Poor Transparency in Flexible Packaging: How to Solve It?

Many factors affect the transparency of laminated film. In actual production, judgments must be made and solutions found based on the apparent phenomena of defective products and the actual conditions.

1. Effect of adhesive color on transparency

In the lamination of flexible packaging, two-component adhesives (alcohol-soluble adhesives, ester-soluble adhesives) or one-component adhesives (water-based adhesives) are mostly used. Whether the adhesive itself is transparent directly affects the transparency of the laminated film. If the adhesive itself has off-color, the corresponding color will remain in the laminated film after lamination. If the colored laminated film is then bonded and wound into a film roll, the change in transparency of the laminated film will be felt even more clearly.

Therefore, when laminating partially transparent or fully transparent flexible packaging, the adhesive used should be inspected to confirm that the adhesive itself has no off-color, and a slightly yellow or colorless adhesive should be selected.

In addition, there is another method for judging the transparency of the adhesive itself: sample the main agent and curing agent in a certain ratio, place them in a transparent cup, cure them in a 50°C oven for 8 hours, and observe light transmission to verify the transparency of the adhesive after the curing reaction.

1. Effect of oven temperature on transparency

In dry lamination, after the plastic substrate is coated with adhesive, the residual solvent in the adhesive liquid is dried through an oven. The oven of a laminator is generally divided into three sections. The first section includes a constant-rate drying stage and part of a falling-rate drying stage, evaporating about 85% of the solvent. Usually the drying rate of the first section must not be too fast, because once it enters the falling-rate drying stage, the drying rate depends on the rate at which the solvent diffuses from inside the coating to the surface.

The second and third sections of the lamination oven are both in the falling-rate drying stage of the coating. As the solvent content of the coating decreases, the drying rate gradually declines. The heat absorbed by the substrate and coating from the nozzle airflow is higher than the heat consumed by solvent evaporation, so the temperature gradually rises until it approaches that of the hot air. Therefore, the temperature setting of the third drying oven must take into account the shrinkage of the substrate due to excessively high temperature and the amount of deformation under the action of heat.

The temperature settings of the three oven sections must go from low to high to ensure that the solvent volatilizes as completely as possible. If the temperature is set improperly—too high when entering the drying tunnel and drying too fast—the solvent on the surface of the adhesive liquid evaporates rapidly, causing a local increase in the temperature of the surface adhesive liquid and skinning on the surface; when the heat penetrates into the interior of the adhesive layer, the solvent beneath the skin film vaporizes and breaks through the adhesive film to form ring-shaped matter, making the adhesive layer uneven and causing opacity. Therefore, temperature control from low to high should be strictly followed, allowing the solvent to evaporate slowly from the inside out, which can avoid the ring-shaped phenomenon and improve transparency.