With the global promotion of low-carbon and environmental protection policies, solvent-free lamination technology has become the core development direction of the plastic packaging industry, thanks to its advantages of zero VOC emissions, energy saving and environmental protection. As the core supporting material of this process, polyurethane (PU) adhesives directly determine the quality, safety and service life of composite packaging. Industry insiders pointed out that mastering scientific construction points is the key to giving full play to the performance of solvent-free PU adhesives and ensuring the stability of packaging products.
The pre-construction preparation is the foundation to avoid construction failures. First of all, the selection of PU adhesives must match the type of plastic substrate. For example, polyester-based PU adhesives are suitable for PET, PA and other substrates, while polyether-based PU adhesives are more adaptable to PE, PP and other non-polar substrates. It is crucial to check the compatibility between the adhesive and the substrate before construction to prevent problems such as poor adhesion and delamination. Secondly, the substrate surface needs to be strictly treated: the surface should be free of oil, dust, water and other impurities, and the surface tension of non-polar substrates such as PE should be increased to more than 38 dyne/cm through corona treatment to improve the wettability of the adhesive.
The control of construction parameters is the core link to ensure the lamination effect. The coating amount of the adhesive should be adjusted according to the type of substrate and the use scenario: for food packaging requiring high barrier property, the coating amount is generally controlled at 1.8-2.5 g/m², while for ordinary packaging, it can be appropriately reduced to 1.2-1.8 g/m². Excessive coating will lead to high cost and poor drying, while insufficient coating will cause insufficient adhesion. Meanwhile, the temperature of the glue tank should be kept at 25-35℃ to ensure the fluidity of the adhesive and uniform coating. The pressure of the lamination roller should be controlled at 0.3-0.5 MPa, and the speed should be matched with the coating amount, generally 150-300 m/min, to avoid air bubbles and uneven lamination caused by too fast speed.
Post-construction curing and quality inspection cannot be ignored. After lamination, the composite film should be cured in a constant temperature environment of 45-55℃ for 24-48 hours. The curing time should be adjusted according to the ambient temperature: the curing time should be prolonged when the temperature is lower than 20℃, and the curing temperature should be properly reduced when the temperature is higher than 35℃ to prevent the adhesive from aging. After curing, quality inspection should be carried out item by item, including adhesion strength (peel strength should not be less than 3.0 N/15mm), appearance (no bubbles, wrinkles, delamination), and heat resistance and cold resistance, to ensure that the product meets the national food packaging safety standards.
Industry experts remind that in the actual construction process, attention should also be paid to the storage of PU adhesives: they should be stored in a cool and dry environment, avoid direct sunlight and high temperature, and use them within the shelf life. In addition, operators need to be professionally trained to master the adjustment skills of construction parameters and the handling methods of common problems. With the continuous upgrading of solvent-free lamination technology, the construction technology of PU adhesives will be more refined, which will further promote the high-quality and low-carbon development of the plastic packaging industry.
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