ELEMENTS OF WOOD FIBER STRUCTURE AND FIBER BONDING

U.S. FOREST SERVICE RESEARCH PAPER FPL 5 MAY 1963

FOREST PRODUCTS LABORATORY U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE = MADISON, WIS SUMMARY

Current concepts of the chemical composition and phy­ sical organization of wood fibers are reviewed. The use of cellulose solvents in studies of the microscopic structure of the cell wall is discussed and illustrated. Types of natural fiber-to-fiberbonding are pictured in color as well as current work on the distribution of additives in linerboard applied for chemical bonding. ELEMENTS OF WOOD FIBER STRUCTURE AND FIBER BONDING by F. A. SIMMONDS, Chemist and G. H. CHIDESTER, Chief Wood Fiber Products Research

ForestProducts Laboratory,1 ForestService U.S. Department of Agriculture

INTRODUCTION

Wood fibers are highly complex in required of modern printing pa­ chemica1composition, distribution pers. Thick-walledfibers add bulk, of chemical components, and phy­ opacity, and tearing strength. sical organization. Their complex structure has a strong bearing on At the microscopical level, the cell their papermaking qualities. In wall consists of several layers of pulp made from any one species, cellulose embedded in lignin and the fibers can vary widely, de­ hemicelluloses. These layers, pending on the pulping process made up of fibrils differently ori­ used. Manufacturers of pulp and ented, comprise three distinct paper are confronted daily with the walls--the primary, the second­ need for a complete understanding ary, and the tertiary. In turn, the ofthe significance of these varying secondary wall consists of two characteristics when striving to in­ layers, S-1and S-2. The S-2layer troduce scientific and engineering is the thickest of the three; it com­ principles into their processes. prises the bulk of the secondary wall, This physical organizationis At the macroscopical level, fiber shown schematically infigure 1, in length, fiber width, and the thick­ whichthe relative thickness of the ness of the fiber wall are impor­ walls is depicted according to pre­ tant. Long fibers improve tearing sent consensus, the ranges of val­ strength. Short, thin-walledfibers ues inmicrons being 0.23 to 0.34, contribute to the smooth surfaces 0.12 to 0.35, 1.77 to 3.68, and 0.10

1Maintained at Madison, Wis., in cooperation with the University of Wisconsin.

FPL-5 1 2 to 0.15 (9). M 122 480 decomposition and differs in its be­ havior to chemicals. The S-2 layer is the richest in cellulose (13) and TERTIARY WALL is the most reactive with respect to SECONDARY WALL (S2) fiber bonding. The physical struc­

SECONDARY WALL (S1) ture is especially critical in the manufactureofhigh-strengthpulps PRIMARY WALL and in subsequent pulp beating and INTERCELLULAR SUBSTANCE papermaking.

At the submicroscopical level, the structure is somewhat less well de­ Figure 1. --The physical organization of a typi­ fined. It includes the crystalline, cal softwood fiber (relative thickness of walls the amorphous, and the "in-be­ to scale). tween"regions of the cellulose, and The term "tertiary wall" refers the hemicelluloses andlignin. The herein to the inner layer of the hydrogen bond cross linkage be­ secondary wall in agreement with tween the cellulose molecules is Bucher (2) and with Liese (11). The generally consideredto be also the latter has demonstrated that the fi­ principal bond between fibers in brillar orientation of this layer is paper. They form between the fi­ interwoven or meshlike in contrast bers and fibrils when these are to those of the S-1 and S-2 layers. dried during the papermaking op­ He also mentioned that the tertiary eration as explained by Campbell wall is more resistant to enzymatic (3).

SOLVENT SEPARATION OF CELL WALL LAYERS

One technique long used in the study more than 50 years earlier (4). of fiber physical structure is the Recently, Jurbergs (10) discussed swelling of fibers with a cellulose this use of swelling agents in some solvent to loosen and separate fi­ detail and also demonstrated the brils and cell wall layers. About value of homogeneous acetylation 1926 at the Forest Products Labo­ in morphological studies. ratory, Ritter used sulfuric acid in his pioneering work on the separa­ Structural features of a Douglas- tion of the layers of the mature cell fir kraft pulp fiber swollen with walls ( 17 ). He was cognizant of the cadmium ethylenediamine are work of the German botanist Dippel. shown in figure 2, upper left. This who had observed primary, sec­ solvent, prepared according to ondary, and inner layers of the cell Henley (8), was diluted with water wall by means of polarized light to a cadmium concentration of about 4.2 percent and used for all swell­ 2 Underlined numbers in parentheses refer to ing treatments reported herein. Literature Cited at the end of this report. For the photographs, polarized

2 FPL-5 Figure 2. --Upper left, chemically swollen Douglas-fir kraft pulp fiber showing the tertiary layer within the balloons of the S-2 layer and the intact S-1 layer at the ends of the balloons (magnification 190X). Upper right, southern pine kraft fiber showing same foregoing structure details (95X). Bottom, sweetgum kraft fiber showing foregoing structure details and lesser resistance to swelling (190X).

light with crossed polars in con­ produced with controlled variations junction with, for the most part, a in characteristics. Ritter and Chi­ first order redplatewas used. The dester (18) found that with decreas­ balloons are areas where the outer ing yield and increasing bursting layer of the secondary wall, S-1, strength, spruce and hemlock sul­ was ruptured, which permitted the fite pulps became less resistant to less resistant inner layer, S-2, to the swelling action of phosphoric swell grossly. A fibrillar strand acid. Also, for a given pulp, swell­ of S-1can be seen at one end of the ing increased during beating . smaller balloon. Microscopical In a comparison based on the ter­ studies of the effects of various tiary wall as revealed by swelling types of mechanical processing on with cupriethylenediamine, Meier this S-1 layer are useful in paper and Yllner (14) reported that this manufacture. Hemicelluloses in wall "is largely destroyed in sul­ this layer have been said to cause fite pulp, but in sulfate pulp, it nonuniformity of reactions during seems as well preserved as in holo­ the chemical conversions of dis­ cellulose. Prehydrolyzed sulfate solving pulps (7). More informa­ pulp shows, if any, a very weak tion is needed on the effects of tertiary wall." This generalization pulping andbleaching processes on apparently stems from observa­ both the chemical and physical tions made on a single pulp of each qualities of S-1. type bleached without a caustic soda extraction stage The tertiary wall is seen as the pronounced screw- or spiral-like Rodgers and coworkers (19) used structure within the fiber and par­ swelling with this reagent (0.18 allel to the long axis of the fiber. as a qualitive measure of This layer in pines, and many other how uniformly and completely dis­ conifers, is characterized by a solving pulp fibers (presumably wart structure that is highly resist­ southernpine) could be expected to ant to chemicals (12, 24). It is plau­ react in the viscose process. The sible that these bodies also cause criterion was essentially the si­ difficulties such as haze in cellulose multaneous dissolution of the ter­ acetate solutions in the chemical tiary and secondary layers of the usage of wood pulps, cell walls. These investigators found, and it seems remarkable, The S-1, S-2, and tertiary layers that this dissolution did not occur of southern pine and sweetgum kraft when the concentration of caustic pulp fibers are shown clearly in soda during cold extraction was up figure 2, upper right and bottom. to 6 percent but at 7 percent, it did. In the sweetgum, the tertiary wall is, in this instance, less pro­ The relative resistance to swelling nounced than in the Douglas-fir and of samples of sweetgum bleached pine pulps kraft and cold-purifiedprehydroly­ sis-kraft pulps produced and de­ Chemical swelling is useful for scribed previously (22) is shown in qualitative comparisons of pulps figure 3. The controlling variable

FPL-5 3 ~ ~

Figure 3. --Relativeresistance to swelling of sweetgum bleached kraft and purified prehydrolysis­

kraft pulps. Upper left, kraft (130X). Upper right, prehydrolysis-kraft, 60-min. hydrolysis

(130X). Lower left and right, 150-min. hydrolysis (130X and 200X).

was duration of water-prehydroly­ ysis-kraftpulp examined by Meier sis at 170° C. Hydrolysis periods and Yllner (14), there was exten­ of 60 and 150 minutes are shown. sive persistance of the tertiary The hydrolysis greatly decreased wall, eventhough a cold extraction resistance to swelling. Although with 5 percent caustic soda solution the 60-minute hydrolysis pulp was was included in the purification only about 100 lower in degree of treatment. polymerization than the kraft pulp, its pentosan content was only about a tenth as much. Compared to this Another phase of chemical swelling pulp, the 150-minutepulpwas about worthy of investigation, especially 500 lower in degree of polymeriza­ for hardwood pulps, is a very mild tion but only about half lower in action to rupture hydrogen bond pentosan content. The effects of cross linkages invarying degree as interfibrillar bonding and chain a sort of internal fibrillation. Ef­ shortening are evident. fects of this treatment on fiber stiffness, strength, and bonding In contrast to the spruce prehydrol­ capacity are of great interest.

MECHANICAL SEPARATION Mechanical processing, such as er, as is shown by the fairly paral­ beating, is another means for re­ lel fibrillar orientation. Adjacent vealing wall structure. Micro­ to this edge is a separated area of scopical studies of the effects of an S-1 layer, identifiable by the various types of mechanical pro­ coaxial orientation of fibrils. cessing on the S-1 layer and their Clearly, the external surface or correlation with paper properties potentialbonding area of this fiber are of practical value. was increased considerably. Pre­ sumably, however, the inherent Themicrograph in figure 4, upper tensile strength and stiffness were left, of a ponderosa pine kraft fiber decreased. beaten to a Canadian standard free­ ness of 270 milliliters shows that An S-1layer completely separated the fiber wall was split open longi­ from a ponderosa pine fiber is tudinally, thus revealing the ter­ shown in figure 4, upper right. The tiary wall. A gross, slightly angled evidence is that this type of sleeve- fibrillar orientation can be seen. like separationis characteristic of The bright left edge is the S-2 lay­ pine kraft pulps.

EFFECT OF PULPING PROCESS ON BEHAVIOR OF FIBER DURING BEATING The pulping process used for sepa­ not only its chemical composition, rating wood fiber affects profoundly strength, and bonding capacity but

FPL-5 4 Figure 4.--Upper left, ponderosa pine kraft fiber showing cell wall layers revealed by beating

(500X). Upper right, the S-1 layer from a beaten ponderosa pine kraft pulp fiber (magnifica­

tion 500X). Lower left, jack pine two-stage sulfite pulp fiber swollen by beating (260X). Lower

right, larger field of the beaten jack pine sulfite pulp (125X).

also its behavior during beating. beaten kraft pulp is shown in figure 5, upper right. The lesser degree A micrograph of a two-stage neu­ of fibrillationis evident, as well as tral sulfite acid sulfite pulp made the preponderance of intact fiber-­ from jack pine in a pulping investi­ a manifestationof the strength su­ gation at the Forest Products Labo­ periority of the pulp. ratory by Sanyer and others (20) is shown also in figure 4, lower left. This difference in swelling charac­ This pulp had been bleached and teristics was attributed (20) largely beaten to a freeness of 235 milli­ to the higher glucomman content liters, Canadian standard. The of this sulfite pulp. Further dis­ point of interest is the central sum­ cussion on certain differences be­ merwood fiber in which a gross tween acid sulfite and kraft pulps swelling of the S-2 layer occurred are quoted from this paper (20) as over an area where the S-1 layer follows: "The fibrillar bundles of was removed during beating. The kraft pulps have been shown to be typical orientation of the S-2 fibrils larger in diameter than those of is evident. This balloonlike swell­ sulfite pulps. In addition, the cry­ ing occurred extensively in this stallites are wider and more per­ pulp. The micrograph in figure 4, fectly developed, and the cellulose lower right, shows a larger area of has a wider lateral order distribu­ the pulp and the extensive fibrilla­ tion in prehydrolysis kraft than in tion induced by the beating. dissolving grade sulfite pulp. Con­ sequently, it is more resistant to Kraft pulp made from the jack pine swelling and has greater mechani­ (20) bleached and beaten to a free­ cal strength. The S-3 layer of the ness of 210 milliliters is shown in secondary wall in sulfite pulp was figure 5, upper left. This pulp was found to be greatly weakened and much stronger than the sulfite pulp. that the cell wall expands or swells Here, the S-1 layer has been fibril­ inwardly without change in external lated and peeled from the S-2 layer fiber diameter. Consequently, the which, in contrast to the weaker apparent density of sulfite fiber is sulfite, did not exhibit balloonlike lower than that of kraft fiber and the swelling. A larger field of the wall is therefore weaker."

FIBER-TO-FIBER BONDING As was mentioned, pulping process known mechanical processing tech- affects the capacity of fibers to bond nique can yield groundwood pulp to one another. For example, in approaching chemical pulp in the manufacture of groundwood strength. The ligninin the ground- pulp, the lignin of the wood is re- wood pulp lacks the bonding capac­ tained, but it is largely removed in ity of the cellulose and hemicellu­ the production of chemical pulp. No loses.

FPL-5 5 Figure 5. --Upperleft, jack pine kraft pulp fibrillated by beating (260X). Upper right, a larger

field of the beaten jack pine kraft pulp opposite (magnification 260X). Lower left, the jack pine

sulfite fiber of figure 3 shrunkenupon drying (260X). Lower right, the shrunken jack pine kraft

pulp fiber above, left (260X).

Before fiber-to-fiber bonds can split open during beating but not fi­ form, fiber surfaces must come brillated and formed the filmlike close enough to one another to be bond clearly evident. The relative within the minute distance through quality of these types of bonds, that which a molecular force such as the exist in papers made from pulps so hydrogen bond can act. This is beaten is not known. brought about by the shrinkage of the fibrous systemwhen dried in the An example of the fibrillar type of presence of a strongly polar liquid, bond observed in a very thin sheet such as water. A point is reached of paper (actually, a handsheet) is when the surface tension of the shown in figure 6, lower left. The water provides the force needed to dynamic behavior of these fibrillar bring the surfaces within the sphere andlamellar bonds during the dry­ of molecular attraction--3 to 5 ing of extremely thin sheets of pa­ Angstroms for the hydrogen bond pers with and without restraint, (3). using the cinnematic technique is under consideration. The micrographin figure 5, lower left, shows the shrinkage of the jack Vertical polarized illumination, as pine sulfite fiber specimen of figure proposed by Page (15), shows pro­ 4 when it was dried on a glass slide mise for the study of areas of fiber without restraint, other than what bonding between crossed fibers in might have resulted from adhesion paper. Inaccordance with his tech­ of fiber to glass. The transverse nique, a micrograph of the surface shrinkage of the summerwood fiber of a handsheet made from a mixture just at the point of the swollen area of dyedandundyed fibers to accen­ was 25 percent. The similar tuate visibility of bonded areas is shrinkage of the pine kraft fiber is shown at the lower right in figure 6 shown at the lower right in figure 5. It may be seen that the bright fiber centrally located crosses a dyed A fibrillar type of fiber-to-fiber fiber at an angle of about 60°. Ac­ bonding between two beaten sweet- cording to hypothesis (15), dark gum kraft pulp fibers is shown in areas like those at which fibers figure 6, upper left. A lamellar or cross one another are areas in film type of bond between two of the which bonds occur. It is not gum fibers is shown at the upper thought, however, that such areas right. Here the fiber at the left was are completely filled with bonds.

LONGITUDINAL SHRINKAGE OF FIBERS IN PAPER

From the pristine wet condition, drying as much as 25 percent in wood pulp fibers may shrink during width but only slightly in length-­

FPL­5 6 Figure 6. --Upperleft, fibrillar bond between two sweetgum kraft pulp fibers (725X). Upper right,

lamellar bond between two sweetgum kraft pulp fibers (magnification 1,250X). Lower left,

fibrillar bonds between fibers in a very thin paper (500X). Lower right, area of fiber bonding

(arrow) between two crossed fibers in paper (500X).

about 1 to 2 percent. several in-machine direction fibers and natural shrinkage can occur In a paper by Smith (23) on the re- over the length of the bridging fi­ lation of curling and cockling of pa- ber, then this may be shortened per to dried-instrains, the practi- much more than 2 percent owing to cal andhypothetical aspects of fiber the diametrical shrinkage of the and sheet shrinkage are dealt with supporting in-machine direction fi­ on the basis of his own data and bers. This was confirmedlater by those of others. Upon consideration Page and Tydeman (16) who ob­ of recoverable dried-in strains he served fibers in the surface of a supposed that when a cross-ma- paper to shrinklengthwise as much chine direction fiber bridges across as 10 percent.

BONDING WITH CHEMICAL ADDITIVES

Military needs for special papers phenol resin was added to relatively and container boards in World War dry board at the size press, a con­ II stimulated research on the use of siderably higher strength was ob­ chemical additives for improving tained than when the chemicals the bonding of fibers, and this field were added to the wet board at the has expanded greatly. Currently, smoothing press, The photomicro­ the Forest Products Laboratory is graphs show that in the stronger developing, with the use of addi­ board the chemical, as revealed by tives, a container board of im­ staining with iodine, penetrated proved stiffness under high humid­ only a little way below each sur­ ity for the Air Force (5). face, but in the weaker board the penetration was consistent through the sheet. The superiority of the The distribution of the chemicals boards only partially penetrated is within the boards is being studied plausibly explained by considering microscopically. The need for them as sandwich constructions. work in this area is recognized widely (21, 6, 1), since differences between treated boards or papers with respect to strength and other The resin-coatedlayers of fibers on qualities can often be explained by the two surfaces act as "skins." the results obtained with this tech­ The center, or core, which had nique. little or no chemical, apparently had sufficient strength at 90 per­ An example of this is shown in fig­ cent relative humidity to prevent ure 7 by means of photomicrographs these skins from buckling. The of cross sections across the ma­ micrograph at the lower left of fig­ chine direction of linerboards-­ ure 7 shows the starch did not pene­ wire side down. When a starch- trate the cell walls of the fibers.

FPL-5 7 Figure 7. --Micrographs of cross-sections showing chemical distribution in linerboard (Iodine

stain shows starch-phenol resin as dark areas). Upper left, unstained cross-section (110X).

Upper right, location of chemicalwhen added at size press of paper machine--30percent strength

increase (110X). Lower left, field from lower right at higher magnification showing no starch

in fiber cell wall (230X). Lower right, location of chemical when added at smoothing press-­

6 percent strength increase (110X).

LITERATURE CITED (1) Boast, William H. Trossett, (8) Henley, D. 1960. The Cellulose Stanley W., Jr. A Scheme for Solvent Cadoxeu, A Prepara­ the Identification of Binders tion, and a Viscosimetric Re­ Generally Used in Mineral lationship With Cupriethylene­ Coatings. Tappi 45 (11): 873­ diamine. Svensk Papperstid­ 877. ning 63 (5): 143-146.

(2) Bucher, Hans. 1957. The Ter­ (9) Jayme, Georg and Fengel, Die­ tiary Wall of Wood Fibers and trich. 1961. Contribution to Its Morphology in Conifer Tra­ the Knowledge of the Micro­ cheids. Holzforschung 11 (1): structure of Springwood Tra­ 1-16. cheids. Holz als Roh- und Werkstoff 19 (2): 50-55, Ta­ (3) Campbell, W. Boyd. 1933. The ble l. Cellulose ate r Relationship in Paper-Making. FS Bul. 84, (10) Jurbergs, K. A. 1960. Mor­ Dept. of Interior, Canada. phological Properties of Cot­ ton and Wood Fibers II. Slash (4) Dippel, Leopold. 1869. Das Mi­ Pine. Tappi 43 (6): 561-568. kroskop, Zweiter theil. Er­ steu Autlage. Secunde Aut­ (11) Liese, Walter. 1960. Die lage. 1898. Vieweg und Sohn, Struktur der Tertiärwand in Braunschweig. Tracheiden und Holzfazern. Holz als Roh-und Werkstoff (5) Fahey, D. J. 1962. Use of 18 296-303. Chemical Compounds to Im­ (12) 1956. Zur systemati­ prove the Stiffness of Contain­ schen Bedeutung der submi­ er Board at High Moisture kroskopischen Warzenstruktur Conditions. Tappi 45 (9): 192A­ bei der Gattung Pinus L. Holz 202A. als Roh-und Werkstoff 14 (1 1): 417. (6) Felton, Clinton D., Thomas, Ed­ ward J., and Clark. 1962. (13) Meier, Hans. 1962. Chemical Ultraviolet Microscopy of Fi­ and Morphological, Aspects of bers and polymers. Tex. Res. the Fine Structure of Wood. Jour. 32 (1): 57-67. Pure and Appl. Chem. Vol. 5, pp. 37-52. (7) Haas, H., Battenburg, and Teves, D. 1952. The Char­ (14) and Yllner, Sven. 1956. acteristics of Wood Pulps for Die Tertiärwand in Fichten­ Use in the Manufacture of Vis­ zellstoff-Tracheiden. Svensk cose. Tappi 35 (3): 116-124. Paperstid 59 (1 1): 395-401.

FPL-5 8 (15) Page, D. H. 1960. Fibre-to- (20) Sanyer, Necmi, Keller, E. L., Fibre Bonds. Part 1 --A Meth­ and Chidester, G. H. 1962. od for Their Direct Observa­ Multistage Sulfite Pulping of tion. Paper Technol. 1 (4): Jack Pine, Balsam Fir, 407-411. Spruce, Oak, and Sweetgum. Tappi 45 (2) 90-104. and Tydeman, P. A. Part 2 --APreliminary Study (21) Seiler, Charles J. 1961. Meth­ of Their Properties in Paper ods for Studying Resin Distri­ Sheets No. 5, pp. 519-530. bution. Tappi 44 (10): 172A­ 175A. (16) Page, D. H. and Tydemann, P. A. 1962. A New Theory of the (22) Simmonds, F. A., Kingsbury, Shrinkage, Structure, and R. M., and Martin, J. S. 1955. Properties of Paper. The For­ Purified Hardwood Pulps for mation and structure of Paper. Chemical Conversion II. Trans. of the Symposium held Sweetgum Prehydrolysis-Sul­ at Oxford, September 1961. fate Pulps. Tappi 38 (3): 178­ London. 186.

(17) Ritter, George J. 1927. Dis­ (23) Smith, S. F. 1950. Dried-in section of Wood Fibers by Strains in Paper Sheets and Chemical Means. Presenta­ Their Relation to Curling, tion before the Division of Cockling and Other Phenome­ Cellulose Chemistry at the na. The Paper Maker and 74th Meeting of the Amer. British Trade Jour. 119 (3) : Chem. Soc. 185-188, 190-192.

(18) and Chidester, G. H. (24) Wardrop, A. B., Liese, 1928. The Microstructure of and Davies, W. 1959. The a Wood Pulp Fiber. Paper Nature of the Wart Structure in Trade Jour. 87 (17): 131-137. Conifer Tracheids. Holzfor­ Pulp and Paper Mag. Can. 26 schung 13 (4): 115-120. (46): 1617-1624.

(19) Rogers, Linwood N., Heath, Merle A., and Gutliph, Earl W. 1959. Hiett. Purified Cel­ lulose Fiber and Process for Same. U.S. Patent 2,878,118.

FPL-5 9 1. -23 FOREST PRODUCTS LABORATORY U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE - MADISON, WIS. In Cooperation with the Universtiy of Wisconsin