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Sheet density
The elimination of fines from the whole pulp decreased the sheet density by about 24%, increasing the sheet’s bulkiness. However, further removal of R200 and R100 did not seem to significantly affect the sheet density. This suggests that the fibrous elements other than the fines had low flexibility and conformability, an important deficiency of mechanical fibres.
Tensile index
Excluding the fines from the whole pulp reduced the tensile index by about 14% (TABLE II). Further exclusion of R100 and R200 fractions further decreased the tensile strength, but less drastically. This could be due to the fact that these two fractions represented a relatively small amount of the whole pulp, about 10 and 7%, respectively (TABLE I). In addition, the R100 had a relatively high freeness of about 450 mL, indicating that these fibres were poorly developed. As such, it would have a relatively less important role in affecting the tensile strength.
Tear index
Removing the short fibres and fines from the whole pulp produced intriguing results in terms of tear strength. First, the loss of fines, about 20%, increased the tear index by about 47% (TABLE II), which could be explained by the conventional thinking that the tear index increases with fibre length. However, the magnitude of the increase in tear fell despite the increase in fibre length when the R200 fraction was also excluded. Similar phenomenon also occurred in the sample from which the R100 was also eliminated (TABLE II). The long fraction composing of R28 and R48 had, in fact, lower tear index compared with the whole pulp. This observation underlines the fact that the increase in fibre length alone does not necessarily improve the tearing resistance, implying an important role of fibre bonding, as earlier discussed. Hence, to produce a pulp with high tensile and tear strengths, fibre length must be preserved and fibre surface has to be well developed, which does not seem achievable by the current refining technology.
PFI Refining of Long Fibres
The concept of fractionation and refining of long fibres is not new. The question remains whether this technique is efficient in developing fibre properties. To shade more light on this issue, the long fractions (R28+R48) of the mill TMPi were refined in a PFI mill using 20k revolutions. The unrefined blend of R28/R48 had a ratio about 1.5, or 60% R28 and 40% R48. Fibre and sheet properties of the resulting pulps are given in TABLE III.
Fibre length distribution
After refining with 20k revolutions the proportion of R28 dropped from 60% to 16%, representing a 57% reduction (Fig. 4). The decrease in R28 indicates a significant fibre shortening effect. For the R48 fraction the change was relatively insignificant, down from 40% to about 38%. In fact, the fibre cutting effect on this fraction is unknown because part of the shortened R28 fibres could remain in the R48 fraction. However, the total loss of R28 and R48 combined was 46%, meaning that nearly half of the fibres had been shortened. As seen in Fig. 4, most of shortened fibres were retained on a 100-mesh screen, about 30% of the total refined pulp. The proportions for the R200 and P200 were comparatively smaller, 9.8 and 6.1%, respectively. The main issue here is if the refining had improved the bonding potential of the long fibres other than shortening fibres and generating fines. This question is treated in the following sections.
Freeness
As a result of the PFI refining (20k revolutions), the freeness fell from 660 mL to 162 mL, a 75% reduction (TABLE III). The freeness of the refined pulp rose back to well above 600 mL when the fines, which represented only 6.1% of the pulp mass, were removed. Further removal of the R200 and R100 fractions had little effect on the freeness, indicating that the fibrous elements other than the fines were not well fibrillated. This also means that fibre breakage is the main refining effect and that there is a possibility that the fibrillar elements of the fines were in fact generated during the fibre shortening process rather than from peeling of fibre surface because the latter would enhance fibre development.