Finding the root cause of your adhesion issues
Product defects can and do occur at any level of the product’s life cycle, from the application of the adhesive or coating, to the base laminate, or in the final package structure. Finding the root cause can be as simple as press side observation or require sophisticated analytical techniques. Often a series of yes or no questions can lead to most probable causes.
If there are several potential factors to consider, experienced technicians can often tell which issue is more common or likely to help speed up troubleshooting. While there are many elements within each major defect category, the two main categories are laminate appearance issues and laminate/package performance issues. Problems seen with adhesives and coatings during application can lead to both appearance and laminate/package performance defects.
One of the more common problems is foaming seen in the pan or sump. Foaming is most commonly caused by over-mixing or improper mixing (wrong blade size or angle) during addition of co-reactant or dilution. Foaming can also be caused by improper configuration of return lines leading to freefall of the adhesive into the sump or recirculate container. While most system can tolerate defoamer additions, often a change in the pan or sump can alleviate the problem.
Another common problem is improper or uneven coating or appearance issues seen on the application cylinder. This can manifest itself as gels or grit, skips, voids or ribbing seen on the cylinder. While this will most commonly lead to laminate appearance complaints, it can also lead to package performance problems if coating weight is variable across web. Common culprits are using mixed adhesive after end of pot life, over shearing, allowing skinning of the adhesive during excessive idle time, and/or not properly mixing and breaking gel structure prior to application.
Voids, skips and ribbing are most commonly caused by running the adhesive/coating outside the suggested application solids range or allowing too much solvent evaporation during application and not monitoring viscosity press side to adjust this. (Note: solventless adhesives are not specifically mentions above but are covered in depth in the article “Improving Appearance with Solventless Adhesives”.
Moving on to appearance issues, the most common defect are voids, skips, dewet or mottled looking lamination. Other appearance issues include hazy lamination and ink smear or ink blead. Ink smear is most typically associated with the use of solventless adhesives which can solvate the ink, causing smear or bleed. It’s a good idea perform ink smear testing with your suppliers to ensure your ink and adhesive systems are compatible. Ensure inks are completely dry by running GC headspace testing on the printed webs and suspend rolls during curing. Hazy laminations are typically caused by under-dried waterbased adhesives or by micro-air entrapment which can be addressed by reducing foaming.
The common issues of voids, skips etc. can come and go in roll or be prevalent throughout roll. There are typically several common fixes for these issues. Follow adhesive manufacturers suggested application solids and monitor throughout run. Use correct volume cylinder for the coating weight desired and ensure cells remain open and unplugged. In some cases, a smoothing bar can help improve flow and leveling of the adhesive (more common but not inclusive with waterbased adhesives). Treat both webs at or above 40 dyne/cm. Even if film is fresh, “bump treating” in line will help to clean surface and promote better flow, leveling and adhesion. Always follow the adhesive manufacturers recommended nip temperature and use a nip roller at or above 85 Shore A hardness.
Laminate or package performance issues may occur early in the process or not show up until packaged product has been on the shelf in service for some time. Issues can be low bond or peel strength, poor heat resistance, tunneling, or delamination in the package after exposure to packaged product or environment. Often it can take relatively sophisticated analytical testing and analysis to determine root cause but keeping good QC records of films and adhesive/coating used during production will be helpful (see article “The Importance of QC Testing”).
Some root cause and common fixes are as follows:
- Ensure the adhesive is the correct adhesive for the end-use package application
- Ensure that it is mixed as recommended and fully cured prior to next converting step
- Ensure coating weight is consistent across web and throughout run and within the target originally set up for the package specification
- Ensure the inks are completely dried by running GC Headspace.
- Some ink solvents can interact with the adhesive compromising its heat and chemical resistance.
- Test to Ensure the adhesive or coating is fully dried as well.
Having good QC data in addition to gathering as many facts as possible upfront will help to solve many issues and make troubleshooting defects significantly faster and easier. Share samples and data with your ink, film and adhesive supplier for analysis. Use your suppliers’ analytical tools as needed to help determine root cause. Sometimes multiple causes could be the culprit and it’s important to have an open dialog to review “most probable” based on experience of the team.