Under neutral and slightly alkaline conditions, the wet-end system in papermaking often contains a significant amount of anionic impurities. Proper application of retention and filtration aids is crucial for enhancing the sizing performance in these conditions. This article shares insights from a senior engineer in a major enterprise regarding the application of retention and filtration systems in neutral sizing.
1) Composition and Function of Retention and Filtration Systems
In recent years, the use of multiple chemical additives in the same paper type has become a trend. Neutral sizing technology itself is a series of synergistic chemical additive applications. Researchers have conducted extensive studies on the papermaking wet-end chemical systems to understand the mechanism of these synergistic additives.
- Multifunctional Wet-End Chemical Systems: For example, the Compozil system developed by Eka-Nobel (Sweden), which consists of BMA (silica sol) and BMB (cationic starch), significantly improves filler retention and improves wet-end forming.
- Dual-Polymer Retention Aids: The PERCOL dual-polymer retention aid and HYDROCOL microparticle retention aids developed by the UK-based Colloid Company are effective in improving both retention and filtration, while also enhancing paper formation.
- Binary Retention Systems: The combination of cationic starch and PAM (Polyacrylamide) in neutral sizing has been widely adopted by papermakers worldwide. It has proven effective in improving the retention of AKD (Alkyl Ketene Dimer), which is known for its low retention rate in neutral sizing systems.
2) Optimization and Application of Retention and Filtration Systems
To effectively enhance retention and filtration performance, it is crucial to avoid excessive flocculation, which could negatively impact paper strength and uniformity. The following points highlight key aspects of applying and optimizing these systems:
- Selection of Cationic Starch: High substitution degree cationic starches exhibit stronger flocculation effects. It is recommended to use cationic starches with a degree of substitution (DS) between 0.03 to 0.04 for optimal retention performance.
- Optimization of Addition Amount: The addition amount of cationic starch should be adjusted based on the specific paper machine, product, and raw material. Typically, the amount of cationic starch added ranges from 0.25% to 1.0%. For thin paper, around 0.5% is ideal. Overuse can lead to excessive flocculation, affecting paper uniformity and strength.
- Ideal Addition Points: The best addition point is typically near the high-position headbox. Other potential points include the inlet and outlet of the rotor screen. The exact location of addition should be adjusted based on actual machine conditions. Generally, the closer the addition point is to the wire section, the better the retention and filtration effect, though this also increases the impact on paper uniformity and strength.
3) Balancing Retention and Strength Enhancement
Improving retention and filtration typically presents a challenge in maintaining paper strength. To simultaneously improve both retention/filtration and paper strength, the following measures can be adopted:
- Adjusting Addition Location: Moving the addition point toward the pulping direction can reduce excessive flocculation and improve the uniformity of starch-fiber contact.
- Increasing Addition Amount: By increasing the addition amount or splitting the addition into two stages, better retention can be achieved while controlling the extent of flocculation.
- Controlling Addition Amount: Ensuring the proper amount of additive is used is crucial, as excess addition can affect paper strength and uniformity.
Under neutral sizing conditions, optimizing the application of retention and filtration aids can effectively improve paper quality and production efficiency. The binary retention system of cationic starch and PAM, with appropriate addition amounts and points, can significantly enhance AKD retention, while maintaining paper strength and uniformity. The balance between retention, filtration, and strength enhancement is key to achieving optimal results in paper manufacturing.