Answered August 30th, 2012 by Expert:
Flagging seems to be an issue that comes up in pressure-sensitive labeling fairly often. As you surmised, there can be a multitude of factors that can contribute to this issue. I will go through some of the more common ones. For flagging to be a consistent problem, it is generally a combination of one or more of these. You probably already realize that, since the labels you reference flag in some instances but not in others, and sometimes the flagged labels tend to “work themselves out.”
Things I would consider:
1. Release agent/mold agent on the PETG bottles.
2. Tension applied to the labelstock in manufacturing.
3. Tension applied to the labelstock in die-cutting/converting process.
4. The actual application of the label specifically the wipe on the trailing edge.
5. The release characteristics of the liner.
6. The coat weight or add/on of adhesive. (See A below.)
7. The actual adhesive. (See B below.)
A. While peel strength is a useful measurement in classifying pressure-sensitive adhesives (PSAs), it typically is not indicative of flagging issues that may occur. Coat weight is measurable in and of itself but a discrepancy in coat weight primarily across the web (label) would more easily be recognized in a tack measurement. Tack is an indication of how an adhesive “wets-out” any given surface; some people also refer to this as “green” adhesion.
B. Having the RIGHT adhesive! Using the correct adhesive for any given application can override many of the other factors to consider in labeling or at a minimum increase the tolerances making the process much more forgiving. Having the right adhesive, however, is not always a simple undertaking: Truly, it is a study in surface interaction. What might work well on a PETG container may not be the best candidate for an HDPE composition. Many of the label converters would agree as we’re seeing an increased trend in backwards integration of converters to coating their own adhesives. Simply put, their customers are demanding the best and it greatly increases their flexibility in meeting specific challenges to be able to work with knowledgeable and reputable adhesive suppliers to provide the best CUSTOM solution.
In summary, using an adhesive that is a better match for the polarity of your substrate could increase the tack/wet-out to the PETG container, alleviating many of the other considerations.
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