AKD Surface Sizing Agents: Enhancing Paper Performance, Sustainability, and Production Efficiency
Introduction
In the pursuit of high-quality paper and board, surface sizing has emerged as a critical process step that not only improves water resistance but also enhances surface strength, printability, and durability. Among the various surface sizing chemistries, AKD (Alkyl Ketene Dimer) surface sizing agents have gained significant traction for their ability to form a thin, hydrophobic layer on the paper surface while simultaneously boosting mechanical properties. Unlike internal (pulp‑added) AKD, which reacts with cellulose fibers throughout the sheet, surface‑applied AKD targets the topographical features of the formed paper, offering faster curing, lower dosage requirements, and synergistic benefits when combined with internal sizing.
This article provides a comprehensive technical overview of AKD surface sizing agents, covering their chemical action, key performance indicators, application methodologies, and emerging industry trends. It draws on both established papermaking knowledge and the latest developments in sustainable barrier technologies, offering practical insights for mill operators, technical managers, and procurement professionals.
1. The Role of Surface Sizing in Modern Papermaking
Surface sizing is applied at the size press or gate‑roll coater after the paper web is formed and partially dried. Its primary functions include:
- Improving surface hydrophobicity – reducing liquid penetration (water, ink, oil) for better printing and packaging performance.
- Enhancing surface strength – reducing dusting, picking, and linting during printing and converting.
- Increasing stiffness and dimensional stability – especially important for lightweight papers and packaging grades.
- Optimising print quality – providing a more uniform surface for ink or toner adhesion.
- Enabling higher filler loads – surface sizing can compensate for the strength loss associated with increased filler content.
AKD surface sizing agents belong to the reactive sizing family. They contain the same β‑lactone ring chemistry as internal AKD, but when applied on the surface, they react primarily with the exposed hydroxyl groups of surface fibres and with starch binder systems, forming covalent ester bonds that anchor hydrophobic alkyl chains right at the sheet interface.
2. Product Features and Quality Parameters
The LY‑AKD surface sizing series (developed as a next‑generation cationic emulsion) is designed for compatibility with both size‑press and gate‑roll applications. The product line offers varying solids content to suit different paper grades and operational requirements.
2.1 Key Performance Attributes
- Efficient surface film formation – the cationic emulsion readily interacts with anionic fibres and starches, creating a continuous, uniform hydrophobic layer that minimises water absorption (measured as Cobb value) and improves ink holdout.
- Enhanced paper strength – besides sizing, the agent contributes to internal bond strength, tensile strength, and surface pick resistance, allowing mills to reduce the use of expensive dry‑strength additives.
- Extended paper permanence – the covalent bonding of AKD is stable over time, reducing yellowing and maintaining performance throughout the paper’s lifecycle.
- Cleaner wet‑end and lower effluent load – because the agent is applied after the forming section, it does not interfere with wet‑end chemistry; it also reduces the need for internal sizing chemicals, lowering the organic load in white water and simplifying wastewater treatment.
- Energy and productivity gains – faster curing at the drying section allows higher machine speeds and fewer breaks, while the reduced need for wet‑end adjustments streamlines grade changes.
2.2 Technical Specifications (Typical Values for LY Series)
The LY‑AKD surface sizing range is available in different solids levels (approximately 15%, 20%, 25%, and 30%) to match the specific demands of various paper types:
| Grade (General) | Solids Content | Recommended Paper Applications |
|---|---|---|
| LY‑AKD‑15 | 15 ± 0.5% | Fine papers, writing & printing grades (cultural papers) |
| LY‑AKD‑20 | 20 ± 0.5% | Higher‑grade cultural papers, lightweight coated base |
| LY‑AKD‑25 | 25 ± 0.5% | Kraft linerboard, testliner, corrugating medium |
| LY‑AKD‑30 | 30 ± 1% | Heavy‑duty packaging boards, high‑strength applications |
Common characteristics across all grades:
| Parameter | Value |
|---|---|
| Appearance | White milky liquid |
| Ionic type | Cationic |
| pH (25°C) | 2.0 – 4.0 |
| Viscosity (25°C) | < 100 mPa·s |
| Solubility | Readily dispersible in cold water |
| Flammability | Non‑flammable |
The cationic nature ensures strong electrostatic attraction to negatively charged paper surfaces, improving retention and film uniformity. The low viscosity facilitates pumping, metering, and mixing with starch solutions.
3. Application Guidelines for Optimal Results
3.1 Addition Point and Mixing
For surface application, the LY‑AKD surface sizing agent is typically metered continuously into the cooked starch solution at the size‑press feed tank or the circulation tank. The recommended addition rate is 5–25% of the starch solution volume (as supplied product), depending on the desired sizing degree and the paper grade.
In practice, the starch–AKD blend is prepared at a coating concentration of 6–8% solids (total solids including starch and sizing agent). The mixture is then applied via a size press or gate‑roll coater at a pickup of approximately 1–5 kg of sizing agent per tonne of paper (active solids basis).
3.2 Internal (Pulp) Addition Option
Although primarily a surface agent, the same product can also be added internally in cases where the mill lacks a size press or wants a combined effect. For internal addition, the agent is dosed directly into the thick stock (at the mixing chest) or into the thin stock (before the pressure screen). When used internally, the dosage is adjusted based on the pulp furnish and target Cobb values, but surface application remains the preferred route for most modern machines.
3.3 Compatibility with Other Additives
The cationic AKD emulsion is compatible with most starches (cationic, anionic, or oxidised), but it is advisable to avoid simultaneous addition with strongly anionic synthetic polymers that could cause precipitation. The pH of the starch–AKD mixture typically ranges between 5.0 and 7.0, which is optimal for AKD reactivity and starch gelatinisation.
4. Storage and Handling Considerations
To preserve emulsion stability and reactivity, the following precautions are essential:
- Temperature control – store at 4–30°C in a cool, well‑ventilated area, away from direct sunlight. Freezing should be avoided, as it can break the emulsion and cause irreversible coagulation.
- Shelf life – the product remains stable for 3 months from the manufacturing date when stored under recommended conditions. Once the container is opened, it should be used promptly to prevent contamination or hydrolysis.
- Packaging – available in 1000 kg plastic IBC totes or in bulk tanker trucks for large‑volume deliveries. The packaging is standardised for easy handling and automated unloading.
5. Synergies with Internal Sizing Systems
One of the most compelling advantages of AKD surface sizing is its ability to work in concert with internal sizing agents. Many mills use a dual‑sizing strategy:
- A reduced dose of internal AKD (or rosin) provides baseline water resistance throughout the sheet thickness.
- The surface AKD adds a strong, fast‑curing top layer that rapidly increases the Cobb value and surface strength.
This combination often allows a 20–30% reduction in total sizing chemical costs while achieving superior performance. Additionally, surface sizing can compensate for variations in internal sizing efficiency caused by furnish changes or wet‑end upsets, providing a more robust process.
6. Environmental and Regulatory Advantages
Modern AKD surface sizing agents align with the industry’s drive toward greener papermaking:
- Neutral/alkaline operation – unlike acid‑rosin sizing, AKD does not require alum, eliminating sulphate discharges and reducing equipment corrosion.
- Lower BOD/COD – because the agent is applied on the surface and fixed to the fibre, less organic matter ends up in the white water circuit, easing the load on biological treatment plants.
- Reduced energy consumption – the faster curing allows lower drying temperatures or higher speeds, cutting energy use per tonne of paper.
- Food‑contact compliance – AKD is approved by the U.S. FDA (FCN) and meets European BfR recommendations for food‑contact paper and board, provided the product is manufactured to appropriate purity standards. Many surface sizing grades are formulated to have low extractable residues, making them suitable for food packaging, paper cups, and liquid packaging board.
7. Market Trends and Innovations in Surface Sizing
7.1 The Shift toward PFAS‑Free Barriers
With increasing regulatory and brand‑owner pressure to eliminate per‑ and polyfluoroalkyl substances (PFAS) from food packaging, AKD‑based surface sizing has become a preferred alternative for grease‑ and water‑resistant papers. Recent developments focus on combining AKD with biopolymers (e.g., chitosan, cellulose nanofibrils) to achieve oil‑ and grease‑resistance comparable to fluorochemicals, without the environmental persistence concerns.
7.2 High‑Solid Emulsions and Micro‑Emulsions
To reduce transport costs and improve film‑forming properties, manufacturers are introducing high‑solid (up to 35–40%) and micro‑emulsion AKD products. These formulations yield finer particle sizes, better spreading, and more uniform coverage, often enabling lower application weights for the same sizing performance.
7.3 Tailored Solutions for Recycled Fibres
Recycled fibre furnishes present challenges due to high ash content, fines, and hydrophobic contaminants. The cationic nature of the LY‑AKD surface series helps overcome these issues by improving retention on recycled fibres and reducing the negative impact of anionic trash. Studies have shown that surface AKD can significantly improve the strength and water resistance of 100% recycled paper and board, supporting the circular economy goals of the industry.
7.4 Digitalization and Process Control
Advanced mills are adopting online sensors and automated dosage control systems that adjust the surface sizing agent feed based on real‑time moisture, Cobb, and surface strength measurements. This not only optimises chemical usage but also ensures consistent product quality, reducing rejected paper and increasing overall equipment effectiveness (OEE).
8. Practical Troubleshooting Tips
| Issue | Likely Cause | Recommended Action |
|---|---|---|
| Poor sizing response (high Cobb) | Insufficient pickup, low starch solids, or excessive pH | Increase dosage, check size‑press nip pressure, or adjust starch concentration; maintain pH < 7.5 in the size‑press mix |
| Foaming in the size‑press pan | Agitation, entrained air, or incompatible additives | Reduce aeration, add a suitable defoamer, or check for contamination |
| Streaks or uneven application | Poor film formation, roll wear, or viscosity variations | Clean the rolls, check metering pumps, and verify viscosity of the starch–AKD blend |
| Yellowing or discoloration | Over‑drying or oxidation | Reduce dryer temperatures or add an antioxidant if necessary |
| Reduced shelf life or emulsion break | Temperature excursions or contamination | Ensure proper storage conditions and use clean transfer lines |
Conclusion
AKD surface sizing agents have evolved from a niche additive to a cornerstone of modern paper and board production. The LY‑AKD surface series, with its cationic emulsion technology and adaptable solids range, offers papermakers a versatile tool to simultaneously improve water repellency, surface strength, printability, and production efficiency. When integrated with internal sizing strategies, these agents provide a cost‑effective, environmentally friendly, and regulation‑compliant solution for a wide spectrum of grades—from premium cultural papers to high‑performance packaging boards.
As the industry continues to embrace sustainability, digitalisation, and the replacement of persistent chemicals, AKD surface sizing will undoubtedly remain a key enabler of high‑quality, responsible papermaking. For technical teams looking to fine‑tune their surface sizing operations, the proven performance and easy application of AKD‑based systems make them a reliable choice for today and the future.
For more information on the LY‑AKD surface sizing series, including sample formulations and application support, please contact our technical service team.
English 
Arabic 
Portuguese 
Spanish 
Vietnamese 
Russian