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Formulating with Polyurethane Prepolymers

SpecialChem / Andy Extance – Jul 2, 2008

As polyurethane adhesive chemists will attest, the reactivity of isocyanates simultaneously makes them both highly useful and highly toxic reagents. However, in today's safety conscious world the use of isocyanates is becoming more restricted. One straightforward chemical modification can eliminate these safety concerns while also enhancing the suitability of the resulting compounds for use in polymerisation. These valuable reagents are polyurethane prepolymers. Prepolymers are usually viscous liquids, mixed together in batch reactors.The isocyanate used is most commonly either diphenylmethane diisocyanate (MDI) or toluene diisocyanate (TDI) and the polyol can be as simple as a glycol. The composition of the reaction mixture is set such that an excess of isocyanate functionality will remain in the matrix that forms after all polyol alcohol groups have reacted. Often the excess of isocyanate to polyol is more than five-fold, with the isocyanate serving partly as a solvent to keep the polyol from gelling.

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