Changes in Viscoelasticity and Elasticity During the Polyurethane Foam Foaming Process

To understand the mechanism of foam formation, it is essential to grasp the viscoelastic and elastic changes of polyurethane materials during foam formation. When preparing flexible polyurethane foam, the liquid raw materials initially undergo a rapid increase in viscosity as they generate bubbles through reaction, gradually transforming into a colloid, and then further evolving into a highly elastic polymer. Similarly, rigid polyurethane foam is formed when liquid raw materials gel to form a thermoset polymer. The changes in viscoelasticity and elasticity of these materials are crucial for foam structure and stability.

During the formation of flexible polyurethane foam, two simultaneous changes occur in the system:

Gas generation causes expansion and growth of the material by forming bubbles that rise within the system.

The raw materials undergo reactions where molecular chains grow, branch, and crosslink, transforming the initially liquid material into a high-viscosity, soluble thermoplastic polymer with moderate molecular weight. Further reactions lead to branching and crosslinking, increasing molecular weight and transitioning from linear to networked molecular structures. In an ideal state, this network structure exhibits minimal viscosity, with dynamic viscosity almost approaching zero.

Research using oscillatory resonance rheometers has studied the viscosity and elasticity changes during polyether prepolymer foaming, revealing trends in foam formation as depicted in the following graph.

While these curve variations may slightly differ from those observed in industrial foam production, they reflect similar trends in polyurethane foam formation, offering practical insights. Based on measurements, the time to reach maximum foaming height is approximately 7 minutes (25 minutes in actual production of polyether prepolymer-based flexible polyurethane foam), maximum dynamic viscosity peaks around 10 minutes, followed by a decline, and achieving maximum elasticity takes around 100 minutes.

The graph also illustrates that during gas generation, the volume of gas produced consistently exceeds the volume growth of the foam. In the final stages of significant gas generation, although the dynamic viscosity of the bubble walls increases rapidly, making their flow difficult, the elasticity growth curve remains in a slow growth phase. At this stage, the bubble walls fail to meet the gas expansion demands, resulting in ruptured bubble walls and the escape of gas, forming an open-cell foam structure.

If, during the peak gas generation, the system's molecular weight growth is slow and the strength of the foam cell walls and network framework is insufficient to withstand the gas expansion pressure, the foam walls inevitably rupture. Due to the low activation energy required for foam wall rupture, once it starts, it spreads further, accelerating the entire foam expansion and causing foam collapse. Localized continuous ruptures result in hollow or cracked foam structures.

Therefore, controlling the occurrence of adverse effects such as hollow or cracked foam structures in polyurethane foam primarily involves controlling the reaction rates of gas generation and molecular weight growth, i.e., gel reaction rates.

In the one-step process for polyurethane foam plastic production, the balance between foaming reactions and gel reactions of molecular chain growth is achieved by varying the type and amount of catalysts used.

Basic studies on catalysts reveal that amine compounds primarily adjust the reaction between isocyanate and water, catalyzing both carbon dioxide gas generation and chain growth reactions. Organic tin catalysts mainly promote the reaction between isocyanate and hydroxyl compounds, catalyzing polymer chain growth reactions.

Adjusting the dosage of these two types of catalysts significantly impacts the preparation of polyurethane foam. For example, when the amount of organic tin catalyst is reduced, the rate of polymer molecular growth slows down, reducing foam cell wall elasticity and making it prone to forming open-cell structures.

By appropriately increasing the amount of organic tin catalyst or reducing the amount of organic amine catalyst, the strength of the foam cell walls can be enhanced at the peak gas generation stage, reducing the occurrence of foam voids or cracking. Hence, the dosage of organic tin catalyst has a significant and sensitive effect on foam structure.

Under normal formulation conditions, insufficient organic tin catalyst dosage results in hollow foam formation, while an appropriate dosage balances gas generation reactions and gel reactions, forming fine and open-cell foam structures. However, an excessively high dosage accelerates polymer chain growth rates, increases foam cell wall strength, and results in the formation of a large number of closed-cell foam structures.

Using a 3000 molecular weight trihydroxy polyether polyol as raw material and employing a one-step foaming process, the influence of dibutyltin dilaurate catalyst dosage on foam cell structure has been specifically studied, as shown in the following graph.

The results are similar to those discussed earlier. Additionally, it is observed that:

Large-pore foams exhibit greater tolerance to organic tin catalysts compared to small-pore foams and are more likely to form open-cell structures.

Conclusions related to closed-cell formation based on polymer growth rates and branching degree have been validated in practical production. Materials with high branching quickly form network structures, increasing the probability of molecular elasticity. Under constant conditions, increasing the activity of isocyanate also increases the closed-cell rate of foam. Similarly, increasing the branching degree of polyol polymers shows a similar tendency. For example, early American imports of polyurethane foam production technology from Germany used 80/20 TDI with higher activity, replacing the original 63/35 TDI formulation. Due to the higher activity of this isocyanate, the polymer chain growth rate increased, resulting in severe foam shrinkage. Later, compensating with lower branching polyether polyols resolved the issue of severe foam shrinkage.

Combined with the first graph, changes in foam structure due to varying rates of dynamic viscosity growth (Curve A) and elasticity growth (Curve B) under constant gas generation rates can be observed.

Increasing Curve A, i.e., increasing the rate of dynamic viscosity growth, has minimal impact when the increase is small. However, a significant increase thickens the foam cell wall, leading to closed-cell structure formation.

Decreasing the rate of dynamic viscosity growth (lowering Curve A) prolongs the flowability of the foam cell walls, increasing the likelihood of closed-cell formation. If the decrease in dynamic viscosity occurs too rapidly, it may result in excessively thin foam cell walls, premature bubble rupture, and foam collapse.

Increasing the rate of bubble elasticity growth (raising Curve B) promotes closed-cell structure formation.

Lowering the rate of bubble elasticity growth (lowering Curve B) leads to inadequate strength in the bubble network, causing the bubble walls to rupture under pressure from gas expansion and subsequent spreading, resulting in hollow or collapsed foam.

Unlike flexible foam plastics, rigid polyurethane foam plastics require foam bodies with high bubble wall strength and closed-cell structures. Therefore, in material selection, polymers with high branching structures are preferred to ensure sufficient foam wall strength and produce foam products with good dimensional stability.