Why Does Flexible PU Foam Fail to Recover After Compression?

Have you ever wondered why some flexible PU foams (commonly called sponges) become thinner and collapse after long-term compression and cannot return to their original shape? This phenomenon in the industry is called “high compression permanent deformation,” a key indicator of material durability and resilience. Its root causes can be attributed to: insufficient strength of the molecular network that forms the basis of foam elasticity, defects in the internal micro-cavities (cells), or improper process control during production. These factors together prevent the material from effectively resisting and recovering from external pressure.

I. Raw material formulation: the inherent weakness of the molecular "skeleton"

The foam’s elasticity originates from its internal three-dimensional network formed by intertwined polymer chains. The components in the formulation are like a building’s blueprint and directly determine the stability of that “structure.”

Polyols: selection and balance of elastic chains

Imbalance in segment flexibility: If polyols with excessively high functionality or too much rigidity are chosen (for example, certain polyester types or sucrose-based polyethers), the polymer chain segments lack sufficient pliability and are prone to irreversible plastic deformation under external force. Conversely, if ethylene oxide (EO) or low-molecular-weight glycols are overused to pursue softness, the polymer chains become overly limp and suffer a sharp drop in strength; under compression, relative permanent slippage between chain segments is likely. Studies have shown that when EO content increases from 10% to 30%, the material’s compression permanent set may rise from 8% to over 15%.

Defects in chain integrity: The “unsaturation” of polyether polyols is a key quality indicator; it reflects the number of incomplete end structures on the polymer chains. When unsaturation exceeds 0.05 mol/kg, these defect points on the chains become weak spots under stress, prone to breakage or deformation, thereby compromising the network’s overall rebound integrity.

Isocyanates: the "adhesive" of the crosslinked network

Incorrect stoichiometry (R-value): If the isocyanate index (R-value, i.e., the NCO/OH molar ratio) falls below 0.95, it means some polyol did not participate in the reaction; these residues act like impurities within the network and significantly reduce crosslink density. As a result, the polymer network becomes loose and cannot effectively constrain chain segment movement. For example, lowering the R-value from 1.0 to 0.9 may increase compression set by 20% to 30%.

Single-type selection: Using only TDI (especially TDI-80) produces urea-based structures that are relatively rigid; if flexible urethane-link-forming MDI is not incorporated in an appropriate amount (e.g., less than 10%), the entire network lacks necessary toughness, leading to reduced rebound performance.

Additives: the overlooked "double-edged sword"

Excess plasticizer: Adding more than 8% plasticizer (such as phthalates) greatly weakens intermolecular interactions; acting like a “lubricant,” it makes chain segments more likely to slide under pressure and remain in the new position.

Catalyst imbalance: When blowing catalysts (e.g., amines) that promote cell formation are used in excess while gelation catalysts (e.g., tin-based) that control molecular chain growth are insufficient, cells expand too quickly and over-stretch cell walls that are not yet firm. These over-stretched structures cannot recover after curing and become initiation points for permanent deformation.

Improper selection of surfactant (silicone oil): Using the wrong type of silicone oil (for example, high-stability grades designed for rigid foam) or adding it in excess (>3%) can form overly rigid films on cell walls that impede elastic deformation. Conversely, too little (<0.5%) will cause cell coalescence and structural unevenness, creating weak points prone to rupture.

II. Cell structure: failure of the microscopic "air chamber" function

Ideal cells should be uniformly distributed, open-celled, and have appropriately thick walls—like micro air-springs. Any structural deviation will lead to functional failure.

High closed-cell content (>10%): A healthy soft foam should be predominantly open-celled so that upon compression air can be easily expelled and upon rebound reintroduced to restore shape. Excessively high closed-cell content traps air within cells; during compression this creates high-pressure zones that rupture cell walls, and on rebound the internal negative pressure cannot replenish air, making recovery difficult. This is often caused by excess silicone oil or excessively high reaction temperatures.

Uneven and overly large cell sizes: When the average cell diameter exceeds 500 μm and size variation is significant, external forces concentrate on large cells (>800 μm) with thin walls (<5 μm), causing them to rupture and collapse first, triggering a chain reaction. Insufficient mixing and excessive blowing agent are the main causes.

Imbalanced cell wall thickness: Walls that are too thin (<3 μm) lack mechanical strength and tear easily; walls that are too thick (>10 μm) lose necessary flexibility and become brittle, failing to effectively distribute stress through deformation under compression.

III. Manufacturing process: "permanent damage" during production

Even an excellent formulation can fail due to poor processing. Every step in production is critical.

Inadequate mixing: Low mixing speed or too-short mixing time can cause local agglomeration of raw materials. Locally there may be regions with excess isocyanate (hard/brittle spots) or excess polyol (soft points), causing uneven and amplified overall deformation under compression due to these weak spots.

Temperature control failures: Raw material or ambient temperatures that are too low (<15 °C) will slow the reaction, allowing unset cells to undergo “pre-deformation” under their own weight and then be locked in. Too high temperatures (>55 °C) make the reaction violent, leading to local scorching or increased closed-cell content.

Insufficient curing: After demolding, the foam needs to continue reacting at 50–60 °C for 24–48 hours; this process is called curing. If time is insufficient or temperature inadequate, the molecular crosslinking reaction will not be completed, and residual monomers, acting like plasticizers, will significantly reduce the material’s final mechanical properties and cause a surge in compression set.

Summary and Countermeasures

The root causes of high compression permanent deformation in flexible PU foam are systemic, involving three dimensions: molecular design (formulation), structure formation (cells), and curing process (processing). The solutions require a systematic approach:

Optimize formulations: Select flexible polyols, balance isocyanate types and R-value, and use additives cautiously.

Control structure: Use processing and additives to ensure the formation of ideal open, uniform cells with appropriate wall thickness.

Standardize processes: Ensure thorough mixing, precise temperature control, and provide adequate post-curing time.

The high compression set of flexible PU foam is a complex issue arising from formulation, structure, and processing. To thoroughly resolve it, a systematic engineering approach is required: optimizing selection at the molecular design level, precisely controlling cell morphology, and strictly executing during production. Coordinated efforts across these three dimensions can effectively minimize compression set, ensuring the product achieves outstanding durability and stable performance to meet the stringent demands of industries worldwide.