Jan Eisby of Vetaphone discusses “shelf life”
Jan Eisby: For the purposes of this discussion, I am referring to the different types of plastic film that are used in the packaging printing industry today. Whether it is a label, shrink sleeve, pouch or any other type of flexible packaging, all film substrates are non-absorbent and must be surface treated to allow the ink , lacquer or adhesive to adhere securely. It starts with the extrusion process, where the substrate being made is corona treated to achieve a certain level of dyne. And it’s what happens to that level of dyne over the next few days, weeks, and months before the substrate is used in production that’s important – that’s what I call its “shelf life.” “.
JE: For a liquid to adhere firmly to a film, the relative surface energy between the two must be adjusted. Untreated film has a very low surface energy, which is measured in dynes, so any liquid applied to it will remain on the surface as small beads. Corona treating the material to increase its dyne level will allow liquid to adhere during the printing or converting process. This will produce high quality salable packaging and significantly reduce the volume of waste produced.
J.E.: No, it’s not. If you look at the production steps involved, you can appreciate the complexity of the situation. The film is first extruded, then printed, then perhaps an adhesive applied for lamination and perhaps a lacquer coating before it becomes the final product in its finished form. There is normally a delay between each of these processes – sometimes short, sometimes long, but whatever it is, time is not a friend of dyne levels as all materials will experience dyne degradation from the when they will be surface treated.
JE: To ensure that each process will work as intended, you should check the dyne level of the substrate before you begin. So check it before printing, then before applying adhesive, and again before coating. This verification applies even to the narrow web sector where many processes are carried out in line in a single pass, therefore without delay. The problem here is that the behavior of the new digital inks which are becoming increasingly popular is different from the flexo inks they are replacing, so again we advise regular testing and checking.
JE: The biggest delay tends to occur between when the film is extruded and when it is used for printing. This can be a matter of a few days, but most often extends over weeks and sometimes months. Thus, the storage of the film is essential. The problem is that the manufacturers of substrates do not give clear indications on this subject in the data sheets relating to the various films which they produce.
JE: For starters, some customers are better equipped to handle storage than others, and dyne decay isn’t necessarily consistent, so every case is different. It is also essential that the film has been properly surface treated during extrusion, as this will impact its shelf life and ability to be processed after storage. Four factors are involved in the decomposition of dyne during storage: time, humidity, additives and contamination. These constitute what we call the “decay profile”. In general terms, the more of each of the four factors you have, the greater the dyne decay will be.
JE: Using a dyne pen. It is a simple, reliable and inexpensive method, especially when you compare it to the cost of attempting the following process with inconsistent surface energy levels between liquid and substrate. You can buy the dyne pens from us through our website, so there is no need to take the risk of getting it wrong.
JE: Let me give you the example of a customer who provided us with different materials to test in our laboratory. These were two types of PE – one with 600 ppm additive, the other with a top coating. Both initially broke down at the same rate, but the topcoated material stabilized after a month at 42 dynes while the other continued to break down over the three months of testing in which it continued to break down. was broken down to 36 dynes. So the choice the customer has to make is either to spend more money upfront on the finish coated material or have the cost and inconvenience of increasing the processing of the cheapest stock after a period of storage.
JE: Plasma is the best form of surface treatment if you need to achieve a high level of dyne that can be retained over a long period of time, typically on films used in agriculture, but also for other applications. Our lab test shows that although corona treatment achieves the desired level of dyne for printing or adhesive application, the decay is relatively rapid compared to ordinary plasma treatment, where the level of dyne reached during processing is higher and the decay is slower and flattens at higher dyne level. Using high power plasma treatment achieves the highest dyne level, and this level is retained for a much longer period of time. But again, it’s a matter of cost – plasma is a much more complex and expensive process and corona is better suited in most cases.
JE: Knowing the parameters of your film is essential – what is its purity, how many ppm of additive does it contain, what is its granulation? These are all key, so you really need to master the decay profile I mentioned earlier or go for JIT production where storage is not an issue. If you are storing film, keep the temperature between 15 and 20 degrees Celsius and the humidity between 50 and 60%. Some transport trucks are air-conditioned, which is good, and be sure to keep the rolls wrapped to avoid dust and dirt. Finally, to understand conversion parameters, you need ink/adhesive surface tension data from your suppliers. If you then measure the surface tension of the film, you can use the curve profile to calculate the amount of boost needed to achieve the correct level of dyne at each stage of the process. There are no shortcuts, but Vetaphone can advise you every step of the way to ensure you get the best use of your time and money.