Some time ago, we had a customer who was printing beverage labels which were being applied in-line at the glass bottle manufacturer. The bottle manufacturer was reporting that the labels were not sticking well to some of the bottles, and they wanted to know why.
Now, glass beverage containers have two coatings on them for lubricity to prevent the bottles from scratching or breaking. The first coating, called hot end coating, is a tin oxide that acts as a primer on the glass. It is applied when the bottles have just come out of the molds and are still very hot. Further down the line, after the glass anneals, a second coating is sprayed on top of the primer. This is a polyethylene-based coating called cold end coating, and it enables labels to adhere well to the glass and keeps the bottles from scratching each other during filling and transport. The point here is that labels don’t stick to the glass; they stick to the polyethylene coating.
In this particular case, the bottle manufacturer started its investigation by doing a bottle soak test in hot water to simulate pasteurization to verify that the labels and ink were properly anchored. At the time, the labels were being printed on one of our clear polypropylenes, so FLEXcon was asked to investigate from our end. We looked at what material lots were used and tested the adhesive performance. The printer/customer gathered information on the lots of glass involved and the label material lots used in labeling them. They also looked at the sequence of labeling at the labeler and determined that the glass in question came from off-line labeling. With this information, the printer made a chart from which they were able to narrow the problems down to a specific timeframe for labeling.
A meeting was requested by the bottle manufacturer with both the printer and FLEXcon due to the large number of bottles in quarantine, so we all flew out to their location for a meeting of the minds. FLEXcon had performed testing on returned labels in inventory and on our retains and had found no issues with the adhesive performance; however, we wanted to support our customer through this issue and help them resolve the problem. In addition, our customer had done a lot of work to narrow down the timing in the manufacture of the glass bottles and was able to trace the issue to a lack of polyethylene coating on the glass – the second coating that enables the labels to stick. Tin oxide – the first coating - does not like water, so it is critical to have the protective polyethylene coating over it. It appeared that the bottle manufacturer had run an entire day of production with just the tin oxide primer coating. Apparently, their systems didn’t alarm when the spray line shut down, and quality control procedures for the consistent mixing of the coating from batch to batch were insufficient. When the labeled bottles with only tin oxide were placed in a wet environment, the water would wick in between the primer and the glass, resulting in failure.
A few days following the meeting, the bottle manufacturer acknowledged that the problem was an internal issue and agreed to put procedures in place for mixing their coatings and to put alarms on their spray lines for when the systems failed to operate properly.