Reflex Blue and other dye based inks come loaded with potential problems, but if you have to use them great care is needed. In the pattern that we have established of alternately covering the activities of the Smartprint Research Centre and the National Printing Laboratory, this month it is a print lab job that prompts this article.
The lab recently received some printed work that comprised a four-colour litho print on one side of a coated sheet, and a light blue stipple on the other. The front side had been UV coated offline.
A problem had shown up in the varnished prints, that in the blue screen on the back of the sheet, some patches had become much lighter in colour than others.
A penny dropped with great rapidity when the identity of the blue print was known, as it turned out to be a Pantone tint of Reflex Blue.
Before proceeding with this particular case, let’s back up a little and explain a bit about the colourants used in printing inks.
With the exception of many ink jet inks and a few specialty flexo or gravure colours, almost all of the colourants used in printing inks are pigments. Pigments are self-coloured chemical compounds that are defined as not being soluble in the medium in which they are dispersed.
Also by definition, dyes are compounds that do dissolve in their carrier.
It may also be generalised that dyes are weaker than pigments of similar shade, much more transparent, give brighter and cleaner colours, but are mostly poorer for light fastness and (importantly) much poorer for chemical resistance.
There are some colours that are part way between pigments and dyes that do find their way into many printing inks. These are made of chemically modified or complexed dyes, and take advantage of the transparency and brightness of hue of dyes, while gaining some improvements in colour strength and light resistance properties.
The most common of these is Reflex Blue, (from a designer’s point of view a highly desirable reddish shade of bright blue), and the (chemically different but physically similar) colours, Rhodamine, Violet, and Purple. These latter colours are technically known as basic dye lakes, and are often referred to under their original trade name, Fanal.
The advantages of these colourants are clear, - pretty, clean strong hues – but all dye derivatives as a colourant type are prone to chemical attack, and cause erratic ink drying (generally slow). These characteristics are often seen with Reflex Blue, which brings us almost back to the start of this piece.
But as with steak knives, there’s more. Reflex Blue also has another unpleasant property, which is responsible for the well-known poor rub-resistance of its prints.
In bloom
When ink made on a modified dye is printed, (regardless of ink type or printing method), tiny crystals of dye migrate to the surface of the print, and cause an effect described as "blooming" or "bronzing". This is seen as a reddish or gold cast or tinge across the surface of the print, (a similar effect to that seen in a butterfly’s wings or across the surface of the fabric known as shot silk).
Because the bloom comprises small crystals of dye, it is readily removed by abrasion, and can easily be wiped away with a wet finger (All of the dye-based colours mentioned above are prone to the same phenomenon, but that very same Reflex Blue is the worst). Thus such colours should never be relied upon to provide rub-resistant prints (In the case of lithography, excess water in the ink can exacerbate both the blooming effect and further retard the already slow drying properties of the colourant, so making the marking or abrasion susceptibility worse).
As noted above, dye based colourants are particularly prone to chemical attack when compared to all other types of pigment.
In particular, Reflex Blue (here we go again) and mixtures containing it, are prone to bleeding into polar solvents such as alcohols or esters, giving rise to a blue halo around the edges of the print, but even more importantly, they go through burnout or drastic colour change under the influence of both alkaline and acid conditions, an effect that is hastened by the presence of any polar solvents. (In extreme cases, the blue colour can disappear altogether to be replaced by a weak, greenish-khaki residue).
Hopefully the point is now becoming crystal clear.
The problem mentioned right at the start was readily reproduced by putting a spot of ammonia on the blue screened print, as the colour rapidly diminished. We thus presumed that some alkaline component, possibly from the UV coating, had penetrated the sheet and had chemically degraded the colour of the blue screen (from behind).
Given the notorious instability of Reflex Blue, and most especially tints of that colour (including greys and greens), the cause of the problem could perhaps have been the use of that blue tint colour in the first place.
So before wrapping up with a plea for all printers to advise their print sales teams and their customers that potential disaster awaits in the use of dye based colours, a few words about how to handle these colours when there is no alternative.
The rub/marking effect from the disturbance of the bloom is best avoided by coating or sealing dye based colours, but extreme care must be taken to ensure that this process does not bring on the dreaded colour burnout problem.
Care and consultation is necessary to ensure that laminating adhesives do not contain alkalis such as ammonia (especially in combination with polar solvents), or that in-line or off-line aqueous coatings are specifically formulated to accommodate colours like Reflex Blue (most are not), and that UV coatings are of a low amine or non-bleaching type. Also due to the drying retardation effect on litho inks, it essential to ensure that litho prints are fully dry before any post processing is attempted. (ref: February article on UV coating).
Difficult behaviour
No doubt, Reflex Blue is a great looking colour but its behaviour must be understood and factored into every job (ditto for all the dye derived colours). The simplest approach for printers and designers for bright blues is to dodge the use of Reflex Blue altogether, and get hold of an alternative colour matched by your ink supplier using Permanent Violet and Process Cyan, as this combination is just about bullet-proof.
Surely, the combination will cost a bit more and will not be as clean in hue, but anyone who has experienced bleeding, rub, or colour burnout will agree with the approach.
