#15 Glazes' repellency phenomena: additives for applications before and after digital printing process
- 1. Digital decoration & new scenarios
- 2. Suspensions applications before digital printing process
a) Excessive or inadequate glaze's drainage
b) Accumulation of water vapor
c) Glaze's uneven absorption
- 3. Application after digital printing process
1. Digital decoration & new scenarios
The ink-jet decoration - that has almost completely replaced traditional decoration systems by means of screen screens or silicone rollers (and for this reason commonly defined as "contact decoration") – has surely eliminated several critical issues bringing out, however, new and more complex problems.
Among them, the most popular concerns the water repellency phenomena that arise due to the lack of chemical compatibility between inks based on non-polar solvents and water – the most representative polar solvent - contained in the glaze suspensions used along the production line before and after digital printing application.
A second important question emerging from the new scenario concerns the inadequate or improper absorption of the glaze on which digital inks are discharged: this can sometimes result in printing definition defects. Let’s start with the latter.
2. Suspension application before digital printing process
THE GLAZE ABSORPTION
In general, it is quite unusual that, along the glazing line, a glaze applied on the ceramic surface reaches the digital printer when is still overly wet: in other words, the percentage of water (that is a polar solvent) within the glaze suspension applied before the printing process tends to largely evaporate before the raw tile is under the printheads from which the non-polar solvent ink is discharged.
This is the reason why inks rarely show repellency phenomena (i.e. incompatibility in terms of chemical nature / difference in polarity and surface tension) with regards to the glaze applied before the digital printing process. However, a particular glaze might be characterized by inappropriate drainage values (poorly or excessively draining) that negatively impact on the following stages of the process. In such cases, a specific category of additives should be used: PRIMERS.
What is a PRIMER?
Primers are (not only in the ceramic field) all those products applied to a surface able to create the technical proper preconditions for the following application.
In our case, we are talking about an additive that must be added within/on the glaze and that can avoid some problems that may arise during the application of solvent-based digital inks.
Below the most frequent:
- Excessive or inadequate glaze’s drainage
- Accumulation of water vapor
- Glaze’s uneven absorption
- A) ECCESSIVE OR IMPROPER GLAZE'S DRAINAGE
In the case of a too draining glaze, the ink might sink too deeply inside the glaze and therefore the amount of ink’s discharge should be increased to reach a good result. For example, when the decoration is on smaltobbio (that is usually white opaque in color), the ink’s penetration in the deepest layers of the glaze leads to a poor visibility of the ink (since it is plunged inside an opaque matrix).
Despite this scenario is quite uncommon, the glaze (that is mostly not very plastic) is in this case characterized by the presence of many inorganic raw materials that, though they are cohesive, are “distant”, thus allowing the drainage/sinking of the ink. To avoid or reduce the problem, it is required a preventive action by adding to the glaze a proper PRIMER, able to create a reticulation between the glaze’s particles, therefore reducing the penetration, both of ink and solvents. The kind and the amount of primer determine the higher or lower closure of the spaces between particles, and so the drainage.
In some cases, the problem can be opposite: the glazed support can be characterized by a very low drainage that turns into a significant loss in terms of printing definition, especially when the graphic project involves the use of high amount of ink. In this scenario the wisest course of action, though drastic, is to carefully analyze the additives in the suspension and then check the glaze’s or engobe’s formulation, reviewing all parameters until the best drainage conditions are achieved. In some cases, it is also useful to decrease the amount of those inorganic raw materials that already in itself tends to reduce the drainage.
The main target is the calibrated formulation of an additive, able to promote the best rheological conditions for the developing of a more open reticulation in the glaze that allows the ink to better sink through the superficial layer (i.e. more drainage). From a chemical point of view, it is recommended to limit the use of binding polymers by giving priority to additives that can make the system cohesive without creating a film-forming effect.
These additives, that can be added directly along the glazing line, “expand” the glaze reticulation providing the glaze with a higher drainage level (we could vulgarly say that they separate the glaze’s inorganic particles).
- B) ACCUMULATION OF WATER VAPOUR IN THE FORM OF CONDENSATION (ON PRINTING BARS)
When tiles covered with a high-water content glaze enter the digital printing machine, reaching the printheads (tiles that normally have a temperature that can range from 50°C to 60°C), a serious problem may occur: the water vapor it produces tends to accumulate on the metal parts of the printing bar until the formation of drops that may fall on the tile, inevitably damaging it. This specific problem can be solved by taking different actions.
When possible, it is recommended to increase the temperature within the dryer to promote and almost finish the evaporation process before the following digital printing process. Nevertheless, it is important at the same time to carefully avoid that the raw ceramic tile reaches the printheads with a too high temperature: this could overheat the printing bar, leading to many different problems such as a partial or full obstruction of the nozzle (due to the thermal shock).
Moreover, if the tile is too warm the ink’s solvent could evaporate just after its discharge by the printhead: more specifically, on the way between the nozzle and the tile support.
What does that mean?
The partial evaporation of the ink’s solvent produces a certain amount of vapor that progressively accumulates under the printing bar in form of very tiny drops that obviously fall on the tile.It should also be said, however, that several digital printers have a blocking mechanism in the event of an out-of-standard temperature being detected.
A second action: increase in the density of the engobe and/or of the glaze, by using a different kind of additivation to minimize the amount of water in application. The target is the developing of the proper conditions in terms of fluidization and rheological stability to work with higher density values without affecting the good leveling of engobes or glazes. The families of additives that can be involved in the process are manifold (dispersant, leveling agents, binders) and, likewise their formulations, they are selected and tailor-made according to the different production parameters. They can be added both during the milling process and directly along the glazing line.
A third option: the use of a PRIMER (to be applied within the glaze but much more frequently on the glaze) that significantly reduces the evaporation by chemically reacting with water, therefore preventing vapor accumulation under the printheads.
- C) GLAZE'S UNEVEN ABSORPTION
In some cases, raw tiles (especially those with textured surfaces) can unevenly absorb the ink, forcing ceramic producers to use specific primers (binders or wetting agents) capable to align the differences.
How do they do it?
Glues or binders can improve the glaze's performance in terms of leveling by increasing drying times (basically providing the glaze with more time to develop a proper leveling) and creating at the same time an organic reticulation that in most cases helps to standardize the drying times of the different parts of the tile.
In other cases, wetting agents - since they reduce the glaze's surface tension - can sometimes reduce even the different value of surface tension between the ink (solvent-based) and the glaze (water-based), leading to a better ink reception by the ceramic support that in turn leads to a more homogeneous absorption. Wetting agents also similarly facilitate the homogeneous distribution of the glaze's solid particles (by a deagglomeration process) therefore improving the application.
3. Application after digital printing process
All applications that follow the digital printing process frequently provide an extra problem often related to the incompatibility between inks' solvents (non-polar polymers) and the end-of-line water-based applications (that contain polar solvents): top glaze, grit suspensions, high-weight glazes (to be use for the following polishing process), etc. etc. Inks' solvent are real oil derivatives that develop a surface film in correspondence of the decoration. The full-field application of the glaze suspension produces real repellency phenomena between the suspension and the ink décor.
The phenomenon is similar to what happen when you try to mix water and cooking oil or when you pour water into dirt oil plate: the two elements are chemically incompatible and therefore they remain separated, without any interaction. In ceramic field, the lack of interaction between ink and glaze leads to the same repellency phenomenon that turns into an improper glaze leveling on the raw ceramic support.
What is the underlying mechanism of the problem and what actions should be taken to solve it?
Once again, the high surface tension of the aqueous system is involved, which necessarily must be reduced by using a proper additive so that the glaze can fully wet the ink’s oily surface that covers the entire tile. More precisely: the lowering of the surface tension present at the interface between the tile’s oily surface and the aqueous surface of the glaze generates greater compatibility and therefore a reduction of the repellent effect.
Going back to the previous example, the pouring of water on the oily plate together with a small amount of detergent (additive that lowers the surface tension) results in a good wettability of the surface and therefore, in this case, in the cleanability/washability of the surface.
IMPORTANT: from a chemical point of view, the proper and good compatibility deriving from the use of additives characterized by a not too high wetting action so to avoid a real solubilization of the ink’s solvent and therefore a worst printing definition.
Although it is rare, the ink applied on the raw ceramic support can be sometimes too humid when the following watery application is ready to be discharged (whether glaze, glue, or grit suspension).
In these cases, the leveling of the aqueous application on the ink is very difficult, unless you use particular and specific additives with a strong surfactant power. However, even if they surely improve the application process, they also can lead to side effects that must be monitored and contained:
- Foam within the system
- Excessive reduction of the aqueous suspension’s surface tension that results, especially with glazes, in a reduction of the glaze’s cohesion
- Ink’s solubilization
In some cases, due to specific aesthetic requirements, repellency phenomena are deliberately sought. There are on the market some additives, commonly named as CURVING, that can be applied by means of digital printheads and that are able to promote compatibility between inks and glazes, limiting the repellency phenomena in specific areas of the ceramic surface. This effect is usually required with some kinds of decoration such as the wood or marble’s veins or geometric designs. After the firing cycle, there is a lack of glaze that aesthetically creates a natural depth effect.
These additives are developed to be strongly non-polar (usually much more than inks) and their application must be preceded by a previous study of the glaze in use as well as of the leveling agent that the glaze contain so to:
- 1. Promote a good and proper leveling and avoid a more general repellency phenomena thanks to the additive’s wetting action
- 2. Keep the repellency phenomena exactly (and only) in the areas where the curving is applied
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