How to Fix a Sticky Surface on Flexible Polyurethane Foam?

1. Nature of the Defect

“Sticky surface” mainly refers to the presence of unreacted raw materials (such as polyols, isocyanates, and catalysts) or residual low-molecular oligomers on the foam surface. The root cause lies in incomplete polymerization or insufficient conversion of raw materials.

2. Core Cause Analysis

2.1. Insufficient Reactivity of the System

Low temperature: When the raw material (feed temperature) or mold temperature is too low, catalyst activity decreases significantly, slowing down the reaction. Typically, when the feed temperature is below 20 °C or the mold temperature is below 45 °C, this risk should be closely monitored (specific thresholds vary by formulation).

Improper catalyst selection: Insufficient dosage or low activity of the catalyst (especially the gelling catalyst) can lead to a slow gelation reaction, causing the polymer network to remain incompletely cured at demolding.

2.2. Improper Selection of Key Raw Materials

Low-activity polyol: Using polyether polyols with high unsaturation (e.g., above 0.06 meq/g for conventional flexible foam polyethers) or low primary hydroxyl content slows down the reaction rate and easily leads to incomplete conversion.

Poor isocyanate purity: When the isocyanate used contains excessive free TDI, unreacted TDI monomers may volatilize during foaming and condense on the cooler foam surface, directly causing surface stickiness.

2.3. Inadequate Post-curing (Maturation)

If the foam is not sufficiently cured after foaming, unreacted low-molecular materials remain on the surface. Inadequate curing time (e.g., only a few hours or 12 hours at room temperature for certain formulations or thick products) does not allow enough time for residuals to migrate and complete secondary reactions.

3. Solutions and Preventive Measures

3.1. Process Optimization (Fundamental Adjustment)

Increase reaction temperature:
Keep feed temperature stable between 25–30 °C and mold temperature between 50–60 °C to provide sufficient activation energy. If the issue originates from the catalyst, consider switching to a more active type (e.g., DABCO 33-LV) and, within safe limits, increase its dosage slightly (avoiding core burning or cracking due to overreaction).

Ensure sufficient curing:
Establish a standard curing process — allow the foam to pre-cure at room temperature for several hours, then transfer it to a 50–60 °C curing room for 12–24 hours (depending on formulation and product size) to ensure full reaction and volatilization of residues.

3.2. Raw Material Adjustment (Fundamental Solution)

Select high-activity polyols:
Replace with high-reactivity polyether polyols (for conventional-density flexible foam, choose products with hydroxyl values of 60–70 mg KOH/g and unsaturation below 0.05 meq/g) to fundamentally improve reaction efficiency and conversion.

Use high-purity isocyanates:
Prefer TDI prepolymers with low free-TDI content (below 0.5%) or higher-purity MDI systems to minimize volatile monomer residues from the source.

3.3. Surface Treatment (Temporary Remedies)

These methods apply to already defective products as emergency measures but cannot permanently eliminate the root cause.

Hot-air drying:
Bake the foam surface at around 80 °C for about 2 hours using circulating hot air to promote further reaction or volatilization of surface residues. Note: Ineffective for deeper internal stickiness.

Light talcum powder coating:
Evenly apply a small amount of food-grade talcum powder to the sticky surface to temporarily remove tackiness through physical adsorption. Note: May affect appearance and subsequent processes such as lamination or gluing.

Conclusion

Stable long-term production must rely on fundamental optimization of raw materials and core process parameters. Emergency treatments can only serve as temporary fixes.