Abstract
An o-nitrobenzyl-protected precursor was used as a phototrigger for the release of the diamine cadaverine in polyurethane adhesives based on castor oil as a renewable source of polyol and organic diisocyanates. This resulted in formulations with suitably controlled curing by photoactivation. This material shows faster curing when UV light is applied as compared to curing in the absence of irradiation, which was in situ monitored by rheological measurements. In addition, the adhesion performance is superior, reaching lap shear strength values of up to 4600 kPa, which is unprecedented for bio-based adhesives. On one hand, the in-depth chemical characterization with FTIR spectroscopy revealed that the slow release of cadaverine yields a well-balanced urethane/urea composition with direct impact on adhesion properties. The photocured bioadhesive was shown to bond a variety of surfaces, such as polyethylene or even wood. On the other hand, the direct one-time addition of cadaverine yields a material with approximately the same viscoelastic properties, which were achieved almost immediately as a consequence of the favored fast formation of urea bonds in detriment of urethanes, however, lacking adhesion properties.
Highlights
One of the pillars of modern chemistry and chemical product design is sustainability
As model systems for our study, we focused on formulations of castor oil with commonly employed organic diisocyanates (HDI or methylenediphenyl diisocyanate (MDI)); see structures in Scheme 1
It can be observed that the signals of 1 vanish, and new resonance signals of the released cadaverine appear with time in the aliphatic region of the 1H nuclear magnetic resonance (NMR) spectrum
Summary
One of the pillars of modern chemistry and chemical product design is sustainability. Diisocyanates may react through one NCO group with the OH groups of castor oil and from urea with photoreleased cadaverine, leading to similar polymer products as those shown in Scheme 2 Another interesting point, which is of importance for the rheological properties and adhesion performance of the formulation, is the supramolecular interaction in the polymer network by means of H-bonding between −NH groups and carbonyl oxygen atoms. This fact and the achieved consistency suggest that these systems could be applied as a coating in this type of non-transparent surfaces These results highlight the excellent response of bio-based adhesives for different surfaces.[5−9] it is worth mentioning that no adhesive properties were obtained by the direct addition of neat cadaverine to MDI- or HDI-based formulations. This result unambiguously indicates that a sensitive balance of urea and urethane formation is required to achieve optimum adhesion
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