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#28 DIGITAL INKS FOR CERAMIC PRODUCTION: WHAT DO WE NEED TO KNOW?

Contents

  1. 1. Introduction
  2. 2. Identikit of an ink
    1. a) Solvent-based VS Water-based
    2. b) Particles' grain size
    3. c) Viscosity
    4. d) Surface tension
    5. e) Drying times

 


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1. Introduction

Thanks to the introduction of some important digital processes, ceramic industry has significantly evolved over the last few years. In this regard, the digital printing system plays one of the most important roles. The use of digital inks has made possible to replace old decoration systems with a technology that surely provide the process with a greater flexibility. The inks’ formulation and, therefore their production, is very delicate and complex, since it must strictly comply with specific parameters.

 

2. Identikit of an ink

What are most important parameters that affect the features of a digital ink?Firstly, to better understand these features it is important to define the ink’s composition: all inks involved in the ceramic production process consist in a solid part dispersed in a liquid. More precisely, the solid part is made up of INORGANIC PIGMENTS. The pigments’ inorganic nature is essential since it more easily withstand the very high temperatures of the ceramic firing cycle that can reach 1.100 / 1.200 °C. Organic pigments, in fact, would burn and disappear during the firing cycle, leaving any visible trace.

 

A) SOLVENT-BASED vs WATER-BASED

At present, two are the main categories of digital inks: solvent-based (the more widespread) and water-based ink products. The most significant difference between the two is based on the different kind of solvent within which the pigment is in suspension: we can talk about non-polar solvents when we refer to solvent-based digital inks and about polar solvents in case of water-based products (in this regard, water is the most representative example of polar solvents). The formula and the composition of a solvent-based digital ink consist in 60/70% of non-polar solvents and chemicals and a 30/40% of pigments while a water-based digital ink is made up of 30/40% of polar solvents and chemicals, 25/35% of pigments and a 30/35% of water.
It is also important to know that the chemical nature of the pigments (the solid part) may change according to the color. For example, a blue ink contains cobalt aluminate while the use of tin and chrome spheres allows you to reach more pinkish shades. The inks’ liquid part (that can be polar or non-polar) is usually developed by using chemicals marked by a dispersant action. In this regard, the quality of the chemicals involved in the formula is a must since they are responsible for the definition and adjustment of the ink’s main parameters:

 

  1. Particles' grain size
  2. Viscosity
  3. Surface tension
  4. Drying times

 

It may seem trivial but is also important to underline that the formulation and the choice of chemicals can change according to the pigment used within the ink suspension.

 

B) PARTICLES' GRAIN SIZE

One of the most important requirements of digital inks is the grain size of the pigments. The piezoelectric application system involves the use of digital printheads provided with nozzles whose diameter is about 50 µm. This is the reason why pigments must be nano-size and, more precisely, the particles’ dimension must be about one micron. Greater dimensions, in fact, may promote agglomeration phenomena, blocking the printhead and therefore the production line, negatively affecting the industrial productivity.

Pigments’ grain size may also affect the proper development of the color, also promoting unexpected sedimentation phenomena. In general, smaller sizes lead to a less color development and to a reduction of possible sedimentation.

 

C) VISCOSITY

Viscosity (or viscous friction coefficient) measures the attitude of a substance (usually a fluid) on generating friction between its layers. It defines the fluid resistance to flow. In liquids, the coefficient of viscosity is inversely proportional to the temperature: the more the temperature increases the more the viscosity decreases. In liquids, the viscosity coefficient is normally inversely proportional to temperature: the more the temperature increases, the lower the viscosity. From a microscopic and therefore particle point of view, the value of viscosity depends both on the shape/size of the molecules and on their kind of interaction.

Speaking of digital inks, viscosity values must range beween very rigid parameters to properly develop the dimension of the ink’s drop and avoid printing defect. At present, considering the general parameter of ceramic industry, the viscosity should range from 10 to 30 cP (Centipoise, unit used to measure viscosity) according to the discharge temperature (that is the temperature of the inks during the discharging from the printheads to the ceramic support).

 

D) SURFACE TENSION
Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. It defines the cohesion force of the particles that are on the superficial part of the fluid.
Even the surface tension is a fundamental value that can affect the proper formation of ink’s drops. Just to give a general indication, the surface tension of a solvent-based digital ink should range between 28 and 35 mN/m. Some producers are already developing digital printheads able to work with higher surface tension values.

 

E) DRYING TIMES
In this regard, since the scenarios can be very different according to the production parameters of each producer and each production line, it is important to point out that the ink’s drying times – that are developed and defined during its formulation – must comply with the following application to promote a correct stratification (and reaction) of the layers (glue, grit, glaze, etc.).

 

In short
Both solvent or water-based digital inks are developed to get a proper balance between all the parameters so to promote the best performance as possible. This is the reason why a preventive study in the R&D labs is mandatory to reach the desired target.

 

Check the orginal post by Zschimmer & Schwarz España

 



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