The market size has made the technology grow leaps and bounds over the last several years. Currently, solventborne inks are primarily used in industrial applications. The small home office inkjet system is exclusively waterborne. New developments in Piezo heads have enabled the use of alternative methods of application, leading to rapid growth of UV Inkjet technology within the printing industry.

UV Inkjet

• absence of volatile organic compounds (VOCs);

• fast cure and low energy requirement;

• good chemical and solvent resistance;

• high gloss and vibrant colors;

• nozzles can be left uncapped for long periods of time as there are no solvents;

• significantly low rate of nozzle failure due to reduced nozzle blocking; and

• excellent adhesion to a wide variety of substrates.

Although UV-curable inks of a full process color system are relatively easy to formulate, a UV-curable white ink poses a real challenge. Herein, we highlight a technological breakthrough in a jettable UV-curable HexiJet™ White ink of high opacity and pigment loading with good settling stability.

Challenges for UV White Inkjet Ink

Formulating an ink with the above requirements is a real challenge since some of them represent antagonistic relationships between performance and physical properties. Because of the small droplet size fired from DOD printheads, inks must be of low viscosity, which requires the use of monomers and low-viscosity oligomers. However, a high-opacity white ink also requires a high loading of inorganic pigments such as TiO2 and ZnS or a combination thereof. Due to the density differential between the monomer and the inorganic pigments, rapid settling is very much an inherent problem in these inks. Hence, settling and supernatant formation are unavoidable but the speed at which this occurs is sensitive to the components present in the ink.

Recently introduced HexiJet™ White ink is UV curable and is engineered for use in a piezo, DOD print head. Inks are formulated so as to provide excellent durability and highly opaque prints at high print speed operations. The unique chemistry in HexiJet™ White ink is optimized to provide good settling stability and excellent cure speed. Upon cure, the ink provides superior end-user properties such as flexibility, gloss, adhesion to various plastic substrates as well as intercoat adhesion. The ink is formulated to be in a low-viscosity range, and the ink's surface tension is optimized to provide good wetting both within the printhead and on the substrate.

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Ink Properties

Viscosity plays a crucial role during the printing process and must be optimized to ensure maximum reliability and high print quality. HexiJet™ White ink has a viscosity range of 10-14 cps at jetting temperatures, which makes it an excellent candidate for piezoelectric printheads. Figure 1 shows a representative temperature/viscosity curve for the HexiJet™ White ink. Based on the high shear rheology experiments, the change in viscosity at the jetting temperatures is relatively minimal, permitting reliable jetting at a variety of temperatures.

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Surface Tension

Dynamic surface tension measurements were carried out on a Kruss BP2 tensiometer. The technique involves measuring the surface tension by varying the surface age for the sample from 10 to 50,000 msecs. It is known that jetting occurs in the microsecond timescale, whereas the lowest surface age measurable with the tensiometer is about 5-10 msecs. This surface age range relates to the inflight timeframe wherein the ink drop has ejected from the printhead nozzle before reaching the substrate. Longer surface ages of around 500-1,000 msecs relates to the timeframe wherein the ink drop has reached the substrate.

Figure 2 shows the dynamic surface tension curve for HexiJet™ White ink. The dynamic surface tension curve can be divided into three parts. The first equilibrium state (Region 1) represents values at low surface age, next a rate limiting stage (Region 2) and lastly a second equilibrium state (Region 3) represented by the static surface tension at high surface age. Ideally, at low surface age the ink should actually start to show a surface tension drop followed by a rapid change in slope. At longer time scale of up to 500 milliseconds the ink should reach an equilibrium value, which essentially is the static surface tension. After reaching this value, it is not expected to change further. All three stages described above will largely be dependent on the type of surfactant and its migratory process at different time scales after the moment the ink is fired from the printhead.

Our state-of-the-art dispersion technology provides the HexiJet™ White ink with surface tension in the range of 24-28 mN/m so as to provide good wetting characteristics both within the printhead and over a wide variety of substrates.

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Jetting Performance

The operating window of the HexiJet™ White ink in the SE 128 printhead at a firing frequency of 16 kHz is shown in Figure 3. It clearly shows that the white ink has a broad operating window. Reliable jetting can be achieved from 30 to 60 ºC and from 90 to 130 V.

In conjunction with jetting reliability, the ink was also investigated using an Optica X Plus vision system (Xennia Technology Ltd.). This system allows for the droplet breakup visualization as it is ejected from the faceplate. An individual nozzle of the faceplate ejecting droplets over a range of strobe delays was examined. The strobe allows one to see droplets in flight. The HexiJet™ White ink was examined with this system, and the image is depicted in Figure 4. It is evident from Figure 4 that the ligaments that are formed upon ejection from the nozzle combine with the drop to form an image without any satellites. The ink trajectory was straight, and the nozzles showed no sign of failure. The ink maintains optimum faceplate wetting to allow efficient drop break up. Currently, the HexiJet™ White ink has also been demonstrated to be applicable for grayscale Leopard printhead (manufactured by TTEC and distributed through Xaar).

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Cure Speed

A 9-micron-thick coating of an ink was cured using a medium-pressure doped mercury (Hg) vapor lamp powered at 300 Watts per inch. Cure speed was determined using FTIR by measuring the peak area at 810 cm-1 for the ink and the cured film. A plot of cure speed versus energy density or dosage for the white ink is shown in Figure 5. The inks were formulated such that the cure is >90% for both top and bottom surfaces at and above an energy density of 200 mJ/cm2.

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Opacity

The concentration of TiO2 can affect the opacity of the cured film. Figure 7 shows the relationship between opacity and percent TiO2 loading. The opacity increases almost linearly until a TiO2 content of 20% and then reaches a plateau. As the TiO2 concentration increases, diffractive light scattering decreases because of TiO2 particle overcrowding.

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Settling Stability

A key factor that leads to the settling of the white ink is the large density difference between the inorganic pigments and the ink vehicle. However, the unique chemistry in HexiJet™ White ink is optimized to provide ink with good settling stability. Figure 8 shows a representative aging profile for the HexiJet™ White ink. The ink was aged at 60 °C for an extended period of time. The overlay of the profiles is used to determine any pigment agglomeration. An ink is considered to have settled when the change in the backscattering is above 5%. The settling stability value obtained for the accelerated aged sample was converted to the equivalent stability at room temperature to obtain a stability of 50 days. It has to be pointed out that a gentle swirl, without the introduction of any air bubbles in the ink will maintain and extend settling shelf life. Particle size characterization also supports the finding that the inks remain stable over the time period.

In order to understand the significance of the sedimentometry results in realistic conditions, jet stability of the white ink was monitored using the Spectra head SE-128 printhead. Figure 9 shows the jet operating window for the 50-day aged sample versus the unaged sample. The results indicate no significant decrease in the jet-operating window over the temperature ranges, indicating a very stable ink.

Good settling stability for the HexiJet™ White ink is an attractive option for the end user since there is no need to invest in special printhead equipment requiring stirring or agitation. In fact, there was no loss of opacity for the printed sample after storage of the HexiJet™ White ink in the printhead for over seven days without any agitation. This ensures uniform end user properties for the cured film.

Summary

Acknowledgement

The authors would like to acknowledge Dr. Stephen Sung of Hexion Specialty Chemicals, an expert in inkjet technology, for his valuable input related to dynamic surface tension and printhead technology.