While there are many variables for ink jet formulations one of the most important functions of ink jet ink is the drop size and is strongly dependent on surface tension.
In order to break up the ink into droplets the surface tension of the ink must be reduced to the lowest level and this is done with a component called a surfactant. The surfactant reduces the surface forces of the liquid. Additionally, another component in the formulation usually referred to as the solvent must be chosen at the lowest energy level. The most commonly used solvents in ink jet ink are water, alcohol, acetone. If water is used as the solvent, surfactants are almost sure to be used due to water having strong surface tension forces. While the above inks are solvent based, other ink jet inks may use other chemical solvents or could be of a different system such as phase change ink that is solid at room temperature but liquid at elevated temperatures. Other systems also have been used for ink jet inks such as oil based inks. The importance in formulation of ink jet ink is selection of components and being able to measure the resultant surface tension along with correlation of these effects and changes with the performance of the print head along with ink on the substrate. The performance is usually considered to be printability.
Additionally, other factors of lesser importance must also be
considered due to surface tensions of the substrate such as matching the energy
level of the substrate. This effect
becomes important when the ink is applied to a nonfibrous material such as
plastic since the resulting ink droplets bead up if the surface tensions do not
match.
There fore being able to measure the surface tension of the
ink is most important in designing ink jet ink. Surface tension is measured by
the contact angle of a droplet with a substrate using the following
relationship gLV COSq = gSV-gSLwhere LV is the inter face of the liquid, vapor and gis the surface energy, the contact angle is cos q. The other interfaces are the energies of the SV solid, vapor and SL,
the solid, liquid. The instruments used are optical and use modern video-based
computers for the droplet measurements. Other instruments have also been used
such as the maximum differential bubble pressure as a bubble of gas emerges
from two different orifices in the test liquid. The difference in pressure
between the two bubbles is directly proportional to the surface tension.
Surface tension has units of dynes per centimeter.
Other Important Components in Ink Jet Ink Formulation
While formulating an ink to operate successfully with a print head and have good printability is important, other requirements are also important such as print fastness to the substrate.
One of the big objections to dye based aqueous inks is that they do not have the water fastness to resist spills of coffee or several drops of rain water that will cause smearing destroying the document. The reason for this is that dye based inks usually do not contain a binder that fixes the absorbed colorant to the substrate. Additionally, binders are polymers and much larger in molecular size and would hinder the absorption drying mechanism. However, ink jet inks often use binders to fix the colorant due to color fastness problems.
There are many types of polymer binders that could be used, however, the inclusion of this component is not with out risk as such a system will increase the viscosity of the system and also increase the surface tension properties. There fore the formulator must carefully balance the components in order to develop a successful ink jet ink formulation. Acrylic polymers are often used since they have good resistance properties. When ink jet ink is printed with a binder system the drying mechanism is no longer by absorption, but occurs mainly on the surface of the substrate. At this point the formulation becomes more complex as properties such as adhesion to the substrate and dry time become important. However, when the print head is a thermal ink jet(TIJ) the formulator becomes much more limited in what component
to use in the formula since the solvent (water) acts as the propellant and is able to change to a gas and at the same time has the thermal properties to transfer heat out of the system in order to be useful. Thus for TIJ print head the formulator is limited to water as the solvent.
Other print heads have there limitations also such as the continuous ink jet system where the
Ink must have dielectric properties so that the droplets will support an electrostatic charge in order to be deflected. However, the drop on demand piezo print head has the greatest formulation latitude.
RESEARCH WORK INDUSTRIES has experience in the complexities of balancing formulations for the desired properties along with capabilities of finding vendors of specialized raw materials, analysis and processing equipment needed for a successful development project.
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