WEDNESDAY 13TH APRIL

Session 2 – Equipment and techniques
Session chair – Dr Thomas Benen, Microtrac

Controlling pigment properties for optimal inkjet results
Dr Thomas Benen
Sales Manager D-A-CH, Microtrac

Thomas explained the importance of controlling pigment dispersion properties for optimal inkjet results. Poor control of pigment particle size can cause printhead blockages and poor image quality. Pigment size controls optical properties, with the optimum opacity with particles in the 250-500 nm range, but colour strength increasing with smaller sizes, meaning that in most cases 50-200 nm particles are desired. Thomas reviewed the solutions for particle size measurement, which each have different applicability, strengths and weaknesses. Laser diffraction is recommended by ISO from 100 nm to 3 mm size, however state-of-the-art instruments can go down as small as 10 nm. Dynamic light scattering is used for nano-inks and is recommended by ISO from 1 nm up to 1 µm, where sedimentation begins. Dynamic image analysis utilises a microscopic camera and measures particles above 1 µm to several mm. The background to each of the techniques was described, including zeta potential measurement for assessing dispersion stability. Thomas also described the different stages of ink production and when particle analysis is used to identify contaminants, assess dispersion performance, monitor pigment milling and test stability of the final ink.

Characterising and optimising particle size for inkjet applications
Dr Anne Virden
Product Technical Specialist – Diffraction, Malvern Instruments

Anne described the effect of the particle size on pigment performance. Pigment-based inks are produced in two stages –first they are pre-mixed and then they are milled to remove agglomerates and reduce the primary particle size. Particle size affects suspension rheology, with smaller particle sizes increasing viscosity, especially at low shear rates, while broader size distributions reduce the viscosity as the particles can move more easily. Anne reviewed the two major particle size analysis techniques – laser diffraction and dynamic light scattering. Anne pointed out that distributions produced by different methods are weighted differently, giving different apparent results with multimodal distributions. Anne also emphasized that techniques requiring dilution of the sample had limited use, as the dilution may change the properties being measured – Malvern’s instrument allows the sample depth to be changed according to the transmission of the sample. The wide dynamic range of laser diffraction allows milling processes to be followed, primary particle size to be characterized and agglomerates to be detected, while dynamic light scattering can measure at high concentrations, allowing the effect of dilution to be investigated.

Innovations in inkjet analysis
Yair Kipman
President, ImageXpert

Yair used a case study of determining correct printhead head to demonstrate how inkjet analysis systems are used. Correct printhead height is important for optimum image quality, and while maximizing this distance has benefits in reducing the risk of printhead impact, allowing printing of more irregular substrates and reducing system design cost, this will also expose print quality issues due to non-optimum drop formation and ejection. One issue that needs to be assessed is the presence and length of any ligament forming behind the main drop – this ligament causes print quality issues in the final print, and varies due to ink and printhead drive parameters. Greyscale printheads work by ejecting several sub-drops which are intended to merge before landing on the substrate, and this can also be assessed using drop visualization. Thirdly, the accuracy of drop ejection can be analysed using drop visualization, allowing the effect of this on print quality to be assessed. As well as analyzing drops during ejection, the positioning of drops on the final print can be assessed automatically using scanners or more complex assessment systems combined with analysis software.

Production concepts for textile inkjet inks
Dr Franz Giger
Market Segment Manager, Bühler

Franz looked at the case of manufacturing a textile inkjet ink to show how milling is used. The grinding process comprises both deagglomeration – the breaking up of aggregates into primary particles – and true grinding where the primary particle size is reduced. Different types of mills are used depending on the product viscosity and final target particle size, with bead mills the most common for inkjet ink use. The physics of the milling process were then described with the dependence of mean particle size on specific energy input presented. Franz showed examples of mills intended for scale-up and full production use, and emphasized the importance of product cooling. The key parameters for bead milling are bead diameter, volume specific power density, mass specific energy, mass flow rate and cooling. The benefits of smaller bead sizes were presented, showing improved transparency, reduced milling time and lower energy usage.

Surface tension and wetting – Experimental approaches and significance in inkjet printing
Dr Maiju Pöysti
Product Manager, Biolin Scientific

Maiju explained the importance of surface tension of inkjet inks, determining both droplet formation and the wetting behaviour on the substrate. The different methods of determining equilibrium and dynamic surface tension were outlined, and their use in inkjet formulation discussed. Equilibrium surface tension is generally used as a quality control method, while critical micelle concentration and dynamic surface tension can be used to guide formulation during development. Surface wetting is characterized by the contact angle, determined by the surface free energy and the liquid surface tension. Surface wetting is influenced by roughness, which increases surface area and introduces more complex processes like capillary action and diffusion, giving a wide range of apparent contact angles with one substrate-liquid combination. The contact angle also exhibits hysteresis under general conditions, with the advancing angle shown to be more stable as a function of droplet volume.

Evening Reception

After the session, the delegates enjoyed a food and drinks reception sponsored by RJA Dispersions and chatted with the other conference sponsors.