Fiber and bonds
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Fiber and Bonds. Introduction. Fibers retain most of their original structure during pulping and papermaking. Their properties determine the properties of paper. Bonding between fibers is also very important. Without bonding, there would be no network.

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Fiber and bonds

Fiber and Bonds


Introduction

Introduction

Fibers retain most of their original structure during pulping and papermaking.

Their properties determine the properties of paper.

Bonding between fibers is also very important.

Without bonding, there would be no network.

During papermaking, bonding arises from the intrinsic tendency of cellulosic fibers to bond to each other when dried from water or other polar liquid.


Fiber and bonds

  • With the obvious importance of fibers and bonds, it is misleading to view paper as a collection of inert fibers attached to one another.

  • The properties of fibers and bonds in paper are intimately connected with the network structure that they form.

  • The mechanical properties of fibers cut from a paper sheet differ from the properties of the same fibers dried individually.


Fiber and bonds

  • The connection between the network structure and the microscopic properties of fibers and bonds develops during paper drying as internal stresses generate in fiber networks.

  • These internal stresses are local and act within and between fibers.

  • Macroscopically, the net stress is zero.

  • The internal stresses depend on the anisotropic shrinkage potential of the fibers and on the macroscopic stresses applied during paper drying.


Fiber properties

Fiber Properties

  • These affect the formation and consolidation of paper structure during the papermaking process.

  • They a responsible for the properties of dry paper.

  • Raw materials determine the range of fiber structures and dimensions that the pulping process modifies.


Fiber and bonds

  • Papermaking pulps consist of mechanical and chemical pulps that have different effects on paper properties.

  • Paper structure depends on fiber dimensions and the mechanical properties of wet fibers.

  • The mechanical properties of dry fibers and paper depend on the stresses under which they were dried.


Raw materials

Raw Materials

  • Paper consists of wood fibers, but nonwood fibers can be important in some specialty papers.

  • Fiber properties very significantly with wood species, growth site, pulping and papermaking treatments.

  • Because of the stochastic nature of these variables, the properties of the pulp may have a wide distribution.


Fiber and bonds

  • Figure 1 illustrates the cell wall of a wood fiber.

  • Often the P and S1 layers are lost in the pulping process.

  • The native structure of

    the fiber and the

    resulting changes to it

    during pulping

    determine how much

    the fiber swells in

    water and how

    conformable it is.

  • The final structure of

    the fibers depends on the papermaking process.


Fiber and bonds

  • If the cell wall is rigid enough, the fiber lumen may survive uncollapsed.

  • The S2 layer is usually the thickest and dominates mechanical properties.

  • In the S2 layer micro fibrils spiral around the fiber axis at a constant fibril angle.

  • The fibrils are small bundles of primarily cellulose molecules.

  • The values of the fibril angles vary, even within a given wood species, but the the chirality of the spiral is always right handed.


Fiber and bonds

  • The S1 and S3 layers also have a spiral fibril structure, but they contribute much less to mechanical properties than S2.

  • In the cell wall, a matrix of amorphous hemicellulose and lignin surround the fibrils.

  • This matrix is ductile compared to the stiff fibrils.

  • The fibrils give the fiber its elastic modulus and tensile strength in the axial direction.

  • Strength improves as the fibril angle decreases.

  • Fibers are plant cells that have a high aspect ratio, as shown in Table 1 on the next slide.


Fiber and bonds

  • The pulp plants have other cells that may strongly influence the pulp quality.

  • For some trees, such as beech, only 40% of the volume is fibers.

  • These nonfiber cells, such as parenchyma cells and vessel elements can have harmful effects on pulping and bleaching processes.

  • They also effect the linting and printability of paper.

  • The absolute length of fibers is one of the most important characteristics of papermaking fibers.


Fiber and bonds

  • A long fiber can have can have more bonds with other fibers and therefore will be held more strongly than a short fiber.

  • The tensile strength of the wet web increases rapidly with fiber length.

  • Likewise, tensile strength, breaking strain and fracture toughness also increase with fiber length.

  • Hardwood fibers are shorter than softwood fibers.


Fiber and bonds

  • Figure 2 shows that hardwood fibers also have a narrower length distribution.

  • It is common to

    express the average

    fiber length as the

    length weighted

    average length.

  • This choice is

    motivated by

    geometric arguments.

  • The probability of two fibers crossing is proportional to the the mean value of fiber length squared.


Fiber and bonds

  • The fiber length times the perimeter of the cross-section defines the outer surface area of a fiber.

  • This outer layer can form inter-fiber bonds or scatter light when not bonded.

  • The cell wall thickness, df, and fiber perimeter, Pf, determine the cross-sectional area of dry cell wall, Af.

  • For circular cross-sections we have the following relationship;

    Af=(Pf-pdf)df


Fiber and bonds

  • This also holds for fibers with noncircular cross-section, providing the cell wall cross-sectional area doesn't change after collapse.

  • Fiber coarseness wf, gives the dry fiber mass per unit length.

  • It is equal to the cross-sectional area times the density of the cell wall.

  • Coarseness tends to

    correlate with fiber

    length in the sense

    that long fibers are

    coarser than short fibers, as shown in Figure 3.


Fiber and bonds

  • Fiber coarseness divided by the fiber width gives the basis weight, bf, of fibers.

  • Basis weight of fibers varies from 3-10 g/m2.

  • For shortwood fibers, the basis weight and coarseness are often related as suggested by Table 2 below.

  • Note that fiber dimensions are usually measured in the wet state.


Fiber and bonds

  • This has little significance for fiber length, but the cross-sectional dimensions of dry fibers are smaller than those of wet fibers.

  • The basis weight of fibers in paper are generally larger than those in Table 2, which are based on wet fibers.

  • The basis weight of fibers is important, because the coverage of paper is equal to b/bf, as we saw in Chapter 1.

  • Likewise the total length of fibers in a sheet of area A is given by

    L=bA/wf


Fiber and bonds

  • Thus, a cm2 of a paper sheet has a total fiber length of 10-100 m, corresponding to 10,000-100,000 fibers.

  • In areas where summer and winter seasons are clearly identifiable, softwoods and some hard woods show a seasonal variation in growth.

  • This is reflected in variation in wood density and fiber dimensions.

  • Early in the growing season, thin walled fibers, springwood or earlywood, grow with large lumen.


Fiber and bonds

  • The fiber dimensions later change gradually, with the fiber wall becoming thicker and the lumen becoming smaller.

  • Fiber perimeter may even decrease.

  • Thick-walled fibers form the summerwood or latewood portion of the growth ring.

  • The variation in coarseness between springwood and summer wood can be greater the the differences in average coarseness for different softwoods.

  • Not the entry for Scots (Scotch) pine in Table 2.


Pulping effects

Pulping effects

  • After the effects of raw materials, the effects of pulping, bleaching and beating are crucial.

  • As we know, the two important classes of pulp are chemical pulp (usually bleached kraft) and mechanical pulp.

  • The differences in fibers for these pulps are illustrated in Figure 4 below.

  • For chemical pulp,

    wood is disintegrated

    into fibers chemically

    by cooking wood chips.


Fiber and bonds

  • For mechanical pulp the process is a mechanical one as with TMP or GW.

  • Some wood species, such as Norway spruce, aspen and radiata pine can be used successfully for both chemical and mechanical pulp.

  • Others such as Scotch pine, eucalyptus and birch are work better for chemical pulping.

  • Chemical and mechanical pulping give distinctly different sheet properties.

  • Paper from bleached kraft pulp has 2-3 times the tensile strength that from mechanical pulp.


Fiber and bonds

  • On the other hand, the opacity and light scattering power form mechanical pulps are superior to those from kraft pulp.

  • This results from the contribution of fines in mechanical pulp.

  • The differences between chemical and mechanical pulps are utilized in many paper and board grades.

  • Newsprint often consists of purely mechanical pulp, while copy paper consists of purely chemical pulp.


Fiber and bonds

  • Mixtures of mechanical and chemical pulps are used in printing papers and multi-ply boards.

  • The differences between chemical and mechanical pulps are illustrated in Table 3.

  • The lignin content in mechanical pulps is approximately 30 %

    and essentially zero

    for bleached kraft pulps.

  • The low lignin content

    gives kraft fibers higher

    wet fiber flexibility,

    collapsibility and

    swelling ability.


Fiber and bonds

  • However, since the lignin and hemicelluloses are removed, chemical pulps give lower raw pulping yields as a percentage of wood mass.

  • As a result, the mechanical pulping process generates fiber fragments and other small particles called fines.

  • In chemical pulps, fines include ray cells and parenchyma cells in addition to particles coming form fiber walls.

  • 20-40% of mechanical pulp and less than 10% of chemical pulp is considered as fines.


Fiber and bonds

  • The fraction of long, intact fibers may be less than 20% in GW pulp, nearly 40% in TMP pulp and as high as 90% for chemical pulp.

  • The weight per unit length of chemical pulp fibers is half that of mechanical pulps because of the difference in yields.

  • For example a 20-80% mixture of kraft pulp and ground wood, respectively has approximately equal numbers of chemical and mechanical long fibers.


Fiber and bonds

  • Mechanical pulping produces pulp that can be used directly, but it often undergoes bleaching.

  • Chemical pulp is also bleached most of the time.

  • The principal use of unbleached pulp is in packaging boards and papers.

  • Even for packaging, the public is demanding a more attractive printable surface.

  • The bleaching process enhances brightness, of course, but also alters the mechanical properties of wet and dry fibers.


Fiber and bonds

  • The bonding ability of fibers may improve, but the tensile strength may deteriorate in bleaching.

  • Chemical pulp is beaten to optimize the contribution to mechanical properties of paper.

  • The beating of chemical pulp loosens the structure of the fiber wall (internal fibrillation).

  • It also may break fragments from the fiber wall.

  • Mechanical pulp produces similar effects.


Fiber and bonds

  • Internal fibrillation involves partial delamination of the fiber wall as shown in Figure 5.

  • The delamination increases the degree of swelling, the flexibility and conformity of the wet fiber wall.

  • This improves

    the inter fiber

    bonding and

    strength at the

    expense of

    optical properties.


Fiber and bonds

  • External fibrillation, where fibrils spread out from the fiber surface, is apparent from Figure 6.

  • External fibrillation enhances sheet consolidation and enforces bonding.


Fiber and bonds

  • The external fibrillation occurs only in aqueous suspension.

  • Removal of water on drying brings the external fibrils back to the fiber surface.

  • The pores close between the lamella and fibrils in the fibril wall.

  • Once dried pulp swells less on rewetting than before drying.

  • This irreversible loss of swelling ability, called hornification, is one way recycled fibers differ form virgin fibers.


Fiber and bonds

  • Recycled fibers are generally shorter, stiffer, more curly and more brittle than virgin fibers.

  • Dewatering tests can assess fines content and the degree of external fibrillation.

  • These tests measure how easily water drains from the papermaking pulp.

  • The Schopper-Riegler number (SR) or Canadian Standard Freeness (CSF) are the most common dewatering tests.

  • The SR number increases with beating and fines content while the CSF decreases.


Fines

Fines

  • The properties of fines differ greatly from the fiber fractions.

  • The common definition of fines is the fraction that passes through a 200 mesh screen.

  • The median size of fines is a few microns.

  • The largest fines particles are fiber fragments and the smallest are fibrils, whose size can be below 1 micron.


Fiber and bonds

  • Fines consist of cellulose, hemicellulose, lignin and extractives.

  • In chemical pulp the hemicellulose content of fines is higher than in the fiber fraction.

  • For mechanical pulps the lignin content of fines is higher than in the fiber fractions.

  • Because of their small size and large surface area, fines can bind more water and swell more.

  • Hydrophobic lignin fines and extractives in mechanical pulps reduce their ability to swell in water relative to fines in chemical pulps.


Fiber and bonds

  • The fines content strongly influences the structure and properties of the fiber network for mechanical pulps.

  • A fine stone GW pulp for magazines can have up to 50% fines, while a newsprint TMP typically are less than 25% fines.

  • The fines content in mechanical pulps grow in the order TMP>PGW>SGW when compared at the same freeness.

  • The amount of fines is lower in chemical pulps than in mechanical pulps.


Fiber and bonds

  • There are two types of chemical pulp fines, primary and secondary fines.

  • Primary fines are present in unbeaten pulps.

  • They consist of parenchyma cells from the wood.

  • Beating creates secondary fines.

  • These include lamellar and fibrillar parts of the fiber wall and some colloidal material.

  • The primary fines content of chemical pulps is usually less than 2%, but beating can increase it to 15%.


Fiber and bonds

  • Mechanical pulp fines are sometimes also divided into primary and secondary fines.

  • The primary fines result from the mechanical disintegration of wood.

  • Secondary fines result from refining of fibers.

  • The two fractions of a GW pulp are Mehistoff (Powders) and Schleimstoff (well-bonding fibrillar particles)

  • Fines have a very large specific surface area, because of their small size.

  • Refining and beating increase the surface area further.


Fiber and bonds

  • In chemical pulps, the specific surface area of primary fines is 4-5 m2/g, while that of secondary fines is 10-20 m2/g.

  • Mechanical pulp finer have 7-8 m2/g, while fibers have a value of about 1 m2/g.

  • Because of their large surface area, fines improve bonding between fibers.

  • Most fines are bonded to fibers when the paper dries.

  • Chemical pulps bond almost completely, while mechanical pulp fines retain some free area.


Wet fiber properties

Wet Fiber Properties

  • The network structure depends on collapsibility, conformability and flexibility of the wet fibers,

  • Conformable fibers bend and match the shape of each other to give a dense and well bonded network.

  • Conformability of fibers depends on their cross-sectional dimensions, internal fibrillation, chemical composition and morphology of the cell wall.


Fiber and bonds

  • Fibers have a circular or rectangular cross-section in wood that may flatten or collapse to a ribbon in the pulping and papermaking process.

  • The perimeter doesn't change with fiber collapse.

  • Fiber collapse is more common with chemical pulp fibers than with mechanical pulp fibers.Springwood fibers collapse more easily than summerwood and sulfite fibers collapse more easily than kraft fibers as shown in Figure 7 on the next slide.


Fiber and bonds

  • For uncollapsed fibers, the lumen scatters light.

  • Fiber collapse has a

    negative effect on the

    optical properties of

    paper.

  • Collapsed ribbon-like

    fibers are flexible and

    can have a higher

    bonded area than

    uncollapsed fibers.

  • Fiber collapse does

    improve strength.


Fiber and bonds

  • Wet fiber flexibility (WFF) or its inverse wet fiber stiffness is sensitive to

    conformability.

  • WFF measurements

    are usually limited to

    long fibers.

  • WFF decreases rapidly

    with increasing fiber

    thickness as shown in

    Figure 8.

  • Thick beams have a

    higher bending

    stiffness than thin.


Fiber and bonds

  • As shown in Figure 9, WFF increases with decreasing yield and increased beating.

  • Mechanical pulp fibers usually have a lower WFF than chemical pulp fibers.

  • The effect of beating

    and yield on fiber

    flexibility probably

    results from an

    increase in the

    porosity or

    delamination of the

    cell wall.


Fiber and bonds

  • Swelling degree of fibers depends on chemical composition and internal fibrillation.

  • Hemicellulose promotes fiber swelling and lignin inhibits it.

  • Chemical pulps swell more than mechanical pulps.

  • The swelling of chemical pulp fibers increases with beating.

  • The swelling is characterized by the WRV, or water retention value.

  • Its measurement gives the amount of water contained by the pulp after centrifugation.


Mechanical properties of dry fibers

Mechanical Properties of Dry Fibers

  • Figure 10 shows the wide distribution of tensile strength values.

  • This variability results from biological raw material and from the nonuniformity of pulping processes.

  • Even selection of

    fibers can cause

    biases, because one

    tends to pick long

    straight fibers for

    testing.


Fiber and bonds

  • Due to the poor statistics of single fiber measurements, the zero span strength is often taken as a measure of fiber strength.

  • Care must be exercised when doing this because the results

    can depend on other

    factors.

  • The load-elongation

    effect of single fibers

    is qualitatively similar

    to paper when the fibril

    angle is small as

    shown in Figure 11.


Fiber and bonds

  • The elastic modulus of the fiber in the axial direction is obtained from the initial linear section of the load elongation curve.

  • The nonlinearity in the load-elongation curves arises from curl and defects, such as crimps, kinks and microcompressions.

  • Fibers with no defects are nearly linearly elastic (Hookian).

  • The cell wall of a fiber can have a large number of dislocations and other inhomogeneities that reduce the elastic modulus, tensile strength and breaking strain.


Fiber and bonds

  • Natural defects, such as pits may also exist.

  • Defects arise from during chipping, pulping and refining.

  • Typical tensile strength values of wood fibers are 100-200 mN as shown in Table 4 for spring and summer wood fibers.

  • Breaking stress values of other natural cellulosic fibers are similar to wood fibers.

  • These are 300-800

    MPa for cotton and

    200-300 MPa for

    viscose.


Fiber and bonds

  • Table 4 shows that the force necessary to break a summerwood fiber is higher than that to break a springwood fiber.

  • This results form summerwood's higher cross-sectional area.

  • They have a higher breaking stress than springwood fibers.

  • The thicker S2 layer and smaller fibril angle in summerwood fibers contribute to their higher tensile strength.


Fiber and bonds

  • Fibers of high fibril angle have a higher breaking strain and lower tensile strength than those of lower fibril angle as shown in Figure 11.

  • Figure 12 shows that the elastic modulus of fibers also increases with decreasing fibril angle.

  • In pulping, gentle

    removal of lignin does

    not change the force

    required to break a fiber.

  • This implies that lignin

    and hemicellulose give

    no contribution to

    tensile strength


Effects of drying stress

Effects of Drying Stress

Jentzen effect:

  • Jentzen showed that

    drying under an axial

    tension increases

    tensile strength and

    decreases breaking

    strain.

  • This is shown in

    Figure 13.

  • The opposite occurs

    when fibers are dried under axial compression.


Fiber and bonds

  • This is shown in Figure 14.

  • The mechanism of these effects are illustrated in Figure 15.


Fiber and bonds

  • With this mechanism, the drying tension;

    • reduces fibril angles,

    • aligns molecules parallel to the external load,

    • straightens the fiber, and

    • pulls out dislocations and other defects.

  • High hemicellulose content is beneficial for the Jentzen effect.

  • The swollen state of the hemicellulose matrix is necessary for the structure and molecular rearrangement of fiber during drying.


Fiber and bonds

  • If dried and rewetted, kraft fibers dried under load show no Jentzen effect.

  • Drying causes irreversible changes in the fiber wall structure.

  • The Jentzen effect shows how much drying conditions can change fiber properties.

  • It demonstrates the importance of internal stresses generated during the drying process.


Bonding

Bonding

  • Inter-fiber bonds were studied extensively during the 1950s and 60s.

  • These studies still retain fundamental value.

  • In this section we will explain the chemical effects of bonding between wood fibers.

  • We will explain the formation of inter-fiber bonds.

  • Finally, we sill discuss the strength of these bonds.


Basic concepts

Basic Concepts

  • There are various types of interactions involving fibers that are sometimes referred to as “bonds”.

  • These are;

  • Normal chemical bonds within cellulose molecules.

  • Intermolecular Van der Waals “bonds”, much weaker than chemical bonds.

  • Entanglements of polymer chains.

  • Inter-fiber bonds: fibers are so close that any of the above may occur.


Molecular bonding

Molecular Bonding

  • Hydrogen bond a special type of almost chemical bond.

  • A covalent bond is

    typically~150-500

    KJ/mol.

  • For hydrogen bonds,

    the energy is typically

    ~8-32 kJ/mol,e.g.

    O-H6…O3 and

    O2-H…O6 , the O…H

    distance ~ 0.17 nm

    versus O-H~0.1nm.


Fiber and bonds

  • Besides hydrogen bonds, van der Waals forces are important for inter-fiber bonding.

  • The van der Waals force is typically~2-8 KJ/mol.

  • This interaction is relatively long range, extending to distances of 0.3-0.5 nm.

  • Much of the cohesion of wet paper comes form van der Waals bonds.

  • In addition covalent and ionic bonds may form between fibers and polymeric mediators.


Structure of inter fiber bonds

Structure of Inter-fiber Bonds

  • Inter fiber bonds in paper form gradually as solids content increases.

  • First, surface tension forces, such as colloidal interactions, and interlocking of fibrils pulls fibers closer together as water is removed from the wet web.

  • This is called the Campbell effect.

  • These same forces give the wet paper web its tensile strength.


Fiber and bonds

  • Fines improve sheet consolidation and bond formation because of their large specific surface area.

  • The Campbell forces increase with the surface area of the pulp.

  • Fiber-water interactions influence inter-fiber bond formation.

  • Water between two cellulosic chains forms hydrogen bonded chains.

  • As water is removed, the chains become shorter and hydrogen bonds form between cellulose molecules.


Fiber and bonds

  • Fibers shrink during drying.

  • The amount of depends on the swelling degree of the fiber wall.

  • Shrinkage in lateral direction and stiffness in longitudinal direction generate shear stresses as shown in Figure 17.

  • Shear stresses at

    inter-fiber bonds

    generate axial

    compressive forces,

    called

    microcompressions on

    the crossing fibers.


Fiber and bonds

  • Nanko and Ohsawa studied the structure of fiber bonds of a bleached kraft pulp, using a transmission electron microscope.

  • Figure 18 summarizes these findings.

  • They identified four main features;

    • Bonding layer,

    • wrinkles,

    • skirt, and

    • covering layer.


Bonding strength

Bonding Strength

  • The strength of inter-fiber bonds usually refers to shear strength.

  • Sometimes the bond strength is characterized in terms of the out of plane delaminating energy.

  • This is generally measured in a peeling test

  • Another important issue is the area of a bond.

  • The specific bond strength is defined as the ratio of bond strength and area.

  • Button modeled the bond between fibers as a lap joint as shown in Figure 19 on the next slide.


Fiber and bonds

  • The stress distribution within a bond is nonuniform when loaded in shear.

  • The highest stresses occur at the edges of the bond.

  • Button concluded that

    bond strength, or

    breaking load, is

    almost independent of

    bond length or area,

    but increases as the

    square root of

    thickness or axial

    strain modulus.


Fibers and bonds in paper

Fibers and Bonds in Paper

  • Deformations in the internal structure of fibers and bonds explain qualitatively the microscopic and macroscopic changes that occur during drying

  • These are usually explained in terms of internal stresses, dried-in stresses or dried-in strains.

  • The dried-in stresses are different in MD and CD directions because of the anisotropy of the process.

  • The effects of web tension and stretching in the MD are very important.


Internal stresses

Internal Stresses

  • During drying, the external macroscopic stresses are distributed in the fiber network largely based on local structure.

  • Wood fibers shrink in the transversal direction primarily, with little change in the longitudinal direction.

  • This transverse shrinkage causes compressive stresses in the axial direction of surrounding fibers.


Fiber and bonds

  • The compressive stresses act on each fiber as illustrated in Figure 20.

  • For a uniaxially restrained sheet, the axial shrinkage of fibers is ten times smaller than the transverse shrinkage.

  • In the lateral direction

    of the sheet, where

    macroscopic shrinkage

    is unconstrained, the

    difference is less, but

    still significant.


Fiber and bonds

  • The drying shrinkage of a fiber is an average over bonded and free segments.

  • For a bonded segment, the balance of the following three forces determines the axial compression:

    • Compressive force by the transverse shrinkage of crossing fiber,

    • Tension or compression arising from the drying stress,

    • Reaction force governed by the axial stiffness of the bonded segment.


Fiber and bonds

  • The compressive force is larger than the macroscopic drying stress and bonded fiber segments generally shrink during drying as illustrated in Figure 21.

  • For free fiber

    segments, the

    compressive force

    arises from the

    shrinkage of

    neighboring bonded

    segments.


Fiber and bonds

  • Thus, the force is smaller than that on bonded segments, because it transmits through the surrounding network.

  • Thus, free segments

    compress less than

    bonded segments.

  • Refining increases

    shrinkage, because it

    increases the swelling of

    wet fibers.

  • The resulting

    distribution of shrinkage for fiber segments is given in Figure 22.


Fiber and bonds

  • Different drying stresses on free and bonded segments affect the three-dimensional structure.

  • If sheet shrinkage is allowed, consolidation may lead to bonds where flexible fibers partially wrap around crossing fibers as illustrated in Figure 23.

  • Bonded segments

    under compression

    have a lower axial

    modulus than free

    segments due to the

    Jentzen effect.


Macroscopic drying shrinkage

Macroscopic Drying Shrinkage

  • The product of the shrinkage potential and the elastic modulus gives the compressive stresses due to drying that are balanced by external applying stresses applied to the web.

  • If the drying rate is low

    or drying temperature

    is high, the drying

    stress is low as

    illustrated in Figure 24.


Fiber and bonds

  • The applied stresses before approximately 60% of solid content have the largest effects on the elastic modulus.

  • The drying shrinkage at the edges is larger than at the center of the web.

  • Figure 25 shows examples of the shrinkage profile on paper machines.


Fiber and bonds

  • The CD drying shrinkage and the amplitude of its variation increases with refining.

  • CD elastic modulus and tensile are lower at the edges than in the center.

  • The CD breaking strain and hygroexpansivity in CD are larger than in the MD.

  • The basis weight is larger at the edges than in the center.

  • Reducing the opening of the slice in edge portion reduces the basis weight variation, but may lead to an orientation of fibers as was discussed previously.


Wet straining in paper web

Wet Straining in Paper Web

  • Stretching in MD causes a contraction in CD as shown in Figure 26.

  • Woodfree furnishes have a smaller Poison ratio than wood-containing furnishes.


Effect on network structure

Effect on Network Structure

  • Stretching tends to amplify the effects of poor formation paper.

  • This results in a decrease in the strength of the wet web.

  • The average basis weight of the paper decreases in proportion to wet strain.

  • In the thickness direction, wet pressing has caused z-directional bends or undulations in fibers that can now be pulled straight.


Fiber and bonds

  • This is illustrated in Figure 27.

  • This mechanism also

    applies to fibers that

    are aligned close to

    the machine direction.

  • However, the shape of

    CD fibers may change

    as well.

  • As fibers are

    straightened,

    thickness increases,

    RBA decreases, modulus increases in MD but decreases in CD.


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