26
2025
-
02
Raw materials and chemical treatment for papermaking
Author:
Wood and non-wood fiber raw materials
The main raw materials for papermaking include wood (such as pine, eucalyptus, etc.) and non-wood fibers (such as bamboo, wheat straw, reeds, etc.). These raw materials are mainly composed of cellulose, hemicellulose, and lignin.
Chemical composition of wood: Wood is mainly composed of cellulose (about 40%-50%), hemicellulose (about 20%-30%), and lignin (about 20%-30%). In addition, it contains small amounts of extractives (such as resins, tannins, etc.) and ash. Cellulose is the main source of strength in paper, hemicellulose helps improve the bonding strength of paper, and lignin needs to be removed as much as possible during the papermaking process to improve the quality of the paper.
Chemical composition of non-wood fibers: The chemical composition of non-wood fibers differs from that of wood. For example, bamboo has a higher cellulose content and a relatively lower lignin content; wheat straw has a higher hemicellulose content. These differences can affect the process parameters and paper performance during papermaking.
Chemical pulping
Purpose: To remove lignin from the raw materials, separate cellulose and hemicellulose, in order to obtain high-quality pulp.
Methods: Mainly the sulfate process (also known as alkaline pulping) and the sulfite process.
Sulfate process: A mixed solution of sodium hydroxide (NaOH) and sodium sulfide (Na₂S) is used to treat the raw materials under high temperature and pressure. In this process, lignin undergoes sulfonation, hydrolysis, and fragmentation reactions, thus being dissolved and removed. The reaction equation is as follows:
Lignin + NaOH + Na₂S → Water-soluble lignin sulfonate + other products
Sulfite process: A sulfite solution (such as calcium sulfite, magnesium sulfite, etc.) is used to treat the raw materials at a certain temperature and pressure. Lignin undergoes sulfonation and hydrolysis reactions under the action of sulfite, generating water-soluble lignin sulfonate. The reaction equation is as follows:
Lignin + H₂SO₃ + Ca²⁺/Mg²⁺ → Water-soluble lignin sulfonate + CaSO₃/MgSO₃ + other products
Mechanical pulping
Principle: The fibers of the raw materials are separated by mechanical force without chemical treatment. This method retains most of the lignin in the raw materials, but the strength and whiteness of the paper are lower.
Main methods: Include stone groundwood pulp (SGW), refiner mechanical pulp (RMP), and thermomechanical pulp (TMP), etc.
II. Chemical principles in the papermaking process
Bleaching of pulp
Purpose: To remove colored substances from the pulp and improve the whiteness of the paper.
Methods: Mainly oxidation bleaching and reduction bleaching.
Oxidation bleaching: Bleaching is performed using chlorine-containing compounds (such as chlorine gas, hypochlorite, etc.), chlorine dioxide, hydrogen peroxide, and other oxidants. These oxidants can destroy the chromophores of colored substances in the pulp, turning them colorless or light-colored. For example, the reaction equation for hypochlorite bleaching is as follows:
Colored substances + NaClO → Colorless substances + NaCl + other products
Reduction bleaching: Bleaching is performed using sodium dithionite (also known as sodium bisulfite) and other reducing agents. Reduction bleaching mainly reduces colored substances to colorless or light-colored substances. The reaction equation is as follows:
Colored substances + Na₂S₂O₄ → Colorless substances + Na₂SO₃ + other products
Pulp refining and additives
Refining: The pulp fibers are separated and fibrillated through mechanical action, increasing the surface area and bonding strength of the fibers. During refining, the cell walls of the fibers undergo a certain degree of rupture and deformation, releasing more hydroxyl and other active groups, which is beneficial for the bonding between fibers.
Additives: Various chemical additives are added to the pulp to improve the performance of the paper. Common additives include:
Sizing agents: Such as rosin, synthetic sizing agents, etc., used to improve the water resistance of the paper. Sizing agents react with hydroxyl groups in the pulp fibers to form a hydrophobic film on the fiber surface, thereby reducing the water absorption of the paper.
Fillers: Such as calcium carbonate, talc powder, etc., used to increase the whiteness, smoothness, and opacity of the paper. Filler particles fill the spaces between fibers, improving the optical and printing properties of the paper.
Strengthening agents: Such as starch, polyacrylamide, etc., used to improve the strength of the paper. Strengthening agents interact with pulp fibers through hydrogen bonds, ionic bonds, etc., enhancing the bonding strength between fibers.
Related news
Inquiry