Degradation mechanism of Acacia mangium tannin in NaOH/urea aqueous solution and application of degradation products in phenolic adhesives

Abstract Tannin-modified phenolic resin has the characteristics of a high molecular weight, high space resistance, high viscosity, and short shelf life. Therefore, it is still very difficult to use concentrated tannins without pretreatment. To utilize the benefits of condensed tannins, the depolymerization of condensed tannins was carried out in a mixed aqueous solution of sodium hydroxide and urea. The effects of different NaOH/urea ratios on depolymerization were investigated. It was found that tannin treatment was optimized when NaOH/urea/water = 12:7:81 (0.2 mol:0.175 mol:4.5 mol). Gel permeation chromatography (GPC) showed that the weight-average molecular weight of the condensed tannins decreased from 1725 Da and 704 Da to 1523 Da and 327 Da. The changes in the molecular structure of the depolymerized products were detected by MALDI-TOF-MS, 13C nuclear magnetic resonance, and infrared spectroscopy. The results showed that the average degree of polymerization of tannin decreased from 7 to 2. The decrease in the degree of polymerization was mainly due to the breaking of C-4 and C-8 chemical bonds. Some ether bonds were broken, which reduced the molecular weight of the tannin and gave it a more uniform distribution in water. In addition, tannin-modified phenolic resin (DTPF) was prepared by the copolycondensation of phenol, tannin depolymerization products, and formaldehyde under alkaline conditions. The molecular structure and properties of DTPF were studied by 13C-NMR, FTIR, DSC, and TG. The results revealed that, compared with untreated tannin-modified phenolic resin (TPF), DTPF resin had lower formaldehyde emission (0.19 mg/L), better thermal stability, and higher bonding strength (1.19 MPa), according to GB/T 17657-2013. It is important to note that this inexpensive and environmentally friendly catalyst can be used as a raw material for the synthesis process; the degradation products do not need to be recovered and purified, thus avoiding the dehydration and purification of the waste liquor, which is safer for the environment.