Tips & Tricks: Attaching Tokens to Bottles of Mineral Water

Outline and impact of problem
Products generally feature details of their contents and other information on their containers or packaging. It is important that the customer finds the packaged goods in flawless condition because investigations carried out by consumer behavior psychologists show there is a close link between a product's external appearance and consumer buying patterns. Nonethe- less, there have been cases where the product looks right, yet complaints surface at a later date regardless of the quality of the contents. For instance, labels are applied to bottles using a special adhesive and have to survive the various phases until final delivery undamaged. The labels are generally firmly attached to the bottle, i.e. fully bonded, and cannot be removed easily. However, there are cases where the label has to be easy to remove. For example, the reverse side of the label could contain information that is important for the buyer or even tokens to collect.

Challenge and solution
As already mentioned, there are applications where the label has to be easily removable by hand and a special release coating has to be applied to ensure this is the case. Correctly coordinating the type of adhesive with the overall system and the minimum application thickness for the type of adhesive also play a major role. If this is completely new territory for print shops, it is recommended that they carry out the appropriate trials in advance.

Case study
The front side of labels for bottles of mineral water was printed in four colors and inline coated, while tokens were printed in one color on the reverse side. The idea behind the promotion was that buyers collecting a hundred of these labels could redeem the value printed on them. To ensure that this system worked, a release coating was applied wet-in-wet to the reverse side of the labels in two printing units. After the labels were dispensed, the coating was in contact with the label adhesive and/or bottle but prevented the label being completely bonded. The complex structure of these labels is shown on the left. However, soon after delivering the bottles to retailers, a complaint was received to the effect that the neck labels on a number of the bottles could not be removed, even though the token collecting offer was clearly visible on the reverse side. Attempts to remove the labels from the bottles led to them being completely destroyed. The complaints became more frequent and the threat of legal ramifications loomed.

Investigation
Visual assessment. The neck labels on the bottles deemed "good" could be removed from the glass surface in tact and without leaving paper behind, whereas the faulty labels were either partially or completely bonded to the surface. Later on, it was no longer possible to determine from the print shop at what point different paper deliveries had been used in the label production process. However, it was established that a particular release coating (D) had been used on the faulty labels. Comparative tests of the papers used for the job gave no clue as to the reason for the problem. Moreover, various chemical analyses of the two release coatings used identified that they essentially contained identical substances. As these  investigations could not unearth the root of the problem, suspicion turned to the influence exerted by the quantity of release coating applied. Sample coatings and adhesion tests were then performed. Batches of weighed print proofs using the materials from the original job (two papers and two release coatings C and D) were produced on a test printing press. The coatings were applied wet-in-wet in quantities of 1.0 grams per square meter - 2.5 grams per square meter - 3.5 grams per square meter.

Adhesion tests
Paper samples with differing applications of release coating and original labels - deemed both "good" and "faulty" - underwent comparative, defined adhesion tests using the label adhesive from the original job. The adhesive was applied to the reverse side of the sample with a blade in thicknesses of ten micrometers and 50 micrometers. The sample was then applied directly after.

Summary assessment of adhesion tests:

1. The quantity of the adhesive applied had no inf luence on the test result.
2. The original labels deemed "good" showed slight adhesion when attempts were made to remove them, however it was still possible to completely peel off the label without leaving paper behind.
3. The faulty labels were completely bonded to the bottles.
4.  When applied in quantities of 2.5 grams per square meter and 3.5 grams per square meter, both release coatings enabled the labels to be removed in tact. The minimum coating application quantity recommended by the manufacturer was 2.5 grams per square meter.
5. When applied in a quantity of 1.0 grams per square meter, coating C enabled the samples to be removed in tact, whereas coating D led to complete bonding under otherwise identical test conditions.
6. The two papers behaved absolutely identically in the tests.

Consequently, the tests showed that the problem was not caused by the quantity of adhesive applied nor the paper quality. Instead, it became clear that coating D only enabled labels to be removed easily when applied in relatively large quantities. Applied in smaller quantities, coating D did not allow the labels to be removed at all.

Surface tension testing
The next step involved performing surface tension tests on the reverse side of coated samples and original labels. These tests were performed with readily available test inks with different in surface tensions. A test ink with a surface tension of 41 mN/m was used.

Results

• The faulty labels and sample coatings with coating D showed spreading of the droplets when small quantities were applied, signifying good wetting (small quantity of coating applied).
• The labels deemed "good" and sample coatings with coatings C and D showed heavy pearling of the droplets when large quantities were applied (large quantities of coating applied), signifying low wetting.
• These tests therefore showed a close correlation between the quantity of release coating applied and surface tension.
• In practice, this test is an effective on-site procedure for quickly identifying possible difficulties arising from too small quantities being applied.
• The illustrations show the surface tension tests on the faulty and good labels.

Conclusion
Labels should generally not peel away from bottles. In this case study, however, the neck labels had to be removed from the bottle in tact as they featured tokens to be collected by customers. Some of the labels produced were completely bonded to the bottles and could not be used as tokens. Investigations showed that the problem was not attributable to either the paper used or the quantity of adhesive applied. Only by producing sample coatings with the two release coatings used in production and performing adhesion tests could the root of the problem be uncovered: The two release coating preparations from the same manufacturer behaved very differently in terms of their release characteristics. While coating C "worked" even when smaller quantities were applied, use of coating D led to complete bonding under the same printing conditions, meaning the tokens could not be used. The investigations came to two conclusions regarding the cause of the problem:

1. Coating D had much poorer release characteristics than coating C from the same manufacturer.
2. In addition, the tests showed that falling short of the minimum application quantity of 2.5 grams per square meter specified by the manufacturer could cause problems in terms of release characteristics.