There are a few different types of foam that can harden into a concrete-like material. The most common are polyurethane foam and polyisocyanurate foam. These foams are used for insulation purposes, but also have structural properties when they cure and harden.
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Polyurethane Foam
Polyurethane foam (PU foam) is a plastic material made by reacting polyols and diisocyanates. The reaction produces bubbles that expand into a rigid foam as it cures. PU foam is an excellent insulator with an R-value of around R-6 to R-6.5 per inch. It also has structural properties once fully cured, able to support weight and resist crushing.
PU foam expands significantly during application, filling cracks and voids. It then cures into a hard, concrete-like mass. The closed cell structure of PU foam resists water absorption and vapor transmission. This makes it suitable for insulating roofs, walls, pipes, tanks, and more.
When used properly, polyurethane foam has compression strength of around 20 psi. This allows it to bear moderate loads, especially when used in thick layers. The tensile strength of PU foam is around 40-60 psi. It can form a durable, rigid structure useful in construction and insulation applications.
Applications of Polyurethane Foam
– Insulating roofs, walls, floors, pipes, and tanks
– Filling gaps and cracks in construction
– Structural support in geosynthetic applications
– Floatation for docks and rafts
– Crafting hard shells and props
– Packaging material
Curing and Hardening Process
Polyurethane foam begins expanding immediately after application due to the reaction between the polyols and diisocyanates. The foam reaches full expansion within 5-30 minutes under normal conditions. It then starts to cure, forming cross-links between polymer chains.
Most PU foams are tack-free within 4-8 hours after application. The curing process continues for 12-24 hours until the foam is fully rigid and concrete-like. Colder temperatures can significantly lengthen the curing time. Heat shortens the curing time.
Properly cured polyurethane foam does not further harden or change properties. It remains rigid and strong for decades. Exposure to sunlight can degrade PU foam over time, causing discoloration and loss of strength.
Polyisocyanurate Foam
Polyisocyanurate (PIR) foam is a thermoset plastic foam similar to polyurethane. It also uses polyols reacted with isocyanates. However, PIR foam has a higher ratio of isocyanurate molecules. This gives it a more rigid cellular structure with improved stability at high temperatures.
PIR foam has an R-value of R-5.6 to R-8 per inch, even better than PU foam. It shares the same closed-cell properties. PIR boards and spray foam are common insulation products. After curing, the foam is exceptionally hard and can support over 50 psi of compression force.
Applications of PIR Foam
– Commercial and industrial roof insulation
– Insulated concrete forms (ICFs)
– Wall insulation
– Structural insulated panels (SIPs)
– Timer frame construction
– Floatation for boats and rafts
– Theater and movie props
Curing and Hardening Process
Like PU foam, polyisocyanurate foam starts expanding and curing immediately after application. It reaches full expansion and sets within 5-20 minutes normally. PIR foam takes about 12 hours to fully harden and achieve closed-cell rigidity.
The extra isocyanurate cross-linking makes PIR foam more dimensionally stable compared to PU foam. It resists deformation from heat better. PIR foam maintains its strength and insulation value long-term provided it is properly installed and not exposed to sunlight or excessive moisture.
Molding vs Spraying Foam
Both polyurethane and polyisocyanurate foam can be created in molded forms or sprayed into place.
Molded foam is produced in a factory and formed into rigid boards or panels. These make installing insulation more convenient. Molded foam has a uniform structure since it cures under controlled conditions.
Spray foam is applied on site using specialized equipment. The foam chemicals are mixed during spraying to expand and cure in place. Spraying allows the foam to reach gaps, seal cracks, and wrap around complex shapes. The structure of spray foam can be less consistent.
Closed-Cell vs Open-Cell Foam
PU and PIR foam almost always have a closed-cell structure. This means the foam has a high percentage of closed foam bubbles rather than open bubbles after expanding.
Closed-cell foam is rigid, strong, and resistant to air and moisture movement. Open-cell foam has more air pockets and a spongy texture. It conforms around objects better but lacks structural strength.
Most construction and insulation applications utilize closed-cell PU or PIR foam. Open-cell polyurethane foam has uses for furniture padding and packaging.
Density of Cured Foam
The cured density of PU and PIR foam depends on the formulation and on the application method. Typical PU foam has a density around 1.5-2.5 pounds per cubic foot (24-40 kg/m3). Higher density foams generally have higher compressive and tensile strength.
Spray polyurethane foam usually results in a density of around 2 pounds per cubic foot (32 kg/m3). Poured or molded PU and PIR foam can achieve densities up to 3 pounds per cubic foot (48 kg/m3).
Higher density does not always equal higher strength. The cellular structure and curing conditions also affect mechanical properties. However, density provides a rough comparison between foams.
Toxicity Concerns
Uncured polyurethane and polyisocyanurate foam can be irritating to the eyes, skin, and respiratory system. Installers should wear protective equipment while spraying or pouring the foam chemicals.
Once fully cured and hardened, PU and PIR foam are considered inert and non-toxic. They do not off-gas or leach chemicals under normal conditions. This makes them safe for insulation in occupied spaces.
Potential health and environmental impacts from manufacture and disposal of foam chemicals should be considered. Following safe handling procedures minimizes risks while taking advantage of the benefits of these plastic foams.
Cost Comparison
Polyurethane and polyisocyanurate foam are cost-effective insulators that also add structural strength. The installed cost is around $1.00-1.50 per square foot for sprayed foam. Molded foam boards cost $0.75-1.25 per square foot.
PU foam is cheaper than PIR foam in most cases. PIR foam can cost up to 30% more due to its higher heat resistance and more complex chemistry. The rigid, strong nature of cured foam offsets some cost by reducing construction time and materials.
Proper installation is critical to get the desired insulation value and prevent problems like moisture accumulation. Hiring experienced contractors is advisable for larger foam insulation projects.
Alternatives
Rigid polyurethane and polyisocyanurate foams have excellent versatility in insulation, construction, and flotation applications. Some alternatives worth considering are:
– Expanded polystyrene (EPS) foam – Lower cost but less structural strength. Used in foam blocks and home insulation.
– Extruded polystyrene (XPS) foam – Higher moisture resistance than EPS. Common as foam board insulation.
– Polyethylene foam – Very buoyant for dock and boat floatation. Weaker and less rigid than PU/PIR foam.
– Mineral wool – Fiber insulation with similar insulation value as foam. Made from molten slag or rock.
– Fiberglass insulation – Traditional fluffy, fabric-like insulator. Not as moisture resistant as foam.
– Natural fiber insulation – Made from cellulose, cotton, hemp, or wool. Renewable but lower R-value than foam.
– Structural insulated panels (SIPs) – Prefab panels with foam cores between wood/metal facings. Highly insulated structural elements.
For most insulation needs, the structural properties and insulation value of PU and PIR foam make them a top choice if cost allows. They are unmatched in versatility among rigid plastic foams. Careful installation and use of protective gear minimize potential health hazards.
Conclusion
Polyurethane and polyisocyanurate foam are unique in their ability to expand into lightweight, rigid plastic insulation and structural elements. The closed-cell nature and high R-value of these foams make them essential for energy efficiency in buildings and cold storage.
With proper spraying or molding equipment and chemical handling procedures, PU and PIR foam provide durability, strength, and insulation unmatched by other materials. Their concrete-like hardness when cured makes them suitable for construction projects and geometric shapes not possible with other insulators.
While PU and PIR foam chemistry requires caution, the inert nature of cured foam makes it suitable for indoor insulation. With rising energy costs and demand for structural versatility, innovative hard foams like these will only become more important construction materials. Their environmental footprint can be reduced by sustainable chemical production methods and recycling systems.
References
| Source | Reference |
| Department of Energy | Polyisocyanurate Insulation Material – Building Technologies Office |
| BuildingGreen.com | Polyisocyanurate Insulation |
| Insulation Institute | Polyisocyanurate Insulation |
| BMC Chemistry | Polyurethane types, synthesis and applications – a review |
| Applied Sciences | Polyurethane Flexible Foam: Processing, Structure, Properties, and Applications |