Dimensionamiento interno de AKD: Una guía práctica para un rendimiento óptimo en la fabricación de papel neutro
Introducción
Alkyl Ketene Dimer (AKD) has established itself as the dominant internal sizing agent for neutral and alkaline papermaking systems. Unlike traditional rosin-alum sizing, which requires acidic conditions and causes equipment corrosion, AKD offers a reactive covalent bonding mechanism that provides durable water resistance while enabling the use of cost-effective calcium carbonate fillers.
This article provides a practical, operationally focused guide to AKD internal sizing—covering the chemistry, optimal process conditions, factors affecting performance, common pitfalls, and troubleshooting strategies. Written for paper mill technical teams, this guide draws on both established industry knowledge and the latest technical insights to help you get the most out of your AKD sizing program.
1. Understanding AKD Chemistry: The Reactive Sizing Mechanism
AKD (Alkyl Ketene Dimer) is a reactive sizing agent characterized by a strained β-lactone ring structure. This ring contains the reactive functional group—a ketene dimer—that enables the molecule to bond covalently with cellulose fibers.
1.1 The Covalent Bonding Mechanism
The sizing mechanism proceeds through three essential steps:
Retención — AKD emulsion particles must be retained on the fiber surface during sheet formation. Because AKD itself has inherently low retention, retention aids (such as cationic starch or dual polymer systems) are essential.
Propagación — During the drying section, the low melting point of AKD (44.5–52°C) allows it to melt and spread uniformly over the fiber surface.
Reacción — The β-lactone ring opens and reacts with the hydroxyl (-OH) groups of cellulose to form a covalent β-keto ester bond. This chemical anchoring creates a durable, water-repellent surface that provides lasting sizing performance.
1.2 The Hydrolysis Challenge
A critical consideration in AKD sizing is the competing hydrolysis reaction—where AKD reacts with water rather than cellulose, producing inactive by-products. The hydrolysis reaction is accelerated by high temperatures and prolonged exposure to water. This is why effective retention and minimal dwell time are essential for maximizing sizing efficiency.
2. Optimal Operating Conditions
2.1 pH Range
AKD performs optimally in a neutral to slightly alkaline environment. While it can function across a wide pH range (7.0–9.0), the optimal pH range is 7.5–8.5.
Key considerations:
- Below pH 7.0: The β-lactone ring may hydrolyze prematurely, reducing its ability to react with fibers.
- Above pH 8.5: Sizing efficiency decreases as hydrolysis accelerates faster than the curing reaction.
2.2 Alkalinity
Beyond pH, total alkalinity plays a significant role in AKD performance. The recommended total alkalinity range is 150–250 mg/L (as CaCO₃). Within this range:
- Sizing efficiency is optimized
- Curing rate is accelerated
- The wet-end chemistry remains stable
2.3 Temperature
AKD emulsions should be stored at 4–30°C, away from direct sunlight. During the papermaking process, the drying section temperature (typically 90–120°C) is sufficient to melt and spread the AKD for reaction with fibers. However, excessive temperatures in the wet end or stock chests should be avoided, as they accelerate hydrolysis.
3. Factors Affecting AKD Sizing Performance
3.1 The Role of Retention Aids
AKD itself has inherently low retention on fibers—only a fraction of the added AKD is actually retained during sheet formation. This is why retention aids are not optional but essential for effective AKD sizing.
Common retention aid systems include:
- Cationic starch: The most widely used retention aid for AKD systems. Cationic starch neutralizes anionic trash and improves AKD retention.
- Dual retention systems: Combinations of cationic starch with CPAM (cationic polyacrylamide) and bentonite have been shown to achieve high sizing efficiency and filler retention simultaneously.
- Advanced systems: Research has shown that dual retention systems with cationic starch and CPAM can improve AKD sizing by up to 60%.
3.2 Chemical Interferences
Several wet-end additives can interfere with AKD sizing performance:
| Additive | Effect on AKD Sizing | Recommendation |
|---|---|---|
| Antiespumantes | Can disrupt emulsion stability | Use with caution; select defoamers compatible with cationic systems |
| Felt cleaners | May contain surfactants that interfere | Avoid addition near AKD addition point |
| Anionic dyes | Compete for cationic sites; reduce AKD retention | Add at separate locations; minimize dosage |
3.3 Alum (Aluminum Sulfate)
Alum is not required for AKD sizing to function. However, in practice, small amounts of alum (3–10 lb/ton of furnish) are sometimes used to:
- Improve retention (particularly of titanium dioxide)
- Enhance drainage
- Reduce press picking problems
- Control anionic trash
Critical note: The alum addition point should be separate from the AKD addition point. Adding alum too close to AKD can cause precipitation and reduce sizing efficiency. Avoid excessive alum addition, as high alum levels can interfere with the AKD reaction.
4. Storage and Stability
4.1 Shelf Life
AKD emulsions have a limited shelf life due to the ongoing hydrolysis reaction. Typical storage recommendations:
| Parámetro | Specification |
|---|---|
| Storage temperature | 4–30°C |
| Shelf life | 1 month (for standard 15% solids emulsions) |
| Storage conditions | Cool, ventilated area; protected from direct sunlight |
| Freezing | Must be prevented (freezing breaks the emulsion) |
AKD emulsions with advanced stabilizers can achieve longer shelf life—up to 6 months for some formulations—but the industry standard remains 1–3 months.
4.2 Hydrolysis During Storage
Hydrolysis is the primary degradation mechanism for AKD emulsions. Even under optimal storage conditions, hydrolysis proceeds slowly, producing inactive by-products that reduce sizing efficiency. Key factors accelerating hydrolysis:
- Elevated temperature: Hydrolysis is quantifiable over 15–30 days at 40–60°C
- Sunlight exposure: Direct UV radiation accelerates degradation
- Contamination: Introduction of bacteria or other contaminants can accelerate breakdown
5. Dosage Guidelines
5.1 Typical Dosage Range
For most paper grades, the recommended AKD dosage is 8–11 kg per tonne of paper. However, the optimal dosage depends on:
- Paper grade: Heavy-sizing applications (archival papers, liquid packaging board) may require higher dosages
- Furnish composition: Recycled fiber typically requires less sizing agent than virgin fiber
- Filler content: Higher filler levels may require adjusted dosage
- Target sizing degree: Higher Cobb value requirements need increased dosage
5.2 Dosage Optimization Principles
More is not always better. Excessive AKD addition can lead to:
- Slip issues: Excess AKD on the paper surface can cause slippage during converting (box-making, printing)
- Economic waste: Unnecessary chemical cost
- Process issues: Increased deposits and runnability problems
The key principle is to find the minimum dosage that achieves the target sizing specification, then fine-tune through systematic trials.
5.3 Effect on Dyes
One benefit of optimized AKD sizing is that dye usage typically decreases. The improved retention and reduced anionic interference mean that less dye is needed to achieve the same color intensity.
6. Curing and Development Time
6.1 The Curing Requirement
Unlike ASA (Alkenyl Succinic Anhydride), which develops sizing immediately, AKD requires a curing period to develop full sizing performance.
6.2 The 24-Hour Rule
For heavy-sizing applications (archival papers, liquid packaging board), the typical recommendation is that sizing continues to develop for up to 24 hours after the paper comes off the machine. Maximum sizing performance is often achieved after this curing period.
This means:
- Off-machine testing at the reel may understate final sizing performance
- Final quality release should account for the curing period
- Customer complaints about initial water resistance may resolve after storage
6.3 Accelerating Curing
While some curing time is inherent to AKD chemistry, several strategies can accelerate development:
- Higher drying temperatures: Within equipment limits, higher temperatures accelerate the reaction
- Optimized pH and alkalinity: Maintaining the 7.5–8.5 pH range and 150–250 mg/L alkalinity maximizes curing rate
- Advanced formulations: Some modern AKD emulsions offer faster curing through optimized particle size and stabilizer systems
7. Laboratory Testing Guidelines
7.1 Sample Preparation for Testing
When testing AKD-sized paper in the laboratory, the curing process must be simulated. The standard practice is:
- Sample the paper immediately after the reel (or from the laboratory handsheet)
- Condition the sample at 105°C for 5–8 minutes to simulate the curing reaction
- Test water absorption using the Cobb test or other standard methods
This accelerated curing protocol provides results that more closely approximate the final, fully cured sizing performance.
7.2 Common Test Methods
| Test | Purpose | Standard |
|---|---|---|
| Cobb test | Measures water absorption (g/m²) | ISO 535, TAPPI T441 |
| HST (Hercules Size Test) | Measures liquid penetration time | TAPPI T530 |
| Sizing degree | Relative measure of hydrophobicity | Various |
8. Troubleshooting Common AKD Sizing Issues
| Issue | Possible Causes | Recommended Actions |
|---|---|---|
| Poor sizing (high Cobb) | Insufficient dosage; pH too low or too high; poor retention; incomplete curing | Verify dosage; check pH (target 7.5–8.5); optimize retention aid system; allow 24-hour curing; test at 105°C for 5–8 min |
| Inconsistent sizing | pH fluctuations; alkalinity variations; retention system instability | Stabilize wet-end chemistry; monitor alkalinity (150–250 mg/L); check retention aid feed consistency |
| Slip issues in converting | Excessive AKD dosage | Reduce dosage; excessive addition does not improve sizing and causes slip |
| Deposits / stickies | Poor retention; excessive AKD hydrolysis; incompatible additives | Improve retention; reduce AKD dosage if excessive; check for incompatible chemicals |
| Short shelf life of emulsion | Temperature excursions; sunlight exposure; contamination | Verify storage conditions (4–30°C, dark, cool); check for freezing; use within stated shelf life |
| Reduced sizing after storage | Hydrolysis during storage | Use fresh emulsion; check storage conditions; verify shelf life |
9. Summary of Key Operating Parameters
| Parámetro | Optimal Range / Value |
|---|---|
| pH | 7.5 – 8.5 |
| Total alkalinity | 150 – 250 mg/L (as CaCO₃) |
| Storage temperature | 4 – 30°C |
| Shelf life | 1 month (standard emulsion) |
| Typical dosage | 8 – 11 kg/ton paper |
| Retention aid | Essential (cationic starch or dual system) |
| Curado | Continues up to 24 hours off-machine |
| Test conditioning | 105°C for 5–8 min before testing |
| Alum | Not required; if used, add separately from AKD |
| Dye usage | Typically decreases with optimized AKD sizing |
Conclusión
AKD internal sizing is a proven, effective technology for neutral and alkaline papermaking. Its covalent bonding mechanism provides durable water resistance, enables the use of calcium carbonate fillers, and reduces equipment corrosion compared to traditional acid sizing systems.
However, AKD is not a “set and forget” chemistry. Success requires careful attention to:
- Wet-end chemistry: Maintaining optimal pH (7.5–8.5) and alkalinity (150–250 mg/L)
- Retención: Using effective retention aid systems to overcome AKD’s inherently low retention
- Storage: Protecting the emulsion from heat, light, and freezing
- Curado: Allowing sufficient time (up to 24 hours) for full sizing development
- Dosage: Finding the minimum effective dosage—more is not better
By understanding the chemistry, controlling the key parameters, and systematically troubleshooting issues, paper mills can achieve consistent, cost-effective AKD sizing performance across a wide range of paper and board grades.
For technical support or product information on LY‑AKD internal sizing agents, please contact our technical service team.
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