21. ***What is the process of making paper?***
A: The papermaking process consists of the following main stages:
Pulping Section: Raw material selection → Cooking and fiber separation → Washing → Bleaching → Re-washing and screening → Concentration or formation of pulp sheets → Storage for later use
Papermaking Section: Pulping → Removal of impurities → Refining → Beating → Preparation of various additives → Mixing of paper materials → Flow of paper materials → Headbox → Wire section → Press section → Drying section → Surface sizing → Drying → Calendering → Winding into paper rolls
Coating Section: Coating base paper → Coating machine → Drying → Winding → Rewinding → Super calendering
Processing Section: Rewinding → Cutting into sheets (or rolls) → Sorting and packaging → Final storage
22. ***What is pulping?***
A: Pulping refers to the production process of using chemical, mechanical, or combined methods to separate plant fibers into decolorized pulp or bleached pulp.
23. ***What is chemical pulping?***
A: Chemical pulping involves treating plant fiber raw materials with a chemical aqueous solution at a specific temperature and pressure. This process removes lignin, non-fiber carbohydrates, fats, resins, etc., while retaining cellulose and varying degrees of hemicellulose to separate fibers into pulp. The most representative methods are the sulfate process and sulfite process.
24. ***What is mechanical pulping?***
A: Mechanical pulping uses mechanical methods to treat fiber raw materials and dissociate fibers. When wood is used as the raw material, it is referred to as mechanical wood pulp, also known as groundwood pulp. Mechanical pulp retains a significant amount of lignin from the raw material, resulting in a higher yield compared to chemical pulp.
25. ***What is the purpose of bleaching pulp?***
A: Pulp produced by chemical or mechanical methods is called colored pulp, which has a certain color (generally yellow or brown). To meet paper usage requirements, the pulp must be bleached to achieve higher whiteness. Bleaching involves using appropriate bleaching agents through oxidation, reduction, or decomposition reactions to dissolve residual lignin in the pulp or decolorize chromophoric substances while retaining lignin.
26. ***What are the common methods for bleaching paper pulp?***
A: There are two primary methods for bleaching pulp:
1. Oxidation Bleaching: Uses oxidative bleaching agents to destroy the structure of lignin, dissolving it to improve pulp purity and achieve whiteness. Common bleaching agents include Cl₂, ClO₂, and hypochlorites. Currently, oxidation bleaching remains the dominant method, but chlorine-containing bleaching agents are gradually being replaced by oxygen-based alternatives due to environmental concerns.
2. Reduction Bleaching: Uses reducing bleaching agents to alter the structure of chromophoric groups and decolorize them. This method does not cause fiber loss and preserves the characteristics of the original pulp, making it suitable for high-yield pulps such as groundwood pulp and chemimechanical pulp. Common bleaching agents include zinc disulfite, sodium trisulfate, hydrogen peroxide, and sodium peroxide. However, reduction-bleached pulp exhibits poor whiteness stability and may revert to its original color after prolonged exposure to air.
In practice, multiple bleaching stages (e.g., three-stage or five-stage bleaching) are often employed to achieve the desired whiteness.
27. ***Why is beating necessary?***
A: After steaming, mechanical grinding, washing, screening, and bleaching, pulp cannot be directly used for papermaking. The fibers in the pulp lack flexibility, and the inter-fiber bonding performance is poor. If used directly, the resulting paper would be loose, porous, rough-surfaced, and low-strength, failing to meet usage requirements. Beating employs mechanical methods to treat fibers in the pulp, causing fibrillation and moderate cutting. More importantly, the fibers swell in water during beating, enhancing their elasticity and plasticity to meet the production requirements of paper machines and ensure the achievement of expected quality indicators.
28. ***What are the methods of making paper sheets?***
A: Paper sheet manufacturing methods can be categorized into dry and wet processes. The primary difference lies in the medium used: the dry process uses air, primarily for synthetic fiber nonwovens (e.g., diapers), while the wet process uses water and is suitable for plant fiber papermaking. Currently, most paper is manufactured using the wet process.
29. ***What are the types of wet paper machines?***
A: Wet paper machines are classified into three categories based on the structure of the forming section: long-net paper machines, round-net paper machines, and clip-net paper machines. Long-net and round-net paper machines use single-sided dewatering during sheet formation, causing differences between the two sides of the paper. Clip-net paper machines employ simultaneous double-sided spraying for dewatering, effectively reducing or eliminating side differences and improving paper uniformity.
30. ***What is the process of making paper sheets?***
A: Using the advanced clip-net paper machine as an example, the paper sheet manufacturing process is as follows:
The paper machine consists of two parts: the wet end and the dry end. The wet end includes the headbox, wire section, and press section. The dry end includes the drying section, calendering section, and reel-up section. Purified paper stock is sprayed uniformly and stably onto the center of two nets (at a 45° or 90° angle) via the headbox. After removing most of the water from both sides of the nets, the sheet is sent to the boot press for pressing and dewatering, improving paper properties and increasing tightness and surface smoothness. The pressed sheet is then sent to the drying section for further dehydration using heat energy to meet the required moisture content. Offset paper undergoes surface sizing in the drying section to enhance surface strength and other properties, meeting high-speed printing requirements. From the drying section, the paper surface remains rough and unsuitable for printing or writing. To improve smoothness and adjust thickness, the paper must pass through one or more metal rollers for calendering treatment (usually one positive and one reverse calender). Finally, the paper can be cut into flat sheets or rolled into reels using a winder.