Paper mills have targets for productivity, quality, costs and efficiency. Eucalyptus
pulps are raw materials for the manufacture of several grades of papers. For each paper grade and for each paper mill design, different may become the pulp
quality requirements. This means that there is no universal pulp, a pulp that may perform well everywhere.
Productivity means fast speed in the paper machine, fast drainage in the wet end, high consistency after wet presses, excellent consolidation in the paper web, and minimum number of paper sheet breaks along the machine. Quality implies in maximum percentage of paper in the specification ranges and minimum generation of broke. Machine operation efficiency is the dream of any paper manufacturer. He wants his machine working smoothly, at the maximum speed as possible, no breaks, no maintenance problems, and achieving the required quality in the manufactured products. The consequence of all this is that the specific unit cost is also here optimized. No doubts that a good pulp is the one able to provide good paper-machine runnability and appropriate quality in
the end product, no matter this paper product is a commodity (toilet
tissue, cut size paper, etc) or a specialty grade (industrial filters, cigarette
Some of the pulp properties are very much related to these performances. For this reason, the papermaker should keep an eye on them. Some of these properties are result of the wood quality, other depends on the conversion of wood to pulp (chipping, cooking, bleaching, pumping, blending, etc), and many are a combination of these two factors influencing the pulp quality. For example, some properties that are related to pulping and bleaching are: viscosity and degradation of cellulose chains, fiber deformations, cell wall integrity, individual fiber strengths, surface charges in fibers, adsorbed ions on pulp surfaces, etc. Some other important pulp properties are related to wood quality and pulp conversion, as the hemicellulose content in the pulp and the microfibril organization in cell wall.
The hemicellulose content depends on how high is the presence of hemicelluloses in the wood and in the ability of the pulping and bleaching processes to preserve them in the pulp fibers. The organization and integrity of the microfibrils in the cell wall depend on how aggressive the pulping process has been to the original fiber wall, and also the type of original wall in the wood (reaction wood, juvenile wood, etc).
There are other properties that are 100% dependent on the wood supply, the pulping and papermaking processes cannot modify them: fiber length, fiber wall thickness, vessel dimensions, etc. There are many pulp properties that are dependent both on wood quality and on pulping/bleaching processes. There are also many cases where many exigencies are placed on the wood quality, when the wood is not the main factor to determine these pulp qualities for paper. It is the case of properties as: WRV – water retention value, WWS – wet web strength, WFF - wet fiber flexibility, fiber bonding and individual fiber strength.
There are many other proper ties related not only to the wood quality.
For example, the fine content of the pulp. Fines in the wood are mainly parenchyma cells, but in the pulp they are also fiber and vessel fragments that are generated in operations such as wood chipping, pulp pumping, pulp dynamic mixing, pulp pressing for dewatering, etc.
THE MOST IMPORTANT EUCALYPTUS PULP AND FIBER PROPERTIES
Today, the pulp and paper laboratories are squeezed by thousands of different types of analyses. Some mills are spending so much time on measuring everything, that the speed to take decisions and actions is completely affected. The laboratory manager has fewer people, due to the constant downsizing obliged by the company’s high administration. He feels lost, but he wants to keep his area working as much as possible to satisfy his customers (paper production manager and commercial manager). Since the number and the type of analyses grow due to the Information Technology support, the time to reflect over the results decreases. The danger is that the quality of the data may also be affected. The following pulp quality parameters are key drivers to distinguish
the different eucalyptus fibrous raw materials and to allow pulp furnish optimizations:
Fiber population or the number of fibers per gram of pulp (associated to fiber coarseness)
The fiber population is related to the weight of each individual fiber, and by extension to the fiber coarseness and to the percentage of fiber wall in the fiber volume. Fiber population is a compound property derived from fiber length, and fiber coarseness. Since fiber length in eucalyptus pulps is a morphological property relatively stable (the weighted average ranges from 0.6 to 0.85 mm maximum), there is a strong correlation between fiber population (Number of fibers/gram) and fiber coarseness.
There is a number of fiber properties associated to fiber population and fiber coarseness: fiber length, cell wall thickness, cell wall cross sectional area, fiber wall fraction (ratio between cell wall thickness and fiber ray), Runkel index, fiber flexibility index (ratio between the lumen diameter and fiber diameter), fiber collapsibility, ratio fiber wall thickness and fiber perimeter, cell wall packing density, wood basic density, wet fiber flexibility, and fines content. Since fiber population and fiber coarseness are definitively some of the most important eucalyptus pulp properties, it is very important to know the type of effects they may impart to papers.
First, it is important to understand the fiber morphological characteristics related to fiber population:
Cell wall thickness (-)
Fiber length (-)
Cell wall fraction (-)
Cell wall cross sectional area (-)
It is clear, that the relationships of these above mentioned morphological characteristics with fiber coarseness are in the opposite way. It is relative simple to understand that higher coarseness in eucalyptus fibers are associated with thicker walled fibers. These fibers produce a more open and loosen paper structure. Fibers are rigid and difficult to be collapsed. Fiber bonding and fiber consolidation are not that favored with such stiff cylindrical fibers. The corresponding papers are more porous, bulkier, and more absorbent. A better consolidation of the paper web is expected with more fibers per gram in the paper structure. As a result, higher the fiber population better is the inter-linkage among fibers, and better are the properties depending on bonding (tensile, folding, surface strength, surface smooth). Paper formation also pleases high fiber population in the furnish.
The number of fiber to fiber crossings in the web at a constant paper grammage and sheet area is expected to be proportional to the fiber population. Because bonding is favored, the paper properties that do not like fiber bonding are affected negatively (bulk, porosity, water absorbency,tactile sensibility or softness, surface roughness). Coarseness in eucalyptus fibers varies from 4.5 to 11 mg/100m. Fiber population, in opposition, varies from 12 to 30 million of fibers per gram of pulp. Both properties have wide ranges of variability, and allow good selections of pulps in the market, depending on the desired end-use.
Fiber population and fiber coarseness are associated to a great number of paper properties. To simplify things, let’s inform the correlation of fiber population with some of the most important paper properties, no matter the type of paper. Obviously, the correlations for fiber coarseness will happen in the opposite way.
Fiber population correlates positively with: formation, surface smoothness, opacity, light scattering coefficient, fiber bonding, surface strength, tensile strength, burst strength, folding, air resistance, water retention value, wet fiber flexibility,wet web strength, wet web stretch, unbeaten Schopper Riegler.
Fiber population correlates negatively with: porosity, stiffness, drainage, bulk, tear strength, dimensional stability, water absorbency, paper softness.
It may be also shortly said that pulps with lower fiber population show better drainage in the wet end, and the paper sheets are more porous, bulkier, more permeable and absorbent. They are very much appreciated by papermakers because they allow faster machine speeds, as far as the furnish may impart enough strengths to the wet paper sheet.
Individual fiber strength
This fiber characteristic is very difficult to be measured in short fibers as those from eucalyptus. There are tests correlated to this strength, as the zero span, very useful for predicting pulp quality and behavior previously to send the furnish to the pulp-machines. Fiber deformations (curl and kinks), micro-fractures in the cell wall, and other fiber defects may contribute to reduction on the individual fiber strength. The consequence of reduced individual fiber strength may be reduced WWS (wet web strength) and dry paper strength properties. Zero span tests provide good indications on the fiber wall resistance.
There are different parameters that the zero span test may evaluate: wet zero span, dry zero span, wet short span, dry short span, B value. These tests provide sound and valuable correlation with fiber specific strength and also with fiber bonding ability (B value). Zero span most valuable tests related to individual fiber strength are the wet and dry zero span. These results are affected by: fiber wall integrity (+), cell wall packing density (+), fiber length (+), pulp viscosity (+) or cellulose chain degradation (-), micellar angle (-), hysteresis (+), fiber wall fraction (+), and fiber chemical composition. Although many times referred as an excellent indicator to individual fiber strength in softwood pulps, the micellar angle does apparently not play so important role for eucalyptus pulps. Most of the eucalyptus fibers have small angles, relatively close to the fiber axis.
In the S2 layer, this angle has been found ranging from 8 to 15º. The intrinsic strength of the fiber may be given either by the dry zero span and/or the wet zero span. For eucalyptus pulps, in unbeaten level, the ranges of results are the following: Wet Zero Span (from 70 to 140Nm/g) ; Dry Zero Span (from 90 to160 Nm/g). B Value is related to increase fiber bonding in the fibrous test pad. Higher the value, better the bonding (results for eucalyptus pulps varies from 1.5 to 3)
Fiber collapsibility is associated to the wet paper sheet compactability. Wet sheet compactability is referred to the readiness the fiber surfaces are adapted and conformed to each other when the wet sheet is pressed. A well conformed wet sheet has more ability to keep fibers and fillers together and conformed after paper drying. The sheet compactability makes the paper sheet denser and places the fiber walls closer to each other during paper manufacturing. As a consequence, fiber bonding is sharply improved, and the strength properties depending on bonding (tensile, burst, folding, surface strength) have their results
On the other hand, some very welcome eucalyptus pulp properties are lost: bulk, porosity, opacity, water absorption, softness, dimensional stability. Fiber collapsibility and wet sheet compactability are result of the fiber resistance to collapse. There are other chances for collapsing fibers in a paper mill, as the operations of paper drying and paper calendaring. However, the wet paper compactability has more effect on collapsing the fiber wall. Collapses convert cylindrical fibers into flat ribbon-like fibers, with better link and contact surface. The lumen collapses because the fiber structure (cell wall and fiber architecture) is unable to support or to absorb the energy applied by the papermaking pressures and forces. In most of the cases, the thick-walled fibers, with higher fiber wall fraction, are stiff, rigid, and more resistant to collapse.
Eucalyptus fiber collapsibility is also negatively related to fiber coarseness. Higher the fiber coarseness, lower is the ability of the eucalyptus fibers for collapsing. The fibers with greater potential for collapsing are usually more difficult to drain the water in the wet end of the paper-machine. Another morphological property very much related to fiber lumen collapse is the cell wall cross section area (area of cell wall in the fiber cross section or diameter). A derived morphological characteristic is the cell wall density or packing density. Itis calculated based on the fiber coarseness and the cell wall cross sectional area. The formula for this calculation is:
Cell wall density (g/cm³)= ( 10 x coarseness) / (wall cross sectional area)
Coarseness in mg/100m
Cell wall cross sectional area in micrometers
One indirect measure of fiber collapsibility is the dry paper sheet bulk (or sheet density) at a given Schopper Riegler or freeness level, or at a given tensile strength. Higher the bulk in a certain level of drainability or tensile, more resistant are the fibers to be collapsed. Higher the densities of the sheet, more collapsed are the fibers, and more compacted the paper sheet. Bulk or sheet density at a given drainability (for example 25 or 30ºSR) or tensile ( for example 50 to 70 Nm/g) and fiber coarseness, are able to give good indications of pulp behavior in relation to fiber collapsibility.
Another interesting property to be measured at a given freeness (or at a given bulk, or at a given tensile) and strongly related to fiber collapsibility and paper compactability is the dry paper porosity (or the opposite property, the air resistance).
Fiber bonding ability and paper network properties
Better the fiber bonding, better is the paper sheet cohesiveness. Cohesiveness and bonding are developed by beating (fibrillation and fiber collapsibility) and by the presence of fines and fiber debris. Bonding may be measured by the dry/wet short span (B value) technique or by other equipment for dry paper bonding tests, as the Scott bond tester or wax picking of paper surface.
Bonding is also related to a number of dry paper strengths, as tensile, burst, folding, and also to sheet apparent density. It is also related to the hemicellulose content of fibers, fiber population, fiber drainability (CSF or ºSR), fines content and fiber collapsibility.
Bonding is very much related to pulp fines content and web consolidation by pressing and collapsing fibrous materials. Higher is the sheet density, the more bonded are the fibers and other anatomical and chemical elements present in the paper structure. Low hemicellulose content and high
coarseness fibers reflect in low cohesiveness and low bonding. Instead of being “glued or linked” to each others, these types of fibers are rigid and stiff: “they tend to touch and to stick one to another, but they are not glued”. Finer fibers with lower coarseness values and higher fiber population make closer-formed and better printing oriented sheets of paper.
Wet fiber flexibility
Wet fiber flexibility (WFF) is one of the most important pulp properties. It is highly related with inter-fiber bonding (+), fiber lumen collapsing (+), water retention value (+), and bulk (-). It has been proved for several researchers that wet fiber flexibility is easily measured and gives a very good indication of paper strengths and paper optical properties. The wet fiber flexibility correlates very well with the paper conformability, paper consolidation, and fiber collapsibility. However, WFF is affected by fiber deformations and cell wall damages. WFF is the opposite property when compared to fiber stiffness.
As a measure of flexibility, the condition is that a force be applied to bend the fiber in its length. What is really very important to papermaking is that WFFcorrelates very well with Wet Web Strength and Wet Web Stretch, two very important properties for predicting machine runnability.
Wet fiber flexibility shows very good correlation with several fiber characteristics, as follows: fiber coarseness (-), cell wall thickness or cell wall fraction (-), paper tensile strength (+), paper sheet density (+), Water Retention Value (+), fiber stiffness (-), paper sheet bulk (-).
Fiber swelling and hydration
This property is very affected by the pulping and bleaching operations during pulp manufacture, and by the pulp hemicellulose content. Several properties are associated to the swelling of fibers: freeness or Schopper Riegler level, WRV - Water Retention Value, fiber charges, carboxyl groups, fines, fiber wall micro-porosity, fiber wall micro-fractures, wet fiber flexibility. Hydration of fibers has to be understood under different approaches. First, we have the cell wall water, the water that is absorbed to the fiber wall due to fiber charges and polarity (carboxylic groups, carbonyl groups, etc) and micro-porosity. Second, we have the water retained by capillarity inside the lumens.
More lumen volume a pulp has, more water will be retained in such way. Third, we have the inter-fiber water, the water that stays held between the fibers in the pulp mat. Water Retention Value is a measure of the affinity of the pulp for water. Depending on the fiber network, and on the number of fibers/gram, more or less water will be retained by the pulp. When Water Retention Value is measured utilizing a pulp pad and a centrifuge to perform the test, the resulting WRV gives a number comprising all these three types of water. It is easy to understand that pulps with fibers containing more lumen total volume, and more fibers have higher WRV results. Small pores (lumens) are able to hold water more strongly than large pores due to the greater surface tension forces associated with the small pores. As a consequence, WRV is strongly related to fiber population (+),
fiber coarseness (), cell wall fraction (-), wet fiber flexibility (+) and fiber saturation point (+). WRV is also influenced by the chemical composition of the fibers.
The hemicelluloses retains more water, thus, fibers with more hemicelluloses are able to retain more water in the fiber walls. On the other hand, most of the pulp extractives are hydrophobic. Pulps with high extractive content have poorer ability to retain water in the fiber wall. It should be well understood that WRV is a combined effect of chemistry (hemicellulose content) and physics (fiber population, total pore volume and pore size distribution). This combined effect will be responsible for the WRV result. You may eventually have a pulp with low
hemicellulose content, but with high WRV due to a huge fiber population.
The high hemicellulose content associated with high fiber population and degraded fibers (low viscosity) will lead to pulps very hard to drain in the paper machine.
These unbeaten pulps have high initial Schopper Riegler degree, proving that drainability is difficult even in the measurement of this property. Paper machine runnability is negatively affected in these conditions. There are eucalyptus pulps with high hemicellulose contents, as example the Eucalyptus globulusmarket pulps.
However, a typical feature of E.globulusis the high fiber coarseness, and low fiber population Under these circumstances, the WRV in
Eucalyptus globulus pulps does not represent problem in normal conditions.
To the papermaker, mainly the one manufacturing porous and absorbent papers, the porosity and capillarity he is concerned are those in the paper sheet. High WRV pulps may have a huge small pore volume (lumens) in the pulp pad used for testing WRV, but these pulps are not recommended to the manufacture of tissue and filter paper. The reason is understandable. These pulps have high WFF, high population of fibers, thin cell walls, high ability to have the lumens collapsed. The paper structure will be dense and compacted, unless the papermaker may have some magic procedure in the manufacture.
Excessive fiber swelling and hydration may be a problem in integrated mills, when the pulp has never been dried prior to its use in paper manufacturing. When a pulp is dried, the hysteresis phenomenon provides a substantial reduction in the WRV of the pulp. Dried pulps, with lower WRV, have a much better drainage in the wet end. However, they are somewhat more difficult to be refined, but the desired strengths are reached in a drainability level that is still convenient to the machine operation. For this reason, in many occasions in an integrated paper mill, the mill manager loves to add some broke (already dried fibers) to the furnish. He believes that some broke is required to improve the performance of the furnish. It is a completely inappropriate thinking, since broke recycling is a loop in his process: he is wasting all the value has being added to the manufacture of this paper. Even worse, he is reducing the paper-machine net production.
The machine has part of its capacity filled by a recycled material, in a loop that consumes resources, reduces capacity and raises costs and inefficiency. In case an integrated mill may eventually have problems due to excessive furnish swelling caused by any reason, the most simple method to improve machine runnability is to buy or to add some dried market pulp in the furnish recipe. The pulp drying operation causes substantial reductions in the WRV and WFF, favoring wet end drainage. However, dried pulps are more difficult to be refined and to develop pulp strengths. Today, there is a trend among pulp buyers to request higher market pulp pH in the pulp sheets. This trend is happening due to alkaline sizing in printing and writing paper manufacture. The papermaker wants to save some money in chemicals for controlling pulp furnish pH and pulp refining. Higher pulp pH means more swelling in the pulp, and more difficulties to drain the pulp sheet in the wet end of the pulp drying machine. The usual market pulp pH has been 5.0 to 5.5 for years. Today, to many customers, the trend has been raising it to 7.0 – 7.5.
Hemicellulose content of the pulp
Hemicellulose content is vital for papermaking. Pulp makers also love to preserve hemicelluloses in the digesting and bleaching operations.
They are improving the pulp yield, and consequently, reducing costs and raising economic margins. Pulps with low hemicellulose content, as the dissolving pulps, are hardly beaten, and fiber bonds are weaker and fewer. Since the great percentage of the eucalyptus pulp hemicelluloses are xylans, the significance of the pentosan content to predict pulp quality is very important. Those pulps containing more hemicelluloses are able to produce paper with better fiber bonding, better strengths properties, better surface smoothness, and lower bulk and porosity. No other fiber chemical component in eucalyptus bleached kraft pulps has so much influence on paper properties than the hemicelluloses, provided these pulps have not been damaged or degraded too much in their manufacture.
The hemicellulose content in pulps can be affected by the today’s operation of the modern digesters. The type and operation of the bleaching sequence is also responsible for removing more or less hemicelluloses. These two factors combined may represent significant improvements or losses in the pulp manufacturing stage. Consequently, the pulp manufacturer must understand very well its role in this process.
The wood raw material, the pulping and bleaching operations, and the pulp mill daily performance may represent opportunities to save or to reduce the hemicellulose content in the pulp.
Hemicelluloses are very hydrophilic organic compounds. The presence of high hemicellulose content in the pulp helps to increase the ability of the fibers to attract and to retain water in the fiber wall. This phenomenon improves the fiber swelling, brings weakening in the microfibril linkages in the cell wall, and favors the pulp refining. Higher the hemicellulose content, better is the wet fiber flexibility, and improved becomes the fiber bonding. Fiber walls may become more plastic and flexible.
As a consequence, for the same level of fiber coarseness and pulp refining, the pulps containing more hemicelluloses give origin to denser paper sheets, and sheets more resistant to the flow of air in the paper Z direction (reduced porosity). Sheet softness is also negatively affected.
Although the hemicelluloses are hydrophilic compounds (“friends of the water”), when they cooperate to reduce paper sheet bulk due to collapsed fibers and better bonding, the paper sheet become less porous. The reduced porosity of the paper structure has lower ability to absorb and to retain water. Curious behavior, intriguing performance: chemically speaking is favorable, but physically may not. Because the characteristics the hemicelluloses impart to the pulps, there are a number of other papermaking performance items being affected.
Drainage in the pulp machine may be negatively affected, but there are cases of opposite behavior. This happens when the pulp refining may be driven to lower Schopper Riegler levels to achieve the same required strengths. In these cases,the machine runnability does not suffer. When the refining is performed to a lower level of ºSR, there are very good improvements in the machine runnability and paper properties: drainage, steam saving, paper dimensional stability, softness, porosity, etc.
For some grades of papers, the effects of hemicelluloses are not completely welcome: tissue, decor, impregnating-base papers, etc. The removal of hemicelluloses may become a solution. This “removal” may be achieved in several ways: utilization of low hemicellulose content wood raw material; more intense cooking to lower kappa number and with more alkalinity in the end of cooking to prevent xylan re-precipitation; removal of hemicellulose by drastic alkaline bleaching stage. When hemicelluloses are partly removed, the weight of each individual fiber is reduced in some extent: this means that the number of fibers per gram of pulp is increased, and coarseness reduced.
The reduction on hemicelluloses in the digester is painful to the pulp maker. The pulp yield drops considerably in this operation. Drastic pulping of wood helps to reduce hemicelluloses to more acceptable levels for tissue manufacturing, but pulp yield may drop about 1.5 to 2.5% based on wood. This means more wood cost in the pulp cost; or reduction on pulp production in case the digester, the causticising or the recovery boiler is bottlenecked in the pulp mill. As an indication of effects: a reduction of 2.5% on the bleached kraft pulp hemicellulose content allows reductions of 10% in tensile strength, and improvements in bulk, porosity and absorption.
In recent days, new opportunities to remove hemicelluloses from wood, and associated to pulp making, are been investigated. The concept of bio-refinery is exactly proposing the utilization of part of the wood hemicelluloses to the manufacture of ethanol, a valuable biofuel. The consequence will be interesting: the possibility to differentiate eucalyptus pulps reducing hemicellulose content without bringing damage to the pulping process or to the environment (more COD in the bleaching filtrates, when hemicelluloses are removed in the bleaching line).
Fiber deformations and cell wall integrity/damages
Eucalyptus pulp fibers are submitted to very hard mechanical forces and stresses. Fiber life is not really easy, believe me. During cooking and bleaching, the alkaline conditions along these processes favor the structural disorganization of the microfibril chains in the cell wall. The removal of wood constituents during pulping and bleaching creates macro-pores in the cell wall. The wall becomes more fragile and damaged in relation to its original organization in the wood. This cell wall fragility enables the fibers to suffer more and to deform when mechanical forces are applied to them.
Deformations and lumen collapse become more frequent. More the fiber is damaged during pulping and bleaching, more sensitive is the fiber wall to be deformed and collapsed. The alkalinity favors fiber hydration and wall swelling; it helps to loose the microfibrils in the wall structure. The fibril network becomes loosen and more porous. This helps pulp refining and improves fiber bonding and wet fiber flexibility. However, the individual fiber strength is reduced. Severe cooking conditions to reduce kappa numbers or to delignify denser woods in general are very harmful to cell walls. At the same time, more hemicelluloses are removed from the fibers. The fibers become weak and sensitive to the mechanical forces, such as those created in presses, pumps, valves, agitators, etc. For these reasons, sometimes we have pulps with the same wall fraction and obtained from the same eucalyptus wood raw material, but with distinct fiber strength and paper performance. This may be explained by an excessive cooking or bleaching in one of them.
Degraded fibers have increased Water Retention Value and Fiber Saturation Point. They absorb water more easily because the microfibrils are more open, loosen and the fibril network more porous. Wet fiber flexibility is also increased, and together, the ability for lumen collapsing. These pulps have fast beating response, the Schopper Riegler degree raises fast. However, this is not followed by an increase in pulp strength (tensile and tear). Fibers are weak, they are more easily broken and form more fiber debris and fines. Wet Web Strength and Zero Span are also harmed. This is a terrible world, but it is a real world in many pulp mills.
Degraded fibers like these do not behave as normal fibers. They are not able to resist the forces of the refining, wet pressing, etc. The papermaker faces a dilemma: “fast refining, high swelling, but no strengths, no bulk, no porosity”.
When fibers are submitted to mechanical forces, they are sensitive to changes in their form, no matter they are low or high viscosity. Deformations in good quality fibers are interesting for a number of reasons.
The deformations in the fibers are measured as curl index, fiber kinks, fiber latency, and fiber micro-fractures in the cell wall. They affect the individual fiber strengths, but they provide substantial improvements in the paper sheet porosity, bulk, softness and water absorption. Fiber deformations are possible to be developed by artificial means at the pulp or the paper mills (shredders, washing presses, etc). Although not completely implemented as a source of pulp and paper differentiation, the utilization of fiber deformations for this particular subject may become more significant in the years to come. Mainly considering that pulp strengths are not the most demanded properties in the eucalyptus pulps. They are important, but not vital.
Fines content in the pulp
Fines are perhaps one of the most important kraft pulp properties, and most of the times, they are seen as a problem, never as a solution. This fundamental property is being neglected by pulp and paper makers; perhaps because fines are no fibers, they are debris or “weak parenchyma cells”. Fines are seen as a filler in the pulp supply. They are created in great extent when the pulp is refined, what means that fines dramatically affect drainability in the wet end section. What I would like to propose to papermakers is really to pay attention to fines in the furnish
The best methodology to measure fines in a pulp stock is the dynamic jar test. It is based on the percentage of the dry weight of a pulp that passes through a 200 mesh wire (openings of 70 micrometers), under constant conditions of time, temperature and consistency. The “management of fines” in the furnish may provide to paper operators one the most simple and convenient methods to control the great majority of the end-product properties.
When a paper mill has two or more machines, the management of fines, via the white waters, may be even more effective, by distributing fines in right dosages in one or another paper machine, according to the paper grade being manufactured. A low fines content eucalyptus pulp is able to better perform in the paper machine. Formation, softness, bulk, porosity, absorption, dimensional stability, and permeability are improved in the paper sheet. Strengths are reduced in opposition to these other gains. In terms of machine performance, the reduction on furnish fine content improves the drainage in the wet end,
the consistency after wet presses, and the consumption of steam in the drying section.
For this reason, fractioning of fines may become an operation to be considered according to the paper end-product being manufactured. In cases of machines bottlenecks, fractioning of fines may also become a solution.