Green Synthesis Of Lignin-Based Phenolic Resin

As one of the three major thermosetting resins,phenolic resin has the advantages of heat resistance,flame resistance,and good chemical stability.However,the raw materials of phenol and formaldehyde used in the production of phenolic resin are highly toxic,and phenol is derived from masonry products and is non-renewable.Biomass-based polymer lignin has both phenolic and alcoholic hydroxyl groups,which has the potential to replace phenol to generate bio-based phenolic resin.However,the large molecular weight,complex structure,and large steric hindrance of natural lignin make it difficult to carry out polymerization.It needs to be modified or depolymerized to replace phenol in the synthesis of phenolic resin.In this work,we first used Ru/C as the catalyst and ethylene glycol as the polymerization inhibitor to depolymerize lignin to obtain a phenolic oil precursor.GPC showed that the number average molecular weight of lignin decreased from1776 to 588 after depolymerization.UV-vis characterization showed that the content of free phenolic hydroxyl groups increased from 0.0524 mmol·g^-1to 0.6299 mmol·g^-1after lignin depolymerization,The results of ¹H-NMR and ¹³C-NMR showed that part of the ether bonds in the lignin structure were broken,the content of phenolic hydroxyl groups increased,and the more increased monomer content were guaiacyl and syringyl.Subsequently,lignin-phenolic oil prepared by hydrogenolysis was used to partially replace(20-50%)phenol to prepare thermoplastic novolac resin.The important properties of phenolic resin,such as softening point,free phenol content,polymerization speed,etc.,all meet the index requirements of industrial products.Using infrared spectroscopy,proton nuclear magnetic resonance spectroscopy,carbon nuclear magnetic resonance spectroscopy,thermogravimetric analysis,differential scanning calorimetry and other methods to analyze the structure and properties of ordinary thermoplastic phenolic resin(PF)and lignin-based phenolic resin(DLPF),etc.A comparative analysis was carried out and the possible formation mechanism of DLPF was proposed.The performance test of 50%-DLPF phenolic molding compound(the substitution rate of lignin-phenolic oil is 50%)shows that its important properties such as bending strength of 101.5 MPa and heat distortion temperature of 208.5°C are much higher than ordinary phenolic resin molds.83.3 MPa and 140.2°C of plastic.In summary,lignin-based phenolic resin is a class of bio-based polymer resin materials with excellent performance.