One of my avocations when I’m not completely immersed in the wonderful world of medical device labeling is kayaking.  Not the shoot the rapids type of whitewater kayaking in a boat that resembles a giant clown shoe, but sea kayaking on Lake Superior for multi day trips in long, skinny boats.  We don’t always find beer (see Shelf Life post earlier) but its always interesting and exciting.  Minnesota, a state that loves a good fee as much as any state, has us register our kayaks and, in exchange for our Krugerrands, give us two nice 4 x 4 labels with a big date on them to stick on our boats.  This allows law enforcement to easily view the date and fine us if it’s expired.  A stickers is issued for each side of the bow.  I noticed the other day that the port sticker had ‘fallen off’ my boat.  This sure looked like a classic adhesive failure to me but which adhesive property caused the failure?

There are  3 basic properties of pressure sensitive adhesives.  The first one is tack or quick stick.  It is the force of the label being pulled directly away from the substrate being labeled.  Depending on what you are labeling, aggressive tack can be good or bad.  Most hot melt adhesives have high initial tack while most acrylics have lower tack and adhesion that builds over time.  If you are slapping a label on a damp 35F milk jug or a small diameter vial then tack is desirable.  If you are labeling an expensive part or have critical positioning where the label might need to be adjusted, then that high initial tack is a bad thing.  Tossing or reworking mislabeled packages is something that line personnel hate almost more than anything.

Shear is the force across the adhesive, the ability to slide on a surface.  Shear is probably the least important of the three main adhesive characteristics for medical device packaging but it can be important.  If the label falls off, it may have really, really low shear properties on that particular substrate.  The one classic example of shear coming into play on a device package involves a cardiac product and a polyprop box that replaced a paper based set up box.  During ETO cycles the paper label wrinkled due to the expansion and contraction of the polyprop box.  By recommending an adhesive with lower shear the label was able to react and move a bit with the box and avoid wrinkling.  It still adhered just fine.

Last but certainly not least we have peel.  Peel is a crucial as well as easily measured characteristic.  There are ASTM tests and various 90 degree and 180 degree peel testers, a couple of which we have in the AW packaging lab.  There is also the time honored standard of picking at the corner of a label with the fingernail, lifting it, and either peeling it off cleanly, wrecking the label, or wrecking the substrate its adhered to.  The ensuing comments from ‘damn that came off easy’ to ‘s**t, I just trashed the label and package’ can be discussed and evaluated. The three of course, are related.  If you think of a teeter totter, tack is the fulcrum.  Peel and shear are generally opposed.  A high peel strength adhesive generally has low shear strength and an adhesive with high shear generally has pretty low peel values.  Either one can have high or low tack.  With most device applications shear is the fat kid on the teeter totter, low on the ground and peel is the skinny kid, way up in the air.  The tack fulcrum can be either high or low depending on how strongly you want that label to stay on the package initially.  Tamper evident seals that go around a 90 degree angle come to mind, especially ones with the reinforced tear strip.  Those need some initial tack to stick reliably in order to give the adhesion time to build to full strength for the tamper evident feature. So what the hell happened to my kayak registration sticker?  Taking the substrate out of the equation (that could be another whole post), most registration/license tab adhesives have very high tack.  The sticker was on there for a few months, plenty of time for the peel properties to build to full strength.  Shear is not an issue because I can’t imagine it sliding off the boat.  That leaves peel.  I suspect that someone, using the manual peel test I described above, heisted the thing because they were too cheap to register their own kayak.  No, you conspiracy theorists out there, I didn’t peel it off to stick it on another one of my kayaks.  I tried peeling another expired one off and it was extremely difficult but not impossible.  The bottom line is that it could not possibly have fallen off and  I guess that’s the end goal for our medical device packages as well.  It can’t fall off.  Peel strength criteria is set, hopefully using a pass/fail criteria of some sort, in the validation and qualification process, and the label and package are on their merry way to the end user.  Now all I have to do is decide whether to spring for the six bucks for duplicate stickers or attempt to talk my way out of a fine if the warden spots me.  Advice will be appreciated.