Adhesion means the force, which keeps the coating sticking on the glass surface. This is in opposition to the cohesion , which is the force, which holds the particles of the coating stick together. Both forces are necessary to have a durable coating. Lack in adhesion is often a result of insufficient precleaning of the glass before the the coating was applied. Adhesion can be tested by a cross cut , followed by a tape test .
Cohesion means the internal forces, which hold the coating together.
To test the adhesion of the coating, it is cut with several crossing cuts. After that, it is tested with the tape test. A standerd concerning the cross cut is DIN EN ISO 2489.
After the application of the paint, solvent evaporation causes it to dry to a dry layer. To achieve the best durability of the coating, the layer needs to be cured. Curing is typically done by heating. By curing, the rest of volatiles is removed from the coating and chemical bondings are formed to make the caoting insoluble.
Washing resistance is an important criteria for decorations on hollow glassware. At tests with houshold dish washing machines, as well as with commercial dish washing machines, the excellent durability of different Ormo paints could be proofed. The biggest factor of influence in washing resistance was the quality of the precleaning of the glass surface before the paint application.
Sometimes it can be observed, that a coating which should be highly transparent and glossy, becomes dull or cloudy after application. This phenomenon is sometimes limited to partial areas of the decoration. Thermal curing does not change the appereance of the coating. There are 2 main reasons for dull coatings: 1. The paint is too old, or it was stored at a too warm place. In this case, the paint is decayed. 2. The humidity of the air is too high, or the glass temperature is too low. In this case the humidity of the air can be reduced by air conditioning, or the glass item can be warmed up before decoration.
Dyes are coloured substances, which are completely soluble in the paint. Paints, which are colorated with dyes give typically highly transparent coatings with practically no light scattering. An alternative to the colouration with dyes is the use of pigments.
Fastness means the duration of the paint against environmental influences, especially against (sun)light.
With the decoration of glass, a frequently asked question is, whether food contact with the decoration may be allowed. Unfortunately the national legislations are very different. An often practized approach is, that any migration from substances of the coating to the food has to be excluded. In doubt, this has to be proofed by practical tests. The proofing can only be done with the decorated glass product. For these reasons, we can not give a general release for food contact of our paints. You are welcome to discuss your concrete questions with us.
Instead of a precleaning of the glass surface before coating, it can be treated with a blue gas flame. This is activating the surface, resulting in an improved adhesion of the coating. Although this treatment can not replace a good cleaning process, it is cheaper to realize than a wet cleaning. Of course the open flame must have enough spatial distance from the place of decoration, to avoid the inflammation of solvent vapours.
The heat soak test is done with tempered glass sheets. With tempered glass, in some cases there happens sponaneuos cracking, which can be provoked by the heat soak test. In the test procedure, the tempered glass sheet is kept for some hours at a temperature of 300°C. If the glass sheet stands this test, it is sure, that there will be no spontanous crack in future. To proof, that the temperature profile of the heat soak test was followed regularily, the glass sheet is marked with the test pencil before the test. Only the correct examination of the heat soak test will give the desired colour change of the marking.
Hot- and coldend coatings are standard at the outer surface of container glassware. The coating consists of a porous inorganic coating (mostly titanium oxide or tin oxide), whose pores are filled with an organic material (polyethylene or a similar, wax-like material). This kind of coating increases the safety against breaking of the glass container. Unfortunately the hot- and coldend coating makes decoration difficult, while the original glass surface is completely covered by this coating.
Hybrid paint is defined by a combination of organic and inorganic binder. The inorganic binder is mostly synthesized according to the solgel procedure. While inorganic coatings excel by hardness and outstanding adhesion properties to the substrate, their chemical resistance as well as their thickness are not suitable for the use as decoration coating. Pure organic coatings have good properties for decoration coatings, while their adhesion to the substate is often poor. Well-done hybrid coatings combine the good properties of both systems.
Our inks can be used for piezo printing heads. We have sucessfully tested the inks with the EPSON 9.000 and EPSON 10.000 printing heads.
Migration means the extraction of parts of the coating (e.g. dyestuff). Migration is an important topic e.g. in food contact applications, because any migration of parts of the coating to the food has to be excluded.
Pigments are little, not soluble particles of a coloured substance, which are dispersed in the paint. In the paint coating these particles are distibuted randomly and generate the colour sensation. Depending on the nature of the pigment, a part of the light may also be scattered at the pigment particles. This is why colouration with pigments will not give completely transparent coatings. By the insolubility of the pigments, the risk of migration of pigments is lower than with the coloration with dyes.
To archieve best durability of the decoration, the glass surface must be clean before the decoration. Although this is a trivial fact, it is often neglected. Often the glass looks clean, although it is not, because disturbing layers are not visible. By a suitable precleaning process, the durability of the decoration can be assured. Sometimes even a flame treatment or heating the glass article in a kiln can be helpful to improve adhesion.
With the solgel technique, inorganic solids are precipated from colloidal (non-crystalline) state. The precursors for the solids are liquids or solutions of solids. The formation of the solid from the precursors is performed by a chemical reaction. The solid is basically generated as macromolecules in the solution. After evaporation of the solvent, the macromolecules grow together to a gellous coating. By a thermal curing, the rest of the volatiles leave the coating and the macromolecules bake together to a solid coating.
The tape test indicates the adhesion of the paint to the glass surface. An adhesive tape is fixed to the decoration and removed again. It is checked, whether parts of the decoration are removed together with the adhesive tape. More information is found in the standard DIN 58196-6.
The fascination of glass is based on a high degree on its transparency. If a colored decoration coating shall appear like coloured glass, it needs to be highly transparent. Oraganic coatings are often cloudy, glass, which is decorated with such a cloudy coating looks like ‘painted’. Also the use of pigments instead of dyes for coloration of the paint can cause light scattering.
Special feltpen, where the flow of the ink to the tip is controlled by a valve.