Avoidance of some color problems in products comprising MDI/PMDI

摘要

PMDA refers to the polyaromatic polyamirte which is formed by acid-catalysed reaction of aniline with formaldehyde in any of its various forms, preferably in the form of an aqueous solution, known as formalin. Many different acids or acidic species have been described in the prior art as catalysts but hydrochloric acid is typically used. The acidic reaction mixture is neutralized, typically with an aqueous solution of NaOH, the organic phase is separated, washed and further worked up by removal of excess aniline and volatile impurities such as but not limited to cyclohexylamine, cyclohexanol, cyclohexanone and nitrobenzene. The resulting PMDA is used for the manufacture of the well-known family of isocyanate products known as MDI/PMDI.