Benchmarking DriLith W2: Sheetfed Offset Water-Washable Waterless Inks
By Emmanuel D. Dimotakis, Michihiko Toya and Anthony Cieri
Daniel J. Carlick Technical Center
Sun Chemical Corporation
The DriLith W2 series of inks has received significant exposure in the printing industry, spurring inquiries from waterless printers around the world.
Benchmarking of DriLith W2 inks has been carried out with both medium and high-end waterless printers using different presses and stocks. It was found that DriLith W2 inks print with general performance characteristics (toning, set off and gloss), dot gain and trap values similar to those of solvent-based inks. Their low tack and soft body permit easier handling and use with a variety of uncoated and coated stock.
In addition, less paper waste at start up, less piling and pulling were also observed during runs.
The waterless printing market consumes about 2 percent of the total offset lithographic plates produced worldwide (2x109 ft2/year) and at the same time provides a much higher profit margin niche business than conventional offset printing. 
Computer to plate technology is also driving the current waterless market. The smallest waterless market is in the U.S., with approximately 50 sheetfed printers producing an average of $14 million to $15 million of printed material per year. Waterless lithography represents 8 percent to 10 percent of the sheetfed market in Japan, 7 percent in Europe and only 1.5 percent in the U.S. 2
In view of our recent market introduction of the first water-washable waterless DriLith W2 inks, [3-9] we decided to take an extensive look at their printability and benchmark them versus our solvent-based waterless inks.
DriLith W2 inks offer the potential to virtually eliminate all pressroom-related volatile organic compounds (VOCs) and hazardous air pollutants (HAPs).
The environmental impact is enormous. Based on U.S. Environmental Protection Agency (EPA) data, a sheetfed printer employing 10 to 20 people produces a total of four tons of VOCs per year from ink, press and blanket wash. Water washability alone offers tremendous benefits. One printer has recently shared data with the Waterless Printing Association where VOCs related to blanket and roller washes alone account for 6.7 tons per year (about 40 percent of the total VOC emissions).
The new inks offer the advantages of water-washability coupled with three to five units lower tack and softer body for easier handling than Sun Chemical’s solvent-based waterless inks.  These advantages, combined with the compatibility between DriLith W2 and zero VOC-containing water-based coatings, has spurred a worldwide interest in DriLith W2 inks, creating a potential for explosive growth in this product line.
The benchmarking of DriLith W2 inks entailed examination of printability parameters such as:
(2) dot gain;
(4) set off; and
(5) rub resistance.
The study was also designed to accommodate:
(1) presses from different manufacturers;
(2) wide and narrow format presses;
(3) stock variety from uncoated board to 100# coated paper; and
(4) optional use of water-based coatings.
At the same time we were working to optimize the magenta-yellow wet trap for our water-washable DriLith W2 customers. It has been suggested that ink film thickness depends on ink color strength and transfer. Ink transfer depends on rheology and formulation. The intent of the experimental design was to find out if the water-washable ink vehicle follows the same rules for ink transfer and trap formation as the solvent-based vehicle.
A Prufbau printability tester with the correct roller pressure and nip gap settings was used for printability testing and trapping experiments. Prints of each DriLith W2 ink were produced at S.W.O.P densities. Densities were measured using an X-Rite 428 densitometer. Gloss of prints at S.W.O.P densities was recorded at a 60° angle using a micro-TRI-gloss glossmeter from BYK Gardner. Set off tests were carried out using a Little Joe and a Josons drying time recorder. Tan∂ curves were obtained using an AR2000 rheometer from TA Instruments.
Two types of trapping experiments were performed, static (print one color at a time: wet on dry ink) and dynamic (print in sequence: wet on wet ink).
In both experiments, three prints of M were produced using the Prufbau printability tester and 130, 170 and 230 mm3 of ink respectively. Y was printed on top of each one of these M prints by using the corresponding volume of Y ink, i.e. 130 mm3 of Y on top of 130 mm3 of M already printed. The traps and mass transferred for both M and Y were measured, recorded and plotted (see Figures related to traps in the next section).
Results and Discussion
General Performance Characteristics (toning, set off and gloss):
A summary of the results from benchmark testing at different customers is provided in Table I. Several printing presses were used with narrow (Adast), medium (Komori, Akiyama, Mitsubishi, OMCSA) and wide format (Komori, Heidelberg, OMCSA). Typical runs produced between 1,000 to 160,000 prints.
No toning at start or pulling was observed, resulting in less paper waste. Uncoated prints provided no set off using both coated and uncoated substrates within the same time frame as the solvent-based waterless inks. The gloss level was the same as that provided by solvent-based inks. Tan∂ values of 1 to 2 were obtained between 0.1-100Hz (Figure 1) with the exception of the black due to its slightly different formulation. These values are consistent with previous observations for good gloss development. 
The new inks allow the use of air-dried water-based coating, thus further reducing the VOCs in the press room. Finally, UV compatibility and good fade resistance were observed after printing.
Dot Gain Analysis of DriLith W2 Prints:
High-end quality printers were used to verify the final dot gain values. Prints were analyzed from the initial and final runs to demonstrate improvement in dot gain with the final formula adjustments. Shown in Table II are dot gains (at 50 percent) for the four process colors at standard ink densities: The dot gain of the black was improved from 29 percent to 23 percent and that of the yellow from 21 percent to 17 percent. The values correspond well with the output variables from GRACoL recommendations shown on the lower part of Table II. The print contrast was also within the recommended values.
Trap Analysis and Improvement:
Failure of ink to trap in wet multicolor printing operations is the result of several factors. [11-12] Most printers associate bad trapping with the second down ink as being tackier than the first.  A review of the scientific literature of offset lithography and wet trapping reveals that several factors can be responsible for bad traps: (1) ink tack, (2) color strength, (3) ink density or mass, (4) film thickness and (5) ink rheology. [11-16] In waterless lithography, there is no fountain solution and knowledge gained from this study can be beneficial to both wet and waterless lithography.
Lithographic ink rheology and press dynamics have been used to simulate DriLith W2 ink operational results in a lab environment. Static and dynamic experiments were carried out using three DriLith W2 yellow inks (two with low, 3990 and 3713 dyn/cm2 and one with high, 6500 dyn/cm2 Duke yield stress) with the same magenta ink (see Table III). The yield stresses were measured by Duke viscometer at 2.5 s-1.
All yellow inks had tack values 0.7-2.0 units lower than magenta and same high shear rate Duke viscosities (within a range of 40 poise at 2500 s-1). Yellow (high ys) and yellow (low ys-B) had the same color strength.
The following results were obtained:
At standard print densities and under dynamic lab conditions, a yellow ink with higher color strength (Figure 2) provided less mass transfer, thinner films and worse trapping (T=25-40) over magenta ink as compared to trapping (T=50-58) obtained with a lower color strength yellow ink (Figure 3). The yellow inks had tack values 0.7-2.0 units lower than magenta. They also had the same high shear Duke viscosities (within a range of 40 poise at 2500 s-1).
These results indicate that the lower tack of the second down ink (yellow) versus that of the first down ink (magenta) is not sufficient to produce high quality trapping. Furthermore, trapping of yellow over magenta under static and dynamic conditions in the lab indicated that press dynamics and magenta leveling also affect the quality of the magenta-yellow trap and other traps as well.
The latter results can not be explained on the basis of yield stress differences in the yellow inks.
Another yellow (low ys-B) with color strength equal to that of high yield stress yellow was tested in trapping experiments. When overprinted on magenta, the trapping was 64 units using 230 mm3 of yellow for standard density (d=1.16). This demonstrates that thicker films of Y ink are transferred on the Prufbau or printing press by using a lower color strength Y ink. Thicker films mean higher volumes of ink since standard yellow densities (d=1.05-1.16) are now obtained using 230 mm3 of yellow DriLith W2 ink versus 130 mm3 for the higher color strength but also of the same low yield stress yellow ink.
When compared to the higher yield stress yellow of the same color strength, it appears that yield stress doesn’t have any significant effect on the trapping. This study indicated that ink trapping is controlled by similar factors, namely ink color strength, as in solvent based inks and helped achieve excellent trapping (70-90 units, table IV) in M-Y, C-M and C-Y traps using water-washable waterless DriLith W2 inks.
Medium and high-end waterless printers have carried out trials using different stocks to benchmark DriLith W2 inks. The following conclusions were drawn:
(1) DriLith W2 inks print with general performance characteristics (toning, set off and gloss), dot gain and trap values similar to those of solvent-based inks.
(2) Their low tack and soft body provide for easy handling and use on a variety of stocks.
(3) Less paper waste at start up, less piling and pulling were observed during runs.
Factors affecting the trap quality of DriLith W2 inks have been examined. Under dynamic lab conditions, a yellow ink with lower color strength provided improved trapping over magenta ink as compared to a higher color strength yellow ink. It appears that trapping is controlled by similar factors as in the printing of solvent-based inks.
This benchmarking study assisted lithographic printing operations in improving various aspects of DriLith W2 ink printability. The inks have received significant exposure in the printing industry, spurring inquiries from waterless printers around the world.
The authors would like to acknowledge the assistance of Richard Drong, marketing manager for sheetfed inks at Sun Chemical, Richard Lovingood and Dan Carroll, senior technicians at our Winston-Salem plant and Tom Gehrke of the Lancaster branch. Sun Chemical Corporation is also acknowledged for allowing publication of this work
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