Lignin Phenol Formaldehyde Resins Synthesised Using South African Spent Pulping Liquor

The current study investigated to which extent phenol could be replaced by lignins to produce lignin phenol formaldehyde (LPF) resins, utilising soda lignin and sodium lignosulphonate as by-products from the South African pulping industry. The lignins were characterised and soda lignin indicated the highest reactivity. It was therefore utilised to produce LPF resins at 60%, 80%, and 100% phenol substitution, using central composite designs to maximise the adhesive strength. A one-pot method allowing direct transition from phenolation to resin synthesis was used for the first time with a pulping lignin at 60% and 80% substitution. Plywood made with LPF60, LPF80, and LPF100 resins attained their highest shear strengths of 0.786, 1.09, and 0.987 MPa, respectively, which adhered to the GB/T 14,732–2013 standard (≥ 0.7 MPa). A substitution level of 68% produced the highest shear strength of 1.11 MPa. High-density particleboard made with this LPF68 resin gave a MOR and MOE of 40 and 3209 MPa, respectively, adhering to the ANSI A208.1 requirements. Thickness swelling and water absorption was 13.5% and 37.2%, respectively. The soda-lignin isolated by precipitation from sugarcane bagasse pulping liquor is the first industrial lignin shown to produce LPF100 resins adhering to standard requirements, without modification or additives.