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Development of sol-gel inkjet inks for printing on glass substrates for architectural applications

Periodic Report Summary - TIGJET (Development of sol-gel inkjet inks for printing on glass substrates for architectural applications)

In the project TIGJET, the research fellow and TIGER Coatings set a goal to develop a new class of sol-gel inkjet inks which opens a new way of manufacturing individually designed flat glass for outdoor applications as currently used screen printing technology is economically inefficient, very inflexible and often of poor image quality. A new class of sol-gel inkjet inks shows on the one hand an excellent adhesion on glass and on the other hand withstands exterior weathering conditions which are typical for permanent usage on facades over years. To achieve this very challenging goal, we have decided to investigate two different routes to develop inks that can fulfil the requirements with regards to adhesion on substrate and weathering performance.

a) Development of inorganic-organic hybrid sol-gel inkjet inks that can be fused into the surface of glass panel at elevated temperatures and can be printed without nozzle failure.
b) Development of 'inorganic' sol-gel inkjet inks; ideally water based.

Development of the sol-gel inkjet inks has been performed according to the description of work in the project proposal. The work was divided into three directions: development of organic-inorganic hybrid inks, inorganic sol-gel inks and water-based sol-gel inks.

Long-term stable dual curable organic-inorganic hybrid sol-gel inkjet inks based on UV-curable organofunctional silanes applicable with major industrial piezo print heads have been developed. The novelty of the developed sol-gel compositions lie in the using of UV-light initiated hydrolysis of organosilanes directly during UV-curing of the inks after their jetting onto glass surface. In this way, it is possible to avoid complications usually occurring when using traditional sol-gel processing of the organosilanes by prehydrolysing of them before inkjet inks composing that leads to the uncontrollable condensation of organosilica networks with time. The last is dangerous for print heads controllable performance due to their blockage during operation or in the stand by mode.

Developed hybrid sol-gel inkjet inks have superior adhesion on glass surface after their thermal treatment at temperatures 160-200 degrees Celsius and extended exposure to high humidity environment (more than 1000 hrs at 40 degrees Celsius, 100 % RH). Hybrid organic-inorganic sol-gel inkjet inks have been developed using set of organic colours (organic pigments) of CMYK and white range. In order to improve outdoor stability of the developed sol-gel ink-jet inks towards UV-light, inorganic pigments were also incorporated in the compositions.

Printing trials of the sol-gel inkjet inks have proved their stable printing characteristics. Print head nozzles' blockage has not been observed even after long breaks between jetting sessions up to 72 hrs.

The results obtained during development of the hybrid organic-inorganic sol-gel inkjet inks have been further used when working on the development of inorganic sol-gel inks for fusion into glass surface at elevated temperatures. As we have already figured out the most applicable organofunctional silanes to compose inkjet inks with long-term stable rheological behaviour, they were used to create on their basis inorganic sol-gel inkjet inks.

Extensive experimental trials in the sol-gel films deposition and their thermal treatment at elevated temperatures (> 650 degrees Celsius) showed that the components reducing melting temperature of silica should be added into the composition to achieve coatings capable for fusing into glass surface.

Inorganic sol-gel inkjet inks have been developed on the basis of UV-curable organofunctional alkoxysilane as inorganic matrix source and glass frit concentrate ensuring melting of the silica matrix and fusing of the coating with inorganic colours into glass. Obtained coatings have extremely high resistance against high humidity environment as well as against UV-light degradation as they are 100 % inorganic systems after firing at temperatures higher than 650 degrees Celsius. As was expected continuous mixing of the systems helps to control sedimentation of the inks due to the high content of the inorganic materials with high density.

To implement ceramic sol-gel inkjet inks into practice further research is necessary to achieve complete stabilisation of inorganic colours in the sol-gel systems as well as to obtain long term rheological stable inks.

Waterborne sol-gel inkjet inks on the basis of inorganic silica binder with organic and inorganic colour sources have been developed. Incorporation of water dispersible acrylic oligomers as well as photoinitiators allows us realising for this type of sol-gel systems 'drop-fixation' mechanism. Sol-gel compositions with organic colours show appropriate long term stability of rheological properties. Addition of high boiling point solvents into the system improves open time properties of the inks.

From the application point of view first two classes of sol-gel inkjet inks are the most perspective as by using developed organic-inorganic hybrid sol-gel inkjet inks one may obtain photographic print quality during wide format inkjet printing on glass surface. Depending on the colour system used in the inks (organic pigments or inorganic pigments) UV-light stability of the printed patterns can be adjusted for indoor or outdoor usage. On the other hand, developed inorganic sol-gel inkjet inks for fusing into glass surface have perspective to be implemented into manufacture of inkjet patterned tempered glass process as an alternative to screen printed tempered glass. Their lower content of the abrasive inorganic particles will lead to longer operation time of the print heads.

Industrial application of the developed sol-gel inkjet inks for exterior applications opens new possibilities of manufacture of individual designed entire facades and this makes the material glass more attractive and will lead to increased business activities for the participating European companies in the worldwide market. There is a global demand of innovative and attractive architectural design. Today, there is no technology available of printing long term stable colourful images on exterior glass. However, there is a very clear market demand for such type of products. European companies will lead the worldwide market for decorative architectural glass for exterior application. If it is becoming feasible to print images by inkjet on glass instead of using screen printing processes there are many obvious advantages. Cost of individual design could significantly be reduced as costly screens are no longer required. Digital direct printing by inkjet will speed up the manufacturing process enormously. Images printed by inkjet should show better resolution and much wider colour space than today available inorganic screen printing inks. These innovations will also boost the entire European glass industry as new market segments for flat glass will be opened up using inkjet printing as finishing step. Both printer manufacturers and raw materials suppliers will also recognize positive economical impacts. While the firsts will be able to sell more printers in the industrial sector, the latter will face an increased demand for certain chemical compounds.