Analysis of the Causes Affecting the Whiteness of Paper
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The main raw material for the production of newsprint is the use of high-yield mechanical pulp. With the continuous development and expansion of the newsprint industry, the shortage of papermaking materials, especially the scarcity of forest resources, has brought many difficulties to the paper industry. Therefore, in the past 20 years, the paper industry has developed the use of recycled fibers in part or in whole to replace mechanical pulp. With the change of the pulp, maintaining the target whiteness, it faces challenges. The production of newsprint in our factory is the same as other manufacturers using recycled fiber. Whiteness has become a major problem. The reasons that affect the whiteness of paper are as follows:
First, the fluctuation of raw material quality
1. After a period of analysis of the existing raw materials, the whiteness of the pulp after deinking is 9#. The US waste is higher than the daily waste, the daily waste is higher than the 8# US waste, and the 8# US waste is higher than the national waste. Because 9# US waste storage time is short, the printing ink is not completely aging and floats on the (fiber) surface of the paper, and is not sucked into the paper, so it is easy to deink. On the contrary, 8# US waste and waste are recycled paper, the storage time varies, and some waste newspapers have been printed for two years or even longer. The ink has been drawn into the paper, and the binder in the ink becomes a fragile structure, thereby forming a plurality of fine ink particles which are not easily removed. Therefore, the length of time the raw materials are stored directly affects the deinking effect and the whiteness of the paper.
2. The decline in the quality of imported waste paper. At present, in some papermaking industries in Europe, the recycling rate of fiber is about 40%, and that of Japan is 50% or even higher. As a result, paper quality has declined and whiteness indicators have also shown a downward trend. The black-and-white newspapers in imported waste papers have lower whiteness than the color-printed newspapers, generally around 51%, and domestic newspapers are lower, generally around 47%.
Second, waste newspaper deinking process ink summer effect
Ink summer effect
The root cause of the summer effect of the ink is that it causes some changes in the ink after the temperature rises. Printing inks are mainly composed of pigments and binders. In addition, there are some fillers and additives. Black inks used for newsprint printing are commonly used as binders for mineral inks, and vegetable oils are commonly used as binders for colored inks.
After the black ink is printed on the paper, the binder (mineral oil) in the ink starts at the surface of the paper, but after a period of time (4-6 months), the mineral oil is almost completely absorbed by the paper due to its The ability to absorb oxygen is poor, the binder remaining on the paper is rarely crosslinked, nor has a stiff structure, and only some fragile and fragile structures form many small ink particles.
After the vegetable oil ink is printed on the paper, the binder (vegetable oil) in the ink is also on the surface of the paper at first, but because of its strong ability to absorb oxygen, after a period of aging, part of the vegetable oil is absorbed by the paper, but there is still Many remain on the paper and glue together. Since vegetable oils have an unsaturated double-stranded structure, some crosslinked rigid structures can be formed and attached to the fibers.
When the old newspaper printed with mineral oil ink and vegetable oil ink is disintegrated in the disintegrator, the mineral oil ink is more likely to be shredded than the vegetable oil ink, so the ERIC value after the reduction is high (effective residual ink particle concentration) and after sufficient washing, The concentration of ink particles that are irreversibly redeposited in the slurry is also large, which has a greater influence on the whiteness of the waste paper pulp.
2. The effect of ink summer effect on whiteness
Canadian deinking pulp whiteness is the lowest in June-August. The whiteness of deinked pulp in the northern United States is the lowest in July-August, which is about 1.4% lower than normal. The bleaching whiteness of our waste paper is also the lowest in July-August. For example, the minimum whiteness pass rate in 2014 is 46.2%, the minimum whiteness pass rate in 99 years is 71.2%, the lowest whiteness pass rate in 2000 is 2.6%, and the lowest whiteness pass rate in 2001 is 11.3%. It can be seen that the summer effect of ink deinking of waste paper is common in that country or region.
Third, the effect of surface sizing on the whiteness of paper
After the surface of the news paper is sized, the whiteness of the paper will continue to decrease. In addition to the fluctuation of the quality of the raw materials and the summer effect of the ink, the oxidized starch glue has a certain influence on the whiteness of the paper. For example, the whiteness of the paper before sizing is 54.4%, and the whiteness after sizing is 53%, which is decreased by 1.4%. After many tests, the whiteness of the paper has decreased by about 2% on average. .
Fourth, discussion
1. The shorter the storage time of waste paper raw materials, the better the deinking is, which is beneficial to improve the whiteness of the pulp.
2. The summer effect of the ink is due to heat drying or aging. Drying promotes the shredding of the ink, allowing more ink particles to be removed during the deinking process, but also increasing the concentration of irreversibly deposited ink particles.
3. Increasing the amount of deinking chemicals to reduce the disintegration time is also an important measure to control the summer effect of the ink.
4. Seeking a new type of surface sizing agent is also an effective way to improve the whiteness of paper.