Polyurethane adhesives for flexible packaging are key components used in flexible composite packaging materials for food, daily chemicals, pharmaceuticals and other products. Its main function is to firmly bond two or more films of different properties (such as BOPP, PET, PA, AL, CPP, PE, etc.) together to form a composite structure with various excellent properties, so as to meet the requirements for barrier properties, weather resistance, aesthetics and mechanical strength.
At present, the mainstream in the market is two-component polyurethane adhesives, which are composed of a main agent and a curing agent. The main agent is usually a polyester or polyether polyol containing hydroxyl groups (-OH), and the curing agent is a compound containing isocyanate groups (-NCO). During the compounding process, the two are mixed and form a tough polyurethane polymer network through chemical reactions, thereby generating a strong adhesive force. This chemical cross-linked structure endows it with excellent high-temperature resistance and resistance to media (such as oil, acid, and spiciness), making it particularly suitable for retaming packaging with harsh contents and packaging of highly oily foods.
In addition to high-performance two-component products, there were also one-component polyurethane adhesives in the early days, which cured by moisture in the air. However, their performance and stability were usually inferior to those of two-component systems, and their application range was relatively narrow.
The advantages of this type of adhesive include:
1. High bonding strength: It has excellent adhesion to various substrates.
2. Good flexibility: After curing, the adhesive film is soft and does not affect the flexing and impact resistance of flexible packaging.
3. Adjustable performance: By altering the raw materials and structure, formulas that can withstand different temperatures (from 121℃ to over 135℃ for steaming) and different media can be designed.
With the strengthening of environmental protection regulations, solvent-free polyurethane adhesives have become an important development direction. It is formed through a 100% solid content reaction, with no VOCs emissions, making it safer and more environmentally friendly. It is gradually replacing traditional solvent-based products. Waterborne polyurethane adhesives are also under development, but their performance and drying speed still pose challenges in some applications at present.
In conclusion, polyurethane adhesives are one of the core technologies for flexible packaging to achieve multi-functionality, high performance and high safety. Their development closely revolves around two major themes: performance improvement and environmental protection requirements.
