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Foam finishing is an advanced paper processing technology that has been developed in recent years. It utilizes foam as a carrier to uniformly apply various finishing agents onto the surface of paper sheets. A typical foam finishing formulation contains only 5%–10% water, with the remaining 90%–95% consisting of air. This significantly reduces the amount of moisture transferred to the paper during application, thereby shortening drying time and reducing energy consumption. Currently, this technology has been successfully applied in fireproofing and flame retardancy, water repellency, whitening, dyeing, and scented paper production.
1. Principle of Foam Finishing
1.1 Foaming Mechanism
A bubble is formed when a liquid film surrounds a gas. Foam refers to a dispersion system in which a large volume of gas is dispersed within a small quantity of liquid, forming an aggregate of microbubbles separated by thin liquid films. Pure liquids are incapable of forming stable foam. Stable foam formation requires the presence of surfactants-known as foaming agents-that impart specific surface-active properties to the solution. These agents form an adsorption layer at the gas–liquid interface, enhancing the elasticity and mechanical strength of the liquid film, thus stabilizing the foam structure. Additionally, mechanical energy-such as bubbling or stirring-is necessary to introduce gas into the liquid phase and generate foam. When air is introduced into a foaming agent solution, bubbles rise through the liquid; upon reaching the air–solution interface, they become enveloped by a bilayer of molecules, forming a stable foam lamella (i.e., liquid film between bubbles).
1.2 Requirements for Foaming Agents
An effective foaming agent should meet the following criteria:
(1) Capable of rapid and uniform foam generation, with good wetting properties that can be adjusted according to concentration to suit different paper surfaces. Once formed, the bubbles should exhibit high stability.
(2) The foaming performance should remain largely unaffected by variations in water quality and temperature, ensuring consistent foam yield under diverse operating conditions.
(3) Highly compatible with various finishing chemicals, without compromising their functional performance upon addition.
(4) Should not affect the color or colorfastness of dyed paper, nor induce yellowing in white paper substrates.
1.3 Foaming Agents and Foam Stabilizers
Commonly used foaming agents are classified into anionic and nonionic types. Sodium dodecyl sulfate (SDS) exhibits the strongest foaming capacity among these. Hydroxyethyl cellulose (HEC) serves as an effective foam stabilizer due to its ability to reinforce the interfacial film structure. Moreover, HEC contributes to surface sizing by forming a robust and flexible film on the paper surface, thereby enhancing paper strength.
2. Equipment Configuration and Process Flow
Foam application is carried out using a dedicated applicator, while the aeration unit is installed separately. The generated foam dispersion is transported via pump to the applicator. Upon contact with the paper surface, the foam rapidly collapses, allowing the active components of the finishing agent to penetrate deeply into the fiber matrix. This ensures efficient and uniform finishing. The overall process flow is as follows:
Wet end → Press section → Double drum drying → Foam applicator → Tension and guide rolls → Cylinder drying section → Calendering → Reeling
2.1 Foam Finishing Process Steps
Preparation of finishing agent stock solution → Foam generation using a foaming machine → Application of foam to paper surface → Defoaming and absorption → Completion of finishing treatment
2.2 Formulation and Process Conditions for Foam Stock Solution
2.2.1 Brightening and Finishing (on dry basis)
Optical brightener BA: 0.4%
Wetting agent JFC: 0.3%
Sodium dodecyl sulfate: 0.2%
Hydroxyethyl cellulose: 0.1%
Liquor pickup rate: 14%–15%
2.2.2 Scented Finishing (on dry basis)
Sandalwood fragrance: 0.2%
Crosslinking fixative: 0.1%
Wetting agent JFC: 0.2%
Sodium dodecyl sulfate: 0.2%
Hydroxyethyl cellulose: 0.1%
Liquor pickup rate: 20%
3. Advantages of Foam Finishing
The application of foam finishing in specialty paper manufacturing has demonstrated multiple advantages through extensive practical implementation:
3.1 Reduced Liquor Pickup: After foam application and pressing, the residual moisture content in the paper prior to drying typically ranges from 10% to 15%, significantly lower than the 22% to 25% observed after conventional size press treatments.
3.2 Increased Machine Speed: Lower moisture load enables higher drying efficiency, allowing for increased production speeds and improved throughput.
3.3 Efficient Use of Chemicals: The targeted delivery mechanism reduces the consumption of finishing agents and additives, contributing to lower production costs.
3.4 Improved Product Quality: Unlike padding methods, which may result in uneven chemical distribution and surface defects, foam finishing ensures uniform coverage and enhances surface clarity and consistency.
3.5 Operational Simplicity and Versatility: The process is straightforward and adaptable to various paper grades, offering broad applicability across different product lines.
3.6 Environmental Benefits: Reduced water usage and lower effluent load contribute to decreased environmental impact and support sustainable manufacturing practices.
In summary, the adoption of foam finishing simplifies processing steps, meets the requirements of diverse specialty papers, reduces energy consumption, and enhances both economic and ecological performance.
