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So far, retirement has been everything I have hoped including some trips, some paddling, chasing deer, a bit of volunteer work, and adult beverages whenever the urge strikes. I do have to suck it up and put in about three days a month working in the sticker business, a schedule I kick myself for not implementing years ago. My most recent gig was attending a supplier mart at one of our customer’s technical conference on the west coast. The more things change the more they stay the same; 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 the damn things stick when you don’t want them to, like when you buy some cool new gadget 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, like the registration sticker on my sea kayak. Both create extreme suction, to use the politically correct phrase, but both can be dealt with by starting out with a very basic two part question:
What does this thing 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 that the label has to adhere to properly and all sorts of nasty things can happen 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 surly.
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 would seem to be key. However, that is not always easy. In the industrial OEM world I don’t know how many times I’ve been told, “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 hate sticking to steel that hasn’t been properly degreased. Lots of times labels hate sticking to steel parts that have been painted and they really, really hate sticking to metals that have been powder coated. But wait, you say, in those cases they really aren’t sticking to the steel, they are sticking to grease, paint, and powder coating. Precisely. A classic label ‘failure’ was at a customer that labeled glass vials. All of a sudden the labels began to fail which of course meant bad labels and a product rejection. Upon investigation, we found out that a zealous new buyer discovered he could save a few fractions of a cent per unit by opting out of the ‘flame treating’ process. He couldn’t figure out how this added any value to the part until his line guys discovered that labels would not stick to the silicone that was typically flamed off at the glass manufacturers. The key, is for us label folks to dig down and find issues like that out before we recommend a label stock. As the doctors say, prescription without diagnosis is malpractice….which is a great segue into medical device substrate issues.
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 pretty simple as well. If you imagine a giant label, it would stick much better to Kansas than 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 about 35 seconds, angering the line operators once again (see flame treating above). Tyvek is a well-known and well-researched medical device packaging surface and at this point in adhesive evolution pretty easy to deal with.
There are, however, many insidious surfaces that want to trip you up. For us, 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 whole new slew of chemistry is introduced when your marketing people 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.
It 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 that we produce specify varnish coating so we know of which we speak. 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 then to smooth, inconsistent coatings. This reliable bond is especially critical for closure type labels that help end users as well as the restock people 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 you are really lucky, then adhesion to Tyvek, a known and consistent surface, will be your biggest challenge. If you are dealing with varnishes, well, good luck! We will do 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, including the sheet-rocking-next-door-to-the-final-pack-area-dust story, and the even more interesting, pacemaker-carton-in-the-back-of-the-car-window-in-Arizona-in-July. Label bubbling caused by outgassing of certain molded plastic parts that are labelled in a rush before the post-cure cycle is completed is one of my favorites, but I gotta save some of this stuff for future blogs.
If you have questions please get in touch, we have seen most of this stuff before and can help you with that diagnosis that needs to happen before the prescription.
Original article: http://www.awtlabelpack.com/2016/11/30/why-the-isnt-this-sticking/