Influence of the storage of reactive urethane quasi-prepolymers in their composition and adhesion properties

In an industrial environment, isocyanate-based adhesives are frequently stored for a long period of time before being used. Due to the presence of unreacted isocyanate (NCO) groups, several undesirable by-products may be formed during the storage period and eventually may affect the adhesion properties. In this work, the effect of the storage of several isocyanate-tipped quasi-prepolymers on their composition and adhesion properties was studied. The functionality of the polyol (2 and 3), the unreacted isocyanate content (14 and 16 wt%), and the concentration of a morpholine-based catalyst (0–7 g/eq polyol) were varied, and two different containers (made from glass and tinplate) were used as storage containers. The influence of the storage conditions on the composition, and properties of the quasi-prepolymers were assessed by plate–plate rheometry, IR spectroscopy and 1H-NMR spectroscopy. The adhesion properties of the moisture-cured quasi-prepolymers have been obtained from single lap shear tests of vulcanized styrene-butadiene rubber/cured quasi-prepolymer joints and of aluminum/cured quasi-prepolymer joints. The properties of the moisture-cured quasi-prepolymers were studied by thermogravimetric analysis and modulated temperature differential scanning calorimetry (MDSC).The viscosity of the quasi-prepolymers increased during the storage period and a reduction in the free isocyanate content was observed, probably due to the participation of NCO groups in secondary reactions. The formation of uretdione and isocyanurate was identified by IR spectroscopy while the allophanate linkages were identified by 1H-NMR. The thermogravimetric analysis and the MDSC results suggest an increase in the crosslink density of the cured quasi-prepolymer after the storage period. Single lap shear strength values of rubber and aluminum/moisture cured quasi-prepolymer joints increased after increasing the storage period of the quasi-prepolymer and was ascribed to the higher cohesion of the higher crosslinked cured quasi-prepolymers.