Effect of Phenolation, Lignin-Type and Degree of Substitution on the Properties of Lignin-Modified Phenol-Formaldehyde Impregnation Resins: Molecular Weight Distribution, Wetting Behavior, Rheological Properties and Thermal Curing Profiles

Here,the effects of substituting portions of fossil-based phenol in phenol formaldehyde resin by renewable lignin from two different sources are investigated using a factorial screening experimental design.Among the resins consumed by the wood-based industry,phenolics are one of the most important types used for impregnation,coating or gluing purposes.They are prepared by condensing phenol with formaldehyde(PF).One major use of PF is as matrix polymer for decorative laminates in exterior cladding and wet-room applications.Important requirements for such PFs are favorable flow properties(low viscosity),rapid curing behavior(high reactivity)and sufficient self-adhesion capacity(high residual curing potential).Partially substituting phenol in PF with bio-based phenolic co-reagents like lignin modifies the physicochemical properties of the resulting resin.In this study,phenol-formaldehyde formulations were synthesized where either 30%or 50%(in weight)of the phenol monomer were substituted by either sodium lignosulfonate or Kraft lignin.The effect of modifying the lignin material by phenolation before incorporation into the resin synthesis was also investigated.The resins so obtained were characterized by Fourier Transform Infra-Red(FTIR)spectroscopy,Size Exclusion Chromatography(SEC),Differential Scanning Calorimetry(DSC),rheology,and measurements of contact angle and surface tension using the Wilhelmy plate method and drop shape analysis.

Impregnated Paper-Based Decorative Laminates Prepared from Lignin-Substituted Phenolic Resins

High Pressure Laminates(HPL)panels consist of stacks of self-gluing paper sheets soaked with phenol-formaldehyde(PF)resins.An important requirement for such PFs is that they must rapidly penetrate and saturate the paper pores.Partially substituting phenol with bio-based phenolic chemicals like lignin changes the physico-chemical properties of the resin and affects its ability to penetrate the paper.In this study,PF formulations containing different proportions of lignosulfonate and kraft lignin were used to prepare paper-based laminates.The penetration of a Kraft paper sheet was characterized by a recently introduced,new device measuring the conductivity between both sides of the paper sheet after a drop of resin was placed on the surface and allowed to penetrate the sheet.The main target value measured was the time required for a specific resin to completely penetrate the defined paper sample(“penetration time”).This penetration time generally depends on the molecular weight distribution,the flow behavior and the polarity of the resin which in turn are dependent on the manufacturing conditions of the resin.In the present study,the influences of the three process factors:(1)type of lignin material used for substitution,(2)lignin modification by phenolation and(3)degree of phenol substitution on the penetration times of various lignin-phenolic hybrid impregnation resins were studied using a complete twolevel three-factorial experimental design.Thin laminates made with the resins diluted in methanol were mechanically tested in terms of tensile and flexural strains,and their cross-sections were studied by light microscopy.