Closed-Cell Reduction Strategies for Polyurethane Foam

The closed-cell content of polyurethane foam directly affects its air permeability, resilience, and hand feel. Excessively high closed-cell content (typically 15%–20%) not only lowers product grade but may also cause difficulties in post-processing. The core of optimizing the foaming process lies in the coordinated adjustment of formulation, process, and equipment to ensure that cells can open naturally or forcibly in the later stages of bubble growth, thereby stably controlling closed-cell content below 5% while maintaining mechanical properties.

I. Formulation Optimization

Polyol: Use polyether polyols with 2–3 functionality and narrow molecular weight distribution (1.1–1.3) to reduce crosslinking density; add 5%–10% polyether–polyester copolymer to adjust cell-wall toughness.

Isocyanate: For flexible foams, it is recommended to use crude MDI with NCO content of 28%–32%, or in an MDI/TDI mixed system keep TDI proportion <30% to avoid excessively fast reactions that cause closed cells.

Blowing agent: Premix physical blowing agent (e.g., cyclopentane) with polyol at 5–10°C, with appropriate dosage to ensure uniform bubbles.

Catalysts: Control the ratio of amine (blowing) to tin (gelation) catalysts at 1:0.8–1.2; may add 0.1%–0.3% delayed‑action amine catalyst to retard gelation.

Cell‑opener: Use physical cell‑openers such as polysiloxane surfactants or talc powder to reduce cell‑wall strength and promote opening.

II. Process Parameters

Temperature: Raw material mixing temperature 20–25°C; foaming environment 23–25°C with humidity <60%; curing in stages: initial stage 60–70°C (to promote opening), later stage 80–90°C (for solidification).

Mixing: High‑speed premixing (1500–2000 rpm, 30–60 s), then after adding isocyanate reduce to medium speed (800–1200 rpm, 15–20 s) to avoid excessive air entrainment.

Pouring and mould: Pour at a uniform speed (50–80 mL/s); provide vent holes of φ1–2 mm in the mould; reserve 1.2 times the expansion ratio space for foam height.

III. Sponge Making Equipment and Post‑Treatment

Automatic continuous foaming machine: High‑precision piston pumps (error <±0.5%), raw material temperature control ±0.5°C.

Post‑treatment: When closed‑cell content >10%, use roller pressing (0.2–0.3 MPa) or needle piercing (needle diameter 0.5–0.8 mm, density 50–100 needles/cm²) to forcibly open cells, then heat‑set at 70–80°C for 2–3 h to stabilise the structure.

IV. Verification and Adjustment

Follow the sequence of "bench‑scale (50–100 g) → pilot‑scale (1–5 kg) → production". Measure closed‑cell content by air permeability method or water displacement method, with target <5%. If fluctuations occur during production, first check raw material batch variations and metering deviations.

In summary, reducing closed‑cell content does not rely on a single measure, but rather on decreasing cell‑wall rigidity, delaying gelation, ensuring sufficient bubble growth and rupture, and then supplementing with physical post‑opening to remedy residual closed cells. Following this approach, closed‑cell content can be stably reduced from 15%–20% to below 5%, while maintaining key indicators such as density and resilience in compliance with standards, providing a reliable guarantee for producing high‑value‑added sponge products.