Formation of Polyurethane Foam

The foaming process of plastic foam can be divided into several stages, as shown in the diagram below. In the initial stage (Interval I), carbon dioxide is generated by the reaction between isocyanate and water, or an external foaming agent (low-boiling solvent) suddenly vaporizes due to the reaction heat, causing a rapid increase in the gas concentration in the reactant material. Once the gas concentration exceeds a certain equilibrium saturation concentration, fine bubbles begin to form in the solution. This process is usually referred to as nucleation.

Relationship between Gas Concentration Changes, Nucleation, and Foam Pore Growth in Solution

This self-nucleation process (in Interval II) continues until the gas concentration reaches a certain range. When nucleation progresses to the point where no more micro-bubbles are generated, the gas concentration in the solution continues to decrease, primarily through diffusion, as gas escapes into the already formed micro-bubbles (Interval III). As the gas concentration gradually decreases and no more gas is produced, it reaches an equilibrium saturation concentration. After this stage, no new bubbles form, and the gas in the small bubbles either merges into larger bubbles through diffusion, forming coalesced bubbles, or expands due to heat.

The beginning of Interval II can be observed from the change in the reactant material turning white during the foaming process. Thus, the time for Interval I roughly corresponds to the cream time in the foaming process, which is typically around 10 seconds. The duration of Interval II is harder to determine precisely but is generally considered to be shorter than the rise time (the time required for the foam to reach its maximum volume). In typical foaming formulations, the rise time is approximately 60 to 120 seconds. When the foaming is complete, Interval III is almost finished as well.

Most foaming formulations include certain foam nucleating agents, such as well-dispersed silicone oil. This is particularly effective in the foaming process of polyether-type polyurethane foam plastics. The role of nucleating agents is to ensure rapid and continuous nucleation even at lower degrees of gas supersaturation, resulting in dense and uniform foam pores.

Additionally, pre-dissolving a certain amount of gas in the reactant material is beneficial for foaming and promotes early nucleation. For example, in industrial foaming machines, a small amount of air is often injected into the mixing head to adjust the pore size. Similarly, installing a larger orifice plate in the mixing head to create negative pressure or increasing the gap in the mixing head sleeve to facilitate air intake can also improve pore structure.

Conversely, in polyether-type prepolymer foaming systems, when foaming is performed with prepolymers typically used in industrial production, degassing the prepolymer before foaming to remove dissolved gases results in foaming that merely boils and ruptures continuously, failing to produce the desired foam plastic, regardless of the concentration of silicone oil (nucleating agent) added. However, when carbon dioxide, air, nitrogen, butane, or well-dispersed solid particles (such as quartz powder) are pre-added to the same prepolymer before foaming, uniformly foamed plastic products are obtained. The main reason is that these additives facilitate foam nucleation.

In the diagram, the relationship between gas concentration changes, nucleation, and foam pore growth also explains why strong gas-producing catalysts (i.e., those with a strong catalytic effect on the reaction between isocyanate and water), such as tetramethyl-1,3-butanediamine, produce better pores compared to ethylmorpholine in prepolymer foaming. Stronger catalysts can quickly increase the gas concentration in the material, speeding up the entry into the self-nucleation zone, promoting rapid nucleation and the formation of more bubbles, resulting in satisfactory pores.

Adding silicone oil or other additives to reduce the liquid surface tension is also very important. It allows for easier self-nucleation at lower gas concentrations compared to materials without reduced surface tension.