Curing Kinetics of Tannin and Lignin Biobased Adhesives Determined by DSC and ABES

The curing process of two biobased adhesives: pine tanninhexamine (TH) and organosolv lignin non-isocyanate polyurethane (NIPU), suitable for interior nonstructural use, were compared with commercial urea-formaldehyde (UF) adhesive. Changes in chemical structure before and after the curing process were observed with Fouriertransform infrared spectroscopy (FTIR). The process of adhesive curing was monitored with differential scanning calorimetry (DSC) and the automated bonding evaluation system (ABES). Both DSC and ABES measurements confirmed UF as the fastest and NIPU as the slowest curing adhesive observed. Taking into account the ABES results, the optimal pressing parameters for the TH adhesive would be 4 min at 175°C, for the NIPU adhesive 7 min at 200°C and for the UF 1.5 min at 100°C. Strong linear correlation was observed between mechanical and chemical curing for the UF and NIPU adhesives, whereas lower correlation was observed for the TH adhesive. At all observed adhesives, the DSC measurements were underestimating the curing process determined by ABES in the first part and overestimating it at the end. The underestimation was the most evident with the TH adhesive and the less with the UF adhesive. When comparing the uncured and cured FTIR spectra of all three types of adhesives, a drastic decrease in the characteristic band of -OH groups at 3330–3400 cm−1 and an increase in the signal intensity at 2920 cm−1 of aliphatic -CH2-groups were observed. For the UF adhesive, the C=O stretching frequency has shifted from 1632 cm−1 for uncured to three different bands at 1766, 1701, and 1655 cm−1 for cured UF. The sharp band for phenolic alcohols at 1236 cm−1 of C–O stretch and hydroxyl O–H functional group at 1009 cm−1 and at 684 cm−1 of uncured TH adhesive diminished during curing, which indicates that a crosslinking reaction occurs via -OH groups. The peak of the C=O group of urethane bridges at 1697 cm−1 for uncured NIPU shifted to lower wavenumber at 1633 cm−1 for cured NIPU.