When selecting an optical brightener for papermaking, customers often compare products according to concentration, whiteness performance and price.
However, another important technical factor is the degree of sulfonation.
Many common stilbene-based optical brighteners used in the paper industry can be grouped into:
Disulfonated optical brighteners
Tetrasulfonated optical brighteners
Hexasulfonated optical brighteners
The number of sulfonic groups influences properties such as water solubility, cellulose affinity, application behavior and suitability for different stages of papermaking.
This is why two optical brighteners with similar appearances can perform very differently in the same paper mill.
1. What Does Sulfonation Mean?
Sulfonic groups increase the water-soluble character of many stilbene-based optical brighteners.
As the degree of sulfonation changes, the balance between:
Water solubility
and
Affinity for cellulose
also changes.
In general, lower-sulfonated grades tend to show stronger affinity for cellulose, while higher-sulfonated grades generally offer greater water solubility and are often used where strong surface-whitening performance is required.
However, sulfonation level should be treated as an important selection factor, not as the only factor determining performance.
Product formulation, concentration, molecular modification, paper chemistry and application process can also affect the final result.
2. Disulfonated Optical Brighteners: Strong Cellulose Affinity
Disulfonated optical brighteners generally have relatively high affinity for cellulose fibers.
This makes them particularly interesting for applications where effective uptake and retention on the fiber are important.
Typical considerations include:
Pulp application
Stock application
Wet-end application
High fiber substantivity
Efficient retention
Controlled application cost
For wet-end use, strong affinity can help more of the optical brightener remain associated with the fibers rather than remaining in the process water.
This can improve application efficiency.
However, stronger affinity also means that process conditions and addition sequence need to be properly controlled to maintain uniform distribution.
3. Why Are Disulfonated Grades Often Used in the Wet End?
The wet end contains a large amount of water.
An optical brightener added at this stage needs to move from the aqueous phase toward the cellulose fibers and remain sufficiently retained during sheet formation.
A grade with strong cellulose substantivity can therefore offer an advantage.
The practical objective is:
Good affinity + good retention + uniform whitening
rather than simply maximum fluorescence in solution.
This is why disulfonated grades are commonly considered when wet-end efficiency is an important requirement.
4. High Affinity Does Not Automatically Mean Better Performance
A common misconception is:
Higher fiber affinity = better optical brightener
This is not always true.
If affinity is very strong, the product may be taken up rapidly by the fiber.
Under poorly controlled conditions, rapid uptake can contribute to:
Uneven distribution
Localized whitening
Lower leveling
Greater sensitivity to the addition point
Therefore, the optical brightener should provide the appropriate balance between affinity and process control.
The ideal product depends on the actual paper machine and wet-end chemistry.
5. Tetrasulfonated Optical Brighteners: A Balanced Option
Tetrasulfonated optical brighteners generally provide a different balance between cellulose affinity and water solubility.
They are widely associated with paper surface applications and can provide good whitening performance in systems such as:
Size press
Surface sizing
Certain coating formulations
They may also be useful where the papermaker needs a balance of:
Solubility
Application flexibility
Whiteness development
Formulation compatibility
Cost efficiency
For many standard surface-whitening requirements, tetrasulfonated grades can provide a practical balance between performance and application cost.
6. Why Are Tetrasulfonated Grades Common at the Size Press?
At the size press, the optical brightener is normally applied in a concentrated aqueous formulation, often together with starch and other additives.
The product therefore needs:
Good water solubility
Compatibility with the sizing formulation
Uniform surface application
Efficient fluorescence
Stable processing behavior
Tetrasulfonated optical brighteners can provide a useful balance of these properties.
However, the result still depends strongly on:
Starch type
Solids content
pH
Application temperature
Other additives
Base-sheet properties
Therefore, not every tetrasulfonated optical brightener will perform identically in the same size-press system.
7. Hexasulfonated Optical Brighteners: High-Whiteness Surface Applications
Hexasulfonated optical brighteners have a higher degree of sulfonation and generally show high water solubility.
They are commonly associated with surface applications where very high levels of whiteness and brightness are required.
Typical applications can include:
High-brightness printing paper
Writing paper
High-whiteness surface-sized paper
Premium paper grades
In conventional paper OBA systems, hexasulfonated grades have often been selected when higher surface whiteness is required.
Their high solubility can be particularly useful in concentrated surface formulations.
8. Does Hexasulfonated Always Give the Highest Whiteness?
No.
This is another important point.
The degree of sulfonation provides a useful indication of product behavior, but it does not mean:
2 sulfonic groups = low performance
4 sulfonic groups = medium performance
6 sulfonic groups = automatically best
Modern modified optical brightener chemistries can change this relationship.
For example, specially modified tetrasulfonated products have been developed to achieve high surface whiteness levels that traditionally required hexasulfonated grades.
Therefore, the actual product should always be evaluated according to its technical performance rather than the sulfonation number alone.
9. Sulfonation Affects Solubility and Affinity Differently
A simplified way to understand the relationship is:
Lower Sulfonation
Generally favors:
Higher cellulose affinity
Higher Sulfonation
Generally favors:
Higher water solubility
This balance influences where the optical brightener may perform best.
For example:
A wet-end process may benefit from stronger cellulose substantivity.
A high-solids size-press formulation may benefit from excellent solubility and surface-whitening efficiency.
However, these are selection principles rather than rigid rules.
10. Which Type Is Better for Pulp Application?
For pulp or wet-end applications, the main considerations include:
Cellulose affinity
Retention
Whiteness gain
Interaction with wet-end chemicals
Required dosage
Process water losses
Disulfonated products are often attractive because of their relatively high cellulose substantivity.
However, the final choice should also consider:
Fiber furnish
pH
fillers
retention aids
cationic additives
recycled-fiber content
A product with high affinity may still perform poorly if it is incompatible with the actual wet-end chemistry.
11. Which Type Is Better for Size Press?
For size-press application, the selection criteria change.
Important factors include:
Water solubility
Compatibility with starch
Surface-whitening efficiency
High-concentration stability
Required whiteness level
Cost efficiency
Tetrasulfonated products are widely used for standard surface-whitening requirements.
Hexasulfonated products have traditionally been used when higher whiteness levels are required.
However, modified tetrasulfonated products can also achieve high levels of surface whiteness, so actual performance trials remain important.
12. Which Type Is Better for Coating?
Coating formulations are more complex than simple aqueous OBA solutions.
They may contain:
Calcium carbonate
Clay
Latex
PVOH
CMC
Starch
Other binders
Shading dyes
Functional additives
Therefore, sulfonation level alone cannot determine coating suitability.
The most important question is:
Is this optical brightener compatible with my complete coating formulation?
Some disulfonated products, for example, can also be used successfully in particular coating formulations when suitable binder systems are present.
This is why paper OBA selection should not be based on a simple rule that disulfonated products are only for the wet end and higher-sulfonated products are only for the surface.
13. Chemical Compatibility Remains Critical
Paper optical brighteners are commonly anionic.
Wet-end and surface formulations may also contain cationic chemicals.
Strong interaction between anionic optical brighteners and cationic additives can affect:
Solubility
Fluorescence
Retention
Formulation stability
Final whiteness
The addition sequence and compatibility should therefore be evaluated before full-scale production.
This is especially important when changing from one OBA chemistry to another, because different affinity levels can change how the product interacts with the paper chemistry.
14. Why Can the Same Dosage Give Different Whiteness?
Two optical brighteners used at the same kilograms per ton do not necessarily provide the same result.
Performance may differ because of:
Active concentration
Degree of sulfonation
Molecular modification
Cellulose affinity
Retention
Application point
Base-sheet brightness
Paper chemistry
Surface formulation
Fluorescent shade
Therefore, dosage should not be compared without considering product efficiency.
The more meaningful question is:
How much product is required to achieve the target whiteness under my actual process conditions?
15. Do Not Compare Only Product Concentration
A more concentrated optical brightener may appear economically attractive.
However, concentration alone does not determine actual application cost.
Consider:
Product concentration + required dosage + retention + whiteness gain + process compatibility
For example, a lower-priced product with poor retention may require more material to achieve the same finished-paper whiteness.
Conversely, a more concentrated or higher-efficiency product may reduce transportation or application requirements.
The comparison should therefore be made on:
Cost per ton of finished paper at the required whiteness
rather than simply price per kilogram.
16. A Practical Selection Guide
A simplified starting point is:
Consider Disulfonated Grades When:
Strong cellulose affinity is important
Wet-end or stock application is used
Retention efficiency is a major consideration
Consider Tetrasulfonated Grades When:
A balanced surface OBA is required
Standard to relatively high whiteness is targeted
Size-press compatibility and cost efficiency are important
Consider Hexasulfonated Grades When:
Very high surface whiteness is required
High water solubility is important
The application is mainly through surface sizing
These are general guidelines rather than absolute formulation rules.
17. How Should You Compare Different Sulfonation Levels?
When testing different products, keep the main production conditions unchanged.
Use the same:
Fiber furnish
Base paper
Application point
Wet-end chemicals
Starch or coating formulation
pH
Drying conditions
Then compare:
Whiteness Gain
How much additional whiteness is achieved?
Retention
For wet-end applications, how efficiently does the OBA remain with the fibers?
Solubility
Does the product remain stable at the required concentration?
Compatibility
Does it work with starch, binders and other process chemicals?
Shade
Does the finished paper have the desired white tone?
Dosage
How much product is required?
Application Cost
What is the real chemical cost per ton of finished paper?
Which Sulfonation Level Should You Choose?
There is no universal rule that one sulfonation level is always superior.
The most suitable optical brightener depends on:
Application Point + Fiber Affinity + Solubility + Required Whiteness + Chemical Compatibility + Dosage + Cost
In general:
Disulfonated grades are often associated with stronger cellulose affinity and wet-end efficiency.
Tetrasulfonated grades provide a useful balance for many surface applications.
Hexasulfonated grades are traditionally associated with very high-whiteness surface applications.
But modern modified products can blur these traditional boundaries.
The final product should therefore always be confirmed through application testing.
Need Help Selecting a Paper Optical Brightener?
If you are comparing different optical brighteners for papermaking, tell us your paper grade, fiber furnish, application point, current OBA, target whiteness and process conditions.
Blue Dolphin can help you evaluate suitable optical brightener options according to your wet-end, size-press or coating application requirements.
Post time: Aug-13-2026