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Manufacturing

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 stiff, ring molecule, which is a reactive intermediate (a tri-isocyanate isocyanurate compound). Remaining MDI and the tri-isocyanate react with polyol to form a complex poly(urethane-isocyanurate) polymer (hence the use of the abbreviation PUI as an alternative to PIR), 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 200C, compared with urethane at 100 to 110C.

PIR typically has an MDI/polyol ratio, also called its index (based on isocyanate/polyol stoichiometry to produce urethane alone), of between 200 and 500. By comparison PUR indices are normally around 100. As the index increases material stiffness but also brittleness also increase, 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

Polyisocyanurate insulation boards

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/hr*ft2*F (0.023 W/m2K) depending on the perimeter:area ratio. PIR foam panels laminated with pure embossed aluminium (U.S. sp. 'aluminum') 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 cored of PIR foam and used extensively as roofing insulation and vertical walls (e.g. for warehousing, factories, office buildings etc). Other typical uses for PIR foams include industrial and commercial pipe insulation, and carving/machining media (competing with expanded polystyrene and rigid polyurethane foams).

Effectiveness of the insulation of a building envelope can be compromised by gaps resulting from shrinkage of individual panels. Manufacturing criteria requires that shrinkage be limited to less than 1% (previously 2%). 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.

References

^ Celeotex GA3000 PIR specification

Health Hazards

PIR insulation can be a mechanical irritant to skin, eyes, and upper respiratory system during fabrication (such as dust). No statistically significant increased risks of respiratory diseases have been found in studies.

External links

Polyisocyanurate Insulation Manufacturers Association

Polyisocyanurate Insulation energy savings, by Center for the Polyurethanes Industry

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Categories: Plastics | Polyurethanes | Building insulation materials | Polymer stubsHidden categories: Wikipedia articles needing clarification from November 2007

by: gaga

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