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THE EFFECT OF REFINING ON PAPER FORMATION

Authors

Omid Ramezani* and Mousa M. Nazhad**

Companies

* Paper Science and Industry Dept., Tehran University, Iran
** Pulp and Paper Technology Dept., Asian Institute of Technology, Thailand

Keywords

refining, formation, PFI, fractionation

 

 

 

 

The objective of this study was to investigate the effect of refining on paper formation. As it was indicated PFI refining didn't imposed a considerable shortening effect on fibers and therefore parameters indicating formation (Specific Formation and formation Value) didn't reveal an increasing or decreasing trend.

INTRODUCTION

1.1 Formation

In addition to fibers, paper consists of fiber fragments, mineral fillers, and chemical additives. In the web formation process, they all settle stochastically on the web. Paper formation is the resulting nonuniform distribution of particles. A more precise definition of formation is the variability of the basis weight of paper. One can easily see the uneven structure of paper with the naked eye at length scales ranging from fractions of a millimeter to a few centimeters.

For practical purposes, a useful definition of formation is the small-scale basis weight variation in the plane of the paper sheet. The definition provides a simple measurement and an unambiguous connection to the structure of paper. Other terms for this property are mass formation, mass distribution, or the distribution of mass density.

Refining creates major changes in pulp properties and therefore may have a significant effect on paper formation. Refining results in internal fibrillation, external fibrillation, fiber shortening , fiber straightening, and fines formation. It has been reported that the shortening effect of refining would improve paper formation due to the decrease in the crowding number that leads to lower propensity to flocculate [1,2] and also smaller flocs existence [3].

An increase in refining intensity reduces both fiber length and fiber coarseness with this effect being most pronounced in double disc refiner systems, not quite so strong in Single disc systems, and weakest in the laboratory using a defibrator system [4]. Kerekes [5] in a comparative study between PFI mill and commercial refiners in refining energy, refining intensity and other factors governing action on pulp, indicated that the PFI mill is a high energy, low intensity refining device which produces a refining effect that differs significantly from an Escher-Wyss laboratory refiner, as a result of his experiment the higher internal fibrillation and lower external fibrillation and fiber shortening are due to the fact that the PFI mill imposes a greater proportion of compressive to shear forces than an industrial refiner.

Nazhad [6] found that for handsheets formed at headbox consistency, Escher Wyss (EW) refining improved consistency, whilst PFI refining did not. He also reported that although fiber shortening by refining improved formation, the other effects of refining worsen formation when paper is produced at headbox consistency and refining takes place at industrial levels of energy and intensity. According to the results of this paper, formation improvement on a paper machine caused by pulp refining occurs when the influence of fiber shortening outweighs the opposing influences of fibrillation and fiber straightening.

Overall, the effect of refining on formation is unclear. This paper tries to study the effects of PFI mill refining on paper formation.

EXPERIMENTAL

The experimental program was consisted of five main parts:

  • Refining under controlled conditions
  • Fractionation
  • Fiber length weighted average determination
  • Forming normal handsheets
  • Measurement of formation

2.1 Refining conditions

PFI mill was used to refining the pulps. Refining was performed under 5 different revolutions of 1000, 3000, 6000, 9000, and 12000.

2.2 Fractionation

The refined pulps were fractionated using Bauer-Mcnet fractionation apparatus and two fractions of 30 and 50 were collected for the following stages of experiment.

2.3. Fiber length

Fiber length weighted average was determined using Kaajani F-200.

2.4 Handsheet forming

The standard handsheet forming technique was employed in this experiment.

2.5 Formation measurement

Ambertec commercial formation tester was used to determine the formation values. Beta transmission is employed to measure the standard deviation in basis weight using a zone size of 1 mmm, which yields a COV for this zone size.

RESULTS AND DISCUSSION

3.1 Effect of refining on Fiber Length Weighted Average

3.1.1 Whole Pulp

The effect of refining on Fibers Length Weighted Average is shown in Figure 1. As it is shown PFI refining has no effect on fiber shortening, therefore, no specific improvement in paper formation could be expected. Also as it was measured the coarseness of fibers did not considerably changed and the values were almost the same in all the refining revolutions.

Figure 1

Fig. 1. The relation between the fibers length weighted average and refining
revolutions in the whole pulp sample

3.1.2 Fractionated Pulp: Fraction 30

No specific decreasing trend was obtained after treating pulps with different refining degrees expressing the no effect of PFI mill refining in fiber shortening (Fig. 2).

Figure 2

Fig. 2. The relation between the fibers length weighted average and refining
revolutions for the fraction 30

3.1.3 Fractionated Pulp: Fraction 50

Similar to fraction 30 no specific decreasing trend was observed in fraction 50. The relation between different refining revolutions and Length Weighted Average changes is shown in Figure 3.

Figure 3

Figure 3

3.2 Effect of refining on paper formation

As it is shown in the Figure 4, the formation values measured for this pulp confirms the approximate similarity in Fibers Length Weighted average, the considerable drop in formation of the handsheet made from Rev. 12000 refining could be attributed to uneven distribution of fibers during handsheets making due to the pulp low drainage.

Figure 4

Fig. 4. The relation between refining revolutions and formation value in fraction 30

Figure 5

Fig. 5. The relation between refining revolutions and specific formation in fraction 30

CONCLUSION

The objective of this study was to investigate the effect of refining on paper formation. As it was indicated PFI refining didn't imposed a considerable shortening effect on fibers and therefore parameters indicating formation (Specific Formation and formation Value) didn't reveal an increasing or decreasing trend.

REFERENCES

1. Kerekes, R.J. and Schell, C.J. "Characterization of Fiber Flocculation Regimes by a Crowding Factor". J. Pulp Pap. Sci. 18(1):32-38 (1992)

2. Kerekes, R.J. and Schell, C.J. "Effect of Fiber Length and Coarseness on Pulp Flocculation". Tappi J. 78(2):133-139 (1995)

3. Kerekes, R.J. "Perspectives on Fiber Flocculation in Papermaking". Proc. Int. Paper physics Conf., Niagara-on-the-lake, ON, Canada, pp.23-31 (1995)

4. Sundstrom L., Brolin A., Hartler N. "Fibrillation and its Importance for the Properties of Mechanical Pulp Fiber Sheets". Nord. Pulp Pap. Res. J. vol. 8,  no.4, Dec. 1993, pp 379-383

5. Kerekes R. "Characterising refining action in PFI mills". 6th Pira International refining conference, Toronto, Canada, 28-29 March 2001, 12pp

6. Stoere P., Nazhad M., Kerekes R. "An Experimental Study of the Effect of Refining on Paper Formation". Tappi J. vol. 84, no. 7, July 2001, p. 52

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