Why Do HR Foam and Memory Foam Have Such Different Gel-to-Cream Time Ratios?

In the production of flexible PU foam, the ratio of gel time to cream time is a key indicator of foaming characteristics. Different types of foam, especially high resilience (HR) foam and slow resilience (memory) High Resilience PU Foamfoam, show significant differences in this ratio. Typically, memory foam has a much higher value compared to HR foam, which is determined by their molecular structures, performance demands, and production processes.

Core Parameter Comparison

To better illustrate this difference, let’s compare the foaming times of these two foams:

High resilience PU foam: Cream time (appearance of milky color) is typically 15–18 seconds, while gel time (foam solidification) is 100–130 seconds. The ratio is about 6.0–7.2.

Slow resilience PU foam: Cream time is shorter, 9–15 seconds, but gel time is longer, 80–100 seconds. The ratio can reach 8.0–11.0.

This clear data gap highlights the fundamental differences in chemical reaction rates, driven by the following factors:

1. Fundamental Molecular Structure Differences

HR foam: Built mainly with low cross-link density flexible chains (e.g., EO/PO polyether). These reactive chains crosslink rapidly once foaming starts, forming a firm network. Thus, gel time quickly follows cream time, yielding a lower ratio.

Memory foam: Designed for viscoelasticity, it incorporates higher cross-link density structures or modified polyurea components, combining rigid crosslinking points with flexible chains. This reduces reactivity, prolonging the time from expansion to final solidification, resulting in a higher ratio.

2. Performance-Driven Foaming Behavior

HR foam: Its main function is fast resilience. Foam must quickly form and solidify bubbles, ensuring structural stability for instant rebound under pressure. This functional demand drives a smaller gel-to-cream ratio.

Memory foam: Known for slow recovery and pressure distribution, it requires prolonged shaping to “memorize” pressure profiles. Longer gel time enables gradual bubble stabilization and stress dispersion, achieved through a larger gel-to-cream ratio.

3. Production Process Adaptations

HR foam: Production favors speed and efficiency. Catalyst adjustments shorten both cream and gel times to match high-speed manufacturing lines.

Memory foam: With inherently slower reactions and lower exotherm, production lines must run slower to allow full foaming and curing. This adaptation reinforces the need for a higher ratio to achieve the desired properties.