Explaining the Stability of Polyurethane Foam

During the foaming process of polyurethane foam, the initial bubbles formed are very similar to those formed in a soap solution system. This similarity does not extend to the later stages of bubble formation due to the rapid increase in the viscosity and elasticity of the liquid phase as the polymerization reaction proceeds, differing significantly from bubbles in a soap solution system. Research on the bubble mechanism in aqueous foam systems suggests that to obtain fine pores, a certain amount of free energy must be added to the system when a certain volume of gas is dispersed in a certain volume of liquid to form bubbles. The relationship is given by:

△F=rA

Where:

△F = free energy;

r = surface tension;

A = total bubble interface area.

From this relationship, it can be seen that without sufficient energy, the liquid foam system tends to reduce interface area, leading to coalescence or rupture of bubbles. Measures such as rapid solidification of the foam before coalescence or rupture, or adding silicone oil to reduce surface tension, can help achieve a larger bubble interface area A under the same energy conditions (ΔF), aiding in the production of finer bubbles.

After foaming and before solidification, the shape of the foam cells typically changes over time due to the gradual diffusion of gas dissolved in the liquid phase into the bubbles and the mutual diffusion of gas between bubbles, causing coalescence. Generally, the internal pressure of gas in spherical cells is higher than the pressure of the surrounding liquid, given by the relationship:

△P=2r/R

Where:

R = radius of the bubble;

r = surface tension.

Usually, the gas pressure in smaller bubbles is higher than that in larger bubbles. The internal pressure difference between bubbles of different radii is:

△P²=2r[(1/R₁)-(1/R₂)

This pressure difference causes gas from smaller bubbles to gradually diffuse into larger bubbles, leading to the disappearance of smaller bubbles and the growth of larger bubbles over time. Reducing surface tension helps decrease the pressure difference between different sized bubbles, contributing to bubble stability and uniform pore size.

Preventing cell rupture before the polymer solidifies is crucial for foam production. In the liquid state, this can be considered through the following concepts:

Regardless of liquid surface tension, stable foam cannot be obtained from any pure liquid alone. To achieve relatively stable foam, two conditions must be met: the system must have at least two components, and these components must preferentially adsorb on the bubble surface. The surface tension is determined by the type and quantity of adsorbed solutes (Gibbs principle):

dr=-∑Γdμ

Where:

μ = chemical potential of the component;

Γ = surface excess of the component.

With a limited amount of solute, increasing the surface area reduces

Γ, thus increasing surface tensionr, which hinders further surface expansion. This relationship prevents the thinning of the cell membrane, contributing to bubble stability.

Temperature also affects bubble stability; higher temperatures reduce surface tension, promoting cell membrane thinning and bubble rupture. Capillary drainage within bubble walls is another factor affecting stability. A magnified cross-section of the bubble wall can be seen in the diagram below. From the diagram we can see capillary action causes drainage from point 3 to points 1 and 2, thinning the bubble wall.

Capillary Drainage Effect in the Early Stage of Foam Formation

Due to the low activation energy for foam rupture, once cell rupture begins, it often accelerates, leading to foam collapse. The electric double-layer effect has little impact on this process. If charges accumulate on both sides of the bubble membrane, they repel each other, limiting membrane thinning. This effect is less significant in organic media with low dielectric constants than in aqueous systems. Van der Waals forces also counteract membrane thinning, as the attraction between surfaces increases as the membrane thins.

In summary, factors preventing membrane thinning and stabilizing foam include:

1. Increasing surface area, thus increasing free energy;

2. Reducing surface excess to increase surface tension;

3. Increasing material viscosity;

4. Minimal impact of the electric double-layer effect on membrane thinning.

Conversely, factors causing membrane thinning and bubble rupture include:

1. Capillary drainage;

2. Gravity-induced drainage;

3. Any factor reducing surface tension, such as hot spots or defoamers;

4. Van der Waals forces between membrane surfaces.

Common measures for producing polyurethane foam include:

1. Adding surfactants to reduce surface tension, aiding in producing uniform microbubbles (△F=r△A), stabilizing bubbles, and reducing gas diffusion (△P=2r[(1/R₁-1/R₂ )]);

2. Rapidly increasing material viscosity to reduce membrane thinning, stabilizing bubbles;

3. Increasing the strength of membrane and bubble junctions;

4. Reducing impurities to prevent local surface tension reduction and local foam collapse, creating voids.