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  • New eco-coatings based on natural dyes and aqueous polyurethane dispersions
    Publication . Sfaxi, Refka; Barreiro, M.F.; Ferreira, Isabel C.F.R.; Mokkadem, Yassin
    At present, there are an increasing interest at industrial level by the N-methyl pyrrolidone (NMP) free polyurethane aqueous dispersion, due to the limitations posed to this solvent in the latest years in Europe. Among the several applications of coatings, the protection and decoration of objects is one of the most used. Decoration requires color control and new ways to generate long-lasting colors. However, these products are traditionally based on synthetic dyes, which have a lack of compatibility with the aqueous medium resulting in phase separation or agglomerations phenomena. Herein, the use of hydrophilic natural dyes can be envisaged as a viable alternative, once these dyes nature increases their compatibility with the aqueous medium, leading to the increasing of the final product stability along the time. In this context, the present work was devoted to the development of new eco-coatings based on natural dyes and in polyurethanes aqueous dispersions exempt of NMP. For this study, a polyurethane aqueous dispersion based on isophorone diisocyanate and propylene glycol polyol was synthesized. After, the dispersion properties like solids content, viscosity, pH and particle size were evaluated. Then, coatings added with carminic acid and spirulina blue dyes were produced by solvent casting, by incorporating dyes contents of 0.2, 0.5, 1.0, 1.5 and 2.0% (w/w) into the dispersion. The films obtained after drying were characterized in terms of chemical structure by Fourier Transform Infrared Spectroscopy, which showed the lack of influence of the dyes chemical structure on the polyurethane films, even for the higher content tested. Differential Scanning Calorimetry evidenced the effect of the dye on the films thermal behavior, once when 2% of dyes were added, the values of melting temperature (Tm) and the melting enthalpy variation (ΔHm) increased for values higher than the base film. Thermogravimetric Analysis showed an increase of the film’s thermal stability as the dyes content was increased. The colorimetric analysis was made in order to inspect the color variation due to the dye type and content used. By comparing the base film color with the dyed films, an increasing on the color variation was detected for both dyes. However, for the films containing Spirulina blue this increased with the dye content rising, reaching a maximum value of 65.58 for 2%Sp. In opposition, for carminic acid, the color variation reached a maximum value of 70.62 for 1.0%Cr, being constant for the higher dye contents. In a general way this study evidences the positive effect of the utilization of natural dyes together with friendly polyurethane aqueous dispersions for the production of innovative coatings.
    2020Dissertação de mestrado Acesso aberto Ver mais