Flexible packaging polyurethane adhesives: The influence of molecular structure on comprehensive performance

The various performance characteristics of flexible packaging polyurethane adhesives originate from the chemical structure of the molecular chains. The types of polyols used in the formula, the molecular weight distribution, and the type of isocyanate directly determine the adhesion, water resistance, aging resistance, and flexibility of the adhesive layer.

 

Polyols are the core components of the main agent. Polyether-type polyols produce polyurethane adhesives with high cohesion, strong adhesion to aluminum foil and nylon substrates, excellent resistance to boiling and oil, and are mostly used in flexible packaging for cooked food preservation; polyester-type polyols have stable ether bonds in their molecular chains and excellent water resistance, but their cohesion is relatively weak, making them more suitable for general packaging composites in high-humidity environments. The molecular weight of polyols also affects the application state. Higher molecular weight leads to an increase in the viscosity of the adhesive solution, resulting in better initial adhesion, but the coating flowability will decrease; lower molecular weight leads to an increase in small molecule substances, which can easily cause residue and odor problems.

The isocyanate structure at the end of the curing agent affects the crosslinking density. Aromatic isocyanates have high reactivity, fast curing speed, and moderate cost, and are the mainstream products available on the market; aliphatic isocyanates do not yellow easily and have excellent light aging resistance, and are mostly used for packaging that requires long-term light preservation, but they have a slow reaction rate and higher raw material costs.

 

The design of molecular structure needs to balance multiple contradictions and strive for high boiling resistance. When the crosslinking density is increased, the adhesive layer becomes harder, which can easily cause the composite film to bend and crack; when focusing on softness and flexibility, some medium-temperature resistance will be sacrificed. At present, the research focus in the industry is to use molecular modification to balance high bonding strength, weather resistance, and processing compatibility, to match the increasingly complex requirements of flexible packaging substrates.