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Understanding substrates
WHAT DO we mean by substrate? The substrate is the surface onto which you can apply a printed image. Some people call it material and others media. To your average general printer substrate is paper or board and the main differentiator is whether the finish is gloss or matte. To a screen printer a substrate is virtually any stable material that can be supported and make contact with the print side of a stencil. As screen printing breaks into more industrial applications, so the range of exotic substrates increases. The substrate can be horizontal, vertical or any angle in between. In reality 99 per cent of all print surfaces are horizontal. It is generally in some forms of container decoration that vertical printing is used. However, there is a UK-based specialist who can screen print onto walls and other vertical surfaces.
The fact that screen printing is an ink-friendly process allows the printer to formulate and use a vast range of ink chemistries. No manufacturer exemplifies this more that Davison Chemographics whose number of ink formulations exceeds 500, the vast majority being screen printing inks (although digital, litho and other processes feature in the markets served by the company). When you examine the list of formulations and substrates they are used on, screen printing is seen as the most versatile system.
Whatever the substrate, it must be wettable by the ink. That means the ink must make full contact with the surface. On most surfaces this is a function of their surface energy and its relationship with the ink's surface tension. Surface tension is the property of a liquid or ink arising from unbalanced molecular forces on or near its surface. If surface tension is higher than the material's surface energy, the ink tends to form droplets. The other name for an ink‘s surface tension properties is its ‘wettability'.
Surface tension is normally measured in energy units called dynes (mN/m). A dyne is the amount of force required to produce an acceleration of 1cm/sec on a mass of 1g. The dyne level of a solid material is called its surface energy.
The surface energy of a substrate can be indicated by applying a liquid of known surface tension and seeing how it reacts. If the surface tension is
lower than the surface energy it will form a continuous film for at least two seconds. If the surface energy is lower, the test liquid will immediately form droplets.
Ideally the surface energy of the substrate should be 10 Dynes greater than the surface tension of the ink to enable complete wetting. Solvent ink is typically 30 to 32Dynes.
There are more accurate means of measuring surface energy. One is to measure the contact angle of a droplet of water. This is done with a Contact Angle Goniometer. A good contact angle is less than 60deg, a poor contact angle approaches 90deg and above. The greater the contact angle the higher the surface tension of the liquid relative to the substrate. There are formulae that will derive the actual surface tension from the measured contact angle.
Surface tension exists because the molecules inside a liquid experience roughly equal cohesive forces in all directions, but molecules at the surface experience larger attractive forces toward the liquid than toward gas.
Contamination in the form of oil, grease, mould release or dust and debris will all compromise the ink's adhesion. Clean paper and board provide a good printing surface as they absorb some of the ink. Once other substrates are used, particularly plastics, matters can change. Many plastics, such as polypropylene and polyethylene, have low surface energies. Helping ink adhere to these surface requires either inks with special chemistries or, better still, substrate pre-treatment to raise its surface energy.
Five methods are used: corona discharge, flaming, cold gas plasma, vacuum plasma and liquid priming. Liquid priming is the least favourite because the liquids contain harmful chemicals that need to be handled and stored very carefully. It is not a recommended process. Far superior but with varying levels of capital cost are the gas phase surface oxidation processes of corona, plasma and flaming. All methods produce a gas plasma comprising free electrons, positive ions and other species. This combination is extremely reactive. Plasma is considered as the forth state of matter: solids, liquids, gas and (with sufficient energy) gas will ionize into plasma.
So, why all this theory about surface tension and surface energy, when all we want to do is select a substrate to print on to? Because, the condition of the surface is all important to the printer and end customer.
Many plastics have low surface tensions (frequently less than 28mN/m). If such materials have to be printed by solvent-based inks they ideally should have surface tensions of over 40mN/m or over 56mN/m in the case of UV-drying systems. If pure water-based ink systems are employed surface tensions of over 72mN/m are needed.
