Whether for use as a label, shrink sleeve, pouch or any other kind of flexible packaging, all filmic substrates are non-absorbent.
As a result, they need to be surface treated to allow ink, lacquer or adhesive to adhere securely.
Matching the surface energy of the ink or lacquer to the substrate is essential. If they don’t match, there will be problems with adhesion. Corona treatment is the way we change the surface structure of the substrate to ensure good adhesion. We measure this surface energy in dynes, which is a function of watts of energy applied per square meter per minute.
All photos courtesy of Vetaphone A/S
Time is no friend to dyne levels. All materials suffer dyne decay from the moment they are surface treated.
Films change over a period of time and under different storage or transport conditions—even from roll to roll of the same substrate. We attribute it to the action of additives or contamination. That’s why it’s essential to test it—the substrate—before you use it. If necessary, re-treat it! Different films are used for different applications and behave differently—It’s a bit of a moving target!
Different rolls from the same supplier can behave differently as well, while supposedly the same material from a different supplier can pose even bigger problems. Typically, rolls of a particular film that vary significantly in price will do the same in terms of performance. Lower purchase price normally indicates the need for more power to reach the correct dyne value and a slower running speed, or both. As with much in life, you get what you pay for!
Eliminate Guesswork
Take all the guesswork out of the equation! Minimize cost implications! That’s my message for printers and converters engaged in everyday production scenarios. The amount of time and money invested at this stage is insignificant compared with getting it wrong in production. If you know exactly what you are dealing with, you don’t make expensive mistakes.
To ensure that each process will work as intended, you need to check the dyne level of the substrate before you start. So, check it before you print and then before you apply adhesive, and again before coating. This checking even applies to the narrow web sector, where many of the processes are done inline in one pass, without any time delay. The issue here is the behavior of new digital inks that are becoming increasingly popular is different from that of the flexographic inks they are replacing, so once again we advise regular testing and checking.
Carry out an advanced contact angle test that analyzes the droplets and gives a highly accurate measurement of the surface energy of the film. This produces a “wetting envelope” that tells you if you have chosen the correct ink or adhesive for the material being used for the job. Similarly, an advanced peel test assesses whether the laminate bonding is good or bad. It measures the force required to tear the product apart and details any issues involved.
“Time is no friend to dyne levels. All materials suffer dyne decay from the moment they are surface treated.”
Take a typical coffee pack for example, which is often a multi-layer material. Corona treatment will be required on the polypropylene (PP) surface to allow it to be printed and subsequently have an adhesive applied before laminating it to a metalized film—in every case, we need to know the dyne level and requirement of each layer before we can recommend the power required and operational production speed.
Production Stages
If you look at the stages of production involved, you can appreciate the complexity of the situation. First the film is extruded, then it is printed, then perhaps has an adhesive applied for lamination and maybe a lacquer coating before it becomes the end-product in its finished form. There is normally a time delay between each of these processes—sometimes short, sometimes long.
As noted, everything begins with the extrusion process, where the substrate being manufactured is corona treated to obtain a certain dyne level. It is what happens to this dyne level over the following days, weeks and months before the substrate is used in production that is important—what I am referring to as its “shelf life.”
For any liquid to adhere securely to a film material, the relative surface energy between the two needs to be adjusted. Untreated film has very low surface energy, so any liquid applied to it will stay on the surface in small beads. Corona treating the material to increase its dyne level will allow the liquid to adhere during the printing or converting process. This will produce packaging of high quality that is commercially sellable and significantly reduce the volume of waste produced.