Dynamic surface tension of digital UV curable inks.

The printing industry has seen huge strides in development of UV curable digital inks. (1) Ink properties crucial for good performance include low viscosity, optimum surface tension, nanometer particle size, fast print speed, good jettability and long-term storage stability. (2,3) The cured inks also needs to provide physical properties such as good substrate and intercoat adhesion, flexibility, hardness, chemical resistance, color gamut and durability. (4)

In addition to viscosity, surface tension is an important property of a digital ink. (5) It can influence ink meniscus recovery and drop formation from the printhead nozzle. For example, low surface tension can result in excessive nozzle faceplate wetting leading to poor ink ejection, thereby affecting jet reliability. On the other hand, a high surface tension can lead to insufficient face plate wetting, which can also impact droplet formation and ejection.

Ink surface tension also affects substrate wetting, print quality and adhesion, and it needs to be optimized to improve jet stability and performance. (6) Hence, an ink formulator needs to balance the demands of good jettability, substrate adhesion, print image quality and drop spread.

Substrate surface energy plays an important role in determining drop spread. Table i shows the surface energy of several substrates. In general, good substrate wetting can be achieved when surface tension of ink is much lower than the substrate. (7) However, if the ink surface tension is too low, it can increase drop spread, reducing resolution. Although it is desirable to use a universal ink for all substrates, the different substrate surface energies make it quite difficult to achieve it since a proper balance of surface tension is required for each substrate.

UV curable digital inks cure rapidly, often before completely wetting out on the substrate. Hence, static surface tension, as measured by techniques such as du-Nouy ring or Wilhelmy plate, is not useful in predicting jetting properties of an ink as it is only relevant to substrate wetting on a larger time scale. In a jettable ink, the time frames involved between newly created surfaces at droplet ejection and contact with substrate followed by UV cure is very rapid, on the order of micro to milliseconds. It is influenced by drop velocity, carriage velocity, reactivity of the ink to UV lamp and distance of printheads to UV lamps. The surface active components need to migrate to the newly formed interface in these short timescales in order to influence surface tension.

For a dynamic process such as inkjet printing, measurement of ink surface tension at extremely short time intervals is critical rather than at longer equilibrium values. The process can best be described using dynamic surface tension measurements which allow a time dependent characterization within milliseconds. (8) However, ink drops are formed in a much shorter timescale compared to the measured surface ages. (9) Although the time interval is much shorter, dynamic surface tension (DST) measurement provides useful information in the timeframe wherein the ink drop resides on the substrate prior to UV cure.

This paper analyzes the effect of surfactant type and concentration on dynamic surface tension of digital inks. The information will be used to correlate image quality of a digital print on substrates with different surface energies. This will be useful in choosing the ink with optimum substrate wetting characteristics.

Digital UV inks have several components such as monomers, oligomers, pigments and additives. However, the surfactant, by far has the highest impact on the surface tension. It is desirable to rapidly lower dynamic surface tension with a suitable surfactant to promote substrate wetting without affecting face plate wetting and image quality.

Dynamic surface tension was determined using a Kruss BP2 Tensiometer which measures the rate at which bubbles are formed. The theory of dynamic surface tension measurement using a bubble tensiometer is explained in detail by Fies et al. (10,11) Gas is bubbled through a 0.3 mm glass capillary immersed in the liquid and the bubble pressure related to surface tension at maximum radius (radius of the capillary tip) is measured. The measurement was carried out at 25[degrees]C for a surface age range from 10 milliseconds to 50 seconds.

Static surface tension measured by the Wilhelmy plate method is an equilibrium surface tension measurement, 12, and it represents the minimum value seen by the dynamic testing. The dynamic testing shows that the surface tension of each sample decreases with time, due to surface active materials in the sample migrating to a freshly developed surface and reaching an equilibrium. However, the surface tension will not decrease below the static (equilibrium) surface tension at any of the surface ages studied by the bubble pressure (dynamic surface tension) technique.

If we consider the complete process of ink drop ejection to curing or pining of the ink drop, there are three time segments. The summation of these individual segments is the time that the drop has to wet-out on the substrate before curing, after which its form will not change.

1.Drop ejection time: Firing frequencies can range from 8-24 kHz in a piezoelectric printhead. A typical frequency is 16 kHz; which is represented on a time scale of 0.063 milliseconds.(9)

2. In-flight travel time: The substrate is recommended to be placed at a distance of 1 mm from the printhead faceplate otherwise drop velocity decreases significantly due to air drag force. Assuming a typical drop velocity of 6 meters/sec from the printhead, the drop will take approximately 0.17 milliseconds (13) to strike the substrate.

3. Dwell time: Distance between the printhead to the UV lamp varies by printer and will also depend on the number of printheads and their arrangement. In one UV inkjet printer, the closest printhead to a UV lamp is 3.5 inches away and the furthest printhead is 15.5 inches away. The carriage speed varies by printer, but a typical speed would approximately be about 24 inches/second. Dwell time for the ink droplet on the substrate prior to UV cure can be calculated based on the above lamp distances and print speed and would range from 146 to 646 milliseconds. (14) It is evident that the ink drop from the printhead that is furthest away from the UV lamp (646 msec) will have the largest time to change its surface tension (contact angle or drop shape on the substrate) compared to the ink drop closest to the UV lamp (146 msec).

[FIGURE 1 OMITTED]

The sum of drop ejection and in-flight travel time is about 0.23 milliseconds. It is difficult to reliably measure dynamic surface tension experimentally with a tensiometer at this time scale since the lowest surface age measured by the tensiometer is about 5 to 10 milliseconds. This surface age range relates to the dwell time wherein the ink drop has ejected from the printhead nozzle and reached the substrate and is in the process of spreading on it before UV cure. Information on surface tension for longer surface ages of above 1000 milliseconds would not be meaningful for the inkjet UV cure process unless it is an extremely slow printing system.

Figure 1 shows the dynamic surface tension curve of a typical and ideal ink sample. It can be divided into three parts: (1) An initial plateau (Region 1) representing values at low surface age; (2) a rate limiting stage due to migration of surface active materials (Region 2), and (3) an equilibrium stage represented by the static surface tension (Region 3).

For an ideal ink, the plateau (Region 1) needs to be short followed by a quick drop (15) (Region 2), upon which the ink should reach the static surface tension. After reaching this value, it is not expected to change further (Region 3). 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 gas is bubbled out of the capillary. The surface tension curve of a typical ink also shows three distinct regions, however, the rate of drop in surface tension is not as rapid as an ideal ink. For an ideal ink, surface tension change in region 2 should occur in less than 0.10 milliseconds, so it is not influenced by any of the three timeframes. However, in reality the surfactant migration to the interface boundary limits the surface tension decrease. Hence, dynamic surface tension drop for a typical ink rarely mimics the ideal behavior.

[FIGURE 2 OMITTED]

Effect of Surfactant Concentration

Dynamic surface tension curve for cyan inks with varying amounts of surfactant are shown in Figure 2. The curves have an initial plateau region followed by a rapid decrease upon which the inks decay to an equilibrium state represented by the static surface tension. The decrease occurs after 500 milliseconds for ink without surfactant. In contrast, the drop in surface tension begins much earlier ca. 100 milliseconds, for inks with the silicone surfactant. The difference between the surface tension at surface age of 10 milliseconds and at 50000 milliseconds is large for inks with surfactant. However, the magnitude of this difference starts to plateau above 100 ppm surfactant level. Surfactants help substrate wetting before UV cure and result in a much better print quality. However, excess surfactant can lead to poor print resolution. Hence, an ink formulator needs to carefully consider type of substrate and the time to cure on the printer prior to optimizing surfactant concentration.

Effect of Surfactant Type