Agentes de colagem superficial na fabricação de papel: tipos, propriedades e aplicações
Introdução
Surface sizing is a critical process in paper manufacturing that significantly enhances the quality and performance of paper and paperboard. After the paper sheet is formed and partially dried, surface sizing agents are applied to create a smooth, uniform layer that cements fibers together, improving both surface strength and bending stiffness. This process results in a paper surface that resists tearing and folding—essential properties for packaging and specialty papers.
The global paper surface sizing agents market was valued at approximately USD 1.11 billion in 2024 and is projected to reach USD 1.50 billion by 2031, growing at a compound annual growth rate (CAGR) of 4.8%. Starch-based sizing agents alone accounted for 61.78% of the global market in 2024. The Asia-Pacific region dominates consumption, accounting for 42% of global demand, while the China market is estimated to grow from USD 285 million in 2024 to USD 410 million by 2031 at a CAGR of 5.59%.
Surface sizing agents are generally classified into two major functional categories: water-resistance (hydrophobic) agents e strength-enhancing agents. This article provides a comprehensive technical overview of the commonly used surface sizing agents in the paper industry, covering their chemical properties, application methods, and suitability for different paper grades.
Part One: Water-Resistance (Hydrophobic) Surface Sizing Agents
Water-resistance surface sizing agents are designed to impart hydrophobic properties to the paper surface, reducing liquid penetration and improving print quality. The two most prominent types in this category are Alkyl Ketene Dimer (AKD) and Styrene-Maleic Anhydride (SMA) copolymers.
1. Alkyl Ketene Dimer (AKD) and Its Modified Products
Chemical Properties
AKD is a reactive sizing agent characterized by a strained β-lactone ring structure. When applied to the paper surface during the drying process, AKD reacts with the hydroxyl (-OH) groups on cellulose fibers to form covalent β-keto ester bonds. This covalent bonding mechanism creates a permanent hydrophobic barrier that significantly enhances water resistance. Unlike traditional rosin-based sizing agents, AKD facilitates neutral sizing, which is more compatible with modern papermaking processes and less corrosive to machinery.
Métodos de aplicação
AKD is primarily utilized in two key areas of paper manufacturing: internal sizing and surface sizing. As an internal sizing agent, AKD is introduced into the paper pulp slurry before sheet formation, ensuring even dispersion throughout the paper web. In surface sizing, AKD is applied to the paper surface after formation and drying, further enhancing water resistance while also improving surface strength, smoothness, and printability.
In practical application, AKD is often used in combination with starch and other strength-enhancing agents. It is suitable for a wide range of paper grades, particularly those requiring high water resistance such as paperboard, liquid packaging board, and food-contact papers.
Key Considerations
- AKD requires proper curing conditions to achieve full sizing development
- The use of cationic starch is necessary to achieve effective AKD sizing when calcium carbonate is used as a filler
- Surface application of AKD reduces hydrolysis compared to internal application, as the agent has less opportunity to react with water before bonding with cellulose
2. Styrene-Maleic Anhydride (SMA) and Related Copolymers
Chemical Properties
SMA is an alternating copolymer of styrene and maleic anhydride. It exhibits excellent film-forming properties and good hydrophobicity. SMA copolymers can significantly improve paper hydrophobicity, water resistance, printability, and surface appearance. Research has demonstrated that surface sizing with SMA copolymer emulsions can effectively enhance the sizing degree and surface strength of paper.
Métodos de aplicação
SMA is primarily used in coated papers, security papers, and greaseproof papers. It improves paper abrasion resistance, strength, and oil absorption properties. Related copolymers such as SMA (styrene-acrylate-maleic anhydride monoester copolymers) offer enhanced performance and represent a technical development direction for paperboard surface sizing.
SMA-based products are low-molecular-weight styrene-maleic anhydride copolymer series products used in paper sizing and surface treatment applications.
Advantages and Innovations
Recent developments include the synthesis of environmentally friendly sizing agents from SMA, such as styrene-maleamic acid polymers containing aldehyde groups (SAFA), which offer sustainable alternatives for paper sizing applications.
Part Two: Strength-Enhancing Surface Sizing Agents
Strength-enhancing surface sizing agents primarily improve the mechanical properties of paper, including surface strength, tensile strength, and stiffness. The most widely used agents in this category include starch and its modified products, carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), polyacrylamide (PAM), and animal glue.
1. Starch and Modified Starches
Chemical Properties
Starch is a natural high-molecular-weight compound. As a surface sizing agent, modified starches—particularly oxidized starch—exhibit superior film-forming properties and strengthening effects. Modified starch is inexpensive, abundantly available, and well-suited to the paper industry’s economic requirements.
Métodos de aplicação
Starch is widely used across various paper grades, especially cultural papers and packaging papers. In papermaking, oxidized starch serves as both a surface sizing agent and a coating adhesive, significantly improving paper printing performance and print quality. It can form a continuous, uniform film on the paper surface, improving oil resistance, rub resistance, smoothness, and gloss.
Performance Benefits
Research has shown that surface sizing with starch positively improves paper physical strength properties and printability. Starch-based sizing addresses common issues such as linting and dusting, reduces fiber consumption, simplifies the sizing preparation process, reduces labor intensity, and enhances paper surface strength and printability.
2. Carboxymethyl Cellulose (CMC)
Chemical Properties
CMC is a white powder with excellent film-forming properties, dispersibility, and binding capacity. It is a cellulose derivative that provides good surface strength enhancement when used as a surface sizing agent.
Métodos de aplicação
When used in combination with starch, CMC significantly improves paper surface strength and oil resistance. CMC also prevents paper warping and deformation during printing processes and enhances printing gloss.
Combination Formulations
Studies have investigated a broad range of size formulations based on CMC, polyvinyl alcohol (PVA), carboxymethyl starch (CMS), and sodium polyacrylate (PA), as well as binary mixtures of these components. CMC is commonly used in combination with starch for surface sizing applications.
3. Polyvinyl Alcohol (PVA)
Chemical Properties
PVA is a synthetic high-molecular-weight compound with excellent adhesive strength and film-forming properties. It provides good surface strength and grease resistance when used as a surface sizing agent.
Application Challenges and Solutions
While PVA offers excellent performance, it has certain limitations: high viscosity, poor coatability, and a tendency to penetrate into the paper interior. To overcome these challenges, phosphate pretreatment technology can be employed. Modified formulations such as oxidized starch-crosslinked PVA have been developed to effectively prevent PVA penetration into the base paper and further enhance water resistance and paper strength.
4. Polyacrylamide (PAM)
Chemical Properties
PAM is a high-molecular-weight compound that, when mixed with glycol, forms a crosslinked network during the drying process that improves paper water resistance.
Métodos de aplicação
While PAM is relatively expensive, it offers excellent quality and is suitable for combination with lower-cost starch to improve paper water resistance. PAM is often used as a surface sizing agent in combination with starch to achieve optimal performance at manageable cost.
5. Animal Glue
Chemical Properties
Animal glue is a protein extracted from animal bones, hides, or sinews. It exhibits good viscosity, coverage, and film-forming properties.
Métodos de aplicação
Animal glue is most suitable for surface sizing of high-grade papers, imparting greater strength to the paper. Historically, animal glue such as gelatin was commonly used for paper sizing, with different amounts used for writing and drawing papers (more thoroughly sized) versus printing papers (less sized).
Limitações
The high cost of animal glue restricts its widespread use, limiting its application primarily to premium paper grades.
Part Three: Novel and Emerging Surface Sizing Agents
Water-Soluble Chitosan Solid Surface Sizing Agents
Chemical Properties
Chitosan is derived from marine natural organic raw materials. When combined with starch, chitosan reacts instantly upon drying and film formation, creating a strong, tough network structure layer. Chitosan exhibits excellent film-forming properties and film strength. It can form strong hydrogen bonds with cellulose carboxyl groups and can react with aldehyde groups to form Schiff base structures, thereby enhancing paper strength.
Métodos de aplicação
Chitosan and its derivatives are used as paper surface sizing agents, auxiliary sizing agents, strength enhancers, retention and drainage aids, and finishing agents in specialty paper applications. Chitosan-based sizing agents offer excellent surface strength properties, moisture resistance, and environmental benefits. They feature low dosage requirements, good compatibility, and serve as excellent alternatives to traditional surface sizing agents.
Environmental Advantages
Chitosan itself also functions as a preservative, and chitosan-based sizing agents are considered environmentally friendly due to their natural origin and biodegradability.
Selection Criteria and Best Practices
Factors to Consider
When selecting a surface sizing agent, paper manufacturers should consider:
- Paper grade and end-use requirements — Different paper grades (cultural papers, packaging papers, specialty papers) have different performance requirements
- Desired properties — Whether the primary goal is water resistance, surface strength, or both
- Production costs — Balancing performance with economic feasibility
- Compatibility with existing processes — Including starch cooking systems and application equipment
- Environmental and regulatory compliance — Including food-contact approvals and sustainability requirements
Application Best Practices
Surface sizing agents are typically applied at the size press or gate-roll coater after the paper sheet has been formed and partially dried. The sizing solution is usually prepared by cooking oxidized starch and then adding the synthetic sizing agent. Key parameters to control include:
- Concentração de amido: Typically 6-8% solids
- Sizing agent dosage: Varies by product type and performance requirements
- pH: Optimal range depends on the specific sizing agent
- Temperature: Affects film formation and curing
- Uniformidade da aplicação: Essential for consistent quality
Market Trends
The surface sizing agents market is experiencing significant shifts driven by several key trends:
Sustainability — The shift toward eco-friendly and biodegradable materials is driving innovation in surface sizing formulations. Starch-based and AKD sizing agents have witnessed 12-15% annual growth as they degrade naturally without releasing harmful byproducts. The European Union’s Circular Economy Action Plan has been particularly influential, with over 60% of regional paper manufacturers now using renewable sizing materials.
E-commerce growth — The explosive growth of e-commerce, projected to maintain 10-12% annual growth through 2032, directly increases demand for high-performance packaging papers requiring surface sizing.
Bio-based innovations — Recent industry developments include Kemira’s 2024 launch of a bio-based sizing agent, reflecting the sector’s shift toward eco-friendly solutions.
Conclusão
Surface sizing agents play an indispensable role in modern papermaking, enabling the production of high-quality paper and paperboard with superior water resistance, surface strength, and printability. From traditional starch and animal glue to advanced synthetic polymers like AKD and SMA, and emerging bio-based materials like chitosan, the range of available surface sizing agents continues to expand, offering paper manufacturers increasingly sophisticated options for meeting diverse performance requirements.
As the industry continues its transition toward sustainability, with growing demand for eco-friendly and biodegradable sizing solutions, the surface sizing agents market is poised for continued growth and innovation. Understanding the properties, applications, and selection criteria for different surface sizing agents is essential for papermakers seeking to optimize product quality, production efficiency, and environmental performance in an increasingly competitive global market.
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