Why use Eastman cellulose esters for coatings and inks?
Eastman cellulose esters (CA,
CAP) are available in various types; viscosities; butyryl, propyl, and acetyl ratios; and hydroxyl contents. Their high Tg, near-Newtonian flow characteristics, and hydroxyl functionality coupled with the fact they are compatible in a wide range of solvents and readily dissolve in many common acrylate monomers assure that Eastman cellulose esters aid formulators as an additive or co-resin for many thermoplastic and thermosetting coating systems.
Nitrocellulose can be used as a raw material to produce several types of coatings and inks, but it is a difficult material to handle and store. It is a considerable fire hazard because it ignites easily and burns fiercely, emitting large quantities of heat and fumes. Improper storage of nitrocellulose increases the risk of explosion. In applications where formulators and manufacturers want to avoid nitrocellulose due to safety or regulatory concerns, cellulose esters may be an appropriate alternative.
A major benefit of using CA, CAB, or CAP in coatings is the control of rheological properties due to their near-Newtonian flow properties. Cellulose esters build viscosity exponentially with evaporation of the solvents, which prevents:
- Film distortions and craters
- Picture framing
- Sags and runs
- Inconsistent gloss control with matting agents
- Poor holdout
- Pigment flooding and floating
- Color separation in multi-pigmented systems
- Poor metal flake orientation
When used in numerous coatings and ink formulations, cellulose esters can provide a significant performance edge. Some of the formulation benefits are explained in more detail below.
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 (near Newtonian at low shear) and surface tension equalization properties, cellulose esters—especially the lower-molecular-weight grades—improve flow and leveling of a coating.
Spray coating usually produces some irregularities in the coating that must level out 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.
Good sag resistance
In many coating formulations, additions of 1%–5% CAB (based on resin solids) will improve flow and leveling while maintaining good sag resistance.
Rapid film formation
CAB can be used as a film former, as a reactive polyol in curing coatings, and 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 cross-link 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.
Fast dry to touch
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.
Enhanced pigments dispersion
Due to their rheological properties, cellulose esters prevent color separation in multi-pigmented systems, commonly called pigment flooding and floating.
Cellulose ester also prevent pigment separation and provides good pigment wetting. Because of good pigment-wetting characteristics (particularly the higher hydroxyl types), CAB is used to enhance pigment dispersion without negative effects on coating performance.
As an illustration, 10%–20% CAB (based on resin solids) is used with both thermoplastic and thermosetting acrylic resins as a pigment dispersion medium.
Outstanding metal finish
Used as viscosity control agents, cellulose esters provide excellent pigment flake orientation; therefore, coatings exhibit a brighter finish with more flop.
Moreover, CAB provides excellent redissolve resistance and prevents strike-in caused by solvents in the topcoat.
Heat and 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 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, and 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. This property is very important for automotive coatings that need superior exterior durability. It is also important when producing white or pastel wood coatings that do not yellow when exposed to sunlight.
Eastman’s technical experts can guide formulators in product selection to help them provide solutions to improve their products to meet the demanding needs of their customers.