U.S. Department of AgricuIture*Forest Service*Forest Products Laboratory*Madison, Wis.

MEASURING SHRINKAGE IN HANDSHEETS DURING DRYING

U.S.D.A.FORESTSERVICE RESEARCH PAPER

FPL131MAY 1970

SUMMARY

Seven pulps and several freenesses of one of the pulps were used to investigate the shrinkage that occurs during drying wet webs. Based on their initial dimensions, the webs decreased an average of about 28 percent in thickness and about 5 percent in width. Shrinkage characteristics were dependent on handsheet moisture content and the amount of fiber bonding. It was also found that pulping process, species composition, and wet or dry lap affected wet-web shrinkage.

ACKNOWLEDGMENTS

The author thanks the following of the Forest Products Laboratory, J. F. Wichmann and R. E. Benson for technical assistance and V. C. Setter-holm for pertinent suggestions,

MEASURING SHRINKAGE IN HANDSHEETS DURING DRYING

By GARY C. MYERS, Research Forest Products Technologist

Forest Products Laboratory,1 Forest Service, U.S. Department of Agriculture

INTRODUCTION

Water, considered the lifeblood of papermaking, is used extensively in the pulping and bleaching processes and in papermaking. Water is needed to carry the fibers through all stages of proces-sing and refining before delivering them to the paper-machine wire, However, when the fibers a re deposited on the wire, water becomes a problem and must be extracted from the web. It is the extraction of water that causes the paper web to shrink.

The mechanisms of water removal, the magni-tude of dimensional changes, and the shrinkage forces exerted by a wet web as it dries, all still not completely understood, are the subjects of much experimentation.

Shrinkage forces developed in wet webs dried

between stationary clamps of a testing machine have been measured and reported (2,3,4,5,10). Most of these measurements were made on wet webs with initial moisture contents below 80 per-cent. Lyne and Gallay (4) measured wet-web tensile strength and elongation at an initially high moisture content of approximately 92 percent; Robertson (la) measured the changes inthickness and correlated paper web specific volume with percent solids content. But thus far nothing has been found in the literature concerning the determination of simultaneous shrinkage force, thickness, and width.

The objective of this work is twofold: To measure the reduction in thickness and width and the simultaneous increase in shrinkage force that occur while a strip of paper is dried, and to gain a better understanding of fiber bonding,

1 Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 2Underlined numbers in parentheses refer to Literature Cited at end of report.

TEST PROCEDURE

Materials

The pulps selected for investigation were an alpha, a bleached southern pine kraft, a bleached gum kraft, a western hemlock bleached sulfite, a northern pine unbleached kraft, and a mixed northern hardwood unbleached kraft, These were commercially made pulps selected to provide a wide distribution of types, including both wet-and dry-lap pulps. In addition, a dry-lapped unbleached southern pine kraft was selected for a series of freeness evaluations.

Beating and Storage of the Pulp

Each pulp (360 grams on a moisture-free basis) was beaten in a standard laboratory beater to approximately 500 milliliters Canadian Stand-ard freeness (table 1). The beaten pulp was diluted to handsheet consistency and stored at 2° C. in covered containers until needed.

Sheetmaking, Pressing, and Storage of

Handsheets weighing about 60 grams per square meter, moisture-free basis, were formed on an 18- by 23-centimeter sheetmold. Test specimens were formed in the wet web by a stainless-steel divided frame placed on the wire of the sheetmold before adding the pulp slurry. The sectioned web, couched from the wire in the usual manner, provided four 2.5- by 20.3-centimeter specimens.

Moisture content of the sheet was adjusted during pressing by interleaving the handsheets with presoaked blotters of known moisture con-tent. A stack of 10 handsheets and blotters, pressed at 4.22 kilograms per square centimeter for 5 minutes, was placed in a plastic bag and stored at 2° C. until needed for testing.

Environmental Conditions

The experimental work, except for sheetmaking and pressing, was conducted in a room main-tained at C. and 50 percent relative humidity.

Table 1.--Pulps, type of lap , and Canadian Standard freeness (CSF)

FPL 131 2 1.5-15

Specimen Preparation

Several hours before testing the specimens, the plastic bag containing the pressed handsheets was removed from cold storage and allowed to reach room temperature.

One handsheet at a time was removed from the packet, only long enough to place identifica-tion numbers on each of the four preformed specimens. One of the specimens was removed for testing. The remaining specimens with the margins of the sheet were immediately returned to the plastic bag to prevent moisture loss.

The ends of a specimen were ironed dry to a distance of 5.7 centimeters with a heat sealer before clamping them in the jaws of the Instron testing machine. The testing machine crosshead

moved manually until 23 gram preload was applied to remove the slack from each specimen before testing. These procedures were followed in preparing the control, thickness, area, and moisture content specimens.

Control Specimen

The first specimen taken from a sheet served as the control to be compared with the specimens used for measuring thickness and area. It was allowed to dry under TAPPI standard humidity conditions without further movement of the testing machine crosshead or disturbance of the speci-men. The shrinkage force exerted by the speci-men while drying was measured to the nearest 0.1 gram with an electronic tension load cell, and continuously recorded on a chart driven at a constant speed. The test was terminated when the moisture content of the specimen came into equilibrium with the atmosphere and the shrinkage force reached a maximum.

Thickness Measurement

Thickness changes during drying were taken on the second specimen tested from each hand-sheet.

Thickness was measured with a special wet-web dial micrometer accurate to 2.5 microns. The anvils of the micrometer were 1.5 centi-meters in diameter, with the stationary anvil having a flat surface and the movable anvil a 5.1-centimeter radius convex surface. The mi-

crometer exerted an average compacting pres-sure of 23 grams per square centimeter on the specimen. It was placed in contact with the specimen only long enough to take a measure-ment at 3-minute intervals until the specimen

in moisture equilibrium with standard room conditions and no change in thickness was indicated on the micrometer.

Area Measurement

Area measurements were taken on the third specimen to be tested from each handsheet.

Because the specimen was delicate and rapidly losing moisture, a photographic method was chosen to record specimen area, To insure accuracy of what was recorded by the camera, a 2.54- by 2.54-centimeter reference square was placed adjacent to and in the same plane as the specimen, and the square and the specimen were photographed simultaneously. The reference square served as a means of calibration and per-mitted accurate determinations of specimen area by weighing without knowing the magnification of the photograph.

The specimen was sandwiched between two strips of plexiglass while photographs were taken to avoid errors caused by any specimen curva-ture. Immediately before taking a photograph, the plexiglass strips were positioned, and they were removed immediately after the photograph was taken.

The time intervals for taking area measure-ments were selected after a close examination of a graph showing the relationship of shrinkage force to time for the control specimen. During the first part of drying when the shrinkage force increased only slightly, measurements were taken every 6 minutes. The interval was reduced to 3 minutes when the shrinkage force began to increase very rapidly.

Shrinkage force-time curves for the control, the thickness, and the area specimens were compared to determine if there was general agreement in shape and magnitude of the three curves. If in agreement, the area of the speci-mens could be measured from the photographs.

Photographic enlargements of approximately 2.3 magnification were made of the test portion of the specimen and the reference square. The prints were conditioned under standard conditions, and the test portion of the specimen and the

3

reference square were carefully cut from the print and weighed on an analytical balance (7).

Moisture Content Determination

The fourth specimen and the margins from each handsheet were used as samples to obtain moisture content and the drying rate. The test specimens were hung to the side of the testing-machine clamps and allowed to dry unrestrained. A part of the margin material was allowed to air-dry for one of the following time intervals: 0, 6, 12, 18, 24, 30, or 36 minutes. When the time interval had elapsed, the wet portion of the specimen was removed, and the moisture content determined by following TAPPI standard method T-412.

Other Measurements

After the specimens had dried in the testing

machine, the ironed-dry portions were removed, and the remaining 8.9-centimeter specimen was used for flatwise tensile strength and density determinations.

Flatwise tensile strength specimens were pre-pared by sandwiching a test specimen between two 2.54-centimeter-diameter steel blocks, which were covered with double-faced, pressure-sensitive tape. The assembly was heated in an oven, pressed while hot, and then allowed to cool. The maximum load required to pull the blocks apart determined the flatwise tensile strength of the material.

RESULTS

Test results presented in tables 2 through 6 include shrinkage force, thickness, area, shrink-age stress, and moisture content for elapsed time intervals of 3 o r 6 minutes o r both. Density and flatwise tensile strength (table 7) were determined on standard conditioned specimens.

Table 2.--Shrinkage force (grams) o f beaten pulps1,2

1Unless indicated otherwise, pulp was beaten to about 500-miIIiIiter Canadian Standard freeness (CSF). 2 Each value represents an average of about 10 measurements.

FPL 131 4

Tabl

e 3.--

Thic

knes

s (m

icro

ns)

of

the

beat

en

pulp

s1,2

~~

Tab

le

4.--

Are

a (s

quar

e ce

nti

me

ters

) o

f th

e

pulp

s1 , 2

Tab

le 5

.--S

hrin

kage

st

ress

(n

ewto

ns p

er

squa

re c

en

tim

ete

r) o

f th

e b

eate

n pu

lps1

,2

--

Tab

le 6

. M

oist

ure c

on

ten

t (p

erc

en

t)

of

the

bea

ten

pulp

s 1

,2

Table 7.--Density and flatwise tensile strength of the beaten pulps1,2

DISCUSSION

Experimental Procedure

The moisture content of the wet webs was decreased to approximately 63 percent during wet pressing; this was the highest moisture con-tent at which the wet webs could be handled without damaging their structure. By starting the drying at that moisture content no data could be obtained in the important region where surface tension predominates.

The biggest advantage of storing the pressed handsheets at C. in plastic bags was that many handsheets could be formed and pressed at one time, and stored without losing moisture. The storage period also allowed the moisture within the handsheet to migrate from areas of high to low concentration and equalize the distribution.

If the ends of each specimen had not been ironed dry before clamping them in the jaws of the testing machine, the specimen would have failed at the jaws during the test because of a high moisture concentration at the clamping edge. Drying the ends of the specimenprevented tearing, but restrained the adjacent wet portion during drying. The central portion of the specimen was

thought to be far enough from the ends to be free from the effects of restraint imposed by drying.

Test results of the control specimen, which was allowed to dry without any disturbance, were used primarily to determine the time intervals between thickness and area measurements. When examined in conjunction with the thickness and area shrinkage force curves, the curves of the control specimen made it possible to judge if any of the tests were invalid and should be discarded. A poorly formed specimen or a very severe disturbance during testing was a reason for rejecting approximately 2 percent of the tests.

Like most dial micrometers, the wet-web mi-crometer exerted a compressive force on the web. The compacting force of 23 grams per square centimeter was chosen as the minimum value believed sufficient to give reliable thickness values without undue compression of the wet specimen.

Because of possible surface irregularities, all thickness measurements were taken at exactly the same location on all specimens. By doing this, it is likely that a cumulative compaction resulted when the specimen had high moisture content. Consequently, the total percent decrease in thickness was probably greater than had actually occurred.

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Positioning and removing the micrometer placed a load on the specimen, as indicated by the x-y recorder. An example of the disturbances caused while taking thickness measurements is illustrated in figure 1. Part of the recorded discontinuity was caused by placing a time mark on the chart when a thickness measurement was taken. A comparison of the drying-shrinkage force curves for the control and the thickness specimens showed no significant difference be-tween them other than limited disturbances caused by the micrometer.

Figure- I .--Relationship of shrinkageforce to time showing disturbances caused by measuring thickness. M 137 142

Two problems occurred in measuring area by the cutout weighing method (7), which, if not taken care of, injected significant errors into the results. Determining the boundaries of a specimen required careful judgment by the ex-perimenter. Because most specimens had fibers protruding from the edges, they presented the first problem; they did not provide a sharp photographic image. Unless the boundaries were properly interpreted, it was not possible to cut the specimen image accurately, and an error as high as 3 percent could result.

The second problem was caused by the unequal shrinkage of the specimens during drying. Cor-rugations formed lengthwise, and the edges curled; thus the specimen appeared to shrink more in width than it actually did. To overcome this problem, it was necessary to sandwich the specimen between plexiglass plates while taking the photographs.

Taking photographs through the plate caused no difficulties, but placing and removing the plates had a disturbing effect on the shrinkage force-time graph. Figure 2 illustrates the magni-tude of shrinkage forces exerted during the area measurements. It is evident that the disturbances caused by taking area measurements are much more severe than those resulting from thickness measurements. The chart was not pipped during area measurements; therefore the loads recorded were undoubtedly the result of placing and re-moving the plexiglass plates.

Figure 2.--Relationship of shrinkageforce to time showing disturbance caused by area. M 137 141

Although only a few of the disturbances were as severe as the figure shows, this curve was selected mainly to illustrate the resiliency of a wet specimen and its ability to recover from a rather severe disturbance without any obvious permanent damage.

Wet-Web Contraction

Wet-web contraction results from three mech-anisms that occur while drying the wet web: Interfiber shrinkage, intrafiber shrinkage, and the twisting of fibers on drying (11). These processes are dependent on moisture content and the characteristics of the constituent fiber.

During drying, a wet web is held together by

FPL 131 10

increasing interfiber contraction forces (6,11), the first type of contraction. The forces are a result of water surface tensions between fibers. As water is removed the surface tension forces become greater and draw the fibers into closer proximity, and reach a maximum at approximately 75 to 80 percent moisture content. From then on, interfiber forces caused by surface steadily decline and are no longer present at complete dryness. In addition to to wet-web tensile strength (1), the main accom-plishment of interfiber shrinkage is to bring the fibers close enough together to enable formation of fiber-to-fiber bonds.

Water removal from the fiber wall causes microstructure changes and subsequent intrafiber shrinkage, the second type of contraction. The fiber-to-fiber bonds, in turn, form an avenue through which intrafiber shrinkage will be trans-ferred from one fiber to another. When an un-supported single fiber is dried, it undergoes about 20 to 30 percent transverse shrinkage and about 1 to 2 percent longitudinal shrinkage. The large difference in transverse and longitudinal shrinkage forms a stress concentration within the bonded area between fibers. When the fiber bonds are strong, the transverse shrinkage forces of one fiber are imposed upon another through the bond, and form microcompressions. The gross longitudinal shrinkage of a fiber, which may be as high as 12 percent, is the summation of transverse shrinkage at the bond sites and of the longitudinal shrinkage of the free fiber be-tween bond sites @.

The third type of contraction of much less importance is a pseudo shortening of the fiber caused by the fibers coiling and twisting during drying. If moisture were added to the twisted fiber, it would straighten. This type of contrac-tion appears only with fibers that are long enough to entangle and lever upon each other and that have no interfiber bonds to interfere with the twisting (11).

Water Removal From the Handsheet

How a handsheet responds to drying can be answered by examining figure 3. The figure shows the relationship of percentage of maximum shrinkage force, thickness, and area to decreasing

Figure 3.--Relationship of percentage of maximum shrinkage force, thickness, and area to decreasing moisture content.

M 137 145

moisture content for the unbleached southern pine kraft at 500 Canadian Standard freeness.

Thickness, area, and shrinkage force behave comparably in response to decreasing moisture content, which is explained as follows. During the initial part of drying, all the curves remain essentially 'on a plateau, although no data were taken for moisture contents in excess of 65 per-cent. At some moisture content, the slope of the curve changes and becomes either ascending or descending. This is the point at which the curves in figure 3 begin. Approaching equilibrium mois-ture content, the slope of the curve changes and returns to another plateau. However, the final plateau does not appear in figure 3 because testing was terminated just prior to this point.

The relationship shown in figure 3 is in agree-ment with results of Lyne and Gallay who found that the thickness of the wet web with increasing solids content is essentially the inverse of the strength (6).

Because the handsheets differed in moisture content, thickness, and specimen width, it was extremely difficult to make any meaningful com-parison between different pulps by examining data as shown in figure 3. Meaningful comparisons, however, could be made by combining shrinkage force, thickness, and width in a single unit of force per cross section of material, hereafter called shrinkage stress.

Differences between pulps become evident when the shrinkage-stress data for all pulps are plotted against moisture content (fig. 4). The softwood

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Figure 4.--Relationship of shrinkage stress to decreasing moisture content.

M 137 143

pulps, as a group, have a greater shrinkage stress than the hardwoods and alpha pulps. The wet-lap kraft exerts a greater shrinkage stress than the dry-lap kraft, which in turn is greater than the dry-lap sulfite pulp.

Beating the unbleached southern pine kraft pulp to five freeness levels caused some dif-ferences in shrinkage-stress characteristics, when compared at the same moisture content (fig. 5). A general trend appears to have been established with decreasing freeness, with the curve for 260-milliliter Canadian Standard free-ness out of sequence. The same pattern would be found if shrinkage force were plotted against moisture content; this prompted a microscopic examination of the pulp to determine if any damage was evident, but none was found.

F i g u r e 5.--Relationship of shrinkage stress to decreasing moisture content of unbleached southern pine kraft at five levels of beating (CanadianStandard freeness, CSF). M 137 146

The maximum shrinkage stress (and highest tensile strength) of the southern pine pulp occurred at the 400-milliliter level of beating. Continued beating to 260 milliliters caused the shrinkage stress to decrease, falling between the 500- and 400-milliliter levels. Wet-webs from the 400-milliliter stock also began developing a shrinkage stress at a much higher moisture content than the webs at other freenesses and attained a higher shrinkage force in a shorter drying

Web contraction caused by fiber shrinkage is closely dependent on the degree of beating (11). Beating increases the fiber surface area available for fiber bonding, and increases the flexibility and conformability of the fibers. An examination of the data shows that the amount of shrinkage in the wet webs is very slight prior to attaining some critical moisture content. At this critical point the thickness and width dimensions begin to change markedly. This is associated with a sudden increase in shrinkage force.

It is believed that this sudden and striking c h an g e in shrinkage characteristics occurs shortly after the fibers have been dried to the fiber saturation point. It should be noted that the moisture content at which this change in shrinkage takes place is different for each kind of pulp and at each freeness, but it generally occurs in the range of 40 to 60 percent. This would indicate that the greatest change in overall shrinkage occurs as a result of intrafiber shrinkage.

Volumetric Shrinkage and Fiber Bonding

The overall shrinkage of paper depends on sheet structure and fiber bonding (9). An open, poorly bonded sheet will have a total shrinkage about equal to that of the longitudinal shrinkage of the fibers. In a well-bonded dense sheet, the total shrinkage approaches that of the transverse shrinkage of the fibers.

The overall shrinkage of most handsheets tested was in excess of the transverse shrinkage re-ported for single fibers because the handsheets were restrained from shrinking in the longitudinal direction. It is known from previous experimental work, that if a handsheet is restrained in length during drying, shrinkage in width will be enhanced. Although the shrinkage force developed is closely associated with intrafiber shrinkage, no force

FPL 131 12

can be imparted to the test machine except by interfiber bonds. Flatwise tensile strength is thought to be a measure of fiber bonding; there-fore, a comparison was made between it and shrinkage force. The plotted data in figure 6 show a good correlation between flatwise tensile strength and volumetric shrinkage of handsheets from all of the pulps.

Shrinkage stress, like volumetric shrinkage, can also be thought of as an indicator of the degree of bonding that takes place within a sheet of paper. Figure 7 shows how closely flatwise tensile strength and shrinkage s t ress are related. If flatwise tensile strength is indeed a measure of fiber bonding, these results illustrate that overall shrinkage and the shrinkage stress of a handsheet also depend on fiber bonding.

Figure 6.--Correlation between volumetric shrinkage and flatgwise tgensile stgrength for all pulps M 137 147

F igu re 7 .- - Corre Ia t ion between shr inkage s t r e s s and f l a t w i s e t e n s i l e s t r e n g t h f o r a l l pu lps. M 137 144

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LITERATURE CITED

1. Brecht, W., and Erfurt, H. 1959. Wet-web strength of mechanical

and chemical pulps of different form composition. Tappi 42: 959-968.

2. Higgins, H. G., Goldsmith, V., and Harring-ton, K. J.

1955. The structure and properties of paper. IV. Drying tensions and dimensional changes in eucalypt kraft papers. Australian J. Appl. Sci. 6: 496-510.

3. Ivarsson, B. W. 1954. Introduction of stress into a paper

sheet during drying. Tappi 37: 634-639.

4. Lyne, L. M., and Gallay, W. 1954. Fiber properties and fiber-water

relationships in relation to the strength and rheology of wet webs. Tappi 37: 581-596.

5. , and Gallay, W. 1954. Measurement of wet web strength.

Tappi 37: 694-698. 6. , and Gallay, W.

1954. Studies in the fundamentals of wet web strength. Tappi 37: 698-704.

7. Morehead, F. F. 1947. Some comparative data on the

cross-sectional swelling of tex-tile fibers. Textile Res. J. 17: 96-98.

8. Page, D. H., and Tydeman, P. A. 1962. A new theory of the shrinkage,

structure, and properties of paper. In “Formation and Struc-ture of Paper,” vol. 1 (F. Bolam, ed.), Trans. Symp., Oxford, 1961, Tech. Sect. Brit. Pap. and Board Makers’ Assoc., pp. 397-413.

9. Pritchard, E. J. 1954. A study of shrinkage and expansion.

Brit. Pap. and Board Makers’ ASSOC., Tech. Sect. Proc. 35: 31-38.

10. Rance, H. F. 1952. The influence of papermaking vari-

ables upon sheet characteristics. Brit. Pap. and Board Makers’ Assoc., Tech. Sect. Proc. 33: 173-195.

11. 1954. Effect of water removal on sheet

properties. The water evaporation phase. Tappi 37: 640-654.

12. Robertson, A. A. 1963. The physical properties of wet

webs. Part 2. Fibre properties and wet web behaviour. Svensk Papperstidn. 66: 477-497,

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