Analysis of the Causes of Paper Flake and Hair Loss in Printing

Dropping and linting refers to the shedding of loose particles on the surface of the paper during the printing process. These particles are mainly composed of single fibers, fillers, rubber compounds and coating particles, or hairs or powders composed of aggregates of these substances. Or scattered on the surface of the paper, or loosely stuck to the surface of the paper, easily fall off.

Paper falling out of powder and hair loss can cause dirty prints on the printed matter, making the printed image text blurry and mixed with white spots (dot). This article focuses on the influence of paper structure and coating structure on powder falling and lint-free printing and some control measures.

The reasons for falling powder and hair loss

The following reasons are summarized in the following reasons:
(1) Poor adhesion between paper fibers or between fibers and fillers;
(2) The surface of the paper has poor fiber bonding or low bonding strength;
(3) During the printing process, electrostatic charges are generated between the paper fiber and the printing plate due to friction. The weak fiber and filler on the surface of the paper are attracted by the positively charged printing plate;
(4) In the embossing zone, the paper is weakened due to the tension and compression of the paper, and the fibers or fillers that are loosely bonded to the surface fall off.

Paper structure effects and control measures

1. The two sides of the paper

The two sides of the paper have a great influence on the quality of the printing paper, which can lead to uneven quality of the printed matter. The smoothness of the front side of the paper is high, and the ink repellent effect is good, but the surface strength is lower than the reverse side, and powder dropping and hair loss are easy during printing. On the contrary, the reverse side of the paper is relatively rough, and the inking effect is worse than the positive one, but the surface strength is high, and it is not easy to lose powder and lose hair in printing. The actual printing also proves that the heap of powder on the blanket in contact with the front side of the paper is much more serious than the reverse side. Since the two sides of the printing paper are not conducive to the quality control of the printed matter, the printing should use a paper with a small difference in two sides.

In order to overcome the two sides of paper, at present paper mills at home and abroad have generally adopted some measures: (1) Adding a stack of nets on a Fourdrinier paper machine to properly dewater the front of the paper sheet so that the properties of the two sides of the paper are closer to each other; (2) using chemical additives; To ensure that the filler and small fibers adhere to the opposite side as firmly as possible to reduce the loss, but also increase the surface bonding strength; 3 rational use of press and calendering equipment, improve the two sides of the paper; 4 foreign papermaking equipment has been used cylinder paper machine, Simultaneous dehydration on both sides greatly reduces the difference between the two sides of the paper.

2. The vertical structure of the paper

The vertical structure of the paper, ie, the structure of the paper in the vertical direction (Z), the most important structural property is the degree of bonding between the fibers, and the bonding state determines the vertical tensile strength of the paper, which is called peel strength or Z-direction strength. . The peeling strength is very important in the printing, especially in web offset printing. The powder dropping and hair loss in the printing process are caused by the lack of Z-direction strength of the paper. The second important property is the distribution of fine fibers and fillers in the Z-direction. During the sheet forming process, due to the loss of fine fibers and fillers on the web, the amount of fine fibers and fillers on the front side of the paper is higher than that of the web, and between them The combination of poor, this is the paper printing powder, hair loss, another reason.

3. Paper formation

The uniformity of the paper indicates the homogeneity of the structure, that is, the distribution (scattering) of fibers and other solid materials (such as fillers) in the paper. The uniformity of the paper also has certain influence on the off-powder and off-fat. The content and distribution of the fine fibers and fillers in the poorly formed paper sheet are not uniform, and the tightness of the sheet after passing through the press section and the calender is inconsistent, resulting in a difference in the bonding strength, causing the paper to lose powder and lose hair. The formation is not good, but also makes the paper leaves produce cloud flowers, the water content of the paper containing clouds is usually uneven, in order to balance the moisture of the paper, but had to dry the paper sheet, and thus easy to make paper appear powder off, hair loss phenomenon .

In recent years, papermaking has used double-layer forming nets in order to better support the paper sheets, so that more fine fibers are retained inside the paper sheets, and the two sides of the paper are tightly structured, so that the phenomenon of powder dropping and hair loss is alleviated. The rotary screen machine is double-sided dewatering, which removes potentially hair-dropping materials (crude fibers and fine fibers) on both sides of the sheet, and can produce paper with a uniform structure and small difference on both sides.

4 paper surface bonding strength

The bond strength of the paper surface refers to the bond strength between the fiber, the filler, and the compound, which reflects the ability of the paper to resist powder loss and hair loss. In the printing process, the ink will tear off some fibers on the surface of the paper, and the force that causes hair loss is usually perpendicular to the surface of the paper. When the paper is separated from the plate, the surface layer may partially fall off. And blistering.

The method of improving the surface bonding strength: rationally controlling the beating, increasing the degree of beating properly, that is, increasing the binding force, improving the tightness of the paper, reasonably controlling the proportion and amount of the filler, improving the pressing process, appropriately increasing the pressure of the press section, and increasing the tightness of the paper; Increase the pressure of calendering to make the paper sheet more compact; apply surface sizing to make the sizing agent penetrate into the interior of the paper sheet; increase the content of long fiber in the pulp, and properly cut the fiber in the beating process to make the fiber fully Relief and deuteration; try to increase the number of free hydroxyl groups, thereby increasing the binding force between the fibers in the paper.

Effect of Coatings on Flake and Hair Loss of Coated Printing Paper

1. Purpose of coating

The purpose of coating the paper is to cover the surface of the paper which is formed by the unevenness of the pulp fibers and the apertured paper with a coating material composed of fine particles and having a good ink absorption property, so as to obtain better uniformity, smoothness and Printing printability and so on. After the paper is coated, the uniformity, tightness and printability of the paper are improved, and the powder dropping and hair loss are reduced.

2. The reason of falling powder and hair loss

For coated paper, the reason for the powder dropping and linting is as follows: 1 The binding force of the base paper is insufficient, and the single fiber and the filler particles fall off from the paper together with the coating. It can be eliminated by the surface sizing method of the base paper, and it can also prevent the adhesive from penetrating from the coating layer into the base paper and cause the coating layer to loosen. 2 powder off is not a problem with the base paper, but the coating layer is peeled off from the surface of the paper and can be eliminated by increasing the amount of the adhesive in the coating layer. 3 The choice or amount of adhesives, coating methods, types of additives, drying, curing, aging conditions, calendering and other factors will also affect the extent of the coated paper off the powder, hair loss.
3. Effect of coating base paper

The coating is based on the base paper for secondary processing, but the physical properties of the base paper are not much improved. Therefore, for the obvious defects in the original paper itself, coating alone cannot solve it.

The absorption performance of the base paper is a factor that affects the strength of the coating and the strength of the bond between the coating and the base paper. After the paint is applied to the surface of the base paper, the moisture and adhesive in the paint are partially transferred to the paper. Overabsorption of the base paper will result in excessive transfer of the adhesive, which will severely impair the bond strength between the filler particles. During the printing, powder and lint will be generated inside the coating. Conversely, if the absorbency of the base paper is too small, the paint adhesive cannot adhere to the paper and enter the base paper, thereby failing to generate sufficient adhesive force. Therefore, the raw paper must have a certain degree of sizing and proper porosity in order to ensure normal absorption.

4. Effect of paint

Adhesive is one of the important components of the coating, and its role is to promote the adhesion of the coating to the surface of the base paper. Too little adhesive will not provide enough adhesive force; too much adhesive will cause the coating to become hard and brittle. The proper application of the adhesive is the assurance that the coated paper coating does not fall off or fall off during printing.
Uniformity of the coating particles is another influencing factor. The uneven coating particles will affect the binding force between the coatings. When the binding force is smaller than the binding force between the printing and the ink, the coating layer will be broken. Therefore, the coating must be broken. The coating has a certain degree of dispersion, so dispersion and screening during coating preparation cannot be ignored. A good filler, with suitable dispersibility, can prevent “sand spots” from appearing during coating, and can avoid peeling off during printing.

There are various factors affecting the printing and lint-free printing. In addition to the above factors, they are related to the base paper, beating method, filler type and amount, and papermaking process. In addition, the type of printing ink and the printing method are also important influencing factors, which need to be considered in many aspects during actual printing.

Source: "Printing Technology"