Analysis of the factors affecting the whiteness of paper

The main raw material for newsprint production is high-yield mechanical pulp. With the continuous development and expansion of newsprint industry, the shortage of papermaking raw materials, especially the shortage of forest resources, has brought many difficulties to the papermaking industry. Therefore, in the past 20 years, the paper industry has developed to use recycled fibers in part or in whole to replace mechanical pulp. With the change of the slurry, maintaining the target whiteness is facing challenges. The production of newsprint in our factory, like other manufacturers using recycled fibers, whiteness has become a major problem. The reasons for affecting the whiteness of paper are as follows:

1. Fluctuation of raw material quality

1. After a period of time, the analysis results of the existing raw materials in our factory show that the whiteness of the pulp after deinking is 9 # US waste is higher than daily waste, daily waste is higher than 8 # US waste, and 8 # US waste is higher than national waste . Because 9 # US waste storage time is short, the printing ink is not completely aging and floats on the surface of the paper (fiber), not sucked into the paper, it is easy to deink. On the contrary, 8 # American waste and national waste are recycled waste paper, and the storage time is different. Some waste newspapers have been printed for two years or even longer. The ink has been absorbed into the paper, and the binder in the ink becomes a fragile structure, thereby forming many 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 paper whiteness.

2. The quality of imported waste paper declined. At present, the paper recycling industry in some European countries has a fiber recycling rate of about 40%, and Japan has reached 50% or even higher. As a result, the paper quality has declined and the whiteness index has also shown a downward trend. Black and white newspapers in imported waste paper have a lower whiteness than color-printed newspapers, generally around 51%, and domestic newspapers are lower, generally around 47%.

2. Ink summer effect of waste newspaper deinking process

1. Ink summer effect

The fundamental reason for the summer effect of ink is that some changes in ink are caused after the temperature rises. Printing inks are mainly composed of pigments and binders, in addition, there are some fillers and additional materials. The black ink used for newsprint printing usually uses mineral oil as a binder, and the colored ink usually uses vegetable oil as a binder.

After the black ink is printed on the paper, the binder (mineral oil) in the ink is initially on the surface of the paper, but after a period of aging (4-6 months), the mineral oil is almost completely absorbed by the paper due to its The ability to absorb oxygen is poor. The binding material left on the paper is rarely cross-linked, and there is no stiff structure, only some fragile and fragile structures, so many small ink particles are formed.

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 the beginning, but due to its strong ability to absorb oxygen, after a period of aging, part of the vegetable oil is absorbed by the paper, but there are still Many remain on the paper surface and stick together with the pigment. Because vegetable oils have an unsaturated double-chain structure, they can form some cross-linked stiff structures and attach to fibers.

When old newspapers printed with mineral oil ink and vegetable oil ink are crushed in a disintegrating machine, mineral oil ink is easier to crush than vegetable oil ink, so the ERIC value after crushing is high (effective residual ink particle concentration) and after sufficient washing, The concentration of irreversible redeposited ink particles in the pulp is also greater, which has a greater impact on the whiteness of waste paper pulp.

2. Ink summer effect on whiteness

The whiteness of Canadian deinked pulp is the lowest from June to August every year. The whiteness of the deinked pulp in the northern United States is the lowest from July to August every year, which is about 1.4% lower than normal. The whiteness of waste paper desizing in our factory is also the lowest from July to August every year. For example, the minimum whiteness pass rate in 1998 was 46.2%, the minimum whiteness pass rate in 1999 was 71.2%, the minimum whiteness pass rate in 2000 was 2.6%, and the minimum whiteness pass rate in 2001 was 11.3%. This shows that no matter which country or region, the summer effect of ink deinking of waste paper is widespread.

3. The effect of surface sizing on paper whiteness

After sizing the surface of newsprint, the whiteness of the finished paper will continue to decline. In addition to factors such as raw material quality fluctuations and the summer effect of ink, oxidized starch glue also has a certain effect on the whiteness of paper. For example, the whiteness of the paper sheet before sizing is 54.4%, and the whiteness after sizing and calendering is 53%, which is a decrease of 1.4%. The results of multiple tests show that the whiteness of the paper sheet after sizing has decreased by about 2% .

4. Discussion

1. The shorter the storage time of the waste paper raw materials, the better the deinking and the whiteness of the pulp.
2. The summer effect of ink is caused by thermal drying or aging. Drying can promote the fragmentation of ink, which facilitates the removal of more ink particles in the deinking process, but also increases the concentration of irreversibly deposited ink particles.
3. Increasing the amount of deinking chemicals and reducing the disintegration time are also important measures 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.

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