NBSIR 75-978

Durability of Paper

E. L. Graminski and E. E. Toth

Polymers Stability and Reactivity Section

Polymers Division

December 15, 1975

Progress Report Covering the Period

January 1 - June 30, 1975

Prepared for

Bureau of Engraving and Printing

U. S. Department of the Treasury

Washington, D. C. 20401

NBSIR 75-978

DURABILITY OF PAPER

E. L. Graminski and E. E. Toth

Polymer Stability and Reactivity Section

Polymers Division

December 15, 1975

Progress Report Covering the Period

January 1 - June 30, 1975

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 the Treasury

Washington, D. C. 20401

U.S. DEPARTMENT OF COMMERCE, Rogers C.B. Morton, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Contents

Page

1. Summary 1

1.1 The Effect of Wet Pressing and Calendering

on the Stiffness Retention of Paper 1

1.2 The Effect of Refining Consistency on the

Morphological Changes of Wood Pulp Fibers. ... 2

1.3 The Physical Properties of Handsheets made

from Classified Currency Paper Furnish 2

2. The Effect of Wet Pressing and Calendering on

the Stiffness Retention of Paper 4

2.1 Background 4

2.2 Experimental 4

2.3 Results and Discussion 5

3. The Effect of Refining Consistency on the

Morphological Changes of Wood Pulp Fibers 8

3.1 Background 8

3.2 Experimental 9

3.3 Results and Discussion 9

4. The Physical Properties of Handsheets made from

Classified Currency Paper Furnish 11

4.1 Background 11

4.2 Experimental 11

4.3 Results and Discussion 12

5. Bibliography 13

i. Appendix—Conversion Table to SI Units 14

1 . SUMMARY

1. 1 Effect of Wet Pressing and Calendering on the Stiffness

Retention of Paper

Previous investigations indicated stiffness retention of

paper increased with increasing paper density at a given weight

per unit area (basis weight) . Increased paper density can be

achieved in various ways, most common of which are wet pressing,

mechanical refining and calendering. The effect of each of

these paper making processes on the density may vary with paper

of different basis weight. As the printing process for currency

is a calendering process it was decided to determine the effect

of wet pressing, calendering and basis weight on the eventual

density and stiffness retention of calendered paper when flexed.

Handsheets were prepared from a kraft wood pulp at three

different basis weights, 50, 75 and 100 g/m2

, and wet pressed

at either the minimum or maximum pressure possible with the

handsheet machine press. Retention of stiffness in the course

of flexing was evaluated on the NBS paper flexer.

The results indicate that fiber compaction during wet

pressing increases with increasing basis weight. As the thickness of the wet mat increases, the opportunity for lateral fiber

movement during wet pressing increases resulting in a more even

distribution of mass and greater density. Calendering further

densifies the paper and the densification appears to be greater

with the higher basis weight papers which were wet pressed at

the higher pressure. Although retention of stiffness decreases

with increasing basis weight the stiffness of the high basis

weight paper after flexing is very much greater than the

stiffness of the lower basis weioht paper. The increase in the

decline of stiffness during flexing for the higher basis weight

papers is not in proportion to the increase of stiffness with

increasing basis weight for the unflexed paper.

The results of this investigation suggest that mill trials

be conducted to determine the effect of wet pressing on paper

density, before and after printing and on the printability of

currency paper. Variation of wet pressing pressure would probably

be most effective in the second and third stages of the press

section.
2

1. 2 The Effect of Refining on the Morphological Changes of

Wood Pulp Fibers

The most common means of obtaining high density wood pulp

papers is by increasing the amount of mechanical refining. Wet

fiber flexibility apparently increases with increasing mechanical

refining and the compactness of the fiber network increases

correspondingly. However, increased refining results in increased

fines production and a decrease in fiber length which is undesirable.

High consistency refining is one possible means for achieving

high wet flexibility without undesirable side effects.

Handsheets were prepared from a kraft wood pulp which had

been mechanically refined to different degrees and at various

consistencies. In addition, some refining was done at a high

consistency, diluted and beaten further at a lower consistency to

determine what effect this procedure may have on the morphological

changes of pulp fibers. The physical properties of the handsheets

were determined.

Fewer fines are produced as the beating consistency increases;

however, the strength properties decline. The two consistency

refining produces some interesting results as folding endurance

increases significantly over paper made from only low consistency

beating.

Apparently significant differences in morphological changes

do occur at different beating consistencies. Identification of

the morphological changes requires microscopic examinations.

Due to the time consumption of available microscopic techniques

further work on this subject will be deferred to a later time

when a more rapid image analysis technique is developed for fiber

analysis

.

1. 3 Physical Properties of Handsheets Made from Classified

Currency Furnish

Papermaking furnishes consist of a wide range of fiber sizes

and shapes. The distribution of the size and shape factors are

of paramount importance to the mechanical properties of paper.

A knowledge of the contribution of the various structural

components to the mechanical properties of paper is of extreme

importance in designing a paper for specific properties. One

means of obtaining this information is to fractionate the fiber

slurry into various components; prepare handsheets from each

fraction and determine the mechanical properties of the handsheets.
A quantity of currency furnish, obtained from the manufacturer

of currency paper, was fractionated into four fractions and

handsheets were made from the first three fractions. The mechanical

properties of these handsheets were compared to those made from

the unfractionated furnish.

The handsheets made from the first fraction containing the

longest fibers had poor mechanical properties. The mechanical

properties of the sheets made from the remaining two fractions

for the most part were better than those from the first fraction.

The best properties were obtained with handsheets made from the

unfractionated furnish. A rather surprising result was the

relatively low percentage of long fibers in the furnish.

A greater indepth analysis of currency furnish should be

performed. The information would most certainly be extremely

helpful in designing a wood pulp paper for currency.
4

2. The Effect of Wet Pressing and Calendering

on the Stiffness Retention of Paper

2. 1 Background

In previous work [1] the effect of mechanical refining

(beating) and wet pressing on the durability of paper was demonstrated. Both processes result in increasing the density of

paper. At constant weights per unit area the retention of bending

stiffness of paper during flexing increases with increasing density.

The fines and debris produced during mechanical refining

form a film-like material (matrix) in the interstices of the

fibers [2] . The matrix serves to restrain the lateral movement

and/or twisting of fibers when paper is deformed. Stresses are

then maintained along the fiber axis instead of being dissipated

in transverse motion. This results in a higher modulus and

bending stiffness. During flexing the matrix cracks and leads to

a large decline in both modulus and stiffness.

As the density of paper increases the free volume of paper

decreases precluding fiber movement when paper is strained. In

essence the importance of matrix to the mechanical properties of

paper decreases as the density increases. Therefore, at a

particular weight per unit area, the more dense a paper is the

smaller should be the decline of mechanical properties when flexed.

A third means for densifying paper, calendering, has not

been investigated previously. The effect of densification by

calendering on the durability of currency needs to be known, as

the dry intaglio printing process for currency is in effect a

calendering process. In this study the effect of calendering on

the durability of wood pulp handsheets was determined.

2 . 2 Experimental

A kraft wood pulp was chosen for this study as it is

relatively easy to refine in comparison to rag pulps. The pulp

was beaten in a PFI laboratory mill at 10 percent consistency,

with no clearance between bedplate and roll for 10 thousand

revolutions at 3. 4 kilograms force and a relative velocity of

roll to bedplate of 6 m/sec. A total of 1200 grams of pulp were

beaten, 40 grams at a time. All of the beaten pulp was combined

in a large stainless vessel and diluted to approximately one

percent consistency. The slurry was stirred for one hour prior

to taking aliquots for handsheet preparation.
5

Handsheets were prepared at either 50, 75 or 100 g/m2

. Wet pressing was done at either the lowest or highest pressure

available on the handsheet machine press. The sheets were dried

on a drum drier at 95°C for approximately 4 minutes.

Calendering was done on a calender stack at a pressure of

37.3 kgf/lin cm at a speed of 3.8 cm/sec.

One half of each handsheet was flexed 1000 times over 3.18 mm

rollers and constrained by a 700g free hanging weight on the NBS

paper flexer. The other half of each handsheet served as the

unflexed control. The results are given in Tables 1, 2, 3 and 4.

2 . 3 Results and Discussion

Increased wet pressing results in an appreciable increase

in breaking strength, elongation to break, folding endurance,

sonic modulus and density while Elmendorf tear, air permeability,

cantilever stiffness and thickness decreased. Calendering results

in a decrease in thickness, air permeability and cantilever

stiffness and a marked increase in initial modulus, sonic modulus

and density. The remaining properties listed in Tables 1 and 3

show little or no change as a consequence of calendering. It

must be cautioned, however, that calendering at other pressures

or by supercalendering may have produced entirely different

results

.

An increase in the weight per unit area (basis weight)

resulted in a substantial increase in breaking strength, energy

to break, Elmendorf tear, folding endurance, cantilever stiffness

and thickness while air permeability decreased. There appears

to be a trend towards a lower modulus and an increase in density

with increasing basis weight. The increase in density may be

due to better fiber compaction at higher basis weight or it may

be an artifact resulting from the thickness measurement.

Paper is not smooth and its thickness is actually a measure

of the highest points on its surfaces. If the roughness of

paper remains the same for all basis weights then the thickness

measurement of the higher basis weight paper would be closer to

its actual average thickness than it would be for the lower

basis weight papers. Consequently the determined volume would

be in greater error for the lower basis weight papers resulting

in a lower estimate of density.

There is also the possibility the density actually increases

with increasing basis weight. If the ability of fibers to move

laterally during wet pressing increases with increasing thickness
6

of the wet web, higher densities would result. The higher

density areas would experience pressure prior to lower density

regions. If the fibrous mass were able to flow laterally, the

regions under high pressure would flow towards the regions of

low pressure. This means that the variance in mass distribution

would decrease and the formation of the paper would be improved.

Improved formation would be very beneficial to the printability

of paper.

In order to prove that mass distribution is more uniform

with increasing basis weight, paper would have to be analyzed

by beta-radiography [3,4]. There is a remote possibility such

an analysis can be made in cooperation with laboratories having

the necessary analytical equipment.

The tensile and compressive stresses at the surfaces of

paper, when bent to a particular radius of curvature, increase

with increasing paper thickness. As a consequence, the magnitude

of structural degradation should increase with increasing

thickness. An excellent measure of structural change as a

consequence of flexing is the increase in air permeability. A

plot of the increase in air permeability after flexing against

the thickness of the paper is shown in Fig. 1. An excellent

correlation exists between the thickness of paper and the

increase in air permeability after flexing.

The increase in air permeability for a given thickness is

greater for the calendered papers which indicates the tensile

and compressive stress are greater in calendered than in

uncalendered paper. This is to be expected as the initial

modulus of the calendered paper is greater than the initial

modulus of the uncalendered paper. Furthermore, the data

suggest that there is a maximum thickness for calendered paper

which should not be exceeded because of extensive structural

degradation when bent to a small radius of curvature. The

maximum thickness, however, is probably substantially greater

than the thickness which would be acceptable for currency.

The increase in air permeability as a consequence of

flexing is probably due to degradation of the matrix. It has

been shown previously that the degradation of the matrix is

probably the greatest factor in the decline of paper stiffness [2]

.

Therefore, the retention of bending stiffness should decrease

with increase of air permeability . A plot of retention of

stiffness against increase in air permeability is shown in Fig. 2.

A good correlation exists between these two variables.

Even though retention of stiffness is lower for the thicker

papers (higher basis weight) the final stiffness of the thicker

papers is much greater than that of the thinner papers (Fig. 3) .
7

Retention of bending stiffness is greater for handsheets

wet pressed at the higher pressure whether calendered or not.

This would be expected as higher wet pressing produces a higher

density and it has been shown previously that stiffness retention

is greater the higher the density [1] . Thickness is only one

important factor in the bending stiffness of paper. The density

is also critical as the importance of matrix to the modulus and

stiffness of paper declines with increasing density.

The results of this study indicate once again that currency

should have maximum density for greatest retention of stiffness.

Wet pressing appears to be a very important manufacturing

variable from the standpoint of achieving the greatest density

for currency. Furthermore, there is the possibility that wet

pressing might affect the uniformity of mass distribution in

paper which is of importance to the printability of currency

paper. Some studies should be made on the effect of wet-pressing

pressures on the formation, density and printability of manufactured

currency paper.
8

3. The Effect of Refining Consistency on the

Morphological Changes of Wood Pulp Fibers

3. 1 Background

Results reported previously [1] and in Section 2.3 of this

report indicate currency paper should have a high density for

optimum stiffness retention during circulation. Generally pulp

is highly mechanically refined whenever a high density paper is

desired. Refining results in greater wet fiber flexibility

which in turn eventuates in a more compact fiber network. The

more compact the network the greater the density.

As fiber compactness increases, the interstices between

the fibers become smaller and smaller and the effect of fines

on the mechanical properties of paper declines since there is

less opportunity for lateral fiber movement during straining

in a compact fiber network. Increased mechanical refining,

which produces greater wet fiber flexibility, also results in

the production of greater amount of fines and a decrease in

fiber length. The increased number of fines causes water

drainage to slow down resulting in a decline in the rate of

paper production. A point is reached in papermaking where the

fines become more of a liability than an asset.

It would be ideal for a refining process to be capable of

producing fibers having high wet flexibility without reducing

the fiber length or producing undesirable quantities of fines.

As mechanical refining progresses, fibers imbibe more water

and become more swollen. Increased swelling makes fibers more

susceptible to damage when subjected to compressive and shear

forces during beating.

One possible means for reducing fiber damage and preserving

their integrity is by reducing available water for swelling in

the beating process. High consistency refining is becoming

increasingly more important in paper manufacture not only

because of the smaller amounts of water used but because high

consistency refining produces desirable morphological changes

in fibers. The morphological changes occurring in high

consistency refining are undoubtedly different from those in

low consistency refining.

As there is interest in developing an all wood currency

paper it was decided to determine the effect of high density

refining of wood pulp on the rheological properties for wood

pulp paper. Hopefully, morphological changes similar to those

occurring with cotton and linen pulps (fiber splitting and

fibrillation) , will occur with wood pulps under certain specific

conditions

.
9

3. 2 Experimental

Forty grams of wood pulp was beaten in a PFI laboratory

mill at 3.4 kgf/cm at a relative velocity of roll to bedplate

of 6 m/sec with no clearance between bedplate and roll for 5

or 10 thousand revolutions at 5, 10, 20 or 40 percent consistency.

Also some beatings involved 5 thousand revolutions at either 20,

30, or 40 percent consistency followed by the addition of sufficient

water to decrease the consistency to 10 percent and beaten for

an additional 5 thousand revolutions.

Six handsheets were made from each of the pulp beatings

and wet pressing was done at the maximum pressure possible on

the handsheet press. The physical properties of the handsheets

were determined and are listed in Table 5.

3. 3 Results and Discussion

As could be expected, the reduction of water in the entire

mechanical refining process results in a decline of paper strength.

As mechanical refining increased paper strength as well as many

other physical properties improved regardless of the consistency.

The ease of water drainage (Canadian Standard Freeness) increases

with increased refining consistency which demonstrates that high

consistency refining does indeed reduce the amount of debris

generated.

The refining done at two different consistencies produced

the most interesting results. The tensile properties of the

handsheets beaten for 5 thousand revolutions at either 20, 30

or 40 percent consistency followed by an additional beating of

5 thousand revolutions at 10 percent consistency were essentially

equivalent to those of handsheets beaten at either 5 or 10 percent

consistency for 10 thousand revolutions. However, the folding

endurance of the handsheets beaten at 40 and 10 percent

consistencies was significantly higher than that of handsheets

beaten at 5 or 10 percent consistency for 10 thousand revolutions

(two sided t test) . It is also noteworthy that the densities

of the paper from the two consistency refining were appreciably

higher than the handsheets obtained from the single consistency

refining.

Evidently significant differences in morphological changes

occur in the two consistency refining resulting in increased

density and folding endurance and a decrease in thickness.

There also appears to be a trend towards a higher elongation to

break in the handsheets from two consistency refining.
10

The results are encouraging in that morphological changes

can be affected by varying the consistency in mechanical refining.

Microscopic investigations are needed to identify the changes.

However, microscopy is long and tedious work and frequently

relies on subjective rather than objective measurements.

Work is commencing in the Applied Mathematics Division on

the image analysis of fibers. When completed, the image analysis

system should permit rapid, quantitative measurement of such

fiber properties as fiber length distribution, coarseness,

fibrillation and curl. It was decided to discontinue further

work on the effect of consistency in refining on the morphological

changes of wood pulp fibers until the image analysis technique

is developed.
11

4. The Physical Properties of Handsheets Made

from Classified Currency Paper Furnish

4. 1 Background

Paper pulp consists of a wide distribution of fiber lengths

and sizes. When filtered on the endless wire section of a

paper machine they form a fiber network which determines the

eventual paper properties. Different papers rely on different

fiber fractions for a particular paper property. To achieve

good formation in paper a short fiber furnish is favored over

long fiber furnish. Folding endurance is enhanced by long

fibers

.

Regardless of the importance of a single fiber fraction

on a particular paper property the remaining fractions have a

strong influence on the final property in question. It has been

postulated numerous times in previous reports [1,2,5,6,7] that

the fines of currency paper are extremely important to the modulus

and bending stiffness of paper. The degradation of that portion

of currency paper, consisting primarily of fines, results in a

substantial decline in modulus and cantilever stiffness.

One method of demonstrating the importance of the various

fiber fractions of paper is to separate the beaten pulp into

individual fractions and to make handsheets from the various

fractions. This was done with a quantity of currency paper

furnish obtained from the manufacturer of currency paper. The

stock was sampled just after the final mechanical treatment and

prior to final dilution for the headbox. The furnish was at

approximately 2.3 percent consistency and contained no wet

strength resin.

4 . 2 Experimental

The currency paper furnish received from the currency paper

manufacturer was filtered to remove excess water. The final

consistency was approximately 20 percent. Aliquots of the

concentrated furnish containing approximately 5 grams of dry

fiber, were fractionated on a commercial fiber classifier. Only

3 screens were used in the classification, 14, 35 and 65 mesh

as the 150 mesh screen constantly plugged causing water overflow. Approximately 75g of dry fiber were fractionated. Handsheets were then made from the various fractions. The physical

properties of the handsheets are given in Table 6.
12

4. 3 Results and Discussion

It was surprising that the long fiber fraction consisted

of only about 16 percent of the total furnish. This is less

than half the amount usually found in a moderately beaten softwood pulp. The second fraction (35 mesh screen) consisted of

approximately 37 percent of the furnish while the third fraction

consisted of about 15 percent of the total. This means that

approximately 32 percent of currency stock consists of very

short fibers and fines.

Despite the low percentage of long fibers in currency paper

it nevertheless has excellent physical properties. Apparently

there are an appreciable number of very long fibers in the long

fiber fraction which have a great effect on the physical properties

of currency paper. A fiber length distribution of this fraction

will be determined in the future as the information may be very

important in the design of a wood pulp paper for currency.

Handsheets made from the first fraction are very porous

and have a low density. The handsheets made from the other two

fractions are less porous and denser. The initial modulus

increases with increasing density demonstrating once again the

importance of fiber compaction on the modulus of paper. The

handsheets made from the unfractionated stock had the highest

modulus, strength, extensibility, folding endurance, and

cantilever stiffness.

While the thickness of paper greatly influences its bending

stiffness it is nevertheless only one factor of importance to

that property. This is demonstrated very nicely when the bending

stiffness of the handsheets from fraction one and the unf ractionated

stock are compared. The thickness of the handsheets from fraction

one are 24 percent higher while the cantilever stiffness is 15

percent lower than the handsheets made from unfractionated stock.

Apparently, the higher density of the unfractionated handsheets

is the primary cause for the higher bending stiffness.

Much could be learned from an in depth study of currency

stock. The information obtained from such a study would greatly

assist in the design of a wood pulp paper for currency. However,

much of the work would involve microscopic investigations of the

fibers. As this work is lengthy and tedious with the available

microscope techniques further work on this problem will be

deferred until fibers can be analyzed automatically (see image

analysis in section 3.3).
13

5. Bibliography

1. Graminski, E. L. and Toth, E. E. , Evaluation of Currency

and Stamp Papers, NBSIR 75-670, February 20, 1975.

2. Graminski, E. L. and Toth, E. E., Evaluation of Currency

and Stamp Papers, NBS Report 10 336, September 17, 19 70.

3. Corte, H. , Das Papier, _24, No. 5, 261 (1970).

4. Hellawell, J. M. , Paper Technology, _14, No. 1, 24 (1973).

5. Graminski, E. L. and Toth, E. E. , Evaluation of Currency

and Stamp Papers, NBSIR 73-274, August 15, 1973.

6. Graminski, E. L. and Toth, E. E. , Evaluation of Currency

and Stamp Papers, NBSIR 74-431, January 2, 1974.

7. Graminski, E. L. and Toth, E. E., Evaluation of Currency

and Stamp Papers, NBSIR 74-751, September 6, 19 74.

USCOMM-NBS-DC
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LOW HIGH LOW HIGH LOW HIGH

WET PRESSING PRESSURE

Figure 3: Bending stiffness of flexed handsheets havin

various weights per unit area.
NBS-114A (REV. 7-73) 1

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 75-978

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Durability of Paper

5. Publication Date

January 1976

6. Performing Organization Code

7. AUTHOR(S)

E. L. Graminski and E. E. Toth

8. Performing Organ. Report No. | NBSIR 75-978

9. PERFORMING ORGANIZATION NAME AND ADDRESS

HATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. |

1 11. Contract /Grant No.

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

Bureau of Engraving and Printing

U. S. Department of the Treasury

Washington, D. C.

13. Type of Report & Period

Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.)

Experimental results indicate that wet pressing of the wet paper web results in

greater densities as the amount of mass being pressed increases. This may be due to

greater freedom for lateral movement of the fibers as the thickness of the wet mat

increases. Greater densities eventuate in increased stiffness retention during flexing

and should result in an improved circulation life of currency. Higher densities with

increasing wet mass indicates more uniform mass distribution which should result in

improved printability . The debris generated during pulp refining is beneficial to the

mechanical properties of paper but its utility decreases as to wet fiber flexibility

and the amount of debris increases with increased mechanical refining. High consistency

refining of wood pulp apparently reduces the amount of fines produced but other

disadvantages arise such as decreased interfiber bonding. The importance of fines to

the mechanical properties of currency paper was demonstrated by forming handsheets

from the various fractions of currency stock and comparing their mechanical properties

with those made from the unfractionated stock.

17. 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 )

"

Calendering; mechanical refining; paper durability; pulp classification; stiffness

retention; wet pressing

18. AVAILABILITY ] Unlimited 19. SECURITY CLASS

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21. NO. OF PAGES

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26

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22. Price

[ j Order From National Technical Information Service (NTIS)

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