In the flexible packaging lamination process, the application of polyurethane adhesives is of vital importance, directly affecting the performance and appearance of the final product. To ensure high-quality compounding, the following key techniques need to be mastered:
1. Substrate pretreatment is the foundation
Before lamination, it is essential to ensure that the surface of the substrate film (such as BOPP, PET, PE, aluminum foil, etc.) is clean, dry, and has sufficient surface tension. It is usually required that the surface tension reach above 38 dynes per centimeter to ensure that the adhesive can be fully wetted and spread, forming a strong bond. For moisture-absorbent films (such as nylon), they should be properly stored and preheated before lamination to remove surface moisture.
2. Precise control of glue mixing and dilution
For two-component polyurethane adhesives, the mixing ratio of the main agent to the curing agent must be strictly in accordance with the process requirements, and high-precision measuring instruments should be used. The purity of the dilution solvent (usually ethyl acetate) is of vital importance. Its moisture and alcohol content must be extremely low (it is recommended that the moisture content be less than 300ppm), otherwise it will consume the curing agent, resulting in incomplete curing of the adhesive and causing problems such as weak bonding and unpleasant odors. The prepared glue solution must be used up within the specified usage time.
3. Optimization of gluing process parameters
The amount of glue applied should be precisely set according to the contents of the package, its structure and the requirements of post-processing. For lightweight and ordinary contents, the amount of glue applied can be appropriately reduced. When used for packaging containing oily, sour, spicy and other corrosive contents or requiring high-temperature steaming, it is necessary to ensure sufficient glue application to guarantee resistance. The uniformity of coating is equally crucial. It is necessary to ensure that the rubber rollers and anilox rollers are clean and free from blockage to avoid defects such as "white spots" or "stripes".
4. The drying tunnel is thoroughly and thoroughly dried
The function of the drying tunnel is to completely evaporate the solvent in the adhesive. It is necessary to set a reasonable gradient temperature (usually low, medium and high three segments) and sufficient air volume to ensure that the solvent is completely evaporated before the film enters the laminating roller. Excessive residual solvents not only cause bubbles and affect the appearance, but also lead to a decrease in adhesion and an unpleasant odor in the packaging bags.
5. The curing conditions are strictly guaranteed
Curing (curing) is a necessary process for polyurethane adhesives to undergo chemical reactions to reach their final strength. Sufficient curing time and temperature must be ensured. If the temperature is too low or the time is too short, the curing will be insufficient, and the adhesive force and heat resistance will be seriously inadequate. Excessively high temperatures may affect the performance of the heat seal layer. Meanwhile, the curing workshop should maintain good ventilation to facilitate the discharge of by-products from the reaction.
