Adhesion: a complicated game of chemical interactions
Adhesion, or lack of it, is one of the most critical attributes for most coatings and inks. Adhesion could be applied to a myriad of substrates, from low to high surface energy. From metallic substrates with high surface energy like copper or aluminum to polyethylene, polypropylene or Teflon with very low surface energy.
When substrates with low surface energy are in contact with coatings or inks of a higher surface tension, the latter are hampered to wet or cover the surface effectively. Instead, low surface energy substrates cause coatings or inks to bead up, limiting their surface coverage and significantly lowering bond strength.
Figure 1: Estimation surface energy and spreading behavior by measuring contact angle
The lack of wetting on low surface energy substrates can be easily overcome by activating them by plasma, corona, RF electric discharge type excimer lamp or other method. (Chemstream has in-house an atmospheric plasma set-up where most substrates can be cleaned and activated, like PP, PE, melamine…)
The activation treatment breaks some of the polymer crystals triggering the formation of new open surface. This new open surface is then ready to interact with an ink or coating. However, normally only a 1% of the C atoms at the surface are functionalized by means of an activation treatment. This is enough to allow a good wetting of an applied coating or ink but largely insignificant to provide a real adhesion. Actually, the most important aspect of a coating or ink to adhere is the selection of the components.
Coatings or inks could be water or solvent based, UV curable or cationic. They all have their advantages or disadvantages and they are chosen depending on the application. Nevertheless, in this case we focus on UV curable coatings and inks due to the fast speed of curing and because they are 100% solids, meaning that everything that is applied, stays. This can be translated into a better control of the film thickness and less waste.
UV curable inks and coatings typically contain reactive monomers, photo initiators, oligomers, pigments and additives that, when introduced to an ultraviolet (UV) lamp, create a rigid film. The reactive monomers in UV instantly crosslink to provide a cured, cross-linked coating. During polymerization, a certain shrinkage occurs when weak Van der Waals forces are replaced by strong, short covalent bonds between the carbon atoms and different monomer units. This causes the UV coating to shrink and pull away from the substrate and, in turn, can result in film failures. Another factor when working with polymers, is the entangling of the polymer chains. The more the chains are entangled, the harder it is to pull them apart.
In order to make an adequate choice of monomers for a coating or ink, ChemStream uses different tools. For instance, HSP (Hansen solubility parameters) provides an elegant and smart way to determine compatibility between polymers. If one tries to mix two polymers with very similar HSP they will entangle perfectly, and adhesion will be good. However, two dissimilar polymers will absolutely not mix, and they will have a very poor adhesion. HSP is ideal to pinpoint the appropriate monomers to obtain the best formulations depending on the specific clients’ applications. (See Case: Solubility parameters).
Figure 2: Curing protective coating with Hg bulb under manufacturing process conditions
Adhesion on metals
Metals are, in general, materials with high surface energy. Therefore, a pretreatment to activate them is not strictly necessary. However, plasma can also be used as a cleaning method. In the case of metals, this results as being very useful to remove any kind of impurities, like oil, that would hamper the adhesion to a coating or ink.
Normally, in the case of metals, the use of monomers is not enough to obtain a good adhesion. Nevertheless, these should still be chosen to minimize shrinking. For metals, the use of adhesion promoters is highly recommended. Phosphate, carboxylic or silane groups are excellent options depending on the type of metal and the conditions that the coatings must bear. Most of these adhesion promoters are acidic compounds, which are not compatible with most of the dispersed pigments. These acidic compounds would destabilize the dispersion, making the pigment particles flocculate. This means that for metals, a clear adhesion primer is frequently a better option followed by the printing or coating of the colored inks on top.
Adhesion on glass
Glass is a very demanded substrate to adhere on and one of the most difficult. Glass is a high surface energy substrate but in general it requires a pre-treatment to remove impurities and to create a more hydrolyzed surface to improve adhesion.
Similar as for metallic substrates, the use of adhesion promoters is needed. In general, the adhesion on glass is more difficult to achieve than on metals, needing either a greater amount of adhesion promoters or stronger ones. If the coating doesn’t need to be in contact with water, the use of phosphates is ideal. They provide an excellent adhesion on glass. However, phosphates desorb in water, destroying the adhesion of the coating.
In case the coating has to be in contact with water (e.g. a washing procedure), silanes are a better option. However, the hydrolysis of silanes (to adhere on the surface) is quite slow. Therefore, the adhesion primer has to be designed in a way that the hydrolysis of the silane compounds is accelerated.
Same as for metals, due to the acidic character of most of the adhesion promoters, a clear adhesion primer is needed, followed by the colored inks.
Figure 5: Illustration of the chemical interaction silanes with glass
Adhesion on plastics
Plastic are substrates with low surface energy. Therefore, their surface has to be activated to obtain a better wetting and adhesion. The chosen monomers should have a rather high Tg to avoid shrinking and with a somewhat large contribution of monofunctionals for a better adhesion.
Figure 3: Activation low energy PP with atmospheric plasma
Figure 4: Cross cut evaluation of UV-cured coatings on Al-substrates