Polyurea/polyurethane (PUa/PU) shell microcapsules (MCs), containing high loadings of isophorone diisocyanate (IPDI) in the core, were developed to enable the production of mono-component, eco-friendly and safer adhesive formulations for the footwear industry. IPDI microencapsulation was obtained via oil–in–water (O/W) microemulsion combined with interfacial polymerization. A methylene diphenyl diisocyanate (MDI) compound (a commercial blend of monomeric and polymeric species), with higher reactivity than IPDI and low viscosity, was added to the O phase to competitively contribute to the shell formation, improving its quality. Four different active H sources were tested, aimed at achieving a high encapsulation yield. The successful encapsulation of IPDI was confirmed by Fourier transformed infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA), while the MCs’ morphology and size distribution were assessed by scanning electron microscopy (SEM). The incorporation of a multifunctional isocyanate silane in the O phase, as “latent” active H source, led to the formation of impermeable PUa/PU-silica hybrid shell MCs with more than 60 wt.% of pure encapsulated IPDI. A proof-of-concept study shows high peeling strength and a structural type of failure of the adhesive joint, revealing an effective IPDI release. These new engineered MCs are found to be promising crosslinkers for mono-component adhesives for high demanding applications.

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We report on the encapsulation of high loadings of liquid isophorone diisocyanate (IPDI) in biodegradable, temperature-responsive poly(ε-caprolactone) (PCL) microcapsules (MCs), aimed to be applied as cross-linking agents for the development of a generation of safer and eco-innovative adhesives, which are one-component and self-reactive. The biodegradable PCL MCs were formed via the solvent evaporation method in combination with an oil-in-oil-in-water (O/O/W) doubleemulsion system. Two PCL grades with different molecular weights (MW), of 45000 and 80000 Da, were tested as the MCs' outer surface (shell) material. The use of a higher MW PCL, for the encapsulation of IPDI, resulted in core−shell MCs of smoother surface and a slightly smaller average shell thickness leading to remarkably high isocyanate loadings, up to 60 wt %. Also, a higher resistance of the MC's shell to air moisture diffusion was revealed by the longer shelf life exhibited by these MCs. The successful IPDI encapsulation and MCs' shell composition were revealed by Fourier transform infrared spectroscopy (FTIR) in combination with thermogravimetric analysis (TGA) and gel permeation chromatography (GPC), while the MCs' morphology and size distribution were assessed by scanning electron microscopy (SEM) and optical microscopy. Differential scanning calorimetry (DSC) in combination with adhesion proof-of-concept studies showed that the developed PCL MCs were able to respond to the external stimuli of temperature and pressure, typically employed during the adhesive application, revealing an effective IPDI release in the adhesive joint. The results obtained confirm the viability of the sustainably engineered MCs to be used as cross-linking agents and therefore enablers of eco-innovative high-performance adhesives. Finally, it should be stressed that the obtained MCs are potentially useful to provide self-healing capability to materials and that the developed technology may be used in the encapsulation of other reactive species by means of a purely physical process using biodegradable polymers.

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