UV Curable Aqueous ~

Dispersions For

The physical and rheological properties

of metal and wood coatings made from

radiation curable acrylated oligomers are put to the test.

n recent years the wood industry has faced increased pressure to I offer environmentally friendly

alternatives to existing technologies. Due to technical advances and increasing pressure to reduce volatile organic compounds (VOCs), UV cur- able water-borne systems are experi- encing rapid growth in the market- place, especially for wood finishing.

The advantages of UV curable water-borne systems for many indus- t r ia l applications have been dis- cussed previouslyl-3. Recent improve- ments in dispersion technology such

By Z. Jeffrey Wang, Jo Ann Arceneaux and Jodi Hall

UCB Chemicals, Radcure Business Unit, Smyrna, Ga.

as the method of surfactant selection, the optimization of various parame- ters in the dispersion process and the enhancement of dispersion stability have made i t possible t o disperse a variety of radiation curable oligomers in water. These oligomers include acrylated epoxy, acrylated urethane and acrylated polyester resins. In this article, we will discuss the physi- cal and rheological properties of these dispersions.

Previously, a common but signifi- cant factor limiting the use of radia- tion curable water-borne coatings on wood was poor performance, due mainly t o the use of water as a sol- vent. Water has poor solvency and a high surface tension, which lead to wetting, leveling, coalescence and other formulation problems. Much work has been done in our laboratory to overcome these problems, with encouraging results.

In this article the performance of coatings made from some of these aqueous dispersions on metal and wood surfaces will be presented and

compared to the properties of coat- ings made from the s a a e - " i s - persed resins.

Also, we will present a new type of aqueous dispersion recent- ly developed i n our laboratory. In addition to water- dilutability and sprayability, this dispersion is char- acterized by being tack-free before cure, allowing early handling and sanding.

Tack-free-before-cure wood coating a re of interest for two reasons: t eliminate waste from dust and othe contamination of coated but uncurec wood, and to allow the repair of coat ing imperfections before cure (rub bable coatings).

Experimental The oligomers (urethane acrylics

epoxy acrylates and polyester acry lates) were dispersed in deionize0 water using appropriate surfactants The resulting dispersions were about 60-65 percent solids.

The viscosities of the dispersions were measured a t 25°C using a Brookfield Synchro-Lectric or Haake viscometer. The particle size and par- ticle size distribution of the disper- sions were observed using ei ther light scattering or microscopic tech- niques.

Darocur 1173 (4 pph on solids) was added to the dispersion or resin sys- tem and mixed until homogenous. Approximately 4-6 mil films were then drawn down with a #5 wire wound rod on Parker Bonderite 40 steel panels.

The viscosity of the formulated dis- persion was adjusted to -200 cps a i room temperature by dilution with deionized water. Wood coatings were then made by spraying these formu- lated dispersions on either maple or oak.

Both primer and top coatings were made from the same formulated dis- persion. The wood was primed, dried, UV cured (this curing step was omit- ted for tack-free-before-cure coat- ings), sanded and topcoated.

For the dispersions, the water was evaporated prior to cure by heating the sample with a n infrared lamp until the film became clear. The dry- ing temperature and time varied

24 August 1996 MODERN PAINT AND COATINGS

urethane acrylate and polyester acry- late-based materials.

Overall, the films made from these dispersions have similar performance properties to the films made from the undispersed resins. As a typical example, Table I1 lists the properties of the film made from the dispersion of an acrylated epoxy oligomer. For comparison purposes, Table I1 also lists the properties of films made from neat acrylated epoxy oligomer and a monomer dilution of acrylated epoxy oligomer.

The viscosities of the dispersions a re much lower than those of the

26

undispersed resins. Fortunately, the viscosity of a dispersion can be easily adjusted to meet the requirements of an application technique through the addition of deionized water. This type of dilution should not affect the final properties of the dispersion. Table I1 compares a water and monomer dilution of epoxy acrylate oligomer with the same viscosity, as well as neat epoxy acrylate oligomer with much higher viscosity.

It was surprising to find that films made from aqueous dispersions can have faster cure speeds that films made from undispersed oligomers. As

films made from neat resins. This improved hardness also could be due to better surface cure from the miti- gation of oxygen inhibition.

The results of the reverse impact tests indicate that films based on dispersions have better flexibility than films made from undispersed oligomers. The cause of th i s in- creased flexibility is not clear. One possible reason is that the surfactant and residual water in the film act as plasticizers and thus flexibilize the film. The increase in adhesion could also be due to the plasticization and the subsequent decrease in shrink-

August 1996 MODERN PAINT AND COATINGS

pending on the film thickness (40- 1°C and 5-10 min). The panels were en cured in air using one or two, 10 wattlin. mercury vapor electrode- ss lamps, a t the maximum belt ,eed that gave a tack free film. The properties of these films were en tested according to the following 3TM4 methods: adhesion (ASTM K- $59-83), adhesion-crosshatch on aple wood (ASTM K-3359-83), irdness (ASTM D3363-74), flexibili- (ASTM D522-88 and D2794-90),

Id gloss a t 60" inclination (ASTM 523-89). In addition, solvent (MEK) id water resistance were tested 3ing the double rub method.

esults A variety of radiation curable igomers have been successfully dis- xsed in water. Chemically, these igomers include acrylated epoxies, romatic and aliphatic urethane

The particle size and particle size distribution of the dispersions can

vary considerably depending on the

chemical and physical properties of the

oligomers.

crylates and acrylated polymers, all f various functionality. Rheologically, these oligomers vary

n viscosity from 80 cps to greater han 1 million cps at room tempera- ure. Table I summarizes typical roperties for these dispersions.

'article Size Dispersions were characterized by

,heir particle size and particle size listribution. In general, the particle ;ize of the dispersions prepared in )ur laboratory ranged from about 1.1-2 pm.

The particle size and particle size listribution can vary considerably lepending on the chemical and phys- ical properties of the oligomers. For 2xample, the dispersion of an acrylat- .d epoxy oligomer has an average particle size of 0.43 pm.

Furthermore, statistical analysis indicates that about 90 percent of the particles fall in th i s particle size

range, within f l o (see Figure 1).

R h eo1 og ica I Be haw or

Flow properties of the aqueous disper- sions were deter- mined. Viscosities of the dispersions (a t about 65% solids) vary over a wide range: 1,200-10,000 cps a t 25°C. The experimental inves- tigations indicate tha t viscosity is closely related to the particle size and par- ticle size distribution of the dispersions.

For those disper- sions containing the same percentage of water , the smaller the average particle size, the greater the viscosity. Under otherwise identical conditions, the viscosity of a dispersion increases with the increasing volume content of the dispersed phase, i . e . , the oligomer phase. Dispersions pre- pared in our laboratory have pseudo- plastic rheological behavior, by which the dispersion viscosity decreases with increasing shear ra te , bu t returns to its original value immedi- ately upon removal of shear.

Figure 2 illustrates this pseudo-

plastic rheological behavior for three dispersions: dispersion of acrylated urethane oligomer (curve W), disper- sion of acrylated epoxy oligomer (curve 0) and dispersion of polyester acrylate oligomer (curve A).

The viscosity of dispersions can be adjusted to a wide range by dilution with water. This unique capability effectively decouples the oligomer molecular weight from the formulat- ing viscosity, thereby offering novel design opportunities for formulation

MODERN PAINT AND COATINGS August 1996 25