Types, Application Points, and Usage Considerations for Internal Sizing Agents in Papermaking

Introduction
Internal sizing agents are key chemicals added directly to paper pulp to impart resistance against liquid penetration (e.g., water and oil) by binding to fibers. In papermaking, the selection and precise addition of these agents are critical for achieving the desired performance and quality. This article discusses the different types of internal sizing agents, their mechanisms of action, optimal addition points in the process, and practical considerations. In addition, it offers insights on balancing effectiveness with production stability and cost.

I. Categories and Mechanisms of Internal Sizing Agents

1. Acidic Sizing Agents

   • Example: Rosin Sizing (Soapified or Enhanced Rosin)
   • Mechanism:
     • Rely on the presence of aluminum sulfate (alum) to form a precipitate that adheres to the fiber surface.
     • Typically applied at a pH range of 4.5–6.0.
   • Characteristics:
     • Low cost but may lead to yellowing and lower durability, making them suitable for lower-grade cultural or packaging papers.

2. Neutral/Alkaline Sizing Agents

   • AKD (Alkyl Ketene Dimer)
     • Mechanism: Reacts with fiber hydroxyl groups to form covalent bonds, providing water and oil resistance.
     • pH Range: 6.5–8.5.
     • Note: Requires a maturation period (typically 24–48 hours) for optimal performance, making it ideal for mid-to-high grade cultural paper and liquid packaging paper.
   • ASA (Alkenyl Succinic Anhydride)
     • Mechanism: Rapidly hydrolyzes and reacts with fibers without a prolonged maturation period.
     • pH Range: 6.0–8.0.
     • Note: Easily emulsified; it must be prepared fresh and is suitable for high-speed paper machines or applications requiring high sizing levels.
   • Synthetic Polymers (e.g., Styrene-Maleic Anhydride Copolymers)
     • Mechanism: Function through electrostatic adsorption or chemical crosslinking, providing water and oil resistance.
     • pH Range: Effective in neutral to alkaline environments.
     • Features: Chlorine-free and environmentally friendly, ideal for specialized paper types.

3. Other Modified Natural Polymers

   • Carboxymethyl Chitosan (CMCS)
     • Mechanism: With a substitution degree greater than 0.6, its water solubility significantly improves, allowing it to interact with fibers via ion-dipole forces.
     • Environmental Advantage: Exhibits a biodegradation rate over 90% (ISO 14855) and produces 65% lower carbon emissions compared to synthetic polymers (Carbon Trust certified).

II. Chemical Kinetic Optimization of Addition Processes

1. Chemical Compatibility in Wet-End Addition

   • Synergistic Effect with Aluminum Salts:
     • The presence of aluminum sulfate (Al₂(SO₄)₃) can increase cationic starch adsorption on fiber surfaces by 2–3 times (Colloids and Surfaces A, 2018).
     • Optimal Al³⁺ concentration: 50–100 ppm (excess can cause over-flocculation).
   • Microbial Degradation Control:
     • To inhibit starch-degrading enzymes, adding 0.05% methylene bis(thiocyanate) (MBT) can help retain more than 95% of the starch’s molecular weight (Bioresource Technology, 2022).

2. Rheological Control in Size Press Applications

   • Oxidized Starch Viscosity Adjustment:
     • The relationship between oxidation degree (carboxyl content) and viscosity can be described as:

       η (mPa·s) = 250 - 15×[COOH] (mmol/g)  
       

     • Common industrial oxidation degree: 0.08–0.12 mmol/g (viscosity range 80–120 mPa·s, Brookfield DV2T, 20°C).
   • Anti-Retrogradation Technique:
     • Adding 0.1%–0.3% hydroxypropylation agents (e.g., propylene oxide) can inhibit amylose recrystallization, with XRD analysis showing up to a 70% reduction in crystallinity.

3. In-Situ Gelatinization in the Formation Section (Spray Application)

   • Thermodynamic Parameters:
     • The activation energy for native starch gelatinization is 120–150 kJ/mol (DSC analysis).
     • Critical gelatinization conditions: Temperature >70°C for 30 seconds (Starch/Stärke, 2020).

III. Production Management and Operational Considerations

1. Controlling Starch Retrogradation

   • Dynamic Storage Stability:
     

     Storage Time (h)	Viscosity Retention (%)	Bonding Strength Retention (%)
     0	100	100
     4	85	80
     8	60	50
     Solution: Add 0.05% sodium citrate (chelates Ca²⁺ to inhibit crosslinking).

2. System Microbial Control

   • Biocide Selection for Starch Preservation:
     

     Type	Minimum Inhibitory Concentration (ppm)	Inhibition Rate of Amylase
     Isothiazolinone	50	92%
     DBNPA	30	88%
     Glutaraldehyde	100	75%

3. Optimal Addition Points and Strategies

   • In the Pulp (Wet-End) Stage:
     • Thick Stock Addition: Ideal for cationic starch, as high pulp consistency (typically 3%–5%) ensures thorough fiber contact and high retention (70%–80%).
       • Considerations: Monitor microbial activity and ensure stable pulp flow, especially during frequent product changes.
     • Dilute Stock Addition: Adding after the pulp pump minimizes residence time and reduces microbial degradation risks, though careful control is needed to maintain fiber contact efficiency.
   • In the Size Press:
     • Sizing agents (e.g., oxidized starch or lower-viscosity cationic/hydroxyethyl starch) are applied to the moist paper web to enhance surface strength and printability.
       • Key Points: Must have proper viscosity to avoid issues like roll blockage and require adequate drying after application.
   • Spray Application in the Formation Section:
     • Uncooked starch suspensions are sprayed onto the wet paper web using nozzles or curtain coating.
       • Advantages: Lower cost native starch is used without pre-cooking, leading to high retention and reduced energy consumption due to the absence of extra drying steps.
       • Applicability: Particularly suitable for thicker boards or high grammage papers with longer dryer residence times, as well as multi-layer boards for improved interlayer bonding.

IV. Conclusion and Future Outlook

The selection and application of internal sizing agents in papermaking must be tailored to specific paper grades, process conditions, and cost considerations. By precisely controlling the addition point, pH, and compatibility with auxiliary agents, manufacturers can balance the sizing effect and production stability. Neutral sizing agents (such as AKD and ASA) offer environmental and aging resistance advantages, making them the mainstream choice for mid-to-high grade papers, while acidic rosin sizing continues to provide cost advantages for lower-grade packaging papers.
Looking ahead, advancements in papermaking technology and the rising demand for eco-friendly solutions will drive further development of high-performance, sustainable internal sizing agents.

References

1. Heinze, T. (2018). Starch Chemistry and Technology. Springer.
2. TAPPI T569 om-15: Internal bond strength of paperboard.
3. Zhang, Y. et al. (2021). “Cationic starch-fiber interactions: A combined QCM-D and AFM study”. Carbohydrate Polymers, 256, 117582.
4. ISO 14855: Determination of the ultimate aerobic biodegradability under controlled composting conditions.