Introduction

In the manufacturing of flexible PU Foam, the dynamic change of internal temperature is a core control variable. Temperature fluctuations directly affect the rates of all chemical reactions, significantly altering the complex interplay of mutual promotion and restraint among reactants. Therefore, accurately grasping the rate of temperature rise inside the foam is critical during formulation design. Drawing on relevant technical literature, this article examines—through a more systematic lens—the generation, evolution, and control strategies of internal foam temperature.

Heat Sources Inside the Foam and the Phased Heat-Release Mechanism

The internal temperature of the foam mainly comes from exothermic chemical reactions. The heat release is not constant; it follows a staged, dynamic pattern.

Stage 1: Intense Heat Release in the Early Foaming Period

This stage occurs primarily while the foam volume rises rapidly. The heat release is dynamic and substantial, yet because the foam system is expanding sharply at the same time, the heat is effectively dispersed. You can imagine a rapidly expanding universe: although countless “stars” (reactions) burn intensely and release energy, the expanding space keeps the average energy density (temperature) per unit volume effectively in check.

Stage 2: Slow Heat Release During Cure and Crosslinking

Once foam expansion stops, the system enters the curing stage. The heat released is smaller and slower but not negligible. This is akin to heating in a closed container: even a small amount of added heat will continue to accumulate because the volume is fixed. In the foam system, any subsequent heat can no longer be dispersed via volumetric expansion, so its cumulative effect on internal temperature becomes more pronounced.

Core Reaction Kinetics and Heat-Control Factors

In the early foaming period, the main heat source is the reaction between water and isocyanate. Its exotherm rate is constrained by multiple factors, including reactant concentrations, the isocyanate index, and catalyst levels. Notably, while releasing a large amount of heat, this stage also generates large volumes of gas.

In the heat balance, the vaporization rate of dichloromethane plays a key role because it directly affects the rate of temperature rise. Although the boiling point of dichloromethane is around 40 °C, practice shows a vaporization lag. Even after foam degassing, when the internal temperature exceeds 140 °C, up to 30% of the dichloromethane may remain unvaporized, trapped in the polymer matrix. Similarly, polyether polyols display a reaction lag: at the time of cell opening, more than half of the polyether can remain unreacted, which relates to the complexity of their polyhydroxy structures.

Because both heat release and gas generation are dynamic variables—and gas volume itself changes with temperature—the foam exhibits a certain degree of self-regulation of temperature in the early stage. A typical example: under different seasons or different initial raw-material temperatures, final product density varies even when the formulation is unchanged. This reflects the system’s self-adjustment to the temperature rise. In the later stage, the main heat source comes from further crosslinking and secondary reactions. The peak exotherm here correlates closely with water content in the formulation and is only weakly related to the isocyanate index.

Isocyanate Index and Temperature-Control Strategies

The Complex Influence of the Isocyanate Index

Raising the isocyanate index does not have a single-direction effect on foam temperature. Two opposing effects occur simultaneously: on one hand, it reduces the water–isocyanate reaction rate, lowering the early-stage temperature rise; on the other, it increases the extent of later crosslinking reactions, intensifying the late-stage temperature rise. Therefore, the net heating or cooling effect depends on the combined outcome of these two effects and must be evaluated with targeted experiments and chemo-thermodynamic models.

Temperature-Control Strategies

Foam heat dissipation is closely tied to cell structure. For example, in large-block, low-density foams, internal temperature typically starts to drop about 40–60 minutes after degassing; as density increases, this cooling time extends significantly and can reach 4–6 hours in high-density foams. In addition, the incorporation of inorganic fillers (e.g., stone powder) can alter the overall exotherm curve by influencing reaction rates.

Summary and Outlook

In summary, the foaming of flexible PU Foam is not a simple thermodynamic equilibrium but a complex transient process jointly governed by reaction kinetics, fluid mechanics (foam expansion), and heat- and mass-transfer phenomena. Future formulation design should go beyond adjusting chemical parameters and, from a process-engineering standpoint, precisely steer the pathway by controlling the reaction environment (e.g., external heat removal, mixing temperature). This enables accurate guidance of the process and precise control of product properties.