On the Causes of Paper Whiteness

Newsprint production, the main raw material 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 greediness of forest resources, has brought many difficulties to the papermaking industry. Therefore, in the past 20 years, the paper industry has developed to use partially or completely recycled fiber instead of mechanical pulp. With the changes in the distribution of pulp, maintaining the target whiteness will face challenges. The production of newsprint at our factory, like other manufacturers that use recycled fiber, has become a major problem. The reasons for affecting paper whiteness are as follows:

First, the fluctuation of raw material quality

1. After analyzing the existing raw materials of our factory for a period of time, the results show that the whiteness of the pulp after deinking is 9# higher than that of the American waste, daily waste is higher than 8# US waste, and 8# US waste is higher than the national waste . Because of the short storage time of 9#, the printing ink is not completely aged and floats on the surface of the paper (fiber), and it is not absorbed into the paper, so it is easy to deink. On the contrary, 8# U.S. Waste and National Waste recycle used paper. The storage time varies, and 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 has become brittle, forming many fine ink particles that are not easily removed. Therefore, the length of storage of raw materials 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 some European countries, the fiber recycling rate is about 40%, and Japan is 50% or even higher. As a result, paper quality declined, and the whiteness index also showed a downward trend. Black and white newspapers in imported waste paper have a lower degree of whiteness than color newspapers. They are generally about 51%, and domestic newspapers are even lower, generally about 47%.

Second, waste newspaper deinking process ink summer effect

Ink summer effect

The root cause of the ink's summer effect is some changes in the ink caused by rising temperatures. Printing inks consist mainly of pigments and binders. In addition, there are fillers and additives. The black ink used for newsprint printing is commonly used as a binder for mineral inks, while colored inks are often used as binders for vegetable oils.

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 time (4-6 months) aging, the mineral oil is almost completely absorbed by the paper due to its The ability to absorb oxygen is poor, and the binder left on the paper is rarely cross-linked, there is no stiff structure, there are only some fragile and fragile structures, so many tiny ink particles are formed.

After the vegetable oil ink is printed on 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 also Many remain on the paper and the pigments stick together. Since vegetable oils all have an unsaturated double-stranded structure, some cross-linked, stiff structures can form and attach to the fibers.

When old newspapers printed with mineral oil and vegetable oil inks are disintegrated in a disintegrator, the mineral oil ink is more apt to be crushed than the vegetable oil ink, so the ERIC value after fragmentation is high (effective residual ink particle concentration) and after sufficient washing, The concentration of irreversibly re-deposited ink particles in the slurry is also greater, which has a greater effect on the whiteness of the waste pulp.

2. Effect of ink summer effect on whiteness

Deinking pulp in Canada has the lowest whiteness every year from June to August. The whiteness of the de-icing pulp in the northern United States is the lowest in each of July-August, about 1.4% lower than normal. The pulp whiteness of our factory is also the lowest from July to August each year. For example, in 1998, the minimum whiteness pass rate was 46.2%, and the minimum whiteness pass rate in 1999 was 71.2%. In 2000, the minimum whiteness pass rate was 2.6%. In 2001, the minimum whiteness pass rate was 11.3%. Thus, no matter the country or region, the summer effect of ink deinking waste paper is widespread.

Third, the effect of surface sizing on paper whiteness

After the newsprint surface sizing, the paper whiteness will continue to decline. In addition to raw material quality fluctuations, ink summer effect and other factors, the oxidized starch glue liquid also has a certain impact on the paper whiteness. For example, the whiteness of the paper before sizing is 54.4%, and the whiteness after sizing calendering is 53%, which is a decrease of 1.4%. After repeated tests, the whiteness of paper sheets after sizing dropped by an average of 2%. .

Fourth, discuss

1. The shorter the storage time of waste paper raw materials is, the more beneficial it is to deinking, which will help increase the whiteness of the pulp.
2. The summer effect of the ink is due to thermal drying or aging. Drying promotes the fragmentation of the ink and facilitates the removal of more ink particles during the deinking process, but also increases the irreversible deposition of the ink particle concentration.
3. Increase the amount of deinking chemicals. Decrease the disintegration time is also an important measure to control the summer effect of the ink.
4. The search for a new type of surface sizing agent is also an effective way to improve the whiteness of the paper.