Pele Oy Flocculation, Formation and Paper Properties Pekka Komulainen 03 September, 2015

Pele Oy Flocculation 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 mm 2 ) 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. 2 Flocculation in approach flow Dispersion in headbox Dispersion on wire Reflocculation on wire

Pele Oy FLOCCULATION 3

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 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. 4 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. 5 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 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 6 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 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. 7 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. 8

Pele Oy FORMATION 9

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. 10 Picture: Jing Yan

Pele Oy Formation measurement Real formation is measured by small scale beta ray absorption (Ambertec). Normally standard deviation of grammage (g/m 2 ) 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. 11

Pele Oy 12 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

Pele Oy 13 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 discussed in this presentation. 14

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. 15

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). 16

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 17 Picture: Innventia

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. 18 Bo Norman Innventia PaperCon 2015