How to Optimize High-Resilience Foam Formulation?

Polyurethane high-resilience foam, prized for load-bearing and comfort, is widely used in automotive seats and premium furniture. In production, companies commonly face two major challenges: (1) core mechanical performance defects (e.g., poor tear strength, insufficient elastic load-bearing) and (2) structural/process defects (e.g., bubbles, deformation, shrinkage). This guide systematically outlines solutions and provides key process-technical background to help users resolve these defects through formulation and process optimization.

Part One: Formulation Optimization Strategies for Core Performance Defects

Tear strength deficiency and insufficient elastic load-bearing are key indicators of durability and comfort; they are mainly resolved by adjusting the formulation’s key raw-material system.

1. Solve poor tear strength: balance toughness and crosslink density

Tear weakness is closely related to polymer crosslink density and chain flexibility.

B-side system and crosslinkers: Strictly control the B-side index deviation within ±2, and select functional crosslinkers. By adjusting their dosage to balance softness and tear strength, appropriate crosslinker blends can improve tear strength by 15%–20%.

MDI system blending: Recommend blending liquefied MDI with crude MDI at a 7:3 ratio. This combination meets product hardness requirements (Shore 30–50D) while avoiding excessive crosslinking and embrittlement from using crude MDI alone.

PAPI and TDI mixed process: By mixing PAPI with TDI at a 3:1 ratio while reducing the TDI index to 105–110, tear performance is significantly improved and rebound rate can be kept stable above 80%.

Low-density product limits: For low-density (≤ 30 kg/m³) products, avoid designing excessive hardness (recommend ≤ 40D), and limit crude MDI addition to ≤30%.

2. Improve resilience and load-bearing: reinforce the sponge skeleton

When high-resilience foam feels soft and lacks load-bearing, strengthen the sponge framework.

Core raw-material upgrade: Use high–molecular-weight rebound polyether (e.g., N6300) — the basis for high elasticity and high load-bearing.

Crosslink enhancement: Increase usage of multi-functional crosslinkers such as Polyol 3610/4640 to raise crosslink density and load-bearing capacity.

Structure optimization: Appropriately add high-solids POP (e.g., 7325) to enlarge cells and increase load-bearing, but control POP dosage to avoid surface cracks.

Additives: Use diethanolamine-based softeners and specific silicone oils to partially improve resilience, but avoid powdered additives which may impair rebound and anti-compression-set performance.

Second Part: Common Structural Defects in Production and Countermeasures

This section covers structural and process defects common to all high-resilience polyurethane foam production technologies, especially in molding processes.

1. Deformation, blowouts and shrinkage: control activity vs. open-cell balance

Deformation, blowouts and shrinkage usually stem from imbalance between foaming and gelling rates, causing internal pressure not to release or to over-release.

Blowouts and closed cells: Reduce polyol and B-side reactivity; use cell-openers appropriately; adjust silicone oil dosage or MC to lower skin and core viscosity.

Deformation and curing: Adjust gel catalysts to extend cure time, ensure full cure before demolding, and perform timely venting (pinhole/rupture) to release internal pressure.

Shrinkage issues: Mainly due to overly stable (closed-cell) foam or insufficient cure. Solutions: intensify cell-breaking, adjust catalyst dosages, and ensure full maturation.

2. Bubbles, hard edges and flash

These defects relate to raw-material dosing, mixing uniformity and temperature control.

Bubbles and flash: Adjust improper pouring methods; add venting holes to improve mold breathability. If isocyanate index is too high or raw-material temperature is improper, adjust mold temperature and reduce B-side index. If rise is too fast and there’s no time to close the mold, reduce catalyst dosage.

Hard edges and flash: Control fill volume to avoid overfilling; strictly control material and mold temperatures; ensure mold parting surfaces are clean and clamping is accurate.

Tearing (during demold): If tearing occurs during demolding, common causes are premature demolding, dirty mold inner walls, or incorrect release agent usage. Extend demold time or increase catalyst dosage, and clean molds regularly.

Part Three: Process-Technical Overview and Production Control Points

1. Advantages and limitations of continuous production line technology

Industrial production of high-resilience foam mainly relies on continuous block-foam foaming technology. Core advantages:

Scale efficiency: Single-line daily output can reach several to dozens of tons.

Performance uniformity: Automated control systems can limit density difference between top and bottom layers to ≤3%, and rebound rate fluctuation to ≤2%.

Raw-material utilization and environmental benefit: Utilization can reach ≥95%, no release agents required, and better environmental performance.

Limitations include high equipment investment and maintenance costs and lower flexibility for process adjustments. Additionally, continuous block-foam edges tend to form hard edges that require trimming, causing 3%–5% material loss.

2. Performance balance principles and quality control

Production of high-resilience soft foam must follow the performance-balance principle: “toughness–elasticity” and “tensile strength–elongation” are dialectically opposed and must be balanced, not maximized simultaneously.

Quality control: Establish strict raw-material traceability, focus on monitoring modified MDI batch NCO content (deviation ≤0.5%) and viscosity to ensure production stability.

Target setting: Set reasonable targets based on end use. For furniture sponge, control tensile strength ≥1.8 MPa and elongation ≥180%; for automotive seat sponge, raise targets to tensile strength ≥2.0 MPa and elongation ≥200%.

Optimizing formulations and solving defects in high-resilience foam is not instantaneous. It requires continuous trial, iteration and experience accumulation. Technicians must combine theoretical understanding with floor-level observation and precise adjustments. Facing variable raw-material batches and tightening product specs, continuous practice, summarization and fine-tuning are the fundamentals to ensure stable output of high-quality products.