Bon Jovi’s now infamous lyrics, “the more things change, the more they stay the same” are true in life, but, as time has proven, is equally true with label adhesion (though Bon Jovi would have never contextualized his song within the labelling industry)! Adhesion–and the lack of it at times–are a continuing topic of discussion.
We have all experienced label issues involving the concept of proper adhesion, a.k.a. ‘sticking’. One end of the spectrum is having labels stick when you don’t want them to, like when purchasing a new electronic and the label comes off in pieces, leaving a nasty, gooey residue. On the other end is the label that’s supposed to stick but doesn’t! Both are equally frustrating, but both scenarios can be dealt with by starting out with a very basic two part question:
What does this actually stick to and what happens to it after it’s stuck?
This is an especially critical question for medical device labeling because there are a number of very different surfaces and materials that labels must to adhere to properly, and all sorts of difficulties can ensue to the label after it is stuck. When issues occur, people in a number of unrelated areas (from manufacturing to regulatory to the end users) tend to become both critical and frustrated.
Adhesion, simply put, is the molecular force of attraction between two different surfaces. Therefore, knowing the two surfaces and knowing the best adhesive to facilitate that molecular level attraction is key. However, that is far easier said than done. In the industrial OEM world, it’s frequently boasted, “Oh yeah, the label sticks to steel”.
Steel has an extremely high dyne level and is one of the easiest things to stick to… with a dozen or so potential pitfalls, of course.
Labels often don’t stick to steel that hasn’t been properly degreased. Many times labels won’t stick to steel that has been painted, and they really won’t stick to metals that have been powder coated. In those cases they really aren’t sticking to the steel, they are sticking to grease, paint, and powder coating. An example of this failure is when a customer labeled glass vials. Almost suddenly, the labels began to fail, which of course meant unusable labels and product rejection. Upon investigation, it was found that a zealous new buyer discovered a few fractions of a cent per unit could be saved by opting out of the ‘flame treating’ process. He couldn’t figure out how this added any value to the part until it was discovered that the labels would not stick to the silicone that was typically flamed off at the glass manufacturers. The key is to uncover issues like this before a label stock is recommended. As the doctors say, prescription without diagnosis is malpractice, and this is equally true with medical device substrates.
Tyvek is typically the long pole in the tent when it comes to label adhesion. Not only does it have an extremely low dyne level, but the caliper of the Tyvek is all over the place. Simply put, dyne is surface tension. A dirty car has a high dyne level and when it rains the water runs off. A newly waxed car has a low dyne level and the water beads up and does not run off or ‘wet out’. We want adhesive to wet out to help form the bond. The caliper issue is simple as well. Imagine, if you will, a giant label; it would stick far better to a state like Kansas than it would to Colorado because the adhesive would only bond to the peaks and not get down in the valleys. So why not add more adhesive like duct tape or Gorilla Tape? Because the ooze created on the label stock would gum up any thermal transfer printer or laser printer in a matter of seconds, causing quality assurance issues (see flame treating above). Tyvek is a well-known and well-researched medical device packaging surface, and at this point in adhesive evolution, easy to deal with.
There are, however, many insidious surfaces that can be problematic. The most common adhesion issues result from the variety of coatings used on the SBS and corrugated cartons used as outer packaging for the sterile package. In addition to the pigments and other additives in the inks, a slew of chemistry-related considerations are introduced when a marketing department then decide to make the carton ‘pretty’ or ‘have the graphics pop’ by adding varnish. Water-based varnish, solvent-based varnish, UV varnish, or the myriad of aqueous coatings, can all add significantly to your adhesion headaches with the outer packaging. By far the simplest solution–one that is 100% effective–is to avoid varnish in the area of your labeling.
This is known in the industry as spot varnishing, where the varnish is printed in specific areas rather than flood coating the entire package surface.
Spot varnishing requires a printing plate up front from the carton printer, but it’s a very low cost option compared to the potential adhesion issues that can arise from variations in the varnishing process and chemistry. With the spot varnish technique, marketing and the end user get that bright shiny package, and manufacturing and package engineering and manufacturing avoid costly and time consuming headaches. Many of the labels produced specify varnish coating. Too much solvent, too little solvent, too light of a coating, too heavy of a coating, UV lights that are getting old, or running too fast for 100% curing, are only a few of the potential issues. It is significantly more reliable for adhesive to bond to paper fibers than to smooth, inconsistent coatings. This reliable bond is especially critical for closure type labels that help end users as well as the restock personnel at the manufacturer tell if a package has arrived intact. The most thorough label qualification and validation process will not pick up the variation in lots of varnished cartons, especially since many validations rely on a three lot sample.
Most medical device label sets need to be imprinted at the manufacturers, typically with thermal transfer or laser, with lot numbers, expiration dates, UDI and other data. These individual labels all adhere to different substrates whether it be pouches, Tyvek, SBS boxes, PETG trays, corrugated, or medical records. If one is fortunate, then adhesion to Tyvek, a known and consistent surface, will present the greatest challenge. When dealing with varnishes, the potential sticking hazards become even more challenging! AWT does our best to help and advise as well as control adhesive type, coat weight, and face stock attributes. What we can’t control are the substrates to which the labels adhere, especially modified ones such as varnished cartons.
There are other anecdotal stories of adhesion issues on device packaging. Label bubbling caused by outgassing of certain molded plastic parts that are labelled in a rush before the post-cure cycle is completed is an infamous story, one that drives home the importance is ensuring of label adhesion.
If you have questions please contact us, we have years of combined experience and expertise when it comes to this issue, and we can help you with diagnosis before the prescription.