Cellulose Esters (CAB/CAP) are additive components for many thermoplastic and thermosetting coating systems. Cellulose esters are well documented as non-yellowing, film forming resins that also provide fast drying and early hardness development, flooding and floating suppression, improvement of metal flake pigment orientation, and crosslinking reactions.
This rheology center provides formulators in the protective coating and related industries with a general discussion of the commercially available cellulose esters:
cellulose acetate (CA),
cellulose acetate propionate (CAP) and cellulose acetate butyrate (CAB) .
Cellulose Esters are useful as a coating additives where they can provide a significant
performance edge. They bring qualities such as:
Good Surface Appearance
Whether the defects are in the paint, on the substrate, or caused by additives, addition of CAB can remove the effects of paint and surface contamination (WD-40 is an example). Good wetting on compromised surfaces yields overall improvement in finished appearance.
Good Flow and Leveling
Due to their rheology profile (Newtonian at low solids), cellulose esters improve flow and leveling of the coating.
Spray coating usually produces some irregularities in the coating that must level out in order to avoid defects such as orange peel, mounding, and cratering once the paint dries. When the coating is too elastic, too viscous, or both, leveling forces are inadequate to overcome these defects and the coating does not flow and level. Using cellulose esters allows you to overcome this problem.
Levels of 1% - 10% CAB (based on resin solids) are used with thermosetting acrylic resins or polyester resins for flow control.
Rapid Film Formation
CAB can be used as a film former, as a reactive polyol in curing coatings, and also as an additive to other film formers.
The reactive hydroxyl groups contained in CABs may be cross-linked with urea-formaldehyde, melamine, or polyisocyanate resins, and provide the ability to formulate a variety of curing coatings and inks. The selection of higher hydroxyl cellulose esters such as Eastman™ CAB-553-0.4 for use in curing systems produces films with high crosslink density and, consequently, excellent chemical and physical properties.
Moreover, due to its high glass transition temperature (Tg), CAB provides excellent hardness and hardness development.
As an example, the addition of 15% - 30% CAB (based on resin solids) in thermosetting acrylic enamels provides a coating that dries like a lacquer on a hard surface and can be sanded to remove orange peel, sags, or embedded dirt. Spot repairs can then be made with the original coating composition. During the final bake at converting temperature, the enamel reflows to eliminate sand marks and provides a glossy thermoset finish.
Due to its high Tg, CAB provides rapid dry-to-touch times facilitating reduced dirt pickup. CAB increases productivity and prevents surface defects with fast dry-to-touch. Reduction in dry time also decreases the occurrence of coating contamination, enables coatings to harden faster for early mar resistance, and speeds turnaround times.
Heat & Moisture Stability
Due to its cellulosic polymer nature and its pendant groups, CAB has good heat and moisture stability (exterior durability). Eastman™ CAB films are considered to be water resistant although they do transmit water vapor to a degree.
Other factors being equal, higher butyryl content leads to higher resistance to moisture. This property may be altered by the addition of plasticizers, waxes, resins, other additives or by a high hydroxyl level. Thus, if any degree of moisture resistance is required from a film of high-hydroxyl butyrate, it is necessary to react the hydroxyls with an isocyanate or amino resin or some other reactive intermediate.
One of the important characteristics of CAB in coatings is its ability to resist degradation due to exposure to ultraviolet light, compared to other cellulose derivatives such as nitrocellulose.