Insulation Materials: Polyisocyanurates

Mel Rasco

January 1, 2008


Polyurethanes (PU) were invented in 1937 by the German scientist Otto Bayer and his coworkers. They recognized that the polyaddition of liquid polyester or polyether diols with liquid diisocyanates yielded products that were superior to existing polyolefin plastics. In 1954, the accidental introduction of water to the reaction mixture was shown to produce flexible foams. Later, this chemistry was modified to produce rigid foams and elastomers. Today, several types of “polyols” and “polyisocyanates” are used to produce polyurethane materials that yield varying properties.

In 1967, the class of materials known as urethane modified polyisocyanurate (PIR) foams was introduced. These compounds are essentially an improvement on PU insulations, offering improved thermal stability, flame resistance, chemical resistance, and dimensional stability. During PIR production, the polyol and polyisocyanate reaction takes place at higher temperatures when compared to PU production. This allows excess isocyanate to react with itself in what is called trimerization, producing strong chains of isocyanurate crosslinks. These crosslinks are stronger than normal PU bonds. Therefore, they are more difficult to break, resulting in the improved properties.

The Manufacturing Process

During the manufacture of PIR insulation, the ratio of polyisocyanate to polyol (index) helps to determine the final properties of the foam, and therefore plays a key role in determining the foam’s application. The low-index foam insulations exhibit behavior closely resembling urethanes, since urethanes are produced at close to a 1/1 polyisocyanate/polyol ratio. As this ratio is increased, the trimerization reaction occurs, and the improved properties associated with PIR insulations become evident. Although all of these properties are important for certain applications, the improvements in dimensional stability, flame resistance, and thermal stability provide the biggest fundamental differences between PU and PIR foams.

Polyisocyanurate insulations are produced in bunstock form, either continuously or individually box poured. Because of better consistency, performance, and quality, PIR insulation produced via the continuous process is usually preferred. These large, rectangular buns are then fabricated into various shapes, including flat boards and pipe shells, which are typically 3 to 4 feet in length. These pipe shells are designed to fit directly over nominal pipe size (NPS) pipe and tubing. Complex shapes can be fabricated to fit tightly around valves, fittings, and other equipment.

Common Applications

Today, PIR foams are an important class of mechanical insulation used in a wide variety of industrial and commercial applications, including air conditioning, refrigeration, food and beverage, pharmaceutical, petrochemical, and liquefied natural gas (LNG). Polyisocyanurate insulation is also used as a core material for composite panels in applications like transportation, building construction, and temporary mobile shelters. These panel core foams are still PIR, but most are made at the lower index range. These low-index foams are better suited for panel core because of better impact resistance, adhesion, and strength properties.


What makes PIR an attractive insulation are its low density, good compressive resistance, excellent thermal conductivity, high closed-cell content, low water absorbance, low water vapor permeability, and excellent flame and smoke performance per American Society for Testing and Materials (ASTM) E84. Values for these properties can be obtained from NIA’s National Insulation Training Program (NITP) chart (see, or by contacting the appropriate manufacturer of PIR insulation. Ease of fabrication and installation are also key characteristics associated with PIR insulation.

For the mechanical insulation market, PIR bunstock is widely used as pipe and vessel insulation. The applicable ASTM material standard for unfaced bunstock PIR is C591. While various grades and densities are produced, the most commonly used are Grade 2, which has an operating temperature range of -297°F to 300°F, and types IV, II, and III, with densities of 2, 2.5, and 3 pounds per cubic foot (lbs/ft3), respectively. For commercial building chilled water pipe and equipment applications, PIR insulation is included in many specifications, such as the Army Corps of Engineers, the Veterans Administration, the General Services Administration, and the private specifications of many mechanical engineering and architectural firms. For industrial applications, PIR insulation is one of the types included in the specs written by or for engineering firms, petrochemical companies, food and beverage manufacturers, and oil and gas producers.


Polyisocyanurate insulation is widely used in the mechanical and industrial insulation industry. Because of some fundamental chemistry changes, it is now filling the needs as a core material in composite panels. Ease of use, combined with excellent physical properties, means that polyisocyanurate insulation will continue to be an important part of insulation systems.

Readers who are interested in learning more about the insulation material featured here should visit the MTL Product Catalog at or visit the NIA Membership Directory at to find a manufacturer.

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