Paper pulp is raw material of papermaking process, how to make paper pulp? Paper pulping means the process of separating fiber materials into single fiber with paper pulping machines, including pulp digester, blow tank. Vacuum drum washer, twin roll press, etc. When we talk about paper making, we must mention pulp beating. As the saying goes: three papermaking, seven beating.
What is pulp beating? Beating is a common step of paper pulp making. It uses the mechanical action to treat pulp fibers suspended in water, and the pulp has obtained the characteristics required by the papermaking process, achieve the desired quality of the paper.
Why beating is needed in the papermaking? After pulp cooking or grounding, screening and bleaching, the pulp usually can’t be applied to manufacture paper. Due to the paper pulp has rich elasticity, if it directly used to paper making without any treatment, the obtained paper will have low hardness. In addition, unbeaten pulp contains connected fiber bundles of varying lengths, which is hard, lack of cutting and separation. The paper made from the unbeaten pulp is loose, porous, rough and low-strength, in a word, it is not suitable for papermaking. The grinding disc is the “heart” of the refiner, which directly produces shear, crushing, stretching, friction and other forces on the pulp fiber, change the fiber morphology and meet the adaptability of the papermaking. The paper produced by the beaten pulp has a tight and uniform structure and a high strength.
Beating terms
- Beating degree: It is the degree of paper filtration and an indicator of the beating situation.
- Fiber length: The average length of the fibers of the paper.
- Wet weight: It is an indicator of fiber length. A certain amount of diluted paper passed through the frame, and the weight of the wet fiber weighed on the frame is the wet weight of the fiber.
- fastigiatedratio: The ratio of fibrillation fiber numbers to the total number of fibers after being beaten.
- water retention value: Under certain conditions, the high-speed centrifuge removes the free water in the paper and retain the bound water in the fiber to indicate the degree of fiber swelling and fibrillation, thereby indirectly measuring the bonding force between the fibers and prediction of paper strength.
Methods of beating
Beating is mainly divided into free beating and wet beating.
- Free beating is mainly based on cutting, and the concentration of pulp is low. The beater of this method has sharper cutting edge, and the tooth pitch of the blade is denser. When the pulp is beaten, the fiber is easily cut, so that the paper has small density, bulkiness, opacity, small stretchability, and easy to adhere the ink. However, the paper produced by this method has a low smoothness and is easy to fluff.
- The wet beating is based on the principle of rupture. The pulp has a higher concentration, and the blade of the beater is more blunt, and the pitch of the blade is sparse. When the fiber is cut, the length of the fiber is prone to be different, so that the paper has strong physical properties. The surface of the paper has good smoothness, high transparency and is not easy to scratch. However, the paper has a large stretchability and a poor ink absorption effect.
Function of beating
- Separate fibers and change the state of surface of fiber.
- Meet the requirement of manufacturing various types of paper with same raw material.
- Meet the requirement of manufacturing same paper with different kind of pulp.
Actions on fibers
The effects of beating on fibers are mainly divided into the following five parts: displacement and deformation of cell walls, breaking of primary and secondary wall, water swelling, fibrillation and cross cut, etc. However, these several functions are not completely separated, but they are intertwined.
displacement and deformation of cell walls
Since the fiber is subjected to mechanical force during the beating process, the microfibers of the secondary wall are bent and deformed, and the gap between the microfibers is increased, which creates conditions for the fiber to absorb more water. In the beginning, this deformation is very small, and gradually becomes larger in the future. Although the displacement of the secondary wall and the further swelling are limited due to the primary and secondary outer layers, the water absorbing deformation makes the fibers soft, and important for removing the primary and secondary outer layers.
breaking of primary and secondary wall
The primary wall is removed in the cooking and bleaching process, but there is still a significant amount. For the fiber who still has the primary wall, it is smooth, very hard, and is not easy to swell. This is because the primary wall contains more lignin and has a network structure. Although it absorbs water, it has a low degree of swelling. The outer layer of the primary wall is very thin, it tightly surrounds the middle layer of the primary wall that can be well swelled. Therefore, it must also be removed in the beating, so that the fine fibers in the middle layer of the secondary wall are separated, finally, the fiber is fully swelled and fibrillated.
water swelling
The fiber swelling is slowly before the primary wall is broken. As the beating progresses, the outer walls of the primary and secondary walls are continuously broken, the w swelling of the fibers are accelerated, and the fiber diameter can be rapidly swollen to twice the original size. The swelling fiber becomes soft and plastic, the external surface area is increased, the internal structure is relaxed, and the intermolecular cohesive force is lowered, which is favorable for the fibrillation.
Fibrillation
In the process of beating, the fiber is mechanically affected by the beating equipment to produce longitudinal splitting of the fiber, and the surface separates the fine fibers, which is called fibrillation.
Cross cut
During the beating process, the fibers are cut due to the shearing action between the blades or the grinding teeth of the beating equipment. At the same time, in the case of a large beating pressure and a high concentration, the fibers rub against each other, which also causes the transverse cutting of the fibers.
