Rheological Aspects of Carboxymethyl Cellulose Acetate Butyrate (CMCABTM) in Waterborne Coatings
Cellulose ester technology has been used in coatings since 1890 when cellulose nitrate (NC) was found to be useful as a coatings resin. Cellulose acetate (CA) and mixed cellulose esters such as cellulose acetate butyrate (CAB) and cellulose acetate propionate (CAP) were developed in the 1930s as a less flammable alternative to nitrate-based coatings for military aircraft. Cellulose esters can be used as binder resins or as additives to coatings to provide application properties and resist film defects.
In waterborne coatings systems, the percent solids increases more slowly due to the slow evaporation rate of water (see Figure 3), especially at higher humidity. This slow evaporation makes it difficult to prevent a waterborne paint from dripping or sagging if it exhibits good flow and leveling. Avoiding these properties requires careful adjustments of the rheological profile of the coating. Rheology-control agents such as alkali-swellable thickeners, associative thickeners and clays are used to build viscosity rapidly to prevent the coating from dripping off vertical substrates. By using these agents, the viscosity increases rapidly due to a decrease in shear rate instead of relying on a rapid increase in solids and viscosity due to evaporation. Rheological additives that prevent dripping also aid in keeping the pigments dispersed. However, the elasticity introduced by such additives can be detrimental to flow and leveling and can propagate other film defects. To compensate for some of these problems, other additives are then needed to reduce the surface tension of the fluid in order to facilitate flow and leveling.2-3 The balancing act between good flow and leveling and sag control can be difficult to achieve.