Part 2: Synergistic effects between P-RC APMP and bleached kraft pulps from Canadian Aspen

Eric C Xu

Preprint of 89th Annual Meeting of the Pulp and Paper Technical Association of Canada, Montreal, Quebec
27-30 January 2003

1. Summary

P-RC APMP and commercial market bleached kraft pulps from Canadian aspen wood were compared both separately and in different combinations in low consistency refining. The results showed that

  • There is synergistic effect on inter-fibre bonding strength between P-RC APMP and bleached kraft pulps from Canadian aspen: the combined pulp has a higher tensile, tensile energy absorption and stretch than the weighted contributions from each component;
  • Replacing a certain percentage of aspen bleached kraft pulp with aspen P-RC APMP improves not only pulp bulk, light scattering, and opacity, but tensile, tensile energy absorption and stretch as well;
  • Addition of aspen P-RC APMP pulp to aspen kraft pulp does note change the freeness/energy curve (trend).

2. Introduction

In papermaking, it is very often that more than one type (or grade) of pulps are used to develop paper sheet properties necessary for both machine runnability and requirement from the end users. In many high end paper grades (e.g., various high brightness printing/writing papers), long fiber from softwood bleached kraft pulp traditionally is often used in combination with short fiber from hardwood bleached kraft pulp (HBKP). In this combination, the long fiber component provides the strength (especially tear resistance) to the paper sheet, while the short fiber helps to improve the paper functional properties, such as sheet smoothness and formation.  More recently, high brightness, high yield chemical mechanical hardwood pulps have been introduced, primarily as a partial replacement of HBKP in those high value added paper grades.  This introduction of hardwood high yield chemical mechanical pulp has been reported to improve sheet (paper) bulk and opacity, and to give paper a better stiffness [1]. The trend of using hardwood high yield chemical mechanical pulp is expected to continue and increase as the industry learns more about the pulp [2].

In production of high yield chemical mechanical pulps, there have been several different pulping technologies developed over the past thirty years.  The most recent development is a process called P-RC APMP [3], which applies alkaline peroxide chemicals before and during the refining process to produce high yield (>85%) and high brightness (>85% ISO) pulp without post bleaching. There have been several reports on some of the latest developments of this technology [4-6] and its application on various hardwood species [7-9].

In this study, two different grades of Canadian aspen P-RC APMP pulps produced at a commercial mill were compared and blended at different percentages with market bleached kraft pulp from Canadian aspen. The objective was to investigate how pulp properties develop from the different pulp furnishes (either separately or in combination) using low consistency refining.

3. Experimental

Standard Canadian aspen market bleached kraft pulp (BKP) and Canadian aspen P-RC APMP pulp from a commercial mill trial were used in this investigation. The pulps were received in 90% dry form. Two high brightness (85% ISO) grades of aspen P-RC APMP pulps were used: high bulk (HB) and high tensile (HT).

The pulps were first refined separately and then in combination at different ratios:

  • 10% P-RC APMP HT and 90% BKP
  • 20% P-RC APMP HT and 80% BKP
  • 10% P-RC APMP HB and 90% BKP
  • 20% P-RC APMP HB and 80% BKP

The refining was done using an Andritz 20" (31 cm) Twin-Flo IIIB Refiner at 4% consistency, 500C. Andritz/Durametal low intensity plates (Pattern 22TA013/014) were used.  For each pulp furnish, four to five specific energies were applied to generate energy curves and to develop pulp properties at different freeness levels for analysis.

The refined pulps were dewatered and hot water disintegrated before pulp testing.  All pulp testes were performed according to TAPPI standards, except freeness, which was by Canadian Standard Freeness (CSF).

Figure 1

Figure 1: Freeness and Energy

Figure 2

Figure 2: Freeness and Specific Edge Load

4. Results and discussion

4.1. Freeness and energy consumption

Figure 1 presents the freeness development at different specific energy consumptions (SEC).  The initial freeness before refining was 550 ml CSF for the kraft pulp, 350 ml CSF for the P-RC APMP high bulk grade (P-RC HB), and 200 ml CSF for the P-RC APMP high tensile grade (P-RC HT). When refined separately, the kraft pulp had different freeness/energy relationship (curve) from the P-RC APMP pulps, which had similar freeness/energy trends. For the pulp blends, although different in their initial freeness, they all had a similar freeness/energy trend as, (parallel to), the kraft pulp, suggesting that adding up to 20% of the P-RC APMP to the kraft had no effect on the freeness development, (amount of freeness change per unit energy, SEC), compared to the kraft itself. This means that for a paper mill that runs aspen BKP in their furnish, replacing up to 20% of the BKP with aspen P-RC APMP will not require any significant adjustment of its refining operation for stock preparation.

Similar trends were also observed for the freeness and specific edge load (SEL) relationship, as shown in Figure 2.  In this investigation, the SEL was controlled under 0.95 w.s/m.

4.2. Pulp physical properties

Bulk
Figure 3 shows how the pulp handsheet bulk property developed for the different furnishes. While it is expected that the high yield P-RC APMP pulps had higher bulk than the kraft pulp, all the blends also had higher bulk than the kraft pulp.  Bulk of the blended pulp increased at a higher P-RC APMP ratio and with a higher initial bulk from the P-RC APMP component.  There was a 0.15-0.20 cm3/g increase from the kraft to 20% replacement with the P-RC APMP HB pulp. The blends from the P-RC APMP HT had half of the gain in bulk compared to that from the high bulk pulp at the same percentage of replacement.

Figure 3

Figure 3: Bulk and Freeness

Figure 4

Figure 4: Tensile and Freeness

Tensile
Of great interest from this study was pulp tensile, (a measure of inter fibre bonding strength), development from those pulp blends. Figure 4 shows that, although the P-RC APMP pulps both had a lower tensile at a given freeness than the kraft pulp, the blends, after being refined to 350 ml CSF or lower, had the same, (from the high bulk, low tensile P-RC APMP HB), or even higher, (from the high tensile P-RC APMP HT), tensile compared to the kraft.  These results suggest that there is a synergistic effect between the P-RC APMP and the kraft pulps: the combined pulp has a high inter-fibre bonding strength than the weighted contributions from each of the components. The blend from the P-RC APMP HT had, on average, approximately 5 points higher tensile index than the kraft, even though the former had a comparable or lower tensile than the latter at a given freeness.

Figure 5

Figure 5: Tensile Energy Absorption (T.E.A.)

Figure 6

Figure 6. Stretch and Freeness

Tensile Energy Absorption
The synergistic effect was also observed in tensile energy absorption (T.E.A.) development, (Figure 5).  Although the P-RC APMP pulps used in this study both had significantly lower T.E.A. than the kraft (by 20 or more points), the blends had either the same (from the P-RC APMP HB) or higher (from the P-RC APMP HT) T.E.A. than the bleached kraft pulp.

Stretch
There was also a synergistic effect on the pulp stretch property from the blend, as shown in Figure 6. Again, the blend had similar or higher stretch than the kraft, although the P-RC APMP pulps both had significant lower stretch than the kraft.  The blends from the P-RC APMP HT, which had higher stretch than the P-RC APMP HB, had higher stretch than the blends from the P-RC APMP HB pulp.

Optical Properties
As expected, the blends all had higher light scattering (Figure 7) and higher opacity (Figure 8) than the kraft pulp.  The blends from the P-RC APMP HB had higher light scattering and opacity than that from the P-RC APMP HT.  The difference (5 points) between the former and the kraft was approximately twice the amount of that between the latter and the kraft.

Figure 7

Figure 7: Light Scattering and Freeness

Figure 8

Figure 8. Opacity and Freeness

5. Discussions

The synergistic effects between the P-RC APMP and the kraft pulps are the most interesting and important observations in this investigation.  This phenomenon may be explained in terms of different fiber distribution and surface chemistry of the P-RC APMP pulp and the kraft. Being chemical mechanical pulp, the P-RC APMP pulps by nature are expected to have a wider fiber size distribution than the kraft pulps, which in turn is expected to help form more "bridges", or "contacts", among the fibers, and hence, improve overall inter-fiber bonding strength. And also, as discussed elsewhere [7, 8, 10], P-RC APMP pulps, like other chemical mechanical pulps, are expected to have higher hemicellulose content on fiber surface than the kraft, as indicated by the fact that the former has a higher pulp intrinsic properties (higher tensile at the same density) than the latter, as reported earlier [7, 8, 10].

Analysis of Figures 4 through 6 indicate that the synergistic effects seem to have more to do with the nature, (high bulk or high tensile), of the P-RC APMP pulps than the amount, or the percentage (10% or 20%), of the P-RC APMP in the blend - the blends having different amounts of the same P-RC APMP pulps had approximately the same amount of the synergistic effect. More work, however, is needed to have a better understanding of the mechanism of the synergistic effects observed.

6. Conclusions

  • There is synergistic effect on inter-fibre bonding strength between P-RC APMP and bleached kraft pulps from Canadian aspen: the combined pulp has a higher tensile, tensile energy absorption and stretch than the weighted contributions from each component.
  • Replacing a certain percentage of aspen bleached kraft pulp with aspen P-RC APMP can improve not only pulp bulk, light scattering, and opacity, but tensile, tensile energy absorption and stretch as well.
  • Addition of aspen P-RC APMP pulp to aspen kraft pulp does note change the freeness/energy curve (trend).

7. References

Reis, R.J. and Nielsen, G., "Aspen BCTMP: Proven Performance", Solutions! 2001 (11): 28 (2001).

Cannell, E., and Cockram, R., "The Future of BCTMP", Pulp and Paper 2002 (5): 61 (2002)

Xu, E.C., "A New Concept In Alkaline Peroxide Refiner Mechanical Pulping", International Mechanical Pulping Conference, Houston, USA, May, 1999.

Xu, E.C. "Some Latest Developments In Alkaline Peroxide Mechanical Pulping, Part 1: Combination of chip pretreatment and refiner bleaching", Tappi Pulping Conference Proceedings, Seattle, WA, Nov. 4-7, 2001.

Xu, E.C, Koefler H. and Antensteiner P., "Some Latest Developments In Alkaline Peroxide Mechanical Pulping, Part 2: Lower Consistency Refining at Secondary", Preprint of 88th PAPTAC Annual Meeting, Montreal, pp. C1. Quebec, (Jan. 28-Feb. 1, 2002).

Authors and literature cited

Authors:

Eric C Xu - Andritz Inc. R&D Lab., Springfield, USA
Dieter Teubner - Andritz AG, Vienna, Austria

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