The main film substrates used for labels today are BOPP, PE and PET. They have differences in mechanical properties but also in how they can successfully be printed. Applying an optimized coating on the film can in many cases improve the print result and in some cases it is the only possibility to achieve an acceptable result.
PE as a label material is sold both as topcoated and non-topcoated, and the reason for using topcoated is a considerably longer shelf-life. BOPP is mainly sold as topcoated due to more difficult corona treatment after aging.
WETTING AND ADHESION
There are measurable properties and theories for wetting and adhesion that has been developed and a few basic concepts will explain how they are linked to the printing of labels (see Figure 11.1).
Figure 11.1 Poor ink wetting, seen at microscopic level
In order for a strong enough interaction to develop between the binder in the printing ink and the top coat, close enough contact on a molecular level (at a level of parts of a nanometer) has to be established. For this to happen the surface energy of the film, the surface tension of the ink and the viscosity/rheology have to support it.
Surface energy is measured by comparing the wetting with standard liquids or by measuring the contact angle and calculating the surface energy (see Figures 11.2 and 11.3).
With liquids the risk of contamination from the surface may reduce the accuracy of the measurement, and with contact angle measurements the result is dependent on which algorithm is used for the calculation. Results will be different based on the method of calculation.
Figure 11.2 Low surface energy substrate
Figure 11.3 High surface energy substrate
The unit for surface energy and surface tension are mNm or Dyn/cm and both units have identical value.
Typically, the surface energy of topcoats is between 38 - 45 mNm and to achieve spontaneous wetting the ink has to have lower surface tension than the substrate. Good wetting can be expected if the contact angle is less than 60 degrees.
Substrates with a low surface energy can still be printed if the lack of spontaneous wetting can be compensated by a printing method which uses a nip that forces the ink to wet the substrate.
Substrates with a very high surface energy are more easily contaminated, as thermodynamically they are likely to adsorb low energy substances that will reduce the total surface energy. There are several mechanisms for adhesion to develop:
By choosing the matching chemistry in ink binder and topcoat formulation, it is possible to greatly improve performance compared to a non-topcoated plastic film.
The final performance is always a combination of the properties of the top coat and the ink chemistry. A functional group that can only create bonds in one of the layers would not improve the adhesion. The printing method and ink type will therefore define the optimal adhesion mechanism.
PRINTING METHODS AND INKS
Topcoats can be designed for a specific combination of printing method and ink system. A standard topcoat for label films usually gives good results using UV inks with flexo, letterpress, screen and inkjet. Water-based flexo also prints well on standard topcoats, but water-based inkjet topcoats are usually designed with special pigments that ‘fix’ the ink drops and absorb the water in a porous matrix that later releases it.
Liquid toner systems such as HP Indigo use toner-specific coatings, nowadays often applied in-line in the printing press.
Dry toner systems are generally less critical, but the heat resistance of the film and the topcoat have to be matched to the fusing temperature.
For solvent-based gravure, non-topcoated films have an advantage, since they reduce the risk of the cells becoming clogged with dissolved materials from the topcoat.
Foiling – either cold foiling or hot foil stamping – is usually designed to work on standard topcoats.
Manufacturers have developed raw materials and formulations which deliver good universal topcoats, but there are some challenges more easily addressed in the printing process. High surface energy is favorable for wet-out but detrimental to negative printing of small fonts.
High build UV curable (screen) inks, can in some cases have a very high shrinkage due to the curing mechanism which generates a stress in the coating/surface interface that might reduce adhesion.
As mentioned above, the optimal situation is to have a good match between topcoat and ink.