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 TreasuryWashington, 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 Calenderingon the Stiffness Retention of Paper 1
1.2 The Effect of Refining Consistency on theMorphological Changes of Wood Pulp Fibers. ... 2
1.3 The Physical Properties of Handsheets madefrom Classified Currency Paper Furnish 2
2. The Effect of Wet Pressing and Calendering onthe 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 theMorphological 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 fromClassified 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 StiffnessRetention of Paper
Previous investigations indicated stiffness retention ofpaper increased with increasing paper density at a given weightper unit area (basis weight) . Increased paper density can beachieved in various ways, most common of which are wet pressing,mechanical refining and calendering. The effect of each ofthese paper making processes on the density may vary with paperof different basis weight. As the printing process for currencyis a calendering process it was decided to determine the effectof wet pressing, calendering and basis weight on the eventualdensity and stiffness retention of calendered paper when flexed.
Handsheets were prepared from a kraft wood pulp at threedifferent basis weights, 50, 75 and 100 g/m 2
, and wet pressedat either the minimum or maximum pressure possible with thehandsheet machine press. Retention of stiffness in the courseof flexing was evaluated on the NBS paper flexer.
The results indicate that fiber compaction during wetpressing increases with increasing basis weight. As the thick-ness of the wet mat increases, the opportunity for lateral fibermovement during wet pressing increases resulting in a more evendistribution of mass and greater density. Calendering furtherdensifies the paper and the densification appears to be greaterwith the higher basis weight papers which were wet pressed atthe higher pressure. Although retention of stiffness decreaseswith increasing basis weight the stiffness of the high basisweight paper after flexing is very much greater than thestiffness of the lower basis weioht paper. The increase in thedecline of stiffness during flexing for the higher basis weightpapers is not in proportion to the increase of stiffness withincreasing basis weight for the unflexed paper.
The results of this investigation suggest that mill trialsbe conducted to determine the effect of wet pressing on paperdensity, before and after printing and on the printability ofcurrency paper. Variation of wet pressing pressure would probablybe most effective in the second and third stages of the presssection.
2
1. 2 The Effect of Refining on the Morphological Changes ofWood Pulp Fibers
The most common means of obtaining high density wood pulppapers is by increasing the amount of mechanical refining. Wetfiber flexibility apparently increases with increasing mechanicalrefining and the compactness of the fiber network increasescorrespondingly. However, increased refining results in increasedfines production and a decrease in fiber length which is undesirable.High consistency refining is one possible means for achievinghigh wet flexibility without undesirable side effects.
Handsheets were prepared from a kraft wood pulp which hadbeen mechanically refined to different degrees and at variousconsistencies. In addition, some refining was done at a highconsistency, diluted and beaten further at a lower consistency todetermine what effect this procedure may have on the morphologicalchanges of pulp fibers. The physical properties of the handsheetswere determined.
Fewer fines are produced as the beating consistency increases;however, the strength properties decline. The two consistencyrefining produces some interesting results as folding enduranceincreases significantly over paper made from only low consistencybeating.
Apparently significant differences in morphological changesdo occur at different beating consistencies. Identification ofthe morphological changes requires microscopic examinations.Due to the time consumption of available microscopic techniquesfurther work on this subject will be deferred to a later timewhen a more rapid image analysis technique is developed for fiberanalysis
.
1. 3 Physical Properties of Handsheets Made from ClassifiedCurrency Furnish
Papermaking furnishes consist of a wide range of fiber sizesand shapes. The distribution of the size and shape factors areof paramount importance to the mechanical properties of paper.A knowledge of the contribution of the various structuralcomponents to the mechanical properties of paper is of extremeimportance in designing a paper for specific properties. Onemeans of obtaining this information is to fractionate the fiberslurry into various components; prepare handsheets from eachfraction and determine the mechanical properties of the handsheets.
A quantity of currency furnish, obtained from the manufacturerof currency paper, was fractionated into four fractions andhandsheets were made from the first three fractions. The mechanicalproperties of these handsheets were compared to those made fromthe unfractionated furnish.
The handsheets made from the first fraction containing thelongest fibers had poor mechanical properties. The mechanicalproperties of the sheets made from the remaining two fractionsfor the most part were better than those from the first fraction.The best properties were obtained with handsheets made from theunfractionated furnish. A rather surprising result was therelatively low percentage of long fibers in the furnish.
A greater indepth analysis of currency furnish should beperformed. The information would most certainly be extremelyhelpful in designing a wood pulp paper for currency.
4
2. The Effect of Wet Pressing and Calenderingon 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 demon-strated. Both processes result in increasing the density ofpaper. At constant weights per unit area the retention of bendingstiffness of paper during flexing increases with increasing density.
The fines and debris produced during mechanical refiningform a film-like material (matrix) in the interstices of thefibers [2] . The matrix serves to restrain the lateral movementand/or twisting of fibers when paper is deformed. Stresses arethen maintained along the fiber axis instead of being dissipatedin transverse motion. This results in a higher modulus andbending stiffness. During flexing the matrix cracks and leads toa large decline in both modulus and stiffness.
As the density of paper increases the free volume of paperdecreases precluding fiber movement when paper is strained. Inessence the importance of matrix to the mechanical properties ofpaper decreases as the density increases. Therefore, at aparticular weight per unit area, the more dense a paper is thesmaller should be the decline of mechanical properties when flexed.
A third means for densifying paper, calendering, has notbeen investigated previously. The effect of densification bycalendering on the durability of currency needs to be known, asthe dry intaglio printing process for currency is in effect acalendering process. In this study the effect of calendering onthe durability of wood pulp handsheets was determined.
2 . 2 Experimental
A kraft wood pulp was chosen for this study as it isrelatively easy to refine in comparison to rag pulps. The pulpwas beaten in a PFI laboratory mill at 10 percent consistency,with no clearance between bedplate and roll for 10 thousandrevolutions at 3. 4 kilograms force and a relative velocity ofroll to bedplate of 6 m/sec. A total of 1200 grams of pulp werebeaten, 40 grams at a time. All of the beaten pulp was combinedin a large stainless vessel and diluted to approximately onepercent consistency. The slurry was stirred for one hour priorto taking aliquots for handsheet preparation.
5
Handsheets were prepared at either 50, 75 or 100 g/m 2.
Wet pressing was done at either the lowest or highest pressureavailable on the handsheet machine press. The sheets were driedon a drum drier at 95°C for approximately 4 minutes.
Calendering was done on a calender stack at a pressure of37.3 kgf/lin cm at a speed of 3.8 cm/sec.
One half of each handsheet was flexed 1000 times over 3.18 mmrollers and constrained by a 700g free hanging weight on the NBSpaper flexer. The other half of each handsheet served as theunflexed control. The results are given in Tables 1, 2, 3 and 4.
2 . 3 Results and Discussion
Increased wet pressing results in an appreciable increasein breaking strength, elongation to break, folding endurance,sonic modulus and density while Elmendorf tear, air permeability,cantilever stiffness and thickness decreased. Calendering resultsin a decrease in thickness, air permeability and cantileverstiffness and a marked increase in initial modulus, sonic modulusand density. The remaining properties listed in Tables 1 and 3
show little or no change as a consequence of calendering. Itmust be cautioned, however, that calendering at other pressuresor by supercalendering may have produced entirely differentresults
.
An increase in the weight per unit area (basis weight)resulted in a substantial increase in breaking strength, energyto break, Elmendorf tear, folding endurance, cantilever stiffnessand thickness while air permeability decreased. There appearsto be a trend towards a lower modulus and an increase in densitywith increasing basis weight. The increase in density may bedue to better fiber compaction at higher basis weight or it maybe an artifact resulting from the thickness measurement.
Paper is not smooth and its thickness is actually a measureof the highest points on its surfaces. If the roughness ofpaper remains the same for all basis weights then the thicknessmeasurement of the higher basis weight paper would be closer toits actual average thickness than it would be for the lowerbasis weight papers. Consequently the determined volume wouldbe in greater error for the lower basis weight papers resultingin a lower estimate of density.
There is also the possibility the density actually increaseswith increasing basis weight. If the ability of fibers to movelaterally during wet pressing increases with increasing thickness
6
of the wet web, higher densities would result. The higherdensity areas would experience pressure prior to lower densityregions. If the fibrous mass were able to flow laterally, theregions under high pressure would flow towards the regions oflow pressure. This means that the variance in mass distributionwould decrease and the formation of the paper would be improved.Improved formation would be very beneficial to the printabilityof paper.
In order to prove that mass distribution is more uniformwith increasing basis weight, paper would have to be analyzedby beta-radiography [3,4]. There is a remote possibility suchan analysis can be made in cooperation with laboratories havingthe necessary analytical equipment.
The tensile and compressive stresses at the surfaces ofpaper, when bent to a particular radius of curvature, increasewith increasing paper thickness. As a consequence, the magnitudeof structural degradation should increase with increasingthickness. An excellent measure of structural change as aconsequence of flexing is the increase in air permeability. Aplot of the increase in air permeability after flexing againstthe thickness of the paper is shown in Fig. 1. An excellentcorrelation exists between the thickness of paper and theincrease in air permeability after flexing.
The increase in air permeability for a given thickness isgreater for the calendered papers which indicates the tensileand compressive stress are greater in calendered than inuncalendered paper. This is to be expected as the initialmodulus of the calendered paper is greater than the initialmodulus of the uncalendered paper. Furthermore, the datasuggest that there is a maximum thickness for calendered paperwhich should not be exceeded because of extensive structuraldegradation when bent to a small radius of curvature. Themaximum thickness, however, is probably substantially greaterthan the thickness which would be acceptable for currency.
The increase in air permeability as a consequence offlexing is probably due to degradation of the matrix. It hasbeen shown previously that the degradation of the matrix isprobably the greatest factor in the decline of paper stiffness [2]
.
Therefore, the retention of bending stiffness should decreasewith increase of air permeability . A plot of retention ofstiffness 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 thickerpapers (higher basis weight) the final stiffness of the thickerpapers is much greater than that of the thinner papers (Fig. 3) .
7
Retention of bending stiffness is greater for handsheetswet pressed at the higher pressure whether calendered or not.This would be expected as higher wet pressing produces a higherdensity and it has been shown previously that stiffness retentionis greater the higher the density [1] . Thickness is only oneimportant factor in the bending stiffness of paper. The densityis also critical as the importance of matrix to the modulus andstiffness of paper declines with increasing density.
The results of this study indicate once again that currencyshould have maximum density for greatest retention of stiffness.Wet pressing appears to be a very important manufacturingvariable from the standpoint of achieving the greatest densityfor currency. Furthermore, there is the possibility that wetpressing might affect the uniformity of mass distribution inpaper which is of importance to the printability of currencypaper. Some studies should be made on the effect of wet-pressingpressures on the formation, density and printability of manufacturedcurrency paper.
8
3. The Effect of Refining Consistency on theMorphological Changes of Wood Pulp Fibers
3. 1 Background
Results reported previously [1] and in Section 2.3 of thisreport indicate currency paper should have a high density foroptimum stiffness retention during circulation. Generally pulpis highly mechanically refined whenever a high density paper isdesired. Refining results in greater wet fiber flexibilitywhich in turn eventuates in a more compact fiber network. Themore compact the network the greater the density.
As fiber compactness increases, the interstices betweenthe fibers become smaller and smaller and the effect of fineson the mechanical properties of paper declines since there isless opportunity for lateral fiber movement during strainingin a compact fiber network. Increased mechanical refining,which produces greater wet fiber flexibility, also results inthe production of greater amount of fines and a decrease infiber length. The increased number of fines causes waterdrainage to slow down resulting in a decline in the rate ofpaper production. A point is reached in papermaking where thefines become more of a liability than an asset.
It would be ideal for a refining process to be capable ofproducing fibers having high wet flexibility without reducingthe fiber length or producing undesirable quantities of fines.As mechanical refining progresses, fibers imbibe more waterand become more swollen. Increased swelling makes fibers moresusceptible to damage when subjected to compressive and shearforces during beating.
One possible means for reducing fiber damage and preservingtheir integrity is by reducing available water for swelling inthe beating process. High consistency refining is becomingincreasingly more important in paper manufacture not onlybecause of the smaller amounts of water used but because highconsistency refining produces desirable morphological changesin fibers. The morphological changes occurring in highconsistency refining are undoubtedly different from those inlow consistency refining.
As there is interest in developing an all wood currencypaper it was decided to determine the effect of high densityrefining of wood pulp on the rheological properties for woodpulp paper. Hopefully, morphological changes similar to thoseoccurring with cotton and linen pulps (fiber splitting andfibrillation) , will occur with wood pulps under certain specificconditions
.
9
3. 2 Experimental
Forty grams of wood pulp was beaten in a PFI laboratorymill at 3.4 kgf/cm at a relative velocity of roll to bedplateof 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 sufficientwater to decrease the consistency to 10 percent and beaten foran additional 5 thousand revolutions.
Six handsheets were made from each of the pulp beatingsand wet pressing was done at the maximum pressure possible onthe handsheet press. The physical properties of the handsheetswere determined and are listed in Table 5.
3. 3 Results and Discussion
As could be expected, the reduction of water in the entiremechanical refining process results in a decline of paper strength.As mechanical refining increased paper strength as well as manyother physical properties improved regardless of the consistency.The ease of water drainage (Canadian Standard Freeness) increaseswith increased refining consistency which demonstrates that highconsistency refining does indeed reduce the amount of debrisgenerated.
The refining done at two different consistencies producedthe most interesting results. The tensile properties of thehandsheets beaten for 5 thousand revolutions at either 20, 30or 40 percent consistency followed by an additional beating of5 thousand revolutions at 10 percent consistency were essentiallyequivalent to those of handsheets beaten at either 5 or 10 percentconsistency for 10 thousand revolutions. However, the foldingendurance of the handsheets beaten at 40 and 10 percentconsistencies was significantly higher than that of handsheetsbeaten at 5 or 10 percent consistency for 10 thousand revolutions(two sided t test) . It is also noteworthy that the densitiesof the paper from the two consistency refining were appreciablyhigher than the handsheets obtained from the single consistencyrefining.
Evidently significant differences in morphological changesoccur in the two consistency refining resulting in increaseddensity and folding endurance and a decrease in thickness.There also appears to be a trend towards a higher elongation tobreak in the handsheets from two consistency refining.
10
The results are encouraging in that morphological changescan 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 frequentlyrelies on subjective rather than objective measurements.
Work is commencing in the Applied Mathematics Division onthe image analysis of fibers. When completed, the image analysissystem should permit rapid, quantitative measurement of suchfiber properties as fiber length distribution, coarseness,fibrillation and curl. It was decided to discontinue furtherwork on the effect of consistency in refining on the morphologicalchanges of wood pulp fibers until the image analysis techniqueis developed.
11
4. The Physical Properties of Handsheets Madefrom Classified Currency Paper Furnish
4. 1 Background
Paper pulp consists of a wide distribution of fiber lengthsand sizes. When filtered on the endless wire section of a
paper machine they form a fiber network which determines theeventual paper properties. Different papers rely on differentfiber fractions for a particular paper property. To achievegood formation in paper a short fiber furnish is favored overlong fiber furnish. Folding endurance is enhanced by longfibers
.
Regardless of the importance of a single fiber fractionon a particular paper property the remaining fractions have a
strong influence on the final property in question. It has beenpostulated numerous times in previous reports [1,2,5,6,7] thatthe fines of currency paper are extremely important to the modulusand bending stiffness of paper. The degradation of that portionof currency paper, consisting primarily of fines, results in a
substantial decline in modulus and cantilever stiffness.
One method of demonstrating the importance of the variousfiber fractions of paper is to separate the beaten pulp intoindividual fractions and to make handsheets from the variousfractions. This was done with a quantity of currency paperfurnish obtained from the manufacturer of currency paper. Thestock was sampled just after the final mechanical treatment andprior to final dilution for the headbox. The furnish was atapproximately 2.3 percent consistency and contained no wetstrength resin.
4 . 2 Experimental
The currency paper furnish received from the currency papermanufacturer was filtered to remove excess water. The finalconsistency was approximately 20 percent. Aliquots of theconcentrated furnish containing approximately 5 grams of dryfiber, were fractionated on a commercial fiber classifier. Only3 screens were used in the classification, 14, 35 and 65 meshas the 150 mesh screen constantly plugged causing water over-flow. Approximately 75g of dry fiber were fractionated. Hand-sheets were then made from the various fractions. The physicalproperties of the handsheets are given in Table 6.
12
4. 3 Results and Discussion
It was surprising that the long fiber fraction consistedof only about 16 percent of the total furnish. This is lessthan half the amount usually found in a moderately beaten soft-wood pulp. The second fraction (35 mesh screen) consisted ofapproximately 37 percent of the furnish while the third fractionconsisted of about 15 percent of the total. This means thatapproximately 32 percent of currency stock consists of veryshort fibers and fines.
Despite the low percentage of long fibers in currency paperit nevertheless has excellent physical properties. Apparentlythere are an appreciable number of very long fibers in the longfiber fraction which have a great effect on the physical propertiesof currency paper. A fiber length distribution of this fractionwill be determined in the future as the information may be veryimportant in the design of a wood pulp paper for currency.
Handsheets made from the first fraction are very porousand have a low density. The handsheets made from the other twofractions are less porous and denser. The initial modulusincreases with increasing density demonstrating once again theimportance of fiber compaction on the modulus of paper. Thehandsheets made from the unfractionated stock had the highestmodulus, strength, extensibility, folding endurance, andcantilever stiffness.
While the thickness of paper greatly influences its bendingstiffness it is nevertheless only one factor of importance tothat property. This is demonstrated very nicely when the bendingstiffness of the handsheets from fraction one and the unfractionatedstock are compared. The thickness of the handsheets from fractionone are 24 percent higher while the cantilever stiffness is 15percent lower than the handsheets made from unfractionated stock.Apparently, the higher density of the unfractionated handsheetsis the primary cause for the higher bending stiffness.
Much could be learned from an in depth study of currencystock. The information obtained from such a study would greatlyassist in the design of a wood pulp paper for currency. However,much of the work would involve microscopic investigations of thefibers. As this work is lengthy and tedious with the availablemicroscope techniques further work on this problem will bedeferred until fibers can be analyzed automatically (see imageanalysis in section 3.3).
13
5. Bibliography
1. Graminski, E. L. and Toth, E. E. , Evaluation of Currencyand Stamp Papers, NBSIR 75-670, February 20, 1975.
2. Graminski, E. L. and Toth, E. E., Evaluation of Currencyand 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 Currencyand Stamp Papers, NBSIR 73-274, August 15, 1973.
6. Graminski, E. L. and Toth, E. E. , Evaluation of Currencyand Stamp Papers, NBSIR 74-431, January 2, 1974.
7. Graminski, E. L. and Toth, E. E., Evaluation of Currencyand Stamp Papers, NBSIR 74-751, September 6, 19 74.
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LOW HIGH LOW HIGH LOW HIGH
WET PRESSING PRESSURE
Figure 3: Bending stiffness of flexed handsheets havinvarious 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 AccessionNo.
3. Recipient's Accession No.
4. TITLE AND SUBTITLE
Durability of Paper
5. Publication Date
January 19766. Performing Organization Code
7. AUTHOR(S)E. L. Graminski and E. E. Toth
8. Performing Organ. Report No.|
NBSIR 75-9789. PERFORMING ORGANIZATION NAME AND ADDRESS
HATIONAL BUREAU OF STANDARDSDEPARTMENT OF COMMERCEWASHINGTON, 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 PrintingU. S. Department of the TreasuryWashington, D. C.
13. Type of Report & PeriodCovered
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 ingreater densities as the amount of mass being pressed increases. This may be due togreater freedom for lateral movement of the fibers as the thickness of the wet matincreases. Greater densities eventuate in increased stiffness retention during flexingand should result in an improved circulation life of currency. Higher densities withincreasing wet mass indicates more uniform mass distribution which should result inimproved printability . The debris generated during pulp refining is beneficial to themechanical properties of paper but its utility decreases as to wet fiber flexibilityand the amount of debris increases with increased mechanical refining. High consistencyrefining of wood pulp apparently reduces the amount of fines produced but otherdisadvantages arise such as decreased interfiber bonding. The importance of fines tothe mechanical properties of currency paper was demonstrated by forming handsheetsfrom the various fractions of currency stock and comparing their mechanical propertieswith 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; stiffnessretention; wet pressing
18. AVAILABILITY ] Unlimited 19. SECURITY CLASS(THIS REPORT)
21. NO. OF PAGES
[_X] For Official Distribution. Do Not Release to NTISUNCLASSIFIED
26
L 1 Order From Sup. of Doc, U.S. Government Printing OfficeWashineton. D.C. 20402. SD Cat. No. CI 3
20. SECURITY CLASS(THIS PAGE)
22. Price
[ j Order From National Technical Information Service (NTIS)Springfield, Virginia 22151 UNCLASSIFIED
USCOMM-DC 29042-P74
