How to Determine Water Dosage in Flexible PU Foam Formulations?

The conventional foam discussed in this article mainly refers to traditional flexible PU foam produced with conventional polyether polyol with a hydroxyl value of approximately 56 and a molecular weight of approximately 3,000, together with TDI-80.

1. Adjusting Foam Density with Water and MC

In the formulation, 1 mole, or 18 g, of water reacts to generate 1 mole, or 22.4 L, of carbon dioxide. One mole, or approximately 85 g, of MC produces 22.4 L of MC vapor after vaporization.

The actual volume contributed by these gases as the foam develops and stabilizes is also affected by factors such as reaction rate, internal foam temperature, and MC vaporization behavior.

Therefore, the amount of water used in a conventional foam formulation is not necessarily fixed.

When the other main formulation and processing conditions are similar, and the TDI dosage is adjusted according to the water level and target TDI index, formulations such as 3.7 parts water + 9 parts MC, 3.8 parts water + 8 parts MC, and 3.9 parts water + 7 parts MC can all work under suitable processing conditions. They can also produce foam with broadly similar densities, even though the water dosage is different.

2. Adjusting the Thermal Balance with Water and MC

The reaction between water and TDI releases heat, while MC absorbs heat during vaporization. Adjusting the amounts of water and MC is therefore an important means of controlling the thermal balance inside the foam.

Using the same three formulations as examples:

• Formula 1: 3.7 parts water + 9 parts MC;
• Formula 2: 3.8 parts water + 8 parts MC;
• Formula 3: 3.9 parts water + 7 parts MC.

All three formulations can work under suitable processing conditions and can produce foam with broadly similar density, but their heat loads are different. In general, Formula 1 has the lowest heat load, followed by Formula 2, while Formula 3 has the highest.

The thermal balance inside the foam further affects the peak internal temperature, the overall reaction rate, and the blow-off time.

Under similar amine catalyst, tin catalyst, and TDI index conditions, Formula 1 will typically show a lower peak internal temperature, a slower overall reaction, and a relatively longer blow-off time.

3. Water and the Catalyst System Jointly Affect the Reaction

The conventional polyether polyols used in this type of foam are relatively low in reactivity, so their reaction with TDI proceeds comparatively slowly.

In medium- and low-density conventional foam formulations, however, the amounts of water and TDI are relatively high. The reactions between water and TDI, including subsequent urea formation, are therefore an important source of heat inside the foam.

Amine catalysts have an important influence on the water–TDI blowing reaction as well as the overall reaction balance. Therefore, in conventional foam formulations, the water dosage needs to be adjusted together with amine catalysts, tin catalysts, and other reaction conditions.

4. Static and Dynamic Analysis

The first three sections mainly examine the relationships among water, MC, and reaction behavior from a relatively static perspective. In actual foaming, however, these factors also need to be understood dynamically.

The first dynamic process is the balance between heat generated by the water–TDI reaction and heat absorbed by MC during vaporization. Material temperature, the catalyst system, and the overall reaction rate all influence how heat develops inside the foam during this stage.

The second dynamic process is the continuous expansion of the foam. As the foam volume increases, MC continues to vaporize and absorb heat while the foam also transfers heat to its surroundings. These processes can slow the rate at which reaction heat raises the internal foam temperature.

The third dynamic process is the overall rise in internal foam temperature as the exothermic reactions continue. As the temperature rises, the rates of the various reactions also increase, while the gases inside the foam undergo thermal expansion. Under approximate conditions, for example, a gas heated from 20°C to 100°C expands to about 1.27 times its original volume.

Based on experience with conventional foam formulations of similar density used in different regions, one noticeable difference is often the ratio between water and MC. The amine catalyst system can then be adjusted to accommodate differences in raw materials, local conditions, and production environments.

In actual production, one noticeable formulation difference between horizontal foaming and vertical foaming is that the water-to-MC ratio is often different.

In vertical foaming, the rising foam needs to develop sufficient structural strength relatively quickly during expansion. This places greater demands on reaction rate and the development of foam structure. In some traditional flexible foam formulations, a higher proportion of water is used to increase urea formation and adjust foaming behavior.

Therefore, in these types of formulations, vertical foaming at the same target density often uses a higher water-to-MC ratio than horizontal foaming.

Overall, once the target density and hardness of a medium- or low-density conventional foam formulation have been established, the water dosage does not necessarily have a single fixed value and can be adjusted within a certain range.

However, changing the water dosage does more than alter the amount of blowing gas. It also affects TDI demand, urea formation, reaction heat, and the overall reaction balance.

For this reason, the final formulation needs to consider MC dosage, TDI index, catalyst system, raw material temperature, and the specific foaming process together, so that it can be adapted to different raw materials, machines, ambient temperatures, oscillating-head foaming, overflow-trough foaming, horizontal foaming, vertical foaming, and other production conditions.