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

1. Introduction: What Are Paper Sizing Agents?

In the paper industry, “sizing” refers to the process of treating paper pulp, paper, or paperboard with chemical agents to impart resistance to fluid penetration. Sizing agents are chemical compounds that, when applied to paper fibers, modify the surface properties of the paper to make it more resistant to water, ink, and other liquids. These agents possess both hydrophilic and hydrophobic groups in their molecular structure; when deposited on fiber surfaces, the hydrophobic groups orient outward, creating a barrier that repels liquids.

Sizing is one of the most fundamental operations in papermaking—with the exception of absorbent grades such as blotting paper, filter paper, wax paper, cigarette paper, and tissue paper, nearly all paper products require some degree of sizing. Proper sizing enhances paper’s resistance to water, oil, and printing inks, while simultaneously improving smoothness, hydrophobicity, and printability.

Sizing agents can be categorized into two primary application methods: internal sizing (also known as beater or engine sizing), where the sizing agent is added to the paper stock at the wet end of the paper machine, and surface sizing, where film-forming substances such as converted starches, gums, and modified polymers are applied to the surface of already-formed paper or board. In practice, many surface-sized papers also contain an internal sizing component.

Depending on the degree of sizing required, papers are generally classified into three categories: unsized (waterleaf—low water resistance, including absorbent papers for blotting), weak sized (slack sized), and strong sized (hard sized).

2. Historical Development of Sizing Technology

The evolution of paper sizing technology reflects the broader technological progress of the paper industry itself.

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

Internal sizing underwent an equally transformative evolution. Traditional rosin-alum sizing, which operates under acidic conditions (pH 4–6), dominated the industry for much of the 20th century. However, the limitations of acidic papermaking—equipment corrosion, paper yellowing and embrittlement, inability to use cost-effective calcium carbonate fillers, and higher environmental load—drove the industry toward neutral and alkaline alternatives.

The introduction of Alkyl Ketene Dimer (AKD) as a neutral sizing agent marked a paradigm shift in the industry. AKD-based sizing agents have been widely adopted in alkaline papermaking systems due to their excellent sizing performance. However, early AKD formulations faced challenges including short storage life, susceptibility to hydrolysis, and the need for a curing period.

To address these limitations, researchers developed redispersible powder sizing agents using starch and polyvinyl alcohol (PVA) as protective colloids, significantly improving storage stability and sizing efficiency. More recently, solvent-free synthesis processes for AKD have been successfully developed, eliminating the residue and environmental pollution issues associated with traditional toluene-based solvent processes.

3. Classification and Types of Sizing Agents

3.1 Internal Sizing Agents

Internal sizing agents are added to the pulp at the wet end and can be broadly divided into three major categories:

Rosin-Based Sizing Agents — The traditional workhorse of the paper industry. Rosin-based sizing agents depend on the formation of electrostatic bonds between the sizing agent and cellulosic fibers. They are typically used with alum (aluminum sulfate) as a precipitant and require acidic conditions (pH 4–6) for effective performance. While cost-effective, rosin systems impose significant limitations on paper quality and environmental performance.

Synthetic Sizing Agents — This category includes Alkyl Ketene Dimer (AKD) and Alkenyl Succinic Anhydride (ASA) , both of which operate under neutral or alkaline conditions (pH 7–9). AKD and ASA are the two most commonly used synthetic internal sizing agents in North America, while AKD is most prominent in Europe. AKD is used primarily when requirements call for hard sizing—for example, in liquid packaging board—and provides good resistance to fruit juice, lactic acid, and other aggressive media. ASA offers the advantage of on-machine sizing development due to its high degree of reactivity with hydroxyl groups on cellulose fibers.

Neutral Sizing Agents — A broader category encompassing AKD and other agents designed for neutral/alkaline papermaking conditions.

3.2 Surface Sizing Agents

Surface sizing agents are applied to the surface of preformed paper at the size press, forming a continuous, unbroken film. Common types include:

• Starch and starch derivatives — The most widely used surface sizing agents, accounting for approximately 61.78% of the global surface sizing agents market. Enzyme-modified starches have recently gained attention for their superior viscosity stability and improved paper surface properties.
• Polyvinyl alcohol (PVA) — Provides excellent film-forming properties and oil resistance.
• Carboxymethyl cellulose (CMC) — Enhances surface strength and printability.
• Synthetic polymers — Including styrene-acrylate emulsions, styrene-acrylic emulsions (SAE), and polyvinylamine.

Surface sizing improves paper surface strength, reduces dusting and linting, enhances printability, and reduces ink feathering. It also improves water resistance by bonding surface fibers.

3.3 The Distinction Between Internal and Surface Sizing

The choice between internal and surface sizing—or a combination of both—depends on the desired properties of the final paper product.

Internal sizing works at the fiber level, slowing water penetration through molecular diffusion rather than capillary action. Surface sizing, by contrast, creates a protective coating on the sheet surface that inhibits absorption and bleeding. Surface-sized papers generally exhibit better writing quality, greater bursting strength, and higher pick resistance than internally sized papers. Importantly, surface sizing techniques enable the use of calcium carbonate as a filler—a filler with excellent optical properties that cannot be used in traditional acidic internal sizing systems.

4. The Chemistry of Sizing: Understanding the Mechanisms

4.1 Rosin Sizing Mechanism

Rosin-based sizing agents rely on electrostatic interactions. The rosin particles, typically dispersed as an anionic emulsion, are precipitated onto fiber surfaces by alum (aluminum sulfate). The sizing effect is achieved through the deposition of hydrophobic rosin particles on fiber surfaces, creating a physical barrier rather than a chemical bond. This mechanism is pH-dependent and requires acidic conditions for optimal performance.

4.2 AKD Sizing Mechanism

AKD represents a fundamentally different approach to sizing. The proposed mechanism involves the formation of a covalent ester bond (β-ketoester bond) between the AKD molecule and cellulose fibers. The long, hydrophobic alkyl chains in AKD molecules become firmly oriented at the fiber surface through this covalent attachment, rendering the paper resistant to liquid wetting and penetration.

This covalent bond is strong enough to resist attack by acids or bases and is almost inert to these chemicals. Research indicates that the establishment of a covalent bond between cellulose and AKD is essential for introducing permanent hydrophobicity to cellulosic surfaces.

The AKD sizing process can be understood through a four-component system:

1. The size — AKD itself
2. The precipitator — Helps deposit size particles onto fibers
3. The catcher — Assists in retention
4. The substrate — The cellulose fibers

For a typical fine paper sized with AKD, the sizing colloids cover only 1–2% of the total surface area of the furnish, at an AKD addition level of just 0.05–0.1% on paper. This remarkable efficiency underscores the power of covalent bonding—very little reacted AKD is required to achieve effective sizing.

However, AKD sizing is not without its challenges. Several factors influence AKD sizing performance, including pH, alkalinity, cationic starch, alum, cationic polymers, filler type, pulp type, and drying conditions. Under papermaking conditions using calcium carbonate as a filler, the use of cationic starch is necessary to achieve effective AKD sizing.

4.3 ASA Sizing Mechanism

ASA (Alkenyl Succinic Anhydride) is another reactive sizing agent that forms covalent bonds with cellulose hydroxyl groups. Unlike AKD, ASA sizing develops rapidly on the paper machine (on-machine sizing), but ASA emulsions are less stable and require more careful handling. While the sizing achieved by AKD is proportional to hydroxyl group concentration, ASA sizing is not—a distinction that has important implications for different pulp types.

4.4 Challenges in AKD Sizing

Despite its widespread adoption, AKD sizing presents several operational challenges:

Deposit Problems — AKD can sometimes cause deposits on the inner side of pipes from the stuff box to the head box, or on press and smoothing rolls. These deposits tend to occur when operating temperatures increase, particularly in summer, and can be mitigated by using higher melting point AKD. Low first-pass retention of solids and AKD is associated with deposit problems, as AKD concentration in the pulp slurry becomes higher and hydrolysis makes emulsion particles unstable.

Hydrolysis — AKD undergoes competitive hydrolysis reactions that affect storage stability, sizing efficiency, and on-machine performance. This is a key reason why AKD emulsions typically have short shelf lives (1–3 months) and require careful storage at 5–32°C.

Curing Time — Traditional AKD formulations require a curing period of 12–24 hours for full sizing development, although modern fast-cure formulations have significantly reduced this requirement.

Sizing Reversion — The hydrophobic properties imparted by AKD can be reversed due to oxidative photodegradation, a phenomenon known as sizing reversion.

Surface Slipperiness — Excessive AKD addition can reduce the friction coefficient of the paper surface, potentially causing handling issues during converting operations.

5. Market Outlook and Industry Trends

5.1 Market Size and Growth

The global sizing agents market is experiencing steady growth, driven by increasing demand from the paper industry and the shift toward sustainable manufacturing practices.

The global Sizing Agents market was valued at approximately USD 4.00 billion in 2025 and is projected to reach USD 5.95 billion by 2032, growing at a compound annual growth rate (CAGR) of 5.82%. Other estimates place the market at USD 5.0 billion in 2025, with expectations of reaching USD 8.1 billion by 2032 at a CAGR of 6.9%.

Within this broader market, the Pulp and Paper Sizing Agents segment was valued at US$ 1,789 million in 2025 and is forecast to reach US$ 2,460 million by 2032, with a CAGR of 4.6%.

The Surface Sizing Agents market specifically was valued at US$ 1,114 million in 2024 and is projected to reach US$ 1,505 million by 2031, at a CAGR of 4.8%.

5.2 Regional Dynamics

The Asia-Pacific region remains the dominant and fastest-growing market, accounting for 42% of global consumption in 2024, due to its large-scale paper production and packaging demand. The China market for paper surface sizing agents is estimated to grow from US$ 285.17 million in 2024 to US$ 410.07 million by 2031, at a CAGR of 5.59%. The Europe market, while more mature, continues to emphasize sustainability upgrades and efficiency improvements, with an estimated increase from US$ 306.22 million in 2024 to US$ 409.80 million by 2031, at a CAGR of 4.43%.

5.3 Key Market Drivers

Sustainability Imperatives — The shift toward eco-friendly and biodegradable materials is driving significant innovation in sizing formulations. Bio-based sizing agents, particularly starch-based and AKD formulations, 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. Consumer demand is equally compelling—74% of consumers are willing to pay more for sustainable packaging options.

E-Commerce and Packaging Growth — The explosive growth of e-commerce, projected to maintain 10–12% annual growth through 2032, directly increases demand for high-performance packaging papers. Major online retailers now specify sizing performance parameters in their packaging standards, forcing converters to invest in advanced sizing solutions.

Shift to Recycled Fibers — The increasing use of recycled fibers in papermaking requires more advanced sizing solutions to maintain quality, driving demand for innovative sizing technologies.

PFAS-Free Alternatives — Growing regulatory pressure on per- and polyfluoroalkyl substances (PFAS) is accelerating the development of alternative sizing technologies that provide grease and water resistance without “forever chemicals”.

5.4 Emerging Technologies

Recent innovations in sizing technology include:

• Nano-scale sizing agents that provide superior barrier properties while using 20–30% less material
• Bio-based sizing agents — Kemira’s 2024 launch of a bio-based sizing agent reflects the industry’s shift toward sustainable solutions
• Enzyme-modified starch surface sizing agents offering improved viscosity stability and mechanical strength
• Novel anionic surface sizing agents using organosilicon RAFT modification to enhance water resistance and mechanical properties
• Digital process controls enabling greater precision in dosage, retention, and drying profiles

6. LY Chem Technology Co., Ltd.: Your Partner in Paper Sizing Solutions

With over two decades of experience in the paper chemicals industry, LY Chem Technology Co., Ltd. offers a comprehensive range of sizing solutions designed to meet the diverse needs of modern papermakers.

6.1 Our Sizing Product Portfolio

Rosin-Based Internal Sizing Agents

• LY-CRS (Cationic Highly Dispersed Rosin Size) — Requires lower aluminum sulfate consumption and enables near-neutral sizing. Ideal for food-grade applications including food board, cup base paper, and liquid packaging paper.
• LY-ARS (Anionic Highly Dispersed Rosin Size) — Tolerant of high-hardness circulating white water with no summer production issues. A complete replacement for traditional saponified rosin size.

AKD Neutral Sizing Series

• LY-AKD301 (Internal AKD Sizing Agent) — Improves paper brightness, water resistance, and aging resistance. Suitable for cultural paper, coated paper, and packaging base paper.
• LY-AKD302 (Surface AKD Sizing Agent) — Used in combination with starch to shorten curing time and improve ring crush strength and burst strength. Specifically designed for corrugating medium and linerboard.

Styrene-Acrylate Surface Sizing Agent

• LY-BB201 — Cationic styrene-acrylate emulsion that can partially replace internal sizing agents. Significantly improves ring crush strength and water resistance of kraft linerboard and white board while reducing overall chemical costs.

6.2 Quality and Compliance

LY Chem’s products are manufactured under ISO 9001 quality management systems and are available with FDA, SGS, and RoHS certifications for export markets. Our food-grade product lines meet stringent food contact safety standards, enabling papermakers to serve the rapidly growing food packaging sector.

6.3 Commitment to Sustainability

LY Chem is committed to supporting the paper industry’s transition toward sustainable papermaking. Our cationic sizing agents are designed for modern neutral/alkaline papermaking systems, reducing equipment corrosion and wastewater discharge. By enabling the use of calcium carbonate fillers and supporting closed-loop white water systems, our products help papermakers reduce their environmental footprint while improving paper quality and production efficiency.

7. Conclusion

Paper sizing agents are essential chemical enablers of modern papermaking, transforming porous fiber networks into functional materials with controlled liquid resistance, strength, and printability. From traditional rosin-alum systems to advanced AKD and ASA technologies, the evolution of sizing chemistry reflects the broader trajectory of the paper industry toward higher quality, greater sustainability, and improved operational efficiency.

The global sizing agents market, valued at approximately USD 4 billion in 2025 and projected to reach nearly USD 6 billion by 2032, continues to expand driven by packaging growth, sustainability mandates, and technological innovation. The shift toward bio-based, PFAS-free, and high-performance formulations presents both challenges and opportunities for chemical suppliers and papermakers alike.

LY Chem Technology Co., Ltd. stands ready to support papermakers in navigating this evolving landscape. With over two decades of technical expertise, a comprehensive product portfolio, and a commitment to quality and sustainability, we are your trusted partner for all paper sizing needs.

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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

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