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85 μm. The ink mark length increased to 5.1 cm due to decreased ink penetration. In contrast, beating displayed a limited effect on the ink absorbability of hardwood paper, revealed by a slight decrease of void size from 70 μm to 90 um to 55 μm to 70 um when the beating revolutions increased from 5000 to 15000 revs. One possible explanation for this result was that the fiber distribution, with small surface voids, was relatively uniform for the shorter hardwood fibers. Effect of Different Pulps on the Ink Absorption The types and sizes of the fibers affected the ink absorption of the paper. The effect of different pulps, softwood pulp and hardwood pulp, on the ink absorption of the prepared papers is shown in Fig. 2.
Fig. 2. The ink marks of different pulp papers Figure 2 shows the changes of ink mark length for different papers prepared with various softwood pulp contents. The ink mark length first increased rapidly from 4.8 cm to 5.9 cm with the increase in softwood pulp content, and then it plateaued at 50% softwood pulp content (reaching a maximum length of 5.9 cm), and subsequently decreased to 4.3 cm. Generally, the fibers in softwood pulp are longer than those in hardwood pulp. Meanwhile, the high crystallinity index of softwood fibers hindered ink penetration and absorption into the paper. In contrast, the fibers in hardwood pulp are generally shorter and weak, which resulted in good ink absorbability. When the softwood pulp and the hardwood pulp were mixed together, particularly with each at 50% content, the prepared papers had longer ink mark lengths ( i . e ., low ink absorption). A possible reason for this result was that the fine fibers filled the network structure from long fibers stacking of softwood pulp to form a smooth and compact surface. Therefore, the ink penetration into the paper was hindered, resulting in low ink absorption. As the softwood pulp content was decreased to 0% (pure hardwood pulp), the smooth surface with large specific surface area and excellent liquid absorption for the obtained papers facilitated the adhesion and spreading of the ink. This led to high ink absorption (with a short ink mark length of 4.3 cm).
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Dong et al . (2020 ). “Ink absorption and overprint,” B io R esources 15(1), 1397-1406.
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