What Are the Raw Materials and Additives for Polyurethane Integral Skin Foam?

Polyether Polyols:

High-activity and high-molecular-weight polyether polyols are used, similar to those in high resilience (HR) foam. The commonly used types include 330N and 3600 (with molecular weights of 4800 and 6000, respectively). Polymer polyols (POP) are also added to improve overall product performance.

POP has a higher viscosity than standard polyether. As the POP content increases, the blend's viscosity rises and moldability worsens. However, styrene-acrylonitrile grafts in POP enhance cell openness.

While POP does not affect the initial reaction speed with isocyanates, it accelerates the later-stage reaction. This maintains mold filling ability while shortening curing and demolding time, boosting production efficiency.

Isocyanates:

Isocyanates used in flexible integral skin foam (ISF) mainly include derivatives of methylene diphenyl diisocyanate (MDI) and toluene diisocyanate (TDI).

MDI derivatives are modified liquid MDIs with slightly higher functionality than pure MDI. TDI is typically used as a urethane-modified semi-prepolymer.

MDI-TDI blends may deteriorate foam properties.

To increase hardness, polymeric MDI can be used. However, MDI and polyether react to form a dense crosslinked structure, making the foam brittle. Thus, MDI needs to be blended or adjusted per product requirements.

Modified MDI often includes carbodiimide or polyether-modified MDI, with free NCO content around 28–30%. This improves reactivity, mechanical strength, and reduces brittleness.

Chain Extenders and Crosslinkers:

These react with isocyanates to form rigid segments, playing a key role in the formability, productivity, and mechanical properties of ISF.

Rigid segment content increases stiffness, heat resistance, and dimensional stability.

For POP systems, chain extenders include diols like ethylene glycol, diethylene glycol, and 1,4-butanediol. Crosslinkers are triols or EO-copolymerized triols.

The types are similar to those used in standard polyether systems, but dosage is slightly reduced.

Catalysts:

Catalysts are critical in balancing the foaming and gelling reactions during integral skin foam production. Imbalances can cause shrinkage or foam collapse.

Strongly basic tertiary amines (e.g., triethylenediamine) promote foaming, while other components accelerate gelling.

Blending the two enables suitable catalytic performance for integral skin systems.

Blowing Agents:

The blowing agent commonly used is 141B, sometimes combined with small amounts of methylene chloride, which aids in skin formation.

A small quantity of water is also added to react with isocyanates and release CO₂, a heat-releasing reaction that accelerates foaming, shortens demolding time, and improves efficiency.

Color Paste:

Aromatic isocyanate-based integral skin foams tend to yellow under UV exposure, so pigments are added, most commonly black pigments.

Black pigment paste is typically made by grinding carbon black with polyether, added at 0.5–2%.

For lighter or brighter colors, UV-stable acrylic or aliphatic topcoats can be used. Oil-soluble pigments ground with polyether can also be added at around 2% of the polyether mass.