Research into mechanical pulping process development can lead to improvements in the properties of printing papers

Making off-the-peg paper from designer pulp

Bread and butter, fish and chips, gin and tonic. Some things just go well together. And so it is with each paper grade and its own particular type of fiber. Most paper properties that are critical for runability and printability can be improved by process development, and in paper terms, this means characterizing and developing fiber properties at each step of the manufacturing process.

Research conducted at KCL in Finland shows that there is still considerable potential for developing the refining and screening processes in the production of thermomechanical pulp (TMP). Recent advances in high intensity refining and fractionation should be further optimized to achieve lower production costs and better quality. Based on trials conducted at the research institute's pilot plant, the four following examples show how process development can be used to improve pulp characteristics. The pilot plant allows each step of the papermaking process to be performed separately, which means process conditions can be varied over a wide range.

Figure 1 - KCL pilot plant flow sheet

Refined solution

At the plant, pulp from each fiber processing stage can be taken separately to the pilot PM. A simulation study on a mill screening process revealed that lightweight coated (LWC) paper made from thermomechanical pulp (TMP) mainline screening accept had high smoothness, gloss and opacity, but exhibited the same fiber roughening as LWC paper prepared from the whole pulp flow1 (Table 1).

This means that the quality of the fiber accept from screening was not optimized as roughening took place with a very low long fiber content. Pilot refiner studies have shown that fiber properties can be developed in the refiner itself, even at lower energy consumption levels. These findings show that there is plenty of scope for improving fiber preparation processes.

Moving to the second point, it is clear that by raising the speed of KCL's 1.1 m pilot refiner to 2,400 rpm, the refining intensity can be increased through shorter pulp residence time in the refiner plate gap. At the same time, the refining temperature can be raised to liberate the fibers more easily and prevent fiber cutting. Changing the refining consistency or installing plates to reduce the residence time in the refining zone can also modify the intensity. With these adjustments, up to 25% of the energy can be saved with retention of fiber length at low freeness (Figure 2).

Figure 2

In our example, tear index was only 6% lower (Figure 3),

Figure 3

but tensile strength was high. High temperature and intensity in refining also gave improved long fiber bonding ability (Figure 4),

Figure 4

which makes it possible to produce smooth paper with low fiber roughening tendency. If the refiner is operated under non-optimum conditions, fiber cutting takes place and fiber bonding ability does not develop2.

In the third case, the fine slotted wedge wire screen basket has made removal of shives and coarse fibers easier than screening with traditional slotted baskets. However, the narrow slots may lower the screening capacity and render screening conditions far from optimum. This can cause a "leak" of stiff fibers or even shives into the accept pulp from screening. These fibers can be more flexible than unrefined reject fibers. But after reject refining, it is the reject fibers that are actually more flexible. As a result, the accept fibers are the most harmful fibers on the paper surface (Figure 5).

Figure 5

Pulp of freeness 30-40 ml can be made free of measurable shives by optimizing the screening conditions. By changing the baskets, feeding consistency and rotor speed with the KCL Valmet TAP 50 screens, all shives and even increased amounts of BMcN -14/+28 fibers can be removed into the reject at a given mass reject rate (Figure 6).

Figure 6

This allows reject refining energy to be directed specifically at harmful shives and long fiber. These fractionating operations ensure that the accept pulp is free of harmful fibers and that reject fibers are subjected to a large amount of energy to make them more flexible.

In mill papers the fibers are oriented, while dewatering and drying on the paper machine differ from the corresponding operations in the laboratory. Tear strength and surface properties are not reliable when measured from handsheets. Even fiber roughening during offset printing differs from water-induced fiber roughening without ink. The ink on the paper surface does not let the fibers draw back onto the paper to the same extent as without ink.

The effect of each fiber fraction flow and final pulp on surface properties can easily be tested by making paper on a pilot paper machine with or without additives. Paper made from each fiber fraction can be coated, calendered and printed on a full-scale heatset press. Gloss development for each component can easily be compared by calendering on the pilot calender. Fiber roughening tendency depends on fiber characteristics, coating structure and coat weight, as well as printing conditions.

A study using pressurised groundwood (PGW) pit pulps screened at KCL's pilot plant showed that pulp fiber quality affects smoothness and roughening behavior as a function of coat weight (Figure 7).

Figure 7

Fiber fractionation gave smoother LWC paper and the roughening diminished along with the coat weight. This showed that fractioning and directing the refining energy at the stiffest fibers improved the smoothness.

Final thoughts

To improve the paper product, we need an accurate characterization for each step of the fiber process. The four examples above show some results of process optimization and development. From these we can see that both refining and screening have a great impact on quality and need to be studied thoroughly.

With current technology it is possible to produce a broad range of different pulps and influence the end-product. But producing the right pulp for a specific paper grade demands a profound understanding of the process.

Ilkka Nurminen is research scientist at KCL Science and Consulting, the Finnish Pulp and Paper Research Institute. He can be contacted on Ilkka.Nurminen@kcl.fi Markus Mannström is vice president at KCL Services. He can be contacted on Markus.Mannstrom@kcl.fi

References

1. Nurminen, I and Sundholm, J, "The Role of Mainline Screening Accept and Refined Reject in Fiber Roughening in LWC Papers". Preprints of International Mechanical Pulping Conference, Stockholm, Sweden, 1997.

    

2. Nurminen, I, "Influence of Refining Temperature and Intensity on TMP Energy Consumption and Pulp and Fiber fraction properties". Preprints of International Mechanical Pulping Conference, Houston, Texas, USA, 1999

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