Editor’s Note: “Importance of Monomer Interfacial Tension for UV Curable Litho Inks Performance” was presented during the 2002 RadTech Technical Conference in Indianapolis, IN. ©2002, RadTech International North America, Inc. Reprinted with permission from the RADTECH 2002 Conference Proceedings.
For many years, the goal of ink makers is to improve the runnability and lithographic behavior, dot gain, cure speed and odor for UV offset inks. Unfortunately, the formulation of UV curable printing inks depends on many variables, both in ink making and in lithography process.
One of the key properties of a good lithographic ink is its ability to emulsify the fountain solution and its water pick-up. The ink-water interaction, in our particular case, is determined by the hydrophilic properties of the various raw materials used in the formulation, such as UV binders, pigments, photoinitiators and UV monomers.
The aim of this paper is to describe the lithographic behavior from different grades of GPTA (glycerol propoxylated triacrylate) at 19% in a magenta UV offset ink. The two GPTAs were tested with various interfacial tension properties which are more hydrophilic for one, and for the other one, more hydrophobic.
In the last 20 years, UV curing inks have been widely used in the field of lithographic ink. According to some market surveys,[1,2] radiation curable inks are forecast to enjoy a 10 percent growth rate.
This growth is also due to a continuous improvement in printing technology. Recent developments in printing presses (sheetfed and web machines in terms of high speed production and inking/dampening units) and drier equipment (nitrogen blanketing and cold lamps) have led to a significant increase in the number of applications in the graphic arts industry, including boxes for cosmetics, food, tobacco, spirits, business forms, direct mail, lottery tickets and credit cards.
Formulation of UV curable printing inks depends on many variables. In this paper, we have tried to highlight the role of the physical behavior of the monomer in an ink recipe. We have fully characterized the monomers in term of interfacial tension in order to anticipate their behavior with water in a lithographic process.
Furthermore, inks have been formulated with these monomers and end-used properties have been compared.
Raw Material Characteristics Monomers
All the monomers used in the study are Cray Valley products. The GPTA monomers have been synthesized in order to change their affinity with water.
UV ink formulation
The printing inks for the laboratory and press investigations have the following formulation (see Table 1).
Experimental Interfacial tensionThe experiments are performed on a Krüss Drop Shape Analysis System DSA 10 at T=23°C.
The interfacial tension is measured using the pending drop method. A drop of liquid is formed at the tip of a syringe needle in a water medium. The drop shape and size depend on the interfacial tension between the liquid and the water medium.
Thus, from the profile of the drop, a software enables the calculation of the value of interfacial tension. It must be noted at this point that the same measurement can be done in air in replacement to water and thus the surface tension of the liquid is obtained.
• Viscosity is measured with a Laray viscometer at 30°C and with a Haake RS100 cone and plate viscometer, fitted with a cone C20/4 (diameter 2 cm) at 30°C.
• The yield value, expressed in Dynes/cm2, is determined using the Laray viscometer.
• Gloss reading is taken from weighed prints at a 60° angle.
• The reactivity is checked by chemical resistance to methyl ethyl ketone (MEK) and ethanol.
• Water pick-up : Duke test
This method is used for the determination of the amount of water picked up by a lithographic printing ink in a laboratory mixer. This laboratory mixer, such as a Duke Ink-Water Emulsification Tester, is equipped with a stainless steel bowl and with mixer blades rotating at90 rounds/min.
For a single-point water pick-up, 50 grams of UV ink are mixed with 50 grams of press fountain solution (Aqualite SM 95 from SICPA + 6 % of Isopropanol) during one minute on the Duke. The water pick-up is determined by weighing the quantity of free water.
• Tack measurements
Tack and misting are measured at 30°C on the Tack-o-Scope instrument from Testprint, using 0,6 g of ink. The water balance of the UV offset inks is checked on the W model. The instrument is fitted with a system allowing the controlled delivery of the fountain solution (or water). This system consists of a roller placed in a duct, which holds the dampening solution from the water duct to the center roller. The roller can be placed in contact with, or removed from, a chromium spiralled brass centre roller by means of a lever.
Thus the water/ink behavior influences the tack value of the emulsified ink, the stability, and the rate of up-take or release of water during the test.
• Lithotronic test method
The lithotronic measuring instrument operates like a rotational viscometer, giving the viscosity behavior of a sample of ink under high shear conditions. A defined amount of printing ink (25 g) is placed in a beaker and the fountain solution is added at a specific stirring speed and flow. The torque at the propeller is measured as a function of the amount of added fountain solution.
• Contact angle
`The contact angle is measured by placing a drop of fountain solution on an ink film and measuring the tangent of the angle at the point of contact between the fountain solution droplet and the ink.
Sheetfed Press Performance
UV offset printability was carried out on a sheetfed offset press Roland Favorit (see Table 2).
Physical Characterization Of the Monomers Interfacial tension
The interface between the fountain solution and the ink is complex: it means that the ink has to exhibit the right hydrophobic/hydrophilic balance.
On the one hand, a certain water uptake is required to achieve good handling features of the ink. For this purpose, the interfacial tension between the ink and the fountain solution must not exceed a value of approximately 10 mN/m.
On the other hand, the ink must not be too soluble in the fountain solution: the interfacial tension must not be lower than 0.5 to 1 mN/m.
Interfacial tension measurements allow us to anticipate the differences observed in ink performances with the same type of monomer.
On Table 3, the values of surface tension and interfacial tension for several monomers are given. The first remark is that there is no correlation between surface tension and interfacial tension with water. Surface tension corresponds to the interface of the monomer with air, which highlights the hydrophobic part of the monomer. On the contrary, the interfacial tension with water is more related to the hydrophilic part of the monomer.
An important factor that influences interfacial tension is the nature of the medium. Examples are given on Table 4, when isopropanol and surfactants are added to water. Addition of isopropanol in water leads to a lower interfacial tension than in pure water.
We have synthesized various GPTA in order to have different behaviors with water (see Table 4). GPTA-SP1 is more hydrophilic as it exhibits a lower interfacial tension with water. GPTA-SP2 is more hydrophobic. The more hydrophilic GPTA (GPTA-SP1) presents a higher hydroxyl value.
Results and Discussion
First the contact angles between the fountain solution and the ink films were measured and compared for both magenta inks. The lower the contact angle between the fountain solution and the ink, the higher the hydrophilicity of the ink.
Unfortunately, no obvious difference was noticed among the contact angle values after 30 seconds to 3 minutes. In this case, the contact angle method can not differentiate between the two grades of GPTA in a static condition (Table 5).
The laboratory results obtained for the UV inks based on GPTA-SP1 and GPTA-SP2 (Table 6) demonstrate that there is no impact of interfacial tension on the rheological properties (viscosity and yield value), wetting characteristic (gloss) and UV reactivity (alcohol and MEK resistance).
On the contrary, the Duke tests and the tack measurements are dependent on interfacial tension.
A correct water uptake is one of the critical properties required to achieve satisfactory lithographic print quality with UV offset printing inks. The aim is to obtain and keep the proper ink-water balance during the printing process. This can be simulated with the Duke and Tack-o-Scope measurements.
The ink-water interaction for GPTA-SP1 confirmed its hydrophilic behavior: the water pick-up with the Duke test increased of 3.6% compared to GPTA-SP2, and the tack value obtained after the first contact with the fountain solution is increased from 140 to about 240 points (+ 70%). In the same conditions, the tack value is increased only 14% in the case of GPTA-SP2.
The time values of de-inking and recovery of the center roller chromium spiral of the Tack-o-scope are presented in Table 7 for both inks.
It can be noted that the ink based on GPTA-SP1 requires much more time to de-ink the spiral. For example, 70 seconds are required for the fountain solution to completely clean the hydrophilic chromium spiral during the first water contact. This is due to the great affinity between the ink and water, as an excessive water compatibility is known to result in serious printing problems like toning (the printing ink is transferred to the non-image areas). For the ink based on GPTA-SP2, the time of de-inking is equivalent to a standard value for UV offset printing inks.
The recovery time is measured between the water withdrawn and the full re-inking of the spiral. Due to its water affinity, the ink based on GPTA-SP1 took more time for the recovery.
The strong fountain solution tinting in the case of GPTA-SP1 demonstrated again its hydrophilic behavior: the magenta pigment (color index PR 57/1 which is normally non-soluble in water) is transferred to the water phase.
Five different water contacts were made during the laboratory trialand an increasing tack was observed after each water contact. A continuous tack increase is related to a lower film thickness due to the transfer of the ink in water. A lower film thickness increases the force required to split the ink film and consequently increases the tack value.
From the lithotronic experiments, we measure the emulsification capacity (EC), which is the percentage (% w/w) of emulsified water in the ink (before the strong drop in torque). The EC for the ink based on GPTA-SP1 is 44,2 % and 47,5 % for the GPTA-SP2 ink, indicating a higher capacity for water emulsification without rheological modification. Moreover, the lithotronic curve of the magenta ink based on GPTA-SP1 gives a flat curve which indicates a sensitive behavior towards offset printing according to our experience (after comparison to laboratory and practical information from our printers). The ink based on GPTA-SP2 displays an increasing curve during the water pick-up which is more in line with “good” UV binders for litho application.
In order to compare all these results, obtained with simple laboratory methods, to the behavior on an offset printing press, printing trials were carried out with the two UV magenta inks on a Man Roland sheetfed printing machine.
The printing trials were performed for both magenta inks with an optical density between 1,4 and 1,7. The red ink based on GPTA-SP2 confirmed a good offset printability. The ink formulated with the monomer GPTA-SP1 displayed a very sensitive runnability, as the lithographic behavior and the water ink balance were not easy to maintain and as some toning appeared during the trials (see Figure 1).
Thanks to various tests (tensiometry, tack measurements, Duke and Lithotronic tests) on UV offset inks based on two GPTAs (GPTA-SP1 and GPTA-SP2), we have succeeded in predicting the water/ink balance and in giving a clear statement of the printability of the UV offset ink.
It appears that interfacial tension is one of the key parameters in the raw material selection for the ink formulator, as it influences the water uptake, the tack value, and the runnability (toning phenomenon).
In the near future, these methods of characterisation based on interfacial tension determination will be used for UV binders having medium to high viscosity.
Thanks to Greet Demeyer and Christine Lacroix from SICPA and Hélène Godelier from Cray Valley for their kind participation.
: G. Faure, “Overview of the UV/EB Inks and Coatings European Market,” p 239, Radtech Europe, 1999.
: European Printing Inks Market, Frost & Sullivan, 2000, n. 3795 – 39.
: K. Alam & M.R. Kamal, p.1955,ANTEC ’99.
: Standard Test Methods for Water Pickup of Lithographic Printing Inks, ASTM, 1989.
: “Studying the Behavior of Emulsions is Valuable and Beneficial for Your Inks,” PPCJ, Sept 1999, 28-30
: J.U. Ziller, Krüss GmbH, Application Note # 207.