1. Definition and Role of Fillers
Filling refers to the process of adding inorganic materials to pulp, which can both improve certain properties of paper and reduce its
production cost. Paper fillers are generally solid microparticles that are essentially insoluble in water. The purpose of adding fillers is to increase the paper’s whiteness, opacity, refractive index, scattering coefficient, etc., while also lowering production costs.
2. Basic Characteristics of Fillers
Ideal paper fillers should have the following characteristics:
- Small and uniform particle size, with high whiteness.
- Chemically stable, not easily reacting with acids or bases, and resistant to oxidation or reduction.
- Insoluble or only slightly soluble in water, allowing effective retention in the paper.
- High opacity, refractive index, and scattering coefficient.
- Rich in resources and cost-effective.
Common fillers include talcum powder, kaolin, titanium dioxide, calcium carbonate, etc., and sometimes special fillers are added to meet specific requirements, such as carbon black for conductive paper.
3. Key Factors Affecting Filler Retention Rate
Filler retention rate refers to the percentage of filler effectively retained in the paper during the production process. Several factors affect the retention rate of fillers, including the following key ones:
3.1 Filler Particle Shape
The shape of filler particles directly affects their retention rate. Generally, non-spherical fillers (such as flaky talcum powder) are less likely to settle and are more easily attached to fibers, forming a tighter fiber network, which results in a higher retention rate. On the other hand, spherical fillers are more likely to be lost from the pulp, leading to a lower retention rate.
3.2 Pulp Beating Degree
The beating degree of pulp significantly impacts filler retention. A higher beating degree results in a tighter paper structure and slower dewatering, increasing the bonding between fibers and thus improving the filler retention rate. Lower beating degrees may lead to more filler loss.
3.3 Pulp Type
Different pulp types exhibit different filler retention rates. Generally, pulp with shorter fibers and higher absorbency (e.g., groundwood pulp) has a higher filler retention rate. In contrast, pulp with lower absorbency (e.g., cotton pulp) results in a lower retention rate. The sequence of filler retention rate from high to low is approximately: cotton pulp < sulfate pulp < sulfite pulp < hemp pulp < groundwood pulp.
3.4 Impact of Beating Method
The beating method also significantly influences filler retention. Pulp that is beaten in a sticky manner (i.e., higher moisture content) increases the surface area of the fibers, making them more tightly bonded and resulting in a higher filler retention rate. In contrast, beating in a free manner (lower moisture content) tends to lower the retention rate.
3.5 White Water Recovery and Utilization
The efficiency of white water recovery directly affects filler retention. By effectively utilizing white water (such as for dilution, concentration adjustment, or pulp refining), the use of fresh water can be minimized, and some filler and fine fibers can be recovered. The loss of white water leads to increased filler loss, so maximizing white water usage is an important method for improving filler retention.
3.6 Use of Retention Aids
The use of retention aids is one of the most important methods to improve filler retention. According to the theory of colloidal adsorption, retention aids interact with fine fibers and fillers in the pulp, significantly reducing their loss and enhancing filler retention. Common retention aids include the following types:
- Cationic Starch: A traditional retention aid, cationic starch can increase filler retention by forming chemical bonds with fibers and fillers, improving paper strength.
- Polyethyleneimine (PEI): Polyethyleneimine is a high-molecular cationic compound widely used in the papermaking industry. It effectively adsorbs negatively charged substances in pulp, enhancing the retention of fillers and fine fibers.
- Polyacrylamide (PAM): A commonly used retention aid with good flocculation and thickening properties, it promotes the bonding between fillers and fibers, reducing loss.
- Alum: Alum is used to improve the wet strength of paper and can also help increase filler retention. It is commonly used in papermaking to adjust pH and promote coagulation in the pulp.
- Bentonite: Bentonite, a natural mineral, is often used to adjust the viscosity of pulp and improve filler retention, especially in the production of specific types of paper.
For example, in the production of dictionary paper, if the amount of titanium white is 10%, the retention rate is 40%. However, if 0.05% polyacrylamide (based on dry weight of the pulp) and 2% alum are added and the pH is adjusted to 6.0–7.0, the retention rate of titanium white can increase to 70%.
On low-speed paper machines, filler retention rates are generally 50–70%, but on high-speed paper machines, due to faster dewatering, filler retention rates are usually lower. Therefore, high-speed paper machines generally require more retention aids to improve filler retention.
Improving filler retention is crucial for enhancing paper quality and reducing production costs. By optimizing filler particle shape, adjusting pulp beating degree, selecting suitable pulp types, improving beating methods, effectively utilizing white water, and using retention aids, filler retention can be significantly improved. Mastering these key factors will help increase production efficiency, reduce resource waste, and ultimately improve paper quality.