Chemical Crosslinking
Chemical reactions between an organic isocyanate and a primary or secondary amine or water produce a highly stable chemical connection. The process of urea linkage formation is a key curing mechanism in polyurethane adhesives used for flexible packaging laminates. This bond type possesses higher thermal resistance and greater stiffness than typical urethane bonds.
Curing Reaction
Polyurethane adhesives used in lamination often react with moisture present in the air or the paperboard substrate. During this curing process, the reaction between isocyanate and water generates amines, which quickly react with more isocyanate to complete the urea linkage formation. This moisture-curing reaction is essential for developing strong adhesion in situations where polyol reactions are slow.
However, it must be carefully controlled because the reaction also produces carbon dioxide gas, which can cause micro-foaming or bubbling within the laminated layers if it is trapped.
Joint Performance
Packaging structures must maintain their bond strength under harsh environment conditions such as hot filling and steam sterilization. The high density of hydrogen bonds in urea linkage formation results in a tough polymer network that resists both heat and chemical attack from acidic food contents. This prevents the laminated layers from peeling apart when the package is heated.
Selecting polyurethanes that optimize this reaction ensures that pouches can withstand boiling and mechanical stress without failure.
