Inkjet Ink Chemistry
In simple terms, an inkjet ink is composed of a colorant and its carrier, or vehicle. For the present purposes aqueous inks are discussed. Solvent-based and UV inks are beyond the scope of this document.
- Colorants -
Colorants may be dyes or pigments. In some cases, dyes and pigments are used together in a hybrid ink system.
Dyes are soluble in the ink vehicle, or more precisely, in water, much like salt or sugar is soluble. It is important for the inkmaker to use inkjet grade dyes that are free of impurities such as salts that are often present in dyes used for fabrics. Dyes are capable of providing very vibrant, saturated colors, and are usually more forgiving and easier to jet. However, most dyes do not have the resistance properties and lightfastness that pigments have, and are therefore unsuited for some applications, such as archival prints.
Pigments are insoluble in the ink vehicle and are thus present as suspended solids (the technical term is colloidal suspension). This means that the pigments have had their particles reduced to very small sizes so they can be stabilized in suspension. Most commonly, pigments that have already been finely dispersed in water are used in aqueous inkjet inks.
Dispersing a pigment is a little like making a hot chocolate with cocoa powder, except you have to put the whole can of cocoa powder into a glass of milk and it has to remain liquid. There are several kinds of pigment dispersions available for inkjet inks, most being referred to as “nano”, meaning that their particle size is very small.
These dispersions, sometimes called SDP's, contain only modified pigment and water. They are characterized by low viscosity and surface tensions very close to water. Manufacturers bring the pigment particle size down to the desired value and they chemically treat the surface of the particles to add certain functionalities. This causes the particles to gain repulsive properties so that they do not reagglomerate.
Core Shell Dispersion
Some refer to these dispersions as encapsulated pigments, perhaps a misnomer since some conventional dispersions could also be called such. The manufacturers grind the pigment down to a small size and they add a monomer (the precursor to a polymer) that attaches to the pigment particle surfaces. A reagent is then added that caused the monomer to polymerize, in effect, coating the particles in plastic. These dispersions usually have lower surface tensions and slightly higher viscosities than self-dispersing pigments. An added benefit is that they usually require less binder than SDP's.
This is an umbrella term for a collection of other technologies used for dispersing pigments. Before the advent of SDP's and core shell dispersions, manufacturers used various dispersants to stabilize their colloidal dispersions of pigment in water. This was done with a myriad of methods such as grinding resins, surfactants, block polymers, graft polymers, and many other variants. A few of these methods are capable of producing nano grade dispersions that are highly stable and can be used for inkjet. However, care must be taken to ensure that compatibility is not compromised in the final product and that all specifications can be met.
The type of dispersion that the inkmaker uses will depend on the end use requirements, and the availability of a given pigment. However, SDP's and core shell types usually offer more flexibility in formulating and are more likely to show better stability than conventional dispersions. Unfortunately, choices are more limited since there are fewer manufacturers of SDP's and core shell dispersions and far fewer pigments being dispersed.
Although pigments are more opaque than dyes and do not possess the same, high chroma, they are more lightfast and less prone to change upon exposure to UV or to ozone that may be present in the air. Care must be taken with pigmented inks to make sure they remain stable and do not show sedimentation over time. Nozzle clogs are more likely with pigmented inks than with dye inks because the tiny pigment particles can accumulate at the nozzle.
- Vehicle -
The ink vehicle contains most, if not all of the following components:
The primary solvent in an aqueous ink is water. Inkmakers use deionized water to ensure that impurities are minimized. Other solvents are used in addition to water but are normally classified by their purpose, as listed below.
It is important that an aqueous inkjet ink be prevented from drying out prematurely, so as to prevent printhead nozzle blockage. Humectants are often slow evaporating solvents such as glycols, glycerine, or other polyols. Under normal conditions they will form azeotropes with water and each other so that evaporation rates are much slower that water alone.
Resinous binders are a normal constituent of inkjet inks. With pigmented inks they help to prevent rub-off, impart gloss, and improve resistance properties. They may also be used with dye-based inks to help with resistance to ozone and other damaging species. Binders also assist with obtaining a consistent appearance across different colors with respect to gloss and bronzing prevention. While there are many binder types used, the most common are acrylics and urethanes.
Most aqueous inkjet inks require that pH, or degree of acidity be controlled. For example, many binders are unstable unless the pH is above neutral, or more preferably, above 8.0. Inks usually contain a small amount of a basic material such as an amine to ensure the pH remains in the required range.
Without the addition of a surfactant, most aqueous inkjet inks would have a surface tension that is too high for jetting and proper droplet formation. This is because of the high surface tension of water. Surfactants are used to lower the surface tension.
Penetrants are normally solvents that permeate rapidly into a substrate. Glycol ethers and pyrrolidones are quite effective for this purpose. Such solvents are usually balanced with other polyols to ensure that the rate of penetration is correct. For example, the humectant glycerine may tend to slow down penetration on some media. It is important that inks penetrate in order to prevent running and smearing.
Some ingredients in inkjet inks, especially binders, can be food for bacteria, fungi, or molds. A small dose of a biocide is used to prevent such growths.
- Physical Properties of Inkjet Ink -
An inkjet ink must perform optimally when run on the target printer, selected print material and must meet the requirements of the user. There are many physical properties that must be maintained and controlled for this to occur. In many cases, these properties are checked with each production lot of ink to ensure that all specifications are met.
All inks will have a range of viscosity within which they must fall. If too low, the ink might drip from the nozzles on some printers; and if too high, the printer may have difficulty ejecting a droplet.
Static Surface Tension
The surface tension of the ink at rest is important since if too low, the ink can leak out of a resting printhead.
Dynamic Surface Tension
This is the property that is in effect when forming a new surface, ie, an ink droplet. In practice, will be higher, sometimes significantly higher than static surface tension. Surfactant choice becomes important to ensure that dynamic surface tension is low enough for droplet formation to reliably occur but not so low that many satellite droplets form.
Electrical conductivity is sometimes used as a test for impurities, usually salts. High conductivity might result from using water that was not properly deionized, from a dye that contained salt, or even from a raw material such as a binder or pigment dispersion that was prepared using tap water. Testing for conductivity is therefore more important with dye inks, especially dye inks for thermal printheads.
It is important to ensure that pH is without the specified range for a given ink formulation, and especially important that it is not below a specified value.
Water and solvents will normally contain dissolved air. If the amount is too high problem may be encountered with some printheads due to the formation of tiny bubbles at the printhead nozzle. Production lots of ink may be degassed and checked by measuring dissolved oxygen.
Vera uses a proprietary technique for measuring the rate of penetration. This rate is compared to a standard to ensure consistency.
Gloss is often checked compared to a standard using a gloss meter. In most cases there is little or no variation in gloss. A significant difference would be an indication of a problem.
Particle size is normally specified for each ink. During quality assurance testing, if a larger than typical size is detected, it might mean that agglomeration is occuring and ink stability would be suspect.
Most, if not all of the above properties might be checked for production lots of ink. However, there are other properties that are checked during the research and development phase:
It is important to know that an ink will remain stable in storage for many months or even years without significant change to viscosity, settling of suspended solids such as pigments or any other deleterious changes that might cause the ink to fail. The normal method is to place a wet sample of the ink in a sealed jar. The jar is then placed in an oven for 6 weeks at 60°C. After exposure the ink is check for any change in its properties.
Depending on end-use requirements, it may be necessary to test a given ink for lightfastness. While there are several means for doing this under accelerated conditions, a typical practice would be to expose a printed sample in xenon lamp testing apparatus. This method takes several weeks and is thus neither necessary, nor practical for production ink lots.
The inkmaker may wish to measure or calculate the amount of material in an ink that remains after all the liquid components have evaporated. This may be important, especially with inks that contain high pigment/binder levels.
This is the density of an ink compared to water. For most inks the value will be a little higher than water. Certain pigments are very heavy and if used in ink will produce high specific gravity. In such cases the viscosity and/or surface tension may have to be adjusted to compensate.
All inks require some degree of this property of viscosity. Large molecules such as binders may impart some viscoelasticity. If too low, an ink may form many satellite droplets when jetted, or even cause the ink to spray or mist.
This is not a specific property but rather, we relate several factors including printer specifications such as nozzle radius and droplet velocity from the nozzle to density, viscosity, and surface tension. Performing certain calculations we arrive at a dimensionless number that can help predict the performance of an ink in a given printer.
- Other Issues Facing Developers -
Coffee Ring Effect
When an inkjet printer deposits a droplet onto media, there is a moment of time when it is resting on the media in a wet state with the perimeter of the droplet pinned to the surface of the media. Before the liquid fraction has penetrated, the pigment particles can sometimes migrate towards the perimeter leaving a printed dot that is pale in the center. It looks like a ring instead of an evenly colored dot, similar to the way a drop of coffee forms a ring when spilled onto a napkin. This phenomenon has been studied and has been found to be caused by a surface tension gradient as liquid evaporates or penetrates at the perimeter of the drop. Inkmakers can detect whether this is occuring by inspecting printed samples under a microscope. The formulator may need to take steps to mitigate this effect.
Humidity is an important factor in printer performance. Problems may occur especially when humidity levels are low. Some printer manufacturers may recommend maintaining relative humidity levels above a certain point, such 35 or 40. When a printer is idle for a few days at low humidity levels there may be a tendency for the inks to gradually lose their volatile components (especially water). When this happens the inks start to dry out causing problems with keeping nozzles clear, buildup on wiper blades, in capping stations and in the flush box.
Metamerism, Gloss Variations, and Bronzing
This is actually a collection of issues related to how an ink appears on different media and under different conditions. Two different inks might appear the same on one paper but different on another. They may have the same gloss on one media but not on another. An ink may appear to have bronzing issues on some media, those odd reflections that give the ink an almost metallic appearance. The inkmaker tries to minimize all of these issues, usually through binder selection, and the balance of different humectants.
This is an issue that is unique to aqueous inks. On the one hand we want an ink that dissolves or rewets in itself in order to prevent nozzle clogs, but on the other hand has water resistance once it it printed. Inkmakers usually look for ways to balance a binder that has good resistance properties with the addition of another binder that has high solubility in water.