Application of Cell-Regulating Agents in Polyurethane Foam Manufacturing

The preparation process of polyurethane foam involves complex chemical reactions. Under the influence of catalysts, the reaction rate between liquid raw materials is rapid, leading to quick growth of polymer molecular chains, a rapid increase in viscosity, and a swift transition to a solid state. Simultaneously, the reaction generates a significant amount of carbon dioxide and/or low-boiling physical blowing agents that vaporize due to reaction heat, causing rapid expansion of the reactive materials. The material gelation and solidification process, along with bubble generation and dispersion within the viscous material, must be balanced to produce uniform foam structures.

Role of Foam Stabilizers (Cell-Regulating Agents)

Foam stabilizers, also known as cell-regulating agents, play a critical role in stabilizing foam due to their unique surface properties, such as surface tension and interfacial orientation. Their molecular structure combines hydrophilic and hydrophobic components, enabling interfacial orientation and emulsification of materials with vastly different hydrophilic and hydrophobic properties into a uniform system.

In early two-step foam production methods, polydimethylsiloxane (dimethyl silicone oil) was used as a stabilizer. The low polarity and strong hydrophobic nature of methyl groups in its side chains result in low intermolecular attraction, often weaker than the cohesive and intercomponent forces within the system. When aligned on the system's surface, these molecules reduce surface tension.

By combining hydrophobic dimethylsiloxane with hydrophilic polyethylene oxide ether or polypropylene oxide ether, the resulting copolymers exhibit excellent surface activity, enabling homogeneous mixing of high-polarity and low-polarity components. These emulsified mixtures react thoroughly, stabilizing the generated polyurethane and polyurea and ensuring uniform gas dispersion. Consequently, silicone-polyether copolymers have become standard foam stabilizers for polyurethane foam.

Mechanism of Foam Stabilization

The foam generation process increases the system's free energy. Lower surface tension reduces the free energy required for gas dispersion, enhancing foam stability. Surfactants effectively lower surface tension, aiding in the formation of uniform, fine bubbles. Reduced surface tension also minimizes force differences between adjacent bubbles. Dynamic tension at the interface forms a surfactant monolayer with higher viscosity than the bulk phase, increasing bubble surface elasticity and preventing coalescence, resulting in uniform and dense foam.

Influence of Copolyether as a Surfactant

Copolyethers themselves are surfactants. The ratio of ethylene oxide to propylene oxide in their molecular structure significantly influences stabilizer performance. Pure ethylene oxide-based polyethers lack effective foam-stabilizing properties. While polyethylene oxide segments provide hydrophilicity and foaming ability, polypropylene oxide segments offer hydrophobicity and permeability, effectively reducing surface tension. Adjusting the proportions of ethylene oxide, propylene oxide, and siloxane yields stabilizers tailored for various polyurethane foams.

Applications in Different Foam Types

Soft Polyurethane Foam

Soft foam stabilization has been a primary focus of early research. In these formulations, the components have limited compatibility, and the reactive groups are relatively sparse. The slower viscosity increase in high-resilience and rigid foam systems necessitates a high open-cell rate, making stabilizers essential.

In water-blown soft foam systems, water constitutes 3%-5% of polyether weight. The reaction between water and isocyanate is faster than that between isocyanate and polyol, resulting in polyurea formation. Polyurea, being highly polar and prone to aggregation, serves as a defoamer that aids in cell opening. Proper timing of cell opening and bursting is crucial for maintaining foam integrity.

High-Resilience Foam

High-resilience systems, which use high-activity polyethers, are less dependent on stabilizers. Low-activity surfactants are typically employed, with specialized cell-regulating agents required for formulations using polymer polyols.

Rigid Polyurethane Foam

Rigid foams, characterized by high crosslinking due to low-molecular-weight, high-functionality polyether polyols, naturally stabilize bubbles. However, the limited flowability of rigid foam formulations necessitates stabilizers with strong emulsification capability, bubble size control, and flowability enhancement. Stability during storage is also critical for combination materials, favoring hydrolysis-resistant silicon-carbon stabilizers.

Performance Evaluation of Stabilizers

The performance of cell-regulating agents is typically assessed based on activity level and process tolerance. Process tolerance refers to the capacity of a formulation or process to accommodate variations without affecting foam properties. Stabilizer dosage, usually 0.5%-2% of the formulation, must be optimized through trials.

Auxiliary Agents for Open-Cell Structures

Foam-opening agents, another class of additives, regulate cell structures but typically complement stabilizers. In some soft foam formulations, additional foam-opening agents improve open-cell rates when existing stabilizers are insufficient. These agents, along with specialized polyol materials, can create rigid polyurethane foam with over 95% open-cell structures.

Research on stabilizers continues globally, with new varieties introduced regularly to enhance foam performance and manufacturing processes.