Dyne Level Loss on Corona Treated Surfaces

Question: I know corona treatment does not create a permanent effect. I’d appreciate any comments you might have on treatment loss, including what causes it and what can be done to minimize it.

Answer: Thanks for the question. Treatment loss is an issue that can have a serious impact on printers, converters, and other material processors, and it can result in disagreements between converters and substrate suppliers. As such, it is critical to have a firm understanding of the phenomenon, and how you can best deal with it.

This post focuses on polymers, specifically films, but some of the comments – especially those relating to contaminants and environmental factors – also apply to metals, composites, and other materials.  Also, while the topic is corona treatment, the factors discussed will also affect the durability of flame or plasma treatment, albeit to a lesser effect.

A variety of factors contribute to treatment loss. The story starts even before the polymer is extruded: Most film feedstock includes slip agents to improve handling characteristics of the extruded film. The concentration of these, as well as their formulation, have major ramifications downstream with regard to treatment loss. Traditional slip agents such as erucamide will by design bloom to the surface, displacing the oxidized layer imparted by corona treatment. High loading of these agents can wreak havoc on treated films. Alternate slip formulations are available (discussed here) which help to minimize this problem.

A second factor related to the compounded resin is the tendency of short chain molecules – oligomers – to migrate to the surface as well. Residual oligomer content will vary depending on the rate at which the material is polymerized (faster polymerization = cheaper material = more oligomers = more treat loss problems). This is the reason why some processors find a strong inverse correlation between film cost and treatment loss. Finally, anti-oxidants and processing aids such as calcium stearate can migrate to the surface over time as well. The latter can also build up on takeup rolls and transfer to the freshly extruded film’s surface directly.

The extrusion process, when surface treatment should ideally be initially applied, is the stage at which the fastest and most dramatic drop in surface energy occurs, though it is hard to quantify, as treatment testing is rarely done prior to takeup at the end of the extrusion line. But the fact remains that the freshly formed polymer matrix is susceptible to changes in molecular orientation – notably at the surface – due to thermodynamic forces, and contact with each metal roll as the film is drawn off tends to neutralize the treatment effect. Higher initial treatment levels result in faster treat loss, and it should be noted that different polymer materials have inherently different rates of change.

Allowing static forces to form between extrusion and finished roll winding can result in film handling difficulties, while attracting airborne contaminants to the film surface, which will further degrade the treatment. Another very common source of contaminants and degraded short-chain polymer molecules comes from the blending of reclaimed material into the feedstock.

Once the film is wound, surface changes continue, especially if the film has been treated on one side only. Contact between the treated and untreated surfaces promotes transfer of energized compounds on the treated surface to the untreated side. A tighter wind increases the intimacy of the interface between the surfaces, and can accelerate this process. And, of course, the presence of slip agents and an excess of oligomers exacerbates the problem.

When processing the extruded film, bump (boost) treating is strongly recommended. This has two beneficial effects: It burns off contaminants and the shortest chain oligomers, and it imparts a fresh layer of treatment on the surface of the polymer. The latter effect is especially helpful, as blooming of additives does not usually occur uniformly – often aggregates of short-chain molecules will form low energy “pools” on the surface.

It is important that the bump treater be placed as close to the print, coating, or lamination station as possible. As is true during extrusion, every roller contact causes treat loss, as well as the potential to pick up contamination from the roller surfaces. Static control is also important at this stage, as these charges will attract airborne contaminants, and may affect liquid flowout. Humidity control is also not to be overlooked: High humidity can carry airborne contaminants, and low humidity promotes static charges.

The preceding discussion presents a litany of reasons for why treatment loss will be a reality for anyone processing corona treated films. So, what can be done to offset this inevitable phenomenon?

Whether you are working with purchased film or are extruding it yourself for future use, it is important to keep storage conditions constant, preferably at 60°-70° Fahrenheit and at 45-60% RH. Higher temperatures will greatly accelerate treatment loss, and low temperatures at high humidity can result in condensation. Temperature and/or humidity cycling will amplify any adverse effects. It is best to keep storage time as constant as possible, and avoid allowing stock to age excessively. If feasible, the relationship among storage time, temperature, and humidity should be determined for each material you process.

For purchased film, always test its dyne level when the material is received. Generate a database that includes the supplier’s surface energy measurements (and ensure that those are generated in a standardized and replicable manner!), your measurements from incoming QC, and what is measured when the rolls go to the converting or printing operation. When possible, it is advisable to work with the supplier to control and monitor storage conditions and duration. Be aware of seasonal differences that affect the rollstock during transport. As noted earlier, inline treating is always strongly recommended, and this is even more true when processing film that was not under your control from the get go.

If you are extruding your own film and storing it in-house prior to printing or converting, all of the above (with the exception of the comments regarding transport) hold true. You might also want to look into reducing the treatment level at extrusion, and increasing the power applied when you bump treat; as noted above, higher treat levels at extrusion are prone to faster treat loss. There will be an optimum balance between the two steps, and this will certainly vary from material to material, and perhaps from process to process.

The immediate effect of treat loss is widely known: poor adhesion and flowout of the ink, coating, or adhesive. This is easily and readily identified in any well-monitored process. But longer term effects can also manifest. Microscopic contaminants introduced after inline treatment but before the print or coating station may not show as defects immediately, but they can create a less than perfect bond between the coating and the film. Gaps or inconsistencies in the interface between the substrate and ink, coating or adhesive act like magnets for any additive or oligomer that has not already bloomed to the surface; if these low molecular weight compounds find their way to those gaps, adhesive failure can occur days, weeks, or even months after the finished product passes final inspection.

One final comment, though it does not relate to treatment loss, is worth noting: Beware of back treatment from your inline bump treater, as it can result in blocking of your wound rolls, and can also cause serious problems if the film is destined for heat sealing at some point downstream.

Consistent Application of Dyne Solution with Cotton Swabs

Question: Currently, we take a Q-tip, dunk it quickly in the bottle of test fluid, then cover a small area of the sample surface and wait a couple of seconds to determine if it is a pass or fail. We believe this may be slightly inaccurate due to an inconsistent amount of solution being applied to the Q-tip. Is this a valid concern?

Answer: Your concern is legitimate. The more test fluid that is absorbed by the cotton applicator tip, the thicker the fluid film will tend to be when applied unless extreme care is taken to barely touch the swab to the surface when applying the test fluid. A thicker film will, due to gravitational forces, push itself outward, resulting in a higher (and less accurate) dyne level reading.

The best way to meter the amount of fluid applied is to use a dropper bottle to apply the fluid to the swab, and specify a given number of drops to apply. For plastic film testing, we usually limit the volume to 4 or 5 drops. Your best method will depend on how large an area you choose to test, as well as empirical feedback from the tester as to which volume is easiest to read with the best replicability.

One tip that may be helpful: I am most able to control the transfer of fluid from the swab to the surface if I hold the swab almost flat, and parallel to the surface, instead of handling it like a pencil or pen. Also, it is very important to use a light touch; bearing down with the swab could scrub off surface contaminants that affect the material’s actual surface energy. And, of course, always use a fresh swab for each test, even if it is at the same dyne level.

This technique suggests purchasing small bottles of test fluid, which will increase your cost per fluid volume, but on the other hand you should save quite a bit of fluid, so you may find there’s actually no extra cost in the long run. But keep in mind the purpose of the test is to produce accurate and repeatable results – the value that accomplishing this objective offers will pay dividends in improved product quality and consistency.