Pele Oy

Flocculation, Formation and Paper

Properties

Pekka Komulainen Pekka.Komulainen@clarinet.fi

13 October, 2016

Pele Oy Flocculation and formation

Flocculation of fibers occur in the approach flow. Headbox tries to destroy flocs and

disperse fibers.

Fiber flocs from the headbox and on the wire fix to the sheet when water removes. This

determines sheet formation, which is measured as small scale basis weight variation

(e.g. 1x1 mm2) by using beta ray absorption.

On the wire fibers reflocculate and disperse very fast again. Dewatering time has a great

effect on this process. Long dewatering time means that there will be more flocculation.

The extent of fiber flocculation or dispersion directly influences the resulting paper

formation.

Good formation may be the only paper property, which has no negative effects on the

final paper properties.

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Flocculation

in approach flow

Dispersion

in headbox

Dispersion

on wire

Reflocculation

on wire

Pele Oy

FLOCCULATION

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Pele Oy Flocculation environment

Ideal suspension of fibers would be so dilute

that no collision between individual fibers

could be possible. Each fiber would then

occupy a sphere, where the sphere diameter

is same as fiber length.

In practice consistencies are higher and there

are always collisions between the fibers.

However, this thinking is the basis of different

theories about flocculation and also very

useful in practice to understand flocculation.

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Picture: Hubbe

Pele Oy Mechanical flocculation structure

Flocks can be formed without any bonds

between fibers. A fiber may only become a

part of a network if it is in contact with at

least three other fibers.

It is easy to make a rigid flock structure from

four wooden sticks, each having three

contact points. Elastic energy between the

bent sticks and friction forces hold the sticks

together.

If fibers are totally dispersed this kind of

flock requires turbulence to be formed.

Turbulence forces can form but also destroy

these flocks and disperse fibers.

Accelerating flow destroys effectively flocks

without forming new flocks. This is very

important in the headbox.

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Flock structure without bonding

d = fiber diameter, L = length

Pele Oy Flocculation variables

Increased flocculation

Long fibers

Low fiber coarseness

Persistently curled fibers

Wide length distribution

Fibrillated fiber surface

Stiff fibers

Low fluid viscosity

Slow dewatering

Small shear forces

Fiber charge close to zero

Decreased flocculation

Short fibers

High fiber coarseness

Straight fibers

Narrow length distribution

Low external fibrillation

Flexible fibers

High fluid viscosity

Fast dewatering

High shear forces

High fiber charge

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The criteria of flocculation for the papermaker is, how high mass consistency

can be used in the headbox. However, in theory volumetric concentration is

important Qualitative effects on flocculation are as follows:

Pele Oy Formation and jet to wire speed ratio

Best formation is normally achieved when jet

and wire speeds are same.

Some other studies conclude that best

formation is, when there is a very small

difference in the jet and wire speeds.

Jet-to-wire speed ratio can have curved CD

profile. This is the reason that formation can

vary very much in the cross machine

direction.

In laboratory sheets good formation correlates

with good tensile strength.

On a paper machine, where good MD tensile

is made with higher jet-to-wire speed ratio,

good tensile strength correlates with bad

formation.

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Pic: JURAJ GIGAC and MÁRIA FIŠEROVÁ

Pele Oy Flocculation tendency of different pulps

Pictures of Huawei Yan after headbox nozzle. Fiber concentration 5 g/l, flow speed 8 m/s.

A = BSKP, B = BHKP, C = TMP and D = SGW.

Formation of groundwood fibers is best and softwood kraft worst. This is not only effected

by fiber length but also by fiber coarseness.

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Crowding factor

where Cm = mass concentration, L= fiber length, ω = fiber coarseness

Pulps formed at 0.5 % consistency. Fiber properties:

Fir: length 2.7 mm, inverse specific perimeter 0.72

Aspen: length 0.8 mm, inverse specific perimeter 0.38

Wood fibers and flocculation

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Picture: Kerekes et Schell , TAPPI Oct 1994

Fir

Crowding factor 95

Aspen

Crowding factor 17

Pele Oy Effect of fiber charge on formation

Adding anionic PAM to the pulp improves formation by increasing negative

charge and preventing flocculation.

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No A-PAM A-PAM

1.7 mg/g

Picture: Lindström et Christiernin,

NPPRJ, Jan 2006

Pele Oy

FORMATION

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Pele Oy Flocculation and dispersion

As shown in the top of the figure, the

random fiber distribution is generated by

the stochastic distribution of fibers in the

plane of paper.

One can see how regions of low and

high grammage are formed by this

natural process.

There is a certain level of flocculation

within random fiber distribution, but they

are not necessarily generated by a

tendency of fiber aggregation through

physical or chemical forces.

The other two figures show flocculated

(left) and dispersed (right) fiber

distributions.

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Picture: Jing Yan

Pele Oy Formation measurement

Real formation is measured by small scale beta ray absorption (Ambertec).

Normally standard deviation of grammage (g/m2) is calculated.

Formation number normalized with respect to the grammage is called

specific formation number, since the formation number is statistically

inversely proportional to the square root of the mean grammage.

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Pele Oy

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Paper quality and formation

It is possible to study paper quality by taking pictures against window and

then treating these digital pictures by adjusting size, colour contrast etc.

These examples are of a Chinese newsprint mill.

Typical formation Wire mark Wire mark

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Some visible paper formation faults

Fluting after coating

Cockling

Flow on wire

Large scale formation

Pele Oy Retention, drainage and formation

Conventional wisdom is that the relationship between retention / drainage

and sheet formation is a tradeoff: Increasing retention produces a decrease

in formation quality and low retention results in better formation.

Frequently when the drainage is improved the retention falls and poor

formation is obtained.

Through the phenomena of adsorption and electrostatic interactions,

retention chemicals are able to develop chemical aggregation mechanisms

by which fillers, fiber fines, and other functional additives are retained in the

sheet.

Chemical retention and flocculation topics are not much discussed in this

presentation.

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Pele Oy Example of refining effects on formation

Sometimes it is not clear how e.g. refining effects on formation.

Normally formation is improved in refining. However, if there is very little

cutting in refining and fines material have more effect on dewatering,

refining can have negative effect on formation.

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Refining effects Explanation Effect

Removal of primary fiber wall Lower fiber coarseness –

Delamination and swelling of fibers (internal fibrillation) More flexible fiber ++

External fibrillation Higher surface friction –

Shortening of fibers (cutting) Shorter fibers need less space +++

Creation of fines Small flocs from fines +

Longer dewatering time –

Dissolving of material (hemicellulose) Lower fiber coarseness –

++Total refining effect on formation:

Pele Oy Flocculation, formation and paper properties

The extent of fiber flocculation or dispersion directly influences the resulting paper

formation. Good formation may be the only paper property, which have no negative

effects on the other paper properties.

Refining produces fine material which is not flocculating, but it increases dewatering

time and can increase flocculation.

Small scale basis weight variation is fixed after wire section and cannot be improved

after that.

Optically measured formation can be improved also in calendering but not mass

formation measured by beta radiation.

Optically measured formation is possible to measure online and also very fast in

laboratory. It is a very common measurement. However, correlation to printing

quality can be very poor, when paper is calendered. Also problems will arise for

highly bleached products and heavy weight products. (Robert Tolkki, KTH).

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Pele Oy Effects of good formation on paper properties

More even print result, less mottling in

offset, less missing dots in rotogravure.

Less print-through

Better paper smoothness

Higher paper gloss

Lower air permeability

Better tensile strength and stiffness

Due to lower calendering need to the

desired smoothness:

Better bulk and stiffness

Better strength properties

Less calender blackening or higher

moisture in calendering

Less dusting and linting

Better opacity and brightness

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Picture: Innventia

Pele Oy Wire shake example Valmet FormMaster 120

FormMaster 120 shakes the breast roll in the cross direction and breaks flocs by

creating shear forces to the web.

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Pele Oy Valmet example on wire shake improvement

Visual appearance of FormMaster improvement on formation of 210 gsm OCC

furnish sheet.

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Pele Oy FormMaster improvement on OCC furnish 210 gsm

Average floc size improvement is 52%. The most improvement is on the largest flocs.

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Pele Oy Three-layer SC paper formation

It is very difficult to get at the same time good formation and retention.

With Aqua-vane headbox, where filler is dosed through the Aq-vanes this is

possible. Lower number in the picture means better formation.

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Bo Norman et al. Innventia

PaperCon 2015

Pele Oy SC-paper

MD

Filler

Filler

Filler

Filler

Filler

Filler

Filler

Filler

Four A4 samples, one from each configuration. The

conclusion of a large set of pretrials was that the most

promising dosage strategy was dosing fillers through the

Aq-vanes only. This strategy was study with reference to

uniform filler dosage across the thickness of the paper.

Bo Norman et al. Innventia, PaperCon 2015 24