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THE EFFECT OF COARSENESS ON PAPER FORMATION
Formation is important not only for the appearance of the paper but also because the non-uniformity of fiber distribution corresponds to the non-uniformity in strength, density and porosity of paper. This study aimed to investigate the effect of fiber coarseness on the formation of paper. It was determined that formation correlates well with coarseness. Formation was improved whenever fiber length and corresponding coarseness was reduced for unbeaten pulp.
Fiber coarseness is defined as weight per fiber length and is normally expressed in units of mg/m or g/m. Coarseness depends on fiber diameter, cell wall thickness, cell wall density and fiber cross section. The coarseness value has a great influence for the paper structure. A high coarseness value indicates a thick fiber wall, giving stiff fibers unable to collapse. Thin walled fibers with low coarseness value give flexible fibers and a denser sheet. The coarser the fibers, the stronger they will be. Coarser fibers make strong paper. This is important for packaging paper and less important for printing paper. Coarse fiber will cause uneven paper surface i.e., bad formation. This is very important for most printing paper and not critical for most packaging paper. Also for this reason, coarse fibers are disadvantageous for printing paper. Coarseness influences fiber failures, fiber strength and inters fiber-bonding . Hardwood fibers have lower fiber coarseness value than softwood fibers, because coarseness increases with the increase of fiber length and fiber wall thickness. Hardwood pulp fibers give better formation than softwood fibers, due to lower coarseness value. Fiber coarseness varies with wood species but the difference in springwood and summerwood fibers also is considerable. The thick walled late wood or summerwood fibers have distinctly higher coarseness value than thin walled early wood fibers.
Coarseness is the most important property for tissue papers, and the lower the coarseness the better, as it gives higher handfeel, higher tensile strength at constant number of fibers, higher absorption and bulk, and better formation.
The generally accepted definition of formation is the small-scale basis weight variation. Formation describes the uniformity of the sheet structure and orientation of the fibers. It is well known that paper formation is strongly affected by furnish composition as well as by operating parameters on the paper machine .
Formation is important not only for the appearance of the paper but also because the non-uniformity of fiber distribution corresponds to the non-uniformity in strength, density and porosity of paper. However, the role of fiber properties on paper formation is still vague and inconclusive.
This study aimed to investigate the effect of fiber coarseness on the formation of paper.
2. MATERIALS AND METHODS
Softwood bleached kraft pulp was used as a raw material.
Bauer McNett fiber classifier was used to fractionate the fiber raw material into four different length classes. This was carried out according to standard SCAN-M 6:69. coarseness of all the samples was examined by using Kajaani FS-200 fiber length analyzer according to standard TAPPI T-271 pm-91. Samples were prepared according to Appendix II.
Fiber suspensions of 0.02% and 0.2% consistency were prepared from each of the four fractions, A, B, C, and D. Handsheets having a basis weight of 60 g/m2 were made from each fraction using standard handsheet (SH) former.
To examine the effect of coarseness on the formation, fractions were made from fiber blends having the same average fiber length, but differing in coarseness.
To obtain two fractions having the same average fiber length but differing in coarseness the equation (1) was employed and results were verified using Kajaani FS-200.
L: length weighed average fiber length
li : average fiber length in fraction i
wi : oven-dry weight of fraction i
To produce fractions (i.e. E and F) with same average fiber length as B, pulp mixtures were produced using 34.72% of fraction A and 65.28% of fraction C for mixture E and 44.25% of fraction A and 55.75% of fraction D for mixture of F.
3.1 Pulp Properties
Fiber length (Length Weighted Fiber Length) and average fiber coarseness of the four fiber fractions, A, B, C and D were determined using Kajaani Fs-200. The results are reported in Table 3.1. It shows that for a given length fraction the longer fiber is also a coarser fiber.
Table 3.1 Summary of pulp properties
Fig 3.1 suggests that fiber length increases if coarseness is increased. The coarseness tended to reduce for shorter fiber length fractions. Johnstone and his colleagues also found the similar relationship of fiber length and coarseness .
Fig 3.1 Relation between fiber length and coarseness
At a constant fiber length, there was a slight difference of fiber coarseness for a mixed fiber fraction in comparison with a single fiber fraction. Fraction E, the mix of fraction A and C, showed a slightly higher coarseness than the fraction B did. This was due to the high amount of long fiber fractions, C required to boost the mean fiber length to the value of fraction B. According to the results, fiber classifiers not only separate pulps into different length classes but also sort pulps according to fiber coarseness. Therefore, both fiber length and coarseness would influence the variation of paper properties focused in this study.
3.2 Formation and fiber properties
Handsheets in different consistencies were made from different fiber length fractions as R30, R50, R100 and R200. Formation of these handsheets was measured by using Beta Radiation, which is not sensitive to the type of furnish. The results are illustrated in Fig 3.2. Lower values of Specific Formation indicate better formation where as higher values represent poor formation.
Fig 3.2 Formation of handsheets made from different fractions
It is clear that papers made from short fibers of fraction 200 had better formation. The values become higher for fractions 100, 50, and 30 as the average fiber length improved. This means that formation deteriorates more and more with fiber length increase.
To study the effect of coarseness on the formation, fractions E and F were made from fiber blends having the same average fiber length equal to fraction B, but differing in coarseness. The coarseness of these blends has been summarized in table 2.3.
Table 3.2 Specific formations of handsheets made of different fractions
Fig 3.3 shows a considerable relation between coarseness and formation. As it is illustrated, coarseness has an adverse effect on formation i.e. the higher amounts of coarseness represent a poor formation in the handsheets.
Fig 3.3 Relationship between coarseness and formation
According to the results specific formation can be a function of fiber length and coarseness. The sheets of longer and coarser fiber had worse formation than the sheets of shorter and slender fiber. The reason is that the long and coarse fibers have a greater tendency to flocculate during sheet making that is detrimental to formation . The number of polygons formed by the intersecting fibers will also be fewer and larger in size. Thus, the network will be a less uniform with more open structure. On the other hand, sheets with shorter and slender fibers could be more uniform because of drainage effects . As shorter and slender fibers are more mobile, therefore, the drainage mechanism, based on the self-healing effect, easily causes the even fiber distribution overall area of sheet.
Formation correlates well with coarseness. Formation was improved whenever fiber length and corresponding coarseness was reduced for unbeaten pulp.
This work was financed by the faculty of Pulp and Paper Technology (PPT) in Asian Institute of Technology (AIT).
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A representative sample of approximately 1.3 g O.D. pulp slurry was taken. The device in Figure 1 was inserted into the Finish hand sheet maker and placed on the wire. The pulp sample and required amount of water were added and agitated. Water was drained and the mini hand sheets to be formed. The Plexiglas was removed carefully from the hand sheet maker and collect the mini hand sheets. Three of the big circular (A1 – A6) sheets were collected into the aluminum container dry in the oven and measure the O.D. weight according the SCAN method. The other three mini sheets were collected into 2-liter volumetric flask and making two-liter suspension by adding water. Since all of the six mini sheets are in same dimensions, O.D. weight of the dried mini sheets are equal to the remaining three sheets those were collected into the two-liter volumetric flask. Fiber length and coarseness of the sample were measured using the suspension of two-liter, by using Kajaani FS200 according to standard T 271 pm-91.
Figure 4 Mini hand sheet maker for coarseness measurements (Diameter of holes A-27 mm, B-16 mm, and C-7.14 mm).