Chemical pulping of wood fibers
1.Pulping of coniferous wood pulp and broad-leaved wood pulp
Wood fibers are generally divided into two major categories: coniferous wood and broad-leaved wood. For the same pulping method, broad-leaved wood pulp requires a higher degree of pulping than coniferous wood pulp to achieve similar physical strength. Moreover, the quality of the pulping process will also indirectly affect the subsequent effect of combining with Polyester Forming Fabric for papermaking. However, the fibers of broad-leaved wood are shorter, and it is very difficult to increase their degree of pulping while avoiding excessive cutting. Therefore, broad-leaved wood pulp can generally only undergo mild pulping to achieve a relatively low physical strength. Generally, broad-leaved wood pulp should not be used alone to make high-quality paper; it is usually combined with coniferous wood pulp or cotton and linen pulp, which are long-fiber pulps, for papermaking to improve the physical strength of the paper and meet the requirements of Polyester Forming Fabric for papermaking, ensuring the stability of the papermaking process.
The fibers of coniferous wood pulp are relatively long, with an average length of 2 to 3.5 mm. During the pulping process, they usually need to be cut down to 0.6 to 1.5 mm to ensure the uniformity of the paper structure and also to better adhere to the Polyester Forming Fabric, thereby reducing the loss of fibers during the papermaking process.
In the wood pulp, the ratio of early wood to late wood varies, which also affects the properties of the pulping process. The cell walls of late wood are thicker and harder, and the primary cell walls are less prone to being damaged. During pulping, the fibers are more likely to be cut short, while it is more difficult for them to absorb water, swell, and become fine fibers. Such pulp materials require higher wear resistance for the Polyester Forming Fabric during papermaking. However, the cell walls of early wood are thinner and have a softer nature. During pulping, they are more likely to separate into individual fibers and undergo fibrillation. During the papermaking process, they have better compatibility with the Polyester Forming Fabric, which can improve the quality of paper formation.
2. Pulping of sulfate wood pulp and sulfite wood pulp
Compared with the pulp of red pine, Korean fir, and fish-scale pine, the pulps of unbleached sulfate larch and ponderosa pine are more difficult to be pulped, and the strength of the resulting paper is also poorer. The cooperation difficulty between these pulps and Polyester Forming Fabric during the papermaking process is also relatively greater. The pulping of larch and ponderosa pine sulfate pulp is difficult and the paper strength is poor mainly because the latewood proportion of larch and ponderosa pine is large. To improve the paper strength of larch and ponderosa pine sulfate pulp, a gradually heavier cutting method should be used during pulping, the pulping concentration should be appropriately increased, the pulping time should be appropriately prolonged, and the pulp consistency can also be appropriately raised. All of these are conducive to increasing the inter-fiber bonding force and improving the physical strength of the paper, thereby optimizing the collaborative papermaking effect with Polyester Forming Fabric.
The unbleached sulfate hard pulp is very tough and is suitable for producing cement bag paper, cable paper, etc. This pulp is difficult to be pulped. If it is pulped with ordinary concentration (such as 4% to 6%), it often requires heavy knives for shortening and loosening, but the pulp consistency increases slowly and the fibers are not easily fibrillated. The polyester forming fabric suitable for this type of pulp needs to have stronger wear resistance and impact resistance. If it is pulped with high concentration (such as 20% to 30%), the swelling degree and softness of the fibers can be appropriately increased, thereby improving the elasticity of the paper and reducing the loss to the polyester forming fabric.
The strength of the unbleached sulfate soft pulp is also quite high, making it suitable for the production of capacitor paper, telephone paper, etc. The pulping method can be achieved by using a light knife for slow beating, with multiple knife drops and a long duration. This results in a viscous pulp that closely adheres to the Polyester Forming Fabric, effectively improving the forming accuracy and surface flatness of the paper.
