Polyisocyanurate, also referred to as PIR, polyiso, or ISO, is a thermoset plastic typically produced as a foam and used as rigid thermal insulation. The starting materials are similar to those used in polyurethane except that the proportion of methylene diphenyl diisocyanate is higher and a polyester-derived polyol is used in the reaction instead of a polyether polyol. The resulting chemical structure is significantly different, with the isocyanate groups on the MDI trimerising to form isocyanurate groups which the polyols link together, giving a complex polymeric structure.
Manufacturing
The reaction of MDI and polyol takes place at higher temperatures compared with 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 stiff, ring molecule, which is a reactive intermediate. Remaining MDI and the tri-isocyanate react with polyol to form a complex poly polymer, which is foamed in the presence of a suitable blowing agent. 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 PIR 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. PIR typically has an MDI/polyol ratio, also called its index, higher than 180. By comparison PUR indices are normally around 100. As the index increases material stiffness the brittleness also increases, although the correlation is not linear. Depending on the product application greater stiffness, chemical and/or thermal stability may be desirable. As such PIR manufacturers can offer multiple products with identical densities but different indices in an attempt to achieve optimal end use performance.
Uses
PIR is typically produced as a foam and used as rigid thermal insulation. Its thermal conductivity has a typical value of 0.16 BTU·in/ depending on the perimeter:area ratio. PIR foam panels laminated with pure embossed aluminium foil are used for fabrication of pre-insulated duct that is used for heating, ventilation and air conditioning systems. Prefabricated PIR sandwich panels are manufactured with corrosion-protected, corrugated steel facings bonded to a core of PIR foam and used extensively as roofing insulation and vertical walls. Other typical uses for PIR foams include industrial and commercialpipe insulation, and carving/machining media. Effectiveness of the insulation of a building envelope can be compromised by gaps resulting from shrinkage of individual panels. Manufacturing criteria require that shrinkage be limited to less than 1%. Even when shrinkage is limited to substantially less than this limit, the resulting gaps around the perimeter of each panel can reduce insulation effectiveness, especially if the panels are assumed to provide a vapor/infiltration barrier. Multiple layers with staggered joints, ship lapped or tongue & groove joints greatly reduce these problems. Polyisocyanurates of isophorone diisocyanate are also been used in the preparation of polyurethane coatings based on acrylic polyols and polyether polyols.
Health hazards
PIR insulation can be a mechanical irritant to skin, eyes, and upper respiratory system during fabrication. No statistically significant increased risks of respiratory diseases have been found in studies.
Fire risk
PIR is at times stated to be fire retardant, or contain fire retardants, but these describe the results of "small scale tests" and "do not reflect hazards under real fire conditions"; the extent of hazards from fire include not just resistance to fire but the scope for toxic byproducts from different fire scenarios. A 2011 study of fire toxicity of insulating materials at the University of Central Lancashire's Centre for Fire and Hazard Science studied PIR and other commonly used materials under more realistic and wide-ranging conditions representative of a wider range of fire hazard, observing that most fire deaths resulted from toxic product inhalation. The study evaluated the degree to which toxic products were released, looking at toxicity, time-release profiles, and lethality of doses released, in a range of flaming, non-flaming, and poorly ventilated fires, and concluded that PIR generally released a considerably higher level of toxic products than the other insulating materials studied. In particular, hydrogen cyanide is recognised as a significant contributor to the fire toxicity of PIR foams. Despite this PIR insulation is generally regarded as being more fire resistant than PUR insulation. PIR insulation board was proposed to be used externally in the refurbishment of Grenfell Tower, London, with vertical and horizontal runs of 100 mm and 150 mm thickness respectively; subsequently "Ipswich firm Celotex confirmed it provided insulation materials for the refurbishment." On 14 June 2017 the block of flats, within 15 minutes, was enveloped in flames from the fourth floor to the top 24th floor. The causes of the rapid spread of fire up the outside of the building have yet to be established. Flames can occupy the cavity between the insulation material and the cladding, and be drawn upwards by convection, elongating to create secondary fires, and do so "regardless of the materials used to line the cavities".
