The reason why soft packaging widely uses polyurethane adhesives is mainly due to their excellent molecular design flexibility and broad adaptability to complex substrates. The specific reasons can be understood from the following four aspects:
1.Wide substrate compatibility
Soft packaging often involves the composite of various materials with distinct properties (such as plastic films, aluminum foil, paper, etc.). Polyurethane molecules contain isocyanate groups (-NCO) and amide groups (-NHCOO-), which can form strong chemical bonds with the active hydrogen groups (hydroxyl and carboxyl) on the surface of the substrate. At the same time, through intermolecular forces, it can achieve excellent physical adsorption. This "combination of rigidity and flexibility" structure enables it to firmly bond highly polar materials like nylon and polyester, and also allows for effective wetting of non-polar polyethylene and polypropylene films through formulation adjustments.
2. Excellent durability and resilience
Soft packaging needs to withstand temperature variations, squeezing, impacts, and erosion from contents (such as acidic or alkaline foods, solvents) during transportation and storage. The main chain of polyurethane adhesives can be designed as a blend of soft segments (providing flexibility and low-temperature resistance) and hard segments (providing cohesion strength and heat resistance). This microphase separation property enables it to remain stable within a wide temperature range of -30°C to 120°C, featuring high peel strength and impact resistance, and effectively preventing packaging from delamination and damage.
3. Process Adaptability and Safety
Polyurethane adhesives can adapt to high-speed composite processes in various forms such as solvent-free, solvent-based or water-based. Particularly, solvent-free polyurethane adhesives, due to their zero VOCs emissions, fast curing speed, and low unit coating volume, have become the mainstream choice for food and pharmaceutical soft packaging, fully meeting the strict requirements for hygiene, safety and environmental protection of modern packaging.
4. Functional customization capability
According to different packaging requirements, by adjusting the structure, molecular weight and functionality of polyols and isocyanates, we can customize special adhesives that are resistant to boiling (121℃ high-temperature sterilization), resistant to freezing, resistant to chemical media or with low migration, precisely meeting the diverse scenarios ranging from casual food packaging to medical sterilization packaging.
