The ceramic tile market is showing increased interest in customized products, including the reproduction of high-quality images on tile surfaces. Ink-jet technology is a very promising decorating technique for this application. Ink-jet is a non-impact method based on projecting ink droplets onto a surface where the smallest droplet volume, compared to other decorating technologies, permits better control of the image quality.¹

Ink-jet technology imposes many constraints on the inks, which must fulfill requirements such as suspension stability, viscosity, surface tension, pigment particle size and color strength. Until now, two types of inks have been developed that do not completely meet market expectations. One ink uses soluble salts dyes, and the achieved colors are poorly saturated. The second method, using micronized pigments, causes problems like clogged nozzles, dispersion instability and loss of color strength.

A New Solution

Research efforts have sought to overcome these limitations. One study has focused on the development of nanosized pigments.² Another found that, in addition to the synthesis technique, it is necessary to control the pigment refractoriness and to optimize the inks' chemical and physical properties, as well as their interaction with the substrate. A ceramic pigment must be highly refractory, capable of withstanding the chemical corrosion of liquid phases formed during the firing of bodies or glazes, and feature suitable optical characteristics.³ Additional requirements include:

• The pigment's suspension in a medium that bestows proper stability on the ink, further ensuring color reproducibility
• Low-viscosity inks that permit a fast flow through the nozzles of the print head and an easier penetration in the capillary channels of porous substrates
• Proper surface tension of the ink to enable the droplet's ejection out of the orifice without dripping caused by gravity⁴

This work has been aimed at optimizing the inks' properties and their interaction with the substrate to ensure high color strength in different ceramic applications. As a result, nano-sized ceramic inks have been developed that are suspensions of oxides or metals in an organic medium (glycol), synthesized by the polyol process according to a patented procedure.⁺² The polyol process is a low-temperature (around 200°C) chemical route in which stable nanoparticles are dispersed in a high boiling alcohol, exploiting both the chelates and the reducing properties of the poly-alcohol. The phase composition of the four inks includes:

• Cyan (C), a cobalt oxide (Co_xO_y)
• Magenta (M), obtained by a gold precursor
• Yellow (Y), obtained by a titania precursor added with Cr and Sb precursors
• Black (B), a CoFe₂O₄

+Performed in CE.RI.COL. laboratories according to Colorobbia Italia's patented procedure.

Put to the Test

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During testing, the nano-inks were characterized to determine the main properties, including viscosity, surface tension, particle size distribution and ζ-potential, that affect the ink-jet printing process (see Table 1). The penetration kinetics of the inks into different ceramic substrates was also studied. The mass fraction absorbed (Ma) with respect to the mass of the deposed nano-inks droplets (Md) was measured, achieving a non-dimensional penetration index (Ip), where Ip = Ma/Md. Finally, the inks' technological properties were tested using a laboratory ink-jet printer on different ceramic tiles (porcelain stoneware, glazed porcelain stoneware and double-fired wall tiles), and the products were then fired in an industrial roller kiln at the proper temperature.

The optimal pigment concentration, which is different for each ink, was calculated taking into account the required chemical and physical properties, and the color yield. The pigment loading in the Y and B inks is comparable to that of currently used inks, but it is lower in the C ink and noticeably lower in the M ink. As no precipitate was detected from suspensions after several months, the four inks can be considered very stable and suitable for tile production purposes.

Transmission electron microscopy (TEM) and dynamic light scattering (DLS) techniques were used to measure the inks' particle sizes. All of the pigments are in the 10-90 nm range, confirming the efficiency of the polyol method in synthesizing nanoparticles regardless of pigment composition.⁵ In the range of shear stresses investigated, the viscosity of the dispersing medium is about 24 mPa at 25°C. In addition, the four nano-inks demonstrate a Newtonian behavior with viscosities at 25°C around 40 mPa (C, M and B inks). Only the Y inks have a relatively high viscosity (≡ 190 mPa), which can be lowered by simply increasing the printing temperature up to 65°C. All of the inks have a surface tension in the 35-45 mN/m range, fulfilling the ink-jet requirements.

For penetration testing, the ink droplets released by the syringe were around 4 µL, while the volume of an ink-jet droplet is generally around 80 pL. It is possible, with these bigger droplets, to study the penetration kinetics and observe the effect of the pigments in suspension. The droplets of glycol were completely adsorbed by the substrate, while the inks' penetration required longer time; in some cases, not all of the ink droplets were adsorbed. Glycol exhibits the quickest penetration, while the Y ink has the lowest penetration index and the C ink penetration is peculiar, being twice all the other inks tested. These inks behave suitably for tile production purposes, since the droplets are similar to those used in the ink-jet printer and are fully absorbed by the ceramic body.

The results showed that the kind of substrate did not significantly affect the penetration kinetics, as the same results were achieved for different compositions of porcelain stoneware. The forming pressure did influence the degree of penetration, however. At increasing forming pressure, the pore size distribution shifted toward lower values, causing more difficult penetration through the porous media since a higher pressure gradient is needed to overcome the surface tension. Dispersed nanoparticles affect ink properties like viscosity and surface tension, as well as the interaction between ink and substrate, and cause a reduction in penetration.

The ink-jet printing tests showed that all of the four inks are suited for use with a commercial ink-jet printer, and the color strength obtained using the inks is very high. In addition, the inks are able to color many ceramic substrates regardless of their chemical composition and firing cycle.

A Promising Future

Nano-sized ceramic inks have been synthesized to optimize their chemical and physical properties. Particle size, phase composition, suspension stability, viscosity and surface tension of the nano-inks have been tailored to obtain suspensions suitable for drop-on-demand ink-jet printers. In addition, the color strength was optimized according to four-color process requirements. Penetration tests demonstrated that pigment nanoparticles and substrate porosity influence the inks' penetration.

Application tests on a laboratory ink-jet printer showed that the inks effectively color most types of ceramic tile substrates, ensuring good reproducibility and high color saturation. The industrial scale-up is in progress, and these ceramic nano-inks will be available for purchase soon.

*Council of National Research - Institute of Science and Technology for Ceramics (CNR-ISTEC), Faenza, Italy

**CE.RI.COL., Colorobbia Research Centre, Vinci, Italy