NBSIR 77-1286

Characterizing the Interfiber

Bond Strength of Paper Pulps

in Terms of A Breaking Energy:

Effect of Beating

Jack C. Smith and Edmond L. Graminski

Polymers Division

Institute for Materials Research

National Bureau of Standards

December 15, 1976

Progress Report Covering the Period

July 1, 1976 through September 30, 1976

Prepared for U. S. Energy Research and Development Administration

Washington, D.C. 20234

NBSIR 77-1286

CHARACTERIZING THE INTERFIBER

BOND STRENGTH OF PAPER PULPS

IN TERMS OF A BREAKING ENERGY:

EFFECT OF BEATING

Jack C. Smith and Edmond L. Graminski

Polymers Division

Institute for Materials Research

National Bureau of Standards

December 15, 1976

Progress Report Covering the Period

July 1, 1976 through September 30, 1976

Prepared for U. S. Energy Research and Development Administration

Washington, D.C. 20234

U S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director
. V'v:'
TABLE OF CONTENTS

Page

SUMMARY AND CONCLUSIONS i

INTRODUCTION 1

EXPERIMENTAL 2

RESULTS AND DISCUSSION 3

REFERENCES 6

i

SUMMARY AND CONCLUSIONS

An energy parameter, E, is being investigated for possible

use in characterizing the adhesion between two paper fibers.

A handsheet in the form of a thin open web is made from the pulp

fibers to be evaluated, and E is determined from a tensile test

that records the force and number of bond breaks, as a specimen

from this web is elongated. Details of the method for measuring E

and preliminary tests to determine its applicability in characterizing Northern and Southern softwood kraft pulps have been described

in a previous report [l]. E was shown to have approximately the

same value for both unbeaten Pulps. Beating to 10,000 revolutions

in a laboratory beater increased this value ninefold. Although it

was not possible to distinguish between the two pulps by means of

the E values alone, some differences were noted in the mechanical

behavior of handsheet specimens from the two different pulps. Webs

from the unbeaten Northern pulp had a tighter structure with more

bonds between fibers than was the case for comparable webs made

from Southern pulp.

In the work reported here the effect of beating has been

examined in more detail. Handsheet specimens were prepared from

Northern and Southern softwood kraft pulps beaten to 1,000 , 2,000 and 5 , 000 revolutions. The method of preparation was the

same as that used previously except that a sheet of filter paper

was used as a forming medium, and the webs were pressed at a lower pressure (44 kPa for 5 min)

The values of E for both unbeaten pulps were comparable,

approximately 0.8x10 ^ j/break. The effect of beating to 5,000

revolutions was to increase E for both pulps fivefold to a value

of approximately 4x10 ^ j/break. E for the Northern pulp increased

rapidly with beating to 2,000 revolutions, and then increased more

slowly. In contrast E for the Southern pulp increased slowly with

beating to 2,000 revolutions, and then increased more rapidly.

There is, however, some uncertainty to these latter results,

because of imprecision in the value of E.

If the elongation of a specimen versus the number of bond

breaks is plotted, the initial slope of this curve, or initial
ii

elongation per break, can be used as a measure of the tightness

of the web structure; i.e., the density of interfiber bonds per

unit area of web. Large values of the initial elongation per break

thus indicate a loose web in which the area density of fibers is

small and in which many of the crossing fibers are not bonded

together. It was found that the unbeaten Northern pulp sample had

an initial elongation per break of 0.l6 %, and that subsequent

beating produced only a slight decrease to a value of 0.12 %/break.

Thus the beating treatment only resulted in a slight tightening of

the web. In contrast the initial elongation per break for the

unbeaten Southern pulp was 0.52 % indicating a relatively loose web.

Beating to 1,000 revolutions brought the value down to 0.l8 %/break,

and further beating produced a small decrease to 0.l6 %/break.

The observed effects of beating on the strength of the interfiber bonds and on the tightness of the fiber web are consistent

with the known morphologies of the two pulps. The unbeaten Northern

pulp fibers form tighter webs because, unlike Southern pulp fibers,

these fibers tend to collapse to a ribbon-like structure and thus

become more conformable. Beating induces the formation of ribbonlike structures, and increases the number and area of the interfiber bonds in the web.

Webs formed from Southern pulp were looser than webs of the

same basis weight (weight per unit area of web) formed from Northern

pulps, even sfter the pulps had been beaten 5,000 revolutions. As

Southern pulp fibers are coarser than Northern pulp fibers, webs of

the Southern pulp would have fewer fibers per unit area in webs of

the same basis weight, and hence fewer bonds on this account. It

seems that better comparisons between pulps might be obtained if

the webs tested had the same number of fibers per unit area rather

than the same basis weight. It is. planned in future work to test

this idea by making measurements of E on a series of webs of

different basis weights in order to obtain a closer match between

webs from different pulps.

The webs used in this work were formed at low pressures. When

they were tested it was difficult to measure E with satisfactory

certainty. It therefore will be necessary to experiment with webs

formed at various pressures, in order to develop webs that are more

suitable fur these measurements, but are still easy to manufacture.
1

INTRODUCTION

If a very thin web of pulp fibers is elongated to break

in a sensitive tensile tester, a force-elongation curve containing

numerous jags is obtained. Each jag is caused by the breakage

of a bond between fibers constituting the paper network. From a plot of work of extension as a function of the number of bond

breaks, the average energy stored in the fibers previously

connected by the broken bond can be found. This energy parameter

E is being investigated for possible use in characterizing the

adhesion between two paper fibers.

Applicability of the energy parameter E in the characterization

of Northern and Southern softwood kraft pulps is being investigated . Preliminary results are given in a previous report [1].

It was shown that the parameter E had the same value for the

unbeaten Northern and Southern pulps, and almost the same value

when both pulps were beaten to 10,000 revolutions. However the

beating treatment produced an increase in the value of E from

__ r7 ^ 7

0.8x10 ' j/break for the unbeaten pulps up to 7x10 j/break for

the beaten pulps. In the work reported here the effect of beating

has been examined in more detail.
2

EXPERIMENTAL

2 Handsheets of basis weight 2.p g/m were prepared from the

batches of Northern and Southern softwood kraft pulps used

previously. The pulps were unbeaten and beaten to 1,000 , 2,000

and d , 000 revolutions in a PFI laboratory beater. The method of

preparation was the same as that reperted previously except for

the following changes: It was found that filter paper (Whatman

No. b4l) could be substituted for the cotton linters pulpshhet

previously used as a forming medium. It was usually not necessary

to treat the filter paper with a fluorocarbon release agent.

Pressing for- five minutes in a hydraulic press was carried out

at 41 zPa (6.4 lb/inp instead of the previous 0.66 Mpa (96 lb/in ) as this produced a coherent web that was easy to separate from

the ferrotype backing plate.

The procedure for preparing, mounting, and testing of specimen

from the handsheet samples was unchanged. Specimen dimensions

were 1 cm width by 2 cm length* The specimens were tested at a cross head speed of 0.2 cm/min and a chart speed of 20 cm/min.

Load cell sensitivities ranged between 0.02 N (2g) full scale

for handsheets of unbeaten pulp and 0.1 N (10 g) full scale

handsheets of pulp beaten to 5000 revolutions.
3

RESULTS AND DISCUSSION

The force-elongation curve obtained by extending fiber web

specimens in a tensile tester can be analyzed to provide two

types of data. Data for a curve of work of extension versus

number of bond breaks can be obtained by integrating under the

force-elongation curve and counting the discontinuities in slope

caused by the bond breaks. The characteristic energy E is

estimated from the minimum slopes of this curve in accordance

with guiding principles given previously [1]. A curve of percent

elongation of the specimen versus number of bond breaks can also

be plotted. The initial slope of this curve, or initial percent

elongation of the specimen per break, is a measure of the tightness

of the web structure, as explained later.

The results from testing five specimens of each of the

handsheet samples are given in tables 1 through 8. It can be

seen from these tables that values of E for both unbeaten pulps

are comparable, about 0.8xl0

- ^ j/break. The effect of beating,

to revolutions is to increase E for both pulps fivefold to

a value of approximately 4xlO

- 7 j/break.

In figure 1 the average values and standard deviations of

the characteristic energy E are plotted to facilitate comparison.

This plot suggests that adhesion between Northern pulp fibers,

as measured by E, improves most rapidly at the start of beating,

and that increased beating produces a lower rate of improvement.

In contrast, adhesion between fibers of Southern pulp does not

increase as rapidly with beating initially, but increases more

rapidly as beating is continued. The standard deviations associated

with the values of E are rather large, however, so this apparent

behavior must be confirmed by further measurements.

The relative tightness of the web structures can also be

determined, according to the following rationale. A tightly

bonded structure is one that has a large number of interfiber

bonds per unit area. The number of these bonds depends
upon the number of fiber crossings and on the number of crossings

that bond together. If the structure is tightly bonded, it can

be seen intuitively that on the average only a small percent

extension is required before the force applied to some one bond

is sufficient to break it. For a loosely bonded network a larger

percent extension on the average is required. Thus a small value

of average percent extension per break would be characteristic

of a tight network, and a larger value of average percent extension

vould be characteristic of a looser network. The average percent

extension per break for a given web specimen is equal to the

initial slope of the curve of percent extension versus number

of breaks obtained by a tensile test.

Values of the initial percent elongation per break are

plotted in Figure 2. It is apparent from this figure that the

unbeaten Northern pulp sample has an initial elongation per break

of 0 .l 6 %, and subsequent beating produces only a slight decrease

to a value of 0.12 %/break. Thus the beating treatment only

resulted in a slight tightening of the web. In contrast the initial

elongation per break for the unbeaten Southern pulp is O .52 % y indicating a relatively loose web. Beating to 1,000 revolutions,

however, brings the value down to 0 .l 8 ^/break, and further beating

to 5*000 revolutions produces a small decrease to 0 .

l

6 $/break.

The effects of beating on the strength of the interfiber

bonds and on the tightness of the fiber web can be explained in

terms of the morphologies of the two kinds of pulp fibers. It is

known that unbeaten Northern pulp fibers have the shape of thinwalled tubes, most of which have collapsed to form a ribbon-like

structure. Therefore most of the junctions between fibers of

unbeaten Northern pulp will have large areas of contact, will

bond easily, and produce a tight web. The effect of beating is

to make the fibers more pliable and to reduce more fibers to a collapsed ribbon-like state. The increase in pliability should

produce more intimate contacts between fibers and increase the

bon strength. The increased number of ribbon-like structures

should provide more bonds and make the web tighter.
5

Fibers from unbeaten Southern pulp are coarser than those

from unbeaten Northern pulp. They have the shape of thick-walled

round hollow tubes, fewer of which have collapsed to a ribbon-like

shape. Therefore fewer fiber junctions will have sufficiently

large areas of contact to produce an effective bond, and the

resultant web will be looser. Only a little beating is needed

to collapse more of the tubes, thus producing more bonds and

providing an appreciably tighter web, but more beating is

j required to make the fibers pliable and increase the bond strength.

Webs from Northern pulp beaten to 5,000 revolutions have

an initial elongation to break of 0.13 %, and webs of the same

basis weight made from Southern pulp with the same degree of

beating have an initial elongation per break of 0.l6 %. Although

specimen variation prevents good resolution, this difference in

value should be expected because of the difference in coarseness

of the two pulps. A web made from the coarser Southern pulp

has fewer fibers and fiber crossovers per unit area than a web of

the same basis weight made from the Northern pulp.

As stated in the experimental section, the handsheet samples

prepared for these experiments were formed at low pressures

I

(44kPa for 5 min). Coherent webs were obtained that were easily

separated from the forming medium and backing plate. However,

the derived curves of work of extension versus number of bond

breaks had a scalloped appearance with many changes in slope.

As a rule the minimum slopes extended over only a few bond breaks,

so the value of the slope was uncertain, and it was difficult to

1

obtain a good estimate of E. This difficulty was not so prevalent

in previous measurements on the webs which were formed at a

higher pressure. It therefore will be necessary to experiment

with webs formed at various pressures, in order to develop webs

that are more suitable for these measurements, but are still

easy to manufacture.
6

REFERENCES

1. J. C. Smith anc} E. L. Graminski, Characterizing the InterFiber Bond Strengths of Paper Pulps in Terms of a BreakingEnergy, Progress report covering the period Jan. 1, 1976 to

June 30 , 1976, NBSIR - 1148. Available from National

Technical Information Service PB 264,689.
7

Table 1.

Tests on Specimens of Northern Pulp, Unbeaten

Specimen E

10-7 j/break

Init. Elong

per break

$/break

1 0.75 0.12

2 0.64 0.14

6 0.70 0.22

8 0.77 0.16

15 O .92 0.18

Average ± standard deviation

E (0.76 ± 0.10)xl0

" 7 j/break.

Initial elongation per break 0.16 ± 0.04 ^/break.

Table 2.

Tests on Specimens of Southern Pulp, Unbeaten

Specimen E Init . Elong

.

per break

10 ‘ j/break ^/break

3 O .76 0.28

4 1.28 0.47

5 0.55 0.51

10 1.12 0.74

15 O .69 0.80

Average ± standard deviation

E (0.88 ± 0.5l)xl0 -7 J/break.

Initial elongation per break 0.52 ± 0.24 $/break
8

Table 3*

Tests on Specimens of Northern Pulp, Beaten 1,000 Revolutions

Specimen

2

3

9

10

11

E

10-7 j/break

Init. Elong.

per Break

%/break

1.42

1.44

1.92

1.32

O .96

0.12

0.11

0.14

0.14

0.12

Average ± standard deviation

E (1.41 ± 0.34) xl0"7 j/break

Initial elongation per break 0.12 ± 0.01 %/break

Table 4.

Tests on Specimens of Southern Pulp, Beaten 1,000 Revolutions

Specimen E

10“7 j/break

Init. Elong

per break

%/break

1 1.40 0.21

2 1.24 0.22

8 1.50 0.18

9 1.00 0.17

10 1.32 0.15

Average ± standard deviation

E (1.29 ± 0.19) xl0

- 7 J/break

Initial elongation per break 0.18 ± 0.09 %/break
9

Table 5*

Tests on Specimens of Northern Pulp, Beaten 2,000 Revolutions

Specimen E

10 7 j/break

Init. Elong

per Break

$/break

1 2.70 0.12

2 1.80 0.12

9 2.10 0.12

10 2,64 0.12

11 2.24 0.12

Average ± standard deviation

E (2.30 ± 0.38) xlO

~ 7 j/break

Initial elongation per break 0.12 ± 0.00 %/break

Table 6 . Tests on Specimens of, Southern Pulp, Beaten 2,000 Revolutions

Specimen E

10-7 j/break

Init. Elong.

per Break

^/break

2 1.80 0.19

5 1.42 0.22

9 1.88 0.16

11 1.72 0.18

12 1.42 0.21

Average ± standard deviation

E (I .63 ± 0.22)xl0 7 j/break

Initial elongation oer break 0.19 ± 0.02 ^/break
10

Table 7»

Teste on Specimens of Northern Pulp, Beaten 3,000 Revolutions

Specimen E

10 ^ j/break

Init. Elong

per Break

$/break

1 4.76 0.17

2 3.16 0.13

9 3.10 0.13

10 2.86 0.13

13 3.02 0.11

Average ± standard deviation

E (3*^6 ± 0.79)xl0 ^ j/break

Initial elongation per break 0.13 ± 0.02 %/break

Table 8 . Tests on Specimens of Southern Pulp, Beaten 5*000 Revolutions

Specimen E Tnit. Elong.

per Break

10 ^ j/break ^/break

3 4.38 0.17

9 4 .04 0.18

10 4 .04 0.18

12 3 • 28 0.15

13 4.70 0.12

Average ± standard deviation

E (4.09 ± 0.53)xl0 -7 J/break

Initial elongation per break 0.l6 ± 0.02 %/break
ENERGY.

E,

10

J/BREAK

BEATING , REVOLUTIONS

Figure 1.

Characteristic Energy E as a Function of Degree of Beating.

Open circles. Northern pulp; solid circles. Southern pulp.

• ;

.

• : '
INITIAL

ELONGATION

PER

BREAK

BEATING , REVOLUTIONS

Figure 2.

Initial Elongation p,*r Break as a Function of Degree of Beating.

Open circles. Northern pulp; solid circles. Southern pulp.
NBS-1 14A mr. v 7 -7 1)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLIC A TTON OR REPORT NO.

NBSIR 77-1286

2. Gov’t Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE

Characterizing the Inter-Fiber Bond Strength of Paper

Pulps in Terms of a Breaking Energy: Effect of Beating

1

5. Publication Date

6. Performing Organization ( od<

7. AUTHOR(S)

Jack C. Smith and Edmond L. Graminski

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unn No.

11. Contract /Grant No.

12, Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

I

..

13. 1 ype of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

j

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

bibliographv or literature survey, mention it here.)

2

Handsheets in the form of a thin web of basis weight 2.5 g/cm were prepared

from Northern and Southern softwood kraft pulps. The pulps were unbeaten and

beaten to 1,000, 2,000 and 5,000 revolutions in a laboratory beater. The relative

density of bonding between fibers and the relative strength of the bonds were

estimated from tensile test data. Bond strengths of the two unbeaten pulps were

equal, but density of bonding was greater in the web made from the Northern pulp.

Bond density and strength for both pulps increased with increases in the degree of

beating to approximately the same value for pulps beaten 5,000 revolutions, but

differences were observed in the rate of change of these quantities.

7. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Adhesion of paper fibers; bonding of paper fibers; paper; paper fibers, bonding;

paper pulps, characterization; paper, tensile tests.

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