Polyisocyanurate versus Polyurethane Composite Foam

PURPOSE

A surprisingly large number of potential buyers of polyurethane (PUR) foam cores for composite applications are unaware of the advantages or even the existence of polyisocyanurate (polyiso or PIR). Some may even think they're the same thing. The fact is that the differences are considerable. Note that while this Bulletin is focused on generic differences between PUR and PIR rigid foam cores for structural composite applications, there are indeed differences between the physical properties and performance characteristics of competing polyiso foam cores. We recommend end-users view the pdf version of this document under our Technical Bulletins, and indeed puruse other Technical Bulletins from Dyplast that address these differences.

POLYURETHANE AND POLYISOCYANURATE CHEMISTRY

Rigid foam cores under the general category “polyurethane” can be made with quite different formulations and therefore quite different physical properties and performance characteristics. The chemistry of ISO-CF® and ISO-CF/HT polyisocyanurate, on the other hand, is based on a modification of the traditional polyurethane formulations. The starting materials are similar to those used in polyurethane except that the proportion of methylene diphenyl diisocyanate (MDI) is higher and a polyester-derived polyol is used in the reaction instead of a polyether polyol.

The reaction of MDI and polyol takes place at higher temperatures compared with the reaction temperature for the manufacture of polyurethane. At these elevated temperatures and in the presence of specific catalysts, MDI will first react with itself, producing a stiff, ring molecule, which is a reactive intermediate (a tri-isocyanate isocyanurate compound). The remaining MDI and the tri-isocyanate react with polyol to form a complex poly(urethane-isocyanurate) polymer, which is foamed in the presence of a suitable blowing agent such as pentane. This isocyanurate polymer has a relatively strong molecular structure, because of the combination of strong chemical bonds, the ring structure of isocyanurate and high cross link density, each contributing to the greater stiffness than found in comparable polyurethanes. The greater bond strength also means these are more difficult to break, and as a result a polyisocyanurate foam is chemically and thermally more stable; breakdown of isocyanurate bonds is reported to start above 200 °C, compared with urethane at 100 to 110 °C.

Polyisocyanurate typically has an MDI/polyol ratio, also called its “index”, higher than 180. By comparison polyurethane indices are normally around 100. As the index increases material rigidity (stiffness) increases. While the majority of structural composite applications benefit from higher-rigidity foam cores, Dyplast can manufacture different polyiso densities - - each with its own rigidities and strengths.

POLYISOCYANURATE VS. POLYURETHANE PROPERTIES

A.    Service Temperature

While incorporating the isocyanurate structure into polyiso does not affect thermal conductivity, it plays a major role in improving most other key properties. One such property is the service temperature. The cyclical ring structure that is unique to polyiso makes it very stable at high temperatures when compared to urethanes. At temperatures above the stable regime (>300°F), the isocyanurate structure causes polyiso to have a tendency to char instead of burn.

B.     Flame Resistance

Another important improvement which is seen with polyiso when compared to a urethane is in the flame resistance. Over the years, urethane-type foams have been reputed to “burn like paper”. But the high index nature of polyiso foams resists this burning. In fact, polyiso foams pass many of the mandatory burn tests without the addition of external flame retardants.

C.    Resistance To Water And Solvents

Since ISO-CF two-pound density foam core is 97% closed cell, it has a low permeance to water (2.5 perm-in) and very low water absorption characteristics (0.5% by volume). This absorption is minimized by the chemical structure of Dyplast’s polyiso formulation. The cross-linked cyclical ring structure also helps it to resist chemicals and solvents. Incidental contact between solvents and polyiso will cause little or no damage to the foam.

D.    Dimensional Stability

Perhaps the biggest improvement polyiso offers over urethanes is the dimensional stability. Because of its highly cross-linked structure, polyiso is very stable at varying climatic conditions. This structure is rigid enough to resist movement by the fabricated foam, making the “growth” and “warp” which has been associated with urethane type foams very negligible with polyiso. This factor also proves advantageous during the handling and installation of polyiso.

SUMMARY: DYPLAST’S ISO-CF AND ISO-CF/HT

Dyplast’s polyisocyanurate brands include a complete line of rigid polyisocyanurate foam bunstock available in a range of densities between 2.0 to 6.0 lb/ft3. Dyplast’s ISO-CF polyiso is ideal for composite foam core temperatures between -297 and +300°F. ISO-CF/HT is suitable for higher temperatures up to 350°F continuous and 375°F intermittent. Dyplast’s ISO products can be fabricated into sheets of various thicknesses and or highly customizable shapes utilizing Dyplast’s CAD and CNC equipment. Very precise dimensions can be achieved to satisfy a variety of specialized or general composite foam core needs.

Thus Dyplast’s ISO line of products is highly differentiated from historical urethane foams. Hopefully, in the future, polyiso brand foams such as ISO-CF will not be confused with the urethanes and will be recognized as a distinctly different and superior foam core for structural composite applications.