Novel Preparation and Characterization of Resol Resin with Phenolated Kraft Lignin

The application of lignin as a sustainable replacement for phenol in resin is one of the main priorities in the polymer industry. The partial substitution of phenol by Kraft lignin in the synthesis of resole resin, a mineral and glass wool insulation binder, was thus investigated. Lignin was activated by phenolation in an alkaline medium at low temperature, followed by reaction with formaldehyde in the same batch. Conducting the phenolation reaction in an alkaline medium allows the synthesis of resol resin to continue without interrupting the process, while the low temperature ensures the low viscosity of the synthesized resol, which is a prerequisite for its use as a binder in the manufacture of thermal insulation products. This is an important innovation that streamlines the production of modified resol. Activated lignin and resole resins were characterized by Fourier transform infrared spectroscopy (FTIR) and gel permeation chromatography (GPC). Phenolation occurs mainly via the binding of phenol to the lignin macromolecule, increasing the molecular weight of activated lignin, with only small amounts of low molecular weight species observed. Resol resins with and without incorporated lignin have identical FTIR spectra and similar molecular weight distributions, which confirms the successful synthesis of lignin-containing resin. With essential relevance for the undisturbed production of thermal insulation products, the most suitable of the resins synthesized with lignin has appropriate viscosity, double the stability of the reference product, and half the amount of tetradimer (tetradimer can cause problems due to precipitation). In addition, this resin results in significantly lower emissions and has increased flexural strength. The synthesis is transferable to industrial practice.