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%\u2014and in some cases even higher\u2014of 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.<\/p>\n\n\n\n
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.<\/p>\n\n\n\n
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\u2014exploring the chemistry, application methods, and the critical role of synergists in achieving optimal performance.<\/p>\n\n\n\n
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:<\/p>\n\n\n\n
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:<\/p>\n\n\n\n
Enhancing water resistance<\/strong> \u2013 creating a hydrophobic barrier that prevents liquid penetration<\/p>\n\n\n\n Improving physical strength<\/strong> \u2013 significantly increasing bursting strength, tensile strength, and surface strength (picking resistance)<\/p>\n\n\n\n Reducing dusting and linting<\/strong> \u2013 effectively eliminating the phenomenon of fiber and filler\u8131\u843d during printing, thereby improving printing efficiency and quality<\/p>\n\n\n\n Enhancing printability<\/strong> \u2013 providing a uniform surface that prevents capillary spreading of ink and improves ink adhesion<\/p>\n\n\n\n Enabling higher filler content<\/strong> \u2013 compensating for the strength loss associated with increased filler loading, allowing mills to maximize cost savings without sacrificing quality<\/p>\n\n\n\n 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.<\/p>\n\n\n\n AKD (Alkyl Ketene Dimer) is a reactive sizing agent characterized by a strained \u03b2-propiolactone ring structure. When applied as a surface sizing agent, AKD functions through a covalent bonding mechanism with cellulose fibers.<\/p>\n\n\n\n The core reaction is an esterification<\/strong> process: the \u03b2-propiolactone ring of the AKD molecule reacts with the hydroxyl (-OH) groups on the cellulose backbone to form a \u03b2-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.<\/p>\n\n\n\n The AKD surface sizing process proceeds through three essential stages:<\/p>\n\n\n\n Stage 1: Retention<\/strong> \u2013 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.<\/p>\n\n\n\n Stage 2: Spreading<\/strong> \u2013 As the paper is dried and heated, the AKD melts (melting point 44.5\u201352\u00b0C) and spreads uniformly over the fiber surfaces.<\/p>\n\n\n\n Stage 3: Covalent Bonding (Reaction)<\/strong> \u2013 At elevated temperatures during the drying section, the strained lactone ring reacts with the hydroxyl groups of cellulose, forming durable \u03b2-keto ester covalent bonds.<\/p>\n\n\n\n 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<\/strong>.<\/p>\n\n\n\n AKD can react with cellulose to form direct \u03b2-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.<\/p>\n\n\n\n When AKD is applied as a surface sizing agent (rather than internally in the wet end), the mechanism offers distinct advantages:<\/p>\n\n\n\n Internal sizing with AKD provides baseline water resistance throughout the sheet thickness. However, surface sizing slows down capillary penetration<\/strong> by forming a barrier layer at the sheet surface, while internal sizing slows down water penetration primarily through molecular diffusion.<\/p>\n\n\n\n AKD surface sizing presents a significant challenge when used alone: AKD is prone to severe hydrolysis and exhibits slow curing<\/strong> without the use of compatible synergists (also called AKD curing promoters, accelerators, or fixing agents).<\/p>\n\n\n\n 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\u2014where AKD reacts with water rather than cellulose\u2014competes with the desired sizing reaction. Without proper stabilization, AKD hydrolysis can significantly reduce sizing efficiency and contaminate the papermaking system.<\/p>\n\n\n\n AKD synergists (curing promoters) function through multiple mechanisms:<\/p>\n\n\n\n Hydrolysis prevention<\/strong> \u2013 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.<\/p>\n\n\n\n Accelerated curing<\/strong> \u2013 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<\/strong> by adding relatively small amounts of AKD reaction accelerators.<\/p>\n\n\n\n Enhanced retention<\/strong> \u2013 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.<\/p>\n\n\n\n Improved emulsion stability<\/strong> \u2013 Synergists stabilize the AKD emulsion, extending shelf life and ensuring consistent performance.<\/p>\n\n\n\n Common AKD synergists include:<\/p>\n\n\n\n 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.<\/p>\n\n\n\n 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<\/strong> to achieve full sizing performance. With the addition of an effective synergist, this curing period can be reduced to approximately one hour<\/strong>.<\/p>\n\n\n\n This dramatic reduction in curing time enables:<\/p>\n\n\n\n Modern papermakers increasingly employ a dual sizing strategy<\/strong> that combines internal and surface AKD application. This approach leverages the strengths of both methods:<\/p>\n\n\n\n Using internal and surface sizing together creates a synergistic effect that improves both the internal structure and surface barrier of the sheet:<\/p>\n\n\n\n For AKD surface sizing applications, the optimal configuration requires:<\/p>\n\n\n\n When AKD surface sizing is used with a compatible synergist, the result is:<\/p>\n\n\n\n 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.<\/p>\n\n\n\n AKD surface sizing, particularly when enhanced with synergists, offers superior performance:<\/p>\n\n\n\n Synergist compatibility<\/strong> \u2013 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.<\/p>\n\n\n\n Application uniformity<\/strong> \u2013 Uneven application leads to inconsistent sizing. Proper size press operation and consistent solution viscosity are essential.<\/p>\n\n\n\n Drying profile<\/strong> \u2013 Adequate heat is required for AKD melting, spreading, and covalent bonding. A rapid curing segment at \u2265110\u00b0C for at least 3 minutes is recommended.<\/p>\n\n\n\n pH control<\/strong> \u2013 Maintaining appropriate pH in the sizing solution prevents premature AKD hydrolysis.<\/p>\n\n\n\n The transition to neutral papermaking with high calcium carbonate filler content has created significant challenges for paper quality\u2014particularly in sizing performance and strength properties. Surface sizing has emerged as the essential technology to address these challenges, and AKD surface sizing\u2014when properly formulated with compatible synergists\u2014represents the state of the art.<\/p>\n\n\n\n The mechanism is clear: AKD forms covalent \u03b2-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:<\/p>\n\n\n\n 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.<\/p>\n\n\n\n <\/p>","protected":false},"excerpt":{"rendered":"
\n\n\n\n2. Understanding AKD Surface Sizing: Mechanism and Chemistry<\/h2>\n\n\n\n
2.1 The Fundamental Principle<\/h3>\n\n\n\n
2.2 The Three-Stage Mechanism<\/h3>\n\n\n\n
2.3 The Debate on Covalent Bonding<\/h3>\n\n\n\n
2.4 Surface vs. Internal Sizing: A Strategic Distinction<\/h3>\n\n\n\n
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\n\n\n\n3. The Critical Role of AKD Synergists (Curing Promoters)<\/h2>\n\n\n\n
3.1 Why Synergists Are Essential<\/h3>\n\n\n\n
3.2 How Synergists Work<\/h3>\n\n\n\n
3.3 Types of Synergists<\/h3>\n\n\n\n
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3.4 The Impact on Curing Time<\/h3>\n\n\n\n
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\n\n\n\n4. The Synergistic Effect: Combining Internal and Surface AKD Sizing<\/h2>\n\n\n\n
4.1 The Dual Sizing Strategy<\/h3>\n\n\n\n
Feature<\/th> X\u00e1c \u0111\u1ecbnh k\u00edch th\u01b0\u1edbc b\u00ean trong<\/th> X\u00e1c \u0111\u1ecbnh k\u00edch th\u01b0\u1edbc b\u1ec1 m\u1eb7t<\/th> Combined System<\/th><\/tr><\/thead> Main Function<\/td> Hydrophobic bonding inside fibers<\/td> Surface film formation<\/td> Total water resistance<\/td><\/tr> Drying Requirement<\/td> \u226524h curing<\/td> \u226595\u00b0C instant fixation<\/td> Efficient under optimized drying<\/td><\/tr> Typical Issues<\/td> False sizing if drying too low<\/td> Uneven sizing if application unstable<\/td> Balanced performance when optimized<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n 4.2 Benefits of the Combined Approach<\/h3>\n\n\n\n
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4.3 AKD Surface Sizing with Synergists: The Optimal Configuration<\/h3>\n\n\n\n
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\n\n\n\n5. AKD Surface Sizing vs. Traditional Surface Sizing<\/h2>\n\n\n\n
Property<\/th> Traditional Surface Sizing<\/th> AKD Surface Sizing (with Synergist)<\/th><\/tr><\/thead> Kh\u1ea3 n\u0103ng ch\u1ed1ng n\u01b0\u1edbc<\/td> Moderate<\/td> Excellent (covalent bonding)<\/td><\/tr> \u0110\u1ed9 b\u1ec1n b\u1ec1 m\u1eb7t<\/td> Good<\/td> Superior<\/td><\/tr> Durability<\/td> Moderate<\/td> Permanent<\/td><\/tr> Curing time<\/td> Immediate<\/td> Accelerated (1 hour with synergist)<\/td><\/tr> Hydrolysis risk<\/td> None<\/td> Controlled by synergist<\/td><\/tr> Compatibility with fillers<\/td> Good<\/td> Excellent<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
\n\n\n\n6. Application Best Practices<\/h2>\n\n\n\n
6.1 Key Operating Parameters<\/h3>\n\n\n\n
Parameter<\/th> Recommended Range<\/th><\/tr><\/thead> \u0110i\u1ec3m \u00e1p d\u1ee5ng<\/td> Size press or gate-roll coater<\/td><\/tr> Starch concentration<\/td> 6\u20138% solids<\/td><\/tr> AKD dosage<\/td> 1\u20135 kg\/ton paper (active solids)<\/td><\/tr> Synergist dosage<\/td> 1\u20133% of AKD liquid<\/td><\/tr> pH of sizing solution<\/td> 5.0\u20137.0<\/td><\/tr> Drying temperature<\/td> \u226595\u00b0C for instant fixation<\/td><\/tr> Curing<\/td> Accelerated to ~1 hour with synergist<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n 6.2 Critical Considerations<\/h3>\n\n\n\n
\n\n\n\n7. K\u1ebft lu\u1eadn<\/h2>\n\n\n\n
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