How to Choose the Right Blowing Agent for Flexible PU Foam?

Controlling the dosage of blowing agents is key to producing high-quality foam. The core lies in synchronizing gas generation with polymer curing. Only when these two processes are balanced can the foam achieve uniform cell structure, target density, and defect-free performance. This requires not just theoretical calculations but also a comprehensive consideration of multiple factors and dynamic adjustments in actual production.

I. Defining the Core Standards of “Ideal Foaming Effect”

Before adjusting the dosage, the target must be clear. The ideal foaming effect is evaluated by the following key indicators:

Cell structure: Uniform pore size, deviation within 20%. Flexible foams require >90% open-cell rate to ensure rebound performance.

Foam density: Actual density should deviate less than 5% from target. For sofa foams, the target is typically 30–40 kg/m³.

Defects: No scorching (burned yellow core), collapse, or shrinkage.

Mechanical properties: Rebound rate, tensile strength, and tear strength must meet design standards. High resilience foam requires >60% rebound.

II. Determining Basic Dosage by Blowing Agent Type

Chemical blowing agent (water): Reacts with isocyanate to generate CO₂. 1 g water produces ~1.24 L CO₂. For conventional flexible foams (30–50 kg/m³), water is usually 1.2–1.8 parts per 100 parts polyol. Adjust based on isocyanate index.

Physical blowing agent (liquid CO₂): Provides uniform cells via rapid evaporation. Typical dosage 0.5–3 parts depending on density, requiring high-pressure injection equipment.

III. Dynamic Adjustment Based on Raw Materials and Process Conditions

1. Raw material characteristics

*Isocyanate activity: MDI reacts faster; reduce water dosage to avoid foam collapse. TDI reacts slower; increase water dosage if needed.

*Polyol molecular weight/functionality: Low molecular weight polyols cure faster → increase blowing agent. High molecular weight, viscous polyols → reduce dosage to avoid large cells.

*Catalyst type: Foaming catalysts accelerate gas generation → reduce dosage. Gelling catalysts speed curing → increase dosage. Maintain foaming/gelling time ratio between 1:1.2–1.5 for optimal structure.

2. Process conditions

*Reaction temperature: +10°C increases reaction rate 1.5–2× → reduce dosage. At low temperature, increase dosage.

*Mixing speed: Low mixing → poor dispersion → reduce dosage. High mixing → more air entrainment → reduce physical blowing agent.

*Pressure: In closed-mold foaming, increase dosage; in open foaming, decrease dosage.

IV. Dosage Adjustment and Synergistic Effects

Heat balance: Reduce water, increase physical blowing agent (heat absorption) to prevent scorching.

Cell refinement: Combine water with liquid CO₂ to achieve fine cell structure.

Ultra-low density foams (<20 kg/m³): Use water + pentane system to balance foaming volume and exotherm.

V. Monitoring and Fine Adjustment in Production

Offline Testing:

Measure density 1h after foaming. If deviation >5%, adjust dosage (±0.1 part per 1 kg/m³ difference). Slice to check cells: too large → reduce dosage; too fine → increase dosage. Low rebound → increase water.

Online Feedback:

Mold edge lack: Insufficient foaming → increase dosage.

Excess overflow: Too much foaming → reduce dosage.

Scorching: Reduce water, increase physical agent.

Collapse: Reduce physical agent or increase gelling catalyst.

Summary

Controlling blowing agent dosage is about balancing gas generation with polymer curing. Overdosage causes cell rupture and low density; underdosage leads to high density and incomplete foam. By combining theoretical calculation, pilot testing, and batch adjustment, manufacturers can achieve uniform cells, accurate density, and defect-free flexible PU foams.