Analytical Techniques for Ink Characterization

Monitoring ink performance is a complicated optimization challenge. It is important to understand how system features, especially the dispersed particle size, affect stability and behavior at each stage of the printing process is critical when trying to achieve the most effective formulation. This article offers guidelines on the use of different analytical methods, including particle size, rheology and zeta potential measurement, to acquire the essential knowledge.

In this article, Malvern Panalytical experts detail analytical strategies supporting formulation, and offer guidance on data collection for:

  • Characterization of ink components
  • Quantification of performance characteristics such as jetability and the possibility of dripping
  • Engineer formulation stability
  • Control manufacture

Case studies provided show the importance of a range of analytical techniques for improving formulation to a successful conclusion.

Innovations in Inks

Advancing ink formulation is an integral part of every development in printing. For example, the invention of the printing press required thick oil-based formulations to adhere to the metal type face, instead of the high viscosity, water-based inks that were present at the time. It is important that present day developers design superior inks that provide an excellent finish for all types of printing technology.

The constituents of modern inks are:

  • Carrier fluid – Either organic or aqueous colorants - pigments or dyes
  • Surfactants – For controlling the surface tension of the ink droplets
  • Binders – For controlling ink elasticity and the manner in which it breaks up when shear is applied
  • Dispersants - For improving the dispersion of colorants in the carrier fluid

The formulation of stable, high performance products depends on the optimal inclusion of each of these constituent components.

Controlling Performance During Printing

Firstly, it is necessary to consider the prerequisites of a printing process, and how to develop a compatible ink. Rheology plays a key role, as rheological properties impact the manner in which an ink jet drips, spreads and atomizes.

Figure 1 shows the use of two inks in a 'drop-on-demand' inkjet printing process. The first shows proper behavior and generates only one discrete droplet subsequent to actuation. Conversely, the second shows improper behavior by generating double droplets. When inks have poorly optimized rheology, they exhibit the latter type of disruptive dripping behavior that affects printing performance.

Ink rheology is directly impacted by the characteristics of the suspended components, such as particle size. Formulation goals, with regard to the finish, dictate particle size specifications for a suspended pigment. It is possible to use polymeric additives to monitor the continuous phase viscosity, and impart properties such as shear thinning behavior or elasticity.

Viscosity

Using simple viscosity measurements, it is possible to determine how an ink will flow through the print head, and how it will atomize. It is essential to have knowledge about the structure-function relationship between additive characteristics, such as molecular weight and molecular weight distribution, and ink performance for successful addition of additives developed to control viscosity. The comprehensive characterization of polymeric additives is possible with techniques such as multi-detector gel permeation chromatography/size exclusion chromatography (GPC/SEC).

Viscosity values need to be determined under conditions that are same as those at the print head. This may be a challenge while developing inks for modern printers with shear rates as high as 105 to 106 s- 1. Using even the most sophisticated rotational rheometers, these shear rates cannot be achieved. Case Study 1 enumerates how microfluidic rheometry, which is a considerably new technique, effectively satisfies the need for high shear viscosity measurement.

Comparison between a proper (top) and improper (bottom) ink within a

Figure 1. Comparison between a proper (top) and improper (bottom) ink within a 'drop on demand' print head illustrates the importance of controlling formulation rheology.

Case Study 1: Using Microfluidic Rheometry to Assess Performance at the Print Head

Research studies recommend a viscosity between 5 and 25 mPa for a ceramic inkjet for good print head performance. Figure 2 shows viscosity measurements made to examine how the viscosity of two commercial ceramic inkjet inks vary as a function of temperature in the 20 °C to 40 °C range. This range shows typical operating temperatures at the time of printer use. By deploying an m-VROCi microfluidic rheometer at an ultra-high shear rate of 30,000 s-1, measurements were taken.

Measurements at high shear (30,000 s-1), made with a microfluidic rheometer, confirm that viscosity remains within the target range of 5 to 25 mPas across an operating temperature range of 20 to 40 °C.

Figure 2. Measurements at high shear (30,000 s-1), made with a microfluidic rheometer, confirm that viscosity remains within the target range of 5 to 25 mPas across an operating temperature range of 20 to 40 °C.

It can be seen from the results that the viscosity of both the inks reduces with an increase in temperature. The shear viscosity of both inks however lies within the preferred range across the test temperature range. This implies that ink flow will be smooth through the print head under typical operating conditions.

Viscoelasticity

Performance at the print head is also directly influenced by the rheological property of viscoelasticity. The measurement and control of viscoelasticity supports the development of an ink which successfully disintegrates into well-defined, discrete droplets at the time of printing, and holds its specific structure on being applied to the substrate.

The poor jetting performance, as shown in Figure 1, indicates a sub-optimal viscoelasticity. The capability of the elongated droplet to disintegrate from the bulk liquid and form a single sphere is directly connected to the property of viscoelasticity, and also to the surface tension of the ink. Microrheology is an advancement that is beneficial for studying viscoelasticity under conditions that offer precious insight into print head performance.

Engineering Stability

It is critical to formulate the ink for successful printing, but it is also important to see how that performance needs to be maintained over time. Developing a robust stable formulation ensures that there is no degradation in ink quality with time, and  it is dependent on maintaining discrete particles in suspension under the encountered conditions in storage and during use. This is probably the most challenging aspect of ink formulation. As shown in Equation 1, the chance of sedimentation in a suspension can be predicted from the ratio of gravitational to Brownian forces:

    a4Δρ / kBT

where a is the particle radius, Δρ is the density difference between the dispersed and continuous phases, g is acceleration due to gravity, kB is the Boltzmann constant and T is the temperature.

In a sub-micron suspension, Brownian motion is normally significant, and the value of this ratio is less than 1. In terms of the practicalities of controlling ink stability, it is possible for formulators to manipulate one or more of these variables:

  • Particle size of the dispersed phase
  • Viscosity of the continuous phase or the suspension.
  • Zeta potential of the system.

Case Study 2: Inducing Stability in a Suspension of Fine Particles

Figure 3 shows the impact of pH on the zeta potential of a suspension of particles having a 3.7 µm average size. A Zetasizer Nano S along with an MPT2 autotitrator gathered these data. There is a drop in Zeta potential below -30 mV at around pH 2, showing that the system is thermodynamically stable at all pH values greater than this. The data showed in Figure 3 shows that the system will be more prone to aggregation.

Varying the pH of a system can help to control the zeta potential to achieve thermodynamic stability.

Figure 3. Varying the pH of a system can help to control the zeta potential to achieve thermodynamic stability.

Flow curves for the suspension reveal that the system has a yield stress at a pH of 2.42 and 3.52 which has been induced by encouraging flocculation/aggregation.

Figure 4. Flow curves for the suspension reveal that the system has a yield stress at a pH of 2.42 and 3.52 which has been induced by encouraging flocculation/aggregation.

To summarize, for this system an increase in pH brings down zeta potential and encourages interaction of particles. This results in a network structure in the suspension that causes stability at low shear in the form of an apparent yield stress.

Squaring the Circle - Rationalizing Performance and Stability Targets

Stability and performance have been treated as distinct goals, but actually, formulators need to satisfy stability and performance targets, instead of one or the other. Inks need to exhibit a rheological profile that meets both of these requirements.

Shear Thinning

Shear-thinning materials have a viscosity based on the magnitude of applied stress, and viscosity decreases with an increase in shear rate. For inks, shear thinning must take place over several years of applied shear because shear rates are very low under storage conditions, but very high at the print head.

Case Study 3: Combining Rotational and Microfluidic Rheometry to Generate Complete Flow Curves for Ink Formulation

Using an m-VROCi microfluidic rheometer, a Kinexus and a rotational rheometer, the viscosities of the commercial ceramic inkjet inks characterized in Case Study 1 were determined across an applied shear rate of 0.5 to 100,000 s-1. Figure 5 shows a single flow curve for each ink, obtained from the combined measurements.

Flow curves can be produced over a very wide range of shear rates by combining rotational (solid squares) and microfluidic (outlined squares) rheometry.

Figure 5. Flow curves can be produced over a very wide range of shear rates by combining rotational (solid squares) and microfluidic (outlined squares) rheometry.

For both the samples, viscosity is comparatively constant across the shear range studied. There is, however, certain evidence of slight non-Newtonian shear thinning behavior, particularly with Ink A. In comparison with 17 mPa.s at 100,000 s-1, the formulation has a measured viscosity of around 22 mPa.s at 1 s-1. This is a tiny yet significant viscosity drop, suggesting the breakdown of a specific degree of the microstructure of the ink upon increasing applied pressure.

Case Study 4: Assessing Thixotropy

The results from tests developed to study thixotropy in one of the weakly structured suspensions produced in Case study 2 is shown in Figure 6. These tests were performed using a Gemini 2 rotational rheometer and a Kinexus Pro. At the beginning of the test, a considerably low shear rate was applied. There was an increase in shear, and a drop in viscosity was observed. Returning to low shear enabled the structure in the sample to rebuild, and viscosity returned to a considerably high value.

An ink that rapidly transitions between high and low viscosity helps to ensure optimal performance on the substrate.

Figure 6. An ink that rapidly transitions between high and low viscosity helps to ensure optimal performance on the substrate.

These results show that the sample structure rebuilds rapidly with a reduction in shear, which is a positive feature for ink performance.

Case Study 5: Monitoring Inkjet Ink Milling Using Laser Diffraction Particle Size Measurement

Inkjet ink production is a two-stage process, which begins with pigment dispersion in a suitable mobile phase through mechanical stirring. For reducing the particles to the required size, and to disintegrate any bound agglomerates, wet milling of this pre-mix was performed.

Particle size distributions,, measured using a Mastersizer 2000 laser diffraction particle size analyzer during the milling of an inkjet ink premix, are shown in Figure 7. The change in Dv10, Dv50 and Dv90 as a function of milling time are shown in Figure 8.

Particle size distributions measured for different milling times in the production of inkjet inks.

Figure 7. Particle size distributions measured for different milling times in the production of inkjet inks.

Laser diffraction particle size data show that milling of this inkjet ink is complete in around 100 minutes.

Figure 8. Laser diffraction particle size data show that milling of this inkjet ink is complete in around 100 minutes.

These results show clearly the ability of laser diffraction to monitor a milling process for a successful completion. After around 100 minutes of milling, a steady, fine particle size distribution is obtained.

Conclusion

Ink formulation has become a very precise science due to the need for high performance inks that work well in emerging and modern printing technology. Previously, formulation required trial and error, however, today a sophisticated knowledge-led approach offers better product control and a more rapid commercialization. Gathering the data needed to support this approach can be based on the application of detailed and effective analytical strategies. Rheological analysis, particle sizing, polymer characterization and zeta potential measurements all play a key role.

This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.

For more information on this source, please visit Malvern Panalytical.

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