Development of MOF-Pigment Hybrids to Implement a Sustainable Coloration of Poly(ε-caprolactone) Composites

A sustainable strategy to improve aesthetic and mechanical properties of poly-(ε-caprolactone) (PCL) is proposed relying on the use of MIL53(Al) metal organic framework (MOF) as filler and hosting matrix for natural dyes extracted from logwood (LW). Three purple pigments are prepared by combining the aqueous extract of LW with MOF, previously prepared and differently dried with three methods to optimize the dye load. The three pigments are obtained with high reaction mass efficiency (RME) values (44–66%). According to the optical and structural characteristics, the color is caused by hematein-aluminum chelates, which are formed when logwood dye molecules interact with the bulk or surface unsaturated aluminum centers of the MOF, depending on how much the matrix has dried out. The pigments are combined in different amounts (1, 3, and 5 wt %) with PCL effectively imparting purple-violet shades (ΔE* ≈ 32–55), while also enhancing its mechanical and thermal characteristics. Tensile characterization results for PCL composites verify the contribution of nanostructured fillers in enhancing stiffness and strength up to 3% by weight, with only a little reduction in ductility. Variable color and tensile qualities are sustainably and effectively given to the PCL polymer by using MOF-LW fillers.