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Dye vs pigmented inks – which ink suits your needs?

By Dr Richard Marsden, Chief Chemist, Linx Printing Technologies Ltd

Choosing the most appropriate ink for your application is a critical decision to ensure that the very best message quality and durability is achieved. Often the type of ink you need will determine which ink jet printer you buy. Therefore understanding the properties of the inks available is important.

Dye-based inks
The most common type of ink in an industrial continuous ink jet (CIJ) printer is dye-based. These inks offer unrivalled performance and reliability. They are ideal for the majority of customers’ applications.

One feature of the dyes used in these types of inks is that they dissolve into a solution (like sugar in tea). This means that when printed they let the colour of the substrate show through. This is fine if you are coding onto a light coloured substrate, but for dark substrates such as some extrusions, a lighter coloured ink (e.g. yellow) is required for contrast. However, a yellow dye-based ink will still appear dark because it is transparent and the dark substrate shows through.

Dye-based inks may also partially dissolve over time (‘bleed’) when printed onto plastics which contain a plasticiser - used to make certain plastics flexible, such as PVC cables. The result is reduced clarity of the printed code over time.

Pigmented inks
Transparency and bleed can mean that dye-based inks are not suitable in certain situations, and the answer to this problem was to develop a new type of ink where the colour is the result of a pigment and not a dye. Unlike dyes, pigments do not dissolve into solution, but exist as tiny particles in suspension (like sand in water). The particles are less than 1 micron in diameter - they need to be small enough to pass through the nozzle of an ink jet printer (which can be as small as 37 microns in diameter) without getting stuck, and without disrupting the ink jet process.

Pigments work by absorbing some of the incoming light (like dyes) but also by scattering the light that hits each particle so that the light cannot reach the substrate. This results in an opaque ink that gives good contrast on dark substrates. The size of the pigment particles must be carefully controlled; if they are too small then the opaqueness (or opacity) will be poor, but if they are too big there is a risk of the nozzle or filters becoming blocked.

As the pigments are insoluble they cannot dissolve into plasticisers, and therefore won’t bleed on PVC cables and other flexible plastics where plasticisers are used.

Light and heat fastness
Dye-based inks have varying (although normally adequate) levels of light-fastness and heat-fastness. Therefore if your application involves a heat process or exposure to light, such as in storage environments or during actual use, then a pigmented ink may offer higher levels of resistance to heat and/or light.

For example, extruded pipes, which are stored outside after coding, or aluminium window frames, which are exposed to sunlight and heat, would retain their code better with a pigmented ink.

Additionally, pigmented inks may offer better chemical resistance as the pigment will not be dissolved by contact with the customer’s product; for example, perfumes or aerosols that may contain solvents.

Maintaining suspension
Pigments in inks need to be kept in suspension for maximum opacity and to reduce the risk of blocked nozzles. This can be achieved by a combination of clever ink chemistry and also by modification of the printer:

   • Stirrers – some ink jet machines include stirrers in the ink tank to prevent the pigments from settling. There are drawbacks with these systems however, as they    can be prone to wear and tear, and mechanical failure. Additionally they may need to be run even when the printer is switched off.
   • Circulating system – some printers keep the pigments in suspension by a vigorous circulating system, a special pump and an innovative ink tank design. This    shape of ink tank causes any settled particles to collect in a small area, thus making their dispersal easier when the powerful pump starts.

Although some pigmented inks may be available on standard ink jet machines, heavily pigmented inks may require a specialist machine to cope with the increased levels of pigment. Whichever machine is used, it is particularly important with pigmented inks to follow recommended maintenance procedures.

Summary
Dye-based inks are the inks of choice for most customer applications, with a wide range of options available. However, for some applications pigmented inks are likely to be more suitable. Deciding between a dye-based ink and a pigmented ink will therefore depend on the following requirements:

   • What is the colour of the product to be coded?
   • What level of contrast of the code is required?
   • Is light (and/or heat) fastness a critical requirement?
   • Are chemicals present in the product that may dissolve the ink?

Taking time to make the correct choice of ink will ensure your coding application is a success!

Dr. Richard Marsden has a degree and Ph.D. in Colour Chemistry from the University of Leeds, followed by postdoctoral fellowships in Synthetic Organic Chemistry at McMaster University in Canada and Hull University, UK.

He first worked on CIJ inks for binary systems with Elmjet, before joining Linx Printing Technologies in 1990. He is a chartered chemist and member of the Royal Society of Chemistry.

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