Surface Sizing in Papermaking: Principles, Applications, and Functional Classification

1. Introduction: The Evolution of Surface Sizing Technology

Surface sizing is an essential process in the paper and paperboard manufacturing sequence, typically positioned at the drying section of the paper machine. In this process, a sizing solution is applied to the paper web—while the sheet is not yet fully dried but has acquired sufficient strength—through a spraying or coating mechanism. Following subsequent drying, a continuous film is formed on the paper surface, thereby modifying the surface properties of the paper or board to meet specific performance requirements.

From the historical perspective of paper industry development, surface sizing has evolved from a niche technology to an indispensable process. In its early stages, due to technological limitations and the specific requirements of available paper grades, surface sizing agents were confined to specialty papers such as banknote paper, security paper, and chart paper. However, as papermaking technology advanced, surface sizing has become a routine paper treatment process across the industry.

The primary purpose of surface sizing is to improve the surface strength, printability, and water resistance of the paper surface. Surface sizing improves the water resistance of paper and bonds surface fibers to it, thereby enhancing printing properties. The growing普及 of printing, copying, and faxing technologies in recent years has imposed increasingly stringent demands on the surface properties, strength, and water resistance of cultural paper, packaging paper, and corrugated board—driving continuous exploration of new surface sizing technologies across the paper industry.

شركة LY Chem Technology المحدودة. , with over two decades of experience in paper chemicals manufacturing, offers a comprehensive range of surface sizing solutions—including our LY-AKD302 surface AKD sizing agent و LY-BB201 styrene-acrylate surface sizing agent—designed to meet the diverse needs of modern papermakers.

2. The Functional Role of Surface Sizing in Papermaking

2.1 Defining Surface Sizing

Surface sizing refers to the application of a sizing formulation to the surface of an already-formed paper web, typically at the size press located near the end of the paper machine’s drying section. Unlike internal sizing—which is applied to the pulp at the wet end and works by slowing water penetration through molecular diffusion—surface sizing creates a protective coating on the sheet surface that inhibits absorption and bleeding.

Surface sizes do not envelop individual fibers but instead add a protective coating to the sheet itself, inhibiting any absorption or bleeding. Surface sizing applies a continuous, unbroken, gelatinous film to the surface of the paper. This fundamental distinction explains why surface-sized papers generally exhibit better writing quality, greater bursting strength, and higher pick resistance than internally sized papers.

2.2 The Mechanism of Surface Sizing

The mechanism of surface sizing operates through a combination of physical and chemical effects. Except for wax-based sizes, surface sizing agents are generally not hydrophobic like internal sizes; instead, they act by sealing the sheet—that is, filling the capillaries and pores with sizing formulation. By forming a continuous film on the paper surface, surface sizing agents:

• Fill surface pores and capillaries, reducing surface porosity
• Bond surface fibers together, reducing dusting and linting
• Create a smooth, uniform surface for improved printability
• Impart controlled hydrophobicity to the paper surface

The advantage of surface sizing in the case of writing and printing papers is that a film is produced on the surface of the paper that will not catch the pen when the paper is written on and will not pick if the paper is printed with tacky inks.

2.3 The Evolution from Specialty to Mainstream

Surface sizing technology originated around 1930 with vertical roll surface sizing presses. In the 1950s, horizontal roll systems providing more uniform size liquor distribution became prevalent. By the 1960s, inclined surface sizing rolls—typically arranged at a 45° angle—became the dominant configuration, offering significant improvements in paper smoothness while reducing contamination in the size bath.

Initially, surface sizing was limited to premium applications such as banknote paper and security paper due to the cost and complexity of the process. However, as printing technologies advanced and quality expectations rose, surface sizing expanded to encompass a broad range of paper grades including cultural paper, packaging paper, and corrugated board.

Today, surface sizing is recognized as an indispensable process in modern papermaking, with the global Paper Surface Sizing Agents market valued at US$ 1,114 مليون في عام 2024 and projected to reach US$ 1,505 مليون بحلول عام 2031, growing at a CAGR of 4.8%.

3. Key Benefits of Surface Sizing

Surface sizing delivers a comprehensive set of benefits that directly impact paper quality, production efficiency, and cost-effectiveness.

3.1 Improved Printability

Surface sizing significantly enhances the printing performance of paper and paperboard. By creating a smooth, uniform surface with controlled porosity and hydrophobicity, surface sizing ensures:

• Sharp print reproduction: Reduced ink feathering and bleeding
• High print density: Improved ink holdout on the surface
• Reduced linting and dusting: Surface fibers are bonded, preventing them from pulling off during printing
• Consistent ink absorption: Uniform surface properties across the sheet

With the proliferation of digital printing technologies—including inkjet, laser, and high-speed offset printing—the demand for papers with superior surface properties has intensified dramatically. Surface sizing directly addresses these requirements.

3.2 Enhanced Water Resistance

Surface sizing improves the water resistance of paper and board by forming a hydrophobic barrier on the surface. This is particularly critical for:

• كرتون تغليف السوائل: Containing fruit juice, milk, and other aqueous liquids
• Outdoor packaging: Withstanding humidity and moisture during transport and storage
• تغليف الأغذية: Preventing moisture migration that could compromise product quality

The surface sizing agent forms a thin film on the paper’s surface after drying, and this film enhances water resistance.

3.3 Increased Physical Strength

Surface sizing improves the physical strength properties of paper and board:

• قوة السطح: Reduces dusting and picking during printing
• Ring crush strength: Particularly important for corrugated board and packaging applications
• Burst strength: Increases resistance to rupture under pressure
• Tensile strength: Improves fiber bonding at the surface

Surface sizing can be used to increase hydrophobicity of the paper/board surface or to improve the strength properties and printability of paper/board.

3.4 Reduced Two-Sidedness

Surface sizing effectively reduces the difference in properties between the top and bottom surfaces of the paper sheet—a phenomenon known as two-sidedness. By applying sizing formulation to both surfaces, papermakers can achieve more uniform:

• Smoothness
• Printability
• مقاومة الماء
• Optical properties

3.5 Consistent Performance Independent of Water Quality

Unlike internal sizing, which can be significantly affected by the quality and temperature of the circulating white water, surface sizing delivers stable, consistent performance regardless of water quality variations. This operational advantage simplifies process control and reduces the risk of quality fluctuations.

3.6 High Retention and Cost Efficiency

Surface sizing offers exceptional retention efficiency—approaching 100%—because the sizing agent is applied directly to the formed sheet rather than being added to the pulp slurry where a portion is inevitably lost with the white water. This high retention translates to:

• Lower chemical consumption per ton of paper
• Reduced chemical costs
• Minimized environmental load from chemical carryover

3.7 Synergy with Internal Sizing

Surface sizing and internal sizing are not mutually exclusive alternatives—they are complementary technologies that work together to achieve optimal paper performance. Surface sizing can effectively compensate for certain deficiencies in internal sizing systems, enabling papermakers to:

• Reduce internal sizing dosage while maintaining overall performance
• Achieve higher filler loadings without compromising strength
• Address specific surface-related quality issues without altering the wet-end chemistry

4. Classification of Surface Sizing Agents by Function

Based on their primary functional role, surface sizing agents can be divided into two broad categories: water-resistant types و strength-enhancing types.

4.1 Water-Resistant Surface Sizing Agents

Water-resistant surface sizing agents are designed to impart hydrophobicity to the paper surface, reducing liquid penetration and improving performance in moisture-sensitive applications.

ثنائي مركب ألكيل كيتين (AKD) : AKD is a reactive sizing agent that forms covalent ester bonds with cellulose fibers, creating a permanent hydrophobic barrier. When applied as a surface sizing agent, AKD:

• Provides excellent water resistance
• Maintains paper recyclability
• Offers superior performance compared to traditional rosin-based alternatives
• Is widely used in various high-grade cultural papers, photographic paper, fruit bag paper, packaging paper, and paperboard

LY Chem’s LY-AKD302 is a surface AKD sizing agent specifically designed for use in combination with starch, shortening curing time while improving ring crush strength and burst strength for corrugating medium and linerboard.

بوليمرات ستايرين-أنهيدريد الماليك المشتركة (SMA) : SMA-based surface sizing agents provide excellent water resistance and are particularly effective in improving the printability of coated papers.

Styrene-Acrylate Emulsions (SAE) : SAE surface sizing agents offer a balance of water resistance and strength enhancement.

LY Chem’s LY-BB201 is a cationic styrene-acrylate emulsion that can partially replace internal sizing agents while significantly improving ring crush strength and water resistance of kraft linerboard and white board.

4.2 Strength-Enhancing Surface Sizing Agents

Strength-enhancing surface sizing agents are primarily used to improve the mechanical properties and surface integrity of paper and board.

Starch and Starch Derivatives: Starch is the most widely used surface sizing agent, accounting for a significant share of the global surface sizing agents market. Oxidized starch, enzyme-modified starch, and cationic starch are commonly employed. The application of enzyme-modified starch in surface sizing provides an effective strategy to improve paper performance.

Carboxymethyl Cellulose (CMC) : CMC is used as a surface sizing agent to improve surface strength and printability. It has been found to have excellent use in sizing for all fibers.

Polyvinyl Alcohol (PVA) : PVA provides excellent film-forming properties and oil resistance, making it suitable for applications requiring high surface strength and grease resistance.

4.3 Emerging Trends: Bio-Based and Sustainable Sizing Agents

The global shift toward sustainable packaging solutions is driving the development of bio-based and environmentally friendly surface sizing agents. Recent innovations include:

• Chitosan-based sizing agents: Biodegradable alternatives derived from natural sources
• Micro-nanofibrillated cellulose (MNFC) blends: Combining cellulose nanomaterials with traditional sizing agents
• Biodegradable waste gelatin-based coatings: Sustainable biomass surface sizing agents

These innovations reflect the industry’s commitment to reducing environmental impact while maintaining or improving paper performance.

5. Surface Sizing vs. Internal Sizing: A Strategic Comparison

5.1 Complementary Mechanisms

Internal sizing and surface sizing operate through fundamentally different mechanisms, and understanding this distinction is essential for optimizing paper performance.

Aspect	تحديد المقاس الداخلي	تحديد حجم السطح
نقطة التطبيق	Wet end (pulp stage)	Size press (near dryer end)
Mechanism	Slows water by molecular diffusion	Seals capillaries and pores
Effect	Bulk water resistance	Surface properties and top-layer barrier
Retention	Partial (some lost in white water)	Near 100%
Dependence on water quality	Significant	Minimal

Surface sizing makes it possible to use calcium carbonate as a filler of excellent optical property—a filler that cannot be used in traditional acidic internal sizing systems.

5.2 Strategic Integration

In modern papermaking, internal and surface sizing are typically used in combination. The practice of adding surface sizing agent at the size press for replacing or supplementing internal sizing has been widely adopted. This integrated approach offers multiple benefits:

• تحسين جودة المنتج: خصائص سطح محسّنة وقوة داخلية
• تقليل التلوث: انخفاض كمية المواد الكيميائية المتبقية في المياه البيضاء
• انخفاض تكاليف الإنتاج: تحسين الاستخدام الإجمالي للمواد الكيميائية
• مرونة أكبر في صياغة الصيغ: القدرة على ضبط خصائص السطح بدقة بشكل مستقل عن التركيب الكيميائي للجزء الرطب

6. Application Scenarios by Paper Grade

The selection of surface sizing approach—and the choice of sizing agent—depends on the specific requirements of the paper grade being produced.

6.1 Cultural Paper and Printing Papers

For writing paper, offset printing paper, and electrostatic copy paper, surface sizing is essential for:

• Achieving high print quality with sharp, clear images
• Preventing ink feathering and bleeding
• Reducing surface dusting that could contaminate printing equipment
• Ensuring consistent ink absorption across the sheet

6.2 Packaging Paper and Board

For linerboard, corrugating medium, and folding carton board, surface sizing provides:

• Enhanced ring crush strength: Critical for box compression resistance
• Improved water resistance: Protecting contents during transport and storage
• Better printability: Enabling high-quality graphics on packaging surfaces

6.3 Food-Contact Papers

For food packaging applications, surface sizing must meet stringent regulatory requirements. Surface sizing agents used in food-contact papers must comply with FDA, BfR, and EU regulations, with specific limits on:

• Migration of sizing agent components into food
• Total extractable substances
• Solvent residues (particularly important for solvent-free AKD products)

6.4 Specialty Papers

For applications such as fruit bag paper, photographic paper, and liquid packaging board, surface sizing delivers specific performance attributes:

• ورق أكياس الفاكهة: مقاومة الماء لحماية الفاكهة من التلف الناتج عن الرطوبة
• Photographic paper: Superior surface smoothness and ink/toner receptivity
• كرتون تغليف السوائل: Barrier properties to contain aqueous liquids

7. Market Trends and Future Directions

7.1 نمو السوق

The global Paper Surface Sizing Agents market continues to expand, driven by increasing demand for high-performance packaging papers and the shift toward sustainable papermaking practices:

• Market size: US$ 1,114 million (2024) , projected to US$ 1,505 million (2031)
• CAGR: 4.8% during the forecast period
• The emulsion-type surface sizing agent market alone was valued at US$ 315 million in 2024, projected to reach US$ 488 million by 2031 at a CAGR of 6.0%

7.2 Key Drivers

نمو التجارة الإلكترونية: With e-commerce packaging volumes projected to grow at over 14% annually, paper-based packaging solutions require high-performance surface sizing agents to enhance water resistance and printability.

متطلبات الاستدامة: The global shift toward sustainable packaging solutions is driving unprecedented growth in the paper and paperboard industry, subsequently boosting demand for surface sizing agents.

Technological Improvements: The enhancement of surface sizing technology—including the formulation of high-performance and greener agents—is supporting market growth.

تطور تكنولوجيا الطباعة: لا يزال انتشار تقنيات الطباعة الرقمية وعالية السرعة يرفع سقف التوقعات المتعلقة بجودة أسطح الورق.

7.3 الابتكارات المستقبلية

Emerging trends in surface sizing technology include:

• النشويات المعدلة إنزيميًّا: يتميز باستقرار اللزوجة وقوة ميكانيكية فائقتين
• التحجيم المعزز بالسيلولوز النانوي: Combining traditional sizing agents with cellulose nanomaterials for improved barrier properties
• عوامل التثبيت ذات الأصل الحيوي: تلبية الطلب على البدائل المتجددة والقابلة للتحلل البيولوجي
• أنظمة تحديد المقاس الذكية: Real-time monitoring and control of sizing application for optimized performance

8. Conclusion: Surface Sizing as a Strategic Capability

Surface sizing has evolved from a niche technology for specialty papers to an indispensable process in modern papermaking. Its ability to simultaneously enhance printability, water resistance, physical strength, and surface uniformity—while offering high retention efficiency and consistent performance independent of water quality—makes it a strategic capability for papermakers serving diverse markets.

النقاط الرئيسية:

• Surface sizing is complementary to internal sizing, not a replacement; the two systems work together to achieve optimal paper performance
• Function determines choice: Water-resistant surface sizing agents (AKD, SMA, SAE) serve different purposes than strength-enhancing agents (starch, CMC, PVA)
• يشهد السوق نمواً قوياً: The global surface sizing agents market is projected to reach US$ 1.5 billion by 2031
• Sustainability is driving innovation: Bio-based and enzyme-modified sizing agents are emerging as key trends
• Application determines strategy: The optimal surface sizing approach varies by paper grade—from cultural paper to packaging board to specialty applications

شركة LY Chem Technology المحدودة. , with over two decades of experience in paper chemicals manufacturing, offers a comprehensive range of surface sizing solutions designed to meet the diverse needs of modern papermakers:

• LY-AKD302 (عامل تحديد الحجم السطحي AKD) : Delivers superior water resistance and strength enhancement when used with starch, specifically designed for corrugating medium and linerboard
• LY-BB201 (عامل تلميع الأسطح من الستايرين والأكريلات) : A cationic emulsion that partially replaces internal sizing while improving ring crush strength and water resistance for kraft linerboard and white board

As the global paper industry continues its transition toward higher quality, greater sustainability, and enhanced performance, surface sizing technology will remain at the forefront of papermaking innovation. LY Chem is committed to supporting papermakers in navigating this evolving landscape with reliable, high-performance chemical solutions.

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For more information about LY Chem’s surface sizing solutions and our complete range of papermaking chemicals, please contact our technical sales team.

المراجع

1. Paper surface sizing application scope and benefits – Patent literature, EPO
2. Surface sizing vs internal sizing comparison – Hand Papermaking, 2024
3. Surface sizing agents types (AKD, starch, PVA, CMC) – Baidu Baike, 2026
4. Global Paper Surface Sizing Agents Market Report 2025 – QY Research, 2025
5. Surface sizing for strength, printability, and water resistance – Amazon Chem
6. Starch and its derivatives for paper coatings: A review – ScienceDirect, 2019
7. AKD surface sizing for packaging applications – TradeKorea / Research and Markets