Conventional epoxy acrylates based on diglycidyl ether bisphenol A are the most frequently used oligomers in the ultraviolet light (UV) curing industry because they are hard, fast curing, and offer excellent abrasion and chemical resistance. Unfortunately, the inherent hardness of these oligomers has several negative side effects, including yellowing over time, high viscosities and poor adhesion.

Recently, a major specialty chemical manufacturer conducted testing to develop new epoxy acrylate oligomers that offer all of the traditional advantages of using an epoxy acrylate without yellowing. In the testing, a diacrylate low viscosity oligomer (CN-132) and a triacrylate low viscosity oligomer (CN-133) were compared to a difunctional bisphenol A-based epoxy acrylate (CN-120). A variety of performance properties were evaluated to determine the utility of these oligomers as replacements for typical epoxy acrylates in wood coating applications.

Cure Methods

Researchers evaluated CN-132, CN-133 and CN-120 without using monomers as diluents. They were cured using 4% of a highly reactive, non-yellowing photoinitiator for the photopolymerization of UV-curable systems. This photoinitiator was composed of a liquid oligomeric alpha hydroxy ketone and 2-hydroxy-2-methyl-1-phenyl propane mixture.

Researchers used an aluminum chromate Q-panel substrate, and drew the films down using a number 40 wire wound rod, which ultimately yielded films with a thickness of 2 mils. The films were cured at 50 feet per minute using a 300-watt per inch mercury lamp.

Performance Testing

Performance properties evaluated include: pencil hardness, chemical resistance, maximum rate of cure speed, adhesion, abrasion resistance and Hoffman scratch.

Pencil Hardness

Pencil hardness is determined using a calibrated set of drawing leads that range from 6B (the softest) to 6H (the hardest). The first pencil that scratches the surface is reported as the coating’s hardness. This is specified as ASTM D 3363.

The results, shown in Figure 1, indicate that CN-133 performs as well as CN-120. CN-132 also performs well but is not as hard. However, the actual difference in pencil hardness is insignificant.

Chemical Resistance

Chemical resistance testing using methyl ethyl ketone (MEK) is an indication of the degree of crosslinking associated with a given film; the higher the MEK resistance the better the cure. It was tested as outlined in Appendix 8 of Radiation Curing Test Methods. Figure 2 shows that testing was ended at 200 MEK double rub cycles, the maximum number of rubs performed in this test. The data reveals that CN-132, CN-133 and CN-120 have excellent MEK resistance and reached the maximum resistance.

Maximum Rate of Cure Speed

The maximum rate of surface cure was determined using a 300-watt per inch Hg bulb. The fastest conveyor line speed at which a tack free cure is still attained is recorded in Figure 3.

The findings represented in Figure 3 indicate that CN-132 and CN-133 outperform CN-120. The difference in cure between CN-132 and CN-133 is significant, but the difference between these two oligomers and CN-120 is even more significant.

Adhesion

Adhesion is a measure of the force required to remove the coating from the substrate. Tests were conducted in accordance with ASTM D 3359 using 610 tape.

Figure 4 illustrates that CN-132 and CN-133 outperform CN-120 again. Adding adhesion promoting monomers or low functionality monomers can enhance adhesion even more.

Abrasion Resistance

Abrasion resistance is one of the most complex properties to achieve, as it is both a surface and sub-surface property. Thus, abrasion resistance can involve different mechanisms that may be interrelated.

Taber abrasion relates to the ability of a coating to resist abrasive wheels. All panels were tested in accordance with ASTM 4060-84. Taber abrasion resistance was tested under varying loads using a CS-17 wheel with a 1,000-gram load for 500 cycle intervals. Milligrams of weight loss per 500 cycles are recorded.

As Figure 5 illustrates, CN-132 failed with the 2,000-gram load. CN-133 showed improvement in abrasion resistance when compared to CN-120 for all of the loading levels.

Hoffman Scratch Resistance

Hoffman scratch resistance is measured by placing a load on a stylus. The weight of the load required to mar and scratch the surface was measured.

As illustrated in Figure 6, CN-132 and CN-133 significantly outperform CN-120 in mar resistance. In addition, the triacrylate oligomer exhibits slightly better scratch resistance than the conventional epoxy acrylate oligomer.

Conclusion

The newly developed, low viscosity, non-yellowing CN-132 and CN-133 oligomers can replace conventional epoxy acrylates in many applications, including wood coatings. These products outperform bisphenol A-based epoxy acrylates (CN-120) in several key performance properties. CN-133 has equivalent pencil hardness and chemical resistance with superior surface cure speed, adhesion, abrasion resistance, and scratch resistance. CN-132 also offers non-yellowing characteristics with extremely low viscosity, superior surface cure speed, and adhesion in comparison to CN-120.

For more information on oligomers, contact Sartomer Co., Oaklands Corporate Center, 502 Thomas Jones Way, Exton, PA 19341; phone 610/363.4199; fax 610/594.0252; e-mail burak@sartomer.com; visit www.sartomer.com.

Links

• Sartomer Co.