Film and Foil The importance of shelf-life
In the third of three articles, Jan Eisby, chief business off icer at Vetaphone A/S, the company that invented Corona treatment, explains the importance of shelf-life on substrate performance.
he various types of plastic fi lm that are used in today’s package printing sector, whether for labels, shrink sleeves, pouches or other kinds of fl exible packaging, are all non-absorbent and need to be surface treated to allow ink, lacquer or adhesive to adhere securely. As I stated previously, this begins with the extrusion process, where the substrate being manufactured is Corona treated to obtain a certain dyne level, but 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; that is what I am referring to as its ‘shelf Life.’
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For any liquid to adhere securely to a fi lmic material, the relative surface energy between the two needs to be adjusted. Untreated fi lm has very low surface energy, measured in dynes, 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 saleable and signifi cantly reduce the volume of waste produced.
The dyne level achieved by surface treatment at extrusion is insuffi cient for the whole downstream production process. First the fi lm 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 fi nished form. There is normally a time delay between each of these processes; time is no friend to dyne levels because all materials will suff er dyne decay from the instant they are surface treated.
To ensure that each process will work as intended, you need to check the dyne level of the substrate before you start. Then, check it before you print and again 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, so without any time delay. The issue here is that the behaviour of the new digital inks that are becoming increasingly popular is diff erent from the fl exo inks they are replacing, so we advise regular testing and checking. The biggest time delay tends to occur between
the fi lm being extruded and when it is used for printing. This can be a matter of a few days, but more often stretches into weeks and occasionally months. So, storage of the fi lm is critical. The problem is that substrate manufacturers do not give clear guidance on this subject in the datasheets relating to the diff erent fi lms they produce. Some customers are better equipped to handle storage than others and dyne decay is not necessarily consistent, so each case is diff erent. It
is essential that the fi lm was correctly surface treated at extrusion because this will have an impact on its shelf life and ability to be boost treated after storage. There are four factors involved in dyne decay during storage: time, humidity, additives and contamination. These make up what we call the ‘decay profi le’. In general terms, the more you have of each of the four factors, the greater the dyne decay will be. Using a dyne pen is a simple, reliable and inexpensive method of testing, especially if you compare it with the cost of attempting the next process with mis-matched surface energy levels between liquid and substrate. Knowing your fi lm parameters is essential; how pure is it, how many ppm of additive does it have, what is its granulation? These are all key elements, so you really need to master the decay profi le or go for ‘JIT’ production where storage is not an issue. If you are storing fi lm, keep the temperature in the 15 - 20 Celsius range and the humidity between 50 – 60 percent. Some transport trucks are air-conditioned; make sure you keep the rolls wrapped to keep out dust and dirt.
Finally, to understand the converting parameters, you need ink/adhesive surface tension data from your suppliers. If you then measure the fi lm surface tension, you can use the curve profi le to calculate the boost dosage needed to reach the correct dyne level at each process stage.
12
Dec 2024 / Jan 2025
www.convertermag.com
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