Some of our customer want to hold the shipping labels on the bag, and protect the labels from rubbing or possible moisture in the warehouse, so they tend to put clear tape over their thermal labels. It is not a particularly good idea to tape over thermal labels. For inkjet or laser, there is no problem with tape over the label, if you use clear tape and are careful to lay the tape down smoothly. If a thermal label in the tape, the tape can make the print hard to read by causing it to fade.
So, why does it fade? Ultimately we got to the bottom of it and found that tape would react with the thermal labels. If a thermal label the chemical in the tape may erase the type on label in a short period of time making the package go lost because both the delivery address and return address can become unreadable. If not a thermal printer it should be no problem. If you print your labels with an ink jet printer, you should tape over the label. Inkjet ink is water soluble and your package can become undeliverable if rained on.
Do you know that if you paste tape on the thermal paper, it will speed up the process of fading? Yes. Any transparent scotch tape can make the text disappear in 1 or 2 weeks. But how does it happen?
One of the reasons is because of the tape itself! Even though tape is a thing that every household has, but we never really know what it is made of. Transparent tapes are made of a thin plastic film called polypropylene.
When the same tape is pasted on the thermal papers, the organic solvent of the adhesive used to make the transparent tape melts the coating of the thermal paper containing the leuco dyes. It doesn’t melt like heat, it melts slowly, so many people don’t think the tape has changed it.
Apart from the heat, UV lights, humidity and chemicals are also factors to blame. For UV lights, it changes the molecules of leuco compounds into colorless pigment reduction products. And most thermal paper materials have limited durability to moisture and degrade once subjected to it. The same goes with chemicals. Expose to chemicals also can cause ink degradation on thermal paper.
The thermal labels are self adhesive. If you still worry the stickiness, you can tape the edges with clear tape. Please not over the printed area, just in case the shipments getting lost from lack of addresses.