The Use of South African Spent Pulping Liquor to Synthesize Lignin Phenol-Formaldehyde Resins

Abstract

Abstract This study aims to investigate the potential of using lignin sourced from South African black liquor as a total phenol substitute in phenol-formaldehyde resins (PFRs), with a particular focus on bonding strength and curing properties. Four South African pulping-based lignins were used to synthesize these lignin-phenol formaldehyde resins (LPF100 resins), namely Eucalyptus Kraft lignin, Pine Kraft lignin, Bagasse Soda lignin, and Bagasse Steam Exploded lignin. Fourier-transform infrared spectroscopy, thermogravimetric analysis, and differential scanning calorimetry were used to determine structural and curing properties. These resins were then used directly (unmodified) as adhesives to test shear bonding strength (R0 LPF100 adhesives). To improve the bonding properties of the unmodified LPF100 adhesives, the LPF100 resins were modified via the addition of a crosslinker (hexamine) as well as a hardener (either glyoxal, R1, or epichlorohydrin, R2). All R0 LPF100 adhesives fell below the GB/T 17657-2013 plywood standard of ≥0.7 MPa, with the Bagasse Soda LPF100 adhesive recording the highest bonding performance of 0.5 MPa, and the lowest curing temperature of 68°C. From the modified adhesives, the best performing were the Pine Kraft (R1) and the Eucalyptus Kraft (R2) LPF100 adhesives, recording 1.4 and 1.3 MPa, respectively. The curing temperatures of both these resins were 71°C and 80°C, respectively. Ultimately, the results of this study indicated that favorable adhesive properties may be obtained with the use of South African pulping-based lignins as a 100 percent phenol substitute in PFRs.