• Paper strength in paper making process always is a annoying problem, it will influence the paper quality and increase production cost, so how to improve paper strength? It is particularly important to identify the reasons that affect paper strength and take solutions.

What is the strength of paper?

• Paper strength refers to the bonding strength between the fibers, adhesives, fillers, or coating particles on the surface of a paper, as well as between the coating and the paper substrate. It represents the ability of a paper to resist ink splitting during the printing process.

Factors affecting paper strength

The impact of slurry types

• Different types of pulp vary in both physical structure and chemical composition. Generally speaking, chemical wood pulp has the highest fiber binding force, followed by cotton pulp, and mechanical wood pulp has the worst.

The Effect of hemicellulose

• The pulp with more hemicellulose is easy to absorb water and swell during beating, which increases the specific surface area and bonding area of the fiber and improves the fiber bonding force. The molecular chain of hemicellulose is shorter than that of cellulose, and it is more hydrophilic. When beating, it is easy to absorb water, swell and fine fibrosis. Therefore, pulp with more hemicellulose is easier to beat, and the binding force between fibers is also greater.​

• Of course, the high content of hemicellulose is not good for beating and paper making. The main reason is that hemicellulose absorbs water and swells too fast, and the beating degree is too high. The paper made is transparent, brittle, and low in strength. On the other hand, with more hemicellulose and short fibers, the strength of the fiber it.

The impact of cellulose

• Generally speaking, cellulose has a long molecular chain, high degree of polymerization, and high fiber strength, making it difficult to cut during pulping. Therefore, by the time it is cut to an appropriate length, the fibers have already achieved sufficient separation and brooming, resulting in a greater bonding force between the fibers.

The influence of lignin

• Lignin is mainly distributed in the outer layers of the primary and secondary walls of fibers. Due to its extremely low hydrophilicity, the presence of lignin affects the swelling and fine fibrosis of fibers. Therefore, pulp with high lignin content is not easy to beat and has poor fiber adhesion.

The influence of paper making additives

• Adding hydrophilic substances such as starch, protein, and plant gum to pulp can increase the bonding strength between fibers. This is because these substances themselves also have polar hydroxyl groups similar to cellulose. Due to the hydrogen bonding of hydroxyl groups, the bonding between fibers is stronger than that of pure fibers.​

• ​​​​Adding hydrophobic substances such as inorganic fillers and alum to pulp can reduce the bonding strength between fibers. This is because adding these substances will separate fibers and reduce their indirect contact surfaces, thereby affecting the bonding strength of fibers.

Hydrogen bonding in carbohydrates in papermaking raw materials

• Hydrogen bonding is an important factor affecting paper bonding. A series of operations in papermaking are efforts to increase hydrogen bonding, such as refining to make fibers soft and expose more fine fibers to increase hydrogen bonding points; During drying, as the water evaporates, the microfibers become closer together, forming hydrogen bonds between the fibers.

• From the above summary, it can be seen that the production process of papermaking is developed around the basic theory mentioned above. Therefore, in order to obtain better paper strength, it is necessary to use good fibers and increase the bonding points of hydrogen bonds.

What are the main reasons of the poor surface strength of paper?

• In order to reduce costs, the production workshop added excessive fillers, which were not firmly bonded in the paper and were prone to detachment.

• The poor organization of the paper machine's web section results in weak fiber bonding, insufficient pressing force, excessively high and rapid drying temperature in the drying cylinder, and loose and fluffy paper surface.

• Due to the movement of the blanket mesh or adhesion of dirt, the surface of the well bonded paper sheet is damaged.

• Overdried paper is prone to electrification under the action of static electricity, causing paper fuzz to remain on the paper surface and fall off during the printing process or when the static electricity disappears.

• The residue of each scraper blade is adsorbed on the paper page, causing hair and powder loss. During paper cutting, the paper fuzz and powder generated by the blunt blade cause the printing plate to paste.

• When coated with copperplate base paper and carbon free copy base paper, the fibers or fillers exposed on the surface of the base paper are not covered by the coating, or although they are covered by the coating, they are not firmly bonded to the base paper. Therefore, during the printing process, they are peeled off, and the peeled fibers often adhere to some of the pigments, increasing the impact of hair loss.

• The powdery substances (usually fillers or pigments) that are weakly bonded to the paper surface in the printing process are peeled off from the paper surface due to the influence of adhesive action, mainly due to improper selection or incorrect dosage of adhesive substances in the coating.

Six Measures to improve the surface strength of paper

Control the addition of fillers and additives

• Increasing the amount of filling can make the slurry easy to dehydrate, thereby reducing the adhesion between the paper web and the drying cylinder. However, excessive ash content can cause a decrease in fiber adhesion and affect surface strength.

Strengthen process control

• In the production process of paper machines, the key is to strengthen the control of dehydration and retention in each department. In the web section, the measure taken is to increase the filler content on the back of the paper web, reducing its adhesion to the drying cylinder. Specifically, a highly dehydrating poly (vinyl acetate) forming mesh was selected, and the arrangement of the mesh case was adjusted to balance the vacuum distribution of the mesh vacuum box and avoid excessive suction as much as possible;

• Strengthen the control of mesh shaking, improve the fiber arrangement of the paper, and make the paper pages less likely to stick when in contact with the front row drying cylinder; Adjust the landing point of the slurry to control the slurry speed and mesh speed within a reasonable range; Improve dehydration.

Reduce static electricity

• Paper with static electricity is prone to adsorbing dust and paper scraps in the air. On the one hand, we have increased the electrolyte content in the paper material to reduce static electricity. On the other hand, before the paper machine coiling cylinder, an electrostatic eliminator is added to eliminate charges on the paper surface by adjusting the discharge voltage. It is also required to increase the moisture content of the paper as much as possible, increase environmental humidity, and reduce static electricity generated by friction.

Strengthen selection and cutting management

• In order to prevent the gray paper from entering the finished paper during the cutting process, we require timely replacement of blunt round and long knives to avoid dust falling due to insufficient sharpness of the knives. We also strengthen the production site management of the paper cutting machine and clean up the dust generated by the knife edges in a timely manner.

• Strictly controlling the quality of talcum powder reduces the impact of fillers on the cutting edge. When there is severe paper dust on the coated paper, on-site wiping of the edges and surface of the finished paper also reduces the impact of paper dust on the printing paste to a certain extent.

Control of coating process

• Coated processing paper has stricter surface strength requirements due to its price and usage. Therefore, we are more cautious when executing the process in the coating and processing paper workshop. To increase the water resistance of the paper, we first strengthened the internal sizing of the paper, strictly controlled the surface absorption of the paper, and strictly controlled the internal sizing process and surface sizing process of the base paper, reducing the moisture absorption deformation of the base paper.

Strengthen quality inspection

• In order to reduce product quality fluctuations and grasp the quality status of the product, we used IGT printing suitability tester and waxing rod to quantitatively test the surface strength of the paper. Adjust the production process based on the inspection results. At the same time, the inspection of indicators such as ash content, moisture content, and surface absorption weight of the paper indirectly indicates the level of surface strength of the paper.

Conclusion

• The surface strength of paper is an important performance of printing paper. With the continuous development of printing technology and the continuous improvement of printing speed, the requirements for the printability of paper, especially the surface strength, are also increasing. Therefore, improving the surface performance of paper and controlling the printing process reasonably during the printing process plays a very important role.