Since labels are our main product, it seems like a brief overview, a little Labels 101, of why they do or don’t stick would be a good first post. There are hundreds of PSA’s, or pressure sensitive adhesives. That means that they don’t need to be licked or have glue slopped on them to get them to stick, they just need to be pressed on firmly. Permanent, removable, and ‘all temp’ are the broad categories of adhesive properties. Hot melt, emulsion acrylic, and solvent acrylic are the pressure sensitive adhesive systems that are generally used for device package labeling. The key to labeling of course, is the actual sticking to the package. What that package is made of, the temperature when the label is applied, and the conditions the package is exposed to are the key to selecting a suitable labeling system.
“We want a label that will stick to the nose tiles of the space shuttle, survive sub-zero orbiting at a zillion miles per hour, and be easily removable upon reentry”. Sometimes it seems to us cynical labeling folk that customers want labels that will do everything. Terminally sterilized device packages are particularly interesting because not only are there a number of different substrates that need to be labeled, such as PET, tyvek (PE), SBS cartons, and Polyprop boxes, but the package is then subject to some form of abuse designed to sterilize it. Not every label material can survive being cooked, bombarded with electrons and radiation, or steamed like a Maine lobster no matter how well is sticks in the first place.
In this post the focus will be the material or substrate that the label needs to bond to. What it’s made of, what the texture of the surface is, and the temperature the label is applied are the key elements. One of the key measures of how certain adhesives behave is the dyne level of the substrate. The simple explanation of dyne is that your dirty car has a fairly high dyne level. Water flows and wets out nicely on your poorly maintained auto’s finish. A nicely washed and waxed car has a fairly low dyne level which makes the water bead up and prevents if from wetting out. Guess which one a label would stick to the best? Some general dyne levels would be stainless steel at 800 or so, PET and ABS in the mid 40’s, polyethylene and polypropylene in the low 30’s and Teflon and powder coated paints at around 20. A second property would be the texture of the surface. The dyne level might be the same but labels stick much better to a nice smooth polyethylene bag than they do to spunbonded polyethylene, eg. Tyvek, If you placed a gigantic label on Kansas it would stick pretty good and contact most of the surface. The same label on Colorado would only be in contact with the tops Pikes Peak, Long’s peak, and the other 14’ers. Same material, very different surfaces. A clean surface is critical as well. I got a call some years back from one of my customers, a start up that had built out a space in a business park. The labeling operation at the tail end of final pack was outside the clean room in one of the finest hallway labeling operations I’ve ever seen. “Your labels don’t stick” was the call from the QA guy. I got over there and by god, he was right. I called our shop and got lot numbers of label stock, retains, etc and it was still a puzzle. Until I walked out the door and saw the sheet rockers that had just finished taping and sanding the bay next to my customers. Sure enough, those nice white boxes had a nice thin white sheen of sheet rock dust on them. Mystery solved.
Surface is key, but so is temperature. In the next post I’ll talk about temperature and some the labeling disasters that I’ve been involved in over the years that involved temperature fluctuation. Any stories sent my way would be appreciated as well.