Purchased by the Forest Products Laboratory, U. S. Department of Agriculture, for official use

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1 Purchased by the Forest Products Laboratory,. S. Department of Agriculture, for official use Reprinted from Tappi, The Journal of the Technical Association of the Pulp and Paper Industry, Vol. 8, No. 11, November Copyright, 1965 by TAPPI, and reprinted by permission of the copyright owner VANCE C. SETTERHOLM and WARREN A. CHILSON Drying Restraint Its Effect on the Tensile Properties of 15 Different Pulps CHANGES in tensile strength and elastic properties of paper as a result of stretching or prevention of shrinkage during drying (often referred to as the effect of restraint during drying) are one of the principal sources of variation in physical properties of paper from any pulp furnish. The other two most important variables in paper manufacture are degree of fiber orientation and drying temperature. Any manufacturing variable that changes the area or thickness of a wet paper web before the web dries will have a sizable effect on sheet properties. Changes in web dimension will produce (1) changes in density and stress distribution of paper under various loading conditions and () alterations in the amount of fiber microcreping at the fiber-to-fiber bonds. VANCE C. SETTERHOLM. Forest Products Technologist and WARREN A: CHILSON, Chemical Engineer Forest Products Laboratory, Forest Service,. S. Department of Agriculture. Maintained at Madison, Wis., in cooperation with the niversity of Wisconsin. The degree that shrinkage is controlled during drying of wet webs has a substantia effect on the visco-elastic and elastic-dependent properties of paper. The re sponse to restraint during drying varies with the kind of pulp from which paper is made. This study shows how tensile properties of handsheets from 15 pulps (mostly commercial pulps in drylap form) when beaten to different freeness values are affected by restraint during drying. It shows certain basic relations which were found between density, pulping process, and the degree of restraint. Modulus of elasticity was found to be dependent on density and the degree of restraint. Tensile strength and strain to failure were dependent on these same factors, and the type of pulp from which the handsheets were made. This paper explains (1) how handsheets from various pulp furnishes respond to the effect of restraint during drying; () permanence of the effects induced by drying restraint; and (3) some basic density-property relationships for handsheets made from widely used papermaking furnishes. PLP FRNISHES Fifteen different commercial pulps (mostly in drylap form) were chosen for this study. These pulps were processed in a standard Valley beater to a freeness considered useful in papermaking. Four of the pulps were given additional processing so that effects of refining might be included. The selection of pulps was based on advice from members of the Boxboard Research and Development Association at Kalamazoo, Mich., who sponsored this project. The pulps selected and their Canadian Standard freeness values are given in Table I. A numerical designation was given to each pulp to identify it in the graphs. 63 Vol. 8, No. 11 November 1965 / Tappi

2 Fig. 1 Frame used to hold wet handsheets. when placed in drying oven. The degree of restraint is controlled by adjusting position of clamp attached to the crank Fig.. Composite graph showing effect of density on modulus of elasticity for different pulps dried under several conditions of restraint TEST PROCEDRES AND APPARATS Samples of pulp (36 g; ovendry basis) were beaten in the test beater to their freeness value (Table I). Handsheets weighing approximately 9 g/sq m (airdry basis) were formed on a 7 9 in. sheet mold and wet-pressed between blotters at 6 psi for 5 min. Two opposite edges of the sheets were dried between heated platens for a distance of approximately 5 / 8 in. One-half inch of the dried ends of these sheets was then clamped in the frames (Fig, 1). Tension was adjusted either to provide various degrees of restraint or to stretch the sheet and then provide restraint. A flat supporting plate was placed under the sheet during clamping to provide support for the wet sheet and thus prevent sagging while the ends were being clamped. The drying frame and sheet were then placed on Table 1. Fifteen Pulps and Their levels of restraint, the following related Freeness Values experiments were performed: Pulp Canadian Standard freeness, ml Western softwood 7,6, bleached kraft 53,355 Western hemlock 7, 6, bleached sulfite,8 Southern pine bleached kraft Sweetgum bleached kraft 3 Aspen bleached cold soda 3 Western softwood 7,6, unbleached kraft 51,33 Southern pine 37 unbleached kraft Manila tabulating card 56 stock Waste newsprint 17 Corrugated boxes 5 Western hemlock unbleached sulfite Spruce groundwood, Milk carton stock 6 Eastern softwood bleachedsulfite Mixed hardwood semichemical pulp edge in an oven maintained at 1 F for at least 1 / hr (twice the time required to remove all free moisture). After drying, the sheets were conditioned at 73 F and % RH for at least 8 hr. The sheets were trimmed to / in. on a paper cutter and weighed on a triple-beam balance to the nearest.1 g. Two tension specimens were cut from the center of each sheet. These were necked specimens, cut from the sheet in the direction in which shrinkage stresses were induced by the applied restraint. Thickness measurements were made at the necked section to the nearest.1 in., with a dial-type micrometer (TAPPI Method T 11 m ). Tension tests of specimens were made on a test machine that used a mechanical drive and a load cell with an electrical resistance-type strain gage for measuring loads. Six specimens of each material were tested with a free length of 5 in. between the grips. Loads were applied through a constant head movement of. in./ min. Because of slippage and the irregular shape of the specimen, the crosshead movement was not used as a measure of strain. An optical, mechanical-type strain gage was used in some tests to obtain accurate strain readings (1). A mechanical-electrical strain gage was developed during the study, and this was used to obtain load-strain data (). All tests were made in a room conditioned to 73 F and % RH. ADDITIONAL TESTS In addition to measuring tensile strength and elastic properties of handsheets from 15 pulps dried under various 1. Effect of moisture content when restraint is applied.. Permanence of the effects of restraint on strength and elastic properties. TEST RESLTS The results of tests are summarized in the following tables: Table II. Tensile properties of handsheets from various pulps that were dried under varying degrees of restraint. Table III. Effect of moisture content at the time restraint was applied on the tensile properties. Table IV. Effect of humidity and water soaking on strength and elastic properties of webs that were prevented from shrinking during drying. EFFECTS OF DRYING RESTRAINT ON PROPERTIES Modulus of Elasticity When wet handsheets are restrained from shrinking in a rigid drying frame, drying stresses develop from surface tension and bond formation and impose a stress on the sheet in the direction of the applied restraint. In theory, this improves stress distribution (3), reduces microcreping at the bondsites (, 5) and increases the modulus of elasticity of individual fibers (6). The application of restraint increases tensile strength or failing load, increases elastic modulus, and decreases strain to failure. There is a small increase in density, which is probably caused by an increase in bonding. Nissan (7) has shown that the cube of modulus of elasticity is a measure of the concentration of hydrogen bonds per unit volume. This increase in density increases with decreased shrinkage allowance and decreases with stretching. A composite of density data is given in Table V. From the increase in density we can infer an increase in fiber bonding. An increase in fiber bonding indicates a greater stress distribution and, consequently, an increase in modulus of elasticity. Improved stress distribution should also result from stretching of the wet web, due to the straightening of fibers. Perhaps most important, the modulus of elasticity is increased with restraint during drying because there is reduced microcreping at the fiber-to fiber bonds and because the modulus of individual fibers has increased. Litt (8) has reported that sheet modulus is dependent on only the modulus of elasticity of fibers and paper density. While this is important, it overlooks the contribution due to stress distribution in the sheet. Tappi / November 1965 Vol. 8, No

3 It was observed that stretching the Table II. Tensile Properties a of Handsheets from Various Pulps That Were wet web 3% lowered handsheet density Dried nder Varying Degrees of Restraint and increased modulus of elasticity. Density Modulus Thus, the data imply a case where of Strain Drying Thickness, lb/ ft / Strength, elastacity, to modulus can be increased despite re- CSf, ml conditions b mils mil g/cm 3 psi psi failure, % duced fiber bonding and that modulus of Western softwood unbleached kraft elasticity need not vary directly with fiber bonding. This phenomenon is probably due to the straightening of fibers and an improved stress distribu tion across the sheet When the modulus of elasticity of all handsheets (Table 11) is plotted against density, a reasonably good correlation is obtained, provided that some dis tinction is made for different levels of restraint or stretching during drying If modulus values are plotted against the cube of density, (Fig. ) the points on the graph define a straight line pass ing through the origin. This shows that such variables as wood species, pulping Bleached western softwood kraft process, and degree of beating are not of primary importance in controlling Young's modulus or the stiffness of paper. Primary factors are density, degree of restraint and drying tempera ture (9). Species, pulping process, and degree of beating affect the elastic properties of paper only to the degree that they change the final sheet density Figure shows the dependency of modulus of elasticity on density that results from the following condition during drying: (a) unrestricted shrinkage Western hemlock bleached sulfite (b) % shrinkage allowance (c) % shrinkage allowance (d) fully restrained (e) 3% stretch, then fully restrained This family of curves represents the modulus-to-density relationship for all the pulps listed in Table 11. Included are several species of hardwood and softwoods, various pulping processes, and different levels of beating. Thus, modulus of elasticity (and stiffness for a given handsheet thickness) depends primarily on density and control of shrinkage during drying. This rela tionship, however, does not hold for Sweetgumbleachedkraft papers that differ in fiber orientation or for those that have been densified by calender pressure on a dry sheet It is apparent that a correlation between properties of handsheets and machine-made sheets will never be Wastenewsprint satisfactory if allowances are not made for effects due to variations in restraint during drying Tensile Strength Corrugatedboardwaste Tensile strength of paper is largely dependent on the same factors (such as fiber bonding and density) that affect modulus of elasticity. However, these properties are not interdependent. The 636 Vol. 8, No. 11 November 1965 / Tappi

4 Table II (Continued) Density Modulus of Strain Drying b Thickness, lb/ ft / Strength, elasticity, to Csf, ml conditions mils mil g/cm 3 psi psi failure, % Tabulating card waste Southern pine unbleached kraft Southern pine bleached kraft Cold soda pulp Milk carton stock Bleached eastern softwood sulfite nbleached western hemlock sulfite nbleached spruce groundwood Mixed hardwood bleached neutral sulfite semichemical Results are average of 6 tests made after conditioning at 75 F and % RH. a b Sample dried unrestrained. Sample dried with a % allowance for shrinkage. Sample dried with a % allowance for shrinkage. Sample dried with no allowance for shrinkage. Sample stretch 3% and dried with a % allowance for shrinkage. Sample stretch 3% and dried with no allowance for shrinkage. Tappi / November 7965 Vol. 8, No. 11 important difference is that tensile strength is mostly a measure of rupture of fibers after the test piece has undergone considerable change in its elastic properties, while modulus of elasticity defines the initial slope of the stressstrain curve before the sheet is stretched beyond the proportional limit. The relationship between tensile strength and density for handsheets from pulps described in Table II is shown in Fig. 3. These data show four distinct classification groups: Group 1. nbleached softwood kraft pulps. Group. Bleached softwood kraft pulps. Group 3. Bleached and unbleached softwood sulfite, as well as bleached hardwood semichemical pulps. Group. Hardwood cold soda, waste news, waste corrugated, and unbleached spruce groundwood pulps. To obtain these curves, data for unrestrained handsheets and those that had been stretched 3% were omitted. Data for unrestrained pulps were omitted, because accurate thickness measurements could not be made on the more or less cockled unrestrained handsheets. Data for handsheets that had been stretched 3% before drying were treated separately. The scatter of the data makes it difficult to present any distinct differences between the levels of restraint. The four data groupings of single curves appear to be fairly logical. For example, the data show that at any given density, unbleached softwood kraft has the highest tensile strength. The data show that tensile strength is primarily dependent on the pulping and bleaching process and on the density achieved in manufacture of 'the handsheets. At the same density, tensile strength is independent of freeness. The principal effect on tensile strength of beating is altering handsheet density. This view was also expressed by R. H. Doughty in 193 (1). One of the basic differences between this study and Doughty's work was the method of drying the handsheets. After pressing wet sheets to a desired density, Doughty dried them between blotters in a vacuum oven, using the blotters to keep the sheets flat. Since no mention was made of the sheets sticking to the blotters, it is assumed that the blotters offered little restraint to the normal shrinking tendencies of his handsheets. The present study shows, as did Doughty, that the most important effect of changing the freeness by beating was to alter the sheet density. The tensile strength and density relationships were studied on unrestrained handsheets to further explore differences in drying conditions. It is fairly certain that thickness measurements on 637

5 Fig.. Showing effect of density on tensile strength for handsheets from different pulps dried in unrestrained condition Fig. 3. Effect of density on tensile strength for handsheets from different pulps dried with no allowance for shrinkage, as well as and % allowance for shrinkage Fig. 5. Composite graph for handsheets from Fig. 6. Composite graph for handsheets from Fig. 7. Composite graph for handsheets from pulps in group 1 showing effect of density on ten- pulps in group 3 showing effect of density on ten- pulps in group showing effect of density on sile strength at all levels of restraint during dry- sile strength at all levels of restraint during dry- tensile strength at all levels of restraint during ing ing drying unrestrained handsheets are usually too imate the true values for unrestrained The curves in Fig. show exactly the high because of the wrinkles in the handsheets. Strength values were ac- same slope as those of the sheets that sheets. However, the density and cordingly adjusted for more realistic were restrained in frames. thickness values of sheets with a % thicknesses, and the strengths were Finally, a composite or family of shrinkage allowance very nearly approx- plotted against the adjusted densities. curves for handsheets from each of the 638 Vol. 8, No. 11 November 1965 / Tappi

6 Table III. Effect of Moisture Content at the Time Restraint Was Applied on Tensile Properties a Drying conditions Thickness, mils Density lb/ ft / mil g/cm 3 Modulus Tensile of strength, elasticity psi psi Stretch. % nrestrained % shrinkage allowance % shrinkage allowance No shrinkage allowance Stretch 3%, then unrestrained Stretch 3%, then % shrinkage Stretch 3%, no shrinkage Dried to % moisture con tent, then fully restrained Dried to % moisture content under restraint, dried unrestrained then Dried to 3% moisture content under full restraint, then dried unrestrained a Average of 13 tests. Specimens conditioned and tested at 75 F and % RH. Fig. 1. Effect of density on tensile strain to failure for handsheets from pulps in group at five levels of restraint or stretching Fig. 8. Composite graph for handsheets from pulps in group showing effect of density on tensile strength at all levels of restraint during drying pulp types shows the relationship between tensile strength and density for Fig. 9. Effect of density on tensile strain to failthe different conditions of restraining ure for handsheets from pulps in group 1 at five and stretching of wet webs used in this levels of restraint or stretching study (Figs. 5-8). As previously shown, increasing the amount of restraint during drying will prove the elastic modulus, it is difficult result in subsequent increases in density to visualize how it will benefit tensile of the dried sheet. Although these strength. changes are not large, they are fairly An interesting aspect of the influence consistent. Density increases as shrink- of restraint during drying on tensile age allowance decreases and then de- strength is the way different pulp groups creases with stretching and restraining respond to variations in restraint and of the wet web. While tensile strength stretching during drying. Kraft pulps is normally (under constant restraint) (bleached or unbleached, Figs. 5 and 6) expected to increase and decrease with are more influenced by restraint than density changes, applying restraint in- sulfite pulps. Figures 5 and 6 illustrate creases the tensile strength above that the greater spread between the 3% that can be accounted for by changes in stretch and unrestrained handsheet density. In fact, tensile strength may tensile values. These differences probeven increase while density decreases. ably reflect basic differences in the It is easy to attribute these effects of ability of the sulfite and kraft pulps to drying restraint to improvements in form fiber-to-fiber bonds or differences the stress distribution, but, while re- in fiber properties. Similarly, bleaching duction of the microcreping may im- reduces the ability of kraft pulps to Fig. 11. Effect of density on tensile strain to failure for handsheets from pulps in group 3 at five levels of restraint or stretching benefit in tensile strength from the effects of drying under restraint. Reclaimed fibers (Fig. 8), such as those from newspapers or pulps containing a high percentage of groundwood, benefit in tensile strength by drying under restraint but not to so great a degree as virgin bleached and unbleached kraft pulps. Also, for equivalent sheetmaking conditions, these handsheets from reclaimed fibers never achieved the same range of densities as handsheets from chemical pulps because they do not have the same capacity for forming fiber-to-fiber bonds. Tappi / November 1965 Vol. 8, No

7 Condition Table IV. Effect of Humidity and Water Soaking on Strength and Elastic Properties a of Restraint-Dried Webs Tensile Modulus of Strain to strength, psi elasticity, psi failure, % Density Cross- Cross- Cross- Exposure, Thickness, lb/ Machine machine Machine machine Machine machine % RH mils ft /mil g/cm 3 direction direction direction direction direction direction nrestrained Restrained Water soak aaverage of 6 tests. Specimens conditioned and tested at 75 F and % RH after exposure shown. Table V. Density Data Average density of all handsheets Condition of lb/ ft / restraint mil g/cm 3 nrestrained.6. % shrinkage.8.5 % shrinkage Restrained % stretch % stretch Fig. 1. Effect of density on tensile strain to failure for handsheets from pulps in group at five levelsof restraint or stretching Fig. 13. Tensile properties of paper as affected by moisture content at time of restraint Strain at Failure in Tension The foregoing discussion concerned the improvements in tensile strength and elastic modulus that are achieved as a consequence of drying handsheets under restraint. In addition, noticeable improvements in dimensional stability (11), which are the result of restrained drying, have been reported. These benefits, however, are obtained at the expense of stretch or toughness of the dried paper. While the loss in strain to failure is immediately evident by examining Table II, the graphical presentation in Figs. 9-1 shows additional comparisons. The figures show strain at failure versus density curves at different levels of restraint for the same pulp grouping that was obtained in the analysis of strength and density data. For these curves the density values shown for unrestrained handsheets were based on values obtained on handsheets where the shrinkage allowance was %. For most of the pulps, the overall percentage reduction in strain to failure from the unrestrained to fully restrained condition was about 7%. Stretching the wet webs 3% resulted in an additional loss of stretch of about 7%. At any given density the unbleached kraft pulps (Fig. 9) showed greater stretch than the bleached kraft pulps (Fig. 1). The bleached kraft pulps, in turn, showed greater stretch than the sulfite pulps (Fig. 11). For unrestrained sheets, the stretch is increased by beating. As greater restraint is applied during drying, the influence of processing becomes less. Thus, in a general way, stretch is influenced more by restraint than it is by refining in the test beater. However, the more beating and the higher the density of handsheets, the greater will be the effect of restraint on stretch. EFFECT OF MOISTRE CONTENT AT THE TIME OF RESTRAINT Results in Table III show that, within the moisture limits of the experiment (-6%), the higher the moisture content at the time the restraining is accomplished, the higher the strength and modulus of elasticity of the dried sheet. This point is illustrated in Fig. 13. PERMANENCE OF EFFECT OF RESTRAINT Data obtained on sheets exposed to high humidity (9% RH) show that much of the improvement in strength and elastic modulus induced by drying under restraint is not seriously reduced by a subsequent high-humidity exposure (Table IV). Although water soaking reduced the elastic modulus greatly, neither the elastic modulus nor the strength curve returned to the original unrestrained levels. LITERATRE CITED 1. Setterholm, V. C. and Kuenzi, E. W.,. S. Forest Prod. Lab. Report 66, Jewett, D. M., An electrical strain gage for the tensile testing of paper,. S. Forest Service Research Note FPL-3, Forest Prod. Lab., Madison, 3. Wis., Schultz, J. H., Tappi (1): 736 (1961).. Page, D. H. and Tydeman, P. A., A New Theory of Shrinkage, Structure, and Properties of Paper in Formation and Structure of Paper, Vol. l. Trans., British Paper and Board Makers Assn. Symposium, Oxford, Rance, H. F., Tappi 37 (1): 6 (195). 6. Jentzen, C. A., Tappi 7 (7): 1 (196). 7. Nissan, A. H., Transactions of Faraday Society 53: 7 (1957). 8. Litt, M., J. Colloid Science 16: 97 (1961). 9. Setterholm, V. C., Chilson, W. A., and Luey, A. T., Effect of temperature and restraint during drying on the tensile properties of handsheets,. S. Forest Prod. Lab. Report 65, Madison, Wis., Doughty, R. H., Paper Trade J. 93 ():39; 93(15):(1931); 95(9):9 (193). 11. Fahey, D. J. and Chilson, W. A., Tappi 6 (7): 393 (1963). RECEIVED MAY 7, Vol. 8, No. 11 November 1965 / Tappi