How to Solve the Large Cell Problem in Flexible PU Foam Sponge?

Large cells in flexible PU foam sponge are a common production challenge that can significantly impair physical properties such as resilience, compression set, and air permeability. To address this issue fundamentally, it is necessary to conduct an in-depth analysis of the complex factors involved. This article systematically examines the causes of large cells from the perspectives of raw material micro-properties, formulation balance, precise process control, and dynamic environmental influences, and proposes corresponding solutions.

1. Influence of Raw Materials on Cell Structure

Polyether Polyol
As the core raw material for flexible foam, the molecular weight distribution of polyether polyol is a key determinant of cell uniformity. A wide molecular weight distribution indicates a high proportion of low-molecular-weight components. These components react much faster than higher-molecular-weight parts, causing localized, intense reactions in the early foaming stage and generating large amounts of gas rapidly. Such uneven reaction speeds lead to asynchronous cell growth, forming large cells in high-reactivity regions. Moreover, high unsaturation in polyether can trigger side reactions, delaying gelation and weakening the strength of the cell walls.

Blowing Agent
Excessive use of blowing agent produces more gas than the cell walls can withstand. The resulting high internal pressure can rupture cell walls before curing, causing cell coalescence—similar to a balloon inflating too fast and bursting, merging smaller balloons into one large one.

Isocyanate
A high isocyanate index means excessive isocyanate usage, which not only accelerates foaming but also reacts with water to produce excess CO₂ gas. This dual acceleration causes cells to expand rapidly in a very short time, exceeding the stabilizing ability of the foam stabilizer and the gelation speed of the cell walls, ultimately leading to rupture and coalescence.

2. Chain Reactions of Formulation Imbalance

Catalyst
Catalysts coordinate the foaming (gas generation) and gelation (cell wall formation) reactions. Insufficient catalyst dosage or low activity delays foaming relative to gelation. If the cell walls have not yet gained enough strength when gas begins to form and expand, they collapse or merge. Inappropriate catalyst selection—such as an imbalanced ratio between amine and organotin catalysts—also disrupts reaction balance and cell uniformity.

Foam Stabilizer
Foam stabilizers, usually silicone-based, act as a “protective film” for cell structure. They form an elastic, strong membrane over cell walls to prevent rupture during growth. Insufficient dosage or improper type results in weak or uneven protective films. When foaming generates gas, these fragile films cannot withstand internal pressure and surface tension, causing rupture and coalescence.

3. Synergistic Effects of Process and Environment

Mixing
Mixing ensures even dispersion of raw materials and complete reaction. Excessive mixing speed can cause over-shearing, leading to localized heat buildup and uneven concentration, triggering local “over-foaming.” This accelerates reactions in certain regions, producing large cells.

Foaming Temperature
Temperature is a key variable controlling reaction speed. Excessively high foaming temperature intensifies the isocyanate–water reaction, generating extra gas and accelerating all reactions, causing cells to over-expand before curing. Conversely, too low a temperature slows reactions, delays gelation, and destabilizes cell structure.

Environmental Factors
High humidity introduces extra water, which reacts with isocyanate to produce excess CO₂, increasing internal pressure and causing rupture. Atmospheric pressure fluctuations also affect cell expansion. In low-pressure environments, reduced external resistance allows cells to over-expand. Advanced foaming technologies, such as variable-pressure foaming, utilize controlled pressure to regulate cell size.

In conclusion, the root cause of large cells in flexible PU foam lies in “imbalance.” Whether it is the imbalance in raw material reactivity, the mismatch between foaming and gelation speeds, or unstable process and environmental control, any of these can disrupt stable cell growth. Therefore, solving this problem requires a systematic approach and meticulous management to ensure every stage—from raw material entry to production completion—is precisely controlled.