Why Does Polyurethane Foam Collapse? Revealing the Imbalance Mechanism Between Gas Generation and Gelation

Blown‐sugar figurines are a traditional Chinese craft: the artisan heats a lump of sticky syrup, blows air into it through a tube while shaping it by hand. The key to success is: air must be blown in while the syrup is transitioning from fluid to viscous to firm.

If the syrup is too thin, bubbles burst instantly; if it is already stiff, air can no longer be blown in.

The polyurethane foaming process is exactly like blown sugar. At its core, it is a dynamic race between gas generation (blowing reaction) and viscosity-building solidification (gelation reaction). Once gas expansion exceeds the real-time mechanical strength of the polymer skeleton, the foam loses structural support—microscopically as cell rupture, locally as cracking, and macroscopically as full-scale collapse.

I. Cell Rupture Analysis: The Microscopic Manifestation of Insufficient Gelation

Cell rupture refers to the breaking of bubble membranes due to inadequate strength during foaming, representing an early typical sign of reaction imbalance.

• Microscopic warning signs: Before rupture occurs, observation of freshly rising foam shows: internal bright spots (cell membranes) decrease, struts lose gloss, then strut breakage appears, eventually evolving into ruptured cells.

• Causation: The core issue is insufficient gelation. If the skeleton does not yet have adequate viscosity and strength, bubble films cannot withstand internal pressure. A moderate increase of stannous octoate (tin catalyst) usually resolves this.

• Physical impact: Rupture triggers bubble coalescence and gas leakage. Severe rupture significantly weakens foam physical properties, causing loss of support force and becoming a precursor to large-scale collapse.

II. Cracking Study: Multi-Factor Stresses Creating Local and Global Damage

Cracking occurs when gas generation and gelation become poorly synchronized, causing structural tearing. Different crack locations indicate different levels of collapse risk.

• Internal cracks, top cracks, and bottom-corner cracks: Excessive instantaneous gas generation (too much amine or water) bursts through a weak, not-yet-shaped gel skeleton. Internal cracks are especially deceptive—surface looks normal, but internal structure is already torn.

• Overall cracking and coordination failure: Poor coordination between fall-plate/sinking plate and the foaming reaction—such as excessively fast rise or improper plate position—causes large structural cracks. These large tears cause rapid pressure loss, directly triggering extensive collapse.

• Mechanical-friction cracking: Poor bottom-paper flatness, excessive conveying resistance, or side-paper misalignment can cause penetrating mechanical tears. When gel strength is low, such physical damage rapidly expands, leading to structural failure.

• Environmental temperature differences: Low ambient or material temperature prolongs the foam’s fragile phase, making all types of cracks more likely.

III. Overall Foam Collapse: Systemic Failure of Core Formulation and Machine Parameters

When local defects spiral out of control or key components are severely insufficient, catastrophic collapse occurs.

• Core formulation balance: Severely insufficient stannous octoate or an overly low TDI index are among the most common causes. Without timely crosslinking, the polymer network cannot trap gas, and foam collapses immediately after rise.

• Effect of solid fillers: In low-density large-cell foams, white oil and calcium carbonate levels are extremely sensitive. Excessive filler weakens or disturbs the stability of uncured bubble membranes, easily triggering chain rupture leading to collapse.

• Continuous foaming machine parameters: Collapse at the foam head often relates to the distance between polyol feed and mixing chamber, and the transient RPM at startup. Any momentary lag in component delivery causes formulation imbalance during rise.

• Shrinkage caused by closed-cell formation: This is a special type of collapse (secondary collapse). When cell membranes harden too quickly and trap the gas, cooling generates negative pressure. If the skeleton solidifies slowly, the foam shrinks inward like vacuum suction.

IV. Scientific Prevention and Process Optimization

To control foaming defects, the key is precise tuning to ensure gelation strength can support and lock in internal gas pressure in real time.

• Increase skeleton viscosity: Appropriately increase stannous octoate or raise the TDI index to ensure structural strength before CO₂ generation reaches its peak.

• Balance internal pressure: Precisely control amine and water levels to prevent overly rapid gas generation from breaking the early skeleton and causing cracks at various positions.

• Optimize startup procedures: Use stepped RPM to reduce material fluctuations during initial mixing and ensure stability in continuous foaming.

• Process diagnosis: Carefully observe the strut structure and bright spots of freshly grabbed foam—one of the most direct and practical methods to distinguish collapse, rupture, and cracking.

Production Overview

Polyurethane foaming is not a simple mixing of components but a rapidly evolving chemical marathon. To handle collapse and cracking, engineers must look beyond single-component adjustments and develop a system-level understanding of reaction balance. Only when gas generation (blowing) and gelation (hardening) remain reasonably synchronized at critical stages can structural collapse be prevented and stable, uniform foam quality ensured.