The secret to developing a good printing UV flexographic (flexo) ink is related to pigment wetting and the rheology of the formulated system. Ultimately, the goal in formulating UV flexographic inks is to have a system that is very low in viscosity and remains low in viscosity under any type of shear or stress (also known as Newtonian).1 Unfortunately, the introduction of pigments into a Newtonian liquid produces considerable deviations from Newtonian behavior. Therefore, by understanding the rheological behavior of an ink system, it is possible to achieve an ink system that is easy to print and has high gloss and excellent color strength. In this paper, we describe a rheology method that is used to measure thixotropy known as the “step-shear” test. This step-shear test simulates the printing process on a flexo press. The rheology data is correlated to information collected from flexo trials performed in house at UCB Chemicals Corp.
Rheology BackgroundThe demands on the quality of UV flexo inks have grown steadily over the past few years due to increasing capabilities of the flexo printing process and the conversion of conventional flexo printers to UV. However, UV flexo inks do not contain solvents, and thus exhibit more complicated rheological properties compared to conventional inks.
In the past, UV flexo inks would thicken with time but would break down when agitated. When the viscosity of a liquid drops while being agitated, it is called “shear thinning.” A shear-thinning liquid, also referred to as thixotropic, is defined by its potential to have its structure rebuild when left at rest for a period of time.1
To effectively evaluate thixotropic behavior, an instrument with time dependent programs and shearing capabilities must be used. Therefore, cone and plate rheometers are ideal for measuring the thixotropy of an UV flexo ink. These rheometers can be used to develop test methods that more accurately simulate the processes that the inks may be subjected to on press.
The rheological characteristics of UV flexo inks determine its performance on the press, and many attempts have been made to relate the rheological parameters of viscosity, yield value and thixotropy both to press performance and to printing quality.2,3
To date, most of the press/rheometer correlations have defied a rigorous quantitative treatment and have provided very limited information. A beneficial rheology program would be one that is close to the practical application. For rheology characterization to be beneficial to UV flexo printers, a program had to be developed that simulated the conditions of a flexo press.
A rheology program was developed that is referred to as a “step-shear” test. For this program, the sample was initially held at a very low shear rate for an extended amount of time. This emulates the process of the ink resting in the pan prior to application. The rheometer was held at a shear rate of 2.5 seconds-1 for 180 seconds. After the ink leaves the pan, it undergoes a tremendous amount of shearing as it passes through the anilox roll to the plate.
During this process, the ink is sheared at higher rates. For the rheology program, the shear rate was increased to 2500.0 seconds-1 and held for 30 seconds. After the ink leaves the plate and is applied to the substrate, it no longer undergoes high shear rates. The shear rate was lowered to 2.5 seconds-1 and held for 600 seconds. The extra time was added to the rheology test to allow the ink to rebuild. A thixotropic liquid is defined by its potential to have its structure rebuild when left at rest for a period of time. The slope of the curve from the step-shear test will give an idea of the speed of recovery. The larger the number for the slope, the faster the ink will “rebuild” and exhibit thixotropic behavior.
The shortness index is calculated by dividing the viscosity measured at 2.5 seconds-1 by the viscosity measured at 2500.0 seconds-1. A shortness index equal to 1.0 indicates Newtonian flow behavior.
The shortness index measures the shear thinning properties of the ink. When the viscosity of a liquid drops when sheared, it is called “shear thinning.” Therefore, a higher shortness index number means more shear thinning of the ink. This step-shear test measures both the extent of the shear thinning and recovery with a time element.
All rheology measurements were performed on a Haake RS150 Cone and Plate rheometer equipped with an aluminum 60mm, 1 degree spindle. The rheometer was thermostated by the use of a Peltier device at 25°C for all measurements. The step shear program, described above, was used to evaluate the UV flexo inks.
Pigment wetting can be described as the interaction between the chemical structure of the binder and the pigment that helps to prevent the flocculation process. The key to producing good ink of any type is primarily the need to overcome the difficulty of wetting the dry organic pigments in the grinding resin. For UV flexo inks, the two most popular classes of grinding resins are epoxy acrylates and polyester acrylates. Epoxy acrylates are popular because of their excellent balance of cure speed, hardness of the final print, and relatively low cost. The polyester acrylates are among the best pigment wetting resins by the use of sophisticated polyols and/or fatty acid modifications. Print quality is affected by the choice of the starting resins, with the polyester acrylates giving better pigment dispersions than standard epoxy acrylates.
A new polyester acrylate, Eb 812, was specifically developed for UV flexographic inks due to its inherent pigment-wetting capabilities and excellent adhesion to plastic substrates. In addition to the development of Eb 812, UCB Chemicals is helping the flexo ink makers to produce higher quality inks by developing a new line of flexo ink vehicles (under the name ViaFlex). ViaFlex 100 is the first product to be launched in this new line of ink vehicles. ViaFlex 100 is a new flexographic pigment-wetting vehicle for pigment grinding and dispersing during the manufacturing of UV flexo inks. By using ViaFlex 100 as the grinding medium, UV flexo ink formulations will exhibit a greater level of color strength, improved rheology, and milling ease. ViaFlex 100 demonstrates low viscosity and has excellent cure response.
UCB has also launched ViaFlex 165, which is a new high-performance pigment-grinding vehicle for UV flexo inks. ViaFlex 165 is low in viscosity and was specifically designed to be used in conjunction with ViaFlex 100.
The photomicrographs in Figure 1 show how ViaFlex 100 effectively prevents flocculation of pigment particles. Optimum deflocculation occurs when primary pigment particles and aggregates are separated from each other as individual moieties. This results in good rheology, high gloss and maximum hiding power. The disadvantage of deflocculation is the tendency of the pigment to “settle out.” Controlled flocculation gives the ink formulator the opportunity to maximize color development. The flocculation process can be observed through microscopy by diluting the inks in a DPGDA/Irgacure 369 formulation. For the flocculation test, the UV flexo ink was diluted 90% in a formulation of 92.5% DPGDA/ 7.5% Irgacure 369 sample. A drawdown (using a #6 drawdown bar) of the final ink was prepared and cured on clear OPP. A sample of the cured ink was removed and observed under a stereo microscope. A Polaroid digital camera was attached to the microscope (400x) and used to collect the photomicrographs.
The UV flexo inks were prepared in two steps. The first step was preparation of the concentrated pigment paste (grind stage) using ViaFlex 165, ViaFlex 100 and pigment.
The dry pigments were obtained from the following sources: Yellow BAW (Ciba), Carbon Black Raven 450 (Columbian Chemicals), Irgalite Blue GLO (Ciba) and Irgalite Rubine L4BD (Ciba). The Anthony 5x12 three-roll mill was used to grind the pigment and oligomer.
The second step was combining the pigment paste with a letdown vehicle to obtain finished inks using several diluting monomers, photoinitiator systems, and additives. For all of the inks, a new liquid photoinitiator vehicle, known as ViaCure DX, was used.
Starting Point Ink Formulations:
Formula A%ViaFlex 16518
Formula B%ViaFlex 16535
Pigment15DPGDA12TMPEOTA30ViaCure DX8Flexo Press Trial
Inks prepared with carbon black pigment and ViaFlex165 vehicle were run on a two-color Aquaflex press. This press is equipped with both air dry and UV lamps and has a maximum speed of 500ft/min. The lamps are Fusion Aetek Ultrapak, 400 watt/inch (160 W/cm). For the work performed, a Harper 550-lines per inch (lpi) ceramic anilox roller was used. Two different doctor blades were used: IKS (25.400 x 0.984 x 0.006”) blade with a 45 single bevel and Allison Systems Corporation (0.75 x 120 x 0.008) Carbon Blue single bevel. An Enercon LM385-02 corona treater was used to surface treat the substrate. The primary substrate used was white polystyrene supplied by MacTac. The digital plate, supplied by Master Graphics, delivers finer highlights and a wider tonal range.
Carbon Black ViaFlex165/ViaFlex100 Results
During a flexo print run, press operators focus their attention on making sure that the ink pan is full, the plates for various colors are aligned correctly and the substrate is feeding properly. However, when non-Newtonian ink is used for the trial, the press operators add an unnecessary variable of ink stirring that must be addressed constantly during the press run.
At a low shear rate (2.5 sec-1), the viscosity of Formula B ink was 9800 cP. When a higher shear rate of 2500.0 sec-1 was applied to the sample, the viscosity dropped to 300 cP. When the shear rate returned to 2.5 sec-1, the viscosity of the ink immediately returned to 9800 cP (Figure 3). Because the ink immediately returned to this high viscosity, it was characterized as a pseudoplastic ink. Since the shortness index is calculated as the viscosity of the ink at low shear rate divided by the viscosity at high shear rate, a shortness index of 32.7 was calculated for this ink. Since Newtonian inks have a shortness index close to 1, this ink was very far from Newtonian. Based on this rheology data, we were able to predict this ink would be very difficult to run on the press and the final image would not look very good.
Formula B ink was added to the ink pan on the flexo press and almost immediately formed a clump in the center of the pan. With steady agitation of the ink, the pan was fully covered with ink. The press operator added two air-powered “vibrators” to the ink pan. Vibrators are very useful but produce significant noise pollution. Once the rollers were started and the anilox was moving through the ink pan, the ink started to “clump” in each corner of the pan. Approximately every 30 seconds, the press operator had to manually stir the ink in the pan. Once the press was started and the initial impression of the plate was made, the operator had to stop the run and make adjustments to the doctor blade because the ink was not being applied to some areas of the plate. A proof of the image was removed from the roll of printed substrate and evaluated. This image can be seen in Figure 2.
From this image, the color strength of the black solid area is very weak. Since a 400-lpi anilox was used to deliver the ink, the solid areas should have had a significant amount of ink. Upon closer inspection of the solid areas, it was clear that the ink did not wet the substrate very well. A lot of work went into producing a very poor image that would never make it to the market.
Formula B ink was reformulated by adding a pigment wetting vehicle (ViaFlex100). Before another trial was started, rheology data was collected on Formula A ink and is shown in Figure 3. Data from step-shear test showed a dramatic decrease in the initial viscosity of the ink when the pigment-wetting vehicle was used. The other major observation was that viscosity slowly increased over time. This ink was labeled as thixotropic, which indicated the ink would be much easier to run on the press.
Once Formula A was added to the pan, very little effort was needed to keep the ink flowing. In fact, the operator did not have to spend time stirring the ink or retrofitting the flexo press with a metering system. A proof of the final image can be seen in Figure 4.
After observation of this image, it is clear the color strength is very strong, it has high gloss and exhibits good pigment wetting. The ink has excellent adhesion to the polystyrene substrate, which could be directly related to a fully cured ink system.
ViaFlex100 Results in Other UCB Resins
Since ViaFlex100 offers improved pigment wetting, this vehicle can be used in combination with the acrylated oligomers to formulate UV flexo inks that result in lower viscosity. As seen in Table 1, ViaFlex 100 is compatible with a variety of UCB Ebecryl oligomers.
The formulation of UV flexo inks of low viscosity and good color strength is a major challenge for the ink maker. As the rheology of UV flexo inks continue to improve, they will become more competitive with conventional inks. The rheology of UV flexo inks can be significantly improved with the right combination of additives and pigment wetting vehicles. With the introduction of the new graphic art “vehicles” and the rheology program outlined in this paper, UCB is providing tools for the ink makers to improve the final rheology of UV flexo inks. Based on the data presented in this paper, the data collected on the rheometer was able to help predict how the ink would perform on press. Flexography is not only a trend; it is the most promising technology in the radiation curing graphic arts market.
The authors gratefully acknowledge the assistance of Dorian Battle for preparation of samples and for her editorial assistance.
1. Eley, Richard R. “Rheology in Coatings, Principles, and Methods,” Encyclopedia of Analytical Chemistry, 1-30 (2000).
2. Leach, R.H. and Pierce, R.J., The Printing Ink Manual (1993).
3. Pangalos, G., Dealy, J.M. and Lyne, M.B., “Rheological Properties of News Inks,” Journal of Rheology, 29, 471-491 (1985). n