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

Abstract

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.