PROPERTIES OF PULP

 

The three most important Parameters which defines the pulp are. 1.) Fiber Length, 2.) Brightness, 3.) Pulping process used. e.g. Northern Soft Wood Bleached Kraft (NSWBK). The Northern Soft Wood tells it is long fiber pulp. Bleached tells, it has high brightness and Kraft tells that Kraft (Sulfate) pulping process is used to produced this pulp. Similarly Southern Hard Wood Unbleached Kraft, will be a short fiber wood unbleached (low brightness) pulp made by kraft process. Link to a few typical Market Pulp data sheets.


Basic Pulp Properties

Pulp Navigator by Metsa Fiber

http://www.canforpulp.com/products/pulp/products/propertysheets.asp (Typical Pulp Properties Data Sheets from Canfor)


Ash Content in Pulp 
Ash content in pulp may consists of various chemicals used during pulping/bleaching, mineral matter from wood or metallic matter from pipes and other machinery. It is not important parameter of pulp.
 
Ash is the residue left after igniting pulp at 525 0C (As per TAPPI T 211). Ash is reported in % of residue to dry pulp basis.
 
The standard procedure of measuring ash content is laid out in  TAPPI  T 211, ISO 1762

Brightness of  Pulp
Brightness of paper is discussed in Paper Properties. The paper brightness is mainly dictated by pulp brightness. There are some modification in stock preparation which can alter paper brightness to some extent such as filler, sizing, whitening agent, dying etc.   
 
Coarseness of Pulp
This is a measure of the average weight of fiber per unit length, often reported in units of mg/m. It is most conveniently measured using an optical analyzer. For fibers of a given average length, it is a measure of the cross sectional area of the fiber. For a given average diameter, it is measure of wall thickness. Coarse fibers are considered to be less conformable than fine fibers and do not bond as readily. Coarser fibers also result in fewer fibers per mass of pulp, which has a significant impact on sheet formation and light scattering potential.
 
Conductivity of Pulp
Electrical grade papers such as cable paper, condenser tissue or insulation paper etc., require very low conductivity to electricity. The presence of metal ion more specifically iron ion contribute to pulp conductivity The pulp used for electrical grades are washed with demineralized water, beater or refiner use lava or other non-metallic bars and contacting surfaces of all equipment are made of stainless steel.

Values for the conductivity of the water extract of the pulp are expressed in µS/m.

         S = Siemen (SI unit of electric conductance) = 1 mho.

 
Dirt in Pulp
Dirt content of pulp particularly of recycled pulp is important for its suitability to make fine paper. Dirt is any foreign material in pulp. TAPPI defines dirt as foreign matter in a sheet which, when examined by reflected, not transmitted light, has a marked contrasting color and has an equivalent black area of 0.04 mm2 or more. 
 
The standard procedure of measuring dirt content is laid out in  TAPPI  T213
Drainage Time of Pulp
Here the drainage time of pulp is discussed in reference to market pulp and/or unrefined pulp. The drainage time of pulp or freeness or slowness of pulp is modified to have some desired properties in the paper, here that is not discussed.
 
Drainage of unrefined pulp which is measured as freeness can give an indication on : 1) Fiber Length of pulp, as long fiber pulps have more freeness compared to short fiber pulps, 2) Damage to fiber during pulping, bleaching or drying as short fibers or fines produced during pulping operation, reduces pulp freeness, 3) Refining energy required to achieve certain slowness during stock preparation.  
 
The standard procedure of measuring pulp drainage is laid out in  TAPPI  T221, T227, ISO 5267-1 and ISO 5267-2
 
Dry Content of Pulp 
Consistency:  is the term used to describe solid content of  pulp during pulp processing. For pulp and paper maker this is the most important process parameters. All equipments are designed to handle pulp at and up to certain consistency. Pulp consistency is roughly divided in to three ranges:
 
Low Consistency: <5%
Medium Consistency: 5 - 15% 
High Consistency: >15%
 
But more preciously actual consistency for these ranges in various unit operation of papermaking are different.
 
Unit Operation Actual Consistency
  Low Consistency (LC) % Medium Consistency (MC) % High Consistency (HC) %
Repulping <6.0 <12.0 12 - 28
Screening <1.5 <4.5 -
Centrifugal Cleaning <1.5 <2.5 2.5 - 6.0
Bleaching - 10 - 15 25 - 35
Refining 3 - 6 10 - 15 28 - 35
Headbox <1.0 - >1.0
 
It is the desire of every pulp maker to keep pulp at the highest possible consistency to minimize dilution water usage and which ends up as effluent. Higher consistency also helps in reducing the bleaching chemical consumption. But there are practical limitation of handling pulp at higher consistency such as high viscosity which make pulp flow very difficult. 
 
The standard procedure of measuring pulp consistency (up to 25%) is laid out in  TAPPI  T240.
 
Moisture Content of Market Pulp:  is important from storage, transportation and handling point of view. Most of the market pulp are sold, stored, transported and used as air dry. The useable part of pulp is dry fiber only, so the tendency is to minimize the moisture content op pulp.
 
Small quantity of pulp is sold as wet lap also. Wet lap pulp is not dried at source and transported at about 50% moisture content. It is feasible for short distance transportation and if pulp is to be used immediately at user end.
 
Extractives (Low Molecular Weight Carbohydrates) in Pulp 
The low molecular carbohydrates indicates an extent of cellulose degradation during pulping and bleaching process, which may effect pulp strength and other properties. Pulp is treated with 1% hot NaOH solution for one hour to estimate loss of yield due to extractives.   
 
The standard procedure of measuring 1% Hot Alkali Solubility is laid out in  TAPPI  T212

Fiber Length of Pulp
Length of fibers  (arithmetic average, weighted average etc.) is one of the most important parameters of pulp. Pulp strength is directly proportional to fiber length and dictates its final use. A long fiber pulp is good to blend with short fiber pulp to optimize on fiber cost, strength and formation of paper. Softwood with pulps in general have longer fiber compared to hard wood pulp. Pulp made from woods grown in cold climate in general  have longer fiber compared to wood grown in warmer climates.
 
Chemical pulps in general have higher fiber length compared to semi chemical pulp and mechanical pulp, when made from same wood. More fibers get damaged/shorten by mechanical action than chemical action.
 
There are several method to measure /report fiber length of pulp. The 'fiber length of pulp by projection' is described in  TAPPI  T232. The 'fiber length of pulp by classification' is described in  TAPPI  T233. "Fiber length of pulp and paper by automated optical analyzer using polarized light' is described in TAPPI T271.
 
The coarseness of  pulp fiber  is described in  TAPPI  T234. 
Effect of Fiber Length Increase/Decrease on Paper Properties
Properties Increases Decreases
Bursting Strength Increases Decreases
Folding Endurance Increases Decreases
Formation Becomes Wilder Becomes less Wild
Print Quality Becomes Poorer Becomes Better
Surface Levelness Decreases Increases
Surface Smoothness Decreases Increases
Tearing Resistance Increases Decreases
Tensile Strength Increases Decreases

 

Fiber Length of Various Raw Material
Raw Material Fiber Length (mm) Raw Material Fiber Length (mm)
Temperate softwoods 2.7-5.6 Esparto grass 1.5
Temperate hardwoods 0.7-1.7 Flax tow, seed 25-30
Tropical mixed hardwoods 0.7-3.0 Hemp, bast 20
Acacia 0.7 Jute, bast 2.5
Eucalyptus 0.7-1.3 Kenaf, bast 2.4-2.6
Gmelina 0.8-1.3 Kenaf, core 0.6
Abaca (ie: manila hemp) 6.0 Ramie 200
Bagasse, depithed 1.0-1.5 Reed 0.5-2.5
Bamboo 1.2-4.0 Rice straw 0.8-1.0
Corn stalk, depithed 1.0-1.5 Sisal 3.0-3.5
Cotton fiber 20-25 Sunn hemp, bast 2.5-3.5
Cotton linters 1.0-3.5 Switchgrass 1.4
Cotton stalks 0.6-0.8 Wheat straw 0.7-1.5

 

Fiber Population of Pulp
Fiber population is defined as the numbers of fibers per gram. It is expressed as Millions fibers/gm.
 
Raw Material Length (mm) Width (mm) Population (Million/gm)
Eucalyptus 0.7 0.016 13-18
Acacia 0.7 0.016 13-18
Scandinavian Birch 1.1   8.0
Scandinavian Pine 3.0   1.5
Tropical Mixed Hardwood 0.9 0.004 5.0
Southern Pine 3.5   1.0
 
 
Fines Content
An additional measure of pulp particle size is the percentage of fines. This consists of particles measuring less than 0.2 mm in length as measured by an optical analyzer, or the weight percentage of the P200 fraction obtained from a Bauer McNett classifier. Fines can have a significant impact on processing, particularly with regard to filtering or drainage operations. Fines content of a kraft pulp may be in the range of 5–15%. For a groundwood mechanical pulp, the fines content may exceed 40%.
 
Kappa Number of Pulp
Kappa number is determination of relative hardness, bleachability or degree of delignification of pulp. It is important parameter of unbleached pulp which is to be bleached. Low Kappa pulps are easier to bleach. High Kappa pulps usually require more energy in refining, but often produce stronger paper or board (particularly with regard to tear strength).
 
A fully bleached pulp can have Kappa # as low as 1 and very high yield pulp may have kappa # as high as 100. Typical  kappa # of some pulps.
  Kappa #
Hardwood Pulp for Bleaching 14 - 20
Softwood Pulp for Bleaching 20 - 30
Wood pulp to be used Unbleached 40 -100
The method to find kappa # of pulp  is described in  TAPPI  T236.
 
Permanganate Number (K Number)
Chemical test performed on pulp to determine the degree of delignification. There is a mathematical relationship between K Number and Kappa Number.
 
Pulping Process
Though the pulping process used is directly not a pulp property but this is one of the most important parameters used in specifying the pulp. As we move  from full mechanical to full chemical pulping process, strength of pulp and bleachability improves. Strength improves due to less degradation of fibers and bleachability as more lignin is removed in chemical than mechanical pulping processes. 
 
Yield: Pulp yield is mainly govern by the pulping process. Mechanical pulping processes which provide high yield, retain almost all constituents of wood. Lignin which is second highest to cellulose, does not bond to itself or cellulose fibers as fibers do, don't contribute to any bonding, resulting in weak pulp. Secondly lignin is brown in color and to maintain high yield of bleached pulp, lignin is not removed during bleaching, but only chemically modified. 
 
Specific Refining Energy
Energy applied per unit weight on oven dry basis (KWH/MT or HPD/Ton ) during refining. The higher value leads to good formation (and thus strength) and plenty of interfiber surfaces (for opacity and brightness). 
 

Grade   Net HP Day/Ton Net KWH/Ton
Fine Paper Hardwood Kraft 2 - 5 36 - 90
Softwood Kraft 3 - 7 54 - 126
Linerboard Base 5 - 7 90 - 126
Top 10 - 12 180 - 216
Newspaper Softwood Kraft 2 - 5 36 - 90
TMP/GWD 1 - 5 18 - 90
GWD Printing Paper Softwood Kraft 3 - 7 54 - 126
  TMP/GWD 3 - 6 54 - 108
An estimate of the specific energy requirement can be made for a given type of pulp if the unrefined pulp freeness and the target freeness level are known. By subtracting the target freeness from the unrefined freeness, the total amount of freeness change is calculated.
 
Values in following table can then be used to predict approximately how much energy should be required to achieve the desired freeness drop.
 
Furnish Freeness Drop Net HP Day/Ton
Bleached Hardwood Kraft 60 - 100 ml
Bleached Softwood Kraft 20 - 40 ml
GWD 3 - 7 ml
OCC 40 - 70 ml
Mixed Office Waste 50 - 70 ml
Newsprint 20 - 35 ml
Tensile Strength of Pulp
This is not the tensile strength of individual fiber, which is even higher than or comparable with steel. The tensile strength discussed here is maximum strength of randomly oriented pulp fiber when formed in a sheet. This tensile strength gives an indication of the maximum possible strength of pulp beaten under ideal condition. This again an indication of what level of tensile strength  can be achieved in real paper making environment.
 
Viscosity of Pulp
Solution viscosity of a pulp gives an estimation of the average degree of polymerization of the cellulose fiber. So the viscosity indicate the relative degradation of cellulose fiber during pulping /bleaching process.
 
Dissolving pulps from wood, which contains a large proportion of  alpha cellulose, give higher viscosity values than paper pulps.
 
The standard procedure of measuring pulp viscosity is laid out in  TAPPI  T230 
 
Zero-Span Tensile
One way of measuring tensile strength of pulp is "zero span breaking strength' described TAPPI  T231. Zero-span tensile data determines the maximum strength of pulp fibers when beaten under idealized laboratory conditions.
Zeta Potential
Zeta potential is the parameter that determines the electrical interaction between particles, a high value, positive or negative, prevents flocculation. Changes to zeta potential can affect retention values, strength, pitch deposition and additive requirements. The zeta potential of pulp and other particles in the process may vary for a number of reasons, e.g. changes in refining, pH, pulp source, broke content and quantities of additives used.
 

Some of the commonly used Pulp & Fiber Test

 
TAPPI Method  number

Test

*Related ISO Method *Related ASTM
T211 Ash in Wood, Pulp, Paper :(525oC) 1762 D1102
T234 Coarseness of Pulp Fibers    
T240 Consistency 4119  
T213 Dirt in Pulp 5350/1, 5350/2  
T221 Drainage time of Pulp 5267-1, 5267-2  
T232 Fiber Length by Projection 16065-1, 16065-2  
T233 Fiber Length by Classification 16065-1, 16065-2  
T271 Fiber Length by Automated Optical analyzer    
T227 Freeness of Pulp (CSF) 5267/2  
T236 Kappa Number 302  
T200 Laboratory Beating (Valley Beater method) 5264  
T248 Laboratory Beating (PFI Mill method) 5264/2  
T220 Physical testing of Pulp Hand sheets 5270  
T204 Solvent Extractives of Wood & Pulp 624 D1107, D1108
T259 Species Identification of Nonwood Plant Fibers    
T230 Viscosity of Pulp (Capillary Viscometer method) 5351/1  
T207 Water Solubility of Wood & Pulp   D1110
T231 Zero Span Breaking Strength (Dry) 15361 D5804

* “Related” does not imply “equivalence.” A “Related Standard” may be a standard for a similar property, but this should not assume identical technical content or matching results.


Go to http://www.tappi.org/content/pdf/standards/subject_index_tms.pdf for a complete list of TAPPI testing standards.

 
 


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