Paper Sizing Agents: A Comprehensive Technical Overview of Chemistry, Classification, and Applications

1. Introduction: The Foundation of Paper Performance

Paper sizing agents are chemical additives essential to the papermaking process, designed to impart resistance to water, ink, and other liquids while simultaneously improving paper strength, smoothness, and printability. Without sizing, most paper products would absorb liquids uncontrollably, making them unsuitable for writing, printing, or packaging applications.

Sizing agents function by modifying the surface properties of cellulose fibers. These chemicals possess both hydrophilic and hydrophobic molecular groups; when applied to paper fibers, the hydrophobic groups orient outward, creating a barrier that repels liquids while maintaining the paper’s structural integrity.

The global paper sizing agents market continues to expand, driven by increasing demand for high-performance packaging papers and the shift toward sustainable papermaking practices. The Paper Surface Sizing Agents market alone was valued at US$ 1,114 million in 2024 and is projected to reach US$ 1,505 million by 2031, growing at a compound annual growth rate (CAGR) of 4.8%.

LY Chem Technology Co., Ltd. , with over two decades of experience in paper chemicals manufacturing, offers a comprehensive range of sizing solutions—including our LY-CRS and LY-ARS rosin-based sizes, LY-AKD301 and LY-AKD302 AKD sizing agents, and LY-BB201 styrene-acrylate surface sizing agent—designed to meet the diverse needs of modern papermakers.

2. Historical Development: From Gelatin to Modern Synthetics

2.1 The Era of Gelatin Sizing

In the eighteenth and early nineteenth centuries, paper sizing was predominantly accomplished using a combination of gelatin and alum (aluminum sulfate). This method, while effective, was expensive and time-consuming, limiting paper production capacity.

2.2 The Rosin-Alum Revolution (1807)

The invention of alum-rosin size by Moritz Friedrich Illig in Germany in 1807 marked a turning point in papermaking history. Rosin, derived from pine trees, proved to be significantly less expensive than gelatin, enabling the rapid expansion of the paper industry.

From approximately 1850 to 1950, paper was internally sized with rosin and papermaker’s alum at an acidic pH, typically between 4 and 5. This acidic system, while economical, imposed significant limitations:

• Paper yellowing and embrittlement: Acidic conditions promoted fiber hydrolysis
• Equipment corrosion: The acidic environment accelerated wear on paper machine components
• Inability to use calcium carbonate fillers: Alkaline fillers could not be used in acidic systems
• Limited paper permanence: Acidic papers degraded over time

2.3 The AKD Revolution (1956)

The development of Alkyl Ketene Dimer (AKD) sizing agents by Hercules Inc. in the 1950s—first introduced commercially in 1955 as Aquapel—represented a paradigm shift in papermaking technology. AKD emulsions gained immediate market acceptance because of their high efficiency and cost-effectiveness compared to traditional rosin-alum size.

Unlike rosin-based sizing, which relies on physical deposition and ionic interactions, AKD forms covalent bonds with cellulose fibers. This chemical bonding mechanism provides:

• Superior and more durable sizing effects compared to rosin
• Neutral/alkaline papermaking conditions, reducing equipment corrosion
• Compatibility with calcium carbonate fillers, lowering production costs
• Improved paper permanence without yellowing or embrittlement

2.4 The Emergence of Surface Sizing

Surface sizing technology originated around 1930 with the introduction of vertical roll surface sizing presses. Initially limited to specialty applications such as banknote paper and security paper, surface sizing has become a routine paper treatment process across the industry.

Today, surface sizing is recognized as an indispensable process in modern papermaking, with applications spanning cultural paper, packaging paper, corrugated board, and food-contact papers.

3. Classification of Sizing Agents

Sizing agents are broadly classified into two categories based on their application method: internal sizing agents and surface sizing agents.

3.1 Internal Sizing Agents

Internal sizing agents are added to the pulp at the wet end of the paper machine. They work by slowing water penetration through molecular diffusion and are distributed throughout the sheet.

Major categories include:

Rosin-Based Sizing Agents — The traditional workhorse of the industry. Rosin sizing typically requires acidic conditions maintained with alum (aluminum sulfate). Dispersed rosin sizes offer significant advantages over traditional saponified rosin sizes, saving 30–50% of both rosin and aluminum sulfate consumption while enabling neutral sizing conditions.

Alkyl Ketene Dimer (AKD) — A reactive sizing agent that forms covalent ester bonds with cellulose fibers. AKD enables neutral/alkaline papermaking, reduces equipment corrosion, and allows the use of calcium carbonate fillers. For bleached pulps, AKD addition typically ranges from 0.10–0.30%.

Alkenyl Succinic Anhydride (ASA) — Another reactive sizing agent that develops sizing rapidly on the paper machine. ASA offers the advantage of on-machine sizing development but requires more careful handling due to its lower emulsion stability.

3.2 Surface Sizing Agents

Surface sizing agents are applied to the surface of already-formed paper at the size press, typically near the end of the drying section. Surface sizing creates a protective film on the sheet surface that inhibits absorption and bleeding, while also bonding surface fibers to reduce dusting and linting.

Common surface sizing agents include:

• Starch and starch derivatives: The most widely used surface sizing agents, offering excellent film-forming properties and strength enhancement
• Carboxymethyl cellulose (CMC) : Improves surface strength and printability
• Polyvinyl alcohol (PVA) : Provides superior film-forming properties and oil resistance
• Styrene-Maleic Anhydride Copolymers (SMA) : Synthetic polymers that significantly improve hydrophobicity, water resistance, printability, and surface appearance

4. Physicochemical Properties of Major Sizing Agents

4.1 Dispersed Rosin Size

Dispersed rosin size offers several advantages over traditional saponified rosin size:

• Chemical savings: Reduces rosin and aluminum sulfate consumption by 30–50%
• Neutral sizing capability: Enables neutral papermaking conditions, improving paper water resistance
• Optimal emulsification temperature: Best results achieved at 140–150°C
• Dispersant requirement: 7–8% dispersant typically sufficient
• Low foaming: Minimal foam generation during production
• Non-toxic production process: Environmentally favorable manufacturing

4.2 AKD (Alkyl Ketene Dimer)

AKD is a low-melting-point (typically 43–55°C) pale yellow waxy solid that is insoluble in water. Key characteristics include:

• Reactive sizing mechanism: Forms covalent β-ketoester bonds with cellulose hydroxyl groups
• High efficiency: Very little reacted AKD is required to achieve effective sizing
• Neutral/alkaline application: Operates effectively at pH 7–9
• Curing requirement: Typically 12–24 hours at room temperature for full sizing development

4.3 Styrene-Maleic Anhydride (SMA) Surface Sizing Agents

SMA copolymers are water-soluble or water-dispersible synthetic polymers that offer:

• Superior hydrophobicity: Excellent water resistance
• Enhanced printability: Improved ink holdout and print quality
• Surface strength improvement: Reduced dusting and picking
• Porosity reduction: Seals surface pores for improved barrier properties

5. Manufacturing Methods

5.1 Rosin-Based Sizing Agents

Lignin-Rosin Sizing Agents: A preparation method involves dissolving lignin and rosin in anhydrous ethanol at a mass ratio of 1:0.25–4, followed by stirring and drying to obtain a powder product. This approach simplifies the preparation process while improving paper strength.

Anionic Rosin Sizing Agents: Produced using self-made anionic acrylic acid copolymers to emulsify rosin, with an optimal rosin-to-emulsifier mass ratio of 1:1.

5.2 AKD Manufacturing

The conventional industrial method for AKD preparation uses triethylamine as an acid-binding agent to facilitate the self-polymerization of fatty acid chlorides, forming ring-structured alkyl ketene dimers (AKD).

Solvent-free AKD synthesis has emerged as the mainstream direction. This process eliminates organic solvents from production, addressing concerns about solvent residues and environmental impact.

5.3 Surface Sizing Agent Manufacturing

Styrene-Maleic Anhydride Copolymers: SMA surface sizing agents are produced through copolymerization of styrene and maleic anhydride. The ammonium salt of styrene-maleic anhydride has been identified as an effective surface sizing agent.

Cationic Sizing Agents: Produced through the reaction of α-olefins with maleic anhydride, followed by imidization with polyamines containing tertiary and primary amino groups, and finally quaternization with epichlorohydrin in the presence of water.

6. Application Strategies and Synergies

6.1 Internal Sizing Applications

Internal sizing is suitable for applications where bulk water resistance is required:

• Liquid packaging board: AKD provides excellent resistance to fruit juice, lactic acid, and other aggressive media
• Corrugating medium: Provides compression resistance and moisture resistance
• Writing and printing papers: Traditional rosin sizing or AKD sizing depending on quality requirements
• Fruit bag paper: Water resistance to protect fruit from moisture damage

6.2 Surface Sizing Applications

Surface sizing offers distinct advantages for applications requiring superior surface properties:

• High-precision printing papers: Enhanced printability and surface smoothness
• Packaging papers with graphics: Improved print quality and surface strength
• Papers requiring reduced two-sidedness: Uniform surface properties
• Food packaging papers: Surface barrier properties with regulatory compliance

6.3 The Synergy of Combined Sizing

In modern papermaking, internal and surface sizing are typically used in combination. This integrated approach offers multiple benefits:

• Improved product quality: Enhanced surface properties and bulk strength
• Reduced pollution: Lower chemical carryover in white water
• Lower production costs: Optimized overall chemical usage
• Greater formulation flexibility: Ability to fine-tune surface properties independently of wet-end chemistry

6.4 The “Lattice” Sizing Mechanism

The sizing mechanism of AKD on paper has been proposed as a “lattice” mechanism. This theory suggests that AKD molecules form a regular, ordered arrangement on the cellulose fiber surface, creating a hydrophobic barrier through molecular organization rather than simply through random deposition.

7. Market Trends and Future Outlook

7.1 Market Growth

The global sizing agents market continues to expand:

• Paper Surface Sizing Agents market: US$ 1,114 million (2024), projected to US$ 1,505 million (2031), CAGR 4.8%
• Surface Sizing Agents market (broader): USD 825.9 million (2025), projected to USD 1.12 billion (2032), CAGR 4.5%
• Europe market: Expected to grow from US$ 306.22 million (2024) to US$ 409.80 million (2031), CAGR 4.43%

7.2 Key Market Drivers

E-Commerce Growth: With e-commerce packaging volumes projected to grow at over 14% annually through 2025, paper-based packaging solutions require high-performance sizing agents to enhance water resistance and printability.

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

Bio-Based Sizing Agents: The rising preference for bio-based sizing agents to meet environmental regulations and sustainability goals is a key market trend.

Printing Technology Evolution: The proliferation of digital and high-speed printing technologies continues to raise quality expectations for paper surfaces.

7.3 Future Innovations

• Enzyme-modified starches: Offering superior viscosity stability and mechanical strength
• Nanocellulose-enhanced sizing: Combining traditional sizing agents with cellulose nanomaterials
• Bio-based sizing agents: Responding to the demand for renewable, biodegradable alternatives
• Solvent-free manufacturing: Eliminating organic solvents from production processes
• Smart sizing systems: Real-time monitoring and control of sizing application

8. Conclusion

Paper sizing agents have evolved dramatically over the past two centuries—from gelatin and alum in the 18th century, through the rosin-alum revolution of 1807, to the AKD revolution of 1956, and continuing today with advanced synthetic and bio-based formulations. This evolution reflects the paper industry’s ongoing pursuit of higher quality, greater sustainability, and improved operational efficiency.

Key takeaways:

• Sizing is essential: With few exceptions, nearly all paper products require sizing
• Two complementary approaches: Internal sizing provides bulk water resistance; surface sizing delivers superior surface properties
• Chemistry matters: Rosin sizing relies on physical deposition; AKD forms covalent bonds with cellulose
• Sustainability drives innovation: Bio-based and solvent-free sizing agents are emerging as key trends
• Market growth is robust: The global sizing agents market continues to expand, driven by packaging growth and sustainability mandates

LY Chem Technology Co., Ltd. , with over two decades of experience in paper chemicals manufacturing, offers a comprehensive range of sizing solutions:

• LY-CRS (Cationic Highly Dispersed Rosin Size) : Lower aluminum sulfate consumption, near-neutral sizing for food-grade applications
• LY-ARS (Anionic Highly Dispersed Rosin Size) : Tolerant of high-hardness white water, complete replacement for traditional saponified rosin size
• LY-AKD301 (Internal AKD Sizing Agent) : Superior water resistance and aging resistance for cultural paper, coated paper, and packaging base paper
• LY-AKD302 (Surface AKD Sizing Agent) : Enhanced ring crush strength and burst strength for corrugating medium and linerboard
• LY-BB201 (Styrene-Acrylate Surface Sizing Agent) : Cationic emulsion that partially replaces internal sizing while improving ring crush strength and water resistance

As the global paper industry continues its transition toward sustainable, high-performance papermaking, LY Chem remains committed to providing innovative chemical solutions that empower papermakers to meet the challenges of tomorrow—today.

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

References

1. Paper sizing with alkyl ketene dimers – ScholarWorks, WMU
2. The Genesis of Alkaline Sizing – cool.culturalheritage.org
3. Rosin sizing history – Wikipedia / Conservation sources
4. AKD sizing history – Hercules Inc., 1956 (De Gruyter / Museum Conservation Institute)
5. Surface sizing compositions – Styrene-maleic anhydride copolymers (Patents / CNKI)
6. Paper Surface Sizing Agents Market Report 2025-2031 – QY Research
7. Surface Sizing Agents Market Outlook 2025-2032 – 24Chemical Research
8. Dispersed rosin size properties – IPPTA
9. AKD synthesis – Solvent-free methods (Patents / Journal sources)
10. Sizing agents classification – Baidu Baike