Inkjet application of water soluble, invisible fluorescent dyes

Inkjet application of water soluble, invisible fluorescent dyes


Centralised personalisation obviously offers a higher level of security: it is possible and reasonable to use the most up-to-date, highly secure and very productive machinery. 

Laser engraving technology suits this system very well. Although, the machinery is quite expensive, taking productivity into consideration the costs per document can be kept at a reasonable level. In this system, data processing and document personalisation are done in a well guarded central building, data transfer is secured by IT (information technology) means, therefore the security level of this personalisation system is really high. 

These advantages are unbeatable as long as we are talking about documents of high security and large quantity, like passports, national IDs and driving licences. 

But what can we do for secure personalisation if the documents require a lower level of security, the quantity is small or medium, the carrier is paper or a paper-like material, or laser engraving is impossible for other reasons (the document should be issued at a consulate, for example)? 

Should we give up the advantages of simple desktop printers and go back to manual personalisation methods, i.e., take a ballpoint pen and record the citizen’s data by hand? Our answer was NO.

But there is a problem with this idea. Commercial desktop printers cannot be considered secure at all. We know that secure personalisation equipment does exist on the market, but with the help of our invisible, UV fluorescent, water soluble dyes it possible to make commercial desktop printers secure by modifying the ink.

Since the technology applied by a certain printer determines the ink composition, we had to concentrate our efforts on types that are suited for office use thus we focussed on so called drop-on demand (DOD) printers.

All printers using this principle generate drops of ink only when it is needed, though the way of drop generation can be different. The two most widely used methods are the piezo and the thermo technology.

The piezo technique is based on the deformation of the piezoceramic material as a consequence of the change in the applied voltage. The electric signal that controls the printing process is sent to the piezo material that changes its shape and acts like the motor of a small pump. The generated droplets fly directly onto the medium.

The “motors” of thermo technique are little heaters within the printing head. These heaters are able to warm up and cool down very quickly. They heat up over the evaporation temperature of the ink carrier material (which is mostly water) within milliseconds. Consequently, a bubble forms within the chamber. This miniature explosion throws out a droplet of the ink to the substrate.  

Although, we are not ink manufacturers we are aware of the most important requirements against ink-jet inks, such as:

  • the created image is clear and sharp with vivid colours,  
  • and presents the original colour of the image,
  • the printed image remains stable for a long period of time,
  • the light and water fastness of the image is excellent,
  • meets the specific requirements of the given printer head,
  • causes no deposition on the thermal elements
  • causes no corrosion on metallic parts,
  • shows no foaming and is not prone to air inclusion,
  • does not sediment even in long storage,
  • makes no coagulates that can clog nozzles,
  • retains its physical properties (surface tension, viscosity, etc.) during even extremely long printing processes,
  • does not evaporate in the printing head but rapidly evaporates on the substrate.


Naturally, all inks should meet the basic requirements regardless of whether they are commercial or security inks.  But what makes any type of ink a security ink? 

Security inks must have a special feature or features that commercial inks do not possess but must ensure the same printability as commercial inks.


Water-based inks can be divided into two remarkably different categories, dye-based and pigment-based inks.

The main difference between these two types is that dye-based inks are true solutions whilst pigment-based inks are disperse systems. This means that dye-based inks practically contain no solid particles, whilst pigment-based inks deliberately contain solid particles that are pigments. This difference determines the different features of the inks. 

Viscosity is a vital parameter of any ink-jet ink. Since the required concentration of dyes, is rather low, consequently the viscosity adjustment is usually fairly easy. Pigment based inks, however, require much higher pigment concentration therefore, it may seriously increase the viscosity of the ink. 

Dye based inks tend to cause depletions in thermo heads due to the applied high molecular weight dies. 

Pigment based inks tend to cause nozzle clogging. 

The better (practically zero) migration of a solid particle is obvious and requires no further explanation.

Luminochem produces large number of different fluorescent material for the security printing industry mainly for offset, flexographic and screen printing technologies but, unfortunately, we do not have the necessary equipment to mill down our pigments to the required 100 nm particle size level. This size range is absolutely necessary to use pigments in ink-jet applications.

But, we are proud of our invisible, fluorescent, water soluble dye line. These dyes are recommended for ink-jet ink applications.


Although the number of water-soluble fluorescent materials is huge, there are only a few that can be used in ink-jet inks for security purposes. The question is what criteria a material should meet if we are to use it. 

  • The material must be completely soluble in water.
  • The substance must form a stable solution for a long period of time; that is, no crystallisation is allowed.
  • The material must keep its fluorescence after drying on the paper. The vast majority of water-soluble fluorescent materials lose their fluorescence as soon as they are not solved. 
  • The fluorescent substance should show a “paper-friendly behaviour”. It means that it should penetrate into the paper deeply enough and should anchor to the paper fibres strongly. These features ensure the good resistance against smearing (which is a common problem with ink-jet inks).
  • The material to be used should be colourless in the concentration it is applied. Otherwise, either the print will be visible in the case of invisible inks or the standard Cyan, Magenta; Yellow and Black (CMYK) colours will be changed resulting in bad colour quality in case our dyes are used in CMYK inks.

What dyes of ours do we recommend for taking these criteria into consideration?


UV365 (UVA) excitable dyes

LUWSBLUE1 has very intensive blue fluorescence. The printed image is completely invisible in normal light and has a brilliant fluorescence under 365 nm UV. 

LUWS66 has intensive yellow fluorescence. You can see the printed image under 365 nm UV.


UV 254 nm excitable dyes

UV 254 nm is the UV C region of ultraviolet radiation. Due to the relative harmfulness of this UV radiation, the use of this region requires devices equipped with appropriate eye protection. Since lots of paper-based documents are overlaminated after personalisation, it must be mentioned that most of these laminates absorb the 254 nm UV irradiation. Therefore, these dyes do not work under laminating films.

LUWS6 has green fluorescence. The printed image is completely invisible either in normal light or in 365 nm UV but has an intensive green fluorescence under 254 nm.

LUWS5 has red fluorescence. The printed image is also invisible either in normal light or in 365 nm UV but has an intensive red fluorescence under 254 nm UV. 


Although the number of plastic based documents increases rapidly, paper-based documents still have and will have a very important role. 

What are the main reasons of this? Why can they retain their importance? 

  • their price is much lower,
  • large production capacities exist worldwide,
  • the production process is simpler,
  • the number of security devices (such as watermarks, security prints, etc.) is much larger,
  • multiple recording is possible,
  • decentralised personalisation is possible.

The security dyes we introduced make the secure personalisation of these documents possible using up-to-date printing technology in an office environment at reasonable costs. 

We offer dyes for ink-manufacturers and system integrators making them able to provide personalisation systems that: 

  • are able to personalise paper-based documents,
  • provide a satisfactory security level,
  • are cheap as far as the required equipment is concerned,
  • need no special expertise to use,
  • are quick and easy to use,
  • work at reasonable costs even if the number of the documents to be personalised is small.


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