A comparison between lignin modified by ionic liquids and glyoxalated lignin as modifiers of urea-formaldehyde resin
Abstract
The aim of this research was to compare the influence of modified
lignin by ionic liquid (IL) on the physical and mechanical properties
of wood-based panels bonded with urea-formaldehyde (UF)
resin with the effect of glyoxalated lignin (GL) on UF properties.
For this purpose, soda bagasse lignin was respectively modified by
1-ethyl-3-methylimidazolium acetate ([Emim][OAc]) IL and glyoxal
and then the various content of modified lignins (10, 15, and 20%)
were added at pH=7 during the UF resin synthesis instead of the
second urea . The changes in the structure and thermal properties
of lignin, after and before modification with glyoxal and IL, were
analyzed by Fourier transform infrared spectrometry (FTIR) and
differential scanning calorimetry (DSC). The physicochemical
properties of the prepared resins as well as the water absorption,
shear strength, and formaldehyde emission of the plywood panels
made with these adhesives were measured according to standard
methods. According to the FTIR spectra, the content of C=O bond
increased in GL while in the IL-treated lignin the content of C–N
bond markedly increased. DSC analysis indicated that lignin
modified by IL had lower glass transition temperature (Tg) value
compared to those modified with glyoxal and unmodified lignin,
respectively. The UF resins containing IL-treated lignin exhibit a
faster gel time compared to those prepared with GL. Equally, the
plywood panels prepared with an IL had lower formaldehyde
emission and higher mechanical strength compared to those
made from UF resin containing GL. There were no significant
differences in dimensional stability of the panels bonded with UFs
modified with GL and those with IL-modified lignin