Danny E. García

Modification of condensed tannins: from polyphenol chemistry to materials engineering

Condensed tannins (CTs) are high molar mass polyphenolic bio-polymers based on flavonol units. CTs have been well-studied due to their respective biological activity. However, the application of CTs in several areas is limited because of their physicochemical properties. The objective of this review is to investigate the state of the art regarding the chemical modification of CTs, and to outline recent and potential applications of tannin derivatives. An overview of the most important reactions is given, and a comprehensive summary of the experimental parameters for modification (chemicals, time, temperature, solvent type, and yield) is presented. The impact of the modification on the physicochemical properties of derivatives in comparison to native tannin behavior is discussed. Finally, the applicability of modified or unmodified CTs is described, referring to academic articles and patents. Future research in terms of modification reaction type, as well as derivatizing agents, is discussed.

Synthesis and physicochemical properties of hydroxypropyl tannins from maritime pine bark (Pinus pinaster Ait.)

Abstract Simple derivatives of condensed tannins are described, which allow the tailoring of gelation (with aldehydes) and thermal degradation behaviors that have been obstacles to the commercial utilization of this abundant bioresource. Hydroxypropyl tannin (HPT) derivatives from bark of Pinus pinaster with varying degrees of substitution (DS 0.1–4.7) were analyzed with regard to molecular and physical characteristics, such as structure, molecular weight, thermal behavior, solubility and gelation. The modification reaction of tannin proceeds stoichiometrically to a mixture of oligomeric derivatives, mostly to trimers and tetramers, as demonstrated by 1H-nuclear magnetic resonance, elemental analysis, and gel permeation chromatography. The derivatives have an improved solubility in organic solvents and increased thermal stability. The glass transition temperature (Tg) was reduced by up to 37°C over unmodified tannin. At DS ≥1.0, the HPT lost the capacity to produce gel with formaldehyde. A structure-property relationship was established by means of multivariate analysis, and the derivatives could be grouped according to their physicochemical profile. HPTs are promising biomacromolecular building blocks for tannin-derived materials.

Condensed Tannin-Based Polyurethane as Functional Modifier of PLA-Composites

ABSTRACT Novel polylactic acid-based composites were prepared for the first time by melt-blending urethanization using bark polyflavonoids, and polymeric methyl diisocyanate. Rheological, morphological, structural, thermal, and mechanical properties were studied. Polymerization between polyflavonoids and isocyanate during the melt-blending was demonstrated by spectroscopy, confocal microscopy, and thermal analysis. Polyflavonoids improved the blend processability and affected the polylactic acid-crystallization, blend-urethanization, and flexural features. Polyphenols act as nucleating agent, and the effect was highly influenced by the polymeric methyl diisocyanate-charge, and the polyflavonoids-content. Low polymeric methyl diisocyanate-loading (10 wt%) degrades the elasticity modulus (E), while the highest polymeric (20 wt%) improved the miscibility between the polylactic acid polymer-matrix and the polyflavonoids. GRAPHICAL ABSTRACT