Polymer Network
Thermosetting synthetic polymers formed by covalent bonds between polyol hydroxyl groups and polyisocyanate functional groups provide permanent structural stability under chemical exposure. When applied to paperboard substrates as a barrier coating, crosslinked polyurethane creates an insoluble film that resists water and organic solvents. Hydroxyl functionality on modified cellulose surfaces can participate in reactions, tying the polymer directly into the top fibre layer.
The chemical network prevents polymer chains from sliding past one another under heat or stress. This thermoset behavior distinguishes the resin from thermoplastic alternatives that soften during downstream converting steps.
Barrier Performance
Fluid resistance in coated packaging materials depends heavily on molecular density and film continuity across surface irregularities. Coating formulations utilizing crosslinked polyurethane exhibit low moisture vapour transmission rates and high resistance to grease penetration. In high-speed gravure application, liquid prepolymers penetrate surface pores before curing into an impermeable barrier layer.
Curing requires precise thermal control during drying to complete the hydroxyl and isocyanate reaction before package creasing occurs. Improperly cured coatings fracture along score lines, exposing underlying paperboard fibres to liquid ingress. The boundary of effective protection fails when mechanical stress exceeds the elongation limits of the cured matrix.
Mechanical Limit
Excessive network density reduces film elongation and causes micro-cracking during folding operations. Formulations of crosslinked polyurethane with high crosslink density maintain hardness but lose flexibility needed for sharp box scores. Adjusting the stoichiometric ratio between reactive groups modulates flexibility without sacrificing solvent resistance.
Standard testing uses grease resistance tests alongside creasing evaluation to establish the boundary of the cured film.
