Neutral sizing agents have become the mainstream choice for mid-to-high-grade paper grades due to their wide pH adaptability, environmental friendliness, and aging resistance. However, challenges such as process sensitivity and high cost require targeted optimization.
I. Core Advantages: Breaking the Limitations of Acidic Sizing
1. Compatibility with Neutral/Alkaline Papermaking Systems to Improve Paper Performance
Filler Upgrade
- Enables the use of calcium carbonate (30%–50% cheaper than talc).
- Paper whiteness increases by +5% to +8% ISO, opacity by +10% to +15%, and bulk improves significantly.
- Ideal for offset paper and coated paper in cultural applications.
Fiber Protection
- Avoids fiber hydrolysis under acidic conditions (pH < 6).
- Final paper strength (tensile index +10%–15%) and durability (folding endurance +20%) are enhanced.
- Suitable for archival and textbook papers requiring long-term preservation.
2. Strong Chemical Bonding, Excellent Water Resistance
AKD
- Forms covalent bonds with cellulose hydroxyl groups.
- Can withstand 100°C hot water soaking for over 2 hours.
- Milk carton board treated with AKD can resist pasteurization (85°C, 30 minutes).
ASA
- Rapid reaction (retention >90% within 3–5 minutes).
- Base paper for paper cups achieves Cobb value <15 g/m², suitable for hot drinks (95°C).
3. Environmentally Friendly Compared to Acidic Systems
No Chloride Pollution
- Replaces acidic rosin + aluminum sulfate systems (reduces aluminum sulfate use by 80%–100%).
- Lowers COD discharge per ton of paper by 40%–60%.
- Compliant with EU REACH and China’s papermaking effluent discharge standards.
Recyclability
- Neutral paper pulp has 30% lower degradation rate than acidic pulp.
- Deinking efficiency improves by 15%–20%.
- Supports green packaging (e.g., e-commerce cartons).
4. Flexible Processing
AKD
- Flexible dosing positions (refiner / headbox).
- Retention rate of 70%–80%.
- Suitable for medium- to low-speed paper machines (<800 m/min).
ASA
- No curing needed (effective within 30 minutes).
- Ideal for high-speed machines (>1500 m/min), such as tissue base paper lines.
II. Key Disadvantages: Process Sensitivity & Cost Challenges
1. Long Curing Time (Specific to AKD)
- Requires 24–48 hours to cure (25°C, 50% humidity).
- Finished paper needs to be stacked, occupying warehouse space.
- Urgent orders difficult to fulfill quickly (acid sizing cures within 30 minutes).
2. Strict Storage and Usage Conditions
AKD
- Storage temperature ≤35°C (40°C half-life is only 7 days).
- Dosing temperature ≤50°C (≥60°C results in >50% degradation).
- Requires insulation during winter transport in northern regions.
ASA
- Emulsion must be freshly prepared and used within 2 hours.
- Daily cleaning of pipelines with 50°C water + 0.1% NaOH is required to prevent gel blockage.
- (A case where failure to clean resulted in 4 hours of downtime.)
3. High Cost
Material Price
- AKD wax powder (15,000–20,000 CNY/ton) is 3–5x the cost of rosin (5,000–8,000 CNY/ton).
- Paper cost increases by 20–50 CNY/ton (at 0.3% dosage).
Additive Dependency
- Must be paired with cationic starch (0.1%–0.3%).
- Additional cost of 5–15 CNY/ton.
4. Complex Equipment Maintenance
Scaling Risk
- AKD easily demulsifies at high temperatures (>120°C) in dryer section, forming wax deposits.
- (One case of coating machine roller scaling led to basis weight deviation >3%.)
- Requires weekly cleaning with 1% NaOH + 0.5% surfactant.
Foam Control
- High foam stability under neutral pH (vs. acidic systems).
- Requires more defoamer (0.02%–0.05%).
- Adds 2–5 CNY/ton in cost.
5. Compatibility Challenges
- Anionic trash (e.g., degraded pulp, lignin) adsorbs sizing agents.
- A hardwood pulp mill with anionic trash >500 μeq/L saw AKD retention drop from 75% to 50%.
- Required 0.1% cationic polyamine addition (+8 CNY/ton).
III. Application Scenarios and Optimization Suggestions
| Paper Grade | Recommended Agent | Key Advantages | Technical Pain Points | Suggested Solutions |
|---|---|---|---|---|
| Offset/Coated Paper | AKD | Neutral papermaking + CaCO₃ filler; +3–5 ISO whiteness | Control freeness (28–32°SR) to reduce anionic sites | Optimize pulp refining stage |
| Liquid Packaging | AKD + PVDC | Water- & oil-resistant; FDA compliant | Monitor curing zone (25±2°C, 50±5% RH) | Use climate-controlled curing warehouses |
| High-Speed Tissue | ASA | Fast reaction, supports 1800 m/min speed | Dosing pump must sync with machine speed (<±0.5%) | Use frequency-controlled dosing systems |
| Food Contact Paper | Synthetic Polymer | No migration risk; EU 10/2011 compliant | Compatibility with food simulants | Test with 50% ethanol simulant prior to production |
Conclusion: Choosing Neutral Sizing Agents Wisely
Neutral sizing technologies like AKD and ASA offer clear advantages—supporting alkaline papermaking, improving hydrophobicity, enabling calcium carbonate fillers, and meeting environmental regulations. These features make them ideal for high-grade paper applications, including textbooks, food packaging, and coated cultural papers.
That said, managing their process sensitivity and curing requirements is essential. By combining real-time system monitoring (e.g., zeta potential, retention rate) with automated storage controls, many mills have successfully shortened AKD curing times and improved efficiency.
For cost-sensitive paper types, acidic rosin sizing may still be a more practical solution.
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