FIBER BONDING Importance of fiber bonding
The bonding is very much important for the development of ● Mechanical strength of paper and ● Run ability of paper machine or printing press.
Paper may be made from glass fibers, ceramic fibers or synthetic textile fibers such as nylon, polyolefin or acrylics do not take fibers in bonding without the addition of adhesives bonding agents or solvents
Factors affecting the fiber bonding are the ● Physical or chemical properties of the fibers are the resultant of the contribution of - the species of tree or plant - the subsequent pulping and bleaching processes. - beating during stock preparation, - wet pressing and - calendaring on the paper machine.
In weak fibers choose stronger fiber from a different source or eliminating excessive degradation resulting from pulping and bleaching errors.
When a bonding additive was used in paper, the fiber breakage increased.
When a surfactant was added, the tensile strength was lower
In a study in which fibers were deliberately weakened by over bleaching, the conclusion was drawn that fiber strength was less important for paper strength than is inter fiber bonding.
A mathematical equation, in which the variables are quantitatively related by the choice of suitable units for the indices:
1 = 1+1 T F B T = tensile strength F = fiber strength index B = the bond strength index
Paper is a net work of crossing fibers, the fiber net work influences the formation and density of fibers, fiber orientation, and the relative bonded area and upon the geometrical and strength properties of the fiber themselves.
When bonding increases strength properties such as tensile and bursting strength increase.
Tearing strength however requires a certain minimum fiber binding after which increased fiber bonding reduces tearing strength.
Folding endurance increases as paper becomes brittle.
Both folding endurance and tearing strength are increased by longer fiber and very sensitive to fiber length as well to the degree of fiber bonding.
Opacity of paper is decreased as fiber bonding increases.
Increased beating will increase the fiber bonding, and, as result, the density.
Wet pressing slightly favors density increase over fiber bonding increase.
Calendaring increase density with little increase in fiber bonding.
Internal bonding also reduces surface ‘fuzz’ a condition caused by loose ends of fibers projecting above the surface of the paper.
One of the consequences of poorly bonded fibers in the surface of paper intended to be printed by the offset process is that the tacky inks will pull out loose surface fibers. The detached fibers adhere to the image transfer blanket and make a defect in the printing on each subsequent sheet of paper.
Theories of fiber bonding Contraction of the web on drying is due to Surface tension of the water film between fibers and Fiber shrinkage The web contracts initially because of surface tension between fibers and then contracts further as drying proceeds because of intra-fiber shrinkage.
Hydrogen bonding and water removal
Effect of surface tension on fiber bonding
Surface tension is important in bringing the fibers together during the drainage and drying of the sheets.
As water is removed, surface tension creates a tremendous force which compacts and draws the fiber into more intimate contact.
When drainage advances to the point that air forms an appreciable proportion of the medium in which the fibers are suspended, the water air interfacial area increases rapidly
The pulp fibers must approach each other within a few tenths of a nanometer to form hydrogen bond. The fibers surfaces approaches to each other when dried from liquids of high surface tension. Tremendous forces from capillary action and hydrogen bonding bring the fiber surfaces very close to gather.
Surfaces tension forces during paper drying
Swanson shows how some of these forces act. The three diagrams are in the order of decreasing water content and increasing forces: a. The web is about 8 % solids or consistency on the Fourdrinier wire in the hivacs. b. The force with a surface tension of 75 dynes/cm for water and a fiber diameter of 0.03 mm would be about 5 kPa; with fibrillation the diameter may be effectively 0.003 mm, giving a force of 50 kPa (7 psi). In the region of 20-25% solids the water becomes discontinuous and the surface tension forces decrease c. As film thickness decreases, the pressure may reach one or two hundred atmospheres.
Surface tension is so important in fiber bonding that the presence, either from poor washing or deliberate addition, of even a small amount of agents that reduce the surface tension will result in a sheet of low density.
This lower density is probably the result of the reduction of contracting forces due to reduce surface tension; however, it may also be the result of the prevention of hydrogen bonding resulting from the adsorption of surfactant which masks the polar grouping on the surface of the fibers. Lyne and Gallay proved that the strength at very low solids content was due almost entirely to the effect of surface tension by making sheets of glass fibers which are non bonding.
Strength of pulp webs with increasing Strength of fiber webs with dryness Increasing dryness Stages for fiber bonding during dewatering of wet webs
Surface tension is the principal factor in sheet consolidation in the early stages of drying up to a solid content of about 20 to 25%
During this period there is a rapid decrease in the caliper of the wet web as water is removed, with increasing air intrusion into the web of fiber and water
It is concluded that up to 20- 25 % solids, the fibers are held together by surface tension forces. Surface tension forces decrease with increasing solids but the hydrogen bonding begins and the strength increases as the water is removed.
The plateau due to reaching the maximum effect of surface tension is clearly seen. In the range of 25 to 45 % solids depending upon the type of pulp. In the low-solids area where surface tension supplies the pressure holding the fibers to gather, the resistance to tensile pull is likely to be frictional.
Types of bonds in dried paper The main forces are:
Primary valence bonds
Polar bond attraction or Van der waals’ forces that is nonpolar, non ionic polar bonds
Paper is not strong as plastic films, which are crossed linked by primary valence bonds. In quantitative terms primary valence bonds require about 30 kcal/mole for disruption compared with about 5 kcal/mole for strong hydrogen bonds, and about 2 kcal/mole for other bonds.
Close proximity of the hydroxyl group is required for hydrogen bonding to occur, approaching to each other to within 2.5 to 3.5Å that is within atomic distances. Hydrogen bonds are formed by functional groups in which the hydrogen is attached to oxygen, most importantly, but alternatively to N2 to F. The groups –OH and –NH2 are important examples.
Water is governed in its properties by hydrogen bonding. The ability of water to disintegrate paper is believed to be the results of penetration in to the web structure, followed by substitution of water to cellulose hydrogen bonds for cellulose to cellulose hydrogen bonds. The ability of water to swell cellulosic structures has the same explanation in hydrogen bonding
Partial solubility theory
A hypothesis is advanced by Urquhart proposed that cellulose CH2OH is soluble in water in certain stages
Campbell extended the idea to beaten fibres supposing that the cellulose crystallites on the surface of the fibres become partially ‘dissolved’ in water and in this state attach themselves to similar crystallites on adjacent fibres
Clark suggested the surface of well-beaten is a ‘two dimensional colloid system’ in which the fibrillate have two dimensions in the colloid range, but are anchored to the fiber in the third dimension
Voyutski proposed ‘Diffusion theory of polymer adhesion’. According to him diffusion aided by the microfibrillar nature of the fiber wall.
Nature of fiber surface
The ability of cellulose fibers to produce fiber-to-fiber bonding is dependent upon the hydrophilic nature of the fiber surface, and the consequent ability to form hydrogen bonds. The presence of hemicellose material is favourable to fiber bonding in that it improves the contact of adjacent hydroxyl- containing surfaces.
The adsorption of hydrophilic materials or surface active agents on the surface of cellulose fibers greatly decreases the amount of fiber to fiber bonding, probably because these agents are adsorbed on the fiber surface in such a way that the large hydrophilic group protrudes from the surface of the fiber and reduces the hydrophilic nature of the fiber surface
In order to soften the paper it is necessary to add long chain amines to the furnishes. The softness is due to decreased fiber to fiber bonding
The sheet forming properties of the pulp are reduced by treatment with tannic acid, probably because of the interface with the hydrophilic character of the fiber surface
Tensile strength of filter paper is much reduced when the paper was dipped in water solution of cetyldimethylbenzyl ammonium chloride and then dried but the tensile strength was unaffected when the paper was dipped in a water solution of Congo red and dried
It is concluded that cationic material was absorbed on the out side of the fibers, thus interfering with bonding, while the dye entered interstices of the cell wall and was adsorbed there, thus leaving the outer bonding surface of the fiber unaffected.
Measurement of fiber bonding and strength of bonds
Fiber bonding is the area of the fibers under contact, the number of bonds within the area of contact, and the strength of each bond. Beating is supposed to increase the bonding of fibers but there is no method for measuring fiber bonding.
Bonded area from light scattering
The reflectance of light from (white) paper is due to light scattering from the unbonded surfaces of the fibers in the paper and that as bonding is increased the light scattering, hence reflectance will decrease.
Surface area of the paper is determined by adsorption of nitrogen using the method of Brunauer, Emmet, and Teller (BET)
Swanson and Steber told that a relative bonded area could be defined, which would be the fiber area bonded divided by the area of the totally bonded area
Ingmanson and Thode explored the idea that the area of the totally unbonded fibers which was actually available for bonding could be determined by changing beating time and wet pressing in otherwise identical papers and plotting the optical scattering coefficient against the tensile strength
The curve could be extrapolated to zero tensile strength, giving a value of scattering coefficient presumably corresponding to zero fiber bonding. The relative bonded area would be then calculated from the scattering coefficients: RBA = (St-Su) where St is the scattering coefficient at zero strength and Su is the scattering coefficient at some measured strength. Bonded area could be converted to absolute values by using the calibration factor obtained from the nitrogen adsorption measurement i.e. scattering coefficient in units’ cm2/gmultiplied by 0.044.
Factor affecting fiber bonding
The degree of bonding between the fibers in a sheet depends upon: ♥ Chemical and physical nature of the fiber surface upon the manner in which the fibers have been formed in to the sheet of the paper ♥ Fibers in actual contact, the number of bonds per unit area and the number of fiber contacts and bonds per unit volume of the fibrous structure
♥ The morphology of pulp fibers, notably the cell wall thickness, spring wood and summer fibers ♥ The effect of beating and fiber morphology, chemical composition of wood in terms of hemicellulose and lignin, degree of polymerization of cellulose ♥ The presence of alumina, sizing, fillers and other additives ♥ Moisture in paper, recycling effect, wet pressing, drying and calendaring 1. Effect of beating on fiber bonding
Unbeaten pulp does not form in to strong sheet. Even small amount of beating is required to develop the tearing strength
However, primary wall is permeable to water but it does not take part bond formation. Removal of primary wall exposes secondary wall which is made up of three components (S1, S2 and S3 layers)
Drainage resistance (water removal resistance) increases
Tensile strength, tensile stiffness, burst strength, internal bonding strength and fracture toughness increases
Tear strength of softwood fibers slightly improve at first but then decreases, where as that of hardwood fibers at first significantly increases but then decreases after prolonged refining
Air permeability, bulk, absorbency, opacity and light scattering decreases
Brightness slightly decreases.
Scott bond: Brushing will enhance hydrogen bonding in z direction of the paper. It is evident from the figure the pliability property higher in pulp having more brushing action and lower in case of fiber cutting.
Effect of fiber cutting and brushing on scott bond
Tensile and Mullen properties: The burst and tensile strength both increase 2-3 times their initial values as refining proceeds. Tensile energy absorption (TEA) also increased significantly, which reflects the combined increases in tensile land stretch. In contrast to those properties that depends largely upon fiber-to-fiber bonding.
Effect of fiber cutting or brushing on tensile and mullen properties
Tear strength: The tearing resistance falls off to less than half the level for the unbeaten pulp. Mill experience supports this effect where it is difficult to obtain a good balance between tear and tensile strength when attempting to maximize both properties.
Effect of fiber cutting or brushing on tear strength
Formation: Refining will produce positive effects on sheet formation. A severe cutting action provides a more rapid improvement in formation; however, many other properties will suffer. Cutting simply makes shorter fibers from long fibers, and the effects are similar to adding more hardwood or filler to furnish. In general controlled uniform refining of stock is the most effective way to improve sheet formation, although chemical formation aids may be needed in special situations.
Effect of fiber cutting or brushing on formation
Vacuum: Higher couch vacuum results from increased refining, which means that the drainage resistance is greater. Uniform gentle refining tends to open up the fibers rather than simply reduce fiber length. The change will be much larger with brushing type refining which more effectively closes up the structure and restricts the movement of air through the web.
Effect of fiber cutting or brushing on vacuum
Steam drying: The steam requirement for drying increases as the amount of refining is increased and brushing action clearly produces the greatest change.
Effect of fiber cutting or brushing on steam drying sheet
Sheet shrinkage: The shrinkage of the fibers and sheet increases as refining increases. Refining places water on the fiber wall. When this water is evaporated, the fiber and sheet shrink
Effect of cutting and brushing on sheet shrinkage
Sheet density and bulk:
Effect of refining on density and bulk of sheet
Porosity:
More uniform, brushing treatment has a strong effect on gurley porosity. In the cutting mode, the fibers are mainly shortened in length so they do not collapse readily to ribbons
Effect of refining on porosity
Smoothness: Cutting mode refining permits some improves packing and distribution of the fibers in the sheet structure to improve sheet smoothness.
Effect of refining on smoothness
Brightness and opacity:
Brightness, opacity, and bulk (caliper) all decrease uniformly increased refining.
Effect of refining on brightness and opacity
2. Effect of conformability, plasticity and swelling upon fiber bonding
An increase in paper density will increase fiber bonding and density is governed by the degree of plasticity developed wet pressing. It depends in turn on the amount and type of beating.
However, delamination will increase plasticity and flexibility.
This is called collapse. The lumens of sulphate pulps collapsed much less then did those of sulphite pulps, which can be explained in terms of stiffer cell wall of sulphate pulps.
3. Effect of fibrillation on fiber bonding
External fibrillation, the partial removal of the fiber wall, leaving it still attached to the fiber. The external fibrillation is the peeling off of the fibrils from the fiber surface and thus the formation of fines.
Internal changes in the wall structure, dislocation within the fiber wall, including the change in fibril, and reduction of crystallinity variously described as delamination, internal fibrillation or swelling.
Over all beating increases the conformability and plasticity of fibers
4. Effect of fibrillation on fiber bonding Fines are divided in to three categories: 1. Primary fines: broken parenchymatous cells generated during pulping 2. Secondary fines Broken primary wall and microfibrillar structure 3. Broken microfibrillar structure during recycling of secondary fibers. Fines are the detached portion of the cellulosic material. Fines may consist of lignin which will have preventive effect on hydrogen bonding. They decrease the strength of paper and acts as filler
4. Effect of hemi cellulose on fiber bonding
Hemicelluloses are water soluble (having primary and secondary hydroxyl groups) and are strongly hydrophilic.
High hemicellulose content favors fiber-to-fiber bonding where as low hemicellulose content favors opacity and tear strength.
It is well established that softwood hemicellulose is much more effective than hard wood cellulose due to the presence of mannose and other hexose content.
β-cellulose is the degraded cellulose produced during cooking and is ineffective during cooking.
Alkali soluble hemicellulose increases the strength properties of paper.
Sulphite pulp wet readily but soda pulp did not when the same species used because alkali in soda process removes alkali soluble hemicellulose which causes the fibers to be brittle and fragmentize on beating instead of developing wetness.
However, wheat straw pulp contains high hemicellulose content than wood pulp still the properties are poor.
The dimensions of fiber, ratio of non-fibrous cells to fibrous cells and physical properties of fiber. Cotton is essentially 95% cellulose The non-cellulosic materials, consisting mostly of waxes, pectinaceous substances, and nitrogenous matter, are located to a large extent in the primary wall, with small amounts in the lumen.
5. Effect of lignin on fiber bonding
6. Effect of recycling on fiber bonding
Recycling results in to decrease in fiber bonding, fiber flexibility and conformability
Burst strength, zero span tensile, tensile strength and double fold decreases whereas tear strength increases due to the effect of drying on fiber stiffness. Opacity and stiffness of fiber increases
After 4th recycling fines contents increased up to 16-35 % and caused a severe reduction in drainage.
7. Effect of water on fiber bonding
Water is essential for the plasticizing of cellulose
Cellulose fibers do not spontaneously dissolve
The never dried wet strength of paper is important to paper machine speed
The rewet strength of paper is important to certain end use
It is tested at a relative humidity of 65 % and temperature 25 0C
8. Effect of alumina, sizing, fillers and fiber bonding
other additives on
Any hydrophobic substances like rosin or paraffin will result in a very great decrease in strength
However, fillers like clay and alumina are hydrophilic in nature will result in a decrease in strength
The loss in strength is less for paper made from slightly beaten or beaten pulp than for papers made from well beaten pulp. Actually rosin will increase the strength of paper if they are slightly beaten
Antifoam added to the paper machine may also reduce inter-fiber bonding
Hydrophilic colloids of which starch, proteins, vegetable gums and water soluble resins particularly polymers of acrylamide
absorbed on the fiber surface where they hold the sheet together through a fiber-to-fiber bonding
The pH slightly effect properties of paper and a very great effect on the permanence of paper
9. Effect of water on fiber bonding
Solvent bonding: Solvents or swelling agents added to gelatinize the fibers which are then bonded by pressure
Thermoplastic fibers: Added as a fiber blend, followed by heat to bond these fibers in the web
Thermoplastic powder: Fine particles (0.002 to 0.005 in) are shifted in to the web. These penetrate by gravity and are bonded at fiber intersection by heat. About 15-30 % binders are used
Printing: Thickened binder (e.g. plasticized polyvinyl acetate)is applied cross wise to the thin web
Saturation: A fluid solution of resin is applied to the web. From 15-50 % binder is used to provide a very high degree of bonding
Foaming: A foamed mixture of binder, emulsifier, foaming agent, and thickener is applied to the web and squeeze d in to it by squeeze rolls
1. What is the impact of fiber bonding on followings: (i) Opacity (ii) Fuzz 2. What is the impact of surface tension fiber bonding? 3. Describe the various views given by Urquhart, Campbell, Clark and Voyutski about partial solubility theory. 4. Tensile strength of filter paper is much reduced when the paper was dipped in water solution of cetyldimethylbenzyl ammonium chloride and then dried but the tensile strength was unaffected when the paper was dipped in a water solution of Congo red and dried. Explain why? 5. Describe various factors which affect fiber bonding. 6.
What is the impact of air permeability, bulk, absorbency, opacity and light scattering and brightness of paper?
7. Describe the impact of fiber cutting and fibrillation on Scot bond, tensile, tear and mullen properties, formation, brightness and opacity, porosity, sheet density and bulk, sheet shrinkage, steam drying and vacuum. 8. Explain the followings: (i) The pH slightly effect properties of paper and a very great effect on the permanence of paper. (ii) Antifoam added to the paper machine may also reduce interfiber bonding. (iii) Clay and alumina are hydrophilic in nature will result in a decrease in strength. (iv) Rosin or paraffin will result in a very great decrease in strength. (v) Up to 4th recycling there is a severe reduction in drainage and increase in tear strength. (vi) Recycling results decrease in fiber bonding, fiber flexibility and conformability. (vii) Wheat straw pulp contains high hemicellulose content than wood pulp still the properties are poor.
