The following presents a chronological overview of the steps Eurocommit has taken in its effort to establish a recommended system of test methods, with the challenges and breakthroughs of each technique noted.
1st Step: Precipitation TemperatureThe precipitation temperature, the temperature at which a resin solution begins to become cloudy, is a key value related to the compatibility of, for example, a resin with a particular oil, and is considered a meaningful parameter for the quality control of incoming supplies. It is also helpful in the overall evaluation of a resin in terms of solubility or mineral oil tolerance.
The designated resin is heated in a printing ink distillate until it dissolves, generally to a temperature above 200 °C. The solution is left to cool and the temperature at which the resin starts to precipitate is determined. In the course of the work a number of uncertainties were identified, such as: sample preparation; rate of heating and cooling; mechanics of stirring; loss of oil/solvent; definition of cloud point; degree of turbidity; and properties of the solvent. In numerous trials, the effects were investigated, leading to a method with solid reproducibility and repeatability.
The Development of Test Oils
It became apparent that the definition of the solvent is one of the major indicators of reproducibility. An exact characterization of individual components, however, is not feasible, as standard-grade printing ink distillates are generally derived from mineral oils through a careful selection of raw materials and the appropriate steps of refining processes, and are complex hydrocarbon mixtures. As a consequence, the analytical characterization, or specification, is not sufficient to fulfill the requirement of a test oil for analytical purposes. On the other hand, the use of a chemically defined solvent of a different nature would not reflect the performance of a resin in the final application. Another constraint arose from the fact that the resins used in printing inks differ considerably in their solubility, while the temperature range that can be measured as the precipitation temperature is limited to between 50 and 150 °C. Test fluids of varying solubility are therefore required.
The problems were solved by the creation of a range of five test oils, each corresponding to commercial printing ink distillate grades. The oils not only address the solubility character of oils in practical use, but are also - in order to meet requirements - standardized batch by batch, beyond the usual quality criteria, through an adjustment of their properties per their compatibility with a selection of resin types. The available grades bear the names Test Oil 6/9, Test Oil 6/9 af, Test Oil 6/9 af new, Test Oil 6/9 ar, Test Oil 6/9 ar blend. They all fall within the boiling range of 260 to 290 °C, with aniline points varying from 40 to 96 °C.
Additional Method Details
As far as sample preparation is concerned, optimal results are obtained by using a resin crushed to particle sizes between 0.5 and 2 mm. For the measurement performance, reliable results can be achieved by using automated equipment that controls the process performance and determines the precipitation temperature by an electronic turbidity measurement (Chemotronic, supplied by Novomatics, Germany, or testprint, Netherlands).
2nd Step: Varnish ViscosityAn additional criterion in the quality control and specification of a resin is the viscosity of a defined varnish or resin solution. The test method involves the preparation of a varnish in a defined oil and the subsequent viscosity measurement. Once again, the work began with an investigation into current practices, revealing that the main variations consisted of the type of solvent, the equipment used to prepare the varnish and, importantly, the temperature profile during the preparation.
The influence of the maximum temperature during varnish preparation turned out to be considerable: when comparing preparations at 190 °C to 230 °C, the higher temperature yields varnish viscosities that are about 10 to 20% lower. Allowing variations in heating and cooling rates results in reduced reproducibility, leading to sigma values around 15 compared to 9 in standardized equipment. These findings led to the recommendation of a suitable automated device for this particular method of quality control (Thermotronic, supplied by Novomatics, Germany, or testprint, Netherlands).
Again, the use of standardized test oils with a specification adapted to resin compatibility was indispensable. Although three test oils would be sufficient for the determination of the precipitation temperature, the full range of five test oils was necessary for the viscosity measurements (in most cases the oils of higher solubility i.e. 6/9 ar blend as standard oil, 6/9 for highly soluble resins, 6/9 ar for low soluble ones).
In order to obtain reliable results on viscosity, the measurement should follow strict rules. A rotation viscometer should be used as standard equipment. At the time of testing, it was agreed to measure at 23 °C and read at a shear rate of 25 s-1, with a narrow definition of dwell times and rate ramps. To avoid negative mechanic effects, the viscosity range is limited between 6 and 60 Pas, preferably 20 to 50 Pas. The resin concentration should be within the limits of 35 to 45%. This can be maintained by choosing the appropriate test oil grade.
When these rules are followed, the method can be applied to approximately 95% of commercially available printing ink resins both for production control, and for quality control and specification.
3rd Step: ViscoelasticityWith increasing demands on printing ink performance and effectiveness, due to faster machines and changing paper quality, the assessment of viscoelasticity is of growing interest. As today's printing processes are characterized by a rapid change between high-shear zones, for instance with the ink passing between rollers, and the low-shear zones in between, oscillatory measurements of varnish viscosity appear to be the optimal methods for an additional characterization of the resin/oil interaction.
This so-called "linear oscillation measurement" can be performed with most of the commercially available rotation rheometers, provided their bearings exhibit a low mechanical resistance (air or magnetic bearing is preferred). The behavior of a varnish is then characterized by the phase shift (tan δ) and the complex viscosity (η*).
True to its principles, Eurocommit did not attempt to evaluate and interpret viscoelastic data, but instead developed a method to measure viscoelasticity, working to define its limits and scope. As recommended earlier, the test varnishes should be prepared by using a standardized procedure and the appropriate test oil. The strain value should be 0.1 (or "10% strain"); with higher values a loss of linearity is probable, and while lower values are possible they bear the risk of inaccuracy due to limited torque resolution. The frequency should be set to 0.1 Hz for higher viscosities (20 to 104 Pas), 1 Hz for lower viscosities (5 to 103 Pas). To avoid mechanical errors, a plate/plate system is recommended, diameter 20 to 50 mm, and a gap of 0.5 to 1.0 mm.
Recently, this test method has been extended to a frequency sweep from 1 to 50 (or 100) Hz and an application to high-viscous media. The findings are still being investigated.
Other Areas of FocusEurocommit has evaluated additional methods such as tack measurement and water balance, and has relied on the support of equipment manufacturers throughout.
To optimize use of the Tack-o-Scope (testprint bv, Netherlands) a program was designed with the manufacturer to improve the reproducibility of the measurements. It consisted of equipment maintenance, a check of roller material, calibration, ambient conditions and sample application, and led to significant improvement of the method.
While lab tests might fulfill their duties in individual laboratories, Eurocommit wished to address the lack of a general method for comparing product and component performance. Of the existing automatic equipment, only the Lithotronic (Novomatics, Germany) was commercially available when the work started. In this device the ink is stirred in a temperature-controlled beaker, water is added at a controlled rate, and the change in torque is measured.
It was concluded that, apart from many other factors, an exact schedule of preconditioning, sample addition and emulsification/stirring is necessary. Furthermore, the mechanics of the equipment are of utmost importance; improvement or exchange of the stirrers can lead to an enormous improvement. With all factors controlled, the method can provide information on water uptake (amount and rate) and water release.