Reactive Resin
Low molecular weight polymers containing unreacted isocyanate groups act as the building blocks for high-performance polyurethane adhesives. An isocyanate prepolymer is formed by reacting an excess of diisocyanate with a polyol, resulting in a stable liquid that can be handled more easily than raw monomers. This intermediate stage allows for the control of viscosity and reaction speed during the lamination of flexible packaging.
When exposed to moisture or a second component, these groups link together to form a tough, thermoset bond.
Crosslinking Chemistry
Reaction control is achieved by balancing the number of available NCO groups against the hydroxyl groups in the curing agent. In solvent-free laminating, the isocyanate prepolymer must have a low enough viscosity to spread evenly across a film at temperatures between 40 and 60 degrees Celsius. If the moisture content in the paper substrate is too high, it can react prematurely with the prepolymer, causing the formation of carbon dioxide bubbles.
These bubbles lead to optical defects or reduced bond strength in the finished laminate.
Processing Safety
Curing continues for several days after the initial application as the molecules continue to organize into a permanent network. Because an isocyanate prepolymer can be sensitive to environmental conditions, it is shipped in sealed containers with a nitrogen blanket to prevent skinning. Final bond integrity depends on the complete conversion of these reactive sites into stable urea or urethane links.
