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Encoding with invisible digital inks
 

ChemStream developed a new set of non-yellowing invisible inkjet inks for different types of drop on demand piezo printheads. The printed image is totally invisible at daylight but it becomes brightly visible when shining with a black light. The inks can be applied in all kinds of sustainable digital workflows, for instance for the purpose of tracking & tracing, encoding, etc. Interesting is the never ending amount of images and text that can be digitally printed, and the relative simplicity of the technique.

Water based formulations:
A water based low viscous pigment ink, based on a high performance nanodispersion of a dedicated fluorescent pigment, has been developed for Epson printheads. Due to their well-designed formulation, these water based inks match the technical requirements for excellent jetting performance and head maintenance The inks are specifically designed for absorbing substrates and perform very well on textile substrates.

UV-curable formulations:
By tuning the spectral response curves of the photoinitiators with the absorption spectra of the dedicated fluorescent dyes and pigments, ChemStream’s new UV-curable invisible inks can be cured by illuminating the ink with UV radiation without destroying the fluorescent characteristics under black light. Two versions have been developed:

- An inkjet ink formulation curable with a UV-bulb and used for very high throughput applications.
- An inkjet ink formulation curable with a UV-LED of 400 nm for moderate throughput applications.

The ink rheology (viscosity and dynamic surface tension) was optimized for several industrial piezo printheads: Xaar, Konica Minolta, Toshiba Tec, Seiko, Fuji-Dimatix, … . Although the UV inks are specifically designed for non-absorbing substrates like metals and plastics, they can also be applied on dedicated textile substrates.

Examples:

Textile printing:
Single Pass and UV-LED curing.
A print was made on a blanco piece of textile using the invisible ink with a Xaar 318 printhead. After printing, the image was immediately cured and fixed with a UV-LED lamp (@ 400 nm). Afterwards, the invisible image at daylight became brightly visible when shining with a black light. No extra after-processing is needed to fix or dry the prints butsome conditions are needed for this technique to be successful: the textile may not contain too many optical brightners to avoid the loss of contrast, and it is preferentially not too porous to avoid the loss of ink into the pores and thus image density. Possibly a customized base coating might be needed in certain cases (an example is shown in the picture below).

invisible inks

 

Metal printing:
Single Pass and Ga/In doped Hg-bulb curing
A digital image can be printed on different metal substrates: Alumina, Copper, Stainless steel,… To optimize the adhesion a customized precoat can also be necessary in this case to improve both the physical and image properties.
In the following example a clear UV-curable precoat of 12 μm has been applied on an Alumina substrate. Before curing, an invisible image was jetted with a Xaar 318 printhead onto the wet layer. Both the precoat and the printed image were simultaneously cured in one pass at 20 m/min with a Ga/In doped mercury bulb.

invisible inks