Interview: The impact of structural ink characteristics on adhesion properties
Bernd Miller, head of technology, Sheetfed & Narrow Web EMEA at Siegwerk Switzerland AG
In an interview with NarrowWebTech Bernd Miller, head of technology, Sheetfed & Narrow Web EMEA at Siegwerk Switzerland AG talks about the adhesion of inks and varnishes, the factor that impact the adhesionand how to improve problems during label printing such as the wetting behaviour of an ink.
Irrespective of the printing process, the adhesion of inks and varnishes depends on different factors. Can you explain which factors we are talking about?
Bernd Miller: There are four factors that impact the adhesion of inks and varnishes to a substrate: the surface tension of the substrate and the wet ink, the swelling capacity of the substrate surface, the drying method of the ink and the roughness of the surface. Depending on each case, these factors can have varying degrees of effect.
You say the surface tension effects the adhesion of ink. What is meant by surface tension?
Bernd Miller: In general, you distinguish between two intermolecular forces of attraction. On the one hand, you have the force of attraction between two different substances, like for example between liquid printing inks and a substrate, which is called adhesion. And on the other hand, you have forces of attraction within a substance, for example in an ink droplet, which are called cohesion. And the cohesion forces on the surface of a liquid are referred to as the surface tension.
And how does the surface tension of the substrate and ink correlate with each other?
Bernd Miller: If the surface tension, or in other words the force by which the molecules mutually attract one another on the surface, is greater than the adhesion force between the droplet and the substrate, the liquid will contract. It tends to assume a spherical shape, this is typically revealed by water forming beads that drip off a waxed surface. But if the surface tension of a liquid is lower than the surface tension of the substrate or the previously printed ink film, this liquid – for example, the freshly applied ink – will tend to spread. And it stands to reason that a printing ink that spreads over the substrate surface and therefore wets this well, will adhere better than if it forms beads that drip off. The surface tension is indicated in dyn/cm (= 1 mN/m). By the way, if referring to solids this force is referred to as surface energy, nevertheless the meaning and units are same as surface tension.
What has to be considered in terms of a good wetting behaviour of the ink on a substrate?
Bernd Miller: For a good wetting behaviour, the surface energy of the substrate must be higher than the surface tension of the ink. If this is not the case, the substrate will repel the ink. Sicura Screen white SF, a silicone-free UV ink, has for instance a very high surface tension of 40 dyn/cm. It, therefore, does not wet properly on an untreated plastic substrate. It is interesting to note that PET (polyethylene terephthalate) has quite a high surface energy of 43 dyn/cm and can thus be easily overprinted with UV flexo inks like for example Sicura Flex 39-8 that has a surface tension of 38 dyn/cm. The lay down and absorption are problem-free. However, if UV inks are printed on PET, ordinary UV flexo inks do not adhere to untreated PET. The higher surface tension of the substrate is only one of the conditions for good adhesion but not a guarantee.
Could the wetting behaviour of an ink be improved?
Bernd Miller: Yes, you always have two possibilities to improve the wetting behaviour of an ink. You can either increase the surface energy of the substrate for example via corona pre-treatment or usage of a primer, or you can reduce the surface tension of the ink by using silicone or other additives. In general, you can say that the simpler solution is the increase of the surface tension by means of corona-treatment because the use of silicones makes further processing like overprinting, hot foil stamping or laminating more difficult.
You’ve mentioned the swelling capacity as another factor for the adhesion of ink to a substrate. What is meant by this?
Bernd Miller: The swelling capacity or also partial solubility of the substrate surfaces plays an important role in anchoring the ink, especially when printing on plastic films. Although the substrate appears to have a smooth surface, the smallest molecules from the ink, such as solvents or UV monomers, can penetrate into the uppermost layer. As a rule of thumb: the smaller the molecules in the ink, the more ‘aggressive’ they are. With regard to UV, the smaller the molecules, the thinner the ink but also the greater the potential for migration. On the other hand, this means that large molecules do not swell the surface of the substrate as easily. It therefore follows that low migration inks tend not to adhere so well to plastic films. PE (polyethylene) and PVC (polyvinylchloride) exhibit good swelling properties. Substrates with poorer or almost no swelling properties in the UV sector are OPP/ PP (polypropylene) and PET.
And how can the ink drying method impact the adhesion of inks and varnishes?
Bernd Miller: In the case of physical drying inks, a solvent like for example water or alcohol evaporates, while chemically drying inks involve a chemical reaction, as happens with UV and EB inks. In both cases, the process is associated with a certain shrinkage of the ink film. In regard to physical drying inks, the solvent, which represents around 50–70% of the ink volume, leaves the ink film in a vertical direction; the dry ink film is thinner than the applied ink layer but adheres well. There is no horizontal shrinkage and no weakening in adhesion due to this. A three-dimensional crosslinking occurs during chemical drying and the molecules move closer together. Depending on the UV system, a shrinkage of
5 to 15% results, that not only has an effect on the thickness of the ink layer but also in the horizontal direction. This explains the effect of curling during UV printing on thin plastic substrates. The tension built up in the ink/film area leads to a reduction in the adhesion properties. The drawback is: The better the cross-linking properties of the UV ink, the greater the shrinkage and the poorer the adhesion.
Which factor has to be considered in terms of adhesion of the inks to substrates like paper and cardboard?
Bernd Miller: In contrast to plastic films, the surface roughness plays an important role with paper and cardboard substrates. If the ink is able to penetrate into cracks and pores on the surface of such absorbing substrates, an improved anchoring will logically be the result. Inks therefore adhere to paper and cardboard practically without any problem. Nevertheless, when it comes to UV inks on paper we are confronted with the problem of “diving” – in other words, immersion or sinking – which has nothing to do with adhesion failure. The liquid components of the UV ink (e.g. binders) are absorbed by the paper and cannot cure under the UV lamp, which means the inks do not adhere. In this case, a primer must be pre-printed in order to fill the pores.
Bernd Miller has almost twenty years of experience in the printing and packaging industries. After completing his chemistry studies, he held various positions within the graphic arts business segment and has been working for Siegwerk Switzerland Ltd for 19 years. He is head of technology in the Siegwerk business units sheet-fed and narrow-web printing inks.