FIBER BONDING AND TENSILE STRESS-STRAIN PROPERTIES OF EARLYWOOD AND LATEWOOD HANDSHEETS

Transcription

1 FIBER BONDING AND TENSILE STRESS-STRAIN PROPERTIES OF EARLYWOOD AND LATEWOOD HANDSHEETS USDA, FOREST SERVICE RESEARCH PAPER FPL U.S. Department of Agriculture, Madison, Wisconsin Forest Products Laboratory, Forest Service

2 Abstract Interfiber bonding, tensile stress-strain properties, fiber collapse, and fiber-to-fiber conformability at fiber crossings were studied on handsheets composed entirely of unbeaten earlywood and latewood loblolly pine kraft fibers. Because of a better stress distribution earlywood sheets showed consistently higher tensile strength and modulus of elasticity at a given level of interfiber bonding than latewood sheets. Microscopical studies showed that earlywood fibers had more uniform fiber-to-fiber conformability and greater degree of fiber collapse than latewood fibers. No differences were detected in cell wall density of dry earlywood and latewood fibers. It is concluded that in this study the amount of bonding developed in the latewood fibers was insufficient to fully utilize their greater fiber strength.

3 FIBER BONDING AND TENSILE STRESS-STRAIN PROPERTIES OF EARLYWOOD AND LATEWOOD HANDSHEETS By WILLIAM E. SMITH 1 AND VON L. BYRD, Forest Products Technologist 2 Forest Products Laboratory, Forest Service U.S. Department of Agriculture Introduction An important morphological attribute of conifers is that they consist almost entirely of fibers having either thin or thick cell walls corresponding to the time of year at which they were laid down. When the wood is pulped, the thinner walled earlywood fibers collapse into ribbonlike forms 40µ to 65µ wide and 10µ to 12µ thick, twice the cell wall thickness (5µ to 6µ). The latewood fibers retain their tubular form and are 25µ to 35µ in diameter with a wall thickness of 10µ to 15µ. Though individual latewood fibers are about 3 three times as strong as earlywood fibers (2), they form bulky, weak sheets whereas earlywood fibers form dense, strong sheets (1, 8). This basic difference in the behavior of earlywood and latewood fibers may account for most of the differences in pulp properties. Because of their ribbonlike nature and greater flexibility, earlywood fibers tend to pack more closely than latewood fibers and to have more contact area at fiber crossings than the less flexible latewood fibers. Thus, though latewood fibers are about three times as strong as earlywood fibers, the latter form stronger, more dense sheets because of their greater fiber-to-fiber conformability resulting in more interfiber bonding at fiber crossings. The lack of conformability of latewood fibers prevents the utilization of the full potential of their greater fiber strength, Also, even though latewood fibers form bulky, weak sheets, they are unsuitable for sanitary paper products because of their stiff nature. The purpose of this study was to find the relationship between interfiber bonding, determined by an electrical conductivity procedure (4, 5), and the tensile stress-strain properties of handsheets composed entirely of either earlywood or latewood loblolly pine fibers. Fiber collapse and fiber-tofiber comformability at fiber crossings were examined microscopically (3). Cell wall density of earlywood and latewood fibers was measured to study the possible influence of this property on sheet characteristics. This study was conducted with unbeaten fibers and the applicability of these data to processed fibers remains to be confirmed. 1 William E. Smith, formerly Forest Products Technologist at Forest Products Laboratory. Present address: Sonoco Products Company, Hartsville, S.C Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 3 Underlined numbers in parentheses refer to literature cited at end of this report.

4 Results and Discussion Interfiber Bonding Maximum tensile stress and modulus of elasticity for earlywood and latewood sheets are plotted against interfiber bond level in figures 1 and 2. The data represent nominal pulp lignin contents of 7.4 and 11.0 percent and wet pressures of 0, 10, 60, 1,000, and 4,350 pounds per square inch, These data, along with tensile strength, modulus, and strain to failure, are given in table 1. The data reveal that as bonding increases, earlywood handsheets rapidly attain high strength and modulus of elasticity followedby alevelingoff of these properties that begins at anintermediate bonding level. In contrast, latewood sheets at low and intermediate bonding levels exhibit lower strengths and moduli. The inflection points in the latewood curves occur at higher bond values than do the corresponding inflections in the earlywood curves, Increasing bonding in latewood sheets by the application of extremely high wet pressures increases the sheet strengths and moduli to values comparable with those of the earlywood sheets, It is apparent that at low and intermediate bonding levels earlywood sheets have higher strength and modulus of elasticity than do latewood sheets. Sheet Density and Conformability Figure 3 reveals that in the earlywood sheet, the modulus of elasticity is greater than that in the latewood sheet when the comparison is made at equal densities until the densities reach approximately 0.60 gram per cubic centimeter. It is interesting that at this sheet density, photomicrographs of transverse sections of handsheets revealed almost complete collapse for both earlywood and latewood fibers. Figure Relationshipof maximum stress (tension) to interfiber bonding of earlywood and latewood handsheets. M Figure Relationship of modulus of elasticity to interfiber bonding of earlywood and latewood handsheets. M FPL 193 2

5 Table 1.--Handsheet properties of earlywood and latewood loblolly pine kraft pulps Figure 3.--ReIationship of modulus of elasticity to sheet density or earlywood and latewood handsheets. Figures 4 and 5 are cross sections of earlywood and latewood sheets, respectively, that have been wet pressed at 60 pounds per square inch. In figure 4, the earlywood fibers are generally flattened though fiber collapse is by no means complete. Buckling of these fibers is common, and the resulting ridges that may occur at any point across the width of the fiber afford interfiber bond sites for neighboring fibers. This means that frequently a crossing of earlywood fibers may be bonded at more than one site across its width, a major factor in uniformly distributing the bonds at the crossing. Thus the stress is uniformly distributed when a load is applied. The latewood sheet in figure 5 is extremely bulky. Curvature of the external surfaces and a low degree of fiber collapse are readily apparent. These curved surfaces when brought into contact with other fibers of similar geometry result in highly localized bonding and nonuniform stress distribution. The centers of the latewood fiber crossings are believed to be the principal bond M

6 sites for moderately pressed sheets, as opposed to the evenly distributed bond sites offered by the corrugated surfaces of earlywood fibers. In figure 6, a cross section of a highly pressed latewood sheet, the striking degree of fiber collapse in the latewood fibers results in flatter external surfaces for bonding than in a less highly pressed sheet. Higher degrees of measured bonding and increased strengths and moduli for these highly pressed latewood sheets substantiate this observation (table 1). The corners of the latewood fiber in the completely collapsed condition offer multiple sites for bonding across the width of the fiber as opposed to the one site offered by the uncollapsed fiber. Uniform distribution of bonding at crossings formed by earlywood fibers in a highly pressed handsheet is suggested by the photomicrographs in figure 7 using the technique of Page (3) for observing contact regions. The bottom fiber was dyed with Chlorazol Black E prior to handsheet formation. Figure 7A, taken with polarized light using vertical illumination, shows the relative positions of the fibers that form the crossing. In figure 7B, the same crossing taken with a second polarizer at 90 to the first, the dark regions in the crossing at are in optical contact, illustrated by the fact that the light is absorbed rather than scattered. It is generally conceded that bond sites occur in the region in optical contact although not all of the area in optical contact is necessarily bonded. This technique for direct observation requires that fiber collapse be complete; thus only highly pressed handsheets can be studied. An example in which the bonding is apparently restricted to the two fiber ridges corresponding to the cell wall corners of a collapsed fiber is seen in figure 8, A and B. The results of microscopic observations of the transverse fiber geometry and of the direct fiberto-fiber crossing are summarized in figure 9, -A-2. Localization of the bonded region in the latewood crossing, as opposed to distribution of the bonds in the earlywood crossing is emphasized. In C, a highly pressed earlywood crossing differs from the lightly pressed crossing shown in A essentially in the degree of bonding. However, the highly pressed latewood crossings shown in D differ from the lightly pressed in B not only in degree of bonding but also in the bond distribution and the lumen geometry. Cell Wall Density of Earlywood and Latewood Fibers The ease with which earlywood fibers collapse may be due to their high-diameter-to-wall thickness ratio rather than to assumed porosity differences in the two fiber types. Theoretically (7), Figure 6.--Latewood handsheet wet pressed at 1,000 pounds per square inch. Fiber collapse is virtually complete, thickened cell corners result in a "dumbbell" transverse fiber shape. M u.r. 5

7 the ratio of initial deflection and unit load of diametrically loaded thin-walled cylinders is proportional to the ratio r 3 /h 3 where r = mean radius and h = wall thickness. Data in the following tabulation illustrate the magnitude of this effect for the earlywood and latewood types of fiber. Sample r 3 per h percent lignin Earlywood 1,150 Latewood percent lignin Earlywood 1,675 Latewood 176 Measurement of the densities of the earlywood and the latewood pulp fiber substances by suspending them in a xylene-carbontetrachloride mixture shows that at the same lignin content the specific gravities of the two fiber types do not differ as seen in figure 10. Extrapolation of these data by linear regression to a lignin content of percent yields a cell wall specific gravity for early- Figure 7.--Bonding at the crossing of two earlywood fibers. Relative positions of the two fibers are shown in A, and darkened. regions in optical contact are shown in B. M Figure 8.--Bonding at crossing of two collapsed latewood fibers; thickened ridges on undyed fiber are in optical contact with underlying fiber. M FPL 193 6

8 wood of compared to a value of found by direct measurement by Stamm and Sanders (6). Similarly, extrapolation to a lignin content of percent yields a cell wall specific gravity for latewood of compared to the measured value of In otherwords, there is essentially no difference in cell wall porosity of dry earlywood and latewood pulp fibers. This does not, however, preclude the possibility of a porosity difference in the water swollen state (i.e., their condition during sheet formation). Such a difference (if it does exist) would also influence conformability. It is therefore reasonable to assume that the difference in collapse behavior of the two fiber types is primarily the result of differences in shape and size of the transverse fiber wall since the effect of cell wall porosity cannot be confirmed at this time. Figure 10.--Relationship between cell wall density and lignin content for earlywood and latewood kraft pulp fibers. (Regression equation: f = x , where x = Iignin content.) M Conclusions Figure 9.--Fiber-to-fiber bonding at crossings: Lightly pressed--earlywood, A; latewood, B; highly pressed--earlywood, C; latewood, D. M Fiber morphology has been shown to governthe distribution of bonds within a fiber-to-fiber crossing. Bond distribution at earlywood fiber-to-fiber crossings is suggested by direct microscopic observation. When compared at the same degree of bonding, moderately pressed earlywood sheets generally exhibited higher moduli of elasticity than did latewood sheets. Interestingly enough, the high modulus of the latewood fiber is not sufficient in itself to override the detrimental effects of bonding and, therefore, the stress concentration in the latewood fiber-to-fiber crossings. Photomicrographs of cross-sections of handsheets show that collapse of fibers is appreciable in earlywood sheets at all wet pressure levels. The more rigid latewood fibers do not collapse at pressures as high as 60 pounds per square inch. However, at 1,000 pounds per square inch, they are almost completely collapsed. This produces flatter external surfaces for bonding. Direct observation of optical contact in fiber-to-fiber crossings and bonding measurements substantiate this as do 7

9 values for moduli of elasticity. No differences in cell wall density between earlywood and latewood fibers were found--an indication that fiber conformability differences in earlywood and latewood pulp fibers are not attributable to cell wall porosity differences. This emphasizes the importance of the combined effects of fiber cross-sectional shape and area on conformability with collapse more common in thinwalled, large diameter cells. Experimental Procedure Kraft pulp was prepared from plantation grown loblolly pine (Pinus taeda L.). The wood was taken from rings 8 to 25 at the 10-foot level of a 30 year-old tree. Chips measuring 5/8 to 3/4 inch longitudinally, 5/32 inch tangentially, and less than 1 inch radially were cooked in a laboratory bomb in circulating oil at 165 C. Because the bomb was not agitated during the cook, a high liquor-to-wood ratio (6:1) and initial liquor concentration (53 g. per active alkali as sodium oxide) were used. To obtain several degrees of delignification, numerous cooks were made in which cooking time was varied. Earlywood and latewood components were manually separated after cooking; the isolated components were washed, separated into individual fibers, and air-dried. Lignin content determinations were made on the separated components. Prior to handsheet preparation, the pulps were deionized as has been described (5). Handsheets were formed on the British sheet mold from 0.10N potassium chloride solutions. The wet handsheets were pressed at 0, 10, 60, 1,000, and 4,350 pounds per square inch. Two necked stress-strain strips were cut from the center of each handsheet. Stress-strain curves to failure were obtained at 50 percent relative humidity and 72 F. A 3.6- by 1.9-centimeter strip was cut from each of the trimmed residual pieces of handsheet for electrical conduction studies. These strips as well as duplicate individual fiber samples were evaluated for conductance at 50 percent relative humidity and 72 F. by measuring the current flow at an applied DC potential of +100 volts (4). Segments of several handsheets were embedded in methyl butyl methacrylate and cross sectioned at 4µ thickness using an ultramicrotome. These cross sections were useful in determining fiber shape and collapse behavior. Dyed fibers were incorporated in some specially prepared handsheets and Page's method for delineating optical contact (3) was used to study fiber-to-fiber crossings. Cell wall density was determined for earlywood and latewood fibers at each degree of delignification by suspension in a calibrated xylene-carbon tetrachloride gradient column. Prior to introduction into the column, the fibers were soaked for 30 hours in xylene. FPL GPO

10 The authors are indebted to Dr. William Gladstone, State University of New York College of Environmental Science and Forestry, Syracuse University, New York, for supplying the pulps used in this study. Literature Cited 1. Jones, E.D., Campbell, R.T., and Nelson, G.G Springwood-summerwood separation of southern pine pulp to improve paper qualities. Tappi 49(9): Determining interfiber bonding by electrical conductivity. P art I, Nature of DC electrical conduction, etc. Tappi 53(10): McIntosh, D.C., and Uhrig, L.O Effect of refining on load-elongation characteristics of loblolly p in e holocellulose and unbleached kraft fibers. Tappi 51(6): Page, D.H Fibre-to-fibre bonds. Part I, A method for their direct observation. Paper Tech. 1(4): Smith, W.E An investigation of a method for measuring interfiber bonding in pulp handsheets based on sheet and fiber DC electrical conductivities. Ph.D. Thesis, North Carolina State Univ., Raleigh, N.C., 124 pp. 6. Stamm, A.J., and Sanders, H.T Specific gravity of the wood Substance of loblolly pine as affected by chemical composition. Tappi 49(9): Timoshenko, S.P Strength of materials, Part II. D. Van Nostrand Co., Inc., New York, 510 pp. 8. Watson, A.J., and Dadswell, H.E Influence of fibre morphology on paper properties, Part II, Earlywood and latewood. Appita 15(6): 116.