Novel Rheology Control Agents

By Gautam S. Haldankar, Mohamad D. Shalati, William J. DeGooyer, Michael A. Gessner, Martin Bosma, Richard H.C. Brinkhuis, Cees A.M. Vijerberg

The requirements for automotive clearcoats have changed significantly in the last two decades. The most dramatic change has been in the VOC reduc­tion of clearcoats. For solventborne clearcoats, VOC levels have changed from 5-5.5 lb/gal to as low as 3.5 lb/gal for OEM applications. The number average molecular weight (Mn) of the resins used for clearcoats was around 5,000 a couple of decades ago and now it is around 2,000-2,500 Daltons.

The rheological behavior of clearcoats has also become increasingly im­portant as the molecular weight of resins has decreased over this period of time. Automotive OEM coatings are applied using automatic electrostatic spray equipment. The applied clearcoats are allowed to flash for 10 min and are baked at 250-280° F for about half an hour. At the time of application, the viscosity of the paint is low (25-30 sec on Ford #4).

However, once applied to the substrate, the viscos­ity of the coating must increase rapidly to prevent sag­ging. Lower molecular weight resins (lower VOC) have a greater tendency to sag during flash-off time and dur­ing the bake cycle because of an inverse relationship between the molecular weight and viscosity.