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

Reprinted from Tappi, The Journal of the Technical Association of the and Industry, Vol. 48, No. 11, November 1965. Copyright, 1965 by TAPPI, and reprinted by permission of the copyright owner

Drying Restraint VANCE C. SETTERHOLM and WARREN A. CHILSON Its Effect on the Tensile Properties of 15 Different Pulps

CHANGES in tensile strength and elastic The degree that shrinkage is controlled during drying of wet webs has a substantia properties of paper as a result of stretch­ effect on the visco-elastic and elastic-dependent properties of paper. The re ing or prevention of shrinkage during sponse to restraint during drying varies with the kind of pulp from which paper is drying (often referred to as the effect of made. This study shows how tensile properties of handsheets from 15 pulps restraint during drying) are one of the (mostly commercial pulps in drylap form) when beaten to different freeness values principal sources of variation in physical are affected by restraint during drying. It shows certain basic relations which properties of paper from any pulp fur­ were found between density, pulping process, and the degree of restraint. Mod­ nish. The other two most important ulus of elasticity was found to be dependent on density and the degree of variables in paper manufacture are restraint. Tensile strength and strain to failure were dependent on these same degree of fiber orientation and drying factors, and the type of pulp from which the handsheets were made. temperature. Any manufacturing variable that This paper explains (1) how hand- processed in a standard Valley beater to changes the area or thickness of a wet sheets from various pulp furnishes re­ a freeness considered useful in paper- paper web before the web dries will have spond to the effect of restraint during making. Four of the pulps were given a sizable effect on sheet properties. additional processing so that effects of Changes in web dimension will produce drying; (2) permanence of the effects refining might be included. The selec­ (1) changes in density and stress dis­ induced by drying restraint; and (3) some basic density-property relation­ tion of pulps was based on advice from tribution of paper under various loading ships for handsheets made from widely members of the Boxboard Research and conditions and (2) alterations in the used furnishes. Development Association at Kalamazoo, amount of fiber microcreping at the Mich., who sponsored this project. fiber-to-fiber bonds. PULP FURNISHES The pulps selected and their Canadian VANCE C. SETTERHOLM. Forest Products Tech­ Standard freeness values are given in nologist and WARREN A: CHILSON, Chemical En­ gineer Forest Products Laboratory, Forest Fifteen different commercial pulps Table I. Service, U. S. Department of Agriculture. Main­ A numerical designation was given tained at Madison, Wis., in cooperation with the (mostly in drylap form) were chosen University of Wisconsin. for this study. These pulps were to each pulp to identify it in the graphs.

634 Vol. 48, No. 11 November 1965 / Tappi Table 1. Fifteen Pulps and Their levels of restraint, the following related Freeness Values experiments were performed: Canadian Standard 1. Effect of moisture content when freeness, restraint is applied. Pulp ml 2. Permanence of the effects of re­ Western softwood 700,650, straint on strength and elastic bleached kraft 530,355 properties. Western hemlock 700, 600, bleached sulfite 500,280 Southern pine bleached 420 kraft TEST RESULTS Sweetgum bleached kraft 430 Aspen bleached cold soda 320 The results of tests are summarized in Western softwood 700,620, the following tables: Fig. 1 Frame used to hold wet handsheets. unbleached kraft 510,330 when placed in drying oven. The degree of re­ Southern pine 370 Table II. Tensile properties of hand- straint is controlled by adjusting position of clamp unbleached kraft sheets from various pulps that were attached to the crank Manila tabulating card 560 dried under varying degrees of restraint. stock Table III. Effect of moisture content Waste 170 Corrugated boxes 520 at the time restraint was applied on the Western hemlock 350 tensile properties. unbleached sulfite Table IV. Effect of humidity and Spruce groundwood 50,100 Milk stock 600 water soaking on strength and elastic Eastern softwood 380 properties of webs that were prevented bleachedsulfite from shrinking during drying. Mixed hardwood 450 semichemical pulp EFFECTS OF DRYING RESTRAINT ON PROPERTIES Modulus of Elasticity edge in an oven maintained at 150°F 1 for at least /2 hr (twice the time re­ When wet handsheets are restrained quired to remove all free moisture). from shrinking in a rigid drying frame, After drying, the sheets were con­ drying stresses develop from surface ditioned at 73°F and 50% RH for at tension and bond formation and impose least 48 hr. The sheets were trimmed a stress on the sheet in the direction of 1 to 5 × 7 /2 in. on a paper cutter and the applied restraint. In theory, this weighed on a triple-beam balance to the improves stress distribution (3), re­ nearest 0.01 g. duces microcreping at the bondsites Two tension specimens were cut from (4, 5) and increases the modulus of the center of each sheet. These were elasticity of individual fibers (6). The necked specimens, cut from the sheet application of restraint increases tensile in the direction in which shrinkage strength or failing load, increases stresses were induced by the applied elastic modulus, and decreases strain to restraint. Thickness measurements failure. There is a small increase in were made at the necked section to the density, which is probably caused by an Fig. 2. Composite graph showing effect of den­ increase in bonding. Nissan (7) has sity on modulus of elasticity for different pulps nearest 0.0001 in., with a dial-type dried under several conditions of restraint micrometer (TAPPI Method T 411 m­ shown that the cube of modulus of elas­ 44). Tension tests of specimens were ticity is a measure of the concentration made on a test machine that used a of hydrogen bonds per unit volume. mechanical drive and a load cell with an This increase in density increases with TEST PROCEDURES decreased shrinkage allowance and de­ AND APPARATUS electrical resistance-type strain gage for measuring loads. creases with stretching. A composite Samples of pulp (360 g; ovendry basis) Six specimens of each material were of density data is given in Table V. were beaten in the test beater to their tested with a free length of 5 in. between From the increase in density we can freeness value (Table I). Handsheets the grips. Loads were applied through infer an increase in fiber bonding. An weighing approximately 90 g/sq m a constant head movement of 0.02 in./ increase in fiber bonding indicates a (airdry basis) were formed on a 7 × 9 in. min. Because of slippage and the greater stress distribution and, con­ sheet mold and wet-pressed between irregular shape of the specimen, the sequently, an increase in modulus of blotters at 60 psi for 5 min. Two crosshead movement was not used as a elasticity. Improved stress distribution opposite edges of the sheets were dried measure of strain. An optical, mechan­ should also result from stretching of the between heated platens for a distance of ical-type strain gage was used in some wet web, due to the straightening of 5 approximately /8 in. tests to obtain accurate strain readings fibers. Perhaps most important, the One-half inch of the dried ends of (1). A mechanical-electrical strain gage modulus of elasticity is increased with these sheets was then clamped in the was developed during the study, and restraint during drying because there is frames (Fig, 1). Tension was adjusted this was used to obtain load-strain data reduced microcreping at the fiber-to either to provide various degrees of (2). All tests were made in a room fiber bonds and because the modulus of restraint or to stretch the sheet and then conditioned to 73°F and 50% RH. individual fibers has increased. Litt (8) provide restraint. A flat supporting has reported that sheet modulus is dependent on only the modulus of plate was placed under the sheet during ADDITIONAL TESTS clamping to provide support for the wet elasticity of fibers and paper density. sheet and thus prevent sagging while the In addition to measuring tensile While this is important, it overlooks the ends were being clamped. The drying strength and elastic properties of hand- contribution due to stress distribution in frame and sheet were then placed on sheets from 15 pulps dried under various the sheet.

Tappi / November 1965 Vol. 48, No. 11 635 It was observed that stretching the Table II. Tensile Propertiesa of Handsheets from Various Pulps That Were wet web 3% lowered handsheet density Dried Under Varying Degrees of Restraint and increased modulus of elasticity. Density Modulus Thus, the data imply a case where of Strain Drying Thickness, lb/1000 ft2/ Strength, elastacity, to modulus can be increased despite re- CSf, ml conditionsb mils mil g/cm3 psi 1000 psi failure, % duced fiber bonding and that modulus of elasticity need not vary directly with Western softwood unbleached kraft 700 U 7.5 2.26 0.436 1850 139 4.62 fiber bonding. This phenomenon is 700 4 7.1 2.48 0.479 2210 204 4.26 probably due to the straightening of 700 2 7.0 2.50 0.483 2330 295 3.34 fibers and an improved stress distribu- 700 0 6.5 2.56 0.494 2490 425 1.66 tion across the sheet. 620 U 72 2.44 0.471 2240 178 4.80 620 4 6.8 2.70 0.521 3180 318 4.60 When the modulus of elasticity of all 620 2 6.0 2.72 0.525 2650 358 3.10 handsheets (Table 11) is plotted against 620 0 6.3 2.76 0.533 3620 534 1.90 density, a reasonably good correlation 620 S-2 7.3 2.64 0.510 3870 513 2.06 is obtained, provided that some dis- 620 S-0 6.2 2.52 0.486 4000 592 1.25 510 U 6.9 2.69 0.519 3390 199 7.20 tinction is made for different levels of 510 4 5.7 2.99 0.577 3980 384 4.70 restraint or stretching during drying. 510 2 5.9 3.02 0.583 5300 583 3.90 If modulus values are plotted against 510 0 5.4 3.05 0.589 4940 640 2.25 the cube of density, (Fig. 2) the points 330 U 7.2 2.76 0.533 4360 186 7.50 330 4 5.5 3.12 0.602 4860 508 5.60 on the graph define a straight line pass- 330 2 5.6 3.18 0.614 5300 596 3.50 ing through the origin. This shows that 330 0 5.3 3.25 0.627 5980 931 2.10 such variables as wood species, pulping Bleached western softwood , and degree of beating are not of 700 U 7.6 2.28 0.440 990 100 3.53 primary importance in controlling 700 4 7.6 2.32 0.448 930 183 2.22 Young's modulus or the stiffness of 700 2 7.6 2.37 0.457 1070 229 1.67 1.10 paper. Primary factors are density, 700 0 7.0 2.44 0.471 1220 230 650 U 6.6 2.67 0.515 1930 159 4.31 degree of restraint and drying tempera- 650 4 6.8 2.69 0.519 2160 269 4.04 ture (9). Species, pulping process, and 650 2 7.1 2.55 0.492 2190 314 2.96 degree of beating affect the elastic 650 0 6.0 2.77 0.535 2300 521 1.64 1.46 properties of paper only to the degree 650 S-2 7.2 2.43 0.469 2730 457 650 S-0 6.6 2.42 0.467 2480 494 0.84 that they change the final sheet density. 530 U 7.0 2.77 0.535 2630 176 5.89 Figure 2 shows the dependency of 530 4 6.8 2.88 0.556 2860 336 4.08 modulus of elasticity on density that 530 2 5.4 2.98 0.575 3410 495 3.10 530 0 5.5 3.09 0.596 4000 660 1.92 results from the following condition 355 U 6.2 2.82 0.544 2440 143 6.58 during drying: 355 4 6.2 3.05 0.589 4680 527 5.12 355 2 5.0 3.24 0.625 5140 582 4.00 355 0 5.3 3.17 0.612 4590 642 2.44 (a) unrestricted shrinkage Western hemlock bleached sulfite (b) 4% shrinkage allowance 700 U 7.2 2.53 0.488 970 130 3.10 (c) 2% shrinkage allowance 700 4 6.4 2.63 0.508 1070 160 2.85 (d) fully restrained 700 2 6.2 2.64 0.510 1140 205 1.90 (e) 3% stretch, then fully restrained 700 0 6.5 2.66 0.513 1350 305 0.98 600 U 6.8 2.78 0.537 1910 182 4.88 600 4 5.5 2.96 0.571 2110 314 3.89 This family of curves represents the 600 2 5.4 3.05 0.589 2430 468 1.63 600 0 5.6 3.13 0.604 2820 588 1.40 modulus-to-density relationship for all 600 S-2 5.1 3.08 0.594 2560 490 1.38 the pulps listed in Table 11. Included 600 S-0 5.8 3.07 0.593 3080 619 1.00 are several species of hardwood and 500 U 6.4 2.87 0.554 2100 187 5.20 softwoods, various pulping processes, 500 4 5.0 3.13 0.604 2730 349 4.92 500 2 5.0 3.22 0.621 3080 457 3.22 and different levels of beating. Thus, 500 0 4.9 3.21 0.620 3580 587 1.67 modulus of elasticity (and stiffness for a 280 U 6.3 2.95 0.569 3000 231 7.30 given handsheet thickness) depends 280 4 5.2 3.58 0.691 3670 514 4.70 primarily on density and control of 280 2 5.0 3.58 0.691 4530 738 3 04 3.69 0.712 4890 851 1.86 shrinkage during drying. This rela- 280 0 4.7 tionship, however, does not hold for Sweetgumbleachedkraft that differ in fiber orientation or 430 U 6.6 2.90 0.560 2770 247 7.38 430 4 5.6 3.17 0.612 3520 437 5.74 for those that have been densified by 430 2 5.6 3.20 0.618 3890 567 3.05 pressure on a dry sheet. 430 0 5.1 3.24 0.625 4610 656 2.25 It is apparent that a correlation 430 S-2 5.4 3.17 0.612 4650 661 2.31 between properties of handsheets and 430 S-0 5.6 3.02 0.583 4680 733 1.67 machine-made sheets will never be Wastenewsprint satisfactory if allowances are not made 170 U 8.4 2.08 0.401 1220 137 2.14 for effects due to variations in restraint 170 4 8.2 2.11 0.407 1140 146 2.37 170 0 8.4 2.09 0.403 1000 147 1.90 during drying. 170 2 8.2 2.01 0.388 1010 156 1.39 170 S-2 8.2 2.02 0.390 1050 160 1.28 170 S-0 8.2 2.05 .396 1250 235 0.85 Tensile Strength Corrugatedboardwaste 520 U 7.9 2.35 0.454 1900 165 5.02 Tensile strength of paper is largely 520 4 7.3 2.36 0.455 2410 224 4.73 dependent on the same factors (such as 520 2 7.5 2.40 0.463 2620 317 2.85 520 0 6.7 2.51 0.484 2960 402 1.81 fiber bonding and density) that affect 520 S-2 7.6 2.34 0.452 2760 395 1.65 modulus of elasticity. However, these 520 S-0 7.9 2.27 0.438 2890 433 1.02 properties are not interdependent. The

636 Vol. 48, No. 11 November 1965 / Tappi Table II (Continued) important difference is that tensile strength is mostly a measure of rupture Density Modulus of Strain of fibers after the test piece has under­ Dryingb Thickness, lb/1000 ft2/ Strength, elasticity, to gone considerable change in its elastic Csf, ml conditions mils mil g/cm3 psi 1000 psi failure, % properties, while modulus of elasticity Tabulating card waste defines the initial slope of the stress- 560 U 7.9 2.62 0.506 2230 232 4.21 strain curve before the sheet is stretched 560 4 6.6 2.60 0.502 2200 243 3.95 beyond the proportional limit. 560 2 6.7 2.61 0.504 2240 263 3.26 The relationship between tensile 7.4 2.78 0.537 2800 403 2.04 560 0 strength and density for handsheets 560 S-2 6.8 2.63 0.508 2600 346 2.01 560 S-0 6.9 2.53 0.488 2580 417 1.12 from pulps described in Table II is shown in Fig. 3. These data show four Southern pine unbleached kraft distinct classification groups: 370 U 7.1 2.56 0.494 3400 147 8.18 Group 1. Unbleached softwood kraft 370 4 6.4 3.10 0.598 4730 390 5.82 370 2 6.1 3.11 0.600 4910 479 3.80 pulps. 370 0 5.5 3.17 0.612 5800 620 2.53 Group 2. Bleached softwood kraft 370 S-2 5.3 3.08 0.594 5640 582 2.46 pulps. 370 S-0 5.7 2.90 0.560 5430 636 1.64 Group 3. Bleached and unbleached Southern pine bleached kraft softwood sulfite, as well as bleached 420 U 6.5 2.86 0.552 4210 250 8.90 hardwood semichemical pulps. 420 4 5.1 3.35 0.647 4860 470 6.24 Group 4. Hardwood cold soda, waste 674 4.10 420 2 5.0 3.48 0.672 5670 news, waste corrugated, and unbleached 420 0 4.8 3.44 0.664 5690 711 3.18 420 S-2 5.5 3.46 0.668 5310 705 2.58 spruce groundwood pulps. 420 S-0 5.3 3.30 0.637 5290 659 1.91 To obtain these curves, data for Cold soda pulp unrestrained handsheets and those that had been stretched 3% were omitted. 320 U 8.8 2.02 0.390 1190 109 3.33 320 4 8.2 2.07 0.400 1250 141 2.42 Data for unrestrained pulps were 320 2 8.4 2.10 0.405 1240 173 1.68 omitted, because accurate thickness 320 0 8.2 2.18 0.421 1750 236 1.24 measurements could not be made on the 320 S-2 8.5 2.09 0.403 1590 228 1.16 more or less cockled unrestrained hand- 320 S-0 8.9 2.02 0.390 1630 243 1.06 sheets. Data for handsheets that had Milk carton stock been stretched 3% before drying were 600 U 7.16 2.64 0.510 2241 170 5.23 treated separately. 600 4 6.72 2.67 0.515 2150 198 4.46 The scatter of the data makes it 600 2 6.25 2.73 0.527 2390 250 3.91 600 0 6.17 2.72 0.525 3060 351 2.80 difficult to present any distinct differ­ 600 S-2 6.63 2.62 0.506 2630 302 2.12 ences between the levels of restraint. 600 S-0 6.48 2.52 0.486 2780 371 1.39 The four data groupings of single Bleached eastern softwood sulfite curves appear to be fairly logical. For example, the data show that at any 380 U 5.5 3.13 0.604 3010 184 6.73 380 4 5.2 3.41 0.658 3100 246 5.92 given density, unbleached softwood 380 2 4.7 3.53 0.681 3540 353 3.58 kraft has the highest tensile strength. 380 0 4.7 3.53 0.681 3690 426 2.34 The data show that tensile strength is 380 S-2 4.7 3.63 0.701 3920 446 2.11 primarily dependent on the pulping 506 1.61 380 S-0 4.6 3.47 0.670 4510 and bleaching process and on the density Unbleached western hemlock sulfite achieved in manufacture of 'the hand- 350 U 6.4 2.99 0.577 2820 172 6.70 sheets. At the same density, tensile 350 4 5.2 3.47 0.670 3740 349 5.40 strength is independent of freeness. 350 2 5.0 3.52 0.679 3870 412 3.67 The principal effect on tensile strength 2.42 350 0 4.9 3.65 0.704 4770 532 of beating is altering handsheet density. 350 S-2 5.0 3.67 0.708 4910 568 2.11 350 S-0 5.0 3.60 0.695 5750 658 1.59 This view was also expressed by R. H. Doughty in 1932 (10). Unbleached spruce groundwood One of the basic differences between 50 U 8.1 2.23 0.430 1550 139 3.06 this study and Doughty's work was the 50 4 8.1 2.21 0.427 1630 147 2.94 50 2 8.0 2.24 0.432 1850 194 2.10 method of drying the handsheets. 50 0 8.1 2.24 0.432 1840 198 1.69 After pressing wet sheets to a desired 50 S-2 8.0 2.17 0.419 1920 219 1.59 density, Doughty dried them between 50 S-0 8.1 2.16 0.417 1960 230 1.29 blotters in a vacuum oven, using the 100 U 9.4 1.94 0.374 1020 108 1.96 100 4 8.9 2.05 0.396 1110 112 2.28 blotters to keep the sheets flat. Since 100 2 8.9 1.94 0.374 1110 121 2.07 no mention was made of the sheets 100 0 8.7 1.96 0.378 1270 160 1.43 sticking to the blotters, it is assumed 100 S-2 9.1 2.02 0.390 1180 149 1.44 that the blotters offered little restraint 100 S-0 9.0 2.00 0.386 1220 152 1.23 to the normal shrinking tendencies of his Mixed hardwood bleached neutral sulfite semichemical handsheets. The present study shows, 450 U 5.8 3.16 0.610 4450 341 3.86 as did Doughty, that the most impor­ 450 4 5.6 3.38 0.652 3950 351 4.00 tant effect of changing the freeness by 450 2 5.4 3.60 0.695 4500 442 3.22 beating was to alter the sheet density. 450 0 5.2 3.71 0.716 5190 632 1.96 The tensile strength and density rela­ a Results are average of 6 tests made after conditioning at 75°F and 50 % RH. b U–Sample dried unrestrained. tionships were studied on unrestrained 4–Sample dried with a 4% allowance for shrinkage. handsheets to further explore differences 2–Sample dried with a 2% allowance for shrinkage. 0–Sample dried with no allowance for shrinkage. in drying conditions. It is fairly S-2–Sample stretch 3% and dried with a 2% allowance for shrinkage. S-0–Sample stretch 3% and dried with no allowance for shrinkage. certain that thickness measurements on

Tappi / November 7965 Vol. 48, No. 11 637 Fig. 4. 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 2 and 4% allow­ ance 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 2 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. 4 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 4% 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. 48, No. 11 November 1965 / Tappi Table III. Effect of Moisture Content at the Time Restraint Was Applied on Tensile Propertiesa Density Modulus Thick- Tensile of ness, lb/1000 ft2 / strength, elasticity Stretch. Drying conditions mils mil g/cm3 psi 1000 psi % Unrestrained 6.6 2.67 0.515 1920 160 4.31 4% shrinkage allowance 6.8 2.69 0.519 2160 269 4.04 2% shrinkage allowance 7.1 2.55 0.492 2190 314 2.96 No shrinkage allowance 6.0 2.77 0.535 2300 523 1.64 Stretch 3%, then unrestrained 6.8 2.45 0.473 1760 183 3.36 Stretch 3%, then 2% shrink- age 7.2 2.43 0.469 2720 455 1.46 Stretch 3%, no shrinkage 6.6 2.42 0.467 2480 495 0.84 Dried to 50% moisture con- tent, then fully restrained 6.3 2.73 0.527 2320 380 1.91 Dried to 50% moisture con- tent under restraint, then dried unrestrained 6.9 2.56 0.494 1840 209 3.27 Dried to 30% moisture con- tent under full restraint, then dried unrestrained 7.6 2.55 0.492 2000 260 3.51 a Average of 13 tests. Specimens conditioned and tested at 75°F and 50% RH.

Fig. 10. Effect of density on tensile strain to failure for handsheets from pulps in group 2 at five levels of restraint or stretching

Fig. 8. Composite graph for handsheets from pulps in group 4 showing effect of density on tensile strength at all levels of restraint during drying

pulp types shows the relationship be­ tween tensile strength and density for Fig. 9. Effect of density on tensile strain to fail- the 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 Fig. 11. Effect of density on tensile strain to amount of restraint during drying will prove the elastic modulus, it is difficult failure for handsheets from pulps in group 3 at result in subsequent increases in density to visualize how it will benefit tensile five levels of restraint or stretching 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 benefit in tensile strength from the age allowance decreases and then de- strength is the way different pulp groups effects of drying under restraint. Re­ creases with stretching and restraining respond to variations in restraint and claimed fibers (Fig. 8), such as those of the wet web. While tensile strength stretching during drying. Kraft pulps from newspapers or pulps containing a is normally (under constant restraint) (bleached or unbleached, Figs. 5 and 6) high percentage of groundwood, benefit expected to increase and decrease with are more influenced by restraint than in tensile strength by drying under density changes, applying restraint in- sulfite pulps. Figures 5 and 6 illustrate restraint but not to so great a degree as creases the tensile strength above that the greater spread between the 3% virgin bleached and unbleached kraft that can be accounted for by changes in stretch and unrestrained handsheet pulps. Also, for equivalent sheet- density. In fact, tensile strength may tensile values. These differences prob­ making conditions, these handsheets even increase while density decreases. ably reflect basic differences in the from reclaimed fibers never achieved It is easy to attribute these effects of ability of the sulfite and kraft pulps to the same range of densities as handsheets drying restraint to improvements in form fiber-to-fiber bonds or differences from chemical pulps because they do not the stress distribution, but, while re- in fiber properties. Similarly, bleaching have the same capacity for forming duction of the microcreping may im- reduces the ability of kraft pulps to fiber-to-fiber bonds.

Tappi / November 1965 Vol. 48, No. 11 639 Table IV. Effect of Humidity and Water Soaking on Strength and Elastic Propertiesa of Restraint-Dried Webs

Tensile Modulus of Strain to strength, psi elasticity, 1000 psi failure, % Density Cross- Cross- Cross- Exposure, Thickness, lb/1000 Machine machine Machine machine Machine machine Condition % RH mils ft2/mil g/cm3 direction direction direction direction direction direction Unrestrained 50 10.4 2.34 0.452 2750 1600 145 70 6.9 8.7 Restrained 50 8.1 2.57 0.496 4180 3190 560 380 2.7 2.4 65 8.4 2.52 0.486 3900 2710 450 365 2.7 2.3 80 8.2 2.54 0.490 4080 2720 430 300 3.0 2.8 90 8.4 2.49 0.481 4100 2650 460 280 3.3 3.1 97 8.6 2.45 0.473 4150 3060 450 300 3.7 3.1 Water soak 8.9 2.37 0.457 3600 2390 270 140 3.5 4.6

aAverage of 6 tests. Specimens conditioned and tested at 75°F and 50% RH after exposure shown.

Table V. Density Data Strain at Failure in Tension and modulus of elasticity of the dried sheet. This point is illustrated in Fig. Average density The foregoing discussion concerned of all handsheets 13. Condition of lb/1000 ft2/ the improvements in tensile strength restraint mil g/cm3 and elastic modulus that are achieved Unrestrained 2.62 0.50 as a consequence of drying handsheets 4% shrinkage 2.82 0.54 under restraint. In addition, noticeable PERMANENCE OF EFFECT 2% shrinkage 2.87 0.55 Restrained 2.92 0.56 improvements in dimensional stability OF RESTRAINT 1% stretch 2.74 0.53 (11), which are the result of restrained 3% stretch 2.66 0.51 drying, have been reported. These Data obtained on sheets exposed to benefits, however, are obtained at the high humidity (90% RH) show that expense of stretch or toughness of the much of the improvement in strength dried paper. While the loss in strain to and elastic modulus induced by drying failure is immediately evident by exam­ under restraint is not seriously reduced ining Table II, the graphical presenta­ by a subsequent high-humidity exposure tion in Figs. 9-12 shows additional (Table IV). Although water soaking comparisons. The figures show strain reduced the elastic modulus greatly, at failure versus density curves at neither the elastic modulus nor the different levels of restraint for the same strength curve returned to the original pulp grouping that was obtained in the unrestrained levels. analysis of strength and density data. For these curves the density values shown for unrestrained handsheets were LITERATURE CITED based on values obtained on handsheets where the shrinkage allowance was 4%. 1. Setterholm, V. C. and Kuenzi, E. W., For most of the pulps, the overall U. S. Forest Prod. Lab. Report 2066, 1956. percentage reduction in strain to failure 2. Jewett, D. M., “An electrical strain from the unrestrained to fully restrained gage for the tensile testing of paper,” condition was about 70%. Stretching U. S. Forest Service Research Note the wet webs 3% resulted in an addi­ FPL-03, Forest Prod. Lab., Madison, tional loss of stretch of about 7%. Wis., 1963. At any given density the unbleached 3. Schultz, J. H., Tappi 44 (10): 736 kraft pulps (Fig. 9) showed greater (1961). 4. , D. H. and Tydeman, P. A., stretch than the bleached kraft pulps “A New Theory of Shrinkage, Struc­ (Fig. 10). The bleached kraft pulps, ture, and Properties of Paper in Forma­ Fig. 12. Effect of density on tensile strain to in turn, showed greater stretch than the tion and Structure of Paper,” Vol. l. failure for handsheets from pulps in group 4 at sulfite pulps (Fig. 11). Trans., British Paper and Board five levelsof restraint or stretching Makers’ Assn. Symposium, Oxford, For unrestrained sheets, the stretch 1961. is increased by beating. As greater 5. Rance, H. F., Tappi 37 (12): 640 restraint is applied during drying, the (1954). influence of processing becomes less. 6. Jentzen, C. A., Tappi 47 (7): 412 Thus, in a general way, stretch is in­ (1964). fluenced more by restraint than it is by 7. Nissan, A. H., Transactions of Fara­ refining in the test beater. However, day Society 53: 700 (1957). the more beating and the higher the 8. Litt, M., J. Colloid Science 16: 297 density of handsheets, the greater will (1961). 9. Setterholm, V. C., Chilson, W. A., and be the effect of restraint on stretch. Luey, A. T., “Effect of temperature and restraint during drying on the tensile properties of handsheets,” EFFECT OF MOISTURE CONTENT U. S. Forest Prod. Lab. Report 2265, AT THE TIME OF RESTRAINT Madison, Wis., 1962. 10. Doughty, R. H., Paper Trade J. 93 Results in Table III show that, within (2):39; 93(15):44(1931); 95(9):29 the moisture limits of the experiment (1932). (20-60%), the higher the moisture 11. Fahey, D. J. and Chilson, W. A., Tappi 46 (7): 393 (1963). Fig. 13. Tensile properties of paper as affected content at the time the restraining is by moisture content at time of restraint accomplished, the higher the strength RECEIVED MAY 27, 1965.

640 Vol. 48, No. 11 November 1965 / Tappi