Q: Our company recently began making custom prepolymers based on 4,4’-MDI (diphenylmethane diisocyanate). We are working on a drum scale during this startup phase and have noticed that our products have a hazy appearance. What is causing this problem?

A: Hazy prepolymers are most often caused by one of two reasons: the presence of MDI dimer in the starting isocyanate or the reaction of the isocyanate with moisture. For a smaller-scale operation, MDI is delivered in drums at ambient temperature. It is a solid with a melting point of 39