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Polyisocyanurate is a form of plastic also known as polyiso or PIR, which is chemically related to polyurethane (PUR) plastic. It is used for many of the same applications as polyurethane, but chiefly is applied as a form of insulating foam board in building construction. It can also be obtained for use as a liquid or sprayed foam that is blown into walls or crawl spaces to fill air cavities as a thermal insulator. As one of the newest insulation materials on the market as of 2011, it has come to be the insulation material of choice in 60% of all commercial construction in the US and Canada due to several characteristics the material possesses making it superior to other types of insulation.
Insulation materials are often placed in interior wall locations where electrical wiring is also run, and this requires that they have heat and flame retardant properties. Polyisocyanurate has a melting point of over 392° Fahrenheit (200° Celsius). When used as foam board, it is a material made up of minute closed cells that contain hydrochlorofluorocarbon (HCFC) gas, which is non-flammable, inert, and an excellent insulator.
These structural and heat-resistant properties give polyisocyanurate a minimum R value for insulation of between 5.6 to 8 per inch, with a standard 2-inch-thick sheet. R value is an industry standard method of measuring resistance to heat flow, and polyisocyanurate R value is generally equivalent to that of polyurethane foam, but far superior to other forms of insulation. Polystyrene insulation has an R value around 4.3 with an equivalent 3.1-inch-thickness sheet, fiberglass batting an R value of 3.3 with a 4-inch thickness and plywood at an R value of 1.25 with a 10.9-inch thickness.
Polyurethane plastic and insulating foam has been manufactured since the 1930s when it was first used by military and aerospace industries. It became commercially popular in the 1970s and polyisocyanurate came onto the market in the US and Europe in the late 1970s. Both types of insulating foam board use thermal barrier coatings of foil or plastic laminate or other materials like gypsum or perlite to increase their ability to retard heat loss and prevent the spread of fires. The key difference between the two materials is that polyisocyanurate passes the Factory Mutual Calorimeter fire test (FM 4450) without such thermal barriers, whereas polyurethane does not.
Both types of foam are also thermosetting plastics, which means that, once they are manufactured or sprayed into a location, they take on a rigid shape that cannot be reformed and reset by melting. Another similarity between both PIR and PUR is that they are produced by reacting methylene diphenyl diisocyanate (MDI) with polyol compounds. Polyisocyanurate has a higher concentration of MDI in the end product than polyurethane, however.
One potential negative characteristic to the use of polyisocyanurate is that, as it ages, it undergoes a process known as thermal drift. This means that, within the first two years of use, some of the HCFC gas contained in the cellular structure escapes into space and is replaced by normal atmosphere, reducing the foam's insulating properties by about 20%. HCFC gasses are also potential ozone depleting gasses, though they have a much weaker effect on the ozone layer than the chlorofluorocarbon family of gasses that have been banned from use worldwide.