Abstract
Tannins are soluble, astringent secondary phenolic metabolites generally obtained from renewable natural resources, and can be found in many plant parts, such as fruits, stems, leaves, seeds, roots, buds, and tree barks, where they have a protective function against bacterial, fungal, and insect attacks. In general, tannins can be extracted using hot water or organic solvents from the bark, leaves, and stems of plants. Industrially, tannins are applied to produce adhesives, wood coatings, and other applications in the wood and polymer industries. In addition, tannins can also be used as a renewable and environmentally friendly material to manufacture bio-based polyurethanes (bio-PUs) to reduce or eliminate the toxicity of isocyanates used in their manufacture. Tannin-based bio-PUs can improve the mechanical and thermal properties of polymers used in the automotive, wood, and construction industries. The various uses of tannins need to be put into perspective with regards to possible further advances and future potential for value-added applications. Tannins are employed in a wide range of industrial applications, including the production of leather and wood adhesives, accounting for almost 90% of the global commercial tannin output. The shortage of natural resources, as well as the growing environmental concerns related to the reduction of harmful emissions of formaldehyde or isocyanates used in the production of polyurethanes, have driven the industrial and academic interest towards the development of tannin-based bio-PUs as sustainable alternative materials with satisfactory characteristics. The aim of the present review is to comprehensively summarize the current state of research in the field of development, characterization, and application of tannin-derived, bio-based polyurethane resins. The successful synthesis process of the tannin-based bio-PUs was characterized by Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), MALDI-TOF mass spectrometry, and gel permeation chromatography (GPC) analyses.
Highlights
Tannins are naturally occurring phenolic compounds consisting of poly-hydroxy polyphenols that are found in various plant parts, including fruits, wood, leaves, and bark
A reaction analogous to the reaction of the CH2 OH group, which is created by the interaction of formaldehyde with tannins, as well as the synthetic amino and phenolic resins, occurs when glyoxal reacts with tannins and with isocyanates in urethane groups [91]
The results demonstrated that a tannin-derived nonisocyanate polyurethane (NIPU) was formed during the manufacture of the tannin-based NIPU resin [102,112], and the peaks of NIPU that related with oligomers were 433.2 Da, 439.2 Da, 451.8 Da, and 556.3 Da
Summary
Tannins are naturally occurring phenolic compounds consisting of poly-hydroxy polyphenols that are found in various plant parts, including fruits, wood, leaves, and bark. It has been mentioned that the pharmacological properties of derivative phlorotannins have been masked by acetylation and methylation [16] Both hydrolysable and condensed tannins include reactive OH groups, which constitute protein complexes such as enzymes and polymers; e.g., cellulose and hemicelluloses [17]. In another study, condensed tannins were extracted from radiata pine (Pinus radiata) barks to produce polyurethane foams These bio-based foams exhibited improved thermal stability and strength, and demonstrated their effectiveness in the construction process [39]. Polyurethane foams with improved thermal and mechanical stability are being pursued in industrial applications, especially in the construction and building industries [50] This aim of this review was to comprehensively summarize the current state of research in the field of development, characterization, and application of tannin-derived, bio-based polyurethane resins
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