The Critical Role of Surface Sizing in Modern Papermaking: Understanding AKD Surface Sizing Technology

Introduction

The paper industry has undergone a fundamental transformation over the past decades with the shift from acidic to neutral and alkaline papermaking systems. This transition has enabled the widespread use of low-cost calcium carbonate fillers, which now constitute 30% to 40%—and in some cases even higher—of the total pulp furnish mass. While this shift has delivered significant cost savings and improved optical properties, it has also introduced a cascade of challenges that threaten both product quality and production efficiency.

As filler content increases, papermakers face deteriorating sizing performance, reduced tensile strength, diminished surface strength, and in severe cases, operational instability. These problems directly impact the economic interests of producers. How to compensate for the defects caused by high filler loading has become a critical concern for paper industry professionals worldwide.

Surface sizing has emerged as the most effective and practical solution to these challenges. This article provides a comprehensive technical overview of surface sizing technology with a focus on AKD (Alkyl Ketene Dimer) surface sizing agents—exploring the chemistry, application methods, and the critical role of synergists in achieving optimal performance.

1. Why Surface Sizing Matters: Compensating for the Limitations of High Filler Loading

1.1 The Filler Challenge

The widespread adoption of calcium carbonate fillers in neutral papermaking has been driven by compelling economic and quality benefits: improved brightness, enhanced opacity, better printability, and significant cost reduction. However, these benefits come at a price. Fillers occupy space between fibers, disrupting the hydrogen bonding that gives paper its strength. The result is a paper sheet with:

Poorer sizing performance – reduced resistance to water and liquid penetration
Lower tensile strength – diminished fiber-to-fiber bonding
Weakened surface strength – increased susceptibility to dusting, picking, and linting during printing
Reduced stiffness and bulk – compromised structural integrity

1.2 Surface Sizing as the Solution

Surface sizing addresses these deficiencies directly by applying a thin coating of sizing solution to the paper surface after the sheet has been formed and dried. The primary functions of surface sizing include:

Enhancing water resistance – creating a hydrophobic barrier that prevents liquid penetration

Improving physical strength – significantly increasing bursting strength, tensile strength, and surface strength (picking resistance)

Reducing dusting and linting – effectively eliminating the phenomenon of fiber and filler脱落 during printing, thereby improving printing efficiency and quality

Enhancing printability – providing a uniform surface that prevents capillary spreading of ink and improves ink adhesion

Enabling higher filler content – compensating for the strength loss associated with increased filler loading, allowing mills to maximize cost savings without sacrificing quality

Research has demonstrated that surface-sized paper exhibits better writing quality, greater bursting strength, and higher pick resistance than internally sized paper. Surface sizing with starch has been shown to positively improve paper physical strength properties and printability.

2. Understanding AKD Surface Sizing: Mechanism and Chemistry

2.1 The Fundamental Principle

AKD (Alkyl Ketene Dimer) is a reactive sizing agent characterized by a strained β-propiolactone ring structure. When applied as a surface sizing agent, AKD functions through a covalent bonding mechanism with cellulose fibers.

The core reaction is an esterification process: the β-propiolactone ring of the AKD molecule reacts with the hydroxyl (-OH) groups on the cellulose backbone to form a β-keto ester covalent bond. This covalent bond anchors the long, hydrophobic alkyl chains of the AKD onto the cellulose surface, creating a permanent barrier to water penetration.

2.2 The Three-Stage Mechanism

The AKD surface sizing process proceeds through three essential stages:

Stage 1: Retention – AKD emulsion particles are retained on the cellulose fibers at the paper surface. The cationic nature of modern AKD emulsions ensures strong electrostatic attraction to negatively charged fiber surfaces.

Stage 2: Spreading – As the paper is dried and heated, the AKD melts (melting point 44.5–52°C) and spreads uniformly over the fiber surfaces.

Stage 3: Covalent Bonding (Reaction) – At elevated temperatures during the drying section, the strained lactone ring reacts with the hydroxyl groups of cellulose, forming durable β-keto ester covalent bonds.

2.3 The Debate on Covalent Bonding

While the covalent bonding mechanism has been extensively studied and debated, a consensus has emerged from decades of research. From a series of indirect techniques, it has been concluded that the establishment of a covalent bond between cellulose and AKD is essential in order to introduce permanent hydrophobicity to cellulosic surfaces.

AKD can react with cellulose to form direct β-keto ester bonds. This covalent bond is strong enough to resist the attack of acids or bases and is almost inert to these chemicals. Both AKD and ASA have a reactive functional group that covalently bonds to cellulose fiber, with hydrophobic tails oriented away from the fiber.

2.4 Surface vs. Internal Sizing: A Strategic Distinction

When AKD is applied as a surface sizing agent (rather than internally in the wet end), the mechanism offers distinct advantages:

Reduced hydrolysis – AKD applied to the surface has less opportunity to react with water before bonding with cellulose
Faster curing – Surface application positions the AKD directly where heat is applied during drying
Targeted performance – The sizing effect is concentrated where it matters most: the paper surface
Cleaner wet-end – Eliminates AKD hydrolysis products from contaminating the wet-end system

Internal sizing with AKD provides baseline water resistance throughout the sheet thickness. However, surface sizing slows down capillary penetration by forming a barrier layer at the sheet surface, while internal sizing slows down water penetration primarily through molecular diffusion.

3. The Critical Role of AKD Synergists (Curing Promoters)

3.1 Why Synergists Are Essential

AKD surface sizing presents a significant challenge when used alone: AKD is prone to severe hydrolysis and exhibits slow curing without the use of compatible synergists (also called AKD curing promoters, accelerators, or fixing agents).

AKD is an active sizing agent that can react with the hydroxyl group of cellulose to form an irreversible covalent bond. However, the hydrolysis reaction—where AKD reacts with water rather than cellulose—competes with the desired sizing reaction. Without proper stabilization, AKD hydrolysis can significantly reduce sizing efficiency and contaminate the papermaking system.

3.2 How Synergists Work

AKD synergists (curing promoters) function through multiple mechanisms:

Hydrolysis prevention – Synergists form a protective barrier around AKD particles, greatly limiting the hydrolysis and migration tendency of AKD. This protection is particularly critical in surface sizing applications where the AKD is exposed to heat and moisture.

Accelerated curing – Synergists speed up the curing reaction, promoting quicker bonding between AKD and cellulose fibers. The curing reaction has been found to be more than 20 times faster by adding relatively small amounts of AKD reaction accelerators.

Enhanced retention – Cationic synergists act as excellent anion capture agents and retention aids in the pulp slurry, quickly precipitating the sizing onto pulp fibers and greatly reducing the loss and hydrolysis of the sizing.

Improved emulsion stability – Synergists stabilize the AKD emulsion, extending shelf life and ensuring consistent performance.

3.3 Types of Synergists

Common AKD synergists include:

PAE (Polyamidoamine-epichlorohydrin) resin – widely used as both a wet strength agent and AKD curing promoter
Cationic polymers – highly branched cationic polyelectrolytes with various functional groups
Quaternary ammonium polymers – specialized cationic salts formulated to optimize AKD emulsion performance

Studies have demonstrated that four types of polymers synthesized as AKD sizing promoters can be effectively applied to AKD sizing systems. All tested polymers improved the rate of cure of AKD emulsions to different degrees.

3.4 The Impact on Curing Time

The practical impact of synergists on production is substantial. Ordinary AKD sizing typically requires more than ten hours of curing after the paper comes off the machine to achieve full sizing performance. With the addition of an effective synergist, this curing period can be reduced to approximately one hour.

This dramatic reduction in curing time enables:

Faster quality release – paper can be tested and released sooner
Reduced warehousing – less inventory of aging paper waiting for full sizing development
Improved customer satisfaction – final product performance is achieved more quickly
Elimination of false sizing – reduces the phenomenon of temporary sizing that disappears over time

4. The Synergistic Effect: Combining Internal and Surface AKD Sizing

4.1 The Dual Sizing Strategy

Modern papermakers increasingly employ a dual sizing strategy that combines internal and surface AKD application. This approach leverages the strengths of both methods:

Feature	Internal Sizing	Surface Sizing	Combined System
Main Function	Hydrophobic bonding inside fibers	Surface film formation	Total water resistance
Drying Requirement	≥24h curing	≥95°C instant fixation	Efficient under optimized drying
Typical Issues	False sizing if drying too low	Uneven sizing if application unstable	Balanced performance when optimized

4.2 Benefits of the Combined Approach

Using internal and surface sizing together creates a synergistic effect that improves both the internal structure and surface barrier of the sheet:

20–30% reduction in total chemical consumption while improving performance
Superior water resistance – internal sizing provides bulk hydrophobicity while surface sizing creates a strong barrier
Enhanced surface strength – the surface layer reinforces the sheet where printing and converting stresses are highest
Process robustness – surface sizing can compensate for variations in internal sizing efficiency

4.3 AKD Surface Sizing with Synergists: The Optimal Configuration

For AKD surface sizing applications, the optimal configuration requires:

AKD surface sizing emulsion – formulated specifically for surface application, typically with higher solids content and optimized particle size
Compatible synergist – added to the sizing solution to prevent hydrolysis, accelerate curing, and enhance retention
Starch carrier – typically oxidized starch that serves as the film-forming binder
Controlled application – metered addition at the size press or gate-roll coater

When AKD surface sizing is used with a compatible synergist, the result is:

Minimal hydrolysis – the synergist protects the AKD from degradation
Rapid curing – full sizing performance develops quickly
Reduced wet-end contamination – hydrolysis products do not enter the wet-end system
Improved overall efficiency – faster machine speeds, fewer breaks, and consistent quality

5. AKD Surface Sizing vs. Traditional Surface Sizing

Traditional surface sizing has relied heavily on starch, animal glue, methyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, and wax emulsions. While these materials provide some surface strength improvement, they offer limited water resistance.

AKD surface sizing, particularly when enhanced with synergists, offers superior performance:

Property	Traditional Surface Sizing	AKD Surface Sizing (with Synergist)
Water resistance	Moderate	Excellent (covalent bonding)
Surface strength	Good	Superior
Durability	Moderate	Permanent
Curing time	Immediate	Accelerated (1 hour with synergist)
Hydrolysis risk	None	Controlled by synergist
Compatibility with fillers	Good	Excellent

6. Application Best Practices

6.1 Key Operating Parameters

Parameter	Recommended Range
Application point	Size press or gate-roll coater
Starch concentration	6–8% solids
AKD dosage	1–5 kg/ton paper (active solids)
Synergist dosage	1–3% of AKD liquid
pH of sizing solution	5.0–7.0
Drying temperature	≥95°C for instant fixation
Curing	Accelerated to ~1 hour with synergist

6.2 Critical Considerations

Synergist compatibility – The synergist must be compatible with the specific AKD emulsion and the starch carrier. PAE resins and cationic polymers are widely used and proven effective.

Application uniformity – Uneven application leads to inconsistent sizing. Proper size press operation and consistent solution viscosity are essential.

Drying profile – Adequate heat is required for AKD melting, spreading, and covalent bonding. A rapid curing segment at ≥110°C for at least 3 minutes is recommended.

pH control – Maintaining appropriate pH in the sizing solution prevents premature AKD hydrolysis.

7. Conclusion

The transition to neutral papermaking with high calcium carbonate filler content has created significant challenges for paper quality—particularly in sizing performance and strength properties. Surface sizing has emerged as the essential technology to address these challenges, and AKD surface sizing—when properly formulated with compatible synergists—represents the state of the art.

The mechanism is clear: AKD forms covalent β-keto ester bonds with cellulose, creating permanent hydrophobicity. Without synergists, AKD surface sizing suffers from severe hydrolysis and slow curing. With the right synergist, AKD surface sizing delivers:

Superior water resistance through permanent covalent bonding
Enhanced surface strength that eliminates dusting and picking
Improved printability through a uniform, hydrophobic surface
Rapid curing – from over 10 hours to approximately 1 hour
Cleaner wet-end operation – hydrolysis products are eliminated from the system
Cost efficiency – enabling higher filler content without sacrificing quality

For papermakers seeking to optimize their operations in the era of high-filler, neutral papermaking, AKD surface sizing with synergists offers a proven, effective, and economically attractive solution. By understanding the chemistry, selecting the right synergist, and following best application practices, mills can achieve the quality, productivity, and profitability that modern papermaking demands.