Over the years, production line speeds have increased due to the advances in both equipment and adhesive technologies. Because of this increase in production efficiency, there have been reduced appearances on certain film structures. This often manifests itself as a mottled appearance that can be interpreted as voids, outgassing, bubbles, etc. However, this is typically caused by reduced flow and leveling of the adhesive onto the printed web, metalized film, etc. The root cause is most often an imbalance of the wet surface tension of the adhesive and the dry surface energy of the substrate leading to reduced wet out.
Many dials can be turned to help alleviate this phenomenon. Often one or more of the fixes will need to be adopted to improve appearance. In some cases, what works for one film or adhesive may not work for another. Since there are many adhesive choices and coating weight, temperature settings, primary and secondary treat levels as well as rewind pressures, taper tension, etc., the lamination process can have many adjustments.
Guideline and Adhesion Choice
Dow has worked on this process by studying both existing adhesive chemistries as well as providing new adhesive choices. Recently, Dow launched PacAcel™, a high run speed adhesive that has many features and has proven to show a marked improvement in appearance at run speeds above 1200 feet/min on a variety of lamination structures. PacAcel™ has found that the adhesive puddle should be about 105°F(41°C) for optimum appearance. In other cases, with more traditional Dow adhesives, we have found that a puddle temperature around 130°F(55°C) is best for appearance. In most cases and application roll temperature of 122°F(50°C) to 140°F(60°C) seems optimum. In addition to that, nip temperature settings of 120°F(49°C) to 130°F(55°C) seem to be optimum. It has been found that this product shows improved appearance on the lower side of the coating weight range at 1 ppr(1.6 gr/m2). Other adhesives may show better appearance at a weight higher in the typical range at 1.4 ppr(2.22 gr.m2). As always, performance versus coating weight should be evaluated but exploring coating weights as it relates to appearance should be explored.
Treat levels should also be closely examined and tested. Most solventless adhesives show optimum wetting when the substrates show a dyne level at or above 40 dyne/cm. Even if films are fresh it is often advisable to bump treat at 1-watt density to clean films of any surface contamination. Check treat with fresh dyne solutions and if below 40 dyne/cm adjust watt density on treater to raise the level at or above 40 dyne/cm. Finally, adjustments on the rewind portion of the machine have been shown to improve flow/leveling/appearance. By winding a tight, warm roll appearance will improve. Rewind tension between 30-40 psi, lay on roll pressure also at 30-40 psi and a taper tension at or below 25% are optimal for many machines.
It is critical to work with both adhesive and machine suppliers to understand optimum settings for each film structure. Once a group of settings has been found for that structure it should be saved in the machine settings and be used for that structure. Often multiple settings will need to be saved as film structures change.