Inkjet Ink and its Important Additives

In Article, Drying, Ink and Fluids, Knowledge Base by Mark Bale0 Comments

From office printers to some of the biggest and fastest production inkjet machines out there, there are some common properties derived from different ingredient choices. “Water” is by far the most common “carrier” used globally for inkjet inks – but there are many other types of ingredients that affect the behavior of ink.

Two of the most important are the pigment dispersion and the resin, which give the ink its end-use properties. These are followed closely by the co-solvents and surfactant that influence the print process itself – such as drying.

In this next ink series post, we outline some of the differences in characteristics that come from various components beyond the carrier itself, so you’ll be better informed next time you have a conversation with your supplier.

Dyes and Pigments
We touched on this in the post “Inkjet Ink – Chemistry Matters.” The single biggest reason pigment has replaced dye for many applications is lightfastness, also known as UV fade resistance. Dyes can develop strong colors but the saturation of these colors can shift as the dye migrates with time and are prone to oxidative deterioration and fading under sunlight through UV exposure. This can be seen clearly in the photo below. The print below shows a faded print logo compared to the original print made with dye-based inks on a desktop printer. The print was displayed behind glass in a south-west facing window for less than 2 months. Print permanence is an important consideration for the retention of records and the increasing use of pigment colorant has helped inkjet compete with electrophotography in this respect.

From an ink-making perspective, pigment colorants take more effort to produce than a dye. Usually a dye can be weighed straight into a formula and readily mixed into the base. For more difficult-to-dissolve dyes, it may be necessary to make an intermediate solution be weighed as a liquid. For pigments, the raw material powder has to be to dispersed into a vehicle to allow it to be milled to the desired particle size. The milling process uses complex and expensive equipment and more energy than simply mixing in the dye.

Many small ink producers actually buy their pigment dispersions from a supplier and will work the dispersions into an ink a bit like a dye. In this case their main focus will be filtration and ensuring they get the right stability with the rest of their materials by monitoring particle size, for example.

For the big players, it is more cost effective to become vertically integrated and make the dispersion, or at least develop it and engage a toll manufacturer. Many inks from the best-known companies can also use proprietary material in the dispersion, thus making their technology specific to the print process. As a result, OEM’s attain complete control over the pigment dispersion formulation and many of its key performance factors. This can be a bigger security feature than any RFID chip on the container.

The main role of the resin is to act as a binder. Resin, which refers to an ink’s polymer, forms the resistance property of the ink layer by forming a film as the water evaporates and often by some additional cross-linking. Resins reduce or eliminate print defects caused by ink rewetting/smearing which can be seen in the image below.

From a jetting perspective the resin binder is also an importance viscosity modifier, since it helps the formulator balance the water content without using too much co-solvent or humectant.

A resin’s polymers are either soluble, like the dyes discussed above, or form a suspension of particles, either as a dispersion or emulsion.  The term ‘latex’ has often been used to refer to the resin in an ink, although that term is somewhat more specific than intended by many using it. Not all resins are man-made plastics; many (inkjet) inks have been made using shellac, for example, which comes from natural sources.

Of all the polymers used in traditional inks, only a minority can be jetted, and of those, most will damage the average print head. For this reason, many major printer suppliers developed their own proprietary materials with the added benefit to the OEM of making them even more difficult to copy.

Humectants and Co-solvents
These materials normally represent the major component in the ink by volume. A humectant is a molecule that holds on to water and can prevent evaporative loss from the nozzle. As a result, they are important for nozzle health. Unfortunately, they usually have a high boiling point so, as printers get faster, the amount of humectant has decreased significantly. Industrial and “production” inks are now quite different in this respect to office inks, which are designed to be easily kept printing-ready between long periods of being idle.

Water-soluble co-solvents also have an important role in controlling the rate of penetration into plain paper.  They may also heavily influence how the resin comes out of solution and forms a film, which can also effect ink bleed. The overall result of trying to balance so many requirements is that typical inks tend to be made up of a mixture of quite a few ingredients.

Since water has a very high surface tension, getting it to jet well, penetrate paper or wet-out on less-absorbent surfaces is where surfactants become important. Surfactants are like the fairy dust of inkjet formulation, where a tiny quantity can make a massive difference to the behaviour, and finding the right one can be the secret to success. This is especially the case for single-pass or page-wide printers because of close dependence of the print quality on the ink spread. As a result, the surfactant(s) in the ink also influence the tendency of ink to bleed.

Because they migrate to new surfaces, many surfactants can also stabilize foam, so it is sometimes necessary to add defoamers to the ink as a counter-balance.

Defoamers can be physical or chemical in nature. Either way, the intention is to break the surface of the bubbles so they collapse. To achieve this, defoamers are generally a bit less compatible in the base. Therefore, if there are both types of material in a given ink, then the balance can be important to avoid irregular wetting (mottle) or even pin-holes.

Rheology Modifiers
Because many print heads require a viscosity much higher than water, special additives called rheology modifiers may be used. Normally added in small quantities, these materials can increase viscosity and help the ink formulator avoid adding too much co-solvent (and affecting the dry speed). Rheological additives are common in paints too, but they have to be used cautiously in inkjet inks because they can have different properties under the high shear condition created by jetting.

Biocide / Fungicide
Finally, one of the most important additives to water-based inks is the biocide / fungicide. The role of this material is to stop the ink from growing biologically (fungus, mould) whilst on the shelf or being transported. The biocides are hazardous so need to be added carefully. Typically used at very low levels, their effectiveness can be influenced by other materials so, shelf-life stability testing is an important part of aqueous ink development.

Inkjet inks are highly complex and it takes years to fully design the chemistry balance which is needed for the type and reliability of a particular print head technology, as well as the substrate it is compatible with. This complexity is why each OEM’s ink’s chemistry is different.

Watch for our next post which will consider how these materials influence the performance when comparing OEM and aftermarket inks.

Mark Bale is the founder of DoDxAct Ltd in Somerset, United Kingdom where he consults in all aspects of inkjet R&D from ink formulation and manufacture through jetting & process integration to final application optimization in production inkjet, wide-format graphics, labels & packaging, decorative surfaces, photovoltaic manufacture and product coding.