Reader responds to Q & A on manufacturing synthetic resins.

TO THE EDITOR:

While the answer to the recent question about manufacturing PUD's, which appears in the June 2004 issue of JCT COATINGSTECH (page 22), provides some general worthwhile advice, there are some specific recommendations that should be followed when manufacturing PUD's.

I assume that the "prepolymer process" is being used, since this is the predominant manufacturing method. In the first step of the process a polyol, an excess of isocyanate, and a dihydroxy acid are heated to form a prepolymer. M-Pyrol is added to control the viscosity of the prepolymer. After the addition of a tertiary amine, to neutralize the acid group, in the second step, the prepolymer is added to a water/diamine mixture in a separate vessel to produce the PUD.

Since there is an excess of isocyanate in the prepolymer, it is very important to be sure all incoming raw materials are as anhydrous as possible. This requires careful analysis of raw materials. Water will produce C[O.sub.2], which in turn will produce foaming. Prepolymer temperature should be monitored, since too high a temperature will produce side reactions that cause high viscosity and off color final products.

Finally pumps, mixers and heated lines should be in good working order. It is important that the prepolymer be pumped at a controlled rate into the water/diamine mixture with no stop-page.

HOWARD S. BENDER, PH.D

President, P.U.D. Technology

Brewster, MA