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The transparency of flexible packaging is poor, how to solve it?

Updated: Jan 7, 2022

1. The effect of adhesive color on transparency

In the compounding of flexible packaging, two-component adhesives (alcohol-soluble adhesives, ester-soluble adhesives) or single-component adhesives (water-based adhesives) are mostly used for processing. Whether the adhesive itself is transparent or not directly affects To the transparency of the composite film. If the adhesive itself has a different color, it will leave the corresponding color in the composite film after compounding. If the colored composite film is laminated and wound to form a film roll, the transparency of the composite film will be more obvious The change.

Therefore, when compounding partially transparent or fully transparent composite packaging, the adhesive used should be checked to confirm that the adhesive itself has no different colors, and choose a yellowish or colorless adhesive.

In addition, there is also a method to judge the transparency of the adhesive itself, that is, the main agent and curing agent are sampled according to a certain ratio, and they are placed in a transparent cup and cured in an oven at 50°C for 8 hours for light transmission observation to verify the adhesiveness. Transparency after curing reaction of the agent.

2. The influence of oven temperature on transparency

In dry compounding, the plastic substrate passes through an oven to dry the residual solvent in the glue solution after the glue is applied. The laminator oven is generally divided into three sections. The first section includes a constant-speed drying section and a partial reduced-speed drying section, which evaporates about 85% of the solvent. Generally, the drying rate of the first section cannot be too fast. Because it enters the reduced-rate drying section, the drying rate depends on the rate at which the solvent diffuses from the inside of the coating to the surface.

The second and third sections of the composite oven are in the coating slow-down drying stage. As the coating solvent content decreases, the drying rate gradually decreases. The heat absorbed by the substrate and the coating from the nozzle airflow is higher than the heat consumed by solvent evaporation, and the temperature gradually rises until it is close to the hot air. Therefore, the temperature setting of the third drying box should consider the shrinkage of the substrate due to the high temperature and the deformation under the action of heat.

The temperature setting of the three-stage oven must be from low to high to ensure that the solvent evaporates as cleanly as possible. If the temperature setting is not appropriate, the temperature is too high when entering the drying tunnel, and the drying is too fast so that the solvent on the surface of the glue will evaporate quickly, causing the surface glue to increase in temperature and the surface will be skinned; when the heat penetrates the glue layer, The solvent under the film vaporizes and breaks through the film to form a ring, making the film uneven and opaque. Therefore, it is necessary to strictly observe the temperature control from low to high and let the solvent evaporate slowly from the inside to the outside to avoid the ring-shaped phenomenon and improve transparency.

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