How to Choose a Polyurethane Foaming Process?

The choice of polyurethane foaming process directly determines product performance. This article compares the core differences among flexible, rigid, and semi-rigid foam processes to help you precisely match your production needs.

1. Flexible Foam Formulation Process

Core Characteristics: High elasticity, open-cell structure, soft touch

①Main Raw Materials:

Polyether Polyol: Highly reactive polyether (e.g., 330N, functionality 3, molecular weight 3000–6000), high hydroxyl content and reactivity, providing high resilience to the foam.

Isocyanate: Mainly TDI (toluene diisocyanate, 80/20 isomer mix), with an isocyanate index of 0.8–1.0 to maintain flexibility by controlling crosslink density.

②Additives:

Blowing Agent:

– Water (chemical blowing agent), 3%–5%, reacts with TDI to generate CO₂ and heat, and participates in chain extension. For low-density foams (<15kg/m³), a small amount of dichloromethane (physical blowing agent, 1%–3%) may be added.

Catalysts (T9, A33):

– Tertiary amines: Promote the foaming reaction (water + TDI), usage 0.1%–0.5%.

– Organotin: Catalyze gel reaction (polyether + TDI), usage 0.05%–0.2%.

Surfactant (Silicone Oil): Usage 0.5%–1.5%, stabilizes the bubble structure, prevents collapse, and ensures uniform open-cell structure.

③Process Essentials:

One-step method: All raw materials are mixed at once and stirred at high speed (1000–3000 rpm) for 10–30 seconds, initiating foaming and gelation reactions. Suitable for continuous foaming machines for sofa cushions, mattresses, etc.

Key Controls:

– Ambient Temperature & Humidity: 20–25°C, 50%–70% humidity to ensure matching speeds of foaming and gelation.

– Foam Density: Adjusting the water and physical blowing agent ratio controls density from 10–100kg/m³ (common foam 30–50kg/m³, high-resilience foam 40–80kg/m³).

④Typical Applications:

Furniture: Sofa cushions (high-resilience foam, rebound rate >60%), mattress filling (super soft foam, IFD 30–80N).

Home Textiles: Pillows (slow rebound memory foam, modified soft foam with special polyether and MDI).

2. Rigid Foam Formulation Process

Core Characteristics: Closed-cell structure, high hardness, low thermal conductivity

①Main Raw Materials:

Polyether Polyol: Low-reactivity polyether (e.g., 4110, functionality 4–8, molecular weight 200–1000), high functionality leads to high crosslinking density and hardness.

Isocyanate: Mainly MDI (polymeric MDI), isocyanate index 1.0–1.2 to ensure excess NCO groups and promote crosslinking.

②Additives:

Blowing Agent:

– Physical: HCFC-141b (phased out), cyclopentane (commonly used but flammable), CO₂ (eco-friendly), dosage 8%–15% to form closed-cell structure.

– Chemical: Water (0.5%–1.5%), assists foaming and density regulation.

Catalysts:

– Tertiary amines: Strong foaming catalysts, 0.1%–0.3%.

– Organotin: Small amounts (0.01%–0.05%) to balance gelation and avoid foam cracking.

Stabilizer (Silicone Oil): Polyester-type surfactant, 0.5%–1.0%, prevents cell coalescence and improves closed-cell rate (>90%).

③Process Essentials:

Mold Foaming: Mixed raw materials are poured into molds and heated (50–80°C) to cure for 30–60 minutes. Suitable for refrigerator insulation, construction sandwich panels, etc.

Key Controls:

– Closed-cell rate: With higher physical blowing agent and fast gelation, rates over 95% are achievable, with thermal conductivity as low as 0.02 W/(m·K).

– Flame Retardancy: Add brominated or phosphorus flame retardants (e.g., phosphate esters), achieving B1 fire rating.

④Typical Applications:

Building Insulation: Exterior wall panels (30–50kg/m³), floor heating insulation.

Cold Chain: Refrigerated truck cabins, insulation boxes (withstand -30°C, thermal conductivity <0.03 W/(m·K)).

3. Semi-Rigid Foam Formulation Process

Core Characteristics: Balance of rigidity and softness, high impact resistance, good dimensional stability

①Raw Material Composition:

Main Raw Materials:

– Polyether Polyol: Medium-reactivity polyether (functionality 3–4, molecular weight 2000–4000), balancing elasticity and strength, e.g., polyether 3628.

Isocyanate: TDI and MDI blend (ratio 7:3 to 5:5), index 0.9–1.1 to maintain moderate crosslinking.

②Additives:

Blowing Agent: Water (1%–2%) + Cyclopentane (3%–5%) blend to control density at 40–80kg/m³.

Catalyst: Mainly secondary amines (e.g., dimethylethanolamine), usage 0.2%–0.6%, balances foaming and gelation.

Fillers: Calcium carbonate, talc (5%–10%), increase strength, dimensional stability, and reduce cost.

③Process Essentials:

Cold Mold Foaming: Mixed materials injected into room-temperature molds, curing by reaction heat (no external heating), short cycle (10–20 minutes), suitable for automotive interior parts.

Key Controls:

– Compression Set: ≤15% (70°C × 22h), ensures long-term shape retention.

– Hardness: Shore A 20–60, adjustable via polyether molecular weight and isocyanate ratio.

④Typical Applications:

Automotive: Dashboard skins (density 50–70kg/m³, strong impact resistance), door panels.

Packaging: Cushioning for precision instruments (EPS alternative, better resilience), sports gear pads.