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Polyurethane absorbent foam is a porous material produced through polyurethane foaming processes. It is widely used in cleaning sponges, absorbent wiping materials, household cleaning products, industrial liquid absorption materials, and other applications requiring water absorption and liquid retention.
Compared with conventional flexible polyurethane foam, absorbent foam requires not only appropriate softness, elasticity, and mechanical strength, but also good hydrophilicity, water absorption speed, absorption capacity, drainage performance, and reusability.
During production, additive selection can influence the foaming process, cell structure, and final water absorption performance.
Common additives used in related polyurethane foam formulations include:
Silicone surfactants: Regulate interfacial behavior, help stabilize foam cells, and influence cell size and open-cell structure.
Hydrophilic modifiers: Improve the wettability of the foam so that water can enter the material more easily.
Cell openers: Help regulate the connectivity between foam cells and improve pathways for liquid penetration and movement.
Foaming catalysts: Regulate the relative rates of the blowing and gelling reactions, influencing foam rise, curing, and cell stability.
Wetting agents: Improve the spreading and penetration of water on the foam surface. Some products can also be introduced through post-treatment processes.
It is important to understand that the water absorption capacity of polyurethane absorbent foam is not determined by a single additive. The selection of polyether or polyester polyols, isocyanate systems, cell structure, crosslinking degree, and manufacturing processes also influence the final performance.
Therefore, a practical selection strategy should begin by defining the target properties of the absorbent foam, identifying which aspects of the formulation need improvement, and then selecting suitable additives for testing.
Silicone surfactants are commonly used additives in polyurethane foam production. They generally include polysiloxane-polyether block or graft copolymers.
Unlike ordinary dimethyl silicone oil, these products contain both polysiloxane and polyether segments in their molecular structures, allowing them to perform specific interfacial functions.
During polyurethane foaming, silicone surfactants mainly provide the following functions:
(1) Improving Compatibility Within the Foaming System
Polyurethane foam formulations contain polyols, water, isocyanates, and other additives. Silicone surfactants can help improve interfacial behavior between certain components and contribute to a more stable foaming process.
(2) Stabilizing Foam Structure
During foaming, bubbles continuously form, expand, and interact. If the foam structure is insufficiently stable, cell coalescence, localized collapse, or other structural defects may occur.
Appropriate silicone surfactants help stabilize the bubbles and reduce the risk of structural instability during foaming.
(3) Regulating Cell Size and Uniformity
Different silicone surfactant structures may have different effects on cell nucleation, stabilization, and cell size distribution.
For absorbent foam, cell uniformity and connectivity can influence how quickly water enters the material and how liquid is distributed throughout the foam.
Selection Recommendations:
When selecting silicone surfactants, consider the following factors:
Whether the product is suitable for flexible polyurethane foam or hydrophilic polyurethane foam systems
Effects on cell size and uniformity
Effects on the degree of cell opening
Foaming stability and resistance to foam collapse
Compatibility with polyols, water, and catalysts
Effects on the final water absorption speed and capacity
It is important to emphasize that ordinary silicone oil is not equivalent to a specialized polyurethane foam silicone surfactant. A product should not be considered suitable for absorbent foam production simply because its name contains the term "silicone oil."
Hydrophilic modifiers are an important category of functional additives to consider in polyurethane absorbent foam formulations.
Even if a polyurethane foam has a relatively high proportion of open cells, it may not necessarily absorb water quickly. If the foam skeleton has insufficient wettability, water may not spread or enter the internal cell channels easily.
Hydrophilic modifiers are mainly used to improve the interfacial behavior between the foam and water.
Depending on the product structure and application method, the following options can be considered:
(1) Hydrophilic Polyether Components
By selecting polyether components with appropriate hydrophilic properties, formulators can adjust the hydrophilicity of the polyurethane system.
However, whether these components effectively improve absorbent foam performance depends on their molecular structure, hydroxyl value, reactivity, and compatibility with the isocyanate system.
(2) Hydrophilic Polymers
Certain hydrophilic polymers can be introduced through formulation or composite modification to improve the water absorption and retention properties of foam.
For example, published research has investigated sodium polyacrylate polymer-modified hydrophilic polyurethane foams, demonstrating that hydrophilic polymers are a potential approach for improving foam wettability.
However, the actual results depend on the polymer type, addition method, foam structure, and formulation conditions. Experimental absorption data from one study should not be directly applied to other production systems.
(3) Surface Hydrophilic Treatment Agents
For polyurethane foam that has already been produced, surface treatment or impregnation can also be considered to improve wettability.
This approach differs from adding hydrophilic components during the foaming stage. It requires evaluation of the distribution of the treatment agent within the foam, resistance to washing, residual substances, and changes in mechanical properties after treatment.
Selection Recommendations:
If the main problem is that water droplets cannot easily enter the foam or the initial wetting speed is slow, hydrophilic modifiers or surface treatment solutions should be evaluated first rather than simply increasing the amount of silicone surfactant.
Polyurethane absorbent foam generally requires a certain degree of open-cell structure.
If the windows between cells are too small or the proportion of closed cells is too high, water may have difficulty entering the material. Even if the foam itself has some hydrophilic properties, its water absorption capacity may not be fully utilized.
Cell openers are mainly used to regulate cell structure and promote connectivity between cells.
Their functions may include:
Regulating cell window structure
Improving connectivity between adjacent cells
Helping create pathways for liquid penetration
Improving the degree of cell opening in specific formulations
However, a higher degree of cell opening is not always better.
Excessive changes to the cell structure may affect mechanical strength, resilience, dimensional stability, and recovery after compression.
Furthermore, the performance of cell openers is influenced by silicone surfactants, catalysts, raw material reactivity, and foaming conditions.
Selection Recommendations:
If absorbent foam has insufficient internal cell connectivity, slow water absorption, or difficulty allowing water to enter its internal structure, cell openers suitable for the specific polyurethane foam system can be evaluated.
Cell morphology, air permeability, water absorption speed, absorption capacity, and compression recovery should be examined together.
Polyurethane foaming generally involves two important reactions:
Water reacts with isocyanate, generating carbon dioxide and contributing to foam expansion.
Polyols react with isocyanates, forming polyurethane structures and gradually establishing the foam skeleton.
Catalysts influence the rates and relative balance of these reactions.
If the blowing reaction is too fast before the foam skeleton develops sufficient strength, structural instability may occur.
If the gelling reaction is too fast, foam expansion may be restricted or the cell structure may be affected.
Therefore, catalyst selection must be considered together with the polyol, isocyanate, water, and silicone surfactant systems.
Selection Recommendations:
For polyurethane absorbent foam, it is not enough to consider whether the foam rises properly. The following parameters should also be evaluated:
Cream time
Rise time
Gel time
Curing condition
Cell uniformity
Occurrence of foam collapse, shrinkage, or localized defects
Catalyst dosage should be determined according to raw material reactivity, formulation, and production equipment. Formulations developed for other polyurethane foam products should not be directly applied without validation.
Wetting agents are mainly used to reduce the resistance to liquid wetting on solid surfaces and improve the spreading and penetration of water droplets.
In certain polyurethane absorbent foam systems, appropriate wetting agents may help water contact the foam skeleton more quickly and enter the open cells.
However, wetting speed and final water absorption capacity are two different performance indicators.
Some wetting agents may improve initial water absorption speed without significantly increasing the actual liquid storage capacity of the material.
Other factors that require evaluation include water solubility, migration, wash resistance, residual substances, and compatibility with the polyurethane system.
Selection Recommendations:
If the main issue is slow initial water absorption, wetting agents can be included as candidates for evaluation.
If the product needs to maintain stable hydrophilicity after repeated washing, the durability of the wetting performance should be tested rather than relying only on initial water absorption results.
Different applications have different requirements for absorbent foam additives.
Household cleaning sponges generally require good water absorption speed, softness, squeezing and drainage performance, and repeated-use durability.
The following additives deserve attention:
Hydrophilic modifiers: Improve water wetting and penetration.
Silicone surfactants: Regulate cell uniformity and foaming stability.
Cell openers: Improve cell connectivity.
Catalysts: Control the balance between blowing and gelling reactions.
For these products, it is not sufficient to pursue high water absorption capacity alone. Drainage during squeezing, recovery, and resistance to damage should also be evaluated.
Industrial absorbent foam may be used for equipment cleaning, liquid wiping, and specific liquid absorption applications.
In addition to water absorption, the following properties may need to be considered:
Absorption performance for different liquids
Liquid absorption speed and retention capacity
Foam mechanical strength
Recovery after compression
Cleaning and reuse requirements
Chemical compatibility with the target liquid
If the material is intended to absorb not only water but also oils, cleaning solutions, or other chemical liquids, its wetting and absorption performance should be evaluated separately for each target liquid.
Hydrophilicity alone does not guarantee good absorption of every type of liquid.
If the target product requires high water absorption capacity and liquid retention, the hydrophilicity, cell structure, and inherent absorption properties of the polymer should all be considered.
Possible formulation development approaches include:
Selecting suitable hydrophilic polyols or hydrophilic modification components
Optimizing silicone surfactants to improve cell stability
Adjusting open-cell structure to create pathways for water penetration
Evaluating hydrophilic polymers or functional composite components
Adjusting the foam skeleton structure to balance absorption capacity and mechanical strength
It should be noted that open cells mainly provide space for liquid entry and storage. They do not automatically mean that the material has a high water absorption ratio.
If higher liquid retention is required after absorption, the polymer structure, capillary effects, and compression state of the material should also be investigated.
For manufacturers of polyurethane absorbent foam, silicone surfactants are among the additives worth evaluating carefully.
However, different polyurethane foam products have different requirements. Silicone surfactants designed for conventional flexible foam are not necessarily suitable for hydrophilic absorbent foam.
The following factors should be considered during selection.
First, identify whether the formulation uses a polyether-based or polyester-based polyurethane system and whether the manufacturing process involves slabstock foaming, molded foaming, or another method.
Different systems may have different raw material compatibility, reaction rates, and cell formation behavior.
Priority should be given to silicone surfactants that the supplier specifically recommends for the corresponding polyurethane foam system.
If the target product requires a high proportion of open cells, evaluate the combined effects of the silicone surfactant on cell stability and cell opening.
Some silicone surfactants provide strong foam stabilization. However, if they are not properly matched with the cell opener, catalyst, and overall formulation, the desired open-cell structure may not be achieved.
Therefore, selecting a surfactant solely because it provides stronger foam stabilization is not an appropriate selection strategy.
If the target product requires fast water absorption, focus on the final foam's water droplet penetration time, capillary absorption speed, and overall water absorption performance.
It is important to understand that silicone surfactants mainly provide interfacial regulation and cell control. Whether the final foam has sufficient hydrophilicity also depends on hydrophilic modifiers, raw material structure, and post-treatment processes.
Silicone surfactant selection should be evaluated according to the performance of the finished foam rather than based solely on individual parameters listed in the raw material TDS.
At a minimum, the following properties should be considered:
Foaming stability
Cell size and uniformity
Degree of cell opening
Water absorption speed
Water absorption capacity
Squeezing, drainage, and recovery performance
Storage stability
To avoid selecting additives solely based on supplier recommendations or product names, a comparative laboratory testing program is recommended.
First, keep the main raw materials and production conditions consistent, including polyols, isocyanates, water, catalysts, and other basic components.
At the same time, define the key target properties of the final product, such as:
Apparent density
Hardness
Cell size
Degree of cell opening
Water absorption speed
Water absorption capacity per unit mass
Liquid retention after absorption
Recovery after compression
Use the existing formulation as the control group and conduct separate single-factor screening tests for hydrophilic modifiers, silicone surfactants, and cell openers.
After identifying promising candidates, investigate the interactions and combinations of different additives.
This approach makes it easier to determine how individual additives affect cell structure and water absorption performance.
It is not recommended to change all additives simultaneously in the first round of testing, as this makes it difficult to identify the actual cause of performance changes.
The following tests are recommended for polyurethane absorbent foam:
| Test item | Main evaluation purpose |
|---|---|
| Water droplet penetration time | Evaluates initial surface wettability |
| Capillary water absorption test | Evaluates the rate of water movement inside the foam |
| Water absorption per unit mass | Evaluates water absorption capacity |
| Water retention test | Evaluates liquid retention after absorption |
| Squeezing and drainage test | Evaluates drainage and reuse performance |
| Cell structure observation | Evaluates cell size, uniformity, and open-cell structure |
| Compression recovery test | Evaluates recovery after compression |
| Water washing resistance test | Evaluates the durability of hydrophilic performance and material properties |
Water absorption capacity per unit mass can be calculated using the following formula:
Water absorption capacity (g/g) = (Wet sample mass − Dry sample mass) ÷ Dry sample mass
For example, if the dry sample weighs 1 g and the wet sample weighs 8 g after water absorption, the water absorption capacity is 7 g/g.
It is important to standardize sample dimensions, drying conditions, immersion time, drainage method, and weighing procedures before testing. Results obtained using different test methods should not be compared directly without considering the differences in test conditions.
Improving water absorption performance should not come at the expense of mechanical strength.
If the degree of cell opening becomes too high or the foam skeleton is too weak, the absorbent material may be damaged during repeated squeezing or use.
Therefore, the following properties should also be tested:
Tensile strength
Tear strength
Compression properties
Resilience
Dimensional stability
Structural changes after repeated water absorption and squeezing
The final formulation should balance water absorption performance with practical service life.
From a formulation development perspective, different additives perform different functions.
| Additive type | Main function | Key selection factors |
|---|---|---|
| Silicone surfactants | Stabilize foam and regulate cell structure | Cell uniformity, open-cell structure, system compatibility |
| Hydrophilic modifiers | Improve wettability between foam and water | Hydrophilic performance, absorption speed, washing resistance |
| Cell openers | Regulate cell windows and connectivity | Cell opening effect, foam strength, resilience |
| Foaming catalysts | Regulate blowing and gelling reactions | Cream time, gel time, foam stability |
| Wetting agents | Improve water spreading on the foam surface | Penetration speed, water resistance, residue, and migration |
In practical production, the initial additive selection should be based on the actual product problem:
If water droplets have difficulty entering the foam, prioritize hydrophilic modifiers and wetting solutions.
If the cells are closed or insufficiently connected, evaluate cell openers and silicone surfactant combinations.
If foam collapse or uneven cell structures occur during production, examine silicone surfactants, catalysts, and reaction balance.
If the foam has high water absorption capacity but is easily damaged during squeezing, further optimize the foam skeleton structure and mechanical properties.
If initial water absorption is good but decreases after repeated washing, investigate the migration, leaching, and post-treatment stability of hydrophilic components.
Silicone surfactants are commonly used foam-stabilizing additives in many polyurethane foam systems. They help control cell structure and the foaming process.
However, whether a silicone surfactant is required and which type should be used depend on the polyurethane system, foaming process, and target cell structure.
Silicone surfactants are not the only source of hydrophilicity in absorbent foam and cannot replace hydrophilic modifiers.
It is not recommended to use ordinary dimethyl silicone oil as a polyurethane foaming surfactant solely based on its product name.
Ordinary dimethyl silicone oil and polysiloxane-polyether copolymer silicone surfactants have different molecular structures and interfacial functions.
Polyurethane foaming generally requires specially designed silicone surfactants to meet foam stabilization and cell regulation requirements.
If ordinary silicone oil is being considered, its effects on foaming stability, cell structure, and final product performance must first be verified.
The first step is to identify the reason for slow water absorption.
If the foam surface is difficult to wet, hydrophilic modifiers or wetting agents should be evaluated first.
If water can wet the surface but cannot easily enter the internal foam structure, cell connectivity should be examined, and cell openers and silicone surfactants should be evaluated.
It is not advisable to increase the dosage of a particular additive without first identifying the underlying problem.
Not necessarily.
Cell openers mainly affect foam structure and connectivity. Increasing the proportion of open cells may improve pathways for water penetration, but absorption capacity also depends on foam density, skeleton structure, hydrophilicity, and available pore volume.
Excessive cell opening may also reduce foam strength and resilience.
Therefore, water absorption capacity, absorption speed, and mechanical properties should be evaluated together.
Their main functions are different.
Silicone surfactants mainly regulate interfacial behavior during foaming, stabilize bubbles, and control cell structure.
Hydrophilic agents mainly improve the wettability between the foam and water, allowing water to enter the material more easily.
Some surfactants may also influence the final foam's wetting behavior, but this does not mean that all silicone surfactants are hydrophilic agents.
Not necessarily.
Polyether-based and polyester-based polyurethane systems may differ in raw material structure, compatibility, and reaction characteristics. Therefore, additive suitability may also differ.
During selection, provide the supplier with the specific polyol type, isocyanate system, foaming process, and target product properties. Suitability should then be confirmed through laboratory testing.
Not necessarily.
Different applications have different requirements for absorption capacity, absorption speed, and liquid retention.
For example, cleaning sponges may place greater emphasis on absorption speed, squeezing and drainage performance, and repeated-use durability. Industrial absorbent materials may focus more on absorption per unit mass and liquid retention.
If only maximum absorption capacity is pursued, the material's strength, resilience, and service life may be overlooked.
Overall performance should be evaluated according to the intended application.
The initial assessment can be based on the material's performance.
If the foam structure is intact and cell connectivity is satisfactory, but water droplets remain on the surface for a relatively long time, surface wettability and the hydrophilic modification system should be examined.
If the cells are visibly uneven, there are too many closed cells, or foam collapse occurs during production, the silicone surfactant, cell opening system, and catalyst should be investigated.
If both problems occur simultaneously, combined formulation testing is recommended.
The root cause cannot be accurately determined from the appearance of the finished foam alone.
Not necessarily.
Adding hydrophilic components during foaming and applying surface treatment after foam production work through different mechanisms.
Components added during foaming may become part of the overall foam formulation. Post-treatment depends on the penetration, deposition, and fixation of the treatment solution within the foam.
Post-treatment also requires evaluation of washing resistance, treatment uniformity, residual substances, and production costs.
The appropriate approach should be selected according to the required durability of hydrophilic performance and the manufacturing process.
Selecting additives for polyurethane absorbent foam requires comprehensive consideration of foam structure, hydrophilicity, water absorption speed, absorption capacity, and mechanical properties.
Silicone surfactants mainly stabilize foam and regulate cell structure during the foaming process. Hydrophilic modifiers improve wettability between the material and water. Cell openers regulate cell connectivity, while catalysts control the balance between blowing and gelling reactions.
These additives interact with one another, and no single product can be expected to achieve all performance objectives.
For polyurethane absorbent foam manufacturers, the recommended approach is to first define the application and target properties, then select additives according to the actual formulation challenges, and finally verify their performance through laboratory trials, cell structure observation, water absorption testing, and mechanical property evaluation.
An effective additive combination should improve water absorption while maintaining stable foam structure, appropriate mechanical properties, and controllable production conditions.
