What Natural Factors Affect Sponge Foam Foaming?

Temperature

The foaming reaction of sponge foam accelerates as the material temperature rises. In sensitive formulations, this may cause core scorching and fire hazards. Typically, the temperatures of the polyol and isocyanate components are kept constant. When foaming, as the foam density decreases, the material temperature should be increased accordingly. For the same formulation, if the material temperature remains unchanged, higher ambient temperatures in summer will speed up the reaction, leading to reduced foam density and hardness, increased elongation, and improved mechanical strength. In summer, the TDI index can be appropriately increased to compensate for the decrease in hardness.

Air Humidity

Increased humidity causes part of the isocyanate groups in the foam to react with moisture in the air, resulting in reduced hardness. Therefore, the TDI dosage can be slightly increased during foaming. However, excessive humidity may lead to excessively high curing temperatures, causing core scorching.

Figure 1: Effect of Atmospheric Pressure on Foam Density

Figure 2: Variation of Atmospheric Pressure with Latitude

The relationship between absolute humidity and foam load hardness is plotted in Figure 3.

Figure 3: Effect of Absolute Air Humidity on Foam Hardness

Notes:

During foam formation, gelation and foaming reactions occur simultaneously but compete with each other. Generally, the foaming reaction is faster than the gelation reaction.

Gelation reaction: Formation of urethane (reaction with -OH).

Foaming reaction: Reaction involving water, producing urea and generating bubbles.

Nucleating agents: Substances that initiate bubble formation, such as fine solid particles, liquids, foam stabilizers, or pre-dissolved micro-bubbles in the material. These include air or nitrogen, carbon dioxide, foam stabilizers, and fillers like carbon black dissolved in polyols and isocyanates. More nucleating agents lead to more bubbles and finer foam cell structure.

The number of bubbles and the size of foam cells depend on the effect of external nucleating agents. More nucleating agents result in more bubbles and smaller cell size.

When temperature rises, gas solubility in the liquid decreases, leading to more bubble formation or expansion of existing bubbles. A longer cream time favors the growth of larger bubbles.

Increasing the catalyst dosage shortens the cream time, promoting finer foam cells due to the competition between gelation and bubble formation.

Whether the foam has an ideal open-cell or closed-cell structure mainly depends on the balance between gelation speed and gas expansion speed during foam formation. This balance can be achieved by adjusting the type and dosage of tertiary amine catalysts and foam stabilizers in the formulation.