The cooking resistance mechanism of flexible packaging polyurethane adhesives

Flexible packaging polyurethane adhesives are the core material for bag laminates during cooking processes. Their cooking resistance properties stem from the stable polyurethane cross-linked network structure within the adhesive layer. The main agent polyol component reacts with the curing agent isocyanate to form amine carbamate chemical bonds. The molecular chains contain both rigid hard segments and flexible soft segments. The hard segments provide strength and heat resistance stability, while the soft segments give the adhesive film flexibility, preventing the adhesive layer from breaking or swelling in a high-temperature and humid environment.

 

The cooking process involves high temperatures and water vapor penetration. Water vapor diffuses towards the composite interface, being the main cause of delamination failure. High-performance polyurethane adhesives can reduce the water absorption rate of the adhesive layer through molecular structure design, minimizing the damage to the bonding interface caused by water molecules. At the same time, the fully cured three-dimensional cross-linked structure can inhibit the softening and flow of the adhesive layer upon heating, maintaining the stability of the film-to-film bonding interface and resisting the invasion of the medium.

The selection of cooking-resistant polyurethane adhesives for different substrates has a significant impact. When aluminum foil and nylon are combined as barrier substrates, higher requirements for the adhesives' resistance to wet heat are imposed, necessitating the use of high cross-linking density systems; for common PET/PE structures, conventional cooking-type polyurethane adhesives can be matched.

 

From a formulation perspective, the type of polyol is crucial. Polyester-type polyols perform better than polyether-type in terms of heat resistance, water resistance, and medium resistance, making them the mainstream raw materials for cooking-grade polyurethane adhesives. In the application field, in addition to the adhesive's own performance, ensuring complete curing to guarantee a complete cross-linking reaction is also an indispensable condition for demonstrating cooking resistance. Unsolidified adhesive layers with insufficient cross-linking degree are prone to bubble formation and delamination during cooking. 

 

With the development of flexible packaging, polyurethane adhesives that can withstand cooking while having low solvent residues have become a research direction. They meet the safety requirements for high-temperature cooking packaging while complying with the environmental protection regulations for food packaging.