Reduction of formaldehyde emission from urea-formaldehyde resins with nanolignin modified by deep eutectic solvent

The aim of the present work was to decrease formaldehyde emission and improve the dimensional stability of the wood-based panels bonded with urea-formaldehyde resin modified by deep eutectic solvent (DES) nanolignin. For this reason a deep eutectic solvent composed of Choline Chloride–ZnCl2 (ChCl-ZnCl2) was employed to pretreat nanolignin. The DES- modified and unmodified nanolignin were used to replace a part of urea (10%, 20%, and 30%) to prepare the lignin-urea- formaldehyde (LUF) wood adhesive. The changes in curing temperature and chemical structure of the LUF resin were determined by Differential Scanning Calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR), respectively. The physico-chemical analysis showed that the UF resin with DES-modified nanolignin had higher solid content, viscosity and faster gel time than those using unmodified nanolignin. FTIR analysis indicated that the methoxy groups decreased and the phenolic hydroxyl groups increased by DES nanolignin modification. The DES-treated nanolignin LUF resin showed a lower peak temperature than those prepared using unmodified nanolignin. Moreover, the panels made from modified nanolignin presented a better mechanical strength (internal bond (IB) strength, flexural strength and flexural modulus) and dimensional stability as well as lower formaldehyde emission than those with unmodified nanolignin. The control UF resin presented a faster gelation time and higher viscosity than both unmodified and modified LUF resins. The mechanical strength of the particleboard bonded with the control UF resin was also better than the panels bonded with both modified and unmodified LUF resins. It should nonetheless be noted that the IB and flexural strengths of the panel bonded with a UF resin with 10 wt% DES-modified nanolignin were comparable to those bonded with the control UF resin. Based on these results, increasing the substitution degree of urea with DES-modified nanolignin significantly improved the dimensional stability and the formaldehyde emission of the particleboards bonded with a UF resin.