AKD sizing agents are key chemicals used in papermaking to enhance paper’s resistance to liquid penetration (such as water and oil). They are applied either within the pulp or at the paper surface to improve the overall strength and surface properties of paper. In this article, we discuss the synthesis of AKD wax powder, the preparation of AKD emulsions, and the guidelines for their application in papermaking. In addition, we address common issues, troubleshooting strategies, and future outlooks based on current industrial research.

I. AKD Wax Powder Synthesis Process

  1. Chemical Reaction Pathway

    • Raw Material and Reaction: Starting with stearic acid, the process involves a phosgenation reaction to produce stearoyl chloride as an intermediate. Subsequently, in a toluene solvent system and catalyzed by triethylamine, a condensation dechlorination reaction occurs, ultimately forming the alkyl ketene dimer (AKD).
    • Final Steps: After vacuum distillation to remove the solvent, the product is processed into wax powder by slicing.

  2. Key Process Control Parameters

    • Phosgenation Temperature: Must be strictly controlled within a specific range (±2°C accuracy recommended).
    • Catalyst Dosage: Triethylamine should be used at 1.5–2.0% of the stearic acid weight.
    • Vacuum Distillation: Maintain a vacuum of –0.095 MPa and a distillation temperature range of 180–200°C.

II. AKD Emulsion Preparation Technology

  1. Traditional Process Route

    • Emulsification: Using cationic starch as the primary emulsifier, molten AKD wax powder is mixed with a gelatinized starch solution under high-shear conditions (2500–3500 rpm).
    • High-Pressure Homogenization: The mixture is then subjected to homogenization at 35–45 MPa before being cooled to form an emulsion.
    • Stabilization: Stabilizers, such as zirconium oxychloride, are added to improve storage stability.

  2. Innovative Process Optimization

    • Advanced Emulsifiers: The introduction of polymeric emulsifiers (e.g., poly(dimethylallyl chloride)) or modified montmorillonite materials can simplify the production process and significantly enhance emulsion stability.
    • Performance Improvements: Optimized formulations have achieved a reduction in the hydrolysis rate by over 30%, with a stable Zeta potential maintained above +25 mV.

  3. Quality Control Indicators

    • Solid Content: 12–16% (by weight).
    • pH Value: 2.0–4.0, which helps inhibit the hydrolysis reaction.
    • Average Particle Size:
      • For pulp internal sizing applications: 0.5–2.0 μm
      • For surface sizing applications: ≤0.2 μm

III. Application Technology for AKD Sizing

  1. Systematic Addition Process

    1.1 Optimizing Addition Location

    • Internal (Wet-End) Sizing:
      • Preferably add in the high-consistency stock (e.g., headbox or mixing tank) to ensure a reaction time of at least 15 minutes.
    • Surface Sizing:
      • Mix with gelatinized oxidized starch and add to the coating system via a metering pump.

    1.2 Chemical Addition Sequence

    • Recommended sequence: Cationic starch (1.5–3.0% of pulp) → AKD emulsion (0.4–7.0% of pulp) → Retention aid (e.g., CPAM, 0.05–0.15% of pulp).
    • It is critical to avoid direct contact between these and any anionic substances.

  2. Troubleshooting Common Issues

    2.1 Emulsion Instability

    • Symptoms: Layer separation (gravity separation in <24 hours), abnormally high viscosity (>500 mPa·s), and flocculation; Zeta potential below 10 mV.
    • Control Measures:
      • Storage Temperature: 5–30°C (optimal 15–25°C).
      • Use Within 8 Hours After Dilution (preferably prepare fresh).
      • Stabilizer Strategy: Use a combination of polyethyleneimine (PEI) and poly(dimethylallyl chloride) emulsifier.

    2.2 Decline in Sizing Efficiency

    • Possible Causes:
      • Inadequate drying temperature (<93°C) leading to incomplete reactions.
      • pH above 8.5, accelerating the hydrolysis reaction.
      • High specific surface area fillers (e.g., PCC) causing adsorption losses.
    • Optimization Measures:
      • Drying Curve: Set a rapid curing stage at 110°C for at least 3 minutes.
      • pH Adjustment: Maintain pH between 7.5–8.5 (using sodium carbonate/sulfuric acid).
      • Filler Combination: Substitute 30% of PCC with GCC.

    2.3 Hydrolysis and Charge Imbalance

    • Hydrolysis Control:
      • Total alkalinity should be maintained between 150–250 ppm (as CaCO₃).
      • Add 0.1–0.3% PAE resin (based on pulp) as a hydrolysis inhibitor.
    • Charge Management:
      • Neutralize anionic impurities using 0.05–0.1% cationic polyamine.
      • Use a dual retention system combining 1.5% cationic starch with 0.1% CPAM.

  3. Foam Issues in Pulp and Surface Sizing

    • Pulp Sizing:
      • Foam thickness >5 cm can occur due to excessive cationic starch or high mixing intensity.
      • Solution: Reduce starch dosage to below 2.5% and add 0.01% polyether defoamer.
    • Surface Sizing Issues:
      • Moisture Rebound: High water vapor transmission (>1500 g/m²·24h) due to insufficient internal water resistance.
        • Increase drying temperature to 110°C and use a high-shear SDH emulsifier (particle size ≤0.15 μm).
      • Uneven Sizing: Surface contact angle differences >15° may indicate improper dilution (below 3% or above 5%) or unstable blade pressure.
        • Adjust dilution concentration to 3.5–4.5% and set blade pressure to 8–12 bar.
      • Slippage: Low friction coefficient (<0.3) may result from AKD hydrolysis products accumulating or low surface tension (<30 mN/m).
        • Add 0.1% zinc stearate and 0.5% surfactant (e.g., AEO-9).
      • Emulsion Breakage: Flocculation or gel formation in the sizing emulsion may occur due to excessive mechanical shear or high electrolyte content (>1000 ppm).
        • Use a gear pump for gentle transport and add 0.2% polyvinyl alcohol for stabilization.

  4. Comparative Analysis of Internal and Surface Sizing Solutions

    Control Dimension Internal Sizing (Pulp) Surface Sizing
    Temperature Control Drying stage temperature ≥110°C Drying stage temperature ≥110°C (via hot-air penetration)
    pH Adjustment Maintain 7.5–8.5 (adjust with Na₂CO₃/Sulfuric acid) 5.0–6.5 (to avoid hydrolysis and film formation issues)
    Emulsion Particle Size 0.5–2.0 μm ≤0.2 μm
    Retention System Cationic starch + CPAM dual system Oxidized starch + PAE blend
    Monitoring Frequency Every 2 hours: Cobb value, Zeta potential Every hour: Surface contact angle, solid content of sizing solution

IV. Conclusion

The selection and application of AKD sizing agents require careful consideration of process conditions, pulp properties, and cost factors. By precisely controlling the addition point, pH, and compatibility with retention aids, manufacturers can optimize sizing effectiveness and production stability. Both internal sizing (with controlled reaction and retention) and surface sizing (for enhanced surface properties) have their unique challenges and solutions. With proper management—including rigorous monitoring and process optimization—AKD sizing agents can significantly improve paper strength, water resistance, and overall quality.

V. Future Outlook

Looking ahead, advancements in emulsification technology, nanomaterial integration, and smart process control will further enhance the performance and sustainability of AKD sizing systems. Innovations such as continuous online monitoring and dynamic adjustment strategies are expected to drive the next generation of papermaking chemicals.

VI. 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.