Abstract
Similar to the 3,4-dihydroxy-L-phenylalanine (DOPA) structures in mussel adhesive proteins (MAPs), tannin contains abundant similar catechol structures in the flavonoid units, which might be developed into high-performance adhesives. In this study, Acacia mangium tannin (AMT) was depolymerized under acidic conditions with 2-methylfuran as a nucleophilic reagent. The depolymerized tannin (DAMT) was combined with polyethyleneimine (PEI) to prepare tannin-based phenolic resins to mimic the MAP systems. The investigation of the chemical structures of tannin indicated the successful reaction of 2-methylfuran onto the flavonoid units to increase the chemical reactivity, as evidenced by the MALDI-TOF-MS, GPC, 1H NMR, and FT-IR analyses. The as-prepared PEI and DAMT-modified phenolic (DTPF-PEI) resin showed a high cross-linking and three-dimensional network, resulting from the high reactivity of DAMT. Complex reactions were observed during the curing of the as-prepared resin, including Michael addition and/or Schiff base formation reactions between the amino groups of PEI and the ortho-quinone of tannins, as well as the condensation reactions between PEI and the phenolic resins. Compared with tannin-phenol-formaldehyde (TPF) resin, the gel time of the DTPF-PEI resin decreased from 630 s to 204 s (decreased by 67.6%), the mass loss after hydrolysis decreased by 26.3%, the bonding strength increased by 63.6% when cured at 135 °C, and the formaldehyde emission decreased by 64.4%. Such high performance might be contributed by the formation of highly cross-linked networks, as well as the hydrogen bonding and pi-cation interactions between the catechol structures and the adhered surfaces. With its outstanding performance, including fast curing rate, excellent adhesion properties, low formaldehyde emission, and high thermal stability, this newly developed adhesive has the potential to be used as an alternative of conventional phenol-formaldehyde resins.
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