NBSIR 75-670

Evaluation of Currency and

Stamp Papers

E. L. Graminski and E. E. Toth

Paper Evaluation Section

Institute for Materials Research

February 20, 1975

Progress report covering the period

July 1 - December 31, 1974

Prepared for Bureau of Engraving and Printing

U.S. Department of the Treasury

Washington, D. C. 20401

NBSIR 75-670

EVALUATION OF CURRENCY AND

STAMP PAPERS

E. L. Graminski and E. E. Toth

Paper Evaluation Section

Institute for Materials Research

February 20, 1975

Progress report covering the period

July 1 - December 31, 1974

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, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts. Director

CONTENTS

Pa9

e

1. SUMMARY . . . . ... . . ..... . 1

1.1 Mechanical Refining, Wet Pressing, and

Modification of Paper with Acrylic Resins . 1

1.2 Modification of Currency Paper 2

2. THE EFFECT OF MECHANICAL TREATMENT ON THE

STIFFNESS RETENTION OF PAPER WHEN FLEXED ..... 3

2 . 1 Background 3

2 . 2 Experimental 3

2.3 Results and Discussion 5

2.3.1 The Importance of Apparent Density

to the Retention of Stiffness with

Flexing 6

2.3.2 The Relationship Between Paper

Modulus and Paper Stiffness 7

2.4 Achievement of Maximum Density for

Currency Paper 9

2.5 Analysis of the Modification of Currency

r aper with Acrylic Resins 11

3. MODIFICATION OF CURRENCY PAPER WITH ACRYLIC

RESINS BY THE SATURATION TECHNIQUE 13

3.1 Background 13

3.2 Experimental 13

3.3 Results and Discussion 14

4. BIBLIOGRAPHY

1. SUMMARY

1 . ] Mechanical Refining, Wet Pressing , and Modification

of Paper with Acrylic Resins

The structure of paper affects the durability of paper.

Mechanical refining (beating), wet pressing, and the addition

of acrylic resins to paper by the beater addition technique

have a significant effect on the structure of paper. The

relationship between the three aforementioned variables on

the durability of paper was determined.

A kraft wood pulp was mechanically refined (beaten) to

various degrees and formed into handsheets which were pressed

at either low or high pressure. Some of the beaten pulp was

modified with acrylic resins known to have varying effects

on the structure of paper. One half of each handsheet was

flexed on the National Bureau of Standards paper f lexer, and

the properties of the flexed paper were compared to those of

the unflexed portion.

Beating increases wet fiber flexibility resulting in a more compact fiber network structure and a denser sheet.

Increasina amounts of wet pressing also compacts the fibers,

resulting in greater paper density. The effects of wet

pressing are greatest at lower levels of pulp refining.

Acrylic resins cause the fibrils and debris to redeposit

onto the fibers ensuing in a more porous sheet.

"'he fines, produced during mechanical refining, form a film-like material (matrix) in the interstices of the fibers

during the formation of paper. This matrix serves to restrain

the lateral movement and the twisting of fibers when paper

is strained, compelling the stresses developed in the fibers

to dissipate axially rather than transversely. Stress dissipation in the direction of least resistance is restrained and,

consequently, the modulus and stiffness are higher. If the

matrix deteriorates when paper is flexed, a significant

decline in stiffness and modulus results.

As mechanical refining and wet pressing increases, the

free volume of paper decreases precluding lateral movement

and/or twisting of fibers, and the matrix contributes less

and less to paper stiffness. If there is little or no

degradation of interfiber bonding during flexing of dense

paper, the free volume does not increase appreciably and

the bending stiffness does not decline as extensively as it

does in less dense papers.
Modification of paper with certain acrylic resins

results in a further improvement of stiffness retention

during flexing. The effect of the acrylic resin on the

structural changes of paper does not diverge at different

levels of mechanical treatment. An analysis of the effect

of acrylic resins on the stiffness retention of paper indicates that the acrylic resin should be located at or near

the top and bottom surfaces of paper for greatest utility.

Additional work must be done to determine the most

effective means for obtaining high density currency paper.

1 . 2 nodi f icat ion of Currency Paper

Laboratory results with handsheets indicated significant

increases in stiffness retention with flexing would result

when paper was modified with certain acrylic resins by the

saturation technique. Results obtained from handsheets,

however, are not always equivalent to results obtained with

machine made paper. Therefore, machine made currency paper

was treated with acrylic resin AC-61 to determine if the

treatment would also improve its stiffness retention. The

reason for choosinq resin AC-61 was because it produced the

best results in handsheets.

The stiffness retention with flexing of currency paper

treated with AC-61 was superior to currency paper normally

sized with a glue-glycerin mixture. The results suggest

that a mill trial should be conducted in which currency

paper would be modified with AC-61 by saturation to enable

a full scale evaluation of the acrylic resin treatment.
3

2. THE EFFECT OF MECHANICAL TREATMENT ON THE

STIFFNESS RETENTION OF PAPER WHEN FLEXED

2 . 1 Background

The mechanical refining of pulp and wet pressing of

paper after formation significantly affect the mechanical

properties of finished paper [1], as both treatments

influence interfiber bonding and fiber strength. The effects

of mechanical refining and wet pressing on the structural

and mechanical properties of paper and on the stiffness

retention of paper with flexing have been evaluated and are

reported here.

Previous studies [2-5] on the modification of paper with

acrylic resins by beater addition indicated that fibrils and

debris redeposited onto the fibers, resulting in a decline

in the material deposited in the interstices of the fibers [4].

The decline in modulus, strength, and retention of stiffness,

which resulted with some of the treatments, could be attributed

to tne accompanying structural change. As acrylic resins in

beater addition might perform differently at different levels

of mechanical refinement and wet pressing, a study has been

made of refining and wet pressing in relation to beater

addition of acrylic resins.

2 . 2 Experimental

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

relatively easy to refine in comparison to rag pulps.

Two acrylic resins were selected for this investigation,

E-631 and TR-407 (same as resin HA-16 except for emulsifier)

.

Resin E-631 has an adverse effect on stiffness retention,

while TR-4 07 was found to give the greatest improvement in

stiffness retention of any of the acrylic resins investigated

in beater addition studies [3]. The use of two greatly different functioning resins would tend to magnify effects and

minimize uncertainties due to sample variation.
The pulp was beaten in a PFI laboratory mill at 10 per — cent consistency, with no clearance between bedplate and

roll, for 1, 2.5, 5, 10, or 20 thousand revolutions at 3.4

kilograms force and a relative velocity of roll to bedplate

of 6 m/sec. Forty grams of pulp were beaten for each of

the variables investigated. Six aliquots were taken from

each beater run sufficient to make a 12 x 12 inch handsheet

of 70 g/m^ . An aliquot of beater stock was diluted with

600 cnw distilled water and agitated 7,500 revolutions in

a British disintegrator. If no further treatment was necessary, as in the preparation of waterleaf control handsheets,

the mixture was transferred to the deckle box of the handsheet machine and a sheet was made as described below. For

acrylic resin treatments, the pH was adjusted to 9 with 1 N

NaOH and a retention aid added to the pulp slurry. Two percent, based on latex solids to be deposited on the fibers,

was added for latex E-631 and 4 percent for latex TR-407.

The retention aid was metered from a 1 percent solution and

diluted with 30 cm-^ distilled water. Two thirds of the

retention aid was added to the pulp suspension and stirred

for 5 minutes in order to exhaust the retention aid from

solution prior to latex addition. The pH of the mixture

was then decreased to 4.0 with 0.5 N H2SO4

.

The pcrylic emulsion was diluted with approximately

50 cm^ distilled water and added to the pulp suspension in

three equal portions with moderate stirring (rapid stirring

removes adsorbed polymer by shearing) . Five minutes was

allowed between each addition to exhaust the acrylic latex.

After all of the latex was added, the remainder of the

retention aid was added and the mixture was stirred for an

additional 5 minutes.

The mixture was then transferred to the deckle box of

the handsheet machine and a sheet was formed. The wire

containing the formed sheet was placed on a blotter, covered

with a felt, and consolidated by pressing the sheet with a 30 cm long roller weighing 22.5 kg. The sheet was removed

from the wire, placed between felts, and passed through the

roll press of the handsheet machine at either the minimum or

maximum pressure possible. The pressed sheet was dried on

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

One half of each sheet was flexed 1,000 times over

3.18 mm rollers and constrained by a 700 g free hanging weight

on the NBS paper flexer. The other half served as a control.

The results are given in Tables 1, 2, 3, and 4.
5

2 . 3 Results and Discussion

Wet pressing and mechanical refinement increase the

tensile and other physical properties of paper with the

exception of Elmendorf tear, cantilever stiffness, air

permeability, and thickness. Mechanical refinement results

in greater fiber wet flexibility eventuating in greater

compaction of fibers before pressing. Therefore, as mechanical refinement is increased, the effect of wet pressing

decreases

.

The chances for stress concentration increases with

density, as when paper is torn, leading to a decrease in

tearing strength. Increased compaction of fibers results

in a less porous sheet leading to lower air permeability

and* decreased thickness. The decrease in thickness is also

responsible for the decline in bending stiffness, as the

stiffness of a material is proportional to the cube of the

thickness. It will be shov/n later, however, that thickness

is not the only factor related to the stiffness of paper.

Beater addition of acrylic resins usually increases the

porosity of paper. The probable causes for this were discussed in a previous report [2] . The effect of the acrylic

resins on the mechanical properties of paper did not appear

to be influenced by mechanical treatment. If the stiffness

retention of paper retrogressed as a result of treatment

with acrylic resin at one set of mechanical treatment conditions, the trend remained unchanged for all the other

mechanical treatment conditions. Stiffness retention, as

well as the retention of other properties, declined with

resin E-631, but improved appreciably with resin TR-407.

The retention of physical properties during flexing

increased as the degree of mechanical refinement increased.

Wet pressing had a similar effect in that retention of

properties with flexing were higher for sheets pressed at

the highest pressure.
6

2.3.1 The Importance of Apparent Density to the

Retention of Stiffness with Flexing

Retention of stiffness increases with increasing

mechanical refining and wet pressing. Both mechanical

processes eventuate higher density paper as the degree of

the process increases. A plot of log percent retention of

stiffness against density for the four papers investigated

indicates a high correlation between the density of the

paper and the retention of stiffness with flexing. The

coefficient of correlation ranges from .89 for paper

treated with E-631 to a high of .99 for the water and

retention aid controls (Figure 1)

.

In the beating of pulp, the following occurs: The

outer layer of the fibers is removed, fibers are cut and

split longitudinally, fines are produced, hemicelluloses and

other low molecular weight celluloses are rearranged and

migrate to the fiber surface or are dissolved, and internal

bonds are broken. The fibrils exhibit a significant axial

orientation which results in an increase in packing density

of the wall [9] , and some fibrils are pulled away from the

fiber but remain attached (fibrillation) while others break

away completely (debris) . These morphological changes , result in increased wet fiber flexibility and increased

f iber strength, stiffness, and modulus upon drying [9]. As

the amount of mechanical refinement increases, dislocations

and other fiber wall damage occur which reduces both fiber

tensile strength and modulus.

Increased wet fiber flexibility results in a more

compact fiber network structure or a denser sheet. The

fines formed during beating also contribute significantly

to sheet density since they produce denser sheets than

fibers alone [9] . As a consequence, increased refining

results in the production of increasingly more dense sheets

with a decreasing amount of free volume, which is the probable

cause for increased stiffness retention.

In less dense sheets, the free volume is greater,

allowing fibers to move laterally or to twist when strained,

permitting the dissipation of stresses in the direction of

least resistance. The matrix, located in the interstices

of fibers in the sheet and formed from the fines generated

during beating, restrains lateral movement and fiber twisting

when paper is strained and results in a higher modulus. On

flexing, the matrix cracks leaving only the fiber network

structure and fiber stiffness contributing to sheet stiffness.
7

As paper density increases, the matrix contributes less

and less to paper stiffness as the decrease in free volume

precludes fiber twisting or lateral movement when the paper

is strained. If there is little or no degradation of interfiber bonding during flexing of dense papers, the free volume

should not increase appreciably and the bending stiffness

should not decline as extensively as it does in less dense

papers

.

In summary, the bending stiffness of paper is dependent

on (1) fiber stiffness, (2) the matrix formed from the fines,

and (3) the fiber network structure. The matrix plays a major role in paper stiffness with less dense papers. Once

the matrix is destroyed in flexing, there is a sharp decline

in stiffness, and the residual stiffness depends upon the

fiber stiffness and fiber network structure. As the fiber

network becomes more compact, the free volume of paper declines,

and the matrix becomes decreasingly important for paper stiffness. The lower the free volume, the lower the opportunity

for fibers to twist and move laterally when paper is strained,

resulting in a higher modulus and bending stiffness.

2.3.2 The Relationship Between Paper Modulus and

Paper Stiffness

When a piece of material, initially straight, is stressed

in bending, the relationships between the applied bending

moment and the deformation, on the one hand, and between the

deformation and the stress at any point in the material, on

the other hand, are given by the ordinary elastic formulae

of strength of materials.

M = E f I R y

bending moment

second moment of area of the section about the

neutral plane

Young's modulus

radius of curvature

the tensile or compressive stress in the material

at any distance (y) from the neutral plane.

where M =

I =

E = R =

f =
8

For a rectangular section, as in the case of paper,

a b3

I =

12

where a = width

b = depth of the cross-section (thickness) [13].

In bending, both the stress and the strain vary linearly

across the section, increasing from zero at the neutral

plane to a maximum at the maximum value of y, providing the

material behaves elastically. The stress distribution is

represented in Figure 5 below.

COMPMLSSIGN

STRESS

Figure 5. Elastic stress distribution in bending

Material on the outside of the bend is in tension while that

on the inside is in compression [13].

For most materials there is a linear relationship

between the modulus and the stiffness of the material, i.e.,

the stiffness increases with increasing modulus. With paper

the reverse is true in that the stiffness decreases with

increasing modulus at a constant cellulose content (weight

per unit area) (Figure 2). The increase in paper density is

achieved by increased beating and wet pressing which results

in a decreased thickness if the weight per unit- area remains

unchanged. As the stiffness of a material is related to the

third power of its thickness, it is only logical to attribute

the inverse relationship of modulus and stiffness of paper to

the decline in thickness.
9

The relationship between the modulus and the stiffness

of flexed paper is the inverse of that for unflexed paper

and identical to that of most materials (Figure 3) . Since

the thickness of flexed and unflexed papers is not significantly different, it becomes apparent that the stiffness

of unflexed paper is highly dependent on a structural

component which deteriorates during flexing. Conceivably,

it is the deterioration of the matrix which has a significant

effect on the original stiffness of paper. The effect of

the matrix on the stiffness of paper could be viewed as

being analogous to the effect of starch on the stiffness of

woven or nonwoven fabrics. The matrix in paper and the

starch in the woven fabric probably function similarly in

increasing the stiffness of the material.

2 . 4 Achievement of Maximum Density for Currency Paper

The most common methods for increasing the apparent

density of paper are (1) mechanical refining (beating),

(2) wet pressing, (3) calendering, (4) selecting pulps with

low fiber coarseness, and (5) additives such as inorganic

fillers

.

The morphological changes that occur in fibers during

beating were enumerated in section 2.3.1 of this report,

^erhaps the most important changes occurring during mechanical refining are the increase in wet fiber flexibility and

production of fines. These two factors have a great effect

on the final density of paper. Since excessive beating

leads to fiber damage, there is a limit to the degree a pulp can be beaten without suffering a decline in strength.

Each pulp has a characteristic response to beating which

must be determined in advance.

The greatest increase in apparent density through wet

pressing is with lightly beaten pulps since these pulps do

not compact well without wet pressing. Excessive wet

pressing may cause fiber damage and a resulting decline in

strength properties. The main advantage in achieving

greater density through wet pressing of lightly refined pulp

is to obtain high interfiber bonding while still maintaining

a lower fiber modulus. In this way, high modulus paper increases its ability to absorb shock and distribute stresses.

There is no information available on the effect of wet

pressing on subsequent calendering. Some work in this area
10

is in progress at NBS . The importance of wet pressing in

optimizing the apparent density of currency paper is unknown.

Calendering is very effective in increasing the apparent

density of paper. However, while calendering makes paper

more uniform in thickness, it is at the expense of uniformity

in density. Calendering is achieved with a set of horizontal

cast iron rolls with chilled hardened surfaces, or a set of

rolls having alternate chilled, cast iron, and soft rolls

( supercalendering)

.

Technically, currency paper is supercalendered during

printing as it is passed twice between a steel roll consisting of the engraved plate and the flexible impression

roll. There is a 20-25 percent reduction in thickness as

a consequence of printing.

In retrospect, the dry process for printing currency

may have a significant effect on the increased circulation

life of currency printed by that method over currency printed

by the wet intaglio process [12]. The apparent density of

currency (calculated from the weight per unit area and thickness of currency) printed by the wet intaglio method is

approximately .791 while currency printed by the dry method

is approximately .901. This represents an increase of almost

14 percent in apparent density. The increase should contribute significantly to the increased circulation life of

currency printed by the dry intaglio process.

m he contribution of density (and, therefore, compressibility) to printability of currency paper has not been

established. This needs further study.

Fiber coarseness, the ratio of fiber length to fiber

diameter, is an important property of papermaking pulps.

It determines the felting characteristics of the fiber.

Included in the coarseness factor are fiber thickness, the

size of the lumen, and the density of the material making

up the fibers. Other factors being equal, the density of the

paper is an inverse function of the coarseness of the pulp.

Pulp coarseness is variable in wood pulps but not in

rag pulps. Coarseness decreases as fibers are split with

beating. However, wood fibers do not easily split apart

lengthwise so that their decrease in coarseness with beating

is minor and not nearly as marked as with cotton and

especially linen fibers. The ability of cotton and linen

to decrease in coarseness during beating probably accounts
11

for the high durability of many rag papers. It is important,

however, to beat rag pulps in a manner which will produce

maximum longitudinal splitting and a minimum of fiber cutting.

The incorporation of fillers in paper usually results

in higher paper densities as the pigments used are much more

dense than cellulose. Unfortunately, as the amount of

filler increases, the strength decreases because the filler

interferes With ihterfiber bonding. Of the five most common

means of increasing paper density (mentioned above) , the

addition of fillers would appear to have the least utility.

The most effective means of achieving maximum densification of currency paper are mechanical refinement and

calendering. Additional information must be obtained on

wet pressing before its role in densif ication can be established .

2 . 5 Analysis of the Modification of Currency Paper

with Acrylic Resins

The main purpose for treating currency paper with

acrylic resins is to produce a more crack resistant matrix.

This has been discussed in section 2.3.1 of this report.

Two methods are commonly used for modifying paper with

latexes: (1) beater addition and (2) saturation. In beater

addition, the fibers essentially are encapsulated with resin

prior to sheet formation. The second technique involves

saturating dry paper with latex, squeezing out the excess,

and drying. The saturation technique deposits resin only

on the exposed portion of the fibers.

A cursory examination of the two methods favors beater

addition over saturation for modifying paper. The intimate

mixing of resin with the fines would appear to be an

excellent means for improving the crack resistance of the

matrix. All that would be necessary is to choose an appropriate resin in sufficient quantity. Unfortunately, treatment of pulps with acrylic resins usually results in a decline

of matrix formation [4]. As a consequence, stiffness retention has declined. A combination of treatments (beater

addition with an acrylic resin followed by treatment with
12

a wet strength resin) has overcome the decline of matrix

formation and conceivably the double treatment could produce

currency paper with a substantial improvement in stiffness

retention

.

When paper is bent, one side of the paper is in tension,

while the other side is in compression (see section 2.3.2

of this report) . The higher the tensile and compression

moduli, the greater the stiffness of a given paper. The

amount of tension and compression also will be dependent

on the radius of curvature of the bend. The degree of

tension or compression through the sheet is in proportion

to the distance from a central neutral plane which is neither

in tension or compression. Therefore, the outermost layers

of paper have the greatest effect on the stiffness of paper,

as these layers are in greatest tension or compression.

From this it is apparent that the outermost layers of paper

should be modified with resin to achieve maximum stiffness

and stiffness retention. When currency paper is modified

with a synthetic resin by the saturation technique, most of

the resin is taken up by the outer layers. For a given

weight gain of acrylic resin, greater stiffness retention

should result from saturation than from beater addition by

virtue of the resin being located in the areas where it is

needed most. In fact, studies have indicated that stiffness

retention improves most when paper is treated with acrylic

resins by saturation [2].
13

3. MODIFICATION OF CURRENCY PAPER WITH ACRYLIC

RESINS BY THE SATURATION TECHNIQUE

3 . 1 Background

All previous work on modifying paper with acrylic

resins by saturation was done with handsheets. This may

not be equivalent to treatment of machine made papers.

Consequently, currency paper was saturated with an acrylic

resin to determine whether improvements in stiffness retention, similar to those observed with acrylic resin modified

handsheets, could be produced.

3 . 2 Experimental

Some nondistinctive , unsized currency paper was obtained

from the manufacturer of U.S. currency paper. The acrylic

resin used in this investigation was AC-61, as it produced

the best results in saturation of all the acrylic resins

evaluated

.

One set of 30 x 30 cm sheets of waterleaf (unsized)

currency paper was glue-sized by the manufacturer of the

paper. As the weight of the sized sheets was approximately

104 g/m* , it was apparent that the glue content was much

greater than normally found in currency paper. Therefore,

a second set of 30 x 30 cm sheets was sized at the National

Bureau of Standards using a glue-glycerin size mixture

employed by the currency paper manufacturer. The sizing

was done with the aid of a laboratory size press. The glueglycerin size mixture was heated and maintained at 63°C.

The paper was passed through the sizing solution at a speed

sufficient to give a residence time in the sizing mixture

of approximately 1.5 seconds. This is the approximate

residence time in the sizing mixture at the paper mill. The

weight of the NBS sized currency paper was 91 g/m^ which is

considerably closer to the normal weight of currency paper.

Saturation with AC-61 was also done with the aid of

the laboratory size press. The emulsion, as received, contained 46.5 percent solids and this was diluted to 20 percent

solids with a 3:1 mixture of water and ethanol. Ethanol was

used to make the diluted emulsion more hydrophobic. Currency

paper has considerable resistance to wetting with water and

the treatment with the diluted emulsion was very nonuniform
14

when ethanol was not used in the dilution. The paper was

passed through the size press at about 5 cm/min. and

at a pressure of 1.4 kg/cm^. it was estimated that the

residence time of the paper in the saturating medium was

about 6 seconds. The saturated sheet was dried on a drum

dryer at 95°C for approximately 4 minutes. Each 30 x 30 cm

sheet was cut in half. One half was designated for flexing

and the remaining half was used for the unflexed control.

Flexing was performed on the NBS paper flexer over

3.18 cm diameter rollers in the cross direction of the

paper for 1,000 double flexes. That portion of the specimen

passing over both rollers was used for subsequent testing.

The air permeability of each specimen was measured in six

different locations with a commercial air permeability

tester. The sample was then cut into eight specimens as

described in NBSIR 74-571 [5]

.

Cantilever stiffness was measured on the CarsonWorthington stiffness tester [11] . These same specimens

were then used for the determination of internal tear

according to TAPPI T414 ts-65. A single specimen was used

for each determination on an Elmendorf tear tester with a

capacity of 200 g. Folding endurance was determined

according to TAPPI T511 su-69 using an MIT folding endurance

tester

.

Load elongation was performed on a constant rate of

elongation apparatus according to TAPPI T404 ts-66, using

a specimen 1.5 cm wide and a span length of 10 cm.

The results are given in Tables 5 and 6.

3 . 3 Results and Discussion

The data indicate that currency paper treated with AC-61

has far better stiffness retention after flexing than when

sized with a glue-glycerin mixture. Furthermore, the acrylic

resin treated currency paper has a higher strength and energy

to break than the glue-sized paper. Retention of stiffness

after 1,000 flexes is only 61 percent for glue-sized paper

as compared to 86 percent for the resin-treated paper

(Figure 4)

.
15

Increasing the glue-glycerin content from approximately

6 to 18 percent results in a softer paper. The stiffness of

the glue-sized papers after flexing are not significantly

different but, as the stiffness of the unflexed paper containing the greater amount of glue-glycerin is much lower,

the retention naturally is much higher (Figure 4) . Ideally,

however, currency should have a high stiffness initially

and it should maintain that stiffness to a high degree when

flexed

.

Perhaps the greatest advantage in glue sizing is the

improvement in folding endurance. However, even though the

higher glue-glycerin content currency paper has a significantly

higher folding endurance, it does not appear to be more

durable. This is an important point to consider in evaluating

potential currency papers. Great emphasis may be placed on

high folding endurance on the assumption that it is synonymous

with durability. Unfortunately, a relatively poor paper

might appear to be high quality if treated with a sufficient

amount of glue-glycerin to impart high folding endurance.

This investigation suggests that a mill trial should

be conducted by modifying currency paper with AC-61 acrylic

resin by saturation. This would enable a full scale evaluation of the paper. Along with the acrylic treated paper,

the evaluation should include some unsized and some gluesized paper from the same lot. A minimum of 10 percent AC-61 resin should be applied to the paper in a uniform

manner. All three papers should be printed.

In addition to evaluating the printability of the paper

and its subsequent durability, there should be an investigation on the repulpability of the resin treated paper. It is

essential that all the currency paper wastes, along with

trimmings from printed currency, be repulpable for reasons

of economy.

If the AC-61 treated currency paper maintains its

superior stiffness retention after printing, a substantial

increase in circulation life of currency might result, providing all other factors remain unchanged. New factors

influencing circulation life may arise as a consequence of

the treatment or because of the extended circulation life.

Therefore, the nev; currency would have to be placed in

circulation and monitored before a final assessment of the

paper could be made.

4. BIBLIOGRAPHY

1. Alexander, S. D. and Marton, R. , Tappi 51 (6): 283 (1968).

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

and Stamp Papers, NBSIR 73-124, January 30, 1973.

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

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

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

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

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

and Stamp Papers, NBSIR 74-571, September 6, 1974.

6. Graminski, E. L. and Toth, E. E .

, Evaluation of Currency

and Stamp Papers, NBS Report 10 336, September 17, 197 0.

7. Graminski, E. L .

, "The Effect of Flexing on the Mechanical

Properties of Paper," The Fundamental Properties of Paper

Related to Its Uses (Technical Section, BP&BMA, London),

in press.

8. Graminski, E. L. and Toth, E. E

.

, Evaluation of Currency

and Stamp Papers, NBS Report 10 465, August 19, 1971.

9 . Alexander, S. D., The Effect of Modifying Single Fiber

Properties on the Relation Between Paper Properties and

Density, Ph.D. dissertation, New York State College of

Forestry, Syracuse, 1966.

10. Fujii, J. S., Molecular Degradation Caused by Vibratory

Ball^Milling of Wood, Ph.D. dissertation, New York State

College of Forestry, Syracuse, 1964.

11. Carson, F. T. and Worthington, V., Tappi 36 (1): 36-42

(1953) .

12. Randall, E. B., Jr. and Mandel, J., Statistical Comparison

of Wear Characteristics of Old and New Dollar Notes,

NBS Report 6933, August 15, 1960.

13. Fenner, A. J., Mechanical Testing of Materials,

Philosophical Library, Inc., New York, NY, pg. 70-71,

1965.

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.7


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NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

ami inc D A DM \C DATA

SHEET

1. PUBLK A 1 [ON OR REPORT NO.

NBSIR 75-670

2. dov't Accession

No.

3. Recipient's Accession No.

4. TI T I. E AND SUBTITLE

NBSIR 75-670, Evaluation of Currency and Stamp

Paper

5. Publication Date February 20. 1975

6. Performing Organization Code

7. AUTHOR(S)

E. L. Graminski and E. E. Toth

3 „ r f or mi o ^ r^3n Report No

NBSIR 75-670

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 3000443

11. Contract/Grant No.

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

Bureau of Engraving & Printing

U.S. Department of the Treasury

Washington, D.C. 20401

13. Type of Report & Period Covered _Progress

,

7/1 - 12/31/74

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.)

The stiffness retention of paper with flexing, which is essential

to the circulation life of currency, can be improved by increasing

the apparent density of paper. An increase in the apparent density

can best be achieved by mechanical refining (beating) of pulp, wet

pressing of the formed sheet, and calendering. Additional work must

be performed to determine which of the three methods is best for

producing a higher density currency paper. Further improvement in

stiffness retention can be achieved by modifying paper with acrylic

resins by the saturation technique. Currency paper, when so modified

with acrylic resin AC-61, appears to have a significantly higher

stiffness retention than regular currency paper.

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

)

Acrylic resins; apparent density; calendering; mechanical refining; stiffness retention; wet pressing

18. AVAILABILITY Q Unlimited

[XXEor Official Distribution. Do Not Release to NTIS

Q Order From Sup. of Doc, U.S. Government Printing Office Washington. D.C. 20402. SD Cat. No. C13

|

1 Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCI ASSIFIED

21. NO. OF PAGES

31

20. SECURH V CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

USCOMM-DC 29042-P74