Polyisocyanurate foam (PIR) is a thermoset closed-cell plastic foam formed by the exothermic reaction between polyisocyanates and polyols. Its unique molecular structure—characterized by a high concentration of thermally stable isocyanurate rings—gives it superior thermal insulation properties and fire resistance compared to standard polyurethane foams (PUR). Developed in the 1960s, PIR foam has become a critical material in construction, LNG transport, aerospace, and industrial insulation due to its ability to operate between -196°C and +150°C while maintaining structural integrity .
Core Properties and Performance Metrics
The molecular architecture of PIR foam enables a combination of physical and thermal properties unmatched by conventional rigid foams.
Rendimiento térmico
Low Thermal Conductivity: Fresh PIR foam samples exhibit thermal conductivity values as low as 0.019 W/(m·K), which remains stable over time (aged samples: ≤0.0218 W/(m·K)). This outperforms PUR foams and mineral wool .
High R-Value: The R-value (thermal resistance per inch) of PIR typically ranges between R-6.0 and R-7.0, making it one of the most efficient insulation materials available.
Wide Service Temperature: PIR retains its properties across extreme temperatures, from cryogenic conditions (-196°C) in LNG storage to +150°C in roofing applications .
Mechanical Strength
Compressive Resistance: At ambient temperatures, PIR foam withstands ≥200 kPa compressive strength at 10% deformation. After immersion in liquid nitrogen (-196°C) for 8 hours, this increases to ≥280 kPa, proving its reliability in cryogenic environments .
Dimensional Stability: Linear thermal expansion is minimal (≤70×10⁻⁶ m/m·K), preventing warping or shrinkage during temperature fluctuations .
Fire Resistance and Safety
Flame Spread: PIR achieves a maximum flame spread rate of <25, meeting the B1 fire rating (per GB8264 standard) and reducing smoke generation .
Oxygen Index: With an oxygen index ≥26, PIR self-extinguishes when the ignition source is removed .
Low Toxicity: Unlike early PUR foams, modern PIR formulations minimize toxic byproducts like aromatic amines through optimized curing .
Environmental Durability
Water Resistance: Water absorption is capped at ≤5%, and its closed-cell structure limits vapor permeability (≤0.8 g/m²) .
Chemical Stability: Resists corrosion, mold, and solvents due to its inert polymer matrix.
Manufacturing Process: Chemistry and Production
Raw Materials
Isocyanates: Methylene diphenyl diisocyanate (MDI) is the primary component due to its reactivity and thermal stability.
Polyols: Petroleum-based polyols (OH number >200) or bio-based alternatives (e.g., rapeseed oil-derived polyols) act as co-reactants .
Catalysts: Potassium acetate or quaternary ammonium salts drive the isocyanurate trimerization reaction, forming robust ring structures .
Blowing Agents: Water reacts with isocyanates to generate CO₂, though low-GWP hydrofluoroolefins (HFOs) are increasingly used.
Reaction Mechanism
PIR synthesis hinges on the trimerization reaction, where three isocyanate groups (NCO) cyclize into isocyanurate rings. This reaction dominates over urethane formation (>70% conversion) when catalysts are optimized. The resulting structure contains urethane, urea, biuret, and isocyanurate linkages .
Production Methods
Continuous Lamination: For insulation boards, chemicals are mixed, deposited onto facers (e.g., aluminum foil), and cured in a double-conveyor oven. Temperature sensors monitor foam rise (140–160°C) to ensure quality .
Spray/Pour Foaming: For on-site applications (e.g., roofing or LNG tanks), a two-component spray system delivers foam that expands and cures within seconds. NASA-developed formulations for space shuttle tanks used pour foam to bond to aluminum at cryogenic temperatures .
Batch Production: Custom shapes are created by pouring foam into molds, followed by oven curing.
Eco-Friendly Innovations
Bio-Polyols: Rapeseed oil-based polyols synthesized via epoxidation or transesterification reduce reliance on fossil fuels .
Flame Retardant Modifiers: Expandable graphite (3–9 wt.%) enhances fire resistance without halogenated additives .
Applications Across Industries
Building and Construction
PIR boards dominate flat roofing and wall insulation due to their high R-value and fire safety. Thicknesses are CNC-cut for precision, minimizing thermal bridging .
Industrial Cryogenic Insulation
Liquefied natural gas (LNG) pipelines and storage tanks (-196°C) rely on PIR for its thermal stability and compressive strength under liquid nitrogen exposure .
Aerospace and Specialized Uses
NASA’s space shuttle external tanks used PIR pour foam as thermal protection. Recent formulations improve adhesion to aluminum and cryogenic performance .
Polyisocyanurate vs. Polyurethane Foam: Critical Differences
Though both are spray-applied or boardstock insulations, PIR’s molecular structure delivers distinct advantages:
Table: PIR vs. PUR Performance Comparison
| Property | PIR Foam | PUR Foam |
|---|---|---|
| Thermal Conductivity | 0.019–0.022 W/(m·K) | 0.022–0.025 W/(m·K) |
| Service Temperature | -196°C to +150°C | -50°C to +120°C |
| Compressive Strength | ≥200–280 kPa | 100–150 kPa |
| Resistencia al fuego | B1 rating, Oxygen Index ≥26 | B2 rating, Oxygen Index ~22 |
| Typical Cost | 10–15% higher | Lower base cost |
Table: Technical Specifications of PIR Foam (Per Industry Standards)
| Parameter | Unit | Value |
|---|---|---|
| Density | kg/m³ | ≥35 |
| Thermal Conductivity | W/(m·K) | ≤0.019 (fresh) |
| Compressive Strength | kPa | ≥200 (ambient) |
| Oxygen Index | — | ≥26 |
| Water Absorption | % | ≤5 |
| Chloride Content | ppm | ≤60 |
Polyisocyanurate foam sets the benchmark for rigid thermoset insulation through its blend of ultra-low thermal conductivity, exceptional fire resistance, and mechanical durability. Innovations in bio-based raw materials and low-GWP blowing agents are enhancing its sustainability profile, while manufacturing advances ensure precision in high-stakes applications from LNG infrastructure to aerospace. Though costlier than PUR, its lifecycle efficiency and safety justify the investment for critical thermal management.
