Neutral Sizing Agents in Papermaking: Types, Mechanisms, and Modern Applications

Introduction

Paper fibers, composed primarily of cellulose, contain a large number of hydroxyl (-OH) groups that readily form hydrogen bonds with water, giving paper a strong hydrophilic tendency. When paper is soaked in water, it loses most of its strength—a desirable property for tissue paper and corrugating medium, but undesirable for most other paper grades such as office paper, food packaging, cosmetic boxes, and food bags.

Sizing is the process by which papermakers impart resistance to water and liquid penetration, preventing the diffusion and penetration of aqueous liquids. There are two primary methods of sizing: internal (or pulp) sizing, where the sizing agent is added to the pulp during beating or stock preparation, and surface sizing, where the agent is applied after the paper sheet is formed. With the exception of absorbent papers, filter paper, wax paper, cigarette paper, and tissue paper, nearly all paper grades require sizing. Sizing improves paper’s resistance to water, oil, and printing ink, while also enhancing smoothness, hydrophobicity, and printability.

Among the various sizing technologies, neutral sizing agents have emerged as the dominant choice for modern papermaking, enabling the transition from acidic to neutral and alkaline papermaking systems. This article provides a comprehensive technical overview of neutral sizing agents—their classification, mechanisms, advantages, limitations, and market trends.

1. What Are Neutral Sizing Agents?

Neutral sizing agents are a subcategory of internal sizing agents used in papermaking. They are designed to function in neutral or alkaline papermaking conditions (pH 6.0–8.5 and above), as opposed to traditional acidic sizing systems that operate at pH 4.0–5.5.

The working principle of neutral sizing agents is based on the synergistic effect of hydrophilic and hydrophobic groups. When used in conjunction with calcium carbonate fillers, neutral sizing agents improve paper whiteness and bulk while reducing the risk of equipment corrosion.

1.1 Why the Shift to Neutral Sizing?

The paper industry’s transition from acidic to neutral/alkaline papermaking has been driven by several compelling factors:

Filler compatibility: Neutral systems enable the use of calcium carbonate—a filler that is 30–50% cheaper than talc and significantly improves paper whiteness (+5–8% ISO), opacity (+10–15%), and bulk
Fiber protection: Acidic conditions (pH < 6) cause fiber hydrolysis, reducing paper strength. Neutral conditions preserve fiber integrity, with tensile index improving by 10–15% and folding endurance by 20%
Equipment longevity: Neutral systems are less corrosive to paper machine equipment, extending service life and reducing maintenance costs
Environmental benefits: Neutral sizing eliminates or greatly reduces the use of aluminum sulfate, lowering COD discharge by 40–60% per ton of paper

2. Classification of Neutral Sizing Agents

Neutral sizing agents are generally classified into three main categories:

Rosin-based sizing agents (cationic dispersed rosin, fortified rosin)
Synthetic sizing agents (AKD, ASA)
Polymer-type sizing agents (cationic polymer dispersions)

Each category has distinct chemical properties, sizing mechanisms, and application characteristics.

3. Rosin-Based Neutral Sizing Agents

3.1 From Acidic to Neutral Rosin Systems

Traditional rosin sizing relies on aluminum sulfate (alum) as a retention aid and precipitant, requiring acidic conditions (pH 4.0–5.5) for effective sizing. Under neutral or alkaline conditions, alum becomes ineffective, limiting the application of conventional rosin sizes.

The development of cationic dispersed rosin sizing agents has overcome this limitation. These agents are self-retaining on negatively charged cellulose fibers due to their cationic nature, eliminating the need for alum as a mordant.

3.2 Properties and Performance

Cationic rosin neutral sizing agents exhibit several key characteristics:

Stability: Some formulations can be stored for over six months
Zeta potential: Typically around +25 mV, indicating strong cationic character
pH range: Effective at pH 6.0–8.0, with some formulations performing well up to pH 8.5
Alum reduction: Compared to traditional fortified rosin sizes, cationic rosin agents can reduce aluminum sulfate consumption by approximately 80%
Sizing efficiency: Twice that of conventional soap rosin sizes

3.3 Applications

Cationic rosin neutral sizing agents are particularly suitable for:

Medium-to-high-grade paper grades where AKD or ASA may be cost-prohibitive
Mills transitioning from acidic to neutral systems that wish to retain rosin-based chemistry
Applications where fast curing is required (rosin sizes cure more quickly than AKD)

Research has shown that the development of neutral dispersed rosin sizing agents aligns with both China’s national conditions and the global trend in papermaking development.

4. Synthetic Neutral Sizing Agents: AKD and ASA

AKD (Alkyl Ketene Dimer) and ASA (Alkenyl Succinic Anhydride) are the two most prevalent synthetic sizing agents used in modern neutral and alkaline papermaking. Both are reactive sizing agents that form covalent bonds with cellulose fibers, but they differ significantly in reactivity, curing requirements, and application conditions.

4.1 AKD (Alkyl Ketene Dimer)

Chemical Mechanism

AKD is a reactive sizing agent characterized by a strained β-propiolactone ring structure. The primary mechanism by which AKD renders cellulose hydrophobic is through a covalent esterification reaction. The strained β-propiolactone ring of the AKD molecule reacts with the hydroxyl groups on the cellulose backbone to form a β-keto ester. This covalent bond anchors the long, hydrophobic alkyl chains of the AKD onto the cellulose surface, creating a permanent barrier to water penetration.

From a series of indirect techniques, researchers have concluded that the establishment of a covalent bond between cellulose and AKD is essential in order to introduce permanent hydrophobicity to cellulosic surfaces. AKD can react with cellulose to form direct β-keto ester bonds that are strong enough to resist the attack of acids or bases and are almost inert to these chemicals.

The Three-Stage Mechanism

The AKD sizing process proceeds through three essential stages:

Retention: AKD emulsion particles are retained on the cellulose fibers in the wet end
Spreading: As the paper is dried and heated, the AKD melts (melting point 44.5–52°C) and spreads over the fiber surfaces
Reaction: At elevated temperatures, the lactone ring of the AKD reacts with the hydroxyl groups of cellulose

Advantages

AKD offers several compelling advantages:

Excellent long-term stability: Provides durable, permanent sizing that ages well
Broad pH compatibility: Performs consistently across a wide range of neutral to alkaline pH levels
Calcium carbonate compatibility: Enables use of inexpensive fillers
Low corrosivity: Less damaging to mill equipment than acidic systems
Environmental friendliness: Contains no organic solvents and is non-toxic

Limitations

Slow curing: Requires 24–48 hours at 25°C and 50% humidity to fully cure, occupying warehouse space
Hydrolysis sensitivity: AKD hydrolyzes in the presence of water, especially at elevated temperatures
Storage constraints: Storage temperature ≤35°C; at 40°C, half-life is only 7 days
Retention aid dependency: Requires cationic starch or polyacrylamide for effective retention

Under papermaking conditions using calcium carbonate as a filler, the use of cationic starch is necessary to achieve effective AKD sizing. Cationic starch helps stabilize the sizing agent and promotes its retention on cellulose.

4.2 ASA (Alkenyl Succinic Anhydride)

Chemical Mechanism

ASA is a reactive neutral sizing agent with high efficiency and activity. It features a unique molecular structure with cyclic dicarboxylic anhydride groups. Like AKD, ASA forms ester bonds with the hydroxyl groups of cellulose fibers.

Key Characteristics

Rapid reactivity: ASA reacts much faster with cellulose fibers than AKD, often during the wet-end stages of papermaking
Immediate sizing: Develops 80%+ of its sizing before the web reaches the size press
No curing required: Unlike AKD, ASA does not require heat treatment or ripening time
pH range: Effective at pH 6.5–8.0

Advantages

ASA offers distinct benefits for specific applications:

Rapid sizing development: Ideal for high-speed paper machines (>1500 m/min)
No curing delay: Finished paper can be processed immediately
Low friction coefficient: No paper slipping issues
Suitable for recycled fiber: Particularly effective for heavy sizing paper based on waste paper

Limitations

Emulsion instability: ASA emulsions are less stable than AKD emulsions and require careful handling
Rapid hydrolysis: ASA hydrolyzes quickly to alkenyl succinic acids
Fresh preparation required: Emulsion must be freshly prepared and used within 2 hours
Equipment maintenance: Requires daily cleaning of pipelines with 50°C water and 0.1% NaOH to prevent gel blockage
Sensitivity: Performance is more sensitive to variations in pH and process conditions

4.3 AKD vs. ASA: A Comparative Summary

Feature	AKD	ASA
Reactivity	Moderate (slower)	High (very fast)
Curing	Requires 24–48 hours	No curing needed
Sizing development	Off-machine	On-machine
Emulsion stability	Good	Poor (requires fresh preparation)
pH range	Broad (neutral to alkaline)	Narrower (6.5–8.0)
Hydrolysis	Slow	Fast
Storage	≤35°C; limited shelf life	Must be used within hours
Best suited for	Medium-to-low speed machines (<800 m/min)	High-speed machines (>1500 m/min)

5. Polymer-Type Neutral Sizing Agents

5.1 Cationic Polymer Dispersions

Cationic polymer dispersions represent a third category of neutral sizing agents. These agents function through a combination of mechanisms:

Electrostatic attraction: The cationic nature of the polymer ensures strong retention on anionic cellulose fibers
Film formation: The polymer forms a hydrophobic film on the fiber surface
Synergistic effects: Often used in combination with polymeric retention aids for enhanced performance

5.2 Applications

Cationic polymer dispersions are used for:

Internal sizing of raw paper pulps at neutral pH
Combination with rosin sizes in neutral-alkaline systems
Specialty paper applications requiring specific surface properties

5.3 Emerging Polymer Technologies

Recent developments include the use of water-soluble chitosan as a sizing agent incorporated in paper composites. Chitosan and its derivatives offer:

Excellent film-forming properties and film strength
Strong hydrogen bonding with cellulose carboxyl groups
Schiff base formation with aldehyde groups
Environmentally friendly characteristics (natural origin, biodegradability)

6. The Role of Retention Aids

A critical factor in the effectiveness of neutral sizing agents—particularly AKD—is the use of retention aids. Cationic starch is the most widely used retention aid for AKD systems. Under papermaking conditions using calcium carbonate as a filler, the use of cationic starch is necessary to achieve effective AKD sizing.

Some cationic polymers work not only to improve AKD sizing performance but also to prevent press and smoothing roll deposits. Surface-active substances such as defoamers and deaerators have no significant effect on size retention, but agglomeration and hydrolysis can be reduced by increased starch addition or by the introduction of long-chain polymers such as polyacrylamides.

For ASA sizing, cationic additives—especially starch and aluminum products—have a very positive effect on sizing performance.

7. Market Landscape and Growth Trends

7.1 Global Market Size

The global sizing agents market is substantial and growing steadily:

The global Sizing Agents market was valued at USD 4.43 billion in 2025 and is expected to register a revenue CAGR of 4.5% during the forecast period
The global Pulp and Paper Sizing Agents market was valued at USD 1,789 million in 2025 and is forecast to reach USD 2,460 million by 2032 at a CAGR of 4.6%
The Neutral Sizing Agent market was valued at USD 1.2 billion in 2024 and is projected to reach USD 2.5 billion by 2034, registering a CAGR of 7.5%

7.2 AKD and ASA Market Share

Among synthetic sizing agents:

AKD holds the largest market share, contributing nearly 38% of global revenue in the paper sizing chemical market in 2024
The global ASA Sizing Agent market is projected to grow from USD 877 million in 2024 to USD 1,159 million by 2031 at a CAGR of 4.4%

7.3 Key Market Drivers

Sustainable Paper Packaging

The shift from plastic packaging toward recyclable fiber-based formats is increasing demand for high-performance sizing chemistry across food packaging, e-commerce shipping, and industrial wrapping applications. Containerboard shipments continue to expand as online retail and packaged food demand increases, directly increasing consumption of AKD and polymer sizing agents because recycled fibers absorb moisture more easily than virgin pulp.

Neutral Sizing as the Mainstream Choice

Neutral sizing agents have become the mainstream choice for mid-to-high-grade paper grades due to their wide pH adaptability, environmental friendliness, and aging resistance. A key trend is the shift toward neutral sizing agents, with AKD leading the charge.

Innovation in Bio-Based Solutions

In September 2024, Kemira (Finland) expanded production capacity for water-based barrier and sizing chemistries at its European paper chemicals network to support food packaging converters shifting away from fluorochemical coatings. In April 2024, Solenis (U.S.) introduced upgraded barrier and sizing solutions for molded fiber packaging applications.

8. Application Guidelines for Neutral Sizing Agents

8.1 Recommended Dosage

The dosage of neutral sizing agents varies by product type and paper grade:

Cationic rosin sizes: 0.3–1.2% based on dry pulp
AKD: Typically 0.6–3.5% based on dry pulp
ASA: Dosage adjusted based on target sizing degree and furnish composition

8.2 Addition Sequence

The typical addition sequence for neutral sizing is:

Neutral sizing agent
Precipitant/retention aid (if required)
Filler

8.3 Critical Process Parameters

Parameter	AKD	ASA	Cationic Rosin
Optimal pH	7.5–8.5	6.5–8.0	6.0–8.0
Curing	24–48 hours	Immediate	Immediate
Retention aid	Essential (cationic starch)	Recommended	Self-retaining
Storage	≤35°C	Use within 2 hours of emulsification	≥6 months

8.4 Troubleshooting Common Issues

Issue	Possible Cause	Recommended Action
Poor sizing (AKD)	Low retention; pH too high or low	Improve retention aid system; optimize pH
Hydrolysis (AKD)	High temperature; prolonged retention	Reduce temperature; improve single-pass retention
Emulsion instability (ASA)	Delayed application; contamination	Prepare fresh emulsion; use immediately
Inconsistent sizing	pH fluctuations; retention variations	Stabilize wet-end chemistry; monitor retention

9. Conclusion

Neutral sizing agents represent a cornerstone technology in modern papermaking, enabling the transition from acidic to neutral and alkaline systems that deliver superior paper quality, reduced environmental impact, and enhanced operational efficiency.

The three main categories—rosin-based, synthetic (AKD/ASA), and polymer-type—offer papermakers a range of options to suit different paper grades, production conditions, and economic considerations. AKD, with its durable covalent bonding and broad pH compatibility, has emerged as the dominant choice, holding nearly 38% of the global market. ASA offers rapid sizing development for high-speed machines, while cationic rosin sizes provide a cost-effective bridge for mills transitioning from acidic systems.

As the paper industry continues to embrace sustainability, with growing demand for recyclable packaging and reduced chemical footprints, neutral sizing technologies—particularly AKD and emerging bio-based alternatives—are well-positioned to meet the challenges of the future. Understanding the chemistry, advantages, and limitations of each neutral sizing agent type is essential for papermakers seeking to optimize product quality, production efficiency, and environmental performance in an increasingly competitive global market.