FLUID-SUBSTRATE SYNERGY

An ink fluid can look robust in formulation and stable in the laboratory, but still fail the moment it meets the substrate. Across packaging, graphics and industrial-print applications, even a well-formulated ink fluid can suffer catastrophic failure when it encounters the intended surface. Common problems include bleeding, excessive spreading or insufficient spreading, reticulation and other related defects.

In print, reticulation is an artifact that manifests as a mottled, grainy or ‘fisheye’ uneven film structure during ink drying. It occurs when adjacent droplets of water-based, inkjet ink pull apart or fail to uniformly coalesce. Reticulation is often driven by a mismatch between the rapid surface-tension dynamics of the droplets, the rheology of the carrier fluid and the impermeable or semi-porous nature of the substrate on which it is applied. See figure 1.

Traditionally, a formulator attempting to address reticulation would adjust ink surface tension and change or rebalance surfactants. However, addressing the surfactant dynamics is not always viable. As aqueous-inkjet printing becomes more widely used across different applications, the surfactant route often yields negligible results. In certain applications, one may resort to the use of a primer layer. However, this can introduce additional process steps and cost implications. For that reason, reticulation should be examined not only as a wetting problem, but also through thermodynamic and rheological perspectives.

While surface wetting plays an important role, it is only one factor in a much more complex process. The key question is not simply whether the ink wets the substrate, but whether the fluid, polymer phase, drying profile and substrate surface energy are working together as a system. Great results stem from fluid-substrate synergy. A systems approach to high-quality print performance means finding a way to have all these aspects function in harmony. This includes the behaviour of the fluid itself, the characteristics of the polymer phase, the rate and conditions of drying and the surface energy of the substrate. When these factors are properly balanced, a uniform coating can form. When they are not balanced, defects such as reticulation can occur. Understanding the interaction of these variables provides a more complete explanation of why reticulation develops and how it can be prevented.

Balancing co-solvent blends, rheology and drying rate becomes the real key in managing reticulation. The main carriers in aqueous ink are water and humectants. However, when water and primary humectants evaporate at different rates it can create a physical phenomenon known as a Marangoni effect. This phenomenon happens when liquids flow from areas of low surface tension to areas of high surface tension. In printing, these surface-tension gradients can drive fluid motion within the wet droplet. This internal movement can pull pigment and polymer particles apart before they have sufficient opportunity to form a continuous matrix. See figure 2.

Changing the ink’s formulation then becomes important. This can achieve a balanced blend of slow-evaporating co-solvents alongside faster-flashing components. It narrows the vapour-pressure gap between water and the solvent system, and can help smooth out drying rates across the droplet boundary. The result is a concise, reliable ink registration. 

The next consideration is how the dispersed polymer or resin behaves upon impact. Formulation chemists are constantly rethinking the polymer role in film formation and how it might affect print performance. The governing factors include particle size, glass transition, film-formation temperature and related properties. Large polymer particles may require extensive packing and high coalescence energy. Shifting to an ultra-fine particle size may significantly increase the inter-particle contact area. This promotes rapid packing and continuous film knitting – even on smooth coated liners. See figure 3.

The ability of polymer particles to undergo or resist deformation at the application temperature is also important. 
If the polymer phase is unable to deform and coalesce during drying, individual micro-droplets may dry as isolated crusts. Therefore, it is important to consider the glass transition temperature and film formation characteristics. This encourages better chain entanglement during the critical phase where water departs.

A great print result starts long before the image reaches the press. Surface energy plays a critical role in how an ink behaves when it contacts a substrate. The balance between the substrate’s surface characteristics and the ink’s surface tension determines how far and how fast an ink droplet spreads. When that balance is off, problems can develop quickly.

To mitigate these risks, it is important to ensure that the polar and dispersive surface-energy components are appropriate for the surface tension of the impinging ink droplet. This is true of treated or untreated substrate surfaces. 
A large surface-energy and surface-tension gap is likely to create an extreme over-wetting condition. See figure 4.

In the event of a mismatch between surface free energy and surface tension, a lower-surface-tension droplet hitting a high-surface-energy surface can experience a very high spreading force. The droplet may aggressively flatten and snap outward almost instantaneously. 
It is also possible that the carrier fluid and surfactant retract unevenly before the high-molecular-weight components can anchor, causing standard pull-back or reticulation. This is where ink formulation becomes especially important.

As innovation drives aqueous-inkjet printing forward at a rapid pace, Nazdar’s R&D lab is sprinting alongside it. It concerns engineering inks to account for the complex interactions between surface energy, surface tension and drying dynamics. By designing formulations that accommodate a wide range of substrates and printing conditions, the lab is able to help converters achieve more consistent wetting, stronger film formation and cleaner print quality. The result is improved reliability on press, fewer defects and greater confidence when printing demanding applications.

Understanding surface energy is not simply to do with avoiding print problems. 
It is about creating the conditions for predictable, repeatable performance. When the ink and substrate are working together – rather than against each other – printers can maximise productivity. At the same time, they can deliver the high-quality results expected by their customers.

“Surfactant adjustment remains an important formulation tool”

Print artifacts – such as reticulation in aqueous-inkjet printing – are rarely caused by one variable alone. They are better understood as the outcome of interactions between fluid rheology, drying dynamics, polymer film formation and substrate surface energy. Surfactant adjustment remains an important formulation tool. However, a broader systems approach gives the formulator more practical routes to improve film uniformity, reduce pull-back and achieve more reliable print performance across demanding substrates. 
For aqueous inkjet systems, the real opportunity lies in treating the ink, substrate and drying environment not as separate variables, but as connected parts of the same print-performance equation. See figure 5.