Insulation of ducts and pipes
Introduction
Polyisocyanurate (in English: polyisocyanurate, AFI: ), also known as PIR, polyiso or ISO, is a thermoset plastic[1] that is usually produced as foam and used as rigid thermal insulation. The starting materials are similar to those used in polyurethane (PUR), 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 "Ether (chemistry)"). The resulting chemical structure is very different, since the isocyanate groups of MDI trimerize to form isocyanurate groups that the polyols link together, giving rise to a complex polymeric structure.[2].
Manufacturing
The reaction of (MDI) and polyol takes place at higher temperatures compared to the reaction temperature for the manufacture of PUR. At these elevated temperatures and in the presence of specific catalysts, MDI will first react with itself, producing a rigid ring molecule, which is a reactive intermediate (a tri-isocyanate isocyanurate compound). The remaining MDI and tri-isocyanate react with the polyol to form a poly(urethane-isocyanurate) complex polymer (hence the use of the abbreviation PUI as an alternative to PIR), which is foamed in the presence of a suitable foaming agent. This isocyanurate polymer has a relatively strong molecular structure, due to the combination of strong chemical bonds, the ring structure of the isocyanurate and the high density of cross-links, which contribute to a higher stiffness than that found in comparable polyurethanes. The higher strength of the "Bond (chemistry)") bonds also means that they are harder to break, and as a result, a PIR foam is more chemically and thermally stable: breaking of isocyanurate bonds has been reported to begin above 200°C, compared to urethane at 100 to 110°C.
The PIR typically has an MDI/polyol ratio, also called its index (based on the isocyanate/polyol stoichiometry to produce urethane alone), greater than 180. In comparison, PUR indices are typically around 100. As the index increases the stiffness of the material also increases the brittleness, although the correlation is not linear. Depending on the product application, greater rigidity and chemical and/or thermal stability may be desirable. Therefore, PIR manufacturers may offer several products with identical densities but different ratings, in an attempt to achieve optimal performance for the end use.