IntroductionOver the past decade, epoxy acrylates and polyester acrylates have become the workhorses for UV litho ink formulating. The Bisphenol-A epoxy acrylates and their modifications are economical and provide the basic litho ink properties such as ink tack, misting and water balance and are suitable for low performance inks. Polyester acrylates have lower viscosities, higher functionalities and are used in high-performance inks.
This study compares the properties and performance of UV litho inks made with the industry standard Bis-A epoxy acrylate versus those made using Bis-A epoxy acrylate modifications such as fatty acids and/or chain extension.
Experimental ApproachThe epoxy acrylates evaluated are commercially available and are commonly used in the formulation of UV lithographic inks. Their descriptions and basic physical properties are given in Table 1.
Table 2 gives the model formula used to prepare cyan and magenta inks with each of the epoxy acrylates. During ink preparation a number of observations were made, including ease of pigment addition and incorporation. Benchtop testing for the inks included viscosity, yield, and ink-water emulsification in accordance with ASTM test methods D-4040, D-4361 and D-4942. Additionally, ink misting and reactivity were also measured.
In this evaluation, total color differences (DE) were used to quantify misting. The degree of misting is indicated by the color difference between the reference, an unexposed chart, and the test charts. The color differences were determined by placing a piece of white chart paper underneath the inkometer rollers during the tack test. Prior to curing, the test area was divided into quadrants. After curing, spectrodensitometer readings were taken in each quadrant and an average color difference calculated for each chart. The reported total color difference for the inks is the grand average of triplicate results.
Ink reactivity was determined by mar resistance of an ink proof (average 0.2-mil dry film thickness). The reactivity was reported as the energy density required to achieve a mar-free film with one 400-watts/inch lamp.
Ink printability was evaluated on a Ryobi CD 2800 duplicator. Make-ready and the presence of typical printing problems such as toning, scumming and plugging were used to characterize the printing properties of the inks. Print contrast and color densities were used as indicators of print quality.
Results and Discussion
Viscosity and Yield
As expected, the Laray viscosities and yields of the inks paralleled the viscosity of the oligomers, except in the case of the Bis-A and the extended modified Bis-A epoxy acrylate inks.
With these inks, a higher ink viscosity and yield were measured with the ink containing the extended modified Bis-A epoxy acrylate, whose oligomer viscosity was actually lower than the Bis-A epoxy acrylate. This can possibly be attributed to the differences in oligomer yield values. The yield value of the extended modified Bis-A epoxy acrylate is approximately four times greater than that of the Bis-A epoxy acrylate.
The Laray viscosity and yield for the fatty acid modified epoxy inks and the flexible Bis-A epoxy acrylate inks were lower, reflecting the use of lower viscosity oligomers. The lowest viscosity ink contained the modified epoxy acrylate.
Tack and Misting
Ink tack was also directly proportional to the oligomer viscosity, again except in the case of the Bis-A and extended Bis-A epoxy acrylate inks. Inks containing the Bis-A epoxy acrylate and the extended, modified Bis-A epoxy acrylate exhibited high ink tacks. The ink tack for the cyan and magenta Bis-A epoxy acrylate inks averaged 34.9 gram-meters, and 46.2 gram-meters for the cyan and magenta inks made with extended, modified Bis-A epoxy acrylate. Most likely, the high yield value of the extended, modified Bis-A epoxy acrylate is responsible for that inks’ high tack.
The ink tacks for the fatty acid modified Bis-A or the flexible Bis-A epoxy acrylate inks were more practical, average were 19.4 gram-meters for the former and 28.0 gram-meters for the latter.
It is well known and accepted that an ink’s tendency to mist is strongly dependent upon its tack. This evaluation supports that theory. The inks with the highest tack, those containing either the Bis-A or the extended, modified Bis-A epoxy acrylate, also had the lowest misting as measured by the total color difference (DE). The modified epoxy acrylate, whose cyan ink tack averaged 5.8 gram-meters displayed excessive misting. Interestingly, the misting of the magenta ink made with the same oligomer was significantly less. While higher DEs were measured with the fatty acid modified and flexible Bis-A epoxy acrylates, the degree of misting was deemed acceptable.
The Bis-A, fatty acid modified Bis-A, extended modified Bis-A and flexible Bis-A epoxy acrylate inks had reactivities of 112-135 mJ/cm2. The fatty acid modified epoxy acrylate exhibited extremely poor reactivity, requiring over 2300 mJ/cm2.
All of the inks exhibited “C” type Surland curves or water pick-up profiles. During emulsification testing, exceptionally high changes in fountain solution conductivity were observed with many of the inks. This was particularly true of the magenta inks where changes in the fountain solution conductivity ranged from 35 percent to 79 percent. With the exception of the fatty acid modified inks (4 percent) and the Bis-A epoxy acrylate inks (48 percent), fountain solution conductivity changes for the cyan inks were less, averaging only 13 percent.
Previous work has shown a correlation between conductivity changes in the fountain solution (during Duke testing), line slopes through the first data points of the Surland curves, and printability. In that evaluation of magenta inks containing either polyester acrylates or epoxy acrylates, acceptable printability was achieved with low changes in conductivity and high line slopes. High conductivity changes and low line slopes resulted in printing difficulties.
A similar correlation is seen in this current evaluation. In general, conductivity changes for the magenta inks were high, and were low for the cyan inks. The calculated line slopes through the first five data points of the Surland curves ranged from 3.2 to 4.1 for the magenta inks and 2.3 to 3.8 for the cyan inks. Inks with lower fountain solution conductivity changes (relative to the color) and low line slopes (below 3.2) tended to exhibit some type of printing difficulty – toning, scumming or plugging. Higher changes in the fountain solution conductivity and line slopes of ~3.7 appear to indicate marginal printability. Inks displayed the best printability when the fountain solution conductivity changes were in the middle of the range (for the color) and the line slopes ranged from 3.3 to 4.1.
The printability of an ink is dependent upon the oligomer and corresponding ink properties such as viscosity, yield and tack. Inks made with the flexible Bis-A or the fatty acid modified Bis-A epoxy acrylate exhibited superior printing qualities. At dry film thicknesses of 0.2-0.3 mil, their color densities were on target (GRACoL- sheetfed, offset wet ink densities). The print contrast, an indication of print quality, and water balance were significantly higher for these inks than for the Bis–A epoxy acrylate inks. Make-ready was straight forward and without complications for these inks. There were no problems maintaining the color density and print contrast while printing on the uncoated stock.
As mentioned earlier, the ink made with the extended, modified Bis-A epoxy acrylate exhibited high viscosity, yield, and ink tack. Its printability was marginal at best. While make-ready was without problems, the achieved color densities were below target and the print contrast narrowly met the 25 percent target. During printing, ink transfer problems were observed particularly with the cyan ink. The high ink tack caused an additional problem – paper picking on the uncoated stock.
The printability of the Bis-A epoxy acrylate inks was marginal as well. Although make-ready was fairly easy and trouble-free, a narrow water window was a major issue with these inks. The color density varied, averaging 1.4 for cyan and 1.45 for magenta. Print contrast was also well within range, ~ 40 – 43% for both inks. It was relatively easy to establish the water window, but maintaining the window with good color density and without scumming or plugging was a challenge.
The “softness” and low tack of the modified epoxy acrylate inks led to extreme printing problems. Make-ready was lengthy and difficult. Roller stripping and bleed-out into the fountain solution were major obstacles. With much difficulty the cyan ink was printed. Plate toning was a problem and the maximum achievable color density was 1.3. Efforts to print the magenta ink were totally unsuccessful.
The results of this evaluation indicate that modifications to the Bis-A epoxy acrylate will influence its usefulness and performance in UV lithographic inks. Some modifications enhance performance such as color development and printability. Other modifications increase the formulating difficulties typically associated with epoxy acrylates.
While the attributes of epoxy acrylates – oligomer/ink tack, ink viscosity and cost cannot be forgotten, polyester acrylates have raised the ink performance bar. Polyester acrylates offer improved pigment wetting, wider water windows and adhesion to various plastics. As ink performance continues to increase, polyester acrylates will become the benchmark for UV lithographic inks.
The author acknowledges with gratitude, the contributions of Charles Henderson of UCB Chemicals.