Abstract
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.
Highlights
IntroductionIsocyanates have been used for over 60 years in polyurethane (PU) and polychloroprene adhesives formulations, mainly because of their high reactivity and capability to promote a good adhesion
Isocyanates have been used for over 60 years in polyurethane (PU) and polychloroprene adhesives formulations, mainly because of their high reactivity and capability to promote a good adhesion.The formation of PUs occurs by the reaction between the extremely reactive polyisocyanates’ NCO groups and any chemical group that contains an active hydrogen (H) atom [1]
Core–shell and free-flowing MCs, of PUa/PU polymeric shell or PUa/PUPerfectly spherical, core–shell and free-flowing MCs, of PUa/PU polymeric shell or PUa/PU-silica silica hybrid shell, with average diameters ranging from 81 ± 4 to 95 ± 7 μm, were synthesized using hybrid shell, with average diameters ranging from 81 ± 4 to 95 ± 7 μm, were synthesized using an an O/W emulsion combined with interfacial polymerization and exhibit a high loading of pure isophorone diisocyanate (IPDI)
Summary
Isocyanates have been used for over 60 years in polyurethane (PU) and polychloroprene adhesives formulations, mainly because of their high reactivity and capability to promote a good adhesion. The formation of PUs occurs by the reaction between the extremely reactive polyisocyanates’ NCO groups and any chemical group that contains an active hydrogen (H) atom [1]. Current commercial polyurethane-based adhesives are bicomponent, i.e., they are composed by two components, a polyol base (OH prepolymer) and a crosslinker. Strong and long-lasting adhesives used in the footwear industries are typically isocyanate-based, which provides appropriate high strength quality adhesive joints when gluing the different parts of the footwear. The high toxicity of isocyanate compounds is restricting their use in the industry, based on current legislation.
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