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1. **What are the raw materials for modern papermaking?**
Answer: The raw materials for modern papermaking include plant fibers (e.g., wood, bamboo, grass), mineral fibers (e.g., asbestos, glass fiber), synthetic fibers (e.g., nylon, metal wire), and high-molecular materials derived from petroleum cracking. Currently, writing, printing, and packaging papers predominantly use plant fibers as the primary raw material.
2. **What types of plant fiber raw materials are used in papermaking?**
Answer: Plant fiber raw materials in the papermaking industry can be categorized into two main groups:
- **Wood fiber raw materials**: Fibers directly extracted from trees.
- **Non-wood fiber raw materials**: These include grass fibers, phloem fibers, and seed hair fibers.
3. **What are the components and functions of commonly used auxiliary materials in paper production?**
Answer: Auxiliary materials refer to additives incorporated into paper to meet specific performance requirements. These include fillers, sizing agents, colorants, and other chemical additives.
- **Fillers**: Enhance paper smoothness and opacity.
- **Sizing agents**: Provide water resistance to paper.
- **Colorants**: Increase paper whiteness or impart color.
- **Other chemical additives**: Modify paper properties for specialized applications.
4. **What is the function of sizing agents?**
Answer: Paper made from plant fibers absorbs water or liquids due to capillary pores within the fibers and hydrophilic hydroxyl groups in cellulose and hemicellulose. Sizing agents, which are colloidal or film-forming substances with liquid resistance, are added to prevent water penetration and diffusion. This process is referred to as "sizing" in the papermaking industry.
5. **What are the methods of applying sizing agents?**
Answer: Based on their effects, sizing methods can be divided into internal sizing and surface sizing.
- **Internal sizing**: Involves adding sizing agents to pulp to produce hydrophobic paper and paperboard. Commonly used sizing agents include rosin sizing agents and synthetic sizing agents.
- **Surface sizing**: Applies a thin layer of sizing agent onto the paper surface to enhance its hydrophobic properties. Commonly used sizing agents include starch, animal glue, and synthetic sizing agents. Surface sizing is primarily used for offset paper, writing paper, and packaging paper with hydrophobic requirements.
6. **What are the effects of surface sizing on printing paper performance?**
Answer:
① Enhances water resistance and surface strength, reducing fiber shedding during offset printing.
② Improves printability; insufficient surface adhesive may result in dull print marks, while excessive adhesive slows ink drying and causes smudging.
③ Increases erasability by reducing ink penetration and enhancing surface anti-friction properties.
④ Boosts paper strength, such as bursting strength, folding endurance, and tensile strength.
⑤ Reduces two-sided differences and paper deformation.
7. **What is the function of fillers?**
Answer: Fillers are added to endow paper with specific properties such as whiteness, opacity, and smoothness. They also reduce internal voids in the paper, improving ink absorbency and uniformity. Insoluble or slightly soluble minerals typically constitute 10% to 25% of printing paper, depending on the type of paper.
8. **What characteristics should fillers for printing paper possess?**
Answer: Printing paper fillers should exhibit high whiteness, high refractive index, fine particle size, low water solubility, low density, and good chemical stability. Additionally, they should be abundant and cost-effective.
9. **What are the common fillers used in printing paper?**
Answer: Common inorganic fillers include talc, clay, titanium dioxide, and calcium carbonate. Titanium dioxide is an expensive, high-quality filler. Organic synthetic fillers consist of high-molecular substances.
10. **What are the effects of fillers on printing paper performance?**
Answer: Adding fillers to printing paper improves printability by increasing whiteness, smoothness, opacity, and stiffness. It enhances ink affinity, softness, and stability, thereby improving printed product quality. However, excessive filler addition reduces paper strength and sizing effectiveness, leading to powdering, fiber shedding, and plate clogging during printing. Precipitated calcium carbonate (PCC) minimizes plate wear.
11. **What is paper opacity?**
Answer: Opacity refers to the property of paper that prevents light transmission. It depends on the paper's light scattering ability, the number of scattering interfaces within the paper, and the refractive index difference at these interfaces. More scattering interfaces and greater refractive index differences increase light scattering and opacity.
12. **How do fillers affect paper opacity?**
Answer: Paper without fillers consists of fibers and air. Air exists in fiber pores, causing light scattering at fiber-air interfaces and imparting low opacity. Adding fillers with higher refractive indices than cellulose increases scattering interfaces, including fiber-air, filler-fiber, and filler-air interfaces. The largest refractive index difference occurs at filler-air interfaces, significantly increasing opacity.
13. **How do fillers improve paper surface smoothness?**
Answer: Fiber interweaving forms porous, uneven surfaces. Adding fine particle fillers enhances paper softness and plasticity, enabling better calendering treatment and improving post-calendering smoothness.
14. **How does filler affect paper strength?**
Answer: Fiber bonding underpins paper strength. Filler addition reduces fiber bonding, significantly decreasing tensile, folding, and bursting strength while slightly reducing tear resistance. There is a limit to filler usage.
15. **How does filler affect paper bulk thickness?**
Answer: Paper fiber density is approximately 1 g/cm³, while filler density ranges from 2.5 to 3.0 g/cm³. Filler addition reduces bulk thickness. However, small amounts of filler increase bulk thickness, particularly noticeable in short-fiber paper.
16. **How does filler affect paper stiffness?**
Answer: Paper stiffness depends on thickness and elastic modulus. Filler effects on bulk thickness and tensile strength must be considered comprehensively. Small amounts of filler have minimal impact on stiffness, while larger amounts significantly decrease stiffness.
17. **What is the role of pigments in paper?**
Answer: Even bleached pulp fibers appear slightly yellow or grayish white due to lignin absorbing purple and blue light (wavelengths 400–500 nm). Higher lignin content results in darker colors. To achieve higher whiteness, blue-purple or red-blue dyes are often added to bleached pulp. Dyeing is essential for colored paper production.
18. **What are the components of colorants?**
Answer: Colorants include pigments and dyes used for paper dyeing and coloring. Most pigments are insoluble inorganic substances, while most dyes are water-soluble or become soluble after chemical treatment. Synthetic dyes have replaced natural dyes.
19. **What are other chemical additives in paper auxiliary materials and their functions?**
Answer: Various non-fibrous additives are incorporated into pulp or paper to meet special application needs. These include:
① **Wet strength agents**: Enhance wet strength (e.g., urea-formaldehyde resin, phenolic resin).
② **Dry strength agents**: Improve dry strength (e.g., cationic starch, polyacrylamide).
③ **Retention aids**: Reduce filler and fine fiber loss during papermaking (e.g., polyacrylamide, polyvinyl oxide).
④ **Defoamers**: Eliminate foam during papermaking (e.g., silicone oil, turpentine).
⑤ **Water repellents**: Provide water resistance (e.g., paraffin, glyoxal).
20. **How can paper be simply classified?**
Answer: Paper is generally classified by application into cultural paper, industrial paper, household paper, and special paper. Cultural paper includes uncoated paper (e.g., newsprint, offset paper) and coated paper (e.g., coated paper, calendar paper).
