in the handsheet formed from the same fibers but no fines.

Possibly the fines improve the bonding so that fiber cross-overs in the sheet of

unfractionated pulp are more efficiently bonded. One might note also that the

characteristic length x is larger in the handsheet made from fraction I than in

c the handsheet made from fractions I, II, III and IV. The fibers of fraction I

are longer so there are fewer fibers per unit mass, and these fibers might form a network with a larger average mesh size.
'
15.

References

1. Eo L. Graminski and E. E. Toth, The Cellulose-Water Interaction in Rag and

Wood Pulps, NBSIR 80 - 1970.

2. J. C. Smith, Characterizing the Interfiber Bonding of Paper Pulps: Rationale

for Bonding Parameters derived From Tensile Test Data, NBSIR 79 - 1722.

Table 1.

Classification of Crane Currency Stock

Weight of Moisture Weight of

Wet Pulp Content Dry Pulp

g 7o g

Unfractionated pulp 924. 76.4 218.

Fraction I (#14 mesh) 365. 78.3 79.

Fraction II (#35 mesh) 260. 78.9 55.

Fraction III (#65 mesh) 94. 74.5 24.

Fraction IV (#150 mesh) 45. 73.1 12.

Fines 48.

1

i

Table 2

Elongation per Break for Currency Pulp Handsheet Specimens

of Various Widths —

Description and Elongation per Break 1 Standard Deviation

Sample Number 1 cm width 0.5 cm width 0.3 cm width

%/break 7>/break 7./break

Unfractionated 1 . 0. 099±0. 009 0.09810.011 0.11510.019

2. 0.098±0.015

Fraction I 1 . 0. 122±0. 016 0.11510.024 0.12310.012

2. 0.09210.011

Fraction II 1 . 0.11010.007 0.09210.011 0.10810.014

2. 0.09610.012

Fraction III 1 . 0.08310.007 0.08710.009 0.08610.011

Fractions 1 . 0.09910.016 0.09710.004 0.08610.019

I, II, III, IV 2. 0.09610.013

Recombined

1. Values given are the averages of determinations from tests on

5 different specimens.

Table 3.

Elongation per Break for Southern Kraft Pulp Handsheet

Specimens of Various Widths —

Description Elongation per Break ± Standard Deviation

1 cm width 0.5 cm width 0.3 cm width

%/break %/break %/break

Unbeaten 1 . 342±0. 544

n , 2/ Beaten — 0. 187±0. 028 0. 245±0. 036 0. 399±0. 132

1. Values given are the averages of determinations from tests on

5 different specimens.

2. Pulp was beaten 5000 revolutions in a laboratory beater.

Table 4

Fit of Curve Between 7 Cm and 13 Cm on the Chart, Figure 7.

X-Head Force Interp

.

Slope Fitted

Distance Force Force

X F F F' F cm mN mN mN/cm mN

.082 4.9 4.95

.084 5.93 515.

.086 7.0 7.06

.088 8.19 613.

. 090 9 0 4 9.45

.092 10o 79 686.

.094 12 o 2 12.14

.096 13.68 760.

.098 15 .2 15.18

.100 16.92 858. . .102 18.6 18.59

.104 20.59 981.

o 106 22.6 22.45

.108 24.81 1128.

.110 27 . 1 26.79

.112 29.37 1152.

.114 31.7 31.68

.116 34.47 1397.

.118 37.3 37.20

.120 40.31 1520.

.122 43.3 43.41

.124 46.78 1716.

.126 50.2 50.42

.128 54.28 2035.

.130 58.4 58.32

Least-squares fit to data in columns 3 and 4: F' = 350.48 + F/. 033434

Least-squares fit to data in columns 1 and 2: F = l o 0636 exp(x/ . 033434) - 11.718

Table 5

Values of C, , C x , 1’ 2 ’ c’ x for the

o Reextension Curves , Figure 7

Curve CrossChart Head Chart

Position c. C 9 X X X

1 2 c o o cm mN mN cm cm cm

3 - 8 1.8405x10 40.193 .046814 .036566 3.7

7 - 13 1.4344 11.718 .033434 .070225 7.0

11 - 18 3.8282x10"

L 12.007 .033305 .11476 11.5

15 - 23 2.7726xl0"

2 6.0772 .029821 .16073 16.1

22 - 29 1.7457xl0“

3

3.8832 .028384 .21876 21.9

27 - 35 2.3254xl0"

5 .32557 .025924 o 24749 24.8

34 - 42 1.6423xl0"

5 .93938 .030272 .33160 33.2

40 - 47 2.4548xl0"

5 1.5966 .035909 .39797 39.8

Table 6.

Characteristic Length for Northern Kraft Pulp Handsheets

of Different Densities

Density

g/m

2 Characteristic

cm

Length

1.25 0.048 + 0.007

1.50 0.042 ± 0.006

1.75 0.034 ± 0.006

2.00 0.029 ± 0 o 004

2.50^ 0.029 ± 0. 003—

2.50 0.029 ± 0.004

Unless otherwise noted handsheets were prepared, from pulp beaten 5000

revolutions in a laboratory beater, and the mats were pressed 5 minutes at

350 kPa. Specimens were 2 cm long and 1 cm wide. Values given are the averages

and standard deviations from measurements on 5 specimens.

1. Pressed at 44 kPa.

2. Average of 9 specimens.
.

'
Table 7.

Characteristic Length x for Northern Kraft Pulp Handsheets

c Made From Beaten Pulps

Amount of Beating Characteristic Length,

cm

1.000 rev 2.000 rev 5.000 rev 5,000 rev^1

10,000 rev^

0.035 ± 0.005

0.030 ± 0.005

0.029 ± 0.003-

0.029 ± 0.004

0.023 ± 0.002

Table 8.

Characteristic Length x^ for Southern Kraft Pulp Handsheets

Made From Beaten Pulps

Amount of Beating

1.000 rev

2.000 rev 5.000 rev

4/ 10,000 rev^

Characteristic Length, x^

cm

0.043 ± 0.008

0.043 ± 0.007

0.040 ± 0.004

0.032 ± 0.005

2 Unless otherwise noted handsheets of 2.5 g/m density were made from pulp

beaten the stated number of revolutions in a laboratory beater. Mats were pressed

5 minutes at 44 kPa. Values given are the averages and standard deviations on

5 specimens 2 cm long and 1 cm wide.

1. Average of 9 specimens.

2. Pressed 5 min at 350 kPa.

3. Pressure not known.

4. Mat laid on cotton linters and pressed 5 min at 660 kPa.

Table 9

Characteristic Length x for Handsheets

c Made From Different Currency Pulp Fractions.

Fraction Characteristic Length, x^

cm

Unfractionated 0.020 ± 0.003-/

Fraction I 0.027 ± 0.006

Fraction II 0.024 ± 0.003

Fraction III 0.014 ± 0.003

I, II, III, IV recombined 0.021 ± 0.003

2

Handsheets were of density 2.5 g/m . Mats were pressed 5 minutes at 350 kPa

Values given are the averages and standard deviations from measurements on

5 specimens 2 cm long and 1 cm wide.

1 0 Average of 6 specimens.

Figure 1.

Currency Pulp, Unfractionated

195 X

Figure 3.

Southern Kraft Pulp, Unbeaten

185 X

Figure 2 0 Currency Pulp,

Recombined Fractions, I, IT, III, IV

180 X

« o

Figure 4.

Southern Kraft Pulp,

Beaten 5000 Revolutions

195 X

o

in

E

o

LU

hI

cr

<

x

o

!\jw ‘ 30303

Figure

5*

Force-elongation

curve

for

a

handsheet

specimen

of

unfractionated

currency

pulp,

2.5

g/m

mass

per

unit

area.

o

ino

inO

cvi m

C\J O

ro

in

roO o

in o<3" o

ro

O

c\J

O ‘ 30d0d o

LlI ><

cr

h-i

cr<xo

Figure 6.

Force-elongation

curve

fora

handsheet

specimen

of

beaten

Southern

kraft

pulp,

2.5

g/m^

mass

per

unit

area.

ooooooooooo O^CON(DLO^rON~

E

o

lu

>

<

crI—

i

cr<

x

o

Nw ‘ 33d0d

Figure

7.

Force-elongation

curve

for

a

handsheet

specimen

of

2

Northern

kraft

pulp,

2.5

g/m

mass

per

unit

area.

FORCE

CHANGE

PER

UNIT

EXTENSION

,

ARBITRARY

UNITS

FORCE , ARBITRARY UNITS

Figure 8„ Plots of force change per unit extension versus

force f,or reextension curves, figure 7.

CHARACTERISTIC

LENGTH

X

r

.

cm

Figure 9. Plot of characteristic lengths for handsheets

of different densities, from table 6.

NBS-1 14A (REV. 2-60

1. PUBLICATION OR

REPORT NO. n

2. Performing Organ. Report No. 3. Publication Date NBSIR 80 - 2190 (\

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

4. TITLE AND SUBTITLE

Characterizing the Interfiber Bonding of Currency Paper Pulps

5. AUTHOR(S)

Jack Co Smith and Elizabeth E. Toth

6. PERFORMING ORGANIZATION (If joint or other than N BS. see in stru ction s)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8.

Type of Report & Period Covered

Annua

1

10/1/79 thru 9/30/80

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

Bureau of Engraving and Printing

Treasury Department

Washington, D.C. 20234

10.

SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here) 2

A beaten currency stock was fractionated by fiber length. Handsheets of 2.5 g/m

mass per unit length were made from the unfractionated stock and the first three

fractions. Similar handsheets were made from beaten and unbeaten kraft woodpulps.

Scanning electron micrographs of these handsheets showed that currency pulp fibers

attached to each other through many fine surface fibrils, and were held together by

hydrophilic fines that acted as a glue. Woodpulp fibers were not fibrillated by

beating nor broken into fines, but bonded by forming intimate contacts between their

ribbon-like structures.

Specimens from the handsheets were extended in a tensile tester. Force-elongation

behaviors were consistent with the behaviors expected from their morphologies. It was

found that the force-elongation curves could be fit by a simple mathematical

expression involving three parameters. These parameters are expected to prove useful

in characterizing the bonding forces between pulp fibers in a paper.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Adhesion of paper fibers; bonding of paper fibers; paper; paper fibers, bonding;

paper pulps, characterization; paper, tensile tests.

13.

AVAILABILITY

| | Uni imited

| xi For Official Distribution. Do Not Release to NTIS

1 Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402. QT Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

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15. Price