HIGH-VISCOSITY INKJET INKS PROMISE INNOVATIONS IN FLEXIBLE PACKAGING PRINTING

High-viscosity (HV), water-based, inkjet inks represent the latest technological innovation from global ink manufacturer, Nazdar and leading printhead technology company, Xaar. Developed with a close-knit team of chemists, the transformative benefits of HV inks for printing, include increased colour density and reduced ink usage. This new ink technology offers challenges and benefits for multiple applications. It may prove to become the ultimate answer to achieving quick drying, without compromising print quality.

Measured in centipoise (cP), the viscosity of water-based inks for inkjet printing, has to be carefully controlled to ensure a jettable consistency. Inkjet printing inks are usually formulated with low viscosities, resulting in highly diluted inks. They have lower pigment concentrations compared to other printing methods. This leads to relatively reduced colour intensity, lower productivity and increased drying energy requirements. 

Conventional inkjet inks usually exist within the ~2–13cP viscosity range. The ultra-high, viscosity ink technology expands to ~30–50cP. This allows HV inks to impart greater colour density during printing. The results lead to greater colour impact and overall improved printing performance. 

“The transformative benefits of high-viscosity inks for printing, include increased colour density and reduced ink usage”

The full commercial potential of HV inks has yet to be unleashed. However, the increased viscosity of the fluids has proved successful across multiple applications. Less absorption into porous substrates, results in the ability to use less ink. In turn, this increases efficiency and creates a whole host of new benefits:

Improved optical density (OD) – Higher colour density and opacity.

Greater hold-out – Less ink absorption in porous media, leading to sharper images.

Potential for single-pass printing – High hold-out means multiple printhead passes may not be needed for high-quality printing.

More functional components – Enhanced ink performance with higher levels of active components.

Reduced carrier content – Less solvent or water for better efficiency.

Higher print speed – Faster drying and single-pass printing facilitates increased production rates.

Enhanced drying mechanism – Faster fixation on porous and non-porous media.

Some of the main challenges in formulating HV inks revolve around fluid stability and rheology. For effective and efficient jetting, there needs to be careful control of the viscous and elastic components of the ink. HV inks tend to have a significantly higher elastic component, which can lead to issues of poor drop formation. It can, additionally, lead to low open time due to partial gelling of the ink in the printhead channels. The increased pigment and binder contents can also bring about unexpected issues of sedimentation, shear sensitivity or ink gelation. It is essential to find the right balance in the formulations, to achieve all the potential benefits of HV inks. 

“This new ink technology offers challenges and benefits for multiple applications”

When it comes to flexible packaging printing, the importance of achieving high-speed printing and quick drying – without compromising print quality – has always posed a challenge. The film substrate is often temperature sensitive. Also, the printing process often needs to be at high speed. Lastly, on these non-porous substrates, all drying has to be through evaporation or curing, without any help from absorption into the media. 

High-viscosity, water-based ink technology, offers reduced water content and higher binder levels. As a result, a unique solution to balance both high-speed drying and durability is available. 

HV inks give thicker films and hence higher colour density and opacity. These inks allow for the use of higher binder levels. This can potentially eliminate the need for a primer.

Increased drying speed and durability on non-porous substrates – combined with a wider colour gamut – means a wider variety of printing available to flexible packaging than ever before. 

Although HV inks are faster drying, they may still not be able to dry fast enough for use in very high-speed printing processes.

As a drying method, near infrared (NIR) is an attractive technology that can also contribute to resolving the issues of drying water-based inks on high speed printing lines. NIR essentially heats and dries the ink – not the substrate – with water vapour removal being managed by a very high airflow through the dryer. This selective heating of the ink, without affecting the substrate, makes it ideal for drying on heat-sensitive film substrates without causing damage.

However, in conventional water-based inks, a lot of water has to be removed before the ink is dry and becomes durable. This can limit the benefits of NIR technology.

Using HV inks that have a lower water content/increased binder levels, with NIR drying technology, gives a strong synergistic combination. Consequently, it can offer a highly effective solution for high-speed drying. An ongoing collaboration between Nazdar and Adphos is working to further explore these combinations.

There are large variations in NIR absorption across the different ink colours (YMCK). Consequently, the colours require different speeds and lamp power when it comes to drying. This makes it very challenging to dry full-colour images. 

Experimental results have shown that using a higher-viscosity ink, with NIR absorbers, can greatly improve the drying performance of all colours. Through a careful choice of NIR absorber loading, it should be possible to balance the drying of YMCK inks. This ought to enable the drying of full-colour images, with simplified lamp arrangements, to be fast and effective.

Backing these new technologies, experimental results are pushing the boundaries of digital printing. Great new possibilities lie ahead for customising flexible packaging printing. 

The ability to print HV inkjet inks is proving to be revolutionary. It opens up new markets and opportunities. HV inkjet inks are paving a way for a greater role of inkjet in the flexible packaging print industry. In addition, the industry is heading towards offering a more sustainable solution for corrugate, textile and advanced manufacturing applications. The benefits are clear. The enhanced viscosity of the fluids reduces substrate absorption (in porous media). This optimises colour vibrancy without, necessarily, the need for increased pigment loading or energy intensive drying processes. With new ongoing research and development to address formulation challenges, the future outlook on HV inks and associated drying technologies is very promising.