Understanding sheet extensibility

Transcription

1 Understanding sheet extensibility By R.S. Seth Abstract: Sheet extensibility or stretch depends primarily on two factors the stretch-potential of fibres, and the degree of bonding between them; better bonding allows greater use of the stretchpotential. The fibre s stretch-potential is controlled by the extent of misalignment of the cellulose fibrils in the fibre wall with respect to the fibre axis. The misalignment may be either natural, as in the case of fibres with high fibril angle, or induced by axial compression of the fibre wall. The misalignment is drawn out when the fibre is strained in tension. Axial compression is induced mostly by mechanical treatment of the pulp, and by shrinkage forces during sheet drying. For a given fibre furnish, the control of these two papermaking processes mechanical treatment and shrinkage allows control of the sheet stretch. ensile strength and extensibility or T stretch are two important failure properties of paper. They are defined by the end-point of the sheet s load-elongation curve (Fig. 1). Individually and together, they are important for many product performance properties. For example, TEA, the tensile energy absorbed by the sheet before failure is proportional to the area under the load-elongation curve. Thus, it depends on both the tensile strength and extensibility of the sheet. A high TEA is desired in sack papers [1]. The bursting strength of paper has been shown to be proportional to the product of tensile strength and the square-root of stretch [2]. The fracture toughness of paper has been found to depend strongly on the sheet s tensile strength and stretch [3, 4]. Sheet stretch has also been regarded as important for paper runnability both at the papermachine s dry-end and in the pressroom [5-8]. Papers with high stretch also seem to have a somewhat higher tearing resistance [9], and folding endurance; they are found to be more dimensionally unstable as well [10]. The factors that control sheet tensile strength are fairly well understood [4]. The tensile strength is high if fibres are strong, long, fine and thin-walled. The fibres should be conformable and have a high fibre-fibre bond strength. The sheet tensile strength is also high if fibres are straight, free from deformations and the sheets are well formed. Otherwise, the stress is unevenly distributed when the sheet is strained, leading to premature failure. This report deals with the factors that control sheet stretch. FACTORS THAT CONTROL SHEET STRETCH A specimen under tensile load extends more, the longer it is. Therefore, extensibility or stretch or strain at failure as a material property, is expressed as a percentage of the original specimen length (Fig. 1). Role of bonding Regardless of how bonding between the fibres is increased by wet pressing, beating or refining, or additives, the sheet stretch of a furnish generally increases with increased fibre-fibre bonding. This is observed for almost all papermaking fibres chemical, mechanical, wood, non-wood, or recycled. The reasons are as follows. Fibres have a certain stretch-potential. However, this potential is realized in paper only when fibres form a bonded network. If the bonding is weak, the network fails before the stretch-potential is realized; the sheet stretch is low. As bonding in the network is increased, the stretch-potential of fibres is increasingly realized, the sheet stretch increases. Since increased inter-fibre bonding also increases sheet tensile strength, an increase in stretch with tensile strength is often observed for handsheets (Figure 2). The stronger the sheet, the more the fibres stretch-potential is utilized. Because of this relationship between tensile strength and stretch, factors such as sheet grammage or formation that tend to affect tensile strength also affect sheet stretch [11]. A comparison of handsheet stretch values at similar tensile strengths provides a meaningful comparison of the stretch-potential of various furnishes. FACTORS THAT CONTROL FIBRES STRETCH-POTENTIAL Fibril angle Most paper is made from wood pulp fibres. A wood fibre consists of helically wound cellulose fibrils held together by a hemicellulose-lignin matrix. Fibril angle is the angle between the longitudinal axis of the fibre and the direction of the cellulose fibrils in the fibre s dominant wall layer. Fibres with high fibril angle are more extensible than those with low fibril angle (Figure 3). This is because the interfibrillar matrix of the fibre wall with a high fibril angle can shear more under a given tensile load than that with a low fibril angle. Note the fibre wall is a composite of highly crystalline elastic fibrils embedded in an amorphous matrix that is capable of flow [12]. The higher stretch-potential of fibres with high fibril angle translates into higher sheet stretch, as demonstrated in the following examples. Figure 4 shows sheet stretch as a function of fibril angle for R.S. SETH Pulp and Paper Research Institute of Canada 3800 Wesbrook Mall Vancouver, BC, Canada V6S 2L9 Pulp & Paper Canada T :2 (2005) 33

2 FIG. 1. Load-elongation curve of paper under in-plane tension. FIG. 2. Increase in sheet extensibility with increased degree of inter-fibre bonding by beating [4]. Each data point is the terminal point of the mean load-elongation curve for a given beating level. FIG. 3. Stress-strain curves of single fibres of different fibril angles [12]. FIG. 4. Dependence of handsheet stretch (corrected for apparent density) on fibril angle for various unbleached, never-dried, unbeaten softwood kraft pulps. Data of Horn [13]. unbeaten, low-yield, unbleached kraft pulps made from various softwoods [13]. The strong dependence of stretch on fibril angle is evident. Figure 5 shows the dependence of sheet stretch on fibril angle for low-yield, unbleached kraft pulps made from various hardwoods [14]. In this case the stretch values are compared at a constant tensile strength of 9.5 km obtained by beating the pulps; the higher the fibril angle, the higher the stretch. The tensile strength - stretch plots in Figure 6 further demonstrate the higher stretch-potential of fibres with higher fibril angle. The pulps in this case were obtained by separately pulping normal and gall woods from the same softwood log. The fibres in the gall wood were somewhat shorter but had a higher fibril angle. The bonding between fibres was increased by increasing the wet-pressing pressure during sheetmaking. It is clear from Figs. 4 to 6 that the fibril angle of the fibre resource will have a large effect on sheet stretch. Axial compressive strain While wood fibres are fairly strong in tension, they easily fail under longitudinal compression. This is because of their helically wound fibrillar structure. The compressive failure initiates with the creation of dislocations or slip planes in the fibre wall (Fig. 7). These are regions where FIG. 5. Dependence of handsheet stretch (measured at 9.5 km tensile strength) on fibril angle for various unbleached hardwood kraft pulps [14]. The pulps were beaten in a PFI mill. the original orientation of the fibrils is locally disturbed; the fibrils remain in parallel alignment, but their direction is locally changed (Fig. 8). A series of closepacked, concertina-like dislocations along the length of a fibre are called microcompressions (Fig. 9). The fibre wall also compresses when bent. Axial compressive strain shortens fibres longitudinally [15-17]; it also changes the tensile load-elongation behaviour of the fibre wall (Fig. 10) [16,18]. The shortening is recovered when the fibre is strained in tension; a fibre shortened by axial compression is more extensible (Fig. 11). Thus, the induction of compressive strain in the fibre wall makes fibres more extensible. Extensible fibres make extensible paper. Let us now consider the various ways papermaking fibres receive axial compressive strain, intentionally or unintentionally, that can change their stretchpotential. WAYS TO INDUCE AND ENHANCE AXIAL COMPRESSIVE STRAIN A. Mechanical treatment of wood and pulps Fibres in wood undergo axial compression when exposed to mechanical stresses FIG. 6. Handsheet stretch as a function of tensile strength for two softwood pulps with different fibril angles. The pulps were unbleached, low-yield, never-dried kraft. The R-14 fractions of Bauer-McNett fibre length classification are also included. such as those encountered during chipping, screw pressing and plug-screw feeding of chips, and defibering. Fibres in pulps are exposed to bending and axial compression in operations such as pumping, mixing, dewatering, fluffing and refining, where the pulp suspension is sheared. The effect of shearing is greater when the pulp consistency is higher. This is because the stresses get transferred directly from fibre to fibre [17]. Although medium (10-15%) or high (15-35%) consistency mechanical treatments are deliberately used to induce dislocations and microcompressions for enhancing the stretch-potential of pulps, the treatments unfortunately also induce many large-scale deformations in the fibre wall. These include nodes or crimps (Fig. 12). These are regions of bending and compressive failures, with a highly localized compressive strain, often associated with delamination of the fibre wall [17]. Fibres have a tendency to develop kinks at these sites, so the direction of the fibre axis changes abruptly at these points and the fibres become curly. While deformations such as dislocations, crimps and kinks increase the stretch-potential of fibres, they decrease the sheet tensile strength. This is because in a fibre network, while straight fibre seg :2 (2005) T 32 Pulp & Paper Canada

3 FIG. 7. Photomicrograph showing slip planes and dislocations. FIG. 8. Schematic representation of a dislocation. FIG. 9. Photomicrographs showing microcompressions in fibres: (a) Softwood chemical pulp, (b) Softwood mechanical pulp. ments are ready to transmit load from one bond to another, segments with deformations do not do so until the deformation is straightened. This creates uneven stress distribution in the network, leading to stress concentration, and premature sheet failure. The extent to which stress is poorly distributed depends on the severity of deformations. Fibres can be straightened and the influence of deformations can be diminished by subjecting them to tensile stresses and swelling without excessive loss of axial compression [19]. This is accomplished by low consistency beating or refining. The latency removal serves a similar purpose for mechanical pulps [17]. Let us now consider some examples of the increase in fibres stretch-potential by mechanical treatment. Figure 13 shows tensile strength - stretch plots for two pulps in which bonding between fibres was increased with increased wet pressing during sheetmaking. The original pulp was a laboratory-made, low-yield unbleached kraft of black spruce. The pulp was kept never-dried at 5% consistency so that fibres remained straight, and any incidental mechanical treatment that could induce deformations in fibres was carefully avoided. A part of this pulp was curlated at 20% consistency in a Hobart kitchen mixer. This treatment induced considerable axial compression in the fibres. Figure 13 shows that the curlated fibres produced highly extensible sheets at a given tensile strength. But at a given stretch, the sheets were weaker in tensile strength. This is because the mechanical treatment also induced large-scale deformations in fibres, which decreased the sheet tensile strength. Beating pulps at high consistencies increases shear between the fibres, thus increasing opportunities for axial compression. Figure 14 shows data from the literature where the beating consistency in a PFI mill was increased from 5 to 35% [20]. A softwood kraft pulp was beaten over a range of levels at each consistency. The sheet stretch at a given tensile strength was higher when the beating consistency was higher (Fig. 14). Axial compression has also been considered responsible for the higher sheet stretch of refiner mechanical pulps compared to stone groundwoods (Fig. 15) [21]. Since refiner pulps are made at a much higher consistency than stone groundwoods, their fibres are more compressed, and therefore more extensible. Pulp fibres can receive unintended axial compression during FIG. 10. Typical stress-strain curves for longleaf pine holocellulose single fibres dried under different axial compressive strains [16]. The axial compressive strain was induced as follows. Wet fibres were aligned between two prestrained porous rubber sheets. The rubber sheets were firmly held together under pressure, and were then allowed to contract. Fibres were dried while held. The prestraining varied between 0 and 20%. FIG. 11. Relationship between axial compressive strain and elongation at break for single fibres. Data of Dumbleton [16]. the discharge of a mill digester [23]. This is confirmed in Fig. 16 where handsheets made from a softwood kraft mill s brown stock were compared with a laboratory kraft pulp that was prepared from the same chip supply and to a similar kappa number as that of the mill pulp. The mill digester was a Kamyr single-vessel hydraulic, operating in conventional mode. The laboratory pulp was made in a 20L indirectly-heated vessel with forced liquor circulation. The bonding in the sheets in Fig. 16 was changed by increasing the wet-pressing pressure during sheetmaking. The sheets made from the mill pulp were more extensible at a given tensile strength. Fibres also receive unintentional mechanical treatment during their passage along the fibre lines of pulp mills including Pulp & Paper Canada T :2 (2005) 35

4 FIG. 12. Photomicrograph showing crimps in fibres. FIG. 13. Handsheet tensile strength against stretch for unbleached softwood laboratory kraft pulps at wet-pressing pressures varying from 70 kpa to 6 MPa. FIG. 14. Handsheet tensile strength against stretch showing the effect of beating consistency. The pulp was beaten over a range of PFI mill revolutions at each consistency. [20]. bleach plants [24,25]. This treatment is imparted by pumps and mixers. The treatment increases the stretch-potential of fibres as shown in Fig. 17. The results were obtained on pulp samples taken at various stages in the bleach plant; sheets from each sample were made at different bonding levels by changing the wet-pressing pressure during sheetmaking. Figure 17 shows that at a given tensile strength, sheets made from the pulp machine couch trim were considerably more extensible than those made from the brown stock. This is because of the cumulative axial compression fibres received during their passage through the bleach plant. The results in Fig. 17 suggest that if a higher stretch-potential in the unbleached pulp is desired, it can be achieved by simply passing the pulp through the bleach plant while withholding bleaching chemicals. It is interesting to note that when the pulps of Fig. 5 were bleached in the laboratory, their sheet stretch at 9.5 km tensile strength increased (Fig. 18). This is ascribed to the unintended axial compression the fibres received during fluffing and mixing stages of the laboratory bleaching. This increased the pulps stretch-potential. Although PFI mill beating at 10% consistency had straightened fibres, sufficient compression in bleached pulps remained to increase the stretch against those of the unbleached pulps. B. Axial compression of fibres by sheet shrinkage during drying Fibres are axially compressed by shrinkage forces during sheet drying. This happens as follows. During the initial stages of sheet drying, as water recedes from the inter-fibre spaces, bonds are formed at fibre-fibre crossings. With further drying, as water evaporates from the fibre wall, the wall begins to shrink. Note that the fibre wall, being an anisotropic structure, has natural shrinkage in the transverse dimensions much greater than that along its length. Since fibres are now already bonded at fibre-fibre crossings, they cannot move relative to one another. When the fibres shrink, the transverse shrinkage of one fibre compresses its crossing fibre in the axial direction, forming microcompressions. This happens at every fibre crossing. Thus, every fibre in the sheet is shortened lengthwise; the sheet shrinks. The sheet can shrink as much as the transverse shrinkage of fibres [26, 27]. If the sheet is restrained from shrinking in any particular direction, the degree of axial compression in that direction is correspondingly reduced. If the wet sheet is subjected to tensile strain or draw, the stretch-potential already existing in the fibres may be pulled out. It is clear that the shrinkage of the sheet during drying increases the stretch-potential of the fibres, and therefore the sheet extensibility. Sheets dried without restraint shrink more than those dried fully restrained. The greater the sheet shrinkage, the higher the sheet stretch (Figure 19). The following examples further demonstrate the relationship between sheet shrinkage and stretch. Figure 20 shows the relationship between sheet shrinkage and stretch for handsheets made from an unbleached, commercial softwood kraft pulp, dried without restraint [28]. The pulp had been given various treatments that included medium consistency refining at 12 and 17% consistency, PFI mill beating, and treatment in a Frotapulper at 32% consistency followed by low-consistency refining. Thus, some of the pulp samples had a high stretch-potential to begin with. The sheet stretch was higher, the greater the sheet shrinkage. The data of Silvy in Fig. 21 demonstrates that increased wet draw decreases sheet stretch [9]. Figure 22 shows the data of Gates and Kenworthy for sheets made under various papermaking conditions from a beaten softwood sulphite pulp [29]. The cross-direction shrinkage was much more than the machine-direction shrinkage, because of the lack of restraint in the cross direction. It is important to note in Fig. 22 that the relationship between sheet shrinkage and stretch is almost independent of fibre orientation. The data of Setterholm and Kuenzi in Fig. 23 also shows the effect of fibre orientation on sheet stretch is almost absent [30]. Htun and Fellers have shown that for sheets dried fully restrained, stretch was identical for machine and cross directions and independent of fibre orientation [31]. Clearly, it is the extent of sheet shrinkage that is relevant for sheet stretch and not the orientation of fibres in the sheet. Recall that during drying, the sheet structure is set as soon as the bonds form at the fibre-fibre crossings. This happens before water begins to evaporate from the fibre walls. Water removal from the walls initiates the shrinkage mechanism. Therefore, fibre orientation is already established when the sheet shrinks. Figure 24 shows the relationship between newsprint machinedirection stretch and machine speed for two sets of furnishes [32]. It is known that the higher the machine speed, the higher the tension exerted on the sheet in the machine direction particularly for machines without web supports. The higher the web tension, the lower the sheet shrinkage, the lower the sheet :2 (2005) T 34 Pulp & Paper Canada

5 FIG. 15. Handsheet tensile strength agains stretch for laboratory mechanical pulps [21]. At a given tensile strength, the refiner pulps, which contain more microcompressions, have a higher stretch. Data of de Montmorency [22]. FIG. 16. Handsheet tensile strength agains stretch for mill and laboratory-made kraft pulps from the same chip supply. FIG. 17. Handsheet tensile strength against stretch for pulps obtained at various stages from a kraft pulp mill bleached plant. The dashed line joins results obtained at the standard 414 kpa wet-pressing pressure. FIG. 18. Results on bleached pulps of Figure 5. stretch. It is also apparent in Fig. 24 that sheets containing TMP, RMP or CTMP had higher residual stretch than those containing groundwood and high-yield sulphite pulps. This is because pulps such as TMP etc. were made at high consistency, and therefore had fibres with higher stretch-potential. It should be noted the relationship between sheet shrinkage and stretch is not unique, but depends on the moisture content of the sheet at which changes from restraint-free drying to restrained drying or vice versa are made [33]. Figure 25 shows the relationship between strain to failure and shrinkage for a series of samples dried under two different drying strategies RF and FR [33]. In the RF strategy, drying was initiated by restrained drying to a given solids, followed by free shrinkage. In the FR strategy, the sample after wet pressing was allowed to dry restraint-free to a given solids, followed by restrained drying. Clearly, for a given shrinkage, the strain to failure depended on the drying strategy employed (see also [34]). In fact, what determines the outcome is the amount of water left to be evaporated from the sheet when changes are made to the restraints. The samples for these experiments were taken from 90 g/m 2 isotropic handsheets of an unbleached softwood kraft pulp, Valley beaten to 22 SR. Factors that control sheet shrinkage In the absence of restraint, sheet shrinkage is affected as follows [27]. The first factor is the transverse shrinkage of fibres. The fibres shrink more when initially more swollen. The swelling in turn depends on the chemical composition of the fibre wall, the drying history of the fibres, the extent of mechanical treatment imparted, and the chemical environment surrounding the fibres [35-38]. Since lignin inhibits swelling, mechanical pulps swell much less than chemical pulps. Hemicellulose-rich chemical pulps tend to swell more because of the greater affinity of hemicellulose for water. Mechanical treatment beating or refining swells fibres by internally delaminating the fibre wall. At a given level of mechanical treatment, never-dried fibres swell more than once-dried or recycled fibres. Fines enhance sheet shrinkage as they are more swollen than fibres. The second factor affecting sheet shrinkage is the extent of fibre-fibre bonding. The greater the bonded area between fibres, the more effective the shrinkage stress transmission between the fibres at the crossings. Thin-walled fibres provide more fibre surface area for bonding per unit fibre mass than thick-walled fibres; they are more conformable as well, form- Pulp & Paper Canada T :2 (2005) 37

6 FIG. 19. Stress-strain curves for handsheets dried with and without restraint. FIG. 20. Relationship between extension at break and shrinkage of freely-dried handsheets of an unbleached, never-dried, commercial softwood kraft pulp. The pulp had been given various high, medium and low-consistency refining treatments. Data of Taraldrud [28]. FIG. 21. Relationship between extension at break, sheet shrinkage and wet draw. Data of Silvy [9]. FIG. 22. Relationship between extension at break and sheet shrinkage for a bleached softwood sulphite pulp beaten to 50 SR [29]. ing larger contacts at crossings. Fibre conformability is also enhanced by swelling. The third factor is the axial compressive resistance of the fibre wall. Thickwalled and lignin-rich fibres offer more axial compressive resistance. Beating or refining decreases fibre wall rigidity and high-consistency refining is particularly effective, as it induces microcompressions in the fibre wall. C. Enforced sheet shrinkage during drying Sheet stretch is greatly increased if the wet sheet, in addition to its natural shrinkage during drying, is forced to shrink more by applying in-plane compressive forces while keeping the sheet restrained from buckling with normal forces. The sheet is compacted and the fibres are further microcompressed. Papers produced by this process have high extensibility often desired for multiwall sacks. In one form of this (Clupak) process, an undried sheet at a suitable moisture content is fed into a nip formed by a heated steel roll and a soft rubber-covered roll. The rolls are independently driven with the surface speed of the soft roll several percent slower than the steel roll. The rubber is stretched in front of the nip. The sheet carried by the steel roll surface enters the nip, adheres to the stretched rubber, slides over the steel surface and is forced to contract with the rubber in the nip. Thus, the sheet is compacted in the machine direction before it leaves the nip. The compaction can be several percent and can be varied by adjusting the speed difference between the two rolls [39,40]. Typical load-elongation curves for commercial compacted papers are shown in Fig. 26. ROLE OF FIBRE S INTERFIBRILLAR MATRIX It is now clear that the origin of a fibre s stretch-potential lies in the misalignment of cellulose fibrils in the fibre wall with respect to the fibre axis. The misalignment may be natural, as in the case of fibres with high fibril angle, or induced by axial compression of the fibre wall. When the fibre is strained in tension, the interfibrillar matrix shears. At a certain shear stress the matrix begins to flow, allowing the misalignment to be drawn out (Figs. 27, 28). The microcompressions may be pulled out; the fibril angle may decrease. Since the matrix consists of hemicellulose and lignin, it is made more mobile by increasing the moisture content or temperature, or decreasing the strain rate. A more mobile matrix increases the sheet stretch. The effects of these factors will depend on the chemical composition of the fibre wall; further discussion of the effects is available in the literature [42-47]. IMPLICATIONS For a given fibre furnish, the two factors that allow a papermaker control over sheet stretch are mechanical treatment of :2 (2005) T 36 Pulp & Paper Canada

7 FIG. 23. Strain at failure as a function of restraint during drying for laboratory-made sheets with different fibre orientations. Data of Setterholm and Kuenzi [30]. FIG. 24. Relationship between newsprint machine-direction stretch and papermachine speed [32]. FIG. 25. Relationship between strain to failure and shrinkage for sheets dried under RF and FR drying strategies. The numbers against the data points refer to the solids content at which the drying restrains were changed [33]. FIG. 26. Typical stress-strain curves for commercial compacted papers [39]. FIG. 27. Shear in the interfibrillar matrix of a high fibril angle under tensile strain [12]. fibres and the extent of shrinkage during sheet drying. While medium or high-consistency mechanical treatments will increase a pulp s stretch-potential, increased wet draw or tension during sheet drying can have the opposite effect. These two factors not only affect sheet stretch, they also influence many other sheet properties [17, 34, 48, 49]. For most papermaking applications, commercial bleached chemical pulp fibres generally have sufficient stretchpotential. This is because of the incidental mechanical treatment they receive during pulping and bleaching. Unfortunately, this occurs usually at the expense of fibre straightness, with a consequent loss in sheet tensile strength. Therefore, fibres have to be given low-consistency refining which subjects them to tensile forces and swells them. Fibres become more conformable and straighter [19]; the sheet tensile strength is mostly recovered. Low-consistency refining is generally carried out at 2-6% consistency. A survey of the literature shows that even at these FIG. 28. Shear in the interfibrillar matrix of a microcompressed fibre under tensile strain [41]. low consistencies, the refining consistency has an effect on sheet stretch [15]; the handsheet stretch of chemical pulps was found to be higher when the refining consistency was higher. It seems that fibres retain more of their stretch-potential when refined at a higher consistency. We have confirmed this by refining a commercial, never-dried, bleached softwood kraft pulp at 2 and 4% consistency in our laboratory Escher-Wyss refiner. The refining intensity and energy in each case were 3 J/m and 100 kwh/t respectively. The Pulp & Paper Canada T :2 (2005) 39

8 pulp refined at 4% consistency had about 10% higher sheet stretch. It appears that opportunities to modify sheet stretch exist even in low-consistency mechanical treatments of pulps (see also [50]). EXPERIMENTAL The standard procedures of PAPTAC were generally followed for handling pulps, and for measuring their physical properties. ACKNOWLEDGEMENTS Ingunn Omholt, Paul Shallhorn and Tetsu Uesaka are thanked for their helpful comments. REFERENCES 1. SANBORN, I.B. and DIAZ, R.J. The stress-stain toughness test for paper and paperboard. Tappi 42 (7): 588 (1959). 2. BÖHMER, E. The analogy between burst testing and conical shells. Norsk Skogind. 16 (9): 382 (1962). 3. SETH, R.S. and PAGE, D.H. Fracture resistance: A failure criterion for paper. Tappi 58 (9): 112 (1975). 4. SETH, R.S. Optimizing reinforcement pulps by fracture toughness. Tappi J. 79 (1): 170 (1996). 5. BARNET, A.J. and HARVEY, D.M. Wet web characteristics and relation to wet end draws. Pulp Paper Can. 81 (11): T306 (1980). 6. MANNSTRÖM, B. Influence of groundwood quality on runnability and printability. Tappi 55 (4): 551 (1972). 7. ELPHICK, J. Establishing control in offset newspaper production. Paper Technol. 12 (4): 316 (1971). 8. UESAKA, T., FERAHI, M., HRISTOPULOS, D., DENG, N. and MOSS, C. Factors controlling pressroom runnability of paper. Science of Papermaking, (C.F. Baker, Ed.), Pulp & Paper Fundamental Research Society, Bury, Lancashire, U.K., 2001, p SILVY, J. Effects of drying on web characteristics. Paper Technol. 12 (6): 445 (1971). 10. NORDMAN, L.S. Laboratory investigations into the dimensional stability of paper. Tappi 41 (1): 23 (1958). 11. SETH, R.S., JANTUNEN, J.T. and MOSS, C.S. The effect of grammage on sheet properties. Appita 42 (1): 42 (1989). 12. PAGE, D.H. and EL-HOSSEINY, F. The mechanical properties of single wood pulp fibres. Part VI. Fibril angle and the shape of the stress-strain curve. J. Pulp Paper Sci. 9 (4): TR99 (1983). 13. HORN, R.A. Morphology of wood pulp fiber from softwood and influence on paper strength. USDA Forest Service Research Paper FPL 242, Forest Products Laboratory, Madison, WI, GURNAGUL, N., PAGE, D.H. and SETH, R.S. Dry sheet properties of Canadian hardwood kraft pulps. J. Pulp Paper Sci. 16 (1): J 36 (1990). 15. PAGE, D.H. The axial compression of fibres - A newly discovered beating action. Pulp Paper Mag. Can. 67 (1): T2 (1966). 16. DUMBLETON, D.F. Longitudinal compression of individual pulp fibers. Tappi 55 (1): 127 (1972). 17. PAGE, D.H., SETH, R.S., JORDAN, B.D. and BARBE, M.C. Curl, crimps, kinks and microcompressions in pulp fibres - their origin, measurement and significance. In Papermaking Raw Materials, (V. Punton, Ed), Mechanical Engineering Publications Ltd., London, 1985, p PAGE, D.H., EL-HOSSEINY, F., KIM, C.Y., WIN- KLER, K., BAIN, R., LANCASTER, P. and VON- DRAKOVA, M. The behaviour of single woodpulp fibres under tensile stress. In Fundamental Properties of Paper Related to its Uses, (F. Bolam, Ed), Tech. Div., BP & BIF, London, 1976, p SETH, R.S. Zero-span tensile strength of papermaking fibres. Paperi Puu 83 (8): 597 (2001). 20. WATSON, A.J., PHILLIPS, F.H., BAIN, R.B. and VENTER, J.S.M. Beating at high concentrations in the PFI mill. Appita 20 (2): 47 (1966). 21. PAGE, D.H. and SETH, R.S. The extensional behaviour of commercial mechanical pulps. Pulp Paper Can. 80 (8): T235 (1979). 22. DE MONTMORENCY, W.H. The relationship of wood characteristics to mechanical pulping. Pulp Paper Mag. Can. 66 (6): T325 (1965). 23. MACLEOD, J.M. and PELLETIER, L.J. Basket cases: kraft pulps inside digesters. Tappi J. 70 (11) 47 (1987). 24. DEGRÂCE, J.H. and PAGE, D.H. The extensional behavior of commercial softwood bleached kraft pulps. Tappi 59 (7): 98 (1976). 25. MACLEOD, J.M., MCPHEE, F.J., KINGSLAND, K.A., TRISTRAM, R.W., O HAGAN, T.J., KOWALSKA, R.E. and THOMAS, B.C. Pulp strength delivery along complete kraft mill fiber lines. Tappi J. 78 (8): 153 (1995). 26. PAGE, D.H. and TYDEMAN, P.A. A new theory of the shrinkage, structure and properties of paper. In Formation and Structure of Paper, (F. Bolam, Ed), Tech. Sect. BP & BMA, London, 1962, p PAGE, D.H. The structure and properties of paper: Part II. Shrinkage, dimensional stability and stretch. Trend No. 18, PAPRICAN, Spring TARALDRUD, O. Medium consistency refining. EUCEPA/ATICELCA 22nd Conference (Florence), Development & Trade in Science & Technology of Pulp & Papermaking, Volume 1, Paper No. 15 (October 6-10, 1986). 29. GATES, E.R. and KENWORTHY, I.C. Effects of drying shrinkage and fibre orientation on some physical properties of paper. Paper Technol. 4 (5): 485 (1963). 30. SETTERHOLM, V. and KUENZI, E.W. Fiber orientation and degree of restraint during drying. Tappi 53(10): 1915 (1970). 31. HTUN, M. and FELLERS, C. The invariant mechanical properties of oriented handsheets. Tappi 65(4): 113 (1982). 32. PAGE, D.H. and HOWARD, R.C. The influence of machine speed on the machine-direction stretch of newsprint. Tappi J. 75 (12): 53 (1992). 33. HTUN, M. and DE RUVO, A. The influence of drying strategies on the relationship between drying shrinkage and strain to failure of paper. In Role of Fundamental Research in Papermaking, Mechanical Engineering Publications, Ltd., London, 1983, p HTUN, M. and DE RUVO, A. Implication of different drying strategies for the mechanical properties of paper. II Latin-American Congress on Pulp and Paper, Torremolinos, Malaga, Spain, June 22-26, 1981, pp EMERTON, H.W. Fundamentals of the Beating Process, BP & BIRA, Kenley, STONE, J.E. and SCALLAN, A.M. Influence of drying on the pore structure of the cell wall. In Consolidation of the Paper Web, (F. Bolam, Ed), Tech. Sect., BP & BMA, London, 1966, p SCALLAN, A.M. The accommodation of water within pulp fibres. In Fibre-Water Interactions in Papermaking, Tech. Div., BP & BIF, London, 1978, p LINDSTRÖM, T. Chemical factors affecting the behavior of fibers during papermaking. Nordic Pulp Paper Res. J. 7 (4): 181 (1992). 39. WELSH, H.S. Fundamental properties of high stretch papers. In Consolidation of the Paper Web, (F. Bolam, Ed), Tech. Sect., BP & BMA, London, 1966, p IHRMAN, C.B. and ÖHRN, O.E. Extensible paper by the double-roll compacting process. In Consolidation of the Paper Web, (F. Bolam, Ed), Tech. Sect., BP & BMA, London 1966, p SETH, R.S. and PAGE, D.H. The stress-strain curve of paper. In Role of Fundamental Research in Papermaking, (J. Brander, Ed), Mechanical Engineering Publications, Ltd., London, 1983, p WINK, W.A. The effect of relative humidity and temperature on paper properties. Tappi 44 (6): 171 A (1961). 43. BENSON, R.E. Effect of relative humidity and temperature on tensile stress-strain properties of kraft linerboard. Tappi 54 (5): 699 (1971). 44. SALMEN, L. Responses of paper properties to changes in moisture content and temperature. In Products of Papermaking, (C.F. Baker, Ed), Pira International, Leatherhead, Surrey, U.K., 1993, p BROUGHTON, G. and MATLIN, N.A. The mechanical behavior of paper - Part I. Tappi 34 (11): 493 (1951) 46. DAVISON, R.W. The weak link in paper dry strength. Tappi 55 (4): 567 (1972) 47. KIMURA, M., USUDA, M. and KADOYA, T. Moisture dependence of impact rupture properties of paper. In Fibre-Water Interactions in Papermaking, Tech. Div., BP & BIF, London, 1978, p Papermaking Science and Technology, Book 9, Papermaking Part 2, Drying. Chapter 10, FAPET, Helsinki, Papermaking Science and Technology, Book 16, Paper Physics, Chapters 2, 5-8, FAPET, Helsinki, LUNDIN, T., LÖNNBERG, B., SOINI, P. and HAR- JU, K. Laboratory LC refining of SBK pulps - Effects of consistency and dispersion. 7th Pira International Refining Conference, Stockholm, March 25-26, 2003, Paper #5. Résumé: L allongement de la feuille dépend principalement de deux facteurs: le potentiel d allongement des fibres et leur degré de cohésion. Une meilleure cohésion offre une meilleure résistance à l allongement. Le potentiel d allongement des fibres est en rapport avec le degré de désalignement des fibrilles de cellulose dans la paroi de la fibre par rapport à l axe de la fibre. Ce désalignement peut être naturel, comme pour les fibres dont l angle des fibrilles est élevé, ou entraîné par la compression axiale de la paroi des fibres. Le désalignement est amplifié lorsque la fibre est soumise à la traction. La compression axiale est entraînée surtout par le traitement mécanique des pâtes, et par la force de rétrécissement pendant le séchage de la feuille. Pour une composition de fabrication donnée, le contrôle de ces deux procédés faisant partie de la fabrication du papier (traitement mécanique et rétrécissement) permet de contrôler l allongement de la feuille. Reference: R.S. SETH. Understanding sheet extensibility. Pulp & Paper Canada. 106(2) 2005: T33-38 (February, 2005). Paper presented at the 90th Annual Meeting in Montreal, QC, Canada, Jan 26, Not to be reproduced without permission of PAPTAC. Manuscript received March 29, Revised manuscript approved for publication by the Review Panel on May 27, Keywords: PAPER SHEETS, STRESS-STRAIN PROPERTIES, STRETCH, BONDING, FIBRE STRUCTURE, FIBRILS, SHRINKAGE, REFINING, COMPRESSION, DEFORMATION :2 (2005) T 38 Pulp & Paper Canada