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Fiber and Pulp Properties for

Papermaking

Pekka Komulainen Pekka.Komulainen@clarinet.fi

20 August, 2015

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Hardwood vs. softwood cells

Hardwood fibers are about third of the softwood fiber length (1 vs. 3 mm) and 2/3 of softwood fiber thickness (20/30 μm). In addition, hardwood includes lot of vessel and ray cells, which can cause so called vessel picking and linting in offset printing.

Picture: Prof. Wimmer

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Wood and fiber properties

The big difference between softwoods and hardwoods is amount of real fibers (tracheids).

Only tracheids can form fiber network and help papermaking.

Biggest problem with nonwood fibers is low share of real fibers (commonly less than 50%).

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Wood properties UnitPicea

abies

Pinus

sylvestris

Pinus

radiata

Populus

tremula

Betula

pendula

Eucalyptus

globulus

Amount of wood volume

Fibers (tracheids) % 95 93 89 61 65 49

Vessels % 0 0 0 26 25 21

Ray cells etc. % 5 7 11 13 10 30

Average fiber dimensions

Length mm 3,3 3,1 3,3 1.0 1.1 1.0

Diameter µm 31 30 40 20 21 16

Cell wall thickness µm 3,1 3,0 7,0 3,2 3,8 3,8

2 x cell wall/width % 20 20 35 32 36 48

Wood Density (dried) kg/m3405 550 515 450 640 820

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Example of softwood fiber basket

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Roles of different papermaking pulps

Softwood chemical pulps

Wet and dry runnability for papermaking, finishing and converting

Ensuring strength and stiffness for packaging materials

Hardwood chemical pulps

Good end use properties of woodfree printing papers and tissues

Good formation, brightness, opacity and printability

Decrease the costs of the fiber furnish

Mechanical pulps

Good runnability and end use properties of mechanical grades

Formation, printability

Better yield and lower costs of the fiber furnish

Bulk and stiffness, especially for filler ply of multilayer products

Recycled and nonwood pulps

Decrease fiber furnish costs, can be more environmentally friendly

Enlarge the raw material base for papermaking

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Roles of different raw materials

Fillers and coating pigments

Improve the end use printability properties of paper

Decrease costs, carbonate widely available

Save forests

Additives

Improve the papermaking process (performance chemicals)

Improve the end use properties of the paper (functional chemicals)

The desired paper properties can be obtained by

Selecting the proper furnish components

Adjusting the fiber furnish composition

Adjusting the properties of the different fiber furnish components used

Properly controlling the total papermaking and finishing processes

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Wood, fiber and paper properties

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FIBER PROPERTIES IN PULPS

Fiber

length

Cell wall thickness

of fibers

Fiber

coarseness

Hemicellulose

content

Nr of Fibers per unit weight

Bulk, Stiffness,

Wet strength

Light scattering coefficient (opacity)

Formation, Porosity, Orientation

Bonding, Density, Dimension stability

Fiber Stiffness

Dry strength Wet Strength

RAW MATERIAL

PROPERTIES

PAPER PROPERTIES

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Pulps and paper grades

Actual fiber furnishes may vary largely and can be quite different especially

in small unintegrated paper mills

Very often the price of fiber seems to be more important than the

performance of fiber in the product; within each end-product the quality

and the price of end-products may vary largely

It is important to understand how each furnish component contributes the

quality of the product and the performance in the paper machine, finishing,

and converting

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Paper Grades Short fibers for printability Long fibers for

runnability

Mechanical grades GW, PGW, TMP, BCTMP, DIP Long fiber softwood

(BSKP) Woodfree grades BHKP, DIP

Non-wood grades Several non-woods (bagasse,

wheat straw etc.) Bamboo, kenaf etc.

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Tensile strength of different pulps

DIP has normally better tensile strength

than TMP even if it contains filler.

Standard newsprint contains 50-100 % DIP.

TMP fibers of Pinus radiata are coarse and

tend to form bulky, low tensile and porous

sheet.

Tensile index of all pulps improve when pulp

is made to lower freeness.

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Effects of refining on fibers

Internal and external fibrillations as well as creation of fines are the main

positive effects of refining.

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Removal of primary

fiber wall and S1 layer

More fiber hairiness

(external fibrillation)

Delamination and

swelling of fibers

(internal fibrillation)

Creation of fines

Fiber cutting and shortening

Dissolving of material

Straightening of fibers

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Final bonding in paper

Picture on the right describes

bonded fibers after refining and

drying.

External fibrils and fines from refining

are an important part of bonding.

Secondary fines (fibrillar) has the

most positive effects.

Collapsed lumen in the ribbon-like

fibers increase bonded area.

Crimped section at fiber crossings

have effect on the rheological

behavior of paper.

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Picture:

Hubbe

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Paper density and strength

How to get bonding without density increase? Dry strength chemicals, surface

sizing and micro-fibrilled cellulose are some possibilities.

Gentle refining or low Specific Edge Load (SEL) gives good bulk and bonding

at the same time. Low SEL for never dried hardwood is < 0.5 J/m.

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Fiber Lumen

More internal fibrillation,

collapsed – good bonding but

low bulk

More external fibrillation,

not collapsed – good bulk and

bonding

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Effect of chemical pulp refining on paper

Positive effects Wet web strength

Fiber bonding and strength

Better formation

Coating coverage

Porosity and ink demand

Smoothness and gloss

Negative effects Water removal and solids content

Bulk and stiffness

Paper compressibility

Opacity and brightness

Drying shrinkage dimension stability

Tear strength

Energy consumption

CAPEX and maintenance costs

Pics: E.Gruber

Internal fibrillation External fibrillation Fiber bonding

+ =

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Wood, fiber and pulp properties for papermaking

It is important to understand the effect of wood properties on paper quality.

Wood density is a simple measure and well suited to predict paper strength.

Fiber length is not the only characteristics correlating with paper strength.

Latest studies show that even more important are external fibrillation and crill content

after stock preparation. Fibers should be easily refined to save energy.

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Main Wood Properties Average dry density

Tracheid, ray cell and

vessel shares

Fibers after Pulping Cell wall thickness

Tracheid length

Coarseness

Fibril angle

Fibers to Paper Machine

Ratio fines to fibers (crill)

External fibrillation

Cell wall collapse, density

Pitch and stickies

Wood for Pulping Cellulose,

hemicellulose

and lignin content

Extractives content

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Pulping

process

External

pressure Pulp

drying

Pulp

refining

Pulp

yield

Lateral

force

Surface

tension

Sheet

density

Surface

properties

Optical

properties

Strength

properties

Lateral

conform-

ability

Wood

vessels

Wood

density

Degree of

collapse

Bonded

area

Wood

extractives

Fiber

diameter

Wall/dia

ratio

Wall

thickness

Forces during papermaking: Fiber processing:

Wood, fiber and paper properties

Wood

properties:

Paper

properties:

Wood

chemistry

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Fiber collapse and paper properties

Differences in relative cell wall thickness have

effect on paper properties.

An example here is Pinus radiata compared to

Norway spruce. TMP fibers of Pinus radiata are

coarse and tend to form bulky, low tensile and

porous sheet with reduced coating and ink

holdout.

Spruce fibers have lot of thin-walled early wood

fibers, which form more dense, smooth and strong

paper.

Dense paper, however, can have lower light

scattering and stiffness.

From hardwoods eucalyptus is more thick-walled

while birch and acacia are more thin-walled.

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Norway spruce

Pinus radiata

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Fiber collapse and flexibility

It is important to increase flexibility and collapsibility of thick-walled fibers.

The main means to improve collapsibility is refining.

When using stratified headbox it is possible to fractionate fibers and put coarse

fraction to the middle layer.

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Good fiber

for printing paper

Suitable fiber for tissue, copy paper and

cartonboard middle ply

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Hardwood chemical pulp

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For woodfree paper grades mainly hardwood kraft is used. Thick-walled

eucalyptus can be better than thin-walled or birch.

On the left side paper is bulky and thicker giving better stiffness, which is

important e.g. for copy paper.

Thick-walled fibers are better in cartonboard middle ply because

smoothness is not as important there as bulk and stiffness.

Picture: Celso Foelkel

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Primary and secondary fines of hardwood

Primary fines of hardwood pulp (ray and vessel cells) cause picking, linting and

reduce bonding.

Secondary fines formed in refining is mostly thin fibrils and enhance bonding.

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Picture: Martin A. Hubbe

Primary fines Secondary fines

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Effect of softwood kraft on paper properties

Positive

Strength properties Increase (also surface, tear and wet strength)

Folding endurance Increases

Negative

Formation Less uniform

Smoothness Decreases

Porosity Increases

Ink holdout Lower

Bulk and stiffness Decrease

Dimensional stability Decreases

Energy consumption Increases

Costs Increase Picture: Canfor

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Fiber type and wet end runnability

Printing papers with mechanical pulp

have better runnability on paper machine

than woodfree grades.

The main reason is that mechanical pulp

fibers are stiffer in wet condition.

The good tensile stiffness of the wet fiber

network is mainly due to elastic pressure

and friction forces between fibers.

It is easy to make a model from four

sticks and note the rigid structure without

bonding.

Good runnability is always more

important than we could imagine.

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No bonding but very rigid structure!

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Dryer section runnability

There should be high residual tension after

first wet strain to avoid web slackness and

breaks.

Low tension after first dryers due to

relaxation may lead to slackening of the

wet web.

This causes wrinkling, bagging, fluttering

and weaving of the web which can lead to

web breaks.

In modern single felted dryer sections, the

problematic areas of paper with low

tension level are mainly found in

converging and diverging gaps between

the dryer cylinders and the fabric.

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Fiber fines and residual tension

The studies of Dr. Retulainen suggest that the residual tension is more closely related to the

tensile stiffness than to the tensile strength. Therefore also the factors, such as the fiber

stiffness, straightness and the activity of the network to bear load, affect the residual tension.

It is interesting to note that stiff TMP fibers blended with only 10 % kraft fines form stronger

wet sheet than kraft fibers and kraft fines.

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On the left the effect of fiber and

fines type on the residual

tension (tension at 1% strain

after 0.475 s relaxation) of wet

sheet (compared at 55%

dryness).

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Advantages of low break frequency

Improved

runnability

Lower raw

material costs

Longer

wire life

Increased

machine

speed

Less steam &

energy/ton Lower

furnish

cost

Higher

press solids

Less starch

etc. needed

Better CD-

profiles

Less shade &

caliper variation

Stronger

paper

Better bulk

& stiffness Better

printability

Constant

filler content

Productivity

Cost

Efficiency

Easy wet end

chemistry Product

Quality

Low Break

Frequency

Less effluent and

fresh water/ton

Better and less variable

raw materials

Less Dry

Broke

Stable and better

paper quality

More Net

Tons

Lower Chemical

Consumption

Decreased use of

chemicals Cleaner

system

Steam & el

used only

once

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TMP fibers and bonding

TMP fibres are normally not very well fibrillated. Fibrillation is needed to get bonding. Sometimes there is still latency left to the paper machine. Latency (curling of fibers) reduces strength and bonding and increases porosity.

Fibrillation can be increased by using fresh/moist wood, alkaline pH, high amount of reject refining and also with post-refining (however may cut fibers too much).

Freeness as such is a measure of fines content, but not a good measure of fibrillation. Fibrillation should be checked from microscope fiber pictures.

Picture: Knowpap

Long TMP fibers with thick cell

wall are normally not very well

collapsed or fibrillated. Internal

bond of paper tends to be low.

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TMP fibrils and flakes

A good quality TMP for printing paper includes fibrils to give bonding and strength as

well as flakes (mainly unbonded material) to improve light scattering and paper opacity.

For cartonboard middle ply flakes are not needed.

Picture: KARI LUUKKO AND HANNU PAULAPURO

TAPPI JOURNAL FEBRUARY 1999

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Effect of TMP fines to paper properties

It is important to note that fibrils increase tensile strength without reducing light scattering and flakes increase light scattering without decreasing tensile strength.

A good TMP includes both flakes and fibrils and thus increases both light scattering and tensile strength.

Picture: KARI LUUKKO AND HANNU

PAULAPURO

TAPPI JOURNAL FEBRUARY 1999

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Fiber wall thickness vs. roughness & opacity

Fiber wall thickness determines very much paper smoothness and opacity.

Increased fiber splitting without fiber cutting (lower freeness) can improve

the situation but not totally.

Picture: PFI

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Optimal mechanical pulp

Optimal mechanical pulp for improved

publication papers should have:

Thin fiber walls (raw material selection)

Fiber fines and large degree of fiber splitting (thin

slot screening and reject refining)

Fibrillated fiber surfaces (reject refining)

Low amount of shives, and especially latewood

shives (reject refining and thin slot screening)

Reasonable fiber length (thin slot screening and

reject refining)

Such fiber properties will give publication papers

with improved print quality, formation and

runnability.

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Fiber Wall Thickness of Norway Spruce

Average fiber wall thickness of Norway spruce TMP is almost 2 µm but there

are some fibers with wall thickness of 3-5 µm.

Mechanical pulps made from mature Pinus radiata can have fiber wall thickness

of about 6 µm, which is three times the Norway spruce value.

Reme, P. A., Kure, K.-A.,

Gregersen, O. W., Helle, T.,

1999 International

Mechanical Pulping Conference

Picea Abies

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Importance of fiber wall thickness

It is important to have several fiber layers in a thin paper to get good formation,

smoothness, opacity and gloss. This correlates with thin fiber wall.

Fiber wall of Norway spruce is about 2 µm while Radiata pine has 4-6 µm. In a 40 gsm

paper there can be only 3-4 fibers of Pinus radiata in the total paper thickness.

Area = Perimeter x Wall Thickness, A=P*T

Fiber volume = Area x Length, V=A*L=P*T*L

Coarseness = Fiber weight/Length, C=W/L

C = Volume*Density/Length, C=V*ρ/L=P*T*L* ρ/L= P*T*ρ

Fiber Grammage (g/m2) = Coarseness/fiber Width = P*T*ρ/P*2 = 2*T*ρ = 3*T (µm)

~ P/2 Fiber Fiber

Wall Grammage

µm g/m2

1 3

2 6

3 9

4 12

5 15

6 18

T

Wall density ~ 1500 kg/m3

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Example: pulp characteristics for containerboards

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There is no common criteria for pulp

quality. It depends on end products

to be manufactured.

Linerboard

Compression strength, burst,

stiffness and porosity are most

important

For white-top grades printing

properties are important

Corrugating medium (fluting)

Compression strength is critical

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What makes strong kraftliner

It is quite common understanding that fiber length and fiber collapse are the most

important fiber properties having effect on important linerboard properties. This

means that kraft pulp must be strongly refined to get lumen collapse and higher

bonded area. But there is also a third variable what Innventia in Sweden has studied.

PulpEye has recently introduced its CrillEye online crill measurement. Crill is finely

divided cellulosic material - finer than external fibrillation - that is liberated during

refining process. The crill particles are about a hundred times thinner than the fibers.

In spite of the fact that only about one per cent of the weight of fibers and other

particles in the furnish is crill, it can correspond to as much as fifty per cent of the

total free surface area. This shows the importance of crill for the strength properties

of paper.

Valmet has also introduced its own online strength measurement, which is based on

external fibrillation or hairiness of fibers.

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Crill and tensile strength

Research studies at Innventia have shown that crill is the single variable

having the strongest connection to paper strength. Laboratory results in

the figure below show a strong correlation to paper tensile strength index.

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Fibers are never identical