Many characteristics of the ink-jet printing technology make it interesting for decoration of ceramic articles. In particular, as far as high resolution images are concerned, the fact that ink-jet has the smallest drop volume, compared to other decorating technologies, permits a better control of image quality; besides, customized products can be obtained at admissible costs because each image can be simply changed by adjusting the software connected to the printer. Nevertheless, ink-jet printing has been applied to ceramic decoration since a few years and up to now it has not become a common tool in the ceramic industry; in fact, design and formulation of suitable inks are the most critical steps of this technology. Nowadays, ink-jet printing on ceramics is carried out by using either soluble dyes or micronized pigments. The former offer a limited colour saturation and palette, while the latter can cause nozzle clogging and dispersion instability, if particle size is not thoroughly controlled. These problems can be overcome through nanosized inks, that are able to increase the image quality and ensure a high reliability to the printing systems. Nanosized inks are two phase fluids, consisting of a dispersion of nanometric particles in a liquid vehicle. This work is aimed at developing suitable nanosized ceramic inks for the ink-jet printing four-colour process. These inks were prepared with various pigment concentrations and were characterized studying the physical and rheological properties required for ink-jet application. In details, particle size distribution (Dynamic Light scattering and TEM), ?-potential and electrical conductivity (electroacoustic spectroscopy), surface tension (optical contact angle) and viscosity (stress-controlled rheometer) were determined. Moreover, penetration kinetics of the four inks into different unfired ceramic tile bodies was experimentally appraised by evaluating the amount of ink absorbed in the unit of time and of surface area by the porous substrate, using an optical contact angle measuring instrument. Results showed that pigment particle size is in the 10-80 nm range, and that all the inks are stable for long time (i.e. more than one year), due to their high values of ?-potential as well as the steric effect of the medium, so that no problems of nozzle clogging may arise from particles sedimentation. These inks fulfil the surface tension requirements for ink-jet printing (i.e. 35-45 mN/m), though values vary significantly with pigment loading. Viscosity values are usually appropriate (lower than 30 mpa s) even if the pigment concentration may cause an excessively viscous behaviour for ink-jet printing; this drawback can be easily overcome heating at temperature lower than 75°C. The electrical conductivity is generally in the 10-100 µs/cm range and is influenced by composition (e.g. inks containing metallic particles exceed 1 s/cm). The penetration kinetics is mostly affected by pigment concentration and composition, though it changes significantly depending on the substrate composition, microstructure and process parameters. In conclusion, the nanosized inks are suitable for ink-jet application as proved by their chemico-physical properties and by application tests that are giving satisfactory results both in terms of colour saturation and image resolution.
Chemico-physical properties of nano-sized ceramic inks for ink-jet printing
Gardini D;Blosi M;Dondi M;Costa AL;Galassi C;Raimondo M;
2006
Abstract
Many characteristics of the ink-jet printing technology make it interesting for decoration of ceramic articles. In particular, as far as high resolution images are concerned, the fact that ink-jet has the smallest drop volume, compared to other decorating technologies, permits a better control of image quality; besides, customized products can be obtained at admissible costs because each image can be simply changed by adjusting the software connected to the printer. Nevertheless, ink-jet printing has been applied to ceramic decoration since a few years and up to now it has not become a common tool in the ceramic industry; in fact, design and formulation of suitable inks are the most critical steps of this technology. Nowadays, ink-jet printing on ceramics is carried out by using either soluble dyes or micronized pigments. The former offer a limited colour saturation and palette, while the latter can cause nozzle clogging and dispersion instability, if particle size is not thoroughly controlled. These problems can be overcome through nanosized inks, that are able to increase the image quality and ensure a high reliability to the printing systems. Nanosized inks are two phase fluids, consisting of a dispersion of nanometric particles in a liquid vehicle. This work is aimed at developing suitable nanosized ceramic inks for the ink-jet printing four-colour process. These inks were prepared with various pigment concentrations and were characterized studying the physical and rheological properties required for ink-jet application. In details, particle size distribution (Dynamic Light scattering and TEM), ?-potential and electrical conductivity (electroacoustic spectroscopy), surface tension (optical contact angle) and viscosity (stress-controlled rheometer) were determined. Moreover, penetration kinetics of the four inks into different unfired ceramic tile bodies was experimentally appraised by evaluating the amount of ink absorbed in the unit of time and of surface area by the porous substrate, using an optical contact angle measuring instrument. Results showed that pigment particle size is in the 10-80 nm range, and that all the inks are stable for long time (i.e. more than one year), due to their high values of ?-potential as well as the steric effect of the medium, so that no problems of nozzle clogging may arise from particles sedimentation. These inks fulfil the surface tension requirements for ink-jet printing (i.e. 35-45 mN/m), though values vary significantly with pigment loading. Viscosity values are usually appropriate (lower than 30 mpa s) even if the pigment concentration may cause an excessively viscous behaviour for ink-jet printing; this drawback can be easily overcome heating at temperature lower than 75°C. The electrical conductivity is generally in the 10-100 µs/cm range and is influenced by composition (e.g. inks containing metallic particles exceed 1 s/cm). The penetration kinetics is mostly affected by pigment concentration and composition, though it changes significantly depending on the substrate composition, microstructure and process parameters. In conclusion, the nanosized inks are suitable for ink-jet application as proved by their chemico-physical properties and by application tests that are giving satisfactory results both in terms of colour saturation and image resolution.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
