How Does the TDI Index Control the Rebound Speed of Memory Foam?

In the production of slow-rebound foam (Memory Foam), the main challenge lies in maintaining stable performance and targeted rebound dynamics. The comfort perceived by consumers essentially depends on the material’s deformation recovery rate, which must be precisely controlled within a narrow range.

We should not treat the TDI index as just a formulation number, but as a core engineering parameter for achieving the desired relaxation time.

I. Performance Boundary: Failure Zones Between TDI Index and Network Integrity

In engineering applications, the selection of the TDI index must avoid two extreme failure boundaries to ensure both the structural integrity and functionality of the foam.

High Index Transition Zone (approaching 100): Rebound Function Failure

When the TDI index increases from 95 toward 100, the conversion rate of isocyanate functional groups rises sharply, leading to higher network point density and tighter topological constraints. Macroscopically, the foam’s compression modulus (hardness) and loss factor gradually increase, and the polymer’s glass transition temperature (Tg) shifts above room temperature.

When crosslink density becomes excessive, the network behaves almost entirely as an elastic solid, leaving insufficient viscous damping to delay recovery—thus eliminating the slow-rebound effect completely.

Low Index Failure Zone (below 80): Structural Integrity Failure

When the TDI index drops below 80, the isocyanate content becomes too low for effective polymerization, preventing stable network formation. This results in weak strength, coarse cell structure, excessive open-cell formation, and irreversible degradation of physical properties. In this state, the foam lacks a stable polymer network and cannot be used in practical applications.

II. Mechanism: How the TDI Index Regulates the Relaxation Time Spectrum

Beyond “crosslink density,” the TDI index controls rebound speed by altering the polymer’s glass transition region (Tg) behavior and chain relaxation time distribution.

By influencing the effective crosslink density and intermolecular interactions, TDI index changes modify the Tg region and relaxation dynamics—ultimately affecting the macroscopic deformation recovery rate.

TDI Index Regulation (80–95):
Within this core range, small adjustments to the TDI index can systematically shift the Tg region of the polymer network to partially overlap with room temperature. When Tg approaches the service temperature, chain segment relaxation becomes maximized, allowing the foam to recover slowly after compression—achieving the desired slow-rebound dynamics.

III. Comparative Analysis: Differences Between TDI and MDI Systems

The slow-rebound foam system is not exclusive to TDI. In MDI (diphenylmethane diisocyanate)-based memory foams, the relationship between isocyanate index and rebound performance differs fundamentally.

The TDI system has lower reactivity and milder exothermic behavior, allowing better control of reaction rate. Its ideal index range for slow rebound is narrow—typically 80–95, with a critical point near 100.

In contrast, the MDI system has higher reactivity and tends to form urea linkages, leading to more complex structures. To introduce sufficient viscous damping and extend relaxation time, MDI formulations often require higher indices, even exceeding 105. This is because the MDI system primarily relies on microphase separation between soft and hard segments for physical crosslinking, rather than purely chemical crosslinking density.

IV. Formulation Synergy: Practical Fine-Tuning of the Index

Although the TDI index is central, its actual effect must be fine-tuned within the entire formulation system.

Influence of Polyols:
High-functionality polyols increase initial crosslink points, shifting the foam toward faster rebound. Conversely, low molecular weight or more flexible polyols require lowering the TDI index to remain within the 80–95 “golden zone” to achieve the same target relaxation time.

Effects of Other Components:
Water and physical blowing agents affect pore structure and density, while surfactants such as silicone oils influence cell wall stability. These factors alter the polymer network’s stress distribution and relaxation behavior, demanding precise calibration of the TDI index by the formulator.

Conclusion

The success of slow-rebound foam lies in achieving specific deformation recovery dynamics.

By adjusting the TDI index, engineers can systematically modify the network structure and Tg region, precisely controlling relaxation time—this is the core engineering principle of memory foam formulation design.