Pulping is to use physical methods to mechanically or fluidly treat the fibers suspended in water, so that the fibers are subjected to shear force, the fiber morphology is changed, and the pulp acquires certain characteristics. The pulping process controls the water filterability of the pulp, adapts to the needs of paper machine production, enables the paper sheet to be well formed, and improves the uniformity and strength of the paper sheet. Therefore, how to ensure the quality of pulping and produce products that meet quality requirements is an important link in the paper production process.

1 Pulping method

During the pulping process, due to the mechanical action of the fibers, the fibers absorb water, swell, and fibrillate, are cut horizontally, crushed, and rubbed. Due to different pulping conditions, pulping can be divided into the following methods, as shown in Table 1.

In the actual operation process, there are semi-free pulping and semi-sticky pulping between free pulping and sticky pulping. The pulping degree and fiber length can also be adjusted according to the paper performance.

2 Factors affecting pulping

The factors affecting pulping mainly include pulp fiber characteristics, pulping equipment and pulping process. For a specific pulping process, the process factors that can be adjusted in time in production are limited. In order to reasonably formulate the pulping process regulations and meet the needs of paper production, the main influencing factors are now introduced.

2.1 Pulp fiber characteristics

2.1.1 Raw material types 

Different raw material types have different components of cellulose, hemicellulose and lignin, different lengths, widths, aspect ratios, wall-cavity ratios of fiber cells, and different arrangements, directions and winding angles of microfibers in the fiber cell walls, and the difficulty of pulping is also different. For example, in wood pulp, the proportions of early wood and late wood are different. Under the same pulping conditions, the pulp quality obtained is also different. Late wood fibers are long, the cell walls are thick and hard, and the primary wall is not easy to be broken. The fibers are easily cut during pulping, and it is difficult to absorb water, swell and fibrillate.

2.1.2 Pulping methods 

Different pulping methods have different pulping effects. For example, KP pulp of wood pulp is not as easy to pulp as SP pulp. Because the degree of polymerization is evenly distributed in KP pulp, but the low degree of polymerization is concentrated in the outermost layer of SP pulp, which is easy to tear and remove.

2.2 Pulping equipment 

There are many types of pulping equipment, which can be divided into two categories: intermittent and continuous according to different production methods. Intermittent pulping equipment is mainly a pulping machine, which has been eliminated by most paper mills. Only a few companies use it as a half-pulper to cut long fiber raw materials such as cotton, hemp, rags, and high-viscosity pulp. Continuous pulping equipment includes conical refiners, disc refiners, and cylindrical refiners. Each type of pulping equipment has different characteristics. When selecting, comprehensive consideration should be given to the performance of the equipment and the pulp requirements.

Table 2 Pulping equipment and characteristics

The choice of blades depends on the pulping process. If the blade is thin, the effective contact area of the pulping is small, the specific pressure is large, which is conducive to the cutting of fibers and suitable for pulping free pulp; if the blade is thick, the contact area is large, and the specific pressure of the pulping is small, it is conducive to the separation, decomposition and fine fiberization of fibers, and suitable for pulping sticky pulp.

2.3 Pulping process

2.3.1 Pulping specific pressure 

The pulping specific pressure is the primary factor that determines the pulping method. Under normal circumstances, a small pulping specific pressure can avoid excessive fiber cutting, but it will prolong the pulping time. Therefore, the general pulping is to appropriately increase the specific pressure under the condition of not affecting the properties of the pulp as much as possible, so as to improve the pulping efficiency and save power consumption. The size of the pulping specific pressure is determined by the type of pulp and the properties of the fiber. Even when the same fiber raw material is used to produce different papers, the required pulping specific pressure is different. For example, for pulping of general cultural and packaging paper produced with grass pulp with shorter fibers, low specific pressure pulping should be used to avoid cutting fibers and improve the strength of paper.

2.3.2 Pulping Concentration

The concentration of pulp has a great influence on the quality of pulping. Pulping concentration can be divided into three types: low concentration, medium concentration and high concentration. It is generally believed that the concentration below 10% is called low concentration pulping, the concentration of 10% to 20% is called medium concentration pulping, and the concentration of high concentration pulping is 20% to 30% or even higher. In the scope of low concentration pulping, the higher the pulping concentration, the more fibers enter between the flying knife and the bottom knife, which is conducive to promoting the squeezing and kneading effect between the fibers, and is conducive to the dispersion, swelling and fine fiberization of the fibers. At the same time, due to the increase in the number of fibers, the pressure on a single fiber is correspondingly reduced, thereby reducing the cutting effect of the fibers. At present, for general continuous pulping equipment, the concentration is mostly around 4%. The medium-concentration pulping of straw pulp (concentration of about 16%), which has been studied more in China, has achieved good results. Generally speaking, the pulping concentration of straw pulp and broadleaf pulp should be higher to prevent cutting; the concentration of coniferous pulp should be slightly lower for cutting. High-concentration pulping is currently less used in China. Its advantages are that it can retain the length of the fiber, can fully and evenly pulp, and can give the paper some excellent properties, such as higher tear strength, elongation and burst resistance.

2.3.3 Throughput

Under the condition of constant pulping pressure and pulping concentration, increasing the pulp throughput will correspondingly shorten the residence time of each fiber in the grinding area, reduce the pulping effect on the fiber, and thus reduce the pulping quality; however, excessively reducing the pulp throughput will cause an increase in unit power consumption. Therefore, in the pulping operation, various process parameters should be controlled, and the pulp throughput should be appropriately increased under the premise of ensuring the pulping quality to reduce the power consumption of pulping.

2.3.4 Pulping time

The length of the pulping time is comprehensively affected by factors such as the pulping method, the type of pulp, the performance of the equipment, the pulping concentration, and the pulping pressure. Generally speaking, free pulping requires a heavy knife at one time and a short pulping time. Sticky pulping requires multiple light knives and a long pulping time.

2.3.5 Pulping temperature 

During pulping, friction heat is generated due to the friction between the fiber and the blade surface and between the fibers. This heat accumulates in the pulp, causing the temperature of the pulp to rise. If the pulp temperature is too high, several side effects may occur: affecting the sizing effect of the pulp and producing false sizing; releasing resin and increasing resin barriers; reducing the water absorption and swelling effect of the fiber, prolonging the pulping time; affecting the bonding strength of the paper.

3 The relationship between pulping and paper performance

Since pulping increases the bonding force of the fiber and reduces the average length of the fiber, it can improve the tensile strength, burst resistance and folding resistance of the paper, while increasing the smoothness, stiffness and tightness of the paper, but on the other hand, it reduces the tear resistance and opacity of the paper and increases the shrinkage of the paper. The performance indicators required for different paper types are also different. Therefore, the pulping process of the pulp should be determined according to the performance requirements of the finished paper.

3.1 Tensile strength

Tensile strength is an important indicator of paper. With the increase of pulping degree, the fiber absorbs water and swells and the degree of fibrillation increases, the number of fine fibers increases, the specific surface area of ​​the fiber increases, more hydroxyl groups are released, the hydrogen bonds between fibers are promoted, and the fiber bonding force continues to increase. Therefore, the pulping degree should be reasonably increased for high-strength paper.

3.2 Burst resistance 

The change of paper burst resistance is generally similar to tensile strength. The main factor affecting it is also the fiber bonding force, followed by the average length of the fiber, the strength of the fiber itself, and the fiber interweaving. At the same time, affected by elongation, for two sheets of paper with the same tensile strength, the paper with a large elongation has a higher burst resistance. However, when the pulping degree is relatively high, the degree of burst resistance decreases greatly.

3.3 Tear resistance 

Generally, the tear resistance of paper decreases with the increase of pulping degree. The main factor is the decrease in the average fiber length. When the pulping degree is the same, the tear resistance of paper made of viscous pulp is higher than that of paper made of free pulp.

3.4 Folding resistance 

The folding resistance of paper is largely affected by the fiber length. The reason why paper fibers are not highly bonded is that repeated bending damages the fibers and causes fiber shedding. Therefore, it is required that the fibers must have a certain degree of bonding to improve the folding resistance. However, when the bonding is too much, it will break quickly due to bending. The folding resistance of paper under high pulping conditions is related to plasticity, and plasticity is closely related to the moisture content of the paper. Within a certain range, increasing the moisture content of paper can increase the flexibility of the fiber and effectively improve the folding resistance of the paper. However, when the moisture content rises to a certain limit, the fiber bonding force will decrease too much, causing the folding resistance to decrease. Therefore, in order to ensure that the paper has a high folding resistance, attention should be paid to increasing the degree of fiber water absorption and swelling and fibrillation during pulping, and avoiding the cutting effect of the fiber as much as possible.

3.5 Porosity 

Paper has a porous mesh structure. Porosity is not only related to absorbency but also to air permeability, which has a great influence on the subsequent processing performance of the paper. During the pulping process, the fiber bonding force gradually increases, the fiber surface area gradually increases, and the pores of the paper sheet become less. Therefore, the absorbency and air permeability decrease. Strength and porosity are contradictory. To ensure a certain strength and obtain porosity, it can be achieved through internal fine fiberization.

3.6 Opacity

The higher the pulping degree, the easier it is for the fibers to get together due to the surface tension when drying on the paper machine, which promotes the formation of hydrogen bonds and the improvement of fiber bonding. At the same time, the paper shrinks, and the pores between the fibers are less, which reduces the scattered light and allows more light to pass through, reducing the opacity of the paper. Therefore, when producing paper with high opacity, it is advisable to use pulp with a high lignin content as the raw material, and the pulping degree of the pulp should be lower.

3.7 Dimensional stability 

The shrinkage of paper depends to a large extent on the pulping characteristics and the type of paper material. Generally speaking, all pulps with long fibers and good pulping have poor dimensional stability after being made into paper.

With the change of environmental humidity, the paper pages will expand and contract in the X, Y, and Z directions, especially in the X and Y directions, which will affect the printing quality. Therefore, if the production requires small expansion and contraction of paper, such as offset printing paper, illustrated newspaper, etc., in addition to selecting a reasonable pulp ratio, a short fiber free pulping process should be adopted, and the pulping degree should not be too high to reduce the expansion and contraction of the paper.

In summary, pulping plays an important role in the performance of paper. In order to more effectively improve the performance of paper and improve product quality, it is necessary to design a reasonable pulping process according to the actual production situation, strictly control the pulping process, and try to avoid its adverse effects on the paper.