How To Make Flexible PU Foam Softer?

To achieve softer flexible PU Foam in production, adjust four core levers—raw-material formulation, foaming process, post-treatments, and functional additives. These variables interact and together determine final physical properties.

I. Raw-Material Formulation: Boost Softness From The Molecular Level

High–Molecular-Weight Polyether Polyols: Use Mn 5,000–6,000 polyethers; longer chains impart superior flexibility and directly lower hardness.

Low-Unsaturation Polyethers: Introduce polyethers with typical unsaturation <0.05 meq/g to cut double bonds, reduce crosslink density, and increase chain mobility—resulting in a softer foam.

Polyester–Polyol Blends: Mix a small fraction of polyester polyol (typically 10%–20% of total) with polyether polyol to enhance hand feel while maintaining elasticity.

Isocyanate Index (TDI Systems): Control the index at 0.95–1.05 to limit crosslink points yet keep structural integrity and avoid collapse from under-crosslinking. Note: MDI systems are generally unsuitable for soft flexible PU foam production.

Plasticizers & Surfactants: Add small-molecule ester plasticizers to intercalate chains, weaken hydrogen bonding, and increase flexibility. Observe regional regulations restricting phthalates; prioritize eco-friendlier options such as acetyl tributyl citrate (ATBC). Open-cell silicone surfactants stabilize foam and lend a smoother surface feel.

II. Foaming Process: Precisely Tune The Microstructure

Density Control: Lowering density is the most direct route to softness. Reducing from 30 kg/m³ to 20 kg/m³ typically cuts 25% IFD (ASTM D3574) by ~40%–50%. Achieve via higher physical blowing-agent loading or foaming-machine speed adjustments. When density <18 kg/m³, add 0.2%–0.5% trimethylolpropane (TMP) to prevent structural collapse.

Cell-Structure Optimization: Reduce nucleator dosage or raise foaming temperature (slabstock 50 ± 2 °C; molded 35 ± 1 °C) to enlarge average pore size for a looser structure. Use open-cell silicone surfactants to push open-cell content to >95%, reducing cell-wall support, enhancing resilience, and softening hand feel.

Reaction-Rate Control: Lower tertiary-amine catalyst levels (e.g., from ~0.3 php to 0.10–0.15 php) to extend cream time so bubbles can expand fully. Introduce delayed-action catalysts to bias blowing ahead of crosslinking/gelation, avoiding premature hardening that hurts softness.

III. Post-Treatments & Functional Additions: Add Special Effects

Chemical & Physical Softening: Mild hydrolysis can cleave a portion of crosslinks, improving softness by ~15%–20%. Physically, thermal stretching or mechanical pre-compression induces plastic deformation, reducing initial hardness while improving both softness and elasticity.

Functional Materials: Disperse small amounts of carboxyl-modified graphene oxide or nano-SiO₂; weak interactions with chains can raise segmental mobility without adding hardness. Keep nano-SiO₂ at 0.5%–1.0%—>1.5% noticeably increases hardness. Use protective gear to avoid nanoparticle inhalation. Introduce bio-based polyols (e.g., castor-oil-based) whose unsaturated fatty-acid segments contribute a distinctive soft feel.

IV. Performance Balance & Compliance

Softness vs. Support: Excess softness can undermine support (e.g., mattress sagging). Use gradient density: 15–20 kg/m³ comfort layer over 25–30 kg/m³ support layer to balance plushness and load-bearing.

Durability: Softening may affect stability. Target:

25% compression set <8% (70 °C/22 h)

Post dry-heat aging hardness change <15% (100 °C/72 h)
Incorporate antioxidants to improve aging resistance.

Safety & Regulations: Ensure all adjustments comply with relevant requirements (e.g., flammability and VOC emissions).

By combining these techniques—and tailoring choices to the end use (furniture, medical, packaging)—you can raise softness while meeting performance and cost targets.