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Flocculation, Formation and Paper Properties

Flocculation, Formation and Paper Properties

Pekka KomulainenPekka.Komulainen@clarinet.fi

03 September, 2015

Pele OyFlocculation and formation

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

Fiber flocks 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 have no negative effects on the final paper properties.

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Flocculationin approach flow

Dispersionin headbox

Dispersionon wire

Reflocculation on wire

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FLOCCULATION

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Pele OyFlocculation 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 is 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 OyMechanical 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 OyFlocculation 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

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

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The criteria of flocculation for the papermaker is, how high mass consistency can be used in the headbox. However, theories often speak about volumetric concentration. Qualitative effects on flocculation are as follows:

Pele OyFormation 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 OyFlocculation 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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FORMATION

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Pele OyFlocculation 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 OyFormation 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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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 formationWire markWire mark

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Some visible paper formation faults

Fluting after coating

Cockling

Flow on wire

Large scale formation

Pele OyRetention, 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 discussed in this presentation.

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Pele OyExample 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 flocks from fines +

Longer dewatering time –

Dissolving of material (hemicellulose) Lower fiber coarseness –

++Total refining effect on formation:

Pele OyFlocculation, 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 OyEffects 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 OyThree-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 Innventia PaperCon 2015