Aerospace coatings must withstand demanding conditions, particularly related to chemical resistance and flexibility. For this reason, 2K polyurethane topcoats have long been the technology of choice. In addition, large aircraft require coatings with good flow properties with balanced drying properties. Eastman cellulose acetate butyrate (CAB) has long been used as a rheology additive in aerospace formulas to improve application properties and physical drying characteristics. CAB also provides hydroxyl functionality, which cross-links with isocyanates to provide a fully cured film without detracting from chemical resistance and flexibility.
Due to its high glass transition temperature (Tg) and viscosity build, CAB can improve physical drying prior to full chemical cure of 2K monocoats. Lower-molecular-weight grades such as Eastman CAB 551-0.01 and Eastman Solus™ 2100 performance additive are typically used, although-higher-molecular-weight grades with lower butyryl content, such as Eastman CAB 381-0.1 or Eastman CAB 381-0.5, may be good options if compatibility and required solids levels are adequate. Use levels are commonly 2%–5%, although much higher levels provide faster drying. One important benefit of the flow properties of CAB is better substrate wetting, which provides good flow and leveling across rivets. Coatings made with cellulose esters help eliminate the common problem of rivet rash, the “window pane” effect that can develop around rivets of the airplane.
In recent years, base coat/clear coat technology has grown for aerospace applications. The same principles described for monocoats apply to clear coats. As with automotive applications where CAB is commonly used, CAB can be used in base coats to provide faster drying and improved redissolve resistance on clear coat application. Redissolve resistance is especially important for mica flake alignment, so that the flat mica flakes are properly oriented for the desired visual effect. The higher viscosity types like CAB 381-2 and CAB 381-20 are typically used. These can also be blended to provide greater solution viscosity customization and subsequently redissolve resistance. The use of CABs also help eliminate the concern of the clear topcoat solvent making the edges of airline companies graphics or logos “fuzzy” as a result of redissolve. Redissolve resistance is more challenging in aerospace as the coatings typically contain a large amount of an aggressive ketone such as MAK. In base coats, lower-molecular-weight grades may be used; depending on viscosity requirements, the CAB 381 series may provide better redissolve resistance. For base coats, typical use levels are 5%–15%.
Formulating tip: CAB is often dissolved as a stock solution to then be added to the final paint. The best solvents for making the highest-solids stock solutions are ketones such as MPK and MAK. With Eastman CAB 551-0.01 and Solus 2100, stock solutions can be prepared at over 40% solids.