De Vries has done research on foam stability in water-phase foam systems, expounding the basic principles and some stabilization points in the formation of polyurethane foam.
The research believes that when a certain volume of gas is dispersed in a unit volume of liquid phase, in order to produce a relatively uniformly dispersed microcellular foam, a certain free energy (ΔF) must be applied to the system, as expressed by the following formula.
△F=yA
where ΔF is the free energy; y is the surface tension; A is the total area of the bubble interface.
Since in the liquid foam system, there is always a tendency for the surface area of the gas-liquid interface to decrease, if the system is not given enough free energy, the bubbles in the liquid phase will merge or collapse. According to the principle of the above formula, adding substances that reduce the surface tension y, such as silicone oil surfactants, can obtain a larger total bubble interface area (A) under the same energy ΔF conditions. Therefore, adding people Appropriate surfactants facilitate the formation of fine bubble dispersions.
Mobay Chemical Company of the United States used a high-speed camera to track and observe the formation process of polyurethane foam. In the process of gradually increasing the viscosity of the material, the shape and volume of the bubbles will change with the increase of time, and the volume of the bubbles will change from small to large. The shape also gradually evolves from spherical to three-dimensional network structure of pentahedron and hexahedron.
According to the classical theory, in the foaming process, the gas pressure in the spherical bubble is larger than the pressure of the surrounding liquid; the gas pressure in the small bubble is larger than that in the large bubble, and the pressure difference between them is △ p and ΔP' represent.
△p=2y/R
△p'=2y(1/R1-1/R2)
where y is the surface tension; R is the bubble radius; R1 and R2 are the radii of small and large bubbles, respectively.
Over time, the small bubbles expand, diffuse and merge into the larger bubbles, giving the gas a progressively larger shape and volume. If an additive that is conducive to reducing the surface tension is added to the formula, it will help to reduce the pressure difference between large and small bubbles, improve the stability of the foam, and produce a foam structure with finer cells.
As the other side of the gas-liquid two-phase, the role of the liquid is another topic in the discussion of bubble stabilization. It is impossible to obtain stable bubbles in a pure liquid, regardless of the surface tension of the liquid. To obtain relatively stable bubbles, the following two conditions must be met:
1. There are at least two or more systems in the system;
2. One component of the system can be preferentially adsorbed on the surface of the bubble. According to Gibbs theory, its surface tension is determined by the type and amount of adsorbed solutes.
dy=—ΣΓdμ
In the formula, Γ is the chemical potential of the component; μ is the surface excess of the component.
According to the above relationship, in the case of a certain amount of solute, the increase of the surface area will reduce the surface excess, and the increase of the surface tension will hinder the further expansion of the surface area, that is, the bubble film will be further thinned. Therefore, increasing the surface tension can prevent the thinning of the bubble wall and contribute to the stability of the bubble.
The liquid on the bubble wall film will produce liquid drainage due to capillary action, which is one of the factors affecting the stability of the cell.
Figure 6-2 is an enlarged cross-sectional model view of the bubble wall part.
According to Laplace and Young's theory, since the pressure of liquid cell wall membrane at ① and ② is lower than the pressure at wall membrane ③, the liquid at wall membrane ③ moves to ①, ② two At the same time, due to the influence of gravity, most of the liquid will flow to ②. The amount of liquid flow is proportional to the distance L between the two ends of ① and ②, that is, the larger the distance L, the greater the amount of liquid flow. As a result of liquid dispersion, the bubble liquid film wall will become thinner, which is more unfavorable to the stability of the bubble. The greater the viscosity of the liquid, the more difficult it is to disperse the liquid. Therefore, the increase in the viscosity of the liquid will undoubtedly play a positive role in the stability of the foam. If some catalysts that can promote the liquid gel reaction are added to the liquid reaction system to accelerate the increase rate of the liquid viscosity, the flow rate of the liquid film will be reduced, which is beneficial to the increase of the stability of the foam. In the same way, when the temperature of the material system increases, the viscosity of the liquid will decrease, the surface tension will decrease, and the tendency of the bubble liquid wall to thin will increase, which will accelerate the rupture of the bubble wall film.
In addition, there is also a double-sided electrical effect in the foam stabilization. For example, the ionic surfactants on both sides of the bubble film wall, due to the expansion of the bubbles, will be trapped inside and outside the liquid film when the liquid film is pulled. There is a charge barrier on the surface. When the wall surface is close, the repulsive force of the two electric properties will prevent the bubble film wall from thinning, and the van der Waals force on both sides of the liquid wall will make the bubble wall film thin under the action of mutual attraction. But this force is relatively weak.
